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

Distinct phenotypes and genotypes of debrisoquine hydroxylation among Europeans and Chinese

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A protocol for the safe administration of debrisoquine in biochemical epidemiologic research protocols for hospitalized patients

The genetically determined ability to metabolize the antihypertensive drug debrisoquine has been proposed as a genetic risk factor for primary carcinomas of the lung. To test this hypothesis, the metabolism of the drug was evaluated in a case control study. The subjects were characterized by their ability to metabolize debrisoquine after receiving a test dose of the drug followed by the collection of an 8-hour urine sample. They were classified by laboratory analysis into one of the following three groups: extensive, intermediate, and poor metabolizers. Poor metabolizers comprise 10% of the population and are unable to hydroxylate the drug. This group was expected to be at highest risk for deleterious effects from this medication. A protocol was created that included patient education and blood pressure monitoring to administer this medication safely to a group of patients with cancer who were already compromised. Although poor metabolizers showed a small decrease in systolic and diastolic blood pressure, no significant hypotensive episodes or clinical sequelae were observed in any of the groups. These data suggest that debrisoquine can be administered safely in a controlled clinical setting and will be useful for the characterization of lung cancer patients in biochemical epidemiology studies.

Flecainide: evidence of non-linear kinetics

The pharmacokinetics of flecainide has been studied in 12 patients with ventricular arrhythmias, both after single administration and during chronic treatment. Both the half-life and the AUC were significantly increased during chronic treatment. This suggests that the kinetics of flecainide might be non-linear also in patients with normal kidney and liver function. The increase in plasma flecainide levels during chronic treatment could not be predicted, so close monitoring of its plasma levels is advisable.

Disposition of the neuroleptic zuclopenthixol cosegregates with the polymorphic hydroxylation of debrisoquine in humans

The pharmacokinetics of a single oral dose of the neuroleptic drug zuclopenthixol (10 or 6 mg) was studied in 6 extensive and 6 poor metabolizers of debrisoquine. The peak plasma concentrations of zuclopenthixol did not differ between the phenotypes, whereas the plasma elimination half-life was significantly longer in poor than in extensive metabolizers (29.9 +/- 6.6 vs 17.6 +/- 6.9 h). Accordingly, the total oral plasma clearance was lower in poor than in extensive metabolizers (0.78 +/- 0.27 vs 2.12 +/- 0.65 1/h/kg). Ten of the volunteers had previously participated in a similar study in which the kinetics of perphenazine, another neuroleptic drug, were studied in poor and in extensive metabolizers of debrisoquine. There was a significant correlation between the oral clearance of perphenazine and that of zuclopenthixol among these 10 subjects. The study indicates that the disposition of zuclopenthixol, as well as that of perphenazine, is related to the genetically determined capacity to hydroxylate debrisoquine. The significance of this polymorphism for the clinical use of neuroleptics is discussed.

Genetic analysis of the interethnic difference between Chinese and Caucasians in the polymorphic metabolism of debrisoquine and codeine

The Far Eastern and Caucasian populations are strikingly different with respect to the debrisoquine/sparteine hydroxylation polymorphism. The number of poor metabolizers, as defined for Caucasians, is very low among Chinese and Japanese. We investigated the molecular basis for this difference by analysis of the CYP2D6 gene in 115 Chinese subjects, combined with phenotypic classification of codeine and debrisoquine metabolism. A correlation between the rates of metabolism of these two drugs and genotype, as analyzed by RFLP using XbaI, was observed among the Chinese. A high frequency (37%) of alleles indicative of gene insertions (reflected by XbaI 44kb fragments) was recorded in the Chinese, but was not associated with the poor metabolizer phenotype, as it is in Caucasians. PCR amplification of part of the CYP2D6 gene with mutation specific primers for CYP2D6A (29A) and CYP2D6B (29B) allelic variants revealed that the XbaI 44kb fragment in Chinese apparently contains a functional CYP2D6 gene, in contrast to the situation among Caucasians. The results provide a molecular explanation of the interethnic difference in the metabolism of drugs affected by the debrisoquine hydroxylation polymorphism.

Pharmacokinetics of codeine and its metabolites in Caucasian healthy volunteers: comparisons between extensive and poor hydroxylators of debrisoquine

1. The kinetics of codeine and seven of its metabolites codeine-6-glucuronide (C6G), norcodeine (NC), NC-glucuronide (NCG), morphine (M), M-3 (M3G) and 6-glucuronides (M6G), and normorphine (NM) were investigated after a single oral dose of 50 mg codeine phosphate in 14 healthy Caucasian subjects including eight extensive (EM) and six poor (PM) hydroxylators of debrisoquine. The plasma and urine concentrations of codeine and the metabolites were measured by h.p.l.c. 2. The mean area under the curve (AUC), half-life and total plasma clearance of codeine were 1020 +/- 340 nmol l-1 h, 2.58 +/- 0.57 h and 2.02 +/- 0.73 l h-1 kg-1, respectively. There were no significant differences between EM and PM in these aspects. 3. PM had significantly lower AUC of M3G, the active metabolites M6G, NM and M (P less than 0.0001), and lower partial metabolic clearance by O-demethylation (P less than 0.0001). In contrast, the PM had higher AUC of NC (P less than 0.05) than the EM. There was no difference between PM and EM in the AUC of C6G and NCG, nor in the partial clearances by N-demethylation and glucuronidation. 4. Among EM, the AUC of C6G was 15 times higher than that of codeine, which in turn was 50 times higher than that of M. The AUCs of M6G and NM were about 6 and 10 times higher than that of M, respectively. The partial clearance by glucuronidation was about 8 and 12 times higher than those by N- and O-demethylations, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxidative polymorphism of debrisoquine is not related to human colo-rectal cancer

The oxidative polymorphism of debrisoquine (DBQ) has been determined in 89 patients with colo-rectal cancer and in 556 normal control subjects. Four patients and 34 controls, with a metabolic ratio greater than 12.6, were classified as poor metabolisers of DBQ (n.s.). No difference was found in the distribution of the frequencies of the MR of DBQ between patients and controls. It is concluded that polymorphic oxidation of DBQ is not related to the risk of developing colo-rectal cancer in human beings.

