Background: ICIs lead to durable response and a significant survival improvement in a limited number of advanced stage Mel and NSCLC pts. The identification of predictive circulating biomarkers could be a promising tool to optimize pts’ selection and outcome for ICIs treatment. Methods: This is a prospective real-world study enrolling advanced stage Mel and NSCLC pts referred to four Italian Centers and treated with ICIs. The primary endpoint is to verify the presence of an association between circulating cytokines (IL-1b, IL-2, IL-4, IL-5, IL-6, IL-8, IL-10, TNFα, GM-CSF) and disease control rate (DCR), progression free survival (PFS) and overall survival (OS). Pts undergo a blood collection, before every cycle for 6 cycles (T1-T6) and at tumor assessment till disease progression (PD) or for 2 years. Biomarker levels were assessed by Luminex xMAP based technology using R&D High Sensitivity kits. Each marker was categorized according to high and low levels by maximizing its discriminative ability, and the association with the outcome was tested in univariate and multiple analyses. Results: We report preliminary results on the T1-T2 blood samples from the first 78 enrolled pts (32 females/46 males; 43 Mel/35 NSCLC; median age 69 years). Serum IL-6, IL-8 and IL-10 were significantly higher at T1 and T2 in pts with PD (Kruskal-Wallis test). The median relative increase (RI) of IL-8 was 32% and 2% in pts with PD and disease control (DC), respectively (p = 0.0001). At multiple logistic analysis, IL-6 and IL-8 at T2 and the RI of IL-8 were independent factors predicting the probability of DC, with an overall accuracy of 83.8%. High levels of IL-6 and IL-8 at T2 were significantly associated with a low probability of DC (OR = 0.13, 95%CI: 0.03-0.52 and OR = 0.09, 95%CI: 0.02-0.37, respectively), and the RI showed a significantly lower probability of DC (OR = 0.14, 95%CI: 0.02-0.58). With a median follow-up of 10.6 months (m), mPFS and mOS were 5.8 m, 95%CI: 2.3-7.4 and 8.3 m, 95%CI: 4.0-13.8 for NSCLC; 6.9 m, 95%CI: 2.8-15.9 and 12.6 m, 95%CI: 4.7-NE for Mel pts, respectively. In the multiple Cox model, elevated IL-6 and IL-8 at T1 (HR = 3.03, 95%CI: 1.55-6.37, HR = 2.86, 95%CI: 1.46-5.63), elevated IL-10 at T2 (HR = 2.86, 95%CI: 1.39-5.94), and a RI of IL-8 (HR = 4.22, 95%CI: 1.85-11.21) remained significantly associated with a worse PFS. Higher levels of IL-6 (HR = 3.85, 95%CI: 1.13-20.0) and IL-8 (HR = 4.29, 95%CI: 1.98-9.83) at T2 and a RI of IL-8 (HR = 3.06, 95%CI: 1.43-6.72) remained significantly associated with a worse OS. Conclusions: High serum levels of IL-8 and IL-6 at T2 of ICI, combined with an increase of IL-8 from baseline, are strong predictors of PD, PFS, OS, in pts with advanced Mel and NSCLC. The role of the other cytokines tested, their time fluctuations and associations with clinical prognostic factors, gender, and immuno-related adverse events will be presented at the meeting.