Viramidine, a Prodrug of Ribavirin, Shows Better Liver-Targeting Properties and Safety Profiles Than Ribavirin in Animals

Advances in Antiviral Delivery Systems and Chitosan-Based Polymeric and Nanoparticulate Antivirals and Antiviral Carriers

Current antiviral therapy research is focused on developing dosage forms that enable highly effective drug delivery, providing a selective effect in the organism, lower risk of adverse effects, a lower dose of active pharmaceutical ingredients, and minimal toxicity. In this article, antiviral drugs and the mechanisms of their action are summarized at the beginning as a prerequisite background to develop relevant drug delivery/carrier systems for them, classified and briefly discussed subsequently. Many of the recent studies aim at different types of synthetic, semisynthetic, and natural polymers serving as a favorable matrix for the antiviral drug carrier. Besides a wider view of different antiviral delivery systems, this review focuses on advances in antiviral drug delivery systems based on chitosan (CS) and derivatized CS carriers. CS and its derivatives are evaluated concerning methods of their preparation, their basic characteristics and properties, approaches to the incorporation of an antiviral drug in the CS polymer as well as CS nanoparticulate systems, and their recent biomedical applications in the context of actual antiviral therapy. The degree of development (i.e., research study, in vitro/ex vivo/in vivo preclinical testing), as well as benefits and limitations of CS polymer and CS nanoparticulate drug delivery systems, are reported for particular viral diseases and corresponding antivirotics.

Broad spectrum antiviral nucleosides-Our best hope for the future

The current focus for many researchers has turned to the development of therapeutics that have the potential for serving as broad-spectrum inhibitors that can target numerous viruses, both within a particular family, as well as to span across multiple viral families. This will allow us to build an arsenal of therapeutics that could be used for the next outbreak. In that regard, nucleosides have served as the cornerstone for antiviral therapy for many decades. As detailed herein, many nucleosides have been shown to inhibit multiple viruses due to the conserved nature of many viral enzyme binding sites. Thus, it is somewhat surprising that up until very recently, many researchers focused more on "one bug one drug," rather than trying to target multiple viruses given those similarities. This attitude is now changing due to the realization that we need to be proactive rather than reactive when it comes to combating emerging and reemerging infectious diseases. A brief summary of prominent nucleoside analogues that previously exhibited broad-spectrum activity and are now under renewed interest, as well as new analogues, that are currently under investigation against SARS-CoV-2 and other viruses is discussed herein.

Remofovir Mesylate: A Prodrug of PMEA with Improved Liver-Targeting and Safety in Rats and Monkeys

Adefovir dipivoxil (Hepsera™), a first-line therapy for chronic hepatitis B, is an esterase-activated prodrug of PMEA. Dose-limiting nephrotoxicity necessitates suboptimal dosing at 10 mg/day. Remofovir mesylate (MB06866Q) (Hepavir B) is a CYP3A4-activated prodrug of PMEA based on the HepDirect™ technology that targets PMEA to the liver. In a whole body autoradiography study in rats after oral dosing (30 mg/kg) of [14C]adefovir dipivoxil or [14C]remofovir mesylate, remofovir yielded 15 times higher concentrations of radioactivity in the liver than adefovir dipivoxil, but only one-third of the concentrations in the kidney. After oral dosing (4 mg/kg) of the same radiolabelled agents in cynomolgus monkeys, remofovir mesylate yielded 60 times higher levels of total radioactivity in the liver, but only two-thirds of total radioactivity levels in the kidney. Thus, remofovir mesylate may provide better efficacy and reduced nephrotoxicity. In portal vein-cannulated rats (30 mg/kg) after a single oral dose of [14C]adefovir dipivoxil or [14C]remofovir mesylate, no PMEA was detectable in rat portal plasma early after dosing, indicating that intestinal CYP3A4 does not play a role in conversion of remofovir mesylate to PMEA. The portal/systemic extraction ratio was quite high in both models, suggesting good liver-targeting properties. Portal and systemic remofovir/PMEA ratio indicates that the liver is the site of conversion of remofovir to PMEA. 28-Day toxicity studies demonstrated renal toxicity in rats at doses of 100 mg/kg or higher with no safety concerns at 30 mg/kg and acceptable safety in monkeys at doses up to 60 mg/kg. Thus, in rats and non-human primates, remofovir mesylate has liver-targeting properties and is safer than adefovir dipivoxil.

Conversion of Viramidine to Ribavirin in Vivo by Adenosine Deaminase and its Inhibition by 2′-Deoxycoformycin

Previously we reported that viramidine is a prodrug of ribavirin and that adenosine deami-nase catalyses viramidine deamination to ribavirin in vivo. This in vivo study explores this prodrug conversion in rats and inhibition by a potent adenosine deaminase inhibitor, 2′-deoxyco-formycin. We found that conversion of viramidine to ribavirin was viramidine dose-dependent in rat plasma. A single intravenous dose of 0.25 mg/kg 2′-deoxycoformycin suppressed orally administered viramidine conversion to ribavirin in plasma by 50%. The inhibition was 2′-deoxycoformycin dose-dependent and a single dose of 2 mg/kg decreased the ribavirin/viramidine area under the concentration—time curve between 0 h and 6 h ratio by 2.5-fold. These findings provide strong evidence that adenosine deaminase plays a major role in converting viramidine to ribavirin in vivo.

