Pharmacogenetic covariation of defective N-oxidation of sparteine and 4-hydroxylation of debrisoquine

Pharmacogenetics of the cytochromes P450: Selected pharmacological and toxicological aspects

With the availability of detailed genomic data on all 57 human cytochrome P450 genes, it is clear that there is substantial variability in gene product activity with functionally significant polymorphisms reported across almost all isoforms. This article is concerned mainly with 13 P450 isoforms of particular relevance to xenobiotic metabolism. After brief review of the extent of polymorphism in each, the relevance of selected P450 isoforms to both adverse drug reaction and disease susceptibility is considered in detail. Bleeding due to warfarin and other coumarin anticoagulants is considered as an example of a type A reaction with idiosyncratic adverse drug reactions affecting the liver and skin as type B. It is clear that CYP2C9 variants contribute significantly to warfarin dose requirement and also risk of bleeding, with a minor contribution from CYP4F2. In the case of idiosyncratic adverse drug reactions, CYP2B6 variants appear relevant to both liver and skin reactions to several drugs with CYP2C9 variants also relevant to phenytoin-related skin rash. The relevance of P450 genotype to disease susceptibility is also considered but detailed genetic studies now suggest that CYP2A6 is the only P450 relevant to risk of lung cancer with alleles associated with low or absent activity clearly protective against disease. Other cytochrome P450 genotypes are generally not predictors for risk of cancer or other complex disease development.

Sequencing the CYP2D6 gene: from variant allele discovery to clinical pharmacogenetic testing

CYP2D6 is one of the most studied enzymes in the field of pharmacogenetics. The CYP2D6 gene is highly polymorphic with over 100 catalogued star (*) alleles, and clinical CYP2D6 testing is increasingly accessible and supported by practice guidelines. However, the degree of variation at the CYP2D6 locus and homology with its pseudogenes make interrogating CYP2D6 by short-read sequencing challenging. Moreover, accurate prediction of CYP2D6 metabolizer status necessitates analysis of duplicated alleles when an increased copy number is detected. These challenges have recently been overcome by long-read CYP2D6 sequencing; however, such platforms are not widely available. This review highlights the genomic complexities of CYP2D6, current sequencing methods and the evolution of CYP2D6 from allele discovery to clinical pharmacogenetic testing.

Guanfu base A, an antiarrhythmic alkaloid of Aconitum coreanum, Is a CYP2D6 inhibitor of human, monkey, and dog isoforms

Guanfu base A (GFA) is a novel heterocyclic antiarrhythmic drug isolated from Aconitum coreanum (Lèvl.) rapaics and is currently in a phase IV clinical trial in China. However, no study has investigated the influence of GFA on cytochrome P450 (P450) drug metabolism. We characterized the potency and specificity of GFA CYP2D inhibition based on dextromethorphan O-demethylation, a CYP2D6 probe substrate of activity in human, mouse, rat, dog, and monkey liver microsomes. In addition, (+)-bufuralol 1'-hydroxylation was used as a CYP2D6 probe for the recombinant form (rCYP2D6), 2D1 (rCYP2D1), and 2D2 (rCYP2D2) activities. Results show that GFA is a potent noncompetitive inhibitor of CYP2D6, with inhibition constant Ki = 1.20 ± 0.33 μM in human liver microsomes (HLMs) and Ki = 0.37 ± 0.16 μM for the human recombinant form (rCYP2D6). GFA is also a potent competitive inhibitor of CYP2D in monkey (Ki = 0.38 ± 0.12 μM) and dog (Ki = 2.4 ± 1.3 μM) microsomes. However, GFA has no inhibitory activity on mouse or rat CYP2Ds. GFA did not exhibit any inhibition activity on human recombinant CYP1A2, 2A6, 2C8, 2C19, 3A4, or 3A5, but showed slight inhibition of 2B6 and 2E1. Preincubation of HLMs and rCYP2D6 resulted in the inactivation of the enzyme, which was attenuated by GFA or quinidine. Beagle dogs treated intravenously with dextromethorphan (2 mg/ml) after pretreatment with GFA injection showed reduced CYP2D metabolic activity, with the Cmax of dextrorphan being one-third that of the saline-treated group and area under the plasma concentration-time curve half that of the saline-treated group. This study suggests that GFA is a specific CYP2D6 inhibitor that might play a role in CYP2D6 medicated drug-drug interaction.

