Analysis of caffeine and paraxanthine in human saliva with ultra-high-performance liquid chromatography for CYP1A2 phenotyping

• Caffeine and paraxanthine are analyzed in human saliva with UHPLC.
• All compounds elute within 6 min.
• Paraxanthine is separated from other caffeine metabolites, such as theophylline.
• The validation demonstrates the suitability of the method for CYP1A2 phenotyping.
• Habitual coffee consumption significantly increases CYP1A2 activity.

Cytochrome P450 1A2 (CYP1A2) plays an important role in drug metabolism. Caffeine (CAF) is converted into paraxanthine (PX) by this enzyme and is used as a xenobiotic substrate to determine the CYP1A2 phenotype in humans. A method for the quantification of CAF and PX in saliva was developed using liquid–liquid extraction with ethyl acetate and analysis with ultra-high-performance liquid chromatography. Peaks from CAF, PX and internal standard were resolved within 6 min. The method was validated from 0.05 to 5 μg mL−1 CAF and 0.025–2.5 μg mL−1 PX. Inter- and intra-day accuracies ranged from 91.2 to 107.2% with precisions <13.5%. The limits of detection were 0.16 and 0.63 ng mL−1 for PX and CAF, respectively. PX/CAF concentration ratios from volunteers were 0.26–1.09 with mean ratios of 0.78 ± 0.26 and 0.38 ± 0.10 for regular and light/non-coffee drinkers, respectively.