Formation and accumulation of pyridyloxobutyl DNA adducts in F344 rats chronically treated with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and enantiomers of its metabolite, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol

Formation and distribution of NNK metabolites in an isolated perfused rat lung

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a lung-specific tobacco carcinogen. Metabolism is critical to its elimination given its lipophilic nature. Although NNK can be metabolized through detoxification pathways that safely eliminate it from the body, it can also be bioactivated, resulting in the formation of potentially carcinogenic DNA adducts. The isolated perfused rat lung (IPRL) system was used to determine the effect of NNK perfusate concentration (0.1 and 1.2 microM) on the formation and distribution of metabolites, the level of individual DNA adducts, and total covalent binding in the lung. Coadministration of the chemopreventive agent phenethyl isothiocyanate (PEITC; 20 microM) was also examined to determine its effect on NNK metabolism. NNK was readily metabolized in the IPRL system. In the 0.1 muM perfusions approximately 55% of metabolites formed were through detoxification pathways, whereas roughly 30% were the result of bioactivation pathways. An increase in NNK concentration increased the percentage of unmetabolized NNK and decreased the apparent metabolic clearance in the lung, but the metabolite profiles remained similar between concentrations. The addition of PEITC reduced the formation of oxidative metabolites and increased 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) formation and the percentage of unmetabolized NNK. PEITC also significantly decreased the formation of DNA adducts in the lung tissue. The level of O(2)-[4-(3-pyridyl)-4-oxobut-1-yl]thymidine (O(2)-POB-dThd) and O(6)-[4-(3-pyridyl)-4-oxobut-1-yl]-2'-deoxyguanosine (O(6)-POB-dGuo) decreased by 70 to 75%, and that of O(6)-methylguanine (O(6)-methyl-Gua) and 7-[4-(3-pyridyl)-4-oxobut-1-yl]guanine (7-POB-Gua) decreased by 40 to 45%. Pyridylhydroxybutyl-DNA adducts were not detected in any of the treatment groups. Thus, the IPRL system is useful in determining pulmonary metabolism and DNA adduct formation separate from other metabolizing organs.

The influence of repair pathways on the cytotoxicity and mutagenicity induced by the pyridyloxobutylation pathway of tobacco-specific nitrosamines

Tobacco-specific nitrosamines, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and N'-nitrosonornicotine, are considered to be human carcinogens. Both compounds are metabolized to pyridyloxobutylating intermediates that react with DNA to form adducts such as 7-[4-(3-pyridyl)-4-oxobut-1-yl]guanine, O(2)-[4-(3-pyridyl)-4-oxobut-1-yl]cytosine, O(2)-[4-(3-pyridyl)-4-oxobut-1-yl]-2'-deoxythymidine (O(2)-pobdT), O(6)-[4-(3-pyridyl)-4-oxobut-1-yl]-2'-deoxyguanosine (O(6)-pobdG), and 4-hydroxy-1-(3-pyridyl)-1-butanone-releasing adducts. The role of specific DNA adducts in the overall genotoxic activity of the pyridyloxobutylation pathway is not known. One adduct, O(6)-pobdG, is mutagenic. To characterize the mutagenic and cytotoxic properties of pyridyloxobutyl DNA adducts, the impact of DNA repair pathways on the cytotoxic and mutagenic properties of the model pyridyloxobutylating agent, 4-(acetoxymethylnitrosamino)-1-(3-pyridyl)-1-butanone (NNKOAc), was investigated in Chinese hamster ovary cell lines proficient or deficient in O(6)-alkylguanine DNA alkyltransferase (AGT), deficient in both AGT and base excision repair (BER), or deficient in both AGT and nucleotide excision repair (NER). The repair of the four pyridyloxobutyl DNA adducts was determined in the same cell lines via sensitive LC-MS/MS methods. NNKOAc was more cytotoxic in the cell lines lacking AGT, BER, and NER repair pathways. It also induced more mutations in the hprt gene in the BER- and NER-deficient cell lines. However, AGT expression did not influence NNKOAc's mutagenicity despite efficient repair of O(6)-pobdG. Analysis of the hprt mutational spectra indicated that NNKOAc primarily caused point mutations at AT base pairs. GC to AT transition mutations were a minor contributor to the overall mutation spectrum, providing a rationale for the observation that AGT does not protect against the overall mutagenic properties of NNKOAc in this model system. The only adduct affected by the absence of effective NER was O(2)-pobdT. Slower repair of O(2)-pobdT in NER-deficient cells was associated with increased AT to TA transversion mutations, supporting the hypothesis that these mutations are caused by O(2)-pobdT. Together, these data support a hypothesis that the pyridyloxobutylation pathway generates multiple mutagenic and toxic adducts.

