Anti-viral agents Flashcards
How big is HIV?
10 000 nucleotides, 10 genes
What is a virus?
Obligate intracellular parasite
Has a genome that has DNA or RNA
What is a viron?
Virus particles
They’re very small- 10nm- 200nm
How is a virus structured?
Has symmetrical protein capsule
Can be non-enveloped or enveloped
What is the central dogma?
DNA -> RNA -> Protein
Reverse transcriptase changes RNA to DNA
Some viruses carry a negative sense which is complementary to mRNA
How does RNA and DNA compare
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
How does a virus replicate?
- Virus attaches to host cell surface via protein/ glycoprotein
- Virus enters cell and capsid falls away
- Viral genome might be converted directly to mRNA, translated by ribosome to protein
- 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
- Virus leaves host by budding and host cell dies
How does HIV replicate?
- Spike protein Gp120 on viron binds to cell surface receptor CD4
- Virus is brought closer to the cell surface where it engages with second coreceptor CCR5 or CXCR4
- Virus membrane and host cell membrane fuse and release core of virus which undergoes reverse transcriptase of genome into double strand DNA
- Viral enzyme integrase, integrates and splices viral DNA into host’s DNA
- Viral genome is transcribed by hosts transcription enzyme into mRNA and translated to form new viral protein
- Proteins come together to form new virus
- Virus buds out of host cell
How does influenza replicate?
- Virus particle latches onto sugars, glycoproteins, and glycolipids on cell surface
- Virus enters cell by endocytosis
- Viral particle fuses its envelope with endosomal lipids releasing its segments of negative sense RNA
- RNA segments enter nucleus where RNA dependent RNA polymerase copies them into mRNA and replicates them into new genomes
- mRNA leaves nucleus and picked up. by host cell ribosome and made into proteins
- New genomes and proteins assembled to make new virus and buds from host cell
What is syncytia?
Viruses with surface proteins that can fuse at neutral pH often fuse cells together
How can we diagnose a virus?
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
What different parts of the HIV cycle can be targeted by anti-virals?
- Inhibit viral entry- fusion inhibitors which stop fusion of virus to the cell membrane
- Reverse transcriptase inhibitors
- Integrase inhibitor- stops HIV from integrating itself into our own DNA
- Protease (normally cleave peptide bonds) inhibitors to stop protein being cut into small pieces during assembly- stops maturation of viral particles
Why is combination therapy effective?
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
What drugs are used against influenza?
Amantadine/ Rimantadine
Relenza + Tamiflu
Baloxavir
How does amantadine/ rimantadine work?
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
How does tamiflu and relenza work?
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
Why is tamiflu preferred over relenza?
It’s offered as an oral pill
How does baloxavir work?
Inhibits polymerase acid endonuclease
A single point mutation in PA I38T makes the virus resistant
It treats influenza A and B
What is acyclovir and how does it work?
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
Why doesn’t acyclovir damage human cells?
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
Why is acyclovir resistance rare?
When virus compromises its ability to separate thymidine kinase it becomes less fit so is not a good selection pressure
What is the name of an anti-viral drug which targets viral enzymes?
Substrate analogue- looks like real substrate to enzyme but has chemical modifications
What’s the difference between prophylaxis and therapy?
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
Why do antivirals need higher selectivity than antibiotics?
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
