Use of an anti-platelet monoclonal antibody F(ab')2 fragment for imaging thrombus.

Ten patients with suspected thrombus have been studied using 111In-labelled F(ab')2 fragments of P256, a monoclonal antibody which recognizes an epitope on the platelet membrane glycoprotein protein IIb/IIIa complex. The F(ab')2 fragment was radiolabelled with 111In via diethylenetriaminepenta-acetic acid to give a specific activity of up to 190 MBq (5 mCi) mg-1 without impairment of immunoreactivity. In vitro platelet aggregation studies showed that the F(ab')2 fragment caused less platelet aggregation than the whole antibody on a molar ratio and was without significant effect upon the sensitivity of platelets to a range of aggregating agents. Platelets were labelled in ten patients by intravenous injection of approximately 100 micrograms P256 F(ab')2. Of the ten patients studied, six showed localization of activity consistent with platelet accumulation. Localization was clearly seen when associated with thrombus of the lower limbs (three patients: deep vein thrombosis; one patient: aortofemoral graft), and was apparent although less marked in two other cases, one of aortic aneurysm and one of carotid stenosis. Use of radiolabelled P256 F(ab')2 offers a means of non-invasive detection of thrombus which, from in vitro studies, would appear to have less direct effect on platelet behaviour than the whole antibody.