Antiviral agents Viruses are non-cellular, infectious agents which take over a host cell to survive and multiply. Can infect human, plant, animals and bacterial cells. More than 400 different types capable to infect human. Responsible for influenza, chicken pox, measles, mumps, viral pneumonia, rubella and smallpox and many other viral infections.
Transcript
Slide 1
Antiviral agents Viruses are non-cellular, infectious agents
which take over a host cell to survive and multiply. Can infect
human, plant, animals and bacterial cells. More than 400 different
types capable to infect human. Responsible for influenza, chicken
pox, measles, mumps, viral pneumonia, rubella and smallpox and many
other viral infections.
Slide 2
Viruses can be transmitted to human by a variety of ways:
Through air by the infected host sneezing or coughing. Through
arthropods or ticks such as yellow fever and Colorado tick fever.
Through physical contact (for some viruses which can not survive
outside the host such as: HIV Cold sore. Genital herpes. Rabies.
Food-borne or water borne viruses such as hepatitis A and E and
viral gastroenteritis.
Slide 3
Viral infections were responsible for the major epidemics
worldwide: Smallpox weakened the Roman Empire. Lethal viruses in
Africa such as Ebola, Lassa. Sever acute respiratory syndrome
(SARS) during 2003 in the Far East. Recently H1N1
Slide 4
The structure of viruses Can be classified as: DNA viruses:
contains either single or double strand DNA RNA viruses: contains
single strand RNA (ssRNA), but some have double strand RNA. The
nucleic acid is protected within a protein coat called capsid. The
capsid contains nucleic acid is called Nucleocapsid. The whole
structure of virus is called virion (the form that the virus takes
when it is outside the host cell)
Slide 5
Capsid Membrane Viral protein Nucleic acid RNA polymerase The
outer surface of viral cell The size of virion can vary from 10nm
to 400nm.
Slide 6
Life cycle of virus Inhibition of RT would prevent conversion
of viral RNA genome into DNA for incorporation into the hosts
replicatory system HIV protease is required to process the
transcribed and translated proteins for the new virions
Slide 7
Vaccination Is the preferred method of protection against viral
diseases. Extremely successful against childhood diseases such as
polio, measles, mumps, smallpox and yellow fever. Works by:
introducing the body to foreign material having molecular
similarity to some component of the virus. Introducing killed or
weakened version of the virus. Or administer fragments of virus
having the characteristics antigen.
Slide 8
Vaccination The body can recognize the molecular fingerprint of
the virus and develop specific antibodies against these antigenic
structure. Vaccines are currently under investigation for HIV,
genital herpes and Ebola virus (causes haemorrhagic fever): These
viruses have rapid gene mutation that results in constant changes
to the amino acid composition of surface glycoprotein (the surface
antigenic component). Patients with weak immune system are not
likely to benefit from vaccination.
Slide 9
Virus is a hard target Most of the time the virus spend in the
host will be inside the host cell. This effectively protect the
viral cell from the host immune system as well as from available
circulating enzymes. Another problem appears in treating viral
infections is the fact that there are limited number of potential
drug targets since viruses use the host biochemical mechanisms to
multiply.
Slide 10
Antiviral agents The first effective antiviral agents appear in
1960s and only three were clinically available: Idoxuridine and
vidarabine for herpes infections. Amantadine for influenza A
infections. Growing interest in finding effective antiviral agents
after that was due to: The need to tackle AIDS spread. The
increased understanding of viral genomic sequence and infectious
mechanisms.
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Antiviral agents for DNA viruses Mainly against herpes viruses
such as cold sore, genital herpes, chicken pox, eye diseases. Three
major mechanisms of actions: 1. Inhibition of viral DNA polymerase.
2. Inhibition of tubulin polymerization. 3. Antisense therapy:
which blocks the translation of viral RNA.
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Acyclovir Acyclovir triphosphate prevents DNA replication By
two ways: Can bind to DNA polymerase instead of deoxyguanosine
triphosphate. Can be incorporated into the growing DNA chain
discontinuation of chain extension due to the absence of 3 hydroxyl
group.
Slide 13
Acyclovir selective toxicity Why Acyclovir does not inhibit DNA
polymerase in normal, uninfected cells: The first step in
phosphorylation requires the viral version of kinase (100 times
more active than the host enzyme). There is a selective uptake of
acyclovir by infected cells. Acyclovir triphosphate is 50 times
more selective on viral DNA polymerase compared to the cellular
polymerase.
Slide 14
Acyclovir analogues Were synthesized mainly to overcome the
poor water solubility and to improve oral availability.
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Valaciclovir Is an L-valyl ester prodrug of acyclovir. Absorbed
more effectively from the gut than acyclovir although it has the
same polarity. That fact that the D-valyl prodrug has poor
absorption suggesting that there is a special transport system
(intestinal oligopeptide and di/tripeptide transporter) required
for valaciclovir absorption. After absorption, valaciclovir will be
hydrolyzed to acyclovir.
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Cidofovir Some viruses do not have thymidine kinase, so resist
the action of acyclovir. Cidofovir is already phosphorylated, so no
need for the kinase, then this will be phosphorylated by cellular
thymidine kinase to give the active Cidofovir triphosphate. Could
be more toxic than acyclovir (why?). Mimic
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Other antiviral agents Are phosphorylated equally by viral and
cellular thymidine kinase, so they are more toxic than acyclovir.
