Metabolic profile of nicotine in subjects whose CYP2A6 gene is deleted

CYP2A6 gene polymorphism and personality traits for NEO-FFI on the smoking behavior of youths

We performed a survey on the relationship between nicotine dependence and psychological (the personality traits using neuroticism extroversion openess-five factor inventory (NEO-FFI)/nicotine metabolism (the CYP2A6 gene polymorphism) factors among Japanese young students to elucidate the mechanism of the development of nicotine dependence. As a result, the frequency of the CYP2A6*4C gene (enzyme activity deficit) was significantly (p<0.05) higher among nonsmokers than smokers, and the Openness score by NEO-FFI was higher among smokers than nonsmokers. However, there were no significant differences in the Neuroticism, Extraversion, Agreeableness, and Conscientiousness scores by NEO-FFI between smokers and nonsmokers. We conclude that the CYP2A6*4C gene and the Openness personality trait may affect the development of the smoking behavior of youth.

Identification of novel CYP2A6*1B variants: the CYP2A6*1B allele is associated with faster in vivo nicotine metabolism

Cytochrome P450 2A6 (CYP2A6) is the human enzyme responsible for the majority of nicotine's metabolism. CYP2A6 genetic variants contribute to the interindividual and interethnic variation in nicotine metabolism. We examined the association between the CYP2A6*1B variant and nicotine's in vivo metabolism. Intravenous infusions of deuterium-labeled nicotine were administered to 292 volunteers, 163 of whom were White and did not have common CYP2A6 variants, other than CYP2A6*1B. We discovered three novel CYP2A6*1B variants in the 3'-flanking region of the gene that can confound genotyping assays. We found significant differences between CYP2A6*1A/*1A, CYP2A6*1A/*1B, and CYP2A6*1B/*1B groups in total nicotine clearance (17.2+/-5.2, 19.0+/-6.4, and 20.4+/-5.9, P<0.02), non-renal nicotine clearance (16.4+/-5.0, 18.5+/-6.2, and 19.8+/-5.7, P<0.01), and the plasma trans-3'-hydroxycotinine/cotinine ratio (0.26+/-0.1, 0.26+/-0.1, and 0.34+/-0.1, P<0.001). There were also differences in total nicotine (29.4+/-12.9, 25.8+/-0.12.9, and 22.4+/-12.4, P<0.01), cotinine (29.2+/-8.1, 32.2+/-9.1, and 33.0+/-6.6, P<0.01) and trans-3'-hydroxycotinine (32.4+/-9.1, 34.2+/-12.3, and 41.3+/-11.3, P<0.001) excreted in the urine. We report evidence that CYP2A6*1B genotype is associated with faster nicotine clearance in vivo, which will be important to future CYP2A6 genotype association studies.

Past and future applications of CYP450-genetic polymorphisms for biomonitoring of environmental toxicants

Cytochrome P450s (CYPs) are a huge gene superfamily of heme enzymes involved in xenobioitc as well as endobiotic metabolism. They play a critical role in adaptation to environmental changes for survival of living organisms. In addition, the huge environmental loads of human-made chemicals are biotransformed into bioactive or detoxified forms by CYPs. Thus, CYPs have been used for biomonitoring of environmental pollutants, screening of their metabolisms and exploring remedy. In particular, the induction or inhibition of CYPs has been applied to exposure monitoring of environmental toxicants, which are biotransformed by CYPs. This review considers past and future applications of CYP-genetic polymorphisms as susceptibility biomarkers for biomonitoring. Furthermore, we suggest the needs for further understanding of the characteristics of each CYP isozyme, consideration of real-life exposures such as mixed contamination with various chemicals, and incorporation of the presence of other phase I and phase II enzymes, for proper applications of CYP polymorphisms on biomonitoring.

