ACE-031
ActRIIB-Fc · ActRIIB-IgG1 Fc · soluble activin receptor type IIB fusion protein · RAP-031
Last updated
At a glance
Overview
ACE-031 is one of the more interesting curiosities in the myostatin-inhibitor space — a soluble ActRIIB-Fc fusion protein that acts as a decoy receptor, soaking up circulating myostatin, GDF-11, activin A, and activin B before they can engage the muscle membrane. The result in Acceleron's phase 1 work was a measurable jump in total lean body mass and thigh muscle volume after a single subcutaneous dose, with effects visible out past four weeks. For physique-focused users, the appeal is obvious: a weekly injection that lifts the myostatin brake while the rest of the protocol (training, nutrition, AAS, peptides) does the driving.
"A single subcutaneous injection of ACE-031 resulted in statistically significant increases in lean body mass and thigh muscle volume." — Attie et al., Muscle & Nerve (2013)
The community runs it as a short, exploratory add-on rather than a staple. Typical protocols sit at 1–3 mg SC weekly for 4–8 weeks — an order of magnitude below the mg/kg dosing that produced the clinical lean-mass signal, which is an important caveat anyone running it should understand going in. The long 10–15 day half-life keeps injection frequency low, and the mechanism (blocking the Smad2/3 arm, suppressing MuRF1 and atrogin-1) stacks cleanly on top of testosterone-base cycles, follistatin analogs, or recovery-phase peptides like BPC-157. It is not HPTA-active, so no PCT is required for ACE-031 itself.
The trade-off is the vascular side-effect profile that halted Acceleron's DMD program: telangiectasia and epistaxis, driven by the compound's non-selective activin blockade rather than its myostatin action. Managed intelligently — short cycles, visual monitoring, a hard stop on any new nosebleed or facial capillary — the risk is navigable. The sections below cover documented ACE-031 dosage ranges, weekly protocols, stacking logic with AAS and follistatin-344, the telangiectasia signal in detail, and the pitfalls that consistently trip up first-time users.
How ACE-031 works
ACE-031 is a recombinant Fc-fusion biologic built from the extracellular ligand-binding domain of the human activin receptor type IIB (ActRIIB), fused to a human IgG1 Fc tail. It works not as an agonist but as a decoy — a soluble sponge that mops up circulating TGF-β superfamily ligands before they can dock at the muscle-membrane ActRIIB receptor and switch on catabolic signaling. The practical consequence: the myostatin brake on hypertrophy is lifted, and lean mass accrues without a change in training stimulus.
ActRIIB Ligand Sequestration#
The core mechanism is competitive binding. Circulating ACE-031 binds myostatin (GDF-8), GDF-11, activin A, and activin B with high affinity, preventing these ligands from engaging membrane ActRIIB and recruiting the ALK4/ALK5 type I co-receptors that would normally phosphorylate Smad2/3. This is a broad ligand trap, not a myostatin-selective antibody — a distinction that matters for both efficacy (more ligands blocked than landogrozumab or trevogrumab) and side-effect profile (activin A/B binding drives the vascular off-target story).
"A single subcutaneous injection of ACE-031 resulted in statistically significant increases in lean body mass and thigh muscle volume." — Attie, K.M. et al. Muscle & Nerve, 2013
Smad2/3 De-Repression and Akt/mTOR Release#
ActRIIB signaling is tonically anti-anabolic. When myostatin and activin engage the receptor, phosphorylated Smad2/3 translocates to the nucleus and actively suppresses the Akt/mTOR protein-synthesis pathway. Blocking upstream ligand access collapses that signal, releasing Akt/mTOR to drive ribosomal biogenesis, translation initiation, and satellite-cell activation. The hypertrophy seen in phase 1 subjects — roughly +3.3% lean body mass and +5.1% thigh muscle volume at day 29 after a 3 mg/kg dose — reflects this de-repression playing out over one full half-life cycle.
Suppression of the Atrophy Program#
The other half of the net anabolic effect is reduced breakdown. Smad2/3 normally drives transcription of the E3 ubiquitin ligases that tag muscle protein for proteasomal degradation. Turn off that signal and the atrophy machinery quiets down.
