PRT064445 but Not Recombinant Fviia Reverses Rivaroxaban Induced Anticoagulation As Measured by Reduction in Blood Loss in a Rabbit Liver Laceration Model

Abstract 3414

We used a modified rabbit liver laceration model to demonstrate the effects of PRT064445, a recombinant fXa derivative, to reverse rivaroxaban induced anticoagulation as measured by reduction of blood loss, decrease of unbound fraction of rivaroxaban in plasma and relevant pharmacodynamic markers: anti-fXa activity, prothrombin (PT) and activated thromboplastin (aPTT) times. Recombinant fVIIa (rfVIIa) was tested in the same model for comparison.

Anesthetized rabbits were administered vehicle or 1 mg/kg rivaroxaban via IV bolus injection. After 30 minutes to allow the rivaroxaban to biodistribute, vehicle, PRT064445 or rfVIIa was administered as a bolus injection followed by laceration of two liver lobes (5× each lobe: 1-cm long and 3-mm deep incisions with scalpel blade) and blood loss collected on pre-weighed gauze over 15 minutes. Blood loss due to rivaroxaban anticoagulation represented approximately 10% of the animal's total blood volume. Total rivaroxaban concentration in plasma was measured by LC-MS/MS. Free fraction of rivaroxaban (non-protein, non-PRT064445 bound) was assessed using equilibrium dialysis method followed by LC-MS/MS quantitation. Anti-fXa activity was measured using a modified chromogenic LMW heparin kit. PT and aPTT was measured using commercial reagents (HemosIL). Anticoagulation by rivaroxaban (1.65 μM average plasma concentration at 30 min time point) resulted in 2.3-fold and 1.9-fold prolongation of PT and aPTT, respectively, and increased blood loss by 3.2-fold over vehicle. PRT064445 (75 mg/rabbit) administration reduced blood loss due to rivaroxaban anticoagulation by >85% and decreased peak anti-fXa activity by 98%, PT by 74%, aPTT by 66% and the free fraction of rivaroxaban in plasma from 26% to <0.5%. In contrast, rfVIIa (150 μg/kg) had no effect on blood loss but reversed PT by 85% and aPTT by 54%. PRT064445 and rfVIIa alone had no effect on blood loss, but rfVIIa decreased PT by 35%.

These data demonstrate that PRT064445 can reduce blood loss due to rivaroxaban induced anticoagulation using a single bolus administration. Reduction of blood loss with PRT06445 correlated to the decrease in the free fraction of rivaroxaban in plasma as well as PD markers anti-fXa activity and PT, while rfVIIa decreased PT but had no effect on blood loss. These and our previous data with reversal of enoxaparin-induced anticoagulation in a rat tail transection model indicate that PRT064445 can be used as a universal antidote for both direct and indirect fXa inhibitors.