Bimagrumab: what it is, the BELIEVE results, side effects, price and when it could reach the UK

Bimagrumab is a fully human monoclonal antibody that binds the activin type II receptors (ActRIIA and ActRIIB) and blocks the signals from myostatin, activin A and related proteins that limit muscle growth. It is owned by Eli Lilly, which bought its developer Versanis Bio in 2023, and it is the most advanced of the muscle-preserving antibodies being tested alongside GLP-1 drugs. This article explains what it is, its long history at Novartis, what the BELIEVE trial with semaglutide reported, its side effects, where the tirzepatide programme stands, what it might cost, when it could reach the UK, and how the research-grade versions sold as laboratory reagents differ from Lilly's antibody. It exists because visitors to our retatrutide page ask about it. Bimagrumab is investigational; ALTA PEPTIDES supplies a research-grade version for laboratory use only and this is not guidance on using anything.

What is bimagrumab?

In shortA Lilly antibody that blocks the muscle brakes, tested as an add-on to weight-loss jabs.

Made byEli Lilly

Developed at Novartis as BYM338, bought by Versanis, then Lilly.

BlocksActRII receptors

Shuts out myostatin, activins and other muscle brakes at once.

TypeAntibody

A large protein given by infusion or injection, not a peptide.

StatusPhase 2

Not approved anywhere, for anything.

From this article.

Most myostatin inhibitors bind a single ligand. Bimagrumab binds the receptor instead, so it blocks myostatin, activin A and related ligands at once. That breadth is why it has a larger effect on body composition than the single-ligand antibodies, and why it is watched for the off-target signals that sank ACE-031, the earlier receptor-based ligand trap halted for bleeding. It was given by intravenous infusion in the trials to date; Lilly has said it has moved to subcutaneous dosing for its own studies. Our myostatin inhibitor pillar explains the pathway and where bimagrumab acts on it.

What did the BELIEVE trial show?

In shortWith semaglutide, 22.1% weight lost and nearly all of it fat. On its own, weight fell and muscle grew.

Semaglutide alone

15.7%

Semaglutide + bimagrumab

22.1%

Bimagrumab aloneall of it fat; lean mass rose

10.8%

Combination92.8% fat

Of all weight lost.

Semaglutide alone71.8% fat

The rest was lean mass.

Bimagrumab aloneLean +2.5%

Weight fell, muscle grew.

Percent body weight lost at 72 weeks, top doses. Heymsfield et al., Nature Medicine 2026.

BELIEVE was a randomised, double-blind, placebo-controlled trial in 507 adults with overweight or obesity, run by Versanis and completed under Lilly. Participants received semaglutide 1.0 or 2.4 mg weekly, intravenous bimagrumab 10 or 30 mg/kg at weeks 1 and 4, then every 12 weeks (weeks 16, 28 and 40), or both, for 48 weeks with an open-label extension to 72 weeks. The primary endpoint was change in body weight, with fat mass, visceral fat, waist circumference and lean mass as secondary endpoints. Results were presented at the American Diabetes Association meeting on 23 June 2025 by Steven Heymsfield of Pennington Biomedical and published in Nature Medicine in March 2026.

• Body weight at 72 weeks: -10.8% (-12.0 kg) with bimagrumab 30 mg/kg alone; -15.7% (-16.5 kg) with semaglutide 2.4 mg alone; -22.1% (-24.2 kg) with the high-dose combination.
• Share of loss from fat: 100% with bimagrumab alone, 71.8% with semaglutide alone, 92.8% with the combination.
• Fat mass: -28.5% with bimagrumab alone, -27.8% with semaglutide alone, -45.7% with the combination.
• Lean mass: +2.5% with bimagrumab alone; preserved with the combination against loss on semaglutide alone.
• Responders: 69.8% of the combination group lost 20% or more of body weight, against 10.9% on bimagrumab alone and 25.0% on semaglutide alone; 94.0% of the combination group lost 30% or more of fat mass.

Bimagrumab alone matched semaglutide on fat loss with minimal change in food intake, which is a different mechanism from appetite suppression. The combination also improved the SF-36 physical functioning score against semaglutide alone.

How does it compare with semaglutide, tirzepatide and retatrutide?

