Metabolic profile of nicotine in subjects whose CYP2A6 gene is deleted

Survey of Secondhand Smoke for 3-Year-Old Children in the Current Era of New Tobacco Products

AIM

To investigate children's exposure to secondhand smoke and aerosols in the current era of new tobacco products.

METHODS

A total of 200 children were recruited from among children who had undergone health checkups for 3-year-olds at the clinic. We investigated the smoking status of their families using a questionnaire and measured urinary cotinine concentrations in their children. The children were divided into four groups based on their families' smoking status: no smokers (n = 114), exclusive combustion cigarette smokers (n = 35), both combustion cigarette and heated tobacco product smokers (n = 12) and exclusive heated tobacco product smokers (n = 39).

RESULTS

Of the 200 children, 53 (26.5%) had detectable urinary cotinine levels. Of these, 12 (6.0%) were classified as having heavy exposure to secondhand smoke (urinary cotinine concentration ≥ 5 ng/mL). Urinary cotinine concentrations among children living with exclusive heated tobacco product smokers were significantly lower than those among children living with exclusive combustion cigarette smokers, but higher than those in children whose family members did not smoke at all. Relative to exclusive combustion cigarette smokers, exclusive heated tobacco product smokers were significantly more likely to smoke indoors.

CONCLUSIONS

Secondhand aerosol exposure from heated tobacco products may be reduced but cannot be completely eliminated.

Dexpanthenol protects against nicotine-induced kidney injury by reducing oxidative stress and apoptosis through activation of the AKT/Nrf2/HO-1 pathway

Dexpanthenol (DEX), a subtype of vitamin B5, plays an important role in anabolic reactions, cellular energy and regeneration in the body. Nicotine has been shown to induce kidney damage through the mechanisms of oxidative stress and apoptosis. The purpose of this study was to investigate the potential protective effects of DEX against nicotine-induced kidney damage through modulation of the AKT/Nrf2/HO-1 signaling pathway. Male rats were intraperitoneally administered with 0.5 mg/kg/day nicotine and/or 500 mg/kg/day DEX for 8 weeks. Following administration, renal function tests were conducted on serum samples, and histopathological examinations and analysis of oxidative stress markers and antioxidant enzymes were performed on tissue samples. Protein levels of Akt, Nrf-2, HO-1, Bcl-xL, and Caspase-9 were also evaluated. Nicotine administration resulted in decreased protein levels of p-Akt, Nrf-2, HO-1, and Bcl-xL and increased Caspase-9 protein levels. In addition, nicotine administration caused an increase in MDA, TOS, and OSI levels and a decrease in GSH, GSH-Px, GST, CAT, SOD, and TAS levels. Additionally, BUN and Creatinine levels increased after nicotine administration. DEX administration positively regulated these parameters and brought them closer to control levels. Nicotine-induced kidney injury caused apoptosis and oxidative stress through Caspase-9 activation. DEX effectively prevented nicotine-induced kidney damage by increasing intracellular antioxidant levels and regulating apoptosis through Bcl-xL activation. These findings suggest that DEX has potential as a protective agent against nicotine-induced kidney damage.

Dexpanthenol exhibits antiapoptotic and anti-inflammatory effects against nicotine-induced liver damage by modulating Bax/Bcl-xL, Caspase-3/9, and Akt/NF-κB pathways

Chronic tobacco use can lead to liver damage and inflammation due to the accumulation of various toxins in the body. This study aimed to investigate the correlation between the molecular mechanisms of nicotine-induced liver injury, the caspase cascade, and the Akt/NF-κB signaling pathway, as well as the protective effects of dexpanthenol (DEX). Male rats were subjected to intraperitoneal injections of nicotine at a concentration of 0.5 mg/kg/day and/or DEX at a concentration of 500 mg/kg/day for 8 weeks. After the treatment period, liver function tests were conducted on serum samples, and tissue samples were analyzed for protein levels of Akt, NF-κB, Bax, Bcl-xL, Caspase-3, and Caspase-9, along with histopathological changes. Additionally, assessments of oxidative stress markers and proinflammatory cytokines were carried out. Nicotine administration led to elevated levels of IL-6, IL-1β, MDA, TOS, and oxidative stress index, accompanied by decreased TAS levels. Moreover, nicotine exposure reduced the p-Akt/Akt ratio, increased NF-κB, Bax, Caspase-3, and Caspase-9 protein levels, and decreased the antiapoptotic protein Bcl-xL levels. DEX treatment significantly mitigated these effects, restoring the parameters to levels comparable to those of the control group. Nicotine-induced liver injury resulted in oxidative stress, inflammation, and apoptosis, mediated by Bax/Bcl-xL, Caspase-3, Caspase-9, and Akt/NF-κB pathways. Conversely, DEX effectively attenuated nicotine-induced liver injury by modulating apoptosis through NF-κB, Caspase-3, Caspase-9, Bax inhibition, and Bcl-xL activation.

Impact of Genetic Variants in the Nicotine Metabolism Pathway on Nicotine Metabolite Levels in Smokers

BACKGROUND

Nicotine metabolism is a major factor in nicotine dependence, with approximately 70% to 80% of nicotine metabolized to cotinine in Caucasians. Cotinine formation is catalyzed primarily by CYP2A6, which also converts cotinine to trans-3'-hydroxycotinine (3HC). The goal of the present study was to examine the effects of CYP2A6 deficiency on nicotine metabolism profiles in vivo and the importance of genetic variants in nicotine-metabolizing enzyme genes on urinary nicotine metabolites levels.

METHODS

Urine samples from 722 smokers who participated in the Singapore Chinese Health Study were analyzed using UPLC-MS/MS to detect nicotine and eight of its urinary metabolites, and a total of 58 variants in 12 genes involved in nicotine metabolism were investigated in 475 of these subjects with informative genotyping data.

RESULTS

Urine samples stratified by the ratio of 3HC/cotinine exhibited a 7-fold increase in nicotine-N'-oxide, a 6-fold increase in nicotine-Glucuronide (Gluc), and a 5-fold decrease in 3HC-Gluc when comparing the lower versus upper 3HC/cotinine ventiles. Significant (P < 0.0001) associations were observed between functional metabolizing enzyme genotypes and levels of various urinary nicotine metabolites, including CYP2A6 genotype and levels of nicotine, nicotine-Gluc, nicotine-N'-oxide and 3HC, UGT2B10 genotype and levels of cotinine, nicotine-Gluc and cotinine-Gluc, UGT2B17 genotype and levels of 3HC-Gluc, FMO3 genotype and levels of nicotine-N'-oxide, and CYP2B6 genotype and levels of nicotine-N'-oxide and 4-hydroxy-4-(3-pyridyl)-butanoic acid.

CONCLUSIONS

These data suggest that several pathways are important in nicotine metabolism.

IMPACT

Genotype differences in several nicotine-metabolizing enzyme pathways may potentially lead to differences in nicotine dependence and smoking behavior and cessation.

CYP2C19 Plays a Major Role in the Hepatic N-Oxidation of Cotinine

The primary mode of metabolism of nicotine is via the formation of cotinine by the enzyme CYP2A6. Cotinine undergoes further CYP2A6-mediated metabolism by hydroxylation to 3-hydroxycotinine and norcotinine, but can also form cotinine-N-glucuronide and cotinine-N-oxide (COX). The goal of this study was to investigate the enzymes that catalyze COX formation and determine whether genetic variation in these enzymes may affect this pathway. Specific inhibitors of major hepatic cytochrome P450 (P450) enzymes were used in cotinine-N-oxidation reactions using pooled human liver microsomes (HLMs). COX formation was monitored by ultrahigh-pressure liquid chromatography-tandem mass spectrometry and enzyme kinetic analysis was performed using microsomes from P450-overexpressing human embryonic kidney 293 (HEK293) cell lines. Genotype-phenotype analysis was performed in a panel of 113 human liver specimens. Inhibition of COX formation was only observed in HLMs when using inhibitors of CYP2A6, CYP2B6, CYP2C19, CYP2E1, and CYP3A4. Microsomes from cells overexpressing CYP2A6 or CYP2C19 exhibited similar N-oxidation activity against cotinine, with maximum reaction rate over Michaelis constant values (intrinsic clearance) of 4.4 and 4.2 nL/min/mg, respectively. CYP2B6-, CYP2E1-, and CYP3A4-overexpressing microsomes were also active in COX formation. Significant associations (P < 0.05) were observed between COX formation and genetic variants in CYP2C19 (*2 and *17 alleles) in HLMs. These results demonstrate that genetic variants in CYP2C19 are associated with decreased COX formation, potentially affecting the relative levels of cotinine in the plasma or urine of smokers and ultimately affecting recommended smoking cessation therapies. SIGNIFICANCE STATEMENT: This study is the first to elucidate the enzymes responsible for cotinine-N-oxide formation and genetic variants that affect this biological pathway. Genetic variants in CYP2C19 have the potential to modify nicotine metabolic ratio in smokers and could affect pharmacotherapeutic decisions for smoking cessation treatments.

