CYP2D6 genotype and phenotyping by determination of dextromethorphan and metabolites in serum of healthy controls and of patients under psychotropic medication

Quantification of Tafenoquine and 5,6-Orthoquinone Tafenoquine by UHPLC-MS/MS in Blood, Plasma, and Urine, and Application to a Pharmacokinetic Study

Analytical methods for the quantification of the new 8-aminoquinoline antimalarial tafenoquine (TQ) in human blood, plasma and urine, and the 5,6-orthoquinone tafenoquine metabolite (5,6-OQTQ) in human plasma and urine have been validated. The procedure involved acetonitrile extraction of samples followed by ultra-high-performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS). Chromatography was performed using a Waters Atlantis T3 column with a gradient of 0.1% formic acid and acetonitrile at a flow rate of 0.5 mL per minute for blood and plasma. Urine analysis was the same but with methanol containing 0.1% formic acid replacing acetonitrile mobile phase. The calibration range for TQ and 5,6-OQTQ in plasma was 1 to 1200 ng/mL, and in urine was 10 to 1000 ng/mL. Blood calibration range for TQ was 1 to 1200 ng/mL. Blood could not be validated for 5,6-OQTQ due to significant signal suppression. The inter-assay precision (coefficient of variation %) was 9.9% for TQ at 1 ng/mL in blood (n = 14) and 8.2% for TQ and 7.1% for 5,6-OQTQ at 1 ng/mL in plasma (n = 14). For urine, the inter-assay precision was 8.2% for TQ and 6.4% for 5,6-OQTQ at 10 ng/mL (n = 14). TQ and 5,6-OQTQ are stable in blood, plasma and urine for at least three months at both -80 °C and -20 °C. Once validated, the analytical methods were applied to samples collected from healthy volunteers who were experimentally infected with Plasmodium falciparum to evaluate the blood stage antimalarial activity of TQ and to determine the therapeutic dose estimates for TQ, the full details of which will be published elsewhere. In this study, the measurement of TQ and 5,6-OQTQ concentrations in samples from one of the four cohorts of participants is reported. Interestingly, TQ urine concentrations were proportional to parasite recrudescence times post dosing To our knowledge, this is the first description of a fully validated method for the measurement of TQ and 5,6-OQTQ quantification in urine.

Physiologically based pharmacokinetic (PBPK) modeling of the role of CYP2D6 polymorphism for metabolic phenotyping with dextromethorphan

The cytochrome P450 2D6 (CYP2D6) is a key xenobiotic-metabolizing enzyme involved in the clearance of many drugs. Genetic polymorphisms in CYP2D6 contribute to the large inter-individual variability in drug metabolism and could affect metabolic phenotyping of CYP2D6 probe substances such as dextromethorphan (DXM). To study this question, we (i) established an extensive pharmacokinetics dataset for DXM; and (ii) developed and validated a physiologically based pharmacokinetic (PBPK) model of DXM and its metabolites dextrorphan (DXO) and dextrorphan O-glucuronide (DXO-Glu) based on the data. Drug-gene interactions (DGI) were introduced by accounting for changes in CYP2D6 enzyme kinetics depending on activity score (AS), which in combination with AS for individual polymorphisms allowed us to model CYP2D6 gene variants. Variability in CYP3A4 and CYP2D6 activity was modeled based on in vitro data from human liver microsomes. Model predictions are in very good agreement with pharmacokinetics data for CYP2D6 polymorphisms, CYP2D6 activity as described by the AS system, and CYP2D6 metabolic phenotypes (UM, EM, IM, PM). The model was applied to investigate the genotype-phenotype association and the role of CYP2D6 polymorphisms for metabolic phenotyping using the urinary cumulative metabolic ratio (UCMR), DXM/(DXO + DXO-Glu). The effect of parameters on UCMR was studied via sensitivity analysis. Model predictions indicate very good robustness against the intervention protocol (i.e. application form, dosing amount, dissolution rate, and sampling time) and good robustness against physiological variation. The model is capable of estimating the UCMR dispersion within and across populations depending on activity scores. Moreover, the distribution of UCMR and the risk of genotype-phenotype mismatch could be estimated for populations with known CYP2D6 genotype frequencies. The model can be applied for individual prediction of UCMR and metabolic phenotype based on CYP2D6 genotype. Both, model and database are freely available for reuse.

Evaluation of CYP2D6 phenotype in the Yoruba Nigerian population

BACKGROUND

There is a lack of information on CYP2D6, a major metabolizing enzyme, in Africa ethnic nationalities. The objective was to determine CYP2D6 phenotype in Yoruba Nigerians using dextromethorphan (DEX).

