Blocking estrogen’s effects: Several types of drugs interfere with estrogen’s ability to stimulate the growth of breast cancer cells:

  • Selective estrogen receptor modulators (SERMs) bind to estrogen receptors, preventing estrogen from binding. Examples of SERMs approved by the FDA are tamoxifen (Nolvadex®), raloxifene (Evista®), and toremifene (Fareston®). Tamoxifen has been used for more than 30 years to treat hormone receptor-positive breast cancer.
    Because SERMs bind to estrogen receptors, they can potentially not only block estrogen activity (i.e., serve as estrogen antagonists) but also mimic estrogen effects (i.e., serve as estrogen agonists). Most SERMs behave as estrogen antagonists in some tissues and as estrogen agonists in other tissues. For example, tamoxifen blocks the effects of estrogen in breast tissue but acts like estrogen in the uterus and bone.
  • Other antiestrogen drugs, such as fulvestrant (Faslodex®), work in a somewhat different way to block estrogen’s effects. Like SERMs, fulvestrant attaches to the estrogen receptor and functions as an estrogen antagonist. However, unlike SERMs, fulvestrant has no estrogen agonist effects. It is a pure antiestrogen. In addition, when fulvestrant binds to the estrogen receptor, the receptor is targeted for destruction.

How is hormone therapy used to treat breast cancer?

There are three main ways that hormone therapy is used to treat hormone-sensitive breast cancer:


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Adjuvant therapy for early-stage breast cancer: Research has shown that women treated for early-stage ER-positive breast cancer benefit from receiving at least 5 years of adjuvant hormone therapy (2). Adjuvant therapy is treatment given after the main treatment (surgery, in the case of early-stage breast cancer) to increase the likelihood of a cure.

Adjuvant therapy may include radiation therapy and some combination of chemotherapy, hormone therapy, and targeted therapy. Tamoxifen has been approved by the FDA for adjuvant hormone treatment of premenopausal and postmenopausal women (and men) with ER-positive early-stage breast cancer, and anastrozole and letrozole have been approved for this use in postmenopausal women.

A third aromatase inhibitor, exemestane, is approved for adjuvant treatment of early-stage breast cancer in postmenopausal women who have received tamoxifen previously.

Until recently, most women who received adjuvant hormone therapy to reduce the chance of a breast cancer recurrence took tamoxifen every day for 5 years. However, with the advent of newer hormone therapies, some of which have been compared with tamoxifen in clinical trials, additional approaches to hormone therapy have become common (3–5). For example, some women may take an aromatase inhibitor every day for 5 years, instead of tamoxifen. Other women may receive additional treatment with an aromatase inhibitor after 5 years of tamoxifen. Finally, some women may switch to an aromatase inhibitor after 2 or 3 years of tamoxifen, for a total of 5 or more years of hormone therapy.

Decisions about the type and duration of adjuvant hormone therapy must be made on an individual basis. This complicated decision-making process is best carried out by talking with an oncologist, a doctor who specializes in cancer treatment.

Treatment of metastatic breast cancer: Several types of hormone therapy are approved to treat hormone-sensitive breast cancer that is metastatic (has spread to other parts of the body).

Studies have shown that tamoxifen is effective in treating women and men with metastatic breast cancer (6). Toremifene is also approved for this use. The antiestrogen fulvestrant can be used in postmenopausal women with metastatic ER-positive breast cancer after treatment with other antiestrogens (7).

The aromatase inhibitors anastrozole and letrozole can be given to postmenopausal women as initial therapy for metastatic hormone-sensitive breast cancer (8, 9). These two drugs, as well as the aromatase inhibitor exemestane, can also be used to treat postmenopausal women with advanced breast cancer whose disease has worsened after treatment with tamoxifen (10).

Neoadjuvant treatment of breast cancer: The use of hormone therapy to treat breast cancer before surgery (neoadjuvant therapy) has been studied in clinical trials (11). The goal of neoadjuvant therapy is to reduce the size of a breast tumor to allow breast-conserving surgery. Data from randomized controlled trials have shown that neoadjuvant hormone therapies—in particular, aromatase inhibitors—can be effective in reducing the size of breast tumors in postmenopausal women. The results in premenopausal women are less clear because only a few small trials involving relatively few premenopausal women have been conducted thus far.

No hormone therapy has yet been approved by the FDA for the neoadjuvant treatment of breast cancer.

Can hormone therapy be used to prevent breast cancer?

Yes. Most early breast cancers are ER-positive, and clinical trials have studied whether hormone therapy can be used to prevent breast cancer in women who are at increased risk of getting the disease.

A large NCI-sponsored randomized clinical trial called the Breast Cancer Prevention Trial found that tamoxifen, taken for 5 years, reduced the risk of developing invasive breast cancer by about 50 percent in postmenopausal women who were at increased risk of getting the disease (12). A subsequent large randomized trial, the Study of Tamoxifen and Raloxifene, which was also sponsored by NCI, found that 5 years of raloxifene reduces breast cancer risk in such women by about 38 percent (13).

As a result of these trials, both tamoxifen and raloxifene have been approved by the FDA to reduce the risk of developing breast cancer in women at high risk of the disease. Tamoxifen is approved for use regardless of menopausal status. Raloxifene is approved for use only in postmenopausal women.

The aromatase inhibitor exemestane has also been found to reduce the risk of breast cancer in postmenopausal women at increased risk of the disease. After 3 years of follow-up in another randomized trial, women who took exemestane were 65 percent less likely than those who took a placebo to develop breast cancer (14). Longer follow-up studies will be necessary to determine whether the risk reduction with exemestane remains high over time, as well as to understand any risks of exemestane treatment. Although exemestane has been approved by the FDA for treatment of women with ER-positive breast cancer, it has not been approved for breast cancer prevention.

What are the side effects of hormone therapy?

The side effects of hormone therapy depend largely on the specific drug or the type of treatment (5). The benefits and risks of taking hormone therapy should be carefully weighed for each woman.

Hot flashes, night sweats, and vaginal dryness are common side effects of hormone therapy. Hormone therapy also disrupts the menstrual cycle in premenopausal women.

Less common but serious side effects of hormone therapy drugs are listed below.

Tamoxifen

  • Risk of blood clots, especially in the lungs and legs (12)
  • Stroke (15)
  • Cataracts (16)
  • Endometrial and uterine cancers (15, 17)
  • Bone loss in premenopausal women
  • Mood swings, depression, and loss of libido
  • In men: headaches, nausea, vomiting, skin rash, impotence, and decreased sexual interest

Raloxifene

  • Risk of blood clots, especially in the lungs and legs (12)
  • Stroke in certain subgroups (15)

Ovarian suppression

  • Bone loss
  • Mood swings, depression, and loss of libido

Aromatase inhibitors

  • Risk of heart attack, angina, heart failure, and hypercholesterolemia (18)
  • Bone loss
  • Joint pain (19–22)
  • Mood swings and depression

Fulvestrant

  • Gastrointestinal symptoms (23)
  • Loss of strength (23)
  • Pain

A common switching strategy, in which patients take tamoxifen for 2 or 3 years, followed by an aromatase inhibitor for 2 or 3 years, may yield the best balance of benefits and harms of these two types of hormone therapy (15).