title: Optimized assays for human UDP-glucuronosyltransferase (UGT) activities: altered alamethicin concentration and utility to screen for UGT inhibitors.
abstract: The measurement of the effect of new chemical entities on human UDP-glucuronosyltransferase (UGT) marker activities using in vitro experimentation represents an important experimental approach in drug development to guide clinical drug-interaction study designs or support claims that no in vivo interaction will occur. Selective high-performance liquid chromatography-tandem mass spectrometry functional assays of authentic glucuronides for five major hepatic UGT probe substrates were developed: β-estradiol-3-glucuronide (UGT1A1), trifluoperazine-N-glucuronide (UGT1A4), 5-hydroxytryptophol-O-glucuronide (UGT1A6), propofol-O-glucuronide (UGT1A9), and zidovudine-5'-glucuronide (UGT2B7). High analytical sensitivity permitted characterization of enzyme kinetic parameters at low human liver microsomal and recombinant UGT protein concentration (0.025 mg/ml), which led to a new recommended optimal universal alamethicin activation concentration of 10 μg/ml for microsomes. Alamethicin was not required for recombinant UGT incubations. Apparent enzyme kinetic parameters, particularly for UGT1A1 and UGT1A4, were affected by nonspecific binding. Unbound intrinsic clearance for UGT1A9 and UGT2B7 increased significantly after addition of 2% bovine serum albumin, with minimal changes for UGT1A1, UGT1A4, and UGT1A6. Eleven potential UGT and cytochrome P450 inhibitors were evaluated as UGT inhibitors, resulting in observation of nonselective UGT inhibition by chrysin, mefenamic acid, silibinin, tangeretin, ketoconazole, itraconazole, ritonavir, and verapamil. The pan-cytochrome P450 inhibitor, 1-aminobenzotriazole, minimally inhibited UGT activities and may be useful in reaction phenotyping of mixed UGT and cytochrome P450 substrates. These methods should prove useful in the routine assessments of the potential for new drug candidates to elicit pharmacokinetic drug interactions via inhibition of human UGT activities and the identification of UGT enzyme-selective chemical inhibitors.
authors.affiliation: Department of Pharmacokinetics, Dynamics, and Metabolism, Pfizer Inc., Groton, Connecticut 06340, USA.
authors.forename: Robert L
authors.initials: RL
authors.lastname: Walsky
authors.forename: Jonathan N
authors.initials: JN
authors.lastname: Bauman
authors.forename: Karine
authors.initials: K
authors.lastname: Bourcier
authors.forename: Georgina
authors.initials: G
authors.lastname: Giddens
authors.forename: Kimberly
authors.initials: K
authors.lastname: Lapham
authors.forename: Andre
authors.initials: A
authors.lastname: Negahban
authors.forename: Tim F
authors.initials: TF
authors.lastname: Ryder
authors.forename: R Scott
authors.initials: RS
authors.lastname: Obach
authors.forename: Ruth
authors.initials: R
authors.lastname: Hyland
authors.forename: Theunis C
authors.initials: TC
authors.lastname: Goosen
mesh_terms: D000408:Alamethicin / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid; D004305:Dose-Response Relationship, Drug; D055808:Drug Discovery / Q000379:methods*; D004791:Enzyme Inhibitors / Q000378:metabolism / Q000494:pharmacology*; D020719:Glucuronides / Q000378:metabolism*; D014453:Glucuronosyltransferase / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism; D015394:Molecular Structure; D011485:Protein Binding; D012710:Serum Albumin, Bovine / Q000494:pharmacology; D013379:Substrate Specificity; D053719:Tandem Mass Spectrometry
chemical_list: D004791:Enzyme Inhibitors; D020719:Glucuronides; D000408:Alamethicin; D012710:Serum Albumin, Bovine; D014453:Glucuronosyltransferase