Lilly paid 1.9 billion for it. Bimagrumab is the activin receptor antibody at the center of the muscle-sparing weight-loss story, and 2026 is the year its real promise and real problems both came into focus.
Key takeaways
- Bimagrumab is a human IgG1 monoclonal antibody that blocks activin type II receptors (ActRIIA and ActRIIB).
- It started at Novartis as BYM338, was spun out into Versanis Bio, then bought by Eli Lilly in July 2023 for 1.9 billion dollars.
- In the BELIEVE trial with semaglutide, the high-dose combination produced 22.1 percent weight loss with only 2.9 percent lean-mass loss.
- The most common side effects are muscle spasms (57 to 64 percent), diarrhea, and acne, with mild eyelid drooping reported in earlier studies.
- Lilly withdrew a planned phase 2b T2DM obesity trial in September 2025 but kept the Zepbound combination program alive.
Quick facts on bimagrumab
| Field | Value |
|---|---|
| Drug name | Bimagrumab (formerly BYM338) |
| Class | Human IgG1 monoclonal antibody |
| Target | Activin type II receptors (ActRIIA and ActRIIB) |
| Sponsor | Eli Lilly (acquired Versanis Bio, originated at Novartis with MorphoSys) |
| Route | Intravenous infusion |
| Half-life | About 4 to 5 weeks; dosed every 4 to 12 weeks |
| Current status (May 2026) | Phase 2 in obesity with semaglutide and tirzepatide; not approved |
| Failed indication | Sporadic inclusion body myositis, phase 2b/3 (2016) |
| Key trial | BELIEVE (NCT05616013) |
For the broader competitive picture, see myostatin inhibitor obesity and the related anti-myostatin antibody class history.
What bimagrumab is and how it works
Bimagrumab is not a pure anti-myostatin drug. It is a fully human IgG1 antibody that binds the activin type II receptors with high affinity, blocking both ActRIIA and ActRIIB.
That receptor sits downstream of multiple ligands: myostatin (GDF8), activin A, activin B, GDF11, and BMP family members. Blocking the receptor is broader than blocking myostatin alone, which is why bimagrumab produces larger lean-mass and fat-mass effects than antibodies that hit only myostatin.
The trade-off is also broader. Hitting activin A means hitting adipose tissue directly, which is why bimagrumab causes meaningful fat loss without changing food intake.
How it got to Lilly
The path was unusual. Novartis developed BYM338 in partnership with MorphoSys, initially betting on it for sarcopenia and sporadic inclusion body myositis (sIBM). FDA granted breakthrough therapy designation in 2013.
The phase 2b/3 sIBM trial failed in 2016 and Novartis paused the program. The asset was spun out into Versanis Bio in 2021, refocused on obesity and body composition, and reported a positive phase 2 T2DM signal in JAMA Network Open the same year.
Eli Lilly acquired Versanis in July 2023 for up to 1.9 billion dollars in cash and milestones, joining tirzepatide (Zepbound) with bimagrumab inside one company. That combination shot is the entire reason the deal happened.
The 2021 JAMA Network Open T2DM trial
The first clean human signal. The 48-week phase 2 trial in adults with type 2 diabetes randomized 75 patients to bimagrumab 10 mg/kg IV every four weeks or placebo, both with standard diet and exercise counseling.
Mean age was 60.4 and baseline BMI was 32.9. After 48 weeks the bimagrumab group:
- Lost 20.5 percent of total body fat mass (7.49 kg) versus 0.5 percent on placebo.
- Gained 3.6 percent in lean mass (1.7 kg) versus a 0.8 percent loss on placebo.
- Dropped HbA1c by 0.76 percentage points versus a small rise on placebo.
- Lost 6.5 percent of body weight overall versus 0.8 percent on placebo.
The side-effect curve was visible even in that small trial: 41 percent reported muscle spasms and 41 percent reported diarrhea. Those two side effects have shadowed every bimagrumab study since.
The BELIEVE trial: bimagrumab plus semaglutide
This is the trial that matters. BELIEVE (NCT05616013) was a phase 2, multicenter, randomized, double-blind study at 26 sites across the United States, Australia, and New Zealand, with 507 adults randomized into nine arms over 72 weeks (48-week primary period plus 24-week extension).
