Bimagrumab
Muscle Growth & Body CompositionAlso known as: BYM338, BYM-338
Mechanism
Bimagrumab is a monoclonal antibody that blocks activin type II receptors (ActRII), preventing myostatin and other TGF-beta family ligands from inhibiting muscle growth. In clinical trials, it produced remarkable simultaneous fat loss and muscle gain — a Phase 2 obesity trial showed 20.5% fat mass reduction with 3.6% lean mass increase over 48 weeks, a body composition change rarely achieved by any single intervention.
Technical detail
Bimagrumab (BYM338) is a fully human IgG1 monoclonal antibody that binds activin type II receptors (ActRIIA and ActRIIB) with high affinity, competitively blocking the binding of myostatin, activin A, GDF-11, and other TGF-β superfamily ligands. This prevents Smad2/3 phosphorylation and downstream anti-myogenic signaling, disinhibiting skeletal muscle hypertrophy and satellite cell differentiation. Phase 2 trials in obese patients with type 2 diabetes demonstrated significant fat mass loss (20.5%) with concurrent lean mass gain (3.6%), improved HbA1c, and metabolic improvements independent of caloric restriction. Phase 2/3 trials ongoing for obesity and body composition optimization.