MDR1 haplotypes do not affect the steady-state pharmacokinetics of cyclosporine in renal transplant patients

This retrospective study investigated the impact of MDR1 haplotypes derived from the single-nucleotide polymorphisms (SNPs) 2677G>T (exon 21) and 3435C>T (exon 26) on the pharmacokinetics of cyclosporine in 98 renal transplant patients. Based on SNPs 2677 and 3435, four different haplotypes and nine different genotypes were identified in the study sample. Frequencies of SNPs, genotypes, and haplotypes were in agreement with previously reported values. Cyclosporine pharmacokinetics were characterized using a 2-hour AUC (AUC0-12), trough concentrations (C0), and blood concentrations 2 hours after cyclosporine administration (C2). No significant differences in dose-corrected AUC0-12, C0, or C2 values were observed between carriers of different SNP variants and genotypes (Kruskal-Wallis test), as well as between carriers and noncarriers of each haplotype (Mann-Whitney U test). Carriers of haplotype 12 (2677G and 3435T), which has previously been associated with increased digoxin AUC values, had a median AUC0-12 of 18.9 micro g*h*L-1 (range: 9.0-35.2) compared to 17.5 micro g*h*L-1 (range: 7.5-37.1) in the noncarrier group. It was concluded that MDR1 haplotypes derived from the SNPs 2677G>T (exon 21) and 3435C>T (exon 26) are not associated with cyclosporine pharmacokinetics in renal transplant patients.

Influence of CYP2C9 genetic polymorphisms on pharmacokinetics of celecoxib and its metabolites

In-vitro data indicate major effects of the genetically polymorphic cytochrome P450 enzyme 2C9 (CYP2C9) on the pharmacokinetics of celecoxib, a nonsteroidal anti-inflammatory drug acting as selective cyclooxygenase-2 inhibitor. Human studies report decreased clearance in heterozygous carriers of the CYP2C9 variant Ile359Leu (*3), but results appeared controversial and only data on single subjects carrying the homozygous CYP2C9*3/*3 genotype have been published. We measured single-dose kinetics of celecoxib and its main metabolites hydroxy- and carboxy-celecoxib in 21 healthy volunteers who were selected as hetero- (n = 4) and homozygous (n = 3) carriers of CYP2C9 variants Arg144Cys (*2) and Ile359Leu (*3). Blood concentrations of celecoxib and its metabolites hydroxy-celecoxib and carboxy-celecoxib were quantified by high-performance liquid chromatography. A more than two-fold reduced oral clearance in homozygous carriers of CYP2C9*3 was seen for celecoxib compared to carriers of the wild-type genotype CYP2C9*1/*1 and heterozygous carriers of one *3 allele were in-between (P = 0.003 for trend), whereas CYP2C9*2 had no significant influence on celecoxib pharmacokinetics. Decreased concentrations of carboxy- and hydroxy-celecoxib in heterozygous and homozygous carriers of CYP2C9*3 were detected which supported the influence of CYP2C9 polymorphisms on celecoxib pharmacokinetic variability. Approximately 0.5% of Caucasians carrying the genotype CYP2C9*3/*3 will have greatly increased internal exposure to celecoxib. It remains to be shown whether this is associated with greater efficacy or with an increased incidence and severity of adverse events.

Rapid assessment of P-glycoprotein inhibition and induction in vitro

PURPOSE

Using rhodamine123 (RH123) cell exclusion. 17 clinically used compounds were screened for their inhibitory effect on P-glycoprotein (P-gp), which was compared with the drugs' inhibitory activity against CYP3A4. The same assay was used to study induction of P-gp activity.

METHODS

P-gp inhibition was assessed using RH123 accumulation into LS180V cells as well as Rh123 transport across Caco-2 mono-layers. Inhibition of CYP3A4 was determined in human liver microsomes using triazolam-4-hydroxylation. Induction of P-gp expression and activity was measured using western blot analysis and RH123 accumulation into LS180V cells, respectively.

