Characterization of multiple products of cytochrome P450 2A6-catalyzed cotinine metabolism

Application of HPLC-QQQ-MS/MS and New RP-HPLC-DAD System Utilizing the Chaotropic Effect for Determination of Nicotine and Its Major Metabolites Cotinine, and trans-3'-Hydroxycotinine in Human Plasma Samples

The routine techniques currently applied for the determination of nicotine and its major metabolites, cotinine, and trans-3′-hydroxycotinine, in biological fluids, include spectrophotometric, immunoassays, and chromatographic techniques. The aim of this study was to develop, and compare two new chromatographic methods high-performance liquid chromatography coupled to triple quadrupole mass spectrometry (HPLC-QQQ-MS/MS), and RP-HPLC enriched with chaotropic additives, which would allow reliable confirmation of tobacco smoke exposure in toxicological and epidemiological studies. The concentrations of analytes were determined in human plasma as the sample matrix. The methods were compared in terms of the linearity, accuracy, repeatability, detection and quantification limits (LOD and LOQ), and recovery. The obtained validation parameters met the ICH requirements for both proposed procedures. However, the limits of detection (LOD) were much better for HPLC-QQQ-MS/MS (0.07 ng mL⁻¹ for trans-3′-hydroxcotinine; 0.02 ng mL⁻¹ for cotinine; 0.04 ng mL⁻¹ for nicotine) in comparison to the RP-HPLC-DAD enriched with chaotropic additives (1.47 ng mL⁻¹ for trans-3′-hydroxcotinine; 1.59 ng mL⁻¹ for cotinine; 1.50 ng mL⁻¹ for nicotine). The extraction efficiency (%) was concentration-dependent and ranged between 96.66% and 99.39% for RP-HPLC-DAD and 76.8% to 96.4% for HPLC-QQQ-MS/MS. The usefulness of the elaborated analytical methods was checked on the example of the analysis of a blood sample taken from a tobacco smoker. The nicotine, cotinine, and trans-3′-hydroxycotinine contents in the smoker’s plasma quantified by the RP-HPLC-DAD method differed from the values measured by the HPLC-QQQ-MS/MS. However, the relative errors of measurements were smaller than 10% (6.80%, 6.72%, 2.04% respectively).

Comprehensive metabolic study of nicotine in equine plasma and urine using liquid chromatography/high-resolution mass spectrometry for the identification of unique biomarkers for doping control

Nicotine is classified as a stimulant, and its use is banned in horse racing and equestrian sports by the International Federation of Horseracing Authorities and the Fédération Équestre Internationale, respectively. Because nicotine is a major alkaloid of tobacco leaves, there is a potential risk that doping control samples may be contaminated by tobacco cigarettes or smoke during sample collection. In order to differentiate the genuine doping and sample contamination with tobacco leaves, it is necessary to monitor unique metabolites as biomarkers for nicotine administration and intake. However, little is known about the metabolic fate of nicotine in horses. This is the first report of comprehensive metabolism study of nicotine in horses. Using liquid chromatography/electrospray ionization high-resolution mass spectrometry, we identified a total of 17 metabolites, including one novel horse-specific metabolite (i.e., 4-hydroxy-4-(3-pyridyl)-N-methylbutanamide), in post-administration urine samples after nasoesophageal administration of nicotine to three thoroughbred mares; eight of these compounds were confirmed based on reference standards. Among these metabolites, N-hydroxymethylnorcotinine was the major urinary metabolite in equine, but it could only be tentatively identified by mass spectral interpretation due to the lack of reference material. In addition, we developed simultaneous quantification methods for the eight target analytes in plasma and urine, and applied them to post-administration samples to establish elimination profiles of nicotine and its metabolites. The quantification results revealed that trans-3'-hydroxycotinine could be quantified for the longest period in both plasma (72 h post-administration) and urine (96 h post-administration). Therefore, this metabolite is the most appropriate monitoring target for nicotine exposure for the purpose of doping control due to its long detection times and the availability of its reference material. Further, we identified trans-3'-hydroxycotinine as a unique biomarker allowing differentiation between nicotine administration and sample contamination with tobacco leaves.

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.

More than Smoke and Patches: The Quest for Pharmacotherapies to Treat Tobacco Use Disorder

Tobacco use is a persistent public health issue. It kills up to half its users and is the cause of nearly 90% of all lung cancers. The main psychoactive component of tobacco is nicotine, primarily responsible for its abuse-related effects. Accordingly, most pharmacotherapies for smoking cessation target nicotinic acetylcholine receptors (nAChRs), nicotine's major site of action in the brain. The goal of the current review is twofold: first, to provide a brief overview of the most commonly used behavioral procedures for evaluating smoking cessation pharmacotherapies and an introduction to pharmacokinetic and pharmacodynamic properties of nicotine important for consideration in the development of new pharmacotherapies; and second, to discuss current and potential future pharmacological interventions aimed at decreasing tobacco use. Attention will focus on the potential for allosteric modulators of nAChRs to offer an improvement over currently approved pharmacotherapies. Additionally, given increasing public concern for the potential health consequences of using electronic nicotine delivery systems, which allow users to inhale aerosolized solutions as an alternative to smoking tobacco, an effort will be made throughout this review to address the implications of this relatively new form of nicotine delivery, specifically as it relates to smoking cessation. SIGNIFICANCE STATEMENT: Despite decades of research that have vastly improved our understanding of nicotine and its effects on the body, only a handful of pharmacotherapies have been successfully developed for use in smoking cessation. Thus, investigation of alternative pharmacological strategies for treating tobacco use disorder remains active; allosteric modulators of nicotinic acetylcholine receptors represent one class of compounds currently under development for this purpose.

