An Antibody Targeting Human FV Promotes Thrombin Generation and Reduces Bleeding in a Hemophilic Mouse Model

Abstract

Hemophilia is a bleeding disorder that results from a deficiency of factor VIII (Hemophilia A; HA) or factor IX (Hemophilia B; HB). Protein replacement therapy ameliorates bleeding but can result in the development of neutralizing antibodies. Innovative nonfactor therapies are in development for hemophilia which target anticoagulant proteins such as tissue factor pathway inhibitor (TFPI), antithrombin, and activated protein C (APC). While these approaches show promise, they influence multiple coagulation factors and pathways which could raise safety concerns. Considering its essential role in coagulation in enhancing thrombin generation (TG), we hypothesized that altering the functional properties of FV/FVa could be an attractive approach for HA and HB treatment. Activated FV (FVa) assembles with FXa on activated cellular surfaces and enhances the kinetics of TG by several orders of magnitude. Here we assess whether monoclonal antibodies targeting all forms of FV are effective in promoting TG in hemophilia using both in vitro and in vivo models in hemophilic mice. Using an initial TG assay screen, we identified a procoagulant antibody (GB5) that binds with high affinity (Kd <0.5 nM) to human (h)FV, FVa, and FV-short and investigated its mechanism of action. Using proteolytic fragments of FV and western blotting experiments, we found that GB5 likely binds in the middle of the light chain region. To evaluate the functional effects of GB5 in vitro, we first employed a plasma-based TG assay using HA or HB plasma triggered with low (0.2 pM) tissue factor. The presence of saturating GB5 increased TG about 3-fold in HA plasma. Similar results were observed with HB plasma. Interestingly, when TG was essentially eliminated in HA and HB plasma with addition of exogenous APC, GB5 restored TG. These results suggest that the procoagulant effect of GB5 neutralizes the anticoagulant effect of APC and enhanced TG in hemophilic plasma To begin to assess the mechanism, the effect of GB5 on FV activation in the TG assay was evaluated. TG assay samples were removed over time, quenched, and analyzed by immunoblotting using labeled FV specific antibodies. In the presence of GB5, ~95% of the starting full-length FV was converted to FVa within 3 min compared to 5 min in the absence of GB5. These results show that FV is activated faster in the presence of GB5 in the TG assay. To extend these data, we next assessed the influence of GB5 in the TG assays using FV-deficient plasma supplemented with either FV or FVa and TFPI⍺. The data show that GB5 had essentially no effect on the TG assay when FV deficient plasma was reconstituted with FVa, but enhanced TG at least 3-fold when supplemented with FV. Consistent with this, the kinetics of prothrombin activation with FXa-FVa-membranes in the presence or absence of GB5 were the same. These data suggest that GB5 impacts reactions that influence FV, with no direct effect on FVa procoagulant function. To evaluate whether GB5 has a procoagulant effect in vivo, a tail vein transection (TVT) model was employed in HA mice. In this model, one lateral tail vein is transected, and blood is collected over 40 min with clots at the vessel wall disrupted every 10 min as needed. Since GB5 interacts with human and not mouse FV, plasma derived human FV was infused into HA mice. We found that blood loss in the TVT model using HA mice infused with human FV alone (5 μg/mouse) or GB5 alone (20 μg/mouse) was no different than mice infused with PBS (n = 3-5 mice/group except GB5 alone where n = 1: ~600 μl blood loss). In contrast to these data, when human FV and GB5 were co-administered, blood loss was reduced ~3-fold (~200 μl) which was statistically significant relative to the controls but did not reach the level of wild-type mice (blood loss ~50 μl). Overall, the results show that GB5 is a procoagulant monoclonal antibody that binds with high affinity to human FV and influences reactions that lead to its activation and inactivation. We hypothesize that the net effect of GB5 is the enhanced production of FVa which has a major impact on TG. In plasma-based systems with and without the protein C system and an in vivo hemophilic model, GB5 is procoagulant and can reduce bleeding. Our work shows that targeting the common coagulation pathway at the level of FV has benefit in the context of hemophilia and may be a viable and safe therapeutic approach.