Genetic polymorphism in cytochrome P450 2D6 (CYP2D6): Population distribution of CYP2D6 activity

Pharmacogenetics of antidepressant treatment in obsessive–compulsive disorder: an update and implications for clinicians

Obsessive–compulsive disorder (OCD) is a chronic neuropsychiatric disorder with high genetic influence. Antidepressants such as serotonin reuptake inhibitors, are widely accepted as the first-line medications for OCD; however, approximately 50% of OCD patients show poor response. Personalized medicine utilizing genetic testing has recently received much attention because the variability of antidepressant response and tolerability are partly due to an individual’s genetic variations. This has led to researchers investigating the role of specific genetic factors on antidepressant response and utility of testing in the clinical realm. Genetic test panels are showing promise for guiding antidepressant treatment to improve outcomes in depression. This article will review the most recent findings in the pharmacogenetics of OCD and its related disorders. Promising results have been reported for several serotonergic and glutamatergic system genes and the cytochrome CYP450 liver enzyme genes, which appear to play an important role in OCD and antidepressant response.

Pharmacogenomics and adverse drug reactions in children

Adverse drug reactions are a common and important complication of drug therapy in children. Over the past decade it has become increasingly apparent that genetically controlled variations in drug disposition and response are important determinants of adverse events for many important adverse events associated with drug therapy in children. While this research has been difficult to conduct over the past decade technical and ethical evolution has greatly facilitated the ability of investigators to conduct pharmacogenomic studies in children. Some of this research has already resulted in changes in public policy and clinical practice, for example in the case of codeine use by mothers and children. It is likely that the use of pharmacogenomics to enhance drug safety will first be realized among selected groups of children with high rates of drug use such as children with cancer, but it also likely that this research will be extended to other groups of children who have high rates of drug utilization and as well as providing insights into the mechanisms and pathophysiology of adverse drug reactions in children.

Pharmacogenomics in children

Historically genetics has not been considered when prescribing drugs for children. However, it is clear that genetics are not only an important determinant of disease in children but also of drug response for many important drugs that are core agents used in the therapy of common problems in children. Advances in therapy and in the ethical construct of children's research have made pharmacogenomic assessment for children much easier to pursue. It is likely that pharmacogenomics will become part of the therapeutic decision making process for children, notably in areas such as childhood cancer where the benefits and risks of therapy are considerable.

Hydromorphone: evolving to meet the challenges of today's health care environment

BACKGROUND

Hydromorphone, a potent analogue of morphine, has long had an important role in pain management and is included in several international guidelines for managing pain. Advances in hydromorphone formulations and the ways in which hydromorphone is being used clinically today warrant a review of the drug's pharmacotherapeutic utility.

OBJECTIVE

The history and recent advances in hydromorphone pharmacotherapy are reviewed. Areas covered include the pharmacologic and metabolic profile of hydromorphone, the role of hydromorphone in pain management, formulations and routes of administration, and issues related to relative opioid potencies, equianalgesic ratios, and opioid rotation. Because hydromorphone, like all opioids, carries a risk of misuse, abuse, and illicit diversion, the related issues of tamper-resistant formulations and "dose-dumping" of extended-release formulations are discussed.

CONCLUSIONS

Due to the epidemic of prescription opioid overdoses associated with prescription opioid abuse in the United States, development of tamper-resistant opioid formulations that avoid dose-dumping issues has become a significant goal of pharmaceutical manufacturers. The current formulation of hydromorphone extended-release potentially provides the benefits of long-acting hydromorphone (ie, continuous pain control, increased quality of life, freedom to perform daily activities) to appropriate patients, while reducing the risks of abuse and without compromising safety.

Genetic variations of human CYP2D6 in the Chinese Han population

Aim: The purpose of this study was to determine the genetic polymorphisms of the CYP2D6 gene and to elucidate the allele distribution pattern in the Chinese Han population. Materials & methods: We used PCR and bidirectional sequencing methods to analyze all nine exons of the CYP2D6 gene in 2129 unrelated, healthy Chinese Han subjects from two geographical locations in China: the northern and southern regions. Results: In total, 165 mutated sites were detected in 2129 participants, of which 67 sites were reported for the first time. Among these novel mutation sites, 22 were nonsynonymous and 12 were named as novel alleles (*87–*93, *94A, *94B and *95–*98) by the Human CYP Allele Nomenclature Committee. In addition, 29 previously reported alleles and 84 genotypes were also detected in 1954 volunteers. Functional prediction of novel variants revealed that eight variants might have a deleterious effect on CYP2D6. Linkage disequilibrium analysis and tagSNP selection were performed separately. By using these methods, distinct differences were found between the two regions. Conclusion: This study provides the most comprehensive data concerning CYP2D6 polymorphisms in the Chinese Han population to date and increases the number of known alleles; these findings may greatly contribute to the development of personalized medicine for the Chinese Han population.

