Variation in the ability of glycoprotein IIb-IIIa antagonists to exert and maintain their inhibitory effects on the binding of fibrinogen.

Tirofiban and eptifibatide dissociate rapidly from glycoprotein IIb-IIIa but have different dissociation constants (KD of tirofiban = 15 nmol/L, that of eptifibatide = 120 nmol/L). Binding of fibrinogen to glycoprotein IIb-IIIa is biphasic, forming an initial reversible complex (KD = 155-180 nmol/L) and a second more stable complex (KD = 20-70 nmol/L). To test whether a comparable extent of inhibition would be maintained by pharmacologic antagonists that exhibit a rapid rate of release, blood from 26 patients with symptomatic coronary artery disease was added to reaction tubes containing a concentration of either agent that had been shown to achieve optimal inhibition (for tirofiban 100 ng/mL, for eptifibatide 1.7 microg/mL) plus a platelet agonist (1 microM adenosine diphosphate [ADP] or 25 microM thrombin receptor agonist peptide [TRAP]), and fluorochrome labeled fibrinogen before analysis by flow cytometry. The extent of inhibition early on (30 seconds to 3 minutes) was similar. By contrast, the extent of inhibition 10 to 15 minutes later was maintained more effectively with tirofiban than eptifibatide (difference in slope P < 0.01). The differences are consistent with the biphasic binding of fibrinogen to GP IIb-IIIa. The clinical implications of this observation merit evaluation to potentially improve care of patients and to guide future drug development.