Comparison of the prevalence of the poor metabolizer phenotype for CYP2D6 between 203 Hmong subjects and 280 white subjects residing in Minnesota

Assessment of 25 CYP2D6 alleles found in the Chinese population on propafenone metabolism in vitro

Cytochrome P450 enzyme 2D6 (CYP2D6) is an important member of the cytochrome P450 enzyme superfamily, with more than 100 CYP2D6 allelic variants being previously reported. The aim of this study was to assess the catalytic characteristics of 25 alleles (CYP2D6.1 and 24 CYP2D6 variants) and their effects on the metabolism of propafenone in vitro. Twenty-five CYP2D6 alleles were expressing in 21 Spodoptera frugiperda (Sf) insect cells, and each variant was evaluated using propafenone as the substrate. Reactions were performed at 37 °C with 1-100 μmol/L propafenone for 30 min. After termination, the product 5-OH-propafenone was extracted and used for signal collection by ultra-performance liquid chromatography (UPLC). Compared with wild type CYP2D6.1, the intrinsic clearance (Vmax and Km) values of all variants were significantly altered. Three variants (CYP2D6.87, CYP2D6.90, CYP2D6.F219S) exhibited markedly increased intrinsic clearance values (129% to 165%), whereas 21 variants exhibited significantly decreased values (16% to 85%) due to increased Km and (or) decreased Vmax values. These results indicated that the majority of tested alleles had significantly altered catalytic activity towards propafenone hydroxylation in this expression system. Attention should be paid to subjects carrying these rare alleles when treated with propafenone.

CYP2D6Genotype and Phenotype in Amerindians of Tepehuano Origin and Mestizos of Durango, Mexico

Although the drug-metabolizing enzyme CYP2D6 has been studied extensively in subjects of differing ethnicities, limited CYP2D6 pharmacogenetic data are available for the Amerindian population and Mestizos of Mexico. Dextromethorphan hydroxylation phenotype was studied in Tepehuano Amerindian (n = 58) and Mestizo (n = 88) subjects, and 195 individuals (85 Tepehuano Amerindians and 110 Mestizos) were genotyped by polymerase chain reaction-restriction fragment length polymorphism methods to identify the frequencies of the CYP2D6*3, *4, *6, and *10 alleles. Tepehuano Amerindian subjects lacked the poor metabolizer (PM) phenotype, whereas in Mestizos the PM phenotype frequency was 6.8%. The CYP2D6*3, *6, and *10 alleles were not found in Tepehuano Amerindians. The CYP2D6*4 allele had a low frequency (0.006) in this Amerindian group. In the Mestizo group, the CYP2D6*3, *4, and *10 alleles had frequencies of 0.009, 0.131, and 0.023, respectively. The CYP2D6*6 allele was not found in Mestizos. The genotype-phenotype association was strongly statistically significant (r(2) = .45; P = .005) in Mestizos. The Tepehuano population was found to have a low phenotypic and genotypic CYP2D6 diversity and differed from other Amerindian groups. On the other hand, the frequencies of the CYP2D6 variant alleles in Mestizos were similar to those reported for whites.

Mixture models and subpopulation classification: a pharmacokinetic simulation study and application to metoprolol CYP2D6 phenotype

Mixture models are applied in population pharmacometrics to characterize underlying population distributions that are not adequately approximated by a single normal or lognormal distribution. In addition to obtaining individualized maximum a posteriori Bayesian post hoc parameter estimates, the subpopulation to which an individual was classified can be determined. However, the accuracy of the classification of subjects to subpopulations is not well studied. We investigated the impact of several factors on the accuracy of classification in mixture models applied to pharmacokinetics using a simulation strategy. The availability of actual subject data allowed us to evaluate mixture model classification in a potentially common application, namely, the classification of clearance into poor metabolizer (PM) or extensive metabolizer (EM) subgroups with the known phenotype status in subjects receiving metoprolol. The factors explored in the simulation study were the magnitude of difference between the clearances in two subpopulations, the between subject variability in clearance, the mixing-fraction, and the population sample size. Populations were simulated at various levels of the above factors and analyzed with a mixture model using NONMEM. The population pharmacokinetics of metoprolol were modeled with the EM/PM phenotype as a known covariate, and without the phenotype covariate using a mixture model. Within the range of scenarios studied, the proportion of subjects classified into the correct subpopulation was high. The simulation-estimation study suggests that a greater separation between two subpopulations, a smaller variability in the parameter distribution, a larger sample size, and a smaller size subpopulation tend to be associated with a greater accuracy of subpopulation classification when a mixture model is applied to pharmacokinetic data. In a population pharmacokinetic analysis of metoprolol, a drug that undergoes polymorphic metabolism, it was possible to correctly identify phenotype status using a mixture model.

