Mechanism of Antivirals Flashcards
Why do we need anti-virals ?
Quick killers e.g. influenza; ebola; MERS; SARS
Slowly, progressive chronic disease leading to cancer
hepatitis B [350,000,000 carriers]
hepatitis C [200,000,000 carriers]
human papilloma viruses
[cervical cancer, second commonest cancer in women]
Human immunodeficiency virus (HIV)
[40 million infected]
Acute imflammatory e.g. herpes
Use of anti-virals
Treatment of acute infection
Influenza ; Chickenpox; herpes infections -(aciclovir)
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. in transplantation
CMV (ganciclovir, foscarnet)
Principles of Anti-Virals
as Therapeutic Agents - selective toxicity
Selective Toxicity
Due to the differences in structure and metabolic pathways between host and pathogen
Harm microorganisms, not the host
Target in microbe, not host (if possible)
Difficult for viruses (intracellular), fungi and parasites
Variation between microbes
Why is it so difficult to develop
effective, non-toxic anti-viral drugs ?
Viruses enter cells using cellular receptors which may have other functions
Viruses must replicate inside cells – obligate intracellular parasites
Viruses take over the host
cell replicative machinery
Virsues have high mutation rate - quasispecies
Anti-virals must be selective in their toxicity
i.e. exert their action only on infected cells
Some viruses are able to remain in a latent state e.g. herpes, HPV
Some viruses are able to integrate their genetic material into host cells
e.g. HIV
Considerations in developing safe anti-viral agents
Can stages of infection be targeted?
Cellular receptor may have other important function
Viral enzymes may be very similar to host
Blocking cellular enzyme may kill cell
Virus Life Cycle
- recognition
- attachment
- penetration
- uncoating
- transcription
- protein synthesis
- replication
- assembly
- lysis and release
Modes of action of selected
anti-virals
Preventing virus adsorption onto host cell
Preventing penetration
Preventing viral nucleic acid replication (nucleoside analogues)
Preventing maturation of virus
Preventing virus release
Discovery of virally encoded enzymes sufficiently different from human counterparts
e.g. and what do they act as
Discovery of virally encoded enzymes sufficiently different from human counterparts
e.g.
Thymidine kinase and HSV/VZV/CMV Protease of HIV Reverse transcriptase of HIV DNA polymerases Neuraminidase of influenza virus
Act as selective targets with minimal effect on host enzymes or processes
Herpes viruses include:
Herpes viruses include: Herpes simplex (HSV), Varicella Zoster Virus (VZV) Cytomegalovirus (CMV) Epstein-Barr virus (EBV)
Selective toxicity of acyclovir - explain what accounts for low toxicity
Requires 2 viral enzymes
= selectively activate ACV
= selectively inhibited
Accounts for low toxicity
Why is aciclovir so effective and safe?
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
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
Aciclovir action in Herpes simplex
Herpes simplex
Treatment of encephalitis
Treatment of genital infection
suppressive therapy for recurrent genital herpes
Aciclovir action in Varicella zoster virus
Varicella zoster virus
Treatment of chickenpox
Treatment of shingles
Prophylaxis of chickenpox
Aciclovir action in CMV/EBV
CMV/EBV
Prophylaxis only
Ganciclovir - use
Active for CMV
- reactivated infection or prophylaxis in organ transplant recipients
congenital infection in newborn
retinitis in immunosuppressed
Ganciclovir - compare with Aciclovir
Structurally similar to aciclovir
CMV does not encode TK but has UL97 kinase
Inhibits CMV DNA polymerase
Foscarnet - decribe action
Foscarnet:
Selectively inhibits viral DNA/RNA polymerases and RTs
No reactivation required
Binds pyrophosphate binding site – a structural mimic
Foscarnet - uses
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)
Cidofovir - describe action
Cidofovir
Chain terminator - targets DNA polymerase
Competes with dCTP
Monophosphate nucleotide analog
Prodrug – phosphorylated by cellular kinases to di-phosphate
drug active against CMV; but MUCH MORE nephrotoxic
Cidofovir - used for
Treatment of retinitis in HIV disease
Resistance to anti-virals in Herpes viruses - list and describe mech.
Two main mechanisms
Thymidine Kinase mutants
DNA polymerase mutants
If occurs in TK, drugs not needing phosphorylation are still effective (e.g. foscarnet, cidofovir)
If occurs in DNA polymerase, all drugs rendered less effective
VERY RARE in immunocompetent patients (low viral load)
Structural features of HIV - list
Envelope protein, gp120
with transmembrane gp41
ds RNA genome
Membrane-
associated
matrix protein
Gag 17
Viral
envelope
Nucleocapsid protein
Gag p24
reverse transcriptase
Name the 7 steps in the life cycle of HIV
Attachment with binding
of viral gp120 via CD4 and CCRX
- Virus assembly and
release by budding - reverse transcription of RNA
into dsDNA - Transcription of viral genes
- Integration into host
chromosome of
proviral DNA - Translation of viral mRNA
into viral proteins - maturation
Anti-reverse transcriptase inhibitors - examples
- Anti-reverse transcriptase inhibitors
nukes -nucleoside/nucleotide RT inhibitors
non-nukes -non-nucleotide RT inhibitors (allosteric)
Protease inhibitors - examples
- Protease inhibitors - multiple types for HIV
Integrase inhibitors - examples
- Integrase inhibitors – POL gene - protease, reverse transcriptase and integrase (IN)
with the 3´end encoding for IN (polynucleotidyl transferase)
Fusion inhibitors - examples
- Fusion inhibitors – gp120/41 - biomimetic lipopeptide
HAART - purpose
Treatment - HAART
Combination of drugs to avoid resistance
Nucleoside reverse transriptase (RT) inhibitors - nukes - describe action
Synthetic analogue of nucleoside thymidine –
when converted to tri-nucleotide by cell enzymes, it blocks RT by
- competing for natural nucleotide substrate dTTP
incorporation into DNA causing chain termination
Others ddI, ddC, d4T, and 3TC (2′,3′-dideoxy-3′-thiacytidine )
Nucleoside reverse transriptase (RT) inhibitors - non-nukes - describe action
Non-competitive inhibitor of HIV-1 RT
Synergistic with NRTI’s such as AZT because of different mechanism
Post-exposure prophylaxis for HIV
PEP – within 72 hours post exposure - take for 28 days.
