Abstract:
A method of coating a medical device, such as a stent is provided. The method can include forming a polymer layer containing a drug on the device, applying a polymer melt free from any solvents to the polymer layer to form a topcoat layer, wherein the during the application of the polymer melt the migration of the drug from the polymer layer is prevented or significantly minimized.

Description:
CROSS REFERENCE 
     This is a divisional application of application Ser. No. 10/772,858, filed on Feb. 4, 2004, now U.S. Pat. No. 7,115,300, which is a continuation-in-part of application Ser. No. 10/040,574, filed on Dec. 28, 2001 and issued as U.S. Pat. No. 6,709,514. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method for coating implantable medical devices, such as stents. 
     2. Description of the Background 
     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 press against the atherosclerotic plaque of the lesion for remodeling of the vessel 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 The balloon is then deflated to a smaller profile to allow the catheter to be withdrawn from the patient&#39;s vasculature.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. Vasospasms and recoil of the vessel wall also threaten vessel closure. Moreover, thrombosis and restenosis of the artery can develop over several months after the procedure, which can 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 can be used not only as a mechanical intervention but also as a vehicle for providing biological therapy. As a mechanical intervention, stents can 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 lumens via catheters, and then expanded to a larger diameter once they are at the desired location. 
     Although stents work well mechanically, the chronic issues of restenosis and, to a lesser extent, thrombosis remain. These events are adversely affected by the mechanical aspects of the stent such as the degree of injury and disturbance in hemodynamics caused by the stent. To the extent that the mechanical functionality of stents has been optimized, it has been postulated that continued improvements could be made by pharmacological therapies. Many systemic therapies have been tried. A challenge is maintaining the necessary concentration of a therapeutic substance at the lesion site for the necessary period of time. This can be done via brute force methods using oral or intravenous administration but the issues of systemic toxicity and side effects arise. Therefore, a preferred route can be achieved by local delivery of a therapeutic substance from the stent itself. Being made of metal, plain stents are not useful for therapeutic substance delivery. Therefore, a coating, usually made from a polymer, is applied to serve as a therapeutic substance reservoir. A solution of a polymer dissolved in a solvent and a therapeutic substance added thereto is applied to the stent and the solvent is allowed to evaporate. Accordingly, a polymeric coating impregnated with a therapeutic substance remains on the surface of the stent. The polymeric coating can include multiple layers. A primer composition, free from any drugs, can be applied on the surface of the device. A polymer solution including the drug can then be applied on the primer layer. To reduce the rate of release of the drug, a topcoat layer can be applied over the reservoir layer. The application of each layer can be performed subsequent to the drying of the previous layer. 
     In order to be effectively applied with conventional spraying or dipping techniques, the coating solution needs to have a low viscosity. Low viscosities can be achieved by adding a higher fraction of solvent to the solution or by changing the composition of the solution with the addition of a “wetting fluid.” Compositions having a low viscosity require multiple applications of the composition and evaporation of the solvent in order to obtain a coating of suitable thickness or weight, as compared to using compositions having greater viscosities. Accordingly, it is desired to use more viscous compositions to reduce the number of application steps and in effect reduce the processing time of forming the coating. 
     The topcoat layer is intended to reduce the rate of release of the drug from the polymer-drug layer. However, when a topcoat composition is applied to the polymer-drug layer, the solvent may extract the drug out from the polymer-drug layer, therefore reducing the effectiveness of the topcoat layer. Accordingly, it is desired to prevent the extraction of the drug out from the drug-polymer layer when a top coat layer is applied. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, a method for coating an implantable medical device is provided, comprising forming a polymer layer containing a drug on the device, and applying a polymer melt free from any solvents to the polymer layer to form a topcoat layer. 
     In accordance with another aspect of the invention, a method of coating an implantable medical device is provided, comprising applying a coating material to a disk member and spinning the disk member about an axis of the disk member such that the force that is applied to the coating material by the disk member discharges the coating material off of the disk member and onto the device. 
