U.S. patent application number 15/234027 was filed with the patent office on 2017-03-09 for drug-delivering composite structures.
This patent application is currently assigned to Ramot at Tel-Aviv University Ltd.. The applicant listed for this patent is Ramot at Tel-Aviv University Ltd.. Invention is credited to Meital ZILBERMAN.
Application Number | 20170065519 15/234027 |
Document ID | / |
Family ID | 37781811 |
Filed Date | 2017-03-09 |
United States Patent
Application |
20170065519 |
Kind Code |
A1 |
ZILBERMAN; Meital |
March 9, 2017 |
DRUG-DELIVERING COMPOSITE STRUCTURES
Abstract
Composite structures composed of a fibril core and a polymeric
coat and designed capable of encapsulating both hydrophobic and
hydrophilic bioactive agents while retaining the activity of these
agents are disclosed. Further disclosed are processes of preparing
such composite structures, and medical devices and disposable
articles made therefrom.
Inventors: |
ZILBERMAN; Meital;
(Tel-Aviv, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ramot at Tel-Aviv University Ltd. |
Tel-Aviv |
|
IL |
|
|
Assignee: |
Ramot at Tel-Aviv University
Ltd.
Tel-Aviv
IL
|
Family ID: |
37781811 |
Appl. No.: |
15/234027 |
Filed: |
August 11, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11634910 |
Dec 7, 2006 |
9446226 |
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15234027 |
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60831200 |
Jul 17, 2006 |
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60742869 |
Dec 7, 2005 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
Y10T 442/2525 20150401;
C12Y 111/01007 20130101; A61K 9/70 20130101; A61K 9/0092 20130101;
A61K 9/0024 20130101; A61K 31/337 20130101; D06M 16/00 20130101;
A61M 31/002 20130101; A61K 47/34 20130101; A61K 38/44 20130101;
A61K 9/19 20130101 |
International
Class: |
A61K 9/00 20060101
A61K009/00; A61K 38/44 20060101 A61K038/44; A61K 31/337 20060101
A61K031/337; A61K 47/34 20060101 A61K047/34; A61M 31/00 20060101
A61M031/00; A61K 9/19 20060101 A61K009/19 |
Claims
1. An article-of-manufacture comprising a structural element core
and a polymeric coat coating at least a part of said structural
element core, said coat comprises at least one bioactive agent
releasably encapsulated therein, wherein said coat has a
microstructure of a freeze-dried water-in-oil emulsion, said
emulsion comprises, prior to freeze-drying, a dispersed aqueous
solution and a continuous organic solution, said organic solution
containing at least one polymer and said aqueous solution or said
organic solution containing said bioactive agent; and wherein a
plurality of droplets of said dispersed aqueous solution freeze-dry
to form microscopic capsules in a solid form of said continuous
organic solution, in a form of a plurality of pores randomly
dispersed within said polymeric porous coat.
2. The article of claim 1, wherein said bioactive agent is
dissolved or dispersed in said aqueous solution or said organic
solution, while retaining an activity thereof during said
freeze-drying.
3. The article of claim 2, wherein said bioactive agent is
releasably encapsulated in said solid form of said continuous
organic solution.
4. The article of claim 2, wherein said bioactive agent is
releasably encapsulated in said pores.
5. The article of claim 1, wherein said polymeric coat is
characterized by an average pore diameter that ranges from about 1
nm to about 1 mm.
6. The article of claim 1, wherein said polymeric coat is
characterized by an average pore diameter that ranges from 1 nm to
1 mm.
7. The article of claim 1, wherein a thickness of said polymeric
coat ranges from about 1 .mu.m to about 2000 .mu.m.
8. The article of claim 1, wherein said at least one polymer is a
biodegradable polymer.
9. The article of claim 8, wherein said biodegradable polymer is
selected from the group consisting of an aliphatic polyester,
poly(glycolic acid), poly(lactic acid), polydioxanone (PDS),
poly(alkylene succinate), poly(hydroxybutyrate), poly(butylene
diglycolate), poly(epsilon-caprolactone) and any co-polymer, blend
and a mixture thereof.
10. The article of claim 9, wherein said aliphatic polyesters is
selected from the group consisting of poly(L-lactic acid),
poly(glycolic acid) and poly(lactic-co-glycolic acid).
11. The article of claim 1, wherein structural element core is
selected from the group consisting of a needle, a tube, a wire, a
thread, a screw, a pin, a tack, a rod and a plate.
12. The article of claim 1, wherein said structural element core is
a medical device.
13. The article of claim 12, wherein medical device is selected
from the group consisting of a needle, a tube, a wire, a thread, a
screw, a pin, a tack, a rod, a plate, a suture, a suture anchor, an
anastomosis clip or plug, a dental implant or device, an aortic
aneurysm graft device, an atrioventricular shunt, a catheter, a
heart valve, a hemodialysis catheter, a bone-fracture healing
device, a bone replacement device, a joint replacement device, a
tissue regeneration device, a hemodialysis graft, an indwelling
arterial catheter, an indwelling venous catheter, a pacemaker, a
pacemaker lead, a patent foramen ovale septal closure device, a
vascular stent, a tracheal stent, an esophageal stent, a urethral
stent, a rectal stent, a stent graft, a synthetic vascular graft, a
vascular aneurysm occluder, a vascular clip, a vascular prosthetic
filter, a vascular sheath and a drug delivery port and a venous
valve.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/634,910, filed on Dec. 7, 2006, which
claims the benefit of priority from U.S. Provisional Patent
Application No. 60/742,869 filed on Dec. 7, 2005, and 60/831,200
filed on Jul. 17, 2006.
[0002] The contents of the above applications are all incorporated
by reference as if fully set forth herein in their entirety.
FIELD AND BACKGROUND OF THE INVENTION
[0003] The present invention relates to the field of material
science and, more particularly, to novel composite structures which
can be used for delivering therapeutic agents.
[0004] Organ and tissue failure or loss is one of the most frequent
and devastating problems still challenging human health care.
Tissue regeneration is a new discipline where living cells, being,
for example, autologous, allogenic, or xenogenic cells, are used to
replace cells lost as a result of injury, disease or birth defect
in a living subject.
[0005] Tissue regeneration typically involves the preparation of
delicate polymeric structures that serve as biodegradable scaffolds
incorporating bioactive molecules and/or cells. Such biodegradable
scaffolds are often further utilized for in vitro studies of
tissues, cells, bioactive agents and the interactions
therebetween.
[0006] An efficient scaffold for tissue regeneration is typically
made of biodegradable structural elements, preferably fibers, in
which biologically active molecules can be incorporated and be
controllably released over time.
[0007] Fibrillar biodegradable scaffolds are ideal particularly
when thin, delicate structures are needed, for example in nerve
regeneration applications. They can also be used to build implants
and other medical devices that combine drug release with other
functions, such as mechanical support for a regenerating tissue or
as stents.
[0008] Polymeric scaffolds that are presently used in tissue
regeneration and other applications are preferably biodegradable,
meaning that over time the polymer breaks down chemically,
metabolically (by biological processes such as hydrolysis or
enzymatic digestion) and/or mechanically.
[0009] Biodegradable structural elements, such as fibers, have been
known and used for many years in many applications such as, for
example, fishing materials, for example, fishing lines and fish
nets; agricultural materials, for example, insect or bird nets and
vegetation nets; cloth fibers and non-woven fibers for articles for
everyday life, for example, disposable women's sanitary items,
masks, wet tissues (wipes), underwear, towels, handkerchiefs,
kitchen towels and diapers; and medical supplies, for example,
operating sutures which are not removed, operating nets and
suture-reinforcing materials. The biodegradability of these
elements renders them highly suitable for constructing medical
devices as well as environmental-friendly products. Ample
description of biodegradable fibers can be found, for example, in
U.S. Pat. Nos. 6,045,908, 6,420,027, 6,441,267, 6,645,622 and
6,596,296.
[0010] Biodegradable fibers are typically produced by conventional
methods such as, for example, solution spinning, electro-spinning
and/or melt-spinning techniques. These fibers are typically made
from a single polymer or a co-polymer or from a blend of polymers
such as, for example, poly(glycolic acid), poly(L-lactic acid),
poly(DL-lactic acid), poly(glycolic-co-lactic acid),
poly(3-hydroxybutyric acid), polycaprolactone, polyanhydride,
chitin, chitosan, sulfonated chitosan, various natural and
derivatized polysaccharide polymers, natural polymers or
polypeptides such as reconstituted collagen or spider silk, as well
as other various aliphatic polyesters consisting of a dibasic acid
and a diol.
[0011] Since non-toxicity is an inherent prerequisite for
biodegradable polymers that are designed for clinical applications,
the starting materials, the final product and the optional
break-down products must be non-toxic and benign. Thus, for
example, degradation of a biodegradable polyester, such as
poly(lactic acid) or poly(glycolic acid), involves a hydrolytic
cleavage which results in carbon dioxide and water as non-toxic and
benign end products.
[0012] The total degradation time of biodegradable polymers can
vary from several days to several years, depending mainly on the
chemical structure of the polymer chains, and physical properties
such density, surface area and size of the polymer. During the
degradation process a controllable release of biological agents
that are attached thereon and/or encapsulated therein can be
effected. Table A below presents the typical degradation time
required for complete loss of mass (in time units of months) of
some commonly used biodegradable polymers.
TABLE-US-00001 TABLE A Degradation time to complete mass loss. Rate
also depends on Polymer part geometry (months) PGA 6 to 12 PLLA
>24 PDLLA 12 to 16 PCL >24 PDO 6 to 12 PGA-TMC 6 to 12 85/15
PDLGA 5 to 6 75/25 PDLGA 4 to 5 65/35 PDLGA 3 to 4 50/50 PDLGA 1 to
2 PGA abbreviates polyglycolide; PLLA abbreviates poly(l-lactide);
PDLLA abbreviates poly(dl-lactide); PDO abbreviates
poly(dioxanone); PGA-TMC abbreviates poly(glycolide-co-trimethylene
carbonate); and PDLGA abbreviates
poly(dl-lactide-co-glycolide).
[0013] When used in clinical applications, the biodegradable
polymer composing a scaffold is selected according to its
properties. Thus, for example, semi-crystalline polymers such as
poly(L-lactic acid) (PLLA) can be used in implants that require
good mechanical properties such as sutures, devices for orthopedic
and cardiovascular surgery, and stents. Amorphous polymers, on the
other hand, such as poly(DL-lactic-co glycolic acid) (PDLGA), are
attractive in drug release applications, where it is important to
have homogenous dispersion of the active species within the
monophasic matrix. The degradation rate of these polymers is
determined by the initial molecular weight, the exposed surface
area, the polymer's degree of crystallinity and (in the case of
co-polymers) quantitative ratio of the two co-monomers.
[0014] Presently known fibrillar scaffolds for, for example, tissue
regeneration are composed of biodegradable fibers that build bulky,
"spaghetti-like" structures, whereby biologically active agents are
trapped in the voids between adjacent fibers. Typically the
scaffold is first prepared and then the biologically active agents
are introduced. Since the bioactive agents are not incorporated
into the biodegradable fibers but are practically soaked into the
fiber-made scaffold, these drug delivery forms display relatively
uncontrolled drug release profiles, a feature that is oftentimes
antithetical to the goal of drug delivery.
[0015] The currently followed paradigm which provides partial
solution to the abovementioned limitations is the use of
drug-loaded fibers, wherein the bioactive agent is incorporated
into the fibers which are used as basic building-blocks of
drug-delivering scaffolds and vehicles.
[0016] The present main obstacle to successful incorporation in and
delivery from biodegradable structures and scaffolds is the
inactivation of bioactive molecules by the exposure to high
temperatures or harsh chemical environments during the production
of the drug-loaded fibers [Thomson, R. C., et al., "Polymer
scaffold processing", in: Lanza R P, Langer R, Vacanti J, editors.
Principles of Tissue Engineering, New York: Academic Press; 2000.
pp. 251-262].
[0017] Nevertheless, few controlled-release fiber systems based on
biodegradable polymers and incorporating bioactive molecules have
been investigated to date. The two basic types of such drug-loaded
fibers are monolithic fibers and reservoir fibers.
[0018] In systems that use monolithic fibers the drug is dissolved
or dispersed throughout the polymer fiber. For example, organic
(hydrophobic) drugs such as curcumin, paclitaxel and dexamethasone
have been melt spun with poly(L-lactic acid) (PLLA) to generate
drug-loaded fibers [Su, S. H., et al., Circulation, 2001, 104: II,
pp. 500-507] and water-soluble (hydrophilic) drugs have been
solution spun with PLLA [Alikacem, N., et al., Invest. Ophthalmol.
Vis. Sci., 2000, 41, pp. 1561-1569]. Various steroid-loaded fiber
systems have demonstrated the expected first order release kinetics
[Dunn, R. L., et al., "Fibrous polymer for the delivery of
contraceptive steroids to the female reproductive track", in Lewis
DH, editor, "Controlled Release of Pesticides and Pharmaceuticals",
New York: Plenum Press, 1981, p. 125-146]. A recently published
work have demonstrated the encapsulation of a limited amount of
partially active (after release) human .beta.-nerve growth factor
(NGF), which was stabilized by a carrier protein, bovine serum
albumin (BSA), in a copolymer of .epsilon.-caprolactone and ethyl
ethylene phosphate (PCLEEP) produced by electro-spinning [Sing, Y.
C. et al., Biomacromolecules, 2005, 6 (4), pp. 2017-2024].
[0019] U.S. Pat. Nos. 6,485,737, 6,596,296 and 6,858,222, U.S.
patent application having the Publication No. 20050106211 and WO
01/10421 teach the fabrication and use of drug-releasing
biodegradable monolithic fibers. The fibers are made by mixing the
bioactive agent in a polymeric solution which in turn is converted
into fibers by extruding the mixture into a coagulating bath. These
fibers are ultimately limited in the mechanical properties as
compared to fibers which are made of similar polymers without the
bioactive agent, and limited in the type of bioactive agents which
can undergo and survive this particular production process.
[0020] The use of monolithic fibers in drug delivery systems thus
suffers several drawbacks including, for example, a limited control
of the drug-release profile, and the incorporation of a foreign,
non-polymeric substance and/or the formation of pores in the core
structure, which adversely affect the strength and/or flexibility
of the fibers and in some cases weaken the infrastructure of the
fibers.
[0021] In systems that use hollow reservoir fibers, drugs such as
dexamethasone and methotrexane are located in a hollowed, internal
section of the fiber [Eenink, M. D. J., et al., J. Control. Rel.,
1987, 6, pp. 225-237; Polacco, G., et al., Polymer International,
2002, 51(12), pp. 1464-1472; and Lazzeri, L., et al., Polymer
International, 2005, 54, pp. 101-107]. These systems also suffer
disadvantages such as a limited control of the drug-release
profile, a weakened infrastructure of the fibers and complicated
production procedure.
[0022] Hence, although the use of fibers in various medical
applications such as tissue regeneration is a promising discipline,
the presently known methods for producing such fibers which can
incorporate and deliver bioactive agents are limited by poor
mechanical properties of the resulting fiber and/or poor drug
loading and/or uncontrollable drug release. Furthermore, many
bioactive agents (for example, proteins) do not tolerate melt
processing, organic solvents and other conditions which are typical
for polymeric fiber production.
[0023] There is thus a widely recognized need for, and it would be
highly advantageous to have biodegradable composite structures,
preferably fibrous structures, which can be loaded with and
controllably-release bioactive agents, while maintaining the
desired mechanical properties of the structure and retaining the
activity of the bioactive agents, and which are devoid of the above
limitations.
SUMMARY OF THE INVENTION
[0024] According to one aspect of the present invention there is
provided a composite structure comprising a fibril core and a
polymeric coat coating at least a part of the fibril core, the
structure being designed such that the coat is capable of
encapsulating at least one bioactive agent while retaining an
activity of the bioactive agent and/or capable of releasing a
bioactive agent encapsulated in the coat in a pre-determined
release rate.
[0025] According to another aspect of the present invention there
is provided a composite structure which includes a fibril core and
a polymeric coat coating at least a part of the fibril core,
wherein the coat includes at least one bioactive agent encapsulated
therein and/or applied thereon.
[0026] According to further features in preferred embodiments of
the invention described below the polymeric coat is a porous
coat.
[0027] According to further features in preferred embodiments of
the invention described below, an activity of the bioactive agent
is at least partially retained.
[0028] According to further features in preferred embodiments of
the invention described below, the coat is capable of releasing the
bioactive agent encapsulated in the coat in a pre-determined
release rate.
[0029] According to still further features in the described
preferred embodiments the structure is a composite fibrous
structure.
[0030] According to still further features in the described
preferred embodiments the fibril core is a polymeric fibril
core.
[0031] According to still further features in the described
preferred embodiments the fibril core is biodegradable.
[0032] According to still further features in the described
preferred embodiments the fibril core is non-degradable.
[0033] According to still further features in the described
preferred embodiments the coat is biodegradable.
[0034] According to still further features in the described
preferred embodiments the fibril core is characterized by a tensile
strength of at least 100 MPa.
[0035] According to still further features in the described
preferred embodiments the porous coat has a pore diameter that
ranges from about 0.001 .mu.m to about 1000 .mu.m.
[0036] According to still further features in the described
preferred embodiments the polymeric coat is characterized by an
average pore diameter that ranges from about 1 nm to about 1
mm.
[0037] According to still further features in the described
preferred embodiments the polymeric coat is characterized by an
average pore diameter that ranges from about 1 nm to about 50
.mu.m.
[0038] According to still further features in the described
preferred embodiments the polymeric coat is characterized by an
average pore diameter that ranges from about 100 nm to about 200
.mu.m.
[0039] According to still further features in the described
preferred embodiments the polymeric coat is characterized by a pore
density that ranges from about 70% of void volume per coat volume
to about 95% of void volume per coat volume.
[0040] According to still further features in the described
preferred embodiments the thickness of the polymeric coat ranges
from about 1 .mu.m to about 2000 .mu.m, and preferably from about
100 .mu.m to about 500 .mu.m.
[0041] According to still further features in the described
preferred embodiments a diameter of the fibril core ranges from
about 1 .mu.m to about 1 cm, and preferably the diameter of the
fibril core ranges from about 50 .mu.m to about 300 .mu.m.
[0042] According to still further features in the described
preferred embodiments the polymeric fibril core comprises at least
one first biodegradable polymer.
[0043] According to still further features in the described
preferred embodiments the polymeric fibril core comprises a
non-biodegradable polymer, preferably nylon.
[0044] According to still further features in the described
preferred embodiments the at least one first biodegradable polymer
is selected from the group consisting of poly(glycolic acid),
poly(lactic acid), polydioxanone (PDS), poly(alkylene succinate),
poly(hydroxybutyrate), poly(butylene diglycolate),
poly(epsilon-caprolactone) and a co-polymer, a blend and a mixture
thereof.
[0045] According to still further features in the described
preferred embodiments the at least one first biodegradable polymer
comprises poly(L-lactic acid).
[0046] According to still further features in the described
preferred embodiments the coat comprises at least one second
biodegradable polymer.
[0047] According to still further features in the described
preferred embodiments the at least one second biodegradable polymer
is selected from the group consisting of poly(glycolic acid),
poly(lactic acid), polydioxanone (PDS), poly(alkylene succinate),
poly(hydroxybutyrate), poly(butylene diglycolate),
poly(epsilon-caprolactone) and a co-polymer, a blend and a mixture
thereof.
[0048] According to still further features in the described
preferred embodiments the at least one second biodegradable polymer
comprises poly(DL-lactic-co-glycolic acid).
[0049] According to still further features in the described
preferred embodiments the coat further comprises at least one
additional agent.
[0050] According to still further features in the described
preferred embodiments the additional agent is selected from the
group consisting of a biodegradation promoting agent, a penetration
enhancer, a humectant, a chelating agent, an occlusive agent, an
emollient, a permeation enhancer, an anti-irritant and a
penetration enhancer.
[0051] According to still further features in the described
preferred embodiments an amount of the bioactive agent ranges from
about 0.00001 weight percentage and about 50 weight percentages of
the total weight of the coat.
[0052] According to still further features in the described
preferred embodiments the bioactive agent is selected from the
group consisting of a hydrophobic bioactive agent and a hydrophilic
bioactive agent.
[0053] According to still further features in the described
preferred embodiments the bioactive agent is selected from a group
consisting of a macro-biomolecule and a small organic molecule.
[0054] According to yet another aspect of the present invention
there is provided a fibrous composition-of-matter comprising any of
the composite structures described herein.
[0055] The fibrous composition-of-matter can be in a form of a
sheet or a mesh.
[0056] According to an additional aspect of the present invention
there is provided a process of preparing a composite structure
which comprises a fibril core and a polymeric coat coating at least
a part of the fibril core, the process is effected by contacting a
fiber and an emulsion of an aqueous solution and an organic
solution, said organic solution containing at least one second
polymer, to thereby obtain the fiber having a layer of an emulsion
applied on at least a part thereof; and freeze-drying the fiber
having a layer applied thereon, thereby obtaining the composite
structure presented herein.
[0057] According to still an additional aspect of the present
invention there is provided a process of preparing a composite
structure which comprises a polymeric fibril core and a polymeric
coat coating at least a part of the fibril core, wherein the coat
comprises at least one bioactive agent encapsulated therein in
and/or applied thereon, the process is effected by contacting a
fiber and an emulsion containing an aqueous solution and an organic
solution, and further containing the at least one bioactive agent
either within the aqueous solution or within the organic solution,
wherein the organic solution containing at least one second
polymer, to thereby obtain a fiber having a layer of an emulsion
applied on at least a part thereof; and freeze-drying the fiber
having the layer applied thereon, thereby obtaining the composite
structure presented herein.
[0058] According to further features in preferred embodiments of
the invention described below, the fibril core is a polymeric
fibril core made from at least one first polymer.
[0059] According to still further features in the described
preferred embodiments providing the polymeric fibril core
comprises: spinning the at least one first polymer, to thereby
obtain a crude fiber; and drawing the crude fiber, to thereby
obtain the polymeric fiber.
[0060] According to still further features in the described
preferred embodiments the at least one first polymer comprises at
least one biodegradable polymer.
[0061] According to still further features in the described
preferred embodiments the at least one first polymer comprises at
least one non-degradable polymer.
[0062] According to still further features in the described
preferred embodiments the spinning is selected from the group
consisting of electro-spinning, gel-spinning, wet-spinning,
dry-spinning, melt-spinning and solution-spinning.
[0063] According to still further features in the described
preferred embodiments the spinning comprises melt-spinning.
[0064] According to still further features in the described
preferred embodiments the drawing is effected at a draw-ratio that
ranges from about 2:1 to about 10:1.
[0065] According to still further features in the described
preferred embodiments the non-degradable polymer comprising the
core is selected from the group consisting of acrylic, aramid,
carbon, cellulose, melamine, nylon, polyacrylonitrile, polyamide,
polyester, polyethylene, polypropylene, polytetrafluoroethylene,
polyvinyl acetate, polyvinyl alcohol, viscose and any co-polymeric
combination thereof.
[0066] According to still further features in the described
preferred embodiments providing the emulsion is prepared by:
dissolving the at least one second polymer in an organic solvent to
thereby obtain the organic solution; contacting the organic
solution and the aqueous solution to thereby obtain a mixture; and
emulsifying the mixture to thereby obtain the emulsion.
[0067] According to still further features in the described
preferred embodiments the organic solvent is selected from the
group consisting of chloroform, dichloromethane, carbon
tetrachloride, methylene chloride, xylene, benzene, toluene,
hexane, cyclohexane, diethyl ether and carbon disulfide.
[0068] According to still further features in the described
preferred embodiments the at least one second polymer comprises at
least one second biodegradable polymer.
[0069] According to still further features in the described
preferred embodiments a concentration of the second biodegradable
polymer in the organic solvent ranges from about 1 weight to volume
percentages to about 50 weight to volume percentages.
[0070] According to still further features in the described
preferred embodiments a ratio of the aqueous solution and the
organic solution in the mixture ranges from about 1 part of the
organic solution to 1 part the aqueous solution to about 20 parts
of the organic solution to 1 part the aqueous solution.
[0071] According to still further features in the described
preferred embodiments the emulsion further contains at least one
bioactive agent and the contacting and/or the emulsifying are
effected at a temperature suitable for retaining an activity of the
bioactive agent.
[0072] According to still further features in the described
preferred embodiments the aqueous solution comprises at least one
component selected from the group consisting of a buffer, an
emulsifying agent, a surfactant, an anti-static agent, a chelating
agent, a preservative, a solubilizer, a viscosity modifying agent,
a biodegradation promoting agent and a penetration enhancer.
[0073] According to still further features in the described
preferred embodiments the organic solution further comprises at
least one component selected from the group consisting of an
emulsifying agent, a surfactant, an anti-static agent, a chelating
agent, a preservative, a solubilizer, a viscosity modifying agent,
a biodegradation promoting agent and a penetration enhancer.
[0074] According to still further features in the described
preferred embodiments an amount of the bioactive agent ranges from
about 0.00001 weight percentage to about 50 weight percentages of
an amount of the at least one second polymer.
[0075] According to still further features in the described
preferred embodiments the amount of the bioactive agent ranges from
about 0.1 weight percentage and about 30 weight percentages of an
amount of the at least one second polymer.
[0076] According to still further features in the described
preferred embodiments the aqueous solution contains a hydrophilic
bioactive agent, and the ratio of the aqueous solution and the
organic solution in said mixture ranges from about 3 parts of the
organic solution to 1 part the aqueous solution to about 20 parts
of the organic solution to 1 part the aqueous solution.
[0077] According to still further features in the described
preferred embodiments a concentration of the bioactive agent in the
aqueous solution ranges from about 1 weight percentage to about 20
weight percentages.
[0078] According to still further features in the described
preferred embodiments the organic solution contains a hydrophobic
bioactive agent, and the ratio of the aqueous solution and the
organic solution in the mixture ranges from about 1 parts of the
organic solution to 1 part the aqueous solution to about 8 parts of
the organic solution to 1 part the aqueous solution.
[0079] According to still further features in the described
preferred embodiments a concentration of the bioactive agent in the
organic solution ranges from about 10 weight percentage to about 30
weight percentages.
[0080] According to further aspects of the present invention there
are provided medical devices comprising the composite structure or
the fibrous composition-of-matter described hereinabove.
[0081] According to further features in preferred embodiments of
the invention described below, the medical device is designed for
transdermal application.
[0082] According to still further features in the described
preferred embodiments the medical device is selected from the group
consisting of a suture, an adhesive plaster and a skin patch.
[0083] According to still further features in the described
preferred embodiments the medical device is designed for topical
application.
[0084] According to still further features in the described
preferred embodiments the medical device is selected from the group
consisting of a suture, an adhesive strip, a bandage, an adhesive
plaster, a wound dressing and a skin patch.
[0085] According to still further features in the described
preferred embodiments the medical device is designed for
implantation in a bodily organ.
[0086] According to still further features in the described
preferred embodiments the medical device is selected from the group
consisting of a plate, a mesh, a screw, a pin, a tack, a rod, a
suture anchor, an anastomosis clip or plug, a dental implant or
device, an aortic aneurysm graft device, an atrioventricular shunt,
a catheter, a heart valve, a hemodialysis catheter, a bone-fracture
healing device, a bone replacement device, a joint replacement
device, a tissue regeneration device, a hemodialysis graft, an
indwelling arterial catheter, an indwelling venous catheter, a
needle, a pacemaker, a pacemaker lead, a patent foramen ovale
septal closure device, a vascular stent, a tracheal stent, an
esophageal stent, a urethral stent, a rectal stent, a stent graft,
a suture, a synthetic vascular graft, a thread, a tube, a vascular
aneurysm occluder, a vascular clip, a vascular prosthetic filter, a
vascular sheath and a drug delivery port, a venous valve and a
wire.
