Mechanisms of Anti-Virals Flashcards
What are the two antimicrobial agents?
β Antibiotics target bacteria
β Antivirals target viruses
Why do we need antivirals?
β for quick killer viruses
β slow progressive chronic diseases
What are the slow progressive viruses that lead to cancer?
β Hep B
β Hep C
β HPV
β HIV
What are the 5 uses of antivirals?
β Treatment of acute infection β Treatment of chronic infection β Post-exposure prophylaxis β Pre-exposure prophylaxis β Prophylaxis for reactivated infection
What are three viruses causing acute infection that antivirals can be used for?
β Influenza
β Chickenpox
β Herpes
What are three viruses that cause chronic infection that antivirals can be used for?
β HCV
β HBV
β HIV
When do you use antivirals prophylactically?
β post exposure HIV
β pre exposure HIV
When do you use antivirals prophylactically for a reactivated infection?
β transplantation
β CMV
Why does selective toxicity work?
β Due to differences in the structure and metabolic pathways between the host and the pathogen
Where should the target for selective toxicity ideally be?
β inside the pathogen
What are the 6 reasons that it is so difficult to develop effective and non-toxic antiviral drugs?
1) Viruses enter cells using cellular receptors which may have other function
2)Viruses must replicate inside cells - obligate intracellular parasites
3)Viruses take over the host cell replicative machinery
4)Viruses have a high mutation rate - quasispecies
5)Antivirals must be selective in their toxicity
I.e exert their actions only on infected cells
6)Some viruses are able to remain in a latent state e.g Herpes, HPV
7)Some viruses are able to integrate their genetic material into host cells e.g HIV
Why is it difficult to make drugs that target fungi and parasites?
β They are eukaryotic organisms
β the targets for the drugs are similar to the host
What happens if you block viral enzymes?
β viral enzymes may be similar to host enzymes
β it may kill the cell
What happens if you block cellular virus receptors?
β It may have an important function and kill the cell
Describe the virus life cycle in 5 steps?
1) Virus attaches to the membrane
2) It enters via membrane fusion or endocytosis
3) When the virus is inside it uncoats and releases the genome
4) The genome replicates itself
5) the virus reassembles and escapes
What are the two ways that a virus can escape the cell?
β Virus reassembles either by budding through the membrane
β Viruses assemble inside the cell and escape by cell lysis
What are the 5 ways in which antivirals work?
β Prevent virus adsorption onto the host cell
β Preventing penetration
β Preventing viral nucleic acid replication (nucleoside analogues)
β Preventing maturation of virus
β Preventing virus release
What was amantadine used to treat?
βInfluenza A
What drugs inhibit nucleic acid polymerisation?
β Acyclovir
β Ganciclovir
β Ribavirin
How do acyclovir and ganciclovir work?
β They inhibit nucleic acid polymerisation by targeting reverse transcriptase or DNA polymerase
How does ribavirin work?
β analogue of GTP
β Compromises the genome replication of the virus
How does Zanamivir work?
β Blocks the release of the virus from the cell
What do protease inhibitors do?
β block particle maturation and assembly of the virus
What are 5 examples of viral targets?
β Thymidine kinase : HSV/VZV/CMV β Protease of HIV β Reverse transcriptase of HIV β DNA polymerases β Neuraminidase of influenza virus
What are 4 examples of human herpesviruses?
β HSV
β VZV
β CMV
β EBV
What are the methods of administration of Aciclovir?
β IV
β oral
β topical
What is Aciclovir used to treat?
β HIV/VZV treatment/prophylaxis
β CMV/EBV prophylaxis
What are the methods of administration of Ganciclovir and what is it used to treat?
β IV/oral
β CMV
What are the methods of administration of Foscarnet and what is it used to treat?
β IV/oral
β CMV
What is the method of administration of Cidofovir and what is it used to treat?
β IV
β CMV
Describe the mechanism of action of Aciclovir
1) When GTP is incorporated into DNA you need a 3β OH group but the aciclovir does not have it
2) It is a chain terminator
3) ACV is first phosphorylated by a viral thymidine kinase (encoded by viral genome)
4) Once the ACV is phosphorylated it remains stable but then it is di and tri phosphorylated by cellular components
5) When it is triphosphorylated it becomes active
6) It then becomes a competitive inhibitor for viral DNA polymerase
7) It competes with normal GTP and stop the viral polymerase from synthesizing the viral genome
What is Aciclovir?
