Mechanism of Antivirals Flashcards
Use of antivirals
Treatment of Acute Infection
- Influenza, Chickenpox, Shingles, Herpes infections
Treatment of Chronic Infection
- HCV, HBV, HIV
Post-exposure prophylaxis and preventing infection
- HIV (PEP)
Pre-exposure prophylaxis
- HIV (PrEP)
Prophylaxis for reactivated infection e.g. in transplantation (immunosuppression)
- Cytomegalovirus (ganciclovir, foscarnet)
What is our aim when developing antiviral drugs?
SELECTIVE TOXICITY:
- drug has selective action against one component and not another
- target in microbe, not host (is possible)
Why is it difficult to develop effective, non-toxic antiviral drugs?
- Viruses enter cells using cellular receptors which may have other functions, producing side effects
- Viruses must replicate inside cells, meaning it is very difficult to identify what is unique about the virus structure and/or viral enzymes to target
- Virus takes over the host cell replicative machinery
- Viruses have high mutation rate (quasispecies), and therefore viruses can escape the effects of antiviral drugs
- Some viruses are able to remain in a latent state e.g. herpes, HPV. If they are latent they are not replicating or expressing any proteins and so the drug won’t have any effects
- Some viruses are able to integrate their genetic material into host cells (e.g. HIV) and therefore it is not possible to remove that integrated genome
Basic virus “life cycle”
· Virus comes in and attaches to the membrane
· It is internalised either by endocytosis or by membrane fusion
· Once the virus is inside, it has to uncoat and release its genome so the genome can replicate itself
· Genome also has to produce mRNAs via transcription, which are translated at the ribosomes of the cell to produce viral proteins
· Virus will then reassemble, either by budding through a membrane, or viruses that assemble completely inside the cell and escape through cell lysis
Mode 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 are examples of selective toxicity viral targets?
virally encoded enzymes sufficiently different from humans counterparts, and are drug targets:
- Thymidine Kinase and HSV/VZV/CMV (Herpes infections)
- Protease of HIV
- Reverse transcriptase of HIV
- DNA polymerases
- Neuraminidase of influenza virus
These act as selective targets, with minimal effect on host enzymes or processes
Anti-herpes virus agents
Aciclovir
- HSV & VZV treatment/prophylaxis
- CMV & EBV prophylaxis (not treatment)
Ganciclovir
-CMV treatment
Foscarnet
-CMV treatment
Cidofovir
-CMV treatment
Action of Aciclovir
Acycloguanosine analogue (GTP analogue) without a 3’OH group, meaning it is a chain terminator
-competes with GTP (competitive inhibitor) for viral DNA polymerase, and once inserted into the DNA, it inhibits polymerisation, stopping viral genome synthesis
Activation of aciclovir
Aciclovir is a pro-drug and has no activity on its own:
1) Phosphorylated by a viral thymidine kinase.
2) Once it is phosphorylated, it remains stable within the cell
3) Then it gets di-phosphorylated and tri-phosphorylated by cellular kinases
4) Only once aciclovir is tri-phosphorylated does it become an active drug.
What are the key features of aciclovir which give it selective toxicity?
Selective Activation
-activated only inside cells that are infected because it needs to be mono-phosphorylated by a viral thymidine kinase
-HSV thymidine kinase has 100x the affinity for aciclovir compared with cellular phosphokinases
Selective Inhibition
-aciclovir tri-phosphate has selective toxicity against the viral DNA polymerase and not host DNA polymerase
-30x more affinity for HSV DNA polymerase compared with cellular DNA polymerase
Active Drug= highly polar
-aciclovir triphosphate is a highly polar compound difficult to leave or enter cells, but aciclovir is easily taken into cells prior to phosphorylation. Therefore, it accumulates in high levels inside the virally infected cell→good
Uses 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
Which herpesvirus does not respond well to aciclovir?
Cytomegalovirus (CMV)
Why does CMV not respond well to aciclovir?
Cytomegalovirus (CMV) does not encode thymidine kinase, therefore it can’t activate aciclovir very effectively
Which anti-viral is used for CMV treatment?
Ganciclovir
Action of Ganciclovir
Ganciclovir is structurally similar to aciclovir
Ganciclovir is phosphorylated by viral UL97 Kinase encoded in the CMV genome. Then, just like aciclovir, it is di-phosphorylated and tri-phosphorylated by cellular kinases and has the same effects of aciclovir. It competes for the natural substrate (GTP) for viral DNA polymerase and blocks the ability of the virus to make its own DNA.
Uses of Ganciclovir
Treatment of CMV:
- Reactivated infection or prophylaxis in organ transplant recipients
- Congenital infection in new-born
- Retinitis in immunosuppressed
Action of Foscarnet
a structural mimic of pyrophosphate which selectively inhibits viral DNA/RNA polymerases by binding to pyrophosphate binding site in viral DNA/RNA polymerases and blocking the action of viral polymerases
- No reactivation required (not a prodrug)
Uses of Foscarnet
CMV Treatment in immunocompromised patients
-e.g. pneumonia in solid organ and bone marrow transplants
May also be used because of ganciclovir resistance
Action of Cidofovir
Monophosphate nucleotide analogue (with no 3’OH group), meaning it is a chain terminator:
-competes with dCTP for viral DNA polymerase and inhibits further polymerisation and synthesis of viral genome
Prodrug- phosphorylated by cellular kinases to di-phosphate
Uses of Cidofovir
Treatment of CMV
-but much more nephrotoxic
Treatment of HIV Retinitis
Lifecycle of HIV
1) Attachment with binding of viral gp120 via CD4 and CCR5X and internalisation
2) Uncoating of the virus, releasing viral genome for reverse transcription of RNA into dsDNA
3) Integration into host chromosome of proviral DNA
4) Transcription of viral genes generating mRNA
5) Translation of viral mRNA into viral proteins at ribosomes and some mRNA encodes the new viral genome
6) Virus assembly and release by budding
7) Post-release maturation of the virus
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 (H275Y; tyrosine replacing histidine)
· Seen in immunocompromised patients; shed virus for weeks/months
· Likely to be selected from among quasispecies during treatment
· Transmissible and virulent
· Remains sensitive to zanamivir
Action of Zanamivir (Relenza) & Oseltamivir (Tamiflu)
· Inhibits virus release from infected cells via inhibition of neuraminidase to prevent transmission of the virus
Neuraminidase is an enzyme on the surface of the flu which cleaves sialic acid residues on receptor so that the virus can be released. Hence, a neuraminidase inhibitor blocks that ability of neuraminidase to cleave off that receptor, and so the virus can’t be released from infected cell
No new virus particles are being produced, and the infected cell will die because it is going to build up virus particles which can’t be released
Uses of Ribavirin
Treat:
- Hepatitis C (HCV)
- Respiratory Syncytial Virus (RSV)
Types of Direct Acting Anti-virals
NS3/4 Protease Inhibitors
- Block protease action on polyproteins e.g. Pol, hence its subsequent viral enzymes are not produced
NS5B Polymerase Inhibitors
- Nucleoside (nukes) or non-nucleotide inhibitors (non-nukes) of polymerases
NS5A Inhibitors
- Block replication and complex assembly formation by blocking the enzymes responsible for budding, maturation and release
