Species variation and stereoselectivity in the metabolism of nicotine enantiomers

Evaluating the Role of Nicotine Stereoisomer on Nicotine Pouch Abuse Liability: A Randomized Crossover Trial

INTRODUCTION

Many oral nicotine pouch (ONP) brands use synthetic nicotine, which typically contains a racemic (50:50) mixture of nicotine's two stereoisomers: S-nicotine and R-nicotine. Because tobacco-derived nicotine contains more than 99% S-nicotine, the effects of R-nicotine in humans are not well known. We compared systemic nicotine exposure and product appeal of ONPs containing more than 99% S-nicotine versus racemic nicotine.

AIMS AND METHODS

N = 18 adult smokers (Mage = 45 years, 66.7% male, 77.8% White) enrolled in a three-visit single-blind, randomized crossover study. During each visit, participants used one wintergreen-flavored, 3 mg nicotine ONP for 30 min following at least12 h nicotine abstinence. Study ONP #1 contained more than 99% S-nicotine and the other two study ONPs contained racemic nicotine (collapsed for analyses). Plasma nicotine assessments and measures of withdrawal relief occurred at t = 0, 5, 15, 30, 60, and 90 min; measures of product appeal were assessed following ONP use.

RESULTS

Using the ONP with more than 99% S-nicotine resulted in greater plasma nicotine concentration from 15 to 90 min (p < .0001) and greater maximum plasma nicotine concentration than the ONPs with racemic nicotine (M = 9.9 ng/mL [SD = 2.5] vs. M = 5.7 ng/mL [SD = 2.8], respectively; p < .0001). Product liking and withdrawal relief were similar across ONPs, although participants reported more "bad effects" when using the ONP with more than 99% S-nicotine.

CONCLUSIONS

Participants reported few subjective differences in ONPs according to nicotine stereoisomer, but plasma nicotine concentration was greater for ONPs using more than 99% S-nicotine. ONPs with more than 99% S-nicotine (vs. racemic nicotine) might be better substitutes for cigarettes, but research into other ONP characteristics (eg flavors, freebase nicotine) is needed to inform regulation.

IMPLICATIONS

Little is known about the effects of racemic (vs. S-) nicotine in humans. In a sample of adults who smoke cigarettes, we identified that oral nicotine pouches containing racemic nicotine exposed participants to less nicotine than oral nicotine pouches containing only S-nicotine, but both types of oral nicotine pouches held similar, moderate appeal. Additional research evaluating the roles that flavorings, total nicotine concentration, and freebase nicotine play in the abuse liability of oral nicotine pouches would inform comprehensive product regulations to support public health.

Identification and Characterization of Synthetic Nicotine Product Promotion and Sales on Instagram Using Natural Language Processing

INTRODUCTION

There has been a rapid proliferation of synthetic nicotine products in recent years, despite newly established regulatory authority and limited research into its health risks. Previous research has implicated social media platforms as an avenue for nicotine product unregulated sales. Yet, little is known about synthetic nicotine product content on social media. We utilized natural language processing to characterize the sales of synthetic nicotine products on Instagram.

METHODS

We collected Instagram posts by querying Instagram hashtags (eg, "#tobaccofreenicotine) related to synthetic nicotine. Using Bidirectional Encoder Representations from Transformers, collected posts were categorized into thematically related topic clusters. Posts within topic clusters relevant to study aims were then manually annotated for variables related to promotion and selling (eg, cost discussion, contact information for offline sales).

RESULTS

A total of 7425 unique posts were collected with 2219 posts identified as related to promotion and selling of synthetic nicotine products. Nicotine pouches (52.9%, n = 1174), electronic nicotine delivery systems (30.6%, n = 679), and flavored e-liquids (14.1%, n = 313) were most commonly promoted. About 16.1% (n = 345) of posts contained embedded hyperlinks and 5.8% (n = 129) provided contact information for purported offline transactions. Only 17.6% (n = 391) of posts contained synthetic nicotine-specific health warnings.

CONCLUSIONS

In the United States, synthetic nicotine products can only be legally marketed if they have received premarket authorization from the Food and Drug Administration (FDA). Despite these prohibitions, Instagram appears to be a hub for potentially unregulated sales of synthetic and "tobacco-free" products. Efforts are needed by platforms and regulators to enhance content moderation and prevent unregulated online sales of existing and emerging synthetic nicotine products.

