Pharmacokinetics of N-propylajmaline in relation to polymorphic sparteine oxidation

Transient Expression of Human Cytochrome P450s 2D6 and 3A4 in Nicotiana benthamiana Provides a Possibility for Rapid Substrate Testing and Production of Novel Compounds

Employment of transient expression of foreign genes for bioconversion of pharmaceutically valuable low-molecular-weight compounds, including plant secondary metabolites, is an enticing trend still scantily explored in plant biotechnology. In the present work, an efficient protocol for rapid assessment of synthetic and plant-derived metabolites as potential substrates for human P450s (CYP2D6 and CYP3A4) via Agrobacterium-mediated transient expression in Nicotiana benthamiana is put forth. Animal P450s with broad substrate specificity are promising candidates for transformation of diverse metabolites. The efficiency of P450s in heterologous surroundings is not always satisfactory and depends on the availability of an associated electron-transfer enzyme. Plants represent an attractive assortment of prospective hosts for foreign P450s expression. The optimal composition of genetic blocks providing the highest transient expression efficiency is designed, an effective substrate administration scheme is validated, and biological activity of the investigated P450s against loratadine and several indole alkaloids with different molecular scaffold structures is tested. A novel indole alkaloid, 11-hydroxycorynanthine, is isolated from N. benthamiana plants transiently expressing CYP2D6 and supplemented with corynanthine, and its structure was elucidated. The proposed technique might be of value in realization of combinatorial biosynthesis concept comprising the junction of heterologous enzymes and substrates in different metabolic surroundings.

Adverse drug reaction monitoring in Jena. Relevance of polymorphic drug metabolizing enzymes for inducing adverse drug reactions

Inter-subject variability in therapeutic drug response and drug toxicity is a major problem in clinical practice. In this field genetic polymorphisms of drug metabolizing enzymes play an important role. In a multicenter study supported by the German Federal Institute for Drugs and Medical Devices (BfArM, Z 12.01-68502-201) adverse drug reactions (ADRs) leading to hospital admission to departments of internal medicine have been registered and evaluated. The aim of the presented part of the study was to look for evident differences in genotypes for polymorphic drug metabolizing enzymes between adverse drug reaction cases and controls. All cases found in the local area--Jena and Weimar--were genotyped for N-acetyl-transferase 2 (NAT2), cytochrom P450 (CYP) 2D6 and 2C19 in comparison to a control population of the same region. The investigation on genotype was carried out for about 2 years (2000-2002). 254 blood samples from patients of the ADR study were analyzed. The genotype of drug metabolizing enzymes was determined by means of polymerase chain reaction using allel specific primers or restriction enzyme analysis. Within all ADRs cases genotyped, no exceptional frequencies for slow acetylators or poor metabolizers (PM) of CYP2D6 or CYP2C19 were found. About 65% of the individuals with ADR genotypically displayed a slow acetylator state. 6.3% PM for CYP2D6, including CYP2D6*3, *4 and *6 alleles, and 2.0% PM frequency for CYP2C19 (*2) have been found in ADR cases. A direct connection between PM genotype and the ADR observed may be assumed only in few of them. Further investigations on genotype and ADR-associated drugs require a much larger sample of patients to obtain more data allowing to focus an association on specific drugs, ADR and polymorphisms genotype of drug metabolizing enzymes might be useful.

The increased intestinal absorption rate is responsible for the reduced hepatic first-pass extraction of propranolol in rats with cisplatin-induced renal dysfunction

The mechanisms responsible for the increased bioavailability of propranolol in renal dysfunction were investigated in rats. Experimental acute renal failure (ARF) was induced by intraperitoneal injection of cisplatin (5 mg kg(-1)). ARF induced a significant increase in blood propranolol concentration after intra-intestinal administration. The extent of bioavailability (F) of propranolol at an intestinal dose of 15 mg kg(-1) was 16.4% and 26.9% in control and ARF rats, respectively, and the F value at a 37.5 mg kg(-1) dose was 54.7% and 81.4% in control and ARF rats, respectively. In contrast, the blood propranolol concentration following intraportal infusion was not increased significantly in ARF rats. The hepatic first-pass extraction (E(h)) was dose-dependent and saturable: E(h) of propranolol in control rats was 58.0% and 18.3% at 8 and 20 mg kg(-1), respectively, and E(h) in ARF rats was 50.8% and 19.9% at 8 and 20 mg kg(-1), respectively. The initial absorption rate of propranolol from the intestine in ARF rats was significantly greater compared with control rats. These results indicated that the increased bioavailability of propranolol in rats with cisplatin-induced renal dysfunction was mainly a result of the increased absorption rate in the intestine followed by the partial saturation of hepatic first-pass metabolism.

