The Tissue Factor Pathway Inhibitor Antibody, PF-06741086, Increases Thrombin Generation in Rare Bleeding Disorder and Von Willebrand Factor Deficient Plasmas
Authors: Rakhe Swapnil , Patel-Hett Sunita R. , Bowley Sheryl , Murphy John E. , Pittman Debra D 
Published: October 2019

Abstract
Hemophilia A and B are hereditary bleeding disorders that result from deficiencies in the intrinsic coagulation pathway leading to insufficient generation of Factor Xa (FXa) and thrombin to promote stable hemostasis. Coagulation defects are also observed in other inherited rare factor bleeding disorders. The extrinsic pathway of coagulation in these disorders cannot generate sufficient levels of FXa due to the regulation by Tissue Factor Pathway Inhibitor (TFPI). TFPI is a Kunitz-type serine protease inhibitor that negatively regulates thrombin generation by inhibiting the FXa/tissue factor (TF)/Factor VIIa (FVIIa) complex. PF-06741086, a fully human inhibitory monoclonal antibody, binds the Kunitz-2 domain and is currently under development as a potential prophylactic treatment to prevent bleeding episodes in hemophilia A and hemophilia B patients with and without inhibitors. The addition of PF-06741086 in vitro to donor plasma from both healthy normal volunteers and hemophilia patients promoted thrombin generation and restored hemostasis in vivo in murine hemophilia bleeding models. Pharmacological effects of PF-06741086 on thrombin generation were also observed in a healthy volunteer Phase 1 study. Other rare disease coagulopathies also result in the insufficient generation of thrombin. In this study, the potential of PF-06741086 to restore thrombin generation in rare disease plasma was explored.
Thrombin generation was measured in citrated platelet poor Factor XI (FXI), Factor V (FV), FVII, von Willebrand Factor (vWF) deficient (Type 1, 2A, 2B and 3) congenital donor plasma following the in vitro addition of PF-06741086 (0, 1, 10 or 100 nM) or a human IgG1 antibody; initiated with 1 pM TF and 4 µM phospholipid. FXI, FV, and FVII donors had less than 1% coagulation factor activity. Non-hemophilic plasma from healthy donors alone was also included in the analysis. In FXI deficient plasmas, a concentration-dependent increase in peak thrombin and a shortening of the lag time was observed with the addition of PF-06741086 normalizing and restoring levels to those observed in the non-hemophilic plasma. A similar response was also observed in all of the vWF deficient plasmas. In one FVII deficient plasma, an increase in peak thrombin was observed at dose of 100 nM PF-06741086, however, the lag time (20 minutes) was significantly extended, relative to healthy volunteer non-hemophilic plasma. As expected, the addition of PF-06741086 to FV deficient plasma did not increase thrombin generation at any concentration.
The in vitro addition of the TFPI antibody, PF-06741086, improved thrombin generation in selected coagulation factor deficient plasmas, including vWF deficiency, to the levels observed in normal plasma. This data suggestion that the inhibition of TFPI may promote hemostasis in rare bleeding disorders such as FXI deficiency and vWF deficiencies.

Disclosures
Rakhe: Pfizer: Employment. Patel-Hett:Pfizer: Employment. Bowley:Pfizer: Employment. Murphy:Pfizer: Employment. Pittman:Pfizer: Employment.