Effects of cytochrome P450 2D6 and 3A5 genotypes and possible coadministered medicines on the metabolic clearance of antidepressant mirtazapine in Japanese patients

Pharmacogenomics for the efficacy and side effects of antihistamines

Antihistamines, especially H1 antihistamines, are widely used in the treatment of allergic diseases such as urticaria and allergic rhinitis, mainly for reversing elevated histamine and anti-allergic effects. Antihistamines are generally safe, but some patients experience adverse reactions, such as cardiotoxicity, central inhibition, and anticholinergic effects. There are also individual differences in antihistamine efficacy in clinical practice. The concept of individualized medicine has been deeply rooted in people's minds since it was put forward. Pharmacogenomics is the study of the role of inheritance in individual variations in drug response. In recent decades, pharmacogenomics has been developing rapidly, which provides new ideas for individualized medicine. Polymorphisms in the genes encoding metabolic enzymes, transporters, and target receptors have been shown to affect the efficacy of antihistamines. In addition, recent evidence suggests that gene polymorphisms influence urticaria susceptibility and antihistamine therapy. Here, we summarize current reports in this area, aiming to contribute to future research in antihistamines and clinical guidance for antihistamines use in individualized medicine.

Phenoconversion of Cytochrome P450 Metabolism: A Systematic Review

Phenoconversion is the mismatch between the individual’s genotype-based prediction of drug metabolism and the true capacity to metabolize drugs due to nongenetic factors. While the concept of phenoconversion has been described in narrative reviews, no systematic review is available. A systematic review was conducted to investigate factors contributing to phenoconversion and the impact on cytochrome P450 metabolism. Twenty-seven studies met the inclusion criteria and were incorporated in this review, of which 14 demonstrate phenoconversion for a specific genotype group. Phenoconversion into a lower metabolizer phenotype was reported for concomitant use of CYP450-inhibiting drugs, increasing age, cancer, and inflammation. Phenoconversion into a higher metabolizer phenotype was reported for concomitant use of CYP450 inducers and smoking. Moreover, alcohol, pregnancy, and vitamin D exposure are factors where study data suggested phenoconversion. The studies reported genotype–phenotype discrepancies, but the impact of phenoconversion on the effectiveness and toxicity in the clinical setting remains unclear. In conclusion, phenoconversion is caused by both extrinsic factors and patient- and disease-related factors. The mechanism(s) behind and the extent to which CYP450 metabolism is affected remain unexplored. If studied more comprehensively, accounting for phenoconversion may help to improve our ability to predict the individual CYP450 metabolism and personalize drug treatment.

A Study to Assess the Proarrhythmic Potential of Mirtazapine Using Concentration-QTc (C-QTc) Analysis

Most new chemical entities with systemic availability are required to be tested in a study specifically designed to exclude drug-induced corrected QT interval (QTc) effects, the so-called thorough QT/QTc study. Mirtazapine (Remeron™) is an antidepressant indicated for the treatment of episodes of major depression, which was originally approved in 1994 without a thorough QT study. To evaluate the proarrhythmic potential of mirtazapine, we performed a QT/QTc study with a novel design including implementation of an analysis of the relationship between drug concentration and the QTc interval as the primary assessment of proarrhythmic potential of mirtazapine. The least squares mean differences of the corrected QT interval between mirtazapine and placebo at the geometric mean maximum concentration of drug in blood plasma (90% confidence interval) were 2.39 milliseconds (0.70, 4.07) at the 45-mg dose and 4.00 milliseconds (1.18, 6.83) at the 75-mg dose level of mirtazapine. Modeling of the concentration/QTc relationship for moxifloxacin confirmed that the assay method was adequately sensitive. This trial showed a positive relationship between mirtazapine concentrations and prolongation of the QTc interval. However, the degree of QT prolongation observed with both 45-mg and 75-mg doses of mirtazapine was not at a level generally considered to be clinically meaningful. This study further demonstrates that analysis of the relationship between drug concentration and the QTc interval may be a reasonable alternative to traditional TQT studies to assess risk of QT prolongation.

Differences in Toxicological and Pharmacological Responses Mediated by Polymorphic Cytochromes P450 and Related Drug-Metabolizing Enzymes

Research over the past 30 years has elucidated the roles of polymorphic human liver cytochrome P450 (P450) enzymes associated with toxicological and/or pharmacological actions. Thalidomide exerts its various pharmacological and toxic actions in primates through multiple mechanisms, including nonspecific modification of many protein networks after bioactivation by autoinduced human P450 enzymes. To overcome species differences between rodents, currently, nonhuman primates and/or mouse models with transplanted human hepatocytes are used. Interindividual variability of P450-dependent drug clearances in cynomolgus monkeys and common marmosets is partly accounted for by polymorphic P450 variants and/or aging, just as it is in humans with increased prevalence of polypharmacy. Genotyping of P450 genes in nonhuman primates would be beneficial before and/or after drug metabolism and toxicity testing and evaluation as well in humans. Genome-wide association studies in humans have been rapidly advanced; however, unique whole-gene deletion of P450 2A6 was subsequently developed to cover nicotine-related lung cancer risk study. Regarding polypharmacy, toxicological research should generally be aimed at identifying the risk of adverse drug events following specific potential drug exposures by examining single or multiple metabolic pathways involving single or multiple drug-metabolizing enzymes. Current and next-generation research of drug metabolism and disposition resulting in drug toxicity would be addressed under advanced knowledge of polymorphic and age-related intra- and/or interspecies differences of drug-metabolizing enzymes. In the near future, humanized animal models combining transplanted hepatocytes and a humanized immune system may be available to study human immune reactions caused by human-type drug metabolites. Such sophisticated models should provide preclinical predictions of human drug metabolism and potential toxicity.

