Activity of ribavirin against Hantaan virus correlates with production of ribavirin-5'-triphosphate, not with inhibition of IMP dehydrogenase

In-cell Western assay to quantify infection with pathogenic orthohantavirus Puumala virus in replication kinetics and antiviral drug testing

Hemorrhagic fever with renal syndrome (HFRS) represents a serious zoonotic disease caused by orthohantaviruses in Eurasia. A specific antiviral therapy is not available. HFRS is characterized by acute kidney injury (AKI) with often massive proteinuria. Infection of kidney cells may contribute to the clinical picture. However, orthohantaviral replication in kidney cells is not well characterized. Therefore, we aimed to perform a reliable high-throughput assay that allows the quantification of infection rates and testing of antiviral compounds in different cell types. We quantified relative infection rates of Eurasian pathogenic Puumala virus (PUUV) by staining of nucleocapsid protein (N protein) in an in-cell Western (ICW) assay. Vero E6 cells, derived from the African green monkey and commonly used in viral cell culture studies, and the human podocyte cell line CIHP (conditionally immortalized human podocytes) were used to test the ICW assay for replication kinetics and antiviral drug testing. Quantification of infection by ICW revealed reliable results for both cell types, as shown by their correlation with immunofluorescence quantification results by counting infected cells. Evaluation of antiviral efficacy of ribavirin by ICW assay revealed differences in the toxicity (TC) and inhibitory concentrations (IC) between Vero E6 cells and podocytes. IC5O of ribavirin in podocytes is about 12-fold lower than in Vero E6 cells. In summary, ICW assay together with relevant human target cells represents an important tool for the study of hantaviral replication and drug testing.

Characterization of the NiRAN domain from RNA-dependent RNA polymerase provides insights into a potential therapeutic target against SARS-CoV-2

Apart from the canonical fingers, palm and thumb domains, the RNA dependent RNA polymerases (RdRp) from the viral order Nidovirales possess two additional domains. Of these, the function of the Nidovirus RdRp associated nucleotidyl transferase domain (NiRAN) remains unanswered. The elucidation of the 3D structure of RdRp from the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), provided the first ever insights into the domain organisation and possible functional characteristics of the NiRAN domain. Using in silico tools, we predict that the NiRAN domain assumes a kinase or phosphotransferase like fold and binds nucleoside triphosphates at its proposed active site. Additionally, using molecular docking we have predicted the binding of three widely used kinase inhibitors and five well characterized anti-microbial compounds at the NiRAN domain active site along with their drug-likeliness. For the first time ever, using basic biochemical tools, this study shows the presence of a kinase like activity exhibited by the SARS-CoV-2 RdRp. Interestingly, a well-known kinase inhibitor- Sorafenib showed a significant inhibition and dampened viral load in SARS-CoV-2 infected cells. In line with the current global COVID-19 pandemic urgency and the emergence of newer strains with significantly higher infectivity, this study provides a new anti-SARS-CoV-2 drug target and potential lead compounds for drug repurposing against SARS-CoV-2.

The on-going coronavirus disease 2019 (COVID-19) pandemic caused by the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is significantly affecting the world health. Unfortunately, over 180 million cases of COVID-19 resulting in nearly 4 million deaths have been reported till June, 2021. In this study, using a combination of bioinformatics, biochemical and mass spectrometry methods, we show that the Nidovirus RdRp associated Nucleotidyl transferase (NiRAN) domain of the RNA-dependent RNA polymerase (RdRp) of SARS-CoV-2 exhibits a kinase like activity. Additionally, we also show that few broad spectrum anti-cancer and anti-microbial drugs dampen this kinase like activity. Of note, Sorafenib, an FDA approved anti-cancer kinase inhibiting drug significantly reduces the SARS-CoV-2 load in cell lines. Our study suggests that NiRAN domain of the SARS-CoV-2 RdRp is indispensible for the successful viral life cycle and shows that abolishing this enzymatic function of RdRp by small molecule inhibitors may open novel avenues for COVID-19 therapeutics.

Characterization of the NiRAN domain from RNA-dependent RNA polymerase provides insights into a potential therapeutic target against SARS-CoV-2

Apart from the canonical fingers, palm and thumb domains, the RNA dependent RNA polymerases (RdRp) from the viral order Nidovirales possess two additional domains. Of these, the function of the Nidovirus RdRp associated nucleotidyl transferase domain (NiRAN) remains unanswered. The elucidation of the 3D structure of RdRp from the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), provided the first ever insights into the domain organisation and possible functional characteristics of the NiRAN domain. Using in silico tools, we predict that the NiRAN domain assumes a kinase or phosphotransferase like fold and binds nucleoside triphosphates at its proposed active site. Additionally, using molecular docking we have predicted the binding of three widely used kinase inhibitors and five well characterized anti-microbial compounds at the NiRAN domain active site along with their drug-likeliness. For the first time ever, using basic biochemical tools, this study shows the presence of a kinase like activity exhibited by the SARS-CoV-2 RdRp. Interestingly, a well-known kinase inhibitor- Sorafenib showed a significant inhibition and dampened viral load in SARS-CoV-2 infected cells. In line with the current global COVID-19 pandemic urgency and the emergence of newer strains with significantly higher infectivity, this study provides a new anti-SARS-CoV-2 drug target and potential lead compounds for drug repurposing against SARS-CoV-2.

Controversies’ clarification regarding ribavirin efficacy in measles and coronaviruses: Comprehensive therapeutic approach strictly tailored to COVID-19 disease stages

BACKGROUND

Ribavirin is a broad-spectrum nucleoside antiviral drug with multimodal mechanisms of action, which supports its longevity and quality as a clinical resource. It has been widely administered for measles and coronavirus infections. Despite the large amount of data concerning the use of ribavirin alone or in combination for measles, severe acute respiratory syndrome, Middle East respiratory syndrome, and coronavirus disease 2019 (COVID-19) outbreaks, the conclusions of these studies have been contradictory. Underlying reasons for these discrepancies include possible study design inaccuracies and failures and misinterpretations of data, and these potential confounds should be addressed.

