Abstract:
A coating for an implantable medical device, such as a stent, is disclosed. The coating comprising a block copolymer obtained by a polymerization reaction of at least two reagents. A method for forming the coating is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is directed to coatings for implantable medical devices, such as drug eluting vascular stents. 
     2. Description of the State of the Art 
     Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease. A catheter assembly having a balloon portion is introduced percutaneously into the cardiovascular system of a patient via the brachial or femoral artery. The catheter assembly is advanced through the coronary vasculature until the balloon portion is positioned across the occlusive lesion. Once in position across the lesion, the balloon is inflated to a predetermined size to radially compress against the atherosclerotic plaque of the lesion to remodel the lumen wall. The balloon is then deflated to a smaller profile to allow the catheter to be withdrawn from the patient&#39;s vasculature. 
     A problem associated with the above procedure includes formation of intimal flaps or torn arterial linings which can collapse and occlude the conduit after the balloon is deflated. Moreover, thrombosis and restenosis of the artery may develop over several months after the procedure, which may require another angioplasty procedure or a surgical by-pass operation. To reduce the partial or total occlusion of the artery by the collapse of arterial lining and to reduce the chance of the development of thrombosis and restenosis, a stent is implanted in the lumen to maintain the vascular patency. 
     Stents are used not only as a mechanical intervention but also as a vehicle for providing biological therapy. As a mechanical intervention, stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically, stents are capable of being compressed, so that they can be inserted through small vessels via catheters, and then expanded to a larger diameter once they are at the desired location. Examples in patent literature disclosing stents which have been applied in PTCA procedures include stents illustrated in U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor. 
     Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. In order to provide an efficacious concentration to the treated site, systemic administration of such medication often produces adverse or toxic side effects for the patient. Local delivery is a preferred method of treatment in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Local delivery thus produces fewer side effects and achieves more favorable results. One proposed method for medicating stents involves the use of a polymeric carrier coated onto the surface of a stent. A solution which includes a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend is applied to the stent. The solvent is allowed to evaporate, leaving on the stent surface a coating of the polymer and the therapeutic substance impregnated in the polymer. Once the stent has been implanted at the treatment site, the therapeutic substance has a sustained release profile from the polymer. 
     The embodiments of the present invention provide for polymers and combination of polymers for coating stents and other implantable medical devices. 
     SUMMARY 
     According to one embodiment of the present invention, a coating for an implantable medical device is provided. The coating comprises a block copolymer obtained by a polymerization reaction of a plurality of reagents. The coating can also include an active agent, such as those used for the treatment of restenosis. The implantable device can be, for example, a stent. In one embodiment of the invention, the reagents for the polymerization comprise an isocyanate-derived compound, for example a product of the reaction between a diisocyanate and a diol, and a hydroxyl-group containing compound, for example, a polyglycol or a polysiloxane. According to another embodiment, the reagents for the polymerization comprise polyesters, such as, for example, poly(alkylene terephthalates), or copolymers of these polyesters with polyglycols, such as, for example, poly(ethylene glycol). 
     A method of fabricating a medical article, such as a stent is also provided. The method comprises coating an implantable medical device with a polymer composition, wherein the polymer composition comprises a block copolymer obtained by a polymerization reaction of a plurality of reagents. The coating can also include an active agent. 
    
