Ultra-rapid cardiotoxicity of the hepatitis C virus protease inhibitor BILN 2061 in the urokinase-type plasminogen activator mouse

Structure-based virtual screening of natural compounds against wild and mutant (R1155K, A1156T and D1168A) NS3-4A protease of Hepatitis C virus

NS3-4A, a serine protease, is a primary target for drug development against Hepatitis C Virus (HCV). However, the effectiveness of potent next-generation protease inhibitors is limited by the emergence of mutations and resulting drug resistance. To address this, in this study a structure-based drug design approach is employed to screen a large library of 7320 natural compounds against both wild-type and mutant variants of NS3-4A protease. Telaprevir, a widely used protease inhibitor, was recruited as the control drug. The top 10 compounds with favorable binding affinities underwent drug-likeness evaluation. Based on ADMET studies, complexes of NP_024762 and NP_006776 were selected for molecular dynamic simulations. Principal component analysis (PCA) was employed to explore the conformational space and protein dynamics of the protein-ligand complex using a Free Energy Landscape (FEL) approach. The cosine values obtained from FEL analysis ranged from 0 to 1, and eigenvectors with cosine values below 0.2 were chosen for further analysis. To forecast binding free energies and evaluate energy contributions per residue, the MM-PBSA method was employed. The results highlighted the crucial role of amino acids in the catalytic domain for the binding of the protease with phytochemicals. Stable associations between the top compounds and the target protease were confirmed by the formation of hydrogen bonds in the binding pocket involving residues: His1057, Gly1137, Ser1139, and Ala1157. These findings suggest the potential of these compounds for further validation through biological evaluation.Communicated by Ramaswamy H. Sarma.

Direct-Acting Antiviral Agents for Hepatitis C Virus Infection-From Drug Discovery to Successful Implementation in Clinical Practice

Today, hepatitis C virus infection affects up to 1.5 million people per year and is responsible for 29 thousand deaths per year. In the 1970s, the clinical observation of unclear, transfusion-related cases of hepatitis ignited scientific curiosity, and after years of intensive, basic research, the hepatitis C virus was discovered and described as the causative agent for these cases of unclear hepatitis in 1989. Even before the description of the hepatitis C virus, clinicians had started treating infected individuals with interferon. However, intense side effects and limited antiviral efficacy have been major challenges, shaping the aim for the development of more suitable and specific treatments. Before direct-acting antiviral agents could be developed, a detailed understanding of viral properties was necessary. In the years after the discovery of the new virus, several research groups had been working on the hepatitis C virus biology and finally revealed the replication cycle. This knowledge was the basis for the later development of specific antiviral drugs referred to as direct-acting antiviral agents. In 2011, roughly 22 years after the discovery of the hepatitis C virus, the first two drugs became available and paved the way for a revolution in hepatitis C therapy. Today, the treatment of chronic hepatitis C virus infection does not rely on interferon anymore, and the treatment response rate is above 90% in most cases, including those with unsuccessful pretreatments. Regardless of the clinical and scientific success story, some challenges remain until the HCV elimination goals announced by the World Health Organization are met.

Breakthroughs in hepatitis C research: from discovery to cure

In the 1970s, an unknown virus was suspected for documented cases of transfusion-associated hepatitis, a phenomenon called non-A, non-B hepatitis. In 1989, the infectious transmissible agent was identified and named hepatitis C virus (HCV) and, soon enough, the first diagnostic HCV antibody test was developed, which led to a dramatic decrease in new infections. Today, HCV infection remains a global health burden and a major cause of liver cirrhosis, hepatocellular carcinoma and liver transplantation. However, tremendous advances have been made over the decades, and HCV became the first curable, chronic viral infection. The introduction of direct antiviral agents revolutionized antiviral treatment, leading to viral eradication in more than 98% of all patients infected with HCV. This Perspective discusses the history of HCV research, which reads like a role model for successful translational research: starting from a clinical observation, specific therapeutic agents were developed, which finally were implemented in national and global elimination programmes.

Humanized Mouse Models for the Study of Hepatitis C and Host Interactions

Hepatitis C virus (HCV) infection is commonly attributed as a major cause of chronic hepatotropic diseases, such as, steatosis, cirrhosis and hepatocellular carcinoma. As HCV infects only humans and primates, its narrow host tropism hampers in vivo studies of HCV-mammalian host interactions and the development of effective therapeutics and vaccines. In this context, we will focus our discussion on humanized mice in HCV research. Here, these humanized mice are defined as animal models that encompass either only human hepatocytes or both human liver and immune cells. Aspects related to immunopathogenesis, anti-viral interventions, drug testing and perspectives of these models for future HCV research will be discussed.

Mice with Chimeric Human Livers and Their Applications

The complete life cycle of the hepatitis C virus (HCV) can be recapitulated in vivo using immunodeficient mice that have had their livers extensively repopulated with human hepatocytes. These human liver chimeric mouse models have enabled the study of many aspects of the HCV life cycle, including antiviral interventions that have helped to shape the curative landscape that is available today. The first human liver chimeric mouse model capable of supporting the HCV life cycle was generated in SCID-uPA mice. Although other human liver chimeric mouse models have since been developed, the SCID-uPA mouse model remains one of the most robust in vivo systems available for HCV studies. This chapter reviews development, validation and application of the SCID-uPA mouse model, and discusses their potential application for studying other liver-centric diseases and pathogens and for the design and testing of vaccine candidates for the eradication of HCV.

