Design, synthesis and biological evaluation of novel 3H-imidazole [4,5-b] pyridine derivatives as selective mTOR inhibitors

Article abstract of DOI:10.1016/j.bmcl.2017.06.010

A series of 3H-imidazo [4,5-b] pyridines derivatives were designed and synthesized as selective mTOR inhibitors. The systematic optimization of the molecules resulted in the identification of two compounds 10d and 10n with nanomolar mTOR inhibitory activity and selectivity over PI3Kα. Besides, compounds 10d and 10n demonstrated attractive potency against human breast cancer cells (MCF-7) and human ovarian cancer cell (A2780).

Full text of DOI:10.1016/j.bmcl.2017.06.010

Bioorganic & Medicinal Chemistry Letters 27 (2017) 3395–3398
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Design, synthesis and biological evaluation of novel 3H-imidazole [4,5-b]
pyridine derivatives as selective mTOR inhibitors
a,b,
Lingzhi Zhang a,c, Tantan Bu a,c, Xiaobo Bao a,b, Tingting Liang a,b, Yiran Ge b, Yungen Xu a,b, , Qihua Zhu
⇑
⇑
a Jiangsu Key Laboratory of Drug Design and Optimization, China Pharmaceutical University, Nanjing 21009, China
b Department of Medicinal Chemistry, China Pharmaceutical University, Nanjing 21009, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 3H-imidazo [4,5-b] pyridines derivatives were designed and synthesized as selective mTOR
Received 20 February 2017
Revised 22 May 2017
Accepted 2 June 2017
Available online 3 June 2017
inhibitors. The systematic optimization of the molecules resulted in the identification of two compounds
10d and 10n with nanomolar mTOR inhibitory activity and selectivity over PI3Ka. Besides, compounds
10d and 10n demonstrated attractive potency against human breast cancer cells (MCF-7) and human
ovarian cancer cell (A2780).
Ó 2017 Elsevier Ltd. All rights reserved.
Keywords:
Mammalian target of rapamycin
mTOR inhibitors
3H-Imidazo [4,5-b] pyridine
Antitumor
Synthesis
Introduction
the replacement of morpholine and aminopyrimidine by a substi-
tuted morpholine and an aryl urea on related imidazolopyrimidine
The mammalian target of rapamycin (mTOR) is a typical serine/
threonine protein kinase including two structural and functional
distinct complexes: mTOR complex 1 (TORC1) and mTOR complex
2 (TORC2). It plays an important role in regulating multiple signal-
ing pathways, including the mitotic signal, the nutrient sensing
signal and the insulin signal. The activity of mTOR is strongly reg-
ulated in response to physiological conditions, and aberrant mTOR
signaling occurs in multiple pathologies, including diabetes, cancer
and neurodegenerative diseases.1 Therefore, mTOR has been con-
sidered as a target for the treatment of human cancer. In the past
decades, a number of mTOR inhibitors have already been approved
or are under clinical development,2 including rapamycin,3 tem-
sirolimus,4 everolimus,5 AZD8055,6 GDC-0349,7 INK1288 and OSI-
0279 (Fig. 1).
scaffold (compound 2) significantly improved mTOR potency and
selectivity over PI3K kinases.11 In order to identify novel mTOR
inhibitor with excellent anti-tumor activity, further studies on
analogous of 2 were carried out in this research. Based on the
SAR of this series that both morpline and urea moiety play impor-
tant roles in the mTOR potency, imidazolopyrimidine scaffold was
replaced by 3H-imidazo [4,5-b] pyridine and a novel series of 3H-
imidazo [4,5-b] pyridine derivatives were synthesized as mTOR
inhibitors in our study.
The preparation of target compounds 10a–p was described in
Scheme 1. The target compounds were synthesized from commer-
cially available 4,6-dichloropyridin-2-amine through six steps.
Nitration of 3 with mixed acids of fuming nitric acid and sulfuric
acid afforded nitro compound 4,12 and then reactions with mor-
pholine or (S)-3-methylmorpholine at room temperature gave
intermediates 5a or 5b. The diamino compounds 6a or 6b, which
obtained by reduction of the nitro group by SnCl2, were heated
with formic acid or acetic acid to give the compounds 7a–d. The
key intermediates 8a–q, which obtained by substitution of amino
group by RI or RBr, underwent standard Suzuki coupling with pina-
col ester 9 to give target compounds 10a–p.
Wendy Lee and coworkers at Genentech Inc. identified imida-
zolopyrimidine 1 as a lead with moderate mTOR potency, low
selectivity versus PI3Ka
and PI3Kd (Fig. 2).10 They reported the first
hit to lead optimization of this compound and demonstrated that
⇑
Corresponding authors at: Jiang Su Key Laboratory of Drug Design and
Optimization, Department of Medicinal Chemistry, China Pharmaceutical
University, 24 Tongjiaxiang, Nanjing 210009, China.
E-mail addresses: xu_yungen@hotmail.com (Y. Xu), zhuqihua@vip.126.com
(Q. Zhu).
All target compounds were evaluated in vitro for their mTOR
enzyme inhibition activity. The results were summarized in Table 1.
As shown in Table 1, all target compounds except compound 10l
c
Both authors contributed equally to this work.
http://dx.doi.org/10.1016/j.bmcl.2017.06.010
0960-894X/Ó 2017 Elsevier Ltd. All rights reserved.

