1. Antiviral Agents Kishore Wary kkwary@uic.edu 312-413-9582
1. A virus is an infectious, obligate intracellular parasite.
2. The genetic material of a virus is either DNA or RNA.
3. The genetic material of a virus enters a host cell and directs the production of the building blocks of new virus particles (called virions).
4. New virions are made in the host cell by assembly of these building blocks.
5. The new virions produced in a host cell then transport the viral genetic material to another host cell or organism to carry out another round of infection.
Viruses are easy to understand when we reduce their properties to simple descriptions such as those listed above. The confounding issues lie in the details – and with viruses, there are too many details…
Three ways to fight virus:
Immunological
Chemotherpy
Natural host defense mechanism (such as Interferons)
Kishore Wary
Bertram G Katzung, 11th Edition. Chapter 55, pp

2. Antiviral Agents Knowledge Objectives:
Know which antiviral agents are used to treat influenza, herpes or HIV. Within each class, the drugs are listed in order of their relative importance.
Know which antiviral agents are not analogs of nucleosides.
Know the rationale for using nucleoside analogs and their mechanisms as antiviral agents.
Know the most common side effects.
Nucleosides are glycosylamines consisting of a nucleobase (often referred to as simply base) bound to a ribose or deoxyribose sugar via a beta-glycosidic linkage. Examples of nucleosides include cytidine, uridine, adenosine, guanosine, thymidine and inosine.
Bertram G Katzung, 11th Edition. Chapter 55, pp

3. Drug List amantadine rimantadine acyclovir valcyclovir famciclovir
penciclovir
ganciclovir
foscarnet
sorivudine
idoxuridine
vidarabine
trifluridine
ribavarine
Anti- HIV Agents - NRTI’s
retrovir
didanosine
zalcitabine
stavudine
lamivudine
abacavar
tenofovir
emtricitabine
Protease Inhibitors
saquinavir
ritonovir
indiavir
nelfinavir
amprenavir
NNRTI’s
nevirapine
delavirdine
efavirenz
1. A virus is an infectious, obligate intracellular parasite.
2. The genetic material of a virus is either DNA or RNA.
3. The genetic material of a virus enters a host cell and directs the production of the building blocks of new virus particles (called virions).
4. New virions are made in the host cell by assembly of these building blocks.
5. The new virions produced in a host cell then transport the viral genetic material to another host cell or organism to carry out another round of infection.
Viruses are easy to understand when we reduce their properties to simple descriptions such as those listed above. The confounding issues lie in the details – and with viruses, there are too many details…
Bertram G Katzung, 11th Edition. Chapter 55, pp

4. Structure of Influenza VirusH N
The influenza virion is roughly spherical. It is an enveloped virus – that is, the outer layer is a lipid membrane which is taken from the host cell in which the virus multiplies. Inserted into the lipid membrane are ‘spikes’, which are proteins – actually glycoproteins, because they consist of protein linked to sugars – known as HA (hemagglutinin) and NA (neuraminidase). These are the proteins that determine the subtype of influenza virus (A/H1N1, for example). The HA and NA are important in the immune response against the virus; antibodies (proteins made by us to combat infection) against these spikes may protect against infection. The NA protein is the target of the antiviral drugs Relenza and Tamiflu.
Also embedded in the lipid membrane is the M2 protein, which is the target of the antiviral adamantanes – amantadine and rimantadine.
Beneath the lipid membrane is a viral protein called M1, or matrix protein. This protein, which forms a shell, gives strength and rigidity to the lipid envelope. Within the interior of the virion are the viral RNAs – 8 of them for influenza A viruses. These are the genetic material of the virus; they code for one or two proteins. Each RNA segment, as they are called, consists of RNA joined with several proteins shown in the diagram: B1, PB2, PA, NP. These RNA segments are the genes of influenza virus. The interior of the virion also contains another protein called NEP.
Influenza hemagglutinin (HA) or haemagglutinin (British English) is a type of hemagglutinin found on the surface of the influenza viruses. It is an antigenic glycoprotein. It is responsible for binding the virus to the cell that is being infected. HA proteins bind to cells with sialic acid on the membranes, such as cells in the upper respiratory tract or erythrocytes.[1]
The name "hemagglutinin" comes from the protein's ability to cause red blood cells (erythrocytes) to clump together ("agglutinate") in vitro.[2]
Vincent Racaniello

