The evolving role of aromatase inhibitors in breast cancer.

Anti-aromatase agents inhibit the cytochrome p-450 component of the aromatase enzyme complex responsible for the final step of estrogen biosynthesis in peripheral tissues. These drugs can be classified into first-generation (e.g., aminoglutethimide), second-generation (e.g., formestane and fadrazole), and third-generation (e.g., anastrozole, letrozole, and exemestane) agents. Anti-aromatase agents can also be divided into type I and type II inhibitors. Type I inhibitors have a steroidal structure similar to androgens and inactivate the enzyme irreversibly by blocking the substrate-binding site, and are therefore known as aromatase inactivators. Type II inhibitors are nonsteroidal and their action is reversible. This article reviews the recent evidence regarding the role of third-generation aromatase inhibitors in the management of breast cancer. Relevant PubMed listed articles and presentations at recent international symposia were reviewed. There is a growing body of evidence supporting the role of third-generation aromatase inhibitors (anastrozole, letrozole, and exemestane) as first-line and second-line therapy for estrogen receptor (ER)- and/or progesterone receptor (PgR)-positive advanced breast cancer in postmenopausal women, and as a neoadjuvant therapy in postmenopausal women with hormone receptor-positive invasive breast cancer unsuitable for breast-conserving surgery. Furthermore, the preliminary results of the ATAC (Arimidex, Tamoxifen, Alone and in Combination) study have shown that adjuvant anastrozole is superior to tamoxifen in terms of disease-free survival (DFS), adverse effects, and prevention of contralateral breast cancer in postmenopausal women with early, ER-positive breast cancer. However, longer follow-up is required to assess the long-term effects of these agents on bone mineral density, cognitive function, and overall survival prior to considering their routine use in the adjuvant setting instead of tamoxifen. The potential role of these drugs in the management of ductal carcinoma in situ (DCIS), premenopausal breast cancer, and breast cancer prevention is worth investigating.