Abstract:
A drug eluting stent is disclosed. The stent includes a stent base; a basecoat formulation comprising Sirolimus in a polymer coating, wherein the polymer coating contains ethylene-co-vinyl acetate and poly-n-butyl methacrylate; and a topcoat disposed on the basecoat formulation.

Description:
CROSS REFERENCE  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 11/132,048 filed on May 17, 2005, which is a divisional application of U.S. patent application Ser. No. 10/678,346 filed Oct. 3, 2003 (U.S. Pat. No. 6,953,560), which is a continuation application of U.S. patent application Ser. No. 09/676,049 filed Sep. 28, 2000 (U.S. Pat. No. 6,716,444). This divisional application claims the priority benefit of U.S. patent application Ser. Nos. 11/132,048, 10/678,346 and 09/676,049, and this divisional application hereby incorporates by reference the complete contents of these applications. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to implantable medical devices that release a drug.  
         [0004]     2. Description of the Background  
         [0005]     Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease. A catheter assembly having a balloon portion is introduced 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 model the arterial lumen. The balloon is then deflated to a smaller profile to allow the catheter to be withdrawn from the patient&#39;s vasculature.  
         [0006]     In treating the damaged vasculature tissue, and to deter thrombosis and restenosis, drugs are commonly administered to the treatment site. For example, anticoagulants are commonly used to prevent thrombosis of the coronary lumen. Antiplatelets are administered to reduce the incidence of major adverse cardiac events. Cytostatic agents are presently used in clinical trials to reduce post-angioplasty proliferation of the vascular tissue.  
         [0007]     Systemic administration of such drugs in sufficient amounts to supply an efficacious concentration to the local treatment site often produces adverse or toxic side effects for the patient. Accordingly, local delivery is a preferred method of treatment since smaller amounts of medication are administered in comparison to systemic dosages, but the medication is concentrated at a specific treatment site. Local delivery thus produces fewer side effects and achieves more effective results.  
         [0008]     A common technique for local delivery of drugs involves coating a stent or graft with a polymeric material which, in turn, is impregnated with a drug or a combination of drugs. Once the stent or graft is implanted within a cardiovascular system lumen, the drug(s) is released from the polymer for the treatment of the local tissues. U.S. Pat. No. 5,605,696 to Eury et al., U.S. Pat. No. 5,464,650 to Berg, et al., and U.S. Pat. No. 5,700,286 to Tartaglia, et al. provide examples illustrating the use of a polymeric coating for the local delivery of a drug or substance.  
         [0009]     Stents are scaffoldings, usually cylindrical or tubular in shape, which are inserted into an anatomical passageway and operate to physically hold open and, if desired, to expand the wall of a passageway. Stents are capable of being crimped onto balloon catheters for insertion through small cavities, positioned in a desired location, and then expanded to a larger diameter. Stents can be either balloon expandable or self-expanding.  
         [0010]     Grafts are typically placed in a blood vessel to either replace a diseased segment that has been removed, or to form a bypass conduit through a damaged segment of the vessel wall as is the case with an aneurysm, for example. The graft has a tubular portion which spans the site of the damaged tissue and through which the blood flows. The graft has sections at both ends of the tube that are used to secure the graft to the inside of a vessel wall. The graft also has an outer surface, portions of which are in contact with an inner surface of the blood vessel wall, and an inner surface in contact with the blood flowing through the vessel.  
         [0011]      FIG. 1  shows an implantable medical device  10 , which may be a stent or graft. Device  10  includes a substrate  12  that may be formed of stainless steel, nickel titanium alloy, or another biocompatible metal. Substrate  12  is covered (usually conformally) by a first layer  14 . First layer  14  includes polymer containing a drug  16 .  
         [0012]     An equation describing the drug release rate per unit area of device  10  is as follows:  
               Φ   ⁡     (   t   )       =       D   p     ⁢       ∑     n   =   0     -     ⁢       b   n     ⁢     exp   (         -     π   2       ⁢     D   p     ⁢       t   ⁡     (       2   ⁢   n     +   1     )       2         4   ⁢     T   2         )     ⁢     (       -   π       2   ⁢       T   ⁡     (       2   ⁢   n     +   1     )       n         )                   (     Equation   ⁢           ⁢   1     )             
 
         [0013]     where Φ(t)=release rate of drug as a function of time, 
        D p =diffusivity of drug  16  in polymer film  14 ,     π=3.14159,     T=thickness of polymer film  14 ,        
 
         [0017]     and b n  
         b   n     =       2   T     ⁢       ∫   O   T     ⁢       C   O     ⁢   cos   ⁢     {       π   ⁢           ⁢     x   ⁡     (       2   ⁢   n     +   1     )           2   ⁢   T       }     ⁢     ⅆ   x               
 
 where Co is the concentration of drug in the polymer at time zero. 
