The ribavirin analog ICN 17261 demonstrates reduced toxicity and antiviral effects with retention of both immunomodulatory activity and reduction of hepatitis-induced serum alanine aminotransferase levels

Anti-HCV and Zika activities of ribavirin C-nucleosides analogues

Ribavirin is an unnatural nucleoside exhibiting broad spectrum of antiviral and antitumor activities, still very widely studied particularly in a repositioning approach. C-triazolyl nucleoside analogues of ribavirin have been synthesized, as well as prodrugs and glycosylated or peptide conjugates to allow a better activity by vectorization into the liver or by facilitating uptake into the cells. The antiviral properties of all synthesized compounds have been evaluated in vitro against two important human viral pathogens belonging to the Flaviviridae family: hepatitis C virus (HCV) and Zika virus (ZIKV). There are no therapeutic options for Zika virus, whereas those available for HCV can be still improved. Our results indicated that compound 2 carrying an N-hydroxy carboxamide function exhibits the most inhibitory activities against both viruses. This compound moderately inhibited the propagation of HCV with an IC₅₀ value of 49.1 μM and Zika virus with an IC₅₀ of 33.2 μM comparable to ribavirin in the Vero cell line. The results suggest that compound 2 and its new derivatives may be candidates for further development of new anti-HCV and anti-ZIKV antiviral drugs.

Critical View on the Usage of Ribavirin in Already Existing Psychostimulant-Use Disorder

Substance-use disorder represents a frequently hidden non-communicable chronic disease. Patients with intravenous drug addiction are at high risk of direct exposure to a variety of viral infections and are considered to be the largest subpopulation infected with the hepatitis C virus. Ribavirin is a synthetic nucleoside analog that has been used as an integral component of hepatitis C therapy. However, ribavirin medication is quite often associated with pronounced psychiatric adverse effects. It is not well understood to what extent ribavirin per se contributes to changes in drug-related neurobehavioral disturbances, especially in the case of psychostimulant drugs, such as amphetamine. It is now well-known that repeated amphetamine usage produces psychosis in humans and behavioral sensitization in animals. On the other hand, ribavirin has an affinity for adenosine A1 receptors that antagonistically modulate the activity of dopamine D1 receptors, which play a critical role in the development of behavioral sensitization. This review will focus on the current knowledge of neurochemical/ neurobiological changes that exist in the psychostimulant drug-addicted brain itself and the antipsychotic-like efficiency of adenosine agonists. Particular attention will be paid to the potential side effects of ribavirin therapy, and the opportunities and challenges related to its application in already existing psychostimulant-use disorder.

Ribavirin suppresses bacterial virulence by targeting LysR-type transcriptional regulators

Targeting bacterial virulence mechanisms without compromising bacterial growth is a promising strategy to prevent drug resistance. LysR-type transcriptional regulators (LTTRs) possess structural conservation across bacterial species and regulate virulence in numerous pathogens, making them attractive targets for antimicrobial agents. We targeted AphB, a Vibrio cholerae LTTR, which regulates the expression of genes encoding cholera toxin and toxin-co-regulated pilus for inhibitor designing. Since AphB ligand is unknown, we followed a molecular fragment-based approach for ligand designing using FDA-approved drugs and subsequent screen to identify molecules that exhibited high-affinity binding to AphB ligand-binding pocket. Among the identified compounds, ribavirin, an anti-viral drug, antagonized AphB functions. Ribavirin perturbed Vibrio cholerae pathogenesis in animal models. The inhibitory effects of the drug was limited to the bacteria expressing wild type AphB, but not its constitutively active mutant (AphB_N100E), which represents the ligand-bound state, suggesting that ribavirin binds to the active site of AphB to exert its inhibitory role and there exists no AphB-independent mechanism of its action. Similarly, ribavirin suppressed the functions of Salmonella Typhi LTTR Hrg, indicating its broad spectrum efficacy. Moreover, ribavirin did not affect the bacterial viability in culture. This study cites an example of drug repurposing for anti-infective therapy.

Challenges for the Development of Ribonucleoside Analogues as Inducers of Error Catastrophe

RNA viruses are responsible for numerous human diseases; some of these viruses are also potential agents of bioterrorism. In general, the replication of RNA viruses results in the incorporation of at least one mutation per round of replication, leading to a heterogeneous population, termed a qua-sispecies. The antiviral nucleoside ribavirin has been shown to cause an increase in the mutation frequency of RNA viruses. This increase in mutation frequency leads to a loss of viability due to error catastrophe. In this article, we review lethal mutagenesis as an antiviral strategy, emphasizing the challenges remaining for the development of lethal mutagenesis into a practical clinical approach.

