Debrisoquine oxidation in an Australian population

Dextromethorphan and debrisoquine metabolism and polymorphism of the gene for cytochrome P450 isozyme 2D50 in Thoroughbreds

OBJECTIVE To characterize polymorphisms of the gene for cytochrome P450 isozyme 2D50 (CYP2D50) and the disposition of 2 CYP2D50 probe drugs, dextromethorphan and debrisoquine, in horses. ANIMALS 23 healthy horses (22 Thoroughbreds and 1 Standardbred). PROCEDURES Single-nucleotide polymorphisms (SNPs) in CYP2D50 were identified. Disposition of dextromethorphan (2 mg/kg) and debrisoquine (0.2 mg/kg) were determined after oral (dextromethorphan) or nasogastric (debrisoquine) administration to the horses. Metabolic ratios of plasma dextromethorphan and total dextrorphan (dextrorphan plus dextrorphan-O-β-glucuronide) and 4-hydroxydebrisoquine concentrations were calculated on the basis of the area under the plasma concentration-versus-time curve extrapolated to infinity for the parent drug divided by that for the corresponding metabolite. Pharmacokinetic data were used to categorize horses into the phenotypic drug-metabolism categories poor, extensive, and ultrarapid. Disposition patterns were compared among categories, and relationships between SNPs and metabolism categories were explored. RESULTS Gene sequencing identified 51 SNPs, including 27 nonsynonymous SNPs. Debrisoquine was minimally detected after oral administration. Disposition of dextromethorphan varied markedly among horses. Metabolic ratios for dextromethorphan ranged from 0.03 to 0.46 (mean, 0.12). On the basis of these data, 1 horse was characterized as a poor metabolizer, 18 were characterized as extensive metabolizers, and 3 were characterized as ultrarapid metabolizers. CONCLUSIONS AND CLINICAL RELEVANCE Findings suggested that CYP2D50 is polymorphic and that the disposition of the probe drug varies markedly in horses. The polymorphisms may be related to rates of drug metabolism. Additional research involving more horses of various breeds is needed to fully explore the functional implication of polymorphisms in CYP2D50.

Desipramine, substrate for CYP2D6 activity: population pharmacokinetic model and design elements of drug-drug interaction trials

AIMS

To develop a population pharmacokinetic model to describe the pharmacokinetics of desipramine in healthy subjects, after oral administration of a 50mg dose. Additional objectives were to develop a semi-mechanistic population pharmacokinetic model for desipramine, which allowed simulation of CYP2D6-mediated inhibition, when using desipramine as a probe substrate, and to evaluate certain study design elements, such as duration of desipramine pharmacokinetic sampling, required sample size and optimal pharmacokinetic sampling schedule for intermediate, extensive and ultrarapid metabolizers of CYP2D6 substrates.

RESULTS

The mean population estimates of the first order absorption rate constant (k(a) ), apparent clearance (CL/F) and apparent volume of distribution at steady state (V(ss) /F) were 0.15h(-1) , 111 lh(-1) and 2950 l, respectively. Further, using the proposed semi-mechanistic hepatic intrinsic clearance model with Bayesian inference, mean population desipramine hepatic intrinsic clearance was estimated to be 262 lh(-1) with between-subject variability of 84%. d-optimal PK sampling times for intermediate metabolizers were calculated to be approximately 0.25, 24, 75 and 200h. Similar sampling times were found for ultrarapid and extensive metabolizers except that the second d-optimal sample was earlier at 14 and 19h, respectively, compared with 24h for intermediate metabolizers. This difference in sampling times between the three genotypes can be attributed to the different intrinsic clearances and elimination rates.

CONCLUSIONS

A two compartment population pharmacokinetic model best described desipramine disposition. The semi-mechanistic population model developed is suitable to describe the pharmacokinetic behaviour of desipramine for the dose routinely used in drug-drug interaction (DDI) studies. Based on this meta-analysis of seven trials, a sample size of 21 subjects in cross-over design is appropriate for assessing CYP2D6 interaction with novel compounds.

