New treatments for chronic hepatitis C: An overview for paediatricians

Pegylated interferon(IFN)α-2a or 2b in combination with ribavirin for children aged 3 years and older is the standard treatment for paediatric chronic hepatitis C.This treatment regimen was developed firstly in adults.In recent years,a number of direct-acting antiviral agents(DAAs)are under development for treatment of chronic hepatitis C virus(HCV)infection.These agents block viral replication inhibiting directly one of the several steps of HCV lifecycle.DAAs are classified into several categories based on their molecular target:HCV NS3/4A protease inhibitors,HCV NS5B polymerase inhibitors and HCV NS5A inhibitors.Other promising compounds are cyclophilin A inhibitors,mi-RNA122and IFN-λ.Several new drugs associations will be developed in the near future starting from the actual standard of care.IFN-based and IFN-free regimens are being studied in adults.In this constantly evolving scenario new drug regimens targeted and suitable for children would be possible in the next future.Especially for children,it is crucial to identify the right combination of drugs with the highest potency,barrier toresistance and the best safety profile.     

Antiviral strategies in hepatitis C virus infection

Resolution of the three-dimensional structures of several hepatitis C virus (HCV) proteins, together with the development of replicative cell culture systems, has led to the identification of a number of potential targets for direct-acting antiviral (DAA) agents. Numerous families of drugs that potently inhibit the HCV lifecycle in vitro have been identified, and some of these molecules have reached early to late clinical development. Two NS3/4A protease inhibitors, telaprevir and boceprevir, were approved in Europe and the United States in 2011 in combination with pegylated interferon (IFN)-α and ribavirin for the treatment of chronic hepatitis C related to HCV genotype 1, in both treatment-na?ve and treatment-experienced patients. Sustained virological response rates in the range of 6675% and 5966% (2988% if the response to the first course of therapy is taken into account) have been achieved in these two patient populations, respectively, with treatment durations of 24 to 48 weeks. A number of other DAAs are at the clinical developmental stage in combination with pegylated IFN-α and ribavirin or with other DAAs in IFN-free regimens, with or without ribavirin. They include second-wave, first-generation, and second-generation NS3/4A protease inhibitors, nucleoside/nucleotide analogue inhibitors and non-nucleoside inhibitorsof HCVRNA-dependent RNA polymerase, inhibitors of nonstructural protein 5A (NS5A) and host-targeted compounds, such as cyclophilin inhibitors and silibinin. The proof of concept that IFN-free regimens may lead to HCV eradication has recently been brought. However, new drugs may be associated with troublesome side effects and drugdrug interactions, and the ideal IFN-free DAA combination remains to be found.     

The Future of HCV Therapy: NS4B as an Antiviral Target

Chronic hepatitis C virus (HCV) infection is a major worldwide cause of liver disease, including cirrhosis and hepatocellular carcinoma. It is estimated that more than 170 million individuals are infected with HCV, with three to four million new cases each year. The current standard of care, combination treatment with interferon and ribavirin, eradicates the virus in only about 50% of chronically infected patients. Notably, neither of these drugs directly target HCV. Many new antiviral therapies that specifically target hepatitis C (e.g. NS3 protease or NS5B polymerase inhibitors) are therefore in development, with a significant number having advanced into clinical trials. The nonstructural 4B (NS4B) protein, is among the least characterized of the HCV structural and nonstructural proteins and has been subjected to few pharmacological studies. NS4B is an integral membrane protein with at least four predicted transmembrane (TM) domains. A variety of functions have been postulated for NS4B, such as the ability to induce the membranous web replication platform, RNA binding and NTPase activity. This review summarizes potential targets within the nonstructural protein NS4B, with a focus on novel classes of NS4B inhibitors.     