Genetic variability in CYP2A6 and the pharmacokinetics of nicotine

Nicotine is the psychoactive substance responsible for tobacco dependence. It is also a therapeutic used to aid smoking cessation. Cytochrome P450 (CYP)2A6 is the human hepatic enzyme that mediates most of nicotine’s metabolic inactivation to cotinine. Genetic variation in the CYP2A6 gene can increase or decrease enzyme activity through altering the protein’s expression level or its structure and function. This article reviews CYP2A6 genetic variation and its impact on in vivo nicotine kinetics, including a description of the individual variants, different phenotyping approaches for assessing in vivo CYP2A6 activity and other sources of variation in nicotine metabolism such as gender. In addition, the effect of CYP2A6 polymorphisms on smoking behavior and tobacco-related lung cancer risk are briefly described. Furthering knowledge in this area will improve interpretation of studies examining smoking behavior, as well as those using nicotine as a therapeutic agent.

Application of nicotine enantiomers, derivatives and analogues in therapy of neurodegenerative disorders

This review gives a brief overview over the major aspects of application of the nicotine alkaloid and its close derivatives in the therapy of some neurodegenerative disorders and diseases (e.g. Alzheimer's disease, Parkinson's disease, Tourette's syndrome, schizophrenia etc.). The issues concerning methods of nicotine analysis and isolation, and some molecular aspects of nicotine pharmacology are included. The natural and synthetic analogues of nicotine that are considered for medical practice are also mentioned. The molecular properties of two naturally occurring nicotine enantiomers are compared--the less-common but less-toxic (R)-nicotine is suggested as a natural compound that may find its place in pharmaceutical practice.

Inhibitory effects of neurotransmitters and steroids on human CYP2A6

Human CYP2A6 catalyzes the metabolism of nicotine, cotinine, and coumarin as well as some pharmaceutical drugs. CYP2A6 is highly expressed in liver and, also, in brain and steroid-related tissues. In this study, we investigated the inhibitory effects of neurotransmitters and steroid hormones on CYP2A6 activity. We found that coumarin 7-hydroxylation and cotinine 3'-hydroxylation by recombinant CYP2A6 expressed in baculovirus-infected insect cells were competitively inhibited by tryptamine (both K(i) = 0.2 microM), serotonin (K(i) = 252 microM and 167 microM), dopamine (K(i) = 49 microM and 22 microM), and histamine (K(i) = 428 microM and 359 microM). Cotinine formation from nicotine was inhibited by tryptamine (K(i) = 0.7 microM, competitive), serotonin (K(i) = 272 microM, noncompetitive), dopamine, noradrenaline, and adrenaline (K(i) = 11 microM, 54 microM, and 81 microM, uncompetitive). Estrogens (K(i) = 0.6-3.8 microM), androgens (K(i) = 60-149 microM), and corticosterone (K(i) = 36 microM) also inhibited cotinine formation, but coumarin 7-hydroxylation and cotinine 3'-hydroxylation did not. Nicotine-Delta(5'(1'))-iminium ion formation from nicotine was not affected by these steroid hormones, indicating that the inhibition of cotinine formation was due to the inhibitory effects on aldehyde oxidase. The nicotine-Delta(5'(1'))-iminium ion formation was competitively inhibited by tryptamine (K(i) = 0.3 microM), serotonin (K(i) = 316 microM), dopamine (K(i) = 66 microM), and histamine (K(i) = 209 microM). Thus, we found that some neurotransmitters inhibit CYP2A6 activity, being related with inter- and intraindividual differences in CYP2A6-dependent metabolism. The inhibitory effects of steroid hormones on aldehyde oxidase may also contribute to interindividual differences in nicotine metabolism.

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.