CYP2A6 gene polymorphism and severity of coronary atherosclerosis in Indonesian male smokers: A pilot study

Nicotine is a toxic alkaloid known to be responsible for the addictive feature of cigarettes. CYP2A6 genetic polymorphism among individuals was suspected to explain the relationship between cigarette smoking and related diseases. CYP2A6 works to slow nicotine metabolism and thus maintain a more prolonged nicotine concentration and increase nicotine exposure to the blood. We aimed to investigate the correlation between the CYP2A6 gene with the severity of coronary atherosclerosis. This cross-sectional study was conducted from April to July 2010 in Makassar Cardiac Centre, Dr Wahidin Sudirohusodo Hospital, Indonesia. Sixty-four male active smokers at the age of ≥45 years, diagnosed with coronary artery disease (CAD), were recruited and asked to smoke the usual number of cigarettes in the last 1 month prior to blood collection for CYP2A6 genotyping. Spearman correlation was performed to analyze the association between the allele variants and coronary stenosis degree, adjusted for CAD risk factors. Furthermore, we estimated the risk ratio to quantify the correlation. Of the 64 male smokers with CAD, the mean duration of smoking was 36.9 ± 8.6 years, and 49 (76.6%) were heavy smokers with >20 cigarettes per day. All 128 alleles were observed. Our results showed that all participants with CYP2A6 variants had a significant correlation with severe coronary artery stenosis (P = .006). Thus, this study suggests that the mutant CYP2A6 gene allele significantly increased the risk of having severe coronary stenosis 1.2 times higher compared to the wild type. This pilot study showed that CYP2A6 gene has an influential role in atherosclerotic development in male smokers. However, our findings should be confirmed with further more extensive studies.

Effects of excessive alcohol drinking on nicotine biotransformation in rats

Alcohol and nicotine (tobacco smoke) are often used together, and taking both addictive substances is associated with an increased risk of certain diseases. It is extremely important to understand the pharmacodynamic and pharmacokinetic mechanisms of the interaction between nicotine and ethanol, which are still not fully understood. The study aimed to evaluate the influence of chronic alcohol consumption on nicotine biotransformation in ethanol-preferring and non-preferring male and female rats. Rats were divided into four groups depending on their alcohol preferences and gender. Nicotine, nornicotine, nicotine N-oxide, cotinine, trans-3'-hydroxycotinine, and cotinine N-oxide in rats plasma were determined by LC-MS/MS after five days of exposure to tobacco smoke. A non-compartmental analysis of nicotine and its metabolites was used for pharmacokinetic parameters calculation. Our experimental results showed that the rate of nicotine elimination depends on gender, regardless of alcohol preferences (significantly slower in females than in males). Mean residence timeof nornicotine, cotinine, and trans-3'-hydroxycotinine were significantly higher in alcohol-preferring male rats than in alcohol preferring female rats. In non-alcohol preferring female rats compared to ethanol-preferring female rats, significantly more nicotine N-oxide (fivefold) and trans-3'-hydroxycotinine (twofold) reached the general circulation unchanged. Drinking ethanol influenced the elimination of nornicotine and cotinine in male rats. Ethanol consumption was identified as a modifier of nicotine pharmacokinetics and this was gender-dependent.

Biochemistry of nicotine metabolism and its relevance to lung cancer

Nicotine is the key addictive constituent of tobacco. It is not a carcinogen, but it drives smoking and the continued exposure to the many carcinogens present in tobacco. The investigation into nicotine biotransformation has been ongoing for more than 60 years. The dominant pathway of nicotine metabolism in humans is the formation of cotinine, which occurs in two steps. The first step is cytochrome P450 (P450, CYP) 2A6–catalyzed 5′-oxidation to an iminium ion, and the second step is oxidation of the iminium ion to cotinine. The half-life of nicotine is longer in individuals with low P450 2A6 activity, and smokers with low activity often decrease either the intensity of their smoking or the number of cigarettes they use compared with those with “normal” activity. The effect of P450 2A6 activity on smoking may influence one's tobacco-related disease risk. This review provides an overview of nicotine metabolism and a summary of the use of nicotine metabolite biomarkers to define smoking dose. Some more recent findings, for example, the identification of uridine 5′-diphosphoglucuronosyltransferase 2B10 as the catalyst of nicotine N-glucuronidation, are discussed. We also describe epidemiology studies that establish the contribution of nicotine metabolism and CYP2A6 genotype to lung cancer risk, particularly with respect to specific racial/ethnic groups, such as those with Japanese, African, or European ancestry. We conclude that a model of nicotine metabolism and smoking dose could be combined with other lung cancer risk variables to more accurately identify former smokers at the highest risk of lung cancer and to intervene accordingly.

Pharmacogenetics factors influencing smoking cessation success; the importance of nicotine metabolism

Introduction: Smoking remains a worldwide epidemic, and despite an increase in public acceptance of the harms of tobacco use, it remains the leading cause of preventable death. It is estimated that up to 70% of all smokers express a desire to quit, but only 3-5% of them are successful.Areas covered: The goal of this review was to evaluate the current status of smoking cessation treatments and the feasibility of implementing personalized-medicine approaches to these pharmacotherapies. We evaluated the genetics associated with higher levels of nicotine addiction and follow with an analysis of the genetic variants that affect the nicotine metabolic ratio (NMR) and the FDA approved treatments for smoking cessation. We also highlighted the gaps in the process of translating current laboratory understanding into clinical practice, and the benefits of personalized treatment approaches for a successful smoking cessation strategy.Expert opinion: Evidence supports the use of tailored therapies to ensure that the most efficient treatments are utilized in an individual's smoking cessation efforts. An understanding of the genetic effects on the efficacy of individualized smoking cessation pharmacotherapies is key to smoking cessation, ideally utilizing a polygenetic risk score that considers all genetic variation.

Molecular mechanisms for nicotine intoxication

Nicotine, one of the more than 4700 ingredients in tobacco smoke, is a neurotoxin and once used as pesticides in agriculture. Although its use in agriculture is prohibited in many countries, nicotine intoxication is still a problem among the workers in tobacco farms, and young children as well as adults due to the accidental or suicidal ingestions of nicotine products. Understanding the mechanism of nicotine intoxication is important not only for the prevention and treatment but also for the appropriate regulatory approaches. Here, we review pharmacokinetics of nicotine and the molecular mechanisms for acute and chronic intoxication from nicotine that might be relevant to the central and the peripheral nervous system. We include green tobacco sickness, acute intoxication from popular nicotine products, circadian rhythm changes, chronic intoxication from nicotine through prenatal nicotine exposure, newborn behaviors, and sudden infant death syndrome.

Nicotine-N'-Oxidation by Flavin Monooxygenase Enzymes

BACKGROUND

The major mode of metabolism of nicotine is by hydroxylation via cytochrome P450 (CYP) 2A6, but it can also undergo glucuronidation by UDP-glucuronosyltransferases and oxidation by flavin monooxygenases (FMO). The goal of this study was to examine the potential importance of FMOs in nicotine metabolism and assess the potential impact of missense polymorphisms in active FMOs on nicotine-N'-oxide (NOX) formation.

METHODS

Urine samples from 106 current Chinese smokers were analyzed for nicotine metabolites by mass spectrometry. Wild-type FMOs 1-5 and their most prevalent nonsynonymous variants were cloned and overexpressed in HEK293 cells, and were tested in oxidation reactions against nicotine.

