The anti-yellow fever virus activity of ribavirin is independent of error-prone replication

Application of the phosphoramidate ProTide approach to the antiviral drug ribavirin

Ribavirin is a nucleoside analogue with broad antiviral activity. Here we report the synthesis and biological evaluation of novel ribavirin ProTides designed to deliver the bioactive ribavirin monophosphate into cells. Some of the compounds display activity similar to the parent nucleoside against a range of viruses. Enzymatic, cell lysate and preliminary modeling studies have been performed to investigate the lack of enhancement of potency by the ProTides, and these indicate a failure at the final, amino acid cleavage step in the ProTide activation process, leading to inefficient release of the nucleoside monophosphate.

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.

Effect of ribavirin on the mutation rate and spectrum of hepatitis C virus in vivo

Their extremely error-prone replication makes RNA viruses targets for lethal mutagenesis. In the case of hepatitis C virus (HCV), the standard treatment includes ribavirin, a base analog with an in vitro mutagenic effect, but the in vivo mode of action of ribavirin remains poorly understood. Here, we test the mutagenic effects of ribavirin plus interferon treatment in vivo using a new method to estimate mutation rates based on the analysis of nonsense mutations. We apply this methodology to a large HCV sequence database containing over 15,000 reverse transcription-PCR molecular clone sequences from 74 patients infected with HCV. We obtained an estimate of the spontaneous mutation rate of ca. 10(-4) substitutions per site or lower, a value within the typically accepted range for RNA viruses. A roughly threefold increase in mutation rate and a significant shift in mutation spectrum were observed in samples from patients undergoing 6 months of interferon plus ribavirin treatment. This result is consistent with the known in vitro mutagenic effect of ribavirin and suggests that the antiviral effect of ribavirin plus interferon treatment is at least partly exerted through lethal mutagenesis.

T-705 (favipiravir) and related compounds: Novel broad-spectrum inhibitors of RNA viral infections

A series of pyrazinecarboxamide derivatives T-705 (favipiravir), T-1105 and T-1106 were discovered to be candidate antiviral drugs. These compounds have demonstrated good activity in treating viral infections in laboratory animals caused by various RNA viruses, including influenza virus, arenaviruses, bunyaviruses, West Nile virus (WNV), yellow fever virus (YFV), and foot-and-mouth disease virus (FMDV). Treatment has in some cases been effective when initiated up to 5–7 days after virus infection, when the animals already showed signs of illness. Studies on the mechanism of action of T-705 have shown that this compound is converted to the ribofuranosyltriphosphate derivative by host enzymes, and this metabolite selectively inhibits the influenza viral RNA-dependent RNA polymerase without cytotoxicity to mammalian cells. Interestingly, these compounds do not inhibit host DNA and RNA synthesis and inosine 5′-monophosphate dehydrogenase (IMPDH) activity. From in vivo studies using several animal models, the pyrazinecarboxamide derivatives were found to be effective in protecting animals from death, reducing viral burden, and limiting disease manifestations, even when treatment was initiated after virus inoculation. Importantly, T-705 imparts its beneficial antiviral effects without significant toxicity to the host. Prompt development of these compounds is expected to provide effective countermeasures against pandemic influenza virus and several bioweapon threats, all of which are of great global public health concern given the current paucity of highly effective broad-spectrum drugs.

Molecular targets for flavivirus drug discovery

Flaviviruses are a major cause of infectious disease in humans. Dengue virus causes an estimated 50 million cases of febrile illness each year, including an increasing number of cases of hemorrhagic fever. West Nile virus, which recently spread from the Mediterranean basin to the Western Hemisphere, now causes thousands of sporadic cases of encephalitis annually. Despite the existence of licensed vaccines, yellow fever, Japanese encephalitis and tick-borne encephalitis also claim many thousands of victims each year across their vast endemic areas. Antiviral therapy could potentially reduce morbidity and mortality from flavivirus infections, but no effective drugs are currently available. This article introduces a collection of papers in Antiviral Research on molecular targets for flavivirus antiviral drug design and murine models of dengue virus disease that aims to encourage drug development efforts. After reviewing the flavivirus replication cycle, we discuss the envelope glycoprotein, NS3 protease, NS3 helicase, NS5 methyltransferase and NS5 RNA-dependent RNA polymerase as potential drug targets, with special attention being given to the viral protease. The other viral proteins are the subject of individual articles in the journal. Together, these papers highlight current status of drug discovery efforts for flavivirus diseases and suggest promising areas for further research.

