Bimagrumab (LY3985863) is a monoclonal antibody that blocks ligand binding to the activin/myostatin type II receptors ActRIIA and ActRIIB on the outer membrane of muscle cells. Myostatin and other activin ligands prevent muscle precursor cells from developing into mature muscle cells and cause existing muscle fibers to atrophy. Conversely, follistatin blocks this interaction, thereby increasing muscle mass. So by preventing myostatin and other ActRII signaling molecules from binding to ActRIIs, bimagrumab removes a systemic brake on muscle growth.
Lilly acquired bimagrumab to improve body composition in combination with its incretin agonist drugs in the treatment of obesity, but it has a prior history of study for more classically age-related conditions such as to improve outcomes after a hip fracture, sporadic inclusion body myositis (sIBM), and sarcopenia.