Synthesis and SAR of derivatives based on 2-biarylethylimidazole as bombesin receptor subtype-3 (BRS-3) agonists for the treatment of obesity

Article abstract of DOI:10.1016/j.bmcl.2010.02.076

This Letter describes a series of potent and selective BRS-3 agonists containing a biarylethylimidazole pharmacophore. Extensive SAR studies were carried out with different aryl substitutions. This work led to the identification of a compound 2-{2-[4-(pyridin-2-yl)phenyl]ethyl}-5-(2,2-dimethylbutyl)-1H-imidazole 9 with excellent binding affinity (IC₅₀ = 18 nM, hBRS-3) and functional agonist activity (EC₅₀ = 47 nM, 99% activation). After oral administration, compound 9 had sufficient exposure in diet induced obese mice to demonstrate efficacy in lowering food intake and body weight via BRS-3 activation.

Full text of DOI:10.1016/j.bmcl.2010.02.076

Bioorganic & Medicinal Chemistry Letters 20 (2010) 2074–2077
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and SAR of derivatives based on 2-biarylethylimidazole as
bombesin receptor subtype-3 (BRS-3) agonists for the treatment of obesity
Jian Liu a, , , Shuwen He a, , Tianying Jian a, Peter H. Dobbelaar a, Iyassu K. Sebhat a, Linus S. Lin a,
*
Allan Goodman b, Cheng Guo b, Peter R. Guzzo b, Mark Hadden b, Alan J. Henderson b, Kevin Pattamana b,
Megan Ruenz b, Bruce J Sargent b, Brian Swenson b, Larry Yet b, Constantin Tamvakopoulos e, Qianping Peng e,
Jie Pan c, Yanqing Kan c, Oksana Palyha c, Theresa M. Kelly c, Xiao-Ming Guan c, Andrew D. Howard c,
Donald J. Marsh c, Joseph M. Metzger d, Marc L. Reitman c, Matthew J. Wyvratt a, Ravi P. Nargund a
a Department of Medicinal Chemistry, Merck Research Laboratories, PO Box 2000, Rahway, NJ 07065, USA
b AMRI, 26 Corporate Circle, Albany, NY12212, USA
c Department of Metabolic Disorders, Merck Research Laboratories, PO Box 2000, Rahway, NJ 07065, USA
d Department of Pharmacology, Merck Research Laboratories, PO Box 2000, Rahway, NJ 07065, USA
e Department of Drug Metabolism, Merck Research Laboratories, PO Box 2000, 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:
This Letter describes a series of potent and selective BRS-3 agonists containing a biarylethylimidazole
pharmacophore. Extensive SAR studies were carried out with different aryl substitutions. This work led
to the identification of a compound 2-{2-[4-(pyridin-2-yl)phenyl]ethyl}-5-(2,2-dimethylbutyl)-1H-imid-
Received 5 January 2010
Revised 16 February 2010
Accepted 18 February 2010
Available online 21 February 2010
azole 9 with excellent binding affinity (IC50 = 18 nM, hBRS-3) and functional agonist activity (EC50
=
47 nM, 99% activation). After oral administration, compound 9 had sufficient exposure in diet induced
obese mice to demonstrate efficacy in lowering food intake and body weight via BRS-3 activation.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Bombesin receptor subtype-3 (BRS-3)
Agonist
Obesity
Obesity is one of the most active therapeutic areas in the phar-
maceutical industry. It is a serious and chronic medical condition
which is growing rapidly through out the world. Moreover, obesity
also causes complications such as diabetes, hypertension, cardio-
vascular disease, cancer and arthritis.1 Currently, sibutramine2
and orlistat3 are the only two drugs approved for the chronic treat-
ment of obesity, with suboptimal tolerability and limited efficacy.
