Identification of a broad-spectrum arenavirus entry inhibitor

Simultaneous screening for selective SARS-CoV-2, Lassa, and Machupo virus entry inhibitors

Emerging highly pathogenic viruses can pose profound impacts on global health, the economy, and society. To meet that challenge, the National Institute of Allergy and Infectious Diseases (NIAID) established nine Antiviral Drug Discovery (AViDD) centers for early-stage identification and validation of novel antiviral drug candidates against viruses with pandemic potential. As part of this initiative, we established paired entry assays that simultaneously screen for inhibitors specifically targeting SARS-CoV-2 (SARS2), Lassa virus (LASV) and Machupo virus (MACV) entry. To do so we employed a dual pseudotyped virus (PV) infection system allowing us to screen ∼650,000 compounds efficiently and cost-effectively. Adaptation of these paired assays into 1536 well-plate format for ultra-high throughput screening (uHTS) resulted in the largest screening ever conducted in our facility, with over 2.4 million wells completed. The paired infection system allowed us to detect two PV infections simultaneously: LASV + MACV, MACV + SARS2, and SARS2 + LASV. Each PV contains a different luciferase reporter gene which enabled us to measure the infection of each PV exclusively, albeit in the same well. Each PV was screened at least twice utilizing different reporters, which allowed us to select the inhibitors specific to a particular PV and to exclude those that hit off targets, including cellular components or the reporter proteins. All assays were robust with an average Z' value ranging from 0.5 to 0.8. The primary screening of ∼650,000 compounds resulted in 1812, 1506, and 2586 unique hits for LASV, MACV, and SARS2, respectively. The confirmation screening narrowed this list further to 60, 40, and 90 compounds that are unique to LASV, MACV, and SARS2, respectively. Of these compounds, 8, 35, and 50 compounds showed IC50 value < 10 μM, some of which have much greater potency and excellent antiviral activity profiles specific to LASV, MACV, and SARS2, and none are cytotoxic. These selected compounds are currently being studied for their mechanism of action and to improve their specificity and potency through chemical modification.

Hexestrol, an estrogen receptor agonist, inhibits Lassa virus entry

Lassa virus (LASV) is the causative agent of human Lassa fever which in severe cases manifests as hemorrhagic fever leading to thousands of deaths annually. However, no approved vaccines or antiviral drugs are currently available. Recently, we screened approximately 2,500 compounds using a recombinant vesicular stomatitis virus (VSV) expressing LASV glycoprotein GP (VSV-LASVGP) and identified a P-glycoprotein inhibitor as a potential LASV entry inhibitor. Here, we show that another identified candidate, hexestrol (HES), an estrogen receptor agonist, is also a LASV entry inhibitor. HES inhibited VSV-LASVGP replication with a 50% inhibitory concentration (IC50) of 0.63 µM. Importantly, HES also inhibited authentic LASV replication with IC50 values of 0.31 µM-0.61 µM. Time-of-addition and cell-based membrane fusion assays suggested that HES inhibits the membrane fusion step during virus entry. Alternative estrogen receptor agonists did not inhibit VSV-LASVGP replication, suggesting that the estrogen receptor itself is unlikely to be involved in the antiviral activity of HES. Generation of a HES-resistant mutant revealed that the phenylalanine at amino acid position 446 (F446) of LASVGP, which is located in the transmembrane region, conferred resistance to HES. Although mutation of F446 enhanced the membrane fusion activity of LASVGP, it exhibited reduced VSV-LASVGP replication, most likely due to the instability of the pre-fusion state of LASVGP. Collectively, our results demonstrated that HES is a promising anti-LASV drug that acts by inhibiting the membrane fusion step of LASV entry. This study also highlights the importance of the LASVGP transmembrane region as a target for anti-LASV drugs.IMPORTANCELassa virus (LASV), the causative agent of Lassa fever, is the most devastating mammarenavirus with respect to its impact on public health in West Africa. However, no approved antiviral drugs or vaccines are currently available. Here, we identified hexestrol (HES), an estrogen receptor agonist, as the potential antiviral candidate drug. We showed that the estrogen receptor itself is not involved in the antiviral activity. HES directly bound to LASVGP and blocked membrane fusion, thereby inhibiting LASV infection. Through the generation of a HES-resistant virus, we found that phenylalanine at position 446 (F446) within the LASVGP transmembrane region plays a crucial role in the antiviral activity of HES. The mutation at F446 caused reduced virus replication, likely due to the instability of the pre-fusion state of LASVGP. These findings highlight the potential of HES as a promising candidate for the development of antiviral compounds targeting LASV.

Treatment of highly virulent mammarenavirus infections-status quo and future directions

INTRODUCTION

Mammarenaviruses are negative-sense bisegmented enveloped RNA viruses that are endemic in Africa, the Americas, and Europe. Several are highly virulent, causing acute human diseases associated with high case fatality rates, and are considered to be significant with respect to public health impact or bioterrorism threat.

AREAS COVERED

This review summarizes the status quo of treatment development, starting with drugs that are in advanced stages of evaluation in early clinical trials, followed by promising candidate medical countermeasures emerging from bench analyses and investigational animal research.

EXPERT OPINION

Specific therapeutic treatments for diseases caused by mammarenaviruses remain limited to the off-label use of ribavirin and transfusion of convalescent sera. Progress in identifying novel candidate medical countermeasures against mammarenavirus infection has been slow in part because of the biosafety and biosecurity requirements. However, novel methodologies and tools have enabled increasingly efficient high-throughput molecular screens of regulatory-agency-approved small-molecule drugs and led to the identification of several compounds that could be repurposed for the treatment of infection with several mammarenaviruses. Unfortunately, most of them have not yet been evaluated in vivo. The most promising treatment under development is a monoclonal antibody cocktail that is protective against multiple lineages of the Lassa virus in nonhuman primate disease models.

Restoration of virulence in the attenuated Candid#1 vaccine virus requires reversion at both positions 168 and 427 in the envelope glycoprotein GPC

Live-attenuated virus vaccines provide long-lived protection against viral disease but carry inherent risks of residual pathogenicity and genetic reversion. The live-attenuated Candid#1 vaccine was developed to protect Argentines against lethal infection by the Argentine hemorrhagic fever arenavirus, Junín virus. Despite its safety and efficacy in Phase III clinical study, the vaccine is not licensed in the US, in part due to concerns regarding the genetic stability of attenuation. Previous studies had identified a single F427I mutation in the transmembrane domain of the Candid#1 envelope glycoprotein GPC as the key determinant of attenuation, as well as the propensity of this mutation to revert upon passage in cell culture and neonatal mice. To ascertain the consequences of this reversion event, we introduced the I427F mutation into recombinant Candid#1 (I427F rCan) and investigated the effects in two validated small-animal models: in mice expressing the essential virus receptor (human transferrin receptor 1; huTfR1) and in the conventional guinea pig model. We report that I427F rCan displays only modest virulence in huTfR1 mice and appears attenuated in guinea pigs. Reversion at another attenuating locus in Candid#1 GPC (T168A) was also examined, and a similar pattern was observed. By contrast, virus bearing both revertant mutations (A168T+I427F rCan) approached the lethal virulence of the pathogenic Romero strain in huTfR1 mice. Virulence was less extreme in guinea pigs. Our findings suggest that genetic stabilization at both positions is required to minimize the likelihood of reversion to virulence in a second-generation Candid#1 vaccine.IMPORTANCELive-attenuated virus vaccines, such as measles/mumps/rubella and oral poliovirus, provide robust protection against disease but carry with them the risk of genetic reversion to the virulent form. Here, we analyze the genetics of reversion in the live-attenuated Candid#1 vaccine that is used to protect against Argentine hemorrhagic fever, an often-lethal disease caused by the Junín arenavirus. In two validated small-animal models, we find that restoration of virulence in recombinant Candid#1 viruses requires back-mutation at two positions specific to the Candid#1 envelope glycoprotein GPC, at positions 168 and 427. Viruses bearing only a single change showed only modest virulence. We discuss strategies to genetically harden Candid#1 GPC against these two reversion events in order to develop a safer second-generation Candid#1 vaccine virus.