 
         [0018]     Equation 1 assumes that: (1) all resistance to drug release is determined by the diffusivity of drug  16  in polymer  14 ; (2) the concentration of drug  16  is uniform throughout; (3) drug  16  does not go into the metallic surface  12 ; and (4) drug  16  is rapidly removed from the surface of polymer  14  as soon as drug  16  is released from polymer  14 .  
         [0019]     The diffusivity of drug  16  in polymer  14 , D P , in turn is determined by certain properties of drug  16  (e.g., molecular weight, size) and physical properties of the polymer  14  through which drug  16  is diffusing (e.g., pore size, crystallinity, glass transition temperature, polarity or hydrophobicity).  
         [0020]      FIG. 2  illustrates the predicted drug release rate curve for a polymer matrix carrying a drug, such as first layer  14 , illustrated in  FIG. 1 . Curve  8  is an exponentially decreasing curve.  
         [0021]     A problem associated with the use of a polymeric coating as a matrix for carrying the drug is that the rate at which the drug is released is highly variable, typically exhibiting a very high rate of release after the medical device is implanted in the patient, followed by a significantly lower rate of release. This may be undesirable in many applications, since the initial concentrations may be too high (causing undesirable side effects or even cell death), the later concentrations may be too low to have any therapeutic effect, and the overall residence time of the drug in the target area may be too short to provide the desired therapeutic effect.  
         [0022]     For example, for certain antiproliferative drugs, a residence time of thirty minutes may be all that is required to achieve a permanent effect, while others may take up to two weeks. Where nitrous oxide (NO) is used as the antiproliferative drug, a residence time of four to eight weeks is desirable, but even longer durations up to twelve weeks may be beneficial, depending on the patient.  
         [0023]     With respect to anti-inflammatory drugs, a long residence time (e.g., several weeks) is desirable, because the anti-inflammatory drug should be delivered until some amount of healing has occurred. Anti-thrombogenic drugs also may require a long residence time, for example, up to five months, since that much time may be required for a stent to become endothelialized.  
         [0024]     Thus, there is a need for a mechanism for controlling the release rate of drugs from implantable medical devices to increase the efficacy of local drug delivery in treating patients.  
       SUMMARY OF THE INVENTION  
       [0025]     The present invention allows for a controlled rate of release of a drug or drugs from a polymer carried on an implantable medical device. The controlled rate of release allows localized drug delivery for extended periods, e.g., weeks to months, depending upon the application. This is especially useful in providing therapy to reduce or prevent cell proliferation, inflammation, or thrombosis in a localized area.  
         [0026]     One embodiment of an implantable medical device in accordance with the present invention includes a substrate, which may be, for example, a metal or polymeric stent or graft, among other possibilities. At least a portion of the substrate is coated with a first layer that includes one or more drugs in a polymer carrier. A barrier coating overlies the first layer. The barrier (which may be considered a coating) reduces the rate of release of the drug from the polymer once the medical device has been placed into the patient&#39;s body, thereby allowing an extended period of localized drug delivery once the medical device is in situ.  
         [0027]     The barrier is necessarily biocompatible (i.e., its presence does not elicit an adverse response from the body), and typically has a thickness ranging from about 50 angstroms to about 20,000 angstroms. It is contemplated that the barrier contains mostly inorganic material. However, some organic compounds (e.g., polyacrylonitrile, polyvinylidene chloride, nylon 6-6, perfluoropolymers, polyethylene terephthalate, polyethylene 2,6-napthalene dicarboxylate, and polycarbonate) may be incorporated in the barrier. Suitable inorganic materials for use within the barrier include, but are not limited to, inorganic elements, such as pure metals including aluminum, chromium, gold, hafnium, iridium, niobium, palladium, platinum, tantalum, titanium, tungsten, zirconium, and alloys of these metals, and inorganic compounds, such as inorganic silicides, oxides, nitrides, and carbides. Generally, the solubility of the drug in the material of the barrier is significantly less than the solubility of the drug in the polymer carrier. Also, generally, the diffusivity of the drug in the material of the barrier is significantly lower than the diffusivity of the drug in the polymer carrier.  
         [0028]     The barrier may or may not be biodegradable (i.e., capable of being broken down into harmless compounds by the action of the body). While it is contemplated that non-biodegradable barrier may be preferable, some biodegradable materials may be used as barriers. For example, calcium phosphates such as hydroxyapatite, carbonated hydroxyapatite, tricalcium phosphate, beta-tricalcium phosphate, octacalcium phosphate, amorphous calcium phosphate, and calcium orthophosphate may be used. Certain calcium salts such as calcium phosphate (plaster of pans) may also be used. The biodegradability of the barrier may act as an additional mechanism for controlling drug release from the underlying first layer.  