Biomarkers of Exposure to Secondhand and Thirdhand Tobacco Smoke: Recent Advances and Future Perspectives

Smoking is the leading preventable disease worldwide and passive smoking is estimated to be the cause of about 1.0% of worldwide mortality. The determination of tobacco smoke biomarkers in human biological matrices is key to assess the health effects related to the exposure to environmental tobacco smoke. The biomonitoring of cotinine, the main nicotine metabolite, in human biofluids-including urine, serum or saliva-has been extensively used to assess this exposure. However, the simultaneous determination of cotinine together with other tobacco biomarkers and the selection of alternative biological matrices, such as hair, skin or exhaled breath, would enable a better characterization of the kind and extent of tobacco exposure. This review aims to perform a critical analysis of the up-to-date literature focused on the simultaneous determination of multiple tobacco smoke biomarkers studied in different biological matrices, due to the exposure to secondhand smoke (SHS) and thirdhand smoke (THS). Target biomarkers included both tobacco-specific biomarkers-nicotine and tobacco specific nitrosamine biomarkers-and tobacco-related biomarkers, such as those from polycyclic aromatic hydrocarbons, volatile organic compounds, metals and carbon monoxide. To conclude, we discuss the suitability of determining multiple biomarkers through several relevant examples of SHS and THS exposure.

Molecular Determinants of Substrate Affinity and Enzyme Activity of a Cytochrome P450BM3 Variant

Cytochrome P450_BM3 catalyzes the hydroxylation and/or epoxidation of fatty acids, fatty amides, and alcohols. Protein engineering has produced P450_BM3 variants capable of accepting drug molecules normally metabolized by human P450 enzymes. The enhanced substrate promiscuity has been attributed to the greater flexibility of the lid of the substrate channel. However, it is not well understood how structurally different and highly polar drug molecules can stably bind in the active site nor how the activity and coupling efficiency of the enzyme may be affected by the lack of enzyme-substrate complementarity. To address these important aspects of non-native small molecule binding, this study investigated the binding of drug molecules with different size, charge, polar surface area, and human P450 affinity on the promiscuous R47L/F87V/L188Q/E267V/F81I pentuple mutant of P450_BM3. Binding free energy data and energy decomposition analysis showed that pentuple mutant P450_BM3 stably binds (i.e., negative ΔG_b°) a broad range of substrate and inhibitor types because dispersion interactions with active site residues overcome unfavorable repulsive and electrostatic effects. Molecular dynamics simulations revealed that 1) acidic substrates tend to disrupt the heme propionate A-K69 salt bridge, which may reduce heme oxidizing ability, and 2) the lack of complementarity leads to high substrate mobility and water density in the active site, which may lead to uncoupling. These factors must be considered in future developments of P450_BM3 as a biocatalyst in the large-scale production of drug metabolites.

Isolation and characterization of the cotinine-degrading bacterium Nocardioides sp. Strain JQ2195

Cotinine, the primary nicotine metabolite, not only more stable and more difficult to degrade in the environment but is a potential health risk to human. To date, little is known about the biodegradation process of cotinine. In this study, a bacterial strain JQ2195 was isolated from municipal wastewater and was identified as Nocardioides sp. based on morphological, physiological characteristics, and 16 S rRNA gene phylogenetic analysis. This strain utilized cotinine as a sole carbon source and degraded 0.5 g L^-1 cotinine completely within 32 h. Optimum degradation of cotinine by JQ2195 was at 30 °C and pH 7.0. Two cotinine degradation intermediates were identified as 6-hydroxy-cotinine and 6-hydroxy-3-succinoylpyridine by UV/VIS spectroscopy and liquid chromatography coupled with time-of-flight mass spectrometry. In addition, about half of cotinine was transformed to 6-hydroxy-3-succinoylpyridine which was a value-added compound for biocatalysis. When 2,6-dichlorophenolindophenol was used as an electron acceptor, the cell-free extract containing the inducible cotinine dehydrogenase could convert cotinine into 6-hydroxy-cotinine with the activity 40 ± 6 mUnmg^-1.

Evaluation of Serum Cotinine Cut-Off to Distinguish Smokers From Nonsmokers in the Korean Population

Background

Cotinine has been widely used as an objective marker to identify current smokers. We conducted this study to address the absence of Korean studies investigating the efficacy of immunoassays and liquid chromatography–tandem mass spectrometry (LC-MS/MS) for the detection of serum cotinine and to determine the optimal serum cotinine cut-off level for differentiating current smokers from nonsmokers.

Methods

Serum specimens were obtained from 120 subjects. They were randomly chosen to represent a broad distribution of urine cotinine levels based on a retrospective review of questionnaires and results of urine cotinine levels. We determined serum cotinine levels using the IMMULITE 2000 XPi Immunoassay System (Siemens Healthcare Diagnostics Inc., USA) and LC-MS/MS (API-4000, Applied Biosystems, USA). Correlation was analyzed between IMMULITE serum cotinine, urine cotinine, and LC-MS/MS serum cotinine levels. ROC curve was analyzed to identify the optimal IMMULITE serum cotinine cut-off level for differentiating current smokers from nonsmokers.

