Antiviral activity of nucleoside analogues against norovirus

Identification of Co-29, a 5-cyano-2-thiacetyl aromatic pyrimidinone, as a potential inhibitor targeting the RdRp of norovirus

BACKGROUND

Human norovirus (HNV) is the predominant pathogen causing outbreaks of acute gastroenteritis globally. Despite significant efforts to combat norovirus infections, there is currently no FDA approved vaccine or antiviral drug available. Consequently, the development of effective antiviral agents is of critical importance.

METHODS AND RESULTS

In this study, a series of 41 5-cyano-2-thiacetyl aromatic pyrimidinone compounds were designed and synthesized. A cell viability-based screening for anti-murine norovirus (MNV) compounds was conducted, revealing that compound 29 (hereafter used as Co-29) exhibited antiviral activity against MNV. Co-29 demonstrated effective inhibition of MNVCW3 RNA replication, exhibiting an EC50 of 58.22 μM. An RdRp enzyme activity assay indicated that Co-29 directly inhibits RdRp activity to both MNV and HNV. Molecular docking studies suggested that Co-29 interacts with the palm region of RdRp via hydrogen bonding with specific residues, which are conserved in RdRps across MNV and HNV norovirus variants.

CONCLUSIONS

In conclusion, our study suggests that the newly synthesized Co-29 may serve as a potential antiviral candidate or lead compound for future studies.

CX-6258 hydrochloride hydrate: A potential non-nucleoside inhibitor targeting the RNA-dependent RNA polymerase of norovirus

Human norovirus (HuNoV), a primary cause of non-bacterial gastroenteritis, currently lacks approved treatment. RdRp is vital for virus replication, making it an attractive target for therapeutic intervention. By application of structure-based virtual screening procedure, we present CX-6258 hydrochloride hydrate as a potent RdRp non-nucleoside inhibitor, effectively inhibiting HuNoV RdRp activity with an IC50 of 3.61 μM. Importantly, this compound inhibits viral replication in cell culture, with an EC50 of 0.88 μM. In vitro binding assay validate that CX-6258 hydrochloride hydrate binds to RdRp through interaction with the "B-site" binding pocket. Interestingly, CX-6258-contacting residues such as R392, Q439, and Q414 are highly conserved among major norovirus GI and GII variants, suggesting that it may be a general inhibitor of norovirus RdRp. Given that CX-6258 hydrochloride hydrate is already utilized as an orally efficacious pan-Pim kinase inhibitor, it may serve as a potential lead compound in the effort to control HuNoV infections.

Further preclinical characterization of molnupiravir against SARS-CoV-2: Antiviral activity determinants and viral genome alteration patterns

The SARS-CoV-2 pandemic has highlighted the need for broad-spectrum antiviral drugs to respond promptly to viral emergence. We conducted a preclinical study of molnupiravir (MOV) against SARS-CoV-2 to fully characterise its antiviral properties and mode of action. The antiviral activity of different concentrations of MOV was evaluated ex vivo on human airway epithelium (HAE) and in vivo in a hamster model at three escalating doses (150, 300 and 400 mg/kg/day) according to three different regimens (preventive, pre-emptive and curative). We assessed viral loads and infectious titres at the apical pole of HAE and in hamster lungs, and MOV trough concentration in plasma and lungs. To explore the mode of action of the MOV, the entire genomes of the collected viruses were deep-sequenced. MOV effectively reduced viral titres in HAE and in the lungs of treated animals. Early treatment after infection was a key factor in efficacy, probably associated with high lung concentrations of MOV, suggesting good accumulation in the lung. MOV induced genomic alteration in viral genomes with an increase in the number of minority variants, and predominant G to A transitions. The observed reduction in viral replication and its mechanism of action leading to lethal mutagenesis, supported by clinical trials showing antiviral action in humans, provide a convincing basis for further research as an additional means in the fight against COVID-19 and other RNA viruses.

New conjugates based on N4-hydroxycytidine with more potent antiviral efficacy in vitro than EIDD-2801 against SARS-CoV-2 and other human coronaviruses

The spread of COVID-19 continues due to genetic variation in SARS-CoV-2. Highly mutated variants of SARS-CoV-2 have an increased transmissibility and immune evasion. Due to the emergence of various new variants of the virus, there is an urgent need to develop broadly effective specific drugs for therapeutic strategies for the prevention and treatment of COVID-19. Molnupiravir (EIDD-2801, MK-4482), is an orally bioavailable ribonucleoside analogue of β-D-N4-hydroxycytidine (NHC), has demonstrated efficacy against SARS-CoV-2 and was recently approved for COVID-19 treatment. To improve antiviral potency of NHC, we developed a panel of NHC conjugates with lipophilic vectors and ester derivatives with amino- and carboxylic-acids. Most of the synthesized compounds had comparable or higher (2-20 times) antiviral activity than EIDD-2801, against different lineages of SARS-CoV-2, MERS-CoV, seasonal coronaviruses OC43 and 229E, as well as bovine coronavirus. For further studies, we assessed the most promising compound in terms of activity, simplicity and cost of synthesis - NHC conjugate with phenylpropionic acid (SN_9). SN_9 has shown high efficacy in prophylactic, therapeutic and transmission models of COVID-19 infection in hamsters. Importantly, SN_9 profoundly inhibited virus replication in the lower respiratory tract of hamsters and transgenic mice infected with the Omicron sublineages XBB.1.9.1, XBB.1.16 and EG.5.1.1. These data indicate that SN_9 represents a promising antiviral drug candidate for COVID-19 treatment, and NHC modification strategies deserve further investigation as an approach to develop prodrugs against various coronaviruses.

Why Certain Repurposed Drugs Are Unlikely to Be Effective Antivirals to Treat SARS-CoV-2 Infections

Most repurposed drugs have proved ineffective for treating COVID-19. We evaluated median effective and toxic concentrations (EC50, CC50) of 49 drugs, mostly from previous clinical trials, in Vero cells. Ratios of reported unbound peak plasma concentrations, (Cmax)/EC50, were used to predict the potential in vivo efficacy. The 20 drugs with the highest ratios were retested in human Calu-3 and Caco-2 cells, and their CC50 was determined in an expanded panel of cell lines. Many of the 20 drugs with the highest ratios were inactive in human Calu-3 and Caco-2 cells. Antivirals effective in controlled clinical trials had unbound Cmax/EC50 ≥ 6.8 in Calu-3 or Caco-2 cells. EC50 of nucleoside analogs were cell dependent. This approach and earlier availability of more relevant cultures could have reduced the number of unwarranted clinical trials.

SARS-CoV-2 resistance to monoclonal antibodies and small-molecule drugs

Over four years have passed since the beginning of the COVID-19 pandemic. The scientific response has been rapid and effective, with many therapeutic monoclonal antibodies and small molecules developed for clinical use. However, given the ability for viruses to become resistant to antivirals, it is perhaps no surprise that the field has identified resistance to nearly all of these compounds. Here, we provide a comprehensive review of the resistance profile for each of these therapeutics. We hope that this resource provides an atlas for mutations to be aware of for each agent, particularly as a springboard for considerations for the next generation of antivirals. Finally, we discuss the outlook and thoughts for moving forward in how we continue to manage this, and the next, pandemic.