Synthesis and evaluation of a new phosphorylated ribavirin prodrug

Ribavirin is an important broad-spectrum antiviral drug. However, its utilization can be limited by its potential to cause hemolytic anemia as well as its variability in dosing levels and efficacy outcomes. To overcome these issues, we report on a new alkoxyalkylphosphodiester prodrug of ribavirin (2) that is designed to release the active ribavirin-monophosphate species selectively in nucleated cells while limiting its exposure in anucleated red blood cells (RBCs). Prodrug 2 displays improved in vitro antiviral activity against the hepatitis C virus replicon and influenza virus. Unlike ribavirin, prodrug 2 does not significantly decrease ATP levels in RBCs. Prodrug 2 demonstrates decreased uptake in RBCs but increased uptake in HepG2 hepatocytes when compared to ribavirin. In vivo, prodrug 2 is orally bioavailable and well-tolerated in rats in which it is processed to ribavirin and accumulates in the liver. These results indicate that prodrug 2 has the potential for safer, lower, less frequent, and less variable administration than ribavirin.

Ascending Multiple-Dose Pharmacokinetics of Viramidine, a Prodrug of Ribavirin, in Adult Subjects With Compensated Hepatitis C Infection

The current study was carried out to evaluate pharmacokinetic profiles of viramidine and ribavirin in patients (n = 8 per dose group) with compensated hepatitis C infection following oral dosing of viramidine (400, 600, or 800 mg bid for 4 weeks). Pharmacokinetic parameters were determined on days 1 and 29 based on plasma, red blood cell, and urine concentrations of viramidine and ribavirin. The results indicate rapid absorption and conversion of viramidine to ribavirin after oral administration of viramidine. Viramidine and ribavirin exposure in plasma and RBCs generally increased from the 400- to 600-mg dose level of viramidine. However, no further increase in exposure was noted at the 800-mg dose. Long half-lives for viramidine (66-76 hours in plasma and 200-420 hours in red blood cells) and ribavirin (340-410 hours in plasma and 360-430 hours in red blood cells) were noted. A negligible amount of viramidine (1%-4% of dose) and a small amount of ribavirin (9%-14% of dose) were excreted in the urine. The renal clearance was low for both viramidine (5-8 L/h) and ribavirin (4-7 L/h). Significant accumulation of viramidine was noted in red blood cells (accumulation factor [R] = 5-8) but not in plasma (R = 2). Extensive accumulation of ribavirin was noted in both plasma (R = 9-17) and red blood cells (R = 77-129). Steady-state levels of ribavirin and viramidine in plasma and red blood cells were achieved by day 22. At steady state, there was extensive conversion of viramidine to ribavirin in both plasma and red blood cells. Both viramidine and ribavirin were preferentially distributed into red blood cells than plasma.

Randomised clinical trial: pre-dosing with taribavirin before starting pegylated interferon vs. standard combination regimen in treatment-naïve patients with chronic hepatitis C genotype 1

BACKGROUND

Combination therapy with the ribavirin (RBV) prodrug taribavirin (TBV) and pegylated interferon (PIFN) has produced lower rates of anaemia than with RBV and PIFN. Studies have demonstrated that the sharpest decline in viral load during TBV therapy occurs at Weeks 4 through 6, when TBV reaches steady-state blood levels.

AIM

The current proof-of-concept study was conducted to examine whether first-order viral kinetics could be influenced by pre-dosing TBV to steady state before introducing PIFN.

METHODS

Therapy-naïve patients with chronic hepatitis C virus (HCV) genotype 1 (G1) were randomised to receive (i) TBV 600 mg BID monotherapy for 4 weeks followed by combination therapy with PIFN [pre-dosing arm (n = 23)] or (ii) TBV administered concurrently with PIFN [standard dosing arm (n = 19)].

RESULTS

More patients achieved undetectable virus or a ≥2-log(10) reduction of HCV RNA at Week 4 in the pre-dosing vs. the standard dosing arm [33% vs. 22% (P = 0.497)]. There was also a trend towards greater reduction in mean log(10) change in HCV RNA in the pre-dosing vs. the standard dosing arm, which was statistically significant at Day 1 [-0.34 ± 0.46 vs. 0.09 ± 0.32 (P < 0.003)] but not at other time points up to Week 24. No significant difference was observed in the rates of anaemia (haemoglobin <10 g/dL) between study arms (4.5% vs. 5.3%).

CONCLUSIONS

Pre-dosing TBV prior to starting PIFN produces a trend towards improved efficacy although statistical significance was not reached in this small patient population. These results warrant larger clinical trials of TBV pre-dosing.

Topical treatment of cutaneous vaccinia virus infections in immunosuppressed hairless mice with selected antiviral substances

BACKGROUND

Certain nucleoside, nucleotide and pyrophosphate analogues may be useful for treating severe complications arising as a result of virus dissemination following smallpox (live vaccinia virus) vaccinations, especially in immunocompromised individuals. We used an immunosuppressed hairless mouse model to study the effects of 10 antiviral agents on progressive vaccinia infections.

METHODS

Hairless mice were immunosuppressed by treatment with cyclophosphamide (100 mg/kg) every 4 days starting 1 day prior to vaccinia virus (WR strain) infection of wounded skin. Topical treatments with antiviral agents were applied twice a day for 7 days starting 5 days after virus exposure.

RESULTS

Topical 1% cidofovir cream treatment was effective in significantly reducing primary lesion severity and decreasing the number of satellite lesions. Topical 1% cyclic HPMPC and 1% phosphonoacetic acid were not quite as active as cidofovir. Ribavirin (5%) treatment reduced lesion severity and diminished the numbers of satellite lesions, but the mice died significantly sooner than placebos. 2-Amino-7-[(1,3,-dihydroxy-2-propoxy)methyl]purine (compound S2242; 1%) moderately reduced primary lesion sizes. Ineffective treatments included 5% arabinosyladenine, 1% arabinosylcytosine, 1% 5-chloro-arabinosylcytosine, 5% arabinosylhypoxanthine 5-monophosphate and 5% viramidine.

