Effects of peroxides on rodent skin: epidermal hyperplasia and tumor promotion.

Free radical generating peroxides are potent skin irritants. After a single topical application of either 10, 20, or 40 mg of lauroyl peroxides or benzoyl peroxide on the dorsal skin of Sencar mice, the epidermal thickness increased markedly. No major inflammatory or vascular alterations were noted. On the other hand, 15 or 30% hydrogen peroxide produced an extensive epidermolysis, as well as inflammation and vascular injury, followed by quick regeneration and epidermal hyperplasia. Both lauroyl peroxide- and benzoyl peroxide-induced hyperplasias were characterized by a sustained production of dark basal keratinocytes, which constituted approximately 10% of the basal cell population during the first week after single topical application. Hydrogen peroxide-induced epidermal hyperplasias also exhibited numerous dark cells, but their presence was less sustained. Although all these peroxides were inactive either as initiators or as complete carcinogens, lauroyl peroxide was as effective as benzoyl peroxide when used as a skin tumor promoter in a two-stage carcinogenesis protocol. In a similar experimental protocol, hydrogen peroxide proved to be a very weak skin tumor promoter.