DETERMINATION OF THE PHOTOPROTECTIVE EFFICACY OF A TOPICAL SUNSCREEN AGAINST UVB-INDUCED DNA-DAMAGE IN HUMAN EPIDERMIS

The ability of a chemical sunscreen with a sun protection factor of te n to protect human skin in situ against UVB-induced DNA damage (cyclob utyl thymine dimers) was evaluated. Biopsies were taken from the left buttock of ten human volunteers prior to UVB (280-315 nm) exposure. Su bsequently, a sunscreen (n=6) or vehicle (n=4) was applied to a deline ated area on the right buttock. After a period of 30 min, the entire b uttock area was irradiated in a UVB cabin with one minimal erythema do se. Immediately after irradiation, biopsy specimens were obtained from the UVB-exposed sunscreen- or vehicle-treated right buttock and from the non-treated UVB-exposed left buttock. Dimers were assayed in skin sections by immunofluorescence microscopy with a monoclonal antibody a gainst the cyclobutyl thymine dimer. The dimer-specific fluorescence f rom the epidermal cell nuclei, identified by counterstaining with prop idium iodide, was quantified through computer-mediated image processin g and analysis in skin sections of one sunscreen-treated and one vehic le-treated volunteer. After a single dose of UVB, significant dimer-sp ecific nuclear fluorescence was observed and measured in the non-treat ed biopsy specimens. No nuclear fluorescence was observed and very lit tle could be measured in the non-UVB-exposed skin and in the sunscreen -treated UVB-exposed skin respectively, indicating that the sunscreen offered good protection against the induction of cyclobutyl thymine di mers by UVB.This visual scoring is in general semiquantitative, but qu antification through computer-mediated image processing was performed in one case for sunscreen-treated skin and in one case for vehicle-tre ated skin. Both assessments resulted in similar conclusions. Thus this technique is an accurate method to determine the efficacy of sunscree ns in protecting skin from UV-radiation-induced DNA damage.