Pyridinesulfonylureas and pyridinesulfonamides as selective bombesin receptor subtype-3 (BRS-3) agonists

Article abstract of DOI:10.1016/j.bmcl.2011.02.011

Bombesin receptor subtype-3 (BRS-3) is an orphan G-protein coupled receptor belonging to the subfamily of bombesin-like receptors. BRS-3 is implicated in the development of obesity and diabetes. We report here small-molecule agonists that are based on a 4

Full text of DOI:10.1016/j.bmcl.2011.02.011

Bioorganic & Medicinal Chemistry Letters 21 (2011) 2040–2043
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Pyridinesulfonylureas and pyridinesulfonamides as selective bombesin
receptor subtype-3 (BRS-3) agonists
Michael M.-C. Lo a, , Harry R. Chobanian a, Oksana Palyha b, Yanqing Kan b, Theresa M. Kelly b,
⇑
Xiao-Ming Guan b, Marc L. Reitman b, Jasminka Dragovic c, Kathryn A. Lyons c, Ravi P. Nargund a, Linus S. Lin a
a Department of Medicinal Chemistry, Merck Research Laboratories, Rahway, NJ 07065, USA
b Department of Metabolic Disorders, Merck Research Laboratories, Rahway, NJ 07065, USA
c Department of Drug Metabolism, Merck Research Laboratories, Rahway, NJ 07065, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Bombesin receptor subtype-3 (BRS-3) is an orphan G-protein coupled receptor belonging to the subfam-
ily of bombesin-like receptors. BRS-3 is implicated in the development of obesity and diabetes. We report
here small-molecule agonists that are based on a 4-(alkylamino)pyridine-3-sulfonamide core. We
describe the discovery of 2a, which has mid-nanomolar potency, selectivity for human BRS-3 versus
the other bombesin-like receptors, and good bioavailability.
Received 30 November 2010
Revised 2 February 2011
Accepted 3 February 2011
Available online 25 February 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Bombesin receptor subtype-3 (BRS-3)
agonists
Orphan G-protein coupled receptor
Pyridinesulfonylureas
Pyridinesulfonamides
Anti-obesity drugs
Bombesin is a tetradecapeptide originally isolated from the skin
of the European frog Bombina bombina.1 Two mammalian bombe-
sin-like peptides, gastrin-releasing peptide (GRP)2 and neurom-
edin-B (NMB),3 have been identified so far, and they modulate
smooth muscle contraction, exocrine and endocrine processes,
metabolism, and behavior.4 Three mammalian bombesin-like
receptors have been described: NMBR for NMB, GRPR for GRP,
and BRS-3.5 BRS-3 is an orphan G-protein coupled receptor, with
relatively low affinities for bombesin, GRP, and NMB, and with
no known high-affinity natural ligands. It is expressed primarily
in the central nervous system, particularly in the hypothalamus,
a brain region that regulates food intake, metabolic rate, and body
weight.6 Indeed, rodent genetics has implicated BRS-3 in the devel-
opment of obesity and diabetes. Mice lacking BRS-3 have a reduced
metabolic rate and become obese and insulin-resistant as they age.
They are also hyperphagic, particularly with a high fat diet.7 The
dual effects on food intake and energy expenditure implicate
BRS-3 agonists as candidate anti-obesity drugs. In addition, BRS-3
is found in developing lungs and certain carcinoma cells, suggest-
ing that the receptor may play a role in cell growth and prolifera-
tion.8 Thus, the development of a BRS-3-selective agonist will be
useful for elucidating the physiologic and pharmacologic functions
of the receptor. Several such agonists have been disclosed,
including peptides9 and peptidomimetics.10 Recently, small-mole-
cule BRS-3-selective agonists have been developed,11,12 including a
series we report here based on a 4-(alkylamino)pyridine-3-sulfon-
amide core. These compounds have allowed exploration of the
physiology resulting from agonism of BRS-3.13
Since the endogenous ligand for BRS-3 remains unidentified, we
relied on high-throughput screening for our initial lead. Compound
1 has a binding IC50 of 600 nM14 and is an agonist with an EC50 of
220 nM15 for human BRS-3. Its structure bears strong resemblance
to the diuretic torsemide (Fig. 1),16 suggesting that this class of
compounds should have desirable pharmacokinetic properties.
Another attractive feature of compound 1 is its modular nature,
which enables the straightforward synthesis of analogs.
All the sulfonylureas reported here were synthesized from
4-chloropyridine-3-sulfonamide (Figs. 2 and 3). Nucleophilic aro-
matic substitution with amines or alcohols afforded the corre-
sponding 4-(alkylamino) or 4-(alkyloxy)pyridine-3-sulfonamides.
N
N
H
H
H
H
N
N
Me
Me
N
N
Me
Me
S
S
O O
O O
NH
O
Me
NH
O
torsemide
1
⇑
Corresponding author. Tel.: +1 732 594 3356; fax: +1 732 594 9545.
E-mail address: michael_lo@merck.com (M.M.-C. Lo).
Figure 1. Initial lead from high-throughput screening.
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.02.011

