Differential Antiplatelet Effects of Various Glycoprotein IIb–IIIa Antagonists

Abstract

The blockade of platelet glycoprotein IIb–IIIa (GPIIb–IIIa) was recently introduced as a new antiplatelet strategy. At present, various GPIIb–IIIa inhibitors are available to treat patients with acute coronary syndrome or when undergoing percutaneous coronary interventions. The current study systematically evaluates the antiplatelet effects of GPIIb–IIIa inhibitors in clinical use. Using conformation-dependent monoclonal antibodies [ligand-induced binding sites (LIBS-1), PMI-1] and flow cytometry, we showed that the GPIIb–IIIa antagonists abciximab, integrelin, lamifiban, and tirofiban, but not EMD 122347 or YM 337, induced LIBS activity of platelet GPIIb–IIIa. The LIBS activity of GPIIb–IIIa antagonists correlates with a proaggregatory response of fixed platelets pretreated with GPIIb–IIIa antagonists (intrinsic activity). All tested GPIIb–IIIa antagonists completely inhibit concentration-dependent ADP (20 μmol/l)-induced aggregation. In contrast, substantial TRAP (25 μmol/l)-induced platelet aggregation still occurs even at high inhibitor concentrations of the tested GPIIb–IIIa antagonists. In addition, we show that GPIIb–IIIa antagonists are poor inhibitors of platelet release reaction (ATP and P-selectin secretion) especially when strong agonists such as TRAP are used to activate platelets. Inhibition of platelet procoagulant activity (thrombin generation) by GPIIb–IIIa antagonists is dependent on the type and concentration of antagonists and on the strength of stimulus (thrombin, tissue factor) used to induce platelet-dependent thrombin generation. The present data show that significant pharmacological differences exist between GPIIb–IIIa antagonists that may have consequences for antithrombotic strategies and for future drug development.