Highlights

  • Mirum reports a reduction in total abnormal bone lesion volume at Week 24.
  • No treated children developed new heterotopic ossification lesions.
  • Clinician-assessed joint mobility improved in the treated group.
  • Mirum plans an sNDA in the first half of 2027 for ages 6 to under 12.

Mirum Pharmaceuticals, Inc. (NASDAQ:MIRM) has announced positive findings from Cohort 2 of its pivotal PROGRESS study of ATEBRIOZ, or zilurgisertib, in children aged six to under 12 with fibrodysplasia ossificans progressiva. The 24-week findings showed lower heterotopic ossification lesion volume, improved clinician-assessed joint mobility and no new lesions among treated patients.

Trial Results Separate Treatment and Placebo

Cohort 2 enrolled 25 children, randomized to receive 75 mg of once-daily oral zilurgisertib or placebo during a 24-week double-blind period. Twelve received treatment and 13 received placebo. Mean total lesion volume declined by 8.09 cm³ in the treated group, while increasing by 9.14 cm³ with placebo. The reported treatment difference favored zilurgisertib, with a nominal p-value of 0.003. No treated patient developed a new lesion; one placebo recipient did.

Joint Mobility Offers a Second Measure

Clinicians assessed mobility using the Cumulative Analogue Joint Involvement Scale, where lower scores indicate less impairment. By Week 24, scores improved by a mean of 0.80 points in the zilurgisertib group compared with 0.09 points in the placebo group. All participants completed the double-blind period and entered the open-label extension. Their mean baseline age was 8.3 years, and they had been diagnosed for an average of five years.

Safety Findings and Existing Approval

Zilurgisertib was generally well tolerated, and the safety profile was consistent with the earlier PROGRESS cohort. No new safety signals were identified, no participants discontinued during the double-blind phase and no treated patients experienced growth plate closure. Most adverse events were mild or moderate. ATEBRIOZ received FDA approval in September 2026 for patients aged 12 and older with FOP; the younger group is not yet covered by that approval.

Regulatory Submission Moves into Focus

Mirum plans to discuss the findings with the FDA and submit a supplemental New Drug Application during the first half of 2027 to seek an expanded indication for children aged six to under 12. Full Cohort 2 results are expected at a future medical meeting. Cohort 3, which is studying patients aged two to under 12, continues enrollment. The latest announcement therefore combines completed 24-week efficacy and safety findings with defined future regulatory and clinical milestones.