Targeting PD-1 in cancer: Biological insights with a focus on breast cancer.

Programmed cell death-1 (PD-1) receptor and its ligands physiologically regulate the activity of the adaptive immune system to limit excessive inflammatory processes, thus preventing normal tissue damage. Tumor cells escape from the host immune surveillance using this pathway, rendering it relevant therapeutic target. Despite the relevant clinical efficacy observed in patients with solid and hematological malignancies, the clinical benefit of these novel treatments is limited to a relatively restricted number of patients. A wide amount of genomic and immune related features is currently under investigation as potential predictive biomarkers for treatment selection. The results obtained so far are encouraging but still imperfect. Combination strategies using different immunotherapeutic agents or with other treatments (such as chemotherapy) are being investigated, showing promising but still not completely satisfactory results. This review aims to shed light on the main principles of targeting PD-1 in breast cancer, from biology through its functional and clinical implications.