Abstract:
A method of coating an implantable prosthesis by applying a composition including a therapeutic substance and a fluid to the prosthesis and removing the fluid is disclosed. The fluid is not capable of significantly dissolving the therapeutic substance. A second fluid in which the therapeutic substance is soluble may be further included in the composition. Methods of providing the composition and a roughened polymeric primer layer are also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to implantable devices, examples of which include stents and grafts. More particularly, the present invention is directed to an improved method of coating an implantable device with a therapeutic substance. 
     2. Description of the Related 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 press against the atherosclerotic plaque of the lesion to remodel 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 The balloon is then deflated to a smaller profile to allow the catheter to be withdrawn from the patient&#39;s vasculature.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 vessel 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 prosthesis, examples of which include stents and grafts, is implanted. 
     Stents are scaffoldings, usually cylindrical or tubular in shape, which function to physically hold open and, if desired, to expand the wall of the vessel. Typically stents are capable of being compressed, so that they may be inserted through small cavities 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. 
     Synthetic vascular grafts are vessel-like configurations that may be positioned into the host blood vessel as a replacement for a diseased or occluded segment that has been removed. Alternatively, a graft may be sutured to the host vessel at each end so as to form a bypass conduit around a diseased or occluded segment of the host vessel. 
     Although stents and grafts are significant innovations in the treatment of occluded vessels, there remains a need for administering therapeutic substances to the treatment site. To provide an efficacious concentration to the treatment site, systemic administration of the therapeutic substance often produces adverse or toxic side effects for the patient. Local delivery is a highly suitable method of treatment, in that smaller levels of therapeutic substances, as compared to systemic dosages, are concentrated at a specific site. Local delivery produces fewer side effects and achieves more effective results. 
     One technique for the local delivery of therapeutic substances is through the use of medicated coatings on implantable devices. A common method for medicating a prosthesis involves the use of a polymeric carrier coated onto the surface of the prosthesis. A composition is formed by dissolving a therapeutic substance in a solution containing a polymer and a solvent. The composition is applied to the prosthesis using conventional techniques, such as spray-coating or dip-coating. The solvent is then removed, leaving on the prosthesis surface a coating of the polymer and the therapeutic substance impregnated in the polymer. 
     A shortcoming of the above-described method is the burst effect, in which an initial rapid release of therapeutic substance upon implantation of the prosthesis is followed by a slower, sustained release of therapeutic substance. Of the various factors which contribute to the burst effect, two factors stem from drying a solution-based coating. First, solution-based coating almost invariably results in an asymmetric distribution of therapeutic substance in the matrix. A higher concentration of therapeutic substance exists at the drying surface, or polymer-air interface, since the therapeutic substance concentrates where the solvent was at the end of the drying process. Additionally, a solvent-dried system may result in the therapeutic substance being in an amorphous phase. When the therapeutic substance is amorphous, as opposed to crystalline, the therapeutic substance exists as individual molecules in the matrix. These molecules may diffuse freely without first having to dissolve into the matrix as they would from a more crystalline phase and therefore contribute to the initial burst of therapeutic substance upon implantation of the prosthesis. 
     Another shortcoming of the above-described method for medicating a prosthesis is that the method does not facilitate processing every therapeutic substance with every polymer and solvent combination. Some therapeutic substances are very delicate and thus cannot tolerate processing in the presence of a solvent and/or a polymer for extended periods of time. This is especially true for peptide-type therapeutic substances, such as actinomycin D and others, having tertiary structure susceptible to transmutation from their native forms. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method by which implantable devices, such as stents and grafts, may be coated with therapeutic substances such that the burst effect and the transmutation of delicate therapeutic substances are minimized. 
     In accordance with one embodiment of the present invention, a method of forming a coating onto a surface of a prosthesis, such as a stent or a graft, is provided. The method includes an act of providing a composition including a therapeutic substance and a first fluid in which the therapeutic substance has limited solubility, such that the therapeutic substance is suspended as particles in the composition. The method additionally includes the acts of applying the composition to a surface of the prosthesis and removing the first fluid from the prosthesis to form a coating. A coating for a prosthesis produced in accordance with the above-described method is also provided. 
     In some embodiments of the method described herein, the composition additionally includes a second fluid capable of dissolving the therapeutic substance, and the act of providing a composition includes the acts of dissolving the therapeutic substance in the second fluid to form a solution and combining the solution with the first fluid. The therapeutic substance thus precipitates out of the solution to form the composition in which the therapeutic substance is suspended as particles. The act of removing the first fluid from the prosthesis also removes the second fluid from the prosthesis. 
