Evidence for an enzymatic defect in the 4-hydroxylation of debrisoquine by human liver

Cytochrome P450 enzymes: interpretation of their interactions with selective serotonin reuptake inhibitors. Part II

The SSRIs have been used as an example to show how one might interpret the available evidence to draw conclusions about the relationships between drugs and P450s. Under what circumstances might one apply the knowledge of such relationships? First, the clinical implications must be considered when drugs with a narrow therapeutic index are coprescribed with other drugs that may affect P450s. For example, good clinical practice demands that before a TCA is coprescribed with another drug, the physician be aware of the potential for the second drug to interact with CYP2D6. Second, it may be helpful to consider P450 enzymes when adverse events occur during polypharmacy. It may happen that a known side effect of one drug occurs. Rather than attributing this to patient sensitivity, the physician should consider the possibility that a pharmacokinetic drug interaction increased plasma drug concentration, which in turn enhanced the probability of such an occurrence. Even when a pharmacokinetic drug interaction is considered as a possible cause, an appreciation of the role of P450s may lead to the realization that an interaction was not only possible but that it was likely. Finally, copharmacy can be used intentionally to produce controlled interactions. Indeed, planned pharmacokinetic drug interactions at the level of P450s have been proposed to reduce cyclosporine dosage requirements, to reduce variability of TCA levels, and to manipulate the contribution of alternative metabolic pathways to minimize toxic effects. As long as pharmaceuticals are metabolized by the P450 system, interactions with the various isozymes will be inescapable. It is fortunate that understanding them is becoming more tractable.

Interindividual variability in the N-sulphation of desipramine in human liver and platelets

1. The activity of N-sulphotransferase (N-ST) with desipramine (DMI) as substrate was measured in 118 human liver specimens, in platelets obtained from 105 subjects, in 12 specimens of human ileum and colon mucosa and in five specimens of human kidney and lung. 2. N-ST activity ranged between 5.71 and 157 pmol min-1 mg-1 protein in human liver and between 0.27 and 17.3 pmol min-1 mg-1 protein in human platelets. 3. Probit analysis was compatible with a unimodal distribution of the data from both liver and platelets. 4. The frequency distribution histograms of N-ST were asymmetric, with a positive skew in data from both liver and platelets. The mode, median and mean of N-ST were 16.4, 30.2 and 40.4 pmol min-1 mg-1 protein in liver, and 2.12, 3.61 and 3.82 pmol min-1 mg-1 protein in platelets, respectively. After logarithmic transformation of N-ST activity, the frequency distribution histogram was symmetric for data from both liver and platelets. 5. In extrahepatic tissues, the average (+/- s.d.) N-ST activity (pmol min-1 mg-1 protein) was 22.2 +/- 22.8 (ileum), 20.9 +/- 26.9 (colon), 12.4 +/- 5.5 (renal cortex), 9.3 +/- 2.8 (renal medulla) and 4.2 +/- 1.1 (lung). N-ST is widely distributed in the body and the intestine is the extrahepatic tissue with the highest N-ST activity.

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

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

Differential foetal development of the O- and N-demethylation of codeine and dextromethorphan in man

1. Codeine and dextromethorphan were N-demethylated in human foetal liver microsomes at high rates which were close to the activities in adult livers. In contrast, foetal liver microsomes did not catalyze the O-demethylation of these drugs at mid-gestation. 2. The metabolic data were in accordance with the absence of P450IID6 and the presence of P450 IIIA as determined by Western blotting with anti-human P450 IID6 (MAb 114/2) and anti-rat P450 IIIA (PCN 2-13-1/C2) monoclonal antibodies, respectively. 3. The inhibitory effects of midazolam and dehydroepiandrosterone support the contention that the N-demethylase is a human foetal form of the cytochrome P450 IIIA family. Consistent with this we found that blotting with the MAb PCN 2-13-1/C2, which recognizes an epitope specific for the P450 III family, correlated well with the N-demethylase activities.