CONCLUSIONS

Of the compounds tested, topically applied cidofovir was the most effective treatment of cutaneous vaccinia virus infections in immunosuppressed mice. Topical treatment with cidofovir could be considered as an adjunct to intravenous drug therapy for serious infections.

Taribavirin: potential future in the treatment of hepatitis C

Taribavirin is a prodrug of ribavirin, studied as a treatment modality in combination with PEGylated interferon for chronic hepatitis C. Taribavirin is catalyzed to ribavirin by adenosine deaminase, which is ubiquitous within the body. Taribavirin has resulted in significantly lower rates of anemia, the main complication of ribavirin and cause of dose reductions or discontinuation. Previous clinical studies failed to demonstrate noninferiority to ribavirin. However, better results were observed when taribavirin was given as a weight-based medicine, as shown in a recently published Phase IIb study.

Peginterferon and ribavirin treatment for hepatitis C virus infection

Pegylated interferon α (IFNα) in combination with ribavirin is currently recommended as a standard-of-care treatment for chronic hepatitis C virus (HCV) infection. This combination therapy has drastically improved the rate of sustained virological response, specifically in difficult-to-treat patients. Recently, individualized treatment, such as response-guided therapy, is being developed based on host-, HCV- and treatment-related factors. Furthermore, modified regimens with currently available medications, novel modified IFNα and ribavirin or combinations with specifically targeted antiviral therapy for HCV agents, are currently being investigated. The purpose of this review is to address some issues and epoch-making topics in the treatment of chronic HCV infection, and to discuss more optimal and highly individualized therapeutic strategies for HCV-infected patients.

Hepatitis C and kidney disease

Multiple extrahepatic manifestations have been associated with chronic hepatitis C, the most important among them being cryoglobulinemia, glomerulonephritis, porphyria cutanea tarda, lichen planus, seronegative arthritis, and lymphoproliferative disorders as in the sudies of Bonkovsky and Mehta (2001) and El-Serag et al. (2002). We will discuss in this paper chronic hepatitis C- related kidney disease and course and management of patients with chronic hepatitis C in special circumstances like hemodialysis and kidney transplantation.

Small molecule drug discovery for Dengue and West Nile viruses: applying experience from hepatitis C virus

There are currently no specific treatments for infection with Dengue virus (DENV) and West Nile Virus (WNV). Drug-discovery programs are underway for both viruses, but as yet no small molecules have advanced to clinical trials. Hepatitis C virus (HCV) is a related flavivirus that has been the focus of intense drug discovery efforts for the last two decades. Many approaches currently being pursued for DENV and WNV have been previously attempted for HCV with varying degrees of success. The experience with HCV may direct DENV and WNV efforts towards approaches with the best chance of success. Based on experience with HCV, the viral polymerase and protease are attractive targets to focus on since these have been most successful to date. Cell-based phenotypic screening may also yield attractive inhibitors. The helicase and methyltransferase enzymes are likely to prove difficult targets and host target approaches are fraught with safety concerns.

Safety and efficacy of viramidine versus ribavirin in ViSER2: randomized, double-blind study in therapy-naive hepatitis C patients

BACKGROUND & AIMS

Pegylated interferon (peg-IFN) plus ribavirin (standard of care for chronic hepatitis C virus [HCV]), can cause dose-limiting anemia in up to 22% of patients. Viramidine is associated with a lower incidence of anemia because of its liver-targeting properties.

METHODS

The efficacy and safety of viramidine versus ribavirin plus peg-IFN alfa-2a was assessed in patients with HCV. Randomized patients received peg-IFN alfa-2a 180 mcg with viramidine 600 mg twice daily or weight-based doses of ribavirin 1000 or 1200 mg/day. Treatment duration was based on HCV ribonucleic acid (RNA) genotype: genotype 2/3 and non-2/3 patients were treated for 24 and 48 weeks, respectively. The primary efficacy end point was the non-inferiority of viramidine versus ribavirin (proportion of patients achieving sustained virologic response at week 24). The primary safety end point was the proportion of patients experiencing a hemoglobin event.

RESULTS

In total, 962 patients received peg-IFN alfa-2a with viramidine (n=644) or ribavirin (n=318). Sustained virologic response was achieved in 40% of viramidine-treated patients and 55% of ribavirin-treated patients (difference of proportions 0.150 [95% CI, 0.09, 0.21]). Improved efficacy was seen with higher viramidine exposure on a mg/kg basis. Viramidine was significantly superior to ribavirin in hemoglobin event rates (54% vs. 80%; p<0.001). Adverse event rates were similar between groups except for diarrhea (viramidine 29.5%; ribavirin 15.7%; p<0.0001).

CONCLUSIONS

Viramidine 600 mg BID did not meet the primary efficacy non-inferiority end point but met the safety end point. Determination of a viramidine dosage that would yield superior efficacy over ribavirin is needed.

Hepatitis C virus infection and immunomodulatory therapies

Hepatitis C virus (HCV) infection remains a large-scale and significant health concern. The combination of subcutaneously administered pegylated interferon and oral ribavirin is the FDA-approved regimen for the treatment of chronic HCV infection. Combination therapy may result in a sustained virologic response leading to HCV eradication, with a reduction in risk for cirrhosis, hepatic decompensation, and hepatocellular carcinoma. However, the combination of PEG-IFN and ribavirin does not universally result in cure in all patients who undergo treatment. In this article, the authors discuss immunomodulatory therapies and clinical trials in the treatment of HCV infection.

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.