     In some embodiments of the above-described method, the composition additionally comprises a suspension stabilizer to minimize flocculation of the therapeutic substance within the composition. 
     In other embodiments of the method, the composition additionally comprises a polymer, such as but not limited to, a bioabsorbable polymer, a biomolecule, or biostable polymer. 
     In still other embodiments, prior to the act of applying the composition to a surface of the prosthesis, the method further includes the act of applying a polymeric primer layer to the surface of the prosthesis. The method may additionally include the act of creating areas of roughness on the polymeric primer layer. 
     In other embodiments, subsequent to the acts of applying the composition to a surface of the prosthesis and removing the first fluid from the prosthesis, the method further includes the act of applying a polymeric topcoat disposed on at least a portion of the composition on the surface of the prosthesis. 
    
    
     These and other aspects of the present invention may be better appreciated in view of the detailed description and drawing of the exemplary embodiments. 
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a flowchart illustrating a method of forming a coating onto a surface of a prosthesis in accordance with the various disclosed embodiments. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Prosthesis 
     The prosthesis used in conjunction with the present invention may be any implantable device, examples of which include self-expandable stents, balloon-expandable stents, and grafs, among other possibilities. The underlying structure of the prosthesis can be virtually any design. The prosthesis can be made of a metallic material or an alloy such as, but not limited to, stainless steel, “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. The prosthesis may also be made from bioabsorbable or biostable polymers. A polymeric prosthesis should be compatible with the composition. 
     Preparation of the Composition 
     FIG. 1 illustrates a method  100  of forming a coating onto a surface of the above-described prosthesis. The coating of the present invention is formed from a selected composition in which a therapeutic substance is suspended as particles. In act  101  of method  100 , such a composition is provided as described below. 
     In accordance with some embodiments, a predetermined amount of a therapeutic substance is added to a predetermined amount of a first fluid. Of the total amount of therapeutic substance added to the first fluid, from about 70% to 100%, or more particularly from about 95% to 100%, of the therapeutic substance should be suspended as particles in the first fluid and from 0% to about 30%, or more particularly from 0% to about 5% of the therapeutic substance may be in solution with the first fluid. Thus, the therapeutic substance has a limited solubility of no more than 30% in the first fluid. When the prosthesis upon which the coating is to be formed is a stent, the size of the suspended particles of therapeutic substance can range from about 0.5 microns in diameter to about 2 microns in diameter. When the prosthesis upon which the coating is to be formed is a vascular graft, the size of the suspended particles of therapeutic substance can range from about 0.5 microns in diameter to about 20 microns in diameter, or more particularly from about 8 microns in diameter to about 13 microns in diameter. The therapeutic substance can be micronized to achieve such particle diameters. 
     In such embodiments, the therapeutic substance can make up from about 0.5% to about 30%, or more particularly from about 5% to about 10%, by weight of the total weight of the composition, and the first fluid can make up from about 70% to about 99.5%, or more particularly from about 90% to about 95%, by weight of the total weight of the composition. Selection of a specific weight ratio is dependent on factors such as the material from which the prosthesis to be coated is made and the geometrical structure of the prosthesis as well as the particular first fluid selected and various physical properties of the selected first fluid, such as contact angle, viscosity, and volatility. Selection of a specific weight ratio is also dependent on factors such as the particular therapeutic substance selected, the duration of the release, the cumulative amount of the release, and the release rate that is desired 
     Therapeutic substances or agents suitable for inclusion in the composition include, but are not limited to, antineoplastic, antimitotic, antiinflammatory, antiplatelet, anticoagulant, antifibrin, antithrombin, antiproliferative, antibiotic, antioxidant, and antiallergic substances as well as combinations thereof. 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 suitable antiinflammatories include glucocorticoids such as dexamethasone, methylprednisolone, hydrocortisone and betamethasone and non-steroidal antiinflammatories such as aspirin, indomethacin and ibuprofen. 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™ (Biogen, Inc., Cambridge, Mass.) Examples of such cytostatic or antiproliferative agents include actinomycin D as well as derivatives and analogs thereof (manufactured by Sigma-Aldrich, Milwaukee, Wis.; or COSMEGEN® available from Merck &amp; Co., Inc., Whitehouse Station, N.J.), 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., Prinivilo 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 that may be used include alphainterferon, genetically engineered epithelial cells, and dexamethasone. While the preventative and treatment properties of the foregoing therapeutic substances or agents are well-known to those of ordinary skill in the art, the substances or agents are provided by way of example and are not meant to be limiting. Other therapeutic substances are equally applicable for use with the disclosed methods and compositions. 
