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Explore  /  Bimagrumab (BYM338)
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Bimagrumab (BYM338)

B
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 p

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 i

Evidence, by outcome

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
B
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
B
Sporadic inclusion body myositis (original indication)
Grew muscle but did not improve function — the "mass ≠ function" wall
Phase 2b/3 (RESILIENT) · Failed primary endpoint
F
Sarcopenia / post-hip-fracture
Same mass-vs-function caveat
Phase 2 · Muscle mass ↑, mobility/strength minimal
C
Metabolic (insulin sensitivity)
Secondary; consistent
Phase 2 (~18% improvement) · Improved
B
Safety
Muscle cramps/spasms, diarrhea, mild pancreatic-enzyme rises, acne; relatively clean vs older ActRII traps
Phase 2 across indications · Generally tolerated

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).
Review status
Not yet reviewed

A credentialed reviewer (PharmD / PhD / MD) will be named before this entry is finalised. Until then, treat it as a working draft. Last updated July 2026 (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)
SemaglutideA
Peptide
TirzepatideA
Peptide
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Bimagrumab — the muscle-sparing antibody: evidence, the story & status · Vallydia