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

Physiologically based pharmacokinetic modeling to predict the pharmacokinetics of codeine in different CYP2D6 phenotypes

Objectives

Codeine, a prodrug used as an opioid agonist, is metabolized to the active product morphine by CYP2D6. This study aimed to establish physiologically based pharmacokinetic (PBPK) models of codeine and morphine and explore the influence of CYP2D6 genetic polymorphisms on the pharmacokinetics of codeine and morphine.

Methods

An initial PBPK modeling of codeine in healthy adults was established using PK-Sim® software and subsequently extrapolated to CYP2D6 phenotype-related PBPK modeling based on the turnover frequency (Kcat) of CYP2D6 for different phenotype populations (UM, EM, IM, and PM). The mean fold error (MFE) and geometric mean fold error (GMFE) methods were used to compare the differences between the predicted and observed values of the pharmacokinetic parameters to evaluate the accuracy of PBPK modeling. The validated models were then used to support dose safety for different CYP2D6 phenotypes.

Results

The developed and validated CYP2D6 phenotype-related PBPK model successfully predicted codeine and morphine dispositions in different CYP2D6 phenotypes. Compared with EMs, the predicted AUC0-∞ value of morphine was 98.6% lower in PMs, 60.84% lower in IMs, and 73.43% higher in UMs. Morphine plasma exposure in IMs administered 80 mg of codeine was roughly comparable to that in EMs administered 30 mg of codeine. CYP2D6 UMs may start dose titration to achieve an optimal individual regimen and avoid a single dose of over 20 mg. Codeine should not be used in PMs for pain relief, considering its insufficient efficacy.

Conclusion

PBPK modeling can be applied to explore the dosing safety of codeine and can be helpful in predicting the effect of CYP2D6 genetic polymorphisms on drug-drug interactions (DDIs) with codeine in the future.

Model-based meta-analysis of ethnic differences and their variabilities in clearance of oral drugs classified by clearance mechanism

In this study, the ethnic ratios (ERs) of oral clearance between Japanese and Western populations were subjected to model-based meta-analysis (MBMA) for 81 drugs evaluated in 673 clinical studies. The drugs were classified into eight groups according to the clearance mechanism, and the ER for each group was inferred together with interindividual variability (IIV), interstudy variability (ISV), and inter-drug variability within a group (IDV) using the Markov chain Monte Carlo (MCMC) method. The ER, IIV, ISV, and IDV were dependent on the clearance mechanism, and, except for particular groups such as drugs metabolized by polymorphic enzymes or their clearance mechanism is not confirmative, the ethnic difference was found to be generally small. The IIV was well-matched across ethnicities, and the ISV was approximately half of the IIV as the coefficient of variation. To adequately assess ethnic differences in oral clearance without false detections, phase I studies should be designed with full consideration of the mechanism of clearance. This study suggests that the methodology of classifying drugs based on the mechanism that causes ethnic differences and performing MBMA with statistical techniques such as MCMC analysis is helpful for a rational understanding of ethnic differences and for strategic drug development.

Epidemiology of adverse events attributed to airway management in paediatric anaesthesia: protocol for the prospective, multicentre, registry-based, cross-sectional Japan Pediatric Difficult Airway in Anesthesia study (J-PEDIA)

INTRODUCTION

Failure to secure an airway during general anaesthesia is a major cause of adverse events (AEs) in children. The safety of paediatric anaesthesia may be improved by identifying the incidence of AEs and their attributed risk factors. The aim of the current study is to obtain real-world data on the incidence of adverse peri-intubation events and assess their association with patient characteristics (including the prevalence of difficult airway features) and choice of anaesthesia management. These data can be used to develop a targeted education programme for anaesthesia providers towards quality improvement activities.

METHODS AND ANALYSIS

This prospective, multicentre, registry-based, cross-sectional study will be conducted in four tertiary care hospitals in Japan from June 2022 to May 2025. Children <18 years of age undergoing surgical and/or diagnostic test procedures under general anaesthesia or sedation by anaesthesiologists will be enrolled in this study. Data on patient characteristics, discipline of anaesthesia providers and methodology of airway management will be collected through a standardised verification system. The exposure of interest is the presence of difficult airway features defined based on the craniofacial appearance. The primary and secondary endpoints are all AEs associated with airway management and reduced peripheral capillary oxygen saturation values. Potential confounders are related to the failure to secure the airway and variations in the anaesthesia providers' levels, adjusted using hierarchical multivariable regression models with mixed effects. The sample size was calculated to be approximately 16 000 assuming a 99% probability of obtaining a 95% Wilson CI with±0.3% of the half-width for the 2.0% of the incidence of critical AEs.

ETHICS AND DISSEMINATION

The study protocol was approved by the Institutional Review Board at Aichi Children's Health and Medical Center (2021051). The results will be reported in a peer-reviewed journal and a relevant academic conference.

TRIAL REGISTRATION NUMBER

UMIN000047351.

