Can GLP-1 Drugs Shrink Breast Tumors? New Study Stirs Up Questions
Industry Buzz
Currently, a lot of the GLP-1 studies in patients who have [had] breast cancer in the past or [who] currently have breast cancer are really looking at the impact on body weight and body composition. [This study] certainly highlights the need for more research in this area.
—Oncologist Eleonora Teplinsky, MD, @drteplinsky
When a study presented at a recent meeting of the Endocrine Society suggested that GLP-1 drugs like tirzepatide (Mounjaro, Zepbound) might shrink breast cancer tumors, it didn’t take long for the headlines—and the questions—to start rolling in.
Eleonora Teplinsky, MD, a board-certified oncologist specializing in breast and gynecologic cancers, took to Instagram to address the buzz. Her key message to physicians? The news is exciting, yes—but it's still early days.
A mouse model, not a mandate
The study in question involved just 16 obese mice with breast tumors, fed a high-fat diet and kept in a warm environment to induce obesity. At week 32, the mice were randomized to receive either tirzepatide or a placebo. []
The mice receiving tirzepatide lost weight—about 20% of their body mass—which aligns with what’s been seen in human trials. Notably, this group also showed reduced breast tumor volume compared to the placebo arm.
But for Dr. Teplinsky, that’s where the excitement should pause. "We really can't extrapolate data from a preclinical study of 16 mice to the impact of how these medications will work on breast cancer in humans," Dr. Teplinsky said.
What GLP-1s do—and what we don’t know yet
GLP-1 receptor agonists, such as tirzepatide and semaglutide, work by mimicking endogenous GLP-1. In people with obesity, this hormone doesn’t function optimally.
By boosting GLP-1 activity, these drugs enhance insulin sensitivity, reduce insulin resistance, suppress appetite, increase satiety, and slow gastric emptying—all of which contribute to weight loss. []
What’s less clear is whether these metabolic shifts can directly or indirectly influence tumor biology, particularly in hormone-sensitive cancers like breast cancer.
"Currently, a lot of the GLP-1 studies in patients who have [had] breast cancer in the past or [who] currently have breast cancer are really looking at the impact on body weight and body composition," Dr. Teplinsky said.
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Dr. Teplinsky also noted that GLP-1s are typically avoided during chemotherapy, due to tolerability and overlapping side effects.
And while this mouse model adds fuel to the hypothesis that metabolic modulation may influence tumor behavior, there’s currently no clinical evidence to support GLP-1 use as an adjunct cancer therapy.
"[This study] certainly highlights the need for more research in this area," Dr. Teplinsky said. "I will say that we should not be giving GLP-1 solely to help with breast cancer treatment. At this time, we follow the FDA-approved indications for prescribing the GLP-1."
Why this matters for oncologists
While no change in practice is warranted now, this study points to a future research opportunity: Could weight loss or metabolic modulation via GLP-1s improve breast cancer outcomes in humans?
If future trials confirm this in a clinical setting, it could open doors to novel multimodal strategies that combine metabolic and oncologic treatment pathways.
Until then, oncologists should continue to manage obesity in breast cancer patients using current standards of care, while monitoring the growing body of literature on GLP-1s.
Related: 'We can now predict breast cancer recurrence just 2 weeks after diagnosis'—could results from a new trial upend SOC?