U.S. patent application number 11/062398 was filed with the patent office on 2005-10-13 for in situ forming hydrogels.
Invention is credited to Hubbell, Jeffrey A., Kornfield, Julia A., Tae, Giyoong.
Application Number | 20050226933 11/062398 |
Document ID | / |
Family ID | 34138157 |
Filed Date | 2005-10-13 |
United States Patent
Application |
20050226933 |
Kind Code |
A1 |
Hubbell, Jeffrey A. ; et
al. |
October 13, 2005 |
In situ forming hydrogels
Abstract
The invention features materials and methods for the liquid to
solid transition of an injectable pre-hydrogel composition to a
hydrogel. These methods can be carried out in situ.
Inventors: |
Hubbell, Jeffrey A.;
(Zumikon, CH) ; Kornfield, Julia A.; (Pasadena,
CA) ; Tae, Giyoong; (Pasadena, CA) |
Correspondence
Address: |
CLARK & ELBING LLP
101 FEDERAL STREET
BOSTON
MA
02110
US
|
Family ID: |
34138157 |
Appl. No.: |
11/062398 |
Filed: |
February 22, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11062398 |
Feb 22, 2005 |
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09559984 |
Apr 26, 2000 |
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6858229 |
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60131164 |
Apr 26, 1999 |
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Current U.S.
Class: |
424/486 ;
424/488 |
Current CPC
Class: |
A61K 9/0024 20130101;
A61K 9/0019 20130101; A61K 47/40 20130101; A61K 47/34 20130101;
A61L 2400/06 20130101 |
Class at
Publication: |
424/486 ;
424/488 |
International
Class: |
A61K 009/14 |
Claims
What is claimed is:
1. A hydrogel precursor composition comprising: (a) a polymer, said
polymer comprising a water soluble polymer domain with at least two
hydrophobic interacting groups attached thereto, said polymer
capable of assembling into a hydrogel under physiological
conditions; and (b) a physical chemical protecting group, said
physical chemical protecting group preventing gelation of said
hydrogel precursor composition.
2. A hydrogel or hydrogel precursor composition comprising: (a) a
polymer, said polymer comprising a water soluble polymer domain
with at least two hydrophobic interacting groups attached thereto,
said polymer capable of assembling into a hydrogel under
physiological conditions; (b) a physical chemical protecting group,
said physical chemical protecting group preventing gelation of said
hydrogel precursor composition or hydrogel; and (c) a molecule that
disrupts an interaction between said physical chemical protecting
group and said hydrophobic interacting groups.
3. The hydrogel or hydrogel precursor composition of claim 1,
wherein said polymer domain comprises poly(ethylene glycol),
poly(vinyl alcohol), poly(vinyl pyrrolidone), poly(ethyl
oxazoline), poly(acrylic acid), poly(acrylamide), poly(styrene
sulfonate), poly(amino acids), polysaccharides, or copolymers
thereof.
4. The hydrogel or hydrogel precursor composition of claim 1,
wherein said chemical protecting group is .beta.-cyclodextrin.
5. The hydrogel precursor composition of claim 1, wherein said
hydrophobic interacting groups are positioned at the termini of
said polymer domain.
6. The hydrogel precursor composition of claim 1, wherein said
hydrophobic interacting groups are positioned within said polymer
domain.
7. The hydrogel precursor composition of claim 1, wherein said
hydrophobic interacting groups are hydrocarbons.
8. The hydrogel precursor composition of claim 5, wherein said
hydrocarbons are perfluorinated hydrocarbons.
9. The hydrogel precursor composition of claim 1, wherein said
physical chemical protecting group is a cyclodextrin.
10. The hydrogel precursor composition of claim 1, wherein said
physical chemical protecting group is a molecule that covalently
binds to said hydrophobic interacting groups.
11. The hydrogel or hydrogel precursor composition of claim 9,
wherein said molecule that covalently binds to said hydrophobic
interacting groups is hydrophilic.
12. The hydrogel or hydrogel precursor composition of claim 1,
wherein said polymer domain comprises poly(ethylene glycol) and
said hydrophobic interacting groups are perfluorinated
hydrocarbons.
13. The hydrogel or hydrogel precursor composition of claim 2,
wherein said molecule that disrupts an interaction between said
physical chemical protecting group and said hydrophobic interacting
groups is a molecule that binds to said physical chemical
protecting group better than said hydrophobic interacting groups
binds to said physical chemical protecting group.
14. A method for forming a hydrogel in contact with a tissue, said
method comprising the steps of: (a) providing a solution, said
solution comprising a polymer, said polymer comprising a water
soluble polymer domain having at least two hydrophobic interacting
groups attached thereto, said polymer capable of assembling into a
hydrogel under physiological conditions, and a physical chemical
protecting group, said physical chemical protecting group
preventing gelation of said polymer; (b) providing a molecule that
disrupts an interaction between said physical chemical protecting
group and said hydrophobic interacting groups; (c) combining said
solution with said molecule that disrupts an interaction between
said physical chemical protecting group and said hydrophobic
interacting groups to form a mixture, wherein prior to, during, or
after said combining, said solution and said molecule that disrupts
an interaction between said physical chemical protecting group and
said hydrophobic interacting groups are contacted with a tissue;
and (d) allowing gelation of the solution of the mixture of step
(c) in contact with said tissue.
15-18. (canceled)
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. Provisional
Application Ser. No. 60/133,164, filed Apr. 26, 1999.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to materials and methods for
inducing in situ transitions of a hydrogel precursor compositions
from an injectable state to a hydrogel.
[0003] A mechanism for gently transitioning a hydrogel precursor
composition from a liquid state to a solid state such that the
transition can be carried out in situ, directly in intimate contact
with sensitive biological materials, is of special interest for
medical purposes. After being delivered in a liquid state, the in
situ formation of a hydrogel at an implantation site has two
potential advantages: the ability to match the morphology of a
material implant to various complex tissue shapes in the body, and
the ability to deliver a large device through a small hole in the
body via minimally invasive surgery (Hubbell, MRS Bulletin,
November issue, 33-35, 1996). In addition, this type of
transitioning system can be used as a carrier for the controlled
release of drugs, for the delivery of living cells in cell
transplantation, as a barrier for the prevention of postoperative
adhesions, or as a structural support at tissue sites.
SUMMARY OF THE INVENTION
[0004] We have developed methods and materials for the transition
of a hydrogel precursor composition to a hydrogel. These methods
and materials are sufficiently gentle that the transition can be
carried out in situ, for example in direct contact with a tissue.
The methods of the invention can be performed without the use of
any complex instrumentation or high temperatures that might
otherwise be harmful to the tissue at the site where the gel forms.
The hydrogels that result from these methods possess high
mechanical strength, and degradation rates that are of therapeutic
use. In addition, these hydrogel precursors can be constructed to
form in a manner that is selective for the intended target site,
i.e., the transition to the precursor composition a hydrogel state
can be controlled so that undesired chemical reactions with
surrounding tissues do not occur.
