In vivo targeting of open prothrombin with the monoclonal antibody POmAb results in anticoagulation without excessive bleeding

BACKGROUND: Antiphospholipid antibodies targeting prothrombin are frequently found in antiphospholipid syndrome (APS), yet their impact on thrombin generation remains unclear. Prothrombin exists in equilibrium between closed and open conformations, influencing its activation to thrombin. We recently identified Prothrombin Open monoclonal Antibody (POmAb), a monoclonal antibody that binds the open form and reduces thrombin generation in plasma. However, the in vivo effects of POmAb on coagulation remain unknown. OBJECTIVES: To investigate the in vivo effects of POmAb on coagulation. METHODS: injury model, and tail bleeding assays. Platelet aggregation was assessed by aggregometry. Prothrombin-binding mechanism was studied by ELISA and surface plasmon resonance. RESULTS: POmAb inhibited thrombin generation in mouse plasma in vitro. Following intravenous administration at 0.5 mg/kg, POmAb localized rapidly to sites of vascular injury and significantly reduced fibrin formation, with nonsignificant effects on platelet accumulation and platelet function ex vivo. POmAb reduced clot weights in an inferior vena cava thrombosis model, but did not alter tail bleeding times. Binding studies revealed a strong preference for surface-bound versus soluble open prothrombin, explaining its localization and anticoagulant profile at such a low dose. CONCLUSION: POmAb exhibits antithrombotic properties in vivo, supporting the potential of POmAb as a novel and safe anticoagulant strategy. Given that POmAb was identified in the context of APS, these findings warrant further investigation into the roles of anti-prothrombin antibodies in APS.