Sunscreen protection against cis-urocanic acid production in human skin

Commercial sunscreens may offer some protection from immunosuppression indu ced by ultraviolet (UV) radiation, but agreement concerning the degree of p rotection is lacking. Cisurocanic acid, formed by the photoisomerization of transurocanic acid is considered an important mediator of the cutaneous im munomodulation resulting from exposure to UV radiation. We investigated the effect of sunscreens on the isomerization of urocanic acid in 17 human sub jects. Two sunscreens containing chemical filters, sun protection factor (S PF)4 and SPF 10, and a SPF 10 sunscreen with a physical filter were applied at a thickness of 2 mg/cm(2). The effect of a thin layer (0.5 mg/cm(2)) of the chemical SPF 10 sunscreen was also evaluated, as the amount of sunscre en applied in practice may be considerably less than recommended. All areas were irradiated with a single UV dose of 3.6 SED (standard erythema doses) . In irradiated unprotected skin the median net production of cis-urocanic acid was 52% (relative amount). In the sites treated with the chemical suns creens, the production of cis-urocanic acid was 7.4% (SPF 4) and 3.5% (SPF 10), and isomerization was thus reduced more efficiently at a higher SPF (p <0.01). The physical sunscreen reduced the formation of cis-UCA to 15%, and was significantly less effective than both the chemical SPF 10 sunscreen ( p<0.01) and the SPF 4 sunscreen (p<0.01), The production of cis-urocanic ac id in the area treated with the thin layer of the chemical SPF 10 sunscreen was 22%, The protection against the production of cis-urocanic acid was th erefore reduced significantly (p<0.01) when the sunscreen was applied in an amount lower than recommended.