Biology Reference
In-Depth Information
treatment was also shown to be similar to that resulting
from siRNA modulation of Chk1. Of importance, this
response appeared to be specific to cancer cells, as prolif-
erating normal fibroblasts did not exhibit the same DNA
damage phenotype on Chk1 inhibition.
In preclinical
in vivo
models (A2780 ovarian xeno-
graft), SCH-900776 showed PD activity in the range
4
patients with advanced solid tumors to assess safety,
tolerability, DLT, and maximal administered dose
(MAD). In the gemcitabine combination trial, a total of
26 patients were treated at doses ranging from 10 to
112 mg/m2 SCH-900776 administered alone and
following gemcitabine (800 mg/m
2
) on days 1 and 8 of
a 21-day cycle. An additional seven patients have also
been treated with a higher dose of gemcitabine
(1000 mg/m
2
) and either 80 or 112 mg/m
2
SCH-900776.
Only one SAE (grade 3 hyperbilirubinemia) was
observed in the monotherapy lead-in, but there were
three reversible DLTs in the combination setting
32 mg/kg, and increased the efficacy of gemcitabine
from 1 log cell kill (150 mg/kg gemcitabine alone) to
approximately 3 log cell kill (150 mg/kg gemcitabine
plus 50 mg/kg SCH-900776). Biomarker responses
were rapidly induced in both xenograft and surrogate
tissue samples, and levels correlated well with increased
efficacy. Importantly the doses of SCH-900776 required
for mechanism based
in vivo
activity were well below
those associated with toxicity, suggesting a good thera-
peutic index. Of particular interest, efficacious doses of
SCH-900776 in combination with gemcitabine were
determined to affect neither the nadir nor rebound
kinetics of hematological parameters in Balb/C mice
when compared to treatment with gemcitabine alone.
Dramatic sensitization of cells to DNA damage
induced by other antimetabolites, for example, treat-
ment with hydroxyurea, has also been demonstrated.
For example, MDA-MB-231 cells were sensitized 100-
fold to hydroxyurea (GI
50
3mMto 30
m
M). This 100-
fold sensitization to hydroxyurea appeared to require
complete inhibition of Chk1 (2
m
M SCH-900776),
although a 10-fold sensitization was still observed at
200 nM SCH-900776. In addition, concentrations of
hydroxyurea that had no effect on checkpoint activation
or growth inhibition alone could still be enhanced by
inhibition of Chk1. SCH-900776 also induced a 50-fold
sensitization to cytarabine, a 10-fold sensitization to
gemcitabine, but no sensitization to 5-fluorouracil.
Hence, there are further subtleties that remain to be
understood as sensitization apparently does not extend
to all antimetabolites. In an MDA-MB-231 cell line
expressing shRNA that resulted in complete knockdown
of Chk1, the full level of sensitization to hydroxyurea
could be achieved at 200 nM SCH-900776, indicating
that the response of tumors may vary with the endoge-
nous level of Chk1, and results from future studies
may be useful to help predict which patients will be
most likely to respond to combination therapy.
122
In contrast to the potentiation seen with antimetabo-
lites, SCH-900776 has been shown to inhibit Chk1 and
overcome SN38-mediated S and G2 arrest at 300 nM in
several carcinoma cell lines, but although it accelerated
the rate of cell killing induced by SN38, the overall level
of cell kill was not increased. Similarly, no overall sensi-
tization to cisplatin was observed.
122
SCH-900776 is currently in phase I clinical trials in two
settings, in combination with cytarabine in patients with
acute leukemias, and in combination with gemcitabine in
e
supra-
ventricular tachycardia with pneumonia (40 mg/m
2
),
atrial fibrillation (112 mg/m
2
) and thrombocytopenia
(112 mg/m
2
). The MADwas 112 mg/m
2
. Signs of clinical
activity were observed in five patients, with two partial
responses in melanoma and cholangiocarcinoma, pro-
longed stable disease in spindle cell sarcoma, and two
instances of stable disease in pancreatic cancer. Worthy
of note is the fact that the pancreatic cancer patients
had previously been treated with gemcitabine, and had
tumors that had progressed in that setting. This suggests
that the combination of DNA damaging agent and
a Chk1 inhibitor may not only enhance efficacy in
chemotherapy-naive patients, but may also be able to
reinstate efficacy in refractory or resistant tumors. PK
analysis showed that exposure to SCH-900776 increased
in a dose-proportional manner (Cmax and AUC) across
the dose range studied, and there was no indication of
a PK interaction with gemcitabine. The mean ½ was
determined to be 6.29
e
9.38 hours. The exposure
threshold for preclinical activity (
e
0.5
m
M Cmax) and
PD evidence of target modulation were observed from
the lowest dose of SCH90076 given (10 mg/m
2
). The
higher dose (1000 mg/m
2
gemcitabine) arm of the trial
is ongoing in order to define the recommended phase
II trial dosing schedule.
123
>
MK1775
Merck has advanced MK1775 (see
Figure 10.2
)into
clinical trials. This is the first Wee1 inhibitor to be
described, and to date is the only inhibitor of this target
in clinical trials as an alternative mechanism to enhance
the efficacy of DNA damaging radio- or chemother-
apies.
73
Early leads were discovered by high
throughput screening of a small chemical library to
find potent inhibitors of Wee1 kinase. Optimization of
these leads led to the identification of MK1775 as
a potent, selective and orally bioavailable Wee1 inhib-
itor (IC
50
5.2 nM). The compound is suggested to bind
in an ATP-competitive manner as evidenced by the
observed increasing linear relationship between the
measured IC
50
value for MK1775 and ATP concentra-
tion in the enzyme assay. The compound is highly