Comparative levels of O6-methylguanine, pyridyloxobutyl-, and pyridylhydroxybutyl-DNA adducts in lung and liver of rats treated chronically with the tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a lung carcinogen in rats and may be a cause of lung cancer in smokers. NNK is metabolized by cytochromes P450 to intermediates that react with DNA forming methyl, pyridyloxobutyl (POB), and pyridylhydroxybutyl (PHB) adducts, which are critical in carcinogenesis. The methyl adduct O(6)-methylguanine (O(6)-methyl-G) has miscoding properties, but there are no reports on levels of this adduct in rats treated chronically with NNK in the drinking water, nor has its levels been compared with those of POB- and PHB-DNA adducts. We used liquid chromatography-electrospray ionization-tandem mass spectrometry-selected reaction monitoring to quantify O(6)-methyl-G in lung and liver DNA of rats treated with a carcinogenic dose of 10 ppm of NNK in the drinking water and sacrificed after 1, 2, 5, 10, 16, and 20 weeks. The maximal level of O(6)-methyl-G in lung DNA, 2550 +/- 263 fmol/mg DNA, was reached at 5 weeks and was significantly greater (P < 0.05) at that point than all other adducts (measured previously) except O(2)-[4-(3-pyridyl)-4-oxobut-1-yl]thymidine. Overall levels of O(6)-methyl-G in lung were intermediate between those of total POB- and PHB-DNA adducts. In liver, the wave of O(6)-methyl-G peaked at 2 weeks while that of total POB-DNA adducts peaked at 10 weeks, and levels of total PHB-DNA adducts were low throughout. The results of this study demonstrate that substantial amounts of O(6)-methyl-G are formed at various time points in lung and liver DNA of rats treated chronically with NNK, supporting its role in carcinogenesis.

Analysis of pyridyloxobutyl and pyridylhydroxybutyl DNA adducts in extrahepatic tissues of F344 rats treated chronically with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and enantiomers of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and its metabolite 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) are potent pulmonary carcinogens in rats. NNK and NNAL require metabolic activation to express their carcinogenicity. Cytochrome P450-catalyzed alpha-hydroxylation at the methyl position of NNK or NNAL generates reactive intermediates, which alkylate DNA to form pyridyloxobutyl (POB)-DNA adducts or pyridylhydroxybutyl (PHB)-DNA adducts. NNK is metabolized to NNAL in a reversible and stereoselective manner, and the tissue-specific retention of (S)-NNAL is believed to be important to the carcinogenicity of NNK. In the present study, we investigated the formation of POB- and PHB-DNA adducts in extrahepatic tissues of F344 rats treated chronically with NNK and (R)- and (S)-NNAL (10 ppm in the drinking water, 1-20 weeks). POB- and PHB-DNA adducts were quantified in nasal olfactory mucosa, nasal respiratory mucosa, oral mucosa, and pancreas of treated rats. Adduct formation in the nasal respiratory mucosa exceeded that in the other tissues. O(2)-[4-(3-Pyridyl)-4-oxobut-1-yl]thymidine (O(2)-POB-dThd) or O(2)-[4-(3-pyridyl)-4-hydroxybut-1-yl]thymidine (O(2)-PHB-dThd) was the major adduct, followed by 7-[4-(3-pyridyl)-4-oxobut-1-yl]guanine (7-POB-Gua) or 7-[4-(3-pyridyl)-4-hydroxybut-1-yl]guanine (7-PHB-Gua). There was a remarkable similarity in adduct formation between the NNK and the (S)-NNAL groups, both of which were distinctively different from that in the (R)-NNAL group. For example, in the nasal olfactory mucosa, POB-DNA adduct levels in the NNK and (S)-NNAL groups were not significantly different from each other, while (R)-NNAL treatment generated 6-33 times lower amounts of POB-DNA adducts than did NNK treatment. In contrast, (R)-NNAL treatment produced significantly higher levels of PHB-DNA adducts than did NNK or (S)-NNAL treatment. Similar trends were observed in the nasal respiratory mucosa, oral mucosa, and pancreas. These results suggest extensive retention of (S)-NNAL in various tissues of NNK-treated rats and support a mechanism in which the preferential metabolism of NNK to (S)-NNAL, followed by sequestration of (S)-NNAL in the target tissues and reoxidation to NNK, is important to NNK tumorigenesis.