Headline results at week 72
| Arm | Weight loss | Fat-mass loss | Lean-mass change | Visceral fat |
|---|---|---|---|---|
| Placebo | Small | About 0 percent | Slight loss | About 0 percent |
| Bimagrumab 30 mg/kg | 10.8 percent (12.0 kg) | 28.5 percent | Plus 2.5 percent | Minus 45.1 percent |
| Semaglutide 2.4 mg | 15.7 percent (16.5 kg) | 27.8 percent | Minus 7.4 percent | Minus 35.8 percent |
| Bim 30 mg/kg + Sema 2.4 mg | 22.1 percent (24.2 kg) | 45.7 percent | Minus 2.9 percent | Minus 58.2 percent |
84.9 percent of patients in the high-dose combination arm lost 15 percent or more of body weight, versus 51.8 percent on semaglutide alone.
Among patients with prediabetes at baseline, 100 percent reached normoglycemia by week 48 in the high-dose combination arm, versus 85 percent on semaglutide.
The lean-mass story
That is the part Lilly bought the drug for. Semaglutide alone took 7.4 percent off lean mass. Adding bimagrumab cut that to 2.9 percent. Bimagrumab monotherapy actually added 2.5 percent lean mass while still producing meaningful weight loss.
For an older adult, that gap is the entire clinical argument for muscle-sparing GLP-1 combinations.
Side effects, in detail
The good news is no surprises. The bad news is no surprises.
Across BELIEVE and the JAMA T2DM trial, the recurring side effects of bimagrumab are:
- Muscle spasms (commonly muscle cramps): 41 to 64 percent across studies.
- Diarrhea: 30 to 41 percent.
- Acne, particularly during the first 12 weeks.
- Mild eyelid drooping (ptosis) reported in some early phase studies but not as a frequent finding in BELIEVE.
- Transient bumps in alkaline phosphatase, creatine kinase, ALT, and AST.
- Occasional acne and skin findings.
Treatment discontinuations due to adverse events were 14 to 21 percent in bimagrumab monotherapy arms, 5 to 13 percent in combination arms, and 3 to 9 percent in semaglutide-only arms. Five participants stopped because of muscle spasms in the bimagrumab monotherapy group; none stopped for that reason in the combination groups.
Three pancreatitis cases were reported across the trial (one each in placebo, bimagrumab 10 mg/kg, and semaglutide 1.0 mg). Four basal or squamous-cell skin cancers were reported, all in monotherapy arms. No deaths occurred.
Why Lilly hit pause on the phase 2b
In September 2025 Lilly withdrew a planned phase 2b trial of bimagrumab in obese T2DM patients before enrollment opened. The 180-patient study had been scheduled to complete in late 2026.
Lilly called it "strategic business reasons" and emphasized that executives "routinely evaluate clinical development programs." Independent reporting (BioPharma Dive, Fierce Biotech) framed the move as Lilly trimming the bet to one combination shot, not killing the program.
The active phase 2 trial pairing bimagrumab with Zepbound (tirzepatide) in non-diabetic obesity is still running, with data expected through 2026. A separate combination phase 2b launched in March 2026 also continues.
The likely reading: Lilly believes in the muscle-sparing thesis but wants the cleanest, simplest registration trial possible, and stripping the diabetes arm reduces complexity and competing endpoints.
How bimagrumab compares to apitegromab and trevogrumab
Three different molecules, three different targets.
| Drug | Target | Specificity | Best human result | Main side effect |
|---|---|---|---|---|
| Bimagrumab | ActRIIA / ActRIIB receptor | Broad (myostatin, activins, GDF11) | 2.5 percent lean gain plus 10.8 percent weight loss | Muscle spasms in 41 to 64 percent |
| Apitegromab | Pro- and latent myostatin | Narrow, myostatin only | 55 percent more lean-mass preservation vs tirzepatide alone | Cleaner profile, lower spasm rate |
| Trevogrumab | Mature and latent myostatin | Narrow, with garetosmab adds activin A | Lean-mass loss cut to 2.0 percent in triple combo | Spasms (9 percent), serious AEs in triple arm |
The honest summary: bimagrumab produces the biggest body-composition swing, but at a real tolerability cost. Apitegromab is cleaner. Trevogrumab plus garetosmab approaches bimagrumab's potency but with an even rougher safety curve in the triple combination.