     In accordance with another aspect of the invention, a method for coating an implantable medical device is provided, comprising forming a polymer layer containing a drug on the device, applying a polymer in a liquid state and free from any solvents to the polymer layer to form a topcoat layer, wherein the during the application of the topcoat layer the migration of the drug from the polymer layer is prevented or significantly minimized. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIGS. 1A-1E  illustrate various embodiments of the coating apparatus. 
         FIGS. 2A-2D  illustrate disk members from various embodiments of the apparatus. 
         FIG. 3  illustrates an embodiment of the apparatus in use. 
         FIGS. 4   a  and  4   b  illustrate possible positions of implantable devices with respect to the apparatus. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1A-1E  illustrate embodiments of an apparatus  10  for coating medical devices, such as stents. Apparatus  10  can include a disk member  12  mounted on a shaft  14 . The shaft  14 , in turn, can be connected to a motor  16  for rotating the disk member  12  in a clockwise or counterclockwise direction. The disk member  12  can be flat ( FIG. 1A ), concave ( FIG. 1B ), convex ( FIG. 1C ) or conical ( FIGS. 1D and 1E ) in shape. The disk member  12  can optionally include a lip  18  disposed about the periphery thereof. The lip  18  can extend in an upwardly direction, towards a nozzle  20 . 
     The disk member  12  can be made from any suitable material or can be coated with the desired material so as to minimize the ability of the composition to adhere to a surface  22  of the disk member  12  on which the composition is applied via the nozzle  20 . One suitable non-stick surface  22  can be TEFLON. A temperature adjustor  24  can also be provided for adjusting the temperature of the composition during the coating process. The temperature adjustor  24  can be used to increase the temperature of the composition that includes a non-volatile solvent (e.g., dimethylsulfoxide (DMSO), dimethylformamide (DMF), and dimethylacetamide (DMAC)), or alternatively, can be used to decrease the temperature of the composition that includes a volatile solvent. A non-volatile solvent is a solvent that has a vapor pressure less than or equal to about 2.338 MPa (17.54 Torr) at ambient temperature. Should a coating composition be used that has no (i.e., 100% free of solvents) to minimal solvent (e.g., less than about 10% solvent by volume), the temperature adjustor  24  can facilitate placing the polymer in a free flowing fluid form. The temperature adjustor  24  can also be used to increase the temperature to or above the melting temperature of the polymer. 
     The diameter of the disk can range from about 5.1 cm (2.0 in.) to about 15 cm (6.0 in.), for example about 10 cm (4.0 in.). Referring to  FIGS. 2A-2D , grooves or channels  28  can be carved within the surface  22  so as to provide control over the direction of the flow of the composition.  FIG. 2A  illustrates grooves  28  extending from the center of the disk member  12  to the outer edge of the disk member  12 .  FIG. 2B  illustrates straight grooves  28 .  FIG. 2C  illustrates circular grooves  28  positioned concentrically to one another. The depth and/or width of the grooves  28  can be incrementally smaller as the grooves  28  move closer to the edge of the disk member  12 . Yet in another embodiment of the invention, groove  28  can be disposed in a spiral or corkscrew like fashion about the surface  22 . 
