Treatment of chronic woodchuck hepatitis virus infection in the Eastern woodchuck (Marmota monax) with nucleoside analogues is predictive of therapy for chronic hepatitis B virus infection in humans

Recent Drug Development in the Woodchuck Model of Chronic Hepatitis B

Infection with hepatitis B virus (HBV) is responsible for the increasing global hepatitis burden, with an estimated 296 million people being carriers and living with the risk of developing chronic liver disease and cancer. While the current treatment options for chronic hepatitis B (CHB), including oral nucleos(t)ide analogs and systemic interferon-alpha, are deemed suboptimal, the path to finding an ultimate cure for this viral disease is rather challenging. The lack of suitable laboratory animal models that support HBV infection and associated liver disease progression is one of the major hurdles in antiviral drug development. For more than four decades, experimental infection of the Eastern woodchuck with woodchuck hepatitis virus has been applied for studying the immunopathogenesis of HBV and developing new antiviral therapeutics against CHB. There are several advantages to this animal model that are beneficial for performing both basic and translational HBV research. Previous review articles have focused on the value of this animal model in regard to HBV replication, pathogenesis, and immune response. In this article, we review studies of drug development and preclinical evaluation of direct-acting antivirals, immunomodulators, therapeutic vaccines, and inhibitors of viral entry, gene expression, and antigen release in the woodchuck model of CHB since 2014 until today and discuss their significance for clinical trials in patients.

Toll-Like Receptor 7 Agonist RG7854 Mediates Therapeutic Efficacy and Seroconversion in Woodchucks With Chronic Hepatitis B

Conventional treatment of chronic hepatitis B (CHB) is rarely curative due to the immunotolerant status of patients. RG7854 is an oral double prodrug of a toll-like receptor 7 (TLR7) agonist that is developed for the treatment of CHB. The therapeutic efficacy, host immune response, and safety of RG7854 were evaluated in the woodchuck model of CHB. Monotreatment with the two highest RG7854 doses and combination treatment with the highest RG7854 dose and entecavir (ETV) suppressed viral replication, led to loss of viral antigens, and induced seroconversion in responder woodchucks. Since viral suppression and high-titer antibodies persisted after treatment ended, this suggested that a sustained antiviral response (SVR) was induced by RG7854 in a subset of animals. The SVR rate, however, was comparable between both treatment regimens, suggesting that the addition of ETV did not enhance the therapeutic efficacy of RG7854 although it augmented the proliferation of blood cells in response to viral antigens and magnitude of antibody titers. The induction of interferon-stimulated genes in blood by RG7854/ETV combination treatment demonstrated on-target activation of TLR7. Together with the virus-specific blood cell proliferation and the transient elevations in liver enzymes and inflammation, this suggested that cytokine-mediated non-cytolytic and T-cell mediated cytolytic mechanisms contributed to the SVR, in addition to the virus-neutralizing effects by antibody-producing plasma cells. Both RG7854 regimens were not associated with treatment-limiting adverse effects but accompanied by dose-dependent, transient neutropenia and thrombocytopenia. The study concluded that finite, oral RG7854 treatment can induce a SVR in woodchucks that is based on the retrieval of antiviral innate and adaptive immune responses. This supports future investigation of the TLR7 agonist as an immunotherapeutic approach for achieving functional cure in patients with CHB.

Combination Treatment with the Vimentin-Targeting Antibody hzVSF and Tenofovir Suppresses Woodchuck Hepatitis Virus Infection in Woodchucks

Current treatment options for patients infected with hepatitis B virus (HBV) are suboptimal, because the approved drugs rarely induce cure due to the persistence of the viral DNA genome in the nucleus of infected hepatocytes, and are associated with either severe side effects (pegylated interferon-alpha) or require life-long administration (nucleos(t)ide analogs). We report here the evaluation of the safety and therapeutic efficacy of a novel, humanized antibody (hzVSF) in the woodchuck model of HBV infection. hzVSF has been shown to act as a viral entry inhibitor, most likely by suppressing vimentin-mediated endocytosis of virions. Targeting the increased vimentin expression on liver cells by hzVSF after infection with HBV or woodchuck hepatitis virus (WHV) was demonstrated initially. Thereafter, hzVSF safety was assessed in eight woodchucks naïve for WHV infection. Antiviral efficacy of hzVSF was evaluated subsequently in 24 chronic WHV carrier woodchucks by monotreatment with three ascending doses and in combination with tenofovir alafenamide fumarate (TAF). Consistent with the proposed blocking of WHV reinfection, intravenous hzVSF administration for 12 weeks resulted in a modest but transient reduction of viral replication and associated liver inflammation. In combination with oral TAF dosing, the antiviral effect of hzVSF was enhanced and sustained in half of the woodchucks with an antibody response to viral proteins. Thus, hzVSF safely but modestly alters chronic WHV infection in woodchucks; however, as a combination partner to TAF, its antiviral efficacy is markedly increased. The results of this preclinical study support future evaluation of this novel anti-HBV drug in patients.

Application of the woodchuck animal model for the treatment of hepatitis B virus-induced liver cancer

This review describes woodchucks chronically infected with the woodchuck hepatitis virus (WHV) as an animal model for hepatocarcinogenesis and treatment of primary liver cancer or hepatocellular carcinoma (HCC) induced by the hepatitis B virus (HBV). Since laboratory animal models susceptible to HBV infection are limited, woodchucks experimentally infected with WHV, a hepatitis virus closely related to HBV, are increasingly used to enhance our understanding of virus-host interactions, immune response, and liver disease progression. A correlation of severe liver pathogenesis with high-level viral replication and deficient antiviral immunity has been established, which are present during chronic infection after WHV inoculation of neonatal woodchucks for modeling vertical HBV transmission in humans. HCC in chronic carrier woodchucks develops 17 to 36 mo after neonatal WHV infection and involves liver tumors that are comparable in size, morphology, and molecular gene signature to those of HBV-infected patients. Accordingly, woodchucks with WHV-induced liver tumors have been used for the improvement of imaging and ablation techniques of human HCC. In addition, drug efficacy studies in woodchucks with chronic WHV infection have revealed that prolonged treatment with nucleos(t)ide analogs, alone or in combination with other compounds, minimizes the risk of liver disease progression to HCC. More recently, woodchucks have been utilized in the delineation of mechanisms involved in innate and adaptive immune responses against WHV during acute, self-limited and chronic infections. Therapeutic interventions based on modulating the deficient host antiviral immunity have been explored in woodchucks for inducing functional cure in HBV-infected patients and for reducing or even delaying associated liver disease sequelae, including the onset of HCC. Therefore, woodchucks with chronic WHV infection constitute a well-characterized, fully immunocompetent animal model for HBV-induced liver cancer and for preclinical evaluation of the safety and efficacy of new modalities, which are based on chemo, gene, and immune therapy, for the prevention and treatment of HCC in patients for which current treatment options are dismal.

Treatment with the Immunomodulator AIC649 in Combination with Entecavir Produces Antiviral Efficacy in the Woodchuck Model of Chronic Hepatitis B

As current interventions for chronic hepatitis B (CHB) rarely induce cure, more effective drugs are needed. Short-term treatment of woodchucks with the novel immunomodulator AIC649, a parapoxvirus-based stimulator of toll-like receptor 9 dependent and independent pathways, has been shown to reduce viral DNA and surface antigen via a unique, biphasic response pattern. The present study evaluated long-term AIC649 treatment in combination with Entecavir for potency and safety in woodchucks. AIC649 monotreatment induced modest reductions in serum viral DNA and surface and e antigens that were associated with the same biphasic response pattern previously observed. Entecavir monotreatment reduced transiently viremia but not antigenemia, while AIC649/Entecavir combination treatment mediated superior viral control. Undetectability of viral antigens and elicitation of antibodies in AIC649/Entecavir-treated woodchucks correlated with the expression of interferons and suppression of viral replication in liver. Combination treatment was well tolerated, and liver enzyme elevations were minor and transient. It was concluded that the AIC649-mediated effects were most likely based on an improvement and/or reconstitution of antiviral immune responses that are typically deficient in CHB. As a combination partner to Entecavir, the antiviral efficacy of AIC649 was markedly enhanced. This preclinical study supports future evaluation of AIC649 for treatment of human CHB.

