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

CYP2D6 genotype determination in the Danish population

CYP2D6 genotyping was carried out by XbaI restriction fragment length polymorphism analysis and polymerase chain reaction in 168 healthy Danish volunteers, 77 extensive metabolizers (EM) and 91 poor metabolizers (PM) of sparteine. All EM were genotyped correctly as heterozygous or homozygous for the functional (wild type) gene, D6-wt. However, the D6-wt gene was apparently also present in 11 (12%) of the PM who accordingly were incorrectly genotyped as EM. The specificity of genotyping PM thus was 100% but the sensitivity was only 88%. The most common allele was the D6-wt with an apparent frequency of 0.741 (0.026) in the Danish population and the second most common allele was the D6-B with an apparent frequency of 0.194 (0.024). The median (range) of the sparteine metabolic ratio (MR) in 47 homozygous D6-wt EM was 0.28 (0.11-4.10) and the corresponding value in heterozygous EM was 0.36 (0.11-9.10). The median difference was 0.09 (95% confidence interval: 0.02-0.16). CYP2D6 phenotyping is a promising tool in tailoring the individual dose of tricyclic antidepressants, some neuroleplics and some antiarrhythmics. However if the genotype test could be improved with regard to both sensitivity in PM and the ability to predict CYP2D6 activity in EM then it would be of even greater clinical value in therapeutic drug monitoring.

Disposition of lupanine and 13-hydroxylupanine in man

1. The in vivo disposition of lupanine and 13-hydroxylupanine was studied in subjects identified as poor metabolizers (PM, n = 4) and extensive metabolizers (EM, n = 7) phenotypes for cytochrome P4502D6 (CYP2D6). 2. After oral administration (40.26 mumol), the half-life (t1/2) of lupanine determined from urinary excretion rate studies in EM subjects was 6.2 +/- 0.5 h (mean +/- SEM) with 95.5 +/- 6.0% of the dose recovered unchanged within 72 h. Similarly, in PM subjects t1/2 = 6.5 +/- 0.9 h and recovery 89.9 +/- 4.5%. 3. For orally administered 13-hydroxylupanine (37.83 mumol) the t1/2 in EM subjects was 6.8 +/- 1.0 h with a recovery of 100.5 +/- 5.3%, and in PM subjects t1/2 = 5.9 +/- 1.6 h with a recovery of 102.5 +/- 4.8%. 4. The t1/2s of both lupanine and 13-hydroxylupanine respectively did not differ significantly between EM and PM phenotypes. In addition, total recovery of dose for both alkaloids was similar between phenotypes. 5. In most subjects, > 76% of lupanine and > 85% of 13-hydroxylupanine was recovered as the unchanged compound. Significant apparent partial dehydroxylation of 13-hydroxy-lupanine was observed in one EM (14% of dose) and one PM (34% of dose) subject. 6. Overall, the finding of a high urinary recovery of unchanged lupanine or 13-hydroxylupanine together with similar t1/2s for both alkaloids in EM and PM CYP2D6 phenotypes suggests that clinical toxicity is unlikely to result from the use of lupin seed in footstuffs.

Genetic differences in drug disposition

Genetic polymorphisms of drug metabolizing enzymes are well recognized. This review presents molecular mechanisms, ontogeny and clinical implications of genetically determined intersubject variation in some of these enzymes. Included are the polymorphic enzymes N-acetyl transferase, cytochromes P4502D6 and 2C, which have been well described in humans. Information regarding other Phase I and Phase II polymorphic pathways, such as glutathione and methyl conjugation and alcohol and acetaldehyde oxidation continues to increase and are also discussed. Genetic factors effecting enzyme activity are frequently important determinants of the disposition of drugs and their efficacy and toxicity. In addition, associations between genetic differences in these enzymes and susceptibility to carcinogens and teratogens have been reported. Ultimately, the application of knowledge regarding these genetic factors of enzyme activity may guide medical therapy and minimize xenobiotic-induced disease.

Acute cholestatic hepatitis and the sudden rise of the pacing threshold 5 weeks after VVI-pacemaker implantation

The patient we describe developed a cholestatic hepatitis and an unusual threshold elevation of about 425%, coincidently, 5 weeks after pacemaker implantation. Lead dislocation, myocardial infarction or metabolic disorder could be excluded as a reason for the pacemaker complication. Neither a biliary obstruction nor an infection of the liver could be found to explain the liver injury. Most probably, both disorders were caused by propafenone (450 mg day-1), withdrawal of the drug resulted in normalization of elevated enzyme and threshold levels.

Genetically determined sparteine oxidation polymorphism in a Polish population

The genetic oxidation polymorphism was determined in 160 healthy Polish volunteers from the south-west of Poland (Wrocław region), using sparteine as a model drug. The results of a Polish population study revealed a bimodal distribution of the sparteine metabolic ratio and showed the existence of two oxidation phenotypes designated as extensive and poor metabolizers. The frequency of poor metabolizers in our study (8.8%) compares well with most results of poor oxidation metabolizers in Caucasian populations.

An enantiomer-enantiomer interaction of (S)- and (R)-propafenone modifies the effect of racemic drug therapy

BACKGROUND

Therapy with racemic compounds produces effects that can be attributed to both (S)- and (R)-enantiomers. Here we have tested the hypothesis that an enantiomer-enantiomer interaction would modulate the effects of treatment with a racemate, the antiarrhythmic propafenone. Previous studies have shown that while the enantiomers of propafenone exert similar sodium channel-blocking (QRS widening) effects, it is the (S)-enantiomer that produces beta-blockade; moreover, we have demonstrated recently that (R)-propafenone inhibits the metabolism of (S)-propafenone in vitro.

