Activin/myostatin type II receptor

Class

Anti-activin/myostatin antibody (lean-mass preservation)

The activin/myostatin type II receptors (ActRIIA and ActRIIB) are cell-surface receptors expressed on skeletal muscle and other tissues that receive signals from members of the TGF-beta superfamily, chiefly myostatin (also called GDF-8) and the activins. Under normal physiology, these ligands act as negative regulators of muscle growth, restraining the size and number of muscle fibers. Diseases marked by muscle wasting or weakness—including spinal muscular atrophy, sarcopenia, cachexia associated with chronic illness, and certain rare bone and connective-tissue disorders—arise in part because this inhibitory signal remains active while muscle-building capacity is reduced. Therapeutic agents in this class work by intercepting either the receptor itself or its upstream ligands, thereby releasing muscle tissue from this restraint and promoting growth in muscle mass and strength. Approaches vary: some antibodies bind the shared type II receptor broadly, blocking multiple ligands at once, while others selectively neutralize a single ligand, such as myostatin or activin A. This distinction matters because the receptor also participates in processes beyond muscle, so the breadth of blockade shapes both the extent of anabolic benefit and the range of biological systems affected.