SARS has 2 "cleavage" events.
I figured Pepsin was probably somehow related to a low-pH environment, since Pepcid is an antacid. But I went to YouTube to look up: What is Pepsin? And if you click on this video below, you'll see how this guy Walter Jahn says Pepsin works best in ACIDIC ENVIRONMENTS.
Lowered pH Leads to Fusion Peptide Release and a Highly-dynamic Intermediate of Influenza Hemagglutinin
Abstract
https://jvi.asm.org/content/84/3/1527.short
ABSTRACT
While the molecular mechanism of membrane fusion by the influenza virus hemagglutinin (HA) protein has been studied extensively in vitro, the role of acid-dependent HA protein activation in virus replication, pathogenesis, and transmission in vivo has not been characterized. To investigate the biological significance of the pH of activation of the HA protein, we compared the properties of four recombinant viruses with altered HA protein acid stability to those of wild-type influenza virus A/chicken/Vietnam/C58/04 (H5N1) in vitro and in mallards. Membrane fusion by wild-type virus was activated at pH 5.9. Wild-type virus had a calculated environmental persistence of 62 days and caused extensive morbidity, mortality, shedding, and transmission in mallards. An N114K mutation that increased the pH of HA activation by 0.5 unit resulted in decreased replication, genetic stability, and environmental stability. Changes of +0.4 and −0.5 unit in the pH of activation by Y23H and K58I mutations, respectively, reduced weight loss, mortality, shedding, and transmission in mallards. An H24Q mutation that decreased the pH of activation by 0.3 unit resulted in weight loss, mortality, clinical symptoms, and shedding similar to those of the wild type. However, the HA-H241Q virus was shed more extensively into drinking water and persisted longer in the environment. The pH of activation of the H5 HA protein plays a key role in the propagation of H5N1 influenza viruses in ducks and may be a novel molecular factor in the ecology of influenza viruses. The data also demonstrate that H5N1 neuraminidase activity increases the pH of activation of the HA protein in vitro.
- Copyright © 2010 American Society for Microbiology
New insights on the antiviral effects of chloroquine against coronavirus: what to expect for COVID-19?
1. Introduction
Chloroquine is an amine acidotropic form of quinine that was synthesised in Germany by Bayer in 1934 and emerged approximately 70 years ago as an effective substitute for natural quinine [1,2]. Quinine is a compound found in the bark of Cinchona trees native to Peru and was the previous drug of choice against malaria [3]. For decades, chloroquine was a front-line drug for the treatment and prophylaxis of malaria and is one of the most prescribed drugs worldwide [4]. Chloroquine and the 4-aminoquinoline drug hydroxychloroquine belong to the same molecular family. Hydroxychloroquine differs from chloroquine by the presence of a hydroxyl group at the end of the side chain: the N-ethyl substituent is β-hydroxylated. This molecule is available for oral administration in the form of hydroxychloroquine sulfate. Hydroxychloroquine has pharmacokinetics similar to that of chloroquine, with rapid gastrointestinal absorption and renal elimination. However, the clinical indications and toxic doses of these drugs slightly differ. In malaria, the indication for chloroquine was a high dose for a short period of time (due to its toxicity at high doses) or a low dose for a long period of time. Hydroxychloroquine was reported to be as active as chloroquine against Plasmodium falciparum malaria and less toxic, but it is much less active than chloroquine against chloroquine-resistant P. falciparum owing to its physicochemical properties. What is advantageous with hydroxychloroquine is that it can be used in high doses for long periods with very good tolerance. Unfortunately, the efficacy of chloroquine gradually declined due to the continuous emergence of chloroquine-resistant P. falciparum strains [5]. Chloroquine is also utilised in the treatment of autoimmune diseases [6]. Yet the activity of the molecule is not limited to malaria and the control of inflammatory processes, as illustrated by its broad-spectrum activity against a range of bacterial, fungal and viral infections [7], [8], [9], [10]. Indeed, in the mid-1990s, due to its tolerability, rare toxicity reports, inexpensive cost and immunomodulatory properties [11], chloroquine repurposing was explored against human immunodeficiency virus (HIV) and other viruses associated with inflammation and was found to be efficient in inhibiting their replication cycle [12].