3396
L. Zhang et al. / Bioorganic & Medicinal Chemistry Letters 27 (2017) 3395–3398
Fig. 1. Structures of representative mTOR inhibitors.
Fig. 2. Structures and design strategy for target compounds 10a–p.
demonstrated varied inhibitory activities against mTOR at the con-
centration of 0.1 M. Among these compounds, compound 10c
Based on the above results, compounds 10d and 10n were cho-
sen to evaluate their PI3K enzyme inhibition activity. As shown in
Table 2, none of them displayed significant inhibition against
PI3K at the concentration of 10.0 M. Therefore, 10d and 10n,
as potent mTOR inhibitors, simultaneously exhibited acceptable
selectivity over PI3K
Encouraged by the favorable enzymatic activity of 10d and 10n,
compounds 10d and 10n were selected to evaluate their anti-pro-
liferative efficacies against several human tumor cell lines, includ-
ing human breast cancer cells (MCF-7) and human ovarian cancer
cell (A2780). GDC-0349, a selective mTOR inhibitor under phase I
clinical trial, was employed as the positive control. As illustrated
in Table 3, compounds 10d and 10n exhibit different degrees of
inhibitory activity against MCF-7 and A2780 cell lines, though less
efficient than the positive drug GDC-0349. Compounds 10n
l
a
(IC50 = 399 nM), which contained an n-propyl group at R position,
showed higher inhibitory activity than methyl and ethyl substi-
tuted compounds 10a and 10b. When the hydrogen atom at R1
position was substituted by methyl, compound 10d (IC50 = 290 nM)
exhibited good inhibitory activity. But with the increasing size of R
(10e–10j), the inhibitory activity decreased, which indicated that
smaller R group was optimal for mTOR enzyme inhibitory activity.
Similar phenomenon was observed when R1 was hydrogen atom
and R2 was methyl, n-propyl substituted analogs 10k showed
higher mTOR inhibitory activity than 10l. When R1 and R2 were
both substituted by methyl, compound 10n (IC50 = 332 nM) with
ethyl substituted at R position showed the highest mTOR enzyme
inhibition activity compared with 10m, 10o and 10p.
a
l
a.

L. Zhang et al. / Bioorganic & Medicinal Chemistry Letters 27 (2017) 3395–3398
3397
Scheme 1. Synthesis route for target compound 10a–p. Regents and conditions: (a) fuming HNO3, conc. H2SO4, 0–5 °C, 12 h; (b) DCM, DIEA, morpholine or (S)-3-
methylmorpholine, r.t., 2 h; (c) SnCl2Á2H2O/HCl, THF, 65 °C, 4 h; (d) formic acid or HOAc/PPA, reflux, 24 h; (e) RI or RBr, K2CO3, DMF, r.t., 12 h; (f) 9, K2CO3, Pd(PPh3)4, DMF,
H2O, 90 °C, 20 h.
Table 1
The structures and mTOR enzyme inhibition activity of compounds 10a–p.
Compound
R
R1
R2
Inhibition (%)a
IC50 (nM)
10a
10b
10c
10d
10e
10f
10g
10h
10i
10j
10k
10l
10m
10n
10o
10p
GDC-0349
Me
Et
n-Pr
n-Pr
n-Bu
EtOCH2
MeOCH2CH2
EtOCH2CH2
CH3OCH2CH2OCH2
Et2NCH2CH2
n-Pr
Bn
Me
Et
H
H
H
H
H
H
H
H
H
H
H
14
20
NDb
422
399
290
425
403
ND
ND
410
ND
ND
ND
ND
332
423
430
23
33
19.5
22.5
16
12.5
20
14
15
2
13
30
19
19
59
Me
Me
Me
Me
Me
Me
Me
H
H
H
Me
Me
Me
Me
Me
Me
H
Me
Me
Me
Me
n-Pr
i-Bu
a
The mTOR inhibition of compounds at 100 nM.
ND: Not determined.
b

3398
L. Zhang et al. / Bioorganic & Medicinal Chemistry Letters 27 (2017) 3395–3398
Table 2
Besides, compounds 10d and 10n were identified to be selective
over PI3K . Further studies will be carried out in the near future.
The inhibitory activities of compounds 10d and 10n against mTOR and PI3Ka.
a
Compound
IC50 (lM)
Acknowledgments
mTOR
PI3Ka
10d
10n
GDC-0349
0.29
0.33
0.039
>10.0
>10.0
4.2
We are thankful for the financial support of Outstanding
Scientific and Technological Innovation Team of Jiangsu Province
of China (2015).
Supplementary data
Table 3
Cellular activity with selected potent inhibitors 10d and 10n.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2017.06.
010.
Compound
IC50
(lM)
MCF-7
A2780
10d
10n
GDC-0349
0.064 0.020
0.018 0.0087
0.0058 0.0028
3.11 1.72
2.71 1.15
0.33 0.14
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In summary, a novel series of 3H-imidazole [4,5-b] pyridine
derivatives as selective mTOR inhibitors have been designed and
synthesized. Two target compounds were evaluated for their
mTOR inhibitory activity. In general, compounds 10d and 10n with
nanomolar mTOR inhibitory activity also demonstrated attractive
potency against the tested cell lines, including MCF-7 and A2780.