Genetically determined oxidation capacity and the disposition of debrisoquine

Oral Yohimbine as a New Probe Drug to Predict CYP2D6 Activity: Results of a Fixed-Sequence Phase I Trial

Objective

Yohimbine pharmacokinetics were determined after oral administration of a single oral dose of yohimbine 5 mg and a microdose of yohimbine 50 µg in relation to different cytochrome P450 (CYP) 2D6 genotypes. The CYP2D6 inhibitor paroxetine was used to investigate the influence on yohimbine pharmacokinetics. Microdosed midazolam was applied to evaluate a possible impact of yohimbine on CYP3A activity and the possibility of combining microdosed yohimbine and midazolam to simultaneously determine CYP2D6 and CYP3A activity.

Methods

In a fixed-sequence clinical trial, 16 healthy volunteers with a known CYP2D6 genotype [extensive (10), intermediate (2) and poor (4) metaboliser] received an oral dose of yohimbine 50 µg, yohimbine 5 mg at baseline and during paroxetine as a CYP2D6 inhibitor. Midazolam (30 µg) was co-administered to determine CYP3A activity at each occasion. Plasma concentrations of yohimbine, its main metabolite 11-OH-yohimbine, midazolam and paroxetine were quantified using validated liquid chromatography-tandem mass spectrometry assays.

Results

Pharmacokinetics of yohimbine were highly variable and a CYP2D6 genotype dependent clearance was observed. After yohimbine 5 mg, the clearance ranged from 25.3 to 15,864 mL/min and after yohimbine 50 µg, the clearance ranged from 39.6 to 38,822 mL/min. A more than fivefold reduction in clearance was caused by paroxetine in CYP2D6 extensive metabolisers, while the clearance in poor metabolisers was not affected. Yohimbine did not alter CYP3A activity as measured by microdosed midazolam.

Conclusions

The pharmacokinetics of yohimbine were highly correlated with CYP2D6, which was further supported by the clearance inhibition caused by the CYP2D6 inhibitor paroxetine. With these data, yohimbine is proposed to be a suitable probe drug to predict CYP2D6 activity. In addition, the microdose can be used in combination with microdosed midazolam to simultaneously evaluate CYP2D6 and CYP3A activity without any interaction between the probe drugs and because the microdoses exert no pharmacological effects.

Clinical Trial Registration

EudraCT2017-001801-34.

Electronic supplementary material

The online version of this article (10.1007/s40262-020-00862-6) contains supplementary material, which is available to authorized users.

Assessment of chimeric mice with humanized liver as a tool for predicting circulating human metabolites

The ability to predict circulating human metabolites of a candidate drug before first-in-man studies are carried out would provide a clear advantage in drug development. A recent report demonstrated that while in vitro studies using human liver preparations reliably predict primary human metabolites in plasma, the predictability of secondary metabolites, formed by multiple reactions, was low, with total success rates of < or =65%. Here, we assess the use of chimeric mice with humanized liver as an animal model for the prediction of human metabolism in vivo. Metabolism studies with debrisoquine and (S)-warfarin demonstrated significantly higher concentrations of their primary human abundant metabolites in serum or plasma in chimeric mice than in control mice. Humanized chimeric mice were also capable of producing human-specific metabolites of several in-house compounds which were generated through more than one metabolism reaction. This model is closer to in vivo human physiology and therefore appears to have an advantage over in vitro systems in predicting complex metabolites in human plasma. However, prediction of human metabolites failed for other compounds which were highly metabolized in mice. Although requiring careful consideration of compound suitability, this model represents a potential tool for predicting human metabolites in combination with conventional in vitro systems.

Evaluation of probe drugs and pharmacokinetic metrics for CYP2D6 phenotyping

INTRODUCTION

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

METHODS

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

CONCLUSION

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

In Vivo and in Vitro Characterization of Chlorzoxazone Metabolism and Hepatic CYP2E1 Levels in African Green Monkeys: Induction by Chronic Nicotine Treatment

