rFIXFc components

Photo courtesy of Biogen

TORONTO—Full results of a phase 3 study support extended-interval dosing with a recombinant factor IX Fc fusion protein (rFIXFc) over FIX products with a standard half-life, according to a speaker at the 2015 ISTH Congress.

Kathelijn Fischer, MD, PhD, of the University Medical Center Utrecht in The Netherlands, reported results with rFIXFc (also known as eftrenonacog alfa and Alprolix), in children with severe hemophilia B who were enrolled on the KIDS B-LONG study.

rFIXFc was successful in preventing and treating bleeding episodes in these patients. Furthermore, the patients did not develop inhibitors, and there were no serious adverse events related to treatment.

Dr Fischer presented these results as abstract LB009. Interim results of this study helped support the US approval of rFIXFc for use in children. The trial was sponsored by Sobi and Biogen, the companies developing rFIXFc.

KIDS B-LONG included 30 boys younger than 12 who had severe hemophilia B. The patients had at least 50 prior exposure days to FIX therapies and no history of inhibitors.

At baseline, all patients were receiving FIX prophylaxis. Seventy-seven percent of patients were receiving 2 or more doses a week.

On day 1 of the study, patients received rFIXFc at 50 IU/kg. They then received weekly prophylaxis at an initial dose of 50 IU/kg to 60 IU/kg. Doses were adjusted throughout the study, but the maximum was 100 IU/kg. The minimum dosing frequency was once a week, and the maximum was twice a week.

Twenty-seven patients (90%) completed the study. The median time spent on study was 49.4 weeks, and 24 patients (80%) received rFIXFc injections on at least 50 separate days.

Safety

None of the patients developed inhibitors or non-neutralizing anti-rFIXFc antibodies. There were no anaphylactic reactions, hypersensitivity reactions, thrombotic events, or deaths.

Adverse events occurred in 86.7% of patients. The most frequent were nanopharyngitis (23.3%) and falls (20%). Eleven serious adverse events occurred in 4 patients. None were considered related to treatment, and none led to study discontinuation.

One adverse event was considered related to rFIXFc. A 3-year-old child experienced decreased appetite.

Efficacy

The median annualized bleeding rate (ABR) was 2.0 overall, 1.1 in children under 6, and 2.1 in children ages 6 to 11.

For spontaneous bleeds, the median ABR was 0, both overall and in the 2 age groups. For joint bleeds, the median ABR was 0 overall and in the younger age group, but it was 1.1 for the older children.

Thirty-three percent of patients had no bleeding episodes while on study, and 63% had no joint bleeds.

Ninety-seven percent of patients receiving rFIXFc prophylaxis had no change in their dosing interval.

For patients under 6, the median prophylactic dose was 59.4 IU/kg/week (range, 53.0-64.8). For patients ages 6 to 11, the median dose was 57.8 IU/kg/week (range, 51.7-65.0)

When patients received rFIXFc to treat bleeding, 75% of bleeds were controlled with 1 infusion, and 91.7% were controlled with 1 or 2 infusions. The median dose per infusion was 63.5 IU/kg (range, 48.9-99.4).

Pharmacokinetics

The terminal half-life of rFIXFc was 66.5 hours for children under 6 and 70.3 hours for children ages 6 to 11. The clearance was 4.4 mL/hour/kg and 3.5 mL/hour/kg, respectively. The incremental recovery (IR) was 0.59 IU/dL per IU/kg and 0.72 IU/dL per IU/kg, respectively.

Compared to pre-study treatment with BeneFIX (recombinant FIX) at 50 IU/kg, rFIXFc at 50 IU/kg had a significantly longer half-life. In children younger than 6, the half-life was 66.5 hours for rFIXFc and 18.2 hours for BeneFIX (P<0.001). In children ages 6 to 11, the half-lives were 71.1 and 19.2 hours, respectively (P<0.001).

There was no significant difference between the treatments with regard to IR for children under 6. IR was 0.59 with rFIXFc and 0.52 with BeneFIX (P=0.109). However, there was a significant difference in IR for children ages 6 to 11—0.70 for rFIXFc and 0.54 for BeneFIX (P=0.003).

Extended-interval dosing appears safe, effective