RESULTS

A strong inverse correlation was observed between the ratio of urinary 3'-hydroxycotinine/cotinine, a measure of CYP2A6 activity, and the urinary levels of NOX alone (r = -0.383; P < 0.001) or NOX measured as a ratio of total nicotine metabolites (r = -0.414; P < 0.001) in smokers. In addition to FMO1 and FMO3, the functional FMO2^427Q isoform was active against nicotine, whereas FMO4 and FMO5 exhibited low activity against nicotine (K _m > 5.0 mmol/L). Significant (P < 0.05) decreases in N'-oxidation activity (V _max/K _m) were observed for the FMO1^I303V, FMO3^N61S, FMO3^D132H, FMO3^V257M, and FMO3^E308G variants in vitro when compared with their respective wild-type isoforms; the truncated FMO2^Q472stop isoform exhibited no enzyme activity.

CONCLUSIONS

These data indicate that increases in nicotine-N'-oxidation occur in subjects with deficient CYP2A6 activity, and that several FMO enzymes are active in nicotine-N'-oxidation.

IMPACT

Several common missense FMO variants are associated with altered enzyme activity against nicotine and may play an important role in nicotine metabolism in low-CYP2A6 activity subjects.

Wastewater-based epidemiology in low Human Development Index states: bias in consumption monitoring of illicit drugs

Wastewater-based epidemiology is a promising approach worldwide, and its application is currently being developed in non-advanced economies. This technology, based on known toxicokinetic data initially used to detect illicit drugs in well-managed and maintained local sewer networks, has been extended to assess other products such as pesticides, alcohol, flame retardants, nicotine, and other substances. This technology is also used in countries with non-advanced economies. The present review aims to support future wastewater-based epidemiology in such countries by providing toxicokinetic data for locally used narcotic drugs that are expected or known to be emerging in developed countries, outlining the excretion differences due to human polymorphism, and summarising the practical obstacles due to the coverage, maintenance efficiency, or type of local sewage network.Case study feedback from Martinique is presented as an example; the Martinique field study complies with the Organisation for Economic Co-operation and Development standards for health issues, but not with regard to population and urban dynamics.

A Comparison of Direct and Indirect Analytical Approaches to Measuring Total Nicotine Equivalents in Urine

Background: Total nicotine equivalents (TNE), the sum of nicotine and metabolites in urine, is a valuable tool for evaluating nicotine exposure. Most methods for measuring TNE involve two-step enzymatic hydrolysis for indirect quantification of glucuronide metabolites. Here, we describe a rapid, low-cost direct LC/MS assay.Methods: In 139 smokers' urine samples, Bland-Altman, correlation, and regression analyses were used to investigate differences in quantification of nicotine and metabolites, TNE, and nicotine metabolite ratio (NMR) between direct and indirect LC/MS methods. DNA from a subset (n = 97 smokers) was genotyped for UGT2B10*2 and UGT2B17*2, and the known impact of these variants was evaluated using urinary ratios determined by the direct versus indirect method.Results: The direct method showed high accuracy (0%-9% bias) and precision (3%-14% coefficient of variation) with similar distribution of nicotine metabolites to literary estimates and good agreement between the direct and indirect methods for nicotine, cotinine, and 3-hydroxycotinine (ratios 0.99-1.07), but less agreement for their respective glucuronides (ratios 1.16-4.17). The direct method identified urinary 3HC+3HC-GLUC/COT as having the highest concordance with plasma NMR and provided substantially better estimations of the established genetic impact of glucuronidation variants compared with the indirect method.Conclusions: Direct quantification of nicotine and metabolites is less time-consuming and less costly, and provides accurate estimates of nicotine intake, metabolism rate, and the impact of genetic variation in smokers.Impact: Lower cost and maintenance combined with high accuracy and reproducibility make the direct method ideal for smoking biomarker, NMR, and pharmacogenomics studies. Cancer Epidemiol Biomarkers Prev; 27(8); 882-91. ©2018 AACR.

P450 Pharmacogenetics in Indigenous North American Populations

Indigenous North American populations, including American Indian and Alaska Native peoples in the United States, the First Nations, Métis and Inuit peoples in Canada and Amerindians in Mexico, are historically under-represented in biomedical research, including genomic research on drug disposition and response. Without adequate representation in pharmacogenetic studies establishing genotype-phenotype relationships, Indigenous populations may not benefit fully from new innovations in precision medicine testing to tailor and improve the safety and efficacy of drug treatment, resulting in health care disparities. The purpose of this review is to summarize and evaluate what is currently known about cytochrome P450 genetic variation in Indigenous populations in North America and to highlight the importance of including these groups in future pharmacogenetic studies for implementation of personalized drug therapy.

A CYP2B6-humanized mouse model and its potential applications

CYP2B6 is a human microsomal cytochrome P450 enzyme with broad substrate selectivity. CYP2B6 is the only functional member of the human CYP2B gene subfamily, which differs from the situation in rodents, such as mouse, where multiple functional Cyp2b genes are expressed. Recent studies with Cyp2b knockout or knockdown mouse models have yielded insights into the in vivo roles of mouse CYP2B enzymes in drug disposition and xenobiotic toxicity. A CYP2B6-humanized mouse model (CYP2A13/2B6/2F1-transgenic/Cyp2abfgs-null), which expresses human CYP2B6 in the liver, and human CYP2A13 and CYP2F1 in the respiratory tract, but not any of the mouse Cyp2b genes, has also been established. In the CYP2B6-humanized mouse, the CYP2B6 transgene is expressed primarily in the liver, where it was found to be active toward prototype CYP2B6 substrate drugs. The regulatory elements of the CYP2B6 transgene appear to be compatible with mouse nuclear receptors that mediate CYP2B induction. Therefore, the CYP2B6-humanized mouse is a valuable animal model for studying the impact of CYP2B6 expression or induction on drug metabolism, drug efficacy, drug-drug interaction, and drug/xenobiotic toxicity. In this mini-review, we provide a brief background on CYP2B6 and the Cyp2b-knockout and CYP2B6-humanized mice, and discuss the potential applications and limitations of the current models.

Effect of UGT2B10, UGT2B17, FMO3, and OCT2 genetic variation on nicotine and cotinine pharmacokinetics and smoking in African Americans

OBJECTIVES

Nicotine metabolism rates differ considerably among individuals, even after controlling for variation in the major nicotine-metabolizing enzyme, CYP2A6. In this study, the impact of genetic variation in alternative metabolic enzymes and transporters on nicotine and cotinine (COT) pharmacokinetics and smoking was investigated.

METHODS

We examined the impact of UGT2B10, UGT2B17, FMO3, NAT1, and OCT2 variation on pharmacokinetics and smoking (total nicotine equivalents and topography) before and after stratifying by CYP2A6 genotype in 60 African American (AA) smokers who received a simultaneous intravenous infusion of deuterium-labeled nicotine and COT.

RESULTS

Variants in UGT2B10 and UGT2B17 were associated with urinary glucuronidation ratios (glucuronide/free substrate). UGT2B10 rs116294140 was associated with significant alterations in COT and modest alterations in nicotine pharmacokinetics. These alterations, however, were not sufficient to change nicotine intake or topography. Neither UGT2B10 rs61750900, UGT2B17*2, FMO3 rs2266782, nor NAT1 rs13253389 altered nicotine or COT pharmacokinetics among all individuals (n=60) or among individuals with reduced CYP2A6 activity (n=23). The organic cation transporter OCT2 rs316019 significantly increased nicotine and COT Cmax (P=0.005, 0.02, respectively) and decreased nicotine clearance (P=0.05). UGT2B10 rs116294140 had no significant impact on the plasma or urinary trans-3'-hydroxycotinine/COT ratio, commonly used as a biomarker of CYP2A6 activity.

CONCLUSION

We found that polymorphisms in genes other than CYP2A6 represent minor sources of variation in nicotine pharmacokinetics, insufficient to alter smoking in AAs. The change in COT pharmacokinetics with UGT2B10 rs116294140 highlights the UGT2B10 gene as a source of variability in COT as a biomarker of tobacco exposure among AA smokers.

