| Dendritic and star-shaped contrast agents for medical devices and bioabsorbable radiopaque bulk material and method for producing same -> Monitor Keywords |
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Dendritic and star-shaped contrast agents for medical devices and bioabsorbable radiopaque bulk material and method for producing sameRelated Patent Categories: Drug, Bio-affecting And Body Treating Compositions, In Vivo Diagnosis Or In Vivo Testing, Magnetic Imaging Agent (e.g., Nmr, Mri, Mrs, Etc.), X-ray Contrast Imaging Agent (e.g., Computed Tomography, Angiography, Etc.)Dendritic and star-shaped contrast agents for medical devices and bioabsorbable radiopaque bulk material and method for producing same description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060292077, Dendritic and star-shaped contrast agents for medical devices and bioabsorbable radiopaque bulk material and method for producing same. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS REFERENCE TO RELATED APPLICATIONS [0001] This Application claims priority under 35 U.S.C. .sctn.119(e) of U.S. Provisional Patent Application No. 60/662,957 filed on Mar. 18, 2005. FIELD OF THE INVENTION [0002] The present invention relates to intravascular devices used in medical treatment and procedures. More specifically, the present invention relates to a new class of organic high intensity X-ray contrast agents suitable for enhancing the imaging of medical devices, particularly polymeric medical devices and polymeric coatings being fabricated from a polymer with the contrast agent dispersed within, conjugated at one or both ends of the polymers, as well as the method of manufacture of such materials and devices. DISCUSSION OF THE RELATED ART [0003] Recently, transluminal prostheses have been widely used in the medical arts for implantation in blood vessels, biliary ducts, or other similar organs of living body. These prostheses are commonly known as stents and are used to maintain, open, or dilate tubular structures. An example of a commonly used stent is given in U.S. Pat. No. 4,733,665 filed by Palmaz on Nov. 7, 1985, which is hereby incorporated herein by reference. Such stents are often referred to as balloon expandable stents. Typically the stent is made from a solid tube of stainless steel, although other metallic materials have been utilized. Thereafter, a series of cuts are made in the wall of the stent. The stent has a first smaller diameter, which permits the stent to be delivered through the human vasculature by being crimped onto a balloon catheter. The stent also has a second, expanded diameter, upon application of a radially, outwardly directed force, by the balloon catheter, from the interior of the tubular shaped member. [0004] However, one concern with such stents is that they are often impractical for use in some vessels such as the carotid artery. The carotid artery is easily accessible from the exterior of the human body, and is close to the surface of the skin. A patient having a balloon expandable stent made from stainless steel or the like, placed in their carotid artery, might be Susceptible to severe injury through day-to-day activity. A sufficient force placed on the patient's neck could cause the stent to collapse, resulting in injury to the patient. In order to prevent this, self-expanding stents have been proposed for use in such vessels. Self-expanding stents act like springs and will recover to their expanded or implanted configuration after being crushed. [0005] One type of self-expanding stent is disclosed in U.S. Pat. No. 4,655,771, which stent has a radially and axially flexible, elastic tubular body with a predetermined diameter that is variable tinder axial movement of the ends of the body relative to each other and which is composed of a plurality of individually rigid but flexible and elastic thread elements defining a radially self-expanding helix. This type of stent is known in the art as a "braided stent" and is so designated herein. Placement of such stents in a body vessel can be achieved by a device that comprises an outer catheter for holding the stent at its distal end, and an inner piston that pushes the stent forward once it is in position. [0006] However, braided stents have many disadvantages. They typically do not have the necessary radial strength to effectively hold open a diseased vessel. In addition, the plurality of wires or fibers used to make such stents could become dangerous if separated from the body of the stent, where they could pierce through the vessel. Therefore, there has been a-desire to have a self-expanding stent that is cut from a tube of metal, which is the common manufacturing method for many commercially available balloon-expandable stents. In order to manufacture a self-expanding stent cut from a tube, the alloy used would preferably exhibit superelastic or psuedoelastic characteristics at body temperature, so that it is crush recoverable. [0007] The prior art makes reference to the use of alloys such as Nitinol (Ni--Ti alloy), which have shape memory and/or superelastic characteristics, in medical devices that are designed to be inserted into a patient's body. The shape memory characteristics allow the devices to be deformed to facilitate their insertion into a body lumen or cavity and then be heated within the body so that the device returns to its original shape. Superelastic characteristics, on the other hand, generally allow the metal to be deformed and restrained in the deformed condition to facilitate the insertion of the medical device containing the metal into a patient's body, with such deformation causing the phase transformation. Once within the body lumen, the restraint on the superelastic member can be removed, thereby reducing the stress therein so that the superelastic member can return to its original un-deformed shape by the transformation back to the original phase. [0008] Alloys having shape memory/superelastic characteristics generally have at least two phases. These phases are a martensite phase, which has a relatively low tensile strength and which is stable at relatively low temperatures, and an austenite phase, which has