Lack of effect of oral ribavirin in naturally occurring influenza A virus (H1N1) infection

Ribavirin for treating Lassa fever: A systematic review of pre-clinical studies and implications for human dosing

Ribavirin is currently the standard of care for treating Lassa fever. However, the human clinical trial data supporting its use suffer from several serious flaws that render the results and conclusions unreliable. We performed a systematic review of available pre-clinical data and human pharmacokinetic data on ribavirin in Lassa. In in-vitro studies, the EC50 of ribavirin ranged from 0.6 μg/ml to 21.72 μg/ml and the EC90 ranged from 1.5 μg/ml to 29 μg/ml. The mean EC50 was 7 μg/ml and the mean EC90 was 15 μg/ml. Human PK data in patients with Lassa fever was sparse and did not allow for estimation of concentration profiles or pharmacokinetic parameters. Pharmacokinetic modelling based on healthy human data suggests that the concentration profiles of current ribavirin regimes only exceed the mean EC50 for less than 20% of the time and the mean EC90 for less than 10% of the time, raising the possibility that the current ribavirin regimens in clinical use are unlikely to reliably achieve serum concentrations required to inhibit Lassa virus replication. The results of this review highlight serious issues with the evidence, which, by today standards, would be unlikely to support the transition of ribavirin from pre-clinical studies to human clinical trials. Additional pre-clinical studies are needed before embarking on expensive and challenging clinical trials of ribavirin in Lassa fever.

nucleoside analogues for the treatment of influenza: history and experience

Aim. In this study we retrospectively evaluated the clinical effectiveness and safety of nucleoside analogues (Triazavirin© and ribavirin) with that of oseltamivir for treating moderate severe influenza in adults. Materials and methods. We have used data from 191 health records of patients with moderate severe PCR confirmed influenza A and B. Control group included 57 patients treated with oseltamivir, comparison groups – 53 and 81 patients, who received Triazavirin© and ribavirin accordingly. We compared infectious intoxication syndrome duration, fever duration, duration of acute rhinitis, acute pharyngitis, acute laryngitis, acute tracheitis, acute bronchitis, cough duration as well as pneumonia occurrence. Results. No statistically significant difference in the duration of developed syndromes in Triazavirin©-treated group and oseltamivir-treated group were observed. We have noticed that fever duration had been significantly longer in ribavirin group compared to control group (4,1±2,22 days vs. 3,1±1,94 days, p<0,05) as well as more frequent pneumonia occurrence (1,2% vs. 0,0%, p>0,05). Conclusion. New nucleoside analogue Triazavirin© showed good efficacy and safety profile in adult patients with influenza. This fact provides the opportunity to recommend it for treatment of influenza along with neuraminidase inhibitors.

Iminosugars: Promising therapeutics for influenza infection

Influenza virus causes three to five million severe respiratory infections per year in seasonal epidemics, and sporadic pandemics, three of which occurred in the twentieth century and are a continuing global threat. Currently licensed antivirals exclusively target the viral neuraminidase or M2 ion channel, and emerging drug resistance necessitates the development of novel therapeutics. It is believed that a host-targeted strategy may combat the development of antiviral drug resistance. To this end, a class of molecules known as iminosugars, hydroxylated carbohydrate mimics with the endocyclic oxygen atom replaced by a nitrogen atom, are being investigated for their broad-spectrum antiviral potential. The influenza virus glycoproteins, hemagglutinin and neuraminidase, are susceptible to inhibition of endoplasmic reticulum α-glucosidases by certain iminosugars, leading to reduced virion production or infectivity, demonstrated by in vitro and in vivo studies. In some experiments, viral strain-specific effects are observed. Iminosugars may also inhibit other host and virus targets with antiviral consequences. While investigations of anti-influenza iminosugar activities have been conducted since the 1980s, recent successes of nojirimycin derivatives have re-invigorated investigation of the therapeutic potential of iminosugars as orally available, low cytotoxicity, effective anti-influenza drugs.

Antivirals for Respiratory Viral Infections: Problems and Prospects

In the past two decades, several newly emerging and reemerging viral respiratory pathogens including several influenza viruses (avian influenza and pandemic influenza), severe acute respiratory syndrome coronavirus (SARS-CoV), and Middle East respiratory syndrome coronavirus (MERS-CoV), have continued to challenge medical and public health systems. Thereafter, the development of cost-effective, broad-spectrum antiviral agents is the urgent mission of both virologists and pharmacologists. Current antiviral developments have focused targets on viral entry, replication, release, and intercellular pathways essential for viral life cycle. Here, we review the current literature on challenges and prospects in the development of these antivirals.

