U.S. patent application number 11/070506 was filed with the patent office on 2005-09-01 for preparation and use of photopolymerized microparticles.
Invention is credited to Anseth, Kristi, Lengsfeld, Corinne, Owens, Jennifer L., Randolph, Theodore.
Application Number | 20050192371 11/070506 |
Document ID | / |
Family ID | 34891034 |
Filed Date | 2005-09-01 |
United States Patent
Application |
20050192371 |
Kind Code |
A1 |
Randolph, Theodore ; et
al. |
September 1, 2005 |
Preparation and use of photopolymerized microparticles
Abstract
Methods of forming crosslinked polymer particles in situ from
polymer precursors such as monomers or oligomers, comprising
exposing a composition comprising at least one polymer precursor, a
solvent or solvent mixture, and an antisolvent or antisolvent
mixture to photoradiation under conditions whereby particles are
formed are provided. The polymer precursor may be photosensitive,
or a separate polymerization initiator may be used. In a preferred
embodiment, the polymer precursor is insoluble in the antisolvent
or antisolvent mixture and the solvent or solvent mixture is
soluble in the antisolvent or antisolvent mixture at the
concentrations used. Crosslinked polymer particles and crosslinked
polymer particles comprising a polymer and a bioactive material are
also provided. The polymer may be erodable, and the polymer
particles formed may be used in a variety of applications,
including controlled release of bioactive materials such as drugs.
Polymer particles formed using the methods of the invention have
low residual solvent levels and high additive encapsulation
efficiencies. The processes of the invention allow control of
particle size and morphology, use low operating temperatures and
are useful for efficient bulk production.
Inventors: |
Randolph, Theodore; (Niwot,
CO) ; Anseth, Kristi; (Boulder, CO) ; Owens,
Jennifer L.; (Boulder, CO) ; Lengsfeld, Corinne;
(Denver, CO) |
Correspondence
Address: |
GREENLEE WINNER AND SULLIVAN P C
4875 PEARL EAST CIRCLE
SUITE 200
BOULDER
CO
80301
US
|
Family ID: |
34891034 |
Appl. No.: |
11/070506 |
Filed: |
March 2, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11070506 |
Mar 2, 2005 |
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10161544 |
Jun 3, 2002 |
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6864301 |
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10161544 |
Jun 3, 2002 |
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09451481 |
Nov 30, 1999 |
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6403672 |
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60110816 |
Nov 30, 1998 |
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Current U.S.
Class: |
522/79 |
Current CPC
Class: |
A61K 9/1694 20130101;
C08F 2/46 20130101; Y10T 428/2989 20150115; Y10T 428/2985 20150115;
C08F 2/18 20130101; Y10T 428/2982 20150115; A61K 9/1635 20130101;
C08F 2/06 20130101; Y10T 428/2984 20150115 |
Class at
Publication: |
522/079 |
International
Class: |
C08K 003/00 |
Goverment Interests
[0002] This invention was made with government support by the
National Institutes of Health under grant number 5 R01 HL59400 and
the National Science Foundation. The federal government may have
certain rights in this invention.
Claims
We claim:
1. Crosslinked polymer particles, wherein the particles are between
0.1 and 200 microns in diameter and have a network mesh size
between about 10 and about 500 Angstroms.
2. The particles of claim 1, wherein the polymer is selected from
the group consisting of: vinyl acetates, vinyl pyrrolidones, vinyl
ethers, olefins, styrenes, vinyl chlorides, ethylenes, acrylates,
methacrylates, nitriles, acrylamides, maleates, epoxies, epoxides,
lactones, ethylene oxides, ethylene glycols, ethyloxazolines, amino
acids, saccharides, proteins, anhydrides, amides, carbonates,
phenylene oxides, acetals, sulfones, phenylene sulfides, esters,
fluoropolymers, imides, amide-imides, etherimides, ionomers,
aryletherketones, amines, phenols, acids, benzenes, cinnamates,
azoles, silanes, chlorides, and epoxides.
3. The particles of claim 1, wherein the polymer comprises a
plurality of converted carbon-carbon double bond functional
groups.
4. The particles of claim 3, wherein the conversion of the
carbon-carbon double bonds is between about 20% and about 100%.
5. The particles of claim 3, wherein the conversion of the
carbon-carbon double bonds is between about 70% and about 100%.
6. The particles of claim 1, wherein the polymer comprises a
plurality of converted acrylate groups.
7. The particles of claim 1, wherein the polymer is poly(ethylene
glycol) (PEG) diacrylate.
8. The particles of claim 1, wherein the polymer comprises a
plurality of converted methacrylate groups.
9. The particles of claim 1, wherein the polymer is methacrlyated
sebacic anhydride (MSA)
10. The particles of claim 1, wherein the polymer is a
copolymer.
11. The particles of claim 1, wherein the polymer is a
copoly(PEG-b-D,L PLA) diacrylate.
12. The particles of claim 1, wherein the polymer is
biodegradable.
13. The particles of claim 1, wherein the particles comprise less
than about 1% of residual solvent.
14. The particles of claim 1, wherein the polymer is a
functionalized polymer having at least one unreacted reactive group
comprising a carbon-carbon double bond.
15. The particles of claim 3, wherein the reactive group is
selected from the group consisting of acrylates, methacrylates,
alkenes and alkynes.
16. Crosslinked polymer particles wherein the particles are between
0.1 and 200 microns in diameter, the polymer comprises a plurality
of converted carbon-carbon double bond functional groups, and the
conversion of the carbon-carbon double bonds is between about 20%
and about 100%.
17. Crosslinked polymer particles wherein a bioactive material is
encapsulated within the polymer.
18. The particles of claim 17 wherein the encapsulation efficiency
of the bioactive material is above about 60%.
19. The particles of claim 17 wherein the polymer comprises a
plurality of converted acrylate groups.
20. The particles of claim 17 wherein the polymer comprises a
plurality of converted methacrylate groups.
21. The particles of claim 17 wherein the polymer is selected from
the group consisting of: poly(ethylene glycol) (PEG) diacrylate,
methacrylated sebacic anhydride (MLA), and copoly(PEG-b-D,L-PLA)
diacrylate.
22. The particles of claim 17 wherein the bioactive material has a
molecular weight less than about 1000 Da.
23. The particles of claim 17 wherein the bioactive material is
selected from the group consisting of: tacrine, erythromycin,
erythromycin estolate, and erythromycin ethylsuccinate.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a division of U.S. patent application
Ser. No. 10,161,544, filed Jun. 3, 2002, now allowed, which is a
continuation-in-part of U.S. patent application Ser. No.
09/451,481, filed Nov. 30, 1999, now U.S. Pat. No. 6,403,672, which
claims the benefit of U.S. Provisional Patent Application No.
60/110,816, filed Nov. 30, 1998, which are hereby incorporated in
their entirety by reference to the extent not inconsistent with the
disclosure herein.
BACKGROUND OF THE INVENTION
[0003] This invention relates generally to polymer particles and
methods of making and using the same.
[0004] Small (micron- and nano-sized) polymer particles are useful
for many applications, including pharmaceutical uses. Polymer
microparticles are useful for injectable and implantable devices
because they have a long circulation time in the body and are
efficient drug, enzyme, and protein carriers (Tom, J. W. et al.
(1993), "Applications of Supercritical Fluids in the Controlled
Release of Drugs," in Supercritical Fluid Engineering Science, pp.
238-257). Crosslinked polymer microparticles have material property
benefits over linear polymer particles including improved
mechanical strength, greater control of transport properties,
material property adjustability and dimensional stability. Some
applications of crosslinked polymers are listed in Cooper, A. L.
and Holmes, A. B. (1998) Proceedings of the 5.sup.th Meeting of
Supercritical Fluids Materials and Natural Products Processing, pp.
843-848. Polymer microparticles (both linear and crosslinked) have
been used in applications such as dental composites, biostructural
fillers and controlled release devices. Some applications of
synthetic bone composites are listed in Popov, V. K. et al. (1998)
Proceedings of the 5.sup.th Meeting of Supercritical Fluids
Materials and Natural Products Processing, pp. 45-50.
[0005] Controlled release devices are useful in many applications,
from medical to agricultural purposes. (Langer, R. (1993),
Polymer-Controlled Drug Delivery Systems," Acc. Chem. Res.
26:537-542; U.S. Pat. No. 5,043,280). Controlled release delivery
systems for drugs have a wide variety of advantages over
conventional forms of drug administration. Some of these advantages
include: decreasing or eliminating the oscillating drug
concentrations found with multiple drug administrations; allowing
the possibility of localized delivery of the drug to a desired part
of the body; preserving the efficacy of fragile drugs; reducing the
need for patient follow-up care; increasing patient comfort; and
improving patient compliance. (Langer, R. (1990), "New Methods of
Drug Delivery," Science 249:1527-1533).
[0006] Crosslinked polymeric release devices have the capability to
modify the release profile of a drug or other chemical by modifying
the structure of the crosslinked polymer network. A crosslinked
polymer network can provide diffusion controlled release of a drug
or other chemical. The rate of diffusion of the drug or other
bioactive material to be released can be influenced by the mesh
size of the network, or the distance between crosslinks, which
depends upon the extent of crosslinking in the network. In a
biodegradable system, the mesh size of the network will increase
with time as the network degrades.