Results

IMMULITE serum cotinine levels correlated with both urine cotinine and LC-MS/MS serum cotinine levels, with correlation coefficients of 0.958 and 0.986, respectively. The optimal serum cotinine cut-off level for distinguishing current smokers from nonsmokers was 13.2 ng/mL (95.7% sensitivity, 94.1% specificity) using IMMULITE.

Conclusions

This is the first study to investigate the use of LC-MS/MS for the measurement of serum cotinine and to determine the optimal serum cotinine cut-off level for the IMMULITE immunoassay. Our results could provide guidelines for differentiating current smokers from nonsmokers in the Korean population.

Quantitative analysis of 3'-hydroxynorcotinine in human urine

INTRODUCTION

Based on previous metabolism studies carried out in patas monkeys, we hypothesized that urinary 3'-hydroxynorcotinine could be a specific biomarker for uptake and metabolism of the carcinogen N'-nitrosonornicotine in people who use tobacco products.

METHODS

We developed a method for quantitation of 3'-hydroxynorcotinine in human urine. [Pyrrolidinone-(13)C4]3'-hydroxynorcotinine was added to urine as an internal standard, the samples were treated with β-glucuronidase, partially purified by solid supported liquid extraction and quantified by liquid chromatography-electrospray ionization-tandem mass spectrometry.

RESULTS

The method was accurate (average accuracy = 102%) and precise (coefficient of variation = 5.6%) in the range of measurement. 3'-Hydroxynorcotinine was detected in 48 urine samples from smokers (mean 393±287 pmol/ml urine) and 12 samples from individuals who had stopped smoking and were using the nicotine patch (mean 658±491 pmol/ml urine), but not in any of 10 samples from nonsmokers.

CONCLUSIONS

Since the amounts of 3'-hydroxynorcotinine found in smokers' urine were approximately 50 times greater than the anticipated daily dose of N'-nitrosonornicotine, we concluded that it is a metabolite of nicotine or one of its metabolites, comprising perhaps 1% of nicotine intake in smokers. Therefore, it would not be suitable as a specific biomarker for uptake and metabolism of N'-nitrosonornicotine. Since 3'-hydroxynorcotinine has never been previously reported as a constituent of human urine, further studies are required to determine its source and mode of formation.

New Insights into the Mechanisms of Action of Cotinine and its Distinctive Effects from Nicotine

Tobacco consumption is far higher among a number of psychiatric and neurological diseases, supporting the notion that some component(s) of tobacco may underlie the oft-reported reduction in associated symptoms during tobacco use. Popular dogma holds that this component is nicotine. However, increasing evidence support theories that cotinine, the main metabolite of nicotine, may underlie at least some of nicotine's actions in the nervous system, apart from its adverse cardiovascular and habit forming effects. Though similarities exist, disparate and even antagonizing actions between cotinine and nicotine have been described both in terms of behavior and physiology, underscoring the need to further characterize this potentially therapeutic compound. Cotinine has been shown to be psychoactive in humans and animals, facilitating memory, cognition, executive function, and emotional responding. Furthermore, recent research shows that cotinine acts as an antidepressant and reduces cognitive-impairment associated with disease and stress-induced dysfunction. Despite these promising findings, continued focus on this potentially safe alternative to tobacco and nicotine use is lacking. Here, we review the effects of cotinine, including comparisons with nicotine, and discuss potential mechanisms of cotinine-specific actions in the central nervous system which are, to date, still being elucidated.

CYP2A6- and CYP2A13-catalyzed metabolism of the nicotine Δ5'(1')iminium ion

Nicotine, the major addictive agent in tobacco, is metabolized primarily by CYP2A6-catalyzed oxidation. The product of this reaction, 5'-hydroxynicotine, is in equilibrium with the nicotine Δ5'(1')iminium ion and is further metabolized to cotinine. We reported previously that both CYP2A6 and the closely related extrahepatic enzyme CYP2A13 were inactivated during nicotine metabolism; however, inactivation occurred after metabolism was complete. This led to the hypothesis that oxidation of a nicotine metabolite, possibly the nicotine Δ5'(1')iminium ion, was responsible for generating the inactivating species. In the studies presented here, we confirm that the nicotine Δ5'(1')iminium ion is an inactivator of both CYP2A6 and CYP2A13, and inactivation depends on time, concentration, and the presence of NADPH. Inactivation was not reversible and was accompanied by a parallel loss in spectrally active protein, as measured by reduced CO spectra. These data are consistent with the characterization of the nicotine Δ5'(1')iminium ion as a mechanism-based inactivator of both CYP2A13 and CYP2A6. We also confirm that both CYP2A6 and CYP2A13 catalyze the metabolism of the nicotine Δ5'(1')iminium ion to cotinine and provide evidence that both enzymes catalyze the sequential metabolism of the nicotine Δ5'(1')iminium ion. That is, a fraction of the cotinine formed may not be released from the enzyme before further oxidation to 3'-hydroxycotinine.

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.

