ACE-031

ACE-031 is an experimental biologic built as a soluble form of the activin receptor type IIB (ACVR2B), fused to a portion of an antibody to extend its presence in the circulation. It was investigated as a way to block signaling molecules that normally restrain muscle growth, with the hypothesized result of increasing muscle mass and strength.

ACE-031 is investigational and is not an approved medicine in any major jurisdiction. Its clinical development program was stopped, and it is best understood today as a research compound rather than a treatment. Claims that present it as an available therapy overstate its regulatory and evidentiary standing.

ACE-031 is described as a "ligand trap." Rather than acting on cells directly, the soluble receptor is designed to bind circulating proteins before they can reach their natural receptors on muscle tissue. High-level themes discussed in the literature include:

• Sequestering myostatin, a negative regulator of skeletal muscle, so that its growth-limiting signal is reduced.

• Binding related ligands such as activins and other members of the same signaling family, which is thought to broaden its effect but also its off-target footprint.

• Shifting the balance of pathways that govern muscle protein accumulation toward growth in preclinical models.

Because ACVR2B receptors and their ligands are involved in processes beyond muscle, including aspects of blood-vessel biology, the same trapping mechanism that is attractive for muscle is also a plausible source of unintended effects.

ACE-031 was explored primarily in the context of muscle-wasting conditions, with early clinical work discussed in relation to Duchenne muscular dystrophy. The rationale was that trapping myostatin might help preserve or build muscle in a population where muscle loss drives disability.

It has no approved indication. Any appearance of ACE-031 in commercial or wellness-oriented settings is outside the bounds of established medicine, and the gap between the exploratory research premise and a validated treatment remains wide.

ACE-031 is named individually on the WADA Prohibited List. Sub-section S4.3, "Agents preventing activin receptor IIB activation," is organised by mechanism and covers activin A-neutralizing antibodies; activin receptor IIB competitors, with ACE-031 given as the example of a decoy receptor; anti-activin receptor IIB antibodies such as bimagrumab; and myostatin inhibitors, including myostatin-binding proteins such as follistatin and myostatin propeptide, and neutralizing antibodies such as apitegromab, domagrozumab, landogrozumab and stamulumab. Substances in S4.3 are non-Specified, so the default first-offence sanction is four years.

Two published lines of work make this enforceable rather than nominal:

• A validated serum assay exists. A multiplexed method for nine inhibitors of the activin receptor signalling pathways in doping-control serum and plasma — using affinity purification, tryptic digestion and LC-HRMS/MS — reaches a limit of detection of 10 ng/mL for ACE-031 (ramatercept), with diagnostic peptides that separate it from the structurally similar luspatercept (Sakellariou et al., Sci Rep 2025).

• Black-market "ACE-031" is not ACE-031. When 14 products sold as ACE-031 were analysed by gel electrophoresis and protease treatment, only 12 contained any ACVR2B-immunoreactive protein at all, and those contained the full-length human activin receptor IIB rather than the ActRIIB-Fc fusion that ACE-031 actually is — the Fc fragment was absent. The products also carried many additional proteins. The same method detected the black-market material in rat serum for up to 48 hours after administration (Reichel et al., Drug Test Anal 2025).

That second finding cuts both ways for a tested athlete: the vial does not contain the drug it claims to, and it is still detectable, still prohibited, and still an unapproved biologic of unknown composition.

Reported and discussed concerns from its clinical program centered on non-muscle effects of blocking this signaling family, including vascular-related findings such as small areas of bleeding (for example, minor nosebleeds and gum bleeding) and changes in blood vessels and skin, alongside more general effects. These signals contributed to the decision to halt development.

As with any investigational biologic, considerations also include potential immune responses to a fused protein, injection-related effects, and the possibility of consequences that only emerge with longer exposure. Robust, long-term human safety data do not exist.

ACE-031 is a fusion protein engineered for a prolonged circulating presence, so discussions of its pharmacology emphasize how a soluble receptor decoy might distribute and persist relative to the ligands it is meant to trap.

Meaningful pharmacologic reasoning about a ligand trap depends on factors such as how much target ligand is present, how tightly it is bound, and how the fused protein is cleared. These are research questions, not the basis for personal use.

ACE-031 is not a peptide that can be described by a sequence. It is a homodimeric fusion protein: the extracellular domain of human ActRIIB joined to the Fc region of human IgG1. In its clinical trials it was given as a subcutaneous injection every two to four weeks, and its terminal half-life in healthy volunteers was 10–15 days — the Fc portion is what buys that duration (Attie et al., Muscle Nerve 2013).

