Inhibitors compounds of the flavivirus replication process

Phenolic compounds of Theobroma cacao L. show potential against dengue RdRp protease enzyme inhibition by In-silico docking, DFT study, MD simulation and MMGBSA calculation

BACKGROUND

Currently, there is no antiviral medication for dengue, a potentially fatal tropical infectious illness spread by two mosquito species, Aedes aegypti and Aedes albopictus. The RdRp protease of dengue virus is a potential therapeutic target. This study focused on the in silico drug discovery of RdRp protease inhibitors.

METHODS

To assess the potential inhibitory activity of 29 phenolic acids from Theobroma cacao L. against DENV3-NS5 RdRp, a range of computational methods were employed. These included docking, drug-likeness analysis, ADMET prediction, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations. The aim of these studies was to confirm the stability of the ligand-protein complex and the binding pose identified during the docking experiment.

RESULTS

Twenty-one compounds were found to have possible inhibitory activities against DENV according to the docking data, and they had a binding affinity of ≥-37.417 kcal/mol for DENV3- enzyme as compared to the reference compound panduratin A. Additionally, the drug-likeness investigation produced four hit compounds that were subjected to ADMET screening to obtain the lead compound, catechin. Based on ELUMO, EHOMO, and band energy gap, the DFT calculations showed strong electronegetivity, favouravle global softness and chemical reactivity with considerable intra-molecular charge transfer between electron-donor to electron-acceptor groups for catechin. The MD simulation result also demonstrated favourable RMSD, RMSF, SASA and H-bonds in at the binding pocket of DENV3-NS5 RdRp for catechin as compared to panduratin A.

CONCLUSION

According to the present findings, catechin showed high binding affinity and sufficient drug-like properties with the appropriate ADMET profiles. Moreover, DFT and MD studies further supported the drug-like action of catechin as a potential therapeutic candidate. Therefore, further in vitro and in vivo research on cocoa and its phytochemical catechin should be taken into consideration to develop as a potential DENV inhibitor.

Dronabinol as an answer to flavivirus infections: an in-silico investigation

Flavivirus infections are common in several parts of the world. Two major types of flaviviruses are dengue and zika viruses. Both these two viral infections have caused many fatalities around the world. There is an absence of a vaccine and an effective medication against these viruses. In this study, we analyzed the ability of dronabinol to act as a potential cure against these viral infections. We performed the docking of dronabinol with several viral proteins followed by molecular dynamics simulation, MM/PBSA and PCA analysis. We checked the ability of the polyphenol dronabinol to interfere with the binding of viral helicases to their cellular targets. We performed 2 D-QSAR studies, drug likeliness, ADMET and target prediction studies. From our study, we observed that dronabinol had the best docking ability against the helicase proteins of dengue and zika. Molecular dynamics simulation and MM/PBSA investigation confirmed the stability of the binding while PCA investigation showed a lowering of molecular motions in response to dronabinol docking to the helicases. Dronabinol interfered in the binding of the helicases to RNA. 2 D QSAR studies revealed a low IC50 value for dronabinol. Dronabinol showed favorable drug-likeness, ADMET properties and target prediction results. Thus we propose dronabinol be further investigated in-vitro as a cure against dengue and zika virus infections.Communicated by Ramaswamy H. Sarma.

Antiviral Activity of Flavonoids from Geopropolis of the Brazilian Jandaira Bee against Zika and Dengue Viruses

Arthropod-borne viruses within the Flaviviridae family such as Zika (ZIKV) and dengue (DENV) are responsible for major outbreaks in tropical countries, and there are no specific treatments against them. Naringenin and 7-O-methyl naringenin are flavonoids that can be extracted from geopropolis, a natural material that the Brazilian Jandaira stingless bee (Melipona subnitida Ducke) produces to protect its nest. Here, these flavonoids were tested against ZIKV and DENV using Vero cells as a cellular model to perform a cytotoxicity assay and to define the effective concentrations of TCID50 as the readout method. The results demonstrated the antiviral activity of the compounds against both viruses upon the treatment of infected cells. The tested flavonoids had antiviral activity comparable with 6-methylmercaptopurine riboside (6-MMPr), used here as a positive control. In addition, to identify the possible action mechanism of the antiviral candidates, we carried out a docking analysis followed by a molecular dynamics simulation to elucidate naringenin and 7-O-methyl naringenin binding sites to each virus. Altogether, these results demonstrate that both flavonoids have potent antiviral effects against both viruses and warrant further in vivo trials.

Broad-Spectrum Small-Molecule Inhibitors Targeting the SAM-Binding Site of Flavivirus NS5 Methyltransferase

Flavivirus infections, such as those caused by dengue virus (DENV), West Nile virus (WNV), yellow fever virus (YFV), and Zika virus (ZIKV), pose a rising threat to global health. There are no FDA-approved drugs for flaviviruses, although a small number of flaviviruses have vaccines. For flaviviruses or unknown viruses that may appear in the future, it is particularly desirable to identify broad-spectrum inhibitors. The NS5 protein is regarded as one of the most promising flavivirus drug targets because it is conserved across flaviviruses. In this study, we used FL-NAH, a fluorescent analog of the methyl donor S-adenosyl methionine (SAM), to develop a fluorescence polarization (FP)-based high throughput screening (HTS) assay to specifically target methyltransferase (MTase), a vital enzyme for flaviviruses that methylates the N7 and 2'-O positions of the viral 5'-RNA cap. Pilot screening identified two candidate MTase inhibitors, NSC 111552 and 288387. The two compounds inhibited the FL-NAH binding to the DENV3 MTase with low micromolar IC50. Functional assays verified the inhibitory potency of these molecules for the flavivirus MTase activity. Binding studies indicated that these molecules are bound directly to the DENV3 MTase with similar low micromolar affinity. Furthermore, we showed that these compounds greatly reduced ZIKV replication in cell-based experiments at dosages that did not cause cytotoxicity. Finally, docking studies revealed that these molecules bind to the SAM-binding region on the DENV3 MTase, and further mutagenesis studies verified residues important for the binding of these compounds. Overall, these compounds are innovative and attractive candidates for the development of broad-spectrum inhibitors for the treatment of flavivirus infections.

Perspectives on the current antiviral developments towards RNA-dependent RNA polymerase (RdRp) and methyltransferase (MTase) domains of dengue virus non-structural protein 5 (DENV-NS5)

Dengue virus (DENV) infection is one of the most emerging arboviral infections in humans. DENV is a positive-stranded RNA virus in the Flaviviridae family consisting of an 11 kb genome. DENV non-structural protein 5 (DENV-NS5) constitutes the largest among the non-structural proteins, which act as two domains, the RNA-dependent RNA polymerase (RdRp) and RNA methyltransferase enzyme (MTase). The DENV-NS5 RdRp domain contributes to the viral replication stages, whereas the MTase initiates viral RNA capping and facilitates polyprotein translation. Given the functions of both DENV-NS5 domains have made them an important druggable target. Possible therapeutic interventions and drug discoveries against DENV infection were thoroughly reviewed; however, a current update on the therapeutic strategies specific to DENV-NS5 or its active domains was not attempted. Since most potential compounds and drugs targeting the DENV-NS5 were evaluated in both in vitro cultures and animal models, a more detailed evaluation of molecules/drug candidates still requires investigation in randomized controlled clinical trials. This review summarizes current perspectives on the therapeutic strategies adopted to target the DENV-NS5 (RdRp and MTase domains) at the host-pathogen interface and further discusses the directions to identify candidate drugs to combat DENV infection.