Cellular Metabolism and Anti-Influenza Activity of 1,3,4-Thiadiazol-2-Ylcyanamide (LY217896)

LY217896 (1,3,4-thiadiazol-2-ylcyanamide) is a 2-substituted thiadiazole that is an effective inhibitor of influenza A and B viruses in vitro and in the mouse infection model. The in vitro anti-influenza activity of LY217896 is reversed by a 10-fold excess amount of guanine or guanosine. LY217896 (1 or 10μg ml−1) effected a selective 60% decrease in the levels of intracellular pools of GTP in MDCK cells. The extent of cytotoxicity of LY217896 is positively correlated with the amount of LY217896 metabolite formed intracellularly. A cell line, derived from parental MDCK cells, was selected for resistance to 50 ng of LY217896 per ml. Unlike parental MDCK cells, the resistant cells were able to undergo log phase replication in LY217896 (25 g ml−1) and were unable to metabolize the compound. Furthermore, LY217896 had no antiviral activity against influenza A/Ann Arbor (IC50 >200μg ml−1) or vaccinia virus (IC50 = 13 μg ml−1) in resistant cells. In contrast, LY217896 inhibited influenza A/Ann Arbor (IC50 = 0.5 μg ml−1) or vaccinia virus (IC50 = 0.13 μg ml−1) in the parental MDCK cells. A thiadiazole, with a guanidinyl group in the 2 position, and ribavirin were active in both the parental cells and resistant cells. Nicotinamide (up to 240-fold excess) did not reverse the anti-influenza activity of LY217896 in vitro or in the mouse infection model. A 10-fold excess of nicotinamide reversed the cytotoxicity of 2-aminothiadiazole but not that of LY217896.

Inhibition of Influenza A and B Viruses by 2′-Deoxy-2′-Fluororibosides

A series of 2′-deoxy-2′-fluororibosides were evaluated for anti-influenza activity in cell culture and in the mouse pneumonia model. Many were found to be potent inhibitors of Influenza A, in chick embryo fibroblast cells (IC50's 0.1–2.9 μM), and in reducing mouse lung virus titres (1–3 log10 units).

Purine analogues proved the most effective, but their activity was an order of magnitude higher in MDCK cells. Anti-influenza activity correlated with intracellular triphosphate levels and with substrate specificity of 2′-deoxycytidine kinase. 2′-deoxy-2′-fluoroguanosine selected for further study was active against all influenza A and B strains tested, including one clinical isolate which proved extremely sensitive when assayed in human tracheal cultures. In vivo, 2′-deoxy-2′-fluoroguanosine (2′-fluorodGuo) was significantly more effective than amantadine or ribavirin in reducing mouse lung virus titre when treatment commenced after infection.

Antiviral combinations for severe influenza

Observational data suggest that the treatment of influenza infection with neuraminidase inhibitors decreases progression to more severe illness, especially when treatment is started soon after symptom onset. However, even early treatment might fail to prevent complications in some patients, particularly those infected with novel viruses such as the 2009 pandemic influenza A H1N1, avian influenza A H5N1 virus subtype, or the avian influenza A H7N9 virus subtype. Furthermore, treatment with one antiviral drug might promote the development of antiviral resistance, especially in immunocompromised hosts and critically ill patients. An obvious strategy to optimise antiviral therapy is to combine drugs with different modes of action. Because host immune responses to infection might also contribute to illness pathogenesis, improved outcomes might be gained from the combination of antiviral therapy with drugs that modulate the immune response in an infected individual. We review available data from preclinical and clinical studies of combination antiviral therapy and of combined antiviral-immunomodulator therapy for influenza. Early-stage data draw attention to several promising antiviral combinations with therapeutic potential in severe infections, but there remains a need to substantiate clinical benefit. Combination therapies with favourable experimental data need to be tested in carefully designed aclinical trials to assess their efficacy.

Characteristics of human infection with avian influenza viruses and development of new antiviral agents

Since 1997, several epizootic avian influenza viruses (AIVs) have been transmitted to humans, causing diseases and even deaths. The recent emergence of severe human infections with AIV (H7N9) in China has raised concerns about efficient interpersonal viral transmission, polygenic traits in viral pathogenicity and the management of newly emerging strains. The symptoms associated with viral infection are different in various AI strains: H5N1 and newly emerged H7N9 induce severe pneumonia and related complications in patients, while some H7 and H9 subtypes cause only conjunctivitis or mild respiratory symptoms. The virulence and tissue tropism of viruses as well as the host responses contribute to the pathogenesis of human AIV infection. Several preventive and therapeutic approaches have been proposed to combat AIV infection, including antiviral drugs such as M2 inhibitors, neuraminidase inhibitors, RNA polymerase inhibitors, attachment inhibitors and signal-transduction inhibitors etc. In this article, we summarize the recent progress in researches on the epidemiology, clinical features, pathogenicity determinants, and available or potential antivirals of AIV.