Evolution of efficacious pangenotypic hepatitis C virus therapies

Hepatitis C compromises the quality of life of more than 350 million individuals worldwide. Over the last decade, therapeutic regimens for treating hepatitis C virus (HCV) infections have undergone rapid advancements. Initially, structure-based drug design was used to develop molecules that inhibit viral enzymes. Subsequently, establishment of cell-based replicon systems enabled investigations into various stages of HCV life cycle including its entry, replication, translation, and assembly, as well as role of host proteins. Collectively, these approaches have facilitated identification of important molecules that are deemed essential for HCV life cycle. The expanded set of putative virus and host-encoded targets has brought us one step closer to developing robust strategies for efficacious, pangenotypic, and well-tolerated medicines against HCV. Herein, we provide an overview of the development of various classes of virus and host-directed therapies that are currently in use along with others that are undergoing clinical evaluation.

[Hepatic decompensation associated with an interferon-free antiviral therapy in patients with HCV-cirrhosis]

Today, there is no complete clarity about the pathogenetic mechanisms of the hepatic decompensation in patients with HCV-cirrhosis during the course of direct-acting antiviral (DAAs) therapy. The current article describes several clinical observations of decompensation (with the development of liver failure and portal hypertension) in cirrhotic patients during the course of DAAs-therapy of hepatitis C. The authors present contemporary views and their own assumptions about the possible mechanisms of the hepatic decompensation associated with DAAs-therapy in patients with liver cirrhosis.

Successful Engraftment of Human Hepatocytes in uPA-SCID and FRG® KO Mice

Mice with humanized chimeric liver are promising in vivo tools to evaluate the efficacy of novel compounds or vaccine induced antibodies directed against pathogens that infect the human liver. In addition they can be used to study the human-type metabolism of medicinal compounds and hepatotoxicity.

Hepatitis E Virus (HEV) Genotype 3 Infection of Human Liver Chimeric Mice as a Model for Chronic HEV Infection

Genotype 3 (gt3) hepatitis E virus (HEV) infections are emerging in Western countries. Immunosuppressed patients are at risk of chronic HEV infection and progressive liver damage, but no adequate model system currently mimics this disease course. Here we explore the possibilities of in vivo HEV studies in a human liver chimeric mouse model (uPA(+/+)Nod-SCID-IL2Rγ(-/-)) next to the A549 cell culture system, using HEV RNA-positive EDTA-plasma, feces, or liver biopsy specimens from 8 immunocompromised patients with chronic gt3 HEV. HEV from feces- or liver-derived inocula showed clear virus propagation within 2 weeks after inoculation onto A549 cells, compared to slow or no HEV propagation of HEV RNA-positive, EDTA-plasma samples. These in vitro HEV infectivity differences were mirrored in human-liver chimeric mice after intravenous (i.v.) inoculation of selected samples. HEV RNA levels of up to 8 log IU HEV RNA/gram were consistently present in 100% of chimeric mouse livers from week 2 to week 14 after inoculation with human feces- or liver-derived HEV. Feces and bile of infected mice contained moderate to large amounts of HEV RNA, while HEV viremia was low and inconsistently detected. Mouse-passaged HEV could subsequently be propagated for up to 100 days in vitro In contrast, cell culture-derived or seronegative EDTA-plasma-derived HEV was not infectious in inoculated animals. In conclusion, the infectivity of feces-derived human HEV is higher than that of EDTA-plasma-derived HEV both in vitro and in vivo Persistent HEV gt3 infections in chimeric mice show preferential viral shedding toward mouse bile and feces, paralleling the course of infection in humans.

IMPORTANCE

Hepatitis E virus (HEV) genotype 3 infections are emerging in Western countries and are of great concern for immunosuppressed patients at risk for developing chronic HEV infection. Lack of adequate model systems for chronic HEV infection hampers studies on HEV infectivity and transmission and antiviral drugs. We compared the in vivo infectivity of clinical samples from chronic HEV patients in human liver chimeric mice to an in vitro virus culture system. Efficient in vivo HEV infection is observed after inoculation with feces- and liver-derived HEV but not with HEV RNA-containing plasma or cell culture supernatant. HEV in chimeric mice is preferentially shed toward bile and feces, mimicking the HEV infection course in humans. The observed in vivo infectivity differences may be relevant for the epidemiology of HEV in humans. This novel small-animal model therefore offers new avenues to unravel HEV's pathobiology.

Animal models for the study of HCV

The development and evaluation of effective therapies and vaccines for the hepatitis C virus (HCV) and the study of its interactions with the mammalian host have been hindered for a long time by the absence of suitable small animal models. Immune compromised mouse models that recapitulate the complete HCV life cycle have been useful to investigate many aspects of the HCV life cycle including antiviral interventions. However, HCV has a high propensity to establish persistence and associated histopathological manifestations such as steatosis, fibrosis, cirrhosis and hepatocellular carcinoma (HCC). Better understanding of these processes requires the development of a permissive and fully immunocompetent small animal model. In this review we summarize the in vivo models that are available for the study of HCV.

Advances in experimental systems to study hepatitis C virus in vitro and in vivo

Hepatitis C virus (HCV) represents a global health concern affecting over 185 million people worldwide. Chronic HCV infection causes liver fibrosis and cirrhosis and is the leading indication for liver transplantation. Recent advances in the field of direct-acting antiviral drugs (DAAs) promise a cure for HCV in over 90% of cases that will get access to these expensive treatments. Nevertheless, the lack of a protective vaccine and likely emergence of drug-resistant viral variants call for further studies of HCV biology. With chimpanzees being for a long time the only non-human in vivo model of HCV infection, strong efforts were put into establishing in vitro experimental systems. The initial models only enabled to study specific aspects of the HCV life cycle, such as viral replication with the subgenomic replicon and entry using HCV pseudotyped particles (HCVpp). Subsequent development of protocols to grow infectious HCV particles in cell-culture (HCVcc) ignited investigations on the full cycle of HCV infection and the virus-host interactions required for virus propagation. More recently, small animal models permissive to HCV were generated that allowed in vivo testing of novel antiviral therapies as well as vaccine candidates. This review provides an overview of the currently available in vitro and in vivo experimental systems to study HCV biology. Particular emphasis is given to how these model systems furthered our understanding of virus-host interactions, viral pathogenesis and immunological responses to HCV infection, as well as drug and vaccine development.

