Myostatin Inhibitors Explained: Drugs, Trials and Status

Myostatin (GDF-8) is a protein that limits muscle growth. Myostatin inhibitors are drugs that block it or the receptors it signals through. One, apitegromab (ISEMBYLD), was approved by the FDA on 2026-09-11 for spinal muscular atrophy [16][17]. Others have failed in muscular dystrophy, sarcopenia and inclusion body myositis trials [9][11][12]. None is approved as a general muscle-building drug.

Key points

• Myostatin is a real brake on muscle mass: mice without it had muscles 2-3 times heavier, and double-muscled cattle, a whippet breed and one German child carry inactive copies [1][2][3][4].
• Drug classes differ in what they block. Some target myostatin only; others block several activin-family ligands, which raises both effect and risk [5].
• The only approval is apitegromab for spinal muscular atrophy (SMA), in people 2 and older already on an SMN2-targeted treatment [16].
• Several programmes raised muscle size on scans but did not improve walking or function in trials [9][11][12].
• Follistatin-344 and ACE-031 sold online are not approved medicines. ACE-031’s own trial was stopped early over nosebleeds and dilated small blood vessels [8].

What myostatin is and how the pathway works

Myostatin is made in skeletal muscle as an inactive precursor. Its propeptide keeps it latent until proteases of the tolloid family cut it. The released protein binds the activin type II receptors (ACVR2A and ACVR2B, mainly ACVR2B), which recruit type I receptors and switch on Smad2 and Smad3 signalling that limits muscle growth [5]. Follistatin, the myostatin propeptide, FLRG and GASP-1 are natural binders that stop myostatin reaching its receptors [5]. Follistatin also binds other family members such as activins [5][6].

This explains the drug designs. A drug can block myostatin activation (apitegromab), bind mature myostatin, block the receptor (bimagrumab), or act as a decoy that soaks up ligands (ACE-031, taldefgrobep alfa). Lee’s 2021 review in the Journal of Clinical Investigation splits them into agents fairly specific to myostatin and agents with broader ligand reach that also block activin A. In healthy adults and patients, the broader group raised thigh muscle volume by 5-9%, against 3-5% for the specific group [5].

Where the idea came from

McPherron, Lawler and Lee (Nature, 1997) disrupted the GDF-8 gene in mice. The animals had individual muscles 2-3 times heavier than normal, through more and larger muscle fibres [1]. The same team found mutations in the myostatin gene of two double-muscled cattle breeds: an 11-nucleotide deletion in Belgian Blue and a missense change in Piedmontese [2].

In dogs, Mosher et al. (PLoS Genetics, 2007) found “bully” whippets carry two copies of a two-base deletion, while dogs with one copy were more muscular and faster in racing than dogs with none [3]. In people, Schuelke et al. (NEJM, 2004) described one German boy with two inactive copies. Ultrasound showed muscles about twice the usual size. His mother, a former professional athlete, carried one copy. The researcher said it was impossible to know whether lacking myostatin could cause problems with age [4].

The drugs, one by one

Drug	What it is	Sponsor	Indication studied	Status found
Apitegromab (ISEMBYLD)	Antibody that blocks myostatin activation [16][22]	Scholar Rock	SMA; obesity (with tirzepatide)	FDA-approved for SMA 2026-09-11 [16][17]
Bimagrumab	Antibody to activin type II receptors [21]	Novartis (RESILIENT), Eli Lilly (BELIEVE)	Inclusion body myositis, sarcopenia, obesity	Investigational; no approval found [9][21]
Trevogrumab, garetosmab	Regeneron antibodies, tested with semaglutide	Regeneron	Weight loss with lean mass preservation	Phase 2; not evaluated by regulators [20]
Taldefgrobep alfa	Engineered protein that binds myostatin [13]	Biohaven (SMA)	DMD, SMA	DMD programme ended 2019; SMA phase 3 missed primary endpoint [13][14]
Domagrozumab	Antibody to myostatin [12]	Not stated in the paper	DMD, limb-girdle dystrophy	No efficacy shown [12]
ACE-031	Activin receptor IIB-Fc decoy [7]	Acceleron [5]	DMD	Stopped early [8]
Follistatin gene therapy	AAV vector carrying FS344 [26]	Academic investigators (sponsor not verified)	Becker dystrophy	Small phase 1/2a only [26]