Impact of hepatitis C virus seropositivity on survival after allogeneic hematopoietic stem cell transplantation for hematologic malignancies

BACKGROUND

Because hepatitis C virus infection causes hepatic and immunological dysfunction, we hypothesized that seropositivity for this virus could be associated with increased non-relapse mortality after allogeneic hematopoietic stem cell transplantation.

DESIGN AND METHODS

We performed a case-control study of the outcomes of patients who were hepatitis C virus seropositive at the time of allogeneic hematopoietic stem cell transplantation (N=31). Patients positive for hepatitis C virus were considered candidates for stem cell transplantation only if they had no significant evidence of hepatic dysfunction. Matched controls (N=31) were seronegative for viral hepatitides and were paired according to age, diagnosis, disease stage, conditioning regimen and donor type. We also compared the hepatitis C virus seropositive patients to all seronegative patients (all controls, N=1800) transplanted during the same period, to adjust for other confounding effects.

RESULTS

The median age of the seropositive patients was 49 (range 26-72); 15 had acute myeloid leukemia/myelodysplastic syndrome, 6 had chronic myeloid leukemia/myeloproliferative disease, 6 non-Hodgkin's lymphoma, 2 myeloma, 1 acute lymphocytic leukemia and 1 Hodgkin's lymphoma; 61% had poor risk disease; 68% had related donors; 68% received reduced intensity conditioning; 7 patients had mildly abnormal alanine transaminase levels (all less than three times the upper limit of normal) and 1 patient had minimally elevated bilirubin. These characteristics were similar to those of the matched control group. Median overall survival was 3, 18 and 20 months, and 1-year survival was 29%, 56% and 56%, in the hepatitis C virus, matched and all controls groups, respectively (hazard ratio for death 3.1, 95% confidence interval 1.9-5.6, p<0.001 in multivariate analysis). Non-relapse mortality at 1 year was 43%, 24% and 23%, respectively (hazard ratio 3.3, 95% confidence interval 1.8-7.1, p<0.01). Disease progression and graft-versus-host disease rates were comparable.

CONCLUSIONS

Hepatitis C virus seropositivity is a significant risk factor for non-relapse mortality after allogeneic hematopoietic stem cell transplantation even in patients with normal or minimally abnormal liver function tests.

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.

Therapeutic issues in HIV/HCV-coinfected patients

The importance of treating hepatitis C virus (HCV)-associated morbidities in a growing population of patients coinfected with human immunodeficiency virus (HIV) has increased since the introduction of highly active antiretroviral therapy. As a result, investigative attention is turning to HCV-related liver disease and treatment-associated issues in coinfection. HIV/HCV-coinfected patients have higher HCV RNA loads and show more rapid progression of fibrosis than do monoinfected patients. Combination therapy with pegylated interferon plus ribavirin (RBV) is the standard of care for HCV in coinfected patients. Therapy slows fibrosis progression, but toxicity prevents identification of the most effective RBV dose. Coinfected patients have about a threefold greater risk of antiretroviral therapy-associated hepatotoxicity than patients with HIV only. Other challenges include anaemia, mitochondrial toxicity, drug-drug interactions and leucopenia. Thus, chronic hepatitis C should be treated in HIV/HCV-coinfected patients, but steps must be taken to prevent and treat potential toxicities. The first European Consensus Conference on the Treatment of Chronic Hepatitis B and C in HIV Co-infected Patients was held March 2005 in Paris to address these issues. This article reviews the peer-reviewed literature and expert opinion published from 1990 to 2005, and compares results with presentations and recommendations from the Consensus Conference to best present current issues in coinfection.

The effects of HCV infection and management on health-related quality of life

Infection with HCV leads to an array of symptoms that compromise health-related quality of life (HRQL). Chronic hepatitis C is treated primarily with pegylated interferon (peg-IFN) and an inosine 5' monophosphate dehydrogenase inhibitor, ribavirin (RBV), with the goal of achieving a sustained virologic response (SVR). SVR reduces the rate of hepatic fibrosis and other disease-related complications and, in turn, increases HRQL. Although combination therapy with peg-IFN and RBV produces SVRs in more than 50% of treated patients, it is associated with side effects that can reduce short-term HRQL, can lead to dose reductions and discontinuations, and may impair treatment response. Fatigue and depression are common symptoms of chronic HCV infection that may also be caused by IFN-based therapy. Hemolytic anemia and IFN-mediated bone marrow suppression are well-known consequences of IFN/RBV therapy, often resulting in dose reductions or discontinuations, and have the potential to affect SVR rates. Management of these symptoms is vital to successful outcomes and generally relies on therapy that is adjunctive to the primary treatment of the viral infection itself. Several new drugs with the potential to increase SVR rates without compromising HRQL are in development.

CONCLUSION

The relationship of chronic HCV infection, treatment, and HRQL is complex. Successful treatment of chronic hepatitis C requires an understanding of the intricacies of this relationship and appropriate management of treatment-related symptoms.

New treatment strategies against hepatitis C viral infection

Treatment of hepatitis C virus infection is currently based on a combination of pegylated interferon and ribavirin. Because efficacy of this therapy remains suboptimal and side effects sometimes problematic, major efforts have been put forward by scientists and the pharmaceutical industry to develop alternative treatments for this chronic infection. Over the past few years, clinical studies performed with some of these new agents have been presented at major international meetings. The present paper aims to review the rationale underlying the development of these new forms of treatment as well as the current available data concerning their clinical efficacy.

