Proteolytically inactive FVIIa binding to EPCR augments the hemostatic activity of a low dose of rFVIIa in hemophilia A (Hem A) mice. (A) Average hemostatic times of wild-type and hemophilia A mice following saphenous vein incision and the effect of varying doses of rFVIIa in restoring hemostasis in hemophilia A mice. Saline or varying doses of rFVIIa (1, 4, or 10 mg/kg body weight) was administered to hemophilia A mice via the tail vein. Five minutes after rFVIIa administration, mice were subjected to saphenous vein incision. Average time to achieve hemostasis was determined as described in “Materials and methods.” *P < .05 compared with hemophilia mice not receiving rFVIIa. (B) Administration of a pharmacological concentration of FVIIa_AI promotes the hemostatic effect of a low dose of rFVIIa. Hemophilia A mice were injected with saline, a low dose of rFVIIa (1 mg/kg), FVIIa_AI (10 mg/kg), or both FVIIa_AI (10 mg/kg) and rFVIIa (1 mg/kg). Five minutes following rFVIIa administration, mice were subjected to saphenous vein incision and the average time to achieve hemostasis was determined (n = 4-7 mice/group). ***P < .001. (C) FVIIa_AI downregulation of APC generation. Wild-type mice were administered with saline or FVIIa_AI (10 mg/kg) via the tail vein. After obtaining the blood sample (pre), mice were injected with saline or thrombin (6 U/kg) via the tail vein to induce activation of protein C in vivo. After 10 minutes, the blood was obtained (post). APC levels in plasma were measured as described earlier.¹⁶  Note: the data shown in panel A are essentially similar to that reported in our earlier publication¹⁹  and was reproduced here to illustrate the rationale behind using 1 mg/kg rFVIIa dose in experiments described in panel B. Hemophilia A mice used in this experiment were in the B6/129S genetic background. All other experiments were performed with mice in the C57BL/6J genetic background. NS, not significant.