Design and synthesis of 3,7-diarylimidazopyridines as inhibitors of the VEGF-receptor KDR

Article abstract of DOI:10.1016/j.bmcl.2003.12.007

3,7-Diarylsubstituted imidazopyridines were designed and developed as a new class of KDR kinase inhibitors. A variety of imidazopyridines were synthesized and potent inhibitors of KDR kinase activity were identified with good aqueous solubility.

Full text of DOI:10.1016/j.bmcl.2003.12.007

Bioorganic & Medicinal Chemistry Letters 14 (2004) 909–912
Design and synthesis of 3,7-diarylimidazopyridines as inhibitors
of the VEGF-receptor KDR
Zhicai Wu,* Mark E. Fraley, Mark T. Bilodeau, Mildred L. Kaufman, Edward S. Tasber,
Adrienne E. Balitza, George D. Hartman, Kathleen E. Coll, Keith Rickert,
Jennifer Shipman, Bin Shi, Laura Sepp-Lorenzino and Kenneth A. Thomas
Departments of Medicinal Chemistry and Cancer Research, Merck Research Laboratories, PO Box 4,
West Point, PA 19486, USA
Received 22 October 2003; revised 24 November 2003; accepted 2 December 2003
Abstract—3,7-Diarylsubstituted imidazopyridines were designed and developed as a new class of KDR kinase inhibitors. Avariety
of imidazopyridines were synthesized and potent inhibitors of KDR kinase activity were identified with good aqueous solubility.
# 2004 Elsevier Ltd. All rights reserved.
Vascular endothelial growth factor (VEGF) is a regu-
lator of vascular permeability and an inducer of endo-
thelial cell proliferation, migration, and survival.
Activation of the VEGF pathway is a fundamental reg-
ulation of angiogenesis, the formation of new capillaries
from established blood vessels. In principle, anti-VEGF
drugs have potential as new therapy for neovascular-
ization-related diseases such as diabetic retinopathy,1
rheumatoid arthritis,2 psoriasis,3 and cancer.4 As eluci-
dated in molecular mechanisms, the mitogenic signal of
VEGF is mediated through the receptor tyrosine kinase
KDR (VEGFR-2).5 Accordingly, the signaling pathway
can be terminated by inhibitors against VEGF or its
receptor KDR. Indeed, antibodies against VEGF6 and
KDR,7 soluble VEGF decoy-receptors,8 and small
molecule inhibitors of KDR kinase9 have been shown to
be efficacious in in vivo tumor xenograft models. These
results have attracted much attention to develop ther-
apeutic reagents from anti-VEGF molecules, especially,
the small molecule KDR kinase inhibitors. Several
classes of KDR kinase inhibitors including indolin-2-
ones, phthalazines, and quinazolines have entered clin-
ical trials. Most recently, bevacizumab, an antibody
against VEGF was shown to significantly prolong
the time to progression of disease in patients with
metastatic renal-cell cancer.10
Chart 1.
As part of an effort to develop inhibitors of KDR, 3,6-
diarylpyrazolo[1,5-a]pyrimidines (1, Chart 1) have been
found to be potent inhibitors of KDR kinase activ-
ity.11,12 Compounds of this series had been hindered by
poor physical properties, including low solubility, high
logP and high protein binding. To address these
limitations, an effort was initiated to explore other
templates including benzimidazoles 2 and imidazo-
pyridines 3. These cores are likely replacements for
the pyrazolo[1,5-a]pyrimidine core since they share the
same key pharmacophore as 1: N-1 is an important
hydrogen bond acceptor and the two aryl substituents
11
form crucial hydrophobic interactions.
Indeed, 1,5-
diarylbenzimidazoles have been identified as potent
inhibitors of KDR kinase.13 In this communication, we
report the development of 3,7-diarylimidazopyridines as
a new class of KDR inhibitors.
Shown in Scheme 1 is the synthesis of the simple diphenyl
imidazopyridine 3. The protected aminopyridine 5
was obtained through the Curtius rearrangement of
* Corresponding author. Tel.: +1-215-652-6331; fax: +1-215-652-
7310; e-mail: zhicai_wu@merck.com
0960-894X/$ - see front matter # 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2003.12.007