Ribavirin inhibits angiogenesis by tetrahydrobiopterin depletion

Ribavirin is a broad-spectrum antiviral drug that is used to treat hepatitis C virus (HCV)-infected patients. The virological response after ribavirin treatment appears to be insufficient to fully explain ribavirin-induced beneficial effects. Angiogenesis plays a pathogenic role in HCV-induced liver damage. Here, we investigated the influence of therapeutic ribavirin concentrations on angiogenesis. Ribavirin inhibited endothelial cell tube formation in vitro and vessel formation in the chick chorioallantoic membrane assay in vivo. Ribavirin inhibits inosine monophosphate dehydrogenase, which causes depletion of cellular GTP and in turn reduction of cellular tetrahydrobiopterin levels. The availability of tetrahydrobiopterin limits NO production by endothelial NO synthase. Ribavirin reduced levels of tetrahydrobiopterin (as revealed by HPLC), NO (as revealed by electron spin resonance spectroscopy), and cGMP (as revealed by RIA) in endothelial cells. Addition of tetrahydrobiopterin or NO prevented ribavirin-induced tube formation inhibition. In conclusion, angiogenesis inhibition by ribavirin has not been described before. This inhibition may contribute to ribavirin-induced pharmacological effects including adverse events.

Activities of oseltamivir and ribavirin used alone and in combination against infections in mice with recent isolates of influenza A (H1N1) and B viruses

Mouse models have been widely used for evaluating potential influenza virus inhibitors. However, the viral strains traditionally used in these models are fairly old and do not represent currently circulating viruses in nature. We developed two new lethal infection models in mice using mouse-adapted influenza A/New Caledonia/20/99 (H1N1) and influenza B/Sichuan/379/99 viruses. Both virus infections were used to study oral treatment with oseltamivir and ribavirin, both alone and in combination. Oral treatments were given twice daily for 5 days starting 4 h before infection in initial studies. Against influenza A, oseltamivir was active at 10, 20, and 40 mg/kg/day, protected 80-100% of mice from death and reduced lung consolidation - ribavirin was similarly effective at 20, 40, and 80 mg/kg/day. When treatments were initiated after virus challenge, delaying treatment with oseltamivir even 1 day caused it to be ineffective. Ribavirin prevented mortality by 50-80% when treatments were delayed 1-4 days after infection. The combination of the two drugs (oseltamivir at 20 mg/kg/day and ribavirin at 40 mg/kg/day) was no better than ribavirin alone. In contrast to what we observed with influenza A virus infections, oseltamivir and ribavirin showed similar dose-related antiviral activities against influenza B virus infections. The compounds both significantly increased survival when treatments started up to 4 days after infection, but ribavirin was more active than oseltamivir (50-80% survival compared to 30-40% survival, respectively, when starting treatments on days 2-4 after infection). By varying the doses of each drug that were used in combination (oseltamivir at 1.25, 2.5 and 5 mg/kg/day; ribavirin at 5, 10 and 20 mg/kg/day) certain dosage combinations were superior to either compound used alone as assessed by decreased mortality, lung virus titre, lung score and lung weight parameters. These activities differed from published results with older, more established virus strains as oseltamivir was less effective and ribavirin was more active than previously reported.

LC-MS/MS method for simultaneous determination of viramidine and ribavirin levels in monkey red blood cells

A high performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) method has been developed for the simultaneous determinations of total viramidine (viramidine, viramidine monophosphate, viramidine diphosphate, and viramidine triphosphate) and total ribavirin (ribavirin, ribavirin monophosphate, ribavirin diphosphate, and ribavirin triphosphate) in monkey red blood cells (RBC). The method involves the addition of internal standards and perchloric acid, conversion of viramidine or ribavirin phosphorylated metabolites to viramidine or ribavirin, purification with an aminopropyl (NH(2)) solid phase extraction (SPE) cartridge, and LC-MS/MS analysis. The MS/MS is selected to monitor m/z 245-->113, 250-->113, 244-->112, and 249-->112 for ribavirin, [(13)C]ribavirin, viramidine, and [(13)C]viramidine, respectively, using positive electrospray ionization. The calibration curves are linear over a concentration range of 100-10,000 ng/mL (0.412-41.2 microM) with a lower limit of quantification (LLOQ) of 100 ng/mL for both compounds. Mean inter-assay recoveries for ribavirin are 101%, 98.9%, and 96.0%, with coefficient of variance (%CV) values between 1.95 and 4.50% for 100, 1000, and 10,000 ng/mL quality control (QC) samples, respectively. Mean inter-assay recoveries for viramidine are 96.3%, 101%, and 102%, with coefficient of variation (%CV) values between 3.61 and 7.22%, for 100, 1000, and 10,000 ng/mL QC samples, respectively. Over-curve dilution QC at 400 microg/mL (1639 microM) for both viramidine and ribavirin are used to ensure the dilution accuracy (25 X dilutions) for monkey samples. The method has been used to simultaneously determine the total concentrations of ribavirin and viramidine in monkey RBC following 5, 15, and 36 weeks dosing of viramidine or ribavirin (60 mg/kg). The concentrations of total ribavirin following ribavirin dosing are 1242 microM at week 5, 1257 microM at week 15, and 1146 microM at week 36. The concentrations of total ribavirin following viramidine dosing are 634 microM at week 5, 716 microM at week 15, and 683 microM at week 36. Only small amounts of viramidine are detected in RBC following viramidine dosing, 7.80 microM at week 5, 6.63 microM at week 15, and 10.4 microM at week 36. The results suggest that ribavirin levels in RBC were at steady state at week 5 of ribavirin or viramidine dosing. At steady state, ribavirin levels in RBC are approximately 2x after ribavirin dosing than viramidine dosing. The relatively small percentage of viramidine in RBC suggests that viramidine either poorly penetrated into RBC or was extensively converted to ribavirin following entry into RBC.