Deletion of the entire cytochrome P450 CYP2D6 gene as a cause of impaired drug metabolism in poor metabolizers of the debrisoquine/sparteine polymorphism

The debrisoquine/sparteine polymorphism is associated with a clinically important genetic deficiency of oxidative drug metabolism. From 5% to 10% of Caucasians designated as poor metabolizers (PMs) of the debrisoquine/sparteine polymorphism have a severely impaired capacity to metabolize more than 25 therapeutically used drugs. The impaired drug metabolism in PMs is due to the absence of cytochrome P450IID6 protein. The gene controlling the P450IID6 protein, CYP2D6, is located on the long arm of chromosome 22. A pseudogene CYP2D8P and a related gene CYP2D7 are located upstream from CYP2D6. This gene locus is highly polymorphic. After digestion of genomic DNA with XbaI endonuclease, restriction fragments of 11.5 kb and 44 kb represent mutant alleles of the cytochrome CYP2D6 gene locus associated with the PM phenotype. In order to elucidate the molecular mechanism of the mutant allele reflected by the XbaI 11.5-kb fragment, a genomic library was constructed from leukocyte DNA of one individual homozygous for this fragment and screened with the human IID6 cDNA. The CYP2D genes were isolated and characterized by restriction mapping and partial sequencing. We demonstrate that the mutant 11.5-kb allele results from a deletion involving the entire functional CYP2D6 gene. This result provides an explanation for the total absence of P450IID6 protein in the liver of these PMs.

Lack of a relationship between the polymorphism of debrisoquine oxidation and lung cancer

1. Determination of debrisoquine oxidation phenotype was carried out in 119 healthy subjects, 135 patients with chronic bronchitis and 153 patients with lung cancer, all of Caucasian origin. 2. A non-Gaussian distribution of the log D/HD ratio was observed in the three groups. 3. Assuming an antimode of 1.12, the proportion of PMs was found to be 6.7% in healthy subjects, 8.9% in chronic bronchitics and 6.5% in patients with lung cancer. These differences were not significant. 4. The presence of a lung tumour itself had no influence on phenotype in a group of 14 patients who were phenotyped before and after surgery. 5. We conclude that a link between debrisoquine phenotype and lung cancer is unlikely.

Debrisoquine oxidation polymorphism in a Tasmanian population

The debrisoquine hydroxylation phenotype was studied in 152 unselected healthy Tasmanian subjects, who were mostly Caucasians of British ancestry. Following a 10 mg oral dose of debrisoquine (D), the ratio of D/4-hydroxydebrisoquine excreted in 8-h urine (metabolic ratio, MR) was determined. MR values were bimodally distributed. Thirteen subjects (8.6%) had MR values from 13.8 to 93.3 and were considered to be poor metabolisers of D, while the others were extensive metabolisers with MR values of 0.04 to 5.4. The D hydroxylation phenotype was not associated with sex. These findings confirm the constancy of D polymorphism in a Caucasian population even after migration to another country.

Ethnic differences in drug disposition and responsiveness

Interethnic differences are important factors accounting for interindividual variations in drug responsiveness. However, these differences in drug response have been a relatively neglected area of investigation, so that similar doses are prescribed to different ethnic populations without consideration of interethnic pharmacokinetic and pharmacodynamic variation. With the increased recognition of genetically determined polymorphism in metabolising ability as an important factor in drug disposition, concern has developed for the importance of individualising drug dose to account for racial differences. The recognition of these differences in drug disposition and responses calls into question the failure of drug licensing authorities to demand information on dosage, efficacy and toxicity in different ethnic groups, and to accept data from limited ethnic groups such as Caucasians. This article reviews the evidence for ethnic differences in drug disposition and sensitivity and should encourage further investigations to elucidate the extent of such differences, their causes and their therapeutic impact.

Concentration-effect relations of 5-hydroxypropafenone in normal subjects

To evaluate the pharmacologic activity of 5-hydroxypropafenone, electrocardiographic changes (PQ and QRS duration) and blood pressure levels were measured in 6 healthy extensive metabolizers of debrisoquine after a single oral dose of 300 mg of this metabolite as a solution in a placebo-controlled, double-blind crossover study. Well-absorbed, with a lag time of 4.4 to 9.8 minutes, 5-hydroxypropafenone reached peak concentrations of 153 to 337 ng/ml after 20 to 51 minutes. The terminal half-life was 506 to 963 minutes. To describe the temporal aspects of the concentration-effect relation, a pharmacokinetic-pharmacodynamic model with a hypothetical effect compartment was applied. The relation between electrocardiographic changes and drug concentration at the effect site could be described by a linear regression model. Significant prolongations of PQ and QRS duration were found in 5 of 6 subjects. There were no changes in QTc interval, blood pressure measurements and heart rate in the supine position. However, blood pressure measurements in the upright position revealed a greater percent decrease of systolic blood pressure than with placebo (mean +/- standard deviation -25.6 +/- 13.8% vs -3.4 +/- 13.1%, p less than 0.05). It is concluded that 5-hydroxypropafenone exerts significant pharmacologic activity in humans as well as animals. Because QRS prolongation in patients treated with class IC antiarrhythmic drugs correlates with the antiarrhythmic effect, our data suggest that 5-hydroxypropafenone may contribute to the therapeutic activity of propafenone in humans.

