Kinetic analyses reveal potent and early blockade of hepatitis C virus assembly by NS5A inhibitors

Daclatasvir Prevents Hepatitis C Virus Infectivity by Blocking Transfer of the Viral Genome to Assembly Sites

BACKGROUND & AIMS

Daclatasvir is a direct-acting antiviral agent and potent inhibitor of NS5A, which is involved in replication of the hepatitis C virus (HCV) genome, presumably via membranous web shaping, and assembly of new virions, likely via transfer of the HCV RNA genome to viral particle assembly sites. Daclatasvir inhibits the formation of new membranous web structures and, ultimately, of replication complex vesicles, but also inhibits an early assembly step. We investigated the relationship between daclatasvir-induced clustering of HCV proteins, intracellular localization of viral RNAs, and inhibition of viral particle assembly.

METHODS

Cell-culture-derived HCV particles were produced from Huh7.5 hepatocarcinoma cells in presence of daclatasvir for short time periods. Infectivity and production of physical particles were quantified and producer cells were subjected to subcellular fractionation. Intracellular colocalization between core, E2, NS5A, NS4B proteins, and viral RNAs was quantitatively analyzed by confocal microscopy and by structured illumination microscopy.

RESULTS

Short exposure of HCV-infected cells to daclatasvir reduced viral assembly and induced clustering of structural proteins with non-structural HCV proteins, including core, E2, NS4B, and NS5A. These clustered structures appeared to be inactive assembly platforms, likely owing to loss of functional connection with replication complexes. Daclatasvir greatly reduced delivery of viral genomes to these core clusters without altering HCV RNA colocalization with NS5A. In contrast, daclatasvir neither induced clustered structures nor inhibited HCV assembly in cells infected with a daclatasvir-resistant mutant (NS5A-Y93H), indicating that daclatasvir targets a mutual, specific function of NS5A inhibiting both processes.

CONCLUSIONS

In addition to inhibiting replication complex biogenesis, daclatasvir prevents viral assembly by blocking transfer of the viral genome to assembly sites. This leads to clustering of HCV proteins because viral particles and replication complex vesicles cannot form or egress. This dual mode of action of daclatasvir could explain its efficacy in blocking HCV replication in cultured cells and in treatment of patients with HCV infection.

Current therapy for chronic hepatitis C: The role of direct-acting antivirals

One of the most exciting developments in antiviral research has been the discovery of the direct-acting antivirals (DAAs) that effectively cure chronic hepatitis C virus (HCV) infections. Based on more than 100 clinical trials and real-world studies, we provide a comprehensive overview of FDA-approved therapies and newly discovered anti-HCV agents with a special focus on drug efficacy, mechanisms of action, and safety. We show that HCV drug development has advanced in multiple aspects: (i) interferon-based regimens were replaced by interferon-free regimens; (ii) genotype-specific drugs evolved to drugs for all HCV genotypes; (iii) therapies based upon multiple pills per day were simplified to a single pill per day; (iv) drug potency increased from moderate (∼60%) to high (>90%) levels of sustained virologic responses; (v) treatment durations were shortened from 48 to 12 or 8 weeks; and (vi) therapies could be administered orally regardless of prior treatment history and cirrhotic status. However, despite these remarkable achievements made in HCV drug discovery, challenges remain in the management of difficult-to-treat patients.

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HCV genotype-specific drugs evolve to pan-genotypic drugs.

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Drug potency increases from moderate (∼60%) to high (>90%) levels of sustained virologic response.

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Treatment durations are shortened from a 48-week to 12-week or 8-week period.

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HCV therapies based upon multiple pills per day are simplified to a single pill per day.

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HCV therapies are administered orally regardless of prior treatment history and cirrhotic status.

Efficacy and Safety of Daclatasvir in Hepatitis C: An Overview

Hepatitis C virus (HCV) infection is a growing public health concern, with 184 million people infected worldwide. During the past decade, interferon has been the backbone of HCV treatment, even though it remains far from ideal. The latest development of the new direct antivirals has drastically changed the treatment approach for chronic hepatitis C (CHC). Inhibitors of the HCV NS5A region have garnered remarkable interest among treating physicians, due to their high potency and favourable safety profile. In particular, treatment with daclatasvir (DCV) has yielded high rates of vriologic response in patients infected with genotype (Gt) 1 and Gt 3, when used in combination with other antivirals of a different class, such as sofosbuvir. Although few data are available for DCV treatment of the other Gts, the results in patients with Gt 2 and Gt 4 infection appear promising, as do those for unique patient populations. NS5A-resistant viral variants can pre-exist or emerge after treatment failure for the HCV NS5A inhibitors. Nonetheless, DCV-resistant viral variants continue to be sensitive to interferon and other classes of antivirals such as NS3/4A and NS5B inhibitors. Herein, we aimed to provide an overview of the current knowledge about DCV in the treatment of CHC.

Discovery of Chromane Containing Hepatitis C Virus (HCV) NS5A Inhibitors with Improved Potency against Resistance-Associated Variants

The discovery of potent and pan-genotypic HCV NS5A inhibitors faces many challenges including the significant diversity among genotypes, substantial potency shift conferred on some key resistance-associated variants, inconsistent SARs between different genotypes and mutants, and the lacking of models of inhibitor/protein complexes for rational inhibitor design. As part of ongoing efforts on HCV NS5A inhibition at Merck, we now describe the discovery of a novel series of chromane containing NS5A inhibitors. SAR studies around the "Z" group of the tetracyclic indole scaffold explored fused bicyclic rings as alternates to the phenyl group of elbasvir (1, MK-8742) and identified novel chromane and 2,3-dihydrobenzofuran derivatives as "Z" group replacements offered good potency across all genotypes. This effort, incorporating the C-1 fluoro substitution at the tetracyclic indole core, led to the discovery of a new series of NS5A inhibitors, such as compounds 14 and 25-28, with significantly improved potency against resistance-associated variants, such as GT2b, GT1a Y93H, and GT1a L31V. Compound 14 also showed reasonable PK exposures in preclinical species (rat and dog).

