Characterization of Uracil Catabolism Variability in Healthy Volunteers

Uracil catabolism is crucial for the pharmacokinetics of the chemotherapeutic 5-fluorouracil (5-FU) since 5-FU is degraded by the same pathway. Decreased activity of the first catabolizing enzyme, dihydropyrimidine dehydrogenase (DPD), is a major predictor of 5-FU toxicity with known risk variants in the DPD gene (DPYD) accounting for ~30% of toxicities. However, not all toxicity cases can be explained by DPYD risk variants. To date, the phenotypic variability in the catabolism downstream of DPD by dihydropyrimidinase (DHP) and β-ureidopropionase (bUP), potentially contributing to 5-FU toxicity, has not been investigated. Thus, we aimed to characterize the baseline phenotypic variability of endogenous metabolites and metabolic ratios of 5-FU catabolism enzymes and to correlate the phenotype with genetic variation in the DHP and bUP genes (DPYS and UPB1). Three variants in DPYS and UPB1 previously associated with 5-FU toxicity were genotyped in 320 healthy volunteers and their plasma uracil, dihydrouracil (UH2), β-ureidopropionic acid (UPA), and β-alanine (BAL) concentrations were determined by LC-MS/MS. High inter-individual variability in all metabolic ratios was observed. Sex-dependent differences were detected at each enzymatic step in the uracil catabolism pathway, with lower metabolite levels (P ≤ 0.007) in women. Moreover, lower UPA/UH2 ratios (P < 0.001) were observed in women, suggesting that reduced 5-Fluoro-UH2 catabolism may contribute to higher fluoropyrimidine toxicity rates observed in females. Furthermore, volunteers carrying DPYS variant c.265-58T>C had lower UH2 plasma levels (P = 0.033) and higher UPA/UH2 ratios (P = 0.036) and carriers of the UPB1 variant c.1-80C>G showed lower BAL plasma levels (P = 0.004). These initial results are in agreement with the previously observed reduced fluoropyrimidine toxicity in c.265-58C carriers and increased toxicity in carriers of c.1-80G, indicating a possible functional effect related to these variants.