Interferon and ribavirin therapy does not select for resistance mutations in hepatitis C virus polymerase

Ribavirin has a minor and transient effect on hepatitis C virus (HCV) replication and has been suggested to select a novel mutation, F415Y, in the RNA-dependent RNA polymerase of subtype 1a viruses. Twenty-nine patients with chronic hepatitis C (subtyped by INNO LiPA as 1a, 17; 1b, 11; 1a/1b, 1) who were nonresponders to interferon-based therapies were identified retrospectively and screened at Baseline, week 24 of treatment, and 24 weeks post-treatment. Selection of resistance mutations, including at amino acid position 415 of the polymerase, was investigated. Using clonal sequencing and pyrosequencing of the NS5B gene, we screened for the F415Y resistance mutation among patients who received combination therapy with ribavirin and interferon α. Of the 15 subtype 1a patients treated with interferon plus ribavirin, only one had the F415Y change at week 24, and an F/Y mixture was still present 24 weeks after therapy. Four additional patients in this group had the F415Y change 24 weeks post-therapy. The NS5B genes were sequenced in order to identify amino acid changes associated with ribavirin therapy, but no evidence was found that ribavirin selects for particular amino acids in the RNA-dependent RNA polymerase. Ribavirin, a weak inhibitor of HCV replication, does not select for resistance mutations in the sequence of the HCV RNA polymerase.

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.

Molecular strategies to inhibit the replication of RNA viruses

There are virtually no antiviral drugs available for the treatment of infections with RNA viruses. This is particularly worrisome since most of the highly pathogenic and emerging viruses are, and will likely continue to be, RNA viruses. These viruses can cause acute, severe illness, including severe respiratory disease, hemorrhagic fever and encephalitis, with a high case fatality rate. It is important to have potent and safe drugs at hand that can be used for the treatment or prophylaxis of such infections. Drugs approved for the treatment of RNA virus infections (other than HIV) are the influenza M2 channel inhibitors, amantadine and rimantadine; the influenza neuraminidase inhibitors, oseltamivir and zanamivir, and ribavirin for the treatment of infections with respiratory syncytial virus and hepatitis C virus. The molecular mechanism(s) by which ribavirin inhibits viral replication, such as depletion of intracellular GTP pools and induction of error catastrophe, may not readily allow the design of analogues that are more potent/selective than the parent drug. Highly pathogenic RNA viruses belong to a variety of virus families, each having a particular replication strategy, thus offering a wealth of potential targets to selectively inhibit viral replication. We here provide a non-exhaustive review of potential experimental strategies, using small molecules, to inhibit the replication of several RNA viruses. Other approaches, such as the use of interferon or other host-response modifiers, immune serum or neutralizing antibodies, are not addressed in this review.

Antiviral Chemistry & Chemotherapy's Current Antiviral Agents FactFile 2008 (2nd Edition): RNA Viruses

Among the RNA viruses, other than the retroviruses (that is, HIV), which are dealt with separately in the current FactFile, the most important targets for the development of antiviral agents at the moment are the orthomyxoviruses (that is, influenza), the hepaciviruses (that is, hepatitis C virus [HCV]) and, to a lesser extent, the picornaviruses. Although the uncoating inhibitors amantadine and rimantadine were the first known inhibitors of influenza A, the neuraminidase inhibitors oseltamivir, zanamivir and peramivir have now become the prime antiviral drugs for the treatment of influenza A and B virus infections. For HCV infections, standard treatment consists of the combination of pegylated interferon-α with ribavirin, but several other antivirals targeted at specific viral functions such as the HCV protease and/or polymerase may be expected to soon take an important share of this important market. Still untapped is the potential of a variety of uncoating inhibitors, as well as protease and/or polymerase inhibitors against the wide spectrum of picornaviruses. While ribavirin has been available for 35 years as a broad-spectrum anti-RNA virus agent, relatively new and unexplored is favipiravir (T-705) accredited with activity against influenza as well as flaviviruses, bunyaviruses and arenaviruses.