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Z. Wu et al. / Bioorg. Med. Chem. Lett. 14 (2004) 909–912
Table 1. SAR of imidazopyridine 3-substituent
Compd
Ar
KDR IC50 (nM)
10a
145Æ37
10b
10c
10d
10e
10f
10g
930
242
2840
Scheme 1. (a) Diphenylphosphoryl azide, Et3N, t-BuOH, reflux, 82%;
(b) PhB(OH)2, Pd(PPh3)4, Na2CO3, dioxane, reflux, 59%; (c) HCl,
EtOAc, 70%; (d) BrCH2CHO, NaHCO3/H2O, 80 ꢀC, 19%; (e) ICl,
AcOH, 72%; (f) PhB(OH)2, Pd(PPh3)4, Na2CO3, dioxane, reflux,
26%.
42Æ0.5
99Æ15
50Æ6
4-iodopicolinic acid. Subsequent Suzuki coupling with
phenylboronic acid produced the diaryl compound 6.
Deprotection followed by cyclization with bromo-
acetaldehyde afforded the imidazopyridine 8. Treatment
with iodine, and then reaction with phenylboronic
acid generated 3.14 The KDR kinase inhibitory
activity of 3 (IC50=98 nM)15 was similar to that of
pyrazolopyrimidine 1 and benzimidazole 2. Encouraged
by the initial result, we decided to examine this new
imidazopyridine class thoroughly.
electronic and steric requirements of region I and gave
the best potency.
Having explored 3-substituents, we turned to position
C-7 of the imidazopyridines. Guided by synthetic
accessibility, we focused on 3-phenylimidazopyridines
while modifying 7-substituents. First, various sub-
stituents were attached to the 7-phenyl group through
the same sequence as in Scheme 1. Instead of using phenyl
boronic acid, 4-formylphenylboronic acid was employed
for the preparation of intermediate 11. The formyl func-
tionality in 11 was then utilized to append various
amines through reductive amination (Scheme 2).
First, we simply extended the chemistry illustrated in
Scheme 1 to replace the 3-phenyl with different aryl
groups via palladium catalyzed cross-coupling reactions
of compound 9. The KDR inhibitory potencies of these
compounds are shown in Table 1. It is worth noting
that 4-pyridyl (10a) maintained similar potency while
3-pyridyl group (10b) displayed decreased potency. This
result was in contrast to 3,6-diarylpyrazolo[1,5-a]-
pyrimidines11,12 in which both 3-pyridyl and 4-pyridyl
gave similar activity when compared to the phenyl
substituent. Moreover, substituted 3-pyridyls (10d)
showed significantly reduced potency. In the 3,6-diar-
ylpyrazolo[1,5-a]pyrimidines series, the 3-thienyl
analogue gave the best intrinsic potency.11 Thus, imida-
zopyridines with 5-membered heterocycles at position
C-3 were examined. We were pleased to find that the
2-(1,3-thiazolyl) and 3-isothiazolyl substituents16 increased
KDR activity 2-fold. These results were consistent with
docking studies using a KDR homology model that
depicted the bicyclic core bound in the adenine region of
the ATP binding site.17 In this model the 3-aryl sub-
stituent occupies the sterically confined hydrophobic
region I. Thus, small steric or electronic modifications
led to very different results (10a vs 10b; 10b vs 10d;
10e, 10g vs 10f). The 2-(1,3-thiazolyl) (10e) and
3-isothiazolyl (10g) substituents met presumably the
The KDR kinase inhibitory activity of 12a–d is pre-
sented in Table 2. We were pleased to find that the
intrinsic potency of these compounds was improved to
Scheme 2. Synthesis of compounds 12a–d.

