Mirum data shows zilurgisertib reduces bone growth in young FOP patients

Cohort 2 of the pivotal PROGRESS study reported positive results for zilurgisertib in pediatric patients aged 6 to less than 12 years with fibrodysplasia ossificans progressiva (FOP). Mirum Pharmaceuticals, Inc. announced on October 26, 2026, that the oral activin receptor-like kinase 2 inhibitor reduced total heterotopic ossification lesion volume and improved clinician-assessed joint mobility at Week 24, with no patients in the treatment group developing new lesions.

The study enrolled 25 pediatric patients who were randomized to receive either once-daily oral zilurgisertib 75 mg or placebo during a 24-week double-blind period. The mean age at baseline was 8.3 years, and the mean time since initial diagnosis was 5.0 years. All patients completed the double-blind period and entered the open-label extension phase.

At Week 24, the mean total heterotopic ossification lesion volume decreased by 8.09 cubic centimeters in patients receiving zilurgisertib, compared with an increase of 9.14 cubic centimeters in the placebo group. The nominal p-value for this difference was 0.003. No patients receiving zilurgisertib developed new heterotopic ossification lesions, while one patient in the placebo group did. Clinician-assessed joint mobility, measured by the Cumulative Analogue Joint Involvement Scale, improved by a mean of 0.80 points for the zilurgisertib group versus 0.09 points for the placebo group.

These findings build on data from PROGRESS Cohort 1, which supported the U.S. Food and Drug Administration approval of ATEBRIOZ (zilurgisertib) in September 2026 for adult and pediatric patients aged 12 years and older with FOP. Mirum plans to submit a supplemental New Drug Application in the first half of 2027 to expand the indication to include pediatric patients aged 6 to less than 12 years.

The company reported that zilurgisertib was generally well tolerated, with a safety profile consistent with Cohort 1 and no new safety signals identified. No patients discontinued treatment during the double-blind period, and no patients receiving zilurgisertib experienced growth plate closure. Most adverse events were mild or moderate in severity. Adverse events occurring in more than 10 percent of zilurgisertib-treated patients at a higher frequency than placebo included arthralgia, cough, pyrexia, and vomiting.

Full results from Cohort 2 are scheduled for presentation at an upcoming medical meeting. Cohort 3 of the PROGRESS study, which evaluates zilurgisertib in pediatric patients aged 2 to less than 12 years, is currently enrolling.