Arylpropionylpiperazines as antagonists of the human melanocortin-4 receptor

Article abstract of DOI:10.1016/j.bmcl.2006.05.088

A series of 3-arylpropionylpiperazines were synthesized as antagonists of the melanocortin-4 receptor. Their potency was found to be increased by replacing the α-methyl substituent of the initial lead 11 with a larger s-Bu or i-Bu group. Further potency enhancement was observed when a glycine or β-alanine was incorporated onto the benzylamine. Some compounds demonstrated good potency, moderate selectivity, and oral bioavailability.

Full text of DOI:10.1016/j.bmcl.2006.05.088

Bioorganic & Medicinal Chemistry Letters 16 (2006) 4674–4678
Arylpropionylpiperazines as antagonists of the human
melanocortin-4 receptor
Wanlong Jiang,a Fabio C. Tucci,a Caroline W. Chen,a Melissa Arellano,a Joe A. Tran,a
Nicole S. White,a Dragan Marinkovic,a Joseph Pontillo,a Beth A. Fleck,b Jenny Wen,d
John Saunders,a Ajay Madan,d Alan C. Fosterc and Chen Chena,*
aDepartment of Medicinal Chemistry, Neurocrine Biosciences, Inc., 12790 El Camino Real, San Diego, CA 92130, USA
bDepartment of Pharmacology, Neurocrine Biosciences, Inc., 12790 El Camino Real, San Diego, CA 92130, USA
cDepartment of Neuroscience, Neurocrine Biosciences, Inc., 12790 El Camino Real, San Diego, CA 92130, USA
dDepartment of Preclinical Development, Neurocrine Biosciences, Inc., 12790 El Camino Real, San Diego, CA 92130, USA
Received 3 May 2006; revised 23 May 2006; accepted 30 May 2006
Available online 13 June 2006
Abstract—A series of 3-arylpropionylpiperazines were synthesized as antagonists of the melanocortin-4 receptor. Their potency was
found to be increased by replacing the a-methyl substituent of the initial lead 11 with a larger s-Bu or i-Bu group. Further potency
enhancement was observed when a glycine or b-alanine was incorporated onto the benzylamine. Some compounds demonstrated
good potency, moderate selectivity, and oral bioavailability.
Ó 2006 Elsevier Ltd. All rights reserved.
The melanocortin-4 receptor (MC4R) is a member of
the G-protein-coupled receptor (GPCR) superfamily,
which plays an important role in the regulation of feed-
ing behavior.1 MC4R antagonists have been shown to
reverse lean body mass loss as well as block the reduc-
tion of food intake in animal models, suggesting the pos-
sible use for the treatment of cancer cachexia.2,3 In
addition, recent studies have also shown that MC4R is
involved in the pathophysiology of anxiety and depres-
sion.4 Therefore, a potent and selective MC4R antago-
nist has the potential to treat these diseases (Fig. 1).
tional antagonists and exhibit anti-cachexia activity in
a rodent animal model.9
By screening a set of compounds designed to target
GPCRs, using a radio-labeled binding assay, a lead
compound was identified.10 Thus, 11 possessed a Ki of
490 nM in competition with [125I]NDP-MSH binding
to cell membranes stably expressing the human melano-
cortin-4 receptor. A detailed structure–activity study
was then conducted to improve its in vitro potency.
Herein, we report on the SAR at both the substituted
phenylpropionyl group and the benzylamine of the lead
compound 11.
Several classes of non-peptide MC4R antagonists have
been reported. The benzamidine 1 has been shown to
be efficacious in a mouse cachexia model.5 Piperazines
such as 2 have demonstrated anxiolytic-like and antide-
pressant-like activities.6 Compound 3 potently inhibits
NDP-MSH binding to the MC4 receptor.7 In addition,
guanidines such as 4 are potent MC4R antagonists.8
We have previously reported that a series of b-Ala-
D(2,4-Cl)Phe dipeptide derivatives, such as 5, are func-
A general synthetic method was developed to quickly
synthesize analogs of 11 (Scheme 1). The piperazine-
benzaldehyde 7, obtained from the corresponding flu-
orobenzene 6, was converted to the imines 8 using
racemic tert-butanesulfinamide. Addition of various
alkyllithium reagents to 8 afforded the sulfinamides 9,
which were selectively deprotected at the Boc-group,
using trifluoroacetic acid, to give the piperazines 10.
Coupling reactions of 10 with 3-(2,4-dichlorophe-
nyl)propionyl chloride, followed by deprotection with
HCl, provided the racemic products 11–14 and 18–19
in good yields.
Keywords: Melanocortin; Antagonist; Arylpropinylpiperazine,
synthesis.
*
Corresponding author. Tel.: +1 858 617 7600; fax: +1 858 617
7967; e-mail: cchen@neurocrine.com
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.05.088

