Medical Microbiology: Mechanism of Antivirals Flashcards
1
Q
Name some types of antimicrobials
A
- Antibiotics
- Anti-virals
- Anti-fungals
- Anti-protosoals
- Anti-helminths
2
Q
Whay do we need anti-virals?
A
-
There are a range of viral infections that kill quickly:
- Influenza
- Ebola
- MERS
- SARS
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There are also slow, progressive, chronic viral infections that can lead to cancer:
- Hepatitis B
- Hepatitis C
- Human papilloma viruses
-
There are highly infectious viral infections:
- Human immunodeficiency virus (HIV)
-
There are acute inflammatory viral infections:
- Herpes
3
Q
What are some of the uses of anti-virals?
A
-
Treatment of acute infection:
- Influenza
- Chickenpox
- Herpes infections
-
Treatment of chronic infection:
- Hepatitis C Virus (HCV)
- Hepatitis B Virus (HBV)
- HIV
-
Post-exposure prophylaxis (PEP) and preventing infection:
- HIV
-
Pre-exposure prophylaxis (PrEP):
- HIV
-
Prophylaxis for reactivated infection: e.g. in transplantation:
- Cytomegalovirus (CMV)
4
Q
What is selective toxicity?
A
- The ability of a drug to target a site specific to a pathogen causing disease
5
Q
What are the principals of the selective toxicity of anti-virals when used as therapeutic agents?
A
- Needs to be differences in structure and metabolic pathways between host and pathogen
- Needs to harm microorganism and have minimal effect on host
- Target needs to be in microbe and not host (if possible)
- Selective toxicity difficult for viruses (intracellular organisms and use cellular processes to replicate) , fungi and parasites (eukaryotic so target for drugs every similar to host cells)
- Variation between microbes - variation even between strains of same species
6
Q
Why is it so difficult to develop effective, non-toxic anti-viral drugs?
A
- Viruses enter cells using cellular receptors which may have other functions - blocking cellular recptors will have toxic effects
- Viruses must replicate inside cells (obligate intracellular pathogens) - makes it difficult to identify unique properties of viral structure/enzymes
- Viruses take over the host cell replicative machinery - Hard to harm virus without harming cellular replication
- Virsues have high mutation rate - Can escape effects of antiviral drugs due to different strains being produced
- Anti-virals must be selective in their toxicity - Only exert their action only on infected cells
- Some viruses are able to remain in a latent state - Not expressing proteins so drug that targets viral enzyme/protein involved inreplication won’t be effcetive e.g. herpes, HPV
- Some viruses are able to integrate their genetic material into host cells - Impossible to remove integrated genetic material from host cell genetic material e.g. HIV
7
Q
Describe the stages of the virus life cycle
A
- Virus recognises host cell and attaches to host cell membrane
- Virus gets internalised via endocytosis or via membrane fusion
- Once inside the host cell the virus uncoats and releases its genome
- The viral genome then intergrates itself into host cell genome and replicate itself
- Once integreated viral genome will be transcribed to produce mRNA
- mRNA travels to ribsome where it is translated to form viral proteins
- At the same time viral genome, along with host cell genome, is replicated
- Virus then reassembles either through budding through the membrane or via lysis of membrane
8
Q
What are some considerations that need to be taken into account when developing safe anti-viral agents?
A
- What stages of infection be targeted?
- Cellular receptors may have other important functions
- Viral enzymes may be very similar to host cell enzymes
- Blocking cellular enzymes may kill host cell
9
Q
What are some modes of action of anti-virals?
A
- Prevent virus adsorption onto host cell
- Prevent penetration
- Prevent viral nucleic acid replication (nucleoside analogues)
- Prevent maturation of virus
- Prevent viral release
10
Q
Give some examples of anti-virals and their actions
A
- Amantadine - Blocks low pH endosome dependent uncoating of influenza A (lots of toxicity associated)
- Acyclovir, Ganciclovir and Ribavarin - Inhibit nucleic acid polymerisation by targeting reverse transcriptases or DNA polymerases (effective aganist HIV/AIDS)
- Ribavarin - Also acts analogue of GTP and so compromises genome replication (effective against RSV)
- HIV protease inhibitors - Inhibit proteases involved in clevage of Gag and Pol polyproteins into individual proteins they encode
- Zanamivir - Prevents influenza release
- Interferons - Blocks viral mRNA translation
11
Q
Give some examples of selective toxicity viral targets
A
- Thymidine kinase - used to herpes virsuses such as HSV/VZV/CMV
- Protease of HIV
- Reverse transcriptase of HIV
- DNA polymerases
- Neuraminidase of influenza virus
- NOTE: All of these are virally encoded enzymes sufficiently different from human counterparts and so can act as selective targtes with minimal effect on host enzymes or processes
12
Q
Name some anti-viral drugs that can be used against herpes viruses
A
-
Aciclovir (IV/oral/topical)
- HSV, VZV treatment/prophylaxis
- CMV/EBV prophylaxis
-
Ganciclovir (IV/oral)
- CMV infection treatment
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Foscarnet (IV/local application)
- For CMV infection treatment
-
Cidofovir (IV)
- For CMV infection treatment
13
Q
Describe the structure of Aciclovir, describe how it is activated and explain how it works
A
- Structure
- Aciclovir is a GTP analogue but is missing the 3’ OH group on the ribose sugar that a normal GTP molecule would have
- This means Aciclovir is a chain terminator - will be inserted into DNA and prevent it from ploymerising
- How it’s activated
- Aciclovir is a pro-drug so needs to be activated
- First phsophorylated by viral thymidine kinase
- Then it gets di- and tri-phosphorylated by cellular kinases
- Only active when tri-phosphorylated
- Mechanism of action
- When active it becomes a competitive inhibitor of viral DNA polymerase
- It competes for GTP and stops viral DNA polymerase from synthesisng viral genome
14
Q
What is it that gives Aciclovir selective toxicity?
