Anti-Viral 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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Key is selectivity

Other problems
Toxicity
Rapid excretion
Rapid metabolism
Poor absorption
Anti-Viral Chemotherapy
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Ideal Drug
Water soluble
Chemically and metabolically stable
Easily absorbed (apolar)
NOT
Toxic
Carcinogenic
Allergenic
Mutagenic
Teratogenic
Anti-Viral Chemotherapy
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Therapeutic index (T.I.)

Minimum dose toxic to cell
Minimum dose toxic to virus
Effective drug: T.I. = 100-1000 at least
Anti-Viral Chemotherapy
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Another consideration:
Disease severity
Rhinovirus v. Symptomatic rabies or Lassa fever
Anti-Viral Chemotherapy
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Reasons for continuing search for anti-virals versus vaccines:
For many established diseases there is still no effective vaccine
Rapid mutation (retroviruses) to resistant forms
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)
Anti-Viral Chemotherapy
At present no drug completely suppresses viral replication (with possible exception of anti-HIV protease inhibitors)
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A successful drug must interfere with:
A specific viral function e.g. enzyme necessary for viral life cycle
Anti-Viral Chemotherapy
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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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)
Anti-Viral Chemotherapy
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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
1990’s Protease inhibitors
Anti-Viral Chemotherapy
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Binding to surface receptor
At present one approved drug in this class
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
Soluble CD4 - May make HIV more infective as results in chemokine receptor being the only necessary receptor
Anti-Viral Chemotherapy
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Maraviroc (brand-named Selzentry, or Celsentri outside the U.S.)

Approved August 2007
Maraviroc
Maraviroc blocks the chemokine receptor CCR5
Because HIV can also use another co-receptor, CXCR4, an HIV tropism test is performed to determine if the drug will be effective
Compared to HAART alone, HAART + Maraviroc gave twice as many patients with HIV <50 copies>13
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-20 FUZEON (enfuvirtide)
Anti-Viral Chemotherapy
T-1249 Next generation: Different site from T-20
Membrane Fusion
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Endosome low pH often necessary for membrane fusion
Lysosomotropic agents
Anti-Viral Chemotherapy
Membrane Fusion
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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
Good alternative to vaccine for:
Elderly
Immunocompromized
Allergic
Where causative strain not the vaccine strain
Affects M2 protein involved in activation of virus before release from cell
Anti-Viral Chemotherapy
Amantadine Rimantadine
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ADAMANTANES
Rapid emergence to amantadine and rimantadine
because of M2 point mutation
CDC
Because of circulating resistant influenza virus strains, the use of Amantadine and Rimantadine is currently (2008) NOT recommended
In the 2005-2006 influenza season, 92% of H3N2 influenza isolates had M2 mutation that would make them resistant
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Uncoating of core of membrane and non-membrane viruses
Uncoating of picornavirus e.g. polio, echo, rhino
Stabilize coated virus?
Anti-Viral Chemotherapy
Arildone
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N
O
O
O
N
CH3
WIN 71711 (Disoxaril)
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
Anti-Viral Chemotherapy
Old formulation worked poorly (oral)
New formulation (nasal spray)
Marketed (2004) by Schering-Plough
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Human rhinovirus with WIN drug – embeds in VP1
VP1
VP1
Anti-Viral Chemotherapy
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Nucleic Acid Synthesis
Polymerases are often virally encoded

Other enzymes in nucleic acid synthesis
e.g. THYMIDINE KINASE in Herpes Simplex
Anti-Viral Chemotherapy
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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
Anti-Viral Chemotherapy
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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
Anti-Viral Chemotherapy
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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
ACG
P
P
P
Anti-Viral Chemotherapy
Cellular TK (where expressed) does not phosphorylate (activate) the drug
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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
Anti-Viral Chemotherapy
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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
Anti-Viral Chemotherapy
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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
Excellent anti-herpes drug
Acyclovir Ganciclovir
Anti-Viral Chemotherapy
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Acyclovir:
Chain terminator

Good anti-herpes drug
Normal DNA synthesis
Anti-Viral Chemotherapy
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Termination
Also inhibits:
Epstein Barr
Cytomegalovirus
Acyclovir:
Chain terminator
Selective:
Virus phosphorylates drug
Polymerase more sensitive
Anti-Viral Chemotherapy
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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
Anti-Viral Chemotherapy
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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
Anti-Viral Chemotherapy
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Adenine arabinoside
HSV encephalitis
Neonatal herpes
Disseminated herpes zoster
Hepatitis B