In shortThose drugs drive the weight loss. Bimagrumab changes what the weight loss is made of.

Bimagrumab vs semaglutide is the only head-to-head that exists, because BELIEVE ran both alone and together in 507 adults over 72 weeks. Bimagrumab 30 mg/kg alone: body weight -10.8%, fat mass -28.5%, lean mass +2.5%, 100% of the loss from fat. Semaglutide 2.4 mg alone: body weight -15.7%, fat mass -27.8%, 71.8% of the loss from fat. High-dose combination: body weight -22.1%, fat mass -45.7%, 92.8% from fat, with lean mass preserved. Semaglutide's own STEP 1 DXA substudy (140 participants, 68 weeks) reported fat mass -19.3% and lean body mass -9.7% with 15.0% weight loss.

Bimagrumab vs tirzepatide has no head-to-head. Tirzepatide's SURMOUNT-1 DXA substudy (160 participants, 72 weeks) reported fat mass -33.9% and lean mass -10.9%, with about 75% of the weight lost being fat and 25% lean. That lean share is lower than semaglutide's in COURAGE (33%) and the same as tirzepatide's in the EMBRAZE apitegromab trial (30%), which is the range the antibodies are being developed to improve on.

Bimagrumab vs retatrutide has no head-to-head either. Retatrutide's phase 2 body composition substudy in type 2 diabetes (Lancet Diabetes and Endocrinology, 2025) reported fat mass reductions at 36 weeks of 4.9%, 15.2%, 26.1% and 23.2% across the 0.5, 4, 8 and 12 mg groups against 4.5% on placebo, with lean mass loss of up to 6.5 kg in the highest-dose groups; the authors described the ratio of fat loss to total loss as consistent with other weight-loss treatments. Retatrutide's phase 2 in obesity reported 24.2% weight loss at 48 weeks at the highest dose, and several phase 3 trials have now reported results; our phase 3 summary has those results. Retatrutide loses far more weight than bimagrumab; bimagrumab loses it with a better composition. Neither number says anything about the two together, because that has not been tested.

Bimagrumab vs retatrutide, side by side

In shortThe key numbers for each, in one table.

Figures are from different trials with different populations and durations and are not directly comparable; they are shown together only to make the pattern visible.

What are the side effects?

In shortMostly muscle spasms, diarrhoea and acne. 9% stopped; no deaths.

Most commonMuscle spasms

Plus diarrhoea and acne.

Blood testsBrief enzyme rises

ALT and lipase, early and temporary.

Stopped for side effects9%

Combination groups, 72 weeks.

DeathsNone

In BELIEVE.

Bimagrumab + semaglutideBELIEVE, 72 wk

9%

Trevogrumab + garetosmab + semaglutideCOURAGE triplet, 26 wk

30.9%

Share stopping because of side effects. BELIEVE: Heymsfield et al., Nature Medicine 2026. COURAGE: Regeneron, 17 Sep 2025.

BELIEVE randomised 507 adults with overweight or obesity to semaglutide, intravenous bimagrumab (10 or 30 mg/kg at weeks 1 and 4, then every 12 weeks) or both, for 48 weeks plus an open-label extension to 72 weeks. Results were presented at the American Diabetes Association meeting on 23 June 2025 and published in Nature Medicine in March 2026. The investigators reported no new safety signals. The common bimagrumab adverse events were muscle spasms, diarrhoea and acne; semaglutide's were nausea, diarrhoea, constipation and fatigue, and the combination groups had both. Bimagrumab-containing groups showed early, transient increases in ALT (a liver enzyme) and in lipase (a pancreatic enzyme), while semaglutide groups showed a sustained lipase increase. Across the combination groups, 9% discontinued for adverse events over 72 weeks. There were no deaths. For comparison, the COURAGE triplet of trevogrumab, garetosmab and semaglutide had a 30.9% discontinuation rate and two deaths over 26 weeks, which is one reason bimagrumab's profile is described as the cleaner of the two combinations despite its broader mechanism; our trevogrumab side effects has that record.

Where does the programme stand now?

In shortStill phase 2. A tirzepatide trial is running; no phase 3 yet.

