PRECLINICAL EVALUATION OF TC-99M PLATELET RECEPTOR-BINDING PEPTIDE

P748 is a dimeric peptide which incorporates two high affinity GPIIb/I IIa receptor-binding domains and a novel Tc-99m binding sequence, whic h provides the platelet imaging agent Tc-99m-P748. The aim of this stu dy was to evaluate (TC)-T-99m-p748 preclinically for use as a hot spot scintigraphic thrombus imaging agent. Methods: Technetium-99m-P748 wa s prepared by either a ligand exchange or a one-vial kit. The oxorheni um congener, [ReO]P748, was prepared by ligand exchange from Bu(4)NReO Br(4). The binding of P748 peptide and [ReO]P748 to GPIIb/IIIa recepto rs on activated platelets was assessed by their inhibition of ADP stim ulated human platelet aggregation in platelet rich plasma (PRP). The l ocalization of Tc-99m-P748 in deep vein and pulmonary thrombi was asse ssed in a canine thrombosis model and the biodistribution of Tc-99m-P7 48 was determined in rats. Results: P748 peptide inhibited the aggrega tion of human platelets in PRP by 50% at a concentration (IC50) of 28 nM and [ReO]P748 had an IC50 of 36 nM showing the high in vitro recept or binding affinity of both the peptide and its rhenium complex (and b y analogy its technetium complex). Technetium-99m-P748 was readily pre pared at room temperature in 15 min in greater than or equal to 90% ra diochemical yield and purity and provided definitive images of femoral vein thrombi within 20 min and pulmonary thrombi within 1 hr in the c anine model. Femoral vein thrombus-to-blood and thrombus-to-muscle rat ios at 4 hr averaged 6.7 and 46, respectively Pulmonary thrombus-to-bl ood and thrombus-to-normal lung ratios at 4 hr averaged 29 and 27, res pectively. Dog and rat studies both showed rapid clearance of the radi otracer from the blood and with no significant hepatobiliary excretion but with notable early kidney retention. Conclusion: The combination of high in vitro receptor-binding affinity, high thrombus uptake and d efinitive in vivo images of both femoral vein and pulmonary thrombi sh ow that Tc-99m-P748 has considerable potential as a clinical imaging a gent for the detection of venous thromboembolism.