Andy Watts - Basic principles of drug design 3 - Neuraminidase Inhibitors Flashcards
What type of drug design was used in the discovery of Neuraminidase inhibitors?
- Structure based rational drug design
What is the function of neuraminidase?
- Releases new virons from infected cells by hydrolysing interactions between HA on the virus and sialyic acid conjugates on host cell membrane so they can reinfect
- Degrades protective mucus layer covering epithelial host cells allowing the virus to reach the surface of epithelial cells
What is the function of haemagglutamin?
- Attaches flu virus to cell surface
Why does resistance occur in the flu virus?
- Flu virus varies amino acids present in NA and HA antigens
In the chronology of Neuraminidase inhibitor design, what first took place?
- What happened?
- Random screening for NA inhibitors?
- No success in screening compounds with potential for NA inhibition
What happened after random screening of compounds took place?
- Decided use mechanism based design
- Started designing a mechanism based transition state inhibitor
- Managed to isolate the enzyme and study it’s crystal structure by X-ray crystallography and molecular modelling
What did isolation and investigations into the NA enzyme reveal?
- That the enzymes active site is made up of 18 AAs which remain constant between each version of NA
- Active site different to comparable mammalian enzymes so any inhibitor designed may be selective antivirals
When the enzyme was crystallised with sialic acid bound to its active site and the structure determined by X-ray crystallography, what happened next?
- A molecular model of the complex was created resembling the crystal structure
- Showed that carboxylate ion of sialic acid was bound to active site through H bonds and ionic interactions
- Also interactions with hydrophobic pocket
- Developed a proposed hydrolysis mechanism of action for release of virion
What did the transition state intermediate possess that sialic acid analogues would’ve benefitted from?
Why would they benefit from this?
What was a disadvantage of this?
How was this disadvantage overcome?
- A double bond between positions C2 and C3
- Caused the same trigonal geometry at C2 as seen in the transition intermediate
- OH bond at C2 was lost resulting in lower H bonds with active site
- The loss of H bond interactions was made up by the fact that the inhibitor did not need to distort from chair shape in order to bind - saving energy
What was done after the addition of a double bond to sialic acid analogues between carbon 2 and 3?
- Molecular modelling software was used to see most favourable conformation for H bond interactions
- Found that binding region that the OH group in the 4 position usually interacts with can also interact with NH2 or guanidinium ions at C4
- Molecules with these groups were modelled in the active site to see if there was room for the groups
What were the results of modelling the sialic acid analogues with NH2 group in place of the OH at position 4?
- Adding the NH2 group made the molecule more potent
- More selective for viral enzyme over mammalian and bacterial
- MORE BASIC GROUP HAS BETTER BINDING
What were the results of modelling the sialic acid analogues with larger guanidinium group in place of the OH at position 4?
What drug is this?
- Adding a larger NH-=NH2-NH2 group (guanidinium)
found that this increased H bonding even more AND VAN DER WAALS - 100 fold increase in activity
- MORE POTENT
- Large group expelled water molecule from binding pocket creating a favourable entropic effect!
ZANAMIVIR!
What is an issue with Zanamivir?
- Polar nature means poor oral bioavailability
- Has to be administered by inhalation
- The drug is excreted unchanged by the kidney which is bad for resistance as there will be more Zanamivir in the environment
How was Tamiflu synthesised?
Step 1
- Based on the structure of Zanamivir
- Dihydropyan oxygen has no important role to play in binding to active site of NA
- Decided to remove it
- This way, the removal of polar oxygen means increased hydrophobicity = increased BA!
How was Tamiflu synthesised?
Step 2
What does this suggest?
- Analoge created with double bond moved to replicate transition state of reaction
- Transition state mimic so expected to bind more strongly and be more potent!
- That the conformation of the ring is crucial for inhibitor activity as the substituents are the same after O removal but just analogue where double bond moved to replicate transition state
