Megestrol Acetate Could Boost Breast Cancer Therapy
A common breast cancer treatment may become even more effective with megestrol acetate, a synthetic hormone similar to progesterone.
New research from the University of Cambridge suggests that adding this drug to standard anti-oestrogen therapy not only eases side effects like hot flushes but may also slow tumour growth.
This could be a major step forward for women with oestrogen receptor (ER)-positive breast cancer, offering treatment that is both gentler on the body and more powerful against the disease.
Targeting ER-Positive Breast Cancer
About 75% of breast cancers are ER-positive, meaning tumour cells rely on oestrogen to grow.
Standard treatment usually involves anti-oestrogen medications like letrozole, which block the hormone’s effects and slow cancer progression.
However, these drugs can trigger menopause-like symptoms such as hot flushes, joint pain, and bone loss. These side effects sometimes lead patients to stop treatment.
Megestrol Acetate Offers Dual Benefits
Megestrol acetate has long been used to ease hot flushes caused by anti-oestrogen therapy.
The PIONEER trial now shows that even a low dose may directly slow tumour growth when combined with anti-oestrogen treatment.
In the study, post-menopausal women with ER-positive breast cancer received letrozole alone or with a low dose (40mg) or high dose (160mg) of megestrol acetate for two weeks before surgery.
Researchers measured tumour growth at the start and end of this period.
Promising Early Results
Patients who received megestrol acetate alongside letrozole showed a greater reduction in actively dividing tumour cells than those on letrozole alone.
Both low and high doses had similar anti-cancer effects, suggesting that lower doses could deliver the benefit while reducing side effects like weight gain or high blood pressure.
This could make long-term use more tolerable while maintaining effectiveness.
Insights from the Lab
Lab studies at the Cancer Research UK Cambridge Institute confirmed that progesterone slows ER-positive breast cancer cells by indirectly blocking the oestrogen receptor.
Mouse studies further showed that combining progesterone with anti-oestrogen therapy slowed tumour growth more effectively than anti-oestrogen treatment alone.
These findings supported testing megestrol acetate in patients, showing that targeting the progesterone receptor can be safe and effective.
Implications for Patients
Megestrol acetate’s dual effect—reducing tumour growth while easing side effects—may improve adherence to anti-oestrogen therapy.
By managing uncomfortable symptoms, more patients could stay on treatment consistently, improving outcomes over time.
The drug is off-patent, making it potentially affordable and accessible for a wide patient population.
Next Steps
Researchers plan to expand studies to larger groups and longer treatment periods to confirm long-term benefits and safety.
If confirmed, megestrol acetate could become a widely used addition to anti-oestrogen therapy, improving both survival and quality of life for women with ER-positive breast cancer.
The PIONEER trial highlights megestrol acetate as a promising companion therapy, combining effectiveness with tolerability.
