Identification and functional characterization of eight CYP3A4 protein variants

CYP3A phenotypes and genotypes in North Indians

OBJECTIVES

To phenotype 200 healthy North Indians for cytochrome P450 3A (CYP3A) activity by measuring urinary ratio of 6beta-OH-cortisol/cortisol (6beta-OH-CS/CS) and to genotype the subjects demonstrating low and high CYP3A activity for the presence of CYP3A4*1B, *2, *4, *5, *6 and *10 alleles.

METHODS

Morning spot urine samples were collected from 200 healthy North Indians. CS and 6beta-OH-CS were extracted and quantified by HPLC. Genotyping was performed by polymerase chain reaction (PCR) followed by restriction fragment length polymorphism (RFLP).

RESULTS

Urinary 6beta-OH-CS/CS ratio demonstrated a mean of 52.0 +/- 46 (1.1-290). North Indians demonstrated unimodal distribution with respect to urinary 6beta-OH-CS/CS ratio. On the basis of phenotypes, the subjects were divided into three groups demonstrating low (n = 50), intermediate (n = 100) and high (n = 50) CYP3A activity. These groups demonstrated 6beta-OH-CS/CS ratio of 13.4 +/- 5.2 (1.1-21.0), 40 +/- 11.9 (21.2-63.2) and 114 +/- 51.0 (66-290), respectively. One hundred subjects, 50 in the low and 50 in the high activity group, were genotyped for CYP3A4*1B, *2, *4, *5, *6 and *10. Only 2 heterozygotes with genotype CYP3A4*1/*1B were found in the high CYP3A activity group. CYP3A4*2, *4, *5, *6 and *10 were not found in the subjects studied.

CONCLUSION

This is the first investigation establishing CYP3A phenotypes and demonstrating the absence of common CYP3A4 genotypes in North Indians.

Functional interaction of nitrogenous organic bases with cytochrome P450: A critical assessment and update of substrate features and predicted key active-site elements steering the access, binding, and orientation of amines

The widespread use of nitrogenous organic bases as environmental chemicals, food additives, and clinically important drugs necessitates precise knowledge about the molecular principles governing biotransformation of this category of substrates. In this regard, analysis of the topological background of complex formation between amines and P450s, acting as major catalysts in C- and N-oxidative attack, is of paramount importance. Thus, progress in collaborative investigations, combining physico-chemical techniques with chemical-modification as well as genetic engineering experiments, enables substantiation of hypothetical work resulting from the design of pharmacophores or homology modelling of P450s. Based on a general, CYP2D6-related construct, the majority of prospective amine-docking residues was found to cluster near the distal heme face in the six known SRSs, made up by the highly variant helices B', F and G as well as the N-terminal portion of helix C and certain beta-structures. Most of the contact sites examined show a frequency of conservation < 20%, hinting at the requirement of some degree of conformational versatility, while a limited number of amino acids exhibiting a higher level of conservation reside close to the heme core. Some key determinants may have a dual role in amine binding and/or maintenance of protein integrity. Importantly, a series of non-SRS elements are likely to be operative via long-range effects. While hydrophobic mechanisms appear to dominate orientation of the nitrogenous compounds toward the iron-oxene species, polar residues seem to foster binding events through H-bonding or salt-bridge formation. Careful uncovering of structure-function relationships in amine-enzyme association together with recently developed unsupervised machine learning approaches will be helpful in both tailoring of novel amine-type drugs and early elimination of potentially toxic or mutagenic candidates. Also, chimeragenesis might serve in the construction of more efficient P450s for activation of amine drugs and/or bioremediation.

Clinical implications ofCYP3Apolymorphisms

Due to their enormous substrate spectrum CYP3A4, -3A5 and -3A7 constitute the most important drug-metabolising enzyme subfamily in humans. CYP3As are expressed predominantly, but not exclusively, in the liver and intestine, where they participate in the metabolism of 45 - 60% of currently used drugs and many other compounds such as steroids and carcinogens. CYP3A expression and activity vary interindividually due to a combination of genetic and nongenetic factors such as hormone and health status, and the impact of environmental stimuli. Over the past several years, genetic determinants have been identified for much of the variable expression of CYP3A5 and -3A7, but not for CYP3A4. Using these markers, an effect of CYP3A5 expression status has been demonstrated beyond doubt for therapies with the immunosuppressive drug tacrolimus. Further associations are likely to emerge for drugs metabolised predominantly by CYP3A5 or -3A7, especially for individuals or tissues with concomitant low expression of CYP3A4. However, as exemplified by the controversial association between CYP3A4*1B and prostate cancer, the detection of clinical effects of CYP3A gene variants will be difficult. The most important underlying problems are the continuing absence of activity markers specific for CYP3A4 and the strong contribution of nongenetic factors to CYP3A variability.

Role of pharmacogenetics in irinotecan therapy

In the treatment of advanced colorectal cancer, irinotecan has become one of the most important drugs, despite its sometimes unpredictable adverse effects. To understand why some patients experience severe adverse effects (diarrhea and neutropenia), while others do not, the metabolic pathways of this drug have to be unraveled in detail. Individual variation in expression of several phase I and phase II metabolizing enzymes and ABC-transporters involved in irinotecan metabolism and excretion, at least partly explains the observed pharmacokinetic interpatient variability. Although the difference in expression-level of these proteins to a certain amount is explained by physiologic and environmental factors, the presence of specific genetic determinants also does influence their expression and function. In this review, the role of genetic polymorphisms in the main enzyme-systems (carboxylesterase, cytochrome P450 3A, and uridine diphosphate-glucuronosyltransferase) and ABC-transporters (ABCB1, ABCC2, and ABCG2) involved in irinotecan metabolism, are discussed. Since at this moment the field of pharmacogenetics and pharmacogenomics is rapidly expanding and simultaneously more rapid and cost-effective screening methods are emerging, a wealth of future data is expected to enrich our knowledge of the genetic basis of irinotecan metabolism. Eventually, this may help to truly individualize the dosing of this (and other) anti-cancer agent(s), using a personal genetic profile of the most relevant enzymes for every patient.

Effect of common CYP3A4 and CYP3A5 variants on the pharmacokinetics of the cytochrome P450 3A phenotyping probe midazolam in cancer patients

PURPOSE

To evaluate the effect of naturally occurring variants in genes encoding the cytochrome P450 (CYP) isoforms CYP3A4 and CYP3A5 in patients with cancer receiving midazolam as a phenotyping probe.

EXPERIMENTAL DESIGN

Five variants in CYP3A4 and CYP3A5 were evaluated in 58 patients (21 women and 37 men) receiving a short i.v. bolus of midazolam (dose, 0.0145 or 0.025 mg/kg). Midazolam concentrations in plasma were determined using liquid chromatography-mass spectrometry, and pharmacokinetic variables were calculated using noncompartmental analysis. Genomic DNA was characterized for the variants by PCR-RFLP, and all genotypes were confirmed by direct nucleotide sequencing.

RESULTS

The mean clearance of midazolam was 24.4 +/- 9.12 L/h, and phenotypic CYP3A activity varied about 4-fold in this population (range, 10.8-44.3 L/h). There were six carriers of the CYP3A4*1B allele (allele frequency, 0.061). No variant alleles for CYP3A4*17, CYP3A4*18A, or CYP3A5*6 were identified. Forty-eight of the 58 patients were homozygous variant for CYP3A5*3C, eight were heterozygous, and two were homozygous wild type (allele frequency, 0.897). No associations were noted between any of the studied genotypes and the phenotypic measures (P > or = 0.16). Likewise, a common variant in exon 26 in the gene encoding P-glycoprotein [i.e., ABCB1 (MDR1) 3435C>T] that was previously reported to be linked to CYP3A4 mRNA levels was unrelated to any of the studied phenotypic measures (P > or = 0.49).

CONCLUSIONS

The studied genetic variants in CYP3A4 and CYP3A5 are unlikely to have an important functional significance to phenotypic CYP3A activity in patients with cancer.