Recently, a novel coronavirus emerged in the Chinese city of Wuhan in December 2019. After human coronavirus 229E (HCoV-229E) (classified in the genus Alphacoronavirus) and HCoV-OC43 (Betacoronavirus lineage 2a member) described in the 1960s, SARS-CoV-1 (Betacoronavirus lineage 2b member) that emerged in March 2003, HCoV-NL63 (Alphacoronavirus lineage 1b member) described in 2004, HCoV-HKU1 (Betacoronavirus lineage 2a member) discovered in 2005, and finally MERS-CoV that emerged in 2012 (classified in Betacoronavirus lineage 2c), the novel coronavirus is the seventh human coronavirus described to date as being responsible for respiratory infection. Evidence was rapidly reported that patients were suffering from an infection with a novel Betacoronavirus tentatively named 2019 novel coronavirus (2019-nCoV) [13,14]. Despite drastic containment measures, the spread of 2019-nCoV, now officially known as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is ongoing. Phylogenetic analysis of this virus indicated that it is different (~80% nucleotide identity) but related to SARS-CoV-1 [15]. Because the world is threatened by the possibility of a SARS-CoV-2 pandemic, the broad-spectrum antiviral effects of chloroquine warranted particular attention for repurposing this drug in the therapy of the disease caused by SARS-CoV-2, named coronavirus disease 2019 (COVID-19).
2. Antiviral properties of chloroquine
In vitro, chloroquine appears as a versatile bioactive agent reported to possess antiviral activity against RNA viruses as diverse as rabies virus [16], poliovirus [17], HIV [12,[18], [19], [20], hepatitis A virus [21,22], hepatitis C virus [23], influenza A and B viruses [24], [25], [26], [27], influenza A H5N1 virus [28], Chikungunya virus [29], [30], [31], Dengue virus [32,33], Zika virus [34], Lassa virus [35], Hendra and Nipah viruses [36,37], Crimean–Congo hemorrhagic fever virus [38] and Ebola virus [39], as well as various DNA viruses such as hepatitis B virus [40] and herpes simplex virus [41].The antiviral properties of chloroquine described in vitro have sometimes been confirmed during treatment of virus-infected patients but have not always been reproduced in clinical trials depending on the disease, the concentration of chloroquine used, the duration of treatment and the clinical team in charge of the trial.
Regarding coronaviruses, the potential therapeutic benefits of chloroquine were notably reported for SARS-CoV-1 [11,42]. Chloroquine was also reported to inhibit in vitro the replication of HCoV-229E in epithelial lung cell cultures [43,44]. In 2009, it was reported that lethal infections of newborn mice with the HCoV-O43 coronavirus could be averted by administering chloroquine through the mother's milk. In vitro experiments also showed a strong antiviral effect of chloroquine on a recombinant HCoV-O43 coronavirus [45]. Although chloroquine was reported to be active against Middle East respiratory syndrome coronavirus (MERS-CoV) in vitro [46], this observation remains controversial [47].
3. Potential antiviral effect of chloroquine against SARS-CoV-2
Because of its broad spectrum of action against viruses, including most coronaviruses and particularly its close relative SARS-CoV-1, and because coronavirus cell entry occurs through the endolysosomal pathway [48], it made sense in a situation of a public-health emergency and the absence of any known efficient therapy to investigate the possible effect of chloroquine against SARS-CoV-2. A recent paper reported that both chloroquine and the antiviral drug remdesivir inhibited SARS-CoV-2 in vitro and suggested these drugs be assessed in human patients suffering from COVID-19 [49].
Recently, the China National Center for Biotechnology Development indicated that chloroquine is one of the three drugs with a promising profile against the new SARS-CoV-2 coronavirus that causes COVID-19. Chloroquine repurposing was investigated in hospitals in Beijing, in central China's Hunan Province and South China's Guangdong Province. According to preliminary reports [50,51] from the Chinese authorities suggesting that approximately 100 infected patients treated with chloroquine experienced a more rapid decline in fever and improvement of lung computed tomography (CT) images and required a shorter time to recover compared with control groups, with no obvious serious adverse effects, the Chinese medical advisory board has suggested chloroquine inclusion in the SARS-CoV-2 treatment guidelines. As a result, chloroquine is probably the first molecule to be used in China and abroad on the front line for the treatment of severe SARS-CoV-2 infections. Although the long use of this drug in malaria therapy demonstrates the safety of acute chloroquine administration to humans, one cannot ignore the minor risk of macular retinopathy, which depends on the cumulative dose [52], and the existence of some reports on cardiomyopathy as a severe adverse effect caused by chloroquine [53,54]. A survey of SARS-CoV-2-infected patients for adverse effects of chloroquine therapy remains to be performed. However, chloroquine is currently among the best available candidates to impact the severity of SARS-CoV-2 infections in humans. Currently, at least ten clinical trials are testing chloroquine as an anti-COVID-19 therapy [55].
