2652 Background: Gut microbiome dysbiosis impairs systemic immune responses and recent evidence suggests its critical role in patients (pts) treated with immune checkpoint inhibitors (ICI). Proton pump inhibitors (PPI) and antibiotics (ATB) may alter the microbiome and their impact on clinical outcomes and toxicities requires further investigation. Methods: This retrospective cohort included consecutive metastatic cancer pts treated with ICI with palliative intent. We reviewed pts' records, concomitant medication and toxicities graded by CTCAE 4.0. Pts with PPI or ATB exposure were analyzed according to previous use (pPPI and pATB, ≤60 days to ICI) and concomitant use (cPPI and cATB). We estimated median overall survival (mOS) and progression free survival (PFS) by Kaplan–Meier and used a Cox proportional-hazards model to adjust for differences in baseline characteristics. Toxicities and ATB/PPI interaction was calculated using Pearson Chi-square method. Results: We enrolled 216 pts with a median age of 59 years, mostly ECOG-PS 0 (34%) or 1 (58%). ICI employed were mostly anti-PD-1 (60.2%), anti-CTLA-4 (16.2%) and anti-PD-L1 (12.5%). Most frequent primary tumor sites were lung n = 39 (18.1%), gastrointestinal n = 34 (15.7%) and melanoma n = 33 (15.2%). Half of the pts (108) received ATB and 114 (52.8%) PPI. Compared to control, pPPI group n = 57 (26.4%) had shorter mOS (11.6m vs 19.7m, p < 0.001) and PFS (2.8m vs 8.5m, p < 0.001), but no statistically significant difference in toxicities grade ≥3 and/or leading to ICI discontinuation (36% vs 29.1%; p = 0.29). cPPI n = 100 (46.3%) depicted a negative impact on mOS (12.1m vs 17.0m; p = 0.01), PFS (4.3m vs 7.1m; p = 0.04) and augmented toxicities (42% vs 19%; p < 0.001). pATB n = 34 (15.7%) had shorter OS (6.9m vs 19.3m, p < 0.001) and PFS (3.2 vs 7.2m, p = 0.005) and higher incidence of toxicities (45.9% vs 28.1%; p = 0.04 ). cATB use n = 92 (42.6%) did not impact OS (12.1m vs 15.6m; p = 0.32) or PFS (5.5m vs 5.9m; p = 0.82), but had a higher incidence of toxicities: 37% vs. 24.2% (p = 0.05). Multivariate analyses confirmed that pATB therapy and pPPI respectively remained as independent prognostic variables associated with OS (HR 2.39; 95% CI, 1.60-3.59; P <.001 and HR 1.73; 95% CI 1.23-2.44; P = 0.002) and PFS (HR 1.72; 95% CI, 1.14-2.61; P = 0.01 and HR 2.36; 95% CI 1.67 - 3.34; P < 0.001) adjusted by performance status, age and line of treatment. Conclusions: These data suggest that concomitant use of PPI and ATB is associated with increased toxicities in ICI treated pts. pPPI and pATB can negatively impact OS and PFS and merit clinician’s attention.