Z. Wu et al. / Bioorg. Med. Chem. Lett. 14 (2004) 909–912
911
Table 2. SAR of 7-phenyl imidazopyridines
Following a standard pyridine N-oxidation, rearrange-
ment sequence the pyridone 17 was obtained. Alkyl-
ation of 17 provided various imidazopyridines.
The activity of a selected set of compounds 18a–d is
shown in Table 3. Indeed, the incorporation of a pyr-
idone substituent with a piperidine basic terminal group
resulted in a compound (18a) that exhibits good intrin-
sic activity as well as cell potency. More importantly,
18a demonstrated increased hydrophilic character with
an aqueous solubility of 3.4 mg/mL at pH 7.4. Other
substituents on the pyridone, such as acyclic amine 18c,
alcohol 18b or substituted piperidine 18d led to
decreased KDR activity.
Compd
R
KDR IC50 (nM)
77
Cell IC50 (nM)
300
12a
12b
12c
12d
52
69
13
410
nd
The selectivity of these imidazopyridines for KDR ver-
sus other closely related receptor tyrosine kinases and a
non-receptor tyrosine kinase is shown for compounds 10e
and 18a in Table 4. In general, these compounds show
modest levels of selectivity against the most KDR-
homologous kinases: Flt-1, Flt-4, and PDGF-receptor
b, and high level of selectivity against FGF receptor-1,
FGF receptor-2, and c-Src. This selectivity profile is
similar to that of the benzimidazole and pyrazolopyr-
imidine analogues.
110
IC50=13 nM (12d). However, the compounds were still
quite lipophilic with low aqueous solubility even though
a basic amine was present. Also, the cell potency of
these compounds (representing the inhibition of VEGF-
stimulated phosphorylation of KDR on tyrosine resi-
dues as determined in human embryonic kidney cells
expressing full length human KDR) was modest.
In summary, we have developed imidazopyrimidines as
a new class of potent KDR inhibitors with good aqu-
eous solubility. Avariety of 3,7-disubstituted imidazo-
pyrimidines were synthesized and their KDR inhibitory
activity tested. It was found that the introduction of
pyridone functionality at position C-7 significantly
improved potency and physical properties. The potency
and polarity of this class of compounds may be further
enhanced by a 3-thiazolyl or isothiazolyl substituent.
To increase the hydrophilicity of these imidazopyr-
idines, we decided to introduce more polarity into the
core. In the benzimidazole class, incorporation of pyr-
idone functionality enhanced cell potency and sig-
nificantly improved polarity and aqueous solubility.
Thus, imidazopyridines with a pyridone group were
prepared (Scheme 3). Starting with bipyridine 13, oxi-
dation and chlorination gave the chloropyridine 14.
Treatment of 14 with ammonium hydroxide at 180 ꢀC
afforded aminopyridine 15, which reacted with phenyl-
bromoacetaldehyde to produce imidazopyridine 16.
Table 3. SAR of imidazopyridines with 7-pyridone functionality
Compd
R
KDR IC50 (nM)
Cell IC50 (nM)
51
18a
28Æ4
18b
18c
OH
120
96
nd
nd
18d
67Æ0
280
Table 4. Fold-selectivity versus a series of kinases
Compd Flt-1 Flt-4 PDGFRb FGFR-1 FGFR-2 c-Src
10e
18a
7.0
3.1
1.7
2.8
1.4
1
47
60
23
35
165
68
Scheme 3. Synthesis of imidazopyridines 18a–d.