Clinical consequences of polymorphic drug oxidation

Many characters are genetically regulated as polymorphisms. This means that discrete groups are seen within the distribution of a certain character. Drug metabolism is no exception and the polymorphism of acetylation is recognised since the 50's. Polymorphic drug oxidation was discovered in the 70's and has been extensively studied. There are two fully established polymorphisms in drug oxidation named as the debrisoquine/sparteine and the s-mephenytoin hydroxylation polymorphisms. The metabolism of a number of important drugs cosegregates with that of debrisoquine. Among these drugs are beta-blockers, antiarrhythmics, tricyclic antidepressants and neuroleptics. Apart from accumulation of parent drug and active metabolite, also reduced formation of active metabolite occur for some drugs in slow metabolisers. There are, however, few cases where the presence of polymorphic drug metabolism is of significant disadvantage. The polymorphisms will add to variability in drug clearance but the potential clinical importance should be evaluated for each drug. The cytochrome P-450 isozyme responsible for debrisoquine hydroxylation is of high affinity-low capacity character, which means that it can be saturated under certain circumstances. This will decrease the difference in drug metabolic rate between rapid and low metabolisers as will inhibitors of the debrisoquine isozyme like cimetidine, quinidine and propafenone. The debrisoquine isozyme is not readily inducible. In cases where a major metabolic route or the formation of an active metabolite are polymorphically controlled, knowledge about a patient's oxidator status might be of practical value for dose adjustments especially if there is a narrow therapeutic ratio or an established concentration-effect relationship. For some drugs it is difficult to differentiate between insufficient therapeutic effect and symptoms of overdosage. Tricyclic antidepressants and neuroleptics meet some of these criteria and patients who get recurrent treatment may benefit if the physician has knowledge about debrisoquine metabolic phenotype. Otherwise, the clinical consequences of polymorphisms in drug oxidation seem so far to be limited, considering that a number of disease conditions have not shown any clear association with oxidation status. The polymorphisms in drug metabolism should be considered as a part of natural variability which could in fact be larger with other drugs that do not show polymorphic elimination.

Is environmental carcinogenesis modulated by host polymorphism?

It is now clear that tobacco smoking, alcohol consumption, dietary factors and occupation can all interact with genetic host factors to place one individual at a greater or lesser risk of a particular cancer than another. Phenotypes which confer significantly elevated risks arise from the human CYP1A1. CYP2D6, GST1 and NAT (N-acetyltransferase) genes. The last is the only one remaining to be cloned. It is envisaged that several of these genes may interact in a given circumstance cooperatively to increase susceptibility. For example CYP1A1, CYP2D6 and GST1 genotypes may have additive or multiplicative risks of bronchogenic carcinoma in cigarette smokers. CYP2D6 and NAT genotypes may interact in bladder cancer. The advent of molecular genetics brings us closer to the day when, for example, factory workers in potentially hazardous environments might be screened using PCR methodology on skin scrapings or buccal swabs for their innate susceptibility to combined workplace and lifestyle cancer risks.

Metabolism of debrisoquine and susceptibility to breast cancer

There may exist an association between the genetically determined oxidation status of the antihypertensive agent debrisoquine (DEB) and the propensity to develop tumours. The metabolism of DEB is extensive in 90% of healthy subjects (metabolic ratio = MR = 0-12.6; MR = % DEB excreted divided by % 4-hydroxy-DEB excreted) and poor in 10% (MR greater than 12.6). In patients with cancer of the lung, urinary bladder, and gastrointestinum, the percentage of high metabolizers is increased to greater than 98%. The poor metabolizer mode is almost devoid of cancer patients. It was investigated whether breast cancer patients show a similar association with respect to the oxidative status of DEB. 108 breast cancer patients and 123 women with benign gynecologic disorders received 1 tablet of 10 mg DEB orally in the evening. Urine was collected for the subsequent 8 hrs and analysed for its content of DEB and its main urinary metabolite 4-OH-DEB by means of HPLC. No decreased amount of poor metabolizers was seen in the cancer group.

Debrisoquine oxidation in Parkinson's disease

Variations in the activities of xenobiotic metabolizing liver enzymes may be involved in the pathophysiology of diseases, including Parkinson's disease. We therefore studied the activity of the debrisoquine metabolizing enzyme in 97 patients with newly diagnosed Parkinson's disease. The urine debrisoquine metabolic ratios (MR) of the patients were compared with a group of 176 healthy subjects. There were 4 poor metabolizers (4.1%) among the parkinsonians. This proportion did not differ from that found in the group of healthy subjects (51%). In contrast to earlier finding, the parkinsonian poor metabolizers (PM) had the onset of the disease later than the parkinsonian extensive metabolizers (EM). In the parkinsonian patients, it was observed that the excretion of debrisoquine and 4-OH-debrisoquine into urine correlated inversely with the actual age and age at disease onset. Our results indicate that in patients with Parkinson's disease, debrisoquine hydroxylation is comparable with healthy subjects.

Is there a link between debrisoquine oxidation phenotype and lung cancer susceptibility?

A link between debrisoquine oxidation phenotype and lung cancer susceptibility has been evoked by some authors but not confirmed by others. For this reason, we compared the frequency of debrisoquine poor metabolizer phenotype in 91 Belgian patients with lung cancer to that studied in 167 healthy Belgian subjects. No significant difference was observed in our study. These results do not support the existence of this link.

Single-dose quinidine treatment inhibits mexiletine oxidation in extensive metabolizers of debrisoquine

Urinary elimination of unchanged mexiletine, p-hydroxymexiletine (PHM), hydroxymethylmexiletine (HMM) and mexiletine N-glucuronide conjugate (MGC) was investigated before and after treatment with quinidine. All subjects were phenotyped as extensive metabolizers for debrisoquine oxidation. The total recovery of mexiletine and metabolites was significantly reduced after quinidine pretreatment. It is concluded that pretreatment with a very low dose of quinidine inhibits markedly the elimination of both major mexiletine metabolites (PHM and HMM) and likely decreases the overall elimination of mexiletine. That should lead to changes in mexiletine disposition and have clinical consequences during combination therapy with both drugs.

