PDCD1 and small cell lung carcinoma: Immunotherapeutic agents for small-cell lung cancer among programmed cell death 1 (PD-1), P.D. ligand 1 (PD-L1), and cytotoxic T lymphocyte-associated protein (CTLA-4) inhibitors et al., some immune checkpoint inhibitors (ICIs), such as serplulimab, atezolizumab, pembrolizumab et al.