Biology Reference
In-Depth Information
apoptosis in response to irradiation.
34,131
e
133
Chk2
/
mice were found to survive significantly longer than
their wild-type littermates following whole body irradi-
ation, and this extended survival was linked to a reduc-
tion in apoptosis across a variety of different cell types,
including spleen, neurons, thymocytes, and hair follicle
cells. It has also been shown that an ATP-competitive
inhibitor of Chk2 was able to protect human CD4(
not always correlate with response. Some studies
have shown that p53 wild-type tumor cells are also
killed by the combination of DNA damage and check-
point abrogation, seemingly at odds with the large
number of studies that have demonstrated a preferen-
tial response in a p53-deficient setting.
134
e
136
This
apparent contradiction is better understood if dosing
schedules and the functional integrity of the p53
pathway in the cell lines being treated are considered.
The exposure of cells that are knocked down for Chk1
to antimetabolites results in growth inhibition and the
response is p53-independent. This argues that p53
wild-type tumors could potentially be sensitive to the
combination of an antimetabolite, such as 5-FU and
a Chk1 inhibitor.
There are undoubtedly also additional mechanisms
that will be found to be important to fully realize the
potential of these agents. For example, Rad51 overex-
pression is known to result in increased HRR as well
as resistance to radiation, and this suggests that tumors
that overexpress Rad51, such as pancreatic tumors,
would rely more heavily on HRR leading to increased
sensitivity to Chk1 inhibition over normal cells.
137
e
138
As p53 is mutated and Rad51 overexpressed in more
than half of all pancreatic carcinomas, both of these
factors anticipate that it will be possible to achieve a ther-
apeutic window for the selective sensitization of pancre-
atic tumor cells by the addition of Chk1 inhibitors to
gemcitabine and/or radiation.
139,140
Recent studies
with AZD7762 in pancreatic models support
)
þ
and CD8(
) cells from apoptosis following radiation
treatment.
35
Thus, the fact that Chk2 is involved in the
regulation of radiation-induced apoptosis in a p53
dependent fashion and that Chk2 null mice survive
whole-body radiation argues that Chk2 inhibition is
radioprotective in normal tissue. Tumor cells should
not be protected from apoptosis as the majority of
them have defects in the p53 pathway. It is less clear
that this protective effect will extend to chemothera-
peutic agents as well as radiation, and it may well be
that some DNA damaging mechanisms benefit while
others, such as those that cause DSBs, do not. Further
work is needed to more fully understand the Chk1/
Chk2 activity that will achieve the most effective balance
of efficacy while avoiding detrimental effects on normal
tissues, and it will be interesting to compare data
from more selective inhibitors in preclinical studies
(for example the Chk2 inhibitor VRX0466617, and
Chk1 selective agents such as EXEL-3611(Chk1 IC
50
2.4 nM, Chk2 IC
50
2400 nM), and to extrapolate from
these in order to more fully understand the importance
of Chk1 selectivity in the outcome of the ongoing clinical
trials.
96,130
þ
this
hypothesis.
104
It also seems likely that the next few years will bring
an even greater understanding and appreciation of the
multiple roles of Wee1 and Chk kinases in the DNA-
damage response and repair networks which provide
additional hypotheses for cancer treatment outside of
the area of chemo- or radiopotentiation. For example,
recent combination experiments with the PARP-1 inhib-
itor olaparib and AZD7762 revealed synergistic activity
in genetic backgrounds with mutations or aberrations
in genes driving DNA damage response and repair
pathways. Studies in two p53 mutant cell lines
(SW620 [colorectal] and NCI-H460dnp53 [lung])
showed that although cells were relatively insensitive
to either olaparib or AZD7762 alone, IC
50
values were
3- to 10-fold lower for the combination.
In vivo
in an
SW620 xenograft model, although there was no statisti-
cally significant difference in the responses to either
single agent compared to vehicle, in combination there
was a statistically significant increase in tumor growth
inhibition compared to either monotherapy or vehicle
.
The combination was well tolerated as no significant
body weight loss was observed, and there were no
signs of overt toxicity. Thus, both
in vitro
and
in vivo
,
preclinical data support the hypothesis that Chk and
Clinical Development Strategies
For both the Chk and Wee1 inhibitors, the tremen-
dous promise of these agents is that they will bring
added efficacy when used in combination with standard
radio- and chemotherapy treatments, and that this
potentiation will be seen across a variety of tumor types.
On the other hand, clinical development is going to be
particularly challenging, given the difficulty in predict-
ing dosing schedules from preclinical data. There are
clearly multiple permutations of doses and schedules
and how to choose the best one is not clear. Translation
of the basic discoveries in this field into clinical practice
will also require a more detailed understanding of
which patient populations are most likely to benefit,
and this in turn will require the concomitant develop-
ment and testing of diagnostic and response biomarkers.
While there are likely to be some common themes,
for example the p53-dependent nature of the increase
in efficacy seen in many preclinical models indicates
that selection for p53-mutated tumors may be a first
step, but a much greater understanding is required to
maximize the utility of these agents as p53 status does