METHODS AND RESULTS

This single-blind, randomized study compared the effects of (R/S)-, (S)-, (R)-propafenone (150 mg q 6 hours for 4 days) and placebo on QRS duration (delta QRS) and on maximum exercise heart rate (delta HRmax), an index of beta-blockade. The clearance of (S)-propafenone was significantly lower (-55 +/- 24%, P < .001) during treatment with (R/S)-propafenone than with the (S)-enantiomer alone, and delta HRmax was significantly altered during (R/S)-propafenone (-8.8 +/- 6.6 beats per minute; P < .01) and during (S)-propafenone (-4.3 +/- 4.8 beats per minute; P < .01) but not during (R)-propafenone (-1.8 +/- 6.4 beats per minute) or placebo (0.3 +/- 7.1 beats per minute). In contrast, (R/S)-, (S)-, and (R)-propafenone all prolonged QRS compared with placebo.

CONCLUSIONS

These data indicate that (R)-propafenone impairs the disposition of (S)-propafenone in humans. As a result, the beta-blocking effects of 150 mg of racemic propafenone (75 mg of the [S]-enantiomer) were more pronounced than those of 150 mg of (S)-propafenone alone. Thus, the effects of racemic drug therapy are not necessarily those predicted by summation of the effects of the individual enantiomers.

An evaluation of cytochrome P450 isoform activities in the female dark agouti (DA) rat: relevance to its use as a model of the CYP2D6 poor metaboliser phenotype

The female dark agouti (DA) rat lacks CYP2D1, the equivalent enzyme in the rat to human CYP2D6 (debrisoquine hydroxylase), and shows impaired metabolism of a number of CYP2D6 substrates. However, from the data available in the literature it is not entirely clear whether the enzyme deficiency in the DA rat is restricted to CYP2D1, and whether factors such as age and substrate concentration are important determinants of interstrain differences in the activity of this enzyme. Given that the female DA rat is used as a model of the human CYP2D6 poor metaboliser phenotype, there is a need for a systematic evaluation of the P450 activities in the DA rat, and of its suitability as a model of the PM phenotype. In the present study metoprolol was used as a probe substrate to investigate CYP2D1 activity since both the alpha-hydroxylation and O-demethylation of this drug are catalysed by CYP2D6 in man. Formation of alpha-hydroxymetoprolol (AHM) and O-demethylmetoprolol (ODM) was 10- and 2.5-fold lower in liver microsomes from female DA rats compared with microsomes from age-matched female Wistar rats, the latter representing the extensive metaboliser strain. Kinetic analysis suggested that in both strains of rat both the alpha-hydroxylation and O-demethylation of metoprolol were catalysed by more than one enzyme. By using quinine as a specific inhibitor of the enzyme, CYP2D1 was identified as an intermediate affinity site in the Wistar strain and was shown to have impaired activity in the DA strain. The activities of lower and higher affinity sites were similar in the two strains. Thus, the only difference between the two strains with respect to both routes of metoprolol metabolism appeared to be in the activity of CYP2D1. Interstrain differences were found to be highly dependent on the choice of substrate concentration, being more marked at lower concentrations. We have also investigated the metabolism of a number of probe compounds for some of the other P450 isoforms commonly involved in drug metabolism to determine the selectivity of the deficiency in the DA strain. p-Nitrophenol hydroxylation and erythromycin N-demethylation were catalysed at higher rates by DA than by Wistar liver microsomes, indicating higher levels of activity of CYP2E1 and CYP3A in the former strain. Felodipine oxidation, tolbutamide hydroxylation and both the hydroxylation and N-demethylation of S-mephenytoin were catalysed at similar rates by microsomes from the two strains, indicating similar activities of enzymes in the CYP2C and CYP3A families.(ABSTRACT TRUNCATED AT 400 WORDS)

Major patterns of human inheritance: relevance to the epilepsies

An understanding of the basic principles of human inheritance patterns is necessary to appreciate the variety and complexity of genetic influences in the epilepsies. The various modes of human inheritance include single gene (mendelian), chromosomal, mitochondrial, and multifactorial/polygenic. Recent insights regarding imprinting, anticipation, and uniparental disomy have been made and have an impact on our understanding of human genetics. All of these phenomena are likely to be of importance in the epilepsies and form the basis for a more detailed analysis of the relevant molecular biology.

Influence of CYP2D6-dependent metabolism on the steady-state pharmacokinetics and pharmacodynamics of metoprolol and nicardipine, alone and in combination

1 The metabolism of metoprolol depends in part on the genetically determined activity of the CYP2D6 isoenzyme. In vitro studies have shown that nicardipine is a potent inhibitor of CYP2D6 activity. Since the combination of metoprolol and nicardipine is likely to be used for the treatment of hypertension, we examined the interaction between these two drugs at steady-state. 2 Fourteen healthy volunteers, seven extensive and seven poor metabolisers of dextromethorphan were studied in a double-blind, randomised cross-over four-period protocol. Subjects received nicardipine 50 mg every 12 h, metoprolol 100 mg every 12 h, a combination of both drugs and placebo during 5.5 days. Steady-state pharmacokinetics of nicardipine and metoprolol were analyzed. Beta-adrenoceptor blockade was assessed as the reduction of exercise-induced tachycardia. 3 During treatment with metoprolol, alone or in combination with nicardipine, its steady-state plasma concentrations were higher in subjects of the poor metaboliser phenotype than in extensive metabolisers. Beta-adrenoceptor blockade was also more pronounced in poor metabolisers than in extensive metabolisers of dextromethorphan during treatment with metoprolol alone or in combination with nicardipine (24.0 +/- 2.4% vs 17.1 +/- 3.5% and 24.1 +/- 2.5% vs 15.4 +/- 2.7% reduction in exercise trachycardia, respectively, P < 0.01 in each case). 4 Nicardipine produced a small increase in plasma metoprolol concentration in extensive metabolisers from 35.9 +/- 16.6 to 45.8 +/- 15.4 ng ml(-1) (P < 0.02), but had no significant effect in poor metabolisers. However, nicardipine did not alter the R/S metoprolol ratio in plasma 3 h after dosing, the plasma concentration of S-(-)-metoprolol 3 h after dosing or the beta-adrenoceptor blockade produced by metoprolol in subjects of both phenotypes. The partial metabolic clearance of metoprolol to alpha-hydroxy-metoprolol was not altered significantly in extensive metabolisers. Plasma nicardipine concentration and beta-adrenoceptor blocking effects did not differ between the phenotypes and were not influenced by metoprolol. We conclude that beta-adrenoceptor blockade during repeated dosing with metoprolol is more pronounced in poor than in extensive metaboliser subjects, that nicardipine decreases a CYP2D6-independent route of metoprolol elimination but does not increase beta-adrenoceptor blockade during repeated dosing with metoprolol.

