ER-Positive Tumors and Hormone Therapy What are aromatase inhibitors? Breast tumors can either be sensitive to estrogen (ER-positive), sensitive to progesterone (PR-positive), sensitive to both (ER-positive and PR-positive), or neither (ER-negative and PR-negative).
If a woman's tumor is hormone-positive (ER-positive and/or PR-positive), her oncologist may recommend that she try a hormonal therapy. Hormone therapies slow or stop cancer's growth by changing the hormonal milieu.
The aromatase inhibitors reduce estrogen by blocking an enzyme called aromatase and keeping it from converting androgens into estrogen. Both pre- and postmenopausal women can use tamoxifen as hormonal therapy. But only postmenopausal women can use an aromatase inhibitor. That's because postmenopausal women get most of their estrogen from the conversion of androgens into estrogen by the aromatase enzyme, while premenopausal women get most of their estrogen directly from their ovaries.
In December 2004, the American Society of Clinical Oncology (ASCO) issued new guidelines on aromatase inhibitors. It is now recommended that most postmenopausal women be treated with an aromatase inhibitor. This means that tamoxifen, which first began to be used in the adjuvant setting in the 1980s, is no longer the standard of care for postmenopausal women. Tamoxifen does, however, remain the standard of care for premenopausal women.
Currently three aromatase inhibitors are approved for use by the US Food and Drug Administration (FDA): anastrozole (brand name Arimidex), letrozole (brand name Femara), and exemestane (brand name Aromasin).
Anastrozole (Arimidex) was initially approved for use as first-line treatment (first drug used) for postmenopausal women with locally advanced or metastatic disease or as second- or third-line treatment after another hormonal therapy has stopped working. In 2002, Arimidex became the first aromatase inhibitor to be approved for use in the adjuvant setting (adjuvant refers to treatment used following surgery) for postmenopausal women with hormone receptor-positive breast cancer.
Letrozole (Femara) was the second aromatase inhibitor approved for treating women with metastatic disease. In October 2004, the FDA approved the use of letrozole as extended adjuvant treatment of early breast cancer in postmenopausal women who had already received five years of tamoxifen. Studies are now underway evaluating the effectiveness of letrozole as first-line treatment in the adjuvant setting. Many of these trials are currently recruiting patients.
Exemestane (Aromasin) is the first and only FDA-approved steroidal aromatase inactivator. It is able to stop the aromatase enzyme's production process permanently, which is why it is called an irreversible aromatase inhibitor. However, the fact that it is "irreversible" has not been found to make it more effective than the two reversible aromatase inhibitors. In October 2005, the FDA approved the use of exemestane as adjuvant treatment in postmenopausal women with early breast cancer who had already had two to three years of tamoxifen to complete five years of hormone therapy. Studies are now underway evaluating the effectiveness of letrozole as first-line treatment in the adjuvant setting in both pre- and postmenopausal women. Many of these trials are currently recruiting patients.
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