Future landscape of hepatitis C research - Basic, translational and clinical perspectives

With the latest all-oral interferon- and ribavirin-free regimens based on direct acting antivirals against the hepatitis C virus (HCV), sustained virological response rates of >90% are achieved, which is equivalent to cure. This has become possible for all genotypes and all subgroups of patients, including many of the most difficult-to-treat populations so far. Since a prophylactic HCV vaccine is not yet available, control of HCV infection will for the time being have to rely on the use of effective and safe antiviral treatments as well as their accessibility and affordability. Different approaches may apply to different parts of the world, eradication of HCV representing a major long-term goal. Whether hepatitis C becomes the first chronic viral infection to be eradicated without a prophylactic vaccine remains to be shown. Here, we briefly summarize advances in the molecular virology of hepatitis C, highlight lessons of biological relevance that were learned through the study of HCV, and its translational and clinical implications. We have also listed selected unsolved challenges, emphasizing that HCV is a unique model and that advances in this direction may yield knowledge of broad biological significance, novel technologies and insights into related important human pathogens.

Structure-activity relationships of proline modifications around the tetracyclic-indole class of NS5A inhibitors

We describe the impact of proline modifications, in our tetracyclic-indole based series of nonstructural protein 5A (NS5A) inhibitors, to their replicon profiles. This work identified NS5A inhibitors with an improved and flattened resistance profiles.

Efficient Suppression of Hepatitis C Virus Replication by Combination Treatment with miR-122 Antagonism and Direct-acting Antivirals in Cell Culture Systems

Direct-acting antivirals (DAAs) against Hepatitis C virus (HCV) show effective antiviral activity with few side effects. However, the selection of DAA-resistance mutants is a growing problem that needs to be resolved. In contrast, miR-122 antagonism shows extensive antiviral effects among all HCV genotypes and a high barrier to drug resistance. In the present study, we evaluated three DAAs (simeprevir, daclatasvir, and sofosbuvir) in combination with anti-miR-122 treatment against HCV genotype 1a in cell cultures. We found that combination treatments with anti-miR-122 and a DAA had additive or synergistic antiviral effects. The EC₅₀ values of simeprevir in simeprevir-resistant mutants were significantly decreased by combining simeprevir with anti-miR-122. A similar reduction in EC₅₀ in daclatasvir-resistant mutants was achieved by combining daclatasvir with anti-miR-122. Combination treatment in HCV-replicating cells with DAA and anti-miR-122 sharply reduced HCV RNA amounts. Conversely, DAA single treatment with simeprevir or daclatasvir reduced HCV RNA levels initially, but the levels later rebounded. DAA-resistant mutants were less frequently observed in combination treatments than in DAA single treatments. In summary, the addition of miR-122 antagonism to DAA single treatments had additive or synergistic antiviral effects and helped to efficiently suppress HCV replication and the emergence of DAA-resistant mutants.

Impact of HCV genotype on treatment regimens and drug resistance: a snapshot in time

The introduction of highly potent direct-acting antivirals (DAAs) has revolutionized hepatitis C virus treatment. Nevertheless, viral eradication worldwide remains a challenge also in the era of DAA treatment, because of the high associated costs, high numbers of undiagnosed patients, high re-infection rates in some risk groups and suboptimal drug efficacies associated with host and viral factors as well as advanced stages of liver disease. A correct determination of the HCV genotype allows administration of the most appropriate antiviral regimen. Additionally, HCV genetic sequencing improves our understanding of resistance-associated variants, either naturally occurring before treatment, acquired by transmission at HCV infection, or emerging after virological failure. Because treatment response rates, and the prevalence and development of drug resistance variants differ for each DAA regimen and HCV genotype, this review summarizes treatment opportunities per HCV genotype, and focuses on viral genetic sequencing to guide clinical decision making. Although approval of the first pan-genotypic DAA-only regimen is expected soon, HCV genetic sequencing will remain important because when DAA therapies fail, genotyping and resistance testing to select a new active DAA combination will be essential. Copyright © 2016 John Wiley & Sons, Ltd.

The safety of daclatasvir for the treatment of hepatitis C

INTRODUCTION

The direct acting antiviral daclatasvir is an NS5A replication inhibitor active against the entire range of hepatitis C virus genotypes. It is a key step in establishing the goal of an all-oral, ribavirin-free, pan-genotypic regimen against hepatitis C.

AREAS COVERED

We review current literature including published abstracts and manuscripts. Evidence was obtained through PubMed/Medline search using listed keywords and through review of published abstracts.

EXPERT OPINION

Daclatasvir introduces a degree of pangenotypic potency currently lacking in other NS5A agents. Emerging literature suggests that daclatasvir in combination with other DAAs will represent a promising option in this difficult to treat populations including posttransplant, genotype 3 and HIV patients.

Viral genome imaging of hepatitis C virus to probe heterogeneous viral infection and responses to antiviral therapies

Hepatitis C virus (HCV) is a positive single-stranded RNA virus of enormous global health importance, with direct-acting antiviral therapies replacing an immunostimulatory interferon-based regimen. The dynamics of HCV positive and negative-strand viral RNAs (vRNAs) under antiviral perturbations have not been studied at the single-cell level, leaving a gap in our understanding of antiviral kinetics and host-virus interactions. Here, we demonstrate quantitative imaging of HCV genomes in multiple infection models, and multiplexing of positive and negative strand vRNAs and host antiviral RNAs. We capture the varying kinetics with which antiviral drugs with different mechanisms of action clear HCV infection, finding the NS5A inhibitor daclatasvir to induce a rapid decline in negative-strand viral RNAs. We also find that the induction of host antiviral genes upon interferon treatment is positively correlated with viral load in single cells. This study adds smFISH to the toolbox available for analyzing the treatment of RNA virus infections.

