Sparteine oxidation is practically abolished in quinidine-treated patients

Oral Adverse Drug Reactions to Cardiovascular Drugs

A great many cardiovascular drugs (CVDs) have the potential to induce adverse reactions in the mouth. The prevalence of such reactions is not known, however, since many are asymptomatic and therefore are believed to go unreported. As more drugs are marketed and the population includes an increasing number of elderly, the number of drug prescriptions is also expected to increase. Accordingly, it can be predicted that the occurrence of adverse drug reactions (ADRs), including the oral ones (ODRs), will continue to increase. ODRs affect the oral mucous membrane, saliva production, and taste. The pathogenesis of these reactions, especially the mucosal ones, is largely unknown and appears to involve complex interactions among the drug in question, other medications, the patient’s underlying disease, genetics, and life-style factors. Along this line, there is a growing interest in the association between pharmacogenetic polymorphism and ADRs. Research focusing on polymorphism of the cytochrome P450 system (CYPs) has become increasingly important and has highlighted the intra- and inter-individual responses to drug exposure. This system has recently been suggested to be an underlying candidate regarding the pathogenesis of ADRs in the oral mucous membrane. This review focuses on those CVDs reported to induce ODRs. In addition, it will provide data on specific drugs or drug classes, and outline and discuss recent research on possible mechanisms linking ADRs to drug metabolism patterns. Abbreviations used will be as follows: ACEI, ACE inhibitor; ADR, adverse drug reaction; ANA, antinuclear antigen; ARB, angiotensin II receptor blocker; BAB, beta-adrenergic blocker; CCB, calcium-channel blocker; CDR, cutaneous drug reaction; CVD, cardiovascular drug; CYP, cytochrome P450 enzyme; EM, erythema multiforme; FDE, fixed drug eruption; I, inhibitor of CYP isoform activity; HMG-CoA, hydroxymethyl-glutaryl coenzyme A; NAT, N-acetyltransferase; ODR, oral drug reaction; RDM, reactive drug metabolite; S, substrate for CYP isoform; SJS, Stevens-Johnson syndrome; SLE, systemic lupus erythematosus; and TEN, toxic epidermal necrolysis.

CYP450 genotype and pharmacogenetic association studies: a critical appraisal

Despite strong pharmacological support, association studies using genotype-predicted phenotype as a variable have yielded conflicting or inconclusive evidence to promote personalized pharmacotherapy. Unless the patient is a genotypic poor metabolizer, imputation of patient's metabolic capacity (or metabolic phenotype), a major factor in drug exposure-related clinical response, is a complex and highly challenging task because of limited number of alleles interrogated, population-specific differences in allele frequencies, allele-specific substrate-selectivity and importantly, phenoconversion mediated by co-medications and inflammatory co-morbidities that modulate the functional activity of drug metabolizing enzymes. Furthermore, metabolic phenotype and clinical outcomes are not binary functions; there is large intragenotypic and intraindividual variability. Therefore, the ability of association studies to identify relationships between genotype and clinical outcomes can be greatly enhanced by determining phenotype measures of study participants and/or by therapeutic drug monitoring to correlate drug concentrations with genotype and actual metabolic phenotype. To facilitate improved analysis and reporting of association studies, we propose acronyms with the prefixes ‘g’ (genotype-predicted phenotype) and ‘m’ (measured metabolic phenotype) to better describe this important variable of the study subjects. Inclusion of actually measured metabolic phenotype, and when appropriate therapeutic drug monitoring, promises to reveal relationships that may not be detected by using genotype alone as the variable.

Pharmacogenetics of drug oxidation via cytochrome P450 (CYP) in the populations of Denmark, Faroe Islands and Greenland

Denmark, the Faroe Islands and Greenland are three population-wise small countries on the northern part of the Northern Hemisphere, and studies carried out here on the genetic control over drug metabolism via cytochrome P450 have led to several important discoveries. Thus, CYP2D6 catalyzes the 2-hydroxylation, and CYP2C19 in part catalyzes the N-demethylation of imipramine. The phenomenon of phenocopy with regard to CYP2D6 was first described when Danish patients changed phenotype from extensive to poor metabolizers during treatment with quinidine. It was a Danish extensive metabolizer patient that became a poor metabolizer during paroxetine treatment, and this was due to the potent inhibition of CYP2D6 by paroxetine, which is also is metabolized by this enzyme. Fluoxetine and norfluoxetine are also potent inhibitors of CYP2D6, and fluvoxamine is a potent inhibitor of both CYP1A2 and CYP2C19. The bioactivation of proguanil to cycloguanil is impaired in CYP2C19 poor metabolizers. The O-demethylation of codeine and tramadol to their respective my-opioid active metabolites, morphine and (+)-O-desmethyltramadol was markedly impaired in CYP2D6 poor metabolizers compared to extensive metabolizers, and this impairs the hypoalgesic effect of the two drugs in the poor metabolizers. The frequency of CYP2D6 poor metabolizers is 2%-3% in Greenlanders and nearly 15% in the Faroese population. The frequency of CYP2C19 poor metabolizers in East Greenlanders is approximately 10%. A study in Danish mono and dizygotic twins showed that the non-polymorphic 3-N-demethylation of caffeine catalyzed by CYP1A2 is subject to approximately 70% genetic control.

Effects of ketoconazole and quinidine on pharmacokinetics of pactimibe and its plasma metabolite, R-125528, in humans

Pactimibe sulfate is a novel acyl coenzyme A:cholesterol acyltransferase inhibitor developed for the treatment of hypercholesterolemia and atherosclerotic diseases. Pactimibe has two equally dominant clearance pathways forming R-125528 by CYP3A4 and M-1 by CYP2D6 in vitro. R-125528 is a plasma metabolite and is cleared solely by CYP2D6 despite its acidity. To evaluate contributions of the cytochrome P450 enzymes on the pharmacokinetics of pactimibe and R-125528 in humans, drug-drug interaction studies using ketoconazole and quinidine were conducted. Eighteen healthy male subjects were given a single dose of pactimibe sulfate without and with 400 mg of ketoconazole (q.d.). With the concomitant treatment, the area under the plasma concentration-time curve (AUC(0-inf)) of pactimibe modestly increased 1.7-fold and AUC(0-tz) of R-125528 decreased by 55%. In addition, 17 healthy male subjects were given a single dose of pactimibe sulfate without and with 600 mg of quinidine (b.i.d.). With the concomitant treatment, the AUC(0-inf) for pactimibe modestly increased 1.7-fold. On the other hand, the AUC(0-tz) of R-125528 was markedly elevated 5.0-fold, although the AUC(0-inf) could not be adequately defined because the terminal elimination phase of R-125528 was not obtained in the study period up to 72 h. As the f(m CYP3A4) and f(m CYP2D6) values of pactimibe estimated from in vitro studies were 0.40 and 0.33, respectively, AUC increase ratios of pactimibe were estimated to be 1.7 with ketoconazole and 1.5 with quinidine. These values were well in accordance with the values observed in this study. Moreover, the f(m CYP2D6) of R-125528 estimated to be almost 1 would well explain the accumulation of R-125528 observed with the quinidine treatment.

