ACE-031: The Myostatin Trap That Rewrites Muscle Biology
Last updated: March 2026
ACE-031 (ActRIIB-Fc) is a soluble decoy receptor engineered by Acceleron Pharma to intercept myostatin, activin, and GDF-11 before they can suppress muscle growth. Phase 2 trials in Duchenne muscular dystrophy demonstrated rapid gains in lean mass and muscle volume — until vascular off-target effects forced a halt.
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Phase 2 Trial Protocol
Smad Pathway Inhibitor
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How ACE-031 Works
ACE-031 is an engineered fusion protein combining the ligand-binding domain of ActRIIB with an IgG1 Fc region for extended half-life. It acts as a circulating decoy receptor, capturing myostatin and related TGF-β family members before they reach muscle tissue — effectively derepressing muscle growth signaling at the pathway level.
ACE-031 consists of the extracellular domain of activin receptor type IIB (ActRIIB) fused to a human IgG1 Fc region. Injected subcutaneously, it circulates in blood and binds target ligands with high affinity — acting as a soluble decoy that competes with cell-surface ActRIIB on muscle fibers. The Fc fusion extends plasma half-life from hours to days, enabling monthly or less-frequent dosing.
Unlike antibodies targeting only myostatin (GDF-8), ACE-031 binds multiple ActRIIB ligands: myostatin, activin A, activin B, GDF-11, and BMP-9/10. This broad-spectrum capture accounts for its potent anabolic effect — but also explains the vascular side effects, since activin and BMP signaling plays roles in endothelial and vascular smooth muscle homeostasis.
Myostatin normally binds ActRIIB on muscle cells, activating the Smad2/3 signaling cascade, which suppresses protein synthesis and promotes atrophy. ACE-031 sequesters myostatin before receptor binding, effectively silencing Smad2/3 inhibition. This shifts the balance toward Smad1/5/8 (anabolic) signaling and allows IGF-1/PI3K/Akt pathways to drive hypertrophy and fiber size without the myostatin brake.
Follistatin is an endogenous binding protein that inhibits activins and myostatin through direct ligand sequestration, but also potently neutralizes FSH-stimulating activins in the hypothalamic-pituitary axis. ACE-031 is receptor-based and more selective for ActRIIB ligands, with no FSH-axis interference. In practice, ACE-031 produced faster and larger lean mass gains than follistatin 344 in comparative preclinical models — at the cost of broader vascular off-target effects.
What the Research Shows
Data from Acceleron Pharma's Phase 2 clinical trials in Duchenne muscular dystrophy and preclinical muscle-wasting models. Human trial data is from small cohorts — interpret with appropriate caution.
Side Effects & Risks
Key Takeaways
- +1.7% lean mass in 29 days from a single 250mg SubQ dose (Phase 2)
- Broad ActRIIB ligand trap: hits myostatin, activin A/B, GDF-11, BMP-9/10
- Smad2/3 derepression drives robust muscle hypertrophy signaling
- Statistically significant MRI-measured thigh muscle volume gains vs. placebo
- Mechanistically distinct from follistatin — receptor-based vs. binding-protein approach
- Most potent myostatin inhibitor tested in human trials to date
- Development halted — epistaxis and telangiectasia in ~25% of subjects
- Off-target BMP-9/10 inhibition disrupts vascular homeostasis
- No approved clinical indication; research-only compound
- Broad ligand binding means unpredictable systemic effects long-term
- Not commercially available; no verified research supply exists
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This page is for educational and research purposes only. It is not medical advice. ACE-031 (ActRIIB-Fc) is not FDA approved for human use. Development was halted by Acceleron Pharma due to adverse vascular events in clinical trials. Research data cited is from small Phase 2 trials; treat all findings as preliminary. Always consult a qualified physician before considering any experimental compound.