W. Jiang et al. / Bioorg. Med. Chem. Lett. 16 (2006) 4674–4678
4675
Cl
N
F
HN
O
O
N
N
O
N
N
Br
N
N
N
N
O
F
O
N
O
Boc
Cl
1
3
2
Cl
Cl
NH
O
F3C
NH2
N
H
N
H
N
Cl
N
N
Br
N
N
O
O
NH
F
O
O
O
O
NH2
4
11
5
Figure 1. Some small molecule MC4R antagonists.
O
X
X
X
S
a
b
N
c
O
O
N
N
F
N
N
Boc
Boc
6
7
8
R1
N
R1
N
O
S
R1
O
S
X
X
X
d
NH2
N
e
N
H
N
H
Cl
Cl
N
N
NH
10a-d, 9h-i
Boc
O
11-14, 18-19
9a-d, 9h-i
Scheme 1. Reagents and conditions: (a) N-Boc-piperazine/DMF/110–140 °C, 32–92%; (b) t-BuSONH2/Ti(OEt)4/THF, rt, 90–99%; (c) R1Li/Me3Al/
THF, À78 °C, 50–80%; (d) TFA/CH2Cl2; (e) 2,4-ClPhCH2CH2COOH/EDC/DMF/CH2Cl2; then HCl/MeOH, 80–90%.
Alternatively, the S-configured compounds S-14–17
were synthesized using S-tert-butanesulfinamide as
shown in Scheme 2.11 20 and 21 were obtained by cou-
pling reactions performed on S-10e and S-10f, respec-
tively, with various arylpropionic acids in the presence
of EDC. Reductive alkylation of the primary amines
S-14–15 with various aldehydes, in the presence of a
reducing agent, or coupling reactions of S-14-15 with
N-Boc-glycine or N-Boc-alanine, gave the products 22-
25. In the cases where the N-side chain contained a
Boc-protecting group, a TFA treatment was required
before final purification through a preparative HPLC
system.12
than the n-butyl analog 12, while the benzoxymethyl
compound 19 only possessed micromolar binding affin-
ity (Table 1).
A comprehensive survey of a set of substituted phenyl-
propionyl groups did not result in compounds with im-
proved binding affinities relative to S-14 or S-15. Thus,
the 4-chlorophenyl analog 20a and the 2-chloro-4-meth-
oxy compound 20b displayed comparable potency to
S-14, while the 2-chloro-3,4-dimethoxy compound 20c
(Ki = 2500 nM) had low binding affinity. A more
detailed study on analogs of compound S-15 resulted
in compounds 21a–u, which displayed low potency
(Table 2).
All compounds were tested in a ligand binding assay as
previously described,13 and the results are listed in Ta-
bles 1–3. The a-n-butyl derivative 12, the isopropyl ana-
log 13, and the s-butyl compound 14 were about 3-fold
better than 11 in binding affinity. The S-isomer of 14
had a Ki value of 75 nM, which was 2-fold better than
the mixture, suggesting that the S-enantiomer is the
more active species. The S-configured isobutyl analog
(S-15, Ki = 74 nM), as well as the 6- and 4-fluoro deriv-
atives S-16 and S-17, displayed comparable binding
affinity. The ethoxymethyl compound 18 was less potent
We then introduced a group on the benzylamino nitro-
gen of S-14 and S-15. While the N-ethyl compound
(22a, Ki = 92 nM) exhibited similar binding affinity to
its parent, other alkylations gave derivatives with lower
potency (22b–f). When an additional amine group was
incorporated, the compounds showed improved potency
in some cases (22g–k). For example, the N-(aminoethyl)
22g had a Ki value of 56 nM, and the N-(3-aminopropyl)
analog (22k, Ki = 30 nM) had about 2-fold improvement
in binding affinity over its parent (Table 3).