Imipramine: a model substance in pharmacokinetic research

The pharmacokinetics of imipramine have been studied over a period of 20 years. Imipramine is rapidly and almost completely metabolized with the formation of desipramine (desmethylation), 2-hydroxy metabolites and subsequent glucoronide coupling. Imipramine has a high clearance (0.8-1.5 l/min) and a corresponding high first pass elimination (30-70%). Volume of distribution is high (650-1100 1) and half-lives accordingly of medium length (6-12 h). The 2-hydroxylation which is the most important step of elimination is mediated via microsomal cytochrome P450 isozyme that exhibit monogenetic polymorphism such that 5-10% of the population have a severely reduced clearance. Co-administration of neuroleptics may also considerably impair the 2-hydroxylation. Steady-state levels of imipramine and desipramine varies considerably among individuals mainly due to variations in metabolism, but also to a smaller extent due to variations in binding to plasma proteins. The variations in steady-state concentrations appear to have clear implication for the clinical effects of imipramine.

Therapeutic use of dextromethorphan: Key learnings from treatment of pseudobulbar affect

A variety of neurological conditions and disease states are accompanied by pseudobulbar affect (PBA), an emotional disorder characterized by uncontrollable outbursts of laughing and crying. The causes of PBA are unclear but may involve lesions in neural circuits regulating the motor output of emotional expression. Several agents used in treating other psychiatric disorders have been applied in the treatment of PBA with some success but data are limited and these agents are associated with unpleasant side effects due to nonspecific activity in diffuse neural networks. Dextromethorphan (DM), a widely used cough suppressant, acts at receptors in the brainstem and cerebellum, brain regions implicated in the regulation of emotional output. The combination of DM and quinidine (Q), an enzyme inhibitor that blocks DM metabolism, has recently been tested in phase III clinical trials in patients with multiple sclerosis and amyotrophic lateral sclerosis and was both safe and effective in palliating PBA symptoms. In addition, clinical studies pertaining to the safety and efficacy of DM/Q in a variety of neurological disease states are ongoing.

Clinical inhibition of CYP2D6-catalysed metabolism by the antianginal agent perhexiline

AIMS

Perhexiline is an antianginal agent that displays both saturable and polymorphic metabolism via CYP2D6. The aim of this study was to determine whether perhexiline produces clinically significant inhibition of CYP2D6-catalysed metabolism in angina patients.

METHODS

The effects of perhexiline on CYP2D6-catalysed metabolism were investigated by comparing urinary total dextrorphan/dextromethorphan metabolic ratios following a single dose of dextromethorphan (16.4 mg) in eight matched control patients not taking perhexiline and 24 patients taking perhexiline. All of the patients taking perhexiline had blood drawn for CYP2D6 genotyping as well as to measure plasma perhexiline and cis-OH-perhexiline concentrations.

RESULTS

Median (range) dextrorphan/dextromethorphan metabolic ratios were significantly higher (P < 0.0001) in control patients, 271.1 (40.3-686.1), compared with perhexiline-treated patients, 5.0 (0.3-107.9). In the perhexiline-treated group 10/24 patients had metabolic ratios consistent with poor metabolizer phenotypes; however, none was a genotypic poor metabolizer. Interestingly, 89% of patients who had phenocopied to poor metabolizers had only one functional CYP2D6 gene. There was a significant negative linear correlation between the log of the dextrorphan/dextromethorphan metabolic ratio and plasma perhexiline concentrations (r(2) = 0.69, P < 0.0001). Compared with patients with at least two functional CYP2D6 genes, those with one functional gene were on similar perhexiline dosage regimens but had significantly higher plasma perhexiline concentrations, 0.73 (0.21-1.00) vs. 0.36 (0.04-0.69) mg l(-1) (P = 0.04), lower cis-OH-perhexiline/perhexiline ratios, 2.85 (0.35-6.10) vs. 6.51 (1.84-11.67) (P = 0.03), and lower dextrorphan/dextromethorphan metabolic ratios, 2.51 (0.33-39.56) vs. 11.80 (2.90-36.93) (P = 0.005).

CONCLUSIONS

Perhexiline significantly inhibits CYP2D6-catalysed metabolism in angina patients. The plasma cis-OH-perhexiline/perhexiline ratio may help to both phenotype patients and predict those in whom perhexiline may be most likely to cause clinically significant metabolic inhibition.

A discordance between cytochrome P450 2D6 genotype and phenotype in patients undergoing methadone maintenance treatment

AIMS

To assess CYP2D6 activity and genotype in a group of patients undergoing methadone maintenance treatment (MMT).

METHODS

Blood samples from 34 MMT patients were genotyped by a polymerase chain reaction-based method, and results were compared with CYP2D6 phenotype (n = 28), as measured by the molar metabolic ratio (MR) of dextromethorphan (DEX)/dextrorphan (DOR) in plasma.

RESULTS

Whereas 9% of patients (3/34) were poor metabolizers (PM) by genotype, 57% (16/28) were PM by phenotype (P < 0.005). Eight patients, who were genotypically extensive metabolizers (EM), were assigned as PM by their phenotype. The number of CYP2D6*4 alleles and sex were significant determinants of CYP2D6 activity in MMT patients, whereas other covariates (methadone dose, age, weight) did not contribute to variation in CYP2D6 activity.

CONCLUSIONS

There was a discordance between genotype and in vivo CYP2D6 activity in MMT patients. This finding is consistent with inhibition of CYP2D6 activity by methadone and may have implications for the safety and efficacy of other CYP2D6 substrates taken by MMT patients.

Physiologically based modelling of inhibition of metabolism and assessment of the relative potency of drug and metabolite: dextromethorphan vs. dextrorphan using quinidine inhibition

AIMS

To define the relative antitussive effect of dextromethorphan (DEX) and its primary metabolite dextrorphan (DOR) after administration of DEX.

