Mass Spectrometric Quantitation of Pyridyloxobutyl DNA Phosphate Adducts in Rats Chronically Treated with N′‑Nitrosonornicotine

The tobacco-specific carcinogens N′-nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) require metabolic activation to exert their carcinogenicity. NNN and NNK are metabolized to the same reactive diazonium ions, which alkylate DNA forming pyridyloxobutyl (POB) DNA base and phosphate adducts. We have characterized the formation of both POB DNA base and phosphate adducts in NNK-treated rats and the formation of POB DNA base adducts in NNN-treated rats. However, POB DNA phosphate adducts in NNN-treated rats are still uncharacterized. In this study, we quantified the levels of POB DNA phosphate adducts in tissues of rats chronically treated with (S)-NNN or (R)-NNN for 10, 30, 50, and 70 weeks during a carcinogenicity study. The highest amounts of POB DNA phosphate adducts were observed in the esophagus of the (S)-NNN-treated rats, with a maximum level of 5400 ± 317 fmol/mg DNA at 50 weeks. The abundance of POB DNA phosphate adducts in the esophagus was consistent with the results of the carcinogenicity study showing that the esophagus was the primary site of tumor formation from treatment with (S)-NNN. Compared to the (R)-NNN group, the levels of POB DNA phosphate adducts were higher in the oral mucosa, esophagus, and liver, while lower in the nasal mucosa of the (S)-NNN-treated rats. Among 10 combinations of all isomers of POB DNA phosphate adducts, Ap­(POB)­C and combinations with thymidine predominated across all the rat tissues examined. In the primary target tissue, esophageal mucosa, Ap­(POB)­C accounted for ∼20% of total phosphate adducts in the (S)-NNN treatment group throughout the 70 weeks, with levels ranging from 780 ± 194 to 1010 ± 700 fmol/mg DNA. The results of this study showed that POB DNA phosphate adducts were present in high levels and persisted in target tissues of rats chronically treated with (S)- or (R)-NNN. These results improve our understanding of DNA damage during NNN-induced carcinogenesis. The predominant POB DNA phosphate isomers observed, such as Ap­(POB)­C, may serve as biomarkers for monitoring chronic exposure of tobacco-specific nitrosamines in humans.