Aromatase Inhibitors for Breast Cancer

Aromatase inhibitors are a type of medication that stops estrogen production in postmenopausal women, especially.

Since aromatase inhibitors were introduced in the late 1990s, the U.S. Food and Drug Administration (FDA) has approved 3 of them for treating breast cancer. They are:

  • Arimidex (anastrozole)

  • Aromasin (exemestane)

  • Femara (letrozole)

What Is Hormone Therapy?

Hormones are chemicals in the bloodstream that control the growth and activity of normal, healthy cells. But certain hormones, such as estrogen, can also fuel the growth of some tumors, including breast cancer.

Estrogen promotes the tumor growth in approximately two out of three breast cancers. Estrogen promotes tumor growth by binding to a protein called an estrogen receptor, which is found in some breast cancer cells. When estrogen binds to this receptor, the cancer cells divide and the tumor grows.

A variety of hormone therapies block estrogen's effect on breast cancer. They do this in several ways.

  • Aromatase inhibitors reduce the amount of estrogen in the body.

  • Nolvadex (tamoxifen) blocks estrogen from binding to its receptor.

Tamoxifen is currently the hormone therapy most commonly used for estrogen-sensitive breast cancers. It is recommended in premenopausal women diagnosed with estrogen-receptor positive (ER+) breast cancer. It is taken in a daily pill form and usually after surgery for about five years.

Hormone therapies work only against tumors that grow in response to estrogen--that is, those that carry estrogen receptors. An ER+ status shows that the tumor may respond to hormone therapy.

How Aromatase Inhibitors Work

If you have not yet gone through menopause, your ovaries still produce most of the estrogen in your body. After menopause, your ovaries no longer make large amounts of estrogen. But your muscles and fat still produce some estrogen from male hormones called androgens.

Aromatase inhibitors work by blocking the production of estrogen from androgens. These drugs interfere with the enzyme aromatase. Its role is to convert androgens, such as testosterone, into estrogen. By interfering with estrogen production, aromatase inhibitors deplete the body of estrogen. This helps slow or stop the growth of breast tumors, sometimes even shrinking them. Researchers have found that the drugs cannot lower estrogen levels enough to affect tumor growth in younger women. That's because their ovaries still make high levels of estrogen. For this reason, these drugs are used only in women who have gone through menopause.

Aromatase inhibitors are not all the same. There are three drugs that stop estrogen production that block the enzyme aromatase, which is responsible for making small amounts of estrogen in postmenopausal women. These drugs are letrozole (Femar), anastrozole (Arimidex), and exemestane (Aromasin). These medications are only effective in postmenopausal women. Toremifene (Fareston) is an anti-estrogen drug that is closely related to tamoxifen. It may be used in postmenopausal women with breast cancer that has spread beyond the breast. It is used in tumors that are estrogen-receptor positive or tumors whose estrogen-receptor status is unknown. Fulvestrant (Faslodex) acts by completely eliminating estrogen. It is often given if the breast cancer is not responding to tamoxifen. It is given by injection once a month to postmenopausal women only. Currently, it is approved for use only in women who have advanced breast cancer.

An Evolving Role

For the last 20 years, tamoxifen has proven to be a powerful drug for treating and preventing breast cancer in many women. But it is not without its problems. Though rare, blood clots and uterine or endometrial cancer can arise as a result of taking tamoxifen.

For postmenopausal women who might respond to hormone therapy, many physicians now recommend using an aromatase inhibitor at some point during adjuvant therapy. Adjuvant therapy is treatment given in addition to the major treatment. Its purpose is to prevent or delay any remaining cancer cells from growing. However, several questions remain unanswered. It is not clear if starting adjuvant therapy with one of these drugs is preferable than giving tamoxifen and then switching to an aromatase inhibitor. It is also not clear how long to give tamoxifen. It has not yet been determined the ideal length of time to give aromatase inhibitors. Research is now being done to answer these questions.

Side Effects and Risks

In general, aromatase inhibitors do not cause serious short-term side effects. These are the most common side effects. They're listed in alphabetical order. Ask your doctor which ones you are most likely to experience.

  • Headache

  • Hot flashes

  • Mild nausea

  • Muscle and joint aches and pains and stiffness

  • Stomach upset

  • Vaginal dryness

These are usually mild. If you do have side effects, you're more likely to have them in the first few weeks. Only in rare instances do side effects require that the treatment be stopped.

In the long term, aromatase inhibitors are less likely to cause endometrial cancer or blood clots than tamoxifen. However, these drugs do increase the risk for bone thinning, which can develop from a lack of estrogen. This can make bones more brittle and likely to break. Talk with your doctor about what you can do to prevent or manage these problems, such as exercising and taking calcium. Drugs called bisphosphonates can prevent or reverse bone loss. Talk with your doctor about whether you need them.

Because estrogen has healthful effects on the heart and on brain function, researchers are looking into the effects these drugs may have on these organs. Ongoing research will more clearly determine the long-term risks and benefits associated with the use of aromatase inhibitors.


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