LAUSR

Bleeding severity in haemophilia usually correlates with the lev‐ els of circulating clotting factor.1 Patients with mild (factor levels 6%‐40% of normal activity) or moderate haemophilia (1%‐5%) bleed infrequently, predominantly due to trauma or after surgery, while individuals with severe disease (<1%) experience frequent spontaneous bleeding, particularly into joints and muscles, or after minor trauma.1 In patients with haemophilia B, prophylaxis with factor IX (FIX) aims to maintain appropriate trough levels to re‐ duce the incidence of bleeds.1 In severe cases, lower trough lev‐ els are associated with an increase in total bleeding events and haemarthroses.2 rIX‐FP (albutrepenonacog alfa, IDELVION®, CSL Behring, King of Prussia, PA, US) is a fusion protein genetically linking recombinant human coagulation FIX with recombinant human albumin and has an extended half‐life compared with standard products, allowing a prolonged dosing interval.3 The safety and efficacy of rIX‐FP was demonstrated in adults/adolescents (7‐, 10‐ and 14‐day prophylaxis treatment regimens) and paediatrics (7‐day prophylaxis only) in two phase III trials of previously treated patients (PTPs) with severe hae‐ mophilia B (FIX activity [FIX:C] ≤2%).3,4 The aim of this analysis was to evaluate mean steady‐state and observed trough FIX:C levels during prophylaxis with rIX‐FP in these two clinical trials and to assess the impact of treatment with rIX‐FP on patients with haemophilia B. The detailed study designs of the adult/adolescent (NCT0101496274) and paediatric (NCT01662531) rIX‐FP phase 3 studies have been described previously.3,4 Subjects were male PTPs with haemophilia B (FIX ≤2%) without inhibitors, and in‐ formed consent was acquired from all patients. Adult and ado‐ lescent patients (≥12 years) received prophylaxis with 35‐50 IU/ kg rIX‐FP every 7 days or 50‐75 IU/kg every 10 or 14 days; pae‐ diatric patients (<12 years) received prophylaxis with 35‐50 IU/ kg rIX‐FP every 7 days.3,4 This analysis is limited to 7‐ and 14‐day regimens. FIX:C was measured at a central laboratory using a validated one‐stage clotting method with Pathromtin SL (Siemens Healthcare Diagnostics, Marburg, Germany) as an activator agent, as previ‐ ously described.3 Adult/adolescent trough FIX:C levels were mea‐ sured every 4 weeks before each infusion over a maximum period of approximately 70 weeks; paediatric trough FIX:C levels were mea‐ sured at 4, 12, 24 and 36 weeks. Steady‐state trough FIX:C included only FIX trough measure‐ ments after three consecutive doses on the 7‐day regimen and two consecutive doses on the 14‐day regimen; if unscheduled doses of FIX product were administered, subsequent trough FIX measure‐ ments occurring within 21 and 28 days for the 7‐day and 14‐day reg‐ imens, respectively, were excluded. Thus, steady‐state trough FIX:C levels reflect the FIX:C values achieved after repeated dosing, where FIX:C levels have reached their maximum plasma concentration for a dosing regimen. Observed trough reflects the measured FIX:C levels at a given time point and includes data from all patients with at least one measurement obtained at observed trough during prophylaxis treatment. Observed trough FIX:C measured during pharmacokinetic (PK), repeated PK or surgical periods, or during an unscheduled visit was not included; if an additional dose was given (eg to treat a bleed) between two prophy‐ laxis doses, the observed FIX:C measurement was excluded. Ninety adult/adolescent (n = 63) and paediatric (n = 27) pa‐ tients with severe haemophilia B (≤2% FIX) were enrolled in the studies. Two paediatric patients did not participate in the PK analysis. The majority of patients had previously received pro‐ phylaxis (71.1%). Baseline demographic data have previously been published.3,4 Adults/adolescents (n = 33) receiving 35‐50 IU/kg rIX‐FP pro‐ phylaxis every 7 days had a mean steady‐state trough FIX:C level of 20.90% (Table 1, Figure 1A), with a mean dose of 41.3 IU/kg. Adults/adolescents (n = 16) receiving 50‐75 IU/kg rIX‐FP prophylaxis every 14 days had a mean steady‐state trough FIX:C level of 12.76% (Table 1, Figure 1B), with a mean dose of 73.5 IU/kg. Including all dose levels, the mean FIX:C trough levels were 22.26% and 12.48% for 7‐day and 14‐day regimens, respectively. Nearly all (96.2%) steady‐state trough measurements were above 5% across all doses and dose intervals. The observed trough values were similar, with mean levels of 22.09% and 12.37% for 7‐day and 14‐day regimens, respectively. Adults/adolescents had a median (IQR) annualized bleeding rate (ABR) of 0.00 (0.00, 1.87) on weekly prophylaxis and 1.08 (0.00, 2.70) on the 14‐day dosing interval. The median annual‐ ized spontaneous bleeding rate (AsBR) on all dosing regimens was