Current and future trends in the prevention, treatment and control of rabies

Rabies remains a global zoonosis of major public health, agricultural and economic significance. Dogs are the major animal reservoirs in developing regions, wildlife maintain cycles of infection even in developed countries and new viral etiological agents continue to emerge. Nearly all human rabies cases are related directly to animal bite and thus, primary disease prevention requires minimization of suspected exposures. Once exposure occurs, modern prophylaxis entails immediate wound care, local infiltration of rabies immune globulin and parenteral administration of modern cell culture vaccines in multiple doses. Pre-exposure vaccination should occur in selected population groups at risk of occupational exposure. Historically, survival from fatal rabies by at least five human patients, vaccinated prior to the onset of clinical signs, signaled initial optimism as to the theoretical utility of medical intervention. Recently, the heroic recovery of an unvaccinated teenager from clinical rabies offers hope of future specific therapy. Canine rabies elimination is the key towards ultimate reduction of the disease burden, as first illustrated in developed countries. Implementation of oral vaccination in free-ranging carnivore hosts demonstrates the feasibility of disease abatement in particular wildlife populations, such as demonstrated in Europe and North America, with an enhanced need for application to developing countries in the Americas, Africa and Eurasia.

Single- and Multiple-Dose Pharmacokinetics of Levovirin Valinate Hydrochloride (R1518) in Healthy Volunteers

R1518 is a valine ester prodrug of levovirin as an investigational new drug for the treatment of hepatitis C virus. Two phase 1, single- and multiple-dose studies were conducted to investigate the pharmacokinetics of R1518 in healthy volunteers. After oral dosing, R1518 was rapidly and exclusively converted to levovirin. Levovirin plasma concentrations peaked at 2 hours, with T(1/2) ranging from 6 to 8 hours. The T(1/2) of R1518 was less than 1 hour, with relative exposures (R1518/levovirin) less than 6%. A high-fat meal did not affect the pharmacokinetics. The female groups in both studies had higher plasma levels than males did due to age and renal function difference. An accumulation ratio of 1.3 to 1.5 was observed with the twice-daily regimen. About 75% to 90% of the levovirin equivalent dose was recovered in urine. Increase in exposure was slightly disproportionate to increase in dose. Significantly improved oral absorption of levovirin was achieved following administration of R1518.

Ribavirin-resistant mutants of human enterovirus 71 express a high replication fidelity phenotype during growth in cell culture

It has been shown in animal models that ribavirin-resistant poliovirus with a G64S mutation in its 3D polymerase has high replication fidelity coupled with attenuated virulence. Here, we describe the effects of mutagenesis in the human enterovirus 71 (HEV71) 3D polymerase on ribavirin resistance and replication fidelity. Seven substitutions were introduced at amino acid position 3D-G64 of a HEV71 full-length infectious cDNA clone (26M). Viable clone-derived virus populations were rescued from the G64N, G64R, and G64T mutant cDNA clones. The clone-derived G64R and G64T mutant virus populations were resistant to growth inhibition in the presence of 1,600 μM ribavirin, whereas the growth of parental 26M and the G64N mutant viruses were inhibited in the presence of 800 μM ribavirin. Nucleotide sequencing of the 2C and 3D coding regions revealed that the rate of random mutagenesis after 13 passages in the presence of 400 μM ribavirin was nearly 10 times higher in the 26M genome than in the mutant G64R virus genome. Furthermore, random mutations acquired in the 2C coding regions of 26M and G64N conferred resistance to growth inhibition in the presence of 0.5 mM guanidine, whereas the G64R and G64T mutant virus populations remained susceptible to growth inhibition by 0.5 mM guanidine. Interestingly, a S264L mutation identified in the 3D coding region of 26M after ribavirin selection was also associated with both ribavirin-resistant and high replication fidelity phenotypes. These findings are consistent with the hypothesis that the 3D-G64R, 3D-G64T, and 3D-S264L mutations confer resistance upon HEV71 to the antiviral mutagen ribavirin, coupled with a high replication fidelity phenotype during growth in cell culture.

Rupprecht C. Design of future rabies biologics and antiviral drugs

In recent years, no major paradigm shifts have occurred in the utilization of new products for the prevention and control of rabies. Development of new cost-effective rabies biologics and antiviral drugs is critical in continuing to prevent and reduce disease. Current rabies vaccines are highly effective but have developed largely based on technical improvements in the vaccine industry. In the future, alternative approaches for improved vaccines, including novel avirulent rabies virus (RABV) vectors, should be pursued. Any rabies vaccine that is effective without the need for rabies immune globulin (RIG) will contribute fundamentally to disease prevention by reducing the cost and complexity of postexposure prophylaxis (PEP). The lack of high quality, affordable RIG is a continuing problem. Virus-specific monoclonal antibodies (mAbs) will soon fulfill the PEP requirement for passive immunity, currently met with RIG. Several relevant strategies for mAb production, including use of transgenic mice, humanization of mouse mAbs, and generation of human immune libraries, are underway. As a result of successful PEP and pre-exposure prophylaxis in developed countries, until recently, no significant focused efforts have been devoted to RABV-specific antiviral agents. To date, combination therapy including broad spectrum antiviral agents has been successful in only one case, and reports of antiviral activity are often conflicting. Current antiviral strategies target either the nucleoprotein or phosphoprotein, but drugs targeting the viral polymerase should be considered. Considering the lag from creation of new concepts to experimental development and clinical trials, many years will likely elapse between today's ideas and tomorrow's practices.

