Bimagrumab
An antibody that blocks the activin type II receptors, tested to preserve muscle during weight loss after failing in a muscle disease
Bimagrumab blocks the receptors that myostatin and activins use to hold muscle growth in check. In trials it added lean mass and took away fat. It did not help people with inclusion body myositis walk further, and a 2026 trial paired it with semaglutide for obesity.
What is it?
Myostatin and related activins act on activin type II receptors to stop muscle growing. Block the receptors and muscle grows, and fat tends to shrink, because the same pathway shapes how the body stores energy. Bimagrumab is an antibody that blocks both receptor types, given by intravenous infusion.
Its first target was muscle disease. In older adults with sarcopenia it raised thigh muscle volume by about 7% to 8% over 16 weeks, and those who walked slowly at the start gained 82 metres on a six-minute walk. In inclusion body myositis, the largest randomised trial of the disease found no improvement in walking distance at any dose.
Then attention moved to fat. In adults with type 2 diabetes and obesity, 48 weeks of infusions cut body fat by about a fifth, added 3.6% lean mass and lowered HbA1c by 0.76 points. A 2026 phase 2 trial combined it with semaglutide: weight fell 9.3 kg on bimagrumab, 14.2 kg on semaglutide and 17.8 kg on both, against 3.3 kg on placebo.
Pooled, the body-composition effect is consistent: about 2 kg more lean mass and 4 to 5 kg less fat than placebo across trials, with only a small HbA1c benefit and a rise in LDL cholesterol. Function is less clear; the walking gain in older adults averaged about 10 metres, and in inclusion body myositis walking kept declining over two years.
The argument for pairing them is that GLP-1 drugs remove some muscle along with fat, and bimagrumab may preserve it. Muscle spasms, diarrhoea and acne are its commonest side effects. It is not approved anywhere. Six months of treatment in older adults did not change heart structure or function, and its pituitary effect, lower FSH in women, reversed on stopping.
Mechanism of Action
Bimagrumab is a monoclonal antibody that blocks activin type II receptors (ActRIIA and ActRIIB), cutting the myostatin and activin signals that restrain muscle.
Muscle growth
Blocking ActRII increases muscle volume and lean mass, though function gains have been small.
Fat loss
The same blockade reduces fat mass and improves insulin sensitivity.
Pituitary activin
It reversibly lowered FSH in women, a sign of activin blockade in the pituitary.
Published Research
These are the trials that describe bimagrumab, read from their PubMed abstracts: two in obesity, one in sarcopenia and one in inclusion body myositis.
With semaglutide
Adults with obesity took bimagrumab, semaglutide 2.4 mg, both, or placebo. At 48 weeks weight fell 9.3 kg, 14.2 kg and 17.8 kg on the high-dose combination against 3.3 kg on placebo, with improvements continuing to week 72.
Type 2 diabetes and obesity
Over 48 weeks of infusions, fat mass fell sharply against placebo, lean mass rose 3.6% against a 0.8% fall, waist shrank 9.0 cm, HbA1c fell 0.76 points and body weight 6.5%. Safety matched earlier studies.
Sarcopenia
In older adults with sarcopenia, thigh muscle volume rose 5% to 8% above baseline through 16 weeks against no change on placebo; slow walkers gained 0.15 m/s gait speed and 82 metres on the six-minute walk.
Inclusion body myositis
In RESILIENT, the largest randomised trial in the disease, monthly infusions at 1, 3 or 10 mg/kg did not improve six-minute walk distance over 52 weeks. Safety was acceptable, and neither death was linked to the drug.
Sarcopenia: muscle without function
In 180 older adults with sarcopenia given exercise and nutrition, bimagrumab raised lean mass 7% against 1% over 6 months, but physical performance, walk distance and gait speed did not improve more than with placebo.
Recovery after hip fracture
In 250 older adults after hip fracture surgery, 210 and 700 mg doses increased lean mass by 1.9 and 2.8 kg against 0.2 kg on placebo over 24 weeks, without a difference in gait speed or physical performance.
