4-Arylphthalazin-1(2H)-one derivatives as potent antagonists of the melanin concentrating hormone receptor 1 (MCH-R1)

Article abstract of DOI:10.1016/j.bmcl.2011.10.111

A novel series of 4-arylphthalazin-1(2H)-one linked to arylpiperidines were synthesized and evaluated as MCH-R1 antagonists. The results of an extensive SAR study probing the effects of substituents on the 4-arylphthalazin-1(2H)-one C-4 aryl group led to the identification of the 4-(3,4-difluorophenyl) derivative as a highly potent MCH-R1 inhibitor with an IC₅₀ = 1 nM. However, further investigations showed that this substance has unacceptable pharmacokinetic properties including a high clearance and volume of distribution.

Full text of DOI:10.1016/j.bmcl.2011.10.111

Bioorganic & Medicinal Chemistry Letters 22 (2012) 427–430
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
4-Arylphthalazin-1(2H)-one derivatives as potent antagonists
of the melanin concentrating hormone receptor 1 (MCH-R1)
Chae Jo Lim a,b, Soo Hee Kim a,b, Byung Ho Lee a, Kwang-Seok Oh a, Kyu Yang Yi a,b,
⇑
a Bio-organic Science Division, Korea Research Institute of Chemical Technology, Yuseong-gu, Daejeon 305-600, 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:
Received 27 July 2011
Revised 28 October 2011
Accepted 31 October 2011
Available online 10 November 2011
A novel series of 4-arylphthalazin-1(2H)-one linked to arylpiperidines were synthesized and evaluated as
MCH-R1 antagonists. The results of an extensive SAR study probing the effects of substituents on the
4-arylphthalazin-1(2H)-one C-4 aryl group led to the identification of the 4-(3,4-difluorophenyl) deriva-
tive as a highly potent MCH-R1 inhibitor with an IC50 = 1 nM. However, further investigations showed
that this substance has unacceptable pharmacokinetic properties including a high clearance and volume
of distribution.
Keywords:
Ó 2011 Elsevier Ltd. All rights reserved.
Melanin concentrating hormone (MCH)
MCH-R1 antagonists
4-Arylphthalazin-1(2H)-one
Obesity
Obesity has become a global epidemic and a major risk factor for
many serious diseases including type 2 diabetes, dyslipidemia, cor-
onary heart disease, stroke and certain cancers.1 Among many cen-
trally-acting neuropeptides, the melanin concentrating hormone
(MCH), a cyclic 19-amino acid polypeptide, has attracted consider-
able recent attention as a target for obesity treatment. MCH is
expressed predominantly in the lateral hypothalamus of the brain
and is known to be involved in both regulation of feeding and
energy homeostasis.2 The effects of MCH are mediated by two types
of G protein-coupled receptors, MCH receptors-1 and -2 (MCH-R1
and -R2).3 The results of previous genetic and pharmacological
studies demonstrated that MCH-R1 plays an important role in the
control food intake and body-weight and suggested that this recep-
tor is one of the most promising targets for the obesity treatment.4
Indeed, numerous MCH-R1 antagonists have been found to have
antiobesity efficacy in diet-induced obesity (DIO) animal models.5
Despite the large efforts of a number of pharmaceutical companies
to develop a variety of pharmacophore derivatives of MCH-R1
antagonists as potential antiobesity therapeutic agents, few have
led to candidates that have advanced to the phase 1 clinical stage
owing to their unsuitable PK profiles and safety concerns.6
aimed at the development of potent MCH-R1 antagonists, we car-
ried out a SAR study of newly prepared pyridazin-3(2H)-ones based
on the lead structure 2, which has an IC50 value of 120 nM. An initial
effort focused on substances containing a modified pyridazin-
3(2H)-one core and explored the effects of fusing rings to the pyri-
dazin-3(2H)-one core. As the data in Table 1 show, introduction of
methyl groups at the 5- and 6-positions of pyridazin-3(2H)-one ring
gave a compound 3 that has a threefold improved affinity for MCH-
R1 compared with unsubstituted analog 2, whereas the cyclohexane
fused analog 4 was found to possess a slightly lower binding affinity.
Interestingly, thiophene ring fused (5) and phthalazin-1(2H)-one
(6) analogs were observed to exhibit sixfold more potent binding
than 2.
Based on the results described above, the phthalazin-1(2H)-one
heterocyclic system was proposed as a potentially interesting scaf-
fold for the development of new MCH-R1 antagonists. The activi-
ties of 4-arylphthalazin-1(2H)-one derivatives, in which a variety
N
Cl
N
In a previous investigation, we discovered that a series of 2-aryl
substituted benzimidazoles that are linked to arylpiperidines serve
as MCH-R1 antagonists.7 Among these substances, the p-chloro-
phenyl derivative 1 was found to exhibit high affinity to MCH-R1
(IC50 = 1 nM) (Fig. 1). As part of a continuing drug discovery program
N
NHCOMe
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. Benzimidazole based MCH-R1 antagonist.
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.10.111

