Practical Guidelines in Antiviral Therapy

341 Pages·2002·19.52 MB·English

by  G.J. Galasso

PREFACE
This text was developed with the practicing physician in mind, but we believe it
will be of considerable interest to the virologist, pharmacologist, chemist and all
scientists interested in antiviral agents. It is proposed that, approximately a year after
its publication, this volume will be available in an online version through the publisher
Elsevier and will be kept updated by the authors. This is to make it readily available
in an updated version for the practicing physician to review what the current status
of antiviral research is, so that he/she can utilize this information in making their
decisions. It is not intended as a recommended treatment syllabus but as a source of
current information. It is also intended for the involved scientist to keep abreast of
developments in subspecialties other than his/her own.
Progress in the field of antiviral development, in the past has been slow, but we are
pleased to see that it is now moving rapidly and we hope that there will be successful
treatment modalities for most viral diseases. The future is indeed bright. However, with
progress, we have also learned of the pitfalls we need be aware of, such as resistance
and toxicity, to which we must remain alert. Research continues at a rapid rate to find
improved drugs and safe treatment regimens. An additional challenge will be to make
treatment for viral disease available for those individuals who live in countries that
cannot afford some of the current antiretroviral drugs.
We are most grateful to the internationally renowned experts who have agreed to
participate in this volume, sharing their information on the latest developments. We have
tried to select experts on both sides of the ocean for each chapter so as to capture the
worldwide practices. We would also like to thank The Macrae Group for helping with
the initial development of this book.
Charles A.B. Boucher
George J. Galasso, Editors
vu
CHAPTER 1
CLASSES OF ANTIVIRAL DRUGS
MIREILLE VAN WESTREENEN and CHARLES A. B. BOUCHER
Table of Contents
Introduction 1
I. Inhibition of Viral Proteins 3
Binding and Entry 3
Replication 4
Release 7
II. Inhibition of Virus Replication By Cellular Enzymes 8
Conclusion 9
References 10
Introduction
Being obligate intracellular parasites, viruses are dependent on the metabolic pathways
of the host cell for their replication. So, virus and host cell are intimately connected and
an effective antiviral agent must be able to distinguish virus related enzymes from host
cell material itself. The search for antitumour agents generated a great deal of interest in
DNA synthesis inhibitors. The first drugs capable of inhibiting viral DNA in vitro were
described in the 50s, but real progress was not made until the 70s. In the last decades
we have come to know more about the biochemistry of viral replication and this has
led to a more rational approach to the search for antiviral agents. When reviewing the
possibilities for antiviral chemotherapy, the best guideline is the viral replicative cycle,
which can be divided in ten steps (Figure 1). First a virus binds to the cell surface of
the host cell and penetrates the cell membrane and shed its protein coat (1-3 binding
and entry). The genetic material of the virus uses the biochemical mechanisms of the
host cell to replicate the genetic material of the virus (4-8 replication). This replication
step differs in RNA and DNA viruses, but mRNA is produced in all. Then the genetic
material is capsulated and the newly formed particles are released out of the host
cell (9-10 release). Although all viruses follow this replicative cycle, different virus
families may differ considerably from one another at one or more steps of the cycle.
Useful inhibitors are generally specific for one family of virus and, in some cases,
to individual members of that family (e.g. particular members of the herpesviruses).
1
Practical Guidelines in Antiviral Therapy Ed. by Charles A.B. Boucher and George J. Galasso. 1 — 12
© 2002 Elsevier Science. Printed in the Netherlands.
M. van Westreenen and C.A.B. Boucher
receptor blockade
1 attachment
immunoglobulins
T-20
2 fusion
betulinic acid
bicyclams
amantadine derivatives
Ij^y 3 uncoating -•
pleconaril
iM 4 early (reverse) (non) nucleoside
transcription (RT) analogues
* ,, 5 early
#
1 translation/ .... integrase inhibitors
»i integration
H
nucleoside analogues
6 replication ---
polymerase inhibitors
V ^ Iste transcription
8 late translation -- oligonucleosides
ribozymes
protease inhibtors
A^^ 9 assembly
amantadine derivates
1
10 budding neuramidase inhibitors
Figure 1. Sites of potential action for most classes of virus inhibitors.
The target of these antiviral agents can be (I) specific viral proteins, (II) host cellular
enzymes or (III) modulation of host immune responses. This chapter will summarize
the various optional compounds, which interact with the viral replicative cycle.
Classes of Antiviral Drugs 3
I. Inhibition of Viral Proteins
Binding and Entry
1. Attachment of The Virus Particle
The first event in viral infection of the host cell is binding of the virus to the cell surface.
