A family and population study of the genetic polymorphism of debrisoquine oxidation in a white British population

Frequency of human CYP2D6 mutant alleles in a normal Chinese population

Substantial differences in the pharmacogenetics of debrisoquine hydroxylation exist between Caucasians and Chinese populations. Among Chinese, not only is the frequency of the poor metabolizer phenotype low, the contribution of the 29B mutation in the CYP2D6 gene is insignificant. By contrast the frequency of D6-J is very common (approximately 0.6) and is clearly related to the activity of debrisoquine hydroxylase. Its presence however, does not predict the poor metabolizer phenotype. The D6-C mutation was also not detectable in our population.

Genetically determined sparteine oxidation polymorphism in a Polish population

The genetic oxidation polymorphism was determined in 160 healthy Polish volunteers from the south-west of Poland (Wrocław region), using sparteine as a model drug. The results of a Polish population study revealed a bimodal distribution of the sparteine metabolic ratio and showed the existence of two oxidation phenotypes designated as extensive and poor metabolizers. The frequency of poor metabolizers in our study (8.8%) compares well with most results of poor oxidation metabolizers in Caucasian populations.

CYP2D6-related oxidation polymorphism in Italy

The distribution of the oxidation polymorphism related to cytochrome CYP2D6 (debrisoquine type) was determined in 246 healthy Italian volunteers. Phenotyping was based on HPLC determination of the dextrometorphan/dextrorphan concentration ratio (metabolic ratio) in urine samples collected over an 8 h interval following a single oral 30 mg dose of dextromethorphan hydrobromide. Urinary excretion of dextromethorphan showed a wide interindividual variability, ranging from < or = 0.04 to 3.9% and from 0.5 to 79.6% of the dose, respectively. Metabolic ratios ranged from < or = 0.001 to 6.6. Eleven of the 246 subjects showed a metabolic ratio greater than 0.30, indicating that 4.5% of the population could be ascribed to the poor metabolizer status. The frequency of the poor metabolizer phenotype in this population is within the range described for other Caucasian ethnic groups.

Is debrisoquine hydroxylation modified during acute viral hepatitis?

Drug metabolism in the liver may be decreased during liver diseases. However, the extent of impairment of specific isozymes of cytochrome P450 is largely unknown. We have studied the debrisoquine hydroxylation capacity of 17 patients with acute viral hepatitis and 106 unrelated healthy subjects. Debrisoquine metabolic ratio was increased in extensive metabolizers (EM) with acute viral hepatitis as compared with healthy EMs (median metabolic ratio: 1.20 vs 0.84, P < 0.05). However, there was no difference in phenotype prevalence between patients and controls. Our results suggest that acute viral hepatitis only has a marginal effect on the activity of CYP2D6 and that substrates of this enzyme may be given in normal therapeutic doses to this category of patients.

Debrisoquine hydroxylase gene polymorphism in meningioma

Cytochrome P450 CYP2D6 polymorphism is an autosomal recessive trait associated with impaired debrisoquine metabolism in 5-10% of caucasian populations. This polymorphism has been associated with susceptibility to Parkinson's disease, bladder cancer, various forms of leukemia and possibly melanoma. In many other cancer forms, the data remained contradictory due to the technical limitations for identifying affected individuals (poor metabolizers). A recently developed polymerase chain reaction-based assay allows convenient screening of approximately 80% of known mutations. We have tested brain tumors correlated with chromosome 22 deviations for genetic polymorphism in the cytochrome P450 CYP2D6 locus localized on chromosome 22q13. Thirty-one meningioma samples were analyzed and the observed frequency of heterozygotes and homozygotes for the G to A mutation did not deviate significantly from the distribution in a normal population. These data are comparable to previous observations in for example breast and colon cancer and indicate that the CYP2D6 locus on chromosome 22q13 is not involved in the pathogenesis of meningiomas.

Polymorphism in the metabolism of drugs, including antidepressant drugs: comments on phenotyping

In neurochemistry there are advantages in determining how patients are likely to react to psychoactive drugs prior to the commencement of drug therapy. Explanations of a patient's nonresponse, or unexpected adverse reactions to drugs are required. In many instances, a knowledge of the drug metabolism status of a patient can be helpful in the selection of a drug and its dosage regimen, and in the prediction of possible drug/drug interactions when two or more drugs have to be administered concomitantly. Important information on these topics may be obtained by phenotyping patients prior to drug therapy. The metabolism of various antidepressant and neuroleptic drugs is catalyzed by CYP2D6, a cytochrome P450 isozyme (also named P450IID6), whereas the metabolism of other drugs may involve different cytochromes P450. The properties of CYP2D6 and four other isozymes (CYP1A1, CYP1A2, CYP2C8/9 and CYP3A4) are described, and substrates identified. Phenotyping of patients for CYP2D6 activity and mephenytoin hydroxylase activity is described.

