Abstract:
The invention relates to a coating composition for an implantable medical device, a method of coating a medical device and a device coated with the composition.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority from UK provisional application No. 0819296.5 entitled “Coating II” filed on Oct. 21, 2008, which is herein incorporated in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a coating composition for an implantable medical device, a method of coating a medical device and a device coated with the composition. 
       BACKGROUND TO THE INVENTION 
       [0003]    Stents are small expandable metal tubes that are implanted in arteries to keep them open in patients whose vessels have become narrowed due to coronary artery disease, the most common cause of death in the Western World. Bare metal stents sometimes become re-narrowed again (restenosis) requiring a re-intervention procedure to re-open them. 
         [0004]    A drug-eluting stent, sometimes referred to as a “coated” or “medicated” stent, is a normal bare metal stent that has been coated with a polymer containing a pharmacologic (drug) that is known to interfere with the process of restenosis (re-narrowing). Restenosis has a number of causes; it is a very complex process and the solution to its prevention is equally complex. However, in the data gathered so far, the drug-eluting stent has been extremely successful in reducing restenosis from the 20-30% range to single digits. 
         [0005]    After stent implantation, in addition to aspirin, the patient must take an anti-clotting or anti-platelet drug, such as clopidogrel or ticlopidine (brand names Plavix and Ticlid) for six or more months after stenting, to prevent the blood from reacting to the new device by thickening and clogging up the newly expanded artery (thrombosis). Ideally a smooth, thin layer of endothelial cells (the inner lining of the blood vessel) grows over the stent during this period and the device is incorporated into the artery, reducing the tendency for clotting. 
         [0006]    Despite the clinical and commercial success of drug-eluting stents, recent evidence has suggested that they result in higher rates of late stage thrombosis, a clot formation at the end of the stent, which, in over 50% of cases, results in either an acute heart attack or death. Many physicians believe that the clot formations are due to the delayed healing of the vessel caused by the cytotoxic drugs that coat the stent or by some biostable polymers that are used to deliver the drug. In response to these concerns some physicians are defaulting to using bare metal stents, where late stent thrombosis is not seen as much of a problem. 
         [0007]    Recently, industry has been working on replacing the biostable polymer coatings used in first generation drug-eluting stents with bioresorbable polymers. Most of the current bioresorbable coatings used for the drug-eluting stents are based on poly(lactide), poly(glycolide) or co-polymers of the two (poly(lactide-co-glycolide)). It is difficult to control the drug-elution profile with these polymers, one problem being a “burst release” of the drug that gives a high initial release of drug followed by a slow release, whereas what is desired is a more steady and sustained release. 
         [0008]    Another problem with coatings on stents is the adhesion of the polymer to the metal surface. Most polymer coatings will not adhere strongly to a normal metal surface and the coating may delaminate when the stent is deployed inside the vessel. 
         [0009]    As a result, there is a requirement for a new generation of drug-eluting stents which have bioresorbable coatings in place of the current stable (non-resorbable) coatings. 
         [0010]    In this way the stent can deliver its payload of a drug from the coating but once the drug has been delivered the coating resorbs so that the stent effectively reverts to a bare metal stent, with lower chance of late stent thrombosis. 
       SUMMARY OF THE INVENTION 
       [0011]    The coating composition of the invention is a drug-eluting bioresorbable coating. The composition of the coating allows control of the elution profile of the drug as well as allowing strong adhesion of the coating to a metal surface and resistance to damage. 
         [0012]    According to a first aspect of the invention there is provided a bioresorbable coating composition for an implantable medical device, wherein the composition comprises a co-polymer of a lactide, a glycolide and s-caprolactone and at least one drug and/or bioactive agent. 
         [0013]    According to a second aspect of the invention there is provided an implantable medical device coated with the bioresorbable coating composition according to the first aspect of the invention. 
         [0014]    According to a third aspect of the invention there is provided a method of applying a bioresorbable coating composition to an implantable medical device comprising the steps of;
       i. providing an implantable medical device,   ii. applying a coating of the bioresorbable coating composition according to the first aspect of the invention to at least part of a surface of the device.       
 
