Author: Howard J. Worman, MD
Author Date: 1/1/2000

As many as 4 million Americans, and 170 million individuals
worldwide, are infected with the hepatitis C virus (HCV).

Chronic infection with HCV can lead to cirrhosis and liver cancer
and is presently the number one indication for liver transplantation
in the United States. Because a small animal model for HCV infection
has not been developed and because robust cell culture systems are
not currently available, extremely little is known about how the
virus replicates, infects cells or damages them. As a result, the
currently available medical treatments for hepatitis C are non-
specific, associated with adverse events and effective in only a
minority of patients. As basic research progresses, however, we
should witness the evolution of more effective treatments in the
next decade.

Short Term (2000 - 2003)

Currently, several preparations of interferon alpha are approved by
the United States Food and Drug Administration (FDA) for the
treatment of chronic hepatitis C. Interferon treatment is associated
with numerous adverse events, most notably flu-like symptoms, low
white blood counts, low platelet counts and depression. Interferon
alpha must be administered by injection three times a week,
something many patients do not like to do. Currently, preparations
of interferon alpha-2b complexed with polyethylene glycol (PEG) are
being studied in clinical trials. These so-called PEG-ylated
interferons are released more slowly and evenly into the bloodstream
and need only be administered by injection once a week. Preliminary
studies indicate that the response rates are superior to non-PEG-
ylated interferon. Less frequent injections should also lead to
better acceptance. PEG-ylated interferon alpha-2b (Schering-Plough)
and alpha-2a (Hoffman La Roche) will likely be approved by the FDA
in 2000.

The combination of interferon alpha-2b plus oral ribavirin
(Rebetron, Schering-Plough) is also approved by the FDA for the
treatment of chronic hepatitis C. Combination treatment result in a
long-term "virological cures" (no detectable virus in blood 6 months
after stopping treatment) in approximately 45% of subjects as
compared to less than 20% for interferon alpha alone. PEG-ylated
interferons, when approved, will therefore likely be used in
combination with ribavirin. The major adverse event associated with
ribavirin is hemolytic anemia (destruction of red blood cells). In
rare cases, anemia can be life-threatening. VX-497 (Vertex
Pharmaceuticals) is a ribavirin-like compound that has been tested
in phase II clinical trials. It appears to decrease blood ALT
activities and to be well-tolerated. Its effectiveness as a single
agent against HCV has not been established (ribavirin as a single
agent is not effective against HCV but is synergistic with
interferon alpha). In the next few years, VX-497 will probably be
studied in combination with interferon alpha to establish if it is
as effective as ribavirin with a similar or preferable adverse
events profile.

Interferon alpha affects the immune system. It is also possible that
other immune-modulatory drugs will be effective in the treatment of
HCV infection. Preliminary studies suggest that one such compound is
interleukin-10 (Schering-Plough). Larger clinical trials of
interluekin-10 for chronic hepatitis C are now getting starting.

In the short term, PEG-ylated interferon plus ribavirin or a similar
compound will probably be the standard of care for the treatment of
chronic viral hepatitis C. Other immune-modulatory compounds such as
interleukin-10 may also be proven useful. Given the preliminary
data, it appears that roughly 50% of individuals will obtain long-
term "virological cures" after such treatments.

Intermediate Term (2004-2010)

The next generation of drugs for the treatment of chronic hepatitis
C will be specifically directed against the virus. To understand the
development of such drugs, a basic understanding of HCV replication
is necessary. Some of the important steps in viral replication
include:

* Viral RNA must be translated into protein.This occurs on
ribosomes, the protein synthesis "factories" within cells. The HCV
RNA genome is translated by internal ribosome binding via a special
sequence known as a IRES. In contrast, cellular RNAs bind ribosomes
via a chemical modification know as a 5' cap. Specific sequences in
HCV RNA, such as the IRES, are potential attack points for anti-
viral drugs.

* HCV RNA must be unwound for translation and replication. This is
catalyzed by a RNA helicase that is part of the viral NS3 protein.
Cells contain different RNA helicases for 5' capped RNA. Therefore,
drugs can be specifically targeted against the HCV NS3 RNA helicase.

* HCV RNA is translated into a polyprotein that must be processed
into smaller proteins. Two viral proteases, NS2 and part of NS3,
catalyze most of the cleavages of the polyprotein. HCV protein NS4A
is a co-factor that binds to NS3 protease. Specific protease
inhibitors (similar drugs are used to attack HIV which has a
protease) may be developed to inhibit HCV replication.

* HCV RNA must be replicated.The viral protein NS5B is an RNA-
dependent RNA polymerase that carries out this function. Plants and
animals have DNA genomes and do not require enzymes that replicate
RNA. Therefore, the NS5B polymerase is a target for anti-viral drugs.

The three-dimensional structures of the HCV NS3 helicase domain, the
NS3 protease domain, the NS3 protease domain complexed with NS4A and
the NS5B polymerase have been determined by X-ray crystallography.
Armed with this knowledge, pharmaceutical chemists can design
compounds that inhibit their functions. This method is called
rational drug design. Rational drug design can be combined with
combinatorial chemistry in which a library of thousands or more of
structurally similar molecules is tested against the target. By
combining rational drug design and combinatorial chemistry with high
throughput screening methods (assays that allow for the automated
rapid testing of compounds), inhibitors of these essential viral
proteins can be discovered and developed into drugs. Many
pharmaceutical and biotechnology companies are currently doing
exactly this. Others are using various methods to identify compounds
that bind to the HCV RNA. These may inhibit the function of the IRES
or the packaging of RNA into viral particles. Molecules known a
ribozymes, catalytic RNAs that splice other RNAs, are also being
tested against HCV.

By the end of the decade, HCV infection will hopefully be treated by
combination therapy with helicase, protease and RNA polymerase
inhibitors. Drugs that attack the viral RNA may also be used.
Interferon or interleukins that affect the immune system may also
continue to have a more limited role along with these more specific
compounds.


Long Term (Beyond 2010)

Its difficult to look more than 10 years into the future, especially
given the rapid pace of modern biomedical research. Even predictions
for the short and intermediate terms may turn out to be wrong as the
result of unanticipated discoveries. Remember that HCV was
discovered only about 10 years ago.

Looking further ahead, advances in technology may lead to a vaccine
against HCV. Vaccine development is presently hindered by an
inability to culture the virus. HCV also replicates and mutates at
an extremely high rate (apparently 1,000 times that of HIV),
enabling it to escape immune system detection. Ten more years of
research may overcome these problems.

The receptors for HCV entry into liver cells are also not presently
known. Viral particles bind to a cell surface protein known as CD81,
however, it is not clear if this binding mediates entry into cells.
The discovery of cell receptors and co-receptors for HCV should
enable the development of drugs that inhibit viral uptake.

It is also not known how HCV damages liver cells. The virus may kill
cells directly. Liver damage may also be mediated by the immune
system attacking cells infected with HCV. Long term research on the
effects of HCV on liver cells, and on the immune response against
the virus, could lead to therapeutic agents that not only attack the
virus but also prevent it from damaging the liver and causing
cirrhosis and cancer.
______________


[This article first appeared in the Spring 2000 edition of the
American Liver Foundation, Greater New York Chapter's, View Points
newsletter]