title: Inhibition of human CYP1A2 activity in vitro by methylxanthines: potent competitive inhibition by 8-phenyltheophylline.
abstract: 1. Humans are exposed in vivo to methylxanthines by dietary ingestion, as well as from their use as therapeutic agents. The inhibitory effect of a series of these compounds on high-affinity phenacetin O-deethylase activity in the human liver microsomal fraction, a measure of CYP1A2 activity, has been evaluated. 2. Paracetamol, the product of phenacetin O-deethylase activity, was analysed by gas chromatography/negative-ion mass spectrometry using a novel bistrifluoromethylbenzoyl/ trimethylsilyl derivative, and incubation conditions for assessing high-affinity phenacetin O-deethylase activity were examined and optimized. 3. 1-Methylxanthine, caffeine, theophylline, 8-methylxanthine, pentoxyfylline and 3isobutyl-1-methylxanthine caused moderate inhibition with IC50 = 260, 140, 120, 100, 62 and 36 microM respectively. 4. 8-Phenyltheophylline was a potent competitive inhibitor of high-affinity phenacetin O-deethylase activity with an IC50 = 0.7 microM and Ki = 0.11 microM. 5. The specificity of inhibition by 8-phenyltheophylline was assessed by measuring its effect on debrisoquine 4-hydroxylase (CYP2D6), terfenadine hydroxylase (CYP3A4), chlorzoxazone 6-hydroxylase (CYP2E1) and tolbutamide 4-hydroxylase (CYP2C9) activities in human liver microsomal fraction. No inhibition of any of these activities was observed. 6. The potency and specificity of 8-phenyltheophylline as an inhibitor of human hepatic CYP1A2 indicate that the compound may be useful as a chemical inhibitor of this enzyme for further in vitro studies.
authors.affiliation: Section on Clinical Pharmacology, Imperial College School of Medicine, London, UK. s.murray@ic.ac.uk
authors.forename: S
authors.initials: S
authors.lastname: Murray
authors.forename: A O
authors.initials: AO
authors.lastname: Odupitan
authors.forename: B P
authors.initials: BP
authors.lastname: Murray
authors.forename: A R
authors.initials: AR
authors.lastname: Boobis
authors.forename: R J
authors.initials: RJ
authors.lastname: Edwards
mesh_terms: D000082:Acetaminophen / Q000494:pharmacology; D001189:Aryl Hydrocarbon Hydroxylases; D001667:Binding, Competitive; D002110:Caffeine / Q000378:metabolism; D002753:Chlorzoxazone / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D019388:Cytochrome P-450 CYP1A2 / Q000378:metabolism*; D065609:Cytochrome P-450 CYP1A2 Inhibitors; D065729:Cytochrome P-450 CYP2C9; D019389:Cytochrome P-450 CYP2D6 / Q000378:metabolism; D019392:Cytochrome P-450 CYP2E1 / Q000378:metabolism; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D008401:Gas Chromatography-Mass Spectrometry; D006801:Humans; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D020128:Inhibitory Concentration 50; D007700:Kinetics; D015636:Magnesium Chloride / Q000494:pharmacology; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism; D006899:Mixed Function Oxygenases / Q000378:metabolism; D008956:Models, Chemical; D010431:Pentoxifylline / Q000378:metabolism; D010615:Phenacetin / Q000378:metabolism; D011485:Protein Binding; D012709:Serum Albumin / Q000494:pharmacology; D039181:Steroid 16-alpha-Hydroxylase; D013250:Steroid Hydroxylases / Q000378:metabolism; D013806:Theophylline / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology*; D013997:Time Factors; D014325:Tromethamine / Q000494:pharmacology; D014970:Xanthines / Q000378:metabolism / Q000494:pharmacology*
chemical_list: D065609:Cytochrome P-450 CYP1A2 Inhibitors; D012709:Serum Albumin; D014970:Xanthines; D014325:Tromethamine; D015636:Magnesium Chloride; C008514:methylxanthine; D000082:Acetaminophen; D002110:Caffeine; C053085:1-methylxanthine; D003577:Cytochrome P-450 Enzyme System; D013806:Theophylline; C028322:8-phenyltheophylline; D006899:Mixed Function Oxygenases; D013250:Steroid Hydroxylases; C450260:CYP2C9 protein, human; D065729:Cytochrome P-450 CYP2C9; D019392:Cytochrome P-450 CYP2E1; D001189:Aryl Hydrocarbon Hydroxylases; C504573:CYP1A2 protein, human; C104464:CYP3A protein, human; D019388:Cytochrome P-450 CYP1A2; D019389:Cytochrome P-450 CYP2D6; D051544:Cytochrome P-450 CYP3A; D039181:Steroid 16-alpha-Hydroxylase; C510163:CYP3A4 protein, human; C053553:tolbutamide 4-hydroxylase; D010615:Phenacetin; D002753:Chlorzoxazone; D010431:Pentoxifylline