428
C. J. Lim et al. / Bioorg. Med. Chem. Lett. 22 (2012) 427–430
Table 1
key intermediates, 4-arylphthalazin-1(2H)-one 8, are either com-
mercially available or were readily prepared by condensation of
2-benzoyl substituted benzoic acids 7 with hydrazine.9 The keto
acids 7 were generated by reaction of the corresponding aryl Grig-
nard reagents with phthalic anhydride. Alternatively, lithiation
reactions of 2-bromobenzoic acids using n-butyllithium followed
by acylation with aroyl esters were used to produce 7.10 Simple
alkylation reactions of 8 with 1-chloro-3-iodopropane, utilizing so-
dium hydride as a base, gave the desired 2-(3-chloropropyl)-4-
arylphthalazin-1(2H)-ones 9. The targets 6 and 10 were then
formed by using coupling reactions of 9 with the known N-[3-(4-
piperidinyl)phenyl]acetamide11 in the presence of sodium
carbonate.
Effects of substituents on the 5- and 6-positions of pyridazin-3(2H)-one derivatives
on MCH-R1 binding affinity
O
X
N
N
N
H
N
Y
O
Cl
a,b
Compound
X
Y
MCH-R1 IC50 (nM)
The binding affinities of the 4-arylphthalazin-1(2H)-one deriva-
tives 6 and 10 to membranes of CHO cells expressing human MCH-
R1 were evaluated. The measurements were performed by using a
competitive binding assay with Eu-labeled MCH and a time-
resolved fluorometric (TRF) assay.12,13 The results show that, in a
2
3
H
Me
H
Me
120
40
4
5
6
170
20
manner that is similar to those of benzimidazole derivatives,7
a
S
three carbon linker between phthalazin-1(2H)-one and arylpiperi-
dine moieties is preferred over two, four, and five carbon linkers
(data not shown). The effects of substituents at 6- and 7-positions
of the phthalazin-1(2H)-one were examined utilizing 4-(p-chloro-
phenyl) substituted phthalazin-1(2H)-one. The results summa-
rized in Table 2 demonstrate that, in general, substituents (R1) at
the 7-position of the phthalazin-1(2H)-one moiety, such as
F-(6a), Cl (6b), and OMe (6c), improved binding affinity slightly.
In addition, the 6-chloro (6e, R2 = Cl) and 6-methyl (6f, R2 = CH3)
analogs were equipotent to the unsubstituted compound 6 while
6-fluoro derivative 6d (R2 = F) was slightly less potent. Finally,
the dichloro analog 6g (R1 = R2 = Cl) also exhibited a good binding
activity. The combined results indicate that non-sterically bulky
substituents at the 6- and 7-positions of the phthalazin-1(2H)-
one group do not greatly affect MCH-R1 binding affinity.
20
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.
of substituents were introduced on the phthalazin-1(2H)-one C-4
aryl moiety, were examined.8 Below, the results of this effort
involving the synthesis, biological evaluation, and structure–activ-
ity relationships of variously substituted 4-arylphthalazin-1(2H)-
one derivatives are described.
The general synthetic routes employed for the preparation of 6
and 10, containing 4-arylphthalazin-1(2H)-one rings linked to aryl-
piperidine through their 2-position, are outlined in Scheme 1. The
Attention was next given to the effects of substituents on the
aryl ring positioned at C-4 of the phthalazin-1(2H)-one containing
hydrogens at C-6 and C-7, and the results are summarized in
Table 3. Removal (10a, R3 = H) or repositioning the chloro
O
R1
O
O
a
R1
R2
R1
O
OH
O
NH
N
R2
c
O
R2
R3
R3
O
b
R1
R2
OH
Br
7
8
O
O
R1
R2
R1
R2
N
N
Cl
N
N
N
H
N
d
e
O
R3
R3
9
6,10
Scheme 1. Reagents and conditions: (a) aryl-MgBr, THF, reflux; (b) n-BuLi, then aryl carboxylic ester, THF; (c) hydrazine, EtOH, reflux; (d) NaH, 1-chloro-3-iodopropane, DMF,
rt; (e) N-[3-(4-piperidinyl)phenyl]acetamide, Na2CO3, NaI (cat), DMF, 100 °C.

