Advancements in Hemophilia Therapy: Exploring the Potential of Factor Xa Variants in Murine Models

Hemophilia A and B are genetic bleeding disorders due to mutations in the genes for FVIII or FIX. The standard treatment is protein replacement, but some patients develop antibodies that hinder the effectiveness of FVIII or FIX. For these patients with inhibitors, alternative treatments like activated prothrombin complex concentrates or recombinant FVIIa are used to bypass the intrinsic pathway and facilitate coagulation.

Traditionally, FXa infusion is considered a potential treatment, but it has drawbacks such as the risk of excessive coagulation and rapid inactivation by plasma inhibitors, leading to a very short half-life. However, recent studies have identified FXa variants with unique characteristics that could overcome these issues. These variants, FXa-I16L and V17A, have a poorly formed active site and low catalytic activity, making them less susceptible to inhibitors. Yet, when they bind to FVa, they efficiently activate prothrombin, suggesting they could function as long-lived, inactive proteases in circulation that become active upon FVa binding.

To test the potential of these variants, in vivo experiments were conducted using FXa-I16L in male HB mice of the Balb/c strain. The modified aPTT clotting assay showed that FXa-I16L administration significantly improved clotting times in untreated HB animals. The protein was well tolerated, with no adverse effects on survival or significant changes in platelet levels. Furthermore, FXa-I16L infusion reduced blood loss in a tail clip assay and restored thrombus formation in various injury models, demonstrating its efficacy in improving hemostasis and clot formation in hemophiliac mice.

The findings indicate that FXa-I16L has the potential to be a superior therapeutic agent for treating hemophilia by enhancing coagulation and restoring thrombus formation at both micro and macrocirculation levels, offering a promising alternative for bypassing deficiencies in the coagulation pathway.