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Home >> Books >> Hepatology 2012 >> HTML

#9002: Hepatology 2012

14 . Hepatitis C: New Drugs

Christian Lange, Christoph Sarrazin

Download the entire book (PDF, 546 pages).

Introduction

Combination therapy with pegylated interferon-a plus weight-based ribavirin leads to sustained virologic response (SVR, defined by undetectable HCV RNA 24 weeks after treatment completion) in approximately 50% of all HCV genotype 1-infected patients, compared to 70-90% of patients infected with HCV genotypes 2 and 3 (Zeuzem 2009). The limited treatment success of this treatment especially in HCV genotype 1 patients, the long treatment durations (up to 72 weeks), as well as the numerous side-effects of PEG-IFN α and ribavirin therapy, and an exploding knowledge of the HCV life cycle and of structural features of the HCV proteins, has spurred the development of many promising directly acting antiviral agents (DAA) (Kim 1996, Lindenbach 2005, Lohmann 1999, Lorenz 2006, Wakita 2005). In principle, each of the four HCV structural and six non-structural proteins, HCV-specific RNA structures such as the IRES, as well as host factors on which HCV depends, are suitable targets for DAA agents. In the following section, DAA compounds currently in clinical development are presented (Table 1, Figure 1).

Table 1. Selected directly acting antiviral agents in the pipeline.

Drug name	Company	Target / Active site		Phase
NS3-4A protease inhibitors
Ciluprevir (BILN 2061)	Boehringer Ingelheim	Active site / macrocyclic		Stopped
Telaprevir (VX-950)	Vertex / Janssen	Active site / linear		IV
Boceprevir (SCH503034)	Merck (S-P)	Active site / linear		IV
Simeprevir (TMC435350)	Janssen / Medivir	Active site / macrocyclic	also in separate Phase II studies with other DAA as part of IFN-free regimens, with and without RBV	III No clinically significant interaction between the investigational HCV protease inhibitor TMC435 and the immunosuppressives cyclosporine and tacrolimus
Danoprevir (R7227)	Roche / InterMune	Active site / macrocyclic		II
Vaniprevir (MK-7009)	Merck	Active site / macrocyclic		Halted/II
MK-5172	Merck	Active site / macrocyclic		II
BI201335	Boehringer Ingelheim	Active site / linear		III
Narlaprevir (SCH900518)	Schering-Plough	Active site / linear		Halted
Asunaprevir (BMS-650032)	Bristol-Myers Squibb	Active site		II
PHX1766	Phenomix	Active site		I
GS-9256	Gilead	Active site		II
GS-9451	Gilead	Active site		I
ABT450	Abbott	Active site		II
IDX320	Idenix	Active site		II
ACH-1625	Achillion	Active site / macrocyclic?		II
Nucleoside analog NS5B polymerase inhibitors Nucleoside analog inhibitors achieves NS5B inhibition NNIs bind distantly to the active centre of NS5B, their application may rapidly lead to the development of resistant mutants in vitro and in vivo. Moreover, mutations at the NNI binding sites do not necessarily lead to impaired function of the enzyme..... active centre of NS5B is a highly conserved region of the HCV genome, NIs are potentially effective against different genotypes. Single amino acid substitutions in every position of the active centre may result in loss of function or in extremely impaired replicative fitness. Thus, there is a relatively high barrier in the development of resistances to NIs.
Valopicitabine (NM283)	Idenix / Novartis	Active site		Stopped gastrointestinal side-effects
Mericitabine (R7128) RO5024048 the most advanced nucleoside polymerase inhibitor all HCV genotypes, and thus far no viral resistance against mericitabine	Roche / Pharmasset	Active site December 2011-March 2014	1. Placebo; Pegasys; Copegus; telaprevir ---------- 2. boceprevir; Pegasys; Copegus	II Enrollment120 treatment up to 48 weeks
R1626 a treatment for HCV, molecule (4'-azidocytidine/PSI-6130 Licensed PSI-6130 pro-drugs also including R7128	Roche internal program to develop R1626 for HCV treatment	Active site	R1626 Trial title : Dengue Completed A Study of Balapiravir in Patients With Dengue Virus Infection	Stopped severe lymphopenia and infectious disease adverse events Drug: balapiravir [RO4588161]; Drug: placebo
PSI-7977 GS-7977 sofosbuvir NCT01054729 NS5B Nucleoside analog inhibitors- impaired replicative fitness NNIs Qualify is based upon stratification for IL-28b status;	Pharmasset/ GSK NNIs bind distantly to the active centre of NS5B	Active site January 21, 2010-May 24, 2012 Last verified: May 2012	100 mg 200 mg 400 mg Placebo Interferon (PEG) Ribavirin (RBV) Eligible patients randomized to one of the 3 active cohorts	II twenty (20) subjects for each, with 16 subjects assigned to active PSI-7977 and 4 subjects assigned to matching placebo in a 4:1 randomization.
PSI-938 demonstrate high antiviral activities ..high genetic barrier to resistance	Pharmasset	Active site		Stopped citing "laboratory abnormalities associated with liver function.
IDX184	Idenix	Active site
IDX189 BMS-986094				Stopped
(HCVet Note: NEW BL-8020 (replicate) acts via a unique mechanism of action, by inhibiting Hepatitis C virus (HCV)-induced autophagy,a catabolic process involving the degradation of a cell's own components through the lysosomal machinery, which differs from the mechanism of currently used anti- HCV agents. Phase I/II clinical trial in Europe to test the drug's safety and efficacy during the first quarter of 2013 BL-8020 inhibits hepatitis-induced autophagy (a mechanism by which cells degrade damaged or unnecessary cellular components, including invading viruses) in the host cells, thereby reducing the ability of the hepatitis virus to replicate... BiolineRX has an exclusive worldwide license for BL-8020 from France Genoscience SA. Non-nucleoside NS5B polymerase inhibitors (NNI) non-nucleoside inhibitors (NNIs) ... achieves NS5B inhibition by binding to different allosteric enzyme sites, which results in conformational protein change before the elongation complex is formed (Beaulieu 2007). For allosteric NS5B inhibition high chemical affinity is required. NS5B is structurally organized in a characteristic "right hand motif", containing finger, palm and thumb domains, and offers at least four NNI-binding sites, a benzimidazole-(thumb 1)-, thiophene-(thumb 2)-, benzothiadiazine-(palm 1)- and benzofuran-(palm 2)-binding site (Lesburg 1999) (Figure 6). Because of their distinct binding sites, different polymerase inhibitors can theoretically be used in combination or in sequence to manage the development of resistance. Because NNIs bind distantly to the active centre of NS5B, their application may rapidly lead to the development of resistant mutants in vitro and in vivo. Moreover, mutations at the NNI binding sites do not necessarily lead to impaired function of the enzyme. Figure 7 shows the structure of selected nucleoside and non-nucleoside inhibitors.
BILB 1941	Boehringer Ingelheim	NNI site 1 / thumb 1		Stopped
BI207127	Boehringer Ingelheim	NNI site 1 / thumb 1		II
MK-3281	Merck	NNI site 1 / thumb 1		II
TMC647055	Janssen	NNI site 1 / thumb 1
Filibuvir (PF-00868554)	Pfizer	NNI site 2 / thumb 2		II
VCH759	ViroChem Pharma	NNI site 2 / thumb 2		II
VCH916	ViroChem Pharma	NNI site 2 / thumb 2		II
VCH222	ViroChem Pharma	NNI site 2 / thumb 2		II
ANA598	Anadys	NNI site 3 / palm 1		II
ABT-072	Abbott	NNI site 3 / palm 1		II
ABT-333	Abbott	NNI site 3 / palm 1		II
HCV-796	ViroPharma / Wyeth	NNI site 4 / palm 2		Stopped
GS-9190	Gilead	NNI site 4 / palm 2		II
IDX375	Idenix	NNI site 4 / palm 2		II
NS5A inhibitor nucleoside Daclatasvir (BMS-790052)	Bristol-Myers Squibb	NS5A domain 1 inhibitor		II
BMS-824393	Bristol-Myers Squibb	NS5A protein		I
PPI-461	Presidio Pharmaceuticals	NS5A protein		I
GS-5885	Gilead	NS5A protein		I
Indirect inhibitors / unknown mechanism of action
Alisporivir (Debio-025)	Debiopharm	Cyclophilin inhibitor		III
NIM811	Novartis	Cyclophilin inhibitor		I
SCY-635	Scynexis	Cyclophilin inhibitor		II
Nitazoxanide		PKR induction (?)		II
Miravirsen	Santaris	miRNA122 antisense RNA		II
Celgosivir	Migenix	Α-glucosidase inhibitor		II

