Title:  Multiblock biodegradable hydrogels for drug delivery and tissue treatment

United States Patent:  6,639,014

Issued:  October 28, 2003

Inventors:  Pathak; Chandrashekhar P. (Austin, TX); Barman; Shikha P. (Bedford, MA); Philbrook; C. Michael (Boston, MA); Sawhney; Amarpreet S. (Lexington, MA); Coury; Arthur J. (Boston, MA); Avila; Luis Z. (Arlington, MA); Kieras; Mark T. (Menlo Park, CA)

Assignee:  Focal, Inc. (Lexington, MA)

Appl. No.:  114722

Filed:  April 2, 2002

Abstract

Gel-forming macromers including at least four polymeric blocks, at least two of which are hydrophobic and at least one of which is hydrophilic, and including a crosslinkable group are provided. The macromers can be covalently crosslinked to form a gel on a tissue surface in vivo. The gels formed from the macromers have a combination of properties including thermosensitivity and lipophilicity, and are useful in a variety of medical applications including drug delivery and tissue coating.

SUMMARY OF THE INVENTION

Macromers are provided which are capable of gelling in an aqueous solution. In one embodiment, the macromers include at least four polymeric blocks, at least one of which is hydrophilic and at least two of which are hydrophobic, and include a crosslinkable group. The polymer blocks may be selected to provide macromers with different selected properties. The macromers can be covalently crosslinked to form a gel on a tissue surface in vivo. The gels formed from the macromers have a combination of properties including thermosensitivity and lipophilicity, and are useful in a variety of medical applications including drug delivery and tissue coating.

DETAILED DESCRIPTION OF THE INVENTION

Macromers are provided which are crosslinkable to form hydrogels which are useful as matrices for controlled drug delivery. In a preferred embodiment, biodegradable macromers are provided in a pharmaceutically acceptable carrier, and are capable of crosslinking, covalently or non-covalently, to form hydrogels which are thermoresponsive. A biologically active agent may be incorporated within the macromer solution or in the resulting hydrogel after crosslinking. The hydrogels have properties, such as volume and drug release rate, which are dependent upon temperature. The hydrogels may be formed in situ, for example, at a tissue site, and may be used for for controlled delivery of bioactive substances and as tissue coatings. The macromers used to form the hydrogels may be fabricated with domains having specific properties including selected hydrophobicity, hydrophilicity, thermosensitivity or biodegradability, and combinations thereof.

Macromers

The macro-monomers ("macromers") which are ionically or covalently crosslinkable to form hydrogels preferably consist of a block copolymer. The macromers can be quickly polymerized from aqueous solutions. The macromers are advantageously capable of thermoreversible gelation behavior, and preferably may be polymerized in a solution state or in a gel state. The macromers are defined as including a hydrophilic block capable of absorbing water, and at least one block, distinct from the hydrophilic block, which is sufficiently hydrophobic to precipitate from, or otherwise change phase while within, an aqueous solution, consisting of water, preferably containing salts, buffers, drugs or polymerizing reagents, at temperatures within or near the physiologically compatible range, for example 0 to 65^oC. The hydrophilic block optionally may be an amphiphilic block. The macromer may include more than one of the same or different hydrophilic or hydrophobic region. Preferably, the macromers include at least three blocks, or more preferably four blocks.

The block copolymers may be linear (AB, ABA, ABABA or ABCBA type), star (AnB or BAnC, where B is at least n-valent, and n is 3 to 6) or branched (multiple A's depending from one B). In these formulae, either A or B may be the hydrophilic block, and the other the amphipathic or hydrophilic block, and the additional block C may be either.

In another embodiment, the macromer includes at least four covalently-linked polymeric blocks, wherein: at least one, or in another embodiment, at least two blocks are hydrophilic, and the hydrophilic blocks individually have a water solubility of at least 1 gram/liter; at least two blocks are sufficiently hydrophobic to aggregate to form micelles in an aqueous continuous phase; and the macromer further includes at least one crosslinkable group. The crosslinkable groups optionally may be separated by at least one degradable linkage capable of degrading under physiological conditions. In one embodiment, at least one hydrophobic block may be separated from any reactive group by at least one hydrophilic block.

