The Function of extravascular coagulation factor IX in haemostasis.

Introduction The majority of clotting factor IX (FIX) resides extravascularly, in the subendothelial basement membrane, where it is important for haemostasis. Aim We summarize preclinical studies demonstrating extravascular FIX and its role in haemostasis and discuss clinical observations supporting this. We compare the in vivo binding of BeneFIX® and the extended half-life FIX, Alprolix® , to extravascular type IV collagen (Col4). Methods Three mouse models of haemophilia were used: the FIX knockout as the CRM- model and two knock-in mice, representing a CRM+ model of a commonly occurring patient mutation (FIXR333Q ) or a mutation that binds poorly to Col4 (FIXK5A ). The murine saphenous vein bleeding model was used to assess haemostatic competency. Clinical publications were reviewed for relevance to extravascular FIX. Results CRM status affects recovery and prophylactic efficacy. Prophylactic protection decreases ~5X faster in CRM+ animals. Extravascular haemostasis can explain unexpected breakthrough bleeding in patients treated with some EHL-FIX therapeutics. In mice, both Alprolix® and BeneFIX® bind Col4 with similar affinities (Kd~20-40 nM) and show dose-dependent recoveries. As expected, the concentration of binding sites in the mouse calculated for Alprolix® (574 nM) was greater than for BeneFIX® (405 nM), due to Alprolix® binding to both Col4 and the endothelial cell neonatal Fc receptor. Conclusion Preclinical and clinical results support the interpretation that FIX plays a role in haemostasis from its extravascular location. We believe that knowing the CRM status of haemophilia B patients is important for optimizing prophylactic dosing with less trial and error, thereby decreasing clinical morbidity.