Around half of all breast cancer patients could be treated by the widely available female hormone progesterone, according to a new study.
Tumours fuelled by the female hormone oestrogen are treated with drugs like tamoxifen to block oestrogen receptors, which cause cancer cells to grow.
Scientists have long known that women whose tumours have progesterone receptors have a better prognosis - but they have been unable to pinpoint the reason behind this observation.
Now, researchers at Cancer Research UK's Cambridge Research Institute and the University of Adelaide have shown how the progesterone interacts with the oestrogen receptor in breast cancer tumours, changing their behaviour and ultimately slowing down tumour growth.
As tumours develop, cells become more sensitive to oestrogen. When oestrogen interacts with the oestrogen receptor, it activates genes that tell the cell to keep dividing.
Researchers studied 'double-positive' breast cancer cells grown in the lab, ensuring they had sufficient oestrogen and progesterone to activate both receptors.
They discovered that the progesterone receptor stuck to the oestrogen receptor, suggesting it was influencing how it works.
Cutting-edge technology was used to pinpoint the genes controlled by the oestrogen receptor. When the scientists added progesterone, they found that it shifted the points at which the oestrogen receptor attached to DNA.
Further experiments on samples from breast cancer patients and on mice implanted with human cancer cells confirmed that the progesterone slowed the growth of tumours.
It is estimated that around half of the 50,000 women who develop breast cancer every year could benefit from the pioneering research.
Cancer Research UK's Dr Jason Carroll, who led the study with Professor Wayne Tilley at the University of Adelaide in Australia, said it provides a case for clinical trials to investigate using progesterone to complement drugs that target the oestrogen receptor.
Dr Emma Smith, senior science communication officer at Cancer Research UK, said: "This exciting study in cells shows how a cheap, safe and widely available drug could potentially improve treatment for around half of all breast cancer patients."