Entry inhibitors as arenavirus antivirals

Arenaviruses belonging to the Arenaviridae family, genus mammarenavirus, are enveloped, single-stranded RNA viruses primarily found in rodent species, that cause severe hemorrhagic fever in humans. With high mortality rates and limited treatment options, the search for effective antivirals is imperative. Current treatments, notably ribavirin and other nucleoside inhibitors, are only partially effective and have significant side effects. The high lethality and lack of treatment, coupled with the absence of vaccines for all but Junín virus, has led to the classification of these viruses as Category A pathogens by the Centers for Disease Control (CDC). This review focuses on entry inhibitors as potential therapeutics against mammarenaviruses, which include both New World and Old World arenaviruses. Various entry inhibition strategies, including small molecule inhibitors and neutralizing antibodies, have been explored through high throughput screening, genome-wide studies, and drug repurposing. Notable progress has been made in identifying molecules that target receptor binding, internalization, or fusion steps. Despite promising preclinical results, the translation of entry inhibitors to approved human therapeutics has faced challenges. Many have only been tested in in vitro or animal models, and a number of candidates showed efficacy only against specific arenaviruses, limiting their broader applicability. The widespread existence of arenaviruses in various rodent species and their potential for their zoonotic transmission also underscores the need for rapid development and deployment of successful pan-arenavirus therapeutics. The diverse pool of candidate molecules in the pipeline provides hope for the eventual discovery of a broadly effective arenavirus antiviral.

Fusogenic structural changes in arenavirus glycoproteins are associated with viroporin activity

Many enveloped viruses enter host cells by fusing with acidic endosomes. The fusion activity of multiple viral envelope glycoproteins does not generally affect viral membrane permeability. However, fusion induced by the Lassa virus (LASV) glycoprotein complex (GPc) is always preceded by an increase in viral membrane permeability and the ensuing acidification of the virion interior. Here, systematic investigation of this LASV fusion phenotype using single pseudovirus tracking in live cells reveals that the change in membrane barrier function is associated with the fusogenic conformational reorganization of GPc. We show that a small-molecule fusion inhibitor or mutations that impair viral fusion by interfering with GPc refolding into the post-fusion structure prevent the increase in membrane permeability. We find that the increase in virion membrane permeability occurs early during endosomal maturation and is facilitated by virus-cell contact. This increase is observed using diverse arenavirus glycoproteins, whether presented on lentivirus-based pseudoviruses or arenavirus-like particles, and in multiple different cell types. Collectively, these results suggest that conformational changes in GPc triggered by low pH and cell factor binding are responsible for virion membrane permeabilization and acidification of the virion core prior to fusion. We propose that this viroporin-like activity may augment viral fusion and/or post-fusion steps of infection, including ribonucleoprotein release into the cytoplasm.

Deletion of the first glycosylation site promotes Lassa virus glycoprotein-mediated membrane fusion

The Lassa virus is endemic in West Africa and causes severe hemorrhagic Lassa fever in humans. The glycoprotein complex (GPC) of LASV is highly glycosylation-modified, with 11 N-glycosylation sites. All 11 N-linked glycan chains play critical roles in GPC cleavage, folding, receptor binding, membrane fusion, and immune evasion. In this study, we focused on the first glycosylation site because its deletion mutant (N79Q) results in an unexpected enhanced membrane fusion, whereas it exerts little effect on GPC expression, cleavage, and receptor binding. Meanwhile, the pseudotype virus bearing GPC_N79Q was more sensitive to the neutralizing antibody 37.7H and was attenuated in virulence. Exploring the biological functions of the key glycosylation site on LASV GPC will help elucidate the mechanism of LASV infection and provide strategies for the development of attenuated vaccines against LASV infection.

A pseudovirus system enables deep mutational scanning of the full SARS-CoV-2 spike

A major challenge in understanding SARS-CoV-2 evolution is interpreting the antigenic and functional effects of emerging mutations in the viral spike protein. Here, we describe a deep mutational scanning platform based on non-replicative pseudotyped lentiviruses that directly quantifies how large numbers of spike mutations impact antibody neutralization and pseudovirus infection. We apply this platform to produce libraries of the Omicron BA.1 and Delta spikes. These libraries each contain ∼7,000 distinct amino acid mutations in the context of up to ∼135,000 unique mutation combinations. We use these libraries to map escape mutations from neutralizing antibodies targeting the receptor-binding domain, N-terminal domain, and S2 subunit of spike. Overall, this work establishes a high-throughput and safe approach to measure how ∼105 combinations of mutations affect antibody neutralization and spike-mediated infection. Notably, the platform described here can be extended to the entry proteins of many other viruses.

Design, Synthesis, and Biological Evaluation of Benzimidazole Derivatives as Potential Lassa Virus Inhibitors

The Lassa virus (LASV) causes Lassa fever, a highly infectious and lethal agent of acute viral hemorrhagic fever. At present, there are still no effective treatments available, creating an urgent need to develop novel therapeutics. Some benzimidazole compounds targeting the arenavirus envelope glycoprotein complex (GPC) are promising inhibitors of LASV. In this study, we synthesized two series of LASV inhibitors based on the benzimidazole structure. Lentiviral pseudotypes bearing the LASV GPC were established to identify virus entry inhibitors. Surface plasmon resonance (SPR) was further used to verify the binding activities of the potential compounds. Compounds 7d-Z, 7h-Z, 13c, 13d, and 13f showed relatively excellent antiviral activities with IC₅₀ values ranging from 7.58 to 15.46 nM and their SI values above 1251. These five representative compounds exhibited stronger binding affinity with low equilibrium dissociation constants (K_D < 8.25 × 10^-7 M) in SPR study. The compound 7h-Z displayed the most potent antiviral activity (IC₅₀ = 7.58 nM) with a relatively high SI value (2496), which could be further studied as a lead compound. The structure-activity relationship indicated that the compounds with lipophilic and spatially larger substituents might possess higher antiviral activity and a much larger safety margin. This study will provide some good guidance for the development of highly active compounds with a novel skeleton against LASV.

Lassa fever - the road ahead

Lassa virus (LASV) is endemic in the rodent populations of Sierra Leone, Nigeria and other countries in West Africa. Spillover to humans occurs frequently and results in Lassa fever, a viral haemorrhagic fever (VHF) associated with a high case fatality rate. Despite advances, fundamental gaps in knowledge of the immunology, epidemiology, ecology and pathogenesis of Lassa fever persist. More frequent outbreaks, the potential for further geographic expansion of Mastomys natalensis and other rodent reservoirs, the ease of procurement and possible use and weaponization of LASV, the frequent importation of LASV to North America and Europe, and the emergence of novel LASV strains in densely populated West Africa have driven new initiatives to develop countermeasures for LASV. Although promising candidates are being evaluated, as yet there are no approved vaccines or therapeutics for human use. This Review discusses the virology of LASV, the clinical course of Lassa fever and the progress towards developing medical countermeasures.

Pseudotyped Viruses for Mammarenavirus

Mammarenaviruses are classified into New World arenaviruses (NW) and Old World arenaviruses (OW). The OW arenaviruses include the first discovered mammarenavirus-lymphocytic choriomeningitis virus (LCMV) and the highly lethal Lassa virus (LASV). Mammarenaviruses are transmitted to human by rodents, resulting in severe acute infections and hemorrhagic fever. Pseudotyped viruses have been widely used as a tool in the study of mammarenaviruses. HIV-1, SIV, FIV-based lentiviral vectors, VSV-based vectors, MLV-based vectors, and reverse genetic approaches have been applied in the construction of pseudotyped mammarenaviruses. Pseudotyped mammarenaviruses are commonly used in receptor research, neutralizing antibody detection, inhibitor screening, viral virulence studies, functional analysis of N-linked glycans, and studies of viral infection, endocytosis, and fusion mechanisms.

