A form of cytochrome P450 in man, orthologous to form d in the rat, catalyses the O-deethylation of phenacetin and is inducible by cigarette smoking

Interindividual Variability in Cytochrome P450 3A and 1A Activity Influences Sunitinib Metabolism and Bioactivation

Sunitinib is an orally administered tyrosine kinase inhibitor associated with idiosyncratic hepatotoxicity; however, the mechanisms of this toxicity remain unclear. We have previously shown that cytochromes P450 1A2 and 3A4 catalyze sunitinib metabolic activation via oxidative defluorination leading to a chemically reactive, potentially toxic quinoneimine, trapped as a glutathione (GSH) conjugate (M5). The goals of this study were to determine the impact of interindividual variability in P450 1A and 3A activity on sunitinib bioactivation to the reactive quinoneimine and sunitinib N-dealkylation to the primary active metabolite N-desethylsunitinib (M1). Experiments were conducted in vitro using single-donor human liver microsomes and human hepatocytes. Relative sunitinib metabolite levels were measured by liquid chromatography-tandem mass spectrometry. In human liver microsomes, the P450 3A inhibitor ketoconazole significantly reduced M1 formation compared to the control. The P450 1A2 inhibitor furafylline significantly reduced defluorosunitinib (M3) and M5 formation compared to the control but had minimal effect on M1. In CYP3A5-genotyped human liver microsomes from 12 individual donors, M1 formation was highly correlated with P450 3A activity measured by midazolam 1'-hydroxylation, and M3 and M5 formation was correlated with P450 1A2 activity estimated by phenacetin O-deethylation. M3 and M5 formation was also associated with P450 3A5-selective activity. In sandwich-cultured human hepatocytes, the P450 3A inducer rifampicin significantly increased M1 levels. P450 1A induction by omeprazole markedly increased M3 formation and the generation of a quinoneimine-cysteine conjugate (M6) identified as a downstream metabolite of M5. The nonselective P450 inhibitor 1-aminobenzotriazole reduced each of these metabolites (M1, M3, and M6). Collectively, these findings indicate that P450 3A activity is a key determinant of sunitinib N-dealkylation to the active metabolite M1, and P450 1A (and potentially 3A5) activity influences sunitinib bioactivation to the reactive quinoneimine metabolite. Accordingly, modulation of P450 activity due to genetic and/or nongenetic factors may impact the risk of sunitinib-associated toxicities.

Exposure to an Environmental Mixture of Polycyclic Aromatic Hydrocarbons Induces Hepatic Cytochrome P450 Enzymes in Mice

Cytochrome P450 enzymes (CYPs) play an important role in bioactivating or detoxifying polycyclic aromatic hydrocarbons (PAHs), common environmental contaminants. While it is widely accepted that exposure to PAHs induces CYPs, effectively increasing rates of xenobiotic metabolism, dose- and time-response patterns of CYP induction are not well-known. In order to better understand dose- and time-response relationships of individual CYPs following induction, we exposed B6129SF1/J mice to single or repeated doses (2-180 μmol/kg/d) of benzo[a]pyrene (BaP) or Supermix-10, a mixture of the top 10 most abundant PAHs found at the Portland Harbor Superfund Site. In hepatic microsomes from exposed mice, we measured amounts of active CYPs using activity-based protein profiling and total CYP expression using global proteomics. We observed rapid Cyp1a1 induction after 6 h at the lowest PAH exposures and broad induction of many CYPs after 3 daily PAH doses at 72 h following the first dose. Using samples displaying Cyp1a1 induction, we observed significantly higher metabolic affinity for BaP metabolism (K_m reduced 3-fold), 3-fold higher intrinsic clearance, but no changes to the V_max. Mice dosed with the highest PAH exposures exhibited 1.7-5-fold higher intrinsic clearance rates for BaP compared to controls and higher V_max values indicating greater amounts of enzymes capable of metabolizing BaP. This study demonstrates exposure to PAHs found at superfund sites induces enzymes in dose- and time-dependent patterns in mice. Accounting for specific changes in enzyme profiles, relative rates of PAH bioactivation and detoxification, and resulting risk will help translate internal dosimetry of animal models to humans and improve risk assessments of PAHs at superfund sites.

Metabolism and Mechanism of Human Cytochrome P450 Enzyme 1A2

Human cytochrome P450 enzyme 1A2 (CYP1A2) is one of the most important cytochrome P450 (CYP) enzymes in the liver, accounting for 13% to 15% of hepatic CYP enzymes. CYP1A2 metabolises many clinical drugs, such as phenacetin, caffeine, clozapine, tacrine, propranolol, and mexiletine. CYP1A2 also metabolises certain precarcinogens such as aflatoxins, mycotoxins, nitrosamines, and endogenous substances such as steroids. The regulation of CYP1A2 is influenced by many factors. The transcription of CYP1A2 involves not only the aromatic hydrocarbon receptor pathway but also many additional transcription factors, and CYP1A2 expression may be affected by transcription coactivators and compression factors. Degradation of CYP1A2 mRNA and protein, alternative splicing, RNA stability, regulatory microRNAs, and DNA methylation are also known to affect the regulation of CYP1A2. Many factors can lead to changes in the activity of CYP1A2. Smoking, polycyclic aromatic hydrocarbon ingestion, and certain drugs (e.g., omeprazole) increase its activity, while many clinical drugs such as theophylline, fluvoxamine, quinolone antibiotics, verapamil, cimetidine, and oral contraceptives can inhibit CYP1A2 activity. Here, we review the drugs metabolised by CYP1A2, the metabolic mechanism of CYP1A2, and various factors that influence CYP1A2 metabolism. The metabolic mechanism of CYP1A2 is of great significance in the development of personalised medicine and CYP1A2 target-based drugs.

RNA sequencing identifies common pathways between cigarette smoke exposure and replicative senescence in human airway epithelia

BACKGROUND

Aging is affected by genetic and environmental factors, and cigarette smoking is strongly associated with accumulation of senescent cells. In this study, we wanted to identify genes that may potentially be beneficial for cell survival in response to cigarette smoke and thereby may contribute to development of cellular senescence.

RESULTS

Primary human bronchial epithelial cells from five healthy donors were cultured, treated with or without 1.5% cigarette smoke extract (CSE) for 24 h or were passaged into replicative senescence. Transcriptome changes were monitored using RNA-seq in CSE and non-CSE exposed cells and those passaged into replicative senescence. We found that, among 1534 genes differentially regulated during senescence and 599 after CSE exposure, 243 were altered in both conditions, representing strong enrichment. Pathways and gene sets overrepresented in both conditions belonged to cellular processes that regulate reactive oxygen species, proteasome degradation, and NF-κB signaling.

CONCLUSIONS

Our results offer insights into gene expression responses during cellular aging and cigarette smoke exposure, and identify potential molecular pathways that are altered by cigarette smoke and may also promote airway epithelial cell senescence.

In vitro analysis of factors influencing CYP1A2 expression as potential determinants of interindividual variation

Individual differences in drug metabolism contribute to interindividual variation that characterizes responses to drugs and risk in exposure to foreign chemicals. Large individual differences are found in expression levels of CYP1A2, a major drug-metabolizing enzyme. Underlying causes for this variation are not well understood. Several factors, including tobacco smoking, consumption of cruciferous vegetables, and sex, have been associated with modulating CYP1A2 expression. To understand factors regulating expression of CYP1A2 in establishing a causal relationship, this study examined effects of cigarette smoke condensate (CSC), indole-3-carbinol (I3C), and 17β-estradiol (estradiol) on CYP1A2 expression in in vitro systems using human liver and lung cells. Treatment with CSC (2-25 μg/mL) significantly increased levels of CYP1A2 in six cell lines examined, in a concentration- and time-dependent manner. Fold changes in expression levels relative to controls varied among cell lines. CYP1A2 enzymatic activity also increased with CSC exposure. Treatment of H1299 and HepB3 cells with dietary agent I3C (50 and 100 μmol/L) increased CYP1A2 expression. In human cell lines H1299 and H1395, treatment with estradiol (10 and 100 nmol/L) significantly reduced expression of CYP1A2. Using ChIP assays, effects of CSC on histone modifications were analyzed. Increases in H3K4me3 and H4K16ac were observed at several segments in the CYP1A2 gene, whereas H3K27me3 decreased, following CSC treatment. These results suggest that CYP1A2 expression is affected epigenetically by CSC. Additional studies will be needed to further establish regulatory mechanisms underlying effects of various environmental, dietary, and endogenous factors on CYP1A2 expression in better predicting individual variation.

