Background: Recent evidence suggests that the gut microbiome is a potent mediator of immune checkpoint inhibitor (ICI) activity in metastatic renal cell carcinoma (mRCC), with both specific bacterial species and cumulative microbial diversity driving response (Routy et al Science 2018; Salgia et al Eur Urol 2020). We examined whether the butyrate-producing bacterium Clostridium butyricum, the key constituent of CBM-588, could modulate the gut microbiome in patients (pts) with mRCC receiving nivolumab/ipilimumab (N/I) and secondarily improve clinical outcome. Methods: An open-label, randomized study was conducted, with key eligibility criteria including confirmed clear cell and/or sarcomatoid mRCC, intermediate/poor risk by IMDC criteria and no systemic therapy for metastatic disease. Patients were randomized 2:1 to receive either N/I+CBM-588 or N/I alone. N/I was dosed at 3 mg/kg and 1 mg/kg IV every 3 weeks for 12 weeks, followed by N at 480 mg IV every 4 weeks. CBM-588 was dosed orally at 80 mg bid. Stool was collected for bacteriomic profiling at baseline and 12 weeks. Metagenomic sequencing was employed using previously published methods (Dizman et al Cancer Med 2020). The primary endpoint of the study was change in Bifidobacterium spp. from baseline to week 12. Secondary endpoints included change in microbial diversity and clinical outcomes including response rate (RR) and progression-free survival (PFS). Results: 30 pts were randomized between April 2019 and Nov 2020; 1 pt was excluded after genomic sequencing clarified a diagnosis of sarcoma. Among 29 evaluable patients (21:8 M:F), median age was 66, 10 pts (34%) had sarcomatoid features and 24 pts (83%) were intermediate risk. Metagenomic sequencing of paired stool specimens showed an 8-fold increase in B. bifidum and a 6-fold increase in B. adolescentis in pts receiving N/I+CBM-588 from baseline to week 12. C. butyricum was detected only in pts receiving CBM-588. Pathogenic species (e.g., Escherichia. coli and Klebsiella spp.) were more prevalent in pts not receiving CBM-588. RR was significantly higher among pts receiving N/I+CBM-588 vs N/I alone (59% vs 11%; P = 0.024). Median PFS was also prolonged with the addition of CBM-588 to N/I (NR vs 11 weeks; P < 0.001). No significant difference in grade 3/4 toxicities were observed between study arms. Conclusions: This is the first randomized, prospective study to suggest enhancement of ICI response with a live bacterial product. The observed clinical impact is corroborated by biologic findings supporting gut modulation by CBM-588. Clinical trial information: NCT03829111