AIM

To determine the confounding factors of ribavirin treatment studies and propose a therapeutic scheme for COVID-19.

METHODS

PubMed database was searched over a period of five decades utilizing the terms “ribavirin” alone or combined with other compounds in measles, severe acute respiratory syndrome, Middle East respiratory syndrome, and COVID-19 infections. The literature search was performed and described according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Articles were considered eligible when they reported on ribavirin dose regimens and/or specified outcomes concerning its efficacy and/or possible adverse-effects. In vitro and animal studies were also retrieved. A chapter on ribavirin’s pharmacology was included as well.

RESULTS

In addition to the difficulties and pressures of an emerging pandemic, there is the burden of designing and conducting well-organized, double-blind, randomized controlled trials. Many studies have succumbed to specific pitfalls, one of which was identified in naturally ribavirin-resistant Vero cell lines in in vitro studies. Other pitfalls include study design inconsistent with the well-established clinical course of disease; inappropriate pharmacology of applied treatments; and the misinterpretation of study results with misconceived generalizations. A comprehensive treatment for COVID-19 is proposed, documented by thorough, long-term investigation of ribavirin regimens in coronavirus infections.

CONCLUSION

A comprehensive treatment strictly tailored to distinct disease stages was proposed based upon studies on ribavirin and coronavirus infections.

Screening and Identification of Lujo Virus Inhibitors Using a Recombinant Reporter Virus Platform

Lujo virus (LUJV), a highly pathogenic arenavirus, was first identified in 2008 in Zambia. To aid the identification of effective therapeutics for LUJV, we developed a recombinant reporter virus system, confirming reporter LUJV comparability with wild-type virus and its utility in high-throughput antiviral screening assays. Using this system, we evaluated compounds with known and unknown efficacy against related arenaviruses, with the aim of identifying LUJV-specific and potential new pan-arenavirus antivirals. We identified six compounds demonstrating robust anti-LUJV activity, including several compounds with previously reported activity against other arenaviruses. These data provide critical evidence for developing broad-spectrum antivirals against high-consequence arenaviruses.

Development of small-molecule inhibitors against hantaviruses

Hantavirus (HV), a pathogen of animal infectious diseases that poses a threat to humans, has attracted extensive attention. Clinically, HV can cause hemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS), between which HFRS is mostly in Eurasia, and HPS is mostly in the Americas. This paper reviews the research progress of small-molecule inhibitors of HV.

Ribavirin-related compounds exert in vitro inhibitory effects toward rabies virus

Rabies remains an invariably fatal neurological disease despite the availability of a preventive vaccination and post-exposure prophylaxis that must be immediately administered to the exposed individual before symptom onset. There is no effective medication for treatment during the symptomatic phase. Ribavirin, a guanine nucleoside analog, is a potent inhibitor of rabies virus (RABV) replication in vitro but lacks clinical efficacy. Therefore, we attempted to identify potential ribavirin analogs with comparable or superior anti-RABV activity. Antiviral activity and cytotoxicity of the compounds were initially examined in human neuroblastoma cells. Among the tested compounds, two exhibited a 5- to 27-fold higher anti-RABV activity than ribavirin. Examination of the anti-RABV mechanisms of action of the compounds using time-of-addition and minigenome assays revealed that they inhibited viral genome replication and transcription. Addition of exogenous guanosine to RABV-infected cells diminished the antiviral activity of the compounds, suggesting that they are involved in guanosine triphosphate (GTP) pool depletion by inhibiting inosine monophosphate dehydrogenase (IMPDH). Taken together, our findings underline the potency of nucleoside analogs as a class of antiviral compounds for the development of novel agents against RABV.

Identification of 2'-deoxy-2'-fluorocytidine as a potent inhibitor of Crimean-Congo hemorrhagic fever virus replication using a recombinant fluorescent reporter virus

Crimean-Congo hemorrhagic fever virus (CCHFV), a tick-borne orthonairovirus, causes a severe hemorrhagic disease in humans (Crimean-Congo hemorrhagic fever, CCHF). Currently, no vaccines are approved to prevent CCHF; treatment is limited to supportive care and the use of ribavirin, the therapeutic benefits of which remain unclear. CCHF is part of WHO's priority list of infectious diseases warranting further research and development. To aid in the identification of new antiviral compounds, we generated a recombinant CCHFV expressing a reporter protein, allowing us to quantify virus inhibition by measuring the reduction in fluorescence in infected cells treated with candidate compounds. The screening assay was readily adaptable to high-throughput screening (HTS) of compounds using Huh7 cells, with a signal-to-noise ratio of 50:1, and Z'-factors > 0.6 in both 96- and 384-well formats. A screen of candidate nucleoside analog compounds identified 2'-deoxy-2'-fluorocytidine (EC₅₀ = 61 ± 18 nM) as having 200 × the potency of ribavirin (EC₅₀ = 12.5 ± 2.6 μM), as well as 17 × the potency of T-705 (favipiravir), another compound with reported anti-CCHFV activity (EC₅₀ = 1.03 ± 0.16 μM). Furthermore, we also determined that 2'-deoxy-2'-fluorocytidine acts synergistically with T-705 to inhibit CCHFV replication without causing cytotoxicity. The incorporation of this reporter virus into the high-throughput screening assay described here will allow more rapid identification of effective therapeutic options to combat this emerging human pathogen.

Insufficient efficacy and safety of intravenous ribavirin in treatment of haemorrhagic fever with renal syndrome caused by Puumala virus

BACKGROUND

Intravenous ribavirin has been reported to be an effective treatment for haemorrhagic fever with renal syndrome (HFRS) caused by Hantaan virus in Asia. However, its therapeutic benefits for HFRS caused by Puumala virus (PUUV) in Europe are still unknown.

