Breast cancer drug keeps working even after you stop taking it

Breast cancer is one of the best examples of a cancer that is hormonally responsive. What does this mean? First, hormones are chemical substances made in one part of the body that circulate throughout the blood stream and affect the function of another portion of the body. In the case of the female sex hormone estrogen, it is made in the ovaries and causes the development of breast tissue, among a host of other effects in the female body. A cancer that is hormonally responsive means that there are some specific chemicals and components either on the surface of the cancer cell, or deep within the insides of the cell itself, that are influenced by hormones that circulate in our bodies. It has been known for a long time that in some people, estrogen influences the development of breast cancer. Removing or blocking estrogen can profoundly stop breast cancer cells from growing. 

There are really three ways that estrogens can be blocked or removed:

1. Taking out the source of the estrogen producing tissue - the ovaries.
2. Block the interaction of estrogen with the chemicals in the breast cancer cell that cause it to grow by taking the drug tamoxifen
3. Block the body's way of producing estrogen with drugs known as aromatase inhibitors.

New research provides important information on the two medication options listed above.

Generally, postmenopausal women who have had surgery or other treatment for early, hormonally responsive breast cancer are given an aromatase inhibitor. These drugs have been shown to better prevent cancer recurrence or spread than the drug tamoxifen. A recent study showed that women who take the drug for five years are protected against cancer spread or recurrence even after they stop taking it.

In this study, more than 9,000 post-menopausal women were randomly assigned to the aromatase inhibitor called anastrozole (Arimidex), tamoxifen, or both drugs for five years. Three years after stopping the drugs, 15% more women who had taken anastrozole were alive and disease-free, compared to the women who had taken tamoxifen.

Women who take anastrozole do have an increased risk of broken bones. However, that risk goes down after they stop taking it. If you or a loved one has breast cancer, ask your doctor about anastrozole. If you are a breast cancer survivor who has not taken this drug, it would be a good idea to ask your doctor whether there is any reason to try this medicine. At this time, most cancer specialists don't think so, but every case is different.

Do you have questions about breast cancer treatments? Or hormonally responsive cancers in general?

Marc Garnick, M.D., is an internationally renowned expert in medical oncology and urologic cancer, with a special emphasis on prostate cancer. He is a Clinical Professor of Medicine at Harvard Medical School and maintains an active oncology practice at Beth Israel Deaconess Medical Center. Dr. Garnick serves as Editor in Chief of Perspectives on Prostate Diseases, a quarterly report from Harvard Health Publications.

Breast Cancer
Medical knowledge about breast cancer is advancing rapidly. What causes it? Why are some people more likely to get it than others? Women now have many more choices to make about imaging tests, biopsies, surgeries, and drug treatments. Breast Cancer: Strategies for Living is a special health report from Harvard Medical School that explains the current state of knowledge regarding diagnosis, treatment and prevention in easy-to-understand language.