Mirum widens beyond Livmarli with a new approval and two phase 3 wins

The technology

Mirum is a rare disease company with a bile acid and liver focus. Livmarli (maralixibat) is an oral inhibitor of the ileal bile acid transporter, approved in the U.S. for cholestatic itch in Alagille syndrome and progressive familial intrahepatic cholestasis. Cholbam and Ctexli (chenodiol) are bile acid medicines for rare disorders. Atebrioz (zilurgisertib), licensed from Incyte, is an oral ALK2 inhibitor that the FDA approved on Sept. 25 for fibrodysplasia ossificans progressiva.

The pipeline reuses the Livmarli mechanism and adds a hepatitis antibody. Volixibat is another bile acid transporter inhibitor, in studies for primary sclerosing cholangitis and primary biliary cholangitis. Brelovitug is a monoclonal antibody against the hepatitis B surface antigen that Mirum is testing in hepatitis delta. Livmarli itself is being extended in the EXPAND study, and the company also lists a phase 2 Fragile X program, MRM-3379.

The odds

Mirum has strong recent data, with regulatory questions on timing. EXPAND met its primary and all five secondary endpoints, with a 0.98-point itch benefit over placebo in children, and a supplemental filing is planned for the first half of 2027. For rare diseases, 93.6% of filed applications are approved in BIO, Informa and QLS data for 2011 to 2020.

Brelovitug cleared AZURE-1, where 56% of weekly-dose patients met the combined endpoint against 0% on delayed treatment, and AZURE-4 is next. Mirum plans a biologics license application in the first half of 2027. Phase 3 programs reach a filing 57.8% of the time overall. Volixibat is the uncertain one: after the VISTAS study, the FDA said it recommended a phase 3 study.

The money

Second-quarter net product sales were $176.2 million, and 2026 guidance is $680 million to $700 million. Operating expenses were $218.8 million in the quarter, so the company is not yet profitable on that measure. Cash and investments were $561.3 million at June 30, up from $391.4 million at the end of 2025.

The balance sheet changed in May. Mirum sold $690.0 million of 0% convertible notes due 2032 with a conversion price near $138.94, and it settled about 75% of its 4.00% notes due 2029 using cash and shares. That removes most of the older debt but adds potential dilution if the new notes convert. Livmarli sales grew 46% year over year in the second quarter.

The risks

Volixibat is the clearest risk. VISTAS met its primary endpoint, but the FDA's recommendation of a phase 3 study could add cost and time before any filing. Livmarli remains the core of sales, with $128.7 million of the $176.2 million in the second quarter, so concentration is high until Atebrioz and brelovitug launch.

Diarrhea is common with this class of drug: it occurred in 64.0% of patients on maralixibat in EXPAND against 11.1% on placebo. Management also changed in medical development in September, when Rob Myers was appointed chief medical officer to succeed Joanne Quan. Atebrioz addresses a small population, about 300 diagnosed patients in the United States by the company's estimate.

Our read

Mirum has a growing commercial base and several late-stage programs, and the second half of 2026 has delivered three positive events: the EXPAND result, the AZURE-1 result and the Atebrioz approval. The FDA response on volixibat is the one disappointment.

The next tests are dated and close. AZURE-4 reports in the fourth quarter, and Atebrioz becomes available in October. If AZURE-4 repeats AZURE-1, Mirum has a hepatitis delta filing for the first half of 2027. If not, the company leans harder on Livmarli growth and the cash raised in May.