What it is
Bimagrumab is not a peptide. It is a fully human monoclonal antibody — a complete IgG immune protein of four chains — designed to block the activin type II receptors, ActRIIA and ActRIIB.
Those receptors are where several growth-limiting signals land. Myostatin is the best known: it tells muscle to stop growing, and animals born without it carry conspicuously more muscle. Activin A and related proteins use the same receptors. Rather than neutralise one signal, as the binding protein follistatin does in part, bimagrumab closes the door they all come through.
It is in this library because it is searched next to muscle and GLP-1 peptides, and because it is the furthest along of the drugs being developed to protect muscle during weight loss.
Seventeen trials, two sponsors, two eras
ClinicalTrials.gov lists 17 studies under the name bimagrumab, with 2,303 planned or actual participants (queried 2026-09-17). They fall into two clearly separate programmes.
| Era | Sponsor and code | Studies | What they tested |
|---|---|---|---|
| 2011–2017 | Novartis, BYM338 | 12 | inclusion body myositis, sarcopenia, hip fracture recovery, cancer cachexia, COPD cachexia, ventilated patients, type 2 diabetes with obesity |
| 2022–2025 | Eli Lilly, LY3985863 | 4 | obesity, with semaglutide or tirzepatide |
| 2025 | Massachusetts General Hospital | 1 | tirzepatide plus bimagrumab: body composition, insulin sensitivity, bone |
Two of the Novartis studies were withdrawn with no participants, as was one Lilly study. PubMed indexes 91 bimagrumab records, 15 of them randomized controlled trials (E-utilities, 2026-09-17).
The first programme: a well-run trial that did not work
Novartis's largest test was RESILIENT (Hanna et al., Lancet Neurology, 2019; DOI 10.1016/S1474-4422(19)30200-5) in inclusion body myositis, the most common muscle disease in people over 50, which has no effective drug.
It was randomised, double-blind and placebo-controlled, at 38 sites, in 251 people given bimagrumab at 1, 3 or 10 mg/kg or placebo by infusion every four weeks. At week 52, the six-minute walking distance did not differ between any dose and placebo (10 mg/kg: +17.6 m, P = 0.22; 3 mg/kg: +18.6 m, P = 0.19; 1 mg/kg: −1.3 m, P = 0.93).
The authors described the safety profile as good; falls were the most frequent adverse event in every arm, placebo included, and muscle spasms were among the events most often reported with bimagrumab. The mechanism added muscle tissue in earlier studies; in this disease, it did not add walking.
The second programme: fat loss that keeps muscle
The obesity story began inside the Novartis programme. A phase 2 trial in 75 adults with type 2 diabetes and obesity (Heymsfield et al., JAMA Network Open, 2021; DOI 10.1001/jamanetworkopen.2020.33457) gave bimagrumab 10 mg/kg (up to 1,200 mg) or placebo every four weeks for 48 weeks, with diet and exercise counselling in both groups:
| Change at 48 weeks | Bimagrumab | Placebo |
|---|---|---|
| Total fat mass | −20.5% (−7.5 kg) | −0.5% |
| Lean mass | +3.6% (+1.7 kg) | −0.8% |
| Body weight | −6.5% | −0.8% |
| HbA1c | −0.76 points | −0.04 points |
Two limits matter: only participants who completed the full regimen were analysed, and 58 of 75 finished. Still, fat falling while lean mass rose is the reverse of what dieting and GLP-1 drugs usually do, and it is why the compound was revived.
Eli Lilly's BELIEVE trial (NCT05616013, 507 participants) then tested it with semaglutide. Its results are posted on ClinicalTrials.gov. Body weight change at 48 weeks:
| Arm | kg |
|---|---|
| Placebo | −3.31 |
| Bimagrumab 30 mg/kg alone | −9.25 |
| Semaglutide 2.4 mg alone | −14.24 |
| Bimagrumab 10 mg/kg + semaglutide 2.4 mg | −14.31 |
| Bimagrumab 30 mg/kg + semaglutide 2.4 mg | −17.79 |
The added weight loss appears only at the higher antibody dose; at 10 mg/kg the combination matched semaglutide alone. Weight is the posted primary outcome. The point of the drug is the composition of the loss — how much is fat and how much is muscle — and that is the figure to read when the full paper is available, not the kilograms.
Lilly's current studies pair it with tirzepatide, including a 252-participant phase 2 trial in obesity and a phase 1 study of a single co-formulated product.
Where it stands
An investigational antibody with a genuine, if mixed, record: one clear null in muscle disease, and consistent fat-loss-with-lean-gain signals in obesity at phase 2. It has no approved product and there is no legitimate consumer source. Anything sold online under this name is not the antibody tested in these trials, which is manufactured in cell culture and given by infusion.
Related entries in this library: trevogrumab takes the narrower route of blocking myostatin itself; follistatin is the body's own myostatin binder; semaglutide, the GLP-1 drug in BELIEVE, and tirzepatide are the drugs it is being tested beside.
Last checked
2026-09-17. The 17 ClinicalTrials.gov records and BELIEVE's posted results were read through the ClinicalTrials.gov v2 API, and the RESILIENT and Heymsfield abstracts through PubMed with DOIs verified by content negotiation, all on that date, as were the PubMed counts.