         [0029]     The one or more drugs contained within the polymer may include, but are not limited to, antineoplastic, anti-inflammatory, antiplatelet, anticoagulant, fibrinolytic, thrombin inhibitor, antimitotic, antiallergic, and antiproliferative substances.  
         [0030]     In accordance with one embodiment of the present invention, the barrier is a homogeneous layer. A homogeneous layer of barrier may be produced by several methods, depending on the type of materials selected from the barrier. For example, nitride barriers, such as titanium nitride and chromium nitride, may be deposited by cathodic arc physical vapor deposition. Oxide barriers, such as silicon dioxide and aluminum oxide, can be produced by reactive sputtering. Metallic barriers, such as aluminum, gold, tungsten, platinum, or alloys of metals, may be produced by sputtering, thermal evaporation, or electron beam evaporation, as well as electroless deposition.  
         [0031]     In accordance with another embodiment of the present invention, the barrier is formed by a number of discrete deposits on the surface of the polymer coating. The release rate of the drug from the polymer coating may be manipulated by controlling the fraction of the surface area covered by the barrier. Such a barrier may be obtained, for example, by cathodic arc sputtering, reactive sputtering, thermal evaporation, and electron beam (e-beam) evaporation of materials such as aluminum, chromium, gold, iridium, niobium, platinum, tantalum, titanium, and tungsten.  
         [0032]     In accordance with another embodiment of the present invention, the barrier is intermixed with the first layer at and near the outer surface of the first layer, rather than being a discrete layer atop the first layer. This embodiment may be produced by several techniques, including for example, ion implantation, plasma ion implantation, alkoxide hydrolysis, and electroless deposition. Ion implantation and plasma ion implantation may produce, for example, titanium and palladium barrier coatings. Alkoxide hydrolysis may produce barrier coatings of titanium oxide, zirconium oxide, and aluminum oxide from titanium alkoxides, zirconium alkoxides and aluminum alkoxides, respectively. Electroless deposition may produce, for example, palladium and gold barrier coatings.  
         [0033]     These and other embodiments and aspects of the present invention may be better understood in view of the drawings and the following detailed description.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]      FIG. 1  is a partial cross-sectional view of a conventional implantable medical device having a polymer coating that carries a drug.  
         [0035]      FIG. 2  illustrates a theoretical drug release rate curve for the implantable medical device of  FIG. 1 .  
         [0036]      FIG. 3  is a partial cross-sectional view of a barrier on the surface of a polymer-coated implantable medical device in accordance with one embodiment of the present invention.  
         [0037]      FIG. 4  is a partial cross-sectional view of a barrier on the surface of a polymer-coated implantable medical device in accordance with a second embodiment of the present invention.  
         [0038]      FIG. 5  is a graph depicting the release of dexamethasone acetate from a silicone polymer and the predicted release of dexamethasone acetate when a barrier is applied to the silicone polymer.  
         [0039]      FIG. 6  is a partial cross-sectional view of various types of barriers on or near the surface of an implantable medical device in accordance with a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0040]      FIG. 3  is a partial cross-sectional view of implantable medical device  11 , which may be a stent or graft, among other possibilities. Medical device  11  includes a substrate  12 . A first layer  24  is present on the outer surface of substrate  12 . First layer  24  includes a polymer and one or more drugs  16  carried within the polymer. First layer  24  can have a thickness ranging from about 2,000 angstroms to about 200,000 angstroms, or more particularly from about 10,000 angstroms to about 100,000 angstroms. Barrier  28  is present as a homogenous layer on the top surface of first layer  24 . Barrier  28  is biocompatible, and typically has a thickness ranging from about 50 angstroms to about 20,000 angstroms, e.g., from about 100 angstroms to about 5,000 angstroms. In this embodiment, barrier  28  is an inorganic material.  
         [0041]     Barrier  28  acts to reduce the rate of delivery of drug  16  to internal target tissue area  50  by at least one of two mechanisms. First, the solubility of drug  16  in barrier  28  is substantially less (i.e., at least two to three orders of magnitude less) than the solubility of drug  16  in first layer  24 . Likewise, the diffusivity of drug  16  in barrier  28  is significantly lower than the diffusivity of drug  16  in first layer  24 . Accordingly, drug  16  will diffuse more slowly through barrier  28  than through first layer  24 . Secondly, barrier  28 , being mostly an inorganic material, is composed of much smaller molecules than those found within the polymer in first layer  24 . The barrier molecules are aligned more tightly and in a more orderly fashion than the polymer molecules, thereby leaving less void space through which drug  16  can travel.  
