Dysfunctional endogenous FIX impairs prophylaxis in a mouse hemophilia B model.

Factor IX (FIX) binds to collagen IV (Col4) in the subendothelial basement membrane. In hemophilia B, this FIX-Col4 interaction reduces the plasma recovery of infused FIX and plays a role in hemostasis. Studies examining the recovery of infused BeneFix (FIX _WT ) in null (cross-reactive material negative, CRM ^- ) hemophilia B mice suggest the concentration of Col4 readily available for binding FIX is ∼405 nM with a 95% confidence interval of 374 to 436 nM. Thus, the vascular cache of FIX bound to Col4 is several-fold the FIX level measured in plasma. In a mouse model of prophylactic therapy (testing hemostasis by saphenous vein bleeding 7 days after infusion of 150 IU/kg FIX), FIX _WT and the increased half-life FIXs Alprolix (FIX _FC ) and Idelvion (FIX _Alb ) produce comparable hemostatic results in CRM ^- mice. In bleeding CRM ^- hemophilia B mice, the times to first clot at a saphenous vein injury site after the infusions of the FIX agents are significantly different, at FIX _WT < FIX _FC < FIX _Alb Dysfunctional forms of FIX, however, circulate in the majority of patients with hemophilia B (CRM ⁺ ). In the mouse prophylactic therapy model, none of the FIX products improves hemostasis in CRM ⁺ mice expressing a dysfunctional FIX, FIX _R333Q , that nevertheless competes with infused FIX for Col4 binding and potentially other processes involving FIX. The results in this mouse model of CRM ⁺ hemophilia B demonstrate that the endogenous expression of a dysfunctional FIX can deleteriously affect the hemostatic response to prophylactic therapy.
(© 2019 by The American Society of Hematology.)