Analysis of imipramine and three metabolites produced by isozyme CYP2D6 expressed in a human cell line

1. A commercially-available human cytochrome P450 isozyme (CYP2D6) preparation was used in imipramine metabolism studies. This isozyme catalysed both aromatic C-oxidation and N-demethylation. 2-Hydroxyimipramine was the major metabolite; desipramine was isolated in a significant amount and 2-hydroxydesipramine was a trace metabolite. 2. To prevent decomposition of metabolites during the analytical procedure, the metabolism mixture was derivatized with acetic anhydride prior to extraction, and the derivatized metabolites were separated and quantified by g.l.c. with N/P detection. The analytical procedure had excellent sensitivity and was capable of routinely quantifying imipramine and its metabolites down to the 0.36 nmol level. 3. In excess of 90% of drug and metabolites was consistently recovered when metabolism was conducted over a 5-60-min duration. 4. The formation of the secondary metabolite, 2-hydroxydesimipramine, from imipramine proceeds by two pathways, via desipramine and via 2-hydroxyimipramine; the former is the preferred pathway. 5. CYP2D6 catalyses C-hydroxylation of imipramine to 2-hydroxyimipramine more efficiently than its N-demethylation to desipramine. Also, the C-hydroxylation of imipramine to 2-hydroxyimipramine proceeds more efficiently than the conversion of desipramine to 2-hydroxydesipramine.

Enantioselective bioanalysis of beta-blocking agents: focus on atenolol, betaxolol, carvedilol, metoprolol, pindolol, propranolol and sotalol

The recent developments in enantioselective HPLC-separation techniques are impressive and are driven by industrial and academic interests; thus there is for instance a high demand for developing stereoselective assays for chiral drugs in biological fluids. The beta-blocking agents, which possess an amino-propanol- or -ethanol side chain with at least one chiral centre, represent one of the most intensively investigated groups of more than 40 drugs introduced world wide. Seven of the most popular beta-blockers were chosen as representatives: atenolol; betaxolol; carvedilol; metoprolol; pindolol; propranolol; and sotalol, these span the whole range of lipophilicity to hydrophilicity (polarity). Enantioselective HPLC bioassays for these beta-blockers published so far, including techniques based on chiral derivatizing agents (CDAs), chiral stationary phases (CSPs) and chiral mobile phase additives (CMPAs) have been reviewed and documented in the light of general aspects together with pharmacokinetic and pharmacodynamic considerations.

Role of human cytochrome P-450s in risk assessment and susceptibility to environmentally based disease

Cytochromes P-450 (P-450s) are a large group of heme-containing proteins that carry out oxidation of numerous chemicals. In mammals, a limited number of P-450s are involved in metabolic pathways of steroid synthesis, while most of these enzymes are involved in metabolism of foreign compounds. The principal beneficial function of P-450s is to convert chemicals into derivatives that can be easily eliminated from the body. This generally occurs through P-450-mediated oxidations of hydrophobic substances followed by conjugation reactions. For many foreign compounds, P-450 metabolism results in production of "activated" metabolites that can cause cell death and gene mutations. During the past several years, it has become widely recognized that marked species differences occur among the foreign compound-metabolizing P-450s. In addition to this interspecies variability in metabolism, marked intraspecies variability, frequently referred to as drug oxidation polymorphisms, occurs in virtually all mammals examined to date. Based on these observations, it is necessary to develop new human P-450-based systems that can be used to study foreign compound metabolism in order to predict human risk. This is being accomplished by use of cDNA-directed expression in B lymphoblastoid cells. These cells can be used to predict how humans will metabolize a chemical and whether it will be metabolically activated to a toxic or mutagenic metabolite. To study human P-450 polymorphisms, polymerase chain reaction (PCR) assays have been developed for diagnosis of known mutant P-450 genes. Molecular probes are also being used to screen populations for levels of expression of carcinogen-activating P-450s in an effort to determine whether expression of certain P-450 forms is associated with increased risk for development of environmentally based disease.

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.

Current status of class III antiarrhythmic drug therapy

Studies in animal models, as well as clinical experience with amiodarone and sotalol, suggest that action potential prolongation may be a useful antiarrhythmic mode of action. A number of agents that produce this class III effect are currently under development. The single greatest liability for further development of this group of drugs is the occasional, and apparently unpredictable, development of exaggerated QT prolongation and polymorphic ventricular tachycardia (torsades de pointes). Available data suggest that QT interval prolongation is not a good indicator of whether or not a class III antiarrhythmic will suppress a target arrhythmia; however, exaggerated QT prolongation is a predictor of torsades de pointes. Further studies to delineate the mechanism underlying the development of torsades de pointes might lead to safer and more effective antiarrhythmic drugs.