Institutional Profile: Karolinska Institutet

Research in pharmacogenomics has been intensive at Karolinska Institutet (KI) for approximately 25 years. Initial initiatives were focused on the identification and characterization of novel CYP2D6 alleles causing ultrarapid or defective drug metabolism. Such discoveries were possible owing to the early implementation of therapeutic drug monitoring and the access to individuals phenotyped with respect to drug metabolism. The translational work at KI has been of utmost importance for successful research, including functional characterization and clinical validation of allelic variants in drug metabolism, as well as discoveries of novel polymorphisms, recent examples being the CYP2C19 and UGT2B17 genes. The clinical pharmacology laboratory at KI campus Huddinge is one of the leading sites for therapeutic drug monitoring in northern Europe and obtains an increasing number of clinical requests, also important for pharmacogenetic research. Furthermore, the recently opened Center for Hematology and Regenerative Medicine, with a clear translational emphasis, offers an opportunity for studying drug metabolism and toxicity in vitro by use of human hepatocytes.

Clinically relevant genetic variations in drug metabolizing enzymes

In the field of pharmacogenetics, we currently have a few markers to guide physicians as to the best course of therapy for patients. For the most part, these genetic variants are within a drug metabolizing enzyme that has a large effect on the degree or rate at which a drug is converted to its metabolites. For many drugs, response and toxicity are multi-genic traits and understanding relationships between a patient's genetic variation in drug metabolizing enzymes and the efficacy and/or toxicity of a medication offers the potential to optimize therapies. This review will focus on variants in drug metabolizing enzymes with predictable and relatively large impacts on drug efficacy and/or toxicity; some of these drug/gene variant pairs have impacted drug labels by the United States Food and Drug Administration. The challenges in identifying genetic markers and implementing clinical changes based on known markers will be discussed. In addition, the impact of next generation sequencing in identifying rare variants will be addressed.

Pharmacogenetics and human genetic polymorphisms

The term pharmacogenetics was first used in the late 1950s and can be defined as the study of genetic factors affecting drug response. Prior to formal use of this term, there was already clinical data available in relation to variable patient responses to the drugs isoniazid, primaquine and succinylcholine. The subject area developed rapidly, particularly with regard to genetic factors affecting drug disposition. There is now comprehensive understanding of the molecular basis for variable drug metabolism by the cytochromes P450 and also for variable glucuronidation, acetylation and methylation of certain drugs. Some of this knowledge has already been translated to the clinic. The molecular basis of variation in drug targets, such as receptors and enzymes, is generally less well understood, although there is consistent evidence that polymorphisms in the genes encoding the β-adrenergic receptors and the enzyme vitamin K epoxide reductase is of clinical importance. The genetic basis of rare idiosyncratic adverse drug reactions had also been examined. Susceptibility to reactions affecting skin and liver appears to be determined in part by the HLA (human leucocyte antigen) genotype, whereas reactions affecting the heart and muscle may be determined by polymorphisms in genes encoding ion channels and transporters respectively. Genome-wide association studies are increasingly being used to study drug response and susceptibility to adverse drug reactions, resulting in identification of some novel pharmacogenetic associations.

Evaluation of probe drugs and pharmacokinetic metrics for CYP2D6 phenotyping

INTRODUCTION

Cytochrome P450 2D6 (CYP2D6) is one of the most important enzymes catalyzing biotransformation of xenobiotics in the human liver. This enzyme's activity shows a high degree of interindividual variability caused in part by its genetic polymorphism, the so-called debrisoquine/sparteine polymorphism. The genetic component influencing CYP2D6 activity can be determined by genotyping. However, genotyping alone is not sufficient to accurately predict an individual's actual CYP2D6 activity, as this is also influenced by other factors. For the determination of the exact actual enzymatic activity ("phenotyping"), adequate probe drugs have to be administered prior to measurements of these compounds and/or their metabolites in body fluids. PROBE DRUGS: Debrisoquine, sparteine, metoprolol or dextromethorphan represent well-established probe drugs while tramadol has been recently investigated for this purpose. The enzymatic activity is reflected by various pharmacokinetic metrics such as the partial clearance of a parent compound to the respective CYP2D6-mediated metabolite or metabolic ratios. Appropriate metrics need to fulfill pre-defined validation criteria.

METHODS

In this review, we have compiled a list of such criteria useful to select the best metrics to reflect CYP2D6 activity. A comprehensive Medline search for reports on CYP2D6 phenotyping trials with the above mentioned probe drugs was carried out.

CONCLUSION

Application of the validation criteria suggests that dextromethorphan and debrisoquine are the best CYP2D6 phenotyping drugs, with debrisoquine having the problem of very limited availability as a therapeutic drug. However, the assessment of the best dextromethorphan CYP2D6 phenotyping metric/procedure is still ongoing.

Development of analytical technology in pharmacogenetic research

Methods used to determine phenotype and genotype for pharmacogenetic polymorphisms are discussed. Phenotyping is mainly applicable to polymorphisms affecting drug disposition rather than drug response and can involve either direct measurement of enzyme activity or administration of a probe drug followed by measurement of drug and/or metabolite levels. Genotyping is now more widely used than phenotyping and can be used to determine genotype for polymorphisms affecting either drug disposition (for example those in the cytochromes P450 or N-acetyltransferases) or drug response (for example those in drug receptors). Most genotyping for known polymorphisms involves use of the polymerase chain reaction and the wide variety of methods based on this technique that are now used for routine genotyping are discussed in detail. In addition, a range of methods that can be used to detect novel polymorphisms, thereby further increasing understanding of interindividual variability in drug disposition and response, is described.

