title: Prolonged and enhanced suppression of thymidylate synthase by weekly 24-h infusion of high-dose 5-fluorouracil.
abstract: We have recently demonstrated that HDFL (high-dose 5-FU 2600 mg m-2 week-1 and leucovorin 500 mg m-2 week-1, weekly 24-h infusion) is highly active in the treatment of gastric cancer. To further clarify the possible mechanism underlying the improved activity of HDFL compared with conventional 5-FU regimens, we conducted in vitro studies examining the effect of these regimens on the differential regulation of thymidylate synthase (TS) in NCI-N87, a human gastric cancer cell line. The expected serum concentrations of 5-FU are 100-200 mM (lasting for less than 30 min) and 5-10 mM (lasting for 24 h) for the conventional 5-FU regimens (bolus injection or short intravenous infusion of 5-FU 370-500 mg m-2) and the HDFL regimens, respectively. Western blot analysis revealed that 24-h exposure of NCI-N87 to 2.5-10.0 mM of 5-FU resulted in a dose-dependent depletion of free TS, lasting for more than 24 h. In contrast, 30-min exposure of NCI-N87 to 200 mM of 5-FU resulted in a less than 12-h depletion of free TS. Moreover, 24-h exposure to 5-FU resulted in a higher S-phase blockade and enhanced cytotoxicity. In both modes of 5-FU treatment, the initial rapid depletion of free TS was accompanied by a rapid increment of a higher-molecular-weight TS molecule, suggesting that rapid formation of the ternary complex was the key mechanism of 5-FU action during this period. Northern blot analysis showed that the steady-state mRNA of TS was not affected by either of the schedules. We conclude that 24-h exposure of gastric cancer cells to low concentration of 5-FU resulted in better suppression of free TS, a higher degree of S-phase blockade, and enhanced cytotoxicity compared to 30-min exposure to high concentration of 5-FU. These in vitro results may help explain the improved clinical efficacy of HDFL regimens compared to conventional 5-FU regimens.
authors.affiliation: Department of Oncology, National Taiwan University Hospital, 7 Chung-Shan South Road, Tapei 100, Taiwan.
authors.forename: K H
authors.initials: KH
authors.lastname: Yeh
authors.forename: S H
authors.initials: SH
authors.lastname: Yeh
authors.forename: C H
authors.initials: CH
authors.lastname: Hsu
authors.forename: T M
authors.initials: TM
authors.lastname: Wang
authors.forename: I F
authors.initials: IF
authors.lastname: Ma
authors.forename: A L
authors.initials: AL
authors.lastname: Cheng
mesh_terms: D000964:Antimetabolites, Antineoplastic / Q000008:administration & dosage*; D015152:Blotting, Northern; D002453:Cell Cycle / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D004334:Drug Administration Schedule; D005472:Fluorouracil / Q000008:administration & dosage*; D006801:Humans; D012333:RNA, Messenger / Q000378:metabolism; D013274:Stomach Neoplasms / Q000188:drug therapy / Q000201:enzymology; D013940:Thymidylate Synthase / Q000037:antagonists & inhibitors* / Q000096:biosynthesis / Q000235:genetics; D014407:Tumor Cells, Cultured
chemical_list: D000964:Antimetabolites, Antineoplastic; D012333:RNA, Messenger; D013940:Thymidylate Synthase; D005472:Fluorouracil