Bioinformatics Research on Inter-racial Difference in Drug Metabolism I. Analysis on Frequencies of Mutant Alleles and Poor Metabolizers on CYP2D6 and CYP2C19

The enzyme activities of CYP2D6 and CYP2C19 show a genetic polymorphism, and the frequency of poor metabolizers (PMs) on these enzymes depends on races. In the present study, the frequencies of mutant alleles and PMs in each race were analyzed based on information from published studies, considering the genetic polymorphisms of CYP2D6 and CYP2C19 as the causal factors of racial and inter-individual differences in pharmacokinetics. As a result, it was shown that there were racial differences in the frequencies of each mutant allele and PMs. The frequencies of PMs on CYP2D6 are 1.9% of Asians and 7.7% of Caucasians, and those of PMs on CYP2C19 are 15.8% of Asians and 2.2% of Caucasians. Based on the results, it was suggested that there would be racial differences in the frequencies of PM subjects whose blood concentrations might be higher for drugs metabolized by these enzymes. Additionally, it was suggested that enzyme activities would vary according to the number of functional alleles even in subjects judged to be extensive metabolizers (EMs). In the bridging study, genetic information regarding CYP2D6 and CYP2C19 of the subjects will help extrapolate foreign clinical data to a domestic population.

Effect of nortriptyline and paroxetine on CYP2D6 activity in depressed elderly patients

This study was performed in elderly patients (1) to assess the degree to which CYP2D6 mediated metabolism of debrisoquine at baseline determines plasma concentration to dose quotients for nortriptyline or paroxetine after 4 weeks of treatment, and (2) to compare the effects of nortriptyline and paroxetine on debrisoquine metabolism after 6 weeks of treatment. CYP2D6 activity was estimated in 66 subjects (71.4 +/- 7.2 years) before initiating treatment and again after 6 weeks of treatment with either nortriptyline or paroxetine under randomized, double-blind conditions according to a standard protocol. CYP2D6 activity was estimated by the debrisoquine recovery ratio in a 6- to 8-hour urine sample collected after oral administration of 10 mg debrisoquine sulfate. Nortriptyline and paroxetine plasma concentrations were obtained weekly. Baseline debrisoquine recovery ratio values were significantly correlated with the plasma concentration to dose quotient at 4 weeks for both nortriptyline ( = -0.75, = 0.0001, N = 29) and paroxetine ( = -0.50, = 0.003, N = 33). Treatment with either nortriptyline or paroxetine was associated with a significant decrease in the median debrisoquine recovery ratio, reflecting inhibition of CYP2D6 metabolism. The percent decrease associated with nortriptyline was significantly smaller than that with paroxetine ( < 0.0001). None of the patients treated with nortriptyline but 19 of the 32 extensive metabolizers treated with paroxetine were converted to phenotypic poor metabolic status. Our observations of CYP2D6 inhibition are consistent with data and results obtained in younger healthy volunteers. The significant correlations between baseline debrisoquine recovery ratio and the plasma concentrations to dose quotients at 4 weeks for both nortriptyline and paroxetine are consistent with CYP2D6 playing a major role in the metabolism of both drugs. CYP2D6 inhibition by paroxetine, which effectively converted 59% of patients to phenotypic PMs, may be especially relevant for elderly patients given their generally higher concentration of paroxetine.