HCV life cycle and treatment targets

HCV is a positive-sense single-stranded RNA virus of approximately 9600 nucleotides. The HCV genome contains a single large open reading frame encoding for a polyprotein of about 3100 amino acids. From this initially translated polyprotein, the structural HCV protein core (C) and envelope glycoproteins 1 and 2 (E1, E2); p7; and the six non-structural HCV proteins NS2, NS3, NS4A, NS4B, NS5A and NS5B, are processed by both viral and host proteases. The core protein forms the viral nucleocapsid carrying E1 and E2, which are receptors for viral attachment and host cell entry. The non-structural proteins are multifunctional proteins essential for the HCV life cycle (Bartenschlager 2004). P7 is a small hydrophobic protein that oligomerises into a circular hexamer, most likely serving as an ion channel through the viral lipid membrane. The large translated section of the HCV genome is flanked by the strongly conserved HCV 3Ž and 5Ž untranslated regions (UTR). The 5ŽUTR is comprised of four highly structured domains forming the internal ribosome entry site (IRES), which plays an important role in HCV replication (Figure 2).

Figure 1. HCV life cycle and targets for directly acting antiviral (DAA) agents.

NS3-4A protease inhibitors

Molecular biology

After receptor-mediated endocytosis, the fusion of HCV with cellular membranes, and uncoating the viral nucleocapsid, the single-stranded positive-sense RNA genome of the virus is released into the cytoplasm to serve as a messenger RNA for the HCV polyprotein precursor. HCV mRNA translation is under the control of the internal ribosome entry site (IRES), formed by domains II-IV of the HCV 5ŽUTR (Moradpour 2007). IRES mediates HCV polyprotein translation by forming a stable complex with the 40S ribosomal subunit, eukaryotic initiation factors and viral proteins.

Figure 2. Genomic organisation of HCV.

From the initially translated HCV polyprotein the three structural and seven non-structural HCV proteins are processed by both host and viral proteases (Moradpour 2007). NS2 is a metalloproteinase that cleaves itself from the NS2/NS3 protein, leading to its own loss of function and to the release of the NS3 protein (Lorenz 2006). NS3 provides a serine protease activity and a helicase/NTPase activity. The serine protease domain comprises two b-barrels and four α-helices. The serine protease catalytic triad - histidine 57, asparagine 81 and serine 139 - is located in a small groove between the two b-barrels (Kim 1996, Kim 1998). NS3 forms a tight, non-covalent complex with its obligatory cofactor and enhancer NS4A, which is essential for proper protein folding (Figure 3). The NS3-4A protease cleaves the junctions between NS3/NS4A, NS4A/NS4B, NS4B/NS5A and NS5A/NS5B. Besides its essential role in protein processing, NS3 is integrated into the HCV RNA replication complex, supporting the unwinding of viral RNA by its helicase activity. Moreover, NS3 might play an important role in HCV persistence via blocking TRIF-mediated toll-like receptor signalling and Cardif-mediated RIG-I signalling, subsequently resulting in impaired induction of type I interferons (Meylan 2005). Thus, pharmacologic NS3 inhibition might support viral clearance by restoring the innate immune response.

Figure 3. Molecular structure of the HCV NS3-4A protease.

The active site of the NS3-4A protease is located in a shallow groove between the two b-barrels of the protease making the design of compound inhibitors relatively difficult. Nevertheless, many NS3-4A protease inhibitors have been developed which can be divided into two classes, the macrocyclic inhibitors and linear tetra-peptide a-ketoamide derivatives. In general, NS3-4A protease inhibitors have been shown to strongly inhibit HCV replication during monotherapy, but also may cause the selection of resistant mutants, which is followed by viral breakthrough. The additional administration of pegylated interferon and ribavirin, however, was shown to reduce the frequency of development of resistance. Future strategies aim for combination therapies with different antiviral drugs to prevent the development of resistance. The most advanced compounds are telaprevir and boceprevir, both approved in 2011.

Ciluprevir (BILN 2061)

The first clinically tested NS3-4A inhibitor was ciluprevir (BILN 2061), an orally bioavailable, peptidomimetic, macrocyclic drug binding non-covalently to the active center of the enzyme (Lamarre 2003) (Figure 4). Ciluprevir monotherapy was evaluated in a double-blind, placebo-controlled pilot study in treatment-naïve genotype 1 patients with liver fibrosis and compensated liver cirrhosis (Hinrichsen 2004). Ciluprevir, administered twice daily for two days at doses ranging from 25 to 500 mg, led to a mean 2-3 log₁₀ decrease of HCV RNA serum levels in most patients. Importantly, the stage of disease did not affect the antiviral efficacy of ciluprevir. The tolerability and efficacy of ciluprevir in HCV genotype 2- and 3-infected individuals was examined in an equivalent study design, but compared to genotype 1 patients, the antiviral activity was less pronounced and more variable in these patients (Reiser 2005). Ciluprevir development has been halted.