Ribavirin analogs

Ribavirin is ineffective against hepatitis C virus as mono-therapy but is critical in attaining both early virologic response and sustained virologic response when combined with pegylated interferon. Ribavirin has significant dose-limiting toxicities, the most important of which is hemolytic anemia. Taribavirin is a ribavirin pro-drug, which targets the liver and has less incidence of anemia, and it may be a promising alternative to ribavirin 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.

Prodrugs of nucleoside analogues for improved oral absorption and tissue targeting

Nucleoside analogues are widely used for the treatment of antiviral infections and anticancer chemotherapy. However, many nucleoside analogues suffer from poor oral bioavailability due to their high polarity and low intestinal permeability. In order to improve oral absorption of these polar drugs, prodrugs have been employed to increase lipophilicity by chemical modification of the parent. Alternatively, prodrugs targeting transporters present in the intestine have been exploited to facilitate the transport of the nucleoside analogues. Valacyclovir and valganciclovir are two successful valine ester prodrugs transported by the PepT1 transporter. Recently, research efforts have focused on design of prodrugs for tissue specific delivery to improve efficacy and safety. This review presents advances of prodrug approaches for improved oral absorption of nucleoside analogues and recent developments in tissue targeting.

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.

Mechanism of action of ribavirin in the treatment of chronic hepatitis C

The current standard treatment for chronic hepatitis C consists of the combination of pegylated interferon and ribavirin. Although interferon is known to have potent antiviral, immunomodulatory, and anti-inflammatory activities against hepatitis C, the mechanism of ribavirin action against this virus is not clearly understood. This article will review proposed mechanisms of ribavirin activity, along with their supporting data, covering ribavirin's roles in the direct inhibition of hepatitis C viral replication, the inhibition of inosine monophosphate dehydrogenase, the induction of mutagenesis to the threshold of error catastrophe, and immunomodulation.

Transport of levovirin prodrugs in the human intestinal Caco-2 cell line

The transport of 10 amino acid ester prodrugs of levovirin (LVV) was investigated in the human intestinal Caco-2 cell line in order to overcome the poor oral bioavailability of LVV, an investigational drug for the treatment of hepatitis C infection. The prodrugs were designed to improve the permeability of LVV across the intestinal epithelium by targeting the di/tri-peptide carrier, PepT1. Caco-2 cell monolayers were employed to study the transport and hydrolysis properties of the prodrugs. Among all mono amino acid ester prodrugs studied, the LVV-5'-(L)-valine prodrug (R1518) exhibited the maximum increase (48-fold) in permeability with nearly complete conversion to LVV within 1 h. Di-amino acid esters did not offer significant enhancement in permeability comparing with mono amino acid esters and exhibited slower conversion to LVV in Caco2 cell monolayers. Pharmacokinetic screening studies of the prodrugs in rats yielded the highest fold increase (6.9-fold) of AUC with R1518 and in general displayed a similar trend to that observed in increases of permeability in Caco-2 cells. Mechanisms involved in the Caco-2 cell transport of R1518 were also investigated. Results of bi-directional transport studies support the involvement of carrier-mediated transport mechanisms for R1518, but not for the LVV-5'-(D)-valine prodrug or LVV. Moreover, the permeability of R1518 was found to be proton dependent. PepT1-mediated transport of R1518 was supported by results of competitive transport studies of R1518 with the PepT1 substrates enalapril, Gly-Sar, valganciclovir, and cephalexin. R1518 was also found to inhibit the permeability of valganciclovir and cephalexin. These results suggest that R1518 is a PepT1 substrate as well as an inhibitor.

Future Therapies for Hepatitis C

Although pegylated interferon-α plus ribavirin has become the standard for treating chronic hepatitis C virus infection, a substantial number of patients do not tolerate therapy and require dose reduction or discontinuation, or do not respond to this combination therapy. Thus, new therapeutic options are needed. An increased knowledge of the hepatitis C virus and an understanding of its replication cycle, as well as advances in biotechnology, have stimulated the development of numerous new antiviral treatments for patients with hepatitis C virus infection. This review focuses on four classes of new agents: new interferons, ribavirin-like molecules, specific small-molecule hepatitis C virus inhibitors and new immune therapies, with particular emphasis on medications in the later stages of development.

