MRM-3379
An investigational treatment for Fragile X Syndrome.
Drug facts
The same compound appears under different names depending on the context. Here is how to identify MRM-3379 wherever you encounter it, plus the key facts at a glance.
- Generic name
- MRM-3379
- Development codes
- MRM-3379, ENT-3379
- Drug class
- PDE4D inhibitor (small molecule)
- Manufacturer
- Mirum Pharmaceuticals (licensed from Enthorin Therapeutics; discovered at Dart NeuroScience)
- How it's taken
- Capsule by mouth, in a trial only
An investigational capsule taken by mouth, from Mirum Pharmaceuticals, for Fragile X syndrome.
It blocks PDE4D, an enzyme that breaks down a messenger inside brain cells called cAMP[7]. It is in BLOOM, a Phase 2 trial in males that is recruiting at 14 US sites and 3 in Spain, and its main goal is to measure safety[1]. It is not FDA approved, and no results from people with Fragile X have been published.
We'll email you when the FDA acts on MRM-3379 and when new trials for Fragile X Syndrome open. Unsubscribe anytime.
- Trial Alerts, when a new trial opens or a trial changes
- Disease Updates, FDA decisions and condition news, usually 1 or 2 emails a month
We never share your email. Unsubscribe anytime. Privacy policy · Check any health site
Where MRM-3379 fits
No medicine is FDA approved to treat Fragile X syndrome itself. Mirum notes there are no approved therapies aimed at its underlying cause[3]. According to NORD, care today centers on special education and speech, occupational and sensory integration therapy, with psychiatric medicines used for specific symptoms when needed[12]. MRM-3379 is 1 of several targeted approaches in trials, and it has not been compared with any other treatment.
How MRM-3379 works
Fragile X syndrome happens when a stretch of the FMR1 gene called a CGG repeat grows to more than 200 copies, which switches the gene off. Without FMR1, cells stop making FMRP, a protein that controls the making of other proteins and helps nerve cells form connections[11]. Inside brain cells, cAMP is a messenger involved in learning and memory, and the PDE4D enzyme breaks it down.
Mirum says blocking PDE4D may restore cAMP levels and help switch on genes that let nerve cell connections strengthen with use[7]. Mirum describes MRM-3379 as an allosteric inhibitor, meaning it attaches to a spot on the enzyme other than its working site, chosen to act selectively on cAMP signaling[7]. Enthorin, which licensed the drug to Mirum, reports that in mice bred without the Fmr1 gene it improved memory and behavior across several tests[8]. Mouse results do not show whether it helps people.
Clinical trial results
BLOOM (NCT07209462) is a randomized, double-blind, placebo-controlled Phase 2 trial, meaning neither participants nor investigators know who gets the drug, for about 60 males with Fragile X syndrome. Males aged 16 to 45 get 1 of 3 doses or placebo for 12 weeks. A separate open-label group of boys aged 13 to under 16 all take the low dose, which Mirum says is meant to explore effects in younger boys closer to the age of diagnosis[1][3].
Participants need a genetic test showing at least 200 CGG repeats, a weight of at least 30kg, the ability to swallow capsules and to do 2 tests from the NIH Toolbox, a set of computer-based thinking tests, and a caregiver who can come regularly[1]. The registered main measure is side effects through week 12[1]. Mirum names the NIH Toolbox Crystallized Cognition Composite, a combined score for vocabulary and reading, as the key secondary measure, with other measures of mood and behavior[3].
ClinicalTrials.gov lists the trial as recruiting at 14 US sites, with a 15th, in Salt Lake City, not yet recruiting, and at 3 hospitals in Spain. It gives an estimated primary completion date of October 2027[1]. Mirum said on November 3, 2026 that topline data are expected in 2027[4], and its pipeline page says the first half of 2027[7].
Mirum also ran a small Phase 1 study (NCT07712718) in 7 healthy men to track how the body absorbs, breaks down and clears a single dose. ClinicalTrials.gov lists it as completed in July 2026[2], and Mirum has not published results.
Another oral PDE4D inhibitor, zatolmilast (BPN14770), was tested in Fragile X first. In a Phase 2 crossover study of 30 men published in Nature Medicine in 2021, it was well tolerated and improved scores on NIH Toolbox language tests[9]. In a May 13, 2026 letter, Shionogi told the community that its 2 later Phase 3 studies, 1 in adults and 1 in adolescents, did not meet their main goals[10]. Those results do not tell us how MRM-3379 will do, since the 2 are different molecules.