Pharmacokinetic Drug Interactions with Tobacco, Cannabinoids and Smoking Cessation Products

Tobacco smoke contains a large number of compounds in the form of metals, volatile gases and insoluble particles, as well as nicotine, a highly addictive alkaloid. Marijuana is the most widely used illicit drug of abuse in the world, with a significant increase in the USA due to the increasing number of states that allow medical and recreational use. Of the over 70 phytocannabinoids in marijuana, Δ9-tetrahydrocannabinol (Δ9THC), cannabidiol (CBD) and cannibinol are the three main constituents. Both marijuana and tobacco smoking induce cytochrome P450 (CYP) 1A2 through activation of the aromatic hydrocarbon receptor, and the induction effect between the two products is additive. Smoking cessation is associated with rapid downregulation of CYP1A enzymes. On the basis of the estimated half-life of CYP1A2, dose reduction of CYP1A drugs may be necessary as early as the first few days after smoking cessation to prevent toxicity, especially for drugs with a narrow therapeutic index. Nicotine is a substrate of CYP2A6, which is induced by oestrogen, resulting in lower concentrations of nicotine in females than in males, especially in females taking oral contraceptives. The significant effects of CYP3A4 inducers and inhibitors on the pharmacokinetics of Δ9THC/CBD oromucosal spray suggest that CYP3A4 is the primary enzyme responsible for the metabolism of Δ9THC and CBD. Limited data also suggest that CBD may significantly inhibit CYP2C19. With the increasing use of marijuana and cannabis products, clinical studies are needed in order to determine the effects of other drugs on pharmacokinetics and pharmacodynamics.

Association between the CYP1A2 rs762551 Polymorphism and Bladder Cancer Susceptibility: a Meta-Analysis Based on Case-Control Studies

BACKGROUND

Previous studies evaluated associations between the CYP1A2 rs762551 polymorphism and bladder cancer risk. However, the results were inconsistent. We therefore performed a meta-analysis of the published case-control studies to assess in detail the association between CYP1A2 rs762551 polymorphism and bladder cancer risk.

MATERIALS AND METHODS

PubMed, Embase and Web of Science were searched to identify relevant studies and the pooled odds ratio (OR) and 95 % confidence interval (95%CI) were calculated.

RESULTS

A total of seven articles including 3,013 cases and 2,771 controls were finally included. Overall, a significant association was found between the CYP1A2 rs762551 polymorphism and bladder cancer susceptibility for CC vs AA (OR=0.82, 95% CI=0.69~0.99), but no significant associations were found for the other three models (AC vs AA: OR=0.91, 95% CI=0.81~1.02; the dominant model: OR=0.90, 95% CI=0.80~1.00; the recessive model: OR=0.84, 95% CI =0.72~1.00). In the subgroup analysis by ethnicity, we detected significant associations between the CYP1A2 rs762551 polymorphism and bladder cancer susceptibility for GA vs GG (OR = 0.78, 95% CI =0.64~0.96) and for the recessive model (OR=0.80, 95% CI=0.66~0.96) in Caucasians, but not for Asians.

CONCLUSIONS

The results from the meta-analysis suggested that the CYP1A2 rs762551 polymorphism is a protective factor for bladder cancer, especially in Caucasians.

Caffeine Cytochrome P450 1A2 Metabolic Phenotype Does Not Predict the Metabolism of Heterocyclic Aromatic Amines in Humans

2-Amino-1-methylimidazo[4,5-b]pyridine (PhIP) and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) are carcinogenic heterocyclic aromatic amines (HAAs) formed in well-done cooked meats. Chemicals that induce cytochrome P450 (P450) 1A2, a major enzyme involved in the bioactivation of HAAs, also form in cooked meat. Therefore, well-done cooked meat may pose an increase in cancer risk because it contains both inducers of P450 1A2 and procarcinogenic HAAs. We examined the influence of components in meat to modulate P450 1A2 activity and the metabolism of PhIP and MeIQx in volunteers during a 4 week feeding study of well-done cooked beef. The mean P450 1A2 activity, assessed by caffeine metabolic phenotyping, ranged from 6.3 to 7.1 before the feeding study commenced and from 9.6 to 10.4 during the meat feeding period: the difference in means was significant (P < 0.001). Unaltered PhIP, MeIQx, and their P450 1A2 metabolites, N(2)-(β-1-glucosiduronyl)-2-(hydroxyamino)-1-methyl-6-phenylimidazo[4,5-b]pyridine (HON-PhIP-N(2)-Gl); N3-(β-1-glucosiduronyl)-2-(hydroxyamino)-1-methyl-6-phenylimidazo[4,5-b]pyridine (HON-PhIP-N3-Gl); 2-amino-3-methylimidazo-[4,5-f]quinoxaline-8-carboxylic acid (IQx-8-COOH); and 2-amino-8-(hydroxymethyl)-3-methylimidazo[4,5-f]quinoxaline (8-CH2OH-IQx) were measured in urine during days 2, 14, and 28 of the meat diet. Significant correlations were observed on these days between the levels of the unaltered HAAs and their oxidized metabolites, when expressed as percent of dose ingested or as metabolic ratios. However, there was no statistically significant correlation between the caffeine P450 1A2 phenotype and any urinary HAA biomarker. Although the P450 1A2 activity varied by greater than 20-fold among the subjects, there was a large intraindividual variation of the P450 1A2 phenotype and inconsistent responses to inducers of P450 1A2. The coefficient of variation of the P450 1A2 phenotype within-individual ranged between 1 to 112% (median = 40%) during the entire course of the study. The caffeine metabolic phenotype for P450 1A2 was a poor predictor of oxidative urinary metabolites of PhIP and MeIQx and may not be a reliable measure to assess the role of HAAs in cancer risk.

Functional characterization of 20 allelic variants of CYP1A2

Genetic variations in cytochrome P450 1A2 (CYP1A2) are associated with interindividual variability in the metabolism and efficacy of many medications. Twenty CYP1A2 variants harboring amino acid substitutions were analyzed for functional changes in enzymatic activity. Recombinant CYP1A2 variant proteins were heterologously expressed in COS-7 cells. Enzyme kinetic analyses were performed with two representative CYP1A2 substrates, phenacetin and 7-ethoxyresorufin. Among the 20 CYP1A2 allelic variants, CYP1A2*4, CYP1A2*6, CYP1A2*8, CYP1A2*15, CYP1A2*16, and CYP1A2*21 were inactive toward both substrates. CYP1A2*11 showed markedly reduced activity, but the changes in Km were different between the substrates. CYP1A2*14 and CYP1A2*20 exhibited increased activity compared to the wild-type enzyme, CYP1A2*1. This comprehensive in vitro assessment provided insight into the specific metabolic activities of CYP1A2 proteins encoded by variant alleles, which may to be valuable when interpreting the results of in vivo studies.

Role of CYP1A2 polymorphisms on lung cancer risk in a prospective study

Cytochrome P4501A2 (CYP1A2) is a key enzyme for lung carcinogen activation and lung inflammation. We studied the interactions of the CYP1A2 functional variants -3860G/A(rs2069514),-2467T/delT(rs3569413),-163C/A(rs762551)] with occupational/environmental carcinogenic exposures in the development of lung cancer in a case-control study nested in the Danish prospective cohort "Diet, Cancer and Health." At enrollment (1993-1997), blood samples for genotype analyses and information on lifestyle were collected 5 (mean value) years before the onset of the disease. The study population included 425 lung cancer cases and 786 subcohort members, who were gender- and age-matched. We found that -163A carriers were at increased risk of lung cancer (P=0.035) in a multivariate COX regression model, which was adjusted for personal habits (i.e., cumulative smoking, passive smoke at home, alcohol intake, and fruit intake) and occupational exposure. Additionally, the interaction between -2467delT and smoking increases lung cancer risk in males, especially light smokers (<21.5 pack-years, P=0.004). The increased lung cancer risk found in -163C carriers, independent of smoking status, and in -2467delT male smokers, suggests that these variants could influence lung cancer development through different mechanisms (i.e. lung carcinogen activation and lung inflammation).