Mechanisms of action of ribavirin against distinct viruses

The nucleoside analogue ribavirin has antiviral activity against many distinct viruses both in vitro and in vivo. Five distinct mechanisms have been proposed to explain the antiviral properties of ribavirin. These include both indirect mechanisms (inosine monophosphate dehydrogenase inhibition, immunomodulatory effects) and direct mechanisms (interference with RNA capping, polymerase inhibition, lethal mutagenesis). Recent concerns about bioterrorism have renewed interest in exploring the antiviral activity of ribavirin against unique viruses. In this paper, we review the proposed mechanisms of action with emphasis on recent discoveries, as well as the implications of ribavirin resistance. Evidence exists to support each of the five proposed mechanisms of action, and distinct virus/host combinations may preferentially favour one or more of these mechanisms during antiviral therapy. Copyright © 2005 John Wiley & Sons, Ltd.

Epoetin alfa for the treatment of combination therapy-induced hemolytic anemia in patients infected with hepatitis C virus

In the United States, about 2.7 million people are chronically infected with the hepatitis C virus, accounting for nearly 1.8% of the population. The current standard of therapy is a combination of pegylated interferon products and ribavirin. A common adverse effect associated with this therapy is anemia, which is frequently referred to as mixed anemia because of the synergistic contribution of the interferons and ribavirin. The effect of ribavirin on the development of anemia is considered greater than that of interferon. The current standard of practice for treating this adverse effect is reduction of the dosages of both drugs, at prespecified hemoglobin levels. However, recent findings underscore the importance of maintaining adequate dosages of interferon and ribavirin, which may be crucial in achieving an early virologic response and a sustained virologic response in treating patients with hepatitis C infection. Treatment with epoetin alfa for this mixed anemia significantly improved hemoglobin levels and quality of life, and enabled adequate dosages of ribavirin to be maintained. Future studies should address several issues: when to start epoetin alfa treatment, the duration of treatment, the drug's optimal dosage, its effects on end-of-treatment and sustained virologic response rates, and a cost analyses.

Viramidine Demonstrates Better Safety Than Ribavirin in Monkeys But Not Rats

The safety of viramidine was evaluated and compared with ribavirin in one-month and six-month studies in rats and one-month and nine-month studies in monkeys. Viramidine administration produced hemolytic anemia, characterized by decreases in hemoglobin (Hgb) concentrations, which was accompanied by erythroid hyperplasia of the bone marrow or increase in reticulocyte counts. In the 1-month study in rats, viramidine or ribavirin dosing at 120 mg/kg/day reduced Hgb concentrations (12-16% and 13-20% compared to controls, respectively) and caused slight erythroid hyperplasia in the bone marrow. In the 6-month study in rats, viramidine or ribavirin dosing at 90 mg/kg/day reduced Hgb concentrations (16-18% and 18% compared to controls, respectively) and increased reticulocyte counts (>25%). Although toxicity effects in monkeys were similar to rats, they occurred at higher doses of viramidine compared to ribavirin. In the 1-month monkey study, viramidine at 600 mg/kg/day caused slight or no reduction (9%-0% in males and females) in Hgb concentrations compared to moderate reduction for ribavirin at 300 mg/kg/day (14%-20% in males and females). There were no signs of erythroid hyperplasia in bone marrow or significant increase in reticulocytes detected after viramidine or ribavirin dosing in monkeys. In the 9-month study in monkeys, viramidine at 180 mg/kg/day slightly reduced Hgb concentrations (5-11%) as compared to ribavirin at 60 mg/kg/day (9-12%). Both treatments had an increase in reticulocytes (49-53% vs. 43-59% compared to controls, respectively). Moderate decrease in Hgb levels (26-29%) together with large increase in reticulocyte counts (203-224%) were seen for viramidine at 600 mg/kg/day. All the changes were reversible after a recovery phase in both rats and monkeys. It is concluded that ribavirin and viramidine produced similar toxicity in rats. However, viramidine is safer than ribavirin in monkeys.

Chronic hepatitis C and no response to antiviral therapy: potential current and future therapeutic options

A significant proportion of chronic hepatitis C patients fails to achieve sustained virologic response even after treatment with the current, more potent, combination of pegylated interferon-alpha (IFNa) plus ribavirin. Such patients represent a rather heterogeneous group and may be divided initially into relapsers and nonresponders. Both the type of previous therapy and of previous response are very important factors for the indication and the type of re-treatment. The combination of pegylated IFNa and ribavirin seems to be a rational approach for patients who failed to respond to IFNa monotherapy. Pegylated IFNa-based regimens appear to induce sustained responses in 40-68% of relapsers but in only 11% of nonresponders to previous therapy with standard IFNa plus ribavirin. Thus, new therapeutic approaches are needed for the latter subgroup of patients as well as those who fail to respond to pegylated IFNa-based regimens. Such new approaches currently under evaluation include the triple combination of pegylated IFNa, ribavirin, and amantadine, alternative types of IFN, use of agents with ribavirin like activity but lesser degrees of side-effects, inhibitors of hepatitis C virus (HCV) replication, mainly inhibitors of NS3 protease or helicase, antisense oligonucleotides, and ribozymes, and several immunomodulators. Moreover, maintenance antifibrotic therapy, mostly with low doses of pegylated IFNa, are under evaluation in patients with advanced fibrosis. Thus, even in the current era of the potent pegylated IFNa-based regimens, the management of these difficult-to-treat patients represents an increasingly frequent problem and perhaps the most challenging therapeutic task in chronic hepatitis C.