METHOD

A total of 89 healthy volunteers received 30 mg of DEX orally followed by blood and urine sample collection at 3-hour and over 8 h post-dose, respectively. DEX and dextrorphan (DOR) concentrations were determined using High Performance Liquid Chromatography (HPLC). The metabolic ratio (MR, DEX/DOR) were plotted for the phenotype determination.

RESULTS

The log MR that separated poor (PMs) from normal metabolizers (NMs) was 0.28 and 0.75 for urine and plasma, respectively. Two subjects (2.3%) identified as PMs had a mean MR of 17 and 3.2 in plasma and urine, significantly higher than that of NMs (p < .0001). A positive correlation between urine and plasma MR was noted.

CONCLUSION

The prevalence of PMs in the Yoruba Nigerians was similar to that reported among blacks.

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.

Addressing phenoconversion: the Achilles' heel of personalized medicine

Phenoconversion is a phenomenon that converts genotypic extensive metabolizers (EMs) into phenotypic poor metabolizers (PMs) of drugs, thereby modifying their clinical response to that of genotypic PMs. Phenoconversion, usually resulting from nongenetic extrinsic factors, has a significant impact on the analysis and interpretation of genotype-focused clinical outcome association studies and personalizing therapy in routine clinical practice. The high phenotypic variability or genotype-phenotype mismatch, frequently observed due to phenoconversion within the genotypic EM population, means that the real number of phenotypic PM subjects may be greater than predicted from their genotype alone, because many genotypic EMs would be phenotypically PMs. If the phenoconverted population with genotype-phenotype mismatch, most extensively studied for CYP2D6, is as large as the evidence suggests, there is a real risk that genotype-focused association studies, typically correlating only the genotype with clinical outcomes, may miss clinically strong pharmacogenetic associations, thus compromising any potential for advancing the prospects of personalized medicine. This review focuses primarily on co-medication-induced phenoconversion and discusses potential approaches to rectify some of the current shortcomings. It advocates routine phenotyping of subjects in genotype-focused association studies and proposes a new nomenclature to categorize study populations. Even with strong and reliable data associating patients' genotypes with clinical outcome(s), there are problems clinically in applying this knowledge into routine pharmacotherapy because of potential genotype-phenotype mismatch. Drug-induced phenoconversion during routine clinical practice remains a major public health issue. Therefore, the principal challenges facing personalized medicine, which need to be addressed, include identification of the following factors: (i) drugs that are susceptible to phenoconversion; (ii) co-medications that can cause phenoconversion; and (iii) dosage amendments that need to be applied during and following phenoconversion.

Contribution of the activities of CYP3A, CYP2D6, CYP1A2 and other potential covariates to the disposition of methadone in patients undergoing methadone maintenance treatment

AIMS

To investigate the influence of different cytochrome P450 (CYP) activities and other potential covariates on the disposition of methadone in patients on methadone maintenance therapy (MMT).

METHODS

Eighty-eight patients (58 male; 21-55 years; 84 White) on MMT were studied. CYP2D6 activity [3 h plasma metabolic ratio of dextromethorphan (DEX) to dextrorphan (DOR)] was determined in 44 patients (29 male; 24-55 years), CYP1A2 activity (salivary caffeine elimination half-life) in 44 patients (21 male; 24-55 years) and CYP3A activity (oral clearance of midazolam) in 49 patients (33 male; 23-55 years). Data on all three CYPs were obtained from 32 subjects. Total plasma concentrations of (RS)-methadone and total and unbound plasma concentrations of both enantiomers were measured by LC/MS. Population pharmacokinetics and subsequent multiple regression analysis were used to calculate methadone oral clearance and to identify its covariates.

RESULTS

Between 61 and 68% of the overall variation in total plasma trough concentrations of (RS)-, (R)- and (S)-methadone was explained by methadone dose, duration of addiction before starting MMT, CYP3A activity and illicit morphine use. CYP3A activity explained 22, 16, 15 and 23% of the variation in unbound (R)-, unbound (S)-, total (RS)- and total (S)-methadone clearances, respectively. Neither CYP2D6 nor CYP1A2 activity was related to methadone disposition.

CONCLUSIONS

CYP3A activity has a modest influence on methadone disposition. Inhibitors and inducers of this enzyme should be monitored in patients taking methadone.