METHODS

Data were analysed from a double-blind, randomized cross-over study in which 22 subjects received the following oral treatments: (i) placebo; (ii) 30 mg DEX hydro-bromide; (iii) 60 mg DEX hydro-bromide; and (iv) 30 mg DEX hydro-bromide preceded at 1 h by quinidine HCl (50 mg). Cough was elicited using citric acid challenge. Pharmacokinetic data from all non-placebo arms of the study were fitted simultaneously. The parameters were then used as covariates in a link PK-PD model of cough suppression using data from all treatment arms.

RESULTS

The best-fit PK model assumed two- and one-compartment PK models for DEX and DOR, respectively, and competitive inhibition of DEX metabolism by quinidine. The intrinsic clearance of DEX estimated from the model ranged from 59 to 1536 l x h(-1), which overlapped with that extrapolated from in vitro data (12-261 l x h(-1)) and showed similar variation (26- vs. 21-fold, respectively). The inhibitory effect of quinidine ([I]/Ki) was 19 (95% confidence interval of mean: 18-20) with an estimated average Ki of 0.017 microM. Although DEX and DOR were both active, the potency of the antitussive effect of DOR was 38% that of DEX. A sustained antitussive effect was related to slow removal of DEX/DOR from the effect site (ke0 = 0.07 h(-1)).

CONCLUSIONS

Physiologically based PK modelling with perturbation of metabolism using an inhibitor allowed evaluation of the antitussive potency of DOR without the need for separate administration of DOR.

Oral lichen planus and intake of drugs metabolized by polymorphic cytochrome P450 enzymes

OBJECTIVE

To study if patients with oral lichen planus (OLP) had a medication profile different from that of a control group without oral mucosal lesions. It was hypothesized that OLP lesions might result from poor drug metabolism (PM) because of genetic variation of the major cytochrome P450-enzymes (CYPs with a PM-risk).

SUBJECTS AND METHODS

Dental records of 172 OLP patients were reviewed in this cross-sectional study and 152 sex- and age-matched subjects served as controls. The measures for the drug profiles were medicine type (ATC-code), mono- and polypharmacy, CYP-enzyme metabolism pattern, and medicine with a potential to induce lichenoid drug eruptions.

RESULTS

Fifty per cent of the OLP patients consumed daily medications as compared with 59% of the controls. The OLP patients more frequently consumed medicines metabolized by CYPs with a PM-risk (P = 0.03). Furthermore, they consumed more medicine with an inhibitory effect on one or more CYPs than the controls (P = 0.01).

CONCLUSION

Confounders like sex, age, systemic diseases, drug distribution into the therapeutic classes, and polypharmacy were similar in the two groups; but the OLP patients consumed more drugs metabolized by CYPs with a PM-risk. The results argue for further investigation of associations between OLP, medication intake and the CYP-enzyme metabolic pathways.

Inhibition of debrisoquine hydroxylation with quinidine in subjects with three or more functional CYP2D6 genes

AIMS

To study whether the CYP2D6 capacity in ultrarapid metabolizers of debrisoquine due to duplication/multiduplication of a functional CYP2D6 gene, can be 'normalised' by low doses of the CYP2D6 inhibitor quinidine and whether this is dose-dependent.

METHODS

Five ultrarapid metabolizers of debrisoquine with 3, 4 or 13 functional CYP2D6 genes were given single oral doses of 5, 10, 20, 40, 80 and 160 mg quinidine. Four hours after quinidine intake, 10 mg debrisoquine was given. Urine was collected for 6 h after debrisoquine administration. Debrisoquine and its 4-hydroxymetabolite were analysed by h.p.l.c. and the debrisoquine metabolic ratio (MR) was calculated.

RESULTS

Without quinidine the MR in the ultrarapid metabolizers ranged between 0.01 and 0.07. A dose-effect relationship could be established for quinidine with regard to the inhibitory effect on CYP2D6 activity. To reach an MR of 1-2, subjects with 3 or 4 functional genes required a quinidine dose of about 40 mg, while the sister and brother with 13 functional genes required about 80 mg quinidine. After 160 mg quinidine, the MRs, in the subjects with 3, 3, 4, 13 and 13 functional genes, were 12.6, 10.1, 9.2, 2.4 and 2.2, respectively.

CONCLUSIONS

A dose-effect relationship could be established for quinidine inhibition of CYP2D6 in ultrarapid metabolizers. The clinical use of low doses of quinidine as an inhibitor of CYP2D6 might be considered in ultrarapid metabolizers taking CYP2D6 metabolized drugs rather than giving increased doses of the drug. Normalizing the metabolic capacity of CYP2D6, by giving a low dose of quinidine, may solve the problem of 'treatment resistance' caused by ultrarapid metabolism.

The antitussive effect of dextromethorphan in relation to CYP2D6 activity

AIMS

To test the hypothesis that inhibition of cytochrome P450 2D6 (CYP2D6) by quinidine increases the antitussive effect of dextromethorphan (DEX) in an induced cough model.

METHODS

Twenty-two healthy extensive metaboliser phenotypes for CYP2D6 were studied according to a double-blind, randomised cross-over design after administration of: (1) Placebo antitussive preceded at 1 h by placebo inhibitor; (2) 30 mg oral DEX preceded at 1 h by placebo inhibitor (DEX30); (3) 60 mg oral DEX preceded at 1 h by placebo inhibitor (DEX60); (4) 30 mg oral DEX preceded at 1 h by 50 mg oral quinidine sulphate (QDEX30). Cough frequency following inhalation of 10% citric acid was measured at baseline and at intervals up to 12 h. Plasma concentrations of DEX and its metabolites were measured up to 96 h by h.p.l.c.

RESULTS

Inhibition of CYP2D6 by quinidine caused a significant increase in the mean ratio of DEX to dextrorphan (DEX:DOR) plasma AUC(96) (0.04 vs 1.81, P<0.001). The mean (+/-s.d.) decrements in cough frequency below baseline over 12 h (AUEC) were: 8% (11), 17% (14.5), 25% (16.2) and 25% (16.9) for placebo, DEX30, DEX60 and QDEX30 treatments, respectively. Statistically significant differences in antitussive effect were detected for the contrasts between DEX60/placebo (P<0.001; 95% CI of difference +80, +327) and QDEX30/placebo (P<0.001, +88, +336), but not for DEX30/placebo, DEX30/DEX60 or DEX30/QDEX30 (P=0.071, -7, +241; P=0.254, -37, +211; P=0.187, -29, +219, respectively).