[0007] Current polymer microparticle manufacturing techniques all
suffer from one or more disadvantages. For example, the spray
drying technique usually requires evaporation of solvent in hot
air. The high temperatures used can degrade sensitive drugs and
polymers. In thermal polymerization, monomer is heated to induce
polymerization. Again, the high temperatures used can cause
degradation (including lowering the activity of biologically active
substances).
[0008] Emulsion and suspension polymerizations (see, for example,
U.S. Pat. No. 5,603,960 (O'Hagan., et al.)) involve combinations of
solvents, emulsifiers, and surfactants where dispersed islands of
monomer polymerize through chemical reaction in a sea of solvent.
These methods often involve operation at high temperatures and thus
have the problems discussed above, use large volumes of solutions
that are often environmentally unfriendly, and permit only minimal
control over particle size and morphology.
[0009] A number of different techniques have been developed to form
small particles of polymers using the solvent power of
supercritical fluids. Supercritical fluids have liquid-like
densities, very large compressibilities, viscosities between those
of liquids and gases, and diffusion coefficients that are higher
than liquids. Due to the high compressibility, the density (and
solvent power) of a supercritical fluid can be adjusted between
gas- and liquid-like extremes with moderate changes in pressure
(Debenedetti, P. G. et al. (1993), "Rapid Expansion of
Supercritical Solutions (RESS): Fundamentals and Applications,"
Fluid Phase Equilibria 82:311-321).
[0010] The Rapid Expansion of Supercritical Solution (RESS)
technique has been used to form small particles of poly(L-lactic
acid) (Debenedetti, P. G. et al. (1993), "Supercritical Fluids: A
New Medium for the Formation of Particles of Biomedical Interest,"
Proceed. Intern. Symp. Control Rel. Bioact. Mater. 20:141-142) and
particles of poly(DL-lactic acid) with embedded lovastatin (Tom, J.
W. et al. (1993), "Applications of Supercritical Fluids in the
Controlled Release of Drugs," in Supercritical Fluid Engineering
Science, pp. 238-257). In the RESS technique, particles of polymer
may be made when a polymer is dissolved in a supercritical fluid
(usually carbon dioxide) followed by rapid expansion of the fluid.
This technique is limited in applicability to compounds that are
soluble in the supercritical fluid. Since most drugs are not
soluble in supercritical fluids and most polymers have very low
solubility in supercritical fluids, the RESS process is not broadly
applicable for drug encapsulation (McHugh, M. and Krukonis, V.
(1994) Supercritical Fluid Extraction, Butterworth-Heinemann).
[0011] In the Precipitation by a Compressed Antisolvent (PCA)
technique (also known as the Gas Antisolvent technique), a solid of
interest is dissolved in a solvent and the resulting solution is
sprayed into a compressed antisolvent (see, for example, U.S. Pat.
Nos. 5,833,891 and 5,874,029). In this technique, the antisolvent
and solvent are soluble, but the solid of interest is not soluble
in the antisolvent. The antisolvent is believed to extract the
solvent, precipitating particles of the solid of interest
(Randolph, T. W. et al. (1993) Biotech. Prog. 9:429-435).
Microparticles of insulin have reportedly been formed using this
technique (Yeo, S. D. et al. (1993), "Formation of Microparticulate
Protein Powders Using a Supercritical Fluid Antisolvent," Biotech.
Bioeng. 41:341-346) and linear polymer microparticles have been
formed using polymer starting materials (Bodmeier, R. et al.
(1995), "Polymeric Microspheres Prepared by Spraying into
Compressed Carbon Dioxide," Pharm. Res. 12 (8):1211-1217; U.S. Pat.
Nos. 5,833,891; 5,874,029).
[0012] There is a need for polymer particles with low residual
solvent levels, high additive encapsulation efficiencies, and
processes of making polymer particles that allow control of
particle size and morphology, with low operating temperatures and
efficient bulk production capability. Formation of polymer
particles with degradable networks, whether by surface or bulk
degradation, are also needed for controlled release of drugs, for
example. In particular, highly crosslinked polymer networks with
degradable chemistries are desired. Preferably, the extent of
crosslinking or mesh size of such highly crosslinked polymer
networks is controlled to tailor the release profile of the drug or
other chemical to be released. In addition, there is a need for a
process that produces polymer particles in situ from polymer
precursors such as monomers or oligomers.
BRIEF SUMMARY OF THE INVENTION
[0013] In a general description of the invention, a method of
forming polymer particles comprising exposing a composition
comprising at least one polymer precursor, a solvent or solvent
mixture, and an antisolvent or antisolvent mixture to
photoradiation under conditions whereby particles are formed is
provided. If the precursor is not photosensitive, at least one
photoinitiator is present in the composition. The solvent is not
required if the polymer precursor is liquid or liquifiable. If
used, the solvent is chosen so that the polymer precursor is
soluble in the solvent at the concentrations used, and the
antisolvent and solvent are soluble in each other at the
concentrations used. The polymer precursor is preferably insoluble
in the antisolvent, but as long as nucleation and particle
formation occur, any solubility condition may be present. Bioactive
materials such as drugs may also be included in the
composition.
[0014] Also provided is a method of forming polymer particles from
a solution comprising contacting a solvent or solvent mixture and
at least one polymer precursor with an antisolvent or antisolvent
mixture under conditions whereby particles are generated; and
exposing said particles to photoradiation, whereby polymer
particles are formed. Preferably the polymer precursor is insoluble
in the antisolvent or antisolvent mixture.
[0015] Also provided are polymer particles prepared by the methods
of the invention that are between about 0.001 .mu.m to about 200
.mu.m in diameter. Each individual particle size and all
intermediate ranges of particle size are included in the invention
In one embodiment, particles are provided that are between about
0.1 .mu.m and about 100 .mu.m in diameter.
[0016] Linear and crosslinked polymer particles may be formed using
the methods of the invention. Crosslinked polymer particles in
which the crosslinked polymer forms a network are also provided.
The mesh size of the network can be between about 10 Angstroms and
about 500 Angstroms. Each individual mesh size and all the
intermediate ranges of mesh sizes are included in the invention.
For example, the mesh size can also be selected to be between about
10 Angstroms and about 100 Angstroms. Crosslinked particles
comprising a multiplicity of converted carbon-carbon double bond
functional groups are provided, wherein the conversion of the
carbon-carbon double bonds is a measure of the extent of
crosslinking. In one embodiment, the carbon-carbon double bond
conversion in the particles is between about 20% and about 100%.
Each individual value of carbon-carbon double bond conversion and
all the intermediate ranges of carbon-carbon double bond conversion
are included in the invention. For example, the carbon-carbon
double bond conversion can be greater than about 70%.
[0017] Also provided is a method of forming polymer particles
comprising: substantially dissolving at least one polymer precursor
in a solvent or solvent mixture to form a solution; contacting said
solution with an antisolvent or antisolvent mixture in which said
polymer precursor is insoluble to form a composition comprising
said precursor, and a substantially soluble mixture of said solvent
or solvent mixture and said antisolvent or antisolvent mixture; and
exposing said composition to sufficient photoradiation to initiate
polymerization whereby polymer particles are formed.
[0018] Also provided is a method of forming polymer particles
comprising: establishing a flow of antisolvent in an optically
accessible chamber; contacting a solution comprising at least one
polymer precursor and at least one polymerization initiator
dissolved in a solvent or solvent mixture with said antisolvent
under conditions whereby particles are formed; and exposing said
particles to photoradiation whereby polymer particles are
formed.
[0019] Also provided is a method for making crosslinked polymer
particles with a desired double bond conversion amount comprising
the steps of: exposing a composition comprising a polymer
precursor, a non-aqueous solvent or solvent mixture, and an
antisolvent or antisolvent mixture to photoradiation under
conditions whereby crosslinked particles of the desired conversion
amount are formed, wherein the antisolvent is a supercritical or
near supercritical fluid in which the polymer precursor is not
substantially soluble.
[0020] Also provided is a method for making crosslinked polymer
particles having a desired network mesh size comprising the steps
of:
[0021] selecting a polymer precursor;
[0022] determining a double bond conversion amount which
corresponds to the desired network mesh size for the polymer;
[0023] exposing a composition comprising the polymer precursor, a
non-aqueous solvent or solvent mixture, and an antisolvent or
antisolvent mixture to photoradiation under conditions whereby
crosslinked particles having the double bond conversion amount are
formed, wherein the antisolvent is a supercritical or near
supercritical fluid in which the polymer precursor is not
substantially soluble and whereby the crosslinked particles have
the desired network mesh size.
[0024] Also provided is a method of forming copolymers comprising:
dissolving or suspending at least two polymer precursors or at
least one polymer precursor and at least one polymer in a solvent
or solvent mixture to form a solution; contacting said solution
with an antisolvent or antisolvent mixture to form a composition
comprising: said precursors or said precursor and polymer; a
soluble mixture of said solvent or solvent mixture and said
antisolvent or antisolvent mixture; and exposing said composition
to photoradiation whereby copolymer particles are formed.
[0025] Copolymers may also be formed where at least one polymer
precursor or at least one polymer are present in a solvent or
solvent mixture, and at least one polymer precursor or at least one
polymer are present in the antisolvent or antisolvent mixture,
providing that at least one polymer precursor is present.
