Background: There is an emerging evidence that the gut microbiome may influence anti-tumor immune response during treatment with checkpoint inhibitors (CPIs). A number of factors can cause gut microbiome dysbiosis including the use of antibiotics (ABX). We hypothesized that the use of ABX during immunotherapy can adversely affect the efficacy of CPIs. We explored the prevalence of ABX use amongst patients (pts) using CPIs, and whether the use of ABX influences the response to CPIs. Methods: We performed an institutional retrospective review of all the pts treated with CPIs from 2/2015-3/2018. A patient was considered to have used ABX if he or she was prescribed ABX within 6 months before or after, initiating CPIs. Statistical analysis was done using logistic regression with overall response rate (ORR) (CR, PR and SD) as the outcome. 9 separate analyses were done: one for each temporality (30 days, 60 days, 6 months) and use-order (before, after, neither) combination. Odds ratios for ORR as univariate analysis, and adjusted for age and sex; and adjusted for age, sex, and tumor type were calculated. Results: Out of 242 pts, 111 were lung, 36 bladder, 35 renal, 16 gastrointestinal and 44 other cancers. 50% (121) pts received nivolumab, 28% (68) pembrolizumab and 21% (52) atezolizumab. 75%, 46% and 32% of the pts received ABX within 6 months, 60 days and 30 days of starting CPIs. Only ABX use in the first 30- or 60-days following CPI initiation was associated with inferior ORR (OR 0.42 [95% CI:0.23,0.76] p = 0.005 for 60-days). ABX use prior to initiation of CPI at any time point, or ABX use in the first 6 months of CPI use did not impact CPI efficacy, Table 1. Conclusions: ABX use within the first 60 days of starting CPI therapy is prevalent (32%). This study suggests that the use of ABX within 60 days following initiation of CPIs significantly negatively impacts the ORR. Unnecessary usage of ABX should be avoided, especially during the early phase of starting CPIs.

*During the same corresponding time period.