Diarylimidazolyl oxadiazole and thiadiazole derivatives as cannabinoid CB1 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2008.10.130

Since the CB1 receptor antagonist SR141716 (rimonabant) was reported to modulate food intake, CB1 antagonism has been considered as a new therapeutic target in the treatment of obesity. Several series of derivatives based on diarylimidazolyl oxadiazole an

Full text of DOI:10.1016/j.bmcl.2008.10.130

Bioorganic & Medicinal Chemistry Letters 19 (2009) 142–145
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Diarylimidazolyl oxadiazole and thiadiazole derivatives as cannabinoid CB1
receptor antagonists
*
Jong Yup Kim, Hee Jeong Seo, Sung-Han Lee, Myung Eun Jung, Kwangwoo Ahn, Jeongmin Kim, Jinhwa Lee
Central Research Laboratories, Green Cross Corporation, 303 Bojeong-Dong, Giheung-Gu, Yongin, Gyeonggi-Do 446-770, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Since the CB1 receptor antagonist SR141716 (rimonabant) was reported to modulate food intake, CB1
antagonism has been considered as a new therapeutic target in the treatment of obesity. Several series
of derivatives based on diarylimidazolyl oxadiazole and thiadiazole scaffolds were synthesized and tested
for CB1 receptor binding affinity. SAR studies directed toward the optimization of imidazole scaffolds
resulted in the discovery of 10s which showed highest potency for CB1 receptor binding affinity
(IC50 = 1.91 nM) prepared to date.
Received 3 September 2008
Revised 28 October 2008
Accepted 30 October 2008
Available online 5 November 2008
Keywords:
Rimonabant
Ó 2008 Elsevier Ltd. All rights reserved.
Anti-obesity
Cannabinoid receptor antagonist
Imidazole
The CB1 cannabinoid receptor belongs to the G-protein-coupled
receptor (GPCR) type and is coupled to inhibitory G proteins (G(i/
o)) to inhibit certain adenylyl cyclase isozymes, leading to de-
creased cAMP production, decreased Ca2+ conductance, increased
K+ conductance, and increased mitogen-activated protein kinase
activity.1 The major physiological effect of cannabinoids is the
modulation of neurotransmitter release via activation of presynap-
tic CB1 receptors located on distinct types of axon terminals
throughout the brain.2
CB1 receptor has been well-documented.13,14 The crucial receptor–
ligand interaction is known to be a hydrogen bond between the
carbonyl group of rimonabant and the Lys192–Asp366 residue of
the CB1 receptor, thereby exerting a stabilizing effect on the
Lys192–Asp366 salt bridge in Figure 1.15
Bioisosteric replacement forms a rational medicinal chemistry
approach for the discovery of new leads or series, based on existing
key ligands. The three-dimensional structure of imidazole main-
tains a high similarity to that of the pyrazole. As a consequence,
they can be regarded as isosteres thereof and have been applied
in order to discover pyrazole bioisosteres.16 Also, we demonstrated
a successful replacement of the key carbonyl group of rimonabant
with imine-type functionality, tetrazole group.17 Among many het-
erocycles involving ‘imine-type’ functionality other than tetrazole,
we were particularly interested in oxadiazole23 or thiadiazole as a
viable surrogate of amide, since modifying the key carbonyl group
of rimonabant into oxadiazole or thiadiazole could furnish a favor-
able balance of potency and physicochemical properties to allow
for further in vivo efficacy evaluation. Herein, we wish to disclose
D
9-Tetrahydrocannabinol ( 9-THC) has been known as active
D
ingredient of Marijuana which has ability to stimulate appetite
by activating the cannabinoid receptor, CB1 in the brain.3
The CB1 receptor antagonists/inverse agonists have shown to be
useful in the suppression of food intake and the reduction of body
weight.4 Food intake suppression mediated by the CB1 inverse ago-
nists has been demonstrated in animal5 and human studies.6 The
first published studies with rimonabant (SR141716) in both
rodents7 and primates8 showed clear differentiation, that is,
marked effects on sweet food intake vs. marginal effects on regular
chow intake or water drinking.
