Cyclosporine A inhibits hepatitis C virus nonstructural protein 2 through cyclophilin A

Bothropstoxins I and II as potent phospholipase A2 molecules from Bothrops jararacussu to impair Hepatitis C virus infection

Hepatitis C is a hepatological disorder induced by the Hepacivirus hominis (Hepatitis C virus, HCV), with approximately 170 million individuals estimated to be presently affected globally. The current treatment for infected patients primarily relies on direct-acting antivirals (DAAs). However, this treatment is marked by its high cost, numerous side effects, and documented instances of antiviral resistance. These challenges underscore the imperative for developing novel therapeutic strategies. In this framework, naturally occurring compounds have exhibited considerable medical significance attributable to their biological functionalities. Compounds extracted from snake venoms have evidenced antiviral efficacy against a variety of viral pathogens including Orthoflavivirus denguei (DENV), Orthoflavivirus flavi (YFV), Orthoflavivirus zikaense (ZIKV), and HCV. Here, the activity of 10 proteins isolated from snakes' venom of Bothrops genus were evaluated against HCV replicative cycle. The full-length JFH-1 HCV system was used to infect the Huh-7.5 cell. Cell viability was measured simultaneously through MTT assay. Eight compounds inhibited up to 99% of HCV infection, being the most potent inhibitory rates observed in BthTX-I and BthTX-II, with an SI of 13.5 and 1736, respectively, being able to block 84.7% and 96% of HCV infectivity, in the same order. BthTX-II also demonstrated a protective effect in cells treated prior to HCV infection of approximately 86.7%. Molecular docking calculations suggest interactions between the two proteins with HCV E1-E2 glycoprotein complex. BthTX-II exhibited stronger interactions, indicated by 22 hydrophobic interactions. In conclusion, these compounds were shown to inhibit HCV infectivity by either acting on the virus particles or protecting the cells against infection.

Natural and Designed Cyclic Peptides as Potential Antiviral Drugs to Combat Future Coronavirus Outbreaks

The COVID-19 pandemic has underscored the need for effective and affordable antiviral drugs. Anthropogenic activities have increased interactions among humans, animals, and wildlife, contributing to the emergence of new and re-emerging viral diseases. RNA viruses pose significant challenges due to their rapid mutation rates, high transmissibility, and ability to adapt to host immune responses and antiviral treatments. The World Health Organization has identified several diseases (COVID-19, Ebola, Marburg, Zika, and others), all caused by RNA viruses, designated as being of priority concern as potential causes of future pandemics. Despite advances in antiviral treatments, many viruses lack specific therapeutic options, and more importantly, there is a paucity of broad-spectrum antiviral drugs. Additionally, the high costs of current treatments such as Remdesivir and Paxlovid highlight the need for more affordable antiviral drugs. Cyclic peptides from natural sources or designed through molecular modeling have shown promise as antiviral drugs with stability, low toxicity, high target specificity, and low antiviral resistance properties. This review emphasizes the urgent need to develop specific and broad-spectrum antiviral drugs and highlights cyclic peptides as a sustainable solution to combat future pandemics. Further research into these compounds could provide a new weapon to combat RNA viruses and address the gaps in current antiviral drug development.

Exploration of the Role of Cyclophilins in Established Hepatitis B and C Infections

Cyclophilin (Cyp) inhibitors are of clinical interest in respect to their antiviral activities in the context of many viral infections including chronic hepatitis B and C. Cyps are a group of enzymes with peptidyl-prolyl isomerase activity (PPIase), known to be required for replication of diverse viruses including hepatitis B and C viruses (HBV and HCV). Amongst the Cyp family, the molecular mechanisms underlying the antiviral effects of CypA have been investigated in detail, but potential roles of other Cyps are less well studied in the context of viral hepatitis. Furthermore, most studies investigating the role of Cyps in viral hepatitis did not investigate the potential therapeutic effects of their inhibition in already-established infections but have rather been performed in the context of neo-infections. Here, we investigated the effects of genetically silencing Cyps on persistent HCV and HBV infections. We confirm antiviral effects of CypA and CypD knock down and demonstrate novel roles for CypG and CypH in HCV replication. We show, furthermore, that CypA silencing has a modest but reproducible impact on persistent HBV infections in cultured human hepatocytes.

Mechanisms of Action of the Host-Targeting Agent Cyclosporin A and Direct-Acting Antiviral Agents against Hepatitis C Virus

Several direct-acting antivirals (DAAs) are available, providing interferon-free strategies for a hepatitis C cure. In contrast to DAAs, host-targeting agents (HTAs) interfere with host cellular factors that are essential in the viral replication cycle; as host genes, they are less likely to rapidly mutate under drug pressure, thus potentially exhibiting a high barrier to resistance, in addition to distinct mechanisms of action. We compared the effects of cyclosporin A (CsA), a HTA that targets cyclophilin A (CypA), to DAAs, including inhibitors of nonstructural protein 5A (NS5A), NS3/4A, and NS5B, in Huh7.5.1 cells. Our data show that CsA suppressed HCV infection as rapidly as the fastest-acting DAAs. CsA and inhibitors of NS5A and NS3/4A, but not of NS5B, suppressed the production and release of infectious HCV particles. Intriguingly, while CsA rapidly suppressed infectious extracellular virus levels, it had no significant effect on the intracellular infectious virus, suggesting that, unlike the DAAs tested here, it may block a post-assembly step in the viral replication cycle. Hence, our findings shed light on the biological processes involved in HCV replication and the role of CypA.

Host Cell Targets for Unconventional Antivirals against RNA Viruses

The recent COVID-19 crisis has highlighted the importance of RNA-based viruses. The most prominent members of this group are SARS-CoV-2 (coronavirus), HIV (human immunodeficiency virus), EBOV (Ebola virus), DENV (dengue virus), HCV (hepatitis C virus), ZIKV (Zika virus), CHIKV (chikungunya virus), and influenza A virus. With the exception of retroviruses which produce reverse transcriptase, the majority of RNA viruses encode RNA-dependent RNA polymerases which do not include molecular proofreading tools, underlying the high mutation capacity of these viruses as they multiply in the host cells. Together with their ability to manipulate the immune system of the host in different ways, their high mutation frequency poses a challenge to develop effective and durable vaccination and/or treatments. Consequently, the use of antiviral targeting agents, while an important part of the therapeutic strategy against infection, may lead to the selection of drug-resistant variants. The crucial role of the host cell replicative and processing machinery is essential for the replicative cycle of the viruses and has driven attention to the potential use of drugs directed to the host machinery as therapeutic alternatives to treat viral infections. In this review, we discuss small molecules with antiviral effects that target cellular factors in different steps of the infectious cycle of many RNA viruses. We emphasize the repurposing of FDA-approved drugs with broad-spectrum antiviral activity. Finally, we postulate that the ferruginol analog (18-(phthalimide-2-yl) ferruginol) is a potential host-targeted antiviral.