CYP2E1 metabolizes compounds, including clinical drugs, organic solvents, and tobacco-specific carcinogens. Chlorzoxazone (CZN) is a probe drug used to phenotype for CYP2E1 activity. Smokers have increased CZN clearance during smoking compared with nonsmoking periods; however, it is unclear which cigarette smoke component is causing the increased activity. The relationships between in vivo CZN disposition, in vitro CZN metabolism, and hepatic CYP2E1 have not been investigated in a within-animal design. In control-treated monkeys (Cercopithecus aethiops), the in vivo CZN area under the curve extrapolated to infinity (AUC(inf)) was 19.7 +/- 4.5 microg x h/ml, t1/2 was 0.57 +/- 0.07 h, and terminal disposition rate constant calculated from last three to four points on the log-linear end of the concentration versus time curve was 1.2 +/- 0.2 /h. In vitro, the apparent Vmax was 3.48 +/- 0.02 pmol/min/mug microsomal protein, and the Km was 95.4 +/- 1.8 microM. Chronic nicotine treatment increased in vivo CZN disposition, as indicated by a 52% decrease in AUC(inf) (p < 0.01) and 52% decrease in Tmax (p < 0.05) compared with control-treated monkeys. The log metabolic ratios at 0.5, 1, 2, and 4 h significantly negatively correlated with CZN AUC(inf) (p = 0.01-0.0001). Monkey hepatic CYP2E1 levels significantly correlated with both in vivo AUC(inf) (p = 0.03) and in vitro (p = 0.004) CZN metabolism. Together, the data indicated that nicotine induction of in vivo CZN disposition is related to the rates of in vitro CZN metabolism and hepatic microsomal CYP2E1 protein levels. Nicotine is one component in cigarette smoke that can increase in vivo CZN metabolism via induction of hepatic CYP2E1 levels. Thus, nicotine exposure may affect the metabolism of CYP2E1 substrates such as acetaminophen, ethanol, and benzene.

Phenobarbital increases monkey in vivo nicotine disposition and induces liver and brain CYP2B6 protein

1. CYP2B6 is a drug-metabolizing enzyme expressed in the liver and brain that can metabolize bupropion (Zyban), a smoking cessation drug), activate tobacco-smoke nitrosamines, and inactivate nicotine. Hepatic CYP2B6 is induced by phenobarbital and induction may affect in vivo nicotine disposition, while brain CYP2B6 induction may affect local levels of centrally acting substrates. We investigated the effect of chronic phenobarbital treatment on induction of in vivo nicotine disposition and CYP2B6 expression in the liver and brain of African Green (Vervet) monkeys. 2. Monkeys were split into two groups (n=6 each) and given oral saccharin daily for 22 days; one group was supplemented with 20 mg kg(-1) phenobarbital. Monkeys were given a 0.1 mg kg(-1) nicotine dose subcutaneously before and after treatment. 3. Phenobarbital treatment resulted in a significant, 56%, decrease (P=0.04) in the maximum nicotine plasma concentration and a 46% decrease (P=0.003) in the area under the concentration-time curve. Phenobarbital also increased hepatic CYP2B6 protein expression. In monkey brain, significant induction (P<0.05) of CYP2B6 protein levels was observed in all regions tested (caudate, putamen, hippocampus, cerebellum, brain stem and frontal cortex) ranging from 2-fold to 150-fold. CYP2B6 expression was induced in specific cells, such as frontal cortical pyramidal cells and thalamic neurons. 4. In conclusion, chronic phenobarbital treatment in monkeys resulted in increased in vivo nicotine disposition, and induced hepatic and brain CYP2B6 protein levels and cellular expression. This induction may alter the metabolism of CYP2B6 substrates including peripherally acting drugs such as cyclophosphamide and centrally acting drugs such as bupropion, ecstasy and phencyclidine.

Cytochrome P450 in vitro reaction phenotyping: a re-evaluation of approaches used for P450 isoform identification

Marker substrates, chemical inhibitors, and inhibitory antibodies are important tools for the identification of cytochrome P450 (P450) isoform responsible for the metabolism of therapeutic agents in vitro. In view of the versatile and nonspecific nature of P450 enzymes, many of the marker substrates and chemical inhibitors used for P450 in vitro reaction phenotyping are isoform selective but not specific. Recently, the use of marker substrate and chemical inhibitors in CYP2D6 in vitro reaction phenotyping was questioned by Granvil et al. (2002). In comparison of a panel of 15 recombinant P450 enzymes, they found that in addition to CYP2D6, CYP1A1 is also capable of catalyzing the formation of 4-hydroxylated metabolite of debrisoquine and that the intrinsic clearance of debrisoquine by CYP2D6-mediated 4-hydroxylation is only about twice that by CYP1A1. In their study, they have also demonstrated that quinidine inhibits both CYP2D6- and CYP1A1-mediated debrisoquine 4-hydroxylation. In view of these important findings, we have reevaluated various approaches used to identify P450 isoform(s) responsible for the metabolism of therapeutic agents. While acknowledging the value of inhibitory antibodies in P450-phenotyping studies, it is our opinion that in well conducted in vitro experiments, isoform-selective chemical inhibitors can also provide valuable and reliable information. Hopefully, future efforts may produce even better P450 isoform-selective marker substrates and inhibitors.