Overview of the pharmacogenetics of HIV therapy

The administration of standard doses of most antiretroviral drugs results in significant variations in plasma drug concentrations among different individuals, influencing antiviral activity as well as incidence of drug-related toxicities. The reasons for this large inter-individual variability in drug levels are multifactorial, and involve differences in metabolism related to gender, concomitant medications, drug compliance, underlying diseases and genetic factors. Pharmacogenetics is the discipline that analyses the genetic basis for the inter-individual variation in the body disposition of drugs. One of its main goals is to give grounds to individualized treatment. The majority of pharmacogenetic traits so far have involved drug metabolism. This is the case for the inherited variation in the pharmacokinetics and pharmacodynamics of drugs such as hydralazine or isoniazid, which is due to polymorphisms in the N-acetyltransferase-2 (NAT2) gene, which allows splitting the population into three categories: slow, intermediate, and fast metabolizers. Pharmacogenetic studies conducted so far with antiretroviral drugs have focussed on metabolizer enzymes at the liver and on transporter proteins on cell membranes. Herein, we review the most relevant metabolizer enzymes and protein transporters, along with the genetic polymorphisms, which seem to influence the pharmacokinetics of antiretroviral drugs, ultimately determining its efficacy and toxicity.

In vitro characterization of the human biotransformation and CYP reaction phenotype of ET-743 (Yondelis®, Trabectidin®), a novel marine anti-cancer drug

ET-743 is a potent marine anti-cancer drug and is currently being investigated in phase I and II clinical trials, e.g. in combination with other anti-cancer agents. To assess the biotransformation and CYP reaction phenotype and their potential implications for human pharmacology and toxicology, the in vitro metabolism of ET-743 was characterized using incubations with human liver preparations, cytochrome P450 (CYP) and uridine diphosphoglucuronosyl transferase (UGT) supersomes.CYP supersomes and liver microsomes showed that ET-743 was metabolized mainly by CYP3A4, but also by CYP2C9, 2C19, 2D6, and 2E1. ET-743 showed the highest affinity for CYP3A4 and the highest maximal metabolic rate for CYP2D6 among the CYPs shown to metabolize ET-743. In addition, the Km value of ET-743 in female microsomes was significantly lower compared to male microsomes, while the Vmax values did not differ. ET-743 glucuronidation, catalyzed by UGT2B15, was observed in microsomes and S9 fraction. In addition, conjugation by glutathione-S-transferase and no sulphation was observed for ET-743 in cytosol and S9 fraction. ET-743 was more extensively metabolized when CYP activity was combined with phase II enzymes UGT and glutathione-S-transferase (GST), indicating that CYP, UGT, and GST simultaneously metabolize ET-743 in the S9 fraction. These results provide evidence that CYP3A4 has a major role in the metabolism of ET-743 in vitro with additional involvement of CYP2C9, 2C19, 2D6, and 2E1. Furthermore, ET-743 is conjugated by UGT and GST. This information could be important for interpretation of the pharmacokinetic data of clinical trials and prediction of drug-drug interactions.

Is tailored therapy feasible in oncology?

Tailored therapy aims to cure a patient who suffers from a specific disease with an effective and safe drug, based on the complex interactions among patient's characteristics, disease physiopathology and drug metabolism. Genomic and proteomic technologies represent promising new useful tools to understand cancer biology and molecular basis of interindividual differences of anticancer drugs efficacy. Genomic profiling seems to be able to re-classifying cancer into new molecular and prognostic homogeneous subgroups. By individual polymorphisms it is possible to identify the patients at higher risk for severe toxicity from those that may gain benefit from a particular treatment. The clinical use of targeted therapy is hampered by several questions, including: optimal biological dose, availability of surrogate biomarkers predictive of activity, schedule of administration, tumor histotype and stage to treat and modalities of combination with chemo/radiotherapy. In addition, further efforts are needed to improve the reliability of genomic and proteomic technologies. These unsolved issues presently make tailored therapy an open challenge.

Phenotype–genotype variability in the human CYP3A locus as assessed by the probe drug quinine and analyses of variant CYP3A4 alleles

The human cytochrome P450 3A (CYP3A) enzymes, which metabolize 50% of currently used therapeutic drugs, exhibit great interindividual differences in activity that have a major impact on drug treatment outcome, but hitherto no genetic background importantly contributing to this variation has been identified. In this study we show that CYP3A4 mRNA and hnRNA contents with a few exceptions vary in parallel in human liver, suggesting that mechanisms affecting CYP3A4 transcription, such as promoter polymorphisms, are relevant for interindividual differences in CYP3A4 expression. Tanzanian (n=143) healthy volunteers were phenotyped using quinine as a CYP3A probe and the results were used for association studies with CYP3A4 genotypes. Carriers of CYP3A4*1B had a significantly lower activity than those with CYP3A4*1 whereas no differences were seen for five other SNPs investigated. Nuclear proteins from the B16A2 hepatoma cells were found to bind with less affinity to the CYP3A4*1B element around -392 bp as compared to CYP3A4*1. The data indicate the existence of a genetic CYP3A4 polymorphism with functional importance for interindividual differences in enzyme expression.

Sequence diversity and haplotype structure at the human CYP3A cluster

The four members of the human CYP3A subfamily play important roles in the clearance of xenobiotics, hormones, and environmental compounds. Many SNPs at the CYP3A locus have been characterized, with several showing large allele frequency differences across populations. In addition to the effects of CYP3A SNPs on drug metabolism, recent studies have highlighted the potential for CYP3A variation in susceptibility to several common phenotypes, including hypertension and cancer. We previously showed that the CYP3A4 and CYP3A5 genes have a strong haplotype structure at varying frequencies across ethnic groups. Here, we extend our re-sequencing survey to the remaining CYP3A genes in the same cluster, CYP3A7 and CYP3A43. Our study identified a large number of SNPs in coding and conserved noncoding sequences, several of which are common. The combined data set allows us to investigate patterns of sequence variation and linkage disequilibrium at the entire CYP3A locus for use in future association studies.

Association of CYP2C8, CYP3A4, CYP3A5, and ABCB1 polymorphisms with the pharmacokinetics of paclitaxel

PURPOSE

To retrospectively evaluate the effects of six known allelic variants in the CYP2C8, CYP3A4, CYP3A5, and ABCB1 genes on the pharmacokinetics of the anticancer agent paclitaxel (Taxol).

EXPERIMENTAL DESIGN

A cohort of 97 Caucasian patients with cancer (median age, 57 years) received paclitaxel as an i.v. infusion (dose range, 80-225 mg/m(2)). Genomic DNA was analyzed using PCR RFLP or using Pyrosequencing. Pharmacokinetic variables for unbound paclitaxel were estimated using nonlinear mixed effect modeling. The effects of genotypes on typical value of clearance were evaluated with the likelihood ratio test within NONMEM. In addition, relations between genotype and individual pharmacokinetic variable estimates were evaluated with one-way ANOVA.

RESULTS

The allele frequencies for the CYP2C8*2, CYP2C8*3, CYP2C8*4, CYP3A4*3, CYP3A5*3C, and ABCB1 3435C>T variants were 0.7%, 9.2%, 2.1%, 0.5%, 93.2%, and 47.1%, respectively, and all were in Hardy-Weinberg equilibrium. The population typical value of clearance of unbound paclitaxel was 301 L/h (individual clearance range, 83.7-1055 L/h). The CYP2C8 or CYP3A4/5 genotypes were not statistically significantly associated with unbound clearance of paclitaxel. Likewise, no statistically significant association was observed between the ABCB1 3435C>T variant and any of the studied pharmacokinetic variables.

CONCLUSIONS

This study indicates that the presently evaluated variant alleles in the CYP2C8, CYP3A4, CYP3A5, and ABCB1 genes do not explain the substantial interindividual variability in paclitaxel pharmacokinetics.