The role of resistance in HCV treatment

The recent development of small molecule compounds that directly inhibit the viral life cycle represents a major milestone for the treatment of chronic hepatitis C virus (HCV) infection. These new drugs that are collectively termed direct-acting antivirals (DAA) include a range of inhibitors of the non-structural (NS) 3/4A protease, NS5B polymerase and NS5A protein. Two NS3/4A protease inhibitors (boceprevir and telaprevir) in combination with pegylated interferon and ribavirin have now been approved for the treatment of chronic HCV genotype 1 infection and cure rates could be increased by 20–30%. However, the majority of DAAs is still in early clinical development. The rapid replication rate of HCV, along with the error-prone polymerase activity leads to a high genetic diversity among HCV virions that includes mutants with reduced susceptibility to DAA-therapy. These resistance-associated variants often occur at very low frequencies. However, during DAA-based treatment, rapid selection of resistance mutations may occur, eventually leading to viral break-through. A number of variants with different levels of resistance have been described in vitro and in vivo for virtually all DAAs. We review the parameters that determine DAA resistance as well as the clinical implications of resistance testing. In addition, the most recent literature and conference data on resistance profiles of DAAs in clinical development and future strategies to avoid the emergence of viral resistance are also discussed.     

The therapeutic approaches for hepatitis C virus: protease inhibitors and polymerase inhibitors

The current standard of care for hepatitis C infection is peginterferon/ribavirin (PegIFN/RBV). We are entering the era where direct-acting antiviral agents (DAAs) will be added to PegIFN/RBV, leading to higher sustained response rates in genotype 1 infected individuals. Currently DAAs are directed toward specific proteins involved in hepatitis C replication with NS3/NS4A protease inhibitors furthest in development. Telaprevir and boceprevir are both NS3/NS4a inhibitors that significantly improve sustained response when added to PegIFN and RBV. The hepatitis C virus (HCV) polymerase inhibitors are another promising DAA class. These molecules are divided into nucleoside/nucleotide polymerase inhibitors and nonnucleotide/nucleoside polymerase inhibitors. Nucleoside/nucleotide polymerase inhibitors have a high barrier to resistance and appear to be effective across a broad range of genotypes. Nonnucleoside polymerase inhibitors have a lower barrier of resistance and appear to be genotype specific. Preliminary data with these compounds are also promising. A third class, NS5A inhibitors, has also shown potent HCV RNA suppression in preliminary studies as monotherapy and with PegIFN and RBV. Combinations of these agents are also entering clinical trials and indeed a preliminary report has demonstrated that the combination of an NS3/4A protease inhibitor and NS5B polymerase inhibitor can effectively suppress virus in genotype 1 individuals. Future studies will concentrate on combinations of direct-acting antiviral agents without and with PegIFN and RBV. Clinicians will need to be familiar with managing side effects as well as resistance as we enter this new era.     

Hepatitis C virus NS5A inhibitors and drug resistance mutations

Some direct-acting antiviral agents for hepatitis C virus(HCV),such as telaprevir and boceprevir have been available since 2011.It was reported that HCV NS5A is associated with interferon signaling related to HCV replication and hepatocarcinogenesis.HCV NS5A inhibitors efficiently inhibited HCV replication in vitro.Human studies showed that dual,triple and quad regimens with HCV NS5A inhibitors,such as daclatasvir and ledipasvir,in combination with other direct-acting antiviral agents against other regions of HCV with or without peginterferon/ribavirin,could efficiently inhibit HCV replication according to HCV genotypes.These combinations might be a powerful tool for"difficult-to-treat"HCV-infected patients."First generation"HCV NS5A inhibitors such as daclatasvir,ledipasvir and ABT-267,which are now in phaseⅢclinical trials,could result in resistance mutations."Second generation"NS5A inhibitors such as GS-5816,ACH-3102,and MK-8742,have displayed improvements in the genetic barrier while maintaining potency.HCV NS5A inhibitors are safe at low concentrations,which make them attractive for use despite low genetic barriers,although,in fact,HCV NS5A inhibitors should be used with HCV NS3/4A inhibitors,HCV NS5B inhibitors or peginterferon plus ribavirin.This review article describes HCV NS5A inhibitor resistance mutations and recommends that HCV NS5A inhibitors be used in combination regimens potent enough to prevent the emergence of resistant variants.     

