An Antibody to Tissue Factor Pathway Inhibitor (PF-06741086) in Combination with Recombinant Factor VIIa Increases Hemostasis in Hemophilia Plasma without Excessive Thrombin Generation

Hemophilia is a hereditary bleeding disorder caused by intrinsic coagulation pathway deficiencies of Factor VIII (hemophilia A) or Factor IX (hemophilia B). Tissue factor pathway inhibitor (TFPI) is a Kunitz-type serine protease inhibitor that negatively regulates thrombin generation within the extrinsic pathway of coagulation. In hemophilia patients the extrinsic pathway remains intact and thus augmentation of this pathway may circumvent the clotting deficiency in hemophilia. PF-06741086, a monoclonal antibody that binds to and neutralizes the inhibitory activity of TFPI is being developed as a potential treatment for bleeding disorders including hemophilia A and hemophilia B with and without inhibitors.

Currently, treatment of inhibitor patients is managed by bypass treatments, such as recombinant Factor VIIa (rFVIIa). The effect of PF-06741086 on thrombin generation in the presence of increasing concentrations of rFVIIa (0.0002 to 20 µg/mL) was studied in severe hemophilia A plasma. A dose-dependent increase in thrombin generation was observed over vehicle control with the addition of rFVIIa to the hemophilia plasma. Addition of a fixed concentration of PF-06741086 (16 µg/mL) in combination with rFVIIa resulted in an increase in thrombin generation including higher peak thrombin and shortening of lag time compared to rFVIIa alone. The TGA profiles with the combination of PF-06741086 and rFVIIa at 0.2, 2, and 20 µg/mL were similar suggesting a saturation of mechanism at these concentrations. The combination of PF-06741086 and rFVIIa restored thrombin generation to normal plasma levels at all rFVIIa concentrations examined.

The TFPI inhibitory activity of PF-06741086 on thrombin generation in the presence and absence of rFVIIa was further studied in additional hemophilia A plasmas, including hemophilia A plasmas with inhibitors and hemophilia B plasma. All donors had less than 1% coagulation factor activity. A rFVIIa concentration of 2 µg/mL was selected because it corresponded to plasma levels that could be observed following dosing of FVIIa and because the thrombin generation response in hemophilia plasma was similar with FVIIa added to hemophilia A plasma at 0.2, 2 and 20 µg/mL. The concentration of PF-06741086 was 16 µg/mL in these studies. The effect of PF-06741086 on thrombin generation was also measured in non-hemophilic plasma which would have the full complement of coagulation factors. The addition of PF-06741086 alone or in combination with rFVIIa to hemophilia A and B plasma resulted in an increase in thrombin generation including higher peak thrombin concentration and shortening of lag time compared to addition of rFVIIa alone. In hemophilic plasma samples with inhibitors (3 - 1261 Bethesda Units), PF-06741086 alone also restored thrombin generation. A minimal additive effect in peak thrombin generation was observed with the combination of PF-06741086 (16 µg/mL) and 2 µg/mL rFVIII. The midpoint peak thrombin levels achieved with PF-06741086 alone or in combination with rFVIIa were similar to those observed in non-hemophilic plasma and did not exceed the level observed in non-hemophilic plasma dosed with PF-06741086. To summarize, use of rFVIIa in combination with PF-06741086 results in increased thrombin generation in hemophilia A, hemophilia B and inhibitor plasmas without inducing excessive coagulation.