Blocking human protein C anticoagulant activity improves clotting defects of hemophilia mice expressing human protein C

Hemophilia A and B are hereditary coagulation defects resulting in unstable blood clotting and recurrent bleeding. Current factor replacement therapies have major limitations such as the short half-life of the factors and development of inhibitors. Alternative approaches to rebalance the hemostasis by inhibiting the anticoagulant pathways have recently gained considerable interest. In this study, we tested the therapeutic potential of a monoclonal antibody, HAPC1573, that selectively blocks the anticoagulant activity of human activated protein C (APC). We generated F8(-/-) or F9(-/-) hemophilia mice expressing human protein C by genetically replacing the murine Proc gene with the human PROC. The resulting PROC+/+;F8(-/-) or PROC+/+;F9(-/-) mice had bleeding characteristics similar to their corresponding F8(-/-) or F9(-/-) mice. Pretreating the PROCC+/+;F8(-/-) mice with HAPC1573 shortened the tail bleeding time. HAPC1573 pretreatment significantly reduced mortality and alleviated joint swelling, similar to those treated with either FVIII or FIX, of either PROCC+/+;F8(-/-) or PROC+/+;F9(-/-) mice in a needle puncture-induced knee-joint bleeding model. Additionally, we found that HAPC1573 significantly improved the thrombin generation of PROC+/+;F8(-/-) mice but not F8(-/-) mice, indicating that HAPC1573 enhanced the coagulant activity of hemophilia mice by modulating human APC in vivo. We further documented that HAPC1573 inhibited the APC anticoagulant activity to improve the clotting time of human plasma deficient of FVIII, FIX, FXI, FVII, VWF, FV, or FX. These results demonstrate that selectively blocking the anticoagulant activity of human APC may be an effective therapeutic and/or prophylactic approach for bleeding disorders lacking FVIII, FIX, or other clotting factors.