Influenza pharmacotherapy: present situation, strategies and hopes

INTRODUCTION

Influenza is a serious health threat for people of all ages. The causative virus is evolving continuously and the risk of an unexpected mutant, which cannot be controlled by seasonal vaccination, is real. New and more effective antiviral drugs are needed.

AREAS COVERED

This review examines the antiviral drugs with confirmed efficacy in combating influenza, as well as newer compounds that are currently undergoing testing and will hopefully be marketed in the near future. A comprehensive, state-of-the-art picture of drug therapy for influenza is presented, including novel solutions and effective strategies for prescribing currently available antiviral drugs, with emphasis on the importance of updated local epidemiological data, clinical assessment and laboratory testing.

EXPERT OPINION

Current anti-influenza drug research is no longer tied solely to viral envelope protein targets like haemagglutinin and neuraminidase. New drugs act on the viral RNA polymerase complex, which is involved in transcription and replication of the viral genome, and can prevent the maturation, replication and dissemination of numerous viral subtypes. Combating this infection and reducing the duration of symptoms also has important socioeconomic implications related to health-care spending (including hospitalization for complications) and sick-leave pay for workers.

Influenza pathogenesis: lessons learned from animal studies with H5N1, H1N1 Spanish, and pandemic H1N1 2009 influenza

Because cases of highly pathogenic influenza are rare, no systematic clinical studies have evaluated different therapeutic approaches. Instead, treatment recommendations are aimed at the alleviation of clinical signs and symptoms, especially the restoration of respiratory function, and at the inhibition of virus replication, assuming viral load is responsible for disease phenotype. Studies of highly pathogenic influenza in different animal models, especially nonhuman primates and ferrets, reproduce many of the key observations from clinical cases. Host-response kinetics reveal a delayed but broad activation of genes involved in the innate and acquired immune responses (innate responses produce inflammatory responses), which continue after the virus has been cleared and may contribute importantly to the clinical signs observed. Experimental animal models point to an important role for immune dysregulation in the pathogenesis of highly pathogenic influenza. The use of these models to develop and validate therapeutic approaches is just beginning, but published studies reveal the importance of early treatment with antivirals and show the potential and limitations of approaches aimed at the host response.

Antiviral Resistance in Influenza Viruses: Clinical and Epidemiological Aspects

Two classes of anti-viral agents, the M2 ion channel inhibitors (amantadine, rimantadine) and neuraminidase (NA) inhibitors (oseltamivir, zanamivir) are available for treatment and prevention of infl uenza in most countries of the world. The principle concerns about emergence of antiviral resistance in infl uenza viruses are loss of drug effi cacy, transmission of resistant variants, and possible increased virulence or transmissibility of resistant variants (1). Because seasonal infl uenza is usually an acute, self-limited illness in which viral clearance occurs rapidly due to innate and adaptive host immune responses, the emergence of drug-resistant variants would be anticipated to have modest effects on clinical recovery, except perhaps in immunocompromised or immunologically naïve hosts, such as young infants or during the appearance of a novel strain. In contrast to the limited impact of resistance emergence in the treated immunocompetent individual, the epidemiologic impact of resistance emergence and transmission could be considerable, including loss of both prophylactic and therapeutic activity for a particular drug, at the household, community, or perhaps global level. Infl uenza epidemiology in temperate climates is expected to provide some protection against widespread circulation of resistant variants, as viruses do not persist between epidemics but rather are re-introduced each season and new variants appear often (2, 3).

