Hydroxylation polymorphisms of debrisoquine and mephenytoin in European populations

10-Hydroxylation of nortriptyline in white persons with 0, 1, 2, 3, and 13 functional CYP2D6 genes

OBJECTIVE

To investigate the disposition and effects of nortriptyline and its major metabolite 10-hydroxy-nortriptyline line in panels of white subjects with different CYP2D6 genotypes, including those with duplicated and multiduplicated CYP2D6*2 genes and to evaluate the contribution of the number of functional C gamma P2D6 alleles to the metabolism of nortriptyline, used here as a model drug for CYP2D6 substrates.

METHODS

Oral single doses of 25 to 50 mg nortriptyline were given to five poor metabolizers of debrisoquin (INN; debrisoquine) with no functional CYP2D6 gene, five extensive metabolizers with one functional CY2D6 gene, five extensive metabolizers with two functional CYP2D6 genes, five ultrarapid metabolizers with duplicated CYP2D6*2 genes, and one ultrarapid metabolizer with 13 copies of the CYP2D6*2 gene. Plasma kinetics of nortriptyline and 10-hydroxynortriptyline were analyzed. Anticholinergic effects (inhibition of salivation and accommodation disturbances), sedation, blood pressure, and effect on supine and erect pulse rate were measured.

RESULTS

There was a clear relation between the C gamma P2D6 genotype and the plasma kinetics of nortriptyline and 10-hydroxynortriptyline. The proportion between the apparent oral clearances of nortriptyline in the groups with 0, 1, 2, 3, and 13 functional genes was 1:1:4:5:17. The proportions between AUC(nortriptyline) to AUC(10-hydroxynortriptyline) ratios in the groups with 0, 1, 2, 3, and 13 functional genes were 36:25:10:4:1. Oral plasma clearance of nortriptyline and AUC(nortriptyline) to AUC(10-hydroxynortriptyline) ratio both correlated significantly with the debrisoquin metabolic ratio (rS = -0.89, p = 0.0001; rS = 0.92, p = 0.0001). Although ultrarapid metabolizer subjects were given double the nortriptyline dose (50 mg), inhibition of salivation was not more pronounced compared with the other genotype groups given 25 mg nortriptyline.

CONCLUSION

The results of this study show the quantitative importance of the CYP2D6 genotype, especially the presence of multiple functional CYP2D6 genes for the pharmacokinetics of nortriptyline and 10-hydroxynortriptyline. Genotyping of subjects with multiple copies of functional genes may be of great value for differentiating ultrarapid metabolizers from patients who do not comply with the prescription and for assuring adequate drug choice and dosage for these patients.

Halofantrine and chloroquine inhibit CYP2D6 activity in healthy Zambians

AIMS

To determine the effect of therapeutic loading doses of halofantrine and chloroquine on CYP2D6 activity in healthy black Zambians.

METHODS

Twenty healthy black male Zambians were phenotyped for CYP2D6 activity by measuring the debrisoquine/4-hydroxydebrisoquine ratio in a 0-8 h urine sample after a 10 mg oral dose of debrisoquine hemi-sulphate. The subjects (all 'extensive metabolizer' phenotype with respect to CYP2D6) were randomized into two groups of 10, and 24 h later one group received 1500 mg halofantrine hydrochloride and the other group 1500 mg chloroquine phosphate both orally in divided doses. All subjects were given further 10 mg doses of debrisoquine at 2 h, 1 week and 2 weeks after the last dose of the antimalarial drug, and phenotyped as described above.

RESULTS

The median debrisoquine/4-hydroxydebrisoquine 0-8 h urinary ratio was increased by halofantrine (1.39 to 6.05; P<0.01; 95% confidence intervals 4.00-11.7) and chloroquine (1.96 to 3.91; P<0.01; 95% confidence intervals 1.34-2.66) when debrisoquine was given 2 h after treatment. The decrease in CYP2D6 activity remained statistically significant for 1 week after both drugs. Halofantrine was a significantly more potent inhibitor of CYP2D6 than chloroquine (P=0.037). Phenocopying occurred in two subjects taking halofantrine and one taking chloroquine (i.e. the debrisoquine/4-hydroxydebrisoquine ratios became consistent with the poor metabolizer phenotype).

CONCLUSIONS

Given in therapeutic loading doses, both halofantrine and chloroquine caused significant inhibition of CYP2D6 activity in healthy black Zambians. With respect to halofantrine, this finding reinforces the recommendation that its combination with other drugs known to prolong the QT interval should be avoided, especially those that are metabolized significantly by CYP2D6.

A dual label oligonucleotide ligation assay for detection of the CYP2C19*1, CYP2C19*2, and CYP2C19*3 alleles involving time-resolved fluorometry

CYP2C19 (S-mephenytoin hydroxylase) is a polymorphically expressed enzyme. Currently, two defective alleles are known--CYP2C19*2 and CYP2C19*3. The authors have developed an oligonucleotide ligation assay to detect these two alleles. This assay combines the hybridization of one common, biotinylated capture probe and two allele-specific probes to the target DNA, with the ability of a DNA ligase to distinguish mismatched nucleotides. The probes are only ligated if they are base paired correctly to the target strand. The biotin is bound to streptavidin, and all DNA not covalently bound to the biotin-labeled capture probe, is removed in a washing procedure. The allele-specific probes are labeled with either europium or samarium, and their emission can be measured simultaneously. The ratio between the emission separates the genotypes. This method was applied on DNA from 19 whites and 21 Vietnamese living in Denmark. All genotypes determined by the assay were consistent with the results from restriction enzyme cleavage. There were 12 poor metabolizers; 10 homozygous CYP2C19*2/CYP2C19*2, one heterozygous CYP2C19*2/CYP2C19*3, and one heterozygous CYP2C19*1/CYP2C19*2. The authors conclude that this assay is well-suited for a high throughput of samples in a routine laboratory. The finding of an apparently heterozygous CYP2C19*1/CYP2C19*2 poor metabolizer, confirms that there are still unknown mutations in CYP2C19.

Genetic analysis of the CYP2D6 gene in patients with Parkinson's disease

To further investigate the association between Parkinson's disease (PD) and genetic polymorphism of the CYP2D6 gene, a mutant allele (CVP2D6J) frequently observed in the Japanese population and related to EM/PM polymorphism (phenotypically, individuals are either extensive metabolizers [EM] or poor metabolizers [PM] of debrisoquine) was investigated. The CYP2D6J gene with a nucleotide substitution from C to T at position 188 (the HphI site in exon 1), which reduces CYP2D6 enzyme activity, was analyzed by polymerase chain reaction (PCR) and by digestion with HphI. No significant relationship was observed between PD patients and controls for this mutation. This suggests that the EM/PM polymorphism of CYP2D6 contributes little to the pathogenesis of PD. To further study the molecular basis for the relationship between PD and CYP2D6, the heterogeneity of CYP2D6 was investigated by combined genotype analysis of the two mutant CYP2D6 genes (ie, CYP2D6J, the HphI site mutation in exon 1, and CYP2D6L, the HhaI site mutation in exon 6). Although some characteristic patterns of the combined genotypes were observed in both PD patients and controls, a strong association between the heterogeneity of the CYP2D6 gene and PD was not shown by combined genotype analysis.

Omission of the deconjugation step in urine analysis and the unaltered outcome of CYP2D6 phenotyping with dextromethorphan

The present study was aimed at determining whether the deconjugation step in chemical analysis could be omitted without altering the outcome of phenotyping CYP2D6 with dextromethorphan. This drug and its metabolite, dextrorphan, were assayed by high-performance liquid chromatography (HPLC) in urine. Urinary levels of dextromethorphan and dextrorphan with and without enzymatic (beta-glucuronidase) treatment of urine and the metabolic ratios for dextromethorphan were determined in 45 subjects. Although the enzymatic treatment did not alter the urinary concentration of dextromethorphan in both phenotypes, it increased the urinary concentration of dextrorphan in both poor and extensive metabolizers by 3.7- and 12.8-fold, respectively. A urinary unconjugated dextromethorphan/unconjugated dextrorphan metabolic ratio of 2.00 and a total dextromethorphan/total dextrorphan metabolic ratio of 0.30, respectively, identified three poor metabolizers. Enzymatic treatment decreased the urinary antimode value. Moreover, the urinary metabolic ratio based on unconjugated dextrorphan and dextromethorphan correlated well with that based on assay of total dextrorphan and dextromethorphan (rs = 0.9458, P < 0.001). The results show that urinary analysis of dextrorphan and dextromethorphan omitting the enzymatic deconjugation step is a fast, reliable and sensitive method and could be used for studying CYP2D6 type genetic polymorphism in man.