4. Mode of action of chloroquine
Chloroquine has multiple mechanisms of action that may differ according to the pathogen studied.
Chloroquine can inhibit a pre-entry step of the viral cycle by interfering with viral particles binding to their cellular cell surface receptor. Chloroquine was shown to inhibit quinone reductase 2 [56], a structural neighbour of UDP-N-acetylglucosamine 2-epimerases [57] that are involved in the biosynthesis of sialic acids. The sialic acids are acidic monosaccharides found at the extremity of sugar chains present on cell transmembrane proteins and are critical components of ligand recognition. The possible interference of chloroquine with sialic acid biosynthesis could account for the broad antiviral spectrum of that drug since viruses such as the human coronavirus HCoV-O43 and the orthomyxoviruses use sialic acid moieties as receptors [58]. The potent anti-SARS-CoV-1 effects of chloroquine in vitro were considered attributable to a deficit in the glycosylation of a virus cell surface receptor, the angiotensin-converting enzyme 2 (ACE2) on Vero cells [59].
Chloroquine can also impair another early stage of virus replication by interfering with the pH-dependent endosome-mediated viral entry of enveloped viruses such as Dengue virus or Chikungunya virus [60,61]. Due to the alkalisation of endosomes, chloroquine was an effective in vitro treatment against Chikungunya virus when added to Vero cells prior to virus exposure [30]. The mechanism of inhibition likely involved the prevention of endocytosis and/or rapid elevation of the endosomal pH and abrogation of virus–endosome fusion. A pH-dependant mechanism of entry of coronavirus into target cells was also reported for SARS-CoV-1 after binding of the DC-SIGN receptor [62]. The activation step that occurs in endosomes at acidic pH results in fusion of the viral and endosomal membranes leading to the release of the viral SARS-CoV-1 genome into the cytosol [63]. In the absence of antiviral drug, the virus is targeted to the lysosomal compartment where the low pH, along with the action of enzymes, disrupts the viral particle, thus liberating the infectious nucleic acid and, in several cases, enzymes necessary for its replication [64]. Chloroquine-mediated inhibition of hepatitis A virus was found to be associated with uncoating, thus blocking its entire replication cycle [22].
Chloroquine can also interfere with the post-translational modification of viral proteins. These post-translational modifications, which involve proteases and glycosyltransferases, occur within the endoplasmic reticulum or the trans-Golgi network vesicles and may require a low pH. For HIV, the antiretroviral effect of chloroquine is attributable to a post-transcriptional inhibition of glycosylation of the gp120 envelope glycoprotein, and the neosynthesised virus particles are non-infectious [19,65]. Chloroquine also inhibits the replication Dengue-2 virus by affecting the normal proteolytic processing of the flavivirus prM protein to M protein [32]. As a result, viral infectivity is impaired. In the herpes simplex virus (HSV) model, chloroquine inhibited budding with accumulation of non-infectious HSV-1 particles in the trans-Golgi network [66]. Using non-human coronavirus, it was shown that the intracellular site of coronavirus budding is determined by the localisation of its membrane M proteins that accumulate in the Golgi complex beyond the site of virion budding [67], suggesting a possible action of chloroquine on SARS-CoV-2 at this step of the replication cycle. It was recently reported that the C-terminal domain of the MERS-CoV M protein contains a trans-Golgi network localisation signal [68].
Beside affecting the virus maturation process, pH modulation by chloroquine can impair the proper maturation of viral protein [32] and the recognition of viral antigen by dendritic cells, which occurs through a Toll-like receptor-dependent pathway that requires endosomal acidification [69]. On the contrary, other proposed effects of chloroquine on the immune system include increasing the export of soluble antigens into the cytosol of dendritic cells and the enhancement of human cytotoxic CD8+ T-cell responses against viral antigens [70]. In the influenza virus model, it was reported that chloroquine improve the cross-presentation of non-replicating virus antigen by dendritic cells to CD8+ T-cells recruited to lymph nodes draining the site of infection, eliciting a broadly protective immune response [71].
