Cutaneous phototoxicity reactions.

This paper reviews some potential mechanisms of cutaneous phototoxicity and discusses selected methods of predicting and ranking phototoxic reactions. Phototoxicity is a non-immunological reaction that is induced by the action of light on a photoactive chemical. The phototoxic chemical may be either exogenous or endogenous, and if exogenous may include topical or systemic agents. Examples of phototoxic agents include psoralens (e.g. 5-methoxypsoralen, 8-methoxypsoralen, as well as the isopsoralens). Psoralens are used therapeutically for the treatment of different skin diseases and psoralen-like chemicals may be present in perfumes and cosmetics. Phototoxic reactions can be evaluated visually in animals. The hairless mouse has also proved useful with skin thickening as a marker of dermal oedema and the induction of polyamine biosynthetic enzymes, as well as other biochemical changes being indicative of cutaneous phototoxicity. In vitro tests of phototoxicity include the Candida albicans assay as well as bacterial mutagenicity. For example, in the latter category different psoralens vary in their ability to induce mutagenicity in Salmonella typhimurium strains, an assay which can provide rapid screening of potential phototoxicity of chemicals. For human testing, phototoxic chemicals can be delivered topically or orally, and in conjunction with appropriate ultraviolet radiation will produce phototoxic reactions which can be evaluated for intensity of response.