The hormones estrogen and progesterone, both found in women, can promote the growth of some types of breast cancer, and these hormones help the cells grow and spread. Endocrine therapy treats the disease by blocking or removing these chemicals.
There are two approaches to endocrine therapy for breast cancer
- Blocking estrogen and progesterone with drugs that help cancer cells grow.
- Stopping the ovaries from producing hormones with drugs or surgery.
Endocrine therapy is different from hormone replacement therapy (HRT), which is the addition of hormones to the body to reduce the effects of menopause.
Which breast cancers require endocrine therapy?
After breast cancer is diagnosed, doctors will test the cells in the tumor to determine if estrogen or progesterone receptors are present on the surface of the cells. If they are present, it means that the cancer cells are dependent on these hormones for growth. For these patients, the doctor may recommend endocrine therapy as part of the treatment plan.
Medications for endocrine therapy of breast cancer
Common drugs that are used in endocrine therapy for breast cancer include:
- Bomacitinib (abemaciclib)
- Anastrozole (anastrozole)
- exemestane (exemestane)
- Fulvestrant
- Goserelin (goserelin)
- Letrozole (letrozole)
- Leuprorelin
- Medroxyprogesterone (megestrol)
- Paboxinib (palbociclib)
- Tamoxifen (tamoxifen)
- Toremifene
Breast cancer and tamoxifen
Tamoxifen is a breast cancer treatment that has been prescribed by doctors for more than 30 years and works by blocking estrogen from entering cancer cells.
Doctors first use tamoxifen to treat people whose breast cancer has metastasized to other parts of the body, because the drug slows or stops the cancer from growing. The drug may also reduce the chance of some early breast cancers coming back and may reduce a woman’s risk of developing breast cancer later in life.
Women at high risk for breast cancer can take tamoxifen to lower their chances of getting cancer, or they can watchfully wait or have surgery to remove their breast, called a mastectomy.
Tamoxifen may be used for:
Tamoxifen is used for
- Combined surgery for early-stage breast cancer, ductal carcinoma in situ (DCIS).
- Treatment of abnormal cells in the breast, i.e., lobular carcinoma in situ (LCIS), to reduce the risk of developing advanced breast cancer.
- Treating estrogen-dependent breast cancer for growth.
- Treating breast cancer that has metastasized to other parts of the body or has come back after treatment.
- Preventing breast cancer in women at high risk of developing it.
Tamoxifen should not be used by the following people:
- Pregnant women
- Women who are planning to become pregnant
- Patients with blood clots or stroke
Physicians should be consulted to determine if tamoxifen is right for you.
For women, the side effects of tamoxifen are similar to some of the symptoms of menopause; the two most common symptoms are hot flashes and vaginal discharge, and other symptoms may include:
- Vaginal dryness or itching
- Irregular menstruation
- Headache
- Nausea and vomiting
- Rash
- Fatigue
- Fluid retention and weight gain
Side effects may be similar to menopause, but tamoxifen does not trigger menopause.
For men, tamoxifen can cause:
- Headache
- Nausea and vomiting
- Rash
- Sexual-related side effects, such as erectile dysfunction or low sex drive
Are there risks associated with taking tamoxifen?
Risks may include:
- Fertility. Tamoxifen may enhance a woman’s fertility for a short time, but it may be harmful to a growing fetus, so it is important to use some form of birth control, such as condoms or diaphragms, while taking it, but not birth control pills, which may affect the action of anti-cancer drugs and affect breast cancer. If you suspect you may be pregnant while taking tamoxifen, you should inform your doctor immediately.
- Blood clots. Women taking tamoxifen may have a slightly higher risk of developing blood clots in their lungs or large veins. For smokers, the risk is even higher.
- Stroke.
- Uterine cancer or sarcoma. The drug may increase the risk of these diseases in women, but the risk is very small and the benefits of tamoxifen for cancer treatment may outweigh the risks. Consult your doctor to be sure.
- Cataracts. Tamoxifen appears to increase the risk of cataracts, and reported eye problems include corneal scarring and retinal changes.
- Medication. Tamoxifen may interfere with the effects of other medications on the body.
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Tamoxifen and breast cancer prevention
In 1998, the National Cancer Institute conducted a large study to clarify whether tamoxifen could reduce the risk of breast cancer in healthy women at high risk. The results of the trial showed a 50% reduction in breast cancer risk in women taking tamoxifen.
The study also showed that tamoxifen reduced the risk of recurrent breast cancer in women with early-stage ductal carcinoma in situ (DCIS).
Can other drugs prevent breast cancer?
Another drug, raloxifene, is similar to tamoxifen and may prevent osteoporosis. Studies have found that raloxifene can be used to prevent breast cancer in women at high risk, but with fewer side effects. The U.S. Food and Drug Administration (FDA) has approved the use of raloxifene to prevent breast cancer.
Other endocrine therapies
Aromatase inhibitors are another type of endocrine therapy that stops the body from breaking down testosterone into estrogen, including anastrozole and letrozole. Aromatase inhibitors prevent breast cancer progression in patients with advanced estrogen-dependent breast cancer and are more effective than tamoxifen for a longer period of time. These drugs can be used to treat cancer in menopausal women, even if the cancer has metastasized to other parts of the body. These drugs are applied once a day. Side effects of aromatase inhibitors include:
- Nausea and vomiting
- Diarrhea and constipation
- Hot flashes
- Headache
- Bone pain
- Severe fatigue
- Rash
- Swelling and fluid retention
- Flu-like symptoms
- Increased bone loss
Another drug, exemestane, can treat postmenopausal patients whose breast cancer has metastasized outside the breast, and is best for people who have taken tamoxifen but have not responded, applied once daily with a meal. Side effects include:
- Nausea
- Fluid retention
- Weight gain
- Headache
- Hot flashes
Pabocinib (palbociclib) and ribociclib are used to block the action of molecules that help cancer cells grow. Doctors will use these drugs in combination with an aromatase inhibitor for patients with advanced postmenopausal cancer who have not received endocrine therapy. The side effects include:
- Red blood cell and white blood cell reduction
- Fatigue
- Nausea and vomiting
- Decreased appetite
- Sore mouth
- Upper respiratory tract infection
Fulvestrant (fulvestrant) is an injectable drug that blocks estrogen from entering cancer cells in postmenopausal women who have human epidermal growth factor receptor-2 (HER-2) protein on their cancer cells and have used endocrine therapy. Common side effects include:
- Injection site pain
- Nausea and vomiting
- Loss of appetite
- Weakness and fatigue
- Hot flashes
- Cough
- Muscle, joint and bone pain
- Constipation
- Shortness of breath
Bolmacinib and paboxinib are sometimes used in combination with fulvestrant, and sometimes bolmacinib can be used alone.
Goserelin and leuprolide are drugs that block hormone production by the ovaries and are indicated for premenopausal patients with hormone-dependent breast cancer. Their side effects include:
- Fluid retention
- Hot flashes
- Irregular menstruation
- Injection site pain