A combination of mutations in the CYP2D6*17 (CYP2D6Z) allele causes alterations in enzyme function

In many black African populations, the capacity for CYP2D6-dependent drug metabolism is generally reduced. A specific variant of the CYP2D6 gene (CYP2D6*17) that carries three functional mutations (T107I, R296C, and S486T) has been found to be present in Zimbabwean subjects with impaired CYP2D6-dependent hydroxylase activity. To evaluate whether the CYP2D6*17 allele was the major cause behind the decreased rate of drug metabolism and to examine the role of the different mutations, CYP2D6 cDNAs containing all eight combinations of the mutations were created. Expression of the cDNAs in COS-1 cells revealed that the CYP2D6 17 enzyme displayed only 20% of the wild-type (CYP2D6 1) activity, whereas the T107I substitution on its own had no significant effect on enzyme function. Expression in yeast showed that the three possible single amino-acid mutant CYP2D6 variants all had properties similar to CYP2D6 1 when the kinetics of bufuralol hydroxylation was examined. However, enzymes containing both the T107I and R296C mutations exhibited a more than 5-fold higher K(m) for bufuralol than the wild-type enzyme, whereas the S486T mutation was of little importance. In contrast, when codeine was used as a substrate, the T107I substitution alone was sufficient to cause a significant increase in the apparent K(m), indicating a differential effect for this substitution depending on the CYP2D6 substrate. In conclusion, the CYP2D6*17 allele represents the first human cytochrome P450 polymorphic variant in which a combination of substitutions is required to alter the enzyme's catalytic properties and is the first case in which a decreased CYP2D6 activity, as monitored in vivo, has been documented to be caused by an enzyme with altered affinity for CYP2D6 substrates.

Metabolic disposition of pantoprazole, a proton pump inhibitor, in relation to S-mephenytoin 4'-hydroxylation phenotype and genotype

OBJECTIVES

To assess the possible relationship between the metabolic disposition of pantoprazole and genetically determined S-mephenytoin 4'-hydroxylation phenotype and genotype.

METHODS

The pharmacokinetic disposition of pantoprazole was investigated in 14 Japanese male volunteers (seven extensive and seven poor metabolizers of S-mephenytoin). All subjects received a single 40 mg oral dose of pantoprazole as the enteric-coated formulation.

RESULTS

An interphenotypic difference in the metabolic disposition of pantoprazole was observed: the mean values for area under the concentration-time curve (AUC), elimination half-life (t1/2), and apparent oral clearance were significantly (p < 0.01) greater, longer, and lower, respectively, in the poor metabolizers than in the extensive metabolizers. The mean AUC of pantoprazole sulfone was greater (p < 0.01) in the poor metabolizers than in the extensive metabolizers, whereas the mean AUC of the main demethylated metabolite (M2) was lower (p < 0.01) in the poor metabolizers than in the extensive metabolizers. A significant negative correlation was observed between the individual values for log10% urinary excretion of 4'-hydroxymephenytoin and AUC of pantoprazole (rs = -0.816; p < 0.005). The CYP2C19 genotyping test results were found to be in a complete accordance with the phenotypes.

CONCLUSION

These data indicated that the metabolic disposition of pantoprazole is under the pharmacogenetic control of S-mephenytoin 4'-hydroxylase (CYP2C19).

Seizures and myoclonus associated with antidepressant treatment: assessment of potential risk factors, including CYP2D6 and CYP2C19 polymorphisms, and treatment with CYP2D6 inhibitors

All adverse drug reaction reports labelled seizures or myoclonus during treatment with antidepressants and stored in the Swedish national database for spontaneous reporting of adverse drug reactions were reviewed in order to evaluate possible risk factors. The reporting physicians were contacted and asked for complementary information, and blood samples for determination of the CYP2D6 and CYP2C19 genotypes were obtained from patients available. In total, 25 cases of seizures and 7 cases of myoclonus were studied. The drugs included were maprotiline (n=8), mianserin (n=7), fluvoxamine (n=6), amitriptyline (n=3), clomipramine (n=3), citalopram (n=2), paroxetine (n=2) and lofepramine (n=1). Previously suggested predisposing factors were identified in all but four cases (87%). None of the 11 patients genotyped were found to be poor metabolizers with respect to the enzymes CYP2D6 or CYP2C19. Thus, neither the CYP2D6 nor the CYP2C19 genotype were found to be associated with the occurrence of seizures/myoclonus during treatment with antidepressants. However, 15 patients (47%) were concomitantly treated with drugs with potential inhibitory effects on CYP2D6, such as neuroleptics and dextropropoxyphene, and the patients might thus have been converted from the extensive metabolizer to the poor metabolizer phenotype during this treatment. Concomitant treatment with drugs decreasing the seizure threshold and/or inhibiting the metabolism of antidepressants appeared to be an important risk factor for the occurrence of seizures/myoclonus.

Quick onset of severe abdominal pain after codeine in an ultrarapid metabolizer of debrisoquine

The authors describe a 33-year-old woman who experienced severe pain in the epigastrium after codeine intake. This side-effect is consistent with that of morphine. Later, the patient was phenotyped and genotyped as an ultrarapid metabolizer with high capacity to metabolize codeine to morphine.

Quantification of the O- and N-demethylated and the glucuronidated metabolites of codeine relative to the debrisoquine metabolic ratio in urine in ultrarapid, rapid, and poor debrisoquine hydroxylators

The O-demethylation of codeine is polymorphic and catalyzed by CYP2D6. The metabolites of codeine formed through O- and N-demethylation as well as glucuronidation were quantified in the ultrarapid metabolizers of debrisoquine and compared with the normal extensive (EM) and poor metabolizers (PM). The urinary codeine and its seven metabolites were detected after 25 mg codeine in 24 healthy Caucasian subjects with low debrisoquine metabolic ratios (MR, < or = 0.11) and a group of 132 subjects tested earlier with codeine and debrisoquine including 114 EMs (MR < 12.6) and 18 PMs (MR > 12.6). Whereas the O-demethylated metabolites accounted for < 0.4% of the total recovery on average in the PMs and 1.7% to 8.7% in the EMs, they accounted for 15.3% in the 24 subjects with ultrarapid metabolism of debrisoquine. This study suggests that the ultrarapid debrisoquine hydroxylators may develop increased O-demethylated metabolite-dependent effects or side-effects of codeine.

Inhibition of the sulfoxidation of omeprazole by ketoconazole in poor and extensive metabolizers of S-mephenytoin

BACKGROUND

The metabolism of omeprazole includes hydroxylation catalyzed by CYP2C19 and, to a minor extent, sulfoxidation, presumably by CYP3A4. Sulfoxidation may be the predominant pathway in individuals devoid of the genetically determined CYP2C19 activity. Ketoconazole is a known CYP3A4 inhibitor in daily doses from 200 to 400 mg. In this study ketoconazole was used as a probe to investigate the extent to which CYP3A4 is involved in omeprazole metabolism in vivo.

METHODS

A single oral 20 mg dose of omeprazole before and after four daily doses of 200, 100, or 50 mg ketoconazole was given to 10 healthy subjects, previously phenotyped as poor or extensive metabolizers of S-mephenytoin. Concentrations of omeprazole, 5-hydroxyomeprazole, omeprazole sulfone, and ketoconazole were analyzed with reversed-phase HPLC methods in plasma samples collected repeatedly for 12 hours after dosing.

RESULTS

After intake of 20 mg omeprazole with 0, 50, 100, and 200 mg ketoconazole, mean values for omeprazole sulfone area under the plasma concentration versus time curve from 0 to 6 hours [AUC(0-6)] were 482, 206, 167, and < 100 nmol/L.hr in extensive metabolizers and 3160, 2430, 937, and 534 nmol/L.hr in poor metabolizers, respectively. Mean omeprazole AUC(0-6) increased from 1660 to 2265 nmol/L.hr in extensive metabolizers and from 7715 to 15319 nmol/L.hr in poor metabolizers after intake of 200 mg ketoconazole.

CONCLUSIONS

An oral daily dose of 100 to 200 mg ketoconazole is sufficient to provide a marked inhibition of the formation of the omeprazole sulfone in both extensive and poor metabolizers and leads to a doubling of omeprazole levels in poor metabolizers, whereas 50 mg ketoconazole provides only partial inhibition. We concluded that CYP3A4 catalyzes the sulfoxidation of omeprazole and that this is the predominant metabolic pathway of omeprazole in poor metabolizers of S-mephenytoin.

Metabolic disposition of lansoprazole in relation to the S-mephenytoin 4'-hydroxylation phenotype status

OBJECTIVE

To assess the possible involvement of CYP2C19 in the metabolism of lansoprazole in vivo.