    
     DETAILED DESCRIPTION 
     The term “block copolymer” is defined as a copolymer containing a linear arrangement of blocks, a block being defined as a portion of a polymer molecule in which the monomeric units have at least one constitutional or configurational feature absent from the adjacent portions. For example, a block copolymer of A and B may be written as . . . -A-A-A-A-B-B-B-B-B-B- . . . . The blocks need not be linked on the ends, since the individual blocks are usually long enough to be considered polymers in their own right. The polymer can be, accordingly, named poly A-block-poly B. The term copolymer is intended to broadly include two or more types of blocks such as triblocks. 
     One example of a block copolymer that can be used to fabricate a coating or a layer for a coating for a stent is isocyanate-diol based block copolymer which includes poly(dimethylsiloxane) blocks or poly(alkylene glycol) blocks. Other examples of block copolymers include poly(alkylene terephthalate)-poly(alkylene glycol) copolymers, polyester-poly(alkylene glycol) copolymers, copolymers based on fluorinated and/or perfluorinated olefins and polyolefin-polystyrene copolymers. 
     The coating can be used as a primer layer, a reservoir layer containing an active agent or a drug, or a topcoat layer for reducing the rate of release of the drug. The block copolymers can be used alone, in combination with, or in mixture with other suitable biocompatible polymers. Poly(ethylene-co-vinyl alcohol) (EVAL) is one example of a polymer than can be employed. EVAL has the general formula —[CH 2 —CH 2 ] m -[CH 2 —CH(OH)] n —. EVAL is a product of hydrolysis of ethylene-vinyl acetate copolymers and may also be a terpolymer including up to 5 molar % of units derived from styrene, propylene and other suitable unsaturated monomers. 
     Other suitable polymers that can also be used in combination with the block copolymer include poly(vinyl alcohol) (PVA) having the general formula —[CH 2 —CH 2 (OH)] m —, and a polymer known under the trade name Bionate® manufactured by The Polymer Technology Group Incorporated of Berkeley, Calif. Bionate® is a thermoplastic polycarbonate-urethane elastomer formed as the product of the reaction between a hydroxyl-terminated polycarbonate, an aromatic diisocyanate, and a low molecular-weight glycol used as a chain extender. Representative examples of other polymers include poly(hydroxyvalerate), poly(L-lactic acid), polycaprolactone, poly(lactide-co-glycolide), poly(hydroxybutyrate), poly(hydroxybutyrate-co-valerate), polydioxanone, polyorthoester, polyanhydride, poly(glycolic acid), poly(D,L-lactic acid), poly(glycolic acid-co-trimethylene carbonate), polyphosphoester, polyphosphoester urethane; poly(amino acids), cyanoacrylates, poly(trimethylene carbonate), poly(iminocarbonate), co-poly(ether-esters) (e.g. PEO/PLA), polyalkylene oxalates, polyphosphazenes, biomolecules (such as fibrin, fibrinogen, cellulose, starch, collagen and hyaluronic acid), polyurethanes, silicones, polyesters, polyolefins, polyisobutylene and ethylene-alphaolefin copolymers, acrylic polymers and copolymers, vinyl halide polymers and copolymers (such as polyvinyl chloride), polyvinyl ethers (such as polyvinyl methyl ether), polyvinylidene halides (such as polyvinylidene fluoride and polyvinylidene chloride), polyacrylonitrile, polyvinyl ketones, polyvinyl aromatics (such as polystyrene), polyvinyl esters (such as polyvinyl acetate), copolymers of vinyl monomers with each other and olefins (such as ethylene-methyl methacrylate copolymers, acrylonitrile-styrene copolymers, ABS resins, and ethylene-vinyl acetate copolymers), polyamides (such as Nylon 66 and polycaprolactam), alkyd resins, other polycarbonates, polyoxymethylenes, polyimides, polyethers, epoxy resins, other polyurethanes, rayon, rayon-triacetate, cellulose, cellulose acetate, cellulose butyrate, cellulose acetate butyrate, cellophane, cellulose nitrate, cellulose propionate, cellulose ethers, soluble fluorinated polymers and carboxymethyl cellulose. 