Human norovirus cultivation systems and their use in antiviral research

Human norovirus (HuNoV) is a major cause of acute gastroenteritis and foodborne diseases, affecting all age groups. Despite its clinical needs, no approved antiviral therapies are available. Since the discovery of HuNoV in 1972, studies on anti-norovirals, mechanism of HuNoV infection, viral inactivation, etc., have been hampered by the lack of a robust laboratory-based cultivation system for HuNoV. A recent breakthrough in the development of HuNoV cultivation systems has opened opportunities for researchers to investigate HuNoV biology in the context of de novo HuNoV infections. A tissue stem cell-derived human intestinal organoid/enteroid (HIO) culture system is one of those that supports HuNoV replication reproducibly and, to our knowledge, is most widely distributed to laboratories worldwide to study HuNoV and develop therapeutic strategies. This review summarizes recently developed HuNoV cultivation systems, including HIO, and their use in antiviral studies.

Modification of N-hydroxycytidine yields a novel lead compound exhibiting activity against the Venezuelan equine encephalitis virus

Nucleoside and nucleobase analogs capable of interfering with nucleic acid synthesis have played essential roles in fighting infectious diseases. However, many of these agents are associated with important and potentially lethal off-target intracellular effects that limit their use. Based on the previous discovery of base-modified 2'-deoxyuridines, which showed high anticancer activity while exhibiting lower toxicity toward rapidly dividing normal human cells compared to antimetabolite chemotherapeutics, we hypothesized that a similar modification of the N4-hydroxycytidine (NHC) molecule would provide novel antiviral compounds with diminished side effects. This presumption is due to the substantial structural difference with natural cytidine leading to less recognizability by host cell enzymes. Among the 42 antimetabolite species that have been synthesized and screened against VEEV, one hit compound was identified. The structural features of the modifying moiety were similar to those of the anticancer lead 2'-deoxyuridine derivative reported previously, providing an opportunity to pursue further structure-activity relationship (SAR) studies directed to lead improvement, and obtain insight into the mechanism of action, which can lead to identifying drug candidates against a broad spectrum of RNA viral infections.

Potential Anti-Mpox Virus Activity of Atovaquone, Mefloquine, and Molnupiravir, and Their Potential Use as Treatments

BACKGROUND

Mpox virus (MPXV) is a zoonotic orthopoxvirus and caused an outbreak in 2022. Although tecovirimat and brincidofovir are approved as anti-smallpox drugs, their effects in mpox patients have not been well documented. In this study, by a drug repurposing approach, we identified potential drug candidates for treating mpox and predicted their clinical impacts by mathematical modeling.

METHODS

We screened 132 approved drugs using an MPXV infection cell system. We quantified antiviral activities of potential drug candidates by measuring intracellular viral DNA and analyzed the modes of action by time-of-addition assay and electron microscopic analysis. We further predicted the efficacy of drugs under clinical concentrations by mathematical simulation and examined combination treatment.

RESULTS

Atovaquone, mefloquine, and molnupiravir exhibited anti-MPXV activity, with 50% inhibitory concentrations of 0.51-5.2 μM, which was more potent than cidofovir. Whereas mefloquine was suggested to inhibit viral entry, atovaquone and molnupiravir targeted postentry processes. Atovaquone was suggested to exert its activity through inhibiting dihydroorotate dehydrogenase. Combining atovaquone with tecovirimat enhanced the anti-MPXV effect of tecovirimat. Quantitative mathematical simulations predicted that atovaquone can promote viral clearance in patients by 7 days at clinically relevant drug concentrations.

CONCLUSIONS

These data suggest that atovaquone would be a potential candidate for treating mpox.

Inhibitory effect of Ephedra herba on human norovirus infection in human intestinal organoids

Human norovirus (HuNoV) is a major cause of acute gastroenteritis and foodborne diseases worldwide with public health concern, yet no antiviral therapies have been developed. In this study, we aimed to screen crude drugs, which are components of Japanese traditional medicine, ''Kampo'' to see their effects on HuNoV infection using a reproducible HuNoV cultivation system, stem-cell derived human intestinal organoids/enteroids (HIOs). Among the 22 crude drugs tested, Ephedra herba significantly inhibited HuNoV infection in HIOs. A time-of-drug addition experiment suggested that this crude drug more preferentially targets post-entry step than entry step for the inhibition. To our knowledge, this is the first anti-HuNoV inhibitor screen targeting crude drugs, and Ephedra herba was identified as a novel inhibitor candidate that merits further study.

Synthesis and biological evaluation of new β-D-N4-hydroxycytidine analogs against SARS-CoV-2, influenza viruses and DENV-2

Drug repurposing approach was applied to find a potent antiviral agent against RNA viruses such as SARS-CoV-2, influenza viruses and dengue virus with a concise strategy of small change in parent molecular structure. For this purpose, β-D-N4-hydroxycytidine (NHC, 1) with a broad spectrum of antiviral activity was chosen as the parent molecule. Among the prepared NHC analogs (8a-g, and 9) from uridine, β-D-N4-O-isobutyrylcytidine (8a) showed potent activity against SARS-CoV-2 (EC50 3.50 μM), Flu A (H1N1) (EC50 5.80 μM), Flu A (H3N2) (EC50 7.30 μM), Flu B (EC50 3.40 μM) and DENV-2 (EC50 3.95 μM) in vitro. Furthermore, its potency against SARS-CoV-2 was >5-fold, 3.4-fold, and 3-fold compared to that of NHC (1), MK-4482 (2), and remdesivir (RDV) in vitro, respectively. Ultimately, compound 8a was expected to be a potent inhibitor toward RNA viruses as a viral mutagenic agent like MK-4482.

Synthesis of 4'-Substituted Carbocyclic Uracil Derivatives and Their Monophosphate Prodrugs as Potential Antiviral Agents

Over the past decades, both 4'-modified nucleoside and carbocyclic nucleoside analogs have been under the spotlight as several compounds from either family showed anti-HIV, HCV, RSV or SARS-CoV-2 activity. Herein, we designed compounds combining these two features and report the synthesis of a series of novel 4'-substituted carbocyclic uracil derivatives along with their corresponding monophosphate prodrugs. These compounds were successfully prepared in 19 to 22 steps from the commercially available (-)-Vince lactam and were evaluated against a panel of RNA viruses including SARS-CoV-2, influenza A/B viruses and norovirus.

Advances in the Development of Small Molecule Antivirals against Equine Encephalitic Viruses

Venezuelan, western, and eastern equine encephalitic alphaviruses (VEEV, WEEV, and EEEV, respectively) are arboviruses that are highly pathogenic to equines and cause significant harm to infected humans. Currently, human alphavirus infection and the resulting diseases caused by them are unmitigated due to the absence of approved vaccines or therapeutics for general use. These circumstances, combined with the unpredictability of outbreaks-as exemplified by a 2019 EEE surge in the United States that claimed 19 patient lives-emphasize the risks posed by these viruses, especially for aerosolized VEEV and EEEV which are potential biothreats. Herein, small molecule inhibitors of VEEV, WEEV, and EEEV are reviewed that have been identified or advanced in the last five years since a comprehensive review was last performed. We organize structures according to host- versus virus-targeted mechanisms, highlight cellular and animal data that are milestones in the development pipeline, and provide a perspective on key considerations for the progression of compounds at early and later stages of advancement.

