Simple liquid chromatographic method for the determination of uracil and dihydrouracil plasma levels: a potential pretreatment predictor of 5-fluorouracil toxicity

5-Fluorouracil (5-FU) is a commonly used anti-cancer drug with notable activity in clinical practice, yet it causes significant unpredictable and often serious toxicity. Both 5-FU and uracil (U) are catabolised by dihydropyrimidine dehydrogenase (DPD) to form dihydrofluorouracil (FUH2) and dihydrouracil (UH2), respectively. A means of predicting toxicity before treatment would be more valuable. Variations in dihydropyrimidine dehydrogenase (DPD) activity between patients are at least partly responsible for variable toxicity. Measurement of the UH2 to U ratio may be a measure of pyrimidine catabolism and thus be utilised to predict subsequent toxicity. We have developed an efficient extraction and detection method using HPLC for the simultaneous measurement of UH2 and U in plasma. A single C18 Spherisorb ODS2 (25 cm) column using isocratic elution was utilised. U, UH2 and the internal standard 4-chlorouracil were detected at wavelengths of 257, 220, and 268 nm, respectively. The chromatographic run time was 45 min which is half that of other methods. The detection limit was 0.02 μM for U and 0.1 μM for UH2 using only 0.5 ml of plasma for both compounds. The basal plasma concentrations of U and UH2 in 23 individuals ranged from 0.025 to 0.27 μM and 0.4–1.7 μM, respectively. This simple method may permit the assessment of pyrimidine catabolism, and therefore allow prediction of the toxicities associated with the use of fluorinated pyrimidines.