A new anti-estrogen drug, anastrozole, was recently approved by the Food and Drug Administration (FDA) for treatment of post-menopausal women with advanced estrogen receptor positive breast cancer.

Breast cancer is a common malignancy occurring in women in the United States, with almost 200,000 new cases diagnosed each year. Breast cancer is characterized by the presence of cancer cells in the tissue or ducts of the breast. Advanced breast cancer refers to cancer that has spread outside the breast to lymph nodes and/or distant locations in the body, often invading vital organs. Optimal treatment of breast cancer often requires several different treatment modalities including surgery, radiation, chemotherapy, and hormonal therapy.

Estrogen receptor positive breast cancer is a type of cancer that is stimulated to grow by a naturally occurring female hormone called estrogen. Specific proteins, called estrogen receptors, cover the surface of these cancer cells, enabling estrogen to enter into the cell, ultimately facilitating cancer growth. Currently, women with estrogen receptor positive breast cancer are often treated with a drug called tamoxifen, which blocks estrogen from entering into a cell, eliminating its source for growth.

In post-menopausal women, estrogen is initially produced by the ovaries and adrenal glands as an

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inactive form. Through a series of biochemical conversions, the inactive form is ultimately converted to the final active form of estrogen, which is the growth stimulant for cancer. The initial conversion step of estrogen is facilitated by a protein called aromatase. The new FDA approved drug, anastrozole, works by inhibiting the protein aromatase, which ultimately blocks the entire conversion process that is responsible for creating the active form of estrogen. This reduces levels of the active form of estrogen in the body so cancer cells are depleted of necessary growth stimulation.

Two large clinical trials in North America and Europe involving over 1,000 women were conducted to evaluate the effectiveness of anastrozole compared to tamoxifen. All women had estrogen receptor positive advanced breast cancer. Half of the women received anastrozole and the other half received tamoxifen. Outcomes between the 2 groups were then directly compared. Patients receiving anastrozole achieved a higher rate of partial or complete cancer disappearance compared to patients receiving tamoxifen. In addition, time to cancer progression was delayed for a longer period of time in patients receiving anastrozole compared to those receiving tamoxifen. These results demonstrate the efficacy of anastrozole for treatment of women with estrogen receptor positive breast cancer.

Post-menopausal women with advanced estrogen receptor positive breast cancer may wish to speak with their doctor about the use of anastrozole or about the risks and benefits of participating in a clinical trial evaluating other promising hormonal treatment strategies.

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