Evidence review: Open Peptide Project (verified against primary PubMed, ClinicalTrials.gov & FDA records) · Last checked 2026-10-27. ← Back to Evidence Library

Bimagrumab (BYM338, LY3985863): what the evidence says

Name: Bimagrumab (BYM338, LY3985863)
Published: 2026-10-27

At a glance

What it is: a fully human monoclonal antibody that blocks activin type II receptors, shutting off myostatin and activin signals that limit muscle growth. Developed by Novartis, then Versanis; owned by Eli Lilly since Aug 2023. Given by IV infusion. Not a peptide drug. ^{R1, R2}

What the evidence says: in a 75-person phase 2 RCT in type 2 diabetes with obesity, it cut fat mass 20.5% and added 1.7 kg lean mass over 48 weeks. ^R3 In the 507-person BELIEVE trial, adding it to semaglutide gave more weight loss than semaglutide alone. ^R4 But in older adults with sarcopenia and in inclusion body myositis, extra muscle did not improve walking or physical function. ^{R5, R1}

Biggest risks: muscle spasms, diarrhea and acne were common in trials. ^{R1, R4} Long-term safety is unknown.

Rating


Human Human evidence	Moderate for fat loss and lean-mass gain (phase 2 RCTs) ^{R3, R4} · Moderate, negative for function in sarcopenia and inclusion body myositis ^{R5, R1} · No human data for healthy lifters
Animal Animal / lab evidence	Mouse work combining it with GLP-1 drugs ^R6
Regulatory	Investigational, not approved anywhere · antibody, so not a 503A/503B bulk substance; not on FDA's safety-risk page · WADA 2026: named under S4.3, prohibited at all times ^{R7, R8, R9, R10}
Established human dose	None (investigational). Study details lists what trials used

Evidence verdict: Fat loss with lean-mass gain in obesity (alone or with semaglutide): Moderate (phase 2 RCTs, investigational) · Muscle function in sarcopenia or inclusion body myositis: no benefit in RCTs · Bodybuilding: No human data

This page reports what studies used. It is not dosing advice.

Legend: HUMAN data · ANIMAL / LAB data · UNKNOWN · UNVERIFIED: not yet confirmed against a primary source

About the evidence: all efficacy trials were sponsored by Novartis or Lilly. The diabetes trial analyzed only completers (58 of 75). ^{R3, R4}

What is it?

Myostatin and activin brake muscle growth through activin type II receptors. Bimagrumab blocks those receptors directly, which is why WADA lists it with ACE-031 and follistatin. ^R10 Related pages: Semaglutide, Tirzepatide.

Does it work?

Human Type 2 diabetes with obesity (Heymsfield 2021): 48 weeks; fat mass −20.5% vs −0.5%, lean mass +3.6% vs −0.8%, waist −9.0 cm, HbA1c −0.76 points, weight −6.5% vs −0.8%. ^{R3, R11}
Human Obesity with semaglutide (BELIEVE, Heymsfield 2026): 507 adults, 48 weeks. Weight change: −9.3 kg bimagrumab (higher dose), −14.2 kg semaglutide, −17.8 kg high-dose combination, −3.3 kg placebo. ^{R4, R12}
Human Sarcopenia (Rooks 2020): 180 adults aged 70+; lean mass +7% vs +1%, but physical performance (SPPB) did not differ from the comparison group, which got optimized standard of care (diet and exercise) plus placebo (p = 0.13). ^R5
Human Inclusion body myositis (RESILIENT): 251 people; 6-minute walk did not improve at any dose. ^R1
Human Meta-analysis: consistent fat loss and lean gain across trials. ^R13

Is it safe?

Human RESILIENT: muscle spasms (51% in the highest-dose group) and diarrhea were the most frequent drug-linked events; falls were common in every group, including placebo. ^R1
Human BELIEVE: muscle spasms, diarrhea and acne. ^R4
Unknown No data beyond about 1.5 years; no data in healthy young people.

Gray-market "bimagrumab": extra risks

A gray-market vial has no quality control and no guarantee of identity, strength or sterility. We test retail vials blind under the Open Peptide Standard; see /tests/. A passing test covers one batch on one date and does not mean the compound is safe. A research-use antibody would need cold storage and sterile IV infusion; we know of no verified retail source and have tested none.