Faldaprevir for the treatment of hepatitis C

The current treatments for chronic hepatitis C virus (HCV) genotype 1 infection are combinations of direct-acting antivirals, and faldaprevir is one of the new generation of HCV NS3/4A protease inhibitors. At the end of 2013, the US Food and Drug Administration (FDA) approved the HCV NS3/4A protease inhibitor simeprevir and the HCV NS5B polymerase inhibitor sofosbuvir. Simeprevir or sofosbuvir in combination with pegylated interferon and ribavirin are available for clinical use. Faldaprevir, another HCV NS3/4A protease inhibitor that also has fewer adverse events than telaprevir or boceprevir, is under development. Of interest, faldaprevir in combination with pegylated interferon and ribavirin, and interferon-free treatment with faldaprevir in combination with deleobuvir plus ribavirin provides high sustained virological response rates for HCV genotype 1 infection. The aim of this article is to review these data concerning faldaprevir. Faldaprevir in combination with pegylated interferon and ribavirin treatment appears to be associated with fewer adverse events than telaprevir or boceprevir in combination with pegylated interferon and ribavirin, and may be one of the therapeutic options for treatment-naive patients with HCV genotype 1. The interferon-free combination of faldaprevir and deleobuvir with ribavirin was effective for HCV genotype 1 infection and may hold promise for interferon-ineligible and interferon-intolerant patients.

Viral Hepatitis B and C. Cure or Treatment?

HBV and HCV infections are among the most important global health problems; both represent also the leading cause of cirrhosis and HCC worldwide. HBV treatment cannot be considered cure but effective viral suppression can be achieved and remains the current principal goal of therapy. Talking about HCV treatment today equals to talking about total cure of the patient, with treatments of very high SVR rates, shorter if not shortest duration, minimal risk for resistance, pangenotypic and practically with no serious adverse events, no fibrosis or previous treatment status limitations, but also with a very high cost.

Vinylated linear P2 pyrimidinyloxyphenylglycine based inhibitors of the HCV NS3/4A protease and corresponding macrocycles

With three recent market approvals and several inhibitors in advanced stages of development, the hepatitis C virus (HCV) NS3/4A protease represents a successful target for antiviral therapy against hepatitis C. As a consequence of dealing with viral diseases in general, there are concerns related to the emergence of drug resistant strains which calls for development of inhibitors with an alternative binding-mode than the existing highly optimized ones. We have previously reported on the use of phenylglycine as an alternative P2 residue in HCV NS3/4A protease inhibitors. Herein, we present the synthesis, structure-activity relationships and in vitro pharmacokinetic characterization of a diverse series of linear and macrocyclic P2 pyrimidinyloxyphenylglycine based inhibitors. With access to vinyl substituents in P3, P2 and P1' positions an initial probing of macrocyclization between different positions, using ring-closing metathesis (RCM) could be performed, after addressing some synthetic challenges. Biochemical results from the wild type enzyme and drug resistant variants (e.g., R155 K) indicate that P3-P1' macrocyclization, leaving the P2 substituent in a flexible mode, is a promising approach. Additionally, the study demonstrates that phenylglycine based inhibitors benefit from p-phenylpyrimidinyloxy and m-vinyl groups as well as from the combination with an aromatic P1 motif with alkenylic P1' elongations. In fact, linear P2-P1' spanning intermediate compounds based on these fragments were found to display promising inhibitory potencies and drug like properties.

HCV animal models and liver disease

The development and evaluation of effective therapies and vaccines for the hepatitis C virus (HCV) and the study of its interactions with the mammalian host have been hindered for a long time by the absence of suitable small animal models. Due to the narrow host tropism of HCV, the development of mice that can be robustly engrafted with human hepatocytes was a major breakthrough since they recapitulate the complete HCV life cycle. This model has been useful to investigate many aspects of the HCV life cycle, including antiviral interventions. However, studies of cellular immunity, immunopathogenesis and resulting liver diseases have been hampered by the lack of a small animal model with a functional immune system. In this review, we summarize the evolution of in vivo models for the study of HCV.

Murine models of hepatitis C: what can we look forward to?

The study of interactions between hepatitis C virus (HCV) with its mammalian host, along with the development of more effective therapeutics and vaccines has been delayed by the lack of a suitable small animal model. HCV readily infects only humans and chimpanzees, which poses logistic, economic and ethical challenges with analyzing HCV infection in vivo. Progress has been made in understanding the determinants that dictate HCV's narrow host range providing a blueprint for constructing a mouse model with inheritable susceptibility to HCV infection. Indeed, genetically humanized mice were generated that support viral uptake, replication and production of infectious virions--albeit at low levels. These efforts are complemented with attempts to select for viral variants that are inherently more capable of replicating in non-human species. In parallel, engraftment of relevant human tissues into improved xenorecipients is being continuously refined. Incorporating advances in stem-cell-biology and tissue engineering may allow the generation of patient-specific humanized mice. Construction of such mouse "avatars" may allow analyzing functionally patient-specific differences with respect to susceptibility to infection, disease progression and responses to treatment. In this review, we discuss the three, before mentioned approaches to overcome current species barriers and generate a small animal model for HCV infection, i.e. genetic modification of mice to increase their susceptibility to the virus; genetic modification of HCV, to increase its pathogenicity for mice; and the introduction of human liver and immune cells into immunodeficient mice, to create "humanized" mice. Although in the foreseeable future there will not be a single model that perfectly mimics the natural course of HCV in humans there is reason for optimism. The spectrum of murine animal models for hepatitis C provides a broad arsenal for analyzing the disease. These models may play an important role by prioritizing vaccine candidates and possibly refining combination anti-viral drug therapies. This article forms part of a symposium in Anti-viral Research on "Hepatitis C: next steps toward global eradication."

