AbstractBackgroundThe use of immune checkpoint inhibitors (ICIs) as first‐line treatment rather than as second‐line treatment makes a big difference in the drug efficacy and progression‐free survival. However, the mechanism for this is still not clear. This study aimed to analyze the effects of the rest period between chemotherapy and immunotherapy on the efficacy of ICIs.MethodsThis study included 100 patients with advanced NSCLC treated with PD‐1/PD‐L1 inhibitors at Chungnam National University Hospital (CNUH) between May 2016 and August 2019. The rest period was defined from the last dose of cytotoxic chemotherapy to the first dose of ICIs. We retrospectively reviewed patients' clinical data and blood test records and analyzed lymphocyte subsets using flow cytometry.ResultsThe median rest period was 64 days. The long rest period group (≥36 days) showed significantly higher clinical benefits than the short rest period group (<36 days) (69.4% vs. 39.5%, p = 0.003). White blood cell (WBC) count, absolute neutrophil count (ANC), absolute lymphocyte count (ALC), and neutrophil‐lymphocyte ratio (NLR) just before chemotherapy were not different between the two groups. However, the blood test after chemotherapy immediately before immunotherapy showed significantly higher ANC and NLR in the short rest period group than in the long rest period group. The frequency of the Th1 subset and PD‐1 + CD8⁺ T cells were significantly higher in the long rest period group than in the short rest period group.ConclusionTime interval from chemotherapy to immunotherapy may affect immune cell status and efficacy of ICIs.