METHODS

Sixteen male Korean subjects, who had been phenotyped as extensive metabolizers and poor metabolizers of S-mephenytoin 4'-hydroxylation polymorphism (n = 8 each) with racemic mephenytoin with use of the 8-hour urine analysis of 4'-hydroxymephenytoin, took an oral dose of 30 mg lansoprazole, and blood samples were collected up to 48 hours after dosing. Lansoprazole and its metabolites were measured by high-performance liquid chromatography with ultraviolet detection.

RESULTS

The mean lansoprazole area under the concentration-time curve (AUC), elimination half-life (t1/2), and apparent oral clearance (CLoral) were significantly (p < 0.001) greater, longer, and lower, respectively, in the poor metabolizer than in the extensive metabolizer group. The mean values for the AUC of hydroxylansoprazole and AUC ratio of hydroxylansoprazole to lansoprazole were significantly (p < 0.01 to p < 0.001) less in the poor metabolizer than in the extensive metabolizer group, whereas those for the AUC of lansoprazole sulfone and ratio of lansoprazole sulfone to lansoprazole were greater (p < 0.001) in the former than in the latter group. In addition, the log10 4'-hydroxymephenytoin excreted in urine correlated significantly (p < 0.01) with the CLoral of lansoprazole.

CONCLUSIONS

These results suggest that the hydroxylation of lansoprazole cosegregates with the genetically determined S-mephenytoin 4'-hydroxylation (CYP2C19) polymorphism in the Korean subjects.

Fluvoxamine and fluoxetine do not interact in the same way with the metabolism of the enantiomers of methadone

Six and seven addicts treated with racemic methadone (MTD) were comedicated with fluvoxamine (FLV) and fluoxetine (FLX), respectively. The plasma concentrations of both (R)- (the active enantiomer) and (S)-MTD were increased by FLV, whereas only (R)-MTD concentrations were increased by the addition of FLX. This suggests that cytochrome P450IID6 (CYP2D6), an enzyme that is strongly inhibited by FLX, preferentially metabolizes (R)-MTD, whereas CYP1A2, which is strongly inhibited by FLV, metabolizes both enantiomers. The choice of a selective serotonin reuptake inhibitor in depressive addicted patients treated with MTD and the possible use of FLX or FLV to potentiate the effects of MTD in some cases of therapeutic failure are discussed.

Pharmacogenetics: a laboratory tool for optimizing therapeutic efficiency

Pharmacogenetics is the study of the linkage between an individual’s genotype and that individual’s ability to metabolize a foreign compound. Differences in metabolism of therapeutics can lead to severe toxicity or therapeutic failure by altering the relation between dose and blood concentration of the pharmacologically active drug. Phenotypes exhibiting poor and ultraextensive metabolism result from genetic variance (polymorphism) of enzymes involved in metabolism. Thus, in pharmacogenetic studies one applies genotyping of polymorphic alleles encoding drug-metabolizing enzymes to the identification of an individual’s drug metabolism phenotype. This knowledge, when applied to dosing or drug selection, can avoid adverse reactions or therapeutic failure and thus enhance therapeutic efficiency. More than 25 commonly prescribed medicines are metabolized by the cytochrome P-4502D6 (CYP2D6) isoenzyme, and polymorphism of the CYP2D6 gene affects the therapeutic management of up to 17% of individuals in some ethnic groups. In this review, we summarize and update information concerning drug-metabolizing genotypes with emphasis on CYP2D6 genotyping techniques that can be applied by the clinical laboratory for linking human genetics to therapeutic management.

The sparteine/debrisoquine (CYP2D6) oxidation polymorphism and the risk of lung cancer: a meta-analysis

OBJECTIVE

To examine the association between the sparteine/debrisoquine (CYP2D6) oxidation polymorphism and the risk of lung cancer.

METHOD

Meta-analysis of case-control studies using a random effects model. The "Main outcome measure" was the odds ratio for the risk of lung cancer, using extensive metabolisers as the reference group.

RESULTS

Thirteen studies were identified. The studies were too heterogeneous to be pooled the size of the odds ratio increased with the sample size. When the analysis was restricted to the largest studies, there was no difference in risk between poor and extensive metabolisers (odds ratio 0.95, 95% confidence interval 0.68-1.33).

CONCLUSION

No association was found between the CYP2D6 oxidation polymorphism and lung cancer risk when sample size bias was taken into account.

Genetic predisposition to drug-induced hepatotoxicity

Drug-induced hepatitis is uncommon and generally unpredictable. Hepatotoxicity may be related to the drug itself, or to chemically reactive metabolites which can bind covalently to hepatic macromolecules and may lead to either idiosyncratic, toxic hepatitis or to immunoallergic hepatitis. There is now evidence indicating that genetic variations in systems of biotransformation or detoxication may modulate either the toxic or sensitizing effects of some drugs. Thus, the genetic deficiency in a particular hepatic cytochrome P 450 isozyme (CYP 2D6) is involved in per-hexiline liver injury. The deficiency in CYP 2C19 might also contribute to Atrium hepatotoxicity. Slow acetylation related to N-acetyltransferase 2 deficiency contributes to sulfonamide hepatitis. The genetic deficiency in glutathione synthetase may increase the susceptibility to several drugs including acetaminophen. A constitutional deficiency in another cell defense mechanism, still not characterized, seems to increase significantly the risk of hepatotoxicity with halothane, phenytoin, carbamazepine, phenobarbital, sulfamides and amineptine.

Antipsychotic drug-induced movement disorders in schizophrenics in relation to CYP2D6 genotype

BACKGROUND

Approximately 5-10% of Caucasians (poor metabolisers) show impaired metabolism of at least 20 therapeutically important drugs, including a number of commonly used antipsychotic agents, because they lack the cytochrome p450 enzyme CYP2D6. The molecular basis of this defect is now well understood and simple genotyping tests using the polymerase chain reaction (PCR) have been developed.

METHOD

To determine whether poor metabolisers are more susceptible to acute dystonic reactions and chronic movement disorders associated with the administration of antipsychotic drugs, we determined CYP2D6 genotypes in a group of 76 schizophrenics using previously described methods involving PCR and restriction fragment length polymorphism analysis.

RESULTS

There was no difference in genotype frequencies between the schizophrenics and a normal control population, suggesting that CYP2D6 genotype was not a factor in determining susceptibility to the disease. However, four of the five poor metabolisers compared with 44% of the remaining subjects were suffering from a movement disorder at the time of the study, although because of the small number of poor metabolisers in the group the difference was not statistically significant. Poor metabolisers were not more likely to suffer an acute dystonic reaction.

CONCLUSIONS

CYP2D6 genotype is not a determinant of susceptibility to acute dystonic reactions but may be a contributory factor in antipsychotic drug-induced movement disorders including tardive dyskinesia.

Molecular mechanisms of genetic polymorphisms of drug metabolism

One of the major causes of interindividual variation of drug effects is genetic variation of drug metabolism. Genetic polymorphisms of drug-metabolizing enzymes give rise to distinct subgroups in the population that differ in their ability to perform certain drug biotransformation reactions. Polymorphisms are generated by mutations in the genes for these enzymes, which cause decreased, increased, or absent enzyme expression or activity by multiple molecular mechanisms. Moreover, the variant alleles exist in the population at relatively high frequency. Genetic polymorphisms have been described for most drug metabolizing enzymes. The molecular mechanisms of three polymorphisms are reviewed here. The acetylation polymorphism concerns the metabolism of a variety of arylamine and hydrazine drugs, as well as carcinogens by the cytosolic N-acetyltransferase NAT2. Seven mutations of the NAT2 gene that occur singly or in combination define numerous alleles associated with decreased function. The debrisoquine-sparteine polymorphism of drug oxidation affects the metabolism of more than 40 drugs. The poor metabolizer phenotype is caused by several "loss of function" alleles of the cytochrome P450 CYP2D6 gene. On the other hand, "ultrarapid" metabolizers are caused by duplication or amplification of an active CYP2D6 gene. Intermediate metabolizers are often heterozygotes or carry alleles with mutations that decrease enzyme activity only moderately. The mephenytoin polymorphism affects the metabolism of mephenytoin and several other drugs. Two mutant alleles of CYP2C19 have so far been identified to cause this polymorphism. These polymorphisms show recessive transmission of the poor or slow metabolizer phenotype, i.e. two mutant alleles define the genotype in these individuals. Simple DNA tests based on the primary mutations have been developed to predict the phenotype. Analysis of allele frequencies in different populations revealed major differences, thereby tracing the molecular history and evolution of these polymorphisms.

