Cellular kinase inhibitors that suppress enterovirus replication have a conserved target in viral protein 3A similar to that of enviroxime

Repurposing Anticancer Drugs Targeting the MAPK/ERK Signaling Pathway for the Treatment of Respiratory Virus Infections

Respiratory virus infections remain a significant challenge to human health and the social economy. The symptoms range from mild rhinitis and nasal congestion to severe lower respiratory tract dysfunction and even mortality. The efficacy of therapeutic drugs targeting respiratory viruses varies, depending upon infection time and the drug resistance engendered by a high frequency of viral genome mutations, necessitating the development of new strategies. The MAPK/ERK pathway that was well delineated in the 1980s represents a classical signaling cascade, essential for cell proliferation, survival, and differentiation. Since this pathway is constitutively activated in many cancers by oncogenes, several drugs inhibiting Raf/MEK/ERK have been developed and currently used in anticancer treatment. Two decades ago, it was reported that viruses such as HIV and influenza viruses could exploit the host cellular MAPK/ERK pathway for their replication. Thus, it would be feasible to repurpose this category of the pathway inhibitors for the treatment of respiratory viral infections. The advantage is that the host genes are not easy to mutate such that the drug resistance rarely occurs during short-period treatment of viruses. Therefore, in this review we will summarize the research progress on the role of the MAPK/ERK pathway in respiratory virus amplification and discuss the potential of the pathway inhibitors (MEK inhibitors) in the treatment of respiratory viral infections.

Evaluation of 3D Human Intestinal Organoids as a Platform for EV-A71 Antiviral Drug Discovery

Enteroviruses are a leading cause of upper respiratory tract, gastrointestinal, and neurological infections. Management of enterovirus-related diseases has been hindered by the lack of specific antiviral treatment. The pre-clinical and clinical development of such antivirals has been challenging, calling for novel model systems and strategies to identify suitable pre-clinical candidates. Organoids represent a new and outstanding opportunity to test antiviral agents in a more physiologically relevant system. However, dedicated studies addressing the validation and direct comparison of organoids versus commonly used cell lines are lacking. Here, we described the use of human small intestinal organoids (HIOs) as a model to study antiviral treatment against human enterovirus 71 (EV-A71) infection and compared this model to EV-A71-infected RD cells. We used reference antiviral compounds such as enviroxime, rupintrivir, and 2'-C-methylcytidine (2'CMC) to assess their effects on cell viability, virus-induced cytopathic effect, and viral RNA yield in EV-A71-infected HIOs and cell line. The results indicated a difference in the activity of the tested compounds between the two models, with HIOs being more sensitive to infection and drug treatment. In conclusion, the outcome reveals the value added by using the organoid model in virus and antiviral studies.

Ligand Recognition by the Lipid Transfer Domain of Human OSBP Is Important for Enterovirus Replication

Oxysterol-binding protein (OSBP), which transports cholesterol and phosphatidylinositol 4-monophosphate (PtdIns[4]P) between different organelles, serves as a conserved host factor for the replication of various viruses, and OSBP inhibitors exhibit antiviral effects. Here, we determined the crystal structure of the lipid transfer domain of human OSBP in complex with endogenous cholesterol. The hydrocarbon tail and tetracyclic ring of cholesterol interact with the hydrophobic tunnel of OSBP, and the hydroxyl group of cholesterol forms a hydrogen bond network at the bottom of the tunnel. Systematic mutagenesis of the ligand-binding region revealed that M446W and L590W substitutions confer functional tolerance to an OSBP inhibitor, T-00127-HEV2. Employing the M446W variant as a functional replacement for the endogenous OSBP in the presence of T-00127-HEV2, we have identified previously unappreciated amino acid residues required for viral replication. The combined use of the inhibitor and the OSBP variant will be useful in elucidating the enigmatic in vivo functions of OSBP.

Enterovirus A71 antivirals: Past, present, and future

Enterovirus A71 (EV-A71) is a significant human pathogen, especially in children. EV-A71 infection is one of the leading causes of hand, foot, and mouth diseases (HFMD), and can lead to neurological complications such as acute flaccid myelitis (AFM) in severe cases. Although three EV-A71 vaccines are available in China, they are not broadly protective and have reduced efficacy against emerging strains. There is currently no approved antiviral for EV-A71. Significant progress has been made in developing antivirals against EV-A71 by targeting both viral proteins and host factors. However, viral capsid inhibitors and protease inhibitors failed in clinical trials of human rhinovirus infection due to limited efficacy or side effects. This review discusses major discoveries in EV-A71 antiviral development, analyzes the advantages and limitations of each drug target, and highlights the knowledge gaps that need to be addressed to advance the field forward.

Picornaviruses: A View from 3A

Picornaviruses are comprised of a positive-sense RNA genome surrounded by a protein shell (or capsid). They are ubiquitous in vertebrates and cause a wide range of important human and animal diseases. The genome encodes a single large polyprotein that is processed to structural (capsid) and non-structural proteins. The non-structural proteins have key functions within the viral replication complex. Some, such as 3D^pol (the RNA dependent RNA polymerase) have conserved functions and participate directly in replicating the viral genome, whereas others, such as 3A, have accessory roles. The 3A proteins are highly divergent across the Picornaviridae and have specific roles both within and outside of the replication complex, which differ between the different genera. These roles include subverting host proteins to generate replication organelles and inhibition of cellular functions (such as protein secretion) to influence virus replication efficiency and the host response to infection. In addition, 3A proteins are associated with the determination of host range. However, recent observations have challenged some of the roles assigned to 3A and suggest that other viral proteins may carry them out. In this review, we revisit the roles of 3A in the picornavirus life cycle. The 3AB precursor and mature 3A have distinct functions during viral replication and, therefore, we have also included discussion of some of the roles assigned to 3AB.

Regulation of the proteostasis network during enterovirus infection: A feedforward mechanism for EV-A71 and EV-D68

The proteostasis network guarantees successful protein synthesis, folding, transportation, and degradation. Mounting evidence has revealed that this network maintains proteome integrity and is linked to cellular physiology, pathology, and virus infection. Human enterovirus A71 (EV-A71) and EV-D68 are suspected causative agents of acute flaccid myelitis, a severe poliomyelitis-like neurologic syndrome with no known cure. In this context, further clarification of the molecular mechanisms underlying EV-A71 and EV-D68 infection is paramount. Here, we summarize the components of the proteostasis network that are intercepted by EV-A71 and EV-D68, as well as antivirals that target this network and may help develop improved antiviral drugs.

