Hormones are chemical messengers that flow through the body and alert the body to respond properly. Estrogen and progesterone are very important to women, affecting female development, menstrual cycles, and pregnancy. But for some women, these hormones can also increase breast cancer risk.
Endocrine therapy can stop breast cancer growth by blocking or removing hormones. Some patients may have surgery to stop the secreted hormones, and if the patient is still having periods, the doctor may recommend removing the ovaries.
When is endocrine therapy used?
Not all breast cancers are hormone-dependent or “hormone-sensitive”. Non-hormone-sensitive breast cancers do not respond to endocrine therapy. If breast cancer is diagnosed, your doctor will test the tumor to determine if it is estrogen or progestin sensitive. If it is sensitive to estrogen or progesterone or both, the doctor will give endocrine therapy.
Patients may also receive endocrine therapy to prevent recurrence if they have already received other treatments, and also to help reduce the risk of new cancer in the opposite breast.
In addition, if you do not have breast cancer, but have a family history or have a related gene that causes an increased risk, your doctor may recommend endocrine therapy to reduce your chances of developing the disease.
What are the endocrine therapy drugs commonly used for breast cancer?
Tamoxifen, which blocks estrogen activity, is used in premenopausal and postmenopausal women. Tamoxifen can also be given to men with hormone-sensitive breast cancer.
Tamoxifen is the standard of care for women with advanced hormone-sensitive cancer. Patients may need to take the drug after surgery if they have early-stage breast cancer or cancer that has invaded other parts of the breast.
Women at high risk for cancer can also take tamoxifen to lower their chances of developing cancer. Another drug approved to reduce the risk of cancer is raloxifene.
Postmenopausal women may be prescribed aromatase inhibitors. After menopause, the main source of estrogen comes from the aromatization process, in which androgens change to estrogens. Aromatase inhibitors can stop cancer growth by blocking the aromatization process. These drugs include anastrozole, exemestane, and letrozole.
Anastrozole is usually used for postmenopausal women with advanced hormone-positive breast cancer, and is an add-on treatment for women with early-stage breast cancer.
Some postmenopausal women use exemestane. If starting the drug, patients will need to stop taking tamoxifen.
Letrozole. If menopausal and hormone-sensitive breast cancer is advanced, your doctor may prescribe this drug as initial and follow-up treatment, in addition to add-on treatment for early-stage breast cancer.
Pabocinib (Palbociclib) is a drug used in combination with letrozole. This drug helps slow the growth of cancer cells. Leukopenia is the most common side effect. Patients should have their blood counts checked before and after treatment.
Ribociclib is used in combination with an aromatase inhibitor as initial endocrine therapy for menopausal women with hormone receptor-positive, HER-2 negative advanced breast cancer.
Patients may be advised to use fulvestrant or toremifene if they do not respond well to other therapies.
What are the side effects?
Side effects of tamoxifen are similar to common menopausal symptoms. Patients may have:
- Hot flashes
- Vaginal discharge
- Fluid retention and swelling
- Disrupted menstrual cycle
- Headaches
- Fatigue
- Nausea
- Vomiting
- Vaginal dryness or itching
- Perivaginal skin irritation and rash
Not all women will experience these symptoms after taking tamoxifen.
Men taking tamoxifen may have headaches, nausea and vomiting, skin rashes, impotence, or decreased sex drive.
There is evidence that tamoxifen increases the risk of uterine cancer. If a patient is taking tamoxifen, they should receive annual pelvic exams to determine if they have symptoms of cancer. If there is vaginal bleeding other than during menstruation, inform your doctor immediately.
Tamoxifen causes a high risk of clotting, especially in women who are receiving chemotherapy. The drug also interferes with other medications being taken.
The side effects of raloxifene are similar to those of tamoxifen, but these side effects are usually milder.
A serious side effect of aromatase inhibitors is osteoporosis, which can lead to fractures. Patients need to have their bone density checked while they are taking these drugs.
The most common side effects of taking letrozole include:
- Mild nausea and vomiting
- Fatigue
- Headache
- Muscle pain and joint pain
- Hot flashes that eventually subside or disappear
Some women may experience hair loss, but the symptoms are usually mild and return to normal after treatment is completed.
For anastrozole, common side effects include:
- Hot flashes
- Nausea
- Lack of energy and weakness
- Back pain
- Bone pain
- Joint pain and stiffness
- Cough
- Flu-like symptoms
- Swelling in the arms and legs