Imaging pulmonary emboli and deep venous thrombi with 99mTc-bitistatin, a platelet-binding polypeptide from viper venom.

UNLABELLED: An imaging test that could locate both pulmonary emboli (PE) and their source, active deep venous thrombi (DVT), would be valuable in patient management. Bitistatin, an 83-amino-acid polypeptide isolated from Bitis arietans venom, binds avidly to the glycoprotein IIb/IIIa receptor on platelets. The goal of this study was to label bitistatin with 99mTc and assess its potential for imaging thrombi and emboli in vivo.

METHODS: Molecular modeling of bitistatin indicated that its primary amines are located on the opposite side of the molecule from the receptor-binding domain. The primary amines were reacted with succinimidyl-4-hydrazino nicotinate hydrochloride to place 2.4 hydrazino nicotinate (Hn) chelating groups per peptide molecule. Hn-bitistatin was labeled by incubation with 99mTc-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: 99mTc-Hn-bitistatin bound to stimulated platelets with a dissociation constant (Kd) = 32 nmol/L, similar to that of 125I-bitistatin (Kd = 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 99mTc-Hn-bitistatin at 4 h after injection was calculated in terms of percentage injected 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 (DVT-to-blood), and 284:1 (DVT-to-muscle). These values were not significantly different from iodinated bitistatin, but uptakes were higher than other tracers tested in the same model.

CONCLUSION: 99mTc-Hn-bitistatin retains the functional activity of the iodinated peptide, has higher DVT and PE uptakes than other thrombus tracers in this standardized model, and has target-to-background characteristics suitable for imaging both PE and DVT in a single test.