Sustained effect of two antioxidants (oxothiazolidine and δ-tocopheryl glucoside) for immediate and long-term sun protection in a sunscreen emulsion based on their different penetrating properties

Objective We investigated the dermal bioavailability and antioxidative properties of a sunscreen formulation containing two antioxidants, oxothiazolidine (OTZ) and delta-tocopheryl glucoside (DTG). OTZ reacts directly with reactive oxygen species to form taurine, while DTG is metabolized in delta-tocopherol to achieve antioxidative activities. Methods After topical application to a hair follicle-derived reconstructed human epidermis (RHE) model, followed by solar-simulated radiation, kinetics of bioavailability and antioxidative responses were measured over 24 h. Markers for oxidative stress were malondialdehyde (MDA), superoxide dismutase (SOD) and catalase activities. Results The two antioxidants had different bioavailability profiles: OTZ was rapidly and extensively absorbed, whereas DTG was slowly absorbed and converted to delta-tocopherol. Compared to OTZ alone, the protection against effects on MDA levels and SOD and catalase activities was higher when DTG was used alone or in combination with OTZ. When used in combination, the degree of protection increased over time and remained constant over 24 h with maximal protection 2 h post-irradiation. DTG slowly penetrated into the skin and was present in the skin at all post-irradiation timepoints, thus allowing a slow but constant supply of delta-tocopherol over at least 24 h. By contrast, the oxidative protection by OTZ was immediate but short-lived due to its rapid penetration through the RHE and into the receptor fluid. Conclusion These results indicate a complementary sunlight protective action of OTZ and DTG with an immediate delivery of OTZ just after topical application of the formulation, and a prolonged skin delivery of delta-tocopherol from the slower penetration and metabolism of DTG.