Original submitted 1 January 2013; Revision submitted 14 August 2013

Cytochrome P450 2D6 polymorphisms and predicted opioid metabolism in African American children with sickle cell disease

The opioid medications codeine and hydrocodone, commonly prescribed in sickle cell disease (SCD), require metabolic conversion by cytochrome P450 2D6 (CYP2D6) to morphine and hydromorphone, respectively, to exert their analgesic effects. The CYP2D6 gene is highly polymorphic, with variant alleles that result in decreased, absent, or ultrarapid enzyme activity. Seventy-five children with SCD were tested for CYP2D6 polymorphisms, and metabolic phenotypes were inferred from the genotypes. The most common variant alleles were CYP2D6*2 (normal activity, 28.7%), CYP2D6*17 (reduced activity, 17.3%), CYP2D6*5 (gene deletion, 8.7%), and CYP2D6*4 (absent function, 8.0%). Normal/extensive metabolizer genotypes were found in 28/75 (37.5%), intermediate metabolism in 33/75 (44.0%), poor metabolism in 4/75 (5.3%), ultrarapid metabolism in 3/75 (4.0%), indeterminate in 6/75 (8.0%). Allele frequencies did not vary significantly among different hemoglobin genotypes. Identification of variant CYP2D6 genotypes may identify individuals with altered metabolism and therefore altered analgesic response to codeine and hydrocodone, thus providing a personalized medicine approach to treatment of pain in SCD. Further pharmacokinetic and pharmacodynamic studies are needed to define the relationship of CYP2D6 and other gene polymorphisms to individual opioid effect in SCD.

Opioid therapy pharmacogenomics for noncancer pain: efficacy, adverse events, and costs

Chronic non-cancer pain is a debilitating condition associated with high individual and societal costs. While opioid treatment for pain has been available for centuries, it is associated with high variability in outcome, and a considerable proportion of patients is unable to attain relief from symptoms while suffering adverse events and developing medication dependence. We performed a review of the efficacy of pharmacogenomic markers and their abilities to predict adverse events, dependence, and associated economic costs, focusing on two genes: OPRM1 and CYP2D6. Data sources were articles indexed by PubMed on or before August 6, 2013. Articles were first selected after review of their titles and abstracts, and full papers were read to confirm eligibility. Initially, fifty-two articles were identified. Of these, 17 were relevant to biological actions of pharmacogenomic markers and their effect on therapeutic efficacy, 16 to adverse events, 15 to opioid dependence, and eight to economic costs. In conclusion, increasing costs of opioid therapy have made the advances in pharmacogenomics an attractive solution to personalize care with unclear repercussions related to the impact on costs, morbidity, and outcomes. This intersection of pharmacoeconomics and pharmacogenomics presents a unique platform to further examine current advances in clinical medicine and their utility in cost-effective treatment of chronic pain.

Addressing human variability in next-generation human health risk assessments of environmental chemicals

BACKGROUND

Characterizing variability in the extent and nature of responses to environmental exposures is a critical aspect of human health risk assessment.

OBJECTIVE

Our goal was to explore how next-generation human health risk assessments may better characterize variability in the context of the conceptual framework for the source-to-outcome continuum.

METHODS

This review was informed by a National Research Council workshop titled "Biological Factors that Underlie Individual Susceptibility to Environmental Stressors and Their Implications for Decision-Making." We considered current experimental and in silico approaches, and emerging data streams (such as genetically defined human cells lines, genetically diverse rodent models, human omic profiling, and genome-wide association studies) that are providing new types of information and models relevant for assessing interindividual variability for application to human health risk assessments of environmental chemicals.

DISCUSSION

One challenge for characterizing variability is the wide range of sources of inherent biological variability (e.g., genetic and epigenetic variants) among individuals. A second challenge is that each particular pair of health outcomes and chemical exposures involves combinations of these sources, which may be further compounded by extrinsic factors (e.g., diet, psychosocial stressors, other exogenous chemical exposures). A third challenge is that different decision contexts present distinct needs regarding the identification-and extent of characterization-of interindividual variability in the human population.