The Combination of Grazoprevir, a Hepatitis C Virus (HCV) NS3/4A Protease Inhibitor, and Elbasvir, an HCV NS5A Inhibitor, Demonstrates a High Genetic Barrier to Resistance in HCV Genotype 1a Replicons

The selection of resistance-associated variants (RAVs) against single agents administered to patients chronically infected with hepatitis C virus (HCV) necessitates that direct-acting antiviral agents (DAAs) targeting multiple viral proteins be developed to overcome failure resulting from emergence of resistance. The combination of grazoprevir (formerly MK-5172), an NS3/4A protease inhibitor, and elbasvir (formerly MK-8742), an NS5A inhibitor, was therefore studied in genotype 1a (GT1a) replicon cells. Both compounds were independently highly potent in GT1a wild-type replicon cells, with 90% effective concentration (EC90) values of 0.9 nM and 0.006 nM for grazoprevir and elbasvir, respectively. No cross-resistance was observed when clinically relevant NS5A and NS3 RAVs were profiled against grazoprevir and elbasvir, respectively. Kinetic analyses of HCV RNA reduction over 14 days showed that grazoprevir and elbasvir inhibited prototypic NS5A Y93H and NS3 R155K RAVs, respectively, with kinetics comparable to those for the wild-type GT1a replicon. In combination, grazoprevir and elbasvir interacted additively in GT1a replicon cells. Colony formation assays with a 10-fold multiple of the EC90 values of the grazoprevir-elbasvir inhibitor combination suppressed emergence of resistant colonies, compared to a 100-fold multiple for the independent agents. The selected resistant colonies with the combination harbored RAVs that required two or more nucleotide changes in the codons. Mutations in the cognate gene caused greater potency losses for elbasvir than for grazoprevir. Replicons bearing RAVs identified from resistant colonies showed reduced fitness for several cell lines and may contribute to the activity of the combination. These studies demonstrate that the combination of grazoprevir and elbasvir exerts a potent effect on HCV RNA replication and presents a high genetic barrier to resistance. The combination of grazoprevir and elbasvir is currently approved for chronic HCV infection.

Coordination of Hepatitis C Virus Assembly by Distinct Regulatory Regions in Nonstructural Protein 5A

Hepatitis C virus (HCV) nonstructural protein (NS)5A is a RNA-binding protein composed of a N-terminal membrane anchor, a structured domain I (DI) and two intrinsically disordered domains (DII and DIII) interacting with viral and cellular proteins. While DI and DII are essential for RNA replication, DIII is required for assembly. How these processes are orchestrated by NS5A is poorly understood. In this study, we identified a highly conserved basic cluster (BC) at the N-terminus of DIII that is critical for particle assembly. We generated BC mutants and compared them with mutants that are blocked at different stages of the assembly process: a NS5A serine cluster (SC) mutant blocked in NS5A-core interaction and a mutant lacking the envelope glycoproteins (ΔE1E2). We found that BC mutations did not affect core-NS5A interaction, but strongly impaired core–RNA association as well as virus particle envelopment. Moreover, BC mutations impaired RNA-NS5A interaction arguing that the BC might be required for loading of core protein with viral RNA. Interestingly, RNA-core interaction was also reduced with the ΔE1E2 mutant, suggesting that nucleocapsid formation and envelopment are coupled. These findings argue for two NS5A DIII determinants regulating assembly at distinct, but closely linked steps: (i) SC-dependent recruitment of replication complexes to core protein and (ii) BC-dependent RNA genome delivery to core protein, triggering encapsidation that is tightly coupled to particle envelopment. These results provide a striking example how a single viral protein exerts multiple functions to coordinate the steps from RNA replication to the assembly of infectious virus particles.

Hepatitis C virus (HCV) nonstructural protein (NS)5A is an enigmatic RNA-binding protein that appears to regulate the different steps from RNA replication to the assembly of infectious virus particles by yet unknown mechanisms. Assembly requires delivery of the viral RNA genome from the replication machinery to the capsid protein to ensure genome packaging into nucleocapsids that acquire a membranous envelope by budding into the lumen of the endoplasmic reticulum. In this study, we provide genetic and biochemical evidence that the viral nonstructural protein (NS)5A contains two regulatory determinants in its domain (D)III that orchestrate virus assembly at two closely linked steps: (i) recruitment of replication complexes to core protein requiring a serine cluster in the C-terminal region of DIII and (ii) RNA genome delivery to core protein requiring a basic cluster in the N-terminal region of DIII. This RNA transfer most likely triggers encapsidation, which is tightly coupled to particle envelopment. These results provide a striking example for a multi-purpose viral protein exerting several distinct functions in the viral replication cycle, thus reflecting genetic economy.

Identification of a resveratrol tetramer as a potent inhibitor of hepatitis C virus helicase

BACKGROUND AND PURPOSE

Hepatitis C virus (HCV) infection is responsible for various chronic inflammatory liver diseases. Here, we have identified a naturally occurring compound with anti-HCV activity and have elucidated its mode of antiviral action.

EXPERIMENTAL APPROACH

Luciferase reporter and real-time RT-PCR assays were used to measure HCV replication. Western blot, fluorescence-labelled HCV replicons and infectious clones were employed to quantitate expression levels of viral proteins. Resistant HCV mutant mapping, in vitro NS3 protease, helicase, NS5B polymerase and drug affinity responsive target stability assays were also used to study the antiviral mechanism.

KEY RESULTS

A resveratrol tetramer, vitisin B from grapevine root extract showed high potency against HCV replication (EC50 = 6 nM) with relatively low cytotoxicity (EC50 >10 μM). Combined treatment of vitisin B with an NS5B polymerase inhibitor (sofosbuvir) exhibited a synergistic or at least additive antiviral activity. Analysis of a number of vitisin B-resistant HCV variants suggested an NS3 helicase as its potential target. We confirmed a direct binding between vitisin B and a purified NS3 helicase in vitro. Vitisin B was a potent inhibitor of a HCV NS3 helicase (IC50 = 3 nM). In vivo, Finally, we observed a preferred tissue distribution of vitisin B in the liver after i.p. injection in rats, at clinically attainable concentrations. Conclusion and Implications Vitisin B is one of the most potent HCV helicase inhibitors identified so far. Vitisin B is thus a prime candidate to be developed as the first HCV drug derived from natural products.

