Biology Reference
In-Depth Information
DNA. Topoisomerase I poisons freeze the cleavable
complex in the broken state resulting in the accumulation
of DNA SSBs. Since the number of frozen complexes/
SSBs determine the level of cytotoxicity, cells with more
active topoisomerase I will experience the greatest cyto-
toxicity. Early studies demonstrated that NU1025 mark-
edly enhanced camptothecin-induced DNA breaks and
cytotoxicity and that both NU1025 and NU1085 potenti-
ated topotecan in a panel of human cancer cell lines.
95,96
The mechanism of sensitization has not been fully deter-
mined, but PARP-1 co-localizes and activates topoisomer-
ase I. Automodification of PARP-1 in response to DNA
damage disrupts this association, and hence stimulation
of topoisomerase I, and since cytotoxicity is related to
topoisomerase I activity PARP-1 might reduce topoiso-
merase I poison-induced damage.
97
Poly(ADP-ribosyl)
ated PARP-1 and PARP-2, but not the unmodified
enzymes, block the formation of topoisomerase I
hypothesis is supported by the recent demonstration
that the PARP inhibitor 4-amino-1,8-naphthalimide
preferentially sensitized S-phase cells to IR and that
following exposure to IR there was an accumulation of
DNA double-strand breaks (DSB) in PARP inhibited
cells.
56
PARP inhibitors also enhanced very low dose
IR (0.05
0.3 Gy) in exponentially growing, but not
confluent cultures of T989G human glioma cells.
52
The growth-arrested hypoxic fraction within a tumor
is largely radioresistant and can re-populate the tumor
after radiotherapy.
104,105
In vitro
models of this situation
measure recovery from potentially lethal damage
(PLD), where the increased survival of growth-arrested
cells is assessed after a recovery period, in comparison
with the survival of cells without the recovery period.
PD128763 blocked PLD recovery and approximately
doubled X-ray-induced cell kill in both proliferating
and stationary cultures,
55
NU1025 retarded the rejoining
of IR-induced DNA strand breaks, prevented recovery
from PLD, and enhanced the cytotoxicity of
g
-irradiation
in exponentially growing L1210 cells,
53
and AG14361
caused a more marked sensitization of cells in the
radio-resistant growth-arrested state.
79
AG14361 doubled
the antitumor effect of fractionated X-rays in mice
bearing human colon cancer xenografts
79
and GPI
15427 significantly enhanced the radiation-induced inhi-
bition of the growth of human head and neck cancer
xenografts.
106
The PARP inhibitor E7016 (formerly GPI
21016) sensitized human glioblastoma and pancreatic
cell lines to ionising radiation with a dose-enhancement
ratio of 1.4 to 1.7, caused the persistence of DNA DSBs
and had greater than additive effects in combination
with temozolomide and ionizing radiation against
glioma xenografts.
107
PARP null mice and cells derived from them are
sensitive to DNAmonofunctional alkylating agents, top-
oisomerase I poisons and ionizing radiation validating
the observed sensitization by PARP inhibitors to these
agents. To date there are no reports of cells and mice
deficient in either PARP-1 or PARP-2 being sensitive to
other cytotoxic drugs, which gives less support to
reports of their chemosensitization by PARP inhibitors.
Investigation of the role of PARP in the response to plat-
inum agents have resulted in some curious and conflict-
ing data. Cisplatin and carboplatin induce DNA intra
and inter-strand cross links that are repaired by nucleo-
tide excision repair and homologous recombination
(HR) repair. PARP-1 has been shown to be activated by
cisplatin-mediated DNA damage.
108
Studies with the
benzamides gave conflicting results in that they
enhanced cisplatin cytotoxicity has been observed in
ovarian and cervical cancer cells
109
but no enhancement
was seen in a study of mammary carcinoma cells
110
and
some of the potent inhibitors identified by Banasik failed
to potentiate cisplatin in a panel of ovarian cancer cell
e
DNA
covalent complexes, inhibit DNA cleavage by topoiso-
merase I and accelerate the removal of camptothecin-
stabilized topoisomerase I
e
DNA cleavable complexes
in
in vitro
biochemical assays.
98
Alternatively, PARP-1
could be involved in the repair of topoisomerase I-associ-
ated DNA damage. PARP-1 deficient and inhibited cells
repaired topoisomerase I poison-induced DNA strand
breaks more slowly and were more sensitive to topoiso-
merase I-induced cytotoxicity.
99
The mechanism was
proposed to be via an effect on BER as BER-defective cells
are hypersensitive to camptothecin,
100,101
suggesting
a role for BER in the response to topoisomerase I poisons,
resistance to camptothecin (due to over-expression of
XRCC1) can be reversed by the PARP inhibitor, 3-AB
102
and BER defective cells were not sensitized by PARP inhi-
bition and PARP inhibition did not affect topoisomerase
Iactivity.
99
PARP-1 may act via recruitment of XRCC1
to the topoisomerase-1-associated DNA breaks, XRCC1
in turn recruits TDP-1, which can remove topoisomerase
I from the DNA.
103
In antitumor activity studies, CEP-
6800 caused a 60% enhancement of irinotecan-induced
delay in the growth of human colon cancer xenografts,
AG14361 increased the irinotecan-induced tumor growth
delay by 2
e
3-fold in a human colon cancer model
76
and
e
GPI
15427
also
enhanced
irinotecan
antitumor
activity
78,79
confirming the
in vitro
data.
Radiotherapy
Radiotherapy, which causes a plethora of DNA
damage including base damage and single- and
double-strand DNA breaks, all of which depend on
PARP activity for their repair, is a major cancer treatment
modality. The general assumption has been that radio-
sensitization by PARP inhibitors is caused by inhibition
of SSBR/BER resulting in conversion of DNA SSB to
DSB on collision with the replication fork. This
