ABSTRACT:The metabolism and disposition of [ 14 C]apixaban, an orally bioavailable, highly selective, and direct acting/reversible factor Xa inhibitor, was investigated in 10 healthy male subjects without (group 1, n ‫؍‬ 6) and with bile collection (group 2, n ‫؍‬ 4) after a single 20-mg oral dose. Urine, blood, and feces samples were collected from all subjects. Bile samples were also collected for 3 to 8 h after dosing from group 2 subjects. There were no serious adverse events or discontinuations due to adverse effects. In plasma, apixaban was the major circulating component and O-demethyl apixaban sulfate, a stable and water-soluble metabolite, was the significant metabolite. The exposure of apixaban (C max and area under the plasma concentration versus time curve) in subjects with bile collection was generally similar to that in subjects without bile collection. The administered dose was recovered in feces (group 1, 56.0%; group 2, 46.7%) and urine (group 1, 24.5%; group 2, 28.8%), with the parent drug representing approximately half of the recovered dose. Biliary excretion represented a minor elimination pathway (2.44% of the administered dose) from group 2 subjects within the limited collection period. Metabolic pathways identified for apixaban included O-demethylation, hydroxylation, and sulfation of hydroxylated O-demethyl apixaban. Thus, apixaban is an orally bioavailable inhibitor of factor Xa with elimination pathways that include metabolism and renal excretion.Thromboembolic events, including acute myocardial infarction, unstable angina, deep vein thrombosis, pulmonary embolism, and ischemic stroke continue to be the leading cause of morbidity and mortality in the United States and other Western countries (Heit et al., 2005;Rosamond et al., 2007). Current therapies for the treatment and prevention of thromboembolic events, such as vitamin K antagonists (e.g., warfarin), heparin, and low-molecular-weight heparin (e.g., enoxaparin), are suboptimal (O'Donnell and Weitz, 2004;Wittkowsky, 2004;Campbell, 2006). However, the requirement for intravenous or subcutaneous injection and/or the need for careful monitoring because of the risk of excessive bleeding or unpredictable/inconsistent pharmacokinetics (PK) can complicate administration and present barriers to the use of these agents (O'Brien and Caro, 2002;Wittkowsky, 2004;Campbell, 2006). Therefore, new, orally active anticoagulants with predictable pharmacokinetic profiles that can be administered with a reduced need for monitoring are needed.Factor Xa is a key serine protease in the coagulation cascade and is a promising target enzyme for new therapeutic agents for the treatment and prevention of arterial and venous thrombosis (Kaiser, 2002;Samama, 2002;Walenga et al., 2003). In particular, factor Xa plays a critical role in blood coagulation, serving as the juncture between the extrinsic (tissue factor initiated) and intrinsic (surface activation and amplification) systems (Mann et al., 2003). Factor Xa forms the prothrombinase complex with phospholi...

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To cite this article: Wong PC, Crain EJ, Xin B, Wexler RR, Lam PYS, Pinto DJ, Luettgen JM, Knabb RM. Apixaban, an oral, direct and highly selective factor Xa inhibitor: in vitro, antithrombotic and antihemostatic studies. J Thromb Haemost 2008; 6: 820-9.Summary. Background: Apixaban is an oral, direct and highly selective factor Xa (FXa) inhibitor in late-stage clinical development for the prevention and treatment of thromboembolic diseases. Objective: We evaluated the in vitro properties of apixaban and its in vivo activities in rabbit models of thrombosis and hemostasis. Methods: Studies were conducted in arteriovenous-shunt thrombosis (AVST), venous thrombosis (VT), electrically mediated carotid arterial thrombosis (ECAT) and cuticle bleeding time (BT) models. Results: In vitro, apixaban is potent and selective, with a K i of 0.08 nM for human FXa. It exhibited species difference in FXa inhibition [FXa K i (nM): 0.16, rabbit; 1.3, rat; 1.7, dog] and anticoagulation [EC 2· (lM, concentration required to double the prothrombin time): 3.6, human; 2.3, rabbit; 7.9, rat; 6.7, dog]. Apixaban at 10 lM did not alter human and rabbit platelet aggregation to ADP, cthrombin, and collagen. In vivo, the values for antithrombotic ED 50 (dose that reduced thrombus weight or increased blood flow by 50% of the control) in AVST, VT and ECAT and the values for BT ED 3· (dose that increased BT by 3-fold) were 0.27 ± 0.03, 0.11 ± 0.03, 0.07 ± 0.02 and > 3 mg kg warfarin, respectively. Conclusions: In summary, apixaban was effective in the prevention of experimental thrombosis at doses that preserve hemostasis in rabbits.

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Efforts to identify a suitable follow-on compound to razaxaban (compound 4) focused on modification of the carboxamido linker to eliminate potential in vivo hydrolysis to a primary aniline. Cyclization of the carboxamido linker to the novel bicyclic tetrahydropyrazolopyridinone scaffold retained the potent fXa binding activity. Exceptional potency of the series prompted an investigation of the neutral P1 moieties that resulted in the identification of the p-methoxyphenyl P1, which retained factor Xa binding affinity and good oral bioavailability. Further optimization of the C-3 pyrazole position and replacement of the terminal P4 ring with a neutral heterocycle culminated in the discovery of 1-(4-methoxyphenyl)-7-oxo-6-(4-(2-oxopiperidin-1-yl)phenyl)-4,5,6,7-tetrahydro-1H-pyrazolo[3,4-c]pyridine-3-carboxamide (apixaban, compound 40). Compound 40 exhibits a high degree of fXa potency, selectivity, and efficacy and has an improved pharmacokinetic profile relative to 4.