Inosine monophosphate dehydrogenase as a probe in antiviral drug discovery

Inosine monophosphate (IMP) dehydrogenase (IMPDH) is a significant enzyme in the purine nucleotide biosynthetic pathway. IMPDH is viewed as an important biological target in the quest for drugs in the antiviral therapeutic area. This review article is focused on the chemistry and biology of IMPDH inhibitors and the use of IMPDH inhibition data as a probe in antiviral drug discovery. Examples of both inosine 5' monophosphate and NAD+ site-directed inhibitors are presented. Correlation of antiviral activities with IMPDH inhibition is discussed.

Analysis of ribavirin mutagenicity in human hepatitis C virus infection

The addition of ribavirin to alpha interferon therapy significantly increases response rates for patients with chronic hepatitis C virus (HCV) infection, but ribavirin's antiviral mechanisms are unknown. Ribavirin has been suggested to have mutagenic potential in vitro that would lead to "error catastrophe," i.e., the generation of nonviable viral quasispecies due to the increment in the number of mutant genomes, which prevents the transmission of meaningful genetic information. We used extensive sequence-based analysis of two independent genomic regions in order to test in vivo the hypothesis that ribavirin administration accelerates the accumulation of mutations in the viral genome and that this acceleration occurs only when HCV replication is profoundly inhibited by coadministered alpha interferon. The rate of variation of the consensus sequence, the frequency of mutation, the error generation rate, and the between-sample genetic distance were measured for patients receiving ribavirin monotherapy, a combination of alpha interferon three times per week plus ribavirin, or a combination of alpha interferon daily plus ribavirin. Ribavirin monotherapy did not increase the rate of variation of the consensus sequence, the mutation frequency, the error generation rate, or the between-sample genetic distance. The accumulation of nucleotide substitutions did not accelerate, relative to the pretreatment period, during combination therapy with ribavirin and alpha interferon, even when viral replication was profoundly inhibited by alpha interferon. This study strongly undermines the hypothesis whereby ribavirin acts as an HCV mutagen in vivo.

Structural effects on the phosphorylation of 3-substituted 1-beta-D-ribofuranosyl-1,2,4-triazoles by human adenosine kinase

The conversion of ribavirin to the monophosphate by adenosine kinase is the rate-limiting step in activation of this broad spectrum antiviral drug. Variation of the 3-substituents in a series of bioisosteric and homologated 1-β-d-ribofuranosyl-1,2,4-triazoles has marked effects on activity with the human adenosine kinase, and analysis of computational descriptors and binding models offers insight for the design of novel substrates.

Selective inhibitors of hepatitis C virus replication

Worldwide over 170 million people are chronically infected with the hepatitis C virus and hence at high risk to develop fatal liver disease. There is no vaccine available and the standard therapy [(pegylated) interferon alfa plus ribavirin] is only effective in 50-60% of patients and is associated with important side-effects. The discovery of novel antiviral strategies to selectively inhibit HCV replication has long been hindered by the lack of convenient cell culture models for the propagation of HCV. This hurdle has been overcome first with the establishment of the HCV replicon system in 1999 and, in 2005, with the development of robust HCV cell culture models. In recent years also mouse models have been elaborated that will be instrumental in assessing the in vivo efficacy of novel drugs. The viral serine protease and the viral RNA dependent RNA polymerase have shown to be excellent targets for selective anti-HCV therapy. Clinical studies with a limited number of HCV protease and polymerase inhibitors resulted in encouraging results. However, and not unexpected, preclinical evidence suggest that the virus may become rapidly resistant to such inhibitors. Combination therapy of drugs with different mode of action and resistance profiles may thus be required. Alternative strategies, such as the use of non-immunosuppressive cyclosporin A analogues with potent anti-HCV activity, may prove important, in particular since such compounds may have a resistance profile that is very different from that of protease or polymerase inhibitors.