Pharmacogenetics and psychopharmacotherapy

Response to drugs can vary between individuals and between different ethnic populations. The biological (age, gender, disease and genetics), cultural and environmental factors which contribute to these variations are considered in this review. The most important aspect is the genetic variability between individuals in their ability to metabolize drugs due to expression of 'polymorphic' enzymes. Polymorphism enables division of individuals within a given population into at least two groups, poor metabolisers (PMs) and extensive metabolisers (EMs) of certain drugs. The two most extensively studied genetic polymorphisms are those involving cytochrome P450 2D6 (CYP2D6) and CYP2C19. CYP2D6 metabolizes a number of antidepressants, antipsychotics, beta-adrenoceptor blockers, and antiarrhythmic drugs. About 7% of Caucasians and 1% of Asians are PMs of CYP2D6 substrates. CYP2C19 enzyme participates in the metabolism of omeprazole, propranolol and psychotropic drugs such as hexobarbital, diazepam, citalopram, imipramine, clomipramine and amitriptyline. The incidence of PMs of CYP2C19 substrates is much higher in Asians (15-30%) than in Caucasians (3-6%). Variations in metabolism of psychotropic drugs result in variations in their pharmacokinetic parameters. This may lead to clinically significant intra- and inter-ethnic differences in pharmacological responses. Such variations are discussed in this review. Differential receptor-mediated response may play a role in ethnic differences in responses to antipsychotics and tricyclic antidepressants, but such pharmacodynamic factors remain to be systematically investigated. The results of studies of ethnic differences in response to psychopharmacotherapy appear to be discrepant, most probably due to limitations of study design, small sample size, inadequately defined study sample, and lack of control of confounding factors. The clinical value of understanding pharmacogenetics is in its use to optimize therapeutic efficacy, to prevent toxicity of those drugs whose metabolism is catalysed by polymorphic isoenzymes, and to contribute to the rational design of new drugs. Finally, applications and impact of pharmacogenetics in the field of psychopharmacotherapy are discussed.

Propafenone in the treatment of cardiac arrhythmias. A risk-benefit appraisal

Propafenone is a potent antiarrhythmic agent effective in either supraventricular or ventricular tachyarrhythmias. For proper utilisation, some important pharmacological aspects must be considered, such as nonlinear pharmacokinetics, inability in some patients (poor debrisoquine metabolisers) to oxidise the drug in the liver, existence of at least one active metabolite (5-hydroxy-propafenone) and ability to exert a slight beta-blocking activity. Like all the other antiarrhythmic drugs, propafenone may be associated with adverse effects and may exert proarrhythmic effects. For this reason, its usage must be based on a careful analysis of the risk-benefit ratio, by considering the patient's profile as well as the characteristics of the arrhythmia and its prognostic significance. Propafenone appears to be very effective, and has a favourable risk-benefit profile in the treatment of all supra-ventricular arrhythmias. Particularly, it is effective in converting atrial fibrillation to sinus rhythm and in preventing atrial fibrillation recurrences, and is very effective in the pharmacological control of the arrhythmias of the Wolff-Parkinson-White syndrome. Propafenone is also effective in suppressing ventricular premature complexes and nonsustained ventricular tachycardias. However, because of potential proarrhythmic effects, its use in these arrhythmias must be considered after a careful analysis of the risk-benefit profile, which could be favourable in some patients, but less favourable in others (e.g. patients with coronary artery disease and ventricular dysfunction). In malignant ventricular arrhythmias, further studies are needed to define the limitations of antiarrhythmic drugs in comparison with non-pharmacological treatments, mainly cardioverter/defibrillators. At present, like the other class I antiarrhythmic agents, propafenone does not seem to be a first choice prophylactic agent for malignant ventricular arrhythmias, although more data from controlled studies are needed.

Risk factors and genetic susceptibility

Tobacco use is such a major determinant of head and neck cancer risk that classical epidemiologic techniques were more than adequate to document and characterize the etiologic association. However, the new emphasis in epidemiologic research is multidisciplinary, centering on the role of interindividual differences in susceptibility to carcinogenic exposures and particularly on the interaction of genetic susceptibility and environmental forces, that is, ecogenetics. For upper aerodigestive tract cancers, measurements of carcinogen metabolic activation and DNA repair capability are especially relevant. These susceptibility markers will enable the identification of high-risk population subgroups who can be targeted for the most intensive primary and secondary preventive strategies.

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.

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.