Lassa antiviral LHF-535 protects guinea pigs from lethal challenge

LHF-535 is a small molecule antiviral currently in development for the treatment of Lassa fever, a zoonotic disease endemic in West Africa that generates significant morbidity and mortality. Current treatment options are inadequate, and there are no approved therapeutics or vaccines for Lassa fever. LHF-535 was evaluated in a lethal guinea pig model of Lassa pathogenesis, using once-daily administration of a fixed dose (50 mg/kg/day) initiating either 1 or 3 days after inoculation with a lethal dose of Lassa virus. LHF-535 reduced viremia and clinical signs and protected all animals from lethality. A subset of surviving animals was rechallenged four months later with a second lethal challenge of Lassa virus and were found to be protected from disease. LHF-535 pharmacokinetics at the protective dose in guinea pigs showed plasma concentrations well within the range observed in clinical trials in healthy volunteers, supporting the continued development of LHF-535 as a Lassa therapeutic.

Lassa Virus Countermeasures

Lassa Fever (LF) is a viral hemorrhagic fever endemic in West Africa. LF begins with flu-like symptoms that are difficult to distinguish from other common endemic diseases such as malaria, dengue, and yellow fever making it hard to diagnose clinically. Availability of a rapid diagnostic test and other serological and molecular assays facilitates accurate diagnosis of LF. Lassa virus therapeutics are currently in different stages of preclinical development. Arevirumab, a cocktail of monoclonal antibodies, demonstrates a great safety and efficacy profile in non-human primates. Major efforts have been made in the development of a Lassa virus vaccine. Two vaccine candidates, MeV-NP and pLASV-GPC are undergoing evaluation in phase I clinical trials.

A pseudovirus system enables deep mutational scanning of the full SARS-CoV-2 spike

A major challenge in understanding SARS-CoV-2 evolution is interpreting the antigenic and functional effects of emerging mutations in the viral spike protein. Here we describe a new deep mutational scanning platform based on non-replicative pseudotyped lentiviruses that directly quantifies how large numbers of spike mutations impact antibody neutralization and pseudovirus infection. We demonstrate this new platform by making libraries of the Omicron BA.1 and Delta spikes. These libraries each contain ~7000 distinct amino-acid mutations in the context of up to ~135,000 unique mutation combinations. We use these libraries to map escape mutations from neutralizing antibodies targeting the receptor binding domain, N-terminal domain, and S2 subunit of spike. Overall, this work establishes a high-throughput and safe approach to measure how ~10 5 combinations of mutations affect antibody neutralization and spike-mediated infection. Notably, the platform described here can be extended to the entry proteins of many other viruses.

Human LAMP1 accelerates Lassa virus fusion and potently promotes fusion pore dilation upon forcing viral fusion with non-endosomal membrane

Lassa virus (LASV) cell entry is mediated by the interaction of the virus glycoprotein complex (GPC) with alpha-dystroglycan at the cell surface followed by binding to LAMP1 in late endosomes. However, LAMP1 is not absolutely required for LASV fusion, as this virus can infect LAMP1-deficient cells. Here, we used LASV GPC pseudoviruses, LASV virus-like particles and recombinant lymphocytic choriomeningitis virus expressing LASV GPC to investigate the role of human LAMP1 (hLAMP1) in LASV fusion with human and avian cells expressing a LAMP1 ortholog that does not support LASV entry. We employed a combination of single virus imaging and virus population-based fusion and infectivity assays to dissect the hLAMP1 requirement for initiation and completion of LASV fusion that culminates in the release of viral ribonucleoprotein into the cytoplasm. Unexpectedly, ectopic expression of hLAMP1 accelerated the kinetics of small fusion pore formation, but only modestly increased productive LASV fusion and infection of human and avian cells. To assess the effects of hLAMP1 in the absence of requisite endosomal host factors, we forced LASV fusion with the plasma membrane by applying low pH. Unlike the conventional LASV entry pathway, ectopic hLAMP1 expression dramatically promoted the initial and full dilation of pores formed through forced fusion at the plasma membrane. We further show that, while the soluble hLAMP1 ectodomain accelerates the kinetics of nascent pore formation, it fails to promote efficient pore dilation, suggesting the hLAMP1 transmembrane domain is involved in this late stage of LASV fusion. These findings reveal a previously unappreciated role of hLAMP1 in promoting dilation of LASV fusion pores, which is difficult to ascertain for endosomal fusion where several co-factors, such as bis(monoacylglycero)phosphate, likely regulate LASV entry.

Lassa virus (LASV) enters cells via fusion with acidic endosomes mediated by the viral glycoprotein complex (GPC) interaction with the intracellular receptor LAMP1. However, the requirement for LAMP1 is not absolute, as LASV can infect avian cells expressing a LAMP1 ortholog that does not interact with GPC. To delineate the role of LAMP1 in LASV entry, we developed assays to monitor the formation of nascent fusion pores, as well as their initial and complete dilation to sizes that allow productive infection of avian cells by LASV GPC pseudoviruses. This novel approach provided unprecedented details regarding the dynamics of LASV fusion pores and revealed that ectopic expression of human LAMP1 in avian cells leads to a marked acceleration of fusion but modestly increases the likelihood of complete pore dilation and infection. In contrast, human LAMP1 expression dramatically enhanced the propensity of nascent pores to fully enlarge when LASV fusion with the plasma membrane was forced by exposure to low pH. Thus, whereas the role of LAMP1 in LASV fusion is confounded by an interplay between multiple endosomal factors, the plasma membrane is a suitable target for mechanistic dissection of the roles of host factors in LASV entry.

The Virus–Host Interplay in Junín Mammarenavirus Infection

Junín virus (JUNV) belongs to the Arenaviridae family and is the causative agent of Argentine hemorrhagic fever (AHF), a severe human disease endemic to agricultural areas in Argentina. At this moment, there are no effective antiviral therapeutics to battle pathogenic arenaviruses. Cumulative reports from recent years have widely provided information on cellular factors playing key roles during JUNV infection. In this review, we summarize research on host molecular determinants that intervene in the different stages of the viral life cycle: viral entry, replication, assembly and budding. Alongside, we describe JUNV tight interplay with the innate immune system. We also review the development of different reverse genetics systems and their use as tools to study JUNV biology and its close teamwork with the host. Elucidating relevant interactions of the virus with the host cell machinery is highly necessary to better understand the mechanistic basis beyond virus multiplication, disease pathogenesis and viral subversion of the immune response. Altogether, this knowledge becomes essential for identifying potential targets for the rational design of novel antiviral treatments to combat JUNV as well as other pathogenic arenaviruses.

Screening Repurposed Antiviral Small Molecules as Antimycobacterial Compounds by a Lux-Based phoP Promoter-Reporter Platform

The emergence of multidrug-resistant strains and hyper-virulent strains of Mycobacterium tuberculosis are big therapeutic challenges for tuberculosis (TB) control. Repurposing bioactive small-molecule compounds has recently become a new therapeutic approach against TB. This study aimed to identify novel anti-TB agents from a library of small-molecule compounds via a rapid screening system. A total of 320 small-molecule compounds were used to screen for their ability to suppress the expression of a key virulence gene, phop, of the M. tuberculosis complex using luminescence (lux)-based promoter-reporter platforms. The minimum inhibitory and bactericidal concentrations on drug-resistant M. tuberculosis and cytotoxicity to human macrophages were determined. RNA sequencing (RNA-seq) was conducted to determine the drug mechanisms of the selected compounds as novel antibiotics or anti-virulent agents against the M. tuberculosis complex. The results showed that six compounds displayed bactericidal activity against M. bovis BCG, of which Ebselen demonstrated the lowest cytotoxicity to macrophages and was considered as a potential antibiotic for TB. Another ten compounds did not inhibit the in vitro growth of the M. tuberculosis complex and six of them downregulated the expression of phoP/R significantly. Of these, ST-193 and ST-193 (hydrochloride) showed low cytotoxicity and were suggested to be potential anti-virulence agents for M. tuberculosis.

