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

Development of a V79 cell line expressing human cytochrome P450 2D6 and its application as a metabolic screening tool

Expression of human cytochrome P450 (CYP) in heterologous cells is a means of specifically studying the role of these enzymes in drug metabolism. The complete cDNA encoding CYP2D6-VAL(374) was inserted into an expression vector containing the strong mycloproliferative sarcoma virus promotor in combination with the enhancer of the cytomegalovirus and stably expressed in V79 Chinese hamster cells. The presence of genomically integrated CYP2D6 cDNA was confirmed by polymerase chain reaction analysis. The protein expression was shown by Western blotting. Functional expression could be demonstrated by O-demethylation of dextromethorphan to dextrorphan in live cells. The enzymatic activity of 154 ± 16 pmol min(-1) mg(-1) protein was comparable with dextromethorphan-O-demethylation activities of human liver. The metabolism of two dopaminergic ergoline derivatives was investigated in whole recombinant V19 cells. Both lisuride and terguride were monodeethylated; in case of lisuride a correlation to the in vivo situation was demonstrated comparing poor and extensive metabolizers.

Pharmacogenetics: a laboratory tool for optimizing therapeutic efficiency

Pharmacogenetics is the study of the linkage between an individual’s genotype and that individual’s ability to metabolize a foreign compound. Differences in metabolism of therapeutics can lead to severe toxicity or therapeutic failure by altering the relation between dose and blood concentration of the pharmacologically active drug. Phenotypes exhibiting poor and ultraextensive metabolism result from genetic variance (polymorphism) of enzymes involved in metabolism. Thus, in pharmacogenetic studies one applies genotyping of polymorphic alleles encoding drug-metabolizing enzymes to the identification of an individual’s drug metabolism phenotype. This knowledge, when applied to dosing or drug selection, can avoid adverse reactions or therapeutic failure and thus enhance therapeutic efficiency. More than 25 commonly prescribed medicines are metabolized by the cytochrome P-4502D6 (CYP2D6) isoenzyme, and polymorphism of the CYP2D6 gene affects the therapeutic management of up to 17% of individuals in some ethnic groups. In this review, we summarize and update information concerning drug-metabolizing genotypes with emphasis on CYP2D6 genotyping techniques that can be applied by the clinical laboratory for linking human genetics to therapeutic management.

Pharmacogenetics in pediatrics. Implications for practice

Cumulative experience with pharmacotherapy in children indicates that it is difficult to prescribe medications rationally solely on the basis of patient age. Furthermore, the apparent drug biotransformation phenotype may be influenced by disease (e.g., infection), environmental factors (e.g., diet and environmental contaminants), and concurrent medications. Therefore, characterization of drug biotransformation pathways during development and, at a given developmental stage, the effects of known modulators of drug biotransformation are essential for optimum treatment. This is particularly true when one considers that altered drug biotransformation may contribute significantly to therapeutic failure (e.g., graft rejection with inadequate serum and tissue concentrations of cyclosporin and myelotoxicity consequent to a relative inability to metabolize normal doses of certain antineoplastic agents). Accordingly, the goals of coordinated clinical and basic investigations should be to characterize important drug biotransformation pathways for compounds under development and intended for use in pediatrics and to identify the population extremes or "outliers" to aid in selection of an appropriate dosage range for efficacy studies. Acquired knowledge should then be incorporated into the drug-design process to further maximize the efficacy-toxicity ratio. The development of acceptable, preferably noninvasive, phenotyping procedures for all age ranges including neonates, infants, and older children is a major challenge for investigators but, if met, will be rewarded with improved pediatric pharmacotherapy.

Genetic predisposition to drug-induced hepatotoxicity

Drug-induced hepatitis is uncommon and generally unpredictable. Hepatotoxicity may be related to the drug itself, or to chemically reactive metabolites which can bind covalently to hepatic macromolecules and may lead to either idiosyncratic, toxic hepatitis or to immunoallergic hepatitis. There is now evidence indicating that genetic variations in systems of biotransformation or detoxication may modulate either the toxic or sensitizing effects of some drugs. Thus, the genetic deficiency in a particular hepatic cytochrome P 450 isozyme (CYP 2D6) is involved in per-hexiline liver injury. The deficiency in CYP 2C19 might also contribute to Atrium hepatotoxicity. Slow acetylation related to N-acetyltransferase 2 deficiency contributes to sulfonamide hepatitis. The genetic deficiency in glutathione synthetase may increase the susceptibility to several drugs including acetaminophen. A constitutional deficiency in another cell defense mechanism, still not characterized, seems to increase significantly the risk of hepatotoxicity with halothane, phenytoin, carbamazepine, phenobarbital, sulfamides and amineptine.

Molecular mechanisms of genetic polymorphisms of drug metabolism

One of the major causes of interindividual variation of drug effects is genetic variation of drug metabolism. Genetic polymorphisms of drug-metabolizing enzymes give rise to distinct subgroups in the population that differ in their ability to perform certain drug biotransformation reactions. Polymorphisms are generated by mutations in the genes for these enzymes, which cause decreased, increased, or absent enzyme expression or activity by multiple molecular mechanisms. Moreover, the variant alleles exist in the population at relatively high frequency. Genetic polymorphisms have been described for most drug metabolizing enzymes. The molecular mechanisms of three polymorphisms are reviewed here. The acetylation polymorphism concerns the metabolism of a variety of arylamine and hydrazine drugs, as well as carcinogens by the cytosolic N-acetyltransferase NAT2. Seven mutations of the NAT2 gene that occur singly or in combination define numerous alleles associated with decreased function. The debrisoquine-sparteine polymorphism of drug oxidation affects the metabolism of more than 40 drugs. The poor metabolizer phenotype is caused by several "loss of function" alleles of the cytochrome P450 CYP2D6 gene. On the other hand, "ultrarapid" metabolizers are caused by duplication or amplification of an active CYP2D6 gene. Intermediate metabolizers are often heterozygotes or carry alleles with mutations that decrease enzyme activity only moderately. The mephenytoin polymorphism affects the metabolism of mephenytoin and several other drugs. Two mutant alleles of CYP2C19 have so far been identified to cause this polymorphism. These polymorphisms show recessive transmission of the poor or slow metabolizer phenotype, i.e. two mutant alleles define the genotype in these individuals. Simple DNA tests based on the primary mutations have been developed to predict the phenotype. Analysis of allele frequencies in different populations revealed major differences, thereby tracing the molecular history and evolution of these polymorphisms.

Pharmacokinetic changes in the elderly. Do they contribute to drug abuse and dependence?