Enterovirus Replication Organelles and Inhibitors of Their Formation

Enteroviral replication reorganizes the cellular membrane. Upon infection, viral proteins and hijacked host factors generate unique structures called replication organelles (ROs) to replicate their viral genomes. ROs promote efficient viral genome replication, coordinate the steps of the viral replication cycle, and protect viral RNA from host immune responses. More recent researches have focused on the ultrastructure structures, formation mechanism, and functions in the virus life cycle of ROs. Dynamic model of enterovirus ROs structure is proposed, and the secretory pathway, the autophagy pathway, and lipid metabolism are found to be associated in the formation of ROs. With deeper understanding of ROs, some compounds have been found to show inhibitory effects on viral replication by targeting key proteins in the process of ROs formation. Here, we review the recent findings concerning the role, morphology, biogenesis, formation mechanism, and inhibitors of enterovirus ROs.

Antivirals and vaccines for Enterovirus A71

Enterovirus A71 (EV-A71) is an important emerging virus posing a threat to children under five years old. EV-A71 infection in infants or young children can cause hand-foot-and-mouth disease, herpangina, or severe neurological complications. However, there are still no effective antivirals for treatment of these infections. In this review, we summarize the antiviral compounds developed to date based on various targets of the EV-A71 life cycle. Moreover, development of a vaccine would be the most effective approach to prevent EV-A71 infection. Therefore, we also summarize the development and clinical progress of various candidate EV-A71 vaccines, including inactivated whole virus, recombinant VP1 protein, synthetic peptides, viral-like particles, and live attenuated vaccines.

Convergent evolution in the mechanisms of ACBD3 recruitment to picornavirus replication sites

Enteroviruses, members of the family of picornaviruses, are the most common viral infectious agents in humans causing a broad spectrum of diseases ranging from mild respiratory illnesses to life-threatening infections. To efficiently replicate within the host cell, enteroviruses hijack several host factors, such as ACBD3. ACBD3 facilitates replication of various enterovirus species, however, structural determinants of ACBD3 recruitment to the viral replication sites are poorly understood. Here, we present a structural characterization of the interaction between ACBD3 and the non-structural 3A proteins of four representative enteroviruses (poliovirus, enterovirus A71, enterovirus D68, and rhinovirus B14). In addition, we describe the details of the 3A-3A interaction causing the assembly of the ACBD3-3A heterotetramers and the interaction between the ACBD3-3A complex and the lipid bilayer. Using structure-guided identification of the point mutations disrupting these interactions, we demonstrate their roles in the intracellular localization of these proteins, recruitment of downstream effectors of ACBD3, and facilitation of enterovirus replication. These structures uncovered a striking convergence in the mechanisms of how enteroviruses and kobuviruses, members of a distinct group of picornaviruses that also rely on ACBD3, recruit ACBD3 and its downstream effectors to the sites of viral replication.

Enteroviruses are the most common viruses infecting humans. They cause a broad spectrum of diseases ranging from common cold to life-threatening diseases, such as poliomyelitis. To date, no effective antiviral therapy for enteroviruses has been approved yet. To ensure efficient replication, enteroviruses hijack several host factors, recruit them to the sites of virus replication, and use their physiological functions for their own purposes. Here, we characterize the complexes composed of the host protein ACBD3 and the ACBD3-binding viral proteins (called 3A) of four representative enteroviruses. Our study reveals the atomic details of these complexes and identifies the amino acid residues important for the interaction. We found out that the 3A proteins of enteroviruses bind to the same regions of ACBD3 as the 3A proteins of kobuviruses, a distinct group of viruses that also rely on ACBD3, but are oriented in the opposite directions. This observation reveals a striking case of convergent evolutionary pathways that have evolved to allow enteroviruses and kobuviruses (which are two distinct groups of the Picornaviridae family) to recruit a common host target, ACBD3, and its downstream effectors to the sites of viral replication.

Modulation of proteolytic polyprotein processing by coxsackievirus mutants resistant to inhibitors targeting phosphatidylinositol-4-kinase IIIβ or oxysterol binding protein

Enteroviruses (e.g. poliovirus, coxsackievirus, and rhinovirus) require several host factors for genome replication. Among these host factors are phosphatidylinositol-4-kinase IIIβ (PI4KB) and oxysterol binding protein (OSBP). Enterovirus mutants resistant to inhibitors of PI4KB and OSBP were previously isolated, which demonstrated a role of single substitutions in the non-structural 3A protein in conferring resistance. Besides the 3A substitutions (i.e., 3A-I54F and 3A-H57Y) in coxsackievirus B3 (CVB3), substitution N2D in 2C was identified in each of the PI4KB-inhibitor resistant CVB3 pools, but its possible benefit has not been investigated yet. In this study, we set out to investigate the possible role of 2C-N2D in the resistance to PI4KB and OSBP inhibition. We show that 2C-N2D by itself did not confer any resistance to inhibitors of PI4KB and OSBP. However, the double mutant (i.e., 2C-N2D/3A-H57Y) showed better replication than the 3A-H57Y single mutant in the presence of inhibitors. Growing evidence suggests that alterations in lipid homeostasis affect the proteolytic processing of the poliovirus polyprotein. Therefore, we studied the effect of PI4KB or OSBP inhibition on proteolytic processing of the CVB3 polyprotein during infection as well as in a replication-independent system. We show that both PI4KB and OSBP inhibitors specifically affected the cleavage at the 3A-3B junction, and that mutation 3A-H57Y recovered impaired proteolytic processing at this junction. Although 2C-N2D enhanced replication of the 3A-H57Y single mutant, we did not detect additional effects of this substitution on polyprotein processing, which leaves the mechanism of how 2C-N2D contributes to the resistance to be revealed.

Investigation of the Role of Protein Kinase D in Human Rhinovirus Replication

Picornavirus replication is known to cause extensive remodeling of Golgi and endoplasmic reticulum membranes, and a number of the host proteins involved in the viral replication complex have been identified, including oxysterol binding protein (OSBP) and phosphatidylinositol 4-kinase III beta (PI4KB). Since both OSBP and PI4KB are substrates for protein kinase D (PKD) and PKD is known to be involved in the control of Golgi membrane vesicular and lipid transport, we hypothesized that PKD played a role in viral replication. We present multiple lines of evidence in support of this hypothesis. First, infection of HeLa cells with human rhinovirus (HRV) induced the phosphorylation of PKD. Second, PKD inhibitors reduced HRV genome replication, protein expression, and titers in a concentration-dependent fashion and also blocked the replication of poliovirus (PV) and foot-and-mouth disease virus (FMDV) in a variety of cells. Third, HRV replication was significantly reduced in HeLa cells overexpressing wild-type and mutant forms of PKD1. Fourth, HRV genome replication was reduced in HAP1 cells in which the PKD1 gene was knocked out by clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9. Although we have not identified the molecular mechanism through which PKD regulates viral replication, our data suggest that this is not due to enhanced interferon signaling or an inhibition of clathrin-mediated endocytosis, and PKD inhibitors do not need to be present during viral uptake. Our data show for the first time that targeting PKD with small molecules can inhibit the replication of HRV, PV, and FMDV, and therefore, PKD may represent a novel antiviral target for drug discovery.

