Mechanism of Antivirals Flashcards
What are the uses of antivirals?
- Treatment of acute infection
- Treatment of chronic infection
- Post-exposure prophylaxis and preventing infection
- Pre-exposure prophylaxis
- Prophylaxis for reactivated infection
Outline the acute infections we can treat using antivirals
Influenza; Chickenpox; herpes infections are treated using aciclovir
Give examples of chronic infections treated using antivirals
HCV, HBV, HIV are treated with numerous different agents
Give examples of antiviral use in prophylaxis in reactivated infections
In immunosuppressed patients: e.g. in transplantation; CMV (ganciclovir, foscarnet)
What provides the selective toxicity of antivirals?
Selective toxicity due to differences in structure and metabolic pathways between host and pathogen
Enables harm to microorganisms, not host
Why is it so difficult to produce a non-toxic antiviral drug?
Difficult to achieve as viruses:
- Enter cells using cellular receptors which may have other functions
- Replicate inside cells
- Take over host replicative machinery
- Have high mutation rate
What is a major principle of antiviral agents?
Antivirals must be selective in their toxicity
i.e. exert their action only on infected cells
Why can it be difficult to target viruses in hosts?
Some viruses able to remain in latent state e.g. herpes, HPV
Some are able to integrate genetic material into host cells e.g. HIV
What factors must we consider when developing a safe antiviral drug?
We can target stages of viral infection however:
- Cellular receptor may have other important function
- Viral enzymes may be similar to host
- Blocking cellular enzyme may kill cell
Describe the general life cycle of a virus
- Virus infects cell and attaches to membrane
- Internalisation via endocytosis / membrane fusion
- Uncoats and releases genome
- Genome replication and
- Reassembly of viral particles
- Budding and release out of cell
Describe the different mechanisms of action of selected antivirals
- Preventing virus adsorption onto host cell
- Preventing penetration
- Preventing viral nucleic acid replication (nucleoside analogues)
- Preventing maturation of virus
- Preventing virus release
What is mantadine used to treat?
- used to treat influenza infections
- blocks viral uncoating mechanism (inhibits fusion of influenza virus to endosome)
- associated toxicity
Which antivirals inhibit viral genomic replication from occurring?
Acyclovir, Ganciclovir, Ribavirin etc.
- HIV/AIDS AZT drugs
Inhibit nucleic acid polymerisation via RT / DNA pol inhibition
Describe the effects of Ribavarin
Ribavirin is an analogue of GTP - compromises viral genome replication as not enough precursors available
What is the mechanism of action of HIV Protease inhibitors?
HIV protease inhibitors block particle maturation and viral assembly
Outline the effect of Zanamivir on flu virus
Zanamivir blocks release of Flu virus - prevents infection of other cells; used more now
What component of viruses are good selective toxicity targets for antivirals?
Virally encoded enzymes sufficiently different from human counterparts
Outline the different Viral enzyme targets
- Thymidine kinase + HSV / VZV / CMV
- Protease of HIV
- Reverse transcriptase of HIV
- DNA polymerases
- Neuraminidase of influenza virus
Why are viral enzymes good targets for antivirals?
Act as selective targets with minimal effect on host enzymes or processes
What is the consequence of Herpes virus infection?
Causes muco-cutaneous lesions
Which viruses are included within the herpes virus family?
- Herpes simplex (HSV),
- Varicella Zoster Virus (VZV)
- Cytomegalovirus (CMV)
- Epstein-Barr virus (EBV)
Which Herpes virus’ does Ganciclovir treat?
IV/oral
For CMV
When is Aciclovir administered?
IV/oral/topical
For HSV, VZV treatment/prophylaxis
CMV/EBV prophylaxis
What is the use of Cidofovir?
IV for CMV
When is Foscarnet used?
IV/local application
For CMV
Describe the mechanism of action of Aciclovir
GTP analogue; is a chain terminator - inserts into DNA to prevent polymerisation
How is aciclovir activated within host cells?
Aciclovir is phosphorylated by Viral Thymidine Kinase (TK)
Tri-phosphorylated by cellular kinases to become active
Substantially more in infected cells
What accounts for aciclovir’s low toxicity?
Aciclovir selective toxicity requires 2 viral enzymes
= to selectively activate ACV
= to selectively inhibit
Why does aciclovir have such high efficacy rates?
HSV thymidine kinase (TK) has 100x the affinity for ACV compared with cellular phosphokinases
Aciclovir triphosphate has 30x the affinity for HSV DNA polymerase compared with cellular DNA polymerase
What makes aciclovir so efficient and safe?
Aciclovir triphosphate is a highly polar compound - difficult to leave or enter cells (but aciclovir is easily taken into cells prior to phosphorylation)
DNA chain terminator
When is aciclovir used for herpes simplex virus?
Treatment of encephalitis
Treatment of genital infection
suppressive therapy for recurrent genital herpes