Poor Metabolizers Of Debrisoquine Research Articles

Debrisoquine oxidation phenotype in psychiatric patients.

Between 5% and 10% of Caucasian populations exhibit poor metabolism of debrisoquine and sparteine (Clark 1985). Results obtained by our group indicate that among the Spanish population the percentage of poor metabolizers of debrisoquine is around 6% (Benítez et al. 1988). Generally the poor-metabolizer phenotype is at increased risk of toxicity when treated with drugs subjected to oxidative metabolism by the debrisoquine/sparteine type of path-way (Idle et al. 1983). A significant correlation between nortriptyline clearance and debrisoquine metabolic ratio has been demonstrated by Bertilsson et al. (1980). Steady-state concentrations of desipramine have also been shown to be highly correlated with the debrisoquine metabolic ratio (Bertilsson and Aberg-Wistedt 1983; Spina 1987).KeywordsPsychiatric PatientPoor MetabolizersExtensive MetabolizersMetabolic RatioNeuroleptic DrugThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Metoprolol-induced hepatitis: is the rate of oxidation related to drug-induced hepatotoxicity?

Metoprolol is as widely used beta-adrenoreceptor antagonist. There is no report of hepatitis involving this drug as well as other beta-adrenoreceptor antagonists. Metoprolol is metabolized by three major oxidation pathways. Two of them, Odealkylation and alpha-hydroxylation, undergo genetically controlled polymorphisms correlated with that of debrisoquine oxidation. This finding strongly suggests that metoprolol oxidation depends at least partly on the same isozyme of cytochrome P-450 involved in the genetic polymorphism of oxidation of debrisoquine and other drugs such as sparteine, perhexiline, and other beta-adrenoreceptor antagonists. Poor metabolizers of debrisoquine and metoprolol exhibit abnormal pharmacokinetics of metoprolol, which can result in an overdose after administration of a standard dose. There is a clear association between impairment of debrisoquine oxidation and susceptibility to develop liver injury with perhexiline. Does such an association exist for metoprolol? We report the first case of hepatitis induced by metoprolol and the result of drug oxidation phenotyping.

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.

Alfentanil pharmacokinetics and metabolism in humans.

The metabolism of alfentanil was studied in three healthy subjects after a 1-h infusion of 2.5 mg alfentanil-3H. One of the subjects was a poor hydroxylator of debrisoquine. Pharmacokinetic parameters were similar in the three subjects and were in the same range as those reported for volunteers. The majority of the administered radioactivity was excreted in the urine (90% of the dose), but unchanged alfentanil represented only 0.16-0.47% of the dose. Alfentanil and metabolites were characterized by HPLC co-chromatography with reference compounds and/or by mass spectrometry and quantified by GLC and radio-HPLC. The main metabolic pathway was N-dealkylation at the piperidine nitrogen, with formation of noralfentanil (30% of the dose). Other Phase I pathways were aromatic hydroxylation, N-dealkylation of the piperidine ring from the phenylpropanamide nitrogen, O-demethylation, and amide hydrolysis followed by N-acetylation. Glucuronic acid conjugation of aromatic or aliphatic hydroxyl functions was the main Phase II pathway. The second major metabolite was the glucuronide of N-(4-hydroxyphenyl) propanamide (14% of the dose). The metabolite pattern in these subjects was qualitatively very similar to that described previously in rats and dogs. Differences in the mass balance of urinary metabolites between the three subjects were very small, and there was no qualitative or quantitative evidence for a deficiency in the metabolism of alfentanil in the subject who was a poor metabolizer of debrisoquine.

Absence of hepatic cytochrome P450bufI causes genetically deficient debrisoquine oxidation in man.

The common genetic deficiency of drug oxidation known as debrisoquine/sparteine-type polymorphism was investigated with bufuralol as prototype substrate. In human liver microsomes the 1'-hydroxylation of bufuralol is catalyzed by two functionally distinct P-450 isozymes, the high-affinity/highly stereoselective P450bufI and the low-affinity/nonstereoselective P450bufII. We demonstrate that P450bufI is unique in hydroxylating bufuralol in a cumene hydroperoxide (CuOOH) mediated reaction whereas P450bufII is active only in the classical NADPH- and O2-supported monooxygenation. In microsomes of liver biopsies of in vivo phenotyped poor metabolizers of debrisoquine or sparteine, the CuOOH-mediated activity was drastically reduced. Rabbit antibodies against a rat P-450 isozyme with high bufuralol 1'-hydroxylase activity (P450db1) precipitated exclusively P450bufI-type activity from solubilized microsomes. Western blotting of microsomes with these antibodies revealed a close correlation between the immunoreactive protein and CuOOH-mediated (+)-bufuralol 1'-hydroxylation. No immunoreactive protein was detected in liver microsomes of in vivo phenotyped poor metabolizers. These data provide evidence for a specific deficiency of P450bufI and are consistent with the complete or almost complete absence of this protein in the liver of poor metabolizers.