IMPORTANCE Picornaviruses remain an important family of human and animal pathogens for which we have a very limited arsenal of antiviral agents. HRV is the causative agent of the common cold, which in itself is a relatively trivial infection; however, in asthma and chronic obstructive pulmonary disease (COPD) patients, this virus is a major cause of exacerbations resulting in an increased use of medication, worsening symptoms, and, frequently, hospital admission. Thus, HRV represents a substantial health care and economic burden for which there are no approved therapies. We sought to identify a novel host target as a potential anti-HRV therapy. HRV infection induces the phosphorylation of PKD, and inhibitors of this kinase effectively block HRV replication at an early stage of the viral life cycle. Moreover, PKD inhibitors also block PV and FMDV replication. This is the first description that PKD may represent a target for antiviral drug discovery.

Identifying novel antiviral targets against enterovirus 71: where are we?

Human enterovirus 71 (HEV71) has been considered as an essential human pathogen, which causes hand, foot and mouth disease in young children. Several HEV71 outbreaks have been observed in many Asia-Pacific countries for the past two decades with significant fatalities. However, there are no competent vaccines or antivirals against HEV71 infection to date. Thus, it is of critical priority to delve into the search for anti-HEV71 agents. Prior to this, there is a need to gain knowledge about the distinct targets of HEV71 that are available and that have been exploited for antiviral therapy. This review aims to provide a better understanding of HEV71 virology and feature potential antivirals for progressive clinical development with respect to their elucidated mechanistic actions.

Suramin interacts with the positively charged region surrounding the 5-fold axis of the EV-A71 capsid and inhibits multiple enterovirus A

Suramin was previously shown to bind to the EV-A71 capsid through its naphthalenetrisulfonic acid groups, thereby reducing virus-cell binding and inhibiting viral replication. Here, we identify VP1-145 as the critical amino acid that accounts for the differential sensitivity of EVA-71 viruses to suramin. A single Q or G to E substitution at VP1-145 results in an approximately 30-fold shift of IC₅₀ or IC₉₀ values reproducing the inhibition profile observed with field isolates expressing either the 145Q or E mutation. Our data support the conclusion that suramin binds to the positively charged region surrounding the 5-fold axis of the capsid and consequently blocks the virus attachment and entry into host cells. In order to assess the antiviral-spectrum of suramin, we analyzed 18 representative enteroviruses: A (n = 7), B (n = 5), C (n = 5) and D (n = 1). We show that suramin potency is restricted to enterovirus A species. Clinical development of suramin is further supported by pharmacokinetic data demonstrating bioactive plasma levels after a single dose intramuscular administration in macaques. Altogether, our findings support the clinical development of suramin as a novel entry inhibitor for the treatment of enterovirus A infections.

Poliovirus Studies during the Endgame of the Polio Eradication Program

Since the beginning of Global Polio Eradication Initiative in 1988, poliomyelitis cases caused by wild poliovirus (PV) have been drastically reduced, with only 74 cases reported in 2 endemic countries in 2015. The current limited PV transmission suggests that we are in the endgame of the polio eradication program. However, specific challenges have emerged in the endgame, including tight budget, switching of the vaccines, and changes in biorisk management of PV. To overcome these challenges, several PV studies have been implemented in the eradication program. Some of the responses to the emerging challenges in the polio endgame might be valuable in other infectious diseases eradication programs. Here, I will review challenges that confront the polio eradication program and current research to address these challenges.

In Vitro Assessment of Combinations of Enterovirus Inhibitors against Enterovirus 71

Enterovirus 71 (EV-A71) is a major causative pathogen of hand, foot, and mouth disease (HFMD) epidemics. No antiviral therapies are currently available for treating EV-A71 infections. Here, we selected five reported enterovirus inhibitors (suramin, itraconazole [ITZ], GW5074, rupintrivir, and favipiravir) with different mechanisms of action to test their abilities to inhibit EV-A71 replication alone and in combination. All selected compounds have anti-EV-A71 activities in cell culture. The combination of rupintrivir and ITZ or favipiravir was synergistic, while the combination of rupintrivir and suramin was additive. The combination of suramin and favipiravir exerted a strong synergistic antiviral effect. The observed synergy was not due to cytotoxicity, as there was no significant increase in cytotoxicity when compounds were used in combinations at the tested doses. To investigate the potential inhibitory mechanism of favipiravir against enterovirus, two favipiravir-resistant EV-A71 variants were independently selected, and both of them carried an S121N mutation in the finger subdomain of the 3D polymerase. Reverse engineering of this 3D S121N mutation into an infectious clone of EV-A71 confirmed the resistant phenotype. Moreover, viruses resistant to ITZ or favipiravir remained susceptible to other inhibitors. Most notably, combined with ITZ, rupintrivir prevented the development of ITZ-resistant variants. Taken together, these results provide a rational basis for the design of combination regimens for use in the treatment of EV-A71 infections.

Dynamic lipid landscape of picornavirus replication organelles

Picornavirus infection induces rapid reorganization of the cellular membrane architecture and appearance of novel membranous structures associated with the viral RNA replication and virion assembly-replication organelles. Recent studies significantly advanced our understanding of their lipid composition and cellular mechanisms involved in their development. Picornaviruses activate synthesis of both structural and signaling lipids and reroute cellular cholesterol trafficking pathways to create unique membranous domains favoring viral replication. Rapidly replicating picornaviruses rely on posttranslational activation and/or specific recruitment of cellular proteins rather than on modulation of expression of cellular genes to create favorable membrane microenvironment. At the same time picornaviruses demonstrate remarkable adaptability to changes in the lipid landscape which should be taken into account when developing novel antiviral strategies.

Mutations in Encephalomyocarditis Virus 3A Protein Uncouple the Dependency of Genome Replication on Host Factors Phosphatidylinositol 4-Kinase IIIα and Oxysterol-Binding Protein

Positive-strand RNA viruses modulate lipid homeostasis to generate unique, membranous “replication organelles” (ROs) where viral genome replication takes place. Hepatitis C virus, encephalomyocarditis virus (EMCV), and enteroviruses have convergently evolved to hijack host phosphatidylinositol 4-kinases (PI4Ks), which produce PI4P lipids, to recruit oxysterol-binding protein (OSBP), a PI4P-binding protein that shuttles cholesterol to ROs. Consistent with the proposed coupling between PI4K and OSBP, enterovirus mutants resistant to PI4KB inhibitors are also resistant to OSBP inhibitors. Here, we show that EMCV can replicate without accumulating PI4P/cholesterol at ROs, by acquiring point mutations in nonstructural protein 3A. Remarkably, the mutations conferred resistance to PI4K but not OSBP inhibitors, thereby uncoupling the levels of dependency of EMCV RNA replication on PI4K and OSBP. This work may contribute to a deeper understanding of the roles of PI4K/PI4P and OSBP/cholesterol in membrane modifications induced by positive-strand RNA viruses.