Discovery of ZFD-10 of a pyridazino[4,5-b]indol-4(5H)-one derivative as an anti-ZIKV agent and a ZIKV NS5 RdRp inhibitor

Zika virus (ZIKV) infection is associated with the birth defect microcephaly and Guillain-Barré syndrome in adults. There is no approved vaccine or specific antiviral agent against ZIKV. ZFD-10, a novel structural skeleton of 1H-pyridazino[4,5-b]indol-4(5H)-one, was firstly synthesized and discovered to be a potent anti-ZIKV inhibitor with very low cytotoxicity. ZFD-10's anti-ZIKV potency is independent of cell lines and ZFD-10 mainly targets the post-entry stages of ZIKV life cycle. Time-of-addition and time-of-withdrawal assays showed that 10 μM ZFD-10 displayed the ability to decrease mainly at the RNA level and weakly the viral progeny particle load. Furthermore, ZFD-10 could protect ZIKV NS5 from thermal unfolding and aggregation and increase the Tagg value of ZIKV NS5 protein from 44.6 to 49.3 °C, while ZFD-10 dose-dependently inhibits ZIKV NS5 RdRp activity using in vitro RNA polymerase assays. Molecular docking study suggests that ZFD-10 affects RdRp enzymatic function through interfering with the fingers and thumb subdomains. These results supported that ZFD-10's cell-based anti-ZIKV activity is related to its anti-RdRp activity of ZIKV NS5. The in vivo anti-ZIKV study shows that the middle-dose (4.77 mg/kg/d) of ZFD-10 protected mice from ZIKV infection and the viral loads of the blood, liver, kidney and brain in the middle-dose and high-dose (9.54 mg/kg/d) were significantly reduced compared to those of the ZIKV control. These results confirm that ZFD-10 has a certain antiviral effect against ZIKV infection in vivo.

Antiviral effects of the fused tricyclic derivatives of indoline and imidazolidinone on ZIKV infection and RdRp activities of ZIKV and DENV

The prevalence and ravages of Zika virus (ZIKV) seriously endanger human health, especially causing significant neurological defects in both neonates as pediatric microcephaly and adults as Guillain-Barré syndrome. In this work, we studied anti-ZIKV effects of the fused tricyclic derivatives of indoline and imidazolidinone and discovered that some of them are valuable leads for drug discovery of anti-ZIKV agents. The current results show that certain compounds are broad-spectrum inhibitors of ZIKV- and dengue virus (DENV)-infection while distinctive compounds are selective ZIKV inhibitors or selective DENV inhibitors. Compounds of 12, 17 and 28 are more active against Asian ZIKV SZ-VIV01 strain than African ZIKV MR766 strain. It is valued that silylation makes six TBS compounds of 4-nitrophenyl hydrazine series and phenyl hydrazine series more active against ZIKV infection than their phenols. Time-of-addition and withdrawal studies indicate that compound 12 majorly acts on post-infection of RNA synthesis stage of ZIKV life cycle. Moreover, compounds of 12, 17 and 18 are anti-ZIKV agents with the inhibitory activities to ZIKV NS5 RdRp while 12 doesn't inhibit DENV infection even though it is a DENV RdRp inhibitor, 17 is an active agent against DENV infection but is only a weak DENV NS5 RdRp inhibitor, and 28 is inactive against DENV infection and not a DENV NS5 RdRp inhibitor. As a result, a compound's antiviral difference between ZIKV and DENV is not always related to anti-RdRp difference between ZIKV RdRp and DENV RdRp, and structural features of a compound play important roles in executing antiviral and anti-RdRp functions. Further discovery of highly potent broad-spectrum or selective agents against infection by ZIKV and DENV will be facilitated.

Exploration of novel hexahydropyrrolo[1,2-e]imidazol-1-one derivatives as antiviral agents against ZIKV and USUV

Zika virus (ZIKV) and Usutu virus (USUV) are two emerging flaviviruses mostly transmitted by mosquitos. ZIKV is associated with microcephaly in newborns and the less-known USUV, with its reported neurotropism and its extensive spread in Europe, represents a growing concern for human health. There is still no approved vaccine or specific antiviral against ZIKV and USUV infections. The main goal of this study is to investigate the anti-ZIKV and anti-USUV activity of a new library of compounds and to preliminarily investigate the mechanism of action of the selected hit compounds in vitro. Two potent anti-ZIKV and anti-USUV agents, namely ZDL-115 and ZDL-116, were discovered, both presenting low cytotoxicity, cell-line independent antiviral activity in the low micromolar range and ability of reducing viral progeny production. The analysis of the structure-activity relationship (SAR) revealed that introduction of 2-deoxyribose to 3-arene was fundamental to enhance the solubility and improve the antiviral action. Additionally, we demonstrated that ZDL-115 and ZDL-116 are significantly active against both viruses when added on cells for at least 24 h prior to viral inoculation or immediately post-infection. The docking analysis showed that ZDL-116 could target the host vitamin D receptor (VDR) and viral proteins. Future experiments will be focused on compound modification to discover analogues that are more potent and on the clarification of the mechanism of action and the specific drug target. The discovery and the development of a novel anti-flavivirus drug will have a significant impact in a context where there are no fully effective antiviral drugs or vaccines for most flaviviruses.

Synthesis and evaluation of anti-yellow fever virus activity of new 6-aryl-3-R-amino-1,2,4-triazin-5(4H)-ones

Yellow fever disease is one of public health concerns in the tropics. Despite its significant medicinal and economic impact among large groups of the population, there is a lack of effective treatment against yellow fever. In this regard, here we describe the synthesis of a series of new 6-aryl-3-R-amino-1,2,4-triazin-5(4H)-ones and evaluation of their in vitro inhibitory activity against yellow fever virus. Among all tested compounds 4 derivatives possessing strong inhibitory activity at μM concentrations were identified. All the active compounds revealed a good toxicity profile. These facts make the compounds interesting candidates for further evaluation of their efficacy in the treatment of yellow fever virus infection in vivo.

