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.
 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.
 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
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)
Membrane
Nucleic acid
Capsid
Viral protein
RNA polymerase
The size of virion can vary
from 10nm to 400nm.
The outer surface of viral cell
Life cycle of virus
HIV protease is required
to process the transcribed
and translated proteins for
the new virions
Inhibition of RT
would
prevent conversion of
viral RNA genome
into
DNA for
incorporation
into the host’s
replicatory
system
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.
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.
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.
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.
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.
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.
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.
Acyclovir analogues
 Were synthesized mainly to overcome the poor water
solubility and to improve oral availability.
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.
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
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).
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.
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.
Life cycle of HIV virus
HIV protease is required
to process the transcribed
and translated proteins for
the new virions
Inhibition of RT
would
prevent conversion of
viral RNA genome
into
DNA for
incorporation
into the host’s
replicatory
system
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.
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.
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 antiviral agents:
 Nitrogen base + Deoxyribose sugar.
 Should be phosphorylated three times (by cellular
kinases) to form the active nucleotide triphosphate.
Nucleoside Reverse Transcriptase Inhibitors (NRTIs)
O
O
NH
NH
N
N
O
HO
HO
O
O
H
H
OH
H
H
Deoxythymidine
H
H
Non-nucleophilic
Azido group
H
H
N3
H
H
Zidovidine (AZT)
O
GROWING
CHAIN OF NEW
STRAND
GROWING
CHAIN OF NEW
STRAND
GUANINE
GUANINE
O
O
O
O
O
P
REVERSE
TRANSCRIPTASE
O
H
O
H
OH
H
O
O
P
O
H
O
H
H
O
O
O
H
P
H
O
NH
H
N
O
NH
N
O
O
O
H
H
N3
H
H
O
O
O
P
O
P
O
O
O
P
O
O
O
H
H
N3
H
H
RT uses zidovidine triphosphate
in place of thymidine triphosphate
as complementary base to Adenine
In template strand
NON-NUCLEOPHILIC
FUNCTIONAL GROUP
O
3'
5'
-
O
O
O
O
-
P
O
O
O-
O
P
O
O
O
O
O
P
O
O
O
O
A
G
O
C
A
T
C
G
T
O
O
O
O
O
O
P
O
O
OH
O
N3
O
O-
O
P
No further
nucleic acid
extension
O
O
-
O
P
O
O
-
P
O
O
O-
5'
3'
Other NRTIs
Non-nucleoside reverse
transcriptase inhibitors (NNRTIs).
 They are hydrophobic molecules bind to the allosteric
binding site which is hydrophobic in nature (noncompetitive reversible inhibitors).
 Rapid resistance emerges due to mutation in the
NNRTI binding site.
First generation
Second generation
Allosteric site vs. catalytic site
Nevirapine binding to the NNRTI allosteric site
NNRTI binding transmits a
conformational change
through the RT protein
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
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
The role of the Proline residue
Tertiary amide
Induces turn
in the backbone
Mechanism of hydrolysis by
protease enzyme
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 PhePro or Tyr-Pro motif.
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.
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?).
The Lead PI
N and C-terminal
protecting groups
prevent hydrolysis
by exopeptidases
HO
O
O
OH
Asn
HN
NH
O
NH2
Pro
O
HN
O
O
NH
H
N
N
O
O
O
Tyr
OH
O
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
Improve binding
To the active site
Better water
Solubility and
availabiliy