CONCLUSIONS

A significant antitussive effect was demonstrated after 60 mg dextromethorphan and 30 mg dextromethorphan preceded by 50 mg quinidine using an induced cough model. However, although the study was powered to detect a 10% difference in cough response, the observed differences for other contrasts were less than 10%, such that it was possible only to imply a dose effect (30 vs 60 mg) in the antitussive activity of DEX and enhancement of this effect by CYP2D6 inhibition.

The visceral and somatic antinociceptive effects of dihydrocodeine and its metabolite, dihydromorphine. A cross-over study with extensive and quinidine-induced poor metabolizers

AIMS

Dihydrocodeine is metabolized to dihydromorphine via the isoenzyme cytochrome P450 2D6, whose activity is determined by genetic polymorphism. The importance of the dihydromorphine metabolites for analgesia in poor metabolizers is unclear. The aim of this study was to assess the importance of the dihydromorphine metabolites of dihydrocodeine in analgesia by investigating the effects of dihydrocodeine on somatic and visceral pain thresholds in extensive and quinidine-induced poor metabolizers.

METHODS

Eleven healthy subjects participated in a double-blind, randomized, placebo-controlled, four-way cross-over study comparing the effects of single doses of placebo and slow-release dihydrocodeine 60 mg with and without premedication with quinidine sulphate 50 mg on electrical, heat and rectal distension pain tolerance thresholds. Plasma concentrations and urinary excretion of dihydrocodeine and dihydromorphine were measured.

RESULTS

In quinidine-induced poor metabolizers the plasma concentrations of dihydromorphine were reduced between 3 and 4 fold from 1.5 h to 13.5 h after dosing (P < 0.005) and urinary excretion of dihydromorphine in the first 12 h was decreased from 0.91% to 0.28% of the dihydrocodeine dose (P < 0.001). Dihydrocodeine significantly raised the heat pain tolerance thresholds (at 3.3 h and 5 h postdosing, P < 0.05) and the rectal distension defaecatory urge (at 3.3 h and 10 h postdosing, P < 0.02) and pain tolerance thresholds (at 3.3 h and 5 h postdosing, P < 0.05) compared with placebo. Premedication with quinidine did not change the effects of dihydrocodeine on pain thresholds, but decreased the effect of dihydrocodeine on defaecatory urge thresholds (at 1.5 h, 3.3 h and 10 h postdosing, P < 0.05).

CONCLUSIONS

In quinidine-induced poor metabolizers significant reduction in dihydromorphine metabolite production did not result in diminished analgesic effects of a single dose of dihydrocodeine. The metabolism of dihydrocodeine to dihydromorphine may therefore not be of clinical importance for analgesia. This conclusion must however, be confirmed with repeated dosing in patients with pain.

Use of in vitro and in vivo data to estimate the likelihood of metabolic pharmacokinetic interactions

This article reviews the information available to assist pharmacokineticists in the prediction of metabolic drug interactions. Significant advances in this area have been made in the last decade, permitting the identification in early drug development of dominant cytochrome P450 (CYP) isoform(s) metabolising a particular drug as well as the ability of a drug to inhibit a specific CYP isoform. The major isoforms involved in human drug metabolism are CYP3A, CYP2D6, CYP2C, CYP1A2 and CYP2E1. Often patients are taking multiple concurrent medications, and thus an assessment of potential drug-drug interactions is imperative. A database containing information about the clearance routes for over 300 drugs from multiple therapeutic classes, including analgesics, anti-infectives, psychotropics, anticonvulsants, cancer chemotherapeutics, gastrointestinal agents, cardiovascular agents and others, was constructed to assist in the semiquantitative prediction of the magnitude of potential interactions with drugs under development. With knowledge of the in vitro inhibition constant of a drug (Ki) for a particular CYP isoform, it is theoretically possible to assess the likelihood of interactions for a drug cleared through CYP-mediated metabolism. For many agents, the CYP isoform involved in metabolism has not been identified and there is substantial uncertainty given the current knowledge base. The mathematical concepts for prediction based on competitive enzyme inhibition are reviewed in this article. These relationships become more complex if the inhibition is of a mixed competitive/noncompetitive nature. Sources of uncertainty and inaccuracy in predicting the magnitude of in vivo inhibition includes the nature and design of in vitro experiments to determine Ki, inhibitor concentration in the hepatic cytosol compared with that in plasma, prehepatic metabolism, presence of active metabolites and enzyme induction. The accurate prospective prediction of drug interactions requires rigorous attention to the details of the in vitro results, and detailed information about the pharmacokinetics and metabolism of the inhibitor and inhibited drug. With the discussion of principles and accompanying tabulation of literature data concerning the clearance of various drugs, a framework for reasonable semiquantitative predictions is offered in this article.

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.

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.

Impact of quinidine on plasma and cerebrospinal fluid concentrations of codeine and morphine after codeine intake

OBJECTIVE

The analgesic effect of codeine depends on its O-demethylation to morphine via sparteine oxygenase (CYP2D6) in the liver and presumably also via this enzyme in the CNS. We studied the ability of quinidine, which is a potent inhibitor of CYP2D6, to penetrate the blood brain barrier and its possible impact on codeine O-demethylation in CNS.

METHODS

The study comprised 16 extensive and one poor metaboliser of sparteine, who underwent spinal anaesthesia for urinary tract surgery or examination. Eight patients were given an oral dose of 125 mg codeine and 9 patients (including the poor metaboliser) were given 200 mg quinidine 2 h before the same dose of codeine. Plasma and spinal fluid samples were collected 2 h after codeine intake.

RESULTS

Free concentrations of quinidine were 11-times lower in cerebrospinal fluid than in plasma, and ranged from 9-15 nmol.l-1. Morphine concentrations were significantly lower in patients pre-treated with quinidine, both in plasma (median 1.45 nmol.l-1, range 0.74-1.95 nmol.l-1 vs 9.86 nmol.l-1, range 4.59-28.4 nmol.l-1) and in cerebrospinal fluid (0.23, 0.16-0.61 nmol.l-1 vs 3.63, 0.6-8.09 nmol.l-1). The morphine/codeine concentration ratio in plasma (3.07 x 10 (-3), 1.68-3.68 x 10 (-3) vs 19.87 x 10 (-3), 9.87-66.22 x 10 (-3) and in cerebrospinal fluid (0.83 d 10 (-3), 0.58-1.45 x 10 (-3) vs 7.19 x 10 (-3), 2.03-17.7 x 10 (-3) was also lower. The morphine/codeine concentration ratios were significantly lower in cerebrospinal fluid both without and with quinidine, but the difference between the plasma and spinal fluid ratio was significantly smaller with quinidine than without (p = 0.0002).

