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

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