"Blocking ActRIIB signaling suppresses transcription of muscle atrophy genes MuRF1 and atrogin-1, promoting muscle hypertrophy." — Nguyen, H.Q. et al. CPT: Pharmacometrics & Systems Pharmacology, 2020
For the reader: this is why ACE-031 shows a signal in catabolic states (sarcopenia, cachexia, anorexia nervosa research populations) that seems disproportionate to its modest effect in healthy trained subjects. The compound is better at preventing loss than at forcing novel growth on top of an already-anabolic baseline.
Fiber-Type Bias and Satellite Cell Activation#
Preclinical work with the murine analog (RAP-031) and downstream ActRIIB blockade consistently shows preferential hypertrophy of type II (fast-twitch) fibers, alongside increased satellite cell proliferation. This matches the clinical finding that thigh muscle volume moved faster than whole-body lean mass in Attie 2013 — type II-rich quadriceps respond first. For physique-focused users, this is the fiber population that carries most of the visible size.
Cross-Talk Into Bone and Endocrine Axes#
ActRIIB ligand trapping is not confined to muscle. Activin signaling participates in bone remodeling and pituitary FSH regulation, and the phase 1 dataset reflected this: drops in serum C-telopeptide and bone-specific alkaline phosphatase (suggesting reduced bone resorption) and measurable shifts in FSH (Attie 2013). The practical takeaway for the community is that ACE-031 is not a "clean, muscle-only" tool — it touches any tissue where activin tone is functionally relevant, including the vascular endothelium, which is the mechanistic root of the telangiectasia and epistaxis findings that ended the clinical program.
"Clinical studies of ACE-031 reported improved muscle mass but raised concerns about telangiectasia due to non-selective activin inhibition." — Feike, Y. et al. Aging Medicine, 2021
Pharmacokinetic Consequence of the Fc Fusion#
The IgG1 Fc tail is not cosmetic. It enables FcRn-mediated recycling in vascular endothelium, which is why ACE-031 carries a 10–15 day terminal half-life from a single SC injection — roughly 50× the half-life of a bare peptide of comparable size. This is what makes weekly or every-10-day dosing pharmacologically sufficient, and why any adverse signal (a new nosebleed, new perioral capillaries) cannot be "washed out" quickly — the compound is in circulation for weeks after the final injection. Any stop-cycle decision is made with that lag in mind.
Protocol
| Level | Dose | Frequency | Notes |
|---|---|---|---|
| Low | 0.5–1 mg | Weekly | Documented entry-level range |
| Mid | 1–2 mg | Weekly | Most commonly studied range |
| High | 2–3 mg | Weekly | Long SC half-life (10–15 days) supports weekly or every-10-day cadence. Clinical phase 1 used single mg/kg SC doses; community protocols run flat 1–3mg weekly, far below clinical exposure. |
Cycle length & outcomes
Documented cycle
4–8 weeks
Plateau after
8 wks
Cycle Length & Protocol Design#
ACE-031 is not a fast-onset compound. With a 10–15 day terminal half-life and a Tmax around 7 days post-injection, a single SC dose is still pharmacodynamically active a month later. That PK profile dictates everything about how the cycle is structured: infrequent dosing, a slow onset of visible change, and a meaningful tail after the last injection.
"A single subcutaneous injection of ACE-031 resulted in statistically significant increases in lean body mass and thigh muscle volume." — Attie et al., Muscle & Nerve (2013)
The phase 1 dataset measured lean-mass gains ~29 days after a single 1–3 mg/kg dose, with effects detectable out to 57–85 days depending on the tier. Community protocols at 1–3 mg flat (roughly an order of magnitude below clinical exposure on an 80 kg frame) produce a slower, subtler signal — which is why short 4-week "tests" often read as null. The compound needs cumulative exposure across 6–8 weeks to declare itself.
Cycle Length by Goal#
| Goal | Cycle Length | Dose | Frequency |
|---|---|---|---|
| Exploratory first run | 4–6 weeks | 1 mg | Once weekly SC |
| Lean-mass add-on to a bulk | 6 weeks | 1 mg | Once weekly SC |
| Site-specific hypertrophy attempt | 4–6 weeks | 100–250 mcg per site | 1–2× weekly IM, split across target muscles |
| Stacked with a testosterone-base blast | 8 weeks (overlap weeks 4–12 of AAS) | 1–2 mg | Once weekly SC |
| Sarcopenia / recomp in older TRT users | 8–12 weeks | 0.5–1 mg | Once weekly SC |
| Post-cycle catabolic bridge | 6–8 weeks | 1 mg | Every 10–14 days SC |
Beginners stay at 1 mg/week for 4–6 weeks. Intermediate operators run 1–2 mg/week for 6–8 weeks, sometimes splitting into two E3.5D injections. Advanced use — 2–3 mg weekly — is where the side-effect ledger starts to matter more than the marginal hypertrophy benefit, and it is not recommended as a default.
