The drug targets the hormone oestrogen
Use of a new class of breast cancer drug has the potential to save 1,000 lives a year in the UK, trials suggest.
Aromasin cut the risk of death among postmenopausal women with hormone-sensitive primary breast cancer by 17% compared with standard therapy.
Instead of five years of the drug tamoxifen, the women were switched to Aromasin after two to three years.
Details of the UK-led study were presented to an American Society of Clinical Oncology conference.
There are about 41,000 new cases of breast cancer in the UK each year, and around 12,000 women a year die from the disease.
Aromasin is one of a class of drugs called aromatase inhibitors.
It works by blocking production of the female sex hormone oestrogen.
Preliminary guidance from the National Institute for Health and Clinical Excellence (NICE), which vets new treatments, recommends the use of aromatase inhibitors alongside tamoxifen.
But Nice said there is no evidence aromatase inhibitors improve survival compared with tamoxifen alone.
Currently, five years of treatment with tamoxifen is viewed as the "gold-standard" treatment for postmenopausal women with breast cancer.
Many cancer specialists are already treating women with aromatase inhibitors.
But at about £1,000 per patient per year, aromatase inhibitors are ten times more expensive than tamoxifen.
The Intergroup Exemestane Study (IES) examined 2,352 postmenopausal women with early-stage breast cancer who switched to Aromasin after two to three years of tamoxifen treatment.
They were compared with another group of 2,372 women treated with tamoxifen alone for about five years.
Women given Aromasin, which has the generic name exemestane, had a 15% lower risk of dying than those restricted to tamoxifen.
When only women with hormone-sensitive cancers were included, the improvement rose to 17%.
Safe and effective
Lead researcher Professor Raoul Coombes, from Imperial College London, said: "These results show that switching to exemestane after two to three years of tamoxifen is safe and can improve the cure rate in postmenopausal women with breast cancer."
As well as improving survival, switching to Aromasin cut the risk of cancer spreading by 17% and lowered the rate of tumours appearing in the opposite breast by 44%.
Professor Timothy Cooke, a breast cancer specialist at Glasgow Royal Infirmary, said it was possible that two years of treatment with tamoxifen softened up cancer cells, so they were more vulnerable to the effects of aromatase inhibitors.
The IES trial was co-ordinated by the charity Cancer Research UK.
Professor John Toy, the charity's medical director, said: "This is very encouraging but we must continue to follow the health of the women in the trial in order to secure long-term conclusions."
Jeremy Hughes, chief executive of the charity Breakthrough Breast Cancer, said: "This sounds like excellent news for women with breast cancer.
"Women tell us that a choice of treatment is increasingly important, particularly as all drugs have some side effects which can impact their quality of life.
"Results from clinical trials like this further support NICE's preliminary recommendation for the use of Aromasin and other aromatase inhibitors in early breast cancer."