5. B. Viral Replication Mammalian cell Uncoating Early protein synthesis
Penetration
Nucleic acid synthesis
Late protein synthesis and processing
Blocked by
e.g., enfuvirtide (HIV)
Maraviroc (HIV)
Docosanol (HSV)
Palivizumab (RSV)
Amantadine
(influenza A)
Fomivirsen
(CMV)
Blocked by NRTIs (HIV),
NNRTIs (HIV)
Acyclovir (HSV)
Foscarnet (CMV)
Entecavir (HBV)
Protease inhibitors
(HIV)
Packaging and
assembly
Neuraminidase
Inhibitors
(influenza)
Viral release
Blocked by Interferon-a (HBV, HCV)
Mammalian
cell
Figure: The Major sites of antiviral drug action (Katzung, Chapter 55)

6. C. Anti-influenza Agents
Amantadine (Symmetrel) and Rimantadine
Primary use: respiratory infections caused by influenza A
b. Mechanism of action: inhibits viral uncoating – interacts with viral M2 protein (ion channel). Blocks entry of protons into virions, prevents uncoating.
c. Good oral absorption; excreted by kidney unmetabolized
d. Side effects: Minor dose-related CNS effects (less with Rimantadine) and GI effects
2. Zanamivir & Oseltamivir
a. Primary use: Treatment of uncomplicated influenza infection; types A & B
b. Mechanism of action: Inhibit neuraminidase which is required for viral replication and release
c. Side effects: Well tolerated
Amantadine
Zanamivir INN (play /zəˈnæmɨvɪər/) is a neuraminidase inhibitor used in the treatment and prophylaxis of influenza caused by influenza A virus and influenza B virus. Zanamivir was the first neuraminidase inhibitor commercially developed. It is currently marketed by GlaxoSmithKline under the trade name Relenza as a powder for oral inhalation.
According to the Centers for Disease Control and Prevention (CDC), no flu, seasonal or pandemic, has shown any signs of resistance to zanamivir.[1]
Zanamivir works by binding to the active site of the neuraminidase protein, rendering the influenza virus unable to escape its host cell and infect others.[9] It is also an inhibitor of influenza virus replication in vitro and in vivo. In clinical trials it was found that zanamivir was able to reduce the time to symptom resolution by 1.5 days if therapy was started within 48 hours of the onset of symptoms.
Bertram G Katzung, 11th Edition. Chapter 55

7. D. Anti-herpes and anti-CMV Agents Nucleoside Analogs:
Analogs of naturally occurring nucleosides
Must be converted to the triphosphate analog in order to be active
Triphosphate competes with native nucleoside for incorporation into viral DNA
Triphosphate inhibits viral DNA polymerase
Frequently cause DNA chain termination
Anti-herpes Agents
1. Acyclovir (Zovirax) – it is a nucleoside analog
Guanosine analog used against herpes simplex 1 and 2 and varicella-zoster
Mechanism of action: the dGTP analog and is incorporated into DNA and causes DNA chain termination; the terminated chain inhibits viral DNA polymerase.
Acyclovir (9-[2-hydroxymethyl]guanine) is a nucleoside analog which selectively inhibits the replication of HSV (types 1 and 2) and VZV. After intracellular uptake, it is converted to acyclovir monophosphate by virally-encoded thymidine kinase; this step does not occur to any significant degree in uninfected cells and thereby lends specificity to the drug's activity. The monophosphate derivative is subsequently converted to acyclovir triphosphate by cellular enzymes.
Acyclovir triphosphate, acting as an analog to deoxyguanosine triphosphate (dGTP), competitively inhibits viral DNA polymerase; incorporation of acyclovir triphosphate into DNA results in chain termination because the absence of a 3' hydroxyl group prevents the attachment of additional nucleosides. Acyclovir triphosphate has a much higher affinity for viral DNA polymerase than for the cellular homolog, yielding a high therapeutic ratio which makes the inhibition of native DNA polymerase clinically unimportant [1,2].
Spectrum of activity — Acyclovir is active against, in descending order of susceptibility, HSV types 1 and 2 (HSV-1, HSV-2), VZV, and Epstein-Barr virus (EBV) [1]. Cytomegalovirus (CMV), which does not encode thymidine kinase, is resistant at clinically achievable levels. Activity versus human herpes viruses 6, 7 and 8 is not well defined.
Bertram G Katzung, 11th Edition. Chapter 55