     The embodiments of the present invention are described in connection with a stent, e.g., balloon expandable or self-expandable stents; however, other implantable medical devices can also be coated with the described block copolymers. Examples of such implantable devices include stent-grafts, grafts (e.g., aortic grafts), artificial heart valves, cerebrospinal fluid shunts, pacemaker electrodes, and endocardial leads (e.g., FINELINE and ENDOTAK, available from Guidant Corp. of Santa Clara, Calif.). The underlying structure of the device can be of virtually any design. The device can be made of a metallic material or an alloy such as, but not limited to, cobalt chromium alloy (ELGILOY), stainless steel (316L), “MP35N,” “MP20N,” ELASTINITE (Nitinol), tantalum, nickel-titanium alloy, platinum-iridium alloy, gold, magnesium, or combinations thereof. “MP35N” and “MP20N” are trade names for alloys of cobalt, nickel, chromium and molybdenum available from Standard Press Steel Co. of Jenkintown, Pa. “MP35N” consists of 35% cobalt, 35% nickel, 20% chromium, and 10% molybdenum. “MP20N” consists of 50% cobalt, 20% nickel, 20% chromium, and 10% molybdenum. Devices made from bioabsorbable or biostable polymers could also be used with the embodiments of the present invention. 
     For the reservoir layer, the coating can include an active agent or a drug. The active agent can be aimed at inhibiting abnormal or inappropriate migration and/or proliferation of smooth muscle cells for the inhibition of restenosis. The active agent can also include any substance capable of exerting a therapeutic or prophylactic effect in the practice of the present invention. For example, the agent can be for enhancing wound healing in a vascular site or improving the structural and elastic properties of the vascular site. Examples of agents include antiproliferative substances such as actinomycin D, derivatives and analogs thereof (manufactured by Sigma-Aldrich of Milwaukee, Wis.), or COSMEGEN available from Merck &amp; Co. Inc. of Whitehouse Station, N.J. Synonyms of actinomycin D include dactinomycin, actinomycin IV, actinomycin 11, actinomycin X 1 , and actinomycin C 1 . The active agent can also fall under the genus of antineoplastic, anti-inflammatory, antiplatelet, anticoagulant, antifibrin, antithrombin, antimitotic, antibiotic, antiallergic and antioxidant substances. Examples of such antineoplastics and/or antimitotics include paclitaxel (e.g. Taxol® by Bristol-Myers Squibb Co. of Stamford, Conn.), docetaxel (e.g. Taxotere®, from Aventis S. A. of Frankfurt, Germany) methotrexate, azathioprine, vincristine, vinblastine, fluorouracil, doxorubicin hydrochloride (e.g. Adriamycin® from Pharmacia &amp; Upjohn, of Peapack N.J.), and mitomycin (e.g. Mutamycin® from Bristol-Myers Squibb Co. of Stamford). Examples of such antiplatelets, anticoagulants, antifibrin, and antithrombins include sodium heparin, low molecular weight heparins, heparinoids, hirudin, argatroban, forskolin, vapiprost, prostacyclin and prostacyclin analogues, dextran, D-phe-pro-arg-chloromethylketone (synthetic antithrombin), dipyridamole, glycoprotein IIb/IIa platelet membrane receptor antagonist antibody, recombinant hirudin, and thrombin inhibitors such as Angiomax made by Biogen, Inc., of Cambridge, Mass.). Examples of such cytostatic or antiproliferative agents include angiopeptin, angiotensin converting enzyme inhibitors such as captopril (e.g. Capoten® and Capozide® from Bristol-Myers Squibb Co. of Stamford), cilazapril or lisinopril (e.g. Prinivil® and Prinzide® from Merck &amp; Co., Inc. of Whitehouse Station, N.J.); calcium channel blockers (such as nifedipine), colchicine, fibroblast growth factor (FGF) antagonists, fish oil (omega 3-fatty acid), histamine antagonists, lovastatin (an inhibitor of HMG-CoA reductase, a cholesterol lowering drug, brand name Mevacor® from Merck &amp; Co., Inc., of Whitehouse Station, N.J.), monoclonal antibodies (such as those specific for Platelet-Derived Growth Factor (PDGF) receptors), nitroprusside, phosphodiesterase inhibitors, prostaglandin inhibitors, suramin, serotonin blockers, steroids, thioprotease inhibitors, triazolopyrimidine (a PDGF antagonist), and nitric oxide. An example of an antiallergic agent is permirolast potassium. Other therapeutic substances or agents which may be appropriate include alpha-interferon, genetically engineered epithelial cells, rapamycin, dexamethasone, and functional analogs and structural derivatives thereof. 
     Embodiments of the present invention are further illustrated by the following examples. 
     EXAMPLE 1 
     A composition can be prepared by mixing the following components:
         (a) between about 0.1 and about 15% (all % are by mass), for example, about 2.0% of a matrix polymer compound, for example, Bionate® polymer;   (b) between about 0.01 and about 1.5%, for example, between about 0.2 and 1.0% of a block-copolymer;   (c) between about 0.05 and about 1.0%, for example, about 0.7% of an active agent, for example, Everolimus (available from Novartis); and   (d) the balance, a solvent, for example, N,N-dimethylacetamide (DMAC) having the formula CH 3 —CO—N(CH 3 ) 2 .       