The macromer further may include five total blocks having the same or different properties such as thermal sensitivity, hydrophilicity or hydrophobicity. Each block also may have a combination of properties. For example, a block may be hydrophilic and also thermosensitive. Additionally, the multiblock macromer may include chemically distinct blocks or may incorporate more than one of the same identical block. The macromer is fabricated with a structure and with properties suitable for different applications. For example the macromer may include a central block of dimer fatty acid which includes central hydrocarbon chain of about 30 carbon atoms and two terminal carboxy groups which are esterified with a thermosensitive poloxamer, such as Pluronic L1050. This central molecule further is polylactated at each hydroxy terminus, and end capped with acryloyl chloride. An another embodiment is a poloxamer including polyhydroxy groups polymerized on each end, and wherein the molecule is end capped at each end with a reactive group such as an acrylate or a secondary isocyanate.

The configuration of the macromers may be preselected depending on the use of the macromer. The macromers may include at least two hydrophobic blocks, separated by a hydrophilic block. The macromers also may be fabricated with a crosslinkable group which is separated by a degradable group from any other crosslinkable group. One preferred embodiment is wherein the dry macromer absorbs at least about 10% in weight of water. The molecular weight of the macromer preferably is at least 1000 Daltons, or optionally is at least 2000 Daltons, or in an alternative embodiment, at least 4000 Daltons.

In a preferred embodiment, the macromer includes at least one thermally sensitive region, and an aqueous solution of the macromer is capable of gelling either ionically and/or by covalent crosslinking to produce a hydrogel with a temperature dependent volume. This permits the rate of release of a drug incorporated in the hydrogel to change depending upon the volume of the hydrogel. Useful macromers are those which are, for example, capable of thermoreversible gelation of an aqueous solution of the macromer at a concentration of at least 2% by weight, and wherein the gelation temperature is between about 0^oC. and about 65^oC. The macromer also may have a phase transition temperature in the range of 0 to 100^oC., and wherein the transition temperature is affected by the ionic composition of an aqueous solution of the macromer or the concentration of macromer in the aqueous solution.

The macromers may be formed by modification of materials and methods described in the prior art. Macromers including a central chain of polyethylene glycol, with oligomeric hydroxy acid at each end and acrylic esters at the ends of the hydroxy acid oligomer are described in Sawhney A. S. et al., Macromolecules, 26: 581 (1993); and PCT WO 93/17669 by Hubbell J. A. et al., the disclosures of which are incorporated herein by reference. U.S. Pat. No. 5,410,016 to Hubbell et al., the disclosure of which is incorporated herein by reference, discloses that biodegradable, water-soluble macromers can be crosslinked in situ to form barrier coatings and depots or matrices for delivery of biologically active agents such as therapeutic drugs. In addition to the materials and methods described in U.S. Pat. No. 5,410,016, materials and methods described by Dunn (U.S. Pat. No. 4,938,763), DeLuca (U.S. Pat. Nos. 5,160,745; and 4,818,542), Zalipsky (U.S. Pat. No. 5,219,564), Cohn (U.S. Pat. No. 4,826,945), Nair (U.S. Pat. Nos. 5,078,994; and 5,429,826), the disclosures of which are incorporated herein by reference, are useful to form the macromers described herein.

For example, the macromer may include a poloxamer backbone extended with hydrophobic materials, such as oligolactate moieties, which serve as the biodegradable segment of the molecule, wherein the PEO-PPO-PEO-lactate copolymer is terminated by acrylate moieties. The materials can be combined with, then delivered and photopolymerized in situ, onto target organs to conform to a specific shape.

The macromers and hydrogels formed therefrom preferably are biocompatible, preferably not causing or enhancing a biological reaction when implanted or otherwise administered within a mammal. The macromers, and any breakdown products of the hydrogels or macromers, preferably are not significantly toxic to living cells, or to organisms. The hydrogels also may have liquid crystalline properties for example at high concentration, which are useful in controlling the rate of drug delivery. Ionic properties can be provided in the backbone of the macromers, conferring the further property of control of delivery and/or physical state by control of the ionic environment, including pH, of the macromer or gel. In one embodiment, the critical ion composition is the hydrogen ion concentration. For example, when a poloxamine, such as a Tetronic surfactant, is used as the core of the macromer, then the resulting macromer has the ionic groups (amines) in the core, and the macromers' ability to gel upon changes in temperature is affected by the pH of the solution.