CONCLUSION

Quinidine penetrates the blood brain barrier poorly, but quinidine pre-treatment leads to pronounced lowering of the cerebrospinal fluid concentration of morphine after codeine intake. However, the O-demethylation of codeine in CNS may not be totally blocked by quinidine.

Lack of relationship between quinidine pharmacokinetics and the sparteine oxidation polymorphism

Quinidine is a very potent inhibitor of CYP2D6, but the role of the enzyme in the biotransformation of quinidine has only been investigated in a single in vitro study and in two small in vivo experiments, with contradictory results. The present investigation was designed to present definite evaluation of whether quinidine is metabolised by CYP2D6. Eight poor metabolizers (PM) and 8 extensive metabolizers (EM) of sparteine each took one oral dose of 200 mg quinidine. In the EM, the total clearance, the clearance via 3-hydroxylation and the clearance via N-oxidation, were 33, 3.7 and 0.23 l.h-1, respectively. In the PM, the corresponding values were 29, 3.1 and 0.18 l.h-1, respectively. There were no statistically significant differences between EM and PM in any of these pharmacokinetic parameters. It is concluded that CYP2D6 is not an important enzyme for the oxidation of quinidine.

Determination of quinidine, dihydroquinidine, (3S)-3-hydroxyquinidine and quinidine N-oxide in plasma and urine by high-performance liquid chromatography

A specific and sensitive method for the quantitation of quinidine, (3S)-3-hydroxyquinidine, quinidine N-oxide, and dihydroquinidine in plasma and urine has been developed. The method is based on a single-step, liquid-liquid extraction procedure, followed by isocratic reversed-phase high-performance liquid chromatography, with fluorescence detection. After extraction from 250 microliters plasma and 100 microliters urine, the limit of determination is 10 nM and 25 nM, respectively. For the use as standards, commercially available quinidine was purified from dihydroquinidine; quinidine N-oxide was synthesized.

Are poor metabolisers of sparteine/debrisoquine less pain tolerant than extensive metabolisers?

It has recently been shown that O-demethylation of the opioid drug codeine to morphine depends on the sparteine/debrisoquine oxygenase (CYP2D6) which in man exhibits genetic polymorphism. Morphine may be an endogenously formed substance in mammalians. Therefore, it may be hypothesized that the final step in an endogenous synthesis of morphine from codeine also depends on CYP2D6. CYP2D6, which is present in the liver and presumably also in the brain, is not expressed in subjects who are poor metabolisers of the sparteine/debrisoquine type. We have determined sensitivity to painful stimuli in 94 extensive metabolisers and 82 poor metabolisers of sparteine in 2 phasic (pain thresholds to heat and pressure) and 1 tonic (cold pressor test) experimental pain model. Extensive and poor metabolisers did not differ significantly in the 2 phasic pain models neither with respect to pain detection nor pain tolerance thresholds. However, for the cold pressor test, peak pain ratings and area under the pain rating-time curve during 2 min were significantly higher in poor than in extensive metabolisers (P = 0.0024 and 0.044). Furthermore, a substantially higher fraction of poor metabolisers prematurely withdrew their hand from the ice water during the cold pressor test due to intolerable pain (32 vs. 18%, P = 0.0545). We conclude that poor metabolisers of sparteine may be less tolerant to tonic pain than extensive metabolisers, and we hypothesize that this may be related to an inherited defect in endogenous synthesis of morphine via CYP2D6 in the brain.

The effect of a low dose of quinidine on the disposition of flecainide in healthy volunteers

We have studied the effects of quinidine on ECG intervals and on the pharmacokinetics of flecainide and its two metabolites in 6 healthy men in an open randomized crossover study. Flecainide acetate (150 mg) was given as a constant rate i.v. infusion over 30 min. Quinidine (50 mg orally), given the previous evening, did not change the volume of distribution of flecainide (7.9 vs 7.4 l.kg-1), but significantly increased its half-life (8.8 vs 10.7 h). This was attributable to a reduction in total clearance (10.6 vs 8.1 ml.min-1 x kg-1), most of it being accounted for by a reduction in non-renal clearance (7.2 vs 5.2 ml.min-1 x kg-1). The excretion of the metabolites of flecainide over 48 h was significantly reduced. These findings suggest that quinidine inhibits the first step of flecainide metabolism, although it may also reduce its renal clearance, but to a lesser extent (3.5 vs 2.9 ml.min-1 x kg-1). The effects of flecainide on ECG intervals were not altered by quinidine. Thus, quinidine tends to shift extensive metabolizer status for flecainide towards poor metabolizer status and may also alter its renal excretion.

The effect of quinidine on the analgesic effect of codeine

We have studied the hypoalgesic effect of codeine (100 mg) after blocking the hepatic O-demethylation of codeine to morphine via the sparteine oxygenase (CYP2D6) by quinidine (200 mg). The study was performed in 16 extensive metabolizers of sparteine, using a double-blind, randomized, four-way, cross-over design. The treatments given at 3 h intervals during the four sessions were placebo/placebo, quinidine/placebo, placebo/codeine, and quinidine/codeine. We measured pinprick pain and pain tolerance thresholds to high energy argon laser stimuli before and 1, 2, and 3 h after codeine or placebo. After codeine and placebo, the peak plasma concentration of morphine was 6-62 (median 18) nmol.l-1. When quinidine pre-treatment was given, no morphine could be detected (less than 4 nmol.l-1) after codeine. The pin-prick pain thresholds were significantly increased after placebo/codeine, but not after quinidine/codeine compared with placebo/placebo. Both placebo/codeine and quinidine/codeine increased pain tolerance thresholds significantly. Quinidine/codeine and quinidine/placebo did not differ significantly for either pin-prick or tolerance pain thresholds. These results are compatible with local CYP2D6 mediated formation of morphine in the brain, not being blocked by quinidine. Alternatively, a hypoalgesic effect of quinidine might have confounded the results.

Stereoselective genetically-determined interaction between chronic flecainide and quinidine in patients with arrhythmias

1. Recent reports have indicated a role for the P450IID6 polymorphism in the stereoselective disposition of single doses of the antiarrhythmic flecainide. 2. In this study, we evaluated the effects of adding low dose quinidine, a potent inhibitor of P450IID6, to chronic flecainide therapy in patients with arrhythmias. 3. In five extensive metabolizer patients, quinidine significantly reduced the clearance of R-(-)-flecainide, from 395 +/- 121 (s.d.) to 335 +/- 88 ml min-1. This change was attributable to a decrease in metabolic clearance, was accompanied by decreased formation of the two major metabolites of flecainide and was not observed in a poor metabolizer subject. The renal clearance of R-(-)-flecainide rose significantly. 4. Quinidine did not alter the clearance of S-(+)-flecainide. 5. The pharmacologic effects of flecainide therapy (QRS widening, % arrhythmia suppression) were slightly, but not significantly, increased. 6. In extensive metabolizer patients receiving chronic flecainide, increased plasma concentrations will develop if P450IID6 is inhibited.