IMPLICATIONS

There is limited clinical understanding of synthetic nicotine's unique health risks and how these novel products are changing over time due to regulatory oversight. Despite synthetic nicotine-specific regulatory measures, such as the requirement for premarket authorization and FDA warning letters issued to unauthorized sellers, access to and promotion of synthetic nicotine is widely occurring on Instagram, a platform with over 2 billion users and one that is popular among youth and young adults. Activities include direct-to-consumer sales from questionable sources, inadequate health warning disclosure, and exposure with limited age restrictions, all conditions necessary for the sale of various tobacco products. Notably, the number of these Instagram posts increased in response to the announcement of new FDA regulations. In response, more robust online monitoring, content moderation, and proactive enforcement are needed from platforms who should work collaboratively with regulators to identify, report, and remove content in clear violation of platform policies and federal laws. Regulatory implementation and enforcement should prioritize digital platforms as conduits for unregulated access to synthetic nicotine products and other future novel and emerging tobacco products.

Synthetic nicotine has arrived

The introduction of a new product line of the popular disposable electronic cigarette brand Puffbar, advertised as containing synthetic nicotine, has drawn attention to the increasing use of synthetic nicotine in marketed products and its uncertain regulatory status. A search of the Truth Tobacco Industry Documents revealed that the industry considered using synthetic nicotine already in the 1960s, efforts that were abandoned due to high costs and insufficient purity. Recent patents revealed renewed efforts to develop more efficient strategies for the synthesis of nicotine. Nicotine exists as two stereoisomers, S-nicotine and R-nicotine. While S-nicotine is the prevalent (>99%) form of nicotine in tobacco, a market-leading form of synthetic nicotine contains both stereoisomers at equal amounts, raising concerns about inaccurate labelling and the poorly understood health effects of R-nicotine. Other manufacturers, including a leading vendor of pharmaceutical grade nicotine, developed stereospecific strategies to synthesise pure S-nicotine, now added to electronic cigarette products marketed in the USA and UK. While S-nicotine and R-nicotine can be differentiated by enantioselective High Performance Liquid Chromatography (HPLC), differentiation of synthetic (fossil-derived) from tobacco-derived S-nicotine will require development of methods to measure carbon isotope (¹⁴C or ¹³C) content. Vendors claim that the FDA has no authority to regulate synthetic nicotine as a tobacco product, allowing them to circumvent the premarket tobacco product application process. However, legal analysis suggests that FDA may have the authority to regulate synthetic nicotine as a drug. Alternatively, Congress needs to include nicotine from any source within the legal definition of tobacco products.

Sex differences in nicotine-induced impulsivity and its reversal with bupropion in rats

BACKGROUND

Enhancement in cognitive impulsivity and the resulting alterations in decision making serve as a contributing factor for the development and maintenance of substance-use disorders. Nicotine-induced increases in impulsivity has been previously reported in male humans and rodents. Although the potential for sex differences in nicotine-induced impulsivity has not been examined.

AIMS AND METHODS

In the present study, male and female Sprague Dawley rats were submitted to a delay discounting task, in which several consecutive measures of self-control were taken. Firstly, rats were tested with vehicle, and next with nicotine doses of 0.4 and 0.8 mg/kg. Thereafter, chronic treatment with bupropion started, and the animals were tested again. Half the animals continued to receive 0.8 mg/kg of nicotine, while the rest received nicotine and also a daily dose of 30 mg/kg of bupropion.

RESULTS

When the animals were first tested with nicotine, female rats showed a significant nicotine dose dependent increase of impulsive behaviour, whereas male rats only showed a decrease on their elections of the larger but delayed reward under the highest dose of 0.8 mg/kg of nicotine. Treatment with bupropion blocked the effect of nicotine on decision making in female rats, as they showed results close to their baseline levels. On the other hand, bupropion did not affect the nicotine-induced delay discounting in male rats.

CONCLUSION

These findings demonstrate sexually dimorphic effects of nicotine on cognitive impulsivity which may help to shed light on nicotine use vulnerabilities observed in women.

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.

