Examination of percutaneous application in a 26-week carcinogenicity test in CB6F1-TG rasH2 mice.

We examined the possibility of expanding applications of rasH2 mice, which are genetically manipulated mice for short-term carcinogenicity tests, to percutaneous application. A 26-week short-term carcinogenicity study was performed on a total of 300 mice including 75 male and female rasH2 mice each, and 75 male and female non-Tg mice each from the same litter as the rasH2 mice divided into untreated group, an ethanol group, a white Vaseline group, an acetone group, and a phorbol 12-myristate 13-acetate (TPA) group. Only shaving of dorsal skin was performed on the untreated mice. As a positive control, TPA was administered percutaneously at a dose of 2.5 microg/kg and 3 times/week for 26 weeks based on the protocol for Tg.AC mice in the ILSI/HESI international validation study. In the ethanol, white Vaseline, and acetone groups, no tumorous changes were observed on the skin at the administration site. In the TPA group, nodular changes at the administration site were observed from seven weeks after the start of administration in rasH2 mice, and the incidence in males and females was 50.0% (7/14) and 53.3% (8/15), respectively. In a pathological examination, nodules in 21.4% (3/14) of males and 46.7% (7/15) of females were diagnosed as skin papilloma or keratoacanthoma, and the rest as squamous cell hyperplasia. In the non-Tg mice, no nodules or tumorigenic changes were observed at the administration site. These findings show that percutaneous application in rasH2 mice is possible in 26-week carcinogenicity tests.