SEC Filings

FDA Clears Mirum's Atebrioz For Rare Bone-Hardening Disease, And A Separate Hepatitis Delta Trial Hits Its Primary Goal

The FDA approved Atebrioz on Friday; the same day, Mirum's late-stage AZURE-1 hepatitis delta study met its primary endpoint at week 24.

Matteo Parrino Jonathan Ponciano

Matteo Parrino, Jonathan Ponciano

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FDA Clears Mirum's Atebrioz For Rare Bone-Hardening Disease, And A Separate Hepatitis Delta Trial Hits Its Primary Goal

Mirum Pharmaceuticals landed two clinical wins on Friday: the FDA approved Atebrioz (zilurgisertib) for a rare disorder in which soft tissue turns to bone, and a separate late-stage trial of its hepatitis delta drug brelovitug met its primary endpoint — setting up a potential second commercial launch as early as the fourth quarter of 2027. MIRM shares fell about 3% to $81.71 in Friday trading, according to market data.

What The FDA Cleared — And The Trial Data Behind It

The FDA's approval covers Atebrioz tablets to reduce new abnormal bone formation in adults and pediatric patients aged 12 and older with fibrodysplasia ossificans progressiva, a rare genetic disorder in which mutations in the ACVR1 gene cause muscles, tendons, ligaments and other soft tissues to be progressively replaced by bone. As those bone lesions accumulate, they restrict movement and can lead to severe disability.

The drug works by blocking a protein called ALK2 — the receptor whose abnormal activation drives the bone-forming process in the disease — in a once-daily oral tablet at a recommended dose of 100 mg.

The approval rested on data from the PROGRESS study, which enrolled adults and pediatric patients aged 12 and older. Over 24 weeks, patients receiving zilurgisertib saw mean new bone lesion volume fall by 3.2 cubic centimeters, while patients on placebo saw an increase of 24.6 cubic centimeters, according to the SEC filing. Those treatment effects held through week 48 of an open-label extension. The most common side effects were headache, joint pain, upper respiratory tract infection, nosebleeds and nausea; no adverse events led to treatment discontinuation or dose reduction.

Atebrioz was developed by Incyte and licensed to Mirum for worldwide commercialization. Mirum paid $16 million upfront for those global commercial rights, with Incyte eligible for up to $63 million in additional milestone payments plus royalties. The drug is expected to be commercially available in the U.S. in October, with eligible patients paying as little as $0 per month through Mirum's patient support program. A marketing authorization application for zilurgisertib is also under review by the European Medicines Agency. Mirum estimates the total annual revenue potential for Atebrioz in this indication at more than $200 million, according to the filing.

The Hepatitis Delta Results — And What They Mean For A Second Launch

On Monday, Mirum announced that the late-stage portion of its AZURE-1 study of brelovitug — a fully human monoclonal antibody that binds to hepatitis B surface antigen — met its primary endpoint at week 24 in chronic hepatitis delta.

The late-stage study enrolled 153 treatment-naive patients split roughly equally across three arms: a 300 mg weekly injection group, a 900 mg monthly injection group, and a delayed-treatment group that began dosing at week 24. The primary endpoint combined two measures: a meaningful reduction in the hepatitis delta virus in the blood (at least a 2-log reduction from baseline, or undetectable levels) and normalization of a liver enzyme called ALT, which signals reduced liver inflammation.

At week 24, 56% of patients in the weekly arm and 45% in the monthly arm hit that combined endpoint, against 0% in the delayed-treatment group. The p-value for both comparisons was less than 0.0001. Virologic response alone — the blood-level measure — reached 86% in the weekly arm and 85% in the monthly arm, also against 0% in the delayed group. No patients in either active treatment arm experienced a serious adverse event or discontinued the study drug through week 24.

Separately, 48-week data from an earlier mid-stage portion of AZURE-1 showed deepening viral suppression over time: the share of patients in the weekly arm achieving the combined primary endpoint rose from 45% at week 24 to 55% at week 48, with the monthly arm moving from 35% to 55% over the same period.

Mirum now estimates total annual revenue potential for brelovitug in chronic hepatitis delta at more than $1 billion, according to the filing. The company plans to submit a Biologics License Application — the FDA approval application for a biologic drug — in the first half of 2027, targeting a U.S. commercial launch in the fourth quarter of 2027.

What To Watch Next

AZURE-1 is one of two late-stage studies intended to support Mirum's FDA approval package for brelovitug. Topline results from the second pivotal study, AZURE-4, are expected in the fourth quarter of 2026. Results from two additional studies, AZURE-2 and AZURE-3, are expected in the second half of 2027. Full AZURE-1 late-stage data will be presented at an upcoming medical congress, the company said.

On the Atebrioz side, enrollment is complete in a cohort of children aged 6 to under 12 years in the PROGRESS program, and enrollment is underway in a cohort of children aged 2 to under 12 years, extending the drug's development into younger patients.

The AZURE-4 topline readout, expected before year-end, will determine whether Mirum has the two-study registration package it needs to file for brelovitug approval in the first half of 2027.

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