Thermosensitive Regions

The macromers may be provided with one or more regions which have properties which are thermoresponsive. As used herein, thermoresponsiveness is defined as including properties of a hydrogel, such as volume, transition from a liquid to a gel, and permeability to biologically active agents, which are dependent upon the temperature of the hydrogel. In one embodiment, the macromers are capable of reversible gelation which is controlled by temperature. The reversible gel further optionally may be crosslinked in situ into an irreversibly and covalently crosslinked gel. This permits the macromer to be applied reliably in surgical applications on a specific area of tissue without running off or being washed off by body fluids prior to gelation or crosslinking.

In one preferred embodiment, the macromers are capable of gelling thermoreversibly, for example, due to the content of poloxamer regions. Since gelling is thermoreversible, the gel will dissipate on cooling. The macromers may further include crosslinkable groups which permit the gel to be further covalently crosslinked for example by photopolymerization. After crosslinking, the gels are irreversibly crosslinked. However, they retain other significant thermoresponsive properties, such as changes in volume and in permeability.

By appropriate choice of macromer composition, hydrogels can be created in situ which have thermosensitive properties, including volume changes and drug release which are dependent upon temperature, which can be used to control drug delivery from the hydrogel. Control of drug delivery can be further controlled by adjustment of properties such as hydrophobicity of amphiphilic or other regions in the gel. Change in volume of the hydrogel may readily be measured by examination of macroscopic unrestrained samples during temperature excursions. Changes in excess of 100% in volume may be obtained with hydrogels formed from the macromers, such as an acrylate-capped polyglycolide-derivatized poloxamer of about 30% PPO (polypropylene oxide) content, the expansion occurring gradually on change of the temperature from about 0^oC. to body temperature (37^oC.). Changes of more than 5% in any linear dimension may be effective in altering the release rate of a macromolecular drug

The macromers preferably include thermogelling macromers, such as "poloxamers", i.e., poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) ("PEO-PPO-PEO"), block copolymers. Aqueous polymeric solutions of poloxamers undergo microphase transitions at an upper critical solution temperature, causing a characteristic gel formation. This transition is dependent on concentration and composition of the block copolymer. Alexandridis et al., Macromolecules, 27:2414 (1994). The segmental polyether portion of the molecule gives water solubility and thermosensitivity. The material also advantageously have been demonstrated to be biocompatible.

For example, the macromer may include a poloxamer backbone extended with hydrophobic materials, such as oligolactate moieties, which serve as the biodegradable segment of the molecule, wherein the PEO-PPO-PEO-lactate copolymer is terminated by acrylate moieties. The materials can be combined with a bioactive agent, then delivered and photopolymerized in situ. In addition to poloxamer cores, meroxapols, such as "reversed Pluronics" (PPO-PEO-PPO copolymers) and poloxamines, such as Tetronic.TM. surfactants, may be used.

Other polymer blocks which may be provided in the monomer which are capable of temperature dependent volume changes include water soluble blocks such as polyvinyl alcohol, polyvinyl-pyrrolidone, polyacrylic acids, esters and amides, soluble celluloses, peptides and proteins, dextrans and other polysaccharides. Additionally, polymer blocks with an upper critical point may be used, such as other polyalkylene oxides, such as mixed polyalkylene oxides and esters, derivatized celluloses, such as hydroxypropylmethyl cellulose, and natural gums such as konjac glucomannan.

In another embodiment, the macromer is defined as having an optically anisotropic phase at a concentration at or below the maximal solubility of the macromer in an aqueous solution, at a temperature between about 0 and 65^oC.

Crosslinkable Groups

The macromers preferably include crosslinkable groups which are capable of forming covalent bonds with other compounds while in aqueous solution, which permit crosslinking of the macromers to form a gel, either after, or independently from thermally dependent gellation of the macromer. Chemically or ionically crosslinkable groups known in the art may be provided in the macromers. The crosslinkable groups in one preferred embodiment are polymerizable by photoinitiation by free radical generation, most preferably in the visible or long wavelength ultraviolet radiation. The preferred crosslinkable groups are unsaturated groups including vinyl groups, allyl groups, cinnamates, acrylates, diacrylates, oligoacrylates, methacrylates, dimethacrylates, oligomethoacrylates, or other biologically acceptable photopolymerizable groups.

Other polymerization chemistries which may be used include, for example, reaction of amines or alcohols with isocyanate or isothiocyanate, or of amines or thiols with aldehydes, epoxides, oxiranes, or cyclic imines; where either the amine or thiol, or the other reactant, or both, may be covalently attached to a macromer. Mixtures of covalent polymerization systems are also contemplated. Sulfonic acid or carboxylic acid groups may be used.

