O-Dealkylation of fluoxetine in relation to CYP2C19 gene dose and involvement of CYP3A4 in human liver microsomes

Analyzing the metabolic fate of oral administration drugs: A review and state-of-the-art roadmap

The key orally delivered drug metabolism processes are reviewed to aid the assessment of the current in vivo/vitro experimental systems applicability for evaluating drug metabolism and the interaction potential. Orally administration is the most commonly used state-of-the-art road for drug delivery due to its ease of administration, high patient compliance and cost-effectiveness. Roles of gut metabolic enzymes and microbiota in drug metabolism and absorption suggest that the gut is an important site for drug metabolism, while the liver has long been recognized as the principal organ responsible for drugs or other substances metabolism. In this contribution, we explore various experimental models from their development to the application for studying oral drugs metabolism of and summarized advantages and disadvantages. Undoubtedly, understanding the possible metabolic mechanism of drugs in vivo and evaluating the procedure with relevant models is of great significance for screening potential clinical drugs. With the increasing popularity and prevalence of orally delivered drugs, sophisticated experimental models with higher predictive capacity for the metabolism of oral drugs used in current preclinical studies will be needed. Collectively, the review seeks to provide a comprehensive roadmap for researchers in related fields.

Assessing the Mechanism of Fluoxetine-Mediated CYP2D6 Inhibition

Fluoxetine is still one of the most widely used antidepressants in the world. The drug is extensively metabolized by several cytochrome P450 (CYP450) enzymes and subjected to a myriad of CYP450-mediated drug interactions. In a multidrug regimen, preemptive mitigation of drug-drug interactions requires knowledge of fluoxetine actions on these CYP450 enzymes. The major metabolic pathway of fluoxetine leading to the formation of its active metabolite, norfluoxetine, is mediated by CYP2D6. Fluoxetine and norfluoxetine are strong affinity substrates of CYP2D6 and can inhibit, potentially through various mechanisms, the metabolism of other sensitive CYP2D6 substrates. Remarkably, fluoxetine-mediated CYP2D6 inhibition subsides long after fluoxetine first passes through the liver and even remains long after the discontinuation of the drug. Herein, we review pharmacokinetic and pharmacogenetic information to help us understand the mechanisms underlying the prolonged inhibition of CYP2D6 following fluoxetine administration. We propose that long-term inhibition of CYP2D6 is likely a result of competitive inhibition. This is due to strong affinity binding of fluoxetine and norfluoxetine to the enzyme and unbound fluoxetine and norfluoxetine levels circulating in the blood for a long period of time because of their long elimination half-life. Additionally, we describe that fluoxetine is a CYP2C9 substrate and a mechanism-based inhibitor of CYP2C19.

Drug Interaction of Fluvoxamine and Fluoxetine with Nevirapine in HIV-1-Infected Individuals

OBJECTIVE

To evaluate the possible pharmacokinetic interactions between nevirapine and fluvoxamine or fluoxetine in patients with HIV-1 infection.

PATIENTS AND METHODS

Patients who were using fluvoxamine or fluoxetine concomitantly were chosen from an unselected cohort (n = 173) of HIV-1-infected individuals using a nevirapine-containing regimen (study group). HIV-1-infected patients using nevirapine without fluvoxamine or fluoxetine and non-HIV-infected individuals who were using fluvoxamine and fluoxetine were included as controls. The influence of fluvoxamine and fluoxetine on the pharmacokinetics of nevirapine was investigated with a previously developed population pharmacokinetic model. Concomitant use of fluvoxamine or fluoxetine was tested independently as covariate for apparent clearance (CL/F) of nevirapine using nonlinear mixed-effect modelling (NONMEM). Furthermore, to explore the influence of nevirapine on the pharmacokinetics of fluvoxamine and fluoxetine, dose-normalised concentrations of fluvoxamine and fluoxetine from the study group were compared with those of the controls.

RESULTS

Of the 173 HIV-1-infected individuals, 14 were using fluoxetine (n = 7) or fluvoxamine (n = 7) simultaneously with nevirapine. In addition, 17 and 29 individuals were identified as controls for the fluoxetine- and fluvoxamine-group, respectively. Concomitant use of fluvoxamine resulted in a significant reduction of 33.7% in CL/F of nevirapine; this reduction in CL/F appeared to be dose-dependent. Concomitant use of fluoxetine had no influence on the pharmacokinetics of nevirapine. Conversely, nevirapine significantly lowered plasma levels of fluoxetine plus norfluoxetine (seproxetine). In contrast, no significant difference was observed in dose-normalised concentrations of fluvoxamine when the controls were compared with the study group.

