Searching for new antiviral therapies

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Candidates for new therapeutic approaches
The current Standard of Care (SoC) combination therapy for
chronic hepatitis C (CHC) is limited by its insufficient efficacy in
some patient groups, the drug side-effects and contraindications
and the high associated costs. There are an increasing number of
therapeutic failures, with patients who do not respond or who
relapse with the available SoC. Assuming there are no changes in
the type of treatment, the projection for the next 20 years is that
the total number of patients with advanced liver disease will be
4-fold higher than today, with nonresponders far exceeding
those actively treated and total medical costs being expected to
triple.
New therapeutic approaches will be especially important for
– Patients with significant adverse events (AEs) associated
with SoC therapy. In clinical trials, AEs imposed dosereduction
in more than 60% of the cases and treatment
withdrawal in 10–15% of cases; in clinical practice, the rate
of treatment discontinuation is substantially higher.
– Treatment-naive and challenging populations. These
include patients infected with viral genotypes 1 and 4
(which are refractory to the current SoC), especially those

with unfavorable pre-treatment characteristics (high VL,
advanced fibrosis, IL28B unfavorable genotypes CT or TT), as
well as other “difficult-to-treat” populations detailed in
chapter 3.
– Relapsers and nonresponders of all genotypes.
Different treatment options that can either augment the
efficacy of current therapy or potentially result in PegIFNand/
or ribavirin (RBV)-sparing regimens are being extensively
studied. New emerging therapies include
– improved interferon (IFN) alfa formulations (to enhance
efficacy and ease of administration)
– alternative RBV-like molecules (to reduce toxicity)
– direct-acting antivirals (DAAs) that target specific key steps
of the viral life cycle
New IFN formulations
New interferons are currently being developed to offer
enhanced activity, improved AE profiles and, hopefully, better
tolerability compared with currently available ones (Table 4.1).
Given the dependence of treatment success on patients
adherence, the development of longer-acting IFN formulations,
with improved pharmacokinetic profiles, is an important focus
of HCV therapy. Their main advantages consist in maintenance
of viral suppression across a longer dosing interval, avoidance of
inter-dose trough, and reduced dosing frequencies (twice or
even once per month compared to once per week for the current
pegylated interferons (PegIFNs). Although studies about
improved formulations of interferons have been focused on HCV
genotype 1, their administration can be also valuable for
genotype 2 or 3 infected patients. In easy-to-treat patients
(infected with genotype 2 or 3), the duration of treatment can be
reduced to 12 weeks if a rapid virologic response (RVR) is
obtained. This can translate into a very convenient therapeutic
regimen of only 3 injections, if longer-acting IFNs, with monthly
dosing, are going to be used.

However, not all patients may benefit from these new types of
IFNs. In particular, it seems unlikely that patients with strong
contraindications to the current IFNs will be eligible for
treatment with newer formulations, even if the AEs profiles of
the new IFNs are milder.
Table 4.1 – New interferons in the treatment of Chronic Hepatitis C*
Interferons/
Manufacturer
Description Clinical
trial phase
Interferon alfacon (consensus
interferon- INFERGEN®)
Three Rivers Pharmaceuticals
www.3riverspharma.com
Bio-engineered IFN, consisting
of the most frequently observed
amino acid in each
corresponding position in the
natural alfa IFN
approved
IFN formulations with improved pharmacokinetic profiles
Albinterferon (Zalbin™)
Human Genome Sciences
www.hgsi.com
Recombinant IFN alfa-2b
fused with human albumin
III
IFN preparations with improved side-effect profiles
Pegylated Interferon lambda
Zymogenetics/Bristol-Myers
Squibb
www.zymogenetics.com
Type III interferon with
restricted receptor distribution
(especially on hepatocytes)
II
Controlled-release recombinant interferon systems
Locteron®
BiolexTherapeutics
www.octoplus.nl
Recombinant IFN alfa-2b in
polyetherester microspheres
II
Interferon alfa-2b XL
Flamel Technologies
www.flamel.com
Recombinant IFN alfa-2b with
Medusa® nanoparticles delivery
system
II
Omega Interferon
Intarcia Therapeutics
www.intarcia.com
Delivered with Omega DUROS®-
continuous micropump infusion
system
II
* According to data from: Hepatitis C new drug pipeline
(http://www.hcvdrugs.com, accessed on 4/29/2011); low dose oral interferon
(Amarillo Bioscience) and oral Belerofon (Nautilus) are not included.
Interferon alfacon or consensus interferon (CIFN) (Infergen®,
Three Rivers Pharmaceuticals) is a recombinant, bio-engineered
interferon, consisting of the most frequently observed amino
acid in each corresponding position in the natural alfa IFN. It