METHODS

A randomized, open-label study of efficacy and safety of intravenous ribavirin in the treatment of HFRS was conducted in the European part of Russia. Seventy-three patients with suspected HFRS within 4 d of the onset of the disease were randomized to receive either intravenous ribavirin (33 mg/kg, followed by 16 mg/kg given every 6 h for 4 d and by 8 mg/kg given every 8 h for 3 d) plus standard therapy (n = 37) or standard therapy alone (n = 36). The primary outcome was the average change from baseline in viral load over time estimated as area under the viral load curve minus baseline (AUCMB). Fifty-five patients with HFRS confirmed by nested reverse transcriptase - polymerase chain reaction (PCR) assay were included in the assessment of the efficacy. All patients entered into the clinical trial were included in the assessment of the safety.

RESULTS

PUUV was detected in all cases of confirmed HFRS. Viral load kinetics were similar in both treatment groups. Significantly more patients receiving ribavirin than standard therapy experienced low haemoglobin level (95% vs 36%), hyperbilirubinemia (81% vs 3%), sinus bradycardia (43% vs 14%), and rash (19% vs 0%).

CONCLUSIONS

Results of the study showed insufficient efficacy and safety of intravenous ribavirin in the treatment of HFRS caused by PUUV.

Mutagenic Effects of Ribavirin on Hepatitis E Virus-Viral Extinction versus Selection of Fitness-Enhancing Mutations

Hepatitis E virus (HEV), an important agent of viral hepatitis worldwide, can cause severe courses of infection in pregnant women and immunosuppressed patients. To date, HEV infections can only be treated with ribavirin (RBV). Major drawbacks of this therapy are that RBV is not approved for administration to pregnant women and that the virus can acquire mutations, which render the intra-host population less sensitive or even resistant to RBV. One of the proposed modes of action of RBV is a direct mutagenic effect on viral genomes, inducing mismatches and subsequent nucleotide substitutions. These transition events can drive the already error-prone viral replication beyond an error threshold, causing viral population extinction. In contrast, the expanded heterogeneous viral population can facilitate selection of mutant viruses with enhanced replication fitness. Emergence of these mutant viruses can lead to therapeutic failure. Consequently, the onset of RBV treatment in chronically HEV-infected individuals can result in two divergent outcomes: viral extinction versus selection of fitness-enhanced viruses. Following an overview of RNA viruses treated with RBV in clinics and a summary of the different antiviral modes of action of this drug, we focus on the mutagenic effect of RBV on HEV intrahost populations, and how HEV is able to overcome lethal mutagenesis.

Ribavirin: a drug active against many viruses with multiple effects on virus replication and propagation. Molecular basis of ribavirin resistance

Ribavirin has proven to be effective against several viruses in the clinical setting and a multitude of viruses in vitro. With up to five different proposed mechanisms of action, recent advances have begun to discern the hierarchy of antiviral effects at play depending on the virus and the host conditions under scrutiny. Studies reveal that for many viruses, antiviral mechanisms may differ depending on cell type in vitro and in vivo. Further analyses are thus required to accurately identify mechanisms to more optimally determine clinical treatments. In recent years, a growing number of ribavirin resistant and sensitive variants have been identified. These variants not only inform on the specific mechanisms by which ribavirin enfeebles the virus, but also can themselves be tools to identify new antiviral compounds.

Involvement of the N-terminal portion of influenza virus RNA polymerase subunit PB1 in nucleotide recognition

The influenza virus PB1 protein functions as a catalytic subunit of the viral RNA-dependent RNA polymerase and contains the highly conserved motifs of RNA-dependent RNA polymerases together with putative nucleotide-binding sites. PB1 also binds to viral genomic RNAs and its replicative intermediates through the promoter regions. The detail function and interplay between functional domains are not clarified although a part of structures and functions of PB1 have been clarified. In this study, we analyzed the function of PB1 subunit in the sense of nucleotide recognition using ribavirin, which is a nucleoside analog and inhibits viral RNA synthesis of many RNA viruses including influenza virus. We screened ribavirin-resistant PB1 mutants from randomly mutated PB1 cDNA library using a mini-replicon assay, and we identified a single mutation at the amino acid position 27 of PB1 as an important residue for the nucleotide recognition.

Ribavirin protects Syrian hamsters against lethal hantavirus pulmonary syndrome--after intranasal exposure to Andes virus

Andes virus, ANDV, harbored by wild rodents, causes the highly lethal hantavirus pulmonary syndrome (HPS) upon transmission to humans resulting in death in 30% to 50% of the cases. As there is no treatment for this disease, we systematically tested the efficacy of ribavirin in vitro and in an animal model. In vitro assays confirmed antiviral activity and determined that the most effective doses were 40 µg/mL and above. We tested three different concentrations of ribavirin for their capability to prevent HPS in the ANDV hamster model following an intranasal challenge. While the highest level of ribavirin (200 mg/kg) was toxic to the hamster, both the middle (100 mg/kg) and the lowest concentration (50 mg/kg) prevented HPS in hamsters without toxicity. Specifically, 8 of 8 hamsters survived intranasal challenge for both of those groups whereas 7 of 8 PBS control-treated animals developed lethal HPS. Further, we report that administration of ribavirin at 50 mg/kg/day starting on days 6, 8, 10, or 12 post-infection resulted in significant protection against HPS in all groups. Administration of ribavirin at 14 days post-infection also provided a significant level of protection against lethal HPS. These data provide in vivo evidence supporting the potential use of ribavirin as a post-exposure treatment to prevent HPS after exposure by the respiratory route.

