9017 Background: Dysregulation of the gut microbiota induced by antibiotic therapy (Abx) may alter the anticancer immune response. Multiple small studies have associated Abx use with inferior immune checkpoint inhibitor (ICI) efficacy in patients with non-small cell lung cancer (NSCLC). We aimed to study the impact of Abx in a larger population of NSCLC patients treated with ICI within the Veterans Health Administration. Methods: We conducted a nested cohort study of Veterans who were diagnosed with NSCLC between 2010 & 2018 and treated with ICI. Two exposures to Abx were specified and separately analyzed: prior Abx (pAbx) was defined as receipt of an Abx prescription within 30 days prior to initiation of ICI, and concurrent Abx (cAbx) was defined as receipt of an Abx prescription within 60 days following ICI initiation. A landmark analysis of 2 months from ICI start was applied to the cAbx analysis to exclude any Veterans with an OS event before that time point. OS was measured from start of ICI using Cox proportional hazard multivariate analyses (MVA). Results: 3,634 Veterans received ICI, mostly nivolumab (59.3%) or pembrolizumab (35.1%). Their median age was 69, and a plurality had male gender (97.0%), white race (73.0%), comorbidity count ≥1 (60.4%), adenocarcinoma (47.8%), and stage IV disease at diagnosis (40.9%). Of the 762 (21.0%) Veterans prescribed pAbx, beta-lactams, quinolones, and macrolides were the most common classes. These patients had shorter OS than those without pAbx (median 7 versus 10 months). Receipt of pAbx was also associated with lower OS on MVA (HR 1.31, p<0.01). In the propensity-matched cohort analysis, Veterans receiving pAbx had lower OS (HR 1.27, p<0.01) (Table top). For the cAbx analysis, 3,223 Veterans survived to the 2-month landmark, of whom 970 (30.1%) received cAbx. These Veterans had shorter OS than those without cAbx (median 7 versus 10 months). Lower OS with cAbx was also observed both on Cox MVA (HR 1.33, p<0.01) and in the matched cohort (HR 1.32, p<0.01) (Table bottom). Conclusions: In the largest analysis to date of Abx use in NSCLC patients receiving ICI, receipt of Abx within either 30 days before or 60 days after start of ICI was associated with lower OS. These findings suggest Abx therapy may have a detrimental effect on immunotherapy outcomes.[Table: see text]
