Agents currently in active clinical development for lung cancer include ipilimumab, which modulates the cytotoxic T-lymphocyte-associated antigen 4 pathway, and multiple agents targeting the programmed death protein 1 (PD-1) pathway, both anti-PD-1 compounds (nivolumab, pembrolizumab [MK-3475]) and those that target programmed death ligand 1 (PD-L1), a key ligand for PD-1 (BMS-936559, MPDL3280A).