Carcinogenicity of Methylurea or Morpholine in Combination with Sodium Nitrite in a Rat Multi‐organ Carcinogenesis Bioassay

For carcinogenic risk assessment of combinations of N‐nitroso precursors in man, the effects of feeding methylurea (MU) or morpholine (Mor) plus sodium nitrite (NaNO(2)) were investigated using a multi‐organ carcinogenesis model. In experiment 1, to initiate multiple organs, groups of 10 or 20 male F344 rats were treated with 6 carcinogens targeting different organs. Starting a week after completion of this initiation phase, animals were given 0.1% MU or 0.5% Mor in their food and/or 0.15% NaNO(2) in their drinking water for 23 weeks. The induction of tumors and/or preneoplastic lesions in the forestomach and esophagus was significantly increased in the group receiving MU plus NaNO(2). The numbers and areas of liver glutathione S‐transferase placental form (GST‐P)‐positive foci were significantly elevated with MU or Mor plus NaNO(2). Experiment 2 was conducted to assess formation of N‐nitroso compounds in the stomach, and to detect DNA adduct generation in target organs by immunohistochemical staining. Groups of 5 or 14 animals were starved overnight, then given 0.4% MU or 2.0% Mor in the diet, or basal diet alone for 1 h. Then NaNO(2) or distilled water was given intragastrically. The mean gastric N‐methyl‐N‐nitrosourea yield in the MU plus NaNO(2) group was 7700 μg at 2 h after combined administration. The mean N‐nitrosomorpholine yield in the group given Mor plus NaNO(2) was 6720 μg. Immunohistochemically, N7‐methyldeoxyguanosine‐positive nuclei were evident in the forestomach epithelium at 8 h after the combination treatment with MU plus NaNO(2).