The first specific cannabinoid CB1 receptor antagonist or inverse
agonist, rimonabant, was discovered in a high-throughput screen-
ing program at Sanofi-Synthelabo in 1994.9 Several CB1 receptor
antagonists including rimonabant, SLV319 (ibipinabant),10
CP-945,598 (otenabant)11 and MK-0364 (taranabant)12 have been
reported to be late phase of clinical trials. A pharmacophore model
for the binding of a low energy conformation of rimonabant in the
Trp255
Typ 275
Asp366-Lys192
Cl
Val196
A
Phe278
O D
Phe170
Leu387
C
N
HN
N
N
B
Trp279
Trp356
Cl
Met384
Cl
Phe200
* Corresponding author. Tel.: +82 31 260 9892; fax: +82 31 260 9870.
E-mail address: jinhwalee@greencross.com (J. Lee).
Figure 1. Rimonabant and its receptor–ligand interaction.
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.10.130

J. Y. Kim et al. / Bioorg. Med. Chem. Lett. 19 (2009) 142–145
143
Table 1
the synthesis and evaluation of the oxadiazole–diarylimidazole
Structures and binding affinities of selected ligands for rat CB1 receptors
and thiadiazole–diarylimidazole as novel CB1 receptor antagonists.
The carboxylic acid derivative 6 was prepared by a conventional
method, for example, by reacting a benzonitrile 1 with an aniline
derivative 2 using sodium bis(trimethylsilyl)amide (NaHMDS) to
produce a corresponding arylbenzamidine 3. Subsequent reaction
Cl
CH3
N
N
N
N
Z
R2
Cl
Cl
of the resulting arylbenzamidine 3 with
a-bromoketone 4 gave
an intermediate ethyl 1,2-diaryl-5-alkyl-1H-imidazole-4-carboxyl-
ate 5. An acid form 6 was transformed from the intermediate 5
using lithium hydroxide, followed by acidification, as shown in
Scheme 1.16
The target compounds 9 and 10 were prepared by reaction of a
carboxylic acid derivative 6 with hydrazide compound 7 in the
presence of EDCI and HOBt in DCM, and cyclization of the resulting
acyl hydrazide compound 8 using Burgess reagent18 to provide an
1,3,4-oxadiazole compound 9, or Lawesson’s reagent19 to yield a
1,3,4-thiadiazole compound 10 as shown in Scheme 2. The hydra-
zide derivative 7 was prepared by treating an ester or a carboxylic
acid with hydrazine.
The target compounds of structures 9 and 10 were evaluated
in vitro in a rat CB1 binding assay.20,24 The results are shown in
Table 1. Since there are a number of literature precedents describ-
ing the importance of lipophilic groups at the C domain of rimo-
nabant as shown in Figure 1, various lipophilic groups have
been focused for investigation. These data demonstrate that the
size of carbocycle appears to affect binding affinity. Thus, 7-mem-
bered ring (9d) is slightly better than the 4- or 5-membered ring,
suggesting that there might be a size requirement for the oxadiaz-
ole alkyl region to attain good binding to CB1 receptor. In the case
of thiadiazole, 5-membered ring (10b) is more active in 2- to 4-
fold than the corresponding oxadiazole (9b). Longer branched ali-
phatic chains are usually better, showing IC50 = 12.9 nM for 9j.
Introduction of additional methyl group at the branch such as t-
butyl 9g elevated CB1 binding affinity of i-propyl 9e in 2-fold. In-
Compound Z
IC50 (nM) for rCB1Ra,b
Z
O
S
R2
O
S
9a
9b
9c
9d
9e
9f
10a
10b
10c
10d
10e
10f
Cyclobutyl
Cyclopentyl
Cyclohexyl
Cycloheptyl
i-Pr
38.7
35.3
115
42.3
10.8
46.0
21.6
22.2
28.1
20.2
12.7
19.7
15.0
16.9
33.5
27.6
25.8
39.4
13.2
14.2
22.3
12.9
31.4
55.1
16.7
15.2
i-Bu
t-Bu
9g
9h
9i
10g
10h
10i
Pentan-2-yl
Pentan-3-yl
Hexan-2-yl
1-Phenylethyl
1-(Triflluoromethyl)cyclopropyl
1-(Triflluoromethyl)cyclobutyl
1-(4-Chlorophenyl)cyclopropyl
Rimonabant
9j
10j
9k
9l
10k
10l
9m
9n
10m
10n
9.66
9.41
5.0 1.0c
CB1 receptor was collected from brain tissue of SD rat.24
These data were obtained by single determinations.26
These data were obtained by in-house assay.