Non-Nicotinic Therapies for Smoking Cessation

Cigarette smoking is the primary cause of numerous preventable diseases; as such, the goals of smoking cessation are both to reduce health risks and to improve the quality of life. Currently, the first-line smoking cessation therapies include nicotine replacement products and bupropion. The nicotinic receptor partial agonist varenicline has recently been approved by the FDA for smoking cessation. A newer product currently under development and seeking approval by the FDA are nicotine vaccines. Clonidine and nortriptyline have demonstrated some efficacy but side effects may limit their use to second-line therapeutic products. Other therapeutic drugs that are under development include rimonabant, mecamylamine, monoamine oxidase inhibitors, and dopamine receptor D3 antagonists. Inhibitors of nicotine metabolism are also promising candidates for smoking reduction and cessation. In conclusion, promising new therapeutic products are emerging and they will provide smokers additional options to assist in achieving smoking cessation.

Isoflavones inhibit nicotine C-oxidation catalyzed by human CYP2A6

The inhibitory effects of isoflavones (daidzein, genistein, and glycitein) on human cytochrome P450 (CYP) 2A6 activities were investigated. Daidzein, genistein, and glycitein uncompetitively inhibited nicotine C-oxidation catalyzed by recombinant CYP2A6 expressed in baculovirus-infected insect cells with Ki values of 1.3 +/- 0.3 microM, 0.7 +/- 0.2 microM, and 5.2 +/- 0.8 microM, respectively, but not coumarin 7-hydroxylation. Effects of the intake of soy isoflavones on in vivo nicotine metabolism were investigated with 7 healthy Japanese homozygotes of CYP2A6*1. The cotinine/nicotine ratio of the plasma concentrations 2 hours after chewing 1 piece of nicotine gum under the basal condition (after abstaining from soy foods for 1 week) was 8.8 +/- 2.6 (4.4-11.4). The ratio was significantly (P < .05) reduced to 6.7 +/- 1.6 (4.0-8.2) after consumption of a soy isoflavone supplement (60 mg of total isoflavones/d) for 5 days. The authors found that isoflavone contained in soy products significantly decreased nicotine metabolism.

CYP2A6, MAOA, DBH, DRD4, and 5HT2A genotypes, smoking behaviour and cotinine levels in 1518 UK adolescents

OBJECTIVES

Smoking is a major cause of death and often initiates in adolescence. Mutations in CYP2A6 slow metabolism of nicotine to cotinine. Haploinsufficiency in adults is associated with lower cigarette consumption, lower cotinine level and higher quit rates. Other genes are also implicated in smoking behaviour. We explored smoking behaviour and cotinine levels in relation to genotypes in adolescents.

METHODS

1518 subjects from the Ten Towns Heart Health Study were genotyped for CYP2A6 alleles *1A, *1B, *2, *4, *5, *9 and *12 to classify predicted nicotine metabolism rate. DBH(rs77905), MAOA(rs1801291+VNTR), DRD4(VNTR) and 5HT2A(rs6313) were also studied. Smoking status was established by questionnaire and salivary cotinine measurement at 13-15 and 18 years.

RESULTS

No significant associations were identified for DBH, MAOA, DRD4 and 5HT2A markers, with smoking status or cotinine level at either age. At age 18, haploinsufficiency (HI) for CYP2A6 was associated with a higher odds of being a current smoker compared with the *1B carriers (WT1B) (OR = 2.23 (1.16, 4.27) for current versus ex); *1A homozygotes (WT1A) were also at slightly higher risk (OR = 1.44 (1.01, 2.06)). Partial haploinsufficiency (PHI) was not associated with being a current smoker. There were no significant associations at age 13-15. PHI and HI were associated with higher cotinine levels amongst smokers at both 13-15 and at 18 years compared with WT1B and WT1A groups.

CONCLUSIONS

CYP2A6 haploinsufficiency increases likelihood of continuing smoking in teenagers. We hypothesize an explanatory 'occupancy' model to explain why haploinsufficiency results in faster progression to nicotine dependence, but lower subsequent consumption.