Disposition and metabolism of codeine after single and chronic doses in one poor and seven extensive metabolisers

1. The pharmacokinetics, metabolism and partial clearances of codeine to morphine, norcodeine and codeine-6-glucuronide after single (30 mg) and chronic (30 mg 8 h for seven doses) administration of codeine were studied in eight subjects (seven extensive and one poor metaboliser of dextromethorphan). Codeine, codeine-6-glucuronide, morphine and norcodeine were measured by high performance liquid chromatographic assays. 2. After the single dose, the time to achieve maximum plasma codeine concentrations was 0.97 +/- 0.31 h (mean +/- s.d.) and for codeine-6-glucuronide it was 1.28 +/- 0.49 h. The plasma AUC of codeine-6-glucuronide was 15.8 +/- 4.5 times higher than that of codeine. The AUC of codeine in saliva was 3.4 +/- 1.1 times higher than that in plasma. The elimination half-life of codeine was 3.2 +/- 0.3 h and that of codeine-6-glucuronide was 3.2 +/- 0.9 h. 3. The renal clearance of codeine was 183 +/- 59 ml min-1 and was inversely correlated with urine pH (r = 0.81). These data suggest that codeine undergoes filtration at the glomerulus, tubular secretion and passive reabsorption. The renal clearance of codeine-6-glucuronide was 55 +/- 21 ml min-1, and was not correlated with urine pH. Its binding to human plasma was less than 10%. These data suggest that codeine-6-glucuronide undergoes filtration at the glomerulus and tubular reabsorption. This latter process is unlikely to be passive. 4. After chronic dosing, the pharmacokinetics of codeine and codeine-6-glucuronide were not significantly different from the single dose pharmacokinetics. 5. After the single dose, 86.1 +/- 11.4% of the dose was recovered in urine, of which 59.8 +/- 10.3% was codeine-6-glucuronide, 7.1 +/- 1.1% was total morphine, 6.9 +/- 2.1% was total norcodeine and 11.8 +/- 3.9% was unchanged codeine. These recoveries were not significantly different (P greater than 0.05) after chronic administration. 6. After the single dose, the partial clearance to morphine was 137 +/- 31 ml min-1 in the seven extensive metabolisers and 8 ml min-1 in the poor metaboliser; to norcodeine the values were 103 +/- 33 ml min-1 and 90 ml min-1; to codeine-6-glucuronide the values were 914 +/- 129 ml min-1 and 971 ml min-1; and intrinsic clearance was 1568 +/- 103 ml min-1 and 1450 ml min-1. These values were not significantly (P greater than 0.05) altered by chronic administration.(ABSTRACT TRUNCATED AT 400 WORDS)

Clinical consequences of polymorphic drug oxidation

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

Impairment of debrisoquine 4-hydroxylase and related monooxygenase activities in the rat following treatment with propranolol

The effect of repetitive oral administration of propranolol on hepatic microsomal drug metabolizing enzyme activities in the rat was investigated. Propranolol ring (4-, 5- and 7-)hydroxylase activities were markedly decreased, but, interestingly, N-desisopropylase activity was increased after propranolol administration. A marked decrease in enzyme activity after propranolol pretreatment was also observed with debrisoquine 4-hydroxylation. In addition, a similar decrease was observed with imipramine 2-hydroxylation which co-segregates with debrisoquine/sparteine type polymorphic drug oxidation, but not with imipramine N-demethylation. These results suggest the selective impairment of debrisoquine 4-hydroxylase by propranolol pretreatment.

Quinidine but not quinine inhibits in man the oxidative metabolic routes of methoxyphenamine which involve debrisoquine 4-hydroxylase

Healthy male volunteers (n = 13) took a single oral dose of 60.3 mg of methoxyphenamine HCl with and without prior administration of either quinidine (250 mg as bisulphate salt) or its diastereomer quinine (300 mg as sulphate salt). Methoxyphenamine and its N-desmethyl, O-desmethyl and aromatic 5-hydroxy metabolites were quantified in the 0-32 h urine. The oxidative routes of methoxyphenamine metabolisms which had been previously shown to involve debrisoquine 4-hydroxylase, namely O-demethylation and 5-hydroxylation were both significantly inhibited by quinidine in the 12 extensive metabolizers. The inhibition was selective in that N-demethylation which does not involve this isozyme was not affected by quinidine. In all but one of these volunteers the methoxyphenamine/O-desmethylmethoxyphenamine ratio changed such that extensive metabolizers could be classified as poor metabolizers due to quinidine pretreatment. No marked change occurred in the renal excretion of methoxyphenamine and its three metabolites either in the extensive metabolizers because of quinine pretreatment or in the poor metabolizer because of treatment with either quinidine or quinine. Thus in the extensive metabolizer phenotype it was demonstrated in one study that enzyme inhibition of quinidine was selective in terms of the metabolic pathways inhibited as well as stereoselective with respect to the inhibitor.

Single-dose quinidine treatment inhibits mexiletine oxidation in extensive metabolizers of debrisoquine

Urinary elimination of unchanged mexiletine, p-hydroxymexiletine (PHM), hydroxymethylmexiletine (HMM) and mexiletine N-glucuronide conjugate (MGC) was investigated before and after treatment with quinidine. All subjects were phenotyped as extensive metabolizers for debrisoquine oxidation. The total recovery of mexiletine and metabolites was significantly reduced after quinidine pretreatment. It is concluded that pretreatment with a very low dose of quinidine inhibits markedly the elimination of both major mexiletine metabolites (PHM and HMM) and likely decreases the overall elimination of mexiletine. That should lead to changes in mexiletine disposition and have clinical consequences during combination therapy with both drugs.

