Bimagrumab

Bimagrumab is an investigational monoclonal antibody—not a peptide—that blocks activin receptors IIA and IIB. Human randomized trials show that bimagrumab weight loss comes mainly from fat and that adding it to semaglutide limits lean-mass loss, but it is not FDA-approved and muscle spasms and acne remain distinct risks.

Key facts

• Evidence: Human randomized trials; promising Phase 2 body-composition results, no Phase 3 verdict.
• U.S. status (July 2026): Investigational and not FDA-approved.
• Research use: Intravenous infusions in BELIEVE; newer trials are testing subcutaneous dosing.
• Main risks: Muscle spasms, acne and diarrhea; long-term obesity safety is not established.
• Sport: Prohibited at all times under WADA class S4.3.

What is bimagrumab?

bimagrumab is a full-size human IgG1 monoclonal antibody, not a peptide, GLP-1 drug or supplement. Its aliases include BYM338, VER201 and Lilly’s development code LY3985863. The antibody was first studied for muscle-wasting diseases, then moved into obesity research after a 75-person randomized trial showed an unusual pairing: fat mass fell while lean mass rose.

That distinction matters on a peptide reference. Bimagrumab appears beside peptides because the target pathway overlaps with follistatin, myostatin and muscle-growth research. Chemically, though, calling it a peptide is like calling a cargo ship a canoe because both float. The verified substance record gives CAS 1356922-05-8, UNII N15SW1DIV8, DrugBank DB12584 and ChEMBL CHEMBL3137353.

How does bimagrumab work?

bimagrumab blocks activin receptors IIA and IIB, preventing activin and myostatin signals from applying their usual brake to skeletal muscle. Muscle cells then increase protein production and reduce protein breakdown. In fat tissue, the same pathway shift promotes lipolysis—the release of stored fat—so body composition can move in two directions at once.

The mechanism is closer to removing a parking brake than pressing a growth-hormone accelerator. Bimagrumab does not raise growth hormone, but the muscle-preservation goal makes the growth-hormone and muscle hub its closest site category. More lean mass is also not automatically more strength: BELIEVE found a grip-strength benefit in one combination group, while most groups looked similar to placebo.

What did the BELIEVE trial actually show?

The BELIEVE trial randomized 507 adults with obesity or overweight plus a complication to nine groups for 48 weeks, followed by an extension to week 72. The headline bimagrumab semaglutide combination worked: high-dose treatment produced an estimated 22.1% weight reduction at week 72, versus 15.7% with semaglutide 2.4 mg and 10.8% with bimagrumab 30 mg/kg alone (Nature Medicine, 2026).

The fine print is more useful than the press-release number. At week 72, 92.2% of high-dose combination weight loss came from fat, not 92.8%. Lean mass rose 2.5% with bimagrumab alone, fell 7.4% with semaglutide, and fell 2.9% with the combination. Bimagrumab reduced semaglutide-associated lean loss; the combination did not preserve every gram.

The 22.1% figure is the week-72 efficacy estimate among data modeled for participants who could stay on treatment. At week 48, the treatment-regimen estimate—closer to what happened regardless of stopping—was 16.4% for the high-dose combination and 13.5% for semaglutide. That estimand distinction is missing from many bimagrumab weight loss summaries.

Does bimagrumab prevent muscle loss from GLP-1 drugs?

bimagrumab reduced, but did not eliminate, lean-mass loss when paired with semaglutide in BELIEVE. This is the clearest human answer so far to the bimagrumab muscle loss question. The combination’s 2.9% lean-mass decline at week 72 was far smaller than semaglutide’s 7.4% decline, while 92.2% of total loss came from fat.

The result supports a muscle-sparing strategy, not a guarantee of stronger muscle. Dual-energy X-ray absorptiometry measures lean tissue, which is broader than contractile muscle, and most grip-strength comparisons were not better than placebo. See why GLP-1 drugs can reduce lean mass for the difference between scale weight, lean mass and physical function.

What are bimagrumab side effects?

bimagrumab side effects in BELIEVE were led by muscle spasms, diarrhea and acne; nausea, constipation and fatigue tracked more closely with semaglutide. Five participants stopped bimagrumab-only treatment because of muscle spasms, four participants stopped bimagrumab-containing treatment because of acne, and the study recorded one severe acne event and five severe muscle-related events.

Treatment-emergent adverse events occurred in 91.1% to 98.2% of active-treatment groups versus 74.5% with placebo. More importantly, adverse-event discontinuations were 14.0% to 21.4% with bimagrumab alone, compared with 3.6% to 8.8% for semaglutide and 5.3% to 12.5% for combinations. No deaths occurred, but an investigational antibody with this discontinuation pattern has not earned a casual “muscle-preserving add-on” label.

Is bimagrumab FDA-approved, and does Lilly still own it?

bimagrumab is not FDA-approved in the United States as of July 16, 2026. FDA’s orphan-drug record says the old inclusion-body-myositis designation was withdrawn or revoked and was never approved for that indication. Lilly completed its Versanis acquisition in August 2023, bringing bimagrumab into Lilly’s obesity pipeline.

The bimagrumab Lilly development story was narrowed, not abandoned. Lilly withdrew NCT06901349, a Phase 2b bimagrumab-plus-tirzepatide trial in people with type 2 diabetes, for strategic business reasons before anyone enrolled. A separate 252-person obesity trial without diabetes, NCT06643728, remained active but not recruiting in May 2026, with primary completion recorded in January 2026. An investigator-led trial was still recruiting in March 2026.

How was bimagrumab used in research?

bimagrumab was given by intravenous infusion in BELIEVE, not injected from a retail peptide vial. The trial used 10 or 30 mg/kg, with doses at baseline and week 4 followed by dosing every 12 weeks; semaglutide arms used 1.0 or 2.4 mg weekly. These are research regimens, not approved dosing instructions.

Newer Lilly studies are testing subcutaneous formulations, but no labeled dose, home-use product or legitimate “research peptide” version exists. Bimagrumab is also prohibited at all times by WADA under S4.3, which explicitly covers anti-activin receptor IIB antibodies. The evidence has reached people; routine clinical use has not.