AntiViral Chemotherapy

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Anti-Viral Chemotherapy

• Bacteria
• Many antibiotics
• Highly selective
• Viruses
• Use host cell metabolism
• Selectivity difficult
• Toxicity

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Anti-Viral Chemotherapy

• Key is selectivity
• Other problems
• Toxicity
• Rapid excretion
• Rapid metabolism
• Poor absorption

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Anti-Viral Chemotherapy
Ideal Drug

• Water soluble
• Chemically and metabolically stable
• Easily absorbed (apolar)
• NOT
• Toxic
• Carcinogenic
• Allergenic
• Mutagenic
• Teratogenic

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Anti-Viral Chemotherapy
Therapeutic index (T.I.) Minimum dose toxic to
cell Minimum dose toxic to virus
Effective drug T.I. 100-1000 at least
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Anti-Viral Chemotherapy
Another consideration Disease severity Rhinovirus
v. Symptomatic rabies or Lassa fever
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Anti-Viral Chemotherapy

• Reasons for continuing search for anti-virals
  versus vaccines
• For many established diseases there is still no
  effective vaccine
• Rapid mutation (retroviruses)
• Or there are problems with the current vaccine
• Reassortment (influenza)
• New and emerging diseases - no vaccine
  available
• Vaccine development takes many years
• Disease that involve immunosuppression (AIDS,
  cancer, transplantation)

At present no drug completely suppresses viral
replication (with possible exception of anti-HIV
protease inhibitors)
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Anti-Viral Chemotherapy

• A successful drug must interfere with
• A specific viral function e.g. enzyme necessary
  for viral life cycle

• A cellular function that the virus needs in
  order to replicate
• If interfere with cellular function either
• It must be crucial to virus but not the cell or
• Only the virus-infected cell must be killed
• (activation of drug in the infected cell only?)

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Anti-Viral Chemotherapy

• Viral enzymes
• Nucleic acid polymerases
• DNA-dependent DNA polymerase - DNA viruses
• RNA-dependent RNA polymerase - RNA viruses
• RNA dependent DNA polymerase (RT) -
  Retroviruses
• Protease (retrovirus)
• Integrase (retrovirus)
• Neuraminidase (orthomyxovirus)

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Anti-Viral Chemotherapy

• 1962 Idoxuridine
• Pyrimidine analog
• Toxic
• Topical - Epithelial herpetic keratitis (cornea)

• 1983 Acyclovir
• Purine analog
• Sugar modification
• Chain terminator
• Anti-herpes
• Selective to virus-infected cells

1990s Protease inhibitors
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Anti-Viral Chemotherapy

• Binding to surface receptor
• At present no effective drugs in this class

• Soluble CD4 - May make HIV more infective as
  results in chemokine receptor being the only
  necessary receptor

CD4-Ig2 (PRO542) - A more stable version of
soluble CD4 is a tetrameric fusion protein of
immunoglobulin G and CD4. It can reduce levels of
virus in vivo
AMD3100 - appears to bind to CXCR4 (fusin)
RFI-641 (biphenyl triazine) active against RSV
fusion
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Anti-Viral Chemotherapy
Membrane Fusion
T-20 FUZEON (enfuvirtide)
Peptides derived from gp41 can inhibit
infection Probably block interaction of gp41
with cell membrane proteins during fusion
T-20 In clinical trials, a nearly two log
reduction in plasma HIV levels achieved
T-1249 Next generation Different site from T-20
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Anti-Viral Chemotherapy
Membrane Fusion

• Endosome low pH often necessary for membrane
  fusion
• Lysosomotropic agents

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Anti-Viral Chemotherapy

• Amantadine 1966
• Rimantadine 1993
• Only effective against flu A (200mg/day)
• Marginally effective therapeutically
• Prophylaxis Reduce flu by 90
• Since disease usually mild and avoidable not used
  much here

Amantadine Rimantadine

• Good alternative to vaccine for
• Elderly
• Immunocompromized
• Allergic
• Where causative strain not the vaccine strain

Lysosomotropic drugs?
Affects M2 Golgi ion channel involved in
activation of virus before release from cell
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Anti-Viral Chemotherapy
Uncoating of core of membrane and non-membrane
viruses

• Uncoating of picornavirus e.g. polio, echo,
  rhino
• Stabilize coated virus?

