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.