Future perspectives and challenges with CDK4/6 inhibitors in hormone receptor-positive metastatic breast cancer

There are three US FDA-approved CDK4/6 inhibitors: palbociclib, ribociclib and abemaciclib for patients with HR-positive, HER2-negative (HR+/HER2-) metastatic breast cancer (MBC). They are all equally effective, so the question becomes how to choose among these agents and how to sequence them. Other areas with active investigation include identifying predictive biomarkers for the selection of patients whom may benefit more from CDK4/6 inhibitors, deciding whether to continue CDK4/6 inhibitors after disease progression on CDK4/6 inhibitors, creating novel treatment combinations and expanding use beyond HR+/HER2- MBC. Here, we review the current use of and potential next directions for CDK4/6 inhibitors in the treatment of patients with HR+/HER2- MBC.