Note for: PARP and PARG inhibitors in cancer treatment
Note for: PARP
and PARG inhibitors in cancer treatment
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Chemotherapy and radiotherapy are nonselectively
means of killing cancer cells.
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PARP is the successful drug targeting DNA damage
response
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It was first approved for treating cancer with
BRCA mutated ovarian and breast cancer – using synthetic lethality approach
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PAPR inhibitor destabilize replication fork
o
Trap PARP to DNA
o
Cell death through replication stress-induced
mitotic catastrophe
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Inhibition of PARG – exacerbate replication
deficiencies of cancer cells
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Highlight 4 PARP inhibitors used in cancer
therapy
o
Olaparib
o
Rucaparib
o
Niraparib
o
Talazoparib
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Cancer cells produce many ROS, thus, having
higher oxidative stress over than normal cells
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Inflammatory cells – macrophages/neutrophil --
> release ROS
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PARPi exploits the defective in HR but PARGi
exploits the defective in replication machinery
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PARP
o
Catalyzing the addition of poly(ADP-ribose) from
NAD+ to protein and release nicotinamide and protein-mono/poly-ADP-ribose as
products
o
PARP1
§
major cellular PAR
§
activated through DNA-binding (binding to DNA
causes the conformational change which the loop is opened for NAD+)
§
Step by step procedures
·
N-terminal bind to DNA through Zn finger domain
·
Induce conformational change – unfolding the helical
domain (HD)
·
NAD+ incorporate into the catalytic pocket
·
Catalysis occur through three key AA
§
Inhibitor
·
1963 – enzyme was discovered
·
1971 – nicotinamide itself
·
1980 – 3-aminobenzamide (3AB)
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Key pharmacophores – nicotinamide/benzamide –
compete with NAD
·
Interaction involved;
o
Hydrogen bonds with Gly, Ser, Glu
o
Hydrophobic interaction with two Tyr in NAD
binding pocket
o
Catalytic AA are conserved among PARP but the AAs
nearby are not, thus, drug design should take this information into
consideration
·
New generation of PARP1 inhibitors
o
Phthalazinone and tetrahyydropyridophthalazinone
§
Scaffold for
·
Olaparib
·
Talazoparib
o
Benzimidazole and indazole carboxamide
§
Scaffold for
·
Veliparib
·
Niraparib
·
Most selective to PARP1/2
o
Veliparib>niraparib (>100 fold selectivity
compared with W/O drug)
§
Based on the formation of hydrogen bond
interaction with regulatory subdomain residue (D766, PARP1)
§
D766 conserved in PARP1/2 not other PARP
o
Olaparib and talazoparib
§
15- to 20-
fold selectivity
o
Rucaparib least selective
o
Off-target report for
§
Rucaparib
·
H6PD (hexose-6-phosphate)
§
Niraparib
·
DCK (deoxycytidine)
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Catalytic inhibitory effects of clinically
relevant PARP inhibitors olaparib, rucaparib, niraparib, and talazoparib are
comparable
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Thiers potency in trapping PARP-DNA complexes
are different
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Thus, the trapping was proposed to relying on
allosteric changes in PARP1 DNA-binding domain upon inhibitor binding
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Highest trapping efficiency
o Talazoparib>>niraparib>olaparib=rucaparib>>veliparib
o
Velaparib is one of the weakest PAPR1/ inhibitor
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PARG
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Hydrolyzes ribose-ribose bonds within PAR
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Has macro domain that binds ADP-ribose moiety
and PARG-specific loop