Current and future antiviral therapy of severe seasonal and avian influenza

The currently circulating H3N2 and H1N1 subtypes of influenza A virus cause a transient, febrile upper respiratory illness in most adults and children (“seasonal influenza”), but infants, the elderly, immunodeficient and chronically ill persons may develop life-threatening primary viral pneumonia or complications such as bacterial pneumonia. By contrast, avian influenza viruses such as the H5N1 virus that recently emerged in Southeast Asia can cause severe disease when transferred from domestic poultry to previously healthy people (“avian influenza”). Most H5N1 patients present with fever, cough and shortness of breath that progress rapidly to adult respiratory distress syndrome. In seasonal influenza, viral replication remains confined to the respiratory tract, but limited studies indicate that H5N1 infections are characterized by systemic viral dissemination, high cytokine levels and multiorgan failure. Gastrointestinal infection and encephalitis also occur. The licensed anti-influenza drugs (the M2 ion channel blockers, amantadine and rimantadine, and the neuraminidase inhibitors, oseltamivir and zanamivir) are beneficial for uncomplicated seasonal influenza, but appropriate dosing regimens for severe seasonal or H5N1 viral infections have not been defined. Treatment options may be limited by the rapid emergence of drug-resistant viruses. Ribavirin has also been used to a limited extent to treat influenza. This article reviews licensed drugs and treatments under development, including high-dose oseltamivir; parenterally administered neuraminidase inhibitors, peramivir and zanamivir; dimeric forms of zanamivir; the RNA polymerase inhibitor T-705; a ribavirin prodrug, viramidine; polyvalent and monoclonal antibodies; and combination therapies.

Avian influenza A (H5N1) infection: targets and strategies for chemotherapeutic intervention

In an avian flu pandemic, which drugs could be used to treat or prevent infection with influenza A (H5N1) virus? Foremost are the viral neuraminidase inhibitors oseltamivir and zanamivir, which have already been used to treat human influenza A (H1N1 and H3N2) and B virus infections. The use of the M2 ion channel blockers amantadine and rimantadine is compounded by the rapid development of drug resistance. Although formally approved for other indications (i.e. treatment of hepatitis C), ribavirin and pegylated interferon might also be useful for controlling avian flu. Combined use of the currently available drugs should be taken into account and attempts should be made to develop new strategies directed at unexplored targets such as the viral proteins hemagglutinin, the viral polymerase (and endonuclease) and the non-structural protein NS1. As has been shown for other viral infections, RNA interference could be a powerful means with which to suppress the replication of avian H5N1.

Antiviral agents active against influenza A viruses

Several drugs are available that could be used, either alone or in combination, for the treatment (prophylaxis or therapy) of an influenza pandemic. These include adamantan(amin)e derivatives (amantadine), neuraminidase inhibitors (zanamivir and oseltamivir), ribavirin and interferon.

Amantadine was the first synthetic compound shown to inhibit influenza-virus replication. It blocks the migration of H⁺ ions into the interior of the virus particles within endosomes, a process that is needed for uncoating to occur.

Neuraminidase inhibitors such as oseltamivir and zanamivir interfere with the release of progeny influenza virions from the surface of infected host cells. In doing so, the neuraminidase inhibitors prevent virus infection of new host cells and thereby halt the spread of infection in the respiratory tract.

Ribavirin targets a cellular enzyme — inosine 5′-monophosphate dehydrogenase, which has a key role in the biosynthesis of GTP and viral RNA synthesis — and is active against both human and avian influenza viruses.

In addition to the available drugs, attempts to further design and develop new antivirals should be intensified, whether based on known molecular targets, such as the neuraminidase or viral uncoating process, or on as-yet relatively unexplored targets such as viral RNA polymerase.

There are currently serious concerns about the control measures that should be taken if a pandemic of influenza A were to strike. De Clercq discusses the therapeutic potential of agents that have been shown to be active against influenza A viruses, and describes emerging strategies for targeting these viruses.

The recent outbreaks of avian influenza A (H5N1) virus, its expanding geographic distribution and its ability to transfer to humans and cause severe infection have raised serious concerns about the measures available to control an avian or human pandemic of influenza A. In anticipation of such a pandemic, several preventive and therapeutic strategies have been proposed, including the stockpiling of antiviral drugs, in particular the neuraminidase inhibitors oseltamivir (Tamiflu; Roche) and zanamivir (Relenza; GlaxoSmithKline). This article reviews agents that have been shown to have activity against influenza A viruses and discusses their therapeutic potential, and also describes emerging strategies for targeting these viruses.

Status presens of antiviral drugs and strategies: Part II: RNA VIRUSES (EXCEPT RETROVIRUSES)

More than 40 compounds have been formally licensed for clinical use as antiviral drugs, and half of these are used for the treatment of HIV infections. The others have been approved for the therapy of herpesvirus (HSV, VZV, CMV), hepadnavirus (HBV), hepacivirus (HCV) and myxovirus (influenza, RSV) infections. New compounds are in clinical development or under preclinical evaluation, and, again, half of these are targeting HIV infections. Yet, quite a number of important viral pathogens (i.e. HPV, HCV, hemorrhagic fever viruses) remain in need of effective and/or improved antiviral therapies.

