Transcript Nucleic Acids - Farmasi Unand

14. Viruses
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14. Viruses
• Viruses are infective agents that are
considerably smaller than bacteria.
They are essentially packages, known as
virions, of chemicals that invade host
cells.
• However, viruses are not independent
and can only penetrate a host cell that
can satisfy the specific needs of that
virus.
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• The mode of penetration varies
considerably from virus to virus. Once
inside the host cell viruses take over
the metabolic machinery of the host
and use it to produce more viruses.
Replication is often lethal to the host
cell, which may undergo lysis to release
the progeny of the virus.
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• However, in some cases the virus
may integrate into the host
chromosome and become dormant.
The ability of viruses to reproduce
means that they can be regarded as
being on the borderline of being
living organisms.
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14.1. Structure and replication
• Viruses consist of a core of either
DNA or, as in the majority of cases,
RNA fully or partially covered by a
protein coating known as the capsid.
The capsid consists of a number of
polypeptide molecules known as
capsomers (Fig.10.43).
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Figure 10.37. (a) Schematic representations of the
structure of a virus (a) without a lipoprotein
envelope (naked virus) and (b) with a lipoprotein
envelope.
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• The capsid that surrounds most viruses
consists of a number of different
capsomers although some viruses will
have capsids that only contain one type
of capsomer.
• It is the arrangement of the capsomers
around the nucleic acid that determines
the overall shape of the virion.
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• In the majority of viruses, the
capsomers form a layer or several
layers that completely surround the
nucleic acids. However, there are
some viruses in which the
capsomers form an open-ended
tube that holds the nucleic acids.
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• In many viruses the capsid is coated
with a protein-containing lipid bilayer
membrane.
• These are known as enveloped viruses.
Their lipid bilayers are often derived
from the plasma membrane of the host
cell and are formed when the virus
leaves the host cell by a process known
as budding.
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• Budding is a mechanism by which a virus
leaves a host cell without killing that
cell. It provides the virus with a
membrane whose lipid components are
identical to those of the host (Fig.
10.43).
• This allows the virus to penetrate new
host cells without activating the host’s,
immune systems.
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• Viruses bind to host cells at specific
receptor sites on the host’s cell
envelope.
• The binding sites on the virus are
polypeptides in its capsid or lipoprotein
envelope. Once the virus has bound to
the receptor of the host cell the virus–
receptor complex is transported into
the cell by receptor-mediated
endocytosis.
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• In the course of this process the
protein capsid and any lipoprotein
envelopes may be removed.
• Once it has entered the host cell the
viral nucleic acid is able to use the
host’s cellular machinery to synthesise
the nucleic acids and proteins required
to replicate a number of new viruses
(Fig. 10.44).
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• A great deal of information is
available concerning the details of
the mechanism of virus replication
but this text will only outline the
main points. For greater detail the
reader is referred to specialist
texts on virology.
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14.2. Classification
• RNA-viruses can be broadly classified into
two general types, namely: RNA-viruses
and RNA-retroviruses.
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• Figure 10.44 A schematic representation
of the replication ofprof.RNA-viruses
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RNA-viruses
• RNA-virus replication usually
occurs entirely in the cytoplasm.
The viral mRNA either forms
part of the RNA carried by the
virion or is synthesised by an
enzyme already present in the
virion.
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• This viral mRNA is used to
produce the necessary viral
proteins by translation using
the host cell’s ribosomes and
enzyme systems.
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• Some of the viral proteins are
enzymes that are used to catalyse
the reproduction of more viral
mRNA. The new viral RNA and viral
proteins are assembled into a
number of new virions that are
ultimately released from the host
cell by either lysis or budding.
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Retroviruses
• Retroviruses synthesise viral DNA
using their viral RNA as a template.
• This process is catalysed by enzyme
systems known as reverse
transcriptases that form part of the
virion. The viral DNA is incorporated
into the host genome to form a socalled provirus.
