Mechanisms of Antivirals Flashcards
Why do we need anti-virals ?
because viruses are quick killers, eg. influenza, ebola, SARS
or viruses can be chronic and slow, leading to cancer, eg. HPV, Hep B, Hep C
To treat human immunodeficiency virus (HIV)
To treat acute inflammatory infections – herpes, shingles, chickenpox
give some uses of anti virals?
- Treating acute (influenza, chickenpox, herpes) and chronic infections (HCV, HIV)
- Post exposure prophylaxis (PEP), taking antiviral medication after being potentially exposed (e.g. to HIV) to prevent infection
- Pre-exposure prophylaxis (HIV)
- Prophylaxis for reactivated infection: e.g. in transplantation CMV (ganciclovir)
Principles of Anti-Virals as Therapeutic Agents
Selective Toxicity - exert their action only on infected cells
-taking into account differences in structure/metabolic pathways of host and pathogen
- drug should harm microorganism but not the host
- drug target should be in the microbe if possible and specific for the microbe
Why is it hard to make anti-virals?
Because they are intracellular organisms that use cellular processes to replicate themselves.
Inhibiting the replicative processes inside the cell will stop the virus from replicating but will also damage the host cell
Viruses enter cells using cellular receptors which may have other functions
Viral enzymes may be very similar to host enzymes, and blocking cellular enzyme may kill the cell
Viruses have high mutation rate - quasispecies (large numbers of mutants of the same isolate of virus in the same human)
Some viruses are able to remain in a latent state e.g. herpes, HPV - difficult to target
Some viruses are able to integrate their genetic material into host cells e.g. HIV
Modes of action of selected anti-virals
Preventing virus adsorption onto host cell
Preventing penetration
Preventing uncoating process, used to be blocked by Amantadine for influenza A, was an effective anti-flu drug BUT had toxic side effects
Preventing viral nucleic acid replication (nucleoside analogues), very effective
-Acyclovir, Ganciclovir, Ribavarin inhibit nucleic acid polymerisation by targeting reverse trancriptases/ DNA polymerases
Preventing virus maturation
Preventing virus release, Zanamivir inhibits Influenza release
Selective toxicity viral targets - examples
Discovery of virally encoded enzymes sufficiently different from human counterparts
e.g.
Thymidine kinase and HSV/VZV/CMV
HIV Protease or Reverse transcriptase
DNA polymerases
Neuraminidase of influenza virus
Act as selective targets with minimal effect on host enzymes or processes
Herpes viruses - examples and how to treat them
Herpes viruses include: Herpes simplex (HSV), Varicella Zoster Virus (VZV) Cytomegalovirus (CMV) Epstein-Barr virus (EBV)
ACICLOVIR
- IV/oral/topical
- good because of its low toxicity
- For HSV, VZV treatment/prophylaxis
GANCICLOVIR, CIDOFOVIR, FOSCARNET
-IV for CMV
aciclovir - mechanism and uses
Acycloguanosine is a GTP analogue
- Phosphorylated by a viral thymidine kinase - makes it stable within the cell
- di and tri phosphorylated by cellular kinases. The tri-phosphate structure of aciclovir is the active drug
- The active drug is a competitive inhibitor with normal GTP for viral DNA polymerase, which is synthesising the viral genome.
- Incorporating aciclovir means termination of the viral DNA polymerase chain, because its a GTP analogue but without the ribose sugar, therefore acts as a chain terminator.
it has low toxicity
TREATS: 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
acyclovir has a low toxicity - how?
- selectively activated in infected cells
- mainly activated by a viral thymidine kinase - unlikely for the drug to be activated by normal cell thymidine kinase
- selectively inhibits viral DNA polymerase, not human DNA polymerase
- Aciclovir triphosphate is a highly polar compound - difficult to leave or enter cells (but aciclovir is easily taken into cells prior to phosphorylation). This means you can accumulate the drug where you need it
Ganciclovir
Active for CMV
- reactivated infection or prophylaxis in organ transplant recipients
- congenital infection in newborn
- retinitis in immunosuppressed (CMV reactivated)
Structurally similar to aciclovir
CMV does not encode TK but has UL97 kinase that activates ganciclovir, which is then di and tri phosphorylated by cellular kinases
Inhibits CMV DNA polymerase (competes with the natural substrate for DNA polymerase and inhibits DNA synthesis)
Foscarnet
– not a pro-drug, doesn’t need to be activated
– May be used because of ganciclovir resistance (TK mutants) - can treat TK mutants in CMV because it targets a different part of the enzyme other than the target site, called the pyrophosphate binding site
selectively inhibits viral DNA/RNA polymerases and RTs
used for CMV infection in the immunocompromised, e.g. pneumonia in solid organ and bone marrow transplants.