Disposition of clozapine in man: lack of association with debrisoquine and S-mephenytoin hydroxylation polymorphisms

A large interindividual variability has previously been demonstrated in the bioavailability, steady-state plasma concentrations and clearance of clozapine, an atypical neuroleptic drug. To evaluate the importance of genetic factors in the metabolism of clozapine, its disposition after a single oral dose of 10 mg was studied in 15 healthy Caucasian volunteers. Five of the subjects were poor metabolisers (PM) of debrisoquine, five were PM of S-mephenytoin, and the remaining five were extensive metabolisers (EM) of both probe drugs. There was a 10-fold interindividual variation in Cmax and a 14-fold variation in AUC(0, 24) of clozapine among the 15 subjects studied. The mean (s.d.) Cmax was 117 (81) nmol l-1 and the mean AUC(0,24) value was 890 (711) nmol l-1 h. The value of t1/2,z varied 3-fold with a mean (s.d.) of 13.3 (5.0) h. There were no significant differences in the plasma concentrations or any of the pharmacokinetic parameters of clozapine between PM and EM of debrisoquine, or between the two S-mephenytoin hydroxylation phenotypes. We conclude that neither of the major genetic polymorphisms of oxidative drug metabolism contribute to the large interindividual variability in clozapine pharmacokinetics.

Dextromethorphan O-demethylation polymorphism in Jordanians

The O-demethylation of dextromethorphan (DMT) to dextrorphan (DRP) was studied in 241 unrelated, healthy Jordanian volunteers (171 males, 70 females). Urine was collected for 8 h following a single oral dose of DMT bromhydrate 30 mg. A thin-layer chromatographic (TLC) technique was used to identify the metaboliser phenotype. The frequency of the poor metaboliser phenotype was found to be 2.9% (approximate 95% confidence interval 0.8-5.0%). Applying the Hardy-Weinberg Law, the frequency of the recessive autosomal gene controlling poor metabolism was 0.17 (95% confidence interval 0.108-0.232).

DNA haplotype dependency of debrisoquine 4-hydroxylase (CYP2D6) expression among extensive metabolisers

Deficient debrisoquine/sparteine type oxidation is inherited as an autosomal recessive trait. Of all Caucasians, 5-10% are poor metabolisers, due to the absence of cytochrome P4502D6. Extensive metabolisers (EMs) exhibit highly variable metabolic activity. We investigated the relationship between CYP2D6 activity and genotypes of the CYP2D locus in a large set of French Caucasian families. Genotypes concern both common mutations affecting the enzyme activity and linked BamHI polymorphisms of the locus. We found, like other authors, that in EMs part of the heterogeneity is explained by a subgroup of individuals heterozygous for a mutant allele. However, a second level of heterogeneity was detected among individuals not carrying mutations, and this was related to a polymorphic BamHI-defined DNA haplotype. Different combinations of haplotypes are associated with differences in CYP2D6 metabolic activity. This finding might help to clarify the conflicting data on the relation between CYP2D6 activity and susceptibility to lung cancer.

Determination of debrisoquine and 4-hydroxydebrisoquine in urine by high-performance liquid chromatography with fluorescence detection after solid-phase extraction

A simple, selective and sensitive method has been developed to determine debrisoquine and 4-hydroxydebrisoquine in human urine. Separation of the analytes was obtained using a mobile phase of 0.1 M sodium dihydrogen phosphate-acetonitrile (87:13, v/v) and a muBondapak C18 column. The column effluent was monitored with fluorescence detection at 210 nm (ex) and 290 nm (em). Rapid sample preparation was achieved by solid-phase extraction columns (Bond Elut CBA, 3 ml capacity) which provided excellent recovery values for both compounds. The cost per sample using this approach could be minimized by column regeneration and re-use. The within-day and the day-to-day reproducibilities were less than 7% for both components. The method was shown to be suitable for the study of the debrisoquine-sparteine type genetic polymorphism in man.

Usefulness of propafenone for supraventricular arrhythmias in infants and children

The relation between propafenone dose, serum level, electrocardiographic parameters, antiarrhythmic drug efficacy and adverse effects was studied in 47 children with symptomatic supraventricular arrhythmias aged 1 day to 10.3 years (median 2.2 months) with a mean follow-up of 14.3 months. Propafenone trough serum levels were measured using gas chromatography. Oral propafenone (mean dose 353 mg/m2/day) was effective in 41 of the 47 patients (87.2%). Serum levels did not differ between patients responding and not responding to propafenone (0.45 +/- 0.40 vs 0.36 +/- 0.41 mg/liter). PR interval and QRS complex duration increased more significantly with propafenone dose increments (p < 0.001), than with propafenone serum levels (p < 0.05). At successful treatment PR interval and QRS complex were prolonged by a mean of 19.2 and 20.5% compared with pretreatment status. Five patients exhibited unexpected marked QRS complex prolongation (50 to 200%) despite low propafenone dosage (< 300 mg/m2/day) and level ranging from 0.05 to 1.33 mg/liter. Three patients (6.1%) were suspected of being "poor" metabolizers of propafenone. Mild chronic elevation of serum liver enzymes was observed in 5 patients treated with a larger dose (mean 448 mg/m2/day, p < 0.001). No proarrhythmia was noted on serial Holter monitors. One patient with Wolff-Parkinson-White syndrome and a normal heart had cardiac arrest after aspiration. Serial monitoring of PR interval and QRS complex duration was more useful for proper dosage adjustment than propafenone serum levels. Serum liver enzymes should be closely monitored when using higher propafenone doses. Malignant proarrhythmia could not be excluded in the 1 patient with cardiac arrest.