[0087] According to still further features in the described
preferred embodiments the organ is selected from the group
consisting of skin, scalp, a dermal layer, an eye, an ear, a small
intestines tissue, a large intestines tissue, a kidney, a pancreas,
a liver, a digestive tract tissue or cavity, a respiratory tract
tissue or cavity, a bone, a joint, a bone marrow tissue, a brain
tissue or cavity, a mucosal membrane, a nasal membrane, the blood
system, a blood vessel, a muscle, a pulmonary tissue or cavity, an
abdominal tissue or cavity, an artery, a vein, a capillary, a
heart, a heart cavity, a male reproductive organ, a female
reproductive organ and a visceral organ.
[0088] According still another aspect of the present invention
there is provided an article-of-manufacture comprising the
composite structure described herein.
[0089] The article of manufacture can be, for example, a fishing
line, a fish net, an insect net, a bird net, a vegetation net, a
cloth fiber, a non-woven fiber, a disposable women's sanitary item,
a mask, a wet tissue (wipe), an underwear, a handkerchief, a towel,
a diaper, a disposable medical supply, a disposable food container
or dish, a disposable item of clothing or a disposable cutlery
item.
[0090] According to yet another aspect of the present invention
there is provided a method for predicting release rate of the
bioactive agent from the composite structure described herein, the
polymeric coat being initially incorporated with an initial
concentration of the bioactive agent. The method is effected by
solving a diffusion equation so as to obtain the concentration
distribution of the bioactive agent as a function of time, and
integrating the concentration distribution so as to obtain an
integrated bioactive agent mass in the polymeric coat as a function
of time. The method further comprises using the integrated
bioactive agent mass for predicting the release rate of the
bioactive agent.
[0091] According to further features in preferred embodiments of
the invention described below, the diffusion equation comprises a
time-dependent diffusion coefficient.
[0092] According to still further features in the described
preferred embodiments the time-dependent diffusion coefficient
comprises a constant term which is proportional to a porosity
characterizing the polymeric coat.
[0093] According to still further features in the described
preferred embodiments the constant term is proportional to the
ratio of the porosity to a tortuosity characterizing the polymeric
coat.
[0094] According to still further features in the described
preferred embodiments the time-dependent diffusion coefficient
comprises a degradation profile characterizing the polymeric
coat.
[0095] The present invention successfully addresses the
shortcomings of the presently known configurations by providing
composite structures which utilize the beneficial mechanical
properties of fibers and allows efficient encapsulation of
bioactive agents therein, and controllable release of these
bioactive agents under physiological conditions.
[0096] Unless otherwise defined, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which this invention belongs. Although
methods and materials similar or equivalent to those described
herein can be used in the practice or testing of the present
invention, suitable methods and materials are described below. In
case of conflict, the patent specification, including definitions,
will control. In addition, the materials, methods, and examples are
illustrative only and not intended to be limiting.
[0097] Unless otherwise defined, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which this invention belongs. Although
methods and materials similar or equivalent to those described
herein can be used in the practice or testing of the present
invention, suitable methods and materials are described below. In
case of conflict, the patent specification, including definitions,
will control. In addition, the materials, methods, and examples are
illustrative only and not intended to be limiting.
[0098] As used herein, the singular form "a", "an" and "the"
include plural references unless the context clearly dictates
otherwise. For example, the term "a protein" or "at least one
protein" may include a plurality of proteins, including mixtures
thereof.
[0099] Throughout this disclosure, various aspects of this
invention can be presented in a range format. It should be
understood that the description in range format is merely for
convenience and brevity and should not be construed as an
inflexible limitation on the scope of the invention. Accordingly,
the description of a range should be considered to have
specifically disclosed all the possible subranges as well as
individual numerical values within that range. For example,
description of a range such as from 1 to 6 should be considered to
have specifically disclosed subranges such as from 1 to 3, from 1
to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as
well as individual numbers within that range, for example, 1, 2, 3,
4, 5, and 6. This applies regardless of the breadth of the
range.
[0100] Whenever a numerical range is indicated herein, it is meant
to include any cited numeral (fractional or integral) within the
indicated range. The phrases "ranging/ranges between" a first
indicate number and a second indicate number and "ranging/ranges
from" a first indicate number "to" a second indicate number are
used herein interchangeably and are meant to include the first and
second indicated numbers and all the fractional and integral
numerals therebetween.
[0101] As used herein throughout, the term "comprising" means that
other steps and ingredients that do not affect the final result can
be added. This term encompasses the terms "consisting of" and
"consisting essentially of".
[0102] As used herein, the phrase "substantially retaining" and/or
"substantially maintaining" refers to a protein's specific
activity, dissolvability and other biochemical properties essential
to its biological activity, which are retained and or maintained at
significant levels subsequent to the chemical modifications,
described in the present invention, carried out so to obtain a
metal-coat on the protein and intermediates to that end.
[0103] The term "method" or "process" refers to manners, means,
techniques and procedures for accomplishing a given task including,
but not limited to, those manners, means, techniques and procedures
either known to, or readily developed from known manners, means,
techniques and procedures by practitioners of the chemical,
pharmacological, biological, biochemical and medical arts.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0104] The patent or application file contains at least one drawing
executed in color. Copies of this patent or patent application
publication with color drawings will be provided by the Office upon
request and payment of the necessary fee.
[0105] The invention is herein described, by way of example only,
with reference to the accompanying drawings. With specific
reference now to the drawings in detail, it is stressed that the
particulars shown are by way of example and for purposes of
illustrative discussion of the preferred embodiments of the present
invention only, and are presented in the cause of providing what is
believed to be the most useful and readily understood description
of the principles and conceptual aspects of the invention. In this
regard, no attempt is made to show structural details of the
invention in more detail than is necessary for a fundamental
understanding of the invention, the description taken with the
drawings making apparent to those skilled in the art how the
several forms of the invention may be embodied in practice.
[0106] In the drawings:
[0107] FIG. 1 presents a schematic illustration of an exemplary
composite fibrous structure according to one of the present
embodiments, showing a dense fibril core, and a biodegradable
porous coat in which bioactive agents can be encapsulated;
[0108] FIG. 2 presents a standard color photograph of a composite
structure according to the present embodiments, demonstrating the
fibrous structure of the composite;
[0109] FIG. 3 presents comparative plots demonstrating the
stress-strain curve of various neat fibers as a function of various
draw ratios (3:1 to 8:1), showing the elastic limit at about 5%
strain shared among all fibers and the different final
stretchability limit and yield point of the various fibers;
[0110] FIGS. 4a-4c present plots demonstrating the yield strength
(FIG. 4a), ultimate tensile strength (FIG. 4a), maximal strain
(FIG. 4b) and Young's modulus, (FIG. 4c) of various neat fibers as
a function of the draw ratio, showing an increase in the yield
strength, the ultimate strength and in Young's modulus, and a
decrease in the maximal strain with the increase in draw ratio;
[0111] FIG. 5 presents a scanning electron micrograph of
cross-section of an exemplary composite fibrous structure according
to the present embodiments, composed of a PLLA-made core fiber and
a 75/25 PDLGA-made porous coat, showing the tight contact between
the core and the coat, and the solid density of the core contrary
to the porous microstructure of the coat;
[0112] FIGS. 6a-6i present a series of SEM micrographs of cross
sections of exemplary composite fibrous structures encapsulating an
enzyme (HRP) according to the present embodiments, showing the
effect of polymer content and HRP loads on the coat's
microstructure at a 4:1 organic-to-aqueous ratio (v/v), wherein the
coat is made from an emulsion having a polymer content of 13% (w/v)
and HRP load of 1% (w/w) (FIG. 6a), the coat is made from an
emulsion having a polymer content of 13% (w/v) and HRP load of 5%
(w/w) (FIG. 6b), the coat is made from an emulsion having a polymer
content of 13% (w/v) and HRP load of 10% (w/w) (FIG. 6c), the coat
is made from an emulsion having a polymer content of 15% (w/v) and
HRP load of 1% (w/w) (FIG. 6d), the coat is made from an emulsion
having a polymer content of 15% (w/v) and HRP load of 5% (w/w)
(FIG. 6e), the coat is made from an emulsion having a polymer
content of 15% (w/v) and HRP load of 10% (w/w) (FIG. 6f), the coat
is made from an emulsion having a polymer content of 19% (w/v) and
HRP load of 1% (w/w) (FIG. 6g), the coat is made from an emulsion
having a polymer content of 19% (w/v) and HRP load of 5% (w/w)
(FIG. 6h), and the coat is made from an emulsion having a polymer
content of 19% (w/v) and HRP load of 10% (w/w) (FIG. 6i);
[0113] FIGS. 7a-7d present a series of SEM micrographs of cross
sections of exemplary composite fibrous structures according to the
present embodiments, showing the effect of polymer content and HRP
loads on the porous coat's microstructure at a 8:1
organic-to-aqueous ratio (v/v), wherein the porous coat is made
from an emulsion having a polymer content of 15% (w/v) and HRP load
of 0% (w/w) (FIG. 7a), the porous coat is made from an emulsion
having a polymer content of 15% (w/v) and HRP load of 5% (w/w)
(FIG. 7b), the porous coat is made from an emulsion having a
polymer content of 19% (w/v) and HRP load of 0% (w/w) (FIG. 7c),
and the porous coat is made from an emulsion having a polymer
content of 19% (w/v) and HRP load of 5% (w/w) (FIG. 7d);
[0114] FIGS. 8a-8i present a series of SEM micrographs of cross
sections of exemplary composite fibrous structures according to the
present embodiments, showing the effect of polymer content and
emulsion phase ratio (O:A) on the porous coat's microstructure at
an HRP load of 5% (w/w), wherein the porous coat is made from an
emulsion having a polymer content of 13% (w/v) and an O:A of 4:1
(v/v) (FIG. 8a), the porous coat is made from an emulsion having a
polymer content of 13% (w/v) and an O:A of 8:1 (v/v) (FIG. 8b), the
porous coat is made from an emulsion having a polymer content of
13% (w/v) and an O:A of 16:1 (v/v) (FIG. 8c), the porous coat is
made from an emulsion having a polymer content of 15% (w/v) and an
O:A of 4:1 (v/v) (FIG. 8d), the porous coat is made from an
emulsion having a polymer content of 15% (w/v) and an O:A of 8:1
(v/v) (FIG. 8e), the porous coat is made from an emulsion having a
polymer content of 15% (w/v) and an O:A of 16:1 (v/v) (FIG. 8f),
the porous coat is made from an emulsion having a polymer content
of 19% (w/v) and an O:A of 4:1 (v/v) (FIG. 8g), the porous coat is
made from an emulsion having a polymer content of 19% (w/v) and an
O:A of 8:1 (v/v) (FIG. 8h), and the porous coat is made from an
emulsion having a polymer content of 19% (w/v) and an O:A of 16:1
(v/v) (FIG. 8i);
[0115] FIGS. 9a-9d present a series of SEM micrographs of the outer
surface of exemplary composite fibrous structures according to the
present embodiments, showing the effect of polymer content and
emulsion phase ratio (O:A) on the porous coat's microstructure at
an HRP load of 5% (w/w), wherein the porous coat is made from an
emulsion having a polymer content of 13% (w/v) and an O:A of 8:1
(v/v), (FIG. 9a), the porous coat is made from an emulsion having a
polymer content of 13% (w/v) and an O:A of 16:1 (v/v) (FIG. 9b),
the porous coat is made from an emulsion having a polymer content
of 19% (w/v) and an O:A of 8:1 (v/v) (FIG. 9c), and the porous coat
is made from an emulsion having a polymer content of 19% (w/v) and
an O:A of 16:1 (v/v) (FIG. 9d);
[0116] FIG. 10 presents comparative plots demonstrating the
cumulative in vitro release of HRP from exemplary composite fibrous
structures according to the present embodiments, as a function of
various HRP contents (1% w/w denoted by white symbols, 5% w/w
denoted by black symbols and 10% w/w denoted gray symbols) and as a
function of various polymer contents (13% w/v denoted rectangles,
15% w/v denoted by circles, and 19% w/v denoted triangle) at a
constant organic-to-aqueous phase ratio of 4:1;
[0117] FIG. 11 is a bar graph demonstrating the release rate of HRP
from various exemplary composite fibrous structures according to
the present embodiments, made of an emulsion having 15% w/v polymer
content, as a function of various HRP loads (white bars denoted 1%
w/v, gray bars denote 5% w/v and black bars denote 10%), during the
first 30 days out of the 90 days of the experiment;
[0118] FIGS. 12a-12c present comparative plots demonstrating the
cumulative in vitro release profiles of HRP from exemplary
composite fibrous structures according to the present embodiments
having a polymer content of 13% w/v (FIG. 12a); 15% w/v (FIG. 12b)
and 19% w/v (FIG. 12c), as a function of the organic-to-aqueous
phase ratio (black triangles denote a 4:1 ratio, blanc rectangles
denote a 8:1 ratio and gray circles denote 16:1 ratio), at a
constant HRP load of 5% w/w
[0119] FIG. 13 presents comparative plots, showing the tensile
stress-strain curves of pre-treated nylon fibril core coated with a
standard reference emulsion containing 17.5% w/v polymer in the
organic solution, 1.43% w/w paclitaxel (relative to the polymer
load), and an organic to aqueous (O:A) phase ratio of 2:1 v/v,
wherein curve "1" corresponds to a surface pre-treated nylon fibril
core, curve "2", considering total diameter, corresponds to a nylon
fibril core coated with said standard reference emulsion, and curve
"3", considering effective diameter, corresponds to a nylon fibril
core coated with said standard reference emulsion;
[0120] FIGS. 14a-14d present a schematic illustration of an
exemplary paclitaxel-eluting fibrous composite structure according
to a preferred embodiment of the present invention (FIG. 14a)
having a nylon core and a biodegradable porous coat loaded with
paclitaxel, and SEM fractographs at various magnifications (FIG.
14b-d) of fibrous composite structures comprising a nylon core
having a diameter in the range of 170-190 .mu.m, and a porous coat
having a thickness of 30-60 .mu.m made from an emulsion containing
17.5% w/v polymer in the organic solution, 1.43% w/w paclitaxel
(relative to the polymer load), and an organic to aqueous (O:A)
phase ratio of 2:1 v/v;
[0121] FIGS. 15a-15d present a series of SEM fractographs
presenting the coat microstructure of various exemplary
paclitaxel-eluting fibrous composite structures, according to
preferred embodiments of the present invention, each having a nylon
core and a coat made from an emulsion containing 17.5% w/v polymer,
1.43% w/w paclitaxel and having a phase ratio of 2:1 O:A (FIG.
15a), a coat made from an emulsion containing 15% w/v polymer,
1.43% w/w paclitaxel and having a phase ratio of 2:1 O:A (FIG.
15b), a coat made from an emulsion containing 17.5% w/v polymer,
2.9% w/w paclitaxel and having a phase ratio of 2:1 O:A (FIG. 15c),
and a coat made from am emulsion containing 17.5% w/v polymer,
1.43% w/w paclitaxel and having a phase ratio of 4:1 O:A (FIG.
15d);
[0122] FIGS. 16a-16c present a series of SEM fractographs
demonstrating the coat's microstructure of exemplary
paclitaxel-eluting fibrous composite structures, each having a
nylon core and a coat made from an emulsion containing no
surfactants (FIG. 16a), a coat made from an emulsion containing 1%
w/w Pluronic.RTM. (FIG. 16b), and a coat made an emulsion
containing 1% w/v PVA (FIG. 16c);
[0123] FIG. 17 presents a cumulative plot showing the paclitaxel
release from the porous coat of an exemplary fibrous composite
structure having a nylon core and a coat made from an emulsion
containing 17.5% w/v polymer in the organic solution, 1.43% w/w
paclitaxel (relative to the polymer load), and an organic to
aqueous (O:A) phase ratio of 2:1 v/v, showing the amount of
released paclitaxel in mg and as the percentage of the released
paclitaxel from the loaded amount, measured over a time period of
four months;
[0124] FIG. 18 presents comparative plots showing the paclitaxel
release profile from the porous coat of exemplary
paclitaxel-eluting fibrous composite structures having a nylon core
and a porous coat made from an emulsion homogenized at a low rate
(marked with blue diamonds), medium rate (marked with magenta
squares) and high rate (marked with green triangles), showing the
effect of the emulsion's homogenization rate on the rate of drug
release;
[0125] FIG. 19 presents comparative plots showing the paclitaxel
release profile from the porous coat of exemplary
paclitaxel-eluting fibrous composite structures according to
preferred embodiments of the present invention having a nylon core
and a porous coat made from an emulsion containing a polymer
content of 15% w/v (marked with blue squares), 17.5% w/v (marked
with magenta circles), and 22.5% w/v (marked with green
triangles);
[0126] FIG. 20 present comparative plots showing the paclitaxel
release profile from the porous coat of exemplary
paclitaxel-eluting fibrous composite structures according to
preferred embodiments of the present invention, having a nylon core
and a porous coat made from an emulsion having a drug content of
0.7% w/w (marked with red diamonds), 1.4% w/w (marked with magenta
circles), 2.9% w/w (marked with blue triangles) and 7.1% w/w
(marked with cyan squares);
[0127] FIG. 21 present comparative plots showing the paclitaxel
release profile from the porous coat of exemplary
paclitaxel-eluting fibrous composite structures according to
preferred embodiments of the present invention having a nylon core
and a porous coat made from an emulsion having an
organic-to-aqueous phase ratio (O:A ratio) of 4:1 v/v (marked with
magenta squares), and 2:1 v/v (marked with green diamonds);
[0128] FIG. 22 present comparative plots showing the drug release
profile from exemplary paclitaxel-eluting fibrous composite
structures according to preferred embodiments of the present
invention, having a nylon core and a porous coat made from an
emulsion containing no surfactant (marked with magenta squares), an
emulsion containing 1% Pluronic.RTM. (marked with blue triangles),
and an emulsion containing 1% PVA (marked with black diamonds);
[0129] FIGS. 23a-23e present five sets of comparative plots and
mean error thereof showing the effect of the emulsion composition
on the predicted HRP release profile (blue curves) as compared to
the experimental release profile (red curves) for a fibrous
composite structure having a biodegradable core (disregarded in the
calculations) and a porous coat made from an emulsion containing an
O:A ratio of 8:1 and a 15% w/v polymer content (FIG. 23a), an O:A
ratio of 8:1, 19% w/v polymer content (FIG. 23b), an O:A ratio of
16:1, 13% w/v polymer content (FIG. 23c), an O:A ratio of 16:1, 15%
w/v polymer content (FIG. 23d) and an O:A ratio of 16:1, 19% w/v
polymer content (FIG. 23e);
[0130] FIG. 24 presents comparative plots showing the normalized
degradation rate of fiber coats made from three types of 75/25
PDLGA polymers (data adopted from Wu et al.), wherein the green
curve represent the degradation rate of a polymer having a 160 kDa
molecular weight, the blue curve represents a polymer of 100 kDa
and the red curve represents a 40 kDa PDLGA polymer;
[0131] FIGS. 25a-25e present five sets of comparative plots showing
the effect of the initial average molecular weight of the polymer
on the predicted HRP release profiles for fibrous composite
structures having a core made from three types of 75/25 PDLGA
polymers having 40 kDa molecular weight (red curves), 100 kDa
molecular weight (blue curves) and 160 kDa molecular weight (green
curves), and made from an emulsion having an O:A ratio of 8:1 and a
polymer content of 15% w/v (FIG. 25a), an O:A ratio of 8:1 and a
polymer content of 19% w/v (FIG. 25b), an O:A ratio of 16:1 and a
polymer content of 13% w/v (FIG. 25c), an O:A ratio of 16:1 and a
polymer content of 15% w/v (FIG. 25d), and an O:A ratio of 16:1 and
a polymer content of 19% w/v (FIG. 25e);
[0132] FIGS. 26a-26b present two comparative plots showing the
effect of the molecular weigh of the bioactive agent on the
predicted release profiles thereof using three model proteins
having a molecular weight of 22 kDa (red curves), 44 kDa (blue
curves) and 160 kDa (green curves), released from the coat of
fibrous composite structures prepared from emulsions of 5% w/w
protein content, a polymer content of 19% w/v and an O:A ratio of
8:1 (FIG. 26a) and an O:A ratio of 16:1 (FIG. 26b).
DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION
[0133] The present invention is of novel composite structures and
processes of preparing same, which can be used as basic structural
elements in the construction of various medical devices and other
articles-of-manufacture. Specifically, the present invention is of
composite fibrous structures which are designed capable of
encapsulating a bioactive agent while retaining the activity of the
bioactive agent as well as the desired mechanical properties of the
structure. The composite structures are further designed so as to
release a bioactive agent encapsulated therein at a desired,
pre-determined release rate. The composite structures can be
beneficially used in the construction of various medical devices
such as wound dressings, stents and devices for tissue
regeneration.
[0134] The principles and operation of the present invention may be
better understood with reference to the drawings and accompanying
descriptions.
[0135] Before explaining at least one embodiment of the invention
in detail, it is to be understood that the invention is not limited
in its application to the details set forth in the following
description or exemplified by the Examples. The invention is
capable of other embodiments or of being practiced or carried out
in various ways. Also, it is to be understood that the phraseology
and terminology employed herein is for the purpose of description
and should not be regarded as limiting.
[0136] As discussed hereinabove, the fields of tissue regeneration,
medical devices in general and implantable medical devices in
particular, call for the development of suitable materials and
structural elements made therefrom, which can satisfy the needs of
modern medicine practices and research. These structural elements
are often required to be made of biodegradable materials, which are
non-toxic and benign both prior to the degradation process and
thereafter (namely, have non-toxic and benign break-down products).
These structural elements are further often required to contain and
controllably release bioactive agents which are necessary for
effecting the desired influence and activity of a particular
device, prevent harmful effects which may be inflicted by the
foreign implant and assist in the healing process. These structural
elements should further preferably be characterized by adequate
mechanical strength and flexibility.
[0137] As discussed hereinabove, fibers are highly suitable for
constructing such elements. However, the presently known
methodologies that utilize structural elements as drug delivery
systems are limited by the requirement to prepare these structures
under conditions that adversely affect the activity and/or the
controllable release of the incorporated active agent on one hand
and by reduced control of the mechanical strength and flexibility
of the structure on the other hand.
[0138] In a search for a novel technique for constructing
structural elements that could be efficiently used as drug delivery
systems, the present inventor has devised and successfully
practiced a novel methodology which enables to produce fibrous
composite structures that are capable of encapsulating and
controllably releasing bioactive agents while combining the
mechanical properties required of a fiber without compromising the
biological activity of the bioactive agents.
[0139] The fibrous structures obtained by this methodology are
based on a core/coat composite structure, or, in other words, on a
system composed of subcomponents, each being prepared by a
different methodology and characterized by different properties.
More specifically, the system designed by the present inventor is
composed of a fibril core, which can be prepared by conventional
methods and provides the structure with the desired mechanical
properties, and a coat coating the core and prepared and applied on
the core under mild conditions that enable to retain an activity of
bioactive agents that can optionally be incorporated therein or
thereon. The present inventor has further showed that by varying
certain parameters during the preparation of these structures, the
performance of these structures, in terms of, for example,
mechanical properties such as strength, porosity, stability and
flexibility, and loading and release profiles of the bioactive
agents, can be finely controlled.
[0140] As is demonstrated and exemplified in the Examples section
that follows, the present inventor has successfully produced
biodegradable polymeric fibers by conventional production methods,
which served as a core for a composite fibrous structure, and
successfully applied thereon a porous polymeric coat made of a
biodegradable polymer and containing a biologically active agent
(e.g., a horseradish peroxidase enzyme, HRP). The present inventor
has further successfully utilized nylon-made sutures as a fibril
core having applied thereon a porous coat made of a biodegradable
polymer and containing a biologically active agent (e.g., a
synthetic drug, paclitaxel).
[0141] The fibril core of the composite fibrous structure
contributed the desired mechanical properties, whereby the porous
coat contributed the capacity to contain and controllably release
the bioactive agent. The release rate of each bioactive agent from
various composite fibers was monitored and several parameters of
the preparation of the coat were examined for their effect on the
release profile.
[0142] Thus, according to one aspect of the present invention,
there is provided a composite structure which comprises a fibril
core and a polymeric coat, coating at least a part of the fibril
core. The composite structure is designed so as to enable the
encapsulation of one or more bioactive agent(s) in or on the coat
while retaining the biological activity of these agents. The
composite structure can also be designed so as to enable the
release of one or more bioactive agent(s) encapsulated in or on the
coat at a pre-determined release rate.
[0143] The composite structure, according to preferred embodiments
of the present invention, is fibrous.
[0144] The term "fiber", as used herein, describes a class of
structural elements, similar to pieces of thread, that are made of
continuous filaments and/or discrete elongated pieces.
[0145] Fibers are often used in the manufacture of other structures
by being spun into thicker fibers, threads or ropes or matted into
sheets or meshes and more bulky structures. Fibers can be obtained
from a natural source such as plants, animal and mineral sources,
or be synthetically man-made from naturally occurring and/or
synthetic substances. Examples of natural fibers include cotton,
linen, jute, flax, ramie, sisal and hemp, spider silk, sinew, hair,
wool and asbestos (the only naturally occurring mineral fiber).
Examples of man-made synthetic fibers include fiberglass, rayon,
acetate, modal, cupro, lyocell, nylon, polyester, acrylic polymer
fibers, polyacrylonitrile fibers and carbon fiber.
[0146] The term "fibril" describes a small, slender and fine fiber
or filament, typically having micro-sized dimension on the scale of
micrometers.
[0147] The term "fibrous" is used herein to describe a fiber-like
shape and structure of a material or structure.
[0148] A fibrous composite structure as presented herein is
therefore composed of two structural elements: a fibril core and a
coat, whereby the structure as a whole adopts the shape of the
fibril core.
[0149] FIG. 1 presents a schematic illustration of an exemplary
composite structure according to preferred embodiments of the
present invention. As can be seen in FIG. 1, the fibrous structure
is composed of a fibril core and a porous coat; whereby the porous
coat can encapsulate or otherwise entrap a bioactive agent. The
fibril core can be, for example, any natural or synthetic fiber as
described hereinabove.
[0150] The fibril core can therefore be made of natural or
synthetic polymeric materials, elemental materials, metallic
substances and any combination thereof. Thus, for example, the
fibril core can be a metallic fibril core, made of metals such as,
for example, stainless steel, platinum, and the like; an elemental
fibril core made of carbon, silicon and the like; or a polymeric
fibril core made of organic and/or inorganic polymers.
[0151] Metallic fibril cores, made, for example, from stainless
steel are useful in applications that require high mechanical
strength and durability. An exemplary application of a composite
structure as described herein, which has a stainless steel fibril
core, is a stent.
[0152] According to preferred embodiments of the present invention,
the fibril core is a polymeric fibril core, made of a polymeric
material. The polymeric fibril core can be either degradable or
non-degradable, as described in detail hereinbelow.
[0153] Thus, according to preferred embodiments of the present
invention, the composite structure includes a polymeric fibril core
made of biodegradable or non-degradable polymers and/or
biodegradable or non-biodegradable co-polymers.
[0154] The fibril core is the part of the composite which bequeath
most of its mechanical properties, having been produced by well
established techniques which are designed to give a fiber with the
desired mechanical properties. These mechanical properties are
typically expressed by tensile strength and elastic modulus, also
known as Young's modulus, as these phrases are defined
hereinbelow.
[0155] The strength and flexibility of the fibril core largely
depend on parameters such as the thickness of the fiber
constituting the fibril core, its chemical composition (namely, the
polymer(s) or other material used to form the fiber) and the
conditions at which it is prepared. By controlling these
parameters, the desired properties of the fibril core can be
obtained.
[0156] Thus, for example, since the thickness of the core has a
direct impact on the strength and flexibility of the composite
structure, the thickness of the core composing the structures
described herein can be selected suitable for the specific
application of the composite structures. For example, certain
orthopedic implants are massive elements which are required to
possess great strength and durability so as to sustain the body's
weight and movements, while sutures used in eye surgery, certain
nano-sized orthopedic implants and devices used for nerve cells
regeneration are typically required to have the most delicate and
thin form.