Repurposing of cyclophilin A inhibitors as broad-spectrum antiviral agents

Cyclophilin A (CypA) is linked to diverse human diseases including viral infections. With the worldwide emergence of severe acute respiratory coronavirus 2 (SARS-CoV-2), drug repurposing has been highlighted as a strategy with the potential to speed up antiviral development. Because CypA acts as a proviral component in hepatitis C virus, coronavirus and HIV, its inhibitors have been suggested as potential treatments for these infections. Here, we review the structure of cyclosporin A and sanglifehrin A analogs as well as synthetic micromolecules inhibiting CypA; and we discuss their broad-spectrum antiviral efficacy in the context of the virus lifecycle.

Cyclosporin A: A Repurposable Drug in the Treatment of COVID-19?

Coronavirus disease 2019 (COVID-19) is now at the forefront of major health challenge faced globally, creating an urgent need for safe and efficient therapeutic strategies. Given the high attrition rates, high costs, and quite slow development of drug discovery, repurposing of known FDA-approved molecules is increasingly becoming an attractive issue in order to quickly find molecules capable of preventing and/or curing COVID-19 patients. Cyclosporin A (CsA), a common anti-rejection drug widely used in transplantation, has recently been shown to exhibit substantial anti-SARS-CoV-2 antiviral activity and anti-COVID-19 effect. Here, we review the molecular mechanisms of action of CsA in order to highlight why this molecule seems to be an interesting candidate for the therapeutic management of COVID-19 patients. We conclude that CsA could have at least three major targets in COVID-19 patients: (i) an anti-inflammatory effect reducing the production of proinflammatory cytokines, (ii) an antiviral effect preventing the formation of the viral RNA synthesis complex, and (iii) an effect on tissue damage and thrombosis by acting against the deleterious action of angiotensin II. Several preliminary CsA clinical trials performed on COVID-19 patients report lower incidence of death and suggest that this strategy should be investigated further in order to assess in which context the benefit/risk ratio of repurposing CsA as first-line therapy in COVID-19 is the most favorable.

Cyclophilins and Their Roles in Hepatitis C Virus and Flavivirus Infections: Perspectives for Novel Antiviral Approaches

Cyclophilins are cellular peptidyl-prolyl isomerases that play an important role in viral infections, with demonstrated roles in the replication of hepatitis C virus (HCV) and other viruses in the Flaviviridae family, such as dengue virus (DENV) and yellow fever virus (YFV). Here, we discuss the roles of cyclophilins in HCV infection and provide a comprehensive overview of the mechanisms underlying the requirement for cyclophilins during HCV replication. Notably, cyclophilin inhibitor therapy has been demonstrated to be effective in reducing HCV replication in chronically infected patients. While the roles of cyclophilins are relatively well-understood for HCV infection, cyclophilins are more recently emerging as host factors for flavivirus infection as well, providing potential new therapeutic avenues for these viral infections which currently lack antiviral therapies. However, further studies are required to elucidate the roles of cyclophilins in flavivirus replication. Here, we review the current knowledge of the role of cyclophilins in HCV infection to provide a conceptual framework to understand how cyclophilins may contribute to other viral infections, such as DENV and YFV. Improved understanding of the roles of cyclophilins in viral infection may open perspectives for the development of cyclophilin inhibitors as effective antiviral therapeutics for HCV and related viruses.

Generation of Combinatorial Lentiviral Vectors Expressing Multiple Anti-Hepatitis C Virus shRNAs and Their Validation on a Novel HCV Replicon Double Reporter Cell Line

Despite the introduction of directly acting antivirals (DAAs), for the treatment of hepatitis C virus (HCV) infection, their cost, patient compliance, and viral resistance are still important issues to be considered. Here, we describe the generation of a novel JFH1-based HCV subgenomic replicon double reporter cell line suitable for testing different antiviral drugs and therapeutic interventions. This cells line allowed a rapid and accurate quantification of cell growth/viability and HCV RNA replication, thus discriminating specific from unspecific antiviral effects caused by DAAs or cytotoxic compounds, respectively. By correlating cell number and virus replication, we could confirm the inhibitory effect on the latter of cell over confluency and characterize an array of lentiviral vectors expressing single, double, or triple cassettes containing different combinations of short hairpin (sh)RNAs, targeting both highly conserved viral genome sequences and cellular factors crucial for HCV replication. While all vectors were effective in reducing HCV replication, the ones targeting viral sequences displayed a stronger antiviral effect, without significant cytopathic effects. Such combinatorial platforms as well as the developed double reporter cell line might find application both in setting-up anti-HCV gene therapy approaches and in studies aimed at further dissecting the viral biology/pathogenesis of infection.

Epstein-Barr Virus Nuclear Antigen 1 Recruits Cyclophilin A to Facilitate the Replication of Viral DNA Genome

Epstein-Barr virus (EBV) nuclear antigen 1 (EBNA1)-mediated DNA episomal genome replication and persistence are essential for the viral pathogenesis. Cyclophilin A (CYPA) is upregulated in EBV-associated nasopharyngeal carcinoma (NPC) with unknown roles. In the present approach, cytosolic CYPA was found to be bound with EBNA1 into the nucleus. The amino acid 376-459 of the EBNA1 domain was important for the binding. CYPA depletion attenuated and ectopic CYPA expression improved EBNA1 expression in EBV-positive cells. The loss of viral copy number was also accelerated by CYPA consumption in daughter cells during culture passages. Mechanistically, CYPA mediated the connection of EBNA1 with oriP (origin of EBV DNA replication) and subsequent oriP transcription, which is a key step for the initiation of EBV genome replication. Moreover, CYPA overexpression markedly antagonized the connection of EBNA1 to Ubiquitin-specific protease 7 (USP7), which is a strong host barrier with a role of inhibiting EBV genome replication. The PPIase activity of CYPA was required for the promotion of oriP transcription and antagonism with USP7. The results revealed a strategy that EBV recruited a host factor to counteract the host defense, thus facilitating its own latent genome replication. This study provides a new insight into EBV pathogenesis and potential virus-targeted therapeutics in EBV-associated NPC, in which CYPA is upregulated at all stages.

Differences across cyclophilin A orthologs contribute to the host range restriction of hepatitis C virus

The restricted host tropism of hepatitis C virus (HCV) remains incompletely understood, especially post-entry, and has hindered developing an immunocompetent, small animal model. HCV replication in non-permissive species may be limited by incompatibilities between the viral replication machinery and orthologs of essential host factors, like cyclophilin A (CypA). We thus compared the ability of CypA from mouse, tree shrew, and seven non-human primate species to support HCV replication, finding that murine CypA only partially rescued viral replication in Huh7.5-shRNA CypA cells. We determined the specific amino acid differences responsible and generated mutants able to fully rescue replication. We expressed these mutants in engineered murine hepatoma cells and although we observed increases in HCV replication following infection, they remained far lower than those in highly permissive human hepatoma cells, and minimal infectious particle release was observed. Together, these data suggest additional co-factors remain unidentified. Future work to determine such factors will be critical for developing an immunocompetent mouse model supporting HCV replication.

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.