Drug metabolism and genetic polymorphism in subjects with previous halothane hepatitis

To test the hypothesis that halothane hepatitis is caused by a combination of altered drug metabolism and an immunoallergic disposition, the metabolism of antipyrine, metronidazole, sparteine, phenytoin, and racemic R- and S-mephenytoin was investigated in seven subjects with previous halothane hepatitis. The HLA tissue types and the complement C3 phenotypes were also determined. The metabolism of antipyrine and metronidazole was within normal range in all subjects, and they were all fast or extensive metabolizers of sparteine, mephenytoin, and phenytoin. HLA tissue types were unremarkable. Five of the seven subjects had complement C3 phenotypes F or FS. In the general population phenotype S is the most common, but the difference in complement C3 phenotypes is not statistically significant (p = 0.07). We conclude, although in a limited number of patients, that subjects with previous halothane hepatitis do not appear to be different from controls with regard to drug metabolism and HLA tissue type. The possibility of a higher frequency of complement C3 phenotype F and FS needs further investigation.

Similar effect of oxidation deficiency (debrisoquine polymorphism) and quinidine on the apparent volume of distribution of (+/-)-metoprolol

The influence of phenotype (debrisoquine type of oxidation polymorphism) and quinidine on (+/-)-metoprolol distribution parameters was investigated in 7 young male volunteers (4 extensive and 3 poor metabolisers). (+/-)-Metoprolol tartrate 20 mg was administered as a 20 min infusion i) alone, ii) 12 h after an oral 50 mg quinidine sulphate capsule, and iii) on the last day of 3 days of treatment with 250 mg quinidine sulphate b.d. as a slow-release tablet. No stereoselectivity was apparent in either poor or extensive metabolizers. When (+/-)-metoprolol was administered alone the apparent volume of distribution at steady-state (Vss) was higher in extensive than in poor metabolisers (4.84 vs 2.83 l.kg-1, respectively). Pre-treatment with low or multiple high doses of quinidine decreased Vss in extensive metabolisers to values comparable to those in poor metabolisers (3.50 and 3.18 l.kg-1, respectively), but had no significant effect in poor metabolisers (3.24 and 3.42 l.kg-1, respectively). Estimation of Vss by noncompartmental analysis or assuming elimination exclusively from the peripheral compartment yielded similar, although somewhat higher, estimates. Despite the small number of subjects, (+/-)-metoprolol distribution appeared to be different both in genetically and environmentally (quinidine)-determined poor metabolisers, and quinidine inhibition was a good, reversible in vivo model of the genetic deficiency in handling (+/-)-metoprolol. Differences both in first pass pulmonary elimination or in tissue binding are logically consistent with these observations, but the amplitude of the effect exceeds expectations from available biological evidence on selective pulmonary metabolic activity and on specific tissue binding sites.

Reproducibility over time of mephenytoin and debrisoquine hydroxylation phenotypes

Mephenytoin and debrisoquine hydroxylation phenotypes were determined twice in 15 Spanish healthy volunteers with an interval of about one year. The phenotype assignment did not change in any subject for either debrisoquine or mephenytoin. Among extensive metabolisers of mephenytoin, there was a slight increase (P = 0.04) of the mephenytoin-S/R enantiomeric ratio over the study period. The family members of a poor metaboliser of mephenytoin were phenotyped, and the heterozygous extensive metabolisers were found to have higher mephenytoin-S/R ratios than other extensive metabolisers suggesting a correlation between the genotype and the S/R ratio.

Oxidative polymorphism of debrisoquine is not related to the risk of Alzheimer's disease

Oxidative polymorphism of debrisoquine has been studied in patients suffering from many spontaneous disorders which show genetic and/or environmental factors in their pathogenesis. To elucidate whether any relationship exists between this genetic polymorphism and the risk of developing Alzheimer disease (AD) we determined the oxidative phenotype and metabolic ratio (MR) of debrisoquine (DBQ) in 47 patients with AD or senile dementia of Alzheimer type (SDAT) and 837 healthy controls. The patients were free of drugs during at least the previous 30 days; all the controls were free of drugs. Three patients (6.38%) and 42 controls (5.02%) were classified as poor metabolizers (PM) of DBQ (non-significant difference). The distribution of MR values in the AD/SDAT patients showed non-significant differences when compared with controls. There was no relation between oxidative polymorphism of DBQ and age at onset of the disease. These results suggest that DBQ oxidative genetic polymorphism cannot be considered as a risk factor for developing AD-SDAT.