M. M.-C. Lo et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2040–2043
2041
N
Reaction of these sulfonamides with alkyl isocyanates under basic
conditions gave most of the desired sulfonylureas.17 The sulfonyl-
urea 2b was obtained by first acylating the sulfonamide with ethyl
chloroformate, followed by displacement of the resultant sulfo-
nylcarbamate with tert-amylamine.
N
H
N
H
N
a, b
NH2
S
R1
S
( )
O O
O O
Cl
NH
O
Table 1 summarizes the binding and functional activity of vari-
ous sulfonylureas for human and mouse BRS-3. No stereochemical
information was known about the initial HTS lead 1. Resynthesis of
compound 1 with diastereomerically pure 2-aminonorbornane
yielded racemic endo-1 and exo-1. Binding and functional data
indicate that the exo stereochemistry yields a slightly better li-
gand. Increasing the size of R1 from isopropyl (exo-1) to tert-butyl
(exo-2a) lowers the human EC50 value below 100 nM, and adding a
methyl group to the tert-butyl group enhances the activity (2b).
Further increase in the size of R1 to 1,1,3,3-tetramethylbutyl (2c)
or adamantyl (2d), however, leads to a loss in potency. Compound
2a is selective for BRS-3 in the human bombesin-like receptor fam-
ily. Its EC50 values in cell-based human NMBR and GRPR assays ex-
exo-2a, 2c, 2d
N
N
H
N
H
N
a, c, d
NH2
R1
S
S
( )
O O
O O
Cl
NH
O
2b
Me
Me
Me
Me
Me
Me
2b
Me Me
Me
R1:
Me
2a
Me
2c
2d
Figure 2. Synthesis of pyridinesulfonylureas—variation of the terminal amino
group. Reagents and conditions: (a) ( )-exo-2-aminonorborane, Et3N, i-PrOH,
100 °C; (b) R1NCO, Et3N, CH2Cl2; (c) ethyl chloroformate, pyridine; (d) tert-
amylamine, toluene.
ceed 10 lM.
In view of the better pharmacokinetic profile of 2a over 2b (vide
infra), tert-butyl was selected as the optimal substituent. Examina-
tion of various tert-butyl derivatives 3 reveals that the SAR of R2 is
quite stringent. A primary amine (R2NHÀ) with a secondary cyclic
R2 is required for activity. The best substituent identified to date is
(1R,2R,4S)-bicyclo[2.2.1]-hept-2-ylamine,18 and even minor
changes to the bicyclic core (3a, 3b) significantly decrease the hu-
man BRS-3 potency. Linear primary amines (3d, 3e) and cyclic sec-
ondary amines (3g) give compounds that are largely inactive.
Introduction of a tertiary nitrogen (3c) or a fused aromatic ring
(3f) also dramatically reduces the activity. Bicyclic alcohols that
are isosteric with the optimal amines lower the activity at human
BRS-3 by approximately 10- to 15-fold (3h).
We have examined the pharmacokinetic profiles of 2a and 2b in
male Sprague–Dawley rats (Table 2).19 As anticipated, both sulfo-
nylureas exhibit moderate clearance rates and good oral exposures.
A comparison of pharmacokinetic parameters shows that 2a exhib-
its a more desirable profile in terms of higher oral exposure and
longer half-life.20
N
N
X
H
H
N
a, b
NH2
N
Me
Me
S
S
O O
O O
Cl
O
Me
R2
3
NH
NH
NH
:
NH
NH
X
R2
( )
(1R,2R,4S)-2a (1S,2S,4R)-2a
NH NH
endo-2a
NH NH
3a
Me
Me
Ph
3d
Ph
N
(1R,2R,4R)-3b (1R,2S,4R)-3b
3c
3e
NH
N
O
O
( )
( )
Based on 2a, we designed 4a–4f to probe the importance of
each hydrogen-bond donor and acceptor to the overall potency
of the molecule (Fig. 4). Compound 4a was synthesized in a similar
fashion as 2a, with 4-chloropyridine-3-sulfonamide replaced by 2-
fluorobenzenesulfonamide. Compounds 4b and 4c were obtained
by acylation of 4-(exo-bicyclo[2.2.1]hept-2-ylamino)pyridine-3-
3g
(1S,2R,4R)-3h
exo-3h
3f
Figure 3. Synthesis of pyridinesulfonylureas—variation of the 4-substituent of
pyridine. Reagents and conditions: (a) R2-XH (X = NH, O), Et3N, tert-amyl alcohol,
120 °C; (b) t-BuNCO, Et3N, CH2Cl2.
Table 1
Binding and functional activity of compounds 1, 2a–2d, 3a–3h at human and mouse BRS-3
a
Compound
Human BRS-3 binding IC50 (nM)
Human BRS-3 agonism EC50, nMa (activity%)
Mouse BRS-3 agonism EC50, nMa (activity%)
exo-1
endo-1
exo-2a
endo-2a
(1R,2R,4S)-2a
(1S,2S,4R)-2a
2b
2c
510 40
820 150
100 20
230 50
440 80 (86)
940 80 (78)
81 12 (96)
160 20 (95)
55 4 (100)
720 110 (89)
51 11 (105)
350 30 (91)
220 20 (44)
450 40 (93)
1700 400 (73)
450 30 (86)
>10,000 (0)
86 11 (88)
98 14 (86)
28 4 (101)
49 5 (91)
21 1 (100)
110 20 (93)
13 2 (108)
100 10 (102)
39 8 (55)
260 50 (90)
90 24 (88)
33 6 (82)
>10,000 (0)
6900 (20)
68
630 50
75
7
5
530 100
310 50
400 70
660 90
300 70
>10,000
>10,000
3000
2d
3a
(1R,2R,4R)-3b
(1R,2S,4R)-3b
3c
3d
7500 (24)
8100 (19)
3e
4300 (43)
3f
3g
3800
6800 (39)
>10,000 (2)
1200 300 (90)
1600 300 (80)
1300 300 (63)
>10,000 (5)
320 90 (92)
320 70 (77)
>10,000
990, 1200
880, 1600
exo-3h
(1S,2R,4R)-3h
a
Values under 2
lM with reported standard errors are means of at least three experiments. Data under 2
lM from duplicate experiments are reported individually.

