Hormone therapy might more accurately be called anti-hormone therapy because it works by blocking hormones that spur certain cancers to grow. Hormones act by attaching to proteins, called receptors, on the outside of cells, resulting in cell or cancer growth. Reducing this type of cancer cell growth by blocking hormones is used most commonly in the treatment of breast, gynecologic, and prostate cancers.
About two-thirds of all breast cancers are hormone-receptor positive, meaning they carry receptors for the female hormones estrogen or progesterone. Two basic types of hormone therapy are available for such tumors: one stops estrogen from acting on breast cancer cells; the other lowers overall estrogen levels in the body or within the cancer cell.
Tamoxifen and similar drugs are used to block the estrogen receptor on tumor cells, preventing them from receiving growth signals. Women with invasive, hormone receptor-positive breast cancer generally take tamoxifen for five to 10 years after surgery to reduce the chances that the cancer will come back. To reduce estrogen production within cancer cells, doctors may prescribe drugs known as aromatase inhibitors, which block an enzyme that helps produce a small amount of estrogen in breast cancer cells, as well as other parts of the body, such as fat and muscle. Because such drugs don’t stop the ovaries from making estrogen, they’re effective only in women whose ovaries no longer function, such as post-menopausal women.
Several studies have compared the effectiveness of aromatase inhibitors and tamoxifen following surgery in post-menopausal patients. The results suggest that aromatase inhibitors, either alone or following tamoxifen treatment, are better than tamoxifen alone at reducing the risk that the cancer will come back after five years. The current standard treatment is to use aromatase inhibitors for about five years in post-menopausal women, or they can be given for five years after tamoxifen to achieve ten years of hormonal treatment. Studies are currently underway to see if taking an aromatase inhibitor for more than five years provides any additional benefit.
Patients diagnosed prior to menopause with an early stage of hormone receptor-positive breast cancer are often treated with tamoxifen, and may receive an aromatase inhibitor if they go through menopause during treatment. In addition, young women may benefit from reducing the amount of estrogen produced from the ovaries by suppressing the ovarian function through medications or surgery. However, tamoxifen or an aromatase inhibitor is always given in addition to ovarian suppression.
Hormone therapy is at times used to treat ovarian cancer. Tumors are often tested to determine if they express the estrogen or progesterone receptors.
Drugs known as luteinizing-hormone-releasing hormone (LHRH) agonists, which switch off estrogen production by the ovaries, are used to lower estrogen levels in premenopausal patients. Tamoxifen is used in some patients to keep circulating estrogens from provoking cancer cell growth. Aromatase inhibitors are sometimes prescribed for post-menopausal patients whose ovarian cancer has come back after treatment.
The main hormone treatment for endometrial cancer, which forms in the inner lining of the uterus, is progesterone or similar drugs called progestins, which can slow the growth of endometrial cancer cells. The anti-estrogen drug tamoxifen may be prescribed for women with advanced or recurrent endometrial cancer to prevent estrogen in the body from spurring cancer cell growth. In women with functioning ovaries, LHRH agonists offer a way to lower estrogen levels. Aromatase inhibitors can be useful as well: even after the ovaries have been removed, estrogen is still made in fat tissue. Aromatase inhibitors can stop this latter estrogen from being formed.
As in breast cancer, there are two basic approaches to hormone therapy for prostate cancer: reducing the production of the male hormone androgen, or blocking androgen from stimulating cancer growth. Together, these treatments are known as androgen suppression or androgen deprivation therapy.
The most common hormone therapies for prostate cancer are treatments that reduce androgen production by the testicles. These include orchiectomy, the surgical removal of one or both testes, which can reduce the level of testosterone – a type of androgen – by 90 to 95 percent; drugs called luteinizing hormone-releasing hormone (LHRH) agonists and antagonists, which prevent the pituitary gland from releasing a hormone that signals androgen production by the testes.
Therapies that block the action of androgen in the body include drugs that prevent androgen production by the adrenal glands and prostate cancer cells themselves. These are known as androgen synthesis inhibitors.
Hormone therapy is used in a variety of different ways in prostate cancer. In many patients, it’s used after surgery and/or radiation therapy to lower the risk that the cancer will return. In others, it’s used before or during radiation therapy, as well as after. The type and timing of hormone therapy delivery depends on a variety of factors including the stage of the disease, the risk that the disease will recur, and a patient’s age and overall health.
The length of treatment depends on an individual’s risk of recurrence. For men with newly diagnosed, localized, intermediate-risk prostate cancer, hormone therapy is generally given for four to six months; for men with higher-risk disease, it’s generally given for two to three years.