Antiviral Agents Flashcards
What is a virus?
Infectious obligate intracellular parasites
Genome comprised of DNA or RNA
Relies on the machinery of the host cell To synthesise the viral particles (Virions) and the proteins that encapsulate them
What is virus morphology?
The shape
Some are symmetrical, some are not
Some have an envelope (derived from the host membrane) some do not
Some are pleiomorphic (undistinguished shape) some have typical shapes
Why do RNA viruses and retroviruses have a high mutation rate?
They use their own polymerase to mutate. These lack the proof reading capacity
Why are RNA virus limited in size?
Because RNA is much more unstable than DNA which limits its size
These viruses often have complex coding strategies to make more proteins than expected from their RNA genome (eg overlapping reading frames)
DNA virus are bigger than RNA viruses, what is an implication of this?
They have plenty of room for accessory genes that can modify the hosts immune response
These genes are often lost in passage in culture
Some viruses have segmented genomes, what is an implication of this?
Allow an easy additional form of recombination known as reassortment, but also impose more difficult packaging strategies
When multiple virus infect a host, they can reassort their segmented genomes (such as in influenza)
What is the generic virus replication cycle?
Virus begins outside the cell (comprised of its genome and its capsid)
Virus attaches to cell using the virus receptor
Once inside the virus capsid falls away, exposing the genome
The genome is converted into mRNA (if necessary) which then undergoes translation
The viral genome uses polymerases (it’s own or the host cells) to replicate its genome to make many more of these proteins and genomes to make new virus particles
The new viruses then leave the cell and infect other cells
What is the replication cycle of HIV
HIV virion has GP120 on its capsid. This attaches to CD4 receptors on host cells
Virus and host cell membranes fuse and viral RNA, reverse transcriptase and integrase and other viral proteins enter the cell.
Viral DNA is formed by reverse transcription
Viral DNA is transported across the nucleus and integrated into the host DNA using integrase
Undergoes transcription woth the genome
New viral RNA is used as genomic RNA and translated to make viral proteins
These proteins move to the surface of the cell and a new immature HIV forms
The virus matures by protease releasing individual HIV proteins
The virus leaves the host cell by budding
What is the cytopathic effect?
The death of the host cell, caused by the virus
The cell triggers apoptosis and is Lysed
This can be due to shut down of the host protein synthesis or accumulation of viral proteins
What forms in cell monolates affected with virus particles?
Plaques
These are clumps of dead cells that originate from a single virus particle. They stain clear as opposed to the living cells which have colour
The plaque assay can be used to quantify how many virus particles are present on a mono layer
Some viruses (eg HIV) don’t form plaques, what do they form instead?
Syncytia
The viruses have surface proteins that can fuse at neutral pH and often fuse all the surrounding cells together
Syncytia can also be assayed time quantify the number of virus particles
What are some methods of viral diagnosis?
Detecting viral genome: PCR, RT-PCR
Detecting viral antigen: IFA, ELISA
Detecting virus particles: EM (electron microscopy), HA
Detecting virus cytopathic effect in cultured cells (virus isolation)
Detecting antibodies to virus (serology)
How can viruses be manipulated?
Virus genomes are so small they can be synthesised in a lab
When introduced into permissive cells they direct synthesis of all their components and new viruses are made de novo
This allows reverse genetics, the creation of viruses at will with engineered mutations in their genomes
What is the rough size range of viruses?
10nm to 1 micro m
Why are there so few effective antivirals?
Viruses are so simple and rely on host cells
Selectivity and specificity must be used to find processes and characteristics unique to the virus particles
What are possible targets for antiviral drugs?
Viral enzymes - increased understanding of the structure of viral components and enzymes can lead to rational drug design
Nucleoside analogues - inhibit or interfere with nucleus acid replication but need to achieve some element of specificity for the viral polymerase (however the human genome may also be suceptible to these nucleoside analogues)
What is the most famous and best antiviral?
Avyclovir
Nucleoside analogue
Looks almost identical to guanosine however it is missing the 3’ hydroxy group. Therefore it acts as a chain terminator as a phosphodiester bond can’t be formed to elongate the chain
What are some examples of nucleoside analogue drugs?
Acyclovir
Zidovudine (AZT)
Ganciclovir
Ribavirin
Why is acyclovir so effective?
It is administered in the form of a pro drug, in the unphosphorylated form
All nucleosides have to be tri phosphorylated before they can be incorporated into the double helix
The addition of the first phosphate to acyclovir to form acyclovir monophosphate is done using the enzyme viral thymidine kinase.
This enzyme is ONLY found in virus particles of the herpes virus family
(Addition of subsequent phosphates can be carried out by cellular enzymes)
ACVTP has a higher affinity for the viral DNA than host DNA
Resistance to acyclovir is rare
What is remdesivir?
A nucleoside analogue of adenosine
Affects the way the strand of DNA twists, causing chain termination 3 nucleotides downstream of incorporation
Originally developed for hepatitis C
Tested against Ebola but doesn’t really work
One of the only agents that works agains SARS covi 2
What is amantidine?
(Has a sister drug rimantidine)
They are cyclic amines woth bully cage like structures
They are byproducts of petroleum refinement
Only effective against influenza A virus.
It works by inhibiting the flow of protons through the M2 channel protein in an endosome. Usually this flow of proteins is what triggers the endosome to release the components of the virus into the cell
However most viruses now are resistant to this. As a single mutation in the structure of M2 can prevent amantidine from binding
What are neuraminidase inhibitors?
Once the crystal structure of the tetramer neuraminidase was discovered, rational drug design could be used.
Scialic acid (on our cells) is usually the substrate for NA however relenza and tamiflu act as substrate analogues that bind and lock into the pocket of viral NA
NA cleaves Scialic acid, which stops HA binding to it and allows the new virion to detach from the host cell and circulate and infect. NA inhibitors prevent the release of new virus particles
What is hepatitis C and what are it’s antiviral treatments?
HCV is a hepatotoxic flavivirus that spread in the 1970s before blood produc screening was implemented
Bcos of that 170 mil people are infected chronically and 4% of these will get a hepatocellular carcinoma
For at least 20 years people relied on interferon treatment with ribavirin, but this was horrible and not very effective
Due to intensive research, we now have some directly acting drugs to treat Hep C that can lead to a total cure. However they mainly treat strains in the western world
What are some treatments for HIV?
There are many that target things such as integrase and reverse transcriptase and prevent HIV from entering host cells
But due to the retrovirus nature of HIV there will always be cells that act as a reservoir and therefore none of these drugs provide a cure
Only two people in the world have been cured of HIV. These both had leukaemia and needed a bone marrow transplant. They both got transplants from people who had a mutation that meant they were naturally resistant to HIV. This is not feasae for the whole population
What are biologicals?
Passive immunotherapy.
Antibodies taken from recovered individuals, or produced from immortalised B cells
Similar to a vaccine, but your own immune system doesn’t make these antibodies, they are simply given
The most effective is palivizumab agains RSV in infants. Reduced hospitalisation by 55%
Where do monoclonal antibodies come from?
Used to be mouse antibodies, these were then altered so they could work in humans.
Now using crispr technology the nice can produce human antibodies straight away