Characterization of Seasonal Pharmacokinetic Variability in Woodchucks

The eastern woodchuck (Marmota monax) is a hibernating species extensively used as an in vivo efficacy model for chronic human hepatitis B virus infection. Under laboratory conditions, woodchucks develop a pseudohibernation condition; thus, the pharmacokinetics (PK) of small-molecule therapeutics may be affected by the seasonal change. The seasonal PK of four probe compounds were characterized over 12 months in seven male and nine female laboratory-maintained woodchucks. These compounds were selected to study changes in oxidative metabolism [antipyrine (AP)], glucuronidation [raltegravir (RTG)], renal clearance [lamivudine (3TC)], and hepatic function [indocyanine green (ICG)]. Seasonal changes in physiologic parameters and PK were determined. Seasonal body weight increases were ≥30%. Seasonal changes in body temperature and heart rate were <10%. The mean AP exposure remained unchanged from April to August 2017, followed by a significant increase (≥1.0-fold) from August to December and subsequent decrease to baseline at the end of study. A similar trend was observed in RTG and 3TC exposures. The ICG exposure remained unchanged. No significant sex difference in PK was observed, although female woodchucks appeared to be less susceptible to seasonal PK and body weight changes. Significant seasonal PK changes for AP, RTG, and 3TC indicate decreases in oxidative metabolism, phase II glucuronidation, and renal clearance during pseudohibernation. The lack of seasonal change in ICG exposure suggests there are no significant changes in hepatic function. This information can be used to optimize the scheduling of woodchuck studies to avoid seasonally driven variation in drug PK. SIGNIFICANCE STATEMENT: Woodchuck is a hibernating species and is commonly used as a nonclinical model of hepatitis B infection. Investigation of seasonal PK changes is perhaps of greater interest to pharmaceutical industry scientists, who use the woodchuck model to optimize the scheduling of woodchuck studies to avoid seasonally driven variation in drug PK and/or toxicity. This information is also valuable to drug metabolism and veterinary scientists in understanding woodchuck's seasonal metabolism and behavior under the pseudohibernation condition.

Animal models for the study of human hepatitis B and D virus infection: New insights and progress

Hepatitis B virus (HBV) is a member of the Hepadnaviridae family and infects hepatocytes, leading to liver pathology in acutely and chronically infected individuals. Co-infection with Hepatitis D virus (HDV), which requires the surface proteins of HBV to replicate, can exacerbate this disease progression. Thus, the >250 million people living with chronic HBV infection, including 13 million co-infected with HDV, would significantly benefit from an effective and affordable curative treatment. Animal models are crucial to the development of innovative disease therapies, a paradigm repeated again and again throughout the fields of immunology, neurology, reproduction, and development. Unfortunately, HBV has a highly-restricted species tropism, infecting limited species including humans, chimpanzees, and treeshrews. The first experimentally controlled studies of HBV infection were following inoculation of human volunteers in 1942, which identified the transmissibility of hepatitis through serum transfer and led to the hypothesis that the etiological agent was viral. Subsequent research in chimpanzees (Desmyter et al., 1971; Lichter, 1969) and later in other species, such as the treeshrews (Walter et al., 1996; Yan et al., 1996), further confirmed the viral origin of hepatitis B. Shortly thereafter, HBV-like viral infections were identified in woodchucks (Summers et al., 1978; Werner et al., 1979) and ducks, and much of our understanding of HBV replication can be attributed to these important models. However, with the exodus of chimpanzees from research and the limited reagents and historical data for treeshrews and other understudied species, there remains an urgent need to identify physiologically relevant models of chronic HBV infection. While large strides have been made in generating such models, particularly over the past two decades, there is still no available model that faithfully recapitulates the immunity and pathogenesis of HBV infection. Here, we discuss recent advancements in the generation of murine and non-human primate (NHP) models of HBV/HDV infection.

Diverse Virus and Host-Dependent Mechanisms Influence the Systemic and Intrahepatic Immune Responses in the Woodchuck Model of Hepatitis B

Woodchuck infected with woodchuck hepatitis virus (WHV) represents the pathogenically nearest model of hepatitis B and associated hepatocellular carcinoma (HCC). This naturally occurring animal model also is highly valuable for development and preclinical evaluation of new anti-HBV agents and immunotherapies against chronic hepatitis (CH) B and HCC. Studies in this system uncovered a number of molecular and immunological processes which contribute or likely contribute to the immunopathogenesis of liver disease and modulation of the systemic and intrahepatic innate and adaptive immune responses during hepadnaviral infection. Among them, inhibition of presentation of the class I major histocompatibility complex on chronically infected hepatocytes and a role of WHV envelope proteins in this process, as well as augmented hepatocyte cytotoxicity mediated by constitutively expressed components of CD95 (Fas) ligand- and perforin-dependent pathways, capable of eliminating cells brought to contact with hepatocyte surface, including activated T lymphocytes, were uncovered. Other findings pointed to a role of autoimmune response against hepatocyte asialoglycoprotein receptor in augmenting severity of liver damage in hepadnaviral CH. It was also documented that WHV in the first few hours activates intrahepatic innate immunity that transiently decreases hepatic virus load. However, this activation is not translated in a timely manner to induction of virus-specific T cell response which appears to be hindered by defective activation of antigen presenting cells and presentation of viral epitopes to T cells. The early WHV infection also induces generalized polyclonal activation of T cells that precedes emergence of virus-specific T lymphocyte reactivity. The combination of these mechanisms hinder recognition of virus allowing its dissemination in the initial, asymptomatic stages of infection before adaptive cellular response became apparent. This review will highlight a range of diverse mechanisms uncovered in the woodchuck model which affect effectiveness of the anti-viral systemic and intrahepatic immune responses, and modify liver disease outcomes. Further exploration of these and other mechanisms, either already discovered or yet unknown, and their interactions should bring more comprehensive understanding of HBV pathogenesis and help to identify novel targets for therapeutic and preventive interventions. The woodchuck model is uniquely positioned to further contribute to these advances.

Liver-Targeted Toll-Like Receptor 7 Agonist Combined With Entecavir Promotes a Functional Cure in the Woodchuck Model of Hepatitis B Virus

Current therapeutics for chronic infection with hepatitis B virus (HBV) rarely induce functional cure due to the immunotolerant status of patients. Small molecule agonists targeting toll-like receptor 7 (TLR7) have been shown to elicit a functional cure in animal models of HBV but sometimes with poor tolerability due to immune-related toxicities. In an effort to increase the therapeutic window of TLR7 agonists to treat chronic hepatitis B (CHB), we developed an oral TLR7 agonist, APR002, designed to act locally in the gastrointestinal tract and liver, thus minimizing systemic exposure and improving tolerability. Here, we describe the pharmacokinetic/pharmacodynamic (PK/PD) profile of APR002 in mice and uninfected woodchucks as well as the safety and antiviral efficacy in combination with entecavir (ETV) in woodchucks with CHB. Treatment of woodchucks chronically infected with woodchuck hepatitis virus (WHV) with weekly oral doses of APR002 was well-tolerated. While APR002 and ETV single agents did not elicit sustained viral control, combination therapy resulted in durable immune-mediated suppression of the chronic infection. These woodchucks also had detectable antibodies to viral antigens, enhanced interferon-stimulated gene expression, and loss of WHV covalently closed circular DNA. Conclusion: APR002 is a novel TLR7 agonist exhibiting a distinct PK/PD profile that in combination with ETV can safely attain a functional cure in woodchucks with chronic WHV infection. Our results support further investigation of liver-targeted TLR7 agonists in human CHB.

In Vitro Anti-hepatitis B Virus Activity of 2',3'-Dideoxyguanosine

2',3'-dideoxyguanosine (DoG) has been demonstrated to inhibit duck hepatitis B virus (DHBV) replication in vivo in a duck model of HBV infection. In the current study, the in vitro antiviral effects of DoG on human and animal hepadnaviruses were investigated. Our results showed that DoG effectively inhibited HBV, DHBV, and woodchuck hepatitis virus (WHV) replication in hepatocyte-derived cells in a dose-dependent manner, with 50% effective concentrations (EC50) of 0.3 ± 0.05, 6.82 ± 0.25, and 23.0 ± 1.5 μmol/L, respectively. Similar to other hepadnaviral DNA polymerase inhibitors, DoG did not alter the levels of intracellular viral RNA but induced the accumulation of a less-than-full-length viral RNA species, which was recently demonstrated to be generated by RNase H cleavage of pgRNA. Furthermore, using a transient transfection assay, DoG showed similar antiviral activity against HBV wild-type, 3TC-resistant rtA181V, and adefovir-resistant rtN236T mutants. Our results suggest that DoG has potential as a nucleoside analogue drug with anti-HBV activity.

