PRT064445 reverses rivaroxaban induced anticoagulation in a rabbit liver laceration "treatment" model

Abstract

We previously demonstrated in a rabbit liver laceration model that prophylactic administration of PRT064445 (PRT), a recombinant, modified fXa molecule that reverses the anticoagulative effect of fXa inhibitors, decreased blood loss (BL), anti-fXa activity and unbound plasma fraction of rivaroxaban (riva) back to baseline levels [Nature Medicine – in press]. In contrast, administration of rfVIIa or PCC had no effect on blood loss but partially reversed increases in prothrombin times (PT) and activated thromboplastin times (aPTT). In this study, PRT was administered after liver laceration to test whether PRT could mitigate the increase in BL due to riva anticoagulation in a setting of active bleeding. Anesthetized rabbits were administered riva (0.5 mg/kg, IV bolus) and after 30 minutes, the liver was lacerated and allowed to bleed for 10 minutes into pre-weighed gauze to measure BL. A new set of gauze was placed and vehicle or PRT (75, 35, 15 or 5 mg/rabbit) was administered as a 5 minute IV infusion and BL measured for an additional 30 minutes. Serial blood samples were obtained for measurement of total and unbound plasma fraction of riva, anti-fXa activity, plasma concentrations of PRT, PT and aPTT. Over the first 10 minutes, mean BL for the five riva treated groups was similar, ranging from 6.4 to 9.1 grams, approximately 2-fold above mean non-riva treated groups. At the end of the 10 minutes, mean riva plasma concentration was 0.63 μM with a 76% and 58% increase in PT and aPTT, respectively. Over the next 35 minutes, riva caused a significant (p≤0.001) increase in BL from 5.0±1.9 grams in vehicle controls to 14.0±6.4 grams. The 75 and 35 mg/rabbit PRT doses demonstrated significant (p≤0.05) decreases (62% and 75%, respectively) in BL and reversal of anti-fXa activity, free fraction of riva, PT and aPTT back to baseline, normal levels. The 15 and 5 mg PRT doses showed no significant reductions in BL. When comparing plasma concentrations of PRT to total riva for the 35 mg/rabbit dose, a significant reduction in BL was achieved at an approximate 1:1 molar ratio. These data demonstrate that PRT064445 can reduce BL in a setting of active visceral bleeding due to riva induced anticoagulation using a single bolus administration and this effect correlates with anti-fXa activity and unbound plasma fraction of the anticoagulant. This study exemplifies the capacity of PRT to reverse anticoagulation of direct fXa inhibitors and the potential to reduce bleeding in situations of acute BL in anticoagulated patients.