antioxidants Review Oxidative Stress, Proton Fluxes, and Chloroquine/Hydroxychloroquine Treatment for COVID-19 Christina B. Klouda and William L. Stone * Department of Pediatrics, East Tennessee State University, Johnson City, TN 37614, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-423-439-8762 Received: 20 August 2020; Accepted: 16 September 2020; Published: 21 September 2020 Abstract: Chloroquine (CQ) and hydroxychloroquine (HCQ) have been proposed as treatments for COVID-19. These drugs have been studied for many decades, primarily in the context of their use as antimalarials, where they induce oxidative stress-killing of the malarial parasite. Less appreciated, however, is evidence showing that CQ/HCQ causes systemic oxidative stress. In vitro and observational data suggest that CQ/HCQ can be repurposed as potential antiviral medications. This review focuses on the potential health concerns of CQ/HCQ induced by oxidative stress, particularly in the hyperinflammatory stage of COVID-19 disease. The pathophysiological role of oxidative stress in acute respiratory distress syndrome (ARDS) has been well-documented. Additional oxidative stress caused by CQ/HCQ during ARDS could be problematic. In vitro data showing that CQ forms a complex with free-heme that promotes lipid peroxidation of phospholipid bilayers are also relevant to COVID-19. Free-heme induced oxidative stress is implicated as a systemic activator of coagulation, which is increasingly recognized as a contributor to COVID-19 morbidity. This review will also provide a brief overview of CQ/HCQ pharmacology with an emphasis on how these drugs alter proton fluxes in subcellular organelles. CQ/HCQ-induced alterations in proton fluxes influence the type and chemical reactivity of reactive oxygen species (ROS). Keywords: oxidative stress; COVID-19; chloroquine; hydroxychloroquine; reactive oxygen species; proton fluxes; SARS-CoV-2 1. Introduction At the writing of this review (mid-2020), there was no Food and Drug Administration (FDA) approved drugs for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA virus infection, which causes the coronavirus disease 2019 (COVID-19). Both chloroquine (CQ) and hydroxychloroquine (HCQ) are 4-aminoquinoline drugs with similar structures, as shown in Figure1. CQ and HCQ have been proposed as potential COVID-19 medical countermeasures [1–6]. The FDA currently approves CQ/HCQ for use as antimalarials, the treatment of rheumatoid arthritis, and systemic lupus erythematosus (SLE) where they act by immunomodulatory mechanisms [7]. HCQ reduces disease activity in SLE patients but has no significant effect on pro-inflammatory cytokines [8]. A recent literature review highlights many immunomodulatory mechanisms influenced by CQ/HCQ and emphasizes our lack of current knowledge concerning their potential effects on immune responses in COVID-19 patients [9]. CQ/HCQ use may lead to unknown alterations in immune responses in COVID-19 patients, including diminished innate immune responses as well as potential modifications of the B and T cell responses to the COVID-19 virus [9]. Antioxidants 2020, 9, 894; doi:10.3390/antiox9090894 www.mdpi.com/journal/antioxidants Antioxidants 2020, 9, x FOR PEER REVIEW 2 of 20 Antioxidants 2020, 9, 894 2 of 19 (a) (b) (c) Figure 1. The organic structures of (a) hydroxychloroquine (HCQ); (b) chloroquine and; (c) heme. Chloroquine can form a membrane bound complex with heme that can promote lipid peroxidation Figure(see Figure 1. The 4 organic and text). structures of (a) hydroxychloroquine (HCQ); (b) chloroquine and; (c) heme. Chloroquine can form a membrane bound complex with heme that can promote lipid peroxidation (seeBoth Figurein vitro 4 andand text).in vivo animal experiments have demonstrated the anti-coronavirus activity of CQ /HCQ [10–12]. Moreover, CQ/HCQ are broadly available, cost-effective, and therefore attractive potentialBoth in therapies vitro and for in viral vivo pandemics animal experiments without an have effective demonstrated vaccine. A the review anti- bycoronavirus Cortegiani activity et al. on ofthe CQ effi /HCQcacy and [10 safety–12]. ofMoreover, CQ for treating CQ/HCQ COVID-19 are broadly concluded available, that available cost-effective, pre-clinical and evidence therefore was attractivesufficiently potential robust therapies to justify for initiating viral pandemics high-quality without clinical an trialseffective [4]. vaccine. Based on A available review by data, Cortegiani the FDA etinitially al. on the published efficacy anand emergency safety of CQ use for authorization treating COVID of CQ-19/HCQ concluded (on 28 Marchthat available 2020) for pre COVID-19-clinical evidencetreatment. was The sufficiently FDA subsequently robust to justify revoked initiating