The elderly frequently use psychoactive drugs including alcohol (ethanol), benzodiazepines and opioid analgesics, which have a propensity to cause abuse and dependence. Theoretically, the changes in pharmacokinetics of these agents in the elderly may modify their abuse and dependence potential. In the elderly, blood alcohol concentrations following an oral dose are higher, alcohol withdrawal syndrome follows a more severe and protracted clinical course and requires treatment with higher doses of chlordiazepoxide than needed for younger adults. However, there is no direct evidence that supports an increased direct abuse and dependence potential of alcohol because of its altered kinetics in the elderly. In the case of oxidatively metabolised benzodiazepine, both age-related pharmacokinetics and pharmacodynamic changes may increase their clinical effects in the elderly. The hypothesis that benzodiazepines have an increased abuse and dependence potential in the elderly has not been tested. Many of the benzodiazepines (e.g. alprazolam, triazolam and midazolam) are metabolised by the cytochrome P450 (CYP)3A subfamily. The pharmacokinetics of these agents may be modified by inhibition of CYP3A due to concurrently administered medications such as selective serotonin reuptake inhibitors. Unfortunately, data on the direct measures of abuse and dependence potential of benzodiazepines are not available in the elderly. Thus, a conclusive statement on the contribution of age-related pharmacokinetic changes to benzodiazepine abuse and dependence cannot be made at the present time. The clinical effects of codeine do not appear to change with age. Codeine is O-demethylated to its active metabolite morphine by the genetically polymorphic CYP2D6 isozyme. The activity of this isozyme is unaltered by age, gender or smoking habits; however, it is subject to potent inhibition by some of the frequently used medications in the elderly, such as the antidepressants paroxetine and fluoxetine. This may result in an impairment in O-demethylation of codeine to morphine and may lead to a decrease in the abuse and dependence potential of codeine. Conversely, those with a very rapid CYP2D6 catalytic activity may have an increased potential for codeine abuse and dependence. The clinical significance of age-related pharmacokinetic changes should be evaluated within the context of clinical practice. Most physicians are inclined to prescribe lower doses to the elderly, which may offset the potential impact of altered pharmacokinetics on the abuse and dependence potential of psychoactive agents. In summary, the available data are not sufficient for a definitive conclusion on whether the pharmacokinetic changes in the elderly translate to an increase in the abuse and dependence potential of alcohol, benzodiazepines or opioids. In particular, the data on age-associated changes in direct measures of abuse potential of these agents are missing. Future comparative systemic pharmacokinetic-pharmacodynamic studies assessing pertinent outcome measures on abuse and dependence potential of commonly used psychoactive drugs are required to resolve the ongoing controversy on risk factors for drug abuse and dependence in the elderly.

Metabolic ratios of four probes of CYP2D6 in Turkish subjects: a cross-over study

The relationships among the metabolic ratios for the standard probe drugs of CYP2D6 activity, such as debrisoquine, sparteine, metoprolol and dextromethorphan, were studied in 32 Turkish subjects. All subjects were randomly selected according to their phenotypes from a group of 111 Turkish subjects whose oxidation status had been tested for debrisoquine previously. All subjects were given a 10 mg debrisoquine tablet, a 100 mg sparteine tablet, a 100 mg. metoprolol tablet and a 20 mg dextromethorphan capsule orally with a wash-out period of at least 1 week between each probe administration. Metabolic ratios were calculated as percentage of dose excreted as parent drug/percentage of dose excreted as its hydroxymetabolite of parent drug in 0-8 h urine. Three poor metabolisers (PM) of debrisoquine were identified. They were also PMs of the other test probes and no misclassification by the 4 phenotyping methods was observed. All six correlations among the metabolic ratios of the 4 probe drugs assessed by Spearman's rank test were highly significant (P < 0.001). The present findings indicate that the oxidative metabolism of debrisoquine, sparteine, metoprolol and dextromethorphan is catalysed by the same cytochrome P450 in the Turkish subjects.

Cytochrome P450 enzymes: interpretation of their interactions with selective serotonin reuptake inhibitors. Part II

The SSRIs have been used as an example to show how one might interpret the available evidence to draw conclusions about the relationships between drugs and P450s. Under what circumstances might one apply the knowledge of such relationships? First, the clinical implications must be considered when drugs with a narrow therapeutic index are coprescribed with other drugs that may affect P450s. For example, good clinical practice demands that before a TCA is coprescribed with another drug, the physician be aware of the potential for the second drug to interact with CYP2D6. Second, it may be helpful to consider P450 enzymes when adverse events occur during polypharmacy. It may happen that a known side effect of one drug occurs. Rather than attributing this to patient sensitivity, the physician should consider the possibility that a pharmacokinetic drug interaction increased plasma drug concentration, which in turn enhanced the probability of such an occurrence. Even when a pharmacokinetic drug interaction is considered as a possible cause, an appreciation of the role of P450s may lead to the realization that an interaction was not only possible but that it was likely. Finally, copharmacy can be used intentionally to produce controlled interactions. Indeed, planned pharmacokinetic drug interactions at the level of P450s have been proposed to reduce cyclosporine dosage requirements, to reduce variability of TCA levels, and to manipulate the contribution of alternative metabolic pathways to minimize toxic effects. As long as pharmaceuticals are metabolized by the P450 system, interactions with the various isozymes will be inescapable. It is fortunate that understanding them is becoming more tractable.

Dextromoramide pharmacokinetics following sublingual administration

The extent of absorption and other pharmacokinetic parameters of dextromoramide following sublingual administration were assessed in five patients receiving chronic opioid analgesia. The use of the standard 5 mg tablet formulation was associated with negligible absorption in two patients, a prolonged time to peak concentration in the other three and substantial variability in clearance. The study concluded that the standard tablet formulation cannot be recommended for sublingual use where reliable, rapid onset analgesia is required.

Attenuation of morphine-induced delirium in palliative care by substitution with infusion of oxycodone

We have observed among patients of the Southern Community Hospice Programme that up to 25% experience acute delirium when treated with morphine and improve when the opioid is changed to oxycodone or fentanyl. This study aimed to confirm by a prospective trial that oxycodone produces less delirium than morphine in such patients. Oxycodone was administered by a continuous subcutaneous infusion, as this allowed more flexible and reliable dosing, and patients were monitored for any adverse reactions to the drug. Thirteen patients completed the study. Statistically significant improvements in mental state and nausea and vomiting occurred following a change from morphine to oxycodone. Pain scores improved but did not reach a level of statistical significance. The phenotype status of the patients was tested to establish their capacity to metabolize oxycodone. One patient who did not achieve adequate pain control proved to be a poor metabolizer. These results show that oxycodone administered by the subcutaneous route can provide effective analgesia without significant side effects in patients with morphine-induced delirium. This treatment allows patients to remain more comfortable and lucid in their final days. A small proportion of patients who do not metabolize oxycodone effectively may not receive this benefit.

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSION

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

Mutation frequencies of the cytochrome CYP2D6 gene in Parkinson disease patients and in families

The frequencies of five mutations of the debrisoquine 4-hydroxylase (CYP2D6) gene (mutations D6-A, B, C, D, and T), corresponding to poor metabolizer (PM) phenotypes, were determined by restriction fragment length polymorphism (RFLP) and polymerase chain reaction (PCR) in 47 patients with Parkinson disease, and compared with the findings in 47 healthy controls. These mutant alleles were about twice as frequent among patients as in controls, with an approximate relative risk ratio of 2.12 (95% confidence interval, 1.41-2.62). There seem to be no significant differences in frequencies of mutant genotypes in patients among gender and modalities of response with levodopa therapy; but frequency of the mutations was slightly enhanced after age-at-onset of 60 years. Mutations D6-B, D, and T were detected in 7 patients belonging to 10 Parkinson pedigrees.

Influence of amino acid residue 374 of cytochrome P-450 2D6 (CYP2D6) on the regio- and enantio-selective metabolism of metoprolol

Cytochrome P-450 2D6 (CYP2D6) is an important human drug-metabolizing enzyme responsible for the oxidation of more than 30 widely used therapeutic agents. The enzymes encoded by the published genomic [Kimura, Umeno, Skoda, Meyer and Gonzalez (1989) Am. J. Hum. Genet. 45, 889-904] and cDNA [Gonzalez, Skoda, Kimura, Umeno, Zanger, Nebert, Gelboin, Hardwick and Meyer (1988) Nature 331, 442-446] sequences of CYP2D6, and presumed to represent wild-type sequences, differ at residue 374 and encode valine (CYP2D6-Val) and methionine (CYP2D6-Met) respectively. The influence of this amino acid difference on cytochrome P-450 expression, ligand binding, catalysis and stereoselective oxidation of metoprolol was investigated by the heterologous expression of the corresponding cDNAs in the yeast Saccharomyces cerevisiae. The level of expression of apo- and holo-protein was similar with each form of CYP2D6 cDNA, and the binding affinities of a series of ligands to CYP2D6-Val and CYP2D6-Met were identical. The enantioselective O-demethylation and alpha-hydroxylation of metoprolol were also similar with each form of CYP2D6, O-demethylation being R-(+)- enantioselective (CYP2D6-Val: R/S, 1.6; CYP2D6-Met: R/S, 1.4), whereas alpha-hydroxylation showed a preference for S-(-)-metoprolol (CYP2D6-Val: R/S, 0.7; CYP2D6-Met: R/S, 0.8). However, although the favoured regiomer overall was O-demethylmetoprolol (ODM), the regioselectivity for O-demethylation of each metoprolol enantiomer was significantly greater for CYP2D6-Val [R-(+)-: ODM/alpha-hydroxymetoprolol (alpha OH), 5.9; S-(-)-: ODM/alpha OH, 2.5) than that observed for CYP2D6-Met [R-(+)-: ODM/alpha OH, 2.2; S-(-)-: ODM/alpha OH, 1.4]. The stereoselective properties of CYP2D6-Val were consistent with those observed for CYP2D6 in human liver microsomes. The difference in the stereoselective properties of CYP2D6-Val and CYP2D6-Met were rationalized with respect to a homology model of the active site of CYP2D6 based on an alignment with the crystal structure of the bacterial cytochrome P-450BM-3' CYP102.