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• Transcription of the provirus
produces new ‘genomic’ viral RNA
and viral mRNA. The viral mRNA is
used to produce viral proteins,
which together with the ‘genomic’
viral RNA are assembled into new
virions.
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• These virions are released by
budding , which in many cases
does not kill the host cell.
Retroviruses are responsible
for some forms of cancer and
AIDS
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DNA-viruses
• Most DNA-viruses enter the host cell’s
nucleus where formation of viral mRNA
by transcription from the viral DNA is
brought about by the host cell’s
polymerases. This viral mRNA is used to
produce viral proteins by translation
using the host cell’s ribosomes and
enzyme systems.
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• Some of these proteins will be enzymes
that can catalyse the synthesis of more
viral DNA.
• This DNA and the viral proteins
synthesised in the host cell are
assembled into a number of new virions
that are ultimately released from the
host by either cell lysis or budding
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14.3. Viral diseases
• Viral infection of host cells is a common
occurrence. Most of the time this
infection does not result in illness as
the body’s immune system can usually
deal with such viral invasion.
• When illness occurs it is often short
lived and leads to long-term immunity.
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• However, a number of viral infections
can lead to serious medical conditions (.
Some viruses like HIV, the aetiological
agent of AIDS, are able to remain
dormant in the host for a number of
years before becoming active, whilst
others such as herpes zoster (shingles)
can give rise to recurrent bouts of the
illness. Both chemotherapy and
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preventative
• Both chemotherapy and preventative
vaccination are used to treat patients.
The latter is the main clinical approach
since it has been difficult to design
drugs that only target the virus.
However, a number of antiviral drugs
have been developed and are in clinical
use.
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AIDS
• AIDS is a disease that progressively
destroys the human immune system. It
is caused by the human
immunodeficiency virus (HIV), which is a
retrovirus. This virus enters and
destroys human T4 lymphocyte cells.
These cells are a vital part of the
human immune system.
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• Their destruction reduces the
body’s resistance to other
infectious diseases, such as
pneumonia, and some rare forms of
cancer.
•.
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• The entry of the virus into the body
usually causes an initial period of acute
ill health with the patient suffering
from headaches, fevers and rashes,
amongst other symptoms.
• This is followed by a period of relatively
good healthy where the virus replicates
in the lymph nodes.
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• This relatively healthy period normally
lasts a number of years before fullblown
• AIDS appears. Full-blown AIDS is
characterised by a wide variety of
diseases such as bacterial infections,
neurological diseases and cancers.
Treatment is more effective when a
mixture of antiviral agents is used
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14.4. Antiviral drugs
• It has been found that viruses utilize a
number of virus-specific enzymes during
replication.
• These enzymes and the processes they
control are significantly different from
those of the host cell to make them a
useful target for medicinal chemists.
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• Consequently, antiviral drugs
normally act by :
• inhibiting viral nucleic acid
synthesis,
• inhibiting attachment to and
penetration of the host cell or
• inhibiting viral protein synthesis.
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Nucleic acid synthesis inhibitors
• Nucleic acid synthesis inhibitors usually
act by inhibiting the polymerases or
reverse transcriptases required for
nucleic acid chain formation.
• However, because they are usually
analogues of the purine and pyrimidine
bases found in the viral nucleic acids,
they are often incorporated into the
growing nucleic acid chain.
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• In this case their general mode of
action frequently involves conversion to
the corresponding 5-triphosphate by
the host cell’s cellular kinases.
• This conversion may also involve specific
viral enzymes in the initial
monophosphorylation step.
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• These triphosphate drug derivatives are
incorporated into the nucleic acid chain
where they terminate its formation.
Termination occurs because the drug
residues do not have the 3-hydroxy
group necessary for the phosphate
ester formation required for further
growth of the nucleic acid chain.
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• This effectively inhibits the
polymerases and transcriptases
that catalyze the growth of the
nucleic acid (Fig. 10.45).
