Hypotensive response to debrisoquine and hydroxylation phenotype

Influence of Inflammation on Cytochromes P450 Activity in Adults: A Systematic Review of the Literature

Background: Available in-vitro and animal studies indicate that inflammation impacts cytochromes P450 (CYP) activity via multiple and complex transcriptional and post-transcriptional mechanisms, depending on the specific CYP isoforms and the nature of inflammation mediators. It is essential to review the current published data on the impact of inflammation on CYP activities in adults to support drug individualization based on comorbidities and diseases in clinical practice. Methods: This systematic review was conducted in PubMed through 7th January 2021 looking for articles that investigated the consequences of inflammation on CYP activities in adults. Information on the source of inflammation, victim drugs (and CYPs involved), effect of disease-drug interaction, number of subjects, and study design were extracted. Results: The search strategy identified 218 studies and case reports that met our inclusion criteria. These articles were divided into fourteen different sources of inflammation (such as infection, autoimmune diseases, cancer, therapies with immunomodulator…). The impact of inflammation on CYP activities appeared to be isoform-specific and dependent on the nature and severity of the underlying disease causing the inflammation. Some of these drug-disease interactions had a significant influence on drug pharmacokinetic parameters and on clinical management. For example, clozapine levels doubled with signs of toxicity during infections and the concentration ratio between clopidogrel's active metabolite and clopidogrel is 48-fold lower in critically ill patients. Infection and CYP3A were the most cited perpetrator of inflammation and the most studied CYP, respectively. Moreover, some data suggest that resolution of inflammation results in a return to baseline CYP activities. Conclusion: Convincing evidence shows that inflammation is a major factor to be taken into account in drug development and in clinical practice to avoid any efficacy or safety issues because inflammation modulates CYP activities and thus drug pharmacokinetics. The impact is different depending on the CYP isoform and the inflammatory disease considered. Moreover, resolution of inflammation appears to result in a normalization of CYP activity. However, some results are still equivocal and further investigations are thus needed.

Importance of Publishing Adverse Drug Reaction Case Reports: Promoting Public Health and Advancing Pharmacology and Therapeutics

This article, which encourages physicians to publish case reports of adverse drug reactions (ADRs), is a review of how well-documented published case reports have contributed to promoting public safety and health and thus served to advance basic pharmacology. The origin of a number of regulatory guidelines can ultimately be traced to safety concerns triggered by such reports. It illustrates how case reports of ADRs, when coupled with simultaneous monitoring of drug pharmacokinetics, have also led to further investigations resulting in major advances in pharmacology, especially pharmacogenetics, mechanisms of drug-drug interactions and modulation of drug metabolism during inflammatory co-morbidities. Published case reports differ significantly from spontaneous case reports since they enjoy quality-compliant peer review and an immediate wider visibility among the readership, triggering others to report similar cases, and ultimately leading to prescribing restrictions on or withdrawals of the drug from the market depending on the risk. Therefore, the reporter should not be discouraged by (a) the unusual or bizarre nature of the reaction; (b) the interval, however long, from commencing drug administration to the onset of the suspected reaction; (c) however well-known the drug or the period for which it has been on the market; and (d) any pressure not to publish. Case reports should be published in reputable journals that are searchable through databases such as PubMed.

The Clinical Significance of the Pharmacogenetics of Cardiovascular Medications

Inter-individual variability in the response to numerous drugs can be traced to a number of sources. One source of variability in drug response is the variability associated with the metabolic capacity of an individual. The component of metabolic capacity that will be the focus of this article is that determined by heredity. Pharmacogenetics is frequently referred to as the study of the effects of heredity on the disposition and response to medications. This article will review the pharmacokinetic and pharmacodynamic significance of pharmacogenetics as it pertains to a select number of cardiovascular agents. The enzyme systems responsible for drug metabolism discussed in this article will be limited to the P-450IID6 and N-acetylation pathways. Given the extensive use of cardiovascular agents in clinical practice that are affected by this genetic polymorphism, it is important for the practicing pharmacist to be aware of this phenomenon and its implications. Hopefully, the knowledge gained from this article will help practicing pharmacists to appreciate the clinical significance of polymorphic drug metabolism and provide a basis for the application of this knowledge to a variety of practice settings.