Figure 4. Molecular structure of selected NS3-4A inhibitors.

Telaprevir (Incivek/Incivo^®) and boceprevir (Victrelis^®)

Telaprevir and boceprevir were approved for the treatment of chronic hepatitis C virus genotype 1 infection by the FDA, EMA and several other countries in 2011. Both telaprevir and boceprevir are orally bioavailable, peptidomimetic NS3-4A protease inhibitors belonging to the class of a-ketoamid derivatives (Figure 4). Like other NS3-4A inhibitors, telaprevir and boceprevir are characterized by a remarkable antiviral activity against HCV genotype 1. However, monotherapy with these agents results in the rapid selection of drug-resistant variants followed by viral breakthrough (Reesink 2006, Sarrazin 2007). Phase II and III clinical studies have shown that the addition of pegylated interferon α plus ribavirin leads to a substantially reduced frequency of resistant mutants and viral breakthrough, and to significantly higher SVR rates in both treatment-naïve and treatment-experienced HCV genotype 1 patients compared to treatment with pegylated interferon α and ribavirin alone (Bacon 2011, Jacobson 2011, Poordad 2011, Sherman 2011, Zeuzem 2011). Therefore, telaprevir- and boceprevir-based triple therapy can be considered the novel standard of care for HCV genotype 1 patients. Results of the Phase III telaprevir and boceprevir approval studies are summarized in Figure 5.

Figure 5. SVR rates in Phase III clinical trials evaluating telaprevir (A) or boceprevir (B) in combination with PEG-IFN α and ribavirin. ADVANCE, ILLUMINATE and SPRINT-2 enrolled treatment-naïve patients, REALIZE and RESPOND-2 enrolled treatment-experienced patients. Telaprevir was administered for 8 or 12 weeks in combination with PEG-IFN α-2a and ribavirin, followed by 12-40 weeks of PEG-IFN α-2a and ribavirin alone. Boceprevir was administered over the whole treatment period of 28 or 48 weeks in combination with PEG-INF α-2b and ribavirin, except of the first 4 weeks of lead-in therapy of PEG-IFN α-2b and ribavirin only. eRVR, extended early virologic response; SOC, standard of care; LI, lead-in (4 weeks of PEG-INF α plus ribavirin only).

Other NS3 protease inhibitors

Other NS3 protease inhibitors are currently in various phases of development (danoprevir (R7227/ITMN191), vaniprevir (MK-7009), BI201335, simeprevir (TMC435350), narlaprevir (SCH900518), asunaprevir (BMS-650032), PHX1766, ACH-1625, IDX320, ABT-450, MK-5172, GS-9256, GS-9451) and will significantly increase treatment options for chronic hepatitis C in the near future. In general, comparable antiviral activities to telaprevir and boceprevir in HCV genotype 1 infected patients were observed during mono- (and triple-) therapy studies (Brainard 2010, Manns 2011, Reesink 2010). Potential advantages of these second and third generation protease inhibitors might be improved tolerability, broader genotypic activity, different resistance profiles, and/or improved pharmacokinetics for once-daily dosage (e.g., TMC435, BI201335). Different resistance profiles between linear tetrapeptide and macrocyclic inhibitors binding to the active site of the NS3 protease have been revealed. However, R155 is the main overlapping position for resistance and different mutations at this amino acid site within the NS3 protease confer resistance to nearly all protease inhibitors currently in advanced clinical development (Sarrazin 2010). An exception is MK-5172, which exhibits potent antiviral activity against variants carrying mutations at position R155. In addition, MK-5172 had potent antiviral activity against both HCV genotype 1 and 3 isolates (Brainard 2010).

Resistance to NS3-4A inhibitors

Because of the high replication rate of HCV and the poor fidelity of its RNA-dependent RNA polymerase, numerous variants (quasispecies) are continuously produced during HCV replication. Among them, variants carrying mutations altering the conformation of the binding sites of DAA compounds can develop. During treatment with specific antivirals, these preexisting drug-resistant variants have a fitness advantage and can be selected to become the dominant viral quasispecies. Many of these resistant mutants exhibit an attenuated replication with the consequence that, after termination of exposure to specific antivirals, the wild type may displace the resistant variants (Sarrazin 2007). Nevertheless, HCV quasispecies resistant to NS3-4A protease inhibitors or non-nucleoside polymerase inhibitors can be detected at low levels in some patients (approx. 1%) who have never been treated with these specific antivirals before (Gaudieri 2009). The clinical relevance of these pre-existing mutants is not completely understood, although there is evidence that they may reduce the chance of achieving an SVR with DAA-based triple therapies if the patient's individual sensitivity to pegylated interferon α + ribavirin is low.

More recently, the Q80R/K variant has been described as conferring low-level resistance to simeprevir (TMC435), a macrocyclic protease-inhibitor. Interestingly, the Q80K variant can be detected in approximately 10% of HCV genotype 1-infected patients (typically in subtype 1a isolates) and a slower viral decline during simeprevir-based triple therapy was observed (Lenz 2011). Table 2 summarizes the resistance profile of selected NS3-4A inhibitors. Although the resistance profiles differ significantly, R155 is an overlapping position for resistance development and different mutations at this position confer resistance to nearly all protease inhibitors (not MK-5172) currently in advanced clinical development (Sarrazin 2010). Importantly, many resistance mutations could be detected in vivo only by clonal sequencing. For example, mutations at four positions conferring telaprevir resistance have been characterized so far (V36A/M/L, T54A, R155K/M/S/T and A156S/T), but only A156 could be identified initially in vitro in the replicon system (Lin 2005). These mutations, alone or as double mutations, conferred low (V36A/M, T54A, R155K/T, A156S) to high (A156T/V, V36M + R155K, V36M + 156T) levels of resistance to telaprevir (Sarrazin 2007). It is thought that the resulting amino acid changes of these mutations alter the confirmation of the catalytic pocket of the protease, which impedes binding of the protease inhibitor (Welsch 2008).

Table 2. Resistance mutations to HCV NS3 protease inhibitors.

