Data collection
We collected data from haemophilia B patients (endogenous FIX activity level ≤ 5 IU/dL) treated with rFIX-Fc (eftrenonacog alfa, Alprolix®) included in the OPTI-CLOT TARGET study (NTR7523)17 or United Kingdom (UK)-EHL Outcome Registry (NCT02938156). Briefly, haemophilia patients in the OPTI-CLOT TARGET study received nine months of PK-guided dosing to investigate the reliability and feasibility of the Bayesian forecasting procedure. FIX samples for PK profiling were obtained pre-infusion and approximately 15-30 minutes, 4, 24, 72-120 and 168h after infusion. During PK-guidance, a minimum of four FIX activity (at nonspecific time points after infusion) levels per patient was collected in a minimum of two visits to validate predicted FIX. The UK-EHL Outcome Registry contains patient characteristics and treatment information, including FIX infusions (timing and doses) and FIX activity level measurements. During PK profiling, FIX activity levels were measured at pre-infusion and approximately 15 minutes, 24, 72, 120 and 168h after infusion. Additional FIX activity levels were sampled during visits at 10 days, 3, 6, 12 and 18 months after initiation of rFIX-Fc treatment. In both studies, no wash-out was required during PK profiling if three prior infusions were documented. Informed consent was obtained from all patients and/or caregivers.