CONCLUSION

We advise that special attention is paid to HIV-1-infected indivi-duals using a nevirapine-containing regimen and fluvoxamine or fluoxetine con-comitantly, since pharmacokinetic interactions have been observed.

Cross-species comparison of fluoxetine metabolism with fish liver microsomes

Pharmaceuticals discharged in municipal wastewater are of emerging concern because of their potential for inducing biological effects in aquatic organisms. Selective serotonin reuptake inhibitors (SSRIs), pharmaceuticals prescribed to treat chronic depression, have been detected in receiving and wastewaters. Fluoxetine is a highly prescribed model SSRI used to assess impacts of antidepressants on aquatic organisms. In this study, in vitro hepaticfluoxetine metabolism was determined in several model fish species: rainbow trout, goldfish, zebrafish and killifish. Incubation of fluoxetine with hepatic microsomes from trout pre-treated with carbamazepine showed a time-dependant loss of fluoxetine, concomitant with an increase in norfluoxetine, the major mammalian demethylated metabolite. However, fluoxetine was not well metabolized in reactions with hepatic microsomes from untreated fish. Fluoxetine loss was greater than norfluoxetine production, indicating that norfluoxetine is not the predominant fluoxetine biotransformation product in fish. Furthermore, norfluoxetine was often undetected, possibly indicating that fluoxetine demethylation is a minor metabolic pathway in fish. Inter-species differences in fluoxetine metabolism were not evident because of high intra-species variability, although killifish appeared to have the highest hepatic metabolic capacity for fluoxetine. Fluoxetine metabolism in mammals is catalyzed by cytochrome P450 (CYP) enzymes. Trout were exposed to knownCYP inducers, carbamazepine and 3-methylcholanthrene, to assess potential induction of hepatic fluoxetine metabolism. Microsomes from 3-methylcholanthrene treated fish did not induce detectable changes in fluoxetine concentrations in vitro, indicating that fish CYP1s are not involved in fluoxetine metabolism; the CYPs involved are still unclear. Identification of metabolites other than norfluoxetine warrants further investigation.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Differential time- and NADPH-dependent inhibition of CYP2C19 by enantiomers of fluoxetine

Fluoxetine [+/--N-methyl-3-phenyl-3-[(alpha, alpha, (-trifluoro-p-tolyl)oxy]-propylamine)] a selective serotonin reuptake inhibitor, is widely used in treating depression and other serotonin-dependent disease conditions. Racemic, (R)- and (S)-fluoxetine are potent reversible inhibitors of CYP2D6, and the racemate has been shown to be a mechanism-based inhibitor of CYP3A4. Racemic fluoxetine also demonstrates time- and concentration-dependent inhibition of CYP2C19 catalytic activity in vitro. In this study, we compared fluoxetine, its (R)- and (S)-enantiomers, ticlopidine, and S-benzylnirvanol as potential time-dependent inhibitors of human liver microsomal CYP2C19. In a reversible inhibition protocol (30 min preincubation with liver microsomes without NADPH), we found (R)-, (S)- and racemic fluoxetine to be moderate inhibitors with IC(50) values of 21, 93, and 27 microM, respectively. However, when the preincubation was supplemented with NADPH, IC(50) values shifted to 4.0, 3.4, and 3.0 microM, respectively resulting in IC(50) shifts of 5.2-, 28-, and 9.3-fold. Ticlopidine showed a 1.8-fold shift in IC(50) value, and S-benzylnirvanol shifted right (0.41-fold shift). Follow-up K(I) and k(inact) determinations with fluoxetine confirmed time-dependent inhibition [K(I) values of 6.5, 47, and 14 microM; k(inact) values of 0.023, 0.085, 0.030 min(-1) for (R)-, (S)-, and racemate, respectively]. Although the (S)-isomer exhibits a much lower affinity for CYP2C19 inactivation relative to the (R)-enantiomer, it exhibits a more rapid rate of inactivation. Racemic norfluoxetine exhibited an 11-fold shift (18-1.5 microM) in IC(50) value, suggesting that conversion of fluoxetine to this metabolite represents a metabolic pathway leading to time-dependent inhibition. These data provide an improved understanding of the drug-interaction potential of fluoxetine.