4676
W. Jiang et al. / Bioorg. Med. Chem. Lett. 16 (2006) 4674–4678
R1
O
S
R1
X
X
X
a
b,c
O
N
H
NH2
N
Cl
Cl
N
N
N
N
NH
Boc
S-10d-g
S-14-17
7
O
d, c
e or f
R1
R1
F3C
X
R2
NH2
N
H
Cl
Cl
N
N
N
Ar
N
20-21
O
O
22-25
Scheme 2. Reagents and conditions: (a) i–S-t-BuSONH2/Ti(OEt)4; ii—R1Li/Me3Al/THF, À78 °C, 50–80%; iii—TFA/CH2Cl2; (b) 2,4-
ClPhCH2CH2COOH/EDC/DMF/CH2Cl2, 80–90%; (c) HCl/MeOH; (d) ArCH2CH2COOH/EDC/DMF/CH2Cl2, 80–90%; (e) aldehyde/
NaB(OAc)3H/CH2Cl2, 80–90%; (f) N-Boc-glycine or N-Boc-b-alanine/EDC/DMF/CH2Cl2; then TFA/CH2Cl2, 70–80%.
Table 1. SAR of a-alkyl benzylamines at the human MC4R
Table 2. SAR of aryl propionyl group at the human MC4R
R1
R1
X
F3C
NH2
NH2
Cl
Cl
N
N
N
N
Ar
20: R1 = sec-Bu
21: R1 = i-Bu
11-19
O
O
Compound
X
R1
Ki (nM)
Compound
R1
Ar
Ki (nM)
11
12
4-CF3
Me
490
190
140
160
75
20a
20b
20c
21a
21b
21c
21d
21e
21f
21g
21h
21i
s-Bu
s-Bu
s-Bu
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
4-ClPh
2-Cl,4-MeOPh
86
57
4-CF3
4-CF3
4-CF3
4-CF3
4-CF3
6-F
n-Bu
i-Pr
13
14
2-Cl,3,4-MeOPh
2-FPh
2-ClPh
2500
2800
1700
3300
1400
2100
2200
4000
>10,000
350
s-Bu
s-Bu
S-14
S-15
S-16
S-17
18
i-Bu
i-Bu
74
2-HOPh
2-MeOPh
110
94
4-F
4-F
i-Bu
EtOCH2
BnOCH2
3-MePh
3-CF3Ph
690
1000
19
4-F
3-MeOPh
4-HOPh
4-MeOPh
4-MeSO2Ph
2,6-ClPh
While the sulfonamide 22l and carbamate 22m were only
weakly active, the acetyl analog 22n had a Ki value of
360 nM. By incorporating an amine into 22n, the result-
ing compounds (22o and 22p) displayed more than 4-
fold increase in potency. These results were confirmed
on the isobutyl analogs 23a–e. Thus, the glycine deriva-
tive 23d had a Ki of 19 nM.
21j
4900
2600
1500
4300
2500
6100
4600
2300
4400
640
21k
21l
3,4-OCH2OPh
3,4-MeOPh
2,5-MeOPh
2,4,5-MeOPh
2-Thienyl
21m
21n
21o
21o
21q
21r
21s
21t
21u
3-Benzothienyl
3-Benzoxazolyl
3-Indolyl
Selected compounds were tested for their selectivity over
the other melanocortin receptor subtypes. For example,
S-16 showed only 10-fold selectivity due to its low affin-
ity at the MC4R. 22p had good selectivity over the MC1
and MC3 receptors, while it still had good binding affin-
ity at the MC5 receptor (Ki = 170 nM, Table 4). In con-
trast, 23d also displayed over 25-fold selectivity at the
MC5R. None of the compounds exhibited significant
stimulation of cAMP release in cells expressing the
MC4 receptor at a 10 lM concentration, demonstrating
that they were not functional agonists. Instead, 22p and
1-Me-3-indolyl
2-Me-3-indolyl
3000
1600
23d showed dose-dependent inhibition of a-MSH-stimu-
lated cAMP production with IC50 values of 1.1 and
1.3 lM, respectively (Table 5).
Due to their desirable in vitro properties, 22p and 23d
were profiled for their pharmacokinetic properties in