[0005] In a first aspect, the invention features a hydrogel
precursor composition comprising a polymer, wherein the polymer
comprises a water soluble polymer domain with at least two
hydrophobic interacting groups attached to it, and wherein the
polymer is capable of assembling into a hydrogel under
physiological conditions. The hydrogel precursor composition also
comprises a physical chemical protecting group that prevents
gelation of the hydrogel precursor composition until desirable.
[0006] In a second aspect, the invention features a hydrogel or
hydrogel precursor composition comprising a polymer, wherein the
polymer comprises a water soluble polymer domain with at least two
hydrophobic interacting groups attached to it, and wherein the
polymer is capable of assembling into a hydrogel under
physiological conditions. The hydrogel or hydrogel precursor
composition also comprises a physical chemical protecting group
that prevents gelation of the hydrogel precursor composition or
hydrogel. The hydrogel or hydrogel precursor composition further
comprises a molecule that disrupts an interaction between the
physical chemical protecting group and the hydrophobic interacting
groups.
[0007] In one embodiment of the above two aspects of the invention,
the polymer domain comprises poly(ethylene glycol) (PEG),
poly(vinyl alcohol), poly(vinyl pyrrolidone), poly(ethyl
oxazoline), poly(acrylic acid), poly(acrylamide), poly(styrene
sulfonate), poly(amino acids), polysaccharides, or copolymers
thereof. Preferably the polymer domain comprises poly(ethylene
glycol). In another embodiment, the hydrophobic interacting groups
are hydrocarbons, preferably perfluorinated hydrocarbons. In yet
another embodiment, the physical chemical protecting group is
cyclodextrin, preferably .beta.-cyclodextrin.
[0008] In other embodiments, the physical chemical protecting group
is a molecule that covalently binds to the hydrophobic interacting
group. Preferably the molecule is hydrophilic. The polymer of the
first or second aspects may be linear or branched, and may comprise
a multi-arm poly(ethylene glycol). The hydrophobic interacting
groups may be positioned at the termini of the polymer domain, or
within the polymer domain. The linkage between the polymer domain
and the hydrophobic interacting groups may be stable or degradable.
Preferably the degradable linkage is an anhydride linkage, an ester
linkage, a carbonate linkage, an amide linkage, or an oligomeric
linkage. In a preferred embodiment, the oligomeric linkage
comprises oligomers of lactic acid, glycolic acid, or
epsilon-caproic acid, or oligomers of trimethylene carbonate, or
co-oligomers thereof.
[0009] In other embodiments, the hydrophobic interacting groups
interact with the physical chemical protecting group through a
noncovalent bond. Preferably the interaction occurs by the
formation of an inclusion complex.
[0010] In still other embodiments, the molecule that disrupts an
interaction between the physical chemical protecting group and the
hydrophobic interacting groups is a molecule that binds to the
physical chemical protecting group better than the hydrophobic
interacting groups bind to the physical chemical protecting group.
Preferably the molecule that disrupts an interaction between the
physical chemical protecting group and the hydrophobic interacting
groups is a one-end modified polymer domain. Most preferably the
one-end modified polymer domain comprises poly(ethylene glycol),
and is modified with a perfluorinated hydrocarbon.
[0011] In still other embodiments, the molecule that disrupts an
interaction between the physical chemical protecting group and the
hydrophobic interacting groups is a molecule that degrades the
linkage between the physical chemical protecting group and the
hydrophobic interacting groups, or is a molecule that degrades the
physical chemical protecting groups themselves. Most preferably a
molecule that degrades the physical chemical protecting group is
.alpha.-amylase or amyloglucosidase.
[0012] In one embodiment of the second aspect of the invention, the
polymer domain comprises poly(ethylene glycol), the hydrophobic
interacting groups are perfluorinated hydrocarbons, and the
chemical protecting group is P-cyclodextrin.
[0013] In a third aspect, the invention features a method for
forming a hydrogel in contact with a tissue, involving providing a
solution comprising a polymer, wherein the polymer comprises a
water soluble polymer domain with at least two hydrophobic
interacting groups attached to it, and wherein the polymer is
capable of assembling into a hydrogel under physiological
conditions, and a physical chemical protecting group that prevents
gelation of the polymer. In addition, the method involves providing
a molecule that disrupts an interaction between the physical
chemical protecting group and the hydrophobic interacting groups.
The solution is combined with the molecule that disrupts an
interaction between the physical chemical protecting group and the
hydrophobic interacting groups, and prior to, during, or after this
combining, the solution and the molecule that disrupts an
interaction between the physical chemical protecting group and the
hydrophobic interacting groups are contacted with a tissue.
Finally, the solution is allowed to gel in contact with the
tissue.
[0014] In a fourth aspect, the invention features a method for
forming a hydrogel in contact with a tissue. The method involves
providing a solution comprising a polymer, wherein the polymer
comprises a water soluble polymer domain with at least two
hydrophobic interacting groups attached to it, and wherein the
polymer is capable of assembling into a hydrogel under
physiological conditions, and a water soluble organic solvent that
prevents gelation of the polymer. The method further involves
removing all or part of the organic solvent from the solution, and
prior to, during, or after this removal, the solution and organic
solvent are contacted with a tissue. Finally, the mixture is
allowed to gel in contact with the tissue.
[0015] In a fifth aspect, the invention features a method for
forming a hydrogel in contact with a tissue. This method involves
providing a solution comprising a polymer, wherein the polymer
comprises a water soluble polymer domain with at least two
hydrophobic interacting groups attached to it, and wherein the
polymer is capable of assembling into a hydrogel under
physiological conditions, and a water soluble organic solvent that
prevents gelation of the polymer. The method also involves
contacting the solution with a tissue, and allowing gelation of the
mixture in contact with the tissue.
[0016] In a sixth aspect, the invention features a method for
incorporating a sensitive biological material into a hydrogel
composition, involving providing a solution comprising a polymer,
wherein the polymer comprises a water soluble polymer domain with
at least two hydrophobic interacting groups attached to it, and
wherein the polymer is capable of assembling into a hydrogel under
physiological conditions, and a physical chemical protecting group
that prevents gelation of the polymer. The method further involves
providing a molecule that disrupts an interaction between the
physical chemical protecting group and the hydrophobic interacting
groups, and providing a sensitive biological material. The
sensitive biological material is combined with either the solution
or with the molecule that disrupts an interaction between the
physical chemical protecting group and the hydrophobic interacting
groups. The solution with the molecule that disrupts an interaction
between the physical chemical protecting group and the hydrophobic
interacting groups and the sensitive biological material are then
combined to form a mixture, and allowed to gel.
[0017] In a seventh aspect, the invention features a method for
incorporating a sensitive biological material into a hydrogel
composition. The method involves, providing a solution comprising a
polymer, wherein the polymer comprises a water soluble polymer
domain with at least two hydrophobic interacting groups attached to
it, and wherein the polymer is capable of assembling into a
hydrogel under physiological conditions, and an organic solvent
that prevents gelation of the polymer. The method also involves
providing a sensitive biological material. The sensitive biological
material is combined with the solution to form a mixture, and prior
to, during, or after, the combining, all or part of the organic
solvent is removed from the solution. Finally, the solution is
allowed to gel.