Influence of mild and moderate hepatic impairment on axitinib pharmacokinetics

OBJECTIVE

To evaluate the effects of hepatic impairment on the pharmacokinetics and safety of a single, oral axitinib dose in subjects with mild or moderate hepatic impairment.

METHODS

In this phase I, open-label, parallel-group study, a total of 24 subjects with either normal hepatic function (n = 8) or with mild (n = 8) or moderate (n = 8) hepatic impairment were administered a single, oral dose of axitinib (5 mg). Blood samples were collected at intervals up to 144 h following dosing, and plasma pharmacokinetics and safety were assessed. Changes in axitinib plasma exposures in subjects with mild or moderate hepatic impairment were predicted using computer simulations and used to guide initial dosing in the clinical study.

RESULTS

Axitinib exposure was similar in subjects with normal hepatic function and those with mild hepatic impairment, but approximately twofold higher in subjects with moderate hepatic impairment. Axitinib exposure weakly correlated with measures of hepatic function but was not affected by smoking status. Axitinib protein binding was similar in the three treatment groups. No significant treatment-related adverse events were reported.

CONCLUSIONS

Compared with subjects with normal hepatic function, moderate hepatic impairment increased axitinib exposure, suggesting that the oral clearance of axitinib is altered in these subjects. In addition, these data indicate a possible need for a dose reduction in subjects who develop moderate or worse hepatic impairment during axitinib treatment. A single 5-mg dose of axitinib was well tolerated in subjects with mild or moderate hepatic impairment.

Pathway-Targeted Pharmacogenomics of CYP1A2 in Human Liver

The human drug metabolizing cytochrome P450 (CYP) 1A2, is one of the major P450 isoforms contributing by about 5–20% to the hepatic P450 pool and catalyzing oxidative biotransformation of up to 10% of clinically relevant drugs including clozapine and caffeine. CYP1A2 activity is interindividually highly variable and although twin studies have suggested a high heritability, underlying genetic factors are still unknown. Here we adopted a pathway-oriented approach using a large human liver bank (n = 150) to elucidate whether variants in candidate genes of constitutive, ligand-inducible, and pathophysiological inhibitory regulatory pathways may explain different hepatic CYP1A2 phenotypes. Samples were phenotyped for phenacetin O-deethylase activity, and the expression of CYP1A2 protein and mRNA was determined. CYP1A2 expression and function was increased in smokers and decreased in patients with inflammation and cholestasis. Of 169 SNPs in 17 candidate genes including the CYP1A locus, 136 non-redundant SNPs with minor allele frequency >5% were analyzed by univariate and multivariate methods. A total of 13 strong significant associations were identified, of which 10 SNPs in the ARNT, AhRR, HNF1α, IL1β, SRC-1, and VDR genes showed consistent changes for at least two phenotypes by univariate analysis. Multivariate linear modeling indicated that the polymorphisms and non-genetic factors together explained 42, 38, and 33% of CYP1A2 variation at activity, protein and mRNA levels, respectively. In conclusion, we identified novel trans-associations between regulatory genes and hepatic CYP1A2 function and expression, but additional genetic factors must be assumed to explain the full extent of CYP1A2 heritability.

Population pharmacokinetics of golimumab, an anti-tumor necrosis factor-alpha human monoclonal antibody, in patients with psoriatic arthritis

The population pharmacokinetics of subcutaneously administered golimumab (50 mg or 100 mg every 4 weeks) were characterized in patients with active psoriatic arthritis (PsA) in GO-REVEAL, a randomized, double-blind, placebo-controlled, phase 3 study. A total of 2029 serum golimumab concentrations from 337 patients were analyzed using NONMEM. A 1-compartment pharmacokinetic model with first-order absorption and elimination was chosen to describe the observed concentration-time data. For a patient of standard weight (70 kg), the population estimates (typical value +/- standard error) for golimumab pharmacokinetic parameters were as follows: apparent clearance = 1.38 +/- 0.04 L/d, apparent volume of distribution = 24.9 +/- 1.04 L, and absorption rate constant = 0.908 +/- 0.121 per day. The between-subject variability was 37.6% in apparent clearance and 37.9% in apparent volume of distribution. Body weight, antibody-to-golimumab status, baseline C-reactive protein level, and smoking status were identified as significant covariates on apparent clearance. Body weight was also a significant covariate on apparent volume of distribution. None of the concomitant medications examined (methotrexate, corticosteroids, and nonsteroidal anti-inflammatory drugs) were significant covariates on apparent clearance, although the median trough golimumab concentration in patients receiving methotrexate was higher than for those not receiving methotrexate. These significant covariates account for part of the variability in systemic exposure to golimumab observed in patients with PsA.

CYP1A2 genotype and rheumatoid arthritis in Koreans

Cytochrome P540 (CYP) 1A2 plays a role in the production of reactive oxygen species (ROS), which have been implicated in the development of rheumatoid arthritis (RA). The objective of this study was to investigate the association between a common polymorphism in the CYP1A2 gene with risk and severity of RA in a Korean population. Cases (n = 1321) with RA and controls (n = 1037) were genotyped for the CYP1A2 -163 A>C polymorphism by real-time PCR. HLA-DRB1 typing and further subtyping of all alleles was performed by PCR, sequence-specific oligonucleotide probe hybridization and direct DNA sequencing analysis. The odds ratio (OR) [(95% confidence interval (CI)] of RA associated with the low inducible C allele was 1.11 (0.80-1.55) among non-shared epitope (SE) carriers, 0.82 (0.56-1.20) among heterozygotes and 0.32 (0.10-1.04) among individuals homozygous for the SE (P = 0.03 for CYP1A2-SE interaction). A protective effect of the low inducibility CYP1A2 C allele among carriers of the SE suggests that a product of CYP1A2-mediated metabolism, such as ROS, may be involved in the development of RA.

Coffee intake, smoking, and pulmonary function in the Atherosclerosis Risk in Communities Study

Coffee contains polyphenolic antioxidants and caffeine, which may favorably affect pulmonary function. Therefore, the authors studied cross-sectional associations (1987-1989) between coffee intake and pulmonary function in the Atherosclerosis Risk in Communities Study, a population-based cohort study (analytic sample = 10,658). They also conducted analyses stratified by smoking status, since smoking is a strong risk factor for respiratory disease and could influence the effects of caffeine and antioxidants. Self-reported coffee intake was categorized as rare/never, <7 cups/week, 1 cup/day, 2-3 cups/day, and >or=4 cups/day. Pulmonary function was characterized by the spirometric measures forced vital capacity (FVC) and forced expiratory volume in 1 second (FEV(1)). After adjustment for demographic factors, lifestyle characteristics, and dietary factors, pulmonary function values increased across increasing categories of coffee consumption in never and former smokers but not in current smokers. In never or former smokers who consumed >or=4 cups of coffee daily, FVC and FEV(1) were 2%-3% greater than in never or former smokers who rarely/never consumed coffee (P(trend) values: in never smokers, 0.04 for FVC and 0.07 for FEV(1); in former smokers, <0.001 for FVC and <0.001 for FEV(1)). These data show a possible beneficial effect of coffee (or a coffee ingredient) on pulmonary function, but it appears to be limited to nonsmokers.

Phenotypic CYP2A6 variation and the risk of pancreatic cancer

OBJECTIVE

Cytochrome P450 2A6 (CYP2A6) is an important metabolic enzyme capable of activating several procarcinogens, including dietary and tobacco-specific nitrosamines, which have been linked to pancreatic cancer. Positive associations between high CYP2A6 activity and lung and colorectal cancers have been reported. This is the first investigation of CYP2A6 activity and pancreatic cancer.