Perspectives from the Society for Pediatric Research: pharmacogenetics for pediatricians

This review evaluates the pediatric evidence for pharmacogenetic associations for drugs that are commonly prescribed by or encountered by pediatric clinicians across multiple subspecialties, organized from most to least pediatric evidence. We begin with the pharmacogenetic research that led to the warning of increased risk of death in certain pediatric populations ("ultrarapid metabolizers") who are prescribed codeine after tonsillectomy or adenoidectomy. We review the evidence for genetic testing for thiopurine metabolism, which has become routine in multiple pediatric subspecialties. We discuss the pharmacogenetic research in proton pump inhibitors, for which clinical guidelines have recently been made available. With an increase in the prevalence of behavioral health disorders including attention deficit hyperactivity disorder (ADHD), we review the pharmacogenetic literature on selective serotonin reuptake inhibitors, selective norepinephrine reuptake inhibitors, and ADHD medications. We will conclude this section on the current pharmacogenetic data on ondansetron. We also provide our perspective on how to integrate the current research on pharmacogenetics into clinical care and what further research is needed. We discuss how institutions are managing pharmacogenetic test results and implementing them clinically, and how the electronic health record can be leveraged to ensure testing results are available and taken into consideration when prescribing medications. IMPACT: While many reviews of pharmacogenetics literature are available, there are few focused on pediatrics. Pediatricians across subspecialties will become more comfortable with pharmacogenetics terminology, know resources they can use to help inform their prescribing habits for drugs with known pharmacogenetic associations, and understand the limitations of testing and where further research is needed.

The impact of CYP2D6 polymorphisms on the pharmacokinetics of codeine and its metabolites in Mongolian Chinese subjects

PURPOSE

Codeine is an analgesic drug acting on μ-opioid receptors predominantly via its metabolite morphine formed almost exclusively by CYP2D6. Genetic polymorphisms in CYP2D6 are associated with diminished pain relief and/or severe opioid side effects. In Chinese individuals, CYP2D6*10 is the most common allele with reduced enzyme activity. In this study, we investigated the effect of this allele on the pharmacokinetics of codeine and its metabolites.

METHOD

A blood sample was collected from healthy Mongolian volunteers for CYP2D6 genotyping using a PCR-RFLP assay. A pharmacokinetic study was then carried out in three groups with CYP2D6*1/*1 (n=10), CYP2D6*1/*10 (n=10) and CYP2D6*10/*10 (n=9) genotypes by collecting serial blood samples for determination of plasma levels of codeine and its metabolites, morphine, morphine 3-glucuronide (M3G) and morphine 6-glucuronide (M6G) before and after a single 30-mg oral dose of codeine phosphate. Codeine and its metabolites were measured by LC-MS/MS.

RESULTS

No significant differences were observed in the pharmacokinetic parameters of codeine in the three genotype groups. However, the C( max) and AUC(0-∞) of morphine, M3G and M6G were significantly different between the study groups (P<0.05). Compared with the *1/*1 group, the AUC(0-∞) for morphine in the *1/*10 and *10/*10 groups decreased by ratios (95 % CI) of 0.93 (0.26-1.59) and 0.494 (0.135-0.853) respectively. Corresponding ratios for M3G were 0.791 (0.294-1.288) and 0.615 (0.412-0.818) and for M6G were 0.643 (0.39-0.957) and 0.423 (0.267-0.579).

CONCLUSION

This study demonstrates that the CYP2D6*10 allele plays an important role in the pharmacokinetics of the O-demethylated metabolites of codeine after oral administration.

Long PCR-based genotyping for a deleted CYP2D6 gene without DNA extraction

In the post-genome era, a simple and inexpensive method for diagnostic analysis is in high demand. Cytochrome P450 (CYP) 2D6 is one of the most widely investigated CYPs in relation to genetic polymorphism. Detection of CYP2D6*5 is difficult since long PCR is used. Especially for samples without DNA extraction, the detection is not sensitive enough for population analysis. Therefore, we developed a CYP2D6*5 genotyping method that involves nested long PCR, directly using human whole saliva as a template without DNA extraction. This method will be very useful for genetic diagnoses and can be an efficient tool for individualization of drug therapy in clinical studies.

Genetic Polymorphisms of Cytochrome P450 Enzymes and the Effect on Interindividual, Pharmacokinetic Variability in Extensive Metabolizers

Genetic polymorphisms of cytochrome P450 (CYP) enzymes are one of the factors that contribute to the pharmacokinetic (PK) variability of drugs. PK variability is observed in the bimodal distribution between extensive metabolizers (EMs) and poor metabolizers (PMs). PK variability may also exist between individuals genotyped as homozygous EMs and heterozygous EMs. This may carry implications for drug dosing and drug response (e.g., risk of therapeutic failure or drug toxicity). Studies have reported significant PK differences between homozygous and heterozygous EMs. Some literature suggests that this distinction may be of clinical relevance. Due to study design limitations and data that are either sparse or conflicting, generalizations regarding the potential impact of the CYP genotype, within EMs, are difficult. Optimally designed clinical trials are needed. This review evaluates the potential impact of CYP genetic polymorphisms on interindividual PK variability of drugs within an EM population.

Ethnic differences in pain and pain management

Considerable evidence demonstrates substantial ethnic disparities in the prevalence, treatment, progression and outcomes of pain-related conditions. Elucidating the mechanisms underlying these group differences is of crucial importance in reducing and eliminating disparities in the pain experience. Over recent years, accumulating evidence has identified a variety of processes, from neurophysiological factors to structural elements of the healthcare system, that may contribute to shaping individual differences in pain. For example, the experience of pain differentially activates stress-related physiological responses across various ethnic groups, members of different ethnic groups appear to use differing coping strategies in managing pain complaints, providers' treatment decisions vary as a function of patient ethnicity and pharmacies in predominantly minority neighborhoods are far less likely to stock potent analgesics. These diverse factors, and others may all play a role in facilitating elevated levels of pain-related suffering among individuals from ethnic minority backgrounds. Here, we present a brief, nonexhaustive review of the recent literature and potential physiological and sociocultural mechanisms underlying these ethnic group disparities in pain outcomes.

