AIM: The present study was designed to define the ki-netic behavior of fluoxetine N-demethylation in human liver microsomes and to identify the isoforms of cy-tochrome P-450 (CYP) involved in this metabolic path-way. METHODS: The kinetics of the formation ofnorfluoxetine was determined in human liver microsomes from six genotyped CYP2C19 extensive metabolizers (EM). The correlation studies between the fluoxetine N-demethylase activity and various CYP enzyme activi-ties were performed. Selective inhibitors or chemical probes of various cytochrome P-450 isoforms were also employed. RESULTS: The kinetics of norfluoxetine formation in all liver microsomes were fitted by a single- enzyme Michaelis-Menten equation (mean Km=32umol/L-7umol/L). Significant correlations were found between N-demethylation of fluoxetine at both 25/anol/L and 100 p.mol/L and 3-hydroxylation of tolbu- tamide at 250 panol/L (rl=0.821, P1=0.001; r2=0.668, P2=0.013), respectively, and S-mephenytoin 4'-hydroxylase activity (r=0.717, P=0.006) at high substrate concentration of 100/zmol/L. S-mephenytoin (SMP)(a CC19 substrate) at high concentration and sulfaphenazole (SUL)(a selective inhibitor of CYP2C9) substantially inhibited norfluoxetine formation. The re-action was minimally inhibited by coincubation with chemical probe, inhibitor of CYP3A4 (tfiacetylolean-domycin, TAO). The inhibition of fluoxetine N-demethylation at high substrate concentration (100ptmoL/L) was greater in PM livers than in EM livers (73% vs4, 5%, P<0.01) when the microsomes were precoincu-bated with SUL plus TAO. CONCLUSION: Cy-tochrome P-450 CYP2C9 is likely to be a major CYP iso-form catalyzing fluoxetine N-demethylation in human liv-er microsomes at a substrate concentration close to the therapeutic level, while polymorphic CYP'2C19 may play a more important role in this metabolic pathway at highsubstrate concentration.

Aims The study was designed to investigate whether genetically determined CYP2C19 activity affects the metabolism of fluoxetine in healthy subjects. Methods A single oral dose of fluoxetine (40mg) was administrated successively to 14 healthy young men with high (extensive metabolizers, n=8) and low (poor metabolizers, n=6) CYP2C19 activity. Blood samples were collected for 5

Aims: To investigate the incidence of the CYP2C19 polymorphism in the Chinese Dai population. Methods: One hundred and ninety-three healthy Chinese Dai volunteers were identified with respect to CYP2C19 by genotype and phenotype analyses. A polymerase chain reaction-restriction fragment length polymorphism method was performed for genotyping procedures. The 4-hydroxymephenytoin (4-OH-MP) and S/R-mephenytoin (S/R-MP) excreted in the urine were determined by high-performance liquid chromatographyand gas chromatography, respectively. Results: Eighteen subjects were identified as poor metabolisers (PMs). The frequency of PMs in the Chinese Dai subjects was 9.3% (95% confidence interval 5.2, 13.4), which is lowerthan that in the Chinese Han population (P<0.05). Chinese Dai subjects had a higher frequency of the mutant CYP2C19*2 allele (0.303) and a lowerfr equency of the mutant CYP2C19*3 allele (0.034). These two mutant alleles could explain all defi-ciencies of CYP2C19 activity in the Chinese Dai subjects. The frequency of the CYP2C193 allele is significantly lowerthan that in the Chinese Han population (P<0.05). The mean S/R ratio was lower in the homozygous extensive metabolisers (EMs) compared with that in heterozygous EMs (P<0.01), and the latter was lowerthan that in the PMs (P<0.01). Furthermore, the mean S/R ratio in CYP2C193/CYP2C192 heterozygous PMs was possibly lower than that in the CYP2C192/CYP2C19*2 homozygous PMs (P<0.05). Conclusion: The frequencies of PMs and CYP2C19*3 allele in the Chinese Dai population are significantly lowerthan those in the Han population. The CYP2C19genotype analysis is largely consistent with the mephenytoin phenotype analysis. The variability of S/R ratios in EMs and PMs shows a gene-dosage e ect.