Toxicological implications of polymorphic drug metabolism

The occurrence of genetic polymorphisms of drug metabolism means that populations contain subgroups (phenotypes) that differ sharply in their abilities to effect a number of metabolic reactions. Because of this, major interphenotype differences occur in responsiveness to drugs and toxic substances. The well established genetic polymorphisms of acetylation and hydrolysis illustrate the important association that exists between phenotype and propensity to develop toxic and exaggerated responses to some substances. Recently, for metabolic oxidation, a new genetic polymorphism of drug metabolism has been described and it promises to provide a better understanding of inter-individual variability in the metabolic handling of, and responsiveness to, drugs and toxic substances. The following effects of the polymorphism are described here: (a) its influence in determining variable presystemic metabolism and hence systemic drug availability; (b) its role in determining alternative toxic pathways of metabolism in individuals who have a genetically determined impairment of oxidative capacity and (c) its influence on the development of agranulocytosis associated with metiamide administration.

Evidence of different metabolic phenotypes in humans

The study of metabolic responses to drugs, environmental changes, and diseases is a new promising area of metabonomic research. Metabolic fingerprints can be obtained by analytical techniques such as nuclear magnetic resonance (NMR). In principle, alterations of these fingerprints due to appearance/disappearance or concentration changes of metabolites can provide early evidences of, for example, onset of diseases. A major drawback in this approach is the strong day-to-day variability of the individual metabolic fingerprint, which should be rather called a metabolic "snapshot." We show here that a thorough statistical analysis performed on NMR spectra of human urine samples reveals an invariant part characteristic of each person, which can be extracted from the analysis of multiple samples of each single subject. This finding (i) provides evidence that individual metabolic phenotypes may exist and (ii) opens new perspectives to metabonomic studies, based on the possibility of eliminating the daily "noise" by multiple sample collection.

LC-MS-based metabolomics in drug metabolism

Xenobiotic metabolism, a ubiquitous natural response to foreign compounds, elicits initiating signals for many pathophysiological events. Currently, most widely used techniques for identifying xenobiotic metabolites and metabolic pathways are empirical and largely based on in vitro incubation assays and in vivo radiotracing experiments. Recent work in our lab has shown that LC-MS-based metabolomic techniques are useful tools for xenobiotic metabolism research since multivariate data analysis in metabolomics can significantly rationalize the processes of xenobiotic metabolite identification and metabolic pathway analysis. In this review, the technological elements of LC-MS-based metabolomics for constructing high-quality datasets and conducting comprehensive data analysis are examined. Four novel approaches of using LC-MS-based metabolomic techniques in xenobiotic metabolism research are proposed and illustrated by case studies and proof-of-concept experiments, and the perspective on their application is further discussed.

Genetic susceptibility to tardive dyskinesia in chronic schizophrenia subjects: III. Lack of association of CYP3A4 and CYP2D6 gene polymorphisms

Tardive dyskinesia is a severe debilitating movement disorder characterized by choreoathetotic movements developing in one-fifth of the patients with schizophrenia. In this study we have investigated the significance of CYP3A4*1B and CYP2D6*4 polymorphisms in TD susceptibility among chronic schizophrenia patients (n = 335) from north India. Tardive dyskinesia was diagnosed in approximately 29% (96/335) of these patients. No significant association of either of the two SNPs with TD (CYP3A4*1B chi2 = 0. 308, df = 1, p = 0.579; CYP2D6*4 chi2 = 0.006, df = 1, p = 0.935) was observed. However a trend towards increased severity of TD in patients heterozygous for the CYP2D6*4 mutation was observed.

Pharmacogenetic aspects of drug-induced torsade de pointes: potential tool for improving clinical drug development and prescribing