Heart safety in older adults
Six months of bimagrumab in healthy older adults did not change left ventricular mass or ejection fraction, while lean mass rose 5.5% and fat fell 14% against placebo.
Insulin resistance
In insulin-resistant adults without diet or exercise changes, bimagrumab added 2.7% lean mass, cut fat 7.9%, left weight unchanged and improved insulin sensitivity by about 20 to 40%.
Recovering from immobilisation
In men whose leg was immobilised in a cast, a single 30 mg/kg infusion restored thigh muscle volume to 5.1% above pre-cast levels by 12 weeks, though strength recovered similarly to placebo.
Intravenous versus subcutaneous
In 91 older adults, subcutaneous bimagrumab had about 40% bioavailability; lean mass rose 4 to 6% with most IV and subcutaneous regimens, except the lowest subcutaneous dose.
Older and obese volunteers
In healthy older and obese adults, adverse events were mostly mild; muscle spasms and rash were common in the obese cohort.
Pituitary hormones
Bimagrumab reversibly lowered FSH in women, consistent with blocking activin at the pituitary, without changing gonadal or adrenal androgens.
RESILIENT extension
In the long-term extension in inclusion body myositis, walking distance declined in all groups through week 104, and falls were somewhat more frequent at 3 mg/kg.
Meta-analysis in obesity and metabolic disease
Pooling 4 trials with 268 participants, bimagrumab reduced weight by 4.85 kg and fat by 4.72 kg, added 1.66 kg of lean mass and lowered HbA1c slightly against placebo, but raised LDL cholesterol.
Body composition meta-analysis
Across 7 randomised trials, bimagrumab increased thigh muscle volume by 5.3% and fat-free mass by 1.9 kg and reduced fat mass by 4.6 kg.
Walking performance
A meta-analysis of 4 studies with 358 older participants found a 10-metre gain in six-minute walk distance over placebo, with wide uncertainty.
Fracture healing in rats
In rats with a fibula osteotomy, bimagrumab briefly affected immature callus but not mature callus, bone density or strength.
Dosing Data
Not approved. Obesity and diabetes trials gave 10 mg/kg (up to 1,200 mg) by intravenous infusion every 4 weeks.
Educational information, not medical advice. Each dose above is tagged with its source: an approved label, a published human study, animal data, or amounts reported by users. None is a recommendation for you. An unapproved compound has no established safe or effective human dose, and products sold for “research use only” are not made or tested for people. Talk to a doctor before acting on anything on this site, including before you start, stop or change any medicine or dose. Full medical disclaimer.
Side Effects & Contraindications
Reported Side Effects
From the trials above.
Contraindications & Cautions
An investigational antibody available only in trials.
Legal Status
Bimagrumab is investigational everywhere.
Frequently Asked Questions
?What is bimagrumab?
An antibody that blocks the activin type II receptors, the brake myostatin uses on muscle growth, given by infusion.
?Does it build muscle?
In trials it added lean mass and muscle volume, but it did not help people with inclusion body myositis walk further.
?Why combine it with semaglutide?
GLP-1 drugs remove some muscle with fat. Paired with semaglutide, it produced more weight loss than either alone in a 2026 trial.
?Is it approved?
No. It is in phase 2 and not approved anywhere.
?Does bimagrumab improve strength or walking?
Less than it builds muscle. Pooled data show about a 10-metre gain in six-minute walk in older adults, strength after immobilisation recovered no faster than on placebo, and walking kept declining in inclusion body myositis.
?Is it safe for the heart?
Six months of treatment in healthy older adults did not change left ventricular mass or ejection fraction. Pooled trials did show a rise in LDL cholesterol.
References
- [pubmed] Heymsfield SB, et al. "Bimagrumab plus semaglutide alone or in combination for the treatment of obesity: a randomized phase 2 trial." Nat Med, 2026;32(3):869-882. PMID: 41772149.
- [pubmed] Heymsfield SB, et al. "Effect of Bimagrumab vs Placebo on Body Fat Mass Among Adults With Type 2 Diabetes and Obesity: A Phase 2 Randomized Clinical Trial." JAMA Netw Open, 2021;4(1):e2033457. PMID: 33439265.
