Purified FXa is obtained after activation with Russell’s viper venom followed by affinity chromatography. The resulting FXa is > 95% pure as judged by sodium dodecylsulfate polyacrylamide gel electrophoresis. The substrate affinity values for FXa, expressed as the Michaelis-Menten-Henri constant (K_m), for human, rabbit, rat and dog FXa are determined using the chromogenic substrate S-2765, and are 36, 60, 240 and 70 μM, respectively. The substrate hydrolysis is monitored by measuring absorbance at 405 nm at 25°C for up to 30 min using a SpectraMax 384 Plus plate reader and SoftMax. FXa activity for each substrate and inhibitor concentration pair is determined in duplicate. The K_i values are calculated by non-linear least-squares fitting of the steady-state substrate hydrolysis rates to the equation for competitive inhibition (Equation 1) using GRAFIT, where v equals reactions velocity in OD min?1, Vmax equals maxiumum reaction velocity, S equals substrate concentration, and I equals inhibitor concentration.

MCE has not independently confirmed the accuracy of these methods. They are for reference only.

Briefly, male New Zealand White rabbits are anesthetized with ketamine (50 mg/kg i.m.) and xylazine (10 mg/kg i.m.), and their femoral artery, jugular vein and femoral vein are catheterized. Thrombosis is induced by an arteriovenous (AV)-shunt device containing a silk thread. Blood flowed from the femoral artery via the AV shunt into the opposite femoral vein for 40 min. The shunt is then disconnected and the silk thread covered with thrombus is weighed. As apixaban has an oral bioavailability of < 5% in rabbits (unpublished result), it is administered intravenously for in vivo studies. To achieve a stable plasma level with minimum experimental variability, apixaban, fondaparinux or vehicle is given by a continuous intravenous infusion 1 h prior to shunt placement. The infusion is continued throughout the experiment. Warfarin or vehicle is dosed orally once daily for 4 days. On the fourth day after the last oral dose of warfarin or vehicle, rabbits are anesthetized 1.5 h later, and the treatment effect is evaluated about 2 h postdose. Arterial blood samples for the determination of clotting times or plasma levels are collected 20 min after shunt placement. Plasma levels of apixaban are measured by a specific and sensitive liquid chromatographic mass spectrometry method (LC/MS/MS). In rabbits treated with apixaban, fondaparinux or warfarin, the antithrombotic effects of these agents are expressed as percentage inhibition of thrombus formation based on the treated vs. the corresponding mean vehicle. The ED₅₀ value (dose that produced 50% inhibition of thrombus formation) is determined as described below. The apixaban group treatment consists of vehicle (10%N,N-dimethylacetamide; 30% 1,2-propanediol; 60% water) (n=4), and apixaban (mg/kg/h) at 0.03 (n=7), 0.1 (n=7), 0.3 (n=7), 1 (n=7), and 3 (n=3). The fondaparinux group treatment consists of vehicle (saline) (n=6), and fondaparinux (mg/kg/h1) at 0.01 (n=5), 0.03 (n=5), 0.1 (n=5), 0.3 (n=5), and 1 (n=5). The warfarin group treatment consists of vehicle (water) (n=6), and warfarin (mg/kg/day) at 0.1 (n=6), 0.3 (n=6), 1 (n=6), and 3 (n=6).

MCE has not independently confirmed the accuracy of these methods. They are for reference only.

• [1]. Pinto DJ, et al. 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, BMS-562247), a highly potent, selective, efficacious, and orally bioavailable inhibitor of blood coagulation factor Xa. J Med Chem. 2007 Nov 1;50(22):5339-56.  [Content Brief]

  [2]. Wong PC, et al. Apixaban, an oral, direct and highly selective factor Xa inhibitor: in vitro, antithrombotic and antihemostatic studies. J Thromb Haemost. 2008 May;6(5):820-9  [Content Brief]

  [3]. Zhang D, et al. Metabolism, pharmacokinetics and pharmacodynamics of the factor Xa inhibitor apixaban in rabbits. J Thromb Thrombolysis. 2010 Jan;29(1):70-80  [Content Brief]

  [4]. He K, et al. Preclinical pharmacokinetics and pharmacodynamics of apixaban, a potent and selective factor Xa inhibitor. Eur J Drug Metab Pharmacokinet. 2011 Sep;36(3):129-39  [Content Brief]