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Aciclovir
• Aciclovir was the first effective
antiviral drug. It is effective against a
number of herpes viruses, notably
simplex, varicella-zoster (shingles),
varicella (chickenpox) and Epstein–Barr
virus (glandular fever).
• It may be administered orally and by
intravenous injection as well as topically.
Orally administered doses have a low
bioavailability.
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• The action of aciclovir is more effective
in virus-infected host cells because the
viral thymidine kinase is a more
efficient catalyst for the
monophosphorylation of aciclovir than
the thymidine kinases of the host cell.
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• This leads to an increase in the
concentration of the aciclovir
triphosphate, which has 100-fold
greater affinity for viral DNA
polymerase than human DNA
polymerase.
• As a result, it preferentially
competitively inhibits viral DNA
polymerase and so prevents the virus
from replicating.
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• However, resistance has been reported
due to changes in the viral mRNA
responsible for the production of the
viral thymidine kinase.
• Aciclovir also acts by terminating chain
formation. The aciclovir–DNA complex
formed by the drug also irreversibly
inhibits DNA polymerase.
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Vidarabine
• Vidarabine is active against herpes
simplex and herpes varicella-zoster.
• However, the drug does give rise to
nausea, vomiting, tremors, dizziness and
seizures. In addition it has been
reported to be mutagenic, teratogenic
and carcinogenic in animal studies.
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• Vidarabine is administered by
intravenous infusion and topical
application. It has a half-life of about
one hour, the drug being rapidly
deaminated to arabinofuranosyl
hypoxanthine (ara-HX) by adenosine
deaminase.
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• This enzyme is found in the serum and red
blood cells. Ara-HX, which also exhibits a
weak antiviral action, has a half-life of about
3.5 hours.
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Zidovudine (AZT)
• Zidovudine was originally synthesised in
1964 as an analogue of thymine by J.
Horwitz as a potential antileukaemia
drug.
• It was found to be unsuitable for use in
this role and for 20 years was ignored,
even though in 1974 W. Osterag et al.
reported that it was active against
Friend leukaemia virus, a retrovirus.
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• However, the identification in 1983
of the retrovirus HIVas the source
of AIDS resulted in the virologist
M. St Clair setting up a screening
programme for drugs that could
attack HIV
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• Fourteen compounds were selected and
screened against Friend leukaemia virus
and a second retrovirus called Harvey
sarcoma virus.
• This screen led to the discovery of
zidovudine (AZT), which was rapidly
developed into clinical use on selected
patients in 1986.
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• AZT is converted by the action of
cellular thymidine kinase to the 5triphosphate.
• This inhibits the enzyme reverse
transcriptase in the retrovirus,
which effectively prevents it from
forming the viral DNA necessary
for viral replication.
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• The incorporation of AZT into the
nucleic acid chain also results in chain
termination because the presence of
the 3-azide group prevents the reaction
of the chain with the 5-triphosphate of
the next nucleotide waiting to join the
chain (Fig. 10.45).
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• AZT is also active against
mammalian DNA polymerase and
although its affinity for this
enzyme is about 100-fold less this
action is thought to be the cause of
some of its unwanted side effects.
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• Zidovudine is active against the
retroviruses (see section 10.14.2) that
cause AIDS (HIV virus) and certain
types of leukaemia.
• It also inhibits cellular a-DNA
polymerase but only at concentrations in
excess of 100-fold greater than those
needed to treat the viral infection.
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• The drug may be administered orally or
by intravenous infusion. The
bioavailability from oral administration
is good, the drug being distributed into
most body fluids and tissues.
• However, when used to treat AIDS it
has given rise to gastrointestinal
disorders, skin rashes, insomnia,
anaemia, fever, headaches, depression
and other unwanted effects.
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Resistance
• Resistance increases with time.
This is known to be due to the virus
developing mutations’ which result
in changes in the amino acid
sequences in the reverse
transcriptase.
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Didanosine
• Didanosine is used to treat some AZT-
resistant strains of HIV. It is also used
in combination with AZT to treat HIV.