CYP2D6 polymorphisms and codeine analgesia in postpartum pain management: a pilot study

BACKGROUND

Codeine, a common opiate prescribed for pain postcesarean section (c-section), is biotransformed by the highly polymorphic Cytochrome P450 enzyme 2D6 (CYP2D6). Ultrarapid metabolizers (UMs), individuals with multiple active copies of CYP2D6, can biotranform up to 50% more codeine into morphine than normal individuals can. In contrast, poor metabolizers (PMs), individuals who have no active CYP2D6 genes, convert almost no codeine into morphine and as a result may take multiple doses of codeine without attaining analgesia.

OBJECTIVE

The aim was to study the relationship between CYP2D6 genotype and codeine analgesia among women recovering from c-section.

METHODS

Forty-five mothers prescribed codeine provided a blood sample for CYP2D6 genotyping and recorded their pain level 4 times a day for 3 days immediately after a c-section. Codeine was used on an as-needed basis; doses and times were recorded. The relationship between CYP2D6 genotype, pain scores, need for codeine, and adverse events was studied. Theoretical morphine dose, based on CYP2D6 genotype, was estimated.

RESULTS

Women at the genotypic extremes reported codeine effects consistent with their genotype: the 2 PMs of codeine reported no analgesia as a result of taking codeine, whereas 2 of the 3 UMs reported immediate pain relief from codeine but stopped taking it due to dizziness and constipation. Much larger numbers are needed to study similar correlations among extensive and intermediate metabolizers.

CONCLUSIONS

In this pilot study, the extreme CYP2D6 genotypes (PMs and UMs) seemed to predict pain response and adverse events. Larger sample sizes are needed to correlate the range of genotypes with pain response.

Racial and ethnic disparities in pain: causes and consequences of unequal care

The purpose of our review is to evaluate critically the recent literature on racial and ethnic disparities in pain and to determine how far we have come toward reducing and eliminating disparities in pain. We examined peer-reviewed research articles published between 1990 and early 2009 that focused on racial and ethnic disparities in pain in the United States. The databases used were PubMed, Medline, Scopus, CINAHL, and PsycInfo. The probable causes of minority group disparities in pain are discussed, along with suggested strategies for eliminating pain-related disparities. This review reveals the persistence of racial and ethnic disparities in acute, chronic, cancer, and palliative pain care across the lifespan and treatment settings, with minorities receiving lesser quality pain care than non-Hispanic whites. Although health and health care disparities attract local, state, and federal attention, disparities in pain care continue to be missing from publicized public health agendas and health care reform plans. Ensuring optimal pain care for all is critically important from a public health and policy perspective. A robust research program on disparities in pain is needed, and the results must be successfully translated into practices and policies specifically designed to reduce and eliminate disparities in care.

PERSPECTIVE

This review evaluates the recent literature on racial and ethnic disparities in pain and pain treatment. Racial and ethnic disparities in acute pain, chronic cancer pain, and palliative pain care continue to persist. Rigorous research is needed to develop interventions, practices, and policies for eliminating disparities in pain.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Impact of CYP2D6*10 on H1-antihistamine-induced hypersomnia

OBJECTIVE

This study investigated the relevance of the cytochrome P450 (CYP) 2D6 genotype to the adverse drug reactions (ADRs) of H1-antihistamines and the level of sedation.

METHODS

Japanese participants in a health screening program were asked to describe any past history of ADRs. Any subjects reporting ADRs induced by H1-antihistamines were then individually interviewed and defined as cases. Excessive daytime sleepiness, which had occurred in the cases as an H1-antihistamine-induced ADR, was assessed by the Epworth sleepiness scale (ESS), and an ESS score >or=12 was considered hypersomnia. CYP2D6*4, *5, *14, and *10 were genotyped by a panel of polymerase chain reaction techniques.

RESULTS

Out of 2,074 participants, 100 cases (M:F = 37:63, mean age 51.9 +/- 9.2 years) were eligible for analysis. The most common etiological drug was chlorpheniramine, which is the most frequently used H1-antihistamine in Japan. CYP2D6*10 allele and genotypes were more frequently found in the cases than in the healthy Japanese population in a large study (P < 0.005 and P = 0.039, respectively), but no difference was observed in the null alleles and genotypes. The ESS scores in 75 cases (M:F=25:50) who had experienced excessive daytime sleepiness were 9.5 +/- 5.5 in men and 12.9 +/- 6.1 in women (P < 0.001, cases vs. 34 subjects without symptoms; P = 0.001 men vs. women). The occurrence of hypersomnia increased as the number of CYP2D6 mutant alleles increased (P = 0.045).

CONCLUSION

The results suggest that the presence of the CYP2D6*10 allele is a risk factor for development of H1-antihistamine-induced ADRs in Japanese.

Differences in drug pharmacokinetics between East Asians and Caucasians and the role of genetic polymorphisms

Interethnic variability in pharmacokinetics can cause unexpected outcomes such as therapeutic failure, adverse effects, and toxicity in subjects of different ethnic origin undergoing medical treatment. It is important to realize that both genetic and environmental factors can lead to these differences among ethnic groups. The International Conference on Harmonization (ICH) published a guidance to facilitate the registration of drugs among ICH regions (European Union, Japan, the United States) by recommending a framework for evaluating the impact of ethnic factors on a drug's effect, as well as its efficacy and safety at a particular dosage and dosage regimen. This review focuses on the pharmacokinetic differences between East Asians and Caucasians. Differences in metabolism between East Asians and Caucasians are common, especially in the activity of several phase I enzymes such as CYP2D6 and the CYP2C subfamily. Before drug therapy, identification of either the genotype and/or the phenotype for these enzymes may be of therapeutic value, particularly for drugs with a narrow therapeutic index. Furthermore, these differences are relevant for international drug approval when regulatory agencies must decide if they accept results from clinical trials performed in other parts of the world.

