Breast cancer tumours can overcome attempts to shut off their oestrogen fuel supply by making the hormone themselves, scientists have learned.
The discovery helps explain why some women with the disease cease responding to drugs called aromatase inhibitors that halt oestrogen production.
In a quarter of women taking the drugs, their tumour cells produce extra copies of genes for the aromatase enzyme which allows them to make oestrogen, a study published in the journal 'Nature Genetics' has shown.
Researcher Dr Luca Magnani, from Imperial College London, said: "For the first time we have seen how breast cancer tumours become resistant to aromatase inhibitors.
"The treatments work by cutting off the tumour's fuel supply - oestrogen - but the cancer adapts to this by making its own fuel supply."
About 70pc of breast cancers are stimulated by oestrogen.
Another drug, tamoxifen, blocks the receptor molecules on tumour cells that allow them to respond to the hormone. Both tamoxifen and aromatase inhibitors eventually stop working in about one-in-three patients and scientists are keen to find out why.
The team is now trying to develop a test that can identify patients whose cancer cells are starting to produce aromatase and oestrogen.
Tumour samples were taken from 150 women whose breast cancers had returned and spread. All the women were treated at the European Institute of Oncology in Italy.
Dr Richard Berks, from UK charity Breast Cancer Now, said: "Once breast cancer spreads it sadly becomes incurable and so we urgently need to tackle drug resistance. It is now critical we find ways to spot, at an early stage, whether a person's breast cancer is becoming resistant to treatment so that they can be moved onto more effective options."