Aromatase Inhibitors
Aromatase inhibitors block aromatase, an enzyme that is a major source of estrogen in many major body tissues including the breast, muscle, liver, and fat. These drugs are showing great promise for treating hormone receptor-positive (also called estrogen-sensitive, hormone-sensitive, or hormone-responsive) breast cancer.
There are currently three aromatase inhibitors approved for treating early-stage, hormone receptor-positive breast cancer in postmenopausal women:
- Anastrazole (Arimidex)
- Exemestane (Aromasin)
- Letrozole (Femara)
Arimidex and Letrozole are approved for treatment after surgery. Exemestane is approved for women who have taken tamoxifen for 2 - 3 years. These drugs are also approved for women with advanced (metastatic) hormone-sensitive breast cancer.
Tamoxifen is another drug used to treat estrogen-sensitive breast tumors, but it works in a different way than aromatase inhibitors. Tamoxifen interferes with tumors’ ability to use estrogen by blocking their estrogen receptors. Aromatase inhibitors reduce the overall amount of estrogen in the body.
Compared to tamoxifen, aromatase inhibitors are less likely to cause blood clots and uterine cancer. However, these drugs are more likely to cause osteoporosis, which can lead to bone loss and fractures. In general, recent studies indicate that aromatase inhibitors may be better than tamoxifen in improving survival in women with hormone-sensitive breast cancer (especially for those with node-positive cancer) and reducing the risk of cancer recurrence.
Selective Estrogen Receptor Downregulators (SERDs)
Selective estrogen receptor downregulators (SERDs) block estrogen in all tissues in the body. Fulvestrant (Faslodex) is one such drug, which is proving to be at least as effective as anastrozole in delaying time to disease progression in women with advanced breast cancer. Side effects generally include gastrointestinal problems and hot flashes.
Progestins
Progestins, particularly megestrol (Megace), have been used as second- or third-line treatment of advanced breast cancer when tamoxifen fails. Some of the aromatase inhibitors, however, are proving to be more effective, and some have fewer side effects, such as weight gain.
Ovarian Ablation
Ovarian ablation literally shuts down estrogen production from the ovaries. Medications can accomplish ovarian ablation, or it can be done by destroying the ovaries with surgery or radiation. (Osteoporosis is one serious side effect of this approach, but several therapies are available to help prevent bone loss.)
Chemical Ovarian Ablation. Drug treatment (non-chemotherapy drugs) to block ovarian production of estrogen is called chemical ovarian ablation. It is often reversible. The primary drugs used are luteinizing hormone-releasing hormone (LHRH) agonists, such as goserelin (Zoladex). (They are also sometimes called GnRH agonists). These drugs block the release of the reproductive hormones LH-RH, which results in the cessation of ovulation and estrogen production.
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