2042
M. M.-C. Lo et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2040–2043
Table 2
C
F
C
Pharmacokinetic properties in rats (1/4 mg kgÀ1 iv/po) for racemic 2a and 2b
H
H
N
a, b
NH2
N
Me
Me
( )
S
S
Parameters
( )-2a
( )-2b
O O
O O
NH
N
O
O
Me
Clp (mL minÀ1 kgÀ1
)
14
0.57
0.66
4.8
3.5
7.0
26
Vdss (L kgÀ1
MRT (h)
t1/2 (h)
)
0.30
0.19
0.25
0.68
2.0
4a
N
M h kg mgÀ1
)
H2
C
H
PO AUCNorm
Cmax
F%
(
l
c
NH2
N
Me
Me
(l
M)
( )
( )
S
O O
S
100
41
O O
NH
NH
Me
e, f
4b
d
Table 3
Binding and functional activity of compounds 4a–4f, 5a–5e at human and mouse BRS-
3
N
N
Me
N
H
H
N
O
Me
Me
N
Me
Me
( )
( )
S
S
Compound Human BRS-3
Human BRS-3
agonism EC50, nM
(activity%)a
Mouse BRS-3
agonism EC50, nM
(activity%)a
O O
O O
binding IC50
,
NH
O
Me
NH
N
O
Me
nMa
4c
4d
4a
4b
4c
4d
4e
4f
6200
6400 (50)
8300 (11)
8200 (44)
640 20 (99)
>10,000 (6)
8300 (22)
340 40 (91)
4900 (61)
590 60 (89)
180 20 (96)
7000 (13)
>10,000
>10,000
650 90
9700
N
H
H
g, h, a
N
N
Me
Me
( )
Cl
S
S
O O
NH
N
NH Me
O O
6700
6500 (49)
Cl
5a
5b
5c
5d
5e
520, 1400
2500
250 60
770 320
290 120
2600 (57)
2100 (81)
4000 (96)
4000 (20)
1200 500 (55)
380 110 (62)
1700 300 (57)
160 50 (102)
200 30 (81)
270 30 (113)
410 110 (72)
170 60 (84)
51 15 (87)
190 20 (88)
4e
Me
H
i
N
N
Me
Me
( )-2a
( )
S
(+ or À)-5e 68
7
O O
NH
O
Me
(À or +)-5e 310 40
a
Values under 2 lM with reported standard errors are means of at least three
4f
experiments. Data under
2
l
M
from duplicate experiments are reported
individually.
Figure 4. Analogs of sulfonylurea 2a, with replacements of hydrogen-bond donor/
acceptor highlighted in red. Reagents and conditions: (a) ( )-exo-2-aminonorborn-
ane, Et3N, tert-amyl alcohol, microwave (160–180 °C); (b) t-BuNCO, Et3N, CH2Cl2;
(c) tert-butylacetyl chloride, Et3N, CH2Cl2; (d) di-tert-butyl dicarbonate, Et3N,
CH2Cl2; (e) ethyl chloroformate, toluene; (f) N-methyl-tert-butylamine, toluene; (g)
1H-pyrazole-1-carboxamidine hydrochloride, Et3N, CH2Cl2; (h) tert-butylamine,
CH2Cl2, 50 °C; (i) 2.0 M TMSCHN2, MeOH.
N
N
H
Cl
S
a, b
N
S
Het
Me
( )
O O
Cl
O O
NH
sulfonamide with tert-butylacetyl chloride and di-tert-butyl dicar-
bonate respectively. Displacement of the corresponding ethyl
sulfonylcarbamate by N-methyl-tert-butylamine afforded 4d. The
synthesis of 4e was achieved in three steps from 4-chlorosulfonyl
chloride: (a) sulfonylation of 1H-pyrazole-1-carboxamidine; (b)
displacement with tert-butylamine; (c) nucleophilic aromatic sub-
stitution by exo-2-aminonorbornane. Finally, 4f was obtained by
methylating racemic 2a with trimethylsilyldiazomethane.
The binding and activity data for compounds 4a–4f (Table 3)
suggest that each hydrogen-bond donor and acceptor in 2a is
essential for activity at mouse and particularly human BRS-3, with
the possible exception of the less acidic sulfonylurea N–H. Given
these findings, we next explored heterocyclic replacements of the
amide group (Fig. 5). Heterocyclic amines were first sulfonylated
with 4-chlorosulfonyl chloride. The resultant products were dis-
placed with racemic exo-2-aminonorborane to afford sulfonamides
5a–5e.
5
Me
Me
Me
Me
N
N
Me
Me
Me
Het :
Me
S
HN N
a
b
c
Me
Me
N
HN
N
HN
N
d
e
Figure 5. Synthesis of pyridinesulfonamides. Reagents and conditions: (a) Het–
NH2, Et3N, CH2Cl2; (b) ( )-exo-2-aminonorbornane, Et3N, tert-amyl alcohol, micro-
wave (160–180 °C).
EC50 value of 380 nM and a full agonist of mouse BRS-3 with an
EC50 value of 51 nM.
In conclusion, we have identified pyridinesulfonylureas and
pyridinesulfonamides with
The phenyl, thiazole, pyrazole, and triazole rings are viable but
mediocre replacements of the amide functionality (Table 3). Com-
pounds 5a–5e are active compounds with EC50 values for human
a 4-(alkylamino)pyridine-3-sulfon-
BRS-3 increased to 1–4 lM. Their potencies at the mouse BRS-3,
amide core that are potent and selective agonists for human and
mouse BRS-3. The SAR defined by the current study is summarized
in Figure 6. In the sulfonylurea class, we have identified compound
2a as a potent human BRS-3 agonist that also has good bioavail-
ability and pharmacokinetic parameters in rats. Replacement of
the hydrogen-bond donors/acceptors in the sulfonylureas led to a
however, are considerably better, with EC50 values in the range
of 150–400 nM and close to full activity. A sterically demanding al-
kyl substituent at the 3-position is essential for potency.21 To
understand the intrinsic activity of the compound, racemic 5e
was separated by chiral HPLC,22 and the faster eluting enantiomer
was determined to be a partial agonist of human BRS-3 with an