Influence of Cyp2D6 genetic polymorphism on ratios of steady-state serum concentration to dose of the neuroleptic zuclopenthixol

One hundred and nineteen psychiatric patients undergoing therapeutic drug monitoring (TDM) of the neuroleptic zuclopenthixol were genotyped with regard to Cyp2D6. Twelve patients (10.1%) were of the poor metabolizer genotype. The extensive metabolizers comprised 58 patients receiving no potentially interacting drugs and 38 patients concomitantly treated with other drugs competing for metabolism by Cyp2D6. Information on the rest (11 patients) was missing. The median steady-state serum concentration-to-dose ratio (C/D) of the PM group (2.00 nmol/L/mg) was close to that of the EM group receiving potentially interacting drugs (1.80) and approximately 60% higher than that of the remaining EM group (1.25) (p < 0.01). When judging the clinical importance of this difference, the total group variability in C/D of nearly 10-fold should be kept in mind (0.5-4.2 nmol/L/mg). In terms of serum concentrations not corrected for dose, the three groups had about similar levels, with median values from 16 to 21 nmol/L. We consider that TDM adequately takes into account dose adjustments for both EM and PM subjects in the context of this neuroleptic.

Genotyping of S-mephenytoin 4'-hydroxylation in an extended Japanese population

OBJECTIVE

To assess the genotype pattern of S-mephenytoin 4'-hydroxylation in an extended Japanese population.

METHODS

One hundred eighty-six unrelated, healthy Japanese subjects were genotyped for S-mephenytoin 4'-hydroxylase (CYP2C19) according to a genotyping technique to identify the wild-type (wt) gene and two mutations, CYP2C19m1 in exon 5 and CYP2C19m2 in exon 4. Fourty-six of the 186 subjects genotyped were phenotyped with racemic mephenytoin using the conventional 8-hour urine analysis of 4'-hydroxymephenytoin.

RESULTS

The frequency of poor metabolizers by the genotyping analysis was 18.8% (35 of the 186 subjects), consisting of 12 homozygous for CYP2C19m1 (m1/m1), three homozygous for CYP2C19m2 (m2/m2), and 20 heterozygous for the two defects (m1/m2). Thus the allele frequencies of CYP2C19m1 and CYP2C19m2 were calculated to be 0.29 and 0.13 (107 and 46 of the total of 372 alleles), respectively. Among the 46 subjects phenotyped, seven were identified as the poor metabolizers, with a log10 urinary excretion of 4'-hydroxymephenytoin of < 0.3% of the racemic dose. These seven subjects were genotyped as the individuals with the m1/m1 (two), m1/m2 (four) or m2/m2 (one) allele combination, indicating a complete concordance between the phenotyping and genotyping tests.

CONCLUSION

The present genotyping test confirmed that the frequency of CYP2C19 mutant gene m1 is about 2.2 times greater than another mutant gene, m2, among Japanese poor metabolizers. The genotyping of CYP2C19 discriminates between the two S-mephenytoin 4'-hydroxylation phenotypes completely in the Japanese subjects.

The hypoalgesic effect of tramadol in relation to CYP2D6

Tramadol inhibits norepinephrine reuptake, stimulates serotonin release, and acts with mu-opioid receptors by way of its metabolite (+)-M1. Formation of M1 seems to depend on the genetic polymorphic CYP2D6. The analgesic effect of 2 mg/kg tramadol was evaluated in 15 extensive and 12 poor metabolizers of sparteine in two parallel, randomized, double-blind, placebo-controlled crossover studies that used experimental pain models. In extensive metabolizers, tramadol increased pressure pain detection (p = 0.03) and tolerance (p = 0.06) thresholds, as well as thresholds for eliciting nociceptive reflexes, after single (p = 0.0002) and repeated (p = 0.06) stimulation of the sural nerve. Peak pain and pain area in the cold pressor test were reduced (p = 0.0006 and 0.0009). In poor metabolizers, only thresholds to pressure pain tolerance (p = 0.02) and nociceptive reflexes after single stimulation (p = 0.04) were increased and the reflex threshold was less increased in poor metabolizers than in extensive metabolizers (p = 0.02). The serum concentration of (+)-M1 2 to 10 hours after tramadol ranged from 10 to 100 ng/L in extensive metabolizers, whereas in poor metabolizers serum concentrations of (+)-M1 were below or around the detection limit of 3 ng/ml. It is concluded that formation of (+)-M1 by way of CYP2D6 is important for the effect of tramadol on experimental pain.

The cytochrome P450 2D6 (CYP2D6) enzyme polymorphism: screening costs and influence on clinical outcomes in psychiatry

OBJECTIVES

This study examined factors that affect cost, reliability, and the value of determining the cytochrome P450 2D6 (CYP2D6) polymorphism in clinical practice.

STUDY DESIGN

The method of deoxyribonucleic acid isolation, sample preparation, oligonucleotide primers, and polymerase chain reaction procedures were scrutinized for their effect on CYP2D6 genotyping efforts. The determination of the CYP2D6 A, B, D, E, and T alleles was used to identify the deficiency in CYP2D6 expression in 161 individuals phenotyped for CYP2D6 activity with dextromethorphan. The CYP2D6 genotype was assessed in 74 outpatients who had received diagnoses of depression. Eighteen of these patients were screened because of an adverse response to a tricyclic or antidepressant known or suspected to be a CYP2D6 substrate.

RESULTS

The CYP2D6 A, B, C, D, E, and T alleles could be detected in 13 hours at a cost of $84 per sample by judicious selection of conditions and procedures. The genotype provided an accurate predictor of CYP2D6 expression in all 134 subjects who expressed the enzyme and in all 27 unrelated individuals phenotyped as deficient in CYP2D6 activity. In the patient group that experienced adverse effects, 44% of all CYP2D6 gene copies contained the A, B, D, E, or T allele(s) associated with inactive CYP2D6 expression. This was more than twice the rate for the occurrence of mutant alleles in the other 56 psychiatric patients (21%) and in 80 random subjects from the general population (20%; p < 0.05).

CONCLUSIONS

Screening psychiatric patients for CYP2D6 expression may distinguish metabolic-based therapeutic problems from drug sensitivity caused by other mechanisms.

Steady-state plasma concentrations of imipramine and desipramine in relation to S-mephenytoin 4'-hydroxylation status in Japanese depressive patients

The steady-state plasma concentrations of imipramine and desipramine were measured after a more than 2-week treatment with 0.39 to 1.39 mg/kg/day of imipramine hydrochloride in 28 Japanese patients with major depression who had been phenotyped simultaneously with mephenytoin (for CYP2C19-related status) and with metoprolol (for CYP2D6-related status) before initiating the antidepressant therapy. Patients consisted of five poor metabolizers (PMs) of CYP2C19 with an extensive metabolizer (EM) phenotype of CYP2D6, whereas the remainder were EMs for both of the phenotypes. The mean respective concentrations (corrected by mg/kg) of imipramine and the sum of imipramine plus desipramine were 2.4 and 1.8 times greater in the CYP2C19-related PM than in the EM group, and these two variables correlated with the log10 urinary excretion of 4'-hydroxymephenytoin (rs = -0.73 and -0.64, both p < 0.01, respectively), but not with the metabolic ratio (MR) of metoprolol/alpha-hydroxymetoprolol. The mean N-demethylation index (MR of desipramine/imipramine) was significantly (p < 0.01) less in the PM than in the EM group. This index correlated with the 4'-hydroxylation of S-mephenytoin (rs = -0.51, p < 0.01), but not with the alpha-hydroxylation of metoprolol, implying that imipramine N-demethylation is under a coregulatory pharmacogenetic control of CYP2C19, but not of CYP2D6. In conclusion, by taking into account that the incidence of the PMs of CYP2C19 is much greater (18-23%) than that of CYP2D6 (< 1%) in Japanese population, the individually predetermined assessment of the CYP2C19-mediated metabolic capacity of imipramine would be more valuable than that of the CYP2D6-mediated capacity for forecasting the steady-state concentrations of imipramine and desipramine in Japanese depressive patients, thereby attaining an individualized optimization of imipramine therapy. Obviously, a pharmacodynamic assessment study conducted simultaneously with predetermined CYP2C19 status is required for supporting this contention.

Ethnic and genetic determinants of omeprazole disposition and effect

OBJECTIVES

To compare the pharmacokinetics and dynamics of omeprazole in white and Chinese subjects.

METHODS

This double-blind two-stage study, performed in the clinical research center of a university hospital, evaluated 15 healthy nonsmoking men (eight white subjects and seven Chinese extensive metabolizers of mephenytoin). Blood samples were obtained over 24 hours after the eighth omeprazole dose (40 mg/day). Omeprazole, omeprazole sulfone, and hydroxyomeprazole pharmacokinetics were calculated from the respective plasma concentration-time curves. Twelve- and 24-hour integrated plasma gastriun (AUCgas12 and AUCgas24) were calculated from the respective plasma gastrin concentrations. A week before the initiation of omeprazole the activities of CYP2D6, CYP2C19, and CYP3A4 were determined by previously established methods.

