ACE-031 looked like the muscle drug bodybuilders had been waiting for. A single subcutaneous dose, lean mass climbs without training, no androgenic side effects. Then the Phase 2 trial was halted because patients started bleeding. The story of why is the most important thing about ACE-031 to understand before deciding what to do with it.
🔑 Key Takeaways
- ACE-031 (also called Ramatercept) is not a peptide in the strict sense. It is a fusion protein: the extracellular domain of the activin receptor type IIB (ActRIIB) fused to a human IgG1 Fc. It works as a decoy receptor that traps myostatin and related muscle-growth inhibitors in circulation before they can reach your muscles.
- The decoy mechanism produces meaningful lean mass gains. Phase 1 in healthy postmenopausal women showed 3.3% lean body mass and 5.1% quadriceps volume gains from a single 250 mg dose at 29 days, with no exercise.
- The same decoy traps BMP-9 and BMP-10, which are required for blood vessel maintenance. Trapping them produced epistaxis (nosebleeds), gingival bleeding, and skin telangiectasia in Phase 2 trial subjects, mimicking a genetic disease called hereditary hemorrhagic telangiectasia (HHT).
- Acceleron Pharma halted the Phase 2 DMD trial in 2013 because of these vascular bleeding events. The drug was not formally pulled, but development was abandoned. Acceleron later pivoted to selective myostatin inhibitors (apitegromab) that avoid BMP binding.
- Half-life is 10 to 15 days due to the Fc fusion. Clinical dosing was 1 to 3 mg/kg every 2 to 4 weeks subcutaneous. Underground community dosing is undocumented and varies wildly.
- Side effect profile in trials: epistaxis (10 to 30%), telangiectasia (visible vascular changes), gum bleeding, FSH reduction in women, injection-site reactions. The bleeding events are dose-dependent and dose-frequency dependent.
- Compared to other myostatin inhibitors, ACE-031 has the cleanest "lean mass gained" record but the worst safety profile because it is the least selective. Apitegromab (Phase 3 for SMA), bimagrumab (mostly halted), and follistatin-based candidates all narrow the binding profile to avoid the bleeding issue.
- If the goal is muscle growth without androgens, peptides that work through the GH axis (CJC-1295 + ipamorelin, sermorelin) deliver more modest but safer gains than ACE-031, and they are far more practical to source.
This page covers what ACE-031 actually is, the muscle data from the trials that did report results, the dosing protocol from clinical use, the side effect picture in detail, why Acceleron halted the program, and how it compares to other myostatin inhibitors and to the GH peptide stack many users actually want.
What Is ACE-031?
A decoy receptor, not a typical peptide.
ACE-031 (development code) is also called Ramatercept (formal name). Acceleron Pharma developed it starting in the late 2000s as a treatment for muscle-wasting diseases. It is structurally distinct from the small peptides most readers associate with this category. Rather than a 7 to 30 amino acid signaling molecule, ACE-031 is a fusion protein: it links the soluble extracellular ligand-binding domain of ActRIIB (the activin receptor type IIB) to a human IgG1 Fc. The Fc fusion gives the drug a long half-life (10 to 15 days) and lets it circulate in the blood as a sponge that grabs muscle-suppressing ligands before they reach their actual receptors.
ACE-031 at a Glance
- Class: Fusion protein (ActRIIB-Fc), not a small peptide
- Other name: Ramatercept (Acceleron Pharma development code)
- Mechanism: Decoy receptor for myostatin (GDF-8), activin A, activin B, GDF-11, BMP-9, BMP-10
- Half-life: 10 to 15 days (Fc fusion extends circulation)
- Route: Subcutaneous injection
- Original developer: Acceleron Pharma (now subsidiary of Merck)
- Trial status: Phase 1 completed (postmenopausal women, lean mass), Phase 2 halted (DMD boys, vascular bleeding)
- Regulatory status: Not approved. No active development as of 2026
How ACE-031 Works
It traps myostatin in the bloodstream before it gets to muscle.
