Prolonged and enhanced suppression of thymidylate synthase by weekly 24-h infusion of high-dose 5-fluorouracil

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 r egimens, we conducted in vitro studies examining the effect of these regime ns 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 (tasting 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. West ern 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 th an 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 mo des of 5-FU treatment, the initial rapid depletion of free TS was accompani ed by a rapid increment of a higher-molecular-weight TS molecule, suggestin g 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-s tate mRNA of TS was not affected by either of the schedules. We conclude th at 24-h exposure of gastric cancer cells to low concentration of 5-FU resul ted 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 eff icacy of HDFL regimens compared to conventional 5-FU regimens. (C) 2000 Can cer Research Campaign http://www.bjcancer.com.