Muscle growth regulation
Activin type IIB ligand trapping
Soluble activin type IIB receptor constructs bind myostatin, activins and related ligands, reducing inhibitory signalling that normally restrains skeletal-muscle growth.
Biological pathway
ActRIIB-Fc ligand binding → sequestration of myostatin and related TGF-beta family ligands → reduced SMAD2 and SMAD3 signalling → increased muscle mass and broader vascular or endocrine effects.
Evidence summary
ACE-031 increased lean mass in early human studies, but development was halted after vascular adverse findings including epistaxis and telangiectasia.
Limitations
The mechanism is not selective for myostatin. Binding multiple ligands can produce off-target vascular and reproductive effects, and increased lean mass does not necessarily improve function.