         [0017]    According to a fourth aspect of the invention there is provided a method of using a device which is coated with the bioresorbable coating composition according to the first aspect of the invention, wherein the method comprises the step of implanting the device in a non-human animal body or human body. 
         [0018]    According to a fifth aspect of the invention there is provided a vehicle, for carrying a drug, wherein the vehicle is defined as the composition of the present invention. 
         [0019]    According to a still further aspect of the invention, there is provided a bioresorbable coating composition, implantable medical implant, vehicle for carrying a drug and methods of producing thereof and methods of use thereof as substantially herein described with reference to the accompanying Examples and Figures. 
         [0020]    In embodiments of the invention the composition comprises a glycolide at between 30-45% by weight of the polymer component of the composition. 
         [0021]    In embodiments of the invention the composition comprises a glycolide at between 5-15% by weight of the polymer component of the composition. 
         [0022]    In embodiments of the invention the composition comprises a glycolide at about 10% by weight of the polymer component of the composition. 
         [0023]    In embodiments of the invention the composition comprise s-caprolactone at between about 10-20% by weight of the polymer component of the composition. 
         [0024]    In embodiments of the invention the composition comprises ε-caprolactone at about 12.5% by weight of the polymer component of the composition. 
         [0025]    In embodiments of the invention the composition comprises a glycolide at between about 10% by weight of the polymer component of the composition and ε-caprolactone at about 12.5% by weight of the polymer component of the composition 
         [0026]    In embodiments of the invention the composition the lactide is D,L-lactide. 
         [0027]    In embodiments of the invention the composition comprises a blend of co-polymers. For example the second co-polymer of the blend is also a co-polymer of a lactide, a glycolide and ε-caprolactone. 
         [0028]    In a specific embodiment of the invention the bioresorbable coating composition comprises a blend of a first co-polymer comprising 47.5% D,L-lactide, 40% glycolide and 12.5% ε-caprolactone and a second co-polymer comprising 57.5% D,L-lactide, 30% glycolide and 12.5% ε-caprolactone. 
         [0029]    A “bioactive agent” or “drug” is herein defined as any biological and/or chemical substance used in the treatment, cure, prevention, or diagnosis of disease or used to otherwise enhance physical or mental well-being. Examples of a suitable drug type for incorporation into the coating composition of the present invention include, an anti-inflammatory agent, a cytotoxic agent, an angiogenic agent, an osteogenic agent, an immunosuppressant, an anti-clotting agent, an anti-platelet agent, an antimicrobial or an antibiotic. 
         [0030]    Suitable anti-platelet agents include clopidogrel, sold as Plavix™ by Bristol Myers-Squibb and ticlopidine, sold as Ticlid™ by Sanofi-Aventis. Suitable anti-clotting agents include heparin. 
         [0031]    Suitable immunosuppressants include rapamycin, also known as Sirolimus available from AG Scientific Inc. 
         [0032]    Suitable antibiotics include gentamicin and vancamycin. 
         [0033]    In embodiments of the invention the composition comprises rapamycin at about 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45% or 50% by weight of the composition. 
         [0034]    In a specific embodiment of the invention the composition comprises rapamycin at between about 20-30% by weight of the composition. 
         [0035]    In a further specific embodiment of the invention the composition comprises rapamycin at about 25% by weight of the composition. 
         [0036]    A bioactive agent is herein defined as any agent that has an effect on, interaction with, or response from living tissue. For example, an antibody or a cell (e.g a stem cell). 
         [0037]    The composition can further comprise at least one additive which is an acid or a derivative thereof selected from the group consisting of hexanoic acid, octanoic acid, decanoic acid, lauric acid, myristic acid, crotonic acid, 4-pentenoic acid, 2-hexenoic acid, undecylenic acid, petroselenic acid, oleic acid, erucic acid, 2,4-hexadienoic acid, linoleic acid, linolenic acid, benzoic acid, hydrocinnamic acid, 4-isopropylbenzoic acid, ibuprofen, ricinoleic acid, adipic acid, suberic acid, phthalic acid, 2-bromolauric acid, 2,4-hydroxydodecanoic acid, butyric acid, monobutyrin, 2-hexyldecanoic acid, 2-butyloctanoic acid, 2-ethylhexanoic acid, 2-methylvaleric acid, 3-methylvaleric acid, 4-methylvaleric acid, 2-ethylbutyric acid, trans-beta-hydromuconic acid, isovaleric anhydride, hexanoic anhydride, decanoic anhydride, lauric anhydride, myristic anhydride, 4-pentenoic anhydride, oleic anhydride, linoleic anhydride, benzoic anhydride, poly(azelaic anhydride), 2-octen-1-yl succinic anhydride and phthalic anhydride. 
         [0038]    Aptly the composition contains the additive in an amount which is not more than 10%, typically not more than 5%, and even more typically not more than 2% by weight of the composition. 
         [0039]    The amount of the additive chosen will also depend upon the rate of degradation desired. In vivo degradation occurs firstly by hydrolytic scission of the polymer chains resulting in the formation of units of increasingly smaller molecular weight until only substantially monomers remain. Thereafter, the monomers are metabolized and absorbed into the body. It is only in the last stages of degradation that mass loss occurs. 
         [0040]    It has been found that a particularly suitable additive for use in the invention is lauric acid. This may be employed as the acid per se or, if desired, as a derivative, for example as the anhydride. 
         [0041]    Advantageously compositions will contain lauric acid a derivative thereof in an amount not more than 10%, more typically not more than 5%, and even more typically not more than 2% by weight of the composition. 
         [0042]    The bioresorbable coating composition of the invention may be applied directly to the implantable medical device. However, most polymeric coatings will not adhere strongly to a normal metal surface and the coating has a tendency to de-laminate. In embodiments of the invention the bioresorbable coating of the invention is chemically and/or physically coupled to the surface of the device. 
         [0043]    In embodiments of the invention “functionalisation” of the metal surface improves the adhesion of the polymeric coating to the surface of the medical device and minimises the risk of delamination. This functionalisation provides a “chemical bridge” between the metal surface and the polymeric coating. 
         [0044]    In the first stage of the functionalisation process a chemical is chosen that reacts well with the inherent functionality of the metal surface. The chemical preferably reacts with the oxides, hydroxides, epoxide or any other surface oxide on metallic surfaces to form strong bonds whilst leaving the rest of the molecule free to react with other species. 
         [0045]    Suitable chemicals for use in the first reaction step include alkoxysilanes of the formula (RO) 3 Si(R 1 X) wherein R represents methyl or ethyl and R 1  represents C 2 -C 10  alkyl in which one or more methylene groups may be replaced by —NH— or —O—, C 2 -C 10  cycloalkyl or cycloalkylalkyl, C 2 -C 10  aralkyl or monocylic or bicyclic aryl and X represents amino, hydroxyl, carboxylic acid or acid anhydride. Preferably R 1  represents C 2 -C 10  alkyl in which one or more methylene groups is optionally replaced by —NH— and X represents —NH 2 , and an example of a suitable priming agent is N-[3-(trimethoxysilyl)propyl]ethylenediamine. 
