lec 6- Antiviral

Question 1

Viral infection and replication

Answer 1

• Glycoprotein envelope- protecting envelope
• mRNA- inhibit polymer synthesis
• Synthesis of structural and non-structural proteins- inhibit polymer synthesis
• Viral assembly
• Interfere with release of new virus as well
• ENZYME inhibitors

Question 2

Challenges to the development of effective antiviral agents

Answer 2

• A myriad number of agents
• Need knowledge of replication at a molecular level to define targets
• Which enzyme is involved in which step: design of enzyme inhibitors
  
  • Problem: viruses as intracellular parasites (inside the body) make targeting more difficult to avoid host toxicity
• Lack of culture systems for some agents hinders development, assay development
• High through-put screening plus ‘rational’ drug design is both labours intensive and expensive

Question 3

Anti-viral assays

Answer 3

• In vivo for cancer assay test molecule in mice with even the ‘right’ human cells in nude mice (allograft, xenograft study)
• Antiviral- test in which animal, problem virus is specific to organism
• In vitro, cannot be tested in an agar plate as antibiotics, need cell culture
• Infect cancer cells with virus, cells killed by virus or test drug
• Seperate Cytotoxic activity from antiviral activity
• Saftey issues, expensive lab

Question 4

MOA 1: work on uncoating of influenza virus

Answer 4

• Blocking the removal of the protective glycoprotein coating
• Envelope binds to the receptor on a human cell, causing glycoproteins to fuse with the membrane this is what allows the viral genome to enter into the infected cell

Question 5

Amantadine and Rimantadine

Answer 5

• Tricyclic amines
• Active vs influenza A ONLY
• High selectivity is a problem
• The problem to build up working concentrations
• Mechanism of action
  
  • Uncoating, coat with glycoproteins = sugar + proteins
• PK pharmacokinetics
  
  • Orally bioavailable
  • Amantadine: renal excretion
  • Rimantadine: hepatic metabolism and renal excretion (Main difference)
• Major toxicity
  
  • Neurotoxicity: amantadine > Rimantadine (main difference)
• Useful for treatment and prophylaxis of influenza A infections
• Resistance mediated by mutations

Question 6

Influenza

Answer 6

• Influenza virus => Absorption => Endocytosis => Endosomal vesicle => Uncoating => RNA replication => Synthesis of viral proteins => Assembly => Budding => Release
  
  • RNA replication => new RNA => Assembly

Question 7

Neuraminidase Inhibitors: Zanamivir and Oseltamivir (Tamiflu)

MOA (mechanism of action) 2 release of virus

Answer 7

• Active vs Influenza A and B
• Binds to viral neuraminidase, enzyme inhibitor
• Mechanism of action- Neuraminidase inhibitors
  
  • Inhibition of viral neuraminidase
  • Neuraminidase catalyzes the cleavage of terminal sialic acid (sugar acid) residues attached to glycoproteins and glycolipids, a process necessary for release of virus from host cell surfaces
  • Oseltamivir is an ester prodrug, ethyl ester

Question 8

Structural similarities of sialic acid Zanamavir and Oseltamivir III

Answer 8

Question 9

From sialic acid to tamiflu

Answer 9

• Sialic acid, sugar, compare glucose very similar, glucose acid
• Amino sugar (glucosamine)
• Biosphere O replaced by carbon
• Zanamivir; pyran acid- very closely related to the sialic acid= enzyme inhibitor
• Oseltamivir; a prodrug, ethyl ester, no pyran, no sugar, more stable
• A good example of drug design, clinically better than placebo

Question 10

Zanamivir and oseltamivir

Answer 10

• PK- Zanamavir (Oral inhalation)
• Oseltamivir, Tamiflu
  
  • Orally bioavailabile
  • Converted from ester prodrug to active form
  • Renally excreted
• Toxicities
  
  • Reactive airway disease by Zanamavir
  • Nausea and vomiting for oseltamivir, risk benefit ratio

Question 11

Zanamivir and Oseltamivir- Indications and resistance

Answer 11

• Indications
  
  • Treatment of influenza A and B within 24-48 hours of symptom onset
  • Prophylaxis
  • Neither drug interferes with Ab response to influenza vaccination
• Resistance
  
  • Reports appearing
  • Incidence may be increasing

Question 12

Anti-Herpesvirus Agents

Answer 12

• Famciclovir
• Idoxuridine
• Ganciclovir
• Trifluridine
• Sugars, look like ribose?
• Enzyme inhibitor
• You either modify sugar or base (NOT BOTH)

Question 13

Anti-herpes Agents

Answer 13

• Idoxuridine: Idoxuridine, uridine containing iodine
• Trifluridine: Uridine base with a trifluoro methyl group- pyrimidine system, DNA can’t continue to build
• Both agents have ribose, original sugar moiety not modified
• Ganciclovir, base OK, sugar ring opened, clinical drug design, glycol mimics sugar

Question 14

Synthetic antiviral agents

Answer 14

• Heterocyclic template, base
• And a sugar, in particular, the simplified sugar molecule
• Acyclovir, right, sugar replaced by glycol side chain (used because it is very cheap), caffeine synthesis combined with sugar
• Principle antiviral design: modified base or simplified sugar molecule

Question 15

DNA basics

Answer 15

• A-T bases = 2 hydrogen bonds
• C-G bases = 3 hydrogen bonds

Question 16

A nucleotide consists of a nitrogen-containing base, a ribose or de-oxyribose sugar a phosphate group

Answer 16

• Chemical targets
• Modified base
• Wrong base
• Enzyme inhibitors

Question 17

Aciclovir mechanism of action

Synthesis of DNA/RNA polymer

Answer 17

• Aciclovir is phosphorylated by herpes virus thymidine kinase with ATP (single phosphate group placed on)
• This phosphorylated Aciclovir is further phosphorylated (addition of 2 more phosphate groups) by ATP and cellular kinases
• This tri-phosphorylated aciclovir is 10X more efficient with herpes virus DNA polymerase

Question 18

Acyclovir I- analogues of nucleotides

Answer 18

• Development represents a watershed in the field of antiviral chemotherapy
• Acyclic guanosine analog
• Active vs HSV, VZV and modestly CMV
• Mechanism of action
  
  • Preferentially taken up by virally infected cells
  • Mono-phosphorylated by viral kinases, selectivity toward viral enzymes
  • Di- and triphosphorylation completed by cellular kinases
    
    • Competative inhibitor of viral DNA polymerase- Enzyme inhibitor
      
      • Cellular DNA polymerases much less susceptible to inhibition
    • Leads to viral DNA chain termination

Question 19

Acyclovir II

Answer 19

• PK, ADME
  
  • Administered by oral, IV and topical routes
  • Oral bioavailability 15-30%
  • T_1/2 3 hours
  • Primarily renally excreted
• Toxicity
  
  • Headache, nausea
  • Renal
  • Neurological
• Resistance
  
  • Mediated by mutations in viral thymidine kinase and/or viral DNA polymerase genes

Question 20

Overview- Action of some important antivirals

Answer 20

• Amantadine blocks Uncoating
• Release oseltamivir blocks neuraminidase
• Synthesis foscarnet block DNA polymerase in DNA virus
• AZT blocks reverse transcriptase in RNA viruses

Question 21

Conclusion

Answer 21

• Field of anti-viral chemotherapy has matured dramatically in past 30+ years
• Greatest progress made for
  
  • Herpes viruses
  • Hepatitis Viruses
  • Respiratory Viruses
  • HIV, not done
• Preventative Vaccination remains the key to global control of viral infection