CYP2A6 AND CYP2B6 are involved in nornicotine formation from nicotine in humans: interindividual differences in these contributions

Nornicotine is an N-demethylated metabolite of nicotine. In the present study, human cytochrome P450 (P450) isoform(s) involved in nicotine N-demethylation were identified. The Eadie-Hofstee plot of nicotine N-demethylation in human liver microsomes was biphasic with high-affinity (apparent K(m) = 173 +/- 70 microM, V(max) = 57 +/- 17 pmol/min/mg) and low-affinity (apparent K(m) = 619 +/- 68 microM, V(max) = 137 +/- 6 pmol/min/mg) components. Among 13 recombinant human P450s expressed in baculovirus-infected insect cells (Supersomes), CYP2B6 exhibited the highest nicotine N-demethylase activity, followed by CYP2A6. The apparent K(m) values of CYP2A6 (49 +/- 12 microM) and CYP2B6 (550 +/- 46 microM) were close to those of high- and low-affinity components in human liver microsomes, respectively. The intrinsic clearances of CYP2A6 and CYP2B6 Supersomes were 5.1 and 12.5 nl/min/pmol P450, respectively. In addition, the intrinsic clearance of CYP2A13 expressed in Escherichia coli (44.9 nl/min/pmol P450) was higher than that of CYP2A6 expressed in E. coli (2.6 nl/min/pmol P450). Since CYP2A13 is hardly expressed in human livers, the contribution of CYP2A13 to the nicotine N-demethylation in human liver microsomes would be negligible. The nicotine N-demethylase activity in microsomes from 15 human livers at 20 microM nicotine was significantly correlated with the CYP2A6 contents (r = 0.578, p < 0.05), coumarin 7-hydroxylase activity (r = 0.802, p < 0.001), and S-mephenytoin N-demethylase activity (r = 0.694, p < 0.005). The nicotine N-demethylase activity at 100 microM nicotine was significantly correlated with the CYP2B6 contents (r = 0.677, p < 0.05) and S-mephenytoin N-demethylase activities (r = 0.740, p < 0.005). These results as well as the inhibition analyses suggested that CYP2A6 and CYP2B6 would significantly contribute to the nicotine N-demethylation at low and high substrate concentrations, respectively. The contributions of CYP2A6 and CYP2B6 would be dependent on the expression levels of these isoforms in any human liver.

Nicotine 5'-oxidation and methyl oxidation by P450 2A enzymes

In smokers, the primary pathway of nicotine metabolism is P450 2A6-catalyzed 5'-oxidation. The nicotine Delta(5'(1'))-iminium ion product of this reaction is further metabolized to cotinine by aldehyde oxidase. Previous investigators have reported kinetic parameters for cotinine formation using human liver cytosol as a source of aldehyde oxidase. Using [5-(3)H]nicotine and radioflow high-performance liquid chromatography analysis, we determined kinetic parameters for nicotine 5'-oxidation by P450 2A6 and the closely related human extrahepatic P450 2A13 as well as the rodent P450s 2A3, 2A4, and 2A5. The formation of both cotinine and nicotine Delta(5'(1'))-iminium ion was monitored. The K(m) and V(max) values for P450 2A6 were 144 +/- 15 muM and 1.30 +/- 0.05 pmol/min/pmol, respectively. Previously reported K(m) values for cotinine formation by P450 2A6 in the presence of cytosol were much lower, ranging from 11 to 45 muM. P450 2A13 was a somewhat better catalyst of nicotine Delta(5'(1'))-iminium formation, with 2-fold lower K(m) and 2-fold higher V(max) values than P450 2A6. The rat P450 2A3 and the mouse P450 2A5, which are 85 and 84% identical to P450 2A6, were much more efficient catalysts of nicotine 5'-oxidation. P450 2A4 was not an efficient catalyst of nicotine metabolism. Whereas 5'-oxidation was the major pathway of nicotine metabolism for all five P450 2A enzymes, these enzymes also catalyzed methyl oxidation. Nornicotine, the product of this reaction was detected as 5 to 15% of the total nicotine metabolites. Nornicotine is the amine precursor to the esophageal carcinogen N'-nitrosonornicotine. Therefore, methyl oxidation of nicotine by P450 2A6 or P450 2A13 followed by nitrosation of nornicotine are possible endogenous pathways of N'-nitrosonornicotine formation.