Development of antivirals against influenza

Morbidity and mortality due to influenza virus infections remain a major problem throughout the world. Yearly, medical costs and loss of productivity resulting from influenza infection are estimated to be in the range of 12 dollars bn in the USA. The predicted increases in the elderly and immune-deficient populations will make influenza an even greater threat in the future. Despite the availability of vaccines, they have been least effective in these high-risk populations. Coupled with the requirement for routine revaccination, the need for effective antiviral agents is illustrated. The currently approved drugs, amantadine, rimantadine and ribavirin (in some countries), have limitations. They are only inhibitory against influenza A viruses, are prone to adverse reactions and quickly give rise to resistant virus. This review examines current drug therapies, antivirals in development and possible future opportunities for anti-influenza drugs.

Approaches and strategies for the treatment of influenza virus infections

Influenza A and B viruses belong to the Orthomyxoviridae family of viruses. These viruses are responsible for severe morbidity and significant excess mortality each year. Infection with influenza viruses usually leads to respiratory involvement and can result in pneumonia and secondary bacterial infections. Vaccine approaches to the prophylaxis of influenza virus infections have been problematic owing to the ability of these viruses to undergo antigenic shift by exchanging genomic segments or by undergoing antigenic drift, consisting of point mutations in the haemagglutinin (HA) and neuraminidase (NA) genes as a result of an error-prone viral polymerase. Historically, antiviral approaches for the therapy of both influenza A and B viruses have been largely unsuccessful until the elucidation of the X-ray crystallographic structure of the viral NA, which has permitted structure-based drug design of inhibitors of this enzyme. In addition, recent advances in the elucidation of the structure and complex function of influenza HA have resulted in the discovery of a number of diverse compounds that target this viral protein. This review article will focus largely on newer antiviral agents including those that inhibit the influenza virus NA and HA. Other novel approaches that have entered clinical trials or been considered for their clinical utility will be mentioned.

Antiviral agents

In recent years, the antiviral armamentarium has expanded considerably. Currently available agents are virustatic, inhibiting specific steps in the process of viral replication. No agent is active against nonreplicating or latent viruses. Acyclovir is useful in the treatment of genital herpes, herpes simplex encephalitis, mucocutaneous herpetic infection, varicella infection in the immunosuppressed host, and herpes zoster infection in the normal and the immunosuppressed host. It can also be used for prevention of herpesvirus infection in immunocompromised patients. Ganciclovir is indicated for the treatment of cytomegalovirus retinitis in patients with acquired immunodeficiency syndrome (AIDS) and is effective in the management of organ-specific cytomegalovirus infection in other immunocompromised patients. Chronic hepatitis C and condyloma acuminatum due to human papillomavirus respond to therapy with interferon alfa-2b. Patients with human immunodeficiency virus infection and CD4 lymphocyte counts of less than 500 cells/mm3 should be treated with zidovudine. Amantadine is useful in a therapeutic and prophylactic role in the management of influenza A virus infection. With the expanded use of and indications for antiviral therapy, clinically significant resistance to these agents has been encountered with increasing frequency.

New aspects of influenza viruses

Influenza virus infections continue to cause substantial morbidity and mortality with a worldwide social and economic impact. The past five years have seen dramatic advances in our understanding of viral replication, evolution, and antigenic variation. Genetic analyses have clarified relationships between human and animal influenza virus strains, demonstrating the potential for the appearance of new pandemic reassortants as hemagglutinin and neuraminidase genes are exchanged in an intermediate host. Clinical trials of candidate live attenuated influenza virus vaccines have shown the cold-adapted reassortants to be a promising alternative to the currently available inactivated virus preparations. Modern molecular techniques have allowed serious consideration of new approaches to the development of antiviral agents and vaccines as the functions of the viral genes and proteins are further elucidated. The development of techniques whereby the genes of influenza viruses can be specifically altered to investigate those functions will undoubtedly accelerate the pace at which our knowledge expands.

Evaluation of the anti-influenza virus activities of 1,3,4-thiadiazol-2-ylcyanamide (LY217896) and its sodium salt

1,3,4-Thiadiazol-2-ylcyanamide (LY217896) and its sodium salt were shown to be effective against influenza A and B viruses in vitro and in the mouse model. In nondividing confluent MDCK cells, the 50% inhibitory concentration of LY217896 ranged from 0.37 to 1.19 micrograms/ml against various strains of influenza A virus and from 0.75 to 1.54 micrograms/ml against various strains of influenza B virus, with no apparent cytotoxicity. However, at a concentration of 0.31 microgram/ml, LY217896 inhibited the replication of dividing MDCK cells. LY217896 (9 mg/m2 of body surface area per day) administered in the diet, in the drinking water, by oral gavage, by intraperitoneal injection, or by aerosolization was well tolerated and protected CD-1 mice infected with a lethal dose of influenza A or B virus. Effective administration of the compound could be delayed for up to 96 h postinfection. Virus titer was reduced by 1 to 2 log10 units in lungs of mice given LY217896 in the drinking water. Mice treated initially with protective levels of LY217896 were resistant to a subsequent challenge of influenza virus in the absence of the compound, indicating that the animals were able to develop immunity to the initial infection. Administration of LY217896 to uninfected mice did not induce interferon-like activity or interfere with natural killer cell function. In the ferret, LY217896 was effective in preventing fever induced by influenza virus.