Enantioselectivity of 4-hydroxylation in extensive and poor metabolizers of debrisoquine.

Debrisoquine (DQ) has no chiral centre, but hydroxylation in position 4 leads to formation of an asymmetric carbon centre with two possible enantiomers, their absolute configuration being R(-) and S(+)-4-hydroxydebrisoquine (4-OHDQ). Since the absolute stereochemistry of the 4-hydroxylation of DQ in man is unknown, the enantioselectivity of this process was studied in panels of extensive (EM) and poor metabolizers (PM) of DQ. In EM subjects 4-hydroxylation of DQ leads almost exclusively to the formation of S(+)-4-OHDQ. In contrast, PM subjects were not only characterized by a decreased total 4-OHDQ formation but also a marked loss of enantioselectivity in product formation. Between 5 to 36% of total 4-OHDQ was excreted as R(-)-4-OHDQ.

Methoxyphenamine O-demethylase and 5-hydroxylase: A GLC-ECD assay to study their activities and their inhibition by debrisoquine and sparteine

A GLC-ECD method is described for the determination of the O-desmethyl, N-desmethyl and aromatic 5-hydroxy metabolites of methoxyphenamine in liver homogenates. The O-desmethyl and 5-hydroxy metabolites are deficient in poor metabolizers of debrisoquine and sparteine and the Dark Agouti rat model of this human phenotype. The present analytical method can be useful in determining methoxyphenamine O-demethylase and 5-hydroxylase activities as well as identifying those substrates which inhibit these and are worthy of further study.

Phenacetin O-deethylase activity of the rat: strain differences and the effects of enzyme-inducing compounds.

Phenacetin O-deethylase activity in microsomal fractions from liver of DA and Fischer rats has been determined. No major sex or strain differences were found. Kinetic analysis revealed two major components of O-deethylase activity in the liver of both strains of rats. Michaelis-Menten analysis revealed no major difference between the strains. Phenacetin O-deethylase activity is inducible by both 3-methylcholanthrene and phenobarbitone in DA and Fischer rats. 3-Methylcholanthrene selectively increases the high-affinity component of activity, by 20- to 25-fold, whereas phenobarbitone selectively increases the low-affinity component, by two- to three-fold. It is concluded that there is no major difference between the DA and Fischer strains in their ability to O-deethylate phenacetin. Thus, unlike poor metabolizers of debrisoquine in the human population, who appear also to have impaired phenacetin O-deethylase activity, the DA rat is deficient in only the former activity.

Quinidine and the identification of drugs whose elimination is impaired in subjects classified as poor metabolizers of debrisoquine.

Quinidine and its diastereoisomer quinine were tested in vitro for their effect on the 4-hydroxylation of debrisoquine, the O-deethylation of phenacetin and the 1'-hydroxylation of bufuralol, by human liver microsomal samples; quinidine was studied for its effect on debrisoquine 4-hydroxylation in vivo. Quinidine was a potent inhibitor of the 4-hydroxylation of debrisoquine and the 1'-hydroxylation of bufuralol, with IC50 values of 0.7 and 0.2 microM, being around 100 times more potent in this respect than quinine. Very much higher (1000-fold) levels of quinidine were required to inhibit the O-deethylation of phenacetin, being rather less potent in this than quinine. Eight subjects were phenotyped for their debrisoquine oxidation status and found to be extensive metabolisers (EM). They were tested again after the co-administration of 50 mg of quinidine with the debrisoquine. The concomitant administration of quinidine increased the metabolic ratios (MRs) by a mean of 26-fold. The effects of quinidine at a dose of only 50 mg, on the metabolism of a new drug in EM subjects may prove a useful method of assessing the contribution of the debrisoquine 4-hydroxylase isozyme to the elimination of the drug tested.