[0026] Also provided is a method of forming particles comprising a
bioactive material and a polymer comprising: exposing a composition
comprising at least one bioactive material, at least one polymer
precursor and an antisolvent or antisolvent mixture to
photoradiation under conditions whereby particles are formed.
[0027] Polymers formed may be erodable or nonerodable,
biodegradable or nonbio-degradable and biocompatible or
nonbiocompatible. Polymer particles formed using the methods of the
invention may be used for controlled release of a desired substance
in an organism or system. Provided is a method of controlled
release of a desired substance comprising: preparing polymer
particles that comprise a degradable polymer and a desired
substance; and exposing said polymer particles to conditions under
which the polymer is degraded.
[0028] Methods of forming degradable particles comprising a
degradable polymer and a pharmaceutical product comprising:
exposing a composition comprising a solvent or solvent mixture, at
least one polymer precursor capable of forming a degradable
polymer, at least one pharmaceutical product, and an antisolvent or
antisolvent mixture to photoradiation whereby polymer particles
that contain a degradable polymer and a pharmaceutical product are
formed are provided.
[0029] A pharmaceutical composition comprising polymer particles
produced by the methods of the invention and a pharmaceutically
acceptable carrier are also provided. Polymer particles comprising
at least one bioactive material and at least one polymer are also
provided.
[0030] Crosslinked polymer particles comprising a degradable
polymer are also provided. Biodegradable crosslinked polymer
particles are also provided. Crosslinked polymer particles further
comprising at least one bioactive material are also provided.
[0031] An apparatus is provided for producing polymer
microparticles which comprises: a reaction chamber; at least one
inlet into said reaction chamber through which an antisolvent or
antisolvent mixture, at least one polymer precursor insoluble in
said antisolvent or antisolvent mixture, and a solvent or solvent
mixture soluble in said antisolvent or antisolvent mixture pass
into said chamber; and a light source optically connected to said
chamber wherein during operation of the chamber said polymer
precursor is polymerized. The apparatus may be used with a
photosensitive polymer precursor, or a polymerization initiator may
be added.
[0032] Advantages of this photopolymerization technique include
morphological control through polymerization rate, process
conditions, and initiation location. Processing time remains short
while processing temperatures remain low. Low operating
temperatures are important since many potential encapsulation
additives degrade at moderate temperatures. In addition, particles
formed using the method of the invention do not require further
processing, for example solvent removal, before use.
[0033] Further objects and advantages of this invention will be
apparent from a consideration of the drawings and description
herein.
[0034] "Microparticles" as used herein means particles that are
less than about 100 .mu.m in diameter. "Nanoparticles" are
particles that are less than about 1 .mu.m in diameter. Both
microparticles, nanoparticles and particles of other sizes may be
produced by the methods of the invention by changing process
parameters and choice of materials. Methods of changing the process
parameters and materials are described herein, or determinable by
one of ordinary skill in the art without undue experimentation.
[0035] "Polymer precursor" means a molecule or portion thereof
which can be polymerized to form a polymer or copolymer. Polymer
precursors include any substance that contains an unsaturated
moiety or other functionality that can be used in chain
polymerization, or other moiety that may be polymerized in other
ways. Such precursors include monomers and oligomers. A
"multifunctional monomer" is a monomer having two or more sites
available for bonding to other molecules during polymerization.
Preferred precursors include those that are capable of being
polymerized by photoradiation. One class of polymer precursors of
the invention are those that are insoluble in the antisolvent or
antisolvent mixture. Another class of polymer precursors of this
invention are photosensitive. If a polymer precursor that
polymerizes photochemically is used (photosensitive polymer
precursor), a separate photoinitator does not need to be used.
Examples of photosensitive polymer precursors include
tetramercaptopropionate and 3,6,9,12-tetraoxatetradeca-1,13-diene.
Another class of precursors that may be used are radically
polymerizable precursors. Another class of precursors that may be
used are ionically polymerizable precursors. Another class of
precursors that are useful in the invention are cationic
precursors.
[0036] Some examples of precursors that are useful in the invention
include ethylene oxides (for example, PEO), ethylene glycols (for
example, PEG), vinyl acetates (for example, PVA), vinyl
pyrrolidones (for example, PVP), ethyloxazolines (for example,
PEOX), amino acids, saccharides, proteins, anhydrides, vinyl
ethers, amides, carbonates, phenylene oxides (for example, PPO),
acetals, sulfones, phenylene sulfides (for example, PPS), esters,
fluoropolymers, imides, amide-imides, etherimides, ionomers,
aryletherketones, olefins, styrenes, vinyl chlorides, ethylenes,
acrylates, methacrylates, amines, phenols, acids, nitriles,
acrylamides, maleates, benzenes, epoxies, cinnamates, azoles,
silanes, chlorides, epoxides, lactones and amides. A preferred
group of precursors includes all the above precursors with the
exception of fluoropolymers.
[0037] Polymer precursors useful for producing crosslinked polymer
particles include multifunctional monomers such as: vinyl acetates
(for example, PVA), vinyl pyrrolidones (for example, PVP), vinyl
ethers, olefins, styrenes, vinyl chlorides, ethylenes, acrylates,
methacrylates, nitriles, acrylamides, maleates, epoxies, epoxides,
and lactones. If a crosslinking agent is involved, polymers useful
for producing crosslinked polymer particles include monomers such
as: ethylene oxides (for example, PEO), ethylene glycols (for
example, PEG), vinyl acetates (for example, PVA), vinyl
pyrrolidones (for example, PVP), ethyloxazolines (for example,
PEOX), amino acids, saccharides, proteins, anhydrides, vinyl
ethers, carbonates, phenylene oxides (for example, PPO), acetals,
sulfones, phenylene sulfides (for example, PPS), esters,
fluoropolymers, imides, amide-imides, etherimides, ionomers,
aryletherketones, olefins, styrenes, vinyl chlorides, ethylenes,
acrylates, methacrylates, amines, phenols, acids, nitriles,
acrylamides, maleates, benzenes, epoxies, cinnamates, azoles,
silanes, chlorides, epoxides, lactones and amides. Crosslinking
agents include reactive groups having photocrosslinkable
carbon-carbon double bonds attached to the ends of the polymer
precursor chains. Such a carbon-carbon double bond can provide two
sites available for bonding to other molecules. Suitable reactive
groups having photocrosslinkable carbon-carbon double bonds
include, without limitation, acrylates, methacrylates, alkenes and
alkynes. Polymers having such reactive groups attached to the
polymer precursor chains are termed "functionalized polymers".
Polymer precursors useful for producing crosslinked polymer
particles also include copolymers of the above monomers. Copolymers
of the above monomers with degradable or erodable polymers may be
used to obtain degradable or erodable crosslinked particles.
[0038] As used herein, "polymer" includes copolymers. "Copolymers"
are polymers formed of more than one polymer precursor. Polymers
that can be formed using the methods of this invention include
those which are prepared from precursors that, in a preferred
embodiment are soluble in a solvent that is soluble in an
antisolvent and can be polymerized with light initiation. One class
of polymers that may be prepared using the method of this invention
includes those that are degradable, preferably biodegradable.
Another class of polymers that may be prepared using the method of
this invention includes those that are not degradable. Another
class of polymers that may be prepared using the method of this
invention includes those that comprise one or more degradable
polymers and one or more nondegradable polymers. Another class of
polymers that may be prepared using the method of this invention
includes poly(lactides), poly(glycolides), and
poly(lactide-co-glycolides). In a preferred embodiment of the
invention, the polymers are degradable or erodable.
[0039] "Degradable or erodable polymers" are those that degrade
upon exposure to some stimulus, including time. Degradable or
erodable polymers include biodegradable polymers. Biodegradable
polymers degrade in a biological system, or under conditions
present in a biological system. Preferred biodegradable polymers
degrade in an organism, preferably a mammal, and most preferably
human. Examples of biodegradable polymers include those having at
least some repeating units representative of at least one of the
following: an alpha-hydroxycarboxylic acid, a cyclic diester of an
alpha-hydroxycarboxylic acid, a dioxanone, a lactone, a cyclic
carbonate, a cyclic oxalate, an epoxide, a glycol, and anhydrides.
Preferred degradable or erodable polymers comprise at least some
repeating units representative of polymerizing at least one of
lactic acid, glycolic acid, lactide, glycolide, ethylene oxide and
ethylene glycol.
[0040] A class of polymers included in this invention are
biocompatible polymers. One type of biocompatible polymers degrade
to nontoxic products. Specific examples of biocompatible polymers
that degrade to nontoxic products that do not need removal from
biological systems include poly(hydro acids), poly(L-lactic acid)
or L-PLA, poly(D,L-lactic acid) or D,L-PLA, poly(glycolic acid) and
copolymers thereof. Polyanhydrides have a history of
biocompatibility and surface degradation characteristics (Langer,
R. (1993) Acc. Chem. Res. 26:537-542; Brem, H. et al. (1995) Lancet
345:1008-1012; Tamada, J. and Langer, R. J. (1992) J. Biomat
Sci.-Polym. Ed. 3:315-353).