shares an 89%, 30% and 60% homology with IFN alfa, IFN beta
and IFN omega, respectively. The CIFN molecule binds to the
IFN-alfa receptor with higher affinity than all other known IFN
alfa molecules (including the natural subtypes, the variants or
recombinants). In vitro it appears to be approximately 5 to 20-
fold more active than PegIFN alfa-2a and alfa-2b (Gonzalez 2009).
Data derived from clinical trials support the use of CIFN for
treatment-naive patients, particularly those with high VL or
genotype 1 infection (Sjogren 2005), as well as in the retreatment
of relapsers and nonresponders (Leevy 2008).
Clinical trials suggested a dose-dependent rate of viral
clearance, however the maximum tolerated dose of daily CIFN in
difficult-to-treat patients is up to 15 μg/day (Bacon 2009).
Administration of an induction dose (up to 18μg/day) or of a
higher dose (24μg/day), did not translate to better rates of SVRs
and was associated with more serious AEs and more
discontinuations (Meyer 2010).
CIFN is approved as monotherapy for CHC in adults with
compensated liver disease, and, from 2010, for retreatment of
CHC, in combination with RBV, being especially effective for
interferon-sensitive patients with lower baseline fibrosis scores.
IFN lambda (IFN-λ) is a type III interferon (comprising of
IL28A, IL28B and IL29), which has previously demonstrated
strong antiviral activity and good tolerability. IFN-λ mediates
antiviral activity through a different signaling pathway than
type I interferons (such as IFN alfa), having a complex binding
mainly through the IL28 receptor, which is present only on
plasmacytoid dendritic cells, peripheral B cell, hepatocytes and
epithelial cells. This restricted distribution compared to that of
IFN-alfa receptor offers the potential for more targeted hepatic
delivery, as well as for a better tolerability and safety profile
than the conventional interferons in terms of bone marrow
suppression (Sommereyns 2008). IFN-λ can enhance the subsaturating
levels of IFN-ɑ and increase its antiviral efficacy. As a
result, the combination of IFNλ and IFN-ɑ may provide additive

therapeutic effects through the complementary roles of the two
types of cytokines (Pagliaccetti 2008). IFN-λ may be used to
target specific cell responses and to avoid the AEs of IFN-ɑs.
Interferon lambda has been pegylated (Zymogenetics/Bristol-
Myers Squibb); its administration in treatment-naive patients
chronically infected with HCV genotypes 1/2/3/4 resulted in
higher rates of RVR and EVR, which extended across all IL28B
host genotypes, and was associated with fewer hematologic
toxicities, flu-like and musculoskeletal symptoms compared with
PegIFNɑ-2a (Zeuzem 2011). IFN lambda might prove to be
increasingly important for the treatment of CHC, due to the
recent findings (see chapter 1) on the impact of host genetics in
the response to therapy (Tanaka 2010).
Albinterferon (Zalbin™, Human Genome Sciences) is a longeracting
IFN, allowing for once or twice/month dosing schedule. It
consists of IFN alfa-2b genetically fused to recombinant human
albumin. Several unique features of albumin make it an ideal
candidate for integration into a drug-design platform, including
its unusually long half-life (~19 days), wide distribution,
negligible potential for confounding enzymatic or
immunological function and its physiological role as a carrier of
blood substances. The pharmacodynamic attributes of
albinterferon, which include the maintenance of viral
suppression across a longer dosing interval, might reduce viral
rebounds, while also improving patient’s compliance.
In phase III trials, in patients with genotype 1 CHC,
albinterferon (900 μg every 2 weeks) achieved noninferiority
compared with PegIFN alfa-2a, indicating that the two drugs are
equivalent (Nelson 2009). Albinterferon was also administered
with good results in combination with RBV in non-responders to
prior IFN therapy and is evaluated for the treatment of HIV/HCV
coinfected patients. However, the preliminary FDA evaluation
indicates that the licensing of this dosing regimen is unlikely,
due to the unfavorable risk-benefit profile, mainly caused by
slightly increased rates of serious pulmonary AEs, coughing and

