Repeated-dose liver micronucleus test of 4,4′-methylenedianiline using young adult rats

Liver micronucleus (MN) tests using partial hepatectomized rats or juvenile rats have been shown to be useful for the detection of hepatic carcinogens. Moreover, Narumi et al. established the repeated-dose liver MN test using young adult rats for integration into general toxicity. In the present study, in order to examine the usefulness of the repeated-dose liver MN test, we investigated MN induction with a 14 or 28 day treatment protocol using young adult rats treated with 4,4′-methylenedianiline (MDA), a known hepatic carcinogen. MDA dose-dependently induced micronuclei in hepatocytes in 14- and 28-day repeated-dose tests. However, although statistically significant increases in micronuclei were observed in bone marrow cells at two dose levels in the 14-day study, there was no dose response and no increases in micronuclei in the 28-day study. These results indicate that the evaluation of genotoxic effects using hepatocytes is effective in cases where chromosomal aberrations are not clearly detectable in bone marrow cells. Moreover, the repeated-dose liver MN test allows evaluation at a dose below the maximum tolerable dose, which is required for the conventional MN test because micronucleated hepatocytes accumulate. The repeated-dose liver MN test employed in the present study can be integrated into the spectrum of general toxicity tests without further procedural modifications.