The CYP2D6 humanized mouse: effect of the human CYP2D6 transgene and HNF4alpha on the disposition of debrisoquine in the mouse

CYP2D6 is a highly polymorphic human gene responsible for a large variability in the disposition of more than 100 drugs to which humans may be exposed. Animal models are inadequate for preclinical pharmacological evaluation of CYP2D6 substrates because of marked species differences in CYP2D isoforms. To overcome this issue, a transgenic mouse line expressing the human CYP2D6 gene was generated. The complete wild-type CYP2D6 gene, including its regulatory sequence, was microinjected into a fertilized FVB/N mouse egg, and the resultant offspring were genotyped by both polymerase chain reaction and Southern blotting. CYP2D6-specific protein expression was detected in the liver, intestine, and kidney from only the CYP2D6 humanized mice. Pharmacokinetic analysis revealed that debrisoquine (DEB) clearance was markedly higher (94.1 +/- 22.3 l/h/kg), and its half-life significantly reduced (6.9 +/- 1.6 h), in CYP2D6 humanized mice compared with wild-type animals (15.2 +/- 0.9 l/h/kg and 16.5 +/- 4.5 h, respectively). Mutations in hepatic nuclear factor 4alpha (HNF4alpha), a hepatic transcription factor known to regulate in vitro expression of the CYP2D6 gene, could affect the disposition of CYP2D6 drug substrates. To determine whether the HNF4alpha gene modulates in vivo pharmacokinetics of CYP2D6 substrates, a mouse line carrying both the CYP2D6 gene and the HNF4alpha conditional mutation was generated and phenotyped using DEB. After deletion of HNF4alpha, DEB 4-hydroxylase activity in CYP2D6 humanized mice decreased more than 50%. The data presented in this study show that only CYP2D6 humanized mice but not wild-type mice display significant DEB 4-hydroxylase activity and that HNF4alpha regulates CYP2D6 activity in vivo. The CYP2D6 humanized mice represent an attractive model for future preclinical studies on the pharmacology, toxicology, and physiology of CYP2D6-mediated metabolism.

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

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

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.

Stereoselective disposition of carvedilol is determined by CYP2D6

Carvedilol is a mixed alpha- and beta-adrenergic receptor antagonist that is administered as a racemic mixture. Although the two isomers are equally potent as alpha 1-blockers the S(-)-isomer is principally responsible for the beta blockade of carvedilol. To determine the role of pharmacogenetics in the metabolism of carvedilol we studied nine extensive metabolizers of both debrisoquin and mephenytoin, seven poor metabolizers of debrisoquin but extensive metabolizers of mephenytoin, and three poor metabolizers of mephenytoin but extensive metabolizers of debrisoquin. The clearance of R-carvedilol was significantly lower than S-carvedilol in both debrisoquin phenotypes. Poor metabolizers of debrisoquin had a significantly lower clearance of R-carvedilol than extensive metabolizers of debrisoquin. The partial metabolic clearance of carvedilol to the two ring-hydroxylated metabolites 4- and 5-hydroxyphenyl carvedilol were significantly reduced in poor metabolizers of debrisoquin. No effect of mephenytoin phenotype on carvedilol kinetics was observed. Thus carvedilol is stereoselectively metabolized in humans, and the clearance of S-carvedilol is higher than that of R-carvedilol. In poor metabolizers of debrisoquin the clearance of R-carvedilol is further reduced, resulting in higher plasma concentrations and perhaps greater alpha-blockade.

