Lack of Hepatocarcinogenicity of Combinations of Low Doses of 2-amino-3, 8-dimethylimidazo[4,5- f ]quinoxaline and Diethylnitrosamine in Rats: Indication for the Existence of a Threshold for Genotoxic Carcinogens

The purposes of the present study were to evaluate the hepatocarcinogenicity ofconcurrent treatment of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline(MeIQx) and diethylnitrosamine (DEN) in rats and to determine whether no effect levels ofcombinations of these two different structural categories of genotoxic hepatocarcinogensexist. Two 16-week rat hepatocarcinogenesis assays were performed using a total of 790male F344 rats. In experiment 1, we evaluated the effects of concurrent treatment of asubcarcinogenic dose of DEN on rat hepatocarcinogenesis induced by various doses of MeIQx.In experiment 2, we determined hepatocarcinogenicities of combinations of MeIQx and DEN atsubcarcinogenic doses, low carcinogenic doses and high carcinogenic doses. Quantitativeanalyses of glutathione S-transferase placental form (GST-P)-positivefoci, a preneoplastic lesion of the liver in rats, revealed that concurrent treatment withsubcarcinogenic doses of DEN did not enhance MeIQx-induced rat hepatocarcinogenicity. Wealso found that concurrent treatment with combinations of subcarcinogenic doses of DEN andMeIQx was not hepatocarcinogenic, indicating that the combined effects of subcarcinogenicdoses of DEN and MeIQx were neither additive nor synergistic. Moreover, concurrenttreatment with low carcinogenic doses of these 2 carcinogens did not show additive orsynergistic effects. Synergetic effects were observed only in rats coadministered highcarcinogenic doses of the 2 carcinogens. These results demonstrate the existence of noeffect levels of combinations of these 2 genotoxic hepatocarcinogens, and provide newevidence supporting our idea that there is a threshold, at least a practical threshold,that should be considered when evaluating the risk of genotoxic carcinogens.