     In other embodiments, the therapeutic substance is a radioactive isotope or a radiopaque substance. Such substances help to facilitate prosthesis usage in radiotherapeutic procedures. Examples of radioactive isotopes include, but are not limited to, phosphoric acid (H 3 P 32 O 4 ), palladium (Pd 103 ), cesium (Cs 131 ), and iodine (I 125 ) An example of a radiopaque substance is gold. 
     The first fluid should be a fluid in which the selected therapeutic substance is insoluble or in which the selected therapeutic substance has limited solubility, i.e., less than or equal to 30%, since the therapeutic substance must be primarily suspended as particles in the first fluid rather than dissolved in the first fluid. Exposure of the therapeutic substance to the first fluid is not permitted to adversely alter the substance&#39;s composition or characteristic. Accordingly, the particular first fluid is selected for chemical compatibility with the selected therapeutic substance. The particular first fluid selected also depends on the material of which the prosthesis is made. For example, if the prosthesis is made of an absorbable polymer, caution should be exercised in the selection of all fluids since some absorbable polymers of which a prosthesis can be made dissolve in fluids that are otherwise suitable for usage in the present invention. In addition, the first fluid should have a high volatility as well as a low contact angle. Exemplary first fluids include, but are not limited to, deionized water, methanol, ethanol, freon, and acetonitrile. 
     In some embodiments, the composition additionally includes a polymer, or a combination of polymers, dissolved in the first fluid. The polymeric material is most suitably biocompatible, including polymers that are non-toxic, non-inflammatory, chemically inert, and substantially non-immunogenic in the applied amounts. The polymer is typically either bioabsorbable or biostable. A bioabsorbable polymer breaks down in the body and is not present sufficiently long after implantation to cause an adverse local response. Bioabsorbable polymers are gradually absorbed or eliminated by the body by hydrolysis, metabolic process, bulk, or surface erosion. Examples of bioabsorbable materials include but are not limited to polycaprolactone (PCL), poly-D, L-lactic acid (DL-PLA), poly-L-lactic acid (L-PLA), poly(lactide-co-glycolide), poly(hydroxybutyrate), poly(hydroxybutyrate-co-valerate), polydioxanone, polyorthoester, polyanhydride, poly(glycolic acid), poly(glycolic acid-cotrimethylene carbonate), polyphosphoester, polyphosphoester urethane, poly (amino acids), cyanoacrylates, poly(trimethylene carbonate), poly(iminocarbonate), copoly(ether-esters), polyalkylene oxalates, polyphosphazenes, polyiminocarbonates, and aliphatic polycarbonates. Biomolecules such as heparin, fibrin, fibrinogen, cellulose, starch, and collagen are typically also suitable. A biostable polymer does not break down in the body, and thus a biostable polymer is present in the body for a substantial amount of time after implantation. Examples of biostable polymers include Parylene®, Parylast®, polyurethane (for example, segmented polyurethanes such as Biospan®), polyethylene, polyethlyene teraphthalate, ethylene vinyl acetate, silicone, polyethylene oxide, and polytetrafluoroethylene (PTFE). 
     In such embodiments, the polymeric material can make up from about 0.1% to about 30%, or more particularly from about 0.1% to about 10%, by weight of the total weight of the composition, the therapeutic substance can make up from about 0.5% to about 30%, or more particularly from about 0.5% to about 10%, by weight of the total weight of the composition, and the first fluid can make up from about 40% to about 99.4%, or more particularly from about 80% to about 99.4%, by weight of the total weight of the composition. In addition to the factors discussed above, selection of a specific weight ratio is dependent on factors such as the particular polymer, or combination of polymers, selected and the solubility of the selected polymer in the selected first fluid. 
     In other embodiments, the therapeutic substance is not initially suspended in the first fluid. Rather, the therapeutic substance is first dissolved in a second fluid to form a solution. The second fluid, therefore, must be selected to be a good solvent for the selected therapeutic substance. The above-described first fluid is then slowly added into the therapeutic substance-containing solution. This process precipitates the therapeutic substance from the composition such that a fine colloidal system is formed in situ. The resulting suspension of the therapeutic substance has a very controlled particle size distribution. The size of the suspended particles of therapeutic substance can range from about 0.5 microns in diameter to about 20 microns in diameter, or more particularly from about 0.5 microns in diameter to about 5 microns in diameter. 