DNA haplotype dependency of debrisoquine 4-hydroxylase (CYP2D6) expression among extensive metabolisers

Deficient debrisoquine/sparteine type oxidation is inherited as an autosomal recessive trait. Of all Caucasians, 5-10% are poor metabolisers, due to the absence of cytochrome P4502D6. Extensive metabolisers (EMs) exhibit highly variable metabolic activity. We investigated the relationship between CYP2D6 activity and genotypes of the CYP2D locus in a large set of French Caucasian families. Genotypes concern both common mutations affecting the enzyme activity and linked BamHI polymorphisms of the locus. We found, like other authors, that in EMs part of the heterogeneity is explained by a subgroup of individuals heterozygous for a mutant allele. However, a second level of heterogeneity was detected among individuals not carrying mutations, and this was related to a polymorphic BamHI-defined DNA haplotype. Different combinations of haplotypes are associated with differences in CYP2D6 metabolic activity. This finding might help to clarify the conflicting data on the relation between CYP2D6 activity and susceptibility to lung cancer.

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.

Molecular heterogeneity of the XbaI defined 44kb allele of the CYP2D locus within the Caucasian population

1. Cytochrome P450 debrisoquine (CYP2D6) activity is polymorphic and under genetic control. Most Caucasians are extensive metabolizers, but 5%-10% are poor metabolizers. 2. Restriction fragment length polymorphism analysis of the CYP2D6 locus identifies a 29kb XbaI fragment, either normal (D6-wt) or mutated, and three mutated XbaI alleles (44kb, 11.5kb and 16 + 9kb). The 44kb allele was initially considered as a poor metabolizer allele owing to a D6-B mutation, but cases of 44kb allele not carrying the D6-B, and therefore potentially functional, have been found. The degree of molecular heterogeneity of this allele was investigated by phenotype and genotype analysis of families. 3. Thirty-one French Caucasian families, representing 117 individuals, possessing at least one 44kb allele in each family were selected. Phenotypes were determined using dextromethorphan, and the XbaI, NcoI and BamH1 RFLPs of 42 independent chromosomes were analyzed. 4. 80% of the XbaI 44kb alleles carried the CYP2D6-B mutation and had an additional NcoI fragment (12.5kb or 4.8kb). The remaining 20% did not carry the CYP2D6-B or A mutations and had no extra NcoI fragment. 5. Information on three families demonstrated that 44kb alleles not carrying the CYP2D6-B mutation were associated with the extensive metabolizer phenotype. 6. We conclude that a substantial percentage of XbaI 44kb alleles is associated with a functional CYP2D gene, and therefore, that the XbaI 44kb allele is not consistently a poor metaboliser allele.

Debrisoquine oxidation polymorphism in a Tasmanian population

The debrisoquine hydroxylation phenotype was studied in 152 unselected healthy Tasmanian subjects, who were mostly Caucasians of British ancestry. Following a 10 mg oral dose of debrisoquine (D), the ratio of D/4-hydroxydebrisoquine excreted in 8-h urine (metabolic ratio, MR) was determined. MR values were bimodally distributed. Thirteen subjects (8.6%) had MR values from 13.8 to 93.3 and were considered to be poor metabolisers of D, while the others were extensive metabolisers with MR values of 0.04 to 5.4. The D hydroxylation phenotype was not associated with sex. These findings confirm the constancy of D polymorphism in a Caucasian population even after migration to another country.

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.

Debrisoquine oxidation in Parkinson's disease

Variations in the activities of xenobiotic metabolizing liver enzymes may be involved in the pathophysiology of diseases, including Parkinson's disease. We therefore studied the activity of the debrisoquine metabolizing enzyme in 97 patients with newly diagnosed Parkinson's disease. The urine debrisoquine metabolic ratios (MR) of the patients were compared with a group of 176 healthy subjects. There were 4 poor metabolizers (4.1%) among the parkinsonians. This proportion did not differ from that found in the group of healthy subjects (51%). In contrast to earlier finding, the parkinsonian poor metabolizers (PM) had the onset of the disease later than the parkinsonian extensive metabolizers (EM). In the parkinsonian patients, it was observed that the excretion of debrisoquine and 4-OH-debrisoquine into urine correlated inversely with the actual age and age at disease onset. Our results indicate that in patients with Parkinson's disease, debrisoquine hydroxylation is comparable with healthy subjects.

