This study was undertaken to compare the platelet binding characteristics and anti-platelet efficacy of a nonpeptide glycoprotein IIb/IIIa antagonist roxifiban with orbofiban in static and dynamic adhesion and aggregation assays. The results indicate that roxifiban binds with higher affinity to glycoprotein IIb/IIIa receptors and exhibits slower dissociation rates than orbofiban. Furthermore, the platelet inhibitory effects of roxifiban, but not orbofiban, were unaffected by changes in plasma calcium concentrations. Both agents reduced, in a concentration-dependent manner, the size of platelet thrombi deposited onto collagen I upon perfusion of heparinized blood at a shear rate of 1,500/s. At a clinically achievable concentration of 60 nM, roxifiban abrogated the formation of thrombi containing > 20 platelets per thrombus, thereby displaying comparable in vitro efficacy to that achieved by the theoretical maximal abciximab blood concentration (3.5 microg/ml) produced after standard treatment. In contrast, orbofiban, even at 500 nM, was only effective in inhibiting the formation of larger platelet thrombi (> or =150 platelets per thrombus). Pretreatment of surface-anchored platelets with roxifiban (100 nM), but not orbofiban (500 nM), inhibited monocytic THP-1 cell attachment under flow. However, this heterotypic adhesion process was also suppressed when orbofiban (500 nM) was maintained in the perfusion buffer during the entire course of flow experiment. These findings demonstrate roxifiban (unlike orbofiban) is a potent glycoprotein IIb/IIIa antagonist with a long receptor-bound lifetime and prolonged anti-platelet efficacy and may thus be beneficial for the treatment and prevention of acute ischemic syndromes.