Assessment of individual CYP2D6 activity in extensive metabolizers with renal failure: comparison of sparteine and dextromethorphan

OBJECTIVES

To examine whether the variability of CYP2D6 activity in patients with chronic renal failure can be assessed, particularly among subjects with the extensive metabolizer phenotype, by use of standard in vivo indexes of CYP2D6 activity derived from oral administration of dextromethorphan and sparteine.

METHODS

A single 100 mg oral dose of sparteine and a single 40 mg oral dose of dextromethorphan were administered on two occasions to 12 patients with chronic renal failure (creatinine clearance ranging from 20 to 70 ml/min) and 12 age- and sex-matched healthy subjects. Sparteine clearances, sparteine metabolic ratio, and urinary recovery of dextrorphan were calculated. Patients and healthy control subjects were not selected on the basis of their CYP2D6 phenotypes.

RESULTS

Chronic renal failure was associated with a decrease in sparteine partial metabolic clearance to dehydrosparteine (median of 322 ml/min and range of 62 to 670 ml/min in patients with renal failure versus median of 635 ml/min and range of 77 to 1276 ml/min in normal subjects; p < 0.02). Sparteine apparent oral clearance (p < 0.03) and renal clearance (p < 0.001) decreased in patients with renal failure. However, sparteine metabolic ratio was not significantly altered in patients with renal failure and showed that all patients were extensive metabolizers of sparteine. Although fractional urinary excretion of dextrorphan decreased in patients with renal failure (median, 24.4%; range, 9.7% to 55.9%) compared with control (median, 47.5%; range, 24.1% to 72.1%) (p = 0.02), it also showed that all subjects were extensive metabolizers of dextromethorphan. The amount of dextromethorphan excreted in urine correlated with creatinine clearance independently from CYP2D6 activity measured as sparteine partial metabolic clearance. However, it did not correlate with sparteine metabolic ratio or with fractional urinary excretion of dehydrosparteine.

CONCLUSION

Assessment of CYP2D6 activity by use of dextromethorphan and sparteine is possible in extensive metabolizer patients with chronic renal failure. However, in these subjects, dextromethorphan and sparteine do not reflect CYP2D6 activity in the same way.

Pharmacokinetics of selective serotonin reuptake inhibitors: clinical relevance

Selective serotonin reuptake inhibitors (SSRIs) are a safe and effective class of drugs for treatment of depressive and obsessive-compulsive disorders. Among this class of drugs, pharmacodynamic actions, antidepressant efficacy and adverse effect profiles are remarkably similar. However, pharmacokinetic profiles of SSRIs are substantially different especially with respect to pharmacokinetically mediated drug-drug interactions. For example, fluoxetine and paroxetine produce clinically significant inhibition of cytochrome P450 2D6 at their usually effective antidepressant dose, whereas citalopram, fluvoxamine or sertraline do not. There is also a substantial difference between SSRIs with respect to their capacity to inhibit other cytochrome P450 enzymes including IA2, 2C19, 3A4 and possibly 2C9/10. The inhibition of these enzymes can reduce the clearance of concomitantly administered drugs which are dependent on oxidative metabolism mediated by these enzymes as a necessary prerequisite for their subsequent elimination. The accumulation of unusually high levels of such drugs can result in an increase in nuisance and/or more serious, even life-threatening, adverse effects depending on the pharmacology of the co-prescribed drug. Knowledge of these issues will enable clinicians to predict and make appropriate dose adjustments to avoid potential drug-drug interactions that otherwise could result in toxicity.

Determination of dextromethorphan metabolic phenotype by salivary analysis with a reference to genotype in Chinese patients receiving renal hemodialysis

BACKGROUND

The polymorphic metabolism of debrisoquin and sparteine by cytochrome P450IID6 (CYP2D6) is genetically determined. Determination of the CYP2D6 metabolic phenotype with conventional urine analytic methods is not feasible in anuric patients with renal failure. The possibility of using salivary analysis, with dextromethorphan as a probe drug, to determine the CYP2D6 metabolic phenotype in patients with renal failure was evaluated.

METHODS AND RESULTS

One hundred four Chinese patients with renal failure were recruited. All 104 patients were receiving hemodialysis. Saliva was collected before and at 3 hours after each patient took a capsule of dextromethorphan hydrobromide (30 mg). Four patients were excluded because of insufficient samples of saliva. The distribution of logarithms of the metabolic ratios (log[MR]) in the 100 patients appeared to be normal. Administration of quinidine sulfate (200 mg twice daily) to nine of the patients significantly and markedly increased the dextromethorphan metabolic ratios. The metabolic ratios of nine patients pretreated with quinidine were higher than any of the 100 patients with renal failure who did not receive quinidine pretreatment. A metabolic ratio of 33 separated these two groups. Genomic deoxyribonucleic acid was extracted from whole blood in a subset of patients. Polymerase chain reaction (PCR)-based methods were used to detect the CYP2D6 and B mutant genes. Mutant B alleles (which are common in white poor metabolizers) of CYP2D6 genes were not detected in any of the 47 subjects tested. A PCR-based test of cytosine (C188) to thymine (T188) polymorphism at 188 base pairs in exon 1 of CYP2D6 genes was performed in 61 patients. Subjects who were homozygous for C188 had significantly (p = 0.0067) lower log[MR] values than those who were homozygous for T188.

CONCLUSIONS

Determination of dextromethorphan metabolic ratios in saliva is feasible in patients with renal failure requiring hemodialysis. All subjects in this study appeared to be "extensive metabolizer" phenotype for CYP2D6, and no poor metabolizer was identified. From the results with quinidine pretreatment, a metabolic ratio of 33 is suggested to be a tentative antimode for identification of poor metabolizers in patients with renal failure.

Prediction of CYP2D6-mediated polymorphic drug metabolism (sparteine type) based on in vitro investigations

Discovery of genetic polymorphism in drug metabolism has contributed a great deal to understanding the variability in dose-concentration relationships introduced by genetic factors, thereby elucidating the mechanisms responsible for unexpected drug reactions. This knowledge should find its way into clinical practice in order to make therapy more efficient and safe. Moreover, genetic factors in drug metabolism should be taken into account during drug development. Therefore, in vitro methods for identifying the metabolic pattern of new compounds during early stages of drug development should be improved. This review summarizes in vitro methods available to identify genetic polymorphism in drug oxidation, in particular the CYP2D6-related polymorphism.

