Advances in Endocrine-Based Therapies for Estrogen Receptor-Positive Metastatic Breast Cancer

Approximately 70% of breast cancers are estrogen-receptor positive. Tamoxifen and aromatase inhibitors have been the mainstay of endocrine therapy and have improved breast cancer survival. However, a large number of patients experience disease recurrence either during or following completion of endocrine therapy. Recent improvements in our understanding of the various mechanisms underlying the development of endocrine resistance have led to a dramatic change in the landscape of current endocrine treatment with the introduction of new drugs targeting molecular pathways involved in endocrine resistance. Over the past years we have witnessed the use of combination endocrine therapy with mammalian target of rapamycin antagonists, whilst most recently the introduction of cyclin-dependent kinase 4/6 inhibitors has significantly improved response to endocrine therapy. Whilst not a formal systematic review, this article will provide historical background and summarise key clinical trials and current strategies in both first-line and second-line endocrine therapy.