What Are Aromatase Inhibitors?

Aromatase inhibitors are a type of hormone therapy for breast cancer patients with estrogen receptor positive breast cancer. Aromatase inhibitors decrease the circulating estrogen in a woman’s body by preventing the aromatase enzyme from converting androgen (androstenedione and testosterone) into estrogen (estrone and estradiol), which is the principal source of estrogen for postmenopausal women. These drugs should only be prescribed to women who are postmenopausal or who have had an oophorectomy.

There are three aromatase inhibitors—Arimidex (anastrozole), Femara (letrazole), and Aromasin (exemestane) that are currently approved by the FDA. These drugs are significantly different from SERMS, or selective estrogen receptor modulators. SERMS, such as Nolvadex (tamoxifen) and Evista (raloxifene), are synthetic hormones that bind to the estrogen receptors in a woman’s breast cells in place of her own estrogen. SERMS “fake out” the estrogen receptors because, unlike natural estrogen, they do not stimulate breast cell growth.

The preliminary results of research comparing the use of tamoxifen alone or tamoxifen plus anastrozole (the ATAC trial) showed a slightly longer disease-free survival for women on anastrozole than for women on tamoxifen, but a longer trial will be needed to enable us to make definitive conclusions. Media hype and pharmaceutical companies’ desires to put new drugs on the market often preempt the thorough study of new drugs before they are released. Thus far, little is known about the long-term effects of aromatase inhibitors. Known side effects of concern include increased risk of bone fractures, elevated cholesterol, musculoskeletal syndrome (aches and pains in soft tissue and joints), and neurocognitive problems (with memory, attention, and word finding).

Like SERMS, aromatase inhibitors are being evaluated as “prevention pills”. Breast Cancer Action has serious concerns about giving otherwise healthy women drugs that can have harmful side effects. The current focus on pills for prevention diverts attention and funding from the true prevention of breast cancer through understanding and eliminating our exposure to toxic chemicals.