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# No Treated Child Developed New Bone Lesions In Mirum's FOP Trial, While One On Placebo Did
- URL: https://www.bullished.co/no-treated-child-developed-new-bone-lesions-in-mirums-fop-trial-while-one-on-placebo-did-f05ba0cf/
- Published: 2026-10-26T21:06:42.000Z
- Updated: 2026-10-28T16:38:54.000Z
- Description: Mirum plans a label-expansion filing in the first half of 2027 after its drug cut abnormal bone growth in children aged 6 to under 12.
- Author: Matteo Parrino
- Tags: SEC Filings, MIRM, Industry: Healthcare

[Mirum Pharmaceuticals](https://www.bullished.co/tag/mirm/), a rare-disease drugmaker, said Monday that children aged 6 to under 12 on its newly approved drug ATEBRIOZ saw [total bone-lesion volume fall 8.09 cm³](https://www.sec.gov/Archives/edgar/data/0001759425/000175942526000054/mirm-20261026.htm?ref=bullished.co) at Week 24, while volume rose 9.14 cm³ on placebo (nominal p=0.003). No child on the drug developed new lesions in the 25-patient cohort of the pivotal PROGRESS study, an [8-K](https://www.bullished.co/learn/8-k-items-explained/) shows.

## Twelve children on drug, thirteen on placebo

The cohort enrolled 25 children with fibrodysplasia ossificans progressiva, a rare disease in which abnormal bone forms in the body. They were randomized 1:1 to once-daily oral zilurgisertib, the drug's generic name, at 75 mg (12 children) or placebo (13) for a 24-week double-blind period. All completed it and moved into an open-label extension. Mean age at baseline was [8.3 years](https://www.businesswire.com/news/home/20261026062788/en/Mirum-Pharmaceuticals-Announces-Positive-Results-from-Cohort-2-of-the-Pivotal-PROGRESS-Study-of-ATEBRIOZ-zilurgisertib-in-Pediatric-Patients-Aged-6-to-12-Years-with-Fibrodysplasia-Ossificans-Progressiva?ref=bullished.co), with a mean of 5.0 years since diagnosis.

The lesion-volume gap was a [treatment difference of 14.0 cm³](https://www.sec.gov/Archives/edgar/data/0001759425/000175942526000054/mirm-20261026.htm?ref=bullished.co) favoring the drug. One placebo patient, or 7.7%, developed new lesions; none of the 12 on zilurgisertib did. The measure counts expansion of existing lesions as well as new ones.

Joint mobility, scored on a 0-to-30 scale where higher means more impairment, improved by a mean 0.80 points on the drug and 0.09 on placebo. Scores in untreated patients are reported to worsen by about 0.5 points per year.

## Safety profile matches the adolescent data

Mirum called the drug generally well tolerated, with a safety profile consistent with Cohort 1 and no new safety signals. No one stopped treatment during the double-blind period, and no treated child had growth plate closure. Most adverse events were mild or moderate, and serious ones were less frequent on the drug than on placebo.

Arthralgia, cough, pyrexia and vomiting were the most common events above placebo rates, at three patients each.

## A wider label is the next step

Mirum, which also sells Livmarli, Cholbam and Ctexli, won FDA approval for ATEBRIOZ in September 2026 for patients 12 and older, based on Cohort 1\. It plans to file a supplemental New Drug Application in the first half of 2027 to cover ages 6 to under 12.

The company said full Cohort 2 results will be presented at an upcoming medical meeting, though its forward-looking language calls that only a potential presentation. A third cohort, covering ages 2 to under 12, is enrolling.

The sNDA, targeted for the first half of 2027, is the next dated step in widening the label below age 12.

Learn

- [Item 8.01: other events →](https://www.bullished.co/learn/8-k-items-explained/#item-801-other-events)  
From 8-K Items Explained: What Every Item Code From 1.01 To 9.01 Tells You, With Examples

### Sources

- [8-K Filing — Mirum Pharmaceuticals, Inc. (MIRM)](https://www.sec.gov/Archives/edgar/data/0001759425/000175942526000054/mirm-20261026.htm?ref=bullished.co) — SEC EDGAR
- [Mirum Pharmaceuticals annonce les résultats de la cohorte 2 de l’étude pivot PROGRESS sur l’ATEBRIOZ1 min de lecture](https://fr.benzinga.com/news/france/general/mirum-pharmaceuticals-annonce-les-resultats-de-la-cohorte-2-de-letude-pivot-progress-sur-latebrioz/?ref=bullished.co) — fr.benzinga.com
- [Mirum Pharmaceuticals to Participate in Upcoming Investor Conferences – Company Announcement - FT.com](https://markets.ft.com/data/announce/full?dockey=600-202610211601BIZWIRE%5FUSPRX%5F%5F%5F%5F20261021%5FBW173855-1&ref=bullished.co) — Financial Times
- [Mirum Pharmaceuticals Announces Positive Results from Cohort 2 of the Pivotal PROGRESS Study of ATEBRIOZ™ (zilurgisertib) in Pediatric Patients Aged 6 to <12 Years with Fibrodysplasia Ossificans Progressiva](https://www.businesswire.com/news/home/20261026062788/en/Mirum-Pharmaceuticals-Announces-Positive-Results-from-Cohort-2-of-the-Pivotal-PROGRESS-Study-of-ATEBRIOZ-zilurgisertib-in-Pediatric-Patients-Aged-6-to-12-Years-with-Fibrodysplasia-Ossificans-Progressiva?ref=bullished.co) — businesswire.com

### More on Mirum Pharmaceuticals

- [Mirum's Livmarli Cuts Itch Versus Placebo In 52-Patient Phase 3 Trial, Sets 2027 Filing Plan](https://www.bullished.co/mirums-livmarli-cuts-itch-versus-placebo-in-52-patient-phase-3-trial-sets-2027-filing-plan-e1cf8237/)
- [FDA Clears Mirum's Atebrioz For Rare Bone-Hardening Disease, And A Separate Hepatitis Delta Trial Hits Its Primary Goal](https://www.bullished.co/fda-clears-mirums-atebrioz-for-rare-bone-hardening-disease-and-a-separate-hepatitis-delta-0c48c39a/)

[All Mirum Pharmaceuticals (MIRM) coverage →](https://www.bullished.co/tag/mirm/)