	36	54	55	80	155	156A	156B	168	170
Telaprevir* (linear)
Boceprevir* (linear)
SCH900518* (linear)
BI-201335* (linear?)
BILN-2061 ** (macrocyclic)
Danoprevir* (macrocyclic)
MK-7009* (macrocyclic)
TMC435* (macrocyclic)
BMS-650032*
(macrocyclic)
GS-9451*
(macrocyclic)
ABT450*
(macrocyclic)
IDX320**
(macrocyclic)
ACH1625**
(macrocyclic)
MK-5172***
(macrocyclic)

36: V36A/M; 54: T54S/A; 55: V55A; 80: Q80R/K; 155: R155K/T/Q; 156A: A156S; 156B: A156T/V; 168: D168A/V/T/H; 170: V170A/T

* mutations associated with resistance in patients

** mutations associated with resistance in vitro

*** no viral break-through during 7 days monotherapy

# Q80 variants have been observed in approximately 10% of treatment-naïve patients and was associated with slower viral decline during simeprevir (TMC435) triple therapy

As shown for other NS3-4A protease inhibitors as well (e.g., danoprevir), the genetic barrier to telaprevir resistance differs significantly between HCV subtypes. In all clinical studies of telaprevir alone or in combination with PEG-IFN α and ribavirin, viral resistance and breakthrough occurred much more frequently in patients infected with HCV genotype 1a compared to genotype 1b. This difference was shown to result from nucleotide differences at position 155 in HCV subtype 1a (aga, encodes R) versus 1b (cga, also encodes R). The mutation most frequently associated with resistance to telaprevir is R155K; changing R to K at position 155 requires 1 nucleotide change in HCV subtype 1a, and 2 nucleotide changes in subtype 1b isolates (McCown 2009).

It will be important to define whether treatment failure due to the development of variants resistant to DAA agents has a negative impact on re-treatment with the same or a different DAA treatment regimen. Follow-up studies of telaprevir and boceprevir Phase III studies have revealed a rapid decline of resistant variants below the limit of detection (>20% of quasispecies) of population sequencing techniques (Barnard 2011, Sherman 2011). However, telaprevir- and boceprevir-resistant variants were detectable by a clonal sequencing approach several years after treatment in single patients who had been treated with telaprevir or boceprevir within smaller Phase Ib studies (Susser 2011).

NS5B polymerase inhibitors

Molecular biology

HCV replication is initiated by the formation of the replication complex, a highly structured association of viral proteins and RNA, of cellular proteins and cofactors, and of rearranged intracellular lipid membranes derived from the endoplasmic reticulum (Moradpour 2007). The key enzyme in HCV RNA replication is NS5B, an RNA-dependent RNA polymerase that catalyzes the synthesis of a complementary negative-strand RNA by using the positive-strand RNA genome as a template (Lesburg 1999) (Figure 6). From this newly synthesized negative-strand RNA, numerous RNA strands of positive polarity are produced by NS5B activity that serve as templates for further replication and polyprotein translation. Because of poor fidelity leading to a high rate of errors in its RNA sequencing, numerous different isolates are generated during HCV replication in a given patient, termed HCV quasispecies. It is reasoned that due to the lack of proofreading of the NS5B polymerase together with the high replication of HCV, every possible mutation is generated each day.

NS5B RNA polymerase inhibitors can be divided into two distinct categories. Nucleoside analog inhibitors (NIs) like valopicitabine (NM283), mericitabine (R7128), R1626, PSI-7977, PSI-938 or IDX184 mimic the natural substrates of the polymerase and are incorporated into the growing RNA chain, thus causing direct chain termination by tackling the active site of NS5B (Koch 2006). Because the active centre of NS5B is a highly conserved region of the HCV genome, NIs are potentially effective against different genotypes. Single amino acid substitutions in every position of the active centre may result in loss of function or in extremely impaired replicative fitness. Thus, there is a relatively high barrier in the development of resistances to NIs.

In contrast to NIs, the heterogeneous class of non-nucleoside inhibitors (NNIs) achieves NS5B inhibition by binding to different allosteric enzyme sites, which results in conformational protein change before the elongation complex is formed (Beaulieu 2007). For allosteric NS5B inhibition high chemical affinity is required. NS5B is structurally organized in a characteristic "right hand motif", containing finger, palm and thumb domains, and offers at least four NNI-binding sites, a benzimidazole-(thumb 1)-, thiophene-(thumb 2)-, benzothiadiazine-(palm 1)- and benzofuran-(palm 2)-binding site (Lesburg 1999) (Figure 6). Because of their distinct binding sites, different polymerase inhibitors can theoretically be used in combination or in sequence to manage the development of resistance. Because NNIs bind distantly to the active centre of NS5B, their application may rapidly lead to the development of resistant mutants in vitro and in vivo. Moreover, mutations at the NNI binding sites do not necessarily lead to impaired function of the enzyme. Figure 7 shows the structure of selected nucleoside and non-nucleoside inhibitors.

Figure 6. Structure of the HCV NS5B RNA polymerase and binding sites.

Figure 7. Molecular structure of selected NS5B polymerase inhibitors.

Nucleoside analogs

Valopicitabine (NM283, 2'-C-methylcytidine/NM107), the first nucleoside inhibitor investigated in patients with chronic hepatitis C, showed a low antiviral activity (Afdhal 2007). Due to gastrointestinal side effects the clinical development of NM283 was stopped.

The second nucleoside inhibitor to be reported in patients with chronic hepatitis C was R1626 (4'-azidocytidine/PSI-6130). A Phase 1 study in genotype 1-infected patients observed a high antiviral activity at high doses of R1626 in genotype 1-infected patients (Pockros 2008). No viral breakthrough with selection of resistant variants was reported from monotherapy or combination studies with pegylated interferon ± ribavirin (Pockros 2008). Due to severe lymphopenia and infectious disease adverse events further development of R1626 was stopped.

Mericitabine (RG7128) is still in development and the most advanced nucleoside polymerase inhibitor. Mericitabine is safe and well-tolerated, effective against all HCV genotypes, and thus far no viral resistance against mericitabine has been observed in clinical studies. Interim results of current Phase II clinical trials in HCV genotype 1-, 2-, 3-infected patients of R7128 in combination with pegylated interferon and ribavirin revealed superior SVR rates of mericitabine-based triple therapy compared to PEG-IFN α alone (Pockros 2011). In an all oral regimen, administration of R7128 in combination with the protease inhibitor R7227/ITMN191 for 14 days, a synergistic antiviral activity of both drugs was observed (Gane 2010). No viral breakthrough with selection of resistant variants has been reported.

Very promising clinical data have been published recently for PSI-7977, a nucleoside analog NS5B inhibitor effective against all HCV genotypes. In HCV genotype 2- and 3- infected patients, PSI-7977 (400 mg once daily) in combination with ribavirin for 12 weeks + PEG-IFN α for 4-12 weeks resulted in 100% RVR and 100% week 12 SVR rates (Gane 2011). No PSI-7977-associated side effects have been reported, and no virologic breakthrough has been observed. A second study evaluated PSI-7977-based triple therapy in treatment-naïve HCV genotype 1-infected patients. In this study, PSI-7977 was administered for 12 weeks, together with PEG-IFN α and ribavirin for 24 or 48 weeks in total, according to whether HCV RNA was below the limit of detection at treatment weeks 4 and 12 or not, respectively (Lawitz 2011). Most patients were eligible for the shortened treatment duration of 24 weeks, and SVR was achieved in approximately 90% of all patients.

