DNA-Repair Enzyme Inhibitors

Question 1

Describe DNA-damaging agents in cancer

Answer 1

Highly efficient in killing proliferating cells
High levels of DNA damage = cell-cycle arrest + death
Double-strand breaks = most toxic of all DNA lesions

Question 2

Describe DNA-damage response pathways

Answer 2

Cells contain mechanism to detect + repair DNA damage
Lesions in DNA enable cell to acquire the properties needed to escape mechanisms = become cancer cell

Question 3

Describe the DNA-damage response pathways (DDR) in normal cells

Answer 3

Have 2 DDR
If 1 pathway eliminated = genome instability = foster evolution of cancer cell
Addition of an inhibitor target 2nd pathway = cell death = stops cancer cell survivor

Question 4

Describe DNA-dependent protein kinase (DNA-PK)

Answer 4

Ku protein component binds directly to DNA ends + help align them for ligation
Binding of Ku protein to DNA = nucleates formation of an active enzyme (contain DNA-PK)
Key role in DNA double-strand break repair

Question 5

Describe DNA-PK inhibitors

Answer 5

Sensitise cancer cells to ionising radiation + chemotherapy
IC50 = 5nM = potent + improved solubility

Question 6

Describe PARP-1

Answer 6

Major role in DNA base-excision repair pathway
Utilises NAD+ as a substrate rather than as coenzyme = triggers polymerisation
Involved in BER DNA-repair pathway

Question 7

Describe PARP inhibition

Answer 7

PARP activation + binding
Administer drug that looks like NAD+
= NO polymerisation
Used in combination with IR + chemo = potentiate activity

Question 8

Describe normal function of PARP

Answer 8

Single strand break
PARP binds
NAD+ = polymerisation
Repair

Question 9

Describe 1st generation PARPi

Answer 9

Low micromolar competitive inhibitors at NAD+ domain
Poor specificity
Common benzamide group
Higher IC50 = better safety profile
eg. Cisapride (5HT4 agonist), Metoclopramide (D2 antagonist), Sulpiride (D2/D3 antagonist) + Nicofetamide (antimuscarinic)

Question 10

What is the good about the benzamide groups in 1st generate PARPi?

Answer 10

Good acceptor carbonyl group = O
Electron-rich aromatic ring = benzene ring
Non-cleavable bond = C-X
At least one free NH group
Carboxamide conformation important for binding

Question 11

Describe Rucaparib

Answer 11

Ring-constrained benzimidazole derivative = highly potent
FDA approved for pre-treated advanced ovarian cancer
Oral tablet