     In such embodiments, the solvent or second fluid can make up from about 50% to about 99%, or more particularly from about 70% to about 99%, by weight of the total weight of the composition, the therapeutic substance can make up from about 0.5% to about 30%, or more particularly from about 0.5% to about 10%, by weight of the total weight of the composition, and the first fluid can make up from about 0.5% to about 20%, or more particularly from about 0.5% to about 20%, by weight of the total weight of the composition. Selection of a specific weight ratio is dependent on factors such as the material from which the prosthesis to be coated is made and the geometrical structure of the prosthesis as well as the particular first and second fluids selected and various physical properties of the selected first and second fluids, such as contact angle, viscosity, and volatility. Selection of a specific weight ratio is also dependent on factors such as the particular therapeutic substance selected, the duration of the release, the cumulative amount of the release, and the release rate that is desired. 
     In some embodiments in which the therapeutic substance is dissolved in a second fluid prior to being suspended in the first fluid, the composition additionally includes a polymer or a combination of polymers dissolved in the second fluid. Examples of such polymeric materials were listed above, and such examples are equally applicable here. In such embodiments, the polymeric material can make up from about 0.1% to about 30%, or more particularly from about 0.1% to about 10%, by weight of the total weight of the composition, the solvent or second fluid can make up from about 20% to about 89.9%, or more particularly from about 70% to about 89.9%, by weight of the total weight of the composition, the therapeutic substance can make up from about 0.5% to about 30%, or more particularly from about 0.5% to about 10%, by weight of the total weight of the composition, and the first fluid can make up from about 0.5% to about 20%, or more particularly from about 0.5% to about 10%, by weight of the total weight of the composition. Typically, the first fluid should not precipitate the polymer from the second fluid. 
     In still other embodiments of the composition, a suspension stabilizer is added to minimize flocculation. Flocculation refers to the clumping and/or settling of suspended particles. In this particular instance, flocculation refers to the clumping and/or settling of therapeutic substance particles in the composition. Exemplary deflocculants include, but are not limited to, pluronic, polyvinyl alcohol (PVA), and stearates. Typically, the stabilizer makes up from about 0.1% to about 1% by weight of the total weight of the composition. 
     Coating the Prosthesis Using the Composition 
     To form a coating on a surface of the prosthesis, the surface of the prosthesis should be clean and free from contaminants that may be introduced during manufacturing. Other than cleaning, the surface of the prosthesis requires no particular surface treatment to retain the applied coating, although the surface may optionally be subjected to physical treatments, such as, but not limited to, the creation of depots in the surface of the prosthesis, to maximize retention of the coating. 
     Referring again to FIG. 1, the above-described composition provided in act  101  of method  100  is applied to the prosthesis in act  102 . Care should be taken to ensure that the particles of therapeutic substance are suspended in the composition, as opposed to settled at the bottom of the composition, prior to the application of the composition to the prosthesis. The addition of a suspension stabilizer to the composition, as described above, can aid in the prevention of settling of therapeutic substance particles. The composition may also be vortexed for approximately 5-30 seconds just prior to application to the prosthesis to ensure proper suspension of the therapeutic substance. 
     The above-described composition may be applied to the surface of a prosthesis in act  102  by any conventional method, such as by spraying the composition onto the prosthesis or immersing the prosthesis in the composition. Variations of spray and immersion techniques are also suitable methods of applying the composition to a surface of a prosthesis. In one such variation, the composition may be applied by spraying or immersing the prosthesis as described above. The composition-coated prosthesis is then centrifuged. The rotation of the prosthesis creates a centrifugal force upon the composition applied to the surface of the prosthesis. This centrifugal force causes excess accumulations of the composition to be more evenly redistributed over the prosthesis and thus provides a more even, uniform coating of the composition on the prosthesis. The rotational speed during centrifugation can be varied. Higher RPM values may provide improved uniformity and a reduction in defects. However, lower RPM values improve the total loading of the composition onto the prosthesis. Increasing the total centrifugation time may also improve the uniformity and reduce defects in the coating of the composition on the prosthesis. 
     The act  102  of applying the composition to the prosthesis is followed by the removal of the first fluid and any second fluid from the composition on the prosthesis in act  103 . The first and second fluids may be removed using techniques such as evaporation at ambient pressure and room temperature in an anhydrous atmosphere for 48 hours, or exposure to mild heat, e.g., 60-65° C., under vacuum conditions. In examples wherein the prosthesis is made of an absorbable polymer, the first and second fluids should be removed relatively quickly to avoid prolonged exposure and thereby ensure the mechanical integrity of the absorbable prosthesis. Upon removal of essentially all of the first fluid and any second fluid, a coating remains on the prosthesis. 