Use of electrospray ionization liquid chromatography-mass spectrometry to study the role of CYP2D6 in the in vitro metabolism of 5-hydroxytryptamine receptor antagonists

An electrospray ionization liquid chromatographic-mass spectrometric (ESI-LC-MS) method has been developed to study the involvement of the cytochrome P450 isoenzyme CYP2D6 in the in vitro metabolism of the indole containing 5-hydroxytryptamine (5-HT(3)++) receptor antagonists tropisetron, ondansetron and dolasetron in human liver microsomes. Compounds were eluted using linear gradients of acetonitrile-20 mM ammonium acetate, solvent A, (10:90, v/v) (pH 6.0) and solvent B, (60:40, v/v) (pH 6.0) and a Nucleosil C(4) column. Microsomal incubations were analysed using selected ion monitoring of the molecular ion of parent drug and the molecular ion of hydroxylated metabolites. The involvement of CYP2D6 in drug metabolism was assessed by inhibition studies using quinidine (5 mu M), a specific inhibitor of human CYP2D6, as well as by incubating compounds with microsomes prepared from cells transfected with cDNA encoding human CYP2D6. Results showed that the oxidation of all three compounds involved CYP2D6, but only that of tropisetron was inhibited by over 90% in the presence of quinidine. The present method can be applied to pre-clinical compounds, at an early stage of drug discovery, to assess the involvement of CYP2D6 in their metabolism and to screen for those compounds where CYP2D6 is the only isoenzyme implicated in the formation of major metabolites.

Plasma levels of the enantiomers of thioridazine, thioridazine 2-sulfoxide, thioridazine 2-sulfone, and thioridazine 5-sulfoxide in poor and extensive metabolizers of dextromethorphan and mephenytoin

Concentrations of total (R) + (S) and of the enantiomers (R) and (S) of thioridazine and metabolites were measured in 21 patients who were receiving 100 mg thioridazine for 14 days and who were comedicated with moclobemide (450 mg/day). Two patients were poor metabolizers of dextromethorphan and one was a poor metabolizer of mephenytoin. Cytochrome P450IID6 (CYP2D6) is involved in the formation of thioridazine 2-sulfoxide (2-SO) from thioridazine and also probably partially in the formation of thioridazine 5-sulfoxide (5-SO), but not in the formation of thioridazine 2-sulfone (2-SO2) from thioridazine 2-SO. Significant correlations between the mephenytoin enantiomeric ratio and concentrations of thioridazine and metabolites suggest that cytochrome P450IIC19 could contribute to the biotransformation of thioridazine into yet-unknown metabolites, other than thioridazine 2-SO, thioridazine 2-SO2, or thioridazine 5-SO. An enantioselectivity and a large interindividual variability in the metabolism of thioridazine have been shown: measured (R)/(S) ratios of thioridazine, thioridazine 2-SO fast eluting (FE), thioridazine 2-SO slow eluting (SE), thioridazine 2-SO (FE+SE), thioridazine 2-SO2, thioridazine 5-SO(FE), and thioridazine 5-SO(SE) were (mean +/- SD) 3.48 +/- 0 .93 (range, 2.30 to 5.80), 0.45 +/- 0.22 (range, 0.21 to 1.20), 2.27 +/- 8.1 (range, 6.1 to 40.1), 4.64 +/- 0.68 (range, 2.85 to 5.70), 3.26 +/- 0.58 (range, 2.30 to 4.30), 0.049 +/- 0.019 (range, (0.021 to 0.087), and 67.2 +/- 66.2 (range, 16.8 to 248), respectively. CYP2D6 is apparently involved in the formation of (S)-thioridazine 2-SO(FE), (R)-thioridazine 2-SO(SE), and also probably (S)-thioridazine 5-SO(FE) and (R)-thioridazine 5-SO(SE).

Venlafaxine oxidation in vitro is catalysed by CYP2D6

1. Several selective 5-HT reuptake inhibitors (SSRIs) are inhibitors of the genetically polymorphic drug metabolizing enzyme, CYP2D6. We studied the interaction of venlafaxine, a new SSRI, with CYP2D6 in human liver microsomes. 2. Venlafaxine was a less potent inhibitor of this enzyme activity in vitro than other SSRIs tested. The average apparent Ki values determined using CYP2D6-dependent dextromethorphan O-demethylation were: 33, 52 and 22 microM for rac-venlafaxine, R(+)-venlafaxine and S(-)-venlafaxine, respectively, vs 0.065 to 1.8 microM for paroxetine, fluoxetine, norfluoxetine, fluvoxamine and sertraline. 3. Microsomes from human livers (n = 3) and from yeast transformed with an expression plasmid containing human CYP2D6 cDNA catalyzed the O-demethylation of venlafaxine, which is the major metabolic pathway in vivo. Intrinsic metabolic clearance values (Vmax/Km) indicated that S(-)-venlafaxine was cleared preferentially via this pathway. 4. In microsomes from CYP2D6-deficient livers (n = 2), Vmax/Km of O-demethylation of venlafaxine was one to two orders of magnitude lower and was similar to the rate of N-demethylation. 5. Studies with chemical probes which preferentially inhibit P450 isoforms suggested that CYP3A3/4 is involved in venlafaxine N-demethylation. 6. These in vitro findings predict phenotypic differences in the kinetics of venlafaxine in vivo, although the clinical importance of this is unclear as O-demethylvenlafaxine is pharmacologically similar to the parent drug. The findings also predict relatively limited pharmacokinetic interaction between venlafaxine and other CYP2D6 substrates.

Relationship between mephenytoin, phenytoin and tolbutamide hydroxylations in healthy African subjects

Mephenytoin, phenytoin and tolbutamide are metabolised by the cytochrome P-450 (CYP) 2C family. Recently, it has been shown that phenytoin and tolbutamide are metabolised by CYP2C9/10 whereas mephenytoin is metabolised by CYP2C19. Until now, in vivo studies were only undertaken in Caucasian subjects and showed a strong relationship between phenytoin and tolbutamide metabolism but no significant relationship between the two drug metabolisms and that of mephenytoin. The metabolism of the three drugs was investigated in eight black Africans by urinary analysis. In this ethnic group, a strong relationship was found between phenytoin and tolbutamide oxidations (rs = -0.83, P = 0.01). On the other hand, no significant relationship was found between mephentoin oxidation and phenytoin or tolbutamide oxidations (rs = 0.31 and rs = -0.33, respectively). This study suggests that, in black Africans, phenytoin and tolbutamide but not mephenytoin are also hydroxylated by similar CYP enzyme(s).

Phenobarbital induces the 2-hydroxylation of desipramine

The kinetics of a single oral dose of desipramine (DMI; 100 mg) were studied in eight epileptic patients chronically treated with phenobarbital (PB) and in eight drug-free healthy controls. All subjects were extensive metabolizers with respect to the genetically determined CYP2D6-related metabolic polymorphism. Compared with controls, epileptic patients exhibited lower peak plasma DMI concentrations (74 +/- 24 vs. 107 +/- 32 nmol/L; means +/- SD, p < 0.05), smaller DMI area-under-the-curve values (1,943 +/- 461 vs. 3,234 +/- 1,145 nmol L-1 h; p < 0.01), and shorter DMI elimination half-lives (15.1 +/- 2.1 vs. 20.6 +/- 3.4 h; p < 0.01). The proportion of the dose excreted as 2-hydroxydesipramine (2-OH-DMI) was significantly higher in the patients (54 +/- 8 vs. 40 +/- 9%; p < 0.05). In one single poor metabolizer volunteer, a 3-week treatment with PB was associated with no major changes in DMI kinetics, but the urinary excretion of 2-OH-DMI tended to increase. These results suggest that PB is an inducer of the 2-hydroxylation of DMI, a reaction primarily catalyzed by CYP2D6, but do not provide further information on the specific P450 isoenzyme(s) being induced.