Identification and quantitative determination of glutathione-related urinary metabolites of fotemustine, a new anti-cancer agent

1. Potential sulphur-containing metabolites of the anticancer agent, fotemustine, were synthesized, namely thiodiacetic acid (TDA), S-2-hydroxyethyl N-acetyl-L-cysteine (2-HE-NAC), N-acetyl-L-cysteine (NAC), S-methyl N-acetyl-L-cysteine (M-NAC), S-carboxymethyl-L-cysteine (CM-Cys), S-carboxymethyl N-acetyl-L-cysteine (CM-NAC), their corresponding sulphoxides and sulphones. Their chemical structures and stabilities were confirmed and derivatization methods were developed for their analysis by sulphur-selective g.l.c. (g.l.c.-FPD) and g.l.c.-mass spectrometry. 2. Four methods for isolation of potential metabolites of fotemustine were developed. Quantification of metabolites, derived in various ways was carried out by g.l.c.-atomic emission detection (AED) or g.l.c.-mass spectrometry. 3. Male Wistar rats (n = 4) were given a single i.p. dose of 40 mg/kg fotemustine. Urine excretion of TDA (18.4 +/- 1.9% in 24 h) and TDA sulphoxide (12.0 +/- 1.6% in 24 h) was significant; 32.7 +/- 4.6% of the fotemustine dose was excreted as TDA, and TDA sulphoxide in 48 h. NAC was excreted in rat urine at 1% of the dose. No other potential glutathione-derived metabolites of fotemustine were excreted. 4. Male Wistar rats (n = 4) were also treated i.p. with fotemustine at 5, 20 and 40 mg/kg, to investigate dose dependency and the time course of excretion of TDA. Excretion of TDA in 48 h urine decreased from 32 +/- 2 to 17 +/- 2% dose (mean +/- SD) with increasing dose of fotemustine.

Similar effect of oxidation deficiency (debrisoquine polymorphism) and quinidine on the apparent volume of distribution of (+/-)-metoprolol

The influence of phenotype (debrisoquine type of oxidation polymorphism) and quinidine on (+/-)-metoprolol distribution parameters was investigated in 7 young male volunteers (4 extensive and 3 poor metabolisers). (+/-)-Metoprolol tartrate 20 mg was administered as a 20 min infusion i) alone, ii) 12 h after an oral 50 mg quinidine sulphate capsule, and iii) on the last day of 3 days of treatment with 250 mg quinidine sulphate b.d. as a slow-release tablet. No stereoselectivity was apparent in either poor or extensive metabolizers. When (+/-)-metoprolol was administered alone the apparent volume of distribution at steady-state (Vss) was higher in extensive than in poor metabolisers (4.84 vs 2.83 l.kg-1, respectively). Pre-treatment with low or multiple high doses of quinidine decreased Vss in extensive metabolisers to values comparable to those in poor metabolisers (3.50 and 3.18 l.kg-1, respectively), but had no significant effect in poor metabolisers (3.24 and 3.42 l.kg-1, respectively). Estimation of Vss by noncompartmental analysis or assuming elimination exclusively from the peripheral compartment yielded similar, although somewhat higher, estimates. Despite the small number of subjects, (+/-)-metoprolol distribution appeared to be different both in genetically and environmentally (quinidine)-determined poor metabolisers, and quinidine inhibition was a good, reversible in vivo model of the genetic deficiency in handling (+/-)-metoprolol. Differences both in first pass pulmonary elimination or in tissue binding are logically consistent with these observations, but the amplitude of the effect exceeds expectations from available biological evidence on selective pulmonary metabolic activity and on specific tissue binding sites.

Kinetics of propiverine as assessed by radioreceptor assay in poor and extensive metabolizers of debrisoquine

A pharmacokinetic study with 30 mg propiverine p.o. was performed in healthy volunteers (10 males, 6 females, age 36-56 years, body weight 55-100 kg, body height 162-184 cm, Broca index 0.96-1.19). 8 of them were poor and 8 extensive metabolizers of the debrisoquine type hydroxylation polymorphism. The total anticholinergic activity of the parent compound and active metabolites was measured with a radioreceptor assay calibrated with the metabolite M2. The affinity of this metabolite to the muscarinic receptors was similar to that of atropine. The urinary excretion of 3 major metabolites was determined with TLC and densitometry. Arterial blood pressure, heart rate, diameter of pupils, accommodation and parotic salivary flow were also measured. The concentrations of anticholinergic equivalents of propiverine were below 1 ng/ml of M2. 1.4-6.0% of the dose were excreted as N-oxidized metabolites into the urine. The poor and extensive metabolizers of debrisoquine did not differ significantly with regard to the concentration time behaviour of the active drug components, pattern of major metabolites, adverse drug reactions or any pharmacodynamic parameters measured.

A preliminary 3D model for cytochrome P450 2D6 constructed by homology model building

A homology model building study of cytochrome P450 2D6 has been carried out based on the crystal structure of cytochrome P450 101. The primary sequences of P450 101 and P450 2D6 were aligned by making use of an automated alignment procedure. This alignment was adjusted manually by matching alpha-helices (C, D, G, I, J, K and L) and beta-sheets (beta 3/beta 4) of P450 101 that are proposed to be conserved in membrane-bound P450s (Ouzounis and Melvin [Eur. J. Biochem., 198 (1991) 307]) to the corresponding regions in the primary amino acid sequence of P450 2D6. Furthermore, alpha-helices B, B' and F were found to be conserved in P450 2D6. No significant homology between the remaining regions of P450 101 and P450 2D6 could be found and these regions were therefore deleted. A 3D model of P450 2D6 was constructed by copying the coordinates of the residues from the crystal structure of P450 101 to the corresponding residues in P450 2D6. The regions without a significant homology with P450 101 were not incorporated into the model. After energy-minimization of the resulting 3D model of P450 2D6, possible active site residues were identified by fitting the substrates debrisoquine and dextrometorphan into the proposed active site. Both substrates could be positioned into a planar pocket near the heme region formed by residues Val370, Pro371, Leu372, Trp316, and part of the oxygen binding site of P450 2D6. Furthermore, the carboxylate group of either Asp100 or Asp301 was identified as a possible candidate for the proposed interaction with basic nitrogen atom(s) of the substrates.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Inhibition by paroxetine of desipramine metabolism in extensive but not in poor metabolizers of sparteine

