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

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.

Novel approach to predicting P450-mediated drug metabolism: development of a combined protein and pharmacophore model for CYP2D6

A combined protein and pharmacophore model for cytochrome P450 2D6 (CYP2D6) has been derived using various computational chemistry techniques. A combination of pharmacophore modeling (using 40 substrates), protein modeling, and molecular orbital calculations was necessary to derive a model which incorporated steric, electronic, and chemical stability properties. The initial pharmacophore and protein models used to construct the combined model were derived independently and showed a high level of complementarity. The combined model is in agreement with experimental results concerning the substrates used to derive the model, with site-directed mutagenesis data available for the CYP2D6 protein, and takes into account the site-directed mutagenesis results for a variety of other 2-family P450s.

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.

Extension of a predictive substrate model for human cytochrome P4502D6

1. Metoprolol, indoramine, codeine, tamoxifen and prodipine, compounds which are clinically used, and MDMA (ecstasy) were fitted in a small molecule model for substrates of human cytochrome P4502D6. 2. For both the R- and S-enantiomer of metoprolol, the R- and S-enantiomer of MDMA, and for indoramine and codeine (all proven substrates of cytochrome P4502D6) an acceptable fit in the substrate model was obtained. 3. For tamoxifen, for which the involvement of cytochrome P4502D6 in the 4-hydroxylation is uncertain, no acceptable fit could be obtained in the substrate model. 4. For prodipine, a competitive inhibitor of P4502D6, for which the involvement of P4502D6 in the metabolism is uncertain, no acceptable fit in the substrate model could be obtained. 5. The substrate model was extended in a direction in which two large known substrates extend from the original substrate model. This extension did not change the flat hydrophobic region of the original substrate model.

"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.

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.

Speculations on the substrate structure-activity relationship (SSAR) of cytochrome P450 enzymes

This brief review attempts to define the SSAR of two families of cytochrome P450. With P4502D catalytic competence is achieved by tight ionic binding which gives the enzyme high regioselectivity. In contrast P4503A achieves catalytic competence by a flexible binding site relying on hydrophobic forces that allow chemically vulnerable sites to be the principal sites of metabolism. In general, the different binding mechanism should be reflected in the enzyme, such that substrates of P4502D should have lower Km values than substrates of P4503A. Thus, routes of metabolism catalysed by P4502D may be saturated at substrate concentrations lower than routes catalysed by P4503A. The apparent differences between P4502D and P4503A in terms of substrate specificity bring into question what relationships govern other families of cytochrome P450. Our analysis of data suggests that the other principal form involved, generally, in the metabolism of pharmaceuticals in humans is P4502C9 (possibly 2C8 and 2C10). The enzyme is responsible for the metabolism of phenytoin, tolbutamide, tienilic acid [4], naproxen, ibuprofen, diclofenac [38], the 7-hydroxylation of S-warfarin [39] and the 7-hydroxylation of delta 1-tetrahydrocannabinol [40]. These compounds all have areas of strong hydrogen bond [4] forming potential (Fig. 8), all distanced 5-10A from the site of metabolism. Moreover the carboxylic acid function of naproxen, ibuprofen and diclofenac (pKa 4.5) and the sulfonylurea of tolbutamide (pKa 5.4) render the compounds ionized at physiological pH. The ionised group is positioned 7-11A from the site of metabolism. It is likely, therefore, that hydrogen bonding and possibly ion-pair interactions play a major role in determining the SSAR of the P4502C isoenzymes. These interactions would suggest that the P4502C enzymes are analogous to P4502D rather than P4503A. In this regard it is noteworthy that P4502C9 is selectively and potently inhibited by sulfaphenazole (IC50 of 0.6 microM), a compound that is structurally related (Fig. 8) to the substrates in terms of potential hydrogen bonding regions [4, 41]. Simplistically we suggest that the SSAR of the various P450 enzymes ranges from the highly selective enzymes dealing with endogenous substrates, through the enzymes metabolising exogenous substrates with narrow substrate structure requirements such as P4502D to P4503A with its broad substrate structure range. It would seem logical that animals and humans would evolve such combinations of isoenzymes to deal with the vast array of exogenous xenobiotics.

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.