Main registered trial: NCT07209462 on ClinicalTrials.gov. Check it for the current status, sites and contacts before asking about enrollment.
Side effects and safety
No safety results from people with Fragile X have been published. Mirum says the drug showed favorable tolerability in healthy volunteers, without giving numbers or listing side effects[3]. BLOOM's main measure is how often side effects happen, how severe they are and how many people stop because of them over 12 weeks[1]. No clinical hold has been reported.
This is not a complete list of side effects. Talk to your doctor or pharmacist about what to expect and when to seek medical attention.
Taking MRM-3379
Taken by mouth as a capsule. In BLOOM, males aged 16 to 45 are randomly assigned to a low, middle or high dose or a matching placebo capsule for 12 weeks, and boys aged 13 to under 16 take the low dose with no placebo[1]. Mirum has not disclosed the dose amounts.
Development history
Dart NeuroScience scientists discovered the drug, then called ENT-3379, and took it through Phase 1[8]. Mirum announced on November 12, 2024 that it had licensed global rights to MRM-3379 for Fragile X syndrome[6], and it later said it owns worldwide rights[5]. ClinicalTrials.gov lists BLOOM's start as November 22, 2025[1], and Mirum announced the first participant enrolled on December 1, 2025[3]. Mirum's May 6, 2026 report says the FDA has granted Fast Track designation for Fragile X syndrome[5]; it does not give the date.
Explore Fragile X Syndrome trials
Common questions about MRM-3379
▸What is MRM-3379?
An investigational capsule taken by mouth, from Mirum Pharmaceuticals, for Fragile X syndrome. It blocks PDE4D, an enzyme that breaks down a messenger inside brain cells called cAMP[7]. It is in BLOOM, a Phase 2 trial in males that is recruiting at 14 US sites and 3 in Spain, and its main goal is to measure safety[1]. It is not FDA approved, and no results from people with Fragile X have been published.
▸How does MRM-3379 work?
Fragile X syndrome happens when a stretch of the FMR1 gene called a CGG repeat grows to more than 200 copies, which switches the gene off. Without FMR1, cells stop making FMRP, a protein that controls the making of other proteins and helps nerve cells form connections[11]. Inside brain cells, cAMP is a messenger involved in learning and memory, and the PDE4D enzyme breaks it down. Mirum says blocking PDE4D may restore cAMP levels and help switch on genes that let nerve cell connections strengthen with use[7]. Mirum describes MRM-3379 as an allosteric inhibitor, meaning it attaches to a spot on the enzyme other than its working site, chosen to act selectively on cAMP signaling[7]. Enthorin, which licensed the drug to Mirum, reports that in mice bred without the Fmr1 gene it improved memory and behavior across several tests[8]. Mouse results do not show whether it helps people.
▸What are the side effects of MRM-3379?
No safety results from people with Fragile X have been published. Mirum says the drug showed favorable tolerability in healthy volunteers, without giving numbers or listing side effects[3]. BLOOM's main measure is how often side effects happen, how severe they are and how many people stop because of them over 12 weeks[1]. No clinical hold has been reported.
▸How is MRM-3379 taken?
Taken by mouth as a capsule. In BLOOM, males aged 16 to 45 are randomly assigned to a low, middle or high dose or a matching placebo capsule for 12 weeks, and boys aged 13 to under 16 take the low dose with no placebo[1]. Mirum has not disclosed the dose amounts.
▸Is MRM-3379 FDA approved?
MRM-3379 is currently in phase 2 clinical trials for Fragile X Syndrome. It has not yet received FDA approval.
▸Is MRM-3379 FDA approved for Fragile X syndrome?
No. MRM-3379 is investigational and available only in the BLOOM clinical trial.
Phase: Phase 2, recruiting.
Designation: Mirum says the FDA granted Fast Track designation, which can speed review but is not an approval.
Results: Mirum expects topline data in 2027.
Source: Mirum releases, May 6 and November 3, 2026; ClinicalTrials.gov NCT07209462.
▸Who can join the MRM-3379 BLOOM trial for Fragile X?
Males aged 13 to 45 with a genetic Fragile X diagnosis of at least 200 CGG repeats.
Age groups: ages 16 to 45 are randomized to 1 of 3 doses or placebo; ages 13 to under 16 all take the low dose.
Other requirements: weight of at least 30kg, able to swallow capsules, able to do 2 NIH Toolbox tests, and a caregiver who can be there regularly.