Perspectives in pharmacogenetics

The historical perspective of pharmacogenetics is presented to a large extent from a personal view, since I happened to get into this field of science at the time of its beginning, and since pharmacogenetics has remained the backbone of my scientific career. Pharmacogenetics initially emphasized observations of interpersonal variability, but the attention on interethnic differences soon followed. Technical advances led to the identification of many responsible gene alterations in both individuals and in populations. Included is a relatively extensive discussion in which the effects and the different consequences of monogenic (mendelian) and multigenic (multifactorial) variation are compared.

Phenotyping of drug-metabolizing enzymes in adults: a review of in-vivo cytochrome P450 phenotyping probes

Cytochrome P450 phenotyping provides valuable information about real-time activity of these important drug-metabolizing enzymes through the use of specific probe drugs. Despite more than 20 years of research, few conclusions regarding optimal phenotyping methods have been reached. Caffeine offers many advantages for CYP1A2 phenotyping, but the widely used caffeine urinary metabolic ratios may not be the optimal method of measuring CYP1A2 activity. Several probes of CYP2C9 activity have been suggested, but little information exists regarding their use, largely due to the narrow therapeutic index of most CYP2C9 probes. Mephenytoin has long been considered the standard CYP2C19 phenotyping probe, but problems such as sample stability and adverse effects have prompted the investigation of potential alternatives, such as omeprazole. Several well-validated CYP2D6 probes are available, including dextromethorphan, debrisoquin and sparteine, but, in most cases, dextromethorphan may be preferred due to its wide safety margin and availability. Chlorzoxazone remains the only CYP2E1 probe that has received much study. However, questions concerning phenotyping method and involvement of other enzymes have impaired its acceptance as a suitable CYP2E1 phenotyping probe. CYP3A phenotyping has been the subject of numerous investigations, reviews and commentaries. Nevertheless, much controversy regarding the selection of an ideal CYP3A probe remains. Of all the proposed methods, midazolam plasma clearance and the erythromycin breath test have been the most rigorously studied and appear to be the most reliable of the available methods. Despite the limitations of many currently available probes, with continued research, phenotyping will become an even more valuable research and clinical resource.

Optimizing drug therapy based on genetic differences: implications for the clinical setting

Differences in drug responses due to gene alterations are rapidly being identified. Gene alterations may inhibit the function of an enzyme so that an active drug accumulates, causing adverse reactions with normal doses. Alternatively, gene alterations may accelerate enzymatic function so that an active drug is rapidly eliminated, causing subtherapeutic responses to normal doses. Mutations and polymorphisms have been identified that affect a person's response to many currently prescribed medications including cardiovascular, anti-infective, chemotherapeutic, psychiatric, and analgesic drugs. The potential exists for drug therapy to be optimized by selecting medication and doses based on a person's genotype rather than by trial and error. In the near future, advanced practice nurses in the acute care setting may be expected to order, provide patient education about, and explain results of genetic tests before initiating a specific drug therapy. Advanced practice nurses must be knowledgeable about what genetic tests are analyzing and their benefits, limitations, and risks.

Pharmacogenetics of the hepatic cytochrome P450 enzyme system: its relevance for prescribing in psychiatry

This article reviews the current knowledge regarding the hepatic cytochrome P450 system, with particular reference to its effect on psychotherapeutic medication. The metabolic processes – by which drugs are broken down in the liver by cytochrome P450 enzymes – are affected by genetic variation between individuals, inhibition and induction of these enzymes by other drugs, disease and age. Genetic influences and enzyme inhibition/induction are discussed in particular detail in this article.

Genetic differences in drug disposition

Genetic polymorphisms of drug metabolizing enzymes are well recognized. This review presents molecular mechanisms, ontogeny and clinical implications of genetically determined intersubject variation in some of these enzymes. Included are the polymorphic enzymes N-acetyl transferase, cytochromes P4502D6 and 2C, which have been well described in humans. Information regarding other Phase I and Phase II polymorphic pathways, such as glutathione and methyl conjugation and alcohol and acetaldehyde oxidation continues to increase and are also discussed. Genetic factors effecting enzyme activity are frequently important determinants of the disposition of drugs and their efficacy and toxicity. In addition, associations between genetic differences in these enzymes and susceptibility to carcinogens and teratogens have been reported. Ultimately, the application of knowledge regarding these genetic factors of enzyme activity may guide medical therapy and minimize xenobiotic-induced disease.

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.

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.