Chloroquine can also act on the immune system through cell signalling and regulation of pro-inflammatory cytokines. Chloroquine is known to inhibit phosphorylation (activation) of the p38 mitogen-activated protein kinase (MAPK) in THP-1 cells as well as caspase-1 [72]. Activation of cells via MAPK signalling is frequently required by viruses to achieve their replication cycle [73]. In the model of HCoV-229 coronavirus, chloroquine-induced virus inhibition occurs through inhibition of p38 MAPK [44]. Chloroquine is a well-known immunomodulatory agent capable of mediating an anti-inflammatory response [11]. Therefore, there are clinical applications of this drug in inflammatory diseases such as rheumatoid arthritis [74], [75], [76], lupus erythematosus [6,77] and sarcoidosis [78]. Chloroquine inhibits interleukin-1 beta (IL-1β) mRNA expression in THP-1 cells and reduces IL-1β release [72]. Chloroquine-induced reduction of IL-1 and IL-6 cytokines was also found in monocytes/macrophages [79]. Chloroquine-induced inhibition of tumour necrosis factor-alpha (TNFα) production by immune cells was reported to occur either through disruption of cellular iron metabolism [80], blockade of the conversion of pro-TNF into soluble mature TNFα molecules [81] and/or inhibition of TNFα mRNA expression [72,82,83]. Inhibition of the TNFα receptor was also reported in U937 monocytic cells treated with chloroquine [84]. In the Dengue virus model, chloroquine was found to inhibit interferon-alpha (IFNα), IFNβ, IFNγ, TNFα, IL-6 and IL-12 gene expression in U937 cells infected with Dengue-2 virus [33].
5. Conclusion
Chloroquine has been shown to be capable of inhibiting the in vitro replication of several coronaviruses. Recent publications support the hypothesis that chloroquine can improve the clinical outcome of patients infected by SARS-CoV-2. The multiple molecular mechanisms by which chloroquine can achieve such results remain to be further explored. Since SARS-CoV-2 was found a few days ago to utilise the same cell surface receptor ACE2 (expressed in lung, heart, kidney and intestine) as SARS-CoV-1 [85,86] (Table 1 ), it may be hypothesised that chloroquine also interferes with ACE2 receptor glycosylation thus preventing SARS-CoV-2 binding to target cells. Wang and Cheng reported that SARS-CoV and MERS-CoV upregulate the expression of ACE2 in lung tissue, a process that could accelerate their replication and spread [85]. Although the binding of SARS-CoV to sialic acids has not been reported so far (it is expected that Betacoronavirus adaptation to humans involves progressive loss of hemagglutinin-esterase lectin activity), if SARS-CoV-2 like other coronaviruses targets sialic acids on some cell subtypes, this interaction will be affected by chloroquine treatment [87,88]. Today, preliminary data indicate that chloroquine interferes with SARS-CoV-2 attempts to acidify the lysosomes and presumably inhibits cathepsins, which require a low pH for optimal cleavage of SARS-CoV-2 spike protein [89], a prerequisite to the formation of the autophagosome [49]. Obviously, it can be hypothesised that SARS-CoV-2 molecular crosstalk with its target cell can be altered by chloroquine through inhibition of kinases such as MAPK. Chloroquine could also interfere with proteolytic processing of the M protein and alter virion assembly and budding (Fig. 1 ). Finally, in COVID-19 disease this drug could act indirectly through reducing the production of pro-inflammatory cytokines and/or by activating anti-SARS-CoV-2 CD8+ T-cells.