Apitegromab

Scholar Rock’s antibody blocks the activation of pro- and latent myostatin rather than the mature protein [22]. The FDA approval covers adults and children 2 and older with SMA who are already on an SMN2-targeted treatment. The pivotal trial (NCT05156320) was 52 weeks, randomized, double-blind and placebo-controlled, with 188 participants who could not move or walk independently. In ages 2-12, one dose arm differed from placebo on the Hammersmith Functional Motor Scale Expanded: 34.2% vs 13.5% reached a clinically meaningful improvement. The page reports no significant result for the other dose arm [16].

The FDA label warns of fractures, including serious ones, and possible fetal harm and effects on reproductive function. Common reactions were upper respiratory infections, vomiting, cough, headache and gastroenteritis [16]. The FDA page is dated “current as of” 2026-09-11, and Scholar Rock announced the approval that day [16][17]. In the EU it is not approved. The EMA review was delayed in July 2026 over a US fill-finish site inspection, not over clinical data, and the company resubmitted its application on 2026-10-08 [18][19].

Bimagrumab

Bimagrumab blocks activin type II receptors, so it affects myostatin and activin A signalling [21]. Its earlier trials were mixed:

• Inclusion body myositis (RESILIENT, Lancet Neurology, 2019): 251 people, 52 weeks. Six-minute walk distance did not differ from placebo at any dose (for example +17.6 m at the top dose, p = 0.22). Muscle spasms and diarrhoea were more common on drug. Funded by Novartis [9]. A two-year extension of 211 people found no benefit in mobility [10].
• Sarcopenia (JAMA Network Open, 2020): 180 adults aged 70 and older, 24 weeks. The primary outcome, Short Physical Performance Battery, rose by 1.34 on drug and 1.03 on placebo (p = .13). Walk distance also did not differ significantly (p = .16) [11].

Lee’s review notes an earlier bimagrumab trial in people 65 and older with sarcopenia that did show higher grip strength [5]. The newer obesity work is in the next section.

Trevogrumab and garetosmab

Regeneron’s phase 2 COURAGE trial added these antibodies to semaglutide in adults with a BMI of 30 or more. Per a trade report on the company’s June 2025 interim data (about half of participants had reached week 26), semaglutide alone lost 7.9 lb of lean mass, 34.5% of weight lost. The company said the combinations preserved 50.8% to 80.9% of that lean mass. Discontinuations for adverse events were 4.6% on semaglutide, 10.6% on high-dose trevogrumab and 28.3% in the triplet with garetosmab. Two deaths occurred in the triplet group with no causal link identified by the company [20]. We could not open the Regeneron release itself.

Taldefgrobep alfa

In Duchenne muscular dystrophy (DMD), a phase 2/3 trial passed no futility threshold on the North Star Ambulatory Assessment and the programme ended in 2019 [13]. In SMA, Biohaven’s phase 3 RESILIENT trial (NCT05337553; 269 enrolled; primary completion 2024-09-25) did not separate from placebo on the primary endpoint at week 48, per the company’s 2024-11-25 update. The company said a subgroup showed benefit and that no treatment-related serious adverse events occurred [14][15]. The registry showed no posted results when checked [15].

Domagrozumab

A phase 2 trial in 120 boys with DMD found no difference from placebo in four-stair-climb time (0.27 seconds, p = 0.94) or secondary measures, though muscle volume rose non-significantly. In limb-girdle dystrophy, 19 patients showed no clear benefit [12].