DESIGN

In this case-control study of cancer of the exocrine pancreas, phenotypic CYP2A6 activity was measured using a ratio of urinary caffeine metabolites. Demographic, smoking, dietary and medical information were obtained by questionnaire. CYP2A6 phenotype, which is not influenced by smoking status, was measured for 90 cases and 470 controls.

RESULTS

When modeled as a continuous variable, and adjusted for age, sex, race, education, current smoking status and chronic pancreatitis, the odds ratio (OR) per one unit of the natural log of the CYP2A6 ratio was 1.52 (95% confidence interval, CI: 1.09-2.12). In an adjusted categorical analysis, subjects in the uppermost quartile (based on controls) of CYP2A6 activity, when compared to the lower three quartiles, carried an 80% greater risk of pancreatic cancer (OR=1.80; 95% CI: 1.07-3.02).

CONCLUSIONS

High levels of CYP2A6 activity, as measured by a caffeine phenotyping assay, were positively associated with pancreatic cancer in this casecontrol study among a Midwestern U.S. population.

A preliminary operational classification system for nonmutagenic modes of action for carcinogenesis

This article proposes a system of categories for nonmutagenic modes of action for carcinogenesis. The classification is of modes of action rather than individual carcinogens, because the same compound can affect carcinogenesis in more than one way. Basically, we categorize modes of action as: (1) co-initiation (facilitating the original mutagenic changes in stem and progenitor cells that start the cancer process) (e.g. induction of activating enzymes for other carcinogens); (2) promotion (enhancing the relative growth vs differentiation/death of initiated clones (e.g. inhibition of growth-suppressing cell-cell communication); (3) progression (enhancing the growth, malignancy, or spread of already developed tumors) (e.g. suppression of immune surveillance, hormonally mediated growth stimulation for tumors with appropriate receptors by estrogens); and (4) multiphase (e.g., "epigenetic" silencing of tumor suppressor genes). A priori, agents that act at relatively early stages in the process are expected to manifest greater relative susceptibility in early life, whereas agents that act via later stage modes will tend to show greater susceptibility for exposures later in life.

Role of cytochrome P450 in drug interactions

Drug-drug interactions have become an important issue in health care. It is now realized that many drug-drug interactions can be explained by alterations in the metabolic enzymes that are present in the liver and other extra-hepatic tissues. Many of the major pharmacokinetic interactions between drugs are due to hepatic cytochrome P450 (P450 or CYP) enzymes being affected by previous administration of other drugs. After coadministration, some drugs act as potent enzyme inducers, whereas others are inhibitors. However, reports of enzyme inhibition are very much more common. Understanding these mechanisms of enzyme inhibition or induction is extremely important in order to give appropriate multiple-drug therapies. In future, it may help to identify individuals at greatest risk of drug interactions and adverse events.

4-Aminobiphenyl and DNA Reactivity: Case Study Within the Context of the 2006 IPCS Human Relevance Framework for Analysis of a Cancer Mode of Action for Humans

The IPCS Human Relevance Framework was evaluated for a DNA-reactive (genotoxic) carcinogen, 4-aminobiphenyl, based on a wealth of data in animals and humans. The mode of action involves metabolic activation by N-hydroxylation, followed by N-esterification leading to the formation of a reactive electrophile, which binds covalently to DNA, principally to deoxyguanosine, leading to an increased rate of DNA mutations and ultimately to the development of cancer. In humans and dogs, the urinary bladder urothelium is the target organ, whereas in mice it is the bladder and liver; in other species, other tissues can be involved. Differences in organ specificity are thought to be due to differences in metabolic activation versus inactivation. Based on qualitative and quantitative considerations, the mode of action is possible in humans. Other biological processes, such as toxicity and regenerative proliferation, can significantly influence the dose response of 4-aminobiphenyl-induced tumors. Based on the IPCS Human Relevance Framework, 4-aminobiphenyl would be predicted to be a carcinogen in humans, and this is corroborated by extensive epidemiologic evidence. The IPCA Human Relevance Framework is useful in evaluating DNA-reactive carcinogens.

Comparative 1-Substituted Imidazole Inhibition of Cytochrome P450 Isozyme-Selective Activities in Human and Mouse Hepatic Microsomes

Inhibition of cytochrome P450(CYP)-selective reactions in a single human and a single mouse hepatic microsome preparation by fourteen 1-substituted imidazoles provides a simultaneous ranking of reaction susceptibility to a specific imidazole and the relative inhibitory potency of the imidazoles for a given reaction. CYP3A4/5 activity was inhibited (IC(50) <5 microM) by the greatest number of imidazoles, followed closely by CYP2C9. Seven imidazoles exhibited IC(50) values for CYP3A4/5 <0.3 microM (none for CYP2C9) and were exclusively above 300 MW. Nafimidone (MW, 236) exhibited an IC(50) value <0.3 microM towards CYP2D6 and CYP1A2 reactions. CYP2E1 and CYP2A6 were exclusively inhibited (IC(50) <5 microM) by imidazoles with MWs below approximately 200. In general, mouse activities exhibited lower IC(50) values than in human microsomes.

Variation in CYP1A2 activity and its clinical implications: influence of environmental factors and genetic polymorphisms

CYP1A2 is involved in the metabolism of several widely used drugs and endogenous compounds, and in the activation of procarcinogens. Both genetic and environmental factors influence the activity of this enzyme. The current knowledge regarding factors influencing the activity of CYP1A2 is summarized in this review. Substrates, inhibitors and inducers of CYP1A2 activity, as well as phenotyping probes, are discussed. The functional significance and clinical importance of CYP1A2 gene polymorphisms are reviewed and interethnic differences in the distribution of CYP1A2 variant alleles and haplotypes are summarized. Finally, future perspectives for the possible clinical applications of CYP1A2 genotyping are discussed.

Inhibition and induction of human cytochrome P450 enzymes: current status

Variability of drug metabolism, especially that of the most important phase I enzymes or cytochrome P450 (CYP) enzymes, is an important complicating factor in many areas of pharmacology and toxicology, in drug development, preclinical toxicity studies, clinical trials, drug therapy, environmental exposures and risk assessment. These frequently enormous consequences in mind, predictive and pre-emptying measures have been a top priority in both pharmacology and toxicology. This means the development of predictive in vitro approaches. The sound prediction is always based on the firm background of basic research on the phenomena of inhibition and induction and their underlying mechanisms; consequently the description of these aspects is the purpose of this review. We cover both inhibition and induction of CYP enzymes, always keeping in mind the basic mechanisms on which to build predictive and preventive in vitro approaches. Just because validation is an essential part of any in vitro-in vivo extrapolation scenario, we cover also necessary in vivo research and findings in order to provide a proper view to justify in vitro approaches and observations.

PhRMA white paper on ADME pharmacogenomics

Pharmacogenomic (PGx) research on the absorption, distribution, metabolism, and excretion (ADME) properties of drugs has begun to have impact for both drug development and utilization. To provide a cross-industry perspective on the utility of ADME PGx, the Pharmaceutical Research and Manufacturers of America (PhRMA) conducted a survey of major pharmaceutical companies on their PGx practices and applications during 2003-2005. This white paper summarizes and interprets the results of the survey, highlights the contributions and applications of PGx by industrial scientists as reflected by original research publications, and discusses changes in drug labels that improve drug utilization by inclusion of PGx information. In addition, the paper includes a brief review on the clinically relevant genetic variants of drug-metabolizing enzymes and transporters most relevant to the pharmaceutical industry.

Effects of gender and moderate smoking on the pharmacokinetics and effects of the CYP1A2 substrate tizanidine

OBJECTIVE

We studied the effects of gender and smoking on the pharmacokinetics and effects of the cytochrome P450 (CYP) 1A2 substrate tizanidine.

METHODS

Seventy-one healthy young volunteers (male and female nonsmokers, male smokers) ingested 4 mg tizanidine. Plasma concentrations and pharmacodynamics of tizanidine were measured, and a caffeine test was performed.

