Drugs used in hormone therapy fall into the following broad classifications:
- Estrogens and antiestrogens
- Androgens and antiandrogens
- Gonadotropin-releasing hormone (GnRH) analogues
- Aromatase inhibitors
Estrogens and Antiestrogens
One of estrogen's normal physiologic functions is to promote the development of female secondary sex characteristics. In adults, estrogen continues to stimulate the growth of cells in the glandular ducts of the breasts and the endometrial lining of the uterus.
Common estrogens and antiestrogens used for hormonal therapy are selective estrogen receptor modulators and diethylstilbestrol (DES).
Selective Estrogen Receptor Modulators (SERMs)
Selective estrogen receptor modulators (SERMs) are both estrogenic and antiestrogenic. That is, they act like estrogen on some parts of the body, such as bones, but block the effect of estrogen on other tissues, such as the breast. A SERM works by binding to estrogen receptors in the body. Not all parts of the body have these receptors, and not all receptors are alike. The estrogen receptors in bone are different than the estrogen receptors in breast tissue. This allows SERMs to have one effect in one kind of tissue and different effect on another type of tissue.
Examples of SERMs include the following:
- Tamoxifen—This is most frequently used in combination with other treatments for women with resected breast cancer, meaning that the initial lesion has been removed.
- Toremifene—This is used in the treatment of metastatic breast cancer.
Raloxifene—This is currently used to treat
osteoporosis. It is also used to help reduce the risk of breast cancer in postmenopausal women with osteoporosis..
DES is a synthetic estrogen. It used to be the primary hormonal therapy for postmenopausal metastatic breast cancer. However, it has largely been replaced by tamoxifen, which has similar efficacy, but lower toxicity. DES is associated with a risk of developing life-threatening blood clots. DES is sometimes used in metastatic breast cancer that is hormonally sensitive, but has not responded to multiple other hormonal therapies. DES is also used in men with metastatic prostate cancer.
DES was prescribed from the early 1940s until 1971 to help pregnant women at risk for premature delivery. Researchers later discovered that DES is linked to an increased risk of clear cell carcinoma of the vagina among daughters of women who used this drug during their pregnancies.
Androgens and Antiandrogens
Androgens are hormones found in both men and women, but they are commonly referred to as male sex hormones. They promote the development and maintenance of male sex characteristics. Androgens play an essential role in reproductive and sexual function in men.
Androgens are used in women with metastatic breast cancer. While their exact mechanism of action is unknown, the theory is that androgen administration redirects the synthesis of estrogens that occurs in the adrenal glands. This indirectly results in a lowering of the estrogen level in the blood.
Examples of androgens include:
Antiandrogens are used in men with metastatic prostate cancer. They are androgen receptor antagonists, meaning that antiandrogens bind to the androgen receptor and prevent dihydrotestosterone from binding. Dihydrotestosterone stimulates new growth of prostate cells, including cancerous prostate cells. These medications are generally used with
Examples of antiandrogens include:
Progestin is the synthetic form of progesterone.
Progesterone is a hormone secreted by the ovaries and endometrial lining of the uterus. Acting with estrogen, progesterone promotes breast development and growth of endometrial cells during the menstrual cycle.
The exact mechanism of progestin action in cancer treatment is unknown. Some theories suggest that progestins may work by suppressing the production of estrogen from the adrenal glands (an alternate source particularly in postmenopausal women), lowering estrogen receptor levels, or altering tumor hormone metabolism. It is also thought that progestin may directly kill tumor cells.
Progestins are commonly used in the management of advanced
uterine cancer. They can also be used for advanced breast cancer, although this use has been less common because of the numerous antiestrogen treatment options available. Occasionally, progestins are used as hormonal therapy for prostate cancer.
Examples of progestins include:
- Progesterone acetate
Gonadotropin-releasing Hormone (GnRH) Analogues
When released from the hypothalamus in the brain, gonadotropin-releasing hormone (GnRH) analogues
stimulate the pituitary gland to produce gonadotropins. Gonadotropins are hormones that stimulate androgen synthesis in the testes and estrogen synthesis in the ovaries (hormones that can go on to promote cancer cell growth). When GnRH analogues are first administered, they can cause an increase in gonadotropin release. But with continued administration, GnRH will block gonadotropin release, and therefore decrease the synthesis of androgen and estrogen.
GnRH analogues are used to treat metastatic prostate cancer. They are also FDA-approved for treatment of metastatic breast cancer in premenopausal women.
Examples of GnRHs include:
In both men and women, a small amount of androgen is continuously converted into estrogen by the enzyme aromatase. Aromatase inhibitors block the action of aromatase, leaving less estrogen available to stimulate estrogen receptors on cancer cells.
Aromatase inhibitors are used primarily in postmenopausal women. Since ovarian production of estrogen is scant in postmenopausal women, androgen conversion becomes a significant alternate source. In premenopausal women, where the primary source of estrogen is the ovary, aromatase inhibitors are less effective.
Examples of aromatase inhibitors include:
Octreotide is used in the therapy of carcinoid syndrome, which is characterized by skin flushing, abdominal cramps, and diarrhea. The syndrome is usually caused by metastatic intestinal carcinoid tumors that secrete excessive amounts of certain hormones, namely serotonin and vasoactive intestinal peptide.
Thyroid Replacement Therapy (Synthroid or Levothyroxine)
Certain thyroid cancers (papillary and follicular cancers) are very responsive to thyroid hormone levels. Thyroid replacement therapy is typically given after surgery and radioactive iodine treatments. Thyroid replacement has two purposes:
- To provide the hormone your thyroid can no longer make, as a result of the surgery
To suppress any
cells from growing back
The surgical approach to hormone therapy is to remove the source of the hormone.
The ovaries are the main source of estrogen in premenopausal women.
Oophorectomy, surgical removal of the ovaries, may be used in premenopausal women to treat advanced breast cancer.
The testes are the main source of testosterone production. For prostate cancer, orchiectomy, removal of the testes, may be considered in advanced cases.