Download Hepatitis C therapy-the future looks bright

eCommons@AKU
Department of Biological & Biomedical Sciences
Medical College, Pakistan
December 2009
Hepatitis C therapy-the future looks bright
Sohail A. Qureshi
Aga Khan University
Humaira Qureshi
Aga Khan University
Anam Hameed
Aga Khan University
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Recommended Citation
Qureshi, S., Qureshi, H., Hameed, A. (2009). Hepatitis C therapy-the future looks bright. European Journal of Clinical Microbiology &
Infectious Diseases, 28(12), 1409-1413.
Available at: http://ecommons.aku.edu/pakistan_fhs_mc_bbs/84
Eur J Clin Microbiol Infect Dis (2009) 28:1409–1413
DOI 10.1007/s10096-009-0798-1
REVIEW
Hepatitis C therapy—the future looks bright
Sohail A. Qureshi & Humaira Qureshi & Anam Hameed
Received: 4 February 2009 / Accepted: 24 July 2009 / Published online: 29 August 2009
# Springer-Verlag 2009
Abstract Hepatitis C virus (HCV) infections affect about
170 million individuals worldwide and can be lifethreatening if left untreated. Over the past three decades,
ribavirin and interferon-α have remained the only available medicines for treating hepatitis C sufferers. Given
that this combination therapy is partially effective at best
and is associated with severe side-effects, there is an
unmet need for new molecular entities which inhibit HCV
replication. By employing a combination of structurebased drug design together with high-throughput screening
approaches, several pharmaceutical companies have been
successful in identifying potentially useful compounds for
treating HCV infections. This article provides an overview
of some of the small-molecule inhibitors that have shown
promise so far in clinical trials and which could reach the
clinic within the next three years.
Introduction
Hepatitis C is the most common chronic blood-borne
infectious disease that is caused by the hepatitis C virus
(HCV). The most common mode of HCV transmission is
through blood transfusions, hemodialysis, and organ transS. A. Qureshi (*) : H. Qureshi
Department of Biological & Biomedical Sciences,
The Aga Khan University Hospital,
Stadium Road,
Karachi 74800, Pakistan
e-mail: [email protected]
A. Hameed
Medical College, The Aga Khan University Hospital,
Stadium Road,
Karachi 74800, Pakistan
plants, but the sharing of used needles, razor blades, and the
use of tattooing guns have also contributed significantly to
virus spread. Although HCV is present in semen, vaginal
fluid, and saliva, the risk of viral transmission from an
infected individual to a non-infected spouse or partner
without the use of condoms over a lifetime is only about 1–
4%. Once in circulation, HCV selectively targets the CD81
and its co-receptor Claudin-1-expressing hepatocytes and
its entry triggers an innate response in infected cells which
promotes interferon (IFN)-α and β synthesis, both of which
act in an autocrine manner to produce a nonpermissive
milieu for virus replication [1]. The acute phase of infection
is mostly asymptomatic and, hence, difficult to detect, but a
small number of patients, however, experience decreased
appetite, fatigue, abdominal pain, jaundice, itching, and flulike symptoms. It is estimated that about one-fifth of the
infected individuals are able to ward off infection completely without relying on any medication, whereas the rest
(∼80%) go on to develop chronic infections which threaten
liver failure. A small percentage of chronically infected
individuals develop hepatocellular carcinoma. The disease
progression of hepatitis C is influenced by numerous
factors, which include age, gender, alcohol consumption,
HIV infection, and fatty liver.
A member of the Flaviviridae family, HCV is approximately 50 nm in diameter and contains a cap-less 9-kb
positive-strand RNA genome that encodes 11 proteins. Two
untranslated regions located at the extreme 5′ and 3′ ends
play important roles in the translation and replication of
viral RNA. The 5′ UTR harbors an internal ribosome
binding site (IRES), from which protein synthesis begins.
Translation of the continuous open reading frame culminates in a ∼3,000 amino acid polyprotein, which is
subsequently processed by cellular as well as two HCVencoded proteases to yield functional proteins.