New Perspectives on Antimicrobial Agents: Molnupiravir and Nirmatrelvir/Ritonavir for Treatment of COVID-19

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has emerged to cause pandemic respiratory disease in the past 2 years, leading to significant worldwide morbidity and mortality. At the beginning of the pandemic, only nonspecific treatments were available, but recently two oral antivirals have received emergency use authorization from the U.S. Food and Drug Administration for the treatment of mild to moderate coronavirus disease (COVID-19). Molnupiravir targets the viral polymerase and causes lethal mutations within the virus during replication. Nirmatrelvir targets SARS-CoV-2's main protease, and it is combined with ritonavir to delay its metabolism and allow nirmatrelvir to inhibit proteolytic cleavage of viral polyproteins during replication, preventing efficient virus production. Both drugs inhibit in vitro viral replication of all variants tested to date. Each is taken orally twice daily for 5 days. When started in the first 5 days of illness in persons at risk for complications due to COVID-19, molnupiravir and nirmatrelvir/ritonavir significantly decreased severe outcomes (hospitalizations and death) with adjusted relative risk reductions of 30% and 88%, respectively, for the two treatments. Molnupiravir should not be used in children or pregnant persons due to concerns about potential toxicity, and reliable contraception should be used in persons of childbearing potential. Nirmatrelvir/ritonavir may cause significant drug-to-drug interactions that limit its use in persons taking certain medications metabolized by certain cytochrome P450 enzymes. Both treatment regimens are important additions to the management of early COVID-19 in at-risk patients in the outpatient setting.

In silico design of a novel nucleotide antiviral agent by free energy perturbation

Nucleoside analogs are the backbone of antiviral therapies. Drugs from this class undergo processing by host or viral kinases to form the active nucleoside triphosphate species that selectively inhibits the viral polymerase. It is the central hypothesis that the nucleoside triphosphate analog must be a favorable substrate for the viral polymerase and the nucleoside precursor must be a satisfactory substrate for the host kinases to inhibit viral replication. Herein, free energy perturbation (FEP) was used to predict substrate affinity for both host and viral enzymes. Several uridine 5'-monophosphate prodrug analogs known to inhibit hepatitis C virus (HCV) were utilized in this study to validate the use of FEP. Binding free energies to the host monophosphate kinase and viral RNA-dependent RNA polymerase (RdRp) were calculated for methyl-substituted uridine analogs. The 2'-C-methyl-uridine and 4'-C-methyl-uridine scaffolds delivered favorable substrate binding to the host kinase and HCV RdRp that were consistent with results from cellular antiviral activity in support of our new approach. In a prospective evaluation, FEP results suggest that 2'-C-dimethyl-uridine scaffold delivered favorable monophosphate and triphosphate substrates for both host kinase and HCV RdRp, respectively. Novel 2'-C-dimethyl-uridine monophosphate prodrug was synthesized and exhibited sub-micromolar inhibition of HCV replication. Using this novel approach, we demonstrated for the first time that nucleoside analogs can be rationally designed that meet the multi-target requirements for antiviral activity.

Inhibitors of dihydroorotate dehydrogenase cooperate with Molnupiravir and N4-hydroxycytidine to suppress SARS-CoV-2 replication

The nucleoside analog N4-hydroxycytidine (NHC) is the active metabolite of the prodrug molnupiravir, which has been approved for the treatment of COVID-19. SARS-CoV-2 incorporates NHC into its RNA, resulting in defective virus genomes. Likewise, inhibitors of dihydroorotate dehydrogenase (DHODH) reduce virus yield upon infection, by suppressing the cellular synthesis of pyrimidines. Here, we show that NHC and DHODH inhibitors strongly synergize in the inhibition of SARS-CoV-2 replication in vitro. We propose that the lack of available pyrimidine nucleotides upon DHODH inhibition increases the incorporation of NHC into nascent viral RNA. This concept is supported by the rescue of virus replication upon addition of pyrimidine nucleosides to the media. DHODH inhibitors increased the antiviral efficiency of molnupiravir not only in organoids of human lung, but also in Syrian Gold hamsters and in K18-hACE2 mice. Combining molnupiravir with DHODH inhibitors may thus improve available therapy options for COVID-19.

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Molnupiravir and DHODH inhibitors are approved drugs, facilitating clinical testing

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The combination may allow lower drug doses to decrease possible toxic effects

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Inhibitors of nucleotide biosynthesis may boost antiviral nucleoside analogs

Comprehensive review on molnupiravir in COVID-19: a novel promising antiviral to combat the pandemic

Despite the progress in the management of COVID-19, effective oral antiviral agents are still lacking. In the present review, the potential beneficial effects of molnupiravir in the management of COVID-19 are discussed. A literature search in Google Scholar, Scopus, PubMed and clinicaltrials.gov for the relevant articles regarding the pharmacokinetics, pharmacodynamics and clinical trials of molnupiravir in the management of COVID-19 is conducted. Most of the preclinical studies and available clinical trials showed a favorable short-term safety profile of molnupiravir; however, given its possible genotoxic effects, further trials are required to confirm the long-term efficacy and safety of molnupiravir in patients with COVID-19.

Antiviral activity of molnupiravir against COVID-19: a schematic review of evidences

BACKGROUND

The study was aimed at encapsulating the evidence of in vitro and in vivo antiviral activities of molnupiravir and its active form against highly pathogenic SARS-CoV-2, the pathogen responsible for COVID-19, and finding out the efficacy and safety of molnupiravir in clinical trials.

MAIN BODY

Information on publications was explored on several databases, gray literature was reviewed, and the outcomes were discussed narratively. Molnupiravir's antiviral efficacy and associated mechanism of action have been verified in vitro against both non-COVID and multiple coronaviruses. Molnupiravir has been tried in preclinical investigations in numerous animal models against non-coronaviruses. Clinical studies in several countries are now being conducted to evaluate its antiviral efficacy in persons infected with COVID-19. The medication displays antiviral effect via generation of copying mistakes during viral RNA replication.

CONCLUSIONS

Molnupiravir is the first oral antiviral medicine to show considerable and convincing antiviral activity in vitro and in animal models. Molnupiravir stops the spread of SARS-CoV-2 in animals that have been infected and in cells grown in a lab. In a clinical research, early molnupiravir treatment reduced hospitalization and death risk in unvaccinated individuals with COVID-19. In the battle against SARS-CoV-2, it could be a potent weapon. However, its role in COVID-19 in moderate to severe cases is still up in the air, and more research is needed.