Genome sequence of the Brown Norway rat yields insights into mammalian evolution

The laboratory rat (Rattus norvegicus) is an indispensable tool in experimental medicine and drug development, having made inestimable contributions to human health. We report here the genome sequence of the Brown Norway (BN) rat strain. The sequence represents a high-quality 'draft' covering over 90% of the genome. The BN rat sequence is the third complete mammalian genome to be deciphered, and three-way comparisons with the human and mouse genomes resolve details of mammalian evolution. This first comprehensive analysis includes genes and proteins and their relation to human disease, repeated sequences, comparative genome-wide studies of mammalian orthologous chromosomal regions and rearrangement breakpoints, reconstruction of ancestral karyotypes and the events leading to existing species, rates of variation, and lineage-specific and lineage-independent evolutionary events such as expansion of gene families, orthology relations and protein evolution.

New therapies for chronic hepatitis C virus infection

Chronic hepatitis C infection is associated with significant morbidity and mortality in addition to substantial social and health-related costs. Since the identification of the virus and determination of the HCV genome over a decade ago, considerable progress has been made in the treatment of chronic hepatitis C infection. However, the current standard combination of interferon-based therapies and ribavirin is effective in only 50% of patients. In addition, this combination is expensive, requires lengthy periods of administration, and is associated with significant side effects. Furthermore, no effective preventive measure, such as vaccination, is currently available. A number of newer therapies, including protease and helicase inhibitors, ribozymes, antisense therapies, and therapeutic vaccines, are in preclinical and clinical development and may significantly enhance existing therapeutic options for the future.

Future trends in managing hepatitis C

Despite recent improvements in the treatment of patients who have chronic hepatitis C, a large proportion of patients do not achieve viral clearance. Treatment regimens are also costly, associated with significant morbidity, require substantial patient commitment, and are not appropriate for all patients. Therefore, it is important to maximize and enhance current therapeutic approaches and to investigate new approaches and therapies. Because the ability to maintain adherence to current treatment is associated with higher sustained virologic response rates (particularly in patients infected with genotype 1), strategies directed at patients and support staff to promote treatment adherence are important. Other strategies to enhance current therapy include alternative interferons (IFNs)/cytokines and new IFN delivery systems. Current therapy may also be enhanced by new ribavirin (RBV) analogs with an improved safety profile or by the addition of other immunomodulatory agents such as inosine 5'-monophosphate dehydrogenase inhibitors, histamine dihydrochloride, thymosin alfa 1, and amantadine. Some of these agents have demonstrated promising results, although further evaluation is required. Greater knowledge of the molecular biology of the hepatitis C virus (HCV) holds promise for the development of targeted therapies such as specific inhibitors of HCV polymerase, protease, or helicase, as well as therapeutic vaccines. Other potential molecular-based therapies include antisense oligonucleotides, ribozymes, and short interfering ribonucleic acid (RNA) molecules. Therapies aimed at reducing or preventing the development of fibrosis are also under investigation. Multiple-drug regimens will likely be required to enhance viral clearance and reduce viral resistance, while providing greater tolerability.

Pleiotropic mechanisms of ribavirin antiviral activities

Renewed interest in the mechanism of action of ribavirin results from its synergistic enhancement of interferon therapy and the need to develop more efficacious agents to treat hepatitis C virus infection. Since the discovery of ribavirin over 30 years ago by scientists at ICN Pharmaceuticals, many mechanisms of action for ribavirin have been proposed. These include inhibition of host inosine monophosphate dehydrogenase by ribavirin monophosphate, inhibition of viral capping enzymes, inhibition of viral RNA synthesis by ribavirin triphosphate, lethal mutagenesis of viral RNA genomes resulting from promiscuous incorporation of ribavirin triphosphate by the viral RNA polymerase, and modulation of the host immune responses. In this article, we will briefly review the evidence for these mechanisms, emphasizing recent findings. In addition, we will discuss strategies for development of nucleoside analogs that may replace ribavirin in the future.