Hepatitis C virus infection and related liver disease: the quest for the best animal model

Hepatitis C virus (HCV) is a major cause of cirrhosis and hepatocellular carcinoma (HCC) making the virus the most common cause of liver failure and transplantation. HCV is estimated to chronically affect 130 million individuals and to lead to more than 350,000 deaths per year worldwide. A vaccine is currently not available. The recently developed direct acting antivirals (DAAs) have markedly increased the efficacy of the standard of care but are not efficient enough to completely cure all chronically infected patients and their toxicity limits their use in patients with advanced liver disease, co-morbidity or transplant recipients. Because of the host restriction, which is limited to humans and non-human primates, in vivo study of HCV infection has been hampered since its discovery more than 20 years ago. The chimpanzee remains the most physiological model to study the innate and adaptive immune responses, but its use is ethically difficult and is now very restricted and regulated. The development of a small animal model that allows robust HCV infection has been achieved using chimeric liver immunodeficient mice, which are therefore not suitable for studying the adaptive immune responses. Nevertheless, these models allowed to go deeply in the comprehension of virus-host interactions and to assess different therapeutic approaches. The immunocompetent mouse models that were recently established by genetic humanization have shown an interesting improvement concerning the study of the immune responses but are still limited by the absence of the complete robust life cycle of the virus. In this review, we will focus on the relevant available animal models of HCV infection and their usefulness for deciphering the HCV life cycle and virus-induced liver disease, as well as for the development and evaluation of new therapeutics. We will also discuss the perspectives on future immunocompetent mouse models and the hurdles to their development.

[Biochemical and pharmacological features of telaprevir]

Telaprevir is the first of a new generation of drugs based on blocking the NS3-4A protease of hepatitis C virus (HCV), which is essential for viral reproduction, and is especially active against genotype 1 HCV. However, to be effective, telaprevir must be combined with pegylated interferon and ribavirin for 12 weeks. Telaprevir has poor solubility in water and tends to crystallize, properties that hamper its formulation as a drug intended for oral delivery. This agent is efficiently absorbed after oral administration, but only if administered with food (not low in fat), since fasting intake markedly reduces systemic exposure. The total daily dose is 2,250 mg. Because of its pharmacokinetics, telaprevir has been designed for administration every 8 hours but efficacy is maintained in a twice-daily dosing regimen. Dose adjustment is not required in compensated liver cirrhosis. Because it is a substrate and potent inhibitor of CYP3A4 and glycoprotein P, telaprevir has multiple drug-drug interactions. The IL-28B genotype has little influence on the likelihood of response to telaprevir triple combination therapy.

The presence of resistance mutations to protease and polymerase inhibitors in Hepatitis C virus sequences from the Los Alamos databank

Several new direct-acting antiviral (DAA) drugs are in development for chronic hepatitis C viral (HCV) infection, and NS3-NS4A serine protease and the NS5B RNA-dependent RNA polymerase have been the major targets. HCV variants displaying drug-resistant phenotypes have been observed both in vitro and during clinical trials. Our aim was to characterize amino acid changes at positions previously associated with resistance in protease (NS3) and polymerase (NS5B) regions from treatment-naïve HCV patients infected with genotypes 1a, 1b and 3a. All 1383 NS3 protease sequences (genotype 1a = 680, 1b = 498 and 3a = 205) and 806 NS5B polymerase sequences (genotypes 1a = 471, 1b = 329, 3a = 6) were collected from Los Alamos databank. Genotype 3a protease sequences showed the typical low-level resistance mutation V36L. NS3 sequences from other genotypes presented mutations on positions 36, 39, 41, 43, 54, 80, 109, 155 and 168 in a frequency lower than 2%, except for the mutation Q80R found in 35% of genotype 1a isolates. Polymerase sequences from genotype 3a patients showed five typical mutations: L419I, I424V, I482L, V499A and S556G. Two positions presented high polymorphism in the NS5B region from genotype 1a (V499A) and genotype 1b (C316N) subjects. Our results demonstrated a natural profile of genotype 3a that can be associated with the pre-existence of HCV variants resistant to first-generation protease inhibitors and to non-nucleoside polymerase inhibitors. Likewise, genotype 1b isolates and genotype 1a sequences exhibited pre-existing mutations associated with resistance to Palm II and Thumb I polymerase inhibitors, respectively.

Statin therapy improves response to interferon alfa and ribavirin in chronic hepatitis C: a systematic review and meta-analysis

The treatment of interferon alfa (IFN-α) and ribavirin for chronic hepatitis C virus (HCV) infection achieves limited sustained virological response (SVR). We conducted a systematic review and meta-analysis to explore the efficacy of adding statins to IFN-α and ribavirin therapy for chronic hepatitis C. Studies with data pertinent to the effect of statins on chronic hepatitis C were reviewed, and randomized controlled trials (RCTs) evaluating the efficacy of the addition of statins to IFN-α and ribavirin were included in meta-analysis. The primary outcome measure was SVR. Secondary outcome measures were rapid virological response (RVR) and early virological response (EVR). The literature was systematically searched through October 2012. After screening of the 1724 non-duplicated entries, 54 potentially relevant studies were fully reviewed. Of those, 18 studies were relevant and 5 RCTs met the inclusion criteria for meta-analysis. In comparison with IFN-α and ribavirin therapy, the addition of statins significantly increased SVR (OR=2.02, 95% CI: 1.38-2.94), RVR (OR=3.51, 95% CI: 1.08-11.42) and EVR (OR=1.89, 95% CI: 1.20-2.98). The SVR increase remained significant for HCV genotype 1 (OR=2.11, 95% CI: 1.40-3.18). There were no significant increases in adverse events and withdrawals with the addition of statins. In conclusion, the addition of statins to IFN-α and ribavirin improves SVR, RVR, and EVR without additional adverse events and thus may be considered as adjuvant to IFN-α and ribavirin for chronic hepatitis C. Statins might also be used for HCV genotypes other than genotype 1, or in patients in whom the use of protease inhibitors is contraindicated or not indicated.