Drug-induced torsade de pointes (TdP) has proved to be a significant iatro-genic cause of morbidity and mortality and a major reason for the withdrawal of a number of drugs from the market in recent times. Enzymes that metabolise many of these drugs and the potassium channels that are responsible for cardiac repolarisation display genetic polymorphisms. Anecdotal reports have suggested that in many cases of drug-induced TdP, there may be a concealed genetic defect of either these enzymes or the potassium channels, giving rise to either high plasma drug concentrations or diminished cardiac repolarisation reserve, respectively. The presence of either of these genetic defects may predispose a patient to TdP, a potentially fatal adverse reaction, even at therapeutic dosages of QT-prolonging drugs and in the absence of other risk factors. Advances in pharmacogenetics of drug metabolising enzymes and pharmacological targets, together with the prospects of rapid and inexpensive genotyping procedures, promise to individualise and improve the benefit/risk ratio of therapy with drugs that have the potential to cause TdP. The qualitative and the quantitative contributions of these genetic defects in clinical cases of TdP are unclear because not all of the patients with TdP are routinely genotyped and some relevant genetic mutations still remain to be discovered. There are regulatory guidelines that recommend strategies aimed at uncovering the risk of TdP associated with new chemical entities during their development. There are also a number of guidelines that recommend integrating pharmacogenetics in this process. This paper proposes a strategy for integrating pharmacogenetics into drug development programmes to optimise association studies correlating genetic traits and endpoints of clinical interest, namely failure of efficacy or development of repolarisation abnormalities. Until pharmacogenetics is carefully integrated into all phases of development of QT-prolonging drugs and large-scale studies are undertaken during their post-marketing use to determine the genetic components involved in induction of TdP, routine genotyping of patients remains unrealistic. Even without this pharmacogenetic data, the clinical risk of TdP can already be greatly minimised. Clinically, a substantial proportion of cases of TdP are due to the use of either high or usual dosages of drugs with potential to cause TdP in the presence of factors that inhibit drug metabolism. Therefore, choosing the lowest effective dose and identifying patients with these non-genetic risk factors are important means of minimising the risk of TdP. In view of the common secondary pharmacology shared by these drugs, a standard set of contraindications and warnings have evolved over the last decade. These include factors responsible for pharmacokinetic or pharmacodynamic drug interactions. Among the latter, the more important ones are bradycardia, electrolyte imbalance, cardiac disease and co-administration of two or more QT-prolonging drugs. In principle, if large scale prospective studies can demonstrate a substantial genetic component, pharmacogenetically driven prescribing ought to reduce the risk further. However, any potential benefits of pharmacogenetics will be squandered without any reduction in the clinical risk of TdP if physicians do not follow prescribing and monitoring recommendations.

Genetic polymorphisms and metabolism of endocrine disruptors in cancer susceptibility

Epidemiological studies have estimated that approximately 80% of all cancers are related to environmental factors. Individual cancer susceptibility can be the result of several host factors, including differences in metabolism, DNA repair, altered expression of tumor suppressor genes and proto-oncogenes, and nutritional status. Xenobiotic metabolism is the principal mechanism for maintaining homeostasis during the body's exposure to xenobiotics. The balance of xenobiotic absorption and elimination rates in metabolism can be important in the prevention of DNA damage by chemical carcinogens. Thus the ability to metabolize and eliminate xenobiotics can be considered one of the body's first protective mechanisms. Variability in individual metabolism has been related to the enzymatic polymorphisms involved in activation and detoxification of chemical carcinogens. This paper is a contemporary literature review on genetic polymorphisms involved in the metabolism of endocrine disruptors potentially related to cancer development.

The genetic basis of variability in drug responses

It is almost axiomatic that patients vary widely in their beneficial responses to drug therapy, and serious and apparently unpredictable adverse drug reactions continue to be a major public health problem. Here, we discuss the concept that genetic variants might determine much of this variability in drug response, and propose an algorithm to enable further evaluation of the benefits and pitfalls of this enticing possibility.

Activation of procarcinogens by human cytochrome P450 enzymes

Enzymatic transformation of most chemical carcinogens is requisite to the formation of electrophiles that cause genotoxicity, and the cytochrome P450 (P450) enzymes are the most prominent enzymes involved in such activation reactions. During the past 15 years the human P450 enzymes have been extensively characterized. Considerable evidence exists that the variation in activity of these enzymes can have important consequences in the actions of drugs. Other studies have been concerned with the activation of procarcinogens by human P450s. Assignments of roles of particular P450s in the metabolism of chemical carcinogens are discussed, along with the current state of evidence for relationships of particular P450s with human cancer.

The influence of CYP2D6 polymorphism and quinidine on the disposition and antitussive effect of dextromethorphan in humans

OBJECTIVES

We studied the disposition of dextromethorphan in extensive and poor metabolizer subjects, as well as the effect of this polymorphism on the antitussive action of dextromethorphan.

METHODS

Six extensive metabolizers were studied on four occasions: (1) after 30 mg dextromethorphan, (2) after 30 mg dextromethorphan 1 hour before 50 mg quinidine, (3) after placebo, and (4) after 50 mg quinidine. Six poor metabolizers were studied on two occasions: (1) after 30 mg dextromethorphan and (2) after placebo. Blood and urine were collected over 168 hours and assayed for dextromethorphan, total (conjugated and unconjugated) dextrorphan, 3-methoxymorphinan, and total 3-hydroxymorphinan. On each occasion at each blood sampling time, capsaicin was administered as an aerosol to provoke cough.

