Bimagrumab (BYM338)
Bimagrumab — the muscle-sparing antibody: evidence, the story & status

Class: protein
Synonyms: bimagrumab
BYM338
Function tags: muscle preservation / anti-ActRII
body composition
metabolic (investigational)
Overall evidence grade: B
WADA prohibited: no
WADA note: Sport: as an agent blocking the myostatin/activin (ActRII) pathway, it falls under WADA S4 (hormone and metabolic modulators / myostatin-pathway agents).
Last updated: 2026-07-13

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.

## 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."

Evidence grades by outcome:
outcome: Body recomposition (fat ↓ + lean ↑), obesity/T2D
grade: B
base: Phase 2 RCT (Heymsfield 2021; fat ~−20%, lean ~+3–4%, insulin sensitivity ↑)
effect: Unique dual effect
caveat: Real human Phase 2, but not approved; single-agent weight change is near-neutral
outcome: Muscle-sparing + fat loss with GLP-1 (the thesis)
grade: B
base: BELIEVE Phase 2b + semaglutide (Nature Med 2026; >90% of loss as fat, lean preserved)
effect: Higher-quality weight loss
caveat: Emerging and positive, but Phase 2b; tirzepatide combos mixed (one terminated)
outcome: Sporadic inclusion body myositis (original indication)
grade: F
base: Phase 2b/3 (RESILIENT)
effect: Failed primary endpoint
caveat: Grew muscle but did not improve function — the "mass ≠ function" wall
outcome: Sarcopenia / post-hip-fracture
grade: C
base: Phase 2
effect: Muscle mass ↑, mobility/strength minimal
caveat: Same mass-vs-function caveat
outcome: Metabolic (insulin sensitivity)
grade: B
base: Phase 2 (~18% improvement)
effect: Improved
caveat: Secondary; consistent
outcome: Safety
base: Phase 2 across indications
effect: Generally tolerated
caveat: Muscle cramps/spasms, diarrhea, mild pancreatic-enzyme rises, acne; relatively clean vs older ActRII traps

Legal status:
region: INT
status: See note
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).

Cosmetic claims:

Note: 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.
Status note: fast-moving — re-check the 2026 tirzepatide-combination readouts

Sources:
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.
Hanna MG, et al. / BELIEVE Phase 2b — bimagrumab + semaglutide (Nature Medicine, 2026; ADA 2025 presentation).
Nunn 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.
Eli Lilly / Versanis acquisition (2023, ~$1.9B); trial registrations NCT05616013 (semaglutide), NCT06643728 (tirzepatide).