Prevention of liver tumor formation in woodchucks with established hepatocellular carcinoma by treatment with cationic liposome-DNA complexes

Background

Approximately 250 million people worldwide are chronically infected with hepatitis B virus (HBV) and more than half of the hepatocellular carcinoma (HCC) cases are attributed to this infection. As HCC has a high mortality rate, and current treatment options are remarkably limited, the development of new therapeutic treatment strategies is warranted.

Methods

In this study, woodchucks infected with woodchuck hepatitis virus (WHV), and with pre-existing liver tumors, were used as a model to investigate if complexes of cationic liposomes and non-coding DNA (JVRS-100) were effective in treatment of HCC.

Results

It was observed that the high serum viral load that is present in a typical chronic WHV infection (i.e., approximately 100-fold higher than human viral loads) results in immune suppression and resistance to treatment with JVRS-100. Treatment of woodchucks with lower serum viral load that more closely matched with the viral load usually seen in human HBV infection appears a better model for immunotherapeutic development based on the responsiveness to JVRS-100 treatment. In the latter case, marked declines in WHV DNA and WHV surface antigen were determined over the 12-week treatment period and WHV markers stayed suppressed during most time points of the 12-week follow-up period. Even more remarkably, the formation of new liver tumors was not observed in woodchucks treated with a well-tolerated dose of JVRS-100, as compared to several new tumors that developed in vehicle-treated control animals.

Conclusions

Although there was little decrease in the volumes of the liver tumors existing at the time of treatment, it is generally accepted that preventing the spread and metastasis of almost always fatal cancers such as HCC and thus, reducing it to a chronic and treatable disease can also be a successful therapeutic approach. The results in woodchucks warrant the investigation of JVRS-100 as an intervention to prevent liver cancer in patients chronically infected with HBV and at high risk for HCC development.

Molecular cloning, characterization and expression analysis of TGF-β and receptor genes in the woodchuck model

Transforming growth factor beta (TGF-β) is an important cytokine with pleiotropic regulatory functions in the immune system and in the responses against viral infections. TGF-β acts on a variety of immune cells through the cell surface TGF-β receptor (University of Duisburg-EssenTGFBR). The woodchuck has been used as a biomedical model for studies of obesity and energy balance, endocrine and metabolic function, cardiovascular, cerebrovascular and neoplastic disease. Woodchucks infected with woodchuck hepatitis virus (WHV) represent an informative animal model to study hepatitis B virus (HBV) infection. In this study, the cDNA sequences of woodchuck TGF-β1, TGF-β2, TGFBR1 and TGFBR2 were cloned, sequenced and characterized. The full-length TGFBR1 cDNA sequence consisted of 1305bp coding sequence (CDS) that encoded 434 amino acids with a molecular weight of 48.9kDa. The phylogenetic tree analysis revealed that the woodchuck TGF-β family genes had a closer genetic relationship with Ictidomys tridecemlineatus. One antibody with cross-reactivity to woodchuck TGFBR1 was identified by flow cytometry. Moreover, the expression of these genes were analyzed at the transcriptional level. The quantitative PCR analysis showed that the TGF-β family transcripts were constitutively expressed in many tissues tested. Altered expression levels of the TGF-β family transcripts in the liver of WHV infected woodchucks were observed. These results serve as a foundation for further insight into the role of the TGF-β family in viral hepatitis in woodchuck model. Our work also possesses the potential value for characterizing the TGF-β family in other related diseases, such as obesity-related diseases, metabolic disorder, cardiovascular disease and cancer.

In vivo models of hepatitis B and C virus infection

Globally, more than 500 million individuals are chronically infected with hepatitis B (HBV), delta (HDV), and/or C (HCV) viruses, which can result in severe liver disease. Mechanistic studies of viral persistence and pathogenesis have been hampered by the scarcity of animal models. The limited species and cellular host range of HBV, HDV, and HCV, which robustly infect only humans and chimpanzees, have posed challenges for creating such animal models. In this review, we will discuss the barriers to interspecies transmission and the progress that has been made in our understanding of the HBV, HDV, and HCV life cycles. Additionally, we will highlight a variety of approaches that overcome these barriers and thus facilitate in vivo studies of these hepatotropic viruses.

Differential association of STAT3 and HK-II expression in hepatitis B virus- and hepatitis C virus-related hepatocellular carcinoma

STAT3 and hexokinase II (HK-II) are involved in viral infection and carcinogenesis of various cancers including hepatocellular carcinoma (HCC). The roles of STAT3 and HK-II in hepatitis B virus (HBV)- and hepatitis C virus (HCV)-related HCC remain largely unclear. This study examined STAT3 and HK-II expression in HBV- and HCV-related HCC, HBV-related liver fibrosis, and normal control liver by using tissue microarray and immunohistochemical method. Results showed that STAT3 expression in HBV-related HCC, HCV-related HCC, and HBV-related liver fibrosis was significantly higher than in control liver (P < 0.001, P = 0.016, and P = 0.005, respectively) and had no significant differences between these three diseased liver tissues. The HK-II expression in HBV-related HCC was significantly higher than that in HCV-related HCC, HBV-related liver fibrosis, and control liver (P = 0.007, P = 0.029, and P = 0.008, respectively) but had no significant elevation in and no significant differences between HCV-related HCC, HBV-related liver fibrosis, and control liver. The HK-II expression was significantly correlated to STAT3 expression in HBV-related HCC (P = 0.022), but no correlation was observed in HCV-related HCC, HBV-related liver fibrosis, and control liver. In conclusion, STAT3 expression is upregulated in both HBV- and HCV-related HCC, while HK-II is predominantly upregulated and correlated to STAT3 in HBV-related HCC. These differential expression and association may suggest the distinct roles of STAT3 and HK-II in hepatocarcinogenesis of HBV and HCV infection. Studies are needed to confirm the relationship of STAT3 and HK-II and to examine the underlying mechanisms. J. Med. Virol. 88:1552-1559, 2016. © 2016 Wiley Periodicals, Inc.

Advances and Challenges in Studying Hepatitis B Virus In Vitro

Hepatitis B virus (HBV) is a small DNA virus that infects the liver. Current anti-HBV drugs efficiently suppress viral replication but do not eradicate the virus due to the persistence of its episomal DNA. Efforts to develop reliable in vitro systems to model HBV infection, an imperative tool for studying HBV biology and its interactions with the host, have been hampered by major limitations at the level of the virus, the host and infection readouts. This review summarizes major milestones in the development of in vitro systems to study HBV. Recent advances in our understanding of HBV biology, such as the discovery of the bile-acid pump sodium-taurocholate cotransporting polypeptide (NTCP) as a receptor for HBV, enabled the establishment of NTCP expressing hepatoma cell lines permissive for HBV infection. Furthermore, advanced tissue engineering techniques facilitate now the establishment of HBV infection systems based on primary human hepatocytes that maintain their phenotype and permissiveness for infection over time. The ability to differentiate inducible pluripotent stem cells into hepatocyte-like cells opens the door for studying HBV in a more isogenic background, as well. Thus, the recent advances in in vitro models for HBV infection holds promise for a better understanding of virus-host interactions and for future development of more definitive anti-viral drugs.

The Woodchuck, a Nonprimate Model for Immunopathogenesis and Therapeutic Immunomodulation in Chronic Hepatitis B Virus Infection

The woodchuck hepatitis virus (WHV) and its host, the eastern woodchuck, is a very valuable model system for hepatitis B virus infection. Many aspects of WHV replication and pathogenesis resemble acute and chronic hepatitis B infection in patients. Since the establishment of immunological tools, woodchucks were used to develop new therapeutic vaccines and immunomodulatory approaches to treat chronic hepadnaviral infections. Combination therapy of nucleos(t)ide analogs, with prime-boost vaccination and triple therapy, including immunomodulatory strategies by blocking the interaction of the programmed death-1 (PD-1) receptor with its ligand inducing a potent T-cell response in chronic WHV carrier woodchucks, suppression of viral replication, and complete elimination of the virus in 30% of the animals. Both strategies may be used for future therapies in patients with chronic hepatitis B.

Determinants of hepatitis B and delta virus host tropism

Hepatitis B virus (HBV) infections are a global health problem afflicting approximately 360 million patients. Of these individuals, 15-20 million are co-infected with hepatitis delta virus (HDV). Progress toward curative therapies has been impeded by the highly restricted host tropism of HBV, which is limited to productive infections in humans and chimpanzees. Here, we will discuss different approaches that have been taken to study HBV and HDV infections in vivo. The development of transgenic and humanized mice has lead to deeper insights into HBV pathogenesis. An improved understanding of the determinants governing HBV and HDV species tropism will aid in the construction of a small animal model with inheritable susceptible to HBV/HDV.

