title: Subnanomolar quantification of caffeine's in vitro metabolites by stable isotope dilution gas chromatography-mass spectrometry.
abstract: A method for the quantification of subnanomolar levels of in vitro metabolites of caffeine by an isotope dilution gas chromatographic-mass spectrometric (GC-MS) assay has been developed and applied. Trideuteromethylated analogs of each primary metabolite were synthesized and added after incubations of caffeine with human liver microsomes high in cytochrome P4501A2. HPLC separation of the metabolites prior to GC-MS quantification allowed the isolation of theobromine and paraxanthine which coeluted by GC and enabled quantification over a larger dynamic range. Quantitative analysis was performed on the n-propylated derivatives by selected-ion monitoring of either the M+. ions for the dimethylxanthines or [M-C3H6]+. ions for 1,3,7-trimethyluric acid. For the least abundant metabolite (1,3,7-trimethyluric acid), the detection level on column was 200 pg. Replicate analyses exhibited intra- and inter-day variability of 4.2 and 7.9%, respectively. This assay has been successfully used in the quantification of caffeine's primary metabolites in more than 180 incubations, at varying substrate concentrations and with multiple enzyme sources.
authors.affiliation: Department of Medicinal Chemistry, School of Pharmacy, University of Washington, Seattle 98195, USA.
authors.forename: K A
authors.initials: KA
authors.lastname: Regal
authors.forename: W N
authors.initials: WN
authors.lastname: Howald
authors.forename: R M
authors.initials: RM
authors.lastname: Peter
authors.forename: C A
authors.initials: CA
authors.lastname: Gartner
authors.forename: K L
authors.initials: KL
authors.lastname: Kunze
authors.forename: S D
authors.initials: SD
authors.lastname: Nelson
mesh_terms: D002110:Caffeine / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D008401:Gas Chromatography-Mass Spectrometry / Q000379:methods*; D006801:Humans; D066298:In Vitro Techniques; D008862:Microsomes, Liver / Q000378:metabolism; D012680:Sensitivity and Specificity
chemical_list: D002110:Caffeine