Effects of low dose mixtures of four N-nitroso compounds on hepatic foci development in the rat

Potential synergism between four N-nitroso compounds (nitrosomorpholine, nitrosodimethylamine, nitrosodiethanolamine, nitroso-oxazolidine) in rat liver carcinogenesis was examined in the medium-term bioassay. Male F344 rats were initially given diethylnitrosamine (DEN, 200 mg/kg, ip) and beginning 2 weeks later received test chemicals for 6 weeks individually at a full or 14 dose of that proven to be carcinogenic individually or in combination. All animals were subjected to partial hepatectomy at week 3 and killed at week 8. Induction of immunohistochemically-demonstrated glutathione S-transferase placental form (GST-P) positive foci was evaluated. The numbers and size of GST-P positive foci were significantly higher than the control levels by the treatment with each nitrosamine at full (11) and one quarter doses (14), excepting nitrosodiethanolamine and by combination of the four chemicals at 14 and 116. Because the dose-response curves were considered non-linear for most nitrosamines, synergistic effects were not apparent for the 14 mixture. Interestingly, however, the values for rats treated with these four chemicals in combination at the 14 dose level were almost the same as the average of four individual treatments at the full dose, and those for the 116 dose mixture were almost the same as the average of 14 individual treatment groups. These results indicate that these nitrosamines worked additively, rather than synergistically, in rat liver carcinogenesis.