Impact of CYP2D6 intermediate metabolizer alleles on single-dose desipramine pharmacokinetics

This study utilized cytochrome P450 2D6 (CYP2D6) genotypes to explain variability of desipramine pharmacokinetics in a cohort of non-poor metabolizer individuals. In an interaction study utilizing desipramine as a probe, genotyping for the CYP2D6*3, *4, *5 and *6 alleles was used to screen out CYP2D6 poor metabolizers. Individuals were categorized according to these and additional alleles (CYP2D6*2, *9, *10, *17, *41 and x2). Genotypes of individuals heterozygous for two or three of *2, *17 and *41 alleles were confirmed by molecular haplotyping. Pharmacokinetic parameters of desipramine were analysed according to CYP2D6 category. Molecular haplotyping was necessary to definitively categorize four of 16 individuals. A subject who had unusually high plasma elimination half-time, exposure and metabolic ratios carried an intermediate metabolizer (IM) *9 allele in combination with a non-functional allele. This combination has a population frequency of less than 1 : 200. Individuals with *1/*1, *1/*2 and *2/*2 genotypes had lower than average plasma elimination half-time, exposure and metabolic ratios. For desipramine, additional genotyping of CYP2D6 IM alleles helped define subgroups of the CYP2D6-positive cohort. This suggests that genotyping for IM alleles will aid in interpretation of clinical trials involving CYP2D6 substrates. Due to the diversity of IM alleles, molecular haplotyping may be necessary to fully characterize CYP2D6 genotype-phenotype relationships.

Toward individualized pharmaceutical care of East Asians: the value of genetic testing for polymorphisms in drug-metabolizing genes

Research into the relationship between genetics and drug response has focused on polymorphisms in genes that encode drug-metabolizing enzymes, particularly the genes of cytochrome P450 superfamily 2, which affect the clearance of the anticoagulant warfarin, proton pump inhibitors, tricyclic antidepressants, and many other clinically relevant drugs. Much of this work has targeted East Asians, a genetically distinguishable and populous group. Researchers have identified polymorphisms that inactivate gene function, compared polymorphism frequencies in East-Asian and Caucasian populations, and determined the effects on the pharmacokinetic parameters of drugs. Detection in an individual of polymorphisms known to inactivate a drug-metabolizing enzyme is predictive of poor metabolism of drugs processed by that pathway, which itself may be predictive of an atypical drug response. Genetic tests can be used to screen for individuals with poor metabolizer phenotypes, with the ultimate goal of predicting the clinical effects of drugs.

Drug interactions of clinical significance for the dermatologist: recognition and avoidance

While it would be impossible for any dermatologist to remember all potential drug interactions, knowledge of the mechanisms of drug interactions can help reduce the risk of serious adverse outcomes. Most drugs are associated with interactions but the majority do not produce significant outcomes. Dealing with drug interactions is a challenge in all clinical practice, including dermatology. New information continues to appear, and dermatologists need to know about the drugs they use.This article focuses on the mechanisms of drug interactions. In particular, the life of a drug in terms of absorption, distribution, metabolism and excretion are reviewed with the focus on points of importance and relevance to drug interactions. The most clinically important drug interactions in dermatological practice are caused by alterations in drug metabolism. The contributions of P-glycoprotein, pharmacogenetic variation and genetic polymorphisms to drug interactions are highlighted, and the best evidence for drug interactions involving drug classes relevant to the dermatologist is presented. Since the initial evidence for clinically relevant drug interactions comes from case reports, prescribing physicians can have a major role in collating information on interactions. By understanding the mechanisms behind drug interactions and staying alert for toxicities, we can help make drug therapy safer and reduce the fear of drug interactions.

Quality of life concerns in patients with breast cancer: evidence for disparity of outcomes and experiences in pain management and palliative care among African-American women

BACKGROUND

African-American women are at higher risk for breast cancer mortality compared with their white counterparts. Furthermore, African-American women present for diagnosis and treatment later in the disease process. It may be expected that this greater disease burden would impose more symptoms compared with women who present with earlier stage disease. However, the effect of breast cancer on the quality of life of African-American women largely has been unexplored.

METHODS

A qualitative literature review was conducted to identify racial disparities in the palliative care of patients with cancer and their impact on quality of life for African-American women. A Medline search was done encompassing the years between 1985 and 2000 and included the following search terms: breast cancer, palliative care, pain management, quality of life, health care disparities, and African Americans. Relevant articles were read and summarized for inclusion in this review.

RESULTS

Differences in treatment patterns, pain management, and the use of hospice care exist between African-American women and women in other ethnic groups. Explanations for these differences have not been researched well. In addition, the emotional, social, and other aspects of quality of life for African-American women with breast cancer are not well understood, in part due to the absence of a standardized quality-of-life measure.

CONCLUSIONS

Physicians and other health care providers must be educated better about pain management and hospice care and, in turn, must inform their patients better about these issues. Physicians' and researchers' considerations of the influence of race and ethnicity on quality of life are critical. Furthermore, future research should be focused on the establishment of a standardized measure for quality of life that better encompasses its social, spiritual, and emotional aspects. Quality-of-life measures should be incorporated into routine health surveillance mechanisms, with an increased emphasis on minority and other under-served populations.