[0157] Therefore, the diameter of the fibril core can range from
about 1 .mu.m to about 1000 .mu.m and in some cases can also be
higher, up to 1 cm. In cases where the structure is designed to be
used to construct, for example, a massive orthopedic implant, thick
cores being from about 500 .mu.m to about 1000 .mu.m and higher in
diameter are preferred. In cases where delicate and thin structures
are desired, the fibril core is preferably from 1 .mu.m to 100
.mu.m in diameter.
[0158] For most applications, structures comprising a fibril core
that has a diameter in the range of from about 50 .mu.m to about
300 .mu.m, and preferably of about 200 .mu.m, are preferred.
[0159] As used herein the term "about" refers to .+-.10%.
[0160] Young's modulus (also known as the modulus of elasticity or
elastic modulus) is a value which serves to determine the stiffness
of a fiber of a given substance. According to Hooke's law the
strain of a fiber is proportional to the exerted stress applied
thereto, and therefore the ratio of the two is a constant that is
commonly used to indicate the elasticity of the substance. Young's
modulus is the elastic modulus for tension, or tensile stress, and
is the force per unit cross section of the material divided by the
fractional increase in length resulting from the stretching of a
fiber. Young's modulus can be experimentally determined from the
slope of a stress-strain curve created during tensile tests
conducted on a sample of the fiber, and expressed in units of force
per unit area (Newton per square meter (N/m.sup.2) or dynes per
square centimeter), namely Pascals (Pa), megaPascals (MPa) or
gigaPascals (GPa).
[0161] The phrase "tensile strength" as used herein describes the
maximum amount of tensile stress that a fiber of a given material
can be subjected to before it breaks. As in the case of Young's
modulus, tensile strength can be experimentally determined from a
stress-strain curve, and is expressed in units of force per unit
area (Newton per square meter (N/m.sup.2) or Pascals (Pa).
[0162] Thus, according to preferred embodiments of the present
invention, the fibril core is characterized by a tensile strength
of at least 100 MPa. According to further preferred embodiments of
the present invention, the fibril core is characterized by higher
tensile strength, for example, higher than 200 MPa, higher than 300
MPa, higher than 400 MPa, higher than 500 MPa, and even higher than
750 MPa or higher than 1 GPa.
[0163] The flexibility of the fibril core can also be controlled so
as to provide the resulting structure with the desired ductility.
While in some applications it is desired that the structure would
have high flexibility and pliancy (for example, stents, sutures
etc.), in other applications more rigid structures are desired (for
example, bone and joint implants, etc.).
[0164] Therefore, according preferred embodiments of the present
invention, the fibril core is characterized by an elasticity
(Young's) modulus of 3 GPa and higher and thus can be
characterized, for example, by an elasticity (Young's) modulus
higher than 4 GPa and even higher than or equal to 5 GPa. The
desired elasticity can be determined, for example, during the
drawing of the fiber, as is detailed hereinbelow.
[0165] Overall, the fibril core in the composite structure
presented herein is characterized by mechanical strength,
elasticity and other properties of typical fibers. These
characteristics can be finely controlled during the preparation of
the fibers constituting the core of the structure, by virtue of the
chemical composition (choice of the polymer or any other substance
composing the fibril core) and the production methods (spinning and
drawing methods), and therefore can have almost any specific
characteristics attributed thereto so as to suit any specific
application.
[0166] Fibers used as the fibril core of the composite structures
can therefore be tailored made so as to provide the composite with
the desired properties, selected in accordance with its intended
use. The fibers can thus be prepared while controlling the
characteristics thereof. Alternatively, commercially or otherwise
available fibers can be utilized as the fibril core in the
composite structure described herein. Such fibers can be utilized
as is or can be subjected to surface treatment prior to use.
[0167] One example of such a commercially available fiber is a
suture. Sutures can serve as the fibril core according to the
embodiments of the present invention where high mechanical strength
is desired.
[0168] The incorporation of the fibril core into the composite
structures presented herein does not weaken or otherwise adversely
affect the properties of the fibril core.
[0169] As mentioned above, in preferred embodiments of the present
invention, the fibril core is a polymeric fibril core. As is
further mentioned hereinabove, the coat coating the fibril core is
further a polymeric coat.
[0170] The term "polymer", as used herein, encompasses organic and
inorganic polymer and further encompasses one or more of a polymer,
a copolymer or a mixture thereof (a blend).
[0171] While any polymer, copolymer or a mixture of polymers and/or
copolymers can be used for producing the core and coat of the
structures described herein, according to preferred embodiments of
the present invention, the coat is made of a biodegradable
polymer.
[0172] The term "biodegradable" as used in the context of the
present invention, describes a material which can decompose under
physiological and/or environmental conditions into breakdown
products. Such physiological and/or environmental conditions
include, for example, hydrolysis (decomposition via hydrolytic
cleavage), enzymatic catalysis (enzymatic degradation), and
mechanical interactions. This term typically refers to substances
that decompose under these conditions such that 50 weight percents
of the substance decompose within a time period shorter than one
year.
[0173] The term "biodegradable" as used in the context of the
present invention, also encompasses the term "bioresorbable", which
describes a substance that decomposes under physiological
conditions to break down to products that undergo bioresorption
into the host-organism, namely, become metabolites of the
biochemical systems of the host-organism.
[0174] The incorporation of a biodegradable coat in the composite
structure described herein, for example, the release of bioactive
agents that are potentially encapsulated in the coat when the
latter is exposed to physiological conditions.
[0175] Further according to preferred embodiments of the present
invention, the core can be either biodegradable or
non-degradable.
[0176] As used herein, the term "non-degradable" describes a
substance which does not undergo degradation under physiological
and/or environmental conditions. This term typically refers to
substances which decompose under these conditions such that more
than 50 percents do not decompose within at least 1 year,
preferably within 2 years, 3 years, 4 years, and up to 10 years and
even 20 or 50 years.
[0177] Structures comprising a non-degradable core are useful, for
example, in applications which require at least part of the
scaffold to be tenable.
[0178] An exemplary non-degradable polymer suitable for use as
fibril core in the context of the present invention is nylon. As
presented in the Examples section that follows, a non-biodegradable
core was prepared from pre-treated nylon suture fibers and was
successfully coated with a porous coat, while maintaining its
physical, chemical and mechanical properties.
[0179] Structures comprising a biodegradable core are desired in
applications where degradation of the whole structure overtime is
desired.
[0180] In embodiments where both the core and the coat are
biodegradable, each is composed of a first and second biodegradable
polymer, respectively.
[0181] Preferred biodegradable polymers according to the present
embodiments are non-toxic and benign polymers. More preferred
biodegradable polymers are bioresorbable polymers which decompose
into non-toxic and benign breakdown products that are absorbed in
the biochemical systems of the subject.
[0182] Non-limiting examples of biodegradable polymers which are
suitable for use as the first and second biodegradable polymers
composing the core and coat of the composite structure described
herein, respectively, include homo-polymers and co-polymers such as
aliphatic polyesters made of glycolide (glycolic acid), lactide
(lactic acid), caprolactone, p-dioxanone, trimethylene carbonate,
hydroxybutyrate, hydroxyvalerate, polypeptide made of natural and
modified amino acids, polyethers made of natural and modified
saccharides, polydepsipeptide, biodegradable nylon co-polyamides,
polydihydropyrans, polyphosphazenes, poly(ortho-esters), poly(cyano
acrylates), polyanhydrides and any combination thereof.
[0183] According to a preferred embodiment of the present
invention, the biodegradable polymer is an aliphatic polyester such
as, for example, poly(glycolic acid), poly(lactic acid),
polydioxanone (PDS), poly(alkylene succinate),
poly(hydroxybutyrate), poly(butylene diglycolate),
poly(epsilon-caprolactone) and a co-polymer, a blend and a mixture
thereof.
[0184] Exemplary aliphatic polyesters that were found suitable for
use in the context of the present invention include poly(L-lactic
acid), poly(glycolic acid) and/or co-polymers thereof such as
poly(DL-lactic-co-glycolic acid).
[0185] According to a preferred embodiment of the present
invention, a polymeric fibril core is made of poly(L-lactic
acid).
[0186] According to another preferred embodiment of the present
invention, the polymeric coat is made of poly(DL-lactic-co-glycolic
acid). An exemplary poly(DL-lactic-co-glycolic acid) that was found
suitable for use in this context of the present invention has a
ratio of DL-lactic acid to glycolic acid of 75 weight percentage to
25 weight percentage respectively. Manipulating the lactic
acid:glycolic acid ratio in the co-polymer, however, can affect the
chemical and physical properties of the coat. Thus, for example,
using polymers with a higher content of glycolic acid (such as for
example a 50:50 lactic acid:glycolic acid ratio) results in a
polymeric porous coat with smaller pore size, while using polymers
with higher contents of lactic acid (such as for example,
poly(lactic acid) results in a polymeric porous coat with larger
pore size.
[0187] The polymeric coat, according to the present embodiments,
can cover the fibril core either partially or, preferably, entirely
by forming a layer on the fibril core surface. The layer can be a
continuous layer along one side of the core fibril, a multitude of
discontinuing patches, and/or a combination thereof, or form a
complete coat which envelops the fibril core all along its long
axis and all around its circumference.
[0188] The thickness of the coat can be tailored so as to suit any
specific application for which the composite structures are used
for. For example, for long-range temporal drug delivery, a large
reservoir of the drug is required, and hence a relatively thick
coat is preferred. A relatively thick coat is also required to
encapsulate large bioactive agents such as virus-shells and cells,
while the entrapment of relatively small drug molecules which are
needed in small locally-distributed amounts may suffice with a
relatively thin coat. Therefore, the thickness of the coat, layered
on the fibril core according to the present embodiments, can range
from about 10 .mu.m to about 2000 microns and in certain cases can
be even up to 1 cm.
[0189] The choice of a certain thickness of the coat may further
depend on the core thickness, the core-coat ratio in the structure
and the desired thickness of the structure as a whole.
[0190] According to a preferred embodiment of the present
invention, the coat has a porous microstructure. As used herein,
the term "porous" refers to a consistency of a solid material, such
as foam, a spongy solid material or a frothy mass of bubbles
embedded and randomly dispersed within a solid matter.
[0191] A porous polymeric coat is highly beneficial since it allows
a controlled release of agents encapsulated therein. In the context
of the present invention, the porosity or porousness (the coat's
microstructure) can be regarded as a combination of three criteria,
namely the density of the pores, the average pore size (diameter),
and the tortuosity which accounts for how many of the pores are
interconnected so as to form a continuous void inside the solid
part of the coat. The tortuosity is correlated to the pore density
and the average pore size since the inter-connectivity or
discreteness of the pores depends on both the size and density
thereof.
[0192] As discussed in detail hereinabove, suitably designed
structural element, designated for medical purposes such as, for
example, constructing medical devices used in tissue regeneration
procedures, are required to have certain mechanical properties such
as tensile strength and elasticity, and chemical properties such as
biodegradability and non-toxicity. In many applications, it is
desired that the structural element will have the capacity to
contain, and controllably release, biologically and
pharmaceutically active agents, collectively referred herein and
throughout as bioactive agents, to their physiological environment
and thus act as a drug delivery system.
[0193] The coat of the composite structure of the present invention
is designed capable of encapsulating, entrapping or enveloping one
or more bioactive agents therein.
[0194] Specifically, the composite structure according to the
present embodiments is designed capable of encapsulating one or
more bioactive agent(s) within the (voids or pores) of the coat.
Alternatively or in addition, the bioactive agent(s) can be
attached to the inner surface of the coat, applied on the outer
surface of the coat and/or encapsulated within the polymeric coat
itself.
[0195] Furthermore, in some embodiments of the present invention,
the bioactive agent(s) can be incorporated into or onto the
biodegradable fibril core, according to methods known in the art,
while recognizing the limitations associated with such
incorporation, as mentioned hereinabove. Encapsulating of a
bioactive agent in the fibril core of the composite structure
described herein allows for late-release of the bioactive
agent.
[0196] Thus, each of the composite structures of the present
embodiments can further comprise one or more bioactive agents. The
bioactive agent can be encapsulated within or attached to or on the
polymeric coat described herein and/or can be encapsulated in the
fibril core described herein.
[0197] Furthermore, the composite structure according to the
present embodiments is designed such that the encapsulation of the
bioactive agent is performed while retaining at least a part and
preferably most or all of the activity of the bioactive agent(s).
Thus, these agents can exert their biological activity and/or
therapeutic effect once the bioactive agent(s) is released to the
physiological environment, as a result of the biodegradation of the
coat, the core and/or the bond used for attaching it to the
coat.
[0198] The release process depends on and controlled by the
degradation process, which in turn is carried out enzymatically,
chemically or via other metabolic reactions in the physiological
environment both in vivo and in vitro. First to degrade would be
the outer surface of the composite fiber, and in most cases, where
the coat forms an entire envelope, the coat would be first to
degrade while being exposed to the physiological environment. As
the coat is degraded and consumed and the pores are gradually
exposed to the physiological environment, the bioactive agent(s)
encapsulated in the coat is released.
[0199] A release process of bioactive agents from the coat can
therefore be controlled by manipulating the composition of the
biodegradable polymer composing the coat, the size, length and
diameter of the composite structure, the thickness of the coat, the
size and density of the pores, and the amount of bioactive agent(s)
encapsulated within or applied on the coat during the preparation
process of the composite structure. These attributes were tested
for their effect on the release profile of two exemplary bioactive
agents, namely an active enzyme (HRP) and a small molecule drug
(paclitaxel), from exemplary composite structures, as is
demonstrated and exemplified in the Examples section that follows
and is further detailed hereinbelow.
[0200] The biodegradation of the coat and/or the core may further
be controlled by the addition of agents which can control and
modify the biodegradation rate of the polymer composing the core
and/or coat. Hence, according to embodiments of the present
invention, the biodegradable coat and/or the biodegradable fibril
core further include a biodegradation promoting agent.
[0201] A biodegradation promoting agent accelerates the chemical
and/or biochemical degradation processes by providing the required
chemical conditions such as pH, ionic-strength, highly-active and
readily activated species and enzymatic co-factors. Non-limiting
examples of biodegradation promoting agents include cellulose
phosphates, starch phosphates, calcium secondary phosphates,
calcium tertiary phosphates and calcium phosphate hydroxide.
[0202] As used herein, the phrase "bioactive agent" describes a
molecule, compound, complex, adduct and/or composite that exerts
one or more biological and/or pharmaceutical activities. The
bioactive agent can thus be used, for example, to promote wound
healing, tissue regeneration, tumor eradication, and/or to prevent,
ameliorate or treat various medical conditions.
[0203] "Bioactive agents", "pharmaceutically active agents",
"pharmaceutically active materials", "therapeutic active agents",
"biologically active agents", "therapeutic agents", "drugs" and
other related terms are used interchangeably herein and include,
for example, genetic therapeutic agents, non-genetic therapeutic
agents and cells. Bioactive agents useful in accordance with the
present invention may be used singly or in combination. The term
"bioactive agent" in the context of the present invention also
includes radioactive materials which can serve for radiotherapy,
where such materials are utilized for destroying harmful tissues
such as tumors in the local area, or to inhibit growth of healthy
tissues, such as in current stent applications; or as biomarkers
for use in nuclear medicine and radioimaging.
[0204] The bioactive agent can be a hydrophilic bioactive agent or
a hydrophobic bioactive agent.
[0205] The term "hydrophilic", as used herein, describes a trait of
a molecule or part of a molecule which renders the molecule
dissolvable, at least in part, in water, aqueous solutions and/or
other polar solvents. The phrase "at least in part" means that the
substance is either completely dissolvable in such solvents or
reaches its maximal saturation concentration in water, aqueous
solutions and/or other polar solvents, while the remainder of the
substance is in the form of a suspension of small solid particles
in the solvent. Hydrophilic agents are therefore typically
water-soluble agents, in which the dissolvability of the molecule
in water, aqueous solutions and polar solvents is higher than its
dissolvability in oils, organic solvents and other non-polar
solvents. The term "hydrophilic", as used and defined herein, also
encompasses amphiphilic or amphiphatic agents, which are
characterized by a part of the molecule that is hydrophilic and
hence renders the molecule dissolvable, at least to some extent, in
water and aqueous solutions.
[0206] The terms "amphiphilic" or "amphiphatic", as used herein,
refer to a trait of a molecule having both hydrophilic and
hydrophobic nature, namely a polar region that can be either ionic,
or non-ionic, and a non-polar region.
[0207] Exemplary hydrophilic substances include, without
limitation, compounds comprising one or more charged or polar
groups such as one or more carboxyl groups (e.g., organic acids),
one or more hydroxyl groups (e.g., alcohols), one or more amino
groups (e.g., primary, secondary, tertiary and quaternary amines),
and any combination thereof. Such groups are present, for example,
in peptides and saccharides and in many other naturally occurring
and synthetic substances.
[0208] Amphiphilic substances also comprise, alongside with charged
or polar groups, also non-polar moieties such as those exhibited in
hydrophobic substances, as these are defined hereinbelow. Exemplary
types of amphiphilic molecules include, without limitation, anionic
molecules (such as sodium dodecyl sulfate), cationic molecules
(such as benzalkonium chloride), zwitterionic molecules (such as
cocamidopropyl betaine) and non-ionic molecules (such as
octanol).
[0209] Representative examples of hydrophilic and/or of amphiphilic
bioactive agents that can be beneficially incorporated in the coat
described herein include, without limitation, amino acids and
peptide- and protein-based substances such as cytokines,
chemokines, chemo-attractants, chemo-repellants, agonists,
antagonists, antibodies, antigens, enzymes, co-factors, growth
factors, haptens, hormones, and toxins; nucleotide-based substances
such as DNA, RNA, oligonucleotides, labeled oligonucleotides,
nucleic acid constructs, and antisenses; saccharides,
polysaccharides, phospholipids, glycolipids, viruses and cells, as
well as hydrophilic or amphiphatic radioisotopes,
radiopharmaceuticals, steroids, vitamins, angiogenesis-promoters,
drugs, anti histamines, antibiotics, antidepressants,
anti-hypertensive agents, anti-inflammatory agents, antioxidants,
anti-proliferative agents, anti-viral agents, chemotherapeutic
agents, co-factors, cholesterol, fatty acids, bile acids, saponins,
hormones, inhibitors and ligands, and any combination thereof.
[0210] The term "hydrophobic", as used herein, refers to a trait of
a molecule or part of a molecule which is non-polar and is
therefore immiscible with charged and polar molecules, and has a
substantially higher dissolvability in non-polar solvents as
compared with their dissolvability in water and other polar
solvents. The phrase "dissolvability" refers to either complete
dissolution of the substance in these solvents or to cases where
the substance reaches its maximal saturation concentration in
non-polar solvents, and the remainder of the substance is in the
form of a suspension of small solid particles in the solvent. When
in water, hydrophobic molecules often cluster together to form
lumps, agglomerates, aggregates or layers on one of the water
surfaces (such as bottom or top). Exemplary hydrophobic substances
include, without limitation, substances comprising one or more
alkyl groups, such as oils and fats, or one or more aromatic
groups, such as polyaromatic compounds.
[0211] Representative examples of hydrophobic bioactive agents that
can be beneficially incorporated in the coat described herein
include, without limitation drugs, anti-coagulants, statins,
hormones, steroids, lipids, antibiotics, antigens, antidepressants,
anti-hypertensive agents, anti-inflammatory agents, antioxidants,
anti-proliferative agents, anti-viral agents, chemotherapeutic
agents, haptens, inhibitors, ligands, radioisotopes,
radiopharmaceuticals, toxins and any combination thereof.
[0212] Each of the hydrophilic and hydrophobic bioactive agents
described herein can be a macro-biomolecule or a small, organic
molecule.
[0213] The term "macro-biomolecules" as used herein, refers to a
polymeric biochemical substance, or biopolymers, that occur
naturally in living organisms. Polymeric macro-biomolecules are
primarily organic compounds, namely they consist primarily of
carbon and hydrogen, along with nitrogen, oxygen, phosphorus and
sulfur, while other elements can be incorporated therein but at a
lower rate of occurrence. Amino and nucleic acids are some of the
most important building blocks of polymeric macro-biomolecules,
therefore macro-biomolecules are typically comprised of one or more
chains of polymerized amino acids, polymerized nucleic acids,
polymerized saccharides, polymerized lipids and combinations
thereof. Macromolecules may comprise a complex of several
macromolecular subunits which may be covalently or non-covalently
attached to one another. Hence, a ribosome, a cell organelle and
even an intact virus can be regarded as a macro-biomolecule.
[0214] A macro-biomolecule, as used herein, has a molecular weight
higher than 1000 dalton (Da), and can be higher than 3000 Da,
higher than 5000 Da, higher than 10 kDa and even higher than 50
KDa.
[0215] Representative examples of macro-biomolecules, which can be
beneficially incorporated in the coat described herein include,
without limitation, peptides, polypeptides, proteins, enzymes,
antibodies, oligonucleotides and labeled oligonucleotides, nucleic
acid constructs, DNA, RNA, antisense, polysaccharides, viruses and
any combination thereof, as well as cells, including intact cells
or other sub-cellular components and cell fragments.
[0216] As used herein, the phrase "small organic molecule" or
"small organic compound" refers to small compounds which consist
primarily of carbon and hydrogen, along with nitrogen, oxygen,
phosphorus and sulfur and other elements at a lower rate of
occurrence. Organic molecules constitute the entire living world
and all synthetically made organic compounds, therefore they
include all natural metabolites and man-made drugs. In the context
of the present invention, the term "small" with respect to a
compound, agent or molecule, refers to a molecular weight lower
than about 1000 grams per mole. Hence, a small organic molecule has
a molecular weight lower than 1000 Da, lower than 500 Da, lower
than 300 Da, or lower than 100 Da.
[0217] Representative examples of small organic molecules, that can
be beneficially incorporated in the coat described herein include,
without limitation, angiogenesis-promoters, cytokines, chemokines,
chemo-attractants, chemo-repellants, drugs, agonists, amino acids,
antagonists, anti histamines, antibiotics, antigens,
antidepressants, anti-hypertensive agents, anti-inflammatory
agents, antioxidants, anti-proliferative agents, anti-viral agents,
chemotherapeutic agents, co-factors, fatty acids, growth factors,
haptens, hormones, inhibitors, ligands, saccharides, radioisotopes,
radiopharmaceuticals, steroids, toxins, vitamins and any
combination thereof.
[0218] One class of bioactive agents which can be encapsulated in
the coat of the composite structures of the present embodiments is
the class of therapeutic agents that promote angiogenesis. The
successful regeneration of new tissue requires the establishment of
a vascular network. The induction of angiogenesis is mediated by a
variety of factors, any of which may be used in conjunction with
the present invention (Folkman and Klagsbrun, 1987, and references
cited therein, each incorporated herein in their entirety by
reference).
[0219] Non-limiting examples of angiogenesis-promoters include
vascular endothelial growth factor (VEGF) or vascular permeability
factor (VPF); members of the fibroblast growth factor family,
including acidic fibroblast growth factor (AFGF) and basic
fibroblast growth factor (bFGF); interleukin-8 (IL-8); epidermal
growth factor (EGF); platelet-derived growth factor (PDGF) or
platelet-derived endothelial cell growth factor (PD-ECGF);
transforming growth factors alpha and beta (TGF-.alpha.,
TGF-.beta.); tumor necrosis factor alpha (TNF-.beta.); hepatocyte
growth factor (HGF); granulocyte-macrophage colony stimulating
factor (GM-CSF); insulin growth factor-1 (IGF-1); angiogenin;
angiotropin; and fibrin and nicotinamide.
[0220] Another important class of bioactive agents which can be
incorporated into the coat of the composite structures of the
present embodiments, especially in certain embodiments which
involve tissue regeneration, implantable devices and healing are
cytokines, chemokines and related factors. Control over these
agents can translate into a successful medical procedure when the
immune system plays a key role. Cytokines are any of several small
non-antibody regulatory protein molecules, such as the interleukins
and lymphokines, which are released by cells of the immune system
population on contact with a specific antigen and act as
intercellular mediators in the generation of an immune response.
Cytokines are the core of communication between immune system
cells, and between these cells and cells belonging to other tissue
types. There are many known cytokines that have both stimulating
and suppressing action on lymphocyte cells and immune response.
They act by binding to their cell-specific receptors. These
receptors are located in the cell membrane and each allows a
distinct signal transduction cascade to start in the cell that
eventually will lead to biochemical and phenotypical changes in the
target cell. Typically, receptors for cytokines are also tyrosine
kinases.
[0221] Non-limiting examples of cytokines and chemokines include
angiogenin, calcitonin, ECGF, EGF, E-selectin, L-selectin, FGF, FGF
basic, G-CSF, GM-CSF, GRO, Hirudin, ICAM-1, IFN, IFN-.gamma.,
IGF-I, IGF-II, IL-1, IL-2, IL-3, IL-4, IL-5, IL-6, IL-7, IL-8,
IL-9, IL-10, M-CSF, MIF, MIP-1, MIP-1.alpha., MIP-1.beta., NGF
chain, NT-3, PDGF-.alpha., PDGF-.beta., PECAM, RANTES, TGF-.alpha.,
TGF-.beta., TNF-.alpha., TNF-.beta., TNF-.kappa. and VCAM-1
[0222] Bioactive agents which can be beneficially incorporated into
the coat of the composite structures of the present embodiments
also include both polymeric (macro-biomolecules, for example,
proteins, enzymes) and non-polymeric (small molecule therapeutics)
agents and include Ca-channel blockers, serotonin pathway
modulators, cyclic nucleotide pathway agents, catecholamine
modulators, endothelin receptor antagonists, nitric oxide
donors/releasing molecules, anesthetic agents, ACE inhibitors,
ATII-receptor antagonists, platelet adhesion inhibitors, platelet
aggregation inhibitors, coagulation pathway modulators,
cyclooxygenase pathway inhibitors, natural and synthetic
corticosteroids, lipoxygenase pathway inhibitors, leukotriene
receptor antagonists, antagonists of E- and P-selectins, inhibitors
of VCAM-1 and ICAM-1 interactions, prostaglandins and analogs
thereof, macrophage activation preventers, HMG-CoA reductase
inhibitors, fish oils and omega-3-fatty acids, free-radical
scavengers/antioxidants, agents affecting various growth factors
(including FGF pathway agents, PDGF receptor antagonists, IGF
pathway agents, TGF-.beta. pathway agents, EGF pathway agents,
TNF-.alpha. pathway agents, Thromboxane A2 [TXA2] pathway
modulators, and protein tyrosine kinase inhibitors), MMP pathway
inhibitors, cell motility inhibitors, anti-inflammatory agents,
antiproliferative/antineoplastic agents, matrix
deposition/organization pathway inhibitors, endothelialization
facilitators, blood rheology modulators, as well as integrins,
chemokines, cytokines and growth factors.
[0223] Additional bioactive agents which can be beneficially
incorporated into the coat of the composite structures of the
present embodiments include cytotoxic factors or cell cycle
inhibitors, including CD inhibitors, such as p53, thymidine kinase
("TK") and other agents useful for interfering with cell
proliferation.
[0224] Additional bioactive agents which can be beneficially
incorporated into the coat of the composite structures of the
present embodiments include genetic therapeutic agents and
proteins, such as ribozymes, anti-sense polynucelotides and
polynucleotides coding for a specific product (including
recombinant nucleic acids) such as genomic DNA, cDNA, or RNA. The
polynucleotide can be provided in "naked" form or in connection
with vector systems that enhances uptake and expression of
polynucleotides. These can include DNA compacting agents (such as
histones), non-infectious vectors (such as plasmids, lipids,
liposomes, cationic polymers and cationic lipids) and viral vectors
such as viruses and virus-like particles (i.e., synthetic particles
made to act like viruses). The vector may further have attached
peptide targeting sequences, anti-sense nucleic acids (DNA and
RNA), and DNA chimeras which include gene sequences encoding for
ferry proteins such as membrane translocating sequences ("MTS"),
tRNA or rRNA to replace defective or deficient endogenous molecules
and herpes simplex virus-1 ("VP22").