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.

A review of the clinical pharmacokinetics and metabolism of the alpha 1-adrenoceptor antagonist indoramin

1. The alpha 1-adrenoceptor antagonist indoramin is rapidly and extensively absorbed after oral administration, but with only low to moderate bioavailability (8-24% median) from the tablet (Baratol). Although plasma protein binding is high (72-86%), the drug is widely distributed into tissues (with median Vz 6.3-7.7 l/kg after i.v. dosage). 2. Elimination of indoramin is rapid in most healthy volunteers, with median plasma clearances of 18-26 ml/min per kg, after i.v. dosage. Elimination occurs principally by metabolism, the major route being indole 6-hydroxylation, followed by sulphate conjugation of 6-hydroxyindoramin. The faecal route of excretion predominates (45-50% of dose), with a further 35-40% in the urine. 3. Extensive variation in single-dose oral pharmacokinetics of indoramin is due largely to the existence of a poor metabolizer phenotype which co-segregates with that of debrisoquine. 4. On repeated administration (37.5 mg twice daily) to healthy volunteers, plasma concentrations of indoramin accumulate 3-4-fold above those anticipated from single-dose kinetics. However, steady state is achieved within the first week of dosing. 5. The pharmacokinetics of indoramin are substantially altered in the elderly. The oral AUC for a 50 mg dose is increased approx. 5-fold and the t1/2 2.5-fold. 6. Cirrhotic liver disease enhances bioavailability and decreases clearance, approx. 2-fold in each case for single oral and i.v. doses of 50 mg and 0.15 mg/kg respectively. 7. After oral indoramin Cmax and AUC are both raised (58% and 25%, respectively, for a 50 mg dose) by co-ingested ethanol (0.5 g/kg). After i.v. indoramin, kinetics are unaffected by alcohol, but indoramin (0.175 mg/kg) slightly increases (26%) blood ethanol concentrations during the first hour after dosing. 8. The pharmacodynamics of indoramin appear to be related to the combined pharmacokinetics of the drug and its 6-hydroxylated metabolite, which contributes to the antihypertensive effect.

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 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.

Pharmacokinetics of indoramin and its 6-hydroxylated metabolite after repeated oral dosing

The pharmacokinetics of indoramin and its active 6-hydroxylated metabolite have been studied in healthy male volunteers after repeated oral dosing with 37.5 mg twice daily for 2 weeks. Plasma concentrations of indoramin accumulated, on average, three to four-fold above those anticipated on the basis of the kinetics after the first dose, though steady state was achieved by the end of the first week and no further increase was observed after 2 weeks. The degree of accumulation was consistent between subjects, with a highly significant (p less than 0.001) correlation between the concentration 2 h after a single dose and the average steady-state concentration. Possible explanations for the accumulation of indoramin are discussed. Plasma concentrations of 6-hydroxyindoramin, in contrast, did not accumulate on multiple dosing. At steady state, concentrations of the metabolite, as represented by the AUC0-8h, were approximately 30-40 per cent of those of the unchanged drug. Since the two compounds are approximately equipotent the metabolite may accounts for 25 per cent of the hypotensive activity of indoramin during a typical clinical regime of 37.5 mg twice daily.

Intra- and inter-subject variation in the pharmacokinetics of indoramin and its 6-hydroxylated metabolite

Intra- and inter-subject variation in the kinetics of indoramin and its active metabolite 6-hydroxyindoramin have been studied in 5 young, healthy, male volunteers administered a single oral dose of the drug on 5 separate occasions. Inter-subject variation represented the main source of variability in indoramin plasma concentrations with, for example, the between-subjects sum of squares (a measure of the contribution to the total variability) representing around 97% of the total sum of squares for Cmax and AUC (0-24). Intra-subject and inter-subject coefficients of variation (C.V.s) were circa 20% and 100% respectively for both these parameters. Variability in 6-hydroxyindoramin concentrations was much lower and was approximately equally derived from intra- and inter-subject variation, with the C.V.s being approximately 44% for both Cmax and AUC (0-24). The results imply that the kinetic behaviour of indoramin within an individual will prove relatively consistent, despite widespread inter-subject variation, once an appropriate dosage regime has been established.