Associations of diet with survival of patients (pts) with metastatic cancer of the urothelium (mUC) and renal cell carcinoma (mRCC) on immune checkpoint blockade (ICB)

4598 Background: The gut microbiome is an emerging modifiable predictor of ICB efficacy. Diet impacts the gut microbiome and fiber intake is associated with longer progression-free survival (PFS) in advanced melanoma treated with ICB. Whether diet impacts ICB benefit in mUC or mRCC remains unknown. Methods: Using a prospective cohort design, baseline dietary data were collected from pts with mUC or mRCC initiating ICB at Memorial Sloan Kettering Cancer Center using the extensively validated Harvard Willett Food Frequency Questionnaire. Tumor mutational burden (TMB) was estimated by next generation sequencing (MSK-IMPACT). The primary endpoint was radiologist assessed PFS, with secondary endpoint of overall survival (OS). Fiber intake was stratified as high or low using the median intake in our cohort. Associations between fiber intake and clinical outcomes were assessed using Kaplan-Meier curves plus univariable (UV) and multivariable (MV) Cox proportional hazards regression. Models were adjusted for prior ICB exposure. Models for mUC controlled for TMB and Bellmunt risk factors. Models for mRCC controlled for histology (clear vs non-clear cell) and IMDC risk score. Results: From 2/2021-6/2022, 88 pts eligible for analysis were enrolled with median follow-up of 10.1 months (mo) (mUC n = 40; mRCC n = 48). Median fiber intake was 17.5 g/day (interquartile range 12.5-24.5). Among pts with mUC, high fiber intake (≥17.5 g/day) was associated with longer PFS (UV hazard ratio [HR] 0.56, 95% CI 0.26-1.19, p = .13; MV HR 0.39, 95% CI 0.16-0.93, p = .03). Similar trends were observed for mRCC. In subgroup analyses, PFS was numerically longer with high fiber intake among mUC pts on maintenance avelumab (median PFS 6.1 vs 3.0 mo, n = 14) and in pts with mRCC treated with first-line (1L) ipilimumab + nivolumab (median PFS 18.0 vs 5.7 mo, n = 14) and 1L ICB + tyrosine kinase inhibitor (median PFS not reached vs 5.1 mo, n = 21). Among pts with mUC on non-maintenance ICB, pts with high fiber intake had numerically higher PFS at 6 mo (38% vs 14%, n = 26). Conclusions: This pilot study indicates that high fiber intake is associated with longer PFS among pts with mUC on ICB. Trends towards longer PFS and OS with high fiber intake were observed among pts with mRCC. Analyses of the gut microbiome are underway to investigate mechanisms of action. [Table: see text]