a
b
c
stead of gem-dimethyl group, cyclopropyl at the position can be
replaced without impeding in vitro CB1 receptor binding affinity,
displaying similar level of binding affinity (9m, 9n). Interestingly,
altering oxadiazoles to the corresponding thiadiazoles improved
CB1 receptor binding affinity in approximately 2-fold, showing
IC50 = 9.66 nM for 10m and IC50 = 9.41 nM for 10n, respectively.
It was encouraging to notice that a considerable CB1 receptor
binding affinity was already observed in a number of compounds.
At this juncture, we decided to proceed to explore R1, X and Y sub-
stituents of the imidazole core by using t-butyl oxadiazole as a
tentative lead scaffold, since (i) our previous research23c on diary-
limidazolyl oxadiazoles as the CB1R antagonists showed that the
t-butyl group is beneficial for the moiety. (ii) The t-butyl group
Cl
NH
NH
O
R1
Cl
O
NH2
CN
NaHMDS
Y
Br
O
X
Y
1
2
3
4
X
X
X
Y
R1
R1
Table 2
O
Structures and binding affinities of selected ligands for rat CB1 receptors
N
O
1) NaHCO3
2) TFA
N
LiOH
N
O
X
N
OH
R1
Cl
Cl
N
N
Y
N
5
6
N
Z
Cl
Scheme 1.
Y
Compound Z
S
IC50 (nM) for rCB1Ra,b
O S
Z
X
Y
O
X
O
X
Y
R1
R1
R1
NH2
R2
N
O
O
H
9g
9o
9p
9q
9r
10g
10o
10p
10q
10r
10s
10t
Cl
Cl
Cl
Cl
Br
Br
Br
Br
Cl
Cl
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
H
Me
Et
n-Pr
Cyclopropyl
Me
Et
n-Pr
i-Pr
Me
Et
Me
Et
13.2
18.1
18.9
79.9
5.81
3.15
71.5
21.2
5.29
20.2
15.8
N
N
N
7
OH
N
HN NH
O
EDCI, HOBt
NMM, DCM
R2
7.94
58.0
3.45
1.91
13.5
6.49
12.7
Cl
Cl
Y
6
8
9s
9t
Lawesson's reagent
Burgess reagent
9u
9v
9w
9x
9y
10u
10v
10w
10x
10y
X
Y
X
Y
c
R1
R1
H
H
H
21.3
31.7
14.7
–
N
10.7
N
N
N
N
N
c
–
N
S
N
O
R2
R2
Rimonabant
5.0 1.0c
Cl
Cl
a
b
c
10
CB1 receptor was collected from brain tissue of SD rat.
These data were obtained by single determinations.
These data were obtained by in-house assay.
9
Scheme 2.