8. D. Anti-herpes and anti-CMV Agents
Zovirax
1. Mechanism of action:
Viral thymidine kinase
ACYCLOVIR
ACYCLOVIR
MONOPHOSPHATE
(Zovirex)
It is a nucleoside analog.
Cellular
GMP kinase
ACYCLOVIR
TRIPHOSPHATE
ACYCLOVIR
DIPHOSPHATE
(Active product)
HERPES VIRUS SPECIFIC BECAUSE PHOSPHORYLATION OF ACYCLOVIR OCCURS TIMES FASTER IN HERPES INFECTED CELLS (DUE TO PRESENCE OF HERPES-SPECIFIC THYMIDINE KINASE)

9. c. Primary uses (Acyclovir)
D. Anti-herpes and anti-CMV Agents
c. Primary uses (Acyclovir)
Topically: primary mucotaneous herpes; genital herpes (less effective than systemic); ineffective in recurrent herpes simplex keratitis.
Orally: severe primary & recurrent genital herpes; varicella-zoster (children).
Intravenously: Treatment of choice for herpes encephalitis and neonatal herpes; severe mucotaneous herpes.
Side effects: local irritation with topical use; headache, nausea and vomiting with oral use; and nephrotoxicity with intravenous use
Resistance: Lack of thymidine kinase for activation.

10. D. Anti-herpes and anti-CMV Agents
Valaciclovir
Valtrex, Zelitrex
2. Valacyclovir: Analog of acyclovir; converted to acyclovir in the body
3. Ganciclovir
Structurally related to acyclovir
Mechanism of action: Same as acyclovir
Primary uses: 100x more active than acyclovir against cytomegalovirus (CMV)
Side effects: Can produce serious myelosuppression
4. Penciclovir and Famciclovir
Mechanism: Converted to the triphosphate form which inhibits viral DNA
polymerase
b) Penciclovir is used topically for genital herpes
c) Famciclovir is given orally and is converted to penciclovir in the body
d) Primary uses: recurrent genital herpes, localized herpes zoster and acute zoster
e) Side effects: headache, diarrhea and nausea
Bertram G Katzung, 11th Edition. Chapter 55, pp

11. D. Anti-herpes and anti-CMV Agents
Other Nucleoside Analogs:
5. Cidofovir
Cytosine analog
Primary uses: I.V. use approved for CMV retinitis
Side effects: Nephrotoxicity
6. Idoxuridine
Iodinated thymidine analog
Primary use: herpes keratitis (topically)
Side effects: Pain, inflammation
7. Vidarabine
Adenosine analog
Primary uses: I.V. for herpes encephalitis and neonatal herpes (most of uses have been replaced by acyclovir)
Idoxuridine
Stoxil
Bertram G Katzung, 11th Edition. Chapter 55, pp

12. D. Anti-herpes and anti-CMV Agents
8. Trifluridine
Fluorinated pyrimidine nucleoside analog
Mechanism: Monophosphate form inhibits thymidylate synthetase and triphosphate is incorporated into host and viral DNA
Primary use: Topically effective against HSV-1 & -2 to treat keratoconjunctivitis and
recurrent epithelial keratitis
Side effects: local irritation
Bertram G Katzung, 11th Edition. Chapter 55, pp