Cyclophilins and cyclophilin inhibitors in nidovirus replication

Cyclophilins (Cyps) belong to the family of peptidyl-prolyl isomerases (PPIases). The PPIase activity of most Cyps is inhibited by the immunosuppressive drug cyclosporin A and several of its non-immunosuppressive analogs, which can also block the replication of nidoviruses (arteriviruses and coronaviruses). Cyclophilins have been reported to play an essential role in the replication of several other RNA viruses, including human immunodeficiency virus-1, hepatitis C virus, and influenza A virus. Likewise, the replication of various nidoviruses was reported to depend on Cyps or other PPIases. This review summarizes our current understanding of this class of nidovirus-host interactions, including the potential function of in particular CypA and the inhibitory effect of Cyp inhibitors. Also the involvement of the FK-506-binding proteins and parvulins is discussed. The nidovirus data are placed in a broader perspective by summarizing the most relevant data on Cyp interactions and Cyp inhibitors for other RNA viruses.

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Nidovirus replication is inhibited by cyclophilin inhibitors.

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Arterivirus replication depends on cyclophilin A.

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Cyclosporin A blocks arterivirus RNA synthesis.

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Using cyclophilin inhibitors against nidoviruses in vivo needs more investigation.

The A-Z of Zika drug discovery

Despite the recent outbreak of Zika virus (ZIKV), there are still no approved treatments, and early-stage compounds are probably many years away from approval. A comprehensive A-Z review of the recent advances in ZIKV drug discovery efforts is presented, highlighting drug repositioning and computationally guided compounds, including discovered viral and host cell inhibitors. Promising ZIKV molecular targets are also described and discussed, as well as targets belonging to the host cell, as new opportunities for ZIKV drug discovery. All this knowledge is not only crucial to advancing the fight against the Zika virus and other flaviviruses but also helps us prepare for the next emerging virus outbreak to which we will have to respond.

Effects of the Mammalian Target of Rapamycin Inhibitor Everolimus on Hepatitis C Virus Replication In Vitro and In Vivo

BACKGROUND

The influence of immunosuppressants on hepatitis C virus (HCV) re-infection after liver transplantation, particularly mammalian target of rapamycin inhibitors, remains unclear. The aim of our study was to analyze the influence of everolimus (EVR) on HCV replication activity in the context of underlying molecular mechanisms, with focus on the pro-myelocytic leukemia protein (PML).

METHODS

HCV viral load was recorded in 40 patients with post-transplant HCV re-infection before and 8 weeks after introduction of EVR. An HCV cell culture replicon system for genotype (GT) 1b, GT2b, and GT3a was used to compare the influence of EVR on HCV replication for the respective genotypes in vitro. Fluorescence-activated cell-sorting analysis was used to test for effects on cell proliferation. PML expression was silenced with the use of small hairpin RNA constructs, and PML expression was quantified by means of quantitative real-time polymerase chain reaction.

RESULTS

In patients with HCV, the viral load of GT1a and GT1b was hardly affected by EVR, whereas the viral load was reduced in patients with GT2a (P ≤ .0001) or GT3 infection (P ≤ .05). In vitro EVR impairs HCV replication activity of GT2a and GT3a up to 60% (P ≤ .0005), whereas in GT1b cells, HCV replication activity is increased by 50% (P ≤ .005). Replicon cell viability was not impaired. HCV replication activity is impaired in the absence of PML, which can be reversed by overexpression of PML isoforms. Furthermore, in the absence of PML, the effect of EVR on HCV replication activity is nearly abrogated.

CONCLUSIONS

The mammalian target of rapamycin inhibitor EVR influences HCV replication via PML. The herein presented results suggest a genotype-dependent benefit for an EVR-based immunosuppressive regimen in patients with GT2a or GT3 re-infection after liver transplantation.

Cyclophilin A as a target in the treatment of cytomegalovirus infections

BACKGROUND

Viruses are obligate parasites that depend on the cellular machinery of the host to regenerate and manufacture their proteins. Most antiviral drugs on the market today target viral proteins. However, the more recent strategies involve targeting the host cell proteins or pathways that mediate viral replication. This new approach would be effective for most viruses while minimizing drug resistance and toxicity.

METHODS

Cytomegalovirus replication, latency, and immune response are mediated by the intermediate early protein 2, the main protein that determines the effectiveness of drugs in cytomegalovirus inhibition. This review explains how intermediate early protein 2 can modify the action of cyclosporin A, an immunosuppressive, and antiviral drug. It also links all the pathways mediated by cyclosporin A, cytomegalovirus replication, and its encoded proteins.

RESULTS

Intermediate early protein 2 can influence the cellular cyclophilin A pathway, affecting cyclosporin A as a mediator of viral replication or anti-cytomegalovirus drug.

CONCLUSION

Cyclosporin A has a dual function in cytomegalovirus pathogenesis. It has the immunosuppressive effect that establishes virus replication through the inhibition of T-cell function. It also has an anti-cytomegalovirus effect mediated by intermediate early protein 2. Both of these functions involve cyclophilin A pathway.

Flavonoids from Pterogyne nitens Inhibit Hepatitis C Virus Entry

Hepatitis C virus (HCV) is one of the leading causes of liver diseases and transplantation worldwide. The current available therapy for HCV infection is based on interferon-α, ribavirin and the new direct-acting antivirals (DAAs), such as NS3 protease and NS5B polymerase inhibitors. However, the high costs of drug design, severe side effects and HCV resistance presented by the existing treatments demonstrate the need for developing more efficient anti-HCV agents. This study aimed to evaluate the antiviral effects of sorbifolin (1) and pedalitin (2), two flavonoids from Pterogyne nitens on the HCV replication cycle. These compounds were investigated for their anti-HCV activities using genotype 2a JFH-1 subgenomic replicons and infectious virus systems. Flavonoids 1 and 2 inhibited virus entry up to 45.0% and 78.7% respectively at non-cytotoxic concentrations. The mechanism of the flavonoid 2 block to virus entry was demonstrated to be by both the direct action on virus particles and the interference on the host cells. Alternatively, the flavonoid 1 activity was restricted to its virucidal effect. Additionally, no inhibitory effects on HCV replication and release were observed by treating cells with these flavonoids. These data are the first description of 1 and 2 possessing in vitro anti-HCV activity.

Multiple effects of toxins isolated from Crotalus durissus terrificus on the hepatitis C virus life cycle

Hepatitis C virus (HCV) is one of the main causes of liver disease and transplantation worldwide. Current therapy is expensive, presents additional side effects and viral resistance has been described. Therefore, studies for developing more efficient antivirals against HCV are needed. Compounds isolated from animal venoms have shown antiviral activity against some viruses such as Dengue virus, Yellow fever virus and Measles virus. In this study, we evaluated the effect of the complex crotoxin (CX) and its subunits crotapotin (CP) and phospholipase A2 (PLA2-CB) isolated from the venom of Crotalus durissus terrificus on HCV life cycle. Huh 7.5 cells were infected with HCVcc JFH-1 strain in the presence or absence of these toxins and virus was titrated by focus formation units assay or by qPCR. Toxins were added to the cells at different time points depending on the stage of virus life cycle to be evaluated. The results showed that treatment with PLA2-CB inhibited HCV entry and replication but no effect on HCV release was observed. CX reduced virus entry and release but not replication. By treating cells with CP, an antiviral effect was observed on HCV release, the only stage inhibited by this compound. Our data demonstrated the multiple antiviral effects of toxins from animal venoms on HCV life cycle.