ACE-031 and follistatin gene therapy

ACE-031 is a fusion of the activin receptor IIB and the Fc part of IgG1. In 48 healthy postmenopausal women, one dose raised lean mass by 3.3% (DXA) and thigh muscle volume by 5.1% (MRI) at day 29 in the top group [7]. In boys with DMD, the ascending-dose trial was stopped after the second dosing regimen over potential safety concerns of epistaxis and telangiectasias. There were no serious adverse events, and the 6-minute walk trend favouring drug was not statistically significant [8]. Lee’s review links these effects to the decoy also blocking BMP-9 and BMP-10, which matter for blood vessels; bimagrumab, which blocks only ligands that signal through the receptor, had not shown them [5].

In the follistatin gene therapy trial (Molecular Therapy, 2014), six men with Becker muscular dystrophy received an AAV vector with FS344 in the thigh muscles. Walk distance improved in four of six, and no adverse effects were reported. With six people and no control group, it is a proof of principle only [26].

How GLP-1 combination trials fit in

The newest interest is preserving lean mass during weight loss on GLP-1 drugs. The phase 2 BELIEVE trial of bimagrumab with semaglutide and the EMBRAZE trial of apitegromab with tirzepatide were company-funded, short and measured DXA lean mass, not function [21][22]. We cover them claim by claim in Myostatin inhibitors and GLP-1 drugs: a claim-by-claim check, and the lean-mass background in GLP-1 weight loss and lean mass.

Regulatory status

• United States (checked 2026-11-09): apitegromab is the only myostatin-pathway drug we found approved, for SMA [16]. Bimagrumab, trevogrumab, garetosmab, taldefgrobep alfa and ACE-031 are investigational or discontinued; we found no approval for them.
• European Union: apitegromab is not approved; the application is under review [18][19]. Other drugs were not checked in the EMA register. UK (MHRA) and Germany (BfArM): not checked.
• FDA compounding lists: the FDA’s page on bulk substances with significant safety risks (current as of 2026-04-22) names 14 category 2 substances and 16 nominated-but-withdrawn ones. Follistatin and ACE-031 appear on neither list [23]. Absence is not a finding of legality. The FDA’s category scheme covers substances nominated for the 503A bulks list [24].
• Sport: a Cologne anti-doping lab summary of the WADA list in force from 2026-01-01 places myostatin inhibitors in section S4, naming agents that reduce myostatin expression, myostatin-binding proteins “e.g., follistatin” and neutralising antibodies [25]. We could not open WADA’s own document, so whether ACE-031 and bimagrumab are named in the 2026 text is unverified.

What the research-peptide products are

“Myostatin inhibitor” listings online are usually follistatin-344 (see follistatin-344) and ACE-031 (see ACE-031). Follistatin-344 is a splice variant that becomes the 315-amino-acid form in blood and was the transgene in the gene therapy trial above, which delivered the gene, not the protein [6][26]. ACE-031 is a discontinued drug candidate whose human trial stopped over bleeding signs [8]. We found no approved product containing either as a medicine.

“Research use only” means a seller labels a product for laboratory work, not for people. It does not make the material a medicine, and it carries no evidence of purity, identity or safety. We did not open seller pages, so we make no claim about specific products. No human trial supports a muscle-building use of injected follistatin protein.

Risks and unknowns

• Bleeding and vessels: epistaxis and telangiectasias stopped the ACE-031 DMD trial [8]. The proposed cause, blocking BMP-9 and BMP-10, applies to decoy receptors with wide ligand reach [5].
• Bimagrumab: muscle spasms, diarrhoea and, in BELIEVE, acne and rises in creatine kinase and liver enzymes [9][21].
• Apitegromab: fractures and possible fetal harm per the FDA [16].
• Reproduction: follistatin binds activin and can suppress FSH, a reason for caution flagged in the follistatin literature [6].
• Unknown: long-term effects of lifelong myostatin loss in people (one child, followed briefly) [4], effects in healthy adults, and whether larger muscles on scans mean better function. Trials in muscular dystrophy, sarcopenia and inclusion body myositis suggest often not [9][11][12].