The murine model for Hantaan virus-induced lethal disease shows two distinct paths in viral evolutionary trajectory with and without ribavirin treatment

In vitro, ribavirin acts as a lethal mutagen in Hantaan virus (HTNV)-infected Vero E6 cells, resulting in an increased mutation load and viral population extinction. In this study, we asked whether ribavirin treatment in the lethal, suckling mouse model of HTNV infection would act similarly. The HTNV genomic RNA (vRNA) copy number and infectious virus were measured in lungs of untreated and ribavirin-treated mice. In untreated, HTNV-infected mice, the vRNA copy number increased for 10 days postinfection (dpi) and thereafter remained constant through 26 dpi. Surprisingly, in ribavirin-treated, HTNV-infected mice, vRNA levels were similar to those in untreated mice between 10 and 26 dpi. Infectious virus levels, however, were different: in ribavirin-treated mice, the amount of infectious HTNV was significantly decreased relative to that in untreated mice, suggesting that ribavirin reduced the specific infectivity of the virus (amount of infectious virus produced per vRNA copy). Mutational analysis revealed a ribavirin-associated elevation in mutation frequency in HTNV vRNA similar to that previously reported in vitro. Codon-based analyses of rates of nonsynonymous (dN) and synonymous (dS) substitutions in the S segment revealed a positive selection for codons within the HTNV N protein gene in the ribavirin-treated vRNA population. In contrast, the vRNA population in untreated, HTNV-infected mice showed a lower level of diversity, reflecting purifying selection for the wild-type genome. In summary, these experiments show two different evolutionary paths that Hantavirus may take during infection in a lethal murine model of disease, as well as the importance of the in vivo host environment in the evolution of the virus, which was not apparent in our prior in vitro model system.

ENT1, a ribavirin transporter, plays a pivotal role in antiviral efficacy of ribavirin in a hepatitis C virus replication cell system

We previously showed that equilibrative nucleoside transporter 1 (ENT1) is a primary ribavirin transporter in human hepatocytes. However, because the role of this transporter in the antiviral mechanism of the drug remains unclear, the present study aimed to elucidate the role of ENT1 in ribavirin antiviral action. OR6 cells, a hepatitis C virus (HCV) replication system, were used to evaluate both ribavirin uptake and efficacy. The ribavirin transporter in OR6 cells was identified by mRNA expression analyses and transport assays. Nitrobenzylmercaptopurine riboside (NBMPR) and micro-RNA targeted to ENT1 mRNA (miR-ENT1) were used to reduce the ribavirin uptake level in OR6 cells. Our results showed that ribavirin antiviral activity was associated with its accumulation in OR6 cells, which was also closely associated with the uptake of the drug. It was found that the primary ribavirin transporter in OR6 cells was ENT1 and that inhibition of ENT1-mediated ribavirin uptake by NBMPR significantly attenuated the antiviral activity of the drug as well as its accumulation in OR6 cells. The results also showed that even a small reduction in the ENT1-mediated ribavirin uptake, achieved in this case using miR-ENT1, caused a significant decrease in its antiviral activity, thus indicating that the ENT1-mediated ribavirin uptake level determined its antiviral activity level in OR6 cells. In conclusion, our results show that by facilitating its uptake and accumulation in OR6 cells, ENT1 plays a pivotal role in the antiviral effectiveness of ribavirin and therefore provides an important insight into the efficacy of the drug in anti-HCV therapy.

In vitro and in vivo activity of ribavirin against Andes virus infection

Pathogenic hantaviruses are a closely related group of rodent-borne viruses which are responsible for two distinct diseases in humans, hemorrhagic fever with renal syndrome and hantavirus pulmonary syndrome (HPS, otherwise known as hantavirus cardiopulmonary syndrome, HCPS). The antiviral effect of ribavirin against Old World hantaviruses, most notably Hantaan virus, is well documented; however, only a few studies have addressed its inhibitory effect on New World hantaviruses. In the present study, we demonstrate that ribavirin is highly active against Andes virus (ANDV), an important etiological agent of HPS, both in vitro and in vivo using a lethal hamster model of HPS. Treatment of ANDV infected Vero E6 cells with ribavirin resulted in dose-dependent reductions in viral RNA and protein as well as virus yields with a half maximal inhibitory concentration between 5 and 12.5 µg ml⁻¹. In hamsters, treatment with as little as 5 mg kg⁻¹ day⁻¹ was 100% effective at preventing lethal HPS disease when therapy was administered by intraperitoneal injection from day 1 through day 10 post-infection. Significant reductions were observed in ANDV RNA and antigen positive cells in lung and liver tissues. Ribavirin remained completely protective when administered by intraperitoneal injections up to three days post-infection. In addition, we show that daily oral ribavirin therapy initiated 1 day post-infection and continuing for ten days is also protective against lethal ANDV disease, even at doses of 5 mg kg⁻¹ day⁻¹. Our results suggest ribavirin treatment is beneficial for postexposure prophylaxis against HPS-causing hantaviruses and should be considered in scenarios where exposure to the virus is probable. The similarities between the results obtained in this study and those from previous clinical evaluations of ribavirin against HPS, further validate the hamster model of lethal HPS and demonstrate its usefulness in screening antiviral agents against this disease.

