Lilly Halts Obesity Trial, Raising Questions About Muscle-Sparing Drugs

Eli Lilly has terminated a mid-stage clinical trial evaluating bimagrumab, a muscle-sparing antibody, in combination with its blockbuster weight-loss drug Zepbound (tirzepatide). The decision, attributed to "strategic business reasons," comes amid growing interest in therapies designed to preserve muscle mass during weight loss treatment.

Trial Termination and Ongoing Research

The Phase IIb trial, which aimed to enroll 180 patients with type 2 diabetes and obesity, was designed to assess changes in body weight, fat, and visceral adipose tissue over 36 weeks. While this particular study has been halted, Lilly confirmed that a separate trial involving non-diabetic individuals with obesity is still underway.

A Lilly spokesperson stated, "We routinely evaluate our clinical development programs to optimize the potential for each product." The ongoing study is expected to yield initial results around April 2026, according to federal clinical trial databases.

Muscle Preservation in Weight Loss: A Growing Field

Bimagrumab, originally discovered by Novartis and later acquired by Lilly through its $1.9 billion purchase of Versanis Bio, works by binding to activin/myostatin type II receptors to promote muscle formation. This mechanism is intended to improve the quality of weight loss by maintaining lean mass.

Several pharmaceutical companies are exploring similar approaches:

• Regeneron Pharmaceuticals reported Phase II data for trevogrumab, showing a 51.3% boost in muscle preservation compared to semaglutide monotherapy.
• Scholar Rock announced that apitegromab, when used with tirzepatide, preserves 55% more muscle mass than tirzepatide alone.
• Other companies, including Biohaven and Veru, are also developing muscle-sparing obesity drugs.

Regulatory Landscape and Future Directions

The FDA appears to be setting higher standards for muscle-sparing obesity drugs. Veru recently disclosed that the agency now guides that "incremental weight loss" over a GLP-1 drug alone is "an acceptable primary endpoint to support approval." This suggests that preserving muscle mass alone may not be sufficient for regulatory clearance.

In light of these developments, pharmaceutical companies are reassessing their strategies in the competitive weight loss market. Lilly's decision to terminate one trial while continuing another reflects the complex landscape of obesity treatment development, where preserving muscle mass must be balanced with overall weight loss efficacy.