Comparison of the inhibitory effects of interferon alfacon-1 and ribavirin on yellow fever virus infection in a hamster model

Antiviral compounds were evaluated for efficacy against yellow fever virus (YFV) in a hamster model of YFV-induced liver disease. Challenge with a 10(2) 50% cell culture infectious doses of YFV resulted in a 50-80% mortality rate in female hamsters. Virus was detected by quantitative real-time RT-PCR (QRT-PCR) in liver, kidney, spleen and serum with peak titers on 4-6 days post-viral challenge (dpi). Serum levels of alkaline phosphatase, alanine aminotransferase (ALT), bilirubin, blood urea nitrogen, potassium and creatinine were significantly elevated, while serum levels of albumin, amylase, glucose, calcium, globulin, phosphorus, sodium and total protein were significantly reduced. Packed cell volume and white blood cell count were significantly elevated during the course of the infection. Intraperitoneal treatment of hamsters with 0.5-5 microg/kg/day interferon (IFN) alfacon-1, 100mg/kg/day viramidine or 50 mg/kg/day ribavirin, initiated 4h prior to YFV challenge, resulted in significant improvement in survival and serum ALT levels. Treatment with IFN alfacon-1 or ribavirin starting 2dpi, also significantly improved survival and serum ALT levels in hamsters challenged with YFV. Pre- and post-virus exposure treatment with IFN alfacon-1 was efficacious in improving disease in YFV-infected hamsters.

Structure and replication of hepatitis delta virus RNA

While this volume covers many different aspects of hepatitis delta virus (HDV) replication, the focus in this chapter is on studies of the structure and replication of the HDV RNA genome. An evaluation of such studies is not only an integral part of our understanding of HDV infections but it also sheds new light on some important aspects of cell biology, such as the fidelity of RNA transcription by a host RNA polymerase and on various forms of post-transcriptional RNA processing. Representations of the replication of the RNA genome are frequently simplified to a form of rolling-circle model, analogous to what have been described for plant viroids. One theme of this review is that such models, even after some revision, deceptively simplify the complexity of HDV replication and can fail to make clear major questions yet to be solved.

Respiratory syncytial virus infections: Recent prospects for control

Respiratory syncytial virus (RSV) infections remain a significant public health problem throughout the world, although recently developed and clinically approved anti-RSV antibodies administered prophylactically to at-risk populations appear to have significantly affected the disease development. Much effort has been expended to develop effective anti-RSV therapies, using both in vitro assay systems and mouse, cotton rat, and primate models, with several products now in various stages of clinical study. Several products are also being considered for the treatment of clinical symptoms of RSV. In this review, updates on the status of the approved anti-RSV antibodies, ribavirin, and recent results of studies with potential new anti-RSV compounds are summarized and discussed.

Assessing evidence from clinical trials in chronic hepatitis C

Hepatitis C is a global problem with significantly associated morbidity and mortality. Although some recent therapeutic advances have shown rates of sustained virologic remission of 50% or higher, combination therapy with interferon and ribavirin is often not well tolerated and is giving rise to a growing number of nonresponders. As a result, a large number of experimental drugs for the treatment of chronic hepatitis C are in development. As the clinical trial reports are made available, physicians need to become familiar with issues related to the design of these studies and to develop strategies to interpret the evidence they yield. The articles in this supplement describe the issues in clinical trial design and the evaluation of evidence from clinical trials in patients with chronic hepatitis C.

Definition and management of anemia in patients infected with hepatitis C virus

Chronic infection with hepatitis C virus (HCV) can progress to cirrhosis, hepatocellular carcinoma, and end-stage liver disease. The current best treatment for HCV infection is combination therapy with pegylated interferon and ribavirin. Although this regimen produces sustained virologic responses (SVRs) in approximately 50% of patients, it can be associated with a potentially dose-limiting hemolytic anemia. Hemoglobin concentrations decrease mainly as a result of ribavirin-induced hemolysis, and this anemia can be problematic in patients with HCV infection, especially those who have comorbid renal or cardiovascular disorders. In general, anemia can increase the risk of morbidity and mortality, and may have negative effects on cerebral function and quality of life. Although ribavirin-associated anemia can be reversed by dose reduction or discontinuation, this approach compromises outcomes by significantly decreasing SVR rates. Recombinant human erythropoietin has been used to manage ribavirin-associated anemia but has other potential disadvantages. Viramidine, a liver-targeting prodrug of ribavirin, has the potential to maintain the virologic efficacy of ribavirin while decreasing the risk of hemolytic anemia in patients with chronic hepatitis C.

Sensitive and specific LC–MS/MS method for the simultaneous measurements of viramidine and ribavirin in human plasma

Viramidine is a prodrug of ribavirin. To facilitate pharmacokinetics studies of viramidine in man, a sensitive and specific LC-MS/MS method for the simultaneous analyses of viramidine and ribavirin in human plasma was developed and validated. The method involved the addition of [13C]viramidine and [13C]ribavirin as internal standards, protein precipitation with acetonitrile, HPLC separation, and quantification by MS/MS system using positive electrospray ionization in the multiple reaction monitoring mode (MRM). The precursor-->product ion transitions were monitored at 245-->113, 250-->113, 244-->112, and 249-->112 for ribavirin, [13C]ribavirin, viramidine, and [13C]viramidine, respectively. The calibration curves for viramidine and ribavirin were linear over a concentration range of 1-1000 ng/mL. For both viramidine and ribavirin, the lower limit of quantification (LLOQ) was 1 ng/mL. For viramidine, intra- and inter-day analyses of QC samples at 1, 5, 250, and 1000 ng/mL indicated good precision (%CV between 1.0 and 7.0%) and accuracy (%bias between -4.3 and 5.2%). For ribavirin, intra- and inter-day analyses of QC samples at 1, 5, 250, and 1000 ng/mL indicated similar precision (%CV between 0.8 and 8.3%) and accuracy (%bias between -5.8 and 9.4%). Both viramidine and ribavirin were stable in human plasma stored at room temperature for at least 3 h, 4 degrees C for at least 6 h, and for at least three freeze-thaw cycles. This accurate and highly specific assay provides a useful method for evaluating the pharmacokinetics of viramidine and ribavirin in man following administration of viramidine.