RESULTS

Omeprazole concentrations were significantly lower (mean area under the plasma concentration time curve extrapolated to infinity [AUCO-infinity] +/- SEM; 7.53 +/- 1.21 versus 12.80 +/- 2.13 mumol.hr.L-1, respectively; p < 0.05) and its oral clearance greater (319 +/- 60 versus 183 +/- 35 ml/min, respectively; p < 0.05) in the white subjects than in the Chinese subjects. Omeprazole and omeprazole sulfone AUCO-infinity values were well correlated with the S/R mephenytoin ratio (r = 0.82 and r = 0.84, respectively; p < 0.001) and with urinary 4'-hydroxymephenytoin (r = -0.58 [p < 0.03] and r = -0.52 [p < 0.02], respectively). Fasting gastrin, AUCgas12, and AUCgas24 were significantly greater in the Chinese subjects than in the white subjects (30.0 +/- 6.4 versus 14.4 +/- 1.2 pmol, respectively [p < 0.02]; 661 +/- 114 versus 334 +/- 38 pmol.hr.L-1, respectively [p < 0.002]; and 1414 +/- 228 versus 747 +/- 99 pmol.hr.L-1, respectively [p < 0.004]). In addition, the S/R mephenytoin ratio and omeprazole AUCO-infinity correlated with the extent of omeprazole induced hypergastrinemia.

CONCLUSION

The metabolism of omeprazole and the rise in gastrin concentration after its administration is genetically determined and ethnically dependent.

Steady-state serum concentrations of the neuroleptic perphenazine in relation to CYP2D6 genetic polymorphism

Steady-state serum concentration to dose ratios of the neuroleptic agent perphenazine were related to CYP2D6 metabolizer status for 96 psychiatric inpatients: 88 extensive metabolizers and eight poor metabolizers. The median concentration per dose of the poor metabolizer group (0.195 nmol/L per milligram) was about twice the median (0.098 nmol/L per milligram) of the 56 extensive metabolizers without interacting medicine (p < 0.01). The rest of the extensive metabolizers (n = 32), who were comedicated with drugs that compete with perphenazine for metabolism by CYP2D6, had an intermediate median value of 0.140 nmol/L per milligram. The range of concentration/dose values for the total extensive metabolizer group extended from 0.025 to 0.688 nmol/L per milligram, that is, an almost thirtyfold variation. The concentration/dose range of the eight poor metabolizer subjects was 0.096 to 0.750 nmol/L per milligram. Serum levels not corrected for dose overlapped to a large degree among the groups, with a total range from 0.5 to 12 nmol/L. This study points toward a limited information value of CYP2D6 genotyping in the context of therapeutic drug monitoring of perphenazine.

Debrisoquin and S-mephenytoin hydroxylation phenotypes and CYP2D6 genotypes in an Estonian population

The polymorphisms of debrisoquin (CYP2D6) and S-mephenytoin (CYP2C19) hydroxylation were studied in 210 unrelated healthy native Estonians by coadministration of mephenytoin and debrisoquin or dextromethorphan. Among the 210 volunteers 21 (10%) were poor metabolizers of debrisoquin/dextromethorphan and two (0.95%) were poor metabolizers of S-mephenytoin. By pooling these data with an earlier study on 156 Estonians, the prevalences of poor metabolizers of debrisoquin/dextromethorphan and poor metabolizers of S-mephenytoin were 7.6% and 2.2%, respectively. The CYP2D6 genotype of 151 subjects was analysed by allele-specific PCR amplification for the defect alleles CYP2D6A and CYP2D6B. All poor metabolizers of debrisoquin carried two defect CYP2D6-alleles. The phenotype (extensive or poor metabolizer) was in all subjects correctly predicted by the genotype. The frequencies of the defect alleles CYP2D6B and CYP2D6A among these 151 subjects (including 14 poor metabolizers-9.3%) were 21.5% and 2.3%, respectively. DNA from 6 subjects with very high CYP2D6 activity (debrisoquin MR < 0.1) was analysed by EcoRI RFLP to identify duplicated or amplified CYP2D6-genes. Two of the subjects were found to carry a duplicated CYP2D6L-gene. In conclusion, the distribution of genetically determined metabolic capacities of CYP2D6 and CYP2C19 in Estonian unrelated subjects did not differ significantly from that in other Caucasian populations. The CYP2D6 phenotype was predicted by PCR-based amplification for the CYP2D6A and CYP2D6B-alleles.

The genetics of pain and pain inhibition

The present review summarizes the current state of knowledge about the genetics of pain-related phenomena and illustrates the scope and power of genetic approaches to the study of pain. We focus on work performed in our laboratories in Jastrzebiec, Poland; Portland, OR; and Los Angeles, which we feel demonstrates the continuing usefulness of classical genetic approaches, especially when used in combination with newly available molecular genetic techniques.

Interethnic differences in drug metabolism: influence of genetic and environmental factors on debrisoquine hydroxylation phenotype

Genetic and environmental factors are determinants of the interindividual and interethnic variability in drug metabolism. The metabolism of several important drugs (e.g. haloperidol) cosegregates with that of debrisoquine. Thus, interethnic differences in debrisoquine hydroxylation polymorphism (CYP2D6) might be partly responsible for the variation in haloperidol disposition between races. The influence of tobacco, ethanol, caffeine, gender, and oral contraceptive use on the debrisoquine metabolic ratio (MR) has been analyzed in 633 Spanish healthy volunteers. MR was also determined in panels of healthy volunteers. 18 smokers were studied during a tobacco abstinence period, and 31 women three times during the same menstrual cycle. Among EMs, debrisoquine MR was significantly (P < 0.05) lower during smoking cessation (mean antilog +/- SD, 0.48 +/- 0.29) compared to a smoking period (0.61 +/- 0.23). During the lutheal phase of the menstrual cycle, debrisoquine MR was also significantly (P < 0.01) lower (0.33 +/- 0.41) compared to the ovulatory-phase (0.41 +/- 0.34) and the phase before ovulation (0.44 +/- 0.36). Among EMs, it is suggested that debrisoquine MR may be modified by tobacco smoking and sexual hormones. The clinical relevance of these findings remains unclear.

Gender differences in pharmacokinetics

There is growing awareness that the underrepresentation of women in clinical trials and in particular in phase I studies may lead to incorrect handling of drugs. Despite the fact that investigations are not informed in a systematic way, there are a number of examples showing pharmacokinetic differences between gender. From the data actually presented, it can be concluded that the activity of CYP 3A4 activity as measured by elimination in vivo is higher in women compared to men. CYP isoenzymes other than CYP 3A4 seem to be more active in men than in woman, as are conjugation reactions, such as glucuronidation. The influence of changing hormonal levels during the lifetime of a woman has been looked at in some drugs but deserves further systematic investigation. The use of oral contraceptives can interfere with the metabolism of many drugs whereas, in pregnancy, the elimination of antiepileptics is increased which, without dose adjustment, leads to an increased number of seizures. The impacts of hormonal replacement therapy (HRT) on the pharmacokinetics of concomitantly given drugs is an important issue, as HRT is increasingly used, but more research is needed in this field.

The effect of age and concomitant treatment with other psychoactive drugs on serum concentrations of citalopram measured with a nonenantioselective method