[0018] In an eighth aspect, the invention features a method for
incorporating a sensitive biological material into a hydrogel
composition, involving providing a solution comprising a polymer,
wherein the polymer comprises a water soluble polymer domain with
at least two hydrophobic interacting groups attached to it, and
wherein the polymer is capable of assembling into a hydrogel under
physiological conditions, and an organic solvent that prevents
gelation of the polymer, and providing a sensitive biological
material. The sensitive biological material is combined with the
solution to form a mixture, and prior to, during, or after the
combining, the solution and/or said sensitive biological material
is contacted with a tissue. Gelation is then allowed to occur.
[0019] In one embodiment of the sixth or seventh or aspect of the
invention, prior to gelation, the mixture is contacted with a
tissue. Preferably prior to, during, or after formation of the
mixture, one or more components of the mixture is contacted with a
tissue.
[0020] In one embodiment of any of the third through eighth aspects
of the invention, the polymer domain comprises poly(ethylene
glycol) (PEG), poly(vinyl alcohol), poly(vinyl pyrrolidone),
poly(ethyl oxazoline), poly(acrylic acid), poly(acrylamide),
poly(styrene sulfonate), poly(amino acids), polysaccharides, or
copolymers thereof. Preferably the polymer domain comprises
poly(ethylene glycol). In another embodiment, the hydrophobic
interacting groups are hydrocarbons, preferably perfluorinated
hydrocarbons. Preferably the polymer domain comprises poly(ethylene
glycol), the hydrophobic interacting groups are perfluorinated
hydrocarbons.
[0021] In other embodiments of any of the third through eighth
aspects of the invention, the polymer is linear or branched. The
branched polymer may comprise a multi-arm poly(ethylene glycol).
The hydrophobic interacting groups may be positioned at the termini
of the polymer domain, or within the polymer domain. The linkage
between the polymer domain and the hydrophobic interacting groups
may be stable or degradable. Preferably the degradable linkage is
an anhydride linkage, an ester linkage, a carbonate linkage, an
amide linkage, or an oligomeric linkage. In a preferred embodiment,
the oligomeric linkage comprises oligomers of lactic acid, glycolic
acid, or epsilon-caproic acid, or oligomers of trimethylene
carbonate, or co-oligomers thereof.
[0022] In preferred embodiments of the third or sixth aspect of the
invention, the physical chemical protecting group is a molecule
that covalently binds to the hydrophobic interacting group.
Preferably the molecule is hydrophilic. In other embodiments, the
hydrophobic interacting groups interact with the physical chemical
protecting group through a noncovalent bond. Preferably the
interaction occurs by the formation of an inclusion complex.
[0023] In still other embodiments of the third or sixth aspects of
the invention, the molecule that disrupts an interaction between
the physical chemical protecting group and the hydrophobic
interacting groups is a molecule that binds to the physical
chemical protecting group better than the hydrophobic interacting
groups bind to the physical chemical protecting group. Preferably
the molecule that disrupts an interaction between the physical
chemical protecting group and the hydrophobic interacting groups is
a one-end modified polymer domain. Most preferably the one-end
modified polymer domain comprises poly(ethylene glycol), and is
modified with a perfluorinated hydrocarbon.
[0024] In still other embodiments of the third or sixth aspect of
the invention, the molecule that disrupts an interaction between
the physical chemical protecting group and the hydrophobic
interacting groups is a molecule that degrades the linkage between
the physical chemical protecting group and the hydrophobic
interacting groups, or is a molecule that degrades the physical
chemical interacting groups themselves. Most preferably the
molecule that degrades the physical chemical interacting groups is
.alpha.-amylase or amyloglucosidase.
[0025] In still another preferred embodiment of the third or sixth
aspect of the invention, the polymer domain comprises poly(ethylene
glycol), the hydrophobic interacting groups are perfluorinated
hydrocarbons, and the chemical protecting group is
.beta.-cyclodextrin.
[0026] In preferred embodiments of the fourth or seventh aspect of
the invention, the organic solvent is removed is by evaporating or
diffusing all or part of it.
[0027] In a preferred embodiment of the fourth, fifth, seventh, or
eighth aspect of the invention, the organic solvent is
N-methylpyrrolidone.
[0028] By a "hydrophobic interacting group" is a group attached to
the water soluble domain of a polymer, that would otherwise not be
soluble under physiological conditions were it not attached to the
water soluble domain of a polymer.
[0029] By a "physical chemical protecting group" is meant a group
or a molecule that interacts with a hydrophobic interacting group
in a manner such that the hydrophobic interacting groups are
prevented from interacting with each other to an extent such that
gelation occurs.
[0030] By "gelation" is meant the formation of a material into a
gelled state. A material is considered to be in a gelled state when
its viscosity is at least 10-fold less than its viscosity when in
the presence of a physical chemical interacting group or an organic
solvent that prevents the hydrophobic interacting molecules of the
material from interacting to an extent such that the material is
not in a liquid state.
[0031] By a "two-end modified polymer domain" is meant a polymer
domain that is modified on each end to contain hydrophobic
interacting groups. Preferably the polymer domain comprises
PEG.
[0032] By a "one-end modified polymer domain" is meant a polymer
domain that is modified on only one end to contain a hydrophobic
interacting group. Preferably the polymer domain comprises PEG.
[0033] By "disrupts" is meant prevents the interaction of two
molecules, for example, two hydrophobic interacting groups of a
polymer. Preferably the interaction between two hydrophobic
interacting groups is sufficient such that the polymer does not
form a hydrogel.
[0034] As used herein, by "prevents" is meant inhibiting the
interaction of hydrophobic interacting groups of a polymer in a
hydrogel precursor composition, thereby inhibiting gelation of the
composition. Preferably the interaction of the hydrophobic
interacting groups is prevented such that the viscosity of the
composition is at least 10-fold less than its viscosity when in the
presence of a physical chemical protecting group or an organic
solvent that inhibits the interaction of the hydrophobic
interacting molecules of the material, to an extent such that the
composition is not in a liquid state.
[0035] By a "stable linkage" is meant a linkage in a material that
is cleaved, whether by hydrolysis or oxidation, at a rate slower
than the rest of the material is degraded, or otherwise cleared
from a site or the body.
[0036] By a "stable linkage" is meant a linkage in a material that
is cleaved, whether by hydrolysis or oxidation, at a rate that is
faster than the rest of the material is degraded or otherwise
cleared from a site or the body. The degradation of an unstable
linkage determines, at least in part, the overall rate of
degradation of the material or its clearance from a site or the
body.
[0037] By an "inclusion complex" is meant a complex between two
components. As used herein, an inclusion complex is formed between
a hydrophobic interacting group(s) and a physical protecting group,
such that the one component (the hydrophobic interacting group) is
partially or wholly surrounded by the second component (the
physical chemical protecting group).
[0038] By a "sensitive biological material" is meant a material
that has biological activity. A sensitive biological material may
include, for example, peptides, polypeptides, proteins, synthetic
organic molecules, naturally occurring organic molecules, nucleic
acid molecules, carbohydrates, lipids, cells, tissues, tissue or
cell aggregates, and components thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1 is a graph illustrating the storage modulus of gel
phases in equilibrium at 298.degree. K.