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HCV genome encodes for 11 proteins: core, F, E1, E2,
p7, NS2, NS3, NS4A, NS4B, NS5A, and NS5B. Core
encodes the viral encapsulating capsid protein, as well as
an F protein (or alternatively reading frame protein; AFRP)
of unknown function that is produced by ribosomal
translational frame-shifting. E1 and E2 are envelope
glycoproteins capable of forming heterodimers with each
other. p7 is a 63-amino acid transmembrane polypeptide
that exhibits properties of an ion channel. NS2 and NS3 are
cysteine and serine proteases, respectively. As a GTPbinding membrane protein localized to the ER, NS4B has
been found to be crucial for HCV replication. NS5A is a
phosphoprotein of unknown function. RNA-dependent
RNA polymerase is the product of the NS5B gene. The
core-E1-E2-p7-NS2 segment of the polyprotein is processed by host cell proteases. NS2 catalyzes the NS2-NS3
junction, whereas NS3-NS4A processes the rest of the
polyprotein [2].
Viral replication involves the synthesis of the negative
strand first, which is subsequently employed as a template
to polymerize multiple copies of the positive-stranded HCV
genome. It is the poor fidelity associated with NS5B which
accelerates viral evolution and leads to the production of
viral quasi-species. Six major genotypes and a number of
subtypes of HCV have been identified so far.
Current treatment regimens
Standard therapy for patients with acute or chronic hepatitis
C is comprised of a combination of a weekly injection of
pegylated IFN-α and twice-a-day oral administration of
ribavirin. Initially, unmodified IFN-α was used, but it had a
short half-life in systemic circulation and, therefore, poor
pharmacokinetic properties. The new generation of polyethylene glycol (PEG) modified IFN-α, such as Peg-Intron
(Schering-Plough, Kenilworth, NJ) and Pegasys (HoffmanLa Roche, Nutley, NJ) are longer lasting and produce a
significantly more efficacious antiviral response than the
unmodified protein. Pegasys has been found to be effective
for treating Peg-Intron non-responders [3].
The two-component combination therapy is significantly
more effective as compared to IFN-α monotherapy. The
mechanism by which ribavirin acts remains elusive, but it is
thought to act either directly by becoming incorporated into
newly synthesized HCV RNA, causing premature chain
termination, or indirectly by inhibiting the inosine monophosphate dehydrogenase (IMPDH) activity. It remains
unclear as to why different HCV genotypes respond
differently to combination therapy. For example, approximately 40% of people with genotype 1 have a sustained
response with peginterferon and ribavirin. In comparison,
patients with genotypes 2 and 3 are significantly more
Eur J Clin Microbiol Infect Dis (2009) 28:1409–1413
responsive to standard therapy. The recommended treatment time is also dependent on the HCV genotype. For
patients with genotypes 2 and 3, a 24-week course of
combination treatment is adequate, whereas for those
carrying genotype 1, a 48-week course is recommended.
The approximate cost of 48-week therapy is US $35,000.
The standard therapy is contraindicated during pregnancy and is associated with adverse effects, including
depression, fatigue, and flu-like symptoms caused by
IFN-α and hemolytic anemia promoted by ribavirin.
Taribavirin (Valeant Pharmaceuticals), a prodrug of
ribavirin, as well as advanced versions of IFN-α such as
the longer-acting Albuferon (Human Genome Sciences)
and controlled-release Locteron (Biolex), are presently
undergoing late-stage clinical trials.
Given the severe side-effects associated with the current
therapy, together with the fact that about one-half of
hepatitis C sufferers do not respond to it, there is an unmet
need for small-molecule drugs with minimal side-effects
that inhibit HCV replication to be brought to market.
Targets for therapeutic intervention
From the 11 different HCV-encoded proteins, NS3 serine
protease and NS5B RNA-dependent RNA polymerase
(RdRP) have emerged as the best targets for drug
development. NS3 has two important functions: it dampens
the host cell innate response by degrading retinoic acid
inducible gene-1 (RIG-1), as well as the TLR3 (Toll-like
receptor) adaptor proteins Cardiff and TRIF (TLR3-interacting factor), and is required for the processing of
polyprotein. The inhibition of NS3 catalytic activity should,
therefore, launch a powerful two-pronged attack against the
virus that is expected to rejuvenate the innate response in
infected cells, as well as promote the accumulation of
unprocessed and, hence, dysfunctional viral proteins incapable of replicating HCV. Similarly, debilitating the activity
of NS5B will attenuate viral replication. The IRES region
within the 5′ UTR also serves as a useful target for
intervention, but the intracellular delivery of nucleic acids
>500 Da in size that are complimentary to this region
across the cell membrane continues to pose a serious
technical hurdle. Cellular proteins and nucleic acids that
facilitate viral replication, such as cyclophilin and
microRNA-122 (miR-122), are also attractive targets for
drug development against HCV.
