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Camila Folle,1 Ana M Marqués,2 Natalia Díaz-Garrido,3– 5 Paulina Carvajal-Vidal,1 Elena Sánchez López,1,6 Joaquim Suñer-Carbó,1,6 Lyda Halbaut,1,6 Mireia Mallandrich,1,6 Marta Espina,1,6 Josefa Badia,3– 5 Laura Baldoma,3– 5 Maria Luisa García,1,6 Ana Cristina Calpena1,6 1Department of Pharmacy and Pharmaceutical Technology and Physical Chemistry, Faculty of Pharmacy and Food Sciences, University of Barcelona, Barcelona, Spain; 2Department of Biology, Healthcare and Environment, Faculty of Pharmacy and Food Sciences, University of Barcelona, Barcelona, Spain; 3Department of Biochemistry and Physiology, Faculty of Pharmacy and Food Sciences, University of Barcelona, Barcelona, Spain; 4Institute of Biomedicine of the University of Barcelona (IBUB), Barcelona, Spain; 5Research Institute Sant Joan De Déu (IR‑SJD), Barcelona, Spain; 6Institute of Nanoscience and Nanotechnology (IN2UB), University of Barcelona, Barcelona, SpainCorrespondence: Camila Folle; Elena Sánchez López, Department of Pharmacy and Pharmaceutical Technology and Physical Chemistry, Faculty of Pharmacy and Food Sciences, University of Barcelona, Barcelona, 08028, Spain, Email camilafolle@ub.edu; esanchezlopez@ub.eduPurpose: Acne vulgaris is one of the most prevalent dermal disorders affecting skin health and appearance. To date, there is no effective cure for this pathology, and the majority of marketed formulations eliminate both healthy and pathological microbiota. Therefore, hereby we propose the encapsulation of an antimicrobial natural compound (thymol) loaded into lipid nanostructured systems to be topically used against acne.Methods: To address this issue, nanostructured lipid carriers (NLC) capable of encapsulating thymol, a natural compound used for the treatment of acne vulgaris, were developed either using ultrasonication probe or high-pressure homogenization and optimized using 22-star factorial design by analyzing the effect of NLC composition on their physicochemical parameters. These NLC were optimized using a design of experiments approach and were characterized using different physicochemical techniques. Moreover, short-term stability and cell viability using HaCat cells were assessed. Antimicrobial efficacy of the developed NLC was assessed in vitro and ex vivo.Results: NLC encapsulating thymol were developed and optimized and demonstrated a prolonged thymol release. The formulation was dispersed in gels and a screening of several gels was carried out by studying their rheological properties and their skin retention abilities. From them, carbomer demonstrated the capacity to be highly retained in skin tissues, specifically in the epidermis and dermis layers. Moreover, antimicrobial assays against healthy and pathological skin pathogens demonstrated the therapeutic efficacy of thymol-loaded NLC gelling systems since NLC are more efficient in slowly reducing C. acnes viability, but they possess lower antimicrobial activity against S. epidermidis, compared to free thymol.Conclusion: Thymol was successfully loaded into NLC and dispersed in gelling systems, demonstrating that it is a suitable candidate for topical administration against acne vulgaris by eradicating pathogenic bacteria while preserving the healthy skin microbiome. Keywords: lipid nanoparticles, thymol, acne vulgaris, NLC, gels, antimicrobial