Nicotine Metabolism and Smoking: Ethnic Differences in the Role of P450 2A6

Nicotine is the primary addictive agent in tobacco, and P450 2A6 (gene name: CYP2A6) is the primary catalyst of nicotine metabolism. It was proposed more than 20 years ago that individuals who metabolize nicotine poorly would smoke less, either fewer cigarettes per day or less intensely per cigarette, compared to smokers who metabolize nicotine more efficiently. These poor metabolizers would then be less likely to develop lung cancer due to their lower exposure to the many carcinogens delivered with nicotine in each puff of smoke. Numerous studies have reported that smokers who carry reduced activity or null CYP2A6 alleles do smoke less. Yet only in Asian populations, both Japanese and Chinese, which have a high prevalence of genetic variants, has a link between CYP2A6, smoking dose, and lung cancer been established. In other ethnic groups, it has been challenging to confirm a direct link between P450 2A6-mediated nicotine metabolism and the risk of lung cancer. This challenge is due in part to the difficulty in accurately quantifying smoking dose and accurately predicting or measuring P450 2A6-mediated nicotine metabolism. Biomarkers of nicotine metabolism and smoking exposure, including the ratio of trans-3-hydroxycotine to cotinine, a measure of P450 2A6 activity and plasma cotinine, or urinary total nicotine equivalents (the sum of nicotine and six metabolites) as measures of exposure are useful for addressing this challenge. However, to take full advantage of these biomarkers in the study of ethnic/racial differences in the risk of lung cancer requires the complete characterization of nicotine metabolism across ethnic/racial groups. Variation in metabolism pathways, other than those catalyzed by P450 2A6, can impact biomarkers of both nicotine metabolism and dose. This is clearly important for smokers with low levels of UGT2B10-catalyzed nicotine and cotinine glucuronidation because the UGT2B10 genotype influences plasma cotinine levels. Cotinine is not glucuronidated in 15% of African American smokers (compared to 1% of Whites) due to the prevalence of a UGT2B10 splice variant. This variant contributes significantly to the higher plasma cotinine levels per cigarette in this group and may also influence the accuracy of the 3HCOT to cotinine ratio as a measure of P450 2A6 activity.

Drug metabolism by flavin-containing monooxygenases of human and mouse

INTRODUCTION

Flavin-containing monooxygenases (FMOs) play an important role in drug metabolism. Areas covered: We focus on the role of FMOs in the metabolism of drugs in human and mouse. We describe FMO genes and proteins of human and mouse; the catalytic mechanism of FMOs and their significance for drug metabolism; differences between FMOs and CYPs; factors contributing to potential underestimation of the contribution of FMOs to drug metabolism; the developmental and tissue-specific expression of FMO genes and differences between human and mouse; and factors that induce or inhibit FMOs. We discuss the contribution of FMOs of human and mouse to the metabolism of drugs and how genetic variation of FMOs affects drug metabolism. Finally, we discuss the utility of animal models for FMO-mediated drug metabolism in humans. Expert opinion: The contribution of FMOs to drug metabolism may be underestimated. As FMOs are not readily induced or inhibited and their reactions are generally detoxifications, the design of drugs that are metabolized predominantly by FMOs offers clinical advantages. Fmo1^(-/-),Fmo2^(-/-),Fmo4^(-/-) mice provide a good animal model for FMO-mediated drug metabolism in humans. Identification of roles for FMO1 and FMO5 in endogenous metabolism has implications for drug therapy and initiates an exciting area of research.

In vivo modulation of the behavioral effects of nicotine by the coumarins xanthotoxin, bergapten, and umbelliferone

RATIONALE

Nicotine, a dominant alkaloid found in tobacco, is responsible for physical dependence, as well as addiction to cigarette smoking; consequently, smoking cessation is a very difficult process. Hepatic cytochrome P-450 2A6 (CYP2A6) is involved in the 70-80 % of the initial metabolism of nicotine and its co-metabolites. As this metabolism is slowed by inhibitors of CYP2A6, this kind of enzymatic inhibition has been proposed as a novel target for smoking cessation.

OBJECTIVES

Nicotine administered alone improved memory acquisition and consolidation as well as exerted antidepressive activity in animal models. These effects persist for 24 h. However, they are completely extinguished 48 h after administration.

METHODS

To investigate if the coumarins prolong the behavioral effects of nicotine, the forced swimming test (FST)-animal models of depression, and passive avoidance (PA) test-memory and learning paradigm were used.

RESULTS

This study revealed that three CYP2A6 inhibitors: two furanocoumarins, xanthotoxin (15 mg/kg) and bergapten (25 mg/kg), and the simple coumarin umbelliferone (25 mg/kg), prolonged the antidepressive and procognitive effects of nicotine.

CONCLUSIONS

These natural products may offer a new approach to the treatment of nicotinism as antidepressant and memory improvement actions are one of the main factors of nicotine dependence.

A Derivative Method with Free Radical Oxidation to Predict Resveratrol Metabolites by Tandem Mass Spectrometry

In this study, we demonstrated an oxidative method with free radical to generate 3,5,4′-trihydroxy-trans-stilbene (trans-resveratrol) metabolites and detect sequentially by an autosampler coupling with liquid chromatography electrospray ionization tandem mass spectrometer (LC-ESI–MS/MS). In this oxidative method, the free radical initiator, ammonium persulfate (APS), was placed in a sample bottle containing resveratrol to produce oxidative derivatives, and the reaction progress was tracked by autosampler sequencing. Resveratrol, a natural product with purported cancer preventative qualities, produces metabolites including dihydroresveratrol, 3,4′-dihydroxy-trans-stilbene, lunularin, resveratrol monosulfate, and dihydroresveratrol monosulfate by free radical oxidation. Using APS free radical, the concentrations of resveratrol derivatives differ as a function of time. Besides simple, convenient and time- and labor saving, the advantages of free radical oxidative method of its in situ generation of oxidative derivatives followed by LC-ESI–MS/MS can be utilized to evaluate different metabolites in various conditions.

Environmental tobacco exposure is associated with vaccine modified measles in junior high school students

Vaccine modified measles (VMM) affects individuals with attenuated vaccine induced immunity. An outbreak of measles occurred in a junior high school, starting from an unvaccinated eighth-grade student who developed natural measles and affected a majority of students who were immunized with a low potent strain of measles vaccine (TD97). To determine whether environmental tobacco smoke (ETS) exposure was associated with the development of VMM in this population, a questionnaire was used asking whether students had VMM symptoms during the outbreak and the smoking status of family members. VMM was defined in the study population as occurrence of fever and/or erythema, along with documented history of measles vaccination. A total of 513 students (85.9%) responded. Overall, the presence of in-house smokers did not differ between VMM students (49.3%) and non-VMM students (50.2%). However, in the ninth grade, presence of an in-house smoker was significantly higher in the family of VMM students (54.0%) than in non-VMM students (36.6%) (P = 0.044). Urinary cotinine levels were also measured in selected students (n = 37). Among families with at least one smoker, urinary cotinine levels were significantly higher in VMM students than in non-VMM students (P = 0.032). Furthermore, a multivariable logistic regression analysis showed that a high urinary cotinine level (>10 ng/mg creatinine; 13.5 percentile) was associated with the development of VMM. Our findings suggest that a high level of ETS exposure may be associated with an increased risk of VMM in a population with attenuated vaccine induced immunity against measles.

CYP2A6 Polymorphisms May Strengthen Individualized Treatment for Nicotine Dependence

Each CYP2A6 gene variant metabolizes nicotine differently depending on its enzymatic activities. The normal nicotine metabolizer CYP2A6(*)1A is associated with high scores of nicotine dependence (5-10) on the Fagerström Test for Nicotine Dependence (FTND) scale because it encodes for enzymes that catalyze nicotine 100%. Slow nicotine metabolizers (i.e., CYP2A6(*)1H, CYP2A6(*)4A, CYP2A6(*)9, and CYP2A6(*)12A) are associated with underrated nicotine metabolizing activity (50%-75%), linking them to low scores for nicotine dependence (0-4) on the FTND scale. In a clinical trial involving the use of bupropion, people who were carriers of slow nicotine metabolizers were found to have a tendency to maintain abstinence 1.7 times longer than people with normal nicotine metabolizers. An overview of CYP2A6 polymorphism enzymatic activities in nicotine dependence etiology and treatment revealed that slow nicotine metabolizers may strengthen the individualized treatment of nicotine dependence.