     The typical polycarbonate glycol intermediate used for the synthesis of Bionate®, poly(1,6-hexyl-1,2-ethyl carbonate)diol, PHECD, is the product of the condensation of 1,6-hexanediol with cyclic ethylene carbonate. The polycarbonate macroglycol product of the above-mentioned condensation is then reacted with aromatic isocyanate, 4,4′-methylene-bisphenyl-diisocyanate (MDI), and chain-extended with 1,4-butanediol. 
     Ingredient (b), the block-copolymer, can be a product of co-polycondensation of an isocyanate-based component and a hydrophobic polymeric component. The isocyanate-based component can be synthesized by reacting a diisocyanate, usually, hexamethylene-1,6-diisocyanate, HMDI, having the formula O═C═N—(CH 2 ) 6 —N═C═O, with, for example, a diol, such as, butane-1,4-diol having the formula HO—(CH 2 ) 4 —OH. 
     This adduct is reacted with the hydrophobic polymeric component, for example, polydimethylsiloxane (PDMS), having a general formula —[O—Si—(CH 3 ) 2 ] n —, to form the block-copolymer. The block-copolymer has an average molecular weight within a range of between about 8,000 and about 50,000. The increase of the molecular weight will improve the protective and the release rate controlling properties, but its solubility may be impaired. Therefore, the molecular weight can be chosen at about 25,000. In case of spraying, the upper limit amount of the block-copolymer can be about 0.5%, and in case of dipping about 1.5%. 
     The above-described block-copolymer (HMDI-butanediol-co-PDMS) is also known under the trade name Elasteon® and is manufactured by AorTech Biomaterials Co. of Chatswood, Australia. 
     In addition to the DMAC, alternative solvents that can be used include N,N-dimethylformamide (DMFA) (H—CON(CH 3 ) 2 ), tetrahydrofuran (THF) (C 4 H 8 O), or dimethylsulfoxide (DMSO)((CH 3 ) 2 C═O). 
     The final composition can be applied on the stent either by spraying or by dipping. In case of spraying, the upper limit amount of Bionate® can be about 5%, and in case of dipping about 15%. 
     In the final product, because PDMS is a surface-active hydrophobic polymer, the PDMS portion of the composition blooms to the polymer-air interface, thus controlling the rate of release of the active agent. The isocyanate portion stays below the PDMA portion. 
     EXAMPLE 2 
     A composition can be prepared by mixing the following components:
         (a) between about 0.1 and about 15%, for example, about 2.0% Bionate®;   (b) between about 0.01 and about 1.5%, for example, between about 0.2 and 1.0% block-copolymer;   (c) between about 0.05 and about 1.0%, for example, about 0.7% active agent, for example, Everolimus; and   (d) the balance, DMAC solvent.       