CONCLUSIONS

Despite these inherent challenges, opportunities exist to incorporate evidence from emerging data streams for addressing interindividual variability in a range of decision-making contexts.

CYP450 phenotyping and accurate mass identification of metabolites of the 8-aminoquinoline, anti-malarial drug primaquine

Background

The 8-aminoquinoline (8AQ) drug primaquine (PQ) is currently the only approved drug effective against the persistent liver stage of the hypnozoite forming strains Plasmodium vivax and Plasmodium ovale as well as Stage V gametocytes of Plasmodium falciparum. To date, several groups have investigated the toxicity observed in the 8AQ class, however, exact mechanisms and/or metabolic species responsible for PQ’s haemotoxic and anti-malarial properties are not fully understood.

Methods

In the present study, the metabolism of PQ was evaluated using in vitro recombinant metabolic enzymes from the cytochrome P450 (CYP) and mono-amine oxidase (MAO) families. Based on this information, metabolite identification experiments were performed using nominal and accurate mass measurements.

Results

Relative activity factor (RAF)-weighted intrinsic clearance values show the relative role of each enzyme to be MAO-A, 2C19, 3A4, and 2D6, with 76.1, 17.0, 5.2, and 1.7% contributions to PQ metabolism, respectively. CYP 2D6 was shown to produce at least six different oxidative metabolites along with demethylations, while MAO-A products derived from the PQ aldehyde, a pre-cursor to carboxy PQ. CYPs 2C19 and 3A4 produced only trace levels of hydroxylated species.

Conclusions

As a result of this work, CYP 2D6 and MAO-A have been implicated as the key enzymes associated with PQ metabolism, and metabolites previously identified as potentially playing a role in efficacy and haemolytic toxicity have been attributed to production via CYP 2D6 mediated pathways.

CYP2D6 gene variants in urban/admixed and Amerindian populations of Venezuela: pharmacogenetics and anthropological implications

BACKGROUND

Differences in genes encoding enzymes involved in the biotransformation of a large number of compounds, such as CYP2D6, are related to inter-individual and inter-ethnic variability in the metabolism of many drugs, which have also been linked to susceptibility to cancer and other health outcomes. Therefore, populations are likely to benefit from inclusion in pharmacogenetic research studies.

AIM

To determine the frequency of functionally important allele variants of CYP2D6 gene in a sample of an Urban/admixed and five Amerindian Venezuelan populations.

SUBJECTS AND METHODS

DNA of 328 unrelated volunteers was analysed for the presence of CYP2D6 *2, *3, *4, *5, *6 and *10 variants.

RESULTS

The frequency in the Urban/admixed population for *2, *3, *4, *5, *6 and *10 alleles was 37.9%, 0%, 13.4%, 2.0%, 1.2% and 4.0%, respectively. In the Bari population, the prevalence of *4 allele associated with decreased enzyme activity was observed in 42.5%, whereas the poor metabolizer genotype *4/*4 was found in 25%. In the Panare, Pemon, Warao and Wayuu populations the *4 allele was found in 5.4%, 2.5%, 1.7% and 4.2%, respectively. The *10 allele frequency found in Amerindians (0.0-6.3%) was lower than reported for Asians.

CONCLUSION

The results are consistent with the known genetic admixture origin of most Venezuela populations. Nevertheless, the observed significant differences among Amerindians highlight the need for pharmacogenetic studies taking into account biogeographical and anthropological considerations.

Pharmacokinetic drug interactions between clobazam and drugs metabolized by cytochrome P450 isoenzymes

STUDY OBJECTIVE

To investigate potential drug-drug interactions between clobazam and cytochrome P450 (CYP) isoenzyme substrates, inhibitors, and inducers.

DESIGN

Two, prospective, open-label, single-center, drug-drug interaction (DDI) studies and a population pharmacokinetics analysis of seven multicenter phase II-III trials.

SETTING

Clinical research unit.

PARTICIPANTS

Fifty-four healthy adult volunteers were enrolled in the two drug-drug interaction studies; 53 completed the studies. The population pharmacokinetics analysis evaluated data from 171 participants from five studies with healthy volunteers and two studies with patients with Lennox-Gastaut syndrome. Participants in these studies received clobazam and stable dosages of the following antiepileptic drugs: phenobarbital, phenytoin, carbamazepine, valproic acid, lamotrigine, felbamate, or oxcarbazepine.

