Discovery of novel imidazo[1,2-a]pyridines as inhibitors of the insulin-like growth factor-1 receptor tyrosine kinase

Article abstract of DOI:10.1016/j.bmcl.2011.06.093

We disclose a novel series of insulin-like growth factor-1 receptor kinase inhibitors based on the 3-(pyrimidin-4-yl)-imidazo[1,2-a]pyridine scaffold. The influence on the inhibitory activity of substitution on the imidazopyridine and at the C5 position of the pyrimidine is discussed. In the course of this optimization, we discovered a potent and selective inhibitor with suitable pharmacokinetics for oral administration.

Full text of DOI:10.1016/j.bmcl.2011.06.093

Bioorganic & Medicinal Chemistry Letters 21 (2011) 4698–4701
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Discovery of novel imidazo[1,2-a]pyridines as inhibitors of the insulin-like
growth factor-1 receptor tyrosine kinase
Richard Ducray a, , Iain Simpson b, Frederic H. Jung a, J. Willem M. Nissink b, Peter W. Kenny b,
⇑
Martina Fitzek b, Graeme E. Walker b, Lara T. Ward b, Kevin Hudson b
a AstraZeneca, Oncology Innovative Medicines, Centre de Recherches, Z.I. la Pompelle, 51689 Reims Cedex 2, France
b AstraZeneca, Oncology Innovative Medicines, Alderley Park, Macclesfield SK10 4TG, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
We disclose a novel series of insulin-like growth factor-1 receptor kinase inhibitors based on the 3-(pyr-
imidin-4-yl)-imidazo[1,2-a]pyridine scaffold. The influence on the inhibitory activity of substitution on
the imidazopyridine and at the C5 position of the pyrimidine is discussed. In the course of this
optimization, we discovered a potent and selective inhibitor with suitable pharmacokinetics for oral
administration.
Received 6 May 2011
Revised 15 June 2011
Accepted 19 June 2011
Available online 29 June 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
IGF1-R
Insulin-like growth factor-1 receptor
Tyrosine kinase inhibitor
The insulin-like growth factor-1 receptor (IGF-1R) has attracted
much attention over the last few years as a therapeutic target for
cancer.1 Upon ligand binding, this receptor tyrosine kinase acti-
vates two important signaling cascades, namely the PI3K-AKT
pathway and the Ras-Raf-MEK pathway. Deregulation of the IGF-
1R axis has been implicated in a variety of solid cancers including
lung, breast, colorectal and prostate. Such a deregulation can occur
through overexpression of the receptor or its ligands (IGF-1 and
IGF-2) or by down-regulation of binding proteins which modulate
the signaling cascade by ligand sequestration.
Inhibition of the IGF-1R pathway has been and continue to be
evaluated as a cancer therapy in clinical trials using either receptor
antibodies such as CP-751,871 (figitumumab)2 or the small mole-
cule kinase inhibitors OSI-9063 and BMS-7548074 (Fig. 1). In addi-
tion, a number of other IGF-1R kinase inhibitors have been
reported.5–8 Herein, we report the discovery and the structure–
activity relationships of a novel series of imidazo-pyridines which
are potent IGF-1R kinase inhibitors.
activity of 1 towards IGF-1R was a key element of our hit-to-lead
strategy. We found that replacing the para-sulfonyl group, favor-
able for CDK2 inhibition, with a N-acetyl-piperidine (2) partially
met this objective by improving IGF-1R potency by nearly 5-fold
while significantly decreasing CDK2 activity. The addition of a
methoxy group on the ortho position of the aniline (3) achieved
an exquisite selectivity by abrogating CDK2 activity with little im-
pact on the IGF-1R IC50. This can be rationalized by a possible steric
interaction between this methoxy group and the side chain of
Phe82 of CDK2 (see crystal structure of 1 bound to CDK2 in Ref.
10a) whereas the corresponding residue in IGF-1R is Leu1078. Inter-
estingly, Emmitte et al.6b have observed a similar effect with their
anilino-pyrimidines in which a methoxy group on the same posi-
tion reduces activity against Aurora B without any detrimental ef-
fect on IGF-1R inhibition. They attributed this difference to the
presence of a tyrosine (Tyr156) in Aurora B in place of Leu1078 in
IGF-1R.
Following a cellular high throughput screening campaign9 of
our compound collection at AstraZeneca, the imidazo[1,2-a]
pyridine 1 (Table 1), a previously described CDK2 inhibitor,10 was
identified as a hit compound. When we measured the kinase inhib-
itory activities of 1 in biochemical assays, this compound appeared
a 1000-fold more potent against CDK2 than IGF-1R. Recognizing
the importance in avoiding inhibition of the Cyclin Dependant Ki-
nases, which interfere with the cell cycle, shifting the strong CDK2
H
NH2
N
N
F
N N
N
HN
O
N
N
N
N
H
N
N
N
HO
OSI-906
BMS-754807
⇑
Corresponding author.
E-mail address: richard.ducray@astrazeneca.com (R. Ducray).
Figure 1. IGF-1R kinase inhibitors in clinical trials.
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.06.093