Systemic absorption of ocular timolol in poor and extensive metabolizers of debrisoquine

The oxidation of timolol exhibits genetic polymorphism of debrisoquine type. After oral administration, poor metabolizers have high timolol concentrations in plasma and show an intensified systemic beta-blockade. Since the contribution of debrisoquine metabolizer status on timolol eyedrop therapy is not known we determined the systemic absorption of ocularly applied timolol in healthy subjects classified either extensive or poor metabolizers. Unlike after oral timolol ocular drug administration caused higher peak drug concentrations in plasma in extensive metabolizers. The variation in the systemic absorption of ocular timolol contributed more than the debrisoquine oxidation phenotype to timolol plasma levels after a single ocular timolol application.

Debrisoquine oxidation phenotype during neuroleptic monotherapy

The debrisoquine oxidation phenotype was determined in 91 schizophrenic patients on monotherapy with different neuroleptics and in 67 untreated healthy volunteers. The prevalence of poor metabolizers of debrisoquine was significantly higher in the patients (46.2%) than in the healthy subjects (7.5%). Treatment with phenothiazine antipsychotics (chlorpromazine, levomepromazine and thioridazine) was associated with a higher debrisoquine metabolic ratio than treatment with haloperidol. On the other hand, treatment with clothiapine appeared not to interfere with debrisoquine oxidation. Oral administration of 50 mg thioridazine daily to 8 healthy subjects resulted in a marked increase in the debrisoquine metabolic ratio and 4 of them were transformed into phenotypically poor metabolizers. The results confirm the fact that phenothiazines, and to a lesser extent haloperidol, inhibit the oxidative metabolism of debrisoquine. They show also that clothiapine administration does not disturb the debrisoquine metabolic ratio.

Comparative effects of the diastereoisomers, quinine and quinidine in producing phenocopy debrisoquine poor metabolisers (PMs) in healthy volunteers

1. A single oral dose (50 mg) of quinidine significantly increased the debrisoquine metabolic ratio in six healthy volunteers. For four of the volunteers the metabolic ratio changed to that typical of the poor metaboliser (PM) phenotype. 2. The effect of quinidine in producing debrisoquine oxidation "poor metaboliser" phenocopies persisted for at least 3 days but had disappeared by 1 week. 3. The debrisoquine metabolic ratios for the same six subjects were not significantly altered by the oral administration of quinine (200 or 400 mg), the diastereoisomer of quinidine. 4. The plasma pharmacokinetic parameters of both nortriptyline and desipramine in healthy volunteers were all changed to those more typical of the debrisoquine PM phenotype following the concomitant administration of quinidine (50 mg). 5. It is concluded that quinidine, but not its diastereoisomer quinine, is a potent selective inhibitor of the in vivo oxidation of debrisoquine and can produce an artifactual PM phenocopy in persons who are phenotypically extensive metaboliser (EM) phenotype status. The clinical implications of this observation are discussed.

Hydroxylation polymorphisms of debrisoquine and mephenytoin in European populations

European data on the polymorphic metabolism of debrisoquine, sparteine, dextromethorphan and mephenytoin have been collected. No significant difference in phenotype frequencies was found between the separate series for debrisoquine, sparteine and dextromethorphan, which supports the claim that these probe drugs reflect the same enzyme polymorphism. The mean frequency of the phenotype slow debrisoquine metaboliser was 7.65% based on 5005 determinations. The overall mean reflecting all three drugs and 8764 determinations was 7.40%. This is consistent with a gene frequency of 0.27 (95% confidence interval 0.26-0.28). The overall mean of the phenotype slow metaboliser of mephenytoin was 3.52% corresponding to a gene frequency of 0.19 (confidence interval 0.17-0.20). The incidence of slow metabolism of debrisoquine and possibly also of S-mephenytoin was homogeneous in the samples from European populations. This is of considerable interest as interethnic differences are now being found both in the phenotypic characters as well as the genotypes of polymorphic drug oxidation.

Genetic polymorphisms of drug metabolism

The molecular mechanisms of 3 genetic polymorphisms of drug metabolism have been studied at the level of enzyme activity, enzyme protein and RNA/DNA. As regards debrisoquine/sparteine polymorphism, cytochrome P-450IID6 was absent in livers of poor metabolizers; aberrant splicing of premRNA of P-450IID6 may be responsible for this. Moreover, 3 mutant alleles of the P-450IID6 locus on chromosome 22 associated with the poor metabolizer phenotype were identified by Southern analysis of leucocyte DNA. The presence of 2 identified mutant alleles allowed the prediction of the phenotype in approximately 25% of poor metabolizers. The additional gene-inactivating mutations which are operative in the remainder of poor metabolizers are now being studied. Regarding mephenytoin polymorphism, although the deficient reaction, S-mephenytoin 4'-hydroxylation, has been well defined in human liver microsomes, the mechanism of this polymorphism remains unclear. All antibodies prepared to date against cytochrome P-450 fractions with this activity recognize several structurally similar enzymes and several cDNAs related to these enzymes have been isolated and expressed in heterologous systems. However, which isozyme is affected by this polymorphism is not known. As regards N-acetylation polymorphism, N-acetyltransferases have been purified from human liver, specific antibodies prepared; it was observed that immunoreactive N-acetyltransferase is decreased or undetectable in liver of "slow acetylators". Two genes that encode functional N-acetyltransferase were characterized. The product of one of these genes has identical activity and characteristics as the polymorphic liver enzyme. Cloned DNA from rapid and slow acetylator individuals has been analyzed to identify the structural or regulatory defect that causes deficient N-acetyltransferase.