The assessment of flavin-containing monooxygenase activity in intact animals

A large number of drug metabolising enzymes with different substrate specificities and induction and inhibition characteristics have been described, suggesting that specific test drugs, i.e. probes, should be used for assessing the activity of distinct metabolising enzymes. The flavin-containing monooxygenase (FMO) and cytochrome P-450 (P-450) are the two main microsomal enzyme systems involved in the oxidation of xenobiotics. FMO is present in liver and other tissues of most vertebrates. It catalyses the oxidation of a wide range of xenobiotics, especially soft nucleophiles bearing nitrogen and sulphur centres. There is substantial information on both in vitro and in vivo probes for cytochrome P-450. For example antipyrine has been widely used for assessing the activity of P-450 in vivo by utilising pharmacokinetic parameters as indices of enzyme activity. In more recent years, isozyme specific probes have also been developed for some of the P-450s. Whereas a number of substrates are available for measuring FMO activity in vitro (e.g. N,N-dimethylaniline), probes for assessing FMO activity in vivo are limited. In this review a background to the use of in vitro and in vivo probes for assessing the activity of FMO is presented, and approaches and criteria for development of potential pharmacokinetic probes for FMO are described. Preliminary data on the development of ethyl methyl sulphide (EMS) and trimethylamine (TMA) as potential pharmacokinetic probes for assessing FMO activity in rats are discussed in detail. Clinical implications of modulation of FMO activity are discussed, and arguments presented as to why the development of FMO probes for use in man will be useful additions to the range of other compounds available for assessment of liver metabolic function.

Sparteine metabolism in a Nigerian population

The oxidative metabolism of sparteine has been investigated in a Nigerian population. The distribution of metabolic capacities was shown to be skewed with two subjects (2/97, 2.1%) being relatively deficient in their ability to produce the dehydrometabolites. These observations afford evidence that sparteine oxidation is under polymorphic control in Nigerians.

Influence of age, renal and liver impairment on the pharmacokinetics of risperidone in man

The pharmacokinetics of the antipsychotic agent risperidone were investigated in healthy young and elderly subjects, cirrhotic patients and patients with moderate and severe renal insufficiency. In a comparative trial, a single oral 1-mg dose was administered to fasting subjects. Plasma and urine concentrations of the parent compound risperidone and the active moiety (i.e. risperidone plus 9-hydroxy-risperidone) were measured by radioimmunoassays. No or only small changes in plasma protein binding were observed in hepatic and renal disease, whereas the protein binding was not influenced by aging. The inter-individual variability in plasma concentrations of the active moiety was much less than the variability in plasma concentrations of risperidone. Three out of six subjects, behaving like poor metabolizers, were on medication (thiethylperazine, amitriptyline, metoprolol) that may inhibit risperidone metabolism by CYP2D6 (debrisoquine 4-hydroxylase). The pharmacokinetics of risperidone in elderly and cirrhotic patients were comparable to those in young subjects, whereas total oral clearance was reduced in renal disease patients. The elimination rate and clearance of 9-hydroxy-risperidone was reduced in elderly and renal disease patients because of a diminished creatinine clearance. The CL(oral) of the active moiety, which is primarily 9-hydroxy-risperidone, was reduced by about 30% in the elderly and by about 50% in renal disease patients. In addition, the t1/2 of the active moiety was prolonged (19 h in young subjects versus about 25 h in elderly and renal disease patients). Based upon the pharmacokinetics of the active moiety, a dose reduction and a cautious dose titration is advised in the elderly and in patients with renal disease. In cirrhotic patients, the single-dose pharmacokinetics were comparable to those in healthy young subjects.

S-mephenytoin hydroxylation phenotypes in a Jordanian population

We tested the ability of 194 unrelated, healthy Jordanian volunteers to metabolize S-mephenytoin. Mephenytoin (100 mg) was coadministered with debrisoquin (10 mg) orally and urine was collected for 8 hours. Mephenytoin metabolism was tested according to three measures: the amount of 4-hydroxymephenytoin, the S/R enantiomeric ratio, and the presence of a polar, acid-labile metabolite in urine collected for 8 hours after the dose. The S/R ratio and the presence of the acid-labile metabolite were determined in the urine of 16 patients who had low amounts of 4-hydroxymephenytoin (log hydroxylation index > or = 1). On examination of these three parameters of oxidation status, nine subjects were found to be poor metabolizers of mephenytoin by all three parameters. Thus 4.6% (95% confidence interval of 1.6% to 7.6%) of Jordanian subjects studied were poor metabolizers of mephenytoin. According to the Hardy-Weinberg Law, the frequency of the recessive autosomal gene controlling the poor metabolizer status of mephenytoin was predicted to be 0.215% (95% confidence interval of 0.146% to 0.283%). These results are on the same order of magnitude as those determined in European white populations and constitute the first report in Arab populations.

Propafenone in a usual dose produces severe side-effects: the impact of genetically determined metabolic status on drug therapy

We report the case of an elderly lady presenting with dizziness, a head injury resulting from a fall and bradycardia. Propafenone 150 mg t.i.d. had been prescribed for atrial fibrillation with tachyarrhythmia, induced by hyperthyroidism, 18 months earlier. A toxic concentration of parent propafenone, and no 5-hydroxy metabolite, was detected in a plasma sample. Symptoms disappeared after the discontinuation of propafenone. The poor metaboliser (PM) phenotype of sparteine/debrisoquine was assumed and subsequently confirmed by phenotyping (sparteine test) and genotyping (allele-specific polymerase chain reaction). The PM phenotype is common in European populations, with a prevalence of about 7%. If drugs with narrow therapeutic ranges undergo genetically polymorphic metabolism, toxicity may arise even with recommended doses. Individualization of doses is required to avoid adverse effects.

Dihydrocodeine: a new opioid substrate for the polymorphic CYP2D6 in humans

BACKGROUND

The opioid dihydrocodeine (DHC) is frequently used as an analgesic and antitussive agent. However, until now there have been no detailed data on dihydrocodeine metabolism in humans. We therefore investigated pathways that contribute to elimination of dihydrocodeine, and we tested the hypothesis that dihydrocodeine O-demethylation to dihydromorphine (DHM) is catalyzed by the polymorphic CYP2D6.

METHODS

A single oral dose of dihydrocodeine was administered to six extensive (metabolic ratio [MR] < or = 1), two intermediate (1 < MR < 20) and six poor metabolizers (MR > or = 20) of sparteine/debrisoquin. Serum concentrations of dihydrocodeine and dihydromorphine were measured up to 25 hours, and urinary excretion of conjugated and unconjugated dihydrocodeine, dihydromorphine, and nordihydrocodeine were determined.

RESULTS

There were no differences in the pharmacokinetics of dihydrocodeine between extensive and poor metabolizers. However, the area under the serum concentration-time curve (AUC), partial metabolic clearance, and total urinary recovery of dihydromorphine were significantly lower in poor metabolizers (10.3 +/- 6.1 nmol.hr/L; 7.0 +/- 4.1 ml/min; 1.3% +/- 0.9% of dose) compared with extensive metabolizers (75.5 +/- 42.9 nmol.hr/L; 49.7 +/- 29.9 ml/min; 8.9% +/- 6.2%; p < 0.01). There was a strong correlation between the AUCDHC/AUCDHM ratio and the urinary metabolic ratio of sparteine (rS = 0.89, p = 0.001). No significant differences between extensive and poor metabolizers were detected in urine for conjugated dihydrocodeine (extensive metabolizers, 27.7% of dose; poor metabolizers, 31.5%), unconjugated dihydrocodeine (extensive metabolizers, 31.1%; poor metabolizers, 31.1%), conjugated nordihydrocodeine (extensive metabolizers, 6.3%; poor metabolizers, 5.4%), or unconjugated nordihydrocodeine (extensive metabolizers, 15.8%; poor metabolizers, 19.5%).

CONCLUSIONS

Dihydrocodeine O-demethylation to dihydromorphine is impaired in poor metabolizers of sparteine. The main urinary metabolites after administration of dihydrocodeine are the parent compound and its conjugates in extensive and poor metabolizers.