Brain uptake kinetics of nicotine and cotinine after chronic nicotine exposure

Blood-brain barrier (BBB) nicotine transfer has been well documented in view of the fact that this alkaloid is a cerebral blood flow marker. However, limited data are available that describe BBB penetration of the major tobacco alkaloids after chronic nicotine exposure. This question needs to be addressed, given long-term nicotine exposure alters both BBB function and morphology. In contrast to nicotine, it has been reported that cotinine (the major nicotine metabolite) does not penetrate the BBB, yet cotinine brain distribution has been well documented after nicotine exposure. Surprisingly, therefore, the literature indirectly suggests that central nervous system cotinine distribution occurs secondarily to nicotine brain metabolism. The aims of the current report are to define BBB transfer of nicotine and cotinine in naive and nicotine-exposed animals. Using an in situ brain perfusion model, we assessed the BBB uptake of [3H]nicotine and [3H]cotinine in naive animals and in animals exposed chronically to S-(-)nicotine (4.5 mg/kg/day) through osmotic minipump infusion. Our data demonstrate that 1) [3H]nicotine BBB uptake is not altered in the in situ perfusion model after chronic nicotine exposure, 2) [3H]cotinine penetrates the BBB, and 3) similar to [3H]nicotine, [3H]cotinine BBB transfer is not altered by chronic nicotine exposure. To our knowledge, this is the first report detailing the uptake of nicotine and cotinine after chronic nicotine exposure and quantifying the rate of BBB penetration by cotinine.

Chronic nicotine exposure alters blood-brain barrier permeability and diminishes brain uptake of methyllycaconitine

Methyllycaconitine (MLA) is reported to be a selective antagonist for the nicotinic acetylcholine receptor alpha7 subtype and has been found in animal behavioral studies to reduce nicotine self-administration and attenuate nicotine withdrawal symptoms. While MLA crosses the blood-brain barrier (BBB), no studies have assessed brain uptake in animals subjected to chronic nicotine exposure. Given that chronic nicotine administration has been reported to alter BBB parameters that may affect the kinetic BBB passage of MLA, we evaluated MLA brain uptake in naive and S-(-)nicotine-exposed rats (4.5 mg/kg/day for 28 days; osmotic minipumps) using in situ rat brain perfusions. Our results demonstrate that in situ(3)H-MLA brain uptake rates in naive animals approximate to intravenous kinetic data (K(in), 3.24 +/- 0.71 x 10(-4) mL/s/g). However, 28-day nicotine exposure diminished (3)H-MLA brain uptake by approximately 60% (K(in), 1.29 +/- 0.4 x 10(-4) mL/s/g). This reduction was not related to nicotine-induced (3)H-MLA brain efflux or BBB transport alterations. Similar experiments also demonstrated that the passive permeation of (14)C-thiourea was diminished approximately 24% after chronic nicotine exposure. Therefore, it appears that chronic nicotine exposure diminishes the blood-brain passive diffusion of compounds with very low extraction rates (i.e. permeability-limited compounds). These findings imply that the pharmacokinetics of neuropharmaceutical agents that are permeability limited may need to be re-evaluated in individuals exposed to nicotine.