Biochemical impact of the host adaptation-associated PB2 E627K mutation on the temperature-dependent RNA synthesis kinetics of influenza A virus polymerase complex

Most avian influenza A viruses, which preferentially replicate at the high temperatures found in the digestive tract of birds, have a glutamic acid at residue 627 of the viral RNA polymerase PB2 subunit (Glu-627), whereas the human viruses, which optimally replicate at the low temperatures observed in the human respiratory tract, have a lysine (Lys-627). The mechanism of action for this mutation is still not understood, although interaction with host factors has been proposed to play a major role. In this study, we explored an alternative, yet related, hypothesis that this PB2 mutation may alter the temperature-dependent enzymatic polymerase activity of the viral polymerase. First, the avian polymerase protein, which was purified from baculovirus expression system, indeed remained significantly active at higher temperatures (i.e. 37 and 42 °C), whereas the human E627K mutant drastically lost activity at these high temperatures. Second, our steady-state kinetics data revealed that the human E627K mutant polymerase is catalytically more active than the avian Glu-627 polymerase at 34 °C. Importantly, the E627K mutation elevates apparent K(cat) at low temperatures with little effect on K(m), suggesting that the E627K mutation alters the biochemical steps involved in enzyme catalysis rather than the interaction with the incoming NTP. Third, this temperature-dependent kinetic impact of the human E627K mutation was also observed with different RNA templates, with different primers and also in the presence of nucleoprotein. In conclusion, our study suggests that the amino acid sequence variations at residue 627 of PB2 subunit can directly alter the enzyme kinetics of influenza polymerase.

Inhibitory effect of a nucleotide analog on infectious salmon anemia virus infection

The infectious salmon anemia virus (ISAV), which belongs to the Orthomyxoviridae family, has been responsible for major losses in the salmon industry, with mortalities close to 100% in areas where Atlantic salmon (Salmo salar) is grown. This work studied the effect of ribavirin (1-β-d-ribofuranosyl-1,2,3-triazole-3-carbaxaide), a broad-spectrum antiviral compound with proven ability to inhibit the replicative cycle of the DNA and RNA viruses. The results show that ribavirin was able to inhibit the infectivity of ISAV in in vitro assays. In these assays, a significant inhibition of the replicative viral cycle was observed with a 50% inhibitory concentration (IC₅₀) of 0.02 μg/ml and an IC₉₀ of 0.4 μg/ml of ribavirin. After ribavirin treatment, viral proteins were not detectable and a reduction of viral mRNA association with ribosomes was observed. Ribavirin does not affect the levels of EF1a, nor its association with polysomes, suggesting that the inhibition of RNA synthesis occurs specifically for the virus mRNAs and not for cellular mRNAs. Moreover, ribavirin caused a significant reduction in genomic and viral RNA messenger levels. The study of the inhibitory mechanism showed that it was not reversed by the addition of guanosine. Furthermore, in vivo assays showed a reduction in the mortality of Salmo salar by more than 90% in fish infected with ISAV and treated with ribavirin without adverse effects. In fact, these results show that ribavirin is an antiviral that could be used to prevent ISAV replication either in vitro or in vivo.

Depletion of GTP pool is not the predominant mechanism by which ribavirin exerts its antiviral effect on Lassa virus

Ribavirin (1-β-d-ribofuranosyl-1,2,4-triazole-3-carboxamide) is the standard treatment for Lassa fever, though its mode of action is unknown. One possibility is depletion of the intracellular GTP pool via inhibition of the cellular enzyme inosine monophosphate dehydrogenase (IMPDH). This study compared the anti-arenaviral effect of ribavirin with that of two other IMPDH inhibitors, mycophenolic acid (MPA) and 5-ethynyl-1-β-d-ribofuranosylimidazole-4-carboxamide (EICAR). All three compounds were able to inhibit Lassa virus replication by ≥2 log units in cell culture. Restoring the intracellular GTP pool by exogenous addition of guanosine reversed the inhibitory effects of MPA and EICAR, while ribavirin remained fully active. Analogous experiments performed with Zaire Ebola virus showed that IMPDH inhibitors are also active against this virus, although to a lesser extent than against Lassa virus. In conclusion, the experiments with MPA and EICAR indicate that replication of Lassa and Ebola virus is sensitive to depletion of the GTP pool mediated via inhibition of IMPDH. However, this is not the predominant mechanism by which ribavirin exerts its in-vitro antiviral effect on Lassa virus.

Maporal virus as a surrogate for pathogenic New World hantaviruses and its inhibition by favipiravir

BACKGROUND

Pathogenic hantaviruses geographically distributed in the Old World cause haemorrhagic fever with renal syndrome (HFRS), whereas New World hantaviruses are the aetiological agents of hantavirus cardiopulmonary syndrome (HCPS). Ribavirin, a drug associated with toxicities, is presently indicated for treatment of HFRS, whereas treatment of the more frequently lethal HCPS is limited to supportive care. Because of the need for safe and effective antivirals to treat severe hantaviral infections, we evaluated favipiravir (T-705) against Dobrava and Maporal viruses as representative Old World and New World hantaviruses, respectively. Dobrava virus causes HFRS in Europe. Maporal virus (MPRLV), recently isolated from western Venezuela, is phylogenetically similar to Andes virus, the principal cause of HCPS in Argentina.

METHODS

Hantavirus replication in the presence of various inhibitors was measured by focus-forming unit assays and quantitative reverse transcriptase PCR. Phylogenetic relationships were assessed by the neighbour-joining and bootstrap consensus methods.

RESULTS

Here, we show that infection of Vero E6 cells with MPRLV is dependent on β3 integrins, similar to that reported for pathogenic hantaviruses. Furthermore, by analysis of molecular determinants associated with the G1 glycoprotein cytoplasmic tail, we show the close genetic proximity of MPRLV to other HCPS-causing hantaviruses in these regions predictive of pathogenicity. We also demonstrate anti-hantavirus activity by favipiravir with inhibitory concentrations ranging from 65 to 93 μM and selectivity indices>50.

CONCLUSIONS

Our data suggest that MPRLV may serve as a safer alternative to modelling infection caused by the highly lethal Andes virus and that hantaviruses are sensitive to the effects of favipiravir in cell culture.