R. Ducray et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4698–4701
4699
Table 1
Table 2
IGF-1R and CDK2 inhibitory activity of compounds 1–3
Inhibition of IGF-1R and CDK2 kinase activity and inhibition of IGF-1R autophospho-
rylation in cell for compounds 3–10
N
N
O
N
N
N
N
N
O
N
O
N
N
H2N
N
S
N
O
N
R
N
N
N
N
H
H
N
N
X
2. X= H
3. X= OMe
O
1
a
a
a
Compd
R
IGF-1R IC50
(lM)
CDK2 IC50
(l
M)
Cell IC50 (lM)
Compd
IGF-1R
CDK2 IC50
(lM)
a
a
3
4
5
6
7
8
9
10
H
F
Cl
Br
Me
CF3
OMe
CN
0.45
>100
2.7
1.1
1.1
10
16
53
27
1.34
0.13
IC50
(
l
M)
0.089
0.010
0.003
0.047
0.015
0.090
0.092
1
2
3
1.26
0.27
0.45
0.001
0.078
>100
0.017
0.009
0.092
0.058
0.19
a
IC50 values are means of at least three experiments, standard deviation is less
than 2.
0.17
a
IC50 values are means of at least two experiments, standard deviation is less
With the improved selectivity profile of compound 3, we turned
than 2.5.
our attention to the optimization of the IGF-1R inhibition by mod-
ifying the 4-(imidazo[1,2-a]pyridin-3-yl)-pyrimidine core. Based
on the binding mode of compound 1 within the CDK2 ATP binding
site,10a we postulated that a small lipophilic substituent on the C5
position of the pyrimidine ring would favorably interact with the
side chain of the methionine gatekeeper of IGF-1R. We were also
interested in substituting the imidazopyridine ring in order to bet-
ter understand the structure–activity relationship in this chemical
series.
Modifications of the starting 4-(imidazo[1,2-a]pyridin-3-yl)-
pyrimidine scaffold have been performed thanks to the general
synthetic route outlined in Scheme 1,11 which represents a more
general and versatile route compared to the one published for
compound 1.10b Starting from the appropriate C5-substituted
2,4-dichloropyrimidine, a vinyl ethyl ether was prepared by a
Suzuki coupling with tri-ethoxyvinyl-borane.12 Alternatively, the
corresponding n-butyl ether was prepared by a Heck coupling with
n-butyl vinyl ether using phosphine-free conditions.13 The (2-chlo-
ropyrimidin-4-yl)-vinyl ether was subsequently transformed into
the desired imidazopyridine in a 2-steps one-pot procedure involv-
ing bromination with NBS followed by cyclocondensation with the
relevant 2-aminopyridine. The resulting 2-chloropyrimidines were
converted into the final products by acid mediated addition of the
2-methoxy-4-(N-acetyl-piperazin-1-yl) aniline.14 The cyano deriv-
ative 10 was obtained from compound 6 via a palladium catalyzed
cyanation and compound 23 was obtained by nucleophilic aro-
matic substitution of the chloro derivative 22 with pyrrolidine.
We started to explore the influence of the pyrimidine C5
substitution (compounds 4–10, Table 2) on the IGF-1R and CDK2
inhibitory activities as well as on the inhibition of IGF-1R auto-
phosphorylation in our cellular assay.9 As expected from the
postulated lipophilic interaction with the gatekeeper, anti IGF-1R
activity was significantly improved by addition of a halogen atom,
the chloro (5) and bromo (6) derivatives being more potent than
the fluoro (4) compound. A methyl group (7), provided an activity
between that of a fluoro (4) and a chloro (5). The more lipophilic
trifluoromethyl derivative (8) displayed a similar kinase inhibition
to chloro (5) but was slightly less potent in the cellular assay.
Methoxy (9) and cyano (10) groups were tolerated, providing the
same level of potency as the fluoro derivative 4. Although the C5
substitution restored some weak CDK2 activity, especially for com-
pounds 4–6, the selectivity ratios remained excellent.
R1
R
R2
R3
O
NH2
Cl
N
R4
N
R4
a
N
N
b
Cl
Cl
N
O
R1
R2
R3
N
N
N
N
N
NH2
R4
O
3-9, 11-22, 24-29
c
Cl
R2
N
With little difference in activity between 5 and 6, we decided to
study the substitution of the imidazopyridine ring in the less lipo-
philic 5-chloropyrimidine series. Because a substituent on the po-
sition 2 or 5 of the imidazopyridine would presumably increase the
torsion angle relatively to the pyrimidine, with a potential detri-
mental effect on the IGF-1R inhibition, we focussed our efforts on
positions C6, 7 and 8 (respectively, R3, R2 and R1 in Table 3). A fluo-
rine atom was well tolerated at all three positions (see 11, 17, 24,
and 29) with no significant impact on either enzymatic or cellular
potency. Similarly a methyl group (see 12, 18, and 25) did not af-
fect potency compared to the parent compound (11). When a
methoxy group was used (see 13, 19, and 26), a reduction in activ-
ity was observed, in particular with compound 13 in cells. The
other substitutions evaluated on position 8 (see 14–16) suggest
N
N
O
O
N
N
N
N
N
N
R4
N
Cl
N
N
N
N
N
H
H
O
O
22 (R2= Cl)
23 (R2= pyrrolidinyl)
6 (R4= Br)
10 (R4= CN)
e
d
Scheme 1. General synthesis of compounds 3–29. Reagents: (a) R = Et: ethynyl
ethyl ether, BH3, THF, then dichloropyrimidine, Pd(PPh3)4, NaOH; R = n-Bu: vinyl
butyl ether, triethylamine, Pd(OAc)2, PEG-400; (b) N-bromosuccinimide, dioxane/
water, then aminopyridine; (c) 2-pentanol, p-toluenesulfonic acid, 130 °C; (d)
ZnBr2, Zn, Pd2dba3, xantphos, DMA; (e) pyrrolidine.

