Kinase experiment:

To measure the inhibition of fXa activity by direct fXa inhibitors and the reversal of its inhibitory effect by r-Antidote, purified human plasma fXa (3 nM) (Haematologic Technologies), varying concentrations of inhibitor (0, 2.5, 5.0 and 7.5 nM) and r-Antidote are added to the assay buffer (20 mM Tris, 150 mM NaCl, 5 mM Ca2+ and 0.1% BSA, pH 7.4). After incubation at room temperature for 30 min, 100 μM Spectrozyme-fXa is added to the mixture, and the initial rate of sub!strate cleavage is monitored continuously for 5 min at 405 nm in a 96-well plate reader. The initial velocity of product formation as a function of inhibitor and r-Antidote concentrations is analyzed by Dynafit to estimate the binding affinity of r-Antidote to each inhibitor[2].

Animal experiment:

Rats[2] Whole-blood INR values (mean±s.d.) in rats infused with Betrixaban (1 mg/kg per hour) or vehicle and then treated with either vehicle or r-Antidote by i.v. bolus (6 mg) over 5 min plus infusion (9 mg/h) for up to 90 min. Circles, vehicle+vehicle; squares, Betrixaban + vehicle; triangles, Betrixaban + r-Antidote. *P≤0.02 compared to the r-Antidote treatment group determined by unpaired two-tailed t test. Whole-blood INR values (mean±s.d.) in rats infused with Apixaban (0.5 mg per kg body weight h–1) or vehicle and then treated with either vehicle or r-Antidote by i.v. bolus (6 mg) over 5 min plus infusion (6 mg/h) for up to 90 min. Circles, vehicle + vehicle; squares, apixaban + vehicle; triangles, apixaban+r-Antidote. *P≤0.01 compared to the r-Antidote treatment group determined by unpaired two-tailed t test.

Betrixaban is a highly potent, selective, and orally efficacious inhibitor of factor Xa with IC50 value of 1.5nM [1].

Betrixaban shows excellent anticoagulant potency in vitro. In the rabbit deep vein thrombosis model, the concentration of betrixaban required to double the rabbit prothrombin time is below 2μM. Betrixaban is selective for fXa and has poor activity for thrombin, trypsin, t-PA and aPC. The patch clamp hERG assay shows that betrixaban has low affinity with hERG suggesting it is safer than other candidate compounds. In addition, betrixaban displays a profile of good oral bioavailability and oral exposure, long half-life in animal models. It has bioavailability of 51.6% at dose of 0.5 mg/kg IV and 2.5 mg/kg PO. Furthermore, the Phase II study has proved betrixaban as an oral fXa inhibitor for prevention of venous thromboembolic events [1].

References:
[1] Zhang P, Huang W, Wang L, Bao L, Jia ZJ, Bauer SM, Goldman EA, Probst GD, Song Y, Su T, Fan J, Wu Y, Li W, Woolfrey J, Sinha U, Wong PW, Edwards ST, Arfsten AE, Clizbe LA, Kanter J, Pandey A, Park G, Hutchaleelaha A, Lambing JL, Hollenbach SJ, Scarborough RM, Zhu BY. Discovery of betrixaban (PRT054021), N-(5-chloropyridin-2-yl)-2-(4-(N,N-dimethylcarbamimidoyl)benzamido)-5-methoxybenzamide, a highly potent, selective, and orally efficacious factor Xa inhibitor. Bioorg Med Chem Lett. 2009 Apr 15;19(8):2179-85.