Myostatin inhibitors reached a milestone in 2026: on September 11, FDA approved apitegromab (Isembyld), an antibody that blocks myostatin activation, as the first muscle-targeted treatment for spinal muscular atrophy (SMA).[5] In its phase 3 trial it improved motor function by a modest 1.8 points on a standard scale versus placebo.[4] No myostatin or follistatin drug is approved for building muscle in healthy people, and follistatin itself has only been tested in humans as an experimental gene therapy in a dozen patients.[9][10]
This page reviews the key studies, from the first myostatin mice to today’s obesity trials. For related compounds, see our muscle and metabolism peptides hub.
Bottom line
| Approach | Example | Best human evidence | Status (Oct 2026) |
|---|---|---|---|
| Anti-myostatin antibody | Apitegromab | Phase 3 SAPPHIRE, 188 patients[4] | FDA-approved for SMA[5] |
| Activin receptor antibody | Bimagrumab | Phase 2 trials, including 507 adults with obesity[7] | Investigational |
| Decoy receptor | ACE-031 | Small trials; Duchenne trial stopped for safety | Discontinued in 2013 |
| Follistatin gene therapy | AAV1-FS344 | Two open-label trials, 6 patients each[9][10] | Experimental |
| Early antibody | MYO-029 | Phase 1/2, 116 adults; no strength gains[3] | Abandoned |
Myostatin and follistatin, explained
Myostatin is a protein that acts as a brake on muscle growth. In 1997, researchers showed that mice lacking the myostatin gene had dramatically larger muscles: individual muscles weighed two to three times more than in normal mice.[1] In 2004, doctors described a German boy born with a mutation that disabled both copies of his myostatin gene. He was unusually muscular from birth, confirming that myostatin limits muscle in humans too.[2]
Follistatin is a natural protein that binds myostatin and related signals (such as activin) and stops them working. Blocking this pathway, by any method, is the idea behind every drug on this page.
The key studies
MYO-029 (2008). The first big test of a myostatin antibody enrolled 116 adults with three types of muscular dystrophy. It was reasonably safe apart from skin reactions at high doses, but there was no improvement in strength or function. The trial was not designed to detect benefits, and only a trend toward bigger muscles was seen.[3]
Apitegromab, SAPPHIRE (2025). This phase 3 trial enrolled 188 people aged 2 to 21 with nonambulatory SMA who were already taking nusinersen or risdiplam. In the main group, aged 2 to 12, motor function scores rose 0.6 points with apitegromab versus a 1.2-point decline with placebo over 12 months, a difference of 1.8 points. The 20 mg/kg dose alone did not reach statistical significance. Side effects were similar to placebo.[4]
Apitegromab approval (2026). FDA approved the drug for SMA patients aged 2 and older who are on an SMN2-targeted treatment, at a recommended dose of 10 mg/kg. The company reported that 34.2% of patients on that dose gained at least 3 points, versus 13.5% on placebo.[5]
Bimagrumab in diabetes and obesity (2021). In 75 adults with type 2 diabetes and obesity, 48 weeks of bimagrumab infusions reduced body fat and increased lean mass compared with placebo.[6]
Bimagrumab plus semaglutide, BELIEVE (2026). In 507 adults with obesity, bimagrumab was tested alone, with semaglutide and against placebo for 48 weeks. The high-dose combination produced the most weight loss. Muscle spasms, diarrhea and acne were common with bimagrumab.[7] A 2026 meta-analysis of bimagrumab trials found it increased lean mass by about 1.66 kg, but it also raised LDL cholesterol and led more people to stop treatment.[8]
Follistatin gene therapy, Becker muscular dystrophy (2015). Six men with Becker muscular dystrophy received injections into both thigh muscles of a virus carrying the follistatin-344 gene. Four of six walked farther on a 6-minute walk test (29 to 125 meters); two showed no change. No adverse effects were reported, and biopsies showed less scarring.[9]
Follistatin gene therapy, inclusion body myositis (2017). Six people with this muscle disease received the same approach plus an exercise program. Walking distance improved 56 meters per year versus a 25.8-meter decline in 8 untreated comparison patients.[10] A Harvard neurologist argued in the same journal that the claimed functional benefits were unfounded, partly because the comparison group wasn’t randomized.[11]
Established vs preliminary
Established:
- Myostatin limits muscle growth in mice and humans.[1][2]
- Blocking the pathway increases muscle mass in people.[8]
- Apitegromab modestly improves motor function in SMA when added to standard treatment.[4]
Preliminary:
- Whether extra muscle translates into meaningful strength in healthy, older or obese adults.
- Whether combining these drugs with GLP-1 medicines protects muscle in the long run.
- Long-term safety, especially blood-vessel effects seen with broader blockers like ACE-031.
Why it took so long
Myostatin was discovered in 1997, yet the first approval came almost 30 years later. Early antibodies like MYO-029 built some muscle but failed to improve strength in muscular dystrophy.[3] Broader blockers such as ACE-031 built more muscle but caused nosebleeds and small dilated blood vessels, because the same signaling pathway also helps maintain blood vessels. Apitegromab succeeded partly by targeting only the inactive precursor form of myostatin and by being tested as an add-on to drugs that already treat the underlying cause of SMA.[4]
Limitations
- Muscle mass isn’t function. Many trials showed bigger muscles but no clear strength gains.[3]
- Small, open-label gene therapy trials. Without randomization, improvement could reflect exercise or chance.[11]
- Gray-market products. A 2026 sports medicine review lists follistatin-344 among unapproved peptides marketed directly to patients, with scarce human safety data.[12]
What’s next
The most watched question is muscle preservation during weight loss. A 252-person Lilly phase 2 trial testing bimagrumab with tirzepatide reached its primary completion date in January 2026 and was listed as active but not recruiting in October 2026; full results had not been posted.[13] Our GLP-1 muscle loss guide explains why this matters and what already helps, mainly resistance training and adequate protein.
For more on muscle-related compounds, see peptide hormones and exercise, IGF-1 LR3 vs MGF and peptides vs SARMs. Myostatin inhibitors are banned in sport at all times; our WADA prohibited list guide has details.
Anyone with a muscle-wasting condition should discuss these treatments, and relevant clinical trials, with a neuromuscular specialist.