Current and future antiviral drug therapies of hepatitis B chronic infection

Despite significant improvement in the management of chronic hepatitis B virus (HBV) it remains a public health problem, affecting more than 350 million people worldwide. The natural course of the infection is dynamic and involves a complex interplay between the virus and the host’s immune system. Currently the approved therapeutic regimens include pegylated-interferon (IFN)-α and monotherapy with five nucleos(t)ide analogues (NAs). Both antiviral treatments are not capable to eliminate the virus and do not establish long-term control of infection after treatment withdrawal. IFN therapy is of finite duration and associates with low response rates, liver decompensating and numerous side effects. NAs are well-tolerated therapies but have a high risk of drug resistance development that limits their prolonged use. The imperative for the development of new approaches for the treatment of chronic HBV infection is a challenging issue that cannot be over-sided. Research efforts are focusing on the identification and evaluation of various viral replication inhibitors that target viral replication and a number of immunomodulators that aim to restore the HBV specific immune hyporesponsiveness without inducing liver damage. This review brings together our current knowledge on the available treatment and discusses potential therapeutic approaches in the battle against chronic HBV infection.

Recent advances in developing nucleic acid-based HBV therapy

Chronic HBV infection remains an important public health problem and currently licensed therapies rarely prevent complications of viral persistence. Silencing HBV gene expression using gene therapy, particularly with exogenous activators of RNAi, holds promise for developing an HBV gene therapy. However, immune stimulation, off-targeting effects and inefficient delivery of RNAi activators remain problematic. Several new approaches have recently been employed to address these issues. Chemical modifications to anti-HBV synthetic siRNAs have been investigated and a variety of vectors are being developed for delivery of RNAi effectors. In this article, we review the potential utility of gene therapy for treating HBV infection.

Unraveling the complexity of hepatitis B virus: from molecular understanding to therapeutic strategy in 50 years

Hepatitis B virus (HBV) is a well-known hepadnavirus with a double-stranded circular DNA genome. Although HBV was first described approximately 50 years ago, the precise mechanisms of HBV infection and effective therapeutic strategies remain unclear. Here, we focus on summarizing the complicated mechanisms of HBV replication and infection, as well as genomic factors and epigenetic regulation. Additionally, we discuss in vivo models of HBV, as well as diagnosis, prevention and therapeutic drugs for HBV. Together, the data in this 50-year review may provide new clues to elucidate molecular mechanisms of HBV pathogenesis and shed new light on the future HBV therapies.

Expression and gene polymorphisms of interleukin 28B and hepatitis B virus infection in a Chinese Han population

BACKGROUND

Recent genome-wide association studies found that genetic polymorphisms near the IL28B gene is strongly associated with sustained viral response and spontaneous viral clearance in chronically infected hepatitis C patients.

AIMS

We aimed to evaluate the effects of IL28B variations on hepatitis B virus (HBV) infection in a Chinese Han population and to explore the association between IL28B polymorphisms and susceptibility to infection, viral clearance, disease progression, viral load and liver inflammation.

METHODS

We determined three IL28B single gene polymorphisms (rs12979860, rs12980275 and rs8099917) in 203 individuals with chronic HBV infection, 203 individuals with self-limited HBV infection and 203 individuals negative for all HBV seromarkers. Interleukin (IL)28B serum levels were evaluated in all subjects. Additionally, peripheral blood mononuclear cells from 42 chronically HBV-infected individuals were subjected to whole-genome expression studies.

RESULTS

The association among genotype, allele and haplotype frequencies of IL28B with alanine aminotransferase levels and HBV DNA was established. However, no significant differences were observed in genotype or allele frequencies among chronically HBV-infected, self-limited and healthy subjects. The serum IL28B level was lower in patients with chronic HBV infection than in the self-limited HBV-infected or healthy subjects. The serum IL28B level was correlated with the subject's genotype. Gene expression micro-array analysis showed enhanced IL28B expression in patients with low HBV viral load.

CONCLUSIONS

Variability at the IL28B locus is associated with HBV viral load and hepatic inflammation. Genetic variation of IL28B may prevent HBV progression by reducing viral load and liver inflammation, providing a valuable gene therapy tool.

Electroporation enhances immunogenicity of a DNA vaccine expressing woodchuck hepatitis virus surface antigen in woodchucks

The development of therapeutic vaccines for chronic hepatitis B virus (HBV) infection has been hampered by host immune tolerance and the generally low magnitude and inconsistent immune responses to conventional vaccines and proposed new delivery methods. Electroporation (EP) for plasmid DNA (pDNA) vaccine delivery has demonstrated the enhanced immunogenicity of HBV antigens in various animal models. In the present study, the efficiency of the EP-based delivery of pDNA expressing various reporter genes first was evaluated in normal woodchucks, and then the immunogenicity of an analog woodchuck hepatitis virus (WHV) surface antigen (WHsAg) pDNA vaccine was studied in this model. The expression of reporter genes was greatly increased when the cellular uptake of pDNA was facilitated by EP. The EP of WHsAg-pDNA resulted in enhanced, dose-dependent antibody and T-cell responses to WHsAg compared to those of the conventional hypodermic needle injection of WHsAg-pDNA. Although subunit WHsAg protein vaccine elicited higher antibody titers than the DNA vaccine delivered with EP, T-cell response rates were comparable. However, in WHsAg-stimulated mononuclear cell cultures, the mRNA expression of CD4 and CD8 leukocyte surface markers and Th1 cytokines was more frequent and was skewed following DNA vaccination compared to that of protein immunization. Thus, the EP-based vaccination of normal woodchucks with pDNA-WHsAg induced a skew in the Th1/Th2 balance toward Th1 immune responses, which may be considered more appropriate for approaches involving therapeutic vaccines to treat chronic HBV infection.

Evaluation of mitochondrial toxicity in Marmota himalayana treated with metacavir, a novel 2',3'-dideoxyguanosine prodrug for treatment of hepatitis B Virus

Metacavir (PNA) is a novel synthetic nucleoside analogue for the treatment of hepatitis B virus (HBV). Our recent studies showed that PNA, a prodrug of 2',3'-dideoxyguanosine (ddG), exhibited lower mitochondrial toxicity in long-term cultures of HepG2 cells. In the current study, we examined the long-term effects of PNA on mitochondrial toxicity in Marmota himalayana (Himalayan marmot). Himalayan marmots were treated daily with oral PNA (50 or 100 mg/kg), ziduvidine (AZT) (100 mg/kg), or water (control) for 90 days. PNA treatment did not alter the body weight or plasma lactate acid level. In livers from the animals treated with PNA at 100 mg/kg/day, histopathology showed mild steatosis or small focal liver cell necrosis. Electron microscopy also showed minor proliferation and partial mitochondrial swelling with crista reduction. Measurement of respiratory chain complex enzyme activity and mitochondrial DNA (mtDNA) content revealed no significant differences in skeletal muscle, liver, and kidney tissues between animals treated with PNA and controls. In contrast, in Himalayan marmots treated with AZT we observed delayed toxicity, including lactic acidosis, severe hepatic steatosis, obvious mitochondrial damage, and significant decreases in respiratory chain complex enzyme activity and mtDNA content. This is similar to the delayed toxicity syndrome observed previously in animals and humans. In summary, PNA treatment did not alter mitochondrial enzyme activity or mtDNA content. This suggests that PNA could pose a very low risk for adverse mitochondrion-related effects. However, long-term hepatotoxic effects of PNA were observed, and this indicates a need for continued monitoring of PNA-associated hepatotoxicity in clinical trials.

Establishing a new animal model for hepadnaviral infection: susceptibility of Chinese Marmota-species to woodchuck hepatitis virus infection

Hepatitis B virus infection (HBV) is a major medical problem in China. The lack of a suitable infection model in China is recognized as an obstacle for research on HBV in China. Chinese Marmota-species is phylogenetically closely related to Marmota monax, thus, it might be suitable to serve as an animal model for HBV infection. Therefore, we attempted to prove the claim about the existence of woodchuck hepatitis virus (WHV)-like viruses in Chinese Marmota-species and to determine the susceptibility of these species to experimental WHV infection. In the present study, 653 sera from three Chinese Marmota-species, Marmota himalayana, Marmota baibacina and Marmota bobak, were screened for WHV-like viruses by serological and molecular assays. The susceptibility to WHV of three species was investigated by experimental infection and monitored by testing of anti-WHc and WHsAg by ELISA, detection of WHV DNA by PCR, and detection of WHV replication intermediates and antigens in liver samples. No evidence for the existence of a genetically closely related virus to WHV in three Chinese Marmota-species was found by serological assays and PCR. M. himalayana was susceptible to WHV infection as inoculated animals became positive for anti-WHc, WHsAg and WHV DNA. Further, WHV replication intermediates and proteins were detected in liver samples. In contrast, M. baibacina remained negative for tested virological parameters. M. bobak species showed a limited susceptibility to WHV. Our data do not support early reports about WHV-like viruses in China. M. himalayana is suitable for the establishment of a model for hepadnaviral infection.

