Checkpoint blockade in the treatment of breast cancer: current status and future directions

There is now accumulating evidence that the host immune system plays an important role in influencing response to treatment and prognosis in breast cancer. Immunotherapy with immune checkpoint inhibitors is a promising and rapidly growing field of interest in many solid tumours, including breast cancer. Trials to date have largely focused on metastatic triple-negative disease, a genomically unstable subtype of breast cancer that is believed to be the most immunogenic and following the development of treatment resistance, has limited treatment options and a particularly poor prognosis. Both checkpoint inhibitor monotherapy and combinations with chemotherapy are being investigated. In this review, we discuss the current evidence for PD-1/PD-L1 blockade in metastatic triple-negative breast cancer (TNBC), HER2+ breast cancer and ER+ disease, as well as the emerging evidence for use in the early-stage (neoadjuvant) setting. We also propose potential ways of improving responses to checkpoint blockade in breast cancer.