Anti-Viral Agents

Question 1

How big are viruses?

Answer 1

Hundreds of nanometers

Question 2

How do virus replicate?

Answer 2

infect something else in order to replicate

Question 3

why is it so difficult to develop drugs which selectively act against viral infections?

Answer 3

1. The antiviral drugs we use have to be highly specific for their purpose
2. Viruses evolve very quickly, and they can evolve to become resistant to the drugs that we use against them.
3. So viruses are obligate intracellular parasites
4. On the outside, they’re completely inert, they don’t do anything
5. When they get inside your cells, they carry in their genome, and their genome can be made of either DNA or RNA. 6.Then once inside the cell, the virus replicates its genome
6. To replicate it has to co-opt cellular machinery which exist inside the cells

Question 4

What do viruses do?

Answer 4

-Need to find unique thing to virus as opposed to parts of the virus which have been borrowed from the cell

Question 5

What is the replication cycle like?

Answer 5

1. The virus is on the outside
2. Virus attaches to the cell and gets in, and then whatever was holding the virus together in the environment falls apart so that the important viral genome is exposed to all the things inside the cell that it’s going to need to borrow, because the virus outside the cell is so small and so simple that it can’t carry everything it needs with it
3. This cell here is just full of all kinds of machinery; ribosomes, other enzymes, vesicles, membranes
4. Virus doesn’t need to make it’s own ones and carry its own ones around, it can just get inside the cell and borrow the ones from the cell
5. Virus is going to replicate itself, it’s going to transcribe messenger RNA which encode it’s proteins, and then later on all of those replicated genomes and new capsids, the coats of the viruses will come together and assemble, and then the virus will leave the cell and go on and do it all again.

Question 6

What happens in real life?

Answer 6

• In real life, there’s more like one going in and about 1,000 going out. So you can see how rapidly the virus expand it’s numbers.
• Of course, if the replication that goes on the inside the cell is somewhat error prone, as to many viruses it is particularly the RNA viruses, where they make mistake as they copy their genomes, then many different versions of the virus can come out from one original one going in
• That’s where evolution comes in because as soon as you have diversity, you could exploit that diversity and put selection on it and then new evolved versions of viruses will arise.

Question 7

How can you control viruses?

Answer 7

Prophylaxis (doing something before it happens) e.g. vaccine condoms

Question 8

When would you give prophylactic drug?

Answer 8

-use drugs prophylactically as well if we know there is an outbreak, we might want to give a drug that would protect against the virus to everybody and then your host cells are full of the antiviral drug before the virus even hits you, that is the very most efficient way of combating a virus

Question 9

What is the most common way to treat infectious disease?

Answer 9

Therapeutics

Question 10

What are antiviral drugs?

Answer 10

-all highly specific for the individual viruses that they treat + very high selectvity, need ton know if ebola or influenza so need to use antivirals and diagnostics

Question 11

What is most important with vaccines?

Answer 11

Safety

Question 12

What screens can you do?

Answer 12

You can setup screens to look for chemicals that are going to work for you or you can be much clever and more hypothesis driven and do what we call rational design for the drugs

Question 13

Are antibiotics selective?

Answer 13

• Antibiotics are moderately selective because we are going to take them in our own bodies, so we don’t want to be taking a drug that’s going to compromise our own host cell
• Bacteria will be affected as different mechanisms but we won’t

Question 14

Why can’t you target ribosomes?

Answer 14

-They are using our machinery so using our ribosomes to translate their mRNAs otherwise every cell in our body is going to stop translating

Question 15

What do you usually target in drug development?

Answer 15

• Enzymes are easier to target in one way or another
• Very often a way to develop a drug that targets an enzyme, is to make what’s known as a substrate analog, which is a chemical which looks like the real substrate to the enzyme but it’s got chemical modifications on it
• Enzyme get socked

Question 16

What is the most obvious substrate analogs to use to target cancer or antivirals?

Answer 16

• Nucleoside analogs
• The things that look like the building blocks of the DNA or the RNA, because all viruses are made of a genome of DNA or RNA

Question 17

What do you need to be careful about with nucleoside analogs?

Answer 17

• our own bodies are copying DNA and RNA all the time, and we don’t want to clog up our own replication machinery, or our own transcription machinery
• So they’re going be cleverly modified so that they target the virus’s ability to replicate but don’t affect our health

Question 18

What is rational drug design?

Answer 18

• If we don’t use nucleoside analogue but we target different enzymes of viruses, and viruses carry several enzymes with them
• Do what we call rational drug design, particularly if we know the structure of the enzyme
• So if we’ve got crystallography on these days Cryo-EM pictures of the three-dimensional structure of the enzyme and we understand intimately how it works
• We can design small molecules that fit into pockets of the enzyme and stop it from working. So we’re going to do a bit of compare and contrast

Question 19

What the very best antiviral agent that we have?

Answer 19

acyclovir

Question 20

What is acyclovir?

Answer 20

Acyclovir is a nucleoside analog

Question 21

What are some different nucleoside analogs that we use as antiviral agent?

Answer 21

• gancyclovir
• zidovudine is quite famous, it’s also known as AZT and it was the very first HIV antiviral to be developed
• dideoxyns which also lack these hydroxy groups

Question 22

How does acyclovir work?

Answer 22

• A chain terminator because it doesn’t have that three prime hydroxy group.
• If acyvlovir gets incorporated into a growing chain of DNA as you copy the viral genome into its copy, the acyclovir, because it lacks this three prime hydroxy group which would normally be where the next base gets attached is the end.

Question 23

Why does acyclovir get attached?

Answer 23

1. There’s no other bases that can be attached there because there’s no chemical hydroxy group for that chemical reaction to take place
2. So the acyclovir is incorporated into the growing strand of DNA as the virus replicates, and then that’s it
3. That’s the end. No more replication, the genome is much shorter than it should have been, the virus doesn’t decode what it needs to do, it doesn’t replicate any further.

Question 24

Why is acyclovir a prodrug?

Answer 24

• Why doesn’t that clog up all the DNA in your cells? As your cell’s replication divide
• How come you can take this drug and you don’t kill yourself? Very, very, beautiful example of specificity, acyclovir is given as what we call a prodrug.

Question 25

In what form is acyclovir given?

Answer 25

Unphosphorylated form

Question 26

What form of a nucleotide is incorporated?

Answer 26

triphosphate form of a nucleotide which is incorporated in the growing strand

Question 27

When will acyclovir be incorporated in?

Answer 27

-It’s not going to be incorporated into the strand until it becomes tri-phosphorylated

Question 28

What happens inside cells?

Answer 28

• Inside the cells, there are enzymes which tri-phosphorylate nucleotides, that’s how our own DNA bio-generation works
• But it just so happens that the host cell enzymes that normally put the first phosphate on to create the mono phosphate don’t work on acyclovir

Question 29

What enzyme will the virus encode?

Answer 29

However, there is an enzyme that the virus itself encodes called thymidine kinase which can put the first phosphate onto acyclovir