Photocarcinogenesis is retarded by a partly photodegraded solution of para-aminobenzoic acid.

A solution of para-aminobenzoic acid (PABA) was exposed to ultraviolet (UV) radiation emitted from a Philips TL 40 W/12 sunlamp and the degree of photodegradation following an exposure of 27 J/cm2 was estimated to be approximately 40%. The formation of the photoproducts was confirmed by mass spectroscopy and UV spectroscopy. The solution was painted on the backs of hairless light-pigmented mice prior to daily UV irradiation by the above sunlamp, and this procedure was continued for 30 weeks. The preirradiated solution of PABA significantly retarded the tumor induction time and reduced significantly the number of squamous cell carcinomas compared with nonprotected controls. This tumor-retarding ability did not differ significantly from the effect achieved when using nonirradiated PABA.