DIFFERENTIAL PHOTOIMMUNOPROTECTION BY SUNSCREEN INGREDIENTS IS UNRELATED TO EPIDERMAL CIS UROCANIC ACID FORMATION IN HAIRLESS MICE

A series of experimental sunscreen preparations based on a common vehi cle, containing increasing concentrations of either octyl-N-dimethyl-p -aminobenzoate (o-PABA) or 2-ethylhexyl-p-methoxycinnamate (2-EHMC) as the ultraviolet B (UVB) absorber, has been tested in the hairless mou se for the ability to protect from erythema, from the systemically sup pressive effects of UVB (280 - 320 nm) radiation on contact hypersensi tivity, and from photoisomerization of epidermal urocanic acid. All th e preparations protected efficiently from the edema component of the e rythema response when mice were exposed to UVB radiation equivalent to three times the minimal erythema dose (MED). However, when mice were exposed to UVB radiation equivalent to 15 X MED, protection from eryth ema was observed only at the higher concentrations of each UVB absorbe r (10% 2-EHMC and 10% or 15% o-PABA). Protection from the UVB-induced suppression of contact hypersensitivity was shown to be dependent on b oth the nature of the UVB absorber and its concentration. Photoimmunop rotection by the sunscreens containing 2-EHMC was evident at lower con centrations (5% and 10% 2-EHMC) than with o-PABA, following both 3 X M ED and 15 X MED of UVB exposure. Photoimmunoprotecaion by o-PABA-conta ining sunscreens was observed only at 15% o-PABA following 3 X MED, an d failed at all tested concentrations after 15 X MED of UVB exposure. Regardless of the photoimmunoprotective capacity, sunscreen preparatio ns containing either of the UVB absorbers prevented the UVB-induced fo rmation of cis urocanic acid in the mouse epidermis and in vitro under all conditions tested. Thus, there appeared to be a correlation betwe en protection from edema and from cis urocanic acid formation at 3 X M ED of UVB, but a dissociation of these variables at 15 X MED of UVB. T here was no relation apparent at either UVB dose between either edema or cis urocanic acid formation and protection from suppression of cont act hypersensitivity.