Characterization of a recombinant factor IX molecule fused to coagulation factor XIII‐B subunit.

Introduction and aim: Severe haemophilia B (HB) is characterized by spontaneous bleeding episodes, mostly into joints. Recurrent bleeds lead to progressive joint destruction called haemophilic arthropathy. The current concept of prophylaxis aims at maintaining the FIX level >3–5 IU/dL, which is effective at reducing the incidence of haemophilic arthropathy. Extended half‐life FIX molecules make it easier to achieve these target trough levels compared to standard FIX concentrates. We previously reported that the fusion of a recombinant FIX (rFIX) to factor XIII‐B (FXIIIB) subunit prolonged the half‐life of the rFIX‐LXa‐FXIIIB fusion molecule in mice and rats 3.9‐ and 2.2‐fold, respectively, compared with rFIX‐WT. However, the mechanism behind the extended half‐life was not known. Materials and methods: Mass spectrometry and ITC were used to study interactions of rFIX‐LXa‐FXIIIB with albumin. Pharmacokinetic analyses in fibrinogen‐KO and FcRn‐KO mice were performed to evaluate the effect of albumin and fibrinogen on in‐vivo half‐life of rFIX‐LXa‐FXIIIB. Finally saphenous vein bleeding model was used to assess in‐vivo haemostatic activity of rFIX‐LXa‐FXIIIB. Results and conclusion: We report here the key interactions that rFIX‐LXa‐FXIIIB may have in plasma are with fibrinogen and albumin which may mediate its prolonged half‐life. In addition, using the saphenous vein bleeding model, we demonstrate that rFIX‐FXIIIB elicits functional clot formation that is indistinguishable from that of rFIX‐WT. [ABSTRACT FROM AUTHOR]