Nine extensive metabolizers (EMs) and eight poor metabolizers (PMs) of sparteine took a single oral dose of 100 mg of desipramine HCI before and while taking paroxetine 20 mg per day. Before paroxetine, the median of the total desipramine clearance was 7 times higher in EMs than in PMs (102 and 15 l.h-1 respectively). This confirms that desipramine is extensively metabolized via the sparteine/debrisoquine oxidation polymorphism i.e. by CYP2D6. During paroxetine, the median clearances were 22 l.h-1 and 18 l.h-1 in EMs and PMs respectively. The 5-fold decrease in clearance in EMs when desipramine was co-administered with paroxetine confirms that paroxetine is a potent inhibitor of CYP2D6. The lack of effect on clearance in PMs shows that paroxetine is a selective inhibitor of CYP2D6, which is absent from the livers of PMs. Before paroxetine, the median of desipramine clearance via 2-hydroxylation was 40-times higher in EMs than in PMs (56 and 1.4 l.h-1 respectively), but during paroxetine, it was only 2-times higher (6 and 2.9 l.h-1 respectively). The increase in this clearance in PMs suggests that paroxetine is an inducer of the alternative, unidentified P450(s) which catalyze(s) the formation of 2-OH-desipramine in this phenotype. Before paroxetine, the median amounts of 2-OH-desipramine glucuronide recovered in urine were 69% and 68% of the total recovery of 2-OH-desipramine in urine in EMs and PMs respectively. During paroxetine, the corresponding values were 77% and 84%.(ABSTRACT TRUNCATED AT 250 WORDS)

The metabolism of aprindine in relation to the sparteine/debrisoquine polymorphism

1. Incubation of the class I antiarrhythmic drug aprindine (AP) with human liver microsomes resulted in the formation of two hydroxylated metabolites (HA1 and HA2) and desethylaprindine which were identified by GC-mass spectrometry. In liver microsomes isolated from a poor metaboliser (PM) of sparteine no hydroxylated metabolites of AP were detected whereas AP N-dealkylation was unimpaired. Thus hydroxylation of AP is mediated by cytochrome P450 2D6 (CYP2D6). 2. AP was found to be a competitive inhibitor of CYP2D6 as indicated by its ability to impair the formation of (2S)-hydroxysparteine, 5,6-didehydrosparteine and 5-hydroxypropafenone by human liver microsomes. 3. These in vitro findings are consistent with a major role of CYP2D6 in the clearance of AP in vivo, with its ability to impair the metabolism of other CYP2D6 substrates in vivo, and an ability to cause phenocopying (conversion of extensive metaboliser phenotypes for sparteine/debrisoquine to apparent 'poor metabolisers).

Polymorphism of debrisoquine and mephenytoin hydroxylation among Estonians

Debrisoquine and S-mephenytoin hydroxylation polymorphisms were studied in 156 unrelated native Estonians. The hydroxylation phenotypes were assessed by coadministration of mephenytoin with debrisoquine or dextromethorphan. The frequency of the poor metaboliser phenotype of debrisoquine/dextromethorphan was 4.5% (95% confidence interval 1.2-7.8%), and that of mephenytoin was 3.9% (95% confidence interval 0.9-6.9%) among Estonians, which is very similar to what has been reported in other Caucasian populations.

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.

Pharmacology, toxicology and human pharmacokinetics of tropisetron

Tropisetron is a novel, potent and highly selective 5-HT3 receptor antagonist, which is active in the treatment of nausea and vomiting induced by highly emetogenic chemotherapeutic drugs such as cisplatin. Tropisetron selectively blocks the excitation of the presynaptic 5-HT receptors of the peripheral neurones involved in the emetic reflex, and may have other direct actions in the CNS on 5-HT3 receptors, mediating the actions of vagal inputs to the area postrema. Toxicological studies show that tropisetron is generally well tolerated by all animal species investigated and no specific organ toxicity was observed, other than slight loss of body weight development. In man, tropisetron metabolism is linked to the cytochrome P-450 2D6 isoenzyme system, which determines the polymorphism of debrisoquine/sparteine metabolism. As a result, there are phenotypical populations of extensive and poor metabolizers. The two main adverse events in human volunteer tolerability studies with tropisetron were headache and constipation. These adverse events tended to be slightly more intense and to last longer in poor metabolizers, compared with extensive metabolizers. Tropisetron had similar pharmacokinetic characteristics in elderly patients and renal patients, compared with healthy subjects. From a toxicological and pharmacokinetic point of view, therefore, daily doses of 5 mg tropisetron can be administered to both poor and extensive metabolizers, as well as to special populations, without any particular precautions.

Inhibition of human cytochrome P450 2D6 (CYP2D6) by methadone

1. In microsomes prepared from three human livers, methadone competitively inhibited the O-demethylation of dextromethorphan, a marker substrate for CYP2D6. The apparent Ki value of methadone ranged from 2.5 to 5 microM. 2. Two hundred and fifty-two (252) white Caucasians, including 210 unrelated healthy volunteers and 42 opiate abusers undergoing treatment with methadone were phenotyped using dextromethorphan as the marker drug. Although the frequency of poor metabolizers was similar in both groups, the extensive metabolizers among the opiate abusers tended to have higher O-demethylation metabolic ratios and to excrete less of the dose as dextromethorphan metabolites than control extensive metabolizer subjects. These data suggest inhibition of CYP2D6 by methadone in vivo as well. 3. Because methadone is widely used in the treatment of opiate abuse, inhibition of CYP2D6 activity in these patients might contribute to exaggerated response or unexpected toxicity from drugs that are substrates of this enzyme.

The role of individual human cytochromes P450 in drug metabolism and clinical response

Recent advances in the study of human cytochromes P450 by protein purification, molecular cloning techniques and analysis of polymorphisms has led to increased understanding of the role of the various forms in the metabolism of clinically important drugs. In particular, the substrate specificity of one form, CYP2D6, is well established. CYP2D6 shows polymorphism, with 5-10% of Caucasians (poor metabolizers) not expressing this enzyme. The molecular basis of this deficiency is now well understood and methods for the detection of poor metabolizers are discussed, as well as the effect of the polymorphism on drug metabolism. Substrate specificities and possible polymorphisms in other cytochromes P450 are also discussed.