Genetic polymorphism of sparteine/debrisoquine oxidation: a reappraisal

Polymorphic oxidation of the sparteine/debrisoquine-type has been shown to account for much of the interindividual variation in the metabolism, pharmacokinetics and pharmacodynamics of an increasing number of drugs, including some antiarrhythmic, antidepressant and beta-adrenoceptor antagonist agents. Impaired hydroxylation of these drugs results from the absence of the enzyme cytochrome P450IID6 in the livers of poor metabolisers, who constitute 6% to 10% of Caucasian populations. The clinical importance of the phenomenon has to be explored further and for most sparteine/debrisoquine-related substrates there is a need for controlled prospective studies to define the consequences to the patient of impaired or enhanced drug oxidation.

The effect of impaired renal function on the pharmacokinetics of metoprolol after single administration of a 14/190 metoprolol OROS system

Renal function is known to sometimes have a significant effect on the pharmacokinetics of drugs or drug metabolites, which are eliminated in appreciable amounts by the kidneys. For this reason, we conducted a study to compare the plasma concentration profiles of metoprolol and its metabolite, alpha-hydroxymetoprolol (OH-metoprolol), in healthy volunteers and in renally impaired patients. Following a single oral dose of a 14/190 metoprolol OROS (oral osmotic) tablet, plasma metoprolol profiles were shown to be similar for both subject groups. However, in renally impaired patients, renal clearance of OH-metoprolol was reduced and mean plasma levels of OH-metoprolol were increased approximately two- to threefold in comparison with healthy volunteers. The accumulation of OH-metoprolol in plasma, however, is unlikely to contribute to the beta-blocking effect of metoprolol, since OH-metoprolol possesses only one tenth the activity of its parent compound.

Recent developments in hepatic drug oxidation. Implications for clinical pharmacokinetics

Cytochrome P450 (P450) is the collective term for a group of related enzymes or isozymes which are responsible for the oxidation of numerous drugs and other foreign compounds, as well as many endogenous substrates including prostaglandins, fatty acids and steroids. Each P450 is encoded by a separate gene, and a classification system for the P450 gene superfamily has recently been proposed. The P450 genes are assigned to families and subfamilies according to the degree of similarity of the amino acid sequences of the protein part of the encoded P450 isozymes. It is estimated that there are between 20 and 200 different P450 genes in humans. The human P450IID6 is a particular isozyme which has been extensively studied over the past 10 years. The P450IID6 is the target of the sparteine/debrisoquine drug oxidation polymorphism. Between 5 and 10% of Caucasians are poor metabolisers, and it has recently been shown that the P450IID6 enzyme is absent in the livers of these individuals. The defect has also been characterised at the DNA and messenger RNA (mRNA) level, and to date 3 different forms of incorrectly spliced P450IID6 pre-mRNAs have been identified in the livers of poor metabolisers. The P450IID6 has a broad substrate specificity and is known to oxidise 15 to 20 commonly used drugs. The metabolism of these drugs is therefore subjected to the sparteine/debrisoquine oxidation polymorphism. The clinical significance of this polymorphism for a particular drug is defined according to the usefulness of phenotyping patients before treatment. It is concluded that this strategy would be of potential value for tricyclic antidepressants, some neuroleptics (e.g. perphenazine and thioridazine) and some anti-arrhythmics (e.g. propafenone and flecainide). The P450IID6 displays markedly stereoselective metabolism and appears uninducible by common inducers like rifampicin and phenazone (antipyrine). With some substrates, such as imipramine, desipramine and propafenone, P450IID6 becomes saturated at therapeutic doses. Finally, its function is potently inhibited by many commonly used drugs, for example, quinidine. The most clinically relevant interaction in relation to P450IID6 function appears to be the potent inhibition by some neuroleptics of the metabolism of tricyclic antidepressants. No drug-metabolising P450 has been so well characterised at the gene, protein and functional levels as the P450IID6. This development is based on an extensive use of specific model drugs, the oxidation of which in vitro and in vivo is dependent on the function of P450IID6; it can be expected that other human drug-metabolising P450s will be similarly characterised in future.

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.

Determination of dextromethorphan metabolizer phenotype in healthy volunteers

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

The influence of cimetidine on debrisoquine 4-hydroxylation in extensive metabolizers

We have studied the effect of cimetidine (800 mg.day-1) administration for three days on debrisoquine 4-hydroxylation in nine healthy extensive metabolizers. The debrisoquine metabolic ratio was significantly increased (p less than 0.01), but the new ratios remained in the extensive metabolizer range (less than 12.6). These data suggest that pre-treatment with cimetidine in usual therapeutic doses will impair debrisoquine 4-hydroxylation, but not enough to alter the apparent oxidation phenotype.

