OC 50.3 - Improved Procoagulant Activity of Hemophilia B Causing Dysfunctional Factor IX Variants with Factor VIII Mimetics

Background: While the FVIII-mimetic emicizumab has been widely adapted for hemophilia A (with subcutaneous administration), prophylaxis for hemophilia B (HB) still requires intravenous FIX administration. Previous studies investigating the effect of emicizumab in HB suggested that emicizumab can enhance the FIX activity of wild-type FIX about tenfold, but HB patients with dysfunctional FIX variants were non-responders to emicizumab (Ogiwara et al. 2017). We recently demonstrated that the mechanism of the high-specific activity of FIX-Padua was due to an enhanced interaction between FIXa/FVIIIa and the increased activity of FIX-Padua compared to FIX-WT is ablated with emicizumab (Samelson-Jones et al 2019).

Aims: We now test the converse hypothesis: HB-causing FIX missense variants due to dysfunctional FIXa/FVIIIa interactions can be rescued with FVIII mimetics.

Methods: Procoagulant activity of recombinant FIX protein and patient samples were assayed by either aPTT assay, one-stage FIX activity, TGA, and ROTEM.

Results: Recombinant protein of high-prevalence HB-causing FIX variants with dysfunctional FIXa/FVIIIa interactions (G93S, R338P, E387K, and I397T) and severe-to-mild HB phenotypes demonstrated increased FIX activity with the addition of therapeutic amounts (300 nM) of emicizumab (Figure-1A). The FIX activities of these recombinant FIX variants without emicizumab is comparable to the clinical data. Emicizumab also corrected the thrombin generation of these variants (Figure-1B). In contrast, HB-causing FIX variants with missense mutations outside FVIIIa-interacting motifs (R248Q and A390V) had no improvement in FIX activity or thrombin generation. In samples from patients with FIX-I397T, emicizumab normalized the aPTT-based clotting time in plasma and substantially improved the ROTEM clot time in whole blood (Figure-1C,D).

Conclusion(s): HB-causing FIX missense variants with dysfunctional FIXa/FVIIIa interactions are highly-susceptible to procoagulant improvement with FVIII-mimetics like emicizumab. Given emicuzmab’s excellent safety record in non-inhibitor HA patients with expanding use into mild HA, consideration of treatment of HB patients with these variants could be considered.