RECEPTOR-BASED RADIOTRACERS FOR CARDIOVASCULAR IMAGING

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The goal of this project is the development of a technetium-labeled radiopharmaceutical for imaging sites of active platelet deposition. Design of this radiotracer will exploit the properties of a class of macromolecules called lectins, which binding specific carbohydrates. A lectin has recently been found, thrombospondin, which mediates the aggregation of thombin-activated platelets. Our laboratory, under support of two NIH-sponsored projects is currently testing the clinical utility of a radiotracer which is targeted to a galactose-specific lectin which resides at the cell surface of hepatocytes. Using the same design strategy as our liver tracer we will attach to human serum albumin a carbohydrate specific to thrombospondin. The resulting albumin derivative, called a neoglycoablumin (NGA), will then be labeled with technetium-99m. Because agglutination of activated platelets by thrombospondin can be reversed by 2-amino-2-deoxy-glycosides, we will synthesize four NGAs by coupling each of the following carbohydrates to albumin: mannosamine, galactosamine, glucosamine, N-acetylglucosamine. Performance of each NGA will be assessed by in vitro binding of each I-125-labeled neoglycoalbumin to thrombin-activated platelets. Measurement of dissociation constants by Scatchard analysis, and reverse-binding rate constants by binding displacement will be carried out. Because complete knowledge of the tissue distribution of thrombospondin is not available we will carry out detailed biodistribution measurements of each Tc-99m-deoglycoalbumin in normal rabbits. Lesion-to-blood ratios at various times will be determined for each Tc-NGA and In-111-labeled platelets in dogs with experimentally induced thrombi and pulmonary emboli. These experiments will be repeated with heparin infusion to investigate the sensitivity of Tc-NGA-thrombospondin binding to anticoagulation therapy. Adequate uptake of a Tc-NGA at platelet deposition sites will provide a clinical test for deep vein thrombi and pulmonary emboli which should exceed the sensitivity of In-111-labeled platelets with the convenience and image quality of a technetium-based radiopharmaceutical.