RESULTS

Dextromethorphan area under the plasma concentration-time curve (AUC) was 150-fold greater in the poor metabolizers than in the extensive metabolizers, and quinidine increased the AUC in extensive metabolizers 43-fold. The median dextromethorphan half-life was 19.1 hours in poor metabolizers, 5.6 hours in extensive metabolizers given quinidine, and 2.4 hours in extensive metabolizers. For dextrorphan (as total), the AUC was reduced 8.6-fold in poor metabolizers; quinidine had no effect on the AUC. The median half-life was 10.1 hours in poor metabolizers, 6.6 hours in extensive metabolizers given quinidine, and 1.4 hours in extensive metabolizers. The apparent partial clearance of dextromethorphan to dextrorphan was 1.2 L/hr in poor metabolizers, 78.5 L/hr in extensive metabolizers given quinidine, and 970 L/hr in extensive metabolizers. There was a strong (r2 = 0.82) and significant (p < 0.01) positive correlation between the prestudy urinary metabolic ratios and the partial clearances of dextromethorphan to dextrorphan. There was very large intersubject variability in responsiveness to capsaicin. There was no difference in the capsaicin-induced cough frequency in the three groups. Dextromethorphan had no antitussive effect in this experimental cough model.

CONCLUSION

The disposition of dextromethorphan was substantially influenced by CYP2D6 status. Capsaicin may not be an ideal agent in experimental cough studies.

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.

Analysis of metoprolol enantiomers in human serum by liquid chromatography on a cellulose-based chiral stationary phase

Metoprolol is a lipophilic, cardioselective beta-adrenergic blocking agent commercially available as a racemic compound. A normal phase high-performance liquid chromatographic method was developed to directly determine individual enantiomeric concentrations of metoprolol in human serum. Separation of the enantiomers was accomplished by a cellulose-tris(3,5-dimethylphenylcarbamate) chiral stationary phase. Metoprolol enantiomers were detected by means of fluorescence with excitation and emission wavelengths of 275 and 315 nm, respectively. Standard curves were linear over the concentration range 12.5-400 ng/ml for each enantiomer. Within-day coefficient of variation was less than 15% at all concentrations and the between-day coefficient of variation ranged from 4.1 to 11.2%. The limit of detection was determined to be 5 ng/ml for each enantiomer and the stereoselective resolution (alpha) of R- and S-metoprolol was 3.08. The assay was employed to determine enantiomeric serum concentrations of metoprolol in healthy male volunteers.

The role of genetically determined polymorphic drug metabolism in the beta-blockade produced by propafenone

Propranolol and the sodium-channel-blocking antiarrhythmic agent propafenone share structural features. Although propafenone's beta-blocking actions are readily demonstrable in vitro, clinically significant beta-blockade occurs inconsistently in vivo. In this study, we tested the hypothesis that genetically determined variations in the biotransformation of propafenone to its 5-hydroxy metabolite account for variations in the drug's beta-blocking action. We assessed beta-blockade by measuring the reduction in tachycardia produced by boluses of isoproterenol and treadmill exercise in 14 normal subjects during treatment with placebo and with 150, 225, and 300 mg of propafenone every eight hours for five days each. Nine subjects (with the extensive-metabolizer phenotype) metabolized most of the propafenone to 5-hydroxy propafenone, and five (with the poor-metabolizer phenotype) did not produce this metabolite. At the lower dosages, beta-blockade was present in both groups but was significantly greater in the subjects with poor metabolism, in whom deficient 5-hydroxylation was associated with higher plasma propafenone levels. At the highest dose, a similar degree of beta-blockade was observed in the two groups. Propafenone also had a higher affinity for beta 2 receptors in vitro than either of its major metabolites. We conclude that the degree of beta-blockade during propafenone therapy reflects genetically determined variations in the metabolism of the parent drug, which is necessary for beta-blockade, and that this action of propafenone is considerably enhanced in patients with deficient 5-hydroxylation of propafenone.