alopecia compared to PegIFN. Development and testing of once
per month dosage is undergoing.
Controlled-release recombinant interferon alfa-2b
formulations were designed to improve the pharmacokinetic
parameters, in order to maintain continuous drug levels and
consequently minimize side effects as compared to current IFNs.
Locteron® (Biolex Therapeutics/OctoPlus) is a recombinant
nonglycosylate IFN alfa-2b produced in polyether-ester
microspheres. This steady controlled-release formulation avoids
fluctuation in IFN levels. A pilot study reported that after
injection of 320 μg Locteron®, the concentration of serum IFN
remained elevated through 14 days (De Leede 2008). Locteron®
can be administered twice monthly, with its trough
concentration between doses maintaining adequate antiviral
activity. Preliminary results of phase IIb studies, showed that in
treatment-naive patients, Locteron®, in combination with RBV,
produced similar viral suppression to that of PegIFN/RBV, with
fewer flu-like side effects and substantially lower rates of
depression.
IFN XL (Flamel Technologies) is an extra-long controlledrelease
formulation of recombinant IFN alfa-2b, based on the
nanoparticles Medusa delivery system, designed for the
tailored delivery of fully-active proteins. Basically this is a
nanoparticle polymer with embedded IFN, which has a slow,
sustained release with increased efficacy. In a phase I study, IFN
XL induced greater reduction in VL after two weeks with fewer
AEs compared to PegIFN (Soriano 2009). A phase IIa study
designed to evaluate IFN XL in combination with RBV in naive
and previous G1 HCV non-responders to SoC is ongoing.
Omega interferon (Intarcia Therapeutics, Inc.) is a type 1
interferon delivered with an osmotic mini-pump implanted
subcutaneously. Omega DUROS® is a drug delivery system that
stabilizes therapeutic proteins, delivering a continuous dose of
omega interferon at a constant rate for 3 months.

Alternative RBV formulation
Optimal RBV dosages are essential in achieving a SVR.
Maintenance of RBV in the therapeutic regimen has been proven
to have an important additive effect in the overall success rate,
leading to both increased RVR and reduced rates of relapses (as
demonstrated by the PROVE-2 trial).
As described in chapter 1, the main impediment in the
administration of high-dose RBV is the dose-dependent
development of hemolytic anemia. Although the addition of
epoetin alfa has been useful in maintaining the highest possible
RBV doses, new RBV-replacement compounds, with an improved
side effects profile, are investigated.
Taribavirin – formerly known as viramidine – (Valeant
Pharmaceuticals International/Kadmon Pharmaceuticals LLC), is
a prodrug of RBV, converted in the active form by adenosine
deaminase. This nucleoside analog was studied for the treatment
of CHC, due to the lower frequency of anemia, a benefit
registered especially within the first 12 weeks of treatment, the
period in which maintenance of the dose of RBV has been shown
to be the most critical. The major conversion site of taribavirin is
in the liver, enabling drug concentration in this location. Due to
its lower uptake in red blood cells, taribavirin causes
significantly less hemolytic anemia compared to RBV. While this
effect was confirmed in several clinical studies, the rates of SVR
were lower with taribavirin.
In two phase III studies, taribavirin failed to prove
noninferiority compared to RBV (SVR rates were 38% and 40%
with taribavirin vs. 52% and 55% with RBV in the VISER 1 and
VISER 2 trials, respectively), even if taribavirin caused lower
rates of severe anemia (5% vs 24%). Suboptimal dosing of
taribavirin (Marcellin 2010) seems to be the explanation, as
recent studies with weight-based dosing of taribavirin confirmed
reduced rates of anemia (7%-15% vs. 24% with RBV), while
acquiring comparable SVR rates and lower relapse rates than
RBV. Whether taribavirin will have a role in the future

combination therapies including DAAs (most of which are also
associated with a certain degree of anemia) remains to be seen.

Direct-Acting Antivirals (DAAs)
Direct-acting antivirals (DAAs), also known as “specifically
targeted antiviral therapy for hepatitis C” (STAT-C), are the most
important new therapeutical options for CHC. In May 2011, two
HCV protease inhibitors Telaprevir (Incivek™) and Boceprevir
(Victrelis™) have been approved by the FDA. For the first time,
we have now drugs with specific anti-HCV activity. Several other
DAAs are at various stages of clinical development, the most
advanced being alternative protease inhibitors and nucleoside
and non-nucleoside polymerase inhibitors. Other tentative
approaches include inhibitors of host cyclophilins, alphaglucosidase
inhibitors, oligonucleotides and immune modulators
(Soriano 2009).
Protease inhibitors (PIs)
A clear understanding of the key sites of action for the newer
antiviral compounds in development is of outmost importance.
HCV is a positive-sense single-stranded RNA virus, meaning that
its genome can function directly as a template for viral protein
synthesis. Consequently, after entering into hepatocytes, HCV
starts its replication by direct translation of the genome into a
large polypeptide that is further processed by the virus NS3
protease. This enzyme has dual activity of serine protease and of
helicase (unwinding the single-strand viral RNA). Together with
the NS4A cofactor, the NS3 protease is responsible for
proteolytic cleavage of its downstream nonstructural proteins
that in turn are critical in forming the replicative complex from
which viral synthesis occurs. Additionally, NS3 protease may
directly impair host IFN responses through the inhibition of
phosphorylation of IFN regulatory factor-3, and administration
of PIs may restore interferon responsiveness.

