What are aromatase inhibitors?

The aromatase inhibitor drugs include anastrozole (Arimidex), exemestane (Aromasin), and letrozole (Femara). Anastrozole is currently being studied for its role in chemoprevention, as part of the IBIS-II trial. Recent data was just reported earlier in June at the ASCO conference that exemestane reduces the risk of an initial breast cancer occurrence. Concerns about using the aromatase inhibitors for breast cancer prevention are mainly related to side effect profiles, including bone pain and joint pain.
Aromatase inhibitors: Three drugs that stop estrogen production in post-menopausal women have been approved to treat both early and advanced breast cancer: letrozole (Femara®), anastrozole (Arimidex®), and exemestane (Aromasin®). They work by blocking an enzyme (aromatase) responsible for making small amounts of estrogen in post-menopausal women. They cannot stop the ovaries of pre-menopausal women from making estrogen, so they are only effective in post-menopausal women. These drugs are taken daily as pills.
Several studies have compared these drugs with tamoxifen as adjuvant hormone therapy in post-menopausal women. Using these drugs, either alone or after tamoxifen, has been shown to better reduce the risk of cancer recurrence than using tamoxifen alone for 5 years.
For post-menopausal women whose cancers are estrogen and/or progesterone receptor–positive, most doctors now recommend using an aromatase inhibitor at some point during adjuvant therapy. But several important questions have not yet been answered. It's not yet clear if starting adjuvant therapy with one of these drugs is better than giving tamoxifen and then switching to an aromatase inhibitor. If tamoxifen is given first, it's not clear how long it should be given. The optimal length of treatment with aromatase inhibitors has not yet been determined, nor has it been shown if any one of these drugs is better than the others. Studies now being done should help answer these questions.
The aromatase inhibitors tend to have fewer serious side effects than tamoxifen -- they don't cause uterine cancers and very rarely cause blood clots. They can, however, cause muscle pain and joint stiffness and/or pain. The joint pain may be similar to a new feeling of having arthritis in many different joints at one time. Because aromatase inhibitors remove all estrogens from women after menopause, they also cause bone thinning, sometimes leading to osteoporosis and even fractures. Many women treated with an aromatase inhibitor are also treated with medicine to strengthen their bones, such as bisphosphonates.

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