Background: Studies have suggested that chemotherapy after immune checkpoint inhibitors may confer an improved response for non-small cell lung cancer (NSCLC). However, potential selection bias in such studies has not been addressed. We therefore applied propensity score analysis to investigate the efficacy of chemotherapy after PD-1 inhibitor treatment (CAP) compared with chemotherapy alone., Methods: We conducted a retrospective observational cohort study for patients treated at 47 institutions across Japan between April 1, 2014 and July 31, 2017. Eligible patients had advanced or recurrent NSCLC who have undergone chemotherapy. Patients subsequently treated with chemotherapy (docetaxel with or without ramucirumab, S-1 or pemetrexed) either after PD-1 inhibitor therapy (CAP cohort) or alone (control cohort) were included. The primary end point was objective response rate (ORR). Inverse probability weighting (IPW) was applied to adjust for potential confounding factors., Results: A total of 1439 patients (243 and 1196 in the CAP and control cohorts, respectively) was available for unadjusted analysis. Several baseline characteristics-including age, histology, EGFR or ALK genetic alterations, and brain metastasis-differed significantly between the two cohorts. After adjustment for patient characteristics with the IPW method, ORR was 18.9% for the CAP cohort and 11.0% for the control cohort (ORR ratio 1.71; 95% CI 1.19 to 2.46; p=0.004). IPW-adjusted Kaplan-Meier curves showed that median progression-free survival (PFS) for the CAP and control cohorts was 2.8 and 2.7 months (IPW-adjusted HR 0.95; 95% CI 0.80 to 1.12; p=0.55), and median overall survival (OS) was 9.2 and 10.4 months (IPW-adjusted HR 1.05; 95% CI 0.86 to 1.28; p=0.63), respectively., Conclusions: After accounting for selection bias by propensity score analysis, CAP showed a significantly higher ORR compared with chemotherapy alone, with the primary end point of ORR being achieved. However, these results did not translate into a PFS or OS advantage, suggesting that prior administration of PD-1 inhibitors may result in a synergistic antitumor effect with subsequent chemotherapy, but that such an effect is transient. CAP therefore does not appear to achieve durable tumor control or confer a lasting survival benefit., Competing Interests: Competing interests: RK has received lecture fees from Bristol-Myers Squibb. HH has received honoraria from AstraZeneca K.K., Boehringer Ingelheim Japan, Bristol-Myers Squibb, Chugai Pharmaceutical, Eli Lilly Japan K.K., MSD K.K., Ono Pharmaceutical, Pfizer Japan, Novartis Pharma K.K., Merck Serono and Taiho Pharmaceutical; and research funding from AbbVie, AC MEDICAL, Astellas Pharma, AstraZeneca K.K., Boehringer Ingelheim Japan, Bristol-Myers Squibb, Daiichi Sankyo, Eisai, Eli Lilly Japan K.K., EPS Associates, GlaxoSmithKline K.K., Japan Clinical Research Operations, Kyowa Hakko Kirin, Merck Serono, MSD K.K., Novartis Pharma K.K., Ono Pharmaceutical, Otsuka Pharmaceutical, PAREXEL International, Pfizer Japan, PPD-SNBL K.K., Quintiles Transnational Japan K.K., Taiho Pharmaceutical, Takeda Pharmaceutical and Yakult Honsha. YC has received honoraria from Chugai Pharmaceutical. DF has received lecture fees from Eli Lilly Japan K.K., Boehringer Ingelheim Japan, Ono Pharmaceutical, Bristol-Myers Squibb, Chugai Pharmaceutical, Taiho Pharmaceutical, Novartis Pharma K.K., MSD K.K. and AstraZeneca K.K.; honoraria from Eli Lilly Japan K.K., Ono Pharmaceutical, Bristol-Myers