A highly conserved G-rich consensus sequence in hepatitis C virus core gene represents a new anti-hepatitis C target

G-quadruplex (G4) is one of the most important secondary structures in nucleic acids. Until recently, G4 RNAs have not been reported in any ribovirus, such as the hepatitis C virus. Our bioinformatics analysis reveals highly conserved guanine-rich consensus sequences within the core gene of hepatitis C despite the high genetic variability of this ribovirus; we further show using various methods that such consensus sequences can fold into unimolecular G4 RNA structures, both in vitro and under physiological conditions. Furthermore, we provide direct evidences that small molecules specifically targeting G4 can stabilize this structure to reduce RNA replication and inhibit protein translation of intracellular hepatitis C. Ultimately, the stabilization of G4 RNA in the genome of hepatitis C represents a promising new strategy for anti-hepatitis C drug development.

Chaperones in hepatitis C virus infection

The hepatitis C virus (HCV) infects approximately 3% of the world population or more than 185 million people worldwide. Each year, an estimated 350000-500000 deaths occur worldwide due to HCV-associated diseases including cirrhosis and hepatocellular carcinoma. HCV is the most common indication for liver transplantation in patients with cirrhosis worldwide. HCV is an enveloped RNA virus classified in the genus Hepacivirus in the Flaviviridae family. The HCV viral life cycle in a cell can be divided into six phases: (1) binding and internalization; (2) cytoplasmic release and uncoating; (3) viral polyprotein translation and processing; (4) RNA genome replication; (5) encapsidation (packaging) and assembly; and (6) virus morphogenesis (maturation) and secretion. Many host factors are involved in the HCV life cycle. Chaperones are an important group of host cytoprotective molecules that coordinate numerous cellular processes including protein folding, multimeric protein assembly, protein trafficking, and protein degradation. All phases of the viral life cycle require chaperone activity and the interaction of viral proteins with chaperones. This review will present our current knowledge and understanding of the role of chaperones in the HCV life cycle. Analysis of chaperones in HCV infection will provide further insights into viral/host interactions and potential therapeutic targets for both HCV and other viruses.

Chronic hepatitis E in solid-organ transplantation: the key implications of immunosuppressants

PURPOSE OF REVIEW

Solid-organ recipients infected with hepatitis E virus (HEV) bear an extremely high risk of developing chronic hepatitis, although this virus only causes acute infection in the general population. Immunosuppressive medication universally used after transplantation to prevent organ rejection appears to be a main risk factor for developing chronic infection. This review aims to overview and emphasize the current clinical and experimental evidence regarding the key implications of immunosuppressants in chronic hepatitis E.

RECENT FINDINGS

Over 60% of organ recipients who are infected with HEV develop chronic hepatitis. Immunosuppressant treatment after transplantation was identified as a key risk factor. Therefore, dose reduction or even withdrawal of immunosuppressants is considered as the first intervention strategy to achieve viral clearance in these patients. Otherwise, ribavirin, as an off-label medication, is considered as an antiviral treatment, with compelling outcomes observed so far. Interestingly, in addition to a common immunosuppression property that can favour HEV infection in general, different types of immunosuppressants may exert differential impacts on the infection course in patients. Furthermore, potential interaction may exist between particular immunosuppressant and ribavirin. With the recent development of a cell culture system for HEV, experimental research has been initiated to investigate how immunosuppressive drugs interact with HEV infection.

SUMMARY

On the basis of the current evidence, it remains impossible to define an optimal immunosuppressive protocol for these HEV-infected patients. However, the realization of this clinical issue and the initiation of translational research using cell culture models of HEV have been represented as milestones in this field.

Cyclophilin polymorphism and virus infection

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Coding non-synonymous SNP's of PPIA are very rare in human.

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3 SNP's result in a destabilization of the CypA protein and an HCV and human coronavirus 229E resistance phenotype.

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Promotor SNP's are associated with rapid disease progression to AIDS.

Viruses are obligate intracellular parasites. All stages of their replication cycle depend on support by host-encoded factors. However, sequence variation also exists in host factors mostly in the form of single nucleotide polymorphisms (SNPs). Several coding and non-coding genetic variants in the PPIA gene encoding for CypA have been described, but there is only limited information about their influence on the course of viral infection. This paper reviews PPIA polymorphisms and what is known about their impact on the replication cycle and course of disease for different viral infections.

Gene therapies for hepatitis C virus

Hepatitis C virus (HCV) is a leading cause of chronic hepatitis and infects approximately three to four million people per year, about 170 million infected people in total, making it one of the major global health problems. In a minority of cases HCV is cleared spontaneously, but in most of the infected individuals infection progresses to a chronic state associated with high risk to develop liver cirrhosis, hepatocellular cancer, or liver failure. The treatment of HCV infection has evolved over the years. Interferon (IFN)-α in combination with ribavirin has been used for decades as standard therapy. More recently, a new standard-of-care treatment has been approved based on a triple combination with either HCV protease inhibitor telaprevir or boceprevir. In addition, various options for all-oral, IFN-free regimens are currently being evaluated. Despite substantial improvement of sustained virological response rates, some intrinsic limitations of these new direct-acting antivirals, including serious side effects, the risk of resistance development and high cost, urge the development of alternative or additional therapeutic strategies. Gene therapy represents a feasible alternative treatment. Small RNA technology, including RNA interference (RNAi) techniques and antisense approaches, is one of the potentially promising ways to investigate viral and host cell factors that are involved in HCV infection and replication. With this, newly developed gene therapy regimens will be provided to treat HCV. In this chapter, a comprehensive overview guides you through the current developments and applications of RNAi and microRNA-based gene therapy strategies in HCV treatment.

The impact of hepatitis E in the liver transplant setting

Hepatitis E virus (HEV) infection has been identified as a cause of graft hepatitis in liver transplant recipients. The true frequency and clinical importance of HEV infections after liver transplantations is a matter of debate. It is proposed that consumption of HEV-contaminated undercooked meat is a main source for HEV infections in developed countries--which might also account for some hepatitis E cases after organ transplantation. However, HEV is also transmitted by transfusion of blood products, likely representing a previously underestimated risk particularly for patients in the transplant setting. HEV infection can take chronic courses in immunocompromised individuals, associated in some cases with rapid progression to cirrhosis within 1-2 years of infection. Diagnosis in transplanted patients is based on HEV RNA testing as antibody assays are not sensitive enough. Selection of immunosuppressive drugs is important as different compounds may influence viral replication and the course of liver disease. Ribavirin has antiviral activity against HEV and should be administered for at least three months in chronically infected individuals; however, treatment failure may occur. HEV infections have also been linked to a variety of extrahepatic manifestations both during and after resolution of infection. In this review we summarize the emerging data on hepatitis E with a particular focus on the importance of HEV infections for liver transplant recipients.