Nicotine kinetics in zebra finches in vivo and in vitro

Nicotine enhances cognitive performance, and in the zebra finch (Taeniopygia guttata), which is a well-established model of cognition, the effects of nicotine on song production have been reported. Nicotine and cotinine plasma levels were assessed in vivo after subcutaneous injection of 0.18 mg/kg nicotine, a dose that elicits changes in song production. The half-life of nicotine elimination was 33 minutes, and levels were undetectable by 4 hours. Average plasma nicotine over 2 hours was 32 ng/ml, similar to levels seen in human smokers and rat models of nicotine behavior. Nicotine brain levels were 30 and 14 ng/g 1 and 2 hours after treatment. To understand the potential for drug interactions and the regulation of nicotine metabolism in zebra finches, we characterized in vitro nicotine metabolism and the hepatic enzyme involved. In humans, cytochrome P450 2A6 metabolizes nicotine to cotinine, and CYP2A-like activity and protein have been reported in some birds. Zebra finch liver microsomes metabolized nicotine and bupropion (a CYP2B substrate) but not coumarin (a CYP2A substrate). Nicotine was metabolized to cotinine with a Michaelis-Menten constant (K(m)) of 96 µM and a V(max) of 56 pmol/min per milligram. Nicotine and bupropion metabolism was inhibited by C-8-xanthate (a specific CYP2B inhibitor) but not by CYP2A-specific inhibitors, and hepatic levels of CYP2B-like but not CYP2A-like proteins correlated with nicotine (r = 0.52; P = 0.04) and bupropion metabolism (r = 0.81; P < 0.001), suggesting CYP2B-mediation of nicotine metabolism as seen in rats. These results will facilitate further investigation of nicotine's effects in zebra finches.

Nicotine metabolism in pregnant and nonpregnant rabbits

Smoking during pregnancy remains a major public health concern and is associated with numerous adverse effects. Recently the clearance of nicotine and cotinine was shown to be substantially increased in pregnant women compared with nonpregnant controls. The present study investigated the usefulness of the rabbit for studying the molecular basis for the observed changes in nicotine and cotinine disposition during pregnancy. Nicotine was largely metabolized to cotinine in rabbit liver microsomes (approximately 50% of total metabolism); significant amounts of nicotine-N'-oxide and nornicotine also were detected. The conversion of nicotine to cotinine also was detected in rabbit placental and fetal liver microsomes, albeit at only a fraction of the rate found in adult rabbit liver microsomes. The major products of cotinine metabolism in rabbit liver microsomes were 5'-hydroxycotinine, cotinine-N'-oxide, and norcotinine. Differences between human and rabbit liver were most apparent for cotinine. The major human metabolite, 3'-hydroxycotinine, was formed at only low levels in rabbit liver microsomes. Pregnancy had no effect on the metabolism of nicotine or on the expression of CYP2A6 immunoreactive proteins in rabbit liver microsomes. These studies provide a complete quantitative assessment of nicotine metabolism in rabbit liver microsomes and suggest that the rabbit may not be an appropriate animal model to study the effects of pregnancy on nicotine and cotinine metabolism. However, a molecular understanding of these effects is essential for prediction of the pharmacological and toxicological consequences of smoking during pregnancy.

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.

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.

Nicotine, its metabolism and an overview of its biological effects

Nicotine is a naturally occurring alkaloid found in many plants. The principal sources of nicotine exposure is through the use of tobacco, nicotine containing gum and nicotine replacement therapies. Nicotine is an amine composed of pyridine and pyrrolidine rings. It has been shown that nicotine crosses biological membranes and the blood brain barrier easily. The absorbed nicotine is extensively metabolized in the liver to form a wide variety of metabolites including nicotine N'-oxide and cotinine N'-oxide. These are the products of mixed function oxidase system. Nicotine is also converted to some biologically important compounds during harvesting. Among these are the nitrosamines specific to tobacco. Nicotine has been shown to affect a wide variety of biological functions ranging from gene expression, regulation of hormone secretion and enzyme activities. The objective of this study was to overview the biological effects and metabolism of nicotine.