Evaluation of probe drugs and pharmacokinetic metrics for CYP2D6 phenotyping

INTRODUCTION

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

METHODS

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

CONCLUSION

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

Assessment of in vivo CYP2D6 activity: differential sensitivity of commonly used probes to urine pH

Drug/metabolite ratios (MRs) are used as in vivo markers of enzyme activity. The ratios are potentially confounded by the renal clearance of the drug (urine-based MRs) or metabolite (plasma-based MRs). The authors have investigated the relative sensitivity of urinary MR of 3 in vivo probe substrates of CYP2D6 debrisoquine (DB), dextromethorphan (DM), and metoprolol (MP) to changes in urine pH. Three groups of healthy volunteers each comprising 12 individuals were given DB (10 mg), DM (25 mg), or MP (100 mg) on 3 occasions. In 1 study arm, urine was acidified by the oral intake of ammonium chloride; in another, it was alkalinized by intake of sodium bicarbonate; and in the third, urine pH was uncontrolled. Urinary MP/alpha-hydroxy-MP, DM/dextrorphan, and DB/4-hydroxy-DB ratios were calculated. The mean(geo) MR for DB was not significantly different in any of the study arms, whereas those for MP and DM were significantly different under acidified and alkalinized urine conditions compared to uncontrolled urine pH (P < .01) and were correlated with urine pH (P < .001). Without control of urine pH, in vivo estimates of CYP2D6 metabolic activity are likely to be less precise using DM or MP as probe substrates compared to DB. Although this is unlikely to cause any problem in distinguishing the large functional differences in CYP2D6 in poor metabolizer (PM) and extensive metabolizer (EM) phenotypes, this may contribute to difficulties in differentiating in vivo metabolic activity among allelic variants within the overall CYP2D6 EM phenotype using MP or DM. However, because DB is not available in many countries (eg, United States), alternative in vivo markers of CYP2D6 with low sensitivity to urine pH should be sought.

DEXTROMETHORPHAN TO DEXTRORPHAN URINARY METABOLIC RATIO DOES NOT REFLECT DEXTROMETHORPHAN ORAL CLEARANCE

Dextromethorphan urinary metabolic ratio is widely used to determine the CYP2D6 phenotype, but its utility to reflect subtle differences in catalytic activity is unclear. We evaluated the capability of dextromethorphan urinary metabolic ratio to predict dextromethorphan oral clearance as a measure of CYP2D6 activity. Data from 10 healthy extensive metabolizers of CYP2D6 were given 30 mg of dextromethorphan hydrobromide orally on two occasions. Blood and urine samples were collected for 72 h. Dextromethorphan and dextrorphan were determined in urine by high-performance liquid chromatography with fluorescence detection and in serum by liquid chromatography-mass spectrometry. The urinary metabolic ratio was very weakly correlated with dextromethorphan oral clearance (r = 0.24; p = 0.04). In contrast, the dextromethorphan oral clearance was highly correlated with the dextromethorphan to dextrorphan area under the concentration-time curve ratio (r = 0.84; p = 0.005) and the 3-h (r = 0.60; p = 0.003), 4-h (r = 0.72, p < 0.001), 6-h (r = 0.67; p < 0.001), and 8-h (r = 0.74; p < 0.001) dextromethorphan to dextrorphan serum ratios. Assuming an effect size of 30%, the number of volunteers required for crossover and cross-sectional studies using the urinary metabolic ratio as the CYP2D6 index was calculated to be 56 and 524, respectively, whereas 14 and 60 subjects are needed if oral clearance is used. Considering the required sample size and the low correlation with oral clearance, urinary metabolic ratio is not recommended as the primary outcome variable in studies requiring the detection of modest changes in CYP2D6 activity.

Dextromethorphan metabolic phenotyping in an Iranian population

OBJECTIVE

CYP2D6 polymorphism of drug metabolism represents an important source of interindividual and interethnic variation in drug response. Since this polymorphism has not been studied in an Iranian population, the present study was undertaken.

METHODS

Two hundred healthy unrelated Iranian subjects participated in this study. Phenotyping was based on high-performance liquid chromatography determination of the dextromethorphan/total dextrorphan molar ratios as metabolic ratios (MRs) in plasma samples collected at 3 h after oral administration of 30 mg dextromethorphan hydrobromide. Since the dextromethorphan detection limit of 5 ng/ml achieved in the simultaneous assay for dextromethorphan and its metabolites was not adequate to identify intermediate metabolizers (IMs), 80 of 200 samples selected randomly were also assayed using a modified, more sensitive procedure with a dextromethorphan detection limit of 1 ng/ml.