We measured citalopram and desmethylcitalopram concentrations in serum from 169 psychiatric patients, who were treated with common therapeutic drug doses. Altogether 202 serum samples were assayed by a nonenantioselective high-performance liquid chromatography (HPLC) method. The results indicate that the kinetic variability (maximum concentration/minimum concentration) in dose- and weight-related serum citalopram (10.6-fold) and desmethylcitalopram (7.2-fold) is large even during monotherapy. Log serum citalopram (r = 0.36, p < 0.05) and desmethylcitalopram (r = 0.51, p < 0.01) concentrations of individual patients increased significantly with increasing drug doses. Dose- and weight-related (calculated as mg/kg dose basis) log serum citalopram (r = 0.29) but not desmethylcitalopram (r = 0.06) concentrations increased with aging (p < 0.001). No sex-related differences were found. Nineteen patients (19 samples) had concomitant treatment with neuroleptics, 84 patients (101 samples) with benzodiazepines, and 18 patients (28 samples) with tricyclic antidepressants. The concentrations in these patients were compared with those of 48 nonsmoking patients (54 samples) without any concomitant psychotropic drug treatment. None of the single neuroleptics alone had a significant effect on dose- and weight-related serum citalopram or desmethylcitalopram concentrations. However, citalopram concentrations increased by 121% (338 +/- 165 vs. 747 +/- 505, mean +/- SD; p < 0.01) and desmethylcitalopram by 85% (124 +/- 53 vs. 229 +/- 138; p < 0.05) when neuroleptics were pooled. Among single benzodiazepines, only alprazolam increased serum citalopram (338 +/- 165 vs. 391 +/- 267; p < 0.01) and desmethylcitalopram (124 +/- 53 vs. 186 +/- 175; p < 0.01) concentrations. When all the benzodiazepines were pooled, they still increased the serum concentration of the parent drug by 23% (338 +/- 165 vs. 414 +/- 303; p < 0.05) and those of the metabolite by 47% (124 +/- 53 vs. 182 +/- 163; p < 0.01). In patients who were simultaneously treated with clomipramine, serum citalopram (338 +/- 165 vs. 655 +/- 409; p < 0.001) and desmethylcitalopram (124 +/- 53 vs. 435 +/- 347; p < 0.001) concentrations were consistently higher than those of the controls. Even when the tricyclic antidepressants were pooled, they increased citalopram concentrations by 44% (338 +/- 165 vs. 486 +/- 312; p < 0.001) and desmethylcitalopram concentrations by 111% (124 +/- 53 vs. 261 +/- 260; p < 0.001). The results suggest that interindividual variability in serum citalopram concentrations is pronounced and that increased serum citalopram levels are related to advancing age and concomitant treatment with other psychotropic drugs. The citalopram dose should therefore ideally be individualized by therapeutic drug monitoring.

Investigation of xenobiotic metabolism by CYP2D6 and CYP2C19: importance of enantioselective analytical methods

Investigations into the genetic polymorphism of drug metabolism have involved specific models to screen poor and extensive metabolisers of xenobiotics. Debrisoquine, sparteine, S-mephenytoin and dextromethorphan are particularly well known. They have been extensively described in the literature and are used to phenotype human subjects before performing investigations with new drugs which are believed to be under the control of a genetic polymorphism. Dextromethorphan, debrisoquine and sparteine are good substrates for CYP2D6, whereas the S-enantiomer of mephenytoin is a good substrate for CYP2C19, both being two isozymes of cytochrome P-450. In many drugs, the hepatic microsomal oxidative metabolism involving stereogenic centres congregates either with CYP2D6 or with CYP2C19 or, in certain cases, with both of them. The availability of both CYP2D6 from poor and extensive metabolisers and an enantioselective assay would allow genetic polymorphism in drug biotransformation to be investigated in vitro ex vivo at an early stage of drug development before the IND (investigational new drug). Single-dose investigations in vivo can also be performed when only minimal pre-clinical toxicological data are available and produce more reliable results than in vitro studies. This paper focuses on the problem of genetic polymorphism in drug development and specifically discusses some relevant knowledge gained in the last two decades on enantioselective bioassays. Specific examples are given.

Chloroguanide metabolism in relation to the efficacy in malaria prophylaxis and the S-mephenytoin oxidation in Tanzanians

S-Mephenytoin and chloroguanide (proguanil) oxidation was studied in 216 tanzanians. The mephenytoin S/R ratio in urine ranged from <0.1 to 1.16. The distribution was skewed to the right, without evidence of a bimodal distribution. Ten subjects (4.6%, 2.2% to 8.3%, 95% CI) with an S/R mephenytoin ratio >0.9, were arbitrarily defined as poor metabolizers of mephenytoin. The chloroguanide/cycloguanil ratio ranged from 0.82 to 249. There was a significant correlation between the mephenytoin S/R ratio and the chloroguanide/cycloguanil ratios (rs = 0.73; p<0.00001). This indicates that cytochrome P4502C19 or CYP2C19 is a major enzyme that catalyzes the bioactivation of chloroguanide to cycloguanil. Chloroguanide is a pro-drug, and hence a low CYP2C19 activity may lead to prophylactic failure caused by inadequate formation of cycloguanil. Fifty-eight women who previously took either 200 mg chloroguanide daily (n = 26) or 200 mg chloroguanide daily plus 300 mg chloroquine weekly (n = 32) in a malaria chemoprophylaxis study showed that there was significant correlation between the number of earlier breakthrough parasitemia episodes and the chloroguanide/cycloguanil ratio (rs = 0.30; p = 0.02). The breakthrough rate did not correlate with the S/R mephenytoin ratio. However, other factors, such as exposure to mosquitoes and sensitivity of the plasmodium to cycloguanil, are probably more important.

Simple and selective assay of 4-hydroxymephenytoin in human urine using solid-phase extraction and high-performance liquid chromatography with electrochemical detection and its preliminary application to phenotyping test

A simple and selective HPLC method for the determination of 4-hydroxymephenytoin (4-OH-M) in human urine, using a controlled potential coulometric detector equipped with a dual working electrode cell of fully porous graphite, has been developed. After acid hydrolysis of urine, 4-OH-M and the internal standard (I.S.), 5-hydroxy-1-tetralone, were extracted from urine by means of a Bond Elut Certify LRC column. The extracts were chromatographed on a reversed-phase mu Bondapak C18 column using methanol-50 mM KH2PO4 (pH 4.0) (30:70, v/v) as the mobile phase at a flow-rate of 1.0 ml/min. Electrochemical detection at applied potential of 800 mV resulted in a limit of quantitation of 0.76 micrograms/ml. The method showed a satisfactory sensitivity, precision, accuracy, recovery and selectivity. The present method was applied to the phenotyping test in thirteen Japanese healthy volunteers who received an oral 100-mg racemic mephenytoin. The phenotypes determined by the present method were found to be in agreement with those obtained with the reported customary assay based on gas chromatography.

Antidepressants and drug-metabolizing enzymes--expert group report

Antidepressant drugs are extensively metabolized. Consequently, the biotransformation pattern of antidepressants has an important influence on their clinical properties, i.e., pharmacokinetics, toxicity, drug-drug interactions, side-effect profile and last but not least therapeutic efficacy. It was against this background that a multidisciplinary group of experts discussed the clinical relevance of the rapidly increasing body of knowledge of antidepressant-metabolizing enzymes. The variability of the response of a given individual to an antidepressant is determined genetically and by the environment. Genetic polymorphism of drug-metabolizing enzymes and inhibition by other substrates may affect the enzymatic biotransformation of antidepressants. In vitro assay techniques allow an estimation of the potential variability in clinical response to antidepressants and a reasonable prediction of the drug-drug interaction patterns. The results of in vitro tests should therefore be considered early in the development of an antidepressant as a background for designing clinical studies (treatment schedules and dosing). Physicians should have an understanding of the relevance of genetic polymorphism for clinical practice. Education is needed in order to fill the existing gaps in knowledge about antidepressant-enzyme interactions and their application in daily treatment practice. The information on potential drug interactions determined by genetic polymorphism and based on studies with enzymes should be increasingly contained in drug compendia.

Paroxetine shifts imipramine metabolism

The combination of selective serotonin reuptake inhibitors with tricyclic antidepressants has proven useful in treatment-resistant depression but has the potential for adverse drug-drug interactions. In the present study, the metabolism of a single dose of imipramine was studied before and after treatment with paroxetine. Paroxetine induced significant elevations of approximately 50% in half-life, area under the curve, and Cmax of imipramine and decreased clearance twofold. The effects on desipramine pharmacokinetics were even more pronounced. These findings indicate a significant interaction of paroxetine with the CYP2D6 isoenzyme.

Dosing of antidepressants--the unknown art

Data from a therapeutic drug monitoring service, in total 2,393 observations in 1,606 patients, were used to analyze factors associated with the prescribed daily doses of the tricyclic antidepressants amitriptyline and nortriptyline. The achieved concentrations in plasma were evaluated in relation to suggested therapeutic ranges. The doses of both drugs were greatly reduced with increasing age, despite the fact that age is of minor importance for their kinetics. Interactions with concomitantly given drugs were not handled by dose adjustments of the antidepressant. Therapeutic drug monitoring increased the proportion of concentrations within the therapeutic range for patients on amitriptyline, but not for those on nor-triptyline. The large interindividual kinetic variability for most antidepressants requires individualized dosing, but this individualization is performed on incorrect grounds.