Characterization of HdnoR, the transcriptional repressor of the 6-hydroxy-D-nicotine oxidase gene of Arthrobacter nicotinovorans pAO1, and its DNA-binding activity in response to L- and D-nicotine Derivatives

Utilization of L-nicotine as growth substrate by Arthrobacter nicotinovorans pAO1 starts with hydroxylation of the pyridine ring at C6. Next, the pyrrolidine ring is oxidized by 6-hydroxy-L-nicotine oxidase, which acts strictly stereo-specific on the L-enantiomer. Surprisingly, L-nicotine also induces the synthesis of a 6-hydroxy-d-nicotine-specific oxidase in the bacteria. Genes of nicotine-degrading enzymes are located on the catabolic plasmid pAO1. The pAO1 sequence revealed that the 6-hydroxy-D-nicotine oxidase gene is flanked by two open reading frames with a similarity to amino acid permeases and a divergently transcribed open reading frame with a similarity to proteins of the tetracycline repressor TetR family. Reverse transcription PCR and primer extension analysis of RNA transcripts isolated from A. nicotinovorans pAO1 indicated that the 6-hydroxy-D-nicotine oxidase gene represents a transcriptional unit. DNA electromobility shift assays established that the purified TetR-similar protein represents the 6-hydroxy-D-nicotine oxidase gene repressor HdnoR and binds to the 6-hydroxy-D-nicotine oxidase gene operator with a Kd of 21 nM. The enantiomers 6-hydroxy-D- and 6-hydroxy-L-nicotine acted in vitro as inducers. In vivo analysis of 6-hydroxy-D-nicotine oxidase gene transcripts from bacteria grown with L- and D-nicotine confirmed this conclusion. The poor discrimination by HdnoR between the 6-hydroxy-L- and 6-hydroxy-D-nicotine enantiomers explains the presence of the 6-hydroxy-D-nicotine-specific enzyme in bacteria grown on L-nicotine.

Sex differences in nicotine effects and self-administration: review of human and animal evidence

Although both the human and animal literatures are notable for the general lack of attention paid to possible sex differences in drug self-administration behavior, evidence is accumulating to suggest that males and females may differ in factors that maintain tobacco smoking or nicotine self-administration. Self-administration of nicotine per se may be less robust in women, and women are less sensitive than men to some effects of nicotine that may be reinforcing. Compared to men, smoking behavior of women may be influenced more by non-nicotine stimuli associated with smoking, suggesting greater conditioned reinforcement of smoking in women. Moreover, nicotine replacement, the current standard treatment for smoking cessation, is sometimes less effective in women, further suggesting the need for greater consideration of non-nicotine factors that may maintain women's smoking. Very recent research on rats also indicates sex differences in nicotine self-administration. However, these differences are complex and suggest that nicotine-seeking behavior is composed of several components, including hedonic, incentive-motivational, and conditioning effects; males and females may differ in one or more of these components. Menstrual or estrous cycle phase effects on the maintenance of nicotine self-administration are not particularly apparent in humans or animals, although cycle phase may influence other stages of dependence (e.g., withdrawal symptoms during cessation). Future research should evaluate further the consistency of results across human and non-human species, identify the conditions and procedures under which sex differences are observed, and elucidate the specific components of reinforcement that may differ between males and females. Studies also should examine the possible generalizability of these sex differences to other drugs of abuse. Identification of specific factors responsible for these sex differences may lead to improved interventions for smoking cessation and other substance abuse in women.

Nicotine enantiomers and oxidative stress

Nicotine affects a variety of cellular processes ranging from induction of gene expression to secretion of hormones and modulation of enzymatic activities. The objective of this study was to characterize the toxicity of nicotine enantiomers as well as their ability to induce oxidative stress in an in vitro model using Chinese hamster ovary (CHO) cells. Colony formation assay has demonstrated that (-)-nicotine is the more toxic of the enantiomers. At 6 mM concentrations, (-)-nicotine was found to be approximately 28- and 19-fold more potent than (+)-, and (+/-)-nicotine (racemic), respectively. Results also indicated that the toxicity of (+/-)-nicotine is higher than that of (+)-nicotine. (-)-Nicotine at a 10 mM concentration substantially decreased glutathione (GSH) levels (46% decrease). In addition, a 3-fold increase in malondialdehyde (MDA) level was evident in cells after exposure to 10 mM (-)-nicotine. Increased lactate dehydrogenase (LDH) activities in the media demonstrated that cellular membrane integrity was disturbed in nicotine treated cells. In the presence of superoxide dismutase (SOD) and catalase (CAT), the LDH activities returned to control value in 24 h with all concentrations of (-)-, (+)-, and (+/-)-nicotine. The decreases in LDH activities in the presence of the radical scavenging enzymes SOD and CAT suggest that membrane damage may be due to free radical generation.