Inhibitory studies of mexiletine and dextromethorphan oxidation in human liver microsomes

The cytochrome P-450dbl isozyme (P-450bdl) is responsible for the genetic sparteine-debrisoquine type polymorphism of drug oxidation in humans. To investigate the relationship between mexiletine oxidation and the activity of this isozyme, cross-inhibition studies were performed in human liver microsomes with mexiletine and dextromethorphan, a prototype substrate for P-450dbl. The formation of hydroxymethylmexiletine and p-hydroxymexiletine, two major mexiletine metabolites, was competitively inhibited by dextromethorphan. Mexiletine competitively inhibited the high affinity component of dextromethorphan O-demethylation. In addition, there was a good agreement between the apparent Km values for the formation of both mexiletine metabolites and the high affinity component of dextromethorphan O-demethylation and their respective apparent Ki values. Several drugs were tested for their ability to inhibit mexiletine oxidation. Quinidine, quinine, propafenone, oxprenolol, propranolol, ajmaline, desipramine, imipramine, chlorpromazine and amitryptiline were competitive inhibitors for the formation of hydroxymethylmexiletine and p-hydroxymexiletine as for prototype reactions of the sparteine-debrisoquine type polymorphism. Amobarbital, valproic acid, ethosuximide, caffeine, theophylline, disopyramide and phenytoin, known to be non-inhibitors of P-450dbl activity, were found not to inhibit the formation of these mexiletine metabolites. Moreover, the formation of both metabolites was strongly inhibited by an antiserum containing anti-liver/kidney microsomes antibodies type I (anti-LKMI) directed against P-450dbl. These data suggest that the formation of two major metabolites of mexiletine is predominantly catalysed by the genetically variable human liver P-450dbl.

Recent developments in hepatic drug oxidation. Implications for clinical pharmacokinetics

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

A dose-effect study of the in vivo inhibitory effect of quinidine on sparteine oxidation in man

1. Twelve healthy extensive metabolisers of sparteine were sparteine tested daily for 6 days (19.00 h to 07.00 h). A small but statistically significant rise in sparteine metabolic ratio (MR) was observed. 2. Following 100 mg quinidine sulphate given to four of the subjects at 16.00 h, sparteine tests were carried out 19.00 h to 07.00 h on the same day and then daily for 6 days. Quinidine caused an immediate twenty-fold increase in sparteine-MR which then gradually returned to normal over the following 4-6 days. Quinidine concentrations in plasma were measurable only up to 20 h after the quinidine test dose. 3. At weekly intervals, all 12 subjects received single doses of quinidine sulphate of 5, 10, 20, 40 and 80 mg at 16.00 h, each time followed by a sparteine test 19.00 h to 07.00 h on the same day. A clear dose-effect relationship was found with a significant rise in the sparteine-MR even after 5 mg quinidine. After 80 mg quinidine, 8 of 12 subjects became phenotypically poor metabolisers (MR greater than 20).

Quinidine kinetics after a single oral dose in relation to the sparteine oxidation polymorphism in man

The kinetics at a single oral dose (400 mg) of quinidine were studied in four extensive metabolizers (EM) and four poor metabolizers (PM) of sparteine. The clearance of quinidine by 3-hydroxylation was significantly lower in PM than in EM, but the difference was small (25-30%). This finding suggests that 3-hydroxylation, in part, is catalyzed by the same isoenzyme of cytochrome P450, P450db1 which oxidizes sparteine. Otherwise, no significant phenotypic differences in total or metabolic clearance were found and it is concluded that the metabolism of quinidine is largely carried out by P450 isoenzymes different from P450db1. A biexponential decline in the log plasma quinidine concentration vs time curves was observed in all subjects, and the mean elimination half-life was 11-12 h. This is about twice as long as generally reported in the literature.

The genetic polymorphism of debrisoquine/sparteine metabolism-molecular mechanisms

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

Testing for bimodality in frequency distributions of data suggesting polymorphisms of drug metabolism--histograms and probit plots

1. The shape of histograms used to illustrate density distributions of indices of polymorphic drug metabolism was shown to be sensitive to the position of the cell divisions. 2. Non-linearity of the probit plot was shown not to indicate bimodality of the original density distribution. Computer simulation was used to generate examples of unimodal density distributions with curvilinear probit plots. 3. Using the same technique probit plots for bimodal density distributions were constructed. Some were shown to differ less from the probit plots of certain unimodal distributions than did the original density distributions. 4. The position of the antimode was shown not to coincide with inflections seen in the probit plots. 5. A new method for determining the linearity of probit plots is suggested.

Metabolic disposition of ajmaline

Urine was collected from six patients receiving a continuous infusion of 20 mg/h ajmaline. Pooled urine was extracted with and without enzymatic conjugate cleavage or hydrolysis with concentrated hydrochloric acid. The extracts were analyzed by gas chromatography/mass spectrometry. Ajmaline and its metabolites in urine were identified in the form of their acetylated derivatives. Twenty two different acetylated derivatives of ajmaline and its metabolites could be detected. Three of these derivatives were artifacts generated by acetylation and/or thermal decomposition. The major metabolic pathways were mono- and di-hydroxylation of the benzene ring with subsequent O-methylation, reduction of the C-21, oxidation of the C-17 and C-21-hydroxyl function, N-oxidation, and a combination of these metabolic steps. Ajmaline and its metabolites were mainly excreted in the form of their conjugates. Furthermore, the interference of sparteine, debrisoquine, quinidine, and nifedipine with ajmaline metabolism was studied with semiquantitative thin-layer chromatography. Ajmaline metabolism was inhibited by co-administration of sparteine or quinidine, but not by debrisoquine or nifedipine. Sparteine most likely competed with ajmaline metabolism. Quinidine probably bound competitively to ajmaline-metabolizing enzymes without being metabolized itself. Additionally, the metabolic ratio of hydroxyajmaline/ajmaline in urine was determined in 9 extensive metabolizers and one poor metabolizer of dextromethorphan. The poor metabolizer had a significantly reduced metabolic ratio of hydroxyajmaline/ajmaline, which indicates that ajmaline metabolism probably co-segregates with polymorphic sparteine/debrisoquine/dextromethorphan metabolism.

Effects of ketoconazole on the polymorphic 4-hydroxylations of S-mephenytoin and debrisoquine

Studies were undertaken in 12 normal, male subjects to determine whether a metabolic interaction occurs between ketoconazole and mephenytoin. A single dose (400 mg) of ketoconazole produced a reduction in the 0-8 h urinary R/S ratio of mephenytoin following oral administration (100 mg) of racemic drug and after 28 daily doses the median value was further reduced to 42.9% of its baseline value. Within 7 days following discontinuation of ketoconazole the enantiomeric ratio had returned to its pre-study value. These findings are consistent with ketoconazole being a potent in vivo inhibitor of mephenytoin's 4-hydroxylation and confirm the ability of such an interaction to be predicted by in vitro studies with human liver microsomes. By contrast, ketoconazole had a much smaller effect on the 0-8 h urinary metabolic ratio of debrisoquine, indicating that ketoconazole has a selective inhibitory effect on different forms of cytochrome P-450.