Differences in phenytoin biotransformation and susceptibility to congenital malformations: a review

The clinical variability of teratogenic response to fetal drug exposure has been well documented. Metabolic differences in biotransformation have been shown to extend to multiple drugs and may involve many steps in drug metabolism with alterations of key intermediates. Although metabolic differences have been reported to be associated with complications of medication use, it has only recently been appreciated that such differences also may be associated in the unborn with the potential for the disruption of normal embryologic development and the production of congenital malformations. It has long been suspected that the teratogenicity of phenytoin may be mediated not only by the parent compound, but also by toxic intermediary metabolites that are produced during the biotransformation of the parent compound. Recent work elucidating differences in isoenzyme forms of cytochrome P-450 enzyme systems, glutathione, and microsomal epoxide hydrolase has provided increased interest in the multiple individual pharmacogenetic differences that may be significant factors affecting increased susceptibility to birth defects in individuals and families with fetal exposure to phenytoin.

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.

Factors affecting the enterohepatic circulation of oral contraceptive steroids

Oral contraceptive steroids may undergo enterohepatic circulation, but it is relevant for only estrogens, because these compounds can be directly conjugated in the liver. Animal studies show convincing evidence of the importance of the enterohepatic circulation, but studies in humans are much less convincing. The importance of the route and the rate of metabolism of ethinyl estradiol are reviewed. Some antibiotics have been reported anecdotally to reduce the efficacy of oral contraceptive steroids, but controlled studies have not confirmed this observation. Although gut flora are altered by oral antibiotics, the blood levels of ethinyl estradiol are not reduced, and one antibiotic at least (cotrimoxazole) enhances the activity of ethinyl estradiol.

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.

Metronidazole and antipyrine as probes for the study of foreign compound metabolism

The aim of the present work was to develop a tool for the study of the enzyme activities relevant for the biotransformation of foreign compounds, their elimination and/or activation to toxic substances. The activity of an enzyme may be assessed by the rate of metabolism of a preferably specific probe or model compound. The cytochrome P450'ies, the key enzymes for the elimination and/or activation of most foreign compounds, exist in multiple forms with variable substrate specificity and regulation. Some cytochrome P450'ies are under genetic control, whereas the activity of others is mainly regulated by the influence from factors in the environment. Only some of the cytochrome P450'ies are relevant for the formation of harmful metabolites. Thus, the activity of as many cytochrome P450 forms as possible should be assessable, preferably simultaneously. The present work evaluated metronidazole in a cocktail with antipyrine as a tool for the study of the regulation of foreign compound metabolism in the liver. The cytochrome P450 catalyzed metabolism of metronidazole and antipyrine was studied in humans and in isolated rat hepatocytes. In humans the influence of dose, route of administration, enzyme induction and inhibition and liver disease was investigated. Rats of either sex were studied with and without pretreatment with specific enzyme inducers and incubations included specific enzyme inhibitors. Evidence was provided that the oxidative formation of the five major metabolites, two from metronidazole and three from antipyrine, depends on different cytochrome P450'ies. In humans it was demonstrated that the clearance of metronidazole and antipyrine could be determined from the same saliva sample collected 16-24 hours after their oral administration and so could the clearance for formation of each metabolite if urine was collected for 48 hours. Thus, with the cocktail of metronidazole and antipyrine and simple non-invasive sampling the activity of five different cytochrome P450'ies can be assessed in vivo. In addition, metronidazole may also be used for assessment of the glucuronidation capacity although this is a minor pathway in man. Because the variation within subjects is much less than between them, the cocktail test is particularly suited for paired designs with measurements before and after an environmental change and the subjects serving as their own control. The metronidazole/antipyrine cocktail may have many applications in the study of the regulation of foreign compound metabolism in man and in animals, in vivo and in vitro.

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.