Identification of CD4+ and CD8+ T cell epitopes of woodchuck hepatitis virus core and surface antigens in BALB/c mice

A therapeutic vaccine against chronic hepatitis B virus (HBV) infection requires the development of a strong and multispecific Th1 cell immune response. Woodchucks chronically infected with the woodchuck hepatitis virus (WHV) closely resemble HBV infection and represent the best animal model for this hepadnavirus-induced disease. Using the BIMAS "HLA Peptide Binding Predictions" program, we have identified and further characterized novel H-2 d-restricted CD8+ epitopes within the WHV core (peptides C#12-21, C#18-32, C#19-27, C#61-69) and surface antigens (peptides preS2#10-18, preS2#27-35, S#76-84, S#133-140 and S#257-265), respectively. These peptides bind to H-2 d with high efficiency and upon immunization of mice with peptide and Freund's adjuvant they induce the development of IFN-gamma producing T cells. More importantly, WHV core peptides C#19-27 and C#61-69 and WHV surface peptides S#133-140 and S#257-265 were also recognized by CD8+ T cells after immunization of mice with DNA/PEI nanoparticles. Direct stimulation of splenocytes obtained from such DNA-immunized mice with peptides C#18-32, S#76-84, and S#257-265 resulted in significant production of IFN-gamma. Thus, we have identified T cell determinants in mice from WHV core and surface antigens that have important value for designing and evaluating an effective vaccine against hepadnavirus infection.

Rapid immunity to vaccination with woodchuck hepatitis virus surface antigen using cationic liposome-DNA complexes as adjuvant

Complexes of cationic liposomes and non-coding DNA (CLDC) have shown promise as vaccine adjuvant. Using the woodchuck animal model of hepatitis B virus (HBV) infection, the immunogenic effects of CLDC were evaluated following vaccination with three doses of woodchuck hepatitis virus surface antigen (WHsAg) adjuvanted with either CLDC or conventional alum and administered intramuscularly (im) or subcutaneously (sc). IM vaccination with WHsAg and CLDC elicited antibodies earlier, in more woodchucks, and with higher titers than WHsAg and alum. After two vaccine doses, antibody titers were higher following im than sc administration. Woodchucks administered two vaccine doses sc received the third vaccine dose im, and antibody responses reached titers comparable to those elicited by im administration. Following the second vaccine dose, im vaccination with WHsAg and CLDC induced T cell responses to WHsAg and selected WHs peptides and expression of the leukocyte surface marker CD8 and of the Th1 cytokines interferon-gamma and tumor necrosis factor alpha in woodchucks. T cell responses and CD8/cytokine expression were diminished in woodchucks from the other groups suggesting that this vaccine regimen induced a skew toward Th1 immune responses. The present study in woodchucks demonstrates that CLDC-adjuvanted WHsAg vaccine administered im resulted in a more rapid induction of humoral and cellular immune responses compared to conventional, alum-adjuvanted WHsAg vaccine. While less rapid, the immune responses following sc administration can prime the im immune responses. This adjuvant activity of CLDC over alum may be beneficial for therapeutic vaccination in chronic HBV infection.

Profound antiviral effect of oral administration of MIV-210 on chronic hepadnaviral infection in a woodchuck model of hepatitis B

MIV-210 is a prodrug of 3'-fluoro-2',3'-dideoxyguanosine with high oral bioavailability in humans and potent activity against hepatitis B virus (HBV). Woodchucks infected with woodchuck hepatitis virus (WHV) represent an accurate model of HBV infection that is utilized for evaluation of the efficacy and safety of novel anti-HBV agents. Oral administration of MIV-210 at 20 or 60 mg/kg of body weight/day induced a rapid virological response in chronically infected woodchucks, reducing serum WHV DNA levels by 4.75 log10 and 5.72 log10, respectively, in 2 weeks. A progressive decline in WHV viremia occurred throughout the 10-week therapy, giving final reductions of 7.23 log10 and 7.68 log10 in the 20- and 60-mg/kg/day groups, respectively. Further, a daily dose of 10 mg/kg decreased the serum WHV load 400-fold after 4 weeks of treatment, and a dose of 5 mg/kg/day was sufficient to maintain this antiviral effect during the following 6-week period. MIV-210 at 20 or 60 mg/kg/day reduced the liver WHV DNA load 200- to 2,500-fold from pretreatment levels and, importantly, led to a 2.0 log10 drop in the hepatic content of WHV covalently closed circular DNA. The treatment with 60 mg/kg/day was well tolerated. Liver biopsy specimens obtained after the 10-week treatment with 20 or 60 mg/kg/day and after the 10-week follow-up showed hepatocyte and mitochondrial ultrastructures comparable to those in the placebo-treated group. It was concluded that MIV-210 is highly effective against chronic WHV infection. These findings, together with the previously demonstrated inhibitory activity of MIV-210 against lamivudine-, adefovir-, and entecavir-resistant HBV variants, make MIV-210 a highly valuable candidate for further testing as an agent against chronic hepatitis B.

Correlation of virus and host response markers with circulating immune complexes during acute and chronic woodchuck hepatitis virus infection

Woodchuck hepatitis virus (WHV) is an established model for human hepatitis B virus. The kinetics of virus and host responses in serum and liver during acute, self-limited WHV infection in adult woodchucks were studied. Serum WHV DNA and surface antigen (WHsAg) were detected as early as 1 to 3 weeks following experimental infection and peaked between 1 and 5 weeks postinfection. Thereafter, serum WHsAg levels declined rapidly and became undetectable, while WHV DNA levels became undetectable much later, between 4 and 20 weeks postinfection. Decreasing viremia correlated with transient liver injury marked by an increase in serum sorbitol dehydrogenase (SDH) levels. Clearance of WHV DNA from serum was associated with the normalization of serum SDH. Circulating immune complexes (CICs) of WHsAg and antibodies against WHsAg (anti-WHs) that correlated temporarily with the peaks in serum viremia and WHs antigenemia were detected. CICs were no longer detected in serum once free anti-WHs became detectable. The detection of CICs around the peak in serum viremia and WHs antigenemia in resolving woodchucks suggests a critical role for the humoral immune response against WHsAg in the early elimination of viral and subviral particles from the peripheral blood. Individual kinetic variation during WHV infections in resolving woodchucks infected with the same WHV inoculum and dose is likely due to the outbred nature of the animals, indicating that the onset and magnitude of the individual immune response determine the intensity of virus inhibition and the timing of virus elimination from serum.

Antiviral effects of lamivudine, emtricitabine, adefovir dipivoxil, and tenofovir disoproxil fumarate administered orally alone and in combination to woodchucks with chronic woodchuck hepatitis virus infection

Adefovir dipivoxil (ADV) and tenofovir disoproxil fumarate (TDF) are nucleotide analogs that inhibit the replication of wild-type hepatitis B virus (HBV) and lamivudine (3TC)-resistant virus in HBV-infected patients, including those who are coinfected with human immunodeficiency virus. The combination of ADV or TDF with other nucleoside analogs is a proposed strategy for managing antiviral drug resistance during the treatment of chronic HBV infection. The antiviral effect of oral ADV or TDF, alone or in combination with 3TC or emtricitabine (FTC), against chronic woodchuck hepatitis virus (WHV) infection was evaluated in a placebo-controlled study in the woodchuck, an established and predictive model for antiviral therapy. Once-daily treatment for 48 weeks with ADV plus 3TC or TDF plus FTC significantly reduced serum WHV viremia levels from the pretreatment level by 6.2 log(10) and 6.1 log(10) genome equivalents/ml serum, respectively, followed by TDF plus 3TC (5.6 log(10) genome equivalents/ml), ADV alone (4.8 log(10) genome equivalents/ml), ADV plus FTC (one survivor) (4.4 log(10) genome equivalents/ml), TDF alone (2.9 log(10) genome equivalents/ml), 3TC alone (2.7 log(10) genome equivalents/ml), and FTC alone (2.0 log(10) genome equivalents/ml). Individual woodchucks across all treatment groups also demonstrated pronounced declines in serum WHV surface antigen, characteristically accompanied by declines in hepatic WHV replication and the hepatic expression of WHV antigens. Most woodchucks had prompt recrudescence of WHV replication after drug withdrawal, but individual woodchucks across treatment groups had sustained effects. No signs of toxicity were observed for any of the drugs or drug combinations administered. In conclusion, the oral administration of 3TC, FTC, ADV, and TDF alone and in combination was safe and effective in the woodchuck model of HBV infection.