[0225] Additional bioactive agents which can be beneficially
incorporated into the coat of the composite structures of the
present embodiments include gene delivery agents, which may be
either endogenously or exogenously controlled. Examples of
endogenous control include promoters that are sensitive to a
physiological signal such as hypoxia or glucose elevation.
Exogenous control systems involve gene expression controlled by
administering a small molecule drug. Examples include tetracycline,
doxycycline, ecdysone and its analogs, RU486, chemical dimerizers
such as rapamycin and its analogs, etc.
[0226] Additional bioactive agents which can be beneficially
incorporated into the coat of the composite structures of the
present embodiments include the family of bone morphogenic proteins
("BMP's") such as BMP-2, BMP-3, BMP-4, BMP-5, BMP-6 (Vgr-1), BMP-7
(OP-1), BMP-8, BMP-9, BMP-10, BMP-11, BMP-12, BMP-13, BMP-14,
BMP-15, and BMP-16. Some of these dimeric proteins can be provided
as homodimers, heterodimers, or combinations thereof, alone or
together with other molecules. Alternatively or, in addition,
molecules capable of inducing an upstream or downstream effect of a
BMP can be provided. Such molecules include any of the "hedgehog"
proteins, or the DNA's encoding them.
[0227] Additional bioactive agents which can be beneficially
incorporated into the coat of the composite structures of the
present embodiments include cell survival molecules such as Akt,
insulin-like growth factor 1, NF-kB decoys, 1-kB, Madh6, Smad6 and
Apo A-1.
[0228] Additional bioactive agents which can be beneficially
incorporated into the coat of the composite structures of the
present embodiments include viral and non-viral vectors, such as
adenoviruses, gutted adenoviruses, adeno-associated virus,
retroviruses, alpha virus (Semliki Forest, Sindbis, etc.),
lentiviruses, herpes simplex virus, ex vivo modified cells (i.e.,
stem cells, fibroblasts, myoblasts, satellite cells, pericytes,
cardiomyocytes, sketetal myocytes, macrophage, etc.), replication
competent viruses (ONYX-015, etc.), and hybrid vectors, artificial
chromosomes and mini-chromosomes, plasmid DNA vectors (pCOR),
cationic polymers (polyethyleneimine, polyethyleneimine (PEI) graft
copolymers such as polyether-PEI and polyethylene oxide-PEI,
neutral polymers PVP, SP1017 (SUPRATEK), lipids or lipoplexes,
nanoparticles and microparticles with and without targeting
sequences such as the protein transduction domain (PTD).
[0229] Additional bioactive agents which can be beneficially
incorporated into the coat of the composite structures of the
present embodiments include chemotherapeutic agents. Non-limiting
examples of chemotherapeutic agents include amino containing
chemotherapeutic agents such as daunorubicin, doxorubicin,
N-(5,5-diacetoxypentyl)doxorubicin, anthracycline, mitomycin C,
mitomycin A, 9-amino camptothecin, aminopertin, antinomycin,
N.sup.8-acetyl spermidine, 1-(2-chloroethyl)-1,2-dimethanesulfonyl
hydrazine, bleomycin, tallysomucin, and derivatives thereof;
hydroxy containing chemotherapeutic agents such as etoposide,
camptothecin, irinotecaan, topotecan, 9-amino camptothecin,
paclitaxel, docetaxel, esperamycin, 1,8-dihydroxy-bicyclo
[7.3.1]trideca-4-ene-2,6-diyne-13-one, anguidine,
morpholino-doxorubicin, vincristine and vinblastine, and
derivatives thereof, sulfhydril containing chemotherapeutic agents
and carboxyl containing chemotherapeutic agents.
[0230] Additional bioactive agents which can be beneficially
incorporated into the coat of the composite structures of the
present embodiments include antibiotic agents. Non-limiting
examples of antibiotic agents include benzoyl peroxide, octopirox,
erythromycin, zinc, tetracyclin, triclosan, azelaic acid and its
derivatives, phenoxy ethanol and phenoxy proponol, ethylacetate,
clindamycin and meclocycline; sebostats such as flavinoids; alpha
and beta hydroxy acids; and bile salts such as scymnol sulfate and
its derivatives, deoxycholate and cholate.
[0231] Additional bioactive agents which can be beneficially
incorporated into the coat of the composite structures of the
present embodiments include non-steroidal anti-inflammatory agents.
Non-limiting examples of non-steroidal anti-inflammatory agents
include oxicams, such as piroxicam, isoxicam, tenoxicam, sudoxicam,
and CP-14,304; salicylates, such as aspirin, disalcid, benorylate,
trilisate, safapryn, solprin, diflunisal, and fendosal; acetic acid
derivatives, such as diclofenac, fenclofenac, indomethacin,
sulindac, tolmetin, isoxepac, furofenac, tiopinac, zidometacin,
acematacin, fentiazac, zomepirac, clindanac, oxepinac, felbinac,
and ketorolac; fenamates, such as mefenamic, meclofenamic,
flufenamic, niflumic, and tolfenamic acids; propionic acid
derivatives, such as ibuprofen, naproxen, benoxaprofen,
flurbiprofen, ketoprofen, fenoprofen, fenbufen, indopropfen,
pirprofen, carprofen, oxaprozin, pranoprofen, miroprofen,
tioxaprofen, suprofen, alminoprofen, and tiaprofenic; pyrazoles,
such as phenylbutazone, oxyphenbutazone, feprazone, azapropazone,
and trimethazone.
[0232] Additional bioactive agents which can be beneficially
incorporated into the coat of the composite structures of the
present embodiments include steroidal anti-inflammatory drugs.
Non-limiting examples of steroidal anti-inflammatory drugs include,
without limitation, corticosteroids such as hydrocortisone,
hydroxyltriamcinolone, alpha-methyl dexamethasone,
dexamethasone-phosphate, beclomethasone dipropionates, clobetasol
valerate, desonide, desoxymethasone, desoxycorticosterone acetate,
dexamethasone, dichlorisone, diflorasone diacetate, diflucortolone
valerate, fluadrenolone, fluclorolone acetonide, fludrocortisone,
flumethasone pivalate, fluosinolone acetonide, fluocinonide,
flucortine butylesters, fluocortolone, fluprednidene
(fluprednylidene) acetate, flurandrenolone, halcinonide,
hydrocortisone acetate, hydrocortisone butyrate,
methylprednisolone, triamcinolone acetonide, cortisone,
cortodoxone, flucetonide, fludrocortisone, difluoro sone diacetate,
fluradrenolone, fludrocortisone, difluro sone diacetate,
fluradrenolone acetonide, medrysone, amcinafel, amcinafide,
betamethasone and the balance of its esters, chloroprednisone,
chlorprednisone acetate, clocortelone, clescinolone, dichlorisone,
diflurprednate, flucloronide, flunisolide, fluoromethalone,
fluperolone, fluprednisolone, hydrocortisone valerate,
hydrocortisone cyclopentylpropionate, hydrocortamate, meprednisone,
paramethasone, prednisolone, prednisone, beclomethasone
dipropionate, triamcinolone, and mixtures thereof.
[0233] Additional bioactive agents which can be beneficially
incorporated into the coat of the composite structures of the
present embodiments include anti-oxidants. Non-limiting examples of
anti-oxidants include ascorbic acid (vitamin C) and its salts,
ascorbyl esters of fatty acids, ascorbic acid derivatives (for
example, magnesium ascorbyl phosphate, sodium ascorbyl phosphate,
ascorbyl sorbate), tocopherol (vitamin E), tocopherol sorbate,
tocopherol acetate, other esters of tocopherol, butylated hydroxy
benzoic acids and their salts,
6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid
(commercially available under the trade name Trolox.RTM.), gallic
acid and its alkyl esters, especially propyl gallate, uric acid and
its salts and alkyl esters, sorbic acid and its salts, lipoic acid,
amines (for example, N,N-diethylhydroxylamine, amino-guanidine),
sulfhydryl compounds (for example, glutathione), dihydroxy fumaric
acid and its salts, lycine pidolate, arginine pilolate,
nordihydroguaiaretic acid, bioflavonoids, curcumin, lysine,
methionine, proline, superoxide dismutase, silymarin, tea extracts,
grape skin/seed extracts, melanin, and rosemary extracts.
[0234] Additional bioactive agents which can be beneficially
incorporated into the coat of the composite structures of the
present embodiments include vitamins. Non-limiting examples of
vitamins include vitamin A and its analogs and derivatives:
retinol, retinal, retinyl palmitate, retinoic acid, tretinoin,
iso-tretinoin (known collectively as retinoids), vitamin E
(tocopherol and its derivatives), vitamin C (L-ascorbic acid and
its esters and other derivatives), vitamin B.sub.3 (niacinamide and
its derivatives), alpha hydroxy acids (such as glycolic acid,
lactic acid, tartaric acid, malic acid, citric acid, etc.) and beta
hydroxy acids (such as salicylic acid and the like).
[0235] Additional bioactive agents which can be beneficially
incorporated into the coat of the composite structures of the
present embodiments include hormones. Non-limiting examples of
hormones include androgenic compounds and progestin compounds such
as methyltestosterone, androsterone, androsterone acetate,
androsterone propionate, androsterone benzoate, androsteronediol,
androsteronediol-3-acetate, androsteronediol-17-acetate,
androsteronediol 3-17-diacetate, androsteronediol-17-benzoate,
androsteronedione, androstenedione, androstenediol,
dehydroepiandrosterone, sodium dehydroepiandrosterone sulfate,
dromostanolone, dromostanolone propionate, ethylestrenol,
fluoxymesterone, nandrolone phenpropionate, nandrolone decanoate,
nandrolone furylpropionate, nandrolone cyclohexane-propionate,
nandrolone benzoate, nandrolone cyclohexanecarboxylate,
androsteronediol-3-acetate-1-7-benzoate, oxandrolone, oxymetholone,
stanozolol, testosterone, testosterone decanoate,
4-dihydrotestosterone, 5.alpha.-dihydrotestosterone, testolactone,
17.alpha.-methyl-19-nortestosterone and pharmaceutically acceptable
esters and salts thereof, and combinations of any of the foregoing,
desogestrel, dydrogesterone, ethynodiol diacetate,
medroxyprogesterone, levonorgestrel, medroxyprogesterone acetate,
hydroxyprogesterone caproate, norethindrone, norethindrone acetate,
norethynodrel, allylestrenol, 19-nortestosterone, lynoestrenol,
quingestanol acetate, medrogestone, norgestrienone, dimethisterone,
ethisterone, cyproterone acetate, chlormadinone acetate, megestrol
acetate, norgestimate, norgestrel, desogrestrel, trimegestone,
gestodene, nomegestrol acetate, progesterone,
5.alpha.-pregnan-3.beta.,20.alpha.-diol sulfate,
5.alpha.-pregnan-3.beta.,20.beta.-diol sulfate,
5.alpha.-pregnan-3.beta.-ol-20-one,
16,5.alpha.-pregnen-3.beta.-ol-20-one,
4-pregnen-20.beta.-ol-3-one-20-sulfate, acetoxypregnenolone,
anagestone acetate, cyproterone, dihydrogesterone, flurogestone
acetate, gestadene, hydroxyprogesterone acetate,
hydroxymethylprogesterone, hydroxymethyl progesterone acetate,
3-ketodesogestrel, megestrol, melengestrol acetate, norethisterone
and mixtures thereof.
[0236] Additional bioactive agents which can be beneficially
incorporated into the coat of the composite structures of the
present embodiments include cells of human origin (autologous or
allogeneic), including stem cells, or from an animal source
(xenogeneic), which can be genetically engineered if desired to
deliver proteins of interest. Cell types include bone marrow
stromal cells, endothelial progenitor cells, myogenic cells
including cardiomyogenic cells such as procardiomyocytes,
cardiomyocytes, myoblasts such as skeletomyoblasts, fibroblasts,
stem cells (for example, mesenchymal, hematopoietic, neuronal and
so forth), pluripotent stem cells, macrophage, satellite cells and
so forth. Cells appropriate for the practice of the present
invention also include biopsy samples for direct use (for example,
whole bone marrow) or fractions thereof (for example, bone marrow
stroma, bone marrow fractionation for separation of leukocytes).
Where appropriate, media can be formulated as needed and included
in the preparation of the fibers of the present invention so as to
maintain cell function and viability. As mentioned herein, the
incorporation of cells into the coat can be preferably effected by
attaching the cells to the surface of the coat, or by employing a
coat that have large pores in the order of at least 100 .mu.m in
diameter or higher.
[0237] As discussed hereinabove, the porosity (microstructure) of
the coat is a key element for determining the release profile of
the bioactive agent therefrom, and it is defined by the average
pore size (diameter) and the density thereof, which also reflect
the level of inter-connectivity of the pores.
[0238] In general, according to preferred embodiments of the
present invention, the porosity of the coat is characterized by an
average pore diameter that can range from 0.001 .mu.m (1 nm) to
1000 .mu.m (1 mm), and a pore density that can range from about 50%
void volume per coat volume to about 95% void volume per coat
volume, preferably from about 70% void volume per coat volume to
about 95% void volume per coat volume, and more preferably from
about 80% void volume per coat volume to about 95% void volume per
coat volume.
[0239] Several factors affect the resulting pore size and density,
including the nature of the bioactive agent which is incorporated
into the coat, and the process of preparing the coat.
[0240] The coat's microstructure strongly affects the rate of
release of the incorporated bioactive agent. According to the
present embodiments, the average pore diameter and density in the
porous coat can be finely controlled so as to enable a particularly
desirable release profile of the encapsulated agent which is
suitable for a particular application. In turn, the nature of the
bioactive agent, namely its capacity to dissolve in water and other
aqueous solutions, affects the coat's microstructure.
[0241] Furthermore, without being bound to any particular theory,
stemming from the process of preparing the coat, presented
hereinbelow, it is assumed that a hydrophobic bioactive agent will
be incorporated into the solid walls of the coat, while hydrophilic
and amphiphilic agents will be incorporated in or on the inner
walls of the pores. Hence, when introduced into a physiological
medium, which is substantially aqueous, a hydrophilic agent will be
exposed to the solvent (water) as soon as the solvent enters the
void constituting a pore, and therefore will be released
immediately upon the exposure of the pore to the physiological
medium. On the other hand, a hydrophobic bioactive agent with
resides inside the solid polymeric walls of the coat will be
released according to the surface area of the solid polymer and at
a rate no faster than the rate of degradation of the solid
polymer.
[0242] When attempting to design the release profile of a bioactive
agent form a composite structure as presented herein, one has to
consider the desirable burst-rate which takes place as soon as the
composite structure is exposed to the host medium, and the
diffusion-controlled rate of release which follows the initial
burst. These stages of release can be controlled by the porosity of
the coat which dictates the surface area exposed to the medium and
the detailed microstructure of the coat.
[0243] For example, a hydrophilic bioactive agent, which is assumed
to be incorporated on the inner walls of the pores, will be all
released as soon as the coat is exposed to an aqueous media in case
the pores are substantially interconnected. In order to lower the
extent of this burst, and allow the agent to be released in a more
prolonged and steady rate, the pores should be discrete so the
inner void of each is exposed to the medium only upon degradation
of its solid polymer wall.
[0244] On the other hand, a burst release of hydrophobic bioactive
agent, which resides within the solid polymer part of the coat,
will be possible if a large surface area of the polymer is exposed
simultaneously to the medium, and therefore the porosity of a
composite structure which incorporates a hydrophobic agent
preferably exhibits interconnected pores so to allow the medium to
penetrate deep into the coat and bring about its degradation more
effectively.
[0245] Regardless of its water-solubility, a relatively large
bioactive agent, such as a virus, an organelle or a cell would
require a suitable pore size to fit its size. Thus, in the case of
a large bioactive agent, the porosity will be characterized by a
large pore size.
[0246] Thus, for example, porous coats designed to encapsulate or
encapsulating a hydrophilic/amphiphilic (water-soluble) bioactive
agent, have a preferred average pore diameter ranges from about 1
nm to about 50 .mu.m, a preferred density ranges from about 70% of
void volume per coat volume to about 90% of void volume per coat
volume, and/or discrete pores.
[0247] Porous coats designed to encapsulate or encapsulating a
hydrophobic (water-insoluble) bioactive agent, have a preferred
average pore diameter ranges from about 1 nm to about 200 .mu.m, a
density that ranges from about 80% of void volume per coat volume
to about 95% of void volume per coat volume, and/or interconnected
pores.
[0248] In cases where the encapsulated agent comprises large
macro-biomolecules, assemblies thereof, organelles or intact cells,
larger pores, having an average pore diameter that ranges from
about 50 .mu.m to about 500 .mu.m and higher are preferred.
[0249] As detailed hereinbelow and is further demonstrated in the
Examples section that follows, a suitable porosity can be adjusted
to almost any bioactive agent by modifying certain parameters in
the process of preparing the composite structures presented herein
and by the use of additional agents and other mechanical and
kinetic factors which contribute to the final microstructure of the
coat, utilizing this flexibility towards a wide range of
therapeutic and other applications.
[0250] One group of additional agents which may contribute to the
final microstructure of the coat includes surfactants or surface
active agents, as these are defined hereinbelow. As demonstrated in
the examples section that follows, the addition of a surfactant at
the preparation stage of the coat material affects the porosity
thereof and in some cases is essential to the formation of the
coat. The requirement of a surfactant is strongly associated with
the nature of the bioactive agent, namely its hydrophobicity or
lack thereof. A hydrophobic bioactive agent and a hydrophilic
bioactive agent may not contribute to the stability of the coat's
precursor, while an amphiphilic bioactive agent, which may act as a
surfactant in most cases, will render the use of an additional
surface active agent unnecessary.
[0251] The coat can further include, in addition to the bioactive
agent, additional agents that may improve the performance of the
bioactive agent. These include, for example, penetration enhancers,
humectants, chelating agents, preservatives, occlusive agents,
emollients, permeation enhancers, and anti-irritants. These agents
can be encapsulated within the pores of a porous coat or can be
doped within the polymer forming the coat.
[0252] Representative examples of humectants include, without
limitation, guanidine, glycolic acid and glycolate salts (for
example ammonium slat and quaternary alkyl ammonium salt), aloe
vera in any of its variety of forms (for example, aloe vera gel),
allantoin, urazole, polyhydroxy alcohols such as sorbitol,
glycerol, hexanetriol, propylene glycol, butylene glycol, hexylene
glycol and the like, polyethylene glycols, sugars and starches,
sugar and starch derivatives (for example, alkoxylated glucose),
hyaluronic acid, lactamide monoethanolamine, acetamide
monoethanolamine and any combination thereof.
[0253] Non-limiting examples of chelating agents include
ethylenediaminetetraacetic acid (EDTA), EDTA derivatives, or any
combination thereof.
[0254] Non-limiting examples of occlusive agents include
petrolatum, mineral oil, beeswax, silicone oil, lanolin and
oil-soluble lanolin derivatives, saturated and unsaturated fatty
alcohols such as behenyl alcohol, hydrocarbons such as squalane,
and various animal and vegetable oils such as almond oil, peanut
oil, wheat germ oil, linseed oil, jojoba oil, oil of apricot pits,
walnuts, palm nuts, pistachio nuts, sesame seeds, rapeseed, cade
oil, corn oil, peach pit oil, poppyseed oil, pine oil, castor oil,
soybean oil, avocado oil, safflower oil, coconut oil, hazelnut oil,
olive oil, grape seed oil and sunflower seed oil.
[0255] Non-limiting examples of emollients include dodecane,
squalane, cholesterol, isohexadecane, isononyl isononanoate, PPG
Ethers, petrolatum, lanolin, safflower oil, castor oil, coconut
oil, cottonseed oil, palm kernel oil, palm oil, peanut oil, soybean
oil, polyol carboxylic acid esters, derivatives thereof and
mixtures thereof.
[0256] Non-limiting examples of penetration enhancers include
dimethylsulfoxide (DMSO), dimethyl formamide (DMF), allantoin,
urazole, N,N-dimethylacetamide (DMA), decylmethylsulfoxide
(C.sub.10 MSO), polyethylene glycol monolaurate (PEGML), propylene
glycol (PG), propylene glycol monolaurate (PGML), glycerol
monolaurate (GML), lecithin, the 1-substituted
azacycloheptan-2-ones, particularly
1-n-dodecylcyclazacycloheptan-2-one (available under the trademark
Azone.RTM. from Whitby Research Incorporated, Richmond, Va.),
alcohols, and the like. The permeation enhancer may also be a
vegetable oil. Such oils include, for example, safflower oil,
cottonseed oil and corn oil.
[0257] Non-limiting examples of anti-irritants include steroidal
and non steroidal anti-inflammatory agents or other materials such
as aloe vera, chamomile, alpha-bisabolol, cola nitida extract,
green tea extract, tea tree oil, licoric extract, allantoin,
caffeine or other xanthines, glycyrrhizic acid and its
derivatives.
[0258] Non-limiting examples of preservatives include one or more
alkanols, disodium EDTA (ethylenediamine tetraacetate), EDTA salts,
EDTA fatty acid conjugates, isothiazolinone, parabens such as
methylparaben and propylparaben, propylene glycols, sorbates, urea
derivatives such as diazolindinyl urea, or any combinations
thereof. The composite structures according to the present
embodiments are particularly beneficial when it is desired to
encapsulate bioactive agents which require delicate treatment and
handling, and which cannot retain their biological and/or
therapeutic activity if exposed to conditions such as heat,
damaging substances and solvents and/or other damaging conditions.
Such bioactive agents include, for example, peptides, polypeptides,
proteins, amino acids, polysaccharides, growth factors, hormones,
anti-angiogenesis factors, interferons or cytokines, cells and
pro-drugs.
[0259] The amount of the bioactive agents that is loaded in the
composite structure is preferably selected sufficient to exert the
desired therapeutic or biological effect. The effective amount of a
bioactive agent therefore depends on the particular agent being
used and can further depend on the desired application of the
resulting structure. Thus, for example, in cases where the
bioactive agent is a growth hormone, minute amounts of the agent
are required so as to exert effective therapy. In cases where the
bioactive agent is a protein or a peptide, medium range amounts of
the agent are required. In cases where the bioactive agent is a
metabolite having a high metabolic turnover rate or a chemical
drugs, larger amounts of the bioactive agent are typically
required.
[0260] Therefore, the amount of the bioactive agent in the
composite structures can range from about 0.00001 weight percentage
to about 50 weight percentages of the amount of the total weight of
the coat, and preferably ranges from about 0.1 weight percentage to
about 30 weight percentages of the amount of the total weight of
the coat, more preferably from about 1 weight percentage to about
20 weight percentages and more preferably from about 1 weight
percentage to about 10 weight percentages of the total weight of
the coat, in cases where the bioactive agent is a biomolecules such
as a peptide. As indicated hereinabove, for bioactive agents such
as growth factors, an amount in the composite structures of from
about 0.00001 to about 0.0001 percents of the total weight of the
coat is sufficient to exert the desired activity, whereby for
bioactive agents such as, for example, synthetic drugs, an amount
in the composite structures of from about 1 to about 30 percents of
the total weight of the coat is preferred. As demonstrated in the
Examples section that follows, an active enzyme (the protein HRP)
and a small hydrophobic organic drug molecule (paclitaxel) were
incorporated into the coat of a composite structure at an amount
that ranges from about 0.00001 to about 10 percents of the total
weight of the coat.
[0261] As is demonstrated in the Examples section that follows,
composite structures containing such relative weights of HRP and
the polymer composing the coat were successfully prepared while
retaining 95% of the activity of the enzyme and achieving a
controllable release thereof. It would be recognized, however, that
lower or higher amounts may be used to achieve efficacious
incorporation and release of other various bioactive agents.
[0262] The amount of the bioactive agent further affects the rate
of release thereof, particularly in cases where the bioactive agent
is encapsulated within the pore voids (a hydrophilic/amphiphatic
agent), due to diffusion-related factors. Hence, the amount of the
bioactive agent can be further manipulated in accordance with the
desired release rate thereof.
[0263] Each of the composite structures described herein can be
further utilized to form a larger, more complex element. The
formation of such an element can be effected, for example, by
assembling a plurality of the composite structures described herein
or by assembling one or more of these composite structures with
other fibers or structures. Such an assembling can be effected, for
example, by twist-spinning a plurality of fibers and/or composites
into cords, weaving a plurality of fibers and/or composites into
meshes, layering a plurality of fibers and/or composites into
sheets and using several of the above techniques in sequence so as
to form more and more complex elements.
[0264] Thus, according to further aspects of the present invention,
there is provided a fibrous composition-of-matter which includes
one or more of the composite structures described herein, either
alone or in combination with other fibers and/or composites. The
composition-of-matter can be, for example, in the form of a cord, a
mesh or a sheet. The composition-of-matter can alternatively be a
three-dimensional structure.
[0265] Being capable of delivering bioactive agents in a controlled
manner, meshes and sheets made from the composite structures of the
present invention can be beneficially used to wound dressing, skin
patches and other medical applications, as discussed in detail
hereinbelow.
[0266] In order to produce the composite structures described
herein, and particularly such structures which combine properties
such as desired mechanical properties together with the capacity to
contain bioactive agents while retaining their activity and to
controllably release these agents, the present inventors have
developed a unique process.
[0267] Thus, according to another aspect of the present invention
there is provided a process of preparing the composite structures
as described herein. The process is effected by providing a fiber
or a fibril; providing an emulsion containing an aqueous solution
and an organic solution which comprises a second polymer;
contacting the fiber and the emulsion to thereby obtain a fiber
having a layer of the emulsion applied on at least a part of the
fiber; and freeze-drying the fiber having the layer applied
thereon.
[0268] The fibers constituting the fibril core of the composite
structures of the present embodiments can be of natural or
synthetic origins, and can be provided ready for use without
further manipulation or preparation procedures or upon surface
treatment thereof.
[0269] Alternatively, the fibers which serve as the fibril core of
the composite structure of the present embodiments can be produced
by conventional fiber-spinning techniques. Such techniques include,
for example, solution spinning, electro-spinning, wet spinning, dry
spinning, melt spinning and gel spinning. Each spinning method
imparts specific physical dimensions and mechanical properties of
the resulting fibers, and can be tuned to give the desired
characteristics according to the required application of the
resulting composite structure.
[0270] Briefly, a fiber spinning technique typically involves the
use of spinnerets. These are similar, in principle, to a bathroom
shower head, and may have from one to several hundred small holes.
As the filaments, or crude fibers, emerge from the holes in the
spinneret, the dissolved or liquefied polymer is converted first to
a rubbery state and then solidified. This process of extrusion and
solidification of "endless" crude fibers is called spinning, not to
be confused with the textile operation of the same name, where
short pieces of staple fiber are twisted into yarn.
[0271] Preferably, the fiber is made of one or more polymer(s),
herein the first polymer. Such polymeric fibers can be produced,
for example, by the fiber spinning processes detailed hereinbelow.
Non-polymeric fibers can be produced, for example, by
melt-spinning.
[0272] Wet spinning is used for fiber-forming substances that have
been dissolved in a solvent. The spinnerets are submerged in a
chemical bath and as the filaments emerge they precipitate from
solution and solidify. Because the solution is extruded directly
into the precipitating liquid, this process for making fibers is
called wet spinning. Fibers such as acrylic, rayon, aramid,
modacrylic and spandex can be produced by this process.
[0273] Dry spinning is also used for fiber-forming substances in
solution, however, instead of precipitating the polymer by dilution
or chemical reaction, solidification is achieved by evaporating the
solvent in a stream of air or inert gas. The filaments do not come
in contact with a precipitating liquid, eliminating the need for
drying and easing solvent recovery. This process may be used for
the production of acetate, triacetate, acrylic, modacrylic, PBI,
spandex and vinyon.
[0274] In melt spinning, the fiber-forming substance is melted for
extrusion through the spinneret and then the crude fibers directly
solidified by cooling. Melt spun crude fibers can be extruded from
the spinneret in different cross-sectional shapes (round, trilobal,
pentagonal, octagonal and others). Nylon (polyamide), olefin,
polyester, saran and sulfar are produced in this manner.