Lassa virus glycoprotein complex review: insights into its unique fusion machinery

Lassa virus (LASV), an arenavirus endemic to West Africa, causes Lassa fever-a lethal hemorrhagic fever. Entry of LASV into the host cell is mediated by the glycoprotein complex (GPC), which is the only protein located on the viral surface and comprises three subunits: glycoprotein 1 (GP1), glycoprotein 2 (GP2), and a stable signal peptide (SSP). The LASV GPC is a class one viral fusion protein, akin to those found in viruses such as human immunodeficiency virus (HIV), influenza, Ebola virus (EBOV), and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). These viruses are enveloped and utilize membrane fusion to deliver their genetic material to the host cell. Like other class one fusion proteins, LASV-mediated membrane fusion occurs through an orchestrated sequence of conformational changes in its GPC. The receptor-binding subunit, GP1, first engages with a host cell receptor then undergoes a unique receptor switch upon delivery to the late endosome. The acidic pH and change in receptor result in the dissociation of GP1, exposing the fusion subunit, GP2, such that fusion can occur. These events ultimately lead to the formation of a fusion pore so that the LASV genetic material is released into the host cell. Interestingly, the mature GPC retains its SSP as a third subunit-a feature that is unique to arenaviruses. Additionally, the fusion domain contains two separate fusion peptides, instead of a standard singular fusion peptide. Here, we give a comprehensive review of the LASV GPC components and their unusual features.

Screening and Identification of Lujo Virus Entry Inhibitors From an Food and Drug Administration-Approved Drugs Library

Lujo virus (LUJV) belongs to the Old World (OW) genus Mammarenavirus (family Arenaviridae). It is categorized as a biosafety level (BSL) 4 agent. Currently, there are no U.S. Food and Drug Administration (FDA)-approved drugs or vaccines specifically for LUJV or other pathogenic OW mammarenaviruses. Here, a high-throughput screening of an FDA-approved drug library was conducted using pseudotype viruses bearing LUJV envelope glycoprotein (GPC) to identify inhibitors of LUJV entry. Three hit compounds, trametinib, manidipine, and lercanidipine, were identified as LUJV entry inhibitors in the micromolar range. Mechanistic studies revealed that trametinib inhibited LUJV GPC-mediated membrane fusion by targeting C410 [located in the transmembrane (TM) domain], while manidipine and lercanidipine inhibited LUJV entry by acting as calcium channel blockers. Meanwhile, all three hits extended their antiviral spectra to the entry of other pathogenic mammarenaviruses. Furthermore, all three could inhibit the authentic prototype mammarenavirus, lymphocytic choriomeningitis virus (LCMV), and could prevent infection at the micromolar level. This study shows that trametinib, manidipine, and lercanidipine are candidates for LUJV therapy and highlights the critical role of calcium in LUJV infection. The presented findings reinforce the notion that the key residue(s) located in the TM domain of GPC provide an entry-targeted platform for designing mammarenavirus inhibitors.

CP100356 Hydrochloride, a P-Glycoprotein Inhibitor, Inhibits Lassa Virus Entry: Implication of a Candidate Pan-Mammarenavirus Entry Inhibitor

Lassa virus (LASV)—a member of the family Arenaviridae—causes Lassa fever in humans and is endemic in West Africa. Currently, no approved drugs are available. We screened 2480 small compounds for their potential antiviral activity using pseudotyped vesicular stomatitis virus harboring the LASV glycoprotein (VSV-LASVGP) and a related prototypic arenavirus, lymphocytic choriomeningitis virus (LCMV). Follow-up studies confirmed that CP100356 hydrochloride (CP100356), a specific P-glycoprotein (P-gp) inhibitor, suppressed VSV-LASVGP, LCMV, and LASV infection with half maximal inhibitory concentrations of 0.52, 0.54, and 0.062 μM, respectively, without significant cytotoxicity. Although CP100356 did not block receptor binding at the cell surface, it inhibited low-pH-dependent membrane fusion mediated by arenavirus glycoproteins. P-gp downregulation did not cause a significant reduction in either VSV-LASVGP or LCMV infection, suggesting that P-gp itself is unlikely to be involved in arenavirus entry. Finally, our data also indicate that CP100356 inhibits the infection by other mammarenaviruses. Thus, our findings suggest that CP100356 can be considered as an effective virus entry inhibitor for LASV and other highly pathogenic mammarenaviruses.

Potent inhibition of arenavirus infection by a novel fusion inhibitor

Several arenaviruses, including Lassa and Lujo viruses in Africa and five New World arenavirus (NWA) species in the Americas, cause life-threatening viral hemorrhagic fevers. In the absence of licensed antiviral therapies, these viruses pose a significant public health risk. The envelope glycoprotein complex (GPC) mediates arenavirus entry through a pH-dependent fusion of the viral and host endosomal membranes. It thus is recognized as a viable target for small-molecule fusion inhibitors. Here, we report on the antiviral activity and pre-clinical development of the novel broad-spectrum arenavirus fusion inhibitors, ARN-75039 and ARN-75041. In Tacaribe virus (TCRV) pseudotyped and native virus assays, the ARN compounds were active in the low to sub-nanomolar range with selectivity indices exceeding 1000. Pharmacokinetic analysis of the orally administered compounds revealed an extended half-life in mice supporting once-daily dosing, and the compounds were well tolerated at the highest tested dose of 100 mg/kg. In a proof-of-concept prophylactic efficacy study, doses of 10 and 35 mg/kg of either compound dramatically improved survival outcome and potently inhibited TCRV replication in serum and various tissues. Additionally, in contrast to surviving mice that received ribavirin or placebo, animals treated with ARN-75039 or ARN-75041 were cured of TCRV infection. In a follow-up study with ARN-75039, impressive therapeutic efficacy was demonstrated under conditions where treatment was withheld until after the onset of disease. Taken together, the data strongly support the continued development of ARN-75039 as a candidate therapeutic for the treatment of severe arenaviral diseases.

Progress in Anti-Mammarenavirus Drug Development

Mammarenaviruses are prevalent pathogens distributed worldwide, and several strains cause severe cases of human infections with high morbidity and significant mortality. Currently, there is no FDA-approved antiviral drugs and vaccines against mammarenavirus and the potential treatment option is limited to an off-label use of ribavirin that shows only partial protective effect and associates with side effects. For the past few decades, extensive research has reported potential anti-mammarenaviral drugs and their mechanisms of action in host as well as vaccine candidates. This review describes current knowledge about mammarenavirus virology, progress of antiviral drug development, and technical strategies of drug screening.

High-Throughput Screening of an FDA-Approved Drug Library Identifies Inhibitors against Arenaviruses and SARS-CoV-2

Arenaviruses are a large family of enveloped negative-strand RNA viruses that include several causative agents of severe hemorrhagic fevers. Currently, there are no FDA-licensed drugs to treat arenavirus infection except for the off-labeled use of ribavirin. Here, we performed antiviral drug screening against the Old World arenavirus lymphocytic choriomeningitis virus (LCMV) using an FDA-approved drug library. Five drug candidates were identified, including mycophenolic acid, benidipine hydrochloride, clofazimine, dabrafenib, and apatinib, for having strong anti-LCMV effects. Further analysis indicated that benidipine hydrochloride inhibited LCMV membrane fusion, and an adaptive mutation on the LCMV glycoprotein D414 site was found to antagonize the anti-LCMV activity of benidipine hydrochloride. Mycophenolic acid inhibited LCMV replication by depleting GTP production. We also found mycophenolic acid, clofazimine, dabrafenib, and apatinib can inhibit the newly emerged severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Owing to their FDA-approved status, these drug candidates can potentially be used rapidly in the clinical treatment of arenavirus and SARS-CoV-2 infection.