Poor in vivo efficacy:
DEAMINATION
Anti-Viral Chemotherapy
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Other sugar modifications:
AZT
azidothymidine
DDI
dideoxyinosine
DDC
dideoxycytidine
Anti-Viral Chemotherapy
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Base change analogs

Altered base pairing

Mutant DNA

Resistant mutants
Trifluorouridine
Viroptic
anti-HSV
Idoxuridine
Anti-Viral Chemotherapy
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Fluoroiodo aracytosine has both a base and a sugar alteration
Anti-Viral Chemotherapy
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Anti-Viral Chemotherapy
Cidofovir
Cidofovir:
DNA chain terminator
DNA polymerase inhibitor

A acyclic nucleoside phosphonate (not a phosphate) - C-O-P bond in a nucleoside monophosphate replaced by a phosphonate (C-P)
More stable
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Anti-Viral Chemotherapy
Cidofovir
Indicated:
CMV retinitis
May be useful:
Pox: molluscum contagiosum virus

Polyoma: JC - progressive multifocal leukoencephalopathy

Adenovirus - gastroenteritis
Vaccinia in immunocompromisation?

Used in on case: Eczema vaccinatum
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Anti-Viral Chemotherapy
Cidofovir
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Prodrugs
e.g. Famciclovir
Taken orally
Converted by
patient’s
metabolism
HSV thymidine kinase
P
Host kinase
P
P
Penciclovir: Available as topical cream
Glaxo-SmithKlein
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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
Anti-Viral Chemotherapy
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Sustiva
(S) -6- chloro-4-(cyclopropylethynyl)-1,4-dihydro-4-(trifluoromethyl)-2H-3, 1-benzoxazin-2-one.
DuPont
Anti-Viral Chemotherapy
Nevirapine
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Nevirapine: Approved for AIDS patients
Good 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
Anti-Viral Chemotherapy
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Anti-Viral Chemotherapy
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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
Anti-Viral Chemotherapy
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May induce mutations in RNA viruses
Anti-Viral Chemotherapy
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Viral Protein synthesis: No inhibitors
Anti-Viral Chemotherapy
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Protein processing:
Proteolysis
Glycosylation
Anti-Viral Chemotherapy
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GAG
Integrase
Polymerase
Protease
GAG/POL polyprotein
Anti-Viral Chemotherapy
Retrovirus --- HIV
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GAG
Integrase
Polymerase
Protease folds and cuts itself free
Anti-Viral Chemotherapy
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GAG
Integrase
Polymerase
Protease cuts at a site between the integrase and polymerase
Anti-Viral Chemotherapy
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GAG
Integrase
polymerase
Anti-Viral Chemotherapy
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Saquinavir
Anti-Viral Chemotherapy
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HIV aspartyl protease inhibitors
Anti-Viral Chemotherapy
Indinavir+AZT+3TC
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Indinavir (Merke)
INDINAVIR + AZT + 3TC
(HAART)
No detectable HIV by PCR

Before: 20,000 - 11,000,000 RNA copies /ml

After: < 200-400 copies
Lasts several years
No replication No resistance
Anti-Viral Chemotherapy
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Anti-Viral Chemotherapy
Integrase Inhibitor
Isentress
Approved for use in adults by the USFDA in October, 2007
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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
Anti-Viral Chemotherapy
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Relenza
Tamiflu (oseltamivir)
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After neuraminidase inhibition, ‘flu hemagglutinin binds to sialic acid on other virus particles: virus clumps
OR virus sticks to mucous in respiratory tract
Anti-Viral Chemotherapy
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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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Zanamivir - Relenza
Neuraminidase inhibitor
Nasal spray
Shortens symptoms by a few days
Tamiflu: Oral neuraminidase inhibitor
Anti-Viral Chemotherapy
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NEURAMINDIASE INHIBITORS
TREATMENT

Oseltamivir is approved for treatment of persons aged >1 year

Zanamivir is approved for treatment of persons aged >7 years

PROPHYLAXIS
Oseltamivir and zanamivir can be used for chemoprophylaxis of influenza

Oseltamivir is licensed for use in persons aged >1 year
Zanamivir is licensed for use in persons aged >5 years
Anti-Viral Chemotherapy
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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
Anti-Viral Chemotherapy
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