Carcinogenicity and DNA adduct formation of 4-(methylnitrosamino)-1-(3-pyridyl)- 1-butanone and enantiomers of its metabolite 4-(methylnitrosamino)-1-(3-pyridyl)-1- butanol in F-344 rats

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is metabolized to enantiomers of 4-(methylnitrosamino)-1-(3-pyridyl)-1- butanol (NNAL), found in the urine of virtually all people exposed to tobacco products. We assessed the carcinogenicity in male F-344 rats of (R)-NNAL (5 ppm in drinking water), (S)-NNAL (5 ppm), NNK (5 ppm) and racemic NNAL (10 ppm) and analyzed DNA adduct formation in lung and pancreas of these rats after 10, 30, 50 and 70 weeks of treatment. All test compounds induced a high incidence of lung tumors, both adenomas and carcinomas. NNK and racemic NNAL were most potent; (R)- NNAL and (S)-NNAL had equivalent activity. Metastasis was observed from primary pulmonary carcinomas to the pancreas, particularly in the racemic NNAL group. DNA adducts analyzed were O²-[4-(3-pyridyl)-4-oxobut-1-yl]thymidine (O²-POB-dThd), 7-[4-(3-pyridyl)-4-oxobut-1-yl]guanine (7-POB-Gua), O⁶-[4-(3- pyridyl)-4-oxobut-1-yl]deoxyguanosine (O⁶-POB-dGuo), the 4-(3-pyridyl)-4-hydroxybut-1-yl (PHB) adducts O²-PHB-dThd and 7-PHB-Gua, O⁶-methylguanine (O⁶-Me-Gua) and 4-hydroxy- 1-(3-pyridyl)-1-butanone (HPB)-releasing adducts. Adduct levels significantly decreased with time in the lungs of rats treated with NNK. Pulmonary POB-DNA adducts and O6-Me-Gua were similar in rats treated with NNK and (S)-NNAL; both were significantly greater than in the (R)-NNAL rats. In contrast, pulmonary PHB-DNA adduct levels were greatest in the rats treated with (R)- NNAL. Total pulmonary DNA adduct levels were similar in (S)- NNAL and (R)-NNAL rats. Similar trends were observed for DNA adducts in the pancreas, but adduct levels were significantly lower than in the lung. The results of this study clearly demonstrate the potent pulmonary carcinogenicity of both enantiomers of NNAL in rats and provide important new information regarding DNA damage by these compounds in lung and pancreas.