Synthesis and SAR study of pyrrolo[3,4-b]pyridin-7(6H)-one derivatives as melanin concentrating hormone receptor 1 (MCH-R1) antagonists

Article abstract of DOI:10.1016/j.bmcl.2013.01.053

The discovery and optimization of novel pyrrolo[3,4-b]pyridin-7(6H)-one MCH-R1 antagonists are described. A systematic SAR study probing the effects of aryl-, benzyl- and arylthio-substituents at the 2-position of the pyrrolo[3,4-b]pyridin-7(6H)-ones led to identification of the 2-[(4-fluorophenyl)thio] derivative 7b as a highly potent MCH-R1 antagonist. This compound also has favorable pharmacokinetic properties along with a high metabolic stability and a minimal impact on CYP isoforms and hERG.

Full text of DOI:10.1016/j.bmcl.2013.01.053

Bioorganic & Medicinal Chemistry Letters xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and SAR study of pyrrolo[3,4-b]pyridin-7(6H)-one
derivatives as melanin concentrating hormone receptor 1 (MCH-R1)
antagonists
Chae Jo Lim a,b, Ji Young Kim a, Byung Ho Lee a, Kwang-Seok Oh a, Kyu Yang Yi a,b,
⇑
a Division of Drug Discovery Research, Korea Research Institute of Chemical Technology, Yuseong-gu, Daejeon 305-343, Republic of Korea
b Department of Medicinal and Pharmaceutical Chemistry, University of Science and Technology, Yuseong-gu, Daejeon 305-350, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
The discovery and optimization of novel pyrrolo[3,4-b]pyridin-7(6H)-one MCH-R1 antagonists are
described. A systematic SAR study probing the effects of aryl-, benzyl- and arylthio-substituents at the
2-position of the pyrrolo[3,4-b]pyridin-7(6H)-ones led to identification of the 2-[(4-fluorophenyl)thio]
derivative 7b as a highly potent MCH-R1 antagonist. This compound also has favorable pharmacokinetic
properties along with a high metabolic stability and a minimal impact on CYP isoforms and hERG.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 8 November 2012
Revised 4 January 2013
Accepted 16 January 2013
Available online xxxx
Keywords:
Melanin concentrating hormone (MCH)
MCH-R1 antagonists
Pyrrolo[3,4-b]pyridin-7(6H)-one
Obesity
The prevalence of obesity has grown and it now has reached
global epidemic levels with more than one billion adults in world
being overweight and at least 200 million being classified as ob-
ese.1 In addition, obesity is a major risk factor for many other dis-
eases including type 2 diabetes, dyslipidemia, coronary heart
disease, stroke and certain cancers.2 Unfortunately, no efficacious
and safe anti-obesity drugs are currently available.3 Among many
centrally acting neuropeptides, the melanin concentrating hor-
mone (MCH) has received recent attention as a target for obesity
treatment. MCH, a cyclic 19-amino acid polypeptide, is primarily
expressed in the lateral hypothalamus and zona incerta areas of
the brain.4 This peptide, which is known to play a physiological
role in both the regulation of feeding and energy homeostasis, is
mediated by two types of G protein-coupled receptors called
MCH receptor 1 and 2 (MCH-R1 and -R2).5 While the exact biolog-
ical functions of MCH-R2 are still unknown, previous genetic and
pharmacological studies have demonstrated that MCH-R1 plays
an essential role in the control of food intake and body weight.6
Therefore, MCH-R1 is considered to be one of the most promising
targets for treating obesity. Indeed, numerous MCH-R1 antagonists
have been found to have anti-obesity efficacy in diet-induced obes-
ity (DIO) animal models.7
few candidates have progressed to the phase 1 clinical stage owing
to their unsuitable PK profiles and safety concerns.8 In particular,
substances possessing diverse heteroaryl fused bicyclic ring sys-
tems, such as thiazolopyridinones, thienopyridazinones, and imi-
dazopyrimidinones, based on the structure of GW856464
containing 3-methoxy-4-[2-(pyrrolidin-1-yl)ethoxy]aniline moiety
have been reported to serve as MCH-R1 antagonists.9 In continuing
efforts aimed at the development of novel and potent MCH-R1
antagonists as anti-obesity agents,10 we recently identified several
pyrrolo[3,4-b]pyridin-7(6H)-ones that have highly potent binding
affinities to MCH-R1 (Fig. 1).11 Herein, we describe the results of
an effort involving the synthesis, biological evaluation, and struc-
ture–activity relationships of several 2-substituted pyrrolo[3,4-
b]pyridin-7(6H)-one derivatives.
