Bimagrumab: Anti-Activin Receptor mAb for Fat Loss with Muscle Preservation
Monoclonal antibody blocking ActRII receptors. Originally for sarcopenia; repurposed for obesity — loses fat while preserving or increasing muscle.
👥 Human studies
- Full name
- Bimagrumab (BYM338)
- Class
- Anti-ActRIIA/B monoclonal antibody
- Half-life
- ~2–3 weeks
- Route
- Intravenous monthly in trials
- Developer
- Novartis → Versanis → Eli Lilly (acquired 2023)
- Regulatory status
- Phase 2b (obesity); orphan drug sIBM (discontinued)
What it is
Bimagrumab is a fully human monoclonal antibody that binds activin type II receptors (ActRIIA and ActRIIB), blocking myostatin and activin A signalling. By removing the inhibitory brake on muscle growth, it increases lean mass and — unexpectedly — reduces fat mass.
How it works
Myostatin normally limits muscle growth. Blocking its receptors increases muscle protein synthesis and muscle fibre size. Fat loss is a downstream effect, possibly through increased resting energy expenditure from larger muscle mass and direct effects on adipocyte browning.
The combination of fat loss + muscle preservation is a major difference from GLP-1 agonists where 20–40% of weight loss is lean tissue.
What the research shows
Phase 2 data in obese adults with T2D showed striking body-composition changes.
Heymsfield SB et al. (2021) — Phase 2 obesity + T2D
Heymsfield S.B. et al., JAMA Network Open 2021;4:e2033457. 👥 Human studies
75 obese adults with T2D received monthly IV bimagrumab or placebo for 48 weeks.
Fat mass reduced by 20.5% (− 7.5 kg), lean mass increased 3.6% (+ 1.7 kg), and HbA1c fell 0.76% — a profile no other agent has replicated.
Limitations: Small (n=75); IV-only administration; cost; mild liver enzyme elevations.
Rooks D. et al. (2017) — sarcopenia in hip-fracture recovery
Rooks D. et al., J Am Geriatr Soc 2017;65:1988–1995. 👥 Human studies
Older adults with mobility impairment received single-dose bimagrumab.
Lean mass and thigh muscle volume increased vs placebo at 8 weeks. Clinical function measures did not consistently improve.
Limitations: Lean mass gains without clear functional benefit limited sarcopenia development.
Safety and limitations
Generally well tolerated in Phase 2: muscle cramps, mild diarrhoea, transient liver enzyme rises. Pancreatitis signals have been monitored without clear excess.
The inherent IV monthly administration limits real-world use; subcutaneous formulations are under investigation by Eli Lilly.