Squibb, Taiho Pharmaceutical and AstraZeneca K.K.; and research funding from AstraZeneca K.K. RT has received honoraria from AstraZeneca K.K., Bristol-Myers Squibb, Chugai Pharmaceutical, Eli Lilly Japan K.K., Kyowa Hakko Kirin, MSD K.K., Boehringer Ingelheim Japan, Nippon Kayaku, Ono Pharmaceutical and Taiho Pharmaceutical. AN has received honoraria from MSD K.K., AstraZeneca K.K. and Chugai Pharmaceutical. TK has received honoraria from Chugai Pharmaceutical, AstraZeneca K.K., Eli Lilly Japan K.K., Taiho Pharmaceutical, Bristol-Myers Squibb, Ono Pharmaceutical, MSD K.K., Pfizer Japan, Kyowa Hakko Kirin, Boehringer Ingelheim Japan, Nippon Kayaku; and research funding from Chugai Pharmaceutical, AstraZeneca K.K., Eli Lilly Japan K.K., Bristol-Myers Squibb, Pfizer Japan and Merck Serono. KT has received honoraria from Chugai Pharmaceutical, AstraZeneca K.K., Boehringer Ingelheim Japan, Eli Lilly Japan K.K., Ono Pharmaceutical. ST has received honoraria from Chugai Pharmaceutical, Novartis Pharma K.K., AstraZeneca K.K. and Taiho Pharmaceutical; and consulting fees from Boehringer Ingelheim Japan and AbbVie. KU has received lecture fees from AstraZeneca K.K., Boehringer Ingelheim Japan, Bristol-Myers Squibb, MSD K.K., Novartis Pharma K.K., Ono Pharmaceutical, Pfizer Japan and Taiho Pharmaceutical. KN has received honoraria from Chugai Pharmaceutical, AstraZeneca K.K., Boehringer Ingelheim Japan, MSD K.K., Bristol-Myers Squibb, Ono Pharmaceutical, Eli Lilly Japan K.K. and Novartis Pharma K.K.; and research funding from Boehringer Ingelheim Japan. OH has received honoraria from Eli Lilly Japan K.K., Novartis Pharma K.K., AstraZeneca K.K. and Boehringer Ingelheim Japan; and research funding from Novartis Pharma K.K., GlaxoSmithKline K.K., Daiichi Sankyo, Bayer Yakuhin, KYORIN Pharmaceutical, Boehringer Ingelheim Japan and Ono Pharmaceutical. YA has received honoraria from AstraZeneca K.K.; and research funding from Chugai Pharmaceutical. NY has received honoraria from AstraZeneca K.K., Boehringer Ingelheim Japan, Bristol-Myers Squibb, Chugai Pharmaceutical, Eli Lilly Japan K.K., MSD K.K., Novartis Pharma K.K., Ono Pharmaceutical, Pfizer Japan, Takeda Pharmaceutical and Taiho Pharmaceutical; research funding from Boehringer Ingelheim Japan, Chugai Pharmaceutical, Eli Lilly Japan K.K. and MSD K.K.; and consulting or advisory fees from AstraZeneca K.K., Boehringer Ingelheim Japan, Bristol-Myers Squibb, Chugai Pharmaceutical, Eli Lilly Japan K.K., MSD K.K., Novartis Pharma K.K., Ono Pharmaceutical, Pfizer Japan and Takeda Pharmaceutical. KN has received honoraria from Astellas Pharma, AstraZeneca K.K., Bristol-Myers Squibb, Chugai Pharmaceutical, Clinical Trial, Eli Lilly Japan K.K., MSD K.K., Nichi-Iko Pharmaceutical, Boehringer Ingelheim Japan, Novartis Pharma K.K., Ono Pharmaceutical, Pfizer Japan, Reno. Medical K.K. and Sym Bio Pharmaceuticals; research funding from A2 Healthcare, AbbVie, Astellas Pharma, Bristol-Myers Squibb, Chugai Pharmaceutical, Daiichi Sankyo, Eisai, Eli Lilly Japan K.K., EP-CRSU, GRITSTONE ONCOLOGY, ICON Japan K.K., inVentiv Health Japan, MSD K.K., Linical, Boehringer Ingelheim Japan, Novartis Pharma K.K., Ono Pharmaceutical, PAREXEL International, Pfizer Japan, Quintiles Quintiles, Taiho Pharmaceutical and Takeda Pharmaceutical; and consulting or advisory fees from Astellas Pharma, Takeda Pharmaceutical and Ono Pharmaceutical., (© Author(s) (or their employer(s)) 2020. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)