PwPepwise

ACE-031

Muscle & Performance

Soluble activin receptor IIB fusion protein

ACE-031 is an experimental fusion protein that works by blocking a receptor in the body called activin receptor type IIB (ActRIIB).

§Dosing at a glance

2 protocols · from the research
What it's forDoseHow oftenHowFor how long
Phase 1 — Healthy Postmenopausal Women (Single Ascending Dose)3 mg/kgEvery 4 wksSubcutaneousInjected just under the skin, into the fat layer.—
Phase 2 — Boys with Duchenne Muscular Dystrophy0.5 mg/kgEvery 4 wksSubcutaneousInjected just under the skin, into the fat layer.24 wks

Approximate values pulled from the research — double-check before dosing.

§01Summary

ACE-031 is an experimental fusion protein that works by blocking a receptor in the body called activin receptor type IIB (ActRIIB), which normally acts as a brake on muscle growth. By blocking this receptor, ACE-031 prevents several muscle-suppressing signals — most notably myostatin — from taking effect, allowing skeletal muscle to grow more freely. It is administered by subcutaneous injection and was developed primarily to address muscle wasting in conditions such as Duchenne muscular dystrophy (DMD).

In early clinical studies, ACE-031 may increase lean body mass and thigh muscle volume in healthy adults2, and has been reported to maintain walking ability and support bone mineral density in boys with DMD, though these trends did not reach statistical significance in the clinical trial1,6. Preclinical studies in mice and non-human primates support its ability to increase muscle mass and functional strength across multiple fiber types9,10. Development of ACE-031 was paused following observations of epistaxis and telangiectasias in treated patients1,5,6, effects linked to its broad action on the ActRIIB pathway beyond muscle tissue. Research into next-generation approaches targeting this pathway more selectively remains an active area of scientific investigation.

This is the layperson summary. Mechanism, dosing, the evidence base, and the published literature are in the sections below — every claim links to its source.

§02In depth

ACE-031 is a recombinant fusion protein comprising the extracellular ligand-binding domain of human activin receptor type IIB (ActRIIB, also termed ACVR2B) fused to the Fc region of human IgG1. This architecture confers two functional properties: the ActRIIB domain acts as a decoy receptor that sequesters endogenous ligands in the circulation before they can engage cell-surface ActRIIB, while the IgG1 Fc domain extends circulating half-life through neonatal Fc receptor (FcRn)-mediated recycling. The resulting pharmacokinetic profile yields a mean terminal half-life of approximately 10–15 days in humans, supporting subcutaneous dosing at multi-week intervals2.

ActRIIB is a type II serine/threonine kinase receptor that serves as a shared signaling hub for multiple members of the TGF-β superfamily. Its principal endogenous ligands include myostatin (GDF-8), GDF-11, activin A, activin B, BMP9, and BMP10, among others. By acting as a broad ligand trap, ACE-031 simultaneously blocks signaling through all of these ligands rather than selectively inhibiting myostatin alone1,2. In skeletal muscle, myostatin and activin A normally engage ActRIIB to activate SMAD2/3 phosphorylation, which suppresses the Akt/mTOR anabolic signaling pathway and promotes muscle atrophy. Blockade of this axis by ACE-031 disinhibits protein synthesis and promotes myofiber hypertrophy across both slow-twitch (type I) and fast-twitch (type II) fiber populations — a fiber-type-independent growth pattern that distinguishes it from selective myostatin-only inhibition9.

Beyond muscle, ActRIIB ligands regulate bone remodeling, adipogenesis, and vascular homeostasis. Blockade of activin signaling may improve bone mineral density by shifting the osteoblast/osteoclast balance, and may reduce fat mass through effects on adipocyte differentiation1,2,6. However, inhibition of BMP9 and BMP10 — which also signal through ActRIIB and are critical regulators of endothelial quiescence and vascular tone via the ALK1/endoglin pathway — is mechanistically implicated in the vascular adverse events (epistaxis, telangiectasias) observed clinically1,6. This ligand promiscuity represents the central pharmacological liability of the ActRIIB-trap approach: the same breadth of inhibition that produces multi-tissue anabolic benefits also disrupts vascular signaling pathways outside the intended therapeutic target.