912
Z. Wu et al. / Bioorg. Med. Chem. Lett. 14 (2004) 909–912
Further efforts along this line will be reported in due
course.
10. Yang, J. C.; Haworth, L.; Sherry, R. M.; Hwu, P.;
Schwartzentruber, D. J.; Topalian, S. L.; Steinberg, S. M.;
Chen, H. X.; Rosenberg, S. A. N. Engl. J. Med. 2003, 349,
427.
11. Fraley, M. E.; Hoffman, W. F.; Rubino, R. S.; Hungate,
R. W.; Tebben, A. J.; Rutledge, R. Z.; McFall, R. C.;
Huckle, W. R.; Kendall, R. L.; Coll, K. E.; Thomas, K. A.
Bioorg. Med. Chem. Lett. 2002, 12, 2767.
12. Fraley, M. E.; Rubino, R. S.; Hoffman, W. F.; Ham-
baugh, S. R.; Arrington, K. L.; Hungate, R. W.;
Bilodeau, M. T.; Tebben, A. J.; Rutledge, R. Z.; Kendall,
R. L.; McFall, R. C.; Huckle, W. R.; Coll, K. E.; Thomas,
K. A. Bioorg. Med. Chem. Lett. 2002, 12, 3537.
Acknowledgements
We thank Kenneth D. Anderson for solubility determi-
nation. We also thank Dr. Art Coddington, Dr. Chuck
Ross and Dr. Harri Ramjit for mass spectral analyses.
References and notes
13. Bilodeau, M. T.; Cunningham, A. M.; Koester, T. J.;
Ciecko, P. A.; Coll, K. E.; Huckle, W. R.; Hungate, R. W.;
Kendall, R. L.; McFall, R. C.; Mao, X.; Rutledge, R. Z.;
Thomas, K. A. Bioorg. Med. Chem. Lett. 2003, 13, 2485.
14. All target compounds were fully characterized by 1H
NMR and mass spectroscopy.
1. Adamis, A. P.; Shima, D. T.; Yeo, K. T.; Yeo, T. K.;
Brown, L. F.; Berse, B.; D’Amore, P. A.; Folkman, J.
Biochem. Biophys. Res. Commun. 1993, 193, 631.
2. Giatromanolaki, A.; Sivridis, E.; Athanassou, N.; Zois,
E.; Thorpe, P. E.; Brekken, R. A.; Gatter, K. C.; Harris,
A. L.; Koukourakis, I. M.; Koukourakis, M. I. J. Path.
2001, 194, 101.
15. The KDR IC50 value represents biochemical inhibition of
phosphorylation of a poly-Glu/Tyr (4:1) peptide substrate
by isolated KDR kinase (cloned and expressed as a GST-
fusion protein), see: Kendall, R. L.; Rutledge, R. Z.; Mao,
X.; Tebben, A. L.; Hungate, R. W.; Thomas, K. A. J.
Biol. Chem. 1999, 274, 6453. Values are reported as single
determinations or as the average of at least two determin-
ationsÆstandard deviation.
3. Detmar, M. Dermatol. Sci. 2000, 24, S78.
4. (a) For reviews, see: Carmeliet, P.; Jain, R. K. Nature
2000, 407, 249. (b) Folkman, J. Nature Med. 1995, 1, 27.
5. (a) Veikkola, T.; Karkkainen, M.; Claesson-Welsh, L.;
Alitalo, K. Cancer Res. 2000, 60, 203. (b) Thomas, K. A.
J. Biol. Chem. 1996, 271, 603.
16. Compounds 10f and 10g were prepared similarly accord-
ing to the following Scheme:
6. Lin, Y. S.; Nguyen, C.; Mendoza, J. L. J. Pharm. Exp.
Ther. 1999, 188, 371.
7. Lu, D.; Jimenez, X.; Zhang, H.; Bohlen, P.; Witte, L.;
Zhu, Z. Int. J. Cancer 2002, 97, 393.
8. Holash, J.; Davis, S.; Papadopoulos, N.; Croll, S. D.; Ho,
L.; Russell, M.; Boland, P.; Leidich, R.; Hylton, D.;
Burova, E.; Ioffe, E.; Huang, T.; Radziejewski, C.; Bailey, K.;
Fandl, J. P.; Daly, T.; Wiegand, S. J.; Yancopoulos, G. D.;
Rudge, J. S. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 11393.
9. (a) For recent reviews, see: Bilodeau, M. T.; Fraley, M. E.;
Hartman, G. D. Expert Opin. Investig. Drugs 2002, 11,
737. (b) Boyer, S. J. Curr. Top. Med. Chem. 2002, 2, 973.
17. Traxler, P.; Furet, P. Pharmacol. Ther. 1999, 82, 195.