Challenges in the endocrine management of breast cancer.

The goal of endocrine therapy in breast cancer is to block the action of estrogen on the tumor cells either by inhibiting estrogen from binding to the specific estrogen receptor or by inhibiting its synthesis. Tamoxifen, a selective estrogen receptor modulator, is the standard endocrine treatment for hormone receptor-positive breast cancer, both in the adjuvant and metastatic settings. Tamoxifen inhibits the binding of estrogen to the receptor, resulting in inhibition of hormone action. However, as tamoxifen is also weakly estrogenic, it may not be optimally effective and increases the risk of endometrial cancer and stroke. Furthermore, patients may be refractory or may become resistant to tamoxifen treatment. Since aromatase inhibitors (AI) block the synthesis of estrogen and have no intrinsic estrogenic activity, they have the potential to be more effective than tamoxifen. Their different mechanism of action and chemical structures may also circumvent tamoxifen resistance. Consequently, AIs are currently being evaluated as an alternative to tamoxifen treatment. A preclinical model has recently been developed to compare the efficacy of AIs and antiestrogens in different treatment schemes and to assist in the design of clinical trials. Current studies with the MCF-7Ca xenograft model are exploring the effects of combination and sequential therapy on tumor growth. The efficacy of AIs in the treatment of hormone receptor-positive breast cancer was first demonstrated in five multicenter second-line trials enrolling several hundreds of postmenopausal patients with metastatic breast cancer who had failed tamoxifen treatment. More recently, anastrozole demonstrated efficacy at least equivalent to that of tamoxifen in first-line randomized, phase III clinical trials in postmenopausal women with hormone receptor-positive or unknown metastatic breast cancer, whereas letrozole demonstrated superiority. The steroidal AI exemestane is currently under evaluation. Letrozole is the only AI to have been studied in a randomized, phase III trial in the neoadjuvant setting. In this trial, more patients underwent breast-conserving surgery with letrozole than with tamoxifen. Smaller phase II studies also suggest that both anastrozole and exemestane are active in the neoadjuvant setting. Because neoadjuvant trials permit temporal sampling of breast tissue, substudies in the phase III trial with letrozole have examined the impact of such biomarkers as estrogen receptor, progesterone receptor and epidermal growth factor receptor family members, HER-1 and HER-2, on patient response. AIs are currently under evaluation in the adjuvant setting, and preliminary results of the Arimidex, Tamoxifen Alone or in Combination (ATAC) trial have been reported. AIs have proven as safe as tamoxifen in trials in patients with metastatic breast cancer. Ongoing clinical trials in the adjuvant setting include companion studies of end-organ effects, particularly bone metabolism and lipid metabolism evaluations. Quality-of-life assessments are also parts of major clinical trials. A head-to-head quality-of-life assessment of anastrozole compared with letrozole demonstrated patient preference for letrozole. These assessments also clearly indicated the eagerness of patients to participate actively in treatment decisions