         [0046]    In the second stage, another chemical is chosen which reacts readily with the functional/reactive groups of the first chemical and which also has a functional group which can react with oxygen containing groups in the polymer, for example, hydroxyl, methoxy and ethoxy groups. A strong bond is therefore formed between the two molecules and the polymer is coupled to the functionalised surface. In this way a strong chemical bond is achieved between the functionalised surface and the polymer, improving the adhesion of the polymer to the metal surface. Any chemical with an alkoxysilyl group on one end and an isocyanate on the other end is suitable for use in the second reaction step. An example of an appropriate chemical is 3-(triethoxysilyl)propylisocyanate. 
         [0047]      FIGS. 1 and 2  illustrate the functionalisation process. 
         [0048]    In embodiments of the invention the bioresorbable coating composition of the first aspect of the invention contains —OH groups to react with the triethoxy groups from the second functionalisation step. 
         [0049]    In further embodiments of the invention an intermediate coating composition is provided between at least part of the surface of the device and the bioresorbable coating composition of the first aspect of the invention to physically “tie” the coating composition in place. This intermediate coating composition can be referred to as a “primer coat” or a “tie coat”. This composition comprises a lower molecular weight polymer of similar or identical chemical composition to the polymeric component of the bioresorbable coating composition of the first aspect of the invention. The polymeric composition of the “tie coat” can be any polymer which is capable of forming a strong adhesion with the polymeric component of the bioresorbable coating composition of the first aspect of the invention. 
         [0050]    In embodiments of the invention the primer coat comprises a co-polymer of a lactide and a glycolide, for example poly(D,L-lactide-co-glycolide), specifically in which the ratio of D,L-lactide to glycolide is 50:50 and the molecular weight of the copolymer is between about 5-15 k. 
         [0051]    The primer coat may be applied directly to at least part of the surface of the device and then the bioresorbable coating composition according to the first aspect of the invention is applied so as to overlay at least part of the primer coat. 
         [0052]    In alternative embodiments of the invention the surface of the device may be chemically functionalised as described above, with the primer coat being applied to this&#39;functionalised surface and then the bioresorbable coating composition of the first aspect of the invention being applied so as to overlay at least part of the primer coat The elution profile of the drug can be further controlled by the use of undrugged top-coat covering, wholly or partially the bioactive/drug-containing layer (i.e the bioresorbable coating composition according to the first aspect of the invention). This composition can comprise a polymer of similar or identical chemical composition to the bioresorbable coating composition according to the first aspect of the invention. For example, the top coat may be a copolymer of lactide, glycolide and ε-caprolactone or it may be a copolymer of D,L-lactide and glycolide. 
         [0053]    The top coat can form between about 1%-50%, or about 1%-45%, or about 1%-40%, or about 1%-35%, or about 1%-30%, or about 1%-25%, or about 1%-20%, or about 1%-15%, or about 1%-10%, or about 1%-5%, or about 5%-10%, or about 5%-15%, or about 5%-20%, or about 10%-20%, or about 15%-20% of the total coating weight (e.g: (i) bioresorbable coating composition according to the first aspect of the invention+top coat or (ii) primer coat+bioresorbable coating composition according to the first aspect of the invention+top coat) 
         [0054]    In further embodiments of the invention the polymer component of all of the polymeric coatings may comprise a blend of two or more polymers that themselves comprise copolymers of lactide, glycolide and s-caprolactone, for example in the ratios outlined above. 
         [0055]    Medical devices for which this coating technology may be advantageous, include but are not limited to stents, orthopaedic implants, dental implants and maxillo-facial implants. 
         [0056]    Examples of stents to which the drug-eluting bioresorbable coating of the invention can be applied include coronary stents, carotid stents, aortic stents, renal stents and venous stents. Other examples of stents include peripheral stents. 
         [0057]    Examples of orthopaedic implants to which the drug-eluting bioresorbable coating can be applied include reconstructive and trauma products, for example, components of hip replacement, components of knee replacements, fracture plates, screws, pins, external fixation plates, intramedullary nails, interference screws, suture anchors. 
         [0058]    Examples of maxillo-facial implants to which the drug-eluting bioresorbable coating of the invention can be applied include plates, screws and meshes. 
         [0059]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purpose of illustration only and are not intended to limit the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0060]    The accompanying drawings, which are incorporated in and, form part of the specification, illustrate the embodiments of the present invention and together with the written description serve to explain the principles, characteristics, and features of the invention. In the following drawings: 
           [0061]      FIG. 1 : Illustrative chemistry 
           [0062]      FIG. 2 : Schematic of surface functionalisation 
           [0063]      FIG. 3 : Percentage of rapamycin released from PLGC polymers (PLGC1, 4, 7, 10, 18) into PBS at 37° C. 
           [0064]      FIG. 4 : Percentage of rapamycin released from PLGC polymers (PLGC12, 13, 14, 15) into PBS at 37° C. 
           [0065]      FIG. 5 : Percentage of rapamycin released from PLGC polymers with undrugged PLGA top-coats (10% of total coat weight) into PBS at 37° C. 
           [0066]      FIG. 6 : Percentage of rapamycin released from 60/40 (PLGC4/8) polymers with 60/40 (PLGC4/8) undrugged top coats of varying thicknesses. 
           [0067]      FIG. 7 : Percentage of rapamycin released from blended PLGC polymers with top coats into PBS at 37° C. 
           [0068]      FIG. 8 : Percentage of rapamycin released from blended PLGC polymers (PLGC4/PLGC8) with PLGC4 top coats into PBS at 37° C. 
           [0069]      FIG. 9 : Percentage of rapamycin released from blended PLGC polymers containing lauric acid into PBS at 37° C. 
           [0070]      FIG. 10 : Adhesion strength of bioresorbable polymer on functionalised and unfunctionalised metal surfaces. 
           [0071]      FIG. 11 : The polymer-coated stent was placed onto the balloon section of a suitably sized commercially available PTCA catheter and crimped by hand. The catheter was fed down a tortuous path in PBS solution to mimic the passage of the stent and catheter in the human arteries. 
           [0072]      FIG. 12 : In vitro rapamycin release from PLGC coated stents in PBS at 37° C. in which the glycolide forms about 10% of the polymeric composition of the coating. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Example 1 
     Polymerisation 
     i) Materials 
     DL-lactide (Purac) 
     Glycolide (Purac) 
     ε-Caprolactone (Aldrich) 
       [0073]    Tin(II) chloride dihydrate (Aldrich)
 