RESULTS

The observed inhibition of RH123 cell exclusion ranged from little or no effect (digoxin, indinavir, fexofenadine) up to a nearly 10-fold increase in RH 123 accumulation (ivermectin, terfenadine). No correlation between P-gp and CYP3A4 inhibition was observed. The rank order in P-gp inhibitory potency for terfenadine, verapamil, ritonavir. and indomethacin was identical in both LS180V and Caco-2 models. Ritonavir and St. John's wort extract showed a concentration-dependent P-gp induction, with good correlation between western blot analysis and RH123 accumulation.

CONCLUSIONS

The RH123 accumulation assay in LS180V cells can be used as a valuable screening tool to study both inhibition and induction of P-gp activity and expression. This assay has the potential to predict P-gp-mediated alterations in intestinal absorption of drugs.

Impact of St John's wort treatment on the pharmacokinetics of tacrolimus and mycophenolic acid in renal transplant patients

BACKGROUND

This study investigated the effect of St John's wort (SJW) extract on the pharmacokinetics of the immunosuppressants tacrolimus (TAC) and mycophenolic acid (MPA).

METHODS

Ten stable renal transplant patients received 600 mg SJW extract for 14 days in addition to their regular regimen of TAC and mycophenolate mofetil.

RESULTS

Dose-corrected AUC((0-12)) of TAC decreased significantly from 180 ng/ml/h at baseline to 75.9 ng/ml/h after 2 weeks of SJW treatment. To maintain therapeutic TAC concentrations, dose adjustments from a median 4.5 mg/day at baseline to 8.0 mg/day under SJW treatment were required. Two weeks after discontinuation of SJW, TAC doses were reduced to a median of 6.5 mg/day. MPA pharmacokinetics remained unaffected by comedication with hypericum extract.

CONCLUSIONS

Administration of SJW extract to patients receiving TAC treatment can result in a serious drug interaction leading to markedly reduced TAC blood concentrations associated with the risk of organ rejection.

Rapid and simple method for the analysis of nateglinide in human plasma using HPLC analysis with UV detection

Nateglinide (NA) is a novel oral mealtime glucose regulator, recently approved for the treatment of type II diabetes mellitus. To facilitate clinical studies investigating the dependence of NA elimination on the genotype of cytochrome P450 isoenzymes, we developed a rapid HPLC method for determination of NA in human plasma samples. The validated limit of quantitation (LOQ) of 0.1 microg/ml is low enough to allow determination of pharmacokinetic parameters of the substance. The intra-assay coefficients of variation (CV) ranged from 1.6 to 12.9% at NA concentrations of 0.5-7.5 microg/ml. The inter-assay variation for the same plasma concentrations ranged from 3.8 to 8.4%. The calibration was linear in the range of 0.1-20 microg/ml. For the quantitation of NA, only 50 microl of plasma were needed. Following protein precipitation in human plasma, the samples were separated by isocratic reversed phase HLPC and analyzed using ultraviolet detection at 210 nm. Sample preparation time and analysis time are both short and allow rapid analysis of large sample sets.

Alterations in cyclosporin A pharmacokinetics and metabolism during treatment with St John's wort in renal transplant patients

AIM

This study investigated the effects of St John's wort extract (SJW) on the pharmacokinetics and metabolism of the immunosuppressant cyclosporin A (CSA).

METHODS

In an open-label study, 11 renal transplant patients received 600 mg SJW extract daily for 14 days in addition to their regular regimen of CSA. Blood concentrations of CSA and its metabolites AM1, AM1C, AM9, AM19, and AM4N were measured by HPLC.

RESULTS

After 2 weeks of SJW coadministration, dose-corrected AUC0-12, Cmax and Ctrough values for CSA decreased significantly by 46%[geometric mean ratio baseline/SJW (95% CI): 1.83 (1.63-2.05)], 42%[1.72 (1.42-2.08)], and 41%[1.70 (1.17-2.47)], respectively. CSA doses were increased from a median of 2.7 mg day(-1) kg(-1) at baseline to 4.2 mg day(-1) kg(-1) at day 15, with the first dose adjustment required only 3 days after initiation of SJW treatment. Additionally, the metabolite pattern of CSA was substantially altered during SJW treatment. Whereas dose-corrected AUC values for AM1, AM1c and AM4N significantly decreased by 59%, 61%, and 23% compared with baseline, AUC values for AM9 and AM19 were unchanged. Following the increase in CSA dose, observed AUC and Cmax values for AM9, AM19, and AM4N increased by 20-51% and 43-90%, respectively.

