Evaluation of a Blood Coagulation Factor IX Variant That Functions Independently of Factor VIII As an Alternative Treatment for Hemophilia A

The serine protease factor IXa (FIXa) serves an important role in coagulation by catalyzing the proteolytic activation of factor X (FX) together with its cofactor VIIIa (FVIIIa). Being a critical protease in coagulation, the FIXa structure has evolved to be subjected to strict regulatory mechanisms. While FIXa displays considerable structural homology with other coagulation serine proteases, its active site is uniquely controlled by the 99-loop that blocks access to the active site pocket. Cofactor-mediated interaction of FIXa with its substrate FX induces a conformational change that allows for active site engagement and substrate catalysis. Previously, the molecular constraints of the 99-loop were lifted due to specific modifications in both the 99-loop (K265A), the S1 active site subpocket (V181I, I383V), and the L6F substitution, thereby generating FIX-FIAV [Quade-Lyssy et al. J. Thromb. Haemost. 2014]. As a result, this variant is capable of functioning independently of factor VIII (FVIII). Moreover, FIX-FIAV was demonstrated to ameliorate the hemophilia A phenotype both in vitro and in vivo.

To further evaluate its therapeutic potential, FIX-FIAV was stably expressed in HEK293 cells and purified by ion-exchange and hydrophobic interaction chromatography. Evaluation of the kinetics of tissue factor-factor VIIa (TF-FVIIa) activation of FIX-FIAV revealed kinetic parameters similar to those of human wild-type FIX(-WT). Analysis of FIX activation intermediates that are formed upon proteolysis by TF-FVIIa or factor XIa demonstrated prolonged formation of FIX-FIAVα, while no FIXa-WTα could be observed. This is consistent with delayed cleavage at position 180, likely resulting from the V181I substitution in FIX-FIAV.

Given that the activation mechanism of FIX-FIAV is unperturbed, we next assessed the specific FVIII clotting activity and demonstrated that FIX-FIAV exhibited significant FVIII-like clotting activity (56 ± 4 U/mg) as opposed to FIX-WT (<13 U/mg). These values correlate with up to 28% of FVIII-independent activity for FIX-FIAV at FIX plasma levels (5 ug/mL), confirming that FIX-FIAV has the potential to enhance thrombin generation in FVIII deficiency. To validate this, tissue factor-initiated (0.5 or 1.0 pM) thrombin generation was assessed in FVIII-immunodepleted plasma, leading to a severely reduced thrombin peak (88% or 81% reduction, respectively) relative to conditions with 100% FVIII. Addition of FIX-FIAV (5 ug/mL) partially restored thrombin generation, demonstrated by an up to ~30% increase in both thrombin peak and endogenous thrombin potential. Evaluation of the FVIII-independent activity of FIX-FIAV in severe hemophilia A patient plasma with or without an inhibitor resulted in an up to 18% or 32% FVIII-like activity, respectively, demonstrating efficacy of FIX-FIAV in the presence of FVIII inhibitors. Although unlikely, it remains to be determined whether specific FVIII-inhibitors may impact FIX-FIAV function. Adding 100% FVIII or low- to mid-range therapeutic concentrations of the bispecific antibody emicizumab to FVIII-deficient plasma incubations with FIX-FIAV resulted in a synergistic enhancement of thrombin generation, demonstrated by a 9-fold increase in thrombin peak. This is consistent with the previously demonstrated hyperactivity of FIX-FIAV in a cofactor-dependent system. In contrast, no synergistic effect on thrombin generation was observed when combining FIX-FIAV with physiologically relevant concentrations of FEIBA or NovoSeven.

Summarizing, FIX-FIAV is characterized by a preserved mechanism of activation in addition to being capable of sustaining therapeutic levels of coagulation activity in FVIII deficiency. This provides support for the use of FIX-FIAV as an alternative treatment for hemophilia A.