Ifosfamide plasma clearance in relation to polymorphic debrisoquine oxidation

Ifosfamide (IF) pharmacokinetics and the plasma (NBP)-alkylating activity were determined in 33 patients with different tumours after the administration of IF as single-agent chemotherapy. All subjects had been phenotyped for debrisoquine oxidation. There is a lack of correlation between the debrisoquine metabolic ratio (DMR) and either the total plasma clearance of IF (CLIF) or the AUC of the plasma NBP-alkylating activity.

Hepatic cytochrome P450-mediated oxidation function in migraine

Hepatic cytochrome P450-dependent oxidation is deficient in 5% to 10% of the Caucasian population. A similar percentage seems to suffer from migraine. The hypothesis was tested that an oxidation deficiency possibly relevant to potential dietary triggers plays a role in the pathogenesis of migraine. In 37 migraine sufferers and 26 controls age- and sex-matched to 26 of these patients, debrisoquine hydroxylation following an oral dose of 10 mg was employed as a marker of oxidation status, determined by calculating the metabolic ratio (MR): urinary debrisoquine/urinary 4-hydroxydebrisoquine. MR was similarly distributed in migraineurs and controls. Three subjects in each group were poor metabolizers (MR greater than 30, versus normal range, 0.1-12). MR in patients did not depend on type of migraine (common versus classic), attack frequency, the presence of trigger factors, smoking or a history of adverse reactions or sensitivity to medicines.

Polymorphism of the 4-hydroxylation of debrisoquine in the San Bushmen of southern Africa

1. The metabolic oxidation of debrisoquine has been studied in a group of 96 San Bushmen. 2. The amounts of debrisoquine and 4-hydroxy-debrisoquine excreted in 0-8 h urine were measured and the metabolic ratio (% dose as debrisoquine/% dose as 4-hydroxy-debrisoquine) calculated. 3. On the basis of Caucasian criteria, that metabolic ratios greater than 12.6 represent poor metabolizers, 19% of the Bushmen were poor metabolizers in contrast to the 8-10% found in Caucasian studies. 4. Probit plots showed four modes may be present in the data, which may represent at least three isozymes of the relevant enzyme which may also differ from the Caucasian isozymes.

Diurnal effects on debrisoquine hydroxylation phenotyping

A study was performed to show whether debrisoquine phenotyping could be performed as an overnight procedure. Phenotyping of 33 normal volunteers was carried out during the day and night. A good correlation was observed between the day- and night-time metabolic ratios, although wide variation was observed in 3 subjects.

Determination of debrisoquine metabolic ratio from hourly urine collections in healthy volunteers

The possibility of simplifying the regimen for the collection of urine samples in the determination of the debrisoquine metabolic ratio (DMR) was explored in 15 normal subjects. In the extensive metaboliser subgroup (EM; n = 11), there was a close correlation between the DMR as determined by an 8 h urine collection and the debrisoquine/4-hydroxydebrisoquine ratio (D/4-OHD) in the hourly samples (excluding the first hour). In the poor metabolisers (PM; n = 4) the phenotype could be identified, but it was not possible to estimate the DMR reliably.

Polymorphism of propafenone metabolism and disposition in man: clinical and pharmacokinetic consequences

The relationship between debrisoquine metabolic phenotype and the pharmacokinetics and pharmacodynamics of propafenone was studied in 28 patients with chronic ventricular arrhythmias (22 extensive metabolizers [EMs] and six poor metabolizers [PMs] of debrisoquine). EMs were characterized by a shorter propafenone elimination half-life (5.5 +/- 2.1 vs 17.2 +/- 8.0, p less than .001), lower average plasma concentration (Cp) (1.1 +/- 0.6 vs 2.5 +/- 0.5 ng/ml/mg daily dosage, p less than .001), and higher oral clearance (1115 +/- 1238 vs 264 +/- 48 ml/min, p less than .001). The active metabolite 5-hydroxypropafenone, assayed in 12 patients, was identified in nine of 10 EMs but in neither of the PMs. A lower incidence of central nervous system side effects was noted in EMs (14% vs 67%, p less than .01). The magnitude of QRS widening at any given propafenone Cp was greater in EMs than PMs. There was no significant difference between EMs and PMs in effective propafenone dose or frequency of antiarrhythmic response. Inhibition of debrisoquine 4-hydroxylation by propafenone was demonstrated both in vivo and in a human liver microsomal system in vitro. We conclude that propafenone is metabolized via the same cytochrome P-450 responsible for debrisoquine's 4-hydroxylation, and that its pharmacokinetics and concentration-response relationships and the incidence of central nervous system side effects are different in patients of different debrisoquine metabolic phenotype.