Preferably, at least a portion of the macromers will have more than one crosslinkable reactive group, to permit formation of a coherent hydrogel after crosslinking of the macromers. Up to 100% of the macromers may have more than one reactive group. Typically, in a synthesis, the percentage will be on the order of 50 to 90%, for example, 75 to 80%. The percentage may be reduced by addition of small co-monomers containing only one active group. A lower limit for crosslinker concentration will depend on the properties of the particular macromer and the total macromer concentration, but will be at least about 3% of the total molar concentration of reactive groups. More preferably, the crosslinker concentration will be at least 10%, with higher concentrations, such as 50% to 90%, being optimal for maximum retardation of many drugs. Optionally, at least part of the crosslinking function may be provided by a low-molecular weight crosslinker. When the drug to be delivered is a macromolecule, higher ranges of polyvalent macromers (ie., having more than one reactive group) are preferred. If the gel is to be biodegradable, as is preferred in most applications, then the crosslinking reactive groups should be separated from each other by biodegradable links. Any linkage known to be biodegradable under in vivo conditions may be suitable, such as a degradable polymer block. The use of ethylenically unsaturated groups, crosslinked by free radical polymerization with chemical and/or photoactive initiators, is preferred as the crosslinkable group.

The macromer may also include an ionically charged moiety covalently attached to the macromer, which optionally permits gellation or crosslinking of the macromer.

Hydrophobic Regions

The macromers further may include hydrophobic domains. The hydrophobicity of the gel may be modified to alter drug delivery or three dimensional configuration of the gel. Amphiphilic regions may be provided in the macromers which in aqueous solution tend to aggregate to form micellar domain, with the hydrophobic regions oriented in the interior of these domains (the "core"), while the hydrophilic domains orient on the exterior ("the corona"). These microscopic "cores" can entrap hydrophobic drugs, thus providing microreservoirs for sustained drug release. Kataoka K., et al., J. Controlled Release, 24:119 (1993). The fundamental parameter of this supramolecular assemblage of amphiphilic polymers in aqueous solution is the Critical Micellar Concentration (CMC), which can be defined as the lowest concentration at which the dissolved macromolecules begin to self-assemble. By selection of the hydrophilic and other domains, drug delivery can be controlled and enhanced.

In one embodiment, the macromers are provided with at least one hydrophobic zone, and can form micelles including a region in which hydrophobic materials will tend to bind and thus tend to reduce escape of the drug from the formed gel. The hydrophobic zone may be enhanced by addition of materials, including polymers, which do not contribute to the formation of a gel network but which segregate into such zones to enhance their properties, such as a fatty acid, hydrocarbon, lipid, or a sterol.

The ability of the macromonomers in one embodiment to form micellar hydrophobic centers not only allows the controlled dissolution of hydrophobic bioactive compounds but also permits the hydrogel to selectively "expand" and "contract" around a transition temperature. This provides an "on-off" thermocontrol switch which permits the thermally sensitive delivery of drugs.

The cell membrane is composed of a bilayer with the inner region being hydrophobic. This bilayer is believed to have a fluid and dynamic structure, i.e., hydrophobic molecules can move around in this structure. A hydrophobic tail incorporated in a macromer can diffuse into this lipid bilayer and result in the rest of the macromonomer (thus, the hydrogel) to better adhere to the tissue surface. The choice of molecular group to be used as hydrophobic tail is guided by the fatty acid composition of the bilayer to assure minimum perturbation of the bilayer structure. Examples of suitable groups are fatty acids, diacylglycerols; molecules from membranes such as phosphatidylserine, and polycyclic hydrocarbons and derivatives, such as cholesterol, cholic acid, steroids and the like. Preferred hydrophobic groups for this purpose are normal constituents of the human body These molecules will be used at a low concentration relative to native molecules in the membrane.

Use of macromers carrying one or more hydrophobic groups can improve the adherence of a hydrogel to a biological material by anchoring a segment of the hydrogel in the lipid bilayer. This anchoring will cause minimal perturbation to the underlying tissue because the insertion of the fatty acid terminal of the macromer into the lipid membrane involves purely physical interaction. The macromer may be applied by using a prewash of the surface with these molecules, in effect `preparing` the surface for coupling and/or an in situ photopolymerization of a mixture of these lipid-penetrating molecules with the crosslinkable macromers.