         [0042]     Some barriers, especially defect-free metallic films and SiO X  films, will be very impermeable, except for microfractures through the barrier coating. While these microfractures may link the top and bottom of the barrier coating, the paths of the microfractures linking the top and bottom of the barrier coating are tortuous, and thus the path that drug  16  must take to exit medical device  11  is much longer than the thickness of barrier  28 . Moreover, since the drug is restricted to diffusing only through the microfractures, the area available for diffusion is greatly reduced as well.  
         [0043]     A simple model for the embodiment illustrated in  FIG. 3  holds that the rate of release of drug  16  from barrier  28  per unit area may be expressed by the following equation:  
               Release   ⁢           ⁢   rate     =           D   1     ⁢   k       T   1       ⁢     C   p   o     ⁢   exp   ⁢     {         -     D   1       ⁢   kt         T   p     ⁢     T   1         }               (     Equation   ⁢           ⁢   2     )             
 
         [0044]     where D 1 =diffusivity of drug  16  in barrier  28 ; 
        k=partition coefficient for drug  16  between first layer  24  and barrier  28 ;     T 1 =thickness of barrier  28 ;     Cp o =initial concentration of drug  16  in barrier  28 ; and     Tp=thickness of first layer  24 .        
 
         [0049]     This equation assumes that all of the resistance to drug release is in the barrier  28 . This is a reasonable assumption since, typically, D 1  is at least two to three orders of magnitude lower than the diffusivity of drug  16  in first layer  24 . As can be seen from Equation 2, variables D 1 , k, and T 1 , are the most significant variables affecting drug release. Of course, the present invention does not depend on this equation or the assumptions discussed above.  
         [0050]     Suitable inorganic materials for barrier  28  include, but are not limited to, silicides, oxides, nitrides, and carbides. Suitable silicides may include suicides of vanadium, zirconium, tungsten, titanium, niobium, and tantalum. Suitable oxides may include oxides of aluminum, barium, calcium, hafnium, niobium, silicon, tantalum, titanium, tungsten, and zirconium. Suitable nitrides may include nitrides of chromium, silicon, titanium, and zirconium. Suitable carbides may include carbides of silicon and titanium. Other suitable materials may include molybdenum disulfide, amorphous diamond, diamond-like carbon, pyrolytic carbon, ultra-low temperature isotropic (ULTI) carbon, amorphous carbon, strontium titanate, and barium titanate. Also suitable for use are pure metals, such as aluminum, chromium, gold, hafnium, iridium, niobium, palladium, platinum, tantalum, titanium, tungsten, zirconium, and alloys of these metals.  
         [0051]     Several methods may be used to deposit barrier  28  on first layer  24 , depending on the application. For example, silicide compounds, such as vanadium disilicide, zirconium disilicide, tungsten disilicide, titanium disilicide, niobium disilicide, tantalum disilicide, vanadium silicide, titanium trisilicide, and tantalum trisilicide may be deposited by sputtering or chemical vapor deposition (CVD).  
         [0052]     Oxide barrier coatings, such as tantalum oxide, titanium dioxide, zirconium oxide, niobium oxide, tungsten oxide, aluminum oxide, and silicon dioxide can be produced by reactive sputtering. The power source used in this method may be AC or DC, and utilizes the pureelement as a target with a sputter gas of argon and low levels of oxygen.  
         [0053]     Nitride barrier coatings, such as titanium nitride, titanium carbonitride, chromium nitride, titanium aluminum nitride, and zirconium nitride can be deposited on first layer  24  at relatively low temperatures (i.e., less than 60° C.) by cathodic arc vacuum deposition. Such a method may be chosen where first layer  24  and drug  16  are temperature-sensitive.  
         [0054]     Films of pure metals (e.g., aluminum, gold, tungsten, platinum) may be produced by physical vapor deposition (PVD) methods such as sputtering, thermal evaporation, or electron beam evaporation. Alloys of these metals can be deposited by sputtering if, for example, an alloy sputtering target is used or multiple metal targets are simultaneously sputtered. Alloys may also be deposited utilizing thermal evaporation or electron beam evaporation if several evaporation sources are used simultaneously.  
         [0055]     Suitable polymer materials for first layer  24  can include, but are not limited to, polyurethanes, polyesterurethanes, silicone, fluoropolymers, ethylene vinyl acetate, polyethylene, polypropylene, polycarbonates, trimethylenecarbonate, polyphosphazene, polyhydroxybutyrate, polyhydroxyvalerate, polydioxanone, polyiminocarbonates, polyorthoesters, ethylene vinyl alcohol copolymer, L-polylactide, D,L-polylactide, polyglycolide, polycaprolactone, copolymers of lactide and glycolide, polymethylmethlacrylate, poly(n-butyl)methacrylate, polyacrylates, polymethacrylates, elastomers, and mixtures thereof.  
