Background  Immunotherapy based on PD-1/PD-L1 inhibitors has made a major breakthrough in the first-line treatment of small cell lung cancer (SCLC), but its efficacy and safety needs to be further verified in a complex real-world context.

  Objective  To analyze the efficacy and safety in SCLC patients with PD-1/PD-L1 inhibitor treatment and its prognostic factors.

  Methods  Clinical data about SCLC patients treated with PD-1/PD-L1 inhibitors in the First Medical Center of Chinese PLA General Hospital from August 2013 to June 2022 were analyzed. The efficacy and the incidence of immune-related adverse events (irAEs) were analyzed. Kaplan-Meier survival curve was used to analyze the effects of different factors on the survival of patients. Univariate and multivariate Cox regression analyses were used to analyze the relationship between different factors and prognosis.

  Results  A total of 250 patients were included in the study, including 213 males (85.2%) and 37 females (14.8%), with a median age of 60.5 years. There were 62 cases (24.8%) of limited stage-SCLC (LS-SCLC) and 188 cases (75.2%) of extensive stage-SCLC (ES-SCLC). The median progression-free survival (PFS) was 7.6 months (95% CI: 6.2 - 9.0), and the median overall survival (OS) was 18.2 months (95% CI: 14.7 - 21.7). The OS of the rechallenge of immunotherapy beyond progression (RIBP) group was significantly longer than that of the discontinuation of immunotherapy beyond progression (DIBP) group (OS: 10.2 months vs 5.1 months, P=0.004). The OS of patients with irAEs of any grade was better than that of patients without irAEs (OS: 29.2 months vs 16.1 months, P=0.003). Multivariate COX analysis showed that liver metastasis (HR=2.427, 95% CI: 1.648 - 3.573, P<0.001), Ki67 index (HR=1.491, 95%CI: 1.009 - 2.203, P=0.045) and treatment lines (HR=1.601, 95% CI: 1.111 - 2.306, P=0.011) were independent prognostic factors for PFS. Liver metastasis (HR=3.325, 95%CI: 2.191-5.046, P<0.001) and disease stage (HR=2.092, 95% CI: 1.163 - 3.761, P=0.014) were independent prognostic factors for OS. The incidence of grade 3 and above general adverse events was less than 24%, and the incidence of grade 3 and above irAEs was less than 11%. No adverse events leading to death occurred.

  Conclusion   Immunotherapy based on PD-1 / PD-L1 inhibitors is effective and safe in patients with SCLC. Liver metastasis and disease stage are independent prognostic factors for OS. After the progress of immunotherapy, immune re-challenge can bring benefits to SCLC patients. The overall survival of patients with any grade of irAEs is better than that of patients without irAEs.