Background: Well-differentiated NETs are highly vascular and hence, inhibition of angiogenesis is of interest. VEGF, a key molecule in promoting angiogenesis, has also been shown to promote an immunosuppressive tumor microenvironment. Nintedanib, an oral inhibitor of FGFR1–3, VEGFR1–3 and PDGFRA, has previously been evaluated in a phase II clinical trial in well-differentiated, non-pancreatic NETs, where 83% of the evaluable patients were progression-free at 16 weeks and median PFS was 11 months. In preclinical studies, nintedanib has been shown to increase PD-L1 expression in tumor cells, which then results in improved outcomes with immune checkpoint inhibition. There are multiple indicators of T-cell exhaustion, including PD-L1 and LAG-3. LAG-3+ T-cells have been found to correlate with poor response to treatment in neuroendocrine neoplasms. Given these findings, this study evaluates serum VEGF and LAG-3 expression to determine potential biomarkers of response in patients treated with nintedanib. Methods: 27 sets of paired samples collected at baseline and 8-weeks post treatment with nintedanib, from clinical trial NCT02399215 were used for this study. With IRB approval, we tested for serum VEGFa and serum LAG-3 using Luminex test kits: HCKP1-11K-01 (LAG-3) and HCYTA-60K-01 (VEGFa). We compared the baseline, 8-week, and percentage change in serum VEGF and serum LAG-3 levels with Rd Cd8+ pAkt+ T-cells (activated cytotoxic T-cells) as a marker of immune activation, Rd CD25+ pAkt+ T-cells (regulatory T-cells) as a marker of immune suppression, and PFS. Results: The median VEGF levels at baseline and 8-weeks were 10.50 pg/mL and 11.90 pg/mL respectively. The median LAG-3 level at baseline and 8-weeks were 59000 pg/mL and 62900 pg/mL respectively. Higher VEGFa levels in the post-treatment samples were associated with poorer progression free survival (HR= 1.009, 95% HR CI: [0.999, 1.019], with P= 0.0760). Higher levels of VEGF at baseline were also associated with lower numbers of Rd CD8+ pAkt + cells (Pearson coefficient = -0.42, p-value 0.04) at baseline. Increased baseline CD25+ pAkt+ levels were positively correlated with percent change in serum LAG-3 levels between baseline and post-treatment samples. (Pearson Co-efficient: 0.45; p=value: 0.02). Conclusions: Elevated serum VEGFa levels at 8-weeks post treatment correlated with lower PFS. Serum VEGFa level is a potential indicator of effector T-cell suppression, as evidenced by low number of activated cytotoxic T-cells in patients with higher VEGFa levels at baseline. Higher baseline regulatory T-cells were associated with a greater increase in serum LAG-3 levels, suggesting that serum LAG-3 may be an indicator of an immunosuppressive tumor microenvironment.