INTERVENTION

In the first drug-drug interaction study, 36 participants received a single oral dose of clobazam 10 mg on day 1, followed by either ketoconazole 400 mg once/day or omeprazole 40 mg once/day on days 17-22, with a single dose of clobazam 10 mg coadministered on day 22, to study the effects of CYP3A4 or CYP2C19 inhibition, respectively, on clobazam and its active metabolite N-desmethylclobazam (N-CLB). In the second study, 18 participants received a drug cocktail consisting of caffeine 200 mg, tolbutamide 500 mg, dextromethorphan 30 mg, and midazolam 4 mg on days 1 and 19, and clobazam 40 mg/day on days 4-19, to study clobazam's effects on CYP1A2, CYP2C9, CYP2D6, and CYP3A4.

MEASUREMENTS AND MAIN RESULTS

In the first DDI study, coadministration of ketoconazole (a CYP3A4 inhibitor) and clobazam increased clobazam's area under the concentration time curve from time zero extrapolated to infinity (AUC(0-∞) ) 54% and decreased clobazam's maximum plasma concentration (C(max) ) by 15% versus administration of clobazam alone, but the combination affected these pharmacokinetic parameters for N-CLB to a lesser degree. The CYP2C19 inhibitor omeprazole increased AUC(0-∞) and C(max) of N-CLB by 36% and 15%, respectively, but did not significantly affect the pharmacokinetics of clobazam. At steady state, N-CLB has 3-4 times greater exposure than clobazam. In the second DDI study, no clinically significant drug-drug interactions were observed between clobazam 40 mg and the CYP probe substrates caffeine or tolbutamide. Exposure to midazolam and its 1-hydroxymidazolam metabolite, however, decreased by 27% and increased 4-fold, respectively. Clobazam increased dextromethorphan (CYP2D6) AUC(0-∞) by 95% and C(max) by 59%. In the population pharmacokinetics analysis, stable dosages of common antiepileptic drugs that induce CYP3A4 or CYP2C19, or inhibit CYP2C19, had negligible effects on clobazam or N-CLB. Clobazam did not affect valproic acid or lamotrigine exposures.

CONCLUSION

These findings suggest no clinically meaningful drug-drug interactions between clobazam and drugs metabolized by CYP3A4, CYP2C19, CYP1A2, or CYP2C9. Concomitant use of drugs metabolized by CYP2D6 may require dosage adjustment. Clobazam may be administered safely as adjunctive therapy in patients with Lennox-Gastaut syndrome, without meaningful changes in clobazam pharmacokinetics that would require dosage adjustment.

Trapping of cis-2-butene-1,4-dial to measure furan metabolism in human liver microsomes by cytochrome P450 enzymes

Furan is a liver toxicant and carcinogen in rodents. It is classified as a possible human carcinogen, but the human health effects of furan exposure remain unknown. The oxidation of furan by cytochrome P450 (P450) enzymes is necessary for furan toxicity. The product of this reaction is the reactive α,β-unsaturated dialdehyde, cis-2-butene-1,4-dial (BDA). To determine whether human liver microsomes metabolize furan to BDA, a liquid chromatography/tandem mass spectrometry method was developed to detect and quantify BDA by trapping this reactive metabolite with N-acetyl-l-cysteine (NAC) and N-acetyl-l-lysine (NAL). Reaction of NAC and NAL with BDA generates N-acetyl-S-[1-(5-acetylamino-5-carboxypentyl)-1H-pyrrol-3-yl]-l-cysteine (NAC-BDA-NAL). Formation of NAC-BDA-NAL was quantified in 21 different human liver microsomal preparations. The levels of metabolism were comparable to that observed in F-344 rat and B6C3F1 mouse liver microsomes, two species known to be sensitive to furan-induced toxicity. Studies with recombinant human liver P450s indicated that CYP2E1 is the most active human liver furan oxidase. The activity of CYP2E1 as measured by p-nitrophenol hydroxylase activity was correlated to the extent of NAC-BDA-NAL formation in human liver microsomes. The formation of NAC-BDA-NAL was blocked by CYP2E1 inhibitors but not other P450 inhibitors. These results suggest that humans are capable of oxidizing furan to its toxic metabolite, BDA, at rates comparable to those of species sensitive to furan exposure. Therefore, humans may be susceptible to furan's toxic effects.