[0275] Gel spinning is a special process used to obtain high
strength or other special fiber properties. The polymer is not in a
true liquid state during extrusion. Not completely separated, as
they would be in a true solution, the polymer chains are bound
together at various points in liquid crystal form. This produces
strong inter-chain forces in the resulting filaments that can
significantly increase the tensile strength of the fibers. In
addition, the liquid crystals are aligned along the fiber axis by
the shear forces during extrusion. The filaments emerge with an
unusually high degree of orientation relative to each other which
increases their strength. The process can also be described as
dry-wet spinning, since the filaments first pass through air and
then are cooled further in a liquid bath. Some high-strength
polyethylene and aramid fibers are produced by gel spinning.
[0276] The fibril core of the composite structure of the present
invention is preferably made by melt spinning or gel spinning. Most
preferably the fibril core is made by melt spinning.
[0277] Electro-spinning is a process used to form very thin fibers.
In this process the fibers are drawn out from a viscous polymer
solution or melt by applying an electric field to a droplet of the
solution, typically at the tip of a metallic needle. The electric
field draws this droplet into a conical structure. If the viscosity
and surface tension of the solution are appropriately tuned,
varicose breakup is avoided without reaching electro-spray and a
stable continuous jet of the liquid polymer is formed. The tendency
to bend results in a whipping process which stretches and elongates
the emerging fiber until its diameter is reduced to few micrometers
or even nanometers, and the fiber is then deposited on a grounded
collector spool.
[0278] The use of solution spinning for preparing fibers which can
have a bioactive agent incorporated therein is described, for
example, in U.S. Pat. Nos. 6,485,737, 6,596,296 and 6,858,222, in
U.S. patent application having the Publication No. 20050106211 and
in WO 01/10421, which are incorporated by reference as if fully set
forth herein. According to the teachings of these patents and
patent applications, the fibers are made by extruding a
water-in-oil emulsion made from a polymer solution and an aqueous
solution, through a dispensing tip and into a coagulation bath. The
coagulation bath contains a solvent which is miscible with the
solvent of the polymer but immiscible with water and is a
non-solvent for the polymer. The resulting fibers are then
collected on a drying spool. These fibers, although capable of
entrapping a bioactive agent therein, are ultimately limited in the
mechanical properties as compared to fibers which are made of
similar polymers but with other spinning techniques.
[0279] As mentioned hereinabove, in some embodiments of the present
invention, the composite structures are biodegradable structures,
comprising a biodegradable core and a biodegradable coat, each
encapsulating one or more bioactive agents. In these cases, the
core fiber containing one or more bioactive agents can be prepared
using any of the methods described in the art and presented
hereinabove, including solution spinning, while recognizing the
compromised made with respect to the mechanical properties and
physical dimension of the resulting fibers.
[0280] As mentioned hereinabove, in addition to bioactive agent(s),
additional ingredients, such as biodegradation promoting agents and
other agents, can be added to the polymer in the process of
preparing the core fibers.
[0281] In cases where the mechanical properties and physical
dimensions of the fibril core require the fiber to be flexible and
thin yet relatively unyielding, the most effective spinning
technique which will achieve these requirements is melt
spinning.
[0282] In the case where melt-spinning is used to produce the
fibers for the fibril core of the present embodiments, the process
is carried out at an elevated temperature so as to melt the
fiber-forming substance and impart a suitable viscosity thereto
prior to its extrusion through the spinneret. When a polymer such
as, for example, poly(L-lactic acid) having a melting point of
173-178.degree. C., is used for the core, the melt-spinning is
effected at a temperature which ranges from about 50.degree. C. to
about 250.degree. C., and preferably at a temperature of
190.degree. C.
[0283] While extruded crude fibers are solidifying, or in some
cases even after they have hardened, the crude fibers may be drawn
to impart strength and other flexibility thereto. As they emerge
from the spinneret, the crude fibers have little molecular
orientation, and their slight birefringence quality (double
refraction), which is used to quantify their degree of internal
molecular orientation and a measure of molecular anisotropy versus
crystallinity, is due to shear forces set up during extrusion
stage.
[0284] In order to achieve desirable properties through molecular
orientation and crystallinity, the newly formed crude fibers must
be drawn. Drawing pulls the molecular chains together and orients
them along the fiber axis, creating a considerably stronger fiber,
much like kneading, which is a form of drawing of the dough,
imparts similar mechanical properties to the resulting noodles and
pasta by reorienting the chains of starch.
[0285] Depending on the specific fiber-forming substance used, the
fibers can be cold drawn or hot drawn. The fibers are drawn to
several times their initial length, and the effect of drawing is
monitored by its effect on birefringence. Along with the tensile
strength of the fiber, the elastic modulus increases significantly
with increasing orientation. Other physical properties, such as
density equilibrium, moisture sorption, tenacity and
elongation-at-break are also affected by drawing.
[0286] The degree of drawing is typically defined by the term
"draw-ratio", which is a measure of the degree of stretching during
the orientation of a fiber or a filament, representing the ratio of
the length of the un-drawn fiber to that of the drawn fiber.
[0287] The required mechanical properties of the final product,
i.e., the composite structure, are substantially determined by the
mechanical properties of the fibers which are used as a core in the
final product. Therefore, the length, thickness, tensile strength
and the elasticity modulus of the final product are partially set
at the stage of spinning, and finally at the stage of drawing of
the fibril core.
[0288] The drawing of the fibers which are used for the core of the
composite structure is preferably effected at an elevated
temperature, or slightly above the glass transition temperature
under which the polymer is rigid and brittle and can undergo
plastic deformation and fracture. The elevated temperature is
determined according to the fiber-forming substance used, and in
cases of where the fiber is a polymeric fiber made of, for example,
poly(L-lactic acid), the drawing temperature preferably ranges from
about 30.degree. C. to about 130.degree. C. and more preferably the
elevated temperature is 70.degree. C.
[0289] The drawing is effected at a draw-ratio ranging from about
2:1 to about 10:1, and more preferably the drawing is effected at a
draw-ratio that ranges from 4:1 to 8:1.
[0290] Once the fibers which are used as a core for the composite
structure of the present embodiments are produced or otherwise
provided, the coat can be formed thereon by means of applying a
layer of an emulsion onto the surface of the fiber. As mentioned
hereinabove, the layer of the emulsion can cover parts of the fiber
or the entire fiber. Discrete patches of the emulsion layer can be
achieved by, for example, spraying, sputtering or brushing the
emulsion on the surface of the fibers. Long continuous streaks
(patches) of the emulsion along the fiber can be achieved, for
example, by partially dipping the fiber in the emulsion without
fully immersing the fiber in the emulsion; and a whole-surface
sheath can be achieved by fully immersing the fiber in the
emulsion.
[0291] The thickness of the coat depends on the viscosity of the
emulsion, namely the more viscous the emulsion, the more it sticks
to the fibril core and thus the thicker the resulting coat is.
Alternatively, the fibril core can be dipped in the emulsion more
than once so as to form a thicker layer of emulsion which turns
into a thicker coat.
[0292] The term "emulsion" as used herein describes a mixture of
two immiscible liquids, typically referred to as phases, such as
water and oil. One liquid (typically referred to as the dispersed
phase) is dispersed in the other (typically referred to as the
continuous phase). Examples of emulsions include milk, butter and
margarine, mayonnaise, the photo-sensitive side of film stock, and
cutting fluid for metalworking. Whether an emulsion turns into a
water-in-oil emulsion or an oil-in-water emulsion depends of the
volume fraction of both phases and on the type of emulsifier used.
Some emulsions are stable, while other emulsions tend to break when
the two phases re-separate if an emulsifier or an emulsion
stabilized is not used. Generally, emulsifiers and emulsifying
particles tend to promote dispersion of the phase in which they do
not dissolve very well, for example, proteins tend to form
oil-in-water emulsions. In milk the continuous phase (water)
surrounds droplets of lipid and protein (oil-in-water emulsion),
and in butter and margarine, a continuous lipid phase surrounds
droplets of water (water-in-oil emulsion).
[0293] The term "emulsifier" (also known as a surfactant or other
surface active material) as used herein, refers to a substance
which stabilizes an emulsion. Most emulsifiers are amphiphilic or
amphiphatic. Proteins and especially lipoproteins are excellent
natural emulsion stabilizers, as can be seen in every-day life food
products such as milk and mayonnaise. Lecithin (found in egg yolk)
is an example of a food emulsifier (in mayonnaise). Detergents of
natural and synthetic origins, such as phospholipids, are another
class of surfactants, and will bind to both oil and water, thus
holding microscopic organic or aqueous droplets in suspension.
[0294] According to preferred embodiments, the emulsion used to
form the porous coat of the composite structures presented herein
is a "water-in-oil" or reversed emulsion, wherein droplets of the
aqueous phase are dispersed in the continuous organic phase.
[0295] The emulsion, according to preferred embodiments of the
present invention, is provided by preparing two solutions, one
being the aqueous phase (water-based phase) and another being the
organic phase (oil-based phase).
[0296] The organic phase is prepared by dissolving one or more
polymer in an organic solvent. The organic solvent is selected
immiscible with an aqueous solution. Examples of such organic
solvents include, without limitation, chloroform, dichloromethane,
carbon tetrachloride, methylene chloride, xylene, benzene, toluene,
hexane, cyclohexane, diethyl ether and carbon disulfide. Preferably
the organic solvent is chloroform, which is immiscible with water,
and suitable for dissolving the abovementioned preferred polymer,
i.e., a biodegradable aliphatic co-polymer such as
poly(DL-lactic-co-glycolic acid) at a ratio of DL-lactic acid to
glycolic acid of about 75 weight percentage to about 25 weight
percentage respectively. The content of the biodegradable polymer
in the organic solvent may range, according to the present
embodiments, from about 1 weight-to-volume percentage to about 50
weight-to-volume percentages, and preferably from about 10
weight-to-volume percentages to about 25 weight-to-volume
percentages.
[0297] The aqueous phase may contain solely water, or may contain
additional substances such as buffer salts, emulsifying agents
(emulsifiers) which may be required to stabilize the emulsion,
surfactants, anti-static agents, chelating agents, preservatives,
solubilizers, viscosity modifying agents, biodegradation promoting
agents, penetration enhancers and other additional agents as
described hereinabove, factors and pharmaceutically acceptable
carriers which may be required for the function of the final
product, such as to preserve and stabilize the activity of the
bioactive agent(s), to improve the performance of the bioactive
agent and/or to carry and affect the rate of its release.
[0298] The bioactive agent can be introduced to either the organic
or the aqueous phase, depending on its nature, namely a hydrophobic
bioactive agent, which is miscible in the solvent of the organic
phase is dissolved or otherwise introduced into the organic phase,
while a hydrophilic/amphiphilic bioactive agent which is
water-soluble, is introduced into the aqueous phase.
[0299] The presence of the bioactive agent in either one of the
phases of the emulsion determines many factors of its release
profile, as discussed hereinabove. A hydrophilic/amphiphilic agent
which is dissolved in the aqueous phase will be found in the
droplets of the dispersed phase and subsequently will be
incorporated to the coat on the inner walls of the pores. A
hydrophobic agent which is dissolved in the organic phase will be
found in the continuous phase and subsequently will be incorporated
to the solid material of coat surrounding the pores.
[0300] The organic or the aqueous phase may further include
additional agents such as, for example, emulsifying agents
(emulsifiers) which may be required to stabilize the emulsion,
surfactants, anti-static agents, chelating agents, preservatives,
solubilizers, viscosity modifying agents, biodegradation promoting
agents, penetration enhancers and other additional agents as
described hereinabove.
[0301] The nature of the bioactive agent may create chemical
conditions which require the use of an emulsifier or surfactant, in
order to stabilize the emulsion. For example, a hydrophobic or a
hydrophilic bioactive agent may alter the relative surface tension
between the two phases such that they no longer form a stable
emulsion. The use of an emulsifier (surfactant, surface-active
agent) may reinstate a relative surface tension suitable for
forming a stable emulsion.
[0302] Amphiphilic bioactive agents form a unique group thereof due
toothier innate capacity to stabilize emulsion, stemming from their
intrinsic surface activity. Proteins are an example of bioactive
agents which also contribute tot the stabilization of the
emulsion.
[0303] Buffer salts which are suitable for use in the preparation
of the emulsion according to embodiments of the present invention
include, but are not limited, to citrate buffers, acetic
acid/sodium acetate buffers and phosphoric acid/sodium phosphate
buffers.
[0304] Emulsifiers which are suitable for use in the preparation of
the emulsion according to embodiments of the present invention
include, but are not limited, to vegetable derivatives, for
example, acacia, tragacanth, agar, pectin, carrageenan and
lecithin; animal derivatives, for example, gelatin, lanolin and
cholesterol; semi-synthetic agents, for example, methylcellulose
and carboxymethylcellulose; and synthetic agents, for example,
Carbopols.RTM.. Other emulsifiers include glycols and polyglycols,
glycerides and polyglycerides, sorbates and polysorbates, sorbitan
isostearate, sorbitan oleate, sorbitan sesquioleate, sorbitan
trioleate, alkyl-amines and alkyl-amides, and esters, salts and
mixtures thereof.
[0305] As used herein, the term "surfactant", which is also
referred to herein interchangeably as "a surface-active agent"
describes a substance that is capable of modifying the interfacial
tension of the liquid in which it is dissolved.
[0306] Surfactants which are suitable for use in the preparation of
the emulsion according to embodiments of the present invention,
include anionic, nonionic, amphoteric, cationic and zwitterionic
surface-active agents. In general, surfactants can include fatty
acid based surfactants; polypeptide based surfactants, for example,
proteins, glycoproteins and other modified polypeptides; and
polyhydroxyl based surfactants. Specific suitable surface-active
agents include but are not limited to triblock copolymer of
ethylene oxide (EO) and propylene oxide (PO), (PEO-PPE-PEO),
poly(vinyl alcohol) (PVA), acyl glutamates, acyl taurates, N-alkoyl
sarcosinates, alkyl alkoxy sulfates, alkyl amidopropyl betaines,
alkyl arylsulfonates, alkyl amine oxides, alkyl betaines, alkyl
carbonates, alkyl carboxyglycinates, alkyl ether carboxylates,
alkyl ether phosphates, alkyl ether sulfates, alkyl ether
sulfonates, alkyl glyceryl ether sulfates, alkyl glycinates, alkyl
phosphates, alkyl succinates, alkyl sulfates, alkyl
sulphosuccinates, ammonium alkyl sulphates, ammonium lauryl
sulphate, and derivatives, esters, salts and mixtures thereof.
[0307] Suitable solubilizers include, but are not limited to,
propylene glycol, 1,3-propylene diol, polyethylene glycol, ethanol,
propanol, glycerine, dimethyl sulphoxide, hexylene glycol,
propylene carbonate, and derivatives, salts and mixtures
thereof.
[0308] Suitable viscosity modifiers include, but are not limited to
carbomer, polyethylene glycol, polypropylene glycol, sodium xylene
sulphonate, urea, acacia, alcohol, ammonium laureth sulfate,
ammonium myreth sulfate, amphoteric-12, amphoteric-7, bentonite,
butylene glycol, cellulose gum, hydroxyethylcellulose,
methylcellulose, hydroxyethyl methylcellulose, hydroxypropyl
methylcellulose, cetyl alcohol, and the likes.
[0309] Examples of other additives that can be added to the aqueous
solution and/or the organic are presented hereinabove.
[0310] As mentioned hereinabove, while preparing composite
structures in which one or more bioactive agent(s) are contained
within the coat, the above-described emulsion contains the
bioactive agent(s). The bioactive agent(s) can be either in the
organic phase and/or in the aqueous phase, depending on its
solubility, stability and other characteristics, and the desired
properties of the resulting structure.
[0311] Thus, for example, water-soluble bioactive agents such as
proteins, peptides, growth factors and the like are dissolved in
the aqueous phase. In these cases, water-in-oil emulsions would
result in polymeric coats in which the bioactive agent is
encapsulated within the pores of the coat. As mentioned
hereinabove, discrete pores are desired so as to affect prolonged
release of the bioactive agent.
[0312] Water-insoluble bioactive agents such as, for example,
cytotoxic drugs, are dissolved in the organic phase. In these
cases, water-in-oil emulsions would result in polymeric coats in
which the bioactive agent is encapsulated within the polymer
composing the coat. As mentioned hereinabove, numerous and
relatively small and interconnected pores are desired so as to
affect efficient release of the bioactive agent via a maximized
exposed surface area.
[0313] A combination of water-soluble bioactive agents that are
encapsulated in the pores and water-insoluble bioactive agents that
are encapsulated in the polymer composing the coat is also within
the scope of the present invention.
[0314] When containing a bioactive agent, the aqueous phase is
preferably prepared at a temperature which would not harm the
bioactive agent, or otherwise jeopardize its activity. Preferably
the temperature of the aqueous phase is kept under 37.degree. C.
Similarly, other parameters of the preparation of the aqueous
solution, such as pH, salinity and other chemical and physical
parameters are kept at such levels as to preserve the activity of
the bioactive agent(s).
[0315] The organic phase, when containing the bioactive agent, is
preferably prepared by selecting a solvent that would not affect
the activity of the agent.
[0316] Once the two solutions, i.e., the organic solution/phase and
the aqueous solution/phase are prepared or otherwise provided, the
two solutions are mixed at a predetermined ratio to thereby obtain
a mixture thereof.
[0317] As is demonstrated in the Examples section that follows, the
ratio between the aqueous and the organic phase in the emulsion may
affect the properties of the resulting structure, as well as the
release profile of an encapsulated bioactive agent.
[0318] According to preferred embodiments of the present invention,
the ratio of the aqueous solution and the organic solution in the
mixture may range from about 1 parts of the organic solution to 1
part the aqueous solution to about 20 parts of the organic solution
to 1 part the aqueous solution. The preferred ratio of organic
solution to aqueous solution depends on the specific requirements
from the final product and its intended use.
[0319] Once the mixture is obtained, the process of emulsification
is effected to thereby obtain the emulsion. The process of
emulsification, which is well known to any artisan skill in the
art, is effected by a mechanical stirrer, mixer or homogenizer
until the desired consistency is achieved.
[0320] The rate (energy input) and the time of emulsification
mixing determine the size of the resulting pores in the coat to a
large extent. Rapid mixing for extended periods of time (typically
using a homogenizer) will result in very fine pores in the porous
coat. Such rapid mixing is typically preferred in cases where the
bioactive agent is dissolved in the organic phase and nano-sized
pores are desired, as is detailed hereinabove.
[0321] As mentioned hereinabove, in cases where the bioactive agent
has a limited solubility in water or is not soluble in water, the
phase which will contain this bioactive agent is the organic
(continuous) phase. In such cases, the aqueous phase may be used in
order to introduce additional components such as buffers,
emulsifying agents, surfactants, anti-static agents, chelating
agents, solubilizers, viscosity modifying agents, biodegradation
promoting agents and penetration enhancers. In these cases the
pores formed by the water droplets in the polymer may be very small
and will accelerate the biodegradation process by increasing the
surface area of the biodegradable polymeric coat.
[0322] When containing a bioactive agent, the emulsification is
effected at a temperature which would not harm the bioactive agent,
or otherwise jeopardize its activity. Preferably the emulsification
is effected at temperature lower than 37.degree. C. Similarly,
other mechanical parameters of the emulsification process are kept
at such levels as to retain the activity of the bioactive
agent(s).
[0323] As presented hereinabove, the resulting emulsion is applied
onto the fiber so as to form a layer of the emulsion thereon. Once
the fiber is fully or partially covered with the emulsion, the
fiber is subjected to freeze-drying so as to solidify the emulsion
and obtain the final composite structure of the present
invention.
[0324] The phrase "freeze drying" (also known as lyophilization) as
used herein is a dehydration process typically used to preserve a
perishable material, or to make the material more convenient for
transport, delivery and storage. Freeze-drying is effected by
deep-freezing the material, typically by flash-freezing in liquid
nitrogen, and then reducing the surrounding pressure to allow the
frozen solvent, typically water and organic solvents in the
material to sublimate directly from the solid phase to gas, and
solidify on a condenser or cold-trap. A cold condenser chamber
and/or condenser plates provide a surface(s), for the vapor to
re-solidify thereon. These surfaces must be colder than the
temperature of the surface of the material being dried, or the
vapor will not migrate to the collector. Temperatures for this ice
of water collection are typically below -50.degree. C. The greatly
reduced water content that results inhibits the action of
microorganisms and enzymes that would normally spoil or degrade a
substance, and greatly reduce the rate of oxidation and other
spontaneous chemical degradation processes.
[0325] If a freeze-dried substance is sealed to prevent the
re-absorption of moisture, the substance may be stored at room
temperature without refrigeration, and be protected against
spoilage for extended periods of time. Freeze-drying tends to
damage the tissue being dehydrated less than other dehydration
methods, which involve higher temperatures. Freeze drying does not
usually cause shrinkage or toughening of the material being dried.
Also, liquid solutions that are freeze-dried can be rehydrated
(reconstituted) more readily because it leaves microscopic pores in
the resulting dried solid. The pores are created by the water
droplets which turned into ice which in turn sublimed, leaving gaps
or pores in its place. This is especially important when it comes
to pharmaceutical manufacturing and uses, as lyophilization also
increases the shelf life of drugs for many years.
[0326] According to preferred embodiments of the present invention,
the process of freeze-drying, which is well known to any artisan
skill in the art, is carried out at reduced temperature and
pressure using conventional methods and tools. A porous coat is
therefore the product of a freeze-dried water-in-oil emulsion
wherein the droplets of the dispersed aqueous phase turn to voids
or pores in the solidified continuous organic phase of the polymer.
In cases where the aqueous phase contains at least one bioactive
agent, the droplets of the dispersed aqueous phase become
microscopic capsules containing the bioactive agent(s) which are
encapsulated, entrapped and embedded in a solid polymer once the
emulsion is freeze-dried.
[0327] Interim summing up, a wide range of bioactive agents can be
incorporated into the coat of the composite structures described
herein. The preparation of the coat does not involve harsh
conditions which typically abolish the activity of many bioactive
agents. The preparation of the coat via the formation of an
emulsion comprising an aqueous phase and an organic phase enables
the incorporation of bioactive agents having a
hydrophilic/amphiphilic nature or a hydrophobic nature, and of a
small organic molecule or a complex macro-biomolecule.
[0328] As is demonstrated in the Example section that follows, a
relatively large and amphiphilic macro-biomolecule in the form of
an intact active enzyme (protein) was successfully incorporated
within the coat of an exemplary fibrous composite structure. Being
amphiphilic in nature, the protein acts as an effective surface
active agent which stabilizes the emulsion made of an aqueous
solution having the protein dissolved therein and an organic phase
comprising the biodegradable polymer.
[0329] The incorporation of a bioactive agent having a more
pronounced solubility trait, such as a small and predominantly
hydrophobic drug molecule paclitaxel, required a different
treatment in order to be incorporated successfully in a composite
fiber as presented herein. A hydrophobic drug molecule, such as
paclitaxel, was intuitively added to the organic phase where it is
more soluble, and the use of surfactants was required in order to
stabilize the emulsion.
[0330] As further presented in the Examples section that follows,
the present inventors have used fine nylon suture fibers as a
fibril core, and coated it with an emulsion containing paclitaxel
so as to form a biodegradable paclitaxel-eluting coat applied on a
non-degradable core. Such paclitaxel-eluting composite structures
combine the strength and ductility of nylon suture fiber, with the
controllably drug-eluting capabilities of the composite structures
presented herein, and therefore can be used in a myriad of medical
applications, including the construction of implantable medical
devices, such as stents.
[0331] As discussed hereinabove, the composite structure of the
present invention is designed suitable for use as a structural
element and/or a drug delivery system in many medical procedures
and devices.
[0332] Hence, according to a further aspect of the present
invention there is provided a medical device which comprises the
composite structure described herein.
[0333] In a preferred embodiment of the present invention, the
medical device is a biodegradable device.
[0334] Generally, the main motivation to have a biodegradable
medical device is to have a device that can be used as an implant
and will not require a second surgical intervention for removal.
Besides eliminating the need for a second surgery, the
biodegradation may offer the advantage of local, functionally
focused drug delivery. For example, a fractured bone that has been
fixated with a rigid, non-biodegradable stainless implant has a
tendency for refracture upon removal of the implant. Since the
stress is borne by the rigid stainless steel, the bone has not been
able to carry sufficient load during the healing process. However,
an implant prepared from biodegradable composite structures as
described herein can be engineered to degrade at a rate that will
slowly transfer load to the healing bone, while steadily delivering
bone-regeneration promoting agent to the locus of the fracture.
[0335] In its simplest form, a biodegradable device having a
bioactive agent delivery capacity consists of a dispersion of the
bioactive agent in a polymeric coat matrix. The bioactive agent is
typically released as the biodegradable polymeric coat biodegrades
in vivo into soluble products that can be absorbed and/or
metabolized and eventually excreted from the body over a period of
time which depends on the polymer and the physical dimensions of
the device.
[0336] The term "delivering" or "delivery" as used in the context
of the present invention refers to the act of enabling the
transport of a substance to a specific location, and more
specifically, to a desired bodily target, whereby the target can
be, for example, an organ, a tissue, a cell, and a cellular
compartment such as the nucleus, the mitochondria, the cytoplasm,
etc.
[0337] In a particularly preferred embodiment, a medical device
comprising the composite structure described herein is used for
implantation, injection, or otherwise placed totally or partially
within the body.
[0338] In preferred embodiments of the present invention, the
medical device is adapted for transdermal and/or topical
applications in a subject. It is particularly important that such
medical device would cause minimal tissue irritation when used to
treat a given tissue.
[0339] Exemplary devices which can be used for transdermal
application include, without limitation, a suture, an adhesive
plaster and a skin patch.
[0340] Exemplary devices which can be used for topical application
include, without limitation, a suture, an adhesive strip, a
bandage, an adhesive plaster, a wound dressing and a skin
patch.
[0341] In more preferred embodiments, the medical device of the
invention is adapted for implanting the medical device in a bodily
organ of a subject. It is particularly important that such medical
device, other than serving its intended purpose, would not evoke an
immune response resulting in systemic failure upon rejection which
may be detrimental and even fatal.
[0342] Exemplary devices which can be used for implanting in a
bodily organ of a subject include, without limitation, a plate, a
mesh, a screw, a pin, a tack, a rod, a suture anchor, an
anastomosis clip or plug, a dental implant or device, an aortic
aneurysm graft device, an atrioventricular shunt, a catheter, a
heart valve, a hemodialysis catheter, a bone-fracture healing
device, a bone replacement device, a joint replacement device, a
tissue regeneration device, a hemodialysis graft, an indwelling
arterial catheter, an indwelling venous catheter, a needle, a
pacemaker, a pacemaker lead, a patent foramen ovale septal closure
device, a vascular stent, a tracheal stent, an esophageal stent, a
urethral stent, a rectal stent, a stent graft, a suture, a
synthetic vascular graft, a thread, a tube, a vascular aneurysm
occluder, a vascular clip, a vascular prosthetic filter, a vascular
sheath and a drug delivery port, a venous valve and a wire.
[0343] Examples of bodily sites where a medical device of the
present invention may be used include, without limitation, skin,
scalp, a dermal layer, an eye, an ear, a small intestines tissue, a
large intestines tissue, a kidney, a pancreas, a liver, a digestive
tract tissue or cavity, a respiratory tract tissue or cavity, a
bone, a joint, a bone marrow tissue, a brain tissue or cavity, a
mucosal membrane, a nasal membrane, the blood system, a blood
vessel, a muscle, a pulmonary tissue or cavity, an abdominal tissue
or cavity, an artery, a vein, a capillary, a heart, a heart cavity,
a male reproductive organ, a female reproductive organ and a
visceral organ.
[0344] Preferred medical devices according to the present invention
include stents, wound dressings, sutures and suture anchors,
interference and general screws, angioplastic plugs, pins and rods,
tacks, plates, meshes, anastomosis clips and rings, dental implants
and guided tissue matrixes.