Human variability in polymorphic CYP2D6 metabolism: is the kinetic default uncertainty factor adequate?

Human variability in the kinetics of CYP2D6 substrates has been quantified using a database of compounds metabolised extensively (>60%) by this polymorphic enzyme. Published pharmacokinetic studies (after oral and intravenous dosing) in non-phenotyped healthy adults, and phenotyped extensive (EMs), intermediate or slow-extensive (SEMs) and poor metabolisers (PMs) have been analysed using data for parameters that relate primarily to chronic exposure (metabolic and total clearances, area under the plasma concentration time-curve) and primarily to acute exposure (peak concentration). Similar analyses were performed with the available data for subgroups of the population (age, ethnicity and disease). Interindividual differences in kinetics for markers of oral exposure were large for non-phenotyped individuals and for EMs (coefficients of variation were 67-71% for clearances and 54-63% for C(max)), whereas the intravenous data indicated a lower variability (34-38%). Comparisons between EMs, SEMs and PMs revealed an increase in oral internal dose for SEMs and PMs (ratio compared to EMs=3 and 9-12, respectively) associated with lower variability than that for non-phenotyped individuals (coefficients of variation were 32-38% and 30% for SEMs and PMs, respectively). In relation to the uncertainty factors used for risk assessment, most subgroups would not be covered by the kinetic default of 3.16. CYP2D6-related factors necessary to cover 95-99% of each subpopulation ranged from 2.7 to 4.1 in non-phenotyped healthy adults and EMs to 15-18 in PMs and 22-45 in children. An exponential relationship (R(2)=0.8) was found between the extent of CYP2D6 metabolism and the uncertainty factors. The extent of CYP2D6 involvement in the metabolism of a substrate is critical in the estimation of the CYP2D6-related factor. The 3.16 kinetic default factor would cover PMs for substrates for which CYP2D6 was responsible for up to 25% of the metabolism in EMs.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

How to manage individualized drug therapy: application of pharmacogenetic knowledge of drug metabolism and transport

Significant fractions of health budgets must be spent for treatment of drug side effects and for inefficient drug therapy. Hereditary variants in drug metabolizing enzymes, drug transporters, and drug targets are important determinants of drug response and toxicity and may therefore aid in selection and dosage of drugs. Today there is extensive knowledge of genetic polymorphisms of cytochrome P450 (CYP) enzymes 2A6, 2C9, 2C19, and 2D6; of phase-2 enzymes such as thiopurine S-methyltransferase; and more recently of drug transporters such as the MDR-1 gene-product P-glycoprotein, affecting a significant share of currently used drugs. However, application of pharmacogenetic knowledge to clinical routine is limited in current practice. To promote the application of pharmacogenetic knowledge in clinical routine, research on genotype-based dose adjustments is still necessary - as is the promotion of faster and cheaper genotype analyses. Furthermore, the benefits of CYP genotype-directed drug therapy should be evaluated in properly designed prospective studies. Once such steps have been successfully taken, drug therapy could well become more prevention-directed and patient-tailored than it is possible today, replacing the current "one drug in one dose for one disease" strategy by a more individualized approach.