Contribution of CNS nicotine metabolites to the neuropharmacological effects of nicotine and tobacco smoking

Nicotine, the principal alkaloid in tobacco products, is generally accepted to be the active pharmacological agent responsible for CNS effects resulting from tobacco use. Arguments are presented in this commentary which take issue with this popular dogma, by providing evidence that nicotine metabolites may also be responsible for the CNS effects commonly attributed to nicotine. CNS effects attributed to nicotine include reinforcing effects, mood elevation, arousal, locomotor stimulant effects, and learning and memory enhancement. The reinforcing and locomotor stimulant effects of nicotine have been suggested to be the result of activation of CNS dopaminergic systems, and nicotine-induced modulation of dopaminergic neurotransmission has been studied in detail. Nicotine acts at a family of nicotinic receptor subtypes composed of multiple subunits; however, the exact composition of the subunits in native nicotinic receptors and the functional significance of the receptor subtype diversity are currently unknown. This nicotinic subtype diversity increases the complexity of the potential mechanisms of action of nicotine and its metabolites. Although peripheral metabolism of nicotine has been studied extensively, metabolism in the CNS has not been investigated to any great extent. Recently, studies from our laboratory have demonstrated that several nicotine metabolites are present in the CNS after acute nicotine administration. Moreover, nicotine metabolites are pharmacologically active in neurochemical and behavioral assays. Thus, CNS effects resulting from nicotine exposure may not be due solely to nicotine, but may result, at least in part, from the actions of nicotine metabolites.

Pharmacokinetics and metabolism of N-(2-hydroxyethyl)-2,5-[14C]-pyrrolidine (HEP, Epolamine) in male healthy volunteers

N-(2-hydroxyethyl)-pyrrolidine (HEP, Epolamine) is a strong base used to salify organic acids of pharmaceutical interest in order to improve their solubility in water. Diclofenac-HEP (Flector) is the first example of an epolamine salt of a drug. In this study, [14C]-HEP was administered by oral route (300 mg, about 50 microCi/subject) to 3 volunteers with the aim to investigate its plasma profile and to calculate the relevant pharmacokinetic parameters. The experimental data correlated with a two-compartment pharmacokinetic model. Total radioactivity in urine and faeces was also measured. The radioactivity was excreted preferentially by the faecal route (about 65% of the dose administered in the 0-72 h collection interval). Urinary excretion accounted for about 30% of the dose and occurred very rapidly (about 22% of the dose was in the 0-8 h collection interval). Metabolic investigations were carried out on urine samples. TLC analysis with radioscan detector indicated a main radioactive zone, accounting for about 98% of the radioactivity in the plate. After scraping off and purification of the radioactive areas, the compound isolated (Met I) was analysed by gas chromatography-mass spectrometry with electron-impact ionization process. The structure of the metabolite was postulated to be pyrrolidine N-oxide.

Role of the lung in accumulation and metabolism of xenobiotic compounds--implications for chemically induced toxicity

The mammalian lung is exposed to and affected by many airborne and bloodborne foreign compounds. This review summarizes the role of lung in accumulation and metabolism of xenobiotics, some of which are spontaneously reactive or are metabolically activated to toxic intermediates. The specific architectural arrangement of mammalian lung favors that so-called pneumophilic drugs are filtered out of the blood and are retained within the tissue as shown in particular for amphetamine, chlorphentermine, amiodarone, imipramine, chlorpromazine, propranolol, local anaesthetics, and some miscellaneous therapeutics. There is strong evidence that intrapulmonary distribution activity and regulation of drug-metabolizing enzymes in lung is distinct from liver. This review focuses on the metabolic rate of selected compounds in lung such as 5-fluoro-2'-deoxyuridine, local anesthetics, nicotine, benzo(alpha)pyrene, ipomeanol, 4-methylnitrosamino-1-(3-pyridyl)-1-butanone. It is widely accepted that the formation of radical species is a key event in the pneumotoxic mechanisms induced by bleomycin, paraquat, 3-methylindole, butylhydroxytoluene, or nitrofurantoin. Finally, methodological approaches to assess the capacity of lung to eliminate foreign compounds as well as biochemical features of the pulmonary tissue are evaluated briefly.

