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Key Words
Breast cancer, anastrozole, aromatase inhibitors, estrogen, tamoxifen. (Definitions of many terms related to cancer can be found in the Cancer.gov Dictionary.)

Summary
In early results from an Italian study, postmenopausal women with breast cancer who were switched from tamoxifen to a newer drug, anastrozole (Arimidex®), were less likely to die or to have a relapse of their disease than women who continued to take tamoxifen. These results are too preliminary, however, to support a conclusion that anastrozole is superior to tamoxifen at preventing breast cancer recurrence.

Source
Abstract presented at San Antonio Breast Cancer Symposium, December 3, 2003.

Background
Following surgery for breast cancer, women whose tumors grow in response to the hormone estrogen usually take the anti-estrogen drug tamoxifen for five years to reduce their risk that the disease will recur. However, tamoxifen does not prevent all recurrences. In addition, the drug increases women's risk for endometrial cancer, severe blood clots, and stroke.

Preliminary results from a large international study published in 2002 - the Arimidex, Tamoxifen, Alone or in Combination study - suggested that anastrozole might be more effective than tamoxifen at reducing the risk of breast cancer recurrence. Anastrozole does not increase the risk of endometrial cancer, blood clots, and stroke, although it appears to increase risk for arthritis, bone loss, and fractures.

Anastrozole belongs to a class of drugs called aromatase inhibitors (AIs). Tamoxifen and AIs both interfere with cancer cells' use of the hormone estrogen to help them grow, but the drugs work in different ways. In another recent international study, postmenopausal women who took letrozole (another drug in the AI class) after completing five years of treatment with tamoxifen were less likely to have a recurrence of breast cancer than women who took a placebo after five years of tamoxifen.

The Study
In the Italian Tamoxifen Arimidex trial, a total of 426 postmenopausal women who had been taking tamoxifen for two years or more were randomly assigned either to continue on tamoxifen until five years had elapsed or to switch to anastrozole for the same period of time. All of the women had had surgery for breast cancer that had spread to the lymph nodes. All had tumors that were sensitive to estrogen. To date, the women have been followed for a median of two years. The team of researchers conducting the trial was led by Francesco Boccardo, M.D., of the University and National Cancer Institute in Genoa, Italy.

Results
So far, of 218 women assigned to continue taking tamoxifen, 26 (12 percent) have had a recurrence of breast cancer, been diagnosed with a second primary cancer, or died. By contrast, of 208 women assigned to take anastrozole, 10 (5 percent) have had a recurrence of breast cancer or been diagnosed with a second cancer, and none has died.

Limitations
Although the results of this study are encouraging, longer follow-up is needed before it can be concluded that anastrozole is superior to tamoxifen, according to Jennifer Eng-Wong, M.D., M.P.H., a medical oncologist at the National Cancer Institute.

Because anastrozole and tamoxifen have different side effects, some women may be better candidates for anastrozole and others for tamoxifen, added Eng-Wong. "The risks associated with each drug need to be considered individually in each patient," she said. "Anastrozole is associated with a higher risk of fractures, so it may not be the best choice for a patient who has osteoporosis and a history of fractures. Alternatively, a patient who is at high risk for blood clots or a stroke is not a good candidate for tamoxifen; anastrozole would be a better choice for her."

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