Mirum Pharmaceuticals Announces First Patient Enrolled in the BLOOM Phase 2 Clinical Study Evaluating MRM-3379 in Fragile X Syndrome

Proof-of-concept study underway for investigational oral therapy in Fragile X syndrome; top-line data from Phase 2 expected in 2027

Mirum Pharmaceuticals, Inc. (NASDAQ:MIRM), a leading rare disease company, today announced that the first participant has been enrolled in the BLOOM Phase 2 clinical study evaluating MRM-3379 in Fragile X syndrome (FXS).

Caused by a mutation of the FMR1 gene, FXS is the most common inherited form of intellectual disability and autism spectrum disorder and affects an estimated 50,000 males in the U.S. and E.U. There are currently no approved therapies for the treatment of this serious, neurodevelopmental condition.

The BLOOM Phase 2 Clinical Study will evaluate the safety, tolerability, and potential clinical benefit of MRM-3379 in male participants with a confirmed genetic diagnosis of FXS.

"Despite its impact, Fragile X syndrome remains without an approved therapy," said Joanne Quan, M.D., Chief Medical Officer at Mirum. "In preclinical models, MRM-3379 shows improvement in multiple cognitive and behavioral domains associated with Fragile X syndrome, as well as favorable tolerability in healthy volunteers. This study is an important next step in evaluating whether MRM-3379 can meaningfully help improve cognition and daily function for people living with FXS."

Michael Tranfaglia, M.D., Medical Director and Co-Founder of the FRAXA Research Foundation, commented: "It's incredibly encouraging to see a company like Mirum step into the Fragile X space with a potential therapy that holds real promise. This study is actively enrolling, and we encourage eligible families to consider participation in this important research."

Hilary Rosselot, Executive Director of the National Fragile X Foundation, added, "The Fragile X community is energized by the launch of this study. Each new clinical program brings hope and potential breakthroughs. We applaud Mirum's commitment to advancing an innovative therapy that could offer a much-needed option for our families."

For more information about the study or to inquire about enrollment, please visit clinicaltrials.gov and search for study ID [NCT07209462].

About MRM-3379

MRM-3379 is an in-licensed investigational oral therapy being evaluated for the treatment of Fragile X syndrome (FXS). It is a selective phosphodiesterase-4D (PDE4D) inhibitor designed to enhance cAMP signaling. MRM-3379 may offer a novel approach to improving cognition, language, and daily function in individuals with FXS.

The BLOOM Phase 2 clinical study of MRM-3379 is currently underway in FXS. Males ages 16 to 45 will be randomly assigned to receive one of three dose levels of MRM-3379 or placebo for 12 weeks. An open-label cohort of boys ages 13 to 16 will receive the lowest dose, in order to explore effects of treatment in younger boys, closer to the age of diagnosis. The study's primary endpoint is safety and tolerability, the key secondary endpoint is the NIH Toolbox Crystallized Cognition Composite (CCC), and several exploratory endpoints will assess potential effects on mood, behavior, and other symptoms that are relevant to this population.

About Fragile X Syndrome (FXS)

Fragile X syndrome (FXS) is a rare, inherited neurological disorder and the leading known genetic cause of intellectual disability and autism spectrum disorder. FXS is caused by a mutation in the FMR1 gene on the X chromosome, which disrupts neuronal signaling and development. The condition is associated with challenges in learning, behavior, language, and daily living. FXS affects an estimated 50,000 males in the U.S. and E.U. Currently, there are no approved therapies that specifically treat the underlying cause of FXS.

About Mirum Pharmaceuticals, Inc.

Mirum Pharmaceuticals (NASDAQ:MIRM) is a leading rare disease company with a global footprint of approved products and a broad pipeline of investigational medicines. Purpose-built to bring forward breakthrough medicines for people with overlooked conditions, Mirum combines deep expertise with strong connections to the rare disease community. The company's commercial portfolio includes LIVMARLI® (maralixibat) for Alagille syndrome (ALGS) and progressive familial intrahepatic cholestasis (PFIC), CHOLBAM® (cholic acid) for bile-acid synthesis disorders, and CTEXLI® (chenodiol) for cerebrotendinous xanthomatosis (CTX). Mirum's clinical-stage pipeline includes volixibat, an IBAT inhibitor in late-stage development for primary sclerosing cholangitis (PSC) and primary biliary cholangitis (PBC), and MRM-3379, a PDE4D inhibitor being evaluated for Fragile X syndrome (FXS). Mirum's success is driven by a team dedicated to advancing high impact medicines through strategic development, disciplined execution and purposeful collaboration across the rare disease ecosystem. Learn more at www.mirumpharma.com and follow Mirum on Facebook, LinkedIn, Instagram and X.

Forward-Looking Statements

Statements contained in this press release regarding matters that are not historical facts are "forward-looking statements" within the meaning of the Private Securities Litigation Reform Act of 1995. Such forward-looking statements include statements regarding, among other things, the results, enrollment, conduct and progress of the BLOOM Phase 2 clinical study evaluating MRM-3379, including any topline data from this study. Because such statements are subject to risks and uncertainties, actual results may differ materially from those expressed or implied by such forward-looking statements. Words such as "expected," "will," "could," "would," "guidance," "potential," "continue" and similar expressions are intended to identify forward-looking statements. These forward-looking statements are based upon Mirum's current expectations and involve assumptions that may never materialize or may prove to be incorrect. Actual results could differ materially from those anticipated in such forward-looking statements as a result of various risks and uncertainties, which include, without limitation, risks and uncertainties associated with Mirum's business in general, the impact of geopolitical and macroeconomic events, and the other risks described in Mirum's Annual Report for the year ended December 31, 2024, filed with the Securities and Exchange Commission on February 26, 2025, and subsequent filings with the Securities and Exchange Commission, which are available at www.sec.gov. All forward-looking statements contained in this press release speak only as of the date on which they were made and estimates as of such date. Mirum undertakes no obligation to update such statements to reflect events that occur or circumstances that exist after the date on which they were made, except as required by law.

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