Positive-strand RNA [(+)RNA] viruses are true masters of reprogramming host lipid trafficking and synthesis to support virus genome replication. Via their membrane-associated 3A protein, picornaviruses of the genus Enterovirus (e.g., poliovirus, coxsackievirus, and rhinovirus) subvert Golgi complex-localized phosphatidylinositol 4-kinase IIIβ (PI4KB) to generate “replication organelles” (ROs) enriched in phosphatidylinositol 4-phosphate (PI4P). PI4P lipids serve to accumulate oxysterol-binding protein (OSBP), which subsequently transfers cholesterol to the ROs in a PI4P-dependent manner. Single-point mutations in 3A render enteroviruses resistant to both PI4KB and OSBP inhibition, indicating coupled dependency on these host factors. Recently, we showed that encephalomyocarditis virus (EMCV), a picornavirus that belongs to the Cardiovirus genus, also builds PI4P/cholesterol-enriched ROs. Like the hepatitis C virus (HCV) of the Flaviviridae family, it does so by hijacking the endoplasmic reticulum (ER)-localized phosphatidylinositol 4-kinase IIIα (PI4KA). Here we provide genetic evidence for the critical involvement of EMCV protein 3A in this process. Using a genetic screening approach, we selected EMCV mutants with single amino acid substitutions in 3A, which rescued RNA virus replication upon small interfering RNA (siRNA) knockdown or pharmacological inhibition of PI4KA. In the presence of PI4KA inhibitors, the mutants no longer induced PI4P, OSBP, or cholesterol accumulation at ROs, which aggregated into large cytoplasmic clusters. In contrast to the enterovirus escape mutants, we observed little if any cross-resistance of EMCV mutants to OSBP inhibitors, indicating an uncoupled level of dependency of their RNA replication on PI4KA and OSBP activities. This report may contribute to a better understanding of the roles of PI4KA and OSBP in membrane modifications induced by (+)RNA viruses.

IMPORTANCE Positive-strand RNA viruses modulate lipid homeostasis to generate unique, membranous “replication organelles” (ROs) where viral genome replication takes place. Hepatitis C virus, encephalomyocarditis virus (EMCV), and enteroviruses have convergently evolved to hijack host phosphatidylinositol 4-kinases (PI4Ks), which produce PI4P lipids, to recruit oxysterol-binding protein (OSBP), a PI4P-binding protein that shuttles cholesterol to ROs. Consistent with the proposed coupling between PI4K and OSBP, enterovirus mutants resistant to PI4KB inhibitors are also resistant to OSBP inhibitors. Here, we show that EMCV can replicate without accumulating PI4P/cholesterol at ROs, by acquiring point mutations in nonstructural protein 3A. Remarkably, the mutations conferred resistance to PI4K but not OSBP inhibitors, thereby uncoupling the levels of dependency of EMCV RNA replication on PI4K and OSBP. This work may contribute to a deeper understanding of the roles of PI4K/PI4P and OSBP/cholesterol in membrane modifications induced by positive-strand RNA viruses.

Phosphatidylinositol 4-kinase III beta is the target of oxoglaucine and pachypodol (Ro 09-0179) for their anti-poliovirus activities, and is located at upstream of the target step of brefeldin A

In recent years, phosphatidylinositol 4-kinase III beta (PI4KB) has emerged as a conserved target of anti-picornavirus compounds. In the present study, PI4KB was identified as the direct target of the plant-derived anti-picornavirus compounds, oxoglaucine and pachypodol (also known as Ro 09-0179). PI4KB was also identified as the target via which pachypodol interferes with brefeldin A (BFA)-induced Golgi disassembly in non-infected cells. Oxysterol-binding protein (OSBP) inhibitor also has interfering activity against BFA. It seems that this interference is not essential for the anti-poliovirus (PV) activities of BFA and PI4KB/OSBP inhibitors. BFA inhibited early to late phase PV replication (0 to 6 hr postinfection) as well as PI4KB inhibitor, but with some delay compared to guanidine hydrochloride treatment. In contrast with PI4KB/OSBP inhibitors, BFA inhibited viral nascent RNA synthesis, suggesting that BFA targets some step of viral RNA synthesis located downstream of the PI4KB/OSBP pathway in PV replication. Our results suggest that PI4KB is a major target of anti-picornavirus compounds identified in vitro for their anti-picornavirus activities and for some uncharacterized biological phenomena caused by these compounds, and that BFA and PI4KB/OSBP inhibitors synergistically repress PV replication by targeting distinct steps in viral RNA replication.

Mechanism of Poliovirus Resistance to Host Phosphatidylinositol-4 Kinase III β Inhibitor

Phosphatidylinositol-4 kinase III β (PI4KB) and oxysterol-binding protein (OSBP) family I have been identified as the major targets of anti-enterovirus drug candidates. Resistance mutations in poliovirus (PV) to these inhibitors have been identified in viral 3A protein, represented by a G5318A (3A-Ala70Thr) mutation, but the mechanism of viral resistance to host PI4KB/OSBP inhibitors remained unknown. In this study, we found that a G5318A mutation enhances the basal levels of phosphatidylinositol 4-phosphate (PI4P) and of the 3A protein and decreases the levels of the 3AB protein during PV replication. The 3A protein acted as a major effector responsible for the resistance to PI4KB inhibitor, but did not enhance the PI4KB activity in vitro in contrast to the 2C, 2BC, 3AB, and 3D proteins. The 3AB protein acted as the primary target of a G5318A mutation and also as an effector. We identified novel resistance mutations to a PI4KB inhibitor [C5151U (3A-T14M) and C5366U (3A-H86Y) mutations] and found that there is a positive correlation between the extent of the resistance phenotype and the levels of the 3A proteins. These results suggested that the 3A protein overproduced by enhanced processing of the 3AB protein with the resistance mutations overcomes the inhibitory effect of PI4KB inhibitor on PV replication independently of the hyperactivation of the PI4KB/OSBP pathway.

