Hormone drug megestrol may slow tumor growth in ER-positive breast cancer

A randomized trial reported that adding megestrol, a synthetic progesterone, to standard anti-estrogen therapy slowed tumor growth over a two-week pre-surgery period in women with ER-positive breast cancer; low and high doses showed similar effects and longer-term benefit is undetermined.

A short randomized trial reported in Nature Cancer found that adding the synthetic progesterone megestrol to standard anti-estrogen therapy slowed tumor growth in women with ER-positive breast cancer during a two-week pre-surgery window. ER-positive cancers rely on estrogen and are commonly treated with drugs that lower estrogen levels, such as letrozole. Researchers compared low-dose (40 mg) and high-dose (160 mg) megestrol alongside letrozole and included a group receiving letrozole alone. Laboratory tests in mice suggested progesterone can indirectly block estrogen receptors and reduce cancer cell division. Key findings: - 198 women with ER-positive breast cancer were randomized to letrozole plus low-dose megestrol, high-dose megestrol, or no megestrol for two weeks before surgery. - Adding megestrol increased letrozole's ability to slow tumor growth in that two-week window, with similar effects from the low and high doses. - Low-dose megestrol is already prescribed for hot flashes and is available as a generic; researchers noted the lower dose may avoid some side effects associated with higher doses. - Follow-up studies are needed to determine whether the observed benefits persist beyond the short treatment period. Summary: The trial indicates that short-term addition of megestrol can enhance the effect of anti-estrogen therapy on tumor growth in ER-positive breast cancer, and a lower dose showed comparable results to a higher dose. Undetermined at this time.