Frequently asked questions

Is there an approved myostatin inhibitor? Yes, one, for a different purpose. The FDA approved apitegromab (ISEMBYLD) for SMA on 2026-09-11 [16][17]. It is not approved for bodybuilding, obesity or ageing.

Why did so many myostatin trials fail? Several raised muscle volume or lean mass but did not improve walking or function: bimagrumab in inclusion body myositis and sarcopenia, domagrozumab in DMD, taldefgrobep alfa in DMD and SMA [9][11][12][13][14]. Why is not settled. The reviews note that picking outcomes for these diseases is hard [5].

Are follistatin-344 and ACE-031 legal or safe? We found neither approved as a medicine. ACE-031’s trial was stopped early over nosebleeds and telangiectasias [8]. Follistatin appears as an example in the WADA S4 myostatin text per a lab summary [25]. We did not assess legality by country.

Do myostatin inhibitors replace GLP-1 muscle-loss care? No such use is approved. The GLP-1 combination data are phase 2, short and company-funded [21][22].

Sources

1. McPherron AC, Lawler AM, Lee SJ, Nature, “Regulation of skeletal muscle mass in mice by a new TGF-beta superfamily member” (1997-05; abstract via Europe PMC). https://europepmc.org/article/MED/9139826
2. McPherron AC, Lee SJ, PNAS, “Double muscling in cattle due to mutations in the myostatin gene” (1997-11; abstract via Europe PMC). https://europepmc.org/article/MED/9356471
3. Mosher DS et al., PLoS Genetics, “A mutation in the myostatin gene increases muscle mass and enhances racing performance in heterozygote dogs” (2007-04-30; abstract via Europe PMC). https://europepmc.org/article/MED/17530926
4. ScienceDaily, report on Schuelke et al., NEJM, “Myostatin mutation associated with gross muscle hypertrophy in a child” (2004-06-24). https://sciencedaily.com/releases/2004/06/040624094836.htm
5. Lee SJ, Journal of Clinical Investigation, “Targeting the myostatin signaling pathway to treat muscle loss and metabolic dysfunction” (2021-05). https://jci.org/articles/view/148372
6. Al-Zaidy SA et al., Journal of Neuromuscular Diseases, “Follistatin Gene Therapy Improves Ambulation in Becker Muscular Dystrophy” (2015-09; abstract via Europe PMC). https://doi.org/10.3233/jnd-150083
7. Attie KM et al., Muscle & Nerve, “A single ascending-dose study of muscle regulator ACE-031 in healthy volunteers” (2012-11-21 online; abstract via Europe PMC). https://doi.org/10.1002/mus.23539
8. Campbell C et al., Muscle & Nerve, “Myostatin inhibitor ACE-031 treatment of ambulatory boys with Duchenne muscular dystrophy” (2017; abstract via Europe PMC). https://doi.org/10.1002/mus.25268
9. Hanna MG et al., Lancet Neurology, “Safety and efficacy of intravenous bimagrumab in inclusion body myositis (RESILIENT)” (2019-09; abstract via Europe PMC). https://doi.org/10.1016/s1474-4422(19)30200-5
10. Amato AA et al., Neurology, “Efficacy and Safety of Bimagrumab in Sporadic Inclusion Body Myositis: Long-term Extension of RESILIENT” (2021-02-17; abstract via Europe PMC). https://doi.org/10.1212/wnl.0000000000011626
11. Rooks D et al., JAMA Network Open, “Bimagrumab vs Optimized Standard of Care for Treatment of Sarcopenia in Community-Dwelling Older Adults” (2020-10-01; abstract via Europe PMC). https://doi.org/10.1001/jamanetworkopen.2020.20836
12. Wagner KR et al., Neuromuscular Disorders, “Randomized phase 2 trial and open-label extension of domagrozumab in Duchenne muscular dystrophy” (2020); Leung DG et al., Muscle & Nerve, domagrozumab in FKRP limb-girdle dystrophy (2021-05-24); abstracts via Europe PMC. https://doi.org/10.1016/j.nmd.2020.05.002 and https://doi.org/10.1002/mus.27259