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

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

The disposition of feprazone and its hydroxylated metabolite in human volunteers

1. 14C-Feprazone administered as a single oral dose (17 mg/subject) to each of 3 human volunteers on the 6th day of repeated dosage with unlabelled feprazone (200 mg/subject, twice daily) was excreted slowly, with only 19-38% of the dose excreted in the urine in 8 days, with a further 27-49% of the dose in the faeces. 2. 14C-Feprazone had a half-life of 30-33 h, similar to that after single dosage of unlabelled feprazone (22-33 h). The half-life for total 14C was not significantly different from that for unchanged feprazone, indicating that no metabolite with a very long half-life was formed. 3. Only feprazone and 4'-hydroxyfeprazone were detected in the plasma of subjects dosed orally with feprazone, the metabolite being characterized by mass spectrometry. The time of peak plasma concentration of feprazone was 4-5 h after dosage, and of 4'-hydroxyfeprazone was approx. 25 h. The urine contained feprazone plus its C-glucuronide, and 4'-hydroxyfeprazone plus its conjugate (glucuronide), in the ratio of approx. 5:1. 4. When 4'-hydroxyfeprazone was administered as a single oral dose to a human volunteer the plasma elimination half-life of the metabolite was 18 h, but after administration of feprazone the half-life of 4'-hydroxyfeprazone was 45 +/- 29 h (10 subjects), indicating the slow hydroxylation of feprazone and the slow excretion of 4'-hydroxyfeprazone. The clearance of feprazone was 5.2 and of 4'-hydroxyfeprazone was 5.5 ml/kg/h. 5. These studies have shown that even though enterohepatic recirculation of the drug in man is indicated, the plasma half-life of feprazone is unchanged on repeated dosage, and accumulation of the drug at a daily dosage of 2 x 200 mg, does not occur.

Adverse drug reaction reporting and retrospective phenotyping for oxidation polymorphism

A genetically determined impairment in the ability to oxidase sparteine and debrisoquine also affects the oxidation of several other drugs. This impairment in oxidation may result in accumulation of the associated drugs and in an increased susceptibility to adverse reactions from these drugs. Dunedin houses the New Zealand national centre for the collation and study of adverse drug reactions. Included among the reporting schemes is an intensified monitoring system for newly released drugs, in which physicians report all clinical events occurring during treatment with the drugs under surveillance. The centre thus has available extensive records of names and addresses of prescribers and patients who have been reported as experiencing an adverse event while receiving drug therapy. We investigated the association between genetically poor oxidation of sparteine and adverse reactions to drugs selected as possibly sharing the sparteine/debrisoquine oxidation pathway; these included perhexiline, metoprolol, debrisoquine, piroxicam, mianserin and nifedipine. A kit containing instructions, a sparteine capsule and a container for urine collection was sent to physicians who reported adverse reactions or events to one of the above drugs for forwarding to the patient. It appeared possible, after assays of returned urine for sparteine and its metabolites, that adverse reactions to nifedipine were associated with genetically poor oxidation.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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

Antiarrhythmic activity, electrocardiographic effects and pharmacokinetics of the encainide metabolites O-desmethyl encainide and 3-methoxy-O-desmethyl encainide in man

Although encainide is an effective antiarrhythmic agent, plasma concentrations and pharmacologic effects are not well correlated. One explanation is the generation of active metabolites: while in most patients (extensive metabolizers; EMs) concentrations of the metabolites O-desmethyl encainide (ODE) and 3-methoxy-O-desmethyl encainide (3MODE) are higher than those of encainide, a small subset (poor metabolizers; PMs) lack the ability to extensively biotransform encainide. Considerable data from studies in vitro and animal studies, as well as indirect evidence in patients, indicate that ODE and 3MODE produce the effects seen during long-term encainide therapy in EMs. We now report the initial direct evaluation of the pharmacologic actions of these metabolites of encainide in man. Nine patients with ventricular arrhythmias, seven of the EM phenotype and two of the PM phenotype, were studied. Chronic high-frequency ventricular arrhythmias were suppressed by encainide therapy in seven of nine; monitoring arrhythmia frequency during withdrawal of encainide allowed definition of plasma concentrations of encainide and metabolites associated with arrhythmia suppression. Intravenous infusions of both ODE and 3MODE suppressed chronic ventricular arrhythmias, while infusions of placebo had no effect. ODE clearance was a function of metabolizer phenotype, with higher clearance (mean 914 ml/min; range 554 to 1,314) in EMs than in PMs (434, 298 ml/min); moreover, 3MODE was detected during ODE infusions in all seven EMs but in neither PM. 3MODE clearance was more uniform (mean 289 ml/min in EMs [range 180-410] vs 300 and 78 ml/min in the two PMs) and ODE was not detected in any subject during 3MODE infusion. Encainide itself was not detected after any infusion of ODE or 3MODE. During withdrawal of encainide therapy, ODE plasma concentration at the time of arrhythmia recurrence was 55 +/- 40 ng/ml (mean +/- SD), while ODE by infusion was effective at a concentration of 37 +/- 15 ng/ml. Similarly, plasma concentration of 3MODE at the time of arrhythmia recurrence after withdrawal of chronic encainide was 116 +/- 35 ng/ml and that during 3MODE infusion was 105 +/- 50 ng/ml. While both compounds prolonged QRS duration, ODE was the more potent, increasing QRS by 9.2 +/- 1.6% per 100 ng/ml vs 1.2 +/- 0.5% per 100 ng/ml for 3MODE. On the other hand, 3MODE prolonged the corrected JT interval by 1.9 +/- 0.6% per 100 ng/ml, while ODE shortened it by 2.7 +/- 1.9% per 100 ng/ml.(ABSTRACT TRUNCATED AT 400 WORDS)