RESULTS

Among nonsmokers, the peak concentration (C(max)) and area under concentration-time curve from 0 to infinity [AUC(0-infinity)] of tizanidine did not differ significantly between females and males. However, the half-life (t(1/2)) was 9% shorter in female nonsmokers than in male nonsmokers (P < 0.05). In male smokers, the t(1/2) was 10% shorter and the weight-adjusted AUC(0-infinity) 33% smaller than in male nonsmokers (P < 0.05). The caffeine/paraxanthine ratio was 35-40% smaller (P = 0.001) in male smokers than in nonsmoking males or females, but did not differ between males and females. Tizanidine lowered blood pressure and caused drowsiness significantly (P < 0.05) more in females than in either male groups. The effects on blood pressure were smallest in male smokers (P < 0.05).

CONCLUSIONS

Gender by itself seems to have no clinically significant effect on the pharmacokinetics of tizanidine, whereas smoking reduces plasma concentrations and effects of tizanidine. Any possible effect of gender and smoking is largely outweighed by individual variability in CYP1A2 activity due to genetic and environmental factors and in body weight. Careful dosing of tizanidine is warranted in small females, whereas male smokers can require higher than average doses.

Tobacco smoking and urinary levels of 2-amino-9H-pyrido[2,3-b]indole in men of Shanghai, China

Carcinogenic heterocyclic aromatic amines (HAA) are formed in cooked meats, poultry, and fish and arise in tobacco smoke. We measured the concentrations of four prevalent HAAs in spot urine samples collected at baseline from 170 participants of the Shanghai Cohort study, a population-based cohort study of adult men recruited during 1986 to 1989 in Shanghai, China. Sixteen (18.6%) of 86 nonsmokers were positive for urinary 2-amino-9H-pyrido[2,3-b]indole (AalphaC) versus 41 (48.8%) of 84 cigarette smokers; the difference was statistically significant (P < 0.001). The number of cigarettes smoked per day was positively and significantly related to urinary levels of AalphaC in study subjects (P < 0.001); the mean level among nonsmokers was 2.54 ng/g creatinine, whereas the means for light (1-19 cigarettes per day) and heavy (20+ cigarettes per day) smokers were 7.50 and 11.92 ng/g creatinine, respectively. 2-Amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline was undetected in the urine of the 170 subjects. Only 5 (2.9%) and 6 (3.5%) subjects, respectively, showed detectable levels of urinary 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline, and smoking status was unrelated to levels of either HAA. Quantitative measurements of HAAs in commonly eaten pork and chicken dishes in Shanghai showed low concentrations of HAAs (<1 ng/g meat). Our data indicate that AalphaC represents a major HAA exposure in adult men of Shanghai, China, and that tobacco smoke is an important point source of their AalphaC exposure.

Interaction between CYP1A2-T2467DELT polymorphism and smoking in adenocarcinoma and squamous cell carcinoma of the lung

This study aimed to identify new genetic characteristics contributing to individual susceptibility to smoke-induced lung cancer. Despite functional evidence of a possible role of cytochrome P450 1A2 (CYP1A2) in lung cancer susceptibility, no studies have evaluated the influence of CYP1A2 genotypes on lung cancer risk. We investigated the interaction between CYP1A2-T2467delT (allele*1D) polymorphism and smoking in Tunisian lung cancer cases (n=101 male smokers) separately for the histological types squamous cell carcinoma (SCC) (n=60) and adenocarcinoma (n=41), and in controls (n=98 male smokers) using a case-only study design. A significant interaction between CYP1A2-T/delT or delT/delT genotypes and tobacco consumption (pack-years) adjusted for age was evident (OR (95% CI) 7.78 (1.52-42.8)) in the SCC cases who smoked relatively less (< or =33 pack-years, I quartile value), but not in adenocarcinoma and controls. Our results suggest that CYP1A2-T2467delT polymorphism has an important role in lung carcinogenesis, especially SCC, among smokers.

CYP2A13 metabolizes the substrates of human CYP1A2, phenacetin, and theophylline

Human cytochrome CYP2A13 shows overlapping substrate specificity with CYP2A6, catalyzing the metabolism of coumarin, nicotine, cotinine, and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone. Recently, it was found that CYP2A13 could catalyze the metabolic activations of 4-aminobiphenyl and aflatoxin B(1), which are known to be catalyzed by human CYP1A2. In the present study, we investigated the substrate specificity of CYP2A13. It was shown that CYP2A13 could catalyze ethoxyresorufin O-deethylation, methoxyresorufin O-demethylation, and phenacetin O-deethylation, which are used as marker activities for human CYP1A2. Although the intrinsic clearances (V(max)/K(m)) of the two former reactions by CYP2A13 were much lower than that of CYP1A2, the value of the last reaction by CYP2A13 was 2-fold higher than that of CYP1A2. Of particular interest was that CYP2A13 has higher affinity toward phenacetin than CYP1A2. In contrast, CYP2A6 hardly catalyzed these reactions, although the amino acid identity with CYP2A13 is as high as 93.5%. Furthermore, we found that CYP2A13 can catalyze theophylline 8-hydroxylation and 3-demethylation, which are known to be mainly catalyzed by human CYP1A2, although the intrinsic clearances were approximately one-tenth that of CYP1A2. CYP2A13 would not contribute to the systemic clearance of these drugs because CYP2A13 is hardly expressed in human liver. However, it may play a role in metabolism in local tissues such as lung or trachea. In conclusion, the results of the present study could extend our understanding of the substrate specificity of CYP2A13.

cDNA cloning and characterization of feline CYP1A1 and CYP1A2

Deficiency of drug glucuronidation in the cat is one of the major reasons why this animal is highly sensitive to the side effects of drugs. The characterization of cytochrome P450 isoforms belonging to the CYP1A subfamily, which exhibit important drug oxidation activities such as activation of pro-carcinogens, was investigated. Two cDNAs, designated CYP1A-a and CYP1A-b, corresponding to the CYP1A subfamily were obtained from feline liver. CYP1A-a and CYP1A-b cDNAs comprise coding regions of 1554 bp and 1539 bp, and encode predicted amino acid sequences of 517 and 512 residues, respectively. These amino acid sequences contain a heme-binding cysteine and a conserved threonine. The cDNA identities, as well as the predicted amino acid sequences containing six substrate recognition sites, suggest that CYP1A-a and CYP1A-b correspond to CYP1A1 and CYP1A2, respectively. This was confirmed by the kinetic parameters of the arylhydrocarbon hydroxylase and 7-ethoxyresorufin O-deethylase activities of expressed CYPs in yeast AH22 cells and by the tissue distribution of each mRNA. However, theophylline 3-demethylation is believed to be catalyzed by CYP1A1 in cats, based on the high V(max) and low K(m) seen, in contrast to other animals. Because feline CYP1A2 had a higher K(m) for phenacetin O-deethylase activity with acetaminophen, which cannot be conjugated with glucuronic acid due to UDP-glucuronosyltransferase deficiency, it is supposed that the side effects of phenacetin as a result of toxic intermediates are severe and prolonged in cats.

Differential inducibility of rat pulmonary CYP1A1 by cigarette smoke and wood smoke

Mainstream cigarette smoke (CS) and wood smoke (WS) were compared in terms of their pulmonary CYP1A1 inducibility. The inducibility was assessed in pulmonary microsomes from rats exposed to freshly generated CS or WS and in rat lung explants treated with extracts of CS or WS total particulate matter (TPM). Mutagenicity in Salmonella typhimurium TA98 and TA100, an effect established for CS and WS in previous studies, was also examined as a test of the biological activity of the smoke samples in the present study. Pulmonary microsomal CYP1A1 activity (as measured by ethoxyresorufin O-deethylase), was induced 4.4-fold and 8.3-fold following exposure of rats to smoke from a single cigarette and three cigarettes, respectively, relative to the activity in control rats. The induction was paralleled by elevated CYP1A1 mRNA level (by northern blot analysis). WS, in contrast to CS, induced neither pulmonary CYP1A1 activity nor mRNA in exposed rats. CYP1A1 protein (by western blot analysis) was induced in cultured rat lung explants by extracts of CS TPM or by a high concentration (496 nM) of benzo[a]pyrene (B[a]P) but not by extracts of WS TPM or a low concentration (0.110 nM) of B[a]P. The induction by high B[a]P concentration was inhibited by extracts of CS or WS TPM, with the inhibition by extracts of WS TPM (75%) being greater than that by extracts of CS TPM (31%). Extracts of CS TPM were as mutagenic as extracts of WS TPM to Salmonella typhimurium TA98 but were more mutagenic than extracts of WS TPM to Salmonella typhimurium TA100. The results show that CS and WS are mutagenic but that WS differs from CS in its inability to induce pulmonary CYP1A1.

