An optimized and validated SPE-LC–MS/MS method for the determination of caffeine and paraxanthine in hair

Caffeine is the probe drug of choice to assess the phenotype of the drug metabolizing enzyme CYP1A2. Typically, molar concentration ratios of paraxanthine, caffeine’s major metabolite, to its precursor are determined in plasma following administration of a caffeine test dose. The aim of this study was to develop and validate an LC–MS/MS method for the determination of caffeine and paraxanthine in hair. The different steps of a hair extraction procedure were thoroughly optimized. Following a three-step decontamination procedure, caffeine and paraxanthine were extracted from 20mg of ground hair using a solution of protease type VIII in Tris buffer (pH 7.5). Resulting hair extracts were cleaned up on Strata-X™ SPE cartridges. All samples were analyzed on a Waters Acquity UPLC® system coupled to an AB SCIEX API 4000™ triple quadrupole mass spectrometer. The final method was fully validated based on international guidelines. Linear calibration lines for caffeine and paraxanthine ranged from 20 to 500pg/mg. Precision (%RSD) and accuracy (%bias) were below 12% and 7%, respectively. The isotopically labeled internal standards compensated for the ion suppression observed for both compounds. Relative matrix effects were below 15%RSD. The recovery of the sample preparation procedure was high (>85%) and reproducible. Caffeine and paraxanthine were stable in hair for at least 644 days. The effect of the hair decontamination procedure was evaluated as well. Finally, the applicability of the developed procedure was demonstrated by determining caffeine and paraxanthine concentrations in hair samples of ten healthy volunteers. The optimized and validated method for determination of caffeine and paraxanthine in hair proved to be reliable and may serve to evaluate the potential of hair analysis for CYP1A2 phenotyping.