Combination therapy more potently regressed tumor blood vessels in SW620 xenograft model. Representative immunofluorescence images (A) show the tumor vasculature in SW620 tumor tissues stained for CD31, a generally conserved endothelial cell marker (green) and VEGFR-2 (red) with DAPI (blue). Most tumor blood vessels in vehicle group were stained and colocalized with both markers, CD31 and VEGFR-2. The area densities of CD31 (B) and VEGFR-2 (C) positive vessels were measured in each group. After irinotecan treatment, CD31 or VEGFR-2 positive tumor blood vessels were slightly regressed compared to vehicle treatment. However, after mABL001 or the combination treatment of mABL001 and irinotecan, CD31 and VEGFR-2 positive tumor vessels were significantly reduced (B,C). VEGFR-2 expression reduced more rapidly on tumor vessels. Scale bar indicates 200 µm. Error bars: mean ± SEM. * p < 0.05, ** p < 0.01, **** p < 0.0001 by Kruskal–Wallis test.