Other nucleoside analogs (e.g., PSI-938 and IDX184) are at earlier stages of clinical development (Sarrazin 2010).

Overall, the newer nucleoside analogs (PSI-7977, PSI-938) also demonstrate high antiviral activities that, together with their high genetic barrier to resistance, suggest that they are optimal candidates for all-oral combination therapies (see below).

Non-nucleoside analogs

At least 4 different allosteric binding sites have been identified for inhibition of the NS5B polymerase by non-nucleoside inhibitors. Currently, numerous non-nucleoside inhibitors are in Phase I and II clinical evaluation (e.g., NNI site 1 inhibitor BI207127; NNI site 2 inhibitors filibuvir (PF-00868554), VCH-759, VCH-916 and VCH-222; NNI site 3 inhibitor ANA598, NNI site 4 inhibitors HCV-796, and ABT-333) (Ali 2008, Cooper 2007, Erhardt 2009, Kneteman 2009). In general, these non-nucleoside analogs display a low to medium antiviral activity and a low genetic barrier to resistance, evidenced by frequent viral breakthrough during monotherapy studies and selection of resistance mutations at variable sites of the enzyme. In line with these experiences in Phase I studies, a Phase II triple therapy study with filibuvir in combination with pegylated interferon and ribavirin showed high relapse and relative low SVR rates (Jacobson 2010). In contrast to nucleoside-analogs, non-nucleoside analogs in general do not display antiviral activity against different HCV genotypes (Sarrazin 2010). Due to their low antiviral efficacy and low genetic barrier to resistance, non-nucleoside analogs will probably not be developed as part of triple therapy but rather as components of quadruple or all-oral regimens (see below).

NS5A inhibitors

The HCV NS5A protein seems to play a manifold role in HCV replication, assembly and release (Moradpour 2007). It was shown that NS5A is involved in the early formation of the replication complex by interacting with intracellular lipid membranes, and it initiates viral assembly at the surface of lipid droplets together with the HCV core (Shi 2002). NS5A may also serve as a channel that helps to protect and direct viral RNA within the membranes of the replication complex (Tellinghuisen 2005). Moreover, it was demonstrated that NS5A is able to interact with NS5B, which results in an enhanced activity of the HCV RNA polymerase. Besides its regulatory impact on HCV replication, NS5A has been shown to modulate host cell signaling pathways, which has been associated with interferon resistance (Wohnsland 2007). Furthermore, mutations within the NS5A protein have been clinically associated with resistance / sensitivity to IFN-based antiviral therapy (Wohnsland 2007).

BMS-790052 was the first NS5A inhibitor to be clinically evaluated. Even low doses of BMS-790052 display high antiviral efficacy against all HCV genotypes in vitro. Monotherapy with BMS-790052 led to a sharp initial decline of HCV RNA concentrations, though its genetic barrier to resistance is relatively low (Gao 2010). According to an interim analysis of a Phase IIb clinical trial in treatment-naïve HCV genotype 1 and 4 patients, treatment with 20 or 60 mg BMS-790052 once daily in combination with PEG-IFN α and ribavirin for 24 or 28 weeks, 54% of all patients achieved an extended RVR, compared to 13% in the control group (Hezode 2011). SVR rates of this study are awaited.

During monotherapy, rapid selection of variants resistant to BMS-790052 occurred (Nettles 2011). The most common resistance mutations in HCV genotype 1a patients were observed at residues M28, Q30, L31, and Y93 of NS5A. In HCV genotype 1b patients, resistance mutations were observed less frequently, predominantly at positions L31 and Y93. These resistance mutations increased the EC50 to BMS-790052 moderately to strongly (Fridell 2011). However, no cross-resistance between BMS-790052 and other DAA agents has been reported. Collectively, BMS-790052 is a highly promising agent for both triple therapy as well as all-DAA combination therapy approaches.

Other NS5A inhibitors (e.g., BMS-824393, PPI-461, GS-5885) are in early clinical development.

Compounds targeting viral attachment and entry

The tetraspanin protein CD81, claudin-1, occludine, scavenger receptor class B type 1 (SR-B1), the low-density lipoprotein (LDL) receptor, glycosaminoglycans and the dendritic cell- /lymph node-specific intercellular adhesion molecule 3-grabbing non-integrin (DC-SIGN/L-SIGN) have been identified as putative ligands for E1 and E2 in the viral attachment and entry steps (Moradpour 2007). HCV entry inhibition might enrich future hepatitis C treatment opportunities, in particular in the prevention of HCV liver graft reinfection. HCV entry inhibition can be theoretically achieved by the use of specific antibodies or small molecule compounds either blocking E1 and E2 or their cellular receptors. So far, only results from clinical trials using polyclonal (e.g., civacir) (Davis 2005) or monoclonal (e.g., HCV-AB 68) (Schiano 2006) HCV-specific antibodies are available. The clinical benefit of these antibodies has been poor, however. The development of small molecule entry inhibitors is in a preclinical stage and is complicated by difficulties in the crystallographic characterization of HCV envelope proteins.

Host factors as targets for treatment

Cyclophilin B inhibitors

HCV depends on various host factors throughout its life cycle. Cyclophilin B is expressed in many human tissues and provides a cis-trans isomerase activity, which supports the folding and function of many proteins. Cyclophilin B enhances HCV replication by incompletely understood mechanisms, like the modulation of NS5B activity. Debio-025 (alisporivir) is an orally bioavailable cyclophilin B inhibitor exerting an antiviral impact on both HCV and HIV replication. In clinical trials in HIV- and HCV-coinfected patients, treatment with 1200 mg Debio-025 twice daily for two weeks led to a mean maximal log₁₀ reduction of HCV RNA of 3.6 and of HIV DNA of 1.0 (Flisiak 2008). Debio-025 was well-tolerated and no viral breakthrough occurred during the 14 days of treatment.

Combination therapy of Debio-025 200 mg, 600 mg or 1000 mg and PEG-IFN a-2a was evaluated in a double-blind placebo-controlled Phase II trial in treatment-naïve patients monoinfected with HCV genotypes 1, 2, 3 or 4. Treatment was administered for 29 days. Mean log₁₀ reductions in HCV RNA at day 29 were 4.75 (1000 mg), 4.61 (600 mg) and 1.8 (200 mg) in the combination therapy groups compared to 2.49 (PEG-IFN a-2a alone) and 2.2 (1000 mg Debio-025 alone) in the monotherapy groups. No differences in antiviral activity were observed between individuals infected with the different genotypes. Debio-025 was safe and well tolerated but led to a reversible bilirubin increase in some of the patients treated with 1000 mg Debio-025 daily (Flisiak 2009). A high genetic barrier to resistance of Debio-025 and a broad HCV genotypic activity highlight the potential of drugs targeting host proteins.