The hydrophobic region may include oligomers of hydroxy acids such as lactic acid or glycolic acid, or oligomers of caprolactone, amino acids, anhydrides, orthoesters, phosphazenes, phosphates, polyhydroxy acids or copolymers of these subunits. Additionally the hydrophobic region may be formed of poly(propylene oxide), poly(butylene oxide), or a hydrophobic non-block mixed poly(alkylene oxide) or copolymers thereof. Biodegradable hydrophobic polyanhydrides are disclosed in, for example, U.S. Pat. Nos. 4,757,128, 4,857,311, 4,888,176, and 4,789,724, the disclosure of which is incorporated by reference herein. Poly L-lactide, or poly D,L-lactide for example may be used. In another embodiment the hydrophobic region may be a polyester which is a copolymer of poly(lactic-co-glycolic) acid (PLGA).

The macromer also may be provided as a mixture including a hydrophobic material non-covalently associated with the macromer, wherein the hydrophobic material is, for example, a hydrocarbon, a lipid, a fatty acid, or a sterol.

Hydrophilic Regions.

Water soluble hydrophilic oligomers available in the art may be incorporated into the biodegradable macromers. The hydrophilic region can be for example, polymer blocks of poly(ethylene glycol), poly(ethylene oxide), poly(vinyl alcohol), poly(vinylpyrrolidone), poly(ethyloxazoline), or polysaccharides or carbohydrates such as hyaluronic acid, dextran, heparan sulfate, chondroitin sulfate, heparin, or alginate, or proteins such as gelatin, collagen, albumin, ovalbumin, or polyamino acids

Biodegradable Regions

Biodegradable molecules or polymers thereof available in the art may be incorporated into the macromers. The biodegradable region is preferably hydrolyzable under in vivo conditions. In some embodiments, the different properties, such as biodegradability and hydrophobicity or hydrophilicity, may be present within the same region of the macromer.

Useful hydrolyzable groups include polymers and oligomers of glycolide, lactide, epsilon-caprolactone, other hydroxy acids, and other biologically degradable polymers that yield materials that are non-toxic or present as normal metabolites in the body. Preferred poly(alpha-hydroxy acids) are poly(glycolic acid), poly(DL-lactic acid) and poly(L-lactic acid). Other useful materials include poly(amino acids), polycarbonates, poly(anhydrides), poly(orthoesters), poly(phosphazines) and poly(phosphoesters). Polylactones such as poly(epsilon-caprolactone), poly(delta-caprolactone), poly(delta-valerolactone) and poly(gamma-butyrolactone), for example, are also useful. The biodegradable regions may have a degree of polymerization ranging from one up to values that would yield a product that was not substantially water soluble. Thus, monomeric, dimeric, trimeric, oligomeric, and polymeric regions may be used.

Biodegradable regions can be constructed from polymers or monomers using linkages susceptible to biodegradation, such as ester, peptide, anhydride, orthoester, phosphazine and phosphoester bonds. The time required for a polymer to degrade can be tailored by selecting appropriate monomers. Differences in crystallinity also alter degradation rates. For relatively hydrophobic polymers, actual mass loss only begins when the oligomeric fragments are small enough to be water soluble. Thus, initial polymer molecular weight influences the degradation rate.

Therapeutic Applications

Biodegradable, temperature responsive hydrogels can be formed in situ and may be use in a variety of therapeutic applications including surgical applications. In one embodiment the gels can be applied topically to the skin to treat a variety of conditions such as abrasion, keratoses, inflammatory dermatoses, injury resulting from a surgical procedure, and disturbed keratinization. The hydrogels may include therapeutic agents such as antibiotics, or antifungals for the localized treatment of different skin conditions.

Macromers which are liquid at room temperature and gel at body temperature, such as macromers including a Pluronic.TM. poloxamer, may be used in treatment of burns and other external injuries. The hydrogels are useful in burn applications, since the hydrogel layer formed on the skin provides local or transdermal delivery of drug to the burn site; maintains high moisture levels on severely burned sites, thus diminishing dehydration; adheres strongly to the damaged tissue, and is elastic, thus minimizing delamination and "peeling" of the hydrogel dressing; and absorbs exudate from the wound. Hydrogels may be selected which dissolve into components which are absorbable and non-toxic, which promote healing, and gel spontaneously and quickly on the burn site, prior to optional further crosslinking.

