Fusion of engineered albumin with factor IX Padua extends half‐life and improves coagulant activity

Summary The short half‐life of coagulation factor IX (FIX) for haemophilia B (HB) therapy has been prolonged through fusion with human serum albumin (HSA), which drives the neonatal Fc receptor (FcRn)‐mediated recycling of the chimera. However, patients would greatly benefit from further FIX‐HSA half‐life extension. In the present study, we designed a FIX‐HSA variant through the engineering of both fusion partners. First, we developed a novel cleavable linker combining the two FIX activation sites, which resulted in improved HSA release. Second, insertion of the FIX R338L (Padua) substitution conferred hyperactive features (sevenfold higher specific activity) as for FIX Padua alone. Furthermore, we exploited an engineered HSA (QMP), which conferred enhanced human (h)FcRn binding [dissociation constant (KD) 0·5 nM] over wild‐type FIX‐HSA (KD 164·4 nM). In hFcRn transgenic mice, Padua‐QMP displayed a significantly prolonged half‐life (2·7 days, P