IntroductionImmune checkpoint inhibitors have had a major impact on cancer treatment. Gut microbiota plays a major role in the cancer microenvironment, affecting treatment response. The gut microbiota is highly individual, and varies with factors, such as age and race. Gut microbiota composition in Japanese cancer patients and the efficacy of immunotherapy remain unknown.MethodsWe investigated the gut microbiota of 26 patients with solid tumors prior to immune checkpoint inhibitor monotherapy to identify bacteria involved in the efficacy of these drugs and immune-related adverse events (irAEs).ResultsThe generaPrevotellaandParabacteroideswere relatively common in the group showing efficacy towards the anti-PD-1 antibody treatment (effective group). The proportions ofCatenibacterium(P = 0.022) andTuricibacter(P = 0.049) were significantly higher in the effective group than in the ineffective group. In addition, the proportion ofDesulfovibrion(P = 0.033) was significantly higher in the ineffective group. Next, they were divided into irAE and non-irAE groups. The proportions ofTuricibacter(P = 0.001) andAcidaminococcus(P = 0.001) were significantly higher in the group with irAEs than in those without, while the proportions ofBlautia(P = 0.013) and the unclassifiedClostridiales(P = 0.027) were significantly higher in the group without irAEs than those with. Furthermore, within the Effective group,AcidaminococcusandTuricibacter(both P = 0.001) were more abundant in the subgroup with irAEs than in those without them. In contrast,Blautia(P = 0.021) andBilophila(P= 0.033) were statistically significantly more common in those without irAEs.DiscussionOur Study suggests that the analysis of the gut microbiota may provide future predictive markers for the efficacy of cancer immunotherapy or the selection of candidates for fecal transplantation for cancer immunotherapy.