Comparison of oral and aerosol ribavirin regimens in the high risk elderly

A comparison of different regiments of ribavirin (R), administered either orally or by aerosol, was performed in 16 elderly subjects (13 men, 3 women, mean age 63 +/- 8 years) considered to be in the "high-risk" category for complications from influenza as defined by the Centers for Disease Control. The subjects were divided into four groups. Group O-600 received 600 mg orally R every 8 hours for 48 hours followed by 200 mg every 8 hours for 72 hours for a total dose of 5.4 g (22.1 mmol). Group O-800 received 800 mg oral R every 8 hours for 24 hours followed by 400 mg every 12 hours for 96 hours for a total dose of 4.1 g (22.9 mMoles). Group A-40 received R (40 mg/ml) aerosolized through a small particle aerosol generator for 6 hours every 12 hours for 96 hours, yielding an average delivered dose of 6.2 g (25.4 mMoles) R. Group A-60 received aerosolized R (60 mg/mL) for 2 hours every 8 hours for 96 hours, yielding an average delivered dose of 4.6 g (18.8 mMoles) R. No hematologic or other laboratory abnormalities were associated with any of the regimens. Group O-800 and O-600 reached mean peak plasma R levels of 11.8 microM and 5.3 microM, respectively, after 18 hours of therapy. Subsequent administration of 20 mg R every 8 hours was sufficient to maintain a plasma R level greater than 7 microM. Among the aerosol groups, group A-40 approached steady state plasma R levels (8-10 microM) more quickly than group A-60.(ABSTRACT TRUNCATED AT 250 WORDS)

Antiviral Chemotherapyand Prophylaxis of Viral Respiratory Disease

Several antiviral agents are currently available for the treatment and prophylaxis of viral respiratory disease. These include oral amantadine for influenza A and aerosolized ribavirin for respiratory syncytial virus infections. Additional agents, such as rimantadine and intranasal interferons, and newer approaches, including thе use of combination chemotherapy, offer promise for the improved management of viral respiratory tract infections.

Oral ribavirin treatment of influenza A and B

A loading dose and short-term administration of oral ribavirin significantly improved symptoms and signs of influenza type A or B infection in 25 patients. The antiviral effect was not significant. No adverse clinical effects or significant laboratory values were observed. Oral treatment of patients with influenza A or B infection might be possible with ribavirin.

Clinical review of ribavirin

The recent approval of ribavirin aerosol for the treatment of severe respiratory syncytial virus (RSV) in infants and young children is a significant addition to the antiviral drugs available today. When administered as an aerosolized form by face mask or mist tent for 20 to 21 hours per day, ribavirin effectively decreases the symptoms of RSV infection and the shedding of RSV virus. Studies of other viral infections such as viral hepatitis, influenza A and B, Lassa fever, genital herpes, and herpes zoster have demonstrated promising, but inconclusive results. Further studies are needed to justify ribavirin therapy for these indications.

Antiviral chemotherapy and chemoprophylaxis

Antiviral compounds have been developed for use in chemoprophylaxis and chemotherapy of a variety of infections in humans, including those caused by influenza viruses, respiratory syncytial virus, and herpesviruses. The efficacy of several of these compounds has been demonstrated in rigorously controlled trials. Advances in molecular virology have led to the identification of biochemically defined, virus-specific functions that serve as appropriate targets for the future development of antiviral compounds. Clinical investigators and practicing physicians are now confronting questions previously raised with the use of antibacterial antibiotics. These questions concern appropriate routes of administration for antiviral compounds, optimal dosage regimens, risks of long-term prophylaxis, and the emergence of resistant organisms.