CONCLUSION

Administration of SJW extract to patients receiving CSA treatment resulted in a rapid and significant reduction of plasma CSA concentrations. Additionally, the substantial alterations in CSA metabolite kinetics observed may affect the toxicity profile of the drug.

Simultaneous determination of celecoxib, hydroxycelecoxib, and carboxycelecoxib in human plasma using gradient reversed-phase liquid chromatography with ultraviolet absorbance detection

A new HPLC method for the simultaneous determination of celecoxib, carboxycelecoxib and hydroxycelecoxib in human plasma samples has been developed. Following a solid-phase extraction procedure, the samples were separated by gradient reversed-phase HLPC (C(18)) and quantified using UV detection at 254 nm. The method was linear over the concentration range 10-500 ng/ml. The intra-assay variability for the three analytes ranged from 4.0 to 12.6% and the inter-assay variability from 4.9 to 14.2%. The achieved limits of quantitation (LOQ) of 10 ng/ml for each analyte allowed the determination of the pharmacokinetic parameters of the analytes after administration of 100 mg celecoxib.

Differential effects of Saint John's Wort (hypericum perforatum) on the urinary excretion of D-glucaric acid and 6beta-hydroxycortisol in healthy volunteers

OBJECTIVE

We investigated the effects of treatment with Saint John's wort (hypericum perforatum) extract on the urinary excretion of D-glucaric acid, 6beta-hydroxycortisol, and free cortisol in order to assess the effect of this extract on the activity of hepatic xenobiotic metabolizing enzymes.

METHODS

Forty-eight healthy volunteers (25 male and 23 female) received a daily dose of 1800 mg hypericum extract for 14 days. Urinary excretion of D-glucaric acid, 6beta-hydroxycortisol, and free cortisol was measured in 24-h urine samples on the day preceding the initiation of hypericum treatment and after 14 days of treatment. D-Glucaric acid was measured enzymatically. Cortisol and 6beta-hydroxycortisol were quantified using high-performance liquid chromatography with ultraviolet detection.

RESULTS

Urinary excretion of D-glucaric acid was unaffected after a 14-day treatment with Saint John's wort extract (26.7 micromol/day vs 27.7 micromol/day; 95% confidence interval of the difference: -1.9 to 3.8). The urinary excretion of 6beta-hydroxycortisol increased from a mean baseline value of 254 microg/day to 369 microg/day (P<0.0001) indicating induction of CYP3A. While the excretion of free cortisol was unaltered, the ratio of 6beta-hydroxycortisol to free cortisol changed significantly from 9.9 at baseline to 14.3 (95% confidence interval of the difference: 2.3-6.5) after Saint John's wort treatment.

CONCLUSIONS

High-dose treatment with Saint John's wort extract induced CYP3A activity in healthy volunteers as evidenced by increased 6beta-hydroxycortisol excretion. This enzyme induction most likely contributes to the decreased bioavailability observed upon co-administration of various drugs with Saint John's wort extract. The D-glucuronic acid pathway appeared unaffected by Saint John's wort.

Differential modulation of P-glycoprotein expression and activity by non-nucleoside HIV-1 reverse transcriptase inhibitors in cell culture

PURPOSE

This study investigated the effects of the non-nucleoside HIV-1 reverse transcriptase inhibitors (NNRTI) nevirapine (NVR), efavirenz (EFV), and delavirdine (DLV) on P-glycoprotein (P-gp) activity and expression to anticipate P-gp related drug-drug interactions associated with combination therapy.