     In some embodiments, a polymeric coating, without a therapeutic substance, is applied to the prosthesis in act  104  prior to the application of the composition to the prosthesis in act  102 . The polymeric layer can serve as a primer to facilitate better adhesion of the composition to the prosthesis and is particularly useful when the composition does not itself contain polymeric material. Examples of suitable polymers were listed above, and such examples are equally applicable here. The polymeric primer can be formed on the prosthesis by any conventional method, such as the immersion or spray techniques described above. By way of example, and not limitation, the polymeric primer can have a thickness of about 0.5 microns to about 2.0 microns. The particular thickness of the layer is dependent on the desired use of the primer and the type of procedure for which the prosthesis is employed. The primer layer should be dried in act  105  prior to the application of the composition onto the primer layer in act  101 . Such drying may be accomplished using conventional methods known to one of ordinary skill in the art, such as those described above with reference to the drying of the composition in act  103 . 
     In some embodiments in which a primer is utilized, asperities, or areas of roughness, are created on the polymeric layer in act  106 . The asperities enable the primer layer to physically entrap the therapeutic substance-containing composition that is subsequently applied atop the primer layer in act  102 . A variety of methods can be used to create the asperities on the primer layer covering the outer surface of the prosthesis. 
     In one method, a pressurized stream of grit material is directed upon the polymeric primer layer after the primer layer has been dried in act  105 . Examples of such processes include bead blasting and sand blasting. Bead blasting refers to the use of pressurized gas to project beads of a relatively uniform diameter at an object at a high velocity. The beads may be made of materials such as, but not limited to, aluminum oxide, silicon oxide, or latex. In sand blasting, the grit projected does not have as uniform a diameter as in bead blasting. Both bead blasting and sand blasting are techniques that are well known to those of ordinary skill in the art. The roughness achieved using a pressurized grit source can be controlled by the size of the grit, e.g., the diameter of the beads, the pressure used, the distance between the grit source and the primer surface, and the length of time the grit is blasted at the primer surface. By way of example and not limitation, the grit can be beads having a diameter of between 10 μm and 50 μm. Pressures of 30 PSI (pounds per square inch) to 60 PSI can be used to project the beads from a distance of approximately 3-10 cm from the prosthesis. 
     In another embodiment, laser etching is used to create asperities on the primer layer after the primer layer has been dried in act  105 . Laser lithographic methods are known to those of ordinary skill in the art. A laser is directed onto the primer layer for a predetermined period of time, which depends on the etch rate and the depth of etch desired. A patterned mask that has openings may be applied over the primer layer before the laser is utilized. The laser is then allowed to etch the primer through the openings of the mask. The use of patterned masks with laser etching is known to those of ordinary skill in the art. 
     In another embodiment, the manner in which the primer is deposited onto the outer surface of the prosthesis in act  104  can create the asperities required in act  106 . The primer may be added via physical deposition processes, for example, sputtering, which is known to those of ordinary skill in the art. In sputtering, an energy beam, for instance an ion beam, is directed at a target formed of the material which is to be deposited on the substrate. The energy beam dislodges atoms of the target material. The target material atoms are transported to the substrate, which in this instance is the primer-coated stent on which asperities are being formed. Once at the substrate, the atoms form “islands”, or small nodules of the deposition material on the substrate. 
     Conventionally, sputtering is used to form a thin film of the deposition material over the substrate, and sputtering conditions are used such that the “islands” grow, spread, and condense on the substrate to form a thin film. To form the asperities, however, the sputtering conditions are set so that instead of creating a uniform film over the substrate, the “islands” do not grow, spread, and condense, leaving a rough surface. Process conditions in which a lower pressure and shorter deposition time than is typically used for thin film deposition are used to form the asperities in the primer layer. 
     In still another embodiment, a polymeric coating, without a therapeutic substance, is deposited in act  107  on the composition-based coating applied in act  102 . The polymeric topcoat can be applied by any conventional method, such as the immersion or spray techniques described above. The polymeric topcoat produces a membrane that reduces the rate of release of the therapeutic substance or substances from the composition-based coating. Examples of suitable polymers were listed above, and such examples are equally applicable here. By way of example, and not limitation, the polymeric topcoat can have a thickness of about 0.25 microns to about 1.5 microns. Typically, the topcoat can have a thickness of about 1 micron. It is understood by one of ordinary skill in the art that the thickness of the polymeric overlayer is based on factors such as the type of procedure for which the prosthesis is employed and the rate of release that is desired. The topcoat should be dried in act  108 . Such drying may be accomplished using conventional methods known to one of ordinary skill in the art, such as those described above with reference to the drying of the composition in act  103 . 
