B‐YOND: safety and efficacy of rFIXFc treatment for haemophilia B

August 2020 Clinical trials Eline Feenstra

Recombinant factor IX Fc fusion protein (rFIXFc) has demonstrated efficacy for treatment of haemophilia B in the Phase 3 B‐LONG and Kids B‐LONG studies. However, long‐term rFIXFc safety and efficacy data have not yet been reported. A new study published in Haemophilia focussed on the ng‐term rFIXFc safety and efficacy in subjects with haemophilia B.

B‐YOND was an open‐label extension for previously treated subjects who completed B‐LONG or Kids B‐LONG. Subjects received ≥1 treatment regimen: weekly prophylaxis (WP), individualized interval prophylaxis (IP), modified prophylaxis or episodic treatment. Subjects could switch regimens at any time. The primary endpoint was inhibitor development.

A total of 93 subjects from B-LONG and 27 from Kids B-LONG (aged 3-63 years) were included. Most subjects received WP (B‐LONG: n = 51; Kids B‐LONG: n = 23). For subjects from B‐LONG, the median treatment duration was 4.0 years and median number of exposure days (EDs) was 146. For paediatric subjects the medium treatment duration was 2.6 years with 132 EDs.

No inhibitor formation

No inhibitors were observed (0 per 1000 subject‐years; 95% CI 0‐8.9) and the overall rFIXFc safety profile was consistent with prior studies. Annualized bleed rates remained low and extended‐dosing intervals were maintained for most subjects. Median dosing interval for the IP group was approximately 14 days for adults and adolescents (n = 31) and 10 days for paediatric subjects (n = 5).

In conclusion, B‐YOND results confirm the long‐term safety and efficacy of rFIXFc treatment for haemophilia B.


Pasi KJ, Fischer K, Ragni M et al. Long-term safety and sustained efficacy for up to 5 years of treatment with recombinant factor IX Fc fusion protein in subjects with haemophilia B: Results from the B-YOND extension study. Haemophilia. 2020 Jun 4. doi: 10.1111/hae.14036.