Cellular mechanisms of PARP and PARG inhibitors (focus PARP1
only)
Functions in DNA repair and replication fork protection
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DNA repair, folk protection, and transcription
regulation
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PARP1 relates in many DNA repair pathways
o
SSB
o
NER
o
Alt-NHEJ
o
HR
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PARP1
o
Under replication stress (stalled
replication fork – can be induced by Misincorporation of ribonucleotides,
Unusual DNA structures, Conflicts between replication and transcription,
Insufficiency of essential replication factors, Common fragile sites,
Overexpression or constitutive activation of oncogenes, Chromatin
inaccessibility)
§
Preserving or replication forks
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PARP1-- > SSB
o
Interacting with scaffold XRCC1 to SSB
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PARP1 -- > response to replication stress
o
Interact with replication machinery
o
Active during S-phase
o
Slow down replication forks to promote fork
reversal by antagonizing REQ1 helicase
Role of PARP1
PARP1 -- > transcription regulation
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Stimulating activity of transcription factor
E2F1 – regulate the expression of replication and HR genes
PARP1 -- > HR
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Promote rapid recruitment of MRE11, EXO1,
BRCA1+2 to DNA damage sites
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Counteract NHEJ – preventing the binding of KU
to DNA end
Actually, other members of the PARP family -- >
implicated in DSB repair and replication fork stability -- > PARP2, PARP3,
PARP10 and PARP14
Genotoxic agents used routinely in cancer therapy -- >
induce high level of DNA damage -- > causing cancer cell to undergo
cell-death because its high proliferative rate (more and more damage
acculation)
Synergistic effects are additionally achieved in patients
with genetic defect in DNA repair pathways ex.
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Pt drugs – improve response rate in BRCA mutated
TNBC
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Genotoxic agents has side effects like
myelosuppression
PARP inhibitor is the successful example of targeted therapy
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Using synthetic lethality concept – targeting
the compensatory pw which cancer relies on
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Compensatory -- > HR defect/replicative
stress defect (genes cause replicative stress)
Not only BRCA1/2 defective – other genes on DNA damage
response (ATM, PRKDC, ATR, RPA1, DSS1, NBN, RAD51, RAD54, CHEK1, CHEK2, FANC,
ERCC1, POLB, FEN1, and CDK12) – shown synthetic lethality in combination with
PARP inhibitors
Amplifying
genomic instability with PARP and PARG inhibitors (focus on PARP1 only)
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Depletion of PARP1 (under replication stress
situation)
o
Increase replication fork speed
o
Impairs replication fork reversal
o
Cause untimely fork restart
o
Consequence -- >
§
Accumulation of DNA damage in S-phase
§
S-phase stalling
§
G2-delay
§
DSB arise
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PARP inhibitor cytotoxicity – correlate with
strength of PARP-DNA entrapment rather than a reduction on PARP1 catalytic
activity
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CSF (common fragile sites) – genome regions
which sensitive to impaired fork progression
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CSF – late replicating and remain
underreplicated at G2/M transition
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PARP inhibitor efficient in
o
HR-defective cells
o
HR-proficient but defect in oxidative stress
system/replication stress
o
PARP-inhibitor efficacy correlate wit basal levels
of replication stress in cancer cells
-
PARP1 also modulates the activity of different
transcription regulators implicated in cancer or inflammation.
