THROMBUS IMAGING WITH A TC-99M-LABELED, ACTIVATED PLATELET RECEPTOR-BINDING PEPTIDE

The objective of this work was the preclinical evaluation of (TC)-T-99 m-P280, a Tc-99m-labeled peptide having high affinity and specificity for the GPIIb/IIIa receptor expressed on activated platelets, for use as a thrombus imaging agent. Methods: The affinity and specificity of P280 peptide for the GPIIb/IIIa receptor was assessed by the inhibitio n of ADP-stimulated human platelet aggregation, the inhibition of the binding of fibrinogen to the GPIIb/IIIa receptor and the inhibition of the binding of vitronectin to the vitronectin receptor. P280 peptide was radiolabeled with Tc-99m by ligand exchange using Tc-99m-glucohept onate. The ability of Tc-99m-P280 to detect thrombi in vivo was assess ed using a canine venous thrombosis model and the biodistribution of T c-99m-P280 was determined in rats and rabbits. Results: P280 peptide h ad an IC50 of 79 nM for the inhibition of aggregation of human platele ts in platelet rich plasma, an IC50 of 6.8 nM for the inhibition of fi brinogen binding to the GPIIb/IIIa receptor and an IC50 of 13 mu M for the inhibition of vitronectin binding to the vitronectin receptor, sh owing the high in vitro receptor binding affinity and specificity of t he peptide. Tc-99m-P280 was readily prepared in greater than or equal to 90% radiochemical yield and purity and provided images of femoral v ein thrombi in the canine model by 1 hr postinjection (thrombus-to-blo od ratio of 4.4 and thrombus-to-muscle ratio of 11 at 4 hr). Dog, rat and rabbit studies all showed rapid clearance of the radiotracer from the blood and rapid renal excretion. Conclusion: The combination of hi gh in vitro receptor-binding affinity and specificity, in vivo thrombu s imaging and fast clearance support the evaluation of Tc-99m-P280 as a clinical imaging agent.