a relatively high tensile strength and which is stable at temperatures higher than the martensite phase. [0009] Shape memory characteristics are imparted to the alloy by heating the metal at a temperature above which the transformation from the martensite phase to the austenite phase is complete, i.e. a temperature above which the austenite phase is stable (the Af temperature). The shape of the metal during this heat treatment is the shape "remembered." The heat-treated metal is cooled to a temperature at which the martensite phase is stable, causing the austenite phase to transform to the martensite phase. The metal in the martensite phase is then plastically deformed, e.g. to facilitate the entry thereof into a patient's body. Subsequent heating of the deformed martensite phase to a temperature above the martensite to austenite transformation temperature causes the deformed martensite phase to transform to the austenite phase, and during this phase transformation the metal reverts back to its original shape if unrestrained. If restrained, the metal will remain martensitic until the restraint is removed. [0010] Methods of using the shape memory characteristics of these alloys in medical devices intended to be placed within a patient's body present operational difficulties. For example, with shape memory alloys having a stable martensite temperature below body temperature, it is frequently difficult to maintain the temperature of the medical device containing such an alloy sufficiently below body temperature to prevent the transformation of the martensite phase to the austenite phase when the device was being inserted into a patient's body. With intravascular devices formed of shape memory alloys having martensite-to-austenite transformation temperatures well above body temperature, the devices can be introduced into a patient's body with little or no problem, but they must be heated to the martensite-to-austenite transformation temperature, which is frequently high enough to cause tissue damage. [0011] When stress is applied to a specimen of a metal such as Nitinol exhibiting superelastic characteristics at a temperature above which the austenite is stable (i.e. the temperature at which the transformation of martensite phase to the austenite phase is complete), the specimen deforms elastically until it reaches a particular stress level where the alloy then undergoes a stress-induced phase transformation from the austenite phase to the martensite phase. As the phase transformation proceeds, the alloy undergoes significant increases in strain but with little or no corresponding increases in stress. The strain increases while the stress remains essentially constant until the transformation of the austenite phase to the martensite phase is complete. Thereafter, further increases in stress are necessary to cause further deformation. The martensitic metal first deforms elastically upon the application of additional stress and then plastically with permanent residual deformation. [0012] If the load on the specimen is removed before any permanent deformation has occurred, the martensitic specimen will elastically recover and transform back to the austenite phase. The reduction in stress first causes a decrease in strain. As stress reduction reaches the level at which the martensite phase transforms back into the austenite phase, the stress level in the specimen will remain essentially constant (but substantially less than the constant stress level at which the austenite transforms to the martensite) until the transformation back to the austenite phase is complete, i.e. there is significant recovery in strain with only negligible corresponding stress reduction. After the transformation back to austenite is complete, further stress reduction results in elastic strain reduction. This ability to incur significant strain at relatively constant stress upon the application of a load, and to recover from the deformation upon the removal of the load, is commonly referred to as superelasticity or pseudoelasticity. It is this property of the material which makes it useful in manufacturing tube cut self-expanding stents. [0013] The prior art makes reference to the use of metal alloys having superelastic characteristics in medical devices that are intended to be inserted or otherwise used within a patient's body. See for example, U.S. Pat. No. 4,665,905 (Jervis) and U.S. Pat. No. 4,925,445 (Sakamoto et al.). However, the prior art has yet to disclose any suitable tube-cut self-expanding stents. In addition, many of the prior art stents lacked the necessary rigidity,or hoop strength to keep the body vessel open. In addition, many of the prior art stents have large openings at their expanded diameter. The smaller the openings are on an expanded stent, the more plaque or other deposits it can trap between the stent and the vessel wall. Trapping these deposits is important to the continuing health of the patient in that it helps prevent plaque prolapse into the vessel, restenosis of the vessel it is implanted into, and strokes caused by the release of embolic particles into the bloodstream. [0014] One additional concern with stents, and with other medical devices, is that they may exhibit reduced radiopacity under X-ray fluoroscopy. To overcome this problem, it is common practice to attach markers made from highly radiopaque materials to the stent, or to use radiopaque materials in plating or coating processes. Those materials are typically gold, platinum, or tantalum. The prior art makes reference to these markers or processes in U.S. Pat. No. 5,632,771 (Boatman et al), U.S. Pat. No. 6,022,374 (Imran), U.S. Pat. No. 5,741,327 (Frantzen), U.S. Pat. No. 5,725,572 (Lam et al), and U.S. Pat. No. 5,800,526 (Anderson et al). However, due to the relative position of these materials in the galvanic series versus the position of the base metal of the stent in the galvanic series, there is a certain challenge to overcome; namely, that of galvanic corrosion. [0015] In addition, biodegradable stents and stents fabricated from polymeric materials that avoid the use of metallic materials must still be able to be visualized under