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Anti-Viral Chemotherapy
N
O
CH3
O
WIN 71711 (Disoxaril)
O
N

• Stabilizes picornaviruses - coated virus remains
  in cytoplasm
• 3-methylisoxazole group inserts in capsid VP1
  and covers ion channel

Similar mechanism Pleconaril Good
bioavailability readily absorbed Crosses
blood-brain barrier
Old formulation worked poorly (oral) New
formulation (nasal spray) Marketed (2004) by
Schering-Plough
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Anti-Viral Chemotherapy
VP1
VP1
Human rhinovirus with WIN drug embeds in VP1
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Anti-Viral Chemotherapy
Nucleic Acid Synthesis Polymerases are often
virally encoded Other enzymes in nucleic acid
synthesis e.g. THYMIDINE KINASE in Herpes Simplex
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Anti-Viral Chemotherapy
Thymidine Kinase Deoxy-thymidine
Deoxy-thymidine triphosphate
Viral or cellular thymidine kinase adds first
phosphate
PO4
PO4
PO4
Cellular kinases add two more phosphates to form
TTP
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Anti-Viral Chemotherapy
Why does Herpes simplex code for its own
thymidine kinase? TK- virus cannot grow in neural
cells because they are not proliferating (not
making DNA) Although purine/pyrimidines are
present, levels of phosphorylated nucleosides are
low Allows virus to grow in cells that are not
making DNA Thymidine kinase is a
misnomer Deoxynucleoside kinase
NON-SPECIFIC
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Anti-Viral Chemotherapy

• Herpes thymidine kinase will phosphorylate any
  deoxynucleoside including drugs as a result of
  its necessary non-specificity
• Nucleoside analog may be given in
  non-phosphorylated form
• Gets drugs across membrane
• Allows selectivity as only infected cell has
  enzyme to phosphorylate the drug

Cellular TK (where expressed) does not
phosphorylate (activate) the drug
ACG
P
P
P
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Anti-Viral Chemotherapy

• Need for activation restricts drug to
• Viruses such as HSV that code for own thymidine
  kinase
• Virus such as cytomegalovirus and Epstein-Barr
  virus that induce cells to overproduce their own
  thymidine kinase
• In either case it is the VIRUS-INFECTED cell
  that activates the drug

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Anti-Viral Chemotherapy

• Thymidine kinase activates drug but
  phosphorylated drug inhibits the polymerase
• Nucleotide analogs
• Sugar modifications
• Base modifications
• Selectivity
• Viral thymidine kinase better activator
• Cellular enzyme may not be present in
  non-proliferating cells
• Activated drug is more active against viral DNA
  polymerase that against cell polymerase

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Anti-Viral Chemotherapy

• Guanine analogs
• Acyclovir acycloguanosine Zovirax
• Ganciclovir Cytovene
• Activated by viral TK
• Activated ACV is better (10x) inhibitor of viral
  DNA polymerase than inhibitor of cell DNA
  polymerase

Acyclovir Ganciclovir
Excellent anti-herpes drug
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Anti-Viral Chemotherapy

• Acyclovir
• Chain terminator
• Good anti-herpes drug

Normal DNA synthesis
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Anti-Viral Chemotherapy

• Acyclovir
• Chain terminator

Termination

• Selective
• Virus phosphorylates drug
• Polymerase more sensitive

• Also inhibits
• Epstein Barr
• Cytomegalovirus

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Anti-Viral Chemotherapy

• Acyclovir very effective against
• Herpes simplex keratitis (topical)
• Latent HSV (iv)
• Fever blisters Herpes labialis (topical)
• Genital herpes (topical, oral, iv)

Resistant mutants in thymidine kinase or DNA
polymerase Appears not to be teratogenic or
carcinogenic Ganciclovir very effective against
cytomegalovirus viral DNA polymerase is very
sensitive to drug activated by cell TK
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Anti-Viral Chemotherapy

• Adenine arabinoside (Ara-A)
• Problems Severe side effects
• Resistant mutants (altered polymerase)
• Chromosome breaks (mutagenic)
• Tumorigenic in rats
• Teratogenic in rabbits
• Insoluble
• Use topical applications in ocular herpes simplex

Competitive inhibitor of virus DNA polymerase
which is much more sensitive than host polymerase
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Anti-Viral Chemotherapy

• Adenine arabinoside
• HSV encephalitis
• Neonatal herpes
• Disseminated herpes zoster
• Hepatitis B
• Poor in vivo efficacy
• DEAMINATION

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Anti-Viral Chemotherapy
Other sugar modifications
AZTazidothymidine
DDCdideoxycytidine
DDIdideoxyinosine
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Anti-Viral Chemotherapy
Base change analogs Altered base pairing Mutant
DNA Resistant mutants
TrifluorouridineViropticanti-HSV
Idoxuridine
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Anti-Viral Chemotherapy
Fluoroiodo aracytosine has both a base and a
sugar alteration
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Prodrugse.g. Famciclovir
Penciclovir Available as topical cream
P
P
P
Taken orally
Converted by patients metabolism
HSV thymidine kinase
Host kinase
Glaxo-SmithKlein
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Anti-Viral Chemotherapy
Non-nucleoside Non-competitive RT
inhibitors Combination therapy with
AZT Resistance mutations will be at different
sites The most potent and selective RT
inhibitors Nanomolar range Minimal toxicity (T.I.
10,000-100,000) Synergistic with nucleoside
analogs (AZT) Good bio-availability Resistant
mutants - little use in monotherapy
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Anti-Viral Chemotherapy
DuPont
Nevirapine
Sustiva
(S) -6- chloro-4-(cyclopropylethynyl)-1,4-dihydro-
4-(trifluoromethyl)-2H-3, 1-benzoxazin-2-one.
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Anti-Viral Chemotherapy