METHODS

NNRTIs were evaluated as P-gp substrates by measuring differential transport across Caco-2 cell monolayers. Inhibition of P-gp mediated rhodaminel23 (Rh123) transport in Caco-2 cells was used to assess P-gp inhibition by NNRTIs. Induction of P-gp expression and activity in LS180V cells following 3-day exposure to NNRTIs was measured by western blot analysis and cellular Rh123 uptake, respectively.

RESULTS

The NNRTIs showed no differential transport between the basolateral to apical and apical to basolateral direction. NNRTI transport in either direction was not affected by the P-gp inhibitor verapamil. DLV inhibited Rh123 transport, causing a reduction to 15% of control at 100 microM (IC50 = 30 microM). NVR caused a concentration-dependent induction of P-gp expression in LS180V cells resulting in a 3.5-fold increase in immunoreactive P-gp at 100 microM NVR. Induction attributable to EFV and DLV was quantitatively smaller. NVR significantly reduced cellular uptake of Rh123 into LS180V cells, indicating increased drug efflux due to induced P-gp activity; effects of EFV and DLV were smaller. Acute DLV treatment of LS180V cells previously induced with NVR or ritonavir did not reverse the decreased Rh123 cell accumulation.

CONCLUSIONS

NNRTIs show differential effects on P-gp activity and expression in vitro. Clinical studies are required to elucidate the clinical importance of potential drug interactions.

Simultaneous determination of ondansetron and tropisetron in human plasma using HPLC with UV detection

A rapid and sensitive HPLC method for the simultaneous quantitation of ondansetron and tropisetron, two serotonin (5-HT) receptor antagonists frequently used in treatment and prevention of nausea and emesis, is described. The procedure involves liquid-liquid extraction of human plasma with dichloromethane coupled with reversed-phase HPLC and UV detection. The lower limits of quantification (LOQ) were 0.62 ng/mL for ondansetron and 1.25 ng/mL or tropisetron. Intra- and inter-assay coefficients of variation ranged from 1.5 to 7.5% and 5.3 to 13.7%, respectively. The sensitivity and precision were sufficient for determination of plasma concentrations after therapeutic administration of both drugs and the method can be used for the estimation of pharmacokinetic parameters.

Decreased plasma levels of amitriptyline and its metabolites on comedication with an extract from St. John's wort ( Hypericum perforatum )

Extracts of St. John's wort ( Hypericum perforatum ) became increasingly popular as easily available remedies for mild to moderate depression. Comedication with hypericum extract was recently shown to drastically reduce plasma concentration of ciclosporin, digoxin, and indinavir. We investigated the possible interaction of hypericum extract LI160 with amitriptyline. Both antidepressants have a high probability of concomitant use. Twelve patients requiring amitriptyline treatment received a single dose of hypericum extract (900 mg) at day 1, continued by a 12-to 14-day treatment with retarded amitriptyline (75 mg twice daily). Then hypericum (900 mg/day) was added for another 14 to 16 days. Steady-state pharmacokinetics of amitriptyline were compared before and after multiple-dose treatment with hypericum extract. Furthermore, comparisons were made for single-dose kinetics of hypericum-extract ingredients hypericin, pseudohypericin, and hyperforin between the first day of concomitant treatment and LI160 alone. Multiple-dose comedication with LI160 led to a statistically significant decrease in the area under the plasma concentration-time curve within one dosing interval of amitriptyline by 22% ( p = 0.03) and nortriptyline by 41% ( p = 0.002), as well as of all hydroxylated metabolites, except for 10-E-hydroxynortriptyline. Plasma levels of amitriptyline and hydroxylated metabolites gradually decreased, whereas nortriptyline concentrations were already markedly decreased after 3 days of cotreatment with hypericum. Cumulative urinary amounts of amitriptyline and metabolites decreased to the same extent as plasma concentrations upon hypericum comedication. Induction of cytochrome P-450 enzymes or drug transporters (P-glycoprotein) by St. John's wort extract may explain this pharmacokinetic interaction. Physicians should be aware of this interaction when treating patients with amitriptyline.

