Evaluation of repeated dose micronucleus assays of the liver using N-nitrosopyrrolidine: a report of the collaborative study by CSGMT/JEMS.MMS.

The repeated dose liver micronucleus (RDLMN) assay has the potential to detect liver carcinogens, and can be integrated into a general toxicological study. To assess the performance of the assay, N-nitrosopyrrolidine (NPYR), a genotoxic hepatocarcinogen, was tested in 14- or 28-day RDLMN assays. NPYR was orally administered to rats at a daily dose of 25, 50 or 100 mg/kg. One day after the last administration, a portion of the liver was removed and hepatocyte micronucleus (MN) specimens were prepared by the new method recently established by Narumi et al. In addition, a bone marrow MN assay and a histopathological examination of the liver were conducted. The detection of Phospho-Histone H3 was performed by immunohistochemistry to evaluate the proliferation rate of hepatocytes. The results showed significant increase in the number of micronucleated hepatocytes and Phospho-Histone H3-positive cells from the lowest dose in both 14- and 28-day RDLMN assays. On the other hand, the bone marrow MN assay yielded a negative result, which was in accordance with the existing report of the bone marrow MN assay using mice. Upon histopathological examination, inflammatory lesions and hypertrophy were noted, which may explain the increase in the hepatocyte proliferation and the enhancement of MN induction by NPYR. Our findings indicate that the RDLMN assay could be a useful tool for comprehensive risk assessment of carcinogenicity by providing information on both genotoxicity and histopathology when integrated into a general repeat dosing toxicity assay.