Modification of 5-methoxypsoralen phototumorigenesis by UVB sunscreens: a statistical and histologic study in the hairless albino mouse.

Sunscreen preparations containing 5-methoxypsoralen at 25 micrograms/ml and 50 micrograms/ml, and the appropriate vehicle only, sunscreen only, and 5-methoxypsoralen only controls, were assessed for tumorigenic potential in hairless albino mice exposed to solar simulated radiation for a period of 44-46 weeks. Some animals were observed for a 15 week post-irradiation period. A wide range of statistical analyses has shown that, during the course of irradiation, the inclusion of sunscreens eliminates the enhancement of phototumorigenesis and malignancy that is normally observed with 5-methoxypsoralen. There was, however, some indication of increased risk at the end of the postirradiation follow-up period. Possible reasons for the protective effect of the sunscreens and the relevance of these data to the assessment of human risk are discussed.