ACE-031 (ramatercept) is a lab-made protein that soaks up myostatin and related signals that normally limit muscle growth. In a 2013 study of healthy women, a single injection increased lean body mass, but a trial in boys with Duchenne muscular dystrophy was stopped early because of nosebleeds and small dilated blood vessels.[1][2] Its developer discontinued it in 2013, and it was never approved anywhere.[4]
This profile covers what ACE-031 is, what the human and animal studies found, side effects, the doses that were tested, how it compares with related drugs that did reach approval, and its legal and anti-doping status.
What is ACE-031?
ACE-031 is a “decoy receptor”. It is made from the outer part of the activin receptor type IIB, the docking site that myostatin and similar growth factors use on muscle cells, fused to the stem (Fc) part of a human antibody.[2] Because it circulates in the blood and grabs those growth factors before they reach muscle, it releases a brake on muscle growth.[1]
It is important to know that ACE-031 is not a short peptide. It is a large, two-chain fusion protein.[7] Under FDA regulations, any amino acid chain longer than 40 amino acids is a protein, which is regulated as a biological product.[14] It is often sold alongside peptides, but it is a different kind of drug.
ACE-031 was developed by Acceleron Pharma in Cambridge, Massachusetts. Shire licensed rights outside North America in 2010.[4] Acceleron is now a subsidiary of Merck.[3]
What the research shows
In animals
- Mice: 28 days of ACE-031 made young mice 16% heavier than controls, and individual leg muscles grew 26% to 46%. Both slow-twitch and fast-twitch fibers got bigger.[5]
- Primates: in a 14-week study in marmosets, treated animals gained lean mass, had larger muscle fibers in the arm and produced more force from leg muscle tested in the lab. Several of its authors were Acceleron scientists, and it was published in 2026.[6]
In people
Three Acceleron-sponsored studies enrolled about 140 people.[1][15][3]
- Single-dose study in healthy women. 48 healthy postmenopausal women received one injection (0.02 to 3 mg/kg) or placebo. At the highest dose, total lean mass rose 3.3% and thigh muscle volume rose 5.1% after 29 days. Blood markers suggested effects on bone and fat metabolism.[1]
- Multiple-dose study in healthy women. A second study enrolled 70 women and tested two to seven doses over one to three months. It was completed in 2011, but we could not find published results.[15]
- Duchenne muscular dystrophy trial. 24 boys who could still walk received ACE-031 (0.5 mg/kg every 4 weeks or 1 mg/kg every 2 weeks) or placebo, on top of steroids.[3] The trial was stopped after the second dose group because of safety concerns. Boys on ACE-031 tended to hold their 6-minute walk distance while the placebo group declined, and they tended to gain lean mass and bone density and lose fat, but none of these differences was statistically significant.[2] An extension study with 11 boys was also ended early.[18]
Why ACE-031 was discontinued
In 2011, the Duchenne program was put on clinical hold after early safety findings in patients. Acceleron and Shire then ran more animal toxicology studies. In April 2013 they concluded that the risk-benefit profile was not appropriate for the intended patients, boys aged four and older, and stopped development. Shire’s partnership ended on June 30, 2013, and Acceleron said it had no plans to continue.[4]
ACE-031 side effects
Results posted for the Duchenne trial show these side effects:[3]
| Side effect | 0.5 mg/kg every 4 weeks (9 boys) | 1 mg/kg every 2 weeks (9 boys) | Placebo (6 boys) |
|---|---|---|---|
| Nosebleeds (epistaxis) | 1 | 5 | 0 |
| Telangiectasia (small dilated blood vessels) | 0 | 5 | 0 |
| Injection-site redness | 3 | 6 | 3 |
| Headache | 1 | 3 | 1 |
There were no serious or severe adverse events.[2] In healthy women, the drug was generally well tolerated; reported side effects included injection-site redness.[1]
Similar side effects appear with sotatercept, an approved drug that also blocks activin signaling. In its main trial, nosebleeds occurred in 22.1% of patients versus 1.9% on placebo, and telangiectasia in 16.6% versus 4.4%.[9] This suggests the blood vessel effects come from blocking this pathway, not from one particular product.
Risks of products sold online
ACE-031 is sold online as a “research chemical”. Austrian and German anti-doping scientists tested 14 such products. Two had no detectable receptor protein. The other 12 contained the full-length natural receptor protein instead of ACE-031, along with many other proteins. None contained the actual drug.[7]
ACE-031 dosage: what was studied
No dose of ACE-031 is established as safe or effective. These are the doses used in research:
| Setting | Dose |
|---|---|
| Healthy women, single dose | 0.02 to 3 mg/kg under the skin[1] |
| Duchenne trial | 0.5 mg/kg every 4 weeks or 1 mg/kg every 2 weeks, under the skin[3] |
Its half-life in people was about 10 to 15 days, so one dose stays in the body for weeks.[1]
ACE-031 vs bimagrumab, apitegromab and sotatercept
Several drugs block the same pathway in different ways, and some have now reached approval:
- Apitegromab (Isembyld) is an antibody that blocks only myostatin activation. FDA approved it in September 2026 for spinal muscular atrophy in adults and children aged 2 and older who are already on SMN2-targeted treatment.[11]
- Sotatercept (Winrevair) is a similar fusion protein built on the type IIA receptor. FDA approved it in March 2024 for pulmonary arterial hypertension, not for muscle.[10][9]
- Luspatercept (Reblozyl) is a modified type IIB receptor fusion protein from Acceleron, the same company that made ACE-031. It was approved in 2019 to treat anemia.[17][19]
- Bimagrumab is an antibody that blocks the type II activin receptors themselves. In a phase 2 trial in people with type 2 diabetes and obesity, it reduced fat mass by 20.5% and increased lean mass by 3.6% over 48 weeks.[12] In a 2026 trial of 507 adults with obesity, it was tested alone and with semaglutide; the high-dose combination produced the most weight loss.[13] It is not approved.
ACE-031 differs from apitegromab mainly in its reach. It traps a wider range of growth factors than myostatin alone.[1] That broad reach may build more muscle, but it also affects blood vessels.
People looking at ACE-031 for muscle often compare it with MK-677, a growth hormone booster, or with other experimental compounds like SLU-PP-332 and 5-Amino-1MQ. Each works differently, and none is approved for building muscle.
Is ACE-031 legal or FDA approved?
- Not FDA-approved. ACE-031 was never approved in any country, and development stopped in 2013.[4]
- Regulated as a biologic. Because it is a protein of more than 40 amino acids, FDA would treat it as a biological product, not a simple peptide drug.[14]
- Banned in sport. WADA lists ACE-031 by name as a decoy activin receptor under S4.3, “agents preventing activin receptor IIB activation”. It is prohibited at all times.[8] German anti-doping researchers flagged it as a possible doping risk as early as 2016,[16] and a lab method has detected black-market versions in rat blood samples.[7]
We did not find trustworthy, non-vendor user reports about ACE-031, so this profile does not include a user-experience section.
This is general information, not legal advice.