Human genetic risk of treatment with antiviral nucleoside analog drugs that induce lethal mutagenesis: The special case of molnupiravir

This review considers antiviral nucleoside analog drugs, including ribavirin, favipiravir, and molnupiravir, which induce genome error catastrophe in SARS-CoV or SARS-CoV-2 via lethal mutagenesis as a mode of action. In vitro data indicate that molnupiravir may be 100 times more potent as an antiviral agent than ribavirin or favipiravir. Molnupiravir has recently demonstrated efficacy in a phase 3 clinical trial. Because of its anticipated global use, its relative potency, and the reported in vitro "host" cell mutagenicity of its active principle, β-d-N4-hydroxycytidine, we have reviewed the development of molnupiravir and its genotoxicity safety evaluation, as well as the genotoxicity profiles of three congeners, that is, ribavirin, favipiravir, and 5-(2-chloroethyl)-2'-deoxyuridine. We consider the potential genetic risks of molnupiravir on the basis of all available information and focus on the need for additional human genotoxicity data and follow-up in patients treated with molnupiravir and similar drugs. Such human data are especially relevant for antiviral NAs that have the potential of permanently modifying the genomes of treated patients and/or causing human teratogenicity or embryotoxicity. We conclude that the results of preclinical genotoxicity studies and phase 1 human clinical safety, tolerability, and pharmacokinetics are critical components of drug safety assessments and sentinels of unanticipated adverse health effects. We provide our rationale for performing more thorough genotoxicity testing prior to and within phase 1 clinical trials, including human PIG-A and error corrected next generation sequencing (duplex sequencing) studies in DNA and mitochondrial DNA of patients treated with antiviral NAs that induce genome error catastrophe via lethal mutagenesis.

Predicting New Anti-Norovirus Inhibitor With the Help of Machine Learning Algorithms and Molecular Dynamics Simulation-Based Model

Hepatitis C virus (HCV) inhibitors are essential in the treatment of human norovirus (HuNoV). This study aimed to map out HCV NS5B RNA-dependent RNA polymerase inhibitors that could potentially be responsible for the inhibitory activity of HuNoV RdRp. It is necessary to develop robust machine learning and in silico methods to predict HuNoV RdRp compounds. In this study, Naïve Bayesian and random forest models were built to categorize norovirus RdRp inhibitors from the non-inhibitors using their molecular descriptors and PubChem fingerprints. The best model observed had accuracy, specificity, and sensitivity values of 98.40%, 97.62%, and 97.62%, respectively. Meanwhile, an external test set was used to validate model performance before applicability to the screened HCV compounds database. As a result, 775 compounds were predicted as NoV RdRp inhibitors. The pharmacokinetics calculations were used to filter out the inhibitors that lack drug-likeness properties. Molecular docking and molecular dynamics simulation investigated the inhibitors' binding modes and residues critical for the HuNoV RdRp receptor. The most active compound, CHEMBL167790, closely binds to the binding pocket of the RdRp enzyme and depicted stable binding with RMSD 0.8-3.2 Å, and the RMSF profile peak was between 1.0-4.0 Å, and the conformational fluctuations were at 450-460 residues. Moreover, the dynamic residue cross-correlation plot also showed the pairwise correlation between the binding residues 300-510 of the HuNoV RdRp receptor and CHEMBL167790. The principal component analysis depicted the enhanced movement of protein atoms. Moreover, additional residues such as Glu510 and Asn505 interacted with CHEMBL167790 via water bridge and established H-bond interactions after the simulation. http://zinc15.docking.org/substances/ZINC000013589565.

Developing a direct acting, orally available antiviral agent in a pandemic: the evolution of molnupiravir as a potential treatment for COVID-19

Despite the availability of vaccines, there remains an urgent need for antiviral drugs with potent activity against SARS-CoV-2, the cause of COVID-19. Millions of people are immune-suppressed and may not be able to mount a fully protective immune response after vaccination. There is also an increasingly critical need for a drug to cover emerging SARS-CoV-2 variants, against which existing vaccines may be less effective. Here, we describe the evolution of molnupiravir (EIDD-2801, MK-4482), a broad-spectrum antiviral agent originally designed for the treatment of Alphavirus infections, into a potential drug for the prevention and treatment of COVID-19. When the pandemic began, molnupiravir was in pre-clinical development for the treatment of seasonal influenza. As COVID-19 spread, the timeline for the development program was moved forward significantly, and focus shifted to treatment of coronavirus infections. Real time consultation with regulatory authorities aided in making the acceleration of the program possible.

Mechanism of molnupiravir-induced SARS-CoV-2 mutagenesis

Molnupiravir is an orally available antiviral drug candidate currently in phase III trials for the treatment of patients with COVID-19. Molnupiravir increases the frequency of viral RNA mutations and impairs SARS-CoV-2 replication in animal models and in humans. Here, we establish the molecular mechanisms underlying molnupiravir-induced RNA mutagenesis by the viral RNA-dependent RNA polymerase (RdRp). Biochemical assays show that the RdRp uses the active form of molnupiravir, β-D-N⁴-hydroxycytidine (NHC) triphosphate, as a substrate instead of cytidine triphosphate or uridine triphosphate. When the RdRp uses the resulting RNA as a template, NHC directs incorporation of either G or A, leading to mutated RNA products. Structural analysis of RdRp-RNA complexes that contain mutagenesis products shows that NHC can form stable base pairs with either G or A in the RdRp active center, explaining how the polymerase escapes proofreading and synthesizes mutated RNA. This two-step mutagenesis mechanism probably applies to various viral polymerases and can explain the broad-spectrum antiviral activity of molnupiravir.

Mechanism of molnupiravir-induced SARS-CoV-2 mutagenesis

Molnupiravir is an orally available antiviral drug candidate currently in phase III trials for the treatment of patients with COVID-19. Molnupiravir increases the frequency of viral RNA mutations and impairs SARS-CoV-2 replication in animal models and in humans. Here, we establish the molecular mechanisms underlying molnupiravir-induced RNA mutagenesis by the viral RNA-dependent RNA polymerase (RdRp). Biochemical assays show that the RdRp uses the active form of molnupiravir, β-D-N4-hydroxycytidine (NHC) triphosphate, as a substrate instead of cytidine triphosphate or uridine triphosphate. When the RdRp uses the resulting RNA as a template, NHC directs incorporation of either G or A, leading to mutated RNA products. Structural analysis of RdRp-RNA complexes that contain mutagenesis products shows that NHC can form stable base pairs with either G or A in the RdRp active center, explaining how the polymerase escapes proofreading and synthesizes mutated RNA. This two-step mutagenesis mechanism probably applies to various viral polymerases and can explain the broad-spectrum antiviral activity of molnupiravir.

Molnupiravir promotes SARS-CoV-2 mutagenesis via the RNA template

The RNA-dependent RNA polymerase of the severe acute respiratory syndrome coronavirus 2 is an important target in current drug development efforts for the treatment of coronavirus disease 2019. Molnupiravir is a broad-spectrum antiviral that is an orally bioavailable prodrug of the nucleoside analogue β-D-N⁴-hydroxycytidine (NHC). Molnupiravir or NHC can increase G to A and C to U transition mutations in replicating coronaviruses. These increases in mutation frequencies can be linked to increases in antiviral effects; however, biochemical data of molnupiravir-induced mutagenesis have not been reported. Here we studied the effects of the active compound NHC 5’-triphosphate (NHC-TP) against the purified severe acute respiratory syndrome coronavirus 2 RNA-dependent RNA polymerase complex. The efficiency of incorporation of natural nucleotides over the efficiency of incorporation of NHC-TP into model RNA substrates followed the order GTP (12,841) > ATP (424) > UTP (171) > CTP (30), indicating that NHC-TP competes predominantly with CTP for incorporation. No significant inhibition of RNA synthesis was noted as a result of the incorporated monophosphate in the RNA primer strand. When embedded in the template strand, NHC-monophosphate supported the formation of both NHC:G and NHC:A base pairs with similar efficiencies. The extension of the NHC:G product was modestly inhibited, but higher nucleotide concentrations could overcome this blockage. In contrast, the NHC:A base pair led to the observed G to A (G:NHC:A) or C to U (C:G:NHC:A:U) mutations. Together, these biochemical data support a mechanism of action of molnupiravir that is primarily based on RNA mutagenesis mediated via the template strand.