C. J. Lim et al. / Bioorg. Med. Chem. Lett. 22 (2012) 427–430
429
Table 2
Table 4
Effects of substituents (R1 and R2) of phthalazin-1(2H)-one derivatives on MCH-R1
Pharmacokinetic profile of 10n in rats
antagonist activity
Parametera
Value
iv t1/2 (h)
Oral AUC (lg h/mL)
iv CL (mL/kg min)
Vdss (L/kg)
%F
5.2
1.0
50
19
25
O
R1
N
N
N
H
N
R2
a
O
Determined in rats by administration of 10 mg/kg, iv and po (n = 3).
substrates possessing p-bromo (10e), p-trifluoromethyl (10f),
p-cyano (10g), p-methoxy (10h), p-methylthio (10i), p-dimethyl-
amino (10j), p-methyl (10k), and p-i-propyl (10l) substituents
all displayed reduced binding activities.
Cl
R1
a,b
Compound
R2
MCH-R1 IC50 (nM)
6
H
F
Cl
OMe
H
H
H
H
H
H
20
14
14
15
30
20
20
20
The effects of disubstitution on the 4-phenyl ring were also inves-
tigated. The 3,4-difluoro derivative 10n exhibited the most potent
binding (IC50 = 1 nM) to this receptor among all of the compounds
studied. Moreover, 2,4-difluoro (10m) and 3-fluoro-4-chloro (10r)
analogs retained high binding affinities as demonstrated by their
respective low IC50 values of 7 and 6 nM. Other disubstituted
substrates, such as the 2,4-dichloro (10o), 3,4-dichloro (10p),
2-fluoro-4-chloro (10q), 2,3,5-trifluoro (10s), and pentafluoro (10t)
derivatives, displayed comparatively lower binding affinities.
The 4-(3,4-difluorophenyl) substituted phthalazin-1(2H)-one
10n that has the most potent MCH-R1 binding ability was sub-
jected to further studies. The results show that this compound does
not inhibit the cytochrome P450 enzymes 2D6 and 3A4 (<10% at
6a
6b
6c
6d
6e
6f
F
Cl
Me
Cl
H
Cl
6g
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.
Table 3
Effects of substituents on the 4-aryl group of phthalazin-1(2H)-one derivatives on
MCH-R1 antagonist activity
10 lM) and has a low hERG binding activity (IC50 = 16 lM). In
addition, compound 10n displays good metabolic stability in hu-
man and rat liver microsomes (62% and 86% for 30 min, respec-
tively). Furthermore, in an in vivo rat iv/po pharmacokinetic
study (10 mg/kg), 10n displays modest oral bioavailability
O
N
N
N
H
N
(F = 25%) with an acceptable plasma level (AUC = 1.0 lg h/mL)
and half-life (t1/2 = 5.2 h) as shown in Table 4. However, compound
10n exhibits a high clearance (Cl = 50 mL/kg min) and a high vol-
ume of distribution (Vd = 19 L/kg).
O
R3
In summary, optimization of the initial pyridazin-3(2H)-one
lead compound 2 led to the discovery of the 4-arylphthalazin-
1(2H)-one derivatives as potent MCH-R1 antagonists. Further
extensive SAR studies probing substituents on the phenyl group
at the 4-position of the phthalazin-1(2H)-one resulted in the iden-
tification of the difluoro derivative 10n as a highly potent MCH-R1
antagonist. This compound also displayed good metabolic stability,
no significant inhibition of CYP450 enzymes, and acceptable hERG
binding activity. Further investigations of 4-arylphthalazin-1(2H)-
one derivatives, concentrating on the improvement of pharmacoki-
netic properties such as clearance and volume of distribution, are
now in progress.
a,b
Compound
R3
MCH-R1 IC50 (nM)
6
4-Cl
H
2-Cl
3-Cl
4-F
4-Br
4-CF3
4-CN
4-OMe
4-SMe
4-NMe2
4-Me
4-i-Pr
2,4-Di-F
3,4-Di-F
2,4-Di-Cl
3,4-Di-Cl
2-F-4-Cl
3-F-4-Cl
2,3,5-F
20
30
96
26
5
50
136
34
179
141
423
91
380
7
10a
10b
10c
10d
10e
10f
10g
10h
10i
10j
10k
10l
10m
10n
10o
10p
10q
10r
10s
10t
1
Acknowledgments
27
12
50
6
27
30
This research was supported by the Ministry of Education,
Science and Technology (2011-0019397) and the R&D program of
MKE/KEIT (10038744), Korea.
2,3,4,5,6-F
a
Binding affinities of compounds for MCH-R1 were determined by using a
Supplementary data
competitive binding with Eu-MCH and a TRF assay.
b
Values are means of at least two measurements.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2011.10.111.
substituent from the para-(6) to the ortho (10b, R3 = o-Cl) or meta
(10c, R3 = m-Cl) positions led to a slight reduction in binding
ability. A further exploration of the effects of substituents at the
para-position of phenyl group revealed that fluoro substituted
derivative 10d (R3 = p-F) has a fourfold higher binding affinity
(IC50 = 5 nM) compared to the p-chloro analog 6. However,
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