Table 1
Human coronavirus (HCoV) receptors/co-receptors as possible targets for chloroquine-induced inhibition of the virus replication cycle
| Coronavirus | Receptora | May also bind | Replication cycle inhibited by chloroquineb |
|---|---|---|---|
| Alphacoronavirus | |||
| HCoV-229E | Aminopeptidase N (APN)/CD13 | Yes | |
| HCoV-NL63 | Angiotensin-converting enzyme 2 (ACE2) | ? | |
| Heparan sulfate proteoglycansc | |||
| Betacoronavirus | |||
| HCoV-OC43 | HLA class Id, IFN-inducible transmembrane (IFITM) proteins in endocytic vesiclese | Sialic acid (O-acetylated sialic acid)f | Yes |
| SARS-CoV-1 | Angiotensin-converting enzyme 2 (ACE2) | DC-SIGN/CD209, DC-SIGNr, DC-SIGN-related lectin LSECting | Yes |
| HCoV-HKU1 | HLA class Ih | Sialic acid (O-acetylated sialic acid) | ? |
| MERS-CoV i | Dipeptidyl peptidase 4 (DPP4)/CD26 | Yes | |
| SARS-CoV-2 | ACE2i | Sialic acid? | Yes |
HLA, human leukocyte antigen.
Schematic representation of the possible effects of chloroquine on the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) replication cycle. SARS-CoV2, like other human coronaviruses, harbours three envelope proteins, the spike (S) protein (180–220 kDa), the membrane (M) protein (25–35 kDa) and the envelope (E) protein (10–12 kDa), which are required for entry of infectious virions into target cells. The virion also contains the nucleocapsid (N), capable of binding to viral genomic RNA, and nsp3, a key component of the replicase complex. A subset of betacoronaviruses use a hemagglutinin-esterase (65 kDa) that binds sialic acids at the surface of glycoproteins. The S glycoprotein determines the host tropism. There is indication that SARS-CoV-2 binds to angiotensin-converting enzyme 2 (ACE2) expressed on pneumocytes [85,99]. Binding to ACE2 is expected to trigger conformational changes in the S glycoprotein allowing cleavage by the transmembrane protease TMPRSS2 of the S protein and the release of S fragments into the cellular supernatant that inhibit virus neutralisation by antibodies [100]. The virus is then transported into the cell through the early and late endosomes where the host protease cathepsin L further cleaves the S protein at low pH, leading to fusion of the viral envelope and phospholipidic membrane of the endosomes resulting in release of the viral genome into the cell cytoplasm. Replication then starts and the positive-strand viral genomic RNA is transcribed into a negative RNA strand that is used as a template for the synthesis of viral mRNA. Synthesis of the negative RNA strand peaks earlier and falls faster than synthesis of the positive strand. Infected cells contain between 10 and 100 times more positive strands than negative strands. The ribosome machinery of the infected cells is diverted in favour of the virus, which then synthesises its non-structural proteins (NSPs) that assemble into the replicase-transcriptase complex to favour viral subgenomic mRNA synthesis (see the review by Fehr and Perlman for details [101]). Following replication, the envelope proteins are translated and inserted into the endoplasmic reticulum and then move to the Golgi compartment. Viral genomic RNA is packaged into the nucleocapsid and then envelope proteins are incorporated during the budding step to form mature virions. The M protein, which localises to the trans-Golgi network, plays an essential role during viral assembly by interacting with the other proteins of the virus. Following assembly, the newly formed viral particles are transported to the cell surface in vesicles and are released by exocytosis. It is possible that chloroquine interferes with ACE2 receptor glycosylation, thus preventing SARS-CoV-2 binding to target cells. Chloroquine could also possibly limit the biosynthesis of sialic acids that may be required for cell surface binding of SARS-CoV-2. If binding of some viral particles is achieved, chloroquine may modulate the acidification of endosomes thereby inhibiting formation of the autophagosome. Through reduction of cellular mitogen-activated protein (MAP) kinase activation, chloroquine may also inhibit virus replication. Moreover, chloroquine could alter M protein maturation and interfere with virion assembly and budding. With respect to the effect of chloroquine on the immune system, see the elegant review by Savarino et al. [11]. ERGIC, ER-Golgi intermediate compartment.
Already in 2007, some of us emphasised in this journal the possibility of using chloroquine to fight orphan viral infections [10]. The worldwide ongoing trials, including those involving the care of patients in our institute [90], will verify whether the hopes raised by chloroquine in the treatment of COVID-19 can be confirmed.
Acknowledgment
The figure was designed using the Servier Medical Art supply of images available under a Creative Commons CC BY 3.0 license.