Hence, obesity remains an active area of research and thus far, a
variety of molecular targets have been evaluated in the search of
more effective obesity treatments.4
The bombesin receptor subtype 3 (BRS-3) is an orphan receptor
which belongs to the bombesin receptor sub-family of G-protein
coupled receptors because of its high sequence homology with
the neuromedin B (NMB-R) (BB1, 47%) and gastrin-releasing pep-
tide (GRP-R) (BB2, 51%) receptors.5 Tissue distribution studies have
shown that BRS-3 mRNA is present primarily in the central nervous
system. BRS-3 mRNA has also been found within secondary
spermatocytes, pregnant uterus, as well as various human lung,
breast, and epidermal cancer cell lines.3,6 BRS-3 is implicated in
the regulation of energy homeostasis because mice lacking func-
tional BRS-3 develop mild obesity, hypertension, and diabetes.7
However, understanding of the role of BRS-3 in physiological or
pathological processes has been hampered by the lack of identifi-
cation of selective ligands, endogenous or exogenous. Although
some progress has been recently made in developing potent and
selective peptides and small molecules that are BRS-3 agonists,8
few of these agents has been successfully used in pharmacological
studies to explore biologic functions of BRS-3 pathways. In this Let-
ter, we present the synthesis and SAR of a series of 2-biarylethy-
limidazole analogues as highly potent and selective BRS-3
agonists. We also discuss results from testing in rodent obesity
models.
In the previous Letter, we described the discovery of a biphenyl
acid analogue 2 as a potent and selective BRS-3 agonist, starting
from our HTS lead compound 1.9 The optimized substituents on
the 4-position of imidazole includes 2,2-dimethyl butyl, cyclopen-
tylmethyl and cyclohexyl methyl. Despite the high potency, com-
pound 2 demonstrated no efficacy on food intake reduction in
rodent models after oral dosing, presumably due to its inability
* Corresponding author. Tel.: +1 732 594 9600; fax: +1 732 594 3007.
E-mail address: jian_liu@merck.com (J. Liu).
Both authors contribute equally to this Letter as senior authors.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.02.076

J. Liu et al. / Bioorg. Med. Chem. Lett. 20 (2010) 2074–2077
2075
to cross the blood–brain barrier to activate the BRS-3 receptors in
I
a
b
OH
O
the brain, since significant food intake reduction was observed
when a close analogue of 2 with tert-butylmethyl on imidazole
was dosed intracerebroventricularly in rat.9 We hypothesized that
the carboxylic acid group in compound 2 is the major reason for its
low brain penetrability. Thus, our effort was focused on identifying
potent and selective BRS-3 agonists with a general structure as 3
which could penetrate the blood–brain barrier by replacing the
benzoic acid moiety with a variety of aryl groups (Fig. 1).
+
Br
Br
5
6
4
Boc
N
HN
c,d
N
N
O
B
Br
O
To facilitate rapid synthesis of these novel analogues, a conver-
gent synthetic route was designed as exemplified by the synthesis
of compound 9 (Scheme 1).10 The Heck reaction of 4-bromoiodo-
benzene 4 with allyl alcohol 5 catalyzed by palladium acetate pro-
vided 3-(4-bromophenyl)propanal 6.11 Condensation between
aldehyde 6 and 1-hydroxy-4,4-dimethylhexan-2-one (A)12 in the
presence of ammonium acetate and copper(II) acetate in acetic
acid afforded a moderate yield of the imidazole product 7. After
the imidazole was protected with a Boc group, 7 was converted
to pinacol boronic ester 8. The protection of the imidazole was
shown to be necessary to effect the boronation reaction. Suzuki
cross coupling of the boronic ester 8 with 2-bromopyridine and
simultaneous removal of the Boc protecting group provided com-
pound 9 in good yield. Starting with different terminal alkenes,
and aryl halides or triflates, this sequence allowed rapid access to
a series of 2-biarylethyl imidazole analogues.
Similarly, starting from key intermediate 7, we were also able
to generate the C–N linked biaryl analogues, exemplified by the
preparation of 11 (Scheme 2). In this case, the imidazole was pro-
tected with a thermally more stable trityl group, since the insta-
bility of the Boc group under the condition required for C–N
coupling failed the reaction. The coupling of aromatic bromide
10 with pyrazole was carried out in DMF catalyzed by copper
powder, and removal of the trityl protection with trifluoroacetic
acid provided excellent yields of compound 11. These above de-
scribed two synthetic routes enabled us to prepare significant
numbers of this series of C–C and C–N linked biaryl imdazoles
for the SAR study.13
8
7
HN
e
N
N
9
Scheme 1. Synthesis of 9. Reagents and conditions: (a) Pd(OAc)2, Bu4NCl, NaHCO3,
DMF, 40 °C, 85%; (b) A, NH4OAc, Cu(OAc)2 in AcOH, 100 °C, 40 min, 45%; (c) Boc2O,
TEA, DMAP in CH2Cl2, 1 h at rt, 80%; (d) bis(pinacolate)diboron, Pd(dppf), KOAc,
DMF, 2 h, 85%; (e) 2-bromopyridine, Pd(dppf), Na2CO3 (2 N), DMF, 85 °C, overnight,
72%.