2x NRTIs + integrase inhibitor
Pre-exposure prophylaxis for HIV
PrEP – pre-exposure - blocks transmission 2x NRTIs (Truvada) two tablets 2 – 24 hours before sex, one 24 hours after sex and a further tablet 48 hours after sex - called ‘on-demand’ or ‘event based’ dosing
NRTIs for HIV
2 x NRTIs = Combination of Nucleoside RTIs emtricitabine (guanosine analog) \+ tenofovir (adenosine analog)
Resistance to anti-virals - describe cause
Use of single agents leads to rapid development of resistance
The drug binding site is altered in structure by as few as one amino acid substitution
Mutation rate - high
Viral load – high
→ resistance
Resistance to anti-virals - describe action
Selection pressure and mutation frequency
Increased mutation rate seen in HIV.
They form a quasispecies within an individual patient:-
A viral swarm
Why do we use HAART for HIV
The error rate in copying viral genome by reverse transcriptase enzyme is 1 base per 10 4-5 incorporations; lacks proof reading capacity. So, for HIV with 10 9-10 viruses produced every day, ALL possible viral variants would be produced
Hence use of combinations of antivirals
e.g. HAART
Amantadine - describe use
Amantadine
Inhibit virus uncoating by blocking the influenza encoded M2 protein when inside cells and assembly of haemagglutinin
Now rarely used
Zanamivir and Oseltamivir (Tamiflu) - describe action
Zanamivir and Oseltamivir (Tamiflu)
Inhibits virus release from infected cells via inhibition of neuraminidase
Oseltamivir –oral
Zanamivir- inhaled or IV - less likely for resistance to develop
Anti’flu agents - Relenza - (zanamivir) and Tamiflu - (oseltamivir) = describe action
target and inhibit NA at highly conserved site (reduce chances of resistance via mutation)
prevent release of sialic acid residues from the cell receptor
preventing virus budding and release and spread to adjacent cells
Neuraminidase inhibitors
Influenza resistance to anti-virals - requires what
Resistance sometimes only requires a single amino acid change - seen recently with swine flu (H1N1) and Tamiflu (oseltamivir)
Influenza resistance to anti-virals - describe example
Point mutation (H275Y; tyrosine replacing histidine)
Seen in immunocompromised patients; shed virus for weeks/months
Likely to be selected from among quasispcies during treatment
Transmissible and virulent
Remains sensitive to zanamivir;
Occupational Infection
Hazards - cause
Exposure prone incidents
Sharps, Splashes and blood-born viruses
Occupational Infection
Hazards - prevention and management
Prevention - Universal Precautions
Management - Emergency Management of exposure prone incidents
Post-exposure prophylaxis for Hep B - action
Hep B
specific Hep B immunoglobulin (passive immunity)
+ vaccination
within 48 hours (HBV treatment includes antivirals 3TC/NRTIs)
Post-exposure prophylaxis for Hep C - action and +ves
Hep C
interferon-γ + ribavarin (anti-viral) for 6 months
within first 2 months of exposure
90% cure rate - now direct acting antivirals
Post-exposure prophylaxis for Hep C - action
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) - transmission
transmitted via blood – infectious (mother to baby)
Hepatitis C virus (HCV) - amount of RNA
9.6 Kb RNA virus, enveloped
Hepatitis C virus (HCV) - treatment
long incubation – 1 - 6 months
vaccination NOT available
early treatment facilitates resolution
Ribavirin - define and describe action
Ribavirin
nucleoside analogue
lock 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
Ribavirin - used for
Treat: RSV and HepC (in combination with pegylated interferon)
Hepatitis C virus (HCV)
and Direct-acting antivirals (DAAs) - use
acts to target specific steps in the HCV viral life cycle
shorten the length of therapy, minimize side effects, target the virus itself, improve sustained virologic response (SVR) rate.
structural and non-structural proteins - replicate and assemble new virions
What is essential for HCV replication and explain why this is useful
All the major HCV-induced enzymes - NS2-3 and NS3-4A proteases, NS3 helicase and NS5B RNA-dependent RNA polymerase (RdRp) are essential for HCV replication
and are potential drug targets.
DAA with different viral targets - effect
DAA with different viral targets, are synergistic in combinations
Many viral infections with no effective therapies - list
Many viral infections with no effective therapies e.g. rabies dengue Common cold viruses Ebola HPV Arbovirsues