Determination of hyperforin, hypericin, and pseudohypericin in human plasma using high-performance liquid chromatography analysis with fluorescence and ultraviolet detection

Hyperforin, hypericin and pseudohypericin are the main ingredients of St. John's wort extract, which is available over the counter for treatment of mild to moderate depression. To facilitate clinical studies we developed two sensitive HPLC methods for determination of hypericin/pseudohypericin and hyperforin, respectively, in human plasma samples. The achieved limits of quantitation of 0.25 ng/ml for hypericin and pseudohypericin and 10 ng/ml for hyperforin were low enough to allow determination of pharmacokinetic parameters of the substances. Following liquid-liquid extraction of human plasma the samples were separated by isocratic reversed-phase HLPC and analyzed using fluorimetric detection for hypericin/pseudohypericin and UV detection for hyperforin.

Saint John's wort: an in vitro analysis of P-glycoprotein induction due to extended exposure

1. Chronic use of Saint John's wort (SJW) has been shown to lower the bioavailability for a variety of co-administered drugs including indinavir, cyclosporin, and digoxin. Decreases in intestinal absorption through induction of the multidrug resistance transporter, P-glycoprotein (P-gp), may explain decreased bioavailability. 2. The present study characterized the response of P-gp to chronic and acute exposure of SJW and hypericin (HYP, a presumed active moiety within SJW) in an in vitro system. Experiments were performed with 3 to 300 microg ml(-1) of methanol-extracted SJW and 0.03 to 3 microM HYP, representing low to high estimates of intestinal concentrations. 3. In induction experiments, LS-180 intestinal carcinoma cells were exposed for 3 days to SJW, HYP, vehicle or a positive control (ritonavir). P-gp was quantified using Western blot analysis. P-gp expression was strongly induced by SJW (400% increase at 300 microg ml(-1)) and by HYP (700% at 3 microM) in a dose-dependent fashion. Cells chronically treated with SJW had decreased accumulation of rhodamine 123, a P-gp substrate, that was reversed with acute verapamil, a P-gp inhibitor. Fluorescence microscopy of intact cells validated these findings. In Caco-2 cell monolayers, SJW and HYP caused moderate inhibition of P-gp-attributed transport at the maximum concentrations tested. 4. SJW and HYP significantly induced P-gp expression at low, clinically relevant concentrations. Similar effects occurring in vivo may explain the decreased bioavailability of P-gp substrate drugs when co-administered with SJW.

Methadone inhibits rhodamine123 transport in Caco-2 cells

This study investigated the effects of racemic methadone (MET) on P-glycoprotein (P-gp) activity in cell culture. MET showed no differential rates of passage between the basolateral to apical (B to A) and apical to basolateral (A to B) direction across Caco-2 cell monolayers in a transwell system. MET transport in either direction was not importantly influenced by the P-gp inhibitor verapamil. However, MET was a potent inhibitor (IC(50) = 7.5 microM) of rhodamine123 B to A transport across Caco-2 cell monolayers, causing a reduction to 25% of control at 100 microM MET. In this model of Caco-2 monolayers, rates of MET passage between B to A and A to B directions could not be distinguished. However, MET can inhibit P-gp activity at intraluminal concentrations that might be achieved clinically. This may lead to increased bioavailability of coadministered compounds.

P-glycoprotein interactions of nefazodone and trazodone in cell culture

This study investigated the effects of nefazodone (NFZ) and trazodone (TZD) on P-glycoprotein (P-gp) activity and expression in cell culture. NFZ and TZD showed no differential transport between the basolateral to apical and apical to basolateral direction across Caco-2 cell monolayers. Transport in either direction was not affected by verapamil. NFZ was a potent inhibitor (IC50 = 4.7 microM) of rhodamine123 (Rh123) B to A transport across Caco-2 cell monolayers, while TZD had minimal effect. Following 72-hour exposure of LS180V cells to NFZ and TZD (10 microM), a twofold increase in immunoreactive P-gp was observed. Rh123 accumulation into these cells was reduced to 65% and 74% of control by NFZ and TZD (10 microM), respectively. It was concluded that differential rates of transport of NFZ and TZD in Caco-2 cells were not evident. However, NFZ is an inhibitor of P-gp activity at clinically relevant in vivo concentrations and may have the potential to increase bioavailability of coadministered compounds that are substrates for transport. Concentrations of NFZ and TZD achieved in the intestine after chronic oral dosing may induce P-gp expression and reduce absorption of coadministered drugs.