Clinical applications of antiviral agents for chemophrophylaxis and therapy of respiratory viral infections

Table III summarizes clinical applications of antiviral agents in respiratory viral infections. (table: see text) For influenza A virus infections, both oral amantadine and rimantadine are effective when used for seasonal prophylaxis and for prophylaxis in institutional populations. Both of these drugs, as well as aerosolized ribavirin, have antiviral and therapeutic effects in uncomplicated influenza. It remains to be determined whether any of these modalities or possibly their combined use [44] will be useful in treating severe influenza hospitalized patients or whether they can prevent the development of complications in high risk patients. Unfortunately, there is no parenteral formulation of amantadine or rimantadine for use in critically ill patients. Aerosolized ribavirin has also been shown to have modest therapeutic effects in influenza B virus infection. However, a major need exists for an antiviral which is active against influenza B virus and which can be used on an outpatient basis. Controlled clinical trials have shown that aerosolized ribavirin therapy improves arterial oxygenation and modifies the severity of respiratory syncytial virus bronchiolitis and pneumonia [3,5]. Its role in treating life-threatening disease or in modifying the long-term sequelae of RSV infections are unknown at the present time. Again, a specific antiviral agent is needed for outpatient use in preventing or treating RSV infections. Finally, after over a decade of work since the original observation that intranasal interferon could prevent experimental rhinovirus infection [11], recent studies have established that intranasal rIFN-a2 is effective in the postexposure prophylaxis of rhinovirus colds in families [42]. This strategy needs to be studied with regard to the prevention of infection and its complications in high risk patients and it remains to be determined whether intranasal interferon will have therapeutic activity in established colds.

Advances in antiviral therapy

Recent developments have increased the options for treatment of viral infections. Vidarabine, an agent originally released for herpes simplex encephalitis, has more recently been shown to be of benefit in neonatal herpes simplex infection and in varicella-zoster infections in immunocompromised hosts. The introduction of acyclovir represents a major advance in antiviral therapy because of its low host toxicity and marked selectivity for herpes simplex and varicella-zoster viruses. Extensive controlled clinical trials demonstrate efficacy in the treatment of infections caused by these viruses in the immunocompromised host and in genital herpes simplex infections in normal hosts. The use of recombinant DNA technology has increased the purity, variety, and availability of interferons for clinical trial. Earlier experience with natural buffy coat-derived alpha interferon showed efficacy in the treatment of varicella-zoster infections in the immunocompromised host, as well as prophylaxis of herpes virus infections in high-risk populations. These results have to be confirmed using the newer interferon preparations. Under development are a variety of new drugs with broadened viral spectrum and improved pharmacokinetic properties. These include nucleoside analogues and novel interferons with modified amino acid sequences. One or more of these agents, used singly or in combination, may prove useful in the more difficult therapeutic problems, such as cytomegalovirus and hepatitis B infections.

Ribavirin

Despite the plethora of antibiotics available for the treatment of bacterial infections, very few agents have been developed to treat viral diseases. Ribavirin (Virazole) is a triazole nucleoside antiviral agent that produces selective antiviral effects against a broad spectrum of RNA and DNA viruses. The drug has been effective in the treatment of naturally occurring influenza A and B infections when administered by aerosol; oral administration has been ineffective. Ribavirin aerosol therapy also has proven effective to reduce symptoms of respiratory syncytial virus infections in young adults and hospitalized neonates. Ribavirin aerosol may be the first antiviral agent to treat these common diseases.

Combined interferon-alpha 2, rimantadine hydrochloride, and ribavirin inhibition of influenza virus replication in vitro

Recombinant DNA-produced human interferon-alpha 2 inhibited the replication of influenza A and B viruses in primary rhesus monkey kidney cells (RMK). Human interferon-alpha 2 interacted additively or synergistically with rimantadine hydrochloride or ribavirin in reducing the yield of clinical isolates of either H3N2 or H1N1 subtype influenza A viruses. The combination of human interferon-alpha 2 and ribavirin also inhibited the replication of an influenza B virus to a greater extent than either single agent. In addition to drug concentration, the virus inoculum and duration of culture were important variables in determining the degree of inhibition. Single drugs or combinations did not significantly inhibit the growth of uninfected RMK cells, which indicated that the observed interactions with respect to antiviral activity were not due to cell cytotoxicity.

Antiviral drugs 1983

A number of antiviral compounds are currently available, and several others are of great interest. Trifluridine, idoxuridine, and vidarabine are effective topically in herpes simplex virus keratoconjunctivitis infection. Vidarabine (and presumably acyclovir) is effective in herpes simplex virus encephalitis and in herpes zoster infections in the immunocompromised host. Acyclovir is effective topically, orally, and intravenously in primary herpes genitalis, and the oral and intravenous forms are effective in recurrent herpes genitalis as well. Amantadine and rimantadine are effective prophylactically and therapeutically in influenza A infections. Ribavirin and interferon, although not licensed, are of great interest. Ribavirin may be useful in respiratory syncytial virus infections, and interferon may be of benefit in common colds and related disorders.