Electron ionization mass spectral study of oxo- and thio derivatives of minor tobacco alkaloids

The electron-ionization mass spectra (EI-MS) of oxo- and thio-derivatives of minor tobacco alkaloids and structurally similar piperidine alkaloids, i.e. thiocotinine (1), 2'-oxo-N-methylanabasine (2), 2'-thio-N- methylanabasine (3), 2'-oxoanabasamine (4) and 2'-thioanabasamine (5) are discussed and general fragmentation routes of their molecular cations are proposed. Comparison of the data obtained for 1 with the EI-MS data of metameric metabolites of nicotine: 3'-hydroxycotinine, (A) and 5'-hydroxycotinine, (B) allows a differentiation between these metamers. The data will be useful for the identification of metabolites of alkaloids of these types in biological matrices.

Characterization and comparison of nicotine and cotinine metabolism in vitro and in vivo in DBA/2 and C57BL/6 mice

DBA/2 and C57BL/6 are two commonly used mouse strains that differ in response to nicotine. Previous studies have shown that the nicotine-metabolizing enzyme CYP2A5 differs in coumarin metabolism between these two strains, suggesting differences in nicotine metabolism. Nicotine was metabolized to cotinine in vitro by two enzymatic sites. The high-affinity sites exhibited similar parameters (Km, 10.7 +/- 4.8 versus 11.4 +/- 3.6 microM; Vmax, 0.58 +/- 0.18 versus 0.50 +/- 0.07 nmol/min/mg for DBA/2 and C57BL/6, respectively). In vivo, the elimination half-lives of nicotine (1 mg/kg, s.c.) were also similar between DBA/2 and C57BL/6 mice (8.6 +/- 0.4 versus 9.2 +/- 1.6 min, respectively); however, cotinine levels were much higher in DBA/2 mice. The production and identity of the putative cotinine metabolite 3'-hydroxycotinine in mice was confirmed by liquid chromatography/mass spectrometry/mass spectrometry. The in vivo half-life of cotinine (1 mg/kg, s.c.) was significantly longer in the DBA/2 mice compared with the C57BL/6 mice (50.2 +/- 4.7 versus 37.5 +/- 9.6 min, respectively, p < 0.05). The in vitro metabolism of cotinine to 3'-hydroxycotinine was also less efficient in DBA/2 than C57BL/6 mice (Km, 51.0 +/- 15.6 versus 9.5 +/- 2.1 microM, p < 0.05; Vmax, 0.10 +/- 0.01 versus 0.04 +/- 0.01 nmol/min/mg, p < 0.05, respectively). Inhibitory antibody studies demonstrated that the metabolism of both nicotine and cotinine was mediated by CYP2A5. Genetic differences in Cyp2a5 potentially contributed to similar nicotine but different cotinine metabolism, which may confound the interpretation of nicotine pharmacological studies and studies using cotinine as a biomarker.

Inactivation of CYP2A6 and CYP2A13 during nicotine metabolism

Nicotine is the major addictive agent in tobacco. The primary catalyst of nicotine metabolism in humans is CYP2A6. However, the closely related enzyme CYP2A13 is a somewhat better catalyst. CYP2A13 is an extrahepatic enzyme that is an excellent catalyst of the metabolic activation of the tobacco-specific carcinogen 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanone (NNK). Here we report that both CYP2A6 and CYP2A13 were inactivated during nicotine metabolism. Inactivation of both enzymes was dependent on NADPH and increased with time and concentration. Alternate substrates for CYP2A6 and CYP2A13 protected these enzymes from inactivation. Inactivation of CYP2A13 was irreversible upon extensive dialysis and seems to be mechanism-based. The K(I) of CYP2A13 inactivation by nicotine was 17 microM, the rate of inactivation, k(inact), was 0.1 min(-1), and the t(1/2) was 7 min. However, the loss in enzyme activity occurred after nicotine metabolism was complete, suggesting that a secondary or possible tertiary metabolite of nicotine may be responsible. [5-(3)H]Nicotine metabolism by CYP2A13 was monitored by radioflow high-pressure liquid chromatography during the course of enzyme inactivation; the major product was the Delta(1'(5'))iminium ion. However, cotinine was a significant metabolite even at short reaction times. The metabolism of the nicotine Delta(1'(5'))iminium ion to cotinine did not require the addition of aldehyde oxidase. CYP2A13 catalyzed this reaction as well as further metabolism of cotinine to 5'-hydroxycotinine, trans-3'-hydroxycotinine, and N-(hydroxymethyl)-norcotinine as enzyme inactivation occurred. Studies are on-going to identify the metabolite responsible for nicotine-mediated inactivation of CYP2A13.

Metabolism and Disposition Kinetics of Nicotine

Nicotine is of importance as the addictive chemical in tobacco, pharmacotherapy for smoking cessation, a potential medication for several diseases, and a useful probe drug for phenotyping cytochrome P450 2A6 (CYP2A6). We review current knowledge about the metabolism and disposition kinetics of nicotine, some other naturally occurring tobacco alkaloids, and nicotine analogs that are under development as potential therapeutic agents. The focus is on studies in humans, but animal data are mentioned when relevant to the interpretation of human data. The pathways of nicotine metabolism are described in detail. Absorption, distribution, metabolism, and excretion of nicotine and related compounds are reviewed. Enzymes involved in nicotine metabolism including cytochrome P450 enzymes, aldehyde oxidase, flavin-containing monooxygenase 3, amine N-methyltransferase, and UDP-glucuronosyltransferases are represented, as well as factors affecting metabolism, such as genetic variations in metabolic enzymes, effects of diet, age, gender, pregnancy, liver and kidney diseases, and racial and ethnic differences. Also effects of smoking and various inhibitors and inducers, including oral contraceptives, on nicotine metabolism are discussed. Due to the significance of the CYP2A6 enzyme in nicotine clearance, special emphasis is given to the effects and population distributions of CYP2A6 alleles and the regulation of CYP2A6 enzyme.