Determination of dextromethorphan metabolizer phenotype in healthy volunteers

The dextromethorphan metabolizer phenotype in 450 healthy volunteers (299 men, 151 women) was determined after oral administration of a 15 mg dose. In 8 h-postdose urine samples the ratio of dextrorphan (DOP) to dextromethorphan (DMP) was measured by HPLC. Urinary excretion of DMP and DOP within 8 h after the dose varied greatly between individuals, ranging from 0-11% and 0.04-100% of dose, respectively. In 143 test subjects the fraction of the dose of DMP in urine was below the detection limit. In the remaining 307 volunteers the metabolic ratio (MR) of DOP to DMP varied from 0.07 to 2906. In 404 test subjects the MR was greater than 10 and they were classified as extensive metabolizers (90% of the entire group). Of the entire group 5% had MRs of 1-10 and less than 1, respectively. Depending on the limit for classification of poor metabolizers, their frequency was 5-10% in the Caucasian population studied. The present data are in agreement with previous findings that the oxidative metabolic polymorphisms of debrisoquin and DMP co-segregate; the frequency of the PM phenotype of dextromethorphan in Caucasian populations varies between 5 and 10%.

Sparteine oxidation polymorphism: phenotyping by measurement of sparteine and its dehydrometabolites in plasma

Phenotyping of the ability to oxidize sparteine was markedly facilitated by analyzing sparteine and dehydrosparteines in a single plasma sample by gas chromatography. The definitive identification of extensive and poor metabolizers was possible only 90 min after ingestion of 100 mg sparteine sulphate. In 121 healthy volunteers determination of the plasma level ratio was compared to the established determination of the metabolic ratio in urine. In each subject the alloted phenotype was the same by both methods. Plasma and urine analysis showed 9.9% of poor metabolizers.

Further analysis of sparteine oxidation in a Japanese population and comparison with data observed in different ethnic populations

1. Data on the oxidation polymorphism of sparteine (SP) studied in 84 unrelated Japanese subjects of whom two (2.4%) were classified as poor metabolizers (PMs) were re-evaluated. The data were obtained from 6-hour urinary excretion ratios of SP to 2- and 5-dehydrosparteines (DHS), after an oral dose of 100 mg of SP sulphate. 2. Urinary excretion of both SP and DHS correlated with the SP/DHS ratio (rs = 0.862 and -0.756, respectively, P less than 0.001). In addition, urinary excretion of 2-DHS, 5-DHS or total DHS discriminated between PMs and extensive metabolizers (EMs). There was also a highly significant correlation (rs = 0.669, P less than 0.001) between the urinary excretion of 2- and 5-DHS. 3. These re-evaluated results on the oxidation polymorphism of SP indicate that 2- and 5-DHS formation from SP shares a common metabolic pathway (presumably via the same P-450 isozyme), and that the SP/DHS ratio, conventionally used as a discriminating index between PMs and EMs, quantitatively reflects the capacity of 2- and 5-DHS formation. 4. The benefit of using a shorter (6 h) collection period for assessing the individual oxidation phenotype of SP and inter-ethnic comparison of SP oxidation is also discussed.

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.

Variability in drug metabolism: importance of genetic constitution

In man wide variability exists in the rate of metabolism of drugs and among factors which contribute to this phenomenon genetic constitution is of major importance. The metabolism of a number of drugs is subject to polymorphism and the frequency distribution of particular pharmacokinetic parameters shows bimodality, with poor (PM) and extensive metabolizers (EM). Acetylation of a number of drugs is known to be polymorphic and the incidence of poor metabolizers varies markedly among different populations. Debrisoquine and sparteine are frequently applied model substrates for the characterization of a polymorphism in oxidative metabolism. Polymorphic drug oxidation may have important clinical implications, because when standard dosage regimens are applied plasma concentrations will reach far above the maximum acceptable in poor metabolizers and consequently side effects may arise. Regarding the multiplicity of the drug oxidizing enzyme system (cytochrome P-450) it could be of interest to combine model substrates in a cocktail to be able to characterize human subjects simultaneously for a number of independent polymorphisms.

Evidence for polymorphic oxidation of sparteine in Japanese subjects

The metabolism of sparteine which exhibits a genetic polymorphism in Caucasians was studied in 84 unrelated Japanese subjects. In contrast to a recent study where debrisoquine was used as a probe and no poor metabolizers could be observed in Japanese involving 100 subjects, two subjects had a urinary metabolic ratio of sparteine greater than 20 and thus were poor metabolizers of sparteine. The incidence of poor metabolizer phenotype of sparteine oxidation of 2% seems to be lower in Japanese as compared with various Caucasian populations where 5 to 10% are poor metabolizers of sparteine. However, this is not conclusive, because the 95% confidence interval of the observed frequency, 0.6 to 8%, covers the range reported in the literature for Caucasians.

