Blood hormone levels key to identifying which post-menopausal women will benefit most from taking anastrozole to prevent breast cancer

This type of drug is recommended by the National Institute of Clinical Care and Excellence (NICE) as an option for preventive therapy in post-menopausal women at high risk of breast cancer. The study, published today (6 December) in Lancet Oncology, could lead to better ways to identify those post-menopausal women who would most benefit from these drugs.

1 in 7 women in the UK will develop breast cancer, with almost 56,000 cases diagnosed every year. Post-menopausal women who have higher concentrations of the hormone oestrogen in their blood stream are at higher risk of developing breast cancer. Aromatase inhibitors stop the production of oestrogen and reduce the amount made in the body. They are currently the most effective preventive agent for oestrogen-receptor positive breast cancer.

Led by Professor Jack Cuzick at Queen Mary University of London, an international team of authors from the UK, Australia, Finland, Germany, Italy, and the USA tested whether measuring oestrogen in the blood could identify which women at increased risk of breast cancer will benefit most from the preventive effects of an aromatase inhibitor. The study was funded by Cancer Research UK, and the National Health and Medical Research Council, Australia, and the Royal Marsden Cancer Charity.

The study analysed data from the IBIS-II prevention trial, an international randomised controlled trial of anastrozole in high-risk post-menopausal women conducted from 2003 to 2012. It found that there was a 55% reduction of risk of developing cancer in three quarters of the women receiving anastrozole. However, women who had the lowest 25% of oestradiol levels showed a much reduced risk. This suggests that inexpensive blood tests to measure hormones could identify those women who would benefit most from this medication, and offer them the best balance of managing cancer risk and side effects.

Professor Jack Cuzick said: "These results are very exciting, and can refine how we choose preventive medication for post-menopausal women at high risk of breast cancer. In our study the 25% of these women with the lowest oestradiol measurements benefitted little from taking anastrozole, while still suffering from the side effects of the drug. A simple blood hormone test could improve the benefit of anastrozole if we use it to select the patients best suited to take it. We now need to routinely assess hormone levels in post-menopausal women at high breast cancer risk before prescribing anastrozole, to identify those who are at greatest risk and will respond well."

Dr David Crosby, head of prevention and early detection at Cancer Research UK, said: "It was really exciting when anastrozole was approved by NICE as a preventive treatment for some woman at high risk of breast cancer. This research now gives us some clues about which women would benefit most from the drug, while identifying women who won't benefit and can be spared unnecessary side effects. Cancer Research UK carried out some of the key work on developing these drugs, known as 'aromatase inhibitors'. It's an area with a lot of potential, and larger trials building on the results in this study will be key to further understanding who is most likely to benefit."

Professor Cuzick is being presented with the William L. McGuire Memorial Lecture Award at the 2023 San Antonio Breast Cancer Symposium (SABCS).