Biology Reference
In-Depth Information
inhibition (TGI) of 70% as compared to vehicle-treated
controls when dosed at 400 mg/kg every fourth day
over four treatment cycles. The activity of gemcitabine
was markedly enhanced in combination, as both doses
of XL-844 (100 mg/kg and 300 mg/kg dosed orally
8 and 24 hours post gemcitabine treatment) showed
increased efficacy over the chemotherapy alone (TGI of
91% and
grade three lipase elevation, although a maximum toler-
ated dose was not reached in the trial.
97
These results
demonstrated for the first time that the preclinical data
suggesting that checkpoint kinase inhibitors could
safely be added to DNA-damaging chemotherapy regi-
mens (in this case gemcitabine treatment) without
a parallel potentiation of unwanted toxicities may trans-
late to the clinical situation.
100% respectively). XL-844 given alone had
no effect on tumor growth at either dose tested. Tumor
regrowth data from the study suggested that not only
does the combination of XL-844 and gemcitabine lead
to supra-additive efficacy, but the duration of the
response is longer than that obtained from treatment
with gemcitabine alone. Both agents were well tolerated
alone, and the only observation in the combination
groups was that of mild, rapidly reversible decreases
in body weight. Importantly, no enhanced hematological
toxicity was observed with combination treatment
providing support for the hypothesis that the chemo-
therapeutic effects of gemcitabine can be potentiated
without a concomitant increase in systemic toxicity,
a major factor for the effective clinical utility of this class
of agent.
Additional
in vivo
studies confirmed a broader poten-
tial scope for the use of XL-844 by demonstrating that
the efficacy of daunorubicin (Dnr) can also be enhanced
by combination with XL-844.
96
In a chronic myeloid
leukemia survival model in nude mice, treatment with
XL-844 in combination with Dnr caused a significant
increase in median survival time (MST) relative to Dnr
alone (MST of 93.4% versus 37.5%, respectively). Of
particular note is the fact that in this study, some animals
that received the combination treatment exhibited
asymptomatic long-term survival, which was in marked
contrast to animals treated with either agent alone.
Development of XL-844 was recently discontinued,
but early indications from the phase I combination trial
with gemcitabine were encouraging. Patients with
advanced malignancies were treated with escalating
doses of XL-844 (0.8
>
AZD7762
AstraZeneca has advanced an intravenous checkpoint
kinase inhibitor, AZD7762 (see
Figure 10.2
), into phase I
clinical trials.
98
e
102
The lead series from which AZD7762
was developed was identified by high throughput
screening that revealed the thiophene urea carboxamides
as potent inhibitors of Chk1. This series was selected as
a number of hits gave a preliminary understanding of
structure-activity relationships (SAR), and the hits gener-
ally had good physicochemical properties, DMPK
characteristics and enzyme potency suggesting that
drug-like properties could be achieved with this class
of compound. Following optimization of Chk1 potency
and selectivity, AZD7762 emerged as the clinical candi-
date. The compound is a relatively selective inhibitor of
the checkpoint kinases 1 and 2 and potently inhibits
both (Chk1 IC
50
5 nM, K
i
3.6 nM, Chk2 IC
50
<
10 nM).
AZD7762 is 10- to 100-fold selective for CHk1 against
the majority of a panel of 164 protein kinases, and impor-
tantly does not inhibit either cyclin-dependent kinases or
protein kinase Cs. This is important as lack of selectivity
against other cell cycle kinases could result in cell cycle
arrest and hence mask the desired phenotype due to
Chk1 inhibition.
In a cell-based assay measuring the ability of
AZD7762 to abrogate the G2/M checkpoint induced
by camptothecin treatment
in HT-29
cells,
the
compound was shown to have an EC
50
of 10 nM.
In vitro
mechanism of action studies demonstrated
that AZD7762 could indeed enhance the activity of
a range of DNA-damaging agents, and that this increase
in activity was generally observed across multiple cell
lines from diverse tumor types. Combination treatment
resulted in an increase in cytotoxic response versus
treatment with DNA-damaging agents alone. In addi-
tion, the response was found to be p53-dependent
.
For
example, in SW620 cells
in vitro
GI
50
and GI
100
values
of 11.9 and 158 nM with gemcitabine alone were
decreased to 2.4 and 4.3 nM respectively, when gemcita-
bine was applied in combination with AZD7762
(300 nM). Topotecan was less efficacious in the same
cell line with a GI
50
of 2.25
m
M, but again the combina-
tion with AZD7762 shifted the GI
50
, in this case to
150 nM. Treatment with AZD7762 alone had no signifi-
cant effect on cell growth in these assays.
2.25 mg/kg/day) twice a week
on the first two days of a six-week cycle, followed by
four once-weekly cycles in combination with gemcita-
bine (800 mg/m2). Gemcitabine was administered eight
hours prior to dosing with XL-844. Serial blood samples
were collected for plasma PK assessment, and tumor
responses assessed approximately every two cycles.
There was no assessment of pharmacodynamic
biomarkers such as pChk1 or
g
-H2AX. Preliminary
results showed the combination to be generally well
tolerated with no apparent PK interaction between
XL-844 and gemcitabine. Interestingly, of 27 evaluable
patients, 10 had stable disease for longer than three
months, while one patient (endometrial cancer) had
a confirmed partial response. Dose-limiting toxicities
were reported to be grade four thrombocytopenia and
e
