Antibiotic use and outcomes with systemic therapy in metastatic renal cell carcinoma (mRCC).

607Background: Antibiotic (Abx) use is shown to alter commensal gut microbiota, a key regulator of immune homeostasis. We conducted the largest investigation to date on the impact of Abx use on outcomes in mRCC patients treated with systemic agents. Methods: Two cohorts were analyzed: an institutional cohort (DFCI, n = 146) of patients receiving PD-1/PD-L1-based immune checkpoint inhibitors (ICI), and an external cohort from pooled phase II/III clinical trials (Pfizer, n = 4144) of patients treated with interferon (IFN, n = 510), mTOR inhibitors (n = 660), and VEGF inhibitors (n = 2974). Abx use was defined as Abx treatment at any time between 8-weeks pre- and 4-weeks post the start of systemic therapy. We examined the associations of Abx use and objective response rate (ORR), progression-free survival (PFS), and overall survival (OS) using Cox/logistic regression models, adjusted for prognostic factors including risk groups. Results: In the DFCI cohort, 15% had non-clear cell histology, 43% had first-lin...