Bimagrumab

Bimagrumab binds the activin type II receptors themselves rather than the circulating ligands, so it blocks myostatin, activin A and GDF-11 signalling at the point of receptor engagement. Removing that Smad2/3 brake increases muscle protein accretion; in humans this reliably produces a 2-3 kg lean mass gain within weeks. The fat-loss effect is the more surprising finding and appears to be partly a consequence of increased muscle mass raising energy expenditure and partly a direct effect on adipose tissue, where ActRII signalling also operates. Its clinical history is instructive: it failed to improve function in sporadic inclusion body myositis despite increasing muscle volume, which is the recurring lesson of this class - bigger muscle is not automatically better muscle. The current thesis is different and more defensible, using it to change the composition of GLP-1-driven weight loss so that a larger share comes from fat.

Targets: Activin receptor type IIA (ActRIIA), Activin receptor type IIB (ActRIIB), Myostatin signalling, Activin A signalling

• · 10 mg/kg monthly by infusion. The figure here is per kilogram of body weight. A subcutaneous co-formulation with tirzepatide is in development.

Work out your exact syringe units →

• very commonMuscle spasms and cramps— The most consistently reported effect across trials.
• commonDiarrhoea
• commonAcne
• commonInjection or infusion reactions
• uncommonElevated liver enzymes and lipase/amylase— Seen in obesity trials; usually asymptomatic but requires monitoring.
• uncommonAnti-drug antibodies— Standard biologic risk.

• Pregnancy and breastfeeding.
• Active malignancy outside a trial context.
• Known hypersensitivity to the antibody or its excipients.

• synergysemaglutide — The central development thesis - in BELIEVE, adding bimagrumab pushed the fat share of weight loss from about 72% to above 90%.
• synergytirzepatide — Being studied as a co-formulated subcutaneous combination by Lilly.
• redundantace-031 — Same receptor axis, different modality.
• redundantapitegromab — Overlapping mechanism; apitegromab is the more selective option.

• · DEXA or comparable body composition imaging - the whole point is the fat-to-lean ratio of weight lost, which the scale cannot tell you.
• · Liver enzymes, lipase and amylase.
• · Functional strength testing, since muscle volume and muscle function have diverged before in this class.

Investigational only; not approved in any jurisdiction. Prohibited in sport at all times by WADA as a myostatin-function modifier.

• Heymsfield et al. 2021, JAMA Network Open - bimagrumab effects on body composition in adults with obesity and type 2 diabetes (trial)
• BELIEVE phase 2b trial of bimagrumab with and without semaglutide in obesity (2025) (trial)
• RESILIENT phase 2b/3 trial of bimagrumab in sporadic inclusion body myositis (negative for the primary endpoint) (trial)

Bimagrumab is the most rigorously characterised muscle-mass drug that exists, and understanding why it works differently from the ligand traps explains most of this class. It is an antibody against the receptors themselves rather than the ligands, so it blocks the entire input to ActRIIA and ActRIIB regardless of which ligand is knocking - myostatin, activin A, activin B, GDF11. Lach-Trifilieff's foundational work showed the effect exceeds myostatin inhibition alone: a mouse version of the antibody produced further hypertrophy even in myostatin-mutant mice, proving that ligands other than myostatin are actively restraining muscle mass. Morvan subsequently showed that blocking both ActRIIA and ActRIIB is required for the maximal effect, which is why bimagrumab is dual-specific and why a pure ActRIIB agent leaves growth on the table. Downstream, receptor blockade prevents ALK4/ALK5 recruitment and Smad2/3 phosphorylation. Lach-Trifilieff demonstrated the consequence directly: bimagrumab prevented myostatin- and activin-A-induced atrophy by inhibiting Smad2/3 phosphorylation and thereby sparing myosin heavy chain from degradation. The clinically fascinating part is the fat effect, which was not the design goal. In the type 2 diabetes trial, 48 weeks produced a 20.5% reduction in body fat mass - 7.5 kg - alongside a 1.7 kg lean mass gain and a 0.76 percentage point HbA1c fall. Nobody fully understands the fat loss. The leading explanations are increased resting energy expenditure from a larger muscle compartment, direct ActRII signalling in adipose tissue, and improved insulin sensitivity redistributing substrate. The BELIEVE trial then established the combination logic that now defines this whole area: bimagrumab plus semaglutide 2.4 mg produced 17.8 kg of weight loss at 48 weeks versus 14.2 kg for semaglutide alone and 9.3 kg for bimagrumab alone.

The honest headline problem with bimagrumab is RESILIENT: in inclusion body myositis it increased muscle mass and did not improve the six-minute walk. Bigger muscle is not automatically better muscle, and if your goal is performance rather than appearance, this class has not yet demonstrated that it delivers. Second, muscle spasms are the most consistently reported adverse effect across trials, alongside diarrhoea and acne in the obesity work, and they are severe enough in some people to stop treatment. Third, the target-mediated disposition means the pharmacokinetics are non-linear - doubling a dose does not double exposure in a predictable way - so dose reasoning by proportion is wrong here in a way it is not for most drugs. Fourth, the 52.5 mg subcutaneous dose was indistinguishable from placebo, so there is a real threshold below which this drug does nothing at all; grey-market microdosing of a monoclonal antibody is simply wasted money. And fifth, this is a genuine monoclonal antibody: producing it requires mammalian cell culture and purification infrastructure that does not exist in the grey-market supply chain, so the probability that an unlicensed vial contains real bimagrumab is low.

