What it is
Bimagrumab is not a peptide. It is a monoclonal antibody, a large protein given by intravenous infusion, that binds and blocks activin type II receptors. It was first developed by Novartis for muscle-wasting conditions such as sporadic inclusion body myositis and sarcopenia, and is now being studied by Eli Lilly alongside the GLP-1 drug semaglutide in obesity.
Mechanism of action
Bimagrumab binds activin type II receptors (ActRII), the cell-surface receptors through which myostatin, activins and related members of the TGF-beta family signal. In skeletal muscle this signaling acts as a brake on growth, so blocking the receptor is expected to allow muscle mass to increase; that is the rationale behind the lean-mass gains measured in trials. The same receptors are present in fat tissue, and in obesity trials receptor blockade has been accompanied by falls in fat mass, although the pathway in fat is less well defined than in muscle. Because bimagrumab blocks the shared receptor rather than one ligand, it interrupts several signals at once, which is broader than approaches aimed at myostatin alone. As an antibody it is long-lived in the body and has been given by intravenous infusion at intervals of weeks, for example every 4 weeks in the 2021 obesity trial (Heymsfield et al. 2021).
What it’s most studied for
Bimagrumab was first studied as a way to build muscle where muscle is lost: sporadic inclusion body myositis, age-related sarcopenia and disuse atrophy. Those trials found gains in lean mass that did not translate into clear improvements in walking or physical performance. Research then moved to obesity, where it reduced fat mass while lean mass rose, and more recently to pairing it with GLP-1 receptor agonists such as semaglutide. The question in the BELIEVE trial was whether adding bimagrumab changes the composition of the weight lost on semaglutide, shifting more of the loss to fat and less to lean tissue.
Notable findings
Each finding carries the species or population it was observed in and the dose that study used. Doses are reported as research context, exactly as published. They are not guidance.
BELIEVE: combined with semaglutide in obesity
Heymsfield et al. (2026) randomized 507 adults with obesity to bimagrumab, semaglutide, both, or placebo. At the primary 48-week endpoint, mean weight change was -17.8 kg with the high-dose combination, -14.2 kg with semaglutide alone, -9.3 kg with bimagrumab alone and -3.3 kg with placebo. In the secondary week-72 analysis, weight fell 22.1% with the combination, 15.7% with semaglutide and 10.8% with bimagrumab; 92.8% of the weight lost on the combination was fat mass versus 71.8% on semaglutide alone, and bimagrumab alone increased total lean mass by 2.5%.
- Dose studied
- Bimagrumab 30 mg/kg by intravenous infusion plus semaglutide 2.4 mg once weekly subcutaneously (high-dose combination arm), adults with obesity; 48-week primary period with follow-up to week 72
[1] Heymsfield SB et al. · 2026 · DOI 10.1038/s41591-026-04204-0 (NCT05616013) ↗
Fat loss in type 2 diabetes
Heymsfield et al. (2021) randomized 75 adults with type 2 diabetes and a BMI of 28 to 40 to bimagrumab or placebo, with diet and exercise counseling for all. After 48 weeks, total fat mass fell 20.5% with bimagrumab versus 0.5% with placebo (-7.5 vs -0.2 kg), while lean mass rose 3.6% versus a 0.8% loss. Body weight fell 6.5% versus 0.8%, waist circumference fell 9.0 cm versus a 0.45 cm increase, and HbA1c fell 0.76 versus 0.04 percentage points.
- Dose studied
- 10 mg/kg (maximum 1,200 mg) intravenously every 4 weeks for 48 weeks, adults with type 2 diabetes and overweight or obesity (phase 2 trial)
[2] Heymsfield SB et al. · 2021 · DOI 10.1001/jamanetworkopen.2020.33457 (NCT03005288) ↗
Sarcopenia: more lean mass, not function
Rooks et al. (2020) randomized 180 community-dwelling adults aged 70 or older with sarcopenia to bimagrumab (n=113) or placebo (n=67), with diet and home-based exercise in both groups. After 24 weeks, lean body mass rose 7% with bimagrumab versus 1% with placebo and fat mass fell, but Short Physical Performance Battery scores did not differ significantly between groups. The authors concluded that bimagrumab did not substantially add to the effect of adequate diet and light exercise on physical function.
- Dose studied
- 700 mg by intravenous infusion once monthly for 6 months, adults aged 70 and older with sarcopenia (randomized trial)
[3] Rooks D et al. · 2020 · DOI 10.1001/jamanetworkopen.2020.20836 ↗
Inclusion body myositis: walking distance unchanged
Hanna et al. (2019) randomized 251 adults with sporadic inclusion body myositis to one of three bimagrumab doses or placebo. At 52 weeks, none of the doses improved 6-minute walk distance relative to placebo, and quadriceps strength showed no consistent difference. Bimagrumab was well tolerated relative to placebo.
- Dose studied
- 1, 3 or 10 mg/kg by intravenous infusion, or placebo, with the primary outcome at 52 weeks (adults with sporadic inclusion body myositis, phase 2b trial)
[4] Hanna MG et al. · 2019 · PMID 31397289 ↗
Study quality
Human evidence for bimagrumab is substantial for an investigational drug, with several randomized, double-blind, placebo-controlled trials: BELIEVE (n=507), the inclusion body myositis trial (n=251), the sarcopenia trial (n=180) and the 2021 obesity trial (n=75). The consistent finding is a body-composition effect, with fat mass down and lean mass up or largely preserved. The functional picture is weaker: the sarcopenia and myositis trials did not show better physical performance or walking distance despite more lean mass. In BELIEVE, the 48-week weight change was the primary endpoint, and the week-72 percentages and fat-share figures come from secondary analyses. Reported adverse events with bimagrumab include muscle spasms, diarrhea and acne, along with transient laboratory changes, and discontinuations were higher in bimagrumab arms. It is an antibody given by intravenous infusion, not a peptide, and no regulator has approved it.
On the research desk
Recent records on Bimagrumab from the Alpha Helix research feed, each with its evidence tier and citation.
References
- 1Bimagrumab plus semaglutide alone or in combination for the treatment of obesity: a randomized phase 2 trial. Nature Medicine. DOI 10.1038/s41591-026-04204-0 (NCT05616013)
- 2Effect of bimagrumab vs placebo on body fat mass among adults with type 2 diabetes and obesity: a phase 2 randomized clinical trial. JAMA Network Open. DOI 10.1001/jamanetworkopen.2020.33457 (NCT03005288)
- 3Bimagrumab vs optimized standard of care for treatment of sarcopenia in community-dwelling older adults: a randomized clinical trial. JAMA Network Open. DOI 10.1001/jamanetworkopen.2020.20836
- 4Safety and efficacy of intravenous bimagrumab in inclusion body myositis (RESILIENT): a randomised, double-blind, placebo-controlled phase 2b trial. The Lancet Neurology. PMID 31397289
Free guide
Want more than the summary?
Get our free research guide, or tell us about your practice and we’ll get back to you.