Clinical Trials Pipeline & In-Development

Bimagrumab

also known as: BYM338, ActRII antibody

⚠ Monoclonal antibody — not a classical peptide. A first-in-class muscle-sparing antibody (fully human IgG1, ~150 kDa) against activin type II receptors. Combined with semaglutide in the BELIEVE Phase 2b trial (Nature Medicine, March 2026), it produced greater fat loss than semaglutide alone while preserving lean mass. Not approved; Lilly developer. Included here because it's widely discussed alongside peptides in the muscle-sparing and myostatin conversations.

A fully human monoclonal antibody against activin type IIA and IIB receptors, originally developed by Novartis, acquired by Versanis Bio, and then by Eli Lilly through the 2023 Versanis acquisition; being developed primarily as a muscle-sparing adjunct to GLP-1 receptor agonist obesity therapy to address the lean-mass loss that accompanies GLP-1-driven weight loss.

Mechanism of action

Binds ActRIIA and ActRIIB with high affinity, blocking signaling from myostatin (GDF-8), activin A, and related TGF-β family ligands at the receptor level. Unlike the ligand-trap approach of ACE-031 (soluble ActRIIB-Fc), bimagrumab blocks the receptor itself rather than sequestering ligands — this receptor-specific blockade appears to avoid the BMP9/BMP10-related vascular toxicities that stopped ACE-031. The resulting myostatin pathway inhibition drives muscle protein synthesis and blunts the lean-mass loss that occurs with caloric restriction.

Primary uses

Muscle preservation during GLP-1 receptor agonist obesity therapy (Phase 2 completed)
Obesity monotherapy (early research)
Historical: sarcopenia and inclusion body myositis (investigational, not pursued to approval)

Typical dosing

10–30 mg/kg monthly (IV) in Phase 2 trials (intravenous (current); SC under investigation)

BELIEVE trial dosed IV at weeks 4, 16, 28, and 40. A subcutaneous formulation is in development.

Regulatory status

Not approved. Eli Lilly developer (via Versanis acquisition, 2023). The BELIEVE Phase 2b trial (Heymsfield et al., Nature Medicine, March 2026; N=507) reported that bimagrumab 30 mg/kg + semaglutide 2.4 mg produced approximately 45.7% total body fat reduction at week 72 vs 27.8% for semaglutide alone, with preservation of lean mass — a markedly higher proportion of weight loss as fat mass. Lilly discontinued a parallel bimagrumab + tirzepatide trial in patients with obesity and T2D in September 2025 for "strategic business reasons," while continuing a bimagrumab + tirzepatide trial in obesity without T2D.

References

[pubmed] Heymsfield SB, et al. "Bimagrumab and semaglutide alone or in combination for the treatment of obesity: a phase 2 randomized clinical trial." Nat Med, 2026.
[news-release] Pennington Biomedical Research Center. "Combination GLP-1 Therapy Shows Fat Mass Loss While Preserving Lean Mass in Adults with Obesity." March 5, 2026.

Related peptides

ACE-031

Acceleron's first-generation myostatin trap — Phase 2 development in Duchenne muscular dystrophy was halted in 2013 after epistaxis and telangiectasia adverse events, but the same scaffold concept led to successor programs at Acceleron, Regeneron, and Lilly.

Follistatin-344

The longer of the two principal follistatin splice variants — binds tissues less avidly than FS-315 and therefore has broader systemic distribution, making it the form used in most gene-therapy and myostatin-inhibition research.

Semaglutide

A once-weekly GLP-1 receptor agonist FDA-approved for type 2 diabetes and chronic weight management.

Tirzepatide

A once-weekly GLP-1/GIP dual agonist that produced unprecedented weight loss in SURMOUNT trials — up to ~21% at highest dose.

Disclaimer

This entry is for educational purposes only and does not constitute medical advice. Dosing information reflects published regulatory or research data and is not a recommendation. Many compounds described here are not approved for human use in the United States. Consult a licensed medical professional before considering any peptide therapy.