A humanized immunoglobulin (Ig) G4 monoclonal antibody directed against the negative immunoregulatory human cell surface receptor programmed cell death 1 (PD-1), with potential immune checkpoint inhibitory and antineoplastic activities. Upon administration, anti-PD-1 monoclonal antibody MEDI0680 binds to and inhibits PD-1 and its downstream signaling pathways. This may restore immune function through the activation of T-cells and cell-mediated immune responses against tumor cells. PD-1, a transmembrane protein in the Ig superfamily expressed on T cells, functions as an immune checkpoint that negatively regulates T-cell activation and effector function when activated by its ligands programmed cell death ligand 1 (PD-L1) or 2 (PD-L2); it plays an important role in tumor evasion from host immunity.