Discovery of New Zika Protease and Polymerase Inhibitors through the Open Science Collaboration Project OpenZika

The Zika virus (ZIKV) is a neurotropic arbovirus considered a global threat to public health. Although there have been several efforts in drug discovery projects for ZIKV in recent years, there are still no antiviral drugs approved to date. Here, we describe the results of a global collaborative crowdsourced open science project, the OpenZika project, from IBM's World Community Grid (WCG), which integrates different computational and experimental strategies for advancing a drug candidate for ZIKV. Initially, molecular docking protocols were developed to identify potential inhibitors of ZIKV NS5 RNA-dependent RNA polymerase (NS5 RdRp), NS3 protease (NS2B-NS3pro), and NS3 helicase (NS3hel). Then, a machine learning (ML) model was built to distinguish active vs inactive compounds for the cytoprotective effect against ZIKV infection. We performed three independent target-based virtual screening campaigns (NS5 RdRp, NS2B-NS3pro, and NS3hel), followed by predictions by the ML model and other filters, and prioritized a total of 61 compounds for further testing in enzymatic and phenotypic assays. This yielded five non-nucleoside compounds which showed inhibitory activity against ZIKV NS5 RdRp in enzymatic assays (IC50 range from 0.61 to 17 μM). Two compounds thermally destabilized NS3hel and showed binding affinity in the micromolar range (Kd range from 9 to 35 μM). Moreover, the compounds LabMol-301 inhibited both NS5 RdRp and NS2B-NS3pro (IC50 of 0.8 and 7.4 μM, respectively) and LabMol-212 thermally destabilized the ZIKV NS3hel (Kd of 35 μM). Both also protected cells from death induced by ZIKV infection in in vitro cell-based assays. However, while eight compounds (including LabMol-301 and LabMol-212) showed a cytoprotective effect and prevented ZIKV-induced cell death, agreeing with our ML model for prediction of this cytoprotective effect, no compound showed a direct antiviral effect against ZIKV. Thus, the new scaffolds discovered here are promising hits for future structural optimization and for advancing the discovery of further drug candidates for ZIKV. Furthermore, this work has demonstrated the importance of the integration of computational and experimental approaches, as well as the potential of large-scale collaborative networks to advance drug discovery projects for neglected diseases and emerging viruses, despite the lack of available direct antiviral activity and cytoprotective effect data, that reflects on the assertiveness of the computational predictions. The importance of these efforts rests with the need to be prepared for future viral epidemic and pandemic outbreaks.

Crystal structure of the Ilheus virus helicase: implications for enzyme function and drug design

Background

The Ilheus virus (ILHV) is an encephalitis associated arthropod-borne flavivirus. It was first identified in Ilheus City in the northeast Brazil before spreading to a wider geographic range. No specific vaccines or drugs are currently available for the treatment of ILHV infections. The ILHV helicase, like other flavivirus helicases, possesses 5ʹ-triphosphatase activity. This allows it to perform ATP hydrolysis to generate energy as well as sustain double-stranded RNA’s unwinding during ILHV genome replication. Thus, ILHV helicase is an ideal target for inhibitor design.

Results

We determined the crystal structure of the ILHV helicase at 1.75-Å resolution. We then conducted molecular docking of ATP-Mn²⁺ to the ILHV helicase. Comparisons with related flavivirus helicases indicated that both the NTP and the RNA-ILHV helicase binding sites were conserved across intra-genus species. This suggested that ILHV helicase adopts an identical mode in recognizing ATP/Mn²⁺. However, the P-loop in the active site showed a distinctive conformation; reflecting a different local structural rearrangement. ILHV helicase enzymatic activity was also characterized. This was found to be relatively lower than that of the DENV, ZIKV, MVE, and ALSV helicases. Our structure-guided mutagenesis revealed that R26A, E110A, and Q280A greatly reduced the ATPase activities. Moreover, we docked two small molecule inhibitors of DENV helicase (ST-610 and suramin) to the ILHV helicase and found that these two molecules had the potential to inhibit the activity of ILHV helicase as well.

Conclusion

High-resolution ILHV helicase structural analysis demonstrates the key amino acids of ATPase activities and could be useful for the design of inhibitors targeting the helicase of ILHV.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-022-00777-8.

Ivermectin Inhibits the Replication of Usutu Virus In Vitro

Usutu virus (USUV) is an emerging mosquito-borne arbovirus within the genus Flavivirus, family Flaviviridae. Similar to the closely related West Nile virus (WNV), USUV infections are capable of causing mass mortality in wild and captive birds, especially blackbirds. In the last few years, a massive spread of USUV was present in the avian population of Germany and other European countries. To date, no specific antiviral therapies are available. Nine different approved drugs were tested for their antiviral effects on the replication of USUV in vitro in a screening assay. Ivermectin was identified as a potent inhibitor of USUV replication in three cell types from different species, such as simian Vero CCL-81, human A549 and avian TME R. A 2- to 7-log10 reduction of the viral titer in the supernatant was detected at a non-cytotoxic concentration of 5 µM ivermectin dependent on the applied cell line. IC₅₀ values of ivermectin against USUV lineage Africa 3 was found to be 0.55 µM in Vero CCL-81, 1.94 µM in A549 and 1.38 µM in TME-R cells. The antiviral efficacy was comparable between the USUV lineages Africa 2, Africa 3 and Europe 3. These findings show that ivermectin may be a candidate for further experimental and clinical studies addressing the treatment of USUV disease, especially in captive birds.

Advancement in the Development of Therapeutics Against Zika Virus Infection

Zika virus (ZIKV), a re-emerging arbovirus, causes teratogenic effects on the fetus and normal nerve functions, resulting in harmful autoimmune responses, which call for the development of therapeutics against ZIKV infection. In this review, we introduce the pathogenesis of ZIKV infection and summarize the advancement in the development of therapeutics against ZIKV infection. It provides guidance for the development of effective therapeutics against ZIKV infection.

The Role of Deubiquitinases in Virus Replication and Host Innate Immune Response

As a critical post-translational modification, ubiquitination is known to affect almost all the cellular processes including immunity, signaling pathways, cell death, cancer development, and viral infection by controlling protein stability. Deubiquitinases (DUBs) cleave ubiquitin from proteins and reverse the process of ubiquitination. Thus, DUBs play an important role in the deubiquitination process and serve as therapeutic targets for various diseases. DUBs are found in eukaryotes, bacteria, and viruses and influence various biological processes. Here, we summarize recent findings on the function of DUBs in modulating viral infection, the mechanism by which viral DUBs regulate host innate immune response, and highlight those DUBs that have recently been discovered as antiviral therapeutic targets.

