Optimized assays for human UDP-glucuronosyltransferase (UGT) activities: altered alamethicin concentration and utility to screen for UGT inhibitors.

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: ß-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.

Investigation into UDP-glucuronosyltransferase (UGT) enzyme kinetics of imidazole- and triazole-containing antifungal drugs in human liver microsomes and recombinant UGT enzymes.

Imidazoles and triazoles represent major classes of antifungal azole derivatives. With respect to UDP-glucuronosyltransferase (UGT) enzymes, the drug metabolism focus has mainly concentrated on their inhibitory effects with little known about azoles as substrates for UGTs. N-Glucuronide metabolites of the imidazole antifungals, tioconazole and croconazole, have been reported, but there are currently no reports of N-glucuronidation of triazole antifungal agents. In this study, evidence for glucuronidation of azole-containing compounds was studied in human liver microsomes (HLM). When a glucuronide metabolite was identified, azoles were incubated in 12 recombinant UGT (rUGT) enzymes, and enzyme kinetics were determined for the UGT with the most intense glucuronide peak. Six imidazole antifungals, three triazoles, and the benzodiazepine alprazolam (triazole) were evaluated in this study. All compounds investigated were identified as substrates of UGT. UGT1A4 was the main enzyme involved in the metabolism of all compounds except for fluconazole, which was mainly metabolized by UGT2B7, probably mediating its O-glucuronide metabolism. UGT1A3 was also found to be involved in the metabolism of all imidazoles but not triazoles. In both HLM and rUGT K(m) values were lower for imidazoles (14.8-144 microM) than for triazoles (158-3037 microM), with the exception of itraconazole (8.4 microM). All of the imidazoles studied inhibited their own metabolism at high substrate concentrations. In terms of UGT1A4 metabolism, itraconazole showed kinetic features characteristic of imidazole rather than triazole antifungals. This behavior is attributed to the physicochemical properties of itraconazole that are similar to those of imidazoles in terms of clogP.