2.2. Dihydropyrimidine Dehydrogenase Enzyme Activity Measurements

For all patients, DPD enzyme activity was determined. This could be either prior to treatment or retrospectively after the occurrence of severe toxicity. DPD enzyme activity measurement in peripheral blood mononuclear cells (PBMCs) [24,25] was used as a reference to assess DPD activity, and has been used previously to determine dosages in DPYD variant-carrying patients [21,26]. A validated method [27] was used, containing radiolabeled thymine as a substrate and consisting of high-performance liquid chromatography (HPLC) with online radioisotope detection using liquid scintillation counting. Normal values for healthy volunteers are 9.9 ± 2.8 nmol/(mg×h), for DPD-deficient patients are 4.8 ± 1.7 nmol/(mg×h), and reference values range from 5.9 to 14 nmol/(mg×h) [28]. Dose reductions based on DPD enzyme activity were performed in a one-to-one ratio, as was previously described by Henricks et al. [21]. Thereafter, toxicity-guided dosing was used.