Hailed as the biggest advance in decades, can this drug prevent 70% of breast cancers?

A drug that could more than halve the incidence of breast cancer is to be tested on 10,000 women in an international trial involving eight countries, doctors said yesterday.

Early trials of the drug, anastrozole, suggest that it may prevent up to 70 per cent of breast cancers, prompting some specialists to speculate that the disease might be eliminated. Professor John Forbes, co-ordinator of the trial in Australia and New Zealand, said: "Our goal is a world without breast cancer. This is not implausible."

There is a long way to go before that point is reached. The £10m trial, launched in London yesterday by Cancer Research UK, will not deliver its results for seven to 10 years.

But scientists are hoping for big gains against the disease, which afflicts more than 40,000 women a year in Britain, more than twice the number for any other cancer, and whose incidence is rising.

Anastrozole, licensed in 1995 for advanced breast cancer, was approved for use in early breast cancer last year. It has been hailed as the biggest advance in drug therapy for the disease in 25 years after studies showed that it extended survival and reduced side-effects, in comparison with the hormonal treatment, tamoxifen.

Specialists predict it will replace tamoxifen, which has been the gold standard since the 1970s, but it costs ten times as much at about £1,000 a year per patient.

Professor Jack Cuzick, leader of the trial and head of statistics for Cancer Research UK, said that finding a way to prevent breast cancer was a significant challenge. "It is not like lung cancer where we know that smoking is the primary cause and if you give up smoking you will not get cancer.

"It is not like cervical cancer where we know that screening gets rid of half to three quarters of cases. With breast cancer we don't really know the primary cause and we can't say, 'If you do this you won't get breast cancer'."

Screening was worthwhile because it could detect breast cancers early but it was not the ultimate answer, he said. As a result scientists had turned to chemoprevention - treating women with drugs to stop the disease occurring before it had developed. Initial efforts have proved disappointing. Researchers have been chastened by poor results from four major trials of tamoxifen to prevent breast cancer, involving 28,000 patients worldwide, which began in 1986.

Tamoxifen has contributed to a 30 per cent fall in deaths in the past decade. But its role in preventing cancer in high-risk women - who have the disease in the family - was unclear.

The four trials showed that tamoxifen did prevent breast cancer, reducing cases by more than a third. But it also increased cases of endometrial cancer (of the lining of the womb) and of blood clots, almost cancelling out the beneficial effect for breast cancer sufferers.

For every 1,000 women treated over five years, tamoxifen reduced the number who got breast cancer from 30 to 19, preventing 11 cases. But it doubled the number who had blood clots, a potentially lethal condition, from six to 12 and the number with endometrial cancer from two to five. For every 11 women who avoided breast cancer thanks to tamoxifen, nine developed endometrial cancer or blood clots because of it.

In the US, scientists were so impressed by the dramatic fall in the incidence of cancer in women on tamoxifen that they stopped the trial 14 months early and offered the drug to those who up to that point had been taking placebo. America is now the only country in the world where tamoxifen is licensed for the prevention of breast cancer.

But in Britain, scientists urged women to continue with the trial to the end of its five-year term to check long-term effects. About 98 per cent agreed and the results, showing the high incidence of side-effects, were published last year.

Professor Cuzick, director of the department of mathematics, statistics and epidemiology at Queen Mary College, London, said: "Tamoxifen is still a very effective treatment but its benefit in prevention is unclear. It may be appropriate for some women but we are not sure which."

The task now was to find a better preventive treatment, he said. Anastrozole, whose brand name is Arimidex, is at least as effective as tamoxifen in cutting cancer. This is based on evidence from women who have been treated for cancer in one breast and seen a 70 per cent reduced occurrence of the disease in the other. Women treated with tamoxifen had a 40 to 50 per cent reduced incidence in the second breast. "Anastrozole is one and a half times as effective at preventing cancer," Professor Cuzick said.

It also causes fewer side- effects. Tamoxifen blocks the action of oestrogen on the cells of the breast while maintaining the beneficial effects of oestrogen on the bone. Anastrozole, one of a new class of drugs known as aromatase inhibitors, works in a different way by preventing the production of oestrogen.

This avoids the oestrogen-like effects of tamoxifen causing endometrial cancer and blood clots but there are worries about the effect of anastrozole on the bones and on cognitive function. Bone scans and mental tests will be conducted as part of the study.

Recruitment has begun among women aged 40 to 70 who are post-menopausal and at high risk - having a mother or sister with the disease or multiple breast cancers in the family or a series of breast lumps or unusual appearance of the breast on mammography.

One of the first to be signed up was Janice Clark, 50, a teaching assistant from Manchester who had five lumps in her breast last year. They were benign but because she had a family history of the disease - an aunt died from breast cancer - she joined the trial.

She said: "I would do anything that I thought might help with the research. I saw ladies with breast cancer when I worked in a hospital and I wanted to do something for them. I have not really worried about myself because I am being closely monitored."

Mary Morrissey, 44, is not in the trial but has been prescribed anastrozole as an alternative to tamoxifen after suffering a pulmonary embolism (blood clot in the lung) following a diagnosis of breast cancer in 1999.

"I was very surprised when my oncologist recommended it. Many of the ladies in my breast cancer support group were on tamoxifen and I knew very little about it. But my doctor said it was a superior drug."

Kate Law, head of clinical trials at Cancer Research UK, said: "With the incidence of breast cancer rising year on year it is vital that we consider ways to prevent the disease. Anastrozole looks like a very promising option but the only way we will know for sure is to carry out this large-scale trial. If the drug lives up to its promise thousands of cancers could be prevented in the UK alone."