Repeated-dose liver micronucleus assay of mitomycin C in young adult rats.

The repeated-dose liver micronucleus (RDLMN) assay has been previously reported to be effective for the detection of hepatocarcinogens and suitable for general toxicology studies. A collaborative study was conducted to evaluate whether this RDLMN assay using young adult rats without collagenase perfusion of the liver can be used to detect genotoxic carcinogens. In this study, we performed the RDLMN assay in young adult rats that received intraperitoneal injections of 0.25, 0.5 or 1.0mg/kg/day of mitomycin C (MMC) for 14 and 28 days. The micronucleus induction in the bone marrow was concurrently measured, and a histopathological examination of the liver was conducted. The results revealed that the frequency of micronucleated hepatocytes (MNHEPs) was significantly increased in all of the treatment groups. However, the highest occurrence of MNHEPs was observed in the low-dose treatment group in both the 14- and the 28-day study periods. In addition, histopathological changes indicating hepatotoxicity were not observed even in the group that received the highest dose of MMC. There was no change in the frequency of metaphase hepatocytes in any of the treatment groups compared with our facility's background data. However, the frequency of proliferating hepatocytes, as assessed by Ki-67 positivity, was decreased at the highest dose, as was the frequency of MNHEPs. Therefore, the decreased induction of MNHEPs in the high-dose groups might be explained by suppression of hepatocyte cell division. In contrast, the frequency of micronucleated immature erythrocytes in the bone marrow significantly increased in a dose-dependent manner in all of the treatment groups in both study periods. Repeated treatment of MMC induced micronuclei in the liver. These results suggest that the novel RDLMN assay can be used to detect MMC genotoxicity in the liver.