W. Jiang et al. / Bioorg. Med. Chem. Lett. 16 (2006) 4674–4678
4677
Table 3. SAR of the N-alkyl or acyl group at the human MC4R
mice. After an intravenous injection at 5 mg/kg, 22p
exhibited a very low clearance (CL = 3.5 mL/min kg)
and low volume of distribution (Vd = 1.6 L/kg), result-
ing in a long half-life (t1/2 = 5.2 h) in this species. At 1-
and 4-h postdosing, the whole brain concentrations were
62 and 70 ng/g, which exhibited low brain/plasma ratio
of 0.02 and 0.03, respectively. After an oral dose of
10 mg/kg, 22p reached a maximal concentration of
1166 ng/mL at 6 h, and an area under the curve
(AUC) of 12,067 ng/mL h, which gave an absolute bio-
availability of 26.1%. The high plasma exposure might
reflect high plasma protein binding of this compound,
indicated by its low Vd value for a highly lipophilic mol-
ecule (measured logD was 4.0).14 In comparison, the
more lipophilic 23d (measured logD of 4.5) had a mod-
erate CL of 26.9 mL/min kg, a high Vd of 8.8 L/kg, and
a moderate t1/2 of 3.8 h. However, despite its high vol-
ume of distribution, the brain penetration of 23d was
still low, presumably caused by efflux mechanism at
the blood–brain barrier.15 23d had a moderate oral bio-
availability of 11.2%.
R1
X
R2
N
N
H
Cl
Cl
N
22-25
R1
O
Compound
X
R2
Ki (nM)
22a
22b
22c
22d
22e
22f
CF3 s-Bu
CF3 s-Bu
CF3 s-Bu
CF3 s-Bu
CF3 s-Bu
CF3 s-Bu
CF3 s-Bu
CF3 s-Bu
CF3 s-Bu
CF3 s-Bu
CF3 s-Bu
CF3 s-Bu
CF3 s-Bu
CF3 s-Bu
CF3 s-Bu
CF3 s-Bu
CF3 i-Bu
CF3 i-Bu
CF3 i-Bu
CF3 i-Bu
CF3 i-Bu
Et
92
2800
780
5900
>10,000
2600
56
CH2CH(Me)CH2CH3
CH2-2-tetrafuranyl
CH2C6H5
CH2C6H4F-2
CH2CH2Ph
22g
22h
22i
CH2CH2NH2
S-CH2CH(NH2)CH3
R-CH2-2-pyrrolidyl
S-CH2-2-pyrrolidyl
CH2CH2CH2NH2
SO2Me
89
110
64
22j
22k
22l
30
1800
2400
360
18
22m
22n
22o
22p
23a
23b
23c
23d
23e
24a
25a
25b
COOMe
COMe
In conclusion, a series of 3-arylpropionylpiperazines
were synthesized and studied as antagonists of the mel-
anocortin-4 receptor. The potency was increased when
the a-methyl of 11 was replaced by a larger s-butyl or
iso-butyl group. Further enhancements were observed
when either a glycine or b-alanine was incorporated
onto the benzylamine. Some compounds demonstrated
good potency, moderate selectivity, and reasonable oral
bioavailability.
COCH2NH2
COCH2CH2NH2
Me
13
1900
170
620
19
Et
CH2CH2Ome
COCH2NH2
COCH2CH2NH2
COCH2CH2NH2
25
a
F
F
F
EtOCH2
300
400
210
a
BnOCH2 CH2CH2NH2
a
BnOCH2 COCH2CH2NH2
a Racemic mixture.
References and notes
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MC4R
MC5R
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22p
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(21%)
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800
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a Binding affinity at the human melanocortin receptors expressed in
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Compound
22p
23d
iv dose (mg/kg)
CL (mL/min kg)
Vd (L/kg)
5
5
3.5
1.6
5.2
26.9
8.8
3.8
t1/2 (h)
AUC (ng/mL h)
Cbrain (ng/g) at 1, 4 h
Cbrain/Cplasma
po dose (mg/kg)
Cmax (ng/mL)
Tmax (h)
23,132
62, 70
3,071
43, 33
0.02, 0.03
0.08, 0.17
10
1,166
6
10
115
2
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AUC (ng/mL h)
F (%)
12,067
687
26.1
11.2
a Average of three animals.

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