RESULTS

Poor and extensive metabolizers (EMs) could be identified distinctly. A 520-fold interindividual variation in dextromethorphan MRs was observed in this study. In contrast to undetectable dextrorphan and hydroxymorphinan concentrations, clearly determinable dextromethorphan concentrations higher than 10 ng/ml were observed in plasma samples of poor metabolizers (PMs). Considering the antimode of 0.3, five (2.5%, 95% confidence interval of 0.34-4.66) volunteers were identified as PMs. Using the more sensitive method, dextromethorphan was quantified in 4 (one PM) of 80 samples. Excluding the PM, a Shapiro-Wilk test indicated a non-normal distribution of MRs (P < 0.01) in the latter population.

CONCLUSIONS

From this study it can be concluded that the frequency of PMs in an Iranian population is 2.5% (95% confidence interval of 0.34-4.66). IMs could be identified using dextromethorphan plasma assays with detection limits of at least 1 ng/ml. However, the phenotype-genotype relationships in this respect remain to be established.

Evaluation of Dextromethorphan Metabolism Using Hepatocytes from CYP2D6 Poor and Extensive Metabolizers

It is important to estimate the defective metabolism caused by genetic polymorphism of drug metabolizing enzymes before the clinical stage. We evaluated the utility of cryopreserved human hepatocytes of CYP2D6 poor metabolizer (PM) for the estimation of the metabolism in PM using dextromethorphan (DEX) as the probe drug for CYP2D6 substrate. The results of low formations of dextrorphan (DXO) and 3-hydroxymorphinan (3-HM) in CYP2D6 PM hepatocytes incubated with dextromethorphan reflected the clinical data. Formation of 3-methoxymorphinan (3-MEM) normalized by CYP3A4/5 activity in the PM hepatocytes reached about 2.8-fold higher than that in CYP2D6 extensive metabolizer (EM) hepatocytes, which clearly showed the compensatory metabolic pathway of O-demethylation catalyzed by CYP2D6 as seen in clinical study. On the contrary, in the condition of the EM hepatocytes with CYP2D6 inhibitors, the enhancement of 3-MEM formation was not observed. In phase II reaction, the glucuronide formation rate of DXO in the PM hepatocytes was lower than that in the EM hepatocytes, which was consistent with clinical data of DXO-glucuronide (DXO-glu) concentration. These results would suggest that CYP2D6 PM hepatocytes could be a good in vitro tool for estimating CYP2D6 PM pharmacokinetics.

Analgesic effect of dextromethorphan in neuropathic pain

BACKGROUND

Dextromethorphan, a clinically available N-methyl-D-aspartic acid (NMDA) receptor antagonist, has an analgesic effect in patients with diabetic neuropathy. The aim of this study was to evaluate the analgesic and adverse effects of a single high dose of dextromethorphan on spontaneous pain in patients suffering long-term neuropathic pain of traumatic origin.

METHODS

Fifteen patients with post-traumatic neuropathic pain participated in this placebo-controlled, double-blind, randomized crossover study. On two separate occasions, the participants received 270 mg of dextromethorphan hydrobromide or placebo. Pain intensity, adverse effects and serum concentrations of dextromethorphan and metabolites were registered.

RESULTS

Dextromethorphan had a statistically significant analgesic effect compared with placebo, but the effect varied markedly among the patients. Light-headedness was the most important adverse effect reported. Extensive metabolizers of dextromethorphan had an apparently better analgesic effect than poor metabolizers.

CONCLUSION

This report indicates that a single high dose of dextromethorphan has an analgesic effect in patients with neuropathic pain of traumatic origin. The main metabolite dextrorphan seems to be important for the analgesic effect. At the relatively high dose studied, the clinical usefulness of dextromethorphan is limited to that portion of the patient population experiencing analgesia without an unacceptable level of adverse effects.

Analysis of pharmacokinetic parameters for assessment of dextromethorphan metabolic phenotypes

In this study, the metabolic ratios of dextromethorphan to dextrorphan (DM/DX) in plasma were calculated at steady state after administering 2 dosage forms (Medicon) and Detusiv) of DM with different release rates. The urinary metabolic ratio for each subject was also determined based on the total drug concentration in the urine. An analysis of pharmacokinetic parameters for determining the DM metabolic phenotype was conducted. Results demonstrate that double logarithmic correlations between the metabolic ratios based on pharmacokinetic parameters of either AUC(0-tau,ss), C(max,ss), C(min,ss), or C(ave,ss) for Medicon and Detusiv and the urinary metabolic ratios were all significant. Probit plots of the metabolic ratios based on these pharmacokinetic parameters revealed 2 clusters of distribution, representing extensive and intermediate metabolizers. An antimode of 2.0 for total drug based on these pharmacokinetic parameters was determined and correspondingly referred to an antimode of 0.02 for the urinary metabolic ratio to delineate extensive and intermediate metabolizers. This model was also verified to be appropriate when using total plasma concentrations of DM and DX at any time during the period of the dosing interval at steady state to calculate the metabolic ratio for identifying extensive and intermediate metabolizers. Therefore, the metabolic ratio based on the pharmacokinetic parameters of either AUC(0-tau,ss), C(max,ss), C(min,ss), or C(ave,ss) and plasma concentrations of DM and DX in a single blood sample at steady state are proposed as an alternative way to identify phenotypes of CYP2D6.