Cytochrome P450 variations in different ethnic populations

INTRODUCTION

Variability of drug response is an important consideration in clinical medicine. A major determinant of drug response variability is hepatic cytochrome P450 oxidase (CYP450)-mediated drug metabolism. Advances in genetics permits genotyping large numbers of patients to identify single nucleotide polymorphisms (SNPs) which may result in variant CYP450 enzyme expression and/or activity. New SNPs with functional impacts are constantly being identified which further explain variability in CYP450 phenotype.

AREAS COVERED

The racial/ethnic distribution of selected CYP450 (CYP1A2, P2C8/9/19, 2D6 and 3A4/5) SNPs are reviewed with an emphasis on the agreement between genotype and phenotype. The reader will gain insight into the SNP distribution by racial/ethnic group and the corresponding relationship between important, highly prevalent, SNPs and their impact on metabolic phenotype.

EXPERT OPINION

Racial/ethnic differences in metabolic phenotype can be explained by differences in SNP distribution. However, overlap in substrate specificity, linkage disequilibrium and previously unidentified SNPs have made phenotypic characterization difficult for CYP3A4/5 and 2C8/9. Studies utilizing newly identified, highly prevalent, racially stratified SNPs and their impact on CYP isoform-specific metabolism will provide new answers.

Chronic pain: reducing costs through early implementation of adherence testing and recognition of opioid misuse

OBJECTIVE

To review the literature on costs associated with chronic pain therapy and to identify key contributing factors. Also, to assess the potential cost-saving benefits of monitoring pain treatment adherence using urine drug tests (UDTs), emphasizing their use in opioid therapy.

RESULTS

Reduced productivity, compensation costs, and treatment of comorbid conditions related to chronic pain contribute to the substantial financial burden of chronic pain management in the United States. The growing use of opioids for chronic pain increases the risk for drug nonadherence and associated drug abuse, potential addiction, and aberrant drug-related behaviors (ADRBs). Treatment of drug abuse increases health care costs; opioid abusers are 25 times more likely to require hospitalization than nonopioid abusers. Early detection of patient nonadherence using UDTs could significantly reduce costs of chronic pain therapy by allowing the physician to identify and treat patients' ADRBs related to controlled substances and drug addiction and abuse problems. Adherence in chronic pain may be determined by point-of-care (POC) tests, and more sensitive laboratory urine tests employing gas chromatography/mass spectrometry with high-performance liquid chromatography tests (LUTs). Cost-benefit studies suggest that the cost of LUTs to optimize adherence may reduce costs associated with nonadherence, such as inpatient clinical care and patient self-release. Current estimates indicate that appropriate use of LUTs could produce decreases up to 14.8-fold in the cost of chronic pain therapy.

CONCLUSIONS

The cost benefits of UDTs can only be fully realized if physicians know how to define and detect various types of drug abuse, addiction, and diversion. Physicians should be educated on the proper implementation of POC tests and LUTs, and interpretation of adherence data. Early monitoring of drug adherence using POC tests and follow-up LUTs may provide substantial cost savings associated with health care issues incurred in nonadherent chronic pain patients, especially those taking opioid therapy.

Usefulness of postmortem biochemistry in forensic pathology: illustrative case reports

The aim of this work is to present some practical, postmortem biochemistry applications to illustrate the usefulness of this discipline and reassert the importance of carrying out biochemical investigations as an integral part of the autopsy process. Five case reports are presented pertaining to diabetic ketoacidosis in an adult who was not known to suffer from diabetes and in presence of multiple psychotropic substances; fatal flecainide intoxication in a poor metabolizer also presenting an impaired renal function; diabetic ketoacidosis showing severe postmortem changes; primary aldosteronism presented with intracranial hemorrhage and hypothermia showing severe postmortem changes. The cases herein presented can be considered representative examples of the importance of postmortem biochemistry investigations, which may provide significant information useful in determining the cause of death in routine forensic casework or contribute to understanding the pathophysiological mechanisms involved in the death process.