Lilly's plan was to pair bimagrumab with its own tirzepatide rather than Novo Nordisk's semaglutide. In March 2025 it registered a nine-arm phase 2b trial in 180 people with obesity and type 2 diabetes; in September 2025 it withdrew the study before enrolling anyone, citing "strategic business reasons", and told reporters that bimagrumab "is still currently being studied in an ongoing phase 2 trial with tirzepatide, alone or in combination, in patients with obesity", with results expected in 2026. No results from that trial had been published at the time of writing, and Lilly has not announced a phase 3. The Nature Medicine paper and continuing enrolment suggest the programme is alive; the withdrawn trial suggests Lilly is deciding how much to invest against retatrutide and its other obesity assets. For the record, TRIUMPH is the name of Lilly's retatrutide phase 3 programme, not a bimagrumab trial.

When could bimagrumab be available in the UK?

In shortNot for years: it needs a phase 3, then an MHRA licence, then NICE.

1. 2025BELIEVE reads out

   Phase 2b with semaglutide.

2. Sep 2025One trial withdrawn

   Lilly drops a planned tirzepatide study before enrolment.

3. Jan 2027Tirzepatide trial ends

   252-person phase 2 due to complete.

4. Not yetPhase 3

   None announced. Needs at least a year.

5. After thatMHRA, then NICE

   UK licence, then NHS appraisal.

Dates from Lilly, ClinicalTrials.gov NCT06643728 and this article. Our estimate for the class: not before about 2030.

Bimagrumab UK availability has two gates. The first is a marketing authorisation from the MHRA, which requires a complete dossier including phase 3 efficacy and safety data; nothing has been submitted. The second, for NHS use, is a NICE technology appraisal, which judges cost-effectiveness against what the NHS already pays for. Wegovy and Mounjaro both went through that process and both are restricted on the NHS to people meeting clinical criteria, with Mounjaro rolled out in phases from 2025 to the highest-need groups first. An antibody added on top of a GLP-1 drug to preserve muscle would have to show NICE that muscle preservation is worth paying for, which no trial has yet measured in terms NICE uses. Private prescribing in the UK would require the MHRA licence first. There is no route by which bimagrumab reaches a UK patient before those steps, and a bimagrumab NHS listing does not exist.

How much will bimagrumab cost?

In shortThere is no price. Similar antibodies for rare diseases cost hundreds of thousands a year.

Garetosmab (Pasatru)rare bone disease, US list

$1.4m a year

Apitegromab (Isembyld)SMA, US net

$310k a year

Rare-disease prices from 2026 launches; not a guide to an obesity price. Bimagrumab has never been priced.

No price exists. What follows is analogy, stated as such. The two approved antibodies in this class are rare-disease infusions priced accordingly: Regeneron put PASATRU at about $1.4 million a year for an average-weight trial patient, and Scholar Rock said ISEMBYLD would cost about $310,000 a year net for a typical patient, with a wholesale acquisition cost of $11,659 per vial. Those are prices for diseases affecting hundreds or thousands of people and tell you nothing about an obesity product for millions, except that antibodies are expensive to make. The better comparators are the drugs bimagrumab would be paired with. In the UK, private prescriptions for Wegovy (semaglutide 2.4 mg) are advertised at roughly £120 to £300 a month depending on dose and provider, and Mounjaro (tirzepatide) at roughly £150 to £340 a month at the higher doses; on the NHS a patient pays the standard prescription charge in England and nothing in Scotland, Wales and Northern Ireland. If bimagrumab were licensed as an add-on, it would be priced on top of those, and Lilly would set it against the value of preserved muscle. Whether that lands at tens of pounds a month or hundreds is unknowable now. Our UK availability timeline records any price the moment one is announced.

Can you buy bimagrumab?

In shortNo. Anything sold online under the name is not the real drug.

Real bimagrumabTrials only

Made in Lilly's plants; not sold.

Research-grade vialsLab reagent

Recombinant antibody from a third-party maker; check the CoA.

ALTAResearch grade

150 mg vials, laboratory research only.

From this article.

Research-grade versions of these antibodies are sold by specialist reagent suppliers: recombinant monoclonal antibodies made from the published sequences by third-party manufacturers, for in-vitro laboratory work. They are not the originator companies' products, they are not approved biosimilars, and they are not medicines; identity and purity rest on the manufacturer's certificate of analysis (for an antibody, methods such as SEC-HPLC and SDS-PAGE), which is the first thing to check. ALTA PEPTIDES supplies research-grade bimagrumab, apitegromab, trevogrumab and garetosmab as 150 mg vials under our research use policy, for laboratory research only and never for human use.

