Pharmacogenetic variants in the DPYD, TYMS, CDA and MTHFR genes are clinically significant predictors of fluoropyrimidine toxicity

Background: Fluoropyrimidine drugs are extensively used for the treatment of solid cancers. However, adverse drug reactions are a major clinical problem, often necessitating treatment discontinuation. The aim of this study was to identify pharmacogenetic markers predicting fluoropyrimidine toxicity. Methods: Toxicity in the first four cycles of 5-fluorouracil or capecitabine-based chemotherapy were recorded for a series of 430 patients. The association between demographic variables, DPYD, DPYS, TYMS, MTHFR, CDA genotypes, and toxicity were analysed using logistic regression models. Results: Four DPYD sequence variants (c.1905+1G4A > c.2846A4T > c.1601G4A and c.1679T4G) were found in 6% of the cohort and were significantly associated with grade 3-4 toxicity (P<0.0001). The TYMS 30-untranslated region del/del genotype substantially increased the risk of severe toxicity (P=0.0123, odds ratio (OR)=3.08, 95% confidence interval (CI): 1.38-6.87). For patients treated with capecitabine, a MTHFR c.1298CC homozygous variant genotype predicted hand-foot syndrome (P=4.1 × 10^-6, OR=9.99, 95% CI: 3.84-27.8). The linked CDA c.-92A4G and CDA c. 451C4T variants predicted grade 2-4 diarrhoea (P=0.0055, OR=2.3, 95% CI: 1.3-4.2 and P=0.0082, OR=2.3, 95% CI: 1.3-4.2, respectively). Conclusion: We have identified a panel of clinically useful pharmacogenetic markers predicting toxicity to fluoropyrimidine therapy. Dose reduction should be considered in patients carrying these sequence variants.