Synergistic antiviral activity of gemcitabine and ribavirin against enteroviruses

Enteroviruses are major causative agents of various human diseases, and some of them are currently considered to be an enormous threat to public health. However, no effective therapy is currently available for the treatment of these infections. We identified gemcitabine, a nucleoside-analog drug used for cancer treatment, from a screen of bioactive chemicals as a novel inhibitor of coxsackievirus B3 (CVB3) and enterovirus 71 (EV71). Gemcitabine potently inhibited the proliferation of CVB3 and EV71, as well as the replication of CVB3 and EV71 replicons, in cells with a low micromolar IC50 (1-5 μM). Its strong inhibitory effect was also observed in cells infected with human rhinoviruses, demonstrating broad-spectrum antiviral effects on enteroviruses. Mechanistically, an extensive analysis excluded the involvement of 2C, 3A, IRES-dependent translation, and also that of polyprotein processing in the antiviral effects of gemcitabine. Importantly, gemcitabine in combination with ribavirin, an antiviral drug currently being used against a few RNA viruses, exhibited a synergistic antiviral effect on the replication of CVB3 and EV71 replicons. Consequently, our results clearly demonstrate a new indication for gemcitabine as an effective broad-spectrum inhibitor of enteroviruses and strongly suggest a new therapeutic strategy using gemcitabine alone or in combination with ribavirin for the treatment of various diseases associated with enterovirus infection.

Itraconazole inhibits enterovirus replication by targeting the oxysterol-binding protein

Itraconazole (ITZ) is a well-known antifungal agent that also has anticancer activity. In this study, we identify ITZ as a broad-spectrum inhibitor of enteroviruses (e.g., poliovirus, coxsackievirus, enterovirus-71, rhinovirus). We demonstrate that ITZ inhibits viral RNA replication by targeting oxysterol-binding protein (OSBP) and OSBP-related protein 4 (ORP4). Consistently, OSW-1, a specific OSBP/ORP4 antagonist, also inhibits enterovirus replication. Knockdown of OSBP inhibits virus replication, whereas overexpression of OSBP or ORP4 counteracts the antiviral effects of ITZ and OSW-1. ITZ binds OSBP and inhibits its function, i.e., shuttling of cholesterol and phosphatidylinositol-4-phosphate between membranes, thereby likely perturbing the virus-induced membrane alterations essential for viral replication organelle formation. ITZ also inhibits hepatitis C virus replication, which also relies on OSBP. Together, these data implicate OSBP/ORP4 as molecular targets of ITZ and point to an essential role of OSBP/ORP4-mediated lipid exchange in virus replication that can be targeted by antiviral drugs.

Phosphatidylinositol-4 kinase III beta and oxysterol-binding protein accumulate unesterified cholesterol on poliovirus-induced membrane structure

Studies on anti-picornavirus compounds have revealed an essential role of a novel cellular pathway via host phosphatidylinositol-4 kinase III beta (PI4KB) and oxysterol-binding protein (OSBP) family I in poliovirus (PV) replication. However, the molecular role for this pathway in PV replication has yet to be determined. Here, viral and host proteins modulating production of phosphatidylinositol 4-phosphate (PI4P) and accumulation of unesterified cholesterol (UC) in cells were analyzed and the role of the PI4KB/OSBP pathway in PV replication characterized. Virus protein 2BC was identified as a novel interactant of PI4KB. PI4KB and VCP/p97 bind to a partially overlapped region of 2BC with different sensitivity to a 2C inhibitor. Production of PI4P and accumulation of UC were enhanced by virus protein 2BC, but suppressed by virus proteins 3A and 3AB. In PV-infected cells, a PI4KB inhibitor suppressed production of PI4P, and both a PI4KB inhibitor and an OSBP ligand suppressed accumulation of UC on virus-induced membrane structure. Inhibition of PI4KB activity caused dissociation of OSBP from virus-induced membrane structure in PV-infected cells. Synthesis of viral nascent RNA in PV-infected cells was not affected in the presence of PI4KB inhibitor and OSBP ligand; however, transient pre-treatment of PV-infected cells with these inhibitors suppressed viral RNA synthesis. These results suggest that virus proteins modulate PI4KB activity and provide PI4P for recruitment of OSBP to accumulate UC on virus-induced membrane structure for formation of a virus replication complex.

New small-molecule inhibitors effectively blocking picornavirus replication

Few drugs targeting picornaviruses are available, making the discovery of antivirals a high priority. Here, we identified and characterized three compounds from a library of kinase inhibitors that block replication of poliovirus, coxsackievirus B3, and encephalomyocarditis virus. Using an in vitro translation-replication system, we showed that these drugs inhibit different stages of the poliovirus life cycle. A4(1) inhibited both the formation and functioning of the replication complexes, while E5(1) and E7(2) were most effective during the formation but not the functioning step. Neither of the compounds significantly inhibited VPg uridylylation. Poliovirus resistant to E7(2) had a G5318A mutation in the 3A protein. This mutation was previously found to confer resistance to enviroxime-like compounds, which target a phosphatidylinositol 4-kinase IIIβ (PI4KIIIβ)-dependent step in viral replication. Analysis of host protein recruitment showed that E7(2) reduced the amount of GBF1 on the replication complexes; however, the level of PI4KIIIβ remained intact. E7(2) as well as another enviroxime-like compound, GW5074, interfered with viral polyprotein processing affecting both 3C- and 2A-dependent cleavages, and the resistant G5318A mutation partially rescued this defect. Moreover, E7(2) induced abnormal recruitment to membranes of the viral proteins; thus, enviroxime-like compounds likely severely compromise the interaction of the viral polyprotein with membranes. A4(1) demonstrated partial protection from paralysis in a murine model of poliomyelitis. Multiple attempts to isolate resistant mutants in the presence of A4(1) or E5(1) were unsuccessful, showing that effective broad-spectrum antivirals could be developed on the basis of these compounds.

IMPORTANCE

Diverse picornaviruses can trigger multiple human maladies, yet currently, only hepatitis A virus and poliovirus can be controlled with vaccination. The development of antipicornavirus therapeutics is also facing significant difficulties because these viruses readily generate resistance to compounds targeting either viral or cellular factors. Here, we describe three novel compounds that effectively block replication of distantly related picornaviruses with minimal toxicity to cells. The compounds prevent viral RNA replication after the synthesis of the uridylylated VPg primer. Importantly, two of the inhibitors are strongly refractory to the emergence of resistant mutants, making them promising candidates for further broad-spectrum therapeutic development. Evaluation of one of the compounds in an in vivo model of poliomyelitis demonstrated partial protection from the onset of paralysis.