13. D. Anti-herpes and anti-CMV Agents
Other Derivatives:
9. Foscarnet
a. Synthetic non-nucleoside analog of pyrophosphate
b. Mechanism: Inhibits herpes DNA polymerase, RNA polymerase and HIV reverse
transcriptase by directly binding to the pyrophosphate binding site; does not
require prior activation
c. Primary uses: Given I.V. for acyclovir resistant herpes; CMV retinitis (synergism withganciclovir)
d. Side effects: Nephrotoxicity, CNS toxicity
10. Fomivirsen
a. Antisense oligonucleotide
b. Mechanism: Binds to mRNA; inhibits protein synthesis and viral replication
c. Primary use: Intravitreal injection for CMV retinitis in AIDS patients
d. Side effects: Increased ocular pressure, ititis & vitreitis
Bertram G Katzung, 11th Edition. Chapter 55, pp

14. Life cycle of HIV AZT, Didanosine, Zalcitabine, Stavudine, Lamivudine,
BLOCKED BY
A. NRTIs
AZT,
Didanosine,
Zalcitabine,
Stavudine,
Lamivudine,
Abacavar,
Tenofovir,
Emtricitabine
B. NNRTIs
Nevirapine
Delavirdine
Efavirenz
Life cycle of HIV. Binding of viral glycoproteins to host cell CD4 and chemokine receptors precedes fusion and entry into the cell. After uncoating, reverse transcription copies the single-stranded HIV RNA genome into double-stranded DNA, which is integrated into the host cell genome.
Gene transcription by host cell enzymes produces mRNA, which is translated into proteins that assemble into immature noninfectious virions that bud from the host cell membrane. Maturation into fully infectious virions is through proteolytic cleavage.

15. Nucleoside Reverse Transcriptase Inhibitors (NRTIs):
retrovir
didanosine
zalcitabine
stavudine
lamivudine
abacavar
tenofovir
emtricitabine
E. Anti-HIV Agents
Nucleoside Reverse Transcriptase Inhibitors (NRTIs):
Are analogs of naturally occurring nucleotides
Require phosphorylation to triphosphate form
Competitively inhibit HIV-1 (and usually HIV-2) reverse transcriptase (RT)
Are incorporated into viral DNA and cause chain termination
Net effect is inhibition of viral DNA synthesis
Block acute infection but are much less active against chronically infected cells
Usually used in combination with other anti-HIV drugs
Bertram G Katzung, 11th Edition. Chapter 55, pp

16. DNA Nucleoside Reverse Transcriptase Inhibitors (NRTIs):
1.Mechanism of action of Zidovudine (Azidothymidine: AZT)
AZT is a nucleoside analog reverse-transcriptase inhibitor (NRTI)
dThd
dTMP
dTDP
dTTP
DNA
Azdd dThd
Azdd TMP
Azdd TDP
Azdd TTP
(Active product)
Bertram G Katzung, 11th Edition. Chapter 55, pp

17. Nucleoside Reverse Transcriptase Inhibitors (NRTIs):
a. Zidovudine (Azidothymidine): AZT is a deoxythymidine analog
b. Mechanism: Inhibits HIV RT and causes DNA chain termination.
c. Metabolized in liver.
d. Primary uses: Management of certain adult patients with symptomatic HIV infections, AIDS and advanced AIDS-related complex (ARC); HIV-infected pregnant women; HIV-infected neonates.
e. Resistance: Usually due to viral mutation.
f. Side effects: Bone marrow depression, headache, abdominal pain, fever and insomnia; clearance reduced 50% in uremic patients; toxicity may increase in patients with advanced hepatic insufficiency.
Bertram G Katzung, 11th Edition. Chapter 55, pp

18. Nucleoside Reverse Transcriptase Inhibitors (NRTIs):
2. Didanosine
a. Deoxyadenosine analog
b. Mechanism: Inhibits HIV RT and causes DNA chain termination
c. Should be taken on empty stomach to decrease degredation by acidic pH
d. Primary uses: Advanced HIV in adults and children (over 6 months); patients intolerant or unresponsive to zidovudine; or who have taken zidovudine for over 4 months
e. Resistance: Viral mutation
f. Side effects: Dose-dependent pancreatic damage; peripheral neuropathy
3. Zalcitabine
a. Deoxycytosine analog
c. Bioavailability reduced by food
d. Primary use: In combination with zidovudine (produces synergistic effects)
f. Side effects: Peripheral neuropathy; oral & esophageal ulcerations