Polymorphic 2-hydroxylation of desipramine. A population and family study

We have studied desipramine hydroxylation capacity, determined as the metabolic ratio of desipramine to 2-hydroxydesipramine in the urine after a single oral dose of 10 mg of desipramine, in 340 Swedish Caucasians, including the members of 45 two-generation families. Desipramine metabolic ratios were bimodally distributed among 237 unrelated subjects and 8% were poor metabolizers. There was a strong correlation between the metabolic ratios for desipramine and debrisoquine in 337 subjects phenotyped with both drugs and there was no dissociation between their capacities to hydroxylate desipramine and debrisoquine. Complex segregation analysis in the 45 families gave evidence for a major locus with incomplete recessivity (d = 0.14) controlling the 2-hydroxylation of desipramine. Similar results were obtained in segregation analysis for debrisoquine. There was evidence for linkage between the CYP2D6 gene and the gene regulating the hydroxylation of desipramine and debrisoquine. This study has provided unequivocal evidence that the capacity to 2-hydroxylate desipramine is polymorphic and under similar genetic control to the hydroxylation of debrisoquine.

Clinical significance of genetic influences on cardiovascular drug metabolism

Inherited differences in metabolism may be responsible for individual variability in the efficacy of drugs and the occurrence of adverse drug reactions. Among the cardiovascular drugs reported to exhibit genetic polymorphism are debrisoquine, sparteine, some beta-adrenoceptor antagonists, flecainide, encainide, propafenone, nifedipine, procainamide, and hydralazine. The implications of genetic differences in the metabolism of these drugs for cardiovascular therapeutics is the subject of this review.

Pharmacokinetic optimisation of tricyclic antidepressant therapy

Pharmacokinetics has greatly contributed to the elucidation of the variability in clinical response to antidepressants in terms of differences in plasma concentrations due to genetic constitution, age, associated diseases and drug interactions. Despite no general agreement, therapeutic and toxic concentrations have been suggested for some tricyclic antidepressants (TCAs) [amitriptyline, nortriptyline, imipramine, desipramine]. Predictive techniques may be implemented on the basis of which starting TCA dosages may be selected to reach more rapidly those concentrations at which efficacy is more probable. Therapeutic drug monitoring may thereafter assist the clinician in refining the individualisation of the dosage regimen.

Food intake increases the relative oral bioavailability of vanoxerine

Each of 12 healthy male subjects received single oral doses of 100 mg vanoxerine (GBR 12909), a dopamine reuptake inhibitor with potential antidepressant activity, on three different occasions (fasting, after a low-fat meal and after a high-fat meal) according to a randomized, cross-over design. The mean tmax value increased from 0.82 h after fasting to 1.44 h after a low-fat meal and to 2.46 h after a high-fat meal. Only modest food effects were seen on mean Cmax values (55 nM, 52 nM and 84 nM, after fasting, after the low-fat meal and after the high-fat meal, respectively) but values of AUC up to the last measurable concentration (AUC(0,t)) increased by 76% (from 110 to 194 nM h) after the low-fat meal and by 255% (from 110 to 391 nM h) after the high-fat meal compared with fasting. All of these effects were statistically significant except for the differences in tmax and Cmax between fasting and the low-fat meal. The mechanism of these changes is unclear, but it seems likely that food may lower the first-pass metabolism of vanoxerine, as has been shown for other lipophilic basic drugs.

Polymorphism of debrisoquine and mephenytoin hydroxylation among Estonians

Debrisoquine and S-mephenytoin hydroxylation polymorphisms were studied in 156 unrelated native Estonians. The hydroxylation phenotypes were assessed by coadministration of mephenytoin with debrisoquine or dextromethorphan. The frequency of the poor metaboliser phenotype of debrisoquine/dextromethorphan was 4.5% (95% confidence interval 1.2-7.8%), and that of mephenytoin was 3.9% (95% confidence interval 0.9-6.9%) among Estonians, which is very similar to what has been reported in other Caucasian populations.