In vitro and in vivo influenza virus-inhibitory effects of viramidine

Viramidine, the 3-carboxamidine derivative of ribavirin, was effective against a spectrum of influenza A (H1N1, H3N2 and H5N1) and B viruses in vitro, with the 50% effective concentration (EC50) ranging from 2 to 32 microg/ml. The mean 50% cytotoxic concentration (CC50) in the MDCK cells used in these experiments was 760 microg/ml. Ribavirin, run in parallel, had a similar antiviral spectrum, with EC50 values ranging from 0.6 to 5.5 microg/ml; the mean CC50 for ribavirin was 560 microg/ml. Oral gavage administrations of viramidine or ribavirin to mice infected with influenza A/NWS/33 (H1N1), A/Victoria/3/75 (H3N2), B/Hong Kong/5/72 or B/Sichuan/379/99 viruses were highly effective in preventing death, lessening decline in arterial oxygen saturation, inhibition of lung consolidation and reducing lung virus titers. The minimum effective dose of viramidine in these studies ranged from 15 to 31 mg/kg/day, depending upon the virus infection, when administered twice daily for 5 days beginning 4 h pre-virus exposure. The LD50 of the compound was 610 mg/kg/day. Ribavirin's minimum effective dose varied between 18 and 37.5 mg/kg/day with the LD50 determined to be 220 mg/kg/day. Viramidine's efficacy was also seen against an influenza A/NWS/33 (H1N1) virus infection in mice, when the compound was administered in the drinking water, the minimum effective dose being 100 mg/kg/day. Delay of the initiation of either viramidine or ribavirin therapy, using the approximate 1/3 LD50 dose of each, was protective as late as 48 h after exposure to the A/NWS/33 virus. While both compounds appear to have similar efficacy against influenza virus infections, when one considers the lesser toxicity, viramidine may warrant further evaluation as a possible therapy for influenza.

Treating HCV with ribavirin analogues and ribavirin-like molecules

Nucleos(t)ide analogues have proven useful in the treatment of viral infections. Ribavirin is a nucleoside, guanosine analogue, whose mechanisms of action include inhibition of inosine monophosphate dehydrogenase (IMPDH), which is the key step in de novo guanine synthesis, a requirement for viral replication. In combination with pegylated interferon alfa, ribavirin is the standard of care for the treatment of chronic hepatitis C today. However, the medication is associated with significant haemolytic anaemia, which may require dose reduction, discontinuation or treatment with recombinant human erythropoietin. Dose reduction also appears to decrease sustained viral clearance rates. Newer IMPDH inhibitors are in various stages of development. Viramidine, a liver-targeting prodrug of ribavirin, has demonstrated significant antiviral activity and erythrocyte-sparing properties. It is currently in Phase 3 trials. Clinical trials of merimepodib, another investigational IMPDH inhibitor, have completed enrolment for a Phase 2b study as a third medication for administration with pegylated interferon plus ribavirin. Although other IMDPH inhibitors also have antiviral activity, these medications appear best suited as immunosuppressive medications at this time.

Phosphorylation of ribavirin and viramidine by adenosine kinase and cytosolic 5'-nucleotidase II: Implications for ribavirin metabolism in erythrocytes

Many nucleoside analog drugs, such as ribavirin and viramidine, are activated or metabolized in vivo through 5'-phosphorylation. In this report, we determined the steady-state kinetic parameters for 5'-monophosphorylation of ribavirin and viramidine by adenosine kinase. The apparent Km for ribavirin is 540 microM, and k(cat) is 1.8 min-1. Its catalytic efficiency of 3.3 x 10(-3) min-1 . microM-1 is 1,200-fold lower than that of adenosine. In contrast to the common belief that ribavirin is exclusively phosphorylated by adenosine kinase, cytosolic 5'-nucleotidase II was found to catalyze ribavirin phosphorylation in vitro. The reaction is optimally stimulated by the physiological concentration of ATP or 2,3-bisphosphoglycerate. In phosphate-buffered saline plus ATP and 2,3-bisphosphoglycerate, the apparent Km for ribavirin is 88 microM, and k(cat) is 4.0 min-1. These findings suggest that cytosolic 5'-nucleotidase II may be involved in ribavirin phosphorylation in vivo. Like ribavirin, viramidine was found to be phosphorylated by either adenosine kinase or cytosolic 5'-nucleotidase II, albeit with a much lower activity. The catalytic efficiency for viramidine phosphorylation is 10- to 330-fold lower than that of ribavirin, suggesting that other nucleoside kinase(s) may be involved in viramidine phosphorylation in vivo. Both ribavirin and viramidine are not phosphorylated by deoxycytidine kinase and uridine-cytidine kinase. The coincidence of presence of high concentrated 2,3-bisphosphoglycerate in erythrocytes suggests that cytosolic 5'-nucleotidase II could play an important role in phosphorylating ribavirin and contribute to anabolism of ribavirin triphosphate in erythrocytes. Elucidation of ribavirin and viramidine phosphorylation mechanism should shed light on their in vivo metabolism, especially the ribavirin-induced hemolytic anemia in erythrocytes.