CDK9 inhibitor FIT-039 prevents replication of multiple DNA viruses

A wide range of antiviral drugs is currently available; however, drug-resistant viruses have begun to emerge and represent a potential public health risk. Here, we explored the use of compounds that inhibit or interfere with the action of essential host factors to prevent virus replication. In particular, we focused on the cyclin-dependent kinase 9 (CDK9) inhibitor, FIT-039, which suppressed replication of a broad spectrum of DNA viruses through inhibition of mRNA transcription. Specifically, FIT-039 inhibited replication of herpes simplex virus 1 (HSV-1), HSV-2, human adenovirus, and human cytomegalovirus in cultured cells, and topical application of FIT-039 ointment suppressed skin legion formation in a murine HSV-1 infection model. FIT-039 did not affect cell cycle progression or cellular proliferation in host cells. Compared with the general CDK inhibitor flavopiridol, transcriptome analyses of FIT-039-treated cells revealed that FIT-039 specifically inhibited CDK9. Given at concentrations above the inhibitory concentration, FIT-039 did not have a cytotoxic effect on mammalian cells. Importantly, administration of FIT-039 ameliorated the severity of skin lesion formation in mice infected with an acyclovir-resistant HSV-1, without noticeable adverse effects. Together, these data indicate that FIT-039 has potential as an antiviral agent for clinical therapeutics.

Rewiring of cellular membrane homeostasis by picornaviruses

Viruses are obligatory intracellular parasites and utilize host elements to support key viral processes, including penetration of the plasma membrane, initiation of infection, replication, and suppression of the host's antiviral defenses. In this review, we focus on picornaviruses, a family of positive-strand RNA viruses, and discuss the mechanisms by which these viruses hijack the cellular machinery to form and operate membranous replication complexes. Studies aimed at revealing factors required for the establishment of viral replication structures identified several cellular-membrane-remodeling proteins and led to the development of models in which the virus used a preexisting cellular-membrane-shaping pathway "as is" for generating its replication organelles. However, as more data accumulate, this view is being increasingly questioned, and it is becoming clearer that viruses may utilize cellular factors in ways that are distinct from the normal functions of these proteins in uninfected cells. In addition, the proteincentric view is being supplemented by important new studies showing a previously unappreciated deep remodeling of lipid homeostasis, including extreme changes to phospholipid biosynthesis and cholesterol trafficking. The data on viral modifications of lipid biosynthetic pathways are still rudimentary, but it appears once again that the viruses may rewire existing pathways to generate novel functions. Despite remarkable progress, our understanding of how a handful of viral proteins can completely overrun the multilayered, complex mechanisms that control the membrane organization of a eukaryotic cell remains very limited.

Recent developments in antiviral agents against enterovirus 71 infection

Enterovirus 71 (EV-71) is the main etiological agent of hand, foot and mouth disease (HFMD). Recent EV-71 outbreaks in Asia-Pacific were not limited to mild HFMD, but were associated with severe neurological complications such as aseptic meningitis and brainstem encephalitis, which may lead to cardiopulmonary failure and death. The absence of licensed therapeutics for clinical use has intensified research into anti-EV-71 development. This review highlights the potential antiviral agents targeting EV-71 attachment, entry, uncoating, translation, polyprotein processing, virus-induced formation of membranous RNA replication complexes, and RNA-dependent RNA polymerase. The strategies for antiviral development include target-based synthetic compounds, anti-rhinovirus and poliovirus libraries screening, and natural compound libraries screening. Growing knowledge of the EV-71 life cycle will lead to successful development of antivirals. The continued effort to develop antiviral agents for treatment is crucial in the absence of a vaccine. The coupling of antivirals with an effective vaccine will accelerate eradication of the disease.

A 3C(pro)-dependent bioluminescence imaging assay for in vivo evaluation of anti-enterovirus 71 agents

Enterovirus 71 (EV71), a member of Picornaviridae, is one of the major pathogens of human hand, foot and mouth disease. EV71 mainly infects children and causes severe neurological complications and even death. The pathogenesis of EV71 infection is largely unknown, and no clinically approved vaccine or effective treatment is available to date. Here we described a novel bioluminescence imaging approach for EV71 detection. In this approach, a plasmid-based reporter was constructed to express the fusion protein AmN(Q/G)BC, a split firefly luciferase mutant, which can be specifically cleaved by EV71 protease 3C(pro). Upon cleavage, the splitting fusion protein restores luciferase activity. Our test confirmed that AmN(Q/G)BC was specifically cleaved by 3C(pro) and EV71 and restored the luciferase activity to a degree that corresponds to the 3C(pro) and virus doses in cells and mice. The anti-EV71 effect of GW5074 and U0126, two mitogen-activated protein kinase (MAPK) inhibitors, was evaluated using this approach to validate its application of screening anti-EV71 agents. We found that the AmN(Q/G)BC reporter efficiently monitored the inhibitory effect of GW5074 and U0126 on EV71 infection under in vitro and in vivo conditions. The data from AmN(Q/G)BC reporter were consistent with Western blotting and histopathology examination. Taken together, this real-time imaging approach can quantitatively monitor the efficacy of anti-EV71 agents and is valuable for anti-EV71 drug screening and evaluation, especially, under in vivo conditions.

Fitness and virulence of a coxsackievirus mutant that can circumnavigate the need for phosphatidylinositol 4-kinase class III beta

Coxsackieviruses require phosphatidylinositol-4-kinase IIIβ (PI4KIIIβ) for replication but can bypass this need by an H57Y mutation in protein 3A (3A-H57Y). We show that mutant coxsackievirus is not outcompeted by wild-type virus during 10 passages in vitro. In mice, the mutant virus proved as virulent as wild-type virus, even when mice were treated with a PI4KIIIβ inhibitor. Our data suggest that upon emergence, the 3A-H57Y mutant has the fitness to establish a resistant population with a virulence similar to that of wild-type virus.

Phosphatidylinositol 4-kinase III beta is essential for replication of human rhinovirus and its inhibition causes a lethal phenotype in vivo

Human rhinovirus (HRV) is the predominant cause of the common cold, but more importantly, infection may have serious repercussions in asthmatics and chronic obstructive pulmonary disorder (COPD) patients. A cell-based antiviral screen against HRV was performed with a subset of our proprietary compound collection, and an aminothiazole series with pan-HRV species and enteroviral activity was identified. The series was found to act at the level of replication in the HRV infectious cycle. In vitro selection and sequencing of aminothiazole series-resistant HRV variants revealed a single-nucleotide mutation leading to the amino acid change I42V in the essential HRV 3A protein. This same mutation has been previously implicated in resistance to enviroxime, a former clinical-stage antipicornavirus agent. Enviroxime-like compounds have recently been shown to target the lipid kinase phosphatidylinositol 4-kinase III beta (PI4KIIIβ). A good correlation between PI4KIIIβ activity and HRV antiviral potency was found when analyzing the data over 80 compounds of the aminothiazole series, covering a 750-fold potency range. The mechanism of action through PI4KIIIβ inhibition was further demonstrated by small interfering RNA (siRNA) knockdown of PI4KB, which reduced HRV replication and also increased the potency of the PI4KIIIβ inhibitors. Inhibitors from two different structural classes with promising pharmacokinetic profiles and with very good selectivity for PI4KIIIβ were used to dissociate compound-related toxicity from target-related toxicity. Mortality was seen in all dosing groups of mice treated with either compound, therefore suggesting that short-term inhibition of PI4KIIIβ is deleterious.