CYP1A2 genetic polymorphisms and adenocarcinoma lung cancer risk in the Tunisian population

AIMS

In this study, the effects of four single nucleotide polymorphisms (SNPs), -3860G>A, -2467delT, -739T>G and -163C>A, of CYP1A2 gene on lung cancer were evaluated in Tunisian population.

MAIN METHODS

Four polymorphisms of CYP1A2 gene were analysed in 109 healthy smokers and in 101 lung cancer cases, including 63 with squamous cell carcinoma (SCC) and 41 with adenocarcinoma (AD). The genotyping for the SNPs -3860 G>A, -2467delT, -739T>G and -163C>A was performed by polymerase chain reaction (PCR)-restriction fragment length polymorphism analysis.

KEY FINDINGS

The results showed that smokers with CYP1A2 gene polymorphisms were associated with an increased risk for the development of lung AD. There was however no significant increased risk of developing lung SCC in smokers having CYP1A2 gene polymorphisms. An increased risk of developing AD was observed in smokers who are carriers of at least one copy of -3680A or -739G giving a significant odds ratio (OR) of 6.02 (CI=2.91-12.9) and 3.01 (CI=1.54-5.98), respectively.

SIGNIFICANCE

These genotyping data are consistent with the hypothesis that tobacco-specific-N-nitrosamines (TSN) such as 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) are major contributors to the development of lung AD and that CYP1A2 gene product plays an important role in the metabolic activation of NNK. This study suggests that SNPs of CYP1A2 could be considered as promising biomarkers in the aetiology of lung AD in smokers.

Mitochondrial DNA adducts in the lung and liver of F344 rats chronically treated with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and (S)-4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol

Two recent studies conducted in our laboratory have demonstrated formation and accumulation of pyridyloxobutyl (POB) and pyridylhydroxybutyl (PHB) adducts in lung and liver total DNA of F344 rats chronically treated with the tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and (R)- and (S)-enantiomers of its metabolite, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL). In this study, we measured POB and PHB adducts in lung and liver mitochondrial DNA (mtDNA), as previous studies suggest a potentially important role of mtDNA in carcinogenesis. Rats were sacrificed after 1, 2, 5, 10, 16, and 20 weeks of treatment with 10 ppm of NNK or (S)-NNAL in drinking water, and mtDNA and nuclear DNA (nDNA) adduct levels in the lung and liver were determined by LC-ESI-MS/MS-SRM. The mean levels of individual POB adducts in mtDNA at all time points were slightly higher than those in nDNA for both NNK and (S)-NNAL-treated rats in the lung (P < 0.001 for both treatments) but not in the liver (P > 0.05). Lung mtDNA of both NNK- and (S)-NNAL-treated rats contained higher concentrations of the sum of three POB adducts (P < 0.001 for both treatments) than nDNA, while the levels of mtDNA and nDNA total POB adducts in the liver were not significantly different in either NNK- or (S)-NNAL-treated rats. Analysis of PHB adducts in mtDNA and nDNA produced results similar to those obtained for POB adducts. The steady accumulation of the lung and liver mtDNA adducts over the course of the study indicates inefficient repair of these adducts in mtDNA. This is the first study to examine the formation of NNK- and (S)-NNAL-derived adducts in rat mtDNA. The results support the hypothesis that preferential binding of tobacco carcinogens to mtDNA of the lung might be functionally important in the development of smoking-induced lung cancer.