Effect of quinidine on the dextromethorphan O-demethylase activity of microsomal fractions from human liver

1. The kinetics of dextromethorphan O-demethylation were measured in microsomes prepared from five human livers, both in the absence and in the presence of quinidine. 2. For each liver and over the concentration range of dextromethorphan examined (4.2-3400 microM), this reaction involved an enzymatic component of high affinity, with an apparent Michaelis-Menten constant (Km) of 4.6 +/- 1.8 microM (mean +/- s.d.) and a maximum velocity (Vmax) of 4.2 +/- 3.5 nmol mg-1 h-1 (mean +/- s.d.). 3. Quinidine was a potent and competitive inhibitor of the activity of this component (mean Ki +/- s.d. of 0.025 +/- 0.008 microM) as it is for other oxidation reactions which have already been found to co-segregate with the debrisoquine-type polymorphism. 4. With microsomes from four of the five livers studied, there was evidence of a second enzymatic component of activity characterized by a similar Vmax and about 20-fold higher Km compared with the high affinity component. The activity of this low affinity component was unaffected by quinidine in the concentrations studied.

Genetic polymorphism in drug oxidation

Of the two clearly established drug oxidation polymorphisms, only the one referred to as debrisoquine polymorphism affects many drugs. The only known polymorphic substrates of mephenytoin hydroxylase are mephenytoin and mephobarbital. Relatively recently discovered drug substrates of debrisoquine hydroxylase are propafenone, diltiazem, and codeine. The list of substrates contains 28 items. The fate of slightly less than half of these is clinically affected in poor metabolizers, and several of the latter drugs are no longer marketed. There are many reasons why a failure of metabolism may not alter the fate of a drug sufficiently to affect its clinical use. Of interest and clinical importance is the inhibition of debrisoquine hydroxylase by inhibitors such as quinidine and by some neuroleptics; also the simultaneous use of two substrates has led to serious toxicity by mutual metabolic inhibition. The study of these oxidation polymorphisms has been instructive not only for formal pharmacogenetics but also for the understanding of problems of therapy in patients without genetic defects.

Metabolic activation of pyrolysate arylamines by human liver microsomes; possible involvement of a P-488-H type cytochrome P-450

Metabolic activating capacity of human livers for carcinogenic heterocyclic arylamines has been studied using a Salmonella mutagenesis test. A large individual variation was observed among 15 liver samples in the capacities of activation of Glu-P-1 (2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole), IQ (2-amino-3-methylimidazo[4,5-f]quinoline) and MeIQx (2-amino-3,8-dimethyl-3 H-imidazo[4,5-f]quinoxaline). The average numbers of revertants induced by the three heterocyclic arylamines were nearly the same or rather higher in the presence of hepatic microsomes from human than those from rat. In high-performance liquid chromatography, formation of N-hydroxy-Glu-P-1 was detected and accounted for more than 80% of the total mutagenicity observed in the human microsomal system with Glu-P-1, indicating that, similarly to experimental animals, N-hydroxylation is a major activating step for heterocyclic arylamines in human. Addition of flavone or 7,8-benzoflavone to human liver microsomes showed effective inhibition of the mutagenic activation of Glu-P-1, although the treatment rather enhanced microsomal benzo[a]pyrene hydroxylation in human livers. Mutagenic activation of Glu-P-1 by human liver microsomes was also decreased by the inclusion of anti-rat P-448-H IgG, and was well correlated with the content of immunoreactive P-448-H in livers, suggesting the involvement of a human cytochrome P-450, which shares immunochemical and catalytic properties with rat P-448-H, in the metabolic activation of heterocyclic arylamines in human livers.

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.