Screening CYP3A single nucleotide polymorphisms in a Han Chinese population with a genotyping chip

Human cytochrome P450 (CYP)3A is a major P450 enzyme found in the liver and gastrointestinal tract. It plays an important role in the metabolism of a wide variety of drugs, some endogenous steroids and harmful environmental contaminants. It has been shown that CYP3A alleles encoding enzymes with little or no activity are largely created by single nucleotide polymorphisms (SNPs) in the sequences of these genes. The most prevalent of these SNPs are often of low allelic frequency, and many are specific to certain ethnic groups. Therefore, an accurate determination of their frequency in any given ethnic population requires investigations involving large sample sizes. A genotyping chip with enzyme-colorimetric detection was developed and used for simultaneous analysis of 22 known CYP3A SNPs in 451 Han Chinese subjects. Following multiplex polymerase chain reaction and allele-specific primer extension labeling, an enzymatic colorimetry detection system was employed to visualize genotype patterns on a nylon membrane. With this robust system, accurate discrimination ratios were obtained, and approximately 9922 genotypes were determined. We found that the major CYP3A SNPs in the Chinese subjects were CYP3A4*4 (allele frequency 2.4%), CYP3A4*5 (0.7%), CYP3A4*18A (2.7%) and CYP3A5*3C (70.2%). Most of the major CYP3A4 SNPs found in other ethnicities were not found in this study. Using these SNPs, 11 haplotypes were identified. Comparison between present and previous studies shows that CYP3A4*4 and CYP3A4*5 alleles were Chinese-specific. The genotyping chip developed in this study is an efficient, economic and accurate system for screening multiple SNPs in a large population. Application of such technology is expected to be less labor intensive and easier to adapt to specific searches when compared with other methodologies.

Sarcomas and pharmacogenetics

Sarcomas are a heterogeneous group of tumors, requiring different chemotherapeutic approaches. Recently, several regimens for metastatic tumors were evaluated with respect to the different responses to conventional chemotherapy of the various histologic subtypes of sarcomas. The impact of pharmacogenetics in the progress of chemotherapy appears to be crucial in defining the clinical response to many drugs, such as anthracycline or alkylating agents, that are widely used in treatment regimens for soft tissue sarcomas (STS) or sarcomas of the bone. Polymorphisms of metabolizing enzymes (e.g., cytochrome P450 and glutathione-S-transferase), transporter proteins (reduced folate carrier and P-glycoprotein) or target proteins (thymidylate synthase, methylenetetrahydrofolate reductase, dihydrofolate reductase, and c-KIT) may be responsible for an altered clinical outcome, in terms of both response and toxicity. The administration of new chemotherapeutic agents, such as imatinib for gastrointestinal tumors (GIST), requires the study of genetic polymorphisms possibly affecting the integrity of the target (c-KIT), which may provide valid information regarding possible developments of therapy. For STS and sarcoma of the bone, the genetic markers, which could be unambiguously predictive of the phenotypic profile of patients, are as yet undetermined.

Cytochrome P450 gene-based drug prescribing and factors impacting translation into routine clinical practice

Pharmacogenetics represents a rapidly advancing, competitive field of investigation. Due to the potential for clinically recognizable interactions between a set of old polymorphic genes and a relatively new environmental insult (drugs), many human geneticists believe that variability in the drug-metabolizing enzyme systems will soon translate into clinical practice across entire populations. Despite this, the field has not yet received widespread clinical acceptance. This article will review the common cytochrome P450 gene polymorphisms and discuss the factors that may facilitate (or attenuate) their translation into clinical practice.

EFFECT OFCYP2D6*10ALLELE ON THE PHARMACOKINETICS OF LORATADINE IN CHINESE SUBJECTS

Loratadine is known to be a substrate for both CYP3A4 and CYP2D6 based on a previous in vitro study. In view of the large interindividual variability in loratadine pharmacokinetics and the greater genetically determined variability of CYP2D6 activity than of CYP3A4 in vivo, we hypothesized that CYP2D6 polymorphisms may contribute to the pharmacokinetic variability of loratadine. The purpose of this study was to evaluate the effect of CYP2D6 genotype (specifically the CYP2D6*10 allele) on the pharmacokinetics of loratadine in Chinese subjects. Three groups of healthy male Chinese subjects were enrolled: group I, homozygous CYP2D6*1 (*1/*1, n=4); group II, heterozygous CYP2D6*10 (*1/*10 or *2/*10, n=6); and group III, homozygous CYP2D6*10 (*10/*10, n=7) carriers. Each subject received a single oral dose of 20 mg of loratadine under fasting conditions. Multiple blood samples were collected over 48 h, and the plasma concentrations of loratadine and its metabolite desloratadine were determined by high-performance liquid chromatography. In comparing homozygous CYP2D6*10 (group III) to heterozygous CYP2D6*10 (group II) to homozygous CYP2D6*1 (group I) subjects, loratadine oral clearance values were 7.17+/- 2.54 versus 11.06+/-1.70 versus 14.59+/-2.43 l/h/kg, respectively [one-way analysis of variance (ANOVA), p<0.01], and the corresponding metabolic ratios [area under the plasma concentration-time curve (AUC)(desloratadine)/AUC(loratadine)] were 1.55+/-0.73 versus 2.47+/- 0.46 versus 3.32+/- 0.49, respectively (one-way ANOVA, p<0.05), indicating a gene-dose effect. The results demonstrated that CYP2D6 polymorphism prevalent in the Chinese population significantly affected loratadine pharmacokinetics.

CYP3A4 and CYP3A5 genotyping by Pyrosequencing

Background

Human cytochrome P450 3A enzymes, particularly CYP3A4 and CYP3A5, play an important role in drug metabolism. CYP3A expression exhibits substantial interindividual variation, much of which may result from genetic variation. This study describes Pyrosequencing assays for key SNPs in CYP3A4 (CYP3A4*1B, CYP3A4*2, and CYP3A4*3) and CYP3A5 (CYP3A5*3C and CYP3A5*6).

Methods

Genotyping of 95 healthy European and 95 healthy African volunteers was performed using Pyrosequencing. Linkage disequilibrium, haplotype inference, Hardy-Weinberg equilibrium, and tag SNPs were also determined for these samples.

Results

CYP3A4*1B allele frequencies were 4% in Europeans and 82% in Africans. The CYP3A4*2 allele was found in neither population sample. CYP3A4*3 had an allele frequency of 2% in Europeans and 0% in Africans. The frequency of CYP3A5*3C was 94% in Europeans and 12% in Africans. No CYP3A5*6 variants were found in the European samples, but this allele had a frequency of 16% in the African samples. Allele frequencies and haplotypes show interethnic variation, highlighting the need to analyze clinically relevant SNPs and haplotypes in a variety of ethnic groups.

Conclusion

Pyrosequencing is a versatile technique that could improve the efficiency of SNP analysis for pharmacogenomic research with the ultimate goal of pre-screening patients for individual therapy selection.

Screening of 12 SNPs of CYP3A4 in a Chinese population using oligonucleotide microarray

Human cytochrome P450 3A4 (CYP34A) plays an important role in the metabolism of many endo- and xenomaterials. It also exhibits a substantial interindividual variation in enzymatic activity. It has been shown that the mutant alleles of CYP3A4 encoding inactive/decreased enzymes are largely caused by single nucleotide polymorphisms (SNPs) in the gene sequence. In the present study, with the goal of detecting the known SNPs of CYP3A4, an oligonucleotide microarray was created. A genotyping standard for this microarray was also established using constructed plasmids as standard templates. The 12 SNPs of CYP3A4 in 387 Chinese DNA samples were screened using this oligonucleotide microarray. Three heterozygous subjects of CYP3A4*/*4, 5 heterozygous subjects of CYP3A4*1/*5, 4 heterozygous subjects of CPY3A4*1/6, and 6 heterozygous subjects of CYP3A4*1/*18 were found. The genotyping results of the 18 heterozygous subjects and 12 wild-type subjects were validated by direct sequencing.