βGTP analog that has lost some ribose sugars
What are the two reasons that Aciclovir has been given selective toxicity?
1) It is selectively activated only inside infected cells because it is activated by a viral thymidine kinase
2) Selective inhibition of DNA polymerase mainly targeting the viral DNA polymerase
Why does aciclovir not affect cellular phosphokinases?
β HSV thymidine kinase has 100x the affinity for aciclovir compared to cellular ones
Why does aciclovir not affect DNA polymerase?
β Aciclovir triphosphate has 30x the affinity for HSV DNA polymerase compared to cellular DNA polymerase
Why is aciclovir so effective?
β It is a highly polar compound
β Difficult to leave or enter cells
What is Aciclovir used for in the treatment of Herpes virus?
β Treatment of encephalitis
β Treatment of genital infection
β Suppressive therapy for recurrent genital herpes
What is Aciclovir used for in the treatment of VZV?
β Treatment of chickenpox
β Treatment of shingles
β prophylaxis of chicken pox
In what way is Aciclovir used in the treatment of EBV/CMV?
β Prophylaxis only
What is Ganciclovir used for in the treatment of CMV?
β Reactivated infection of prophylaxis in organ transplant recipients
β Congenital infection in newborn
β Retinitis in immunosuppressed
What does Ganciclovir inhibit?
β DNA polymerase
What happens to Ganciclovir inside the cell?
β Di and tri phosphorylated within the cell by cellular components
What kinase does CMV use?
β UL97 kinase
What is the function of Foscarnet?
β Selectively inhibits viral DNA/RNA polymerases and reverse transcriptases
What does Foscarnet bind to?
β Pyrophosphate binding site
β structural mimic
What viruses is Foscarnet used for and in what cases?
β Herpes - normal use
β CMV infection in the immunocompromised
β Ganciclovir resistance (TK mutants)
What type of a drug is Cidofovir?
β Chain terminator
β competes with dCTP
β monophosphate nucleotide analogue
What does Cidofovir target?
β DNA polymerase
What is Cidofovir used to treat in other viruses?
β Retinitis in HIV
β CMV but is nephrotoxic
What are Foscarnet and Cidofovir usually used for?
β Herpes
What are the two mechanisms of resistance to antivirals in Herpes?
β Thymidine kinase mutants
β DNA polymerase mutants
What happens if there is a mutation in the Herpes virus DNA polymerase?
β All drugs are rendered less effective
What happens if there is a mutation in the Herpes virus Thymidine Kinase?
β Drugs not needing phosphorylation are still effective
Describe the life cycle of HIV in 7 steps?
1) Attachment with binding of viral gp120 via CD4 and CCRX
2) Reverse transcription of RNA into dsDNA
3) Integration into host chromosome of proviral DNA
4) Transcription of viral genes
5) Translation of viral mRNA into viral proteins
6) Virus assembly and release by budding
7) Maturation
What are the 4 classes of anti HIV drugs?
β Anti reverse transcriptase inhibitors
βProtease inhibitors
β Integrase inhbitors
β Fusion inhibitors
What are the two subclasses of reverse transcriptase inhibitors?
β Nukes : nucleoside/nucleotide RT inhibitors
β Non-nukes : non-nucleoside RT inhibitors (allosteric)
What does the POL gene in HIV encode?
β protease
β reverse transcriptase and integrase
β 3β end encoding for integrase (polynucleotidyl transferase)
What kind of drugs are fusion inhibitors?
β biomimetic lipopeptide
What is HAART?
β combinations of drugs to avoid resistance
What is ziovudine (AZT)?
β synthetic analogue of the nucleoside thymidine
β acts like a chain terminator
How does ziovudine (AZT) work?
β When converted to a tri nucleotide by cell enzymes it blocks reverse transcriptase by:
β competing for the natural nucleotide substrate dTTP
β Incorporation into DNA causing chain termination
What kind of drugs are non-nukes?
β Non-competitive inhibitor of reverse transcriptase
Why are non nukes synergistic with nukes?
β They have different mechanisms
What is the treatment for post exposure prophylaxis HIV?
