Sunscreen Protection Against Ultraviolet Radiation-induced Pyrimidine Dimers in Mouse Epidermal DNA

Pyrimidine dimers were measured in epidermal DNA of SKH:HRI mice following exposure to solar-simulated UV radiation (SSUV, 290–400 nm) or to UVA (320–400 nm). Mice were exposed to SSUV or UVA after topical application (2 mg/cm²) of vehicle, a UVB absorber (5% 2-ethylhexyl p-methoxycinnamate 2-EHMC]), or a broad-spectrum UVA absorber (5% Mexoryl®SX). The rates of induction of pyrimidine dimers in untreated animals were 5.4 ± 0.57 times 10^-4 (mean ± SEM) and 7.6 ± 0.95 times 10^-6 dimers per 10⁸ Da of epidermal DNA per J/m² of SSUV and UVA, respectively. Topical application of Mexoryl®SX reduced the rate of induction of pyrimidine dimers in SSUV-exposed animals to 4.7 ± 0.44 times 10^-5 dimers per 10⁸ Da per J/m² for a dimer induction protection factor (PF) of 11.5 (5.4 times 10 ⁴/4.7 times 10^-5). The rate of dimer induction in Mexoryl®SX-treated, UVA-ex-posed mice was 0.95 ± 0.2 times 10^-6 dimers per 10⁸ Da per J/m² (PF = 8.0). The 2-EHMC at a concentration of 5% (wt/wt) was significantly less effective than Mexoryl®SX in preventing the induction of pyrimidine dimers in animals exposed to either SSUV or UVA. The rates of dimer induction in 2-EHMC-treated mice were 8.2 ± 1.1 times 10^-5 and 3.8 ± 0.33 times 10^-6 dimers per Da per J/m² of SSUV (PF = 6.6) and UVA (PF = 2.0), respectively. Upon normalizing to the efficacy for edema induction, UVA induced approximately one-fourth the number of pyrimidine dimers per equivalent edematous response when compared to SSUV.