N-glucuronidation reactions. II. Relative N-glucuronidation reactivity of methylbiphenyl tetrazole, methylbiphenyl triazole, and methylbiphenyl imidazole in rat, monkey, and human hepatic microsomes
Huskey, S.W.; Doss, G.A.; Miller, R.R.; Schoen, W.R.; Chiu, S.H.
Drug Metabolism and Disposition the Biological Fate of Chemicals 22(4): 651-658
1994
ISSN/ISBN: 0090-9556 PMID: 7956743 Document Number: 426648
The relative intrinsic in vitro N-glucuronidation reactivity of three classes of heterocyclic compounds was compared using model compounds incubated with UDP-glucuronic acid-enriched liver microsomes from rats, monkeys, and humans. These compounds, all methylbiphenyl (MB) derivatives, represent three classes of N-containing heterocycles commonly used in the design of new drug entities (i.e. MB-tetrazole, MB-triazole, (1,2,3- and 1,2,4-), and MB-imidazole (C2- and C4-substituted)). The structures of all respective N-glucuronides generated from microsomal incubations were determined by Nuclear Overhauser Effect difference NMR spectroscopy. The chemical and enzymic stabilities of N-glucuronides were also studied. In general, relatively low reactivity was found at nitrogens located next to substituted carbons in heterocycles such as N3 in MB-C4-imidazole, N3 in MB-1,2,3-triazole, N2 (or N4) in MB-1,2,4-triazole, and N1 (or N4) in MB-tetrazole. MB-C2-imidazole, in which both nitrogens are in immediate neighboring positions of the substituted carbon, was unreactive toward N-glucuronidation. When the rate of N-glucuronidation was compared under optimal reaction conditions for each compound, most compounds showed higher reactivity with liver microsomes from monkeys than those from rats, except for N2-glucuronidation of MB-tetrazole and MB-1,2,3-triazole. However, the trend for the relative N-glucuronidation reactivity of these compounds by liver microsomes from humans is quite different from those by monkeys and rats. For example, MB-1,2,4-triazole and MB-C4-imidazole are the best substrates for N-glucuronidation (both at Nl) with liver microsomes from humans, whereas MB-tetrazole and MB-1,2,3 triazole are the best substrates for N-glucuronidation (both at N2) with that from rats. Moreover, the predominant glucuronide of MB-1,2,3-triazole produced by liver microsomes from monkeys was at N1, whereas it is at N2 by those from rats and humans. In addition, the formation of both N1- and N2-glucuronides of MB-1,2,3-triazole by liver microsomes increased 3-fold in phenobarbitalinduced rats compared with that by untreated and 3-methylcholanthrene-induced rats. However, N1-glucuronidation of MB-1,2,3-triazole decreased by liver microsomes in dexamethasone-induced rats selectively, whereas N2-glucuronidation was unaffected. These results suggest that at least two isozymes of UDP-glucuronosyltransferase are responsible for N-glucuronidation of MB-1,2,3-triazole, and the species difference in the relative reactivity of these compounds may reflect differences in substrate specificity or differences in their distribution of isozymes among the species studied.