Tetrazole-biarylpyrazole derivatives as cannabinoid CB1 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2008.02.061

Cannabinoid CB1 receptors have been the focus of extensive studies since the first clinical results of rimonabant (SR141716) for the treatment of obesity and related metabolic disorders were reported in 2001. To further evaluate the properties of CB recep

Full text of DOI:10.1016/j.bmcl.2008.02.061

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Bioorganic & Medicinal Chemistry Letters 18 (2008) 2385–2389
Tetrazole-biarylpyrazole derivatives as cannabinoid CB1
receptor antagonists
Suk Youn Kang, Sung-Han Lee, Hee Jeong Seo, Myung Eun Jung, Kwangwoo Ahn,
Jeongmin Kim and Jinhwa Lee*
Small Molecule Group, Central Research Institute, Green Cross Corporation, 341 Bojeong-Dong, Giheung-Gu, Yongin,
Gyunggi-Do 446-799, Republic of Korea
Received 23 November 2007; revised 5 February 2008; accepted 23 February 2008
Available online 29 February 2008
Abstract—Cannabinoid CB1 receptors have been the focus of extensive studies since the first clinical results of rimonabant
(SR141716) for the treatment of obesity and related metabolic disorders were reported in 2001. To further evaluate the properties
of CB receptors, we have designed a new series of tetrazole-biarylpyrazoles. The various analogues were efficiently prepared and bio-
assayed for binding to cannabinoid CB1 receptor. Six of the new compounds which displayed high in vitro CB1 binding affinities
were assayed for binding to CB2 receptor. Noticeably, cyclopentyl-tetrazole (9a) demonstrated good binding affinity and selectivity
for CB1 receptor (IC50 = 11.6 nM and CB2/CB1 = 366).
Ó 2008 Elsevier Ltd. All rights reserved.
The World Health Organization (WHO) recently de-
clared that obesity has become a global epidemic, posing
a serious threat to public health because of the increased
risk of associated health problems.1,2 There are growing
evidences that obesity as a chronic disease cannot be
cured only by short-term diet or exercise. Reducing
weight by caloric restriction generally fail as most obese
patients readily regain their lost weight thereafter. There-
fore, medical treatment of obesity became a necessity.3–5
should be taken into consideration for new drug devel-
opment of anti-obesity agent.
The first specific cannabinoid CB1 receptor antagonist,
rimonabant, was discovered in a high throughput
10
´
screening programme at Sanofi-Synthelabo in 1994.
Several CB1 receptor antagonists including rimonabant,
SLV319,11 CP-945,598 and MK-036412 have been re-
ported to be in various phase of clinical trials.1,6,13,14
A pharmacophore model for the binding of a low energy
conformation of rimonabant in the CB1 receptor has
been well-documented.14,15 The key 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
Recent development of obesity drugs reveals that it is
possible to control appetite and reduce weight by block-
ing cannabinoid receptors in the brain, liver or muscle,
via cannabinoid (CB1) receptor antagonists or CB1
receptor inverse agonists.6,7 A cannabinoid CB1 recep-
tor antagonist is designed to block the effects of endog-
enous cannabinoids. This type of drug is particularly
interesting since it not only causes weight loss but also
reverses the metabolic effects of obesity such as insulin
resistance and hyperlipidemia.8 The other cannabinoid
receptor, CB2, is related to immune regulation and neu-
rodegeneration.9 Therefore, the CB1/CB2 selectivity
Trp255
Typ 275
Asp366-Lys192
Cl
Val196
A
Phe278
D
O
Phe170
Leu387
C
N
HN
N
N
B
Trp279
Cl
Met384
Cl
Keywords: Rimonabant; Anti-obesity; Cannabinoid receptor antago-
nist; Tetrazole.
Trp356
Phe200
*
Corresponding author. Tel.: +82 31 260 9892; fax: +82 31 260
9870; e-mail: jinhwalee@greencross.com
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.2008.02.061

