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Dysfunctional Endogenous FIX Impairs Prophylaxis in a Mouse Hemophilia B Model

Brian Cooley, George Broze Jr, David M. Mann, Feng-Chang Lin, Lee G. Pedersen and Darrel W. Stafford

Key Points

  • An endogenous, dysfunctional (CRM+) FIX molecule affects prophylactic FIX efficacy.

  • Recovery studies indicate the amount of extravascular, Col4-bound FIX is several folds greater than the FIX in plasma.

Abstract

Factor IX (FIX) binds to collagen IV (Col4) in the sub-endothelial 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 (FIXWT) 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 374-436 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 following infusion of 150 IU/kg FIX), FIXWT and the increased half-life FIXs Alprolix (FIXFC) and Idelvion (FIXAlb) produce comparable hemostatic results in CRM- mice. In bleeding CRM- hemophilia B mice, the times to first clot at a saphenous vein injury site following the infusions of the FIX agents are significantly different: FIXWT < FIXFC < FIXAlb. 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, FIXR333Q, 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.

  • Submitted November 2, 2018.
  • Revision received April 9, 2019.
  • Accepted April 9, 2019.