Bimagrumab — Safety, Research, Risks & Legal Status

Not medical advice. Bimagrumab is discussed here for informational and harm-reduction purposes only. We do not endorse use, and any dosing context is informational, not a protocol.

Also known as	BYM338; LY3985863; anti-ACVR2B antibody
Category	Research Chemical
Drug class	Fully human monoclonal antibody; dual ActRIIA/ActRIIB antagonist
Developer history	Novartis (BYM338) to Versanis Bio to Eli Lilly (LY3985863, acquired July 2023)
Administration route	Intravenous infusion only (all clinical trials to date)
Failed indication	Sporadic inclusion body myositis - RESILIENT Phase 2b terminated early for lack of efficacy (April 2016)
Active development focus (mid-2026)	Obesity and metabolic disease, including combination trials with tirzepatide
Approval status	Not approved by FDA or any major regulatory authority as of mid-2026
US legal status	Not approved by the FDA or any major regulatory authority. Bimagrumab is an investigational monoclonal antibody; it is not available for legal human use outside of registered clinical trials. A Breakthrough Therapy Designation granted in 2013 for sporadic inclusion body myositis (sIBM) was rendered moot after the Phase 2b trial failed to demonstrate efficacy. No approval pathway has been announced for obesity or metabolic indications as of mid-2026. It is not a dietary supplement. Possession or purchase outside a clinical trial context has no established legal pathway in the US.

What is Bimagrumab?

Bimagrumab is a fully human investigational monoclonal antibody that blocks activin type II receptors (ActRIIA and ActRIIB), the membrane receptors through which myostatin, activin, and related ligands suppress skeletal muscle growth. By competitively binding these receptors with higher affinity than endogenous ligands, bimagrumab disinhibits muscle protein synthesis, producing lean mass gain and - through incompletely understood mechanisms involving brown adipose tissue - concurrent fat mass reduction. Originally developed by Novartis (as BYM338), it was out-licensed to Versanis Bio and subsequently acquired by Eli Lilly in 2023 (re-designated LY3985863). As of mid-2026 it remains investigational, with active Phase 2b trials in obesity and metabolic disease.

How it works

Bimagrumab is a dual antagonist of ActRIIA and ActRIIB. It binds both receptor subtypes with affinity exceeding that of natural ligands (myostatin, activin A, activin B, GDF11), preventing downstream SMAD2/3 phosphorylation and relieving tonic inhibition of skeletal muscle hypertrophy. Secondary effects include altered brown adipose tissue differentiation and off-target signaling through ActRII expression on pituitary, gonadal, and adrenal tissues - the latter being a source of notable safety concern. The net phenotypic effect observed in human trials is increased lean mass, decreased fat mass, and improvement in some metabolic parameters; the mechanisms linking ActRII blockade to fat loss are not fully characterized.

Background & history

Novartis initiated development of BYM338 in the early 2010s, obtaining FDA Breakthrough Therapy Designation in August 2013 for sporadic inclusion body myositis (sIBM), a rare inflammatory muscle disease with no approved therapy. The RESILIENT Phase 2b/3 double-blind trial in sIBM was terminated early in April 2016 after interim analysis showed lack of efficacy, ending the neuromuscular disease program. Novartis subsequently out-licensed the asset to Versanis Bio, a startup that repositioned bimagrumab toward cardiometabolic indications. Phase 2 data in type 2 diabetes and obesity (published 2021) showed significant fat loss and lean mass preservation, generating renewed interest. Eli Lilly acquired Versanis Bio in July 2023 for approximately $1.925 billion, absorbing the compound into their obesity pipeline alongside tirzepatide (Zepbound). Lilly has since run multiple Phase 2b trials including combination studies with tirzepatide; as of mid-2026 at least one combination trial has been terminated, and results from the larger obesity program are pending.

