ACE-031 (international name: ramatercept) is not a peptide in the everyday sense but a large protein made by genetic engineering. It works like a “trap”: it catches myostatin — the body's own brake on muscle growth — in the bloodstream before it can reach the cells. The promise was simple: more muscle without training. In mice and in monkeys that really did happen; in humans very little was ever tested. The clinical programme stopped in 2011, and not for lack of efficacy: boys with muscular dystrophy developed nosebleeds and dilated skin vessels. This compound is not part of the SWISS LAB range, and a separate section below explains why.
What ACE-031 is
ACE-031 is a fusion protein — two different parts stitched into one molecule. The first is the extracellular fragment of the activin receptor type IIB (ActRIIB for short), the “socket” on the cell surface to which myostatin normally attaches. The second is the Fc fragment of human immunoglobulin IgG1, a piece of an antibody that extends the lifetime of the whole construct in the body. The compound was developed by the American company Acceleron Pharma; the literature and the unregulated market mostly use the code ACE-031, while the name ramatercept appears rarely. Nowhere in the world is it an approved medicine.
How it works — myostatin and the “trap” in plain terms
Muscle does not grow without limit, because the body brakes itself. The main brake is myostatin: a protein released by muscle that attaches to the ActRIIB receptor and tells the cell “enough”. ACE-031 is that same receptor, but dissolved in the blood and detached from any cell. It circulates and collects myostatin, so less of it reaches the real receptors on muscle — the brake is partly released.
Here lies the heart of the problem. A free-floating receptor cannot tell the molecules we wanted to catch from those that merely happen to fit it. Besides myostatin, ActRIIB also binds activin A, GDF-11 and — crucially, as it turned out — the protein BMP9, which regulates how blood vessels behave. A myostatin trap is therefore also a trap for a vascular signal.
Structure and origin
The Fc fragment is no ornament: it is what keeps the molecule in the body for weeks rather than minutes. In the study in healthy volunteers the mean half-life was 10-15 days, which is why the trial schedule used subcutaneous injections every two to four weeks. ACE-031 was preceded by a simpler soluble receptor tested in rodents; that line of work grew into a whole family of TGF-beta superfamily “ligand traps”.
What was studied — animals
In mice the effect was striking. In a 2010 paper, eight-week-old C57BL/6 mice received either ACE-031 or vehicle over 28 days. By the end, mean body weight in the treated group was 16% higher than in the controls, and the weights of individual muscles — soleus, plantaris, gastrocnemius and extensor digitorum longus — had risen by 33, 44, 46 and 26% respectively. Fibre cross-sectional area increased both in type I fibres (by 22%) and in type II fibres (by 28%), which differs from selective blockade of myostatin alone, an approach that acts predominantly on type II fibres.
The finding was reproduced in primates. In a paper published in 2026, common marmosets received ACE-031 or vehicle over 14 weeks. Lean body mass in the treated group was significantly greater at the end than at baseline (which was not the case in the vehicle group), the cross-sectional area of both type I and type II fibres in the biceps brachii increased, and muscle tested after the experiment produced more force. It is worth knowing who ran this work: the authors are former employees of Acceleron Pharma, the company that held the patent and no longer exists.
Human data
Human data are scarce and all of it comes from 2008-2011. The first study (published in 2013) enrolled 48 healthy postmenopausal women, randomised 3:1 to a single subcutaneous injection of ACE-031 at doses from 0.02 to 3 mg/kg or to placebo. The protein was generally well tolerated, and the most common adverse event was redness at the injection site. In the highest-dose group, total body lean mass had risen by a mean of 3.3% after 29 days (p = 0.03, measured by DXA) and thigh muscle volume by 5.1% (p = 0.03, by magnetic resonance imaging). Changes that were statistically significant, but small, and after a single dose.
A second phase 1 study in healthy postmenopausal women (70 participants, completed in 2011, number NCT00952887) has no publication indexed in PubMed — its results are not public.
The third and most important study involved ambulatory boys with Duchenne muscular dystrophy. It was randomised, double-blind, placebo-controlled and dose-ascending, and its primary objective was safety. No serious or severe adverse events were recorded, but the study was stopped after the second dosing regimen over safety concerns: epistaxis and telangiectasias (dilated vessels visible through the skin). A trend towards maintenance of the six-minute walk distance was noted against a decline in the placebo group, as were trends towards greater lean mass and bone density and lower fat mass — none of them statistically significant. In the ClinicalTrials.gov registry the core study (NCT01099761) lists 24 participants and a note that it was terminated “based on preliminary safety data”, while the extension (NCT01239758) lists 11 participants and the same note.
Why the programme was stopped — vessels, not muscle
Nosebleeds and telangiectasias are not muscle symptoms. That suggested the trap was catching more than myostatin, and the suspicion is now backed by measurements. A 2021 paper measured exactly what the trap binds in two versions. The version built from two identical parts — which is what ACE-031 is — also bound BMP9, described there as the vascular regulatory ligand. A newer version made of two different receptor fragments bound the ligands that inhibit muscle growth but no longer bound BMP9; the two also had different effects on vessel outgrowth in a cultured retinal explant. In short: the next generation of the trap was designed to avoid BMP9.
The second, independent signal comes from oncology. Dalantercept is a different trap, one that prevents activation of the ALK1 receptor by BMP9 and BMP10. In the first human study it was given to 37 people with advanced cancer: the most common events were peripheral oedema, fatigue and anaemia, the dose-limiting toxicities were oedema and fluid retention, and eight patients developed telangiectasia. So when the BMP9/BMP10 signal is blocked in humans, the very vascular findings that halted the ACE-031 programme appear. One has to be honest about how strong that conclusion is — it is a convergence of two observations, not direct proof. Nobody measured how much BMP9 ACE-031 actually captured in the boys with dystrophy.