In a Phase II clinical trial in treatment-naïve HCV genotype 1 patients, combination therapy with Debio-025, PEG-IFN a-2a and ribavirin for 24-48 weeks resulted in SVR rates of 69-76% compared to 55% in the control group (Flisiak 2011).

Nitazoxanide

Nitazoxanide with its active metabolite tizoxanide is a thiazolide antiprotozoal approved for the treatment of Giardia lamblia and Cryptosporidium parvum infections. In vitro studies have revealed an essential inhibitory impact on HCV and HBV replication by still unknown mechanisms.

Results of two Phase 2 studies evaluating 500 mg nitazoxanide twice daily for 12 weeks followed by nitazoxanide, PEG-IFN α-2a ± RBV for 36 weeks yielded conflicting results with SVR rates of 79% in treatment-naïve genotype 4 patients, but of only 44% in HCV genotype 1 patients (Rossignol 2009). Additional studies are warranted to determine the role of nitazoxanide in the treatment of chronic hepatitis C.

Silibinin

Silymarin, an extract of milk thistle (Silybum marianum) with antioxidant activity, has been empirically used to treat chronic hepatitis C and other liver diseases. Silibinin is one of the six major flavonolignans in silymarin. Surprisingly, recent reports demonstrated that silibinin inhibits HCV at various steps of its life cycle (Ahmed-Belkacem 2010, Wagoner 2010). In addition, intravenous silibinin in non-responders to prior IFN-based antiviral therapy led to a decline in HCV RNA between 0.55 to 3.02 log₁₀ IU/ml after 7 days and a further decrease after an additional 7 days in combination with PEG-IFN α-2a/RBV in the range of 1.63 and 4.85 log₁₀ IU/ml (Ferenci 2008). Ongoing studies will clarify the role of silibinin in the treatment of chronic hepatitis C, including HCV liver graft reinfection.

Miravirsen

MicroRNA-122 (miRNA-122) is a liver-specific microRNA that has been shown to be a critical host factor for HCV (Landford 2010). MiRNA-122 binds to the 5ŽNTR region of the HCV genome, which appears to be vital in the HCV replication process. Miravirsen is a modified antisense oligonucleotide that targets miRNA-122 and thereby prevents binding of miRNA-122 to the HCV genome. In a Phase IIa proof-of-principle study, weekly subcutaneous injections of miravirsen led to a reduction of HCV RNA serum concentration of up to 2.7 log₁₀ IU/mL, indicating that an antisense oligonucleotide-based approach of miRNA-122 inhibition could be a promising modality for antiviral therapy (Janssen 2010). No relevant side effects were seen in this study.

Newer combination therapies

The approval of the HCV protease inhibitors telaprevir and boceprevir in 2011 constitutes a milestone in the treatment of chronic HCV genotype 1 infection. Nevertheless, telaprevir- or boceprevir-based triple therapy has certain limitations. In particular, treatment success still depends on the interferon-sensitivity of individual patients because a slow decline of HCV viral load during triple therapy is associated with a high risk of antiviral resistance development. Consequently, viral breakthrough of drug resistant variants was observed in a significant number of patients with partial or null response to previous treatment with PEG-IFN α and ribavirin, in patients with limited decline of HCV viral load during lead-in treatment with PEG-IFN α and ribavirin alone, or in difficult to cure populations like Blacks or patients with advanced liver fibrosis. In addition, triple therapy is not an option for patients with contraindications to PEG-IFN α or ribavirin, such as patients with decompensated liver cirrhosis or liver transplant failure.

To overcome these limitations, numerous trials have been initiated to investigate the potential of combination therapies with different DAA agents alone (Table 3). As is well established in the treatment of HIV infection, combining DAA agents with different antiviral resistance profiles should result in a substantially decreased risk of viral breakthrough of resistant variants. Nucleoside analog NS5B inhibitors plus drugs targeting host factors such as the cyclophilin inhibitor alisporivir display a high genetic barrier to resistance development and may therefore be key agents for effective DAA combination therapies (Sarrazin 2010). In contrast, NS3-4A and NS5A inhibitors display a low genetic barrier to resistance development, but in view of their high antiviral efficacy they appear to be promising combination partners for nucleoside analogs or cyclophilin inhibitors. Due to their low antiviral efficacy and low genetic barrier to resistance development, the role of non-nucleoside analog NS5B inhibitors is currently less clear. A potential advantage of non-nucleoside analogs is their binding to multiple target sites that may allow simultaneous treatment with several non-nucleoside analogs.

Currently, DAA combination treatment regimens can be classified according to the usage of PEG-IFN α into quadruple therapy regimens and all-oral therapy regimens. Quadruple therapy approaches are based on therapy of PEG-IFN α and ribavirin plus combination of two DAA agents from different classes. In contrast, all-oral treatment comprises interferon-free regimens including combinations of various DAA compounds with or without ribavirin.

Quadruple therapy

Preliminary SVR data of a small but highly informative trial serves as a proof-of-concept for the potential of quadruple therapy approach for patients with previous null response to PEG-IFN α + ribavirin (Lok 2011). In this Phase II study, 11 HCV genotype 1 patients with prior null response were treated with a combination of the NS5A inhibitor BMS-790052 and the protease inhibitor BMS-650032 together with PEG-IFN α and ribavirin for 24 weeks. Quadruple therapy resulted in 100% SVR 12 weeks after treatment completion in both HCV genotype 1a- and 1b-infected patients. Even though the number of patients included in this trial was very limited, this high SVR rate after quadruple therapy seems impressive compared to SVR rates of ~30% that were achieved with telaprevir-based triple therapy in prior null responders (Zeuzem 2011).

A Phase II clinical trial assessed quadruple therapy with the non-nucleoside NS5B inhibitor tegobuvir in combination with the NS3-4A inhibitor GS-9256 + PEG-IFN α and ribavirin for 28 days in treatment-naïve HCV genotype 1 patients (Zeuzem 2011). The primary endpoint of this study was rapid virologic response (RVR), which was achieved in 100% of patients. After 28 days of quadruple therapy, treatment with PEG-IFN α and ribavirin was continued, which led to complete early virologic reponse (cEVR) in 94% of patients (Zeuzem 2011).