     Instead of the Elasteon® copolymer utilized in Example 1, another block-copolymer is used as ingredient (b). The copolymer can comprise an HMDI-butane diol adduct, but instead of PDMS of Example 1, the HMDI-butane diol adduct is co-polycondensed with a polyglycol, typically an alkylene glycol, for instance, polyethylene glycol (PEG) having the general formula H—[O—CH 2 —CH 2 ] n —OH. In this formulation, there is no PDMS, and the PEG portion of the block-copolymer does not bloom to the polymer-air interface, unlike PDMS of Example 1. Instead, the PEG portion migrates to the polymer-water interface, the water being provided by the body fluids surrounding the implantable device. The block copolymer can have an average molecular weight within a range of between about 8,000 and about 50,000, for example, about 25,000. 
     EXAMPLE 3 
     A coating can be formed on a stent according to a two-step procedure: 
     Step 1: a coating is applied on the surface of the device, the coating being made from the following composition:
         (a) between about 0.1 and about 15%, for example, about 2.0% Bionate®;   (b) between about 0.05 and about 1.0%, for example, about 0.7% active agent, for example, Everolimus; and   (c) the balance, DMAC solvent.       

     No block copolymer is used in step 1. Instead, the block copolymer is applied in the second step of the procedure, which is performed following the drying of the coating applied in step 1. 
     Step 2: a topcoat is applied over the coating of step 1. This topcoat is based on a formulation comprising:
         (a) between about 0.1 and about 15%, for example, about 2.0% Bionate®;   (b) between about 0.01 and about 1.5%, for example, between about 0.2 and 1.0% block copolymer; and   (c) the balance, DMAC solvent.       

     The block-copolymer can be the same as the one described in Example 1 above—namely, HMDI-butanediol-co-PDMS product, such as Elasteon®. The block-copolymer can have an average molecular weight within a range of between about 8,000 and about 50,000, for example, about 25,000. Alternative solvents also include DMFA, THF, or DMSO. 
     Higher loading limits (e.g., 15% Bionate®, and 1.5% block-copolymer) can be used when the composition is applied by dipping, and the lower loading limits (e.g., 5% and 0.5%, respectively) can be used when the composition is applied by spraying. 
     EXAMPLE 4 
     A composition can be prepared by mixing the following components:
         (a) between about 0.1 and about 15%, for example, about 2.0% polyester, for example, poly(alkylene terephthalate), such as poly(ethylene terephthalate) (PET) with a general formula —[O—CH 2 —CH 2 —O—CO—C 6 H 4 —O—CO—CH 2 —CH 2 ] n —;   (b) between about 0.01 and about 1.5%, for example, between about 0.2 and 1.0% block-copolymer;   (c) between about 0.05 and about 1.0%, for example, about 0.7% active agent, for example, Everolimus; and   (d) the balance, solvent, for example, trifluoroacetic anhydride (TFAA), having the formula (CF 3 —CO) 2 O.       

     Ingredient (b), the block-copolymer, such as PBT-PEG is a product of co-polycondensation of poly(butylene terephthalate) (PBT), having the general formula —[O—(CH 2 ) 4 —O—CO—C 6 H 4 ′—O—CO—(CH 2 ) 4 ] n — with poly(ethylene glycol) (PEG) having the general formula —[O—CH 2 —CH 2 —O—CH 2 —CH 2 ] n —. A product of polycondensation of monomeric butylene terephthalate and ethylene glycol is also acceptable and may be used as long as the final block-copolymer is formed with a molecular weight of between about 8,000 and about 50,000, for example, about 25,000. 
     Higher loading limits (e.g., 15% PET and 1.5% for the block-copolymer) can be used when the composition is applied by dipping, and lower loading limits (e.g., 5% and 0.5% respectively) can be used when the composition is applied by spraying. 
     EXAMPLE 5 
     The composition according to this example can be made according to the same formulation as in Example 4 with the following exceptions:
         (a) instead of PET in ingredient (a) and instead of PBT in the block-copolymer of ingredient (b), a polymer of L-lactic acid is used (PLLA). This polymer is formed by self-condensation of L-lactic acid having the formula CH 3 —CH(OH)—COOH. The amount of PLLA in the formulation is the same as that of PET or PBT, respectively.       

     (b) the solvent can be 1,1,2-thrichloroethylene having the formula CHCl═CCl 2 . Alternatively, DMSO or methyl chloroform (also known as 1,2,3-trichloroethane), having the formula CH 3 —CCl 3 , can be used. The amounts of the solvents used are the same as those of Example 4. 
     EXAMPLE 6 
     The composition can be made according to the formulation described in Example 1, except that EVAL is used as a matrix polymer, instead of Bionate®. 
     EXAMPLE 7 
     The composition can be made according to the formulation described in Example 1, except that in the soft portion of the block co-polymer a long-chained glycol is used instead of PDMS. The long-chained glycol has the general formula HO—(CH 2 ) n —OH, where “n” is an integer having a value of at least 18. 
     EXAMPLE 8 
     A coating can be formed on a stent according to a two-step procedure: 
     Step 1: a coating is applied on the device to form a drug-polymer layer, the coating being made from the following composition:
         (a) between about 0.1 and about 15%, for example, about 2.0% EVAL;   (b) between about 0.05 and about 1.0% for example, about 0.7% active agent, for example, Everolimus; and   (c) the balance, DMAC solvent.       