• Activin receptor type IIB (ActRIIB) — High-affinity binding; bimagrumab was raised specifically to prevent ligand access to this receptor

  Blocks myostatin, activin A/B and GDF11 signalling. Abolishes Smad2/3 phosphorylation and spares myosin heavy chain from degradation.

• Activin receptor type IIA (ActRIIA) — Also bound - the dual specificity is the point

  Morvan showed dual ActRIIA/IIB blockade is required for maximal hypertrophy; blocking ActRIIB alone leaves an escape route.

• Downstream ALK4/ALK5-Smad2/3 — Not a binding target

  Signalling is prevented upstream. Smad2/3 phosphorylation falls, FoxO-driven MAFbx and MuRF1 transcription falls, and Akt-mTORC1 is disinhibited.

• Effect of Bimagrumab vs Placebo on Body Fat Mass Among Adults With Type 2 Diabetes and Obesity (NCT03005288) Phase 2 · n=75 · 48 weeks · 2021

  Change in body fat mass over 48 weeks with intravenous bimagrumab 10 mg/kg up to 1200 mg every 4 weeks. Fat mass fell 20.5% (-7.5 kg) versus -0.5% (-0.18 kg) on placebo (P<0.001). Lean mass rose 3.6% (1.70 kg) versus -0.8% (-0.4 kg). HbA1c fell 0.76 percentage points versus 0.04 (P=0.005). 58 of 75 randomised subjects completed.

• BELIEVE - Bimagrumab plus semaglutide alone or in combination for the treatment of obesity (NCT05616013) Phase 2 · n=507 · 48 weeks · 2026

  Absolute change in body weight at week 48 across nine arms, with a 24-week open-label extension to week 72. Least-squares mean weight change was -9.3 kg for bimagrumab 30 mg/kg, -14.2 kg for semaglutide 2.4 mg, and -17.8 kg for the combination, versus -3.3 kg for placebo (all P<0.001). Common adverse events were muscle spasms, diarrhoea and acne for bimagrumab; nausea, diarrhoea, constipation and fatigue for semaglutide.

• Bimagrumab vs Optimized Standard of Care for Treatment of Sarcopenia in Community-Dwelling Older Adults Phase 2 · 2020

  Bimagrumab versus optimised standard of care in sarcopenic community-dwelling older adults, assessing lean mass and physical function. Published in JAMA Network Open in 2020.

• RESILIENT - Safety and efficacy of intravenous bimagrumab in inclusion body myositis Phase 2b/3 · 2019

  Change in six-minute walk distance in sporadic inclusion body myositis. The trial did not meet its primary endpoint despite increasing muscle mass - the clearest demonstration in this whole class that added muscle mass does not automatically translate into function.

• NCT06643728 - Weight management with bimagrumab (LY3985863) and tirzepatide, alone or in combination Phase 2 · n=252

  Body composition and weight outcomes with bimagrumab and tirzepatide alone or combined. Listed as active, not recruiting on ClinicalTrials.gov.

Measurable lean mass change within four to eight weeks, and 4-6% lean body mass gain across most regimens in the dedicated PK/PD study. Fat mass changes accumulate over months, with the headline -7.5 kg figure measured at week 48. Muscle spasms, when they occur, tend to appear in the first weeks. HbA1c improvement follows the composition change and is assessed at 12-week intervals.

The stack with real evidence behind it is bimagrumab plus a GLP-1 receptor agonist, and BELIEVE ran it properly: 507 subjects, nine arms, 48 weeks, with the combination reaching 17.8 kg of weight loss against 14.2 kg for semaglutide alone. The mechanism is complementary rather than merely protective - the incretin drives energy deficit, the ActRII blocker shifts what the deficit comes out of, and the combination lost more total weight than either alone. A tirzepatide combination is in phase 2 as well. What you should not do is stack bimagrumab with any other ActRII-pathway agent - ACE-031, follistatin, apitegromab, trevogrumab - because they converge on the same node and you gain nothing but risk. With testosterone or anabolic steroids the mechanisms are genuinely independent and the combination is plausible, but untested. Note the practical hazard of combining with a GLP-1: bimagrumab improves insulin sensitivity substantially, so anyone on insulin or a sulfonylurea needs those doses reviewed before, not after.