Metoprolol oxidation by rat liver microsomes: Inhibition by debrisoquine and other drugs

The oxidative metabolism of metoprolol has been shown to display genetic polymorphism of the debrisoquine-type. The use of in vitro inhibition studies has been proposed as a means of denning whether one or more forms of cytochrome P-450 are involved in the rnonogenically-controlled metabolism of two substrates. We have, therefore, tested the ability of debrisoquine and other substrates to inhibit the oxidation of metoprolol by rat liver microsomes. Debrisoquine and guanoxan were potent competitive inhibitors of the α-hydroxylation and O-desmethylation of metoprolol as well as its metabolism by all routes (measured by substrate disappearance). Cimetidine and ranitidine, drugs which are known to impair the clearance of metoprolol in man, showed an inhibitory action comparable to that of debrisoquine in rat liver microsomes. Antipyrine, a compound whose metabolism is not impaired in poor metabolisers of debrisoquine, was found to be only a weak inhibitor of the metabolism of metoprolol. These findings suggest that the oxidation of metoprolol is linked closely to that of debrisoquine, cimetidine and ranitidine but not to that of antipyrine in the rat.

Debrisoquine oxidation in an Australian population.

The standard laboratory method for determination of debrisoquine phenotype has been modified and shortened with no loss of sensitivity. Debrisoquine metabolic ratios (MR) at 4 and 8 h showed excellent correlation indicating that collection time can also be shortened. Same day phenotyping is therefore possible. One hundred normal, Caucasian Australian subjects were phenotyped (46 males, 54 females) and 6% were poor metabolisers (PM) of debrisoquine. Fifty of the original subjects were also acetylation phenotyped and 34% were fast and 66% slow acetylators. One PM of debrisoquine was a fast acetylator of sulphadimidine and four PM were slow acetylators. This was a non-significant association.

Amitriptyline pharmacokinetics and clinical response: II. Metabolic polymorphism assessed by hydroxylation of debrisoquine and mephenytoin.

A subgroup of 16 out of 30 endogenous depressive inpatients (cf. part I), treated for 3 weeks with 150 mg amitriptyline (AT) daily, participated in a pharmacogenetic study: all were phenotyped with debrisoquine and 3 of them with mephenytoin. Four patients were found to be poor metabolizers (PMs) of debrisoquine and one of mephenytoin. Plasma levels of AT + NT (nortriptyline) were highest in the PMs of debrisoquine, but the ratio of hydroxylated metabolites to the parent compounds appeared to be lower in these subjects. From these data, it is speculated that, in the PM of mephenytoin, the demethylation of AT is impaired. In 12 patients, free plasma 10-hydroxy-AT (ATOH) and 10-hydroxy-NT (NTOH) were found to be bound to a similar extent to plasma proteins, but not so firmly as their parent compounds, by a factor of 6 and 4 respectively. While mean total plasma ATOH reached only 15% of the value of AT, total plasma NTOH was as high as NT. ATOH correlated significantly with its parent compound, but NTOH did not correlate with NT. No drug plasma levels/clinical relationship was found in this small group of patients, even when the hydroxylated metabolites were taken into account. Both poor and extensive metabolizers of debrisoquine responded to treatment. The debrisoquine-test appears to be a useful clinical tool for detecting in patients a genetic deficiency in the hydroxylation of AT-type drugs.

Co-inheritance of the polymorphic metabolism of encainide and debrisoquin.

O-demethylation of the investigational antiarrhythmic encainide was found to be correlated with the genetically determined hydroxylation of debrisoquin in 20 randomly selected and unrelated subjects and in five members of one family. Extensive metabolizers of debrisoquin had a mean (+/- SD) encainide elimination t1/2 of 1.19 +/- 0.98 hours (range 0.25 to 3.4 hours). Poor metabolizers of debrisoquin (two normal subjects and three family members) had a mean t1/2 of 13.2 +/- 0.73 hours (range 7.8 to 22.4 hours). The elimination rate constant of encainide and the fractional excretion of O-desmethyl encainide in urine were linearly related to the fractional urinary excretion of 4-hydroxy-debrisoquin. Poor metabolizers could be identified after a 50 mg dose of encainide by the fractional excretion of O-desmethyl encainide in urine or the absence of (1) measurable ECG changes or (2) O-desmethyl encainide in plasma. The correlation between excretion of O-desmethyl encainide and 4-hydroxy-debrisoquin suggests that significant numbers of the caucasian population (7% to 9%) are likely to be poor metabolizers of encainide and to have markedly different pharmacokinetics and plasma concentration-response relationships than extensive metabolizers.

