ACE-031

A soluble form of the activin type IIB receptor fused to an antibody fragment. It binds myostatin and related activins before they can reach the real receptor. Because ActRIIB also carries signals involved in blood-vessel regulation, blocking it broadly has effects well beyond muscle - which is exactly what the trial found.

A phase 2 human trial was stopped early for safety — participants developed nosebleeds and dilated surface blood vessels (telangiectasia), which is a vascular signal rather than a nuisance side effect. Because development was halted rather than completed, no dose was ever established as usable and we publish no ladder for it. That is not caution on our part: there is genuinely nothing to publish. A compound whose trial was stopped for safety is a different category from one that was never tested at all, and it is the single most useful fact about this one.

The clearest case in this catalogue of tested and stopped. It worked on muscle. It was halted anyway. Both halves are true and both belong in the same breath.

ACE-031 is unusual here in that the community discusses it in the past tense. It is the myostatin story that nearly worked - real lean mass gains in an actual human trial, then a halt for bleeding and visible blood-vessel changes. Very few people report having used it, and most of the discussion that exists is people explaining to each other why they did not. In a space where enthusiasm is the default, that is a meaningful signal. The related conversation worth knowing is about myostatin inhibition in general: community sentiment drifted from excitement to caution, largely because of what happened here.