Differential expression of microRNAs in early-stage neoplastic transformation in the lungs of F344 rats chronically treated with the tobacco carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone

While numerous microRNAs (miRNAs) have been reported to alter their expression levels in human lung cancer tissues compared with normal tissues, the function of these miRNAs and their contribution to the long process of lung cancer development remains largely unknown. We applied a tobacco-specific carcinogen-induced cancer model to investigate the involvement of miRNAs in early lung cancer development, which could also provide information on potential, early biomarkers of lung cancers. Male F344 rats were first chronically treated with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a carcinogen present in tobacco products, for up to 20 weeks. The expression profiles of miRNAs in rat lungs were then determined. As measured by miRNA microarrays and confirmed by Northern blot and real-time polymerase chain reaction analyses, NNK treatment reduced the expression of a number of miRNAs, such as miR-101, miR-126*, miR-199 and miR-34. Significantly, these miRNAs overlap with previously published reports on altered miRNA expression in human lung cancer samples. These miRNAs might, therefore, represent early-response miRNAs that signify the molecular changes associated with pulmonary tumorigenesis. Moreover, we identified cytochrome P450 (CYP) 2A3, a critical enzyme in rat lungs that activates NNK to render it carcinogenic, as a potential target of miR-126*. NNK treatment in rats repressed miR-126* but induced CYP2A3 expression, a mechanism that may potentiate the oncogenic effects of NNK.

Quantitation of pyridylhydroxybutyl-DNA adducts in liver and lung of F-344 rats treated with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and enantiomers of its metabolite 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a potent pulmonary carcinogen in rats and is believed to be one cause of lung cancer in smokers. NNK is metabolized to 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), which is also a strong lung carcinogen in rats and has a chiral center at its 1-carbon. Previous studies have demonstrated that cytochrome P450-catalyzed alpha-hydroxylation of NNK in the lung leading to the formation of methyl and pyridyloxobutyl (POB)-DNA adducts is critical for its carcinogenicity. alpha-Hydroxylation of NNAL would similarly produce pyridylhydroxybutyl (PHB)-DNA adducts, but these have not been previously investigated in vivo. POB- and PHB-DNA adduct levels can indicate the amounts of pyridyloxobutylating and pyridylhydroxybutylating agents present in tissues of NNK- or NNAL-treated rats at any given point. Therefore, in this study, we developed a sensitive and quantitative liquid chromatography-electrospray ionization-tandem mass spectrometry-selected reaction monitoring method to determine levels of the PHB-DNA adducts O(6)-[4-(3-pyridyl)-4-hydroxybut-1-yl]-2'-deoxyguanosine (O(6)-PHB-dGuo, 10b), O(2)-[4-(3-pyridyl)-4-hydroxybut-1-yl]thymidine (O(2)-PHB-dThd, 11b), and 7-[4-(3-pyridyl)-4-hydroxybut-1-yl]-2'-deoxyguanosine (7-PHB-dGuo, 12b), the latter as the corresponding base 7-[4-(3-pyridyl)-4-hydroxybut-1-yl]-Gua (7-PHB-Gua, 14b) in DNA isolated from liver and lung of rats treated with 10 ppm NNK, (S)-NNAL, or (R)-NNAL in the drinking water for 20 weeks and sacrificed at 1, 2, 5, 10, 16, and 20 weeks. PHB-DNA adduct levels were higher in lung than in liver at each time point, consistent with previous studies of POB-DNA adducts in rats treated with NNK and NNAL in the drinking water. The results showed that NNK and (S)-NNAL behaved in a similar fashion, while (R)-NNAL was strikingly different. In the rats treated with NNK or (S)-NNAL, levels of each adduct at each time point were remarkably similar in lung, and levels of O(2)-PHB-dThd were generally greater than 7-PHB-Gua > O(6)-PHB-dGuo. The highest PHB-DNA adduct levels were found in lung and liver of rats treated with (R)-NNAL, suggesting that there are cytochrome P450s that efficiently catalyze the alpha-methyl hydroxylation of this compound. The results of this study provide further support for our hypothesis that (S)-NNAL is rapidly formed from NNK, sequestered at an unknown site in the lung, and then released and reoxidized to NNK with consequent DNA adduct formation resulting in lung carcinogenicity.