Tr t
N
HN
a
N
N
Br
Br
7
10
HN
b,c
N
N
N
11
Scheme 2. Synthesis of 11. Reagents and conditions: (a) TrtCl, TEA, CH2Cl2, 72%; (b)
pyrazole, NaH, Cu, DMF; (c) TFA, 95% (two steps).
All final compounds were evaluated in vitro for BRS-3 binding14
and agonist functional potency.15 Selected SAR are summarized in
Table 1. The carboxylic acid was mapped around the biphenyl ring
(structures not shown); however, all of these compounds lost po-
tency (IC50 >10,000 nM) compared to the original compound 2.
Substitutions on the phenyl ring with halides (fluorine in 3a) led
to slight loss of potency with no improvements of other biological
properties. Replacing phenyl ring with pyridine (3b and 3c) caused
significant loss of potency. Typical acid replacements were also
investigated,16 again no obvious advantages. A breakthrough was
made when heterocyclic aromatic rings were used to replace the
benzoic acid moiety (9, 11 and 3d–3s). With an appropriately posi-
tioned nitrogen, the potency for BRS-3 was maintained. To our
great delight, by switching the benzoic acid to basic aromatic
heterocycles such as pyridine, isothiazole, triazole or pyrazole,
resulted in compounds (3d, 3n, 3p, 9, 11) that crossed the blood–
brain barrier and achieved acceptable brain/plasma ratios. As sum-
marized in Table 2, all five compounds achieved respectable brain
drug level in rats.
Compound 9, 2-{2-[4-(pyridin-2-yl)phenyl] ethyl}-5-(2,2-dim-
ethylbutyl)-1H-imidazole, was selected for further profiling. It is
a potent BRS-3 agonist with excellent binding affinity (human
IC50 = 18 nM) and high selectivity over NMB-R (human IC50
=
7000 nM) and GRP-R (human IC50 = 6400 nM).16,17 It also binds po-
tently to rodent BRS-3 (IC50 = 6.9 and 2.4 nM for mouse and rat,
respectively). Although Compound 9 has limited oral bioavailabil-
ity and short half life in mice,18 when dosed at 50 mg/kg, a brain/
plasma ratio of 0.84 was observed at 1 h, with drug levels of
HN
HN
N
N
0.49 lM and 0.41 lM in the plasma and brain, respectively. With
oral dosing of compound 9, we have demonstrated reduction of
food intake by a single dose and reduction of body weight with
14-day treatment of diet induced obese mice.18
COOH
OH
COOH
2
1
In summary, SAR studies were carried out on a series of 2-bia-
rylethyl imidazole analogues as BRS-3 agonists. A highly selective
and potent BRS-3 agonist 9 was discovered. Compound 9 (Bag-1
in Ref. 18) has acceptable pharmacokinetic properties and brain
penetrability to enable testing of the effects of a BRS-3 agonist
on food intake and body weight. The discovery of compound
Bag-1, with its suitability for in vivo studies, will propel a better
understanding of the biology and physiology of BRS-3.18
HN
N
R
Ar
3
Figure 1.