Disposition kinetics of encainide and metabolites

Interpretation of plasma concentration data during encainide therapy is predicated on an understanding of the role of active metabolites during treatment. In over 90% of patients, encainide is rapidly biotransformed to O-desmethyl encainide (ODE) and 3-methoxy-O-desmethyl encainide (3-MODE), which persist in plasma hours after encainide itself is undetectable. This metabolism occurs in the liver, and encainide clearance is sufficiently high that a significant first-pass effect is seen during oral therapy (bioavailability 30 +/- 7%). In these extensive metabolizers, ODE and 3-MODE appear to mediate the arrhythmia suppression and electrocardiographic changes seen during encainide therapy. In less than 10% of patients, a genetic defect prevents expression of the enzyme responsible for the rapid biotransformation of encainide. In this poor metabolizer subset, the systemic clearance of encainide is 10-fold lower than in extensive metabolizers (0.18 +/- .002 vs 1.9 +/- 0.2 liters/min), the first-pass effect is virtually absent (bioavailability 83% to 88%), plasma concentrations are higher and an antiarrhythmic effect may be seen at usual encainide doses. Minimally effective plasma concentrations appear to be 35 ng/ml (ODE), 100 ng/ml (3-MODE) and 300 ng/ml (encainide), making ODE one of the most potent sodium channel blockers yet used in man. The elimination half-life of encainide is 2.3 +/- 0.3 hours in extensive metabolizer patients. Despite this rapid elimination, encainide can be administered every 8 to 12 hours in both extensive and poor metabolizer subsets; this is because of slowly eliminated metabolites in extensive metabolizers and slower elimination of encainide itself (11.3 +/- 0.3 hours) in poor metabolizers.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Characterization of a human liver cytochrome P-450 involved in the oxidation of debrisoquine and other drugs by using antibodies raised to the analogous rat enzyme

Debrisoquine 4-hydroxylase activity is a prototype for genetic polymorphism in oxidative drug metabolism in humans; approximately 10% of Caucasian populations exhibit the poor metabolizer phenotype, and the clearance of at least 14 other drugs has been shown to be deficient in patients exhibiting this phenotype. Antibodies prepared to a cytochrome P-450 shown to be responsible for debrisoquine 4-hydroxylation in rats were found to inhibit the oxidation of debrisoquine and sparteine, encainide, and propranolol, three other drugs suggested to be associated with this phenotype, in human liver microsomes. The antibodies did not inhibit the oxidation of seven other cytochrome P-450 substrates. The antibodies recognized a single polypeptide of Mr51,000 after combined sodium dodecyl sulfate/polyacrylamide electrophoresis and immunochemical staining of human liver microsomes. The intensity of this band was significantly correlated with debrisoquine 4-hydroxylase activity when liver microsomes from 44 organ donors were examined. Immunoprecipitation of in vitro translation products of total liver RNA revealed major electrophoretic bands corresponding to the cytochrome P-450 in rats and humans. The level of translatable mRNA coding for the debrisoquine-hydroxylating cytochrome P-450 was an order of magnitude less in human liver than in rat liver. The availability of these antibodies provides a biochemical basis for further basic and clinical studies on the role of a particular cytochrome P-450 polymorphism in humans.

Polymorphic hydroxylation of perhexiline maleate in man

Long term perhexiline maleate therapy causes peripheral neuropathy and hepatic damage in certain subjects. An association between these adverse reactions and a genetically determined relative inability to hydroxylate debrisoquine has been described. This association could indicate either that the effects of perhexiline impair debrisoquine oxidation thus producing a phenocopy, or that perhexiline is polymorphically hydroxylated and that the polymorphism is controlled by the same alleles as control the debrisoquine polymorphism. To test the second possibility, a study investigating the hydroxylation status of a population of healthy volunteer subjects has been performed using perhexiline maleate. Hydroxylation phenotyping was performed on 50 normal volunteers. A standard oral dose was given and plasma and urinary perhexiline, 4-monohydroxyperhexiline (MI metabolite), and 4'monohydroxyperhexiline (MIII metabolite) was measured. The 24-hour plasma perhexiline concentration, the 24-hour plasma MI metabolite concentration, and 12 to 24-hour urinary MI metabolite excretion were clearly bimodal, suggesting the existence of a polymorphism for perhexiline hydroxylation. Poor metabolisers represent 6% of the population studied. Known poor metabolisers of debrisoquine are also poor metabolisers of perhexiline, while known extensive metabolisers of debrisoquine are also extensive metabolisers of perhexiline, indicating that in white British subjects the hydroxylation polymorphism is under identical genetic control for both compounds. The poor metaboliser sub-group exhibited the highest plasma perhexiline levels. Perhexiline phenotyping separates the poor and extensive metaboliser phenotypes much more clearly than other tests and defines a sub-group at risk from perhexiline toxicity. Pretreatment phenotyping using this test, followed by exclusion of poor metabolisers from perhexiline therapy, should substantially reduce the incidence of major adverse effects.

