Summary

In addition to inhibition of platelet aggregation, GPIIb-IIIa antagonistsmay reduce thrombotic events via other mechanisms. In a novelwhole blood flow cytometric system, we investigated the effects ofGPIIb-IIIa antagonists, in the presence or absence of thrombin inhibitors,on platelet surface-bound factor V/Va and platelet surface phospholipids.Diluted venous blood was incubated with either buffer ora GPIIb-IIIa antagonist (abciximab, tirofiban, or eptifibatide). Somesamples were pre-incubated with clinically relevant concentrations ofunfractionated heparin (UFH), a low molecular weight heparin, a directthrombin inhibitor, or buffer only. Platelets were then activated andlabeled with mAb V237 (factor V/Va-specific) or annexin V (bindsphosphatidylserine), fixed, and analyzed by flow cytometry. In theabsence of thrombin inhibitors, GPIIb-IIIa antagonists (especiallyabciximab) significantly reduced agonist-induced platelet procoagulantactivity, as determined by reduced binding of V237 and annexin V.At high pharmacologic concentrations, unfractionated heparin andenoxaparin, but not hirudin, further reduced factor V/Va binding to thesurface of activated platelets in the presence of GPIIb-IIIa antagonists.Agonist-induced platelet procoagulant activity was reduced in a patientwith Glanzmann’s thrombasthenia. We conclude that GPIIb-IIIaantagonists reduce platelet procoagulant activity in whole blood andheparin and enoxaparin augment this reduction. Fibrinogen binding toGPIIb-IIIa is important in the generation of platelet procoagulantactivity.