- [pubmed] Rooks D, et al. "Treatment of Sarcopenia with Bimagrumab: Results from a Phase II, Randomized, Controlled, Proof-of-Concept Study." J Am Geriatr Soc, 2017;65(9):1988-1995. PMID: 28653345.
- [pubmed] Hanna MG, et al. "Safety and efficacy of intravenous bimagrumab in inclusion body myositis (RESILIENT): a randomised, double-blind, placebo-controlled phase 2b trial." Lancet Neurol, 2019;18(9):834-844. PMID: 31397289.
- [clinical-trial] Rooks D, et al. "Bimagrumab vs Optimized Standard of Care for Treatment of Sarcopenia in Community-Dwelling Older Adults: A Randomized Clinical Trial." JAMA Netw Open, 2020;3(10):e2020836. PMID: 33074327.
- [clinical-trial] Hofbauer LC, et al. "Bimagrumab to improve recovery after hip fracture in older adults: a multicentre, double-blind, randomised, parallel-group, placebo-controlled, phase 2a/b trial." Lancet Healthy Longev, 2021;2(5):e263-e274. PMID: 36098133.
- [pubmed] Shao C, et al. "Efficacy and Safety of Bimagrumab in Adults With Obesity and Metabolic Dysfunction: A Systematic Review and Meta-Analysis of Randomized Controlled Trials." Diabetes Obes Metab, 2026;28(10):9433-9442. PMID: 42530342.
- [pubmed] Kanbay M, et al. "Effect of Bimagrumab on body composition: a systematic review and meta-analysis." Aging Clin Exp Res, 2024;36(1):185. PMID: 39251484.
- [pubmed] Spitz RW, et al. "Blocking the activin IIB receptor with bimagrumab (BYM338) increases walking performance: A meta-analysis." Geriatr Gerontol Int, 2021;21(10):939-943. PMID: 34405505.
- [pubmed] Rooks D, et al. "Cardiac safety of chronic inhibition of the myostatin-activin pathway with bimagrumab in healthy older adults." J Clin Endocrinol Metab, 2026;111(9):2573-2582. PMID: 41873146.
- [pubmed] Garito T, et al. "Bimagrumab improves body composition and insulin sensitivity in insulin-resistant individuals." Diabetes Obes Metab, 2018;20(1):94-102. PMID: 28643356.
- [pubmed] Rooks DS, et al. "Effect of bimagrumab on thigh muscle volume and composition in men with casting-induced atrophy." J Cachexia Sarcopenia Muscle, 2017;8(5):727-734. PMID: 28905498.
- [pubmed] Petricoul O, et al. "Pharmacokinetics and Pharmacodynamics of Bimagrumab (BYM338)." Clin Pharmacokinet, 2023;62(1):141-155. PMID: 36527600.
- [pubmed] Rooks D, et al. "Safety and pharmacokinetics of bimagrumab in healthy older and obese adults with body composition changes in the older cohort." J Cachexia Sarcopenia Muscle, 2020;11(6):1525-1534. PMID: 33264516.
- [pubmed] Garito T, et al. "Effects of bimagrumab, an activin receptor type II inhibitor, on pituitary neurohormonal axes." Clin Endocrinol (Oxf), 2018;88(6):908-919. PMID: 29566437.
- [pubmed] Amato AA, et al. "Efficacy and Safety of Bimagrumab in Sporadic Inclusion Body Myositis: Long-term Extension of RESILIENT." Neurology, 2021;96(12):e1595-e1607. PMID: 33597289.
- [pubmed] Tankó LB, et al. "Does Activin Receptor Blockade by Bimagrumab (BYM338) Pose Detrimental Effects on Bone Healing in a Rat Fibula Osteotomy Model?." Calcif Tissue Int, 2016;99(3):310-21. PMID: 27167138.
Sources & Citations
Studies were read from their PubMed abstracts. Read October 7, 2026.
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