Biology Reference
In-Depth Information
alone was followed by one of three treatment arms:
in combination with a single dose of gemcitabine
(1000 mg/m
2
), cisplatin (75 mg/m
2
) or carboplatin
(AUC 5). Five BID doses of MK1775 were then explored
in the same three treatment arms, and a cohort expansion
conducted to confirm the MTD. The PD markers
assessed included pCDC2 levels and Wee1 signature.
To date, 91 patients have been treated, 41 with MK1775
as a monotherapy, and 50 in the combination setting.
While no DLTs were identified with MK1775 as a single
agent, 20 patients experienced DLTs (seven after single
dose and 13 after multiple doses) in the combination
arms. The most common adverse events were hemato-
logical, nausea and vomiting, and fatigue. Marker
responses were observed in a number of patients (three
melanoma, one pancreatic, and three ovarian) and there
was evidence of clinical activity as evidenced by
a number of partial responses, and patients with stable
disease. PK was shown to be linear from 25 to 1300 mg
MK1775. The MTD was only determined in the multiple
dose setting for gemcitabine, dose finding with cisplatin
and carboplatin is still ongoing.
for Chk1 than Chk2. SCH-900776 is more selective still,
being 500-fold selective for Chk1.
It seems clear that checkpoint abrogation, and hence
the potentiation of efficacy, is driven by inhibition of
Chk1 rather than Chk2. It was initially supposed that
Chk1 and Chk2 had overlapping or partially redundant
roles in downstream checkpoint responses as early
studies suggested Chk1 and Chk2 shared certain
common substrates known to be involved in the initia-
tion of cell cycle arrest in response to DNA damage,
such as Cdc25 family phosphatases.
66,127
However,
since that time, genetic and biochemical data have
provided increasing evidence that it is Chk1 rather
than Chk2 that is the principal, direct effector of DNA
damage and replication checkpoints, with Chk2 play-
ing an auxiliary role, specifically in response to
DSBs.
31,128
Chk2 function is time-dependent and is
generally limited to DSB-induced checkpoints induced
by irradiation. In contrast, Chk1 is involved in all cell
cycle checkpoints regardless of the mechanism of
induction, including UV and numerous DNA-
damaging agents, as well as in response to DNA repli-
cation stress. The G2 checkpoint is impaired in response
to Chk 2 depletion in some, but not all, cell types. For
example, MEFs and HCT116 cells lacking Chk2 were
found to exhibit normal G2 checkpoint responses,
whereas Chk2 knockout DT40 cells showed a weakened
and delayed G2 arrest.
34,129
While these variations in
checkpoint proficiency in cells genetically deficient for
Chk2 have been interpreted as evidence of pathway
crosstalk and compensation by Chk1, it is equally
possible that the role of Chk2 in the G2 checkpoint
simply varies between cell types.
However, what is clear is that, unlike Chk1 knock-
down, Chk2 knockdown does not sensitize cells broadly
to chemotherapy or radiation treatment. In addition,
studies with a very selective small molecule Chk2 inhib-
itor, VRX0466617 (Chk1 IC
50
>
DISCUSSION
As can be seen from the data discussed above, the
past few years have seen a dramatic increase in interest
in therapeutic targets from the DNA damage detection
and response networks, and the area has been one of
intense research, yielding agents that have advanced
as far as clinical trials. For the Chk and Wee1 inhibitors
in particular, most of the work to date has been focused
on the hypothesis that inhibiting these targets has the
potential to increase the efficacy that can be achieved
with DNA damaging radio- or chemotherapy, without
a concomitant increase in therapy-related toxicity.
Preclinical and early clinical data seem to support this
notion, but there is still much that remains to be under-
stood before the full promise of such agents can be
realized.
10000 nM, Chk2 Ki
11 nM) failed to demonstrate synergy with either gemci-
tabine or doxorubicin.
130
Thus, Chk1 is an extremely
attractive target for multiple reasons and represents
one of the most important targets for anticancer thera-
peutics directed at the DDR network.
Perhaps more relevant in determining the value of
a dual inhibitor of Chk1 and Chk2 over Chk1 alone is
the hypothesis that Chk2 inhibition may lead to the
enhanced effect of sensitizing p53 null cancer cells while
protecting normal cells, and so may play a role in maxi-
mizing the therapeutic window that can be achieved
clinically.
18
This is supported by studies of Chk2-defi-
cient mice. Chk2 knockout mice are developmentally
normal, and are not spontaneously cancer prone. Chk1
knockout, in contrast, results in embryonic lethality.
The Chk2
/
phenotype is dominated by an increased
resistance to radiation, and by a decreased level of
Selectivity Profiles
Particularly for the checkpoint kinase inhibitors
identified to date, there is still a question as to the ideal
balance of target inhibition and whether the optimum
efficacy will be achieved by inhibiting Chk1 and Chk2
in contrast to specific inhibition of Chk1. Although
not all still continuing in clinical trials, all the Chk
inhibitors that reached clinical development are very
potent inhibitors of Chk1, but they exhibit varying
degrees of activity against Chk2. For example,
AZD7762 is essentially equipotent for both targets,
while PF00477736 is around a 100-fold more potent
