Tumorigenic activity of the tobacco-specific nitrosamines 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), 4-(methylnitrosamino)-4-(3-pyridyl)-1-butanol (iso-NNAL) and N′-nitrosonornicotine (NNN) on topical application to Sencar mice

The tumor-initiating activities of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), 4-(methylnitrosamino)-4-(3-pyridyl)-1-butanol (iso-NNAL) and N′-nitrosonornicotine (NNN) were evaluated on the skin of female SENCAR mice. A total initiator dose of 28 μmol/mouse of each nitrosamine was applied in 10 subdoses administered every second day. Promotion commenced 10 days after the last initiator dose and consisted of twice weekly application of 2.0 μg of tetradecanoylphorbol acetate for 20 weeks. NNK induced a 79% incidence of skin tumors with an average of 1.6 tumors/mouse and a 59% incidence of lung adenomas. In contrast, iso-NNAL and NNN were not active as tumor initiators in either the skin or lung of mice. The tumorigenic activity of NNK on SENCAR mouse skin was evaluated at several doses. At a total initiator dose of 28 and 5.6 μmol/mouse, NNK exhibited significant activity (P < 0.005) inducing a 59% and 24% incidence of skin tumors, respectively. In this dose response bioassay, NNK at a total initiator dose of 28 μmol induced a 63% incidence (P < 0.005) of lung adenomas. The numbers of lung adenomas induced at the lower doses employed were not significant. NNK, at a total initiation dose of 1.4 μmol, did not exhibit significant tumorigenic activity (P > 0.05). Analysis of DNA from the skin of mice treated with NNK using HPLC with fluorescence detection failed to detect O⁶- and N-methylguanine (O⁶-MG and N⁷-MG) adducts. These data indicate that NNK can exert a contact carcinogenic effect and suggest that mechanisms other than DNA methylation may be involved in its activation to a tumorigenic agent in mouse skin.