Antimicrobials 4: antiviral
Viruses: what are they?
• cellular parasites –
– require a host cell (which may be prokaryotic or eukaryotic cell) this is unlike
bacteria that can survive on their own
– utilise the host cell’s metabolic machinery to replicate and survive this is because
viruses are just little bits of nucleic acid
• basic components of a virus
– nucleic acid (either RNA or DNA) enclosed in a protein coat or capsid. For a virus to
replicate it needs to generate more RNA and it uses the cell’s equipment to make
the protein coat
– Some viruses may also have a lipoprotein coat surrounding the capsid.
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In essence there are two parts:
o The nucleic acid – which codes for the synthesis of the viruses’ proteins using the
cells’ own equipment
o Protein or lipoprotein
Types of viruses
• DNA viruses (the nucleic acid in the viruses below is DNA)
– poxvirus (smallpox which is pretty much gone from the world)
– herpes (cold sores) – quite common
– varicella zoster virus (VZV)(chicken pox : shingles) – varicella is the component that
causes chicken pox and the zoster component causes shingles
– Adenovirus (sore throat)
– papilloma virus (warts)
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RNA viruses
– orthomyxoviruses (influenza)
– myxoviruses (measles, mumps)
– retroviruses (HIV/ AIDS)
– picornavirus (common cold)
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Understanding this is how we develop effective drugs
Pinocystosis is how the virus penetrates after attachment it creates little vesicles to help it
get into the cell
It then uncoats and travels to the nucleus
He just read each step until 7 where he said that the proteins made in step 6 assemble into
virus particles. The viral particles then bud from the nucleus and are released from the cell
so they can go infect other cells
Many of the drugs we use inhibit the viral DNA polymerase enzymes. They work where the
virus is trying to replicate it’s DNA
RNA viruses – single stranded
• Similar life cycle except that the viral RNA serves as the mRNA to direct synthesis of proteins
(basically many of the initial steps for DNA do not occur with the RNA viruses)
– RNA polymerase replicates the viral RNA
– Also codes for structural proteins required for capsid formation
• RNA may not necessarily enter the nucleus because RNA may be translated in the cytoplasm
unlike the DNA which needs to get into the nucleus
RNA viruses – retroviruses e.g. HIV
• This is a subgroup of RNA viruses as the virion contains a reverse transcriptase (which is now
a drug target)
– converts the RNA into DNA
– new DNA integrated into the genome of the host cell (termed provirus)
– transcribed and translated into new set of viral proteins. So normal RNA viruses just
get straight translated but these goes back to DNA via reverse transcriptase and
then transcribed etc
• Retroviruses may replicate without killing host cells
– may remain within host cell & continue to replicate i.e. they can be dormant for a
long time (HIV – 5-10 years) unlike normal RNA viruses which kill the cell straight
away
– have the capacity to transform host cells into malignant cells as they sit in the cell
for a long time. HIV can cause things like Hodgkins disease
Action of antiviral drugs
• Viruses rely upon many metabolic processes of the host cells
– -----> difficult to find drugs which are selective for the pathogen. As not only is the
virus reliant on the cells machinery to live but the host cell also needs this to live
• some enzymes are specific for viruses
– e.g. reverse transcriptase
Strategies for antiviral action
• Inhibit transcription of viral genome we can inhibit viral replication
– DNA polymerase inhibitors (aciclovir)
– reverse transcriptase inhibitors (zidovudine)
• inhibit post-translational events
– protease inhibitors (saquinavir used for HIV) when the virus first makes its proteins
they are very big and non functional so these big proteins need to be pruned down
by proteases so we can inhibit this and leave the virus with non-functional proteins
• inhibit viral attachment/penetration of host cells - stop step 1
– amantadine
• Immunomodulation (we modify the immune system to try and improve the immune systems
response to the virus)
– interferon
• inhibiting late stage of viral replication
– influenza neuraminidase inhibitors (neuraminidase is used by the virus to escape
from the host cell so by stopping it leaving the cell we stop it affecting other cells
– (zanamivir / oseltamivir). These 2 drugs are used for influenza to inhibit the above
mentioned enzyme
Inhibiting transcription of viral genome (using nucleoside based drugs)
• Inhibitors of DNA polymerase
– aciclovir (a guanosine derivative) – idea is to produce an analogue of the building
blocks of nucleic acid. The drug binds in it’s place and blocks the enzyme
– ganciclovir (a guanosine derivative)
– vidarabine (an adenosine derivative)
• Inhibitors of reverse transcriptase
– zidovudine (a thymidine derivative) – again same idea making a structural analogue
of the building blocks of nucleic acid
Aciclovir (Ac)
• guanosine derivative
• Ac undergoes phosphorylation to Ac-monophosphate (MP) by viral enzyme, viral
thymidylate kinase (TK)
• viral TK much more effective in synthesising Ac-MP than host cell TK – meaning that cells
that are infected by the virus will have a much higher concentration of the Ac-MP
• Ac-MP then has two more phosphates added on to become Ac-triphosphate (TP) and this is
done by host cell kinases.