This binding of the virus particle to the cell-surface molecule, the receptor, involves
numerous interactions between the virion surface and the receptor. A wide variety of
cell-surface molecules that normally serve the host cells as receptors for other molecules
such as protein molecules, carbohydrates, or glycolipids are used by viruses for entry.
Some receptor molecules are widely distributed. To block initial aspecific binding of
virus to cells, polyanionic compounds (i.e. polysulfates, polysulfonates, polycarboxylates,
polyoxometalates) were suggested. Although polyanionic compounds may interfere with
processes other than virus adsorption (particularly virus-cell fusion and the reverse
transcription process), their mechanism of action can be attributed primarily to inhibition
of virus-cell binding. Dextran sulfate, as prototype of the sulfated polysaccharides, was
found to inhibit the in vitro replication of HIV and other enveloped viruses [1]. A series
of sulfated polymers were potent and selective inhibitors of respiratory syncytial virus
and influenza A virus in mice [2]. The fact that the polysaccharides and polymers are
inhibitory to some myxoviruses and retroviruses but not to others seems to depend on
the composition of the amino acid sequences of the viral envelope glycoproteins that are
involved in virus-cell binding and fusion.
Another form of receptor blocking can be achieved by specific molecules such as the
CD4 protein molecule, which serves as the receptor for HIV on T-lymphocytes. Various
forms of recombinant CD4 were assayed against in vitro and in vivo HIV infection [3].
To block the attachment of rhinovirus to the cellular protein, intracellular adhesion
molecule-1 (ICAM-1) was tested. Because 90% of all rhinoviruses use ICAM-1 as a
mechanism to gain entry to the host cell, antagonism of the virus-receptor interaction
would appear to be an effective way to inhibit a broad spectrum of rhinoviruses [4].
Another way of preventing binding of the virus to the host cell is by using
antibodies against the infecting agent. These antibodies are specific immunoglobulins
against an infecting agent and can be obtained by passive or active immunization
(vaccination). Passive immunization with for instance CMV-, hepatitis B-, tetanus-, or
RSV immunoglobulin can only protect the host during a short time. Vaccins against
measles, rubella, mumps, hepatitis A, hepatitis B, influenza and poliomyelitis protects the
host from infection during a time span ranging from a couple of years to lifetime [5].
2. Virus-cell Fusion
After binding to its surface receptor, a virus must enter the cell. Two general pathways
have been defined for virus entry: 1) surface fusion between the viral lipid envelope and
the cell plasma membrane and 2) as an alternative strategy for nonenveloped viruses;
receptor-mediated endocytosis. Some compounds are assumed to interact with the
postbinding virus-cell fusion process by affecting syncytium formation. Syncytia results
from the fusion of the infected cell with neighboring cells, which is a feature of infection
by lentiviruses, paramyxoviruses, some herpesviruses, and other viruses. This may rep-
M. van Westreenen and C.A.B. Boucher
resent an important mechanism of spread which avoids exposure of virions to antibodies.
Several compounds effectively block syncytium formation by interfering with the
junction of the viral envelope glycoprotein (gpl20) and the (CD4) cell-receptor.
Examples of such virus-cell fusion inhibitors are T-20, betulinic acid, bicyclams
and negatively charged albumins [6-9]. The most promising compound in preclinical
development is T-20, a synthetic peptide, which blocks HIV-1 entry into the host cell
by binding to the viral glycoprotein gp41.
3. Uncoating of The Viral Capsid
The process of disassembly or uncoating requires that the virion is stable to survive the
conditions that exist in the new intracellular environment. However, the virion should
not be as stable as to withstand the receptor and pH-induced conformational changes
that must occur to allow efficient DNA or RNA release. The uncoating-blocking agents
retain the virus in the encapsulated state by increasing the stability of the virion, and
in this way the infection cycle is effectively blocked. Uncoating inhibitors such as the
amantadine derivatives (amantadine and rimantadine) specifically prevent release of
influenza A virus in the cells. Amantadine and rimantadine operate through physical
blockade of the proton channels formed by the protein M2, which is a minor component
of the influenza viral envelope that is thought to play a key role in stabilizing the viral
hemagglutinin. This blockade affects the proton flow through the M2 protein channels,
which leads to inhibition of virus uncoating [10].
