Update on the use of aromatase inhibitors in breast cancer

Estrogens are biosynthesised from androgens by the CYP450 enzyme complex called aromatase. Aromatase is expressed in the ovary, placenta, brain, bone, adipose tissue and breast tissue. In breast cancer, intratumoural aromatase is the source for local estrogen production in the tissue. Inhibition of aromatase is an important approach for reducing growth stimulatory effects of estrogens in estrogen-dependent breast cancer. The potent and selective third-generation aromatase inhibitors anastrozole, letrozole and exemestane were introduced to the market as endocrine therapy in postmenopausal patients failing anti-estrogen therapy alone, or multiple hormonal therapies. Anastrozole and letrozole are both non-steroidal aromatase inhibitors that compete with the substrate for binding to the enzyme active site. Exemestane is a mechanism-based steroidal inhibitor that mimics the substrate, is converted by the enzyme to a reactive intermediate, and results in inactivation of aromatase. These third-generation aromatase inhibitors are currently approved as first-line therapy for the treatment of postmenopausal women with metastatic estrogen-dependent breast cancer. The use of an aromatase inhibitor as initial therapy, or after treatment with tamoxifen, is now recommended as adjuvant hormonal therapy for postmenopausal women with hormone-dependent breast cancer. Several clinical studies of aromatase inhibitors focus on the use of these agents in the adjuvant setting, for the treatment of early breast cancer. Recently published results show improved responses with these agents compared with tamoxifen.