Three descendants of the same idea show what "formulation" actually means in this class:

• Luspatercept (Reblozyl) — a modified ActRIIB-Fc trap from the same company that reached FDA approval in 2019, supplied as 25 mg and 75 mg lyophilized single-dose vials for subcutaneous injection. Its indication is anemia in beta thalassemia, not muscle (FDA prescribing information). The trap concept produced a real medicine; it was pointed at red blood cells.

• ActRIIB:ALK4-Fc — a heterodimeric redesign built specifically so it would not stably bind BMP9, the vascular ligand implicated in ACE-031's bleeding signal (Li et al., J Clin Invest 2021).

• Products sold as "ACE-031" — of 14 black-market items analysed, only 12 contained any ActRIIB-immunoreactive protein, and those contained full-length human ActRIIB with no Fc fragment, alongside numerous unrelated proteins (Reichel et al., Drug Test Anal 2025). Whatever is in those vials, it is not the molecule this page describes.

The ACE-031 evidence base consists largely of preclinical work and early-phase clinical study, with attention to whether trapping myostatin and related ligands could increase muscle in humans. Early findings generated interest in the myostatin-inhibition strategy as a whole.

The most consequential part of that story is that development was discontinued, with safety observations playing a central role. Because the program did not advance to established efficacy and safety conclusions, claims about ACE-031 should be treated as hypotheses from an incomplete investigation rather than settled results.

For deeper dives on this peptide, see:

• ACE-031 benefits and areas of interest
• ACE-031 side effects and safety context
• ACE-031 dosing concepts (educational)
• ACE-031 research and evidence overview

Why was ACE-031's development stopped? Because of bleeding. The phase 2 trial in ambulatory boys with Duchenne muscular dystrophy was halted after the second dosing regimen "due to potential safety concerns of epistaxis and telangiectasias" — nosebleeds and dilated small blood vessels in the skin. The paper is explicit that these non-muscle adverse events, not lack of effect, ended the program; the muscle signals were actually trending in the right direction (Campbell et al., Muscle Nerve 2017).

What caused the nosebleeds? The leading explanation is that the trap was not selective enough. ActRIIB-Fc also binds bone morphogenetic protein 9 (BMP9), a ligand that maintains blood-vessel quiescence, and sequestering it produces vascular effects. A later redesign, ActRIIB:ALK4-Fc, was built to interact with BMP9 only transiently, and its authors state directly that BMP9 inhibition "might have been responsible for epistaxis and telangiectasia seen previously in a clinical study of ActRIIB-Fc" (Li et al., J Clin Invest 2021). The same broad binding that made ACE-031 powerful is what made it bleed.

Did ACE-031 actually increase muscle in people? Yes, and quickly. In 48 healthy postmenopausal women given a single subcutaneous dose, the 3 mg/kg group showed a 3.3% increase in total-body lean mass by DXA and a 5.1% increase in thigh muscle volume by MRI at day 29 — from one injection (Attie et al., Muscle Nerve 2013). That result is why the compound is still discussed a decade after it was abandoned. It is also why the safety signal mattered so much: the mechanism worked.

Is there an approved myostatin-blocking drug today? Not for muscle. The nearest approval is luspatercept (Reblozyl), a related ActRIIB-Fc trap approved in 2019 for anemia in beta thalassemia (FDA label). Apitegromab, a myostatin-neutralizing antibody for spinal muscular atrophy, has an accepted FDA application with a 30 September 2026 action date and is not approved as of this page's update (Cure SMA). Thirty years after myostatin was identified, no drug is approved to block it for muscle mass.

Is the ACE-031 sold online actually ACE-031? Almost certainly not. When 14 black-market products were analysed, none contained the ActRIIB-Fc fusion protein that defines ACE-031; the 12 that contained anything recognisable held full-length human ActRIIB without the Fc fragment, plus many additional proteins (Reichel et al., Drug Test Anal 2025). Without the Fc region, the long half-life that made ACE-031 work as a fortnightly injection is absent.

Is ACE-031 banned in sport? Yes, by name. The 2026 Prohibited List places it under S4.3, "Agents preventing activin receptor IIB activation," as the worked example of a decoy activin receptor. S4.3 substances are non-Specified and prohibited in and out of competition, so the default first-offence sanction is four years (WADA Prohibited List).