A Neutralizing Monoclonal Antibody (PF-06741086) Against Tissue Factor Pathway Inhibitor in Combination with Activated Prothrombin Complex Concentrate Increases Hemostasis in Inhibitor Hemophilia Plasmas without Excessive Thrombin Generation

Hemophilia A and B are hereditary bleeding disorders caused by intrinsic coagulation pathway deficiencies of Factor VIII or Factor IX, respectively. Tissue factor pathway inhibitor (TFPI) is a Kunitz-type serine protease inhibitor that negatively regulates thrombin generation within the extrinsic pathway of coagulation. PF-06741086 is a fully human monoclonal antibody which binds the Kunitz-2 domain and neutralizes the inhibitory activity of human tissue factor pathway inhibitor 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. Activated prothrombin complex concentrate (aPCC) is used as bypass treatment for the resolution of bleeding in some hemophilia patients with inhibitors. Hemophilia inhibitor patients receiving PF-06741086 have a possibility to also receive treatment with aPCC. The aim of the current study was to assess the potential additive effect of PF-06741086 with aPCC added in vitro to Hemophilia A and B inhibitor plasmas using a thrombin generation assay (TGA).

Thrombin generation in the presence of 1 pM tissue factor and 4 µM phospholipid, was measured using the calibrated automated thrombogram (CAT) system in citrated platelet poor hemophilia A inhibitor (88-160 Bethesda Units) donor plasma or hemophilia B inhibitor (FIX immune-depleted and spiked with FIX neutralizing antibody, 14 Bethesda Units) plasma following the addition of PF-06741086 or aPCC (FEIBA) either alone or in combination. All donors had less than 1% coagulation factor activity. Non-hemophilic plasma from healthy donors alone or spiked in vitro with 16 µg/mL of PF-06741086 was also included in the analysis. Non-hemophilic plasma would have the full complement of coagulation factors. Dose-dependent increases in peak thrombin were observed with the addition of aPCC alone or PF-06741086 alone to the hemophilia plasmas. For combination studies, the aPCC concentration of 1 Unit/mL was selected to correspond to plasma levels that could be achieved clinically post-dosing. The concentration of PF-06741086 at 16µg/mL in these studies was chosen to approximate the Cmax concentration following a single 300 mg subcutaneous dose. Both PF-06741086 (16 µg/mL) and aPCC (1 Unit/mL) decreased the lag time in hemophilia plasma, however, there was not an additive decrease in the lag time with the combination of PF-06741086 and aPCC. The addition of PF-06741086 in combination with aPCC to hemophilia plasma resulted in an increase in thrombin generation including a higher peak thrombin concentration compared to the addition of either alone, but was within the range reported in studies for non-hemophilic normal plasma. To summarize, the addition of aPCC (1 Unit/mL) in combination with PF-06741086 (16µg/mL) in vitro resulted in increased thrombin generation in hemophilia A and hemophilia B inhibitor plasmas without inducing excessive coagulation.