13. Muntoni F et al., Neurology and Therapy, “The Clinical Development of Taldefgrobep Alfa: An Anti-Myostatin Adnectin for the Treatment of Duchenne Muscular Dystrophy” (2024-01-08; abstract via Europe PMC). https://doi.org/10.1007/s40120-023-00570-w
14. Biohaven, update on taldefgrobep alfa in SMA (2024-11-25; read via a CureSMA copy). https://curesma.org/?p=18665
15. ClinicalTrials.gov, NCT05337553, taldefgrobep alfa in SMA (Biohaven; 269 enrolled; read via API 2026-11-09). https://clinicaltrials.gov/study/NCT05337553
16. FDA, “FDA Approves First Therapy to Target Muscle Loss in Spinal Muscular Atrophy” (content current 2026-09-11). https://www.fda.gov/drugs/news-events-human-drugs/fda-approves-first-therapy-target-muscle-loss-spinal-muscular-atrophy
17. Scholar Rock, “Scholar Rock Announces FDA Approval of ISEMBYLD (apitegromab-mstn)” (2026-09-11; read via a Finviz copy of the Business Wire text). https://finviz.com/news/391334/scholar-rock-announces-fda-approval-of-isembyld-apitegromab-mstn-the-first-and-only-muscle-targeted-treatment-for-children-and-adults-with-spinal-muscular-atrophy-sma
18. Scholar Rock, MAA resubmission to the EMA (2026-10-08; read via a Finviz copy, as in our claim check). https://finviz.com/news/400325/scholar-rock-resubmits-apitegromab-marketing-authorisation-application-maa-to-european-medicines-agency-ema-for-treatment-of-children-and-adults-with-spinal-muscular-atrophy-sma
19. BioPharm International, “EMA review of apitegromab in spinal muscular atrophy delayed” (2026-07-20). https://www.biopharminternational.com/view/ema-review-apitegromab-spinal-muscular-atrophy-delayed
20. Patient Care Online, “Regeneron phase 2b study shows antibodies help preserve lean mass during weight loss with semaglutide” (2025-06-02; trade report of a Regeneron release, which returned HTTP 403). https://www.patientcareonline.com/view/regeneron-phase-2b-study-shows-antibodies-help-preserve-lean-mass-during-weight-loss-with-semaglutide
21. Heymsfield SB et al., Nature Medicine, “Bimagrumab plus semaglutide alone or in combination for the treatment of obesity: a randomized phase 2 trial” (2026-03-02). https://pmc.ncbi.nlm.nih.gov/articles/PMC13004672/
22. Nature Medicine, “Apitegromab for lean mass preservation during tirzepatide-induced weight loss: a randomized, double-blind, placebo-controlled phase 2 trial” (2026-06-08). https://pmc.ncbi.nlm.nih.gov/articles/PMC13375528/
23. FDA, “Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks” (current as of 2026-04-22; read 2026-11-09). https://www.fda.gov/drugs/human-drug-compounding/certain-bulk-drug-substances-use-compounding-may-present-significant-safety-risks
24. FDA, “Bulk Drug Substances Used in Compounding Under Section 503A of the FDC Act” (read 2026-11-09). https://www.fda.gov/drugs/human-drug-compounding/bulk-drug-substances-used-compounding-under-section-503a-fdc-act
25. German Sport University Cologne, Department of Biochemistry, “S4 Hormones and Metabolic Modulators” (reflects WADA 2026 list; read 2026-11-09). https://www.dshs-koeln.de/en/universitaet/organisation/institute-einrichtungen/department-of-biochemistry/doping-substances/prohibited-classes-of-substances-and-methods/s4-hormones-and-metabolic-modulators
26. Mendell JR et al., Molecular Therapy, “A phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy” (2014-10-17; abstract via Europe PMC). https://doi.org/10.1038/mt.2014.200

This article is information, not medical advice.