The polymorphic metabolism of dextromethorphan

The ability of healthy male volunteers to metabolize a 30-mg oral dose of dextromethorphan (DM) was studied in 252 Americans. Two blood samples were collected at four and 24 hours after administration of the dose. The resulting plasma was analyzed for unchanged DM. The volunteers were classified as slow, intermediate, or fast metabolizers on the basis of plasma concentrations of DM. Further differentiation of slow and intermediate metabolizers was achieved by comparing the two-point estimates of elimination-rate constants. In the population studied, 84.3% were fast DM metabolizers, 6.8% were intermediate metabolizers, and 8.8% were slow metabolizers. Previous reports have related the slow DM metabolizers to slow debrisoquin metabolizers, but no such correlations have been achieved with intermediate DM metabolizers. These intermediate DM metabolizers may suggest a new polymorphism not related to debrisoquin or may suggest that "debrisoquin gene" regulation is more complex than originally suggested.

The pharmacokinetics of indoramin and 6-hydroxyindoramin in poor and extensive hydroxylators of debrisoquine

Five poor metabolisers (PM) and seven extensive metabolisers (EM), of debrisoquine, all healthy volunteers, received 50 mg indoramin orally following an overnight fast. Plasma concentrations of indoramin and 6-hydroxyindoramin were determined by HPLC with fluorimetric detection. In PM subjects, mean values of Cmax (158 ng/ml) and AUC(0-24) (2556 ng X h X m-1) for indoramin were substantially elevated and t 1/2 beta (18.5 h) prolonged by comparison with values in the EM subjects (21.6 ng/ml, 151 ng X h X ml-1 and 5.2 h respectively). For 6-hydroxyindoramin, on the other hand, Cmax (12.4 ng/ml) and AUC (0-8) (47.5 ng X h X ml-1) in PM subjects were significantly lower than in the EM subjects (28.2 ng/ml and 94.7 ng X h X ml-1). There was a tendency to a higher incidence of side-effects in the PM group. Although the difference did not achieve statistical significance (0.1 greater than p greater than 0.05), all the PM subjects experienced sedation compared to only two in the EM group. Differences in blood pressure and pulse rate between the two groups were small. It is concluded that the oxidative metabolism of indoramin is subject to genetic polymorphism, which is probably under the control of the same gene locus as that influencing debrisoquine oxidation. The clinical consequences are discussed.

The genetic polymorphism of sparteine metabolism

The formation of the two major metabolites of the antiarrhythmic and oxytocic drug sparteine (2- and 5-dehydrosparteine) exhibits a genetic polymorphism. Two phenotypes, extensive (EM) and poor metabolizers (PM) are observed in the population. The frequency of the PM phenotype in various populations (Caucasian and Japanese) ranges from 2.3 to 9%. The metabolism of sparteine is determined by two allelic genes at a single gene locus. PM subjects are homozygous for an autosomal recessive gene. The metabolism of sparteine is predominantly under genetic control as treatment with drugs such as antipyrine and rifampicin known to induce oxidative drug metabolism elicited only marginal changes in sparteine metabolism. The formation of 2-dehydrosparteine in human liver microsomes from EM and PM subjects showed a more than 40-fold difference in Km between EM and PM subjects. However, Vmax-values were almost identical in both groups. These data indicate that the basis of the differences in oxidative capacity between EM and PM subjects is more likely to be due to a variant isozyme with defective catalytic properties than to a decreased amount of the isozyme.

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.

Genetically determined variability in acetylation and oxidation. Therapeutic implications

The clinical significance of two separate genetic polymorphisms which alter drug metabolism, acetylation and oxidation is discussed, and methods of phenotyping for both acetylator and polymorphic oxidation status are reviewed. Particular reference is made to the dapsone method, which provides a simple means of distinguishing fast and slow - and possibly intermediate - acetylators, and to the sparteine method which allows a clear separation of oxidation phenotypes. Although acetylation polymorphism has been known for some time, definite indications for phenotyping are few. It is doubtful whether acetylator phenotype makes a significant difference to the outcome in most isoniazid treatment regimens, and peripheral neuropathy from isoniazid in slow acetylators is easily overcome by pyridoxine administration. However, in comparison with rapid acetylators, slow acetylators receiving isoniazid have an increased susceptibility to phenytoin toxicity, and perhaps also to carbamazepine toxicity. It is also possible that rapid acetylators receiving isoniazid attain higher serum fluoride concentrations from enflurane and similar anaesthetics than do similarly treated slow acetylators. Thus, when drug interactions of these types are suspected, phenotyping for acetylator status may be advisable. If routine monitoring of serum procainamide and N-acetylprocainamide concentrations is practised, phenotyping of subjects prior to therapy with these agents should not be necessary. Although acetylator phenotype influences serum concentrations of hydralazine, when this drug is given in combination with other drugs acetylator phenotype has not been shown to influence the therapeutic response. Slow acetylator phenotype along with female gender and the presence of HLA-DR antigens appear to be risk factors in the development of hydralazine-induced systemic lupus erythematosus (SLE). Determination of acetylator phenotype may therefore help determine susceptibility to this adverse reaction. In the case of sulphasalazine, adult slow acetylators require a lower daily dose of the drug than fast acetylators in order to maintain ulcerative colitis in remission without significant side effects. It is therefore advisable to determine acetylator phenotype prior to sulphasalazine therapy. Work on the association of acetylation polymorphism with various disease states is also reviewed. It is possible that a higher incidence of bladder cancer is associated with slow acetylation phenotype - especially in individuals exposed to high levels of arylamines. The question as to whether idiopathic SLE is more common in slow acetylators remains unresolved. There appears to be no difference between fa

