Recombinant factor VIIa enhances platelet adhesion and activation under flow conditions at normal and reduced platelet count.

BACKGROUND: Recombinant factor VIIa (rFVIIa), which was developed for treatment of inhibitor-complicated hemophilia, appears suitable as prohemostatic agent in other clinical disorders including patients with thrombocytopenia. It is generally accepted that rFVIIa functions by enhancement of thrombin generation at the site of injury. It is, however, unknown if and how this affects platelet adhesion and aggregation. OBJECTIVES: To determine the effect of rFVIIa-mediated thrombin generation on platelet adhesion and aggregation under flow conditions at normal and reduced platelet counts. METHODS: Washed platelets and red cells were combined to obtain plasma-free blood with different platelet counts. The reconstituted blood was perfused over a collagen- or fibrinogen-coated surface in the absence or presence of a thrombin generating system consisting of purified coagulation factors rFVIIa, factor (F)X and prothrombin. RESULTS: Addition of coagulation factors rFVIIa, FX and prothrombin to washed platelets and red cells enhanced platelet adhesion and aggregation to collagen and adhesion and spreading to fibrinogen at normal platelet count and at platelet numbers as low as 10 000 microL(-1). rFVIIa-mediated thrombin generation enhanced the activation state of platelets as measured by intracellular calcium fluxes, and enhanced the exposure of procoagulant phospholipids as measured by annexin A5 binding. CONCLUSIONS: Taken together, increased platelet adhesion and aggregation by rFVIIa-mediated thrombin formation may explain the therapeutic effects of rFVIIa in thrombocytopenic conditions and in patients with a normal platelet count by (i) enhancement of primary hemostasis and (ii) enhancement of procoagulant surface leading to elevated fibrin formation.