The macromers also may be applied to biological tissue, or on the surface of a medical device, to form hydrogels in a variety of surgical applications for the treatment of tissue or organs. The gel also may be applied between two surfaces, such as tissue surfaces, to adhere the surfaces. The hydrogels may be applied to tissue such as vascular tissue, for example for the treatment of restenosis of the arteries or in angioplasty procedures. A biologically active material may be provided in the gel optionally in the form of particles, microparticles, prodrug conjugates, or liposomes. The macromers may be designed such that the crosslinked gel changes in permeability in response to a change in temperature, ionic concentration or a change in pH, thereby altering the rate of drug release from the hydrogel.

Drug Delivery

The macromers may be crosslinked reversibly or irreversibly to form gels for controlled drug delivery applications. The composition and properties of the macromers can be selected and fabricated to produce hydrogels with desired drug delivery properties. The drug may be provided in the macromer solution prior to or after administration, and either before or after gel formation, depending on the macromer composition.

For example, the gels can be designed to have a selected rate of drug release, such as first order or zero order drug release kinetics. For specific drugs, such as peptides, the composition of the gel may be designed to result in pulsatile or mixed wave release characteristics in order to obtain maximum drug efficacy and to minimize side effects and tolerance development. Bae et al., Pharmaceutical Research, 8: 531 (1991).

The drug release profiles can be selected by the use of macromers and gels formed therefrom that respond to specific external stimuli such as ultrasound, temperature, pH or electric current. For example, the extent of swelling and size of these hydrogels can be modulated. Changes induced in the swelling directly correlate to the rate of release of the incorporated drugs. Through this, a particular release profile may be obtained. The hydrogels are preferably biodegradable so that removal is not required after administration or implantation.

The gels permit controlled drug delivery and release of a biologically active agent in a predictable and controlled manner locally at the targeted site where it is needed, when the tissue to be treated is localized. In other embodiments, the gels also can be used for systemic delivery.

A variety of therapeutic agents can be delivered using the hydrogels. Examples include synthetic inorganic and organic compounds, proteins and peptides, polysaccharides and other sugars, lipids, gangliosides, and nucleic acid sequences having therapeutic, prophylactic or diagnostic activities. Nucleic acid sequences include genes, antisense molecules which bind to complementary DNA to inhibit transcription, and ribozymes. The agents to be incorporated can have a variety of biological activities, such as vasoactive agents, neuroactive agents, hormones, anticoagulants, immunomodulating agents, cytotoxic agents, antibiotics, antivirals, antisense, antigens, and antibodies. Proteins including antibodies or antigens can also be delivered. Proteins are defmed as consisting of 100 amino acid residues or more; peptides are less than 100 amino acid residues. Unless otherwise stated, the term protein refers to both proteins and peptides. Examples include insulin and other hormones.

Specific materials include antibiotics, antivirals, antiinflammatories, both steroidal and non-steroidal, antineoplastics, anti-spasmodics including channel blockers, modulators of cell-extracellular matrix interactions including cell growth inhibitors and anti-adhesion molecules, enzymes and enzyme inhibitors, anticoagulants and/or antithrombotic agents, growth factors, DNA, RNA, inhibitors of DNA, RNA or protein synthesis, compounds modulating cell migration, proliferation and/or growth, vasodilating agents, and other drugs commonly used for the treatment of injury to tissue. Specific examples of these compounds include angiotensin converting enzyme inhibitors, prostacyclin, heparin, salicylates, nitrates, calcium channel blocking drugs, streptokinase, urokinase, tissue plasminogen activator (TPA) and anisoylated plasminogen activator (TPA) and anisoylated plasminogen-streptokinase activator complex (APSAC), colchicine and alkylating agents, and aptomers. Specific examples of modulators of cell interactions include interleukins, platelet derived growth factor, acidic and basic fibroblast growth factor (FGF), transformation growth factor .beta. (TGF .beta.), epidermal growth factor (EGF), insulin-like growth factor, and antibodies thereto. Specific examples of nucleic acids include genes and cDNAs encoding proteins, expression vectors, antisense and other oligonucleotides such as ribozymes which can be used to regulate or prevent gene expression. Specific examples of other bioactive agents include modified extracellular matrix components or their receptors, and lipid and cholesterol sequestrants.