4700
R. Ducray et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4698–4701
Table 3
Table 4
Inhibition of IGF-1R kinase activity and inhibition of receptor autophosphorylation in
Pharmacokinetic parameters16 of compound 5
cell for compounds 11–29
a
b
c
Species
CL
Vdss
iv t½
Fd (%)
Mouse
Rat
Dog
15
6.5
4.7
0.8
0.7
1.9
0.9
4.6
5.5
36
49
30
R1
R2
N
O
R3
a
b
c
Plasma clearance expressed in mL/min/kg.
Volume of distribution at steady state in L/kg.
Terminal half-life in hour.
N
N
N
N
Cl
d
Oral bioavailability (%).
N
N
O
Compds R1
R2
R3
IGF-1R IC50
M)
Cell IC50
M)
a
a
(
l
(l
Table 5
11
12
13
14
15
16
17
18
19
20
21
22
23
F
H
H
H
H
H
H
F
H
H
H
H
H
H
H
H
H
H
H
H
H
0.010
0.013
0.040
0.035
0.035
0.14
0.011
0.009
0.008
0.049
0.060
0.010
0.21
0.022
0.038
0.11
0.054
0.11
In vitro metabolic stability of compound 5
Me
MeO
NH2
CH2OH
CN
H
H
H
H
H
Mouse
32
Rat
15
Dog
6.7
Human
6.9
a
CLint
a
Intrinsic clearance from incubation studies with hepatocytes, expressed in
min/106 cell.
lL/
0.77
0.025
0.026
0.043
0.13
0.19
0.050
0.27
Me
MeO
CH2OH
CN
Cl
a dual kinase inhibitor of IGF-1R and IR such as 5 may prove advan-
tageous over specific IGF-1R targeting with antibodies.
The general kinase inhibition profile was assessed in a panel of
69 kinases and the results are shown as a heat map in Figure 2.
Only three kinases belonging to the Mitogen-Activated Protein Ki-
nase (MAPK) family were inhibited by more than 80% at a com-
pound concentration of
(MAPK9) and ERK8 (MAPK15).
Finally, no significant inhibition of the human ether-a-go-go re-
lated gene (hERG) ion channel was seen in an electrophysiological
H
H
Pyrrolidin-1-
yl
H
H
H
H
H
H
24
25
26
27
28
29
H
H
H
H
H
F
F
Me
MeO
CN
0.007
0.010
0.055
0.053
0.013
0.013
0.066
0.071
0.27
1 lM, namely JNK1 (MAPK8), JNK2
NMe2 0.43
0.022
F
0.024
a
IC50 values are means of at least two experiments, standard deviation is less
than 2.5.
assay (IC50 >30 lM).
The pharmacokinetic parameters of 5 in mouse, rat and dog are
presented in Table 4. This compound displays a low plasma clear-
ance and a low volume of distribution across the three species. Its
oral bioavailability is good, especially in the rat, and its metabolic
stability across species when incubated in the presence of hepato-
cytes (Table 5) predicts a low hepatic clearance in human. This pro-
vides evidence that compound 5 would be a suitable candidate for
further evaluation as an orally active IGF-1R kinase inhibitor.
In summary, we have identified a novel series of imidazo
[1,2-a]pyridines as inhibitors of the IGF-1R tyrosine kinase. Despite
the CDK2 activity of the original hit, modification of the aniline
provided a series with a good selectivity versus CDK2. The lead
compound 5 demonstrates an appropriate cell potency and a good
pharmacokinetic profile in pre-clinical species which enables fur-
ther evaluation in in vivo efficacy models. In the following paper
we report subsequent efforts towards the optimization of the ani-
line moiety in 5 and the resulting structure–activity relationship
against hERG inhibition that emerged.
the activity is governed by size rather than electronic properties
since the small but strongly donating amino group displays a sim-
ilar IC50 to a methyl. The same trend is observed at C7 (see 19–23),
the pyrrolidine derivative being the least active, as well as position
C6 (see 26–28), a dimethyl amino group being disfavoured.
Since none of the modifications performed on the imidazopyri-
dine ring led to a significant improvement over 5, further evaluation
of this lead compound was conducted. As expected from the high
homology of their kinase domains, 5 inhibits the Insulin Receptor
tyrosine kinase to the same extent as IGF-1R, as demonstrated by
its activity in a kinase biochemical assay (IC50 = 9 nM) and in a cel-
lular mechanistic assay15 (IC50 = 12 nM). There is increasing evi-
dence that the Insulin Receptor IR-A isoform is over-expressed in
many cancers and potentially drives tumor progression upon IGF2
binding and/or hetero-dimers formation with IGF-1R.1c Therefore
Figure 2. Heat map showing the inhibitory activity of compound 5 against a panel of 69 kinases at 1 lM. The percentage of inhibition is colored from green (0%) to yellow
(60%) to red (90%).