Influence of the genetic polymorphism in the 5′-noncoding region of the CYP1A2 gene on CYP1A2 phenotype and urinary mutagenicity in smokers

The functional significance of genetic polymorphisms on tobacco smoke-induced CYP1A2 activity was examined. The influence of three polymorphisms of the cytochrome P450 1A2 gene (CYP1A2) (-3860 G-->A (allele *1C), -2467 T-->delT (allele *1D), -163C-->A (allele *1F)), located in the 5'-noncoding promoter region of the gene, on CYP1A2 activity (measured as caffeine metabolic ratio, CMR), was studied in Caucasian current smokers (n=95). Tobacco smoke intake was calculated from the number of cigarettes/day. Also, studied was the influence of these CYP1A2 genotypes on smoking-associated urinary mutagenicity, detected in Salmonella typhimurium strain YG1024 with S9 mix, considering the urinary excretion of nicotine plus its metabolites as an internal indicator of tobacco smoke exposure. Smokers with at least one of the variant alleles CYP1A2 -3860A and -2467 delT showed a significantly increased CYP1A2 CMR (-3860 G/A versus G/G, p<0.05; -2467 delT/delT versus T/delT and T/T, p<0.01). Multiple regression analysis showed that the increase in CYP1A2 CMR (ln values) was again significantly related to the presence of CYP1A2 variants -2467delT and also to variant -163A (p<0.05), but moderately to -3860A (p=0.084). No influence of the number of cigarettes smoked per day by each subject was found. Heavy smokers (n=48, with urinary nicotine plus its metabolites>or=0.69 mg/mmol creatinine) with variant allele -2467delT or -163A had significantly increased urinary mutagenicity (p<0.01 and <0.05). CYP1A2 genetic polymorphisms are shown to influence the CYP1A2 phenotype in smokers, -2467 T-->delT having the main effect. This information is of interest for future studies assessing the possible role of tobacco smoke-inducible CYP1A2 genotypes as individual susceptibility factors in exposure to carcinogens.

Assessment of CYP1A2 Activity in Clinical Practice: Why, How, and When?

The cytochrome P450 enzyme CYP1A2 mediates the rate-limiting step in the metabolism of many drugs including theophylline, clozapine, and tacrine as well as in the bioactivation of procarcinogens. CYP1A2 activity shows both pronounced intra- and interindividual variability, which is, among other factors, related to smoking causing enzyme induction, to drug intake and to dietary factors which may result in induction or inhibition. In contrast to these exogenous factors, genetic influences on enzyme activity seem to be less pronounced. Therefore, phenotyping of CYP1A2, i.e. the determination of the actual activity of the enzyme in vivo, represents a useful approach both for scientific and clinical applications. CYP1A2 is almost exclusively expressed in the liver. Since liver tissue cannot be obtained for direct phenotyping, a probe drug which is metabolized by CYP1A2 has to be given. Proposed probe drugs include caffeine, theophylline, and melatonin. Caffeine is most often used because of the predominating role of CYP1A2 in its overall metabolism and the excellent tolerability. Various urinary, plasma, saliva, and breath based CYP1A2 caffeine metrics have been applied. While caffeine clearance is considered as the gold standard, the salivary or plasma ratio of paraxanthine to caffeine in a sample taken approximately 6 hr after a defined dose of caffeine is a more convenient, less expensive but also fully validated CYP1A2 phenotyping metric. CYP1A2 phenotyping is applied frequently in epidemiologic and drug-drug interaction studies, but its clinical use and usefulness remains to be established.

Interspecies metabolism of heterocyclic aromatic amines and the uncertainties in extrapolation of animal toxicity data for human risk assessment

Heterocyclic aromatic amines (HAAs) are potent bacterial mutagens that are formed in cooked meats, tobacco smokes condensate, and diesel exhaust. Many HAAs are carcinogenic in experimental animal models. Because of their wide-spread occurrence in the diet and environment, HAAs may contribute to some common types of human cancers. The extrapolation of animal toxicity data on HAAs to asses human health risk has many uncertainties, which can lead to tenuous risk assessment estimates. Perhaps the most critical and variable parameters in interspecies extrapolation are the effects of dose, species differences in catalytic activities of xenobiotic metabolism enzymes (XMEs), human XME polymorphisms that lead to interindividual differences in carcinogen metabolism and dietary constituents that may either augment or diminish the carcinogenic potency of these genotoxins. The impact of these parameters on the metabolism and toxicological properties of HAAS and uncertainties in extrapolation of animal toxicity data for human risk assessment are presented in this article.

Effects of Ciprofloxacin on the Stereoselective Disposition of Mexiletine in Man

Mexiletine is extensively metabolized in man, with less than 10% of the dose being excreted unchanged in urine. Clinical drug-drug interaction studies as well as in vitro drug metabolism studies suggest that CYP1A2, in addition to CYP2D6, is involved in the metabolism of mexiletine in man. Therefore, the objective of the study was to determine whether potential inhibition of CYP1A2 by the quinolone antibiotic agent ciprofloxacin would alter the stereoselective disposition of mexiletine. Nineteen healthy men (10 smokers and 9 nonsmokers) received a single 200-mg oral dose of racemic mexiletine hydrochloride on 2 occasions: once alone and once during concomitant administration of ciprofloxacin 750 mg BID (starting 3 days before and up to 2 days after the administration of mexiletine). Serial blood and urine samples were collected for 48 hours, and pharmacokinetic parameters were derived. Total clearances of R-(-)- and S-(+)-mexiletine were 42% and 63% higher in smokers compared with nonsmokers (P < 0.05). This observation is in agreement with increased clearance of mexiletine under conditions of increased CYP1A2 activity. On the other hand, ciprofloxacin administration only marginally decreased R-(-)- and S-(+)-mexiletine clearances (2 to 5 L/h; P < 0.05) secondary to a decrease in mexiletine nonrenal clearance. In conclusion, the increase in mexiletine nonrenal clearance in smokers and its decrease during the combined administration of ciprofloxacin confirm the role of CYP1A2 in the overall clearance of the drug. Nevertheless, results obtained in this study suggest that no major drug interaction is to be expected during the concomitant administration of ciprofloxacin and mexiletine in patients.

Physiologically based pharmacokinetic (PBPK) modeling of caffeine and theophylline in neonates and adults: implications for assessing children's risks from environmental agents

Children's risks can differ from those in adults for numerous reasons, one being differences in the pharmacokinetic handling of chemicals. Immature metabolism and a variety of other factors in neonates can affect chemical disposition and clearance. These factors can be incorporated into physiologically based pharmacokinetic (PBPK) models that simulate the fate of environmental toxicants in both children and adults. PBPK models are most informative when supported by empirical data, but typically pediatric pharmacokinetic data for toxicants are not available. In contrast, pharmacokinetic data in children are readily available for therapeutic drugs. The current analysis utilizes data for caffeine and theophylline, closely related xanthines that are both cytochrome P-450 (CYP) 1A2 substrates, in developing PBPK models for neonates and adults. Model development involved scale-up of in vitro metabolic parameters to whole liver and adjusting metabolic function for the ontological pattern of CYP1A2 and other CYPs. Model runs were able to simulate the large differences in half-life and clearance between neonates and adults. Further, the models were able to reproduce the faster metabolic clearance of theophylline relative to caffeine in neonates. This differential between xanthines was found to be due primarily to an extra metabolic pathway available to theophylline, back-methylation to caffeine, that is not available to caffeine itself. This pathway is not observed in adults exemplifying the importance of secondary or novel routes of metabolism in the immature liver. Greater CYP2E1 metabolism of theophylline relative to caffeine in neonates also occurs. Neonatal PBPK models developed for these drugs may be adapted to other CYP1A2 substrates (e.g., arylamine toxicants). A stepwise approach for modeling environmental toxicants in children is proposed.

