Abstract:
A method for delivering heparin, wherein an implantable stent is coated with a material that attracts heparin and with which heparin forms a bond. The stent is exposed to a heparin containing solution just prior to implantation or is first implanted and then exposed to heparinized blood. As heparin becomes detached from the stent, the implantation site is exposed to heparin to restore an effective level and thereby prevent thrombosis.

Description:
This is a divisional of U.S. patent application U.S. Ser. No. 08/847,763 filed Apr. 24, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to endovascular stents and more particularly pertains to coatings that are applied to stents in order to reduce thrombogenicity. 
     Stents are implanted within blood vessels in an effort to maintain their patency by preventing collapse of the lumen and/or by impeding restenosis. Unfortunately, the presence of a foreign object within the blood flow may have a thrombogenic effect. It has therefore been found to be desirable to use various anti-coagulant drugs in an effort to reduce the likelihood of the development of restenosis and provide an antithrombogenic effect. 
     A drug that has been found to be particularly effective for such purpose is heparin. By maintaining an effective concentration of the drug in and about the implantation site until the stent is encapsulated by tissue, the risk of thrombogenesis is substantially mitigated. To that end, various approaches have been employed in the administration of heparin. 
     While the systemic administration of heparin can cause the implantation site to be subjected to an effective level of heparin, such level of heparin would necessarily also be present throughout the rest of the body which can lead to undesirable side effects such as bleeding. It has therefore been recognized that a regimen wherein the heparin is substantially constrained to the implantation site is far more desirable. An approach that has been devised to achieve such end requires the coating or impregnation of the stent itself with heparin. The heparin is thereby concentrated where it is most needed while its presence, and consequently its effect, throughout the rest of the body is minimized. 
     Disadvantages associated with heretofore known heparinized stents include, the limited shelf life of such devices, the fact the heparin is degraded when the stent is sterilized either by heat or by exposure to ethylene dioxide, the inability of the physician to alter the dosage that the patient is subjected to and the inability to replenish any heparin that may be lost while the device is deployed. Additionally, the cost of heretofore known heparinized stent devices has been very high as it necessarily includes the costs associated with the stringent regulatory requirements attendant a drug containing device. 
     The prior art has been unable to overcome these disadvantages and shortcomings and a new approach is needed to safely, effectively, and economically deliver heparin to an implantation site. 
     SUMMARY OF THE INVENTION 
     The present invention provides for the coating of an implantable endovascular device to facilitate the subsequent loading of heparin onto its surface. Such loading can be achieved in vitro just prior to implantation or preferably, in vivo after the device is in place. As a result, the device has a considerably longer shelf-life than heparin-containing devices, the need for special handling and sterilization procedures associated with heparin-containing devices is obviated, and the dosage of heparin can be readily tailored to an individual patient&#39;s needs including any adjustment that may be required after the device has been deployed. An additional advantage provided by such a device is that it is not subject to the stringent regulatory requirements associated with drug-containing devices. 
     More particularly, the present invention provides for the coating of stent surfaces with a material or combination of materials that are selected for their ability to adhere to the stent surface, to attract heparin and to form preferably an ionic bond therewith. The heparin is attracted by and attaches to functional groups incorporated in the coating which may include primary, secondary, and/or tertiary amines or other functionalities such as carboxyl groups. 
     The heparin-attracting coating may be applied so as to encapsulate the entire stent or alternatively, to cover only selected surfaces thereof. By limiting coverage to only the inner surface of the stent, i.e., the surface that is directly exposed to blood flow, a much higher level of heparin can be loaded onto the stent than would be safe if such level were in direct contact with the vessel wall. A toxic effect on the vessel wall is thereby avoided while the blood is exposed to a more effective concentration of heparin. Alternatively, it may be deemed sufficient to coat only the ends of the stent, i.e., where disturbance of flow is greatest and where thromboses are most likely to occur. 
     The coating may be applied by different processes such as by dipping, spraying or molding. The preferred method is by plasma deposition wherein a base layer, selected for its ability to adhere to the stent surface, is first deposited on the stent followed by the deposition of a top layer thereon that is selected for its ability to bond to the base layer and to avail the appropriate functional groups for bonding to the heparin. 
    
    
     These and other features and advantages of the present invention will become apparent from the following detailed description which, taken in conjunction with the accompanying drawings, illustrates by way of example the principles of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an implantable stent. 