Oxysterol-binding protein family I is the target of minor enviroxime-like compounds

Enviroxime is an antipicornavirus compound that targets host phosphatidylinositol 4-kinase III beta (PI4KB) activity for its antipicornavirus activity. To date, several antipoliovirus (PV) compounds similar to enviroxime that are associated with a common resistance mutation in viral protein 3A (a G5318A [3A-Ala70Thr] mutation in PV) have been identified. Most of these compounds have a direct inhibitory effect on PI4KB activity, as well as enviroxime (designated major enviroxime-like compounds). However, one of the compounds, AN-12-H5, showed no inhibitory effect on PI4KB and was considered to belong to another group of enviroxime-like compounds (designated minor enviroxime-like compounds). In the present study, we performed a small interfering RNA (siRNA) sensitization assay targeting PI4KB-related genes and identified oxysterol-binding protein (OSBP) as a target of minor enviroxime-like compounds. Knockdown of OSBP and OSBP2 increased the anti-PV activities of AN-12-H5 and a newly identified minor enviroxime-like compound, T-00127-HEV2, and also to T-00127-HEV1 to a minor extent, in the cells. A ligand of OSBP, 25-hydroxycholesterol (25-HC), acted as a minor enviroxime-like compound. Minor enviroxime-like compounds induced relocalization of OSBP to the Golgi apparatus in cells. Treatment of the cells with major or minor enviroxime-like compounds suppressed the expression of genes (HMGCS1 and SQLE) in the SREBP/SCAP regulatory pathway and diminished endogenous phosphatidylinositol 4-phosphate (PI4P) at the Golgi apparatus. Our results suggested that minor enviroxime-like compounds are phenotypically identical to 25-HC and that major and minor enviroxime-like compounds suppress the production and/or accumulation of PI4P in PV-infected cells by targeting PI4KB and OSBP family I activities, respectively.

[Exploration for anti-enterovirus compounds and analysis on the mechanism of its inhibitory effect on virus infection]

Poliovirus (PV) is a small non-enveloped virus belonging to the family Picornaviridae, and is the causative agent of poliomyelitis. With established vaccines, the global eradication program for poliomyelitis is ongoing by the World Health Organization since 1988. In the eradication program, antivirals are anticipated to have some roles in the endgame and post-eradication era of PV. During our search for potent anti-PV compounds, we identified candidate compounds that are associated with a common resistance mutation in viral protein 3A similar to enviroxime (designated as enviroxime-like compounds). Recently, PIK93, an inhibitor of host phosphatidylinositol 4-kinase III beta (PI4KB), was identified as a potent anti-enterovirus compound (Hsu et al., Cell 141:799-811). We found that PIK93 is an enviroxime-like compound, and showed that T-00127-HEV1, which is a novel enviroxime-like compound identified in high-throughput screening, is a specific PI4KB inhibitor. We also showed that PI4KB is an enterovirus-specific host factor required for its viral RNA replication. Analysis of anti-enterovirus compounds would unravel novel host factors that could serve as promising antiviral targets of prophylaxis and therapy of the infection.

Antiviral drug discovery for the treatment of enterovirus 71 infections

Enterovirus 71 (EV71) is a small, positive-sense, single-stranded RNA virus in the genus Enterovirus, family Picornavirus. It causes hand, foot and mouth disease in infants and children, which in a small percentage of cases progresses to central nervous system infection, ranging from aseptic meningitis to fatal encephalitis. Sporadic cases of EV71 infection occur throughout the world, but large epidemics have occurred recently in Southeast Asia and China. There are currently no approved vaccines or antiviral therapies for the prevention or treatment of EV71 infection. This paper reviews efforts to develop antiviral therapies against EV71.

The Pathogenesis and Prevention of Encephalitis due to Human Enterovirus 71

Human enterovirus 71 (HEV71) has emerged as a major cause of viral encephalitis in Southeast Asia, with increased epidemic activity observed since 1997. This is reflected in a large increase in scientific publications relating directly to HEV71. New research is elucidating details of the viral life cycle, confirming similarities between HEV71 and other enteroviruses. Scavenger receptor B2 (SCARB2) is a receptor for HEV71, although other receptors are likely to be identified. Currently, the only strategies to prevent HEV71-associated disease are early diagnosis and aggressive supportive management of identified cases. As more information emerges regarding the molecular processes of HEV71 infection, further advances may lead to the development of effective antiviral treatments and ultimately a vaccine-protection strategy. The protective efficacies of several inactivated HEV71 vaccines have been confirmed in animal models, suggesting that an effective vaccine may become available in the next decade.

Coxsackievirus mutants that can bypass host factor PI4KIIIβ and the need for high levels of PI4P lipids for replication

RNA viruses can rapidly mutate and acquire resistance to drugs that directly target viral enzymes, which poses serious problems in a clinical context. Therefore, there is a growing interest in the development of antiviral drugs that target host factors critical for viral replication, since they are unlikely to mutate in response to therapy. We recently demonstrated that phosphatidylinositol-4-kinase IIIβ (PI4KIIIβ) and its product phosphatidylinositol-4-phosphate (PI4P) are essential for replication of enteroviruses, a group of medically important RNA viruses including poliovirus (PV), coxsackievirus, rhinovirus, and enterovirus 71. Here, we show that enviroxime and GW5074 decreased PI4P levels at the Golgi complex by directly inhibiting PI4KIIIβ. Coxsackievirus mutants resistant to these inhibitors harbor single point mutations in the non-structural protein 3A. These 3A mutations did not confer compound-resistance by restoring the activity of PI4KIIIβ in the presence of the compounds. Instead, replication of the mutant viruses no longer depended on PI4KIIIβ, since their replication was insensitive to siRNA-mediated depletion of PI4KIIIβ. The mutant viruses also did not rely on other isoforms of PI4K. Consistently, no high level of PI4P could be detected at the replication sites induced by the mutant viruses in the presence of the compounds. Collectively, these findings indicate that through specific single point mutations in 3A, CVB3 can bypass an essential host factor and lipid for its propagation, which is a new example of RNA viruses acquiring resistance against antiviral compounds, even when they directly target host factors.

