Imaging pulmonary emboli and deep venous thrombi with Tc-99m-bitistatin, aplatelet-binding polypeptide from viper venom

An imaging test that could locate both pulmonary emboli (PE) and their sour ce, active deep venous thrombi (DVT), would be valuable in patient manageme nt. Bitistatin, an 83-amino-acid polypeptide isolated from Bitis arietans v enom, binds avidly to the glycoprotein IIb/IIIa receptor on platelets. The goal of this study was to label bitistatin with Tc-99m and assess its poten tial for imaging thrombi and emboli in vivo. Methods: Molecular modeling of bitistatin indicated that its primary amines are located on the opposite s ide of the molecule from the receptor-binding domain. The primary amines we re reacted with succinimidyl-4-hydrazino nicotinate hydrochloride to place 2.4 hydrazino nicotinate (Mn) chelating groups per peptide molecule. Hn-bit istatin was labeled by incubation with Tc-99m-glucoheptonate to 96 TBq/mmol and then tested for binding to platelets in vitro and for imaging of 24-h- old DVT and PE in a canine model used previously for other thrombus tracers . Results: Tc-99m-Hn-bitistatin bound to stimulated platelets with a dissoc iation constant (K-d) = 32 nmol/L, similar to that of I-125-bitistatin (K-d = 41 nmol/L). In vivo, focal uptake was observed in planar images as early as 30 min (DVT) and 60 min (PE) after injection. Lesion uptake of Tc-99m-H n-bitistatin at 4 h after injection was calculated in terms of percentage i njected dose per gram (%ID/g) of tissue and averaged 0.89 %ID/g PE and 0.79 %ID/g DVT. Lesion-to-background ratios averaged 34:1 (PE-to-lung), 18:1 (D VT-to-blood), and 284:1 (DVT-to-muscle). These values were not significantl y different from iodinated bitistatin, but uptakes were higher than other t racers tested in the same model. Conclusion: Tc-99m-Hn-bitistatin retains t he functional activity of the iodinated peptide, has higher DVT and PE upta kes than other thrombus tracers in this standardized model, and has target- to-background characteristics suitable for imaging both PE and DVT in a sin gle test.