Host-targeting agents for prevention and treatment of chronic hepatitis C - perspectives and challenges

Hepatitis C virus (HCV) infection is a major cause of chronic liver disease and hepatocellular carcinoma worldwide. Furthermore, HCV-induced liver disease is a major indication of liver transplantation. In the past years, direct-acting antivirals (DAAs) targeting HCV enzymes have been developed. DAAs increase the virologic response to anti-HCV therapy but may lead to selection of drug-resistant variants and treatment failure. To date, strategies to prevent HCV infection are still lacking and antiviral therapy in immunocompromised patients, patients with advanced liver disease and HIV/HCV-co-infection remains limited. Alternative or complementary approaches addressing the limitations of current antiviral therapies are to boost the host's innate immunity or interfere with host factors required for pathogenesis. Host-targeting agents (HTAs) provide an interesting perspective for novel antiviral strategies against viral hepatitis since they have (i) a high genetic barrier to resistance, (ii) a pan-genotypic antiviral activity, and (iii) complementary mechanisms of action to DAAs and might therefore act in a synergistic manner with current standard of care or DAAs in clinical development. This review highlights HTAs against HCV infection that have potential as novel antivirals and are in preclinical or clinical development.

The molecular basis of drug resistance against hepatitis C virus NS3/4A protease inhibitors

Hepatitis C virus (HCV) infects over 170 million people worldwide and is the leading cause of chronic liver diseases, including cirrhosis, liver failure, and liver cancer. Available antiviral therapies cause severe side effects and are effective only for a subset of patients, though treatment outcomes have recently been improved by the combination therapy now including boceprevir and telaprevir, which inhibit the viral NS3/4A protease. Despite extensive efforts to develop more potent next-generation protease inhibitors, however, the long-term efficacy of this drug class is challenged by the rapid emergence of resistance. Single-site mutations at protease residues R155, A156 and D168 confer resistance to nearly all inhibitors in clinical development. Thus, developing the next-generation of drugs that retain activity against a broader spectrum of resistant viral variants requires a comprehensive understanding of the molecular basis of drug resistance. In this study, 16 high-resolution crystal structures of four representative protease inhibitors – telaprevir, danoprevir, vaniprevir and MK-5172 – in complex with the wild-type protease and three major drug-resistant variants R155K, A156T and D168A, reveal unique molecular underpinnings of resistance to each drug. The drugs exhibit differential susceptibilities to these protease variants in both enzymatic and antiviral assays. Telaprevir, danoprevir and vaniprevir interact directly with sites that confer resistance upon mutation, while MK-5172 interacts in a unique conformation with the catalytic triad. This novel mode of MK-5172 binding explains its retained potency against two multi-drug-resistant variants, R155K and D168A. These findings define the molecular basis of HCV N3/4A protease inhibitor resistance and provide potential strategies for designing robust therapies against this rapidly evolving virus.

Hepatitis C virus (HCV) infects over 170 million people worldwide and is the leading cause of chronic liver diseases, including cirrhosis, liver failure, and liver cancer. New classes of directly-acting antiviral agents that target various HCV enzymes are being developed. Two such drugs that target the essential HCV NS3/4A protease are approved by the FDA and several others are at various stages of clinical development. These drugs, when used in combination with pegylated interferon and ribavirin, significantly improve treatment outcomes. However HCV evolves very quickly and drug resistance develops against directly-acting antiviral agents. Thus, despite the therapeutic success of NS3/4A protease inhibitors, their long-term effectiveness is challenged by drug resistance. Our study explains in atomic detail how and why drug resistance occurs for four chemically representative protease inhibitors –telaprevir, danoprevir, vaniprevir and MK-5172. Potentially with this knowledge, new drugs could be developed that are less susceptible to drug resistance. More generally, understanding the underlying mechanisms by which drug resistance occurs can be incorporated in drug development to many quickly evolving diseases.

Modeling quasispecies and drug resistance in hepatitis C patients treated with a protease inhibitor

Telaprevir, a novel hepatitis C virus (HCV) NS3-4A serine protease inhibitor, has demonstrated substantial antiviral activity in patients infected with HCV. However, drug-resistant HCV variants were detected in vivo at relatively high frequency a few days after drug administration. Here we use a two-strain mathematical model to explain the rapid emergence of drug resistance in HCV patients treated with telaprevir monotherapy. We examine the effects of backward mutation and liver cell proliferation on the preexistence of the mutant virus and the competition between wild-type and drug-resistant virus during therapy. We also extend the two-strain model to a general model with multiple viral strains. Mutations during therapy only have a minor effect on the dynamics of various viral strains, although they are capable of generating low levels of HCV variants that would otherwise be completely suppressed because of fitness disadvantages. Liver cell proliferation may not affect the pretreatment frequency of mutant variants, but is able to influence the quasispecies dynamics during therapy. It is the relative fitness of each mutant strain compared with wild-type that determines which strain(s) will dominate the virus population. This study provides a theoretical framework for exploring the prevalence of preexisting mutant variants and the evolution of drug resistance during treatment with other HCV protease inhibitors or polymerase inhibitors.