The general synthetic route employed for the preparation of 2-
substituted pyrrolo[3,4-b]pyridin-7(6H)-one derivatives 5, 6 and 7
is outlined in Scheme 1. Methyl 3-methyl-2-picolinate was reacted
with m-CPBA to give the corresponding N-oxide 1, which upon
O
OMe
O
X
N
In spite of the fact that a variety of substances have been ex-
plored as MCH-R1 antagonists in the pharmaceutical industry,
N
Ar
n
N
X = CH2, S; n = 0 or 1
⇑
Corresponding author. Tel.: +82 42 860 7143; fax: +82 42 860 7149.
E-mail address: kyyi@krict.re.kr (K.Y. Yi).
Figure 1. 2-Substituted pyrrolo[3,4-b]pyridin-7(6H)-one derivatives.
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.01.053
Please cite this article in press as: Lim, C. J.; et al. Bioorg. Med. Chem. Lett. (2013), http://dx.doi.org/10.1016/j.bmcl.2013.01.053

2
C. J. Lim et al. / Bioorg. Med. Chem. Lett. xxx (2013) xxx–xxx
O-
N+
O
O
O
O
N
Cl
N
Cl
N
a
b
c
OMe
Me
OMe
Me
OMe
Br
OMe
Me
3
2
1
R1
O
OMe
O
N
e
N
N
O
OMe
O
Cl
N
d
5a-w
N
N
O
OMe
N
X
f or g
N
4
R2
N
O
6a-g:
X = CH2
7a-i: X = S
O2N
OMe
OH
O2N
OMe
H2N
OMe
h
i
N
N
O
O
9
8
Scheme 1. Reagents and conditions: (a) m-CPBA, dichloromethane, rt; (b) POCl3, ClCH2CH2Cl, reflux; (c) N-bromosuccinimide, benzoyl peroxide, CCl4, reflux; (d) 9, acetic acid,
reflux; (e) arylboronic acid, Pd(PPh3)4, 2 M Na2CO3, MeOH and toluene (1:4), reflux; (f) benzylboronic acid pinacol ester, Pd(PPh3)4, Ba(OH)2, 1,4-dioxane, reflux; (g) aryl thiol,
Et3N, DMF, 130 °C; (h) 1-(2-chloroethyl)pyrrolidine hydrochloride, K2CO3, DME and H2O (10:1), reflux; (i) 10% Pd/C, H2(g), MeOH.
treatment with excess phosphorus oxychloride provided a mixture
Table 1
of ortho- and para-chloro regioisomeric products. Separation by
using silica gel column chromatography gave the desired ortho-
chloropyridine 2.12 Reaction of 2 with NBS in the presence of
benzoyl peroxide in CCl4 at reflux afforded the benzyl bromide
derivative 3. In a parallel sequence, 3-methoxy-4-[2-(pyrrolidin-
1-yl)ethoxy]aniline 9 was prepared starting by treatment of
2-methxoy-4-nitrophenol13 with 1-(2-chloroethyl)pyrrolidine,
using sodium carbonate as a base, to generate the ether 8. Reduc-
tion of the nitro group in 8 utilizing catalytic hydrogenation
provided the corresponding aniline derivative 9. In initial studies,
we observed that reaction of 3 with aniline 9 using several bases
did not produce the desired 2-chloro-pyrrolo[3,4-b]pyridin-
7(6H)-one 4. However, when this reaction was carried out in acetic
acid, condensation product 4 was generated in high yield. Finally,
the target 2-substituted pyrrolo[3,4-b]pyridin-7(6H)-ones 5 and 6
were obtained by reaction of 4 with selected arylboronic acids or
benzylboronic acid pinacol esters under standard Suzuki coupling
conditions. In addition, the 2-arylthio derivatives 7 were synthe-
sized by using simple nucleophilic substitution reactions of 4 with
aryl thiols with triethylamine as a base.
