What Is Tamoxifen?

Not all breast cancer is treated in the same way. After a breast cancer diagnosis, your doctor will review the pathology report to determine the molecular subtype of the cancer. This will allow your medical team to devise a treatment plan that is targeted to your specific cancer. Many of these breast cancer treatment plans will include tamoxifen.

What is tamoxifen? It's often used as a treatment for the most common molecular subtype of breast cancer: hormone-receptor-positive cancer. Tamoxifen, which is prescribed in pill form, helps to slow or prevent the growth of breast cancer that is hormone-driven.

Hormone-Receptor-Positive Cancer

Approximately 75–80 percent of breast cancers have hormone receptors for both estrogen (ER) and progesterone (PR), according to the National Breast Cancer Foundation. When we talk about the hormone-receptor status of a cancer, it is commonly abbreviated with the initials for the hormones (ER/PR), combined with a plus or minus that indicates whether or not there were many hormone receptors in the cancer cell. A cancer cell with excessive estrogen and progesterone receptors is referred to as ER+/PR+.

How Hormones Function in the Breast

Breast cells contain proteins called hormone receptors that bind with the hormone. According to Albert Wendt, MD, breast cancer specialist at the University of Arizona Cancer Center at Dignity Health St. Joseph's Hospital and Medical Center, this connection is like a puzzle, and the pieces can bind together only if they are the correct shape, or if estrogen and its receptor have compatible shapes. In the middle of a woman's menstrual cycle, estrogen is abundant; the hormone then connects with the estrogen receptors in the breast cell. When the estrogen molecules bind with a receptor, together they cause the DNA to trigger cell growth, also called cell proliferation. Later in the menstrual cycle, estrogen levels drop significantly. Fewer estrogen-receptor pairs allow the cell proliferation to stop, and the cells start to die off until the cycle repeats. This is a normal hormonal fluctuation within a menstrual cycle.

Dr. Wendt explains that, when there is cancer in the breast, the cancer cells with an excess of estrogen receptors will continue to grow and divide unchecked. This is where tamoxifen helps to treat breast cancer.

How Tamoxifen Interacts with Hormone Receptors

Tamoxifen is a selective estrogen receptor modulator (SERM). It acts in a similar manner to estrogen when it interacts with bones and the uterus, stopping the function of estrogen in the breast, as detailed by the National Cancer Institute. Tamoxifen molecules have a shape that is similar enough to estrogen to bind with the estrogen receptor. It tricks the excessive receptors into binding with the drug, thus preventing the receptor from binding with the estrogen. When the hormone receptor is bound to tamoxifen instead of estrogen, it cannot trigger the cell proliferation. Tamoxifen works by keeping the hormone receptors occupied, and that keeps them from creating unchecked cell growth.

Tamoxifen Use in Breast Cancer Treatment

Tamoxifen is frequently used in the treatment of premenopausal women with hormone-receptor-positive breast cancer, and the pill is often prescribed for a duration of 5–10 years. Some of the side effects of tamoxifen are similar to the side effects of menopause: hot flashes, mood swings, depression, vaginal dryness, and loss of libido. Additionally, there is the risk of blood clots, stroke, bone loss, cataracts, and the possibility of developing uterine and endometrial cancer.

The pathology reports that detail the findings from a biopsy or excision will help the medical team determine whether tamoxifen is likely to be effective in the treatment on an individual basis. This tailored approach to cancer treatment helps to ensure that the cancer is targeted effectively while limiting the side effects of the treatment. According to Dr. Wendt tamoxifen is often highly effective in the treatment of hormone-receptor-positive cancers.