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S. Y. Kang et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2385–2389
stabilizing effect on the Lys192–Asp366 salt bridge as
shown in Figure 1.6
in methylene chloride to afford the corresponding amide
3. Subsequently, nitrile 4 was prepared by condensation
of amide 3 with phosphoryl chloride in a high yield.
Treatment of nitrile 4 with sodium azide efficiently gave
rise to tetrazole 5 in a [2+3]-cycloaddition fashion
(Scheme 1).18,19
So far, various analogues of rimonabant by using strat-
egy such as conformational constraints or scaffold hop-
ping have been designed for the purpose of enhancement
of binding affinity and selectivity for CB1 receptor.6 For
the most part, replacements of pyrazole core to 5- or 6-
membered ring scaffolds were actively studied. We envi-
sioned that the key carbonyl group of rimonabant might
be replaced to the corresponding imine-type functional-
ity or ‘imine’-containing heterocycles. Similar ap-
proaches were already demonstrated successfully with
imidazole16a,b or oxadiazole16c scaffolds.
With requisite tetrazole 5 in hand, the synthesis of 2-
and 3-tetrazole was studied (see Scheme 2). Tetrazole 5
reacted with alkyl bromide or iodide in the presence of
potassium carbonate in DMF to yield alkyl tetrazole 6
and 7, respectively. Tetrazoles 6 and 7 were separated
by silica-gel column chromatography. Alternatively,
Mitsunobu reaction conditions were utilized for alkyl-
ation of tetrazole 5 with aliphatic alcohols to produce
tetrazoles 6 and 7, respectively, as shown in Scheme 2.
Among many heterocycles involving ‘imine-type’ func-
tionality, we were particularly interested in tetrazole as
a viable surrogate of amide, since modification of the
amide moiety into tetrazole could impart a favourable
balance of potency and physicochemical properties to
allow for further in vivo efficacy evaluation. Herein,
we wish to describe the design, synthesis and biological
evaluation of tetrazole-biarylpyrazole analogues as no-
vel CB1 receptor antagonists.
Cl
Cl
N
N
O
N
N
a
b
2
N
N
N
N
NHR
R
Cl
Cl
Cl
Cl
Synthesis of tetrazole-biarylpyrazoles commenced with
the generic acid 1.17 Acid 1 was converted to the acyl
chloride 2 with thionyl chloride, and this intermediate
was then treated with ammonium hydroxide solution
8
6
Scheme 3. Reagents and conditions: (a) NH2R, Et3N, DMAP,
CH2Cl2; (b) PCl5, PhCH3, 10 min; then HN3, rt 1 day then reflux.
Cl
Cl
c
a
COX
CO2H
N
N
N
N
Cl
Cl
Cl
Cl
1
2
3
X= Cl
b
X = NH2
Cl
Cl
d
N
N
N
CN
N
N
N
N
N
H
Cl
Cl
Cl
Cl
5
4
Scheme 1. Reagents and conditions: (a) SOCl2, PhCH3, reflux; (b) ammonium hydroxide solution, CH2Cl2, rt; (c) POCl3, DMF; (d) NaN3, NH4Cl,
DMF, microwave (180 °C, 20 min), 96% overall yield (4 steps).
Cl
Cl
a or b
R
N
N
N
N
N
N
N
N
5
+
N
N
N
N
R
Cl
Cl
Cl
Cl
6
7
Scheme 2. Reagents and conditions: (a) RX, K2CO3, DMF, rt or 80 °C; (b) ROH, PPh3, DIAD, THF, 0 °C to rt.