Discovery of a novel highly potent broad-spectrum heterocyclic chemical series of arenavirus cell entry inhibitors

We identified and explored the structure-activity relationship (SAR) of a novel heterocyclic chemical series of arenavirus cell entry inhibitors. Optimized lead compounds, including diphenyl-substituted imidazo[1,2-a]pyridines, benzimidazoles, and benzotriazoles exhibited low to sub-nanomolar potency against both pseudotyped and infectious Old and New World arenaviruses, attractive metabolic stability in human and most nonhuman liver microsomes as well as a lack of hERG K + channel or CYP enzyme inhibition. Moreover, the straightforward synthesis of several lead compounds (e.g., the simple high yield 3-step synthesis of imidazo[1,2-a]pyridine 37) could provide a cost-effective broad-spectrum arenavirus therapeutic that may help to minimize the cost-prohibitive burdens associated with treatments for emerging viruses in economically challenged geographical settings.

Lassa Virus Treatment Options

Lassa fever causes an approximate 5000 to 10,000 deaths annually in West Africa and cases have been imported into Europe and the Americas, challenging public health. Although Lassa virus was first described over 5 decades ago in 1969, no treatments or vaccines have been approved to treat or prevent infection. In this review, we discuss current therapeutics in the development pipeline for the treatment of Lassa fever, focusing on those that have been evaluated in humans or animal models. Several treatments, including the antiviral favipiravir and a human monoclonal antibody cocktail, have shown efficacy in preclinical rodent and non-human primate animal models and have potential for use in clinical settings. Movement of the promising preclinical treatment options for Lassa fever into clinical trials is critical to continue addressing this neglected tropical disease.

Screening of Botanical Drugs against Lassa Virus Entry

Lassa virus (LASV) belongs to the Old World Mammarenavirus genus (family Arenaviridae). At present, there are no approved drugs or vaccines specific for LASV. In this study, high-throughput screening of a botanical drug library was performed against LASV entry using a pseudotype virus bearing the LASV envelope glycoprotein complex (GPC). Two hit compounds, bergamottin and casticin, were identified as micromolar range inhibitors of LASV entry. A mechanistic study revealed that casticin inhibited LASV entry by blocking low pH-induced membrane fusion. Analysis of adaptive mutants demonstrated that the F446L mutation, located in the transmembrane domain of GP2, conferred resistance to casticin. Furthermore, casticin antiviral activity extends to the New World (NW) pathogenic mammarenaviruses, and mutation of the conserved F446 also conferred resistance to casticin in these viruses. Unlike casticin, bergamottin showed little effect on LASV GPC-mediated membrane fusion, instead inhibiting LASV entry by blocking endocytic trafficking. Notably, both compounds showed inhibitory effects on authentic lymphocytic choriomeningitis virus. Our study shows that both casticin and bergamottin are candidates for LASV therapy and that the conserved F446 in LASV GPC is important in drug resistance in mammarenaviruses.IMPORTANCE: Currently, there is no approved therapy to treat Lassa fever (LASF). Our goal was to identify potential candidate molecules for LASF therapy. Herein, we screened a botanical drug library and identified two compounds, casticin and bergamottin, that inhibited LASV entry via different mechanisms.

Characterizing the Lassa Virus Envelope Glycoprotein Membrane Proximal External Region for Its Role in Fusogenicity

The membrane-proximal external region (MPER) of Lassa virus (LASV) glycoprotein complex (GPC) is critical in modulating its functionality. Till now, the high-resolution structure of the intact GPC, including MPER is not available. In this study, we used alanine substitution to scan all 16 residues located in LASV MPER. Western blotting and quantification fusion assay showed that the residues located at the C terminus of the HR2 (M414 and L415) and N terminus of the MPER (K417 and Y419) are critical for GPC-mediated membrane fusion function. Furthermore, cell surface biotinylation experiments revealed that M414A, K417A and Y419A expressed similar levels as WT, whereas L415A mutant led to a reduction of mature GPC on the cell surface. Moreover, substitution of these residues with the similar residue such as M414L, L415I, K417R and Y419F would partly compensate the loss of the fusion activity caused by the alanine mutant in these sites. Results from this study showed that several key residues in the MPER region are indispensable to promote the conformational changes that drive fusion events and shed light on the structure analysis of LASV GPC and anti-LASV therapeutics.

Identification of the dietary supplement capsaicin as an inhibitor of Lassa virus entry

The limited treatment options for the increasing occurrence of Lassa hemorrhagic fever in West Africa poses an urgent need for the discovery and development of novel therapeutics. Dietary supplements, especially natural products that are edible and safe for human use, are a good source of drug discovery with potential for uncovering novel applications. In this study, we tested 40 natural products of dietary supplements and identified capsaicin, a common dietary supplement abundant in chili peppers, as an inhibitor of Lassa virus (LASV) entry with EC50 of 6.9-10.0 μmol/L using an HIV based pseudovirus platform. Capsaicin inhibits the entry of five LASV strains but not against the Old World arenavirus lymphocytic choriomeningitis virus (LCMV), showing a preferential activity against LASV. Capsaicin inhibits LASV entry by blocking the pH dependent viral fusion through affecting the stable signal peptide (SSP)-GP2 transmembrane (GP2TM) region of the LASV surface glycoprotein. Mutational study revealed the key residues Ala25, Val431, Phe434 and Val435 in SSP-GP2TM region in capsaicin's antiviral effect. This study for the first time reveals a direct acting antiviral effect of capsaicin against the hemorrhagic fever causing LASV, providing detailed interaction hot spots in the unique SSP-GP2TM interface of LASV glycoprotein that is crucial in fusion inhibition, and offering a new strategy in discovering and developing antivirals from natural products that are safe for human use.

SAR studies of 4-acyl-1,6-dialkylpiperazin-2-one arenavirus cell entry inhibitors

Old World (Africa) and New World (South America) arenaviruses are associated with human hemorrhagic fevers. Efforts to develop small molecule therapeutics have yielded several chemical series including the 4-acyl-1,6-dialkylpiperazin-2-ones. Herein, we describe an extensive exploration of this chemotype. In initial Phase I studies, R1 and R4 scanning libraries were assayed to identify potent substituents against Old World (Lassa) virus. In subsequent Phase II studies, R6 substituents and iterative R1, R4 and R6 substituent combinations were evaluated to obtain compounds with improved Lassa and New World (Machupo, Junin, and Tacaribe) arenavirus inhibitory activity, in vitro human liver microsome metabolic stability and aqueous solubility.

Assessment of antiviral therapeutics in animal models of Lassa fever

Lassa virus (LASV) is an emerging zoonotic virus endemic in West Africa that can cause severe haemorrhagic Lassa fever (LF) in humans. LF recently gained international attention as a prominent infectious disease, leading to increasingly severe outbreaks in Nigeria over the past three years. Morbidity and mortality associated with LF disease in Nigeria continue to rise with 106 deaths reported in 2016, 143 in 2017 and 562 in 2018. Despite the significant health impact LF imposes on West Africa there are currently no FDA-approved therapeutics or vaccines available for treatment and prevention. This review focuses on the assessment and current state of LF antiviral therapeutics in animal models and their potential role in reducing disease burden throughout West Africa.

The ReFRAME library as a comprehensive drug repurposing library to identify mammarenavirus inhibitors

Several mammarenaviruses, chiefly Lassa virus (LASV) in Western Africa and Junín virus (JUNV) in the Argentine Pampas, cause severe disease in humans and pose important public health problems in their endemic regions. Moreover, mounting evidence indicates that the worldwide-distributed mammarenavirus lymphocytic choriomeningitis virus (LCMV) is a neglected human pathogen of clinical significance. The lack of licensed mammarenavirus vaccines and partial efficacy of current anti-mammarenavirus therapy limited to an off-label use of the nucleoside analog ribavirin underscore an unmet need for novel therapeutics to combat human pathogenic mammarenavirus infections. This task can be facilitated by the implementation of "drug repurposing" strategies to reduce the time and resources required to advance identified antiviral drug candidates into the clinic. We screened a drug repurposing library of 11,968 compounds (Repurposing, Focused Rescue and Accelerated Medchem [ReFRAME]) and identified several potent inhibitors of LCMV multiplication that had also strong anti-viral activity against LASV and JUNV. Our findings indicate that enzymes of the rate-limiting steps of pyrimidine and purine biosynthesis, the pro-viral MCL1 apoptosis regulator, BCL2 family member protein and the mitochondrial electron transport complex III, play critical roles in the completion of the mammarenavirus life cycle, suggesting they represent potential druggable targets to counter human pathogenic mammarenavirus infections.