Cyclophilin inhibition as potential therapy for liver diseases

The cyclophilins are a group of proteins with peptidyl-prolyl isomerase enzymatic activity, localised in different cellular compartments and involved in a variety of functions related to cell metabolism and energy homeostasis, having enhanced expression in inflammation or malignancy. Cyclophilin A (CypA), the most abundantly expressed cyclophilin, is present mainly in the cytoplasm and is a host factor involved in the life cycle of multiple viruses. The extracellular fractions of CypA and CypB are potent pro-inflammatory mediators. CypD, located in mitochondria, is a key regulator of mitochondrial permeability transition pores, and is critical for necrotic cell death. Cyclosporines are the prototype cyclophilin inhibitors. Cyclic peptides, which bind and inhibit cyclophilins without having immunosuppressive properties, have been generated by chemical modifications of cyclosporin A. In addition, cyclophilin inhibitors that are structurally different from cyclosporines have been synthesized. The involvement of cyclophilins in the pathogenesis of different liver diseases has been established using both in vitro and in vivo investigations, thus indicating that cyclophilin inhibition may be of therapeutic benefit. This review summarises the evidence for potential therapeutic applications of non-immunosuppressive cyclophilin inhibitors, alone or in combination with other agents, in virus-induced liver diseases like hepatitis C, B or Delta, liver inflammation and fibrosis, acetaminophen-induced liver toxicity and hepatocellular carcinoma.

Hepatitis C virus core antigen testing in liver and kidney transplant recipients

HCV RNA levels correlate with the long-term outcome of hepatitis C in liver transplant recipients. Nucleic acid testing (NAT) is usually used to confirm HCV reinfection and to examine viral loads after liver transplantation. HCV core antigen (HCVcoreAg) testing could be an alternative to NAT with some potential advantages including very low intra- and interassay variabilities and lower costs. The performance of HCVcoreAg testing in organ transplant recipients is unknown. We prospectively studied 1011 sera for HCV RNA and HCVcoreAg in a routine real-world setting including 222 samples obtained from patients after liver or kidney transplantation. HCV RNA and HCVcoreAg test results showed a consistency of 98% with a very good correlation in transplanted patients (r > 0.85). The correlation between HCV RNA and HCVcoreAg was higher in sera with high viral loads and in samples from patients with low biochemical disease. Patients treated with tacrolimus showed a better correlation between both parameters than individuals receiving cyclosporine A. HCV RNA/HCVcoreAg ratios did not differ between transplanted and nontransplanted patients, and HCV RNA and HCVcoreAg kinetics were almost identical during the first days after liver transplantation. HCVcoreAg testing can be used to monitor HCV viral loads in patients after organ transplantation. However, the assay is not recommended to monitor antiviral therapies.

Predictability of IL-28B-polymorphism on protease-inhibitor-based triple-therapy in chronic HCV-genotype-1 patients: A meta-analysis

AIM: To investigate the predictability of interleukin-28B single nucleotide polymorphism rs12979860 with respect to sustained virological response (SVR) in chronically hepatitis C virus (HCV) genotype-1 patients treated with a protease-inhibitor and pegylated interferon-α (Peg-INF-α) based triple-therapy.

METHODS: We searched PubMed, the Cochrane Library and Web of Knowledge for studies regarding the interleukin 28B (IL-28B)-genotype and protease-inhibitor based triple-therapy. Ten studies with 2707 patients were included into this meta-analysis. We used regression methods in order to investigate determinants of SVR.

RESULTS: IL-28B-CC-genotype patients achieved higher SVR rates (odds 5.34, 95%CI: 3.81-7.49) than IL-28B-non-CC-genotype patients (1.88, 95%CI: 1.43-2.48) receiving triple-therapy. The line of therapy (treatment-naïve or -experienced for Peg-INF-α) did not affect the predictive value of IL-28B (P = 0.1). IL-28B-CC-genotype patients treated with protease inhibitor-based triple-therapy consisting of Boceprevir, Simeprevir, Telaprevir or Vaniprevir showed odds of 3.38, 14.66, 7.84 and 2.91, respectively. The odds for CC genotype patients treated with Faldaprevir cannot be quantified, as only a single study with a 100% SVR rate was available.

CONCLUSION: IL-28B-SNP predicts the outcome for chronic HCV genotype-1 patients receiving protease inhibitor-based triple-therapy. The predictive value varies between the different protease inhibitors.

Cyclophilin A associates with enterovirus-71 virus capsid and plays an essential role in viral infection as an uncoating regulator

Viruses utilize host factors for their efficient proliferation. By evaluating the inhibitory effects of compounds in our library, we identified inhibitors of cyclophilin A (CypA), a known immunosuppressor with peptidyl-prolyl cis-trans isomerase activity, can significantly attenuate EV71 proliferation. We demonstrated that CypA played an essential role in EV71 entry and that the RNA interference-mediated reduction of endogenous CypA expression led to decreased EV71 multiplication. We further revealed that CypA directly interacted with and modified the conformation of H-I loop of the VP1 protein in EV71 capsid, and thus regulated the uncoating process of EV71 entry step in a pH-dependent manner. Our results aid in the understanding of how host factors influence EV71 life cycle and provide new potential targets for developing antiviral agents against EV71 infection.

Stearoyl coenzyme A desaturase 1 is associated with hepatitis C virus replication complex and regulates viral replication

The hepatitis C virus (HCV) life cycle is tightly regulated by lipid metabolism of host cells. In order to identify host factors involved in HCV propagation, we have recently screened a small interfering RNA (siRNA) library targeting host genes that control lipid metabolism and lipid droplet formation using cell culture-grown HCV (HCVcc)-infected cells. We selected and characterized the gene encoding stearoyl coenzyme A (CoA) desaturase 1 (SCD1). siRNA-mediated knockdown or pharmacological inhibition of SCD1 abrogated HCV replication in both subgenomic replicon and Jc1-infected cells, while exogenous supplementation of either oleate or palmitoleate, products of SCD1 activity, resurrected HCV replication in SCD1 knockdown cells. SCD1 was coimmunoprecipitated with HCV nonstructural proteins and colocalized with both double-stranded RNA (dsRNA) and HCV nonstructural proteins, indicating that SCD1 is associated with HCV replication complex. Moreover, SCD1 was fractionated and enriched with HCV nonstructural proteins at detergent-resistant membrane. Electron microscopy data showed that SCD1 is required for NS4B-mediated intracellular membrane rearrangement. These data further support the idea that SCD1 is associated with HCV replication complex and that its products may contribute to the proper formation and maintenance of membranous web structures in HCV replication complex. Collectively, these data suggest that manipulation of SCD1 activity may represent a novel host-targeted antiviral strategy for the treatment of HCV infection.

IMPORTANCE

Stearoyl coenzyme A (CoA) desaturase 1 (SCD1), a liver-specific enzyme, regulates hepatitis C virus (HCV) replication through its enzyme activity. HCV nonstructural proteins are associated with SCD1 at detergent-resistant membranes, and SCD1 is enriched on the lipid raft by HCV infection. Therein, SCD1 supports NS4B-mediated membrane rearrangement to provide a suitable microenvironment for HCV replication. We demonstrated that either genetic or chemical knockdown of SCD1 abrogated HCV replication in both replicon cells and HCV-infected cells. These findings provide novel mechanistic insights into the roles of SCD1 in HCV replication.