• Ac-TP inhibits viral DNA polymerase
– Terminates viral replication
• So it’s a partnership between the virus and host cell that activates the drug as at first its the
viral enzyme then second host cell enzyme
• Ac-TP rapidly broken down (has a short duration of action)
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both of these are guanosine derivatives
Ganciclovir has a slightly longer side chain – still needs phosphylation
Aciclovir: viral selectivity
• viral DNA polymerase is 30x more susceptible to Ac-tP than host cells (which gives
protection to host cell enzymes)
• high specificity for (DNA viruses) so good for :
– herpes simplex (cold sores)
– varicella-zoster (chicken pox - shingles) [less susceptible to acyclovir then herpes but
still effective]
– EB virus (glandular fever – another herpes type virus) [less again so harder to treat
this with this drug]
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Shows how acyclovir works
First of all it is taken up by the host cell
The VTK does the first phosphyrylation before the cellular enzymes put on the next two
phosphates
The Ac-triphosphate is then taken up into the nucleic acid and inhibits the enzyme.
The inhibition terminates the nucleic chain from extending further which leaves a
dysfunctional nucleic acid
Aciclovir continued
• If there is mutations in the viral DNA polymerase acyclovir will no longer be effective as it
can’t bind to the enzyme kind of like certain antibacterial resistances
• resistant herpes simplex virus has caused pneumonia, encephalitis. There is a small
percentage of herpes viruses that are resistant to acyclovir
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above are the choices for treating herpes
short half life means frequent dosing for acyclovir
both points for the pro drugs is that they are converted in the body and so can be given once
daily while maintain efficacy
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valaciclovir is a pro-drug of acyclovir
contains the ring structure and an amino acid substitution on the chain
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a hydrolase enzyme cleaves off the L-valine and leaves us with aciclovir
the hydrolysis is done quite slowly which results in aciclovir being released progressively and
allows for once daily dosing
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shows aciclovir levels after we have given various formulations
a modest increase in valaciclovir dosage results in considerable increase in AUC
so basically valaciclovir is a better way of administering aciclovir
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problem with herpes viruses is they can be latent (sit around for a significant period of time)
you can see the virus doing what it normally does giving the characteristical blister of herpes
but it can also move into nerves and sit within the primary afferent of the peripheral nerve
(just before it joins up with the spinal cord)
it can sit there so that it still exists even after the blister heals and after a period of time can
bring up another blister. So we need to treat both the latent virus and blister
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as usual the lower the IC50 the more susceptible the virus/potent the drug is for that virus
herpes 1 and 2 are most susceptible
varicella is 10 fold higher so need a 10 fold increase in dose
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the human viruses 6,7 and 8 basically can’t be treated with this drug
so we mainly treat herpes simplex virus and we can treat chicken pox and shingles with
slightly higher doses
Efficacy in genital herpes simplex
• Antivirals reduce the impact of genital herpes infections:
– Episodically (tablets taken for 5 days when outbreaks occur):
• Abortion of lesion development/abort the progression of skin lesions
• reducing the pain associated with the infection (because it prevents the
progression to blisters & ulceration)
• reducing the time to lesion healing
– Suppressively (daily tablets taken):
