ACE-031 is not a small peptide you reconstitute from a vial. It is a recombinant fusion protein: the extracellular part of the human activin receptor type IIB (ActRIIB, the protein encoded by ACVR2B) stitched to the Fc region of human IgG1. It was developed by Acceleron Pharma, and in some references it carries the generic name ramatercept.
The idea behind it is a decoy. Myostatin (also called GDF-8) and several related TGF-beta-family proteins normally signal through ActRIIB to put a brake on muscle growth. ACE-031 floats in the circulation and soaks up those ligands before they reach the real receptor on muscle, releasing the brake. Acceleron described it as "a soluble form of activin receptor type IIB" that "promotes muscle growth by binding to myostatin and other negative regulators of muscle mass" (Attie et al., *Muscle & Nerve*, 2013).
What the human studies actually showed
Unlike most compounds sold to the research-chemical market, ACE-031 went into real, registered clinical trials.
The first published human study was a single ascending-dose trial in 48 healthy postmenopausal women (Attie et al., 2013). At the 3 mg/kg dose, by day 29 the investigators reported a statistically significant rise in total body lean mass (about 3.3%) and in thigh muscle volume (about 5.1%), along with biomarker changes pointing toward shifts in bone and fat metabolism. The mean half-life was long, roughly 10 to 15 days, which fits an Fc-fusion protein. The main side effect at lower doses was injection-site redness.
The lead therapeutic target was Duchenne muscular dystrophy. A randomized, double-blind, placebo-controlled ascending-dose study gave ACE-031 subcutaneously every 2 to 4 weeks to ambulatory boys with DMD (Campbell et al., *Muscle & Nerve*, 2017). There were signals worth noting: a trend toward maintaining six-minute walk distance versus a decline on placebo, plus trends toward more lean mass and bone mineral density and less fat. None of those efficacy trends reached statistical significance.
Why it was stopped
The DMD trial was halted after the second dosing regimen. The reason was not muscle-related. Investigators saw epistaxis (nosebleeds) and telangiectasias (small dilated blood vessels), and these non-muscle adverse events drove the decision to discontinue (Campbell et al., 2017). The mechanism that has been proposed is off-target: ActRIIB decoys can also trap BMP9 and BMP10, ligands that matter for normal blood-vessel (endothelial) function, which lines up with the vascular and bleeding effects observed. A drop in follicle-stimulating hormone was also reported in the healthy-volunteer work, consistent with interference in activin signaling.
In May 2013, Acceleron Pharma and its partner Shire announced they had ended the collaboration on ACE-031 and would not restart development of the program. It was never approved by the FDA or any other regulator.
Status for buyers and athletes
ACE-031 is an investigational biologic that failed in development. It is not an approved medicine anywhere, and anything sold under this name is research material, not something cleared for human use. Nothing here is medical or dosing advice.
A myostatin-pathway inhibitor like this falls squarely in the category of substances the World Anti-Doping Agency prohibits. WADA's list bans "agents preventing activin receptor IIB activation," naming activin receptor competitors and anti-myostatin agents, so athletes in tested sport should treat ACE-031 as prohibited.
One more practical point. ACE-031 is a glycosylated Fc-fusion protein produced in mammalian cell culture, not a chemically synthesized peptide. That makes meaningful third-party verification hard. The purity and identity COAs that vendors publish for synthetic peptides, and the independent rater scores this site aggregates, are built around small peptides; they do not establish that a product labeled "ACE-031" is the correctly folded, correctly glycosylated protein Acceleron studied. Treat any such product with that gap in mind.