[0041] Another class of polymers that may be prepared using the
method of this invention include particles that are a suitable size
for injection or administration orally or incorporated in a
preparation suitable for oral administration. For oral or
injectable delivery, it is preferred that most particles are less
than 50 microns in diameter. Another class of particles that may be
prepared using the method of this invention include those that are
a suitable size for inhalation or pulmonary delivery. For pulmonary
delivery, it is preferred that greater than about 90 weight percent
of all solid particles in an administered pharmaceutical
formulation are of a size smaller than about 10 microns and more
preferably at least about 90 weight percent are smaller than about
6 microns, and even more preferably at least about 90 percent of
all solid particles are from about 1 micron to about 6 microns.
Particularly preferred for pulmonary delivery applications are
particles of from about 2 microns to about 5 microns in size. Other
classes of particles of suitable size for various applications are
included in the methods of the invention.
[0042] Solvents useful in the invention include those that dissolve
some portion of a polymer precursor and are preferably at least
partially soluble in the antisolvent used. Preferably the solvent
is miscible in the antisolvent or antisolvent mixture at the
temperature and pressure of operation. Preferred solvents are not
water. Some examples of preferred solvents include methylene
chloride, methanol, toluene, propanol, ethanol, acetone, ethers,
hexanes, heptane, tetrahydrofuran, methyl ethyl ketone, chloroform,
carbon tetrachloride, butanone, dimethyl sulfoxide, isopropanol,
ethyl acetate, methyl acetate, n-methylpyrrolidine, propylene
carbonate, alkanes, and acetonitrile. If a liquid or liquidizable
polymer precursor is used, a solvent is not necessary. One solvent
or a mixture of solvents may be used.
[0043] Photoinitiators that are useful in the invention include
those that can be activated with light and initiate polymerization
of the polymer precursor. Preferred initiators include
azobisisobutyronitrile, peroxides, phenones, ethers, quinones,
acids, formates. Cationic initiators are also useful in the
invention. Preferred cationic initiators include aryldiazonium,
diaryliodonium, and triarylsulfonium salts. Most preferred
initiators include Rose Bengal (Aldrich), Darocur 2959
(2-hydroxy-1-[4-(hydroxyethoxy)phenyl]-2-methyl-1-propanone, D2959,
Ciba-Geigy), Irgacure 651 (2,2-dimethoxy-2-phenylacetophenone,
1651, DMPA, Ciba-Geigy), Irgacure 184 (1-hydroxycyclohexyl phenyl
ketone, 1184, Ciba-Geigy), Irgacure 907
(2-methyl-1-[4-(methylthio)phenyl]-2-(4-morphol- inyl)-1-propanone,
1907, Ciba-Geigy), Camphorquinone (CQ, Aldrich), isopropyl
thioxanthone (quantacure ITX, Great Lakes Fine Chemicals LTD.,
Cheshire, England). CQ is typically used in conjunction with an
amine such as ethyl 4-N,N-dimethylaminobenzoate (4EDMAB, Aldrich)
or triethanolamine (TEA, Aldrich) to initiate polymerization.
[0044] The wavelengths and power of light useful to initiate
polymerization depends on the initiator used or the wavelength (or
wavelengths) will activate the photosensitive precursor. A
combination of photosensitive precursor(s) and photoinitiator(s)
may be used. When Rose Bengal is used as the initiator, a visible
light source is preferably used. Light used in the invention
includes any wavelength and power capable of initiating
polymerization. Preferred wavelengths of light include ultraviolet
or visible. Any suitable source may be used, including laser
sources. The source may be broadband or narrowband, or a
combination. The light source may provide continuous or pulsed
light during the process.
[0045] Chamber windows made from various materials may be used in
the method of this invention. In addition, a filter may used to
block a wavelength from reaching the chamber, or allow a selected
wavelength or wavelengths of light to reach the chamber. The
chamber windows themselves may act as this filter, or a separate
filter or filters may be used in conjunction with the chamber
windows.
[0046] In one embodiment, a broadband light source may be used, and
by selecting chamber window compositions and/or filter
combinations, the selected wavelength or wavelengths of light may
pass through the chamber. Light of different selected wavelengths
may pass through the same chamber at various locations. This
feature may be used to activate more than one photoinitiator.
[0047] As used herein, "antisolvent" is a substance in which the
polymer precursor is substantially not soluble. It should be
understood that it is possible that the antisolvent may be capable
of dissolving some amount of the precursor without departing from
the scope of the present invention. The antisolvent is, however,
preferably incapable of dissolving a significant portion of the
precursor such that at least a significant portion of precursor is,
in effect, not soluble in the antisolvent. Preferably, the
antisolvent precipitation is conducted under thermodynamic
conditions which are near critical or supercritical relative to the
antisolvent fluid. The antisolvent preferably comprises any
suitable fluid for near critical or supercritical processing. These
fluids include carbon dioxide, ammonia, nitrous oxide, methane,
ethane, ethylene, propane, butane, pentane, benzene, methanol,
ethanol, isopropanol, isobutanol, fluorocarbons (including
chlorotrifluoromethane, monofluoromethane, hexafluoraethane and
1,1-difluoroethylene), toluene, pyridine, cyclohexane, m-cresol,
decalin, cyclohexanol, o-xylene, tetralin, aniline, acetylene,
chlorotrifluorosilane, xenon, sulfur hexafluoride, propane and
others. Carbon dioxide, ethane and propane are preferred
antisolvents. Most preferably, carbon dioxide is used as the
antisolvent. One antisolvent, or a mixture of different
antisolvents may be used.
[0048] As used herein, "supercritical or near supercritical fluid"
means a substance that is above its critical pressure and
temperature or is substantially near its critical pressure and
temperature.
[0049] Components that are "contacted" with each other refers to
two or more components physically near each other. Components that
are contacted with each other are preferably in intimate contact
with each other so that they may react with each other or affect
each other. Contact may include emulsions or microemulsions.
[0050] A "bioactive" material is any substance which may be
administered to any biological system, such as an organism,
preferably a human or animal host, and causes some biological
reaction. Bioactive materials include pharmaceutical substances,
where the substance is administered normally for a curative or
therapeutic purpose. The bioactive material may comprise a protein
or other polypeptide, an analgesic or another material. In one
embodiment, the bioactive material has a molecular weight less than
1000 Da. Suitable bioactive material includes, without limitation,
tacrine, erythromycin, erythromycin estolate, and erythromycin
ethylsuccinate.
[0051] A "polymer shell" may be a continuous coating of polymer
over some substance, but the coating is not required to be
continuous. The polymer shell may have nonhomogeneous regions where
there is no coating, or regions where the coating is thicker than
in other areas. The polymer shell may be composed of different
materials. Preferably, the polymer shell is a homogeneous coating
with uniform thickness. "Encapsulated" is intended to indicate a
substance, such as a bioactive material, is homogeneously
distributed throughout the polymer.
[0052] "Linear polymers" are those polymers that are composed of
individual polymer chains that do not have bonds connecting the
chains. "Crosslinked polymers" are those polymers that have bonds
between polymer chains. Branched polymers are also included in the
invention.
[0053] "Soluble" does not necessarily mean completely soluble. As
long as some portion of one substance dissolves in another
substance, the substances are soluble in each other.
[0054] Likewise, "insoluble" does not necessarily mean that no
amount of one substance will dissolve in another substance.
[0055] A "composition" of substances is not intended to mean the
substances are soluble or miscible with each other, or react with
each other. A "composition" is merely intended to mean all listed
substances are present.
BRIEF DESCRIPTION OF THE DRAWINGS
[0056] FIG. 1 is a schematic diagram of a photopolymerization
system.
[0057] FIG. 2 is a schematic diagram of a continuous
photopolymerization system.
[0058] FIG. 3 is a UV-vis spectrum of the chamber window, Rose
Bengal (3.8.times.10.sup.-6 wt percent) in methylene chloride and
Rose Bengal bis(triethylammonium) salt (0.001 wt percent) in
methylene chloride.
[0059] FIG. 4 is a Fourier transform infrared (FTIR) spectra of
methacrylated sebacic anhydride oligomer and poly(methacrylated
sebacic anhydride).
[0060] FIG. 5 is a scanning electron microscopy (SEM) photograph of
poly(methacrylated sebacic anhydride) microparticles precipitated
by spraying and photopolymerizing a 10 wt % MSA solution through a
100 .mu.m capillary nozzle into CO.sub.2 at a temperature of
37.degree. C. and pressure of 8.5 MPa.
[0061] FIG. 6 shows cloud point data for various molar
concentrations (consistent with general operating conditions) of
MSA monomer in methylene chloride at 37.degree. C.).
[0062] FIG. 7 is an SEM micrograph of PMSA precipitated by spraying
and photopolymerizing a 5 wt % MSA solution through an ultrasonic
atomizing nozzle into CO.sub.2 at a temperature of 25.degree. C.
and pressure of 8.5 MPa.
[0063] FIG. 8A-8C are SEM micrographs for 5% (A), 10% (B) and 20%
(C) MSA.
[0064] FIGS. 9A and 9B are SEM micrographs showing triacrylate
polymers under two experimental conditions.
[0065] FIGS. 10A and 10B are SEM micrographs showing triacrylate
polymers under two experimental conditions.
[0066] FIG. 11 is a SEM micrograph for 5% MSA/5% PLA copolymer.
[0067] FIG. 12 is a fluorescence micrograph of PMSA particles
encapsulated with tacrine.
