Skin tumorigenesis and carcinogenesis studies with 7,12-dimethylbenz[a]anthracene, ultraviolet light, benzoyl peroxide (Panoxyl gel 5%) and ointment gel.

In a previous paper it was demonstrated on hairless mouse skin that 5% benzoyl peroxide (BP) in a gel (Panoxyl), or gel alone, applied just before UV radiation had a protective effect against UV-induced tumorigenesis, but both enhanced 7,12-dimethylbenz[a]anthracene (DMBA)-induced tumorigenesis. Groups of hairless (hr/hr) mice were therefore given ultraviolet (UV) irradiation with or without additional treatment with Panoxyl or gel in order to see whether Panoxyl or the gel given long time before, or after, irradiation influenced UV-induced tumorigenesis. Consequently, in some animals Panoxyl or gel was applied in the evening and the mice were irradiated the next day; in others, Panoxyl or gel was applied 5-30 min after UV irradiation. Enhancement of DMBA-induced carcinogenesis in hr/hr mice by the gel alone (assumed to be inert) was unexpected, and hence one group of hr/hr mice was first given 51.2 micrograms DMBA in acetone and thereafter treated twice a week with gel alone. All mice were tested and observed for skin tumors and other lesions for 52 weeks. Neither Panoxyl nor gel influenced UV tumorigenesis or carcinogenesis under these experimental conditions. In hr/hr mice there was this time no enhancement of DMBA-induced tumorigenesis by the gel, and a slight reduction of carcinogenesis. In addition, several groups of SENCAR mice (which have been bred for high sensitivity to skin carcinogenesis) were also treated, with acetone alone, with a single application of DMBA alone, with Panoxyl alone, or with DMBA followed by treatment with the ointment gel or with Panoxyl twice a week throughout the experiment. In SENCAR mice there was no difference between the results of treatment with DMBA followed by Panoxyl, or DMBA followed by gel, and both substances tended to reduce the tumorigenicity of DMBA alone, and Panoxyl or gel showed no tumorigenicity of their own. The total dose of UV used in this study was lower than that used in the first study. This reduction in dose significantly increased the tumorigenic effect of UV.