ABSTRACT
The hypothesis that glycoprotein (GP) IIb/IIIa antagonists stimulate platelets is controversial. Here, we report the results of flow cytometric measurements of platelet activation markers in a phase I dose optimization study of Roxifiban, an orally active GP IIb/IIIa antagonist. Whole blood was collected at pre-dose and during the dosing interval directly into citrate fixative so that circulating levels of platelet activation could be assessed. P-selectin expression and fibrinogen binding of single platelets were unchanged at any of the dosing intervals compared to the pre-dose values, whereas microaggregate formation was reduced. Blood was also collected in hirudin to maintain physiological calcium concentrations and stimulated with platelet agonists to test whether GP IIb/IIIa antagonists lower the threshold for platelet activation. After stimulation with a concentration range of ADP and TRAP, P-selectin expression was not altered by Roxifiban administration compared to pre-dose levels. Fibrinogen binding and microaggregate formation were reduced by Roxifiban dosing in a dose-dependent manner. Inhibition of both parameters was retained at trough and no increase above pre-dose values was observed at any time. This study provides evidence for a dose-dependent inhibition of platelet functions by an orally active GP IIb/IIIa antagonist and does not detect paradoxical activation of platelets by a GP IIb/IIIa antagonist in humans.