What investors and patients are watching
Five things will decide bimagrumab's fate:
- Whether the Zepbound combination phase 2 reproduces the BELIEVE lean-mass numbers.
- Whether the muscle-spasm rate can be lowered with dosing tweaks (every 12 weeks instead of every 4).
- Whether the skin-cancer signal in monotherapy disappears in combination data.
- Whether the FDA accepts an "incremental weight loss" or "lean-mass preservation" endpoint rather than absolute weight loss.
- Whether Lilly prioritizes bimagrumab or one of its in-house programs once retatrutide and orforglipron data mature.
Realistic timeline
A bimagrumab plus GLP-1 launch in obesity is unlikely before 2027 to 2028. Phase 3 will need to run roughly 18 to 24 months with a registration package after. Combination pricing is expected to land in the 1,500 to 2,000 dollar per month range in the United States.
That timeline puts bimagrumab among the first muscle-sparing GLP-1 add-ons that could plausibly reach pharmacies, alongside trevogrumab (Regeneron) and apitegromab (Scholar Rock).
Where to read next
- Myostatin inhibitors and GLP-1 muscle loss for the cross-drug comparison.
- Taldefgrobep alfa for the adnectin alternative.
- Anti-myostatin antibody for MYO-029, domagrozumab, and earlier failures.
- Myostatin inhibitor drug for the broader clinical map.
- Best myostatin inhibitor for ranking by use case.
Sources and notes
This article was built from DuckDuckGo and Bing SERP review, full-page competitor checks, and peer-reviewed trial data:
- Bimagrumab plus semaglutide alone or in combination - Nature Medicine 2026
- Effect of Bimagrumab vs Placebo on Body Fat Mass - JAMA Network Open 2021
- Bimagrumab - Wikipedia
- Lilly stops trial of muscle-sparing obesity drug - BioPharma Dive
- Eli Lilly withdraws phase 2b obesity trial - Fierce Biotech
- Effect of Bimagrumab on body composition: a systematic review and meta-analysis - PMC
- Pharmacokinetics and Pharmacodynamics of Bimagrumab (BYM338)
- Bimagrumab: Novel Medical Therapy for Inclusion Body Myositis - Cardiology in Review
- Safety and Efficacy of Bimagrumab and Semaglutide - NCT05616013
Frequently Asked Questions
Is bimagrumab approved by the FDA?
No. Bimagrumab is investigational. It is in phase 2 trials for obesity in combination with semaglutide and tirzepatide as of May 2026. There is no approved indication.
How is bimagrumab different from semaglutide?
Semaglutide is a GLP-1 receptor agonist that reduces appetite centrally. Bimagrumab is an antibody that blocks activin type II receptors in muscle and fat tissue, producing fat loss and lean-mass gain without changing food intake. The two mechanisms add up: BELIEVE showed greater fat loss and far less lean-mass loss when combined.
What are the main side effects of bimagrumab?
Muscle spasms (cramps) hit 41 to 64 percent of patients across trials, diarrhea about 30 to 41 percent, and acne is common in the first 12 weeks. Transient elevations in liver enzymes and creatine kinase have been reported. In early studies, mild eyelid drooping was noted; it has not been a frequent finding in BELIEVE.
Why did Lilly withdraw the phase 2b bimagrumab trial?
In September 2025 Lilly withdrew a planned phase 2b in obese T2DM patients before enrollment opened, citing "strategic business reasons." The Zepbound combination phase 2 remains active. Analysts read the pull-back as Lilly simplifying its registration plan, not abandoning the program.
How does bimagrumab compare to apitegromab?
Bimagrumab blocks the receptor and produces larger body-composition swings (about 2.5 percent lean gain plus 10.8 percent weight loss as monotherapy). Apitegromab blocks only pro- and latent myostatin, which gives a smaller but cleaner effect: about 55 percent more lean-mass preservation versus tirzepatide alone, with a lower muscle-spasm rate.
When might bimagrumab actually launch?
The earliest plausible approval is 2027 to 2028, assuming phase 3 reproduces BELIEVE. Combination pricing in the United States is likely to be 1,500 to 2,000 dollars per month, with infusion or injection burden on top.
This article is for educational purposes only and is not medical advice. Bimagrumab is an investigational antibody that is not approved for any indication and is only available within a registered clinical trial. Talk with a qualified clinician before changing any medication, supplement, or weight-loss strategy.
