Endocrine therapy for ER-positive/HER2-negative metastatic breast cancer.

The estrogen receptor (ER) has been targeted for breast cancer treatment for over a century, but many challenges persist. ER-positivity identifies the largest breast cancer subgroup, and ER-directed therapies prolong survival and improve symptoms in the advanced setting with a very favorable side effect profile. Treatment strategies have included decreasing estrogen synthesis and modulating or degrading the ER. However, ER+ breast cancer once diagnosed in the advanced setting still represents an incurable condition. Many efforts are ongoing to circumvent resistance mechanisms with a few strategies already incorporated into clinical practice such as the combination of endocrine agents with drugs that interfere with other signaling pathways and cell-cycle progression. Important questions remain as how best to select each available strategy, how to sequence them and ultimately how to extend benefits to the largest number of patients in need.