CHARACTERIZATION OF THE IN-VITRO GLUCURONIDATION OF FLURBIPROFEN ENANTIOMERS

To investigate the glucuronidation of the R- and S-enantiomers of the nonsteroidal antiinflammatory drug, flurbiprofen, by liver microsomes of several mammals, including humans, a new and reliable HPLC method f or the separation and quantification of the corresponding diastereoiso meric glucuronides has been developed. Interspecies comparison reveale d that the glucuronidation of flurbiprofen was highly efficient with l iver microsomes of humans, monkeys, rats, and guinea pigs (in decreasi ng ranking order). Gunn rats, which present a genetic defect in the bi lirubin UDP-glucuronosyltransferase (UGT) isoforms, were still able to glucuronidate the drug. The R-enantiomer was glucuronidated faster th an the S-form by liver microsomes of rats and humans. Although the K-M of glucuronidation of R- and S-enantiomers by rat liver UGT were in s ame order of magnitude (apparent K-M 0.52 and 0.57 mM, respectively), the apparent V-max's were significantly different (9.34 and 5.48 nmol/ min.mg of protein). Regardless of the enantiomer considered, the glucu ronidation of flurbiprofen was strongly increased up to 5-fold by trea tment of rats with phenobarbital and, at a lower extent, by 3-methyich olanthrene. In contrast, the treatment of rats with ciprofibrate marke dly decreased the activity. Glucuronidation of R-flurbiprofen was more enhanced by phenobarbital than that of the S-antipode. Each flurbipro fen enantiomer could weakly inhibit the glucuronidation of its antipod e in a noncompetitive way. The apparent K-i was 0.51 mM with R-flurbip rofen as a substrate, and 0.37 mM with S-enantiomer. On the other hand , the rat liver UGT2B1 isoform, stably expressed in V79 cells, could g lucuronidate flurbiprofen in an appreciable amount. Results indicated that more than one UGT isoforms are involved in the glucuronidation of R- and S-flurbiprofen in rat liver microsomes.