5,6-Dihydrouracil is a catabolite of uracil .1 It is formed from uracil by dihydropyrimidine dehydrogenase. The plasma ratio of 5,6-dihydrouracil to uracil has been used as a phenotypic marker of dihydropyrimidine dehydrogenase activity.2
1.Büchel, B., Rhyn, P., Schürch, S., et al.LC-MS/MS method for simultaneous analysis of uracil, 5,6-dihydrouracil, 5-fluorouracil and 5-fluoro-5,6-dihydrouracil in human plasma for therapeutic drug monitoring and toxicity prediction in cancer patientsBiomed. Chromatogr.27(1)7-16(2013) 2.Chavani, O., Jensen, B.P., Strother, R.M., et al.Development, validation and application of a novel liquid chromatography tandem mass spectrometry assay measuring uracil, 5,6-dihydrouracil, 5-fluorouracil, 5,6-dihydro-5-fluorouracil, α-fluoro-β-ureidopropionic acid and α-fluoro-β-alanine in human plasmaJ. Pharm. Biomed. Anal.142125-135(2017)