Evaluation of the rodent micronucleus assay by a 28-day treatment protocol: Summary of the 13th Collaborative Study by the Collaborative Study Group for the Micronucleus Test (CSGMT)/Environmental Mutagen Society of Japan (JEMS)-Mammalian Mutagenicity Study Group (MMS)

To examine whether micronucleus tests can be incorporated into general toxi cology assays, we performed micronucleus tests applying the treatment proto cols typically used in such assays. In this 13th Collaborative Study of the CSGMT, both rats and mice were tested, although mts were used in the major ity of the studies. Fifteen mutagens were tested in rats, mainly by oral (p .o.) administration. Micronucleus induction was evaluated 2, 3, and 4 days, and 1, 2, 3, and 28 days after the beginning of the treatment in the perip heral blood, and at 28 days in the bone marrow. Of the 15 chemicals that in duced micronuclei in rats in short-term assays, two chemicals (1,2-dimethyl hydrazine 2HCl and mitomycin C) were negative in all our experiments, possi bly because of insufficient dose levels. The remaining 13 were positive wit hin the estimated dose range of a general toxicology assay, suggesting the possibility of integrating the micronucleus assay into general toxicology a ssays. Three patterns were observed in micronucleus induction during the pe riod of repeated treatment: (1) gradual increases in micronucleus frequency with sequential doses, (2) a peak at 3-5 days Followed by gradual decrease s in micronucleus frequency with sequential doses, and (3) a rapid increase in micronucleus frequency followed by a plateau. We evaluated factors that might have been involved in those patterns, such as the spleen function, t arget organ exposure, extramedullary hematopoiesis, hypothermia, and hypoxi a. Another factor we considered was dosage. Because the dosages employed in a general toxicity assay ore usually lower than those used in short-term m icronucleus assays, this discrepancy was considered the greatest potential problem for integrating the micronucleus assay into general toxicology assa ys. Our results indicate that the integration of the micronucleus assay int o a 28-day toxicological assay is feasible. To serve this purpose, blood sa mples collected 4 days after the beginning of treatment and blood and bone marrow samples collected at autopsy should be examined. Furthermore, althou gh it is recognized that mice may be suitable for performing independent mi cronucleus assays, we propose that rats can provide biologically important and relevant information regarding potential chemical mutagens that can be evaluated under conditions used in the conduct of general toxicology studie s. (C) 2001 Wiley-Liss, Inc.