Against everything else in this class, bimagrumab is the one with real evidence: 507 randomised subjects in BELIEVE, 75 in the diabetes trial, dedicated pharmacokinetics, and a published negative trial that tells you what it does not do. Against ACE-031, which is the decoy-receptor version of the same idea, bimagrumab has not produced the telangiectasia and epistaxis that stopped that programme - the antibody appears not to disturb BMP9/BMP10 endothelial signalling the way a broad ActRIIB trap does. Against apitegromab and trevogrumab, which block myostatin selectively, bimagrumab blocks the whole receptor and gets a larger effect at the cost of activin-related adverse effects; the selective antibodies are cleaner and, on current data, smaller in effect. Against IGF-1 analogues and growth hormone, the comparison is not close - bimagrumab produces documented, randomised, placebo-controlled body-composition change, while LR3 produces forum reports. Against a GLP-1 alone for weight loss, bimagrumab alone loses less weight but changes composition more favourably, and the combination beats both.

Bimagrumab is not approved for any indication and has no market price. It is in phase 2 development under Eli Lilly as LY3985863. Any material offered outside a trial is of unknown provenance and cannot be sensibly priced.

• ?The amino acid sequence is not given. Bimagrumab is a fully human IgG1/lambda antibody but the CDR sequences were not resolved from a primary source in this session.
• ?Numeric clearance, volume of distribution and time to steady state were not available; target-mediated disposition makes single values misleading in any case.
• ?The mechanism of the fat loss is genuinely unresolved. Increased energy expenditure from a larger muscle compartment, direct ActRII signalling in adipose tissue and improved insulin sensitivity are all plausible and none is established.
• ?Long-term safety beyond 72 weeks is unknown, and chronic blockade of activin signalling has endocrine implications - FSH in particular - that the published trial summaries do not detail.
• ?Whether the muscle mass gained is functionally useful remains open. RESILIENT says it was not in inclusion body myositis; whether that generalises to healthy or obese people is untested.
• ?Effects on FSH, testosterone and fertility from chronic ActRII blockade were not resolved from the trial reports reviewed here.

• Effect of Bimagrumab vs Placebo on Body Fat Mass Among Adults With Type 2 Diabetes and Obesity: A Phase 2 Randomized Clinical Trial Heymsfield SB, Coleman LA, Miller R, Rooks DS, Laurent D, Petricoul O, Praestgaard J, Swan T, Wade T, Perry RG, Goodpaster BH, Roubenoff R, JAMA Network Open, 2021 · PMID 33439265

  The trial that turned bimagrumab from a muscle drug into a body-composition drug. Source of the -7.5 kg fat, +1.70 kg lean and -0.76 percentage point HbA1c figures.

• Bimagrumab plus semaglutide alone or in combination for the treatment of obesity: a randomized phase 2 trial Heymsfield SB, et al., Nature Medicine, 2026 · PMID 41772149

  BELIEVE. The 507-subject trial establishing that ActRII blockade adds to GLP-1 weight loss rather than merely protecting lean mass - 17.8 kg combined versus 14.2 kg for semaglutide alone.

• Pharmacokinetics and Pharmacodynamics of Bimagrumab (BYM338) Petricoul O, et al., Clinical Pharmacokinetics, 2023 · PMID 36527600

  Source of the ~40% subcutaneous bioavailability and the target-mediated disposition finding, plus the demonstration that weekly subcutaneous dosing matches monthly intravenous dosing. Lean body mass rose 4-6% (1.5-2 kg) and fat mass fell 2-3 kg across most regimens; the lowest subcutaneous dose, 52.5 mg, was indistinguishable from placebo.

• An antibody blocking activin type II receptors induces strong skeletal muscle hypertrophy and protects from atrophy Lach-Trifilieff E, Minetti GC, Sheppard K, Ibebunjo C, Feige JN, Hartmann S, Brachat S, Rivet H, Koelbing C, Morvan F, Hatakeyama S, Glass DJ, Molecular and Cellular Biology, 2014 · PMID 24298022

  The mechanism paper. Shows the effect exceeds myostatin inhibition alone, including further hypertrophy in myostatin-mutant mice, and identifies Smad2/3 inhibition sparing myosin heavy chain as the proximate mechanism.

• Blockade of activin type II receptors with a dual anti-ActRIIA/IIB antibody is critical to promote maximal skeletal muscle hypertrophy Morvan F, Rondeau JM, Zou C, Minetti G, Scheufler C, Scharenberg M, Jacobi C, Brebbia P, Ritter V, Toussaint G, Koelbing C, Leber X, Schilb A, Witte F, Lehmann S, Koch E, Geisse S, Glass DJ, Lach-Trifilieff E, Proceedings of the National Academy of Sciences USA, 2017 · PMID 29109273

  Why bimagrumab targets both receptors rather than ActRIIB alone. Directly relevant to why decoy-receptor approaches behave differently.

• Safety and efficacy of intravenous bimagrumab in inclusion body myositis (RESILIENT): a randomised, double-blind, placebo-controlled phase 2b trial Hanna MG, et al., Lancet Neurology, 2019 · PMID 31397289

  The negative trial, and the most important one to read. Muscle mass increased and function did not, which is the central caveat for every compound in this class.

• Safety and pharmacokinetics of bimagrumab in healthy older and obese adults with body composition changes in the older cohort Rooks D, et al., Journal of Cachexia, Sarcopenia and Muscle, 2020 · PMID 33264516

  Safety and pharmacokinetics in the older and obese populations where the drug is actually being developed.