Other uncoating inhibitors have been shown to prevent the uncoating of picornavi-
ruses [4]. These compounds can prevent uncoating of the virion by binding within a
hydrophobic pocket of the virus. In this way these agents increase the stability of the
viral capsid to receptor and pH induced conformational changes which normally occur
during the process of cellular entry. These events prevent the disassembly and release
of viral ribonucleic acid. Pleconaril is one such antipicornaviral agent with potential
therapeutic applications in the treatment of viral meningitis, upper respiratory disease,
and other enteroviral infections [11, 12]. Conformational changes in the presence of
calcium ions are also suggested by data indicating that calcium mediates a tighter
interaction of hepatitis A receptors with hepatitis A particles. Calcium ions were
found to destabilize hepatitis A virus particles [13]. Hepatitis A virus replication in
cultured cells was studied in the presence of other potential uncoating inhibitors. Strong
inhibition was observed in the presence of chlorpromazine and chloroquine.
Replication
Different events occur in the replication cycle of DNA or RNA viruses. The genetic
material of DNA viruses is translated into mRNA by RNA polymerase. The genetic
material of RNA viruses is transcribed into mRNA by a virus-coded RNA polymerase or
the RNA is modulated by a viral reverse transcriptase into viral DNA, which is integrated
into the host cell DNA. After activation of the cell, viral mRNA can be produced. Antiviral
drugs that inhibit the replication steps can be therefore divided into reverse transcriptase
inhibitors (4), integrase inhibitors (5), nucleoside analogues, DNA polymerase inhibitors or
RNA polymerase inhibitors (6) and viral protein synthesis (mRNA) antagonists (7, 8).
Classes of Antiviral Drugs
4. Reverse Transcriptase
The emergence of HIV and of human cancer caused by retroviruses quickened interest
in the search for inhibitors of the crucial retroviral enzym, reverse transcriptase (RT).
Two classes of drugs act on the reverse transcriptase; the nucleoside reverse transcriptase
inhibitors (NRTIs) and the non-nucleoside reverse transcriptase inhibitors (NNRTIs).
The first acts at the substrate (dNTP) binding site and is active against retroviruses.
NRTIs either directly inhibit the enzyme or serve as alternative substrates for catalysis.
The earliest compounds to display sufficient antiviral activity in vivo were nucleoside
analogues; the first to be licensed for human use was zidovudine (AZT). Abacavir,
didanosine (ddl), lamivudine (3TC), stavudine (d4T), and zalcitabine (ddC) are all
dideoxynucleoside analogues. Nucleoside analogues can either act by inhibiting viral
reverse transcription (HIV), inhibiting viral DNA or RNA polymerases synthesis. For
example, lamivudine is also used as monotherapy in the treatment of hepatitis B virus
infection [14]. The acyclic nucleoside phosphonate (ANP) analogues such as cidofovir
(HPMPC), adefovir (PMEA) and PMPA (tenofovir) act also with the dNTP binding
site of HIV-1 reverse transcriptase and inhibit the replication of other retroviruses and
hepadnaviruses (e.g. hepatitis B). The ANP analogues, if incorporated into the growing
DNA chain, terminate the chain growth and thus act as DNA chain terminator [15].
The second class of drugs inhibits reverse transcriptase by binding to sites other than
which normally interact with the nucleosides. These compounds act at the aspecific
binding site and are active against HIV-1 but not other retroviruses. The NNRTIs bind
to HIV reverse transcriptase and disrupt the catalytic site to block viral replication.
They do not inhibit human polymerases, which makes them less toxic than the NRTIs.
The NNRTIs have shown to be potent partners for antiretroviral combined therapies,
resistance may develop early when given alone. A benzodiazepine-like drug with the
shortened name TIBO was the first of the NNRTIs to be identified. Subsequently several
other NNRTIs were found to behave like TIBO: HEPT, nevirapine, delaviridine
and many others [16].
5. Early Translation/Integration
During integration, viral DNA is inserted into the host genome in a process catalyzed by
the virus-encoded integrase. Thus, integration is leading to the formation of progeny viral
particles. Integration is also essential for maintenance of persistent infection by keeping
the pro virus in the host cell genome, ready for production of new infectious particles [17].
HIV integrase is being intensively explored as a potential target for anti-HIV agents [18].
As no cellular homologue of HIV integrase has been described, potential inhibitors could
be relatively nontoxic. Development of HIV-l integrase inhibitors could have favorable
implication for combination therapy, as well as prevention of the chronic carrier state
and the emergence of resistant mutants. Although several classes of putative integrase
inhibitors has been described, still no clinically useful anti-integration drugs are available.
It is the structural and functional complexity of the integration process together with
the limitations of the available in vitro assays that has made it problematic to develop
inhibitors of the HIV integrase [19].