Inhibition of CYP2D6 activity by bupropion

The purpose of this study was to assess the effect of bupropion on cytochrome P450 2D6 (CYP2D6) activity. Twenty-one subjects completed this repeated-measures study in which dextromethorphan (30-mg oral dose) was administered to smokers at baseline and after 17 days of treatment with either bupropion sustained-release (150 mg twice daily) or matching placebo. Subjects quit smoking 3 days before the second dextromethorphan administration. To assess CYP2D6 activity, urinary dextromethorphan/dextrorphan metabolic ratios were calculated after an 8-hour urine collection. Thirteen subjects received bupropion, and 8 received placebo. In those receiving active medication, the dextromethorphan/dextrorphan ratio increased significantly at the second assessment relative to the first (0.012 +/- 0.012 vs. 0.418 +/- 0.302; P < 0.0004). No such change was observed in those randomized to placebo (0.009 +/- 0.010 vs. 0.017 +/- 0.015; P = NS). At baseline, all subjects were phenotypically extensive CYP2D6 metabolizers (metabolic ratio <0.3); after treatment, 6 of 13 subjects receiving bupropion, but none of those receiving placebo, had metabolic ratios consistent with poor CYP2D6 metabolizers. Bupropion is therefore a potent inhibitor of CYP2D6 activity, and care should be exercised when initiating or discontinuing bupropion use in patients taking drugs metabolized by CYP2D6.

Temporal changes in drug metabolism (CYP1A2, CYP2D6 and CYP3A Activity) during pregnancy

OBJECTIVE

The purpose of this study was to determine whether drug metabolism (CYP1A2, CYP2D6 and CYP3A) activity varies in the pregnant state compared with the nonpregnant state.

STUDY DESIGN

Subjects were studied at 14 to18 weeks of gestation, 24 to 28 weeks of gestation, and 36 to 40 weeks of gestation and again at 6 to 8 weeks after the delivery. Twenty-five subjects completed all 4 study periods and had evaluable data. Salivary caffeine clearance was used as a measure of CYP1A2 activity; dextromethorphan O- and N-demethylation were used to assess CYP2D6 and CYP3A activity, respectively.

RESULTS

CYP1A2 activity was significantly reduced at all periods of the pregnancy as compared with the postpartum period during the first (-32.8% +/- 22.8%), second (-48.1% +/- 27%), and third periods (-65.2% +/- 15.3%), respectively. In contrast, CYP2D6 activity was increased significantly throughout the pregnancy (25.6% +/- 58.3% at 14-18 weeks of gestation, 34.8% +/- 41.4% at 24-28 weeks of gestation, and 47.8% +/- 24.7% at 36-40 weeks of gestation) as compared with the postpartum period. CYP3A activity was consistently, significantly increased (35%-38%) during all stages of the pregnancy.

CONCLUSION

Opposing changes in drug metabolism occur during pregnancy, with CYP1A2 activity decreased and CYP2D6 and CYP3A activities increased. The direction of dosing adjustments during pregnancy will depend on the drug and the enzyme that is responsible for its metabolism.

Development of a chromatographic bioreactor based on immobilized β-glucuronidase on monolithic support for the determination of dextromethorphan and dextrorphan in human urine

We here reported the development and application of an immobilized enzyme reactor (IMER) based on beta-glucuronidase to the on-line determination of urinary molar ratios of dextromethorphan (DOMe)/dextrorphan (DOH) for the assessment of the metabolic activity of CYP2D6, a genetically variable isoform of cytochrome P-450 (CYP). beta-Glucuronidase was immobilized on an HPLC monolithic aminopropyl silica support. Catalytic activity and stability of the chromatographic reactor were evaluated using 8-hydroxyquinoline glucuronide (8-HQG) as substrate. The IMER was coupled through a switching valve to a reversed-phase column (C8) for the simultaneous determination of dextromethorphan and its main metabolite dextrorphan. On purpose a selective reversed-phase ion pair HPLC method coupled with fluorescence detection was developed. Urine samples were first centrifuged to remove insoluble materials and then aliquots of the supernatants were injected into the coupled-column analyser. Linearity, precision and accuracy of the method were established. The method reliability was verified by comparing our data with previous data of a phenotyping study carried out by the Poison Control Centre of Pavia-Clinical Toxicology Division.

