Background: The NIVOREN GETUG-AFU 26 study launched a translational research program to quantify baseline circulating soluble factors levels and correlate them with outcomes to nivolumab in mRCC pts. We previously identified on a training set (n = 80, 40 responders/40 progressors) several soluble factors significantly associated with worse overall survival (OS), progression-free survival (PFS) and response (IL-8 and VEGF), or with worse OS only (IL-6, IL-7) (Carril-Ajuria et al. ASCO GU. 2022). Our aim was to confirm these findings using an independent validation set. Methods: The remaining pts (n = 233) included in the translational-program of the NIVOREN study were included in this validation set. Based on previous results (training set), a panel of 7 different soluble factors (VEGF, VCAM-1, IL-6, IL-7, IL-8, BAFF, CXCL13) were quantified for each plasma sample using the MSD electrochemiluminescence assay. The association between baseline soluble-factors levels and response, PFS and OS was evaluated using previously identified cut-off values. Results: Two hundred thirty-three pts were included in the validation set. Baseline characteristics were similar to the overall trial population. The IMDC risk score breakdown was 17.7% good, 57.3% intermediate and 25.0% poor. With a median follow-up of 21.8 months (mo), the OS rate was 69.6% at 12 mo and median PFS was 3.0 mo. IL-8 (cut-off: 17.9 pg/ml) and IL-6 (cut-off: 8.7 pg/ml), involved in inflammation, confirmed an association with worse OS (IL8: HR = 2.57, p < 0.0001, IL-6: HR = 3.28,p < 0.0001) and worse PFS (IL-8: HR = 1.61,p = 0.0008, IL-6: HR = 1.68, p = 0.0021). VEGF (cut-off: 48.3 pg/ml) confirmed the association with worse OS (HR = 1.56, p = 0.0176), but not with PFS (p = 0.2068). IL-7, involved in T and B cells development, did not show a significant association with OS or PFS when using 8.6 pg/ml (training set) as cut-off (p = 0.0675 and p = 0.7818, respectively). IL-7 was the only cytokine to show an association with response (p = 0.044). Interestingly, circulating CXCL13 (cut-off: 106.4 pg/ml) and BAFF (cut-off: 1122.6 pg/ml), involved in B cell differentiation/survival, were significantly associated with worse OS (HR = 2.09, p = 0.0001 and HR = 2.34, p = 0.0001, respectively); BAFF showed a trend for worse PFS (HR = 1.29, p = 0.0920). Conclusions: Using the cut-off values previously identified in the training set, we confirmed a significant association between baseline blood concentration of IL-6/IL-8 and worse OS/PFS, and of VEGF with worse OS. Non-responders presented lower baseline circulating IL-7 concentrations. CXCL13 and BAFF were significantly associated with worse OS. Multivariate analyses are ongoing.