Enhancement of Chemosensitivity to Fluoropyrimidines by Retroviral Transduction of Thymidine Phosphorylase cDNA: an in vitro and in vivo Study

Thymidine phospholyrase (TP) is an essential enzyme in activating 5’-deoxy-5-fluorocytidine (5’- DFUR) into 5-fluorouracil (5-FU) and for the conversion of 5-FU into 5-fluoro-2’-deoxyuridine. The purpose of this study was to examine the therapeutic efficacy of the retroviral vector-mediated TP gene in the sensitivity to fluoropyrimidines, 5-FU and its prodrugs, 5’-DFUR and capecitabine, in MC38 murine colon adenocarcinoma cells in vitro and in vivo. After retroviral infection with or without human TP cDNA, we obtained MC38 cells having the stable expression of TP (MC38-TP) and control-vector transfected cells (MC38-Neo). There was no significant difference in the doubling time in vitro and tumor growth rate in vivo among parental MC38 cells (MC38-P), MC38- Neo and MC38-TP, demonstrating that the TP gene was not directly toxic. The in vitro study showed significant increases in sensitivities to 5-FU, 5’-DFUR and capecitabine in MC38-TP cells. The 50% growth inhibitory concentration (IC50) of MC38-TP cells to 5-FU, 5’-DFUR and capecitabine, respectively, was about 10-fold, 800-fold and 40-fold higher than that of MC38-P cells and MC38-Neo cells. The in vivo study showed significant increases in sensitivities to 5-FU, 5’- DFUR and capecitabine in MC38-TP tumors. There was no significant difference in the sensitivities to 5-FU, 5’-DFUR and capecitabine between MC38 and MC38-Neo tumors. The tumor-cure rate in MC38-TP tumors treated with capecitabine was 100% and that treated with 5’-DFUR was 63%. In conclusion, our results demonstrate that the stable expression of TP gene by using recombinant retroviral vector could dramatically increase the anticancer effect of fluoropyrimidines.