What the research says

Human evidence is Phase 2 level only; no Phase 3 data in any indication have been reported. Key data points: (1) A Phase 2 randomized trial in adults with type 2 diabetes and obesity (48 weeks, intravenous dosing) demonstrated statistically significant reductions in fat mass and increases in lean mass versus placebo, with improvements in insulin sensitivity and lipids (PubMed PMID 30733376, PMC7807292). (2) The RESILIENT Phase 2b trial in sIBM was a double-blind, placebo-controlled study terminated early for lack of efficacy - bimagrumab did not meaningfully slow disease progression in that population (PubMed PMID 31451179). (3) A Phase 2a/b multisite randomized trial evaluated bimagrumab for functional recovery after hip fracture in older adults; results were published in The Lancet Healthy Longevity (2021) and showed some lean mass benefit but mixed functional outcomes. (4) Cardiac safety was assessed in a dedicated study of healthy older and obese adults over 6 months, finding no significant adverse effects on cardiac structure, blood pressure, or resting heart rate (Journal of Clinical Endocrinology and Metabolism, 2024). (5) Phase 2b obesity combination trials with tirzepatide (NCT06643728, NCT06890611) were initiated by Lilly; as of mid-2026 at least one of these trials has been reported terminated, with the full dataset not yet publicly available. Preclinical data in animal models show robust dose-dependent skeletal muscle hypertrophy and fat mass reduction; rat toxicology studies identified reproductive safety signals including reduced FSH and testicular findings. Human reproductive safety remains incompletely characterized.

Reported effects

Increased skeletal muscle lean mass (demonstrated in Phase 2 human trials)
Reduced fat mass, including visceral fat (demonstrated in Phase 2 human trials)
Improved insulin sensitivity and glycemic parameters in type 2 diabetes context (Phase 2)
Improved lipid parameters in obesity/diabetes trial
Muscle spasms (frequent adverse effect attributed to rapid myocyte growth)
Acne and skin rash (common; attributed to ActRII expression in skin and hair follicles)
Potential off-target effects on pituitary-gonadal-adrenal axis (mechanistic concern, incompletely characterized in humans)

Dosing & administration (informational)

Human trials used intravenous infusions at varying doses and intervals (e.g., 10 mg/kg every 4 weeks in some Phase 2 protocols). These ranges are reported here for informational reference only and do not constitute dosing guidance. Bimagrumab is not approved for any indication, is not available for self-administration, and requires clinical infrastructure for administration. Anyone with a clinical question about bimagrumab should consult a licensed clinician or inquire about enrollment in a registered trial at ClinicalTrials.gov.

This is general research/context information, not medical advice or a recommended protocol.

Safety & side effects

Not approved; safety profile is incomplete and derived from Phase 2 trial populations only
Muscle spasms: frequent, attributed to rapid skeletal muscle growth and increased neuromuscular excitability; described as manageable in trials
Acne and rash: common adverse event; mechanism likely ActRII expression on skin and follicular epithelium
Gastrointestinal effects: diarrhea and nausea reported across trials
Reproductive and endocrine signals: animal studies show reduced FSH and testicular atrophy with teratogenic effects; human reproductive safety not fully characterized
Cardiovascular: no significant signal in 6-month healthy adult study, but long-term cardiovascular safety data are absent
Off-target pituitary-gonadal-adrenal signaling is a mechanistic concern given ActRII expression across these tissues; monitoring data not publicly detailed

Drug & supplement interactions

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Who should avoid it

Self-administration outside a registered clinical trial - no legitimate supply and no safe route without clinical infrastructure
Use without reproductive safety counseling given unresolved signals from preclinical studies
Concurrent use with hormonal agents affecting the gonadal axis without endocrine monitoring
Any source claiming to supply injectable bimagrumab to consumers - these are counterfeit or unverifiable products
Use by individuals of reproductive potential without awareness that human teratogenicity risk is uncharacterized

How it is commonly combined

No stacking data exist for bimagrumab in the human literature. The only combination data available are from Lilly-sponsored trials pairing bimagrumab with tirzepatide (a GLP-1/GIP dual agonist), which showed potential synergy in fat reduction with lean mass preservation; however, at least one of these trials has been terminated and full results are not publicly available as of mid-2026. Combination with any other agent is entirely speculative and outside the scope of available evidence.

