ACE-031
Activin receptor fusion protein · No brand name. Also written ACVR2B-Fc
- UK status
- Unlicensed. Trials halted on safety.
- Availability
- Research use only
- Evidence base
- Human trials halted on safety grounds
- Controlled drug
- No
Sequence
A fusion protein of the activin receptor type IIB and an antibody fragment. Not a short peptide.
The short version
What it does
Sold to remove the brake on muscle growth. Your body actively limits how much muscle you build, and this is meant to block that limit.
How it does it
A protein called myostatin tells muscle to stop growing. This is a decoy that soaks it up before it can deliver the message, along with related signals in the same family.
The catch
This is the one where the trials were stopped. Human studies in boys with muscular dystrophy were halted early because participants developed nosebleeds and visibly dilated blood vessels in the skin.
Everything below goes into the detail, with sources.
Most compounds in this register carry the risk that nobody knows what they do. ACE-031 is different: a proper trial found out, and the answer was bad enough to stop the trial.
At a glance
- Compound type
- Fusion protein, activin receptor type IIB linked to an antibody fragment
- Drug class
- Myostatin and activin pathway inhibitor
- Developed by
- Acceleron Pharma
- Studied for
- Duchenne muscular dystrophy
- Development status
- Discontinued. Trials stopped early on safety grounds.
- Reported reason
- Nosebleeds and dilated blood vessels in the skin
- UK status
- Unlicensed. Supply for human use prohibited.
- In sport
- Prohibited at all times. WADA lists myostatin inhibitors under S4.
01What it is
Myostatin limits muscle growth. Animals and rare humans lacking it develop extraordinary muscle mass, which made blocking it an obvious target for muscle-wasting diseases and, inevitably, for physique.
ACE-031 is not a peptide in the usual sense. It is a fusion protein: the part of the activin receptor type IIB that normally catches myostatin, joined to an antibody fragment so it lasts in the bloodstream. It acts as a decoy, binding myostatin and related signals before they reach real receptors.
The activin receptor does not only handle myostatin. It handles a family of related signalling proteins involved in blood vessel formation and other processes. A decoy that soaks up the family does not confine itself to muscle, and that is the leading explanation for what went wrong.
02How it came about
ACE-031 was developed by Acceleron Pharma and taken into clinical trials for Duchenne muscular dystrophy, a severe inherited muscle-wasting condition affecting boys, where increasing muscle mass would be a genuine benefit.
Trials were stopped early. Reported reasons were non-muscle effects including epistaxis, nosebleeds, and telangiectasia, small dilated blood vessels visible in the skin, consistent with the compound affecting blood vessel signalling. Development for that indication was discontinued.
03UK legal and licensing status
ACE-031 has no marketing authorisation anywhere. Supplying it for human use in the UK is prohibited. It is not a controlled drug. It is sold as a research chemical.
Athletes: WADA prohibits agents affecting myostatin function, including myostatin inhibitors, at all times. Prohibited in and out of competition.
04How it works
It binds myostatin and related members of the same signalling family, preventing them from activating their receptors. With the brake released, muscle mass increases.
Because the same family is involved in blood vessel formation and other processes, the effects are not confined to muscle. That is not speculation: it is the most likely account of the trial findings.
05What it is not
Claims that attach themselves to this compound and do not hold up.
- Not a compound with unknown risks. The risks are partly known, which is worse. Trials found them and stopped.
- Not a peptide. A fusion protein, much larger, engineered for a long half-life.
- Not abandoned for commercial reasons. This one was stopped on safety.
- Not the same as the myostatin gene mutations you have read about. People born without functional myostatin developed that way. Blocking it in an adult is a different intervention with different consequences.
06What the evidence shows
The evidence is a halted clinical programme, and that is more informative than most records here.
Trials in Duchenne muscular dystrophy are conducted in a population with a severe progressive disease, where investigators and families accept meaningful risk because the alternative is grim. For a trial in that setting to be stopped early over nosebleeds and dilated blood vessels tells you those findings were taken seriously.
Anyone selling this for physique is selling a compound whose human trials were halted, to people with no disease at all.
Related molecules in the same pathway have continued in development for other conditions, with different designs aimed at more selective targeting. That work does not validate ACE-031.
Whether any of the vascular effects are reversible, what happens with prolonged exposure in healthy adults, and long-term consequences of suppressing a whole signalling family. None of it was answered, because the programme stopped.
07Reported harms
The documented findings from the halted trials are epistaxis and telangiectasia, consistent with effects on blood vessel signalling. There is no completed safety profile, no approved label and no frequency table.
Because it is a fusion protein rather than a small peptide, immunogenicity, meaning an immune response against the drug, is an additional recognised concern for this class of molecule.
Unregulated supply risks apply in full, and a large engineered protein is considerably harder to manufacture correctly than a short peptide.
08Interactions and cautions
No data. Anyone on anticoagulants or with a bleeding disorder should note that the documented trial findings involved bleeding.
09Stopping
Not documented. Whether vascular changes resolve after stopping was not established.
10Monitoring
Nothing exists. In the trials, participants were monitored by clinicians, which is how the problems were found in the first place.
11Questions worth asking
Take these to a doctor or pharmacist. A good clinician will not mind being asked, and the answers are specific to you in a way nothing on this page can be.
- Do I understand that human trials of this were stopped early on safety grounds?
- Am I taking anything that affects bleeding?
- What would I accept as a reason not to take something, if a halted trial is not one?
12Common questions
Why were ACE-031 trials stopped?
Because of effects outside muscle: nosebleeds and telangiectasia, small dilated blood vessels visible in the skin, consistent with the compound affecting blood vessel signalling. Development for Duchenne muscular dystrophy was discontinued.
Does it build muscle?
Blocking myostatin increases muscle mass, and that pathway is well established. The question was never whether it works, it was what else it does, and the trials answered that.
Is it legal in the UK?
It has no marketing authorisation, so supplying it for human use is prohibited. It is not a controlled drug. It is sold as a research chemical.
Are other myostatin drugs safer?
Other molecules targeting this pathway have continued in development with designs aimed at more selective targeting. Whether any of them prove safe is a separate question, and none of that work makes ACE-031 safe.
13Sources
Links go to the authoritative source. Where a document sits behind a paywall or moves around, the full citation is given so you can find it yourself.
- ACE-031 clinical trial recordsClinicalTrials.gov
- ACE-031 in Duchenne muscular dystrophy, published findingsPubMed
- Prohibited List, S4 hormone and metabolic modulatorsWADA
- Unlicensed medicines guidanceMHRA
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