Reduced Dosing Frequency Following a Switch to Rix-FP for the Treatment of Hemophilia B: Results from the Athn 2 Study

Introduction: The approval of extended half-life recombinant factor IX (rFIX) replacement products has expanded the range of therapeutic options available for the treatment of hemophilia B. Compared with standard half-life FIX, these products allow for extended dosing intervals while maintaining appropriate bleed control. One such extended half-life product is rIX-FP (IDELVION; CSL Behring), a recombinant fusion protein linking rFIX with recombinant albumin, offering the possibility of dosing up to every 21 days in adults. The ATHN 2: Factor Switching Study provides information on patient outcomes following a switch from a previous FIX product to rIX-FP.

Methods: ATHN 2 was a factor-switching study sponsored by the American Thrombosis and Hemostasis Network (ATHN) conducted in participants across the US hemophilia treatment center (HTC) network. This was a multi-center, longitudinal, observational study with two arms: a prospective arm following participants for up to one year after switching factor replacement product, and a retrospective arm including participants who have switched products within the 50 weeks prior to enrollment with retrospective and/or prospective assessment for up to one year. Male and female children and adults (≥2 years) with FIX clotting activity ≤5% of normal who had previously been treated with plasma-derived or recombinant FIX for at least 50 exposure days were eligible for inclusion. Treatment was administered at the discretion of the participant's hemophilia caregiver. Data was collected at study/clinic visits, or via a telephone interview conducted every three months. Baseline demographic data was collected for all participants. The prescribed dosing frequency for each participant was collected before and after the switch to rIX-FP, including dosing frequency taken from the first and last treatment records taken during the study following the switch. Participants were also asked to rank their satisfaction with rIX-FP upon the completion or early termination of the study.

Results: A total of 41 participants were included in this analysis; 27 in the prospective arm and 14 in the retrospective arm. The mean (SD) age across all participants was 22.5 (17.1) years, ranging from 2 to 71 years. The median age was 18 years old and most participants had severe hemophilia B (FIX activity <1%; n=26, 63%). Prior to the switch to rIX-FP, 76% (n=31) of participants were receiving prophylaxis and 24% (n=10) received episodic treatment. The majority of participants (62%) receiving prophylaxis were treated twice a week (Table 1). Following the treatment switch, 93% of participants were initially assigned to a once-weekly or less frequent dosing regimen. This proportion remained stable over the course of the study, with 89% of participants on once-weekly or less frequent prophylaxis by the time of the last record taken. Among 23 participants with complete data on their prophylaxis dose interval before and after treatment switch, 70% of the participants were able to extend their dose frequency and maintain the extended dose frequency through the study. Thirty-seven participants responded to the satisfaction survey, with the majority (n=33, 89%) being somewhat or very satisfied with rIX-FP treatment.

Conclusions: Switching to rIX-FP allowed participants to extend their prophylaxis dosing interval and a majority of participants were able to maintain the extended dose interval through the study period. In addition, a majority of participants were satisfied with rIX-FP treatment, altogether suggesting that extended half-life factor replacement with rIX-FP offers a valuable treatment option for hemophilia B.

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