Mirum Pharmaceuticals announced the start of the BLOOM Phase II study of MRM‑3379, a brain‑penetrant phosphodiesterase‑4D inhibitor, targeting cognition and behavior in Fragile X syndrome. The announcement was made on January 9, 2026, marking the first human evaluation of the compound.
MRM‑3379 was discovered by Dart Neuroscience, advanced through a Phase 1 safety study by Enthorin Therapeutics, and licensed to Mirum in October 2025 for an upfront payment of $7.5 million, with milestone payments up to $217.5 million and royalties on future sales. The Phase 1 trial demonstrated a favorable safety profile and dose‑dependent improvements in preclinical models.
The BLOOM study will enroll male patients with Fragile X syndrome, randomizing them to 12 weeks of MRM‑3379 or placebo. Primary endpoints focus on safety and tolerability, while secondary endpoints assess changes in cognitive function and adaptive behavior. Top‑line data are expected in 2027.
Fragile X syndrome affects roughly 1 in 4,000 males and 1 in 8,000 females worldwide, representing an orphan market with no approved disease‑modifying therapies. The launch of a Phase 2 program positions Mirum to diversify beyond its cholestatic liver disease portfolio and tap a sizable unmet need.
The trial launch signals Mirum’s strategic pivot toward neurodevelopmental disorders, leveraging its rare‑disease commercialization expertise. Successful outcomes could open a new revenue stream and broaden the company’s product pipeline, potentially accelerating future earnings growth.
CEO Chris Peetz said the company is “well positioned heading into 2026 with strong commercial momentum and multiple upcoming trial readouts.” Chief Medical Officer Joanne Quan highlighted that preclinical data show “improvement in multiple cognitive and behavioral domains” and that the study will evaluate whether MRM‑3379 can meaningfully improve daily function for people living with FXS.
Following the announcement, analysts noted the positive sentiment driven by the strategic diversification and the potential for a first‑in‑class therapy in a disease with no approved treatments. The market reaction was amplified by institutional buying and the company’s recent revenue growth from its core liver‑disease assets.
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