Extravascular factor IX after gene therapy in hemophilia B, does it matter?

Gene therapy will very likely change the treatment paradigm of hemophilia B in the coming years. For the majority of patients, adjunctive exogenous factor (F)IX clotting factor concentrate will continue to be needed in case of surgery or bleeding. However, there is insufficient evidence on the optimal FIX product to be used in these circumstances, given the differences in body distribution between the currently available products. Unknown factors include the behavior of FIX Padua in the extravascular space and its contribution to hemostasis. Other issues are the potential importance of the presence of cross-reactive material and the discrepancies between different assays in measuring FIX activity. In conclusion, even after gene therapy, the differences between different FIX products remain relevant for optimal bleeding and perioperative management. Thus, real-world data on the use of exogenous FIX after gene therapy are needed to determine the preferred exogenous FIX concentrate.