Up to a thousand women a year suffering from breast cancer could have their lives saved thanks to one of the new generation of breast cancer drugs, results from a clinical trial published today suggest.

The findings of a five-year trial into Aromasin, one of a new breed of drugs called aromatase inhibitors, provide the first proof that women who switch to this after taking the current treatment, tamoxifen, for two to three years are more likely to be alive after five years.

Post-menopausal women with early-stage breast cancer have a 17% lower risk of dying if they take both drugs in sequence, compared with those who just take tamoxifen, the new research shows.

The actual risk of dying after five years is 6.6% for women given both drugs, compared with a death rate of 8.3% with tamoxifen and 11.9% with no drugs at all. But experts stress the difference is crucial.

Mary McCormack, a consultant clinical oncologist at University College hospital in London, said: "We're talking about many hundreds of potential lives being saved purely by switching drugs at this early stage. We think up to a thousand."

The findings, announced in the US, will make it easier for every breast cancer sufferer who is eligible to lobby for the drug - which at £1,000 a year is 10 times more expensive than tamoxifen.

Aromasin (also known by its generic name, exemestane) is an aromatase inhibitor, aimed at women with breast cancer who have been through the menopause. Last month, the National Institute for Health and Clinical Excellence (Nice) provisionally recommended the use of aromatase inhibitors - which include Arimidex and Femara -alongside the "gold standard" tamoxifen. But in its draft guidance, Nice pointed out there was no evidence the new drugs could improve survival compared with tamoxifen alone.

The new data, from the Intergroup Exemestane Study, provides the first proof that women live longer when their treatment includes an aromatase inhibitor.

The research, led by Charles Coombes, professor of cancer medicine at Imperial College London, compared 2,352 postmenopausal women with early-stage breast cancer who switched to Aromasin after two to three years on tamoxifen with 2,372 treated with tamoxifen alone.

Women given Aromasin had a 17% lower risk of dying than those restricted to taxomifen. The new drug also cut the risk of the cancer spreading by 17% and lowered the incidence of tumours in the opposite breast by 44%. The overall risk of any breast cancer recurrence or any new breast cancer was reduced by 24%.

At a glance:
The impact Aromasin could have on cancer sufferers:

11.9% - risk of dying of breast cancer after five years if no treatment at all

8.3% - risk of dying after five years if given tamoxifen

6.59% - risk if given tamoxifen and then Aromasin, also known as by its generic name, exemestane