M. M.-C. Lo et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2040–2043
2043
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N
N
Me
H
N
Me
D
Me
S
C
O O
H
optimal activity achieved with tert-butyl,
allows some variation in steric demand.
A
N-H important
cycloaliphatic amine, (1R,2R,4S)-bicyclo[2.2.1]
as the best substituent for the amino group;
potency quite sensitive to small changes
Figure 6. SAR summary of pyridinesulfonamides as BRS-3 agonists.
series of sulfonamides that are good mouse BRS-3 agonists, and
one of the enantiomeric triazolyl sulfonamide 5e shows moderate
human BRS-3 activity.
Acknowledgments
The authors thank I. Capodanno, J. Fenyk-Melody, M. Ferguson,
J. Hausamann, J. Mane, C. Nunes, X. Shen, K. Vakerich, Z. Yao for
dosing the animals used in pharmacokinetic experiments.
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value determined with an internal standard. 100% activation is defined as the
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17. All compounds reported in this letter were characterized by both 1H NMR and
LC–MS.
18. The absolute stereochemistry of the amine was determined by stereospecific
synthesis from (+)-endo-2-norborneol.
19. The blood samples were collected at various time points into lithium heparin
tubes and centrifuged. The plasma samples were kept at À70 °C until analysis.
The plasma samples were extracted by protein precipitation and analyzed by
LC/MS/MS.
20. We used a mouse PD model to determine target engagement in the CNS from
our compounds (see Ref. 13b for details of the model). Compound 2a was
tested active at 10 mpk when dosed orally.
21. Sulfonamides with heterocycles lacking sterically demanding 3-substituents
are inactive.
22. Chiralpak AD column, 12:88 ethanol/hexanes as eluent, tR = 11.4, 12.6 min.