The effects of beta-adrenoceptor antagonists and levomepromazine on the metabolic ratio of debrisoquine

The in vivo inhibitory effect of five beta-adrenoceptor antagonists and levomepromazine on debrisoquine metabolism was assessed in 37 subjects. The debrisoquine phenotyping test was performed before and after 7 days' treatment with oxprenolol (40 mg three times daily), propranolol (20 mg three times daily), timolol (10 mg twice daily), pindolol (5 mg twice daily), metoprolol (50 mg twice daily) or levomepromazine (10 mg daily), each of which was given to six-seven subjects. No clear change in the urinary metabolic ratio of debrisoquine/4-OH-debrisoquine (MR) was seen with any of the single beta-adrenoceptor antagonist treatments, but the MR value increased significantly when all beta-adrenoceptor blocker treatments were considered together. Debrisoquine metabolism was clearly impaired after levomepromazine 10 mg daily for 7 days; the mean MR increased from 1.24 +/- 1.6 to 4.70 +/- 5.23 (P = 0.018) and the excretion of 4-hydroxydebrisoquine decreased from 0.92 +/- 0.46 mg to 0.31 +/- 0.19 mg (P = 0.043). Thus, levomepromazine changes MRs towards those characteristic of phenotypically poor metabolizers, but beta-adrenoceptor antagonists at the doses examined have only a marginal effect.

Genetic polymorphism of sparteine/debrisoquine oxidation: a reappraisal

Polymorphic oxidation of the sparteine/debrisoquine-type has been shown to account for much of the interindividual variation in the metabolism, pharmacokinetics and pharmacodynamics of an increasing number of drugs, including some antiarrhythmic, antidepressant and beta-adrenoceptor antagonist agents. Impaired hydroxylation of these drugs results from the absence of the enzyme cytochrome P450IID6 in the livers of poor metabolisers, who constitute 6% to 10% of Caucasian populations. The clinical importance of the phenomenon has to be explored further and for most sparteine/debrisoquine-related substrates there is a need for controlled prospective studies to define the consequences to the patient of impaired or enhanced drug oxidation.

Polymorphic oxidation of debrisoquine in bladder cancer

The oxidative polymorphism of debrisoquine has been determined in 125 patients with bladder cancer and in 556 healthy control subjects; 96.6% of patients and 93.9% of controls with a metabolic ratio of debrisoquine less than 12.6 were classified as extensive metabolizers of debrisoquine (P = NS). The distribution of frequencies of metabolic ratio values tended to have lower values in the patients (P less than 0.05), reflecting a higher oxidative rate of debrisoquine in urothelioma patients that cannot be explained solely in terms of enzymatic induction by drugs, tobacco or alcohol. Patients with a high occupational risk for urothelioma had lower metabolic ratio values (P = 0.03). Our results suggest that oxidative polymorphism of debrisoquine might be related to the pathogenesis of bladder cancer.

Clinical pharmacokinetics of imipramine and desipramine

The pharmacokinetics of imipramine and desipramine have been extensively investigated with recent studies designed to understand sources of intersubject variability and to study discrete clinical populations rather than healthy volunteers. Sources of intersubject variability in pharmacokinetics are both genetic (oxidative phenotype) and environmental. Oxidative phenotype has an important impact on first-pass metabolism. In individuals with poor metabolism, systemic availability for imipramine is increased. Intrinsic clearance of desipramine is reduced 4-fold in individuals with poor metabolism. Recent pharmacokinetic studies in diverse patient populations such as the depressed elderly, children and alcoholics have revealed decreased clearance of imipramine in the elderly and increased clearance of both imipramine and desipramine in chronic alcoholics. In at least a third of the population, nonlinear pharmacokinetics of desipramine may be observed at steady-state plasma concentrations above 150 micrograms/L. These nonlinear changes in desipramine pharmacokinetics are not associated with age or sex, but are associated with higher desipramine 2-hydroxydesipramine concentration ratios. Hydroxylated metabolites of imipramine and desipramine may possess both antidepressants and cardiotoxic activity but their formation is rate limited and plasma concentrations tend to follow the parent compound with little accumulation. The potent cardiovascular effects of the hydroxymetabolites may be particularly relevant for the elderly and in acute overdose.

Rapid screening for polymorphisms in dextromethorphan and mephenytoin metabolism

1. The phenotyping parameters for dextromethorphan and mephenytoin were assessed in 48 normal male volunteers following administration of each metabolic probe drug on separate occasions and together according to a randomized 3-way crossover design. 2. Neither the urinary S-/R-mephenytoin ratio nor the dextromethorphan metabolic ratio were altered significantly by coadministration of the probe drugs. 3. Five-hundred and nineteen subjects were screened for expression of mephenytoin 4-hydroxylase and dextromethorphan O-demethylase activity following the coadministration of mephenytoin and dextromethorphan. The activity was determined in each case by methods not requiring any quantitative measurements. 4. Nineteen (3.7%) of the subjects were identified as poor metabolizers (PMs) of mephenytoin and 35 subjects (6.7%) as PMs of dextromethorphan. 5. All PMs of dextromethorphan were confirmed by more rigorous evaluation of the metabolic ratio.

