Region — International. Neutral scientific reference — not offered for sale here.

Set region

Explore Shop Tools Journal Trust

Explore  /  Bimagrumab (BYM338)

Research reference — not for sale

Bimagrumab (BYM338)

lead outcome

Body recomposition (fat ↓ + lean ↑),…

grades vary by outcome ↓

Protein (not a peptide)

also called — bimagrumab · BYM338

muscle preservation / anti-ActRIIbody compositionmetabolic (investigational)

Reference / reference entry — not sold here, and — being an antibody — not something the gray market can even make. It's here because it sits at the hottest theme in obesity right now: preserving muscle during GLP-1 weight loss. And it has one of the best redemption stories in the field. No dosing published here.

In brief

Bimagrumab is a monoclonal antibody that blocks the activin type II receptors, producing a unique effect — fat loss with simultaneous muscle gain. Originally a failed muscle-wasting drug, it was revived for obesity (Versanis → Lilly's ~$1.9B buyout) and is now the lead candidate for the field's hottest goal: preserving muscle during GLP-1 weight loss. Phase 2 data (alone and with semaglutide) are genuinely promising, but it is investigational, not approved, still carries the old "builds muscle without always improving function" caveat, and — as an antibody — exists only as a pharma asset, not a sellable or gray-market product.

Legal standing, by region

International

See note

- Everywhere — investigational, not approved. Multiple Phase 2/2b obesity trials ongoing (semaglutide combo positive; tirzepatide combos mixed; readouts 2026). - Gray market: effectively none — a monoclonal antibody requires complex biologic manufacturing and cannot be casually synthesised or sold the way peptides are. It's a pure pharma-pipeline asset, and in this register a notable marker of the "muscle-sparing" theme. - ⚠ Sport: as an agent blocking the myostatin/activin (ActRII) pathway, it falls under WADA S4 (hormone and metabolic modulators).

Evidence, by outcome

How we grade →

An honest grade per outcome — drawn from the evidence, not any catalogue. Hype and undemonstrated marketing claims grade low.

OutcomeEvidence base · effectGrade

Body recomposition (fat ↓ + lean ↑), obesity/T2D

Real human Phase 2, but not approved; single-agent weight change is near-neutral

Phase 2 RCT (Heymsfield 2021; fat ~−20%, lean ~+3–4%, insulin sensitivity ↑) · Unique dual effect

Muscle-sparing + fat loss with GLP-1 (the thesis)

Emerging and positive, but Phase 2b; tirzepatide combos mixed (one terminated)

BELIEVE Phase 2b + semaglutide (Nature Med 2026; >90% of loss as fat, lean preserved) · Higher-quality weight loss

Sporadic inclusion body myositis (original indication)

Grew muscle but did not improve function — the "mass ≠ function" wall

Phase 2b/3 (RESILIENT) · Failed primary endpoint

Sarcopenia / post-hip-fracture

Same mass-vs-function caveat

Phase 2 · Muscle mass ↑, mobility/strength minimal

Metabolic (insulin sensitivity)

Secondary; consistent

Phase 2 (~18% improvement) · Improved

Safety

Muscle cramps/spasms, diarrhea, mild pancreatic-enzyme rises, acne; relatively clean vs older ActRII traps

Phase 2 across indications · Generally tolerated

—

Disclosure

Vallydia sells its own cosmetic serums, and some ingredients graded here belong to the same categories as those products. Grades are drawn from the published evidence by the method we publish, and applied to our own ingredients on the same terms — our copper-peptide serum is graded no more kindly than the peptides it competes with. We disclose the interest so you can weigh it.

Trade names shown here (for example Matrixyl, Argireline, Syn-Ake) are the property of their respective owners and are used only to identify the ingredient under discussion. Their appearance implies no affiliation with, or endorsement by, the proprietor.

Evidence changes

Get notified if this changes

We track new research and update these grades as the evidence moves. Leave your email and we'll tell you if the evidence for Bimagrumab (BYM338) changes - nothing else.

Identity

a fully human monoclonal antibody (~150 kDa, modified IgG1) that binds the activin type II receptors (ActRIIA and ActRIIB) with far higher affinity than their natural ligands — thereby blocking myostatin, activin and related signals that limit muscle growth. Given roughly once monthly (originally IV, now also subcutaneous). This is the same myostatin/activin pathway as Follistatin (#40) — but reached by blocking the receptor with an antibody rather than mopping up ligands with a protein.

Mechanism (as proposed)

bimagrumab competitively occupies ActRIIA/ActRIIB, blocking myostatin, activin A and GDF-11 from signalling → skeletal-muscle hypertrophy, plus increased fat oxidation and improved insulin sensitivity. The fat loss is driven both by the metabolic lift from added muscle and by direct effects on adipose tissue — which is how it uncouples "lose fat" from "lose weight."

Sources — 4 cited

01Heymsfield 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.

02Hanna MG, et al. / BELIEVE Phase 2b — bimagrumab + semaglutide (Nature Medicine, 2026; ADA 2025 presentation).

03Nunn E, et al. Antibody blockade of activin type II receptors preserves skeletal muscle mass and enhances fat loss during GLP-1 receptor agonism. Mol Metab. 2024.

04Eli Lilly / Versanis acquisition (2023, ~$1.9B); trial registrations NCT05616013 (semaglutide), NCT06643728 (tirzepatide).

Updated 2026-07-13 (fast-moving — re-check the 2026 tirzepatide-combination readouts)

Grades reflect the published evidence, not our interest. No dosing, reconstitution, or administration is published for research compounds — that restraint is deliberate.

Related compounds

Follistatin (FST-344)F

Protein (not a peptide)

Explore by goal

Metabolic →GH & endocrine →

Vallydia

A neutral reference and a lawful-lane shop. Information for those who seek it — never promotion.

Region — International

ExploreRegister The Register — full index Categories Trust & COA How we grade Open data Brands

ShopCosmetic peptides Journal Quizzes

TermsPrivacyCookiesReturnsShippingImprint

This site provides neutral scientific reference and sells only products lawful in your region. Nothing here is medical advice, a recommendation, or an offer to supply unapproved medicines. No dosing or administration is published for research compounds. Cosmetic peptides per Regulation (EC) 1223/2009. Unapproved injectable peptides are neither sold nor advertised in the EU (Directive 2001/83/EC, Title VIII). © 2026 Vallydia.