An gas chromatography-electron-capture detection method has been developed for simultaneous determination of fluoxetine and p-trifluoromethylphenol (TFMP), an O-dealkylated metabolite of fluoxetine in human liver microsomes. Prior to the analysis, aliquots of alkalinized microsomal mixture were extracted with ethyl acetate solvent containing acetonitrile (10%, v/v) and the derivatizing reagent, pentafluorobenzenesulfonyl chloride (0.1%, v/v). The organ phase was retained and taken to dryness, the residue was reconstituted in methanol, and the aliquot of extracts was injected directly into a gas chromatograph equipped with an electron-capture detector. 2, 4 Dichlorophenol was added to the initial incubation mixture and carried through the procedure as the internal standard. The method provided the mean recoveries of up to 103% for fluoxetine and 104% for TFMP. Acceptable relative standard deviations were found for both within-run and day-to-day assays. The practical limit of detection (signal-to-noise ratio53) was 1.62ng/ml for TFMP and 6.92ng/ml for fluoxetine in human liver microsomes, and the limit of quantitation was 8.1 pg for TFMP and 34.6 pg for fluoxetine. The assay is rapid and sensitive and has been applied successfully to simultaneous quantification of fluoxetine and TFMP in human liver microsomes with different CYP2C19 genotypes.

Ethnic differences in drug metabolism are well documented for a number of drugs. The molecular mechanisms responsible for ethnic differences in drug metabolism have been partly clarified because of the advances in molecular biology in recent years. Gene dosage determines the drug metabolism as demonstrated for S-mephenytoin and diazepam metabolism. Genotype analysis indicates a different frequency for the mutant alleles in different ethnic populations, which results in variations in the frequency of subjects who are homozygous for the mutant allele among the extensive metabolizers in different ethnic populations. Ethnic differences in drug metabolism may result from differences in distribution of a polymorphic trait and mutations which code for enzymes with abnormal activity which occur with altered frequency in different ethnic groups.

The study was designed to define the contribution of cytochrome P450 2C19 (CYP2C19) and cytochrome P450 3A4 (CYP3A4) to citalopram N-demethylation and to evaluate the relationship between the disposition of citalopram and CYP2C19 genotype. A single oral 40mg dose of citalopram was administered to eight extensive metabolizers and five poor metabolizers recruited from 77 healthy Chinese volunteers whose genotypes and phenotypes were predetermined. The plasma concentrations of citalopram and desmethylcitalopram were N determined by high-performance liquid chromatography. It was found that the genotype of CYP2C19 had a significant effect on the N-demethylation of citalopram. Poor metabolizers with m1 mutation had higher area under the plasma concentration versus time curve (AUC03) values than did extensive metabolizers. Terminal eliminationelimination half-life (t1/2) values of citalopram in poor metabolizers were significantly higher than the values in extensive metabolizers who were either homozygous or heterozygous with CYP2C19*1. The oral clearance (CLoral) of citalopram in poor metabolizers was significantly lower than that of extensive metabolizers. The AUC03 and maximum plasma concentration (Cmax) of desmethylcitalopram in poor metabolizers were significantly lower than the values of extensive metabolizers. The results show that CYP3A4 is not the major enzyme in the N-demethylation of citalopram among extensive metabolizers. The polymorphism of CYP2C19 plays an important role in the Ndemethylation of citalopram in vivo. The extensive metabolizers and poor metabolizers of CYP2C19 had significant difference in disposition of citalopram in vivo.

Objectives: Our objectives were to determine whether the Gly389 polymorphism of the β1-adrenergic receptor exhibits reduced responsiveness in vivo and to test the hypothesis that the Gly389Arg polymorphism affects the blood pressure and heart rate response to metoprolol. Methods: β1-Adrenergic receptor genotype was determined by polymerase chain reaction-restriction fragment length polymorphism assay. Exercise-induced heart rate increases were compared to determine the functional significance in vivo in 8 healthy Chinese men homozygous for Gly389 and 8 homozygous for Arg389. All of the subjects were given 25, 50, or 75mg of metoprolol every 8 hours; the dosages were given in a random order, and each dosage was given for β1 day. The degree of β-blockade was measured as the reduction in resting and exercise heart rates and blood pressures. Plasma metoprolol concentrations were measured by the use of HPLC-fluorescence detection. Results: Exercise led to a workload-dependent increase in heart rate. There were no differences in exerciseinduced heart rate increases between Arg389 and Gly389 homozygotes. Oral metoprolol caused significant dose-dependent decreases in both resting and exercise heart rates in both groups. The reductions in the resting heart rate in 3 dosage levels of metoprolol were 6.3%±0.8% versus 4.1% 0.7%, 10.1%±1.0% versus 6.2%±1.1%, and 14.4%±1.4% versus 10.9%±1.3% in homozygous Arg389 subjects and Gly389 subjects, respectively (P=.008). We also found differences with respect to the exercise heart rate (8.9%±0.5%, 14.0%±0.9%, and 20.1%±1.5% in Arg389 subjects and 6.6%±0.7%, 11.7%±1.0%, and 16.4%±1.3% in Gly389 subjects; P=.017) and systolic pressure (5.9%±0.7%, 9.2%±1.0%, and 11.6% 1.2% in Arg389 subjects and 4.6%±0.5%, 6.0%±0.8%, and 9.9%±0.9% in Gly389 subjects; P=.011). However, the difference in the fall in diastolic pressure was not statistically significant (P=.442).