Determinants of PARP inhibitor sensitivity in cancer cells
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PARP1 inhibitor is very effective with cell
which defective in BRCA1/2 or cell with phenocopies BRCA1/2 (BRCAness)
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Biomarkers used to identify the patients
o
HR deficiency (mutation in DNA repair genes)
o
DNA repair expression levels
o
Replication stress markers
o
Transcriptome profiles (to check the PARP
inhibitor response)
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Mutational signature is used for preliminary
screening for PARP1- receiver
o
Defective in BRCA1/2 caused special feature in
genetic defects (due to the repair through NHEJ as backup pathway)
o
This cannot be used in case of HR restoration
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Genomic signature for HR deficiency
o
LOH
o
Telomeric allelic imbalances (TAI)
o
Large scale state transition (LSTs)
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HRDetect
and HRD scores (at DNA
level) -- > used to predict PAPR inhibitor sensitivity in clinical setting
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Gene expression profile of BRCA mutated ovarian
cancers could be also used as the predictor of PARP sensitivity
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Cytological signature of HR deficiency – number
of RAD51 foci (reduced in Rad51 foci – markers for PARP sensitivity)
Replication stress markers
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Loss of TP53+RB1+amplification of MYC -- >
replication stress -- > sensitize small cell lung cancers
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Schlafen – marker for the replication stress
Clinically recommended does and severity of side effects
correlate with PARP inhibitor trapping potency
Olaparib – approve 2017 – maintenance therapy in
Pt-sensitive high-grade ovarian cancer patients irrespective of BRCA status
Rucaparib – approve 2016 – advanced ovarian cancer with
germline and somatic mutation of BRCA1/2
Niraparib and rucaparib – approve 2017, 2018 – maintenance
treatment of recurrent, epithelial ovarian fallopian tube, primary peritoneal
cancer irrespective of BRCA status
Talazoparib – approve 2018 – BRCA mutated and HER2-negative
breast cancer
No PARG inhibitor reached clinical trials – low metabolic
stability
Resistant mechanism (anything that causes PARP dissociation
easily -- > resistant to PARP)
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Restoration of HR pathway
o
Secondary mutation caused the restoration of HR
o
Changing in gene expression levels in HR
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Inactivation of different NHEJ-promoting factors
that inhibit DNA end resection can restore HR in BRCA1-deficienct cells
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Stabilization of replication forks in
BRCA1/2-deficienct cells
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PARP1 mutation – R591C -- > WGR domain --
> critical for interdomain communication between WGR and DNA-binding domain
-- > it dissociates very quick
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Drug efflux
o
Overexpression of ATP-binding cassette (ABC) --
> drug transporters -- > associate with drug resistant
o
Olaparib and rucaparib
§
Substrate of ATP-dependent drug efflux
P-glycoprotein (P-gp) pump (MDR1)
§
Long term treat -- > increase in MDR1 -- >
intracellular PARP inhibitor is reduced
PARP inhibitor combination therapy
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Combination of PARP inhibitors with cell cycle check-point
inhibitors -- > promising strategy -- > fight against PARP inhibitor
resistance caused by replication fork stabilization
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Inhibition of S/G2 checkpoint kinases -- >
ATR and CHK1 -- > unscheduled replication origin firing, exhausts nuclear
RPA pools (excess ssDNA), depletes dNTP -- > underreplicated DNA +
unrepaired DNA damage entering mitotic -- > cell death by mitotic catastrophe
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Inhibition of BRD4 (global transcription
regulator) – synthetic lethal with PARP inhibitors
o
BRD4 inhibition -- > induces HR deficiency --
> lowering the expression of CTIP, BRCA1 and RAD51
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Histone deacetylase (HDAC – epigenetic modifiers)
– synergize with PARP inhibitors
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Agents that pharmacologically induce HR
deficiency like androgen receptor inhibitors, PI3K-AKT-mTOR inhibitors, and
antiangiogenic agents
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Combination with immune checkpoint inhibitors --
> DNA damage caused by PARP inhibitor -- > activate cGAS-STING pathway --
> trigger expression of immune checkpoint in cancer cells (PD1) -- > administration
of anti-PD-L1 -- > helping T-cell to proliferate and release cytokine --
> kill cancer cells
Conclusion
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Cancer cell characteristic
o
High level of replication stress
§
Destability of replication folk
o
Harbor germline or somatic mutation in DNA
damage response genes
§
Mutation in HR – prevent restoration of fork
stability
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PARP and PARG inhibitors
o
Exploit these underlying mechanisms
§
Destabilizing replication folks
§
Inducing DNA damage
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Synthetic lethality for PARP
o
Besides BRCA1/2 extent to HR deficiency genes
o
Replication stress
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Future research on PARP and PARG
o
Dynamic between these two proteins in regulating
replication folk stability
o
How replication stress and genomic instability
-- > induce mitotic defect and cell death through replication and mitotic
catastrophe
o
Looking for the marker to indicate PARP
sensitivity
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