Bimagrumab is an investigational monoclonal antibody that blocks activin type II receptors. The 507-person BELIEVE phase 2 trial tested it alone, semaglutide alone, and four bimagrumab-plus-semaglutide dose combinations in adults with obesity.
The peer-reviewed BELIEVE report replaces the earlier unpublished framing of these results. Under the treatment-regimen estimand at week 48, the high-dose combination group lost 17.8 kg on average, compared with 14.2 kg in the semaglutide 2.4 mg group and 3.3 kg with placebo. Under the efficacy estimand at week 72, average body-weight reduction was 22.1% with the high-dose combination and 15.7% with semaglutide 2.4 mg.
Bimagrumab is not FDA-approved, and BELIEVE does not justify saying it prevented muscle loss or increased muscle in every combination group.
Sponsorship history
Novartis originally developed bimagrumab for muscle disorders. Versanis Bio later advanced it for obesity and sponsored BELIEVE. Lilly completed its acquisition of Versanis in 2023, bringing bimagrumab into Lilly's pipeline.
Calling it simply a "Novartis drug" misses the current development history.
How bimagrumab differs from semaglutide
Semaglutide, the active drug in Ozempic and Wegovy, activates the GLP-1 receptor and reduces energy intake. Bimagrumab binds activin type II receptors, affecting signaling in skeletal muscle and adipose tissue.
The mechanism explains why researchers measured body composition, but it does not prove a clinical benefit by itself. Lean mass measured by DXA includes more than skeletal muscle, and a body-composition change does not automatically mean improved strength or physical function.
BELIEVE trial design
BELIEVE randomized 507 adults with obesity to nine groups:
- Placebo.
- Two bimagrumab doses.
- Semaglutide 1.0 mg or 2.4 mg.
- Four dose combinations of bimagrumab and semaglutide.
The primary treatment period ran for 48 weeks, followed by an extension through week 72. Only 74.4% of participants completed the primary period, so discontinuation and the trial's estimand matter when reading the averages.
Weight results
At week 48, average absolute body-weight changes under the treatment-regimen estimand included:
| Group | Average change at week 48 (treatment-regimen estimand) |
|---|---|
| High-dose bimagrumab + semaglutide 2.4 mg | -17.8 kg |
| Semaglutide 2.4 mg | -14.2 kg |
| Placebo | -3.3 kg |
At week 72, the efficacy-estimand analysis found average percentage weight reduction of 22.1% with the high-dose combination and 15.7% with semaglutide 2.4 mg. These are group averages from an investigational trial, not predicted individual outcomes.
Lean-mass results
Under the week 48 treatment-regimen estimand, average lean-mass reductions across the four combination groups were about 0.8% to 2.3%. The semaglutide-only groups had reductions of about 4.7% to 6.9%.
That finding supports the narrower statement that BELIEVE observed less lean-mass loss with the combinations. It does not support saying bimagrumab fully prevented muscle loss. The combination groups still had average lean-mass reductions, and DXA lean mass is not identical to muscle strength or function.
At week 48, the treatment-regimen-estimand analysis in kilograms showed the same direction: lean mass fell by 0.6 to 1.3 kg across the combination groups and by 2.6 to 3.9 kg in the semaglutide-only groups.
Safety and discontinuation
During the first 48 weeks, adverse-event discontinuation ranged from 5.3% to 12.5% across combination groups, 3.6% to 8.8% in semaglutide groups, and 14.0% to 21.4% in bimagrumab-only groups. Placebo discontinuation due to adverse events was 3.6%.
The trial was phase 2 and was not designed to establish long-term safety or show fewer fractures, disability, or other clinical outcomes related to muscle health.
Tirzepatide combination study
Lilly is also studying bimagrumab with tirzepatide. NCT06643728 is a 252-person phase 2 trial in adults with obesity or overweight without type 2 diabetes. It includes bimagrumab, tirzepatide, combination, and placebo groups and measures body weight, fat mass, and lean mass.
A trial registration shows what researchers plan to measure. It does not establish that the combination works or is safe.
What people can do now
Bimagrumab is not available as an approved treatment. There is no evidence-based launch year to offer. People concerned about lean mass during weight loss can ask a clinician or registered dietitian about an individualized nutrition and resistance-training plan, particularly if they have frailty, kidney disease, limited mobility, or another condition that changes what is safe.
A single fixed protein target is not appropriate for every reader. Needs vary with body size, age, health, activity, total energy intake, and kidney function.
This article is for informational purposes only and does not constitute medical advice. Individual results vary.
Frequently asked questions
What is bimagrumab?
Bimagrumab is an investigational monoclonal antibody that blocks activin type II receptors. It was originally developed by Novartis, later advanced by Versanis Bio, and entered Lilly’s portfolio when Lilly acquired Versanis in 2023.
Did bimagrumab prevent muscle loss with semaglutide?
No trial can support the absolute word prevent. In BELIEVE, average lean-mass reductions were smaller in the bimagrumab-plus-semaglutide groups than in the semaglutide-only groups, but lean mass still declined in the combination groups.
Is bimagrumab FDA-approved?
No. Bimagrumab remains investigational. No reliable approval or launch date has been established.