Benzhydrylpiperazine compounds inhibit cholesterol-dependent cellular entry of hepatitis C virus

Hepatitis C virus (HCV) remains a serious global health problem that lacks an effective cure. Although the introduction of protease inhibitors to the current standard-of-care interferon/ribavirin therapy for HCV infection has improved sustained virological response of genotype 1-infected patients, these inhibitors exacerbate already problematic side effects. Thus, new HCV antivirals are urgently needed. Using a cell-protection screen previously developed in our laboratory, we evaluated 30,426 compounds for inhibitors of potentially any stage of the HCV life cycle and identified 49 new HCV inhibitors. The two most potent hits, hydroxyzine and chlorcyclizine, belong to the family of benzhydrylpiperazines and were found to inhibit the entry of cell culture-produced HCV with IC50 values of 19 and 2.3 nM, respectively, and therapeutic indices of >500 and >6500. Both compounds block HCV entry at a late stage of entry prior to viral fusion and their inhibitory activities are highly dependent on the host's cholesterol content. Both compounds are currently used in the clinic for treating allergy-related disorders and the reported peak plasma level (160 nM) and estimated liver concentration (1.7 μM) of hydroxyzine in humans are much higher than the molecule's anti-HCV IC90 in cell culture (64 nM). Further studies are therefore justified to evaluate the use of these molecules in an anti-HCV therapeutic regimen.

Calcineurin inhibitors stimulate and mycophenolic acid inhibits replication of hepatitis E virus

BACKGROUND & AIMS

Many recipients of organ transplants develop chronic hepatitis, due to infection with the hepatitis E virus (HEV). Although chronic HEV infection is generally associated with immunosuppressive therapies, little is known about how different immunosuppressants affect HEV infection.

METHODS

A subgenomic HEV replication model, in which expression of a luciferase reporter gene is measured, and a full-length infection model were used. We studied the effects of different immunosuppressants, including steroids, calcineurin inhibitors (tacrolimus [FK506] and cyclosporin A), and mycophenolic acid (MPA, an inhibitor of inosine monophosphate dehydrogenase) on HEV replication in human hepatoma cell line Huh7. Expression of cyclophilins A and B (the targets of cyclosporin A) were knocked down using small hairpin RNAs.

RESULTS

Steroids had no significant effect on HEV replication. Cyclosporin A promoted replication of HEV in the subgenomic and infectious models. Knockdown of cyclophilin A and B increased levels of HEV genomic RNA by 4.0- ± 0.6-fold and 7.2- ± 1.9-fold, respectively (n = 6; P < .05). A high dose of FK506 promoted infection of liver cells with HEV. In contrast, MPA inhibited HEV replication. Incubation of cells with guanosine blocked the antiviral activity of MPA, indicating that the antiviral effects of this drug involve nucleotide depletion. The combination of MPA and ribavirin had a greater ability to inhibit HEV replication than MPA or ribavirin alone.

CONCLUSIONS

Cyclophilins A and B inhibit replication of HEV; this might explain the ability of cyclosporin A to promote HEV infection. On the other hand, the immunosuppressant MPA inhibits HEV replication. These findings should be considered when physicians select immunosuppressive therapies for recipients of organ transplants who are infected with HEV.

Inhibition of HCV replication by cyclophilin antagonists is linked to replication fitness and occurs by inhibition of membranous web formation

BACKGROUND & AIMS

Replication of hepatitis C virus (HCV) requires host cell factors, such as cyclophilin A (CypA). CypA binds to HCV's nonstructural protein (NS)5A to promote replication of viral RNA. CypA antagonists, such as cyclosporines, are potent inhibitors of HCV replication. NS2 modulates sensitivity of HCV to cyclosporines. We investigated why cyclosporines require NS2 to increase their inhibitory effect and how they block HCV replication.

METHODS

We determined replication fitness and sensitivity of various HCV replicons, containing or lacking NS2, to cyclosporine and other direct-acting antiviral agents. We also analyzed the effects of cyclosporine on membranous web formation by electron microscopy.

RESULTS

NS2-5B replicons of genotype 2a (JFH1), but not genotype 1b, had increased sensitivity to cyclosporine. This difference was lost with replication-attenuated NS3-5B JFH1 RNAs, showing that cyclosporine sensitivity is linked to reduced replication fitness of NS2-containing HCV RNAs. Fitness also determined sensitivity to a nucleoside analogue and an NS5A inhibitor, but not to telaprevir. Cyclosporine blocked de novo formation of the membranous web, but had little effect on established membranous replication factories. This block was prevented by cyclosporine resistance mutations in NS5A.

CONCLUSIONS

Cleavage at the NS2/3 junction is a rate-limiting step in replication of particular HCV isolates and determines their sensitivity to CypA inhibitors. These drugs target de novo formation of the membranous web and RNA replication.

The clinically approved drugs amiodarone, dronedarone and verapamil inhibit filovirus cell entry

Objectives

Filoviruses such as Ebola virus and Marburg virus cause a severe haemorrhagic fever syndrome in humans for which there is no specific treatment. Since filoviruses use a complex route of cell entry that depends on numerous cellular factors, we hypothesized that there may be drugs already approved for human use for other indications that interfere with signal transduction or other cellular processes required for their entry and hence have anti-filoviral properties.

Methods

We used authentic filoviruses and lentiviral particles pseudotyped with filoviral glycoproteins to identify and characterize such compounds.

Results

We discovered that amiodarone, a multi-ion channel inhibitor and adrenoceptor antagonist, is a potent inhibitor of filovirus cell entry at concentrations that are routinely reached in human serum during anti-arrhythmic therapy. A similar effect was observed with the amiodarone-related agent dronedarone and the L-type calcium channel blocker verapamil. Inhibition by amiodarone was concentration dependent and similarly affected pseudoviruses as well as authentic filoviruses. Inhibition of filovirus entry was observed with most but not all cell types tested and was accentuated by the pre-treatment of cells, indicating a host cell-directed mechanism of action. The New World arenavirus Guanarito was also inhibited by amiodarone while the Old World arenavirus Lassa and members of the Rhabdoviridae (vesicular stomatitis virus) and Bunyaviridae (Hantaan) families were largely resistant.

Conclusions

The ion channel blockers amiodarone, dronedarone and verapamil inhibit filoviral cell entry.

A novel porcine reproductive and respiratory syndrome virus vector system that stably expresses enhanced green fluorescent protein as a separate transcription unit

Here we report the rescue of a recombinant porcine reproductive and respiratory syndrome virus (PRRSV) carrying an enhanced green fluorescent protein (EGFP) reporter gene as a separate transcription unit. A copy of the transcription regulatory sequence for ORF6 (TRS6) was inserted between the N protein and 3′-UTR to drive the transcription of the EGFP gene and yield a general purpose expression vector. Successful recovery of PRRSV was obtained using an RNA polymerase II promoter to drive transcription of the full-length virus genome, which was assembled in a bacterial artificial chromosome (BAC). The recombinant virus showed growth replication characteristics similar to those of the wild-type virus in the infected cells. In addition, the recombinant virus stably expressed EGFP for at least 10 passages. EGFP expression was detected at approximately 10 h post infection by live-cell imaging to follow the virus spread in real time and the infection of neighbouring cells occurred predominantly through cell-to-cell-contact. Finally, the recombinant virus generated was found to be an excellent tool for neutralising antibodies and antiviral compound screening. The newly established reverse genetics system for PRRSV could be a useful tool not only to monitor virus spread and screen for neutralising antibodies and antiviral compounds, but also for fundamental research on the biology of the virus.