Pipeline (sponsor, trials, expected readouts)

Sponsor: Eli Lilly (acquired with Versanis). ^R2
NCT06643728: bimagrumab with tirzepatide, phase 2; primary completion listed Jan 2026, results not yet published as of Oct 26, 2026. ^R14
NCT05933499: academic phase 2 with tirzepatide (body composition, bone); primary completion Dec 2028. ^R15
UNVERIFIED Lilly's phase 3 plans; no phase 3 trial found on CT.gov.

What we don't know

Unknown Whether the extra muscle improves strength or function in obesity
Unknown Long-term heart, bone and reproductive safety

Is it legal?

US (FDA): investigational only; no approved use. ^R14
Athletes: WADA 2026 names bimagrumab as an anti-activin receptor IIB antibody under S4.3, prohibited at all times. ^R10

Study details

These are amounts researchers used, not recommendations.

Study	People / model	Route	Amount used	Follow-up	Reported	Citation
Heymsfield 2021	75 adults, T2D + BMI 28-40	IV every 4 weeks	10 mg/kg (max 1,200 mg)	48 weeks	Fat −20.5%, lean +3.6%	PMID 33439265
BELIEVE 2026	507 adults with obesity	IV every 12 weeks (± weekly SC semaglutide 1.0 or 2.4 mg)	10 or 30 mg/kg	48 weeks	Combination −17.8 kg	PMID 41772149
Rooks 2020	180 adults 70+ with sarcopenia	IV monthly	700 mg	24 weeks	Lean +7%; function not better	PMID 33074327
RESILIENT 2019	251 adults with IBM	IV every 4 weeks	1, 3 or 10 mg/kg	≥48 weeks	Walk test not improved	PMID 31397289

Full reference list (PubMed, CT.gov, FDA and WADA checked Oct 26, 2026)

^R1 Hanna MG, et al. Lancet Neurol. 2019. PMID 31397289
^R2 Eli Lilly and Company, Lilly Completes Acquisition of Versanis Bio, Aug 14, 2023. https://investor.lilly.com/news-releases/news-release-details/lilly-completes-acquisition-versanis-bio
^R3 Heymsfield SB, et al. JAMA Netw Open. 2021. PMID 33439265
^R4 Heymsfield SB, et al. Nat Med. 2026. PMID 41772149
^R5 Rooks D, et al. JAMA Netw Open. 2020. PMID 33074327
^R6 Nunn E, et al. Mol Metab. 2024. PMID 38218536
^R7 FDA, 503A bulk drug substances categories list (updated May 14, 2026). https://www.fda.gov/media/94155/download
^R8 FDA, 503B bulk drug substances categories list (updated Mar 21, 2025). https://www.fda.gov/media/94164/download
^R9 FDA, Certain bulk drug substances for use in compounding that may present significant safety risks (content current Apr 22, 2026). https://www.fda.gov/drugs/human-drug-compounding/certain-bulk-drug-substances-use-compounding-may-present-significant-safety-risks
^R10 WADA. The 2026 Prohibited List, S4.3. https://www.wada-ama.org/sites/default/files/2025-09/2026list_en_final_clean_september_2025.pdf
^R11 ClinicalTrials.gov NCT03005288, Novartis, bimagrumab in type 2 diabetes with obesity, phase 2, completed (CT.gov API v2, checked Oct 26, 2026). https://clinicaltrials.gov/study/NCT03005288
^R12 ClinicalTrials.gov NCT05616013, Eli Lilly (Versanis), BELIEVE, bimagrumab and semaglutide in obesity, phase 2, completed (CT.gov API v2, checked Oct 26, 2026). https://clinicaltrials.gov/study/NCT05616013
^R13 Kanbay M, et al. Aging Clin Exp Res. 2024. PMID 39251484
^R14 ClinicalTrials.gov NCT06643728, Eli Lilly, bimagrumab and tirzepatide for weight management, phase 2, active not recruiting, primary completion Jan 2026 (CT.gov API v2, checked Oct 26, 2026). https://clinicaltrials.gov/study/NCT06643728
^R15 ClinicalTrials.gov NCT05933499, Massachusetts General Hospital, tirzepatide and bimagrumab on body composition and bone, phase 2, recruiting, primary completion Dec 2028 (CT.gov API v2, checked Oct 26, 2026). https://clinicaltrials.gov/study/NCT05933499

Disclaimer

This page is for education and harm reduction, not medical advice or a recommendation to use Bimagrumab (BYM338, LY3985863). It is an investigational IV antibody given only in trials. Talk to a clinician you trust, especially if you are pregnant, have heart disease, or are a tested athlete. Report problems to FDA MedWatch (https://www.fda.gov/safety/medwatch).