Identification of a clinical compound losmapimod that blocks Lassa virus entry

Lassa virus (LASV) causes Lassa hemorrhagic fever in humans and poses a significant threat to public health in West Africa. Current therapeutic treatments for Lassa fever are limited, making the development of novel countermeasures an urgent priority. In this study, we identified losmapimod, a p38 mitogen-activated protein kinase (MAPK) inhibitor, from 102 screened compounds as an inhibitor of LASV infection. Losmapimod exerted its inhibitory effect against LASV after p38 MAPK down-regulation, and, interestingly, had no effect on other arenaviruses capable of causing viral hemorrhagic fever. Mechanistic studies showed that losmapimod inhibited LASV entry by affecting the stable signal peptide (SSP)-GP2 subunit interface of the LASV glycoprotein, thereby blocking pH-dependent viral fusion. As an aryl heteroaryl bis-carboxyamide derivative, losmapimod represents a novel chemical scaffold with anti-LASV activity, and it provides a new lead structure for the future development of LASV fusion inhibitors.

Hemorrhagic Fever-Causing Arenaviruses: Lethal Pathogens and Potent Immune Suppressors

Hemorrhagic fevers (HF) resulting from pathogenic arenaviral infections have traditionally been neglected as tropical diseases primarily affecting African and South American regions. There are currently no FDA-approved vaccines for arenaviruses, and treatments have been limited to supportive therapy and use of non-specific nucleoside analogs, such as Ribavirin. Outbreaks of arenaviral infections have been limited to certain geographic areas that are endemic but known cases of exportation of arenaviruses from endemic regions and socioeconomic challenges for local control of rodent reservoirs raise serious concerns about the potential for larger outbreaks in the future. This review synthesizes current knowledge about arenaviral evolution, ecology, transmission patterns, life cycle, modulation of host immunity, disease pathogenesis, as well as discusses recent development of preventative and therapeutic pursuits against this group of deadly viral pathogens.

Identification of Clotrimazole Derivatives as Specific Inhibitors of Arenavirus Fusion

Arenaviruses are a large family of emerging enveloped negative-strand RNA viruses that include several causative agents of viral hemorrhagic fevers. For cell entry, human-pathogenic arenaviruses use different cellular receptors and endocytic pathways that converge at the level of acidified late endosomes, where the viral envelope glycoprotein mediates membrane fusion. Inhibitors of arenavirus entry hold promise for therapeutic antiviral intervention and the identification of "druggable" targets is of high priority. Using a recombinant vesicular stomatitis virus pseudotype platform, we identified the clotrimazole-derivative TRAM-34, a highly selective antagonist of the calcium-activated potassium channel KCa3.1, as a specific entry inhibitor for arenaviruses. TRAM-34 specifically blocked entry of most arenaviruses, including hemorrhagic fever viruses, but not Lassa virus and other enveloped viruses. Anti-arenaviral activity was likewise observed with the parental compound clotrimazole and the derivative senicapoc, whereas structurally unrelated KCa3.1 inhibitors showed no antiviral effect. Deletion of KCa3.1 by CRISPR/Cas9 technology did not affect the antiarenaviral effect of TRAM-34, indicating that the observed antiviral effect of clotrimazoles was independent of the known pharmacological target. The drug affected neither virus-cell attachment, nor endocytosis, suggesting an effect on later entry steps. Employing a quantitative cell-cell fusion assay that bypasses endocytosis, we demonstrate that TRAM-34 specifically inhibits arenavirus-mediated membrane fusion. In sum, we uncover a novel antiarenaviral action of clotrimazoles that currently undergo in vivo evaluation in the context of other human diseases. Their favorable in vivo toxicity profiles and stability opens the possibility to repurpose clotrimazole derivatives for therapeutic intervention against human-pathogenic arenaviruses.IMPORTANCE Emerging human-pathogenic arenaviruses are causative agents of severe hemorrhagic fevers with high mortality and represent serious public health problems. The current lack of a licensed vaccine and the limited treatment options makes the development of novel antiarenaviral therapeutics an urgent need. Using a recombinant pseudotype platform, we uncovered that clotrimazole drugs, in particular TRAM-34, specifically inhibit cell entry of a range of arenaviruses, including important emerging human pathogens, with the exception of Lassa virus. The antiviral effect was independent of the known pharmacological drug target and involved inhibition of the unusual membrane fusion mechanism of arenaviruses. TRAM-34 and its derivatives currently undergo evaluation against a number of human diseases and show favorable toxicity profiles and high stability in vivo Our study provides the basis for further evaluation of clotrimazole derivatives as antiviral drug candidates. Their advanced stage of drug development will facilitate repurposing for therapeutic intervention against human-pathogenic arenaviruses.

Diagnostics for Lassa fever virus: a genetically diverse pathogen found in low-resource settings

Lassa fever virus (LASV) causes acute viral haemorrhagic fever with symptoms similar to those seen with Ebola virus infections. LASV is endemic to West Africa and is transmitted through contact with excretions of infected Mastomys natalensis rodents and other rodent species. Due to a high fatality rate, lack of treatment options and difficulties with prevention and control, LASV is one of the high-priority pathogens included in the WHO R&D Blueprint. The WHO LASV vaccine strategy relies on availability of effective diagnostic tests. Current diagnostics for LASV include in-house and commercial (primarily research-only) laboratory-based serological and nucleic acid amplification tests. There are two commercially available (for research use only) rapid diagnostic tests (RDTs), and a number of multiplex panels for differential detection of LASV infection from other endemic diseases with similar symptoms have been evaluated. However, a number of diagnostic gaps remain. Lineage detection is a challenge due to the genomic diversity of LASV, as pan-lineage sensitivity for both molecular and immunological detection is necessary for surveillance and outbreak response. While pan-lineage ELISA and RDTs are commercially available (for research use only), validation and external quality assessment (EQA) is needed to confirm detection sensitivity for all known or relevant strains. Variable sensitivity of LASV PCR tests also highlights the need for improved validation and EQA. Given that LASV outbreaks typically occur in low-resource settings, more options for point-of-care testing would be valuable. These requirements should be taken into account in target product profiles for improved LASV diagnostics.

Two Point Mutations in Old World Hantavirus Glycoproteins Afford the Generation of Highly Infectious Recombinant Vesicular Stomatitis Virus Vectors

Human hantavirus infections cause hantavirus pulmonary syndrome in the Americas and hemorrhagic fever with renal syndrome (HFRS) in Eurasia. No FDA-approved vaccines and therapeutics exist for these deadly viruses, and their development is limited by the requirement for high biocontainment. In this study, we identified and characterized key amino acid changes in the surface glycoproteins of HFRS-causing Hantaan virus that enhance their incorporation into recombinant vesicular stomatitis virus (rVSV) particles. The replication-competent rVSVs encoding Hantaan virus and Dobrava-Belgrade virus glycoproteins described in this work provide a powerful and facile system to study hantavirus entry under lower biocontainment and may have utility as hantavirus vaccines.

Rodent-to-human transmission of hantaviruses is associated with severe disease. Currently, no FDA-approved, specific antivirals or vaccines are available, and the requirement for high biocontainment (biosafety level 3 [BSL-3]) laboratories limits hantavirus research. To study hantavirus entry in a BSL-2 laboratory, we set out to generate replication-competent, recombinant vesicular stomatitis viruses (rVSVs) bearing the Gn and Gc (Gn/Gc) entry glycoproteins. As previously reported, rVSVs bearing New World hantavirus Gn/Gc were readily rescued from cDNAs, but their counterparts bearing Gn/Gc from the Old World hantaviruses, Hantaan virus (HTNV) or Dobrava-Belgrade virus (DOBV), were refractory to rescue. However, serial passage of the rescued rVSV-HTNV Gn/Gc virus markedly increased its infectivity and capacity for cell-to-cell spread. This gain in viral fitness was associated with the acquisition of two point mutations: I532K in the cytoplasmic tail of Gn and S1094L in the membrane-proximal stem of Gc. Follow-up experiments with rVSVs and single-cycle VSV pseudotypes confirmed these results. Mechanistic studies revealed that both mutations were determinative and contributed to viral infectivity in a synergistic manner. Our findings indicate that the primary mode of action of these mutations is to relocalize HTNV Gn/Gc from the Golgi complex to the cell surface, thereby affording significantly enhanced Gn/Gc incorporation into budding VSV particles. Finally, I532K/S1094L mutations in DOBV Gn/Gc permitted the rescue of rVSV-DOBV Gn/Gc, demonstrating that incorporation of cognate mutations into other hantaviral Gn/Gc proteins could afford the generation of rVSVs that are otherwise challenging to rescue. The robust replication-competent rVSVs, bearing HTNV and DOBV Gn/Gc, reported herein may also have utility as vaccines.

