LONDON -- A new study calls into question widely publicized U.S. results, released earlier this year, suggesting that the drug tamoxifen prevents breast cancer in women at high risk of developing the disease.
British scientists who conducted the longest-running study of tamoxifen have concluded there isn't enough evidence the drug prevents breast cancer, according to research published Friday in the medical Journal Lancet. They said that only longer-term trials would provide information about which women would benefit most, the significance of side effects, the effectiveness of the drug and its impact on death rates.
Although scientists agree tamoxifen fights the recurrence of breast cancer in women who already have had surgery, they are divided over whether it prevents the disease from occurring in the first place. Part of the caution over use of the drug relates to its side effects, which include doubling the risk of getting uterine cancer and tripling the risk of developing a blood clot in the lungs, a potentially fatal disorder.
In the latest study, researchers from the Royal Marsden Hospital in London found that after six years the incidence of breast cancer among 2,494 women with a family history of the disease was the same, regardless of whether they took the drug: 34 of the women on tamoxifen developed breast cancer, compared with 36 women on a placebo. A separate study following 5,408 women for four years, conducted by the European Institute of Oncology in Milan came to a similar conclusion.
A U.S. National Cancer Institute study released in April said tamoxifen reduced the chances of breast cancer in high-risk women by 45%. It abandoned its trial of 13,388 women early to give every woman involved the option of taking the drug.
"There are significant numbers of women requesting to take tamoxifen since the American study and I have grave concerns about the widespread use of it in healthy women," said Dr. Trevor Powles, leader of the British study. Tamoxifen, produced by a unit of British drug company Zeneca Group PLC, is known as an "antiestrogen" because it blocks the effects of the hormone in some tissues and retards growth of cancer cells that depend upon estrogen.
Dr. Powles said the U.S. study results, which followed women for an average of three years, could mean tamoxifen was merely treating existing undetectable tumors rather than preventing new ones, and it is unclear how effective the drug would have been if the study had continued for several more years.
Barnett Kramer, deputy director of the National Cancer Institute in Bethesda, Md., said the British and Italian studies don't shake his confidence in the findings of the U.S. study. "We are extremely confident, because of the number of women involved, that the data stand, and the chance that our results occurred by chance were 1 in 10,000," he said.
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