The Medical Research Council has refused to support the trial because of evidence that the drug, tamoxifen, can cause liver cancer in rats. The council, which is conducting its own toxicity studies, says it will have results in a year.
Other studies have found evidence that tamoxifen can also increase the risk of cancer of the womb, to one in 200 women, although researchers stress that this has not been fully evaluated.
The United Kingdom Co-ordinating Committee of Cancer Research, which represents the major cancer research organisations, announced yesterday that the Department of Health had approved the Tamoxifen Prevention Trial in which 15,000 women at increased risk of breast cancer will receive the drug or a placebo for five years. Dr Jack Cuzick, of the Imperial Cancer Research Fund and chair of the trial working group, said the risk benefit ratios had been carefully weighed. In the UK women have a lifetime risk of breast cancer of one in 12.
'The women in the trial will be more than twice as likely to develop breast cancer as the average woman. Some will be three or four times as likely. Against that I calculate their risk of developing liver cancer at something less than one in 10,000. The Medical Research Council's decision not to join the trial at this stage is disappointing.'
The majority of women in the trial will have double the risk of having an immediate relative who has developed breast cancer, and 10 per cent will have three or four times the risk with a higher incidence in relatives at a younger age. Dr Cuzick said that giving tamoxifen to 10,000 women aged 50 - with two and a half times the risk of breast cancer - for five years would mean, on average, 200 fewer cases of breast cancer, 90 fewer cases of heart disease and 50 fewer pelvic fractures because of tamoxifen's beneficial effects.
The MRC says too little is known about the action of the drug to give it to women who are not at very high risk. 'We should be in a better position in a year's time,' said Dr Lewis Smith, director of the MRC Toxicology Unit in Leicester. Experiments have shown the increased liver cancer in rats, but not in mice. The trial is the first in which healthy people with an increased cancer risk will be given a powerful drug.Reuse content