Pharmacokinetics and metabolism of [(14)C]ribavirin in rats and cynomolgus monkeys

Absorption, pharmacokinetics, distribution, metabolism, and excretion of [(14)C]ribavirin were studied in rats (30 mg/kg of body weight) and cynomolgus monkeys (10 mg/kg) after intravenous (i.v.) and oral administration. The oral absorption and bioavailability were 83 and 59%, respectively, in rats and 87 and 55%, respectively, in monkeys. After i.v. administration, the elimination half-life (t([1/2])) was 9.9 h in rats and 130 h in monkeys and the total body clearance was 2,600 ml/h/kg in rats and 224 ml/h/kg in monkeys. The apparent volume of distribution was 11.4 liter/kg in rats and 29.4 liter/kg in monkeys. There was extensive distribution of drug-derived radioactivity into red blood cells and extensive metabolism of ribavirin in rats and a lesser degree of metabolism in monkeys. Excretion of total radioactivity in urine from rats accounted for 84% of the i.v. dose and 83% of the oral dose, whereas that from monkeys accounted for 47% of the i.v. dose and 67% of the oral dose. Several metabolites were observed in plasma and urine from both species. The amount of unchanged ribavirin in urine from both species was quite small after either i.v. or oral administration.

New therapies on the horizon for hepatitis C: are we close?

A myriad of new therapies for treating hepatitis C are in various stages of preclinical and clinical development. As reviewed here, these include nucleic acid-based approaches (antisense and ribozymes), small molecule inhibitors of essential hepatitis C virus (HCV)-encoded enzymes (protease, helicase, and polymerase), immune modulation, and immunotherapy. As more details of the HCV lifecycle are elucidated, new targets and approaches will be discovered. Drug development is difficult, expensive, and always agonizingly slow for patients in need and their physicians. Nonetheless, a broad effort has been mounted for HCV, and substantial progress has been achieved. The prospects for new HCV treatments are bright.

Ribavirin in cancer immunotherapies: controlling nitric oxide augments cytotoxic lymphocyte function

Either ribavirin (RBV) or cyclophosphamide (CY) can shift an immune response from Th2 toward a Th1 cytokine profile. CY is used in this role in various current cancer immunotherapy attempts but with mixed success. More potent and reliable immunoadjuvants and Th1 response biasing methods are needed. RBV is used today mainly to augment interferon-alpha treatment of hepatitis C. RBV shifts an immune response from Th2 toward Th1 more effectively than CY and may be a safe and useful adjuvant for current cancer immunotherapeutic efforts. RBV is thought to act by inhibition of tetrahydrobiopterin synthesis. Tetrahydrobiopterin is an essential cofactor for all known isoforms of nitric oxide synthase. Lowered nitric oxide favors Th1 development as high levels favor Th2 weighting.

Hepatitis C virus therapies: current treatments, targets and future perspectives

Chronic hepatitis C virus (HCV) infection is the cause of an emerging global epidemic of chronic liver disease. Current combination therapies are at best 80% efficacious and are often poorly tolerated. Strategies to improve the therapeutic response include the development of novel interferons, nucleoside analogues with reduced haemolysis compared with ribavirin and inosine 5'-monophosphate dehydrogenase inhibitors. Compounds in preclinical or early clinical trials include small molecules that inhibit virus-specific enzymes (such as the serine proteases, RNA polymerase and helicase) or interfere with translation (including anti-sense molecules, iRNA and ribozymes). Advances in understanding HCV replication, obtaining a sub-genomic replicon and contriving potential small animal models, in addition to solving crystallographic structures for the replication enzymes, have improved prospects for developing novel therapies. This review summarizes current and evolving treatments for chronic hepatitis C infection. In addition, progress in HCV targets and drug discovery tools valuable in the search for novel anti-HCV agents is detailed.

Effect of ribavirin, levovirin and viramidine on liver toxicological gene expression in rats

The ribavirin/interferon-alpha combination is currently the standard therapy for patients with chronic hepatitis C. However, ribavirin causes hemolytic anemia as a significant side-effect. Levovirin, an L-enantiomer of ribavirin, possesses similar immunomodulatory activity to ribavirin but lacks direct antiviral activity or hemolytic anemia. Viramidine is a liver-targeting prodrug of ribavirin with much less potential for hemolytic anemia. The aim of the present study is to profile the hepatic toxicological gene response to ribavirin, levovirin and viramidine. Rats were dosed orally with 120 mg kg(-1) day(-1) of ribavirin and viramidine and 2000 mg kg(-1) day(-1) of levovirin for 8 days. Ribavirin did not cause any significant change (>threefold) in gene expression as analyzed by the Affymetrix GeneChip technique. Levovirin decreased the mRNA level of CYP7A1 by fourfold but did not affect the expression of CYP27/CYP7B1 that functions as an alternative pathway for cholesterol metabolism. Viramidine down-regulated both expressed sequence tag 233569 and heat shock protein 86 genes threefold. The changes at mRNA level of these genes were confirmed by the reverse transcription competitive polymerase chain reaction technique. None of the compounds changed the liver/body weight ratio, the major cytochrome P-450 protein levels or enzyme activities. The data indicated that a high dose of ribavirin, levovirin or viramidine did not cause significant change at the transcription level of most of the liver toxicological genes.