• preventing recurrences/outbreaks in genital herpes by getting to those
latent viruses
• reducing the transmission of genital herpes to partners
Efficacy in shingles (VZV)
• Antivirals reduce the pain of shingles and prevent post herpetic neuralgia (if taken within 72
hours of rash development – this only prevents the post herpetic neuralgia)
• Post herpetic neuralgia is ongoing pain in the area where the blisters were
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Guanosine analogue
Used to treat cytomegalovirus (CMV) infection
activated by conversion to Ga-triphosphate (TP) – just like aciclovir
– CMV viral kinase Ga ---> Ga –MP (first step done by viral kinase)
– cellular kinases (induced by CMV infection) i.e. infected cells have higher amounts of
cellular kinases so there is substantial enzyme activity to convert the
monophosphate to the triphosphate
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Ga-MP ------> Ga-TP (this form blocks the enzyme)
Ga-TP concentration 100x greater in CMV infected cells as there is a lot of enzyme activity in
these cells
Ga-TP inhibits viral DNA synthesis by:
– competitive inhibition of DNA polymerase
– direct incorporation into viral DNA
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------> termination of DNA elongation. Blocks further development of nucleic
acid. No effective DNA synthesized
cellular DNA polymerase susceptible
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but much higher Ga-TP concentrations than viral (he didn’t explain this
point)
persists for 18 -20 hr in infected cells
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CMV infection occurs in particular in immunocompromised patients (AIDS, transplant)
– We measure antibodies in transplant patients before the transplant and if they have
CMV antibodies we give them Ga prophlactically
Nucelotide based drugs
• Viral DNA Polymerase Inhibitors
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acyclovir, ganciclovir,
– vidarabine, idoxuridine
• Viral RNA polymerase
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tribavirin
• Reverse Transcriptase
– zidovudine (AZT)
– didanosine (dideoxyinosine)
– Zalcitabine (dideoxycytidine)
Other nucleotide based drugs
• Vidarabine – adenosine analogue (adenine arabinoside)
– Similar mechanism to aciclovir – phosphorylation by cellular kinases to form the
triphosphate
– selective inhibitor of viral DNA polymerase
– useful in herpes simplex and varicella-zoster (shingles)
– a second line drug – aciclovir is the preferred drug. Use vidarabine if there is
intolerance to aciclovir
• Idoxuridine and bromovinyldeoxyuridine
– analogues of thymidine
– otherwise a similar mechanism of action of aciclovir
• Tribavirin
– guanosine analogue
– mechanism of action is not very clear
– may inhibit viral mRNA synthesis and also nucleotide synthesis (for viral and host cell
DNA and RNA)
– effective against a wide range of DNA and RNA viruses
Interferon
• synthesised from T lymphocytes and is part of the normal immune response
• it can be Synthesised by recombinant DNA technology so we now have interferon to give to
patients
• Induces host ribosomes to synthesise enzymes that inhibit the translation of viral RNA into
viral proteins
• Halts viral replication. Is not standard therapy but if someone is acutely ill in hospital or they
have a transplant and we are worried about viruses we use it
• Has to be given parenterally because it is a big protein
• Side effects (“flu” like symptoms)
• Part of the normal body response to a virus is to produce interferon and some of the
flu like symptoms you get when you’re sick is due to the circulating interferon
Key points
• More difficult to develop than anti-bacterials
– Viruses use host cell “machinery” to replicate so there are a limited number of
targets
• Viral DNA polymerase inhibitors (nucleoside analogues)
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– aciclovir, valaciclovir (Herpes)
– ganciclovir (CMV infections)
Non specific immune therapy (boost immune system)
– Interferon only used in hospital patients that are desperately ill