The binding affinities of the 2-substituted pyrrolo[3,4-b]pyri-
din-7(6H)-one derivatives 5, 6 and 7 to membranes of CHO cells
expressing human MCH-R1 were determined by using a competi-
tive binding assay with Eu-labeled MCH and a time-resolved fluo-
rometric (TRF) assay.14 The initial SAR study focused on exploring
the effects of various substitutents on the 2-phenyl group of the
pyrrolo[3,4-b]pyridin-7(6H)-ones. As the results summarized in
Table 1 show, while the substance having a methoxy group at
the para-position (5d) exhibits moderate binding affinity for
MCH-R1, those in which this group is either removed (5a, R1 = H)
or repositioned from the para to ortho (5b) or meta (5c) positions
have significantly decreased binding affinities. In addition, binding
of the chloro (5e–g), fluoro (5h–i), methyl (5l–m) and ethyl (5n–
o)-substituted derivatives also displays similar trends. A further
exploration of the effects of substitutents at the para position of
aryl group demonstrated that substances containing fluoro (5i),
Effects of substituents on the 2-phenyl group of pyrrolo[3,4-b]pyridin-7(6H)-one
derivatives on MCH-R1 binding affinity
R1
O
OMe
O
N
N
N
5
a,b
Compound
R1
MCH-R1 IC50 (nM)
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
5m
5n
5o
5p
5q
5r
H
10,000
7110
2010
340
510
5650
440
4130
3100
540
10,000
1040
80
2000
50
210
2-OMe
3-OMe
4-OMe
2-Cl
3-Cl
4-Cl
2-F
4-F
4-CF3
4-CN
2-Me
4-Me
2-Et
4-Et
4-n-Pr
4-n-Bu
4-i-Pr
2780
90
5s
5t
4-Vinyl
4-t-Bu
130
320
5u
5v
5w
2,3-Di-Me
2,5-Di-Me
3,4-Di-Me
190
300
180
a
Binding affinities of compounds for MCH-R1 were determined by using a
competitive binding with Eu-MCH and a TRF assay.
b
Values are means of at least two measurements.
trifluoromethyl (5j) and nitrile (5k) moieties at this position all
have low binding affinities. In contrast, introduction of a methyl
group at the para-position (5m) was found to result in a high
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C. J. Lim et al. / Bioorg. Med. Chem. Lett. xxx (2013) xxx–xxx
3
Table 2
Table 3
Effects of substituents on the 2-benzyl and 2-arylthio groups of pyrrolo[3,4-b]pyridin-
7(6H)-one derivatives on MCH-R1 binding affinity
In vivo pharmacokinetic profiles of 7a and 7b in rats
Parametera
7a
7b
O
OMe
O
t1/2 (h)
2.8
3.4
5.7
1.6
8.3
92
X
N
N
R2
N
Oral AUC (
l
g h/mL)
0.32
25.7
22.7
35
iv CL (mL/kg min)
Vdss (L/kg)
F (%)
6, X = CH2; 7, X = S
a
Determined in rats by administration of 10 mg/kg, iv and po (n = 3).
Compound
X
R2
MCH-R1 IC50 (nM)
a,b
6a
6b
6c
6d
6e
6f
6g
7a
7b
7c
7d
7e
7f
CH2
CH2
CH2
CH2
CH2
CH2
CH2
S
S
S
S
S
H
2-Me
3-F
4-F
4-Cl
4-Me
4-t-Bu
H
4-F
4-Cl
4-Me
4-OMe
2,4-Di-F
3,4-Di-F
2-Cl-4-F
3-Cl-4-F
70
720
410
80
120
160
1910
20
30
40
60
100
60
(7i) derivatives, also exhibited high binding activities. Interest-
ingly, replacement of the arylthio group (R2PhS) with an arylsulfo-
nyl group (R2PhSO2) led to complete loss of MCH-R1 binding
activity (data not shown).
