Mirum reports positive Cohort 2 data for ATEBRIOZ in younger FOP patients

Mirum Pharmaceuticals announced on October 26, 2026 positive results from Cohort 2 of the pivotal PROGRESS study of ATEBRIOZ (zilurgisertib), an oral activin receptor-like kinase 2 inhibitor, in pediatric patients aged 6 to under 12 years with fibrodysplasia ossificans progressiva.¹ At Week 24, zilurgisertib reduced total heterotopic ossification lesion volume and improved clinician-assessed joint mobility, and no zilurgisertib-treated patients developed new HO lesions.¹

The results build on data from PROGRESS Cohort 1, which supported FDA approval of ATEBRIOZ in September 2026 for adult and pediatric patients aged 12 years and older with FOP, and the company plans a supplemental New Drug Application submission in the first half of 2027 to expand the indication to patients aged 6 to under 12 years.¹

Cohort 2 enrolled 25 pediatric patients aged 6 to under 12 years with FOP, randomized 1:1 to once-daily oral zilurgisertib 75 mg (n=12) or placebo (n=13) over a 24-week double-blind period followed by an open-label extension.¹ Mean total HO lesion volume decreased by 8.09 cm³ in the zilurgisertib group versus an increase of 9.14 cm³ with placebo, a nominal p=0.003.¹ CAJIS joint mobility scores improved by a mean of 0.80 points with zilurgisertib compared with 0.09 points with placebo.¹

On safety, zilurgisertib was generally well tolerated with a profile consistent with Cohort 1, no new safety signals, no discontinuations during the double-blind period, and no growth plate closures among treated patients.¹ Adverse events occurring in more than 10% of zilurgisertib-treated patients and at higher frequency than placebo included arthralgia, cough, pyrexia, and vomiting (3 subjects each), as well as alopecia, epistaxis, fall, iron deficiency, and nasopharyngitis (2 subjects each).¹

Full Cohort 2 results will be presented at an upcoming medical meeting, while Cohort 3, evaluating patients aged 2 to under 12 years, is currently enrolling.¹