Molecular heterogeneity of the XbaI defined 44kb allele of the CYP2D locus within the Caucasian population

1. Cytochrome P450 debrisoquine (CYP2D6) activity is polymorphic and under genetic control. Most Caucasians are extensive metabolizers, but 5%-10% are poor metabolizers. 2. Restriction fragment length polymorphism analysis of the CYP2D6 locus identifies a 29kb XbaI fragment, either normal (D6-wt) or mutated, and three mutated XbaI alleles (44kb, 11.5kb and 16 + 9kb). The 44kb allele was initially considered as a poor metabolizer allele owing to a D6-B mutation, but cases of 44kb allele not carrying the D6-B, and therefore potentially functional, have been found. The degree of molecular heterogeneity of this allele was investigated by phenotype and genotype analysis of families. 3. Thirty-one French Caucasian families, representing 117 individuals, possessing at least one 44kb allele in each family were selected. Phenotypes were determined using dextromethorphan, and the XbaI, NcoI and BamH1 RFLPs of 42 independent chromosomes were analyzed. 4. 80% of the XbaI 44kb alleles carried the CYP2D6-B mutation and had an additional NcoI fragment (12.5kb or 4.8kb). The remaining 20% did not carry the CYP2D6-B or A mutations and had no extra NcoI fragment. 5. Information on three families demonstrated that 44kb alleles not carrying the CYP2D6-B mutation were associated with the extensive metabolizer phenotype. 6. We conclude that a substantial percentage of XbaI 44kb alleles is associated with a functional CYP2D gene, and therefore, that the XbaI 44kb allele is not consistently a poor metaboliser allele.

The Role of Pharmacogenetics in Cancer Chemotherapy and the Development of Malignancy

The role of pharmacogenetics in the area of cancer chemotherapy and the development of malignancy has not been well defined. Only four chemotherapeutic agents have been evaluated for toxicity or clinical response based on genetic differences in metabolism. These include 5-fluorouracil, 6-mercaptopurine, amonafide, and cyclophosphamide. Severe toxicity of 5-fluorouracil and amonafide due to individual differences in drug metabolism has been reported in the literature. Tumor response in leukemic children may be associated with genetic differences in metabolism of 6-mercaptorpurine. The development of malignancy may be secondary to an individual's ability to detoxify carcinogens found in the environment. For example, the incidence of bladder cancer appears higher in subjects who have occupational exposure to aromatic amines and the slow acetylator phenotype. Some evidence also exists that smokers who are very extensive metabolizers of debrisoquin may be more prone to developing lung cancer. Strong evidence for an association between other cancer types and pharmacogenetics requires more study.

Relationship between personality and debrisoquine hydroxylation capacity. Suggestion of an endogenous neuroactive substrate or product of the cytochrome P4502D6

We administered the Karolinska Scales of Personality to 225 healthy subjects in Spain selected from a group of 925 individuals previously phenotyped with regard to their capacity to hydroxylate debrisoquine. A significant relationship was found between the scores in as many as 4 of the 15 subscales (psychic anxiety, psychasthenia, inhibition of aggression and socialization) and the debrisoquine hydroxylation capacity. Poor metabolizers were more anxiety-prone and less successfully socialized than extensive metabolizers of debrisoquine. This and a previous study among subjects in Sweden suggest that there may be a relationship between personality and the activity of the enzyme hydroxylating debrisoquine (cytochrome P4502D6). This polymorphic enzyme may have an endogenous neuroactive substrate or product, such as a biogenic neurotransmitter amine.

Genetically Determined Polymorphisms in Drug Metabolism

Many factors can influence the metabolism and disposition of drugs. Genetically determined differences in an individual's capacity to metabolize drugs are known causes of interindividual and interethnic variabilities in drug disposition and response. In general, a poor metabolizer for a specific metabolic pathway would likely develop adverse effects, and an extensive metabolizer for the same metabolic pathway might have less than optimal response. Although there are different types of polymorphism in drug metabolism, polymorphisms in debrisoquine-type oxidation, S-mephenytoin oxidation, and N-acetylation have been the most extensively studied. This article will present the basic concepts of pharmacogenetics, review the major types of metabolic polymorphisms, outline ways to determine phenotyping and genotyping differences in metabolizing enzyme activities, and discuss how these differences relate to drug metabolism, response, and toxicity. When evaluating drug response and adverse reactions in individual patients, an awareness of genetic differences in metabolic capacities would help contribute to optimization in drug therapy.

Prediction of phenotype for acetylation and for debrisoquine hydroxylation by DNA-tests in healthy human volunteers

The debrisoquine/sparteine-type polymorphism of drug oxidation and the polymorphism for acetylation are two common inherited variations in human drug metabolism. The phenotypes for hydroxylation and acetylation can be predicted be newly developed methods based on mutation-specific amplification of DNA by the polymerase chain reaction (PCR), which also allow for identification of heterozygous carriers of one mutant allele. In the present study, the results of genotyping of 81 healthy European volunteers were compared with the phenotype obtained by the classical biochemical approach using debrisoquine and caffeine as probe drugs. Genotyping correctly predicted all 73 extensive metabolisers (EMs) and 6 out of 8 poor metabolisers (PMs) of debrisoquine. All 48 rapid acetylators and 33 of 35 slow acetylators were predicted. Overall, the DNA analysis result matched the in vivo phenotype in 97.5% of individuals.