Examples of proteins further include cytokines such as interferons and interleukins, poetins, and colony-stimulating factors. Carbohydrates include Sialyl lewis^x which has been shown to bind to receptors for selectins to inhibit inflammation. A "Deliverable growth factor equivalent" (abbreviated DGFE), a growth factor for a cell or tissue, may be used, which is broadly construed as including growth factors, cytokines, interferons, interleukins, proteins, colony-stimulating factors, gibberellins, auxins, and vitamins; further including peptide fragments or other active fragments of the above; and further including vectors, i.e., nucleic acid constructs capable of synthesizing such factors in the target cells, whether by transformation or transient expression; and further including effectors which stimulate or depress the synthesis of such factors in the tissue, including natural signal molecules, antisense and triplex nucleic acids, and the like. Exemplary DGFE's are vascular endothelial growth factor (VEGF), endothelial cell growth factor (ECGF), basic fibroblast growth factor (bFGF), bone morphogenetic protein (BMP), and platelet derived growth factor (PDGF), and DNA's encoding for them. Exemplary clot dissolving agents are tissue plasminogen activator, streptokinase, urokinase and heparin.

Drugs having antioxidant activity (i.e., destroying or preventing formation of active oxygen) may be provided in the hydrogel, which are useful, for example, in the prevention of adhesions. Examples include superoxide dismutase, or other protein drugs include catalases, peroxidases and general oxidases or oxidative enzymes such as cytochrome P450, glutathione peroxidase, and other native or denatured hemoproteins.

Mammalian stress response proteins or heat shock proteins, such as heat shock protein 70 (hsp 70) and hsp 90, or those stimuli which act to inhibit or reduce stress response proteins or heat shock protein expression, for example, flavonoids, may be provided in the hydrogel.

The macromers may be provided in pharmaceutical acceptable carriers known to those skilled in the art, such as saline or phosphate buffered saline. For example, suitable carriers for parenteral adminstration may be used.

Administration of Macromers

Modern surgical procedures which provide access to a variety of organs using minimally invasive surgical devices may be used to apply the macromers. Using techniques such as laparoscopy/endoscopy, it is possible to deposit a macromonomer solution at a localized site and subsequently polymerize it inside the body. This method of "on-site" polymerization offers unique advantages such as conformity to specific organs and adherence to underlying tissue. Hill-West J. L. et al., Obstetrics & Gynecology, 83:59 (1994). Catheter delivery systems available in the art also may be used as described, for example, in U.S. Pat. Nos. 5,328,471 and 5,213,580 to Slepian. The macromer also may applied during surgery conducted through the cannula of a trocar.

Formation of Microspheres

In one embodiment, the biodegrabable macromers are crosslinked, either reversibly or nonreversibly to form microspheres. As used herein, the term "microspheres" includes includes particles having a uniform spherical shape or an irregular shape, and microcapsules (having a core and an outer layer of polymer) which generally have a diameter from the nanometer range up to about 5 mm. In a preferred embodiment, the microspheres are dispersed in biocompatible, biodegradable hydrogel matrices. The microspheres are useful for controlled release and targeted delivery of drugs within the body.

The microspheres are formed in one embodiment by aggregation and subsequent polymerization of portions of the macromers which are similar in charge properties such as hydrophilicity. This results in a matrix which consists of spontaneously-assembled "nodes", which may be crosslinked covalently, and may be further covalently linked to hydrophilic bridges of the macromers to form a hydrogel.

When the macromer is amphiphilic and includes hydrophobic and hydrophilic domains, in an aqueous environment, at or above a certain concentration, the molecules to arrange themselves into organized structures called micelles, at the critical micellar concentration (CMC). These micelles can be of different shapes and sizes, though are generally spherical or elliptical shape. When the solution is water, the hydrophobic portions are at the center of the micelle while the hydrophilic tails orient themselves toward water. The interior core of a typical surfactant has a size from 10-30 Angstroms. Pluronic.TM. poloxamer based biodegradable macromers, as described in Example, undergo micellization in an aqueous environment with CMC values ranging between 0 and 5% (w/v). After photopolymerization and gelation, this micellar structure is preserved in the crosslinked gel. On a microscopic level, the gel contains micelles which are interconnected by covalent bonds to form the gel. These micellar domains or microspheres can be used for the controlled or sustained release of drugs. Controlled, pseudo-zero order release of small compounds such as chlorohexidine is possible from such hydrogels.

