CDK4/6 inhibitors: mechanisms of resistance and potential biomarkers of responsiveness in breast cancer

Hormone receptor (HR)-positive, HER2-negative tumors represent the most common form of metastatic breast cancer (MBC), and endocrine therapy has been the mainstay treatment for several decades. Recently, a novel drug class called CDK4/6 inhibitors in combination with endocrine therapy have remarkably improved the outcome of patients with HR-positive, HER2-negative MBC by targeting the cell cycle machinery and overcoming aspects of endocrine resistance. Several potential cell-cycle-specific and nonspecific mechanisms of resistance to CDK4/6 inhibitors have been reported in recent studies. This review discusses potential resistance mechanisms to CDK4/6 inhibitors, the use of biomarkers to guide treatment for HR-positive, HER2-negative MBC and possible approaches to overcome resistance to CDK4/6 inhibitors.Approximately 70% of breast cancers are hormone receptor (HR)-positive. A CDK4/6 inhibitor combined with endocrine therapy is the first-line standard of care for patients with HR-positive, HER-2 negative advanced breast cancer. Markers to predict the efficacy of CDK4/6 inhibitors in HR-positive, HER2-negative advanced breast cancer are limited. In this review, we summarize the use of CDK4/6 inhibitors in breast cancer, as well as possible approaches to overcome resistance to CDK4/6 inhibitors.