Association of the antagonism of von Willebrand factor but not fibrinogen by platelet αIIbβ3 antagonists with prolongation of bleeding time

Summary. Background: The αIIbβ3 antagonists inhibit platelet aggregation and are used as antithrombotic agents for cardiothrombotic disease. The present study investigates the correlation of inhibition of fibrinogen and von Willebrand factor (VWF) binding by αIIbβ3 antagonists with the inhibition of platelet aggregation and prolongation of bleeding time (BT). Methods: Inhibition of fibrinogen and VWF binding were assessed in a purified αIIbβ3-binding assay. As an in vitro cell-based assay, platelet aggregation and VWF-mediated adhesion studies were performed using human platelets. In vivo effects on BT were measured using a template device in dogs at the same time as an ex vivo aggregation study was performed. Results: In vitro studies demonstrated that the antiaggregatory effects of αIIbβ3 antagonists correlate with their inhibition of fibrinogen binding, but not VWF. Interestingly, the effects of αIIbβ3 antagonists on BT could be differentiated from the inhibition of platelet aggregation. Furthermore, this differentiation was strongly correlated with the different inhibitory potencies between fibrinogen and VWF binding, as well as that between VWF-mediated adhesion and aggregation. Conclusions: Our study provides novel evidence showing that the inhibitory effect of αIIbβ3 antagonists on VWF, but not fibrinogen binding, correlates with their ability to prolong BT.