Molecular epidemiology of lung cancer in female passive smokers

To clarify etiology of lung cancer in nonsmoker females, various studies have been done. Particularly, host factors and environmental tobacco smoking (ETS) of females have been emphasized. However, traditional epidemiological data showed controversial results of sex or gender differences in lung cancer susceptibility and suggest presence of some confounders. One of them is that most of epidemiology studies are based on self-reports for ETS. To prevent misestimate effects of ETS via the self-report, exposure monitoring of ETS is required. On the other hand, focusing on genetic polymorphisms in metabolic enzymes and DNA repair, molecular epidemiological studies have been done in nonsmoker females. Therefore, this review considered: 1. gender differences in lung cancer; 2. effects of passive smoking on lung cancer; 3. exposure monitoring of ETS including genetic risks of lung cancer to clarify etiology of lung cancer in the nonsmoker females with molecular epidemiological discussion.

Trans-3'-hydroxycotinine O- and N-glucuronidations in human liver microsomes

Trans-3'-hydroxycotinine is a major metabolite of nicotine in humans and is mainly excreted as O-glucuronide in smoker's urine. Incubation of human liver microsomes with UDP-glucuronic acid produces not only trans-3'-hydroxycotinine O-glucuronide but also N-glucuronide. The formation of N-glucuronide exceeds the formation of O-glucuronide in most human liver microsomes, although N-glucuronide has never been detected in human urine. Trans-3'-hydroxycotinine N-glucuronidation in human liver microsomes was significantly correlated with nicotine and cotinine N-glucuronidations, which are catalyzed mainly by UDP-glucuronosyltransferase (UGT)1A4 and was inhibited by imipramine and nicotine, which are substrates of UGT1A4. Recombinant UGT1A4 exhibited substantial trans-3'-hydroxycotinine N-glucuronosyltransferase activity. These results suggest that trans-3'-hydroxycotinine N-glucuronidation in human liver microsomes would be mainly catalyzed by UGT1A4. In the present study, trans-3'-hydroxycotinine O-glucuronidation in human liver microsomes was thoroughly characterized, since trans-3'-hydroxycotinine O-glucuronide is one of the major metabolites of nicotine. The kinetics were fitted to the Michaelis-Menten equation with a K(m) of 10.0 +/- 0.8 mM and a V(max) of 85.8 +/- 3.8 pmol/min/mg. Among 11 recombinant human UGT isoforms expressed in baculovirus-infected insect cells, UGT2B7 exhibited the highest trans-3'-hydroxycotinine O-glucuronosyltransferase activity (1.1 pmol/min/mg) followed by UGT1A9 (0.3 pmol/min/mg), UGT2B15 (0.2 pmol/min/mg), and UGT2B4 (0.2 pmol/min/mg) at a substrate concentration of 1 mM. Trans-3'-hydroxycotinine O-glucuronosyltransferase activity by recombinant UGT2B7 increased with an increase in the substrate concentration up to 16 mM (10.5 pmol/min/mg). The kinetics by recombinant UGT1A9 were fitted to the Michaelis-Menten equation with K(m) = 1.6 +/- 0.1 mM and V(max) = 0.69 +/- 0.02 pmol/min/mg of protein. Trans-3'-hydroxycotinine O-glucuronosyltransferase activities in 13 human liver microsomes ranged from 2.4 to 12.6 pmol/min/mg and were significantly correlated with valproic acid glucuronidation (r = 0.716, p < 0.01), which is catalyzed by UGT2B7, UGT1A6, and UGT1A9. Trans-3'-hydroxycotinine O-glucuronosyltransferase activity in human liver microsomes was inhibited by imipramine (a substrate of UGT1A4, IC(50) = 55 microM), androstanediol (a substrate of UGT2B15, IC(50) = 169 microM), and propofol (a substrate of UGT1A9, IC(50) = 296 microM). Interestingly, imipramine (IC(50) = 45 microM), androstanediol (IC(50) = 21 microM), and propofol (IC(50) = 41 microM) also inhibited trans-3'-hydroxycotinine O-glucuronosyltransferase activity by recombinant UGT2B7. These findings suggested that trans-3'-hydroxycotinine O-glucuronidation in human liver microsomes is catalyzed by mainly UGT2B7 and, to a minor extent, by UGT1A9.

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.