Research summary
January 2008
Study confirms long-term superiority of anastrozole over tamoxifen in preventing breast cancer recurrence
New data shows anastrozole is better than tamoxifen in preventing cancer recurrence in post-menopausal women with hormone-sensitive early breast cancer. The benefits of anastrozole over tamoxifen continue to increase up to four years after active treatment has finished.
Background
One of the main aims of breast cancer treatment is to stop the disease from returning (recurrence). A recurrence can occur in the same breast, in the other breast or elsewhere in the body. A woman’s chance of surviving breast cancer is reduced if the cancer returns.
Tamoxifen and anastrozole are hormonal therapies used to treat women who have hormone receptors on their breast cancer cells (also known as ‘hormone-sensitive’ or ‘hormone receptor positive’ breast cancer). This means the growth of the cancer cells is affected by female hormones. About two-thirds of women with breast cancer have hormone receptors on the breast cancer cells.
Clinical trials have shown hormonal therapies lower the risk of breast cancer recurrence.
Anastrozole (also known as Arimidex®) is a type of hormonal therapy called an aromotase inhibitor. It stops breast cancer cells from growing by blocking the production of female hormones. Specifically, it blocks the conversion of androgens to oestrogen. Tamoxifen is an anti-oestrogen that works by stopping breast cancer cells from getting oestrogen.
Research findings
Researchers have been examining the benefits and safety of anastrozole versus tamoxifen through the ATAC* trial.
The trial consisted of three randomly assigned treatment arms:
- the first group of participants received tamoxifen for five years
- the second group received anastrozole for five years
- the third group received a combination of tamoxifen and anastrozole over five years.
Data from the ATAC trial has previously been reported after 68 months follow-up (just over five years).
New data from the trial was presented at the San Antonio Breast Cancer Symposium in the United States in December 2007 and reported in the January 2008 edition of The Lancet. The data showed that after 100 months follow-up (more than eight years), anastrozole is better than tamoxifen in reducing the risk of breast cancer returning in post-menopausal women with hormone-sensitive early breast cancer.
This finding confirms the results reported at 68 months but show that even up to four years after a woman’s treatment ends, the benefits of anastrozole over tamoxifen in preventing recurrence continue to increase.
After 68 months follow up:
- 9.7% of women taking anastrozole had experienced a recurrence
- 12.5% of women taking tamoxifen had experienced a recurrence.
After 100 months follow up:
- 17.0% of women taking anastrozole had experienced a recurrence
- 21.8% of women taking tamoxifen had experienced a recurrence.
The researchers concluded that after a median follow-up of 100 months, compared to tamoxifen, anastrozole significantly:
- reduces the risk of all recurrences by 24 per cent
- improves disease free survival by 15 per cent.
The results also showed there was no significant difference in the risk of cardiovascular problems or deaths between the treatment groups. Additionally, the increase in fracture rates previously seen in patients on anastrozole returned to the levels of those on tamoxifen once their active treatment had ceased.
Endometrial cancer rates were previously noted to be higher for women taking tamoxifen. This remained the case for these women after their tamoxifen treatment had stopped.
*ATAC: Arimidex® (anastrozole), Tamoxifen, Alone or in Combination.
National Breast Cancer Centre comment
The results after 100 months follow-up show that the benefits of anastrozole in preventing recurrence of breast cancer are maintained up to four years after treatment has stopped with no additional safety concerns.
Overall, there was a 24 per cent decrease in risk of recurrence with anastrozole compared to tamoxifen. However, there is still no difference noted between anastrozole and tamoxifen in terms of overall survival.
The National Breast Cancer Centre has developed Clinical practice guidelines about the use of aromatase inhibitors for post-menopausal women with hormone-receptor positive early breast cancer. An Information booklet is also available to assist women with breast cancer in making decisions about the use of hormonal therapies.
Source
The Arimidex®, Tamoxifen, Alone or in Combination (ATAC) Trialists’ Group. Effect of anastrozole and tamoxifen as adjuvant treatment for early-stage breast cancer: 100 month analysis of the ATAC trial. The Lancet 2008; 9(1):45-53.