Steady-state plasma levels of clomipramine and its metabolites: impact of the sparteine/debrisoquine oxidation polymorphism. Danish University Antidepressant Group

After an initial placebo week, 37 depressed inpatients were treated with the fixed dose of 75 mg clomipramine b.d. A sparteine test was carried out during the placebo period and again during the second week of active therapy. Blood for drug assay was collected at the end of the inter-dose interval in the (morning) at weekly intervals. Clomipramine and four metabolites (desmethylclomipramine, didesmethylclomipramine, 8-hydroxyclomipramine, and 8-hydroxydesmethylclomipramine) in plasma were assayed by reversed phase HPLC. The clomipramine and desmethylclomipramine steady-state plasma levels varied by factors of 11 and 9, respectively, and the clomipramine/8-hydroxyclomipramine and desmethylclomipramine/8-hydroxydesmethylclomipramine ratios both varied by 7-fold. During the placebo week, 36 patients were phenotyped as extensive metabolizers (EM) (metabolic ratio, MR, 0.1-2.0), and one patient was phenotyped as a poor metabolizer (PM) (MR > 300). During clomipramine treatment, one patient changed phenotype from EM to PM (MR = 140). In the EM, the median of the MR increased from 0.4 to 2.3. There was a statistically significant correlation between the MR before and during clomipramine treatment, even when the PM was excluded. Neither the steady-state plasma clomipramine levels nor the clomipramine/desmethylclomipramine ratios showed a significant correlation with the MR. In contrast, the desmethylclomipramine and didesmethylclomipramine steady-state levels and the desmethylclomipramine/8-hydroxydesmethylclomipramine and clomipramine/8-hydroxyclomipramine ratios showed a significant positive correlation with the MR. The PM had the highest steady-state plasma desmethylclomipramine level and the highest desmethylclomipramine/8-hydroxydesmethylclomipramine ratio. These correlation coefficients (rs) were generally increased when the correlation analyses were based on the MR obtained during clomipramine treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Debrisoquine hydroxylation in Parkinson's disease

Debrisoquine (DBQ) metabolism was studied in 80 Parkinson's disease (PD) patients, 26 of whom had young onset Parkinson's disease (YOPD), and in 143 controls. There was no significant difference between the proportion of poor metabolisers of DBQ among YOPD patients compared either to other parkinsonians, or to controls. Nor was there a significant correlation between the age of disease onset and DBQ metabolic ratio (MR). The results do not support the suggestion that impairment of DBQ metabolism (and hence cytochrome P450) is a primary defect in YOPD. However, in comparison with controls, MR values were modestly but significantly higher in PD patients, even in those not treated with drugs known to affect DBQ metabolism.

Debrisoquine metabolism in Chinese patients with Alzheimer's and Parkinson's diseases

We determined the oxidative phenotype and metabolic ratio of debrisoquine in 96 Chinese patients with Alzheimer's disease (n = 12), Parkinson's disease (n = 55), and using patients with stroke and cervical spondylosis as controls (n = 29). We did not find any difference in debrisoquine metabolic phenotype among Parkinson's disease, Alzheimer's disease, and control patients as judged by chi-square analysis. In addition, the metabolic ratio of all our patients was less than 12.6. The result suggested that Chinese patients with Parkinson's disease and Alzheimer's disease metabolize debrisoquine at a velocity not different from that of their Western counterparts even though the frequency distribution of debrisoquine metabolism phenotyping in these two populations is quite different.

Debrisoquine hydroxylase gene polymorphism and susceptibility to Parkinson's disease

The pathogenesis of Parkinson's disease may be influenced by genetic and environmental factors. Cytochrome P450 mono-oxygenases help to protect against toxic environmental compounds and individual variations in cytochrome P450 expression might, therefore, influence susceptibility to environmentally linked diseases. The frequency of mutant CYP2D6 alleles was studied in 229 patients with Parkinson's disease and 720 controls. Individuals with a metabolic defect in the cytochrome P450 CYP2D6-debrisoquine hydroxylase gene with the poor metaboliser phenotype had a 2.54-fold (95% Cl 1.51-4.28) increased risk of Parkinson's disease. Determination of CYP2D6 phenotype and genotype may help to identify those at greatest risk of Parkinson's disease and may also help to identify the environmental or metabolic agents involved in the pathogenesis of this disease.

Stereoselective genetically-determined interaction between chronic flecainide and quinidine in patients with arrhythmias

1. Recent reports have indicated a role for the P450IID6 polymorphism in the stereoselective disposition of single doses of the antiarrhythmic flecainide. 2. In this study, we evaluated the effects of adding low dose quinidine, a potent inhibitor of P450IID6, to chronic flecainide therapy in patients with arrhythmias. 3. In five extensive metabolizer patients, quinidine significantly reduced the clearance of R-(-)-flecainide, from 395 +/- 121 (s.d.) to 335 +/- 88 ml min-1. This change was attributable to a decrease in metabolic clearance, was accompanied by decreased formation of the two major metabolites of flecainide and was not observed in a poor metabolizer subject. The renal clearance of R-(-)-flecainide rose significantly. 4. Quinidine did not alter the clearance of S-(+)-flecainide. 5. The pharmacologic effects of flecainide therapy (QRS widening, % arrhythmia suppression) were slightly, but not significantly, increased. 6. In extensive metabolizer patients receiving chronic flecainide, increased plasma concentrations will develop if P450IID6 is inhibited.