A potent Lassa virus antiviral targets an arenavirus virulence determinant

Arenaviruses are a significant cause of hemorrhagic fever, an often-fatal disease for which there is no approved antiviral therapy. Lassa fever in particular generates high morbidity and mortality in West Africa, where the disease is endemic, and a recent outbreak in Nigeria was larger and more geographically diverse than usual. We are developing LHF-535, a small-molecule viral entry inhibitor that targets the arenavirus envelope glycoprotein, as a therapeutic candidate for Lassa fever and other hemorrhagic fevers of arenavirus origin. Using a lentiviral pseudotype infectivity assay, we determined that LHF-535 had sub-nanomolar potency against the viral envelope glycoproteins from all Lassa virus lineages, with the exception of the glycoprotein from the LP strain from lineage I, which was 100-fold less sensitive than that of other strains. This reduced sensitivity was mediated by a unique amino acid substitution, V434I, in the transmembrane domain of the envelope glycoprotein GP2 subunit. This position corresponds to the attenuation determinant of Candid#1, a live-attenuated Junín virus vaccine strain used to prevent Argentine hemorrhagic fever. Using a virus-yield reduction assay, we determined that LHF-535 potently inhibited Junín virus, but not Candid#1, and the Candid#1 attenuation determinant, F427I, regulated this difference in sensitivity. We also demonstrated that a daily oral dose of LHF-535 at 10 mg/kg protected mice from a lethal dose of Tacaribe virus. Serial passage of Tacaribe virus in LHF-535-treated Vero cells yielded viruses that were resistant to LHF-535, and the majority of drug-resistant viruses exhibited attenuated pathogenesis. These findings provide a framework for the clinical development of LHF-535 as a broad-spectrum inhibitor of arenavirus entry and provide an important context for monitoring the emergence of drug-resistant viruses.

Lassa fever is a viral hemorrhagic fever disease that is transmitted to humans primarily through contact with the urine or feces of infected rodents. The disease is endemic in West Africa, and an unusually large outbreak occurred in Nigeria in early 2018. The case fatality rate was 25% among confirmed cases, underscoring the need for an effective antiviral therapy. Here, we evaluated the small-molecule drug LHF-535, which targets the arenavirus envelope glycoprotein, for broad-spectrum activity against Lassa viruses of different lineages and related arenaviruses that cause hemorrhagic fever diseases in South America. We also selected for LHF-535-resistant viruses and characterized their genotype and phenotype. Using a combination of surrogate systems and wild-type viruses, we determined that all tested Lassa virus strains and New World hemorrhagic fever arenaviruses were sensitive to LHF-535. Sensitivity to the drug was modulated by specific amino acid changes in the viral envelope glycoprotein, and the majority of emerging drug-resistant viruses were attenuated for virulence. Similarly, the live-attenuated vaccine strain for Argentine hemorrhagic fever was also resistant to LHF-535. These findings indicate that LHF-535 targets a viral virulence determinant, the mutation of which may result in the emergence of drug-resistant viruses, but with reduced capacity for virulence.

Metal chelators for the inhibition of the lymphocytic choriomeningitis virus endonuclease domain

Arenaviridae is a viral family whose members are associated with rodent-transmitted infections to humans responsible of severe diseases. The current lack of a vaccine and limited therapeutic options make the development of efficacious drugs of high priority. The cap-snatching mechanism of transcription of Arenavirus performed by the endonuclease domain of the L-protein is unique and essential, so we developed a drug design program targeting the endonuclease activity of the prototypic Lymphocytic ChorioMeningitis Virus. Since the endonuclease activity is metal ion dependent, we designed a library of compounds bearing chelating motifs (diketo acids, polyphenols, and N-hydroxyisoquinoline-1,3-diones) able to block the catalytic center through the chelation of the critical metal ions, resulting in a functional impairment. We pre-screened 59 compounds by Differential Scanning Fluorimetry. Then, we characterized the binding affinity by Microscale Thermophoresis and evaluated selected compounds in in vitro and in cellula assays. We found several potent binders and inhibitors of the endonuclease activity. This study validates the proof of concept that the endonuclease domain of Arenavirus can be used as a target for anti-arena-viral drug discovery and that both diketo acids and N-hydroxyisoquinoline-1,3-diones can be considered further as potential metal-chelating pharmacophores.

Tangeretin, an extract from Citrus peels, blocks cellular entry of arenaviruses that cause viral hemorrhagic fever

The family Arenaviridae consists of numerous enveloped RNA viruses with ambisense coding strategies. Eight arenaviruses, including Lassa virus, are known to cause severe and fatal viral hemorrhagic fever (VHF) in humans, yet vaccines and treatments for disease caused by arenaviruses are very limited. In this study, we screened a natural product library consisting of 131 compounds and identified tangeretin, a polymethoxylated flavone widely present in citrus fruit peels, as a Lassa virus entry inhibitor that blocks viral fusion. Further analyses demonstrated the efficacy of tangeretin against seven other VHF-causing arenaviruses, suggesting that this compound, which has a history of medical usage, could be used to develop an effective therapeutic to treat infection and disease caused by Lassa virus and related viruses.

Screening and Identification of Lassa Virus Entry Inhibitors from an FDA-Approved Drug Library

Lassa virus (LASV) belongs to the Mammarenavirus genus (family Arenaviridae) and causes severe hemorrhagic fever in humans. At present, there are no Food and Drug Administration (FDA)-approved drugs or vaccines specific for LASV. Here, high-throughput screening of an FDA-approved drug library was performed against LASV entry by using pseudotype virus bearing LASV envelope glycoprotein (GPC). Two hit compounds, lacidipine and phenothrin, were identified as LASV entry inhibitors in the micromolar range. A mechanistic study revealed that both compounds inhibited LASV entry by blocking low-pH-induced membrane fusion. Accordingly, lacidipine showed virucidal effects on the pseudotype virus of LASV. Adaptive mutant analyses demonstrated that replacement of T40, located in the ectodomain of the stable-signal peptide (SSP), with lysine (K) conferred LASV resistance to lacidipine. Furthermore, lacidipine showed antiviral activity against LASV, the closely related Mopeia virus (MOPV), and the New World arenavirus Guanarito virus (GTOV). Drug-resistant variants indicated that V36M in the ectodomain of the SSP mutant and V436A in the transmembrane domain of the GP2 mutant conferred GTOV resistance to lacidipine, suggesting the interface between SSP and GP2 is the target of lacidipine. This study shows that lacidipine is a candidate for LASV therapy, reinforcing the notion that the SSP-GP2 interface provides an entry-targeted platform for arenavirus inhibitor design.IMPORTANCE Currently, there is no approved therapy to treat Lassa fever; therefore, repurposing of approved drugs will accelerate the development of a therapeutic stratagem. In this study, we screened an FDA-approved library of drugs and identified two compounds, lacidipine and phenothrin, which inhibited Lassa virus entry by blocking low-pH-induced membrane fusion. Additionally, both compounds extended their inhibition against the entry of Guanarito virus, and the viral targets were identified as the SSP-GP2 interface.