Another Phase II clinical trial investigated a response-guided approach during quadruple therapy containing the non-nucleoside NS5B inhibitor VX-222 (100 mg or 400 mg) in combination with the NS3-4A inhibitor telaprevir + PEG-IFN α and ribavirin in treatment-naïve HCV genotype 1 patients (Nelson 2011). Quadruple treatment was administered for 12 weeks. All treatment was stopped after 12 weeks in patients who were HCV RNA-negative at treatment weeks 2 and 8. Patients in whom HCV RNA was detectable at treatment week 2 or 8 received an additional 12 weeks of PEG-IFN α and ribavirin alone. Up to 50% of patients met the criteria for the 12-week treatment duration. Of those, 82-93% achieved an SVR 12 weeks after treatment completion. In patients who were treated with an additional 12 weeks of PEG-IFN α and ribavirin, the end-of-treatment response was 100%.

Collectively, the quadruple therapy approach appears to be highly promising in patients with limited sensitivity to interferon-α, even in patients with HCV subtype 1a.

All-oral therapy without ribavirin

A first interferon-free clinical trial (the INFORM-1 study) evaluated the combination of a polymerase inhibitor (R7128) and an NS3 inhibitor (R7227/ITMN191). In this proof of principle study, patients were treated with both compounds for up to 2 weeks (Gane 2010). HCV RNA concentrations decreased by up to 5.2 log₁₀ IU/ml, viral breakthrough was observed in only one patient (although no resistant HCV variants were identified), and HCV RNA was undetectable at the end of dosing in up to 63% of treatment-naïve patients. However, the fundamental question of whether an SVR can be achieved with combination therapies of different DAA compounds without PEG-IFN α and ribavirin was not answered by this trial.

SVR data are available for a Phase II clinical trial investigating therapy with the NS5A inhibitor BMS-790052 in combination with the NS3-4A protease inhibitor BMS-60032 for 24 weeks in 10 HCV genotype 1 patients with a previous null response to PEG-IFN α and ribavirin (Lok 2011). 36% of patients achieved an SVR 24 weeks after treatment completion. All patients with viral breakthrough were infected with HCV genotype 1a, and in all of them HCV variants with resistance mutations against both agents were detected. Although data of longer follow-up periods are needed, this trial constitutes a proof-of-principle that SVR can be achieved via all-oral regimens, even in patients infected with HCV subtype 1b. This was confirmed with a 100% SVR rate in a small study evaluating the same agents (BMS-790052 and BMS-60032) in Japanese HCV genotype 1b previous null responders (Chayama 2011).

Another trial has investigated 12 weeks of PSI-7977 monotherapy (400 mg once daily) in HCV genotype 2- and 3-infected patients (n=10). 100% of patients achieved an RVR and EOTR, which translated into an SVR in 60% of patients (Gane 2011).

All-oral therapy with ribavirin

Two trials evaluated all-oral DAA combination therapies with ribavirin. In one of them, combination therapy of the NS3-4A inhibitor BI-201335, the non-nucleoside NS5B inhibitor BI-207127 (400 or 600 mg TID) and ribavirin for 4 weeks was assessed (Zeuzem 2011). Virologic response rates in patients treated with 600 mg TID of BI-207127 were 82%, 100% and 100% at treatment days 15, 22, and 29, respectively (Zeuzem 2011). In patients who received the lower dose of BI-207127, virologic response rates were significantly lower, and in these patients lower virologic response rates were observed for patients infected with HCV subtype 1a compared to subtype 1b.

Another trial compared tegobuvir (a non-nucleoside NS5B inhibitor) + GS-9256 (a NS3-4A inhibitor) with or without ribavirin in treatment-naïve HCV genotype 1 patients (Zeuzem 2011). Importantly, tegobuvir + GS-9256 + ribavirin led to a higher HCV RNA decline after 28 days of treatment compared to tegobuvir + GS-9256 alone (-5.1 log₁₀ vs. -4.1 log₁₀, respectively), indicating that ribavirin might be an important component of interferon-free DAA combination therapies. SVR data of these and additional combination therapy regimens are expected in the near future.

Additional trials investigated all-oral combination regimens with ribavirin in HCV genotype 2 and 3 patients. 12 weeks of PSI-7977 plus ribavirin resulted in 100% RVR, EOTR, and SVR rates in a small number of treatment-naïve patients (n=10) (Gane 2011). In contrast, during treatment with the cyclophilin A inhibitor alisporivir in combination with ribavirin, only approximately 50% of HCV genotype 2 and 3 patients became HCV RNA-negative at treatment week 6 (Pawlotsky 2011). Nevertheless, these data highlight the impressive potential of all-oral regimens, when agents with little risk of antiviral resistance development such as nucleoside analog NS5B inhibitors are used in combination with ribavirin.

Table 3. Selected trials evaluating DAA combination therapies.

DAAs combined	Additional medication	Phase
BMS-650032 (NS3-4A inhibitor)	+ / - PEG-IFN α and ribavirin	II
+ BMS-790052 (N5A inhibitor)
BI-201335 (NS3-4A inhibitor)	+ ribavirin + / - PEG-IFN α	II
+ BI-207127 (non-nuc. NS5B inhibitor)
GS-9190 (non-nuc. NS5B inhibitor)	+ / - ribavirin + / - PEG-IFN α	II
+ GS-92568 (NS3-4A inhibitor)
Danoprevir (NS3-4A inhibitor)	followed by PEG-IFN α and ribavirin	II
+ RG-7128 (nuc. NS5B inhibitor)
Telaprevir (NS3-4A inhibitor)	+ / - ribavirin + / - PEG-IFN α	II
+ VX-222 (non-nuc. NS5B inhibitor)
PSI-938 (purine nuc. NS5B inhibitor)	-	II
+ PSI-7977 (pyrimidine nuc. NS5B inhibitor)

Novel interferons

Over the last years, attempts have been made to reduce side effects and treatment discomfort of PEG-IFN α. However, interferons with longer half-life and sustained plasma concentrations (e.g., albinterferon, a fusion protein of IFN α 2b with human albumin) have so far shown no overall benefit with respect to SVR rates (Zeuzem 2010). Still promising is the development of pegylated interferon lambda 1 (PEG-IFN lambda 1). Like other type 3 interferons, IFN lambda 1, which is also called interleukin-29 (IL-29), binds to a different receptor than IFN α, but downstream signaling pathways of IFN lambda and IFN α are largely comparable. The IFN lambda receptor is predominantly expressed in hepatocytes. Thus, interferon-related side effects may be less frequent during PEG-IFN lambda treatment. A Phase I clinical trial evaluating pegylated interferon lambda with or without ribavirin was completed (Muir 2010). Interferon lambda was well-tolerated and the majority of patients achieved a greater than 2 log₁₀ decline of HCV RNA by 4 weeks. According to an interim analysis of a subsequent Phase II clinical trial, PEG-IFN lambda (240 ug, 180 ug, or 120 ug once weekly) was compared to PEG-IFN α-2a. PEG-IFN lambda at doses of 240 or 180 ug resulted in approximately 10% higher RVR and approximately 20% higher cEVR rates, a lower frequency of flu-like symptoms, but with more frequent aminotransferase and bilirubin elevations than PEG-IFN α-2a (Zeuzem 2011).