peptidomimetic PIs that bind reversibly and block the protease
catalytic site.
However, monotherapy with PIs is not an option, due to
early emergence of resistance. Minor resistant populations
preexist at baseline in all HCV-infected patients and are rapidly
selected with PIs monotherapy. Therefore, boceprevir and
telaprevir still require a platform of PegIFN/RBV. When
administered in this triple therapy combination, each of the two
PIs substantially increases the rates of SVR in both treatmentnaive
and treatment-experienced patients.
Triple therapy
Triple therapy with a PI was shown to almost double the
success rate in treatment-naive patients infected with HCV
genotype 1 from 38-44% obtained with SoC to 63-75% (Poordad
2011). The increase in SVR rate is even higher in previous
nonresponders-from 17-21% with SoC to 59-66% with triple
therapy (Bacon 2011). Nevertheless, the addition of a new agent
to an existing treatment regimen will pose substantial challenges
in terms of drug interactions and adherence, due to the
associated side effects and risk of resistance emergence.
Maximizing tolerance of future PIs based regimens will be
extremely important to achieve optimal treatment outcomes.
Telaprevir (Incivek™, Vertex Pharmaceuticals) was approved
by FDA for treatment of genotype 1 CHC in adult naive patients
with compensated liver disease, including cirrhosis, and in prior
null responders, partial responders, and relapsers, only in
combination with PegIFN/RBV.
Preliminary studies have demonstrated that 14 days
monotherapy, while inducing a VL median decline of more than
4.4 log10 units in patients with CHC G1 infection, was limited by
the appearance of resistance mutation as early as 4-7 days after
initiation. Interestingly, the mutations were subsequently
suppressed by administration of PegIFN/RBV. Consequently,
telaprevir was administered in combination with PegIFN/RBV

for 12 weeks, followed by SoC therapy alone for another 24 - 48
weeks. The recommended dose of Incivek is 750 mg orally 3
times a day.
Several phase II and III studies have assessed the efficacy and
safety of telaprevir in treatment naive G1 patients, concluding
that triple therapy yields a higher rate of SVR than current SoC
and lower rates of relapse. The SVR for patients treated with
Incivek across all studies, and across all patient groups, was
between 20 and 45% higher than the current SoC (Hézode 2009,
McHutchison 2009). RBV was shown to be an essential part of the
therapeutic regimen, playing a critical role both in achieving
superior RVR and SVR and in reducing the rates of virologic
breakthrough due to drug resistance.
The results of a response-guided therapy (RGT) study,
ILLUMINATE (Sherman 2010) support a shorter course of
treatment (from 48 to 24 weeks) for rapidly responsive naivepatients.
Sixty percent of previously untreated patients achieved
an EVR and received only 24 weeks of treatment. The SVR for
these patients was 90%. In order to identify patients who may
benefit from shorter duration of therapy, a new predictor of
treatment response was proposed: extended RVR (eRVR),
defined as undetectable HCV RNA at week 4 and 12. Among
patients who achieved an eRVR, rates of SVR were comparable
between those treated for a total duration of 24 or 48 weeks (92%
vs. 88%, respectively). Among those who did not achieve eRVR,
but continued treatment for 48 weeks, the SVR rate was lower,
but still significant (64%). More recent studies have evaluated
the use of triple therapy including telaprevir as a retreatment
option for nonresponders and relapsers to previous SoC therapy,
demonstrating synergistic effects in viral reduction and
decreased emergence of resistance. SVR rates were higher
among patients who previously experienced relapse versus
nonresponders (McHutchison 2010).
Rashes, pruritus, anemia and nausea were the most commonly
reported AEs with the use of telaprevir. AEs rates resulting in
treatment withdrawal were about 10% higher in telaprevir arms