Funding: This study was supported by IHU–Méditerranée Infection, University of Marseille and CNRS (Marseille, France). This work has benefited from French state support, managed by the Agence nationale de la recherche (ANR), including the ‘Programme d'investissement d'avenir’ under the reference Méditerranée Infection 10-1AHU-03.
Competing interests: None declared.
Ethical approval: Not required.
References
- Correspondence
- Open Access
- Published:
Hydroxychloroquine, a less toxic derivative of chloroquine, is effective in inhibiting SARS-CoV-2 infection in vitro
Cell Discovery 6, Article number: 16 (2020)
Dear Editor,
The outbreak of coronavirus disease 2019 (COVID-19) caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2/2019-nCoV) poses a serious threat to global public health and local economies. As of March 3, 2020, over 80,000 cases have been confirmed in China, including 2946 deaths as well as over 10,566 confirmed cases in 72 other countries. Such huge numbers of infected and dead people call for an urgent demand of effective, available, and affordable drugs to control and diminish the epidemic.
We have recently reported that two drugs, remdesivir (GS-5734) and chloroquine (CQ) phosphate, efficiently inhibited SARS-CoV-2 infection in vitro1. Remdesivir is a nucleoside analog prodrug developed by Gilead Sciences (USA). A recent case report showed that treatment with remdesivir improved the clinical condition of the first patient infected by SARS-CoV-2 in the United States2, and a phase III clinical trial of remdesivir against SARS-CoV-2 was launched in Wuhan on February 4, 2020. However, as an experimental drug, remdesivir is not expected to be largely available for treating a very large number of patients in a timely manner. Therefore, of the two potential drugs, CQ appears to be the drug of choice for large-scale use due to its availability, proven safety record, and a relatively low cost. In light of the preliminary clinical data, CQ has been added to the list of trial drugs in the Guidelines for the Diagnosis and Treatment of COVID-19 (sixth edition) published by National Health Commission of the People’s Republic of China.
CQ (N4-(7-Chloro-4-quinolinyl)-N1,N1-diethyl-1,4-pentanediamine) has long been used to treat malaria and amebiasis. However, Plasmodium falciparum developed widespread resistance to it, and with the development of new antimalarials, it has become a choice for the prophylaxis of malaria. In addition, an overdose of CQ can cause acute poisoning and death3. In the past years, due to infrequent utilization of CQ in clinical practice, its production and market supply was greatly reduced, at least in China. Hydroxychloroquine (HCQ) sulfate, a derivative of CQ, was first synthesized in 1946 by introducing a hydroxyl group into CQ and was demonstrated to be much less (~40%) toxic than CQ in animals4. More importantly, HCQ is still widely available to treat autoimmune diseases, such as systemic lupus erythematosus and rheumatoid arthritis. Since CQ and HCQ share similar chemical structures and mechanisms of acting as a weak base and immunomodulator, it is easy to conjure up the idea that HCQ may be a potent candidate to treat infection by SARS-CoV-2. Actually, as of February 23, 2020, seven clinical trial registries were found in Chinese Clinical Trial Registry (http://www.chictr.org.cn) for using HCQ to treat COVID-19. Whether HCQ is as efficacious as CQ in treating SARS-CoV-2 infection still lacks the experimental evidence.
To this end, we evaluated the antiviral effect of HCQ against SARS-CoV-2 infection in comparison to CQ in vitro. First, the cytotoxicity of HCQ and CQ in African green monkey kidney VeroE6 cells (ATCC-1586) was measured by standard CCK8 assay, and the result showed that the 50% cytotoxic concentration (CC50) values of CQ and HCQ were 273.20 and 249.50 μM, respectively, which are not significantly different from each other (Fig. 1a). To better compare the antiviral activity of CQ versus HCQ, the dose–response curves of the two compounds against SARS-CoV-2 were determined at four different multiplicities of infection (MOIs) by quantification of viral RNA copy numbers in the cell supernatant at 48 h post infection (p.i.). The data summarized in Fig. 1a and Supplementary Table S1 show that, at all MOIs (0.01, 0.02, 0.2, and 0.8), the 50% maximal effective concentration (EC50) for CQ (2.71, 3.81, 7.14, and 7.36 μM) was lower than that of HCQ (4.51, 4.06, 17.31, and 12.96 μM). The differences in EC50 values were statistically significant at an MOI of 0.01 (P < 0.05) and MOI of 0.2 (P < 0.001) (Supplementary Table S1). It is worth noting that the EC50 values of CQ seemed to be a little higher than that in our previous report (1.13 μM at an MOI of 0.05)1, which is likely due to the adaptation of the virus in cell culture that significantly increased viral infectivity upon continuous passaging. Consequently, the selectivity index (SI = CC50/EC50) of CQ (100.81, 71.71, 38.26, and 37.12) was higher than that of HCQ (55.32, 61.45, 14.41, 19.25) at MOIs of 0.01, 0.02, 0.2, and 0.8, respectively. These results were corroborated by immunofluorescence microscopy as evidenced by different expression levels of virus nucleoprotein (NP) at the indicated drug concentrations at 48 h p.i. (Supplementary Fig. S1). Taken together, the data suggest that the anti-SARS-CoV-2 activity of HCQ seems to be less potent compared to CQ, at least at certain MOIs.