Resistance to Direct-Acting Antivirals

Viral resistance corresponds to the selection, during treatment, of pre-existing viral variants less susceptible to the drug??s inhibitory activity because they bear amino acid substitutions altering the drug target. Hepatitis C virus (HCV) drugs in development can be split into two groups according to their barrier to resistance. Direct-acting antiviral drugs with a low barrier to resistance include first-generation NS3-4A protease inhibitors, non-nucleoside inhibitors of HCV RNA-dependent RNA polymerase and first-generation NS5A inhibitors. HCV drugs with a high barrier to resistance include nucleoside/nucleotide analogues, possibly second-generation protease and NS5A inhibitors, and host-targeted agents, such as cyclophilin inhibitors or microRNA-122 antagonists. This article reviews recent findings that add to our knowledge and understanding of HCV resistance to direct-acting antiviral drugs and discusses them in the context of new therapeutic strategies, with and without pegylated interferon-?? and/or ribavirin.     

Second Generation Protease Inhibitors and Nucleotide Inhibitors

The hepatitis C virus (HCV) protease and polymerase inhibitors are in rapid phases of development. Following closely behind the approval of the inhibitors of the HCV RNA NS3/4A protease, boceprevir and telaprevir, include both a) more potent protease inhibitors and b) the development of the viral NS5B RNA-dependent RNA polymerase inhibitors. Protease inhibitors generally have a low barrier to resistance and extensive cross-resistance between agents. NS5B polymerase inhibitors are generally considered to have a high barrier to resistance and the potential for use across all genotypes. However, in the absence of ribavirin or of other direct-acting antivirals these agents may predispose to the selection of resistant variants or for viral relapse following treatment completion. Optimal combination of agents and therapy durations remain major challenges in clinical research.     

Emerging Therapies for Hepatitis C

The combination of pegylated interferon (PEG-IFN) and ribavirin (RBV), the current therapy for hepatitis C virus (HCV) infection, has saved the lives of many HCV-infected patients. Direct-acting antivirals (DAAs) target several sites of HCV nonstructural proteins, resulting in the cessation of viral replication. The first NS3/4A protease inhibitors consisted of boceprevir and telaprevir, which have shown superior efficacy against genotype 1 HCV infection when combined with PEG-IFN/RBV compared with the standard therapy in both treatment-naive and -experienced patients. Simeprevir, faldaprevir, and asunaprevir are second-wave, first-generation NS3/4A inhibitors that have already been or will soon be approved. Second-generation protease inhibitors are in clinical trials. Daclatasvir is the first approved DAA belonging to the class of NS5A replication complex inhibitors. The potency of daclatasvir is very high, and this drug is an important and essential component of combination regimens for all genotypes. Sofosbuvir, the first approved NS5B polymerase inhibitor, is characterized by high potency and genetic barriers to resistance. Sofosbuvir combined with RBV achieved an interferon-free regimen in genotype 2 or 3 patients with a reduced treatment duration. It can also be used in combination with PEG-IFN/RBV in genotype 1 patients for 12 weeks. DAAs have provided new hope for curing HCV infections with a short treatment duration and acceptable adverse events.     

Virologic Tools for HCV Drug Resistance Testing

Recent advances in molecular biology have led to the development of new antiviral drugs that target specific steps of the Hepatitis C Virus (HCV) lifecycle. These drugs, collectively termed direct-acting antivirals (DAAs), include non-structural (NS) HCV protein inhibitors, NS3/4A protease inhibitors, NS5B RNA-dependent RNA polymerase inhibitors (nucleotide analogues and non-nucleoside inhibitors), and NS5A inhibitors. Due to the high genetic variability of HCV, the outcome of DAA-based therapies may be altered by the selection of amino-acid substitutions located within the targeted proteins, which affect viral susceptibility to the administered compounds. At the drug developmental stage, preclinical and clinical characterization of HCV resistance to new drugs in development is mandatory. In the clinical setting, accurate diagnostic tools have become available to monitor drug resistance in patients who receive treatment with DAAs. In this review, we describe tools available to investigate drug resistance in preclinical studies, clinical trials and clinical practice.     

