Extravascular factor IX pool fed by prophylaxis is a true hemostatic barrier against bleeding

Factor (F)IX can bind to type IV collagen in the endothelial basement membrane and diffuse into extravascular spaces. Previous studies in rodents have reported a large biodistribution of FIX.

The aim of the study was to evaluate the potential hemostatic activity of extravascular FIX and its role in protecting against joint bleeds.

The capacity of 4 different FIX molecules (plasma-derived and recombinant) to bind type I and type IV collagen was studied here. FIX molecules were also administered intravenously at doses of 50 to 3000 IU/kg in FIX knockout mice.

A specific FIX signal was detected in immunohistochemistry in the liver as well as in muscles and knee joints with recombinant FIX molecules injected at 1000 and 3000 IU/kg but not at the usual clinical doses of 50 to 100 IU/kg, while plasma-derived FIX generated a FIX signal at all doses, including 50 IU/kg. Such a signal was also detected after five 100 IU/kg daily infusions of recombinant FIX, suggesting that FIX can accumulate in the extravascular space during prophylaxis. The extravascular procoagulant activity of FIX, assessed in saphenous vein bleeding assays, was significantly higher in hemophilia B mice after these 5 days of prophylaxis compared to a single infusion of 100 IU/kg of FIX and assessment of FIX activity 7 days later.

Taken together, these results show that in individuals with severe hemophilia B receiving regular prophylaxis with FIX, extravascular accumulation of FIX over time may have a significant impact on the coagulation capacity and protection toward bleeding.