GPIIb-IIIa Antagonist-induced Reduction in Platelet Surface Factor V/Va Binding and Phosphatidylserine Expression in Whole Blood
Mark I. Furman (1), (2) , Lori A. Krueger (1), (3) , A. L. Frelinger III (1), (3) , Marc R. Barnard (1), (3) , Mary Ann Mascelli (5) , Marian T. Nakada (5) , Alan D. Michelson (1), (3), (4)
From the (1) Center for Platelet Function Studies, (2) Division of Cardiovascular Medicine, Departments of Medicine, (3) Pediatrics and (4) Surgery , University of Massachusetts Medical School, Worcester, MA and (5) Centocor, Inc., Malvern PA, USA
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.