Synergistic drug combination (SDC) of axitinib, erlotinib, and dasatinib synergistically inhibits cancer cell metabolic activity. (A) Dose response curves of axitinib, erlotinib, and dasatinib in the CRC (SW620, HT29, DLD-1) cell lines. (B) Metabolic activity of the SDC compared to the two-drug combinations. CI indicates the Combination Index that was assessed using the Chou–Talalay method [27]. CI values are showed below the graph. CI < 0.9 indicates synergism, CI = 0.9 indicates an additive effect, and CI > 1 indicates antagonism. Significance (* p < 0.05, ** p < 0.01) for two-drug combinations and monotherapies is indicated as compared to the SDC. (C) Metabolic activity of the SDC in ovarian carcinoma (A2780), breast cancer (MDA-MB-231), and prostate cancer (PC3) cells. Significance (* p < 0.05, ** p < 0.01) is indicated as compared to control. (A,B) Color coded drug doses include axitinib 16 μM, erlotinib 20 μM, and dasatinib 0.2 μM and are based on our previous study [20]. Metabolic activity was assessed after 72 h of drug administration by the CellTiter-Glo® luminescence assay and are represented as a percentage of the control. Cells treated with 0.1% DMSO were used as a control (ctrl). Error bars indicate SEM.