T. K. Fröhlich¹, U. Amstutz², S. Aebi³, M. Jörger⁴, G. Andrey¹, G. Mäder¹, C. R. Largiadèr¹

¹Institute of Clinical Chemistry, Inselspital, Bern University Hospital, and University of Bern, Switzerland, ²Pharmaceutical Outcomes Programme, Child & Family Research Institute, University of British Columbia, Vancouver, Canada, ³Department of Medical Oncology, Cantonal Hospital, Luzern, Switzerland, ⁴Department of Oncology, Cantonal Hospital, St. Gallen, Switzerland

The fluoropyrimidine 5-fluorouracil (5-FU) is among the most commonly used chemotherapeutic agents for the treatment of solid carcinomas. Genetic factors are thought to account for a major part of severe to life-threatening 5-FU-related toxicity. We used a targeted pathway approach investigating the predictive potential of polymorphisms in multiple genes in the 5-FU metabolism and in the closely connected folate pathway with respect to 5-FU toxicity. A total of 300 patients receiving 5-FU-based chemotherapy were genotyped for 17 regulatory or functional polymorphisms in 12 genes and additionally, for four risk variants of the major target of 5-FU pharmacogenetics, the dihydropyrimidine dehydrogenase gene (DPYD). Associations with 5-FU-related toxicity and interactions with clinical and demographical factors (sex, age and concomitant drugs) were assessed using ordinal logistic regression models. Univariate association with toxicity was observed for a regulatory 19 bp deletion in the first intron of the dihydrofolate reductase (DHFR; P=0.026). A sex-dependent effect was found for the thymidylate synthase gene (TYMS) promoter region tandem repeat 2R2R (P=0.037) and the methylenetetrahydrofolate dehydrogenase (MTHFD1) c.1958AA genotypes (P=0.009). In addition, the co-administration of cis- or carboplatin was strongly associated with the occurrence of severe 5-FU toxicity (P<0.0001). A predictive model containing a simple additive pharmacogenetic index and the therapy-related effect correctly identified 40% (20/50) with severe 5-FU toxicity with a specificity of 0.75. Including the risk variants of DPYD to the model, 78% (39/50) of severe toxicity cases were identified with a specificity of 0.77, PPV of 0.41 and NPV of 0.95. Our results suggest that functional polymophisms in the folate pathway contain important information for the prediction of 5-FU toxicity, which need validation in an independent cohort. Furthermore, as some of the genetic effects appeared to differ according to sex or co-administered drugs, the predictive power of a pharmacogenetic model may be substantially improved by inclusion of clinical and demographical factors.