LONDON (AP) -- British scientists heading a major international breast cancer study criticized U.S. researchers on Tuesday for cutting short a drug trial and claiming that a drug halves the risk of developing the disease.
The Britons, including the doctor who pioneered trials with the drug, tamoxifen, accused the National Cancer Institute in Bethesda, Md., of acting prematurely, jeopardizing the U.S. study, and perhaps raising false hopes.
At a news conference, they said the American researchers will now be unable to establish whether tamoxifen actually prevents cancer in high-risk women and saves lives, or delays the onset of the disease.
"Our emphasis is to try and get long-term data from this trial," said Tony Howell of Christie's Hospital in Manchester, co-chairman of the seven-nation British-led study, called the International Breast Intervention Study.
"Unfortunately, the Americans will not be able to do that now because, to be frank, they are prematurely stopping their trial," he said.
The British-led study, so far involving 4,500 women in Britain, Australia, New Zealand, Spain, Finland, Switzerland and Belgium, began four years ago.
The researchers hope a total of 7,000 women will participate in the study, with the results not expected for several years.
On Monday, officials at the institute defended their decision to end the 6-year-old tamoxifen drug trial.
Dr. Leslie Ford said that the agency was following the standards set for the tamoxifen trial when it started. She said the trial was designed to ask whether tamoxifen could prevent breast cancer. "We all felt the question had been answered," she said.
She said a statistical evaluation showed that there was a clear difference in the incidence of breast cancer among women taking the drug compared to women who were not. Since half of the 13,388 women in the trial were getting placebo, the study was stopped so that all of them could benefit from the drug.
But experts in London felt the NCI action was precipitous.
"The Americans have unblinded the trial, which means it will be unbalanced and they will not be able to answer many questions," said Dr. Trevor Powles of London's Royal Marsden Hospital.
Powles, who in 1986 headed the first pilot study on the tamoxifen's supposed anti-carcinogenic effects, said "it looks" as though the benefits of the drug are likely to substantially outweigh the risks, but it was too early to be sure.
Dangers include an increased risk of getting endometrial cancer -- cancer of the lining of the uterus -- and blood clots in the lung.
Among high-risk women are those with close relatives who developed breast cancer young. Age increases the risk in all women.
The drug, long used to try to stop the recurrence of breast cancer, blocks the effects of estrogen and retards the growth of cancer cells that depend on the hormone.
The Pittsburgh-based study found there were 45 percent fewer cases of invasive breast cancer among the tamoxifen-takers. The federal researchers also warned of the risk of serious side-effects.
One danger for the British-led study now is that women will pull out and simply demand tamoxifen.
Shares in the British-based Zeneca Group, which produces the drug, rose Monday on the American announcement.
"You start to wonder what the hidden agenda is," Michael Baum of London's University College Hospital, the other co-chairman of the British-led study, said of the American announcement. "Is the National Cancer Institute of America trying to defend its budget or something like that? And I don't think this is just sour grapes or British conservatism."
Ford said the American trial was halted upon the recommendation of a monitoring committee that concluded tamoxifen was effective against breast cancer. The committee is independent of NCI.
Londoner Patricia Campbell, 52, whose grandmother and whose 41-year-old sister died of breast cancer, said she will stay in the British trial. Like the others, she gets examined regularly and does not know whether she's taking a placebo or tamoxifen.
"To stop it now would be very frustrating," she said. "I don't feel that I am a guinea pig. In fact, taking part makes me feel safer."
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