Genetic polymorphisms of human N-acetyltransferase, cytochrome P450, glutathione-S-transferase, and epoxide hydrolase enzymes: relevance to xenobiotic metabolism and toxicity

In this review, an overview is presented of the current knowledge of genetic polymorphisms of four of the most important enzyme families involved in the metabolism of xenobiotics, that is, the N-acetyltransferase (NAT), cytochrome P450 (P450), glutathione-S-transferase (GST), and microsomal epoxide hydrolase (mEH) enzymes. The emphasis is on two main topics, the molecular genetics of the polymorphisms and the consequences for xenobiotic metabolism and toxicity. Studies are described in which wild-type and mutant alleles of biotransformation enzymes have been expressed in heterologous systems to study the molecular genetics and the metabolism and pharmacological or toxicological effects of xenobiotics. Furthermore, studies are described that have investigated the effects of genetic polymorphisms of biotransformation enzymes on the metabolism of drugs in humans and on the metabolism of genotoxic compounds in vivo as well. The effects of the polymorphisms are highly dependent on the enzyme systems involved and the compounds being metabolized. Several polymorphisms are described that also clearly influence the metabolism and effects of drugs and toxic compounds, in vivo in humans. Future perspectives in studies on genetic polymorphisms of biotransformation enzymes are also discussed. It is concluded that genetic polymorphisms of biotransformation enzymes are in a number of cases a major factor involved in the interindividual variability in xenobiotic metabolism and toxicity. This may lead to interindividual variability in efficacy of drugs and disease susceptibility.

"It's the genes, stupid". Molecular bases and clinical consequences of genetic cytochrome P450 2D6 polymorphism

In this review we highlight the information available on the genetic polymorphism of cytochrome P4502D6 expression in man. An absent function of this enzyme is observed in 7-10 percent of the Caucasian population which are referred to as Poor metabolizers as opposed to the remainder of the population (Extensive Metabolizers). More than 30 widely used drugs have been identified as substrates for CYP2D6. Disposition and action of these compounds depend on the individual phenotype. Both the molecular bases of the variable enzyme activity and the consequences for drug therapy are outlined. While mutations on the DNA level have been investigated in great detail larger scale clinical trials are lacking and information on therapeutic consequences of CYP2D6 mediated polymorphic drug oxidation is restricted to case reports. Besides these implications for drug metabolism several lines of evidence indicate that CYP2D6 could be involved in biotransformation of endogenous compounds.

Fatal poisoning with detajmium: identification of detajmium and its metabolites and artifacts by gas chromatography-mass spectrometry and quantification by high-performance liquid chromatography

After ingestion of an unknown dose of detajmium, a 14-year-old female collapsed with asystolia. Resuscitation efforts were not successful. A medicolegal autopsy was carried out, and blood, liver and gastric content were extracted and analyzed by gas chromatography-mass spectrometry (GC-MS). After derivatization with acetic anhydride, detajmium and twelve of its derivatives and metabolites were identified. The main metabolic pathways include hydroxylation and subsequent O-methylation of the indol ring, and oxidation as well as reduction of the C-21 hydroxyl function. Cleavage of the N-alkyl side-chain is a further, possibly non-enzymatic degradation pathway. Artifact formation induced by acetylation included dehydratation of the hydroxyl function of C-21 and the N-alkyl side-chain. The detajmium concentration in blood of the decreased was determined by high-performance liquid chromatography with fluorimetric detection (12 micrograms/ml).

Clinical significance of genetic influences on cardiovascular drug metabolism

Inherited differences in metabolism may be responsible for individual variability in the efficacy of drugs and the occurrence of adverse drug reactions. Among the cardiovascular drugs reported to exhibit genetic polymorphism are debrisoquine, sparteine, some beta-adrenoceptor antagonists, flecainide, encainide, propafenone, nifedipine, procainamide, and hydralazine. The implications of genetic differences in the metabolism of these drugs for cardiovascular therapeutics is the subject of this review.