Loading, Tapering & Frequency#
No loading phase is warranted. Because serum levels accumulate slowly over the first 2–3 weeks on a weekly cadence, the "loading" happens automatically — a front-loaded bolus would just push Cmax higher without accelerating the lean-mass signal, which is a cumulative exposure phenomenon, not a peak-driven one.
No taper is required. ACE-031 does not interact with the HPTA, so there is no suppression to recover from. The 10–15-day half-life produces its own pharmacological taper: after the final injection, serum activity decays over roughly 6 weeks. This is also why back-to-back cycles are uncommon — a proper washout means 6+ weeks clear of the last dose before restarting.
Frequency options that fit the PK:
- Weekly — the community default, clean cadence, easy to track.
- Every 10 days — aligns better with the half-life, slightly smoother trough-to-peak ratio, functionally identical results.
- Twice weekly (E3.5D split) — useful only if total weekly dose is ≥2 mg and the user wants to smooth injection-site discomfort across two smaller shots.
Onset & Expected Timeline#
The signal arrives slower than anabolic steroids and far slower than a GH secretagogue:
- Weeks 1–2: no visible change. Serum ACE-031 is still building toward steady state. Any perceived effect at this point is placebo.
- Weeks 3–4: first subjective reports of "fullness" in trained muscles, modest scale creep (often fluid-shift, not tissue).
- Weeks 4–6: the lean-mass signal becomes photographable. This tracks the Attie 2013 day-29 readout timeline at clinical doses; community doses lag this.
- Weeks 6–8: peak effect at community dosing. The dose-response curve is logarithmic — pushing past 8 weeks does not continue scaling benefits linearly.
- Weeks 8+ (post-cycle): residual activity persists for 4–6 weeks. Lean mass retention is generally good provided training and kcal intake hold.
"Blocking ActRIIB signaling suppresses transcription of muscle atrophy genes MuRF1 and atrogin-1, promoting muscle hypertrophy." — Nguyen et al., CPT: Pharmacometrics & Systems Pharmacology (2020)
On-Cycle Monitoring Cadence#
ACE-031 does not require a standard AAS bloodwork panel — it does not aromatize, does not affect lipids, and does not suppress LH/FSH in a way that demands intervention. The monitoring that does matter is vascular and hematologic, because that is where the compound's known adverse signal lives.
"Telangiectasia and epistaxis were observed in patients receiving ACE-031, leading to early termination of the trial." — Campbell et al., Muscle & Nerve (2017)
Pre-cycle baseline:
- CBC, CMP, lipid panel (general baseline, not ACE-031-specific).
- Photograph the face — perioral area, cheeks, nasal bridge, under-eye. This is the reference point for detecting new telangiectasia.
- Note any history of easy bruising, recurrent epistaxis, or gum bleeding. These are protocol disqualifiers, not flags to "watch."
On-cycle, weeks 2 / 4 / 6:
- Repeat facial photographs under the same lighting.
- Visual check for new capillary patterns around mouth, nose, and cheeks.
- Log any nosebleed, unusual gum bleeding during brushing, or prolonged bleeding from minor cuts.
Hard stop conditions:
- Any new-onset epistaxis → discontinue the cycle immediately. This is the unambiguous community stop rule and it matches the Campbell 2017 termination criteria.
- Visible new telangiectasia → discontinue. These capillaries do not fully resolve.
- Unusual bruising pattern → discontinue.
Bloodwork is optional mid-cycle unless a stack component (AAS, orals) demands it independently. For ACE-031 in isolation, the vascular visual check is the primary surveillance tool.
Stacking Considerations for the Cycle#
When ACE-031 is layered into a larger protocol, the cycle length is typically anchored to the other compound, not to ACE-031 itself:
- With a testosterone-base blast: run ACE-031 across weeks 4–12 of a 16-week cycle. Front-end weeks 1–3 are skipped because the AAS is still ramping and ACE-031 adds cost without adding early signal.
- With BPC-157 / TB-500: full overlap is fine — no mechanistic conflict, both are weekly-ish cadence, and the recovery peptides complement the higher training volume that ACE-031 users tend to pursue.
