XV454 demonstrated high potency (IC50 = 14-25 nM) in inhibiting human platelet aggregation induced by adenosine diphosphate (ADP, 10 μM), thrombin receptor agonist peptide (TRAP) (10 μM), or collagen (20 μg/ml). XV454 exhibited a high degree of selectivity for platelet aIIbβ3 in comparison with c7E3, which is a nonspecific antagonist for both aIIbβ3 and avb3. Both XV454 and c7E3 bind with high affinity to either activated (A) or unactivated (U) human, baboon, or canine platelets. XV454 binds with a relatively higher affinity [Kd = 0.5 nM (A), 0.6 nM (U)] as compared with c7E3 [Kd = 9.1 nM (A), 9.2 (U) nM]. XV454 demonstrated a tight association with human, baboon, and, to a lesser extent, with canine platelets (t1/2 of dissociation = 110 ± 6, 80 ± 10, and 23 ± 2 min, respectively). Both c7E3 and XV454 associate tightly with a slower dissociation rate with unactivated human platelets: t1/2 of 42 and 116 min, respectively. In nonhuman primates, oral (0.1 mg/kg, p.o.) and intravenous (0.05 mg/kg, i.v. bolus administration of XV454 methyl ester prodrug resulted a long-lasting maximal antiplatelet efficacy for ≤72 h with significant but reversible prolongation of bleeding time and without effects on platelet count, clinical chemistry, or hemodynamic profile. In conclusion, XV454 represents a potent antiplatelet agent in inhibiting platelet aggregation along with a high affinity and relatively slow dissociation rate from human platelet GPIIb/IIIa receptors that allow a long-lasting antiplatelet efficacy after single i.v. or oral administration.