Identification and structure-activity relationships of a new series of Melanocortin-4 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2006.01.016

The identification and optimization of a series of acylguanidine-based melanocortin-4 receptor antagonists is discussed.

Full text of DOI:10.1016/j.bmcl.2006.01.016

Bioorganic & Medicinal Chemistry Letters 16 (2006) 2302–2305
Identification and structure–activity relationships of a new series
of Melanocortin-4 receptor antagonists
Tricia J. Vos,* Suresh Balani, Christopher Blackburn, Ryan W. Chau, M. Diana Danca,
Stacey V. Drabic, Cheryl A. Farrer, Michael A. Patane, Stephen G. Stroud,
David L. Yowe and Christopher F. Claiborne
Millennium Pharmaceuticals, Inc. 40 Landsdowne St. Cambridge, MA 02139, USA
Received 9 November 2005; accepted 6 January 2006
Available online 3 February 2006
Abstract—The identification and optimization of a series of acylguanidine-based melanocortin-4 receptor antagonists is discussed.
Ó 2006 Elsevier Ltd. All rights reserved.
The melanocortins are a family of five G-protein cou-
pled receptors that play important physiological roles
including energy homeostasis, body weight regulation,
sexual behavior, anxiety, and pain.1 The melanocortin-
4 receptor is an important therapeutic target for the
treatment of a variety of disorders.2 Many peptidic
modulators of the melancortin receptors have been dis-
covered, but only in recent years have non-peptidic
small molecule agonists and antagonists been reported.3
secondary amines gave the acylguanidines 6. Alterna-
tively, N,N0-disubstituted acylguanidines 8 were pre-
pared by conversion of carboxylic acids 4 to amides 7
and then sequential treatment of the amide with sodium
Cl
X
H
H
N
N
NH
O
NH2
N
Br
O
Br
O
1
2
X = F
X = Cl
3
We recently reported a series of amidine-based MC4R
antagonists (e.g., 1 and 2).3c During the course of investi-
gations leading to the discovery of 1, acylguanidine 3 was
synthesized and found to be an active MC4R antagonist
(Ki = 0.70 lM, cAMP IC50 = 2.58 lM) which had re-
duced hERG binding (Ki = 3.15 lM for 3 vs 0.84 lM
for 2). We describe herein the optimization and struc-
ture–activity relationships of analogs of compound 3
(see Chart 1).
Chart 1. First- and second-generation small molecule MC4-R
antagonists.
A
A
A
H
H
H
H
a,b
c
X
N
N
X
N
N
R1
R2
X
OH
O
S
O
N
O
B
B
B
4
5
6
Analogs of 3 were prepared from the corresponding car-
boxylic acids4 as shown in Scheme 1. Acylguanidines
with substitution on the terminal nitrogen were prepared
by treatment of the appropriate carboxylic acid 4 with
fluoro-N,N,N0-tetramethylformamidinium hexafluoro-
phosphate (TFFH)5 followed by 2-methyl-2-thiopseu-
dourea sulfate to give the acylthioguanidine 5.
Displacement of the thiomethyl group with primary or
d, e
A
A
N
f, g, h
H
X
NH2
X
N
N
O
O
N
B
B
H
R
7
8
Scheme 1. Reagents and conditions: (a) SOCl2, reflux, 1 h; (b) 2-
methyl-2-thiopseudourea sulfate, 1 N NaOH, 0 °C, 3.5 h; (c) HNR1R2,
Et3N, o-xylene, 145 °C, 4 h; (d) DIPEA, TFFH, DMF, rt, 1.5 h; (e)
NH3(g), 5 min; (f) NaH, DMF, rt, 5 min; (g) RNCS, 60 °C, 0.5 h;
(h) 2-amino-5-diethylaminopentane, HgCl2, rt, 10 min.
Keywords: Melanocortin; Antagonist; Non-peptide; Small molecule.
*
Corresponding author. Tel.: +1 617 551 8692; e-mail:
tricia.vos@mpi.com
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.01.016