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.

Effects of G169R and P34S substitutions produced by mutations of CYP2D6*14 on the functional properties of CYP2D6 expressed in V79 cells

CYP2D6 is a polymorphic enzyme that catalyzes the oxidation of various drugs. At least 40-mutant alleles of CYP2D6 have been reported. CYP2D6*14, which is one of them found in Asian populations, causes deficient activity of CYP2D6. Four amino acid substitutions, P34S, G169R, R296C, and S486T, are present in the protein encoded by CYP2D6*14 (CYP2D6 14). Among them, G169R is thought to be a definitive substitution because it is unique to CYP2D6 14. However, a previous study showed that the activity of G169R-substituted CYP2D6 was about 40% of wild-type CYP2D6, suggesting that a combination of G169R and other substitutions may be required to abolish the activity of CYP2D6. In the present study, we examined the effects of combined substitutions of G169R and P34S on the functional properties of CYP2D6 and compared them with those of a single substitution of G169R or P34S using a cDNA expression system of V79 cells. The results showed that a combined substitution of G169R and P34S reduced the activities of CYP2D6 to less than the detection limit of our analytical method for bufuralol 1'-hydroxylation and dextromethorphan O-demethylation. However, these activities were not completely abolished by a single substitution of P34S or G169R. The findings suggest that simultaneous substitution of G169R and P34S is crucial for almost completely abolishing the activity of CYP2D6 at least in V79 cells, although whether the absence of metabolism is due to the absence of functional protein or catalytic incompetency remains unclear because the levels of CYP2D6 protein expressed in V79 cells were too low to be determined by difference CO-reduced spectra.

The molecular and enzyme kinetic basis for the diminished activity of the cytochrome P450 2D6.17 (CYP2D6.17) variant. Potential implications for CYP2D6 phenotyping studies and the clinical use of CYP2D6 substrate drugs in some African populations

In this study, the basis for the diminished capacity of CYP2D6.17 to metabolise CYP2D6 substrate drugs and the possible implications this might have for CYP2D6 phenotyping studies and clinical use of substrate drugs were investigated in vitro. Enzyme kinetic analyses were performed with recombinant CYP2D6.1, CYP2D6.2, CYP2D6.17 and CYP2D6.T107I using bufuralol, debrisoquine, metoprolol and dextromethorphan as substrates. In addition, the intrinsic clearance of 10 CYP2D6 substrate drugs by CYP2D6.1 and CYP2D6.17 was determined by monitoring substrate disappearance. CYP2D6.17 exhibited generally higher K(m) values compared to CYP2D6.1. The V(max) values were generally not different except for metoprolol alpha-hydroxylation with the V(max) value for CYP2D6.17 being half that of CYP2D6.1. CYP2D6.1 and CYP2D6.2 displayed similar kinetics with all probe drugs except for dextromethorphan O-demethylation with the intrinsic clearance value of CYP2D6.2 being half that of CYP2D6.1. CYP2D6.17 exhibited substrate-dependent reduced clearances for the 10 substrates studied. In a clinical setting, the clearance of some drugs could be affected more than others in individuals with the CYP2D6(*)17 variant. The CYP2D6(*)17 allele might, therefore, contribute towards the poor correlation of phenotyping results when using different probe drugs in African populations. To investigate effects of CYP2D6(*)17 mutations on the structure of the enzyme, a homology model of CYP2D6 was built using the CYP2C5 crystal structure as a template. The results suggest an alteration in position of active-site residues in CYP2D6.17 as a possible explanation for the reduced activity of the enzyme.

Drug interactions: Proteins, pumps, and P-450s

The two major concerns in drug safety are adverse drug reactions and drug interactions. When multiple drug therapies are prescribed, drug interactions become an important consideration for patients and physicians. The life of a drug is reviewed with emphasis on absorption, distribution, metabolism, and excretion. Pharmacokinetic and pharmacodynamic mechanisms for drug interactions are reviewed. The contributions of P-glycoprotein, pharmacogenetic variation, and genetic polymorphisms to drug interactions are highlighted. Prediction of drug interactions is possible with knowledge of which agents are likely to cause alterations in drug metabolism. (J Am Acad Dermatol 2002;47:467-84.)

LEARNING OBJECTIVE

At the conclusion of this learning activity, participants should have an understanding of the life of a drug. This knowledge should help predict important potential drug interactions.

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.

Molecular basis of ethnic differences in drug disposition and response

Ethnicity is an important demographic variable contributing to interindividual variability in drug metabolism and response. In this rapidly expanding research area many genetic factors that account for the effects of ethnicity on pharmacokinetics, pharmacodynamics, and drug safety have been identified. This review focuses on recent developments that have improved understanding of the molecular mechanisms responsible for such interethnic differences. Genetic variations that may provide a molecular basis for ethnic differences in drug metabolizing enzymes (CYP 2C9, 2C19, 2D6, and 3A4), drug transporter (P-glycoprotein), drug receptors (adrenoceptors), and other functionally important proteins (eNOS and G proteins) are discussed. A better understanding of the molecular basis underlying ethnic differences in drug metabolism, transport, and response will contribute to improved individualization of drug therapy.