Urinary caffeine metabolites in man. Age-dependent changes and pattern in various clinical situations

In an exploratory study the 24-h urinary excretion pattern of caffeine and 14 of its major metabolites was studied in 32 volunteers (adults, adolescents and children), 14 patients either with end stage renal disease or liver cirrhosis, 7 heavy smokers and 27 patients on therapy with cimetidine, allopurinol, theophylline or phenytoin. Caffeine and its metabolites were quantified by UV-absorption after liquid/liquid-extraction and HPLC-separation, which ensured proper analysis of 1-methyluric acid. In adults the renal excretion of caffeine derivatives corresponded to an intake of 509 mg caffeine/day, with 1-methyluric acid as the predominant metabolite. About 69% of caffeine was degraded by the paraxanthine pathway, and theobromine- (19%) and the theophylline pathway (14%) were less important. The ratio of paraxanthine formation to urinary caffeine concentration (= clearance equivalent) was about 2.2 ml.min-1.kg-1 in adults, and the corresponding ratios for theophylline and theobromine were 0.43 ml.min-1.kg-1 and 0.59 ml.min-1.kg-1, respectively. As expected, caffeine degradation was impaired in patients with cirrhosis and was increased in persons who smoked heavily or who were on phenytoin therapy. The results document the possibility of noninvasively investigating gross differences in caffeine disposition by analysis of the urinary pattern of its metabolites.

The human hepatic cytochromes P450 involved in drug metabolism

The cytochromes P450 are a superfamily of hemoproteins that catalyze the metabolism of a large number of xenobiotics and endobiotics. The type and amount (i.e., the animal's phenotype) of the P450s expressed by the animal, primarily in the liver, thus determine the metabolic response of the animal to a chemical challenge. A majority of the characterized P450s involved in hepatic drug metabolism have been identified in experimental animals. However, recently at least 12 human drug-metabolizing P450s have been characterized at the molecular and/or enzyme level. The characterization of these P450s has made it possible to "phenotype" microsomal samples with respect to their relative levels of the various P450s and their metabolic capabilities. The purpose of this review is to compare and contrast the human P450s involved in drug metabolism with their related forms in the rat and other experimental species.

Idiosyncratic reactions to antidepressants: a review of the possible mechanisms and predisposing factors

Antidepressants, a widely used group of drugs, are associated with a range of idiosyncratic reactions affecting in particular the liver, skin and both the hematological and central nervous systems. These reactions seem to be mediated by chemically reactive metabolites formed by the cytochrome P450 enzyme system, the toxicity occurring either directly or indirectly via an immune mechanism. Individual susceptibility is determined by factors, both genetic and environmental, which result in inadequate detoxication of the chemically reactive metabolite. Prevention of such reactions will depend on either the development of new compounds which are not converted to toxic metabolites or by prediction of individual susceptibility prior to drug administration.

The metabolism of mexiletine in relation to the debrisoquine/sparteine-type polymorphism of drug oxidation

1. The relationship between the metabolism of the antiarrhythmic drug mexiletine and the debrisoquine/sparteine-type polymorphism was studied in vitro, using microsomes from six human livers, and in vivo, in nine healthy drug-free volunteers with wide variation in their ability to hydroxylate debrisoquine. 2. There was a strong and similar correlation between the formation rate of both major mexiletine metabolites, p-hydroxymexiletine (PHM) and hydroxymethylmexiletine (HMM), and the high affinity component of dextromethorphan O-demethylase activity in human liver microsomes (rs = 0.94; P less than 0.01). 3. There were marked interindividual differences in the amounts of PHM and HMM excreted in the urine over 48 h after a single 200 mg oral dose of mexiletine hydrochloride. Recoveries of both metabolites were correlated inversely with the debrisoquine/4-hydroxydebrisoquine (D/HD) urinary metabolic ratio (rs = -0.83; P = 0.006 and rs = -0.85; P = 0.004, respectively) and were lower in poor metabolisers of debrisoquine (PM) than in extensive metabolisers (EM). Moreover, PM had the highest values of mexiletine/PHM and mexiletine/HMM urinary ratios. In addition, there was a strong correlation between these two indices of mexiletine hydroxylation and the D/HD metabolic ratios (rs = 0.92; P = 0.001 and rs = 0.90; P = 0.001, respectively). 4. After mexiletine pretreatment, the values for D/HD ratio were significantly increased in EM while corresponding values in PM were similar. 5. These findings are in accordance with previous in vitro data suggesting that PHM and HMM formation is predominantly catalyzed by the genetically variable human liver cytochrome P450IID6 isoenzyme responsible for the debrisoquine/sparteine-type polymorphism of drug oxidation.(ABSTRACT TRUNCATED AT 250 WORDS)