Metabolic profile of nicotine in subjects whose CYP2A6 gene is deleted

Generally, 70-80% of absorbed nicotine is mainly metabolized to cotinine by cytochrome P450 (CYP) 2A6. There is genetic polymorphism in the human CYP2A6 gene. Among several mutated alleles, CYP2A6*4 allele is a whole deleted type. The purpose of the present study was to clarify the metabolic profile of nicotine in subjects whose CYP2A6 gene is deleted. We developed a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for nicotine and its nine metabolites. Excretion levels of nicotine and its metabolites in 24 h accumulated urine after the chewing of one piece of nicotine gum were evaluated in five Japanese subjects whose CYP2A6 genotype was determined. In three subjects with CYP2A6*1A/CYP2A6*1A, CYP2A6*1A/CYP2A6*1B, and CYP2A6*1A/CYP2A6*4 (group I), nicotine was mainly excreted as cotinine, trans-3'-hydroxycotinine, and their glucuronide (approximately 60%). In contrast, in two subjects with CYP2A6*4/CYP2A6*4 (group II), trace levels of cotinine, cotinine N-glucuronide, and cotinine 1'-N-oxide were detected. Trans-3'-hydroxycotinine and its O-glucuronide were not detected. The excretion levels of nicotine itself, nicotine N-glucuronide, and nicotine 1'-N-oxide were higher than those in the other three subjects. The total excretion levels of these three compounds were approximately 95% in group II versus 35% in group I. However, the sum of the excretion levels of nicotine and all metabolites was similar among these five subjects. This is the first report of the metabolic profile of nicotine in subjects whose CYP2A6 gene is deleted.

Interindividual Variability in Nicotine Metabolism: C-Oxidation and Glucuronidation

Nicotine has roles in the addiction to smoking, replacement therapy for smoking cessation, as a potential medication for several diseases such as Parkinson's disease, Alzheimer's disease, and ulcerative colitis. The absorbed nicotine is rapidly and extensively metabolized and eliminated to urine. A major pathway of nicotine metabolism is C-oxidation to cotinine, which is catalyzed by CYP2A6 in human livers. Cotinine is subsequently metabolized to trans-3'-hydroxycotinine by CYP2A6. Nicotine and cotinine are glucuronidated to N-glucuronides mainly by UGT1A4 and partly by UGT1A9. Trans-3'-hydroxycotinine is glucuronidated to O-glucuronide mainly by UGT2B7 and partly by UGT1A9. Approximately 90% of the total nicotine uptake is eliminated as these metabolites and nicotine itself. The nicotine metabolism is an important determinant of the clearance of nicotine. Recently, advances in the understanding of the interindividual variability in nicotine metabolism have been made. There are substantial data suggesting that the large interindividual differences in cotinine formation are associated with genetic polymorphisms of the CYP2A6 gene. Interethnic differences have also been observed in the cotinine formation and the allele frequencies of the CYP2A6 alleles. Since the genetic polymorphisms of the CYP2A6 gene have a major impact on nicotine clearance, its relationships with smoking behavior or the risk of lung cancer have been suggested. The metabolic pathways of the glucuronidation of nicotine, cotinine, and trans-3'-hydroxycotinine in humans would be one of the causal factors for the interindividual differences in nicotine metabolism. This review mainly summarizes recent results from our studies.

Metabolism of nicotine and cotinine by human cytochrome P450 2A13

Nicotine, a major constituent of tobacco, plays a critical role in smoking addiction. In humans, nicotine is primarily metabolized to cotinine, which is further metabolized to trans-3'-hydroxycotinine. Recently, we have demonstrated that heterologously expressed human CYP2A13 is highly active in the metabolism of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a nicotine-derived carcinogen. In the present study, CYP2A13-catalyzed NNK metabolism was found to be inhibited competitively by nicotine and N'-nitrosonornicotine (NNN), suggesting that both nicotine and NNN are also substrates of CYP2A13. We have further demonstrated that human CYP2A13 is indeed an efficient enzyme in catalyzing C-oxidation of nicotine to form cotinine, with the apparent K(m) and V(max) values of 20.2 microM and 8.7 pmol/min/pmol, respectively. CYP2A13 also catalyzes the 3'-hydroxylation of cotinine to form trans-3'-hydroxycotinine, with the apparent K(m) and V(max) values of 45.2 microM and 0.7 pmol/min/pmol, respectively. The importance of CYP2A13-catalyzed nicotine and cotinine metabolism in vivo remains to be determined.

