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Updated: Aug 21, 2026

Tail Vein Transection Bleeding Model in Fully Anesthetized Hemophilia A Mice
Published on: September 30, 2021
Substitution-induced allosteric activation of coagulation factor IX as a potential alternative hemophilia A treatment
Viola J F Strijbis1, Ka Lei Cheung1, Dorothea Gobbo1
1Dept. of Internal Medicine, Div. of Thrombosis and Hemostasis, Einthoven Laboratory for Vascular and Regenerative Medicine, Leiden University Medical Center, Leiden, the Netherlands.
Abstract:
Cofactor-independent activity of FIX has previously been achieved in FIX-IDAV and FIX-FIAV variants containing L6F, V181I, E185D, K265A, and I383V substitutions. Cofactor-dependent FIX hyperactivity was attained through the V10K, R338L, and S377W (KLW) modifications. We evaluated whether combining IDAV/FIAV and KLW substitutions could enhance cofactor-independent activity and elucidated the biochemical mechanisms underlying FVIII-independent FIX-FIAV function. Recombinant FIX variants, expressed in HEK293 cells, were analyzed for FIX-specific and FVIII-equivalent clotting activities. Highly purified FIX-FIAV was characterized using purified component assays and FXIa-triggered thrombin generation in FVIII-deficient plasma. FIX-IDAV-KLW and FIX-FIAV-KLW exhibited 20-28-fold higher FIX-specific activity and a modest twofold increase in FVIII-equivalent clotting activity. Owing to thrombotic safety concerns, KLW combinations were not further pursued. FIX-FIAV displayed normal FIX-specific activity and fourfold enhanced FVIII-equivalent clotting activity. The FIAV substitutions did not alter activation by the extrinsic (TF-FVIIa) or intrinsic (FXIa) pathways, nor cofactor-dependent FX activation, but conferred FVIII-independent FX conversion. Kinetic analyses revealed a modified active site conformation with a trend toward reduced antithrombin inhibition. In FVIII-deficient plasma, FIX-FIAV dose-dependently increased FXIa-triggered thrombin generation by shortening the lag time and time to peak and shifted coagulation phenotype categories based on FVIII-equivalent thrombin generation (TG) activity from severe toward moderate/mild ranges in plasma-based models, without exceeding normal FVIII-equivalent TG activity. FIX-FIAV represents a FIX variant with FVIII-independent activity, normal FIX-specific activity, and reduced inactivation by antithrombin. Targeted modification of regions surrounding the FIXa active site enables allosteric activation, providing a potential foundation for novel bypassing strategies in hemophilia A therapy.
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