Specifically targeted antiviral therapy for hepatitis C virus

Hepatitis C virus （HCV） infection affects 180 million people worldwide with the predominant prevalence being infection with genotype 1, followed by genotypes 2 and 3. Standard anti-HCV therapy currently aims to enhance natural immune responses to the virus, whereas new therapeutic concepts directly target HCV RNA and viral enzymes or influence host-virus interactions. Novel treatment options now in development are focused on inhibitors of HCV- specific enzymes, NS3 protease and NS5B polymerase. These agents acting in concert represent the concept of specifically targeted antiviral therapy for HCV （STAT-C）. STAT-C is an attractive strategy in which the main goal is to increase the effectiveness of antiviral responses across all genotypes, with shorter treatment duration and better tolerability. However, the emergence of resistant mutations that limit the use of these compounds in monotherapy complicates the regimens. Thus, a predictable scenario for HCV treatment in the future will be combinations of drugs with distinct mechanisms of action. For now, it seems that interferon will remain a fundamental component of any new anti-HCV therapeutic regimens in the near future; therefore, there is pressure to develop forms of interferon that are more effective, less toxic, and more convenient than pegylated interferon.     

Antiviral activity of the hepatitis C virus polymerase inhibitor filibuvir in genotype 1-infected patients

More effective and better-tolerated therapies are needed for chronic hepatitis C virus (HCV) infection. Among the direct-acting anti-HCV agents in development is the nonstructural 5B protein (NS5B polymerase) non-nucleoside inhibitor filibuvir. We investigated the antiviral activity, pharmacokinetics, safety, and tolerability of multiple doses of filibuvir in treatment-naive and treatment-experienced patients who were chronically infected with HCV genotype 1 in two phase 1b clinical studies (study 1 was a randomized, placebo-controlled dose escalation study and study 2 was a nonrandomized, open-label study). The filibuvir doses evaluated ranged from 200-1400 mg daily, and the duration of dosing ranged from 3-10 days. Genotypic changes in the NS5B nucleotide sequence following short-term filibuvir therapy were also assessed. Filibuvir potently inhibited viral replication in a dose-dependent manner. Mean maximum HCV RNA change from baseline ranged from -0.97 log(10) IU/mL with filibuvir given at 100 mg twice daily to -2.30 log(10) IU/mL with filibuvir given at 700 mg twice daily in treatment-naive patients. In treatment-experienced patients, an HCV RNA reduction of 2.20 log(10) IU/mL was achieved with filibuvir given at 450 mg twice daily. Filibuvir was well tolerated in both studies. Adverse events were mild or moderate in severity. No discontinuations, serious adverse events, or deaths were reported. NS5B sequencing identified residue 423 as the predominant site of mutation after filibuvir dosing. CONCLUSION: Filibuvir administration resulted in significant reductions in HCV RNA concentrations at doses that were well tolerated in patients infected with HCV genotype 1. Filibuvir is currently being evaluated in combination with pegylated interferon alfa 2a plus ribavirin in treatment-naive patients.     

Boceprevir and telaprevir in the management of hepatitis C virus-infected patients

Recent approval of direct-acting antiviral agents (DAAs) against hepatitis C virus (HCV) offers a major advance in the management of HCV infection. These DAAs, boceprevir and telaprevir, when given with pegylated interferon alfa (Peg-IFN) and ribavirin (RBV), result in a much higher sustained virologic response rate compared with Peg-IFN and RBV. The DAA-containing regimens are approved for HCV genotype 1 infection in HCV treatment-naive and HCV treatment-experienced patients. In this review, we present an overview of pharmacology, efficacy, adverse events, and emergence of resistance-associated variants with the use of these agents. As with all drugs, especially newly approved drugs, clinicians must consult the package insert for detailed prescribing information, list of all reported adverse events, contraindications, and drug interactions.     