Oxidative metabolism of cinnarizine in rat liver microsomes

The oxidative metabolism of cinnarizine (CZ) [1-(diphenylmethyl)-4-(3-phenyl-2-propenyl)-piperazine] to 1-(diphenylmethyl)piperazine (M-1), 1-(diphenylmethyl)-4-[3-(4'-hydroxyphenyl)-2-propenyl]piperazine (M-2), benzophenone (M-3) and 1-[4'-hydroxyphenyl)-phenylmethyl]-4-(3- phenyl-2-propenyl)piperazine (M-4) has been studied in rat liver microsomes. In Wistar rats, kinetic analysis revealed sex differences (male > female) in the Km values for formation of all the metabolites and the Vmax values for the formation of M-1, M-3 and M-4. The reactions required NADPH, and were inhibited by carbon monoxide and SKF 525-A. Only M-2 formation was suppressed by sparteine or metoprolol, and was significantly lower in female Dark Agouti rats than in Wistar rats of both sexes. The results suggest that CZ is oxidized by cytochrome P450, and M-2 formation is related to debrisoquine/sparteine-type polymorphic drug oxidation.

Advances in the diagnosis of adverse drug reactions

The clinician plays a key role in the detection and diagnosis of adverse drug reactions (ADRs). The diagnosis of ADRs, however, is a complex task. In the past, there were no systematically applied diagnostic criteria for ADRs, no formal methods of case analysis, no standardized epidemiologic approaches, and a limited knowledge of mechanisms. This resulted in the overdiagnosis of ADRs, which has negative consequences such as limiting treatment options. Recently, there have been various improvements in the diagnosis of ADRs, such as the development of standardized decision aids and of in vitro diagnostic tests. This article briefly reviews some of this knowledge, discusses the role of in vivo and in vitro rechallenge, and summarizes a probabilistic approach for collecting relevant information and diagnosing ADRs. The intention is to increase awareness of the different approaches for diagnosing ADRs as well as to stimulate researchers to continue to collect pharmacoepidemiologic information, study the pharmacologic, immunologic, and genetic factors involved in the pathogenesis of drug reactions, and develop and test new diagnostic instruments under various clinical conditions.

Absence of CYP3A genetic polymorphism assessed by urinary excretion of 6 beta-hydroxycortisol in 102 healthy subjects on rifampicin

A previous study has demonstrated that the urinary level of 6 beta-hydroxycortisol is a marker of liver CYP3A content after induction by rifampicin. To put in evidence an eventual genetic polymorphism for this cytochrome, the frequency distribution of 6 beta-hydroxycortisol excretion was investigated in 102 healthy Caucasians before and after 6 days of oral rifampicin administration (600 mg daily). After rifampicin treatment, a wide interindividual distribution was observed but no clear bimodality. Moreover the mean 6 beta-hydroxycortisol level was higher in women (n = 38) than in men (n = 64). These observations do not favour the existence of a CYP3A genetic polymorphism based on 6 beta-hydroxycortisol excretion but evoke a sexual dimorphism. However, CYP3A is composed of at least four enzymes and as the enzyme(s) responsible for cortisol 6 beta-hydroxylation is (are) not perfectly known, it can not be excluded that a genetic polymorphism does exist for one enzyme of this family.

Inhibitors of imipramine metabolism by human liver microsomes

1. The aromatic 2-hydroxylation of imipramine was studied in microsomes from three human livers. The kinetics were best described by a biphasic enzyme model. The estimated values of Vmax and Km for the high affinity site ranged from 3.2 to 5.7 nmol mg-1 h-1 and from 25 to 31 microM, respectively. 2. Quinidine was a potent inhibitor of the high affinity site for the 2-hydroxylation of imipramine in microsomes from all three human livers, with apparent Ki-values ranging from 9 to 92 nM. This finding strongly suggests that the high affinity enzyme is CYP2D6, the source of the sparteine/debrisoquine oxidation polymorphism. 3. The selective serotonin reuptake inhibitors (SSRI), paroxetine, fluoxetine and norfluoxetine were potent inhibitors of the high affinity site having apparent Ki-values of 0.36, 0.92 and 0.33 microM, respectively. Three other SSRIs, citalopram, desmethylcitalopram and fluvoxamine, were less potent inhibitors of CYP2D6, with apparent Ki-values of 19, 1.3 and 3.9 microM, respectively. 4. Among 20 drugs screened, fluvoxamine was the only potent inhibitor of the N-demethylation of imipramine, with a Ki-value of 0.14 microM. 5. Neither mephenytoin, citalopram, diazepam, omeprazole or proguanil showed any inhibition of the N-demethylation of imipramine and the role of the S-mephenytoin hydroxylase for this oxidative pathway could not be confirmed.

The metabolism of tramadol by human liver microsomes

The metabolism of tramadol was investigated in vitro using microsomal fractions of human liver. The parent compound and its main metabolites were determined by a newly developed high performance liquid chromatography assay. O-demethylation of tramadol was found to be stereoselective. The Vmax of the O-demethylation of (-)-tramadol was 210 pmol.mg-1.min-1, whereas (+)-tramadol was O-demethylated with a Vmax of 125 pmol.mg-1.min-1. The Km for both enantiomers was determined to be 210 microM. O-demethylation was inhibited competitively by quinidine (ki = 15 nM) and propafenone (ki = 34 nM). N-demethylation was also stereoselective, preferentially metabolizing the (+)-enantiomer. Whereas O-demethylation displayed monophasic Michaelis-Menten kinetics, N-demethylation was best described by a two-site model. Competitive inhibition of the O-demethylation both by quinidine and propafenone suggests that O-demethylation is carried out by P-450IID6.