Specific, sensitive and accurate liquid chromatographic-tandem mass spectrometric method for the measurement of ribavirin in rat and monkey plasma

Ribavirin is a purine nucleoside analog with broad spectrum activity against a spectrum of DNA and RNA viruses. To facilitate pharmacokinetics studies, a LC-MS-MS method for the analysis of ribavirin in rat and monkey plasma was developed and validated. The method involved the addition of acyclovir as an internal standard and protein precipitation with acetonitrile followed by separation by an Intertsil Silica column and quantification by a MS-MS equipped with a positive electrospray ionization in the multiple reaction monitoring mode. The MS-MS reaction was selected to monitor the 245-->113 and 226-->152 transitions for ribavirin and internal standard, respectively. The calibration curve was linear over a concentration range of 10-5000 ng/ml. The lower limit of quantitation was 10 ng/ml, the coefficient of variation (CV) was 8-11%, and the bias was 1-3%. Intra-day and inter-day analysis of QC samples at 30, 1500 and 3500 ng/ml indicate that the method was precise (CV<18%) and accurate (bias<13%). Ribavirin in rat and monkey plasma was stable at 5 degrees C for at least 24 h, 0 degrees C for at least 4 h, and after three freeze-thaw cycles. This specific, accurate and precise assay is useful in the study of the pharmacokinetics of this compound.

Treatment of recurrent hepatitis C

1. Treatment of established recurrent hepatitis C with interferon-alpha monotherapy does not achieve sustained virologic response (SVR). 2. Treatment of established recurrent hepatitis C with combination interferon plus ribavirin achieves SVR rates of 17% to 27%, but dropout rates approach 30%. 3. Pretransplant prophylaxis against recurrent hepatitis C with combination interferon plus ribavirin is poorly tolerated in patients with decompensated hepatitis C cirrhosis. 4. Posttransplant prophylaxis with combination interferon plus ribavirin prevents both recurrent viremia and hepatitis in 15% to 20% of patients, but dropout rates approach 50%. 5. Hepatitis C virus genotype is the best predictor of response to antiviral prophylaxis and treatment of recurrent hepatitis C. 6. Interferon-alpha therapy is not associated with an increased risk of allograft rejection in liver transplant recipients. 7. Ribavirin therapy is associated with increased hemolysis in liver transplant recipients. 8. Preliminary data suggest pegylated interferon monotherapy will have similar efficacy but better tolerability than combination interferon plus ribavirin. 9. In a recent study, posttransplant immunoprophylaxis with polyclonal hepatitis C immunoglobulin had no effect on recurrent viremia or hepatitis.

Specific, sensitive and accurate LC-MS/MS method for the measurement of levovirin in rat and monkey plasma

Levovirin is a guanosine nucleoside analogue and the L-enantiomer of ribavirin. Levovirin has a better safety profile than ribavirin, exerts similar immunomodulatory effects in a mouse efficacy model, and may provide a better therapeutic option than ribavirin in patients with chronic hepatitis C virus (HCV) infection. To facilitate pharmacokinetic studies, a LC-MS/MS method for the analysis of levovirin in rat and monkey plasma was developed and validated. The method involved adding ICN 10537 as an internal standard, protein precipitation with acetonitrile followed by separation on an Intersil Silica column, and quantification by a MS/MS system equipped with positive electrospray ionization (ESI) in the multiple reaction monitoring (MRM) mode. The MS/MS reaction was selected to monitor the 245-->113 and 259-->128 transitions for levovirin and internal standard, respectively. The calibration curve was linear over a concentration range of 10-5000 ng/ml. The limit of quantitation was 10 ng/ml, the coefficient of variation (CV) was 3-5%, and the bias was 3-6%. Intra- and inter-day analysis of QC samples at 30, 1500 and 3500 ng/ml indicated that the method was precise (CV<6%) and accurate (bias <9%). Levovirin in rat and monkey plasma was stable at 5 degrees C for at least 24 h, 0 degrees C for at least 4 h, and after three freeze-thaw cycles. This specific, accurate and precise assay is useful in the study the pharmacokinetic characteristics of this compound.

Immunological and virological effects of ribavirin in hepatitis C after liver transplantation

BACKGROUND

Hepatitis C recurring after liver transplant may cause progressive liver dysfunction, and available treatment regimens are unsatisfactory. A better understanding of the mechanisms of action of drugs currently used to manage hepatitis C would be helpful.