Cyclophilin inhibitors as antiviral agents

Cyclophilins (Cyps) are ubiquitous proteins that effect the cis-trans isomerization of Pro amide bonds, and are thus crucial to protein folding. CypA is the most prevalent of the ~19 human Cyps, and plays a crucial role in viral infectivity, most notably for HIV-1 and HCV. Cyclophilins have been shown to play key roles in effective replication of a number of viruses from different families. A drug template for CypA inhibition is cyclosporine A (CsA), a cyclic undecapeptide that simultaneously binds to both CypA and the Ca(2+)-dependent phosphatase calcineurin (CN), and can attenuate immune responses. Synthetic modifications of the CsA scaffold allows for selective binding to CypA and CN separately, thus providing access to novel, non-immunosuppressive antiviral agents.

Hepatitis C virus proteins: from structure to function

Great progress has been made over the past years in elucidating the structure and function of the hepatitis C virus (HCV) proteins, most of which are now actively being pursued as antiviral targets. The structural proteins, which form the viral particle, include the core protein and the envelope glycoproteins E1 and E2. The nonstructural proteins include the p7 viroporin, the NS2 protease, the NS3-4A complex harboring protease and NTPase/RNA helicase activities, the NS4B and NS5A proteins, and the NS5B RNA-dependent RNA polymerase. NS4B is a master organizer of replication complex formation while NS5A is a zinc-containing phosphoprotein involved in the regulation of HCV RNA replication versus particle production. Core to NS2 make up the assembly module while NS3 to NS5B represent the replication module (replicase). However, HCV proteins exert multiple functions during the viral life cycle, and these may be governed by different structural conformations and/or interactions with viral and/or cellular partners. Remarkably, each viral protein is anchored to intracellular membranes via specific determinants that are essential to protein function in the cell. This review summarizes current knowledge of the structure and function of the HCV proteins and highlights recent advances in the field.

Cyclophilins as modulators of viral replication

Cyclophilins are peptidyl‐prolyl cis/trans isomerases important in the proper folding of certain proteins. Mounting evidence supports varied roles of cyclophilins, either positive or negative, in the life cycles of diverse viruses, but the nature and mechanisms of these roles are yet to be defined. The potential for cyclophilins to serve as a drug target for antiviral therapy is evidenced by the success of non-immunosuppressive cyclophilin inhibitors (CPIs), including Alisporivir, in clinical trials targeting hepatitis C virus infection. In addition, as cyclophilins are implicated in the predisposition to, or severity of, various diseases, the ability to specifically and effectively modulate their function will prove increasingly useful for disease intervention. In this review, we will summarize the evidence of cyclophilins as key mediators of viral infection and prospective drug targets.

DNA methyltransferases 1 and 3B are required for hepatitis C virus infection in cell culture

DNA methyltransferases (DNMTs) are responsible for establishing and maintaining DNA methylation, which are dysregulated in hepatitis C virus (HCV)-associated hepatocellular carcinoma (HCC). In this report, using lentivirus-mediated shRNA interference technology, we identified DNMT1 and DNMT3B as host factors involved in HCV propagation. Our results demonstrated that down-regulation of DNMT1 or DNMT3B expression in Huh7.5.1 cells severely impaired cell culture-produced HCV (HCVcc) infection. Furthermore, knockdown of DNMT1 or DNMT3B did not affect HCV entry and internal ribosome entry site (IRES)-directed translation but did inhibit subgenomic replication. In contrast, knockdown of DNMT3A had no significant effect on HCV infection, entry, translation, or replication, which suggested that DNMT3A did not play a significant role in HCV life cycle. Moreover, we showed that DNMT inhibitors 5-Aza-C and 5-Aza-dC significantly suppressed HCVcc infection, viral RNA replication, and protein expression. These results suggest that DNMTs are critical for HCV replication and may represent potent targets for the treatment of HCV infection.

Optimization of Cyclophilin Inhibitors for Use in Antiviral Therapy

Cyclophilins are members of the Propyl Peptidase Isomerase (PPIase) family of proteins and have recently been found to be required for efficient replication and/or infectivity of several viruses. Cyclosporine A (CsA), the prototypical inhibitor of cyclophilins has shown good activity against several key viruses, including HIV‐1 and HCV, however the immunosuppressive activity of CsA precludes its use as an effective anti‐viral agent. Structural information derived from the ternary complex formed by CsA, cyclophilin A and calcineurin has allowed the design of non‐immunosuppressive derivatives of CsA that retain, and in some cases improve, antiviral activity toward hepatitis C. Chemical modification of CsA has led to compounds with improved pharmacokinetic properties and with reduced drug‐drug interaction potential. Non‐CsA derived inhibitors of cyclophilin A have recently been identified and hold promise as synthetically more tractable leads for cyclophilin‐based discovery projects.

Emerging therapeutic targets for hepatitis C virus infection

Therapy for hepatitis C virus (HCV) is a rapidly evolving field wherein traditional treatment with the nonspecific antiviral agents pegylated interferon (IFN)-alfa and ribavirin has been and will continue to be supplanted by combinations of targeted therapies against HCV with and without concomitant pegylated IFN and/or ribavirin, resulting in markedly superior rates of viral clearance. Exhaustive study of HCV structure and replication through the development of in vitro systems has enabled the development of numerous novel direct acting antiviral agents that currently are undergoing clinical trials. As our understanding of the HCV virus and its antiviral targets increases, the future of HCV therapy holds the promise of high rates of viral eradication in all patient populations, many or all of whom will be treatable with IFN-free combinations of all-oral agents.

Subtype specific differences in NS5A domain II reveals involvement of proline at position 310 in cyclosporine susceptibility of hepatitis C virus

Hepatitis C virus (HCV) is susceptible to cyclosporine (CsA) and other cyclophilin (CypA) inhibitors, but the genetic basis of susceptibility is controversial. Whether genetic variation in NS5A alters cell culture susceptibility of HCV to CypA inhibition is unclear. We constructed replicons containing NS5A chimeras from genotypes 1a, 2a and 4a to test how variation in carboxy terminal regions of NS5A altered the genotype 1b CsA susceptibility. All chimeric replicons including genotype 1b Con1LN-wt replicon exhibited some cell culture sensitivity to CsA with genotype 4a being most sensitive and 1a the least. The CypA binding pattern of truncated NS5A genotypes correlated with the susceptibility of these replicons to CsA. The Con1LN-wt replicon showed increased susceptibility towards CsA when proline at position 310P was mutated to either threonine or alanine. Furthermore, a 15 amino acid long peptide fused N terminally to GFP coding sequences confirmed involvement of proline at 310 in CypA binding. Our findings are consistent with CypA acting on multiple prolines outside of the previously identified CypA binding sites. These results suggest multiple specific genetic variants between genotype 1a and 1b in the C-terminus of NS5A alter the CsA susceptibility of replicons, and some variants may oppose the effects of others.

Curing a viral infection by targeting the host: the example of cyclophilin inhibitors

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Viruses exploit multiple host cell mechanisms for their own replication.

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These mechanisms may serve as targets for antiviral therapy.

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Host-targeted therapies may have a high barrier to resistance.

