Schedule-dependent interaction between raltitrexed and 5-fluorouracil in human colon cancer cell lines in vitro

Raltitrexed (Tomudex) is a novel thymidylate synthase inhibitor with signif icant activity against advanced colorectal cancer. We studied the cytotoxic interactions of raltitrexed and 5-fluorouracil (5-FU) in four human colon cancer cell lines on various schedules. The cell growth inhibition after 5 days was determined using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetraz olium bromide (MTT) assay. The cytotoxic interactions at the IC80 level wer e evaluated by the isobologram method. Simultaneous exposure to raltitrexed and 5-FU for 5 days produced additive to synergistic effects in Colo201 ce lls, and produced additive effects in Colo321, LoVo, and WiDr cells. Simult aneous exposure to raltitrexed and 5-FU for 24 h produced additive effects in Colo201, LoVo, and WiDr cells, and produced antagonistic effects in Colo 320 cells. Sequential exposure to raltitrexed for 24 h followed by 5-FU for 24 h produced additive effects in Colo201, Colo320, and LoVo cells, and pr oduced antagonistic effects in WiDr cells. The reverse sequence produced ad ditive effects in Colo201 cells, and produced antagonistic effects in Colo3 20, LoVo, and WiDr cells. Simultaneous exposure to raltitrexed and 5-FU for 4 h and sequential exposure to raltitrexed for 4 h followed by 5-FU for 4 h with a 20-h interval produced additive effects, while the reverse sequenc e produced antagonistic effects in LoVo and WiDr cells. These findings sugg est that the simultaneous administration of raltitrexed and 5-FU or the seq uential administration of raltitrexed followed by 5-FU may be the optimal s equence, while the reverse sequence may be inappropriate. Preclinical and c linical studies of the simultaneous administration of raltitrexed and 5-FU and the sequential administration of raltitrexed followed by 5-FU are requi red to better understand the antitumor, toxic, and pharmacokinetic interact ions of this combination in order to develop the combination chemotherapy o f raltitrexed and 5-FU.