144
J. Y. Kim et al. / Bioorg. Med. Chem. Lett. 19 (2009) 142–145
Table 3
Structures and binding affinities of selected ligands to rat CB1 and human CB2 receptors, and CB2/CB1 selectivity of the ligands
Br
Et
N
N
N
N
Z
R2
Cl
Cl
Compound
R2
Z
IC50 (nM) for rCB1Ra,c
IC50 (nM) for hCB2Rb,c
9z
9aa
9ab
9ac
9ad
9ae
1-Phenylcyclopropyl
O
O
O
O
O
O
S
S
S
S
S
9.14
18.6
8.38
8.05
9.05
13.6
8.29
9.21
5.72
–
–
1-(4-Methoxyphenyl)cyclopropyl
1-(4-Methylphenyl)cyclopropyl
1-(2,4-Dichlorophenyl)cyclopropyl
1-(4-Chlorophenyl)cyclopropyl
1-(4-Chlorophenyl)cyclobutyl
1-Phenylcylcopropyl
1-(4-Methoxyphenyl)cyclopropyl
1-(4-Methylphenyl)cyclopropyl
1-(2,4-Dichlorophenyl)cyclopropyl
1-(4-Chlorophenyl)cyclopropyl
1-(4-Chlorophenyl)cyclobutyl
Rimonabant
>10,000
>10,000
–
>10,000
>10,000
>10,000
>10,000
>10,000
>10,000
>10,000
1760d
10z
10aa
10ab
10ac
10ad
10ae
6.57
4.53
S
4.79
5.0 1.0d
a
CB1 receptor was collected from brain tissue of SD rat.
CB2 receptor was recombinant human receptor expressed in CHO cell.
These data were obtained by single determinations.
b
c
has played an outstanding role in the field of medicinal chemistry
due to its unique properties.21
interchangeable with a pyrazole ring in rimonabant as shown in
Refs. 15 and 16. Importantly, these analogs also display good selec-
tivity for CB1R over CB2R. Thus, the diarylimidazolyl oxadiazole or
thiadiazole class of compounds possesses promising therapeutic
possibility as a CB1 receptor antagonist for the treatment of
obesity.
The binding affinity data of selected key diarylimidazolyl oxadi-
azoles and thiadiazoles for the CB1 receptor are shown in Table 2.
When methyl was replaced by ethyl 9o or n-propyl 9p, it
showed similar level of CB1 receptor binding affinity, respectively,
but binding potency for cyclopropyl 9q deteriorated in 4-fold, indi-
cating that even small ring such as cyclopropyl is not tolerated for
the region. This is also observed in the thiadiazole 10q. Replace-
ment of 1-(4-chlorophenyl) 9g (IC50 = 13.2 nM) with 1-(4-bromo-
phenyl) 9r (IC50 = 5.81 nM) improved CB1 receptor binding
affinity in more than 2-fold. This phenomenon is clearly demon-
strated by comparing 9o (IC50 = 18.1 nM) versus 9s
(IC50 = 3.15 nM). However, this effect is not observed any longer
as the number of carbon on imidazole core is increased beyond
ethyl as shown in n-propyl 9t (IC50 = 71.5 nM).
Acknowledgments
Financial support was provided by Green Cross Corporation
(GCC). We are grateful to Dr. Chong-Hwan Chang for his many
helpful discussions throughout small molecule programs at GCC.
Also we thank Dr. Eun Chul Huh, Mr. Jung Ho Kim and Ms. Jae-
Young Jang at GCC Office of R&D planning and coordination for
their supports and services.
As observed previously, thiadiazoles turned out to be more ac-
tive than the corresponding oxadiazoles. Of note is that among our
compounds tested, 2-(1-(4-bromophenyl)-2-(2,4-dichlorophenyl)-
5-ethyl-1H-imidazol-4-yl)-5-t-butyl-1,3,4-thiadiazole 10s was
shown to be most potent in vitro CB1 diarylimidazolyl heterocycle
receptor ligand (IC50 = 1.91 nM) prepared to date.27
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2008.10.130.
References and notes
The binding affinity data of selected key 2-(1-(4-bromophenyl)-
2-(2,4-dichlorophenyl)-5-ethyl-1H-imidazol-4-yl)-1,3,4-oxa(thia-
)diazole for the CB1 receptor is shown in Table 3. Cyclopropanes or
cyclobutanes at the benzylic position are also well tolerated with-
out detriment to in vitro CB1 receptor binding affinity as displayed
in Table 3. Chlorine or methoxy substitution on the phenyl ring
also maintained similar receptor binding affinity, indicating that
a halogen atom or methoxy group appears to be well tolerated at
this position. Binding affinity was also measured for the CB2 recep-
tor expressed in CHO cells, employing [3H]WIN-55,212-2 as a
radio-ligand.22,24 Virtually all of our imidazole-based compounds
were devoid of activity in this CB2 receptor binding assay, indicat-
ing high selectivity for CB1 over CB2 for these analogs. Selected
examples of the above analogs are summarized in Table 3.
