Mounjaro shows potential to slow breast cancer growth

Mounjaro may significantly reduce breast cancer tumour growth in obese patients, according to early-stage research in mice.

The weight loss drug, which contains tirzepatide, showed what researchers described as a “beneficial impact” on cancer outcomes, though they stress more testing is needed.

In a study involving 16 nine-week-old mice with breast cancer tumours who were fed a high-fat diet to induce obesity, those treated with the drug had substantially smaller tumours than those given a placebo.

At 32 weeks of age – roughly middle age for a mouse – the obese animals were split into two groups.

One group received tirzepatide injections every other day for 16 weeks, while the other received a placebo. Scientists monitored the mice’s weight and tumour growth twice a week.

The mice given tirzepatide lost around 20 per cent of their body weight – similar to average long-term weight loss in humans using the medication.

The fat loss occurred mainly in adipose tissue, the body’s fat-storing cells.

More notably, their tumour size was significantly reduced compared to the control group.

Amanda Kucinskas, research fellow in obesity and breast cancer risk, said: “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.”

The study found a clear association between lower body weight and reduced tumour size.

Total fat mass was also “strongly linked” to tumour growth, researchers said.

However, scientists said it remains unclear how tirzepatide may be slowing tumour growth, and human trials would be needed to test the effect.

They add to growing evidence that GLP-1 agonists – the class of drugs that includes tirzepatide – may have cancer-related benefits beyond weight loss.

Data presented in May at the American Society for Clinical Oncology conference suggested GLP-1s could reduce the risk of 14 obesity-related cancers – including breast cancer – in people with diabetes.

That study found diabetes patients taking GLP-1s had a 7 per cent lower risk of developing obesity-related cancers than those on DPP-4 inhibitors, another diabetes medication.

When other health benefits were factored in, they were also 8 per cent less likely to die over a 10-year period.

Separate research from the University of Texas, presented in December at the San Antonio Breast Cancer Symposium, found obese patients who took GLP-1s for just over a year after finishing breast cancer treatment had a “significantly improved” chance of living longer.

However, the same study noted that patients taking hormone drugs such as tamoxifen still gained weight despite using the jabs – possibly because hormone therapy typically causes weight gain that counteracts the drug’s effects.