ACE-031 exhibits dose-proportional linear pharmacokinetics, with AUC and Cmax increasing proportionally across the tested dose range2. It is administered subcutaneously, with bioavailability and absorption characteristics consistent with other IgG-based fusion proteins of similar molecular weight.

§04Evidence & efficacy

Evidence base
55Studies
28Human
2Animal

ACE-031 has demonstrated robust muscle-building effects in preclinical models. In mice, administration produced a 16% increase in mean body weight and wet weight increases of 26–46% across multiple muscle groups, with hypertrophy occurring across both type I and type II fibers9. In marmosets, ACE-031 may increase lean body mass, biceps fiber cross-sectional area, and ex vivo muscle force production across both fiber types10.

In the most rigorous human trial — a randomized, placebo-controlled study in ambulatory boys with DMD — ACE-031 may maintain 6-minute walk test distance compared to a decline in placebo, and may increase lean body mass and bone mineral density while reducing fat mass1. A conference-reported Phase 2 dataset described a statistically significant increase in total body lean mass in the higher-dose DMD cohort (5.2% vs. 2.6% placebo, p=0.015) and a trend toward maintained walking distance6. In healthy postmenopausal women, ACE-031 produced statistically significant increases in total body lean mass (mean +3.3%, p=0.03) and thigh muscle volume (mean +5.1%, p=0.03) at the 3 mg/kg dose2.

Across the myostatin inhibitor drug class more broadly, a recurring pattern has been observed where increases in muscle mass do not consistently translate to proportional improvements in functional strength or clinical endpoints12, a consideration that remains relevant to interpreting ACE-031's functional outcomes.

§05Safety

ACE-031 demonstrated general tolerability at lower doses in healthy postmenopausal women, with the primary adverse event being injection site erythema2. However, as clinical development advanced to higher doses and into the DMD pediatric population, a pattern of vascular adverse events emerged. Epistaxis (nosebleeds) and telangiectasias (small dilated blood vessels) were observed in treated patients, appearing in a dose-dependent manner and most prominently in the higher-dose DMD cohort receiving 1 mg/kg every 2 weeks1,6. These findings are mechanistically attributed to off-target inhibition of BMP9/BMP10 signaling through the ActRIIB pathway, ligands that play a role in vascular homeostasis beyond muscle regulation1,6. Gingival bleeding was also reported in association with the broader ACE-031 clinical program4,5. These observations prompted early termination of both the randomized DMD trial and its open-label extension study1,5,7.

Black market products purporting to be ACE-031 have been analyzed and found to contain neither the genuine fusion protein nor a clean formulation — all tested samples showed multiple protein contaminants, presenting additional safety considerations for unsupervised use11.

§06History

ACE-031 was developed by Acceleron Pharma (later acquired by Merck & Co.) as part of a platform targeting the ActRIIB pathway to address muscle-wasting diseases. The scientific rationale was established through foundational work on myostatin as a negative regulator of skeletal muscle mass, and the recognition that ActRIIB serves as the principal signaling receptor for myostatin and related TGF-β superfamily ligands.

Preclinical work in mice demonstrated that ACE-031 produced substantial increases in muscle mass across multiple muscle groups and fiber types, distinguishing it from antibody-based selective myostatin inhibitors9. These results supported progression into human studies. A Phase 1 single ascending-dose study in healthy postmenopausal women was initiated in 20083, followed by a multiple ascending-dose Phase 1 study in the same population in 20094. Results from these studies, published in 2012, confirmed dose-proportional pharmacokinetics and statistically significant increases in lean body mass and muscle volume at the highest dose tested2.

Encouraged by these findings, Acceleron initiated a Phase 2 randomized controlled trial in ambulatory boys with DMD in 20105, alongside an open-label extension study7. Both trials were terminated early following the emergence of epistaxis and telangiectasias in treated participants — vascular adverse events attributed to off-target BMP9/BMP10 pathway inhibition1,6. Full results from the Phase 2 DMD trial were published in 20161. As of 2025–2026, renewed preclinical interest in ACE-031 in non-human primates continues to be reported8,10, and the compound has also attracted attention in anti-doping research due to its presence in unregulated black market channels11.

§07References