Cell type mediated resistance of vesicular stomatitis virus and Sendai virus to ribavirin

Ribavirin (RBV) is a synthetic nucleoside analog with broad spectrum antiviral activity. Although RBV is approved for the treatment of hepatitis C virus, respiratory syncytial virus, and Lassa fever virus infections, its mechanism of action and therapeutic efficacy remains highly controversial. Recent reports show that the development of cell-based resistance after continuous RBV treatment via decreased RBV uptake can greatly limit its efficacy. Here, we examined whether certain cell types are naturally resistant to RBV even without prior drug exposure. Seven different cell lines from various host species were compared for RBV antiviral activity against two nonsegmented negative-strand RNA viruses, vesicular stomatitis virus (VSV, a rhabdovirus) and Sendai virus (SeV, a paramyxovirus). Our results show striking differences between cell types in their response to RBV, ranging from virtually no antiviral effect to very effective inhibition of viral replication. Despite differences in viral replication kinetics for VSV and SeV in the seven cell lines, the observed pattern of RBV resistance was very similar for both viruses, suggesting that cellular rather than viral determinants play a major role in this resistance. While none of the tested cell lines was defective in RBV uptake, dramatic variations were observed in the long-term accumulation of RBV in different cell types, and it correlated with the antiviral efficacy of RBV. While addition of guanosine neutralized RBV only in cells already highly resistant to RBV, actinomycin D almost completely reversed the RBV effect (but not uptake) in all cell lines. Together, our data suggest that RBV may inhibit the same virus via different mechanisms in different cell types depending on the intracellular RBV metabolism. Our results strongly point out the importance of using multiple cell lines of different origin when antiviral efficacy and potency are examined for new as well as established drugs in vitro.

A novel nucleoside analog, 1-beta-d-ribofuranosyl-3-ethynyl-[1,2,4]triazole (ETAR), exhibits efficacy against a broad range of flaviviruses in vitro

Antiviral therapies are urgently needed to control emerging flaviviruses such as dengue, West Nile, and yellow fever. Ribavirin (RBV) has shown activity against flaviviruses in cultured cells, but efficacy in animal models has generally been poor. In a preliminary screen of novel, synthetic 1-beta-d-ribofuranosyl-azole analogs, two compounds, 1-beta-d-ribofuranosyl-3-ethynyl-[1,2,4]triazole (ETAR) and 1-beta-d-ribofuranosyl-4-ethynyl-[1,3]imidazole (IM18), significantly reduced the replication of dengue virus serotype 2 (DENV-2) in cultured Vero cells. In the current study we demonstrated that the effective concentration 50 (EC(50)) of ETAR for DENV-2 is substantially lower than both IM18 and RBV. Moreover, ETAR reduced the replication of five additional flaviviruses, including DENV serotypes 1, 3 and 4, Langat virus and Modoc virus, > or =1000-fold relative to untreated controls. Addition of exogenous guanosine to DENV-2 infected cells negated the antiviral effects of both RBV and ETAR, indicating that GTP depletion is a major mechanism of action for both drugs. ETAR represents a promising drug candidate for the treatment of flavivirus infections.

Understanding the alphaviruses: recent research on important emerging pathogens and progress towards their control

The alphaviruses were amongst the first arboviruses to be isolated, characterized and assigned a taxonomic status. They are globally very widespread, infecting a large variety of terrestrial animals, insects and even fish, and circulate both in the sylvatic and urban/peri-urban environment, causing considerable human morbidity and mortality. Nevertheless, despite their obvious importance as pathogens, there are currently no effective antiviral drugs with which to treat humans or animals infected by any of these viruses. The EU-supported project-VIZIER (Comparative Structural Genomics of Viral Enzymes Involved in Replication, FP6 PROJECT: 2004-511960) was instigated with an ultimate view of contributing to the development of antiviral therapies for RNA viruses, including the alphaviruses [Coutard, B., Gorbalenya, A.E., Snijder, E.J., Leontovich, A.M., Poupon, A., De Lamballerie, X., Charrel, R., Gould, E.A., Gunther, S., Norder, H., Klempa, B., Bourhy, H., Rohayemj, J., L'hermite, E., Nordlund, P., Stuart, D.I., Owens, R.J., Grimes, J.M., Tuckerm, P.A., Bolognesi, M., Mattevi, A., Coll, M., Jones, T.A., Aqvist, J., Unger, T., Hilgenfeld, R., Bricogne, G., Neyts, J., La Colla, P., Puerstinger, G., Gonzalez, J.P., Leroy, E., Cambillau, C., Romette, J.L., Canard, B., 2008. The VIZIER project: preparedness against pathogenic RNA viruses. Antiviral Res. 78, 37-46]. This review highlights some of the major features of alphaviruses that have been investigated during recent years. After describing their classification, epidemiology and evolutionary history and the expanding geographic distribution of Chikungunya virus, we review progress in understanding the structure and function of alphavirus replicative enzymes achieved under the VIZIER programme and the development of new disease control strategies.

Another ten stories in antiviral drug discovery (part C): "Old" and "new" antivirals, strategies, and perspectives

The ten stories told here deal with (i) ribavirin as an inhibitor of IMP dehydrogenase and (ii) ribavirin, in combination with pegylated interferon, as the present "standard of care" for hepatitis C; (iii) S-adenosylhomocysteine hydrolase inhibitors as antiviral agents; (iv) new adamantadine derivatives for the treatment of influenza A virus infections; (v) 5-substituted 2'-deoxyuridines (i.e. IDU, TFT) for the treatment of herpes simplex virus (HSV) infections; (vi) acyclic guanosine analogues (e.g. acyclovir) for the treatment of HSV infections; (vii) OMP decarboxylase inhibitors (i.e. pyrazofurin) and CTP synthetase inhibitors (i.e. cyclopentenylcytosine) as possible antiviral agents; (viii) the future of cidofovir (and alkoxyalkyl esters thereof) and ST-246 as potential antipoxvirus agents; (ix) the two decade journey from tivirapine to rilpivirine in the ultimate therapy of HIV infections; and (x) the extension of the therapeutic application of tenofovir disoproxil fumarate (Viread) to the treatment of hepatitis B virus infection, in addition to HIV infection.

