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Objective To investigate the effect of rigosertib (RGS) combined with classic chemotherapy drugs including 5-fluorouracil, oxaliplatin, and irinotecan in colorectal cancer. Methods Explore the synergy effects of RGS and 5-fluorouracil (5-FU), oxaliplatin (OXA), and irinotecan (IRI) on colorectal cancer by subcutaneously transplanted tumor models of mice. The mice were randomly divided into control group, RGS group, 5-FU group, OXA group, IRI group, 5-FU+ RGS group, OXA+ RGS group and IRI+ RGS group. The synergy effects of RGS and OXA on KRAS mutant colorectal cancer cell lines in vitro was detected by CCK-8. Ki-67 immunohistochemistry and TdT-mediated dUTP nick-end labeling (TUNEL) staining were performed on the mouse tumor tissue sections, and the extracted tumor tissue was analyzed by western blot. The blood samples of mice after chemotherapy and RGS treatment were collected, blood routine and liver and kidney function analysis were conducted, and H&E staining on liver sections was performed to observe the side effects of chemotherapy and RGS. Results The subcutaneously transplanted tumor models were established successfully in all groups. 55 days after administration, the fold change of tumor size of OXA+ RGS group was 37.019±8.634, which is significantly smaller than 77.571±15.387 of RGS group (P=0.029) and 92.500±13.279 of OXA group (P=0.008). Immunohistochemical staining showed that the Ki-67 index of tumor tissue in control group, OXA group, RGS group and OXA+ RGS group were (100.0±16.8)%, (35.6±11.3)%, (54.5±18.1)% and (15.4±3.9)%, respectively. The Ki-67 index of OXA+ RGS group was significantly lower than that in control group (P=0.014), but there was no significant difference compared to OXA group and RGS group (OXA: P=0.549; RGS: P=0.218). TUNEL fluorescence staining showed that the apoptotic level of OXA+ RGS group was 3.878±0.547, which was significantly higher than 1.515±0.442 of OXA group (P=0.005) and 1.966±0.261 of RGS group (P=0.008). Western blot showed that the expressions of apoptosis related proteins such as cleaved-PARP, cleaved-caspase 3 and cleaved-caspase 8 in the tumor tissues of mice in the OXA+ RGS group were higher than those in control group, OXA group and RGS group. After the mice received RGS combined with chemotherapy drugs, there was no significant effect on liver and kidney function indexes, but the combined use of oxaliplatin and RGS significantly reduced the white blood cells [(0.385±0.215)×109/L vs (5.598±0.605)×109/L, P<0.001] and hemoglobin[(56.000±24.000)g/L vs (153.333±2.231)g/L, P=0.001] of the mice. RGS, chemotherapy combined with RGS and chemotherapy alone did not significantly increase the damage to liver cells. Conclusions The combination of RGS and oxaliplatin has a stronger anti-tumor effect on KRAS mutant colorectal cancer. RGS single agent will not cause significant bone marrow suppression and hepatorenal injury in mice, but its side effects may increase correspondingly after combined with chemotherapy. [ABSTRACT FROM AUTHOR]