Virtual Screening of Drug-Like Compounds as Potential Inhibitors of the Dengue Virus NS5 Protein

Dengue virus (DENV) is the causative agent of dengue fever. Annually, there are about 400 million new cases of dengue worldwide, and so far there is no specific treatment against this disease. The NS5 protein is the largest and most conserved viral protein among flaviviruses and is considered a therapeutic target of great interest. This study aims to search drug-like compounds for possible inhibitors of the NS5 protein in the four serotypes of DENV. Using a virtual screening from a ∼642,759-compound database, we suggest 18 compounds with NS5 binding and highlight the best compound per region, in the methyltransferase and RNA-dependent RNA polymerase domains. These compounds interact mainly with the amino acids of the catalytic sites and/or are involved in processes of protein activity. The identified compounds presented physicochemical and pharmacological properties of interest for their use as possible drugs; furthermore, we found that some of these compounds do not affect cell viability in Huh-7; therefore, we suggest evaluating these compounds in vitro as candidates in future research.

Current Trends and Limitations in Dengue Antiviral Research

Dengue is the most prevalent arthropod-borne viral disease worldwide and affects approximately 2.5 billion people living in over 100 countries. Increasing geographic expansion of Aedes aegypti mosquitoes (which transmit the virus) has made dengue a global health concern. There are currently no approved antivirals available to treat dengue, and the only approved vaccine used in some countries is limited to seropositive patients. Treatment of dengue, therefore, remains largely supportive to date; hence, research efforts are being intensified for the development of antivirals. The nonstructural proteins, 3 and 5 (NS3 and NS5), have been the major targets for dengue antiviral development due to their indispensable enzymatic and biological functions in the viral replication process. NS5 is the largest and most conserved nonstructural protein encoded by flaviviruses. Its multifunctionality makes it an attractive target for antiviral development, but research efforts have, this far, not resulted in the successful development of an antiviral targeting NS5. Increase in structural insights into the dengue NS5 protein will accelerate drug discovery efforts focused on NS5 as an antiviral target. In this review, we will give an overview of the current state of therapeutic development, with a focus on NS5 as a therapeutic target against dengue.

Small-molecule endoplasmic reticulum proteostasis regulator acts as a broad-spectrum inhibitor of dengue and Zika virus infections

Flaviviruses, including dengue and Zika, are widespread human pathogens; however, no broadly active therapeutics exist to fight infection. Recently, remodeling of endoplasmic reticulum (ER) proteostasis by pharmacologic regulators, such as compound 147, was shown to correct pathologic ER imbalances associated with protein misfolding diseases. Here, we establish an additional activity of compound 147 as an effective host-centered antiviral agent against flaviviruses. Compound 147 reduces infection by attenuating the infectivity of secreted virions without causing toxicity in host cells. Compound 147 is a preferential activator of the ATF6 pathway of the ER unfolded protein response, which requires targeting of cysteine residues primarily on protein disulfide isomerases (PDIs). We find that the antiviral activity of 147 is independent of ATF6 induction but does require modification of reactive thiols on protein targets. Targeting PDIs and additional non-PDI targets using RNAi and other small-molecule inhibitors was unable to recapitulate the antiviral effects, suggesting a unique polypharmacology may mediate the activity. Importantly, 147 can impair infection of multiple strains of dengue and Zika virus, indicating that it is suitable as a broad-spectrum antiviral agent.

Semisynthesis of Dolabellane Diterpenes: Oxygenated Analogues with Increased Activity against Zika and Chikungunya Viruses

Brown algae and soft corals represent the main marine sources of dolabellane diterpenes. The antiviral activity of dolabellanes has been studied for those isolated from algae, whereas dolabellanes isolated from soft corals have been barely studied. In this work, a collection of dolabellane diterpenes consisting of five natural and 21 semisynthetic derivatives was constructed, and their antiviral activities against Zika (ZIKV) and Chikungunya (CHIKV) viruses were tested. Dolabellatrienone (1) and (1R,7R,8R,11S)-7,8-epoxy-13-keto-dolabella-3,12(18)-diene (2), isolated from Eunicea genus soft corals, were employed to obtain 21 dolabellane and dolastane diterpenes by reactions such as allylic oxidations, reductions, acid-catalyzed epoxide ring opening, and acetylations. All of the compounds were identified by a combination of one- and two-dimensional NMR, mass spectrometry, and X-ray diffraction experiments. The cytotoxicites against Vero cells and the antiviral activities against ZIKV and CHIKV was tested to calculate the half-maximal effective concentration (EC₅₀) and selectivity indexes (SIs). In general, the addition of oxygen-containing functional groups improved the bioactivity of dolabellane and dolastane diterpenes against ZIKV and CHIKV replication. Compound 9 showed an EC₅₀ = 0.92 ± 0.08 μM and SI = 820 against ZIKV.

Antiviral Drug Targets of Single-Stranded RNA Viruses Causing Chronic Human Diseases

Ribonucleic acid (RNA) viruses associated with chronic diseases in humans are major threats to public health causing high mortality globally. The high mutation rate of RNA viruses helps them to escape the immune response and also is responsible for the development of drug resistance. Chronic infections caused by human immunodeficiency virus (HIV) and hepatitis viruses (HBV and HCV) lead to acquired immunodeficiency syndrome (AIDS) and hepatocellular carcinoma respectively, which are one of the major causes of human deaths. Effective preventative measures to limit chronic and re-emerging viral infections are absolutely necessary. Each class of antiviral agents targets a specific stage in the viral life cycle and inhibits them from its development and proliferation. Most often, antiviral drugs target a specific viral protein, therefore only a few broad-spectrum drugs are available. This review will be focused on the selected viral target proteins of pathogenic viruses containing single-stranded (ss) RNA genome that causes chronic infections in humans (e.g. HIV, HCV, Flaviviruses). In the recent past, an exponential increase in the number of available three-dimensional protein structures (>150000 in Protein Data Bank), allowed us to better understand the molecular mechanism of action of protein targets and antivirals. Advancements in the in silico approaches paved the way to design and develop several novels, highly specific small-molecule inhibitors targeting the viral proteins.

Natural Products with Potential to Treat RNA Virus Pathogens Including SARS-CoV-2

Three families of RNA viruses, the Coronaviridae, Flaviviridae, and Filoviridae, collectively have great potential to cause epidemic disease in human populations. The current SARS-CoV-2 (Coronaviridae) responsible for the COVID-19 pandemic underscores the lack of effective medications currently available to treat these classes of viral pathogens. Similarly, the Flaviviridae, which includes such viruses as Dengue, West Nile, and Zika, and the Filoviridae, with the Ebola-type viruses, as examples, all lack effective therapeutics. In this review, we present fundamental information concerning the biology of these three virus families, including their genomic makeup, mode of infection of human cells, and key proteins that may offer targeted therapies. Further, we present the natural products and their derivatives that have documented activities to these viral and host proteins, offering hope for future mechanism-based antiviral therapeutics. By arranging these potential protein targets and their natural product inhibitors by target type across these three families of virus, new insights are developed, and crossover treatment strategies are suggested. Hence, natural products, as is the case for other therapeutic areas, continue to be a promising source of structurally diverse new anti-RNA virus therapeutics.