T. J. Vos et al. / Bioorg. Med. Chem. Lett. 16 (2006) 2302–2305
2303
hydride, an isothiocyanate, and an amine in the presence
of mercury (II) chloride.6
Additional analogs were prepared in order to further
investigate the effects of a distal basic group on MC4R
activity. Increasing the length of the tether between the
acylguanidine and the basic center from 2 to 3 methy-
lene units significantly increased MC4R binding and
function (13). Installation of an additional basic center
(14) or the presence of a secondary rather than tertiary
amine as the basic group (15) did not further enhance
potency. An additional increase in tether length from 3
Initial optimization of 3 focused on exploring the effects
of substituents on the terminal nitrogen atom (Table 1).
Small linear, branched, or cyclic alkyl substitutents (9,
10, and 11) had little effect on activity. An enhancement
of MC4R binding affinity was seen when a basic group
was included in the acylguanidine substituent (12).
Table 1. SAR of terminal substituted acylguanidine analogs
F
H
N
R1
R2
O
N
Br
O
N
H
Compound
R1
R2
H
MC4-R binding affinity (Ki, lM)a
MC4-R antagonism (IC50, lM)b
9
0.92
ND
10
11
H
H
0.77
2.6
8.5
ND
12
13
14
15
16
17
18
19
20
21
22
23
H
H
H
H
H
H
H
H
H
H
H
H
0.19
4.5
N
N
N
0.0032
0.064
0.080
0.0043
0.00058
0.021
0.026
0.0051
0.057
0.013
0.0028
0.33
0.67
5.7
N
N
H
0.13
0.070
0.39
0.65
0.18
1.2
N
N
N
N
N
N
0.031
0.097
NH
N
a Values are means of at least three experiments (ND, not determined); results obtained in MC4-R membrane filtration assay.
b Values are means of at least three experiments (ND, not determined); results obtained in cAMP functional assay.

2304
T. J. Vos et al. / Bioorg. Med. Chem. Lett. 16 (2006) 2302–2305
Table 2. N,N0-Disubstituted acylguanidine SAR
A
N
H
R
X
N
N
O
N
B
H
Compound
Ring A
X
R
H
Ring B
MC4-R binding affinity (Ki, lM)a
MC4-R antagonism (IC50, lM)a
F
16
CH2
0.0043
0.13
Br
Br
Br
O
Cl
24
25
CH2
CH2
H
H
0.00083
0.011
0.26
0.26
O
O
S
F
F
F
F
F
F
F
F
F
26
27
28
29
30
32
33
34
35
O
H
0.90
2.5
0.53
17
Br
Br
Br
Br
Br
O
O
O
O
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
i-Pr
Ph
Me
H
4.1
ND
ND
0.66
>33
0.17
1.5
6.3
0.11
26
Cl
H
Cl
O
H
0.014
0.089
0.69
O
H
H
1.8
a Values are averages of at least two determinations.
to 4 methylene units provided a boost in not only MC4R
binding but also functional activity (16, 17) Increasing
lipophilicity in the distal region provided analogs (18,
18, 20, and 21) which were less active than M. Binding
and functional activity were further enhanced when
the tether was constrained (22). A distal NH appears
to be important (23).
B-ring analogs (33–35) displayed varying levels of MC4R
activity, but none had activity superior to that of 16.
Pharmacokinetic profiles of compounds 13 and 22 were
compared head to head with 1. When dosed in rats, the
oral bioavailability of 1 and 13 are similar (4.6% F and
6.3% F, respectively) however their plasma half-lives
after iv dosing are extremely disparate (1 h for 1 vs
31 h for 13). Compounds in this series also have longer
brain half-lives than 1 as exemplified by comparison
with 22 (Fig. 1).
Using compound 16 as a starting point, SAR about the
remaining portions of the molecule were investigated
(Table 2). Changes to Ring A (24, 25) did not significant-
ly affect MC4R activity. Substitution of one methylene
unit by oxygen in the linker between the A and B rings
resulted in a moderate decrease in activity (26). N,N0-di-
substituted analogs (27–29) were very poor MC4R bind-
ers. B-ring analog 30 was slightly less active than 16,
while 32 was inactive in the antagonist assay. Other
In summary, we have identified a new series of small
molecule melanocortin-4 antagonists-based on an acyl-
guanidine scaffold. These compounds have improved
pharmacokinetic profiles relative to previously reported
amidine-based antagonists.

T. J. Vos et al. / Bioorg. Med. Chem. Lett. 16 (2006) 2302–2305
2305
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1000
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10
1 plasma
1 brain
22 plasma
22 brain
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0
5
10
15
20
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Time (hr)
Figure 1. Concentration vs. time profiles in brain and plasma for
compounds 1 and 22 after a 20 mg/kg po dose in mice. Each time point
represents the average of three animals.
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