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Cyclophilin inhibitors have shown promise in curing chronic hepatitis C.

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Cyclophilin inhibitors may potentially be used to treat other viral infections.

Every step of the viral life cycle is dependent on the host, which potentially can be explored for antiviral targets. Historically, however, drug discovery has focused mainly on viral targets, because of their perceived specificity. Efforts to pursue host targets have been largely hampered by concern over potential on-target toxicity, the lack of predictive cell culture and animal models, and the complexity of host–virus interactions. On the other hand, there are distinct advantages of targeting the host, such as creating a high barrier to resistance, providing broad coverage of different genotypes/serotypes and possibly even multiple viruses, and expanding the list of potential targets, when druggable viral targets are limited. Taking hepatitis C virus (HCV) as the example, there are more than 20 inhibitors of the viral protease, polymerase and NS5A protein currently in advanced clinical testing. However, resistance has become a main challenge with these direct-acting antivirals, because HCV, an RNA virus, is notoriously prone to mutation, and a single mutation in the viral target may prevent the binding of an inhibitor, and rendering it ineffective. Host cyclophilin inhibitors have shown promising effects both in vitro and in patients to prevent the emergence of resistance and to cure HCV infection, either alone or in combination with other agents. They are also capable of blocking the replication of a number of other viral pathogens. While the road to developing host-targeting antivirals has been less traveled, and significant challenges remain, delivering the most effective antiviral regimen, which may comprise inhibitors of both host and viral targets, should be well worth the effort.

Cyclophilin involvement in the replication of hepatitis C virus and other viruses

In recent months, there has been a wealth of promising clinical data suggesting that a more effective treatment regimen, and potentially a cure, for hepatitis C virus (HCV) infection is close at hand. Leading this push are direct-acting antivirals (DAAs), currently comprising inhibitors that target the HCV protease NS3, the viral polymerase NS5B, and the non-structural protein NS5A. In combination with one another, along with the traditional standard-of-care ribavirin and PEGylated-IFNα, these compounds have proven to afford tremendous efficacy to treatment-naíve patients, as well as to prior non-responders. Nevertheless, by targeting viral components, the possibility of selecting for breakthrough and treatment-resistant virus strains remains a concern. Host-targeting antivirals are a distinct class of anti-HCV compounds that is emerging as a complementary set of tools to combat the disease. Cyclophilin (Cyp) inhibitors are one such group in this category. In contrast to DAAs, Cyp inhibitors target a host protein, CypA, and have also demonstrated remarkable antiviral efficiency in clinical trials, without the generation of viral escape mutants. This review serves to summarize the current literature on Cyps and their relation to the HCV viral life cycle, as well as other viruses.

Phenotypic analysis of NS5A variant from liver transplant patient with increased cyclosporine susceptibility

Hepatitis C virus (HCV) replication is limited by cyclophilin inhibitors but it remains unclear how viral genetic variations influence susceptibility to cyclosporine (cyclosporine A, CsA), a cyclophilin inhibitor. In this study HCV from liver transplant patients was sequenced before and after CsA exposure. Phenotypic analysis of NS5A sequence was performed by using HCV sub genomic replicon to determine CsA susceptibility. The data indicates an atypical proline at position 328 in NS5A causes increases CsA sensitivity both in the context of genotype 1a and 1b residues. Point mutants mimicking other naturally occurring residues at this position also increased (Ala) or decreased (Arg) replicon sensitivity to CsA relative to the typical threonine (genotype 1a) or serine (genotype 1b) at this position. This work has implications for treatment of HCV by cyclophilin inhibitors.

Is there a role for cyclophilin inhibitors in the management of primary biliary cirrhosis?

Autoimmune hepatitis (AIH) and primary biliary cirrhosis (PBC) are poorly understood autoimmune liver diseases. Immunosuppression is used to treat AIH and ursodeoxycholic acid is used to slow the progression of PBC. Nevertheless, a proportion of patients with both disorders progress to liver failure. Following liver transplantation, up to a third of patients with PBC experience recurrent disease. Moreover a syndrome referred to as "de novo AIH" occurs in a proportion of patients regardless of maintenance immunosuppression, who have been transplanted for disorders unrelated to AIH. Of note, the use of cyclosporine A appears to protect against the development of recurrent PBC and de novo AIH even though it is a less potent immunosuppressive compared to tacrolimus. The reason why cyclosporine A is protective has not been determined. However, a virus resembling mouse mammary tumor virus (MMTV) has been characterized in patients with PBC and AIH. Accordingly, we hypothesized that the protective effect of cyclosporine A in liver transplant recipients may be mediated by the antiviral activity of this cyclophilin inhibitor. Treatment of the MMTV producing MM5MT cells with different antivirals and immunosuppressive agents showed that both cyclosporine A and the analogue NIM811 inhibited MMTV production from the producer cells. Herein, we discuss the evidence supporting the role of MMTV-like human betaretrovirus in the development of PBC and de novo AIH and speculate on the possibility that the agent may be associated with disease following transplantation. We also review the mechanisms of how both cyclosporine A and NIM811 may inhibit betaretrovirus production in vitro.

Suppression of viral RNA binding and the assembly of infectious hepatitis C virus particles in vitro by cyclophilin inhibitors

Nonstructural protein 5A (NS5A) of hepatitis C virus (HCV) is an indispensable component of the HCV replication and assembly machineries. Although its precise mechanism of action is not yet clear, current evidence indicates that its structure and function are regulated by the cellular peptidylprolyl isomerase cyclophilin A (CyPA). CyPA binds to proline residues in the C-terminal half of NS5A, in a distributed fashion, and modulates the structure of the disordered domains II and III. Cyclophilin inhibitors (CPIs), including cyclosporine (CsA) and its nonimmunosuppressive derivatives, inhibit HCV infection of diverse genotypes, both in vitro and in vivo. Here we report a mechanism by which CPIs inhibit HCV infection and demonstrate that CPIs can suppress HCV assembly in addition to their well-documented inhibitory effect on RNA replication. Although the interaction between NS5A and other viral proteins is not affected by CPIs, RNA binding by NS5A in cell culture-based HCV (HCVcc)-infected cells is significantly inhibited by CPI treatment, and sensitivity of RNA binding is correlated with previously characterized CyPA dependence or CsA sensitivity of HCV mutants. Furthermore, the difference in CyPA dependence between a subgenomic and a full-length replicon of JFH-1 was due, at least in part, to an additional role that CyPA plays in HCV assembly, a conclusion that is supported by experiments with the clinical CPI alisporivir. The host-directed nature and the ability to interfere with more than one step in the HCV life cycle may result in a higher genetic barrier to resistance for this class of HCV inhibitors.

New targets for treatment against HCV infection

Owing to the tremendous effort from both academia and industry, drug development for hepatitis C virus (HCV) infection has been flourishing, with a range of pipeline compounds at various stages of development. Although combination of the recently launched serine protease inhibitors will further improve the response rate of current interferon-based therapy, some intrinsic limitations of these compounds and the tendency of resistance development by the virus, urge the development of alternative or additional therapeutic strategies. In this article we provide an overview of different host and viral factors which have emerged as new potential targets for therapeutic intervention using state-of-the-art technologies.