Oxidative polymorphism of debrisoquine in Parkinson's disease

Oxidative phenotype and metabolic ratio (MR) of debrisoquine (DBQ) have been determined in 87 patients with Parkinson's disease and in 556 healthy control subjects. Three patients (3.45%) and 34 control subjects (6.12%), having an MR greater than 12.6, were classified as poor metabolisers (PM) of DBQ (ns). The distribution of MR values in the 84 Parkinsonian patients classified as extensive metabolisers (EM) showed a less efficient oxidative rate when compared with controls of the same phenotype (p less than 0.001). This difference may be due to enzymatic inhibition caused by drug treatment in 40 of these patients. As in patients not taking any drug known to inhibit the oxidation of DBQ, distribution of MR values was not different from that in controls. A negative correlation (r = -0.36, p less than 0.02) was found between MR of DBQ and age at onset of disease in patients free of drugs known to interact with DBQ metabolism. A higher rate of DBQ oxidation could be a genetic factor that delays the clinical onset of Parkinson's disease in predisposed people.

Inhibitory studies of mexiletine and dextromethorphan oxidation in human liver microsomes

The cytochrome P-450dbl isozyme (P-450bdl) is responsible for the genetic sparteine-debrisoquine type polymorphism of drug oxidation in humans. To investigate the relationship between mexiletine oxidation and the activity of this isozyme, cross-inhibition studies were performed in human liver microsomes with mexiletine and dextromethorphan, a prototype substrate for P-450dbl. The formation of hydroxymethylmexiletine and p-hydroxymexiletine, two major mexiletine metabolites, was competitively inhibited by dextromethorphan. Mexiletine competitively inhibited the high affinity component of dextromethorphan O-demethylation. In addition, there was a good agreement between the apparent Km values for the formation of both mexiletine metabolites and the high affinity component of dextromethorphan O-demethylation and their respective apparent Ki values. Several drugs were tested for their ability to inhibit mexiletine oxidation. Quinidine, quinine, propafenone, oxprenolol, propranolol, ajmaline, desipramine, imipramine, chlorpromazine and amitryptiline were competitive inhibitors for the formation of hydroxymethylmexiletine and p-hydroxymexiletine as for prototype reactions of the sparteine-debrisoquine type polymorphism. Amobarbital, valproic acid, ethosuximide, caffeine, theophylline, disopyramide and phenytoin, known to be non-inhibitors of P-450dbl activity, were found not to inhibit the formation of these mexiletine metabolites. Moreover, the formation of both metabolites was strongly inhibited by an antiserum containing anti-liver/kidney microsomes antibodies type I (anti-LKMI) directed against P-450dbl. These data suggest that the formation of two major metabolites of mexiletine is predominantly catalysed by the genetically variable human liver P-450dbl.

Recent developments in hepatic drug oxidation. Implications for clinical pharmacokinetics

Cytochrome P450 (P450) is the collective term for a group of related enzymes or isozymes which are responsible for the oxidation of numerous drugs and other foreign compounds, as well as many endogenous substrates including prostaglandins, fatty acids and steroids. Each P450 is encoded by a separate gene, and a classification system for the P450 gene superfamily has recently been proposed. The P450 genes are assigned to families and subfamilies according to the degree of similarity of the amino acid sequences of the protein part of the encoded P450 isozymes. It is estimated that there are between 20 and 200 different P450 genes in humans. The human P450IID6 is a particular isozyme which has been extensively studied over the past 10 years. The P450IID6 is the target of the sparteine/debrisoquine drug oxidation polymorphism. Between 5 and 10% of Caucasians are poor metabolisers, and it has recently been shown that the P450IID6 enzyme is absent in the livers of these individuals. The defect has also been characterised at the DNA and messenger RNA (mRNA) level, and to date 3 different forms of incorrectly spliced P450IID6 pre-mRNAs have been identified in the livers of poor metabolisers. The P450IID6 has a broad substrate specificity and is known to oxidise 15 to 20 commonly used drugs. The metabolism of these drugs is therefore subjected to the sparteine/debrisoquine oxidation polymorphism. The clinical significance of this polymorphism for a particular drug is defined according to the usefulness of phenotyping patients before treatment. It is concluded that this strategy would be of potential value for tricyclic antidepressants, some neuroleptics (e.g. perphenazine and thioridazine) and some anti-arrhythmics (e.g. propafenone and flecainide). The P450IID6 displays markedly stereoselective metabolism and appears uninducible by common inducers like rifampicin and phenazone (antipyrine). With some substrates, such as imipramine, desipramine and propafenone, P450IID6 becomes saturated at therapeutic doses. Finally, its function is potently inhibited by many commonly used drugs, for example, quinidine. The most clinically relevant interaction in relation to P450IID6 function appears to be the potent inhibition by some neuroleptics of the metabolism of tricyclic antidepressants. No drug-metabolising P450 has been so well characterised at the gene, protein and functional levels as the P450IID6. This development is based on an extensive use of specific model drugs, the oxidation of which in vitro and in vivo is dependent on the function of P450IID6; it can be expected that other human drug-metabolising P450s will be similarly characterised in future.

A dose-effect study of the in vivo inhibitory effect of quinidine on sparteine oxidation in man

1. Twelve healthy extensive metabolisers of sparteine were sparteine tested daily for 6 days (19.00 h to 07.00 h). A small but statistically significant rise in sparteine metabolic ratio (MR) was observed. 2. Following 100 mg quinidine sulphate given to four of the subjects at 16.00 h, sparteine tests were carried out 19.00 h to 07.00 h on the same day and then daily for 6 days. Quinidine caused an immediate twenty-fold increase in sparteine-MR which then gradually returned to normal over the following 4-6 days. Quinidine concentrations in plasma were measurable only up to 20 h after the quinidine test dose. 3. At weekly intervals, all 12 subjects received single doses of quinidine sulphate of 5, 10, 20, 40 and 80 mg at 16.00 h, each time followed by a sparteine test 19.00 h to 07.00 h on the same day. A clear dose-effect relationship was found with a significant rise in the sparteine-MR even after 5 mg quinidine. After 80 mg quinidine, 8 of 12 subjects became phenotypically poor metabolisers (MR greater than 20).

