Biology Reference
In-Depth Information
Monofunctional DNA-Methylating Agents
Monofunctional DNA-methylating agents are the
most potent activators of PARP-1 (and -2) and all conven-
tional PARP inhibitors can be considered modulators of
resistance to anticancer DNA-methylating agents such
as 5-(3,3-dimethyl-1-triazeno)imidazole-4-carboxamide
(DTIC) and temozolomide. These drugs methylate
DNA at the
O
6
- and N
7
-position of guanine and the
N
3
-position of adenine.
69
Although a minor lesion, the
most cytotoxic lesion is
O
6
-methylguanine, because,
unless it is repaired bymethylguanine methyltransferase
prior to replication, it will mispair triggering the
mismatch repair (MMR) proteins to initiate futile repair
cycles, resulting in apoptosis.
70,71
The N-methylpurines,
which are much more numerous, are targets for SSBR/
BER and hence PARP-1 and 2 play a role in their repair.
Several studies have investigated temozolomide chemo-
sensitization by second- and third-generation PARP
inhibitors (see reviews cited above). In the first of such
studies, the PARP inhibitors PD128763 and NU1025
increased temozolomide-induced DNA strand breakage
and caused a four- to seven-fold potentiation of temozo-
lomide cytotoxicity.
72
Drug scheduling experiments with
NU1025 revealed that PARP inhibition during the repair
phase alone was sufficient for potentiation of MTIC cyto-
toxicity.
53
The subsequent development of much more
potent PARP inhibitors using crystal structure-based
drug design and conventional structure-activity relation-
ships has allowed in vivo antitumor activity studies, as
well as cell-based studies to be performed. A panel of
phthalazinone PARP inhibitors increased MMS-induced
cytotoxicity with an 18-fold enhancement was demon-
strated for the most cellularly active inhibitors.
73,74
Ulti-
mately this series generated AZD2281, olaparib, which is
in an advanced stage of clinical evaluation and which
showed good enhancement of temozolomide efficacy
in the SW620 model.
75
CEP-6800 and GPI 15427
increased temozolomide-induced DNA damage and
cytotoxicity or growth inhibition in human glioblastoma
cells and enhanced the antitumor activity of temozolo-
mide in mice bearing gliomas, including intracranially
implanted tumors.
76
e
78
Several of the potent benzimid-
azoles and tricyclic lactam benzimidazoles and indoles,
including the first PARP inhibitor to enter clinical trial,
AG-014699 (PF-01367338) and its predecessor AG14361
potentiated temozolomide-induced growth inhibition of
human colon cancer cells by up to 10-fold and showed
remarkable activity with temozolomide against SW620
human colon cancer xenografts, resulting in complete
and durable tumor regressions.
79
ABT-888, another
PARP inhibitor currently undergoing clinical evaluation,
has shown excellent activity against murine melanoma,
rat glioma and orthotopic human breast and prostate
xenografts, models of metastasis to bone and brain and
spontaneous lung metastases as well as subcutaneous
xenografts of pancreatic, ovarian and non-small-cell
lung cancer.
80,81
The only studies to be conducted to
date with preclinical models of pediatric cancers demon-
strate that AG-014699/PF-01367338 enhances the anti-
tumor activity of temozolomide against neuroblastoma
and medulloblastoma xenografts.
82,83
These more potent inhibitors have helped reveal
details regarding the mechanism of the potentiation.
For example, studies with ABT-888 in synchronized cells
demonstrated that sensitization of temozolomide cyto-
toxicity was much greater in S-phase; the hypothesis
that this was due to unrepaired single-strand breaks
converting to DNA DSBs during replication was
confirmed by direct measurement of DSB by neutral
comet assay.
84
A major mechanism of cellular resistance to temozolo-
mide in experimental studies is loss of MMR
85
and in
glioma patients MMR deficiency was related to poor
response to temozolomide.
86
Cancer-susceptibility
syndromes and sporadic cancers of the colon and ovary
are associated with MMR defects.
87,88
Because MMR
defects confer tolerance to
O
6
-methylguanine theN-meth-
ylpurines that are dependent on PARP-mediated repair
are the major cytotoxic lesions. PARP inhibitors have
been shown to preferentially sensitize MMR-defective
cells to temozolomide. The first studies demonstrated
that 3AB, PD128763 and NU1025 enhance temozolomide
cytotoxicity in MMR-deficient cell lines.
89
e
91
Subse-
quently, in studies with isogenic pairs of MMR-proficient
and -deficient human cancer cells the PARP inhibitor
AG14361 caused greater temozolomide sensitization in
MMR-deficient cells than in MMR-proficient cells,
completely overcoming temozolomide resistance.
92
GPI
15427, which sensitized both MMR-proficient and -defi-
cient cell lines to temozolomide only caused significant
enhancement of the antitumor activity of temozolomide
against a MMR-defective xenograft.
78
Similarly, INO-
1001 enhanced temozolomide-induced tumor growth
delay to a greater extent in MMR defective glioma xeno-
grafts.
93
ABT-888 was also a more effective sensitizer of
temozolomide activity against MMR-defective leukemia
cells.
94
Since only tumors lack MMR, PARP inhibition, in
combination with temozolomide, represents a potentially
selective therapeutic approach.
Topoisomerase I Poisons
Topoisomerase I poisons are more widely used than
temozolomide and DTIC in the treatment of cancer. Top-
oisomerase I reduces the torsional strain in DNA result-
ing from local unwinding for replication and
transcription by forming a transient complex with DNA
catalysing the cleavage, unwinding and relegation of