High-throughput screening and structure-based drug
design approaches have been used to identify several
small molecules that specifically target the serine protease
and RdRP. A number of these compounds are in clinical
trials, but we discuss only those entities that have shown
promise (Table 1).
Eur J Clin Microbiol Infect Dis (2009) 28:1409–1413
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RNA-dependent RNA polymerase inhibitors
Over the past decade, a number of putative RdRP inhibitors
have entered the clinical pipelines of various companies,
but their development had to be aborted due to the lack of
efficacy or toxicity or the establishment of adaptive
mutations in the NS5B gene. At present, R1626 (Roche)
is the most promising oral RdRP inhibitor that is in phase II
clinical trials. R1626 has a good safety profile and has
shown a profound antiviral effect when combined with
standard therapy. The results from a trial conducted earlier
in 2008 found that 84% of HCV genotype 1-infected
patients responded positively to 4 weeks of R1626,
Pegasys, and ribavirin triple therapy, followed by 44 weeks
of standard therapy. Even at high doses, R1626 does not
appear to promote any adaptive mutations in the HCV
quasispecies [4, 5].
As an oral cytidine nucleoside analog, R7128 (Pharmasset/
Roche) has shown a superior profile over R1626 in terms
of both reduced toxicity and efficacy. Data from phase I
clinical trials have revealed that, when treatment-naïve
chronically infected HCV genotype 1 subjects were coadministered R7128 (1,500 mg twice-daily) together with
Pegasus and ribavirin over a 4-week period, HCV RNA
was undetectable in almost 85% of the patients, demonstrating that R7128 is a potent inhibitor of HCV
replication. Moreover, preliminary results from another
phase 1 study have shown that 4 weeks of the above triple
therapy is also effective in treating patients infected with
HCV genotypes 2 and 3, demonstrating that R7128
possesses significant antiviral activity against a broad
spectrum of HCV variants. The emergence of R7128induced resistance mutations in the HCV genome has not
been observed [6]. MK0608 (Merck) is another nucleoside
inhibitor that is presently in phase I trials [7].
Development of the non-nucleoside RdRP inhibitor PF
868,544 (Pfizer; phase II) is also underway. This compound
inhibits the enzymatic activity by binding to an allosteric
site away from the catalytic site and has an in vitro EC50 of
59 nM against HCV genotype 1. How well it performs in
patients who are not responsive to standard therapy or those
who have relapsed remains to be assessed [8].
Serine protease inhibitors
The development of potent NS3-4A inhibitors has been a
challenge. Initial efforts to identify useful lead compounds
by screening sensitive biochemical assays against a large
library of compounds did not culminate in much success.
Moreover, resolving the three-dimensional structure of the
enzyme also did not offer much hope to medicinal chemists
because the substrate-binding region of protease was
solvent-exposed, shallow, and did not contain any exploitable binding pockets. These issues notwithstanding, several
promising NS3-4A inhibitors have been identified and their
therapeutic utility is currently being evaluated.
The most advanced inhibitor of the serine protease is
Telaprevir (VX-950; Vertex Pharmaceuticals/Johnson &
Johnson), a peptidomimetic compound harboring an αketoamide group optimized to form a reversible covalent
bond with the catalytic serine 139. Telaprevir can be taken
orally and has produced impressive results so far in clinical
trials for treating genotype 1-infected patients who have
failed prior therapy. Having successfully undergone phase I
and II clinical trials, it is currently in phase III clinical
evaluation. Interim results from phase II (PROVE 3)
Table 1 Anti-hepatitis C virus (HCV) drugs in the pipeline
Drug
Company
Phase
Target
R1626
R7128
MK0608
PF868544
Roche
Pharmasset/Roche
Merck
Pfizer
II
I
I
II
NS5B
NS5B
NS5B
NS5B
Telaprevir (VX-950)
Vertex Pharma/
Johnson
Schering-Plough
III
Schering-Plough
II
TMC435350
Tibotec Medivir
II
ITMN-191
InterMune/Roche
I
NS-3 (serine
protease)
NS-3 (serine
protease)
NS-3 (serine
protease)
NS-3 (serine
protease)
NS-3 (serine
protease)
Boceprevir
(SCH503034)
SCH900518
*Experimental dose
II
(RdRP)
(RdRP)
(RdRP)
(RdRP)
Genotype
Dosage*
Route
1
1, 2, 3
1,2
1
Oral
Oral
Oral
Oral
1, 2 (also 3)
1,500–3,000 mg BID
1,500 mg BID
1 mg/kg
100–450 mg BID or 300 mg
TID
750 mg/8 h
1
800 mg TID
Oral
1
200–600 mg QD
Oral
1, 2, 4, 5, 6 (also
3)
1
25–400 mg daily
Oral
900 mg BID
Oral
Oral
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clinical trials have indicated that, when genotype 1-infected
standard therapy non-responders and relapsers were administered Telaprevir plus pegylated interferon and ribavirin for 12 weeks followed by another 12 weeks of
Pegasys and ribavirin only, 41% (27 of 66) and 73% (29
of 40) of patients showed an absence of HCV RNA
12 weeks post-treatment (SVR12), respectively. Telaprevir has also demonstrated potent reduction of HCV RNA
following 14 or 28 days of therapy, prompting Vertex to
assess the ability of Telaprevir to shorten treatment
duration to 12 weeks [9].
