PDCD1 and esophageal squamous cell carcinoma: Similarly, other PD-1 antibodies, such as nivolumab, pembrolizumab, camrelizumab, and sintilimab, demonstrated consistently that they as the second-line monotherapy antibodies significantly improved OS compared with the investigator-chosen chemotherapy in the overall group or subgroup population of pretreated advanced or metastatic ESCC [38–41].