Meclizine metabolism and pharmacokinetics: formulation on its absorption

Meclizine, an antihistamine, has been widely used for prophylactic treatment and management of motion sickness. However, the onset of action of meclizine was about 1 hour for the treatment of motion sickness and vertigo. A new suspension formulation of meclizine (MOS) was developed with the intention to achieve a rapid effect. To investigate the pharmacokinetics of the new MOS formulation versus the marketed meclizine oral tablet (MOT), a phase 1 pharmacokinetic study was performed in 20 healthy volunteers. In addition, an in vitro metabolic study using human hepatic microsome and recombinant CYP enzyme was also performed to determine the metabolic pathway in the human body. The plasma concentration of MOS appeared more rapidly in comparison to the MOT. The geometric mean ratios (90% confidence interval) of AUC(0-24) and AUC(0-∞) indicated no significant difference in bioavailability between the 2 formulations. CYP2D6 was found to be the dominant enzyme for metabolism of meclizine, and its genetic polymorphism could contribute to the large interindividual variability. In view of the similar bioavailability with a much shorter peak time of the plasma meclizine concentration from the MOS formulation, this new formulation is expected to produce a much quicker onset of action when used for the management of motion sickness.

Pain, analgesia and genetics

OBJECTIVES

In the clinical setting, there is marked intersubject variability in the intensity of pain reported by patients with apparently similar pain states, as well as widely differing analgesic dosing requirements between individuals to produce satisfactory pain relief with tolerable side-effects. Genetic and environmental factors as well as their interaction are implicated, and these are discussed in this review.

KEY FINDINGS

Pioneering work undertaken in mice more than a decade ago, showed a strong genetic contribution to levels of nociception/hypersensitivity as well as levels of antinociception produced by commonly available analgesic agents. To date more than 300 candidate 'pain' genes have been identified as potentially contributing to heritable differences in pain sensitivity and analgesic responsiveness in animals and humans, with this information available in a publicly accessible database http://www.jbldesign.com/jmogil/enter.html. Since then, many genetic association studies have been conducted in humans to investigate the possibility that single nucleotide polymorphisms (SNPs) in an individual gene may explain drug inefficacy or excessive toxicity experienced by a small subset of the whole population who have the rare allele for a particular SNP.

SUMMARY

Despite the fact that SNPs in more than 20 genes that affect pain sensitivity or contribute to interindividual variability in responses to analgesic medications have been identified in the human genome, much of the data is conflicting. Apart from deficiencies in the design and conduct of human genetic association studies, recent research from other fields has implicated epigenetic mechanisms that facilitate dynamic gene-environment communication, as a possible explanation.

Toxicokinetics of kava

Kava is traditionally consumed by South Pacific islanders as a drink and became popular in Western society as a supplement for anxiety and insomnia. Kava extracts are generally well tolerated, but reports of hepatotoxicity necessitated an international reappraisal of its safety. Hepatotoxicity can occur as an acute, severe form or a chronic, mild form. Inflammation appears to be involved in both forms and may result from activation of liver macrophages (Kupffer cells), either directly or via kava metabolites. Pharmacogenomics may influence the severity of this inflammatory response.

Genetic polymorphism in metabolism and host defense enzymes: implications for human health risk assessment

Genetic polymorphisms in xenobiotic metabolizing enzymes can have profound influence on enzyme function, with implications for chemical clearance and internal dose. The effects of polymorphisms have been evaluated for certain therapeutic drugs but there has been relatively little investigation with environmental toxicants. Polymorphisms can also affect the function of host defense mechanisms and thus modify the pharmacodynamic response. This review and analysis explores the feasibility of using polymorphism data in human health risk assessment for four enzymes, two involved in conjugation (uridine diphosphoglucuronosyltransferases [UGTs], sulfotransferases [SULTs]), and two involved in detoxification (microsomal epoxide hydrolase [EPHX1], NADPH quinone oxidoreductase I [NQO1]). This set of evaluations complements our previous analyses with oxidative and conjugating enzymes. Of the numerous UGT and SULT enzymes, the greatest likelihood for polymorphism effect on conjugation function are for SULT1A1 (*2 polymorphism), UGT1A1 (*6, *7, *28 polymorphisms), UGT1A7 (*3 polymorphism), UGT2B15 (*2 polymorphism), and UGT2B17 (null polymorphism). The null polymorphism in NQO1 has the potential to impair host defense. These highlighted polymorphisms are of sufficient frequency to be prioritized for consideration in chemical risk assessments. In contrast, SNPs in EPHX1 are not sufficiently influential or defined for inclusion in risk models. The current analysis is an important first step in bringing the highlighted polymorphisms into a physiologically based pharmacokinetic (PBPK) modeling framework.