Metabolic polymorphisms

Polymorphisms have been detected in a variety of xenobiotic-metabolizing enzymes at both the phenotypic and genotypic level. In the case of four enzymes, the cytochrome P450 CYP2D6, glutathione S-transferase mu, N-acetyltransferase 2 and serum cholinesterase, the majority of mutations which give rise to a defective phenotype have now been identified. Another group of enzymes show definite polymorphism at the phenotypic level but the exact genetic mechanisms responsible are not yet clear. These enzymes include the cytochromes P450 CYP1A1, CYP1A2 and a CYP2C form which metabolizes mephenytoin, a flavin-linked monooxygenase (fish-odour syndrome), paraoxonase, UDP-glucuronosyltransferase (Gilbert's syndrome) and thiopurine S-methyltransferase. In the case of a further group of enzymes, there is some evidence for polymorphism at either the phenotypic or genotypic level but this has not been unambiguously demonstrated. Examples of this class include the cytochrome P450 enzymes CYP2A6, CYP2E1, CYP2C9 and CYP3A4, xanthine oxidase, an S-oxidase which metabolizes carbocysteine, epoxide hydrolase, two forms of sulphotransferase and several methyltransferases. The nature of all these polymorphisms and possible polymorphisms is discussed in detail, with particular reference to the effects of this variation on drug metabolism and susceptibility to chemically-induced diseases.

P450 enzymes. Inhibition mechanisms, genetic regulation and effects of liver disease

Multiple hepatic P450 enzymes play an important role in the oxidative biotransformation of a vast number of structurally diverse drugs. As such, these enzymes are a major determinant of the pharmacokinetic behaviour of most therapeutic agents. There are several factors that influence P450 activity, either directly or at the level of enzyme regulation. Drug elimination is decreased and the incidence of drug interactions is increased when there is competition between 2 or more drugs for oxidation by the same P450 enzyme. The available knowledge concerning the relationship between the presence of certain functional groups within the drug structure and inhibition of P450 activity is increasing. In many instances, it is possible to associate inhibition with certain drug classes, e.g. antimycotic imidazoles and macrolide antibiotics. Disease states, especially those with hepatic involvement, and the genetic makeup of the individual are conditions in which some P450s may be downregulated (that is, the enzyme concentrations in liver are decreased), with associated slower rates of drug elimination. In these individuals, dosages of drugs that are substrates for downregulated P450s should be decreased. Exposure to environmental pollutants as well as a large number of lipophilic drugs can result in induction (upregulation) of P450 enzyme activity. This raises the issue of previous approaches to the study of P450 induction in vivo. The use of human hepatocyte preparations in culture is a promising new direction that could assist the determination of modifications to drug therapy necessitated by exposure to inducing agents. Until such information is obtained, however, the use of drugs known to increase the microsomal expression of particular P450s, and increase associated drug oxidation capacity in humans, should be used with caution.

Monitoring of ajmaline in plasma with high-performance liquid chromatography

A rapid, reliable and sensitive assay for routine determination of ajmaline in plasma by high-performance liquid chromatography with fluorimetric detection is presented. A low limit of detection in plasma (less than 1 ng/ml ajmaline) could be achieved by the extraction of plasma samples and the use of fluorimetric detection. Deproteinization of the plasma sample instead of extraction, or the use of an ultraviolet detector, yielded a higher limit of detection (less than 50 ng/ml). Two different eluents were studied. Eluent 1 allowed clear separation of ajmaline from isoajmaline and sandwicine, but did not separate isoajamaline from sandwicine. With eluent 2, separation of isoajmaline and sandwicine was achieved, but separation of ajmaline from sandwicine was less optimal than with eluent 1. Therefore, eluent 1 was used for further clinical studies. No interference was observed from therapeutic doses of other commonly co-administered drugs, such as acetylsalicylic acid, digoxin, digitoxin, ranitidine, dopamine, dobutamine, furosemide, captopril or glycerol trinitrate. In addition, the chemical stability of ajmaline and a possible rearrangement of ajmaline to its stereoisomers isoajmaline and sandwicine was studied in vivo and in vitro. Ajmaline proved to be unusually stable under both in vivo and in vitro conditions.

