Anti-viral agents

Question 1

How big is HIV?

Answer 1

10 000 nucleotides, 10 genes

Question 2

What is a virus?

Answer 2

Obligate intracellular parasite

Has a genome that has DNA or RNA

Question 3

What is a viron?

Answer 3

Virus particles

They’re very small- 10nm- 200nm

Question 4

How is a virus structured?

Answer 4

Has symmetrical protein capsule

Can be non-enveloped or enveloped

Question 5

What is the central dogma?

Answer 5

DNA -> RNA -> Protein
Reverse transcriptase changes RNA to DNA
Some viruses carry a negative sense which is complementary to mRNA

Question 6

How does RNA and DNA compare

Answer 6

RNA viruses and retroviruses use their own polymerase to replicate. These lack proofreading so have a high mutation rate

RNA viral genomes are limited. Largest RNA is coronavirus RNA which often use complex coding strategies to make more proteins

DNA is long and has space for accessory genes which modify host immune response

Question 7

How does a virus replicate?

Answer 7

1. Virus attaches to host cell surface via protein/ glycoprotein
2. Virus enters cell and capsid falls away
3. Viral genome might be converted directly to mRNA, translated by ribosome to protein
4. Viral genome can use its own polymerase or polymerase of host cell to replicate itself and make all different proteins and genomes to form new viruses
5. Virus leaves host by budding and host cell dies

Question 8

How does HIV replicate?

Answer 8

1. Spike protein Gp120 on viron binds to cell surface receptor CD4
2. Virus is brought closer to the cell surface where it engages with second coreceptor CCR5 or CXCR4
3. Virus membrane and host cell membrane fuse and release core of virus which undergoes reverse transcriptase of genome into double strand DNA
4. Viral enzyme integrase, integrates and splices viral DNA into host’s DNA
5. Viral genome is transcribed by hosts transcription enzyme into mRNA and translated to form new viral protein
6. Proteins come together to form new virus
7. Virus buds out of host cell

Question 9

How does influenza replicate?

Answer 9

1. Virus particle latches onto sugars, glycoproteins, and glycolipids on cell surface
2. Virus enters cell by endocytosis
3. Viral particle fuses its envelope with endosomal lipids releasing its segments of negative sense RNA
4. RNA segments enter nucleus where RNA dependent RNA polymerase copies them into mRNA and replicates them into new genomes
5. mRNA leaves nucleus and picked up. by host cell ribosome and made into proteins
6. New genomes and proteins assembled to make new virus and buds from host cell

Question 10

What is syncytia?

Answer 10

Viruses with surface proteins that can fuse at neutral pH often fuse cells together

Question 11

How can we diagnose a virus?

Answer 11

Detecting viral genome: PCR, RT-PCR
Detecting viral antigen: IFA, ELISA
Detecting virus particles: EM, HA
Detecting virus cytopathic effect in cultured cells
Detecting antibodies to virus

Question 12

What different parts of the HIV cycle can be targeted by anti-virals?

Answer 12

1. Inhibit viral entry- fusion inhibitors which stop fusion of virus to the cell membrane
2. Reverse transcriptase inhibitors
3. Integrase inhibitor- stops HIV from integrating itself into our own DNA
4. Protease (normally cleave peptide bonds) inhibitors to stop protein being cut into small pieces during assembly- stops maturation of viral particles

Question 13

Why is combination therapy effective?

Answer 13

Extremely unlikely that one viral genome will accumulate all the correct mutations to make it resistant to multiple antivirals acting on it at the same time

Question 14

What drugs are used against influenza?

Answer 14

Amantadine/ Rimantadine
Relenza + Tamiflu
Baloxavir

Question 15

How does amantadine/ rimantadine work?

Answer 15

They’re cyclic amines with bulky cage like structures
Byproducts of petroleum refinement
Active against influenza A only
Block replication of influenza:
-Sits in and blocks the tetrameric M2 proton channel so that protons can’t get through and unlock viral core by uncoating virus
-virus locked in endosomes and wont initiate infection
A single amino acid change in M2 can make the virus resistant
H3N2 subtype causes the vast majority of seasonal flu and all have a S31N mutation - makes amantadine useless

Question 16

How does tamiflu and relenza work?

Answer 16

It inhibits neuraminidase enzyme which normally removes sialic acid from infected cells which allows them to leave the host cell and migrate- NA inhibitors made to look like sialic acid

Without sialic acid cleavage virus is unable to leave the host and infect other cells

Relenza modifies the sialic acid so that it sticks more strongly than normal sialic acid therefore the enzyme cannot cleave it

Tamiflu blocks the enzyme but with a different chemistry

Question 17

Why is tamiflu preferred over relenza?

Answer 17

It’s offered as an oral pill

Question 18

How does baloxavir work?

Answer 18

Inhibits polymerase acid endonuclease
A single point mutation in PA I38T makes the virus resistant
It treats influenza A and B

Question 19

What is acyclovir and how does it work?

Answer 19

It’s a nucleoside analogue antiviral often used to treat coldsores (HSV-1) and chickenpox (varicella zoster)
It mimics guanosine but lacks the 3’ hydroxy group so acts as a chain terminator- prevents phosphodiester bond formation in viral genomic replication

Question 20

Why doesn’t acyclovir damage human cells?

Answer 20

It’s delivered as an unphosphorylated form
Its converted to acyclovir triphosphate only in HSV infected cells due to presence of thymidine kinase
Acyclovir triphosphate inhibits DNA polymerase and prevents viral DNA synthesis

Acyclovir triphosphate also has a high affinity for viral DNA polymerase

Question 21

Why is acyclovir resistance rare?

Answer 21

When virus compromises its ability to separate thymidine kinase it becomes less fit so is not a good selection pressure

Question 22

What is the name of an anti-viral drug which targets viral enzymes?

Answer 22

Substrate analogue- looks like real substrate to enzyme but has chemical modifications

Question 23

What’s the difference between prophylaxis and therapy?

Answer 23

Prophylaxis is the prevention of a disease before the aetiological agent is required through vaccination or giving drug before infection
Therapy is treating the disease after host has been infected

Question 24

Why do antivirals need higher selectivity than antibiotics?

Answer 24

Bacteria targeted by AB have large number of distinct differences to our cells e.g. 50S ribosome
Viruses however use host cell machinery for reproduction so we need to target the virus without harming the host