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.

Enzymatic activation of chemicals to toxic metabolites

A variety of enzymes function in the oxygenation, oxidation-reduction, conjugation, and hydrolysis of drugs and other foreign chemicals. Often these enzymes detoxicate chemicals to prevent detrimental effects. In this review we will, however, concentrate on cases in which metabolism activates chemicals to reactive species which cause cellular damage. Particular attention will be given to mixed-function oxidases, which carry out a variety of oxygenations, as well as other reactions. (We will focus on cellular toxicity as opposed to initiation of tumorigenesis in this review.) In many cases, considerable circumstantial evidence exists linking these enzymes to enhanced toxicity of chemicals, although causal relationships have seldom been demonstrated. Further, in very few cases is the explicit cause of toxicity known. Modification of critical protein residues is suspected, although oxidative stress may also be involved in some cases. We discuss general aspects of mechanisms of toxic action, briefly list all cases in which metabolism is suspected to play a role in enhancing toxicity, and review a few examples in detail where substantial chemical and enzymatic information is available. The latter instances would involve knowledge of the enzymes involved, chemical evidence on the structures of the reactive metabolites, identification of adducts, and some inference into the biological processes which are effected to elicit toxicity. We consider, in this regard, vinyl halides (which have been a focus in our own laboratory), acetaminophen, pyrrolizidine alkaloids, and fluoroxene.

Characterization of a common genetic defect of cytochrome P-450 function (debrisoquine-sparteine type polymorphism)--increased Michaelis is Constant (Km) and loss of stereoselectivity of bufuralol 1'-hydroxylation in poor metabolizers

In order to define the mechanism of the debrisoquine-sparteine type genetic polymorphism of drug oxidation we studied the kinetics of bufuralol 1'-hydroxylation in liver microsomes from extensive and poor metabolizers and in a purified reconstituted human cytochrome P-450 isozyme with high activity for bufuralol 1'-hydroxylation, P-450[buf]. In extensive metabolizer microsomes the enzymatic reaction displayed apparent Michaelis-Menten kinetics and the (+)-isomer was preferentially metabolized. By contrast, the enzymatic reaction in poor metabolizer microsomes was characterized by a 4- to 5-fold increase in Km and by a loss of stereoselectivity. In a non-membraneous reconstituted system containing NADPH cytochrome P-450 reductase, a NADPH regenerating system and phospholipids, P-450[buf] exhibited an almost complete substrate stereoselectivity for (+)-isomer 1'-hydroxylation. It is concluded that the purified cytochrome P-450[buf] is the target of the debrisoquine-sparteine type oxidation polymorphism and that poor metabolizers have a quantitative or qualitative deficiency of this isozyme.

High affinity of quinidine for a stereoselective microsomal binding site as determined by a radioreceptor assay

The techniques of the radioreceptor binding assay were applied to detect stereoselective binding of quinidine and quinine to a site on human liver microsomes. Binding of 3H-dihydroquinidine was 50% inhibited by 20-100 nM quinidine, while its enantiomer quinine did not displace the 3H-ligand at concentrations up to 500 nM. This stereoselectivity agreed with the affinity values measured by functional enzyme assays of cytochrome P450 activity using sparteine or debrisoquine as substrates.

Urinary bile acid and bile alcohol excretion does not reflect the genetic polymorphism of debrisoquine hydroxylation

Excretion of the major urinary bile alcohol 27-nor-5 beta-cholestane-3 alpha, 7 alpha, 12 alpha, 24,25- pentol , and of cholic, chenodeoxycholic, deoxycholic and lithocholic acid was measured in 24 h urine collections of 10 extensive and seven poor metabolizers of debrisoquine. There was no significant difference of the excretion of these cholesterol metabolites between the two groups, indicating that cholesterol hydroxylation to bile alcohols and bile acids is probably not controlled by the same genes responsible for the 'debrisoquine-type' hydroxylation polymorphism.