vs PegIFN/RBV, the most severe being rash, that resolved with
discontinuation of therapy. Serious skin reactions, including
Drug Rash with Eosinophilia and Systemic Symptoms (DRESS)
and Stevens-Johnson Syndrome were reported in less than 1% of
subjects who received telaprevir combination treatment
compared to none who received PegIFN/RBV alone. A sequential
discontinuation of drugs was proposed for the management of
moderate or severe rash.
Boceprevir (Victrelis™, Merck) is another potent HCV NS3 PI
with antiviral activity against genotype 1 HCV. Boceprevir is FDA
approved for the treatment of CHC genotype 1 infection, in
combination with PegIFN/RBV, in patients aged 18 years of age
and older with compensated liver disease, including cirrhosis,
who are previously untreated or who have failed previous
interferon and ribavirin therapy. Boceprevir is administered
orally, at a dose of 800 mg three times daily.
The safety and efficacy of triple therapy with oral boceprevir
plus PegIFN/RBV vs PegIFN/RBV alone were demonstrated in a
phase III registration trial for treatment-naive patients, SPRINT-
2 (Poordad 2011) and in previously partial responders and
relapsers to SoC (RESPOND-2) (Bacon 2011). Boceprevir, in
combination with PegIFN/RBV has not been studied in patients
documented to be historical null responders (less than 2 log10
HCV RNA decline by treatment week 12) during prior therapy
with PegIFN/RBV.
The treatment strategy is different from telaprevir, Boceprevir
being administered in triple combination therapy for 24-44
weeks only after a 4 weeks lead-in phase with PegIFN/RBV alone.
Therefore, RVR was defined as undetectable HCV RNA at week 4
of boceprevir-containing therapy (meaning week 8 of all
therapy, including the lead-in period). In theory, a lead-in phase
may provide the additional advantage of reducing viral
replication and, consequently, the rate of resistance emergence.
However in phase III clinical trials, patients with poor response
to PegIFN/RBV, defined as <1 log10 decline after 4 weeks lead-in,
had a higher incidence of resistance mutations. Nevertheless, the

virologic response at the end of the lead-in phase is highly
predictive for the final outcome of therapy. Substantially higher
SVR rates are obtained in patients showing more than 1 log10
decline in HCV RNA at this time point. Even in patients with a
poor response to interferon, addition of boceprevir can generate
a SVR in up to 34% of the patients. This is an important
information, arguing for the utility of a lead-in phase in the
previously treated nonresponders or relapsers. For naive
patients, the lead-in period can further serve to test both
compliance and tolerability before exposure to PIs.
The most commonly reported AEs with boceprevir were anemia
(almost twice as many boceprevir recipients had Hb levels <9.5
mg/ml compared to controls) and dysgeusia (more than twice as
often in boceprevir recipients than in controls). Serious AEs were
reported in 11% of patients receiving boceprevir in combination
with PegIFN/RBV compared with 8% of patients receiving
PegIFN/RBV alone. The most common reason for dose reduction
in the trials was anemia.
Other investigational HCV PIs
A series of additional PIs are in development and preliminary
studies confirm their superior antiviral effectiveness in
combined triple therapy over the SoC in treatment-naive
patients. Unlike telaprevir or boceprevir, which are active only
on genotypes 1 and 2 and have to be dosed three times a day,
investigational second-generation PIs, mainly non-covalent
inhibitors of the NS3/4A, seem to be active against different HCV
genotypes, as well as against resistant HCV variants previously
selected by other PIs. Also, they have a longer half-life which
enables more convenient once-daily dosing. In addition, they
may provide improved safety and efficacy as well as shortened
treatment duration for a higher proportion of patients. An
example is BI 201335, a once-daily HCV NS3/4A protease
inhibitor optimised to target genotype-1 HCV, with strong in
vitro activity also against GTs 4-6. Phase II studies showed
BI 201335 to have strong efficacy, with overall SVR rates

