Background: We have previously demonstrated that the live bacterial product CBM588 may augment clinical outcomes in pts with mRCC receiving 1st-line (1L) therapy with dual immune checkpoint inhibitors (ICIs; Dizman et al Nature Medicine 2022). As tyrosine kinase inhibitors (TKIs) in combination with ICIs also represent a 1L standard of care in mRCC, we sought to determine if CBM588 would augment clinical outcome with the TKI cabozantinib (cabo) with nivo. Methods: Eligible pts were ≥18 yrs old (Karnofsky performance status ≥70%) with histologically verified (clear-cell, papillary or sarcomatoid component) mRCC and no prior systemic therapy for metastatic disease. Patients were randomized 1:2 to receive either cabo/nivo at the standard dose/schedule (40mg PO QD and 480mg IV monthly, respectively) alone or with CBM588 dosed at 80mg PO BID. The primary endpoint was the relative abundance of Bifidobacterium spp. in stool specimens at baseline and after 12 weeks of treatment. Secondary endpoints included assessment of response rate, overall survival, progression-free survival (PFS), systemic immunomodulation, and toxicity. A two-group t-test with a one-sided type I error of 0.05 was used to assess the study's primary endpoint, hypothesizing an increase in Bifidobacterium spp. with CBM588 therapy. Results: A total of 30 (20:10 M:F) pts were recruited; median age was 65 (36-84) and 5 pts (17%) had sarcomatoid features and 2 pts (7%) had predominant papillary histology. 12 (40%), 12 (40%) and 6 pts (20%) had good, intermediate and poor risk, respectively. Metagenomic sequencing of paired stool specimens showed a significant decrease in diversity from baseline to week 12 in patients receiving cabo/nivo (P=0.02); no significant difference was observed in those treated with cabo/nivo+CBM588. No significant change in Bifidobacterium spp. was observed. RR was 63% in pts receiving cabo/nivo+CBM588 vs 33% in pts receiving cabo/nivo alone. Median PFS was not reached in pts receiving CBM588 vs 5.8 mos in pts receiving cabo/nivo along (P=0.03). Grade 3/4 toxicities were observed in 44% and 42% of patients on control and experimental arms, respectively. Conclusions: In the second prospective trial assessing the addition of CBM588 to ICI-based therapy in mRCC, a consistent result with improved PFS and RR was observed. Further translational efforts are underway to characterize the mechanism through which CBM588 augments clinical activity. Clinical trial information: NCT05122546.