GLYCOPROTEIN IIB IIIA RECEPTOR ANTAGONISTS INHIBIT THE DEVELOPMENT OFPLATELET PROCOAGULANT ACTIVITY/

We examined the effects of glycoprotein IIb/IIIa (GPIIb/IIIa) antagoni sts c7E3 Fab and DMP728 on the development of platelet prothrombinase (PT) activity, c7E3 Fab dose-dependently inhibited the rate of thrombi n-stimulated thrombin generation over a 1-minute reaction time. The IC 50 was 11 nM with an IC90 of 1000 nM. DMP728 inhibited PT activity max imally by 60% at 100 nM. A similar profile was observed for the inhibi tion of platelet tenase activity. Inhibition was platelet specific up to approximately 200 nM c7E3 Fab. Above 200 nM, inhibition was platele t independent, as shown by the inhibition of activity assembled on PS/ PC vesicles. c7E3 Fab and DMP728 did not inhibit calcium ionophore-ind uced activity. DMP728 potency diminished with reaction time (over 6 mi nutes) whereas c7E3 Fab potency did not. Inhibition by 2 mu M DMP728 w as not further increased by 20 nM c7E3 Fab. Heparin inhibition of plat elet PT activity was additive to that of c7E3 Fab. Studies with added von Willebrand factor (vWf) indicate that in the context of thrombin a ctivation vWf activates platelets through mechanisms independent of GP IIb/IIIa to promote PT activity. Thrombin activation induced binding o f FITC-AnnexinV to a subpopulation of platelets which was reduced by a pproximately 50% by pretreatment with either c7E3 Fab or DMP728. Toget her, these data indicate that c7E3 Fab and DMP728 inhibit the developm ent of GPIIb/IIIa-mediated platelet PT activity at events during plate let activation. The inhibitory activities are not additive, suggesting these agents compete for the same site or inhibit via the same mechan ism. Inhibition accompanies a reduction in the number of phosphatidyls erine binding sites, implying that GPIIb/IIIa receptor antagonists red uce platelet membrane scrambling induced by thrombin. The additivity o f inhibition with heparin by c7E3 Fab suggests a combination of these agents might have a greater bleeding liability than the use of either agent alone. (C) 1998 DuPont Merck Pharmaceutical Company. Published b y Elsevier Science Ltd.