Characterization of (+/-)-bufuralol hydroxylation activities in liver microsomes of Japanese and Caucasian subjects genotyped for CYP2D6

Twenty-four genetic polymorphisms in the CYP2D6 gene were analysed in liver DNA samples of 39 Japanese and 44 Caucasians and compared with CYP2D6 protein levels and bufuralol 1'- and 6-hydroxylation activities in liver microsomes of these human samples. We detected 13 types of CYP2D6 genetic polymorphisms and classified these into 20 genotypes; nine types were found in Japanese and 14 types in Caucasian samples. CYP2D6*10B, but not CYP2D6*10A, was the most frequent (34.6%) in Japanese. In Caucasians, several CYP2D6 polymorphisms including CYP2D6*4, *4D, *4E, *4L, *3, *9, *5 and *2E (frequencies of 6.8, 3.4, 4.5, 9.1, 1.1, 2.3, 2.3 and 4.5%, respectively) were detected. A Caucasian having CYP2D6*3/*5 had a protein with slower gel mobility (immunoblotting with anti-CYP2D6 antibody) and very low activity for bufuralol 1'-hydroxylation. Five Caucasian samples (CYP2D6*4/*4, *4/*4L, or *4D/*4L) had no measurable CYP2D6 protein and very low bufuralol 1'-hydroxylation activities. Seven Japanese subjects with CYP2D6*10B/*10B had CYP2D6 protein at levels of approximately 20% of those present in humans with CYP2D6*1 and *2 and catalysed bufuralol 1'-hydroxylation at low rates. Kinetic analysis of bufuralol 1'- and 6-hydroxylation indicates that (i) the Km values for 1'-hydroxylation were lower in individuals with CYP2D6*1/*1, *1/*2, *1/*2X2, and *2/*2 than those with CYP2D6*4/*4, *4/*4L, *4D/*4L, or *10B/*10B and Vmax values tended to be higher in the former groups (*1, *2), and (ii) individuals with heterozygous CYP2D6*1/*4D, *1/*4L, and *1/*5 had relatively high Vmax/Km ratios, whereas individuals with heterozygous CYP2D6*1/*9, *2/4D, *2/*5, *2/*10B, *2E/*4E, *3/*5, *4L/*9, and *10B/*39 had lower Vmax/Km ratios for bufuralol 1'-hydroxylation. Quinidine inhibited bufuralol 1'-hydroxylation in liver microsomes, particularly at low substrate concentrations, in individuals with CYP2D6*1/*1, and 1/1*2, but not those with CYP2D6*4/*4 and very slightly in individuals with CYP2D6*10B/*10B. The latter two groups were found to be more sensitive to alpha-naphthoflavone than the former groups, indicative of the contribution of CYP1A2. These results support the view that CYP2D6*3, *4, *4D, and *4L are major genotypes producing poor metabolizer phenotypes in CYP2D6 in Caucasians, whereas CYP2D6*10B is a major factor in decreased CYP2D6 protein expression and catalytic activities in Japanese.

The prolonged duration of rocuronium in Chinese patients

We compared the potency and duration of action of rocuronium in Chinese and Caucasian patients during general anesthesia. Thirty-six women (18 Caucasian and 18 Chinese) and 36 children (18 Caucasian and 18 Chinese) were evaluated during the administration of propofol/fentanyl anesthesia. Patients in each age group were randomized into three subgroups to receive single doses of 0.06, 0. 12, or 0.18 mg/kg rocuronium (adults) or 0.12, 0.18, or 0.24 mg/kg rocuronium (children). Neuromuscular blockade was assessed by electromyography of the adductor pollicis after train-of-four (TOF) stimulation of the ulnar nerve. Dose response curves were constructed when maximum neuromuscular depression of the first twitch of the train (T(1)) was obtained. A second bolus dose of rocuronium was then administered to a total dose of 0.6 mg/kg. The times of spontaneous recovery to T(1) 10%, 25%, and 90% of control and to TOF 0.25, 0.50, and 0.70 were recorded. For both adults and children, recovery occurred later in Chinese than in Caucasian patients (P<0.05 for T(1) of 10%, 25%, 75%, and 90% and TOF to 0.7). The 50% effective dose was smaller in Chinese adults (125+/-63 vs. 159+/-66 microg/kg) and Chinese children (171+/-43 vs. 191+/-46 microg/kg) than in Caucasian adults and children, but the difference was not statistically significant. In adults, time to 25% T(1) recovery was 43+/-13 min in Chinese patients and 33+/-10 min in Caucasian patients (P<0.05). The corresponding values were more rapid for children: 30+/-10 and 24+/-6 min (P<0.05). We conclude that the recovery from rocuronium neuromuscular blockade was longer in Chinese compared with Caucasian patients and in adults compared with children.

The decreased in vivo clearance of CYP2D6 substrates by CYP2D6*10 might be caused not only by the low-expression but also by low affinity of CYP2D6

