(S)OXAZEPAM GLUCURONIDATION IS INHIBITED BY KETOPROFEN AND OTHER SUBSTRATES OF UGT2B7

1,4-Benzodiazepine anxiolytics such as diazepam and halazepam are conv erted in vivo to oxazepam, an active metabolite with a hydroxyl group at the asymmetric C3 position, D-glucuronic acid couples with the C3 h ydroxyl group of oxazepam to form pharmacologically inactive diastereo meric glucuronide conjugates. Conjugation with glucuronic acid is cata lysed by the microsomal UDP-glucuronosyltransferase (UGT) enzyme syste m, which includes an undetermined number of isozymes, Although 1,4-ben zodiazepines are ultimately cleared as oxazepam glucuronide, little is known about the particular UGT isozyme(s) responsible for the conjuga tion at the C3 position of these molecules. Microsomal preparations fr om three human livers were used to study the glucuronidation of (R,S)o xazepam in vitro, The predominant formation of the S- over the R-glucu ronide was reflected by the kinetic parameters: For (S)oxazepam glucur onide, the constants were K-m = 0.18 +/- 0.02 mM and V-max = 202.6 +/- 25.0 nmol min(-1) per mg protein; for (R)oxazepam glucuronide, they w ere K-m = 0.22 +/- 0.02 mM, V-max = 55.4 +/- 9.5 nmol min(-1) per mg p rotein. Inhibition studies suggest that the two diastereomeric glucuro nidations are catalysed by different UGT isozymes. That is, there was competitive inhibition of (S)oxazepam glucuronidation by non-steroidal anti-inflammatory drugs (NSAIDs), including ketoprofen while (R)oxaze pam glucuronidation was not equally inhibited by these compounds. The order of potency of inhibitors of (S)oxazepam glucuronidation in this study was the same as the rank order of substrates conjugated by UGT2B 7; hyodeoxycholic acid, estriol, (S)naproxen, ketoprofen, ibuprofen, f enoprofen, clofibric acid, and morphine (in descending order), The inh ibition profile of (S)oxazepam glucuronidation suggests that UGT2B7 is the catalysing enzyme.