reaching 83% in GT1 patients at once-daily dosages of 240 mg (in
combination with PegINF+RBV). BI 201335 is now in phase III
trials in combination with PegINF+RBV and in phase II as part of
the interferon-free combination with the polymerase inhibitor,
BI 207127, in genotype-1 HCV patients.
Other notable examples are MK-5172, a competitive inhibitor
of HCV NS3/4A protease that has demonstrated in vitro activity
against genotypes 1b, 2a, 2b, and 3a, and proved to be active in
vivo against genotypes 1 and 3; and TMC435 (that can also be
administered once-daily), active in therapy-naive patients with
HCV G4 infection. Moreover, TMC435 antiviral activity was
similar, irrespective of the IL28B genotype. Some compounds,
such as Danoprevir (formerly R7227/ITMN-191) are being
studied in combination with low-dose ritonavir (a pharmacologic
booster used for HIV protease inhibitors) in order to improve
pharmacokinetics, without increasing toxicity. Whether such
complex therapies have the potential to minimize the risk of
viral breakthrough and the selection of resistance mutations,
remains to be evaluated.
HCV polymerase inhibitors
The HCV NS5B enzyme is an RNA-dependent RNA polymerase
essential for viral replication. As the enzyme is highly conserved
across all HCV genotypes, the inhibitors are expected to have
pan-genotypic activity. The structure of NS5B, like many other
viral polymerases (HIV reverse transcriptase included),
resembles the shape of a hand consisting of finger, thumb and
palm domains. There are two major classes of polymerase
inhibitors: nucleoside analogs and non-nucleoside analogs. The
enzyme has a catalytic site for nucleoside binding and at least
four other sites to which a non-nucleoside molecule could bind
and cause allosteric alteration. Inhibitors of NS5B polymerase
have advanced to the phase II of clinical development. These
agents have demonstrated potent antiviral efficacy, achieving
multi-log reductions in HCV RNA with short-term treatment.

Nucleoside analogs target the catalytic sites of the enzyme by
competing with natural substrates and, once incorporated, act as
chain terminators stopping the further extension of viral RNA
nascent strand. This drug class is considered to have the
broadest genotypic coverage as well as a high resistance barrier.
This is due to the fact that mutations at the active site also affect
the viral polymerase fitness.
Several early developed compounds were discontinued because
of high toxicity (gastrointestinal or neutropenia related,
respectively).
The current most advanced compound in development is the
nucleoside analog mericitabine (R7128), an investigational
nucleoside inhibitor of NS5B HCV polymerase with antiviral
activity against HCV genotypes 1-6. The compound is a prodrug
of an oral cytidine nucleoside analog (PSI-6130). A phase IIb trial
in therapy-naive patients with genotype 1 or 4 HCV infection
demonstrated that a combination of mericitabine and
PegIFN/RBV achieves high rates of both rapid and complete
early virologic responses. Mericitabine has a safety profile
similar to SoC and, importantly, does not seem to be associated
with treatment-emergent viral breakthrough or resistance. The
combination of this NS5B polymerase inhibitor with an NS3
protease inhibitor (Danoprevir, R7227), administered without
additional PegIFN/RBV, for 14 days in treatment-naive, genotype
1-infected patients, demonstrated sustained viral suppression,
absence of PI resistant mutations and acceptable safety and
tolerability (INFORM 1 trial). The combination is associated with
a lower risk of relapse during SoC.
There are several others compounds in early stages of clinical
development, that are designed to achieve higher concentrations
of the active substance in the liver, reducing systemic exposure
and limiting the potential side effects.
The non-nucleoside polymerase inhibitors are a very
promising class of molecules, because they target multiple
distinct domains on the NS5B polymerase, acting through

allosteric inhibition. HCV polymerase has at least four allosteric
binding pockets for nonnucleosidic inhibitors, unlike the HIV
reverse transcriptase where there is only a single one. Therefore,
if patients do not respond to one non-nucleoside inhibitor, there
is enough differentiation between the binding sites to allow the
use of a different drug within the class. Several non-nucleoside
HCV polymerase inhibitors are in clinical development. Most of
these investigational agents are active only against HCV
genotypes 1a and 1b and show a relatively high rate of
resistance, as well as an increased frequency of specific sideeffects.
These observations suggest that their use could be
limited to combination with other DAAs (Table 4.2). Such an
approach was investigated for a low potent non-nucleoside
polymerase inhibitor (tegobuvir, formerly known as GS-9190) in
combination with a protease inhibitor (GS-9256) in treatmentnaive
G1 HCV patients. The combination alone, without SoC was
not effective due to virologic rebound and selection of dual
resistance mutations that existed before treatment. Addition of
RBV alone significantly reduced the virologic breakthrough
rates.
Table 4.2 – Combinations of DAAs tested with or without PegIFN/RBV
Company DAA combination Phase
Vertex Telaprevir (PI*) + VX-222 (NNI†) II
Boehringer Ingelheim BI 201335 (PI) + BI 207127 (NNI) IIb
Bristol-Myers-Squibb BMS-650032 (PI)
+ BMS-790052 (NS5A inhibitor) II
Gilead GS-9256 (PI) + GS-9190 (NNI) II
Hoffmann-La Roche Danoprevir (R7227) (PI) + R7128 (NI‡) I
* PI: protease inhibitor
† NNI: non-nucleoside (polymerase) inhibitor
‡ NI: nucleoside (polymerase) inhibitor
NS5A inhibitors
NS5A is a membrane-associated phosphoprotein involved in
both the formation of the replication complex and in the virus
assembly. The most potent HCV NS5A inhibitor reported to date