Nicotine-related alkaloids and metabolites as inhibitors of human cytochrome P-450 2A6

S-(-)-Nicotine and 13 of the most prevalent nicotine-related alkaloids and metabolites (i.e., S-(-)-nornicotine, myosmine, beta-nicotyrine, S-cotinine, S-norcotinine, S-(-)-nicotine N-1'-oxide, S-(-)-nicotine Delta1'-5'-iminium ion, S-(-)-anabasine, S-(-)-N-methylanabasine, anabaseine, S-(-)-anatabine, nicotelline, and 2,3'-bipyridyl) were evaluated as inhibitors of human cDNA-expressed cytochrome P-450 2A6 (CYP2A6) mediated coumarin 7-hydroxylation. Tobacco alkaloids myosmine, S-(-)-nornicotine, S-cotinine, S-norcotinine, S-(-)-nicotine N-1'-oxide, S-(-)-nicotine Delta1'-5'-iminium ion, S-(-)-N-methylanabasine, anabaseine, and nicotelline had Ki values for inhibition of coumarin 7-hydroxylation ranging from 20 microM to more than 300 microM whereas nicotine and S-(-)-anatabine were much more potent (i.e. 4.4 and 3.8 microM, respectively). The tobacco alkaloids 2,3'-bipyridyl (7.7 microM) and S-(-)-anabasine (5.4 microM), were somewhat less potent compared with S-(-)-nicotine or S-(-)-anatabine in inhibition of human CYP2A6. beta-Nicotyrine, in which the N-methylpyrrolidino moiety of nicotine was replaced by the aromatic N-methylpyrrole ring, was shown to inhibit human CYP2A6 with much greater potency (Ki=0.37 microM) compared with S-(-)-nicotine. Among the compounds examined, only nicotine and beta-nicotyrine were mechanism-based inhibitors of human CYP2A6. The potency of the mechanism-based CYP2A6 inhibitors suggests that, for smokers, modulation of CYP2A6 may be greater than that predicted on the basis of serum concentration of these alkaloids. Our results indicate that the prominent nicotine-related alkaloid beta-nicotyrine present after smoking potently inhibits human CYP2A6.

Human variability for metabolic pathways with limited data (CYP2A6, CYP2C9, CYP2E1, ADH, esterases, glycine and sulphate conjugation)

Human variability in the kinetics of a number of phase I (CYP2A6, CYP2C9, CYP2E1, alcohol dehydrogenase and hydrolysis) and phase II enzymes (glycine and sulphate conjugation) was analysed using probe substrates metabolised extensively (>60%) by these routes. Published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and available data on subgroups of the population (effects of ethnicity, age and disease) were abstracted using parameters relating primarily to chronic exposure [metabolic and total clearances, area under the plasma concentration time-curve (AUC)] and acute exposure (C(max)). Interindividual differences in kinetics for all these pathways were low in healthy adults ranging from 21 to 34%. Pathway-related uncertainty factors to cover the 95th, 97.5th and 99th centiles of healthy adults were derived for each metabolic route and were all below the 3.16 kinetic default uncertainty factor in healthy adults, with the possible exception of CYP2C9*3/*3 poor metabolisers (based on a very limited number of subjects). Previous analyses of other pathways have shown that neonates represent the most susceptible subgroup and this was true also for glycine conjugation for which an uncertainty factor of 29 would be required to cover 99% of this subgroup. Neonatal data were not available for any other pathway analysed.

Clinical considerations in study designs that use cotinine as a biomarker

Subjects enrolled in studies are not always screened for routine habits such as smoking. Personal history is not always reliable and therefore an objective biomarker is necessary to screen for smokers. The objectives of this article were to review the metabolism of nicotine and other metabolic considerations associated with smoking; to review some of the routine methods used to assess exposure to nicotine-containing products; to revisit cotinine breakpoints utilized to distinguish smokers from non-smokers during screening for clinical trials; to assess the utility of screening questions regarding smoking practices; and to recommend standards for clinical pharmacology studies. The results indicated that cotinine levels serve as a useful biomarker of tobacco exposure; racial issues may be clinically relevant in determining smoking status; cessation of smoking should occur at least 14 days prior to the start of the study; adverse effects from nicotine withdrawal such as craving, hunger and weight gain may persist for more than 6 months; potential metabolic interactions via cytochrome P2A6 and P1A2 need to be considered when designing a study; and the use of a single calibrator as a breakpoint is acceptable if a categorical outcome such as 'smoker' versus 'non-smoker' is desired. Nicotine from food products is not expected to impact assay sensitivity or to be clinically relevant; a serum cotinine concentration of 10 ng ml(-1) be employed as a breakpoint for non-smokers versus smokers; other non-invasive alternatives are collection of urine, saliva, or hair (with suggested breakpoints of 200 ng ml(-1), 5 ng ml(-1) and 0.3 ng mg(-1), respectively; screening questions be accompanied by testing for cotinine; and the inclusion of smokers in studies should be considered once the impact of smoking on the targeted population is understood.

Nicotine metabolism, human drug metabolism polymorphisms, and smoking behaviour

Large interindividual differences occur in human nicotine disposition, and it has been proposed that genetic polymorphisms in nicotine metabolism may be a major determinant of an individual's smoking behaviour. Hepatic cytochrome P4502A6 (CYP2A6) catalyses the major route of nicotine metabolism: C-oxidation to cotinine, followed by hydroxylation to trans-3'-hydroxycotinine. Nicotine and cotinine both undergo N-oxidation and pyridine N-glucuronidation. Nicotine N-1-oxide formation is catalysed by hepatic flavin-containing monooxygenase form 3 (FMO3), but the enzyme(s) required for cotinine N-1'-oxide formation has not been identified. trans-3'-Hydroxycotinine is conjugated by O-glucuronidation. The uridine diphosphate-glucuronosyltransferase (UGT) enzyme(s) required for N- and O-glucuronidation have not been identified. CYP2A6 is highly polymorphic resulting in functional differences in nicotine C-oxidation both in vitro and in vivo; however, population studies fail to consistently and conclusively demonstrate any associations between variant CYP2A6 alleles encoding for either reduced or enhanced enzyme activity with self-reported smoking behaviour. The functional consequences of FMO3 and UGT polymorphisms on nicotine disposition have not been investigated, but are unlikely to significantly affect smoking behaviour. Therefore, current evidence does not support the hypothesis that genetic polymorphisms associated with nicotine metabolism are a major determinant of an individual's smoking behaviour and exposure to tobacco smoke.