Bioinformatics Research on Inter-racial Difference in Drug Metabolism II. Analysis on Relationship between Enzyme Activities of CYP2D6 and CYP2C19 and their Relevant Genotypes

The enzyme activities of CYP2D6 and CYP2C19 show a genetic polymorphism, and the frequency of poor metabolizers (PMs) on these enzymes depends on races. We have analyzed frequencies of mutant alleles and PMs based on the published data in previous study (Shimizu, T. et al.: Bioinformatics research on inter-racial difference in drug metabolism, I. Analysis on frequencies of mutant alleles and poor metabolizers on CYP2D6 and CYP2C19.). The study shows that there were racial differences in the frequencies of each mutant allele and PMs. In the present study, the correlation between genotypes and drug-metabolizing enzyme activities was investigated. The result showed that enzyme activities varied according to the genotypes of subjects even in the same race. On the other hand, if subjects had the same genotypes, almost no racial differences were observed in drug-metabolizing enzyme activities. From these results, it was supposed that the racial differences in activities of these enzymes could be explained by the differences in distribution of genotypes. It would be possible to explain the racial differences in drug-metabolizing enzyme activities based on the differences on individual pharmacogenetic background information, not merely by comparison of frameworks such as races and nations.

CYP2D6 genotyping as an alternative to phenotyping for determination of metabolic status in a clinical trial setting

The emerging application of pharmacogenomics in the clinical trial setting requires careful comparison with more traditional phenotyping methodologies, particularly in the drug metabolism area where phenotyping is used extensively. The research objectives of this study were 1) to assess the utility of cytochrome P450 2D6 (CYP2D6) genotyping as an alternative to traditional phenotyping as a predictor of poor metabolizer status; 2) to identify issues for consideration when implementing CYP2D6 genotyping in clinical trials; and 3) to outline the advantages and disadvantages of CYP2D6 genotyping compared with phenotyping. DNA samples obtained from 558 previously phenotyped individuals were blindly genotyped at the CYP2D6 locus, and the genotype-phenotype correlation was then determined. The CYP2D6 genotyping methodology successfully predicted all but 1 of the 46 poor metabolizer subjects, and it was determined that this 1 individual had a novel (presumably inactive) mutation within the coding region. In addition, we identified 2 subjects with CYP2D6 genotypes indicative of poor metabolizers who had extensive metabolizer phenotypes as determined by dextromethorphan/dextrorphan ratios. This finding suggests that traditional phenotyping methods do not always offer 100% specificity. Our results suggest that CYP2D6 genotyping is a valid alternative to traditional phenotyping in a clinical trial setting, and in some cases may be better. We also discuss some of the issues and considerations related to the use of genotyping in clinical trials and medical practice.

Glucuronidation and sulphation of paracetamol in HIV-positive patients and patients with AIDS

AIMS

To gauge the effect of disease state and disease progression on the glucuronidation and sulphation of paracetamol (APAP) among HIV-positive patients and patients with AIDS.

METHODS

The extent of APAP glucuronidation and APAP sulphation was assessed using a spot urine sample collected 4 h after the oral administration of 500 mg of APAP to 108 patients with AIDS or HIV infection. The molar concentrations of APAP and its glucuronide and sulphate metabolites were determined using a validated h.p.l.c. method and glucuronidation and sulphation indices were constructed using APAP metabolite/APAP molar concentration ratios.

RESULTS

No effect of disease state, AIDS vs asymptomatic HIV positive vs control, on APAP glucuronidation or sulphation was observed. The patient population was studied over time and disease progression also did not significantly alter the calculated glucuronidation and sulphation indices. The effect of the concomitant administration of other therapeutic agents was assessed and in the cross sectional portion of the study dapsone appeared to significantly decrease APAP sulphation as did lamivudine. In the longitudinal portion of the study the latter effect was not observed but zidovudine was seen to increase APAP glucuronidation. The data also indicates that APAP glucuronidation may be reduced in patients who are >10% below their ideal body weight.

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.