An Antibody to Tissue Factor Pathway Inhibitor (TFPI) Restores Hemostasis after the Onset of Bleeding in Hemophilic a Mouse Injury Models

Hemophilia is a family of rare bleeding disorders characterized by inadequate levels of intrinsic coagulation factors, Factor VIII (FVIII) in hemophilia A and Factor IX (FIX) in hemophilia B. This leads to insufficient thrombin generation for the conversion of fibrinogen to fibrin for the development of a stable clot. Replacement factor therapies are provided as a prophylactic treatment to prevent bleeds or for on-demand treatment to treat an active bleed. Some patients develop inhibitory antibodies making them refractory to treatment. Although hemophilia patients have defects in the intrinsic pathway, the extrinsic pathway remains intact. An alternative approach to therapy would be to augment the extrinsic tissue factor pathway. TFPI is a Kunitz domain type inhibitor that negatively regulates thrombin generation within the extrinsic pathway of coagulation by rapidly inactivating the protease functions of Factor Xa and the Factor VIIa/Tissue Factor complex. PF-06741086 is a fully human monoclonal antibody directed against TFPI. Here, we investigated the activity of PF-06741086 in controlling bleeding in hemophilic mouse injury models when administered after a bleeding injury.

PF-06741086 restores hemostasis prior to the onset of an injury. As a model for on demand treatment of bleeds, the effect of PF-06741086 on restoring hemostasis in hemophilia A mice following the induction of a severe bleed was monitored. PF-06741086 (1, 3, and 6 mg/kg), recombinant FVIII (200 U/mg) or vehicle were administered to mice, at 2 minutes into an active bleed after the 3 mm tail transection. A dose-dependent decrease in blood loss was observed with the infusion of PF-06741086, 51 % (3 mg/kg), and 76 % (6 mg/kg) compared to vehicle treated mice. A decrease in blood loss (81 %) was also observed for mice receiving recombinant FVIII, compared to vehicle treated mice. The magnitude of the effect in mice that received PF-06741086 (6 mg/kg) at 2 minutes post injury was similar to hemostatic effects of recombinant FVIII administered 2 minutes after the onset of bleeding.

In a second model, the cremaster microvessel laser injury model was used to investigate the hemostatic effect of PF-06741086 in hemophilia A mice after the induction of a vessel injury. In each individual mouse, an injury was made without administration of PF-06741086 and data was recorded for platelet accumulation and fibrin generation. A second laser injury was made in the same mouse and a single intravenous dose of PF-06741086 was infused at 6 mg/kg immediately following the laser injury. When compared to untreated mice, enhanced platelet accumulation and fibrin deposition at the site of injury was observed when PF-06741086 was administered in hemophilia A mice during an ongoing bleed induced with laser injury.

In summary, PF-06741086, an inhibitor of TFPI, restores hemostasis in on demand hemophilia mouse injury models when administered after the onset of a bleeding injury. PF 06741086 is being developed for the treatment of hemophilia A and hemophilia B, with and without inhibitors.

All experiments were within guidelines and were reviewed and approved by Pfizer Institutional Animal Care and use Committee.