SARS-CoV-2 RNA-dependent RNA polymerase as a therapeutic target for COVID-19

Introduction: The current SARS-CoV-2 pandemic urgently demands for both prevention and treatment strategies. RNA-dependent RNA-polymerase (RdRp), which has no counterpart in human cells, is an excellent target for drug development. Given the time-consuming process of drug development, repurposing drugs approved for other indications or at least successfully tested in terms of safety and tolerability, is an attractive strategy to rapidly provide an effective medication for severe COVID-19 cases.Areas covered: The currently available data and upcominSg studies on RdRp which can be repurposed to halt SARS-CoV-2 replication, are reviewed.Expert opinion: Drug repurposing and design of novel compounds are proceeding in parallel to provide a quick response and new specific drugs, respectively. Notably, the proofreading SARS-CoV-2 exonuclease activity could limit the potential for drugs designed as immediate chain terminators and favor the development of compounds acting through delayed termination. While vaccination is awaited to curb the SARS-CoV-2 epidemic, even partially effective drugs from repurposing strategies can be of help to treat severe cases of disease. Considering the high conservation of RdRp among coronaviruses, an improved knowledge of its activity in vitro can provide useful information for drug development or drug repurposing to combat SARS-CoV-2 as well as future pandemics.

Repurposing existing drugs for the treatment of COVID-19/SARS-CoV-2 infection: A review describing drug mechanisms of action

The outbreak of a novel coronavirus (SARS-CoV-2) has caused a major public health concern across the globe. SARS-CoV-2 is the seventh coronavirus that is known to cause human disease. As of September 2020, SARS-CoV-2 has been reported in 213 countries and more than 31 million cases have been confirmed, with an estimated mortality rate of ∼3%. Unfortunately, a drug or vaccine is yet to be discovered to treat COVID-19. Thus, repurposing of existing cancer drugs will be a novel approach in treating COVID-19 patients. These drugs target viral replication cycle, viral entry and translocation to the nucleus. Some can enhance innate antiviral immune response as well. Hence this review focuses on comprehensive list of 22 drugs that work against COVID-19 infection. These drugs include fingolimod, colchicine, N4-hydroxycytidine, remdesivir, methylprednisone, oseltamivir, icatibant, perphanizine, viracept, emetine, homoharringtonine, aloxistatin, ribavirin, valrubicin, famotidine, almitrine, amprenavir, hesperidin, biorobin, cromolyn sodium, and antibodies- tocilzumab and sarilumab. Also, we provide a list of 31 drugs that are predicted to function against SARS-CoV-2 infection. In summary, we provide succinct overview of various therapeutic modalities. Among these 53 drugs, based on various clinical trials and literature, remdesivir, nelfinavir, methylpredinosolone, colchicine, famotidine and emetine may be used for COVID-19. SIGNIFICANCE: It is of utmost important priority to develop novel therapies for COVID-19. Since the effect of SARS-CoV-2 is so severe, slowing the spread of diseases will help the health care system, especially the number of visits to Intensive Care Unit (ICU) of any country. Several clinical trials are in works around the globe. Moreover, NCI developed a recent and robust response to COVID-19 pandemic. One of the NCI's goals is to screen cancer related drugs for identification of new therapies for COVID-19. https://www.cancer.gov/news-events/cancer-currents-blog/2020/covid-19-cancer-nci-response?cid=eb_govdel.

Targeting viral genome synthesis as broad-spectrum approach against RNA virus infections

Zoonotic spillover, i.e. pathogen transmission from animal to human, has repeatedly introduced RNA viruses into the human population. In some cases, where these viruses were then efficiently transmitted between humans, they caused large disease outbreaks such as the 1918 flu pandemic or, more recently, outbreaks of Ebola and Coronavirus disease. These examples demonstrate that RNA viruses pose an immense burden on individual and public health with outbreaks threatening the economy and social cohesion within and across borders. And while emerging RNA viruses are introduced more frequently as human activities increasingly disrupt wild-life eco-systems, therapeutic or preventative medicines satisfying the “one drug-multiple bugs”-aim are unavailable. As one central aspect of preparedness efforts, this review digs into the development of broadly acting antivirals via targeting viral genome synthesis with host- or virus-directed drugs centering around nucleotides, the genomes’ universal building blocks. Following the first strategy, selected examples of host de novo nucleotide synthesis inhibitors are presented that ultimately interfere with viral nucleic acid synthesis, with ribavirin being the most prominent and widely used example. For directly targeting the viral polymerase, nucleoside and nucleotide analogues (NNAs) have long been at the core of antiviral drug development and this review illustrates different molecular strategies by which NNAs inhibit viral infection. Highlighting well-known as well as recent, clinically promising compounds, structural features and mechanistic details that may confer broad-spectrum activity are discussed. The final part addresses limitations of NNAs for clinical development such as low efficacy or mitochondrial toxicity and illustrates strategies to overcome these.

Application of Molecular Dynamics Simulations to the Design of Nucleotide Inhibitors Binding to Norovirus Polymerase

The RNA-dependent RNA polymerase (RdRp) of norovirus is an attractive target of antiviral agents aimed at providing protection against norovirus-associated gastroenteritis. Here, we perform molecular dynamics simulations of the crystal structure of norovirus RdRp in complex with several known binders, as well as free-energy simulations by free-energy perturbation (FEP) to determine binding free energies of these molecules relative to the natural nucleotide substrates. We determine experimental EC₅₀ values and nucleotide incorporation efficiencies for several of these compounds. Moreover, we investigate the mechanism of inhibition of some of these ligands. Using FEP, we screened a virtual nucleotide library with 121 elements for binding to the polymerase and successfully identified two novel chain terminators.

Anti-norovirus activity of C7-modified 4-amino-pyrrolo[2,1-f][1,2,4]triazine C-nucleosides

Synthetic nucleoside analogues characterized by a C-C anomeric linkage form a family of promising therapeutics against infectious and malignant diseases. Herein, C-nucleosides comprising structural variations at the sugar and nucleobase moieties were examined for their ability to inhibit both murine and human norovirus RNA-dependent RNA polymerase (RdRp). We have found that the combination of 4-amino-pyrrolo[2,1-f][1,2,4]triazine and its 7-halogenated congeners with either a d-ribose or 2'-C-methyl-d-ribose unit resulted in analogues with good antiviral activity against murine norovirus (MNV), albeit coupled with a significant cytotoxicity. Among this series, 4-aza-7,9-dideazaadenosine notably retained a strong antiviral effect in a human norovirus (HuNoV) replicon assay with an EC₅₀ = 0.015 μM. This study demonstrates that C-nucleosides can be used as viable starting scaffolds for further optimization towards the development of nucleoside-based inhibitors of norovirus replication.