Human cytochromes P-450

Studies in vivo have provided evidence for a multiplicity of cytochromes P-450 in man, some of which are under independent monogenic control. Although the activity of cytochromes P-450 in man are generally lower than those of rat, this is by no means always the case. There are several important exceptions including the N-hydroxylation of 2-acetamidofluorene. Studies in vitro by a number of different techniques have confirmed the evidence from studies in vivo that there are multiple forms of human cytochrome P-450. In addition to differences in Vmax, the different forms of cytochrome P-450 may also exhibit marked differences in their apparent Km values. The implications that this may have for pharmacokinetics and toxicology are discussed. The polymorphism in the 4-hydroxylation of debrisoquine observed in vivo has been shown to be due to a defect in a specific form of cytochrome P-450 which appears to be under monogenic regulation. Cross-inhibition studies have enabled the specificity of this isozyme to be characterized. Such studies have also enabled the contribution of this isozyme of cytochrome P-450 to the oxidation of other substrates to be determined. Compounds investigated include bufuralol and phenytoin. Evidence from studies both in vivo and in vitro suggest that selective induction of different forms of cytochrome P-450 can occur in man. However, the number of different classes of inducer in man is not yet known. Human cytochromes P-450 have been purified to near homogeneity in several laboratories. Different forms of cytochrome P-450 purified from the same liver sample vary in molecular weight, chromatographic characteristics and substrate specificities.

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.

Impaired oxidation of debrisoquine in patients with perhexiline neuropathy

The use of perhexiline maleate as an antianginal agent is occasionally associated with side effects, particularly neuropathy and liver damage. The reason why some individuals develop these toxic reactions is not clear, though some evidence suggests that they may result from impaired oxidative metabolism, due to genetic or hepatic factors, and consequential accumulation of the drug in toxic concentrations. Drug oxidation was measured with an oxidation phenotyping procedure in 34 patients treated with perhexiline, 20 of whom had developed neuropathy and 14 of whom had not. Most of the 20 patients with neuropathy, but not the unaffected patients, showed an impaired ability to effect metabolic drug oxidation. This impairment was independent of hepatic function, concurrent drug therapy, or tobacco or alcohol consumption. The fact that the ability to oxidise several drugs is genetically controlled points to a genetic susceptibility to developing neuropathy in response to perhexiline. Routine determination of the drug oxidation phenotype might lead to safer use of perhexiline by predicting patients who may be more at risk of developing a neuropathic reaction associated with its long-term use.

Some observations on the oxidation phenotype status of Nigerian patients presenting with cancer

The hypothesis is being explored that there may be an association between genetically determined oxidation status and propensity to develop carcinoma in response to environmental chemical carcinogens. For this purpose, the genetic structure of a normal, healthy Nigerian population with respect to oxidation status, has been compared with that found for a group of 59 Nigerian patients presenting with carcinoma of the liver and gastrointestinal tract. Genetically determined oxidation status was assessed by measuring the extent of oxidation of a probe drug, debrisoquine, to its major metabolite, 4-hydroxydebrisoquine. The cancer group contained a disproportionately large number of individuals who were extensive oxidizers compared to the controls (2 P = 0.0045). The findings support the view that genetically determined oxidation status may be an important host factor in influencing responsiveness to chemical carcinogens that require oxidative metabolic activation.

A study of the debrisoquine hydroxylation polymorphism in a Nigerian population

1. The metabolic oxidation of debrisoquine has been studied in a group of 123 Nigerian volunteers. 2. All subjects excreted unchanged drug together with five oxidation products, namely, 4-, 5-, 6-, 7- and 8-hydroxy-debrisoquine. 3. The 4-hydroxylation reaction exhibits polymorphism; ten subjects were defective in their ability to effect this reaction. 4. The incidence (q) of the allele governing impaired 4-hydroxylation (DL) among Nigerians was calculated as being 0.28 (95% confidence limit of 0.20-0.37). 5. An association was demonstrated between the ability to effect 4-hydroxylation and 6- and 7-hydroxylation of debrisoquine, suggesting that the alleles controlling alicyclic oxidation also influence aromatic hydroxylation.

