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Flashcards in Mechanisms of Anti-virals Deck (42)
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1

Why do we need anti-virals? 

  • Some viruses kill quickly 
    • Influenza, Ebola, MERS, SARS 
  • Some viral infections can be slow and progressing chronic infections leading to cancer 
    • HepB, HepC, HPV (can cause cervical cancer) 
  • Some viral infections are highly infectious such as HIV 
  • Acute inflammatory diseases e.g Herpes 

2

Give some examples of anti-microbials 

Antibiotics (target bacteria), antivirals, anti-fungals, anti-protosoals, anti-helminths.

3

What are some uses of anti virals? 

  • Treatment of acute infection
    • Infuenza, chickenpox, shingles, herpes infections – (acyclovir)
  • Treatment of chronic infection
    • HCV, HBV, HIV (numerous different agents)
  • Post-exposure prophylaxis and preventing infection
    • HIV (PEP)
  • Pre-exposure prophylaxis
    • HIV (PrEP)
  • Prophylaxis for reactivated infection
    • E.g. transplantation, CMV (ganciclovir, forscarnet)

4

What is selective toxicity? 

When a drug has a selective action against one component and not another 

5

Elaborate on selective toxicity 

  • Is achieved due to differences in structure and metabolic pathways between host and pathogen 
  • It harms the microorganisms, not the host 
  • We want the target to be in the microbe (not host) if possible 
  • Difficult for viruses as they are intracellular and use cellular processes to replicate themselves
  • Variation between microbes (even strains of same species) 
    • E.g HIV has a high mutation rate as no DNA repair mechanism 

6

Why is it difficult to develop effective, non-toxic anti-viral drugs? 

  • Virus may enter the cells using cellular receptors which may have other functions 
  • Viruses must replicate inside cells → obligate intracellular parasites 
  • Viruses take over host cell replicative machinery 
  • Viruses = HIGH MUTATION RATE → Quasispecies 
  • Anti-virals must be selective in their toxicity 
    • Should only exert their action on infected cells
  • Some viruses can remain in a latent state e.g Herpes 
  • Some viruses are able to integrate their genome into the host genome 

7

Provide a recap of the virus life cycle 

  1. Virus will attach itself to the membrane and internalise via membrane fusion (remember HIV glyprotein fusion) or endocytosis 
  2. Virus will then uncoat, removal of capsid and release its genome 
  3. The genome will replicate itself and make mRNA which will enter the ribosomes of the cell and make viral proteins
  4. The newly synthesised viral proteins will assemble at the membrane = new virus particle 
  5. The processed and assembled virus will exit the cell via budding through the membrane 
  6. Some viruses will assemble inside the cell and escape via cell lysis 

8

What are some considerations when developing safe anti-viral agents? 

  • Cellular receptor may have other important functions 
  • Viral enzymes may be very similar to host 
  • Blocking cellular enzyme may kill the cell 

9

What is the mode of action of selected anti-virals? 

  • Preventing virus adsorption onto host cell 
  • Preventing penetration 
  • Preventing viral NA replocation (through nucleoside analogue, usually a terminator) 
  • Preventing virus maturation 
  • Preventing virus release 

10

What is the mode of action of selected anti-virals? 

  • Amantadine → blocks the uncoating process in the influenza A virus,however not used anymore because it had toxicity associated with it).
  • Acyclovir, Ganciclovir, Ribavarin → Inhibit NA polymerisation by inhibiting reverse transcriptase’s or DNA polymerases
  • Zidovudine/Azidothymidine (AZT) → thymidine analogue, selectively inhibits HIV’s reverse transcriptase
  • Ribavarin → Acts as an analogue of GTP, compromises the genome replication
  • Protease inhibitors → Block particle maturation
  • Zanamivir → Is an anti influenza drug, blocks the mature release of the particle from the cell

11

List some selective toxicity viral targets 

  • Thymidine kinases of HSV/VZV/CMV
  • Proteases of HIV 
  • Reverse transcriptases of HIV 
  • DNA polymerases 
  • Neuraminidases of Influenza virus 

12

List the herpes viruses

  • Herpes simplex (HSV) 
    • HSV1 causes muco-cutuneous lesions on the lip 
  • Varicella zoster virus (VZV) 
  • Cytomegalovirus (CMV) 
  • Epstein-Barr Virus (EBV) 

13

List some drugs for treating herpes viruses 

  • Aciclovir → for HSV, VZV treatment, prophylaxis of CMV 
  • Ganciclovir → for CMV 
  • Foscarnet → for CMV 
  • Cidofir → for CMV 

14

What are other anti-herpes virus agents? Foscarnet 

Foscarnet - also treats CMV 

  • Selective inhibits viral DNA/RNA polymerases by competing with pyrophosphate
  • Binds to pyrophosphate binding site à structural mimic
  • No reactivation required (not a prodrug like aciclovir and ganciclovir)
  • Used for CMV infection in the immunocompromised
  • E.g. pneumonia in solid organ and bone marrow transplants
  • May be used because of ganciclovir resistance (TK mutants)

15

What is aciclovir? 

Acycloguanosine analogue (GTP analogue), has a missing 3' OH acting as a chain terminator, is inserted into DNA and prevents it from replicating 

Competitive inhibitor of viral DNA polymerase, competes with standard GTP 

16

What is the mechanism of action of aciclovir? 