Pharmacogenetic differences in the inhibitory effect of cimetidine on the metabolism of antipyrine

The relationship between acetylator phenotype and the inhibitory effect of cimetidine on the hepatic metabolism of antipyrine has been studied in 20 subjects. Cimetidine, 1,0 g/day resulted in a significant decrease in the metabolic clearance rate of antipyrine, but only in slow acetylators, as fast acetylators were less affected. No sex difference was observed. No major change occurred in the urinary excretion of D-glucaric acid, which means that cimetidine had not-affected that Phase II reaction. It did significantly decrease the urinary partial clearance rate of norantipyrine, leaving that of antipyrine and 4-OH-antipyrine unchanged, which suggests that cimetidine had preferentially inhibited the P450 isozyme that catalyses norantipyrine formation.

Polymorphic drug oxidation: pharmacokinetic basis and comparison of experimental indices

The pharmacokinetic basis for using various experimental indices, (urinary drug: metabolite and metabolite:drug + metabolite ratios, urinary metabolite recovery and AUC values), for detecting polymorphic oxidative drug metabolism was examined. Pharmacokinetic determinants in addition to partial metabolic clearance down the polymorphic route were identified in each index. The ability of the various indices to discriminate bimodality in population data was assessed using a computer simulation. With the exception of the AUC data, bimodality was apparent to varying extents in all of the frequency distributions and, in general, logarithmic transformation allowed clearer visualisation of the two phenotypic groups. Simulated distributions were compared with those observed experimentally for metoprolol and its alpha-hydroxy metabolite. Detailed pharmacokinetic data from controlled studies in small numbers of volunteers can form the basis of the input to the simulation programme. Inspection of the output may help in the design of further studies in larger numbers of subjects in whom only limited data collection is possible.

Oxidative metabolism of encainide: polymorphism, pharmacokinetics and clinical considerations

The 8-h urinary metabolic profiles of encainide and its oxidized metabolites, O-desmethyl- (ODE), 3-methoxy-O-desmethyl- (MODE), N-desmethyl- (NDE) and N, O-didesmethyl- (DDE) encainide were studied in a group of 112 normal Caucasians. Nine of these subjects (8%) were defective in their ability to 4-hydroxylate debrisoquine. The cumulative frequency distribution of the 8-h recovery ratio of encainide/ODE indicated two distinct populations in complete concordance with the debrisoquine phenotyping. The subjects with an 'extensive metabolizer' (EM) phenotype had a ratio from 0.003 to 0.9 whereas the PM group had values from 7.4 to 48. In addition, no MODE was detected in the urine from 'poor metabolizers' (PM). The oxidative metabolism of encainide, specifically the O-demethylation pathway, is, therefore, polymorphically distributed and controlled by the same genetic factor(s) that determine the 4-hydroxylation of debrisoquine. In EM subjects, ODE and MODE are the major metabolites in plasma and their concentrations are much greater than those of unchanged drug. As ODE is a more potent antiarrhythmic agent than encainide and MODE is at least equipotent, these metabolites significantly contribute to the overall antiarrhythmic effect in EM patients. The low plasma concentrations of ODE and MODE in PM subjects would be expected to result in inefficacious therapy when usual doses of encainide are administered. However, in such individuals, chronic oral therapy results in accumulation of unmetabolized encainide to far higher levels than in EM subjects. As encainide itself has intrinsic antiarrhythmic activity at these concentrations, this generally results in the desired clinical response. Despite pronounced interphenotypic differences in encainide's disposition and pharmacokinetics, the polymorphic oxidative metabolism appears to have limited consequences for the drug's clinical efficacy.