     In the second step of the procedure, which is performed following the drying of the drug-polymer layer of step 1, a topcoat can be applied over the drug-polymer layer. 
     Step 2: the topcoat can be based on a formulation comprising:
         (a) between about 0.1 and about 15%, for example, about 1.5% block copolymer such as Elasteon® copolymer; and   (b) the balance, DMAC solvent.       

     The block-copolymer can have an average molecular weight within a range of between about 8,000 and about 50,000, for example, about 25,000. Alternative solvents include DMFA, THF, or DMSO. 
     EXAMPLE 9 
     A coating can be formed on a stent according to a three-step procedure. The first and the second steps are the same as the steps described in Example 8. In the third step of the procedure, which is performed following the drying of the topcoat layer, a finishing coat is applied over the topcoat. The finishing coat can be based on a formulation comprising between about 0.1 and about 15%, for example, about 2.0% PBT-PEG block copolymer described in Example 4, and the balance, DMAC solvent. 
     EXAMPLE 10 
     A coating can be formed on a stent according to a three-step procedure: 
     Step 1: a coating is applied on the device to form a drug-polymer layer. The coating is made from the following composition:
         (a) between about 0.1 and about 15%, for example, about 2.0% EVAL;   (b) between about 0.05 and about 1.0%, for example about 0.7% active agent, for example, Estradiol; and   (c) the balance, DMAC solvent.       

     In the second step of the procedure, which is performed following the drying of the drug-polymer layer, a topcoat can be applied over the drug-polymer layer. 
     Step 2: the topcoat can be based on a formulation comprising:
         (a) between about 0.1 and about 15%, for example, about 1.8% block copolymer such as Kraton® G-1650 copolymer; and   (b) the balance, THF solvent.       

     Kraton® G-1650, manufactured by Shell Chemicals Co. of Houston, Tex., is a three block copolymer with hard polystyrene end blocks and a thermoplastic elastomeric poly(ethylene-butylene) soft middle block. Kraton® G-1650 contains about 30 mass % of polystyrene blocks. 
     In the third step of the procedure, which is performed following the drying of the topcoat layer, a finishing coat can be applied over the topcoat. 
     Step 3: the finishing coat can be based on a formulation comprising:
         (a) between about 0.1 and about 15%, for example, about 2.0% of PBT-PEG block copolymer described in Examples 4 and 9; and   (b) the balance, DMAC solvent.       

     EXAMPLE 11 
     A coating can be formed on a stent according to a two-step procedure: 
     Step 1: a coating is applied on the device to form a drug-polymer layer. The coating can be made from the following composition:
         (a) between about 0.1 and about 15%, for example, about 2.0% Elasteon® block-copolymer;   (b) between about 0.05 and about 1.0%, for example, about 0.7% active agent, for example Everolimus; and   (c) the balance, DMAC solvent.       

     The Elasteon® block-copolymer has the average molecular weight within a range of between about 8,000 and about 50,000, for example, about 25,000. Alternative solvents include DMFA, THF, or DMSO. 
     In the second step of the procedure, which is performed following the drying of the drug-polymer layer, a topcoat layer is applied over the drug-polymer layer. 
     Step 2: the topcoat layer can be based on a formulation comprising:
         (a) between about 0.1 and about 15%, for example, about 1.5% poly(butyl methacrylate) (PBMA); and   (b) the balance, DMAC solvent.       

     EXAMPLE 12 
     The composition can be made according to the formulation described in Example 11, except that instead of PBMA, a copolymer of vinylidene fluoride and hexafluoropropene, poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP) can be used as a topcoat polymer. 
     While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications can be made without departing from this invention in its broader aspects. Therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.