Bioactivation of the narcotic drug codeine in human liver is mediated by the polymorphic monooxygenase catalyzing debrisoquine 4-hydroxylation (cytochrome P-450 dbl/bufI)

Codeine O-demethylation to its active moiety morphine was investigated in human liver microsomes from 1 poor and 5 extensive metabolizer subjects (debrisoquine-type of oxidation polymorphism). Apparent Km of the reaction in one extensive metabolizer's microsomes was 149 microM and Vmax 17.6 nmol X mg P-1 X hour-1 versus greater than 1 mM and 1.6 nmol X mg P-1 X hour-1 respectively in one poor metabolizer. In vitro morphine production was competitively inhibited by quinidine (Ki 15 nM), the selective inhibitor of cytochrome P-450 dbl/bufI. There was also an excellent correlation between dextromethorphan O-demethylation, a prototype reaction for cytochrome P-450 dbl/bufI activity, and codeine O-demethylation. These data allow to conclude that codeine bioactivation to morphine is dependent on the polymorphic monooxygenase known as cytochrome db1/bufI.

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.

Enzymatic basis of the debrisoquine/sparteine-type genetic polymorphism of drug oxidation. Characterization of bufuralol 1'-hydroxylation in liver microsomes of in vivo phenotyped carriers of the genetic deficiency

The genetically controlled polymorphic oxidation of debrisoquine and sparteine is caused by the absence or functional deficiency of a cytochrome P-450 isozyme. In order to elucidate the mechanisms underlying the differences in cytochrome P-450 function we have studied the 1'-hydroxylation of the prototype drug bufuralol in human liver microsomes of individuals phenotyped in vivo as extensive metabolizers (EM, N = 10), poor metabolizers (PM, N = 5) and in subjects with an intermediate rate of metabolism (IM, N = 4). PM- as compared to EM-microsomes were characterized by a decreased Vmax for (+)-bufuralol 1'-hydroxylation (7.51 +/- 2.03 nmol X mg-1 X hr-1 vs 11.95 +/- 4.80 nmol X mg-1 X hr-1) but not for (-)-bufuralol 1'-hydroxylation (4.72 +/- 0.87 nmol X mg-1 X hr-1 vs 5.55 +/- 1.49 nmol X mg-1 X hr-1). The apparent Km for (+)-bufuralol 1'-hydroxylation was increased in PM microsomes (118 +/- 84.9 microM vs 17.9 +/- 6.30 microM). Inhibition of bufuralol 1'-hydroxylation by quinidine was biphasic in EM microsomes, providing further support for the involvement of at least two cytochrome P-450 isozymes. Quinidine acted as a competitive inhibitor of only the high affinity/stereoselectivity component of the reaction. Our data suggest that the debrisoquine/sparteine type of oxidation polymorphism is caused by an almost complete loss of a minor cytochrome P-450 isozyme which has a high affinity and stereoselectivity for (+)-bufuralol and a high sensitivity to inhibition by quinidine.

Tolbutamide 4-hydroxylase activity of human liver microsomes: effect of inhibitors

Eight samples of human liver have been characterised for microsomal protein content, cytochrome P-450 content, tolbutamide 4-hydroxylase and ethinyloestradiol 2-hydroxylase activities. Cytochrome P-450 content correlated significantly with ethinyloestradiol 2-hydroxylase activity but not with tolbutamide 4-hydroxylase activity. There was no significant correlation between ethinyloestradiol 2-hydroxylase and tolbutamide 4-hydroxylase activities. The maximum tolbutamide 4-hydroxylase activity was 0.45 nmol min-1 mg-1 microsomal protein, with a Km value of 74 microM. A number of compounds were tested for their ability to inhibit tolbutamide metabolism. All the compounds showing inhibition were either non-competitive or mixed non-competitive inhibitors of tolbutamide 4-hydroxylation. These studies suggest that tolbutamide is metabolised by an isozyme of cytochrome P-450 which appears to be distinct from those isozymes metabolising many other drugs.

