Evaluation of percutaneous absorption of the repellent diethyltoluamide and the sunscreen ethylhexyl p‐methoxycinnamate‐loaded solid lipid nanoparticles: an in‐vitro study

Objectives Diethyltoluamide and ethylhexyl p‐methoxycinnamate (OMC) are two active ingredients in insect repellent and sunscreen products, respectively. The concurrent application of these two substances often increases their systemic absorption, compromising the safety and efficiency of the cosmetic product. In this study, diethyltoluamide and OMC were incorporated into solid lipid nanoparticles, a colloidal drug delivery system, to reduce percutaneous absorption and avoid toxic effects and also maintain the efficacy of the two active compounds on the skin surface for a long duration. Methods Solid lipid nanoparticles were prepared based on an ultrasonication technique and characterized by differential scanning calorimetry (DSC) analyses. In‐vitro studies determined the percutaneous absorption of diethyltoluamide and OMC. Key findings DSC data carried out on unloaded and diethyltoluamide‐ and/or OMC‐loaded solid lipid nanoparticles highlighted that diethyltoluamide and OMC modified the temperature and the enthalpy change associated to the calorimetric peak of solid lipid nanoparticles. The concurrent presence of the two compounds in the solid lipid nanoparticles caused a synergic effect, indicating that the lipid matrix of nanoparticles guaranteed a high encapsulation of both diethyltoluamide and OMC. Results from the in‐vitro study demonstrated that the particles were able to reduce the skin permeation of the two cosmetic ingredients in comparison with an oil‐in‐water emulsion. Conclusions This study has provided supplementary evidence as to the potential of lipid nanoparticles as carriers for topical administration of cosmetic active compounds.