Immune Checkpoint Inhibitors and Novel Immunotherapy Approaches for Breast Cancer

Abstract

Purpose of review: To critically review the existing evidence on immune checkpoint inhibitors (ICIs) in early-stage and metastatic breast cancer and discuss emerging strategies in the different breast cancer subtypes.

Recent findings: Immunotherapy has become one of the major milestones in contemporary oncology, revolutionizing the treatment of multiple solid tumors. ICI agents combined with chemotherapy have demonstrated significant efficacy in both early-stage and metastatic triple-negative breast cancer. However, only a subgroup of patients responds to those agents and some associated toxicities, although infrequent, can be life-disabling. Emerging data from immunotherapy studies in advanced hormone receptor-positive (HR +) breast cancer as well as HER2-positive disease are arising with mixed results. Although breast cancer has not classically been considered a hot tumor, ICIs have proven to be effective in a subset of breast cancer patients. However, much remains to be learned, and the identification of new biomarkers beyond PD-L1 expression is essential not only to improve the efficacy of ICI but also to identify patients who can avoid them, together with their toxicities and costs.

Keywords: Biomarkers; Breast cancer; Early stage; HER2-positive breast cancer; Hormone receptor–positive breast cancer; Immune checkpoint blockade; Immune checkpoint inhibitors; Immune response; Immunotherapy; Metastatic breast cancer; PD-1; PD-L1; Triple-negative breast cancer; Tumor mutational burden; Tumor subtype; Tumor-infiltrating lymphocytes.