CYP2D6 exhibits genetic polymorphism with interindividual differences in metabolic activity. We have found a significant influence on the pharmacokinetics of venlafaxine by the CYP2D6*10 allele in a Japanese population. CYP2D6.10, which is translated from CYP2D6*10, has two amino acid substitutions: Pro34 --> Ser and Ser486 --> Thr. In this study, CYP2D6.10 was expressed in Saccharomyces cerevisiae and its catalytic activity for CYP2D6 substrates was investigated. The CYP2D6*10B- and *10C-associated cDNA were isolated from human lymphocyte genotyped as CYP2D6*10. In addition, three forms of CYP2D6, Pro34/Thr486 (PT), Ser34/Ser486 (SS), and Pro34/Ser486 (wild type, CYP2D6.1), were constructed by PCR-site mutagenesis to clarify the effects of the two amino-acid substitutions. The expression of CYP2D6 protein was confirmed by immunoblotting using CYP2D antibody. The absorbance at 450 nm was measured by CO-reduced difference spectra from five all microsome preparations. The CYP2D6 forms with Pro34 --> Ser amino acid substitution were at a lower expression than CYP2D6.1 from the findings of immunoblotting and spectral analysis. The apparent K(m) values of CYP2D6.1, CYP2D6.10A, and CYP2D6.10C were 1.7, 8.5, and 49.7 microM, respectively, for bufuralol 1'-hydroxylation, and 9.0, 51.9, and 117.4 microM, respectively, for venlafaxine O-demethylation, respectively. The V(max) values were not significantly different among the three variants. These findings suggest that the decreased in vivo clearance by CYP2D6*10 was caused not only by low expression of but also the increased K(m) value of CYP2D6.

Frequencies of CYP2D6 mutant alleles in a normal Japanese population and metabolic activity of dextromethorphan O-demethylation in different CYP2D6 genotypes

AIMS

To determine the frequencies of 11 CYP2D6 mutant alleles (CYP2D6*2, *3, *4, *5, *8, *10, *11, *12, *14, *17 and *18), and their relation to the metabolic capacity of CYP2D6 in Japanese subjects.

METHODS

One hundred and sixty-two unrelated healthy Japanese subjects were genotyped with the polymerase chain reaction amplification method and 35 subjects were phenotyped with dextromethorphan.

RESULTS

The frequencies of CYP2D6*2,*5, *10 and *14 were 12.9, 6.2, 38.6 and 2.2% in our Japanese subjects, respectively. CYP2D6*3, *4, *8, *11, *12, *17 and *18 were not detected. The mean log metabolic ratio of dextromethorphan in subjects with genotypes predicting intermediate metabolizers was significantly greater than that of heterozygotes for functional and defective alleles.

CONCLUSIONS

CYP2D6*5 and CYP2D6*14 are the major defective alleles found in Japanese subjects. In addition, CYP2D6*10 may play a more important role than previously thought for the treatment of Japanese patients with drugs metabolized by CYP2D6.

Polymorphic human cytochrome P450 enzymes: an opportunity for individualized drug treatment

Approximately 40% of human P450-dependent drug metabolism is carried out by polymorphic enzymes, which can cause abolished, quantitatively or qualitatively altered or enhanced drug metabolism. The latter situation is due to stable duplication, multiduplication or amplification of active genes, most likely in response to dietary components that have resulted in a selection of alleles with multiple non-inducible genes. Several examples exist where subjects carrying certain alleles suffer from a lack of drug efficacy due to ultrarapid metabolism or, alternatively, adverse effects from the drug treatment due to the presence of defective alleles. Knowledge in this field has grown rapidly and can now be applied to both drug development and clinical practice. This is facilitated by the recent development of high-throughput methods for mutation detection and oligonucleotide chips array technology for the identification of a multitude of mutations in the genes encoding drug-metabolizing enzymes. The outcome will allow for safer and more efficient drug therapies.

A puzzling case of seizures and visual hallucinations during clomipramine treatment with a high dose but causing a low serum concentration

We present a puzzling case of a 25-year-old depressive man suffering from seizures and visual hallucinations during clomipramine treatment with a high dose but causing a low serum concentration. We examined alleles of cytochrome P450 (CYP) isozymes. It was revealed that he was not an ultrarapid metabolizer for CYP2D6, and that the genotypes were homozygous for CYP2D6J and heterozygous for CYP2C19m1. Throughout the treatment period, his compliance was good. Since he was a smoker, it seems likely that his low clomipramine level was due to smoking-induced CYP1A2 activity. These findings suggest that smoking-induced CYP1A2 activity overcomes the possibly inhibiting effects of homozygosity for CYP2D6J and heterozygosity for CYP2C19m1, and that high-dose clomipramine is not always a direct cause of seizures.

Effect of the CYP2D6*10 genotype on venlafaxine pharmacokinetics in healthy adult volunteers

AIMS

Interindividual differences in the pharmacokinetics of venlafaxine, a new antidepressant, were shown during early clinical trials in Japan. Venlafaxine is metabolized mainly by CYP2D6 to an active metabolite, O-desmethylvenlafaxine (ODV). Therefore, the influence of the CYP2D6 genotypes on venlafaxine pharmacokinetics was examined in a Japanese population.

METHODS

Twelve adult Japanese men in good health participated in this study. Genomic DNA was isolated from peripheral lymphocytes, and the CYP2D6 genotypes were determined by codon 188C/T, 1934G/A, 2938G/A and 4268G/C mutations using endonuclease tests based on PCR and by Xba I-RFLP analysis. Subjects were categorized into the following 3 groups (n=4 in each group); Group1: CYP2D6*10/*10, *5/*10, Group2: CYP2D6*1/*10, *2/*10 and Group3: CYP2D6*1/*1, CYP2D6*1/*2. Venlafaxine (25 mg, n=6; 37.5 mg, n=6) was administered orally at 09.00 h following an overnight fast. Plasma concentrations of venlafaxine and ODV were monitored by h.p.l.c. for 48 h.

RESULTS

The Cmax and AUC of venlafaxine were 184% and 484% higher in the group 1 subjects than in the group 3 subjects, and 101% and 203% higher in the group 1 than in the group 2, respectively.

