Combined therapy with a thymidylate synthase-inhibiting vector and S-1 has effective antitumor activity against 5-FU-resistant tumors

High levels of intratumoral thymidylate synthase (TS) expression are associated with resistance to 5-fluorourcil (5-FU). In order to establish a new treatment method for 5-FU-resistant tumors, the efficacy of gene therapy was investigated using an adenoviral vector expressing short hairpin RNA (shRNA) targeting TS. A replication-deficient recombinant adenoviral vector expressing shRNA targeting TS was constructed under the control of the human U6 promoter (Ad-shTS). Three 5-FU-resistant cancer cell lines, DLD-1/5FU, KM12C/5FU and NUGC-3/5FU, were used. Transduction with Ad-shTS effectively downregulated TS expression in all three 5-FU-resistant tumor cells. MTT assays demonstrated that treatment with Ad-shTS significantly inhibited the growth of all three 5-FU-resistant tumor cells. Furthermore, combined treatment with Ad-shTS and 5-FU demonstrated significantly greater inhibition of tumor cell growth in comparison to 5-FU treatment alone and Ad-shTS treatment alone. S-1, a combination of tegafur, gimeracil and oteracil potassium, was used for the 5-FU treatment by in vivo experiments. The combined treatment of Ad-shTS and S-1 was found to have the strongest antitumor effect against 5-FU-resistant DLD-1/5FU xenografts in nude mice in comparison to S-1 treatment alone and Ad-shTS treatment alone. Furthermore, the apoptotic index in tumors treated with combined Ad-shTS and S-1 was significantly higher in comparison to that in tumors treated with S-1 alone and that in tumors treated with Ad-shTS alone. Consequently, the combined treatment of the TS-inhibiting adenoviral vector and S-1 has effective antitumor activity against 5-FU-resistant tumors.