Potent inhibition of yeast-expressed CYP2D6 by dihydroquinidine, quinidine, and its metabolites

The inhibitory effects of dihydroquinidine, quinidine and several quinidine metabolites on cytochrome P450 2D6 (CYP2D6) activity were examined. CYP2D6 heterologously expressed in yeast cells O-demethylated dextromethorphan with a mean Km of 5.4 microM and a Vmax of 0.47 nmol/min/nmol. Quinidine and dihydroquinidine both potently inhibited CYP2D6 metabolic activity (mean Ki = 0.027 and 0.013 microM, respectively) in yeast microsomes and in human liver microsomes. The metabolites, 3-hydroxyquinidine, O-desmethylquinidine and quinidine N-oxide also inhibited CYP2D6, but their Ki values (0.43 to 2.3 microM) were one to two orders of magnitude weaker than the values for quinidine and dihydroquinidine. There was a trend towards an inverse relationship between Ki and lipophilicity (r = -0.90, N = 5, P = 0.07), as determined by the retention-time parameter k' using reverse-phase HPLC. Thus, although the metabolites of quinidine have the capacity to inhibit CYP2D6 activity, quinidine and the impurity dihydroquinidine are the important inhibitors of CYP2D6.

Metabolism of methoxyphenamine and 2-methoxyamphetamine in P4502D6-transfected cells and cell preparations

1. Control and P4502D6-transfected human B-lymphoblastoid cell lines (cHol and h2D6v2 respectively) were used to study 2D6-mediated metabolism of methoxyphenamine (MPA) and 2-methoxyamphetamine (2MA). The main metabolites were products of O-dealkylation and aromatic hydroxylation at the 5-position. In addition, N-desmethyl-methoxyphenamine (NDMP) was also identified as a minor metabolite of MPA in extracts of these cells, confirming previous reports of 2D6-mediated N-demethylation of MPA. 2. An additional ring-hydroxylated metabolite of MPA and 2MA has been tentatively identified as the corresponding 3-hydroxy-2-methoxy derivative. 3. MPA metabolism in whole cells was time dependent, with approximately 30% of the MPA metabolized after 72 h. A 35% conversion of MPA was achieved on average with cell lysates. Only 18% 2MA was metabolized. By contrast, control cells (cHol) showed no evidence of any MPA or 2MA metabolites even after 96-h incubation. 4. Continuous presence of haemin/dimethylsulphoxide (DMSO) throughout the 4-day incubation with MPA resulted in a shift in the metabolite profile towards the production of NDMP at the expense of the other products. 5. In summary, h2D6v2 cells, lysates and microsomes can form all metabolites of MPA and can be used in drug interaction studies.

Amitriptyline metabolism in elderly depressed patients and normal controls in relation to hypothalamic-pituitary-adrenal system function

The pharmacokinetics of amitriptyline (AMI) have been extensively studied, and a large interindividual variability between oral dose and concentration of the drug in plasma has been documented. The aim of this study was twofold: first, to compare AMI kinetics in depressed patients with those of healthy controls and, second, to describe the relationship between AMI levels in plasma and hypothalamic-pituitary-adrenal (HPA) system changes during depression. Thirty-eight patients with a DSM-III-R diagnosis of major depression and 13 healthy control persons received 75 mg of AMI daily for 6 weeks. Levels of AMI and nortriptyline in plasma were determined, and neuroendocrine testing with the combined dexamethasone-suppression/CRH-stimulation test (DST) was done before AMI administration and after weeks 1, 3, and 6 of medication. AMI levels in plasma were significantly higher in the patient group compared with controls during the entire treatment period, whereas nortriptyline levels did not differ between the two groups. Drug levels correlated significantly with age, but gender had no effect on the concentration of the drug in plasma. Twenty-two patients remitted after treatment. There was no difference in drug levels between responders and nonresponders. Fifteen patients were DST nonsuppressors before treatment; 23 patients and all controls suppressed cortisol after dexamethasone. DST suppressors had significantly higher AMI levels in plasma at weeks 3, 5, and 6 compared with DST nonsuppressors. In comparison to patients with high AMI levels in plasma, those with low drug concentration had higher postdexamethasone cortisol and adrenocorticotropic hormone levels and an increased hormone release after additional CRH.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of the enantiomers of thioridazine, thioridazine 2-sulfone, and of the isomeric pairs of thioridazine 2-sulfoxide and thioridazine 5-sulfoxide in human plasma

Thioridazine is a commonly prescribed phenothiazine drug administered as a racemate and it is believed that its antipsychotic effect is mainly associated with (R)-thioridazine. A method based on high-performance liquid chromatography has been developed for the determination of the enantiomers of thioridazine and thioridazine 2-sulfone (THD 2-SO2 or sulforidazine) and of the enantiomers of the diastereoisomeric pairs of thioridazine 2-sulfoxide (THD 2-SO or mesoridazine) and thioridazine 5-sulfoxide (THD 5-SO) in the plasma of thioridazine-treated patients. The method involves sequential achiral and chiral HPLC. The limits of quantitation for total (R) + (S) concentrations were found to be 15 ng/ml for thioridazine and 5 ng/ml for its metabolites. The limits for the determination of the (R)/(S) ratios were found to be 60 ng/ml for racemic THD and 10 ng/ml for racemic THD 2-SO, THD 2-SO2, THD 5-SO (FE) and THD 5-SO (SE). The method has been used to determine the concentrations of the enantiomers of thioridazine and of its metabolites in the plasma of a patient treated with 100 mg of racemic thioridazine hydrochloride per os per day for 14 days. The results show a high enantioselectivity in the metabolism of this drug: the (R)/(S) ratios for THD, THD 2-SO (FE), THD 2-SO (SE), THD 2-SO2, THD 5-SO (FE) and THD 5-SO (SE) were found to be 3.90, 1.22, 6.10, 4.10, 0.09 and 28.0, respectively.

Influence of renal function on the steady-state pharmacokinetics of the antiarrhythmic propafenone and its phase I and phase II metabolites

The aim of this study was to investigate the disposition of propafenone and its Phase I and II metabolites in relation to kidney function under steady-state conditions. The mechanism of the renal handling of propafenone glucuronides (filtration, secretion) was also examined. Racemic (R/S) propafenone was administered to 7 young volunteers, to 5 older patients with a normal glomerular filtration rate and to 4 patients with chronic renal failure. No difference was found in the plasma concentrations of propafenone and 5-hydroxypropafenone between the three groups. The propafenone glucuronide (PPFG) concentration was elevated in the older compared to the younger subjects (S-PPFG: 544 vs. 222 nmol.ml-1.mol-1; R-PPFG: 576 vs. 304 nmol.ml-1.mol-1). Although Glomerular filtration rate did not differ, the renal clearance of propafenone glucuronides was reduced in the former group, which could be attributed to their impaired renal secretion. A dramatic increase in propafenone glucuronide concentration was observed in the patients with renal failure (S-PPFG: 2783 nmol.ml-1.mol-1; R-PPFG: 7340 nmol.ml-1.mol-1). In summary, the disposition of propafenone and of its active metabolite 5-hydroxypropafenone was not affected by kidney dysfunction, indicating that no dose adjustment is necessary in patients with renal failure. The accumulation of drug glucuronides in older patients with apparently normal kidney function should be taken into account as a possible factor modifying drug therapy.

