Review of the development of letrozole and its use in advanced breast cancer and in the neoadjuvant setting.

Five years of adjuvant endocrine therapy with the antiestrogen tamoxifen has been shown to significantly reduce the risk of recurrence in women with early breast cancer and has thus been the standard of adjuvant therapy for this malignancy over the last two decades. Tamoxifen has also been used for the first-line treatment of advanced or metastatic breast cancer, and it was studied in the neoadjuvant setting to promote breast-conserving surgery in those patients who may be otherwise ineligible. However, comparative clinical trials involving the more recently approved third-generation aromatase inhibitor drugs (anastrozole, letrozole, and exemestane) have challenged tamoxifen as first-line therapy in advanced/metastatic breast cancer as well as in the neoadjuvant setting. Although trials with other AIs have shown improved efficacy and better tolerability over tamoxifen, letrozole has consistently demonstrated superiority over tamoxifen when used as first-line treatment for advanced/metastatic breast cancer or as neoadjuvant therapy. The efficacy of letrozole in the neoadjuvant setting further extends to those tumors with positive human epidermal growth factor receptor (HER1) and/or HER2 expression, which are often less responsive to tamoxifen. The encouraging results of such trials identify letrozole as a better alternative to tamoxifen in improving responses rates in the treatment of advanced breast cancer and as neoadjuvant therapy, which allows breast-conserving surgery in women with inoperable breast cancer or who were not candidates for breast-conserving surgery.