Debrisoquine hydroxylase gene polymorphism and susceptibility to Parkinson's disease

The pathogenesis of Parkinson's disease may be influenced by genetic and environmental factors. Cytochrome P450 mono-oxygenases help to protect against toxic environmental compounds and individual variations in cytochrome P450 expression might, therefore, influence susceptibility to environmentally linked diseases. The frequency of mutant CYP2D6 alleles was studied in 229 patients with Parkinson's disease and 720 controls. Individuals with a metabolic defect in the cytochrome P450 CYP2D6-debrisoquine hydroxylase gene with the poor metaboliser phenotype had a 2.54-fold (95% Cl 1.51-4.28) increased risk of Parkinson's disease. Determination of CYP2D6 phenotype and genotype may help to identify those at greatest risk of Parkinson's disease and may also help to identify the environmental or metabolic agents involved in the pathogenesis of this disease.

In vitro forecasting of drugs that may interfere with codeine bioactivation

The O-demethylation of codeine (methylmorphine) into morphine is mediated by the polymorphic cytochrome P450 DB1 (P450 IID6). By means of in vitro screening in human liver microsomes we have studied the effect on codeine bioactivation of several drugs used as analgesics or as adjuvants for pain control. In microsomes from an extensive metabolizer subject, paracetamol (acetaminophen) and NSAIDs (acetylsalicylic acid, diclofenac, indomethacin, piroxicam, and pirprofen), benzodiazepines (chlordiazepoxide, clonazepam, diazepam, flunitrazepam, and midazolam), and anticonvulsants (carbamazepine and phenytoin) did not alter the reaction. There was marked inhibition of in vitro morphine production by neuroleptics (chlorpromazine, haloperidol, levomepromazine, and thioridazine), metoclopramide, and tricyclic antidepressants (amitriptyline, clomipramine, desipramine, imipramine, and nortriptyline). Enzyme kinetics showed competitive inhibition by neuroleptics (chlorpromazine Ki = 0.5 microM) and antidepressants (clomipramine Ki = 6.8 microM), which are substrates of the polymorphic monooxygenase. Due to the low affinity of codeine for P450 DB1 (Km = 100-200 microM), its bioactivation in extensive metabolizers, and thus its analgesic efficacy, is liable to vary greatly when it is combined with any drug that has a high affinity for the polymorphic isozyme.

Relative expression of cytochrome P450 isoenzymes in human liver and association with the metabolism of drugs and xenobiotics

Cytochrome P450s play a central role in the metabolism and disposition of an extremely wide range of drugs and chemical carcinogens. Individual differences in the expression of these enzymes may be an important determinant in susceptibility to adverse drug reactions, chemical toxins and mutagens. In this paper, we have measured the relative levels of expression of cytochrome P450 isoenzymes from eight gene families or subfamilies in a panel of twelve human liver samples in order to determine the individuality in their expression and whether any forms are co-regulated. Isoenzymes were identified in most cases on Western blots based on the mobility of authentic recombinant human cytochrome P450 standards. The levels of the following P450 proteins correlated with each other: CYP2A6, CYP2B6 and a protein from the CYP2C gene subfamily, CYP2E1 and a member of the CYP2A gene subfamily, CYP2C8, CYP3A3/A4 and total cytochrome P450 content. Also, the levels of two proteins in the CYP4A gene subfamily were highly correlated. These correlations are consistent with the relative regulation of members of these gene families in rats or mice. In addition, the level of expression of specific isoenzymes has also been compared with the rate of metabolism of a panel of drugs, carcinogens and model P450 substrates. These latter studies demonstrate and confirm that the correlations obtained in this manner represent a powerful approach towards the assignment of the metabolism of substrates by specific human P450 isoenzymes.

Thebaine O-demethylation to oripavine: genetic differences between two rat strains

1. Codeine O-demethylation to morphine is mediated by cytochrome P450 IID1 (rat), or P450 IID6 (man), and exhibits genetic polymorphism. Thebaine is a precursor in the formation of endogenous morphine and codeine in man, being O-demethylated to oripavine. 2. The objective of the present study was to ascertain whether the O-demethylation of thebaine to oripavine was mediated by cytochrome P450 IID1 in rat liver microsomes. 3. Thebaine O-demethylation showed strain differences in female Sprague-Dawley (SD) and female Dark-Agouti (DA) rats, which serve as a model for the human debrisoquine/sparteine metabolism phenotypes. 4. The total intrinsic clearance of thebaine to oripavine was high (19.7 ml/h per mg protein) in SD rats, indicating that oripavine is a major metabolite of thebaine. A 3-fold lower intrinsic clearance was observed in DA rats (6.7 ml/h per mg protein). 5. Thebaine O-demethylation was inhibited by quinine and known substrates of cytochrome P450 IID1/P450 IID6, supporting the major involvement of cytochrome P450 IID1 in oripavine formation in rats.

Metoprolol oxidation polymorphism in a Korean population: comparison with native Japanese and Chinese populations

We examined metoprolol oxidation capacity in 218 unrelated, healthy Korean subjects using the 8 h urinary metabolic ratio (MR) of metoprolol to alpha-hydroxymetoprolol after an oral dose of 100 mg metoprolol tartrate. The results were compared with those from 295 Japanese and 107 mainland Chinese whose metoprolol oxidation capacities were assessed in a similar manner. The frequency of occurrence of poor metabolisers (PMs) was 0.5% (1/218) in the Korean, 0.7% (2/295) in the Japanese and 0% in the Chinese population. However, the respective mean (+/- s.d.) MRs (0.84 +/- 1.14 and 0.87 +/- 0.90) in the Korean and Japanese extensive metabolisers (EMs) were significantly (P less than 0.001) less than that in the Chinese EMs (2.81 +/- 2.35), and the mode of the distribution histogram and the probit plot of data for Chinese EMs were shifted to the right compared with those for Korean and Japanese EMs. The results indicate that Koreans, like Japanese and mainland Chinese, have a much lower frequency of the PM phenotype of debrisoquine/sparteine-type oxidation compared with that reported for Caucasian populations. Chinese EMs appear to have a lower capacity to metabolise metoprolol to alpha-hydroxymetoprolol compared with Korean or Japanese EMs.