Myostatin (also called GDF-8, growth differentiation factor 8) is a muscle-growth brake your body produces in muscle tissue. It exists to prevent uncontrolled muscle growth. When myostatin binds to its receptor (ActRIIB on muscle cells), it activates the Smad2/3 signaling pathway, which suppresses protein synthesis and limits muscle hypertrophy. Genetic disruption of myostatin produces dramatically muscled cattle ("double-muscled" Belgian Blues) and the rare human cases like the famous "Liam Hoekstra" myostatin-related hypermuscular phenotype.
ACE-031 is the soluble extracellular portion of that ActRIIB receptor. Injected into circulation, it binds myostatin (and related ligands) in the blood, where they cannot reach the actual receptors on muscle cells. The result is the same as deleting myostatin: the muscle-growth brake comes off.
The off-target problem (and why it matters)
ActRIIB does not bind myostatin alone. It is a "promiscuous" receptor that binds multiple TGF-beta family ligands:
- Myostatin (GDF-8) — primary muscle target, the intended one
- Activin A — reproductive and immune signaling
- Activin B — pituitary FSH regulation
- GDF-11 — aging and tissue regeneration signaling
- BMP-9 — vascular endothelium maintenance, critical for blood vessel integrity
- BMP-10 — vascular endothelium maintenance, similar
The first three are the muscle-and-hormone effects. The last two are the safety problem. Trapping BMP-9 and BMP-10 disrupts vascular endothelial maintenance, causing blood vessel walls to weaken in a pattern that looks exactly like an inherited disease called hereditary hemorrhagic telangiectasia (HHT). HHT patients are born with mutations in the BMP-9/10 signaling pathway and have lifelong nosebleeds, gum bleeding, and visible vascular malformations (telangiectasia) in skin and mucous membranes. ACE-031 chemically reproduced HHT in trial subjects.
ACE-031 Benefits
What the trial data actually showed.
Lean mass gains in healthy adults (Phase 1)
In a Phase 1 study of healthy postmenopausal women given a single 250 mg subcutaneous dose, ACE-031 produced 3.3% lean body mass gains and 5.1% quadriceps muscle volume gains by day 29. The participants did not exercise. The drug alone produced these gains. Compared to typical peptides that drive 0.5 to 1% body composition shift over weeks, ACE-031 effects are roughly 5 to 10 times larger per unit time. Bone density also increased (about 3.4% lumbar spine), which is a meaningful side benefit and a contrast to follistatin (which can decrease bone density via GDF-11 inhibition).
Muscle effects in DMD boys (Phase 2, halted early)
The Phase 2 trial in Duchenne muscular dystrophy (DMD) boys aged 4+ used 1 mg/kg escalating to 3 mg/kg dosing. The trial was halted at 12 weeks. Before halting, lean mass gains of 3 to 5% and thigh muscle volume gains of about 3.1% over placebo were observed. This is consistent with the Phase 1 mechanism, but the trial was too short to assess functional improvement (walking, falls, fatigue), which were the primary endpoints.
Bone density
An unexpected benefit. The lumbar spine bone density increase of about 3.4% in Phase 1 caught attention because it suggested ActRIIB inhibition has a bone-anabolic effect, not just a muscle-anabolic one. This is opposite to follistatin, which through GDF-11 inhibition can reduce bone formation. ACE-031's bone profile is one of its potentially distinctive features.