[0345] In a world where environmental conservation becomes
critical, biodegradable products which are not necessarily for
medical purposes and uses are of great importance and need. Many
disposable products are turned environmentally-friendly by using
biodegradable compounds in their production. As known in the art,
there are many such products and raw materials available, yet the
use of the composite structure of the present invention to produce
disposable goods and products has an added benefit stemming from
the presence of bioactive agents therein.
[0346] Thus, according to another aspect of the present invention
there is provided an article-of-manufacture which comprises one or
more of the composite structures described herein.
[0347] Such articles-of-manufacture may include, without
limitation, fishing lines and nets, insect and bird nets,
vegetation nets, woven and non-woven cloths and fibers, disposable
women's sanitary items, disposable facial masks (as used by
surgeons), wet "paper" tissues (wipes), disposable underwear,
disposable handkerchiefs, towels and diapers, disposable medical
supplies, disposable food containers or dishes, disposable items of
clothing, disposable cutlery items and other disposable consumer
and industrial products.
[0348] The rate of release of bioactive agents from the composite
structure of the present embodiments depends on various parameters,
including, without limitation, the composition of the core and/or
coat and the process employed for preparing the emulsion for the
coat. As demonstrated in the Examples section that follows,
empirical data can be accumulated so as to obtain release rates
corresponding to different combinations and sub-combination of
materials and manufacturing possesses. Additionally or
alternatively, the release rate can be predicted by constructing a
mathematical-physical model of the release mechanism, and solving
the equations governing such model by an appropriate mathematical
method or by performing a mathematical simulation. Prediction of
the release rate using a mathematical-physical model is
particularly useful in the design phase of the composite structure
because such model can enable fast evaluation and fine tuning of
the various parameters for achieving an optimal or improved release
profile, while reducing the typically costly and time consuming
laboratory procedures.
[0349] Many models for predicting diffusion systems from objects
and degrading surfaces have been developed. To this end see, for
example, Gopferich A. et al. in Biomaterials 1996; 17: 103-114;
Siepmann J. et al. in Advanced Drug Delivery Reviews 2001; 48:
229-247; Charlier A. et al. in International Journal of
Pharmaceutics 2000; 200: 115-120; Faisant N. et al. in European
Journal of Pharmaceutical Sciences 2002; 15: 355-366; and Zhang M.
et al. in Journal of Pharmaceutical Sciences 2003; 92:
2040-2056.
[0350] Sagiv A. et al. in Annals of Biomedical Engineering 2003;
31: 1132-1140, developed a specific model for predicting protein
release from monolithic PLLA fibers. However, this model assumes a
constant diffusion coefficient and is therefore applicable only for
relatively slowly degrading materials such as PLLA or the like. It
is therefore an object of the present invention to provide a
technique for predicting the release rate for fast degrading
materials, such as, but not limited to PDLGA, PGA, PLLA, PDLLA,
PCL, PDO and PGA-TMC, wherein 50/50 PDLGA is considered a fast
degrading polymer and PCL and PLLA are considered slower degrading
polymers in the context of the present invention (for degradation
time to complete mass loss and abbreviations see, Table A
hereinabove).
[0351] Hence, according to another aspect of the present invention,
there is provided a method for predicting release rate of the
bioactive agent from the composite structure.
[0352] The method of the present embodiments employs a
mathematical-physical model which is based on diffusion phenomena.
In various exemplary embodiments of the invention the model uses
the structural characteristics of the polymeric coat and/or its
degradation and swelling capabilities. In preferred embodiments of
the present invention, the mathematical-physical model is based on
the molecular weight of the bioactive agent and/or host polymer.
Preferably, the method of the present embodiments is capable of
adjusting the mathematical-physical model based on the emulsion's
formulation parameters.
[0353] In various exemplary embodiments of the invention a
diffusion equation is solved so as to obtain the concentration
distribution of the bioactive agent as a function of time.
[0354] In general, the diffusion equation is preferably in
accordance with Fick's second law of diffusion, which has the form
.differential.C/.differential.t=D.gradient..sup.2C, where C=C(x, t)
is a time-dependent concentration distribution function describing
the concentration C of the bioactive agent at a three-dimensional
spatial location x within the polymeric coat and time t, D=D(x, t)
is the diffusion coefficient of the bioactive agent at location x
within the polymeric coat and time t, and .gradient..sup.2 is the
Laplace operator.
[0355] The coordinate system at which the diffusion equation is
presented depends on the geometrical shape of the composite
structure. For example, when the composite structure has a
cylindrical shape, a cylindrical coordinate system is preferred;
when the composite structure has a spherical shape, a spherical
coordinate system is preferred; and when the composite structure
has a disc shape, a polar coordinate system is preferred. Also
contemplated are other coordinate systems, such as, but not limited
to, elliptic coordinate system, elliptic cylindrical coordinate
system, ellipsoidal coordinate system, parabolic coordinate system,
parabolic cylindrical coordinate system, toroidal coordinate system
and the like.
[0356] While the embodiments below are described with a particular
emphasis to a composite structure having a cylindrical shape (for
example, a fiber), it is to be understood that more detailed
reference to cylindrical shape is not to be interpreted as limiting
the scope of the invention in any way.
[0357] Hence, in cylindrical coordinates (r, .theta., z), the
diffusion equation has the form:
.differential. C .differential. t = 1 r { .differential.
.differential. r ( rD .differential. C .differential. r ) +
.differential. .differential. .theta. ( D r .differential. C
.differential. .theta. ) + .differential. .differential. z ( rD
.differential. C .differential. z ) } . ( EQ . 1 ) ##EQU00001##
[0358] In various exemplary embodiments of the invention, a
circular symmetry is employed. In these embodiments the bioactive
agent concentration distribution is substantially isotropic and
therefore the partial derivative with respect to the angular
coordinate .theta. can be neglected:
.differential. C .differential. .theta. = 0. ( EQ . 2 )
##EQU00002##
[0359] When the composite structure of the present embodiments has
an elongated shape in which the radius is significantly smaller
than the length (for example, a fiber), end effects can be
neglected. Thus, in various exemplary embodiments of the invention
symmetry with respect to the longitudinal axis z is assumed:
.differential. C .differential. z = 0. ( EQ . 3 ) ##EQU00003##
[0360] Employing the above symmetries, the diffusion equation has
the reduced form:
.differential. C .differential. t = 1 r { .differential.
.differential. r ( rD .differential. C .differential. r ) } . ( EQ
. 4 ) ##EQU00004##
[0361] Generally, the diffusion coefficient D can be a function of
time and/or space. It was found by the Inventors of the present
invention that it is sufficient to use a time-dependent diffusion
coefficient which is homogenous with respect to the radial
coordinate r. Thus, according to a preferred embodiment of the
present invention the diffusion coefficient is a one-variable
function D(t). In this embodiment, the diffusion equation has the
form:
.differential. C .differential. t = 1 r { .differential.
.differential. r ( rD .differential. C .differential. r ) } = 1 r {
D .differential. C .differential. r + rD .differential. 2 C
.differential. r 2 } = 1 r D .differential. C .differential. r +
.differential. 2 C .differential. r 2 D , ( EQ . 5 )
##EQU00005##
where for clarity of presentation the arguments of the functions
D(t) and C(r, t) were omitted. A preferred expression for the
time-dependent diffusion coefficient D(t) according to various
exemplary embodiments of the present invention is provided
hereinunder.
[0362] Equation 5 can be rearranged as follows:
.differential. C .differential. t = D ( 1 r .differential. C
.differential. r + .differential. 2 C .differential. r 2 ) . ( EQ .
6 ) ##EQU00006##
[0363] The reduced diffusion equation 6, or any other form of
diffusion equation (for example, Equations 1 or 4) can be solved
using any known technique for solving partial-differential
equation. Generally, the solution includes selecting appropriate
initial and boundary conditions and applying a numerical procedure
(for example, semi-discretisation method, Euler method,
Crank-Nicholson method, Monte-Carlo simulation, Lagrangian method,
wavelets, etc.) to obtain the function C(r, t) which describes the
concentration distribution of the bioactive agent as a function of
the time.
[0364] In various exemplary embodiments of the invention the
initial condition for the diffusion equation comprises the initial
concentration C.sub.0(r)=C(r, 0) of the bioactive agent, as
incorporated initially with the polymeric coat. C.sub.0(r) can also
be a homogenous function which does not vary with the radial
coordinate. In this embodiment, the initial contrition is
preferably:
C=C.sub.0@t=0,r.sub.1<r<r.sub.2, (EQ. 7)
where r.sub.1 is the radius of the fibril core and r.sub.2 is the
radius of the composite structure (see, FIG. 1).
[0365] The boundary conditions for the diffusion equation are
preferably, but not obligatorily: (i) a "no flux" condition at
r=r.sub.1, and (ii) a "perfect sink" condition at r=r.sub.2. The
"no flux" condition indicates that the bioactive agent within the
coat diffuses toward the surface of the coat but not toward the
core. The "perfect sink" condition indicates that the concentration
of bioactive agent in the medium outside the composite structure is
zero. Mathematically, the two boundary conditions can be written in
the form:
C = 0 @ r = r 2 , t > 0 ( EQ . 8 ) .differential. C
.differential. r = 0 @ r = r 1 , t > 0 ( EQ . 9 )
##EQU00007##
[0366] Once the diffusion equation is solved with the appropriate
initial and boundary conditions (for example, conditions 7-9) the
method preferably continues to an additional step in which the
concentration distribution is integrated so as to obtain the
integrated bioactive agent mass M(t) in the coat as a function of
the time. Mathematically, the integration can be expressed as
follows:
M ( t ) = .intg. r 1 r 2 S * C ( r , t ) r = .intg. r 1 r 2 2 .pi.
rL * C ( r , t ) r = 2 .pi. L .intg. r 1 r 2 r * C ( r , t ) r ( EQ
. 10 ) ##EQU00008##
where, S is the cross-sectional area of the composite structure,
and L is the total length of the composite structure. Knowing the
initial mass M(t=0) of the bioactive agents in the polymeric coat,
the released mass M.sub.released can be calculated by subtracting
the integrated mass M(t) from the initial mass M(t=0):
M.sub.released(t)=M(t=0)-M(t). (EQ. 11)
[0367] The release rate of the bioactive agent can then be obtained
from the calculated released mass, for example, by numerically
differentiating M.sub.released with respect to time, or by
calculating the difference between two values of M.sub.released at
predetermined time intervals.
[0368] Following is a description of a preferred time-dependent
diffusion coefficient, according to various exemplary embodiments
of the present invention.
[0369] Previous reports on drug delivery systems based on porous
matrices revealed that the bioactive agent is released much more
slowly than would be expected from the simplest consideration of
aqueous diffusion. The porous structure of the coat partially
suppresses the diffusion of the bioactive agents because their
percolate through a relatively long tortuous path on their way to
the matrix surface. On the other hand, the suppression of the
diffusion decreases with the degradation of the coat. Thus, the
diffusion coefficient of the bioactive agent in preferably an
increasing function of the time. For example, the time-dependence
of the diffusion coefficient can be expressed in terms of the
degradation profile M.sub.w1 of the biodegradable polymeric coat,
which is preferably defined as:
M wl ( t ) = M w ( t = 0 ) - M w ( t ) M w ( t = 0 ) , ( EQ . 12 )
##EQU00009##
where M.sub.w(t) is a function describing the molecular weight of
the biodegradable polymeric coat as a function of the time t.
[0370] According to a preferred embodiment of the present invention
it is assumed that the degradation of the biodegradable polymeric
coat follows first-order kinetics.
[0371] First order kinetics implies that the molecular weight
M.sub.w is proportional to the rate by which the molecular weight
changes with time. Mathematically, first order kinetics implies
that B dM.sub.w/dt=-M.sub.w, where B is referred to as the decay
constant of M.sub.w. As will be appreciated one of ordinary skill
in the art, such behavior is described by an exponentially
decreasing function M.sub.w(t)=M.sub.w(t=0) exp(-t/B). The ratio
M.sub.w(t)/M.sub.w(t=0) is referred to as the "normalized molecular
weight", and denoted {tilde over (M)}.sub.w(t).
[0372] According to the percolation theory, the diffusion rate in a
porous structure characterized by a given average tortuous path and
a given average porosity is, to a good approximation, inversely
proportional to the average tortuous path and directly proportional
to the average porosity.
[0373] The average tortuous path of the composite structure of the
present embodiments is preferably parameterized as
.tau.(r.sub.2-r.sub.1), where .tau. is the so-called "tortuosity
factor" [Gopferich A., Macromolecules 1997; 30: 2598-2604;
Geankoplis CJ., Transport process and unit operations, second
edition, 1983, Englewood Cliffs, N.J.: Prentice Hall, ch. 6.; and
Pismen LM., Chemical Engineering Science 1974; 29: 1227-1236]. In
this embodiment, the initial value of the time-dependent diffusion
coefficient, denoted D.sub.0, is proportional to the ratio
.epsilon./.tau., where both .epsilon. and .tau. are used as input
parameters characterizing the initial state of the biodegradable
coat in terms of average porosity and average tortuosity path,
respectively.
[0374] Mathematically, D.sub.0 can be written as:
D 0 = D w .tau. , ( EQ . 13 ) ##EQU00010##
where D.sub.w is some asymptotic diffusion coefficient of the
bioactive agent in a given medium. In various exemplary embodiments
of the invention D.sub.w is the diffusion coefficient of the
bioactive agent in water.
[0375] There are many known techniques for determining the values
of .epsilon. and .tau. of a given structure. Typically, but not
obligatorily, .epsilon. and .tau. are determined by means of
stochastic geometry (for example, stereology sampling). For
example, a cross-sectional image of the structure can be obtained,
for example, using two dimensional scanning electron microscope. A
grid of points can be defined over the image and a point-counting
estimation technique can be employed to characterize the structure
in terms of average porosity and average tortuosity path. More
specifically, the porosity of the structure can be estimated by
calculating the ratio between the number of points that overlap the
pores of the structure and the total number of points that occupy
the cross-section of the structure, and the average tortuosity path
can be estimated by. In another technique, a three-dimensional
image of the structure is used. The three-dimensional image can be
inputted to an appropriate simulation algorithm which defines
"walkers" percolating through the pores until they escape the
structure. Knowing the velocity of the walkers and the percolation
time, the algorithm can calculate the average tortuosity path. The
porosity can be estimated by calculating the probability that an
arbitrarily chosen voxel of the three-dimensional image is a
pore.
[0376] According to a preferred embodiment of the present invention
the time-dependence of the diffusion coefficient D(t) is obtained
by combining the constant term D.sub.0 and the function
M.sub.w1(t), substantially according to the following equation:
D(t)=D.sub.0+(D.sub.w-D.sub.0)*M.sub.w1t), (EQ. 14)
where M.sub.w1 is the degradation profile of the biodegradable
polymeric coat, which is preferably given by Equation 12 above.
Thus, the diffusion coefficient of the bioactive agent within the
biodegradable polymeric coat evolves from an initial has low
"effective" value (the constant term D.sub.0 in Equation 14), to
the characteristic diffusion coefficient of the bioactive agent in
water, D.sub.w.
[0377] Any suitable value can be used for the asymptotic diffusion
D.sub.w. A preferred expression for D.sub.w is the semi-empirical
equation of Polson [Saltzman WM., Drug delivery: engineering
principles for drug therapy, 2001, Oxford, Oxford University Press;
He L. and Niemeyer, B., Biotechnology Progress 2003; 19: 544-548;
and Tyn MT. and Gusek TW., Biotechnology and Bioengineering 1990;
35: 327-338]:
D w = A * T .mu. M wBA 1 / 3 , ( EQ . 15 ) ##EQU00011##
where M.sub.wBA is the bioactive agent's molecular weight, T is the
absolute temperature, .mu. is the viscosity of the external fluid
medium, which is typically an aqueous medium, and A is a constant
which is specific to the bioactive agent.
[0378] The diffusion rate of the bioactive agent depends, inter
alia, on the concentration of the polymer in the biodegradable
polymeric coat. A higher polymer concentration results in a more
viscous organic phase, thus creating a more stable emulsion.
Typical polymer concentration used in the context of the present
invention, expressed in % w/v in the organic phase were 13%, 15%
and 19%, as presented in the Examples section that follows. This
higher viscosity, along with the higher density, is expected to
create the following hindering effects on the diffusion rate:
[0379] (i) slowing the matrix degradation rate due to more dense
solid matrix and a lower "readiness" to water penetration; and
[0380] (ii) reducing the free volume available for bioactive agent
diffusion, leading to a shorter initial burst effect in the release
profile.
[0381] The time-dependence of the normalized molecular weight of
the biodegradable polymeric coat can be parameterized using any
known procedure. For example, FIG. 24 shows the normalized
molecular weight of three biodegradable polymers as a function of
time. The data were taken from Wu et al., Synthesis,
characterization, biodegradation, and drug delivery application of
biodegradable lactic/glycolic acid polymers. Part II:
Biodegradation. Journal of Biomaterials Science--Polymer edition
2001; 12(1): 21-34. Shown in FIG. 24, are the time-dependences of
the normalized molecular weights of a 75/25 PDLGA with initial
molecular weights of 40 kDa, 100 kDa and 160 kDa. As shown, the
normalized molecular weights decrease with time. The
time-dependence of the normalized molecular weight can therefore be
parameterized by fitting experimental data of the biodegradable
polymer (such as the experimental data shown in FIG. 24 to an
exponential decreasing function and extracting the decay constant B
from the obtained fit. Any fitting procedure can be employed,
including, without limitation, .chi..sup.2 minimization or the
like.
[0382] The degradation profile M.sub.w1 can then be written in the
form:
M wl ( t ) = 1 - M ~ w ( t ) = 1 - exp ( - t B ) = 1 - exp ( - C p
B t ) , ( EQ . 16 ) ##EQU00012##
where C.sub.P is a dimensionless parameter which is proportional to
the concentration of the biodegradable polymer in the coat. Typical
values of C.sub.P are from about 0.2 to about 1.5.
[0383] Additional objects, advantages, and novel features of the
present invention will become apparent to one ordinarily skilled in
the art upon examination of the following examples, which are not
intended to be limiting. Additionally, each of the various
embodiments and aspects of the present invention as delineated
hereinabove and as claimed in the claims section below finds
experimental support in the following examples.
EXAMPLES
[0384] Reference is now made to the following examples, which
together with the above descriptions, illustrate the invention in a
non limiting fashion.
Materials and Experimental Methods
[0385] Poly(L-lactic acid) (PLLA, cat. RESOMER L210, inherent
viscosity=3.6 dL per gram in CHCl.sub.3 measured at 30.degree. C.),
obtained from Boehringer Ingelheim, Germany, was used to form a
biodegradable fibril core composed of a relatively high molecular
weight PLLA.
[0386] Poly(DL-lactic-co-glycolic acid), 75%/25%, (PDLGA, cat.
75DG065, inherent viscosity=0.65 dL per gram in CHCl.sub.3 measured
at 30.degree. C., molecular weight of approximately 118,000 grams
per mole), obtained from Absorbable Polymer Technologies, Inc, USA,
was used to form a biodegradable porous coat.
[0387] Horseradish peroxidase (HRP) with an initial enzymatic
activity of 500 U/mg, was obtained from Aldrich, and served as a
protein model.
[0388] A BCA.TM. Protein Assay Kit, obtained from Pierce, was used
for measuring the protein content of solutions with a relatively
high (20-2000 .mu.g/ml) protein content, and a Micro BCA.TM.
Protein Assay Kit, obtained from Pierce, was used for measuring the
protein content of solutions with a relatively low (0.5-40
.mu.g/ml) protein content.
[0389] A 1-Step.TM. Slow TMB ELISA Kit, obtained from Pierce, was
used for measuring HRP enzymatic activity.
[0390] Absorbance in enzymatic assays was measured using a
SpectraMax 340PC384 plate reader spectrophotometer.
[0391] Paclitaxel (Genexol.TM.) was purchased from Sam Yang Corp,
Seoul, Korea.
##STR00001##
[0392] Melt-spinning was performed on a piston/cylinder one shot
spinning system obtained from Alex James Inc. of Greer S.C. The
spinnerette capillary was 0.024 inch in diameter. Extrusion rate
was 0.5 grams to 4 grams per minute.
[0393] Drawing was performed manually by stretching the fibers on a
hot plate at temperature of 70-80.degree. C.
[0394] Ethilon.TM. monofilament nylon sutures (model W597), Ethicon
Inc., USA, having a diameter of approximately 200 .mu.m, were used
as core fibers for paclitaxel-eluting fibers.
[0395] Surface active agents for stabilizing the emulsions used for
the paclitaxel-eluting fibers were Pluronic.RTM. L121TM, a triblock
copolymer of ethylene oxide (EO) and propylene oxide (PO),
(PEO-PPE-PEO), with a mean molecular weight of about 4,400 Da,
which was received as a gift from BASF, USA; and Poly(vinyl
alcohol) (PVA), 87% to 89% hydrolyzed, molecular weight ranging
from 13,000 to 23,000 Da, which was purchased from Sigma.
[0396] SEM measurements were performed using a Jeol JSM-6300
scanning electron microscope set at an accelerating voltage of 5
kV.
[0397] The mechanical properties of the fibers were measured at
room temperature in unidirectional tension at a rate of 50 mm per
minute on an ASTM D 638-98 device, using a Universal Testing System
machine obtained from MTS Systems Corporation, Eden Prairie, Minn.
The tensile strength was defined as the maximum strength in the
stress-strain curve; the maximal strain as the breaking strain; the
Young's modulus as the slope of the stress-strain curve in the
elastic (linear) region. Five samples were tested for each
point.
Enzyme-Eluting Composite Structures
[0398] Preparation of Biodegradable Core Fibers:
[0399] Poly(L-lactic acid) (PLLA) (10 grams) was melt spun at
190.degree. C. in a batch mode using a piston/cylinder one shot
spinning system, and then drawn at 70.degree. C. to a draw ratios
of 3:1 to 8:1, so as to create fibers with various mechanical
properties. The final diameter of the drawn fibers was 200
.mu.m.
[0400] FIG. 3 presents the stress-strain curves of the fibers drawn
at various ratios. As can be seen in FIG. 3, fibers made at various
draw ratios share a similar transition point from elastic behavior
to plastic behavior at about the 5% strain point (in this case of
tension, the strain is stretch), and an elastic limit. The fibers
actually stretch, which is indicative of a high ductility. The
point where the curve bends is known as the proportional limit; up
to this point the relationship between stress and strain is
proportional, after this point, the fibers do not regain their
original shape after the strain is removed. As expected, fibers
drawn at 8:1 ratio are more brittle and less stretchable than those
drawn at lower ratios, up to the more stretchable fiber drawn at a
3:1 ratio.
[0401] FIGS. 4a-c present plots of the yield strength, ultimate
tensile strength, maximal strain and Young's modulus, as a function
of the draw ratio. As can be seen in FIGS. 4a-c, yield strength
(FIG. 4a), ultimate strength (FIG. 4a) and Young's modulus (FIG.
4b) increase with the increase in draw ratio while the maximal
strain (FIG. 4c) decreases while increasing the draw. The 8:1 drawn
fibers exhibited the highest tensile strength of 980 MPa and
modulus of 4.9 GPa together with good ductility and flexibility
estimated by 50% strain. Hence, in composite structures that are
designed to be used in applications that require high strength of
the structure (for example, in stents), fibers drawn at 8:1 ratio
are used. In applications such as tissue engineering and in the
following experiments, fibers drawn at 4:1 ratio were used.
[0402] Preparation of Emulsions for the Biodegradable Porous
Coat:
[0403] Poly(DL-lactic-co-glycolic acid) (PDLGA) (0.5 gram, 0.6 gram
or 0.75 gram) was dissolved in chloroform (4 ml) to form an organic
phase (corresponding to a polymer content of 13%, 15% and 19% w/v
respectively). Horseradish peroxidase (HRP) in quantities that
enabled to obtain contents of 1, 5, and 10 weight percentage (w/w)
relative to the polymer quantity, was dissolved in water. The
organic phase was placed in a test tube and an aqueous solution
containing HRP was poured into the test tube. The volume of the
aqueous phase used was 0.25 ml, 0.5 ml and 1 ml, which enabled
organic-to-aqueous phase ratios of 16:1, 8:1 and 4:1, respectively.
Homogenization of the emulsion was thereafter performed using a
hand-held 7 mm rotor homogenizer (Omni International, Inc.)
operated at 5,000 rpm for 3 minute. These processing conditions
were experimentally found to be optimal for preserving the
enzymatic activity of HRP, and yielded homogenous emulsions for all
examined formulations.
[0404] The content of the polymer in the organic phase is expressed
in weight of PDLGA per volume of chloroform. The ratio of HRP
content in the aqueous phase to polymer content in the organic
phase is referred to herein as the HRP load, expressed in weight
per weight percentage (w/w) ratio. The ratio of organic phase (O)
to aqueous phase (A) is referred to herein as O:A, expressed in
volume to volume percentage (v/v) ratio.
[0405] Coating Biodegradable Fibril Cores with a Biodegradable
Porous Coat:
[0406] The core PLLA fibrils were stretched delicately on special
holders, then dipped and coated in fresh emulsions, and immediately
thereafter flash-frozen in a liquid nitrogen bath. The holders and
samples were then placed in a pre-cooled freeze dryer (VirTis model
101) equipped with a liquid nitrogen trap and capable of sustaining
organic solvents. The freezing temperature of the condenser was
approximately -105.degree. C.
[0407] Freeze drying was performed in the following three
stages:
[0408] i) For the first 12 hours, the cold condenser plate served
as a cold trap and a temperature gradient developed between the
coated fibers and the condenser.
[0409] ii) The condenser operation was stopped, and its plate
temperature was allowed to increase slowly to room temperature. The
liquid nitrogen trap was activated simultaneously. The chloroform
and water, which accumulated on the condenser plate during the
first drying stage, were sublimed and transferred onto the nitrogen
trap's surface along with residual liquids from composite
fibers.
[0410] iii) Final drying was achieved by vacuum drying for an
additional 24 hours at room temperature.
[0411] The samples were stored in desiccators until further
use.
[0412] Freeze-dried emulsions were fabricated in the same manner,
without being applied as coating on core fibrils. These emulsions
were poured onto aluminum plates (5 cm diameter) and freeze-dried
as described hereinabove. These samples were used for determining
the effects of several processing parameters on the microstructure
of the biodegradable porous layer as presented hereinbelow.
[0413] FIG. 2 presents a standard color photograph of an exemplary
composite fibrous structure according to the present embodiments,
showing a thin elongated fiber prepared according to the methods
presented herein from a fibril core and a porous coat.
[0414] Characterization of the Composite Structures
[0415] The effect of varying several characteristics of the
emulsion's composition on the microstructure of the resulting
porous coat, coating the fibril core of the composite fibrous
structure, was studied. These characteristics include the HRP load
as a function of polymer quantity (weight percentage), the polymer
content in the organic phase (weight percentage), and the organic
to aqueous phase ratio in the emulsion.
[0416] For clarity, it is stated that the polymer content and other
polymer parameters mentioned hereinbelow refer to the polymer used
in the preparation of the emulsion (coat-polymer), which is not to
be confused with the polymer used in the preparation of the fibril
core (core-polymer) discussed hereinabove.
[0417] Morphological Characterization:
[0418] The relationship between various parameters of the emulsion
composition and the microstructure (morphology) of the resulting
porous coat, coating the fibril core, was examined by analyzing SEM
images of cryogenically fractured surfaces (cross-sections) of the
composite fibrous structures.
[0419] The following emulsion parameters were examined:
[0420] i) The HRP load relative to the coat-polymer quantity
(expressed in weight per weight percentage, w/w);
[0421] ii) The coat-polymer content in the organic phase (expressed
in weight per volume percentage, w/v); and
[0422] iii) The organic to aqueous phase ratio in the emulsion
(O:A, expressed in volume per volume percentage, v/v).
[0423] The SEM samples were stained with gold and the dimensions of
the observed features were calculated using the Image Pro Plus
software.