The tobacco carcinogen NNK is stereoselectively reduced by human pancreatic microsomes and cytosols

BACKGROUND/AIMS

Cigarette smoking increases the risk of cancer of the pancreas. The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is the only known environmental compound that induces pancreatic cancer in laboratory animals. Concentrations of NNK are significantly higher in the pancreatic juice of smokers than in that of nonsmokers. The chiral NNK metabolite, (R,S)-4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) is itself a potent pancreatic carcinogen in rats. The carcinogenicity of NNAL is related to its stereochemistry; (S)-NNAL is a more potent lung tumorigen in the A/J mouse than is (R)-NNAL. In this study, we determined the potential of the human pancreas to convert NNK into NNAL.

MATERIALS AND METHODS

Human pancreatic microsomes and cytosols were incubated with [5-(3)H]NNK, and the metabolic products were determined by high-performance liquid chromatography (HPLC).

RESULTS

(S)-NNAL was the predominant isomer formed in all cytosolic incubations. In ten microsomal samples, NNAL was formed at an average rate of 3.8 +/- 1.6 pmol/mg/min; (R)-NNAL was the predominant isomer in this group. The average rate of NNAL formation in 18 other microsomal samples was significantly lower, 0.13 +/- 0.12 pmol/mg/min (p < 0.001); (S)-NNAL was the predominant isomer formed in this group.

CONCLUSION

In human pancreatic tissues, there is intraindividual variability regarding the capacity for, and stereoselectivity of, carbonyl reduction of NNK.

Progress and challenges in selected areas of tobacco carcinogenesis

Tobacco use continues to be a major cause of cancer in the developed world, and despite significant progress in this country in tobacco control, which is driving a decrease in cancer mortality, there are still over 1 billion smokers in the world. This perspective discusses some selected issues in tobacco carcinogenesis focusing on progress during the 20 years of publication of Chemical Research in Toxicology. The topics covered include metabolism and DNA modification by tobacco-specific nitrosamines, tobacco carcinogen biomarkers, an unidentified DNA ethylating agent in cigarette smoke, mutations in the K-RAS and p53 gene in tobacco-induced lung cancer and their possible relationship to specific carcinogens, secondhand smoke and lung cancer, emerging issues in smokeless tobacco use, and a conceptual model for understanding tobacco carcinogenesis. It is hoped that a better understanding of mechanisms of tobacco-induced cancer will lead to new and useful approaches for the prevention of lung cancer and other cancers caused by tobacco use.

Development of liquid chromatography electrospray ionization tandem mass spectrometry methods for analysis of DNA adducts of formaldehyde and their application to rats treated with N-nitrosodimethylamine or 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone

Reaction of formaldehyde with DNA in vitro produces a variety of adducts among which are observed the cross-link di-(N(6)-deoxyadenosyl)methane (dAdo-CH 2-dAdo, 1) and the hydroxymethyl adduct N(6)-hydroxymethyl-dAdo (N(6)-HOCH 2-dAdo, 2). While the structures of these adducts have been known for decades, there have been no reports of their formation in vivo. Formaldehyde is released during intracellular metabolism of carcinogenic N-nitrosomethyl compounds such as N-nitrosodimethylamine (NDMA) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), but DNA adducts formed by this pathway have not been previously characterized. In this study, we addressed these questions by developing highly sensitive liquid chromatography-electrospray ionization-tandem mass spectrometry-selected reaction monitoring methods for quantitation of adducts 1 and 2, the latter as N(6)-methyl-dAdo ( 3). Considerable effort was devoted to the problem of artifactual formation of 1, which can occur during storage of DNA samples by reaction of dAdo with 2. This problem was solved by the addition of adenosine deaminase during the DNA hydrolysis step, thus removing dAdo as a reactant. The instability of adduct 2 was another potential block to analysis, and this was solved by converting it to 3 with NaBH 3CN. Separate aliquots of DNA were analyzed for adducts 1 and 2, using the [(15)N]-labeled adducts as internal standards. The application of these methods to rat hepatic DNA to which adducts 1 and 3 were added demonstrated accuracy and precision. The detection limits for adducts 1 and 3 were 1-4 adducts per 10 (9) nucleotides using 100-150 microg of DNA. The method was applied to analyze hepatic and pulmonary DNA from rats treated with NDMA and NNK. The results clearly demonstrated the dose-dependent presence of N(6)-HOCH 2-dAdo ( 2) in all DNA samples. The cross-link adduct dAdo-CH 2-dAdo ( 1) was observed in hepatic DNA of NNK-treated rats, with lower amounts in pulmonary DNA. Levels of these adducts were generally less than those of DNA adducts formed by the classical diazohydroxide pathway of nitrosamine metabolism. The results of this study demonstrate for the first time the presence of formaldehyde DNA adducts in tissues of rats treated with carcinogenic nitrosamines and suggest that these adducts may play a role in cancer induction by these compounds.

