Peptides
Bimagrumab
An investigational activin-receptor antibody with human fat-loss and lean-mass findings, including a newer obesity trial alongside semaglutide.
Randomized phase 2 obesity trials; body composition is not a proven functional outcome
Also known as: BYM338
Practical use
Evidence interpretation and clinical context
- Check the denominator
- Write down whether a percentage describes fat, lean tissue or whole-body weight.
- Keep completion rates visible
- The 58 of 75 completion figure affects how confidently the early result generalizes.
- Compare like populations
- Do not treat the diabetes trial and the later trial excluding diabetes as identical settings.
Overview
Bimagrumab blocks type II activin receptors and affects signaling involved in muscle and fat biology. Its mechanism differs from GLP-1 receptor agonism. Lilly’s current development pipeline continues to list the molecule in clinical development.
Risks and limitations
Muscle spasms, diarrhea and acne were common bimagrumab adverse events in the newer trial. Combining mechanisms adds separate tolerability questions. Phase 2 studies cannot fully characterize uncommon or long-latency harms.
How to read this
Fat mass, scale weight and lean mass answer different questions. A fall in fat mass with preservation or gain of lean mass can look different on the scale from weight loss that includes both tissues.
Research explained
Effect of Bimagrumab vs Placebo on Body Fat Mass Among Adults With Type 2 Diabetes and Obesity: A Phase 2 Randomized Clinical Trial.
Who was studied
75 adults with overweight or obesity and type 2 diabetes randomized for 48 weeks; 58 completed the study.
What was found
Fat mass changed −20.5% with bimagrumab versus −0.5% with placebo; lean mass changed +3.6% versus −0.8%. Both groups received diet and exercise counseling.
What it cannot establish
The reported analysis relied on completers, and sex distribution differed between groups. The trial did not establish strength, disability or cardiovascular-event benefit.
Read the original studyBimagrumab plus semaglutide alone or in combination for the treatment of obesity: a randomized phase 2 trial.
Who was studied
507 adults with obesity, excluding diabetes, randomized across nine groups for 48 weeks, followed by an open-label extension.
What was found
Selected higher-exposure groups lost 9.3 kg with bimagrumab, 14.2 kg with semaglutide and 17.8 kg with their combination, versus 3.3 kg with placebo at week 48.
What it cannot establish
These are selected trial arms, not a universal expected response or a treatment recipe. The later extension was open label.
Read the original studyCurrent status
The sponsor lists bimagrumab in its clinical-development pipeline. The phase 2 publication describes it as investigational; neither source establishes an approved obesity indication.
Supporting sourceDo not turn a scan into a strength claim
The earlier trial’s main endpoint was fat mass. An increase in lean mass on body-composition testing needs separate evidence before being described as stronger, more capable muscle.
Supporting sourceClaims versus findings
Not established
The lean-mass increase proves better athletic performance.
The reported endpoints do not demonstrate better sports performance or long-term physical function.
Check the evidenceCommon questions
Can the percentage fat change be read as percentage body-weight loss?
No. The denominator is fat mass, not total body weight.
SourceSources
Current status — primary or official source
Sources checked 2026-10-01.
General education; not a personal treatment plan.