Predicting Fluorouracil Toxicity: Can We Finally Do It?

Currently, no reliable markers of sensitivity or resistance to fluorouracil (FU) have been validated to permit their use as a standard of care for the management of patients with cancer, despite the large number of studies attempting to identify useful molecular predictors of response to treatment. This may be attributed to the complexity of the involved molecular events and/or incomplete understanding of the role of downstream cell signaling pathways in response to chemotherapy-induced stress. In addition and most important is the lack of a comprehensive, well-designed, standardized molecular approach that could evaluate all of these events. To date, the majority of identified predictive markers of response to FU in patients with cancer have been at relatively upstream levels of drug metabolism (eg, dihydropyrimidine dehydrogenase [DPD] enzyme), drug target (eg, thymidylate synthase enzyme [TYMS]), and DNA repair pathways. The fact that molecular events might have overlapping or antagonistic function and could possibly be activated after chemotherapy-induced stress renders the use of a single marker of response unlikely in the prediction of outcome. Because early detection of patients with cancer who are at risk of developing life-threatening toxicity to FU might allow dose reductions or selection of an alternative treatment regimen, various genotypic and phenotypic methods have been developed to predict toxicity and/or response. None, however, has proven to be reliable in prospective clinical studies, demonstrating a great margin of uncertainty. In the study by Schwab et al in the current issue of the Journal of Clinical Oncology, the authors have examined a few of the previously suggested markers of FU toxicity in a large, albeit diverse group of patients. The experimental design focused on an examination of only a few sequence variants in three selected genes (DPYD, the gene that encodes DPD enzyme; TYMS; and methylene tetrahydrofolate reductase [MTHFR]), coupled with nongenetic factors (sex and diet) previously suggested to be associated with severe toxicity from FU. Although the use of pharmacogenetic methods in the risk evaluation of patients undergoing chemotherapy is to be commended, without a comprehensive approach in which all of the key regulatory mechanisms and drug targets are also evaluated, there is a risk of oversimplification and misinterpretation. Given the nomogram proposed in the study by Schwab et al, 2 to estimate the probability that a given patient will develop toxicity, the question arises whether the experimental approach to predict toxicity in such a diverse group of cancer patients (colon cancer, other gastrointestinal tumors, cancer of unknown origin, and breast cancer) can be expected to have reliable true predictive value with the limited number of selected genetic/ nongenetic parameters. Clinical studies, prospectively using genetic tests to predict FU toxicity, have raised numerous concerns because of the lack of inclusion of important factors that could contribute to FU toxicity. This has highlighted the necessity of a thorough re-evaluation of the parameters used (genetic/nongenetic) in screening tests to decrease the incidence of false positive or negative predictions. For example, on the catabolic side of FU metabolism, routine detection of DPD deficiency in patients with cancer likely to undergo FU therapy has been examined. However, DPD deficiency has been observed in a relatively small percentage of patients with grade 3 to 4 FU toxicity, leaving a large number of patients with an unexplained molecular basis of toxicity. Although the assessment of DPD activity is informative about the likelihood of DPD deficiency, and a few DPYD alleles (eg, DPYD*2A and DPYD*13) have been associated with decreased DPD activity, DPD deficiency has also been detected in patients with wild-type DPYD. Scientific evidence supporting the role of epigenetic mechanisms, mainly methylation, in regulating DPD enzyme activity has been observed in a subset of patients who had no sequence abnormalities in their DPYD gene. In these patients (in contrast to the findings by Schwab et al), the regulatory elements in the DPYD promoter region were methylated, suggesting that genetic and epigenetic mechanisms could act separately or in concert to downregulate DPD activity. Recently, it has been reported that individuals with mutations in genes downstream of DPD (dihydropyrimidinase [DHP] and beta-ureidopropionase [BUP1]) had altered activity of these two enzymes, which resulted in impairment of the uracil catabolic pathway. This finding is supported by a recently developed uracil breath test that evaluates the integrity of the entire catabolic pathway (DPD, DHP, and BUP1 enzymes). Although the frequency of deficiency in DHP and BUP1 enzymes has been previously reported to be low, recent studies suggest that both might occur more frequently than previously believed. On the anabolic side of FU metabolism, there is also complexity owing to both the multiple enzymatic steps (eg, orotate phosphoribosyl transferase thymidine phosphorylase, and uridine phosphorylase, and thymidine kinase) that must convert FU to FU nucleotides and the multiple sites of action (TYMS inhibition, incorporation into DNA and RNA). In the present article, the authors have neglected other genes important for FU anabolic activity as well as other sites of action and have chosen to examine only selected mutations in TYMS, the variable number tandem repeats at the 5 UTR and JOURNAL OF CLINICAL ONCOLOGY E D I T O R I A L VOLUME 26 NUMBER 13 MAY 1 2008