METHODS

In a pilot, uncontrolled clinical trial, we treated 12 patients with post-liver transplantation hepatitis C with 1000-1200 mg qd of ribavirin, given as a monotherapy. We measured the transaminases levels, the liver disease grading and staging scores, the intrahepatic interferon-gamma, tumor necrosis factor-alpha, interleukin (IL)-4 and IL-10 mRNA levels, the serum and liver hepatitis C virus (HCV) RNA titers, and the intrahepatic HCV envelope 2 protein staining score before and after 12 weeks of ribavirin monotherapy.

RESULTS

Ribavirin induced a significant amelioration of the transaminases levels. This biochemical response was not associated with a distinct change in the intrahepatic T helper 1/T helper 2 cytokine mRNA profile. Furthermore, some histological parameters, such as the portal inflammation and the fibrosis scores, worsened significantly even in the short term. A slight, albeit not significant, decrease of serum HCV RNA level and intrahepatic HCV antigen staining score was observed. Intrahepatic genomic-strand (but not negative-strand) HCV RNA titer decreased significantly (P=0.024).

CONCLUSIONS

Contrary to what is suggested by experimental data, administration of ribavirin alone to patients with recurrent hepatitis C after liver transplantation is not accompanied by a specific change of the intrahepatic interferon-gamma, tumor necrosis factor-alpha, IL-4, or IL-10 mRNA transcription profile.

Mechanisms of action of ribavirin in antiviral therapies

Although ribavirin was originally synthesized over 30 years ago and has been used to treat viral infections as monotherapy (respiratory syncytial virus and Lassa fever virus) or with interferon-alpha (IFN-alpha) as combination therapy (hepatitis C virus), the precise mechanism of its therapeutic activities remains controversial. In this review we focus on two main biological properties of ribavirin: its indirect and direct antiviral activities (with particular emphasis on its efficacy against chronic hepatitis C infection). Each property could individually or collectively account for its clinical efficacy against viral infections. First, with emphasis on the evidence for indirect activities of ribavirin, we will review the clinical observations that suggest that the immunomodulatory properties of ribavirin can in part account for its antiviral activities in vivo. We will then describe the mode of ribavirin's direct antiviral activities. These direct activities can be ascribed to several possible mechanisms, including the recently described activity as an RNA mutagen, a property that may be important in driving a rapidly mutating RNA virus over the threshold to 'error catastrophe'.

Evolution of hepatitis C virus genome in chronically infected patients receiving ribavirin monotherapy

Recent results of clinical trials suggest that combination of interferon and ribavirin exhibits an enhanced antiviral effect in the treatment of chronic hepatitis C. To investigate the effect of ribavirin on hepatitis C virus (HCV) infection, we analysed the evolution of the genetic heterogeneity of HCV in relation to the anti-HCV humoral response in patients treated by ribavirin alone. The study population included 35 patients with liver biopsy proven chronic hepatitis C infected with HCV genotype 1. Among them, 26 were treated with ribavirin for at least 12 months and nine untreated patients served as a control group. Serum samples were analysed before and at 6 and 12 months of therapy. Three regions of the HCV genome, i.e. HVR1, a domain of NS5A including part of the interferon sensitivity determining region (ISDR), and a segment of NS5B, were amplified by RT-PCR using specific primers. The PCR products were then studied using single-strand conformation polymorphism (SSCP) analysis followed by either direct sequencing, or cloning and sequencing. In parallel, the humoral anti-E1 response was studied using an ELISA (Innotest HCV E1Ab, Innogenetics). The results of HCV genome analysis showed no significant effect on the amino acid sequence evolution of the HVR1, NS5A and NS5B regions of HCV. Analysis of a phylogenetic tree from the major quasispecies variants showed the absence of correlation with ribavirin response, and the absence of selection of viral strains during ribavirin treatment. A trend towards a decrease in the anti-E1 Ab response was also observed. Altogether these results suggest that ribavirin may not exhibit a direct antiviral effect, but may trigger a favourable response to interferon by modulating the immune response against HCV.

Monocyclic L-nucleosides with type 1 cytokine-inducing activity

A series of 1,2,4-triazole L-nucleosides were synthesized and evaluated for their ability to stimulate type 1 cytokine production by activated human T cells in direct comparison to the known active agent ribavirin. Among the compounds prepared, 1-beta-L-ribofuranosyl-1,2,4-triazole-3-carboxamide (5, ICN 17261) was found to be the most uniformly potent compound. Conversion of the 3-carboxamide group of 5 to a carboxamidine functionality resulted in 1-beta-L-ribofuranosyl-1,2,4-triazole-3-carboxamidine hydrochloride (10), which induced cytokine levels comparable to 5 for two of the three type 1 cytokines examined. Modification of the carbohydrate moiety of 5 provided compounds of reduced activity. Significantly, ICN 17261 offers interesting immunomodulatory potential for the treatment of diseases where type 1 cytokines play an important role.