Oxidation of tricyclic antidepressant drugs, debrisoquine and 7-ethoxyresorufin, by human liver preparations.

Data obtained from human studies in vivo show that the dispositions of the tricyclic antidepressant drugs desmethylimipramine (DMI) and nortriptyline are related to the debrisoquine hydroxylation phenotype. To obtain insight into the enzymic mechanisms behind this, the metabolism of debrisoquine and antidepressant drugs by human liver preparations have been studied. The 2-hydroxylation of DMI in vitro correlates with the 4-hydroxylation of debrisoquine among various livers (rs = 0.90). Debrisoquine inhibits DMI hydroxylation competitively, and DMI inhibits debrisoquine hydroxylation, suggesting that DMI hydroxylation is catalysed by the debrisoquine hydroxylase in human liver. By monitoring the hydroxylation of DMI in various fractions during separation and purification of cytochrome P-450 from human liver microsomes we have purified a cytochrome P-450 which efficiently hydroxylates this drug. The apparently electrophoretically homogeneous enzyme had a molecular weight of 51,500 and hydroxylated DMI and debrisoquine at rates of up to 0.95 and 0.45 nmol/min . nmol P-450, respectively. This is probably the major debrisoquine hydroxylating cytochrome P-450 in man. Nortriptyline 10-hydroxylation correlates strongly (r = 0.96) with debrisoquine hydroxylation in human liver microsomes. Nortriptyline inhibits DMI-hydroxylation competitively, and the drug also inhibits the 4-hydroxylation of debrisoquine. Thus it is probable that nortriptyline is hydroxylated by debrisoquine hydroxylase. Imipramine N-demethylation did not correlate significantly (P greater than 0.1) with debrisoquine hydroxylation among microsomes from nine livers. However, if a liver from a subject, which was a poor metabolizer of debrisoquine in vivo, was included, a correlation was obtained (r = 0.79, P less than 0.01, N = 10). Imipramine demethylation also correlated with DMI-hydroxylation only if the 'poor metabolizer' liver was included (r = 0.75, P less than 0.05, N = 10). Debrisoquine inhibited imipramine demethylation competitively. The data indicate that imipramine can interact with debrisoquine- and DMI-hydroxylase, but it is uncertain if this enzyme plays an important quantitative role in its demethylation. Ethoxyresorufin O-deethylation correlated with DMI hydroxylation (r = 0.80) in human liver preparations, and DMI inhibited the former reaction in what is probably a mixed competitive-non-competitive inhibition. Liver preparations from a subject who was a poor oxidizer of debrisoquine both in vivo and in vitro had unusually low capacity to metabolize ethoxyresorufin. Thus ethoxyresorufin, at least partly, seems to interact with an enzyme that can metabolize DMI in human liver.(ABSTRACT TRUNCATED AT 400 WORDS)

Metoprolol metabolism and debrisoquine oxidation polymorphism--population and family studies.

The metabolism of metoprolol was studied in 143 unselected hypertensive patients and in 10 families. The log10 metoprolol to alpha-hydroxymetoprolol urinary ratio was bimodally distributed and was correlated with the debrisoquine oxidation phenotype (rs = 0.81, P less than 0.001). The results of the pedigree study were compatible with poor hydroxylation of metoprolol being inherited as an autosomal recessive trait. The major urinary metabolite of metoprolol metabolism was H117-04, the end-product of O-dealkylation. The distribution of the log10 metoprolol to H117-04 (M/H117-04) urinary ratio was unimodal. However, there was a significant correlation between this ratio and the debrisoquine oxidation phenotype (rs = 0.68, P less than 0.001) and poor metabolisers of debrisoquine (PMs) were concentrated at the upper end of the range of M/H117-04 values. These results indicate that both the alpha-hydroxylation and O-dealkylation of metoprolol are under polymorphic control of the debrisoquine type. Plasma concentrations of metoprolol were about three times higher in PMs than in extensive metabolisers of debrisoquine (EMs) at 3 h after dosing. In a sub-group of 24 subjects, all seven PMs but only two EMs showed more than a 10% reduction in post-exercise heart rate at 24 h after dosing.

ECOGENETICS OF PARKINSON'S DISEASE: 4-HYDROXYLATION OF DEBRISOQUINE

It is postulated that Parkinson's disease is the result of environmental factors acting on genetically susceptible individuals against a background of normal ageing. Many potentially neurotoxic xenobiotics are detoxified by hepatic cytochrome P450. The function of one such system was studied in forty patients with Parkinson's disease and forty normal control subjects. Significantly more parkinsonian than control subjects had partially or totally defective 4-hydroxylation of debrisoquine. Poor metabolisers of debrisoquine tended to have had earlier onset of disease.

