R296C and other CYP2D6 mutations in Chinese

Evaluation of the Drug Interaction Potential of Aplaviroc, a Novel Human Immunodeficiency Virus Entry Inhibitor, Using a Modified Cooperstown 5 + 1 Cocktail

Aplaviroc is a novel CCR5 antagonist, a class of compounds under investigation as viral entry inhibitors for the treatment of human immunodeficiency virus infection. A modified Cooperstown 5+1 cocktail was used to assess the drug interaction potential of aplaviroc. Fifteen healthy subjects were administered single oral doses of caffeine (CYP1A2), warfarin (CYP2C9), omeprazole (CYP2C19), dextromethorphan (CYP2D6), and midazolam (CYP3A) alone (reference treatment) and during steady-state administration of aplaviroc (400 mg every 12 hours, test treatment). Metabolite-to-parent area under the plasma concentration versus time curve (AUC) ratios (paraxanthine/caffeine and 5-hydroxyomeprazole/omeprazole), oral clearance (S-warfarin), AUC (midazolam), and metabolite-to-parent urinary excretion ratio (dextrorphan/dextromethorphan) were determined. The test-to-reference treatment ratios (geometric mean ratio and 90% confidence interval) were caffeine, 1.06 (0.97-1.17); S-warfarin, 0.93 (0.76-1.15); omeprazole, 1.07 (0.98-1.16); dextromethorphan, 1.17 (0.97-1.42); midazolam, 1.30 (1.04-1.63). No significant inhibition of CYP1A2, CYP2C9, CYP2C19, or CYP2D6 enzyme activity was observed. Mild inhibition of CYP3A isozymes should not preclude the use of concomitant CYP3A substrates in future clinical studies with aplaviroc.

Debrisoquine 4-hydroxylase (CYP2D6) genetic polymorphisms and susceptibility to schizophrenia in Chinese patients from Taiwan

Debrisoquine 4-hydroxylase (CYP2D6) is one of the cytochrome P450 enzyme families that metabolize many compounds. Polymorphic activities of debrisoquine 4-hydroxylase were suggested to be associated with some complex diseases, such as cancer and Parkinson's disease. Schizophrenia is also a complex disorder, and hence we are interested in understanding if the CYP2D6 gene is a susceptibility gene for schizophrenia in Chinese. We determined the genotype and allele frequencies of four molecular variants of CYP2D6 gene (i.e. 188C/T, 1934G/A, 2938C/T and 4268C/G) in 162 Chinese schizophrenic patients and 94 non-psychotic control subjects from Taiwan. No significant differences of allele or genotype frequencies of three polymorphisms (i.e. 188T/C, 2938C/T and 4268C/G) were detected between patients and control subjects. The 1934A allele, which accounts for the majority of poor metabolizers in Caucasians, was not detected in either patients or control subjects, indicating that the 1934A allele is very rare in Chinese. Our data suggest that the CYP2D6 gene may not be a susceptibility gene for schizophrenia in Chinese schizophrenic patients.

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.

Genetic Analysis of the CYP2D6 Locus in a Hong Kong Chinese Population

Background: The cytochrome P450 CYP2D6 enzyme debrisoquine 4-hydroxylase metabolizes many different classes of commonly used drugs, such as tricyclic antidepressants and neuroleptics. Genetic polymorphism of the CYP2D6 gene is responsible for pronounced interindividual and interracial differences in the metabolism of these drugs. The CYP2D6*10 allele and its variants are the most frequent alleles found in Orientals, and they are responsible for diminished debrisoquine 4-hydroxylase activity because of the presence of a C188→T mutation in exon 1.

Methods: One hundred nineteen Hong Kong Chinese subjects were genotyped by means of allele-specific PCR, PCR, and restriction enzyme analysis for 10 CYP2D6 alleles (CYP2D6*1, *2, *4D, *5, *8/*14, *10A, *10B, *15, *16, and J9).

Results: CYP2D6*10B was the most prevalent allele, and CYP2D6*10/CYP2D6*10 was the most frequent genotype, representing 46.22% of the population.

Conclusions: There was no significant difference in the prevalence of the alleles analyzed between our study and the Chinese populations genotyped previously. This is the largest study in terms of the number of CYP2D6 alleles analyzed in an Oriental population and the first one conducted in a Hong Kong Chinese population.

Genetic association analysis between CYP2D6*2 allele and tardive dyskinesia in schizophrenic patients

Previous studies have shown a possible association between tardive dyskinesia (TD) and debrisoquine 4-hydroxylase (CYP2D6) polymorphisms, which result in absent enzyme activity. We have recently found a positive association between TD and the CYP2D6*10 allele, which codes for the intermediate metabolizer (IM) phenotype and is characterized by decreased but not absent CYP2D6 activity in Japanese schizophrenic patients. In addition, the CYP2D6* 2 allele with the HhaI site mutation in exon 6 has also been reported to be an IM allele and a risk factor for Parkinson's disease (PD) in the Japanese population. In the present study, we investigated potential contributions of the CYP2D6*2 allele to TD using case-control and regression analysis in 99 schizophrenic patients. No significant differences in genotypic and allelic frequencies were found between patients with and without TD. Even after using regression analysis to adjust for the confounding variables, there was no significant association of the CYP2D6*2 genotype with either outcome variable, the occurrence of TD or the total AIMS score. These results suggest that the CYP2D6*2 allele may not contribute to the pathogenesis of TD.

Formation of morphine from codeine in Chinese subjects of different CYP2D6 genotypes

Codeine and morphine pharmacokinetics among different CYP2D6 genotypes was compared in this study. Polymerase chain reaction tests were used to determine CYP2D6 genotypes in leukocyte deoxyribonucleic acid in 32 unrelated volunteers. Based on the genotypes, subjects were categorized into three groups: homozygous C/C188 (n = 8), heterozygous C/T188 (n = 12), and homozygous T/T188 (n = 12). Each subject was given a single oral dose of 30 mg codeine phosphate tablet after overnight fasting. Plasma concentration of codeine and 24-hour urinary morphine recovery were measured with HPLC. All three genotypes of subjects showed almost identical time profiles of plasma codeine. Urinary morphine glucuronide was hydrolyzed with beta-glucuronidase. The total recovered amount of morphine and glucuronides was 4349 +/- 646, 2564 +/- 242, and 1127 +/- 164 nmol (mean +/- SEM), respectively, for C/C188, C/T188, and T/T188 subjects (p < 0.05). The significant lower amount of urinary morphine but identical codeine plasma concentration suggested a lower partial clearance of the formation of morphine from codeine in T/T188 subjects. The results suggest a future study to assess the analgesic effect of codeine in different genotypes of CYP2D6 extensive metabolizers.