Topical nanostructured lipid carriers/inorganic sunscreen combination for alleviation of all-trans retinoic acid-induced photosensitivity: Box-Behnken design optimization, in vitro and in vivo evaluation

All-trans retinoic acid is a natural retinoid and the physiologically active metabolite of vitamin A. The aim of the present study is to develop and optimize a nanostructured lipid carrier formulation to enhance the photostability of all-trans retinoic acid and alleviate its skin photosensitivity. Box-Behnken design was used for optimizing dependent variables such as particle size, zeta potential and viscosity. The total lipid (%), liquid lipid (%) and total surfactant (%) were selected as independent variables. The optimized formulation was characterized by particle size of 151 nm, zeta potential of -31 mV and viscosity of 2064 cps. In vitro photoprotection effect of the optimized formulation containing different types and concentrations of inorganic sunscreens was evaluated employing Transpore (R) tape assay. Sun protection factor and other spectroscopic indices revealed that 6% titanium dioxide was the best choice to be combined with the optimized formulation. After 6 h of ultraviolet A exposure, the optimized formulation and the optimized formulation combined with 6% titanium dioxide enhanced the photostability of all-trans retinoic acid by about 1.5 and 2 times, respectively, compared to its methanolic solution. In vivo photoprotection effect of the developed formulations was conducted on mice exposed to direct sun light for 4 days. Photographs of the mice's skin, biochemical analysis of the pro-inflammatory cytokines in the skin as well as histopathological examination, depicted that the optimized formulation promoted an obvious alleviation of the all-trans retinoic acid-induced photosensitivity, which was further potentiated by the addition of 6% titanium dioxide, compared to the marketed product.