Females: the trial enrolls males only.
Where: 14 US sites recruiting, 1 more in Salt Lake City not yet recruiting, and 3 in Spain.
Source: ClinicalTrials.gov NCT07209462, updated September 29, 2026.
▸How is MRM-3379 different from zatolmilast for Fragile X?
Both are oral PDE4D inhibitors, but they are different molecules from different companies, and no trial has compared them.
MRM-3379: Mirum's drug, now in the Phase 2 BLOOM trial with results expected in 2027.
Zatolmilast: Shionogi's drug; Shionogi said in May 2026 that its 2 Phase 3 studies did not meet their main goals.
What it means: zatolmilast's results do not predict how MRM-3379 will do.
Source: Mirum release, November 3, 2026; Shionogi community letter, May 13, 2026; Berry-Kravis et al., Nature Medicine 2021.
Sources and references
Every factual claim on this page is drawn from the public sources listed below. Click any reference to open the original document.
- ClinicalTrials.gov. Study of MRM-3379 in Male Participants With Fragile X Syndrome (BLOOM). https://clinicaltrials.gov/study/NCT07209462
- ClinicalTrials.gov. A Study to Evaluate the Absorption, Distribution, Metabolism, and Excretion of [14C]-MRM-3379. https://clinicaltrials.gov/study/NCT07712718
- Mirum Pharmaceuticals · 2025-12-01. Mirum Pharmaceuticals Announces First Patient Enrolled in the BLOOM Phase 2 Clinical Study Evaluating MRM-3379 in Fragile X Syndrome. https://ir.mirumpharma.com/news/news-details/2025/Mirum-Pharmaceuticals-Announces-First-Patient-Enrolled-in-the-BLOOM-Phase-2-Clinical-Study-Evaluating-MRM-3379-in-Fragile-X-Syndrome/default.aspx
- Mirum Pharmaceuticals (BioSpace wire copy) · 2026-11-03. Mirum Pharmaceuticals Reports Third Quarter 2026 Financial Results and Provides Business Update. https://www.biospace.com/press-releases/mirum-pharmaceuticals-reports-third-quarter-2026-financial-results-and-provides-business-update
- Mirum Pharmaceuticals · 2026-05-06. Mirum Pharmaceuticals Reports First Quarter 2026 Financial Results and Provides Business Update. https://ir.mirumpharma.com/news/news-details/2026/Mirum-Pharmaceuticals-Reports-First-Quarter-2026-Financial-Results-and-Provides-Business-Update/default.aspx
- Mirum Pharmaceuticals (BioSpace wire copy) · 2024-11-12. Mirum Pharmaceuticals Reports Third Quarter 2024 Financial Results and Provides Business Update. https://www.biospace.com/press-releases/mirum-pharmaceuticals-reports-third-quarter-2024-financial-results-and-provides-business-update
- Mirum Pharmaceuticals. Pipeline | Mirum Pharmaceuticals. https://mirumpharma.com/our-science/pipeline/
- Enthorin Therapeutics (Business Wire, BioSpace copy) · 2026-01-12. Enthorin Therapeutics Announces Initiation of Phase II Clinical Trial of MRM-3379 (Formerly ENT-3379) for Fragile X Syndrome by Licensing Partner Mirum Pharmaceuticals. https://www.biospace.com/press-releases/enthorin-therapeutics-announces-initiation-of-phase-ii-clinical-trial-of-mrm-3379-formerly-ent-3379-for-fragile-x-syndrome-by-licensing-partner-mirum-pharmaceuticals
- Nature Medicine · 2021-05. Inhibition of phosphodiesterase-4D in adults with fragile X syndrome: a randomized, placebo-controlled, phase 2 clinical trial. https://pubmed.ncbi.nlm.nih.gov/33927413/
- Shionogi Inc. (posted by FRAXA Research Foundation) · 2026-05-13. EXPERIENCE Update Community Letter. https://www.fraxa.org/wp-content/uploads/2026/05/EXPERIENCE-Update-Community-Letter-Final-FRAXA-05.13.26.pdf
- MedlinePlus, National Library of Medicine. Fragile X syndrome: MedlinePlus Genetics. https://medlineplus.gov/genetics/condition/fragile-x-syndrome/
- National Organization for Rare Disorders · 2026-03-31. Fragile X Syndrome - Symptoms, Causes, Treatment | NORD. https://rarediseases.org/rare-diseases/fragile-x-syndrome/