Imaging vascular thrombosis with 99mTc-labeled fibrin alpha-chain peptide

An agent that permits scintigraphic detection of chronic deep venous thrombosis (DVT) or pulmonary embolism (PE) would be a welcome addition to the armamentarium of nuclear medicine. Because fibrin is the integral part of each clot, old or fresh, we hypothesized that a 99mTc-labeled fibrin alpha-chain N-terminal peptide, Gly-Pro-Arg-Pro-Pro, that binds to the C-terminal portion of the gamma-chain of fibrin can detect DVT and PE.

The peptide was modified to Gly-Pro-Arg-Pro-Pro-Aba-Gly-Gly-(D)-Ala-Gly to permit efficient binding of 99mTc (99mTc-TP 850). The stability of the peptide was examined in vitro as well as in vivo. The ability of the agent to bind to rabbit, dog, and human fibrin and to inhibit adenosine diphosphate-induced platelet aggregation was examined. Blood clearance and 3-h tissue distribution were studied. DVT was induced in 8 rabbits using a stimulating electrode and in 2 rabbits by inserting a thrombin-soaked suture. PE was induced in 6 additional rabbits by introducing tantalum-impregnated blood clots into the right atrium, and the rabbits were radiographed to locate the emboli. 99mTc-TP 850 was then injected through a lateral ear vein, and each rabbit was imaged for up to 3 h. The rabbits were then killed, the heart and lungs were dissected and radiographed and the clots were harvested so that clot-to-blood radioactivity ratios could be determined.

The peptide analog permitted efficient incorporation of 99mTc, which was stable in vitro and in vivo. The blood clearance was biphasic, with an alpha phase half-life of approximately 4 min (20%) and a beta phase half-life of approximately 13 min (88%). The mean binding of 99mTc-TP 850 to human, dog, and rabbit fibrin was 46% +/- 2%, 60% +/- 3%, and 56% +/- 2.5%, respectively, and the inhibitory concentration of 50% for dog and rabbit platelet aggregation was 236 pm and 167 pm, respectively. All clots, including 24-h-old pulmonary emboli, were delineated. The radioactivity associated with clots varied from 0.01 to 0.09 %ID/g, with clot-to-blood radioactivity ratios ranging from 1.2 to 12.0. However, 48-h-old pulmonary emboli had lysed and were seen neither by radiography nor by scintigraphy.

A fibrin alpha-chain, N-terminal peptide that binds to the C-terminal portion of the gamma-chain of fibrin has been modified and labeled with 99mTc. The resultant peptide is stable in vitro and in vivo; binds to human, dog, and rabbit fibrin in large quantities; and inhibits platelet aggregation. The peptide clears rapidly from the blood and delineates experimental DVT and PE in rabbits. This agent is worthy of further investigation.