Genetic predictors of the maximum doses patients receive during clinical use of the anti-epileptic drugs carbamazepine and phenytoin

Phenytoin and carbamazepine are effective and inexpensive anti-epileptic drugs (AEDs). As with many AEDs, a broad range of doses is used, with the final "maintenance" dose normally determined by trial and error. Although many genes could influence response to these medicines, there are obvious candidates. Both drugs target the alpha-subunit of the sodium channel, encoded by the SCN family of genes. Phenytoin is principally metabolized by CYP2C9, and both are probable substrates of the drug transporter P-glycoprotein. We therefore assessed whether variation in these genes associates with the clinical use of carbamazepine and phenytoin in cohorts of 425 and 281 patients, respectively. We report that a known functional polymorphism in CYP2C9 is highly associated with the maximum dose of phenytoin (P = 0.0066). We also show that an intronic polymorphism in the SCN1A gene shows significant association with maximum doses in regular usage of both carbamazepine and phenytoin (P = 0.0051 and P = 0.014, respectively). This polymorphism disrupts the consensus sequence of the 5' splice donor site of a highly conserved alternative exon (5N), and it significantly affects the proportions of the alternative transcripts in individuals with a history of epilepsy. These results provide evidence of a drug target polymorphism associated with the clinical use of AEDs and set the stage for a prospective evaluation of how pharmacogenetic diagnostics can be used to improve dosing decisions in the use of phenytoin and carbamazepine. Although the case made here is compelling, our results cannot be considered definitive or ready for clinical application until they are confirmed by independent replication.

Pharmacogenetics and Human Molecular Genetics of Opiate and Cocaine Addictions and Their Treatments

Opiate and cocaine addictions are major social and medical problems that impose a significant burden on society. Despite the size and scope of these problems, there are few effective treatments for these addictions. Methadone maintenance is an effective and most widely used treatment for opiate addiction, allowing normalization of many physiological abnormalities caused by chronic use of short-acting opiates. There are no pharmacological treatments for cocaine addiction. Epidemiological, linkage, and association studies have demonstrated a significant contribution of genetic factors to the addictive diseases. This article reviews the molecular genetics and pharmacogenetics of opiate and cocaine addictions, focusing primarily on genes of the opioid and monoaminergic systems that have been associated with or have evidence for linkage to opiate or cocaine addiction. This evidence has been marshalled either through identification of variant alleles that lead to functional alterations of gene products, altered gene expression, or findings of linkage or association studies. Studies of polymorphisms in the mu opioid receptor gene, which encodes the receptor target of some endogenous opioids, heroin, morphine, and synthetic opioids, have contributed substantially to knowledge of genetic influences on opiate and cocaine addiction. Other genes of the endogenous opioid and monoaminergic systems, particularly genes encoding dopamine beta-hydroxylase, and the dopamine, serotonin, and norepinephrine transporters have also been implicated. Variants in genes encoding proteins involved in metabolism or biotransformation of drugs of abuse and also of treatment agents are reviewed.

CYP3A4 is a vitamin D-24- and 25-hydroxylase: analysis of structure function by site-directed mutagenesis

Studies were performed to identify the microsomal enzyme that 24-hydroxylates vitamin D, whether 25-hydroxylation occurs, and structure function of the enzyme. Sixteen hepatic recombinant microsomal cytochrome P450 enzymes expressed in baculovirus-infected insect cells were screened for 24-hydroxylase activity. CYP3A4, a vitamin D-25-hydroxylase, and CYP1A1 had the highest 24-hydroxylase activity with 1 alpha-hydroxyvitamin D(2) (1 alpha OHD(2)) as substrate. The ratio of rates of 24-hydroxylation of 1 alpha-hydroxyvitamin D(3) (1 alpha OHD(3)), 1 alpha OHD(2), and vitamin D(2) by CYP3A4 was 3.6/2.8/1.0. Structures of 24-hydroxyvitamin D(2), 1,24(S)-dihydroxyvitamin D(2), and 1,24-dihydroxyvitamin D(3) were confirmed by HPLC and gas chromatography retention time and mass spectroscopy. In characterized human liver microsomes, 24-hydroxylation of 1 alpha OHD(2) by CYP3A4 correlated significantly with 6 beta-hydroxylation of testosterone, a marker of CYP3A4 activity. 24-Hydroxylase activity in recombinant CYP3A4 and pooled human liver microsomes showed dose-dependent inhibition by ketoconazole, troleandomycin, alpha-naphthoflavone, and isoniazid, known inhibitors of CYP3A4. Rates of 24- and 25-hydroxylation of 1 alpha OHD(2) and 1 alpha OHD(3) were determined in recombinant wild-type CYP3A4 and site-directed mutants and naturally occurring variants expressed in Escherichia coli. Substitution of residues showed the most prominent alterations of function at residues 119, 120, 301, 305, and 479. Thus, CYP3A4 is both a 24- and 25-hydroxylase for vitamin D(2), 1 alpha OHD(2), and 1 alpha OHD(3).

Human variability in xenobiotic metabolism and pathway-related uncertainty factors for chemical risk assessment: a review

This review provides an account of recent developments arising from a database that defined human variability in phase I metabolism (CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, hydrolysis, alcohol dehydrogenase), phase II metabolism (N-acetyltransferases, glucuronidation, glycine conjugation, sulphation) and renal excretion. This database was used to derive pathway-related uncertainty factors for chemical risk assessment that allow for human variability in toxicokinetics. Probe substrates for each pathway of elimination were selected on the basis that oral absorption was >95% and that the metabolic route was the primary route of elimination of the compound (60-100% of a dose). Intravenous data were used for compounds for which absorption was variable. Human variability in kinetics was quantified for each compound from published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and other subgroups of the population using parameters relating to chronic exposure (metabolic and total clearances, area under the plasma concentration-time curve (AUC)) and acute exposure (Cmax) (data not presented here). The pathway-related uncertainty factors were calculated to cover 95%, 97.5% and 99% of the population of healthy adults and of each subgroup. Pathway-related uncertainty factors allow metabolism data to be incorporated into the derivation of health-based guidance values. They constitute an intermediate approach between the general kinetic default factors (3.16) and a chemical-specific adjustment factor. Applications of pathway-related uncertainty factors for chemical risk assessment and future refinements of the approach are discussed. A knowledge-based framework to predict human variability in kinetics for xenobiotics showing a threshold dose below which toxic effects are not observed, is proposed to move away from default assumptions.

Ile118Val genetic polymorphism of CYP3A4 and its effects on lipid-lowering efficacy of simvastatin in Chinese hyperlipidemic patients

OBJECTIVES

To determine the frequencies of CYP3A4 alleles (CYP3A4*4,*5 and *6) in Chinese hyperlipidemic patients and to observe the impact of CYP3A4*4 (Ile118Val) genetic polymorphism on the lipid-lowering effects of simvastatin and on the activity of CYP3A4.

METHODS

From hospitalized and non-hospitalized patients, 211 unrelated hyperlipidemic patients were recruited for genotyping. CYP3A4 genotypes were determined by means of polymerase chain reaction and restriction fragment length polymorphism analysis. Of the non-hospitalized hyperlipidemic patients, 8 with CYP3A4*1/*1 and 8 with CYP3A4*1/*4 genotypes were selected to be treated with 20 mg simvastatin daily for 4 weeks. Serum triglycerides (TG), cholesterol (CHO) and low-density lipoprotein (LDL) levels were determined using an automated analyzer (Hitachi 747, Boehringer Mannheim, Mannheim, Germany). CYP3A4 activity was determined by the ratio of 6-hydroxycortisol to free cortisol (6-OHC/FC) in the morning spot urine with a high-throughput liquid chromatography-tandem mass spectrometry method.

RESULTS

Of 211 subjects, 14 (allele frequency 3.32%) were heterozygous for CYP3A4*4 (Ile118Val). Nevertheless, no subjects with a CYP3A4*5 or CYP3A4*6 allele or homozygous for CYP3A4*4 were identified. The ratio of 6beta-OHC/FC was 9.9 +/- 13.7 and 56.6 +/- 35.7 in subjects with the Ile118Val variant (n = 8) and in CYP3A4 wild-type subjects (n = 8), respectively (P = 0.0039). After oral intake of simvastatin 20 mg daily for 4 weeks, the change of serum lipids in CYP3A4*1/*1 and CYP3A4*1/*4 groups showed a significant difference, with a mean decrease in triglycerides and total cholesterol of 38.1 +/- 7.6% versus 25.1 +/- 8.3% (P = 0.034) and of 35.8 +/- 9.6% versus 22.0 +/-20.4% (P = 0.0015) (means +/- SD), respectively. We found no statistically significant difference in the reductions of LDL between subjects carrying the *1 and *4 genotypes (29.0 +/- 7.4% versus 36.8 +/- 8.8%, P = 0.0721).