Antibody vs peptide: why they are different kinds of drug

In shortAntibodies are big proteins grown in cells; peptides are small chains made by chemistry.

Semaglutide activates the GLP-1 receptor; tirzepatide activates the GIP and GLP-1 receptors; retatrutide activates GIP, GLP-1 and glucagon receptors. All three are synthetic peptides of a few thousand daltons, given by weekly injection, and their weight loss comes from reduced appetite and food intake, with glucagon adding energy expenditure in retatrutide's case. Our GLP-1 explainer covers the class. Bimagrumab is a fully human antibody of about 150 kilodaltons that binds the activin type IIA and IIB receptors on skeletal muscle and elsewhere, blocking myostatin, activin A and related ligands that limit muscle growth. It was given by intravenous infusion in the trials to date; Lilly has said it moved to subcutaneous injection for its own studies. Its weight loss comes from fat loss with minimal change in food intake, a different route entirely. Our pillar article explains the pathway.

Why test it alongside tirzepatide?

In shortLilly owns both, and wants weight loss that keeps muscle.

Lilly owns both molecules, and the logic is the same as Regeneron's with trevogrumab and semaglutide or Scholar Rock's with apitegromab and tirzepatide: the incretin drives the loss and the antibody protects the muscle. BELIEVE showed the principle with semaglutide, but Lilly has no interest in selling an add-on for a Novo Nordisk drug. The tirzepatide programme has been uneven. A phase 2b in obesity with type 2 diabetes was registered in March 2025 and withdrawn in September 2025 before enrolment, for "strategic business reasons"; a separate phase 2 in obesity continues with results Lilly said to expect in 2026, which had not appeared at the time of writing. Whether the antibody eventually pairs with tirzepatide, with retatrutide, or with neither is a commercial decision Lilly has not announced. The price and availability section tracks the timeline, and the side effects section covers the safety record on each side of the comparison.

Why the way it works matters for safety

In shortIt blocks a whole receptor, so it can affect more than muscle.

Most myostatin inhibitors bind one ligand. Trevogrumab binds myostatin, garetosmab binds activin A, apitegromab binds the inactive precursor of myostatin. Bimagrumab binds the activin type IIA and IIB receptors, so it blocks myostatin, activin A and any other ligand that uses those receptors at once. That is why it moves body composition further than the single-ligand antibodies, and why its safety is scrutinised: activins and their receptors have roles outside muscle, in the gonads, the pituitary, the skin and the vasculature. The earlier receptor-based drug ACE-031, a soluble ActRIIB decoy, was halted in 2011 after nosebleeds, gum bleeding and telangiectasias attributed to it also trapping BMP9 and BMP10. Bimagrumab is an antibody to the receptor rather than a decoy receptor, and does not bind BMP9 or BMP10 directly; whether that distinction is why it has avoided the bleeding problem is a reasonable inference, not a proven one. Our pillar article explains the pathway, and the Novartis section below covers the drug's history.

The Novartis years: myositis, sarcopenia and diabetes

In shortIt started as a muscle-wasting drug, failed there, then found a second life in obesity.

Bimagrumab began as Novartis's BYM338. Its first target was sporadic inclusion body myositis, a progressive muscle disease, where it received FDA breakthrough designation. In April 2016 Novartis announced that the phase 2b/3 RESILIENT trial had failed to meet its primary endpoint, and the long-term extension later confirmed a good safety profile without clinical benefit on mobility. A sarcopenia trial in older adults, published in JAMA Network Open in 2020, found muscle gains without a clear functional advantage over optimised standard care, the same result that had met every earlier myostatin drug.

The turn came from a study nobody had designed for weight loss. In a phase 2 trial published in JAMA Network Open on 13 January 2021, 75 adults with type 2 diabetes and obesity received bimagrumab or placebo for 48 weeks. Total body fat mass fell by 20.5% on bimagrumab against 0.5% on placebo, with lean mass rising, and insulin sensitivity improved. Novartis licensed the antibody to Versanis Bio in 2021 to pursue obesity, and in July 2023 Lilly agreed to buy Versanis for up to $1.925 billion, completing the deal in August 2023.

