Technetium-99m labeled 50H.19 antibody fragments: interaction of the antibody with platelets.

The monoclonal antibody 50H.19 recognized three antigens (Mr = 31-, 40-, 45-K) on normal and thromboasthenic platelets, but only one (Mr = 31-K) on Bernard-Soulier platelets. The intact antibody and its F(ab')2 fragments, had direct platelet-aggregating activity, and induced the platelet release reaction. The intact antibody potentiated platelet aggregation induced by platelet-activating factor or thrombin. Additions of indomethacin did not inhibit aggregation: addition of PGI2, or a calcium channel blocker completely inhibited aggregation. A reduced amount of platelet-aggregating activity was observed with antibody fragments prepared for labeling with 99mTc by pre-exposure to stannous ions, and herein used in biodistribution studies and elsewhere in thrombus imagining studies (J. Nucl. Med. 27: 1315; 1986). Antibody fragments radiolabeled with 99mTc bound to isolated platelets and to clots containing platelets.