[0068] FIG. 13 shows release data from PMSA microparticles
encapsulated with tacrine.
[0069] FIG. 14 shows release data from particles containing Rose
Bengal incorporated in a MSA matrix and polymerized into disks and
release data from Rose Bengal homogeneously incorporated into MSA
disks and polymerized.
[0070] FIG. 15 shows the variation of double bond conversion with
the residence time of the poly(ethylene glycol) diacrylate (PEGDA)
particles in the apparatus.
[0071] FIGS. 16A-D are SEM micrographs of PEGDA particles formed
with photoinitiator concentrations of 1 (A), 1.5 (B), 2 (C), and 4
(D) percent by weight of the monomer.
[0072] FIGS. 17A-C are SEM micrographs of PEGDA particles prepared
with average incident light intensities of 3 (A), 4 (B), and 6.25
(C) W/cm.sup.2.
DETAILED DESCRIPTION OF THE INVENTION
[0073] A process for photopolymerizing polymer particles in-situ
with antisolvent precipitation is provided. Photopolymerization
occurs when solutions of polymer precursor and solvent are exposed
to light of sufficient power and of a wavelength capable of
initiating polymerization while being contacted with an antisolvent
at reduced temperature (T.sub.r) and pressure (P.sub.r). The
polymerization may be initiated by a polymerization initiator
activated by light, or a photosensitive polymer precursor may be
used. If a photosensitive polymer precursor is used, a separate
photoinitiator is optional. The polymer precursor and solvent
solution may be homogeneous, but that is not required. This type of
polymerization results in particles with a wide range of
morphologies, sizes and physical characteristics adjustable by
changing the process conditions. The polymer particles produced by
the methods of the invention do not require any further processing
(for example solvent removal) before they may be used.
[0074] Not wishing to be bound by any theory, it is believed that
the chamber conditions coupled with the antisolvent properties
(high diffusivity, low viscosity, and high solvating capacity)
facilitates the extraction of the solvent from the solution leaving
mostly precursor and initiator (if used). At the same time, these
precursor/initiator particles receive photons from the UV source,
initiating the polymerization.
[0075] The reduced temperature is the ratio of the operating
temperature over the critical temperature for the antisolvent. 1 T
r = T operating T critical
[0076] The reduced pressure is the ratio of the operating pressure
over the critical pressure for the antisolvent. 2 P r = P operating
P critical
[0077] In the methods of the invention, T.sub.r=0.7 to 1.3,
preferably T.sub.r=0.9 to 1.1. In the methods of the invention,
P.sub.r=0.5 to 2, preferably, P.sub.r=0.75 to 1.5.
[0078] The precursor/initiator/solvent should remain at a
temperature such that the initiators (if used), precursors and any
desired additives are not degraded to an extent that is
unacceptable for the particular application. The methods of this
invention may be used where the antisolvent is not at reduced
pressure and temperature to produce polymer particles, as long as
particle formation occurs. The methods of the invention may also be
used to produce polymer particles when the solvent or solvent
system used is not completely miscible in the antisolvent or
antisolvent mixture, or when the precursor is soluble to some
extent in the antisolvent or antisolvent mixture.
[0079] Polymers with many different morphologies and physical
properties may be produced using the methods of this invention. The
morphology changes in polymers formed by changing conditions in the
PCA experiment have been studied (Dixon, D. J. and Johnston, K. P.
(1993), "Formation of Microporous Polymer Fibers and Oriented
Fibrils by Precipitation with a Compressed Fluid Antisolvent," J.
Appl. Polym. Sci. 50:1929-1942).
[0080] Polymers with improved mechanical strength,
polymer-encapsulated bioactive materials where the biomaterial has
controlled transport properties through the polymer, and polymers
that are capable of degrading or remaining substantially intact in
a given system, for example an organism such as a human or other
mammal, may be prepared using the methods of this invention.
Nondegradable polymers may be formed using the methods of this
invention. Polymer particles may be formed using the methods of the
invention for use in many applications such as agricultural
controlled release of fertilizer, use as fillers, and other
applications. In addition to polymer particles, polymer fibers and
porous polymer particles, for example, are achievable by changing
one or more process parameters such as the solvent flow rate,
polymer precursor type and functionality, photoinitiator
concentration, initiation rate, chamber operating temperature or
pressure, among other parameters.
[0081] For example, increasing the concentration of polymer
precursor in the solution increases the diameter of the polymer
particles formed. In addition, initiating polymerization some
distance after the particles have been formed increases the
diameter of the polymer particles formed. The diameter of the
polymer particles formed can be increased by manipulating the
nozzle size, increasing the concentration of monomer, increasing
the temperature. Polymer fibers as opposed to more spherical
particles can be formed by using slower flow rates of the
precursor/initiator/solvent.
[0082] Copolymers may also be made using this method, as well as
bioerodable polymer particles which can be used, for example, in
controlled release applications.
[0083] Polymers with crosslinked polymer networks may also be
formed using the methods of this invention. In a photoinitiated
process, the extent of crosslinking may be controlled by, for
example, controlling the carbon-carbon double bond or other
reactive functional group concentration in the polymer precursor,
the intensity of the light source, the time of exposure to the
light source, and the photoinitiator concentration (if present).
The time of exposure to the light source, or residence time, is
controlled by the combination of the antisolvent and solution flow
rates. The extent of cross-linking is preferably large enough to
provide gelation of the polymer and prevent agglomeration of the
particles once they are formed. In general, less reactive polymers
will require more exposure time, higher light intensity, and a
higher photoinitiator concentration. Those skilled in the art can
assess the relative reactivities of monomers based on their
molecular weights and functional groups.
[0084] For precursors having C.dbd.C functional groups, formation
of crosslinks between molecules involves conversion of C.dbd.C
bonds to C--C bonds. Bonds which have been converted from C--C
double bonds can be termed "converted C--C double bonds."
Similarly, those functional groups containing C--C double bonds
which have been converted to C--C single bonds can be termed
"converted functional groups", e.g. converted acrylate or
methacrylate groups. The extent of crosslinking can be measured by
FTIR analysis of the double bond conversion in the polymer
particles (referenced to double bond measurements of the unreacted
precursor) or by other methods as known to the art. Uncertainty in
this method of double bond conversion can be .+-.10%, but is
typically .+-.5%. Similar measures of the extent of crosslinking
are known to the art for precursors having other types of
functional groups.
[0085] For polymer networks which swell in solvent, the mesh size
of the polymer network in the particles for a given extent of
double bond conversion can be estimated from network mesh sizes for
a bulk polymer having the same extent of double bond conversion.
Mesh sizes for the bulk polymer can be calculated from measurements
of the swelling of the bulk polymer as described by Lu and Anseth,
(2000), "Release Behavior of High Molecular Weight Solutes from
Poly(ethylene glycol)-Based Degradable Networks", Macromolecules,
pp 2509-2515.
[0086] For polymer networks which do not swell in solvent, the mesh
size of the particles can be estimated statistically assuming an
ideal network and a monodisperse monomer molecular weight. For
macromers having C.dbd.C functional groups which are 100% converted
to C--C kinetic chains or crosslinks, the sum of the bond lengths
of the repeating unit (excluding side groups) can multiplied by the
number of monomer units to obtain the length of one side of the
mesh. The length of the other side of the mesh can be estimated to
be the same as the kinetic chain length or C--C bond length. With
an ideal network (100% conversion, no cyclization) the mesh size is
the average of these 2 lengths. To calculate for other conversions,
the system can be idealized further. For example, for 50%
conversion, it can be estimated that every other monomer unit,
there will be a kinetic chain link, so the length of one side of
the mesh can be estimated as twice the sum of the bond lengths of
the starting monomer multiplied by the number of monomer units. The
length of the other side of the mesh can be estimated as the C--C
bond length as before.
[0087] Additives of various sorts may be added to the
precursor/initiator/solvent solution or the antisolvent. These
additives may include, but are not limited to: plasticizers,
coloring agents, encapsulation agents, bioactive materials such as
drugs of various kinds, and other inert or bioactive particles. As
used herein, encapsulation efficiency refers to the amount of drug
encapsulated into a quantity of particles (which can be calculated
from release data) divided by the amount of drug loaded into an
equivalent quantity of precursor. The methods of the invention
allow improved encapsulation efficiency of additives such as
bioactive materials by allowing a wide range of polymer precursors
to be used. The polymer precursor can then be selected which is
compatible with the drug. For example, a hydrophobic bioactive
material can be paired with a relatively hydrophobic polymer.
[0088] The degradability of these materials can further be
controlled by varying polymer composition and morphology. This
permits tuning degradation devices to match a desired release rate
or release profile. Homogeneous encapsulation of a drug, for
example, into polymer particles in a single manufacturing step is
possible using the methods of this invention. Changing the size and
morphology of the degradable particles allows control over the dose
and duration of the drug delivery.
[0089] A variety of embodiments of the invention are possible. For
example, one drug may be encapsulated in a polymer particle using
the methods of the invention. Then, a second polymer precursor,
initiator and drug may be used to encapsulate a second drug over
the first particle. This will result in a particle that has two or
more different bioactive materials with different release profiles.
This is useful in a variety of different therapeutic
applications.
[0090] Methods of determining appropriate dosages for bioactive
materials are well known to one of ordinary skill in the art.
