Antiviral Agents

Question 1

What is a virus?

Answer 1

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

Question 2

What is virus morphology?

Answer 2

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

Question 3

Why do RNA viruses and retroviruses have a high mutation rate?

Answer 3

They use their own polymerase to mutate. These lack the proof reading capacity

Question 4

Why are RNA virus limited in size?

Answer 4

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)

Question 5

DNA virus are bigger than RNA viruses, what is an implication of this?

Answer 5

They have plenty of room for accessory genes that can modify the hosts immune response

These genes are often lost in passage in culture

Question 6

Some viruses have segmented genomes, what is an implication of this?

Answer 6

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)

Question 7

What is the generic virus replication cycle?

Answer 7

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

Question 8

What is the replication cycle of HIV

Answer 8

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

Question 9

What is the cytopathic effect?

Answer 9

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

Question 10

What forms in cell monolates affected with virus particles?

Answer 10

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

Question 11

Some viruses (eg HIV) don’t form plaques, what do they form instead?

Answer 11

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

Question 12

What are some methods of viral diagnosis?

Answer 12

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)

Question 13

How can viruses be manipulated?

Answer 13

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

Question 14

What is the rough size range of viruses?

Answer 14

10nm to 1 micro m

Question 15

Why are there so few effective antivirals?

Answer 15

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

Question 16

What are possible targets for antiviral drugs?

Answer 16

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)

Question 17

What is the most famous and best antiviral?

Answer 17

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

Question 18

What are some examples of nucleoside analogue drugs?

Answer 18

Acyclovir

Zidovudine (AZT)
Ganciclovir
Ribavirin

Question 19

Why is acyclovir so effective?

Answer 19

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

Question 20

What is remdesivir?

Answer 20

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

Question 21

What is amantidine?

Answer 21

(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

Question 22

What are neuraminidase inhibitors?

Answer 22

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

Question 23

What is hepatitis C and what are it’s antiviral treatments?

Answer 23

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

Question 24

What are some treatments for HIV?

Answer 24

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

Question 25

What are biologicals?

Answer 25

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%

Question 26

Where do monoclonal antibodies come from?

Answer 26

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