Clinical consequences of polymorphic drug oxidation

Many characters are genetically regulated as polymorphisms. This means that discrete groups are seen within the distribution of a certain character. Drug metabolism is no exception and the polymorphism of acetylation is recognised since the 50's. Polymorphic drug oxidation was discovered in the 70's and has been extensively studied. There are two fully established polymorphisms in drug oxidation named as the debrisoquine/sparteine and the s-mephenytoin hydroxylation polymorphisms. The metabolism of a number of important drugs cosegregates with that of debrisoquine. Among these drugs are beta-blockers, antiarrhythmics, tricyclic antidepressants and neuroleptics. Apart from accumulation of parent drug and active metabolite, also reduced formation of active metabolite occur for some drugs in slow metabolisers. There are, however, few cases where the presence of polymorphic drug metabolism is of significant disadvantage. The polymorphisms will add to variability in drug clearance but the potential clinical importance should be evaluated for each drug. The cytochrome P-450 isozyme responsible for debrisoquine hydroxylation is of high affinity-low capacity character, which means that it can be saturated under certain circumstances. This will decrease the difference in drug metabolic rate between rapid and low metabolisers as will inhibitors of the debrisoquine isozyme like cimetidine, quinidine and propafenone. The debrisoquine isozyme is not readily inducible. In cases where a major metabolic route or the formation of an active metabolite are polymorphically controlled, knowledge about a patient's oxidator status might be of practical value for dose adjustments especially if there is a narrow therapeutic ratio or an established concentration-effect relationship. For some drugs it is difficult to differentiate between insufficient therapeutic effect and symptoms of overdosage. Tricyclic antidepressants and neuroleptics meet some of these criteria and patients who get recurrent treatment may benefit if the physician has knowledge about debrisoquine metabolic phenotype. Otherwise, the clinical consequences of polymorphisms in drug oxidation seem so far to be limited, considering that a number of disease conditions have not shown any clear association with oxidation status. The polymorphisms in drug metabolism should be considered as a part of natural variability which could in fact be larger with other drugs that do not show polymorphic elimination.

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.

Clinical course and outcome in class IC antiarrhythmic overdose

120 cases of class IC antiarrhythmic overdose, including propafenone, flecainide, ajmaline and prajmaline overdose, were evaluated with respect to clinical course, therapy and outcome. Whereas drug overdose in general has an overall mortality of less than 1%, intoxication with antiarrhythmic drugs of class IC was associated with a mean mortality of 22.5%. Nausea, which occurred within the first 30 minutes after ingestion, was the earliest symptom. Spontaneous vomiting probably led to self-detoxication in about half the patients. Cardiac symptoms including bradycardia and, less frequently, tachyrhythmia occurred after about 30 minutes to 2 hours. Therapeutic measures included administration of activated charcoal, gastric lavage and a saline laxative, catecholamines, and in some patients, hypertonic sodium bicarbonate, insertion of a transvenous pacemaker and hemoperfusion. Fatal outcome was mainly due to cardiac conduction disturbances progressing to electromechanical dissociation or asystolia. Resuscitation, which had to be performed in 29 patients, was successful in only two of them. No correlation was found between fatal outcome, the type of antiarrhythmic, and ingested dose. Since a specific treatment is not available and resuscitive procedures including sodium bicarbonate and insertion of a pacemaker are of limited therapeutic value, early diagnosis and primary detoxification are most important for prevention of fatal outcome.

The genetic polymorphism of debrisoquine/sparteine metabolism-molecular mechanisms

The genetic polymorphism of debrisoquine/sparteine metabolism is one of the best studied examples of a genetic variability in drug response. 5-10% of individuals in Caucasian populations are 'poor metabolizers' of debrisoquine, sparteine and over 20 other drugs. The discovery and the inheritance of deficient debrisoquine/sparteine metabolism are briefly described, followed by a detailed account of the studies leading to the characterization of the deficient reaction and the purification of cytochrome P-450IID1, the target enzyme of this polymorphism. It is demonstrated by immunological methods that deficient debrisoquine hydroxylation is due to the absence of P-450IID1 protein in the livers of poor metabolizers. The cloning and sequencing of the P-450IID1 cDNA and of IID1 related genes are summarized. The P-450IID1 cDNA has subsequently led to the discovery of aberrant splicing of P-450IID1 pre-mRNA as the cause of absent P-450IID1 protein. Finally, the identification of mutant alleles of the P-450IID1 gene (CYP 2D) by restriction fragment length polymorphisms in lymphocyte DNA of poor metabolizers is presented.

