Developments in Breast Cancer Hormonal Treatment

How Does It Work?

Tamoxifen works by blocking estrogen from binding to cancer cells. Anastrozole works by halting the production of estrogen in post-menopausal women, so there is essentially no estrogen available to promote the cancerous growths. Anastrozole is just one of a few drugs in a class called aromatase inhibitors, named after the way they stop estrogen production.

Although tamoxifen blocks estrogen from reaching the cancer cells, it also has some partial estrogen-like properties that account for its side effects. These include serious effects such as ]]>blood clots]]> , ]]>strokes]]> , and ]]>uterine cancers]]> , as well as more minor but often irritating side effects like hot flashes, weight gain, and mood swings. Since anastrozole works differently than tamoxifen, patients taking it have much less of these side effects. In the scientific studies, patients on anastrozole experienced more ]]>bone fractures]]> and musculoskeletal disorders than women taking tamoxifen.

Hope and Caution

Although the data on anastrozole looks good, the comparison studies have only followed its use in patients for an average of three years.

Many breast cancer specialists are wary that three years is not long enough to assess possible long-term side effects and not long enough to fairly compare it to the standard five years of tamoxifen. “Tamoxifen has been used routinely for twenty-five years,” says Dr. Jones. “We don’t want to suddenly replace tamoxifen. It is the gold standard. We understand about tamoxifen, we know how it works, and we know in whom it works.” Dr. Jones says that cancer specialists would feel more comfortable seeing what the results are after five or ten years.

“While researchers await more mature results and findings from additional trials using the aromatase inhibitors in the adjuvant setting, many patients and physicians have elected to use the third-generation aromatase inhibitors either as initial therapy in the adjuvant setting or following a course of tamoxifen,” Dr. Winer says. Women who switch to an aromatase inhibitor may continue this therapy for 2-3 more years, but no longer than five years. Women are advised that the result of treatment with an aromatase inhibitor for longer than five years has not been studied and should only be done in the context of a clinical trial.

Current Assessments

Adjuvant therapy for post-menopausal women with hormone receptor-positive breast cancer should include an aromatase inhibitor in order to lower the risk of tumor recurrence, according to an updated technology assessment from the American Society of Clinical Oncology (ASCO). The three types of aromatase inhibitors are anastrozole (Arimidex), letrozole (Femara), and exemestane (Aromasin). The long-term side effects of aromatase inhibitors are not known. Early data suggest that when compared with tamoxifen, aromatase inhibitors may reduce the chance of blood clots and uterine cancer and may increase the risk of ]]>osteoporosis]]> and fractures.

Women who develop invasive hormone receptor-positive breast cancer while taking tamoxifen for breast cancer risk reduction, and women who cannot take tamoxifen because of high risk of severe side effects, or who have tried tamoxifen and had to stop because of severe side effects, might be advised to consider using an aromatase inhibitor.