Irreversible binding and metabolism of propranolol by human liver microsomes--relationship to polymorphic oxidation

Studies were performed to investigate the irreversible binding and oxidative metabolism of propranolol in human liver microsomes and the relationship of binding and metabolism to the polymorphic oxidation of debrisoquine. Incubation of microsomes with 14C-labelled propranolol in the presence of a NADPH-generating system gave rise to irreversible binding which increased linearly with time and became saturated at high substrate concentrations. The extent of binding was decreased by the exclusion of cofactors, boiling, anaerobic conditions, and the addition of reduced glutathione and SKF-525A. Trichloropropene oxide had a negligible effect on cofactor-dependent binding. However, debrisoquine, antipyrine and phenacetin decreased binding to a considerable extent. The latter compound abolished cofactor-dependent binding completely at the concentration used (1 mM). Electrophoresis of microsomes which had been incubated with tritiated propranolol revealed that binding was probably occurring to a large number of proteins particularly in the 40,000-90,000 molecular weight range. Glutathione, debrisoquine and antipyrine did not inhibit the 4'-hydroxylation and N-deisopropylation of propranolol. In contrast, phenacetin exerted a very potent inhibitory action on both routes of metabolism. It is concluded that a product or products of propranolol oxidation bind irreversibly but non-selectively to human liver microsomal protein, the enzyme system responsible for the activation of propranolol appears to be related more closely to the cytochrome P-450 system which metabolizes phenacetin than to that metabolising debrisoquine, and radiolabelled propranolol is not a sufficiently specific probe for studying these cytochrome P-450 systems.

High-performance liquid chromatographic assays for bufuralol 1'-hydroxylase, debrisoquine 4-hydroxylase, and dextromethorphan O-demethylase in microsomes and purified cytochrome P-450 isozymes of human liver

Bufuralol, debrisoquine, and dextromethorphan are three prototype substrates of the common genetic deficiency of oxidative drug metabolism in man known as debrisoquine/sparteine-type polymorphism. We describe assays for the in vitro metabolism of (+)- and (-)-bufuralol, debrisoquine, and dextromethorphan in human liver microsomes and reconstituted purified cytochrome P-450 isozymes. These assays combine nonextractive sample preparation by precipitation of protein with perchloric acid with reversed-phase inorganic ion-pair HPLC and fluorescence detection. The minimal detectable levels of the major metabolites formed are 1'-hydroxybufuralol, 0.1 ng/ml; 4-hydroxydebrisoquine, 0.8 ng/ml; and dextrorphan, 0.1 ng/ml. Formation of these metabolites is linear for at least 45 min and between 1 and 100 micrograms of microsomal protein. Comparative kinetic analysis of the three monooxygenase reactions in human liver microsomes revealed an apparent biphasicity of (+)- and (-)-bufuralol 1'-hydroxylation and dextromethorphan O-demethylation but monophasic formation of 4-hydroxydebrisoquine in the substrate concentration range (less than 1 mM) studied. These data, in combination with those obtained by purified human cytochrome P-450 isozymes indicate the involvement of the same enzyme in the metabolism of all three substrates investigated. However, additional and distinct activities contribute to the metabolism of (+)- and (-)-bufuralol and dextromethorphan.

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.

Bufuralol 1'-hydroxylase activity of the rat. Strain differences and the effects of inhibitors

The kinetics of bufuralol 1'-hydroxylase activity of hepatic microsomal fractions have been determined in female DA and Fischer 344 rats, strains between which there is a large difference in debrisoquine 4-hydroxylase activity. Two components of bufuralol 1'-hydroxylase activity could be observed in both strains. Although there were differences between the strains in Vmax and Km of both components of activity, these were much less marked than the differences previously reported for debrisoquine 4-hydroxylase (Kahn et al., Drug Metab. Dispos. 13, 510 (1985)). The kinetics of bufuralol 1'-hydroxylase activity were such that the difference in activity between the strains varied with the concentration of bufuralol, 4-5-fold at 2.5 microM, no difference at 100 microM Competitive inhibitors of debrisoquine 4-hydroxylase activity in man were competitive inhibitors of bufuralol 1'-hydroxylase activity in the Fischer 344 rat, but not in the DA rat. The Ki for inhibition of bufuralol 1'-hydroxylase activity by debrisoquine in the Fischer 344 rat was 184 microM, compared with a Km for the 4-hydroxylation of this compound of 10.5 microM. It is concluded that the major isozyme of cytochrome P-450 catalysing the 1'-hydroxylation of bufuralol in the rat is different from that catalysing debrisoquine 4-hydroxylation (P-450UT-H).

