Biology Reference
In-Depth Information
components of essentially every S-phase as a consequence of inherent errors
that frequently arise at sites of replication, particularly when cells experience
oncogene-driven replication stress. ATR and Chk1 prevent new replication
origins from firing during the S-phase
59,60
and are involved in the intra-S and
G2/M checkpoints where Chk1 phosphorylation of Cdc25A facilitates its pro-
teosomal degradation and Chk1 phosphorylation of Cdc25B/C induces 14-3-3
binding.
32,33,61
In p53-deficient cells, knockdown of Chk1 has been shown to
induce loss of both these cell cycle checkpoints.
62
1. I
NHIBITION OF
C
HK
1
Because Chk1 and its upstream activator ATR are essential in mammals,
63-65
functional studies of this pathway have been limited mainly to RNAi-mediated
knockdown approaches or the use of inhibitors. These inhibitors are often not
specific for the kinase of interest, although more selective inhibitors are currently
in development. The largest body of clinical data to date comes from studies with
UCN01 (7-hydroxystaurosporine), a molecule originally isolated as an inhibitor of
protein kinase C (PKC) and also known to inhibit a number of other kinases
including Chk1, MK2, phosphoinositide-dependent kinase-1 (Pdk1), and several
cyclin-dependent kinases (Cdks).
26,66-69
In combination with DNA-damaging
agents or during replicative stress, UCN-01 causes a bypass of intra-S and G2
checkpoints in cells with defective p53 function,
70-72
which phenocopies the loss
of Chk1 using RNAi.
62
As UCN-01 also potently inhibits MK2, some of its efficacy
may result from the simultaneous disruption of two critical checkpoint pathways in
p53-defective cells.
26
In phase I clinical trials, UCN-01 used either as a single agent or in
combination with several DNA-damaging agents including Irinotecan
(CPT-11), an analog of the topoisomerase I inhibitor camptothecin,
73
showed
initial promise. Two patients with p53-deficient triple-negative breast
cancers responded dramatically to therapy, confirming preclinical data that
show a synergistic effect between UCN-01 and DNA-damaging agents
in tumor cells with defective p53 function.
70-72
However, due to adverse
toxicities and poor pharmacokinetics, UCN-01 has not been further clinically
developed.
In recent years, novel inhibitors with a higher specificity for Chk1 have
been tested successfully in preclinical models and are currently being evaluat-
ed in clinical phase I studies in combination with DNA-damaging agents or
antimetabolites, which interfere with DNA replication (reviewed in Ref.
74
).
54
These newer inhibitors, including AZD7762, PF477736, and SCH900776, are
more selective for Chk1 than UCN-01, although none of them is completely
exempt from inhibition of other kinases.