Characterization of CYP2B6 in a CYP2B6-humanized mouse model: inducibility in the liver by phenobarbital and dexamethasone and role in nicotine metabolism in vivo

The aim of this study was to further characterize the expression and function of human CYP2B6 in a recently generated CYP2A13/2B6/2F1-transgenic (TG) mouse model, in which CYP2B6 is expressed selectively in the liver. The inducibility of CYP2B6 by phenobarbital (PB) and dexamethasone (DEX), known inducers of CYP2B6 in human liver, was examined in the TG mice, as well as in TG/Cyp2abfgs-null (or "CYP2B6-humanized") mice. Hepatic expression of CYP2B6 mRNA and protein was greatly induced by PB or DEX treatment in both TG and TG/Cyp2abfgs-null mice. Function of the transgenic CYP2B6 was first studied using bupropion as a probe substrate. In PB-treated mice, the rates of hepatic microsomal hydroxybupropion formation (at 50 μM bupropion) were >4-fold higher in TG/Cyp2abfgs-null than in Cyp2abfgs-null mice (for both male and female mice); the rate difference was accompanied by a 5-fold higher catalytic efficiency in the TG/Cyp2abfgs-null mice and was abolished by an antibody to CYP2B6. The ability of CYP2B6 to metabolize nicotine was then examined, both in vitro and in vivo. The rates of hepatic microsomal cotinine formation from nicotine were significantly higher in TG/Cyp2abfgs-null than in Cyp2abfgs-null mice, pretreated with PB or DEX. Furthermore, systemic nicotine metabolism was faster in TG/Cyp2abfgs-null than in Cyp2abfgs-null mice. Thus, the transgenic CYP2B6 was inducible and functional, and, in the absence of mouse CYP2A and CYP2B enzymes, it contributed to nicotine metabolism in vivo. The CYP2B6-humanized mouse will be valuable for studies on in vivo roles of hepatic CYP2B6 in xenobiotic metabolism and toxicity.

Pharmacogenetics of nicotine and associated smoking behaviors

This chapter summarizes genetic factors that contribute to variation in nicotine pharmacokinetics and nicotine's pharmacological action in the central nervous system (CNS), and how this in turn influences smoking behaviors. Nicotine, the major psychoactive compound in cigarette smoke, is metabolized by a number of enzymes, including CYP2A6, CYP2B6, FMOs, and UGTs, among others. Variation in the genes encoding these enzymes, in particular CYP2A6, can alter the rate of nicotine metabolism and smoking behaviors. Faster nicotine metabolism is associated with higher cigarette consumption and nicotine dependence, as well as lower quit rates. Variation in nicotine's CNS targets and downstream signaling pathways can also contribute to interindividual differences in smoking patterns. Binding of nicotine to neuronal nicotinic acetylcholine receptors (nAChRs) mediates the release of several neurotransmitters including dopamine and serotonin. Genetic variation in nAChRs, and in transporter and enzyme systems that leads to altered CNS levels of dopamine and serotonin, is associated with a number of smoking behaviors. To date, the precise mechanism underpinning many of these findings remains unknown. Considering the complex etiology of nicotine addiction, a more comprehensive approach that assesses the contribution of multiple gene variants, and their interaction with environmental factors, will likely improve personalized therapeutic approaches and increase smoking cessation rates.

The contribution of common genetic variation to nicotine and cotinine glucuronidation in multiple ethnic/racial populations

BACKGROUND

The lung cancer risk of smokers varies by race/ethnicity even after adjustment for smoking. Evaluating the role of genetics in nicotine metabolism is likely important in understanding these differences, as disparities in risk may be related to differences in nicotine dose and metabolism.

METHODS

We conducted a genome-wide association study in search of common genetic variants that predict nicotine and cotinine glucuronidation in a sample of 2,239 smokers (437 European Americans, 364 African Americans, 453 Latinos, 674 Japanese Americans, and 311 Native Hawaiians) in the Multiethnic Cohort Study. Urinary concentration of nicotine and its metabolites were determined.

RESULTS

Among 11,892,802 variants analyzed, 1,241 were strongly associated with cotinine glucuronidation, 490 of which were also associated with nicotine glucuronidation (P < 5×10(-8)). The vast majority were within chromosomal region 4q13, near UGT2B10. Fifteen independent and globally significant SNPs explained 33.2% of the variation in cotinine glucuronidation, ranging from 55% for African Americans to 19% for Japanese Americans. The strongest single SNP association was for rs115765562 (P = 1.60 × 10(-155)). This SNP is highly correlated with a UGT2B10 splice site variant, rs116294140, which together with rs6175900 (Asp67Tyr) explains 24.3% of the variation. The top SNP for nicotine glucuronidation (rs116224959, P = 2.56 × 10(-43)) was in high LD (r(2) = 0.99) with rs115765562.

CONCLUSIONS

Genetic variation in UGT2B10 contributes significantly to nicotine and cotinine glucuronidation but not to nicotine dose.

IMPACT

The contribution of genetic variation to nicotine and cotinine glucuronidation varies significantly by racial/ethnic group, but is unlikely to contribute directly to lung cancer risk.

Online monitoring oxidative products and metabolites of nicotine by free radicals generation with Fenton reaction in tandem mass spectrometry

In general, over 70% absorbed nicotine is metabolized to cotinine and trans-3′-hydroxycotinine by cytochrome oxidase P450, and nicotine is also a major addictive and the psychoactive component in cigarettes. As a xenobiotic metabolism, hydrophobic compounds are usually converted into more hydrophilic products through enzyme systems such as cytochrome oxidase P450, sulfotransferases, and UDP-glucuronosyltransferases to deliver drug metabolites out of the cell during the drug metabolic process. In this study, an electrodeless electrochemical oxidation (EEO) reaction via Fenton reaction by producing free radical to react with nicotine to immediately monitor the oxidative products and metabolic derivatives of nicotine by tandem mass spectrometer (MS) is done. Fenton reaction generates free radicals via ferrous ion (Fe²⁺) and hydrogen peroxide (H₂O₂) to oxidize DNA and to degrade proteins in cells. In the EEO method, the oxidative products of nicotine including cotinine, cotinine-N-oxide, trans-3′-hydroxycotinine, nornicotine, norcotinine, 4-oxo-4-(3-pyridyl)-butanoic acid, 4-hydroxy-4-(3-pyridyl)-butanoic acid, and nicotine-N′-oxide were detected by tandem mass spectrometer to simulate the changes of nicotine and its derivatives in a time-dependent manner.

Effects upon in-vivo nicotine metabolism reveal functional variation in FMO3 associated with cigarette consumption

BACKGROUND

Flavin-containing monooxygenases (FMO) catalyze the metabolism of nucleophilic heteroatom-containing drugs and xenobiotics, including nicotine. Rare mutations in FMO3 are responsible for defective N-oxidation of dietary trimethylamine leading to trimethylaminuria, and common genetic variation in FMO3 has been linked to interindividual variability in metabolic function that may be substrate specific.

METHODS

A genetic model of CYP2A6 function is used as a covariate to reveal functional polymorphism in FMO3 that indirectly influences the ratio of deuterated nicotine metabolized to cotinine following oral administration. The association is tested between FMO3 haplotype and cigarette consumption in a set of nicotine-dependent smokers.

RESULTS

FMO3 haplotype, based on all common coding variants in Europeans, significantly predicts nicotine metabolism and accounts for ∼2% of variance in the apparent percent of nicotine metabolized to cotinine. The metabolic ratio is not associated with FMO2 haplotype or an FMO1 expression quantitative trait locus. Cross-validation demonstrates calculated FMO3 haplotype parameters to be robust and significantly improve the predictive nicotine metabolism model over CYP2A6 genotype alone. Functional classes of FMO3 haplotypes, as determined by their influence on nicotine metabolism to cotinine, are also significantly associated with cigarettes per day in nicotine-dependent European Americans (n=1025, P=0.04), and significantly interact (P=0.016) with CYP2A6 genotype to predict cigarettes per day.

CONCLUSION

These findings suggest that common polymorphisms in FMO3 influence nicotine clearance and that these genetic variants in turn influence cigarette consumption.

Cigarette smoking substantially alters plasma microRNA profiles in healthy subjects

Circulating microRNAs (miRNAs) are receiving attention as potential biomarkers of various diseases, including cancers, chronic obstructive pulmonary disease, and cardiovascular disease. However, it is unknown whether the levels of circulating miRNAs in a healthy subject might vary with external factors in daily life. In this study, we investigated whether cigarette smoking, a habit that has spread throughout the world and is a risk factor for various diseases, affects plasma miRNA profiles. We determined the profiles of 11 smokers and 7 non-smokers by TaqMan MicroRNA array analysis. A larger number of miRNAs were detected in smokers than in non-smokers, and the plasma levels of two-thirds of the detected miRNAs (43 miRNAs) were significantly higher in smokers than in non-smokers. A principal component analysis of the plasma miRNA profiles clearly separated smokers and non-smokers. Twenty-four of the miRNAs were previously reported to be potential biomarkers of disease, suggesting the possibility that smoking status might interfere with the diagnosis of disease. Interestingly, we found that quitting smoking altered the plasma miRNA profiles to resemble those of non-smokers. These results suggested that the differences in the plasma miRNA profiles between smokers and non-smokers could be attributed to cigarette smoking. In addition, we found that an acute exposure of ex-smokers to cigarette smoke (smoking one cigarette) did not cause a dramatic change in the plasma miRNA profile. In conclusion, we found that repeated cigarette smoking substantially alters the plasma miRNA profile, interfering with the diagnosis of disease or signaling potential smoking-related diseases.