Stereoselective disposition of flecainide in relation to the sparteine/debrisoquine metaboliser phenotype

1. The disposition of the enantiomers of the antiarrhythmic drug flecainide has been studied in five extensive (EM) and five poor (PM) metabolisers of sparteine/debrisoquine after administration of 50 mg of racemic flecainide acetate under conditions of high urinary flow rate and acidic urinary pH. 2. In the EM subjects there were no significant differences in the oral clearance, half-life or urinary excretion of (+)-S- and (-)-R-flecainide. 3. In the PM subjects differences in the pharmacokinetics of S- and R-flecainide were observed. The oral clearance of R-flecainide (467 +/- 109 ml min-1) was less (P less than 0.03) than that of the S-enantiomer (620 +/- 172 ml min-1). The half-life of R-flecainide (12.9 h) was longer (P less than 0.03) than that of S-flecainide (9.8 h). The renal clearance of the two enantiomers was, however, comparable and similar to that observed in the EM subjects. The urinary recovery of R-flecainide (15.6 +/- 3.7 mg) was greater (P less than 0.03) than that of the S-enantiomer (12.0 +/- 3.7 mg). The enantioselective disposition observed in PMs is therefore due to greater impairment in the metabolism of R- than S-flecainide. 4. The urinary recoveries of two major metabolites of flecainide, meta-O-dealkylated flecainide (MODF) and the meta-O-dealkylated lactam of flecainide (MODLF) were lower (P less than 0.05) in PMs, 12.0% +/- 3.1% and 8.2% +/- 3.2% of the dose administered, respectively, than in EMs of 17.7% +/- 3.3% and 16.5% +/- 3.3%, respectively. 5. One PM subject had a greatly diminished flecainide metabolic capacity and a rare genotype, as assigned by Xbal RFLP analysis.

Stereoselective disposition and pharmacologic activity of propafenone enantiomers

Propafenone is an antiarrhythmic drug that produces a variable degree of beta-blockade in humans and is administered as a racemate. To examine the relative contribution of the individual enantiomers to pharmacologic effects seen during treatment with propafenone, we assessed the steady-state plasma concentrations of (+)-S-propafenone and (-)-R-propafenone in seven patients who were on long-term oral therapy, and we evaluated the electrophysiologic and beta-blocking properties of both enantiomers in vitro. The metabolism of propafenone is known to be polymorphic and to cosegregate with that of debrisoquine-4-hydroxylation. Among five patients with the extensive metabolizer phenotype (EM), the ratio of the area under the plasma concentration-time curve of (+)-S-propafenone to (-)-R-propafenone was 1.73 +/- 0.15 (mean +/- SD). In the other two patients, who had the poor metabolizer phenotype (PM), the concentrations of both enantiomers were elevated but the S/R ratios were similar to those seen in patients with EM. In canine cardiac Purkinje fibers, both enantiomers produced similar frequency-dependent depression of maximum upstroke of phase 0. In contrast, the affinity of the human lymphocyte beta 2-adrenoceptor was approximately 100-fold greater for (+)-S-propafenone (Ki, 7.2 +/- 2.9 nM) than for the (-)-R-enantiomer (Ki, 571 +/- 141 nM). We conclude that during long-term oral therapy, propafenone undergoes stereoselective disposition in patients with either EM or PM. beta-Blockade during propafenone therapy is likely related to accumulation of (+)-S-propafenone.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics and antiarrhythmic efficacy of intravenous ajmaline in ventricular arrhythmia of acute onset

21 patients with acute myocardial infarction and ventricular arrhythmia of Lown class II-IIIB of acute onset received a short infusion of (50 mg/5 min) ajmaline (Gilurytmal). 6 of the patients had normal kidney and liver function (Group 1), 4 patients had acute renal failure and hemodialysis treatment (Group 2), 4 patients had impaired hepatic function (Group 3), 3 patients had cardiogenic shock (Group 4), and 4 patients had been pretreated with phenobarbital for seizures for at least 5 days (Group 5). A distribution half-life of 6 +/- 1 min and an elimination half-life of 95 +/- 6 min was determined in Group 1. The total plasma clearance was significantly lower in patients with impaired liver or cardiac function and significantly higher in Group 5, whereas impaired renal function did not affect total plasma clearance. After short infusion, ventricular arrhythmia of Lown II-IIIB completely disappeared for at least 16 to 36 min (mean: 19 min), which was associated with an ajmaline plasma level of 0.1-0.45 micrograms/ml. Additionally, steady-state plasma levels of ajmaline were determined after continuous infusion of 10-50 mg/h to 16 patients (Group 6) with ventricular arrhythmia of acute onset (Lown class IVA-V). Ventricular arrhythmia completely disappeared or at least changed to lower Lown classes at ajmaline plasma levels of 0.4-2.0 micrograms/ml. The ajmaline plasma protein binding was 76 +/- 9%. Ajmaline had a special affinity to alpha 1-acid glycoprotein.

