Abstract 1055: Enhanced efficacy of trifluridine for ionizing radiation in human colorectal cancer cell lines

Background: Trifluridine/tipiracil is a combination of trifluridine (FTD), a nucleoside metabolic inhibitor, and tipiracil, a thymidine phosphorylase inhibitor, in a molecular ratio of 1:0.5. Trifluridine/tipiracil is indicated for the treatment of patients with metastatic colorectal cancers refractory to standard chemotherapies. Chemotherapy can make radiation therapy more effective against some colorectal cancers. In this study, we investigated whether cytotoxicity was enhanced when FTD was sequentially used with ionizing radiation (IR). Method: The colorectal cancer cell lines HT-29, HCT-15, and HCT 116, showing low, middle and high sensitivity for IR, respectively, were treated with the combinations of FTD and IR as follows, 1) sequence 1 (pre-radiation): exposure to 2-8 Gy radiation alone and no treatment for 24 h followed by exposure to 0, 2.0, or 4.0 µM FTD for 24 h, 2) sequence 2 (post-radiation): exposure to 0, 2.0, or 4.0 µM FTD for 24 h followed by 2-8 Gy radiation alone and no treatment for 24 h. Cells were irradiated using a cabinet X-ray irradiation system and all treatments were evaluated by the clonogenic survival assay. The dose modification factors (DMFs) were calculated from clonogenic survival curves as the ratio of radiation doses (control radiation dose divided by the FTD-treated radiation dose). To evaluate DNA strand-breaks, alkaline comet assays were performed and the comet tail moment was determined. Results: DMFs of sequence 1 at 8 Gy for 2.0 µM FTD were 1.55, 1.24, and 1.03 and for 4.0 µM FTD were 1.75, 1.26, and 1.02 for HT-29, HCT-15, and HCT 116, respectively, and of sequence 2 at 8 Gy for 2.0 µM FTD were 1.82, 1.38, and 1.03 and for 4.0 µM FTD were 2.73, 1.41, and 1.09 for these cell lines, respectively. The DMFs increased in an FTD dose-dependent manner compared with IR-only treatment. In particular, FTD enhanced the efficacy of radiation in HT-29 and HCT-15 indicating low and middle sensitivity to IR. Compared with IR-only treatment, the median comet tail moments of sequence 1 at 8 Gy for 2.0 µM FTD, respectively, were 13.8-, 14.4-, and 2.0-fold and for 4.0 µM FTD were 16.6-, 12.6-, and 3.6-fold in HT-29, HCT-15, and HCT 116, respectively, and of sequence 2 at 8 Gy for 2.0 µM FTD were 4.8-, 4.3-, and 1.8-fold and for 4.0 µM were 7.5-, 14.3-, and 2.6-fold in these cell lines, respectively. Taken together, FTD promoted DNA strand-breaks and enhanced the efficacy of radiation in these three cell lines, regardless of both sequ