Synergistic Activity of Combined NS5A Inhibitors

Daclatasvir (DCV) is a first-in-class hepatitis C virus (HCV) nonstructural 5A replication complex inhibitor (NS5A RCI) that is clinically effective in interferon-free combinations with direct-acting antivirals (DAAs) targeting alternate HCV proteins. Recently, we reported NS5A RCI combinations that enhance HCV inhibitory potential in vitro, defining a new class of HCV inhibitors termed NS5A synergists (J. Sun, D. R. O'Boyle II, R. A. Fridell, D. R. Langley, C. Wang, S. Roberts, P. Nower, B. M. Johnson F. Moulin, M. J. Nophsker, Y. Wang, M. Liu, K. Rigat, Y. Tu, P. Hewawasam, J. Kadow, N. A. Meanwell, M. Cockett, J. A. Lemm, M. Kramer, M. Belema, and M. Gao, Nature 527:245-248, 2015, doi:10.1038/nature15711). To extend the characterization of NS5A synergists, we tested new combinations of DCV and NS5A synergists against genotype (gt) 1 to 6 replicons and gt 1a, 2a, and 3a viruses. The kinetics of inhibition in HCV-infected cells treated with DCV, an NS5A synergist (NS5A-Syn), or a combination of DCV and NS5A-Syn were distinctive. Similar to activity observed clinically, DCV caused a multilog drop in HCV, followed by rebound due to the emergence of resistance. DCV-NS5A-Syn combinations were highly efficient at clearing cells of viruses, in line with the trend seen in replicon studies. The retreatment of resistant viruses that emerged using DCV monotherapy with DCV-NS5A-Syn resulted in a multilog drop and rebound in HCV similar to the initial decline and rebound observed with DCV alone on wild-type (WT) virus. A triple combination of DCV, NS5A-Syn, and a DAA targeting the NS3 or NS5B protein cleared the cells of viruses that are highly resistant to DCV. Our data support the observation that the cooperative interaction of DCV and NS5A-Syn potentiates both the genotype coverage and resistance barrier of DCV, offering an additional DAA option for combination therapy and tools for explorations of NS5A function.

Mechanisms of Hepatitis C Viral Resistance to Direct Acting Antivirals

There has been a remarkable transformation in the treatment of chronic hepatitis C in recent years with the development of direct acting antiviral agents targeting virus encoded proteins important for viral replication including NS3/4A, NS5A and NS5B. These agents have shown high sustained viral response (SVR) rates of more than 90% in phase 2 and phase 3 clinical trials; however, this is slightly lower in real-life cohorts. Hepatitis C virus resistant variants are seen in most patients who do not achieve SVR due to selection and outgrowth of resistant hepatitis C virus variants within a given host. These resistance associated mutations depend on the class of direct-acting antiviral drugs used and also vary between hepatitis C virus genotypes and subtypes. The understanding of these mutations has a clear clinical implication in terms of choice and combination of drugs used. In this review, we describe mechanism of action of currently available drugs and summarize clinically relevant resistance data.

Hepatitis C virus drug resistance-associated substitutions: State of the art summary

Hepatitis C virus (HCV) drug development has resulted in treatment regimens composed of interferon-free, all-oral combinations of direct-acting antivirals. While the new regimens are potent and highly efficacious, the full clinical impact of HCV drug resistance, its implications for retreatment, and the potential role of baseline resistance testing remain critical research and clinical questions. In this report, we discuss the viral proteins targeted by HCV direct-acting antivirals and summarize clinically relevant resistance data for compounds that have been approved or are currently in phase 3 clinical trials.

CONCLUSION

This report provides a comprehensive, systematic review of all resistance information available from sponsors' trials as a tool to inform the HCV drug development field.

Resistance-Associated NS5A Variants of Hepatitis C Virus Are Susceptible to Interferon-Based Therapy

BACKGROUND & AIMS

The presence of resistance-associated variants (RAVs) of hepatitis C virus (HCV) attenuates the efficacy of direct acting antivirals (DAAs). The objective of this study was to characterize the susceptibility of RAVs to interferon-based therapy.

METHODS

Direct and deep sequencing were performed to detect Y93H RAV in the NS5A region. Twenty nine genotype 1b patients with detectable RAV at baseline were treated by a combination of simeprevir, pegylated interferon and ribavirin. The longitudinal changes in the proportion of Y93H RAV during therapy and at breakthrough or relapse were determined.

RESULTS

By direct sequencing, Y93H RAV became undetectable or decreased in proportion at an early time point during therapy (within 7 days) in 57% of patients with both the Y93H variant and wild type virus at baseline when HCV RNA was still detectable. By deep sequencing, the proportion of Y93H RAV against Y93 wild type was 52.7% (5.8%- 97.4%) at baseline which significantly decreased to 29.7% (0.16%- 98.3%) within 7 days of initiation of treatment (p = 0.023). The proportion of Y93H RAV was reduced in 21 of 29 cases (72.4%) and a marked reduction of more than 10% was observed in 14 cases (48.7%). HCV RNA reduction was significantly greater for Y93H RAV (-3.65±1.3 logIU/mL/day) than the Y93 wild type (-3.35±1.0 logIU/mL/day) (p<0.001).

CONCLUSION

Y93H RAV is more susceptible to interferon-based therapy than the Y93 wild type.

Ribavirin Contributes to Hepatitis C Virus Suppression by Augmenting pDC Activation and Type 1 IFN Production

Ribavirin is used as a component of combination therapies for the treatment of chronic hepatitis C virus (HCV) infection together with pegylated interferon and/or direct-acting antiviral drugs. Its mechanism of action, however, is not clear. Direct antiviral activity and immunomodulatory functions have been implicated. Plasmacytoid dendritic cells (pDCs) are the principal source of type 1 interferon during viral infection. The interaction of pDCs with HCV-infected hepatocytes is the subject of intense recent investigation, but the effect of ribavirin on pDC activation has not been evaluated. In this study we showed that ribavirin augments toll-like receptors 7 and 9-mediated IFNα/β expression from pDCs and up-regulated numerous interferon-stimulated genes. Using the H77S.3 HCV infection and replication system, we showed that ribavirin enhanced the ability of activated pDCs to inhibit HCV replication, correlated with elevated induction of IFNα. Our findings provide novel evidence that ribavirin contributes to HCV inhibition by augmenting pDCs-derived type 1 IFN production.

The yin and yang of hepatitis C: synthesis and decay of hepatitis C virus RNA

Hepatitis C virus (HCV) is an unusual RNA virus that has a striking capacity to persist for the remaining life of the host in the majority of infected individuals. In order to persist, HCV must balance viral RNA synthesis and decay in infected cells. In this Review, we focus on interactions between the positive-sense RNA genome of HCV and the host RNA-binding proteins and microRNAs, and describe how these interactions influence the competing processes of viral RNA synthesis and decay to achieve stable, long-term persistence of the viral genome. Furthermore, we discuss how these processes affect hepatitis C pathogenesis and therapeutic strategies against HCV.

