Weight-Loss Drug Mounjaro Shrinks Breast Cancer Tumors in Mice

In a revealing preclinical study, the weight-loss drug tirzepatide—already known for fighting obesity and diabetes—unexpectedly slowed the growth of breast cancer tumors in obese mice.

Researchers found that as the mice shed fat, their tumors shrank too, suggesting a direct link between weight loss and reduced cancer progression.

Promising Obesity Drug Curbs Breast Cancer in Mice

A recent study presented at ENDO 2025, the annual meeting of the Endocrine Society in San Francisco, Calif., found that tirzepatide, a medication approved for type 2 diabetes under the brand name Mounjaro and for weight loss as Zepbound, slowed the growth of breast cancer linked to obesity in a mouse model.

“Obesity is a significant risk factor for breast cancer, and while it is very preliminary data, our studies in mice suggest that these new anti-obesity drugs may be a way to reduce obesity-associated breast cancer risk or improve outcomes,” said study author Amanda Kucinskas, B.S., a Ph.D. candidate in the labs of Drs. Erin Giles and Kanakadurga Singer at the University of Michigan in Ann Arbor, Mich.

Challenges of Traditional Weight Loss for Cancer Prevention

Scientists have long known that obesity can lead to worse outcomes for people with breast cancer, while weight loss has been associated with better prognoses. However, achieving and maintaining weight loss through conventional methods remains difficult for many individuals.

To explore alternatives, Kucinskas and her team turned to tirzepatide. This medication belongs to a new class of drugs that activate both GLP-1 (glucagon-like peptide 1) and GIP (glucose-dependent insulinotropic polypeptide) receptors, which are involved in regulating blood sugar and appetite. The researchers aimed to determine whether this drug could also slow the progression of breast cancer tied to obesity.

Inside the Mouse Study Design

This mouse study included 16 mice. The 9-week-old C57BL/6 mice were fed a 40% high-fat diet and housed in a warm environment to induce obesity. At 32 weeks of age, the mice with obesity were randomly assigned injections of tirzepatide or a placebo every other day for 16 weeks. Tumor volumes were measured twice weekly.

The researchers found that the anti-obesity drug reduced body weight and body fat by approximately 20% in mice, similar to the amount of weight loss achieved by women on this drug. They found this was primarily due to a loss of adipose mass, with a reduction in adipose depot weights compared to controls.

Correlating Body Weight With Tumor Growth

The anti-obesity drug also reduced tumor volume compared to the controls. At the end of the study, the researchers found that tumor volume was significantly correlated with body weight, total adipose mass and the amount of fat stored in the liver.

“While these are very preliminary results, they suggest that this new anti-obesity drug may also have a beneficial impact on breast cancer outcomes,” Kucinskas said.

Ongoing studies are underway in collaboration with Dr. Steve Hursting’s lab at the University of North Carolina at Chapel Hill to separate the weight loss from the tumor-specific effects of tirzepatide.

Meeting: ENDO 2025

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