The role of the woodchuck model in the treatment of hepatitis B virus infection

Experimental studies of animals with chronic hepadnavirus infection could provide valuable insight into optimal therapeutic strategies for individuals with chronic HBV infection. In this review, we focus on the contributions of the woodchuck model to our understanding of HBV biology and on its role in the development of antiviral drug. Furthermore, we consider the implications of studies focusing on the natural history of WHV infection for the management of HBV and the capacity of treatment to prevent complications of chronic hepatitis B infection.

Chemoimmunotherapy of chronic hepatitis B virus infection in the woodchuck model overcomes immunologic tolerance and restores T-cell responses to pre-S and S regions of the viral envelope protein

Treatment of chronic hepatitis B virus (HBV) infection could combine potent antiviral drugs and therapeutic vaccines to overcome immunological tolerance and induce the recovery phenotype to protect against disease progression. Conventional vaccination of woodchucks chronically infected with the woodchuck hepatitis virus (WHV) elicited differential T-cell response profiles depending on whether or not carriers were treated with the potent antiviral drug clevudine (CLV), which significantly reduces viral and antigen loads. The differential T-cell responses defined both CLV-dependent and CLV-independent epitopes of the pre-S and S regions of the WHV envelope protein. Only combined treatment involving CLV and conventional vaccine therapeutically restored the T-cell response profile of chronic WHV carrier woodchucks to that seen in prophylactic vaccination and in recovery from acute WHV infection. The results have implications for mechanisms of immunological tolerance operating in chronic HBV infection and suggest that such combined chemoimmunotherapy may be useful for treatment of humans with chronic HBV infection.

Antiviral effect of orally administered (-)-beta-D-2-aminopurine dioxolane in woodchucks with chronic woodchuck hepatitis virus infection

(-)-beta-D-2-Aminopurine dioxolane (APD) is a nucleoside prodrug that is efficiently converted to 9-(beta-D-1,3-dioxolan-4-yl)guanine (DXG). DXG has antiviral activity in vitro against hepatitis B virus (HBV) but limited aqueous solubility, making it difficult to administer orally to HBV-infected individuals. APD is more water soluble than DXG and represents a promising prodrug for the delivery of DXG. A placebo-controlled, dose-ranging efficacy and pharmacokinetic study was conducted with woodchucks that were chronically infected with woodchuck hepatitis virus (WHV). APD was efficiently converted to DXG after oral and intravenous administrations of APD, with serum concentrations of DXG being higher following oral administration than following intravenous administration, suggestive of a considerable first-pass intestinal and/or hepatic metabolism. APD administered orally at 1, 3, 10, and 30 mg/kg of body weight per day for 4 weeks produced a dose-dependent antiviral response. Doses of 3 and 10 mg/kg/day reduced serum WHV viremia by 0.4 and 0.7 log(10) copies/ml, respectively. The 30-mg/kg/day dose resulted in a more pronounced, statistically significant decline in serum WHV viremia of 1.9 log(10) copies/ml and was associated with a 1.5-fold reduction in hepatic WHV DNA. Individual woodchucks within the highest APD dose group that had declines in serum WHV surface antigen levels, WHV viremia, and hepatic WHV DNA also had reductions in hepatic WHV RNA. There was a prompt recrudescence of WHV viremia following drug withdrawal. Therefore, oral administration of APD for 4 weeks was safe in the woodchuck model of chronic HBV infection, and the effect on serum WHV viremia was dose dependent.

Rapid emergence of a viral resistant mutant in WHV chronically infected woodchucks treated with lamivudine and a pre-S/S CHO-derived hepatitis B virus vaccine

To determine whether the addition of a pre-S/S human vaccine increases the antiviral activity of lamivudine, four woodchucks were treated with a daily dose of 100 mg/kg lamivudine and four 50 microg doses of CHO-derived pre-S/S human vaccine. WHV DNA titres decreased up to two logarithms in three woodchucks. At week 4, in three of the animals, the sequence analysis showed a predominant strain containing a nucleotide change from A to T at position 1696 of domain B of the WHV DNA polymerase. Vaccination did not further suppress WHV DNA, despite anti-HBs production in three animals. The woodchuck remains a useful model for characterising the biology and kinetics of the emergence of drug-resistant variants and could be used for pre-clinical studies of combinations of new antiviral drugs.

Targets of emerging therapies for viral hepatitis B and C

Viral hepatitis B and C, structurally two completely different viruses, commonly infect human hepatocytes and cause similar clinical manifestations. Since their discovery, IFN has been a pillar in the treatment. However, because of the different natures of the viruses, therapeutic approaches diverge and new treatment targets are tailored specifically for each virus. Herein, the authors analyse therapeutic approaches for hepatitis B virus (HBV) and hepatitis C virus (HCV) and focus on emerging concepts that are under clinical evaluation. In particular, promising viral inhibitors for HBV and HCV are reviewed and the current status of research for gene therapy for HCV is described. Immune therapy is a fast-moving field with fascinating results which include therapeutic vaccines and toll-like receptor agonists that could improve tomorrow's treatment approaches.

Immunosuppression reactivates viral replication long after resolution of woodchuck hepatitis virus infection

Resolution of hepatitis B virus (HBV) infection is characterized by coordinated humoral and cellular immune responses. Immunity is durable over decades, protecting the host from reinfection and potential activation of residual HBV. Woodchucks infected at birth with woodchuck hepatitis virus (WHV) cleared viremia and developed antibodies to surface antigen (anti-WHs). Woodchucks became seronegative for anti-WHs 3-6 years later, but in some, WHV DNA was detected in serum, liver, and/or peripheral blood mononuclear cells (PBMCs). Those with WHV DNA had increased in vitro cellular immune responses to viral antigens, CD4 and CD8 markers, and Th1-type cytokines, suggesting active WHV-specific T lymphocytes. Immunosuppression for 12 weeks using cyclosporine A in such woodchucks resulted in transient reactivation of WHV replication. Serum of 1 woodchuck that became positive for WHV DNA during immunosuppression was inoculated into WHV-susceptible woodchucks, and a productive infection was demonstrated. The results indicate that after infection durable cellular immunity to WHV is essential for the long-term control of viral replication and is probably maintained by continuous priming from residual virus.

CONCLUSION

These experimental observations demonstrate the potential of immunosuppression to reactivate HBV after resolution of infection.

The woodchuck as an animal model for pathogenesis and therapy of chronic hepatitis B virus infection

This review describes the woodchuck and the woodchuck hepatitis virus (WHV) as an animal model for pathogenesis and therapy of chronic hepatitis B virus (HBV) infection and disease in humans. The establishment of woodchuck breeding colonies, and use of laboratory-reared woodchucks infected with defined WHV inocula, have enhanced our understanding of the virology and immunology of HBV infection and disease pathogenesis, including major sequelae like chronic hepatitis and hepatocellular carcinoma. The role of persistent WHV infection and of viral load on the natural history of infection and disease progression has been firmly established along the way. More recently, the model has shed new light on the role of host immune responses in these natural processes, and on how the immune system of the chronic carrier can be manipulated therapeutically to reduce or delay serious disease sequelae through induction of the recovery phenotype. The woodchuck is an outbred species and is not well defined immunologically due to a limitation of available host markers. However, the recent development of several key host response assays for woodchucks provides experimental opportunities for further mechanistic studies of outcome predictors in neonatal- and adult-acquired infections. Understanding the virological and immunological mechanisms responsible for resolution of self-limited infection, and for the onset and maintenance of chronic infection, will greatly facilitate the development of successful strategies for the therapeutic eradication of established chronic HBV infection. Likewise, the results of drug efficacy and toxicity studies in the chronic carrier woodchucks are predictive for responses of patients chronically infected with HBV. Therefore, chronic WHV carrier woodchucks provide a well-characterized mammalian model for preclinical evaluation of the safety and efficacy of drug candidates, experimental therapeutic vaccines, and immunomodulators for the treatment and prevention of HBV disease sequelae.