[0424] FIG. 5 presents a SEM micrograph, showing a typical
cross-section of an exemplary composite fibrous structure according
to the present embodiments. This particular image is of a composite
fibrous structure which was prepared using an emulsion having a
ratio of HRP to coat-polymer of 5%, coat-polymer content of 15% and
organic to aqueous phase ratio of 4:1. As can be seen in FIG. 5,
the interface between the dense core PLLA fiber and the porous
75/25 PDLGA porous coat, created by freeze drying of the emulsion,
exhibits excellent tight contact, which allows strong adhesion
between the fibril core and the porous coat. Since both parts are
made of aliphatic poly(a-hydroxy acids), their similar surface
tensions contribute to good adhesion at the interface.
[0425] Effect of HRP Load on the Microstructure of the Porous
Coat:
[0426] The relationship between the emulsion composition, for
example, the HRP load, and the microstructure of the resulting
porous coat, coating the fibril core, was examined by electron
microscopy.
[0427] FIGS. 6a-6i present a series of SEM micrographs, showing the
effect of various HRP loads (1%, 5% and 10% w/w) and various
coat-polymer contents (13%, 15%, and 19% w/v) of the emulsion, on
the resulting porous coat's microstructure (cross section), coating
the fibril core of the composite fibrous structures. The tested
coats were prepared from emulsions having a constant O:A ratio of
4:1. Table 2 below presents the indices of the SEM micrographs of
FIG. 6.
TABLE-US-00002 TABLE 2 HRP load relative to polymer Polymer
contents in the organic phase (w/v) content (w/w) 13% 15% 19% 1%
FIG. 6a FIG. 6d FIG. 6g 5% FIG. 6b FIG. 6e FIG. 6h 10% FIG. 6c FIG.
6f FIG. 6i
[0428] As can be seen in FIGS. 4a-4i, for any given structure
(prepared using the same coat-polymer content), as the HRP content
was increased, the porous coat structure changed from a dual pore
population (coexistence of large and small pores) to a relatively
uniform pore population. The surface of the large pore population
in the samples prepared from emulsions having 1% HRP consisted of
smaller pores, whereas a more uniform pore size was achieved when
HRP content was increased to 5% and 10% w/w. This effect is
attributed to the emulsion-stabilizing effect of the protein,
acting as a surfactant.
[0429] FIGS. 7a-7d present a series of SEM micrographs, showing the
effect of the coat-polymer content and protein (HRP) load of the
emulsion on the morphology of pore size distribution of the
resulting porous coat. The tested coats were prepared from
emulsions having a constant O:A phase ratio of 8:1. Table 3 below
presents the indices of the SEM micrographs of FIG. 7 and the
results obtained in this study.
[0430] As can be seen in FIGS. 7a-7d and Table 3, a similar
phenomenon to that observed in coats prepared from emulsion of an
O:A ratio of 4:1 was observed in samples prepared from emulsions
having an O:A phase ratio of 8:1, namely, the pore size
distribution narrowed and their average size decreased as the
protein (HRP) content increased.
TABLE-US-00003 TABLE 3 HRP load relative Polymer contents in the
organic phase to polymer content (w/v) (w/w) 15% 19% 0% 5.30 .+-.
2.80 .mu.m 5.50 .+-. 2.60 .mu.m (FIG. 7a) (FIG. 7c) 5% 3.02 .+-.
1.13 .mu.m 2.40 .+-. 1.10 .mu.m (FIG. 7b) (FIG. 7d)
[0431] As can be further seen in Table 3, the mean pore diameter of
samples prepared from emulsions containing a 15% w/v coat-polymer
content decreased from 5.3 .mu.m in samples prepared from emulsions
without HRP to 3.0 .mu.m in samples containing 5% w/w HRP, and the
mean pore diameter of samples prepared from emulsions containing
19% w/v coat-polymer content decreased from 5.5 .mu.m in samples
prepared from emulsions without HRP to 2.4 .mu.m in samples
prepared from emulsions containing 5% w/w HRP.
[0432] These results can be explained by the following:
[0433] The emulsion used in the coating procedure is
thermodynamically complex as it is stabilized by both the polymer
which is dissolved in the organic phase, and by the HRP protein
molecules which are dissolved in the aqueous phase. The co-polymer
PDLGA is an aliphatic polyester and its chains do not have a
designated anchoring region at the organic/aqueous interface, like
in amphiphilic substances. Stabilization of the emulsion therefore
occurs only through weak interactions at the organic/aqueous
interface [Tadros, T. F. et al., Adv. Colloid Interface Sci., 2004,
108-109, 207-226]. In contradistinction, proteins such as HRP,
which contain defined hydrophobic/hydrophilic regions and an
electrostatic charge [Piazza, R., Curr. Opin. Colloid Interface
Sci., 2004, 8, 515-522], have a natural tendency to adsorb to the
organic/aqueous interface. Proteins thus act similarly to
block-co-polymer surfactants, which are widely used as emulsifiers.
Although an emulsion was obtained also in the absence of HRP, the
decrease in pore diameter due to HRP incorporation supports the
phenomenon of emulsion stabilization by HRP. The emulsions'
stabilization effect correlates with the HRP load. Fibrous
structures prepared from emulsions with an HRP load of 1%
demonstrated a dual pore population obtained by coalescence of the
original dispersed aqueous drops prior to its liquid nitrogen
fixation, whereas structures prepared from emulsions with HRP loads
of 5% and 10% had much more homogeneous pore characteristics (see,
FIG. 4); an indication of an improvement in the emulsion stability.
Similar effects of pore size reduction were previously described
for PDLGA freeze-dried bulky scaffolds containing bovine serum
albumin [Whang, K., et al., Biomaterials, 2000, 21, 2545-2551].
[0434] Effect of the Organic-to-Aqueous Phase Ratio and the Polymer
Content in the Emulsion on the Microstructure of the Porous
Coat:
[0435] The relationship between the emulsion composition, for
example, the organic-to-aqueous phase ratio, and the microstructure
of the resulting porous coat, coating the fibril core, was examined
by electron microscopy.
[0436] FIGS. 8a-8i present a series of SEM micrographs, showing the
effect of various organic-to-aqueous phase ratios in the emulsion
(O:A of 4:1, 8:1 and 16:1) and various coat-polymer contents (13%,
15%, and 19% w/v) on the resulting coat's microstructure (cross
section) of composite fibrous structures. The tested coats were
prepared from emulsions having a constant HRP load of 5% w/w. Table
4 below presents the indices of the SEM micrographs of FIG. 8 and
the average pore size measured in each structure.
[0437] As can be seen in FIG. 8, for any given structure
(containing various coat-polymer contents), as the ratio between
the organic phase and the aqueous phase increased, the resulting
coat's microstructure changed progressively from having a highly
dense and partially interconnected pores to having a relatively low
density of pores separated by thick polymer walls.
TABLE-US-00004 TABLE 4 O:A Polymer contents in the organic phase
(w/v) ratio 13% 15% 19% 4:1 (FIG. 8a) (FIG. 8d) (FIG. 8g) 8:1 2.47
.+-. 1.08 .mu.m 1.67 .+-. 0.58 .mu.m 1.28 .+-. 0.63 .mu.m (FIG. 8b)
(FIG. 8e) (FIG. 8h) 16:1 3.19 .+-. 1.12 .mu.m 1.60 .+-. 0.65 .mu.m
1.50 .+-. 0.78 .mu.m (FIG. 8c) (FIG. 8f) (FIG. 8i)
[0438] As can be seen in Table 4, the mean pore size measured in
various coats prepared from emulsions with relatively high O:A
phase ratio of 16:1, decreased from 3.19 .mu.m to 1.60 .mu.m with
the increase in coat-polymer content in the organic phase from 13%
to 15% w/v. This phenomenon can also be attributed to an increase
in the emulsion's stability. In fact, it has been shown that such
an effect is not prominent at relatively low emulsion viscosities.
In studies conducted with a similar series of structures prepared
in a 4:1 O:A ratio, no significant effect of the polymer content on
the coat's structure was observed (data not shown).
[0439] The Microstructure of the Surface of the Composite Fibrous
Structures:
[0440] The relationship between the emulsion composition and the
microstructure of the outer surface of the porous coat, coating the
fibril core, was examined by electron microscopy.
[0441] FIGS. 9a-9d present a series of SEM micrographs, showing the
effect of various organic-to-aqueous phase ratios (O:A of 8:1 and
16:1) and various coat-polymer contents (13% and 19% w/v) in the
emulsion, on the surface structure of the resulting coats. The
tested coats were prepared from emulsions having a constant load of
5% w/w HRP. Table 5 below presents the indices of the SEM
micrographs of FIG. 9.
TABLE-US-00005 TABLE 5 Polymer contents in the organic phase (w/v)
O:A ratio 13% 19% 8:1 (FIG. 9a) (FIG. 9c) 16:1 (FIG. 9b) (FIG.
9d)
[0442] As can be seen in FIG. 9, all the tested structures have an
outer surface with relatively small pore size (1-2 .mu.m). It
appears that the O:A phase ratio and coat-polymer content of the
emulsion had a negligible effect on the pore size at the outer
surface of the resulting coats. The decrease in pore density with
the increase in O:A phase ratio was observed as expected, due to a
decrease in the emulsion's aqueous content.
[0443] As described hereinabove, during the preparation of a
composite fibrous structure of the present invention, the fibril
core, coated by the emulsion of the coating material, is exposed to
liquid nitrogen. This procedure, together with surface tension
forces, may create a "skin" on the outer surface of the porous
coat. As can be seen in FIGS. 6 and 8, this "skin" is a thin layer
which appears to be slightly different in density than the inner
part of the coat and very thin compared to the thickness of the
coat. As can also be seen in FIGS. 6 and 8, apart from the "skin",
the coat's bulk microstructure remains alike, indicating that
flash-freezing the emulsion preserves its microstructure.
[0444] In conclusion, it has been shown that the HRP load and the
organic-to-aqueous phase ratio in the emulsion used for preparing
the fibrous structures have a significant effect on the
microstructure of the porous coat, whereas the polymer content in
the organic phase of the emulsion affected these fiber
characteristics only marginally and per specific conditions.
[0445] Activity Assays:
[0446] In order to determine the capability of a composite fibrous
structures to deliver a relatively sensitive bioactive agent (for
example, an enzyme) both qualitatively (activity) and
quantitatively (rate), the release profile and activity of HRP, as
an exemplary protein, encapsulated in various composite fibrous
structures were monitored and measured over a time period of 90
days.
[0447] The relationship between various parameters of the emulsion
composition used to prepare the coat, coating the fibril core, and
the release profile of HRP was determined by measuring the activity
and rate of release.
[0448] As in the morphological analysis, the following emulsion
parameters were examined:
[0449] i) The HRP load relative to the coat-polymer quantity
(expressed in weight per weight percentage, w/w);
[0450] ii) The coat-polymer content in the organic phase (expressed
in weight per volume percentage, w/v); and
[0451] iii) The organic to aqueous phase ratio in the emulsion
(O:A, expressed in volume per volume percentage, v/v).
[0452] HRP Activity:
[0453] The enzymatic activity of HRP which was released or
extracted from the composite fibrous structures was determined
using the HRP calibration curve according to a previously described
method [Woo B. H. et al., Pharm. Res., 2001, 18(11), pp
1600-1605].
[0454] Briefly, an HRP calibration curve was obtained using HRP
stock solutions with concentrations ranging from 0.1 .mu.g/ml to 10
.mu.g/ml. A substrate stock solution was prepared with a slow TMB
reagent (Pierce). A 1N sulfuric acid (H.sub.2SO.sub.4) served as
the reaction quenching solution.
[0455] TMB reagent (0.4 ml) was placed in a 2 ml Eppendorf tube.
The enzymatic reaction was initiated by adding 5 .mu.l of solutions
in the range of 0.1 .mu.g/ml to 10 .mu.g/ml HRP concentration to
the tube containing the substrate. Sulfuric acid (0.4 ml) was added
to the tube after 2 minutes to terminate the reaction and
absorbance was measured at 450 nm.
[0456] Composite fibrous structures prepared using emulsions
containing 15% w/v coat-polymer, 5% w/w HRP and organic-to-aqueous
phase ratios of 4:1, 8:1 and 16:1 were tested.
[0457] The specific activity assays of the HRP encapsulated in
composite fibrous structures prepared using various emulsion
formulations, were performed using the procedure described
hereinabove.
[0458] All the examined samples preserved at least 95% of the
original specific enzymatic activity, indicating that the
emulsification, core fiber coating and coat freeze-drying processes
had negligible effect on the enzymatic activity of HRP.
[0459] In-Vitro Protein Release Studies:
[0460] Various samples of HRP-containing composite fibrous
structures were used to determine the release kinetics of HRP over
a time period of 90 days. The HRP release studies were conducted in
closed 1 ml glass vessels in which the HRP-containing composite
fibrous structures were immersed in 1 ml sterile double-distilled
water containing sodium azide as preservative (0.05% w/w) at
37.degree. C. The entire aqueous medium was replaced periodically
by fresh medium and HRP content in the removed medium was
determined by the micro BCA assay method, by measuring absorbance
at 595 nm.
[0461] Cumulative HRP release profiles were determined relative to
the initial amount of HRP in each of the tested structures, i.e.,
the amount of HRP released during the incubation period and the
residual HPR remaining in the structures. All experiments were
performed in triplicate.
[0462] Effect of HRP Load:
[0463] FIG. 10 presents comparative plots of cumulative in vitro
release of HRP from various composite fibrous structures, as a
function of various HRP contents (1%, 5% and 10% w/w) and as a
function of various coat-polymer contents (13%, 15%, and 19% w/v)
at a constant organic-to-aqueous phase ratio of 4:1 in the emulsion
used to prepare the coat coating the composite fibrous structures.
Table 6 below presents the symbol markers of the in vitro release
plots as appear in FIG. 10.
TABLE-US-00006 TABLE 6 HRP content relative to polymer Polymer
contents in the organic phase (w/v) content (w/w) 13% 15% 19% 1%
White rectangles White circles White triangles 5% Black rectangles
Black circles Black triangles 10% Gray rectangles Gray circles Gray
triangles
[0464] As can be seen in FIG. 10, all composite fibrous structures
exhibited HRP release profiles characterized by an initial burst
effect followed by a decreased release rate over time for the first
30 days. In most samples the release rate was constant from day 30
to day 90.
[0465] The burst effect increased with the increase in HRP load,
due to a higher driving force for diffusion. A substantial change
was observed between 1% and 5% w/w HRP load, where the initial
burst increased from 20% to a mean of 70%. The constant release
rate decreased with the increase in HRP load. The coat-polymer
content did not exhibit a significant effect on the release
profile, which stands in agreement with the absence of its effect
in the structure morphology, as show in FIG. 6.
[0466] FIG. 11 presents comparative results of the HPR release
assays, showing the rate of the release from composite fibrous
structures made of an emulsion having 15% w/v coat-polymer, as a
function of various HRP loads (initial burst values are not
included) during the first 30 days of the experiments.
[0467] As can be seen in FIG. 11, the release rate decreased for
all samples, yet the composite fibrous structures loaded with 1%
w/w HRP (marked with white bars in FIG. 11) exhibited a much more
moderate decrease. This phenomenon was also observed in samples
made with coat-polymer contents of 13% w/v (data not shown).
[0468] In summary, the initial burst effect greatly increased with
the increase in HRP load, due to a higher driving force for
diffusion. Since HRP also acts as a surfactant, an HRP load of 5%
and 10% w/w stabilizes the emulsion used for the coat and decreases
the pore size of the coat. The release rate decreases with the
increase in HRP load, probably further due to these structural
changes.
[0469] The HRP load had a dominant effect on its own release
profile (see, FIG. 10), due to the driving force for diffusion. The
dramatic decrease in burst release and total release of HRP from
composite fibrous structures having coats loaded with a 1% w/w HRP
relative to 5% and 10% w/w loads may also be related to HRP-PDLGA
interactions, such as hydrogen bonds. Protein-polymer interactions
have also been previously reported for emulsion systems containing
other proteins, such as bovine serum albumin (BSA) [Verrecchia, T.
et al., J. Biomed. Mater. Res., 1993, 27(8), pp 1019-28] and
lysozyme [Jiang, G. et al., J. Control. Release, 2002, 79(1-3), pp
137-145 and Diwan, M. and Park, T. G., J. Control. Release., 2001,
73(2-3), pp 233-244]. These publications demonstrate that
incubation of lysozyme in the presence of PLGA results in protein
adsorption as compared with its load in the surrounding medium. It
has also been shown that adsorption is a function of PDLGA
microparticle surface area, and that some of the BSA molecules are
irreversibly bound regardless of incubation conditions.
[0470] Effect of Organic-to-Aqueous Phase Ratio:
[0471] FIGS. 12a-12c present comparative plots of cumulative in
vitro release profiles of HRP composite fibrous structures as a
function of the organic-to-aqueous phase ratio of the emulsion (4:1
in black triangles, 8:1 in white rectangles and 16:1 in gray
circles), and as a function of coat-polymer contents in the
emulsion (13% w/v--FIG. 12a, 15% w/v--FIG. 12b and 19% w/v--FIG.
12c) at a constant HRP load of 5% w/w of the emulsion used to
prepare the coat.
[0472] As can be seen in FIGS. 12a-12c, all release profiles
exhibited a characteristic pattern of an initial burst effect
accompanied by a decrease in release rate over time. All samples
released at least 90% of the active enzyme during the 90 day
experiment. An increase in the organic-to-aqueous phase ratio of
the emulsion used to prepare the coat resulted in a significant
decrease in the initial burst release as well as in a more moderate
release curve, for all coat-polymer contents.
[0473] These trends in the cumulative release profiles are
attributed mainly to changes in the coat microstructure. Thus,
manipulation of the emulsion's O:A phase ratio served as a powerful
tool for achieving a variety of protein release profiles, while
preserving a constant HRP load (see, FIG. 12). The change in the
characteristic structure from a dense and partially interconnected
pore population for the 4:1 O:A phase ratio formulations to a less
dense population with a closed pore pattern in the 16:1 O:A phase
ratio resulted in a sharp decrease in HRP diffusion from the porous
coat, dramatically reducing the burst effect from 70-80% to only
about 10-20%.
[0474] In summary, as the organic-to-aqueous phase ratio increased,
the porous coat's microstructure changed from dense partially
interconnected pores to a relatively low density porous structure
with the pores being separated by thick coat-polymer walls. These
structural changes resulted in a sharp decrease in HRP diffusion
and led to a smaller initial burst effect and a more moderate
release profile.
[0475] Effect of Polymer Content:
[0476] FIGS. 10 and 12 present the results discussed hereinabove,
which also show the effect of the emulsion's coat-polymer content
on the HRP release profile.
[0477] As can be seen in FIG. 10, the HRP release profile from
composite fibrous structures, in which the coat were made from
emulsions with three different coat-polymer content values and a
4:1 organic-to-aqueous phase ratio, exhibited similar release
profiles in all studied formulations.
[0478] Although two-dimensional variations on both O:A ratio and
coat-polymer content showed a higher sensitivity to the variations
in O:A ratio, the effect of the variation in coat-polymer content
were more pronounce at the 8:1 and 16:1 O:A phase ratios. As can be
seen in FIGS. 12a-12c, the HRP release assays of the structures
prepared from emulsions having 8:1 and 16:1 O:A phase ratios
exhibited a decrease in the burst effect as the coat-polymer
content in the emulsion's organic phase increased. The burst effect
observed from the structures made from emulsions having 8:1 O:A
ratio (white rectangles in FIGS. 12a-12c) decreased from 66% to 26%
and that of the 16:1 O:A ratio samples (gray circles in FIGS.
12a-12c) decreased from 20% to 10% and the overall profile was more
moderate. These results correspond with the observed morphological
changes (see, FIGS. 6 and 8). Thus, the pore size of the coats
prepared from emulsion having 4:1 O:A phase ratio did not
demonstrate a significant change with increasing coat-polymer
content (see, FIG. 6), whereas the pore size of the samples
prepared from emulsions having 8:1 and 16:1 O:A ratios decreased
with the increase in the emulsion's coat-polymer content (see, FIG.
8 and Table 3). This decrease in pore size and pore density results
in lower HRP diffusion and therefore was expressed as a decrease in
the burst release.
[0479] Residual Protein Recovery from Composite Fibrous
Structures:
[0480] Residual protein recovery from spent composite fibrous
structure samples used in the abovementioned in-vitro release
experiments was conducted according to a previously described
method [Jeffery H et al., Pharm. Res., 1993, 10(3), pp
362-368].
[0481] Briefly, composite fibrous structures were extracted in 1 ml
sodium dodecyl sulfate (SDS)/NaOH 5%/0.1 M solution for 48 hours at
37.degree. C. Following extraction, the HRP concentration was
estimated using a micro BCA assay method as described hereinabove.
Based on these assays, the exact amount of the HRP loaded in each
structure was determined and served for calculating the percentages
cited in the assays above.
[0482] In summary, although the coat-polymer content determines the
emulsion's viscosity, it affects the resulting coat's
microstructure and the HRP release profile only at relatively high
organic-to-aqueous phase ratios. In such formulations, an increase
in the coat-polymer content in the emulsion decreases the resulting
coat pore size via increased emulsion stability, resulting in a
lower burst release and a more moderate release profile. The
release profiles of the HRP-loaded fibers, which were, generally
exhibited an initial burst effect accompanied by a decrease in
release rates with time, as typical for diffusion-controlled
systems.
[0483] These assays demonstrate that an appropriate selection of
the emulsion's parameters used to prepare the coat of the composite
fibrous structure of the present invention can yield structures
that have the desired protein release behavior, stemming from the
coat's microstructure, as well as other mechanical properties.
Drug-Eluting Composite Structures
[0484] Preparation of Nylon Core Fibers:
[0485] The nylon suture fibers, used as core fibers for the
preparation of paclitaxel-eluting fibrillar structures, were
surface-treated in order to dispose of the original fiber's coating
and to enhance the adhesion between the core fiber and the coating.
The nylon fibers were slightly stretched on special holders and
dipped in a 75/25 v/v formic acid/ethanol solution for 15 seconds.
The fibers were thereafter washed and dried in a vacuum oven at
65.degree. C. for 80 minutes.
[0486] Preparation of Emulsions for the Composite
Paclitaxel-Eluting Porous Coat:
[0487] For the preparation of paclitaxel-eluting fibrous
structures, paclitaxel, a water insoluble (hydrophobic) drug, was
incorporated into the organic phase of the emulsion, and surface
active agents were used in order to stabilize the emulsion.
[0488] 75/25 Poly(DL-lactic-co-glycolic acid) (75/25 PDLGA) (0.5
gram, 0.6 gram or 0.75 gram) was dissolved in chloroform (4 ml) to
form an organic phase (corresponding to a polymer content of 13%,
15% and 19% w/v respectively) to form an organic solution and
paclitaxel was added to the solution. Double-distilled water was
poured into the organic phase in a test tube and homogenization of
the emulsion was performed using a hand-held homogenizer (OMNI TH,
7 mm rotor) operating at 16,500 rpm (medium rate) for 3 minutes,
for most samples. In order to evaluate the effect of processing
conditions on the porous coat structure, some samples were prepared
using homogenization rates of 5,500 rpm (low rate) or 25,000 rpm
(high rate) and homogenization durations of 1 minutes and 4
minutes.
[0489] A standard reference sample was prepared with 17.5% w/v
polymer in the organic solution, 1.43% w/w paclitaxel (relative to
the polymer load), and an organic to aqueous (O:A) phase ratio of
2:1 v/v. The emulsion used in this sample is also referred to
herein as a standard reference emulsion, and fibrous structures
made with this emulsion are referred to herein as standards
reference fibers. Other samples were prepared, for example, with
emulsions containing 15% and 22.5% w/v polymer, 0.71%, 2.86% and
7.14% w/w paclitaxel and O:A phase ratios of 4:1 and 1.3:1.
[0490] All the tested formulations used for preparing the emulsions
are presented in Table 8 below.
[0491] Some samples were made from emulsions that further contain a
surfactant. Pluronic.RTM. (1% w/w relative to the polymer quantity)
was added to a polymer solution and PVA (1% w/v relative to the
water quantity) was added to the water.
[0492] Coating Nylon Core Fibers with a Biodegradable Porous
Paclitaxel-Eluting Coat:
[0493] The treated nylon core fibers were dip-coated, while placed
on holders, in fresh emulsions and then frozen immediately in a
liquid nitrogen bath. The holders holding the samples were
thereafter placed in a pre-cooled freeze dryer (Virtis 101 equipped
with a nitrogen trap) set at -105.degree. C. and capable of working
with organic solvents. The samples were freeze dried in order to
preserve the microstructure of the emulsion-based core/coat fiber
structures.
[0494] Freeze drying was performed in the following two stages:
[0495] i. The freeze dryer chamber pressure was reduced to 100
mTorr, while the temperature of the condenser remained at
-105.degree. C.
[0496] ii. The condenser was turned off and its plate temperature
slowly increased to room temperature, while the pressure was
monitored between 100 mTorr and 700 mTorr. During this step the
liquid nitrogen trap condensed the excess water and solvent
vapors.
[0497] The samples were stored in desiccators until use.
[0498] Tensile and Mechanical Properties of the Composite Fibrous
Structures:
[0499] The composite structures' tensile mechanical properties were
measured at room temperature under unidirectional tension at a rate
of 50 mm per minute according to the standard method of tensile
strength ASTM D 3379, using a 5500 Instron machine. Briefly, the
tensile strength was defined as the maximum strength in the
stress-strain curve, whereas the maximal strain was defined as the
breaking strain and Young's modulus was defined as the slope of the
stress-strain curve in the elastic (linear) region. Six samples
were tested for each point, and the means and standard deviations
were calculated using the SPSS 10 software. ANOVA (Tukey-Kramer)
was used for group comparison.
[0500] The nylon suture fibers were surface-treated, as described
hereinabove, in order to dispose of the fiber's original
manufacturer incrustation and to enhance the adhesion between the
core fiber and the coating. Two methods were used for evaluating
the mechanical properties of the core/coat fibers: one considering
the total diameter of the fiber including the coat's thickness, and
one considering the effective diameter, which is actually the
treated core fiber without the added thickness of the coat,
assuming that the coat contributes only marginally to the
macroscopic mechanical properties of the composite structures.
[0501] FIG. 13 presents comparative plots, showing the tensile
stress-strain curves of the treated nylon fibers and of fibers
coated with the standard reference emulsion described hereinabove,
wherein curve "1" corresponds to a surface treated nylon core
fiber, curve "2", considering total diameter, corresponds to a
standard reference fibrous structure, and curve "3", considering
effective diameter, corresponds to a standard reference fibrous
structure. As can be seen in FIG. 13, some decrease in the strength
and Young's modulus was observed in the treated core fiber possibly
upon treatment of the nylon core and the freezing and freeze-drying
process.
[0502] Table 7 below presents the fiber's macroscopic mechanical
properties as measured for five types of fibers, namely:
[0503] Uncoated treated nylon core fibers;
[0504] Nylon core fibers coated with the standard reference
emulsion described hereinabove and considering total fiber
diameter, denoted "Composite type A*";
[0505] Nylon core fibers coated with the standard reference
emulsion described hereinabove and considering effective fiber
diameter, denoted "Composite type A**";
[0506] Nylon core fibers coated with a more viscous emulsion (22.5%
w/v polymer as compared to 17.5% w/v of the standard emulsion) and
considering effective fiber diameter, denoted "Composite type B**";
and
[0507] Nylon core fibers coated with a less viscous emulsion
(higher solvent volume of 5 ml, which gives rise to 14% w/v polymer
content, instead of 4 ml, which gives rise to 17.5% w/v polymer
content) and considering effective fiber diameter, denoted
"Composite type C**".
[0508] As can be seen in Table 7, the measured macroscopic
mechanical properties, calculated for nylon core fibers coated with
a standard emulsion, while considering the effective diameter of
the composite structures, show that the actual effect of the
coating results is a 18% decrease in tensile strength and a 20%
decrease in Young's modulus. These results demonstrate that the
process of fiber coating, which includes exposure to the emulsion,
quenching by immersing in liquid nitrogen and freeze drying,
results in minor decrease in the tensile strength and modulus of
the composite structure, as compared to the non-coated fiber, while
the fibers remained strong and flexible.
