Evaluating the drug-target relationship between thymidylate synthase expression and tumor response to 5-fluorouracil. Is it time to move forward?

Abstract

Thymidylate synthase is a target of 5-fluoruracil, a pyrimidine analog used to treat gastrointestinal and other cancers. The 5-fluorouracil metabolite, fluoro-deoxyuridine monophosphate, forms a ternary complex with thymidylate synthase and 5,10-methylene tetrahydrofolate. The purpose of this study was to evaluate the time-honored connection between thymidylate synthase and 5-fluorouracil. From our literature search spanning reports from 1995 to 2007 published in journals having an impact factor greater than 2, we stratified the tumors within each article, according to low versus high thymidylate synthase expression. These groups were subdivided into responders, stable disease or disease progression. The relationship between thymidylate synthase expression and 5-fluorouracil response was analyzed for the overall group, as well as for subsets. Overall, the literature supported an approximately 2-fold inverse relationship between thymidylate synthase expression and response to 5-fluoruracil. We found no change in the trend for a relationship between thymidylate synthase and 5-fluorouracil when the literature was stratified by date of publication, impact factor of the journal in which the report was published, or substrate (mRNA versus protein) for measuring thymidylate synthase expression. Of note, there is no significant change in the trend when comparing 5-fluorouracil treatment alone or in combination with leucovorin. We found a decline of this trend when certain chemotherapeutics were used in combination with 5-fluorouracil. In sum, the connection between thymidylate synthase expression and patient response to 5-fluorouracil does not satisfy expectations for an effective drug-target relationship; and thus, studies of the thymidylate synthase tandem repeat status might only be clinically valuable in regards to patient toxicity. Thus, we question the reliability of thymidylate synthase expression as a clinical predictor of 5-fluorouracil response. Future research could perhaps be directed towards alternate targets and metabolites of 5-fluorouracil, in an effort to find a clinically relevant biomarker panel for response and to optimize fluoropyrimidine-based therapy.

Figure 1

History and metabolites of 5-FU. (A) A timeline of the genesis of the relationship between 5-FU and TS. (B) The metabolic pathway to FdUMP. On the left of the figure we show the main metabolic pathways to the 5-FU metabolite FdUMP. TP is thymidine phosphorylase; TK is thymidine kinase; OPRT is orate phosphoribosyltransferase; RR is ribonucleotide reductase. Note that the common pathway in cancer cells metabolize 5-FU to FdUMP is through ribonucleotide reductase. On the right is a simple schematic showing the central dogma of the most publicized 5-FU resistant mechanism. Increase numbers of TS molecules (top) in a cell will be able to resist FdUMP; while low amounts of TS in a cell will be inhibited by FdUMP (bottom) causing cellular death.

Figure 2

The pharmacogenetic window of 5-FU compared to imatinib. (A) Preclinical foundation for imatinib, demonstrating promising pharmacogenetic window. In this in vitro model of CML, imatinib exposure inhibited cell growth selectively in leukemia cells expressing the bcr-abl protein. Adapted with permission from Druker et al. (B) Clinical results of imatinib (left) versus previous standard therapy (right) for newly diagnosed chronic-phased CML, as reported by O’Brien et al. Imatinib was developed for human trials based upon preclinical data (A) that demonstrated promising activity in cell lines expressing the bcr-abl protein. CCR = complete cytogenetic response. Other = less than complete cytogenetic response.

Figure 3

(A) Cumulative data for tumors organized according to TS expression level and response to 5-FU chemotherapy. For the subset of journals in which articles with an impact factor greater than five, 35% of tumors with high TS expression demonstrate a response to 5-FU versus 53% of tumors with low TS expression. For tumors treated with 5-FU in combination with leucovorin, 83% of tumors with low TS expression had a response to treatment whereas 64% of tumors with low TS expression had no response to this combination treatment. R Response; NR No response. (B) Percent difference of response rates predicted by low TS expression. For tumors with low TS expression from 29 selected papers, the differences between percentages of responders minus non-responders. The differences between these percentages are displayed in descending order. The results above the x-axis support the ability of low TS levels to predict response to 5-FU, while the results below the x-axis contradict this conclusion. Each bar is marked with the reference number of the article it represents. (C) Percent difference of nonresponse rates predicted by high TS expression. For tumors with high TS expression from 29 selected papers, the differences between percentages of non-responders minus responders. The differences between these percentages are displayed in descending order. The results above the x-axis support the ability of high TS levels to predict resistance to 5-FU, while the results below the x-axis contradict this conclusion. Each bar is marked with the reference number of the article it represents.