Intratumoural thymidylate synthase and dihydropyrimidine dehydrogenase activities are good predictors of 5-fluorouracil sensitivity in colorectal cancer

To identify factors that influence the clinical response to 5-fluorouracil (5-FU), we studied the correlation between in vitro sensitivity to 5-FU and the expression of seven biological markers. The markers, thymidylate synthase (TS), dihydropyrimidine dehydrogenase (DPD), pyrimidine nucleoside phosphorylase, p53 (wild/mutant), p21, cyclo-oxygenase-2, and inducible nitric oxide synthase were measured in tumour tissues from 32 colorectal. cancer patients. The activities of TS and DPD were significantly lower in the tumours sensitive to 5-FU compared with those that were not sensitive to 5-FU. In tumours with TS < 3.7 pmol/min per mg protein and DPD < 98 pmol/min per mg protein, the percentage of cases sensitive to 5-FU (67%) and the mean percentage inhibition of tumour cells by S-FU (42.8%) were significantly higher than in the other tumours (0% and 13.1%, respectively). The other biological markers did not correlate with in vitro sensitivity to 5-FU. Tumour sensitivity to S-FU can be more precisely predicted by taking the activities of both TS and DPD into consideration than by using either alone.