Genetics of drug transformation

Biotransformations of drugs are controlled or strongly affected by genetic factors. During the past few years several genetic deficiencies of drug-metabolizing reactions catalyzed by members of the family of cytochrome P-450 were observed. Choice of the appropriate drug to study and attention to urinary metabolites have been the essential ingredients for the recent discovery of genetic deficiencies of drug metabolism in man which include recessive deficiency of debrisoquine/sparteine metabolism and of mephenytoin metabolism. The clinical significance of these defects is discussed. Ethanol after metabolism to acetaldehyde is further metabolized to acetic acid by aldehyde dehydrogenase. Numerous isozymes of aldehyde dehydrogenase exist, one of which possesses a high affinity for acetaldehyde. Approximately 40% of the Oriental population lack this high affinity isozyme so that in these individuals who may have symptoms of flushing and other unpleasant effects the acetaldehyde formed is destroyed only at high plasma concentrations.

Brief debrisoquin administration to assess central dopaminergic function in children

Central dopaminergic (DA) function in children was assessed by monitoring plasma-free homovanillic acid (pHVA) levels after brief (18 hour) administration with debrisoquin sulfate, a peripherally active antihypertensive agent that blocks peripheral, but not central, HVA production. Brief debrisoquin administration resulted in marked reductions in pHVA in each of six patients studied. In five of the six patients, post-debrisoquin pHVA levels remained relatively stable over the six-hour period of observation. No significant cardiovascular or behavioral side effects of debrisoquin were observed. The brief debrisoquin administration method appears to be a safe, simple, and potentially valid peripheral technique for evaluating aspects of central dopaminergic function in children with neuropsychiatric disorders. Additional work is needed to further establish this method's validity and reliability.

Assessment of central dopaminergic function using plasma-free homovanillic acid after debrisoquin administration

Central dopaminergic (DA) function in children and adults was assessed by monitoring plasma-free levels of the dopamine metabolite homovanillic acid (pHVA) before and after a single oral dose and chronic oral administration of debrisoquin. Debrisoquin inhibits peripheral metabolism of dopamine to HVA and does not cross the blood-brain barrier. By reducing peripheral formation of HVA through the use of debrisoquin, the remaining HVA in plasma more accurately reflects central DA activity. Debrisoquin administration resulted in marked reductions of pHVA in each of 12 patients studied. Eleven of the 12 subjects tolerated debrisoquin without physical or behavioral side effects. The debrisoquin administration method appears to be a safe and potentially valid technique for evaluating aspects of central dopaminergic function in children and adults.

Plasma concentrations of nortriptyline and its 10-hydroxy metabolite in depressed patients--relationship to the debrisoquine hydroxylation metabolic ratio

In 20 depressed patients treated with nortriptyline (NT) there was a significant relationship between the plasma concentration of NT and the debrisoquine metabolic ratio (rs = 0.77; P less than 0.01). (The debrisoquine test was performed after stopping NT treatment). This is in agreement with the hypothesis that the hydroxylations of NT and debrisoquine are mediated by similar enzymatic mechanisms. In contrast there was no significant relationship between the debrisoquine metabolic ratio and the plasma concentrations of the active metabolite 10-hydroxy-nortriptyline. In 11 of the patients the debrisoquine metabolic ratio was significantly higher during than after NT treatment. This may be due to an inhibition of the debrisoquine hydroxylation by NT.

Oral morphine in cancer patients: in vivo kinetics and in vitro hepatic glucuronidation

The kinetics of morphine and formation of the main metabolite, morphine-3-glucuronide (M3G) after single and intravenous doses of morphine were studied in six cancer patients and compared with the formation rate of M3G in vitro in microsomes isolated from liver biopsies obtained from the same patients at palliative laparotomy. The results showed that high formation rates of M3G in vitro in microsomes isolated from liver biopsies were associated both with high apparent oral clearance values and high M3G/morphine AUC (area under the concentration vs time curve) ratios as measured in vivo in the same patients. In accordance with previous results marked interindividual differences were seen in the kinetics of morphine; the oral bioavailability varied between 30 and 69% and the systemic plasma clearance between 18.6 and 34.0 ml min-1 kg-1. This variation correlated with the variation in morphine metabolism as assessed in vitro. In vivo, a high M3G/morphine AUC ratio predicted a high oral clearance. Hepatic UDP-glucuronyl transferase activity is thus an important determinant of the in vivo kinetics of orally administered morphine.

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