Lack of In Vivo Mutagenicity of Acetamide in a 13-Week Comprehensive Toxicity Study Using F344 gpt Delta Rats

Acetamide, a food contaminant, has been shown to induce hepatocellular tumors in rats. However, the mode of action underlying acetamide-induced hepatocarcinogenesis remains unclear. In the current study, we aimed to examine the possible involvement of in vivo mutagenicity in hepatocarcinogenesis of acetamide and evaluate its toxicological profile using a comprehensive medium-term toxicity study in gpt delta rats. Six-week-old male F344 gpt delta rats were given a basal diet containing 0%, 0.625%, 1.25%, or 2.5% acetamide for 13 weeks. In general toxicologic assessment, hepatotoxic parameters in serum, such as aspartate aminotransferase and alanine aminotransferase were significantly changed at the 1.25% group and higher. Histopathological examination of the liver revealed that various changes related to hepatic injury were observed at the 1.25% group and higher. Interestingly, Feulgen-positive cytoplasmic inclusion was frequently observed in hepatocytes in these groups. In the hematopoietic system, red blood cell parameters in plasma, such as mean corpuscular volume and mean corpuscular hemoglobin were significantly changed at the 1.25% group and higher, and decrease of erythroblast in the spleen was observed histopathologically in the 2.5% group. Thus, the no-observed-adverse-effect level of acetamide in this study was 0.625% (equivalent to 394 mg/kg body weight/day). In vivo mutation assays showed that acetamide induced no changes in gpt and red/gam gene mutant frequencies, even at the carcinogenic target site. In contrast, Ki67-positive hepatocytes were increased significantly at carcinogenic doses. Therefore, these results suggested that cell proliferation activity, but not mutagenicity, played crucial roles in acetamide-induced hepatocarcinogenesis in rats.