Simultaneous determination of thymine and its sequential catabolites dihydrothymine and β-ureidoisobutyrate in human plasma and urine using liquid chromatography-tandem mass spectrometry with pharmacokinetic application

► HPLC-MS/MS assay of thymine, dihydrothymine and β-ureidoisobutyrate in plasma/urine is presented. ► This method can be applied to endogenous assay of these compounds, and in pharmacokinetic studies. ► Time profiles of the three compounds in two healthy adults given a 250 mg oral dose of thymine are shown. ► This method may be valuable for a pharmacokinetic screen to predict fluoropyrimidine toxicity.