Diethylene glycol (Aldrich)
 
       Dichloromethane (Fisher) 
     ii) Initiator-Catalyst System 
       [0074]    Tin(II) chloride dihydrate (0.5 g) was added to diethylene glycol (1.46 g) and heated gently to dissolve the tin chloride 
         [0075]    iii) Polymer Production 
         [0076]    D,L-lactide, glycolide and s-caprolactone were weighed into a Wheaton vial, total weight of 40 g. 10 microlitres of tin chloride in diethylene glycol was added to each vial together with 100 microlitres of diethylene glycol. The vials were placed in an oil bath set at 150° C. and stirred until the viscosity became so great that the magnetic follower wouldn&#39;t go round after which time the vials were placed in the oven at 150° C. and left for ˜16 hours. To remove the polymer from the vials dichloromethane was added and the vials rolled until the polymer dissolved. The polymer solution was poured onto siliconised paper and the dichloromethane evaporated off. To remove residual monomer the polymers were melted under vacuum to boil the monomer off. 
       Characterisation of Polymers 
     (i) Molecular Weight 
       [0077]    Molecular weight distributions were determined by conventional Gel Permeation Chromatography (GPC) in chloroform. Column calibration was achieved using narrowly disperse polystyrene standards 
         [0078]    Samples were dissolved in GPC grade chloroform with 0.1% v/v toluene as flow rate marker. 
       Column: PLgel MIXED-D 300×7.5 mm 
       [0079]    Mobile phase: Chloroform (GPC grade)
 
Flow rate: 1 mL/min
 
Column temp: 30° C.
 
Sample conc: approx 2 mg/mL
 
Injection volume: 100 μL
 
         [0080]    The results are shown in Table 1. 
         [0000]    ii) Glass transition temperature 
         [0081]    The glass transition temperatures of the polymers were measured by Differential Scanning calorimetry using a Perkin Elmer DSC7. The heating rate was 10° C./min. The results are shown in Table 1. 
         [0082]    The ratios of the lactide, glycolide and ε-caprolactone in the different are shown in Table 1. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Properties of polymers 
               
             
          
           
               
                 Ref 
                 Composition (%) 
                   
                 Molecular weight 
               
             
          
           
               
                 Code 
                 Lactide 
                 Glyolide 
                 Caprolactone 
                 Tg 
                 Mw 
                 Mn 
                 PD 
               
               
                   
               
               
                 PLGC1 
                 40   
                 40   
                 20   
                 17.7 
                 41600 
                 24900 
                 1.67 
               
               
                 PLGC2 
                 45   
                 45   
                 10   
                 n/a 
                 n/a 
                 n/a 
                   
