# Bimagrumab > An antibody that blocks the activin type II receptor itself — shutting off myostatin AND activin signaling at once; famous for adding muscle while cutting fat. - Also known as: BYM338 - Class: Growth Hormone, Metabolic - FDA approved: No - Canonical page: https://www.americanpeptide.com/catalog/bimagrumab ## Overview Bimagrumab is a fully human monoclonal antibody that blocks the activin type II receptors (ActRIIA/ActRIIB) — the receptor every ligand on this axis must use. By acting at the receptor rather than on a single ligand, it shuts down myostatin and activin signaling simultaneously, which is why it produces the axis’s most striking body-composition effect: in trials it has both reduced fat and increased lean mass, an unusual combination for a weight-affecting drug. Bimagrumab’s history maps the whole field. It was developed (as BYM338, Novartis) for muscle-wasting conditions — sporadic inclusion body myositis (where its pivotal trial failed), sarcopenia, and COPD-related wasting — before a notable Phase 2 in type 2 diabetes showed it cut fat mass substantially while adding lean mass. That fat-down/muscle-up profile made it a centerpiece of the obesity muscle-preservation thesis: Versanis advanced it, and Eli Lilly acquired Versanis for roughly $2 billion in 2023 to pair it with tirzepatide. The honest update is that the path has not been smooth: a combination Phase 2 with semaglutide showed weight loss with preserved/added lean mass, but Lilly subsequently terminated a mid-stage obesity study pairing bimagrumab with its own incretin therapy. As the receptor-level blocker of the axis, bimagrumab is the broadest-acting and the most-watched — and a reminder that even the most mechanistically compelling agent has to clear real trials. It is investigational for these uses. ## Mechanism Binds ActRIIA/ActRIIB and prevents their natural ligands (myostatin, activins, and related factors) from signaling. Blocking the shared receptor disinhibits muscle growth broadly while reducing fat mass. ## Chemistry | Property | Value | | --- | --- | | Molecular weight | 150000 Da | ## Research areas Studied in: Obesity, Type 2 diabetes, Muscle wasting, Body composition. Guides on this site: - [Weight Loss & Metabolic Health](https://www.americanpeptide.com/research-areas/weight-loss): Incretin and metabolic peptides studied for glycemic control and fat loss. - [Growth Hormone & Body Composition](https://www.americanpeptide.com/research-areas/growth-hormone-axis): Secretagogues studied for GH release, IGF-1, and body composition. ## Key research - Receptor-level blockade — targets ActRIIA/B, blocking myostatin and activin signaling together (broadest mechanism on the axis). - Fat down, muscle up — Phase 2 in type 2 diabetes reduced fat mass while increasing lean mass, an unusual profile. - GLP-1 combination — bimagrumab + semaglutide preserved/added lean mass alongside weight loss in Phase 2. - Commercial bet and setback — Lilly acquired Versanis (~$2B, 2023) but later terminated a mid-stage obesity combination study. - Muscle-wasting origins — earlier developed for inclusion body myositis (pivotal trial failed), sarcopenia, and COPD. ## Storage, handling & synthesis **Storage.** Stored refrigerated (2–8 °C), protected from light and freezing, not shaken; investigational handling per trial protocol. **Handling.** A large glycosylated antibody sensitive to freezing, heat, and agitation, which can cause aggregation. **Synthesis.** Bimagrumab is a fully human recombinant monoclonal antibody (~150 kDa) produced in mammalian cell culture, characterized with antibody-grade analytics (glycan/charge-variant profiling, mass spectrometry, receptor-binding/cell-based potency) — not a synthetic peptide. ## FAQs ### What is bimagrumab? A monoclonal antibody that blocks the activin type II receptor, shutting off both myostatin and activin signaling. It is notable for reducing fat while increasing muscle mass, and has been studied in obesity and type 2 diabetes. ### How is it different from the myostatin antibodies? The myostatin antibodies neutralize a ligand; bimagrumab blocks the shared receptor those ligands use, so it acts more broadly across the whole activin/myostatin axis. ### Is bimagrumab approved for obesity? No. Despite encouraging body-composition data and a major acquisition, it remains investigational, and at least one mid-stage obesity combination study was terminated. ### Is this medical advice? No — this is a research and educational reference. Bimagrumab is an investigational antibody, not an approved drug. ## Latest research Recent trials and publications mentioning Bimagrumab, pulled automatically from ClinicalTrials.gov and PubMed (unfiltered search results, refreshed daily). ### Recent trials - [A Study to Investigate Weight Management With Bimagrumab (LY3985863) and Tirzepatide (LY3298176), Alone or in Combination, in Adults With Obesity or Overweight](https://clinicaltrials.gov/study/NCT06643728) — ACTIVE_NOT_RECRUITING · PHASE2 · NCT06643728 - [Effect of Tirzepatide and Bimagrumab on Body Composition, Insulin Sensitivity, and Bone in Adults With Obesity](https://clinicaltrials.gov/study/NCT05933499) — RECRUITING · PHASE2 · NCT05933499 - [A Study of Bimagrumab Alone (LY3985863) and Bimagrumab With Tirzepatide (LY900042) in Healthy Participants](https://clinicaltrials.gov/study/NCT06890611) — COMPLETED · PHASE1 · NCT06890611 - [A Study of Bimagrumab (LY3985863) and Tirzepatide (LY3298176), Alone or in Combination, in Participants With Obesity or Overweight With Type 2 Diabetes](https://clinicaltrials.gov/study/NCT06901349) — WITHDRAWN · PHASE2 · NCT06901349 - [Safety and Efficacy of Bimagrumab and Semaglutide in Adults Who Are Overweight or Obese](https://clinicaltrials.gov/study/NCT05616013) — COMPLETED · PHASE2 · NCT05616013 - [Safety, Pharmacokinetics and Efficacy of Bimagrumab in Overweight and Obese Patients With Type 2 Diabetes](https://clinicaltrials.gov/study/NCT03005288) — COMPLETED · PHASE2 · NCT03005288 --- Source: AmericanPeptide.com — https://www.americanpeptide.com/catalog/bimagrumab Data license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Attribution: AmericanPeptide.com. Research reference only — computational and educational content, not medical advice.