The hydrogel thus is formed in one embodiment by providing a solution of macromer in aqueous solution (with or without drug); "freezing" the micellar structure of the macromer by a chemical crosslinking via a chemical reaction; adding the drug to the crosslinked macromer if it has not been already added; and using the resultant dispersed composite, containing microspheres consisting of drug-attracting micellar cores, for drug delivery.

In addition to photopolymerization, crosslinking can be implemented by, for example, isocyanate-amine chemistry, or hydroxy- or aldehyde-amine chemistry, to freeze micellar structure. For example, isocyanate terminated poloxamer lactate diol can react in water to form crosslinked polyurethane based networks. This is an advantageous method of forming a drug delivery device for local or systemic delivery, because the formation of the delivery-controlling micropheres and the microsphere-confining gel is accomplished simultaneously, and may be accomplished at the site of delivery in a few seconds by photopolymerization.

In one embodiment the macromer includes PEO segments, and hydrophobic "ends" containing reactive groups, and the micellar domains are hydrophobic and are interlinked by the PEG segments to form a hydrogel. Reversible gelling microsphere-forming macromers also may be made from Pluronics.TM. (PEG-PPO-PEG), lactylated and acrylate-capped, which are gelled and reacted in a non-aqueous phase. A hydrophilic drug then may be added (while in the hydrophobic solvent) which partitions to the hydrophilic core. Because the micelles have been cross-linked in the hydrophobic environment, they will not be able to revert to the conformation which they would normally assume in a hydrophilic environment. The trapped hydrophilic drug molecules then need to diffuse through a relatively hydrophobic region to escape from the nanoparticle. This permits flexibility in the formation of microspheres.

They may be hydrophilic or hydrophobic depending on the solvent in which they are polymerized, and on the composition of the macromers.

In other embodiments, physical or chemical crosslinking to form hydrogels (or organogels) can occur in zones other than those responsible for the primary sustained release characteristics of the matrix. For example, "single-ended" materials could have alternative reaction sites on the non-micellar ends, which could subsequently reacted to form a gel. Since matrix-controlled drug delivery is a function of both diffusion from the micelles and of matrix degradation, manipulation of the macromolecular backbone can also control matrix degradation. This can occur through stabilization of hydrolytic groups by their chemical and physical environment (for example, macromers based on reverse Pluronic.TM. gels are more stable than normal Pluronic.TM. gels, in aqueous solution). It is possible that the increased hydrophobicity of the environment of the lactide ester bonds, due to the adjacent block being PPO rather than PEO, inhibits hydrolysis of the bond.

Alternatively, and particularly in gel-forming compositions, the cross-linking reactive groups or biodegradable groups may be in the hydrophilic portions of the macromers, so that the hydrophobic domains would not be locally crosslinked in the hydrophobic regions, while the micelles would still be stabilized by the crosslinking of the material, and particular hydrophobic sections of macromers would be sterically restricted to one or only a few different micelles. In either of these cases, the hydrophobic zones are not rigidly crosslinked, but are connected to crosslinks via the hydrophilic blocks, which may be very flexible. The hydrophobic blocks thus can associate above or below a critical temperature, and dissociate on change in temperature. This allows, for example, both thermosensitive gelation and thermosensitive variation in drug diffusion rate.

The hydrogels may be designed to be biodegradable by incorporation of a group such as a lactide, glycolide or other self-degrading linkage. Alternatively, this is not necessary when non-gelled nanospheres are formed, since these are small enough to be removed by phagocytosis. Control of the rates of delivery of both small and large molecules can be obtained by control of the hydrophobicity of the associating hydrophobic domains of amphipathic hydrogels.

The crosslinked microspheres containing a biologically active agent, in either gel or dispersion form, can be made in a single step. In addition to drug delivery applications, the method is suitable for non-medical uses including delivery of agricultural materials such as herbicides and pesticides and in water treatment.

Claim 1 of 15 Claims

What is claimed is:

1. A composition formed from an aqueous solution of a macromer or mixtures thereof, wherein the macromer comprises a total of five or ten covalently linked polymeric blocks, wherein:

a) at least one block is hydrophilic and each hydrophilic block individually has a water solubility of at least 1 gram/liter;

b) at least two blocks are sufficiently hydrophobic to cause the macromer to aggregate to form micelles in art aqueous continuous phase;

c) the macromer comprises at least one crosslinkable group; and

d) the macromer comprises at least one thermally sensitive region,

wherein the crosslinkable groups are covalently crosslinked.

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