         [0056]     Representative elastomers include, but are not limited to, a thermoplastic elastomer material available under the trade name “C-FLEX” from Concept Polymer Technologies of Largo, Fla., polyetheramide thermoplastic elastomer, fluoroelastomers, fluorosilicone elastomer, sytrene-butadiene rubber, butadiene-styrene rubber, polyisoprene, neoprene (polychloroprene), ethylene-propylene elastomer, chloro-sulfonated polyethylene elastomer, butyl rubber, polysulfide elastomer, polyacrylate elastomer, nitrile, rubber, polyester, styrene, ethylene, propylene, butadiene and isoprene, polyester thermoplastic elastomer, and mixtures thereof.  
         [0057]     The choice of the polymer in first layer  24  typically depends on the selection of the particular drug  16 . For a drug  16  that is to be dissolved in the polymer, the factors to be considered in selecting an appropriate polymer include the molecular structure of both the polymer and the drug, the crystallinity or amorphousness of the polymer, the molecular weight of the drug (which is indicative of its size on a molecular level), and the solubility of the drug in the polymer. Examples of drug/polymer pairings include dexamethasone in silicone or polyurethanes or fluoroelastomer; taxol in ethylene vinyl acetate or L-polylactide or polycaprolactone or polyurethanes; rapamycin in polymethylmethacrylate or poly(n-butyl)methacrylate; tranilast in polyurethanes or ethylene vinyl acetate; and fluorouracil in polyurethanes.  
         [0058]     Examples of drug  16  include 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 antiinflammatory substances include estradiol. Examples of such antiplatelets, anticoagulants, antifibrin, and antithrombins include sodium heparin, low molecular weight heparins, heparinoids, hirudin, argatroban, forskolin, vapiprost, prostacyclin and prostacyclin analogs, 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 antiallergic substances include permirolast potassium and tranilast. 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, mitomycin, 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, fibrolast growth factor (FGF) antagonists, fish oil (omega 3-fatty acid), histamine antagonists, lovastatin (an inhibitor of HMB-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.  
         [0059]     Other drugs or agents that may be used include, for example, alpha-interferon, genetically engineered epithelial cells, and dexamethasone. While the preventative and treatment properties of the foregoing drugs 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 drugs that may be carried in a polymer coating are equally applicable for use with the present invention.  
         [0060]     According to the present invention, the release rate of the drug typically will be controlled in part by the material selected for the barrier. For instance, the barrier material may be an inorganic compound, which is typically polar or ionic in nature. Generally, if the selected drug  16  is likewise polar or ionic in nature (e.g., cisplatin, fluoracil), then the solubility of drug  16  in barrier  28  will be higher, resulting in a higher diffusivity, and thus a faster rate of delivery of drug  16  through barrier  28  to internal target tissue area  50 . Conversely, if the selected drug  16  is nonpolar, (e.g., TAXOL®, dexamethasone), then the solubility of drug  16  in barrier  28  will be lower, resulting in a lower diffusivity, and thus a lower rate of delivery of drug  16  through barrier  28  to internal target tissue area  50 . However, this is not always the case. Titanium dioxide is a polar inorganic compound, but it has such a high degree of crystallinity, and an affinity for itself that other polar compounds (e.g., the drug to be delivered) do not necessarily dissolve into titanium dioxide very well.  
         [0061]      FIG. 4  illustrates another embodiment of the present invention, wherein barrier  28  is composed of a number of discrete deposits above first layer  24 . When using certain deposition methods, the first few atoms or molecules of barrier  28  deposited on first layer  24  form small “nucleation sites,”  30  on the surface  29  of first layer  24 . Barrier  28  may be made discontinuous by arresting the deposition process at the nucleation stage, as illustrated in  FIG. 4 . However, if the deposition is allowed to continue, nucleation sites  30  expand in surface area and eventually coalesce into a continuous film, as barrier  28  is illustrated in  FIG. 3 .  
         [0062]     In the embodiment illustrated in  FIG. 4 , the rate of drug delivery from first layer  24  to internal tissue target area  50  is reduced primarily by the presence of nucleation sites  30 . Nucleation sites  30  reduce the surface area available for drug  16  to diffuse from first layer  24  toward the internal tissue target area  50 , because the vast majority of drug  16  will preferentially diffuse between nucleation sites  30  at spaces  32 . While drug  16  may diffuse through nucleation sites  30 , it will do so at a much slower rate. Drug  16  situated beneath nucleation site  30  will not proceed directly to tissue  50  through nucleation site  30 , but will instead be directed by the diffusion gradient formed toward a space  32 . Nucleation sites  30  thus may also increase the length and tortuosity of the average diffusion path taken by molecules of drug  16 , again resulting in a slower rate of diffusion of drug  16  from first layer  24 .  