Synthesis and biological evaluation of novel flexible nucleoside analogues that inhibit flavivirus replication in vitro

Flaviviruses, such as Dengue (DENV) and Zika (ZIKV) viruses, represent a severe health burden. There are currently no FDA-approved treatments, and vaccines against most flaviviruses are still lacking. We have developed several flexible analogues ("fleximers") of the FDA-approved nucleoside Acyclovir that exhibit activity against various RNA viruses, demonstrating their broad-spectrum potential. The current study reports activity against DENV and Yellow Fever Virus (YFV), particularly for compound 1. Studies to elucidate the mechanism of action suggest the flex-analogue triphosphates, especially 1-TP, inhibit DENV and ZIKV methyltransferases, and a secondary, albeit weak, effect on the DENV RNA-dependent RNA polymerase was observed at high concentrations. The results of these studies are reported herein.

Sustainable, three-component, one-pot procedure to obtain active anti-flavivirus agents

The mosquito-borne viruses belonging to the genus Flavivirus such as Dengue virus (DENV) and Zika virus (ZIKV) cause human infections ranging from mild flu-like symptoms to hemorrhagic fevers, hepatitis, and neuropathies. To date, there are vaccines only for few flaviviruses while no effective treatments are available. Pyridobenzothiazole (PBTZ) derivatives are a class of compounds endowed with a promising broad-spectrum anti-flavivirus activity and most of them have been reported as potent inhibitors of the flaviviral NS5 polymerase. However, synthesis of PBTZ analogues entails a high number of purification steps, the use of hazardous reagents and environmentally unsustainable generation of waste. Considering the promising antiviral activity of PBTZ analogues which require further exploration, in this work, we report the development of a new and sustainable three-component reaction (3CR) that can be combined with a basic hydrolysis in a one-pot procedure to obtain the PBTZ scaffold, thus reducing the number of synthetic steps, improving yields and saving time. 3CR was significantly explored in order to demonstrate its wide scope by using different starting materials. In addition, taking advantage of these procedures, we next designed and synthesized a new set of PBTZ analogues that were tested as anti-DENV-2 and anti-ZIKV agents. Compound 22 inhibited DENV-2 NS5 polymerase with an IC₅₀ of 10.4 μM and represented the best anti-flavivirus compound of the new series by inhibiting DENV-2- and ZIKV-infected cells with EC₅₀ values of 1.2 and 5.0 μM, respectively, that translates into attractive selectivity indexes (SI - 83 and 20, respectively). These results strongly reaffirm PBTZ derivatives as promising anti-flavivirus agents that now can be synthesized through a convenient and sustainable 3CR in order to obtain more potent compounds for further pre-clinical development studies.

Broad-Spectrum Flavivirus Inhibitors: a Medicinal Chemistry Point of View

Infections by flaviviruses, such as Dengue, West Nile, Yellow Fever and Zika viruses, represent a growing risk for global health. There are vaccines only for few flaviviruses while no effective treatments are available. Flaviviruses share epidemiological, structural, and ecologic features and often different viruses can co-infect the same host. Therefore, the identification of broad-spectrum inhibitors is highly desirable either for known flaviviruses or for viruses that likely will emerge in the future. Strategies targeting both virus and host factors have been pursued to identify broad-spectrum antiflaviviral agents. In this review, we describe the most promising and best characterized targets and their relative broad-spectrum inhibitors, identified by drug repurposing/libraries screenings and by focused medicinal chemistry campaigns. Finally, we discuss about future strategies to identify new broad-spectrum antiflavivirus agents.

Target Identification Using Homopharma and Network-Based Methods for Predicting Compounds Against Dengue Virus-Infected Cells

Drug target prediction is an important method for drug discovery and design, can disclose the potential inhibitory effect of active compounds, and is particularly relevant to many diseases that have the potential to kill, such as dengue, but lack any healing agent. An antiviral drug is urgently required for dengue treatment. Some potential antiviral agents are still in the process of drug discovery, but the development of more effective active molecules is in critical demand. Herein, we aimed to provide an efficient technique for target prediction using homopharma and network-based methods, which is reliable and expeditious to hunt for the possible human targets of three phenolic lipids (anarcardic acid, cardol, and cardanol) related to dengue viral (DENV) infection as a case study. Using several databases, the similarity search and network-based analyses were applied on the three phenolic lipids resulting in the identification of seven possible targets as follows. Based on protein annotation, three phenolic lipids may interrupt or disturb the human proteins, namely KAT5, GAPDH, ACTB, and HSP90AA1, whose biological functions have been previously reported to be involved with viruses in the family Flaviviridae. In addition, these phenolic lipids might inhibit the mechanism of the viral proteins: NS3, NS5, and E proteins. The DENV and human proteins obtained from this study could be potential targets for further molecular optimization on compounds with a phenolic lipid core structure in anti-dengue drug discovery. As such, this pipeline could be a valuable tool to identify possible targets of active compounds.

Discovery of Dengue Virus Inhibitors

To date, there is still no approved anti-dengue agent to treat dengue infection in the market. Although the only licensed dengue vaccine, Dengvaxia is available, its protective efficacy against serotypes 1 and 2 of dengue virus was reported to be lower than serotypes 3 and 4. Moreover, according to WHO, the risk of being hospitalized and having severe dengue increased in seronegative individuals after they received Dengvaxia vaccination. Nevertheless, various studies had been carried out in search of dengue virus inhibitors. These studies focused on the structural (C, prM, E) and non-structural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B and NS5) of dengue virus as well as host factors as drug targets. Hence, this article provides an overall up-to-date review of the discovery of dengue virus inhibitors that are only targeting the structural and non-structural viral proteins as drug targets.

Bauhinia holophylla (Bong.) Steud. leaves-derived extracts as potent anti-dengue serotype 2

Dengue virus (DENV) is the most prevalent mosquito-borne viral pathogen and made the disease a major health concern worldwide. However, specific antiviral drugs against this arbovirose or vaccines are not yet available for treatment or prevention. Thus, here we aimed to study the antiviral activity of hydroethanolic extract, fraction ethyl acetate and subfractions of the leaves of Bauhinia holophylla (Fabaceae:Cercideae), a native plant of the Brazilian Cerrado, against DENV-2 by methylthiazolyldiphenyl-tetrazolium bromide (MTT) method in mammalian cells culture. As results, the hydroethanolic extract showed the most potent effect, with an inhibitory concentration (IC₅₀) of 3.2 μg mL^-1 and selectivity index (SI) of 27.6, approximately 16-times higher anti-DENV-2 activity than of the ribavirin (IC₅₀ 52.8 μg mL^-1). Our results showed in this study appointed that B. holophylla has a promising anti-dengue activity, which was associated mainly with the presence of flavonoids.