[0509] As can further be seen in Table 7, the other two composite
structures exhibited mechanical properties similar to those
obtained for the fibers that were coated with the standard
emulsion, indicating that the emulsion's viscosity has no essential
effect on the fibers' mechanical properties.
TABLE-US-00007 TABLE 7 Strength Modulus Strain Fiber type (MPa)
(MPa) (%) Treated nylon core 396 .+-. 50 880 .+-. 15 48.0 .+-. 5.5
fibers Composite type A* 267 .+-. 32 590 .+-. 7 47.4 .+-. 4.8
Composite type A** 325 .+-. 40 700 .+-. 12 47.9 .+-. 5.0 Composite
type B** 331 .+-. 35 713 .+-. 17 37.8 .+-. 5.3 Composite type C**
337 .+-. 41 695 .+-. 21 39.0 .+-. 4.9
[0510] Morphological Characterization:
[0511] The morphology of the composite structures (cryogenically
fractured surfaces) was evaluated using a Jeol JSM-6300 scanning
electron microscope (SEM) at an accelerating voltage of 5 kV.
Briefly, the samples were Au sputtered prior to observation. The
mean pore diameter and porosity of the observed morphologies was
analyzed using Sigma Scan Pro software and statistics were drawn
using SPSS 10 software. Statistical significance was determined
using the ANOVA (Tukey-Kramer) method.
[0512] In order to evaluate the porosity of the samples of each of
the SEM fractographs, the area occupied by the pores was
calculated, using the Sigma Scan Pro software, and the porosity was
determined as the area occupied by the pores divided by the total
area.
[0513] The effects of the emulsion's composition and processing
parameters on the microstructure were studied by examining the
following parameters:
[0514] i. emulsion formulation (polymer content, % w/v, measured
relative to the solvent volume);
[0515] ii. paclitaxel content (% w/w, measured relative to the
polymer weight);
[0516] iii. aqueous to organic phase ratio (v/v);
[0517] iv. PDLGA co-polymeric ratio;
[0518] v. addition of surface active agents; and
[0519] vi. duration and rate of homogenization.
[0520] The characterization microstructure was based on the
following parameters:
[0521] i. mean pore diameter and distribution;
[0522] ii. porosity and pore structure; and
[0523] iii. coating thickness and adhesion quality.
[0524] The results of these studies are presented in Table 8
below.
[0525] FIG. 14a presents a schematic illustration of an exemplary
paclitaxel-eluting composite fiber, showing a nylon core, and a
biodegradable porous coat in which paclitaxel is encapsulated.
[0526] FIGS. 14b-14d present SEM fractographs of fibrous composite
structures prepared with a standard reference emulsion as described
hereinabove, showing the overall morphology thereof. The diameter
of the treated core fiber was in the range of 170-190 .mu.m and
coat thickness of 30-60 .mu.m was obtained for most emulsion
formulations. Relatively high contents of hydrophobic components,
such as PDLGA and paclitaxel, resulted in an increase in coat
thickness, due to higher emulsion's viscosity. As can be seen in
FIG. 14, there are no gaps between core and coat, indicating that
the quality of the interface between the fiber and the porous
coating is high, and that the surface treatment enabled good
adhesion therebetween. The coat's porous structure in all studied
samples contained round-shaped pores, usually within the 5-10 .mu.m
in diameter, with a porosity exceeding 80% (see, Table 8 below).
The coat's microstructure was uniform in each sample, presumably
due to rapid quenching of the emulsion, which enabled preservation
of its microstructure. As can further be seen in FIG. 14b, the
pores were partially interconnected by smaller inner pores.
TABLE-US-00008 TABLE 8 Porosity Process Mean pore (% .+-. Coating
Parameters Amount size [.mu.m] 10%) thickness [.mu.m] Polymer
content 15 5.8 .+-. 2.3 85 27.7 .+-. 3.6 [% w/v] 17.5 6.5 .+-. 2.3
85.2 104 .+-. 31.4 22.5 5.4 .+-. 2.1 82 64.2 .+-. 32.4 Paclitaxel 0
6.9 .+-. 1.9 N/A 42.2 .+-. 3 content [% w/w] 0.71 5.4 .+-. 2.6 89
74.2 .+-. 9.9 1.43 6.5 .+-. 2.3 85.2 104 .+-. 31.4 2.86 21.2 .+-. 6
85 81 .+-. 37.7 7.14 79.1 .+-. 17 N/A 192.8 .+-. 90.7 Organic to
4:1 6.1 .+-. 3.1 87.6 52.3 .+-. 12.5 Aqueous phase 2:1 6.5 .+-. 2.3
85.2 104 .+-. 31.4 ratio [v/v] 1.3:1 7.8 .+-. 3.8.quadrature. 94.2
64.6 .+-. 24.1 Surfactant None 6.5 .+-. 2.3 85.2 104 .+-. 31.4
content [1% w/v] Pluronic .RTM. 8.2 .+-. 3.0 88 204.1 .+-. 129.3
PVA 6.2 .+-. 2.8.quadrature. 87.5 77.5 .+-. 24.7 Homogenization 60
7 .+-. 3.7 86.8 23.8 .+-. 1.3 duration [Sec] 180 6.5 .+-. 2.3 85.2
104 .+-. 31.4 240 5.9 .+-. 2.6 81.6 90.2 .+-. 44.7 Homogenization
5,500 7.7 .+-. 3.5 92.7 114.6 .+-. 33.2 rate [rpm] 16,500 6.5 .+-.
2.3 85.2 104 .+-. 31.4 25,000 5.8 .+-. 1.9 86 65.7 .+-. 20.7
[0527] Effect of Emulsion Formulation:
[0528] SEM measurements indicated that higher drug content results
in a larger pore size, presumably due to emulsion instability.
FIGS. 15a-15d present SEM fractographs of various
paclitaxel-eluting composite fibrous structures, all having a nylon
core and made using various emulsions, which demonstrate the effect
of the emulsion's formulation on the resulting coat's
microstructure. FIG. 15a shows a composite fibrous structure made
with a standard reference emulsion containing 17.5% w/v polymer,
1.43% w/w paclitaxel and having a phase ratio of 2:1 O:A. FIG. 15b
shows a composite fibrous structure made with an emulsion
containing 15% w/v polymer as compared to the standard reference
fiber. FIG. 15c shows a composite fibrous structure made with an
emulsion containing 2.9% w/w paclitaxel as compared to the standard
reference emulsion. FIG. 15d shows a composite fibrous structure
made with an emulsion having an O:A ratio of 4:1 as compared to the
standard reference emulsion.
[0529] As can be seen in FIGS. 15a-15d, the pore size was almost
unaffected by the emulsion's polymer content (see also, Table 8
hereinabove), but less dense "polymeric walls" appeared to be
created between adjacent pores. It is suggested that a relatively
low polymer content reduces the binding region between the matrix
and paclitaxel. This features may affect the release of the drug,
resulting in a higher diffusion coefficient which enables more
effective drug release, as discussed hereinbelow.
[0530] Effect of Surfactants:
[0531] Pluronic.RTM. type surfactants are block copolymers based on
ethylene oxide and propylene oxide. They can function as
antifoaming agents, wetting agents, dispersants, thickeners and
emulsifiers.
[0532] FIGS. 16a-16c present a series of SEM fractographs
demonstrating the coat's microstructure of exemplary
paclitaxel-eluting composite fibrous structures, each having a
nylon core and a coat made from an emulsion that contains no
surfactants (FIG. 16a), a coat made from an emulsion containing 1%
w/w Pluronic.RTM. (FIG. 16b), and a coat made an emulsion
containing 1% w/v PVA (FIG. 16c).
[0533] As presented in Table 8 hereinabove, incorporation of
Pluronic.RTM. in the emulsion resulted in an increase in the pore
size and porosity.
[0534] As can be seen in FIGS. 16a-16c, relatively large voids
appeared between domains of the regular porous structure as a
result of the presence of Pluronic.RTM. surfactant observed in FIG.
16b, instead of the regular homogenous structure observed in FIG.
16a. These large voids between the regular porous regions introduce
local continuous paths for drug diffusion and hence may result in
increase in the drug release rate and quantity. On the other hand,
the presence of PVA surfactant had almost no effect on the coat's
morphology, as can be seen in FIG. 16c.
[0535] In Vitro Paclitaxel Release Studies:
[0536] Cumulative release of paclitaxel from samples of composite
fibrous structures was monitored and followed over a time period of
four months. Samples of composite structures were immersed in PBS
at 37.degree. C. for 112 days. The medium was entirely removed
periodically and assayed for drug release, and fresh medium was
introduced. The paclitaxel content of each medium sample was
determined using Agilent 1100 High Performance Liquid
Chromatography (HPLC). The paclitaxel-eluting composite structures
maintained their mechanical integrity throughout the entire test
period, without visible cracking or discharge of core degradation
products to the medium.
[0537] The paclitaxel release profile obtained for most studied
structures during the test period exhibited a low initial burst
effect, accompanied by a decrease in release rate over time.
[0538] FIG. 17 presents cumulative plot of paclitaxel release from
an exemplary composite fibrous structure made with a standard
reference emulsion as described hereinabove, showing the amount of
released paclitaxel in mg and as the percentage of the released
paclitaxel from the loaded amount, as measured over a time period
of four months.
[0539] As can be seen in FIG. 17, the release rate of paclitaxel
exponentially decreased with time, and a minor burst effect of less
than 3% was observed during the first days of release. Such a
release profile is typical of diffusion-controlled systems. The
paclitaxel release from the porous coat was relatively slow, mainly
due to the fact that paclitaxel is hydrophobic in nature and
therefore resides within the slow-dissolving/biodegrading polymer.
The exponential drop in release rate may be caused by the
progressively longer distance the drug has to pass through the
coat.
[0540] These results corroborate that the drug release profile of
paclitaxel from these composite fibrous structures is controlled
mainly by diffusion and that the degradation rate of the coat's
biodegradable polymer has a minor effect on drug release profile.
The partial amount of the loaded drug that was released, is within
the desired amount that corresponds to a therapeutically effective
amount of the drug that is required in many applications such as
implantable medical devices, (e.g., a stent).
[0541] Effect of Coat Processing Conditions:
[0542] The kinetic parameters of the coating process include the
rate and the duration of homogenization of the emulsion containing
the drug prior to freezing and subsequent freeze drying thereof. As
presented hereinabove, the emulsions were typically homogenized by
a hand-held homogenizer operating at a medium rate of 16,500 rpm
for 3 minutes (referred to herein as a moderate rate). The effect
of processing conditions on the drug release rate from the coat was
examined for a low rate of homogenization (5,500 rpm) and a high
rate (25,000 rpm), and for homogenization durations of 1 minutes
and 4 minutes.
[0543] FIG. 18 presents comparative plots showing the drug release
from the porous coat of paclitaxel-eluting composite structures,
wherein the various emulsion used in the preparation of the porous
coat was homogenized at a low rate (marked with blue diamonds),
medium rate (marked with magenta squares) and high rate (marked
with green triangles), showing the effect of the emulsion's
homogenization rate on the rate of release. As can be seen in FIG.
18, the homogenization rate had some effect on the release profile,
while increased homogenization rate resulted in increased drug
release rate and quantity. Taken together with the results
presented in Table 8, it is suggested that while an increase in
homogenization rate results in a slight decrease in pore size, the
presence of smaller pores enable some increase in drug release rate
and quantity.
[0544] The homogenization duration did not have a significant
effect on paclitaxel release profile for samples prepared using
homogenization durations which exceeded 180 seconds. This is in
agreement with the similarity in pore size and shape as presented
in Table 8 hereinabove. However, at relatively short homogenization
times, such as 60 seconds, resulted in local continuous paths in
the coat microstructure, presumably due to instability of the
emulsion, enabling drug diffusion and therefore higher release
rates.
[0545] Effect of Polymer Content in the Emulsion Formulation:
[0546] In general, the stability of the emulsion used in the
preparation of the paclitaxel-eluting fibrous composite structures
determines the porous structure, as a more hydrophobic organic
phase is expected to exhibit a porous structure with larger pores,
due to higher interfacial tension leading to coalescence of aqueous
domains. Such an increase in pore size is expected to result in a
decreased surface area and a lower diffusion rate. A more
hydrophobic organic phase is therefore expected to enable lower
drug release rates and quantities.
[0547] FIG. 19 presents comparative plots of the drug release
profile from paclitaxel-eluting fibrous composite structures,
showing the effect of the polymer content in the emulsion
formulation on the drug release from the composite structures,
wherein the drug release profile from emulsions having a polymer
content of 15% w/v is marked with blue squares, 17.5% w/v is marked
with magenta circles, and 22.5% w/v is marked with green
triangles.
[0548] As can be seen in FIG. 19, the release rate and the amount
of drug release increased with the decrease in polymer content. The
quantity released from the formulation containing 15% w/v polymer
was significantly higher than that obtained for 17.5% w/v and 22.5%
w/v formulations. Since the pore size was almost unaffected by the
emulsion's polymer content, as can be seen in FIGS. 15a and 15b and
in Table 8 hereinabove, it is suggested that less dense "polymeric
walls" are created between adjacent pores in the coats prepared
from emulsions having a relatively low polymer content, and
therefore a higher diffusion rate is observed with such composite
structures.
[0549] The effect of the polymer content of the organic phase was
found to affect mostly the emulsion viscosity, with only a marginal
indirect effect on the release of paclitaxel.
[0550] Effect of Drug Content in the Emulsion Formulation:
[0551] Since paclitaxel is a hydrophobic drug, a higher paclitaxel
content in the organic phase of the emulsion is expected to result
in higher interfacial tension, namely a greater difference between
the surface tension of the organic and aqueous phases, leading to a
less stable emulsion with a larger pore size. This expectation is
corroborated with the finding presented in Table 8 hereinabove.
Larger pores are expected to reduce the release rate for a given
porosity and interconnectivity.
[0552] FIG. 20 presents comparative plots showing the drug release
profile from paclitaxel-eluting fibrous composite structures,
demonstrating the effect of the drug content in the emulsion
formulation on the drug release from the composite fibers, wherein
the drug release profile from emulsions having a drug content of
0.7% w/w is marked with red diamonds, 1.4% w/w is marked with
magenta circles, 2.9% w/w is marked with blue triangles and 7.1%
w/w is marked with cyan squares.
[0553] As can be seen in FIG. 20, the drug content has a
significant effect on the release profile. Both the release rate
and the amount of drug released increased with the increase in
paclitaxel content, mainly due to a higher drug concentration
gradient between the coat matrix and the surrounding medium.
Furthermore, a relatively large burst effect was observed for the
high drug content samples. Fibers coated with emulsion containing
7.14% w/w paclitaxel released 7% during the first 24 hours compared
to 3% from samples prepared with emulsions containing 2.9% w/w
paclitaxel.
[0554] It was concluded that the driving force for diffusion has a
greater effect than the morphological changes, since the release
rate in this system increased with the drug content, in spite of
the morphological changes which favor the opposite drug release
behavior.
[0555] Effect of Organic-to-Aqueous Ratio in the Emulsion
Formulation:
[0556] The release profile as well as the pore size and porosity
exhibited little sensitivity to a change in the organic-to-aqueous
phase ratio (O:A ratio) range, as can be seen in Table 8
hereinabove. It should be mentioned that the relatively narrow O:A
range of 2:1 and 4:1 O:A ratio was practiced due to emulsion
stability considerations.
[0557] FIG. 21 presents comparative plots showing the drug release
profile from paclitaxel-eluting fibrous composite structures,
demonstrating the effect of the organic-to-aqueous phase ratio (O:A
ratio) in the emulsion formulation on the drug release from the
composite structures, wherein the drug release profile from
emulsions having a O:A ration of 4:1 v/v O:A is marked with magenta
squares, and 2:1 v/v O:A is marked with green diamonds.
[0558] As can be seen in FIG. 21, the drug release from structures
made from an emulsion having a 4:1 v/v O:A ratio is significantly
lower than the release rate from structures made from an emulsion
of 2:1 v/v O:A ratio. It is suggested that the porosity of samples
derived from emulsions with O:A ratios higher than 4:1, may not be
high enough so as to enable effective release of water-insoluble
agents such as paclitaxel. On the other hand, samples derived from
emulsions with O:A ratios less than 2:1 are not stable enough to
sustain the production process. For example, the composite
structures prepared with emulsions having a 1.3:1 O:A ratio were
not stable enough and exhibited a relatively large pore
distribution with a porosity of 94.2%, as presented in Table 8
hereinabove.
[0559] Effect of Surfactants:
[0560] The effect of the incorporation of surfactants into the
preparation of the emulsions used to make the composite fibers was
investigated for two surfactants, PVA and Pluronic.RTM.. Both
surfactants were incorporated at a concentration of 1% w/w.
[0561] FIG. 22 presents comparative plots showing the drug release
profile from paclitaxel-eluting composite structures, demonstrating
the effect of the incorporation of a surfactant to the emulsion
formulation on the drug release from the composite structures,
wherein the drug release profile from structures made from
emulsions having no surfactant is marked with magenta squares,
emulsions having 1% Pluronic.RTM. is marked with blue triangles,
and emulsions having 1% PVA is marked with black diamonds.
Pluronic.RTM. was also incorporated at a concentration of 10% w/w,
but did not further increase the release rate (data not shown).
[0562] As can be seen in FIG. 22, the incorporation of
Pluronic.RTM. in the emulsion resulted in an increase in the drug
release rate and quantity, whereas incorporation of PVA resulted in
a decrease in both parameters as compared to structures made from
emulsions having no surfactant added. As presented and discussed
hereinabove, the incorporation of the Pluronic.RTM. surfactant to
the emulsion changed the coat's microstructure (see, FIG. 16b),
causing the introduction of relatively large voids between domains
of a regular porous structure, instead of the regular homogenous
structure overall. These large voids between the regular porous
regions, expressed as increased pore size and porosity, as
presented in Table 8, introduced local continuous paths for drug
diffusion and it is suggested that these paths enabled some
increase in release rate and quantity in the case of Pluronic.RTM..
On the other hand, the PVA surfactant had almost no effect on the
coat's morphology, as seen in FIG. 16c, but still resulted in a
decrease in the release rate.
[0563] In conclusion, it is shown that the internal surface area of
the pores in the porous coat affects the release rate of
hydrophobic small-molecule bioactive agents such as paclitaxel,
from the composite structures described herein. A higher internal
surface area of the coat can be achieved by adjusting the emulsion
formulation and preparation process so as to obtain smaller and
more interconnected pores.
A Model for Predicting the Release of a Bioactive Agent from
Composite Fibers
[0564] The ability to predict the rate of release of a bioactive
agent from the composite structures presented herein is of high
importance in the design stage of preparing a composite structure
according to the present invention. To this end, the present
inventors have developed a mathematical-physical model which uses
physical values of various key parameters that govern the rate of
release of a bioactive agent from a composite structure as
presented herein. These parameters include the relative
concentration of the bioactive agent in the coat, the tortuosity
factor which is closely related to the porosity of the coat, the
physical dimensions of the core and the coat and the coat-polymer
composition.
[0565] The mathematical model is presented in detail hereinabove,
and the experimental data used to validate this model were taken
from the examples for HRP-eluting composite structure presented
hereinabove. Data taken from a research by Wu et al. [Part II:
Biodegradation. Journal of Biomaterials Science--Polymer edition
2001; 12(1): 21-34] were used for interpolation in order to obtain
a good estimation for the degradation profile of 75/25 PDLGA with
an initial molecular weight of 100 kDa.
[0566] Prediction of Rate of Release as a Function of the Emulsion
Characteristics:
[0567] FIGS. 23a-23e present five sets of comparative plots and
mean error thereof showing the predicted HRP release profile (blue
curves) as compared to the experimental release profile (red
curves) for each of the following composite fibrous structures: a
structure having a biodegradable core (not included in the
calculations) and a coat made from an emulsion containing an O:A
ratio of 8:1 and a 15% w/v polymer content (FIG. 23a), an O:A ratio
of 8:1, 19% w/v polymer content (FIG. 23b), an O:A ratio of 16:1,
13% w/v polymer content (FIG. 23c), an O:A ratio of 16:1, 15% w/v
polymer content (FIG. 23d) and an O:A ratio of 16:1, 19% w/v
polymer content (FIG. 23e).
[0568] As can be seen in FIGS. 23b-23e, a very good fit between
predicted and experimentally measured data was generally obtained
for all studied structures (see, FIG. 23a). Hence, these results
support the first model assumption regarding prediction adequacy of
a model based on Fick's laws.
[0569] Table 9 below presents the emulsion parameters of the five
structures used in the studies presented in FIG. 23 along with
their semi-empirical polymer concentration C.sub.P and the
tortuosity factor .tau. values of each sample fiber.
TABLE-US-00009 TABLE 9 Emulsion Polymer content in Fiber
organic:aqueous the organic phase type phase ratio (O:A) (% w/v)
C.sub.P .tau. A 8:1 15% 0.29 3.3 B 8:1 19% 0.58 7.0 C 16:1 13% 0.54
8.0 D 16:1 15% 0.8 11.0 E 16:1 19% 1.4 21.0
[0570] As discussed hereinabove, two basic emulsion types were
prepared by using a constant organic phase volume with two
different aqueous phase volumes, namely an O:A ratio of 8:1 and
16:1. The structures fabricated with a higher O:A ratio of 16:1
exhibit a more tortuous diffusion path, leading to higher values of
the tortuosity factor as seen in Table 9 hereinabove. Furthermore,
the tortuosity factor within both O:A ratio of 8:1 and 16:1
increases with the increase in the polymer content. Therefore,
either increasing the emulsion's O:A ratio, namely decreasing the
aqueous phase volume, or increasing the polymer content, resulted
in a decrease in the free space available for diffusion, leading to
a higher tortuosity factor, which in turn leads to a lower release
rate of the bioactive agent from the structure's coat. These
results are in agreement with the second model assumption, that
emulsion formulation parameters affect the bioactive agent release
profile.
[0571] Since a higher polymer content leads to an emulsion with a
more viscous and dense organic phase, it was assumed that the
resulting solid porous structure will tend to absorb less water,
resulting in slower hydrolysis and hence slower degradation,
leading to a shorter and more moderate burst effect. Following this
assumption, C.sub.P was introduced into the model so as to alter
the porous structure's degradation rate. This was also supported by
the experimental results, which demonstrate that as the polymer
content increases, the coat's matrix degradation decreases, leading
to a smaller initial burst release.
[0572] Comparisons of the C.sub.P and .tau. values of different
composite structures lead to the elucidation of the effect of
processing conditions on these parameters as assessed by examining
their microstructure. For example, as can be seen in Table 9
hereinabove, the C.sub.P value of a sample prepared with an
emulsion having a O:A ratio of 16:1 and a polymer content of 15%
w/v is 2.8 times higher than that of a sample prepared with an
emulsion having a O:A ratio of 8:1, and for structures made with a
polymer content of 19% w/v the C.sub.P value of the O:A ratio of
16:1 sample is 2.4 times higher than that of the sample made with
an emulsion having an O:A ratio of 8:1.
[0573] A similar tendency was observed for .tau., as can be seen in
Table 9 hereinabove, wherein the .tau. value of the sample made
with an emulsion having an O:A ratio of 16:1 and a polymer content
of 15% w/v is 3.3 times higher than that of the 8:1 sample, and for
fibers made with a polymer content of 19% w/v the t value of the
sample made with an emulsion having an O:A ratio of 16:1 is 3.0
times higher than that of the fibers made from an emulsion having
an O:A ratio of 8:1. This consistent behavior of both parameters as
a function of the polymer concentration simplify the model and
corroborate its validity and prediction capacity when combined with
certain experimental calibration curves and/or mathematical
functions which may be developed in order to further simplify the
model.
[0574] Prediction of Rate of Release as a Function of the Polymer's
Molecular Weight:
[0575] The effect of the PDLGA molecular weight on the release rate
was examined using the degradation profiles of polymers with
initial average molecular weights of 40 kDa and 160 kDa, in
addition to that of the standard average molecular weight of 100
kDa actually used in the experiments presented hereinabove. These
degradation profiles were obtained using interpolations based on
the experimental results of Wu et al., and are presented in FIG.
24.
[0576] FIG. 24 presents comparative plots showing the degradation
rate of fiber coats made from three types of PDLGA polymers (data
adopted from Wu et al.), wherein the green curve represent the
degradation rate of a polymer having a 160 kDa molecular weight,
the blue curve represents a polymer of 100 kDa and the red curve
represents a 40 kDa PDLGA polymer.
[0577] FIGS. 25a-25e present five sets of comparative plots showing
the predicted HRP release profiles for composite structures made
with three types of 75/25 PDLGA polymers having 40 kDa molecular
weight (red curves), 100 kDa molecular weight (blue curves) and 160
kDa molecular weight (green curves), and made from emulsions having
an O:A ratio of 8:1 and a polymer content of 15% w/v (FIG. 25a), an
O:A ratio of 8:1 and a polymer content of 19% w/v (FIG. 25b), an
O:A ratio of 16:1 and a polymer content of 13% w/v (FIG. 25c), an
O:A ratio of 16:1 and a polymer content of 15% w/v (FIG. 25d), and
an O:A ratio of 16:1 and a polymer content of 19% w/v (FIG.
25e).
[0578] As can be seen in FIGS. 25a-25e, the decrease in initial
molecular weight resulted in an increased HRP release rate in all
tested samples. This prediction is logical and consistent with
experimental results, since a lower initial molecular weight
polymer will result in shorter polymer chains as degradation
proceeds, giving rise to an enhanced drug release rate. It should
be noted that the burst release values in these predictions is
almost unaffected with the initial average molecular weight mainly
because the only parameter that was changed in the calculation for
these predictions is the matrix degradation profile, leaving the
same tortuosity factor which was calculated for the 100 kDa fiber
type. However, the tortuosity factor is expected to increase with
an increase in the molecular weight.
[0579] Prediction of Rate of Release as a Function of the Protein's
Molecular Weight:
[0580] The effect of the bioactive agent's molecular weight,
corresponding to its size, on its release profile from the various
fibrous composite structures was also studied using the
mathematical model presented herein.
[0581] FIGS. 26a-26b present two comparative plots showing the
effect of the molecular weight of the bioactive agent on the
predicted release profiles thereof using three model proteins
having a molecular weight of 22 kDa (red curves), 44 kDa (blue
curves) and 160 kDa (green curves), released from the coat of
composite structures prepared from emulsions of 5% w/w model
protein, a polymer content of 19% w/v and an O:A ratio of 8:1 (FIG.
26a) and an O:A ratio of 16:1 (FIG. 26b).
[0582] As can be seen in FIGS. 26a-26b, the predicted profiles
demonstrate that the protein release rate decreases with the
increase in its molecular weight, namely high molecular weight
proteins exhibit a lower diffusion coefficient, which results in
lower mobility in water. Since protein release occurs by means of
diffusion in water, this lower diffusion coefficient should result
in a lower release rate. These results support the second model
assumption, stating that the release profile is affected by the
sizes of both system's components, namely the bioactive agent and
the coat, and that the effect of the bioactive agent's size on its
release profile is apparently higher than that of the host
polymer's initial average molecular weight.
[0583] It is appreciated that certain features of the invention,
which are, for clarity, described in the context of separate
embodiments, may also be provided in combination in a single
embodiment. Conversely, various features of the invention, which
are, for brevity, described in the context of a single embodiment,
may also be provided separately or in any suitable
subcombination.
[0584] Although the invention has been described in conjunction
with specific embodiments thereof, it is evident that many
alternatives, modifications and variations will be apparent to
those skilled in the art. Accordingly, it is intended to embrace
all such alternatives, modifications and variations that fall
within the spirit and broad scope of the appended claims. All
publications, patents and patent applications mentioned in this
specification are herein incorporated in their entirety by
reference into the specification, to the same extent as if each
individual publication, patent or patent application was
specifically and individually indicated to be incorporated herein
by reference. In addition, citation or identification of any
reference in this application shall not be construed as an
admission that such reference is available as prior art to the
present invention.
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