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 defining whether one or more forms of cytochrome P-450 are involved in the monogenically-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 alpha-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.

A sensitive capillary GC assay for the determination of sparteine oxidation products in microsomal fractions of human liver

A sensitive method for the assay of sparteine oxidase activity in vitro by microsomal fractions of human liver is described. The activity of sparteine oxidase was assessed by the formation of 2- and 5-dehydrosparteines, which were estimated by capillary gas chromatography with N2-FID detection. The limit of detection of the two metabolites, 2- and 5-dehydrosparteine, was 10 pmol (2.3 ng) per sample. Sparteine oxidase activity was linear with microsomal protein concentration ranging from 25 to 200 ug and with incubation times between 5 and 60 minutes. Omission of NADPH on incubation under an atmosphere of carbon monoxide inhibited formation of both metabolites, thus indicating that aforementioned metabolites arise in reaction catalyzed by cytochrome P-450. In three liver samples from humans classified as extensive (EM) metabolizers the formation of 2- and 5-dehydrosparteines was observed, 2-dehydrosparteine being the major metabolite. In these samples sparteine oxidase activity was characterised by Vmax = 136 +/- 53 pmol/min/mg and Km = 44 +/- 12 microM for 2-dehydrosparteine formation. For 5-dehydrosparteine formation the following values were obtained: Vmax = 57 +/- 18 pmol/min/mg and Km = 42 +/- 26 microM. In a liver sample from a poor metabolizer (PM) only the formation of 2-dehydrosparteine was detected with the method of analysis used. In this sample a great increase in Km (Km PM = 3033 microM) was noted, while Vmax was very similar to those obtained for 2-dehydrosparteine formation in EM subjects (Vmax PM = 147 pmol min/mg).

Human-liver cytochromes P-450 involved in polymorphisms of drug oxidation

Nine forms of cytochrome P-450 have been purified to electrophoretic homogeneity from human-liver microsomes. These include the enzymes involved in debrisoquine 4-hydroxylation, phenacetin O-deethylation and mephenytoin 4-hydroxylation, three reactions which are characterized by genetic polymorphism in humans. Evidence for the involvement of the above enzymes comes from reconstituted immunochemical inhibition studies with human-liver microsomes. These and other lines of evidence are consonant with the view that different forms of cytochrome P-450 are involved in the three reactions. The debrisoquine 4-hydroxylase has been studied most extensively in terms of its substrate specificity. In addition, an analogous rat enzyme shows some homology and serves as a useful model. The use of antibodies raised to the rat-liver enzyme in immuno-inhibition studies with human-liver microsomes provides a means of determining the extent to which this enzyme participates in other reactions. Translation of rat-liver mRNA in vitro yields the intact debrisoquine 4-hydroxylase; studies with human mRNA suggest a lower frequency than in rats. The basis for impaired catalytic activity in phenotypically poor human metabolizers appears to be an altered enzyme in all three cases, as opposed to a decreased level of a single enzyme. Using antibody screening of fusion proteins expressed in a cDNA library, it has been possible to isolate cDNA probes for all three of these cytochromes P-450 for use in screening individuals and ultimately determining the basis of these polymorphisms.

The relationship between the acetylator and the sparteine hydroxylation polymorphisms

Thirty-eight healthy white British Caucasian subjects were hydroxylator phenotyped with sparteine and acetylator phenotyped with sulphadimidine. The results showed that there was no significant difference in the mean sparteine metabolic ratio between eight rapid acetylator extensive hydroxylators and 27 slow acetylator extensive hydroxylators.

Is the activation of aflatoxin B1 catalysed by the same form of cytochrome P-450 as that 4-hydroxylating debrisoquine in rat and/or man?