In vitro characterization of cytochrome P450 catalysed metabolism of the antiemetic tropisetron

The new 5-hydroxytryptamine type 3 (5HT3) receptor antagonist tropisetron is used in the treatment of chemotherapy-related nausea. The drug is extensively metabolized in man, with the enzymes involved in tropisetron biotransformation being unknown. Identification of these enzymes would make it possible to predict both interindividual variability in plasma concentrations and metabolic interaction potential. The present in vitro study was therefore aimed at identifying and characterizing the cytochrome P450 enzymes catalysing tropisetron metabolism. Enzyme kinetics for formation of 5-hydroxy (5-OH-ICS), 6-hydroxy (6-OH-ICS) and N-demethyl tropisetron (N-De-ICS) were studied in the microsomal fraction of eight human livers (seven livers from extensive metabolizer (EM), one liver from a poor metabolizer (PM) for CYP2D6). Formation of 5-OH-ICS and 6-OH-ICS was biphasic with a high (5-OH: Km 3.9 +/- 2.1 microM; Vmax 1.88 +/- 0.73 pmol/mg/min; 6-OH: Km 4.66 +/- 1.84 microM; Vmax 4.00 +/- 1.77 pmol/mg/min) and low (5-OH: Km 172 +/- 51 microM; Vmax 17.0 +/- 9.4 pmol/mg/min; 6-OH: Km 266.0 +/- 76.0 microM; Vmax 81.4 +/- 27.9 pmol/mg/min) affinity component. The high-affinity component was identified as CYP2D6 which exhibits a genetic polymorphism in man. This component was absent in the PM liver. The low-affinity component was present in EM and PM livers and was identified as CYP3A4. LKM1 antibodies directed against CYP2D6 completely inhibited the high affinity component. Quinidine (0.5 microM) inhibited 5- and 6-hydroxylation at 10-80 microM tropisetron concentrations competitively by 70% with Ki values of 10 and 18 nM, respectively. Stably-expressed CYP2D6 catalysed the formation of both 5-OH-ICS and 6-OH-ICS. Both inhibition experiments and use of stably-expressed enzymes revealed formation of N-De-ICS to be mediated by CYP3A4. Based on in vitro intrinsic clearances CYP2D6-catalysed 5-OH-ICS and 6-OH-ICS is the predominant route of tropisetron elimination. Large phenotype-related differences in total clearance are to be expected after administration of tropisetron. However, in view of the wide therapeutic index of tropisetrone and the rather high Ki for inhibition of the metabolism of other drugs by tropisetron, both the interindividual variability and the interaction potential appear to be of no clinical relevance.

Exploration of the metabolism of dihydrocodeine via determination of its metabolites in human urine using micellar electrokinetic capillary chromatography

After single-dose administration of 40 or 60 mg of dihydrocodeine (DHC, in a slow-release tablet) to four healthy individuals known to be extensive metabolizers of debrisoquine, the urinary excretion of DHC and its four major metabolites, dihydrocodeine-6-glucuronide, nordihydrocodeine, dihydromorphine and nordihydromorphine, was assessed using micellar electrokinetic capillary chromatography (MECC). DHC and two of its metabolites (dihydrocodeine-6-glucuronide and nordihydrocodeine) could be analyzed by direct urine injection, whereas the metabolic pattern was obtained by copolymeric bonded-phase extraction of the solutes from both plain and hydrolyzed urine specimens prior to analysis. The total DHC equivalents excreted within 8 and 24 h were determined to be 30.4 +/- 7.7% (n = 5) and 63.8 +/- 6.1% (n = 2), respectively, and only about 4% of the excreted DHC equivalents were identified as morphinoids. Furthermore, almost no morphinoid metabolites of DHC could be found after administration of quinidine (200 mg of quinidine sulfate) 2 h prior to DHC intake.

Interindividual variability of chlorzoxazone 6-hydroxylation in men and women and its relationship to CYP2E1 genetic polymorphisms

Chlorzoxazone 6-hydroxylation is mediated by CYP2E1, and its measurement provides an in vivo probe of the enzyme's activity. To determine the population distribution of such activity, the disposition of chlorzoxazone and its 6-hydroxy metabolite was determined after oral administration to 70 white subjects (40 men and 30 women) residing in middle Tennessee. Both oral (330 +/- 111 ml.min-1, mean +/- SD) and fractional (213 +/- 86 ml.min-1) clearances varied fourfold to fivefold within the population and were unimodally distributed in a visually normal fashion. Clearance values were one-third greater in men than in women, but such differences were less striking after normalization according to body weight. Attempts to develop a single-time-point measure of 6-hydroxylating ability on the basis of plasma levels or urinary excretion of chlorzoxazone or its metabolite were unsuccessful. Genetic polymorphisms (Pst I and Rsa I restriction fragment length polymorphisms) in the 5'-flanking region of CYP2E1 deoxyribonucleic acid obtained from peripheral leukocytes were not associated with differences in the disposition of chlorzoxazone. Similarly, no major effects on 6-hydroxylation were associated with mutations in intron 6 associated with a Dra I restriction fragment length polymorphism. The interindividual variability in CYP2E1 activity as measured in vivo in healthy subjects appears to be considerably less than that expected based on in vitro studies. Whether such variability is associated with individual susceptibility to CYP2E1-mediated toxicity remains to be determined.

Identification of human CYP isoforms involved in the metabolism of propranolol enantiomers--N-desisopropylation is mediated mainly by CYP1A2

1. Studies using human liver microsomes and six recombinant human CYP isoforms (i.e. CYP1A2, 2A6, 2B6, 2D6, 2E1 and 3A4) were performed to identify the cytochrome P450 (CYP) isoform(s) involved in the ring 4-hydroxylation and side-chain N-desisopropylation of propranolol enantiomers in humans. 2. alpha-Naphthoflavone and 7-ethoxyresorufin (selective inhibitors of CYP1A1/2) inhibited the N-desisopropylation of R- and S-propranolol by human liver microsomes by 20 and 40%, respectively, while quinidine (a selective inhibitor of CYP2D6) abolished the 4-hydroxylation of both propranolol enantiomers almost completely. In contrast, sulphaphenazole (CYP2C8/9 inhibitor), S-mephenytoin (CYP2C19 inhibitor), troleandomycin (CYP3A3/4 inhibitor) and diethyldithiocarbamate (CYP2E1 inhibitor) elicited only weak inhibitory effects on propranolol metabolism via the two measured metabolic pathways. 3. Significant (P < 0.01) correlations were observed between the microsomal N-desisopropylation of both propranolol enantiomers and that for the O-deethylation of phenacetin among the 11 different human liver microsome samples (r = 0.98 and 0.77 for R- and S-propranolol, respectively). A marginally significant (r = 0.60, P congruent to 0.05) correlation was also observed between N-desisopropylation of S-, but not of R-propranolol and the 4'-hydroxylation of S-mephenytoin. No significant correlations were observed between the N-desisopropylation of propranolol enantiomers and the 2-hydroxylation of desipramine, the hydroxylation of tolbutamide or the 6 beta-hydroxylation of testosterone. 4. Significant (P < 0.01) correlations were observed between the microsomal 4-hydroxylation of R- and S-propranolol and the 2-hydroxylation of desipramine (r = 0.85 and 0.98, respectively). A weak (r = 0.66), albeit significant (P < 0.05) correlation was observed between the 4-hydroxylation of R-, but not of S-propranolol and the hydroxylation of tolbutamide. No significant correlations were observed between the 4-hydroxylation of propranolol enantiomers and the oxidation of other substrates for CYP1A2, 2C19, and 3A3/4. 5. Recombinant human CYP1A2 and CYP2D6 exhibited comparable catalytic activity with respect to the N-desisopropylation of both propranolol enantiomers; only expressed CYP2D6 exhibited a marked catalytic activity with respect to the 4-hydroxylation of both propranolol enantiomers.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibitory effects of antiparasitic drugs on cytochrome P450 2D6

The interaction of antiparasitic drugs with the polymorphic cytochrome P450 2D6 was studied in human liver microsomes. Of ten different drugs tested, three quinolines, oxamniquine, primaquine and chloroquine inhibited microsomal CYP2D6-catalysed formation of 1'hydroxybufuralol at concentrations that might have clinical consequences in drug use. These drugs inhibited competitively bufuralol metabolism with Ki values of 22, 23 and 15 microM, respectively, indicative of high affinity for the CYP2D6-active site. The results imply that oxamniquine, primaquine and chloroquine could be substrates of cytochrome P4502 D6 or that they are potent non-substrate inhibitors of the enzyme similar to quinidine. In either case, the inhibition of CYP2D6 by these agents could lead to interference with in vivo population-phenotyping procedures in the tropical regions where treatment with the drugs is common.