Utilizing toxicogenomic data to understand chemical mechanism of action in risk assessment

The predominant role of toxicogenomic data in risk assessment, thus far, has been one of augmentation of more traditional in vitro and in vivo toxicology data. This article focuses on the current available examples of instances where toxicogenomic data has been evaluated in human health risk assessment (e.g., acetochlor and arsenicals) which have been limited to the application of toxicogenomic data to inform mechanism of action. This article reviews the regulatory policy backdrop and highlights important efforts to ultimately achieve regulatory acceptance. A number of research efforts on specific chemicals that were designed for risk assessment purposes have employed mechanism or mode of action hypothesis testing and generating strategies. The strides made by large scale efforts to utilize toxicogenomic data in screening, testing, and risk assessment are also discussed. These efforts include both the refinement of methodologies for performing toxicogenomics studies and analysis of the resultant data sets. The current issues limiting the application of toxicogenomics to define mode or mechanism of action in risk assessment are discussed together with interrelated research needs. In summary, as chemical risk assessment moves away from a single mechanism of action approach toward a toxicity pathway-based paradigm, we envision that toxicogenomic data from multiple technologies (e.g., proteomics, metabolomics, transcriptomics, supportive RT-PCR studies) can be used in conjunction with one another to understand the complexities of multiple, and possibly interacting, pathways affected by chemicals which will impact human health risk assessment.

Age and sex dependent changes in liver gene expression during the life cycle of the rat

Background

Age- and sex-related susceptibility to adverse drug reactions and disease is a key concern in understanding drug safety and disease progression. We hypothesize that the underlying suite of hepatic genes expressed at various life cycle stages will impact susceptibility to adverse drug reactions. Understanding the basal liver gene expression patterns is a necessary first step in addressing this hypothesis and will inform our assessments of adverse drug reactions as the liver plays a central role in drug metabolism and biotransformation. Untreated male and female F344 rats were sacrificed at 2, 5, 6, 8, 15, 21, 52, 78, and 104 weeks of age. Liver tissues were collected for histology and gene expression analysis. Whole-genome rat microarrays were used to query global expression profiles.

Results

An initial list of differentially expressed genes was selected using criteria based upon p-value (p < 0.05) and fold-change (+/- 1.5). Three dimensional principal component analyses revealed differences between males and females beginning at 2 weeks with more divergent profiles beginning at 5 weeks. The greatest sex-differences were observed between 8 and 52 weeks before converging again at 104 weeks. K-means clustering identified groups of genes that displayed age-related patterns of expression. Various adult aging-related clusters represented gene pathways related to xenobiotic metabolism, DNA damage repair, and oxidative stress.

Conclusions

These results suggest an underlying role for genes in specific clusters in potentiating age- and sex-related differences in susceptibility to adverse health effects. Furthermore, such a comprehensive picture of life cycle changes in gene expression deepens our understanding and informs the utility of liver gene expression biomarkers.

Approaches to advancing quantitative human health risk assessment of environmental chemicals in the post-genomic era

The contribution of genomics and associated technologies to human health risk assessment for environmental chemicals has focused largely on elucidating mechanisms of toxicity, as discussed in other articles in this issue. However, there is interest in moving beyond hazard characterization to making more direct impacts on quantitative risk assessment (QRA)--i.e., the determination of toxicity values for setting exposure standards and cleanup values. We propose that the evolution of QRA of environmental chemicals in the post-genomic era will involve three, somewhat overlapping phases in which different types of approaches begin to mature. The initial focus (in Phase I) has been and continues to be on "augmentation" of weight of evidence--using genomic and related technologies qualitatively to increase the confidence in and scientific basis of the results of QRA. Efforts aimed towards "integration" of these data with traditional animal-based approaches, in particular quantitative predictors, or surrogates, for the in vivo toxicity data to which they have been anchored are just beginning to be explored now (in Phase II). In parallel, there is a recognized need for "expansion" of the use of established biomarkers of susceptibility or risk of human diseases and disorders for QRA, particularly for addressing the issues of cumulative assessment and population risk. Ultimately (in Phase III), substantial further advances could be realized by the development of novel molecular and pathway-based biomarkers and statistical and in silico models that build on anticipated progress in understanding the pathways of human diseases and disorders. Such efforts would facilitate a gradual "reorientation" of QRA towards approaches that more directly link environmental exposures to human outcomes.