5045 Background: Although recent evidence has suggested that patients who receive antibiotics (ABX) during the course of ICI treatment might decrease overall survival (OS) (1), our previous analysis did not support a difference in OS in urothelial cancer patients who did and did not use ABX during the course of ICI treatment without regard to timing (2). This updated analysis aims to addresses the question of timing; specifically, use of ABX in the 30-day window pre- or post- initiation of ICI treatment. Methods: We pooled data from 7 trials that led to drug approval and which included 1747 patients with advanced urothelial cancer treated with an ICI. Five trials enrolled patients who received prior platinum and 2 enrolled cisplatin-ineligible patients. Concomitant medication datasets were searched for systemic ABX use. The association between ABX use and survival was evaluated using Kaplan-Meier estimates and Cox proportional hazards regression models stratified by study. Results: Overall, 56% of patients were exposed to antibiotics (ABX+) and 43% were not exposed (ABX-). In an exploratory analysis, median OS was similar between arms: 9.7 vs. 9.3 months in ABX+ vs. ABX- patients, respectively (HR 0.96). However, OS results differed in the 27% of patients who were exposed to antibiotics in the 30-day window pre- or post- initiation of ICI treatment, for whom median OS was 4.7 months vs. 11.5 months in the ABX+ vs. ABX- patients, respectively (HR 1.8). This remained true after controlling for baseline risk prognostic factors (Bajorin and Bellmunt scores). Similar trends were observed for progression-free survival (PFS). Conclusions: Patients treated with ABX while on therapy with an ICI for urothelial cancer had similar OS outcomes to those not treated with ABX. However, in an exploratory analysis looking at ABX use in the 30-day window pre- or post-initiation of ICI treatment, OS appeared decreased in ABX+ vs ABX- patients. Our exploratory analyses appear to show an association of OS/PFS with timing of antibiotics. References: 1) Routy B, Science (2017) 2) Weinstock C, ASCO 2019, abstract. [Table: see text]