Development of antiviral therapy for severe acute respiratory syndrome

A new disease, the severe acute respiratory distress syndrome (SARS), caused by the SARS coronavirus (SARS-CoV), emerged at the beginning of 2003 and rapidly spread throughout the world. Although the disease had disappeared in June 2003 its re-emergence cannot be excluded. The development of vaccines against SARS-CoV may take years. Therefore, the availability of effective antiviral drugs against SARS-CoV may be crucial for the control of future SARS outbreaks. In this review, experimental and clinical data about potential anti-SARS drugs is summarised and discussed. Animal model studies will be needed to help to determine which interventions warrant controlled clinical testing.

Treatment of mannan-enhanced influenza B virus infections in mice with oseltamivir, ribavirin and viramidine

Mannan, a polysaccharide preparation from Saccharomyces cerevisiae, has previously been shown to enhance influenza virus replication in mice by inhibiting host defense collectins. The use of mannan in infections may serve to broaden the types of influenza viruses that can be studied in rodent infection models. When mannan was co-administered with influenza B/Sichuan/379/99 virus to mice, the animals died from the infection, whereas mice infected with only virus survived. Three types of influenza A (H1N1) and another influenza B (Hong Kong/330/01) virus infection were also enhanced by mannan, but not four types of influenza A (H3N2) viruses. Mannan was used at 0.16 or 0.5 mg/mouse for optimal disease-enhancing activity using influenza B/Sichuan/379/99 virus. Using this model, influenza B/Sichuan/379/99 infections were treated with oseltamivir, ribavirin or viramidine (the carboxamidine derivative of ribavirin). When oral gavage treatments started 4 h before virus and mannan challenge, oseltamivir was effective at 2.5, 5 and 10 mg/kg/day. Ribavirin was active at 20, 40 and 80 mg/kg/day. Viramidine was effective at 80 and 160 mg/kg/day but not at 40 mg/kg/day. Active drug doses improved lung consolidation scores and lung weights, with decreases in lung virus titres also noted. Arterial oxygen saturation values in treated groups were significantly better than those of the placebo group on days 7-11 of the infection. Oseltamivir (5 mg/kg/day) and ribavirin (40 mg/kg/day) were used alone and in combination to determine how late after infection they could be beneficially administered. Ribavirin alone was very effective (90-100% survival of mice) when treatments started as late as 3 days after infection. Forty percent survival was evident even when treatments started 4 days post-infection. Oseltamivir was active starting treatments 1 day after virus exposure, but lost considerable efficacy when treatments began after that time. The combination of ribavirin and oseltamivir appeared to be no better than ribavirin alone, due to the stronger beneficial effect of ribavirin in this model. The overall results demonstrate that mannan can be used to enhance certain non-lethal influenza virus infections sufficiently to allow antiviral studies.

Recent developments in target identification against hepatitis C virus

Chronic hepatitis C is a leading cause of liver cirrhosis and hepatocellular carcinoma worldwide. Recent progress in the understanding of the molecular virology of hepatitis C has allowed the identification of novel antiviral targets. Moreover, in vitro and in vivo model systems have been developed that allow the systematic evaluation of new therapeutic strategies. Exciting results from proof-of-concept clinical studies have now been reported for a specific hepatitis C virus serine protease inhibitor. These and other novel antiviral strategies may complement existing therapeutic modalities in the future.

Dual-action mechanism of viramidine functioning as a prodrug and as a catabolic inhibitor for ribavirin

An investigational nucleoside analogue drug, viramidine, has recently emerged as a potentially safer alternative to ribavirin for the treatment of hepatitis C viral infection. We have reported that viramidine mainly functions as a prodrug of ribavirin that is enriched in the liver. This in vitro study further explores viramidine's activity against nucleoside phosphorylase, a host enzyme that is responsible for phosphorolysis of ribavirin in vivo. Our experiments show that viramidine inhibits ribavirin phosphorolysis with a K(i) of 2.5 microM. This result suggests that viramidine may act through a dual-action mechanism by serving as a prodrug of ribavirin and concomitantly as an inhibitor for nucleoside phosphorylase catabolism of ribavirin.

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.

Disposition and metabolic profiles of [14C]viramidine and [14C]ribavirin in rat and monkey red blood cells and liver

The disposition and metabolic profiles of [14C]viramidine and [14C]ribavirin were compared in rat and monkey red blood cells and liver. Our data reveal that the total ribavirin-related components (ribavirin plus its mono-, di-, and triphosphate metabolites) may account for most of the drug in monkey liver following prolonged oral administration of viramidine.

Viral hepatitis in 2003

PURPOSE OF REVIEW

This article discusses over 40 studies in the field of viral hepatitis that were published in 2003. Studies that reported novel findings of clinical relevance were selected after a MEDLINE search. This article focuses on hepatitis B and hepatitis C.

RECENT FINDINGS

In the United States, all genotypes of hepatitis B virus were reported, and precore and core promoter mutations were found in approximately a third of patients. The response to lamivudine was less durable than that induced by interferon. Adefovir dipivoxil was safe and effective for treatment-naive patients as well as for lamivudine-resistant patients. In acute hepatitis C infection, close monitoring of hepatitis C virus RNA and treatment of patients with persistent viremia is a reasonable alternative to immediate treatment of all patients. Noninvasive indices to predict liver fibrosis are currently being tested. Failure to achieve an early virologic response to pegylated interferon plus ribavirin can identify more than 95% patients who will not have a sustained response.

SUMMARY

The clinical significance of hepatitis B virus genotypes and viral variants continues to be elucidated worldwide. Long-term lamivudine treatment is safe, despite a rising rate of resistance. Adefovir appears to be safe and effective as initial therapy for chronic hepatitis B virus infection or for the management of lamivudine-resistant hepatitis B infection. Noninvasive indices for liver fibrosis require validation but may reduce the need for liver biopsy in the future. Newer agents for the treatment of hepatitis C virus infection show promise.