         [0063]     The release rate associated with the embodiment of  FIG. 4  may be generally described by the following equation: 
 
Release Rate=(1− A )Φ( t )  (Equation 3) 
 
         [0064]     where A=fraction of surface area covered by nucleation sites; and  
         [0065]     Φ(t) is as defined in Equation 1.  
         [0000]     Equation 3 assumes that there is no diffusion through nucleation sites  30 . Again, this equation is provided as an explanatory aid, and does not in any way limit the invention.  
         [0066]     Advantageously, in this embodiment, the rate of drug delivery (and thus, the effective treatment time) may be manipulated by controlling the number of nucleation sites  30  that are permitted to form on surface  29  of first layer  24 . The number of nucleation sites may be controlled by controlling various deposition process parameters, such as deposition rate, substrate temperature, energy of the deposited atoms, substrate surface cleanliness and defects, angle of the incident atom flux, and substrate surface roughness.  
         [0067]     The embodiment illustrated in  FIG. 4  may be achieved by performing deposition processes that deposit layers of material by way of nucleation, such as cathodic arc sputtering, reactive sputtering, thermal evaporation and electron beam (e-beam) evaporation of materials such as gold, palladium, and aluminum.  
         [0068]     The embodiment illustrated in  FIG. 4  may also be achieved by depositing a continuous film, and then creating holes in that film. For example, metal films can be deposited with differing amounts of grain structure. An etching chemical (e.g., typically mixtures of mineral acids) may be used to preferentially etch between grains and remove some of the metal film. Alternately, a continuous film could be deposited, and holes made in that continuous film by, for example, ion milling, a laser, or electron beam machining. Another method for achieving this embodiment would be deposition of a continuous film, followed by mechanically stressing the film, so that the continuous film fractures at preselected locations.  
         [0069]      FIG. 6  illustrates another embodiment of the invention, wherein first layer  24 , and the barrier coating are not two discrete layers Instead, the interface between first layer  24  and barrier  40  is irregular, because barrier  40  is blended with first layer  24  to form an intermixed zone  42  at and near the top surface of first layer  24 . Intermixing of the barrier  40  with first layer  24  may result when barrier  40  is deposited using one of several methods, whereby the deposited inorganic materials actually penetrate into first layer  24 , rather than merely adhering to the surface of first layer  24 . These methods include, for example, ion implantation, plasma ion implantation, alkoxide hydrolysis, and electroless deposition. Intermixed zone  42  may include isolated bodies  43  of barrier material formed within first layer  24  and/or fingers  45  of barrier coating material extending into first layer  24 .  
         [0070]     The rate of drug delivery from first layer  24  to internal target tissue area  50  may be reduced in large part by at least two mechanisms in this embodiment. First, if drug  16  diffuses through the barrier material in intermixed zone  42 , then drug  16  may have to follow a more tortuous diffusion path than drug  16  would otherwise follow, decreasing the diffusion rate. Second, if the barrier material in intermixed zone  42  acts as a complete barrier to diffusion, then the area normal to the direction of diffusion is significantly reduced, thus reducing the diffusion rate, in a manner similar to the nucleation sites in the embodiment illustrated in  FIG. 4 .  
         [0071]     Meaningful quantitative or semiquantitative descriptions of drug release rates from the embodiment illustrated in  FIG. 6  are difficult to provide, since the geometry of the intermixed zone  42  is highly variable. However, it can be stated that, regardless of the geometry of the intermixed zone, the rate of release of drug  16  from first layer  24  will be significantly reduced.  
         [0072]     One of ordinary skill in the art will appreciate that, for all of the embodiments described herein, the thickness of the barrier used may be varied, with a corresponding change in the drug release rate. Generally, the thicker the barrier, the greater the reduction in the drug release rate. However, there is a practical limit on the thickness of the barrier to be applied to first layer  24 . If the barrier becomes too thick, then, when subjected to stress from mechanical flexing or changes in temperature, internal stresses will build up within the barrier. When these internal stresses exceed the adhesion between the barrier and the underlying film, the barrier may fracture throughout. Small pieces of the barrier may then be lifted off from first layer  24 . The pieces could enter the lumen of a blood vessel, and act as an embolus, with potentially very harmful effects on the patient.  
