VGA039 as a protein S-targeted hemostatic promoting monoclonal antibody, promotes in-vitro thrombin generation in plasma samples from subjects across a broad range of bleeding disorders, including von Willebrand disease, hemophilia A, hemophilia B an

Abstract Introduction: VGA039 is a fully human IgG4 monoclonal antibody that targets protein S (PS), a negative regulator of coagulation. VGA039 prevents PScofactor activity for tissue factor pathway inhibitor-α (TFPIα) and activated protein C (APC), which respectively regulate thrombin generation during the initiation and propagation phases of coagulation. VGA039 is currently being evaluated in clinical studies in von Willebrand Disease (VWD) and has shown marked reductions in bleeding rates associated with VGA039 concentrations ≥25 µg/mL (NCT05776069). Since VGA039 promotes both primary and secondary hemostasis independently of coagulation pathway factor levels and/or activities, it has the potential to restore hemostasis across a broad range of bleeding disorders. Here we show that VGA039 promotes in vitro TG in a concentration dependent manner and to the same extent in plasma samples from patients with von Willebrand Disease (VWD) (regardless of VWD type), Hemophilia A (HA), Hemophilia B (HB), and Hemophilia C (HC). Methods: In vitro thrombin generation studies were conducted on plasma samples from healthy volunteers, and patients with either VWD, HA, HB, or HC (deficiencies in VWF, factor VIII, IX or XI respectively). Samples were acquired from commercial sources or directly from patients with approvals by ethics committees and obtained with informed consent. A thrombin generation assay (TGA) was used to assess, thrombin generation (TG) in plasma samples, inhibition of TG by exogenous APC, and restoration of APC-inhibited TG by VGA039. Coagulation was initiated by the addition of phospholipid membranes and tissue factor (TF), and TG was monitored over time. VGA039 reversal of APC-mediated inhibition of TG (%APC Reversal) was calculated and an EC50 determined. A total of 12 healthy, 12 HA, 4 HB, 5 HC, and 45 VWD samples (8, 13 and 24 with type 1-3 respectively) were analyzed in the TGA. Results: As expected, baseline TG in bleeding disorder samples was lower than healthy samples, and the addition of exogenous human APC decreased TG from baseline across all bleeding disorders (range of 67% to 87%). The addition of VGA039 reversed APC mediated inhibition of TG in all bleeding disorder samples in a concentration dependent manner. In VWD samples, the EC50 for VGA039 reversal of APC inhibition of TG was 20.1 µg/mL (22.3 µg/mL, 22.1 µg/mL and 18.4 µg/mL in type 1-3 respectively) which is near the same concentration as those associated with marked reductions in bleeding rates in clinical studies (≥25 µg/mL). The VGA039 EC50 was also similar across all additional bleeding disorder samples evaluated; 26.2 µg/mL in HA, 21.0 µg/mL in HB, and 33.9 µg/mL in HC. VGA039 at 25 µg/mL restored TG in the presence of APC by 5.2-fold in HA, 4.3-fold in HB, 5.1-fold HC, compared to 4.3-fold in VWD (4.2-fold, 5.2-fold and 3.9-fold in type 1-3 respectively). Further increasing the concentration of VGA039 to 200 µg/mL reached a TGA plateau, with TG reaching 8.5-fold in HA, 6.6-fold in HB, 10.8-fold in HC, and 6.4-fold in VWD (6.5-fold, 8.6-fold and 5.4-fold in type 1-3 respectively). Conclusions: In a prior single-ascending-dose clinical study in VWD, VGA039 induced marked reductions in bleeding rates associated with VGA039 plasma concentrations ≥25 µg/mL. Here, VGA039 added to plasma samples derived from patients with Hemophilia A, Hemophilia B or Hemophilia C reversed exogenous APC mediated inhibition of thrombin generation with a similar potency and to a near equivalent extent as plasma samples from von Willebrand Disease (including type 1-3). These data show the broad hemostatic potential of VGA039 and support the potential clinical utility of VGA039 across multiple congenital bleeding disorders.