Aims To identify the cytochrome P450 (CYP) and UDP-glucuronosyltransferase (UGT) isoforms responsible for the formation of the primary metabolite(s) of zaltoprofen, and to predict possible drug interactions by investigating the inhibition of CYP isoforms in vitro . Methods The metabolism of zaltoprofen was studied in vitro using recombinant CYP and UGT isoform cDNA-expression systems. The effects of selective isoform inhibitors on zaltoprofen metabolism were studied using human liver microsomes. The inhibitory effects of zaltoprofen on the metabolism of selective probe substrates for CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4 were also determined in human liver microsomes. Results Zaltoprofen was extensively metabolized by CYP2C9 and UGT2B7. CYP2C9 catalysed sulphoxidation but not hydroxylation of zaltoprofen. In the human liver microsomal metabolism study, zaltoprofen metabolism was markedly inhibited by sulphaphenazole, a selective inhibitor of CYP2C9. In the drug interaction study, negligible inhibition ( < 15%) of the activities of CYP1A2, CYP2C19, CYP2D6, CYP2E1 and CYP3A4 was apparent at 5 µ g ml − 1 , the maximum plasma concentration observed in humans after oral administration of an 80 mg zaltoprofen tablet. However, zaltoprofen inhibited CYP2C9 by 26% at 5 µ g ml − 1 . At higher concentrations, zaltoprofen produced some inhibition of CYP2C9 (I C 50 = 19.2 µ g ml − 1 ; 64.4 µ M ) and CYP3A4 (I C 50 = 53.9 µ g ml − 1 ; 181 µ M ). The free drug concentrations in plasma (0.02 µ g ml − 1 , 67.0 n M ) at the C max of the clinically effective doses are much lower than the I C 50 values corrected for the nonspecific binding ratio of zaltoprofen to microsomal protein (15.5 µ g ml − 1 for CYP3A4, 49.5 µ g ml − 1 for CYP3A4). Furthermore, the maximum free drug concentrations in the hepatic intracellular was calculated to be 0.068 µ g ml − 1 and the increase in the AUC in the presence of zaltoprofen was estimated to be only 0.4% for CYP2C9 substrates and 0.1% for CYP3A4 substrates, respectively. Conclusions Zaltoprofen is predominantly metabolized by CYP2C9 and UGT2B7, and is considered unlikely to cause significant drug interactions in vivo when coadministered with CYP substrates at clinically effective doses.Keywords: CYP2C9, human liver microsomes, in vitro metabolism, UGT2B7, zaltoprofen S. Furuta et al. 296

The synthesis of [2, leucine-enkephalin by the catalytic tritiation of [2,6-dibromo-Tyr'] leucine-enkephalin is described. The precursor amino acid, 2,6-dibromo-~~-tyrosine, was synthesized in three steps from 2,6-di bromo-4-methoxytoluene. The protected [ 2,6-di bromo-Tyrl] leucine-enkephalin derivative was prepared by solid-phase synthesis, followed by epimeric resolution on H PLC. The peptide was tritiated catalytically to yield [3H -Tyrl] leucine-enkephalin with a specific radioactivity of 1.37 TBq/mmol. The distribution of tritium was investigated by H PLC with radioisotope detection following enzymatic hydrolysis, and confirmed that the tritium label was entirely located at the tyrosine residue.