a Cytotoxicity and antiviral activities of CQ and HCQ. The cytotoxicity of the two drugs in Vero E6 cells was determined by CCK-8 assays. Vero E6 cells were treated with different doses of either compound or with PBS in the controls for 1 h and then infected with SARS-CoV-2 at MOIs of 0.01, 0.02, 0.2, and 0.8. The virus yield in the cell supernatant was quantified by qRT-PCR at 48 h p.i. Y-axis represents the mean of percent inhibition normalized to the PBS group. The experiments were repeated twice. b, c Mechanism of CQ and HCQ in inhibiting virus entry. Vero E6 cells were treated with CQ or HCQ (50 μM) for 1 h, followed by virus binding (MOI = 10) at 4 °C for 1 h. Then the unbound virions were removed, and the cells were further supplemented with fresh drug-containing medium at 37 °C for 90 min before being fixed and stained with IFA using anti-NP antibody for virions (red) and antibodies against EEA1 for EEs (green) or LAMP1 for ELs (green). The nuclei (blue) were stained with Hoechst dye. The portion of virions that co-localized with EEs or ELs in each group (n > 30 cells) was quantified and is shown in b. Representative confocal microscopic images of viral particles (red), EEA1+ EEs (green), or LAMP1+ ELs (green) in each group are displayed in c. The enlarged images in the boxes indicate a single vesicle-containing virion. The arrows indicated the abnormally enlarged vesicles. Bars, 5 μm. Statistical analysis was performed using a one-way analysis of variance (ANOVA) with GraphPad Prism (F = 102.8, df = 5,182, ***P < 0.001).
Both CQ and HCQ are weak bases that are known to elevate the pH of acidic intracellular organelles, such as endosomes/lysosomes, essential for membrane fusion5. In addition, CQ could inhibit SARS-CoV entry through changing the glycosylation of ACE2 receptor and spike protein6. Time-of-addition experiment confirmed that HCQ effectively inhibited the entry step, as well as the post-entry stages of SARS-CoV-2, which was also found upon CQ treatment (Supplementary Fig. S2). To further explore the detailed mechanism of action of CQ and HCQ in inhibiting virus entry, co-localization of virions with early endosomes (EEs) or endolysosomes (ELs) was analyzed by immunofluorescence analysis (IFA) and confocal microscopy. Quantification analysis showed that, at 90 min p.i. in untreated cells, 16.2% of internalized virions (anti-NP, red) were observed in early endosome antigen 1 (EEA1)-positive EEs (green), while more virions (34.3%) were transported into the late endosomal–lysosomal protein LAMP1+ ELs (green) (n > 30 cells for each group). By contrast, in the presence of CQ or HCQ, significantly more virions (35.3% for CQ and 29.2% for HCQ; P < 0.001) were detected in the EEs, while only very few virions (2.4% for CQ and 0.03% for HCQ; P < 0.001) were found to be co-localized with LAMP1+ ELs (n > 30 cells) (Fig. 1b, c). This suggested that both CQ and HCQ blocked the transport of SARS-CoV-2 from EEs to ELs, which appears to be a requirement to release the viral genome as in the case of SARS-CoV7.
