Evaluation of sunscreen protection in human melanocytes exposed to UVA or UVB irradiation using the alkaline comet assay

The in vivo assessment of sunscreen protection does not include the photoge notoxicity of UVA or UVB solar radiation. Using the comet assay we have dev eloped a simple and rapid technique to quantify sunscreen efficacy against DNA damage induced by UV light. Cutaneous human melanocytes from primary cu ltures were embedded in low-melting point (LPM) agarose and exposed to UVA (0.8 J/cm(2)) or to UVB (0.06 J/cm(2)) through a quartz slide covered with 10 muL volumes of sunscreens. DNA single-strand breaks induced directly by UVA at 4 degreesC and indirectly through nucleotide excision repair by UVB following a 35 min incubation period at 37 degreesC were quantified using t he comet assay. Tail moments (TM) (tail length X % tail DNA) of 100 cells/s ample were determined by image analysis. DNA damage was evaluated with a no nlinear regression analysis on the normalized distribution frequencies of T M using a chi (2) function. The coefficients or genomic protection (CGP) we re defined as the percentage or inhibition or DNA lesions caused by the sun screens. Twenty-one sunscreens were evaluated, and the calculated CGP were compared with the in vivo sun protective factor (SPF) and with the protecti on factor UVA (PFA). Nonlinear relationships were found between SPF and CGP (UVB) and between PFA and CGP(UVA).