Smoking impact on CYP1A2 activity in a group of patients with schizophrenia

The purpose of the present study was to assess the impact of smoking on the metabolism of psychotropic drugs in a group of patients with schizophrenia, by measuring CYP1A2 activity. This activity was assessed by the molar ratio (MR) of caffeine metabolites in urine [(AFMU+1U+1X)/17U] and saliva (17X/137X). Participants were 40 patients with schizophrenia: 30 current cigarette smokers and 10 nonsmokers. The two groups (smokers and nonsmokers) differed significantly in their ratio of men to women (83% men and 17% women were among smokers compared with 50% men and 50% women nonsmokers). No other group differences were found regarding age, level of education, PANSS, extrapyramidal symptoms, age of symptoms onset, antipsychotic doses (chloropromazine equivalents), and anticholinergic drug used. Smokers had significant higher MR in urine (P<0.001) as well as in saliva (P=0.001) than nonsmokers, suggesting a higher activity of CYP1A2 dependent on smoking. When gender was used as a covariate, the differences between the two groups remained significant for MR. Cigarette smoking may be a factor influencing the plasma levels of antipsychotics that metabolized through CYP1A2. Clinicians should weight the possibility that smoking and the subsequent modulation of antipsychotic metabolism may be the main reason of treatment resistance. Furthermore, any attempt to reduce or cease smoking in patients with schizophrenia necessitates close monitoring of drug doses, because untoward adverse effects may emerge.

Tight-binding inhibition by alpha-naphthoflavone of human cytochrome P450 1A2

Human cytochrome P450 (P450) enzymes exhibit remarkable diversity in their substrate specificities, participating in oxidation reactions of a wide range of xenobiotic drugs. Previously, we reported that alpha-naphthoflavone (ANF) is bound to the recombinant P450 1A2 tightly and stabilizes an overall enzyme conformation. The present study is designed to determine the type of P450 1A2 inhibition exerted by ANF, using two different substrates of P450 1A2, 7-ethoxycoumarin (EOC) and 7-ethoxyresorufin (EOR). ANF is generally known as a competitive inhibitor of the enzyme. However, in our tight-binding enzyme kinetics study, ANF acts as noncompetitive inhibitor in 7-ethoxycoumarin O-deethylation (ECOD) (K(i)=55.0 nM), but as competitive inhibitor in 7-ethoxyresorufin O-deethylation (EROD) (K(i)=1.4 nM). Based on homology modeling studies, ANF is positioned to bind to a hydrophobic cavity next to the active site where it may cause a direct effect on substrate binding. It is agreed with the predicted binding site of ANF in P450 3A4, in which ANF is rather known as a stimulating modulator. Our results suggest that ANF binds near the active site of P450 1A2 and exhibits differential inhibition mechanisms, possibly depending on the molecular structure of the substrate.

Genotyping of four genetic polymorphisms in the CYP1A2 gene in the Egyptian population

AIMS

The goal of this study was to determine the frequencies of CYP1A2*1C, *1D, *1E and *1F variants in the Egyptian population and compare frequencies with other populations.

METHODS

Genotyping was performed in a total of 212 unrelated Egyptian subjects using polymerase chain reaction-restriction fragment length polymorphism assay.

RESULTS

The frequencies of CYP1A2*1C, *1D, *1E and *1F variants in the Egyptian population were 0.07, 0.40, 0.03 and 0.68, respectively. The Egyptians have a lower frequency of CYP1A2*1C, and CYP1A2*1E than the Japanese (0.07 vs 0.21 and 0.03 vs 0.08, respectively), while the frequencies of CYP1A2*1D and CYP1A2*1F did not differ significantly between the two groups. CYP1A2*1F (0.68) frequency in Egyptians was identical to that observed in Caucasians (0.68 among 236 German individuals).

CONCLUSIONS

The present study is the first to describe the frequencies of four known allelic variants of CYP1A2 among the Egyptian population. CYP1A2*1C and *1E occurred at frequencies significantly lower than that in Japanese, while similar frequencies were observed for CYP1A2*1D and *1F. The CYP1A2*1F frequency appeared to be identical to that of Caucasians. This does not exclude the possibility of the presence of new mutations relatively specific to the Egyptian population that have not been identified.

Role of specific cytochrome P450 enzymes in the N-oxidation of the antiarrhythmic agent mexiletine

1. Mexiletine is extensively metabolized in man by C- and N-oxidation and the aim of the present study was to characterize major cytochrome P450 enzyme(s) involved in the formation of N-hydroxymexiletine. 2. Incubations with genetically engineered microsomes indicated that the formation rate of N-hydroxymexiletine was highest in the presence of microsomes expressing high levels of either CYP1A2 or CYP2E1 and the formation of N-hydroxymexiletine by human liver microsomes was inhibited about 40% by antibodies directed against CYP1A1/1A2 or CYP2E1. Additional incubations demonstrated that formation of N-hydroxymexiletine was decreased 47 and 51% by furafylline, 40 microm and 120 microm, respectively, and decreased 55 and 67% by alpha-naphthoflavone, 1 microm and 3 microm, respectively (all p < 0.05 versus control). 3. The formation rate of N-hydroxymexiletine in human liver microsomes was highly correlated with CYP2B6 (RS-mexiletine, r = 0.7827; R-(-)-enantiomer, r = 0.7034; S-(+)-enantiomer, r = 0.7495), CYP2E1 (S-(+)-enantiomer, r = 0.7057) and CYP1A2 (RS-mexiletine, r = 0.5334; S-(+)-enantiomer, r = 0.6035). 4. In conclusion, we have demonstrated that CYP1A2 is a major human cytochrome P450 enzyme involved in the formation of N-hydroxymexiletine. However, other cytochrome P450 enzymes (CYP2E1 and CYP2B6) also appear to play a role in the N-oxidation of this drug.

Evaluation of cytochrome P450 probe substrates commonly used by the pharmaceutical industry to study in vitro drug interactions

Pharmaceutical industry investigators routinely evaluate the potential for a new drug to modify cytochrome p450 (p450) activities by determining the effect of the drug on in vitro probe reactions that represent activity of specific p450 enzymes. The in vitro findings obtained with one probe substrate are usually extrapolated to the compound's potential to affect all substrates of the same enzyme. Due to this practice, it is important to use the right probe substrate and to conduct the experiment under optimal conditions. Surveys conducted by reviewers in CDER indicated that the most common in vitro probe reactions used by industry investigators include the following: phenacetin O-deethylation for CYP1A2, coumarin 7-hydroxylation for CYP2A6, 7-ethoxy-4-trifluoromethyl coumarin O-dealkylation for CYP2B6, tolbutamide 4'-hydroxylation for CYP2C9, S-mephenytoin 4-hydroxylation for CYP2C19, bufuralol 1'-hydroxylation for CYP2D6, chlorzoxazone 6-hydroxylation for CYP2E1, and testosterone 6 beta-hydroxylation for CYP3A4. We reviewed the validation information in the literature on these reactions and other frequently used reactions, including caffeine N3-demethylation for CYP1A2, S-mephenytoin N-demethylation for CYP2B6, S-warfarin 7'-hydroxylation for CYP2C9, dextromethorphan O-demethylation for CYP2D6, and midazolam 1'-hydroxylation for CYP3A4. The available information indicates that we need to continue the search for better probe substrates for some enzymes. For CYP3A4-based drug interactions it may be necessary to evaluate two or more probe substrates. In many cases, the probe reaction represents a particular enzyme activity only under specific experimental conditions. Investigators must consider appropriateness of probe substrates and experimental conditions when conducting in vitro drug interaction studies and when extrapolating the results to in vivo situations.

The effect of the CYP1A2 *1F mutation on CYP1A2 inducibility in pregnant women

AIMS

To investigate the influence of the CYP1A2*1F mutation on CYP1A2 activity in smoking and nonsmoking pregnant women.

METHODS

Pregnant women (n = 904) who served as control subjects in a case-control study of early fetal loss were investigated. They were phenotyped for CYP1A2 using dietary caffeine and the urinary ratio AFMU + 1X + 1 U/1,7 U. An assay for CYP1A2*1F using 5'-nuclease assay (Taqman) was developed to genotype the population.