Animal models for the study of hepatitis C virus infection and related liver disease

Hepatitis C virus (HCV) causes liver-related death in more than 300,000 people annually. Treatments for patients with chronic HCV are suboptimal, despite the introduction of directly acting antiviral agents. There is no vaccine that prevents HCV infection. Relevant animal models are important for HCV research and development of drugs and vaccines. Chimpanzees are the best model for studies of HCV infection and related innate and adaptive host immune responses. They can be used in immunogenicity and efficacy studies of HCV vaccines. The only small animal models of robust HCV infection are T- and B- cell deficient mice with human chimeric livers. Although these mice cannot be used in studies of adaptive immunity, they have provided new insights into HCV neutralization, interactions between virus and receptors, innate host responses, and therapeutic approaches. Recent progress in developing genetically humanized mice is exciting, but these models only permit studies of specific steps in the HCV life cycle and have limited or no viral replication.

Discovery and development of telaprevir: an NS3-4A protease inhibitor for treating genotype 1 chronic hepatitis C virus

Infection with hepatitis C virus (HCV) is a major medical problem with over 170 million people infected worldwide. Substantial morbidity and mortality are associated with hepatic manifestations (cirrhosis and hepatocellular carcinoma), which develop with increasing frequency in people infected with HCV for more than 20 years. Less well known is the burden of HCV disease associated with extrahepatic manifestations (diabetes, B-cell proliferative disorders, depression, cognitive disorders, arthritis and Sjögren's syndrome). For patients infected with genotype 1 HCV, treatment with polyethylene glycol decorated interferon (peginterferon) α and ribavirin (PR) is associated with a low (40-50%) success rate, substantial treatment-limiting side effects and a long (48-week) duration of treatment. In the past 15 years, major scientific advances have enabled the development of new classes of HCV therapy, the direct-acting antiviral agents, also known as specifically targeted antiviral therapy for hepatitis C (STAT-C). In combination with PR, the HCV NS3-4A protease inhibitor telaprevir has recently been approved for treatment of genotype 1 chronic HCV in the United States, Canada, European Union and Japan. Compared with PR, telaprevir combination therapy offers significantly improved viral cure rates and the possibility of shortened treatment duration for diverse patient populations. Developers of innovative drugs have to blaze a new path with few validated sign posts to guide the way. Indeed, telaprevir's development was once put on hold because of its performance in a standard IC(50) assay. Data from new hypotheses and novel experiments were required to justify further investment and reduce risk that the drug might fail in the clinic. In addition, the poor drug-like properties of telaprevir were a formidable hurdle, which the manufacturing and formulation teams had to overcome to make the drug. Finally, novel clinical trial designs were developed to improve efficacy and shorten treatment in parallel instead of sequentially. Lessons learned from the development of telaprevir suggest that makers of innovative medicines cannot rely solely on traditional drug discovery metrics, but must develop innovative, scientifically guided pathways for success.

Animal models in the study of hepatitis C virus-associated liver pathologies

It is estimated that more than 170 million individuals worldwide are chronically infected with hepatitis C virus (HCV), with approximately 20% of the cases developing cirrhosis. Each year, between 1 and 4% of patients exhibiting cirrhosis develop hepatocellular carcinoma. Chronic HCV infection is also linked with the development of several metabolic disorders, including hepatic steatosis and insulin resistance. Research into HCV-related pathologies is hampered by a relative paucity of small animal models. As a result, little is known about the molecular mechanisms involved, and much of our current knowledge is drawn by inference from in vitro studies using overexpressed proteins. In this article, we will review the currently available animal models for the study of HCV pathogenesis, with an emphasis on murine models. Then, we will provide an overview of how these models have contributed to the deciphering of the molecular mechanisms underlying dysregulated lipid metabolism and hepatocellular carcinoma during HCV infection.

Animal model for study of human hepatitis viruses

Human hepatitis B virus (HBV) and hepatitis C virus (HCV) infect only chimpanzees and humans. Analysis of both viruses has long been hampered by the absence of a small animal model. The recent development of human hepatocyte chimeric mice has enabled us to carry out studies on viral replication and cellular changes induced by replication of human hepatitis viruses. Various therapeutic agents have also been tested using this model. In the present review, we summarize published studies using chimeric mice and discuss the merits and shortcomings of this model.

Discovery of (7R)-14-cyclohexyl-7-{[2-(dimethylamino)ethyl](methyl) amino}-7,8-dihydro-6H-indolo[1,2-e][1,5]benzoxazocine-11-carboxylic acid (MK-3281), a potent and orally bioavailable finger-loop inhibitor of the hepatitis C virus NS5B polymerase

Infections caused by hepatitis C virus (HCV) are a significant world health problem for which novel therapies are in urgent demand. The polymerase of HCV is responsible for the replication of viral genome and has been a prime target for drug discovery efforts. Here, we report on the further development of tetracyclic indole inhibitors, binding to an allosteric site on the thumb domain. Structure-activity relationship (SAR) studies around an indolo-benzoxazocine scaffold led to the identification of compound 33 (MK-3281), an inhibitor with good potency in the HCV subgenomic replication assay and attractive molecular properties suitable for a clinical candidate. The compound caused a consistent decrease in viremia in vivo using the chimeric mouse model of HCV infection.