               
               
                 PLGC3 
                 42.5 
                 42.5 
                 15   
                 29.8 
                 52600 
                 24600 
                 2.13 
               
               
                 PLGC4 
                 47.5 
                 40   
                 12.5 
                 28.9 
                 36100 
                 20700 
                 1.74 
               
               
                 PLGC5 
                 45   
                 40   
                 15   
                 30.2 
                 59800 
                 32100 
                 1.86 
               
               
                 PLGC6 
                 42.5 
                 40   
                 17.5 
                 30.6 
                 64400 
                 32300 
                 1.99 
               
               
                 PLGC7 
                 47.5 
                 37.5 
                 15   
                 24.9 
                 46900 
                 25300 
                 1.85 
               
               
                 PLGC8 
                 57.5 
                 30   
                 12.5 
                 38   
                 56500 
                 29900 
                 1.89 
               
               
                 PLGC9 
                 47.5 
                 40   
                 12.5 
                 27.8 
                 60508 
                 26887 
                 2.26 
               
               
                 PLGC10  
                 57.5 
                 30   
                 12.5 
                 32    
                 70024 
                 31939 
                 2.19 
               
               
                 PLGC11 
                 50   
                 37.5 
                 12.5  
                 31.4  
                 38700 
                 20500 
                 1.89 
               
               
                 PLGC12  
                 52.5 
                 35   
                 12.5  
                 25.9  
                 38100 
                 21200 
                 1.8  
               
               
                 PLGC13 
                 55   
                 32.5 
                 12.5  
                 29.1  
                 50300 
                 25600 
                 1.96 
               
               
                 PLGC14  
                 47.5 
                 40   
                 12.5 
                 23   
                 33600  
                 18400 
                 1.83 
               
               
                 PLGC15  
                 57.5 
                 30   
                 12.5  
                 33.9  
                 41500 
                 21600 
                 1.92 
               
               
                 PLGC16 
                 40   
                 40   
                 20   
                 n/a 
                 38000  
                 19300 
                 1.97 
               
               
                 PLGC17  
                 47.5  
                 37.5 
                 15   
                 n/a 
                 44200 
                 19100 
                 2.31 
               
               
                 PLGC18  
                 47.5 
                 35   
                 17.5 
                 n/a 
                 36400  
                 15000 
                 2.43 
               
               
                   
               
             
          
         
       
     
       Example 2 
     i) Materials 
       [0083]    Sodium hydrogen carbonate: Sigma Aldrich
 
N[3-(trimethoxysilyl)propyl]ethylenediamine (TMSPEA): Sigma Aldrich
 
Glacial acetic acid: Sigma Aldrich
 
3-(triethoxysilyl)propyl isocyanate (TESPI): Sigma Aldrich
 
       Poly(D,L-lactide-co-glycolide) (PLGA1) 50:50, Mw 5-15 k: Sigma Aldrich 
     Poly(D,L-lactide-co-glycolide) (PLGA2) 50:50, Mw 10 k: Durect 
       [0084]    Lauric acid (LA): Sigma Aldrich
 