Mining a Kröhnke Pyridine Library for Anti-Arenavirus Activity

Several arenaviruses cause hemorrhagic fever (HF) disease in humans and represent important public health problems in their endemic regions. In addition, evidence indicates that the worldwide-distributed prototypic arenavirus lymphocytic choriomeningitis virus is a neglected human pathogen of clinical significance. There are no licensed arenavirus vaccines, and current antiarenavirus therapy is limited to an off-label use of ribavirin that is only partially effective. Therefore, there is an unmet need for novel therapeutics to combat human pathogenic arenaviruses, a task that will be facilitated by the identification of compounds with antiarenaviral activity that could serve as probes to identify arenavirus-host interactions suitable for targeting, as well as lead compounds to develop future antiarenaviral drugs. Screening of a combinatorial library of Krönhke pyridines identified compound KP-146 [(5-(5-(2,3-dihydrobenzo[ b][1,4] dioxin-6-yl)-4'-methoxy-[1,1'-biphenyl]-3-yl)thiophene-2-carboxamide] as having strong anti-lymphocytic choriomeningitis virus (LCMV) activity in cultured cells. KP-146 did not inhibit LCMV cell entry but rather interfered with the activity of the LCMV ribonucleoprotein (vRNP) responsible for directing virus RNA replication and gene transcription, as well as with the budding process mediated by the LCMV matrix Z protein. LCMV variants with increased resistance to KP-146 did not emerge after serial passages in the presence of KP-146. Our findings support the consideration of Kröhnke pyridine scaffold as a valuable source to identify compounds that could serve as tools to dissect arenavirus-host interactions, as well as lead candidate structures to develop antiarenaviral drugs.

Baseline mapping of Lassa fever virology, epidemiology and vaccine research and development

Lassa fever (LF) is a zoonotic disease associated with acute and potentially fatal hemorrhagic illness caused by the Lassa virus (LASV), a member of the family Arenaviridae. It is generally assumed that a single infection with LASV will produce life-long protective immunity. This suggests that protective immunity induced by vaccination is an achievable goal and that cell-mediated immunity may play a more important role in protection, at least following natural infection. Seropositive individuals in endemic regions have been shown to have LASV-specific T cells recognizing epitopes for nucleocapsid protein (NP) and glycoprotein precursor (GPC), suggesting that these will be important vaccine immunogens. The role of neutralizing antibodies in protective immunity is still equivocal as recent studies suggest a role for neutralizing antibodies. There is extensive genetic heterogeneity among LASV strains that is of concern in the development of assays to detect and identify all four LASV lineages. Furthermore, the gene disparity may complicate the synthesis of effective vaccines that will provide protection across multiple lineages. Non-human primate models of LASV infection are considered the gold standard for recapitulation of human LF. The most promising vaccine candidates to date are the ML29 (a live attenuated reassortant of Mopeia and LASV), vesicular stomatitis virus (VSV) and vaccinia-vectored platforms based on their ability to induce protection following single doses, high rates of survival following challenge, and the use of live virus platforms. To date no LASV vaccine candidates have undergone clinical evaluation.

Determining the Virus Life-Cycle Stage Blocked by an Antiviral

Among the members of the Arenaviridae family, Junín virus and Lassa virus represent important human health threats generating annual outbreaks of severe human hemorrhagic fever (HF) in endemic areas of Argentina and Western Africa, respectively. Given the lack of a specific and safe chemotherapy, the search for effective antiviral compounds is a continuous demanding effort. During the last two decades, academic research studies originated important results identifying novel molecules to be considered for further in vivo characterization. This chapter summarizes experimental in vitro approaches used to determine the possible mechanism of action of these antiviral agents.

Studies of Lassa Virus Cell Entry

Host cell entry is the first and most fundamental step of every virus infection and represents a major barrier for zoonotic transmission and viral emergence. Targeting viral entry appears further as a promising strategy for therapeutic intervention. Several cellular receptors have been identified for Lassa virus, including dystroglycan, TAM receptor tyrosine kinases, and C-type lectins. Upon receptor binding, LASV enters the host cell via a largely unknown clathrin- and dynamin-independent endocytotic pathway that delivers the virus to late endosomes, where fusion occurs after engagement of a second, intracellular receptor, the late endosomal/lysosomal resident protein LAMP1. Here, we describe a series of experimental approaches to investigate LASV cell entry and to test candidate inhibitors for their action at this early and decisive step of infection.

Epistastic Interactions within the Junín Virus Envelope Glycoprotein Complex Provide an Evolutionary Barrier to Reversion in the Live-Attenuated Candid#1 Vaccine

The Candid#1 strain of Junín virus was developed using a conventional attenuation strategy of serial passage in nonhost animals and cultured cells. The live-attenuated Candid#1 vaccine is used in Argentina to protect at-risk individuals against Argentine hemorrhagic fever, but it has not been licensed in the United States. Recent studies have revealed that Candid#1 attenuation is entirely dependent on a phenylalanine-to-isoleucine substitution at position 427 in the fusion subunit (GP2) of the viral envelope glycoprotein complex (GPC), thereby raising concerns regarding the potential for reversion to virulence. In this study, we report the identification and characterization of an intragenic epistatic interaction between the attenuating F427I mutation in GP2 and a lysine-to-serine mutation at position 33 in the stable signal peptide (SSP) subunit of GPC, and we demonstrate the utility of this interaction in creating an evolutionary barrier against reversion to the pathogenic genotype. In the presence of the wild-type F427 residue, the K33S mutation abrogates the ability of ectopically expressed GPC to mediate membrane fusion at endosomal pH. This defect is rescued by the attenuating F427I mutation. We show that the recombinant Candid#1 (rCan) virus bearing K33S GPC is viable and retains its attenuated genotype under cell culture conditions that readily select for reversion in the parental rCan virus. If back-mutation to F427 offers an accessible pathway to increase fitness in rCan, reversion in K33S-GPC rCan is likely to be lethal. The epistatic interaction between K33S and F427I thus may minimize the likelihood of reversion and enhance safety in a second-generation Candid#1 vaccine.IMPORTANCE The live-attenuated Candid#1 vaccine strain of Junín virus is used to protect against Argentine hemorrhagic fever. Recent findings that a single missense mutation in the viral envelope glycoprotein complex (GPC) is responsible for attenuation raise the prospect of facile reversion to pathogenicity. Here, we characterize a genetic interaction between GPC subunits that evolutionarily forces retention of the attenuating mutation. By incorporating this secondary mutation into Candid#1 GPC, we hope to minimize the likelihood of reversion and enhance safety in a second-generation Candid#1 vaccine. A similar approach may guide the design of live-attenuated vaccines against Lassa and other arenaviral hemorrhagic fevers.

A DNA vaccine delivered by dermal electroporation fully protects cynomolgus macaques against Lassa fever

Lassa virus (LASV) is an ambisense RNA virus in the Arenaviridae family and is the etiological agent of Lassa fever, a severe hemorrhagic disease endemic to West and Central Africa.^1,2 There are no US Food and Drug Administration (FDA)-licensed vaccines available to prevent Lassa fever.^1,2 in our previous studies, we developed a gene-optimized DNA vaccine that encodes the glycoprotein precursor gene of LASV (Josiah strain) and demonstrated that 3 vaccinations accompanied by dermal electroporation protected guinea pigs from LASV-associated illness and death. Here, we describe an initial efficacy experiment in cynomolgus macaque nonhuman primates (NHPs) in which we followed an identical 3-dose vaccine schedule that was successful in guinea pigs, and a follow-on experiment in which we used an accelerated vaccination strategy consisting of 2 administrations, spaced 4 weeks apart. In both studies, all of the LASV DNA-vaccinated NHPs survived challenge and none of them had measureable, sustained viremia or displayed weight loss or other disease signs post-exposure. Three of 10 mock-vaccinates survived exposure to LASV, but all of them became acutely ill post-exposure and remained chronically ill to the study end point (45 d post-exposure). Two of the 3 survivors experienced sensorineural hearing loss (described elsewhere). These results clearly demonstrate that the LASV DNA vaccine combined with dermal electroporation is a highly effective candidate for eventual use in humans.