Debrisoquine/sparteine hydroxylation genotype and phenotype: analysis of common mutations and alleles of CYP2D6 in a European population

Four different mutations of the cytochrome P450 CYP2D6 gene associated with the poor metabolizer phenotype (PM) of the debrisoquine/sparteine polymorphism were analyzed by Xba I restriction fragment length polymorphism (RFLP) analysis and a polymerase chain reaction (PCR)-based DNA amplification method in DNA of 394 healthy European subjects; 341 of these were phenotyped by sparteine or debrisoquine administration and urinary metabolic ratios (MR). Our study demonstrates the efficiency of the PCR-test for phenotype prediction; 96.4% of individuals were correctly predicted, i.e., 100% of the extensive metabolizers (EMs) and 86.0% of the poor metabolizers (PMs). In contrast, Xba I RFLP analysis was far less informative, predicting the phenotype in only 26.8% of PMs. By combining both DNA tests, the prediction rate of the PM phenotype increased to 90.6%. A point mutation at a splice-site consensus sequence termed D6-B represented the most common mutant CYP2D6 gene and accounted for more than 75% of mutant alleles. In addition, other known mutations such as D6-D (14%), D6-A (5%), and the rare D6-C mutation bring the identified mutant alleles to greater than 95% of all mutant PM-alleles. Most of Xba I 44-kb alleles were confirmed as mutant alleles carrying the D6-B mutation. However, 9.7% did not have this mutation and may express a functional CYP2D6 gene. Moreover, all Xba I 16 + 9-kb alleles contained the D6-B mutation. Heterozygous EM individuals had a significantly higher MR when compared to homozygous EMs. Genotyping provides an important advantage for investigations of the influence of CYP2D6 activity on drug therapy and its association with certain diseases.

Determination of debrisoquine and metabolites in human urine by gas chromatography-mass spectrometry

A gas chromatographic-mass spectrometric analysis has been developed for the determination of debrisoquine and its metabolites in the urine of healthy individuals (controls) and patients with chronic renal failure. The sensitive and specific assay comprises selected-ion monitoring of the drug and the metabolites 4-hydroxydebrisoquine and 8-hydroxydebrisoquine using guanoxan as the internal standard. The limit of detection is ca. 0.2 microgram/ml. The clinical study shows that the healthy individuals and patients with chronic renal failure can be divided in two groups of extensive metabolizers and poor metabolizers, respectively. The extensive metabolizers excreted large amounts of 4-hydroxydebrisoquine and minor amounts of 8-hydroxydebrisoquine. The poor metabolizers excreted small amounts of 4-hydroxy metabolite, and no 8-hydroxydebrisoquine was detected in the urine.

Phenotypic debrisoquine 4-hydroxylase activity among extensive metabolizers is unrelated to genotype as determined by the Xba-I restriction fragment length polymorphism

1. The major pathway for 4-hydroxylation of debrisoquine in man is polymorphic and under genetic control. More than 90% of subjects (extensive metabolizers, EMs) have active debrisoquine 4-hydroxylase (cytochrome P450IID6) while in the remainder (poor metabolizers, PMs), cytochrome P450IID6 activity is greatly impaired. 2. Within the EM group, cytochrome P450IID6-mediated metabolism of a range of substrates varies widely. Some of this intra-phenotype non-uniformity may be explained by the presence of two subsets of subjects with different genotypes (heterozygotes and homozygotes). 3. Cytochrome P450IID6 substrates have not differentiated between these two genotypes. However, a restriction fragment length polymorphism (RFLP) which identifies mutant alleles of cytochrome P450IID6 locus has been described and can definitively assign genotype in some heterozygous EM subjects. 4. In this study, we used RFLP analysis and encainide as a model substrate to determine if non-uniformity in cytochrome P450IID6 activity among EMs is related to genotype. We tested the hypothesis that heterozygotes exhibit intermediate metabolic activity and that homozygous dominants exhibit the highest activity. We proposed encainide as a useful substrate for this purpose since cytochrome P450IID6 catalyzes not only its biotransformation to O-desmethyl encainide (ODE) but also the subsequent metabolism of ODE to 3-methoxy-O-desmethyl encainide (MODE). 5. A single 50 mg oral dose of encainide was administered to 139 normal volunteers and 14 PMs were identified. Urinary ratios among encainide, ODE and MODE in the remaining 125 EM subjects revealed a wide range of cytochrome P450IID6 activity. However, Southern blotting of genomic DNA digested with XbaI identified obligate heterozygotes in both extremes of all ratio distributions.(ABSTRACT TRUNCATED AT 250 WORDS)