Clevudine inhibits hepatitis delta virus viremia: a pilot study of chronically infected woodchucks

In a small controlled study, clevudine, a potent inhibitor of hepadnaviruses, including hepatitis B virus and woodchuck hepatitis virus, suppressed hepatitis delta virus (HDV) viremia in chronically infected woodchucks. Suppression was correlated with the marked reduction of woodchuck hepatitis virus surface antigen in individual animals, consistent with the concept that repression of surface antigen expression may be a useful antiviral strategy for HDV.

Characterization of hepatitis B virus inhibition by novel 2'-fluoro-2',3'-unsaturated beta-D- and L-nucleosides

The clinical emergence of lamivudine and adefovir resistance mutations on prolonged therapy further necessitates the development of additional drugs for the treatment of hepatitis B virus (HBV) infections. We have evaluated a number of novel 2'-fluoro-2',3'-unsaturated D- and L-nucleosides for their anti-HBV activity in the HepG2-2.2.15 cell system. The most potent nucleosides were beta-L-2'-fluoro-2',3'-dideoxy-2',3'-didehydrocy-tidine (L-2'-Fd4C) and beta-L-2'-fluoro-2',3'-dideoxy-2',3'-didehydro-5-fluorocytidine (L-2'-Fd4FC) with median effective concentrations (EC50) of 0.002 microM and 0.004 microM, respectively. The D-enantiomers of the 2'-fluoro-substituted cytidine analogues in this series showed activity, with the 5-fluorocytidine (D-2'-Fd4FC) being the most potent (EC50 = 0.05 microM). The active compounds were not cytotoxic to a number of cell lines or to bone marrow progenitor cells. Furthermore, mitochondrial DNA synthesis and function were not affected by these nucleosides. L-2'-Fd4C did not affect viral transcription, implying that it does not inhibit cellular RNA polymerase II. Studies with the HBV polymerase in core particles revealed that the 5'-triphosphates of L-2'-Fd4C and D-2'-Fd4FC produced a dose-dependent inhibition of the incorporation of 32P-dCTP into the HBV DNA, indicating that the mechanism of action of these compounds is through specific inhibition of viral DNA synthesis. This class of nucleosides, which exhibit potent antiviral activity and a favourable safety profile, have potential for the treatment of HBV infections and warrant further development.

Animal models for the study of HBV replication and its variants

Enormous progresses in hepatitis B virus research have been made through the identification of avian and mammalian HBV related viruses, which offer ample opportunities for studies in naturally occurring hosts. However, none of these natural hosts belongs to the commonly used laboratory animals, and the development of various mouse strains carrying HBV transgenes offered unique opportunities to investigate some mechanisms of viral pathogenesis. Furthermore, the need to perform infection studies in a system harbouring HBV-permissive hepatocytes has lately led researchers to create new challenging human mouse chimera models of HBV infection. In this review, we will overview the type of animal models currently available in hepadnavirus research.

Antiviral effect of oral administration of tenofovir disoproxil fumarate in woodchucks with chronic woodchuck hepatitis virus infection

Tenofovir disoproxil fumarate (TDF) is a nucleotide analogue approved for treatment of human immunodeficiency virus (HIV) infection. TDF also has been shown in vitro to inhibit replication of wild-type hepatitis B virus (HBV) and lamivudine-resistant HBV mutants and to inhibit lamivudine-resistant HBV in patients and HBV in patients coinfected with the HIV. Data on the in vivo efficacy of TDF against wild-type virus in non-HIV-coinfected or lamivudine-naïve chronic HBV-infected patients are lacking in the published literature. The antiviral effect of oral administration of TDF against chronic woodchuck hepatitis virus (WHV) infection, an established and predictive animal model for antiviral therapy, was evaluated in a placebo-controlled, dose-ranging study (doses, 0.5 to 15.0 mg/kg of body weight/day). Four weeks of once-daily treatment with TDF doses of 0.5, 1.5, or 5.0 mg/kg/day reduced serum WHV viremia significantly (0.2 to 1.5 log reduction from pretreatment level). No effects on the levels of anti-WHV core and anti-WHV surface antibodies in serum or on the concentrations of WHV RNA or WHV antigens in the liver of treated woodchucks were observed. Individual TDF-treated woodchucks demonstrated transient declines in WHV surface antigen serum antigenemia and, characteristically, these woodchucks also had transient declines in serum WHV viremia, intrahepatic WHV replication, and hepatic expression of WHV antigens. No evidence of toxicity was observed in any of the TDF-treated woodchucks. Following drug withdrawal there was prompt recrudescence of WHV viremia to pretreatment levels. It was concluded that oral administration of TDF for 4 weeks was safe and effective in the woodchuck model of chronic HBV infection.

Suppression of lamivudine-resistant B-domain mutants by adefovir dipivoxil in the woodchuck hepatitis virus model

Adult woodchucks (Marmota monax) chronically infected with woodchuck hepatitis virus (WHV) were treated orally with lamivudine (15 mg/kg per day) for 57 weeks. After 20 weeks of treatment a 2-3 log reduction in serum WHV DNA was detected. Serum titers of WHV then increased gradually, in the presence of lamivudine treatment, reaching pre-treatment values by week 40. Viral recrudescence was associated with development of mutations in the B domain of the WHV polymerase gene. Mutations observed in the highly conserved FLLA motif of the B domain were L564V, L565M, and A566T, with A566T being the most frequently observed. Beginning on week 57 of lamivudine treatment, one group (n = 3) was treated orally with adefovir dipivoxil at a dose of 15 mg/kg per day plus lamivudine, and a second group (n = 3) was treated with H2O placebo plus lamivudine. In woodchucks treated with adefovir dipivoxil, two had the A566T mutation, and one had both A566T and L565V. In the group maintained on lamivudine monotherapy, A566T alone was present in one animal, another carried both A566T and L565V, and in the third, no B-domain mutations were detected. There was a 4.5 log reduction in serum WHV DNA after 12 weeks of treatment with the adefovir/lamivudine combination, while in the lamivudine monotherapy controls, WHV DNA decreased by only 0.83 log (P > 0.001). A slight recurrence in serum titers of WHV DNA was observed one week after withdrawal of adefovir treatment but no further increase in viral load was observed during the remainder of the 12-week post-treatment follow-up period. The results demonstrate that supplemental adefovir dipivoxil treatment is effective in suppressing replication of lamivudine-resistant B-domain mutants in the woodchuck model of hepatitis B virus infection.

Review article: Nucleoside analogues for the treatment of chronic hepatitis B

Current accepted treatment for chronic hepatitis B uses either the immunomodulator interferon alpha or nucleoside analogues lamivudine or adefovir. Interferon has side effects which mean it is often poorly tolerated. Long-term use of lamivudine is associated with increasing viral resistance for each year it is taken and the rebound viraemia that can occur when the drug is stopped is also of concern to many. Adefovir appears to have less of the resistance issues of lamivudine but is still a relatively new drug and at present its use is principally limited to patients with lamivudine-resistant disease. A number of other nucleoside analogues are currently being developed with some now at the stage of early clinical trials. A proportion share the significant resistance problems of lamivudine but many appear to have more potent anti-viral effect than the drugs currently available. If some of these newer anti-viral agents are approved for use in chronic hepatitis B, the potential for prolonged suppression of hepatitis B virus replication with resultant stabilization or improvement in liver disease may be achieved.

Korean Medicinal Plant Extracts Exhibit Antiviral Potency Against Viral Hepatitis

OBJECTIVES

Investigation of natural ethnopharmacologic extracts exhibiting antiviral potential may lead to the discovery of new therapeutics for the treatment of chronic viral hepatitis infections. Traditional Korean medicinal herbs have been identified that exhibit potency against hepatitis B virus (HBV) and hepatitis C virus (HCV) infections. Research on the antiviral potential of naturally derived extracts is facilitated through the use of appropriate animal and liver cell culture models for these hepatotrophic pathogens. Objectives of this study were to demonstrate antiviral activity of an aqueous extract of herbal formulation KYH-1 in surrogate in vitro assays for HBV and HCV and identify mechanisms of action.

METHODS

Antiviral potency of KYH-1 was measured in tissue culture systems that support replication of the woodchuck hepatitis virus (WHV), and the bovine viral diarrhea virus (BVDV). These assays serve as surrogate models for HBV and HCV, respectively. A recombinant HBV polymerase gene expression assay was used to define a molecular target.