Valosin-containing protein (VCP/p97) is required for poliovirus replication and is involved in cellular protein secretion pathway in poliovirus infection

Poliovirus (PV) modifies membrane-trafficking machinery in host cells for its viral RNA replication. To date, ARF1, ACBD3, BIG1/BIG2, GBF1, RTN3, and PI4KB have been identified as host factors of enterovirus (EV), including PV, involved in membrane traffic. In this study, we performed small interfering RNA (siRNA) screening targeting membrane-trafficking genes for host factors required for PV replication. We identified valosin-containing protein (VCP/p97) as a host factor of PV replication required after viral protein synthesis, and its ATPase activity was essential for PV replication. VCP colocalized with viral proteins 2BC/2C and 3AB/3B in PV-infected cells and showed an interaction with 2BC and 3AB but not with 2C and 3A. Knockdown of VCP did not suppress the replication of coxsackievirus B3 or Aichi virus. A VCP-knockdown-resistant PV mutant had an A4881G (a mutation of E253G in 2C) mutation, which is known as a determinant of a secretion inhibition-negative phenotype. However, knockdown of VCP did not affect the inhibition of cellular protein secretion caused by overexpression of each individual viral protein. These results suggested that VCP is a host factor required for viral RNA replication of PV among membrane-trafficking proteins and provides a novel link between cellular protein secretion and viral RNA replication.

The PI3K/Akt pathway contributes to arenavirus budding

Several arenaviruses, chiefly Lassa virus (LASV), cause hemorrhagic fever (HF) disease in humans and pose a significant public health concern in regions where they are endemic. On the other hand, evidence indicates that the globally distributed prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) is a neglected human pathogen. The phosphatidylinositol 3-kinase (PI3K)/Akt pathway participates in many cellular processes, including cell survival and differentiation, and also has been shown to play important roles in different steps of the life cycles of a variety of viruses. Here we report that the inhibition of the PI3K/Akt pathway inhibited budding and to a lesser extent RNA synthesis, but not cell entry, of LCMV. Accordingly, BEZ-235, a PI3K inhibitor currently in cancer clinical trials, inhibited LCMV multiplication in cultured cells. These findings, together with those previously reported for Junin virus (JUNV), indicate that targeting the PI3K/Akt pathway could represent a novel antiviral strategy to combat human-pathogenic arenaviruses.

Combating enterovirus replication: state-of-the-art on antiviral research

Enteroviruses form an important genus within the large family of Picornaviridae. They are small, non-enveloped (+)RNA viruses, many of which are important pathogens in human and veterinary science. Despite their huge medical and socio-economical impact, there is still no approved antiviral therapy at hand for the treatment of these infections. Three capsid-targeting molecules (pleconaril, BTA-798 and V-073) are in clinical development. Pleconaril and BTA-798 are in phase II clinical trials for the treatment of enterovirus-induced sepsis syndrome and rhinovirus-induced aggravation of pre-existing asthma or COPD respectively. V-073 is in preclinical development for the treatment of poliovirus infections in the context of the worldwide polio eradication program. The capsid binding molecules have shown good in vitro potency against a number of enterovirus species, but lack activity against others. Another potential drawback of capsid inhibitors in the clinical setting could be the rapid emergence of drug resistance. It will therefore be important to develop inhibitors that affect other stages in the viral replication cycle. Several viral proteins, such as the viral 3C protease, the putative 2C helicase and the 3D RNA-dependent RNA polymerase may be/are excellent targets for inhibition of viral replication. Also host cell factors that are crucial in viral replication may be considered as potential targets for an antiviral approach. Unraveling these complex virus-host interactions will also provide better insights into the replication of enteroviruses. This review aims to summarize and discuss known inhibitors and potential viral and cellular targets for antiviral therapy against enteroviruses.

Phosphatidylinositol 4-kinase III beta is a target of enviroxime-like compounds for antipoliovirus activity

Enviroxime is an antienterovirus compound that targets viral protein 3A and/or 3AB and suppresses a step in enterovirus replication by unknown mechanism. To date, four antienterovirus compounds, i.e., GW5074, Flt3 inhibitor II, TTP-8307, and AN-12-H5, are known to have similar mutations in the 3A protein-encoding region causing resistance to enviroxime (a G5318A [3A-Ala70Thr] mutation in poliovirus [PV]) and are considered enviroxime-like compounds. Recently, antienterovirus activity of a phosphatidylinositol 4-kinase III beta (PI4KB) inhibitor, PIK93, was reported, suggesting that PI4KB is an important host factor targetable by antienterovirus compounds (N. Y. Hsu et al., Cell 141:799-811, 2010). In this study, we analyzed the inhibitory effects of previously identified enviroxime-like compounds (GW5074 and AN-12-H5) and a newly identified antienterovirus compound, T-00127-HEV1, on phosphoinositide (PI) kinases. We found that T-00127-HEV1 inhibited PI4KB activity with a higher specificity for than other PI kinases, in contrast to GW5074, which had a broad specificity for PI kinases. In contrast, AN-12-H5 showed no inhibitory effect on PI4KB activity and only moderate inhibitory effects on PI 3-kinase activity. Small interfering RNA (siRNA) screening targeting PI kinases identified PI4KB is a target of GW5074 and T-00127-HEV1, but not of AN-12-H5, for anti-PV activity. Interestingly, T-00127-HEV1 and GW5074 did not inhibit hepatitis C virus (HCV) replication, in contrast to a strong inhibitory effect of AN-12-H5. These results suggested that PI4KB is an enterovirus-specific host factor required for the replication process and targeted by some enviroxime-like compounds (T-00127-HEV1 and GW5074) and that enviroxime-like compounds may have targets other than PI kinases for their antiviral effect.

Developments towards antiviral therapies against enterovirus 71

Enterovirus 71 (EV71) has emerged as a clinically important neurotropic virus that can cause acute flaccid paralysis and encephalitis, leading to cardiopulmonary failure and death. Recurring outbreaks of EV71 have been reported in several countries. The current lack of approved anti-EV71 therapy has prompted intense research into antiviral development. Several strategies--ranging from target-based chemical design to compound library screenings--have been employed, while others revisited compound series generated from antiviral developments against poliovirus and human rhinoviruses. These efforts have given rise to a diversity of antiviral candidates that include small molecules and non-conventional nucleic-acid-based strategies. This review aims to highlight candidates with potential for further clinical development based on their putative modes of action.

Recent advances in research on human enterovirus 71

Over the past decade, Human enterovirus (HEV)71 has emerged as a highly significant cause of viral encephalitis in the south-east Asian region. A pattern of increased epidemic activity has been observable since 1997, the cause of which is unclear. Ongoing investigations into the molecular basis of HEV71 infection and virulence, in particular viral translation and replication, have confirmed similarities between HEV71 and other enteroviruses, including the prototype species Poliovirus, but more work is required in this field. Although several putative receptors for HEV71 have been identified, it remains likely that other, as yet unidentified, receptors exist. Work in several established animal models for HEV71 infection has confirmed the protective efficacy of several inactivated vaccines. As more information emerges regarding the molecular processes involved in HEV71 infection, further advances may lead to the development of more effective antiviral treatments and, ultimately, a vaccine protection strategy.