  Citation: Ramalho, Í.M.d.M.; Bezerra, G.S.; Ostrosky, E.A.; Ferrari, M.; Oliveira, V.d.S.; Wanderley Neto, A.d.O.; Quintans, J.d.S.S.; Passos, F.R.S.; Heimfarth, L.; Quintans-Júnior, L.J.; et al. Chrysin-Loaded Microemulsion: Formulation Design, Evaluation and Antihyperalgesic Activity in Mice. Appl. Sci. 2022, 12, 477. https://doi.org/10.3390/ app12010477 Academic Editors: Artur Turek and Eva Martins Received: 11 December 2021 Accepted: 22 December 2021 Published: 4 January 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). applied sciences Article Chrysin-Loaded Microemulsion: Formulation Design, Evaluation and Antihyperalgesic Activity in Mice Ízola Morais de Medeiros Ramalho 1 , Gabriela Suassuna Bezerra 1 , Elissa Arantes Ostrosky 1 ,Márcio Ferrari 1 , Verônica da Silva Oliveira 1 , Alcides de Oliveira Wanderley Neto 2 , Jullyana de Souza Siqueira Quintans 3 , Fabiolla Rocha Santos Passos 3 , Luana Heimfarth 3 , Lucindo José Quintans-Júnior 3 , Bolí var Ponciano Goulart de Lima Damasceno 4 , Attilio Converti 5, * and Ádley Antonini Neves de Lima 1 1 Department of Pharmacy, Federal University of Rio Grande do Norte, Natal 59012-570, RN, Brazil; izola.ramalho.069@ufrn.edu.br (Í.M.d.M.R.); gabriela.suassuna.070@ufrn.edu.br (G.S.B.); elissa.arantes@ufrn.br (E.A.O.); marcio.ferrari@ufrn.br (M.F.); veronica.oliveira.082@ufrn.edu.br(V.d.S.O.); adley.lima@ufrn.br (Á.A.N.d.L.) 2 Institute of Chemistry, Federal University of Rio Grande do Norte, Natal 59012-570, RN, Brazil; alcides.wanderley@ufrn.br 3 Laboratory of Neuroscience and Pharmacological Assays, Department of Physiology, Federal University of Sergipe, São Cristóvão 49100-000, SE, Brazil; jullyanas@academico.ufs.br (J.d.S.S.Q.); fabiollarsp@academico.ufs.br (F.R.S.P.); luahei@academico.ufs.br (L.H.); lucindo@academico.ufs.br (L.J.Q.-J.) 4 Center for Biological and Health Sciences, Department of Pharmacy, State University of Paraíba, Campina Grande 58429-600, PB, Brazil; bolivarpgld@servidor.uepb.edu.br 5 Department of Civil, Chemical and Environment Engineering, Pole of Chemical Engineering, University of Genoa, I-16145 Genoa, Italy * Correspondence: converti@unige.it; Tel.: +39-010-3352593 Abstract: Chrysin is a bioactive flavonoid found in pollens, passion flowers, honey, royal jelly, and propolis, which is commonly used as an ingredient in natural food supplements and is primarily re- sponsible for their pharmacological properties. A transparent chrysin-loaded microemulsion (CS-ME) prepared through a ternary phase diagram was evaluated for use as an antihyperalgesic formulation. It was formulated with 40% Labrasol ® (surfactant), 5% isopropyl myristate (oil phase) and 55% water (aqueous phase) and classified as an oil-in-water (O/W) microsized system (74.4 ± 15.8 nm). Its negative Zeta potential (−16.1 ± 1.9 mV) was confirmed by polarized light microscopy and dynamic light scattering analysis. In vitro studies in Franz-type static diffusion cells showed that chrysin release from CS-ME followed zero-order kinetics. Oral administration of CS-ME in mice resulted in a statistically significantly reduction (p < 0.05) in carrageenan-induced mechanical hyperalgesia compared to the control group. Treatment with CS-ME also showed anti-inflammatory activity by significantly decreasing the TNF-α level (p < 0.01) and increasing that of IL-10 (p < 0.05) com- pared to the control group. These results suggest that the proposed microsystem is a promising vector for the release of chrysin, being able to improve its capacity to modulate inflammatory and nociceptive responses. Keywords: microemulsion; drug delivery system; flavonoid; inflammation; pain; mechanical hyperalgesia 1. Introduction Defined as dispersions between oil, surfactant, aqueous phase, and, frequently, co- surfactants, microemulsified systems were discovered in 1943 by Hoar and Schulman, who observed that an opaque emulsion stabilized by a surfactant became clear after the addition of a medium-chain length alcohol [1,2]. By changing the proportions of compo- nents, it is possible to prepare different microstructures that can be classified into three categories according to their physicochemical properties, namely oil-in-water, bicontinuous, and water-in-oil microemulsions [3]. Several advantages are provided by microemulsions Appl. Sci. 2022, 12, 477. https://doi.org/10.3390/app12010477 https://www.mdpi.com/journal/applsci