A possible association between the metabolic activation of aflatoxin B1 (AFB1) to a mutagen and the 4-hydroxylation of debrisoquine, which shows genetic variation both in man and in the rat, was investigated. Hepatic microsomal fractions from female DA and Fischer rats catalyse, at the same rate, the conversion of AFB1 to a mutagenic and arylating metabolite, that bound covalently to microsomal proteins. Debrisoquine was without effect on either of these reactions. In contrast, metyrapone did inhibit the mutagenic activation of AFB1. Microsomal fraction from human liver was also capable of activating AFB1 to a mutagenic and arylating metabolite. Again, debrisoquine was without appreciable effect on these reactions. In contrast, cimetidine caused profound inhibition of the mutagenic activation of AFB1. It is concluded that the activation of AFB1 to a mutagenic, and presumably carcinogenic, metabolite is catalysed by a different form of cytochrome P-450 from that catalysing the 4-hydroxylation of debrisoquine, both in rat and in man.

Stereo- and regioselectivity of hepatic oxidation in man--effect of the debrisoquine/sparteine phenotype on bufuralol hydroxylation

The influence of the debrisoquine/sparteine-type of oxidation polymorphism on plasma bufuralol concentration and the pattern of urine metabolites was studied in extensive and poor metabolizer subjects. (+)- and (-)-bufuralol, and (+)- and (-)-OH-bufuralol in plasma were determined by enantioselective HPLC, and urinary bufuralol and its metabolites were assayed by gas chromatography-mass spectrometry. Three hours after administration of racemic bufuralol the plasma (-)/(+) isomeric ratio for unchanged bufuralol was 1.84 in extensive metabolizers, indicating preferential clearance of the (+)-isomer through aliphatic 1'-hydroxylation and glucuroconjugation, while the (-)-isomer was mainly eliminated by aromatic 4-hydroxylation. Poor metabolizers were characterized by impaired 1'- and 4-hydroxylation, with almost total abolition of the stereoselectivity of these reactions. The data strongly suggest that both 1'- and 4-hydroxylation are catalyzed by the same enzyme. These in vivo observations are in agreement with recent in vitro data obtained in human liver microsomes from phenotyped patients and support the concept of deficiency of a highly stereoselective cytochrome P-450 isozyme as the cause of this polymorphism.

The polymorphic oxidation of beta-adrenoceptor antagonists. Clinical pharmacokinetic considerations

Wide variability in response to some drugs such as debrisoquine can be attributed largely to genetic polymorphism of their oxidative metabolism. Most beta-blockers undergo extensive oxidation. Anecdotal reports of high plasma concentrations of certain beta-blockers in poor metabolisers (PMs) of debrisoquine have claimed that the oxidation of these drugs is under polymorphic control. Subsequently, 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, timolol and bopindolol. The poor metaboliser phenotype is associated with increased plasma drug concentrations, a prolongation of elimination half-life and more intense and sustained beta-blockade. Phenotypic differences have also been observed in the pharmacokinetics of the enantiomers of metoprolol and bufuralol. In vivo and in vitro studies have identified some of the metabolic pathways which are subject to the defect, viz. alpha-hydroxylation and O-demethylation of metoprolol and 1'- and possibly 4- and 6-hydroxylation of bufuralol. In contrast, the overall pharmacokinetics and pharmacodynamics of propranolol, which is also extensively oxidised, are not related to debrisoquine polymorphism, although 4'-hydroxypropranolol formation is lower in poor metabolisers. As anticipated, the disposition of atenolol which is eliminated predominantly unchanged by the kidney and in the faeces, is unrelated to debrisoquine phenotype. The clinical significance of impaired elimination of beta-blockers is not clear. If standard doses of beta-blockers are used in poor metabolisers, these subjects may be susceptible to concentration-related adverse reactions and they may also require less frequent dosing for control of angina pectoris.

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.