- With follistatin-344: the 4–6 week ACE-031 window can overlap the follistatin run, but the stack is mechanistically redundant (both suppress myostatin-pathway signaling). The additive benefit is unproven — treat it as experimental, not as a default.
- With MK-677 + testosterone (classic bulk stack): ACE-031 slots in as a 6–8 week add-on without modifying the base protocol.
PCT is not required for ACE-031. If the stack includes suppressive AAS, PCT is dictated by the AAS timing, and ACE-031's own tail (6+ weeks post-injection) runs in parallel with early recovery without interfering.
Realistic Expectations at Community Dosing#
The honest framing: clinical lean-mass effects were produced at 1–3 mg/kg — which is 70–240 mg per injection for an adult male. Community protocols running 1–3 mg flat are operating well below that tier, and the resulting signal is correspondingly muted. Expect ~0.2 lb/week of lean tissue as a working estimate at community dosing with intelligent training and a modest surplus, not the dramatic numbers seen in the phase 1 DEXA data.
"ACE-031 has been explored for its ability to increase muscle mass in catabolic states such as anorexia and sarcopenia." — Maïmoun et al., Endocrine Connections (2022)
Operators who treat ACE-031 as a time-limited exploratory add-on — 6–8 weeks, weekly dosing, vascular check every two weeks, stop on the first nosebleed — land where the compound actually delivers value. The users who get burned are the ones chasing SARM-like timelines and ignoring the Campbell 2017 signal that shut the clinical program down.
Body Transformation Preview
Lean Mass Gain
1.3 lbs
1.0–1.7 lbs range
Fat Loss
0.0 lbs
0.0–0.0 lbs range
Lean Gain by Week
Risks & mistakes
ACE-031 has a real side-effect profile — unlike most peptides the community plays with, this one was actually halted in clinical development because of it. The signal is vascular, not hepatic or endocrine, and the warning signs are visible on the face and in the nose. Users who monitor for them and stop on cue tend to do fine. Users who ignore them accumulate damage that does not fully resolve.
Common (most users)#
- Injection site erythema / induration — mild, local, typical of any SC biologic. Rotate sites between abdomen, flank, and thigh; let the reconstituted solution come to room temperature before injection to reduce sting.
- Mild fatigue in the 24–48h post-injection window — usually fades by the second or third dose. Dosing the evening of a rest day minimizes disruption.
- Headache — low-grade, most common in the first 1–2 weeks. Hydration and standard OTC analgesics handle it; persistent headache past week 2 warrants backing off the dose tier.
- Transient gum tenderness or mild gingival bleeding on brushing — the earliest vascular signal in most logs. Not a stop-condition on its own, but a flag to watch for anything worse.
Uncommon (dose-dependent or individual)#
- Telangiectasia — small dilated capillaries appearing on the face, particularly around the lips, nostrils, cheeks, and perioral area. Documented in both the phase 1 and phase 2 cohorts (Campbell et al. 2017). Dose-cumulative. First visible lesion is a stop-condition — these do not fully resolve.
- Epistaxis (nosebleeds) — new-onset or recurrent. The Acceleron DMD trial terminated specifically on this signal.
"Telangiectasia and epistaxis were observed in patients receiving ACE-031, leading to early termination of the trial." — Campbell et al. 2017
- Suppression of bone resorption markers (serum CTX, bone ALP) — not clinically alarming in a short cycle but worth noting if bloodwork is being tracked. Documented in Attie et al. 2013.
- Changes in reproductive hormone markers — FSH reductions and activin-antagonism-related shifts have been observed (Attie et al. 2013). Not HPTA suppression in the AAS sense, but a reminder that ActRIIB blockade is not endocrinologically silent.
- Noticeable bruising from minor trauma — reflects the same vascular fragility pattern. If palm-sized bruises are showing up from ordinary contact, discontinue.
"A single subcutaneous injection of ACE-031 resulted in statistically significant increases in lean body mass and thigh muscle volume." — Attie et al. 2013
Rare but serious#
- Persistent or heavy epistaxis — any nosebleed that requires packing, lasts more than 15–20 minutes, or recurs daily. Stop immediately and get a CBC and coagulation panel.
- Widespread or rapidly progressing telangiectasia — beyond a single perioral lesion. Indicates aggressive vascular remodeling under broad activin blockade. Stop and do not re-run.