     The apparatus  10  can be used for coating any suitable medical substrate that can be implanted in a human or veterinary patient. Examples of such implantable devices include balloon expandable stents, self-expandable stents, stent-grafts, grafts (e.g., aortic grafts), artificial heart valves, cerebrospinal fluid shunts, anastomosis devices, pacemaker electrodes, and endocardial leads (e.g., FINELINE and ENDOTAK, available from Guidant Corporation). 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., 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 
     The composition or coating material that can be applied by the nozzle  20  can include a polymer or combination of polymers such as in blend form or conjugated form. The polymer or combination of polymers can be in a liquid state or fluid flow form such as in a melt form. Polymer melt is defined as a polymer or a combination in a non-solid state or having been modified by exposure to temperature equal to or greater than a melting temperature (T m ) of the polymer. If a combination of polymers is used, the highest melting temperature must be reached or surpassed. In some embodiments, the composition or coating material is free from any solvents. Free from solvents or solvent free is defined as (1) completely (100%) free from any solvents or (2) having very minimal amount of a solvent(s) including less than 20%, less than 15%, less than 10%, less than 5% or alternatively less than 1% by volume. In some embodiments, the polymer melt is solvent free. For the application of a topcoat layer over polymer-drug layer, the composition can be free from any solvents. In some embodiments, the polymer or the combination can be dissolved in a suitable amount of solvent, for example greater than 50% by volume of a solvent or combination of solvents. In some embodiments, polymeric material can also be emulsified in a solid concentration in a carrier such as water with about 10 weight percent to about 50 weight percent polymeric material. 
     In some embodiments, a therapeutic substance can be added therein. A therapeutic substance or drug is included in the polymer-drug reservoir layer. If an optional primer layer is applied on the surface of the device, the primer composition should be free from any drugs. However, some drugs may migrate into the primer layer from the reservoir layer. The topcoat layer can be with or without a therapeutic substance. If a therapeutic substance or drug is added to topcoat layer, the drug can be different that the drug used in the polymer-drug layer so as to provide for a cocktail formulation. 
     Representative examples of polymers that can be used include ethylene vinyl alcohol copolymer (commonly known by the generic name EVOH or by the trade name EVAL), 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); copoly(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; polycarbonates; polyoxymethylenes; polyimides; polyethers; epoxy resins; polyurethanes; rayon; rayon-triacetate; cellulose; cellulose acetate; cellulose butyrate; cellulose acetate butyrate; cellophane; cellulose nitrate; cellulose propionate; cellulose ethers; and carboxymethyl cellulose. 
     A solvent can be defined as a liquid substance or composition that is compatible with the polymer and/or drug and is capable of dissolving the polymer and/or drug at the concentration desired in the composition. Examples of solvents include, but are not limited to, dimethylsulfoxide (DMSO), chloroform, acetone, water (buffered saline), xylene, methanol, ethanol, 1-propanol, tetrahydrofuran, 1-butanone, dimethylformamide, dimethylacetamide, cyclohexanone, ethyl acetate, methylethylketone, propylene glycol monomethylether, isopropanol, isopropanol admixed with water, N-methyl pyrrolidinone, toluene, and combinations thereof. 
     The therapeutic substance, drug or active agent can be for inhibiting the activity of vascular smooth muscle cells. More specifically, 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. 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, or derivatives and analogs thereof (manufactured by Sigma-Aldrich 1001 West Saint Paul Avenue, Milwaukee, Wis. 53233; or COSMEGEN available from Merck). Synonyms of actinomycin D include dactinomycin, actinomycin IV, actinomycin I 1 , 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., Stamford, Conn.), docetaxel (e.g. Taxotere®, from Aventis S.A., Frankfurt, Germany) methotrexate, azathioprine, vincristine, vinblastine, fluorouracil, doxorubicin hydrochloride (e.g. Adriamycin® from Pharmacia &amp; Upjohn, Peapack N.J.), and mitomycin (e.g. Mutamycin® from Bristol-Myers Squibb Co., Stamford, Conn.). 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/IIIa platelet membrane receptor antagonist antibody, recombinant hirudin, and thrombin inhibitors such as Angiomax ä (Biogen, Inc., 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., Stamford, Conn.), cilazapril or lisinopril (e.g. Prinivil® and Prinzide® from Merck &amp; Co., Inc., 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., 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 can be appropriate include alpha-interferon, genetically engineered epithelial cells, rapamycin and dexamethasone. 
