N-Glucuronidation of Some 4-Arylalkyl-1H-Imidazoles by Rat, Dog, and Human Liver Microsomes

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Abstract
N-Glucuronidation in vitro of six 4-arylalkyl-1H-imidazoles (both enantiomers of medetomidine, detomidine, atipamezole, and two other closely related compounds) by rat, dog, and human liver microsomes and by four expressed human UDP-glucuronosyltransferase isoenzymes was studied. Human liver microsomes formed N-glucuronides of 4-arylalkyl-1H-imidazoles with high activity, with apparent Vmax values ranging from 0.59 to 1.89 nmol/min/mg of protein. In comparison, apparentVmax values for two model compounds forming the N-glucuronides 4-aminobiphenyl and amitriptyline were 5.07 and 0.56 nmol/min/mg of protein, respectively. Atipamezole showed an exceptionally low apparent Kmvalue of 4.0 μM and a high specificity constant (Vmax/Km) of 256 compared with 4-aminobiphenyl (Km, 265 μM;Vmax/Km, 19) and amitriptyline (Km, 728 μM;Vmax/Km, 0.8).N-Glucuronidation of medetomidine was highly enantioselective in human liver microsomes; levomedetomidine exhibited a 60-fold Vmax/Kmvalue compared with dexmedetomidine. Furthermore, two isomeric imidazole N-glucuronides were formed from dexmedetomidine, but only one was formed from levomedetomidine. Dog liver microsomes formed N-glucuronides of 4-arylalkyl-1H-imidazoles at a low rate and affinity, with apparent Vmax values ranging from 0.29 to 0.73 nmol/min/mg of protein and apparentKm values from 279 to 1640 μM. Rat liver microsomes glucuronidated these compounds at a barely detectable rate. Four expressed human UDP-glucuronosyltransferase isoenzymes (UGT1A3, UGT1A4, UGT1A6, and UGT1A9) were studied for 4-arylalkyl-1H-imidazole-conjugating activity. Only UGT1A4 glucuronidated these compounds at an activity of about 5% of that measured for 4-aminobiphenyl. The observed activity of UGT1A4 does not explain the high efficiency of glucuronidation of 4-arylalkyl-1H-imidazoles in human liver microsomes.