title: High-throughput radiometric CYP2C19 inhibition assay using tritiated (S)-mephenytoin.
abstract: A rapid and sensitive radiometric assay for assessing the potential of drugs to inhibit cytochrome P450 (P450) 2C19 in human liver microsomes is described. The new assay, which does not require high-performance liquid chromatography (HPLC) separation or mass spectrometric detection, is based on the release of tritium as tritiated water that occurs upon CYP2C19-mediated 4'-hydroxylation of (S)-mephenytoin labeled with tritium in the 4' position. Because this reaction is subject to an NIH shift, tritium was also introduced into the 3'- and 5'-positions of the tracer to enhance formation of a tritiated water product. Tritiated water was separated from the substrate using 96-well solid-phase extraction plates. The reaction is NADPH-dependent and sensitive to CYP2C19 inhibitors. IC(50) values for 15 diverse drugs differed less than 2.5-fold from those determined by quantification of the unlabeled 4'-hydroxy-(S)-mephenytoin product, using HPLC coupled to mass spectrometric detection. All of the steps of the new assay, namely incubation, product separation, and radioactivity counting, are performed in a 96-well format and can be automated. This assay represents a non-HPLC, high-throughput version of the classic (S)-mephenytoin 4'-hydroxylation assay, which is the most widely used method to assess the potential for CYP2C19 inhibition of new chemical entities.
authors.affiliation: Department of Pharmacology, Istituto di Recherche di Biologia Molecolare, Merck Research Laboratories, Rome, Italy.
authors.forename: Annalise
authors.initials: A
authors.lastname: Di Marco
authors.forename: Antonella
authors.initials: A
authors.lastname: Cellucci
authors.forename: Ashok
authors.initials: A
authors.lastname: Chaudhary
authors.forename: Massimiliano
authors.initials: M
authors.lastname: Fonsi
authors.forename: Ralph
authors.initials: R
authors.lastname: Laufer
mesh_terms: D001189:Aryl Hydrocarbon Hydroxylases / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D065731:Cytochrome P-450 CYP2C19; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D006801:Humans; D008617:Mephenytoin / Q000378:metabolism*; D008862:Microsomes, Liver / Q000378:metabolism; D006899:Mixed Function Oxygenases / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D011874:Radiometry; D011994:Recombinant Proteins / Q000378:metabolism; D014316:Tritium / Q000032:analysis; D014867:Water
chemical_list: D004791:Enzyme Inhibitors; D011994:Recombinant Proteins; D014867:Water; D014316:Tritium; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; C045793:CYP2C19 protein, human; D065731:Cytochrome P-450 CYP2C19; D008617:Mephenytoin