RESULTS

KYH-1 exhibited potent antiviral activity against WHV and to a lesser extent against BVDV. KYH-1 and its constituent components inhibited HBV polymerase priming in vitro. Additionally, KYH-1 suppressed HBV replication in a human hepatoblastoma cell line.

CONCLUSION

Evaluation of naturally derived products for antiviral activity against HBV and HCV in standardized surrogate assays provides a scientific basis for potential use as complementary or alternative medicines. This study provides significant results justifying preclinical evaluation of KYH-1 as an antiviral therapy for HBV infections.

Hepatocellular carcinoma in the woodchuck model of hepatitis B virus infection

The Eastern woodchuck ( Marmota monax ) harbors a DNA virus (Woodchuck hepatitis virus [WHV]) that is similar in structure and replicative life cycle to the human hepatitis B virus (HBV). Like HBV, WHV infects the liver and can cause acute and chronic hepatitis. Furthermore, chronic WHV infection in woodchucks usually leads to development of hepatocellular carcinoma (HCC) within the first 2-4 years of life. The woodchuck model has been important in the preclinical evaluation of safety and efficacy of the antiviral drugs now in use for treatment of HBV infection and continues to serve as an important, predictive model for innovative forms of therapy of hepatitis B using antiviral nucleosides and immune response modifiers alone or in combination. Almost all woodchucks that become chronic WHV carriers after experimental neonatal inoculation develop HCC with a median HCC-free survival of 24 months and a median life expectancy of 30-32 months. The woodchuck model of viral-induced HCC has been used effectively for the development of new imaging agents for enhancement of detection of hepatic neoplasms by ultrasound and magnetic resonance imaging. The chemoprevention of HCC using long-term antiviral nucleoside therapy has been shown in the woodchuck, and "proof of principal" has been established for some of the innovative, molecular methods for treatment of HCC. The model is available for fundamental investigations of the viral and molecular mechanisms responsible for hepatocarcinogenesis and should have substantial value for future development of innovative methods for chemoprevention and gene therapy of human HCC.

GB virus B infection of the common marmoset (Callithrix jacchus) and associated liver pathology

GB virus B (GBV-B) is a flavivirus that is related closely to hepatitis C virus (HCV) and induces an acute hepatitis when inoculated into several species of New World primates. Common marmosets (Callithrix jacchus) are a widely available, non-endangered primate species that is susceptible to GBV-B infection and develops a characteristic acute hepatitis. Here, animals were found to be susceptible to serially passaged serum and GBV-B transcripts. Hepatic pathology and peripheral viraemia could be quantified biochemically, immunophenotypically and morphologically, and persisted for periods of up to 6 months in some animals. Hepatitis was characterized by a marked influx of CD3+ CD8+ T lymphocytes and CD20+ B cells within the first 2 months of primary infection. The results of this study document the marmoset as another small, non-human primate species in which the pathogenesis of GBV-B can be studied and used as a surrogate model of HCV infection for investigation of pathogenesis and antiviral drug development.

Effect of a combination of clevudine and emtricitabine with adenovirus-mediated delivery of gamma interferon in the woodchuck model of hepatitis B virus infection

Our aim was to evaluate the antiviral effect of a combination of two nucleoside reverse transcriptase inhibitors, emtricitabine (FTC) and clevudine (L-FMAU), with the addition of an adenovirus-driven delivery of recombinant gamma interferon (IFN-gamma) in the woodchuck model of hepatitis B virus infection. Six woodchuck hepatitis virus (WHV)-infected woodchucks received L-FMAU (10 mg/kg) plus FTC (30 mg/kg) intraperitoneally for 8 weeks; six other animals received in addition an intravenous injection of a recombinant adenovirus vector expressing woodchuck IFN-gamma (Ad-IFN) at weeks 4 and 8. In the control group, two animals received Ad-IFN alone, two received adenovirus vector expressing the green fluorescent protein reporter gene, and one remained untreated. In less than 2 weeks, all woodchucks that received L-FMAU plus FTC showed a rapid and marked inhibition of viral replication, with a 4-log(10) drop in serum WHV DNA. In two animals, viremia remained suppressed for several months after the end of treatment. Similarly, a dramatic decrease in intrahepatic replicative intermediates of viral DNA was observed in the L-FMAU/FTC-treated groups. The additional administration of Ad-IFN led to increased inflammation in the liver but did not enhance the antiviral effect of the L-FMAU/FTC combination. In conclusion, therapies combining L-FMAU and FTC in WHV-infected woodchucks resulted in a potent and sustained antihepadnaviral effect both in the liver and in the blood circulation. However, no extra benefit of adding IFN-gamma gene transduction to the L-FMAU/FTC combination could be detected.

Cell culture and animal models of viral hepatitis. Part I: hepatitis B

Despite the existence of a preventative vaccine, HBV represents a substantial threat to public health, suggesting the need for research to develop new treatments to combat the disease. The authors review the available in vitro and in vivo models, including recently developed transgenic and chimeric mouse models.

Interferon-gamma-associated responses to woodchuck hepatitis virus infection in neonatal woodchucks and virus-infected hepatocytes

Acute hepatitis and recovery from woodchuck hepatitis virus (WHV) infection involves increased intrahepatic expression of interferon-gamma (IFN-gamma) and tumour necrosis factor-alpha (TNF-alpha) mRNAs. In the present study, recovery correlated with increased intrahepatic expression of mRNAs for major histocompatibility complex class 1 (MHC1), beta(2)-microglobulin, 2'5'-oligoadenylate synthetase (2'5'-OAS), and indoleamine dioxygenase (IDO). By comparison, acute WHV infection progressing to chronicity was associated with diminished expression of these IFN-gamma-associated mRNAs in liver. Transfection of WHV-infected primary hepatocytes (WPH) from WHV carriers with an IFN-gamma-expressing plasmid (pIFN-gamma) resulted in dose-dependent accumulations of MHC1, TNF-alpha, 2'5'-OAS, and IDO mRNAs within 96 h. Markers of T cells and immune-mediated cytotoxicity that accumulate in recovering liver were not apparent in WPH based on the relative lack of CD3, CD4, Fas ligand, perforin, and granzyme B mRNAs. Expression of pIFN-gamma, and TNF-alpha-expressing plasmid (pTNF-alpha), did not affect total WHV RNA, or fully double-stranded WHV DNA in WPH, but each reduced some of the replicative intermediate (RI) species of WHV DNA synthesis. WPH treated with recombinant IFN-alpha protein had a higher fold induction of 2'5'-OAS mRNA associated with partial reductions in WHV RNAs and the major RI species. Thus, IFN-gamma expression in carrier WPH induced several host responses often observed in liver of recovering woodchucks, and impaired a stage of WHV DNA synthesis by a non-cytolytic mechanism mediated by TNF-alpha. Local enhancement of IFN-gamma-associated responses in chronic WHV-infected hepatocytes may promote therapeutic antiviral effects, but additional effector mechanisms evident during recovery appear necessary for more complete clearance of WHV infection.

Clevudine Therapy with Vaccine Inhibits Progression of Chronic Hepatitis and Delays Onset of Hepatocellular Carcinoma in Chronic Woodchuck Hepatitis Virus Infection

We examined a rational approach to therapy of chronic hepatitis B virus (HBV) infection that utilized the reduction of viral load combined with appropriately timed immune modulation/stimulation. In a placebo-controlled study, chronic woodchuck hepatitis virus (WHV) carrier woodchucks received clevudine (l-FMAU), previously shown to have especially potent and sustained antiviral activity in woodchucks, for 32 weeks followed by WHV surface antigen (WHsAg) alum-adjuvanted vaccine at 32, 36, 40 and 48 weeks. Clevudine induced significant reductions in viraemia, surface antigenaemia, hepatic WHV nucleic acids, and hepatic core and surface antigens. Viral replication markers remained markedly suppressed in 75% of the clevudine-treated woodchucks following drug withdrawal, but remained at high levels in the vaccine monotherapy and placebo groups. Combination drug and vaccine therapy had benefits based on sustained reduction of viraemia, antigenaemia, and hepatic WHV DNA and RNA; inhibition of progression of chronic hepatitis; reduced frequency of chronic liver injury; and delayed onset of hepatocellular carcinoma (HCC). Combination therapy contributed to prevention of HCC in up to 38% of treated carriers, although the growth rate of established HCC was not affected. This study demonstrates enhanced benefits of combination chemo-immunotherapy against viral load and disease progression in chronic hepadnaviral infection, and provides a platform for further development of such treatment regimens.