Legal & regulatory. Not approved by the FDA or any major regulatory authority. Bimagrumab is an investigational monoclonal antibody; it is not available for legal human use outside of registered clinical trials. A Breakthrough Therapy Designation granted in 2013 for sporadic inclusion body myositis (sIBM) was rendered moot after the Phase 2b trial failed to demonstrate efficacy. No approval pathway has been announced for obesity or metabolic indications as of mid-2026. It is not a dietary supplement. Possession or purchase outside a clinical trial context has no established legal pathway in the US.

Quality & harm reduction

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Frequently asked questions

Is bimagrumab approved or available to purchase?

No. Bimagrumab is not approved by the FDA or any major regulatory body and is not legally available outside registered clinical trials. It is produced only as an investigational biologic. Any product sold as bimagrumab through consumer channels cannot be verified for identity or safety.

What is the dosing protocol for bimagrumab?

Dosing is outside the scope of this reference entry. Clinical trial protocols used intravenous infusions at doses such as 10 mg/kg on defined intervals; these are informational data points from published trials, not a protocol or recommendation. Bimagrumab cannot be safely self-administered and is not available outside clinical settings. Consult a licensed clinician or search ClinicalTrials.gov for active trials.

Did bimagrumab work for muscle-wasting disease?

It failed in the primary indication pursued. The RESILIENT Phase 2b double-blind trial in sporadic inclusion body myositis was terminated early in 2016 due to lack of efficacy. The drug did not meaningfully slow disease progression in that population despite the mechanistic rationale. The FDA Breakthrough Therapy Designation for sIBM is no longer active.

What is the current evidence for bimagrumab in obesity?

Phase 2 data in adults with type 2 diabetes and obesity (published 2021, PMID 30733376) showed statistically significant fat mass reductions and lean mass gains versus placebo over 48 weeks, with metabolic improvements. This is encouraging but Phase 2 level only. Eli Lilly is running Phase 2b trials including combinations with tirzepatide; at least one combination trial has been reported terminated as of mid-2026, and no Phase 3 data exist.

What are the main safety concerns with bimagrumab?

The most frequently reported adverse events in trials are muscle spasms and acne or skin rash. Gastrointestinal effects including diarrhea and nausea have also been reported. A more significant concern is the reproductive and endocrine safety signal: rat toxicology studies found reduced FSH, smaller testes, and teratogenic effects. Human reproductive safety is not fully characterized. Off-target ActRII effects on the pituitary-gonadal-adrenal axis are a mechanistic concern that has not been resolved in the published human data.

How does bimagrumab differ from myostatin inhibitors targeting a single ligand?

Bimagrumab acts at the receptor level rather than targeting a single ligand. It blocks both ActRIIA and ActRIIB, which are shared receptors for myostatin, activin A, activin B, GDF11, and related ligands. This broader blockade may explain both its more pronounced fat-loss effects compared to some ligand-specific approaches and the wider safety footprint, since ActRIIs are expressed in pituitary, gonadal, adrenal, and skin tissues beyond skeletal muscle.

References & further reading

PubMed PMID 39385353 - bimagrumab obesity review 2024
PubMed PMID 30733376 - Phase 2 type 2 diabetes and obesity trial outcomes; PMC7807292
PubMed PMID 31451179 - RESILIENT Phase 2b sporadic inclusion body myositis trial
ClinicalTrials.gov NCT02250443 - sIBM long-term safety study (terminated early)
ClinicalTrials.gov NCT06643728 - obesity/overweight bimagrumab plus tirzepatide Phase 2b; NCT06890611 - healthy volunteer study bimagrumab with/without tirzepatide