is BMS-790052, currently in phase II clinical trial in combination
with SoC. It was also used in combination with a protease
inhibitor (BMS-650032) for retreatment of previous nonresponders
to SoC with good results, but only in association with
PegIFN/RBV. Exclusion of SoC from the therapeutic regimen
resulted in high rates of viral breakthrough through week 12.
Host cyclophilins inhibitors
Another interesting therapeutic approach is directed at host
factors important in the viral life cycle. The most promising
target are cyclophilins, a family of highly conserved cellular
peptidyl-prolyl isomerases (PPIase) involved in many cellular
processes such as protein folding and trafficking. Cyclophilin
inhibitors block the interaction of cyclophilins with HCV
proteins and hence the formation of a functional viral
replication complex. Currently, several non-immunosuppressive
cyclosporin analogs are being tested. The most potent seems to
be Alisporivir (Debio-025), tested in both HCV monoinfected
and HIV/HCV coinfected patients with promising results. The
combination of Debio 025 and PegIFN-ɑ2a showed a significant
VL reduction after 28 days in patients infected with genotypes 1,
3 and 4 (Flisiak 2009). Such host protein-targeting compounds
have the advantage of higher genetic barriers to resistance and
could be instrumental in future IFN-free regimens (Table 4.3).
Emergence of drug resistant mutations
High levels of baseline drug resistance mutations in the NS3
protease or NS5B polymerase were identified in a significant
number of viral isolates from treatment-naive patients.
Moreover, there seem to be differences between HCV
genotypes/subtypes in terms of the frequencies of baseline
mutations and natural polymorphisms which can translate into
distinct susceptibility to DAAs. An overlap of immune escape and
drug resistance profiles has also been reported (Gaudieri 2009).

The majority of DAAs have a low genetic barrier to resistance,
with the possible exception of nucleoside analogs inhibitors of
HCV polymerase.
There is broad cross resistance between drugs in the same
class, as has been shown for the two approved PIs, telaprevir and
boceprevir. Possible exceptions are the non-nucleoside
inhibitors of HCV polymerase that might be administered in
additive or synergistic combinations. The majority of patients
with virologic breakthrough during triple therapy with PIs
presented high-level resistant variants, these emerged more
frequently in the HCV genotype 1a patients (Kuntzen 2008);
predominant mutations were V36M and R155K compared to
A156T in genotype 1b. There is no information regarding the
possible archiving of drug resistant mutants in cellular
sanctuaries, as is the case for HIV. Emergence of resistance may
be limited by optimized pharmacokinetics of the DAAs and by
their use in combinations.
What does the future hold?
In the near future, trials of SoC plus STAT-C will be initiated in
difficult-to-treat populations (patients with advanced liver
disease, cirrhosis, recipients of liver transplantation or patients
with major comorbidities such as HIV coinfection). It remains to
be seen if there are safer regimens with less drug interactions,
especially with antiretroviral drugs (Seden 2010). As shown in
chapter 1, race is an important determinant of the therapy
response; as a consequence new HCV therapies should be also
studied in Asian, Afro-american and Latino populations in order
to fully characterize their efficacy and safety.
The predictive value of on-treatment viral kinetics will require
re-evaluation for the DAAs and their combinations. Although
evaluation of SVR at 6 months after treatment completion will
remain the gold standard for treatment success, there is growing
evidence indicating that SVR at 12 weeks after treatment
completion may be enough to predict long-term viral clearance.

Preliminary data show that DAAs induce a more rapid decline in
the VL than the one seen with PegIFN/RBV.
Table 4.3 – The most promising new therapeutical options for CHC
(as of June 2011) *
Category Mechanism Example Manufacturer Phase
BI201335 Boehringer III
TMC435 Medivir/Tibotec III
GS-9256, -9451 Gilead II
Danoprevir Intermune/Roche II
Vaniprevir Merck II
NS3/NS4A
protease
inhibitors
ACH-1625 Achillon Pharm. II
ABT 450 Abbott/Enanta II
BMS-650032 Bristol-Myers Squibb IIa
Mericitabine Roche/Pharmaset II
PSI-7977 Pharmaset II
IDX 184 Idenix II
NS5B polymerase
inhibitors,
nucleoside
analogs
Filibuvir Pfizer II
GS-9190 Gilead II
VX 222 Vertex II
ABT 333, -072 Abbott II
Setrobuvir Anadys Pharm. II
NS5B polymerase
inhibitors,
non-nucleoside
analogs
BMS-790052 Bristol-Myers Squibb II
ABT 267 Abbott II
AZD 7295 AstraZeneca II
Direct-acting
antivirals
NS5A inhibitors
Cyclophilins
inhibitors
Alisporivir Novartis/Debiopharm III
MBL-HCV1
human
monoclonal
antibody
The University of
Massachusetts Medical
School
Virus entry II
inhibitors
ITX 5061 iTherX II
Host
targeting
agents
Bavituximab Peregrine Pharm. II
* For more information, see http://hcvdrugs.com and the manufacturers' web
sites presented at the end of the chapter.

