Metastatic breast cancer: Endocrine therapy landscape reshaped

Endocrine therapy (ET) of hormone receptor (HR)-positive and human epidermal growth factor receptor 2-(HER2)-negative metastatic breast cancer (MBC) historically focused on estrogen deprivation and antagonism. The identification of several intracellular pathways promoting resistance to antiestrogen therapy led to the introduction of novel endocrine drug combinations that reformed treatment schema and expanded therapeutic options. There is no doubt that efforts to overcome or delay resistance to ET are fruiting, particularly with the introduction of cyclin-dependent kinase 4/6 inhibitors such as palbociclib and ribociclib, and mechanistic target of rapamycin inhibitors such as everolimus. Although still considered incurable by currently available treatment modalities, many patients with MBC nowadays enjoy several years of good quality life coupled with decent tumor control. The diversity of therapies and unusual pattern of side effects can be quite perplexing to the treating physician. The sequence of variable agents and management of side effects, in addition to the timing of initiation of cytotoxic chemotherapy, is among the challenges faced by oncologists. In this review, we shed a spotlight on mechanisms of resistance to ET, and provide a review of landmark studies that have recently reshaped the landscape of treatment options for patients with metastatic HR-positive, HER2-negative MBC. A suggested treatment strategy for newly diagnosed patients is also discussed herein.