Novel Snake Venom Derived Hemocoagulase Reverses Anticoagulant Effect of Factor Xa Inhibitors, Restores Hemostatic Clot Formation, and Limits Bleeding in Vitro and In Vivo

Background: Direct oral anticoagulants (DOACs) such as rivaroxaban and apixaban are factor Xa (FXa) inhibitors widely used to treat thromboembolism and prevent stroke. Despite improvements in patient outcomes compared to vitamin K antagonists, major bleeding still occurs and there is an urgent unmet need for reversal agents in the event of major bleeding or when emergency surgery is needed. Slounase is a snake venom derived batroxobin (a thrombin-like enzyme) containing FXa that converts fibrinogen to fibrin in a manner distinctly different from thrombin. We recently reported slounase enhances platelet-fibrin clot formation in vivo in heparin-anticoagulated mice suggesting slounase may bypass coagulation to restore hemostasis and prevent bleeding. Here, we investigate if slounase reverses the anticoagulant effect of FXa inhibitors and restores hemostatic clot formation as a novel bypassing agent. Methods: In this study we used in vitro thromboelastography (TEG) to test the effect of slounase on whole blood from healthy human donors spiked with a range of apixaban or rivaroxaban doses. Clot formation kinetics were recorded for 60 minutes and the parameters reaction time and maximum amplitude (MA) were analyzed. The effect of rivaroxaban and apixaban on hemostasis in vivo was determined in wild type (WT) mice pretreated with apixaban (30mg/kg) or rivaroxaban (10mg/kg) via oral gavage using intravital microscopy laser-induced cremaster arteriole thrombosis model. WT mice pretreated with apixaban or rivaroxaban were further treated with slounase (1U/kg) and reversal of anticoagulant effect of FXa inhibition was determined in vivo under intravital microscopy. The dynamics of platelet recruitment and fibrin formation within the growing clot were analyzed for change in fluorescent intensity over the course of thrombus formation in response to vascular injury. Additionally, mice were pretreated with FXa inhibitors via oral gavage and then intravenously dosed slounase (1U/kg) 10 minutes prior to tail bleeding assay to test the effect on bleeding time and blood loss. Results: A high dose of apixaban and rivaroxaban spiked in human whole blood caused full inhibition of the TEG parameters and an inability to form a stable thrombus in vitro. However, slounase treatment dose dependently restored the parameters of clot formation into the range of control. The in vivo laser-induced cremaster arteriole model was used to test slounase as an intervention to apixaban and rivaroxaban pretreatment. The severe hemostatic defect was determined by limited platelet accumulation and fibrin formation at the site of vascular injury in both groups. Additionally, platelet adherence was transient and fibrin formation was unstable in mice treated with rivaroxaban and apixaban. Intravenous intervention with slounase (1U/kg) increased platelet accumulation and fibrin formation at the site of vascular injury. Slounase treatment reversed the anticoagulant effect of FXa inhibitors in vivo and restored the hemostatic clot formation in response to vascular injury in mice treated with FXa inhibitors. In the in vivo tail bleeding assay WT mice pretreated with FXa inhibitors bled continually until intervention via cauterization. Slounase pretreatment significantly decreased blood loss and bleeding time into the range of WT controls. Conclusions: Slounase is a potential bypassing agent for reversal of anticoagulation by DOACs rivaroxaban and apixaban. Using TEG, our data demonstrated the anticoagulant effect of FXa inhibitors in vitro and the reversal of anticoagulation by slounase. Slounase dose dependently restored clot formation in human whole blood pretreated with an inhibitory dose of FXa inhibitors. Slounase intervention improved hemostasis in vivo by increasing platelet accumulation and fibrin formation at the site of vascular injury. Additionally, slounase treatment decreased bleeding and blood loss in an in vivo bleeding assay demonstrating its potential to quickly reverse effects of DOACs and prevent bleeding episodes. This study further demonstrates slounase is a novel bypassing agent that promotes platelet procoagulant activity and converts fibrinogen to fibrin independent of thrombin activity.