Synthesis and anti-Hantaan virus activity of N(1)-3-fluorophenyl-inosine

As part of an ongoing effort to develop new antiviral nucleoside analogs, our interest was drawn to N(1)-aryl purines as a novel structural class and potential scaffold for drug discovery. Herein, we describe the synthesis of N(1)-3-fluorophenyl-inosine (FPI) and N(1)-3-fluorophenyl-hypoxanthine (FP-Hx) and their antiviral activity against hantaviruses. The EC(50) for FPI and FP-Hx were 94 and 234microM, respectively, against Hantaan virus. FPI was not toxic to mammalian cells at concentrations that exhibited antiviral activity. Analysis of its metabolism revealed a low conversion of FPI in Vero E6 or human cells to a 5'-triphosphate, and it was a poor substrate for human purine nucleoside phosphorylase. Further, the compound did not alter GTP levels indicating FPI does not inhibit inosine monophosphate dehydrogenase. With respect to the virus, FPI did not decrease viral RNA levels or increase the mutation frequency of the viral RNA. This suggests that the antiviral activity of FPI might be solely due to the interaction of FPI or its metabolites with viral or host proteins involved in post-replication events that would affect the levels of infectious virus released. Synthesis of other compounds structurally similar to FPI is warranted to identify more potent agents that selectively abrogate production of infectious virus.

Antivirals: current state of the art

Most of the antiviral agents that have been approved, and are currently used in the treatment of virus infections, are targeted at HIV, HBV, herpes simplex virus (HSV), varicella-zoster virus (VZV), cytomegalovirus (CMV) and HCV or influenza virus. Additional compounds for HIV, HBV, HSV, VZV, CMV, HCV, influenza virus and several other viral infections, for example poxvirus (e.g., variola, vaccinia and monkeypox), respiratory syncytial virus, hemorrhagic fever virus (e.g., Lassa, Rift Valley and Ebola) and enterovirus (e.g., polio, Coxsackie and echo), are still in the experimental stage, that is, under clinical or preclinical development.

Ribavirin in the treatment of chronic hepatitis C

BACKGROUND AND AIM

Current practice guidelines recommend that individuals chronically infected with the hepatitis C virus (HCV) be treated with pegylated interferon plus ribavirin. Ribavirin, however, is associated with serious adverse events (AE), especially anemia. We review its mechanism of action, its importance in treating chronic hepatitis C (CHC) patients, the AE associated with its use, and techniques used to lessen these AE.

METHODS

Medline searches were performed using the keywords ribavirin and hepatitis, together with the keywords mechanism, anemia, liver transplant, renal function, pharmacokinetics, and dose reduction. Searches of abstracts of recent Digestive Diseases Week, American Association for the Study of Liver Diseases, and European Association for the Study of Liver Diseases meetings were also performed.

RESULTS

Ribavirin may be effective in treating CHC by affecting the virus or the host; for example by inducing viral mutations, blocking cellular enzymes, or affecting the host immune response. Although the pegylated interferons are the primary drugs used to treat CHC, a combination with ribavirin is more effective than pegylated interferon alone. Ribavirin-associated AE may be lessened by ribavirin dose reductions and by maintenance of the hematocrit.

CONCLUSIONS

Treatments of ribavirin toxicities, especially anemia, can allow patients to continue full-dose combination therapy with peginterferon and ribavirin, enhancing their probability of attaining a sustained virologic response (SVR). Treatment of CHC should be tailored to individual patients, especially those with renal dysfunction, and should include agents that treat the side-effects of CHC treatment. Monitoring of plasma ribavirin concentrations during treatment may help in the future.

Low Response to Pegylated Interferon plus Ribavirin in HIV-Infected Patients with Chronic Hepatitis C Treated with Abacavir

Background

There is little information about the influence of antiretroviral drugs on the antiviral activity of pegylated interferon (PEG-IFN) plus ribavirin (RBV) against hepatitis C virus (HCV).

Methods

All HIV-infected patients with chronic hepatitis C who received first-line PEG-IFN plus RBV were retrospectively analyzed. Only patients in whom virological stopping rules were applied and who did not change their antiretrovirals were chosen. Plasma RBV concentrations were measured at week 4.

Results

A total of 493 patients (78% males, mean age 41 years, 78% on antiretroviral therapy, mean CD4+ T-cell count 561 cells/μl) fit the study inclusion criteria. Mean baseline serum HCV RNA was 5.89 log10 IU/ml, 65% were infected by genotypes 1 or 4 and 40% had advanced liver fibrosis (Metavir F3F4). The overall rate of sustained virological response (SVR) was 38%. Factors associated with lack of SVR in the multivariate analyses (odds ratio [95% confidence interval], P-value) were higher baseline serum HCV RNA (2.42 per log10 IU/ml [1.31–4.46], 0.005), HCV genotypes 1 or 4 (5.95 [2.50–14.29], <0.001) and lower RBV plasma trough concentrations (1.74 per μg/ml [1.15–2.63], 0.009). Interestingly, a trend was noticed for abacavir use (2.22 [0.91–5.40], 0.08), which become significant when only considering the subset of patients with RBV plasma levels <2.3 μg/ml (7.63 [1.39–41.67], 0.02).

Conclusions

The use of abacavir might interfere with the anti-HCV activity of PEG-IFN plus RBV. As both antivirals are guanosine analogues, an inhibitory competition between abacavir and RBV might explain this observation, which is more prominent in patients with lower RBV exposure.

