Lecture 12 - New targets in Chemo Flashcards
Fundamental principles of the outcomes of DNA damaging therapies
- If you can recover the damage by a repair mechanism from the drug then you have failed
- If you can inhibit the repair mechanisms you can get a better result
Example of a drug & target where there is direct inhibition of a repair enzyme
Enzyme targeted: MGMT
Drug: O6-Benzylguanine, patrin
Examples of targets of inhibitors of DNA control systems
- PARP-1
- Checkpoint kinase
- DNA dependent protein kinases
Example of PARP-1 inhibitors
- Olaparib
- rucaparib
- veliparib
- niraparib
Where do alkylating agents preferentially alkylate in DNA?
N7 position
What is another site which DNA alkylators target in DNA, other than the most common one?
O6-alkyl guanine
What removes O6-alkyl guanine in DNA?
MGMT enzyme
How can resistance to O6-alkylators be overcome and drugs for this?
- By MGMT depletion
- Thus making the DNA more susceptible to damage from the O6 lesions
- O6-Benzylguanine and Partin in clinical trials
What is the issue with depleting MGMT?
- Can cause myelosuppression
- lower doses of alkylating agents would be required
What is PAR?
The third nucleic acid in mammalian cell
- Poly(ADP-ribose)
Features of PAR
- Polyanionic polymer
- linear or branched
- built from ADP-ribose units derived from NAD+
- Usually build onto Glu side chains in target proteins
What substrate do PARPs use?
NAD+
What is PARP-1?
- Poly(ADP-ribose)polymerase-1
- Found in nuclei
- essential for initiating DNA repair
What is the reaction of PARP-1?
- PARP-1 uses NAD+
- Forms intermediate, then lone pair of electron on Oxygen forms a double bond
- Break the carbon bond
- Oxygen quenched by reaction with an alcohol
- Forms polymer chain
What is the structure of PARP-1?
N terminal –> C terminal
- DNA binding domain with 2 ZNF
- Auto modification domain containing GLUTAMATE with Nucleuar localisation signal and caspase-3 cleavage site
- NAD+ binding domain
Where does polymerisation occur in PARP-1?
at NAD+ binding domain
What role does PARP-1 play?
Repairing damaged DNA
Steps in repair of DNA by PARP-1
- SSB
- PARP-1 recruited to damage, forms PAR chain on histone near damage
- Histone removed from DNA
- Repair enzymes bought to damage
- PARP-1 adds PAR chains to itself, removal of PARP-1
- PARG snips PAR chains off PARP-1 and histone
- return back to repaired DNA with histone and PARP-1 back in place
- Inhibition of PARP-1 = no base repair
What is the cancer therapeutic application of inhibition of PARP-1?
- Radiotherapy, cytotoxic electrophilic drugs and inhibitors of topisomerase II cause DNA SSB
- If you can repair via PARP-1 then cells survive
- Inhibit PARP-1 to inhibit repair and potentiate these therapies
What is the other therapeutic applications of inhibition of PARP-1?
- Reduction of organ damage following reperfusion injury
Regarding drug design of inhibitors of PARP, what important obstacle was discovered which prompted new drug design of PARP-1 inhibitors
Full analogues of NAD+ such as TAD do not inhibit PARP activity as they are inacitve
When designing a drug to inhibit PARP-1, what features of simple analogues DONT make a difference when wanting to increase potency of the inhibitor?
- Polarity of 3-subsituent
- Hydrogen bonding
- Size
When designing a drug to inhibit PARP-1, what features of simple analogues are favoured to increase potency of the inhibitor?
- electron neutral or electron donating group in the ring (pushes the electron density towards)
- Side chain on 3rd position of ring (not 2nd or 4th)
- Amide group > sulphur group as side chain
- Control of steric clashes/conformation by building another ring/benzene ring to lock amide in place
- A bulkier subsitent
When designing a drug to inhibit PARP-1, what features of simple analogues are disfavoured to increase potency of the inhibitor?
- Electron withdrawing groups which pull electron denisity away from the ring (DONT WANT A NITROGEN)
- Side chain on 2,4 position of ring
- Steric clashes