Modelling hepatitis C therapy--predicting effects of treatment

Mathematically modelling changes in HCV RNA levels measured in patients who receive antiviral therapy has yielded many insights into the pathogenesis and effects of treatment on the virus. By determining how rapidly HCV is cleared when viral replication is interrupted by a therapy, one can deduce how rapidly the virus is produced in patients before treatment. This knowledge, coupled with estimates of the HCV mutation rate, enables one to estimate the frequency with which drug resistant variants arise. Modelling HCV also permits the deduction of the effectiveness of an antiviral agent at blocking HCV replication from the magnitude of the initial viral decline. One can also estimate the lifespan of an HCV-infected cell from the slope of the subsequent viral decline and determine the duration of therapy needed to cure infection. The original understanding of HCV RNA decline under interferon-based therapies obtained by modelling needed to be revised in order to interpret the HCV RNA decline kinetics seen when using direct-acting antiviral agents (DAAs). There also exist unresolved issues involving understanding therapies with combinations of DAAs, such as the presence of detectable HCV RNA at the end of therapy in patients who nonetheless have a sustained virologic response.

Utility of hepatitis C viral load monitoring on direct-acting antiviral therapy

BACKGROUND

Hepatitis C virus (HCV) RNA loads serve as predictors of treatment response during interferon-based therapy. We evaluated the predictive ability of HCV RNA levels at end of treatment (EOT) for sustained virologic response (SVR12) during interferon-sparing direct-acting antiviral therapies.

METHODS

HCV genotype 1-infected, treatment-naive patients were treated with sofosbuvir and ribavirin for 24 weeks (n = 55), sofosbuvir and ledipasvir for 12 weeks (n = 20), sofosbuvir, ledipasvir, and GS-9669 for 6 weeks (n = 20), or sofosbuvir, ledipasvir, and GS-9451 for 6 weeks (n = 19). Measurements of HCV RNA were performed using the Roche COBAS TaqMan HCV test and the Abbott RealTime HCV assay. Positive predictive value (PPV) and negative predictive value (NPV) of HCV RNA less than the lower limit of quantification (

RESULTS

All 55 patients treated with sofosbuvir and ribavirin had HCV RNA

CONCLUSIONS

Contrary to past experience with interferon-containing treatments, low levels of quantifiable HCV RNA at EOT do not preclude treatment success.

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Key Words

• HCV RNA
• direct-acting antiviral
• hepatitis C
• viral load

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MESH Headings

• Adult
• Aged
• Antiviral Agents/pharmacology
• Antiviral Agents/therapeutic use
• Drug Monitoring/methods
• Female
• Hepacivirus/drug effects
• Hepatitis C/drug therapy
• Hepatitis C/epidemiology
• Hepatitis C/virology
• Humans
• Male
• Middle Aged
• Predictive Value of Tests
• RNA, Viral/blood
• Viral Load/drug effects

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Grants

• HHSN261200800001C NCI NIH HHS
• HHSN261200800001E NCI NIH HHS
• HHSN261200800001E NCI NIH HHS

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Affiliation(s)

Sreetha Sidharthan Critical Care Medicine Department, National Institutes of Health Clinical Center, National Institutes of Health, Bethesda
Anita Kohli Critical Care Medicine Department, National Institutes of Health Clinical Center, National Institutes of Health, Bethesda
Zayani Sims Critical Care Medicine Department, National Institutes of Health Clinical Center, National Institutes of Health, Bethesda
Amy Nelson Institute of Human Virology, University of Maryland, Baltimore Laboratory of Immunoregulation, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland
Anu Osinusi Laboratory of Immunoregulation, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland Gilead Sciences Inc, Foster City, California
Henry Masur Critical Care Medicine Department, National Institutes of Health Clinical Center, National Institutes of Health, Bethesda
Shyam Kottilil Institute of Human Virology, University of Maryland, Baltimore Laboratory of Immunoregulation, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland

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NS5A phosphorylation: its functional role in the life cycle of hepatitis C virus

ABSTRACT 

Hepatitis C virus (HCV) is a major cause of liver disease. HCV RNA replicates in a membrane-associated replication complex. Nonstructural protein 5A (NS5A) is phosphorylated on multiple serine and threonine residues and exists in basally phosphorylated and hyperphosphorylated forms. To date, studies have identified several serine/threonine kinases responsible for NS5A phosphorylation. Although NS5A has no known enzymatic activity, it is a multifunctional protein required for HCV RNA replication and virion assembly. The phosphorylation status of NS5A is considered to have a significant impact on its function and the viral life cycle. Furthermore, NS5A inhibitors represent a new class of direct acting antivirals and have become a key component for effective combination therapies against HCV infection.

HCV Kinetic Models and Their Implications in Drug Development

Chronic infection with hepatitis C virus (HCV) affects about 170 million people worldwide and is a major cause of liver complications. Mathematical modeling of viral kinetics under treatment has provided insight into the viral life cycle, treatment effectiveness, and drugs' mechanisms of action. Here we review the implications of viral kinetic models at the different stages of development of anti-HCV agents.

Fast hepatitis C virus RNA elimination and NS5A redistribution by NS5A inhibitors studied by a multiplex assay approach

While earlier therapeutic strategies for the treatment of hepatitis C virus (HCV) infection relied exclusively on interferon (IFN) and ribavirin (RBV), four direct-acting antiviral agents (DAAs) have now been approved, aiming for an interferon-free strategy with a short treatment duration and fewer side effects. To facilitate studies on the mechanism of action (MOA) and efficacy of DAAs, we established a multiplex assay approach, which employs flow cytometry, a Gaussia luciferase reporter system, Western blot analysis, reverse transcription-quantitative PCR (RT-qPCR), a limited dilution assay (50% tissue culture infectious dose [TCID50]), and an image profiling assay that follows the NS5A redistribution in response to drug treatment. We used this approach to compare the relative potency of various DAAs and the kinetics of their antiviral effects as a potential preclinical measure of their potential clinical utility. We evaluated the NS5A inhibitors ledipasvir (LDV) and daclatasvir (DCV), the NS3/4A inhibitor danoprevir (DNV), and the NS5B inhibitor sofosbuvir (SOF). In terms of kinetics, our data demonstrate that the NS5A inhibitor LDV, followed closely by DCV, has the fastest effect on suppression of viral proteins and RNA and on redistribution of NS5A. In terms of MOA, LDV has a more pronounced effect than DCV on the viral replication, assembly, and infectivity of released virus. Our approach can be used to facilitate the study of the biological processes involved in HCV replication and help identify optimal drug combinations.

