Evidence Grade C — Moderate human evidence. 74 published studies, 52 human. 13 registered clinical trials.
Medically reviewed by a licensed medical professional
Bimagrumab is a monoclonal antibody — not a peptide — that simultaneously reduces body fat while increasing lean muscle mass. Acquired by Eli Lilly for $1.9 billion, it is being developed as a partner to GLP-1 drugs like tirzepatide. Its unique value is addressing the muscle loss that typically accompanies weight loss with current obesity medications — ensuring patients lose fat, not muscle.
Bimagrumab is also known by these brand and alternate names:
74 published studies: 52 human, 5 animal, 2 in-vitro, 31 reviews
Bimagrumab is in Phase II/III development (not yet approved). A Phase II trial (75 patients with type 2 diabetes and obesity) showed 20.5% fat mass reduction with 3.6% lean mass increase — essentially reshaping body composition without net appetite suppression. The BELIEVE Phase IIb trial (507 patients) showed that bimagrumab plus semaglutide achieved 22% weight loss with the weight lost being 88% fat — significantly better body composition than semaglutide alone.
Bimagrumab is not a peptide. Its strategic significance is as a combination partner to address the muscle loss problem inherent in weight loss with current obesity drugs. Eli Lilly's acquisition positions it alongside tirzepatide.
Bimagrumab blocks the activin type II receptors that normally transmit muscle-limiting signals from myostatin, activin, and GDF-11. By removing these growth-restraining signals, the antibody promotes muscle protein synthesis and fat mobilisation simultaneously. Crucially, bimagrumab does not suppress appetite — it works through a fundamentally different mechanism from GLP-1 agonists, making it a natural combination partner.
The BELIEVE trial (507 patients) showed that bimagrumab combined with semaglutide achieved 22% total weight loss, with 88% of the weight lost being fat — significantly better body composition than semaglutide alone. In an earlier trial, it produced 20.5% fat mass reduction alongside a 3.6% increase in lean muscle mass. This body composition advantage addresses a genuine limitation of current obesity treatments: rapid weight loss typically includes significant muscle loss, which can reduce metabolic rate and physical function. Phase 2 trials combining bimagrumab with tirzepatide are underway, but Phase 3 has not yet been initiated. Side effects include muscle spasms, acne, and temporary liver enzyme elevations. An earlier trial in inclusion body myositis (a muscle disease) failed its primary endpoint.
PeptideTrace tracks 13 registered clinical trials for Bimagrumab sourced from ClinicalTrials.gov.
Safety and Efficacy of Bimagrumab and Semaglutide in Adults Who Are Overweight or Obese
Safety, Pharmacokinetics and Efficacy of Bimagrumab in Overweight and Obese Patients With Type 2 Diabetes
An Extension Study of the Efficacy, Safety and Tolerability of BYM338 (Bimagrumab) in Patients With Sporadic Inclusion Body Myositis Who Previously Participated in the Core Study CBYM338B2203
A 24-week Off-drug Extension Study in Sarcopenic Elderly Who Completed Treatment in the 6-month Core Study
Dose Range Finding Study of Bimagrumab in Sarcopenia
Bimagrumab (BYM338) is a fully human IgG1 monoclonal antibody targeting activin type II receptors (ActRIIA and ActRIIB). NOTE: This is a full monoclonal antibody, NOT a peptide. Molecular weight approximately 150 kDa. Originally developed by Novartis/MorphoSys; licensed to Versanis Bio; Eli Lilly acquired Versanis in July 2023 for approximately $1.9 billion. Currently administered as intravenous infusion, with a subcutaneous formulation under development by Lilly.
Bimagrumab blocks signaling of myostatin, activin A, and GDF11 through activin type II receptors, disinhibiting skeletal muscle protein synthesis and hypertrophy while simultaneously promoting fat mobilization and brown adipose tissue differentiation. Critically, the compound does NOT suppress appetite, distinguishing its mechanism from all incretin-based therapies. The result is simultaneous fat mass reduction and lean mass preservation or increase, addressing the fundamental body composition limitation of GLP-1-based weight loss therapies.
Phase 2 T2D/obesity (Heymsfield et al., JAMA Network Open 2021; N=75): fat mass decreased -20.5% versus -0.5% placebo (P<0.001); lean mass increased +3.6% versus -0.8% (P<0.001); HbA1c improved -0.76% versus +0.04% (P=0.005). BELIEVE Phase 2b (N=507; ADA 2025; Nature Medicine 2026): bimagrumab 30 mg/kg plus semaglutide 2.4 mg achieved -22.1% weight loss at 72 weeks versus -15.7% with semaglutide alone. Body composition quality: 92.8% of weight loss was fat in the combination group versus 71.8% with semaglutide alone. Lean mass loss was only 2.6% (combination) versus 7.9% (semaglutide alone), representing 67% more muscle preservation. Bimagrumab monotherapy produced -10.8% weight loss with 100% derived from fat and +2.5% lean mass gain.
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Semaglutide is one of the most extensively studied peptide medications available today, with clinical trial data spanning over 50,000 patients. It is sold as Ozempic for type 2 diabetes (approved 2017), Rybelsus as an oral tablet for diabetes (approved 2019), and Wegovy for weight management (approved 2021). Wegovy has since received additional approvals for reducing heart attack and stroke risk, for use in adolescents aged 12 and older, and for liver disease related to obesity. In clinical trials, patients taking Wegovy lost an average of 14.9% of their body weight over 68 weeks, and the SELECT trial demonstrated a 20% reduction in major cardiovascular events such as heart attack and stroke. Research is now exploring its potential in early Alzheimer's disease. The main side effects are gastrointestinal, particularly nausea, and a key limitation is that most weight tends to return after stopping treatment.
Pemvidutide is in Phase II/III development (not yet approved). In the MOMENTUM Phase II trial (391 patients, 48 weeks), the 2.4 mg dose achieved 15.6% weight loss. Body composition analysis showed 78% of weight lost was fat mass, with only 22% lean mass — a favourable ratio. A Phase IIb MASH trial (212 patients) showed MASH resolution in up to 75% of patients at the highest dose. Pemvidutide's balanced receptor profile and no-titration dosing regimen represent potential practical advantages. The high rate of MASH resolution in Phase II positions it as a candidate for liver disease indications alongside obesity.
MariTide is in Phase II/III development (not yet approved). Phase II results (592 patients, 52 weeks) showed 16–20% weight loss across doses. Body composition analysis showed 26–37% fat mass reduction with 8–13% lean mass reduction. Notably, weight loss appeared to continue beyond the 52-week treatment period without plateau. MariTide's monthly dosing would be a significant convenience advantage if efficacy is confirmed in Phase III. The ongoing weight loss trajectory without plateau is particularly interesting and distinguishes it from some GLP-1 agonists where weight loss plateaus. Amgen is conducting Phase III trials.
Evidence Reviews
Timelines
This content is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before making decisions about your health.