Intra- and interethnic variability in reduced haloperidol to haloperidol ratios

Steady-state haloperidol and reduced haloperidol concentrations were measured in 250 schizophrenic patients from 4 ethnic groups: 39 Blacks, 66 Caucasians, 82 Chinese, and 63 Mexican Americans. The distribution of the reduced haloperidol to haloperidol concentration (RH/HL) ratios was bimodal in all ethnic groups, with the antimode determined by probit plot as 0.46, 0.51, 0.36, and 0.76, respectively. With these antimodes, the proportion of patients with low RH/HL ratios were 41%, 42%, 73%, and 57% in the four ethnic groups, respectively. Compared with the other three ethnic groups, in the Chinese patients the ratio was lower. The mean RH/HL ratio in the Chinese was 0.34 compared with 0.81 to 0.87 in the non-Chinese groups. In 53 patients who were treated with two or more haloperidol dosage regimens, steady-state haloperidol and reduced haloperidol drug concentrations obtained from the different regimens were positively correlated with the haloperidol dose (R = .79 and R = .62, respectively). Our data suggest not only the existence of a bimodal distribution in the RH/HL ratio, but also that the antimode separating the low and high ratio subgroups is different among the various ethnic groups.

The application of in vitro models of drug metabolism and toxicity in drug discovery and drug development

In vitro models are being used increasingly during all phases of the drug development process in concert with the more traditional in vivo toxicological and pharmacokinetic evaluations. These in vitro models may be classified empirically as either validated in vitro screens, value-added screens or 'ad-hoc' mechanistic screens. The application of these screens is discussed with respect to their level of validation, standardization, uses of human tissue, level of iteration with in vivo studies, regulatory position and utility in the drug discovery and development process. The predictability and reproducibility of these screens is discussed, as well as future trends in regard to emerging technology and its application.

The effect of carbamazepine on the 2-hydroxylation of desipramine

The effect of carbamazepine (CBZ, 200 mg twice daily for 28 days) on the kinetics of a single oral dose of desipramine (DMI, 100 mg) was investigated in six healthy volunteers. Compared with a control session, treatment with CBZ caused a marked increase in DMI apparent oral clearance (from 1.05 +/- 0.40 to 1.38 +/- 0.52 1 h per kg, means +/- SD, P < 0.01) and a significant shortening in DMI half-life (from 22.1 +/- 3.5 to 17.8 +/- 3.5 h, P < 0.01). The amount of 2-hydroxydesipramine (2-OH-DMI) excreted in urine over a 24-h period was significantly increased during CBZ intake (from 75 +/- 15 to 92 +/- 16 mumol, P < 0.01). These findings suggest that CBZ induces the 2-hydroxylation of DMI, a reaction primarily catalyzed by the polymorphic CYP2D6 isozyme. This interaction may have considerable practical significance.

Simultaneous determination of dihydrocodeine and dihydromorphine in serum by gas chromatography-tandem mass spectrometry

A sensitive and specific method was developed for the determination of dihydrocodeine and its metabolite dihydromorphine in human serum using codeine and morphine as internal standards. Measurement is performed with GC-tandem MS after one simple extraction step and derivatization to the pentafluoropropionic esters. Sensitivity of the method is excellent and allows for the reproducible quantification of dihydrocodeine and dihydromorphine with limits of quantification of 2 ng/ml and 40 pg/ml serum, respectively. The method is therefore well suited for investigation of the pharmacokinetics and the metabolism of dihydrocodeine.

The role of cytochrome P450 enzymes in hepatic and extrahepatic human drug toxicity

The human cytochrome P450 enzyme system metabolises a wide array of xenobiotics to pharmacologically inactive metabolites, and occasionally, to toxicologically active metabolites. Impairment of cytochrome P450 activity, which may be either genetic or environmental, may lead to toxicity caused by the parent compound itself. In practise, this usually only applies to drugs that have a narrow therapeutic index and when their clearance is critically dependent upon the fraction normally metabolised by that pathway. P450 enzymes may also convert the drug to a chemically reactive metabolite, which, if not detoxified, may lead to various forms of hepatic and extrahepatic toxicity, including cellular necrosis, hypersensitivity, teratogenicity, and carcinogenicity, depending on the site of formation and the relative stability of the metabolite, and the cellular macromolecule with which it reacts. Variation in the regulation and expression of the drug metabolising enzymes may play a key role in both interindividual variation in sensitivity to drug toxicity and tissue-specific damage. Avoidance of toxicity may be possible in rare instances by prediction of individual susceptibility or by designing new chemical entities that are metabolised by a range of enzymes (both cytochromes P450 and others) and do not undergo bioactivation.

Pharmacokinetics of the newer antidepressants: clinical relevance

The newer antidepressants are a diverse group of compounds with distinct pharmacokinetic properties. The selective serotonin reuptake inhibitors (SSRIs)--paroxetine, sertraline, and fluvoxamine--have elimination half-lives of 15-26 hours. The extended half-life of fluoxetine (4-6 days) and its active metabolite, norfluoxetine (4-16 days), results in an extended time to steady-state and a prolonged washout period when dosing is discontinued. The SSRIs are administered as a single daily dose. Venlafaxine and nefazodone have short half-lives, 2-5 hours, and are dosed > or = 2 times daily. The newer antidepressants are all highly cleared from the body through hepatic metabolism. The biotransformation of all the drugs except paroxetine and fluvoxamine results in the formation of pharmacologically active metabolites. The newer antidepressants display a broad variability similar to the tricyclic antidepressants (TCAs) in steady-state drug concentrations. Due largely to a safer toxicity profile, the variability in clearance is of lesser importance with the newer antidepressants than with the TCAs. No useable concentration versus therapeutic effect relationship has been found with the newer drugs, and widely varying concentrations appear to have little relationship to adverse effects. Knowledge of kinetic characteristics is important for designing dosage regimens and avoiding potentially serious drug-drug interactions that are mediated through inhibition of specific hepatic cytochrome P450 enzyme pathways. Each of the SSRIs inhibits at least one cytochrome P450 enzyme, and all of the SSRIs increase serum concentrations of concomitantly administered TCAs.

Debrisoquine and mephenytoin oxidation in Sinhalese: a population study

The frequency distributions of the 0-8 h urinary metabolic ratios of debrisoquine and mephenytoin were measured in 111 healthy, unrelated Sinhalese resident in Sri Lanka. Blood samples were taken from 77 of these subjects for CYP2D6 genotyping. Bimodality in the distribution of the log10 debrisoquine/4-hydroxydebrisoquine ratio was not evident from visual inspection and by kernel density analysis. The results of genotyping indicated that 82% of the population were either homozygous for the wild-type CYP2D6 gene or heterozygous for the wild type allele and the whole gene deletion. Eighteen per cent of the Sinhalese population were heterozygous for the CYP2D6B mutation and the wild-type allele. All of these genotypes give rise to the extensive metaboliser phenotype in white Caucasians. No CYP2D6A mutations were identified and no individuals who were homozygous for the mutant alleles were detected, which is in accord with an absence of phenotypic poor metabolisers of debrisoquine. The mutant CYP2D6 allele frequency in Sinhalese (9%) is only half that observed in white Caucasians. The S/R-mephenytoin ratio ranged from 0.09 to 2.27 (median 0.38). By visual inspection and kernel density analysis the distribution of the S/R-mephenytoin ratio was bimodal and, using a value of 0.9 for the antimode, 16 (14%) subjects were poor metabolisers. In conclusion, the prevalence of the poor metaboliser phenotype in Sinhalese appears much lower for debrisoquine and higher for mephenytoin than in white Caucasians. These findings are similar to those observed in Indians living in Bombay and in Oriental populations.