The tumorigenic activities and DNA methylating abilities in F344 rats of the tobacco specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and the structurally related nitrosamine N-nitrosodimethylamine (NDMA) were compared. Groups of 30 male rats were given 60 s.c. injections of 0.0055 mmol/kg of either NNK or NDMA over a 20-week period (total dose, 0.33 mmol/kg). The experiment was terminated after 104 weeks. The numbers of rats with tumors were as follows for NNK and NDMA, respectively: liver, 10 and 6; lung 13 and 0; and nasal cavity, 6 and 1. NNK was significantly more tumorigenic than was NDMA toward the lung (P < 0.01) and nasal cavity (P < 0.05). Groups of rats were treated with a single s.c. injection of 0.39 mmol/kg or 0.055 mmol/kg of NNK or NDMA and the levels of 7-methylguanine and O6-methylguanine were measured in liver, lung, and nasal mucosa 1–48 h after treatment. In liver and lung, levels of 7-methylguanine and O6-methylguanine in DNA were 3–22 times (P < 0.001) greater in NDMA treated rats than in NNK treated rats. Levels of methylation induced by NDMA and NNK in the nasal mucosa were similar. The results of this study demonstrate that NNK is a more potent tumorigen than NDMA in the F344 rat and suggest that DNA methylation alone does not account for its strong tumorigenicity in rat lung and nasal mucosa.

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Supported by National Cancer Institute Grant CA-21393. This is Paper 88 in “A Study of Chemical Carcinogenesis.”

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