Palmatine inhibits Zika virus infection by disrupting virus binding, entry, and stability

Zika virus (ZIKV) is an emerging vector-borne virus that is associated with severe congenital cerebral anomalies in fetuses and paralytic Guillain-Barré syndrome in adults. In the current global health crisis, there are no vaccines or therapeutics available for the treatment of ZIKV infection. In the present study, we evaluated the efficacy of the protoberberine alkaloid, palmatine, in inhibiting ZIKV and Japanese encephalitis virus (JEV). Palmatine was shown to bind to restricted viruses, inhibit ZIKV infection, and resist ZIKV-induced cytopathic effects. Palmatine was also shown to inhibit JEV infection in multiple cell lines. Overall, the effects of palmatine in disrupting ZIKV binding, entry, and stability indicate that this small molecule would be a good starting point for the development of treatments aimed at inhibiting ZIKV infection.

Zika Virus NS5 Forms Supramolecular Nuclear Bodies That Sequester Importin-α and Modulate the Host Immune and Pro-Inflammatory Response in Neuronal Cells

The Zika virus (ZIKV) epidemic in the Americas was alarming because of its link with microcephaly in neonates and Guillain-Barré syndrome in adults. The unusual pathologies induced by ZIKV infection and the knowledge that the flaviviral nonstructural protein 5 (NS5), the most conserved protein in the flavivirus proteome, can modulate the host immune response during ZIKV infection prompted us to investigate the subcellular localization of NS5 during ZIKV infection and explore its functional significance. A monopartite nuclear localization signal (NLS) sequence within ZIKV NS5 was predicted by the cNLS Mapper program, and we observed localization of ZIKV NS5 in the nucleus of infected cells by immunostaining with specific antibodies. Strikingly, ZIKV NS5 forms spherical shell-like nuclear bodies that exclude DNA. The putative monopartite NLS ³⁹⁰KRPR³⁹³ is necessary to direct FLAG-tagged NS5 to the nucleus as the NS5 ³⁹⁰ARPA³⁹³ mutant protein accumulates in the cytoplasm. Furthermore, coimmunostaining experiments reveal that NS5 localizes with and sequesters importin-α, but not importin-β, in the observed nuclear bodies during virus infection. Structural and biochemical data demonstrate binding of ZIKV NS5 with importin-α and reveal important binding determinants required for their interaction and formation of complexes that give rise to the supramolecular nuclear bodies. Significantly, we demonstrate a neuronal-specific activation of the host immune response to ZIKV infection and a possible role of ZIKV NS5's nuclear localization toward this activation. This suggests that ZIKV pathogenesis may arise from a tissue-specific host response to ZIKV infection.

Insect anionic septapeptides suppress DENV replication by activating antiviral cytokines and miRNAs in primary human monocytes

Dengue viruses (DENVs) have threatened 2/3 of the world population for decades. Thus, combating DENV infection with either antiviral therapy or protective vaccination is an urgent goal. In the present study, we investigated the anti-DENV activity of insect cell-derived anionic septapeptides from C6/36 mosquito cell cultures persistently infected with DENV. These molecules were previously shown to protect C6/36 and Vero cells against DENV infection. We found that treatment with these septapeptides strongly and rapidly downregulated the multiplication of DENV-1 16007, DENV-3 16562, and DENV-4 1036 but not that of DENV-2 16681 in primary human monocytes. This inhibitory effect was likely mediated through various routes including the increased production of antiviral cytokines (IFN-I), activation of mononuclear cell migration, and upregulation of the expression of antiviral miRNAs (has-miR-30e*, has-miR-133a, and has-miR-223) and inflammation-related miRNAs (has-miR-146a and has-miR-147). In conclusion, anionic septapeptides exerted anti-DENV activity in human monocytes through the upregulation of innate immune responses and the activation of several previously reported antiviral and inflammation-related miRNAs.

Discovery and Computational Analyses of Novel Small Molecule Zika Virus Inhibitors

Zika virus (ZIKV), one of the flaviviruses, has attracted worldwide attention since its large epidemics around Brazil. Association of ZIKV infection with microcephaly and neurological problems such as Guillain–Barré syndrome has prompted intensive pathological investigations. However, there is still a long way to go on the discovery of effective anti-ZIKV therapeutics. In this study, an in silico screening of the National Cancer Institute (NCI) diversity set based on ZIKV NS3 helicase was performed using a molecular docking approach. Selected compounds with drug-like properties were subjected to cell-based antiviral assays resulting in the identification of two novel lead compounds (named Compounds 1 and 2). They inhibited ZIKV infection with IC₅₀ values at the micro-molar level (8.5 μM and 15.2 μM, respectively). Binding mode analysis, absolute binding free energy calculation, and structure–activity relationship studies of these two compounds revealed their possible interactions with ZIKV NS3 helicase, suggesting a mechanistic basis for further optimization. These two novel small molecules may represent new leads for the development of inhibitory drugs against ZIKV.

Potent Inhibition of Zika Virus Replication by Aurintricarboxylic Acid

Zika virus (ZIKV) is one of the recently emerging vector-borne viruses in humans and is responsible for severe congenital abnormalities such as microcephaly in the Western Hemisphere. Currently, only a few vaccine candidates and therapeutic drugs are being developed for the treatment of ZIKV infections, and as of yet none are commercially available. The polyanionic aromatic compound aurintricarboxylic acid (ATA) has been shown to have a broad-spectrum antimicrobial and antiviral activity. In this study, we evaluated ATA as a potential antiviral drug against ZIKV replication. The antiviral activity of ATA against ZIKV replication in vitro showed median inhibitory concentrations (IC₅₀) of 13.87 ± 1.09 μM and 33.33 ± 1.13 μM in Vero and A549 cells, respectively; without showing any cytotoxic effect in both cell lines (median cytotoxic concentration (CC₅₀) > 1,000 μM). Moreover, ATA protected both cell types from ZIKV-induced cytopathic effect (CPE) and apoptosis in a time- and concentration-dependent manner. In addition, pre-treatment of Vero cells with ATA for up to 72 h also resulted in effective suppression of ZIKV replication with similar IC₅₀. Importantly, the inhibitory effect of ATA on ZIKV infection was effective against strains of the African and Asian/American lineages, indicating that this inhibitory effect was not strain dependent. Overall, these results demonstrate that ATA has potent inhibitory activity against ZIKV replication and may be considered as a potential anti-ZIKV therapy for future clinical evaluation.