Benzydamine N-oxidation as an index reaction reflecting FMO activity in human liver microsomes and impact of FMO3 polymorphisms on enzyme activity

AIMS

The role of flavin containing monooxygenases (FMO) on the disposition of many drugs has been insufficiently explored. In vitro and in vivo tests are required to study FMO activity in humans. Benzydamine (BZD) N-oxidation was evaluated as an index reaction for FMO as was the impact of genetic polymorphisms of FMO3 on activity.

METHODS

BZD was incubated with human liver microsomes (HLM) and recombinant enzymes. Human liver samples were genotyped using PCR-RFLP.

RESULTS

BZD N-oxide formation rates in HLM followed Michaelis-Menten kinetics (mean Km = 64.0 microM, mean Vmax = 6.9 nmol mg-1 protein min-1; n = 35). N-benzylimidazole, a nonspecific CYP inhibitor, and various CYP isoform selective inhibitors did not affect BZD N-oxidation. In contrast, formation of BZD N-oxide was almost abolished by heat treatment of microsomes in the absence of NADPH and strongly inhibited by methimazole, a competitive FMO inhibitor. Recombinant FMO3 and FMO1 (which is not expressed in human liver), but not FMO5, showed BZD N-oxidase activity. Respective Km values for FMO3 and FMO1 were 40.4 microM and 23.6 microM, and respective Vmax values for FMO3 and FMO1 were 29.1 and 40.8 nmol mg-1 protein min-1. Human liver samples (n = 35) were analysed for six known FMO3 polymorphisms. The variants I66M, P135L and E305X were not detected. Samples homozygous for the K158 variant showed significantly reduced Vmax values (median 2.7 nmol mg-1 protein min-1) compared to the carriers of at least one wild type allele (median 6.2 nmol mg-1 protein min-1) (P < 0.05, Mann-Whitney-U-test). The V257M and E308G substitutions had no effect on enzyme activity.

CONCLUSIONS

BZD N-oxidation in human liver is mainly catalysed by FMO3 and enzyme activity is affected by FMO3 genotype. BZD may be used as a model substrate for human liver FMO3 activity in vitro and may be further developed as an in vivo probe reflecting FMO3 activity.

Scaling drug biotransformation data from cDNA-expressed cytochrome P-450 to human liver: a comparison of relative activity factors and human liver abundance in studies of mirtazapine metabolism

The present study represents a comparison of three approaches to transform recombinant cytochrome P-450 (rCYP) enzyme kinetic data to human liver activity using mirtazapine (MIR) biotransformation as a model. MIR metabolite rCYP formation rates were corrected using I) relative activity factors (RAFs) determined on site, II) RAFs based on activity data provided by the rCYP manufacturer, and III) immunologically determined human liver abundance of CYP isoforms reported in the literature. For 2.5, 25, and 250 microM MIR, predictions of 1) the relative contribution of CYP isoforms to a particular reaction, 2) absolute metabolite formation rates, 3) the relative contribution of each pathway to net MIR biotransformation, and 4) the relative contribution of CYP isoforms to net MIR biotransformation were generated, and the results were compared with data obtained with human liver microsomes (HLM). We found that RAFs determined on site most accurately predict the results observed in HLM. Estimations based on liver abundance systematically underestimated CYP1A2 and overestimated CYP3A and CYP2C9 contributions to MIR metabolism and, therefore, seem less suitable to predict CYP isoform involvement in a particular reaction. Normalized RAFs calculated from the manufacturer activity data fell within the range of RAFs determined on site and lead to similar results for CYP isoform contribution to metabolic reactions and to net MIR biotransformation. Considering the time and resource-intensive step of RAF determination, manufacturer RAFs are an alternative to RAFs determined on site for the transformation of rCYP enzyme kinetic data; both of them provide more accurate estimations than immunologically determined human liver CYP isoform content.