Cotinine: Beyond that Expected, More than a Biomarker of Tobacco Consumption

A greater incidence of tobacco consumption occurs among individuals with psychiatric conditions including post-traumatic stress disorder (PTSD), bipolar disorder, major depression, and schizophrenia, compared with the general population. Even when still controversial, it has been postulated that smoking is a form of self-medication that reduces psychiatric symptoms among individuals with these disorders. To better understand the component(s) of tobacco-inducing smoking behavior, greater attention has been directed toward nicotine. However, in recent years, new evidence has shown that cotinine, the main metabolite of nicotine, exhibits beneficial effects over psychiatric symptoms and may therefore promote smoking within this population. Some of the behavioral effects of cotinine compared to nicotine are discussed here. Cotinine, which accumulates in the body as a result of tobacco exposure, crosses the blood-brain barrier and has different pharmacological properties compared with nicotine. Cotinine has a longer plasma half-life than nicotine and showed no addictive or cardiovascular effects in humans. In addition, at the preclinical level, cotinine facilitated the extinction of fear memory and anxiety after fear conditioning, improved working memory in a mouse model of Alzheimer’s disease (AD) and in a monkey model of schizophrenia. Altogether, the new evidence suggests that the pharmacological and behavioral effects of cotinine may play a key role in promoting tobacco smoking in individuals that suffer from psychiatric conditions and represents a new potential therapeutic agent against psychiatric conditions such as AD and PTSD.

CYP2A6 and CYP2B6 genetic variation and its association with nicotine metabolism in South Western Alaska Native people

OBJECTIVES

Alaska Native (AN) people have a high prevalence of tobacco use and associated morbidity and mortality when compared with the general USA population. Variations in the CYP2A6 and CYP2B6 genes, encoding enzymes responsible for nicotine metabolic inactivation and procarcinogen activation, have not been characterized in AN and may contribute toward the increased risk.

METHODS

AN people (n=400) residing in the Bristol Bay region of South Western Alaska were recruited for a cross-sectional study on tobacco use. They were genotyped for CYP2A6*1X2A, *1X2B, *1B, *2, *4, *7, *8, *9, *10, *12, *17, *35 and CYP2B6*4, *6, *9 and provided plasma and urine samples for the measurement of nicotine and metabolites.

RESULTS

CYP2A6 and CYP2B6 variant frequencies among the AN Yupik people (n=361) were significantly different from those in other ethnicities. Nicotine metabolism [as measured by the plasma and urinary ratio of metabolites trans-3'-hydroxycotinine to cotinine (3HC/COT)] was significantly associated with CYP2A6 (P<0.001), but not CYP2B6 genotype (P=0.95) when controlling for known covariates. It was noteworthy that the plasma 3HC/COT ratios were high in the entire Yupik people, and among the Yupik CYP2A6 wild-type participants, they were substantially higher than those in previously characterized racial/ethnic groups (P<0.001 vs. Caucasians and African Americans).

CONCLUSION

Yupik AN people have a unique CYP2A6 genetic profile that associated strongly with in-vivo nicotine metabolism. More rapid CYP2A6-mediated nicotine and nitrosamine metabolism in the Yupik people may modulate the risk of tobacco-related diseases.

Cotinine: a potential new therapeutic agent against Alzheimer's disease

Tobacco smoking has been correlated with a lower incidence of Alzheimer's disease (AD). This negative correlation has been attributed to nicotine's properties. However, the undesired side-effects of nicotine and the absence of clear evidence of positive effects of this drug on the cognitive abilities of AD patients have decreased the enthusiasm for its therapeutic use. In this review, we discuss evidence showing that cotinine, the main metabolite of nicotine, has many of the beneficial effects but none of the negative side-effects of its precursor. Cotinine has been shown to be neuroprotective, to improve memory in primates as well as to prevent memory loss, and to lower amyloid-beta (Aβ)) burden in AD mice. In AD, cotinine's positive effect on memory is associated with the inhibition of Aβ aggregation, the stimulation of pro-survival factors such as Akt, and the inhibition of pro-apoptotic factors such as glycogen synthase kinase 3 beta (GSK3β). Because stimulation of the α7 nicotinic acetylcholine receptors (α7nAChRs) positively modulates these factors and memory, the involvement of these receptors in cotinine's effects are discussed. Because of its beneficial effects on brain function, good safety profile, and nonaddictive properties, cotinine may represent a new therapeutic agent against AD.

The contribution of common CYP2A6 alleles to variation in nicotine metabolism among European-Americans

OBJECTIVE

To study the association between cytochrome P450 2A6 (CYP2A6) genotype and metabolism of nicotine to cotinine, identify functional polymorphisms, and develop a predictive genetic model of nicotine metabolism.

METHODS

The conversion of deuterated (D2)-nicotine to D2-cotinine was quantified in 189 European-Americans and the contribution of CYP2A6 genotype to variability in first-pass nicotine metabolism was assessed. Specifically, (i) single time point measures of D2-cotinine/(D2-cotinine+D2-nicotine) after oral administration were used as a metric of CYP2A6 activity; (ii) the impact of CYP2A6 haplotype was treated as acting multiplicatively; (iii) parameter estimates were calculated for all haplotypes in the subject pool, defined by a set of polymorphisms previously reported to affect function, including gene copy number; and (iv) a minimum number of predictive polymorphisms were justified to be included in the model based on statistical evidence of differences between haplotypes.

RESULTS

The final model includes seven polymorphisms and fits the phenotype, 30-min after D2-nicotine oral administration, with R=0.719. The predictive power of the model is robust: parameter estimates calculated in men (n=89) predict the phenotype in women (n=100) with R=0.758 and vice versa with R=0.617; estimates calculated in current smokers (n=102) predict the phenotype in former-smokers (n=86) with R=0.690 and vice versa with R=0.703. Comparisons of haplotypes also demonstrate that CYP2A6*12 is a loss-of-function allele indistinguishable from CYP2A6*4 and CYP2A6*2 and that the CYP2A6*1B 5'-untranslated region conversion has negligible impact on metabolism. After controlling for CYP2A6 genotype, modest associations were found between increased metabolism and both female sex (P=4.8×10) and current smoking (P=0.02).

CONCLUSION

Among European-Americans, seven polymorphisms in the CYP2A6 gene explain the majority of variability in the metabolism of nicotine to cotinine after oral administration. Parameters determined from this in-vivo experiment can be used to predict nicotine metabolism based on CYP2A6 genotype.

Nicotine exposure and metabolizer phenotypes from analysis of urinary nicotine and its 15 metabolites by LC-MS

Smokers who inhale less deeply are exposed to lower amounts of the toxic substances present in tobacco smoke. In order to more rigorously assess tobacco smoke exposure, it is necessary to have an accurate method for quantifying nicotine and all of its known metabolites.

METHODS

A stable-isotope dilution LC-MRM/MS assay has been developed for quantification of urinary nicotine and the 15 possible metabolites that could arise from known metabolic pathways. Nicotine, cotinine, trans-3´-hydroxy-cotinine, nicotine-N-oxide, cotinine-N-oxide, nornicotine, norcotinine and 4-hydroxy-4-(3-pyridyl)butanoic acid were quantified by direct analysis. The corresponding glucuronide metabolites were quantified after urine hydrolysis with β-glucuronidase.

RESULTS

Nicotine and all 15 nicotine metabolites were quantified by LC-MRM/MS in most urine samples from 61 tobacco smokers. Urinary nicotine and metabolite concentrations ranged from 7.9 to 337.8 µM (mean 75.5 ± 67.8 µM). Three nicotine metabolizer phenotypes were established as reduced metabolizers (ratio < 8), normal metabolizers (ratio 8-30), and extensive metabolizers (ratio > 30). 4-hydroxy-4-(3-pyridyl)butanoic acid, which has not been quantified previously, was an abundant metabolite in all three phenotypes.