Clinical significance of the sparteine/debrisoquine oxidation polymorphism

The sparteine/debrisoquine oxidation polymorphism results from differences in the activity of one isozyme of cytochrome P450, the P450db1 (P450 IID1). The oxidation of more than 20 clinically useful drugs has now been shown to be under similar genetic control to that of sparteine/debrisoquine. The clinical significance of this polymorphism may be defined by the value of phenotyping patients before treatment. The clinical significance of such polymorphic elimination of a particular drug can be analyzed in three steps: first, does the kinetics of active principle of a drug depend significantly on P450db1?; second, is the resulting pharmacokinetic variability of any clinical importance?; and third, can the variation in response be assessed by direct clinical or paraclinical measurements? It is concluded from such an analysis that, in general, the sparteine/debrisoquine oxidation polymorphism is of significance in patient management only for those drugs for which plasma concentration measurements are considered useful and for which the elimination of the drug and/or its active metabolite is mainly determined by P450db1. At present, this applies to tricyclic antidepressants and to certain neuroleptics (e.g. perphenazine and thioridazine) and antiarrhythmics (e.g. propafenone and flecainide). Phenotyping should be introduced in to clinical routine under strictly controlled conditions to afford a better understanding of its potentials and limitations. The increasing knowledge of specific substrates and inhibitors of P450db1 allows precise predictions of drug-drug interactions. At present, the strong inhibitory effect of neuroleptics on the metabolism of tricyclic antidepressants represents the best clinically documented and most relevant example of such an interaction.

Genetic polymorphism in drug oxidation

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

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.

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

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

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.

Pharmacokinetics of n-propyl-ajmaline-bitartrate in elderly patients with ventricular ectopic activity

11 patients (9m, 2f, median age 59 years) with ventricular ectopic activity of at least Lown grade III received 20 mg N-Propyl-ajmaline-bitartrate (N-PAB) p.o. Plasma concentrations of N-PAB were determined with HPLC from blood samples within 26 hours after administration. An open two-compartment model was used. In 8 patients with normal function of the liver and the kidneys, the median clearance of N-PAB was 6.86 ml/min/kg and the median volume of distribution was 1.56 l/kg. Two patients had a clearly diminished clearance of 1.58 ml/min/kg without obvious impairment of liver or renal function. One patient with chronic glomerulonephritis (plasma creatinine 3.4 mg/dl) had a N-PAB clearance of 2.79 ml/min/kg. None of the Spearman rank correlation coefficients between the pharmacokinetic parameters of N-PAB with age, plasma albumin/globulin-quotient, plasma creatinine and cholin-esterase were significant. All calculated parameters were in the range determined in young subjects. It is concluded that physiological changes with age do not lead to significant changes of the pharmacokinetics of N-PAB. On the other hand in patients with increased levels of plasma creatinine a diminished clearance of N-PAB can be expected. It is also possible that patients without an obvious impairment of liver or renal function may have diminished N-PAB clearance.

Pharmacokinetics and metabolism of quinidine in extensive and poor metabolisers of sparteine

The pharmacokinetics and metabolism of quinidine were investigated in extensive and poor metabolisers of sparteine. No differences in plasma clearance, terminal half life, volume of distribution or cumulative urinary excretion of quinidine, 3-hydroxyquinidine and quinidine-N-oxide were observed between phenotypes. Thus, it is unlikely that quinidine metabolism is controlled by the sparteine/debrisoquine gene locus.