S-mephenytoin hydroxylation phenotypes in a Jordanian population

We tested the ability of 194 unrelated, healthy Jordanian volunteers to metabolize S-mephenytoin. Mephenytoin (100 mg) was coadministered with debrisoquin (10 mg) orally and urine was collected for 8 hours. Mephenytoin metabolism was tested according to three measures: the amount of 4-hydroxymephenytoin, the S/R enantiomeric ratio, and the presence of a polar, acid-labile metabolite in urine collected for 8 hours after the dose. The S/R ratio and the presence of the acid-labile metabolite were determined in the urine of 16 patients who had low amounts of 4-hydroxymephenytoin (log hydroxylation index > or = 1). On examination of these three parameters of oxidation status, nine subjects were found to be poor metabolizers of mephenytoin by all three parameters. Thus 4.6% (95% confidence interval of 1.6% to 7.6%) of Jordanian subjects studied were poor metabolizers of mephenytoin. According to the Hardy-Weinberg Law, the frequency of the recessive autosomal gene controlling the poor metabolizer status of mephenytoin was predicted to be 0.215% (95% confidence interval of 0.146% to 0.283%). These results are on the same order of magnitude as those determined in European white populations and constitute the first report in Arab populations.

An unequal cross-over event within the CYP2D gene cluster generates a chimeric CYP2D7/CYP2D6 gene which is associated with the poor metabolizer phenotype

1. The study of the CYP2D genotype and phenotype of a Caucasian family revealed that a XbaI-9 kb allele was associated with the poor metabolizer phenotype. 2. A Polymerase Chain Reaction (PCR)-based assay showed that the previously described mutations D6A and D6B are not associated with the XbaI-9 kb allele. 3. To explore the molecular basis of the poor metabolizer phenotype associated with the XbaI-9 kb allele, complete sequencing of the nine exons and intron-exon boundaries of the CYP2D6 gene was undertaken after amplification by PCR. 4. All the exons were successfully amplified using CYP2D6 gene-specific primers except exon 1 which required a combination of CYP2D7 gene-specific 5' primer and a CYP2D6 gene-specific 3' primer. 5. Sequence data derived from this amplified product revealed that the XbaI-9 kb allele corresponds to a novel rearrangement of the locus. This involved a deletion of an approximately 20 kilobase (kb) DNA segment generating a hybrid 5' CYP2D7/CYP2D6 3' gene. 6. The chimeric gene is non-functional presumably due to an insertion in exon 1 (characteristic of the exon 1 of the CYP2D7 gene) which causes a shift in the reading frame with premature termination of translation.

Geographical/interracial differences in polymorphic drug oxidation. Current state of knowledge of cytochromes P450 (CYP) 2D6 and 2C19

The isoenzymes which catalyse the polymorphic hydroxylations of debrisoquine/sparteine and S-mephenytoin are cytochromes P450 2D6 and P450 2C19 (CYP2D6 and CYP2C19), respectively. CYP2D6 is involved in the stereospecific metabolism of several important groups of drugs, for example antiarrhythmics, antidepressants and neuroleptics. About 7% of Caucasians but only 1% of Orientals are poor metabolisers (PMs) of debrisoquine. The most common mutated allele CYP2D6B in Caucasian PMs is almost absent from their Oriental counterparts. On the other hand, the mean activity of CYP2D6 in Oriental extensive metabolisers (EMs) is lower than that in Caucasian EMs. This is due to the frequent distribution of a partially deficient CYP2D6 allele causing a Pro34-->Ser amino acid exchange in as many as 50% of Oriental alleles. This is the molecular genetic basis for slower metabolism of antidepressants and neuroleptics observed in Oriental compared with Caucasian people, and consequently for the lower dosages of these drugs used. While CYP2D6 catalyses the metabolism of lipophilic bases only, CYP2C19 is involved in the metabolism of acids (e.g. S-mephenytoin), bases (e.g. imipramine and omeprazole) and neutral drugs (e.g. diazepam). About 3% of Caucasians and 12 to 22% of Orientals are PMs of S-mephenytoin. Polymerase chain reaction-based genotyping techniques recently became available for the two CYP2C19 mutated alleles m1 and m2, which cause no enzyme to be expressed. M1 accounts for about 80% of the mutations responsible for the PM phenotypes in Caucasians, Oriental and Black people. Diazepam is partially demethylated by CYP2C19, and the high frequency of mutated alleles in Orientals is probably the reason why such populations have a slower metabolism and are treated with lower doses of diazepam than Caucasians. Omeprazole is to a major extent hydroxylated by CYP2C19, and there is an approximately 10-fold difference in oral clearance between EMs and PMs of S-mephenytoin. The separation of Caucasians from Orientals is fairly recent in the evolutionary process (40,000 to 60,000 years ago); the separation of Black from Caucasian/Oriental people occurred much earlier, about 150,000 years ago. As pronounced differences have been found between Caucasians and Orientals in the CYP2D6 and CYP2C19 enzymes, it might be expected that Black people will show even greater differences in this respect. Some studies have been performed with Black participants, but the picture is not clear. The mean CYP2D6 activity in Black EMs seems to be lower than that in Caucasian EMs and similar to that of Oriental EMs.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for the polymorphic oxidation of debrisoquine and proguanil in a Khmer (Cambodian) population

The frequency distributions of the urinary metabolic ratios of debrisoquine and proguanil were measured in a population of unrelated Khmers. Out of 98 Khmer subjects studied, two were identified as poor metabolisers of debrisoquine when a metabolic ratio of 12.6 was used as the cut off point. This represents a prevalence of debrisoquine poor metabolisers of 2.1% (95% confidence interval 0.25-7.3%) which is similar to other Asian populations. Based on the distribution of the ratio of proguanil to cycloguanil excreted in urine, and using an antimode value of 10, the prevalence of poor metabolisers of proguanil in a Khmer population was estimated to be 18.4% (95% confidence interval 10.9-28.1%). The frequency of poor metabolisers of proguanil in Khmers was higher than that described for Caucasian populations, but similar to most reported results in Asian populations.

Comparison of the interaction potential of a new proton pump inhibitor, E3810, versus omeprazole with diazepam in extensive and poor metabolizers of S-mephenytoin 4'-hydroxylation

OBJECTIVE

To compare the interaction potential of E3810, [(+/-)-sodium 2-[[4-(3-methoxpropoxy)-3-methylpyridin-2-yl]methylsulfinyl] -1H-benzimidazole] a new proton pump inhibitor, and omeprazole with diazepam in relation to S-mephenytoin 4'-hydroxylation status.

STUDY DESIGN

Fifteen healthy male volunteers consisting of six poor metabolizers and nine extensive metabolizers of S-mephenytoin 4'-hydroxylation participated in the study, where two poor and three extensive metabolizers each as a group were randomly allocated to one of the three different treatment sequences with a 3-week washout period among the three trial phases. Each volunteer received an oral once-daily dose of E3810 (20 mg), omeprazole (20 mg), or placebo for 23 days and an intravenous dose (0.1 mg/kg) of diazepam on posttreatment day 8. Plasma concentrations of diazepam and demethyldiazepam were measured up to 16 days after the administration of diazepam.

RESULTS

Diazepam was more slowly metabolized in the poor metabolizers than in the extensive metabolizers. No significant effects of E3810 and omeprazole on any kinetic parameters of diazepam were observed in the poor metabolizers. In the extensive metabolizers, omeprazole significantly decreased the mean clearance of diazepam and increased its half-life, area under the plasma concentration-time curve, and mean residence time compared with E3810 and placebo (p < 0.05 or 0.01), whereas no changes in these kinetic parameters were observed during the treatment with E3810. Omeprazole significantly increased the mean area under the plasma concentration-time curve (0-16 days) of demethyldiazepam in the extensive metabolizers compared with placebo (p < 0.01), whereas E3810 significantly increased it in the poor metabolizers compared with omeprazole or placebo (p < 0.05).

CONCLUSION

The results indicate that E3810 as a substrate goes less toward S-mephenytoin 4'-hydroxylase (CYP2C19) and has a much weaker, if any, potential to interact with diazepam compared with omeprazole.

Comparison of the kinetic disposition and metabolism of E3810, a new proton pump inhibitor, and omeprazole in relation to S-mephenytoin 4'-hydroxylation status

We studied the kinetic disposition and metabolism of E3810 [(+/-)-sodium 2-[[4-(3-methoxypropoxy)-3-methylpyridin-2-yl]methylsulfinyl ]-1H- benzimidazole], a new proton pump inhibitor, and omeprazole in 15 Japanese male volunteers, six of whom were poor metabolizers and nine of whom were extensive metabolizers of S-mephenytoin. All received once-daily 20 mg doses of E3810 or omeprazole for 7 days in a randomized crossover manner, with a 3-week washout period between the two trial phases. The parent drugs and their principal metabolites in plasma and urine were measured on days 1 and 7 after drug administration. The mean values for area under the plasma concentration-time curve (AUC) of omeprazole were 6.3- and 4.4-fold greater, whereas those of E3810 were 1.8- and 1.9-fold greater in poor metabolizers than in extensive metabolizers after the first and final doses, respectively. Although the mean AUC values for both drugs were significantly (p < 0.01 or p < 0.05) greater in poor metabolizers than in extensive metabolizers, the difference in the AUC between the two groups was smaller after E3810 than after omeprazole administration. The AUC of omeprazole tended to increase with the repeated doses in extensive metabolizers, whereas no such change was observed for E3810. The urinary excretions of the principal metabolite(s) of two proton pump inhibitors also reflected the data derived from plasma samples in relation to S-mephenytoin 4'-hydroxylation status. We conclude that the metabolism of two proton pump inhibitors is under coregulatory control of S-mephenytoin 4'-hydroxylase (CYP2C19), but that the magnitude of CYP2C19-mediated metabolism appears to differ between the two drugs. In contrast to omeprazole, the metabolism of E3810 is less saturable in extensive metabolizers during the repetitive dosings.