Competitive inhibition of sparteine oxidation in human liver by beta-adrenoceptor antagonists and other cardiovascular drugs

The rate of oxidation of sparteine by the 9000 x g supernatant fraction of a human liver was measured in the presence of various drugs which exert cardiovascular effects. Hexamethonium, ouabain, caffeine and isoproterenol had no effect on this rate, while alprenolol, metoprolol, oxprenolol, propranolol, timolol, pindolol, lidocaine, mexiletine, 17-n-pentyl-sparteine, tolazoline, quinine, quinidine, cinchonine and cinchonidine inhibited the in vitro reaction competitively. Stereoselective inhibition was observed between quinine (Ki = 15 microM) and quinidine (Ki = 0.06 microM). Genetic evidence suggests that the primary metabolism of sparteine depends on a single species of cytochrome P450. In vitro competitive inhibition of sparteine oxidation by a drug indicates that this drug is capable of occupying the same enzymatic site as sparteine. This may mean that the competing drug is also metabolized at that site and thereby subject to the same genetic variation as sparteine's oxidation; absence of inhibition excludes this possibility.

Pharmacogenetics of mephenytoin: a new drug hydroxylation polymorphism in man

Inherited deficiency in mephenytoin hydroxylation was observed in a family study. It is important that the propositus was of the extensive metabolizer phenotype for the genetically controlled hydroxylation of debrisoquine. Thus, a genetic polymorphism of drug hydroxylation was suspected for mephenytoin. A population study of mephenytoin hydroxylation, combined with identification of extensive and poor debrisoquine hydroxylation phenotypes, was carried out in 221 unrelated normal volunteers. Twelve of them (5%) exhibited defective aromatic hydroxylation of mephenytoin, and 23 (10%) could be identified as poor metabolizers of debrisoquine. Amongst these 35 subjects with a drug hydroxylation deficiency, 3 (or 0.5%; 1 female, 2 males) displayed both defects simultaneously. A panel study of 10 extensive and 10 poor metabolizers of mephenytoin showed that the ability to perform aromatic hydroxylation of the demethylated mephenytoin metabolite nirvanol (5-phenyl-5-ethylhydantoin) was co-inherited with the mephenytoin hydroxylation polymorphism. Family studies suggested that poor metabolizer phenotypes of nirvanol and mephenytoin were most likely to have the homozygous genotype for an autosomal recessive allele of deficient aromatic drug hydroxylation. Intra-subject comparison of the debrisoquine and mephenytoin hydroxylation phenotypes in these subjects indicated that deficiency in the two drug hydroxylations occurred independently. Consequently, the co-inheritance of extensive and poor hydroxylation of mephenytoin and nirvanol, respectively, represents a new drug hydroxylation polymorphism in man.

The contribution of genetically determined oxidation status to inter-individual variation in phenacetin disposition

The oxidative O-de-ethylation and aromatic 2-hydroxylation of phenacetin have been investigated in panels of extensive (EM, n = 13) and poor (PM, n = 10) metabolizers of debrisoquine. The EM group excreted in the urine significantly more paracetamol (EM: 40.8 +/- 14.9% dose/0-8 h; PM: 29.2 +/- 8.7% dose/0-8 h, 2P less than 0.05) and significantly less 2-hydroxylated metabolites (EM: 4.7 +/- 2.3% dose/0-8 h; PM: 9.7 +/- 3.5% dose/0-8 h, 2P less than 0.005) than the PM group. Apparent first-order rate constants, calculated from pooled phenotype data, for overall elimination of phenacetin (k) and formation of paracetamol (kml) were higher in the EM group (EM: k = 0.191 +/- 0.151 h-1; kml = 0.091 +/- 0.025 h-1; PM: k = 0.098 +/- 0.035 h-1, 2P less than 0.05, kml = 0.052 +/- 0.019 h-1, 2P less than 0.05) than the PM group. The apparent first-order rate constant for 2-hydroxylation displayed no significant inter-phenotype differences. Correlation analysis demonstrated that genetically determined oxidation status accounted for approximately 50% of the inter-individual variability in phenacetin disposition encountered in this study.

Genetically determined oxidation capacity and the disposition of debrisoquine

1 The disposition in urine of debrisoquine and its hydroxylated metabolites has been studied in subjects of the 'extensive metabolizer' (EM; n = 5) and 'poor metabolizer' (PM; n = 5) phenotypes. The 4-hydroxylation of debrisoquine by PM subjects following a 10 mg oral dose was capacity-limited and displayed significant dose-dependency over a range of 1-20 mg. In contrast, the EM subjects' ability to perform this metabolic oxidation did not deviate from first-order kinetics over a dose range of 10-40 mg. 2 The disposition of debrisoquine in plasma following a 10 mg oral dose has been studied in EM (n = 4) and PM (n = 3) subjects. Whilst PM subjects displayed significantly higher plasma levels of debrisoquine at all time points following 1 h post-dosing, and higher values for areas under the plasma concentration-time curve (EM: 105.6 +/- 7.0 ng ml-1 h; PM: 371.4 +/- 22.4 ng ml-1 h, 2P less than 0.0001), neither debrisoquine plasma half-life (EM: 3.0 +/- 0.5 h; PM: 3.3 +/- 0.4 h) nor renal clearance of the drug (EM: 152.8 +/- 30.3 ml min-1; PM: 137 +/- 4.5 ml min-1) displayed significant inter-phenotype differences. 3 The results of these investigations show that the phenotyping of individuals for debrisoquine oxidation status by means of a 'metabolic ratio' derived from a single 0-8 h urine sample has a sound kinetic basis. The kinetic differences between the two phenotypes would strongly suggest that the metabolic defect manifested in PM subjects is one of pre-systemic elimination capacity.

