Thrombus imaging with an I-123-labeled F(ab')2 fragment of an anti-human fibrin monoclonal antibody in a rabbit model.

The murine monoclonal antibody MA-15C5 binds to cross-linked fibrin but not to fibrinogen. Fab or F(ab')2 fragments of MA-15C5, labeled with iodine-123, were injected intravenously into nine rabbits with a nonocclusive 0.2-mL human plasma clot in the jugular vein and into eight control rabbits. Scintigrams obtained at hourly intervals after injection of the F(ab')2 fragment revealed a significantly higher accumulation of tracer in the clot region than in the contralateral region. The relative excess of tracer in the jugular vein region was significantly greater in the study animals than in the control animals throughout the study period. Blind reading of serial scans revealed no visible tracer accumulation in seven of eight control rabbits, whereas the scans of seven of nine study rabbits showed clearly visible accumulation (sensitivity = 77%, specificity = 87%). The Fab fragment of MA-15C5 had a lower affinity and bound less avidly to the blood clot in vivo. These results suggest that the F(ab')2 fragment has better imaging properties than the Fab fragment and that it may be useful for imaging blood clots in man.