An anti-oestrogen drug called letrozole is more effective at preventing breast cancer from coming back than tamoxifen in post-menopausal women, US researchers have said.

Oestrogen, the female sex hormone, encourages hormone-sensitive breast cancer cells to divide. Tamoxifen works by blocking oestrogen's effects on cells, whereas letrozole, which belongs to a class of drugs called aromatase inhibitors, works by preventing the body from making oestrogen in the first place.

The Breast International Group (BIG) 1-98 study looked at 8,010 post-menopausal women with hormone-sensitive breast cancer. The first analysis of the data was published in 2005, and showed that letrozole reduces the risk of disease re-appearing compared with tamoxifen.

These updated results show that women who were prescribed letrozole for five years after surgery had a 20 per cent reduced chance of their breast cancer coming back compared with women who received tamoxifen. This was after an average eight years of follow up.

The researchers also found that the number of women who had died by this point was 21 per cent lower among women who received letrozole than among women who received tamoxifen.

Study author Meredith Regan, of the International Breast Cancer Study Group Statistical Centre at Dana-Farber Cancer Institute, Boston, said: "This updated analysis shows that letrozole offers long-term protection over tamoxifen in these patients."

Aromatase inhibitors can be used as treatment after surgery for most post-menopausal women who have hormone-receptor positive tumours - which account for around 60 per cent of all breast cancers. They can be used alone or alongside tamoxifen.

Professor Jack Cuzick, a Cancer Research UK epidemiologist based at Queen Mary, University of London, said: "This study confirms what we've known for some time - that the aromatase inhibitors are more effective than tamoxifen at delaying the return of breast cancer in postmenopausal women with hormone receptor-positive early breast cancer. It also adds important evidence that this leads to a reduced breast cancer death rate.

"Tamoxifen has been one of the most effective breast cancer treatments ever developed, and is still a useful treatment for women with hormone-sensitive breast cancer who haven't yet gone through menopause. But these results further support the use of aromatase inhibitors as the first line treatment for post-menopausal women with hormone-sensitive breast cancer."

Source: CRUK