Some breast cancer treatments may limit effectiveness of weight loss medications

Breast cancer medications, called aromatase inhibitors, may lessen the effect of weight loss drugs, according to a new study being presented Friday at ENDO 2023, the Endocrine Society's annual meeting in Chicago, Ill.

The study found that weight loss medications are less effective in breast cancer survivors who are treated with aromatase inhibitors, compared with women without a history of breast cancer who are not taking aromatase inhibitors.

Aromatase inhibitors are used to treat some types of breast cancer or to keep it from coming back. They may also be used to help prevent breast cancer in some women who are at a high risk of developing it. These medications block the activity of an enzyme called aromatase, which the body uses to make estrogen in the ovaries and other tissues. Blocking aromatase lowers the amount of estrogen made by the body, which may stop the growth of cancer cells that need estrogen to grow.

"Weight gain is a common concern in breast cancer survivors," said lead researcher Sima Fansa, M.D., of the Mayo Clinic in Rochester, Minn. She said studies have shown that aromatase inhibitors may be a risk factor for weight gain. Weight gain and obesity, in addition to being associated with breast cancer recurrence and breast cancer-related death, can lead to heart disease and heart-related death.

"This is the first study assessing response to weight loss medications in a subgroup of breast cancer survivors taking aromatase inhibitors," Fansa said. "Our results highlight the need to develop better approaches to manage weight gain in patients with a history of breast cancer taking aromatase inhibitors. Preventing weight gain in this group or treating obesity effectively will improve breast cancer outcomes, prevent further health complications, and improve quality of life for these people."

The study included 99 patients—63 women with a history of breast cancer taking aromatase inhibitors and weight loss medications (liraglutide, semaglutide or phentermine); and 36 women without a history of breast cancer who were not taking aromatase inhibitors but were taking weight loss medications.

The women in the breast cancer group lost less weight at 3, 6 and 12 months compared with the women without a history of breast cancer (3.7% vs. 5.6% at 3 months; 3.9 vs. 9.5% at 6 months; and 5.2% vs. 10.5% at 12 months).

"We believe that this difference may be explained, at least partially, by the anti-estrogen effect of aromatase inhibitors," Fansa said. "Aromatase inhibitors can lead to decreased muscle mass, increased fat mass, and changes in energy expenditure. This may affect the body's response to weight loss interventions, including weight loss medications."