The news: Eli Lilly quietly cancelled a clinical trial for an experimental drug to slow muscle loss in obesity patients taking its GLP-1 Zepbound.

How we got here: Lilly was investigating a combination of the experimental drug bimagrumab with Zepbound in a midstage trial. The study (expected to last 13 months) was halted after less than a month, although Lilly did not give a specific reason why, per Bloomberg.

• Lilly acquired bimagrumab as part of a $2 billion deal for Versanis Bio in 2023.
• Novo Nordisk, Roche, Regeneron and Veru are also conducting trials for potential treatments that would slow the lean muscle loss that often goes hand-in-hand with rapid weight loss.

Why it matters: Healthcare experts, physicians and patients are concerned about the decrease in muscle mass as a side effect of GLP-1 weight loss drugs.

• Studies have shown that in a nine to 18 month period, GLP-1 users lost between 25% and 39% of their lean muscle mass. That’s far higher than the 10% to 30% decreases people experience from more traditional weight loss methods like calorie restriction, per a Lancet article in November 2024.
• Muscle mass is important for longevity, with studies showing links to lowered risk of type 2 diabetes and cognitive decline, and improved cardiovascular and mental health, per Massachusetts General Hospital.

Our take: Lilly’s kiboshed trial is a reminder that weight loss drugmakers need to look beyond drug solutions to support current patients’ efforts to maintain muscle mass. Proactively packaging more tried-and-true methods like fitness coaching with strength training downloadable apps for GLP-1 users may be more successful. Drugmakers could also partner with physicians and employers to support their GLP-1 behavioral programs with compliance-safe resources around lifestyle needs like fitness and nutrition.

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