CYP2A6 genetic variation and potential consequences

Human cytochrome P450 2A6 (CYP2A6) has been shown to have large interindividual and interethnic variability in levels of expression and activity. This is thought to be largely due to genetic polymorphisms. In recent years, 13 genetic variants (CYP2A6*1-*11 and the gene duplication, *1 x 2) of CYP2A6 have been identified and a number of these have been shown to result in altered CYP2A6 enzyme activity. For example, there are alleles which result in variants that are in inactive (e.g. due to a gene deletion), have decreased activity (e.g. altered enzyme structure or transcriptional activity) or have increased activity (e.g. due to gene duplications). The resulting interindividual variation in metabolic activity may affect the metabolism of CYP2A6 substrates including nicotine, cotinine (the major metabolite of nicotine), several tobacco-specific procarcinogens, coumarin and many toxins. The frequencies of the CYP2A6 alleles vary considerably among different ethnic populations, which may partially explain the interethnic variability found in CYP2A6-related metabolic activity (e.g. nicotine metabolism), behaviors (i.e. smoking) and disease (i.e. lung cancer). Investigations of the genetic variation of CYP2A6 and its resulting effects on metabolism and health consequences are still fairly early; this review summarizes what is presently known about CYP2A6, its genetic variants and their clinical consequences.

N-glucuronidation of nicotine and cotinine in human: formation of cotinine glucuronide in liver microsomes and lack of catalysis by 10 examined UDP-glucuronosyltransferases

Two predominant human glucuronide metabolites of nicotine result from pyridine nitrogen atom conjugation. The present objectives included determination of the kinetics of formation of S(-)-cotinine N1-glucuronide in pooled human liver microsomes and investigation of the UDP-glucuronosyltransferases (UGTs) involved in N-glucuronidation of nicotine isomers and S(-)-cotinine by use of recombinant enzymes (UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A7, UGT1A8, UGT1A9, UGT1A10, UGT2B7, and UGT2B15). Quantification was by radiochemical high-performance liquid chromatography with use of radiolabeled substrates. S(-)-Cotinine N1-glucuronide formation in human liver microsomes was proven by comparing the chromatographic behaviors and electrospray ionization-mass spectral characteristics of the metabolite with a synthetic reference standard. This glucuronide was formed by one-enzyme kinetics with K(m) and V(max) values of 5.4 mM and 696 pmol/min/mg, respectively, and the apparent intrinsic clearance value (V(max/Km)) was 9-fold less than that previously determined for S(-)-nicotine N1-glucuronide (0.13 versus 1.2 microl/min/mg) using the same pooled microsomes. This comparison of values is consistent with the observation that on smoking cigarettes, although the average S(-)-cotinine plasma levels usually far exceed S(-)-nicotine levels, the urinary recovery of S(-)-cotinine N1-glucuronide only averages 3-fold greater than for S(-)-nicotine N1-glucuronide. None of the UGTs examined catalyzed the N-glucuronidation of S(-)-nicotine, R(+)-nicotine, and S(-)-cotinine, including UGT1A3 and UGT1A4, the only isoforms known to catalyze many substrates at a tertiary amine. Also, neither S(-)-nicotine or S(-)-cotinine affected enzyme inhibition of trifluoperazine, a UGT1A4 substrate. It would appear that the same, as yet unexamined, UGT catalyzes the N-glucuronidation of both cotinine and nicotine.

Genetic variation in CYP2A6-mediated nicotine metabolism alters smoking behavior

Approximately 50% of the initiation of tobacco dependence is genetically influenced, whereas maintenance of dependent smoking behavior and amount smoked have approximately 70% genetic contribution (1-5). Determining the variation in nicotine's inactivation is important because of nicotine's role in producing tobacco dependence and regulating smoking patterns (6-11). The genetically polymorphic CYP2A6 enzyme is responsible for the majority of the metabolic inactivation of nicotine to cotinine (12-14). Both in vitro and in vivo studies have demonstrated considerable interindividual variation in CYP2A6 activity (15-17). CYP2A6 is genetically polymorphic, individuals carrying inactive CYP2A6 alleles have decreased nicotine metabolism, are less likely to become smokers and if they do, they smoke fewer cigarettes per day (13,18,19). The decrease in smoking behavior was confirmed by measuring carbon monoxide (CO, a measure of smoke inhalation) levels, plasma and urine nicotine and cotinine levels, and cigarette counts (13,18,19). A duplication variant in the CYP2A6 gene locus has been identified which increases nicotine inactivation and increases smoking (19). CYP2A6 can also activate tobacco smoke procarcinogens (e.g. NNK, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone); current studies are investigating the role of CYP2A6 in risk for lung cancer. Based on these epidemiologic data it was postulated that inhibition of CYP2A6 activity might be useful in a therapeutic context. Kinetic studies in humans indicated that selective CYP2A6 inhibitors decrease the metabolic removal of nicotine. It was also shown that inhibiting CYP2A6 in vivo (phenocopying, or mimicking the genetic defect) in smokers results in decreased smoking, making nicotine orally bioavailable, and the rerouting of procarcinogens to detoxifying pathways (20-22).

