A clinical trial run by Cambridge University found that a low dose of a drug that mimics the hormone progesterone may not only lower the menopause-like side effects of breast cancer treatment but also help slow down the growth of tumours. This eases long-standing concerns about using hormones in these patients. 

The results are from the PIONEER study, which took place in ten hospitals in the UK and was published today in Nature Cancer. Researchers looked at women who had post-menopausal oestrogen receptor (ER)-positive breast cancer. The condition is the most common type of breast cancer, making up about 75% of all cases. 

Oestrogen, a hormone that women normally have, makes ER-positive cancers grow. To prevent this from happening, patients are often given anti-oestrogen drugs like letrozole, which lower oestrogen levels in the body. These drugs work well, but they cause hot flashes, joint pain, and bone thinning, which are all symptoms like those of menopause. This phenomenon makes some women stop taking them early. 

The PIONEER trial tried adding low-dose megestrol acetate, a man-made form of progesterone, to see if it could help. Megestrol is already known to lower hot flashes in women who are getting anti-oestrogen medicine.

One hundred ninety-eight patients were randomly selected to receive either letrozole alone, letrozole plus 40 mg of megestrol every day, or letrozole plus 160 mg of megestrol every day. Researchers were able to see how the rates of cancer growth changed after giving two weeks of treatment before surgery to remove the tumour. 

Patients who got the combined treatment had a bigger drop in cancer cells that were still growing compared to patients who only got letrozole. Importantly, the smaller dose worked just as well as the higher one. 

Dr Richard Baird from the University of Cambridge said, "Overall, anti-oestrogens are very good treatments." " But some people have side effects that make their lives worse. "Taking something for a long time changes your body in a big way, even if the side effects are small." 

Doing work in the lab helped make sense of the results. Professor Jason Carroll and his team discovered that progesterone can indirectly block the oestrogen receptor, thereby slowing down the division of cancer cells. He said, "Progesterone might make tumours grow, and people have been worried about that." "Our results indicate that it actually helps stop it in this case." 

Dr Rebecca Burrell, a joint first author, added that it could be crucial to use a lower amount. She said, "Even a quarter of the licensed dose had the desired effect, with fewer expected side effects such as weight gain or high blood pressure." "This could help people stick with their treatment and get better results." 

Researchers suggest a longer follow-up period to confirm the long-term benefits and safety. However, if successful, the method could prove beneficial and accessible due to the low cost and unprotected nature of megestrol.