Introduction: In clinical trials, recombinant factor IX fusion protein (rFIXFc) has demonstrated safety, efficacy and prolonged activity with extended dosing intervals for treatment of haemophilia B. Aim: To assess the real-world clinical utility of rFIXFc in a variable patient population and routine clinical practice. Methods: A multicentre, retrospective chart review was conducted of patients with haemophilia B who had received rFIXFc prophylaxis or on-demand treatment for ≥6 months across six sites in the United States. Results: Sixty-four eligible patients were identified who had a median (range) duration on rFIXFc of 2.7 (0.5-5.0) years. Of 32 patients on rFIXFc prophylaxis who switched from prophylaxis with another factor treatment (ie pre-rFIXFc) and had a known pre-rFIXFc dosing interval, the initial dosing interval was lengthened for 26 (81%) patients and maintained for the remaining 6 (19%) patients. Most (n = 48 [91%]) patients who received rFIXFc prophylaxis from the beginning to the end of the chart review period (n = 53) maintained or lengthened the dosing interval from first through last dose of rFIXFc. For patients receiving rFIXFc prophylaxis, there was an approximate 50% reduction in weekly factor consumption compared with pre-rFIXFc prophylaxis. Overall annualized bleed rates, annualized spontaneous bleed rates and annualized joint bleed rates decreased after switching to rFIXFc prophylaxis (n = 24 with bleed data). Compliance to recommended treatment improved or remained stable in most patients with available data (30/31). Conclusion: Recombinant factor IX fusion protein prophylaxis improved bleed control, reduced overall consumption, reduced frequency of infusion and improved compliance for patients with haemophilia B in a real-world setting. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

Background Few have assessed physical activity (PA) and annual bleed rates (ABRs) among people with hemophilia on extended half‐life (EHL) factors (recombinant factor VIII Fc [rFVIIIFc]/recombinant factor IX Fc [rFIXFc]) and conventional factors (recombinant factor VIII [rFVIII]/recombinant factor IX [rFIX]). Objective To assess changes in PA and ABR at consecutive annual visits in individuals with severe hemophilia A and B (HA/HB) on prophylactic treatment with rFVIIIFc/rFIXFc versus rFVIII/rFIX. Patients/Methods We conducted a retrospective chart review of 344 people with severe HA/HB (ages 6‐35) receiving prophylaxis with rFVIIIFc/rFIXFc (EHL factors) or rFVIII/rFIX (conventional factors) for ≥6 months in 2014‐2015. Differences in changes in outcomes from 2014 to 2015 were compared across the treatment groups. Results Baseline characteristics and adherence to the prophylactic regimen were similar across the treatment groups. Greater increase in weekly PA frequency and duration were observed among all EHL groups, except for children treated with rFIXFc. The increase in PA frequency was greater among the children on rFVIIIFc group, adults on rFVIIIFc group, and adults on rFIXFc group by 1.2, 1.2, and 1.4 events/week, respectively, compared to their rFVIII/rFIX counterparts. The increases in PA duration were 44, 60, and 80 min/wk greater among the children on rFVIIIFc, adults on rFVIIIFc, and adults on rFIXFc groups, respectively. Larger reductions in total ABR were observed in children and adults treated with rFVIIIFc compared to rFVIII (0.4 and 0.7 fewer bleeds). Larger reductions were also observed in spontaneous ABR in adult rFVIIIFc and rFIXFc groups (0.8 and 0.3 fewer bleeds, respectively). Conclusions This study suggests that rFVIIIFc/FIXFc agents can positively impact PA while maintaining low ABRs.

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Introduction:Monitoring of factor IX (FIX) replacement therapy in haemophilia B relies on accurate coagulation assays. However, considerable interlaboratory variability has been reported for one-stage clotting (OSC) assays. This study aimed to evaluate the realworld, interlaboratory variability of routine FIX activity assays used in clinical haemostasis laboratories for the measurement of recombinant FIX Fc fusion protein (rFIXFc) activity.Methods: Human FIX-depleted plasma was spiked with rFIXFc at 0.80, 0.20 or 0.05 IU/mL based on label potency. Participating laboratories tested samples using their own routine OSC or chromogenic substrate (CS) assay protocols, reagents and FIX plasma standards. Laboratories could perform more than one measurement and method, and were not fully blinded to nominal activity values. Results:A total of 142 laboratories contributed OSC results from 175 sample kits using 11 different activated partial thromboplastin time (aPTT) reagents. The median recovered FIX activity for the 0.80, 0.20 and 0.05 IU/mL samples was 0.72 IU/mL, 0.21 IU/mL and 0.060 IU/mL, respectively. Across all OSC reagents, interlaboratory variability (% CV) per aPTT reagent ranged from 9.4% to 32.1%, 8.2% to 32.6% and 12.2% to 42.0% at the 0.80, 0.20 and 0.05 IU/mL levels, respectively. CS results showed excellent median recoveries at all nominal levels (87.5% to 115.0%; n = 11) with low interlaboratory variability (CV 3.6% to 15.4%). Conclusion:This large, real-world data set indicates that rFIXFc activity in plasma samples can be accurately measured with the majority of routine OSC and CS assay methods. Given the variation in FIX assay procedures between sites, it is important that individual laboratories qualify their in-house methods for monitoring of rFIXFc activity. K E Y W O R D S chromogenic substrate assay, factor IX replacement therapy, haemophilia B, one-stage clotting assay, recombinant factor IX Fc fusion protein | 351 SOMMER Et al.