Interethnic difference in omeprazole's inhibition of diazepam metabolism

OBJECTIVES

To compare the effect of omeprazole, a substrate and inhibitor of CYP2C19, on diazepam metabolism in white and Chinese subjects.

SUBJECTS AND METHODS

The study, which took place at a clinical research center in a University Hospital, was designed as a double blind, crossover, two-stage study; each stage lasted 21 days and was separated by 4 weeks. Subjects were eight white and seven Chinese men who were extensive metabolizers of debrisoquin and mephenytoin. The subjects received, in a randomized order, omeprazole, 40 mg/day, and placebo for 21 days, followed by a 10 mg oral dose of diazepam. Diazepam and desmethyldiazepam plasma concentrations were determined by HPLC during a 26-day period after diazepam administration.

RESULTS

In white subjects omeprazole treatment decreased diazepam clearance by 38% +/- 4.4% and increased desmethyldiazepam area under the plasma concentration-time curve (AUC) by 42.4% +/- 7.0%. In contrast, diazepam oral clearance decreased by only 20.7% +/- 7.3% and desmethyldiazepam AUC decreased by 25.4% +/- 4.6% in the Chinese group. The decrease in diazepam clearance and the prolongation in diazepam and desmethyldiazepam elimination half-lives after administration of omeprazole were significantly greater in the white group than in the Chinese group (p < 0.03, p < 0.001, and p < 0.004, respectively). In the absence of omeprazole, diazepam oral clearance was marginally greater (mean +/- SEM) (34.4 +/- 2.8 ml/min versus 25.2 +/- 3.5 ml/min, p = 0.057, respectively) and the AUC of desmethyldiazepam was significantly lower (8794 +/- 538 micrograms/L.hr versus 16,358 +/- 2985 mg/L.hr, p = 0.04, respectively) in the white subjects compared with the Chinese subjects.

CONCLUSION

The extent of the inhibitory effect of omeprazole on diazepam metabolism is dependent on ethnicity. Further studies are needed to determine the mechanism responsible for this phenomenon.

Comparison of the prevalence of the poor metabolizer phenotype for CYP2D6 between 203 Hmong subjects and 280 white subjects residing in Minnesota

Genetic polymorphism of the P450IID6 (CYP2D6) enzyme system can be an important component of the variability in response to drug therapy. Interpopulation differences in the prevalence of deficiencies of drug-metabolizing enzymes may be clinically important in the selection and dosage of drug therapies for patients. Since 1980, the State of Minnesota has had more than a 1000% increase in population of Hmong refugees from Laos. The Hmong are frequently treated in our institution's international clinic with virtually no systematically acquired knowledge about the ability of this relatively ethnically pure population to metabolize commonly used Western medications. To further our knowledge of drug metabolism in this population, we identified the prevalence of the poor metabolizer phenotype for CYP2D6 in a sample population of Hmong subjects and compared this prevalence to that in a sample population of white subjects. Urine collected after ingestion of dextromethorphan in 237 healthy Hmong and 280 healthy white volunteers was analyzed by HPLC. Based on probit plots of the metabolic ratios (dextro-methorphan/dextrorphan), 8.9% of Hmong subjects and 6.1% of white subjects were assigned the poor metabolizer phenotype (difference not significant). Weak associations were found between body surface area and metabolic ratio for both Hmong and white men and between smoking status and metabolic ratio for white subjects only. We conclude that the prevalence of poor metabolizers for the CYP2D6 enzyme system is similar between Hmong subjects and white subjects residing in Minnesota and that an antimode of 0.3 for metabolic ratio appears to be reasonable for the populations studied.

A PET study of D2 and 5-HT2 receptor occupancy induced by risperidone in poor metabolizers of debrisoquin and risperidone

The hydroxylation of the new antipsychotic drug risperidone to its main, active metabolite 9-hydroxyrisperidone is catalyzed by the hepatic cytochrome P450 enzyme CYP2D6, and cosegregates with the polymorphic hydroxylation of debrisoquin. We have previously examined central D2 dopamine and 5-HT2 receptor occupancy after 1 mg risperidone orally in three healthy subjects who were extensive metabolizers (EM) of debrisoquin, using positron emission tomography and the radioligands [11C]raclopride and [11C]NMSP. In this study, the same experimental design was repeated in two healthy poor metabolizers (PM) of debrisoquin to compare the D2 and 5-HT2 receptor occupancy induced by risperidone in EM and PM. The two PM had much higher plasma concentrations and longer elimination half-lives of risperidone than the three EM. Plasma concentrations of the sum of risperidone and 9-hydroxyrisperidone partly overlapped among the EM and PM. D2 receptor occupancy was 50% and 54% in the two PM, as compared to 40%, 43% and 55% in the EM. 5-HT2 receptor occupancy was 63% and 73%, as compared to 45%, 56% and 68% in the EM. These findings support the view that the active 9-hydroxyl metabolite of risperidone contributes to the in vivo effects of risperidone in humans, and thus partly counterbalances the marked variability in the disposition of risperidone.

Polymorphic drug metabolism in schizophrenic patients with tardive dyskinesia

The metabolism of many neuroleptics cosegregates catalyzed by the polymorphic cytochrome P450 CYP2D6. The population can be phenotyped into extensive metabolizers (EM) and poor metabolizers (PM) with respect to this enzyme's activity. PM are likely to achieve higher than average concentrations of neuroleptic drugs in plasma, with an increased risk of extrapyramidal side effects, possibly including tardive dyskinesia. Sixteen white schizophrenic patients who had developed tardive dyskinesia during long-term neuroleptic treatment were phenotyped with debrisoquine and genotyped by CYP2D6-specific DNA amplification and EcoRI restriction fragment length polymorphism analysis. Only 1 (6%) of the 16 patients had a PM genotype, 8 (50%) were homozygous, and 7 (44%) were heterozygous EM. None had a CYP2D6 genotype indicative of ultrarapid debrisoquine hydroxylation capacity. The patients were also phenotyped with mephenytoin, a probe drug for another polymorphic cytochrome P450, CYP2C19. One patient was a PM of S-mephenytoin, which corresponds to the frequency found in healthy white volunteers. In conclusion, there was no overrepresentation of PM of debrisoquine or of S-mephenytoin among the 16 patients with neuroleptic-induced tardive dyskinesia. However, the PM of debrisoquine had the highest score on the Simpson-Angus Rating Scale and the second highest on the Abnormal Involuntary Movement Scale, despite a very low neuroleptic dose. Also, the debrisoquine MR correlated significantly with the SARS score (rs = 0.685, p < 0.05, N = 10), indicating a relationship between the degree of impaired CYP2D5 activity and the severity of extrapyramidal side effects during neuroleptic treatment.

Phenotyping and genotyping of S-mephenytoin hydroxylase (cytochrome P450 2C19) in a Shona population of Zimbabwe

The S-mephenytoin hydroxylase has recently been identified as cytochrome P450 2C19 (CYP2C19). This enzyme metabolizes mephenytoin, diazepam, omeprazole, and citalopram and has been shown to be polymorphically distributed. One clinical implication of CYP2C19-dependent drug metabolism for persons who reside in tropical regions is in the use of the antimalarial drug chloroguanide hydrochloride, which is apparently biotransformed to its active metabolite by this isozyme. In this investigation we studied mephenytoin metabolism in 103 black Zimbabwean Shona subjects. Four were identified as poor metabolizers (4%). This prevalence is comparable to that in white subjects (2% to 5%) but lower than the 15% to 20% incidence of poor metabolizers among Oriental subjects. Of the subjects phenotyped, 84 were genotyped for the G-->A mutation in exon 5 of CYP2C19, which creates a cryptic splice site, causing the production of a nonfunctional protein. Three of the four poor metabolizers were homozygous for this mutation, whereas the fourth one was heterozygous. The G-->A mutation has been shown to predict the incidence more than 60% of poor metabolizers among white subjects and Japanese subjects, and in the current investigation we also obtained a similar relationship in the black population.