Effect of quinidine on the dextromethorphan O-demethylase activity of microsomal fractions from human liver

1. The kinetics of dextromethorphan O-demethylation were measured in microsomes prepared from five human livers, both in the absence and in the presence of quinidine. 2. For each liver and over the concentration range of dextromethorphan examined (4.2-3400 microM), this reaction involved an enzymatic component of high affinity, with an apparent Michaelis-Menten constant (Km) of 4.6 +/- 1.8 microM (mean +/- s.d.) and a maximum velocity (Vmax) of 4.2 +/- 3.5 nmol mg-1 h-1 (mean +/- s.d.). 3. Quinidine was a potent and competitive inhibitor of the activity of this component (mean Ki +/- s.d. of 0.025 +/- 0.008 microM) as it is for other oxidation reactions which have already been found to co-segregate with the debrisoquine-type polymorphism. 4. With microsomes from four of the five livers studied, there was evidence of a second enzymatic component of activity characterized by a similar Vmax and about 20-fold higher Km compared with the high affinity component. The activity of this low affinity component was unaffected by quinidine in the concentrations studied.

Genetically-determined interaction between propafenone and low dose quinidine: role of active metabolites in modulating net drug effect

1. Quinidine is a potent inhibitor of the genetically-determined debrisoquine 4-hydroxylation. Oxidation reactions of several other drugs, including the 5-hydroxylation of the new antiarrhythmic drug propafenone, depend on the isozyme responsible for debrisoquine 4-hydroxylation. 2. The effect of quinidine on the debrisoquine phenotype-dependent 5-hydroxylation and the pharmacological activity of propafenone was studied in seven 'extensive' metabolizers and two 'poor' metabolizers of the drug receiving propafenone for the treatment of ventricular arrhythmias. 3. In patients with the extensive metabolizer phenotype, quinidine increased mean steady-state plasma propafenone concentrations more than two fold, from 408 +/- 351 (mean +/- s.d.) to 1096 +/- 644 ng ml-1 (P less than 0.001), decreased 5-hydroxypropafenone concentrations from 242 +/- 196 to 125 +/- 97 ng ml-1 (P less than 0.02) and reduced propafenone oral clearance by 58 +/- 23%. 4. Despite these changes in plasma concentrations, electrocardiographic intervals and arrhythmia frequency were unaltered by quinidine coadministration, indicating that 5-hydroxypropafenone contributes to the pharmacologic effects of propafenone therapy in extensive metabolizers. 5. In contrasts, plasma concentrations of propafenone and 5-hydroxypropafenone remained unchanged in the two patients with the poor metabolizer phenotype. 6. Biotransformation of substrates for the debrisoquine pathway can be markedly perturbed by even low doses of quinidine; interindividual variability in drug interactions may have a genetic component.

Selective in vivo inhibition by quinidine of methoxyphenamine oxidation in rat models of human debrisoquine polymorphism

1. Lewis and Dark Agouti (DA) rat strains (n = 4), models of human extensive and poor metabolizer phenotypes of debrisoquine/sparteine, respectively, were dosed with methoxyphenamine with and without prior administration of quinidine. Methoxyphenamine and its three metabolites, namely N-desmethylmethoxyphenamine, O-desmethylmethoxyphenamine and 5-hydroxymethoxyphenamine were quantified in 0-24 h urine. 2. The oxidative metabolic routes of methoxyphenamine which had been previously shown to involve the debrisoquine/sparteine isozyme, namely O-demethylation and aromatic 5-hydroxylation, were both significantly inhibited by quinidine in the two rat strains. 3. The oxidative metabolic route of methoxyphenamine which had been previously shown to not involve the debrisoquine/sparteine isozyme, namely N-demethylation, was not significantly inhibited by quinidine in either rat strain. 4. The Lewis strain pretreated with quinidine resembled the DA strain without such pretreatment in terms of O-desmethylmethoxyphenamine and 5-hydroxymethoxyphenamine in that the mean percentages of the dose excreted as these two metabolites and the mean O-desmethylmethoxyphenamine/methoxyphenamine and 5-hydroxymethoxyphenamine/methoxyphenamine ratios were similar to one another. 5. Ten days after quinidine administration to the Lewis strain of rat, all parameters of methoxyphenamine and its metabolites returned to normal. 6. A protocol involving substrate administration to Lewis strain rats with and without prior administration of quinidine could be developed as an attractive approach to screen substrates for metabolism in vivo by the debrisoquine/sparteine isozyme. Such an approach obviates interstrain differences.

Substantial rise in sparteine metabolic ratio during haloperidol treatment

A sparteine test was carried out in 14 patients suffering from acute schizophrenic psychoses before and 1-2 times during oral haloperidol treatment in doses of 10-40 mg day-1. In patients classified as extensive metabolisers (sparteine MR less than 20 before treatment), haloperidol treatment resulted in a rise in sparteine MR that correlated with the serum-haloperidol concentration both within and between patients. At the highest serum haloperidol concentrations (60-80 nM) an increase in sparteine MR by a factor 15-50 was seen, but no patients were transformed into phenotypically poor metabolisers. The steady state concentration of haloperidol on the initial standard dose of 10 mg day-1 was the same in one patient classified as a sparteine poor metaboliser (MR = 112) as in eleven patients classified as extensive metabolisers (MR:0.22-1.47).

Quinidine inhibits the 2-hydroxylation of imipramine and desipramine but not the demethylation of imipramine

On separate occasions 6 extensive metabolizers of sparteine took a single oral dose of 100 mg imipramine and desipramine before and during the intake of quinidine sulphate 200 mg/day. During quinidine the total oral clearance of imipramine on average was reduced by 35%, and that of desipramine by 85%. The clearance of imipramine via demethylation was not significantly reduced during quinidine administration, whereas its clearance by other pathways, largely 2-hydroxylation, was reduced by more than 50%. 2-OH-Imipramine and 2-OH-desipramine were detected in plasma before (maximum concentrations 30-100 nmol.l-1) but not during quinidine. It appears that quinidine is a potent inhibitor of the sparteine/debrisoquine oxygenase, P450dbl, which is responsible for the 2-hydroxylation of imipramine and desipramine, but not of the P450 isozyme responsible for the demethylation of imipramine.

Clinical significance of the sparteine/debrisoquine oxidation polymorphism

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