What the trial data did not show
- Functional muscle improvement in DMD boys (trial halted before primary endpoint)
- Performance benefits in healthy athletes (no athletic trial was conducted)
- Long-term safety at any dose (longest exposure was 12 weeks)
- Reversibility of muscle gains after stopping (no sustained-effect data)
ACE-031 Dosage
Clinical doses are documented. Community doses are not.
| Setting | Dose | Frequency | Notes |
|---|---|---|---|
| Phase 1 (healthy women) | 250 mg flat (single dose) | Once | Lean mass +3.3% at 29 days. Subcutaneous. |
| Phase 2 (DMD boys) | 1 mg/kg, escalated to 3 mg/kg | Every 2 to 4 weeks | Halted at 12 weeks for safety. |
| Underground community | Highly variable, often 100 to 500 mcg per dose | Weekly to biweekly | No published dose-response data. Most reports are anecdotal. |
Why the community dose is so different from the clinical dose
Two reasons. First, ACE-031 is expensive: at typical underground vendor pricing of $80 to $150 per mg, a single 250 mg clinical dose would cost $20,000 to $40,000. Few users can afford it. Second, the bleeding side effects are dose-dependent. Lower community doses (100 to 500 mcg) reduce both cost and side-effect risk, but they also produce minimal documented muscle effect at those doses. The honest framing: at the doses most users can actually access and afford, ACE-031 may not produce measurable benefit, while still carrying tail risk of vascular events.
Half-life and dosing frequency
The 10 to 15 day half-life means weekly or biweekly dosing produces accumulating drug levels. This is unlike short peptides where each dose is largely cleared before the next. Once you start ACE-031, drug levels build over the first 4 to 6 weeks and remain elevated for weeks after stopping. This matters because side effects can emerge after stopping, when accumulated drug is still in circulation.
ACE-031 Side Effects
The ones that halted Acceleron's program.
| Side effect | Frequency in trials | Mechanism | Severity |
|---|---|---|---|
| Epistaxis (nosebleeds) | 10 to 30% in Phase 2 | BMP-9/10 trapping, vascular fragility | Mild to moderate, but persistent and recurrent |
| Gingival bleeding | Common in Phase 2 | Same vascular mechanism | Mild but unsettling |
| Telangiectasia (skin) | Variable, dose-dependent | Capillary dilation | Cosmetic and persistent |
| FSH suppression (women) | Documented in Phase 1 | Activin B trapping | Hormonal, partially reversible |
| Injection site reactions | Common | Subcutaneous injection | Mild, self-limited |
| Flu-like symptoms | Reported in community use | Systemic protein dosing | Mild, transient |
The HHT phenocopy explained
Hereditary hemorrhagic telangiectasia is an inherited disease caused by mutations in genes that encode parts of the BMP-9/10 receptor system (ENG, ALK1, MADH4). HHT patients have weakened blood vessel walls, recurrent nosebleeds from childhood, vascular malformations in skin and lungs, and elevated risk of internal hemorrhage. ACE-031 reproduces this phenotype pharmacologically. Subjects in the Phase 2 trial were not genetically HHT, but they developed the symptoms because their BMP-9/10 signaling was being chemically blocked by the decoy receptor.
This is why Acceleron halted the trial. It was not a single severe adverse event. It was the realization that the side-effect mechanism was inherent to the drug's design, not a dosing artifact, and that no realistic dose adjustment would resolve it.
Who should not use ACE-031
- Anyone with personal or family history of bleeding disorders
- Anyone with known HHT or BMP-9/10 pathway mutations
- Anyone on anticoagulation (warfarin, apixaban, rivaroxaban, dabigatran)
- Anyone with active gastrointestinal ulcers or vascular malformations
- Pregnancy or breastfeeding (no data, avoid)
- Anyone with cardiovascular disease or stroke history
- Younger users (especially adolescents, given the DMD trial was specifically halted in this population)
ACE-031 vs Other Myostatin Inhibitors
The category exists. The selectivity matters.
