A new analysis published in The Lancet Oncology finds that, for the last 20 years, the preventive effects of the breast cancer drug tamoxifen have remained constant, having reduced breast cancer rates by almost a third.
"Tamoxifen is a well-established and effective treatment for certain breast cancers, but we now have evidence of its very long-term preventive benefits," says lead author Prof. Jack Cuzick, head of the Centre for Cancer Prevention at Queen Mary University of London, UK.
"The preventive effect of tamoxifen is highly significant with a reduction in breast cancer rates of around a third," he adds, "and this impact has remained strong and unabated for 20 years."
The new analysis expanded upon the International Breast Cancer Intervention Study (IBIS-I), which weighed the long-term risks and benefits of taking tamoxifen to prevent breast cancer in women considered to be at high risk. The IBIS-I participants were 7,154 pre- and post-menopausal women aged between 35 and 70 - most of them had a family history of breast cancer.
In IBIS-I, the women were randomized to receive either 20 mg of tamoxifen or a placebo every day for 5 years. The health of all participants was monitored after treatment was completed, with an average follow-up time of 16 years (the maximum follow-up was at 22 years).
In the extended analysis, which was presented at the San Antonio Breast Cancer Symposium, it is revealed that 251 women in the tamoxifen group and 350 women in the placebo group developed breast cancer. This works out as a reduced rate of breast cancer in the tamoxifen group of 29%.
The estimated risk of developing breast cancer after 20 years of follow-up was found to be 8% among the tamoxifen group, compared with 12% in the placebo group.
Women taking tamoxifen 'at increased risk for endometrial cancer'
However, this benefit was diminished among women who took hormone replacement therapy during the 5 years of treatment with tamoxifen.
Also, the women who took tamoxifen were at increased risk of endometrial cancer. This cancer - which is a known side effect of tamoxifen - was 3.8 times more common in the tamoxifen group during treatment, though the researchers found no increased risk in the follow-up period. Five women in the tamoxifen group died from endometrial cancer.
The researchers did not find a significant reduction in deaths specific to breast cancer following treatment with tamoxifen. Deaths from other causes were similar across both groups.
The extended IBIS-I analysis follows the first results of the IBIS-II trial, which were released last year. That trial reported that taking the aromatase inhibitor anastrozole for a period of 5 years reduced the risk of breast cancer among post-menopausal, high-risk women by 53%, compared with women who took a placebo.
Prof. Cuzick says of the team's findings:
"We hope these results will stimulate more women, particularly younger women, to consider treatment options for breast cancer prevention if they have a family history of the disease or other major risk factors."
Breast cancer is the most common cancer in women, with an estimated 1.6 million cases per year worldwide. However, Prof. Cuzick says there has been a clear and continuing reduction in breast cancer rates, although there has not yet been a reduction in breast cancer deaths.
"We will need to continue monitoring these women for a further decade to get a clearer picture of the impact of tamoxifen on death rates," he says. "Some of the side effects of tamoxifen are also cause for concern and need continued monitoring - specifically the increased occurrence of endometrial cancer."