Genetic polymorphisms in the Hmong population

BACKGROUND

The Hmong, an isolated, agrarian people from southern China, migrated to the mountainous regions of what are today Vietnam, Cambodia, and Laos. Minnesota has the second largest Hmong population in the United States. The authors compared frequencies of common genetic polymorphisms believed to influence risk of malignancy to determine whether frequencies in the Hmong are different from those in other Asian populations and in white Minnesotans.

METHODS

Genotyping for glutathione S-transferase micro1 (GSTM1), glutathione S-transferase theta1 (GSTT1), myeloperoxidase (MPO) (C(-)463T), nicotinamide adenine dinucleotide phosphate:quinone oxidoreductase (NQO1) (C609T), 5,10-methylenetetrahydrofolate reductase (MTHFR) (C677T), MTHFR (A1298C), methionine synthase reductase (MTRR) (A66G), X-ray repair cross complementing 1 (XRCC1) 194 (Arg194Trp), XRCC1 280 (Arg280His), and XRCC1 399 (Arg399Gln) alleles was performed by TaqMan analysis using DNA isolated from newborn heel-stick spots provided by the Minnesota Department of Health.

RESULTS

The Hmong had significantly higher frequencies of the NQO1 T allele and the XRCC1 Trp polymorphism (Arg194Trp) and had significantly lower frequencies of the G allele in MTRR (A66G) and the T allele in MTHFR (C677T) compared with white Minnesotans. The Hmong also were significantly more likely to lack the GSTM1 and GSTT1 genes compared with whites (82% vs. 54% and 61% vs. 18%, respectively). Genotype frequencies were similar for MTHFR (A1298C), MPO (C(-)463T), and XRCC1 (Arg280His, Arg399Gln). Genotype frequencies at these loci also were compared with those reported for other Asian populations and showed notable differences between the Hmong and Chinese/Taiwanese, Korean, and Japanese populations.

CONCLUSIONS

The genetic differences identified have implications for both cancer etiology and prognosis in this unique population.

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 I. Analysis on Frequencies of Mutant Alleles and Poor Metabolizers on CYP2D6 and CYP2C19

The enzyme activities of CYP2D6 and CYP2C19 show a genetic polymorphism, and the frequency of poor metabolizers (PMs) on these enzymes depends on races. In the present study, the frequencies of mutant alleles and PMs in each race were analyzed based on information from published studies, considering the genetic polymorphisms of CYP2D6 and CYP2C19 as the causal factors of racial and inter-individual differences in pharmacokinetics. As a result, it was shown that there were racial differences in the frequencies of each mutant allele and PMs. The frequencies of PMs on CYP2D6 are 1.9% of Asians and 7.7% of Caucasians, and those of PMs on CYP2C19 are 15.8% of Asians and 2.2% of Caucasians. Based on the results, it was suggested that there would be racial differences in the frequencies of PM subjects whose blood concentrations might be higher for drugs metabolized by these enzymes. Additionally, it was suggested that enzyme activities would vary according to the number of functional alleles even in subjects judged to be extensive metabolizers (EMs). In the bridging study, genetic information regarding CYP2D6 and CYP2C19 of the subjects will help extrapolate foreign clinical data to a domestic population.

Evaluation of CYP2D6 oxidation of dextromethorphan and propafenone in a Chinese population with atrial fibrillation

The objective of this study was to determine the percentage of patients with paroxysmal atrial fibrillation who were poor metabolizers of CYP2D6 in a Chinese population from Hong Kong and to assess the relationship between the dextromethorphan/dextrorphan ratio and the propafenone/5-hydroxypropafenone ratio or the steady-state propafenone concentration. Patients (n = 60) were recruited from the Arrhythmia Clinic at the University of Hong Kong and given dextromethorphan 30 mg. The dextromethorphan and dextrorphan concentrations in urine over the next 8 hours were used to determine metabolizer status. If the metabolic ratio was greater than 0.3, the patient was determined to be a poor metabolizer. In phase 2, patients (n = 38) were given propafenone 150 mg twice daily, and at steady state, the propafenone and 5-OH propafenone plasma concentrations were determined. It was found that 15% of the patients were poor metabolizers of dextromethorphan. There was a significant correlation between the metabolic ratios of dextromethorphan/dextrorphan and propafenone/5-OH propafenone (r = 0.49, p = 0.0019) and between the dextromethorphan/dextrorphan ratio and the concentration of propafenone (r = 0.32, p = 0.05). No correlations were found in the extensive or poor metabolizer subgroups. It was concluded that the percentage of poor metabolizers in atrial fibrillation patients from Hong Kong was much larger than in previous studies of Chinese patients who were not from Hong Kong. The ability to metabolize dextromethorphan to dextrorphan is related to the ability to metabolize propafenone to 5-hydroxypropafenone.