2076
J. Liu et al. / Bioorg. Med. Chem. Lett. 20 (2010) 2074–2077
Table 1
The potency of BRS-3 agonists in human and mice BRS-3 receptors
a
a
Compound
Ar
R
hBRS-3 binding
IC50 (nM)
hBRS-3 function EC50
(nM) (activation%)b
mBRS-3 Function EC50
(nM) (activation%)b
a
HN
N
R
Ar
2
11
25 (101)
63 (103)
1263 (95)
9.6 (94)
4.7 (96)
14 (97)
COOH
F
F
3a
3b
36
COOH
560
N
COOH
N
3c
2821
5234 (15)
305 (90)
COOH
3d
9
177
18
425 (98
47 (99)
22 (103)
2.8 (78)
N
N
3e
3f
7.5
131 (98)
305 (95)
4.3 (97)
15 (103)
N
101
N
3g
3h
6.3
51
105 (102)
169 (100)
19 (106)
11 (91)
N
N
OH
N
3i
7.3
37 (97)
3.9 (119)
NH2
N
3j
140
19
193 (95)
66 (97)
26 (115)
16 (105)
CN
3k
N
F
3l
11
45
10
51
33
340
63 (102
3.1 (103)
11 (109)
6.9 (119)
11 (107)
9.0 (113)
12 (121)
S
N
S
N
3m
3n
3o
11
3p
538 (104)
44(100)
N
S
N
N
N
431 (100)
135 (106)
570 (89)
N
N
N
N
N

J. Liu et al. / Bioorg. Med. Chem. Lett. 20 (2010) 2074–2077
2077
Table 1 (continued)
a
a
Compound
Ar
R
hBRS-3 binding
IC50 (nM)
hBRS-3 function EC50
(nM) (activation%)b
mBRS-3 Function EC50
(nM) (activation%)b
a
HN
N
R
Ar
N
3q
3r
390
205
28
641 (93)
422 (89)
171 (108)
57 (111)
25 (99)
N
N
N
N
N
N
N
3sc
28 (133)
a
The reported data are the average of at least three repeated experiments.
The percentages of activation are the maxim activation of tested compounds relative to that of [
Compound with piperidine as a saturated ring maintains most potency.
b
c
D
-Tyr6,b-Ala11,Phe13,Nle14]-bombesin (6–14).
Tamvakopoulos, C.; Peng, Q.; Guan, X.; Reitman, M. L.; Patchett, A. A.;
Wyvratt, M. J.; Nargund, N. P. Bioorg. Med. Chem. Lett. 2010, 20, 1913.
10. Dobbelaar, P. H.; Franklin, C. L.; Goodman, A.; Guo, C.; Guzzo, P. R.; Hadden, M.;
He, S.; Henderson, A. J.; Jian, T.; Lin, L. S.; Liu, J.; Nargund, R. P.; Ruenz, M.;
Sargent, B. J.; Sebhat, I. K.; Yet, L. PCT Int. Appl. WO 2008051405, 2008.
11. (a) Gibson, S. E.; Jones, J. O.; McCague, R.; Tozer, M. J.; Whitcombe, N. J. Synlett
1999, 954; (b) Taylor, E. C.; Liu, B. Tetrahedron Lett. 1999, 40, 4023.
12. 1-Hydroxy-4,4-dimethylhexan-2-one (A) was prepared from commercially
available 4,4-dimethylhex-1-ene by treatment with potassium permanganate
in a mixture of acetic acid, acetone and water.
Table 2
Plasma and brain levelsa of BRS-3 agonists 1 h following IV or PO dose in rats
Compd
Dose
Plasma (
l
M)
Brain (
lM)
b/p ratio
3d
9
3n
11
3p
1 mg/kg IV
50 mg/kg PO
1 mg/kg IV
1 mg/kg IV
1 mg/kg IV
0.039
0.89
0.54
0.03
0.20
0.030
0.15
0.23
0.02
0.08
0.77
0.17
0.42
0.67
0.40
13. The synthesis of some compounds needed additional transformation of
functional groups, such as ester to acid (2, 3a, 3b and 3c).
a
Data are averages of three repeated experiments.
