Background: Combination therapy with the immune checkpoint inhibitor (ICI) nivolumab (nivo) and the tyrosine kinase inhibitor cabozantinib (cabo) is a new standard of care for first line treatment of patients with clear cell mRCC. However, despite the improved clinical benefit obtained with this regimen, a subgroup of patients still presents with progressive disease as best response (Choueiri et al NEJM 2021). There is now evidence supporting the role of the gut microbiome in mediating ICI activity (Routy et al Science 2018) and certain bacterial species, such as Bifidobacterium spp. in predisposing clinical response in patients with mRCC receiving ICIs (Salgia et al Eur Urol 2020). Moreover, recent evidence from a phase I clinical trial suggests that the addition of CBM588, a live probiotic comprised primarily of Clostridium butyricum, can enhance clinical response in patients with mRCC receiving nivolumab plus ipilimumab without incurring added toxicity (Meza et al ASCO, 2021). Herein we present the study design of an ongoing phase I study evaluating the biological effect of CBM588 in combination with cabozantinib plus nivolumab in patients with mRCC. Methods: This is an open label, randomized, single institution phase 1 trial for patients with confirmed mRCC with clear cell, papillary, or sarcomatoid components, who have not received prior systemic therapy for metastatic disease. A total of 30 eligible patient will be randomized 1:2 to receive either cabo/nivo at the standard dose/schedule (40mg PO QD and 480mg IV /4wks, respectively) alone or with CBM588 dosed at 80mg PO bid. The primary objective of the study is to determine the biologic effect of CBM588 with cabo/nivo in the modulation of the gut microbiome. This will be done by assessing the changes in Bifidobacterium spp. abundance and Shannon index (a measure of microbiome diversity) in stool specimens. Stool will be collected for bacteriomic profiling at baseline and after 12 weeks of treatment. Metagenomic sequencing will be performed using previously published methods (Dizman et al Cancer Med 2020). Secondary objectives include determining the effect of CBM588 on (1) clinical efficacy, through overall survival, response rate, and progression-free survival; (2) systemic immunomodulation, through assessment of changes in circulating Tregs, circulating cytokines/chemokines, etc; and (3) toxicities. A two-group t-test with a one-sided type I error of 0.05 will be used to assess the study primary endpoint. Clinical trial information: NCT05122546.