[0040] FIG. 2 is graph illustrating the loss modulus of gel phases
in equilibrium at 298.degree. K.
[0041] FIG. 3 is a graph illustrating the viscosity change of 10KC8
in aqueous solution, induced by addition of N-methylpyrrolidone
(NMP) to disrupt the association of 10KC8.
[0042] FIG. 4 is a graph illustrating the re-establishment of the
associated state of 10KC8 by solvent exchange, from NMP to
water.
DETAILED DESCRIPTION
[0043] The present invention features hydrogels formed by the
physical association of polymers in a hydrogel precursor
composition. The hydrogel may comprise any hydrophilic (soluble)
and biocompatible polymer domain, modified with any hydrophobic
interacting groups at two or more sites along the chain (e.g., at
the ends or in the domain of the polymer). These hydrophobic
interacting groups bind strongly to each other in an interchain
manner to form a gel matrix in situ.
[0044] An injectable state of the polymer matrix is produced either
by the addition of molecules termed "physical chemical protecting
groups" that act to disrupt association among the hydrophobic
interacting groups of the polymer matrix, or by changing the
solvent state to disrupt association among the hydrophobic
interacting groups of the polymer matrix.
[0045] The injectable state of the hydrogel precursor composition
can be switched to a solid hydrogel state by removal of the
physical chemical protecting groups after or during delivery to the
desired site so that association among the hydrophobic interacting
groups is re-established. The physical chemical protecting groups
may be removed by their degradation, using, for example, an enzyme,
or by addition of a competitor that binds the physical chemical
protecting groups, transferring them away from the association
sites of the polymer matrix. A PEG molecule with one end modified
with a hydrophobic interacting group is one example of a competitor
that may be used. The physical chemical protecting groups may also
be removed by disrupting the bonds formed between the hydrophobic
interacting groups and the physical chemical protecting groups.
[0046] The injectable state of the hydrogel precursor composition
can also be switched to a solid hydrogel state by changing the
solvent conditions to replace a solvent that does not permit
association of the hydrophobic interacting groups with a solution
that does permit such association. For example, an organic solvent,
such as N-methylpyrrolidone (NMP) does not permit association of
the hydrophobic interacting groups, but replacing the solvent with
an aqueous solution, for example that of a tissue or other body
fluid, or evaporating the organic solvent off does permit
association.
[0047] This novel approach to making polymeric compositions that
transition from a liquid state to a solid state is advantageous for
the following reasons. It is safe and economical, because it does
not involve chemical reactions or the transfer of heat, and it does
not require the use of complex instruments and surgical devices
that supply both fluids and light to a site. In addition, the
hydrogel precursor composition may be applied to a site, for
example, a tissue, and formed to the morphology of the site.
Another advantage of this method is that a large amount of material
may be delivered to a site using minimally invasive surgery,
because the material is in a liquid, injectable state.
[0048] Polymer Domains
[0049] Any polymer domain that is substantially water-soluble may
be used in the present invention. Examples of such polymer domains
include, but are not limited to, poly(ethylene glycol), poly(vinyl
alcohol), poly(vinyl pyrrolidone), poly(ethyl oxazoline),
poly(acrylic acid), poly(acrylamide), poly(styrene sulfonate),
poly(amino acids), polysaccharides, and copolymers thereof. Each of
these polymers presents numerous opportunities for attachment of
the hydrophobic interacting groups. For example, initiation and
termination of polymerization can be performed so as to obtain good
control over the identity of polymer end groups, allowing the
hydrophobic interacting groups to be attached thereto.
[0050] Alternatively, the hydrophobic interacting groups can be
attached as side groups on the polymer domain, either directly by
coupling to the side group on the polymer domain (e.g., coupling to
the carboxylic acid side groups on poly(acrylic acid)) or
indirectly, by coupling to side groups incorporated into the
polymer domain by copolymerization (e.g., coupling to carboxylic
acid side groups on poly(acrylamide-co-acrylic acid)). For use in
the present invention, PEG homopolymers that are approximately
4,000 to 10,000 g/mol are particularly useful.
[0051] Hydrophobic Interacting Groups
[0052] Perfluorinated hydrocarbons are the hydrophobic interacting
groups that provide for the desired gelation transitions.
Preferably the perfluorinated hydrocarbons have the formula
C.sub.nF.sub.2n+1CH.sub.2CH.- sub.2OH, where n=6 to 10. Other
hydrocarbon groups can also provide these desired gelation
interactions, and may be used, although they interact with less
affinity than the corresponding perfluorinated hydrocarbon
groups.
[0053] Connecting Schemes Between the Polymer and the Hydrophobic
Interacting Groups
[0054] The linkage between the hydrophobic interacting groups and
the polymer domain may be selected to be relatively stable or
readily degradable. For example, the hydrophobic interacting groups
can be attached via anhydride, ester, carbonate, or amide linkages,
to make them susceptible to hydrolysis. Oligomeric linkages (e.g.
oligomers of lactic, glycolic, or epsilon-caproic acid or oligomers
of trimethylene carbonate) can also be incorporated between the
polymer chain and the hydrophobic interacting groups. This allows
for the regulation of degradation by a process that is
hydrolytically controlled. Also, the design and incorporation of
such degradable linkages will lead to more predictable toxicology
and pathways for elimination of the polymer from the body.
[0055] Polymer Conformations
[0056] The polymers of the present invention may be linear or
branched. A branched conformation may lead to more effective gel
formation due to the existence of multiple points for interaction.
Thus, multi-arm PEGs (e.g., those PEGs having more than 2 arms) are
effective polymer domains. Even more complex branching can be
included in the polymer conformations of the invention.
[0057] The polymers of the present invention may possess terminal
hydrophobic interacting groups or the hydrophobic interacting
groups may be incorporated along the polymer domain, either by
copolymerization or by copolymerization of a site for secondary
grafting of the hydrophobic interacting group. Incorporation of
hydrophobic interacting groups along the polymer domain provides
for a greater density of hydrophobic interacting groups.
[0058] Physical Chemical Protecting Groups
[0059] The physical chemical protecting groups may interact with
the hydrophobic interacting groups in various ways. For example,
the physical chemical protecting groups and the hydrophobic
interacting groups may exist as an inclusion complex. Examples of
physical chemical protecting groups include, but are not limited
to, cyclodextrins, for example, .alpha.-, .beta.-, or
.gamma.-cyclodextrin. The physical chemical protecting group may be
removed by an enzyme, for example, a cyclodextrinase, thus exposing
the hydrophobic interacting groups.
[0060] Alternatively, a hydrophilic bulky group (the physical
chemical protecting group) can be attached beside or on the
terminus of the hydrophobic interacting group, with a
hydrolytically sensitive linkage. Rapid hydrolysis then triggers a
transition from the sol (soluble state) to the gel state. Such a
hydrophilic group may be a PEG chain, for example, and the linkage
may be a hydrolytically sensitive ester anhydride, amide,
carbonate, or oligomeric linkage. This linkage may also be an
enzymatically cleavable site, which results in degradation (and
thus gelation) after addition of the appropriate enzyme.