2-Hydroxyimino-6-aza-pyrimidine nucleosides: synthesis, DFT calculations, and antiviral evaluations

Synthesis, DFT calculations, and antiviral evaluation of a series of novel 2-hydroxyimino-6-aza-pyrimidine ribonucleosides is reported. The hydrogen bonding between the C²N–OH moiety and N³–H and/or N³ moieties shapes the pyrimidine nucleoside as purine.

Small-Molecule Antiviral β-d-N4-Hydroxycytidine Inhibits a Proofreading-Intact Coronavirus with a High Genetic Barrier to Resistance

Coronaviruses (CoVs) have emerged from animal reservoirs to cause severe and lethal disease in humans, but there are currently no FDA-approved antivirals to treat the infections. One class of antiviral compounds, nucleoside analogues, mimics naturally occurring nucleosides to inhibit viral replication. While these compounds have been successful therapeutics for several viral infections, mutagenic nucleoside analogues, such as ribavirin and 5-fluorouracil, have been ineffective at inhibiting CoVs. This has been attributed to the proofreading activity of the viral 3'-5' exoribonuclease (ExoN). β-d-N4-Hydroxycytidine (NHC) (EIDD-1931; Emory Institute for Drug Development) has recently been reported to inhibit multiple viruses. Here, we demonstrate that NHC inhibits both murine hepatitis virus (MHV) (50% effective concentration [EC50] = 0.17 μM) and Middle East respiratory syndrome CoV (MERS-CoV) (EC50 = 0.56 μM) with minimal cytotoxicity. NHC inhibited MHV lacking ExoN proofreading activity similarly to wild-type (WT) MHV, suggesting an ability to evade or overcome ExoN activity. NHC inhibited MHV only when added early during infection, decreased viral specific infectivity, and increased the number and proportion of G:A and C:U transition mutations present after a single infection. Low-level NHC resistance was difficult to achieve and was associated with multiple transition mutations across the genome in both MHV and MERS-CoV. These results point to a virus-mutagenic mechanism of NHC inhibition in CoVs and indicate a high genetic barrier to NHC resistance. Together, the data support further development of NHC for treatment of CoVs and suggest a novel mechanism of NHC interaction with the CoV replication complex that may shed light on critical aspects of replication.IMPORTANCE The emergence of coronaviruses (CoVs) into human populations from animal reservoirs has demonstrated their epidemic capability, pandemic potential, and ability to cause severe disease. However, no antivirals have been approved to treat these infections. Here, we demonstrate the potent antiviral activity of a broad-spectrum ribonucleoside analogue, β-d-N4-hydroxycytidine (NHC), against two divergent CoVs. Viral proofreading activity does not markedly impact sensitivity to NHC inhibition, suggesting a novel interaction between a nucleoside analogue inhibitor and the CoV replicase. Further, passage in the presence of NHC generates only low-level resistance, likely due to the accumulation of multiple potentially deleterious transition mutations. Together, these data support a mutagenic mechanism of inhibition by NHC and further support the development of NHC for treatment of CoV infections.

Identification of novel non-nucleoside vinyl-stilbene analogs as potent norovirus replication inhibitors with a potential host-targeting mechanism

Norovirus (NV), is the most common cause of acute gastroenteritis worldwide. To date, there is no specific anti-NV drug or vaccine to treat NV infections. In this study, we evaluated the inhibitory effect of different stilbene-based analogs on RNA genome replication of human NV (HNV) using a virus replicon-bearing cell line (HG23). Initial screening of our in-house chemical library against NV led to the identification of a hit containing stilbene scaffold 5 which on initial optimization gave us a vinyl stilbene compound 16c (EC₅₀ = 4.4 μM). Herein we report our structure-activity relationship study of the novel series of vinyl stilbene analogs that inhibits viral RNA genome replication in a human NV-specific manner. Among these newly synthesized compounds, several amide derivatives of vinyl stilbenes exhibited potent anti-NV activity with EC₅₀ values ranging from 1 to 2 μM. A trans-vinyl stilbenoid with an appended substituted piperazine amide (18k), exhibited potent anti-NV activity and also displayed favorable metabolic stability. Compound 18k demonstrated an excellent safety profile, the highest suppressive effect, and was selective for HNV replication via a viral RNA polymerase-independent manner. Its potential host-targeting antiviral mechanism was further supported by specific activation of heat shock factor 1-dependent stress-inducible pathway by 18k. These results suggest that 18k might be a promising lead compound for developing novel NV inhibitors with the novel antiviral mechanism.

Calicivirus RNA-Dependent RNA Polymerases: Evolution, Structure, Protein Dynamics, and Function

The Caliciviridae are viruses with a positive-sense, single-stranded RNA genome that is packaged into an icosahedral, environmentally stable protein capsid. The family contains five genera (Norovirus, Nebovirus, Sapovirus, Lagovirus, and Vesivirus) that infect vertebrates including amphibians, reptiles, birds, and mammals. The RNA-dependent RNA polymerase (RdRp) replicates the genome of RNA viruses and can speed up evolution due to its error-prone nature. Studying calicivirus RdRps in the context of genuine virus replication is often hampered by a lack of suitable model systems. Enteric caliciviruses and RHDV in particular are notoriously difficult to propagate in cell culture; therefore, molecular studies of replication mechanisms are challenging. Nevertheless, research on recombinant proteins has revealed several unexpected characteristics of calicivirus RdRps. For example, the RdRps of RHDV and related lagoviruses possess the ability to expose a hydrophobic motif, to rearrange Golgi membranes, and to copy RNA at unusually high temperatures. This review is focused on the structural dynamics, biochemical properties, kinetics, and putative interaction partners of these RdRps. In addition, we discuss the possible existence of a conserved but as yet undescribed structural element that is shared amongst the RdRps of all caliciviruses.

The Adenosine Analogue NITD008 has Potent Antiviral Activity against Human and Animal Caliciviruses

The widespread nature of calicivirus infections globally has a substantial impact on the health and well-being of humans and animals alike. Currently, the only vaccines approved against caliciviruses are for feline and rabbit-specific members of this group, and thus there is a growing effort towards the development of broad-spectrum antivirals for calicivirus infections. In this study, we evaluated the antiviral activity of the adenosine analogue NITD008 in vitro using three calicivirus model systems namely; feline calicivirus (FCV), murine norovirus (MNV), and the human norovirus replicon. We show that the nucleoside analogue (NA), NITD008, has limited toxicity and inhibits calicivirus replication in all three model systems with EC₅₀ values of 0.94 μM, 0.91 µM, and 0.21 µM for MNV, FCV, and the Norwalk replicon, respectively. NITD008 has a similar level of potency to the most well-studied NA 2′-C-methylcytidine in vitro. Significantly, we also show that continual NITD008 treatment effectively cleared the Norwalk replicon from cells and treatment with 5 µM NITD008 was sufficient to completely prevent rebound. Given the potency displayed by NITD008 against several caliciviruses, we propose that this compound should be interrogated further to assess its effectiveness in vivo. In summary, we have added a potent NA to the current suite of antiviral compounds and provide a NA scaffold that could be further modified for therapeutic use against calicivirus infections.