A
- It’s selectively activated only inside cells that are infected because it’s mainly activated by a viral thymidine kinase
- Although there are cellular thymidine kinases they have poor activity against aciclovir so unlikely to be activated in uninfected cell
- Tri-phosphorylated aciclovir has selective toxicity aganist viral DNA polymerase as it’s at least 30x more active against viral DNA ploymerase compared to host DNA polymerase
15
Q
Why is aciclovir so effective and safe?
A
- Aciclovir triphosphate is a highly polar compound - difficult to leave or enter cells so will accumulate inside infected cells
- Aciclovir is easily taken into cells prior to phosphorylation)
- DNA chain terminator - When it’s incorportaed into viral DNA strand it causes termination of strand synthesis
16
Q
What infections can Aciclovir be used to treat?
A
-
Herpes simplex
- Treatment of encephalitis
- Treatment of genital infection
- Suppressive therapy for recurrent genital herpes
-
Varicella zoster virus
- Treatment of chickenpox
- Treatment of shingles
- Prophylaxis of chickenpox
-
CMV/EBV
- Prophylaxis only
17
Q
What conditions/complications can cytomegalovirus (CMV) cause?
A
- Reactivated infection or prophylaxis in organ transplant recipients
- Congenital infection in newborns
- Retinitis in immunosuppressed patients
18
Q
Describe the structure of Ganciclovir
A
- Structurally similar to aciclovir
- CMV does not encode Thymidine Kinase but has UL97 kinase (CMV phosphotransferase) which activates ganciclovir slightly better than aciclovir
19
Q
Why is Ganciclovir used to treat CMV instead of Aciclovir?
A
- CMV does not encode Thymidine Kinase but has UL97 kinase (CMV phosphotransferase) which activates ganciclovir slightly better than aciclovir
20
Q
How does Ganciclovir work?
A
- Gets phosphorylated by UL97 kinase
- Then gets di- and tri-phosphorylated by cellular kinases
- Once tri-phosphorylated it inhibits CMV DNA polymerase so viral genome can’t be synthesised
21
Q
Describe the characteristics of foscarnet
A
- Selectively inhibits viral DNA/RNA polymerases
- Doesn’t require cellular activation - not a pro-drug
- Competes with pyrophosphate for binding to pyrophosphate binding site on DNA/RNA polymerase - a structural mimic
- Used for CMV infection in the immunocompromised
- May be used because of ganciclovir resistance in CMV TK mutants
22
Q
Describe the characteristics of Cidofovir
A
- Chain terminator (doesn’t have 3’ OH group) - targets DNA polymerase (has very high affinity for it)
- Competes with dCTP
- Monophosphate nucleotide analog
- Prodrug – phosphorylated by cellular kinases to di-phosphate
- Active against CMV; but MUCH MORE nephrotoxic than Ganciclovir
- Used for treatment of retinitis in HIV disease
26
Q
Briefly describe the structure of HIV
A
- Viral envelop - lipid bilayer
- Within viral envelope there’s Envelope protein gp120 and transmembrane protein gp41
- Capsid has 3 components:
- Memebrane associated matrix protein
- Nucleocapsid protein
- Capsid protein
- ds RNA genome
- Contains 3 viral enzymes:
- Reverse transcriptase
- Integrase
- Protease
27
Q
Describe the life cycle of HIV
A
- Attachment of HIV to host cell membrane via binding of viral gp120 to CD4 and CCRX
- Reverse transcription of RNA into dsDNA
- Integration into host chromosome of proviral DNA
- Transcription of viral genes
- Translation of viral mRNA into viral proteins
- Virus assembly and release by budding
- Post-release maturation
30
Q
Explain how Zidovudine (AZT) works as a nucleoside reverse transcriptase inhibitor (nuke)
A
- Synthetic analogue of nucleoside thymidine - contains azide group instead of hydroxyl group
- When converted to tri-nucleotide by cell enzymes, it blocks reverse transcriptase by:
- Competing for natural nucleotide substrate of reverse transcriptase, dTTP (Deoxythymidine triphosphate)
- Incorporation into DNA causing chain termination - doesn’t have 3’ OH group so rest of strand can’t be synthesised
39
Q
How does neuraminidase allow for influenza virus release?
A
- Initially when influenza virus buds off the cell it’s still attached to the sialic acid-containing receptors on the cell surface
- Neuraminidase then cleaves off sialic acid from the cell surface receptor allowing influenza virus to be released from the cell
40
Q
How do neuraminidase inhibitors prevent release of influenza from the host cell?
A
- Target and inhibit neuraminidase at highly conserved site (reduces chances of resistance via mutation)
- This prevents release of sialic acid residues from the cell receptor
- This prevents virus budding and release and spread to adjacent cells
45
Q
How does Ribavirin work?
A
- Ribavirin needs to phosphorylated in order to become active
- Once phophorylated it blocks RNA synthesis by inhibiting inosine 5’-monophosphate (IMP) dehydrogenase
- This blocks the conversion of IMP to XMP (xanthosine 5’-monophosphate) and thereby stops GTP synthesis and, consequently, RNA synthesis
49
Q
Name some Direct-acting antivirals and their mechanism of action
A
-
NS3/4 protease inhibitors
- Block polyprotein processing
- If they can’t process the polyproteins, they cant make the enzymes necessary to assemble the virus
-
NS5B polymerase inhibitors (nukes/non-nukes)
- Block RNA replication
-
NS5A inhibitors
- Block the replication complex formation and assembly of the viruses by blocking the enzymes necessary for the process of budding, maturation and release.
