Combination studies of antifolates with 5-fluorouracil in colon cancer cell lines

The combined cytotoxic effects of the thymidylate synthase (TS) inhibitors 5-fluorouracil (5FU) and different antifolates were studied in seven colon cancer cell lines. Growth inhibition of the antifolates, Nolatrexed, Raltitrexed, GW1843U89, or MTA in combination with 5FU, was determined and multiple drug effect analysis showed that the drugs acted mostly additively. The only synergistic interaction was found for 5FU and Nolatrexed in the LS174T cell line. Also Raltitrexed and 5FU were slightly synergistic in WiDr/F cells grown at low folate levels, but for the other cell lines grown at high folate levels this combination was more antagonistic. GW1843U89 and 5FU were mainly additive, while 5FU and MTA showed antagonism in WiDr and additivity in LS174T. The effect of the drugs at their target was evaluated by in situ TS inhibition. We observed lower TS activity in all cells when two drugs were used instead of one. Statistical analysis revealed that none of the values of the combinations was higher or lower than could be expected from the product of the effect of single drugs. We concluded that the effects on TS inhibition were additive for all 5FU/antifolate combinations in all cell lines. DNA strand break formation, as a result of TS inhibition, was measured by means of a fluorometric analysis of DNA unwinding. Raltitrexed-induced DNA damage was significantly increased by 5FU in WiDr cells single agent: 67% double stranded (ds) DNA, combination: 39% ds DNA, P<0.0001]. In LS174T a trend for antagonistic effects was observed for combinations of MTA, GW1843U89, or Raltitrexed and 5FU. The combinations showed additive effects in WiDr/F cells. The overall conclusion of the three assays in each of the cell lines indicated that 5FU and antifolate combinations were predominantly additive in colon cancer cells.