Evidence for the polymorphic oxidation of debrisoquine in the Thai population

Debrisoquine polymorphism has been studied extensively in Caucasian populations. The prevalence of the poor metaboliser phenotype is 3-10% in European and American Caucasian populations but appears to be very low in Asian populations. This study was carried out to determine the metabolic oxidation status in 173 Thai subjects. Phenotyping was performed using the metabolic ratio (MR) calculated as the 0-8 h urinary output of debrisoquine/0-8 h urinary output of 4-hydroxydebrisoquine after oral administration of 10 mg debrisoquine hemisulphate. Two subjects (1.2%) were phenotyped as poor metabolisers; they had MR values of 13.17 and 92.04. The incidence of the poor metaboliser phenotype of debrisoquine oxidation of 1.2% seems to be lower in the Thai population compared with that in various Caucasian populations.

Minimal effective concentration values of propafenone and 5-hydroxy-propafenone in acute and chronic therapy

We evaluated the antiarrhythmic efficacy and the minimal effective concentrations of propafenone and its metabolite 5-hydroxy-propafenone during a) acute intravenous infusion (1.5 mg/kg in bolus followed by 45 minutes infusion), b) an acute oral single-dose test (450 mg), and c) 14-day chronic therapy (300 mg tid) followed by a washout. Oxidative metabolism was assessed by a debrisoquine oral test in every patient. Eleven patients with stable ventricular premature beats (VPBs) greater than or equal to 300/hr and Lown class greater than or equal to 3 completed the study. The main results emphasized a certain discrepancy between the clinical effect of the acute intravenous infusion (efficacy in 5 out of 11 patients) and of the acute oral test and chronic therapy (efficacy in 11/11), with a time lag of the ECG changes during the acute intravenous infusion. The minimal effective concentrations were lower after acute oral administration compared with chronic treatment both for propafenone (200 +/- 189 ng/ml vs. 492 +/- 530 ng/ml; p less than 0.05) and for 5-hydroxy-propafenone (82 +/- 40 ng/ml vs. 149 +/- 80 ng/ml; p less than 0.02). A linear correlation was demonstrated between drug/metabolite ratios of propafenone and debrisoquine, either after acute oral (r = 0.91) or after chronic administration (r = 0.84). The pharmacokinetics of propafenone was nonlinear and showed wide interindividual variations.(ABSTRACT TRUNCATED AT 250 WORDS)

Quinidine kinetics after a single oral dose in relation to the sparteine oxidation polymorphism in man

The kinetics at a single oral dose (400 mg) of quinidine were studied in four extensive metabolizers (EM) and four poor metabolizers (PM) of sparteine. The clearance of quinidine by 3-hydroxylation was significantly lower in PM than in EM, but the difference was small (25-30%). This finding suggests that 3-hydroxylation, in part, is catalyzed by the same isoenzyme of cytochrome P450, P450db1 which oxidizes sparteine. Otherwise, no significant phenotypic differences in total or metabolic clearance were found and it is concluded that the metabolism of quinidine is largely carried out by P450 isoenzymes different from P450db1. A biexponential decline in the log plasma quinidine concentration vs time curves was observed in all subjects, and the mean elimination half-life was 11-12 h. This is about twice as long as generally reported in the literature.

Cytochrome P450dbl phenotypes in malignant and benign breast disease

129 female patients with breast cancer and 79 controls undergoing biopsy for benign breast conditions had debrisoquine hydroxylator phenotype established. 129 female hospital patients with known hydroxylator phenotype were used as another control group. The breast cancer cases differed significantly from the benign controls in their debrisoquine phenotype, with 10% being poor metabolisers compared with none of the controls (P less than 0.01). However, while a comparison of the distributions of metabolic ratio (an inverse measure of debrisoquine metabolism) of breast cancer patients and hospital controls showed a significant difference by rank, there was no significant difference in the proportion of poor metabolisers in these two groups. The cases with benign disease differed from the hospital controls in both metabolic ratio distribution (P less than 0.001) and frequency of poor metabolisers (P less than 0.05). Although there was a shift in metabolic ratio distribution, debrisoquine hydroxylator phenotype was not a genetic marker for breast cancer. Why no patients undergoing biopsies for benign conditions were poor metabolisers is unknown.

The genetic polymorphism of debrisoquine/sparteine metabolism--clinical aspects

It has been established that the metabolism of more than twenty drugs, including antiarrhythmics, beta-adrenoceptor antagonists, antidepressants, opiates and neuroleptics is catalyzed by cytochrome P-450dbl. The activity of this P-450 isozyme is under genetic rather than environmental control. This article discusses the therapeutic implications for each of the classes of drugs affected by this genetic polymorphism in drug metabolism. Not only are the problems associated with poor metabolizers who are unable to metabolize the compounds discussed, but it is also emphasized that it is difficult to attain therapeutic plasma concentrations for some drugs in high activity extensive metabolizers.

Debrisoquine oxidation phenotype and susceptibility to lung cancer

1. It has been suggested that poor metabolisers of debrisoquine are at reduced risk of developing lung cancer from smoking cigarettes. This has been investigated in 82 patients with established cancer of the lung. 2. The frequency of poor metaboliser subjects was not different from that in the normal population. 3. There was no tendency for subjects with lung cancer to metabolise debrisoquine more rapidly than non-cancer subjects. 4. It is concluded that debrisoquine metabolic phenotype is not a good predictor of risk of developing lung cancer in the population at large.