Phenacetin O‐deethylase: an activity of a cytochrome P‐450 showing genetic linkage with that catalysing the 4‐hydroxylation of debrisoquine?

Phenacetin O-deethylase activity was impaired, both in vivo and in vitro, in poor metabolisers of debrisoquine, consistent with the work of others. No impairment was observed in the oxidation of acetanilide, amylobarbitone or antipyrine in the PM phenotype. There was a good correlation (r = 0.804) between the high affinity component of phenacetin O-deethylase and debrisoquine 4-hydroxylase activities. No such correlation was observed with the low affinity component of phenacetin O-deethylase activity. Although debrisoquine was a competitive inhibitor of phenacetin O-deethylase activity, phenacetin was without effect on debrisoquine 4-hydroxylation. There was also marked differences in the effects of sparteine, guanoxan and alpha-naphthoflavone on the two activities. Cigarette smoking was associated with a significant, two-fold, increase in phenacetin O-deethylase activity whilst debrisoquine 4-hydroxylase activity was not affected. It is concluded that the high affinity component of phenacetin O-deethylase and debrisoquine 4-hydroxylase activities are catalysed by different isozymes of cytochrome P-450 but that these are most probably regulated by closely linked genes.

Methoxyphenamine metabolism in rat models of human debrisoquine phenotypes.

The metabolism of the beta 2-adrenoceptor agent methoxyphenamine was investigated in rats of the Lewis and Dark Agouti strains, which are proposed models for human extensive and poor metabolizers of debrisoquine, respectively. Following oral ingestion of 20 mg kg-1 of methoxyphenamine, Dark Agouti excreted, on the average, significantly more methoxyphenamine and less O-demethylmethoxyphenamine and 5-hydroxymethoxyphenamine in 0- to 24-h urine than Lewis. In contrast, the N-demethylation of methoxyphenamine showed no interphenotype differences between the two strains. It is possible that in rats, the form of cytochrome P-450, which controls the 4-hydroxylation of debrisoquine, may also control the O-demethylation and aromatic 5-hydroxylation of methoxyphenamine.

Oxidation phenotype and the metabolism and action of beta-blockers.

Variability in response to some drugs such as debrisoquine can be attributed to genetic polymorphism of their oxidative metabolism. Most beta-adrenoceptor antagonists (beta-blockers) are extensively metabolised via oxidative routes. Anecdotal reports of high plasma concentrations of certain beta-blockers in poor metabolisers of debrisoquine (PM) have claimed that their oxidation is under polymorphic control. Controlled studies have shown that debrisoquine oxidation phenotype is a major determinant of the metabolism, pharmacokinetics and some of the pharmacological actions of metoprolol, bufuralol and timolol. The PM phenotype is associated with an increased drug bioavailability, a prolongation of elimination half-life and more intense and sustained beta-blockade. Phenotypic differences were also noted in the pharmacokinetics of the enantiomers of metoprolol. In vivo and in vitro work has identified some of the metabolic pathways which are subject to the defect, namely, the alpha-hydroxylation and the O-dealkylation of metoprolol and the 1'-hydroxylation of bufuralol. In contrast, the pharmacokinetics and pharmacodynamics of propranolol which is also extensively oxidised, are not related to debrisoquine polymorphism, although 4'-hydroxypropranolol formation is lowered in PM subjects. The clinical significance of impaired elimination of beta-blockers is unclear. If standard doses of beta-blockers are used in PM subjects, they may be susceptible to concentration-related adverse reactions and they may also require lower and less frequent dosing for control of angina pectoris.

Effect of oxidative polymorphism (debrisoquine/sparteine type) on hepatic first-pass metabolism of bufuralol.

Bufuralol is a beta-adrenoceptor blocking drug whose oxidative metabolism is under the same genetic control as debrisoquine and sparteine. The pharmacokinetics of bufuralol were studied in 10 healthy subjects (7 extensive and 3 poor metabolizers of debrisoquine) after oral and intravenous administration. In extensive metabolizers the systemic availability of bufuralol was 43%. Poor metabolizers were characterized by a considerable increase in systemic availability due to a corresponding decrease in hepatic first-pass metabolism. After oral administration of bufuralol non-linear kinetics may occur.