Didanosine is administered orally in
dosage forms that contain antacid
buffers to prevent conversion by the
stomach acids to hypoxanthine
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• However, in spite of the use of buffers
the bioavailability from oral
administration is low.
• The drug can cause nausea, abdominal
pain and peripheral neuropathy, amongst
other symptoms. Drug resistance occurs
after prolonged use.
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• Didanosine is converted by viral and
cellular kinases to the monophosphate
and then to the triphosphate.
• In this form it inhibits reverse
transcriptase and in addition its
incorporation into the DNA chain
terminates the chain because the drug
has no 3-hydroxy group (Fig. 10.45).
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Host cell penetration inhibitors
• The principal drugs that act in this
manner are amantadine and rimantadine
(Fig. 10.46).
• Both amantadine and rimantadine are
also used to treat Parkinson’s disease.
However, their mode of action in this
disease is different from their action
as antiviral agents.
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Amantadine hydrochloride
• Amantadine hydrochloride is
effective against influenza A virus
but is not effective against the
influenza B virus. When used as a
prophylactic, it is believed to give
up to 80 per cent protection
against influenza A virus infections
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• The drug acts by blocking an ion
channel in the virus membrane
formed by the viral protein M2.
This is believed to inhibit the
disassembly of the core of the
virion and its penetration of the
host (see section 10.14.1).
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• Amantadine hydrochloride has a good
bioavailability on oral administration,
being readily absorbed and distributed
to most body fluids and tissues.
• Its elimination time is 12–18 hours.
However, its use can result in
depression, dizziness, insomnia and
gastrointestinal disturbances, amongst
other unwanted side effects.
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Rimantadine hydrochloride
• Rimantadine hydrochloride is an
analogue of amantadine
hydrochloride.
• It is more effective against
influenza A virus than amantadine.
Its mode of action is probably
similar to that of amantadine.
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• The drug is readily absorbed when
administered orally but undergoes
extensive first-pass metabolism.
However, in spite of this, its
elimination half-life is double that
of amantadine. Furthermore, CNS
side effects are significantly
reduced.
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Inhibitors of viral protein
synthesis
• The principal compounds that act as
inhibitors of protein synthesis are the
interferons.
• These compounds are members of a
naturally occurring family of
glycoprotein hormones (RMM 20 000–
160 000), which are produced by nearly
all types of eukaryotic cell.
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• Three general classes of interferons
are known to occur naturally in
mammals, namely: the α-interferons
produced by leucocytes, β-interferons
produced by fibroblasts and γinterferons produced by T lymphocytes.
At least twenty α-, two β- and two γinterferons have been identified
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• Interferons form part of the human
immune system. It is believed that the
presence of virions, bacteria and other
antigens in the body switches on the
mRNA that controls the production and
release of interferon.
• This release stimulates other cells to
produce and release more interferon.
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• Interferons are thought to act by
initiating the production in the cell
of proteins that protect the cells
from viral attack.
• The main action of these proteins
takes the form of inhibiting the
synthesis of viral mRNA and viral
protein synthesis.
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• a- Interferons also enhance the
activity of killer T cells
associated with the immune
system. (see section 14.5.5).
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• The main action of these proteins
takes the form of inhibiting the
synthesis of viral mRNA and viral
protein synthesis.
• α- Interferons also enhance the
activity of killer T cells associated
with the immune system.
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• A number of a-interferons have
been manufactured and proven to
be reasonably effective against a
number of viruses and cancers.
• Interferons are usually given by
intravenous, intramuscular or
subcutaneous injection.
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• However, their administration can cause
adverse effects, such as headaches,
fevers and bone marrow depression,
that are dose related.
• The formation and release of interferon
by viral and other pathological
stimulation has resulted in a search for
chemical inducers of endogenous
interferon.
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• Administration of a wide range of
compounds has resulted in the
induction of interferon production.
However, no clinically useful
compounds have been found for
humans’ although tilorone is
effective in inducing interferon in
mice.
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