| Compound | Selectivity | Status | Notes |
|---|---|---|---|
| ACE-031 (Ramatercept) | Pan-ActRIIB ligand trap (broad) | Halted 2013 | Largest muscle effect, worst bleeding profile |
| Apitegromab | Selective for latent myostatin only | Phase 3 for SMA | No off-target BMP-9/10 effect, much cleaner safety |
| Bimagrumab | ActRIIB antibody, broader binding | Mostly halted | Tested in obesity, sarcopenia. Some bleeding observed. |
| Trevogrumab | Selective myostatin antibody | Phase 2/3 | Cleaner profile than ACE-031 |
| Follistatin 344/315 | Endogenous activin/myostatin binder | Underground use | Bone density loss concern, no clinical approval |
| SARMs (ostarine, ligandrol) | Androgen receptor selective | Underground use | Different mechanism (anabolic), cardiovascular and liver concerns |
The comparison with what actually works in practice
For users seeking lean muscle gain without androgens, the practical and safer alternatives are growth hormone-releasing peptides:
- CJC-1295 + ipamorelin stack (the standard "GH pulse" combo). Drives endogenous GH release, which raises IGF-1, which promotes muscle protein synthesis. Smaller per-month effect than ACE-031 (1 to 3% body composition shift over 8 to 12 weeks vs ACE-031's 3 to 5% in 4 weeks), but with a fraction of the safety risk. See our CJC-1295 vs ipamorelin guide and ipamorelin + CJC-1295 dosage guide.
- Sermorelin: pure GHRH analog, slower acting but longer safety record. See our does sermorelin work guide.
- Tesamorelin: stronger GHRH analog with visceral fat data. See our tesamorelin guide.
Why Acceleron Halted ACE-031 (And What It Means)
The decision was scientific, not commercial.
Acceleron had a viable Phase 1 efficacy signal (lean mass gains in healthy adults). They had a real medical need (Duchenne muscular dystrophy, where any muscle preservation matters enormously). They had a viable commercial path. What they did not have was a way to remove the BMP-9/10 binding without removing the myostatin binding, because both ligands bind the same domain of ActRIIB. The decoy mechanism that worked for muscle was inseparable from the mechanism that broke blood vessels.
Acceleron pivoted. Their later compound apitegromab targets latent (precursor) myostatin specifically, before activation, which avoids the BMP-9/10 issue entirely. Apitegromab is now in Phase 3 for spinal muscular atrophy (SMA) and shows the cleaner safety profile that ACE-031 could not achieve. The lesson the field took from ACE-031 is that selectivity matters: a pan-ligand trap looks like a feature for efficacy and turns out to be a fatal flaw for safety.
Where to Buy ACE-031 (And Why Most Sources Are Sketchy)
This section is honest about a hard sourcing problem.
ACE-031 was never commercialized. Acceleron stopped manufacturing after the Phase 2 halt. The drug exists today only as research-grade material made by various peptide synthesis labs and sold by online vendors as an unapproved compound. Quality is highly variable. The molecule itself (an Fc fusion protein) is harder to manufacture cleanly than a typical 7 to 30 amino acid peptide. Many "ACE-031" products on the market are misidentified, low-purity, or in some cases entirely unrelated peptides relabeled.
What to verify before buying (knowing it is hard)
- Mass spectrometry showing the expected molecular weight (around 80 kDa for the Fc fusion)
- SDS-PAGE showing single-band protein at expected size
- HPLC purity 95%+ minimum
- Endotoxin testing (relevant for any injectable protein)
- Source country (most reliable manufacturing for fusion proteins is in regulated facilities, not contract synthesis labs)
- Vendor reputation on independent peptide and bodybuilding forums
For broader sourcing standards see our peptide vendor guide and how to read a peptide COA.
Frequently Asked Questions
Medical disclaimer. This article is informational only and does not replace individualized medical advice. ACE-031 is not approved for any human use by the FDA. Phase 2 development was halted in 2013 due to vascular safety concerns. The bleeding mechanism is intrinsic to the drug's design and cannot be safely titrated away. Anyone considering use should understand they are operating without clinical supervision, without a validated safe dose, and without long-term safety data.