慢性丙型肝炎直接抗病毒药物的研究进展和应用前景

针对HCV感染的标准治疗方案是聚乙二醇干扰素α联合利巴韦林。按此治疗方案部分HCV感染者是可治愈的。但仍有一小部分患者不能治愈。随着分子生物学的进展,针对HCV生活周期中病毒蛋白靶向特异性治疗的许多小分子化合物的研究得到了迅速发展,提高了抗病毒的疗效。这些药物统一命名为抗HCV的直接抗病毒药物(direct-acting antiviralagents,DAAs),包括非结构蛋白(non-structural protein,NS)3/4A蛋白酶抑制剂、NS5B聚合酶抑制剂和NS5A蛋白抑制剂等。本文对慢性丙型肝炎DAAs的研究进展、临床应用、应用中的问题及应用前景进行概述。     

Core as a novel viral target for hepatitis C drugs

Hepatitis C virus (HCV) infects over 130 million people worldwide and is a major cause of liver disease. No vaccine is available. Novel specific drugs for HCV are urgently required, since the standard-of-care treatment of pegylated interferon combined with ribavirin is poorly tolerated and cures less than half of the treated patients. Promising, effective direct-acting drugs currently in the clinic have been described for three of the ten potential HCV target proteins: NS3/NS4A protease, NS5B polymerase and NS5A, a regulatory phosphoprotein. We here present core, the viral capsid protein, as another attractive, non-enzymatic target, against which a new class of anti-HCV drugs can be raised. Core plays a major role in the virion's formation, and interacts with several cellular proteins, some of which are involved in host defense mechanisms against the virus. This most conserved of all HCV proteins requires oligomerization to function as the organizer of viral particle assembly. Using core dimerization as the basis of transfer-of-energy screening assays, peptides and small molecules were identified which not only inhibit core-core interaction, but also block viral production in cell culture. Initial chemical optimization resulted in compounds active in single digit micromolar concentrations. Core inhibitors could be used in combination with other HCV drugs in order to provide novel treatments of Hepatitis C.     

What Are the Promising New Therapies in the Field of Chronic Hepatitis C After the First-Generation Direct-Acting Antivirals?

A number of promising new hepatitis C virus (HCV) antiviral regimens have emerged during the last few years, with a trend toward increased efficacy, safety, and tolerability, when compared with currently available therapies. The focus of recent HCV antiviral drug development has been on inhibition of HCV replication, largely by targeting specific components of the HCV replication complex itself. A significant effort has been put into generating drugs that inhibit the NS5B polymerase. A number of such drugs have been developed, and NS5B polymerase inhibitors can be divided into nucleoside polymerase inhibitors and nonnucleoside polymerase inhibitors, with each group carrying specific pharmacologic and clinical characteristics. Additional research has explored the efficacy of drugs that inhibit the HCV replication complex via other mechanisms. Second-generation NS3-4A protease inhibitors have been developed, which have generally improved on the efficacy of the currently available FDA-approved first-generation agents. NS5A inhibitors have also been studied. These medications impede HCV replication and viral particle assembly and enhance host immune activation via novel mechanisms. Alternatively, medications that target a host protein, cyclophillin B, are under evaluation. These medications block HCV replication via modification of the effects of NS5B and via other poorly understood mechanisms. Detailed below are the most important HCV antiviral agents under development, many of which show promise for use within the next few years.     

The Hepatitis C Virus Nonstructural Protein 2 (NS2): An Up-and-Coming Antiviral Drug Target

Infection with Hepatitis C Virus (HCV) continues to be a major global health problem. To overcome the limitations of current therapies using interferon-α in combination with ribavirin, there is a need to develop drugs that specifically block viral proteins. Highly efficient protease and polymerase inhibitors are currently undergoing clinical testing and will become available in the next few years. However, with resistance mutations emerging quickly, additional enzymatic activities or functions of HCV have to be targeted by novel compounds. One candidate molecule is the nonstructural protein 2 (NS2), which contains a proteolytic activity that is essential for viral RNA replication. In addition, NS2 is crucial for the assembly of progeny virions and modulates various cellular processes that interfere with viral replication. This review describes the functions of NS2 in the life cycle of HCV and highlights potential antiviral strategies involving NS2.     