  • First it has to be activated by viral thymidine kinases
  • It will then be phosphorylated by cellular guanylate GDP kinases = remains stable in the cell 
  • Once it has been tri phosphorylated =  active drug 
  • Competitve inhibitor for viral DNA polymerase, it competes for standard GTP preventing viral polymerase from synthesising viral genome 

17

What two things provide aciclovirs selective toxicity? 

  • Activated to an active drug substantially more in infected cells as it is activated by thymidine kinase = selective activation of aciclovir 
    • Cells do contain thymidine kinase but they have low activity against aciclovir 
  • Active triphosphorylated form in 30 times more active against viral DNA polymerases than host DNA polymerases = selective inhibition of polymerases  

18

Why is aciclovir so effective + safe? 

  • HSV thymidine kinase has 100x the affinity for aciclovir compared with cellular phosphokinases 
  • Aciclovir triphosphate has 30x the affinity for HSV DNA polymerase compared with cellular DNA polymerase
  • Aciclovir triphosphate is a highly polar compound à difficult to leave or enter cells (but aciclovir is easily taken into cells prior to phosphorylation)
  • DNA chain terminator

19

What are treatments of aciclovir? 

  • 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

20

What for what type of treatment is ganciclovir used for? 

  • Used to treat cytomegalovirus (most people have been infected by this + causes mild flu like symptoms) 
    • Reactivated infection or prophylaxis in organ transplant 
    • Congenital infection in new-born 
    • Can damage retina and cause retinitis in immunosuppressed patients 

21

Describe gangciclovir

  • Structurally similar to aciclovir 
  • CNV does not encode TK but has UL97 kinase 
  • Inhibits CMV DNA polymerase 

22

What is the mechanism of action of gangciclovir? 

  • Gangciclovir will be phosphorylated by UL97 kinase gene encoded in the CMV genome 
  • It will then be diphosphorylated and triphosphorylated inside the cell via cellular kinases 
  • It then has the same effects as aciclovir by competitive inhibition 
  • Similar to ACV it will compete for the natural substrate DNA polymerase and block the abillity to make its own DNA

23

What are other anti-herpes virus agents? (CIDOFOVIR) 

  • Chain terminator – targets DNA polymerase
  • Competes with dCTP
  • Monophosphate nucleotide analogue
  • Prodrug à phosphorylated by cellular kinases to di-phosphate
  • Drug active against CMV à MUCH MORE nephrotoxic
  • Used when you can’t use ganciclovir
  • Treatment of retinitis in HIV disease

24

How does resistance to anti-virals occur in herpes viruses? 

  1. Thymidine kinase mutants 
  2. DNA polymerase mutants 

 

  • If it occurs in TK, drugs not needing phosphorylation are still effective e.g foscarnet, cidofovir 
  • If occurs in DNA polymerase all drugs will be rendered less effective 
    • However this is very rare in immune competent patients 

25

What is the structure of HIV? 

  • Double stranded RNA genome encaspulated by nucleocapsid (Gag p24) 
  • Outer layer consists of lipid bilayer consisting of protuding protein spikes which mediate entry into cell 
    • Envelope glycoproteins gp120 (receptor ligand) and gp41 (transmembrane) 
  • Inside envelope lie Gag proteins (membrane associated matrix protein Gag 17) 
  • Virus will encode particular viral enzymes 
    • Integrase 
    • Protease 
    • Reverse Transcriptase 

26

Describe the steps in the HIV life cycle 

  1. Attachment with binding of viral gp120 via CD4 and CCRX
  2. Reverse transcription of RNA into dsDNA
  3. DNA provirus integrates into host chromosome
  4. Transcription of host viral genes which form mRNA
  5. Translation of mRNA to make viral proteins or regeneration of RNA to form the genome in new virus particle
  6. Virus assembly and release by budding
  7. Post-release maturation

27

List some Anti-HIV drugs 

  1. Anti-reverse transcriptase inhibitors
    1. Nukes – nucleoside/nucleotide RT inhibitors (block RT by mimicking nucleotides/nucleosides)
    2. Non-nukes – non-nucleotide RT inhibitors (allosteric)
  2. Protease Inhibitors – multiple types
  3. Integrase inhibitors – POL gene – protease, reverse transcriptase and integrase (IN) with the 3’ end encoding for IN (polynucleotidyl transferase)
    1. Remember the three main genes encoded by the HIV genome Gag (MA, CA, NC), POL (PR, RT, IN) and Env (envelope glycoproteins)
  4. Fusion inhibitors – gp120/41 – biomimetic lipopeptide
    • Can make the mimics of the fusion mechanism so that gp120 is used and blocks infection of the cell

28

What type of therapy do we use with someone with HIV? 

We use HAART (highly active antiretroviral therapy) 

When treating someone with HIV we will use a combination of drugs, because if we just use one that targets one of the mechanisms, this virus will overcome that by rapid mutation 

29

Describe AZT as an example of a nuke 

  • Is a nucleoside reverse transcriptase inhibitor → Nukes
  • Is a synthetic analogue of nucleoside thymidine
    • When converted to trinucleotide by cell enzymes it blocks RT by:
  • Competing for natural nucleotide substrate dTTP
  • Incorporation into DNA causing chain termination!!!

30

Describe Nevirapine as an example of a non-nuke 

  • Is a non-nucleoside reverse transcriptase inhibitor → Non-nuke
  • Is a non-competitive inhibitor of HIV-1 RT
  • Synergistic with NRTI’s such as AZT because of different mechanism
    • Many HIV patients will have a nuke + non-nuke and then a protease/integrase inhibitor

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