Polymer particles and compositions comprising bioactive materials
are administered by methods well known in the art, or by adapting
methods well known in the art.
[0091] This invention is useful for other controlled release of
materials other than drugs. Other applications include controlled
release of fragrances and pesticides. Particles may be made using
the methods of the invention that release corrosion inhibitors over
a specified time. This may be useful in pipeline applications.
Other uses are readily apparent to one of ordinary skill in the art
without undue experimentation.
[0092] To circumvent potential problems associated with
solubilizing a hydrophilic drug in an organic solvent such as
microphase separation and consequent burst effects, the
photopolymerization technique described herein can be combined with
a solubilization technique known as hydrophobic ion-paring (HIP) to
form homogeneous solutions of drug, monomer and initiator in an
organic solvent and photopolymerized drug-encapsulated
microparticles. HIP is described in U.S. Pat. Nos. 5,981,474 and
5,771,559, hereby incorporated by reference to the extent not
inconsistent with the disclosure herein. HIP is a technique whereby
ionic pharmaceutical agents can be directly solubilized in organic
solvents. HIP consists of pairing charges on the molecule with
oppositely charged, hydrophobic organic ions, effectively
increasing the molecule's solubility in low-dielectric organic
solvents. The photopolymerization method described herein may be
used in combination with HIP to encapsulate a therapeutic agent in
polymer particles.
[0093] Parts per billion residual solvent levels have been obtained
for PCA processing of linear poly(lactic) acid, in which the
particles are washed with several volumes of CO.sub.2 after
processing (Falk and Randolph (1998) Pharmaceutical Research, 15,
8,1233-1237). If the particles of the present invention are washed
with supercritical fluid such as CO.sub.2 after processing,
residual solvent levels can be reduced below 1%.
[0094] Apparatus for Polymerization Experiments
[0095] FIG. 1 illustrates an apparatus of this invention for
providing polymer particles. The apparatus has a chamber (45) with
one or more inlets (85, 95) that allow substances to pass into
chamber (45). In a particular embodiment, antisolvent (20) is
connected to optional oxygen scrubber (80) through connecting
tubing (50). Oxygen scrubber (80) is connected to pump (15) through
connecting tubing (50). Pump (15) is connected to valve (5) with
connecting tubing (50). Valve (5) is connected to inlet (85)
through connecting tubing (50). Inlet (85) allows antisolvent (20)
to enter chamber (45). Pump (15) is used to pump solution (10)
comprising one or more polymer precursors, one or more initiators
and one or more solvents to valve (5) through connecting tubing
(70). Solution (10) is pumped to inlet (95) through connecting
tubing (70). Inlet (95) allows solution (10) to enter injector (25)
inside chamber (45). Light pipe (35) passes light from light source
(30) into chamber (45). After polymer particle formation, particles
(90) pass out of chamber (45) to filter (40) and valve (5).
[0096] The embodiment described by FIG. 1 illustrates more than one
inlet (85 and 95). In an alternative embodiment, the antisolvent
and solution pass into the chamber through one inlet. The
precursor/initiator/solvent may also be sprayed into a stream of
antisolvent or antisolvent mixture. In another alternative
embodiment, there are multiple inlets for various components.
[0097] In operation, the following preferred procedure is used.
Antisolvent (20) is optionally deoxygenated with oxygen scrubber
(80). Antisolvent (20) is pumped with one or more pumps (15)
through connecting tubing (50) to valve (5). Antisolvent (20) is
then pumped into optically accessible high pressure chamber (45)
through inlet (85). The flow rate of antisolvent (20) into chamber
(45) is typically about 25 ml/min. An optional heating or cooling
source (not shown) may be positioned in any suitable location to
provide any necessary heating or cooling to the antisolvent, the
chamber, or any part of the apparatus or any component thereof.
Antisolvent (20) is preferably pressurized and heated so that it is
at or above its critical pressure and critical temperature. Chamber
(45) is allowed to equilibrate at the desired temperature and
pressure (preferably at or above the critical temperature and
pressure of the antisolvent). At least one polymer precursor and at
least one photoinitiator are dissolved or suspended in a suitable
solvent to form solution (10). Solution (10) is pressurized to the
desired pressure with pump (15). Solution (10) is pumped through
connecting tubing (70) to valve (5) and pumped through connecting
tubing (70) to inlet (95). Antisolvent (20) can be co-flowed
coaxially with solution (10). Solution (10) passes through inlet
(95) into injector (25) into chamber (45). In one embodiment,
injector (25) is a stainless steel tube with a 100 .mu.m opening
which injects solution (10) into chamber (45). In another
embodiment, injector (25) is any type of injector known in the art,
including ultrasonic nozzles and laser drilled holes. Many
different nozzles may be used, including a stainless steel
capillary tube, a quartz capillary tube, a sonicated nozzle, and a
converging diverging nozzle with a premixing chamber. The injector
and inlet are not required to be separate components. The flow rate
of solution (10) through injector (25) is typically about 0.1 to
about 1 ml/min. Light source (30) provides the necessary photons to
initiate photopolymerization at a desired distance (in one
embodiment, 2-3 cm) below solution injector (25). In one
embodiment, light source (30) is a ultraviolet or visible light
source at about 800 to about 6300 mW/cm.sup.2 (30). The light is
transferred from light source (30) into chamber (45) using any
suitable means, including optical fiber (35). Particles (90) may be
collected by any suitable means, including the use of filter (40).
A 0.2 .mu.m filter is used in one embodiment, but any suitable pore
size may be used. The size of the pores of the filter needed will
depend on the size of the particles formed and the desired size of
particles collected. Particles may be transferred from filter (40)
through valve (5).
[0098] The chamber itself in a preferred embodiment is a
5".times.4".times.9" long stainless steel chamber with a volume of
100 ml. Tempered borosilicate windows (3.5" long each) are used.
The chamber weighs about 50 pounds. Other embodiments of the
chamber may be used.
[0099] Alternate Apparatus for Continuous Processing
[0100] Alternatively, the apparatus may be operated in a continuous
mode. This is shown in FIG. 2. In this embodiment, injector loop
(60) with injection port (65) such as those used in an HPLC
apparatus may be added to the apparatus to allow processing of
multiple solutions or multiple samples of the same solution without
the time consuming pressurizing and depressurizing cycles that
would otherwise be required. In this embodiment, a flow of solvent
(40) is maintained through connecting tubing (70), a flow of
antisolvent (20) is maintained through connecting tubing (50), and
a solution of polymer precursor/initiator and solvent is injected
into the solvent flow through injection port (65).
[0101] A series of valves and filters may be used to enable
particle collection without depressurizing the system. This is also
shown in FIG. 2. The flow path exiting the chamber is split.
Particles may be collected on filter (125) by closing valve (100)
and opening valve (105). By closing valve (105) and opening valve
(100), particles may be collected on filter (120). While particles
are being collected on filter (120), filter (125) may be replaced.
This way particles may be collected continuously by routing the
flow. This allows a continuous, not batch, process to be
maintained, and greater amounts of polymer particles may be
produced.
[0102] The apparatuses described above are only some of the
possible apparatuses that may be used to carry out the invention.
Other embodiments of the apparatus or components of the apparatus
will be readily apparent to those of ordinary skill in the art. For
example, the solution of polymer precursor(s) and photoinitiator(s)
may pass into the chamber through other diameter injectors or
injector types other than those mentioned specifically. The
solution of polymer precursor(s) and photoinitiator(s) may be
optionally heated or cooled in any suitable location. Any suitable
light source may be used, and any suitable method of bringing light
to the chamber may be used. The light is brought into the chamber
at any desired location. The range of possible modifications is
well known to one of ordinary skill in the art without undue
experimentation.
[0103] The invention will be further understood by reference to the
following examples intended as illustrations, not limitations.
EXAMPLES
Preparation of Methacrylated Sebacic Anhydride
[0104] The monomer, methacrylated sebacic anhydride (MSA), was
prepared by combining 40 g sebacic acid (Aldrich) with 88 ml
methacrylic anhydride (Aldrich) and refluxing for approximately 1
hour. This process, shown in Scheme 1, converts the dicarboxylic
acid to the anhydride monomer which is subsequently dissolved in
dry methylene chloride (Fisher) and precipitated in petroleum ether
(Aldrich) for purification and recovery (U.S. Pat. No. 4,789,724).
1
[0105] Proton NMR spectroscopy (Varian VXR-300S) was used to verify
the existence of the characteristic methacrylate end-capped
.dbd.CH.sub.2 protons that give peaks at 5.8 and 6.2 ppm. Infrared
spectroscopy (IR) shows the presence of the methacrylate double
bond group at 1635 cm.sup.-1 and confirmed the conversion of the
acid groups to the anhydride (Muggli, D. S. et al. (1998)
Macromolecules 31:4120-4125). After forming the dimethacrylated
monomeric anhydride, the monomer can be oligomerized through a
condensation polymerization under vacuum at a temperature of
60.degree. C. A ratio of the integrated area of the .dbd.CH.sub.2
proton peaks to the internal protons in the MSA backbone from the
NMR analysis suggests a number average degree of oligomerization of
-6 repeat units.