[0061] Solvents That Prevent Interactions Between Hydrophobic
Interacting Groups
[0062] Organic solvents may be used to prevent hydrophobic
interacting groups from associating, thus prevent gel formation.
The injectable state of the hydrogel precursor composition can be
switched to a solid hydrogel state by changing the solvent
conditions to replace a solvent that does not permit association of
the hydrophobic interacting groups with a solution that does permit
such association. For example, an organic solvent, such as
N-methylpyrrolidone (NMP) does not permit association of the
hydrophobic interacting groups. But replacing the organic solvent
with an aqueous solution, including that of a tissue or other body
fluid, or evaporating the organic solvent off does permit
association of the hydrophobic interacting groups. When the solvent
exchange is done in vivo, the preferred solvent is NMP (because of
the low toxicity). When in vitro solvent exchange is conducted, a
number of organic solvents may be used, including, for example,
ethyl acetate. Such the solvents may be removed prior to
introduction of the hydrogel to an in vivo site.
[0063] Alternatively, the organic solvent may be removed by
evaporation, thus allowing the precursor hydrogel solution to form
a hydrogel. For example, the organic solvent may be evaporated from
a solution of polymer and NMP or methylene chloride, resulting in
formation of a polymer matrix. Then the polymer matrix may be
rehydrated in water, either in vitro or in vivo.
[0064] Hydrogels in Contact with Tissues
[0065] The hydrogels of the present invention may be formed in
contact with a tissue. Preferably the tissue is within a tumor,
subcutaneous, intramuscular, adjacent to a tooth, upon the inner or
outer surface of an artery or vascular graft, or upon any tissue
surface when used to prevent postoperative adhesions.
[0066] Incorporation of a Sensitive Biological Material
[0067] A sensitive biological material may be incorporated into a
hydrogel through the practice of this invention. Examples of
sensitive biological materials include, but are not limited to
drugs, proteins, peptides, RNA, DNA, inorganic and organic
molecules, carbohydrates, lipids, cells, tissues, tissue or cell
aggregates, and combinations thereof.
[0068] Specific examples of cells that may be incorporated into the
hydrogel include, but are not limited to, chondrocytes, endothelial
cells, muscle cells, fibroblasts, skin cells, islets of Langerhans,
and genetically modified cells for protein delivery.
[0069] Specific examples of sensitive biological materials that may
be incorporated into the hydrogels include, enzymes, antibiotics,
antineoplastic agents, local anesthetics, hormones, antiangiogenic
agents, antibodies, neurotransmitters, psychoactive drugs, drugs
affecting reproductive organs, oligonucleotides, including
antisense oligonucleotides, vasoactive agents, anticoagulants,
immunomodulators, cytotoxic agents, antiviral agents, and
combinations thereof.
[0070] Exemplary sensitive biologicals materials which may be
incorporated into the hydrogels of the present invention include
growth hormone, for example, human growth hormone, calcitonin,
granulocyte macrophage colony stimulating factor (GMCSF), ciliary
neurotrophic factor, and parathyroid hormone. Other specific
therapeutic agents include parathyroid hormone-related polypeptide,
somatostatin, testosterone, progesterone, estradiol, nicotine,
fentanyl, norethisterone, clonidine, scopolomine, salicylate,
salmeterol, formeterol, albeterol, and valium.
[0071] Drugs for the treatment of pneumonia may be used, including
pentamidine isothionate. Drugs for the treatment of pulmonary
conditions, such as asthma may be used, including albuterol
sulfate, .beta.-agonists, metaproterenol sulfate, beclomethasone
dipropionate, triamcinolone acetamide, budesonide acetonide,
ipratropium bromide, flunisolide, cromolyn sodium, ergotamine
tartrate, and protein or polypeptide drugs such as TNF antagonists
or interleukin antagonists.
[0072] Other therapeutic agents include cancer chemotherapeutic
agents, such as cytokines, chemokines, lymphokines, and
substantially purified nucleic acids, and vaccines, such as
attenuated influenza virus. Substantially purified nucleic acids
that can be incorporated include genomic nucleic acid sequences,
cDNAs encoding proteins, expression vectors, antisense molecules
that bind to complementary nucleic acid sequences to inhibit
transcription or translation, and ribozymes. For example, genes for
the treatment of diseases such as cystic fibrosis, for example,
cystic fibrosis transmembrane regulator can be administered.
Polysaccharides, such as heparin, can also be administered.
[0073] Further therapeutic agents include tissue plasminogen
activator (t-PA), superoxide dismutase, catalase luteinizing
hormone releasing hormone (LHRH) antagonists, IL-11 platelet
factor, IL-4 receptor, enbrel, IL-1 receptor antagonists, TNF
receptor fusion proteins, megakaryocyte growth and development
factor (MGDF), stemgen, anti-HER-2 and anti-VEGF humanized
monoclonal antibody, anti-Tac antibody, GLP-1 amylin, and GLP-1
amylin analogues.
[0074] Additional therapeutic agents include atrial natriuretic
factor, atrial natriuretic peptide, beta-human chorionic
gonadotropin, basic fibroblast growth factor, bovine growth
hormone, bone morphogenetic protein, B cell stimulating factor-1, B
cell stimulating factor-2, bovine somatotropin, carcinobreaking
factor, cartilage induction factor, corticotropin releasing factor,
colony stimulating factor, differentiating factor-1, endothelial
cell growth factor, erythroid differentiation factor, elongation
factor 1-alpha, epidermal growth factor, erythropoietin,
thrombopoietin, thymopoietin, fibroblast growth factor, follicle
stimulating hormone, granulocyte colony stimulating factor, glial
fibrillary acidic protein, growth hormone releasing factor, human
alpha-1 antitrypsin, human atrial natriuretic factor, human
chorionic gonadotropin, human leukemia inhibitory factor,
hemopoietin-1, hepatocyte growth factor, human transforming growth
factor, human thyroid-stimulating hormone, interferon,
immunoglobulin A, immunoglobulin D, immunoglobulin E, insulin-like
growth factor-1, insulin-like growth factor-II, immunoglobulin G,
immunoglobulin M, interleukin-1, interleukin-2, interleukin-3,
interleukin-4, interleukin-5, interleukin-6, kidney plasminogen
activator, lectin cell adhesion molecule, luteinizing hormone,
leukemia inhibitor factor, monoclonal antibody, macrophage
activating factor, macrophage cytotoxic factor, macrophage colony
stimulating factor, megakaryocyte colony stimulating factor, tumor
necrosis factor, macrophage inhibitory factor, Mullerian inhibiting
substance, megakaryocyte stimulating factor, melanocyte stimulating
factor, neutrophil chemotactic factor, nerve growth factor, novel
plasminogen activator, nonsteroidal anti-inflammatory drug,
osteogenic factor extract, antitumor lymphokine, prostate-specific
antigen, anti-platelet activating factor, plasminogen activator
inhibitor, platelet-derived growth factor, platelet-derived wound
healing formula, plasmatic human interleukin inducing protein,
tumor angiogenesis factor, tissue control factor, T cell growth
factor, T cell modulatory peptide, transforming growth factor,
tumor growth inhibitor, tumor inhibiting factor, tissue inhibitor
of metalloproteinases, tumor necrosis factor, tissue plasminogen
activator, thyroid stimulating hormone, urokinase-plasminogen
activator, vascular endothelial growth factor, and vasoactive
intestinal peptide.