Norovirus antivirals: Where are we now?

Human noroviruses inflict a significant health burden on society and are responsible for approximately 699 million infections and over 200 000 estimated deaths worldwide each year. Yet despite significant research efforts, approved vaccines or antivirals to combat this pathogen are still lacking. Safe and effective antivirals are not available, particularly for chronically infected immunocompromised individuals, and for prophylactic applications to protect high-risk and vulnerable populations in outbreak settings. Since the discovery of human norovirus in 1972, the lack of a cell culture system has hindered biological research and antiviral studies for many years. Recent breakthroughs in culturing human norovirus have been encouraging, however, further development and optimization of these novel methodologies are required to facilitate more robust replication levels, that will enable reliable serological and replication studies, as well as advances in antiviral development. In the last few years, considerable progress has been made toward the development of norovirus antivirals, inviting an updated review. This review focuses on potential therapeutics that have been reported since 2010, which were examined across at least two model systems used for studying human norovirus or its enzymes. In addition, we have placed emphasis on antiviral compounds with a defined chemical structure. We include a comprehensive outline of direct-acting antivirals and offer a discussion of host-modulating compounds, a rapidly expanding and promising area of antiviral research.

Targeting the Viral Polymerase of Diarrhea-Causing Viruses as a Strategy to Develop a Single Broad-Spectrum Antiviral Therapy

Viral gastroenteritis is an important cause of morbidity and mortality worldwide, being particularly severe for children under the age of five. The most common viral agents of gastroenteritis are noroviruses, rotaviruses, sapoviruses, astroviruses and adenoviruses, however, no specific antiviral treatment exists today against any of these pathogens. We here discuss the feasibility of developing a broad-spectrum antiviral treatment against these diarrhea-causing viruses. This review focuses on the viral polymerase as an antiviral target, as this is the most conserved viral protein among the diverse viral families to which these viruses belong to. We describe the functional and structural similarities of the different viral polymerases, the antiviral effect of reported polymerase inhibitors and highlight common features that might be exploited in an attempt of designing such pan-polymerase inhibitor.

Therapeutics and Immunoprophylaxis Against Noroviruses and Rotaviruses: The Past, Present, and Future

Background:

Noroviruses and rotaviruses are important viral etiologies of severe gastroenteritis. Noroviruses are the primary cause of nonbacterial diarrheal outbreaks in humans, whilst rotaviruses are a major cause of childhood diarrhea. Although both enteric pathogens substantially impact human health and economies, there are no approved drugs against noroviruses and rotaviruses so far. On the other hand, whilst the currently licensed rotavirus vaccines have been successfully implemented in over 100 countries, the most advanced norovirus vaccine has recently completed phase-I and II trials.

Methods:

We performed a structured search of bibliographic databases for peer-reviewed research litera-ture on advances in the fields of norovirus and rotavirus therapeutics and immunoprophylaxis.

Results:

Technological advances coupled with a proper understanding of viral morphology and replication over the past decade has facilitated pioneering research on therapeutics and immunoprophylaxis against noroviruses and rotaviruses, with promising outcomes in human clinical trials of some of the drugs and vaccines. This review focuses on the various developments in the fields of norovirus and rotavirus thera-peutics and immunoprophylaxis, such as potential antiviral drug molecules, passive immunotherapies (oral human immunoglobulins, egg yolk and bovine colostral antibodies, llama-derived nanobodies, and anti-bodies expressed in probiotics, plants, rice grains and insect larvae), immune system modulators, probiot-ics, phytochemicals and other biological substances such as bovine milk proteins, therapeutic nanoparti-cles, hydrogels and viscogens, conventional viral vaccines (live and inactivated whole virus vaccines), and genetically engineered viral vaccines (reassortant viral particles, virus-like particles (VLPs) and other sub-unit recombinant vaccines including multi-valent viral vaccines, edible plant vaccines, and encapsulated viral particles).

Conclusions:

This review provides important insights into the various approaches to therapeutics and im-munoprophylaxis against noroviruses and rotaviruses..

Synthesis and antiviral evaluation of novel peptidomimetics as norovirus protease inhibitors

A series of tripeptidyl transition state inhibitors with new P1 and warhead moieties were synthesized and evaluated in a GI-1 norovirus replicon system and against GII-4 and GI-1 norovirus proteases. Compound 19, containing a 6-membered ring at the P1 position and a reactive aldehyde warhead exhibited sub-micromolar replicon inhibition. Retaining the same peptidyl scaffold, several reactive warheads were tested for protease inhibition and norovirus replicon inhibition. Of the six that were synthesized and tested, compounds 42, 43, and 45 potently inhibited the protease in biochemical assay and GI-1 norovirus replicon in the nanomolar range.

Selective Inhibition of Enterovirus A Species Members' Reproduction by Furano[2, 3-d]pyrimidine Nucleosides Revealed by Antiviral Activity Profiling against (+)ssRNA Viruses

The rational design of broad-spectrum antivirals requires data on antiviral activity of compounds against multiple viruses, which are often not available. We have developed a panel of (+)ssRNA viruses composed of Enterovirus and Flavivirus genera members allowing to study these activity spectra. Antiviral activity profiling of a set of nucleoside analogues revealed N 4-hydroxycytidine as an efficient inhibitor of replication of coxsackieviruses and other enteroviruses, but ineffective against tick-borne encephalitis virus. Furano[2, 3-d]pyrimidine nucleosides with n-pentyl or n-hexyl tails showed selective inhibition of Enterovirus A representatives. 5-(Tetradec-1-yn-1-yl)-uridine showed selective inhibition of tick-borne encephalitis virus at the micromolar level.

β-d-N 4-Hydroxycytidine Is a Potent Anti-alphavirus Compound That Induces a High Level of Mutations in the Viral Genome

Venezuelan equine encephalitis virus (VEEV) is a representative member of the New World alphaviruses. It is transmitted by mosquito vectors and causes highly debilitating disease in humans, equids, and other vertebrate hosts. Despite a continuous public health threat, very few compounds with anti-VEEV activity in cell culture and in mouse models have been identified to date, and rapid development of virus resistance to some of them has been recorded. In this study, we investigated the possibility of using a modified nucleoside analog, β-d-N ⁴-hydroxycytidine (NHC), as an anti-VEEV agent and defined the mechanism of its anti-VEEV activity. The results demonstrate that NHC is a very potent antiviral agent. It affects both the release of genome RNA-containing VEE virions and their infectivity. Both of these antiviral activities are determined by the NHC-induced accumulation of mutations in virus-specific RNAs. The antiviral effect is most prominent when NHC is applied early in the infectious process, during the amplification of negative- and positive-strand RNAs in infected cells. Most importantly, only a low-level resistance of VEEV to NHC can be developed, and it requires acquisition and cooperative function of more than one mutation in nsP4. These adaptive mutations are closely located in the same segment of nsP4. Our data suggest that NHC is more potent than ribavirin as an anti-VEEV agent and likely can be used to treat other alphavirus infections.IMPORTANCE Venezuelan equine encephalitis virus (VEEV) can cause widespread epidemics among humans and domestic animals. VEEV infections result in severe meningoencephalitis and long-term sequelae. No approved therapeutics exist for treatment of VEEV infections. Our study demonstrates that β-d-N ⁴-hydroxycytidine (NHC) is a very potent anti-VEEV compound, with the 50% effective concentration being below 1 μM. The mechanism of NHC antiviral activity is based on induction of high mutation rates in the viral genome. Accordingly, NHC treatment affects both the rates of particle release and the particle infectivity. Most importantly, in contrast to most of the anti-alphavirus drugs that are under development, resistance of VEEV to NHC develops very inefficiently. Even low levels of resistance require acquisition of multiple mutations in the gene of the VEEV-specific RNA-dependent RNA polymerase nsP4.