this emergency high-quality use clinical authorization trials [4]. (on Based15 June on available 2020) due data,to new the clinicalFDA initially data suggestingpublished an CQ emergency/HCQ were use ine authorizationffective and raisingof CQ/HCQ concerns (on 28 regarding March 2020) “serious for COVIDcardiac-19 adverse treatment. events The and FDA other subsequently potential seriousrevoked side this eemergencyffects” [13]. use Nevertheless, authorization the (on FDA 15 June states 2020)“additional due to new clinical clinical trials data continue suggesting to evaluate CQ/HCQ the were potential ineffective benefit and of rais theseing concerns drugs in regarding treating or “seriouspreventing cardiac COVID-19. adverse “events and other potential serious side effects” [13]. Nevertheless, the FDA states “Theaddi moleculartional clinical basis trials for continue the potential to evaluate antiviral the activity potential of benefit CQ/HCQ of these are not drugs fully in understood,treating or preventingand multiple COVID mechanisms-19. “ have been proposed [9]. This review will explore the hypothesis that alterationsThe molecular in proton basis fluxesfor the andpotential redox antiviral physiology activity induced of CQ/HCQ by CQ are/HCQ not fully are understood, relevant to and their multiplepotential mechanisms antiviral activity have been and proposed side-effects. [9]. This The review production will explore of reactive the hypothesis oxygen species that alterations (ROS) and incellular proton/subcellular fluxes and protonredox physiology fluxes are interdependent induced by CQ/HCQ factors modulatingare relevant manyto their antimicrobial potential antiviral immune activityresponses and [side14].- Weeffects. will The also production highlight concernsof reactive that oxygen CQ/HCQ-induced species (ROS) oxidativeand cellular/subcellul stress couldar be protonproblematic fluxes are in theinterdependent treatment of factors critically modulating ill COVID-19 many patients. antimicrobial The “cytokine immune storm”responses occurring [14]. We in willsome also severe highlight forms concerns of viral infection that CQ/HCQ (e.g., influenza-induced Aoxidative viruses) isstress associated could withbe problematic increased oxidative in the treatmentstress as of well critically as increased ill COVID morbidity-19 patients. and mortality The “cytokine [15]. s Accumulatingtorm” occurring evidence in some suggests severe forms that thisof viralcytokine infection storm (e.g., is a significantinfluenza A cause viruses) of ARDS is associated and multiorgan with increased failure in COVID-19oxidative stress patients as [well16]. as increasedConsiderable morbidity evidenceand mortality suggests [15]. thatAccumulating parasite-specific evidence oxidative suggests stress that this induced cytokine by storm CQ/HCQ is a treatmentsignificant accountscause of ARDS for the and antimalarial multiorgan activity failure in of COVID these drugs-19 patients [17]. Less[16]. appreciated, however, is evidenceConsiderable (see below)evidence showing suggests that that CQ /parasiteHCQ, by-specific themselves, oxidative are pro-oxidantsstress induced that by can CQ/HCQ increase treatmentoxidative accounts stress parameters for the antimalarial [18–21]. Viral activity infections of these are drugs also generally[17]. Less accompaniedappreciated, however, by oxidative is evidestressnce with (see potential below) pathophysiologicalshowing that CQ/HCQ, consequences by themselves, [22]. As mentioned are pro-oxidants above, compelling that can increase evidence oxidativesupports stress the view parameters that patients [18–21]. with Viral COVID-19 infections are are at also increased generally risk accompanied of developing by ARDSoxidative and stresssubsequent with potential death from pathophysiological respiratory failure [23consequences]. Decades of [2 research2]. As havementioned demonstrated above, the compelling central role evidenceof oxidative supports stress the in ARDSview that pathophysiology patients with COVID [24–27].-19 Understanding are at increased the risk potential of developing role of oxidative ARDS andstress subsequent in COVID-19 death is critical,from respiratory since potential failure anti-COVID-19 [23]. Decades drug of research candidates have that demonstrated increase oxidative the centralstress haverole of the oxidative potential stress to exacerbate in ARDS ARDS pathophysiology pathophysiology. [24–27]. Understanding the potential role Antioxidants 2020, 9, x FOR PEER REVIEW 3 of 20 of oxidative stress in COVID-19 is critical, since potential anti-COVID-19 drug candidates that increase oxidative stress have the potential to exacerbate ARDS pathophysiology. 2. CQ/HCQ Pharmacology and Alterations in Subcellular Organelles