Synthesis of 1-beta-D-ribofuranosyl-3-ethynyl-[1,2,4]triazole and its in vitro and in vivo efficacy against Hantavirus

There are no FDA approved drugs for the treatment of hemorrhagic fever with renal syndrome (HFRS), a serious human illnesses caused by hantaviruses. Clinical studies using ribavirin (RBV) to treat HFRS patients suggest that it provides an improved prognosis when given early in the course of disease. Given the unique antiviral activity of RBV and the lack of other lead scaffolds, we prepared a diverse series of 3-substituted 1,2,4-triazole-beta-ribosides and identified one with antiviral activity, 1-beta-d-ribofuranosyl-3-ethynyl-[1,2,4]triazole (ETAR). ETAR showed an EC(50) value of 10 and 4.4 microM for Hantaan virus (HTNV) and Andes virus, respectively. ETAR had weak activity against Crimean Congo hemorrhagic fever virus, but had no activity against Rift Valley fever virus. Intraperitoneally delivered ETAR offered protection to suckling mice challenged with HTNV with a approximately 25% survival at 12.5 and 25mg/kg ETAR, and a MTD of 17.1+/-0.7 days. ETAR was phosphorylated in Vero E6 cells to its 5'-triphosphate and reduced cellular GTP levels. In contrast to RBV, ETAR did not increase mutation frequency of the HTNV genome, which suggests it has a different mechanism of action than RBV. ETAR is an exciting and promising lead compound that will be elaborated in further synthetic investigations as a framework for the rational design of new antivirals for treatment of HFRS.

New and Old World hantaviruses differentially utilize host cytoskeletal components during their life cycles

Recently we reported that the N protein of the Old World hantavirus, Hantaan (HTNV), traffics on microtubules to the ER-Golgi intermediate compartment (ERGIC) prior to its movement to the Golgi and requires an intact ERGIC for viral replication. We have extended these studies to the New World hantaviruses, Andes virus (ANDV) and Black Creek Canal virus (BCCV), and an additional Old World hantavirus, Seoul virus (SEOV). These studies support microtubule-dependent trafficking of the N protein to ERGIC within the perinuclear region for the New and Old World hantaviruses. However, we observed that early entry events were distinct for HTNV and ANDV with respect to the pathway for entry and the dependence on an intact actin (ANDV) versus microtubule (HTNV) cytoskeleton for viral replication. These studies show for the first time that while Old and New World hantaviruses share common features in their pathways, they have evolved differences in their interaction with host cell machinery.

Ribavirin reveals a lethal threshold of allowable mutation frequency for Hantaan virus

The broad spectrum of antiviral activity of ribavirin (RBV) lies in its ability to inhibit IMP dehydrogenase, which lowers cellular GTP. However, RBV can act as a potent mutagen for some RNA viruses. Previously we have shown a lack of correlation between antiviral activity and GTP repression for Hantaan virus (HTNV) and evidence for RBV's ability to promote error-prone replication. To further explore the mechanism of RBV, GTP levels, specific infectivity, and/or mutation frequency was measured in the presence of RBV, mycophenolic acid (MPA), selenazofurin, or tiazofurin. While all four drugs resulted in a decrease in the GTP levels and infectious virus, only RBV increased the mutation frequency of viral RNA (vRNA). MPA, however, could enhance RBV's mutagenic effect, which suggests distinct mechanisms of action for each. Therefore, a simple drop in GTP levels does not drive the observed error-prone replication. To further explore RBV's mechanism of action, we made a comprehensive analysis of the mutation frequency over several RBV concentrations. Of importance, we observed that the viral population reached a threshold after which mutation frequency did not correlate with a dose-dependent decrease in the level of vRNA, PFU, or [RTP]/[GTP] (where RTP is ribavirin-5'-triphosphate) over these same concentrations of RBV. Modeling of the relationship of mutation frequency and drug concentration showed an asymptotic relationship at this point. After this threshold, approximately 57% of the viral cDNA population was identical to the wild type. These studies revealed a lethal threshold, after which we did not observe a complete loss of the quasispecies structure of the wild-type genome, although we observed extinction of HTNV.

Dynein-dependent transport of the hantaan virus nucleocapsid protein to the endoplasmic reticulum-Golgi intermediate compartment

In contrast to most negative-stranded RNA viruses, hantaviruses and other viruses in the family Bunyaviridae mature intracellularly, deriving the virion envelope from the endoplasmic reticulum (ER) or Golgi compartment. While it is generally accepted that Old World hantaviruses assemble and bud into the Golgi compartment, some studies with New World hantaviruses have raised the possibility of maturation at the plasma membrane as well. Overall, the steps leading to virion assembly remain largely undetermined for hantaviruses. Because hantaviruses do not have matrix proteins, the nucleocapsid protein (N) has been proposed to play a key role in assembly. Herein, we examine the intracellular trafficking and morphogenesis of the prototype Old World hantavirus, Hantaan virus (HTNV). Using confocal microscopy, we show that N colocalized with the ER-Golgi intermediate compartment (ERGIC) in HTNV-infected Vero E6 cells, not with the ER, Golgi compartment, or early endosomes. Brefeldin A, which effectively disperses the ER, the ERGIC, and Golgi membranes, redistributed N with the ERGIC, implicating membrane association; however, subcellular fractionation experiments showed the majority of N in particulate fractions. Confocal microscopy revealed that N was juxtaposed to and distributed along microtubules and, over time, became surrounded by vimentin cages. To probe cytoskeletal association further, we probed trafficking of N in cells treated with nocodazole and cytochalasin D, which depolymerize microtubules and actin, respectively. We show that nocodazole, but not cytochalasin D, affected the distribution of N and reduced levels of intracellular viral RNA. These results suggested the involvement of microtubules in trafficking of N, whose movement could occur via molecular motors such as dynein. Overexpression of dynamitin, which is associated with dynein-mediated transport, creates a dominant-negative phenotype blocking transport on microtubules. Overexpression of dynamitin reduced N accumulation in the perinuclear region, which further supports microtubule components in N trafficking. The combined results of these experiments support targeting of N to the ERGIC prior to its movement to the Golgi compartment and the requirement of an intact ERGIC for viral replication and, thus, the possibility of virus factories in this region.