[0106] Initiator Selection
[0107] The photoinitiators used in these experiments were Rose
Bengal and Rose Bengal bis(triethyl ammonium) salt obtained from
Aldrich, although other initiators may be used. Interestingly, the
Rose Bengal and its ammonium salt have dramatically different
absorbance spectra in methylene chloride. FIG. 3 shows that the
large peak at -550 nm seen in the Rose Bengal salt is not present
in Rose Bengal. Since the chamber window used in these experiments
absorbed wavelengths below 350 nm, visible light initiators were
used, and the triethyl ammonium Rose Bengal salt, whose absorbance
spectrum is shown in FIG. 3 was used in these experiments.
[0108] Particle Production.
[0109] 5-20 wt % methacrylated sebacic anhydride was dissolved in
methylene chloride along with 2% photoinitiator by monomer weight.
The chamber was pressurized with deoxygenated CO.sub.2 by two ISCO
compressed gas pumps and allowed to equilibrate to the desired
temperature and pressure. The monomer-solvent solution was then
pressurized to the desired pressure by a third ISCO pump. The
solution was injected into the pressurized chamber environment at a
constant flow rate (1 ml/min) through the nozzle while the CO.sub.2
flowed at a constant rate of 25 ml/min. A high powered light source
(1-4 W/cm.sup.2) (EFOS, Novacure) with a visible filter (350-650
nm) and a fiber optic liquid light guide was used to initiate the
photo-polymerization below the nozzle. A 5 cm Light Line (EFOS) was
used to spread out the beam from the light source to give a longer
initiating time in the chamber.
[0110] After spraying and polymerization, the system was allowed to
settle for half an hour before slow depressurization (.about.30
min) at the operating temperature. This slow depressurization
increased the number of particles collected on the scanning
electron microscopy (SEM) stub mounted inside the chamber. After
depressurization, samples were also taken from both the inside of
the chamber and the 0.2 .mu.m filter. The resulting particles were
examined using SEM to determine their size and morphology.
[0111] The poly(methacrylated sebacic anhydride) (PMSA) particles
were also viewed using a fluorescence microscope (data not shown).
The Rose Bengal photoinitiator is a fluorescent dye for the
microparticles, with an excitation peak at 540 nm and an emission
band between 550-600 nm, so the distribution of photoinitiator in
the polymerized particles can be visibly characterized.
[0112] Polymerization of the multifunctional anhydride monomers
during the particle processing was confirmed through Fourier
transform infrared (FTIR) spectra of the particles compared to the
oligomer (FIG. 4). The peak at 1635 cm.sup.-1 is assigned to the
carbon-carbon double bond stretching in MSA and is largely reduced
in relative intensity in the spectrum of (PMSA). The reduction of
the peak to immeasurable levels further suggests nearly complete
reaction possibly due to added mobility from the solvent.
[0113] FIG. 5 is a scanning electron microscopy (SEM) photograph of
the poly (methacrylated sebacic anhydride) microparticles magnified
2200 times. These particles were sprayed as described above with a
nozzle consisting of a 100 .mu.m stainless steel capillary tube and
an operating temperature of 37.degree. C. The particles, although
not perfectly spherical, exhibit a round or substantially spherical
morphology with diameters ranging from 5 to 15 .mu.m. This narrow
size distribution is an important advantage of this processing
technique since many applications of polymer microparticles require
a narrow size distribution, especially biomedical applications
where the body may absorb or reject the particles based on their
size. The size distribution can also control the release
kinetics.
[0114] Cloud Point Measurements
[0115] Cloud point experiments were performed to measure the
solubility of the monomer solution in CO.sub.2 at the initial
operating conditions of 8.5 MPa and 37.degree. C. A 3 L view cell
was injected with a methylene chloride/MSA solution and a pump
system insured complete mixing. At a constant temperature of
37.degree. C., the cell was then pressurized with CO.sub.2 using a
hand pump up to .about.95 bar. Next, the cell was slowly
depressurized and the pressure at which the monomer solution
mixture became visibly insoluble was recorded. The process was
repeated three times for each MSA solution concentration and the
cloud point pressures were averaged. These results are shown in
FIG. 6.
[0116] FIG. 6 shows the cloud point pressure for this system, which
is far below the initial operating pressure implying that the
monomer solution is in a gaseous state at the initial operating
conditions of 37.degree. C. and 8.5 MPa. If the monomer remains in
a gaseous state during photopolymerization, this would
significantly dilute the monomer concentration and decrease the
rate of polymerization and the probability of microparticle
formation during the exposure time. A rapid polymerization rate and
a low gel point conversion are desired for this system because the
exposure time after atomization is short (4.times.10.sup.-2 sec).
To circumvent this problem, the operating temperature was lowered
to 25.degree. C. such that the operating pressure was below the
cloud point. After this adjustment, the monomer polymerized in the
chamber and particles with a high C.dbd.C conversion were
obtained.
[0117] FIG. 7 is an SEM micrograph of PMSA particles magnified 2000
times that were precipitated from a 5 wt % MSA solution using an
ultrasonicated atomizing nozzle and an operating temperature of
25.degree. C. Consistent product particles with the same size
(again 5 to 15 .mu.m) and morphology were formed at these operating
conditions. Although not spherical, the particle morphology is
strikingly similar to the cusped surfaces formed in low pressure
PCA with linear polymers. These particle surface features are
speculated to be a result of slow drying or surface-only
polymerization. A surface-only polymerization would likely result
in these flat, petal-like particles because polymerization would
occur on one face of the droplet which might give the "dark" side
of the droplet time diffuse into the CO.sub.2 phase.
Monomer-solvent solubilities and operating conditions may be
manipulated to obtain solid, spherical particles.
[0118] FIG. 8 shows SEM micrographs of PMSA particles prepared from
5% (FIG. 8A), 10% MSA (FIG. 8B) and 20% MSA (FIG. 8C). The
experiment was performed at a temperature of 37.degree. C. and a
pressure of 85 bar. Notice the particle size increases with
increasing concentration of MSA. The same magnification is used for
all micrographs of FIG. 8 (500 times).
[0119] Triacylate Polymerization
[0120] Particles of triacrylate were also formed using the system
as described above, using the following two compositions:
[0121] A: 10% 1,1,1-trimethylol propane triacrylate, 10% (by
monomer weight) DMPA photoinitiator, 90% methylene chloride.
[0122] B. 15% triacrylate, 6% (by monomer weight) DMPA
photoinitiator, 85% methylene chloride.
[0123] The experiments were carried out using carbon dioxide as the
antisolvent, using a pressure of 85 bar and a temperature of
37.degree. C.
[0124] FIG. 9A shows a SEM micrograph of particles formed using
condition A above at 10,000 times magnification. FIG. 9B shows a
SEM micrograph of particles formed using condition B above at
10,200 times magnification.
[0125] FIG. 10A shows a SEM micrograph of particles formed using
condition A above at 5,200 times magnification. FIG. 10B shows a
SEM micrograph of particles formed using condition B above at 5,100
times magnification.
[0126] Copolymer Formation
[0127] Copolymers of methacrylated sebacic anhydride (MSA) and
polylactic acid (PLA) were formed using the methods of the
invention using the following two compositions:
[0128] A: 2.5% MSA, 2.5% PLA, 95% methylene chloride, 20% (by
monomer weight) DMPA photoinitiator.
[0129] B. 5% MSA, 5% PLA, 90% methylene chloride, 20% (by monomer
weight) DMPA photoinitiator.
[0130] The experiments were carried out using carbon dioxide as the
antisolvent, using a pressure of 85 bar and a temperature of
37.degree. C.
[0131] The SEM of particles from system B are shown in FIG. 11 at
4700 times magnification.
[0132] Hydrophobic Ion Pairing
[0133] The drug tacrine, given to sufferers of Alzheimers disease,
has been encapsulated homogeneously into these microparticles and
the release behavior of the drug has been studied.
[0134] A 0.1 M aqueous solution of dodecyl sulfate, sodium salt
(SDS) was prepared. In addition, a 5-mg/ml aqueous solution of
Tacrine was also prepared. 2
9-amino-1,2,3,4-tetrahydroacridine (Tacrine)
[0135] Tacrine has one charged site available, and therefore
requires a one to one pairing to a surfactant. The appropriate
volumes of each solution were combined to obtain this
stoichiometric ratio of SDS molecules to Tacrine molecules. The
solution was then mixed vigorously for approximately 2 minutes. A
cloudy solution results from the ion-paired precipitate and aqueous
phases. Centrifuging the solutions for approximately 15 minutes at
6000 rpm separates the precipitate from the aqueous phase, allowing
the aqueous phase to be removed easily. The wet precipitate was
then dried under vacuum for 24 hrs before use in any further
experimentation.
[0136] Fluorescence imaging allowed the particles to be examined
using a different technique since the Rose Bengal initiator is also
a fluorescent material. FIG. 12 is a fluorescence microscopy image
of PMSA microparticles encapsulated with tacrine magnified 20
times. Although not perfectly spherical, these particles exhibit a
relatively round shape. From the fluorescence images of the
particles, Rose Bengal appears to be evenly distributed on the
particle surface. This image suggests that the Rose Bengal is
dispersed throughout the particles, which may provide many
initiation sites for a given particle. Many initiation sites could
result in a non-uniform surface, where particle formation is
dominated by nucleation and growth, rather than by atomization.