From non-A, non-B hepatitis to hepatitis C virus cure

The hepatitis C virus (HCV) was discovered in the late 1980s. Interferon (IFN)-α was proposed as an antiviral treatment for chronic hepatitis C at about the same time. Successive improvements in IFN-α-based therapy (dose finding, pegylation, addition of ribavirin) increased the rates of sustained virologic response, i.e. the rates of curing HCV infection. These rates were further improved by adding the first available direct-acting antiviral (DAA) drugs to the combination of pegylated IFN-α and ribavirin. An IFN-free era finally started in 2014, yielding rates of sustained virologic response over 90% in patients treated for 8 to 24 weeks with all-oral regimens. Major challenges however remain in implementation of these new treatment strategies, not only in low- to middle-income countries, but also in high-income countries where the price of these therapies is still prohibitive. Elimination of HCV infection through treatment in certain areas is possible but raises major public health issues.

A novel method for the measurement of hepatitis C virus infectious titres using the IncuCyte ZOOM and its application to antiviral screening

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Hepatitis C virus (HCV) is a significant human pathogen, causing severe liver disease.

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Accurate quantification of viral titres is essential for the majority of assays.

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The current methods of HCV titration and quantification are laborious and imprecise.

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We report a novel method for calculating infectious HCV titres using the IncuCyte ZOOM.

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This method has applications for screening of novel antiviral compounds.

Hepatitis C virus (HCV) is a significant human pathogen infecting 3% of the world population. An infectious molecular clone capable of replicating and releasing infectious virions in cell culture has only been available since 2005, leaving a significant knowledge gap concerning post-RNA replication events such as particle assembly, trafficking and release. Thus, a fast, efficient and accurate method of measuring infectious viral titres is highly desirable. Current methods rely upon manual counting of infected cell foci and so are both labour-intensive and susceptible to human error. Here, we report a novel protocol, which utilises the IncuCyte ZOOM instrument and related software to accurately count infected cells and extrapolation of this data to produce an infectious titre, reported as infectious units per millilitre (IU/mL). This method reduces cost, time and error in experiments. We also demonstrate that this approach is amenable to high-throughput compound screening, thereby expediting the identification of novel antivirals.

Protease Inhibitors Block Multiple Functions of the NS3/4A Protease-Helicase during the Hepatitis C Virus Life Cycle

Hepatitis C virus (HCV) NS3 is a multifunctional protein composed of a protease domain and a helicase domain linked by a flexible linker. Protease activity is required to generate viral nonstructural (NS) proteins involved in RNA replication. Helicase activity is required for RNA replication, and genetic evidence implicates the helicase domain in virus assembly. Binding of protease inhibitors (PIs) to the protease active site blocks NS3-dependent polyprotein processing but might impact other steps of the virus life cycle. Kinetic analyses of antiviral suppression of cell culture-infectious genotype 1a strain H77S.3 were performed using assays that measure different readouts of the viral life cycle. In addition to the active-site PI telaprevir, we examined an allosteric protease-helicase inhibitor (APHI) that binds a site in the interdomain interface. By measuring nucleotide incorporation into HCV genomes, we found that telaprevir inhibits RNA synthesis as early as 12 h at high but clinically relevant concentrations. Immunoblot analyses showed that NS5B abundance was not reduced until after 12 h, suggesting that telaprevir exerts a direct effect on RNA synthesis. In contrast, the APHI could partially inhibit RNA synthesis, suggesting that the allosteric site is not always available during RNA synthesis. The APHI and active-site PI were both able to block virus assembly soon (<12 h) after drug treatment, suggesting that they rapidly engage with and block a pool of NS3 involved in assembly. In conclusion, PIs and APHIs can block NS3 functions in RNA synthesis and virus assembly, in addition to inhibiting polyprotein processing.

IMPORTANCE

The NS3/4A protease of hepatitis C virus (HCV) is an important antiviral target. Currently, three PIs have been approved for therapy of chronic hepatitis C, and several others are in development. NS3-dependent cleavage of the HCV polyprotein is required to generate the mature nonstructural proteins that form the viral replicase. Inhibition of protease activity can block RNA replication by preventing expression of mature replicase components. Like many viral proteins, NS3 is multifunctional, but how PIs affect stages of the HCV life cycle beyond polyprotein processing has not been well studied. Using cell-based assays, we show here that PIs can directly inhibit viral RNA synthesis and also block a late stage in virus assembly/maturation at clinically relevant concentrations.

Cyclophilin and NS5A inhibitors, but not other anti-hepatitis C virus (HCV) agents, preclude HCV-mediated formation of double-membrane-vesicle viral factories

Although the mechanisms of action (MoA) of nonstructural protein 3 inhibitors (NS3i) and NS5B inhibitors (NS5Bi) are well understood, the MoA of cyclophilin inhibitors (CypI) and NS5A inhibitors (NS5Ai) are not fully defined. In this study, we examined whether CypI and NS5Ai interfere with hepatitis C virus (HCV) RNA synthesis of replication complexes (RCs) or with an earlier step of HCV RNA replication, the creation of double-membrane vesicles (DMVs) essential for HCV RNA replication. In contrast to NS5Bi, both CypI and NS5Ai do not block HCV RNA synthesis by way of RCs, suggesting that they exert their antiviral activity prior to the establishment of enzymatically active RCs. We found that viral replication is not a precondition for DMV formation, since the NS3-NS5B polyprotein or NS5A suffices to create DMVs. Importantly, only CypI and NS5Ai, but not NS5Bi, mir-122, or phosphatidylinositol-4 kinase IIIα (PI4KIIIα) inhibitors, prevent NS3-NS5B-mediated DMV formation. NS3-NS5B was unable to create DMVs in cyclophilin A (CypA) knockdown (KD) cells. We also found that the isomerase activity of CypA is absolutely required for DMV formation. This not only suggests that NS5A and CypA act in concert to build membranous viral factories but that CypI and NS5Ai mediate their early anti-HCV effects by preventing the formation of organelles, where HCV replication is normally initiated. This is the first investigation to examine the effect of a large panel of anti-HCV agents on DMV formation, and the results reveal that CypI and NS5Ai act at the same membranous web biogenesis step of HCV RNA replication, thus indicating a new therapeutic target of chronic hepatitis C.