[0075] Drugs may be dissolved or suspended as precipitates within
the polymer form in its dissociated state. This dissociated state
can be converted into the associated hydrogel state by any of the
methods described above, e.g., by solvent exchange, by drying, by
degradation of a protecting group, or by competitive displacement
of a protecting group.
[0076] As a specific example, the associating polymers are
dissolved in dichloromethane at about 40% by weight and a protein
drug is added as a suspension. The solution is dried by evaporation
to form a film or particles. The dry polymer-protein depot is then
re-hydrated by addition of a limited amount of buffered saline
(e.g., an amount necessary to bring the material to its equilibrium
swelling state). The material is injected, for example, as a
particulate, or placed in a tissue site to release its drug.
[0077] As a second specific example, the associating polymers are
dissolved in NMP at about 50% by weight and the protein is added as
a suspension. The polymer-protein-NMP mixture is injected into a
tissue site, whereupon diffusion of the NMP from the system and
counter-diffusion of water into the system results in a swollen gel
depot. Alternatively, the NMP is exchanged against water away from
a tissue site, to produce a swollen material that is then injected
as a particulate, or placed in a tissue site.
[0078] In both of the above examples, the protein is released by
diffusion from the depot, with some contribution to the release
process also being given by dissolution of the material from the
surface of the depot.
EXAMPLE 1
Synthesis of End-Group Modified PEGs
[0079] Poly (ethylene glycol) (PEG) of nominal molecular weight
6000 g/mol (6K) (from Fluka), 10K (from Aldrich), and 20K (from
Fluka) were used. Three different fluorinated alcohols
(C.sub.nF.sub.2n+1CH.sub.2CH.sub.2OH- , where n=6, 8, 10) were
purchased from Lancaster Synthesis Inc. Isophorone diisocyanate
(IPDI), dibutyltin diacetate and anhydrous tetrahydrofuran (THF)
were purchased from Aldrich.
[0080] The method of Glass et al. (Kaczmarski and Glass,
Macromolecules, 26:5149-5156, 1993) was used to attach the
perfluorinated end groups to the terminal hydroxyls of PEG. PEG was
dried by azeotropic distillation in toluene, and was reacted with
100 fold molar excess (with respect to end-groups) of
vacuum-distilled IPDI in anhydrous THF for 48 hours. This
intermediate was precipitated in anhydrous ethyl ether to remove
unreacted IPDI, and was subsequently reacted with a 10-fold excess
of perfluoroalcohol in anhydrous THF for 48 hours. Dibutyltin
diacetate was added for the second step. The reaction mixture was
precipitated in anhydrous ethyl ether, then dissolved in THF, and
reprecipitated to form the final two-end modified PEG molecules
that contain hydrophobic interacting groups. All reactions were
done under argon purge.
[0081] One-end modified PEG molecules can be generated using a
monomethoxy PEG, and keeping the molar ratios in the reaction the
same as those described above.
[0082] The degree of substitution was determined by .sup.19F NMR
using CF.sub.3COOH or CF.sub.3SO.sub.3Na as an internal standard
with a 5 second delay time (i.e., long enough to get the integral
value independent of the delay time between pulses). The samples
prepared for this study are described in Table 1, where nKCm is the
sample, in which nK denotes the PEG molecular weight and Cm denotes
the length of the C.sub.mF.sub.2m+1CH.sub.2CH.sub.2OH group.
[0083] For a given PEG, each sample modified with C.sub.10F.sub.21
was checked by reverse phase HPLC. A C18 column was used with the
Water HPLC system with a gradient input of mixed solvent (ranging
from 20:80 of acetonitrile:ethanol to 100% acetonitrile) that can
separate unmodified, one-end modified, and two-end modified
samples. Good agreement between the values obtained by HPLC (in
parenthesis in the final column of Table 1) and the values obtained
by .sup.19F NMR support the reliability of the NMR method.
1TABLE 1 Reaction extent of two-end modified PEGs Degree of
substitution Sample PEG-block End-group (%) 6KC10 6 kg/mol
--(CH.sub.2).sub.2--C.sub.10F.sub.21 97 (97) 6kC8 6 kg/mol
--(CH.sub.2).sub.2--C.sub.8F.sub.17 89 6KC6 6 kg/mol
--(CH.sub.2).sub.2--C.sub.6F.sub.13 99 10KC10 10 kg/mol
--(CH.sub.2).sub.2--C.sub.10F.sub.21 94 (96) 10KC8 10 kg/mol
--(CH.sub.2).sub.2--C.sub.8F.sub.17 94 20KC10 20 kg/mol
--(CH.sub.2).sub.2--C.sub.10F.sub.21 97 (92) 20KC8 20 kg/mol
--(CH.sub.2).sub.2--C.sub.8F.sub.17 96
EXAMPLE 2
Formation of Hydrogel Phases
[0084] The phase behavior of two-end modified PEGs was governed by
the relative length of the PEG chain and the perfluorinated
hydrocarbon end groups (Table 2). Some of the two-end modified PEGs
showed phase separation and others did not. This phase separation
phenomenon can provide useful applications for these transition
systems; when this system is used as a delivery device in the open
system, it will maintain the high modulus matrix of the equilibrium
composition, compared to the systems using materials which do not
show the phase separation, since the matrix formed from these would
be dissolved with continuous lowering of the modulus for the same
concentration of polymers.
[0085] Among the two-end modified PEGs synthesized, 6KC10 did not
exist as a homogeneous phase in water, rather only as a slightly
swollen precipitate. At the other extreme, 20KC10 and 20KC8 existed
as homogeneous solutions over the whole range of polymer
concentrations, though the viscosity increased drastically as the
concentration of polymer increased. Polymers that were in between
6KC10 and 20KC8 in terms of the relative length of PEG to the
hydrophobic end groups, for example, 6KC6, 6KC8, 10KC8, and 10KC10,
showed phase separation into a gel coexisting with a sol (soluble
liquid phase) in water and in phosphate buffered saline (PBS)
solution. Increasing the temperature did not lead to any noticeable
change of the gel phase concentration for the phase separating
systems; the phase boundary was almost temperature-invariant. But,
some increase of the gel phase concentration was observed for 10KC8
above 60.degree. C.
[0086] The gel properties, such as modulus and transport properties
(diffusion coefficient and viscosity) were sensitive to the degree
of swelling of the gel (the inverse of the gel phase
concentration). Increasing the length of the PEG chain increased
the swelling ratio of the gel, since it is analogous to reducing
the crosslink density. When the PEG length was fixed and the length
of the hydrophobic interacting group was varied, the swelling ratio
was nearly constant (compare 10KC10 to 10KC8; and compare 6KC8 to
6KC6). The swelling ratio increased, however, with PEG length
(compare 6KC8 to 10KC8).