Anti-flavi: A Web Platform to Predict Inhibitors of Flaviviruses Using QSAR and Peptidomimetic Approaches

Flaviviruses are arboviruses, which comprises more than 70 viruses, covering broad geographic ranges, and responsible for significant mortality and morbidity globally. Due to the lack of efficient inhibitors targeting flaviviruses, the designing of novel and efficient anti-flavi agents is an important problem. Therefore, in the current study, we have developed a dedicated prediction algorithm anti-flavi, to identify inhibition ability of chemicals and peptides against flaviviruses through quantitative structure–activity relationship based method. We extracted the non-redundant 2168 chemicals and 117 peptides from ChEMBL and AVPpred databases, respectively, with reported IC₅₀ values. The regression based model developed on training/testing datasets of 1952 chemicals and 105 peptides displayed the Pearson’s correlation coefficient (PCC) of 0.87, 0.84, and 0.87, 0.83 using support vector machine and random forest techniques correspondingly. We also explored the peptidomimetics approach, in which the most contributing descriptors of peptides were used to identify chemicals having anti-flavi potential. Conversely, the selected descriptors of chemicals performed well to predict anti-flavi peptides. Moreover, the developed model proved to be highly robust while checked through various approaches like independent validation and decoy datasets. We hope that our web server would prove a useful tool to predict and design the efficient anti-flavi agents. The anti-flavi webserver is freely available at URL http://bioinfo.imtech.res.in/manojk/antiflavi.

Investigational drugs for the treatment of Zika virus infection: a preclinical and clinical update

INTRODUCTION

The Zika virus (ZIKV) infection results in severe neurological complications and has emerged as a threat to public health worldwide. No drugs or vaccines are available for use in the clinic and the need for novel and effective therapeutic agents is urgent.

AREAS COVERED

This review describes the latest progress of antiviral development for the treatment of ZIKV infection; it primarily focuses on the literature describing 20 potential anti-ZIKV drugs/agents currently being tested in vivo or in clinical trials. The paper also discusses the need for novel ZIKV inhibitors and the critical issues for successful antiviral drug development.

EXPERT OPINION

So far, 20 compounds have been tested in vivo and three in the clinical trials; progressing these compounds to the clinic is a challenge. Novel ZIKV inhibitors that target virus or host factors are urgently needed. Knowledge-driven drug repurposing, structure-based discovery, RNA interference, long noncoding RNAs, miRNAs, and peptide inhibitors may pave the way for the discovery of such novel agents.

Roles and Prospects of Dengue Virus Non-structural Proteins as Antiviral Targets: An Easy Digest

Dengue is a neglected disease caused by the infection of dengue virus which is transmitted by Aedes mosquitoes and to some, it could be fatal. Regardless of the enormous work devoted to research for the treatment of dengue, to this day there is no cure, and treatment is solely limited to supportive care by treating the symptoms. The inhibition of the viral RNA non-structural enzymes has been the most popular approach amongst the strategies applied to the search and development of dengue antivirals. This review is a compact digest of what is already known of the roles and the prospects of the dengue virus non-structural proteins NS1, NS2BNS3, NS4A, NS4B and NS5 as the targets for antiviral studies including the recent progress that has been published regarding their roles.

Using bioinformatics tools for the discovery of Dengue RNA-dependent RNA polymerase inhibitors

Background

Dengue fever has rapidly manifested into a serious global health concern. The emergence of various viral serotypes has prompted the urgent need for innovative drug design techniques. Of the viral non-structural enzymes, the NS5 RNA-dependent RNA polymerase has been established as a promising target due to its lack of an enzymatic counterpart in mammalian cells and its conserved structure amongst all serotypes. The onus is now on scientists to probe further into understanding this enzyme and its mechanism of action. The field of bioinformatics has evolved greatly over recent decades, with updated drug design tools now being publically available.

Methods

In this study, bioinformatics tools were used to provide a comprehensive sequence and structural analysis of the two most prominent serotypes of Dengue RNA-dependent RNA polymerase. A list of popular flavivirus inhibitors were also chosen to dock to the active site of the enzyme. The best docked compound was then used as a template to generate a pharmacophore model that may assist in the design of target-specific Dengue virus inhibitors.

Results

Comparative sequence alignment exhibited similarity between all three domains of serotype 2 and 3.Sequence analysis revealed highly conserved regions at residues Meth530, Thr543 Asp597, Glu616, Arg659 and Pro671. Mapping of the active site demonstrated two highly conserved residues: Ser710 and Arg729. Of the active site interacting residues, Ser796 was common amongst all ten docked compounds, indicating its importance in the drug design process. Of the ten docked flavivirus inhibitors, NITD-203 showed the best binding affinity to the active site. Further pharmacophore modeling of NITD-203 depicted significant pharmacophoric elements that are necessary for stable binding to the active site.

Discussion

This study utilized publically available bioinformatics tools to provide a comprehensive framework on Dengue RNA-dependent RNA polymerase. Based on docking studies, a pharmacophore model was also designed to unveil the crucial pharmacophoric elements that are required when constructing an efficacious DENV inhibitor. We believe that this study will be a cornerstone in paving the road toward the design of target-specific inhibitors against DENV RdRp.

Polymeric Prodrugs Targeting Polyamine Metabolism Inhibit Zika Virus Replication

The Zika virus (ZIKV) is primarily transmitted via an infected mosquito bite, during sexual intercourse, or in utero mother to child transmission. When a fetus is infected, both neurological malformations and deficits in brain development are frequently manifested. As such, there is a need for vaccines or drugs that may be used to cure ZIKV infections. Metabolic pathways play a crucial role in cell differentiation and development. More importantly, polyamines play a key role in replication and translation of several RNA viruses, including ZIKV, Dengue virus, and Chikungunya virus. Here, we present polyamine analogues (BENSpm and PG11047) and their corresponding polymer prodrug derivatives for inhibiting ZIKV infection by intersecting with polyamine catabolism pathways. We tested the compounds against ZIKV African (MR766) and Asian (PRVABC59) strains in human kidney epithelial (Vero) and glioblastoma derived (SNB-19) cell lines. Our results demonstrate potent inhibition of ZIKV viral replication in both cell lines tested. This antiviral effect was mediated by the upregulation of two polyamine catabolic enzymes, spermine oxidase, and spermidine (SMOX)/spermine N1-acetyltransferase (SAT1) as apparent reduction of the ZIKV infection following heterologous expression of SMOX and SAT1. On the basis of these observations, we infer potential use of these polyamine analogues to treat ZIKV infections.

Toll-like receptor agonist R848 blocks Zika virus replication by inducing the antiviral protein viperin

Zika virus (ZIKV) is an emerging pathogen linked to neurological disorders for which there is currently no targeted therapy. To identify host innate immune response proteins that restrict ZIKV replication, we treated monocytes and macrophages with toll-like receptor (TLR) agonists. Of those tested, the TLR7/8 agonist R848 (resiquimod) was the most potent inhibitor of ZIKV replication. RNA-seq analysis identified several genes strongly induced by R848 in monocytes. Testing of several of these for their ability to restrict ZIKV replication identified viperin, an interferon-induced gene active against several viruses. Transduction of microglial CHME3 cells with a viperin lentiviral expression vector rendered them resistant to ZIKV infection, preventing the synthesis of viral RNA and protein. CRISPR/Cas9 knock-out of viperin in macrophages relieved the block to infection, demonstrating that viperin is a major innate immune response protein able to block ZIKV replication. TLR agonists may be useful for the prophylactic or therapeutic treatment for ZIKV.

