-Guinea pigs are the most frequently used animals in phototoxicity studies. However, general toxicity studies most often use Sprague-Dawley (SD) rats. To reduce the number of animals needed for drug development, we examined whether skin phototoxicity studies could be performed using SD rats. A total of 19 drugs that had previously been shown to have phototoxic potential and 3 known phototoxic compounds were administered transdermally to guinea pigs and SD rats. Eleven of the potentially phototoxic drugs and 2 of the known phototoxic compounds were also administered orally to guinea pigs and SD rats. After administration, the animals were irradiated with UV-A (10 J/cm 2 ) and UV-B (0.25 J/cm 2 in guinea pigs and 0.031 J/cm 2 in SD rats) with doses based on standard phototoxicity study guidelines and the results of a minimum erythema dose test, respectively. In the transdermal administration study, all of the known phototoxic compounds and 7 of the drugs induced phototoxic reactions. In the oral administration study, both known phototoxic compounds and 5 drugs induced phototoxic reactions in both species; one compound each was found to be toxic only in SD rats or guinea pigs. The concordance rate of guinea pigs and SD rats was 100% in the transdermal administration study and 85% in the oral administration study. This study demonstrated that phototoxicity studies using SD rats have the same potential to detect phototoxic compounds as studies using guinea pigs.

The Pig-a gene is involved in the synthesis of glycosylphosphatidylinositol (GPI) anchors. Pig-a gene mutations can be detected by identifying the presence of CD59, the GPI anchor protein, on the surface of erythrocytes (RBC Pig-a assay) and reticulocytes (PIGRET assay) and can be identified using flow cytometry. The usefulness of these Pig-a gene mutation assays has been confirmed in multilaboratory trials with referenced mutagens. Although 4, 4′ -methylenedianiline (MDA) is an aromatic amine and has been identified as a potent hepatic carcinogen, in vivo micronucleus tests for MDA in hematopoietic cells determined that it was negative to weakly positive for genotoxicity. In the present study, we examined the mutagenicity of MDA in the peripheral blood of rats after 1-and 28-day MDA dosing using the Pig-a gene mutation assays. We also examined the utility of the RBC Pig-a and PIGRET assays. No changes in mutation frequency were observed after one-day MDA administration. Repeated dosing caused a moderate increase in mutation frequency compared to vehicle control at days 14 and 28, as measured by the RBC Pig-a assay and at day 14 by the PIGRET assay. The highest mutation frequency was found on days 7 and 14 by the PIGRET and RBC Pig-a assays, respectively.In this study, we detected the mutagenicity of MDA in peripheral blood samples using gene mutation assays and judged to be positive for the MDA mutagenicity since a significant increase in mutation frequency was observed at high dose. These assays are expected to be easily integrated into general toxicity tests and to be combined with existing genotoxicity studies.

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