     Referring to  FIG. 3 , a medical device, for example a stent  30 , can be positioned in close proximity to the edge of the disk member  12 , for example at a distance of about 0.5 mm (0.02 in.) to about 5.0 mm (0.20 in.). For disk member speeds between about 1,000 rpm and about 20,000 rpm, the stent  30  can be placed to the side of the disk member  12 , with the longitudinal axis of the stent  30  above or slightly above the surface  22  of the disk member  12 , as illustrated in  FIG. 4   a . For speeds below about 1,000 rpm, the stent  30  should be placed so that the longitudinal axis is below the surface  22 , as illustrated in  FIG. 4   b . For coating stents, the stent  30  can be positioned on a mandrel  32  for rotating the stent  30  about the longitudinal axis of the stent  30 . The composition can be applied to the disk member  12  while the disk member  12  is spinning at about 100 rpm to about 20,000 rpm. The flow rate of the from the nozzle  20  can be from about 1.0 g/min. (0.16 lbs./hour) to about 31 g/min. (5.0 lbs./hour). With the use of apparatus  10 , the polymer content can be greater than about 80% of the coating material, more narrowly greater than about 90%, in one embodiment greater than about 95%, without the increased viscosity affecting the coating process of the devices. In some embodiments, the coating material can be melt of a polymer (or blend of polymers or chemical bonded polymers) that is solvent free. The solvent free coating or polymer melt application is most suitable with deposition of a topcoat layer to prevent solvent extraction of the drug from the polymer-drug layer. The centripetal force applied to the coating material causes the material to flow off the disk member  12  and onto the stent  30 . The temperature of the material can be, for example, about 140° C. to about 240° C., more narrowly about 140° C. to about 200° C., yet more narrowly about 140° C. to about 190° C. 
     In some embodiments, a primer layer can be formed on the surface of the device. A composition including a polymer an optionally a solvent can be applied to the surface of the device. The composition is free from any drugs. The polymer can be deposited in a liquid state or melt form with the apparatus of the present invention or can alternatively be deposited by conventional techniques such as spraying or dipping. The primer layer should be dried or allowed to solidify before advancing to the application of the reservoir layer. A reservoir composition of a polymer or mixture of polymers and a drug or combination of drugs can be applied to the primer layer or to the surface of the device. The composition can be solvent free such that the polymer is in a free flowing fluid form. The apparatus of the present invention can apply the reservoir composition to the device. In some embodiments, a topcoat can be applied to the dried or solidified reservoir layer. The top coat composition can include a polymer or combination of polymers (e.g., in blend or bonded form) that is in fluid state or a melt form. The composition can be solvent free. The advantages of this formulation of the topcoat include preventing or at least minimizing surface extraction of the drug from the reservoir layer. The amount of solvent in a solvent free coating formulation can be directly correlated to the amount of surface extraction that can occur. Accordingly, the less solvent, the more preferable the formulation for preventing drug extraction. Other advantages of solvent free formulations include prevention of degradation of the drug caused by the solvent; a more predictable drug release rate profile; and the polymer selection would be independent of solubility of the polymer with solvents. Additionally, with the use of solvent-free systems, more polymer can be deposited in a shorter duration of time, therefore increasing manufacturability. 
     Alternative coating systems and methods that allow for coating materials that are entirely free of any solvents or have relatively little amount of solvent can also be used with the practice of the present invention. For example, a polymer composition can be atomized prior to or subsequent to contact with a carrier gas. The contact can occur under conditions such that vaporization of substantially all of the coating material occurs. The vapor can flow to and condense on the device to form a coating. 
     Although the invention has been disclosed in terms of particular embodiments and applications, one of ordinary skill in the art, in light of this teaching, can generate additional embodiments and modifications without departing from the spirit of or exceeding the scope of the claimed invention. Accordingly, it is to be understood that the drawings and descriptions herein are proffered by way of example to facilitate comprehension of the invention and should not be construed to limit the scope thereof.