Sodium dodecyl sulphate (SDS): Sigma Aldrich
 
Rapamycin: LC laboratories
 
       Phosphate Buffered Saline (PBS): Sigma Aldrich 
     Solvents: Sigma Aldrich 
       [0085]    ii) Coating of Stents with Polymer Composition 
       Stage 1: Cleaning 
       [0086]    Commercially available stainless steel stents were cleaned by sonication for 15 minutes in a 7.5% w/w solution of aqueous sodium hydrogen carbonate, rinsing in deionised water, sonication for 15 minutes in 2-propanol and sonication for 15 minutes in deionised water. The samples were then dried at 100° C. for 16 hours, followed by drying at 50° C. for 30 minutes. 
       Stage 2: Functionalisation 
       [0087]    2.5 ml glacial acetic acid solution in toluene (1.0% w/w, 0.4 mmol) was added to 200 ml toluene followed by 1.6 ml TMSPEA and mixed. The samples were removed from the 50° C. oven and immersed in the solution for 5 minutes. The samples were removed and kept at 50° C. for 20 hours. 
         [0088]    The samples were rinsed in a series of solvents by rotating sequentially for 15 minutes in each of toluene, methanol, deionised water, methanol and deionised water. Finally, the samples were rinsed for 5 minutes in methanol and then dried at 50° C. for 2 hours. 
         [0089]    Anhydrous toluene was added, under nitrogen, into a measuring cylinder being purged with nitrogen. Enough TESPI was added to give a 4% v/v solution in toluene. The samples (dried at 50° C. for 15 minutes and allowed to cool for two minutes before use) were immersed in the solution on a holder and rotated under nitrogen for 15 minutes. The samples were then rinsed in anhydrous toluene under nitrogen and dried under vacuum for 16 hours. 
       Stage 3: Applying a Primer Coat 
       [0090]    The stents were attached to a mandrel and coated with a primer solution containing 0.5% w/w PLGA1 in CHCl 3  on a Sonotek MediCoat Benchtop Coater. The parameters used were: 0.075 ml/min flow rate, 0.8 W ultrasonic power, 2 passes, 40 rpm rotation, 0.13 cm/s horizontal travel and 25 mm from stent to spray head. After priming, the stents were left for 16 hours at 100° C. 
       Stage 4: Preparing the Drug-Containing Biresorbable Coating 
       [0091]    Solutions were prepared by dissolving the polymers listed in Table 1 in CHCl 3  with 25% (by weight of the solid polymer) rapamycin. 
       Stage 5: Coating 
       [0092]    After cooling for 5 minutes, the stents were attached to a mandrel and coated on the Sonotek MediCoat Benchtop Coater with a 0.5% w/w solution in CHCl 3  of polymer/drug (75/25). The parameters used were: 0.09 ml/min flow rate, 1.0 W ultrasonic power, 16 passes, 40 rpm rotation, 0.13 cm/s horizontal travel and 25 mm from stent to spray head. After coating, the stents were dried under vacuum for 16 hours at 40° C. 
       Stage 6: Elution Testing 
       [0093]    The stents were released into phosphate buffered saline solution (PBS) at 37° C. and the elution monitored by UV/vis spectroscopy. Fresh buffer solution was added after each reading and the cumulative absorbance at 279 nm was recorded. 
       Results 
       [0094]      FIG. 3  shows the effect of varying the proportion of ε-caprolactone in the polymer on the release of rapamycin; increasing the fraction of ε-caprolactone increases the rate at which rapamycin is released. 
         [0095]      FIG. 4  shows the effect of changing the ratio of D,L-lactide:glycolide with a fixed amount of ε-caprolactone (12.5%) on release of rapamycin; increasing the amount of glycolide increases the rate at which rapamycin is released. 
       Example 3 
       [0096]    Commercially available stainless steel stents were functionalised, a primer coat applied and overlaid with polymer PLGC1 containing 25% rapamycin in an identical manner to Example 2 Stages 1-5. An “undrugged top coat” of poly(D,L-lactide-co-glycolide) 50:50 (PLGA2) was then applied in the following manner. 
         [0097]    The stents were attached to a mandrel and coated on the Sonotek MediCoat Benchtop Coater with a 0.5% w/w solution in CHCl 3  of PLGA2. The parameters used were: 0.09 ml/min flow rate, 1.0 W ultrasonic power, 2 passes, 40 rpm rotation, 0.13 cm/s horizontal travel and 25 mm from stent to spray head. After coating, the stents were dried under vacuum for 16 hours at 40° C. The top-coat made up 10% of the total coating weight. 
         [0098]    The elution of rapamycin from the stents was tested in an identical manner to that described in Example 2 Stage 6. 
         [0099]    The results, shown in  FIG. 5 , show that the use of an undrugged top coat can control and slow the release of rapamycin from the coating. 
       Example 4 
       [0100]    Polymers PLGC4 and PLGC8 were blended together in CHCl 3  in the ratio 60% PLGC4:40% PLGC8 (60/40 PLGC4/8). To this blend was added 25% rapamycin (based on dry weight of polymer i.e. 75% polymer:25% rapamycin). 
         [0101]    Commercially available stainless steel stents were functionalised, primed and coated with polymer 60/40 PLGC4/8 containing 25% rapamycin in an identical manner to Example 2 Stages 1-5. 
         [0102]    An undrugged top-coat of 60/40 PLGC4/8 was applied in an identical manner to that described in Example 3. The top-coats made up zero, 10 or 20% of the total coating weight. 
         [0103]    The elution of rapamycin from the stents was tested in an identical manner to that described in Example 2 Stage 6. 
         [0104]    The results, shown in  FIG. 6 , show how top-coats of different weights can further control drug release, with greater coating weight reducing the rate of elution of rapamycin. 
       Example 5 
       [0105]    Polymers PLGC4 and PLGC8 were blended together in CHCl 3  with the ratio of PLGC4:PLGC8 varying as follows: 100:0, 80:20, 70:30, 60:40, 50:50. To these blends were added 25% rapamycin (based on dry weight of polymer i.e. 75% polymer:25% rapamycin). 
         [0106]    Commercially available stainless steel stents were functionalised, primed and coated with polymer blends containing 25% rapamycin in an identical manner to Example 2 Stages 1-5. 
         [0107]    An undrugged top-coat of PLGC4 and PLGC8 blended in the same ratios as their respective main coats was applied in an identical manner to that described in Example 3. The top-coats made up 10% of the total coating weight. 
         [0108]    The elution of rapamycin from the stents was tested in an identical manner to that described in Example 2 Stage 6. 
         [0109]    The results, shown in  FIG. 7 , show how blending of the polymers can be used to control the elution profile of the drug. Specifically, in this case, increasing the ratio of PLGC4 in the blend increases the rate of elution of rapamycin. 
       Example 6 
       [0110]    Polymers PLGC4 and PLGC8 were blended together in CHCl 3  with the ratio of PLGC4:PLGC8 varying as follows: 90:0, 80:20. To these blends were added 25% rapamycin (based on dry weight of polymer i.e. 75% polymer:25% rapamycin). 
         [0111]    Commercially available stainless steel stents were functionalised, primed and coated with polymer blends containing 25% rapamycin in an identical manner to Example 2 Stages 1-5. 
         [0112]    An undrugged top-coat of PLGC4 was applied in an identical manner to that described in Example 3. The top-coats made up 10% of the total coating weight. 
         [0113]    The elution of rapamycin from the stents was tested in an identical manner to that described in Example 2 Stage 6. 
         [0114]    The results, shown in  FIG. 8 , show again how drug release can be controlled by blending of the polymers. 
       Example 7 
       [0115]    Polymers PLGC1 and PLGC4 were blended with 25% rapamycin and either 0, 0.75, 1.5 or 3% lauric acid (LA) to produce the following compositions: 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                 PLGC4 (%) 
                 Rapamycin (%) 
                 Lauric acid (%) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 75 
                 25 
                 0 
               
