The Future of ER+/HER2- Metastatic Breast Cancer Therapy: Beyond PI3K Inhibitors

Most breast cancers express the estrogen receptor (ER) receptor and are negative for the human epidermal growth factor receptor 2 (HER2) receptor. ER+/HER2- cancers are treated with hormone-based therapies in the adjuvant setting and derive significant survival benefit from these therapies in the metastatic setting. However, hormone resistance develops in most metastatic patients. An increased understanding of the biology of ER+/HER2- breast cancers has led to the development of new therapies for this disease including CDK4/6 inhibitors and PI3K inhibitors. Several other neoplastic processes are targeted by novel drugs in clinical development, addressing cancer vulnerabilities. These include newer ways to block the ER and targeting the HER2 receptors in ER+/HER2- cancers expressing HER2 in low levels not qualifying for clinical positivity. In addition, promising therapeutic options include targeting other surface receptors or their downstream pathways, as well as targeting the apoptotic machinery and boosting the immune response which is initially insufficient in these cancers. A selection of new drugs in advanced development for ER+/HER2- breast cancer will be discussed in this review.