19. Nucleoside Reverse Transcriptase Inhibitors (NRTIs):
4. Stavudine
a. Thymidine analog
b. Mechanism: Inhibits HIV RT and causes DNA chain termination
c. Bioavailability not reduced by food
d. Primary use: Advanced HIV in patients unresponsive to other therapies
e. Resistance: Not frequently observed
f. Side effects: Peripheral sensory neuropathy
5. Lamivudine
a. Cytosine analog
d. Primary uses: Usually used in combination with with other RT inhibitors for HIV-1 treatment;
also approved for chronic hepatitis B infection
e. Resistance: Viral mutation
f. Side effects: Generally well tolerated

20. Nucleoside Reverse Transcriptase Inhibitors (NRTIs):
6. Abacavir
a. Guanosine analog
b. Newer agent that seems to be more effective than earlier NRTIs
c. Mechanism: Inhibits HIV RT and causes DNA chain termination
d. Good oral oral absorption; bioavailability not reduced by food
e. Resistance: Develops more slowly because it requires three concomitant HIV mutations
f. Side effects: Hypersensitivity reactions (may be fatal)

21. Non-Nucleoside Reverse Transcriptase Inhibitors (NNRTIs)
Bind to RT at a different site than nucleoside reverse transcriptase inhibitors (NRTIs)
Do not require phosphorylation to inhibit the HIV RT
Do not compete with nucleoside triphosphates for incorporation into DNA
Bind to RT’s active site and block RNA- and DNA-dependent DNA polymerase
No cross resistance with NRTIs or protease inhibitors (below)
Rapid development of resistance by viral mutation
Used in combination antiretroviral therapy
Metabolized by cytochrome P450 enzyme complex
Interactions with drugs which are metabolized by certain cytochrome P450 enzymes
Frequently require dosage reduction in patients with compromised liver function

22. Non-Nucleoside Reverse Transcriptase Inhibitors (NNRTIs)
1. Nevirapine
a. Excellent oral bioavailability not reduced by food therapy
b. Side effects: Severe rash, hepatic diamage, fever, nausea
2. Delavirdine
a. Good oral bioavailability; reduced by antacids
b. Side effects: Skin rash, can be teratogenic (avoid pregnancy during therapy)
3. Efavirenz
c. Good oral bioavailability with long half-life
a. Side effects: Generally well tolerated; CNS effects, skin rash

23. Protease Inhibitors:
HIV requires specific protease to generate essential structural proteins of the mature virion core as well as RT itself.
Protease inhibitors block this enzyme and consequently the development of mature infectiousvirions during HIV replication.
Are effective in both acutely and chronically infected cells
High potential for resistance through viral mutation.
Produce synergistic effects when used in combination with RT inhibitors.
Metabolized by cytochrome P450 enzyme complex.
Interactions with drugs which are metabolized by certain cytochrome P450 enzymes.
Frequently require dosage reduction in patients with compromised liver function

24. Protease Inhibitors:
1. Saquinavir
a. Poor to adequate oral bioavailability
b. Side effects: Fairly well tolerated with mild GI discomfort
c. Usually used in combination with Ritonavir (see below)
2. Ritonavir
a. Good oral bioavailability when given with food
b. Side effects: GI disturbances, peripheral or oral sensations, elevated serum triglycerides and aminotransferase levels
3. Indinavir
a. Excellent oral bioavailability when given on empty stomach
b. Side effects: Hyperbilirubinemia and nephrolithiasis (crystals forming in the kidneys)

25. Protease Inhibitors:
4. Nelfinavir
a. Oral bioavailability increased with food
b. Side effects: Diarrhea
5. Amprenavir
a. Good oral bioavailability when given with or without food
b. Efficacy increases when combined with two nucleoside RT inhibitors
c. Side effects: GI disturbances & rashes