CONCLUSION

Using this assay it will now be possible to determine whether there are relationships between nicotine exposure and/or metabolizer phenotype with exposure to toxic substances that are present in tobacco smoke and/or to biological response biomarkers to tobacco smoking. This will help in identifying individuals at high risk for developing smoking-related diseases as well as those amenable to smoking cessation programs.

Genetic polymorphisms of drug-metabolizing phase I enzymes CYP2E1, CYP2A6 and CYP3A5 in South Indian population

CYP2E1, CYP2A6 and CYP3A5 enzymes belong to phase I group of drug-metabolizing enzymes, which are involved in the metabolism of various compounds and xenobiotics. Presence of polymorphisms in the genes coding for these enzymes results in interindividual variations in drug metabolism, therapeutic response and susceptibility towards various diseases. The frequencies of these variants in genes differ considerably between ethnic groups. This study was carried out to estimate the allele and genotype frequencies of common variants in CYP2E1, CYP2A6 and CYP3A5 in South Indian population. Six hundred and fifty-two unrelated healthy volunteers of South Indian origin (Andhra Pradesh, Karnataka, Kerala and Tamil Nadu) were included in this study. Polymerase chain reaction-restriction fragment length polymorphism, allele-specific PCR, real-time PCR, SNaPshot and gene sequencing methods were used for the identification of gene polymorphisms. The frequencies of CYP2E1*1B, CYP2E1*5B and CYP2E1*6 alleles in South Indian population were 14.3, 1.3 and 22.4%, respectively. The frequencies of CYP2A6*2, CYP2A6*4A and CYP2A6*5 alleles were found to be 1, 8.9 and 0.7%, respectively. The distribution of CYP3A5*3 allele was 63.5%. There were no variant alleles of CYP3A5*2, CYP3A5*4 and CYP3A5*6 in South Indian population. The frequencies of CYP2E1, CYP2A6 and CYP3A5 in the South Indian population are distinct from Caucasians, Chinese, Japanese, African Americans and other compared populations. This is the first study conducted in the South Indian population with a larger sample size. The findings of our study provide the basic genetic information for further pharmacogenomic investigations in the population.

The Role of Genetics in Nicotine Dependence: Mapping the Pathways from Genome to Syndrome

Nicotine dependence continues to be a major public health problem worldwide and there is unequivocal evidence that genetics play a substantial role in its etiology. This review provides an overview of the evidence for genetic influences and recent advances in the field. Traditional quantitative genetics studies have revealed nicotine dependence is heritable and molecular genetics studies are providing increasing evidence that the genes responsible for nicotine's pharmacokinetics and pharmacodynamics are particularly important. Despite considerable progress, a number of significant complexities and challenges remain. These include determining the specificity of genetic influences and clarifying the role of interactive contributions. One promising strategy for addressing these issues is an intermediate phenotype approach that attempts to identify the intervening proximal mechanisms that confer differential genetic risk. Understanding these mechanisms may permit more precision in understanding genetic influences and may also identify novel targets for intervention or prevention.

Human blood concentrations of cotinine, a biomonitoring marker for tobacco smoke, extrapolated from nicotine metabolism in rats and humans and physiologically based pharmacokinetic modeling

The present study defined a simplified physiologically based pharmacokinetic (PBPK) model for nicotine and its primary metabolite cotinine in humans, based on metabolic parameters determined in vitro using relevant liver microsomes, coefficients derived in silico, physiological parameters derived from the literature, and an established rat PBPK model. The model consists of an absorption compartment, a metabolizing compartment, and a central compartment for nicotine and three equivalent compartments for cotinine. Evaluation of a rat model was performed by making comparisons with predicted concentrations in blood and in vivo experimental pharmacokinetic values obtained from rats after oral treatment with nicotine (1.0 mg/kg, a no-observed-adverseeffect level) for 14 days. Elimination rates of nicotine in vitro were established from data from rat liver microsomes and from human pooled liver microsomes. Human biomonitoring data (17 ng nicotine and 150 ng cotinine per mL plasma 1 h after smoking) from pooled five male Japanese smokers (daily intake of 43 mg nicotine by smoking) revealed that these blood concentrations could be calculated using a human PBPK model. These results indicate that a simplified PBPK model for nicotine/cotinine is useful for a forward dosimetry approach in humans and for estimating blood concentrations of other related compounds resulting from exposure to low chemical doses.

Association between nicotine metabolism and CYP2A6*1 and CYP2A6*4 genotypes in an Iranian population

The cytochrome P450 (CYP) family is the principal enzyme system involved in the metabolism of xenobiotics and endogenous compounds. Among this family, CYP2A6 is one of the most important enzymes for metabolism of nicotine. In this study, the linkage of CYP2A6*1 and CYP2A6*4 genotypes with nicotine metabolism was investigated. A single polymerase chain reaction-restriction fragment length polymorphism was used to resolve the genotypes into CYP2A6*1 (wild type), CYP2A6*2, or CYP2A6*3. The population studied consisted of 200 healthy smokers from Mashhad city, North East of Iran. The urinary cotinine as the principal metabolite of nicotine was analyzed for 12 subjects (7 subjects with CYP2A6*1 as controls and 5 subjects with CYP2A6*4). The results indicated that cumulative urinary cotinine excretion in CYP2A6*4 genotype was about one-eighth compared with the control group (wild type). Cotinine formation from nicotine has individual and ethnic variability that correlated with the level of CYP2A6 expression. Moreover, urinary cotinine level was drastically lower in CYP2A6*4 subjects than in CYP2A6*1 subjects.

Biomonitoring of urinary cotinine concentrations associated with plasma levels of nicotine metabolites after daily cigarette smoking in a male Japanese population

Human biomonitoring of plasma and urinary levels of nicotine, cotinine, and 3′-hydroxycotinine was conducted after daily cigarette smoking in a population of 92 male Japanese smokers with a mean age of 37 years who had smoked an average of 23 cigarettes per day for 16 years. Members of the population were genotyped for the nicotine-metabolizing enzyme cytochrome P450 2A6 (CYP2A6). The mean levels of nicotine, the levels of its metabolites cotinine and 3′-hydroxycotinine, and the sum of these three levels in subjects one hour after smoking the first cigarette on the sampling day were 20.1, 158, 27.7, and 198 ng/mL in plasma and 846, 1,020, 1,010, and 2,870 ng/mL in urine under daily smoking conditions. Plasma levels of 3′-hydroxycotinine and urinary levels of nicotine and 3′-hydroxycotinine were dependent on the CYP2A6 phenotype group, which was estimated from the CYP2A6 genotypes of the subjects, including those with whole gene deletion. Plasma cotinine levels were significantly correlated with the number of cigarettes smoked on the day before sampling (r = 0.71), the average number of cigarettes smoked daily (r = 0.58), and the Brinkman index (daily cigarettes × years, r = 0.48) under the present conditions. The sum of nicotine, cotinine, and 3′-hydroxycotinine concentrations in plasma showed a similar relationship to that of the plasma cotinine levels. Urinary concentrations of cotinine and the sum of nicotine metabolite concentrations also showed significant correlations with the plasma levels and the previous day’s and average cigarette consumption. The numbers of cigarettes smoked per day by two subjects with self-reported light smoking habits were predicted by measuring the urinary cotinine concentrations and using linear regression equations derived from above-mentioned data. These results indicate that biomonitoring of the urinary cotinine concentration is a good, easy-to-use marker for plasma levels of cotinine and the sum of nicotine metabolites in smokers independent of genetic polymorphism of CYP2A6.

UGT2B10 genotype influences nicotine glucuronidation, oxidation, and consumption

BACKGROUND

Tobacco exposure is routinely assessed by quantifying nicotine metabolites in plasma or urine. On average, 80% of nicotine undergoes C-oxidation to cotinine. However, interindividual variation in nicotine glucuronidation is substantial, and glucuronidation accounts for from 0% to 40% of total nicotine metabolism. We report here the effect of a polymorphism in a UDP-glucuronsyltransferase, UGT2B10, on nicotine metabolism and consumption.

METHODS

Nicotine, cotinine, their N-glucuronide conjugates, and total trans-3'-hydroxycotinine were quantified in the urine (n = 327) and plasma (n = 115) of smokers. Urinary nicotine N-oxide was quantified in 105 smokers. Nicotine equivalents, the sum of nicotine and all major metabolites, were calculated for each smoker. The relationship of the UGT2B10 Asp67Tyr allele to nicotine equivalents, N-glucuronidation, and C-oxidation was determined.