Structure(s), function(s), and inhibition of the RNA-dependent RNA polymerase of noroviruses

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This review summarizes current knowledge on the norovirus RdRp.

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Multiple X-ray structures of norovirus RdRp show important conformational changes.

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Norovirus RdRp recognizes specific promotor sequences to initiate RNA synthesis.

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Anti-HCV nucleoside analogs such as 2CM-C also inhibit Norovirus RdRp.

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Suramin and its analogs act as allosteric non-nucleoside polymerase inhibitors.

Noroviruses belong to the Caliciviridae family of single-stranded positive-sense RNA viruses. The genus Norovirus includes seven genogroups (designated GI-GVII), of which GI, GII and GIV infect humans. Human noroviruses are responsible for widespread outbreaks of acute gastroenteritis and represent one of the most common causes of foodborne illness. No vaccine or antiviral treatment options are available for norovirus infection. The RNA-dependent RNA polymerase (RdRp) of noroviruses is a key enzyme responsible for transcription and replication of the viral genome. Here, we review the progress made in understanding the structures and functions of norovirus RdRp and its use as a target for small molecule inhibitors. Crystal structures of the RdRp at different stages of substrate interaction have been determined, which shed light on its multi-step catalytic cycle. The in vitro assays and in vivo animal models that have been developed to identify and characterize inhibitors of norovirus RdRp are also summarized, followed by an update on the current antiviral research targeting different regions of norovirus RdRp. In the future, structure-based drug design and rational optimization of known nucleoside and non-nucleoside inhibitors of norovirus RdRp may pave the way towards the next generation of direct-acting antivirals.

Antiviral targets of human noroviruses

Human noroviruses are major causative agents of sporadic and epidemic gastroenteritis both in children and adults. Currently there are no licensed therapeutic intervention measures either in terms of vaccines or drugs available for these highly contagious human pathogens. Genetic and antigenic diversity of these viruses, rapid emergence of new strains, and their ability to infect a broad population by using polymorphic histo-blood group antigens for cell attachment, pose significant challenges for the development of effective antiviral agents. Despite these impediments, there is progress in the design and development of therapeutic agents. These include capsid-based candidate vaccines, and potential antivirals either in the form of glycomimetics or designer antibodies that block HBGA binding, as well as those that target essential non-structural proteins such as the viral protease and RNA-dependent RNA polymerase. In addition to these classical approaches, recent studies suggest the possibility of interferons and targeting host cell factors as viable approaches to counter norovirus infection. This review provides a brief overview of this progress.

Inhibition of human norovirus by a viral polymerase inhibitor in the B cell culture system and in the mouse model

The recently developed human norovirus (HuNoV) B cell culture and mouse models hold promise for drug discovery and development but their suitability for antiviral studies has not been assessed. We demonstrate the inhibitory effect of the nucleoside analogue 2'-C-methylcytidine (2CMC) on HuNoV replication in the human B cell BJAB cell line and in Balb/c Rag/gamma chain-deficient (Rag-γc(-/-)) mice. These data suggest the applicability of both models for future study and development of antiviral drugs for the treatment of HuNoV infections.

Anti-norovirus therapeutics: a patent review (2010-2015)

INTRODUCTION

Human noroviruses are the primary causative agents of acute gastroenteritis and are a pressing public health burden worldwide. There are currently no vaccines or small molecule therapeutics available for the treatment or prophylaxis of norovirus infections. An improved understanding of norovirus biology, as well as the pathogenic mechanisms underlying the disease, has provided the impetus for a range of intense exploratory drug discovery efforts targeting viral and host factors.

AREAS COVERED

An overview of norovirus inhibitors disclosed in the patent literature (2010-present) and Clinicaltrials.gov is presented. The review is further enriched and supplemented by recent literature reports.

EXPERT OPINION

Seminal discoveries made in recent years, including a better understanding of the pathobiology and life cycle of norovirus, the identification and targeting of multiple viral and host factors, the advent of a replicon system and a small animal model for the preclinical evaluation of lead compounds, and the availability of high resolution X-ray crystal structures that can be utilized in structure-based drug design and lead optimization campaigns, collectively suggest that a small molecule therapeutic and prophylactic for norovirus infection is likely to emerge in the not too distant future.

Viruses in Rodent Colonies: Lessons Learned from Murine Noroviruses

Noroviruses (NoVs) are highly prevalent, positive-sense RNA viruses that infect a range of mammals, including humans and mice. Murine noroviruses (MuNoVs) are the most prevalent pathogens in biomedical research colonies, and they have been used extensively as a model system for human noroviruses (HuNoVs). Despite recent successes in culturing HuNoVs in the laboratory and a small animal host, studies of human viruses have inherent limitations. Thus, owing to its versatility, the MuNoV system-with its native host, reverse genetics, and cell culture systems-will continue to provide important insights into NoV and enteric virus biology. In the current review, we summarize recent findings from MuNoVs that increase our understanding of enteric virus pathogenesis and highlight similarities between human and murine NoVs that underscore the value of MuNoVs to inform studies of HuNoV biology. We also discuss the potential of endemic MuNoV infections to impact other disease models.

Recent Advances in the Discovery of Norovirus Therapeutics

Noroviruses are members of the family Caliciviridae. Norovirus infections are a global health burden that impacts >20 million individuals annually in the U.S. alone. Noroviruses are associated with high morbidity among vulnerable populations, particularly immunocompromised patients. This perspective highlights recent developments related to the discovery and development of norovirus-specific small-molecule therapeutics as well as recent advances in our understanding of norovirus biology and pathogenesis. Most of the work in this area is at the early discovery stage and has been primarily focused on inhibitors of norovirus 3C-like protease and RNA dependent RNA polymerase. However, recent discoveries emanating from basic studies in norovirus research have resulted in the identification of new host-related drug targets that can be exploited. A repurposed compound has been advanced to human clinical studies.

Repurposing of rutin for the inhibition of norovirus replication

Drug repurposing is a strategy employed to circumvent some of the bottlenecks involved in drug development, such as the cost and time needed for developing new molecular entities. Noroviruses cause recurrent epidemics and sporadic outbreaks of gastroenteritis associated with significant mortality and economic costs, but no treatment has been approved to date. Herein, a library of molecules previously used in humans was screened to find compounds with anti-noroviral activity. Antiviral testing for four selected compounds against murine norovirus infection revealed that rutin has anti-murine norovirus activity in cell-based assays.