Debrisoquine/sparteine hydroxylation genotype and phenotype: analysis of common mutations and alleles of CYP2D6 in a European population

Four different mutations of the cytochrome P450 CYP2D6 gene associated with the poor metabolizer phenotype (PM) of the debrisoquine/sparteine polymorphism were analyzed by Xba I restriction fragment length polymorphism (RFLP) analysis and a polymerase chain reaction (PCR)-based DNA amplification method in DNA of 394 healthy European subjects; 341 of these were phenotyped by sparteine or debrisoquine administration and urinary metabolic ratios (MR). Our study demonstrates the efficiency of the PCR-test for phenotype prediction; 96.4% of individuals were correctly predicted, i.e., 100% of the extensive metabolizers (EMs) and 86.0% of the poor metabolizers (PMs). In contrast, Xba I RFLP analysis was far less informative, predicting the phenotype in only 26.8% of PMs. By combining both DNA tests, the prediction rate of the PM phenotype increased to 90.6%. A point mutation at a splice-site consensus sequence termed D6-B represented the most common mutant CYP2D6 gene and accounted for more than 75% of mutant alleles. In addition, other known mutations such as D6-D (14%), D6-A (5%), and the rare D6-C mutation bring the identified mutant alleles to greater than 95% of all mutant PM-alleles. Most of Xba I 44-kb alleles were confirmed as mutant alleles carrying the D6-B mutation. However, 9.7% did not have this mutation and may express a functional CYP2D6 gene. Moreover, all Xba I 16 + 9-kb alleles contained the D6-B mutation. Heterozygous EM individuals had a significantly higher MR when compared to homozygous EMs. Genotyping provides an important advantage for investigations of the influence of CYP2D6 activity on drug therapy and its association with certain diseases.

Omeprazole drug interaction studies

This review examines the literature on drug interactions with omeprazole. Different mechanisms have been proposed as potential causes for such interactions. First, the absorption of some drugs might be altered due to the decreased intragastric acidity resulting from omeprazole treatment. There was no effect of omeprazole on the absorption of amoxycillin, bacampicillin and alcohol, while the amount of digoxin and nifedipine absorbed was increased by 10 and 21%, respectively, both increases probably being of no clinical significance. Secondly, the metabolism of high clearance drugs might be altered by changes in liver blood flow, although that is not affected by omeprazole, as indicated by the unchanged elimination of indocyanine green. In addition, the clearance of intravenously administered lidocaine (lignocaine) [a high clearance drug] was unaffected by omeprazole, further indicating that the latter does not alter liver blood flow. Thirdly, since omeprazole is a substituted benzimidazole, it might have the potential to interfere with the metabolism of other drugs by altering the activity of drug metabolising enzymes in the cytochrome P450 system, through either induction or inhibition. There is no indication of induction of this enzyme system in any interaction study with omeprazole. As regards inhibition, on the other hand, there is now considerable information available which indicates that omeprazole has the potential to partly inhibit the metabolism of drugs metabolised to a great extent by the cytochrome P450 enzyme subfamily IIC (diazepam, phenytoin), but not of those metabolised by subfamilies IA (caffeine, theophylline), IID (metoprolol, propranolol) and IIIA (cyclosporin, lidocaine, quinidine). Since relatively few drugs are metabolised mainly by IIC compared with IID and IIIA, the potential for omeprazole to interfere with the metabolism of other drugs appears to be limited.

Genetic analysis of the interethnic difference between Chinese and Caucasians in the polymorphic metabolism of debrisoquine and codeine

The Far Eastern and Caucasian populations are strikingly different with respect to the debrisoquine/sparteine hydroxylation polymorphism. The number of poor metabolizers, as defined for Caucasians, is very low among Chinese and Japanese. We investigated the molecular basis for this difference by analysis of the CYP2D6 gene in 115 Chinese subjects, combined with phenotypic classification of codeine and debrisoquine metabolism. A correlation between the rates of metabolism of these two drugs and genotype, as analyzed by RFLP using XbaI, was observed among the Chinese. A high frequency (37%) of alleles indicative of gene insertions (reflected by XbaI 44kb fragments) was recorded in the Chinese, but was not associated with the poor metabolizer phenotype, as it is in Caucasians. PCR amplification of part of the CYP2D6 gene with mutation specific primers for CYP2D6A (29A) and CYP2D6B (29B) allelic variants revealed that the XbaI 44kb fragment in Chinese apparently contains a functional CYP2D6 gene, in contrast to the situation among Caucasians. The results provide a molecular explanation of the interethnic difference in the metabolism of drugs affected by the debrisoquine hydroxylation polymorphism.

Codeine O-demethylation: rat strain differences and the effects of inhibitors

The oxidative metabolism of more than 20 drugs (e.g. sparteine, debrisoquine, dextromethorphan) is mediated by cytochrome P450IID6. Codeine O-demethylation to morphine was recently demonstrated to co-segregate with the polymorphic metabolism of debrisoquine and dextromethorphan. The female Dark-Agouti rat (DA) is an animal model for the poor metabolizer phenotype (PM) using debrisoquine or dextromethorphan as substrates. Studies were carried out to evaluate codeine metabolism in liver microsomes from female DA and Sprague-Dawley (SD) rats. The intrinsic clearance of codeine to morphine was 10-fold lower in DA rats due to a 5-fold higher Km (287 vs 49 microM) and a 2-fold lower Vmax (48 vs 94 nmol/mg/hr). Nineteen drugs were tested for inhibition of codeine O-demethylation. The four most potent competitive inhibitors were dextromethorphan (Ki = 2.53 microM), propafenone (Ki = 0.58 microM), racemic methadone (Ki = 0.3 microM) and quinine (Ki = 0.07 microM). The differences in morphine formation from codeine between SD and DA rats and the inhibition results show that this animal model appears to be a suitable model for the human EM and PM phenotypes, respectively. These strains could be used to study the pharmacodynamic consequences of the genetic polymorphism in codeine O-demethylation, and the effects of metabolic inhibitors. The outcome of these studies could impact on the therapy of pain control.