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What Happens During Hormone Therapy for Breast Cancer

Hormones are chemicals your body makes to control the growth and activity of normal cells. Hormones can also speed the growth of some types of cancer. For example, the hormones estrogen and progesterone can stimulate the growth of some breast cancers.

Not all breast cancers have estrogen or progesterone receptors. In general, hormone therapy works best for treating breast tumors that have hormone receptors and doesn't work for tumors that do not have them. Your doctor will test your tumor to see if it grows in response to hormones. If it doesn’t, you probably won’t get hormone therapy.

Hormone therapy is used to prevent or block hormones from speeding up the growth of cancer cells.

Hormone treatment may be given in two ways: drug therapy or ovarian ablation.

Drug therapy is when you undergo hormone therapy by taking special medicines.

Ovarian ablation is a procedure used to stop the ovaries from making female hormones. This type of hormone therapy may be done surgically by removing the ovaries. It may also be done by damaging the ovaries with radiation.

How you get hormone therapy in drug form

Three classes of drugs are used for hormone therapy:

  • Selective estrogen receptor modulators (SERMs)

  • Aromatase inhibitors

  • Estrogen receptor downregulators

Each works a little differently and causes different side effects.

SERMs

How they work. The SERMs class of drugs keep hormones from binding to their receptors. They do this by mimicking the shape of the hormone and filling the space in the receptor. That leaves the cancer-promoting hormone with no place to bind to the cancer cells.

Common types. There are three common SERMs used for breast cancer. You take these as pills:

  • Tamoxifen (most widely prescribed SERM)

  • Fareston (toremifene)

  • Raloxifene (approved for use only in postmenopausal women, and has similar effects compared with tamoxifen) 

Aromatase inhibitors

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

How they work. These drugs interfere with the enzyme called aromatase. Its role is to convert androgens, such as testosterone, into estrogen. By affecting how much estrogen is made, aromatase inhibitors deplete the body of estrogen. This helps slow or stop the growth of breast tumors that are sensitive to estrogen, 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.

Common types. These are aromatase inhibitors that the FDA has approved for breast cancer:

  • Arimedex (anastrozole)

  • Femara (letrozole)

  • Aromasin (exemestane) 

Aromatase inhibitors are not all the same. Two major classes of these drugs work in slightly different ways. Type-1 inhibitors include exemestane, which has a lasting effect on estrogen production. Once a woman takes it, she will not be able to make estrogen again. Type-2 inhibitors include anastrozole and letrozole. They work only while you are taking the drug. Once you stop taking the drug, your body can make estrogen again. All three drugs are available in pill form.

Many studies have compared aromatase inhibitors with tamoxifen as adjuvant hormone therapy in postmenopausal women. Using these drugs, either alone or after tamoxifen, has been shown to better reduce the risk of cancer recurrence than using only tamoxifen for five years. 

Estrogen-receptor downregulators

These drugs attack the tumor's estrogen receptors. They damage the receptors so that they are unable to bind to estrogen. The only drug of this type that is approved to be used for breast cancer is Faslodex (fulvestrant). You get it by a monthly injection into a muscle. Your doctor may prescribe it for you if tamoxifen or toremifene doesn't work. 

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