Drug resistance against HCV NS3/4A inhibitors is defined by the balance of substrate recognition versus inhibitor binding

Hepatitis C virus infects an estimated 180 million people worldwide, prompting enormous efforts to develop inhibitors targeting the essential NS3/4A protease. Resistance against the most promising protease inhibitors, telaprevir, boceprevir, and ITMN-191, has emerged in clinical trials. In this study, crystal structures of the NS3/4A protease domain reveal that viral substrates bind to the protease active site in a conserved manner defining a consensus volume, or substrate envelope. Mutations that confer the most severe resistance in the clinic occur where the inhibitors protrude from the substrate envelope, as these changes selectively weaken inhibitor binding without compromising the binding of substrates. These findings suggest a general model for predicting the susceptibility of protease inhibitors to resistance: drugs designed to fit within the substrate envelope will be less susceptible to resistance, as mutations affecting inhibitor binding would simultaneously interfere with the recognition of viral substrates.

Factors determining successful engraftment of hepatocytes and susceptibility to hepatitis B and C virus infection in uPA-SCID mice

BACKGROUND & AIMS

The human liver-uPA(+/+)-SCID mouse is currently the best small animal model available for viral hepatitis infection studies.

METHODS

We identify critical factors affecting animal survival, engraftment efficacy, kinetics of liver repopulation and virological outcome by analysing the data from 400 human hepatocyte transplantations and 115 subsequent HBV and/or HCV inoculations in this mouse model.

RESULTS

Almost one third of animals succumbed during the first week after hepatocyte transplantation. Only during this critical period, liver necrosis due to embolization of donor cells in the portal vein was observed. This may have caused a fatal acute liver failure that complicated the pre-existing chronic liver disease. From the second week onwards, confluent hepatocyte clusters repopulated the liver and restored its synthetic functions as evidenced by increasing human albumin levels in plasma. Xenogenic repopulation by human cells proceeded approximately 4-times slower compared to allogenic mouse hepatocytes. All HBV inoculations were successful, even in animals with low graft take. HCV infection rate varied substantially, although every donor cell type yielded infectable animals. A reproducible 100% HCV infectivity was reached with high quality inocula in animals with human albumin plasma levels >1 mg/ml. Superior animal survival, adequate liver engraftment and a high viral infection rate were observed after transplanting cryopreserved commercial human hepatocytes.

CONCLUSIONS

Our findings favour the use of commercially available, cryopreserved human hepatocytes for the humanization of the uPA(+/+)-SCID liver. While HBV infectivity criteria are less stringent, human albumin plasma levels exceeding 1 mg/ml are required for a consistent HCV infection in chimeric mice.

New antiviral therapies for chronic hepatitis C

Chronic hepatitis C is an important health issue worldwide. The current standard therapy is based on a combination of pegylated-interferon (pegIFN) and ribavirin (RBV), but this treatment leads to only ~50% sustained virological response (SVR) in patients with HCV genotype 1 and high viral loads, who were mostly null-responders or relapsers. Among HCV genotypes other than HCV genotype 1, especially HCV genotype 4 patients show only 40-70% SVR by this treatment. Although new drugs also depend on the combination of pegIFN and RBV, it appears that these drugs improve not only rapid virological response (RVR) but also early virological response, leading to SVR in these patients. In the near future, we predict higher SVR rates in chronic hepatitis C patients treated with these new drugs.

MicroRNA-196 represses Bach1 protein and hepatitis C virus gene expression in human hepatoma cells expressing hepatitis C viral proteins

Hepatitis C virus (HCV) directly induces oxidative stress and liver injury. Bach1, a basic leucine zipper mammalian transcriptional repressor, negatively regulates heme oxygenase 1 (HMOX1), a key cytoprotective enzyme that has antioxidant and anti-inflammatory activities. microRNAs (miRNAs) are small noncoding RNAs ( approximately 22 nt) that are important regulators of gene expression. Whether and how miRNAs regulate Bach1 or HCV are largely unknown. The aims of this study were to determine whether miR-196 regulates Bach1, HMOX1, and/or HCV gene expression. HCV replicon cell lines (Con1 and 9-13) of the Con1 isolate and J6/JFH1-based HCV cell culture system were used in this study. The effects of miR-196 mimic on Bach1, HMOX1, and HCV RNA, and protein levels were measured by way of quantitative real-time polymerase chain reaction (qRT-PCR) and Western blotting, respectively. The Dual Glo Luciferase Assay System was used to determine reporter activities. miR-196 mimic significantly down-regulated Bach1 and up-regulated HMOX1 gene expression and inhibited HCV expression. Dual luciferase reporter assays demonstrated that transfection of miR-196 mimic resulted in a significant decrease in Bach1 3'-untranslated region (UTR)-dependent luciferase activity but not in mutant Bach1 3'-UTR-dependent luciferase activity. Moreover, there was no detectable effect of mutant miR-196 on Bach1 3'-UTR-dependent luciferase activity.

CONCLUSION

miR-196 directly acts on the 3'-UTR of Bach1 messenger RNA and translationally represses the expression of this protein, and up-regulates HMOX1. miR-196 also inhibits HCV expression in HCV replicon cell lines (genotype 1b) and in J6/JFH1 (genotype 2a) HCV cell culture system. Thus, miR-196 plays a role in both HMOX1/Bach1 expression and the regulation of HCV expression in human hepatocytes. Overexpression of miR-196 holds promise as a potential novel strategy to prevent or ameliorate hepatitis C infection, and to protect against liver injury in chronic HCV infection.