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

Polymorphisms of CYP2A6 and its practical consequences

CYP2A6 is an hepatic enzyme predominantly with some expression in specialized extrahepatic cell types. The CYP2A6 enzyme has a somewhat restricted active site, accepting only a few xenobiotics as substrates. Interest in CYP2A6 has risen considerably after nicotine and some tobacco specific nitrosamines were established as high-affinity substrates for this enzyme. Recently, the organization and structures of the CYP2A gene cluster and several polymorphic alleles of the CYP2A6 gene have been characterized. Two alleles with a point mutation and at least three different types of gene deletion, all leading to deficient gene function, have been found. The frequencies of these alleles vary considerably among different ethnic populations, the deletion alleles being most common in Orientals (up to 20%). The frequency of point mutations are low in all populations studied thus far (< 3%). Several case-control studies have addressed the relationship between CYP2A6 status and smoking habits as well as the role of CYP2A6 polymorphism in lung cancer risk. Studies in Japanese suggest that CYP2A6 poor metabolizer genotypes result in altered nicotine kinetics and may lower cigarette smoking elicited lung cancer risk, whereas similar studies in Caucasian populations have not revealed any clear associations between variant CYP2A6 genotypes and smoking behaviour or lung cancer predisposition.

Identification of a single nucleotide polymorphism in the TATA box of the CYP2A6 gene: impairment of its promoter activity

Human cytochrome P450 2A6 (CYP2A6) constitutes the major nicotine oxidase, and large interindividual differences are seen in the levels of this enzyme, to a great extent caused by the distribution of several different polymorphic gene variants mainly located in the open reading frame (ORF). In the present study, we report a common polymorphism located in the 5' flanking region of CYP2A6 affecting its expression. DHPLC analysis and complete sequence of the open reading frame of the gene from a Turkish individual revealed a -48T > G substitution disrupting the TATA box. Using dynamic allele-specific hybridization (DASH), genotyping of this novel variant (named CYP2A6*9) was carried out in 116 Swedish, 132 Turkish, and 102 Chinese subjects, and the allele frequencies were found to be 5.2, 7.2, and 15.7%, respectively. The significance of the polymorphism was investigated by the construction of luciferase reporter plasmids containing 135 or 500 bp of the 5'-upstream region of the gene transfected into human hepatoma B16A2 cells. The constructs carrying the -48T > G mutation were only expressed at about 50% of the wild-type alleles. It is concluded that the CYP2A6*9 allele might be one of the most common CYP2A6 variants in Caucasians that alters the levels of enzyme expression.

Duplications and defects in the CYP2A6 gene: identification, genotyping, and in vivo effects on smoking

In humans, 80% of nicotine is metabolized to the inactive metabolite cotinine by the enzyme CYP2A6, which can also activate tobacco smoke procarcinogens (e.g., 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone). Previously, we demonstrated that individuals who are nicotine-dependent and have defective CYP2A6 alleles (*2, *3) smoked fewer cigarettes; however, we recognize that the genotyping method used for the CYP2A6*3 allele gave a high false-positive rate. In the current study we used improved genotyping methods to examine the effects of the defective CYP2A6*2 and CYP2A6*4 alleles on smoking behavior. We found that those with the defective alleles (N = 14) smoked fewer cigarettes per day than those homozygous (N = 277) for wild-type alleles (19 versus 28 cigarettes per day, P <.001). In addition, we identified a duplicated form of the CYP2A6 gene, corresponding to the gene deletion CYP2A6*4 allele, developed a genotyping assay, assessed the gene copy number, and examined its prevalence in Caucasian smokers (N = 296). We observed an ascending rank order for plasma cotinine and breath carbon monoxide levels (an index of smoke inhalation) in individuals with null (CYP2A6*2 and CYP2A6*4) alleles (N = 14), those homozygous for wild-type (CYP2A6*1/*1) alleles (N = 277), and those with our newly identified CYP2A6 gene duplication (N = 5). The phenotype, as determined by plasma nicotine/cotinine ratios, had a descending rank order for these three genotype groups that did not reach significance. Although further characterization is required for the duplication gene variant, these results extend our previous findings and suggest a substantial influence of CYP2A6 genotype and phenotype on smoking behavior.

Liquid chromatography-mass spectrometry in the study of the metabolism of drugs and other xenobiotics

The application of liquid chromatography-mass spectrometry (LC/MS) to the study of metabolism of drugs and other xenobiotics is reviewed. Original research papers covering the period from 1998 to early 2000 and concerning the use of LC/MS in the study of xenobiotic metabolism in humans and other mammalian species are reviewed. LC/MS interfaces, sample preparation steps, column types, mobile phases and additives, and the type of metabolites detected are summarized and discussed in an attempt to identify the current and future trends in the use of LC/MS for metabolism studies. Applications are listed according to the parent xenobiotic type and include substances used in therapeutics, drug candidates, compounds being evaluated in clinical trials, environmental pollutants, adulterants and naturally occurring substances.