The 2-phenylthio- (7a) and 2-[(4-fluorophenyl)thio]- (7b) pyr-
rolo[3,4-b]pyridin-7(6H)-ones that display the most potent MCH-
R1 binding affinity were subjected to further studies. The pharma-
cokinetic properties of the two derivatives, determined employing
an iv/po pharmacokinetic study (10 mg/kg) in rats, are shown in
Table 3. While both compounds showed acceptable half-lives and
clearances, 7b has a relatively better plasma level and volume of
distribution than 7a. In addition, 7b displayed excellent oral bio-
availability (F = 92%) and it exhibits good metabolic stability in hu-
man and rat liver microsomes (97% and 79% for 30 min,
respectively), moderate permeability (1.9 Â 10À6 cm/s, PAMPA)
S
S
S
S
7g
7h
7i
50
80
50
a
Binding affinities of compounds for MCH-R1 were determined by using a
competitive binding with Eu-MCH and a TRF assay.
b
Values are means of at least two measurements.
and high aqueous solubility (104
bit the cytochrome P450 enzymes 2D6 and 3A4 (<10% at 10
and it has a low hERG binding activity (IC50 = 25 M).
l
g/mL). Finally, 7b does not inhi-
lM)
l
MCH-R1 binding activity (IC50 = 80 nM). Moreover, the ethyl deriv-
ative 5o was found to have a 1.6-fold more potent binding affinity
(IC50 = 50 nM) than the p-methyl analog 5m. When the size of the
alkyl group was increased from ethyl (5o) to n-propyl (5p) and n-
butyl (5q), a large decrease in MCH-R1 binding activity took place.
Other analogs having similar two carbon units such as i-propyl
(5r), vinyl (5s) and t-butyl (5t) displayed comparatively lower
binding affinities. The effects of disubstitution on the phenyl group
were also investigated. Incorporation of two methyl groups at the
2,3- (5u), 2,5- (5v) and 3,4- (5w) positions led to reduced binding
affinities compared to the 4-methyl analog 5m. The SAR results de-
scribed above indicate that the properties of substitutents on phe-
nyl group at the 2-position of the pyrrolo[3,4-b]pyridin-7(6H)-one
ring system play significant roles in determining binding to MCH-
R1.
In summary, the studies described above have led to the discov-
ery of pyrrolo[3,4-b]pyridin-7(6H)-one derivatives that act as
MCH-R1 antagonists. An extensive, systematic SAR investigation
probing the effects of substituents on the 2-aryl, 2-benzyl and
2-arylthio groups on the pyrrolo[3,4-b]pyridin-7(6H)-one ring sys-
tem resulted in the identification of the 2-[(4-fluorophenyl)thio]-
pyrrolo[3,4-b]pyridin-7(6H)-one 7b as a highly potent MCH-R1
antagonist. This compound was observed to have good pharmaco-
kinetic properties, metabolic stability and minimal impact on CYP
isoforms and hERG. Further investigations aimed at evaluating the
in vivo efficacy of 7b in animal models are underway.
Acknowledgments
Our attention next turned to an exploration of the effects of
substituents on the aromatic ring of the 2-benzyl group. In a man-
ner that is similar to the results of the SAR performed on members
of the 2-phenyl-pyrrolo[3,4-b]pyridin-7(6H)-one family, the bind-
ing affinities of simple benzyl (6a), o-methyl (6b) and m-fluoro
(6c) analogs were lower than that of the p-ethylphenyl analog 5o
(Table 2). In addition, other benzyl-substrates possessing p-fluoro
(6d), p-chloro (6e), p-methyl (6f) and p-t-butyl (6g) substituents
were found to display comparatively lower binding affinities than
that of 6a.
This research was supported by a Grant from the Ministry of
Education, Science and Technology (2011-0019397) and the R&D
program of MKE/KEIT (10038744), Korea.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2013.01.
053.
The effect of replacement of the benzyl group with an arylthio
group at the 2-position of the pyrrolo[3,4-b]pyridin-7(6H)-ones
was evaluated. The results, summarized in Table 2, show that, in
general, the arylthio compounds 7 have higher binding affinities
than the corresponding aryl (5) and benzyl (6) derivatives. The
phenylthio-analog 7a was observed to have the most potent
MCH-R1 binding activity among all substances tested
(IC50 = 20 nM). The p-fluoro (7b) and p-chloro (7c) arylthio analogs
bind slightly less tightly with IC50 values of 30 and 40 nM, respec-
tively, while other substrate such as those containing 4-methyl
(7d) and 4-methoxy (7e) substituents have reduced binding activ-
ities. Additionally, disubstitued substrates, such as the 2,4-difluoro
(7f), 3,4-difluoro (7g), 2-chloro-4-fluoro (7h) and 3-chloro-4-fluoro
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