RESULTS

The frequencies of *1 A and *1F alleles among Swedish women were 0.29 and 0.71, respectively. There was no statistically significant difference in CYP1A2 activity between the genotypes, although a trend towards enhanced activity was observed in *1F/*1F (log MRc 0.77) and *1F/*1 A (log MRc 0.82) genotypes compared with the *1 A/*1 A genotype (log MRc 0.71) (anovaP = 0.07). The mean difference between the *1 A homozygotes and the heterozygotes was 0.11 [95% confidence interval of the difference: (-0.21, -0.01)] and that between the *1 A and *1F homozygotes was 0.05 [95% confidence interval of the difference: (-0.13, 0.03)]. No significant effect (P = 0.22) of the *1F on CYP1A2 activity was observed in smokers, tested using an interaction term (smoking * genotype) in the anova model (*1F/*1F log MRc 0.79, *1F/*1 A log MRc 0.86, and *1 A/*1 A log MRc 0.73). In smokers, there was no difference in ratio between homozygotes for the *1 A and *1F alleles [mean difference -0.06; 95% confidence interval of the difference: -0.22, 0.11] or between *1 A/*1 A and *1 A/*1F genotypes [mean difference -0.13; 95% confidence interval of the difference: -0.29, 0.04].

CONCLUSIONS

The effect of the CYP1A2*1F mutation on CYP1A2 activity in smoking pregnant women could not be confirmed.

The interindividual differences in the 3-demthylation of caffeine alias CYP1A2 is determined by both genetic and environmental factors

This study investigated the role of genetic factors (CYP1A2) in caffeine metabolism. The CYP1A2 activity was determined in 378 Danish twins following oral intake of a single dose of 200 mg caffeine and subsequent determination of the caffeine ratio (AFMU+1MU+1MX)/17DMU in a 6-h urine sample. The mean (+/- SD) caffeine ratio was 5.9 +/- 3.4. The caffeine ratio was statistically significantly higher in men compared to women, in smoking men and women compared to non-smoking persons of the same gender and in women not taking oral contraceptives compared with women on oral contraceptives. Thus, we confirmed that CYP1A2 is more active in men than in women, that it is induced by smoking and inhibited by oral contraceptives. In the subsequent analysis of heritability, we included 49 monozygotic twin pairs and 34 same gender dizygotic twin pairs concordant for non-smoking and non-use of oral contraceptives. The intraclass correlation coefficient was 0.798 (95% confidence interval, 0.696-0.900) and 0.394 (95% confidence interval, 0.109-0.680) in the monozygotic and dizygotic twins, respectively. The correlation was statistically significantly higher (P = 0.0015) in the former compared with the latter. A biometrical model for the caffeine ratio including only additive genetic factors and unique environmental factors was the overall best fitting model. Estimates based on this model gave a heritability estimate of 0.725 (95% confidence interval 0.577-0.822). Unique environmental effects seem to account for the remainder 0.275 (95% confidence interval, 0.178-0.423). Our study shows that the CYP1A2 activity is mainly governed by genetic factors.

No role of protected region B of human cytochrome P4501A2 gene (CYP1A2) as an AP-1 response element

Cytochrome P4501A2 (CYP1A2) is a member of the cytochrome P450 family of isozymes involved in the phase I drug metabolism of vertebrates. CYP1A2 is responsible for the activation of a number of aromatic amines to mutagenic and carcinogenic forms. Thus, the level of CYP1A2, which varies among different populations, may determine an individual's susceptibility to these chemicals. We have previously reported on the importance of a cis element named PRB (protected region B) in the regulation of human Cytochrome P4501A2 (CYP1A2) gene, which appeared to act as a positive regulatory element. Closer examination of the PRB sequence (-2218 to -2187 bp) revealed a putative AP-1 binding site, TGACTAA, at -2212 bp (Chung and Bresnick, 1997). To elucidate the role of AP-1 in CYP1A2 regulation, we transiently overexpressed c-Jun and c-Fos transcription factors in human hepatoma HepG2 cells, and examined their influence on the CYP1A2 promoter activity by reporter gene assays. Cotransfection of the c-Jun and the c-Fos expression vectors increased the induced transactivation by five to six fold from the CYP1A2 promoter constructs. However, deletion of the PRB element did not affect the degree of activation by the c-Jun and the c-Fos. Therefore, it is unlikely that the c-Jun and the c-Fos activate the CYP1A2 promoter through this AP-1 consensus-like sequence in the PRB region.

Plasma caffeine metabolite ratio (17X/137X) in vivo associated with G-2964A and C734A polymorphisms of human CYP1A2

Either G-2964 or A734 in the human CYP1A2 gene was confirmed to be associated with high inducible enzyme activity in smokers, but not in nonsmokers. In this study, for the first time, we observed an association between phenotypes and genotypes of CYP1A2 with respect to the two genetic polymorphisms in 163 healthy Chinese volunteers living in Qidong. The ratio of plasma 17X/137X at 6 h after oral administration of 300 mg caffeine was employed in CYP1A2 phenotyping analysis, while genotyping analysis was carried out by polymerase chain reaction-restriction fragment length polymorphism. The allele frequencies of A at -2964 and A at 734 in 139 non-smoking subjects were 0.25 and 0.67, respectively. The A/A-2964C/C734, G/A-2964C/C734 or A/A-2964C/A734 genotype that was thought to have lower inducibility/activity of CYP1A2 than the other genotypes did not exist in the tested Chinese subjects. The ratio of 17X/137X was 0.46 +/- 0.26 in G/G-2964A/A734 genotypes (n = 22) and 0.36 +/- 0.19 in non-G/G-2964A/A734 (n = 117). In addition, there was significant difference between them (P = 0.036). A similar result was also achieved in 24 smokers. Since Qidong is a special region with particularly high incidence of hepatocellular carcinoma in China, the association of phenotypes with genotypes of CYP1A2 in the Qidong population might result from some inducible environmental factors such as those of cigarettes in smokers.

Interindividual variability in inhibition and induction of cytochrome P450 enzymes

Drug interactions have always been a major concern in medicine for clinicians and patients. Inhibition and induction of cytochrome P450 (CYP) enzymes are probably the most common causes for documented drug interactions. Today, many pharmaceutical companies are predicting potential interactions of new drug candidates. Can in vivo drug interactions be predicted accurately from in vitro metabolic studies? Should the prediction be qualitative or quantitative? Although some scientists believe that quantitative prediction of drug interactions is possible, others are less optimistic and believe that quantitative prediction would be very difficult. There are many factors that contribute to our inability to quantitatively predict drug interactions. One of the major complicating factors is the large interindividual variability in response to enzyme inhibition and induction. This review examines the sources that are responsible for the interindividual variability in inhibition and induction of cytochrome P450 enzymes.

Influence of ligand binding to human cytochrome P-450 1A2: conformational activation and stabilization by alpha-naphthoflavone

Human cytochrome P-450 (P-450) 1A2 expressed in Escherichia coli is readily converted into non-native cytochrome P-420 (P-420) in the presence of detergents. alpha-Naphthoflavone (ANF) has been used to prevent P-450 1A2 inactivation to P-420 during purification. However, the mechanism by which ANF modulates P-450 1A2 is not clearly understood. We observed that recombinant human P-450 1A2 prepared in the absence of ANF has an approx. 5 times higher maximum catalytic activity in the O-deethylation of 7-ethoxycoumarin than that in the presence of ANF, with the same K(m) values. The results revealed that the enzyme purified with ANF is not catalytically fully active, indicating that ANF tightly binds to the enzyme, only to be dissociated by heat denaturation. Furthermore, the inactive P-420 form of the enzyme could be reconverted to P-450 by ANF in high concentrations of detergents. The reconversion was concentration-dependent, confirming ANF-induced regeneration of active P-450 1A2. The reconversion coincided with the conformational change of the enzyme including increased alpha-helix content. The conformation of P-450 1A2 was also stabilized by ANF, resulting in an approx. 5 degrees C increase in thermal stability.