The first nucleoside-based antiviral agents (inhibit both viral DNA
polymerase and thymidylate synthetase).
Slide 18
AIDS Acquired Immune Deficiency Syndrome caused by human
immunodeficiency virus (HIV virus). Immune deficiency because the
virus attacking the T- cells which are crucial to the immune
system. Acquired because with weakened immune system, the patient
will be more susceptible for opportunistic secondary diseases.
infection by opportunistic pathogens (e.g. pneumonia, TB)
ultimately kills the host not the virus itself in most of the
cases.
Slide 19
HIV virus Is one of the RNA retroviruses. Two types: HIV-1: is
responsible for AIDS in America, Europe, and Asia. HIV-2: prevalent
in Western Africa. Most clinically useful antiviral agents against
AIDS are either: Reverse transcriptase inhibitors. HIV protease
inhibitors.
Slide 20
Life cycle of HIV virus Inhibition of RT would prevent
conversion of viral RNA genome into DNA for incorporation into the
hosts replicatory system HIV protease is required to process the
transcribed and translated proteins for the new virions
Slide 21
Antiviral agents against HIV Until 1987, no anti-HIV drug was
available. Extensive studies carried out on the life cycle of HIV
have led to identifying possible drug targets within the viral
cell: Reverse transcriptase. Protease. Unfortunately, HIV undergoes
mutation extremely easily, which results in rapid development of
resistance. For that, current therapy depends on the use of
combination of reverse transcriptase inhibitors and protease
inhibitors.
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The ideal anti-HIV agent Must have high affinity for its
target. Activity range in picomolar. Be effective in preventing the
virus multiplying and spreading. Show low activity against host
enzymes. Safe and well tolerated. Has a broad antiviral activity.
It must be inexpensive since it will be used for the life time of
the patient.
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Nucleoside Reverse Transcriptase Inhibitors (NRTIs) This enzyme
is unique to the virus so it is an ideal target. However, it still
a DNA polymerase like, so there is a possibility that its inhibitor
might affect the cellular DNA polymerase. Nucleoside-like drugs
have been proved as useful anti- viral agents: Nitrogen base +
Deoxyribose sugar. Should be phosphorylated three times (by
cellular kinases) to form the active nucleotide triphosphate.
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Nucleoside Reverse Transcriptase Inhibitors (NRTIs) Zidovidine
(AZT) Deoxythymidine Non-nucleophilic Azido group
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RT uses zidovidine triphosphate in place of thymidine
triphosphate as complementary base to Adenine In template
strand
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No further nucleic acid extension
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Other NRTIs
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Non-nucleoside reverse transcriptase inhibitors (NNRTIs). They
are hydrophobic molecules bind to the allosteric binding site which
is hydrophobic in nature (non- competitive reversible inhibitors).
Rapid resistance emerges due to mutation in the NNRTI binding site.
First generationSecond generation
Slide 29
Allosteric site vs. catalytic site
Slide 30
Nevirapine binding to the NNRTI allosteric site Relative
orientation of nucleophilic 3-OH from growing strand and
electrophilic 5-phosphate from nucleotide triphosphate substrate
altered and can no longer form a bond elongation stopped NNRTI
binding transmits a conformational change through the RT
protein
Slide 31
Viral protease Has a broad substrate specificity, can cleave
variety of peptide bonds in viral polypeptides. Mostly it can
cleave the peptide bond next to proline residue and an aromatic
residue (phenylalanine and tyrosine). This cleavage (next to
proline) is not common with mammalian proteases such as renin and
pepsin.. This results in better selectivity against HIV protease
over the mammalian proteases. HIV-1 protease is 50% similar to
HIV-2 homologue, the differences is far from the active site so
high possibility to get inhibitors against both a the same
time
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The role of the Proline residue Tertiary amide Induces turn in
the backbone
Slide 33
Mechanism of hydrolysis by protease enzyme
Slide 34
Targeting HIV protease HIV protease is a much smaller enzyme
than the equivalent host aspartate proteases. Cleaves substrates at
the N-terminal to proline residues unlike mammalian proteases.
Peptides from infected cells suggested that Tyr-Pro sites were the
likely cleavage sites. Rationale for inhibitor design was based on
Phe- Pro or Tyr-Pro motif.
Slide 35
HIV Protease Inhibitors (PIs) Are not prodrugs and do not need
to be activated. So can be tested in-vitro to test activity
(IC50.the concentration of drug required to inhibit the enzyme by
50%) especially after knowing that viral protease can easily
isolated. Low IC50 does not mean a good antiviral activity (Why?).
Most of them are derived from peptide lead compounds.
Slide 36
HIV Protease Inhibitors (PIs) They are oligopeptides in
general: Less well absorbed. Susceptible to first pass metabolism
by cytochrome P- 450, which may results in drug-drug interactions
with many other drugs taken by AIDS patients such as ketoconazole,
rifampicin and astemizole. Rapidly excreted (Why?). High plasma
protein binding (Why?).
Slide 37
The Lead PI Asn Tyr Pro N and C-terminal protecting groups
prevent hydrolysis by exopeptidases
Slide 38
Possible structural modification on the lead PI The size of the
compound: di, tri, tetra, or pentapeptide. The amino acid motif:
tyr-pro or phe-pro. The nature of C and N-terminal protecting
group. The use of proline analogues
Slide 39
Improve binding To the active site Better water Solubility and
availabiliy