ACE-031

Structure: A dimeric fusion protein: the extracellular ligand-binding domain of the activin receptor type IIB (ActRIIB/ACVR2B) joined to the Fc portion of human IgG1. It never enters the cell. It works purely as a decoy, binding circulating ligands so they cannot reach the real receptor on muscle. Because ActRIIB is a promiscuous receptor, the decoy binds not only myostatin (GDF-8) but also activin A, GDF-11, BMP-9 and BMP-10. That lack of selectivity is the whole story of this compound.

Dosage:

• Clinical trial range: 0.02 to 3mg/kg SubQ, single dose, in the healthy volunteer study; 1 to 3mg/kg SubQ every 2 to 4 weeks in the Duchenne muscular dystrophy trials.
• Grey-market protocols: typically quoted as 1mg/kg every 2 to 4 weeks, extrapolated from the above. There is no dose that has been shown to be safe for long-term use in a healthy adult.
• Duration: the DMD trials were stopped before any long-term dosing schedule was established.

Administration:

• SubQ injection, insulin syringe, abdomen or thigh
• Dosing intervals are measured in weeks, not days, because of the Fc half-life. Stacking doses on a shorter interval accumulates exposure faster than most users assume.

Key Notes:

• The trials were stopped for safety, and that is the single most important fact about this compound. The Duchenne muscular dystrophy program was halted after participants developed telangiectasia (visible dilated small blood vessels, often on the face and torso), gum bleeding, and nosebleeds. These were not trivial cosmetic findings; they were considered serious enough to end development of a drug for a fatal disease with no good alternatives.
• The likely mechanism for the bleeding is off-target blockade of BMP-9 and BMP-10. That signalling axis (BMP9/10 acting through ALK1 and endoglin) maintains vascular endothelial integrity, and loss-of-function mutations in the same pathway cause hereditary haemorrhagic telangiectasia, a disease defined by exactly these symptoms. In other words, the side effect is a predictable consequence of the receptor's promiscuity, not bad luck.
• The muscle effects are real. A single dose in healthy postmenopausal women produced a measurable increase in lean body mass and a decrease in fat mass within a month, alongside changes in bone and fat-tissue markers. This is not a compound that does nothing. It is a compound that does something and is dangerous anyway.
• Sourcing is a serious problem. ACE-031 is a large, glycosylated, dimeric Fc-fusion protein. Producing it correctly requires mammalian cell expression and purification, not the solid-phase peptide synthesis used by the labs that supply the research chemical market. Material sold under this name is frequently something else, or nothing at all. Paying for a mislabelled vial is the common outcome; getting the real thing is the outcome with the bleeding risk.
• Related molecules from the same class did reach market for other indications (activin receptor traps used in anaemia and pulmonary arterial hypertension), which tells you the class is druggable. It also tells you that the specific ActRIIB construct was the one that got dropped.
• Comparison with other myostatin approaches: Follistatin works on the same axis with the same evidence problems, and YK-11 is often marketed as a myostatin inhibitor on very thin grounds. None of these has a human safety record worth relying on.

Bloodwork Monitoring:

• Haemoglobin and Haematocrit: activin receptor traps drive erythropoiesis. If you are already on testosterone and sitting at the top of the range, this is the panel that moves first and the one most likely to cause a real problem.
• FSH and LH: the healthy-volunteer study showed FSH suppression, since activin is a physiological driver of FSH secretion. Expect a confusing gonadotropin picture that has nothing to do with your AAS use.
• Platelets and clotting studies: worth a baseline, but understand what they do and do not tell you. The bleeding seen in trials was vascular, not a clotting-factor or platelet defect, so a normal coagulation screen does not mean you are safe. Visible telangiectasia, gum bleeding when brushing, or recurrent nosebleeds are the actual warning signs, and they are clinical, not laboratory.
• ALT and AST: no specific signal, but include them if stacking with orals.
• Baseline before starting and a repeat at 4 to 6 weeks is the minimum, and given the Fc half-life a single dose commits you to weeks of exposure you cannot withdraw.