Antipyrine metabolism in relation to polymorphic oxidations of sparteine and debrisoquine

Thirty-five healthy subjects who had been classified as extensive or poor metabolizers of both sparteine and debrisoquine were given a single oral dose of antipyrine. Saliva concentration of antipyrine and urinary excretion of its three major oxidation metabolites were measured. All the parameters of antipyrine metabolism which were estimated had similar distributions in both the 28 EM and 7 PM genetic phenotypes defined by the metabolism of sparteine and debrisoquine. The clearance of antipyrine by the formation of 4-hydroxy-antipyrine and 3-hydroxy-antipyrine respectively were closely correlated (r = 0.83, P less than 0.001) and both were significantly higher in smokers than in non-smokers. Demethylation of antipyrine also seemed to be influenced by smoking, but not to a statistically significant extent. These findings confirm the influence of the environmental factor of smoking in antipyrine oxidative biotransformations.

Inhibition of sparteine oxidation in human liver by tricyclic antidepressants and other drugs

Testing for competitive inhibition of sparteine oxidation in the 9000 x g supernatant fraction from human liver provides an in vitro means to identify drugs which can bind to the same form of cytochrome P450 which oxidizes sparteine. There has so far been only two outcomes of this test: either the drug examined competed with sparteine for a common binding site, or it did not inhibit the reaction. The results of such in vitro testing implicated the involvement of guanoxan, nortriptyline, desipramine, imipramine, amitriptyline and chlorpromazine with this enzyme. Amobarbital, tolbutamide and guanethidine in therapeutic concentrations did not interfere with sparteine oxidation by this preparation.

Contribution of the genetic status of oxidative metabolism to variability in the plasma concentrations of beta-adrenoceptor blocking agents

The oxidative metabolism of bufuralol is under the same genetic control as that of debrisoquine and sparteine. 154 fasting volunteers received a 30 mg tablet of bufuralol and a blood sample was taken 3 h later. In poor metabolizers (8% of the sample) the plasma bufuralol concentrations were very high and the metabolite concentrations were low. The genetic oxidative status is a major source of interindividual variation in the plasma concentration of drugs that undergo oxidative metabolism.

Biotransformation of amitriptyline in alcoholic depressive patients

The biotransformation of amitriptyline (AMT) during steady state conditions was studied in plasma and urine from 11 nonalcoholic and 10 alcoholic depressive inpatients treated with oral AMT. The 2 groups of patients had a different pattern of biotransformation. The Demethylation of AMT was lower in alcoholic than in nonalcoholic depressive patients, and conjugation and hydroxylation of AMT were also more marked in the former group. The results may be of clinical relevance since the conjugates of AMT are inactive.

Spectral binding studies of the polymorphically metabolized drugs debrisoquine, sparteine and phenformin by cytochrome P-450 of normal and hydroxylation deficient rat strains

The mechanisms of polymorphic drug hydroxylation of debrisoquine, sparteine and related drugs in vivo have been investigated using Cyt P-450 preparations of inbred rat strains as an in vitro model of the poor and extensive metabolizer phenotypes found in various rat strains and in man. Optical difference spectroscopy with debrisoquine, sparteine, phenformin and three other drugs (selected test compounds with proven or suspected hydroxylation polymorphisms in man) exhibited Type 1 binding in normal Sprague-Dawley, Fischer and Lewis Cyt P-450, whereas no Type I drug binding was found in the hydroxylation deficient DA rat liver Cyt P-450. Cyt P-450 content and Type II drug binding of metiamide was the same in normal and hydroxylation deficient rat liver microsomes. The pronounced Type I drug binding in extensive hydroxylation Cyt P-450 and the defective Type I binding in DA Cyt P-450 in vitro, therefore, closely parallels the polymorphic hydroxylation pattern of these test drugs found in the four rat strains studied in vivo. Consequently, missing binding properties of Cyt P-450 or of its micro-environment might represent the enzymatic defect underlying the genetically determined hydroxylation deficiency of polymorphically metabolized drugs in the poor metabolizer phenotype in the DA rat and, by inference, in man.