Quantitation of uridine and L-dihydroorotic acid in human plasma by LC–MS/MS using a surrogate matrix approach

•A validated LC–MS/MS assay for the quantitation of endogenous analytes, Uridine and L-Dihydroorotic Acid, in human plasma.•A surrogate matrix approach with stable isotope labeled internal standards was used.•Parallelism was successfully demonstrated between the authentic matrix and the surrogate matrix.•The assay was applied to a first-in-man clinical trial to help assessing target engagement and pharmacodynamic effect.