The polymorphic 4-hydroxylation of debrisoquine in a Saudi arab population

1. Debrisoquine 4-hydroxylation was polymorphic in 102 Saudi arab volunteers, the population comprising one phenotypically poor metabolizer and 101 phenotypically extensive metabolizers of debrisoquine. 2. Mean urinary recoveries of drug and metabolite were low in Saudis (15 +/- 10% dose, mean +/- S.D.), which compared well with previously studied populations of Egyptians (16%) and Ghanaians (18%), but which were lower than those seen in a UK white population (41%). 3. Saudis, like Egyptians, were more extensive metabolizers of debrisoquine than UK whites, as judged by the metabolic ratio (% dose as debrisoquine/% dose as 4-hydroxy-debrisoquine eliminated in the urine). 4. Neither sex, urine collection period nor urinary recovery of drug and metabolite had any statistically significant effect upon the distribution of metabolic ratios in Saudis. 5. The frequency of the DL allele controlling the recessive poor metabolizer trait was 0.099 +/- 0.049 (+/- S.E.M.) in Saudis, which compared well to Egyptians (0.118 +/- 0.059) but was significantly lower than that for UK whites (0.298 +/- 0.030). 6. These findings raise questions regarding the efficacy and safety in Saudis of drugs undergoing oxidative metabolism which have been evaluated for usage in European white subjects.

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

A population survey of 258 unrelated white British subjects showed a polymorphism for the 4-oxidation of debrisoquine. "Extensive metabolisers" (EM) and "poor metabolisers" (PM) are recognisable, 8.9% of the population being PM. Nine pedigrees ascertained through PM probands show that the PM phenotype is an autosomal Mendelian recessive character. The EM phenotype is dominant and the degree of dominance has been estimated at 30%. PM subjects are more prone to hypotension during debrisoquine therapy. The alleles controlling this polymorphism appear to control the oxidation of other drugs.

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

Two subjects from each of the three groups of homozygous rapid, heterozygous, and homozygous non-metabolizers (N-oxidation) of sparteine received a single oral dose of debrisoquine. The urinary ratio of debrisoquine/4-hydroxy-debrisoquine, reflecting the individual's capacity to C-hydroxylate debrisoquine, was closely related to his phenotype for sparteine metabolism. This indicates that the two metabolic reactions are controlled by similar if not identical genetic factors.

Interindividual and interspecies variation in the metabolism of the hallucinogen 4-methoxyamphetamine

1. The qualitative and quantitative aspects of the urinary elimination of orally administered 4-methoxy[14C]amphetamine have been examined in the rat and guinea-pig and in three volunteer human subjects, to determine interspecies and interindividual variations in disposition of the drug. 2. Both rat and guinea-pig excreted 70--80% of the administered dose(6 mg/kg) in the urine within 24 h, mainly as metabolites. 3. In the guinea-pig, the drug was metabolized by O-demethylation to give 4-hydroxyamphetamine, which was excreted free (4% dose) and conjugated (73%). No other metabolite was detected. 4. The rat metabolizes the drug both by O-dealkylation and by side-chain oxidation, the products being 4-hydroxyamphetamine (5% of dose free and 60% conjugated) and 1-(4'-methoxyphenyl)propan-2-one oxime (5% dose, free and conjugated). 5. In man the drug (dose 5 mg) is metabolized by O-demethylation and by side-chain oxidation. Marked intersubject variations were observed both in the array and quantitative aspects of metabolite excretion. Two subjects excreted mainly 4-hydroxyamphetamine (free and conjugated) together with smaller amounts of 1-(4'-methoxyphenyl)propan-2-one oxime and 4-hydroxynorephedrine. The third subject, however, who was previously known to exhibit a genetically determined defect in drug oxidation, was defective in O-dealkylation of 4-methoxyamphetamine, and the main excretion products were the unchanged drug together with products of side-chain oxidation, namely, 1-(4'-methoxyphenyl)propan-2-one oxime, 1-(4'-methoxyphenyl)propan-2-one and 4-methoxybenzoic acid. 6. Inter-individual differences in oxidative O-demethylation of the drug are discussed in relation to current theories on the aetiology of schizophrenia and reported fatalities arising from abuse of the drug.