DNA adduct profiles: chemical approaches to addressing the biological impact of DNA damage from small molecules

Diverse small molecules alkylate DNA and form covalently linked adducts that can influence crucial biological processes, contributing to toxicity and mutation. Understanding the chemical reactivity dictating DNA alkylation and interactions of adducts with biological pathways can impact disease prevention and treatment. The ambident reactivity of DNA-alkylating small molecules, and of DNA itself, often results in formation of multiple adducts. Determining which structures impart biological responses is important for understanding the underlying relationships between small-molecule structure and biology. With application of sensitive and structure-specific experimental and analytical methodology, such as heteronuclear NMR spectroscopy and mass spectrometry, there are increasing numbers of studies that evaluate DNA alkylation from the perspective of resulting adduct profiles. DNA adduct profiles have been examined for both exogenous and endogenous reactive small molecules. Examples of recent findings are in the areas of tobacco-specific carcinogens, lipid peroxidation products, environmental and dietary chlorophenols, and natural-product-derived antitumor therapies. As more profile data are obtained, correlations with biological impact are being observed that would not be identified by a simplified single agent/single adduct approach.

Analysis of pyridyloxobutyl DNA adducts in F344 rats chronically treated with (R)- and (S)-N'-nitrosonornicotine

NNN (1) is an esophageal carcinogen in rats. 2'-Hydroxylation of NNN is believed to be the major bioactivation pathway for NNN tumorigenicity. (S)-NNN is preferentially metabolized by 2'-hydroxylation in cultured rat esophagus, whereas there is no preference for 2'-hydroxylation versus 5'-hydroxylation in the metabolism of (R)-NNN. 2'-Hydroxylation of NNN generates the reactive intermediate 4-oxo-4-(3-pyridyl)butanediazohydroxide (8), resulting in the formation of pyridyloxobutyl (POB)-DNA adducts. On the basis of these observations, we hypothesized that (S)-NNN treatment would produce higher levels of POB-DNA adducts than that by (R)-NNN in the rat esophagus. We tested this hypothesis by treating male F344 rats with 10 ppm of (R)-NNN or (S)-NNN in drinking water. After 1, 2, 5, 10, 16, or 20 weeks of treatment, POB-DNA adducts in esophageal, liver, and lung DNA were quantified by HPLC-ESI-MS/MS. In the rat esophagus, (S)-NNN treatment generated levels of POB-DNA adducts 3-5 times higher than (R)-NNN treatment, which supports our hypothesis. 7-[4-(3-Pyridyl)-4-oxobut-1-yl]guanine (7-POB-Gua, 14) was the major adduct detected, followed by O2-[4-(3-pyridyl)-4-oxobut-1-yl]thymidine (O2-POB-dThd, 11) and O2-[4-(3-pyridyl)-4-oxobut-1-yl]cytosine (POB-Cyt, 15). O6-[4-(3-Pyridyl)-4-oxobut-1-yl]-2'-deoxyguanosine (O6-POB-dGuo, 10) was not detected. The total POB-DNA adduct levels in the esophagus were 3-11 times higher than those in the liver for (R)-NNN and 2-6 times higher than those for (S)-NNN. In contrast to the esophagus and liver, (R)-NNN treatment produced more POB-DNA adducts than (S)-NNN treatment in the rat lung, which suggested an important role for cytochrome P450 2A3 in NNN metabolism in the rat lung. In both the liver and lung, O2-POB-dThd was the predominant adduct and accumulated during the experiment. The results of this study demonstrate that individual POB-DNA adducts form and persist in the esophagi, livers, and lungs of rats chronically treated with NNN enantiomers and demonstrate that (S)-NNN produces higher levels of POB-DNA adducts in the esophagus than (R)-NNN, suggesting that (S)-NNN is more tumorigenic than (R)-NNN to the rat esophagus.