Daclatasvir inhibits hepatitis C virus NS5A motility and hyper-accumulation of phosphoinositides

Combinations of direct-acting antivirals (DAAs) against the hepatitis C virus (HCV) have the potential to revolutionize the HCV therapeutic regime. An integral component of DAA combination therapies is HCV NS5A inhibitors. It has previously been proposed that NS5A DAAs inhibit two functions of NS5A: RNA replication and virion assembly. In this study, we characterize the impact of a prototype NS5A DAA, daclatasvir (DCV), on HCV replication compartment formation. DCV impaired HCV replicase localization and NS5A motility. In order to characterize the mechanism behind altered HCV replicase localization, we examined the impact of DCV on the interaction of NS5A with its essential cellular cofactor, phosphatidylinositol-4-kinase III α (PI4KA). We observed that DCV does not inhibit PI4KA directly, nor does it impair early events of the NS5A-PI4KA interaction that can occur when NS5A is expressed alone. NS5A functions that are unaffected by DCV include PI4KA binding, as determined by co-immunoprecipitation, and a basal accumulation of the PI4KA product, PI4P. However, DCV impairs late steps in PI4KA activation that requires NS5A expressed in the context of the HCV polyprotein. These NS5A functions include hyper-stimulation of PI4P levels and appropriate replication compartment formation. The data are most consistent with a model wherein DCV inhibits conformational changes in the NS5A protein or protein complex formations that occur in the context of HCV polyprotein expression and stimulate PI4P hyper-accumulation and replication compartment formation.

Hepatitis C virus life cycle and lipid metabolism

Hepatitis C Virus (HCV) infects over 150 million people worldwide. In most cases HCV infection becomes chronic, causing liver disease ranging from fibrosis to cirrhosis and hepatocellular carcinoma. HCV affects the cholesterol homeostasis and at the molecular level, every step of the virus life cycle is intimately connected to lipid metabolism. In this review, we present an update on the lipids and apolipoproteins that are involved in the HCV infectious cycle steps: entry, replication and assembly. Moreover, the result of the assembly process is a lipoviroparticle, which represents a peculiarity of hepatitis C virion. This review illustrates an example of an intricate virus-host interaction governed by lipid metabolism.

Hepatitis C virus NS5A: enigmatic but still promiscuous 10 years on!

Since one of us co-authored a review on NS5A a decade ago, the hepatitis C virus (HCV) field has changed dramatically, primarily due to the advent of the JFH-1 cell culture infectious clone, which allowed the study of all aspects of the virus life cycle from entry to exit. This review will describe advances in our understanding of NS5A biology over the past decade, highlighting how the JFH-1 system has allowed us to determine that NS5A is essential not only in genome replication but also in the assembly of infectious virions. We shall review the recent structural insights - NS5A is predicted to comprise three domains; X-ray crystallography has revealed the structure of domain I but there is a lack of detailed structural information about the other two domains, which are predicted to be largely unstructured. Recent insights into the phosphorylation of NS5A will be discussed, and we shall highlight a few pertinent examples from the ever-expanding list of NS5A-binding partners identified over the past decade. Lastly, we shall review the literature showing that NS5A is a potential target for a new class of highly potent small molecules that function to inhibit virus replication. These direct-acting antivirals (DAAs) are now either licensed, or in the late stages of approval for clinical use both in the USA and in the UK/Europe. In combination with other DAAs targeting the viral protease (NS3) and polymerase (NS5B), they are revolutionizing treatment for HCV infection.

Resistance patterns associated with HCV NS5A inhibitors provide limited insight into drug binding

Direct-acting antivirals (DAAs) have significantly improved the treatment of infection with the hepatitis C virus. A promising class of novel antiviral agents targets the HCV NS5A protein. The high potency and broad genotypic coverage are favorable properties. NS5A inhibitors are currently assessed in advanced clinical trials in combination with viral polymerase inhibitors and/or viral protease inhibitors. However, the clinical use of NS5A inhibitors is also associated with new challenges. HCV variants with decreased susceptibility to these drugs can emerge and compromise therapy. In this review, we discuss resistance patterns in NS5A with focus prevalence and implications for inhibitor binding.

Asymmetric binding to NS5A by daclatasvir (BMS-790052) and analogs suggests two novel modes of HCV inhibition

Symmetric, dimeric daclatasvir (BMS-790052) is the clinical lead for a class of picomolar inhibitors of HCV replication. While specific, resistance-bearing mutations at positions 31 and 93 of domain I strongly suggest the viral NS5A as target, structural mechanism(s) for the drugs' activities and resistance remains unclear. Several previous models suggested symmetric binding modes relative to the homodimeric target; however, none can fully explain SAR details for this class. We present semiautomated workflows to model potential receptor conformations for docking. Surprisingly, ranking docked hits with our library-derived 3D-pharmacophore revealed two distinct asymmetric binding modes, at a conserved poly-proline region between 31 and 93, consistent with SAR. Interfering with protein-protein interactions at this membrane interface can explain potent inhibition of replication-complex formation, resistance, effects on lipid droplet distribution, and virion release. These detailed interaction models and proposed mechanisms of action will allow structure-based design of new NS5A directed compounds with higher barriers to HCV resistance.

Daclatasvir-like inhibitors of NS5A block early biogenesis of hepatitis C virus-induced membranous replication factories, independent of RNA replication

BACKGROUND & AIMS

Direct-acting antivirals that target nonstructural protein 5A (NS5A), such as daclatasvir, have high potency against the hepatitis C virus (HCV). They are promising clinical candidates, yet little is known about their antiviral mechanisms. We investigated the mechanisms of daclatasvir derivatives.

METHODS

We used a combination of biochemical assays, in silico docking models, and high-resolution imaging to investigate inhibitor-induced changes in properties of NS5A, including its interaction with phosphatidylinositol-4 kinase IIIα and induction of the membranous web, which is the site of HCV replication. Analyses were conducted with replicons, infectious virus, and human hepatoma cells that express a HCV polyprotein. Studies included a set of daclatasvir derivatives and HCV variants with the NS5A inhibitor class-defining resistance mutation Y93H.

RESULTS

NS5A inhibitors did not affect NS5A stability or dimerization. A daclatasvir derivative interacted with NS5A and molecular docking studies revealed a plausible mode by which the inhibitor bound to NS5A dimers. This interaction was impaired in mutant forms of NS5A that are resistant to daclatavir, providing a possible explanation for the reduced sensitivity of the HCV variants to this drug. Potent NS5A inhibitors were found to block HCV replication by preventing formation of the membranous web, which was not linked to an inhibition of phosphatidylinositol-4 kinase IIIα. Correlative light-electron microscopy revealed unequivocally that NS5A inhibitors had no overall effect on the subcellular distribution of NS5A, but completely prevented biogenesis of the membranous web.