[0087] The low concentration of the dilute phase (sol phase) means
that a small driving force for the degradation of these gels would
be present when they are exposed to an open system (e.g., as an
implant) in the case of diffusion in the dilute phase to be
rate-determining step, compared to the systems which do not show
phase separation for the same concentration of polymers.
[0088] The behavior of the polymers in deionized water versus
phosphate buffered saline (PBS) showed that the gel concentration
was slightly higher in PBS than in water, and the sol concentration
was consistently lower in PBS than in water. This difference was
due to the decrease of solvation of PEG chains from a salting out
effect and the increase of aggregation tendency of fluorocarbon end
groups by the added salts in the PBS solution (Zhang et al,
Abstract of the American Chemical Society Meeting, 213, 236, 1997;
and Bailey et al., J. Appl. Polym. Sci. 1:56-62, 1959). The effect
of dissolved electrolytes will be present in vivo, with the
beneficial effects of increasing the modulus and reducing the rate
of dissolution.
2TABLE 2 Phase behaviors and compositions of phases of modified
PEGs equilibrium equilibrium compositions compositions in water (wt
%) in PBS (wt %) sample type of phase gel conc., sol conc., gel
conc., sol conc., in PBS (wt %) behavior C.sub.gel, eq C.sub.sol,
eq C.sub.gel, eq C.sub.sol, eq 20KC8 1 phase N/A N/A N/A N/A 20K10
1 phase N/A N/A N/A N/A 10KC8 2 phase 6.5 .+-. 0.2 0.075 .+-. 0.005
7.8 .+-. 0.2 0.055 .+-. 0.002 10KC10 2 phase 6.8 .+-. 0.7 0.019
.+-. 0.008 8.1 .+-. 0.7 0.011 .+-. 0.003 6KC6 2 phase 9.5 .+-. 0.5
0.066 .+-. 0.007 10.5 .+-. 0.6 0.038 .+-. 0.002 6KC8 2 phase 11.0
.+-. 0.3 0.042 .+-. 0.007 12.5 .+-. 0.3 0.017 .+-. 0.001 6KC10
insoluble N/A N/A N/A N/A
EXAMPLE 3
Rheological Properties of Gel Phases
[0089] Rheological measurements were made to gain initial insight
into gel structure (Table 3). Previous work by Annable, et al., (J.
Rheology 37:695-726, 1993) showed that the PEG systems modified
with hydrocarbon tails were governed by a single relaxation time.
Thus, these systems can be well described by a simple Maxwell
model.
[0090] The gel phases of all the systems showing phase separations
were still governed by the single relaxation behavior. A similar
order of magnitude of infinite modulus (G.sub..infin., 10.sup.4 Pa)
was observed for 10KC10 and 10KC8, indicating the similar density
of physical junctions within these two gels, which coincide with
similar values of swelling ratios. A higher value was observed for
6KC8, meaning a higher density of physical junctions was present,
which also agrees with the smaller swelling ratio. The large
difference in relaxation time between 10KC10 and 10KC8 showed that
the addition of one CF.sub.2 unit significantly increases the
strength of physical junctions, resulting in a longer relaxation
time.
3TABLE 3 Relaxation times of gel phases in equilibrium at 298K
10KC10 10KC8 6KC8 Relaxation time (sec) 1.14 0.028 0.021
EXAMPLE 4
Disruption of a Gel by .beta.-CD (Induction to the Injectable
State)
[0091] Cyclodextrins (CDs) are cyclic starches consisting of 6, 7,
or 8 .alpha.-1,4-linked glucose monomers called .alpha., .beta.,
and .gamma.-cyclodextrin, respectively. These molecules are ring or
torus-shaped and possess a hydrophobic cavity and a hydrophilic
exterior. The partial hydrophobic nature of CD allows it to
associate with nonpolar organic moieties or molecules to form
inclusion complexes (Shieh et al., Pure Appl. Chem. A33:673-683,
1996).
[0092] Complex formation between .alpha., .beta., and .gamma.-CD
and perfluorocarbon surfactants showed that .beta.-CD has the
largest association constants among the cyclodextrins for a given
hydrophilic head. For long fluorocarbon surfactants
(C.sub.mF.sub.2m+1Na, where m.gtoreq.7), it is even possible for
two .beta.-CD molecules to bind to each surfactant molecule (Guo et
al., Langmuir 8:446-451, 1992). Based on the association between
.beta.-CD and the fluorocarbon surfactants, the addition of
.beta.-CD to solutions of one end-modified PEG (modified with
perfluorinated groups) reduced the viscosity of the solution
(Zhang, et al., Abstract of the American Chemical Society Meeting,
211:166-Poly, 1996).
[0093] If the complexation of .beta.-CD to the fluoro end groups of
two-end modified PEG is sufficient to hide the hydrophobicity of
the end groups, the gel phase will not be formed. Mixing a
saturated aqueous solution of .beta.-CD and the gel phase of 10KC10
caused the disappearance of the gel-phase. In addition, adding a
one-fold molar ratio excess of .beta.-CD to end groups to 10KC10,
applying water, and shaking the solution resulted in low viscosity
solutions. These results indicated that .beta.-CD can effectively
prevent the hydrophobic interacting groups of the polymer from
strongly associating with each. The solution was not clear,
especially for the higher concentration of solutes, and the
apparent viscosity was much higher than the same concentration of
unmodified pure PEG solution, so it appears that there may be weak
or local associations among the .beta.-CD-complexed polymers.
Nevertheless, the addition of cyclodextrin to the polymers is
enough to transition the gel to an injectable state.
EXAMPLE 5
Reformation of a Gel by Enzymatic Degradation of .beta.-CD
[0094] There are several sources of enzymes that can degrade
cyclodextrin. Most of them are from microbial sources, but enzymes
from saliva and the pancreas can also effectively degrade
.gamma.-CD and to a lesser extent .beta.-CD (Saha et al.,
Starch/Starke 44:312-315, 1992). .alpha.-amylase from aspergillus
oryzae can degrade .beta.-CD (Jordal et al, Starch/Starke
36:104-143, 1984), although it is a relatively poor
cyclodextrinase.
[0095] Two enzymes were tested for their ability to degrade
cyclodextrin in the system described herein: .alpha.-amylase from
aspergillus oryzae (crude powder), and amyloglucosidase from
aspergillus niger (solution in 1 M glucose), both purchased from
Sigma. In one study, 0.008 g of .alpha.-amylase (from aspergillus
oryzae) was added to 0.55 g of the homogeneous complex solution of
10KC8 and .beta.-CD (7.73 weight % for 10KC8, and 3.35 weight % for
.beta.-CD). After shaking to mix, the sample was kept at 37.degree.
C. The sample started to become viscous upon mixing, and after 20
minutes, it exhibited a gel-like structure.
[0096] In a second study, 0.065 g of the enzyme solution
amyloglucosidase (from aspergillus niger) was added to 0.513 g of
the precursor solution (7.59 weight % for 10KC8, and 3.27 weight %
for .beta.-CD). After 30 minutes, the sample started to become
viscous, and after 70 minutes, it became insoluble.