Interestingly, we found that CQ and HCQ treatment caused noticeable changes in the number and size/morphology of EEs and ELs (Fig. 1c). In the untreated cells, most EEs were much smaller than ELs (Fig. 1c). In CQ- and HCQ-treated cells, abnormally enlarged EE vesicles were observed (Fig. 1c, arrows in the upper panels), many of which are even larger than ELs in the untreated cells. This is in agreement with previous report that treatment with CQ induced the formation of expanded cytoplasmic vesicles8. Within the EE vesicles, virions (red) were localized around the membrane (green) of the vesicle. CQ treatment did not cause obvious changes in the number and size of ELs; however, the regular vesicle structure seemed to be disrupted, at least partially. By contrast, in HCQ-treated cells, the size and number of ELs increased significantly (Fig. 1c, arrows in the lower panels).
Since acidification is crucial for endosome maturation and function, we surmise that endosome maturation might be blocked at intermediate stages of endocytosis, resulting in failure of further transport of virions to the ultimate releasing site. CQ was reported to elevate the pH of lysosome from about 4.5 to 6.5 at 100 μM9. To our knowledge, there is a lack of studies on the impact of HCQ on the morphology and pH values of endosomes/lysosomes. Our observations suggested that the mode of actions of CQ and HCQ appear to be distinct in certain aspects.
It has been reported that oral absorption of CQ and HCQ in humans is very efficient. In animals, both drugs share similar tissue distribution patterns, with high concentrations in the liver, spleen, kidney, and lung reaching levels of 200–700 times higher than those in the plasma10. It was reported that safe dosage (6–6.5 mg/kg per day) of HCQ sulfate could generate serum levels of 1.4–1.5 μM in humans11. Therefore, with a safe dosage, HCQ concentration in the above tissues is likely to be achieved to inhibit SARS-CoV-2 infection.
Clinical investigation found that high concentration of cytokines were detected in the plasma of critically ill patients infected with SARS-CoV-2, suggesting that cytokine storm was associated with disease severity12. Other than its direct antiviral activity, HCQ is a safe and successful anti-inflammatory agent that has been used extensively in autoimmune diseases and can significantly decrease the production of cytokines and, in particular, pro-inflammatory factors. Therefore, in COVID-19 patients, HCQ may also contribute to attenuating the inflammatory response. In conclusion, our results show that HCQ can efficiently inhibit SARS-CoV-2 infection in vitro. In combination with its anti-inflammatory function, we predict that the drug has a good potential to combat the disease. This possibility awaits confirmation by clinical trials. We need to point out, although HCQ is less toxic than CQ, prolonged and overdose usage can still cause poisoning. And the relatively low SI of HCQ requires careful designing and conducting of clinical trials to achieve efficient and safe control of the SARS-CoV-2 infection.
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Acknowledgements
We thank Professor Zhengli Shi and Dr. Xinglou Yang from Wuhan Institute of Virology and Professor Fei Deng from National Virus Resource Center for providing SARS-CoV-2 strain (nCoV-2019BetaCoV/Wuhan/WIV04/2019); Professor Xiulian Sun for kind help in statistical analysis; Professor Zhenhua Zheng for kindly providing the anti-LAMP1 rabbit polyclonal antibody; Prof. Zhengli Shi for kindly providing the anti-NP polyclonal antibody; Beijing Savant Biotechnology Co., ltd for kindly providing the anti-NP monoclonal antibody; Min Zhou and Xijia Liu for their assistance with this study; Jia Wu, Jun Liu, Hao Tang, and Tao Du from BSL-3 Laboratory and Dr. Ding Gao from the core faculty of Wuhan Institute of Virology for their critical support; Professor Gengfu Xiao, Professor Yanyi Wang and other colleagues of Wuhan Institute of Virology and Wuhan National Biosafety Laboratory for their excellent coordination; and Dr. Basil Arif for scientific editing of the manuscript. This work was supported in part by grants from the National Science and Technology Major Projects for “Major New Drugs Innovation and Development” (2018ZX09711003 to W.Z.), the National Natural Science Foundation of China (31621061 to Z.H.), and the Hubei Science and Technology Project (2020FCA003 to Z.H.).
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Z.H., M.W., and W.Z. conceived and designed the experiments and provided the final approval of the manuscript. J.L., R.C., M.X., X.W., H.Z., H.H., and Y.L. participated in multiple experiments; all the authors analyzed the data. M.W., R.C., J.L., and Z.H. wrote the manuscript.
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