Comparative metabolism of debrisoquine, 7-ethoxyresorufin and benzo(a)pyrene in liver microsomes from humans, and from rats treated with cytochrome P-450 inducers

The metabolism of debrisoquine, 7-ethoxyresorufin and benzo(a)pyrene has been studied in human liver microsomes. There was a significant correlation (r = 0.70, P less than 0.05) between debrisoquine hydroxylation and 7-ethoxyresorufin 0-deethylation among various livers, and debrisoquine inhibited 7-ethoxyresorufin deethylation competitively. These results suggest that debrisoquine and 7-ethoxyresorufin may be metabolised by a common P-450 form in human liver. The effect of cytochrome P-450 inducers on the metabolism of the three substrates was also examined in rat liver. Debrisoquine hydroxylation was not enhanced by phenobarbitone, beta-naphthoflavone or isosafrole.

Inhibition of desmethylimipramine 2-hydroxylation by drugs in human liver microsomes

The 2-hydroxylation of desmethylimipramine (DMI) correlates strongly with the 4-hydroxylation of debrisoquine (D) both in human volunteers and in vitro comparing human liver microsomes from different individuals. D competitively inhibits the 2-hydroxylation of DMI in vitro suggesting that DMI is hydroxylated by the 'debrisoquine hydroxylase' which is under monogenic control in man. We have characterized the effect of drugs on the hydroxylation of DMI in human liver microsomes by measuring the formation of 2-OH-DMI with HPLC using fluorescence detection. Amitriptyline, nortriptyline and metoprolol inhibited the hydroxylation of DMI competitively indicating interaction with the catalytical site for DMI 2-hydroxylation. Antipyrine and amylobarbitone at concentrations similar to their Km-values for metabolism did not inhibit DMI-hydroxylation. Thus, for these compounds there was a good correspondence between the drugs' capacity to inhibit DMI 2-hydroxylation competitively in vitro and their apparent metabolism by the 'debrisoquine hydroxylase' in vivo in man. Thioridazine, chlorpromazine, quinidine and quinine also inhibited DMI-hydroxylation competitively. Thioridazine was an unusually potent inhibitor (apparent inhibition constant Ki = 0.75 microM). Quinidine was also an unusually potent inhibitor (Ki = 0.27 microM) and much more efficient than its isomer quinine (Ki = 12 microM). Theophylline could inhibit DMI hydroxylation but with atypical kinetics. We suggest that this simple DMI in vitro test as well as earlier described inhibition tests with debrisoquine, sparteine and bufuralol can be used to screen if drugs interact with the 'debrisoquine hydroxylase' in human liver.

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.

Attempts to phenotype human liver samples in vitro for debrisoquine 4-hydroxylase activity

Twenty-eight samples of human liver have been characterised for cytochrome P-450 content, aldrin epoxidase, debrisoquine 4-hydroxylase and bufuralol 1'-hydroxylase activities. Evidence is presented here and elsewhere that bufuralol 1'-hydroxylase and debrisoquine 4-hydroxylase are activities catalysed by the same form of cytochrome P-450 in man, and that this form is different from that catalysing the epoxidation of aldrin. Attempts to phenotype liver samples in vitro, in the absence of any metabolic data in vivo for debrisoquine 4-hydroxylation status, met with limited success. A combination of enzyme assays will most probably be required in any such phenotyping of human liver samples.

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.

Genetic polymorphism in drug oxidation: in vitro studies of human debrisoquine 4-hydroxylase and bufuralol 1'-hydroxylase activities

A sensitive, specific assay utilizing fluorescence-HPLC has been developed for determining the 1'-hydroxylation of bufuralol by human liver. The 1'-hydroxylation of the isomers of bufuralol varied threefold, both the Vmax and the Km for the (+) isomer being greater than the corresponding values for the (-) isomer. Debrisoquine was a competitive inhibitor of the 1'-hydroxylation of both isomers and of the racemate of bufuralol. Both isomers and the racemate of bufuralol were competitive inhibitors of debrisoquine 4-hydroxylase activity. The competitive inhibition of debrisoquine and bufuralol of each other's metabolism, together with the similarity in the values for Km and Ki, support the conclusion that the same form of cytochrome P-450 catalyses these two reactions.