β 2 nukes + integrase inhibitors
β within 72 hours post exposure - take for 28 days
What is the treatment for pre exposure prophylaxis HIV and what is this called ?
β 2 x nukes (NRTIs) (truvada) β two tablets 2-24 hours before sex β one 24 hours after sex β a further tablet 48 hours after sex β on demand or event based dosing
What the combination of NRTIs used for pre exposure prophylaxis?
β emitricitabine (guanosine analogue)
β Tenofovir (adenosine analogue)
What makes it more likely for resistance to occur?
β High mutation rate and high viral load
β use of single agens
Why do you use a combination of antivirals in HIV?
βThe error rate in copying viral genome by reverse transcriptase enzyme is 1 base per 10 ^4-5 incorporation
βlacks proofreading capacity
βSo for HIV with 10 ^9-10 viruses produced every day all possible variants would be produced
What does HIV form within an individual patient?
β A quasispecies
What three drugs are used to treat the influenza virus?
β Amantadine
β Zanamivir
β Oseltamivir
What are the methods of administration of Zanamivir and Oseltamivir (tamiflu)?
β Zanamivir - inhaled or IV
β Oseltamivir - oral
How do Zanamivir and Oseltamivir work?
β Inhibit the virus release from infected cells via the inhibition of neuraminidase
How do haemagglutinin and Neuraminidase help with the budding process?
βHemagglutinin allows the receptor interaction
βThe neuraminidase is an enzyme that cleaves off sialic acid from the receptor associated with the cell surface
βWhen the virus buds out of the cell it is still attached to the sialic acid receptor
βThe neuraminidase cleaves the process and allows the virus to be released
What happens if you use a neuraminidase inhibitor?
β It blocks the ability of the neuraminidase to cleave the sialic acid so the virus canβt be released from the infected cell
β the cell will die because it gets filled with virus particles
What mutation occurred with tamiflu?
β Point mutation H275Y
How does amantadine work?
β Inhibits virus uncoating by blocking the influenza encoded M2 protein when inside cells and assembly of haemagglutinin
How are mutations selected in influenza?
β selected from quasispecies during treatment
What are influenza mutations still sensitive to?
β Zanamivir
What is the post exposure prophylaxis treatment for Hep B?
β Specific hep B immunoglobulin (passive immunity) + vaccination within 48 hours
β Antivirals NRTIs
What is the post exposure prophylaxis treatment for Hep C?
βInterferon gamma + ribavirin (anti-viral) for 6 months within the first 2 months of exposure
What is the cure rate for post exposure prophylaxis in Hep C?
β 90%
How is Hep C transmitted?
β via blood
β mother to baby
What % of drug users are Hep C positive?
β 20%
What is a major cause of chronic liver disease?
β Hep C
How many people are infected worldwide with Hep C?
β 170 million
What are the occupational risk groups for Hep C?
β Healthcare workers
What is the needle stick risk for Hep C?
β 3% to sero-conversion
β chronic carriage 85%
What is the incubation period for Hep C?
β 1-6 months
What family is Hep C from?
β Flaviviridae
What kind of a drug is Ribavarin?
β nucleoside analogue
How does Ribavirin work?
β Blocks RNA synthesis by inhibiting inosine 5β monophosphate dehydrogenase
β this blocks the conversion of IMP to XMP (xanthosine 5β monophosphate)
β this stops GTP synthesis and consequently RNA synthesis
β this stops GTP synthesis and consequently RNA synthesis
What is Ribavirin used to treat?
β RSV and Hep C in combination with pegylated interferon
What is the aim of direct antivirals in the treatment of HepC?
β Shorten the length of therapy
β minimize side effects
β target the virus itself
β improved sustained virologic response
How do direct acting anti virals (DAAβs) work?
β It binds fuses and uncoats
βThe NS3/4 protease inhibitors blocks the protein translation and poly processing so the virus canβt make its own proteins
βThere can also be inhibitors of Hep C polymerases
βYou can block the enzymes that are responsible for replication, budding and release
What are the 4 major HCV induced enzymes?
β NS2-3
β NS3-4A proteases
β NS3 helicase
β NS5B RNA dependent RNA polymerase
What 6 viruses do not have effective therapies?
β Rabies β Dengue β Common cold β Ebola β HPV β Arboviruses