CONCLUSIONS

The allele frequency of CYP3A4*4 was 3.32% among the hyperlipidemic patients from the Chinese mainland. CYP3A4*4 was an allelic variant related to a functional decrease of CYP3A4 activity, and *4 expression seemed to increase the lipid-lowering effects of simvastatin.

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 inter-individual differences in drug metabolism and pharmacokinetics on safety evaluation

Safety evaluation aims to assess the dose-response relationship to determine a dose/level of exposure for food contaminants below which no deleterious effect is measurable that is 'without appreciable health risk' when consumed daily over a lifetime. These safe levels, such as the acceptable daily intake (ADI) have been derived from animal studies using surrogates for the threshold such as the no-observed-adverse-effect-level (NOAEL). The extrapolation from the NOAEL to the human safe intake uses a 100-fold uncertainty factor, defined as the product of two 10-fold factors allowing for human variability and interspecies differences. The 10-fold factor for human variability has been further subdivided into two factors of 10(0.5) (3.16) to cover toxicokinetics and toxicodynamics and this subdivsion allows for the replacement of an uncertainty factor with a chemical-specific adjustment factor (CSAF) when compound-specific data are available. Recently, an analysis of human variability in pharmacokinetics for phase I metabolism (CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, hydrolysis, alcohol dehydrogenase), phase II metabolism (N-acetyltransferase, glucuronidation, glycine conjugation, sulphation) and renal excretion was used to derive pathway-related uncertainty factors in subgroups of the human population (healthy adults, effects of ethnicity and age). Overall, the pathway-related uncertainty factors (99th centile) were above the toxicokinetic uncertainty factor for healthy adults exposed to xenobiotics handled by polymorphic metabolic pathways (and assuming the parent compound was the proximate toxicant) such as CYP2D6 poor metabolizers (26), CYP2C19 poor metabolizers (52) and NAT-2 slow acetylators (5.2). Neonates were the most susceptible subgroup of the population for pathways with available data [CYP1A2 and glucuronidation (12), CYP3A4 (14), glycine conjugation (28)]. Data for polymorphic pathways were not available in neonates but uncertainty factors of up to 45 and 9 would allow for the variability observed in children for CYP2D6 and CYP2C19 metabolism, respectively. This review presents an overview on the history of uncertainty factors, the main conclusions drawn from the analysis of inter-individual differences in metabolism and pharmacokinetics, the development of pathway-related uncertainty factors and their use in chemical risk assessment.

Allelic expression imbalance of the human CYP3A4 gene and individual phenotypic status

The human cytochrome P450 3A4 (CYP3A4) plays a dominant role in the metabolism of numerous clinically useful drugs. Alterations in the activity or expression of this enzyme may account for a major part of the variation in drug responsiveness and toxicity. However, it is generally accepted that most of the known single nucleotide polymorphisms in the coding and 5'-flanking regions are not the main determinants for the large inter-individual variability of CYP3A4 expression and activity. We show that the allelic variation is critically involved in determining the individual total hepatic CYP3A4 mRNA level and metabolic capability. There exists a definite correlation between the total CYP3A4 mRNA level and allelic expression ratio, the relative transcript level ratio derived from the two alleles. Individuals with a low expression ratio, exhibiting a large difference of transcript level between the two alleles, revealed extremely low levels of total hepatic CYP3A4 mRNA, and thus low metabolic capability as assessed by testosterone 6beta-hydroxylation. These results present a new insight into the individualized CYP3A4-dependent pharmacotherapy and the importance of expression imbalance to human phenotypic diversity.

Homozygous CYP2B6 *6 (Q172H and K262R) correlates with high plasma efavirenz concentrations in HIV-1 patients treated with standard efavirenz-containing regimens

Efavirenz (EFV) is metabolized by cytochrome P450 2B6 (CYP2B6) in the liver. We analyzed the genotypes of CYP2B6 and their contribution to plasma EFV concentrations in 35 EFV-treated patients in International Medical Center of Japan. The mean plasma EFV concentration of patients with CYP2B6 *6/*6 (Q172H and K262R) (25.4+/-7.5 microM, +/-SD, n = 2) was significantly higher than that of patients with genotypes *6 heterozygote (9.9+/-3.3 microM, n = 10) or without alleles *6 (8.0+/-2.6 microM, n = 23) (p < 0.0001). To confirm our result, we further analyzed nine patients (three with high EFV concentrations and arbitrarily selected six with normal EFV concentrations) treated in Osaka National Hospital, and it resulted that the only three patients with the high concentrations were the *6/*6 holder. EFV dose could be decreased in those patients harboring the genotype to reduce toxicity with compromising potency, representing the first step of the Tailor-Made therapy of HIV-1 infection.

CYP3A4 and MDR1 alleles in a Portuguese population

Polymorphisms in cytochrome P450 CYP3A4 and multidrug resistance (MDR) 1 genes coding for the important drug-metabolising CYP3A4 and the ATP-binding cassette (ABC) transporter P-glycoprotein (Pgp) are poorly documented in the Portuguese population. In this study we have determined the frequencies of CYP3A4 and MDR1 alleles in Portuguese Caucasians. Both genes were simultaneously analysed as these genes are known to be frequently co-induced and their products to show a pronounced overlap of substrates. CYP3A4 A-392G (CYP3A4*1B), T673C (CYP3A4*2) and MDR1 T-129C, G2677T and C3435T single nucleotide polymorphisms (SNPs) were analysed in 100 individuals from the southern region of the country. We observed a frequency of 4.0% for CYP3A4*1B, not significantly different from that reported on other Caucasian European populations. CYP3A4*2 was found at an allele frequency of 4.5%, constituting the first report of the presence of this allele outside the Finnish population. Significant differences were found concerning the MDR1 C3435T SNP frequency (64.5%) compared with other European populations, while no differences were found concerning G2677T (47.5%) or T-129C (5%) SNPs. Linkage between the C3435T and G2677T SNPs was observed, although not as evidently as documented in other Caucasian populations. No preferential associations were detected between CYP3A4 and MDR1 alleles.

The influence of pharmacogenetics on the time to achieve target tacrolimus concentrations after kidney transplantation

Previously, we reported that, at 3 months after renal transplantation, individuals with CYP3AP1 genotype CYP3AP1*1 (linked to CYP3A5*1 and strongly associated with expression of CYP3A5) required twofold higher doses of tacrolimus to achieve target blood concentrations than individuals with the genotype CYP3AP1*3/*3 (CYP3A5 nonexpressors). This study assesses the relationship between concentration-controlled dosing during the early period after transplantation, the time to achieve target concentrations and genotype in 178 renal transplant recipients (CYP3AP1*1/*3 or *1/*1: n = 53, CYP3AP1*3/*3: n = 125). Patients with CYP3AP1*1/*3 or *1/*1 had lower mean tacrolimus concentrations during the first week (Median 13.5 vs. 18.5 microg/L, p < 0.0001) with significant delay in achieving target concentrations (15-20 microg/L during week 1, then 10-15 microg/L). More CYP3AP1*3/*3 patients had tacrolimus concentrations above target during the first week (73.6% vs. 35.8%, p = 0.003). There was no difference in the rate of biopsy-confirmed acute rejection, but rejection occurred earlier in the CYP3AP1*1/*3 or *1/*1 group (median 7 d vs. 13 d, p = 0.005). In conclusion, an initial dosing regimen for tacrolimus based on knowledge of the CYP3AP1 genotype and subsequently guided by concentration measurements has the potential to increase the proportion of patients achieving target blood concentrations early after transplantation.