Cidofovir
Chain terminator - targets DNA polymerase and competes with dCTP, acts as a monophosphate nucleotide analogue
Prodrug – phosphorylated by cellular kinases to di-phosphate
drug active against CMV, but MUCH MORE nephrotoxic
Treatment of retinitis in HIV disease
Resistance to anti-virals in Herpes viruses
Two main mechanisms
Thymidine Kinase mutants
DNA polymerase mutants
If occurs in TK, no longer activate the pro drug, but drugs not needing TK phosphorylation are still effective (e.g. foscarnet, cidofovir)
If mutation occurs in DNA polymerase, all drugs rendered less effective as they cannot bind
VERY RARE in immune competent patients (low viral load) - Both of these mutants are only easily generated when you have a high viral load
HIV stands for?
Human immunodeficiency virus
Name the 7 steps in the life cycle of HIV
- Attachment to host membrane, binding of viral gp120 via CD4 and CCRX
- reverse transcription of RNA into dsDNA
- Integration of proviral DNA into host chromosome
- Viral gene transcription
- Translation of viral mRNA into viral proteins
- Virus assembly and release by budding
- maturation
Anti-HIV drugs
- Anti-reverse transcriptase inhibitors - nukes block RT by mimicking nucleotides/nucleosides, non nukes block RT via allosteric sites
- Protease inhibitors - Virus cant assemble itself. Virus proteases have a completely different structure to host proteases, so you block the viral ones without affecting the host ones – important part of selective toxicity
- Integrase inhibitors – no cross linking of viral DNA into host DNA, POL gene codes for 3 separate components including integrase
- Fusion inhibitors - prevent virus fusing with host cell membrane, preventing virus particle internalisation, complex long lipopeptides act as biomimicks and interrupt gp120-gp141 interaction.
HAART, highly active anti-viral therapy - combination of drugs to avoid resistance
example of a nuke and non nuke?
Zidovudine
-analogue of nucleoside thymidine, acts as a chain terminator because they can’t be incorporated into synthesising chain
Nevirapine
Non-competitive inhibitor of HIV-1 RT
Synergistic with NRTI’s such as AZT because of different mechanism
Nothing to do with chain termination
Resistance to anti-virals
use of single agents leads to rapid development of resistance, drug binding site is altered in structure by as few as one amino acid substitution
High mutation rate and viral load —–> resistance
Some mutations are lethal to the virus and it will die, but some mutations will confer resistance - selection pressure
Increased mutation rate seen in HIV due to lack of proofreading ability
Formation of quasispecies within an individual patient:-A viral swarm
Influenza virus - drugs
Amantadine
- Inhibits virus uncoating by blocking the influenza encoded M2 protein when inside cells and assembly of haemagglutinin
- M2 protein involved in early endosome acidification when the virus gets into the cell - blocking M2 means virus capsid not released into cytoplasm
- rarely used because toxic + not very effective
Zanamivir/Relenza (IV/inhaled) and Oseltamivir/Tamiflu (oral)
- Inhibits virus release from infected cells
- targets and inhibits Neuraminidase at highly conserved site (reduces chances of resistance via mutation)
- prevents virus budding and release
- prevent release of sialic acid residues from the cell receptor
Influenza resistance to anti-virals
Resistance sometimes only requires a single amino acid change - seen recently with swine flu (H1N1) and Tamiflu (oseltamivir)
Point mutation seen in immunocompromised patients; shed virus for weeks/months
Likely to be selected from among quasi species during treatment
antigenic shift and drift
Occupational Infection - Hazards
Exposure prone incidents
-Sharps, Splashes and blood-born viruses
Hepatitis B
Hepatitis C
HIV
take universal precautions
Post-exposure prophylaxis
Emergency, requiring rapid treatment
Hep B
specific Hep B immunoglobulin (passive immunity)
+ vaccination within 48 hours
Hep C
interferon- + ribavarin (anti-viral) for 6 months
within first 2 months of exposure
90% cure rate - now direct acting antivirals
HIV 80% protection i.e. no sero-conversion must be FAST – hours antiviral drug treatment – 28 days 2xNRTI + protease or integrase inhibitor
Hepatitis C virus (HCV)
RNA virus, enveloped
transmitted via blood – infectious (mother to baby)
major cause of chronic liver disease
occupational risk groups – healthcare workers
needle-stick risk
vaccination NOT available
Ribavirin
nucleoside analogue
HCV treatment
Ribavirin blocks precursor GTP pathway, blocks GTP synthesis and therefore blocks RNA synthesis
Inhibits IMP dehydrogenase, blocks conversion of IMP to XMP, stopping GTP synthesis and consequently RNA synthesis
HCV - new treatments?
Direct-acting antivirals (DAAs) - new class of medication, targets specific steps in HCV life cycle
aim: shorten length of therapy, minimize side effects, target the virus itself
DAA with different viral targets, are synergistic in combinations
examples
NS3/4 protease inhibitors - block polyprotein (GAG) processing and protease synthesis
NS5A inhibitors block replication and assembly
are all viral infections treatable?
no