Cytochrome P-450-dependent hydroxylation in migraine

The hypothesis was tested that an acute oxidation deficiency related to potential dietary trigger factors plays a role in the migraine attack. Migraine sufferers (14F and 4M), fulfilling the criteria for migraine with and without aura according to the classification of the International Headache Society, were coadministered oral mephenytoin (100 mg) and debrisoquine (10 mg) during the initial phase of a typical migraine attack. This was repeated during a period without migraine. The hydroxylation of mephenytoin and debrisoquine hydroxylation did not differ during and without the migraine attack. We conclude that hydroxylation, via cytochrome P-450 (2D6, 2C8 and 9), is not reduced during the migraine attack. The results do not support the hypothesis that oxidation deficiency is involved in the pathophysiology of migraine.

Metabolism of paracetamol and phenacetin in relation to debrisoquine oxidation phenotype

The metabolism of paracetamol and phenacetin has been studied in subjects previously phenotyped as either extensive or poor metabolisers of debrisoquine (EM and PM, respectively), in order to examine the relationship between phenacetin and paracetamol activation and debrisoquine oxidation status. In separate experiments, paracetamol and phenacetin were administered orally to groups of 5 EM and 5 PM subjects, and the excretion of metabolites measured for 24 h. There were no differences between EM and PM subjects in the excretion of metabolites. After phenacetin, 0.82 of the dose was recovered in urine, mostly as paracetamol glucuronide (51%) and sulphate (30%), with smaller amounts of free paracetamol (4%) and the mercapturate (5%) and cysteine conjugates (5%), 2-hydroxyphenetidine (5%) and N-hydroxyphenacetin (0.5%). Following paracetamol, 0.87 of the dose was recovered, with similar proportions of paracetamol-derived metabolites. It is concluded that the debrisoquine oxidation phenotype is unrelated to either the metabolic activation of phenacetin and paracetamol, or to their overall metabolic clearance.

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.

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.

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.

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.

The distribution of debrisoquine metabolic phenotypes and implications for the suggested association with lung cancer risk

Debrisoquine hydroxylation exhibits wide inter-individual variation in Caucasian populations. After similar doses of the drug, extensive metabolizers excrete up to several hundred times more of the urinary metabolite 4-hydroxy-debrisoquine than do poor metabolizers. The phenotypes have traditionally been defined by the metabolic ratio (MR), or the molar ratio of debrisoquine to its chief metabolite recovered in an aliquot of an eight hour urine sample, after a test dose of the drug. Deficient metabolism is inherited as an autosomal recessive condition. We have reanalyzed previously published data from a study of lung cancer patients and controls using a computerized optimization method to more accurately estimate the parameters describing the three phenotypic distributions. Using these new distributions to categorize controls, we show that Hardy-Weinberg conditions are now fulfilled. When the newly defined phenotype parameters are employed to assign the phenotypes of cases and controls, a highly significant difference in phenotype distribution between cases and controls is still observed. This result supports the hypothesis that the debrisoquine metabolic phenotype may be associated with lung cancer susceptibility.

Debrisoquine oxidation in a Finnish population: the effect of oral contraceptives on the metabolic ratio

1. The debrisoquine hydroxylation phenotype was assessed in 211 healthy subjects of Finnish Caucasian origin. 2. A bimodal distribution was observed in the urinary debrisoquine/4-hydroxydebrisoquin ratio. Eleven subjects (about 5%) were poor metabolizers of debrisoquine, when a ratio of 12.6 was used as the cut-off point. 3. The mean metabolic ratio of 35 extensive metabolizers taking oral contraceptives was slightly lower than that of extensive metabolizers not taking contraceptives. This indicates that oral contraceptives may change the phenotyping near the metabolic ratio of 12.6. 4. The metabolic ratios were determined in eight subjects using both 4 h and 6 h urine collection times. The metabolic ratios corresponded well with each other indicating the usefulness of the shorter 4 h collection time.