Conclusions

Telaprevir- and boceprevir-based triple therapy of treatment-naïve and treatment-experienced HCV genotype 1 patients results in substantially increased SVR rates compared to PEG-INF-α and ribavirin alone. The approval of these agents represents a major breakthrough in the treatment of chronic hepatitis C. However, successful use of these drugs will require a precise classification of response patterns to previous treatment, careful on-treatment monitoring of HCV viral load and emergence of antiviral resistance as well as of additional side effects and numerous possible drug-drug interactions. Next-generation NS3-4A protease inhibitors and NS5A inhibitors may have even more favorable properties than telaprevir and boceprevir in terms of HCV genotype coverage, safety profiles, less pronounced drug-drug interactions, or possible once-daily administration. However, the triple therapy approach has several limitations. First of all, concomitant IFN α and ribavirin are necessary to avoid the development of antiviral resistance. Consequently, the efficacy of triple therapy was limited in prior null responders to PEG-IFN α and ribavirin, and triple therapy cannot be administered to patients with contraindications to PEG-IFN α or ribavirin. Recent data indicate that the development of DAA combination therapies in all-oral or quadruple treatment regimens will likely be a very potent option for these patients. In such DAA combination regimens, the inclusion of drugs with a high genetic barrier to resistance such as nucleoside NS5B inhibitors or drugs targeting host factors such as alisporivir may be important.

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Reiser M, Hinrichsen H, Benhamou Y, et al. Antiviral efficacy of NS3-serine protease inhibitor BILN-2061 in patients with chronic genotype 2 and 3 hepatitis C. Hepatology 2005;41:832-5. (Abstract)

Rossignol JF, Elfert A, El-Gohary Y, Keeffe EB. Improved virologic response in chronic hepatitis C genotype 4 treated with nitazoxanide, peginterferon, and ribavirin. Gastroenterology 2009;136:856-62. (Abstract)

Sarrazin C, Kieffer TL, Bartels D, et al. Dynamic hepatitis C virus genotypic and phenotypic changes in patients treated with the protease inhibitor telaprevir. Gastroenterology 2007;132:1767-77. (Abstract)

Sarrazin C, Rouzier R, Wagner F, et al. SCH 503034, a novel hepatitis C virus protease inhibitor, plus pegylated interferon alpha-2b for genotype 1 nonresponders. Gastroenterology 2007;132:1270-8. (Abstract)

Sarrazin C, Zeuzem S. Resistance to direct antiviral agents in patients with hepatitis C virus infection. Gastroenterology 2010;138:447-62. (Abstract)

Schiano TD, Charlton M, Younossi Z, et al. Monoclonal antibody HCV-AbXTL68 in patients undergoing liver transplantation for HCV: results of a phase 2 randomized study. Liver Transpl 2006;12:1381-9. (Abstract)

Sherman KE, Flamm SL, Afdhal NH, et al. Response-guided telaprevir combination treatment for hepatitis C virus infection. N Engl J Med 2011;365:1014-24. (Abstract)

Sherman KE, Sulkowski M, Zoulim F, Alberti A. Follow-up of SVR durability and viral resistance in patients with chronic hepatitis C treatetd with telaprevir-based regimens: interim analysis of the extend study. Hepatology 2011;54:1471.

Shi ST, Polyak SJ, Tu H, Taylor DR, Gretch DR, Lai MM. Hepatitis C virus NS5A colocalizes with the core protein on lipid droplets and interacts with apolipoproteins. Virology 2002;292:198-210. (Abstract)

Susser S, Vermehren J, Forestier N, et al. Analysis of long-term persistence of resistance mutations within the hepatitis C virus NS3 protease after treatment with telaprevir or boceprevir. J Clin Virol 2011. (Abstract)

Tellinghuisen TL, Marcotrigiano J, Rice CM. Structure of the zinc-binding domain of an essential component of the hepatitis C virus replicase. Nature 2005;435:374-9. (Abstract)

Wagoner J, Negash A, Kane OJ, et al. Multiple effects of silymarin on the hepatitis C virus lifecycle. Hepatology 2010;51:1912-21. (Abstract)

Wakita T, Pietschmann T, Kato T, et al. Production of infectious hepatitis C virus in tissue culture from a cloned viral genome. Nat Med 2005;11:791-6. (Abstract)

Welsch C, Domingues FS, Susser S, et al. Molecular basis of telaprevir resistance due to V36 and T54 mutations in the NS3-4A protease of the hepatitis C virus. Genome Biol 2008;9:R16. (Abstract)

Wohnsland A, Hofmann WP, Sarrazin C. Viral determinants of resistance to treatment in patients with hepatitis C. Clin Microbiol Rev 2007;20:23-38. (Abstract)

Zeuzem S, Andreone P, Pol S, et al. Telaprevir for retreatment of HCV infection. N Engl J Med 2011;364:2417-28. (Abstract)

Zeuzem S, Arora S, Bacon B, Box T, Charlton M. Pegylated interferon-lambda shows superior viral response with improved safety and tolerability versus pegIFN-alfa-2a in hCV patients (G1/2/3/4): merge phase IIB trhough week 12. J Hepatol 2011;54:538.

Zeuzem S, Asselah T, Angus P, et al. Efficacy of the Protease Inhibitor BI 201335, Polymerase Inhibitor BI 207127, and Ribavirin in Patients With Chronic HCV Infection. Gastroenterology 2011, in press. (Abstract)

Zeuzem S, Berg T, Moeller B, et al. Expert opinion on the treatment of patients with chronic hepatitis C. J Viral Hepat 2009;16:75-90. (Abstract)

Zeuzem S, Buggisch P, Agarwal K, et al. The protease inhibitor GS-9256 and non-nucleoside polymerase inhibitor tegobuvir alone, with RBV or peginterferon plus RBV in hepatitis C. Hepatology 2011, in press. (Abstract)

Zeuzem S, Sulkowski MS, Lawitz EJ, et al. Albinterferon Alfa-2b was not inferior to pegylated interferon-alpha in a randomized trial of patients with chronic hepatitis C virus genotype 1. Gastroenterology 2010;139:1257-66. (Abstract)

Design:
Attilio Baghino

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FAIR USE NOTICE

Christian Lange, Christoph Sarrazin Download the entire book (PDF, 546 pages).

HCV life cycle and treatment targets

Ciluprevir (BILN 2061)

Telaprevir (Incivek/Incivo®) and boceprevir (Victrelis®)

NS5A inhibitors

Host factors as targets for treatment

Christian Lange, Christoph Sarrazin

Download the entire book (PDF, 546 pages).

Telaprevir (Incivek/Incivo^®) and boceprevir (Victrelis^®)