X-ray fluoroscopy. For these types of devices a major challenge exists in how to impart/increase the radiopacity of these devices with out the use of radiopaque markers or coatings. The prior art makes reference to one such method in U.S. Pat. No. 4,935,019 (Papp), in which a radiopaque, polymeric composition suitable for printing onto surgical fabrics provides an X-ray detectable marker, said marker is obtained by dispersing a heavy metal salt such as barium sulfate in a liquid polymer carrier. In Papp, the barium sulfate has an average particle size greater than about 5 microns and is present in an amount of from about 15 to 90% by weight of total solids of said composition. Papp indicates that barium sulfate comprising from about 60 to 90% by weight of solids of said composition is preferred. However addition of barium sulfate in large percentage quantities such as this may affect the integrity of the base material, reducing strength, and adversely affecting other mechanical properties and characteristics. In biodegradable polymers, the impact of radiopaque additives may also affect properties such as degradation rates of bioabsorbable polymers, elasticity, while potentially adding the presence of stress risers in and around-any localized concentration of barium sulfate particles within the material. Furthermore, inorganic contrast agents such as barium sulfate and zirconium oxide do not readily dissolve or do not easily disperse in organic solvents, which are commonly used to dissolve non-degradable and biodegradable polymers. [0016] Accordingly, there is a need for a radiopaque material or agent that can be easily added to biostable polymeric and biodegradable polymeric materials which readily dissolves into the polymer so that the resulting composite material is adequately radiopaque and which will not adversely affect the material or mechanical properties of the material one desires to make radiopaque. BRIEF SUMMARY OF THE INVENTION [0017] The high intensity X-ray contrast agent in accordance with the present invention overcomes the disadvantages and shortcomings of what is currently available and satisfies the unmet needs of imaging medical devices, particularly non-metallic medical devices by maximizing the intensity of the x-ray contrast agent both through primary and secondary effects. Primary effects include incorporating the radiopaque element and maximizing the content of this element in the contrast agent through chemistry, while secondary effects include optimizing the location of the radiopaque element within the polymer. Essentially by selectively maximizing and incorporating the iodine content within and dispersed throughout the polymer one can tune the radiopacity of polymeric materials to levels previously not available. Moreover, the creation and optimization of this contrast agent allows for improved processing characteristics when combined with polymeric materials and as such may further reduce manufacturing costs while providing a polymeric material with improved high intensity radiopacity with a satisfactory degradation profile. [0018] The present invention relates to a high intensity dendritic or starshaped contrast agent suitable for use with implantable polymeric medical devices or for a polymeric coating of an implantable medical device. Multivalent hydroxyl or amine containing organic compounds such as pentaerythritol, bis-pentaerythritol glycerol, polyhydric mono- and di-saccharides, etc., can be used to react with an iodine containing aromatic compounds such as 2,3,5-triiodobenzoic acid to form such high iodine containing compounds. Each such compound may contain a multiple of three (3) iodine atoms, greatly intensifying the x-ray image of a medical device fabricated from a material containing such a compound. The iodine content in such a high intensity dendritic contrast agent may be as high as 85% using commercially available dendritic polyamine precursors. [0019] In an exemplary embodiment of the present invention, the contrast agent may contain a multiplicity of iodine atoms or bromine atoms or a combination of both in a single molecule in order to enhance the x-ray image produced by dispersing the agent throughout the material that either the device will be fabricated from or applied as a coating to the device. In accordance with the present invention, the contrast agent can be constructed from any core of dendrimer containing free functional groups such as amine, hydroxyl, sulfhydryl, isocyante, and result in a molecule containing a multiple of three (3) iodine or bromine or a combination of both atoms with each additional conjugation of small iodine or bromine containing building block, such as triiodobenzoic acid or as triiodobenzoic acid chloride. When constructed in this fashion, the contrast agent may be substantially soluble in common organic solvent such as acetone, dimethylacetamide (DMA), dimethylsulfoxide (DMSO), acetone, THF, 1,4-dioxane, DCM etc. and also has substantially good miscibility with common organic polymers such as PLGA, PLA etc. The contrast agent in accordance with the present invention can form a solid solution with a polymer matrix that can then form the basis of a medical device. The contrast agent in accordance with the present invention is substantially biocompatible and can be added to polymer or polymer mixtures, and/or inorganic/organic composite materials to enhance its X-ray image quality. [0020] In another exemplary embodiment of the present invention, the contrast agent may be mixed with the bulk material by various means such as solvent casting, injection and/or compression molding in order to form a medical device or a coating for a medical device. The bulk form can then be processed to final size and shape by traditional fabrication methods. Alternatively, the polymeric coating with the contrast agent included can be applied to the surface of an implantable medical device employing traditional coating methods Continue reading about Dendritic and star-shaped contrast agents for medical devices and bioabsorbable radiopaque bulk material and method for producing same... 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