     FIG. 2 is a greatly enlarged, schematic, cross-sectional view of a portion of the stent of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A wide variety of different stent configurations have been devised to address various issues inherent in their use and function. Additionally, various materials have been employed in their construction including metals and polymers. Both the degree of turbulence caused by a particular stent configuration when subjected to blood flow as well as the material from which it is constructed affects the degree of thrombogenicity associated with a particular stent device. The present invention provides a coating for such stents to which heparin becomes attached and thus serves to reduce or eliminate thrombosis formation. Moreover, the stent&#39;s coating allows the heparin to be loaded thereon immediately before the implantation procedure or after the stent is in place. 
     Critical requirements for the coating of the present invention include that it adheres to the stent surface and that it has functional groups that attract heparin and to which heparin bonds. Functional groups that are known to have the requisite affinity for heparin include primary, secondary, and tertiary amines wherein primary amines are preferred due to their enhanced affinity. Alternatively, carboxyl groups may be used. The functional groups must include positively charged entities that serve to attract the negatively charged entities associated with the heparin. Such attraction facilitates the formation of an ionic bond. 
     The coating can be applied by different processes such as by dipping, spraying, molding or by plasma deposition. Plasma deposition is preferred and first requires the deposition of a base layer or primer that prepares the surface of the stent to receive the functionality group containing substance. In the preferred embodiment, a metallic stent is first plasma deposited with methane gas leaving a film on the surface of the stent wherein the methane molecules are oriented with the carbon against the stent and the hydrogen exposed. A top layer that includes the desired functionalities is then deposited on the base layer. Such second layer may be formed by the plasma deposition of ammonia gas to leave the primary amine functional groups extending from the stent surface. Other chemicals such as alkylamine, nitrile compounds or amine containing monomers can also be used to plasma deposit amine functionalities on the surface. In the event a mixture of primary, secondary, and tertiary amines is deposited by such methods it is preferred that the primary amine constitutes a greater percentage of the mixture due to its greater affinity for heparin. Alternatively, the deposition of carboxyl functional groups can be achieved by the plasma deposition of monomers like methyl methacrylate or acrylic acid. 
     The resulting coating thickness should be 0.001 inch or less while a thickness less than 1 micron is preferred. Although it may be desirable to have a uniform concentration of functional groups extending from the surface, it is not critical to the function of the coating. On the other hand, a concentration of at least 54 picamoles/stent must be achieved in order to ensure that heparin becomes attached at an effective level. 
     The coating may be applied to the entire stent or just to selected surfaces thereon. FIG. 1 generally illustrates a stent  12  in its deployed state and serves to identify the vessel wall-facing surface  14 , the blood flow-facing surface  16 , its upstream edge  18 , and its downstream edge  20 . By coating only the surfaces facing the blood flow, a concentration of heparin can be loaded thereon that would be toxic to the vessel wall tissue if it were to be present on the surfaces in direct contact with the vessel wall. Alternatively, it may be sufficient to exclusively coat the upstream and/or downstream edges of the stent for a particular stent configuration implanted in a particular patient as thrombosis is most likely to occur at such interfaces due to turbulence induced by their presence in the blood flow. 
     After the coating process is completed, the coated stent is cleaned and sterilized and appropriately packaged for long-term storage. Due to the absence of any degradable drugs or substances on the stent, a fairly extended shelf-life can be expected. 
     The stent of the present invention can be used in two different ways. A first use calls for the stent to be implanted in the form in which it had been stored, without having heparin loaded thereon. Once in place, it is contacted with heparinized blood, either by an injection of heparin via a catheter extended to a position just upstream of the implantation site or by IV. As the heparin macromolecules  22  pass by the functional groups  24  in the coating  26 , the heparin is attracted thereto and becomes attached (FIG.  2 ). Heparin that does not attach, quickly becomes diluted downstream of the implantation site to levels that do not adversely affect the patient. Subsequent heparin flow past the implantation site can cause more and more heparin molecules to be pulled from the blood flow until the stent coating is saturated. Once attached, heparin can inhibit coagulation by binding with anti-thrombin III and/or other factors of the coagulation cascade. Should a heparin molecule become detached, it is replaced by other heparin molecules still present in the blood flow. Alternatively, an additional dosage of heparin can be administered. 
     Alternatively, the physician may pre-treat the stent prior to implantation by flushing it with, for example, a heparinized saline solution. In this way, the. physician can easily and precisely adjust the heparin level by controlling the concentration of the heparin in the saline solution and/or controlling the exposure time thereto. Once implanted, the heparin level can be increased or replenished by introducing heparin into the blood flow upstream of the implantation site as was described above. The heparin level is maintained on the stent until the natural healing processes cause the stent surfaces to be completely covered by tissue at which point thrombogenicity ceases to be of concern. 
     While a particular form of the invention has been illustrated and described, it will also be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited except by the appended claims.