26. F. Combination therapy for HIV
1. Atripla
a. New combination HIV therapy that combines three different anti- HIV drugs in a single pill.
b. Emtricitabine: an NRTI analog of cytosine
c. Tenofovir: an NRTI analog of adenosine monophosphate
d. Efavirenz: an NNRTI
ATRIPLA is a combination of three HIV medicines: SUSTIVA® (efavirenz), EMTRIVA® (emtricitabine) and VIREAD® (tenofovir disoproxil fumarate).
ATRIPLA is a prescription medication used alone as a complete regimen, or with other anti-HIV-1 medicines, to treat HIV-1 infection in adults and children at least 12 years old who weigh at least 40 kg (88 lbs).
ATRIPLA is the #1 prescribed HIV regimen.*
ATRIPLA is the only 1 pill daily HIV regimen with 3 DHHS-preferred† meds for patients new to therapy.
ATRIPLA is proven to lower viral load to undetectable‡ and helps raise T-cell (CD4+) count to help control HIV through 3 years of a clinical study in patients new to therapy.
Through 3 years, 71% of patients taking the meds in ATRIPLA had undetectable‡ viral loads versus 58% for COMBIVIR (lamivudine/zidovudine) + SUSTIVA
At Year 3, the average increase of CD4+ cell count was 312 cells/mm3 for patients taking the meds in ATRIPLA versus 271 cells/mm3 for COMBIVIR + SUSTIVA
The long-term effects (beyond 3 years) of ATRIPLA are not known at this time. People taking ATRIPLA may still get opportunistic infections or other conditions that happen with HIV-1 infection.

27. G. Interferons
a. A family of small antiviral proteins produced as earliest response of body to viral infections
b. Three classes: alpha, beta and gamma
c. Alpha and beta are produced by all body cells in response to various timuli, e.g., viruses, endotoxins, bacteria, cytokines, etc.
d. Gamma produced by T-lymphocytes and natural killer cells
e. Mechanism: inhibits viral protein synthesis by blocking the translation of viral messenger RNA; other actions include inhibition of viral penetration, uncoating or synthesis of mRNA as well as inhibition of virion assembly and release
f. Primary uses: chronic hepatitis C; Kaposi’s sarcoma (in HIV infected patients); hairy cell leukemia, chronic myelogenous leukemia, malignant melanoma, papillomavirus; herpes simplex, varicella, herpes keratitis.
g. Side effects: Bone marrow suppression, acute influenza-like syndrome

28. H. Miscellaneous Agents
1. Ribavirin
a. Guanosine analog
b. Mechanism: Phosphorylated to triphosphate by host enzymes, and inhibits RNA-dependent
RNA polymerase, viral RNA synthesis, and viral replication
c. Primary uses: severe RSV (respiratory syncytial virus) bronchiolitis and pneumonia in
hospitalized children; chronic hepatitis C (plus interferon)
d. Side effects: conjunctival irritation, rash (in aerosol form); dose-related hemolytic anemia
(systemically); teratogenic and mutagenic potential
2. Palivizumab
a. Humanized monoclonal antibody
b. Mechanism: Targets F glycoprotein on surface of RSV
c. Primary use: Approved for prevention of RSV in high-risk infants and children
d. Side effects: Elevation in serum aminotransferase levels

30. HIV entry
Entry. Trimers of HIV-1 Env recognize and bind CD4 receptor molecules on the surfaces of target cells (1). After a conformational change, gp120 can then bind coreceptor molecules (2), triggering the insertion of the gp41 fusion peptide into the target cell membrane (3). This causes the formation of the six-helix bundles leading to membrane fusion (4), thus allowing the viral core to enter the cell (5).
Cellular Microbiology Volume 7, Issue 5, pages , 18 MAR 2005 DOI: /j x

31. HIV exit
Exit. HIV-1 is targeted to the membranes of multivesicular bodies (MVBs) as well as the plasma membrane in different cell types. Members of the ESCRT family of proteins have been shown to be essential for HIV-1 budding. The ESCRT machinery, normally associated with MVBs, can also be recruited to the plasma membrane to mediate viral budding. The enlarged area shows a diagram of the key players involved in HIV-1 budding.
Cellular Microbiology Volume 7, Issue 5, pages , 18 MAR 2005 DOI: /j x