In conclusion, we investigated a series of diarylimidazolyl oxa-
diazole and thiadiazole derivatives as antagonists to the cannabi-
noid CB1 and CB2 receptors. Several of the compounds in these
series exceeded or maintained the potency of known CB1 antago-
nists,25 validating that (i) a 1,3,4-oxadiazole/thiadiazole ring could
act as a bioisostere of the amide moiety and (ii) an imidazole ring is
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receptor) or cell membranes (containing CB2 receptor) were incubated in
96-well plate with TME buffer containing 0.5% essentially fatty acid free bovine
serum albumin (BSA), 3 nM [3H]WIN55,212-2 (for CB2 receptor, NEN; specific
activity 50–80 Ci/mmol) or 3 nM ([3H]CP55,940, [3H]2-[(1S,2R,5S)-5-hydroxy-
2-(3-hydroxypropyl)cyclohexyl]-5-(2-methyloctan-2-yl)phenol, for CB1 recep-
tor, NEN; specific activity 120–190 Ci/mmol) and various concentrations of the
synthesized cannabinoid ligands in a final volume of 200 lL. The assays were
incubated for 1 h at 30 °C and then immediately filtered over GF/B glass fiber
filter (Perkin-Elmer Life and Analytical Sciences, Boston, MA) that had been
soaked in 0.1% PEI for 1 h by a cell harvester (Perkin-Elmer Life and Analytical
Sciences, Boston, MA). Filters were washed five times with ice-cold TBE buffer
containing 0.1% essentially fatty acid free BSA, followed by oven-dried for
60 min and then placed in 5 mL of scintillation fluid (Ultima Gold XR; Perkin-
Elmer Life and Analytical Sciences, Boston, MA), and radioactivity was
quantitated by liquid scintillation spectrometry. In CB1 and CB2 receptor
competitive binding assay, nonspecific binding was assessed using
rimonabant and 1 M WIN55,212-2, respectively. Specific binding was defined
as the difference between the binding that occurred in the presence and
absence of 1 M concentrations of rimonabant or WIN55,212-2 and was 70–
1 lM
l
l
80% of the total binding. IC50 was determined by nonlinear regression analysis
using Graph-Pad PRISM. All data were collected in triplicate and IC50 was
determined from three independent experiments.
25. Several compounds in this series were highly efficacious in in vivo efficacy on
the DIO mouse model. One of the compounds in the series showed 25 1%
reduction in body weight after 14 days at 10 mg/kg via po. That is a notable
piece of information that this series of compounds are indeed CB1R antagonists
or inverse agonists, unpublished results.
24. CB1 and CB2 receptor binding assay. For the CB1 receptor binding studies, rat
cerebellar membranes were prepared as previously described by the methods
of Kuster et al.20 Male Sprague–Dawley rats (200–300 g) were sacrificed by
decapitation and the cerebella rapidly removed. The tissue was homogenized
in 30 volumes of TME buffer (50 mM Tris–HCl, 1 mM EDTA, 3 mM MgCl2, pH
7.4) using a Dounce homogenizer. The crude homogenates were immediately
centrifuged (48,000g) for 30 min at 4 °C. The resultant pellets were
resuspended in 30 volumes of TME buffer, and protein concentration was
26. In this communication, CB1R data were obtained by single determinations.
Whenever we use rimonabant as our reference in our in-house assay, the CB1R
binding affinity for rimonabant has showed a certain number in the close range
(IC50 = 5.0 1.0 nM) in each different assay (>1500 compounds tested).
Therefore, we believe that all SAR discussions in the manuscript are
scientifically meaningful.
27. The compound 10s shows cLogP = 8.0, indicating the highly lipophilic
character of 10s just as many other known CB1R ligands are so.