       EXAMPLE 1  
     Use of SiO, Film as a Barrier Coating to Reduce the Rate of Release of Nitric Oxide from a Polymer Coated Medical Device  
       [0073]     Healthy endothelial cells located along the intima (the innermost layer of cells lining the blood vessels) produce a nearly constant supply of nitric oxide. Nitric oxide molecules signal the surrounding smooth muscle cells. We currently believe nitric oxide molecules also prevent the smooth muscle cells from migrating and/or proliferating, which, it is postulated, would reduce the incidence of restenosis following PTCA. To be effective in local delivery, nitric oxide would ideally be released over a period of weeks (e.g., four to eight weeks, generally, and in some cases, up to twelve weeks, depending on the patient). In accordance with one embodiment of the invention, a homogeneous film of non-stoichiometric silicon dioxide (known as Si0 x  is deposited as barrier coating  28  on first layer  24 . First layer  24  contains drug  16 , which in this case is nitric oxide or a molecule which releases nitric oxide.  
         [0074]     Experimental data collected by others has shown that a polycarbonate film having an average molecular weight in the range of 20,000 to 40,000 g/mole has an oxygen (02) transmission rate of 1000 cm 3 /m 2 -atm-day. See, Erlat, et al., “Morphology and Gas Barrier Properties of Thin SiO x  Coatings on Polycarbonate: Correlations with Plasma-enhanced Chemical Vapor Deposition Conditions,” J. Mater. Res., Vol. 15, No. 3, March 2000. By depositing a SiO x  coating having a thickness of approximately 30 nanometers (nm) on the polycarbonate, using plasma-enhanced chemical vapor deposition, the oxygen transmission rate was reduced by one order of magnitude to approximately 100 cm 3 /m 2 -atm-day. By increasing the thickness of the SiO x  coating deposited to 60 nanometers, the oxygen transmission rate was further reduced by another order of magnitude, to approximately 10 cm 3 /m 2 -atm-day. A less dramatic reduction in oxygen transmission was observed with further increases in SiO, coating thickness.  
         [0075]     Since oxygen (O 2 ) and nitric oxide (NO) are similar to each other in size and molecular weight, we anticipate that an implanted medical device (e.g., a stent or graft) having a SiO x  layer provided atop a polymeric coating containing nitric oxide would cause a similar reduction in the rate at which nitric oxide is released from the polymeric material into a target tissue area. That is, the release rate of nitric oxide from the polymeric carrier could be reduced by a factor of 10 by depositing a SiO x  coating that is 30 nanometers thick on the polymeric carrier, and reduced by a factor of 100 by depositing a SiO x  coating 60 nanometers thick on the polymeric carrier. As a result, for a given mass of nitric oxide loaded into the polymer carrier, the period of time over which nitric oxide is released could be extended by factors of 10 and 100, respectively. Accordingly, the implantable medical device having such a polymer coating and barrier coating would be useful in providing localized drug delivery for an extended period to a patient after PTCA.  
       EXAMPLE 2  
     Use of Discontinuous Barrier Coating to Reduce the Rate Of Release of Dexamethasone from a Silicone Polymer  
       [0076]     Line A of  FIG. 5  illustrates the release of dexamethasone acetate (C 22 H 29 F 05 ), a synthetic steroid used as an anti-inflammatory agent, from collars made from a silicone polymer over a period of 25 days when the polymeric collars were immersed in saline solution at a temperature of 37° C. Each polymeric collar was loaded with approximately 0.25 mg of dexamethasone acetate. A total of twenty polymeric collars were tested, and the averaged results are shown. After 25 days, a total of 0.018 mg of dexamethesone had been released, on average, from each polymeric collar.  
         [0077]     Assuming nucleation sites  30  of a barrier material were deposited on the silicone polymer in a manner similar to that illustrated in  FIG. 4 , so as to cover 50% of the available surface area, then the release rate would be reduced by 50%, with the result that the drug release curve would be predicted to appear as shown by line B of  FIG. 5 . That is, after 25 days, an average of 0.009 mg of dexamethasone would be released. Advantageously, the reduction in the drug release rate by 50% essentially means that the drug would be locally delivered to the body for a period of time twice as long, significantly extending the time of drug release.  
         [0078]     Similarly, the process parameters could be controlled so that nucleation sites  30  cover 75% of surface  29  of first layer  24  (which  FIG. 4  approximates). This would result in a drug delivery rate that is approximately 25% lower than that associated with the homogeneous film embodiment illustrated in  FIG. 3 .  
       EXAMPLE 3  
     Method for Producing a Titanium Oxide Barrier Coating on a Polymer-Coated Medical Device by Alkoxide Hydrolysis  
       [0079]     One method by which the embodiment illustrated in  FIG. 6  may be produced is alkoxide hydrolysis. If an implantable medical device having a first layer  24  is exposed to a humid environment and then immersed in a metal alkoxide solution, within seconds, metal oxide will be formed at or near the top surface of first layer  24  by reaction of the alkoxide with the water in the polymer coating. As long as the polymer contains some moisture (e.g., at least 1% by weight), the reaction can go forward. Examples of suitable metal alkoxides include titanium ethoxide and titanium propoxide.  
