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Adenosine diphosphate directly induces platelet aggregation via the G-protein coupled P2Y1 and P2Y12 receptors. P2Y12, but not P2Y1, receptor antagonists are available in the clinic. The relevance of the P2Y1 receptor as an antiplatelet target has been studied in rodents, but not in higher species. We therefore examined effects of the pharmacological blockade of the P2Y1 receptor with its selective antagonist MRS2500 in monkey models of electrolytic-mediated arterial thrombosis (ECAT) and kidney bleeding time (KBT). Abciximab, a GPIIb-IIIa antagonist, and cangrelor, a P2Y12 antagonist, were utilized to validate these monkey models. Compounds were given IV at 15-60 min before thrombosis initiation in anesthetized monkeys. Scanning electron microscopy showed the luminal surface of thrombotic artery covered with platelet aggregates and fibrin network. Administration of abciximab at 0.25 and 0.7 mg/kg IV significantly reduced thrombus weight by 71 ± 1 and 100 ± 0 %, and increased KBT by 10.0 ± 0.1- and 10.1 ± 0-fold, respectively (n = 3/dose). Likewise, cangrelor at 0.6 and 2 mg/kg/h IV significantly reduced thrombus weight significantly by 72 ± 9 % and 100 ± 0 % and increased KBT by 2.1 ± 0.1- and 9.8 ± 0.2-fold, respectively (n = 3/dose). MRS2500 [mg/kg + mg/kg/h IV] at 0.09 + 0.14 and 0.45 + 0.68 significantly reduced thrombus weight by 57 ± 1 % and 88 ± 1 % and increased KBT by 2.1 ± 0.3- and 4.9 ± 0.6-fold, respectively (n = 4/dose). In summary, MRS2500 prevented occlusive arterial thrombosis at a dose that moderately prolonged KBT, indicating a role of P2Y1 receptors in arterial thrombosis and hemostasis in monkeys. Thus P2Y1 receptor antagonism provides a suitable target for drug discovery.

Apixaban has similar affinity for human and rabbit factor Xa (FXa). Rabbits are commonly used in development of thrombosis disease models; however, unlike in other species, apixaban demonstrated poor oral bioavailability (F = 3%) and a high clearance rate (2.55 l/h/kg) in rabbits. Oxidative metabolism of [14C] apixaban by liver microsomes was approximately 20 times faster in rabbits than in rats or humans. Following an intravenous (IV) dose of 5 mg/kg, circulating levels of [14C] apixaban decreased from the earliest sampling time (5 min) to undetectable at 4 h. After an oral dose of 30 mg/kg, levels of [14C] apixaban were only detected at 1 and 4 h. Radioactivity profiling showed that apixaban was a significant component in plasma only after IV administration; O-demethyl apixaban (M2), O-demethyl apixaban glucuronide (M14) and O-demethyl apixaban sulfate (M1) were prominent metabolites after both IV and oral administration. Studies of apixaban in rabbits showed a good correlation between apixaban concentrations and inhibition of FXa activity, prolongation of prothrombin time and modified prothrombin time, with no lag time between these ex vivo pharmacodynamic markers and plasma drug levels. The apixaban concentration required for 50% inhibition (IC50) of FXa activity ex vivo (0.22 +/- 0.02 microM) agreed with the IC50 from in vitro experiments in rabbit and human plasma. In summary, apixaban shows similar affinity for human and rabbit FXa. It produces a rapid onset of action, predictable concentration-dependent pharmacodynamic responses, and, unlike rats or humans, a rapid hepatic metabolism in rabbits.