Medicinal Chemistry Thaise Martins 1 , Vera L.M. Silva 2 , Artur M.S. Silva 2 , José L.F.C. Lima 1 , Eduarda Fernandes 1,* and Daniela Ribeiro 1,* 1 LAQV, REQUIMTE, Laboratory of Applied Chemistry, Department of Chemical Sciences, Faculty of Pharmacy, University of Porto, Porto, Portugal; 2 LAQV, REQUIMTE, Department of Chemistry, University of Aveiro, Aveiro, Portugal Abstract: Aims: This study evaluates the ability of chalcones to scavenge hypochlorous acid (HOCl) and modulate oxidative burst. Background: The chemistry of chalcones has long been a matter of interest to the scientific commu- nity due to the phenolic groups often present and to the various replaceable hydrogens that allow the formation of a broad number of derivatives. Due to this chemical diversity, several biological activi- ties have been attributed to chalcones, namely anti-diabetic, anti-inflammatory and antioxidant. Objectives: Evaluate the ability of a panel of 34 structurally related chalcones to scavenge HOCl and/or suppress its production through the inhibition of human neutrophils’ oxidative burst, followed by the establishment of the respective structure-activity relationships. Methods: The ability of chalcones to scavenge HOCl was evaluated by fluorimetric detection of the inhibition of dihydrorhodamine 123 oxidation. The ability of chalcones to inhibit neutrophils’ oxida- tive burst was evaluated by chemiluminometric detection of the inhibition of luminol oxidation. Results: It was observed that the ability to scavenge HOCl depends on the position and number of hydroxy groups on both aromatic rings. Chalcone 5b was the most active with an IC 50 value of 1.0 ± 0.1 µM. The ability to inhibit neutrophils’ oxidative burst depends on the presence of a 2’-hydroxy group on A-ring and on other substituents groups, e.g. methoxy, hydroxy, nitro and/or chlorine at- om(s) at C-2, C-3 and/or C-4 on B-ring, as in chalcones 2d, 2f, 2j, 2i, 4b, 2n and 1d, which were the most actives with IC 50 values ranging from 0.61 ± 0.02 µM to 1.7 ± 0.2 µM. Conclusion: The studied chalcones showed high activity at a low micromolar range, indicating their potential as antioxidant agents and to be used as a molecular structural scaffold for the design of new anti-inflammatory compounds. A R T I C L E H I S T O R Y Received: July 01, 2020 Revised: September 14, 2020 Accepted: October 05, 2020 DOI: 10.2174/1573406417666201230093207 Keywords: Chalcones, hypochlorous acid, scavenging activity, reactive species, human neutrophils, antioxidant activity. 1. INTRODUCTION Neutrophils, also known as polymorphonuclear leuko- cytes, are the primary phagocytic cells to be recruited and actively act in the innate immune host defense [1]. Due to a chemical gradient of chemokines discharged by infectious agents or other inflammatory cells, neutrophils are enrolled to migrate to sites of injury or infection, to resolve the nox- ious stimuli and initiate the healing process [2]. These * Address correspondence to these authors at the LAQV, REQUIMTE, La- boratory of Applied Chemistry, Department of Chemical Sciences, Faculty of Pharmacy, University of Porto Rua de Jorge Viterbo Ferreira n.º 228, 4050-313 Porto, Portugal; Tel: +351 220428662; +351 220428675; E-mails: dsribeiro@ff.up.pt; egracas@ff.up.pt cells can act by multiple means, both intra- and extracellular: by releasing antibacterial proteins (e.g. defensins and lyso- zymes) and neutrophil extracellular traps (NETs), and by the production of reactive oxygen species (ROS) [3]. The rapid release of high levels of ROS during phagocytosis or in re- sponse to an inflammatory stimulus is known as oxidative burst. In neutrophils, the oxidative burst and the related pro- duction of ROS are mainly catalyzed by the activation of adenine dinucleotide phosphate (NADPH) oxidase, a multi- subunit enzyme, with components found in the cytoplasm and membranes [4]. This process is initiated by the transfer of electrons from cellular NADPH to molecular oxygen (O 2 ), generating superoxide anion radical (O 2 ●‒ ). This RS has a short life (10 6 ns) and can produce H 2 O 2 by spontaneous dismutation or via a reaction catalyzed by superoxide dis- 1875-6638/22 $65.00+.00 © 2022 Bentham Science Publishers Send Orders for Reprints to reprints@benthamscience.net 88 Medicinal Chemistry, 2022, 18, 88-96 RESEARCH ARTICLE Chalcones as Scavengers of HOCl and Inhibitors of Oxidative Burst: Structure-Activity Relationship Studies