Understanding the estrogen receptor signaling pathway: focus on current endocrine agents for breast cancer in postmenopausal women

Current status of hormone therapy in patients with hormone receptor positive (HR+) advanced breast cancer

The natural history of HR+ breast cancer tends to be different from hormone receptor-negative disease in terms of time to recurrence, site of recurrence and overall aggressiveness of the disease. The developmental strategies of hormone therapy for the treatment of breast cancer have led to the classes of selective estrogen receptor modulators, selective estrogen receptor downregulators, and aromatase inhibitors. These therapeutic options have improved breast cancer outcomes in the metastatic setting, thereby delaying the need for chemotherapy. However, a subset of hormone receptor-positive breast cancers do not benefit from endocrine therapy (intrinsic resistance), and all HR+ metastatic breast cancers ultimately develop resistance to hormonal therapies (acquired resistance). Considering the multiple pathways involved in the HR network, targeting other components of pathologically activated intracellular signaling in breast cancer may prove to be a new direction in clinical research. This review focuses on current and emerging treatments for HR+ metastatic breast cancer.