     In still another embodiment, both a polymeric primer and a polymeric topcoat are employed via acts  104  and  107 , respectively. In this embodiment, a polymeric primer is applied to a clean prosthesis prior to the application of the composition as described above. After the primer layer has been dried, the composition is applied atop the dried primer layer. The polymeric topcoat is applied and dried in acts  107  and  108 , respectively, subsequent to the application and drying of the composition in acts  102  and  103 , respectively. Such an embodiment facilitates the initial immobilization of the therapeutic substance on the prosthesis followed by the controlled release of the therapeutic substance. 
     Methods of Use 
     An implanted stent or graft, having the above-described therapeutic coating, is useful for treating occluded regions of blood vessels caused by thrombosis and/or restenosis, among other possible uses. 
     Stents may be placed in a wide array of blood vessels, both arteries and veins. Briefly, an angiogram is first performed to determine the appropriate positioning for stent therapy. Angiograms are typically accomplished by using a catheter to inject a radiopaque contrasting agent into an artery or vein as an X-ray is taken. A guidewire is then advanced through the lesion or proposed site of treatment. Over the guidewire is passed a delivery catheter which allows a stent in its collapsed configuration to be inserted into the passageway. The delivery catheter is inserted either percutaneously or by surgery into the femoral artery, brachial artery, femoral vein, or brachial vein and advanced into the appropriate blood vessel by steering the catheter through the vascular system under fluoroscopic guidance. A stent having the above described covering may then be expanded at the desired area of treatment. A post-insertion angiogram may also be utilized to confirm appropriate positioning. 
     Vascular grafts may be used to replace, bypass, or reinforce diseased or damaged sections of a vein or artery. The general procedure for implantation includes the step of pre-clotting, wherein the graft is immersed in the blood of the patient and allowed to stand for a period of time sufficient for clotting to ensue. After pre-clotting, hemorrhaging is less likely to occur when the graft is implanted, and thus the growth of tissue is not impeded. Grafts may be placed either through invasive surgery or non-invasively through percutaneous endoluminal transport. Percutaneous delivery of a graft avoids the complications and risks of surgery. The graft may be attached to the vessel at each end of the diseased region, thus bypassing the diseased region. Alternatively, the diseased region may be removed and replaced by the graft. 
     EXAMPLES 
     Exemplary embodiments of the invention are illustrated below. These examples are being given by way of illustration only and not by way of limitation. The parameters given, including percentages of fluids, therapeutic substance, polymers, temperature, duration of time, and other data, are exemplary, not limiting. 
     Example 1 
     The following is an exemplary method in which the composition includes a therapeutic substance and a first fluid. A polymeric primer layer and a polymeric topcoat are also employed. 
     Multi-Link Duet™ stents (available from Guidant Corporation) are cleaned by placement in an ultrasonic bath of isopropyl alcohol solution for 10 minutes. The stents are air-dried. A primer coating of ethylene vinyl alcohol copolymer (EVOH) is made with 1 gram of EVOH and 5 grams of dimethyl sulfoxide (DMSO), yielding an EVOH:DMSO ratio of 1:5. The EVOH is dissolved by placing the vial of EVOH:DMSO solution on a roller. bottle apparatus in an oven at 60° C. for 6 hours. The EVOH:DMSO solution is sprayed onto the cleaned Multi-Link Duet™ stents. The primer-coated stents are placed in an oven at 60° C. for 4 hours. 
     A 5% suspension of actinomycin D is made by adding 0.05 gram of actinomycin D to 1 gram of methanol. The primer-coated stents are dipped into the suspension for 5 seconds. The actinornycin D-coated stents are then air-dried for 15 minutes. 
     An EVOH topcoating, which is prepared in the same manner as the primer coating above, is sprayed onto the actinomycin D-coated stents. The coated stents are placed in a convection oven at 50° C. for 4 hours. The average amount of actinomycin D retained on the stents is about 39 micrograms. 
     Example 2 
     The following is another exemplary method in which the composition includes a therapeutic substance and a first fluid. A polymeric primer layer and a polymeric topcoat are also employed. 
     Multi-Link Duet™ stents are cleaned by placement in an ultrasonic bath of isopropyl alcohol solution for 10 minutes. The stents are air-dried. A primer coating of EVOH is made with 1 gram of EVOH and 7 grams of DMSO, yielding an EVOH:DMSO ratio of 1:7. The EVOH is dissolved by placing the vial of EVOH:DMSO solution on a roller bottle apparatus in an oven at 60° C. for 6 hours. The EVOH:DMSO solution is applied to the cleaned Multi-Link Duet™ stents by a modified dip method. The primer-coated stents are placed in an oven at 60° C. for 4 hours. 