[0137] Drug Release Protocol
[0138] Approximately 2.5 mg of PMSA particles were placed into a
1.5 ml-capacity plastic centrifuge vial and filled with
phosphate-buffered saline (PBS) (pH=7.4) at 37.degree. C. The
centrifuge tube was shaken to disperse the particles throughout the
solution and placed in a 37.degree. C. temperature bath for 3
minutes. The tube was then immediately centrifuged, and the buffer
was drawn off and analyzed for drug concentration. The centrifuge
tube was refilled with 37.degree. C. PBS, and the cycle was
repeated for about 3 hours. The tubes remained in the temperature
bath for 3 minutes for the first hour, and 5-10 minutes for the
remainder of the time data was collected. UV-Vis spectrophotrometry
(Model 8452, Hewlett Packard) was used to determine the
concentration of tacrine (absorbance was measured at 322 nm) in
experiment samples.
[0139] Drug release experiments illustrate the release properties
of the microparticles formed in the photopolymerization PCA
process. In all studies almost all of the particles degraded within
2 hours and this time scale is consistent with what can be
calculated from the degradation kinetic constant of PMSA
investigated by Muggli et al (Muggli, D. S. et al. (1999) J.
Biomed. Mater. Res. 46:271-278). A 5-15 .mu.m particle would
degrade in about an hour, according to the calculated kinetic
constant, but since oligomerized MSA was used, a longer degradation
time should be expected. FIG. 13 shows the release profile of
tacrine (absorbance measured at 322 nm (upper data)) and Rose
Bengal (absorbance measured at 550 nm (lower data)) from the
particles into PBS buffer. The tacrine release profile follows the
curve for surface erosion of a sphere as expected. This profile
concurs with SEM photographs that show that the particles have a
round shape. The shape of the release profile of Rose Bengal
indicates there is some influence of diffusion in the release. The
Rose Bengal release profile is also affected by photobleaching of
particles in room light. This may explain the lag time at the
beginning of the Rose Bengal release in FIG. 13.
[0140] Release of Particles from an MSA Matrix
[0141] Crosslinked particles containing Rose Bengal were
incorporated in a MSA matrix and polymerized into disks. Rose
Bengal was also homogeneously incorporated into another set of MSA
disks and polymerized. Advantages of using a particle-polymer
composite include multi-mode degradation and release possible
through the use of different biocompatible polymers and drugs, ease
of control of surface degradation, and in one particular
application, bone growth is facilitated by the resulting porous
structures.
[0142] Disk samples were 0.25 to 0.3 grams, 13 mm diameter and 1.5
mm thick. Disks were placed in 10 ml of PBS buffer at 37.degree. C.
to monitor the degradation.
[0143] FIG. 14 shows release behavior for both sets of disks as a
function of the mass fraction of the disks that had degraded. The
absorbance of Rose Bengal was measured at 550 nm, and the disks
were weighed to monitor the degradation. FIG. 14 shows a linear
relationship between degradation and release for both homogeneous
and heterogeneous disks.
[0144] One particular application of the particles is in bone
cements. Particles release drugs over time as the bone cement
(formed of degradable material) degrades and bone regrows.
Diacrylated Poly(ethylene glycol) (PEGDA) Polymerization
[0145] Particles of Poly(ethylene glycol) Diacrylate (PEGDA) were
formed using the methods of the invention using the PEG1000DA
monomer (PEG1 kDA, Monomer-Poymer and Dajac Laboratories,
Southhampton, Pa.). The experiments were carried out using
methylene chloride as the solvent and carbon dioxide as the
antisolvent, using a pressure of 85 bar and a temperature of
35.degree. C. The photoinititator used was
2,2-dimethoxy-2-phenlyacetophenone (DMPA, Ciba Geigy, Tarrytown,
N.Y.). 3
Poly(ethyleneglycol) Diacrylate
[0146] FIG. 15 shows that the double bond conversion varied with
the residence time of the particles in the apparatus. The solution
used in these experiments had 25% PEG1000DA and 2% (by monomer
weight) DMPA. The light intensity was 6 W/cm.sup.2. The value of
the double bond conversion indicates the extent of cross-linking.
The particles corresponding to approx. 20% conversion were
agglomerated. The residence time of the particles in the apparatus
was varied by manipulating the combination of the antisolvent and
solution flow rates.
[0147] The double bond conversion of the particles was determined
through FTIR analysis compared to that of the PEG1000DA monomer.
Particles were combined with mineral oil and crushed uniformly
using a mortar and pestle to create a smooth paste of oil and
crushed particles. A spectrum (64 scans averaged) was acquired in
the mid-IR region of the resulting paste sandwiched between two KBR
crystals. The same technique was used to make samples of the
PEG1000DA monomer. Fractional conversion was calculated by
subtracting the area of the carbon-carbon double bond peak of the
reacted sample from that of the unreacted monomer and then dividing
by the peak area of the unreacted monomer.
[0148] Experiments were also performed to assess the influence of
the amount of photoinitiator on the process. FIGS. 16A-D,
respectively, show SEM micrographs of the particles formed with
photoinitiator concentrations of 1, 1.5, 2, and 4 percent by weight
of the monomer (25 wt % monomer, 6 W/cm.sup.2 incident light
intensity). The solution flow rate was 1 ml/min. As shown in FIGS.
16B and 16C, The 1.5 and 2 wt % initiator samples formed very
smooth, spherical particles with diameters in the range 0.5 to 50
microns. In contrast, the samples processed with 1 and 4 wt %
initiator generally look like agglomerated particles with little
uniformity in structure, as is shown in FIGS. 16A and 16D. In the
case of the 1 wt % DMPA photoinitiator sample, the rate of
polymerization was likely insufficient to produces significant
conversion that would prevent particle agglomeration. In the case
of the 4 wt % DMPA photoinitiator sample, an excess of initiator
likely led to decreased double bond conversion (Lovell, L. G.;
Berchtold, K. A.; Elliott, J. E.; Lu, H.; Bowman, C. N. Polym. Adv.
Technol. 2001, 12, 335-345. and Kloosterboer, J. Adv. Poly. Sci.
1988, 84, 1-61).
[0149] The effect of varying light intensity during the process was
also examined. FIGS. 17A-C, respectively, show particles prepared
with average incident light intensities of 3, 4, and 6.25
W/cm.sup.2. The light intensities were measured using an EFOS
Novacure Radiometer, Mississauga, Ontario, Canada. Other
experimental conditions were 25 wt % monomer and 2 wt %
photoinitiator relative to monomer. As shown in FIG. 17A, samples
prepared at the low light intensity (3 W/cm.sup.2) show little to
no structure, judging from the large agglomerated masses and
minimal evidence of particle formation. The medium intensity
experiment (4 W/cm.sup.2) exhibits an increased amount of particle
formation but still significant numbers of aggregates (FIG. 17B).
The high intensity experiment (6.25 W/cm.sup.2) produced nicely
formed spheres in size ranges from 1 to 50 microns (FIG. 17C).
These results suggest that greater light intensity is more
effective in producing crosslinked spherical particles in the
limited time that the particles have to polymerize before they will
interact with other particles in the high-pressure chamber due to
fluid mixing that occurs in the process.
[0150] Calculation of Mesh Sizes for PEGDA Networks
[0151] The mesh sizes were calculated statistically assuming an
ideal network and a monodisperse monomer molecular weight. For the
starting macromer, PEG (--C--C--O--).sub.n, the PEG based chain had
a bond length of 4.34 Angstroms for 2 C--O bonds at 1.4 angstroms
each and one C--C bond at 1.54 Angstroms. For 100% double bond
conversion, the length one side of the mesh was estimated as n*4.34
Angstroms. The length of the other side of the mesh was estimated
to be the same as the kinetic chain length or C--C bond length. For
an ideal network (100% conversion, no cyclization) the mesh size
was estimated as the average of these 2 lengths.
[0152] For example, for 50% conversion, it was estimated that there
will be a kinetic chain link every other monomer unit. The length
of one side of the mesh was estimated as twice the sum of the bond
lengths of the starting monomer multiplied by the number of monomer
units. The length of the other side of the mesh can be estimated as
the C--C bond length as before.
[0153] For a completely reacted system (100% double bond
conversion) the calculated mesh sizes were 12.0 Angstroms for
PEG200DA, 30 Angstroms for PEG 600DA, and 50.4 Angstroms for
PEG1000DA. For a partially reacted system with 50% double bond
conversion the calculated mesh sizes were 23.3 Angstroms for
PEG200DA, 59.4 Angstroms for PEG 600DA, and 100.0 Angstroms for
PEG1000DA.
[0154] Encapsulation Efficiencies
[0155] The encapsulation efficiency measured for tacrine in MSA was
92.+-.4%, that in PEG10000DA was 31.+-.7%, that for PEG600DA was
26.+-.7%, and that for PEG200DA was 85.+-.24%.
[0156] Although the description above contains many specificities,
these should not be construed as limiting the scope of the
invention but as merely providing illustrations of some of the
presently preferred embodiments of the invention. For example,
antisolvents other than carbon dioxide may be used. Other
embodiments and uses are readily apparent to one of ordinary skill
in the art without undue experimentation. Thus, the scope of the
invention should be determined by the appended claims and their
legal equivalents, rather than by the examples given. All
references cited herein are hereby incorporated by reference to the
extent not inconsistent with the disclosure herewith.
* * * * *