Bimagrumab
Bimagrumab (anti-activin receptor antibody)
A muscle-preserving antibody that blocks activin receptors to reduce fat while increasing lean mass, being studied alongside GLP-1 drugs.
Overview
Bimagrumab is a monoclonal antibody that blocks activin type II receptors, releasing the brake on muscle growth. In trials it reduced body fat while increasing lean muscle mass, an unusual profile for a weight-loss agent. It is being studied in obesity, including in combination with semaglutide, to preserve muscle during weight loss. It is investigational for this use. Update, September 2026: the BELIEVE Phase 2 (Nature Medicine, 2026) reported 22.1% weight loss with semaglutide, 92.8% of it from fat, and 10.8% weight loss with lean mass up 2.5% for bimagrumab alone.
Mechanism of action
Blocks activin type II receptors, neutralizing myostatin and activin signaling to build muscle and reduce fat mass.
Key studies & citations
- Human2021
Effect of bimagrumab vs placebo on body fat mass among adults with type 2 diabetes and obesity: a phase 2 randomized clinical trial
48 weeks of ActRII blockade produced significant fat-mass loss, lean-mass gain and metabolic improvement.
JAMA Network Open - Human2026
Bimagrumab plus semaglutide alone or in combination for the treatment of obesity: a randomized phase 2 trial (BELIEVE)
507 adults: weight change at 48 weeks was -17.8 kg for high-dose combination, -14.2 kg semaglutide alone, -9.3 kg bimagrumab alone, -3.3 kg placebo.
Nature Medicine
Frequently asked questions
Why combine bimagrumab with GLP-1 drugs?
GLP-1 drugs cause some muscle loss along with fat loss. Bimagrumab builds muscle, so combining them is being tested to preserve lean mass during weight loss.