               
                 74.25 
                 25 
                 0.75 
               
               
                 73.5 
                 25 
                 1.5 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                 PLGC1 (%) 
                 Rapamycin (%) 
                 Lauric acid (%) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 75 
                 25 
                 0 
               
               
                 73.5 
                 25 
                 1.5 
               
               
                 72 
                 25 
                 3 
               
               
                   
               
             
          
         
       
     
         [0116]    Commercially available stainless steel stents were functionalised, primed and coated with the above polymer the blends in an identical manner to Example 2 Stages 1-5. 
         [0117]    The elution of rapamycin from the stents was tested in an identical manner to that described in Example 2 Stage 6. 
         [0118]    The results, shown in  FIG. 9 , show that lauric acid can be added to the polymer coatings as a degradation accelerant but has a minimal affect on the rate of release of rapamycin. 
       Example 8 
     Stage 1: Cleaning 
       [0119]    Stainless steel samples (50 mm×50 mm×0.25 mm annealed finish) were cleaned by sonication for 15 minutes in a 7.5% w/w solution of aqueous sodium hydrogen carbonate, rinsing in deionised water, sonication for 15 minutes in 2-propanol and sonication for 15 minutes in deionised water. The samples were then dried at 100° C. for 16 hours, followed by drying at 50° C. for 30 minutes. 
       Stage 2: Functionalisation 
       [0120]    2.5 ml glacial acetic acid solution in toluene (1.0% w/w, 0.4 mmol) was added to 200 ml toluene followed by 1.6 ml TMSPEA and mixed. The samples were removed from the 50° C. oven and immersed in the solution for 5 minutes. The samples were removed and kept at 50° C. for 20 hours. 
         [0121]    The samples were rinsed in a series of solvents by rotating sequentially for 15 minutes in each of toluene, methanol, deionised water, methanol and deionised water. Finally, the samples were rinsed for 5 minutes in methanol and then dried at 50° C. for 2 hours. 
         [0122]    Anhydrous toluene was added, under nitrogen, into a measuring cylinder being purged with nitrogen. Enough TESPI was added to give a 4% v/v solution in toluene. The samples (dried at 50° C. for 15 minutes and allowed to cool for two minutes before use) were immersed in the solution on a holder and rotated under nitrogen for 15 minutes. The samples were then rinsed in anhydrous toluene under nitrogen and dried under vacuum for 16 hours. 
         [0123]    This stage was omitted in half the samples in order to create unfunctionalised control samples for comparison. 
       Stage 3: Applying a Primer Coat 
       [0124]    The samples were placed at an angle of approximately 45° and primed with 0.5% w/w PLGA1 solutions in CHCl 3 . The priming was performed using a handheld spray gun working at 10 psi from a distance of approximately 15 cm. Between 4 and 8 passes were needed, depending on the speed of movement, to achieve a primer coat weight of between 50 and 100 μg per cm2. The samples were then cured for 16 hours at 100° C. 
       Stage 4: Coating 
       [0125]    After curing at 100° C., the samples were cooled for 5 minutes and then placed at an angle of approximately 45° and coated with a 0.5% w/w PLGC4 solution in CHCl 3 . The coating was performed using a handheld spray gun working at 10 psi from a distance of approximately 15 cm. Between 30 and 60 passes were needed, depending on the speed of movement, to achieve a coat weight of 600-700 μg per cm2. The samples were then dried under vacuum at 50° C. for 16 hours. 
       Stage 5: Testing 
       [0126]    The adhesion tests were performed using the Elcometer 110 PATTI (Pneumatic Adhesion Tensile Test Instrument). Prior to testing all samples were glued to a fixed surface to prevent moving during the procedure. An aluminium pull stub is glued to the test surface using acrylic based super glue. The glue is applied to the stub then placed quickly onto the sample. Taking care to keep the stub still, an activator is sprayed directly onto the interface, whilst simultaneously maintaining pressure on the stub. A pulling piston is attached and a pressurised control module applies increasing pressure to the pull stub until it becomes detached from the test surface. The control module registers the maximum pressure (psig) attained which can be converted into MPa or bond strength (POTS). 
         [0127]    The results, shown in  FIG. 10 , show that functionalising the surface significantly improves the adhesion of the PLGC to the plates&#39;. 
       Example 9 
     Cleaning 
       [0128]    A commercially available stainless steel stent was cleaned by sonication for 15 minutes in a 7.5% w/w solution of aqueous sodium hydrogen carbonate, rinsing in deionised water, sonication for 15 minutes in 2-propanol and sonication for 15 minutes in deionised water. The stent was then dried at 100° C. for 16 hours, followed by drying at 50° C. for 30 minutes. 