CONCLUSIONS

These results suggest that CYP2D6*10 influences the pharmacokinetics of venlafaxine in a Japanese population.

Genetic polymorphisms of human N-acetyltransferase, cytochrome P450, glutathione-S-transferase, and epoxide hydrolase enzymes: relevance to xenobiotic metabolism and toxicity

In this review, an overview is presented of the current knowledge of genetic polymorphisms of four of the most important enzyme families involved in the metabolism of xenobiotics, that is, the N-acetyltransferase (NAT), cytochrome P450 (P450), glutathione-S-transferase (GST), and microsomal epoxide hydrolase (mEH) enzymes. The emphasis is on two main topics, the molecular genetics of the polymorphisms and the consequences for xenobiotic metabolism and toxicity. Studies are described in which wild-type and mutant alleles of biotransformation enzymes have been expressed in heterologous systems to study the molecular genetics and the metabolism and pharmacological or toxicological effects of xenobiotics. Furthermore, studies are described that have investigated the effects of genetic polymorphisms of biotransformation enzymes on the metabolism of drugs in humans and on the metabolism of genotoxic compounds in vivo as well. The effects of the polymorphisms are highly dependent on the enzyme systems involved and the compounds being metabolized. Several polymorphisms are described that also clearly influence the metabolism and effects of drugs and toxic compounds, in vivo in humans. Future perspectives in studies on genetic polymorphisms of biotransformation enzymes are also discussed. It is concluded that genetic polymorphisms of biotransformation enzymes are in a number of cases a major factor involved in the interindividual variability in xenobiotic metabolism and toxicity. This may lead to interindividual variability in efficacy of drugs and disease susceptibility.

Pharmacokinetics of nortriptyline and its 10-hydroxy metabolite in Chinese subjects of different CYP2D6 genotypes

OBJECTIVES

To study the impact of the CYP2D6*10 allele on the disposition of nortriptyline in Chinese subjects.

METHODS

A single dose of 25 mg nortriptyline was given orally to 15 healthy Chinese volunteers who were classified as extensive metabolizers after phenotyping with debrisoquin (INN, debrisoquine) and who were genotyped by allele-specific polymerase chain reaction. Five subjects were homozygous for CYP2D6*1, 5 subjects were homozygous for CYP2D6*10, and 5 subjects were heterozygous for these 2 alleles. Plasma concentrations of nortriptyline and its main metabolite 10-hydroxynortriptyline were measured by liquid chromatography-mass spectrometry, and the pharmacokinetics were studied during 168 hours after the dose.

RESULTS

Subjects who were homozygous for CYP2D6*10 had significantly higher total areas under the plasma concentration-time curve (AUC), lower apparent oral clearances, and longer mean plasma half-life of nortriptyline than subjects in the CYP2D6*1/*1 and the heterozygous groups. For 10-hydroxynortriptyline, the AUC was lower and the plasma half-life was longer in subjects who were homozygous for CYP2D6*10 than in subjects in the other 2 groups.

CONCLUSION

The CYP2D6*10 allele in Chinese subjects was associated with significantly higher plasma levels of nortriptyline compared with the CYP2D6*1 allele because of an impaired metabolism of nortriptyline to 10-hydroxynortriptyline, particularly in the subjects with the CYP2D6*10/*10 genotype. The results suggest that genotyping of CYP2D6 may be a useful tool in predicting the pharmacokinetics of nortriptyline.

Pharmacogenetics and cancer chemotherapy

Cancer chemotherapy is limited by significant inter-individual variations in responses and toxicities. Such variations are often due to genetic alterations in drug metabolising enzymes (pharmacokinetic polymorphisms) or receptor expression (pharmacodynamic polymorphisms). Pharmacogenetic screening prior to anticancer drug administration may lead to identification of specific populations predisposed to drug toxicity or poor drug responses. The role of polymorphisms in specific enzymes, such as thiopurine S-methyltransferases (TPMT), dihydropyrimidine dehydrogenase (DPD), aldehyde dehydrogenases (ALDH), glutathione S-transferases (GST), uridine diphosphate glucuronosyl-transferases (UGTs) and cytochrome P450 (CYP 450) enzymes in cancer therapy are discussed in this review.

Correlation of morphine sulfate in blood plasma and saliva in pediatric patients

This study sought to determine whether saliva concentrations of morphine correlate with plasma levels of morphine in pediatric patients receiving morphine analgesia for severe pain, and to evaluate whether the measurement of saliva morphine concentrations would be a useful, noninvasive, clinical tool to diagnose systemic exposure to morphine. Fifteen pediatric patients were enrolled; for the control group, 18 adult volunteers were recruited. Patients received continuous morphine drips to ameliorate pain caused by a sickle cell vasoocclusive crisis (range, 10-40 micrograms/kg.h). Control subjects were randomized into those receiving acetaminophen with either 8 mg (n = 13) or 30 mg (n = 5) of codeine. All participants fasted at least 2 hours before sample collection. Blood and saliva samples were collected simultaneously. All samples were analyzed by radioimmunoassay for morphine. There was no correlation between saliva and plasma morphine concentrations in either the patients receiving intravenous morphine (r = 0.04, P = 0.89) or in the controls receiving codeine (r = 0.43, P = 0.08). There was no observed difference in the mean counts per minute (CPM) for saliva samples in the pH range 3.96 to 8.06. Saliva concentrations of morphine cannot be used to predict the plasma concentration of morphine in children or adults. However, the concentration of morphine in saliva may be used as a qualitative indicator of systemic exposure to morphine in a subject.