RESULTS

Individuals heterozygous for the Asp67Tyr allele excreted less nicotine or cotinine as their glucuronide conjugates than did wild-type, resulting in a 60% lower ratio of cotinine glucuronide to cotinine, a 50% lower ratio of nicotine glucuronide to nicotine, and increased cotinine and trans-3'-hydroxycotinine. Nicotine equivalents, a robust biomarker of nicotine intake, were lower among Asp67Tyr heterozygotes compared with individuals without this allele: 58.2 (95% confidence interval, 48.9-68.2) versus 69.2 nmol/mL (95% confidence interval, 64.3-74.5).

CONCLUSIONS

Individuals heterozygous for UGT2B10 Asp67Tyr consume less nicotine than do wild-type smokers. This striking observation suggests that variations in nicotine N-glucuronidation, as reported for nicotine C-oxidation, may influence smoking behavior.

IMPACT

UGT2B10 genotype influences nicotine metabolism and should be taken into account when characterizing the role of nicotine metabolism on smoking.

Enzyme-linked immunosorbent assay of nicotine metabolites

INTRODUCTION

The level of cotinine in biological specimens, such as serum, urine, and saliva, measured by gas or liquid chromatography is the most validated and reliable indicator of exposure to tobacco smoke. However, chromatographic methods are not always suitable for all types of situations.

METHODS

We validated a commercially available enzyme-linked immunosorbent assay (ELISA) that uses a polyclonal antibody to cotinine as a practical alternative to chromatographic methods.

RESULTS

The cotinine antibody cross-reacts to 3-hydroxycotinine (3HC) and its glucuronide, thus generating a value for immunoreactive (IR) cotinine, which is a complex comprising cotinine, 3HC, and 3HC-glucuronide. The levels of IR cotinine in the urine of kindergarten children closely correlated with those of cotinine measured by gas chromatography-mass spectrometry (GC-MS) and reflected the smoking behavior of their parents more precisely than cotinine levels determined by GC-MS.

DISCUSSION

Our findings showed that the cotinine-based ELISA can be a practical biomarker of exposure to tobacco smoke.

Polymorphism in cytochrome P450 2A6 and glutathione S-transferase P1 modifies head and neck cancer risk and treatment outcome

A case control study was carried out to investigate the association of functionally important polymorphism in cytochrome P450 2A6 (CYP2A6) and glutathione S-transferase P1 (GSTP1) genes with head and neck squamous cell carcinoma (HNSCC) and treatment response in cases receiving a combination of chemo-radiotherapy. The study group consisted of 350 males suffering from HNSCC and an equal number of male controls. Multivariate logistic regression analysis revealed statistically significant decrease in risk to HNSCC in cases with variant genotypes (CYP2A6*1B and CYP2A6*4C) of CYP2A6 (OR: 0.78; 95% CI: 0.43-1.22; P=0.04) or GSTP1 (OR: 0.71; 95% CI: 0.51-1.00; P=0.05). The risk associated with these variant genotypes was found to be further decreased in cases carrying a combination of variant genotypes of CYP2A6 and GSTP1 (OR: 0.40; 95% CI: 0.25-0.65; P=0.00). A similar decrease in risk was observed in cases with variant genotypes of CYP2A6 (OR: 0.59; 95% CI: 0.40-0.86; P=0.00) or GSTP1 (OR: 0.62; 95% CI: 0.42-0.91; P=0.01) and who were regular tobacco users (cigarette smokers or tobacco chewers). Interestingly, only 27% of the cases carrying the variant forms of CYP2A6 (*1A/*4C+*1B/*4C+*4C/*4C) responded to the treatment for HNSCC when compared to those with wild-type genotype (69%). However with GSTP1, cases with homozygous mutant genotype (Val/Val) showed a superior treatment response (75%) when compared to cases with wild-type genotype (25%). Further, cases carrying a combination of variant genotype of CYP2A6 and wild-type genotype of GSTP1 exhibited a very poor treatment response demonstrating that polymorphisms in CYP2A6 and GSTP1 not only modified the risk to HNSCC but also played a major role in determining the chemotherapeutic response.

A novel CYP2A6 allele (CYP2A6*35) resulting in an amino-acid substitution (Asn438Tyr) is associated with lower CYP2A6 activity in vivo

Cytochrome P450 2A6 (CYP2A6) is the primary human enzyme involved in nicotine metabolism. The objective of this study was to characterize two nonsynonymous single nucleotide polymorphisms in CYP2A6(*)24, 594G>C (Val110Leu) and 6458A>T (Asn438Tyr). We determined their haplotype, allele frequencies, effect on CYP2A6 activity in vivo, as well as their stability and ability to metabolize nicotine in vitro. CYP2A6(*)35 (6458A>T) occurred at a frequency of 2.5-2.9% among individuals of black African descent, 0.5-0.8% among Asians and was not found in Caucasians. In addition, we identified two novel alleles, CYP2A6(*)36 (6458A>T and 6558T>C (Ile471Thr)) and CYP2A6(*)37 (6458A>T, 6558T>C and 6600G>T (Arg485Leu)). In vivo, CYP2A6(*)35 was associated with lower CYP2A6 activity as measured by the 3HC/COT ratio. In vitro, CYP2A6.35 had decreased nicotine C-oxidation activity and thermal stability. In conclusion, we identified three novel CYP2A6 alleles (CYP2A6(*)35, (*)36 and (*)37); the higher allele frequency variant CYP2A6(*)35 was associated with lower CYP2A6 activity.

Molecular genetics of nicotine metabolism

The molecular genetics of nicotine metabolism involves multiple polymorphic catalytic enzymes. Variation in metabolic pathways results in nicotine disposition kinetics that differ between individuals and ethnic groups. Twin studies indicate that a large part of this variance is genetic in origin, although environmental influences also contribute. The primary aim of this chapter is to review the current knowledge regarding the genetic variability in the enzymes that metabolize nicotine in humans. The focus is on describing the genetic polymorphisms that exist in cytochromes P450 (CYPs), aldehyde oxidase 1 (AOX1), UDP-glucuronosyltransferases (UGTs), and flavin-containing monooxygenase 3 (FMO3). Genetic studies have demonstrated that polymorphisms in CYP2A6, the primary enzyme responsible for nicotine breakdown, make a sizable contribution to the wide range of nicotine metabolic capacity observed in humans. Thus, special attention will be given to CYP2A6, because slower nicotine metabolism requires less frequent self-administration, and accordingly influences smoking behaviors. In addition, the molecular genetics of nicotine metabolism in nonhuman primates, mice, and rats will be reviewed briefly.

Novel and established CYP2A6 alleles impair in vivo nicotine metabolism in a population of Black African descent

Cytochrome P450 2A6 (CYP2A6) is a human enzyme best known for metabolizing tobacco-related compounds, such as nicotine, cotinine (COT), and nitrosamine procarcinogens. CYP2A6 genetic variants have been associated with smoking status, cigarette consumption, and tobacco-related cancers. Our objective was to functionally characterize four nonsynonymous CYP2A6 sequence variants with respect to their haplotype, allele frequency, and association with in vivo CYP2A6 activity. In vivo, nicotine was administered orally to 281 volunteers of Black African descent. Blood samples were collected for kinetic phenotyping and CYP2A6 genotyping. In vitro, nicotine C-oxidation catalytic efficiencies of heterologously expressed variant enzymes were assessed. The four uncharacterized sequence variants were found in seven novel alleles CYP2A6(*)24A&B ; (*)25, (*)26, (*)27, and *28A&B, most were associated with impaired in vivo CYP2A6 activity. Nicotine metabolism groupings, based on the in vivo data of variant alleles, were created. Mean trans-3'-hydroxycotinine/cotinine (3HC/COT) differed (P<0.001) between normal (100%), intermediate (64%), and slow (40%) groups. Systemic exposure to nicotine following oral administration also differed (P<0.001) between normal (100%), intermediate (139%), and slow (162%) metabolism groups. In addition, alleles of individuals with unusual phenotype-genotype relationships were sequenced, resulting in the discovery of five novel uncharacterized alleles and at least one novel duplication allele. A total of 7% of this population of Black African descent had at least one of the eight novel characterized alleles and 29% had at least one previously established allele. These findings are important for increasing the accuracy of association studies between CYP2A6 genotype and behavioral, disease, or pharmacological phenotypes.