Variables affecting nicotine metabolism

Nicotine metabolism is exceedingly sensitive to perturbation by numerous host factors. To reduce the large variations and discrepancies in the literature pertaining to nicotine metabolism, investigators in future studies need to recognize and better control these host factors. Recent advances in the understanding of nicotine metabolism have suggested new approaches to elucidating underlying mechanisms of certain toxic effects associated with cigarette smoking.

Metabolism of S-nicotine in noninduced and aroclor-induced rats

The urinary excretion of nicotine and its metabolites in noninduced and Aroclor-induced male and female rats has been determined following intravenous administration of 2'-[14C]-labeled S-nicotine at a dose of 4.6 mumol/kg. Complete recovery of the administered radioactivity was achieved: 95% in urine and 4% in feces over 96 h and 1% remaining in the body. More than 40 nicotine metabolites were found by radio-HPLC; 19 were identified including the cis/trans-diastereomers of nicotine-N'-oxide and 3'-hydroxycotinine. The urinary metabolite profile and excretion kinetics of nicotine and its metabolites were significantly different between noninduced and Aroclor-induced rats. The major urinary nicotine metabolite in the noninduced rat was cis-nicotine-N'-oxide. In the Aroclor-induced rat, cotinine metabolites were the major metabolites found. Sex differences were found for the urinary nicotine metabolite profile, mainly expressed in the excretion of cis-nicotine-N'-oxide, 29% in the male and 17% in the female noninduced rat, and the excretion of cotinine, 5% in the male and 12% in the female noninduced rat. High stereoselectivity was found for the formation of the cis/trans-diastereomers of nicotine-N'-oxide as well as of 3'-hydroxycotinine, the stereoselectivity being more pronounced in male rats.

Identification of cis-3'-hydroxycotinine as a urinary nicotine metabolite

1. cis-3'-Hydroxycotinine was detected as an S(-)-nicotine metabolite in the urine of smokers as well as in the urine of rats and hamsters dosed with nicotine. 2. The excreted amount of cis-3'-hydroxycotinine is lower than that of the trans-isomer.

Metabolism of nicotine by hepatocytes

The profile of nicotine metabolites produced by freshly isolated hepatocytes from rats, hamsters, guinea pigs, mice and humans was investigated after a 30-min exposure to nicotine ([2-14C]pyrrolidine). Large species differences occurred in the extent of nicotine metabolism; these ranged from 95% metabolism in guinea pig hepatocytes to only 30% metabolism in human and rat hepatocytes. The spectrum of metabolites formed also varied widely in different species. In hepatocytes from obese human subjects, nicotine was metabolized most extensively in smokers, least in nonsmokers, and to an intermediate degree in exsmokers, suggesting that cigarette smoking enhances the rate of nicotine metabolism. Pretreatment of all nonhuman species studied with phenobarbital and beta-naphthoflavone and with Aroclor in rats produced distinctive inductive patterns. Phenobarbital pretreatment of nonsmokers for 2 days prior to liver biopsy doubled the extent of nicotine conversion to cotinine by their hepatocytes. Rat and hamster hepatocytes exhibited sex and stereoselectivity differences in nicotine metabolism. Collectively, these studies indicate that hepatocytes offer some advantages over in vivo systems in investigating certain aspects of nicotine metabolism.

Effect of continuous administration of nicotine on urinary histamine and N tau-methylhistamine levels in the guinea pig

The effect of continuous subcutaneous administration of S-(-)- and R-(+)-nicotines on urinary excretion levels of histamine and N tau-methylhistamine in guinea pigs, over a 23-day period, has been studied. Urinary levels of these endogenous compounds were measured utilizing paired-ion reversed-phase high performance liquid chromatography with flow-through electrochemical detection. Urinary histamine levels of animals that had been administered either of these nicotine isomers were not significantly different from control values. Initial levels of urinary N tau-methylhistamine (days 2-3) in R-(+)- and S-(-)-nicotine-treated animals were, 2-fold and 8-fold higher, respectively than control levels but in both cases these levels returned to control values over the remainder of the time course examined (days 6-23). These results suggest that exposure to S-(-)-nicotine results in initial histamine release and/or inhibition of histamine uptake. However, longer term exposure to S-(-)-nicotine may not result in significantly altered levels of circulating histamine.