VALIDATED ASSAYS FOR HUMAN CYTOCHROME P450 ACTIVITIES

The measurement of the effect of new chemical entities on human cytochrome P450 marker activities using in vitro experimentation represents an important experimental approach in drug development. In vitro drug interaction data can be used in guiding the design of clinical drug interaction studies, or, when no effect is observed in vitro, the data can be used in place of an in vivo study to claim that no interaction will occur in vivo. To make such a claim, it must be assured that the in vitro experiments are performed with absolute confidence in the methods used and data obtained. To meet this need, 12 semiautomated assays for human P450 marker substrate activities have been developed and validated using approaches described in the GLP (good laboratory practices) as per the code of U.S. Federal Regulations. The assays that were validated are: phenacetin O-deethylase (CYP1A2), coumarin 7-hydroxylase (CYP2A6), bupropion hydroxylase (CYP2B6), amodiaquine N-deethylase (CYP2C8), diclofenac 4'-hydroxylase and tolbutamide methylhydroxylase (CYP2C9), (S)-mephenytoin 4'-hydroxylase (CYP2C19), dextromethorphan O-demethylase (CYP2D6), chlorzoxazone 6-hydroxylase (CYP2E1), felodipine dehydrogenase, testosterone 6 beta-hydroxylase, and midazolam 1'-hydroxylase (CYP3A4 and CYP3A5). High-pressure liquid chromatography-tandem mass spectrometry, using stable isotope-labeled internal standards, has been applied as the analytical method. This analytical approach, through its high sensitivity and selectivity, has permitted the use of very low incubation concentrations of microsomal protein (0.01-0.2 mg/ml). Analytical assay accuracy and precision values were excellent. Enzyme kinetic and inhibition parameters obtained using these methods demonstrated high precision and were within the range of values previously reported in the scientific literature. These methods should prove useful in the routine assessments of the potential for new drug candidates to elicit pharmacokinetic drug interactions via inhibition of cytochrome P450 activities.

Therapeutic Monitoring of Antidepressants in the Era of Pharmacogenetics Studies

As for other drugs, there is a large interindividual variability of the plasma concentrations of antidepressants for a given dose. Within the last 2 decades, a very large number of pharmacogenetic studies have made it possible to understand the importance of genetic factors on the disposition of drugs in the organism, many of them at the levels of drug metabolism. Polymorphism of CYP2D6 and of other drug-metabolizing enzymes may thus lead to very large differences in drug exposure between patients and possibly also to toxicity or ineffective drug concentrations in some subjects. In consequence, dose recommendations of antidepressants based on genotypes, justified by the principle of administering bioequivalent individualized drug doses, are now proposed. However, blood (and thus possibly brain) concentrations also depend on other factors than the genetic makeup of the patients. Therapeutic drug monitoring of antidepressants allows us to take into account the influence of factors such as comedications, diet, smoking habit, impaired organ function, and compliance. Therapeutic drug monitoring and genotyping are thus complementary, and their combined use contributes to improve pharmacotherapy with antidepressants and other drugs.

Genetics of the Variable Expression of CYP3A in Humans

CYP3A isozymes participate in the metabolism of 45-60% of currently used drugs and of a variety of other compounds such as steroid hormones, toxins, and carcinogens. The CYP3A expression status is a major determinant of drug efficacy and safety, and it may also affect an individual's predisposition to certain cancers. The inter- and intraindividual expression of CYP3A is variable because of a complex interplay between genetic and environmental factors. Markers predictive of the individual CYP3A activity could improve therapies with CYP3A substrates by personalised dose adjustments, but their development has been slower than for other drug-metabolizing enzymes. Here we summarize the recent progress in genomics and regulation of CYP3A. The recently described markers of the CYP3A5 and CYP3A7 polymorphisms should facilitate the development of isozyme-specific activity markers for the individual CYP3A isozymes, including CYP3A4.

Haemodynamic crisis and reversible multiorgan failure caused by HIV post-exposure prophylaxis after needle-stick injury in a health care worker

We report the case of a 59-year-old nurse from our HIV ward who developed a severe haemodynamic crisis with concomitant acute multiorgan failure after initiation of a post-exposure prophylaxis (PEP) with zidovudine/lamivudine (CombivirTM) and lopinavir/ritonavir (KaletraTM) after a needle-stick injury with an HIV-contaminated needle. Although serious and life-threatening adverse effects of post-exposure prophylaxis have been documented in several cases, this is the first report of a severe acute cardiovascular incident following PEP initiation.

Human drug metabolising cytochrome P450 enzymes: properties and polymorphisms

The cytochrome P450s are responsible for about 75% of phase I dependent drug metabolism and for the metabolism of a huge amount of dietary constituents and endogenous chemicals. The human has 59 active genes, and 6 of those encode important drug metabolising enzymes. About 40% of cytochrome P450 dependent drug metabolism is catalysed by polymorphic enzymes and such drug P450 interactions are frequently seen in adverse drug reaction reports. In this contribution an update of human cytochrome P450 enzymology and pharmacogenetics is given with particular emphasis on CYP1B1, CYP2B6, CYP2E1 and CYP3As.

CYP3A5*3 and *6 single nucleotide polymorphisms in three distinct Asian populations

OBJECTIVE

To determine the frequencies of two functional single nucleotide polymorphisms, CYP3A5*3 and CYP3A5*6, in the CYP3A5 gene in three distinct Asian ethnic groups, namely, the Chinese, Malays and Indians.

METHODS

Single nucleotide polymorphism analyses of CYP3A5*1, *3 and *6 were performed in 296 healthy subjects (108 Chinese, 98 Malays and 90 Indians) using the polymerase chain reaction-restriction fragment length polymorphism method.

RESULTS

The *1 allele frequency was 25% in Chinese compared with 40% in Malays and Indians ( P=0.001). The *3 allele frequency was also higher in the Chinese population, being 76% versus 60% in the Malays and Indians ( P=0.001). The Malays and Indians also had allele frequencies significantly different from Caucasian, Japanese and African-American populations (each P<or=0.001) previously published in the literature. The *6 allele was not detected in any the three Asian ethnic groups analysed.

CONCLUSION

These results seem to suggest that genetic polymorphisms in CYP3A5 in Asians, in particular Malays and Indians but also Chinese although to a lesser extent, may be an important genetic contributor to interindividual as well as interethnic differences in clearance of CYP3A substrates.

Quinine 3-hydroxylation as a biomarker reaction for the activity of CYP3A4 in man

OBJECTIVE

To investigate the usefulness of the 3-hydroxylation of quinine as a biomarker reaction for the activity of CYP3A4 in man and to study the interindividual variation in the metabolic ratio (MR), i.e. quinine/3-hydroxyquinine.

METHODS

Data from a previous study (A) was used for determination of the MR of quinine in plasma and urine at different time points. In study B, 24 healthy Swedish subjects received 250 mg quinine hydrochloride first alone and later together with four other CYP probe drugs [losartan (CYP2C9), omeprazole (CYP2C19), debrisoquine (CYP2D6) and caffeine (CYP1A2)] administered on the same day. Plasma and urine samples were collected before quinine intake and 16 h thereafter and analysed for quinine and 3-hydroxyquinine using high-performance liquid chromatography. Plasma and/or urine were collected for the other probes at different time points. MRs of all the probes were determined and correlations to quinine MR were studied.

RESULTS

In study A, the MR in plasma was stable over 96 h. The ratio increased from 5.8 to 12.2 (P=0.006) during co-administration with ketoconazole, whereas no significant difference (P=0.76) was observed during co-administration with fluvoxamine (from 5.8 to 6.0). In study B, there was no significant difference (P=0.36) between the mean MRs when quinine was given alone (4.7) or together with the four other drugs (4.5). There was a significant correlation between the MR of quinine and omeprazole sulphone formation (r=0.52, P<0.01), but not to the MRs of the other probes. There was a fivefold interindividual variability in the MR.

CONCLUSIONS

The MR of quinine in plasma or urine may serve as a stable measure of the activity of CYP3A4 in man. These results together with in vitro data show that quinine is also a specific CYP3A4 probe.

