Evaluation of the RBC Pig-a assay and the PIGRET assay using benzo[a]pyrene in rats.

The red blood cell (RBC) Pig-a assay has the potential to detect the in vivo mutagenicity of chemicals. Recently, use of the Pig-a assay with reticulocytes (the PIGRET assay) reportedly enabled the in vivo mutagenicity of chemicals to be detected earlier than using the RBC Pig-a assay. To evaluate whether the PIGRET assay is useful and effective as a short-term test, compared with the RBC Pig-a assay, we performed both assays using benzo[a]pyrene (BP), which is a well-known mutagen. BP was used to dose 8-week-old male rats orally at 0, 75.0, 150, and 300mg/kg administered as a single administration. Peripheral blood samples were then collected on days 0, 7, 14, and 28 after treatment and were used in both assays. In the treatment groups receiving 150mg/kg of BP or more, both the RBC Pig-a assay and the PIGRET assay detected the in vivo mutagenicity of BP. In the 300mg/kg treatment group, in which a significant increase in the mutant frequency (MF) was observed at all the sampling points using both the RBC Pig-a assay and the PIGRET assay, the reticulocyte (RET) Pig-a MF was higher than the RBC Pig-a MF on days 7 and 14 after treatment; nevertheless, the negative control RET Pig-a MF was comparable to the negative control RBC Pig-a MF. In addition, the RET Pig-a MF began to increase after day 7 and reached a maximum value on day 14 after treatment, whereas the RBC Pig-a MF increased continuously from day 7 until day 28 after treatment. These results indicate that the PIGRET assay has a higher sensitivity than the RBC Pig-a assay and that the PIGRET assay is useful for the earlier detection of the in vivo mutagenicity of chemicals, compared with the RBC Pig-a assay.

Evaluation of the repeated-dose liver, bone marrow and peripheral blood micronucleus and comet assays using kojic acid.

The repeated-dose liver micronucleus assay has the potential to detect liver carcinogens and could be integrated into general toxicological studies. To assess the performance of this assay, kojic acid was tested in 14-day and 28-day liver micronucleus assays. We evaluated the incidence of micronucleated cells in liver, bone marrow and peripheral blood and performed comet assays in both the liver and peripheral blood (comet assay was performed only for 14-days). Kojic acid, a skin-whitening agent used in cosmetic products, was orally dosed in six-week-old male rats at 250, 500 and 1000mg/kg/day for 14 days, and at 125, 250 and 500mg/kg/day for 28 days. Organ weight and histopathology were examined at the end of the experiment. Neither a clear, positive response in micronucleus (MN) incidence nor changes in the percent of tail DNA in the comet assays was noted in liver and bone marrow. An increase of relative liver weight was observed in 1000mg/kg/day for 14 days. In histopathology, minimal hypertrophy of hepatocytes was found at 1000mg/kg/day for 14 days. The results of both the micronucleus assay and the comet assay indicate that 14-day and 28-day repeated dosing of kojic acid are non-genotoxic in the liver and bone marrow. Kojic acid has been known to act as a tumor-promoter in thyroid carcinogenesis but has not been shown to have initiation activities in liver carcinogenesis. Findings in this study are consistent with the evidence that kojic acid is not an apparent initiator of liver carcinogenesis. Therefore, the liver micronucleus assay is simple and sensitive to detect genotoxic liver carcinogens.