Safety in the myositis trial (RESILIENT)

In shortIn older patients with a muscle disease, the side effects were mostly mild.

Bimagrumab's largest safety dataset comes from sporadic inclusion body myositis, where Novartis ran the phase 2b/3 RESILIENT trial in 251 participants randomised to placebo or bimagrumab at 1, 3 or 10 mg/kg by infusion every four weeks. The trial, published in The Lancet Neurology in 2019, missed its primary endpoint on six-minute walking distance, but its safety reporting is the most detailed in the class. The most frequent adverse events were muscle spasms (40% to 68% across the bimagrumab groups, against 21% on placebo) and diarrhoea (32% to 52%, against 18%). Adverse events leading to discontinuation occurred in four participants (6%) in each bimagrumab group against one (2%) on placebo. Serious adverse events were reported by 33% of the 10 mg/kg group, 17% of the 3 mg/kg group, 32% of the 1 mg/kg group and 32% on placebo, in an older population with a progressive muscle disease. No significant adverse cardiac effects were recorded on electrocardiography or echocardiography. A long-term extension published in Neurology in 2021 confirmed a safety profile the authors described as good, without clinical benefit on mobility.

Safety in the sarcopenia and diabetes trials

In shortSmaller trials showed the same pattern: spasms, diarrhoea, nothing new.

Two smaller Novartis trials complete the record. In sarcopenia, a phase 2 published in JAMA Network Open in 2020 compared bimagrumab with optimised standard care in older adults and found muscle gains without a clear functional advantage; its safety findings were in line with RESILIENT. In type 2 diabetes with obesity, 75 adults received bimagrumab or placebo for 48 weeks in a phase 2 published in JAMA Network Open in January 2021; fat mass fell 20.5% on bimagrumab against 0.5% on placebo, lean mass rose, and the authors reported an acceptable safety profile with no deaths. Muscle spasms and diarrhoea appeared again. Across all of these trials, several hundred people have received bimagrumab for up to two years, and the pattern has not changed.

What is still not known about its safety

In shortLong-term effects, and effects in large groups, are still unknown.

Bimagrumab has no label, no regulatory review, and no data beyond about two years of treatment. The transient ALT and lipase rises are described as transient because they settled during the trials; their meaning over longer use is unknown. Lilly has said it moved to subcutaneous injection for its own studies, and the safety of that route has not been published. No trial has combined bimagrumab with tirzepatide or retatrutide in published form, so the side effects of those combinations are unknown; Lilly's tirzepatide phase 2 continues with results not yet reported. Effects on fertility and pregnancy have not been characterised in people, which matters for a drug that blocks activin receptors. Price, timing and the fake listings are covered earlier in this article, as is the comparison with the GLP-1 drugs; for the class overview, see the UK availability timeline. ALTA PEPTIDES supplies research-grade versions of these antibodies as laboratory reagents under our research use policy: recombinant antibodies made by a third-party manufacturer, not the products of the originator companies.

Why is there no brand name?

In shortBrand names come near approval. Bimagrumab isn't close yet.

A medicine gets a brand name when a regulator approves it, and bimagrumab has not been approved by any regulator for any use. Its names are development codes: BYM338 under Novartis, which developed it for inclusion body myositis and sarcopenia, and LY3985863 under Lilly, which bought its licensee Versanis Bio in 2023 for up to $1.925 billion. The two approved antibodies in this class show how naming works: garetosmab became PASATRU when the FDA approved it for fibrodysplasia ossificans progressiva in August 2026, and apitegromab became ISEMBYLD when it was approved for spinal muscular atrophy in September 2026. Bimagrumab will get a brand name on the day it is approved and not before. Nothing sold now under a brand name is Lilly's bimagrumab.

