The effect of aging on the repeated-dose liver micronucleus assay using N-nitrosodipropylamine, quinoline, and carbendazim

The repeated dose liver micronucleus (RDLMN) assay has been sufficiently validated in terms of the numbers and types of chemicals studied. However, it remains unclear whether aging affects assay results. The OECD Test Guideline 407 (Repeated Dose 28-Day Oral Toxicity Study in Rodents) indicates that dosing should begin as soon as feasible after weaning and in any event before 9 weeks of age. Therefore, it is particularly important to determine whether there are age-related differences between 6 and 8 weeks of age at the start of dosing when considering the possibility of integrating this assay into a 4-week repeated dose general toxicity study. We evaluated the impact of the rats’ age on the RDLMN assay with three chemicals: N-nitrosodipropylamine, quinoline, and carbendazim. There were no significant age-related differences for the first two chemicals, whereas a markedly higher frequency of micronucleated hepatocytes (MNHEPs) was observed in younger rats for carbendazim. However, regardless of the age of animals, micronucleus induction was detected in all three chemicals. Combined with the previous reports on clofibrate and diethylnitrosamine, we concluded that animals of any age from 6 to 8 weeks could be used in the RDLMN assay.