         [0080]     A solution of titanium ethoxide is created in dry ethanol at concentrations from about 0.1% to 10%. A polymer coated stent is exposed to an environment with controlled relative humidity between 20% and 100% for between 0.5 and 60 minutes. The stent is then immersed in the alkoxide solution for between 1 and 60 seconds, and then removed. The excess alkoxide solution is rinsed off using ethanol or water. The stent is then dried in a suitable environment (temperature from 20° to 70° C. for 1 to 60 minutes).  
       EXAMPLE 4  
     Methods for Producing Barrier Coatings on a Polymer by Electroless Plating  
       [0081]     Another method by which the embodiments illustrated in  FIGS. 3 and 6  may be produced is reductive deposition in solution, or electroless plating of a material onto a polymer coated surface of an implantable medical device. Electroless plating uses a chemical reduction reaction to deposit metal on a substrate without the passage of an electric current.  
         [0082]     Nonconductive substrates, such as polymers, are readily plated by this method if the polymer is first catalytically activated. The polymer may be catalytically activated by, for example, immersing it in a colloidal solution containing palladium. This colloidal solution provides nucleation sites of palladium (in this instance) on the polymer prior to electroless deposition of the metal. Certain polymers require an etching step before activation.  
         [0083]     Biocompatible inorganic barriers that may be deposited by electroless deposition include palladium (Pd) and gold (Au). Reducing agents for palladium include hypophosphite, amineborane, and hydrazine. Reducing agents for gold include borohydride, amineborane, and hydrazine. The principal reactions that are believed to occur during electroless palladium plating,  
         [0084]     using hypophosphite as a reducing agent in one commercial application, are shown below. 
 
2H 2   − PO 2 +2H 2 O→2HPO 3   −2 +4H + +2H  (1) 
 
Pd ++ +2H − →Pd 0 +H 2   (2) 
 
2H 2 PO 2   − +2H→2P+4OH − +H 2   (3) 
 
         [0085]     “Training Course in Electroplating and Surface Finishing,” ©1998 by American Electroplaters and Surface Finishers Society, Inc., AESF, 12644 Research Parkway, Orlando, Fla. 32826-3298.  
         [0086]     According to the authors, the hypophosphite ion is thought of as being catalytically oxidized (dehydrogenated) on the surface to be plated to release hydride ions (H—) which are then available for reduction of palladium ions at the surface. The products of the reactions are thus the palladium-phosphorus deposit, phosphite ions, hydrogen ions, and hydrogen gas.  
         [0000]     The phosphorus may be codeposited with palladium to the extent of 1 to 15 percent by weight by a mechanism shown in reaction (3) above.  
         [0087]     For example, to provide a palladium barrier coating, an implantable medical device having a polymer coating with a finely etched surface is immersed in an aqueous solution of 5 g/L SnC 12  and 5 mL/L HCL for a period of about 5 minutes at 35° C. The implantable medical device is then immersed in a plating solution at a temperature of 50° C., the plating solution including 2 g/L of palladium chloride, 160 mL/L of ammonium hydroxide (27%), 26 g/L of ammonium chloride, and 10 g/L of sodium hypopphosphite.  
         [0088]     To provide a gold barrier coating, an implantable medical device having a polymer coating with a finely etched surface is immersed in an aqueous solution of 5 g/L SnCl 2  and 5 mL/L HCL for a period of about 5 minutes at 35° C. The implantable medical device is then immersed in a plating solution at a temperature of 75° C., the plating solution including 5.8 g/L of potassium gold cyanide, 13.0 g/L of potassium cyanide, and 21.6 g/L of potassium borohydride.  
         [0089]     Codeposition of phosphorus and boron may occur in the electroless plating process givenabove. In another process which is commercially available, a proprietary reducing reagent is used that yields an electroless deposit of pure palladium that is free of phosphorus or boron. The process is known as the Pallatect™ process, and is available from Atotech USA, Inc. of Somerset, N.J. This process reportedly involves the steps of acid cleaning, microetching, activation, and electroless deposition of palladium.  
         [0090]     While particular embodiments of the present invention have been shown and described, various changes and modifications can be made without departing from this invention in its broader aspects. For instance, while we have discussed the invention in conjunction with stents and grafts, artisans will appreciate that the invention may be used with any implantable medical device that can be used in conjunction with a drug-loaded polymer for localized drug delivery. For example, such barrier coatings may be applied to drug-loaded polymer coatings on catheters, cerebrospinal fluid shunts, drug delivery pacemaker leads, defibrillation electrodes, artificial hips and bones, breast implants, and other implanted devices. 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.