We evaluated the expression of 5-FU pathway genes in prechemotherapeutic fresh frozen samples obtained from primary tumors to predict response and survival of 59 metastatic gastric cancer patients treated with S-1 monotherapy as first line treatment. Five 5-FU pathway genes, including thymidylate synthase (TS), dihydropyrimidine dehydrogenase (DPD), orotate phosphoribosyltransferase (OPRT), thymidine phosphorylase (TP) and uridine phosphorylase (UP), were analyzed by the quantitative real-time reverse transcriptional PCR method. Median values of each gene were selected for cut-off values separating high and low gene expressions. In univariate analyses, low TS, high OPRT and low TP were significantly associated with a tumor shrinkage and a long survival, whereas DPD and UP gene expressions did not correlate with response and survival. Multivariate analyses revealed that independent variables were OPRT and TS for response and TS and TP for survival. When OPRT and TS were combined, a significantly increased accuracy rate of 91.5% was seen for response. Similarly, an increased hazard ratio of 10.29 was observed for survival in patients possessing low TS and low TP, compared with those with high TS or high TP. The simple combinations of 2 genes, OPRT and TS for response and TS and TP for survival, may allow identification of gastric cancer patients who will benefit from S-1 chemotherapy. ' 2006 Wiley-Liss, Inc.Key words: orotate phosphoribosyltransferase; thymidylate synthase; thymidine phosphorylase; dihydropyrimidine dehydrogenase; S-1 The significant survival benefit of 5-fluorouracil (5-FU)-based chemotherapy for metastatic gastric cancer compared with best supportive care is reported.1,2 5-FU combined with cisplatin has been referred to as standard chemotherapy for metastatic gastric cancer, with the median survival time ranging from 7.3 to 10.5 months.1 During the last few years, additional drugs, including oxaliplatin, taxanes, irinotecan, and oral fluoropyrimidines such as S-1, have been introduced into chemotherapy regimens for the improvement of survival. 2,3 This approach has resulted in the increase of response rate, but has not changed the median survival time under 12 months, often with a higher frequency of severe adverse events, even acceptable.1,2 Thus, there is a need for diagnostic methods that allow prediction of clinical outcome and enable a pretherapeutic discrimination of treatment effect.The antitumor effect of 5-FU is the inhibitor of thymidylate synthase (TS), an essential DNA synthetic enzyme, by 5-fluoro-2 0 -deoxyuridine-5 0 -monophosphate (FdUMP), as well as incorporation of 5-FU metabolites into RNA and DNA. 4 The initial metabolism of 5-FU into nucleotides is essential for its action by one or more of the following 3 pathways: (i) directly to 5-fluorouridine 5 0 -monophosphate (FUMP) by orotate phosphoribosyltransferase (OPRT); (ii) indirectly to FUMP in a sequence of reactions with conversion of 5-FU to 5-fluorouridine phosphorylated by uridine phosphorylase (UP); (iii) indirectly to ...