What are Aromatase Inhibitors?

Aromatase inhibitors (AIs) are a class of medication used to treat breast cancer and ovarian cancer in post-menopausal women. They work by inhibiting the action of the enzyme aromatase, which is responsible for the synthesis of the hormone estrogen. Estrogen stimulates breast and uterine tissue, thereby promoting the growth of cancer in those tissues. Aromatase inhibitors therefore help slow the growth of cancer by limiting the amount of estrogen in the body.

Before menopause, most of the estrogen in a woman's body is produced in the ovaries, but after menopause, most estrogen is produced instead in the adrenal gland by aromatization, whereby the hormone androgen is converted into estrogen by aromatase. Because aromatization is not a major source of estrogen in pre-menopausal women, aromatase inhibitors are only used to treat cancer in post-menopausal patients. If used in pre-menopausal women, AIs can actually exacerbate cancer, because the ovaries increase estrogen production in response to the inhibition of aromatization.

Some aromatase inhibitors are currently used to treat endometriosis, and to stop or slow early onset puberty in children. Current research is aimed at whether AIs can be used to stimulate ovulation, or to help adolescents with growth defects reach a normal adult height. Aromatase inhibitors are also sometimes used by bodybuilders taking anabolic steroids to prevent the conversion of excess testosterone, an androgen, into estrogen, which can cause undesirable effects including gynecomastia, or male breast growth. However, this use is not medically recognized as effective or safe.

Possible side effects of aromatase inhibitors include joint disorders such as arthritis and arthralgia, or joint pain. In patients already suffering from joint disorders, AIs can cause an increase in symptoms. Aromatase inhibitors can also worsen osteoporosis, and are associated with an increase in hip, spine, and wrist fractures. Hypercholesterolemia, or high cholesterol levels in the blood, are another possible side effect. The possible risks of taking AIs increases with prolonged use.

Women taking aromatase inhibitors are sometimes also prescribed bisphosphonates, a class of medication used to prevent bone loss, to address the side effect of osteoporosis. However, bisphosphonates are associated with another serious side effect, osteonecrosis of the jaw, characterized by bone damage and death in the jaw due to insufficient blood supply. Statins, drugs that lower cholesterol, can help address both cardiovascular risks and osteoporosis when taken along with an AI, and they do not carry the risk of osteonecrosis of the jaw.