Safety and the limits of the evidence
The state of knowledge is quickly summarised: fewer than 150 people in total received this compound in studies, for at most a few months, and the only study in the target population was halted. The 2017 publication does not report what happened to the vascular findings after withdrawal, so it cannot tell us whether they resolve.
The contents of vials on the unregulated market are a separate matter. In 2025 anti-doping laboratories from Austria and Germany examined 14 products sold as ACE-031. Only 12 contained any protein reactive with an antibody against the ActRIIB receptor — and those 12, after mass spectrometry and immunoblotting, turned out to contain the full-length human ActRIIB receptor rather than ACE-031. The absence of the Fc part was confirmed by the failure of the IdeS protease, which recognises that part, to cut the products. The authors themselves judged giving these products to humans to be unethical and ran their detection experiment in rats. ACE-031 is also a substance banned in sport — chapter S4.3 of the 2024 World Anti-Doping Agency list. We deliberately give no methods of use and no doses.
Why it is not part of our range
Four reasons, each sufficient on its own. First: the only study in the group the compound was designed for was stopped over vascular findings and never completed. Second: the mechanism behind those findings has independent human confirmation — blocking the BMP9/BMP10 axis also produced telangiectasia in an oncology study with a different molecule. Third: there is no dose with a demonstrated safety margin in humans, because the programme stalled at phase 2. Fourth: in the only analytical study of products from the unregulated market, none of the 14 contained this molecule, so a buyer has no way of establishing what they are holding. This is a description of the evidence, not a judgement of the reader — but it is not a basis on which a responsible offer can be built.
The wider context — releasing the brake versus pressing the accelerator
Muscle pharmacology has two routes. The first is to add a growth signal: growth hormone and insulin-like growth factor (we cover them in our pieces on somatropin and on IGF-1 LR3). The second is to release the brake, which is what myostatin traps do. The story of ACE-031 shows where that second route trips up: brakes in the body are not assigned to a single function, and a receptor that catches myostatin also catches signals meant for blood vessels. Newer molecules in this family are now designed to bind a narrower set of ligands.
Summary
ACE-031 really does increase muscle mass — unambiguously in mice, reproducibly in marmosets and to a small degree in healthy women after a single dose. Its development stalled not on efficacy but on blood vessels: the study in boys with dystrophy was halted after nosebleeds and telangiectasias, and later work pointed to a plausible mechanism — the trap also binds BMP9. The literature on ACE-031 itself effectively ends in 2017. The evidence lets us say that the compound acts on muscle, but not that it is safe for humans — and it was precisely that asymmetry that ended its development.
Sources
- Campbell C, McMillan HJ, Mah JK, et al. Myostatin inhibitor ACE-031 treatment of ambulatory boys with Duchenne muscular dystrophy: Results of a randomized, placebo-controlled clinical trial. Muscle & Nerve. 2017;55(4):458-464. PMID: 27462804. DOI: 10.1002/mus.25268. pubmed.ncbi.nlm.nih.gov/27462804
- Attie KM, Borgstein NG, Yang Y, et al. A single ascending-dose study of muscle regulator ACE-031 in healthy volunteers. Muscle & Nerve. 2013;47(3):416-423. PMID: 23169607. DOI: 10.1002/mus.23539. pubmed.ncbi.nlm.nih.gov/23169607
- Bendell JC, Gordon MS, Hurwitz HI, et al. Safety, pharmacokinetics, pharmacodynamics, and antitumor activity of dalantercept, an activin receptor-like kinase-1 ligand trap, in patients with advanced cancer. Clinical Cancer Research. 2014;20(2):480-489. PMID: 24173543. DOI: 10.1158/1078-0432.CCR-13-1840. pubmed.ncbi.nlm.nih.gov/24173543
- Li J, Fredericks M, Cannell M, et al. ActRIIB:ALK4-Fc alleviates muscle dysfunction and comorbidities in murine models of neuromuscular disorders. The Journal of Clinical Investigation. 2021;131(4):e138634. PMID: 33586684. DOI: 10.1172/JCI138634. pubmed.ncbi.nlm.nih.gov/33586684
- Cadena SM, Bogdanovich S, Khurana TS, et al. ACE-031, a soluble activin type IIB receptor, increases muscle mass and strength in the common marmoset (Callithrix jacchus). PLoS One. 2026;21(2):e0342666. PMID: 41686840. DOI: 10.1371/journal.pone.0342666. pubmed.ncbi.nlm.nih.gov/41686840
- Cadena SM, Tomkinson KN, Monnell TE, et al. Administration of a soluble activin type IIB receptor promotes skeletal muscle growth independent of fiber type. Journal of Applied Physiology. 2010;109(3):635-642. PMID: 20466801. DOI: 10.1152/japplphysiol.00866.2009. pubmed.ncbi.nlm.nih.gov/20466801
- Reichel C, Filip T, Gmeiner G, Thevis M. Gel Electrophoretic Detection of Black Market ACE-031. Drug Testing and Analysis. 2025;17(10):1934-1946. PMID: 40312924. DOI: 10.1002/dta.3898. pubmed.ncbi.nlm.nih.gov/40312924
For in-vitro laboratory research only. It is not a human medicine and is not for treatment.