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  Polymeric coating for reducing the rate of release of a therapeutic substance from a stentUSPTO Application #: 20070016284Title: Polymeric coating for reducing the rate of release of a therapeutic substance from a stent Abstract: A stent for delivery of a therapeutic agent is disclosed. The stent includes a polymer coating for reducing the rate of release of the therapeutic agent. The polymer has a crystalline structure wherein the polymer is capable of significantly maintaining the crystalline lattice structure while the therapeutic agent is released from the stent such that the aqueous environment to which the stent is exposed subsequent to the implantation of the stent does not significantly convert the crystalline lattice structure of the polymer to an amorphous structure. (end of abstract) Agent: Squire, Sanders & Dempsey LLP - San Francisco, CA, US Inventor: Stephen D. Pacetti USPTO Applicaton #: 20070016284 - Class: 623001460 (USPTO) Related Patent Categories: Prosthesis (i.e., Artificial Body Members), Parts Thereof, Or Aids And Accessories Therefor, Arterial Prosthesis (i.e., Blood Vessel), Having Plural Layers, Coating The Patent Description & Claims data below is from USPTO Patent Application 20070016284. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATION [0001] This is a divisional application of U.S. application Ser. No. 09/948,513, filed on Sep. 7, 2001, the teachings of which are incorporated herein in their entirety by reference. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] A medical device, such as a stent, for delivering a therapeutic substance is disclosed. The stent includes a polymeric coating for reducing the rate of release of the therapeutic substance. [0004] 2. Description of the Background [0005] Blood vessel occlusions are commonly treated by mechanically enhancing blood flow in the affected vessels, such as by employing a stent. Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically stents are capable of being compressed, so that they can be inserted through small lumens via catheters, and then expanded to a larger diameter once they are at the desired location. Examples in the patent literature disclosing stents include U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor. [0006] Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. Local delivery of a therapeutic substance is a preferred method of treatment because the substance is concentrated at a specific site and thus smaller total levels of medication can be administered in comparison to systemic dosages that often produce adverse or even toxic side effects for the patient. [0007] One method of medicating a stent involves the use of a polymeric carrier coated onto the surface of the stent. A composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend is applied to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent strut surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer. [0008] Depending on the physiological mechanism targeted, the therapeutic substance may be required to be released at an efficacious concentration for an extended duration of time. Increasing the quantity of the therapeutic substance in the polymeric coating can lead to poor coating mechanical properties, inadequate coating adhesion, and overly rapid rate of release. Increasing the quantity of the polymeric compound by producing a thicker coating can perturb the geometrical and mechanical functionality of the stent as well as limit the procedures for which the stent can be used. [0009] It is desirable to increase the residence time of a substance at the site of implantation, at a therapeutically useful concentration, without the addition of a greater percentage of the therapeutic substance to the polymeric coating and without the application of a significantly thicker coating. SUMMARY OF THE INVENTION [0010] The present invention discloses a stent for delivery of a therapeutic agent. The stent includes a polymer coating for reducing the rate of release of the therapeutic agent. The polymer has a crystalline lattice structure, wherein the polymer is capable of significantly maintaining the crystalline lattice structure while the therapeutic agent is released from the stent such that the aqueous environment to which the stent is exposed subsequent to the implantation of the stent does not significantly convert the crystalline lattice structure of the polymer to an amorphous structure. [0011] The coating can contain the therapeutic agent. In one embodiment, the melting point of the polymer is greater than or equal to about 135.degree. C. at ambient pressure. In another embodiment, the polymer is a hydrophobic polymer having a solubility parameter not greater than about 10.7 (cal/cm.sup.3).sup.1/2. [0012] Also disclosed is a method of forming a coating for a stent. The method includes applying a first composition including a polymeric material to at least a portion of the stent to form a polymer coating supported by the stent. The polymer has a crystalline structure, wherein the aqueous environment to which the coating is exposed subsequent to the implantation of the stent does not significantly convert the crystalline structure of the polymer to an amorphous structure for the duration of time which the agent is released from the stent. [0013] The present invention additionally discloses a composition for coating a stent. The composition includes a fluid and a polymer dissolved in the fluid. The polymer includes a crystalline structure during the duration of delivery of an active agent from the stent, and the aqueous environment to which the stent is exposed subsequent to the implantation procedure does not significantly change the crystalline stricture to an amorphous structure. [0014] Also disclosed is a stent for delivering a therapeutic agent to an implanted site. The stent includes a radially expandable body structure and a polymeric coating supported by the body structure for extending the residence time of the therapeutic agent at the implanted site. The polymeric coating is made from a hydrophobic polymer having a degree of crystallinity that remains at or above about 10% at least until a significant amount of the therapeutic substance has been released from the stent. DETAILED DESCRIPTION Embodiments of the Rate-Reducing Coating [0015] One mechanism through which the release rate of an active agent from a medical device can be controlled is the crystallinity of the polymer with which the medical device is coated. A polymer in which the molecules are arranged in a highly ordered and regular pattern formed by folding and stacking of the polymer chains is said to be crystalline. By contrast, amorphous polymers have molecules that are arranged randomly with no regularity of orientation with respect to one another. Among the factors that affect polymer crystallinity are the stereoregularity of the polymer, the tacticity of the polymer, the presence of branching, the degree of polymerization, and the strength of the intermolecular forces between the polymer chains. [0016] The structural arrangement and regularity of a polymer is an important factor in the determination of polymer crystallinity. A regular arrangement along the polymer chains provides the polymer structure with a high degree of symmetry, allowing the chains to pack into crystals. Irregularity along the polymer chains, however, prevents the chains from packing closely to one another, thereby decreasing crystallinity. Polymers with regular, linear, and rigid structures tend to form ordered crystals. By contrast, polymers with large side groups, mixed tacticity or an atactic structure, a mix of side or functional groups, or composed of more than one monomer tend not to pack well into crystalline structures. [0017] The degree of polymerization also contributes to the determination of the crystallinity of a polymer. Relatively short chains organize themselves into crystalline structures more readily than longer molecules, as longer molecules tend to become tangled and thus have difficulty arranging themselves in an ordered manner, resulting in a more amorphous structure. [0018] Also influencing polymer crystallinity is the presence of intermolecular forces. The presence of polar and hydrogen bonding groups favors crystallinity because such groups promote dipole-dipole and hydrogen bonding intermolecular forces. Such strong interchain forces hold the polymer chains in a tightly packed configuration, thereby promoting crystallinity. By contrast, polymers with little or no intermolecular forces will tend to have random, non-crystalline structures as a result of thermal motion. [0019] Typically, as the crystallinity of a polymer increases, so too does the polymer's ability to reduce the rate at which an active agent is released from a medical device coated with the polymer. This is because it is more difficult for an active agent to diffuse through a tightly packed, crystalline polymer than a more loosely packed, amorphous polymer. The purpose of the coating of the present invention is to decrease the rate of release of an active agent therefrom. Accordingly, the polymer for forming the rate-reducing coating should be selected to have sufficient crystallinity such that the active agent may not readily diffuse therethrough. Continue reading... Full patent description for Polymeric coating for reducing the rate of release of a therapeutic substance from a stent Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Polymeric coating for reducing the rate of release of a therapeutic substance from a stent patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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