Development of an intergenotypic hepatitis C virus (HCV) cell culture method to assess antiviral susceptibilities and resistance development of HCV NS3 protease genes from HCV genotypes 1 to 6

Protease inhibitors (PIs) of hepatitis C virus (HCV) provide an additional or alternative therapy for chronic infection. However, assessment of their efficacy and ability to inhibit replication of different genotypes is hampered by the lack of a convenient animal model or a method for in vitro culture of HCV other than the type 1/2-based replicons and the infectious genotype 2a clone JFH1. To address this problem, we constructed a panel of replication-competent chimeric Jc1 (pFK JFH1/J6/C-846) clones containing protease and NS4A coding sequences from all six major genotypes, enabling the determination of replication and the susceptibility to PIs. Chimeras showed substantial variability in replication kinetics, attributable in part to naturally occurring polymorphisms and differing requirements for adaptive mutations in NS3 and NS4A. Through calculation of 50% inhibitory concentrations (IC(50)s) of BILN 2061, measuring reduction in the number of focus-forming units/ml (FFU/ml) and replication inhibition, consistent genotype-associated differences in antiviral susceptibilities were observed. IC(50)s for genotype 1b, 4a, and 6a-derived chimeras (1 to 3 nM) were approximately 100-fold lower than those for genotypes 2a, 3a, and 5a (range, 80 to 720 nM), implying major differences in response to therapy. In vitro passage in increasing concentrations of BILN 2061 rapidly induced resistance-associated mutations at position 168 in chimeras of all 6 genotypes and at position 156 in genotypes 1b and 4a, each with substantial variability in the identity of substituted amino acids. The system will allow future comprehensive phenotypic characterization of naturally occurring and treatment-induced mutations for PIs in trial or entering clinical use.

Experimental models for hepatitis C viral infection

Hepatitis C virus (HCV) infection is a leading cause of chronic liver disease. The majority of infected individuals develop a persistent infection, which is associated with a high risk of liver cirrhosis and hepatocellular carcinoma. Since its discovery 20 years ago, progress in our understanding of this virus has been suboptimal due to the lack of good model systems. However, in the past decade this has greatly accelerated with the development of various in vitro cell culture systems and in vivo small-animal models. These systems have made a major impact on the field of HCV research, and have provided important breakthroughs in our understanding of HCV infection and replication. Importantly, the in vitro cell culture systems and the small-animal models have allowed preclinical testing of numerous novel antiviral compounds for the treatment of chronic HCV infection. In this article, we give an overview of current models, discuss their limitations, and provide future perspectives for research directed at the prevention and cure of hepatitis C.

Towards a small animal model for hepatitis C

Hepatitis C virus (HCV) causes chronic liver disease and affects an estimated 3% of the world's population. Options for the prevention or therapy of HCV infection are limited; there is no vaccine and the nonspecific, interferon-based treatments now in use are frequently ineffective and have significant side effects. A small-animal model for HCV infection would significantly expedite antiviral compound development and preclinical testing, as well as open new avenues to decipher the mechanisms that underlie viral pathogenesis. The natural species tropism of HCV is, however, limited to humans and chimpanzees. Here, we discuss the prospects of developing a mouse model for HCV infection, taking into consideration recent results on HCV entry and replication, and new prospects in xenotransplantation biology. We highlight three independent, but possibly complementary, approaches towards overcoming current species barriers and generating a small-animal model for HCV pathogenesis.

HCV animal models: a journey of more than 30 years

In the 1970s and 1980s it became increasingly clear that blood transfusions could induce a form of chronic hepatitis that could not be ascribed to any of the viruses known to cause liver inflammation. In 1989, the hepatitis C virus (HCV) was discovered and found to be the major causative agent of these infections. Because of its narrow tropism, the in vivo study of this virus was, especially in the early days, limited to the chimpanzee. In the past decade, several alternative animal models have been created. In this review we review these novel animal models and their contribution to our current understanding of the biology of HCV.

Cell therapy for the diseased liver: from stem cell biology to novel models for hepatotropic human pathogens

It has long been known that hepatocytes possess the potential to replicate through many cell generations because regeneration can be achieved in rodents after serial two-thirds hepatectomy. It has taken considerable time and effort to harness this potential, with liver regeneration models involving hepatocyte transplantation developing over the past 15 years. This review will describe the experiments that have established the models and methodology for liver repopulation, and the use of cells other than adult hepatocytes in liver repopulation, including hepatic cell lines and hematopoietic, cord blood, hepatic and embryonic stem cells. Emphasis will be placed on the characteristics of the models and how they can influence the outcome of the experiments. Finally, an account of the development of murine models that are competent to accept human hepatocytes is provided. In these models, liver deficiencies are induced in immunodeficient mice, where healthy human cells have a selective advantage. These mice with humanized livers provide a powerful new experimental tool for the study of human hepatotropic pathogens.

Resistance mechanisms in HCV: from evolution to intervention

Recent advances in our understanding of the HCV life cycle and the functions of virally encoded proteins enabled the development of specifically targeted antiviral therapies for HCV, which directly inhibit HCV replication. Early clinical trials show great efficacy; however, from the first trials it became evident that, similar to HIV and HBV, selection of resistant variants will be problematic. Error-prone replication of HCV, resulting in a complex quasispecies population within each infected individual, enables rapid adaptation to changing environments. In this review, the evolutionary mechanisms involved in the selection process resulting in drug resistance are discussed. We give an overview of the resistance profiles to recently developed HCV protease and polymerase inhibitors and discuss potential implications for future treatment developments.

Mouse models for the study of HCV infection and virus-host interactions

Hepatitis C virus (HCV) is a major cause of chronic liver disease including steatosis, cirrhosis and hepatocellular carcinoma. The development of transgenic mice expressing HCV proteins and the successful repopulation of SCID/Alb-uPA mice with human hepatocytes provides an important tool for unraveling virus-host interactions in vivo. Several of these mouse models exhibit aspects of HCV-related liver disease. Thus, these in vivo models play an important role to further understand the pathogenesis of HCV infection and to evaluate the pre-clinical safety and efficacy of new antiviral compounds against HCV. This review summarizes the most important mouse models currently used to study HCV pathogenesis and infection. Finally, the perspective of these models for future HCV research as well as the design of novel small animal models is discussed.