Genetic polymorphism of CYP2D6 and CYP2C19 metabolism determined by phenotyping Israeli ethnic groups

Genetic polymorphism of the cytochrome P450 isoenzymes CYP2D6 and CYP2C19 was determined by phenotyping four ethnic groups of the Israeli population. The groups consisted of Ethiopian subjects, Yemenite subjects, and Russian subjects representing first-generation new immigrants and an Israeli Arab group. Dextromethorphan was used as the probe for CYP2D6 activity and mephenytoin was used for CYP2C19 activity. The two drugs were administered simultaneously and urine samples were collected over a period of 8 hours. The CYP2D6 phenotype was determined from the ratio of dextromethorphan conversion to dextrorphan and the CYP2C19 phenotype from the ratio of S-mephenytoin and R-mephenytoin. The used liquid chromatographic method was able to completely separate dextrorphan and dextromethorphan. Fluorescence detection allowed dextromethorphan quantification at 1 ng/mL. Mephenytoin enantiomers were completely separated in high-performance liquid chromatography and the respective fractions were collected and analyzed using a gas chromatography/mass spectrometry system with selective ion monitoring. The prevalence of poor metabolizer phenotype of dextromethorphan (CYP2D6) in the Yemenite (0%) and Ethiopian groups (0%) was significantly different from the prevalence in the Russian (17%) and Israeli Arab (9%) groups. A significant difference was also found in the distribution of the metabolic ratio of the extensive metabolizer phenotype between the Ethiopian group and the Russian and Yemenite groups. No significant difference was found in the prevalence of poor mephenytoin metabolizer phenotype (CYP2C19) between the Yemenite (8%), Ethiopian (6%), Russian (9%), and Israeli Arab (8%) groups. No difference was observed in the distribution of metabolic ratio within the extensive metabolizer phenotype subgroups of the four ethnic groups.

Evaluation of the influence of diabetes mellitus on antipyrine metabolism and CYP1A2 and CYP2D6 activity

STUDY OBJECTIVE

To evaluate the metabolism of antipyrine, a general metabolic probe, caffeine, a probe for cytochrome P450 (CYP) 1A2 and N-acetyltransferase activity, and dextromethorphan, a specific probe for CYP2D6 activity in patients with type 1 or 2 diabetes mellitus.

DESIGN

Prospective, controlled study.

SETTING

Research facility. Patients. Fifteen patients with type 1 and 16 with type 2 diabetes, and 16 healthy controls.

INTERVENTION

Each subject simultaneously received antipyrine 10 mg/kg, caffeine 100 mg, and dextromethorphan 30 mg.

MEASUREMENTS AND MAIN RESULTS

The pharmacokinetics of antipyrine and its primary metabolites were determined from saliva and urine samples. Type 1 diabetes had marked effects on antipyrine metabolism whereas type 2 disease did not alter the metabolism of any of the probe drugs. The apparent oral clearance of antipyrine was increased 72% in patients with type 1 disease compared with controls (p=0.0001). In addition, formation clearances of 4-hydroxyantipyrine and 3-hydroxymethylantipyrine were increased by 74% and 137% in those patients relative to controls. The caffeine metabolic index (paraxanthine/caffeine) was increased 34% (p=0.11), and N-acetylation and CYP2D6 phenotype were not altered.

CONCLUSION

The metabolism of antipyrine is increased in patients with type 1 diabetes. Based on in vitro reports of antipyrine metabolism and current caffeine metabolic index data, the predominant effect of type 1 diabetes appears to be an increase in CYP1A2 activity. Assessment of the effect of the disease on other specific CYP metabolic pathways is warranted.