The 125th Lys and 145th Thr Amino Acids in the GTPase Domain of Goose Mx Confer Its Antiviral Activity against the Tembusu Virus

The Tembusu virus (TMUV) is an avian pathogenic flavivirus that causes a highly contagious disease and catastrophic losses to the poultry industry. The myxovirus resistance protein (Mx) of innate immune effectors is a key antiviral “workhorse” of the interferon (IFN) system. Although mammalian Mx resistance against myxovirus and retrovirus was witnessed for decades, whether or not bird Mx has anti-flavivirus activity remains unknown. In this study, we found that the transcription of goose Mx (goMx) was obviously driven by TMUV infection, both in vivo and in vitro, and that the titers and copies of TMUV were significantly reduced by goMx overexpression. In both primary (goose embryo fibroblasts, GEFs) and passaged cells (baby hamster kidney cells, BHK21, and human fetal kidney cells, HEK 293T), it was shown that goMx was mainly located in the cytoplasm, and sporadically distributed in the nucleus. The intracellular localization of this protein is attributed to the predicted bipartite nuclear localization signal (NLS; 30 residues: the 441st–471st amino acids of goMx). Intuitively, it seems that the cells with a higher level of goMx expression tend to have lower TMUV loads in the cytoplasm, as determined by an immunofluorescence assay. To further explore the antiviral determinants, a panel of variants was constructed. Two amino acids at the 125th (Lys) and 145th (Thr) positions in GTP-binding elements, not in the L4 loop (40 residues: the 532nd–572nd amino acids of goMx), were vital for the antiviral function of goMx against TMUV in vitro. These findings will contribute to our understanding of the functional significance of the antiviral system in aquatic birds, and the development of goMx could be a valuable therapeutic agent against TMUV.

Structure and function of Zika virus NS5 protein: perspectives for drug design

Zika virus (ZIKV) belongs to the positive-sense single-stranded RNA-containing Flaviviridae family. Its recent outbreak and association with human diseases (e.g. neurological disorders) have raised global health concerns, and an urgency to develop a therapeutic strategy against ZIKV infection. However, there is no currently approved antiviral against ZIKV. Here we present a comprehensive overview on recent progress in structure-function investigation of ZIKV NS5 protein, the largest non-structural protein of ZIKV, which is responsible for replication of the viral genome, RNA capping and suppression of host interferon responses. Structural comparison of the N-terminal methyltransferase domain and C-terminal RNA-dependent RNA polymerase domain of ZIKV NS5 with their counterparts from related viruses provides mechanistic insights into ZIKV NS5-mediated RNA replication, and identifies residues critical for its enzymatic activities. Finally, a collection of recently identified small molecule inhibitors against ZIKV NS5 or its closely related flavivirus homologues are also discussed.

Dengue virus NS2 and NS4: Minor proteins, mammoth roles

Despite the ever-increasing global incidence of dengue fever, there are no specific chemotherapy regimens for its treatment. Structural studies on dengue virus (DENV) proteins have revealed potential drug targets. Major DENV proteins such as the envelope protein and non-structural (NS) proteins 3 and 5 have been extensively investigated in antiviral studies, but with limited success in vitro. However, the minor NS proteins NS2 and NS4 have remained relatively underreported. Emerging evidence indicating their indispensable roles in virus propagation and host immunomodulation should encourage us to target these proteins for drug discovery. This review covers current knowledge on DENV NS2 and NS4 proteins from structural and functional perspectives and assesses their potential as targets for antiviral design. Antiviral targets in NS2A include surface-exposed transmembrane regions involved in pathogenesis, while those in NS2B include protease-binding sites in a conserved hydrophilic domain. Ideal drug targets in NS4A include helix α4 and the PEPEKQR sequence, which are essential for NS4A-2K cleavage and NS4A-NS4B association, respectively. In NS4B, the cytoplasmic loop connecting helices α5 and α7 is an attractive target for antiviral design owing to its role in dimerization and NS4B-NS3 interaction. Findings implicating NS2A, NS2B, and NS4A in membrane-modulation and viroporin-like activities indicate an opportunity to target these proteins by disrupting their association with membrane lipids. Despite the lack of 3D structural data, recent topological findings and progress in structure-prediction methods should be sufficient impetus for targeting NS2 and NS4 for drug design.

Pharmacological Inhibition of Protein Kinase C Reduces West Nile Virus Replication

Flaviviruses are relevant animal and human pathogens that include West Nile virus (WNV), Japanese encephalitis virus, dengue virus, or Zika virus, among others. Currently, no licensed therapy is available to fight flaviviral infections. Protein kinases C (PKCs) constitute a family of multifunctional lipid-dependent isoenzymes that regulate a wide variety of cellular processes (apoptosis, differentiation, proliferation, cellular transformation, motility, adhesion, etc.) being currently considered at the front line of drug development for the treatment of diverse human disorders. PKCs have also been implicated in different steps during viral replication; however, nowadays, results regarding their role in flavivirus replication are controversial. Here we demonstrate that calphostin C and chelerythrine, two broad-PKC inhibitors that target conventional, novel and atypical PKCs, significantly inhibit WNV multiplication in cell culture without affecting cell viability. A reduction of viral yields was observed in treated cells when compared with mock-treated cells. Likewise, immunofluorescence detection of viral enveloped E protein was reduced in treated cells, as was the amount of viral RNA released to the supernatant, mainly in those treated with chelerythrine. On the other hand, two PKC inhibitors specific for conventional and novel isoforms (staurosporine and enzastaurine) did not show any significant effect in WNV multiplication. These results suggested that PKCs, more probably atypical PKCs, are likely involved in WNV multiplication, although both broad-spectrum tested drugs seem to act through different mechanisms, and point to them as potential antiviral candidates for WNV, as well as for other related flaviviruses.

Modulation of Lipid Droplet Metabolism-A Potential Target for Therapeutic Intervention in Flaviviridae Infections

Lipid droplets (LDs) are endoplasmic reticulum (ER)-related dynamic organelles that store and regulate fatty acids and neutral lipids. They play a central role in cellular energy storage, lipid metabolism and cellular homeostasis. It has become evident that viruses have co-evolved in order to exploit host lipid metabolic pathways. This is especially characteristic of the Flaviviridae family, including hepatitis C virus (HCV) and several flaviviruses. Devoid of an appropriate lipid biosynthetic machinery of their own, these single-strand positive-sense RNA viruses can induce dramatic changes in host metabolic pathways to establish a favorable environment for viral multiplication and acquire essential components to facilitate their assembly and traffic. Here we have reviewed the current knowledge on the intracellular life cycle of those from the Flaviviridae family, with particular emphasis on HCV and dengue virus (DENV), and their association with the biosynthesis and metabolism of LDs, with the aim to identify potential antiviral targets for development of novel therapeutic interventions.