CYP enzyme polymorphisms and susceptibility to HCV-related chronic liver disease and liver cancer

Cancer risk can be influenced by the exposure to endogenous or environmental toxins. Polymorphic enzymes involved in the metabolic activation/detoxification of carcinogens may account for individual variations of risk. We studied the polymorphisms of five enzymes of the P450 superfamily, CYP1A1, CYP1A2, CYP2D6, CYP2E1 and CY3A4, as risk factors for liver disease progression and cancer in hepatitis C virus-infected patients. CYP genotyping was performed by polymerase chain reaction (PCR) restriction fragment length polymorphism or allele-specific PCR. Different stages of disease were considered, as follows: 90 asymptomatic carriers and 87 chronic hepatitis, 92 cirrhosis and 91 hepatocellular carcinoma (HCC) cases. Reference allele frequencies were obtained from 99 blood donors. Allele distributions among categories were compared using the chi(2) test. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated to express relative risks. Independent associations were modeled by correspondence analysis and logistic regression. Frequencies of the CYP1A1 highly inducible alleles, MspI m2 and Val, were increased in liver disease patients compared with carriers; no specific association with HCC was found. The high-activity CYP2E1 c2 allele was underrepresented among HCC patients with respect to other HCV categories, including cirrhosis. CYP2D6 poor metabolizer (PM) genotypes were significantly more frequent in healthy subjects (7.1%) and carriers (11.1%) than in hepatitis/cirrhosis (4.6%) and HCC (1.2%) patients. This was confirmed by multivariable analysis. PM genotypes protected against progressive disease as ORs reduced proportionally to stage. The age at diagnosis for HCC was anticipated in non-PM individuals. No differences were seen for CYP1A2 and CYP3A4 genes. Polymorphic variants of CYP genes may contribute to the progression of liver disease and HCC risk in HCV-infected subjects.

Human variability in CYP3A4 metabolism and CYP3A4-related uncertainty factors for risk assessment

CYP3A4 constitutes the major liver cytochrome P450 isoenzyme and is responsible for the oxidation of more than 50% of all known drugs. Human variability in kinetics for this pathway has been quantified using a database of 15 compounds metabolised extensively (>60%) by this CYP isoform in order to develop CYP3A4-related uncertainty factors for the risk assessment of environmental contaminants handled via this route. Data were analysed from published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and other subgroups using parameters relating primarily to chronic exposure [metabolic and total clearances, area under the plasma concentration-time curve (AUC)] and acute exposure (Cmax). Interindividual variability in kinetics was greater for the oral route (46%, 12 compounds) than for the intravenous route (32%, 14 compounds). The physiological and molecular basis for the difference between these two routes of exposure is discussed. In relation to the uncertainty factors used for risk assessment, the default kinetic factor of 3.16 would be adequate for adults, whereas a CYP3A4-related factor of 12 would be required to cover up to 99% of neonates, which have lower CYP3A4 activity.

Analysis of homotropic and heterotropic cooperativity of diazepam oxidation by CYP3A4 using site-directed mutagenesis and kinetic modeling

The structural basis for the cooperativity of diazepam oxidation catalyzed by human cytochrome P450 3A4 (CYP3A4) and 40 mutants has been investigated. An ordered two-site model in which substrates bind first to a catalytic/effector site and then to the catalytic site was used to explain sigmoidal kinetics for temazepam formation but hyperbolic kinetics for nordiazepam formation. In this model diazepam binds to the enzyme-substrate complex with a greater affinity (K(S2)=140 microM) than to free enzyme (K(S1)=960 microM). Residues 107, 119, 211, 301, 304, 309, 369, 370, and 373 play an important role in determining regioselectivity of diazepam oxidation. Interestingly, S119F and A370F displayed sigmoidal kinetics for nordiazepam formation, whereas I301F exhibited hyperbolic kinetics for both products. In the presence of increasing concentrations of testosterone, K(S1) for diazepam decreased, whereas K(S2) increased. The data suggest that three sites exist within the active pocket.

Polymorphism of cytochrome P450 and xenobiotic toxicity

The majority of human P450-dependent xenobiotic metabolism is carried out by polymorphic enzymes which can cause abolished, quantitatively or qualitatively altered or enhanced 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. An updated list of variant CYP alleles is present at the Home Page of the Human Cytochrome P450 (CYP) Allele Nomenclature Committee (http://www.imm.ki.se/CYPalleles/). 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. Dosage requirements for several commonly used drugs that have a narrow therapeutic range can differ more than 20-fold dependent on the genotype or the enzyme expression status. By contrast, carcinogen metabolising cytochrome P450s are less polymorphic and no firm relationships have been established linking increased risk for cancer with any specific P450 polymorphism. In the present overview recent aspects of cytochrome P450 polymorphism and xenobiotic toxicity are discussed.

A wide interindividual variability of urinary 6beta-hydroxycortisol to free cortisol in 487 healthy Japanese subjects in near basal condition

The frequency distribution of CYP3A activity was investigated by measuring ratios of urinary 6beta-hydroxycortisol to free cortisol in 487 healthy subjects to determine whether a genetic polymorphism for this cytochrome enzyme exists in "native-born" Japanese persons. Spot urine samples (from 9:00 am to 12:00 pm) were collected for measurement of 6beta-hydroxycortisol and free cortisol by high-performance liquid chromatography with a CN column after extracting with a solid-phase column (Bond-Elut C18). The frequency distribution of the urinary 6beta-hydroxycortisol to free cortisol was widely distributed among subjects but with no clear bimodality by a probit plot. Furthermore, the frequency distribution assessed on a new normal test variable plot indicated the possible existence of a CYP3A sexual dimorphism. Mean 6beta-hydroxycortisol levels were higher in women (n = 249) than in men (n = 238) by 1.7-fold, and this difference was statistically significant (P < 0.01). These results show that a CYP3A genetic polymorphism in Japanese persons, based on 6beta-hydroxycortisol excretions, likely does not exist, but there appears to be a broad unimodal distribution of enzyme activity in the population.

Genetic contribution to variable human CYP3A-mediated metabolism

The human CYP3A subfamily plays a dominant role in the metabolic elimination of more drugs than any other biotransformation enzyme. CYP3A enzyme is localized in the liver and small intestine and thus contributes to first-pass and systemic metabolism. CYP3A expression varies as much as 40-fold in liver and small intestine donor tissues. CYP3A-dependent in vivo drug clearance appears to be unimodally distributed which suggests multi-genic or complex gene-environment causes of variability. Interindividual differences in enzyme expression may be due to several factors including: variable homeostatic control mechanisms, disease states that alter homeostasis, up- or down-regulation by environmental stimuli (such as smoking, drug intake, or diet), and genetic mutations. This review summarizes the current understanding and implications of genetic variation in the CYP3A enzymes. Unlike other human P450s (CYP2D6, CYP2C19) there is no evidence of a 'null' allele for CYP3A4. More than 30 SNPs (single nucleotide polymorphisms) have been identified in the CYP3A4 gene. Generally, variants in the coding regions of CYP3A4 occur at allele frequencies <5% and appear as heterozygous with the wild-type allele. These coding variants may contribute to but are not likely to be the major cause of inter-individual differences in CYP3A-dependent clearance, because of the low allele frequencies and limited alterations in enzyme expression or catalytic function. The most common variant, CYP3A4*1B, is an A-392G transition in the 5'-flanking region with an allele frequency ranging from 0% (Chinese and Japanese) to 45% (African-Americans). Studies have not linked CYP3A4*1B with alterations in CYP3A substrate metabolism. In contrast, there are several reports about its association with various disease states including prostate cancer, secondary leukemias, and early puberty. Linkage disequilibrium between CYP3A4*1B and another CYP3A allele (CYP3A5*1) may be the true cause of the clinical phenotype. CYP3A5 is polymorphically expressed in adults with readily detectable expression in about 10-20% in Caucasians, 33% in Japanese and 55% in African-Americans. The primary causal mutation for its polymorphic expression (CYP3A5*3) confers low CYP3A5 protein expression as a result of improper mRNA splicing and reduced translation of a functional protein. The CYP3A5*3 allele frequency varies from approximately 50% in African-Americans to 90% in Caucasians. Functionally, microsomes from a CYP3A5*3/*3 liver contain very low CYP3A5 protein and display on average reduced catalytic activity towards midazolam. Additional intronic or exonic mutations (CYP3A5*5, *6, and *7) may alter splicing and result in premature stop codons or exon deletion. Several CYP3A5 coding variants have been described, but occur at relatively low allelic frequencies and their functional significance has not been established. As CYP3A5 is the primary extrahepatic CYP3A isoform, its polymorphic expression may be implicated in disease risk and the metabolism of endogenous steroids or xenobiotics in these tissues (e.g., lung, kidney, prostate, breast, leukocytes). CYP3A7 is considered to be the major fetal liver CYP3A enzyme. Although hepatic CYP3A7 expression appears to be significantly down-regulated after birth, protein and mRNA have been detected in adults. Recently, increased CYP3A7 mRNA expression has been associated with the replacement of a 60-bp segment of the CYP3A7 promoter with a homologous segment in the CYP3A4 promoter (CYP3A7*1C allele). This mutational swap confers increased gene transcription due to an enhanced interaction between activated PXR:RXRalpha complex and its cognate response element (ER-6). The genetic basis for polymorphic expression of CYP3A5 and CYP3A7 has now been established. Moreover, the substrate specificity and product regioselectivity of these isoforms can differ from that of CYP3A4, such that the impact of CYP3A5 and CYP3A7 polymorphic expression on drug disposition will be drug dependent. In addition to genetic variation, other factors that may also affect CYher factors that may also affect CYP3A expression include: tissue-specific splicing (as reported for prostate CYP3A5), variable control of gene transcription by endogenous molecules (circulating hormones) and exogenous molecules (diet or environment), and genetic variations in proteins that may regulate constitutive and inducible CYP3A expression (nuclear hormone receptors). Thus, the complex regulatory pathways, environmentally susceptible milieu of the CYP3A enzymes, and as yet undetermined genetic haplotypes, may confound evaluation of the effect of individual CYP3A genetic variations on drug disposition, efficacy and safety.

