Dysfunctional endogenous FIX impairs prophylaxis in a mouse hemophilia B model

Area of Science:

• Biochemistry
• Hematology
• Pharmacology

Background:

• Factor IX (FIX) interacts with collagen IV (Col4) in the subendothelial basement membrane, influencing its plasma recovery and hemostasis in hemophilia B.
• The concentration of available Col4 for FIX binding in mice is estimated around 405 nM, suggesting a significant vascular reservoir.
• Dysfunctional FIX variants are common in hemophilia B patients (CRM+), complicating treatment strategies.

Purpose of the Study:

• To investigate the impact of the Factor IX-collagen IV interaction on the efficacy of different FIX therapies in hemophilia B mouse models.
• To compare the hemostatic performance of wild-type FIX (FIXWT), FIX-Fc (FIXFC), and FIX-albumin (FIXAlb) in both CRM- and CRM+ hemophilia B models.
• To evaluate the effect of endogenous dysfunctional FIX on the response to prophylactic FIX therapy.

Main Methods:

• Utilized hemophilia B mouse models (CRM- and CRM+) to assess hemostasis via saphenous vein bleeding after FIX infusions.
• Administered FIXWT, FIXFC, and FIXAlb at a prophylactic dose (150 IU/kg) and measured time to first clot.
• Assessed the competition for Col4 binding by endogenous dysfunctional FIX (FIXR333Q) in CRM+ mice.

Main Results:

• In CRM- mice, FIXWT, FIXFC, and FIXAlb showed comparable hemostatic results, with time to clot differing significantly: FIXWT < FIXFC < FIXAlb.
• In CRM+ mice expressing dysfunctional FIXR333Q, none of the infused FIX products improved hemostasis.
• Endogenous dysfunctional FIX competed with infused FIX for Col4 binding, impairing the hemostatic response.

Conclusions:

• The interaction between FIX and Col4 is a critical factor in FIX pharmacokinetics and hemostasis.
• Current prophylactic FIX therapies are less effective in hemophilia B patients with endogenous dysfunctional FIX due to competition for binding sites.
• Developing FIX therapies that overcome competition from endogenous dysfunctional FIX is crucial for improving treatment outcomes in a significant portion of hemophilia B patients.