The genetic polymorphism of debrisoquine/sparteine metabolism-molecular mechanisms

The genetic polymorphism of debrisoquine/sparteine metabolism is one of the best studied examples of a genetic variability in drug response. 5-10% of individuals in Caucasian populations are 'poor metabolizers' of debrisoquine, sparteine and over 20 other drugs. The discovery and the inheritance of deficient debrisoquine/sparteine metabolism are briefly described, followed by a detailed account of the studies leading to the characterization of the deficient reaction and the purification of cytochrome P-450IID1, the target enzyme of this polymorphism. It is demonstrated by immunological methods that deficient debrisoquine hydroxylation is due to the absence of P-450IID1 protein in the livers of poor metabolizers. The cloning and sequencing of the P-450IID1 cDNA and of IID1 related genes are summarized. The P-450IID1 cDNA has subsequently led to the discovery of aberrant splicing of P-450IID1 pre-mRNA as the cause of absent P-450IID1 protein. Finally, the identification of mutant alleles of the P-450IID1 gene (CYP 2D) by restriction fragment length polymorphisms in lymphocyte DNA of poor metabolizers is presented.

Correlations among the metabolic ratios of three test probes (metoprolol, debrisoquine and sparteine) for genetically determined oxidation polymorphism in a Japanese population

The study was aimed at defining the relationships among the oxidative capacities for three prototype drugs, metoprolol, debrisoquine and sparteine, used for assessing genetically determined polymorphism of drug oxidation in a Japanese population. Among 292 unrelated healthy Japanese subjects who had been defined as extensive (EMs, n = 291) or poor (PM, n = 1) metabolisers of metoprolol oxidation, 55 subjects (EMs = 54 and PM = 1) were selected. One PM of metoprolol oxidation was also identified as a PM not only of debrisoquine but also of sparteine, and no misclassification by the three phenotypic methods was observed. All three correlations among the metabolic ratios of the three test probes assessed by Spearman's rank test were highly significant (P less than 0.001). These findings indicate that in Japanese subjects the oxidation capacities of metoprolol, debrisoquine, and sparteine are closely related. It appears that in Japanese the polymorphic oxidation of the three drugs is co-regulated, either by the same enzyme or gene-controlling system.

Codeine O-demethylation co-segregates with polymorphic debrisoquine hydroxylation

1. A single oral dose of codeine (25 mg) was given to 132 healthy Swedish Caucasians who had previously been phenotyped with respect to debrisoquine hydroxylation. The 'metabolic ratios' (MR) in urine of codeine O-demethylation (codeine/(morphine (M) + morphine-3- and 6-glucuronides (M3G and M6G) + normorphine], N-demethylation (codeine/(norcodeine (NC) + norcodeine glucuronide + normorphine (NM]) and glucuronidation (codeine/codeine-6-glucuronide (C6G] were calculated following h.p.l.c. analysis of urine samples collected over 8 h. 2. There was a significant correlation between the log MR for debrisoquine hydroxylation and the log MR for codeine O-demethylation (rs = 0.77, P less than 0.001). The poor debrisoquine hydroxylators had MRs of codeine O-demethylation between 8.3 and 55.1, while the values for extensive hydroxylators were between 0.4 and 5.5. 3. The poor debrisoquine hydroxylators excreted significantly less M, M3G, M6G and NM, while the urinary recovery of C6G and NC was significantly higher in these subjects compared to the extensive hydroxylators. 4. The MRs for glucuronidation and N-demethylation did not exhibit a bimodal distribution, and were not related to the MR of debrisoquine hydroxylation. 5. No associations were found between sex, body-weight, smoking habits, age, urine volume or urine pH and the O-demethylation of codeine. 6. The O-demethylation of codeine to form M appears to be under the same polymorphic genetic control as the 4-hydroxylation of debrisoquine.

Testing for bimodality in frequency distributions of data suggesting polymorphisms of drug metabolism--histograms and probit plots

1. The shape of histograms used to illustrate density distributions of indices of polymorphic drug metabolism was shown to be sensitive to the position of the cell divisions. 2. Non-linearity of the probit plot was shown not to indicate bimodality of the original density distribution. Computer simulation was used to generate examples of unimodal density distributions with curvilinear probit plots. 3. Using the same technique probit plots for bimodal density distributions were constructed. Some were shown to differ less from the probit plots of certain unimodal distributions than did the original density distributions. 4. The position of the antimode was shown not to coincide with inflections seen in the probit plots. 5. A new method for determining the linearity of probit plots is suggested.

Interindividual and interethnic differences in the demethylation and glucuronidation of codeine

1. The 8 h urinary excretion of codeine and seven of its metabolites was compared in 149 healthy Swedish Caucasians and 133 healthy Chinese following a single oral dose of 25 mg codeine phosphate. 2. The total 8 h urinary recovery of drug-related material was 74 +/- 24% in the Caucasians and 60 +/- 14% in the Chinese (P less than 0.001). The excretion of unchanged codeine was significantly higher in the Chinese (7.2%) compared with the Caucasians (4.3%, P less than 0.001). 3. The Caucasians excreted significantly greater proportions of codeine-6-glucuronide (C6G) (62%) than the Chinese (44%) (P less than 0.001). The frequency distribution of the log metabolic ratio (MR) for glucuronidation (codeine/C6G) was shifted towards higher values in the Chinese population. Males in both groups and Chinese smokers had significantly lower glucuronidation MRs than females and non-smokers in the respective populations (P less than 0.001). 4. The frequency distribution of the MR for O-demethylation (codeine/morphine (M) + M-3 and M-6-glucuronide (M3G and M6G) + normorphine (NM) was highly skewed in the Caucasians, suggestive of a bimodal distribution. There was a 160-fold interindividual variation in this MR. A unimodal distribution of the log O-demethylation MR was observed in Chinese. The Caucasians excreted less M and more M6G than did the Chinese (P less than 0.001). 5. Significantly more norcodeine (NC) and less NC-glucuronide (NCG) were excreted in the Chinese compared with the Caucasians (P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)