CONCLUSIONS

Highly potent inhibitors of NS5A, such as daclatasvir, block replication of HCV RNA at the stage of membranous web biogenesis-a new paradigm in antiviral therapy.

Current and future targets of antiviral therapy in the hepatitis C virus life cycle

ABSTRACT 

Advances in our understanding of the hepatitis C virus (HCV) life cycle have enabled the development of numerous clinically advanced direct-acting antivirals. Indeed, the recent approval of first-generation direct-acting antivirals that target the viral NS3–4A protease and NS5B RNA-dependent RNA polymerase brings closer the possibility of universally efficacious and well-tolerated antiviral therapies for this insidious infection. However, the complexities of comorbidities, unforeseen side effects or drug–drug interactions, viral diversity, the high mutation rate of HCV RNA replication and the elegant and constantly evolving mechanisms employed by HCV to evade host and therapeutically implemented antiviral strategies remain as significant obstacles to this goal. Here, we review advances in our understanding of the HCV life cycle and associated opportunities for antiviral therapy.

A pharmacokinetic/viral kinetic model to evaluate the treatment effectiveness of danoprevir against chronic HCV

BACKGROUND

Viral kinetic models have proven useful to characterize treatment effectiveness during HCV therapy with interferon (IFN) or with direct-acting antivirals.

METHODS

We use a pharmacokinetic/viral kinetic (PK/VK) model to describe HCV RNA kinetics during treatment with danoprevir, a protease inhibitor. In a Phase I study, danoprevir monotherapy was administered for 14 days in ascending doses ranging from 200 to 600 mg per day to 40 patients of whom 32 were treatment-naive and 8 were non-responders to prior pegylated IFN-α/ribavirin treatment.

RESULTS

In all patients, a biphasic decline of HCV RNA during therapy was observed. A two-compartment PK model and a VK model that considered treatment effectiveness to vary with the predicted danoprevir concentration inside the second compartment provided a good fit to the viral load data. A time-varying effectiveness model was also used to fit the viral load data. The antiviral effectiveness increased in a dose-dependent manner, with a 14-day time-averaged effectiveness of 0.95 at the lowest dose (100 mg twice daily) and 0.99 at the highest dose (200 mg three times daily). Prior IFN non-responders exhibited a 14-day time-averaged effectiveness of 0.98 (300 mg twice daily). The second phase decline showed two different behaviours, with 30% of patients exhibiting a rapid decline of HCV RNA, comparable to that seen with other protease inhibitors (>0.3 day(-1)), whereas the viral decline was slower in the other patients.

CONCLUSIONS

Our results are consistent with the modest SVR rates from the INFORM-SVR study where patients were treated with a combination of mericitabine and ritonavir-boosted danoprevir.

NS5A inhibitors impair NS5A-phosphatidylinositol 4-kinase IIIα complex formation and cause a decrease of phosphatidylinositol 4-phosphate and cholesterol levels in hepatitis C virus-associated membranes

The hepatitis C virus (HCV) nonstructural (NS) protein 5A is a multifunctional protein that plays a central role in viral replication and assembly. Antiviral agents directly targeting NS5A are currently in clinical development. Although the elucidation of the mechanism of action (MOA) of NS5A inhibitors has been the focus of intensive research, a detailed understanding of how these agents exert their antiviral effect is still lacking. In this study, we observed that the downregulation of NS5A hyperphosphorylation is associated with the actions of NS5A inhibitors belonging to different chemotypes. NS5A is known to recruit the lipid kinase phosphatidylinositol 4-kinase IIIα (PI4KIIIα) to the HCV-induced membranous web in order to generate phosphatidylinositol 4-phosphate (PI4P) at the sites of replication. We demonstrate that treatment with NS5A inhibitors leads to an impairment in the NS5A-PI4KIIIα complex formation that is paralleled by a significant reduction in PI4P and cholesterol levels within the endomembrane structures of HCV-replicating cells. A similar decrease in PI4P and cholesterol levels was also obtained upon treatment with a PI4KIIIα-targeting inhibitor. In addition, both the NS5A and PI4KIIIα classes of inhibitors induced similar subcellular relocalization of the NS5A protein, causing the formation of large cytoplasmic NS5A-containing clusters previously reported to be one of the hallmarks of inhibition of the action of PI4KIIIα. Because of the similarities between the effects induced by treatment with PI4KIIIα or NS5A inhibitors and the observation that agents targeting NS5A impair NS5A-PI4KIIIα complex formation, we speculate that NS5A inhibitors act by interfering with the function of the NS5A-PI4KIIIα complex.

NS2 is dispensable for efficient assembly of hepatitis C virus-like particles in a bipartite trans-encapsidation system

Infectious hepatitis C virus (HCV) particle production in the genotype 2a JFH-1-based cell culture system involves non-structural proteins in addition to canonical virion components. NS2 has been proposed to act as a protein adaptor, co-ordinating the early stages of virion assembly. However, other studies have identified late-acting roles for this protein, making its precise involvement in infectious particle production unclear. Using a robust, bipartite trans-encapsidation system based upon baculovirus expression of HCV structural proteins, we have generated HCV-like particles (HCV-LP) in the absence of NS2 with overt similarity to wild-type virions. HCV-LP could transduce naive cells with trans-encapsidated subgenomic replicon RNAs and shared similar biochemical and biophysical properties with JFH-1 HCV. Both genotype 1b and JFH-1 intracellular HCV-LP were produced in the absence of NS2, whereas restoring NS2 to the JFH-1 system dramatically enhanced secreted infectivity, consistent with a late-acting role. Our system recapitulated authentic HCV particle assembly via trans-complementation of bicistronic, NS2-deleted, chimeric HCV, which is otherwise deficient in particle production. This closely resembled replicon-mediated NS2 trans-complementation, confirming that baculovirus expression of HCV proteins did not unduly affect particle production. Furthermore, this suggests that separation of structural protein expression from replicating HCV RNAs that are destined to be packaged alleviates an early stage requirement for NS2 during particle formation. This highlights our current lack of understanding of how NS2 mediates assembly, yet comparison of full-length and bipartite systems may provide further insight into this process.