Characterization and analyses of multidrug resistance-associated protein 1 (MRP1/ABCC1) polymorphisms in Chinese population

OBJECTIVE

To explore the distribution frequencies of four common single nucleotide polymorphisms (SNPs) of MRP1/ABCC1 in a mainland Chinese population and investigate whether these SNPs affect the expression and function of the MRP1/ABCC1.

METHODS

The genotype of 208 healthy volunteers was determined using PCR-restriction fragment length polymorphism. The four candidated SNPs were recreated by site-directed mutagenesis and tested for their effect on MRP1/ABCC1 expression and multidrug resistance function in stable transfected HEK293 and CHO-K1 cell lines. Real-time PCR, western blot and confocal microscopy were used to determine the mRNA, protein expression, and protein trafficking. At last, the effect of mutations on MRP1/ABCC1-mediate drug resistance was determined using methyl thiazolyl tetrazolium assay.

RESULTS

The allelic frequencies of Cys43Ser (128G>C), Thr73Ile (218C>T), Arg723Gln (2168G>A), and Arg1058Gln (3173G>A) in mainland Chinese were 0.5, 1.4, 5.8, and 0.5%, respectively. None of these mutations had any effect on MRP1/ABCC1 expression and trafficking, but that Arg723Gln mutation significantly reduced MRP1/ABCC1-mediated resistance to daunorubicin, doxorubicin, etoposide, vinblastine, and vincristine. The Cys43Ser mutation did not affect all tested drug resistance. In contrast, the Thr73Ile mutation reduced resistance to methotrexate and etoposide, whereas the Arg1058Gln mutation increased the response of two anthracycline drugs and etoposide in HEK293 and CHO-K1 cells as well as vinblastine and methotrexate in CHO-K1 cells.

CONCLUSION

The allelic frequency of the Arg723Gln mutation is relatively higher than other SNPs in mainland Chinese population and therefore this mutation significantly reduces MRP1/ABCC1 activity in multidrug resistance.

Influence of CYP2C9, 2C19 and 2D6 genetic polymorphisms on the steady-state plasma concentrations of the enantiomers of fluoxetine and norfluoxetine

The antidepressant fluoxetine is administered as racemic mixture of two enantiomers (S- and R-fluoxetine). While S- and R-fluoxetine are equipotent in blocking serotonin reuptake, the enantiomers of the demethylated metabolite, norfluoxetine, show marked differences in pharmacological activity, S-norfluoxetine being about 20 times as potent as R- norfluoxetine as a serotonin reuptake inhibitor. In vitro and in vivo data suggest that the metabolism of fluoxetine to norfluoxetine is stereoselective and mediated, at least in part, by the polymorphic cytochrome P450 (CYP) isoenzymes CYP2D6, CYP2C9 and CYP2C19. In the present study, the influence of CYP2D6, CYP2C9 and CYP2C19 polymorphisms on the steady-state plasma concentrations of fluoxetine and norfluoxetine enantiomers was evaluated in 78 patients receiving chronic fluoxetine treatment (10-60 mg/day). The plasma concentrations of fluoxetine and norfluoxetine enantiomers were measured and CYP2D6, CYP2C9 and CYP2C19 genotypes were analyzed. No statistically significant relationship was identified between CYP2D6 or CYP2C19 genotypes and the dose normalised plasma concentrations of any of the enantiomers or the active moiety (i.e. the sum of S-fluoxetine, R-fluoxetine and S-norfluoxetine). However, the plasma concentration of S-norfluoxetine was very low in the only CYP2D6 poor metaboliser. Furthermore, the median S-norfluoxetine/S-fluoxetine ratios were higher in homozygous than in heterozygous extensive metabolisers (P<0.05). Among homozygous extensive metabolizers for CYP2D6, patients homozygous for CYP2C9*1 had lower dose-normalized R-fluoxetine concentrations and lower active moiety levels compared with those carrying detrimental CYP2C9 alleles (P<0.05). These results suggest that CYP2D6 and CYP2C9 polymorphisms contribute to the interindividual variability in fluoxetine pharmacokinetics at steady-state.

Simultaneous determination of fluoxetine and its metabolite p-trifluoromethylphenol in human liver microsomes using a gas chromatographic-electron-capture detection procedure

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 ratio=3) was 1.62 ng/ml for TFMP and 6.92 ng/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.