• Nevirapine Approved for AIDS patientsGood
  blocker of mother to child transmission
  peri-natal - breast feeding
• Single dose at delivery reduced HIV transmission
  by 50
• Single dose to baby by 72 hours
• Efavirenz (Sustiva, DMP266)
• In combination therapy will suppress viral load
  as well as HAART and may be better Approved for
  AIDS patients

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Anti-Viral Chemotherapy
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Anti-Viral Chemotherapy

• Ribavirin
• Guanosine analog
• Non-competitive inhibitor of RNA polymerase in
  vitro
• Little effect on flu in vitro
• Often good in animals but poor in humans
• Aerosol use respiratory syncytial virus
• i.v./oral reduces mortality in Lassa fever,
  Korean and Argentine hemorrhagic fever

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Anti-Viral Chemotherapy
May induce mutations in RNA viruses
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Anti-Viral Chemotherapy
Ribozymes RNA molecules that have catalytic
properties among which are the specific cleavage
of nucleic acids Heptazyme Ribozyme that
cleaves hepatitis C RNA at highly conserved
regions Recognizes and cuts all known types of
the hepatitis C virus, thereby stopping viral
replication Poor in clinic
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Anti-Viral Chemotherapy
Viral Protein synthesis No inhibitors
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Anti-Viral Chemotherapy

• Protein processing
• Proteolysis
• Glycosylation

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Anti-Viral Chemotherapy
GAG/POL polyprotein
GAG
Integrase
Polymerase
Protease
Retrovirus --- HIV
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Anti-Viral Chemotherapy
GAG
Integrase
Polymerase
Protease folds and cuts itself free
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Anti-Viral Chemotherapy
GAG
Integrase
Polymerase
Protease cuts at a site between the integrase and
polymerase
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Anti-Viral Chemotherapy
GAG
Integrase
polymerase
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Anti-Viral Chemotherapy
Saquinavir
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Anti-Viral Chemotherapy
IndinavirAZT3TC
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Anti-Viral Chemotherapy
Indinavir (Merke) INDINAVIR AZT
3TC (HAART) No detectable HIV by PCR Before
20,000 - 11,000,000 RNA copies /ml After lt
200-400 copies Lasts several yearsNo replication
No resistance
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Anti-Viral Chemotherapy
Influenza Requires neuraminidase to escape from
cell Requires neuraminidase to penetrate
mucus Zanamivir - RELENZA (fall
1997) Neuraminidase inhibitor Active against
Influenza A and Influenza B
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Tamiflu (oseltamivir)
Relenza
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Anti-Viral Chemotherapy
After neuraminidase inhibition, flu
hemagglutinin binds to sialic acid on other virus
particles virus clumps OR virus sticks to mucous
in respiratory tract
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Neuraminidase of virus removes sialic acid from
cell surface thereby releasing virus
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Virus hemagglutinin sticks new virus particle to
sialic acid on cell surface Virus cannot escape
from infected cell after neuraminidase inhibition
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Anti-Viral Chemotherapy
Zanamivir - Relenza Neuraminidase inhibitor Nasal
spray Shortens symptoms by a few days
Tamiflu Oral neuraminidase inhibitor
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Anti-Viral Chemotherapy

• NEURAMINDIASE INHIBITORS
• TREATMENT
• Oseltamivir is approved for treatment of persons
  aged gt1 year
• Zanamivir is approved for treatment of persons
  aged gt7 years
• PROPHYLAXIS
• Oseltamivir and zanamivir can be used for
  chemoprophylaxis of influenza
• Oseltamivir is licensed for use in persons aged
  gt1 year
• Zanamivir is licensed for use in persons aged gt5
  years

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Anti-Viral Chemotherapy
ADAMANTANES Rapid emergence to amantadine and
rimantadine because of M2 point mutation

• Among influenza virus- infected participants in
  10 clinical trials, the risk for pneumonia among
  those participants receiving oseltamivir was
  approximately 50 lower than among those persons
  receiving a placebo

• NEURAMINIDASE INHIBITORS
• Development of viral resistance to zanamivir and
  oseltamivir during treatment has been identified
• No transmission of neuraminidase
  inhibitor-resistant viruses in humans has been
  documented to date
• Data are limited concerning the effectiveness of
  zanamivir and oseltamivir for treatment of
  influenza among persons at high risk for serious
  complications of influenza