Sources

• Heymsfield et al., Nature Medicine, March 2026: "Bimagrumab plus semaglutide alone or in combination for the treatment of obesity: a randomized phase 2 trial" (BELIEVE, 507 participants, 72-week weight, fat and lean mass results).
• BELIEVE symposium, American Diabetes Association Scientific Sessions, 23 June 2025, and the ADA press release of the same date (responder rates, adverse events, discontinuations).
• Heymsfield et al., JAMA Network Open, 13 January 2021: "Effect of Bimagrumab vs Placebo on Body Fat Mass Among Adults with Type 2 Diabetes and Obesity" (75 participants, 48 weeks, 20.5% fat mass reduction).
• Rooks et al., JAMA Network Open, 2020: bimagrumab versus optimised standard of care in sarcopenia; Novartis announcement, April 2016: RESILIENT trial in sporadic inclusion body myositis did not meet its primary endpoint.
• Eli Lilly press releases, July and August 2023: agreement to acquire and completion of the acquisition of Versanis Bio for up to $1.925 billion.
• Fierce Biotech, 25 September 2025: Lilly withdraws phase 2b trial of bimagrumab with tirzepatide (NCT06901349) before enrolment; Lilly statement on the continuing phase 2 trial.
• Hanna et al., The Lancet Neurology, 2019: "Safety and efficacy of intravenous bimagrumab in inclusion body myositis (RESILIENT)" (251 participants; muscle spasms 40% to 68% and diarrhoea 32% to 52% across the bimagrumab arms, against 21% and 18% on placebo; discontinuations; serious adverse events by arm; cardiac monitoring).
• Amato et al., Neurology, 2021: long-term safety and tolerability of bimagrumab in sporadic inclusion body myositis (RESILIENT extension).
• Heymsfield et al., JAMA Network Open, 13 January 2021: bimagrumab versus placebo in adults with type 2 diabetes and obesity (75 participants, 48 weeks, 20.5% fat mass reduction, safety).
• Rooks et al., JAMA Network Open, 2020: bimagrumab versus optimised standard of care in sarcopenia.
• Muscular Dystrophy Association, April 2011: ACE-031 trials halted for bleeding-related adverse events.
• Heymsfield et al., Nature Medicine, March 2026: BELIEVE phase 2 trial of bimagrumab and semaglutide.
• Eli Lilly press releases, July and August 2023: acquisition of Versanis Bio for up to $1.925 billion.
• Fierce Biotech and BioPharma Dive, September 2025: Lilly withdraws phase 2b trial NCT06901349 of bimagrumab with tirzepatide before enrolment; Lilly statement on the continuing phase 2 with results expected in 2026.
• Eli Lilly press release, September 2026: data to be presented at EASD 2026 (Foundayo, retatrutide and eloraTZP; no bimagrumab presentation listed).
• Managed Healthcare Executive, August 2026: PASATRU annual list price of about $1.4 million; Reuters and Scrip, 14 September 2026: ISEMBYLD net annual cost of about $310,000 and wholesale acquisition cost of $11,659 per vial.
• UK online pharmacy price guides for Wegovy and Mounjaro, September 2026 (private prescription ranges by dose); NHS England guidance on tirzepatide and semaglutide eligibility.
• Heymsfield et al., Nature Medicine, March 2026: BELIEVE phase 2 trial of bimagrumab and semaglutide (weight, fat and lean mass by arm, share of loss from fat).
• Wilding et al., STEP 1 DXA substudy, presented at ENDO 2021, the Endocrine Society annual meeting, March 2021 (140 participants; weight -15.0%, fat mass -19.3%, lean body mass -9.7%).
• Look et al., Diabetes, Obesity and Metabolism, 2025: body composition changes with tirzepatide in the SURMOUNT-1 DXA substudy (160 participants; fat mass -33.9%, lean mass -10.9%; about 75% fat and 25% lean).
• Lancet Diabetes and Endocrinology, 2025: effects of retatrutide on body composition in people with type 2 diabetes, a substudy of the phase 2 trial (fat mass by dose, lean mass, fat-loss ratio).
• Jastreboff et al., New England Journal of Medicine, 2023: phase 2 trial of retatrutide in obesity (24.2% weight loss at 48 weeks, 12 mg).
• Regeneron press release, 17 September 2025 (COURAGE lean share on semaglutide); Scholar Rock press release, 18 June 2025 (EMBRAZE lean share on tirzepatide).
• Fierce Biotech, 25 September 2025: Lilly withdraws phase 2b bimagrumab and tirzepatide trial; statement on the continuing phase 2.

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