R. Ducray et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4698–4701
4701
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We would like to thank the following people for their contribu-
tion to the synthesis of the compounds described herein: Pascal
Boutron, Myriam Didelot, Hervé Germain, Franck Lach, Maryannick
Lamorlette, Antoine Legriffon, Mickael Maudet, Morgan Ménard,
Georges Pasquet, Fabrice Renaud, and Gail Young. We are also
grateful to Jonathan Wingfield, Tuseef Chaudhary, Katy Lynaugh
and Natalie Lahan for generating the biological screening data as
well as Margaret Veldman-Jones and Catherine Trigwell for their
work on in vitro assays development.
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the In Cell 3000 imaging system. The cellular IC50 values quoted in the present
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with a 12 points dose–response format on 384 well plates. In this case, the
Acumen reader platform was used to detect the signal and nuclear co-staining
was not required.
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14. Careful control of the reaction is necessary to avoid solvolysis of the acetamide.
However, when de-acetylation occured, the piperazine product was easily
converted back to the N-Ac compound by reaction with acetic anhydride.
15. Measuring inhibition of the Insulin Receptor auto-phosphorylation after
insulin stimulation in CHO cells transfected with the human gene of IR.
16. Mouse PK: nude mice dosed at 4
l
mol/kg iv and 10
lmol/kg po; rat PK: male
Han Wistar rats dosed at 4 mol/kg iv and 10
l
lmol/kg po; dog PK: mean values
for one male and one female beagle dogs dosed at 0.8
kg po.
lmol/kg iv and 10 lmol/