All Oral HCV Therapies of the Future

Hepatitis C virus (HCV) therapy has revolved around pegylated interferon-alpha (PEG IFN) and ribavirin (RBV), with the recent addition of the direct-acting antivirals (DAA), telaprevir or boceprevir, to the regimen for genotype 1 patients. Further progress has been made with the identification of multiple targets, allowing for additional DAAs, including other protease inhibitors, NS5A inhibitors and NS5B polymerase inhibitors. Initial studies have shown these drugs to provide more effective therapy than the current standard of care when combined with PEG IFN and RBV. More recently, however, even without PEG IFN, combinations of DAAs have been shown to allow for shorter treatment duration, increased efficacy, and better safety profiles. These regimens have the potential to allow HCV to be eradicated with all oral therapy in the near future.     

Dual daclatasvir and sofosbuvir for treatment of genotype 3 chronic hepatitis C virus infection

Chronic hepatitis C virus (HCV) infection is one of the most common etiologies of liver-related mortality throughout the world. Traditionally, therapy has been focused on pegylated interferon in combination with ribavirin, with clinical trials demonstrating that HCV genotype 1 had the lowest response rate (40–50%), while genotype 3 had an intermediate response rate (60–70%). Recently, significant advances have been made with all-oral direct-acting antiviral (DAA) therapy, which have significantly improved cure rates for HCV genotype 1. Accordingly, HCV genotype 3 is now potentially the most difficult to treat. One of the most potent DAA medications is sofosbuvir, a pan-genotypic nucleotide analogue that inhibits the NS5B polymerase of HCV. Daclatasvir, a pan-genotypic inhibitor of the HCV NS5A replication complex, was recently approved in the United States for treatment of HCV genotype 3 in conjunction with sofosbuvir. This combination may provide a powerful tool in the treatment of HCV genotype 3.     

Chronic hepatitis C: treatments of the future

The launch of first-generation protease inhibitors (PIs) was a major step forward in hepatitis C virus (HCV) treatment. However, this major advance has, up to now, only been applicable to genotype-1 patients. Second-wave and second-generation PIs appear to achieve higher antiviral potency, with pan-genotype activities, fewer side-effects and potential activity against PI-resistant mutation by second-generation PIs, through more convenient daily administration. Other direct-acting antivirals (DAAs) include NS5B inhibitors such as nucleoside/nucleotide inhibitors (NIs) and non-nucleoside inhibitors (NNIs). NIs have similar efficacy across all genotypes and present with the highest barrier to resistance of all DAAs to date. PSI-7977, a pyrimidine nucleotide analogue, also has highly potent antiviral activity across all HCV genotypes. In combination with ribavirin in an interferon-free regimen, it can achieve a 100% sustained viral response (SVR) rate in genotype 2/3 treatment-na?ve patients. In association with pegylated interferon and ribavirin (PR), it achieves an SVR of 91% in genotype-1 na?ve patients. NNIs in association with PR appear to be less potent, but they may nonetheless play a key role in many of the combination trials including either PIs or NIs. NS5A inhibitors also exhibit highly potent antiviral activity. Evaluation of their activity in combination with PIs demonstrated for the first time that an interferon-free regimen can cure genotype-1b null-responder patients. Furthermore, quadruple therapy with PR can achieve a 100% SVR in genotype-1 null-responder patients. Other players in the field, such as cyclophilin inhibitors and therapeutic vaccines, may have a role in combination with DAAs. The near future of HCV treatment looks promising. However, whether or not DAA combinations will lead to an interferon-free regimen for all patients remains an open question.