The binding of Telaprevir to genotype 1a NS3-4A is a
slow-on, slow-off process (half life ∼1 h), with a steadystate inhibition constant (Ki) of 7 nM. In comparison,
Telaprevir binding to genotypes 2a and 3a serine proteases
yields inhibition constants of 30–50 nM and 300 nM,
respectively, indicating that the drug’s potency against
each of these enzymes is reduced by 4–7-fold and 40-fold,
respectively [10].
Boceprevir (SCH-503034; Schering-Plough) is another
oral ketoamide inhibitor of serine protease found to prevent
HCV polyprotein maturation and HCV replication. Its
binding leads to adduct formation between the carbonyl
carbon of its ketoamide group and serine-139 located in the
active site of the enzyme. Boceprevir is a potent inhibitor of
the NS3 protease, with an estimated Ki of 14 nM. The
results from a phase II study have shown that a 48-week
treatment regimen comprised of pegylated interferon and
ribavirin for 4 weeks prior to including Boceprevir
achieved an astounding 74% sustained virological response
after 24 weeks (SVR24) in genotype 1-infected treatmentnaïve patients as compared to the control group, in which
48 weeks of pegylated interferon and ribavirin treatment
yielded a 38% SVR24. Currently, the effectiveness of
Boceprevir in treating non-responders is under evaluation.
Additionally, a next-generation compound SCH-900518
has also entered clinical trials. Encouraging results from
phase II studies have shown that SCH 900518 is an order of
magnitude more potent than Boceprevir, could exhibit
activity against HCV strains resistant to other protease
inhibitors, and can potentially be dosed once per day [11,
12].
The macrocyclic serine protease inhibitor TMC 435350
(Tibotec/Medivir; phase II) has also shown promise in that
it is well tolerated and produces a dose-dependent rapid
antiviral response in genotype 1-infected subjects. It binds
non-covalently to its target with a fast on-rate, but has a
slow off-rate, and has been found to inhibit NS3 serine
protease from genotypes 1, 2, 4, 5, and 6 with an IC50 of
less than 10 nM; against the genotype 3 enzyme, however,
its IC50 is approximately 100 nM [13, 14].
Albeit in an early stage of development, ITMN-191
(InterMune/Roche; phase I) is another NS3 inhibitor that is
Eur J Clin Microbiol Infect Dis (2009) 28:1409–1413
tolerated well as monotherapy and significantly reduces
viral load in chronically infected HCV patients who have
not responded well to the standard combination therapy. At
present, the effectiveness of ITMN-191 together with
standard therapy is underway. Interestingly, data obtained
from the HCV replicon system suggest that ITMN-191,
when combined with the RdRP inhibitors R1626 or R7128,
not only potently inhibits viral replication, but also reduces
or suppresses the emergence of HCV variants that are drugresistant [15, 16].
Conclusion
With an estimated 3% of the world population infected
with hepatitis C virus (HCV), there is a dire need to
develop not only potent molecules that will inhibit HCV
replication, but also efficacious vaccines that will protect
individuals from becoming infected. Towards that end,
tremendous progress has been made in these areas over
the past decade, which is likely to culminate in the
approval of at least one compound within the next three
years. Given that the various drugs currently in the
pipeline preferentially target the North American HCV
genotype 1, the obvious challenge for pharmaceutical and
biotechnology companies is to develop a pan-HCV
inhibitor that will be useful for treating infections caused
by all HCV variants. The rapid rate at which HCV evolves
will undoubtedly make the discovery of such a molecule
challenging, as well as time-consuming. Until such a
magic bullet is discovered, it is likely that a combination
therapy consisting of a cocktail of one or more anti-HCV
drugs together with standard therapy will be used for
treating hepatitis C infections.
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