Comparative activity of amantadine and ribavirin against influenza virus in vitro: possible clinical relevance

The activities of amantadine and ribavirin against influenza A viruses were compared against low-multiplicity (plaque inhibition) and high-multiplicity (protein synthesis inhibition) infections. Our results suggest that the predictive value of in vitro data for the clinic may be improved by consideration of tests against a high-multiplicity infection.

Comparative therapeutic effect of aerosolized and oral rimantadine HCl in experimental human influenza A virus infection

Thirty-six adult volunteers were inoculated intranasally with 7.2 log10 egg infectious doses 50% of an attenuated A/Khabarovsk/77/H1N1 virus. Twenty-four hours later volunteers were begun on both aerosol treatments (rimantadine HCl 25 mg in saline or saline, 10 min exposure twice daily) and oral medications (rimantadine HCl 50 mg or placebo every 6 h, three times daily) which were administered for 5 days. Virus-positive volunteers receiving placebo by both of the two routes had a peak in clinical illness scores on the second treatment day (mean score 5.3), which was not observed in either the aerosol rimantadine (0.6) or oral rimantadine (0.9) treated volunteers. On the second treatment day, the proportion of virus-positive volunteers with elevated axillary temperature measurements and the mean peak temperature measurement were also significantly reduced in both drug groups. No significant effects on the duration of virus shedding were noted. In experimental influenza A virus infection, characterized by mild clinical illness and short duration of virus shedding, low doses of aerosolized rimantadine had a therapeutic effect comparable to that found with larger doses of oral rimantadine.

Ribavirin and inosiplex: a review of their present status in viral diseases

A considerable amount of information has accumulated during the past 10 years in the search for antiviral agents. Ribavirin and inosiplex are 2 interesting developments to come out of this search. Ribavirin, a synthetic nucleoside, has an unusually wide spectrum of antiviral activity, especially when tested in vitro. A large number of RNA and DNA viruses are sensitive, especially herpes viruses, poxvirus, influenza, parainfluenza, reovirus, togavirus, and RNA tumour viruses. The in vivo antiviral spectrum of activity is much narrower, with activity against herpes virus, influenza, parainfluenza, measles and adenoviruses. However, controlled clinical trials have not been uniformly successful in treating influenza, hepatitis, herpes simplex and herpes zoster. Inosiplex has been shown to have antiviral activity in vivo against influenza, herpes simplex, rhinovirus and vaccinia virus infections. However, antiviral activity has not been consistently demonstrated, and this observation led to further studies which revealed its immunomodulating effects. The accumulated evidence has indicated that inosiplex is more a prohost agent rather than an antiviral drug. Immune functions which are depressed during viral infection can be restored to normal by inosiplex therapy. At present, neither ribavirin nor inosiplex alone has been shown to be uniformly successful in the treatment of human viral diseases. Nevertheless, their potential place in chemotherapy should not be neglected, although further data are needed to determine what this place will be. Whether combining them with other antiviral agents such as interferon, acyclovir, Ara-A, and so on, would produce a potentiation of action and improved antiviral chemotherapy, will be an interesting area for further study.

Enhancement of activity against influenza viruses by combinations of antiviral agents

In an investigation of alternative therapeutic approaches for the treatment of influenza virus infections, the antiviral activities of rimantadine hydrochloride, amantadine hydrochloride, ribavirin, and combinations of these drugs were assessed in vitro. Madin-Darby canine kidney cell monolayers were inoculated with recent isolates of influenza viruses at low multiplicities of infection, and virus titers were determined after 24 h. The combination of rimantadine and ribavirin resulted in an enhanced antiviral effect (a decrease in virus titer of > 1.0 log10 plaque-forming unit per ml at 24 h relative to the maximal effect of a single drug) against A/USSR/90/77/H1N1, A/Texas/1/77/H3N2, A/New Jersey/76/HSW1N1, and A/PR/834/H0N1 viruses. The degree of inhibition depended on the virus strain used, the drug concentrations, and the virus inoculum. Amantadine and ribavirin showed enhanced activity. Ribavirin in combination with high (50 micrograms/ml), but not low (1.56 to 25 micrograms/ml), concentrations of rimantadine showed an enhanced antiviral effect against B/Hong Kong/72 virus. An assay of Madin-Darby canine kidney cell proliferation in the presence of drugs showed that the enhanced inhibitory effect of drug combinations was not due to increased cytotoxicity.