       Functionalisation 
       [0129]    2.5 ml glacial acetic acid solution in toluene (1.0% w/w, 0.4 mmol) was added to 200 ml toluene followed by 1.6 ml TMSPEA and mixed. The stent was removed from the 50° C. oven and immersed in the solution for 5 minutes. The stent was removed and kept at 50° C. for 20 hours. 
         [0130]    The stent was rinsed in a series of solvents by rotating sequentially for 15 minutes in each of toluene, methanol, deionised water, methanol and deionised water. Finally, the stent was rinsed for 5 minutes in methanol and then dried at 50° C. for 2 hours. 
         [0131]    Anhydrous toluene was added, under nitrogen, into a measuring cylinder being purged with nitrogen. Enough TESPI was added to give a 4% v/v solution in toluene. The stent (dried at 50° C. for 15 minutes and allowed to cool for two minutes before use) was immersed in the solution on a holder and rotated under nitrogen for 15 minutes. The stent was then rinsed in anhydrous toluene under nitrogen and dried under vacuum for 16 hours. 
       PLGA Coupling 
       [0132]    The stent was attached to a mandrel and coated with a primer solution containing 0.5% w/w PLGA1 in CHCl 3  on a Sonotek MediCoat Benchtop Coater. The parameters used were: 0.075 ml/min flow rate, 0.8 W ultrasonic power, 4 passes, 40 rpm rotation, 0.13 cm/s horizontal travel and 25 mm from stent to spray head. After priming, the stent was left for 16 hours at 100° C. 
       Coating 
       [0133]    After cooling for 5 minutes, the stent was attached to a mandrel and coated on the Sonotek MediCoat Benchtop Coater with a 0.5% w/w solution in CHCl 3  of PLGC4, PLGC8 and rapamycin (45:30:25). The parameters used were: 0.09 ml/min flow rate, 1.0 W ultrasonic power, 16 passes, 40 rpm rotation, 0.13 cm/s horizontal travel and 25 mm from stent to spray head. After coating, the stent was dried under vacuum for 16 hours at 40° C. 
       Top Coat 
       [0134]    After cooling for 5 minutes, the stent was attached to a mandrel and coated on the Sonotek MediCoat Benchtop Coater with a 0.5% w/w solution in CHCl 3  of PLGC4 and PLGC8 (60:40). The parameters used were: 0.09 ml/min flow rate, 1.0 W ultrasonic power, 2 passes, 40 rpm rotation, 0.13 cm/s horizontal travel and 25 mm from stent to spray head. After coating, the stent was dried under vacuum for 16 hours at 40° C. 
       Tracking and Deployment 
       [0135]    The stent was placed onto the balloon section of a suitably sized commercially available PTCA catheter and crimped by hand. The catheter was fed down a tortuous path in PBS solution at 37° C. to mimic the passage of the stent and catheter in the human arteries. This was repeated five times. Finally, the stent was expanded at the nominal pressure of the device (10 bar). The expanded stent was rinsed in deionised water, dried and observed under an optical microscope. No pitting, cracking or delamination of the coating was observed (see  FIG. 11 ), including at the expansion points of the stent struts. 
       Example 10 
       [0136]    Polymers were prepared following the same methods as described in Example 1 using the ratios of lactide, glycolide and ε-caprolactone shown below: 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Composition (%) 
               
             
          
           
               
                   
                 Ref Code 
                 Lactide  
                 Glycolide 
                 Caprolactone 
               
               
                   
                   
               
             
          
           
               
                   
                 PLGC21 
                 77.5 
                 10 
                 12.5 
               
               
                   
                 PLGC22 
                 75 
                 10 
                 15 
               
               
                   
                 PLGC23 
                 76.75 
                 10 
                 13.25 
               
               
                   
                   
               
             
          
         
       
     
         [0137]    Stents were cleaned, functionalised and a primer coat of PLGA1 applied as described in Example 2 Stages 1-3. The stents were then coated with polymers PLGC21, PLGC22 and PLGC23 containing 25% rapamycin as described in Example 2, Stages 4 and 5. The elution of rapamycin from the stents was tested in an identical manner to that described in Example 2, Stage 6. 
         [0138]    The results, shown in  FIG. 12 , show that the upturn in release caused by the onset of degradation can be delayed, and drug can be released over a longer timescale, with a lower glycolide content. Furthermore for a constant glycolide content, drug release can be extended by lowering the caprolactone content. In this example drug release can be extended to around 1000 hours (42 days). 
         [0139]    Overall, the system described can be designed to tailor drug release profiles between about 100 and 1000 hours with a reasonably consistent rate of drug release.