Individual differences in in vitro and in vivo metabolic clearances of the antipsychotic drug olanzapine from non-smoking and smoking Japanese subjects genotyped for cytochrome P4502D6 and flavincontaining monooxygenase 3

OBJECTIVE

The antipsychotic olanzapine is reportedly metabolized by inducible human cytochrome P450 (CYP) 1A2 and variable copy-number CYP2D6 and polymorphic flavin-containing monooxygenase 3 (FMO3) in different pathways. We investigated individual differences in the metabolite formation and clearance of olanzapine in vitro and in vivo.

METHODS

Human liver microsomal olanzapine oxidation activities were evaluated, and plasma concentrations of olanzapine were determined in 21 Japanese patients (mean age: 50 years, range: 32-69 years, 14 male and 7 female, including 6 smokers) genotyped for CYP2D6 (*1, *5, and *10) and FMO3 (E158K, C197fsX, R205C, V257M, E308G, and R500X).

RESULTS

Furafylline (a CYP1A2 inhibitor), quinidine (a CYP2D6 inhibitor), and heat treatment (inactivates FMO3) suppressed liver microsomal metabolic clearance of olanzapine by approximately 30%. Olanzapine N-demethylation and N-oxygenation were found to be catalyzed by CYP1A2 and CYP2D6 and by CYP2D6 and FMO3, respectively, in experiments using liver microsomes and recombinant enzymes. Plasma concentrations and clearance of olanzapine were not affected by CYP2D6 or FMO3 genotypes or smoking behavior.

CONCLUSIONS

Olanzapine clearance was not affected by CYP2D6 or FMO3 genotypes or smoking behavior as a single factor under the present conditions because olanzapine clearance is mediated by multiple enzymes involved in two major and one minor pathways.

Individual differences in in vitro and in vivo metabolic clearances of antipsychotic risperidone from Japanese subjects genotyped for cytochrome P450 2D6 and 3A5

OBJECTIVE

There are conflicting reports regarding the effects of cytochrome P450 (P450, CYP) genotypes on the plasma concentrations of risperidone and its pharmacologically active metabolite, 9-hydroxyrisperidone (paliperidone), in clinical patients. The aim of this study was to investigate individual differences in the metabolic clearance of risperidone in vitro and in vivo.

METHODS

In vitro liver microsomal risperidone 9-hydroxylation activities and in vivo plasma concentrations of risperidone and paliperidone were investigated in 15 male and 12 female Japanese subjects (mean age 52 years, range: 24-75 years) genotyped for CYP2D6 and CYP3A5.

RESULTS

CYP2D6 intermediate and poor metabolizers showed significantly lower liver microsomal risperidone 9-hydroxylation activities than extensive metabolizers did at 5 μM of risperidone; this difference was not evident at 50 μM of risperidone. The recombinant CYP3A5 Vmax/Km value for risperidone 9-hydroxylation was 30% that of CYP3A4, and liver microsomes from CYP3A5 expressers had similar risperidone 9-hydroxylation activities to those of CYP3A5 poor expressers. The plasma concentration/dose ratios for risperidone and paliperidone in 27 Japanese patients were not significantly influenced by the CYP2D6 or CYP3A5 genotypes.

CONCLUSIONS

Individual differences in metabolic clearance of risperidone under the present conditions were not significantly influenced by the genotypes of CYP2D6 or CYP3A5.

Effects of 22 novel CYP2D6 variants found in Chinese population on the metabolism of dapoxetine

Background

CYP2D6 is one of the most important members of the cytochrome P450 superfamily. Its genetic polymorphism significantly influences the efficacy and safety of some drugs, which might cause adverse effects and therapeutic failure.

Methods and results

The aim of this research was mainly to explore the catalytic activities of 22 newly reported CYP2D6 isoforms (2D6*87, *88, *89, *90, *91, *92, *93, *94, *95, *96,*97, *98, *R25Q, F164L, E215K, F219S, V327M, D336N, V342M, R344Q, R440C, R497C) on dapoxetine in vitro. The research was designed with an appropriate incubation system in test tubes and carried out in the constant temperature water. Through detecting its two metabolites desmethyldapoxetine and dapoxetine-N-oxide, the available data were obtained to explain the influence of CYP2D6 polymorphism on the substrate drug dapoxetine. As a result, the intrinsic clearance (V_max/K_m) values of most variants were significantly altered when compared with the counterpart of CYP2D6*1, with most of these variants exhibiting either reduced V_max and/or increased K_m values. For dapoxetine demethylation pathway (which produces desmethyldapoxetine), 2D6*89 and E215K exhibited no markedly decreased relative clearance of 92.81% and 97.70%, respectively. The relative clearance of rest 20 variants exhibited decrease in different levels, ranging from 20.44% to 90.90%. For the dapoxetine oxidation pathway (which produces dapoxetine-N-oxide), the relative clearance values of three variants, 2D6*90, *94, and V342M, exhibited no markedly increased relative clearance of 106.17%, 107.78%, and 109.98%, respectively; the rest 19 variants exhibited significantly decreased levels ranging from 27.56% to 84.64%. In addition, the kinetic parameters of two CYP2D6 variants (2D6*92 and 2D6*96) could not be detected, due to the defect of the CYP2D6 gene.

Conclusion

As the first report of all aforementioned alleles for dapoxetine metabolism, these data may help in the clinical assessment of the metabolic elimination of dapoxetine and may provide fundamental information for further clinical studies.