EXAMPLE 6
Reformation of a Gel by Transfer of CD to a One-End Modified
PEG
[0097] PEGs modified to contain a hydrophobic interacting group on
only one end will form a micelle-like structure in aqueous
solutions, and in this structure they are injectable even when
present below the critical transition concentration. Furthermore,
the affinity of a one-end modified PEG having a small molecular
weight of a PEG for CD is greater than the affinity of a two-end
modified PEG having a large molecular weight of PEG for CD (Amiel
et al., J. Inclusion Phen. & Mol. Recog., 25:61-67, 1996).
Thus, mixing a CD-complexed, two-end modified PEG solution and an
appropriate amount of a one-end modified PEG solution will result
in the transfer of the majority of .beta.-CD from the two-end
modified PEG to the one-end modified PEG by mass action and the
higher tendency to make inclusion complexes with the one-end PEG.
The removal of .beta.-CD from the two-end modified PEG then reveals
the hydrophobic interacting groups of the two-end modified PEGs,
and, if the concentration of the added one-end modified PEG is not
so high as to break down the physical junction by the surfactant
action of the excess bare one-end modified PEG, the mixture will
form a gel structure again.
[0098] With 10KC10 as a gel-forming agent, 5K-M-C10 and 2K-M-C10
(where M denotes that only one end is modified with a
perfluorinated group) were explored as CD-transfer inducing agents.
First, a 5 weight % solution of 10KC10, coupled with CDs, and a
10.2 weight % solution of 5K-M-C10 solution were mixed together in
equal amounts. The mixture exhibited a marked enhancement of
viscosity, but did not form a gel state (where the gel state was
determined by whether there was a noticeable flow of solution when
the vial containing it was inverted).
[0099] Next, using 2K-M-C10 and 10KC10, the mixing ratios were
varied from 1:1 to 1:3 (10KC10:2K-M-C10, in molar concentration),
keeping the total concentrations of the reactants constant at 6.3
weight %. Among these mixtures, the 1:2 ratio gave the most
gel-like state, which was maintained up to a temperature of
37.degree. C. A 1:2 molar ratio mixture of 10KC10-CD complex
solution (0.073 g/ml, polymer/water) and 18.2 weight % of 2K-M-CIO
solution resulted in reversion to a gel structure. For 10KC8, a 1:1
molar ratio was sufficient to induce the gel phase since CD
transfers more easily from the C8 end group of the two-end modified
PEG to the C10 end group of the one-end modified PEG.
EXAMPLE 7
Dissolution Characteristics of Gel Phases
[0100] Dissolution rates of gel phases are measured by direct
measurements of dissolved amounts of polymers, or by the shift of
the surface plasmon resonance angle of ultrathin gold film coated
with the thin film of the polymer matrix that is exposed to the
flow of water (Aust et al., TIP 2:313-32, 1994). For the transition
system showing phase separations, the compositions of the polymer
matrix are the equilibrium gel concentrations. To compare the phase
separation system with the system with no phase separation, the
dissolution rates of 10 weight % of 20KC10 and 12.8 weight % of
one-end modified 5K-M-C10, which shows a lyotropic gel phase
transition at that concentration, were measured (Table 4).
4TABLE 4 Dissolution rates of polymer matrix 10KC10 10KC8 6KC8
20KC10 5K-M-C10 Conc. (wt %) (6.8 wt %) (6.5 wt %) (11.0 wt %)
(10.0 wt %) (12.8 wt %) Dissolution not 1.67 .times. 10.sup.-3 3.33
.times. 10.sup.-4 0.168 0.201 rate measurable (mg/cm.sup.2/hr)
[0101] As expected, the systems which did not show phase separation
(20KC10 and 5K-M-C 10) exhibited much faster dissolution rates than
those with phase-separation (.about.100 times faster rates of
20KC10 than that of 10KC8). Of the phase-separating systems
examined, 6KC8 showed around 5 times slower dissolution rates than
10KC8, and the rate of 10KC10 dissolution was much slower than
6KC8. The absolute small value of dissolution rates for the phase
separating species confirmed that these species can be used as
delivery carriers in the open system. Also, by choosing the right
ratio of hydrophilic and hydrophobic groups, the degradation rate
of the matrix can be controlled.
[0102] Another feature of polymer matrix degradation is whether the
matrix is degraded homogeneously or heterogeneously. Maintaining
the constant resonance angle for the phase separating species until
the film is thin enough so that the thickness affects the resonance
angle denotes that no change in the refractive index of the polymer
matrix. This means that the hydrogels degrade heterogeneously
(i.e., from the surface inward). Such a heterogeneous degradation
characteristic is beneficial for the application of delivery of a
sensitive biological material, because the drug can exhibit a liner
(i.e., zero order) release profile. Thus using the systems of this
invention, the constant release of a drug is achievable.
EXAMPLE 8
Disruption of a Gel by Addition of Organic Solvents
[0103] Associative interactions of polymers through their
hydrophobic interacting groups may be disrupted by altering the
characteristics of the solvent that the polymers are contained in,
and these interactions may be re-established by altering the
solvent again. Disrupting the associative interactions of the
polymer can be achieved, for example, by dissolving the gel-forming
polymer in a water mixture with a water-soluble organic solvent,
such as NMP, or in the organic solvent neat, and then converting
the non-associative state into the associated state by removal of
the solvent and addition of water. This may be accomplished in a
number of ways. A flowable solution of the polymer in NMP or an
NMP-water mixture (in the non-associated state) may be contacted
with an aqueous environment, permitting the diffusion of the NMP
from the polymer solution and its corresponding replacement by
water, thus converting the material into an associative state.
[0104] Alternatively, the material in the NMP or NMP-water mixture
may be injected into a tissue site, and the NMP allowed to exchange
with the aqueous component of the body fluids, to achieve the same
end. When the exchange is conducted in vivo, the preferred solvent
is NMP, the toxicity of which is very low, although other solvents,
including ethyl acetate, may also be useful. When the exchange is
conducted in vitro, for example, for the encapsulation of drugs, a
wide variety of solvents are available, since the solvent may be
removed before introduction into the body. Alternatively, the
solvent may be removed by evaporation, such as by drying a solution
of the polymer from NMP or methylene chloride, followed by
rehydration in water, either in vitro or in vivo.
[0105] As evidence that the associated state may be disrupted by
the addition of an organic solvent, the viscosity of a 8% solution
of 10KC8 in water was measured. Varying concentrations of NMP were
added to this solution, and a dramatic reduction in the viscosity
of the system was observed, as illustrated in FIG. 3.
[0106] As evidence that the associated state may be restored by the
exchange of an organic solvent, a 50% solution of 10KC8 in NMP was
placed within a reservoir of water, the thickness of the initial
sample was approximately 1.5 mm. For this system, the steady
viscosity of the sample increased dramatically by more than two
orders of magnitude, as shown in FIG. 4.
[0107] All publications mentioned in this specification are herein
incorporated by reference to the same extent as if each independent
publication or patent application was specifically and individually
indicated to be incorporated by reference.
* * * * *