Metabolism of the antidepressant mirtazapine in vitro: contribution of cytochromes P-450 1A2, 2D6, and 3A4

The metabolism of the antidepressant mirtazapine (MIR) was investigated in vitro using human liver microsomes (HLM) and recombinant enzymes. Mean K(m) values (+/-S.D., n = 4) were 136 (+/-44) microM for MIR-hydroxylation, 242 (+/-34) microM for N-demethylation, and 570 (+/-281) microM for N-oxidation in HLM. Based on the K(m) and V(max) values, MIR-8-hydroxylation, N-demethylation, and N-oxidation contributed 55, 35, and 10%, respectively, to MIR biotransformation in HLM at an anticipated in vivo liver MIR concentration of 2 microM. Recombinant CYP predicted a 65% contribution of CYP2D6 to MIR-hydroxylation at 2 microM MIR, decreasing to 20% at 250 microM. CYP1A2 contribution increased correspondingly from 30 to 50%. In HLM, quinidine and alpha-naphthoflavone reduced MIR-hydroxylation to 75 and 45% of control, respectively, at 250 microM MIR. A >50% contribution of CYP3A4 to MIR-N-demethylation at <1 microM MIR was indicated by recombinant enzymes. In HLM, ketoconazole (1 microM) reduced N-desmethylmirtazapine formation rates to 60% of control at 250 microM. Twenty percent of MIR-N-oxidation was accounted for by CYP3A4 at 2 microM MIR, increasing to 85% at 250 microM, while CYP1A2 contribution decreased from 80 to 15%. Ketoconazole reduced MIR-N-oxidation to 50% of control at 250 microM. MIR did not substantially inhibit CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP1E2, and CYP3A4 activity in vitro. Induction/inhibition or genetic polymorphisms of CYP2D6, CYP1A2, and CYP3A4 may affect MIR metabolism, but involvement of several enzymes in different metabolic pathways may prevent large alterations in in vivo drug clearance.

Cytochrome P-450 enzymes and FMO3 contribute to the disposition of the antipsychotic drug perazine in vitro

RATIONALE

Perazine (PER) is a phenothiazine antipsychotic drug frequently used in Germany that undergoes extensive metabolism.

OBJECTIVES AND METHODS

To anticipate metabolic drug interactions and to explore the relevance of polymorphisms of metabolic enzymes, perazine-N-demethylation and perazine-N-oxidation were investigated in vitro using human liver microsomes and cDNA expressed enzymes.

RESULTS

CYP3A4 and CYP2C9 were identified as the major enzymes mediating PER-N-demethylation. At 10 microM PER, a concentration consistent with anticipated in vivo liver concentrations, CYP3A4 and CYP2C9 contributed 50% and 35%, respectively, to PER-N-demethylation. With increasing PER concentrations, contribution of CYP2C9 decreased and CYP3A4 became more important. In human liver microsomes, PER-N-demethylation was inhibited by ketoconazole (>40%) and sulfaphenazole (16%). Allelic variants of recombinant CYP2C9 showed differences in PER-N-demethylase activity. The wild type allele CYP2C9*1 was the most active variant. Maximal activities of CYP2C9*2 and CYP2C9*3 were 88% and 18%, respectively, compared to the wild type activity. Perazine-N-oxidation was mainly mediated by FMO3. In the absence of NADPH, heat treatment of microsomes abolished PER-N-oxidase activity. Methimazole inhibited PER-N-oxidation, while CYP specific inhibitors had no inhibitory effect. Perazine is a potent inhibitor of dextromethorphan-O-demethylase, S-mephenytoin-hydroxylase, alprazolam-4-hydroxylase, phenacetin-O-deethylase and tolbutamide-hydroxylase activity in human liver microsomes.

CONCLUSIONS

Alterations in the activity of CYP3A4, CYP2C9 and FMO3 through genetic polymorphisms, enzyme induction or inhibition bear the potential to cause clinically significant changes in perazine clearance. PER may alter the clearance of coadministered compounds metabolized by CYP2D6, CYP2C19, CYP2C9, CYP3A4 and CYP1A2.