14. For human BRS-3 binding assays, 1 to 4
NFAT-CHO cells expressing the receptor were incubated with 0.3 pM [125I]-[
Tyr6,b-Ala11,Phe13,Nle14]-bombesin (6–14) 125I-dY-peptide) and various
concentrations of test compounds in 200 L of binding buffer (50 mM Tris,
lg of membrane protein obtained from
D
-
(
l
Reference and notes
pH 7.2, 5 mM MgCl2, 0.1% BSA). After a 2 h incubation at room temperature, the
binding reaction was terminated by filtering through a GF/c filter and washing
1. Olshansky, S. J.; Passaro, D. J.; Hershow, R. C.; Layden, J.; Carnes, B. A.; Brody, J.;
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the filter with PBS using
a Packard 96-well Harvester. The amount of
radioligand bound to the receptor was measured by liquid scintillation
counting of the radioactivity on the filter. The nonspecific binding was
defined as the binding in the presence of 100 nM unlabeled dY-bombesin.
The data, as %inhibition of binding, was plotted versus the log molar
concentration of receptor ligand (compound). The IC50 was reported as the
inflection point of the resulting sigmoidal curve.
15. The functional assay is an aequorin bioluminescence assay. It was performed in
a
96-well format using a Wallac Microbeta luminometer equipped with
microinjector module. Compounds in DMSO (0.5% final concentration) were
titrated in the plates at two times concentration in a volume of 0.1 mL ECB
buffer (20 mM HEPES, pH 7.4, 140 mM NaCl, 20 mM KCl, 1 mM MgCl2, 1 mM
CaCl, 5 mM glucose, 0.1 mg/ml BSA). The HEK293AEQ cells from lines
expressing either human, rat or mouse BRS3 (20,000 per well) were charged
with coelenterazine (molecular probes) and then injected in 0.1 mL ECB buffer
into the compound containing wells. The bioluminescence was monitored for
30 s, or alternatively, total bioluminescence was determined over 10 min. The
bioluminescent readings were plotted vs. the log molar concentration of
receptor ligands (compounds). The EC50 for activation was reported as the
inflection point of the resulting sigmoidal curve. The percentages of activation
are the maxim activations of tested compounds relative to that of dY-peptide.
16. Extensive SAR with various acid isosteres were carried out and the results will
be published separately.
6. (a) Sano, H.; Feighner, S. D.; Hreniuk, D. L.; Iwaasa, H.; Sailer, A. W.; Pan, J.;
Reitman, M. L.; Kanatani, A.; Howard, A. D.; Tan, C. P. Genomics 2004, 84, 139;
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(c) Weber, D.; Berger, C.; Heinrich, T.; Eickelmann, P.; Antel, J. J. Pept. Sci. 2002,
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Iijima, H.; Shimada, H.; Arai, T.; Ueno, H.; Usui, Y.; Sakaki, T.; Wada, E.; Wada, K.
J. Pept. Sci. 2005, 11, 136; (e) Carton, D. L.; Collin-Smith, L. J.; Alejandro, J. D.;
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17. The binding protocols for human NMB-R and GRP-R are the same as for BRS-3
except that less protein (membrane) is needed for these two receptors. Both
used 0.5 lg per well instead of the 2 lg typically used for BRS-3.
18. For more detailed pharmacological profiling of 9 (referred to therein as Bag-1),
see: Guan, X-M.; Chen, H.; Dobbelaar, P. H.; Dong, Y.; Fong, T. M.; Gagen, K.;
Gorski, J.; He, S.; Howard, A. D.; Jian, T.; Jiang, M.; Kan, Y.; Kelly, T. M.; Kosinski,
J.; Lin, L. S.; Liu, J.; Marsh, D. J.; Metzger, J. M.; Miller, R.; Nargund, R. P.; Palyha,
O.; Shearman, L.; Shen, Z.; Stearns, R.; Strack, A. M.; Stribling, S.; Tang, Y. S.;
Wang, S.-P.; White, A.; Yu, H.; Reitman, M. L. Cell Metab. 2010, 11, 101.
9. He, S.; Dobbelaar, P. H.; Liu, J.; Jian, T.; Sebhat, I. K.; Lin, L. S.; Goodman, A.; Guo,
C.; Guzzo, P. R.; Hadden, M.; Henderson, A. J.; Ruenz, M.; Sargent, B. J.; Yet, L.;
Kelly, T. M.; Palyha, O.; Kan, Y.; Pan, J.; Chen, H.; Marsh, D. J.; Shearman, L. P.;
Strack, A. M.; Metzger, J. M.; Feighner, S. D.; Tan, C.; Howard, A. D.;