     A 5% suspension of actinomycin D is made by adding 0.05 gram of actinomycin D to 1 gram of methanol. The primer-coated stents are dipped into the suspension for 5 seconds. The actinomycin D-coated stents are then air-dried for 15 minutes. 
     An EVOH topcoating, which is prepared in the same manner as the primer coating above, is sprayed onto the actinomycin D-coated stents. The coated stents are placed in a convection oven at 50° C. for 4 hours. The average amount of actinomycin D retained on the stents is about 15 micrograms. 
     Example 3 
     The following is an exemplary method in which the composition includes a therapeutic substance, a first fluid, a polymer, and a suspension stabilizer. 
     A Multi-Link Duet™ stent is cleaned by placement in an ultrasonic bath of isopropyl alcohol solution for 10 minutes. The stent is air-dried. An EVOH:DMSO solution is made with 1 gram of EVOH and 4 grams of DMSO, making a solution having a EVOH:DMSO ratio of 1:4. Actinomycin D is added to the EVOH:DMSO solution to form a suspension. Actinomycin D constitutes 10% by weight of the total weight of the suspension. Pluronic, a suspension stabilizer, is added to the suspension to slow the flocculation rate of the suspended actinomycin D particles. The suspension stabilizer constitutes 0.5% by weight of the total weight of the suspension. The suspension is vortexed for 10 seconds. The viscosity of the EVOH:DMSO solution is adequate to slow the flocculation rate of the suspended actinomycin D particles for approximately 30 minutes. The cleaned Multi-Link Duet™ stent is attached to mandrel wires and dipped into the suspension. The coated stent is then placed in a vacuum oven at 60° C. for 24 hours. It is predicted that the coating will remain intact on the stent. 
     Example 4 
     The following is an exemplary method in which the composition includes a therapeutic substance, a first fluid, a second fluid, and a polymer. 
     A Multi-Link Duet™ stent is cleaned by placement in an ultrasonic bath of isopropyl alcohol solution for 10 minutes. The stent is air-dried. An actinomycin D:tetrahydrofuran (THF) solution is made with 0.32 gram of actinomycin D and 4 grams of THF, making a solution having an actinomycin D:tetrahydrofuran (THF) ratio of 2:23. An EVOH:DMSO solution is made with 1 gram of EVOH and 4 grams of DMSO, making a solution having a EVOH:DMSO ratio of 1:4. The EVOH:DMSO solution is gradually added into the actinomycin D:THF solution. This process precipitates the actinomycin D from the actinomycin:THF solution. Actinomycin D constitutes 3.4% by weight of the total weight of the suspension. The suspension is vortexed for 10 seconds. The viscosity of the EVOH:DMSO:THF solution is adequate to slow the flocculation rate of the suspended actinomycin D particles. The cleaned Multi-Link Duet™ stent is attached to mandrel wires and dipped into the suspension. The coated stent is then placed in a vacuum oven at 60° C. for 24 hours. It is predicted that the coating will remain intact on the stent. 
     Example 5 
     The following is an exemplary method in which the composition includes a therapeutic substance, a first fluid, and a suspension stabilizer. A polymeric primer layer is also employed. 
     A synthetic graft made from polytetrafluoroethylene (PTFE) is cleaned by placement in an ultrasonic bath of isopropyl alcohol solution for 10 minutes. The graft is air-dried. An EVOH solution is made with 1 gram of EVOH and 7 grams of DMSO, making an EVOH:DMSO ratio of 1:7. The mixture is placed in a warm water shaker bath at 60° C. for 24 hours. The solution is cooled and vortexed. The cleaned graft is dipped in the EVOH:DMSO solution. The coated graft is then placed in a oven at 60° C., under vacuum condition, and for 24 hours. The coating will provide a primer layer to facilitate adhesion of the following actinomycin D layer. 
     A suspension of actinomycin D in DMSO:THF:EVAL is made with 0.32 gram of actinomycin D and 9 grams of DMSO:THF:EVAL, such that actinomycin D constitutes 3.4% by weight of the total weight of the suspension. The suspension is vortexed for 10 seconds. Pluronic, a suspension stabilizer, is added to the suspension to slow the flocculation rate of the suspended actinomycin D particles. The suspension stabilizer constitutes 0.5% by weight of the total weight of the suspension. The primer-coated graft is attached to mandrel wires and dipped into the suspension. The coated graft is then placed in a vacuum oven at 60° C. for 24 hours. It is predicted that the coatings will remain intact on the graft. 