Hepatic cytochrome P450-mediated oxidation function in migraine

Hepatic cytochrome P450-dependent oxidation is deficient in 5% to 10% of the Caucasian population. A similar percentage seems to suffer from migraine. The hypothesis was tested that an oxidation deficiency possibly relevant to potential dietary triggers plays a role in the pathogenesis of migraine. In 37 migraine sufferers and 26 controls age- and sex-matched to 26 of these patients, debrisoquine hydroxylation following an oral dose of 10 mg was employed as a marker of oxidation status, determined by calculating the metabolic ratio (MR): urinary debrisoquine/urinary 4-hydroxydebrisoquine. MR was similarly distributed in migraineurs and controls. Three subjects in each group were poor metabolizers (MR greater than 30, versus normal range, 0.1-12). MR in patients did not depend on type of migraine (common versus classic), attack frequency, the presence of trigger factors, smoking or a history of adverse reactions or sensitivity to medicines.

Comparison of gas chromatographic and high-performance liquid chromatographic assays for the determination of debrisoquine and its 4-hydroxy metabolite in human fluids

A comparison of an established gas chromatographic assay for 4-hydroxydebrisoquine and debrisoquine and a modified high-performance liquid chromatographic assay was made. Both assays used guanoxan as the internal standard and required derivatization of all three compounds with acetylacetone at 96 degrees C for 2.5 h and subsequent ethereal extraction and cleaning steps before chromatographic analysis. For detailed pharmacokinetic studies the gas chromatographic assay was more sensitive in the measurement of low concentrations in plasma, but the liquid chromatographic assay was adequate for phenotyping the 4-hydroxylation of debrisoquine in a population. In the latter assay a mobile phase consisting of 70% methanol in water at pH 3.5 (adjusted with orthophosphoric acid containing 10 mM 1-pentanesulphonic acid was employed (flow-rate 1.5 ml/min) with a pre-column (C8) linked to a reversed-phase muBondapak C18 cartridge in a Z-module. The eluate was detected at 248 nm. With this assay it was observed that the buccal absorption of debrisoquine and 4-hydroxy-debrisoquine was affected by the pH of the buccal medium. This indicates that urinary pH may influence the excretion of both substances at high pH. The debrisoquine-to-4-hydroxydebrisoquine ratio may be dose-dependent.

Determination of debrisoquine metabolic ratio from hourly urine collections in healthy volunteers

The possibility of simplifying the regimen for the collection of urine samples in the determination of the debrisoquine metabolic ratio (DMR) was explored in 15 normal subjects. In the extensive metaboliser subgroup (EM; n = 11), there was a close correlation between the DMR as determined by an 8 h urine collection and the debrisoquine/4-hydroxydebrisoquine ratio (D/4-OHD) in the hourly samples (excluding the first hour). In the poor metabolisers (PM; n = 4) the phenotype could be identified, but it was not possible to estimate the DMR reliably.

Lack of relationship between tolbutamide metabolism and debrisoquine oxidation phenotype

The oxidative metabolism of tolbutamide was studied in 13 healthy subjects of known debrisoquine phenotype. Three were poor (PM) and ten were extensive (EM) metabolisers of debrisoquine. The mean values for total plasma clearance, elimination half-life, and metabolic clearance were 0.26 ml.min-1.kg-1, 3.4 h, and 0.17 ml.min-1. kg-1 in PM subjects and 0.22 ml.min-1.kg-1, 4.3 h and 0.15 ml.min-1.kg-1 in EM subjects. Total urinary recovery (% of dose) and ratio of hydroxy- to carboxytolbutamide were 69.4% and 0.219 respectively in PM subjects and 70.9% and 0.226 in EM subjects. There were no statistically significant differences between EM and PM metabolisers for any of these parameters. In addition there was no correlation between the debrisoquine metabolic ratio and tolbutamide urinary metabolite recovery or plasma clearance. These data indicate that hydroxylation of debrisoquine and tolbutamide are not catalyzed by the same enzyme. The ratio of hydroxy- to carboxytolbutamide in our subjects, and in other recent studies, suggests that some previous publications were inaccurate and their conclusions about the genetic control of tolbutamide metabolism were incorrect.