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

BACKGROUND Factor IX (FIX) 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. OBJECTIVE the aim of the study was to evaluate the potential hemostatic activity of extravascular FIX and its role in protecting against joint bleeds. METHODS The capacity of 4 different FIX molecules (plasma derived [pd] and recombinants) 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 knock-out mice. RESULTS 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 50-100 IU/kg, while pdFIX 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 rFIX, 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 five days of prophylaxis compared to a single infusion of FIX 100 IU/kg and assessment of FIX activity seven days later. CONCLUSION 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 towards bleeding.