Effects of irinotecan/5-fluorouracil combination on thymidylate synthase and cell cycle in human colon carcinoma cell line

ABSTRACT

Thymidylate synthase (TS) is responsible for the de novo synthesis of thymidylate,which is required for DNA synthesis and repair TS and is an important target for 5-fluorouracil (5-FU). In this study we investigated whether the greater cytotoxicity of irinotecan (CPT-11) followed by 5-FU in HT-29 and SNU-C4 cell lines was related to TSmRNA expression or activity. Thus, cells were exposed for 24 h to each drug, alone or in combination, and assessed for colony formation, TS catalytic activity, TS-mRNA expression and cell cycle distribution. Pre-treatment with CPT-11 at IC20 increased the 5-FU cytotoxicity in HT-29 and SNU-C4 cells. TS catalytic activity and TS-mRNA expression suggested that the differences in sensitivity to 5-FU among the cell lines were not correlated to TS profile. When we exposed cells to CPT-11 at IC20 and subsequently to 5-FU at IC50, the impact on TS activity and mRNA expression were the same as observed with 5-FU alone. CPT-11 induced G2/M arrest, while 5-FU arrested cells in S-phase in both cell lines. When cells were treated with CPT-11 followed by 5-FU we observed an increased in 5-FU-inducible S-phase arrest in both cell lines. Our findings suggested that the greater cytotoxicity of CPT-11/5-FU combination in HT-29 and SNU-C4 cell lines are not related to interference with thymidylate synthase.