Resistance testing is likely to become a part of the treatment
algorithm with the introduction of DAAs. Extensive knowledge of
the impact of these mutations on the phenotypic characteristics
and on the replicative fitness of the viral population will be
important (Kuntzen 2008) in order to tailor therapeutic decisions
for the management of the HCV infected patient.
It is expected that the HIV model of development of highly
active combined therapies, consisting of at least 3 drugs with
different mechanisms of action will be reproduced for HCV, in an
attempt to obtain effective interferon-free regimens. With such
combinations, HCV may become the first chronic viral infection
to be cured. While sufficient suppression of HIV RNA and HBV
DNA can only be achieved by long-term administration of potent
antiviral drugs, HCV RNA may be completely eradicated from the
infected individual after a limited duration of treatment. This is
foreseeable due to the fact that, unlike HIV (that replicates
through a proviral DNA subsequently integrated into the
lymphocytes nucleus), or HBV (that replicates through a cccDNA
that may integrate into the hepatocyte nucleus), HCV replication
is entirely intra-cytoplasmic and is not accompanied by the
establishment of extrahepatic reservoirs. In a viral kinetic model
for the pharmacokinetics of telaprevir, a rapid decrease in the
second slope of viral decline was found, four fold higher than
with standard interferon therapy. According to these data, a
combination triple therapy administered for 7-10 weeks might
be sufficient to eradicate the virus in fully compliant patients
(Guejd 2011). Patients who ultimately fail to clear the virus with
combination STAT-C regimens may still have improvements in
liver histology that can be further sustained by introduction of a
separate group of anti-fibrotic agents.
Outlook
The SoC for first-line treatment of HCV genotype 1 will most
likely soon become a triple combination of a PI, either
boceprevir or telaprevir, with PegIFN/RBV. Individualized
treatment must take into account baseline viral, host and disease

characteristics, as well as reviewed on-treatment predictors and
detection of resistant mutations. The importance of genetic
markers such as the IL28B polymorphism on the SVR during
triple therapy is not yet known.
According to the available data, the combinations of DAAs will
still require a backbone of PegIFN/RBV in order to attain
complete viral suppression and to avoid virologic breakthrough
and resistance. However, this will be affected by costs, increased
toxicities and emergence of viral resistance. For this reason, a lot
of effort is directed to the parallel development of multidrug
regimens that may offer independence from PegIFN/RBV,
providing new hope for patients who are intolerant or have
contraindications to to PegIFN/RBV. Future treatment strategies
will include combinations of several DAAs with different
mechanisms of action, together with host modulators and drugs
addressing innate immunity against HCV.
Links
– European Agency for Medicines: www.ema.europa.eu
– U.S. Food and Drug Administration (FDA):
www.fda.gov/Drugs
– EASL: 5th Clinical Practice Guidelines on the Management of
Hepatitis C Virus Infection:
www.easl.eu/_clinical-practice-guideline
– Treatment for chronic hepatitis C and co-infection with
HIV/HCV: www.hivandhepatitis.com
– Hepatitis C Medication: http://pharmexec.findpharma.com
– Abbott: www.abbott.com
– Achillion Pharmaceuticals: www.achillion.com
– Anadys Pharmaceuticals: www.anadyspharma.com
– AstraZeneca : www.astrazeneca.com
– BMS: www.bms.com
– Boehringer Ingelheim: www.boehringer-ingelheim.com
– Gilead: www.gilead.com
– IDX: www.idenix.com

– iTherX: www.itherx.com
– Merk: www.merck.com
– Novartis: www.novartis.com
– Peregrine Pharmaceuticals: www.peregrineinc.com
– Pharmasset: www.pharmasset.com
– Pfizer: www.pfizer.com
– Roche: www.roche.com
– Tibotec: www.tibotec.com
– Vertex: www.vrtx.com