     Example 6 
     The following is an exemplary method in which the composition includes a therapeutic substance and a first fluid. A polymeric topcoat is also employed. 
     A stent having cavities formed in the surface thereof is cleaned by placement in an ultrasonic bath of isopropyl alcohol solution for 10 minutes. The stent is air-dried. Cisplatin is micronized using a mortar and pestle assembly to yield particles ranging in size from 1-2 microns. A suspension of cisplatin in dimethyl formamide (DMF) is made such that cisplatin constitutes 5% by weight of the total weight of the suspension. The suspension is vortexed vigorously for 10 seconds to prevent flocculation. The cleaned stent is attached to mandrel wires and dipped into the suspension. The excess suspension is removed by lightly touching a highly porous, lint-free cloth to the stent for 30 seconds. 
     The cisplatin-coated stent is dipped into a 1:4 EVOH:DMSO solution, free from cisplatin. The stent is placed in the oven to dry as previously described. The top coating will provide a barrier layer for controlling the release of cisplatin from the cisplatin layer. It is predicted that the coatings will remain intact on the stent. 
     Example 7 
     The following is an exemplary method in which the composition includes a therapeutic substance and a first fluid. A polymeric primer layer and a polymeric topcoat are also employed. 
     A Multi-Link Duet™ stent is cleaned by placement in an ultrasonic bath of isopropyl alcohol solution for 10 minutes. The stent is air-dried. An EVOH:DMSO solution is made with 1 gram of EVOH and 7 grams of DMSO, making an EVOH:DMSO ratio of 1:7. The mixture is placed in a warm water shaker bath at 60° C. for 24 hours. The solution is cooled and vortexed. The cleaned Multi-Link Duet™ stent is dipped in the EVOH:DMSO solution. The EVOH-coated stent is placed in a oven at 60° C., under vacuum condition, and for 24 hours. The coating will provide a primer layer for the following cisplatin layer. 
     Cisplatin is micronized using a mortar and pestle assembly to yield particles ranging in size from 1-2 microns. A suspension of cisplatin in dimethyl formamide (DMF) is made such that cisplatin constitutes 5% by weight of the total weight of the suspension. The suspension is vortexed vigorously for 10 seconds to prevent flocculation. The primer-coated stent is attached to mandrel wires and dipped into the suspension. The excess suspension is removed by lightly touching a highly porous, lint-free cloth to the stent for 30 seconds. 
     The cisplatin-coated stent is dipped into a 1:4 EVOH:DMSO solution, free from cisplatin. The stent is placed in the oven to dry as previously described. The top coating will provide a barrier layer for controlling the release of cisplatin from the cisplatin layer. It is predicted that the coatings will remain intact on the stent. 
     Example 8 
     The following is an another exemplary method in which the composition includes a therapeutic substance and a first fluid. A polymeric primer layer and a polymeric topcoat are also employed. 
     A synthetic graft made from expanded polytetrafluoroethylene (ePTFE) is cleaned by placement in an ultrasonic bath of isopropyl alcohol solution for 10 minutes. The graft is air-dried. An EVOH:DMSO solution is made with 1 gram of EVOH and 7 grams of DMSO, making an EVOH:DMSO ratio of 1:7. The mixture is placed in a warm water shaker bath at 60° C. for 24 hours. The solution is cooled and vortexed. The cleaned graft is dipped in the EVOH:DMSO solution. The EVOH-coated graft is placed in a oven at 60° C., under vacuum condition, and for 24 hours. The coating will provide a primer layer for the following cisplatin layer. 
     Cisplatin is micronized using a mortar and pestle assembly to yield particles ranging in size from 1-2 microns. A suspension of cisplatin in methanol is made such that cisplatin constitutes 5% by weight of the total weight of the suspension. The suspension is vortexed vigorously to prevent flocculation. The primer-coated graft is dipped into the suspension. The excess suspension is removed by lightly touching a highly porous, lint-free cloth to the stent for 30 seconds. 
     The cisplatin-coated graft is dipped into a 1:4 EVOH:DMSO solution, free from cisplatin. The graft is placed in the oven to dry as previously described. The top coating will provide a barrier layer for controlling the release of cisplatin from the cisplatin layer. It is predicted that the coatings will remain intact on the graft. 
     While particular embodiments, applications, and examples of the present invention have been shown and described, it will be apparent to those of ordinary skill 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.