Expression of paclitaxel-inactivating CYP3A activity in human colorectal cancer: implications for drug therapy

Cytochrome P450 3A is a drug-metabolising enzyme activity due to CYP3A4 and CYP3A5 gene products, that is involved in the inactivation of anticancer drugs. This study analyses the potential of cytochrome P450 3A enzyme in human colorectal cancer to impact anticancer therapy with drugs that are cytochrome P450 3A substrates. Enzyme activity, variability and properties, and the ability to inactivate paclitaxel (taxol) were analysed in human colorectal cancer and healthy colorectal epithelium. Cytochrome P450 3A enzyme activity is present in healthy and tumoral samples, with a nearly 10-fold interindividual variability. Nifedipine oxidation activity+/-s.d. for colorectal cancer microsomes was 67.8+/-36.6 pmol min(-1) mg(-1). The K(m) of the tumoral enzyme (42+/-8 microM) is similar to that in healthy colorectal epithelium (36+/-8 microM) and the human liver enzyme. Colorectal cancer microsomes metabolised the anticancer drug paclitaxel with a mean activity was 3.1+/-1.2 pmol min(-1) mg(-1). The main metabolic pathway is carried out by cytochrome P450 3A, and it is inhibited by the cytochrome P450 3A-specific inhibitor ketoconazole with a K(I) value of 31 nM. This study demonstrates the occurrence of cytochrome P450 3A-dependent metabolism in colorectal cancer tissue. The metabolic activity confers to cancer cells the ability to inactivate cytochrome P450 3A substrates and may modulate tumour sensitivity to anticancer drugs.

High-performance liquid chromatographic assay for amiodarone N-deethylation activity in human liver microsomes using solid-phase extraction

A selective and sensitive assay for amiodarone N-deethylation activity in human liver microsomes by high-performance liquid chromatography (HPLC) with UV detection is reported. The extraction of desethylamiodarone from incubation samples was performed by means of an original solid-phase extraction (SPE) procedure using a polymeric reversed-phase sorbent (Oasis HLB). The method was validated for the determination of desethylamiodarone with respect to specificity, linearity, precision, accuracy, recovery, limit of quantitation and stability. Amiodarone N-deethylation activity from low to high substrate concentrations using human liver microsomes was precisely determined without a concentration step. This method is applicable to the study in vitro of the metabolism of amiodarone.

Co-regulation of CYP3A4 and CYP3A5 and contribution to hepatic and intestinal midazolam metabolism

We recently demonstrated that a variant allele of CYP3A5 (CYP3A5*3) confers low CYP3A5 expression as a result of improper mRNA splicing. In this study, we further evaluated the regulation of CYP3A5 in liver and jejunal mucosa from white donors. For all tissues, high levels of CYP3A5 protein were strongly concordant with the presence of a wild-type allele of the CYP3A5 gene (CYP3A5*1). CYP3A5 represented greater than 50% of total CYP3A content in nearly all of the livers and jejuna that carried the CYP3A5*1 wild-type allele. Overall, CYP3A5 protein content accounted for 31% of the variability in hepatic midazolam hydroxylation activity. Improperly spliced mRNA (SV1-CYP3A5) was found only in tissues containing a CYP3A5*3 allele. Properly spliced CYP3A5 mRNA (wt-CYP3A5) was detected in all tissues, but the median wt-CYP3A5 mRNA was 4-fold higher in CYP3A5*1/*3 livers compared with CYP3A5*3/*3 livers. Differences in wt-CYP3A5 and CYP3A4 mRNA content explained 53 and 51% of the interliver variability in CYP3A5 and CYP3A4 content, respectively. Hepatic CYP3A4 and CYP3A5 contents were not correlated when all livers were compared. However, for CYP3A5*1/*3 livers, levels of the two proteins were strongly correlated (r = 0.93) as were wt-CYP3A5 and CYP3A4 mRNA (r = 0.76). These findings suggest that CYP3A4 and CYP3A5 genes share a common regulatory pathway for constitutive expression, possibly involving conserved elements in the 5'-flanking region.

Pharmacogenetics in diverse ethnic populations--implications for drug discovery and development

It is widely acknowledged that the vast quantities of data now publicly available as a result of the human genome initiative have the potential to revolutionize the pharmaceutical industry. More tangibly to the drug development business, the dawn of the pharmacogenetics era has the potential to impact not only the discovery of new medicines but also the safety and efficacy of pharmaceutical agents. Coincident with these scientific advances is the emergence of new markets for pharmaceutical agents. Japan, which represents the world's second biggest market, is a good example. With the ICH E5 agreement in 1998 and a rapid change in the drug registration process in Japan, there are increasing opportunities to improve access to more medicines in all parts of the world. However, it is increasingly clear that significant genetic variation still exists between populations, with a host of data on interethnic variation in drug metabolizing enzyme and drug transporter activity. Evidence suggesting that this genetic variation may play an important role in defining some of the interethnic variation in drug response to currently marketed compounds is reviewed here, and future possibilities of using such information to better streamline the drug development process are discussed.

Mutation analysis of the human CYP3A4 gene 5' regulatory region: population screening using non-radioactive SSCP

Human CYP3A4 is the major cytochrome P450 isoenzyme in adult human liver and is known to metabolise many xenobiotic and endogenous compounds. There is substantial inter-individual variation in the hepatic levels of CYP3A4. Although, polymorphic mutations have been reported in the 5' regulatory region of the CYP3A4 gene, those that have been investigated so far do not appear to have any effect on gene expression. To determine whether other mutations exist in this region of the gene, we have performed a new population screen on a panel of 101 human DNA samples. A 1140 bp section of the 5' proximal regulatory region of the CYP3A4 gene, containing numerous regulatory motifs, was amplified from genomic DNA as three overlapping segments. The 300 bp distal enhancer region at -7.9kb containing additional regulatory motifs was also amplified. Mutation analysis of the resulting PCR products was carried out using non-radioactive single strand conformation polymorphism (SSCP) and confirmatory sequencing of both DNA strands in those samples showing extra SSCP bands. In addition to detection of the previously reported CYP3A4*1B allele in nine subjects, three novel alleles were found: CYP3A4*1E (having a T-->A transversion at -369 in one subject), CYP3A4*1F (having a C-->G tranversion at -747 in 17 subjects) and CYP3A4*15B containing a nine-nucleotide insertion between -845 and -844 linked to an A-->G transition at -392 and a G-->A transition in exon 6 (position 485 in the cDNA) in one subject. All the novel alleles were heterozygous. No mutations were found in the upstream distal enhancer region. Our results clearly indicate that this rapid and simple SSCP approach can reveal mutant alleles in drug metabolising enzyme genes. Detection and determination of the frequency of novel alleles in CYP3A4 will assist investigation of the relationship between genotype, xenobiotic metabolism and toxicity in the CYP3A family of isoenzymes.