Dextromethorphan phenotyping and haloperidol disposition in schizophrenic patients

This study examined the relationship between the metabolic ratios of dextromethorphan/dextrorphan, haloperidol disposition, and the incidence of extrapyramidal side effects in schizophrenic patients. Eighteen schizophrenic patients were phenotyped with a test dose of dextromethorphan prior to the initiation of haloperidol treatment. The metabolic ratio of dextromethorphan/dextrorphan was determined in each patient. Patients were treated with oral haloperidol 10 mg/day for 2 weeks. Blood samples for haloperidol and reduced haloperidol were obtained at week 2 of haloperidol treatment. Haloperidol and reduced haloperidol plasma concentrations were assayed by HPLC with electrochemical detection. Significant correlations of dextromethorphan/dextrorphan metabolic ratios vs. plasma haloperidol concentrations, reduced haloperidol concentrations, and reduced haloperidol/haloperidol ratios were found (r = 0.726, P = 0.0007; r = 0.782, P = 0.0001; and r = 0.619, P = 0.006, respectively). Ten patients who experienced extrapyramidal side effects had higher reduced haloperidol concentrations and reduced haloperidol/haloperidol ratios than the other patients (2.49 +/- 1.42 [S.D.] ng/ml vs. 1.10 +/- 0.46 ng/ml, P = 0.014 and 0.287 +/- 0.102 vs. 0.192 +/- 0.065, P = 0.030). The former also had a trend to have higher haloperidol concentrations and dextromethorphan/dextrorphan ratios than the latter (8.04 +/- 2.91 ng/ml vs. 5.83 +/- 1.79 ng/ml, P = 0.066 and 0.023 +/- 0.017 vs. 0.011 +/- 0.010, P = 0.077). Phenotyping patients has the potential to assist clinicians in predicting plasma drug concentrations during the subsequent neuroleptic drug treatment. Further research with phenotyping and psychotropic drug metabolism in psychiatric patients is needed.

Predominance of slow acetylators of N-acetyltransferase in a Hmong population residing in the United States

Pharmacogenetics can be an important determinant of pharmacologic response. To learn more about interpopulation differences in drug metabolism between ethnically diverse populations of subjects cared for by an International Clinic, a study was conducted to describe the prevalence of fast or slow acetylators of N-acetyltransferase (NAT2) in a population of Hmong residing in Minnesota. Ninety-eight healthy Hmong refugees from Laos volunteered to take caffeine as an oral probe drug to establish acetylator phenotype. Participants were classified as either rapid or slow acetylators based on the urinary molar ratio of select metabolites of caffeine. Assignment of phenotype was based on results from analysis of urine collected subsequent to ingestion of caffeine. The ratio of 5-acetylamino-6-formylamino-3-methyluracil (AFMU) to the combined products of the 7-demethylation pathway of paraxanthine (AFMU, 1-methylxanthine (1X), and 1-methylurate (1U)] formed the basis for this determination. A probit plot of the data collected in our subjects qualified a metabolic ratio of 0.34 as an acceptable cut point for phenotype assignment. Participants with an AFMU/(AFMU + 1X + 1U) ratio of < 0.34 were classified as slow acetylators and all others as rapid acetylators. Analysis of the data suggested a bimodal distribution with an excess (74.5%) of slow acetylators in the population. The predominance of slow acetylators found in the Hmong contrast with the prevalence of slow acetylators seen in other ethnic groups. These findings may have important clinical implications given the large number of Hmong treated each year in our International Clinic and the increasing use of medications metabolized by NAT2 in this population.

Interethnic differences in drug metabolism: influence of genetic and environmental factors on debrisoquine hydroxylation phenotype

Genetic and environmental factors are determinants of the interindividual and interethnic variability in drug metabolism. The metabolism of several important drugs (e.g. haloperidol) cosegregates with that of debrisoquine. Thus, interethnic differences in debrisoquine hydroxylation polymorphism (CYP2D6) might be partly responsible for the variation in haloperidol disposition between races. The influence of tobacco, ethanol, caffeine, gender, and oral contraceptive use on the debrisoquine metabolic ratio (MR) has been analyzed in 633 Spanish healthy volunteers. MR was also determined in panels of healthy volunteers. 18 smokers were studied during a tobacco abstinence period, and 31 women three times during the same menstrual cycle. Among EMs, debrisoquine MR was significantly (P < 0.05) lower during smoking cessation (mean antilog +/- SD, 0.48 +/- 0.29) compared to a smoking period (0.61 +/- 0.23). During the lutheal phase of the menstrual cycle, debrisoquine MR was also significantly (P < 0.01) lower (0.33 +/- 0.41) compared to the ovulatory-phase (0.41 +/- 0.34) and the phase before ovulation (0.44 +/- 0.36). Among EMs, it is suggested that debrisoquine MR may be modified by tobacco smoking and sexual hormones. The clinical relevance of these findings remains unclear.