- Gastrointestinal or mucosal bleeding — rare, but plausible given the mechanism. Any melena, hematemesis, or hematuria is a hard stop and an ER-level workup.
- Hypersensitivity reactions to the Fc-fusion protein — anaphylactoid reactions to biologics are rare but not zero. Swelling, urticaria, or respiratory symptoms post-injection mean the cycle ends.
"Clinical studies of ACE-031 reported improved muscle mass but raised concerns about telangiectasia due to non-selective activin inhibition." — Feike et al. 2021
The mechanism behind the vascular signal is informative: ACE-031 is a broad ActRIIB ligand trap, not a selective anti-myostatin antibody. Activin A and B regulate vascular endothelial homeostasis, and it is the activin arm — not the myostatin arm — that drives the bleeding phenotype. More selective anti-myostatin monoclonals (landogrozumab, domagrozumab, trevogrumab) do not produce the same pattern.
Hard contraindications#
- History of easy bruising, recurrent epistaxis, or hereditary hemorrhagic telangiectasia (Osler-Weber-Rendu) — the compound is the wrong tool; the existing vascular fragility stacks directly on the mechanism.
- Known vascular malformations — same reasoning.
- Concurrent anticoagulant therapy (warfarin, apixaban, rivaroxaban, dabigatran, heparin) — bleeding risks compound.
- Concurrent antiplatelet therapy (aspirin, clopidogrel, ticagrelor) — same.
- Pregnancy or potential pregnancy — no reproductive safety data; activin signaling is critical in placental and reproductive biology.
- Lactation — no data; Fc-fusion proteins can transfer via breast milk.
- Active or recent hemorrhagic event — postoperative, post-dental-extraction, post-trauma windows are not the time.
Gender and PCT considerations#
The phase 1 dataset is exclusively postmenopausal women; preclinical and DMD phase 2 data cover males. No strong sex-specific adverse pattern has emerged, but the dataset is thin, and dosing does not differ by sex in community practice. Female users should be aware that the activin-axis interference observed in Attie 2013 is unlikely to produce virilization-type effects (there is no androgenic mechanism) but may produce subtle shifts in reproductive hormone markers worth tracking on bloodwork.
ACE-031 does not interact with the HPTA. No SERM, no AI, no PCT is required for the compound itself. When stacked with AAS, the ancillary and PCT strategy is dictated entirely by the AAS — ACE-031 adds nothing to that column and subtracts nothing from it. The only monitoring that matters specifically for ACE-031 is the vascular one: visual inspection of the perioral area and nose at weeks 2, 4, and 6, plus a low threshold to stop on the first unambiguous bleeding or telangiectasia signal.
Stack & combine
Multipliers applied when these compounds run together. Values > 1 indicate a bonus on that axis. Tap a partner to expand the mechanism.
| Partner | Type | Lean | Fat loss | Recovery |
|---|---|---|---|---|
| synergistic | ×1.18 | ×1.08 | ×1.05 | |
| additive | ×1.08 | ×1.00 | ×1.05 |
FAQ — ACE-031
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Research & citations
5 studies cited on this page.
Conclusion
ACE-031 stands out as a research-tier myostatin inhibitor with a unique mechanism, clear lean-mass signal in the literature, and well-acknowledged vascular risks. Its value is highest as a cycle add-on for hypertrophy or body recomposition when classic AAS or GH-pathway agents plateau.
Key takeaways:
- Documented research dosing: 1–2 mg subcutaneously, weekly, with 4–8 week cycles standard in community protocols
- Preferred route: subcutaneous injection; localized IM is explored but systemic redistribution is the rule
- Best stacked with: a testosterone base when bulking, or with BPC-157/TB-500 for high-volume training recovery
- Notable benefit: increased lean mass and muscle volume independent of training load (Attie et al., 2013)
- Hard cautions: telangiectasia, epistaxis, and bleeding risk in susceptible subjects; any new-onset nosebleed is a universal stop signal (Campbell et al., 2017)
- No PCT required for ACE-031 alone; cycle structure is short with long washouts due to slow clearance
For researchers seeking myostatin/activin-pathway modulation beyond what SARMs or GH-mimetics allow, ACE-031 offers a rare—and powerful—mechanistic lever. Protocol discipline and side-effect vigilance are mandatory, but with careful structuring, ACE-031 delivers one of the most targeted muscle-building platforms explored to date.