S. Y. Kang et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2385–2389
2387
Table 1. Inhibitions of rCB1 receptor of alkyl substituted tetrazolea,b
As another method, synthesis of alkyl tetrazole 6 pro-
gressed via different pathway as shown in Scheme 3.20
Thus, treatment of acyl chloride 2 with primary, sec-
ondary and aryl amines in the presence of triethyl-
amine in methylene chloride generated the
corresponding amides 8. Preparation of tetrazole 6
from amide 8 was accomplished efficiently by the ac-
tion of hydrazoic acid and phosphorus pentachloride
as shown in Scheme 3.
Cl
Cl
Cl
R
N
N
N
N
N
N
N
N
N
N
N
N
R
Cl
Cl
Cl
6
7
As depicted in Scheme 4, each regioisomer of tetrazole
6s or 7s was unmistakably assigned by preparation of
tetrazoles 6s with different reaction pathways as previ-
ously described, and subsequent comparison of 1H
NMR spectra of each sample. All other tetrazole com-
pounds shown in Table 1 were also identified in an anal-
ogous fashion.
b
b
R
Compound IC50
Compound IC50
Me
Et
6a
6b
6c
6d
6e
6f
1060
7a
7b
7c
7d
7e
>1000
>1000
62.1
202
68.9
166
n-Pr
i-Pr
101
87.7
n-Bu
tBu
87.8
361
n-Pentyl
n-Octyl
Phenyl
Benzyl
6g
6h
6i
184
7g
7h
122
50.6
The inhibitions data of several key tetrazole-biarylpy-
razoles for the CB1 receptor are shown in Table 1.21
Basically, methyl tetrazole 6a and 7a had poor in vitro
activity for CB1 receptor. But when methyl was super-
seded with ethyl 6b or n-propyl 6c, it showed more than
fivefold increase in binding potency compared to 6a. 1-
Ethyl-1H-tetrazole 6b showed approximately fivefold
more potent than 2-ethyl-2H-tetrazole 7b. But the rela-
tionship between relative orientation of substituents of
tetrazole and in vitro binding activity became less obvi-
ous when the alkyl chain became elongated (see Table 1:
6c, 6e, 6g and 6h, or 7c, 7e, 7g and 7h). Better in vitro
inhibition of binding displayed when straight alkyl
chains on tetrazole than branched alkyl chains if the
same number of carbons was counted (see Table 1: 6c
vs 6d and 6e vs 6f, or 7c vs 7d and 7e vs 7f). Among ali-
phatic alkyl chains tested, the best result for alkyl substi-
tuted tetrazole was obtained when n-octyl was alkylated
to 1H-tetrazole (6h) or 2H-tetrazole (7h). They showed
good binding affinity for rat CB1R (IC50 ꢀ 50 nM),
indicating the importance of non-polar moiety in order
to optimally bind to a hydrophobic area of CB1 recep-
tor. Interestingly, 1-cyclohexyl-1H-tetrazole 6s could
not be observed because of poor solubility in DMSO.
47.3
33.8
94.3
105
6j
7j
105
93
2-Methylpyridine
3-Methylpyridine
4-Methylpyridine
2-Methylfuran
6k
6l
7k
7l
103
100
135
93.8
195
77.6
6m
6n
6o
88.2
53.8
105
7m
7n
7o
7p
3-Methylfuran
3-Methylthiophene 6p
39.8
181
Cyclopropyl
Cyclobutyl
6q
6r
6s
6t
42.3
26.5
Cyclopentyl
Cyclohexyl
7s
67.9
75.8
Insoluble 7t
26.7
7v
Cycloheptyl
Cyclohexylmethyl
6u
85.3
a CB1 receptor was collected from brain tissue of SD rat.
b These data were obtained by single determinations.
methylpyridines (6k, 6l, 6m) seemed to be tolerated for
the replacement to a degree (IC50 ꢀ 100 nM), but none
appeared more potent than the simple phenyl group.
However, hydrophobic methylthiophene (6p) appears
to restore similar level of inhibition of phenyl group
for the region. The carbocycle derivatives (6r, 6s, 6u) dis-
played good binding inhibitions (IC50 = 42.3, 26.5,
26.7 nM, respectively), indicating that there might be
some SAR potential for other carbocycles. The surpris-
ingly poor result for cyclopropyl (6q, IC50 = 181 nM)
The 1-phenyl-1H-tetrazole 6i displayed an improvement
in inhibition of binding compared to the simple aliphatic
chains (IC50 = 33.8 nM). Heteroaryl groups such as
Cl
Cl
N
N
N
N
N
N
a
N
N
5
N
N
+
N
N
Cl
Cl
Cl
Cl
7s
6s
b, c
2
Scheme 4. Reagents and conditions: (a) cyclohexylbromide, K2CO3, DMF, 16% (6s) and 61% (7s); (b) cyclohexylamine, Et3N, CH2Cl2, rt; (c) PCl5,
PhCH3, 10 min; then HN3, rt 1 day then reflux, 74% overall yield.

2388
S. Y. Kang et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2385–2389
Table 2. Inhibitions of CB1 and CB2 receptors of cycloalkyl substi-
tuted tetrazolea,b
In conclusion, we investigated a series of tetrazole-bia-
rylpyrazole derivatives for their inhibition of binding
for cannabinoid CB1 and CB2 receptors. We have iden-
tified a novel tetrazole-based series of small molecule
cannabinoid CB1 antagonists that show potency compa-
rable to that of known CB1 antagonists. Several com-
pounds in this series exhibited potent CB1 receptor
binding affinities, validating the hypothesis that tetra-
zole could replace amide functionality to act as a bioiso-
stere of amide moiety of rimonabant.
X
R'
N
N
N
N
N
N
R
Cl
Cl
9
c
Compound rCB1 IC50 hCB2 IC50
c
R
R0
Rimonabant
X
Tetrazoles substituted with alkyl chains, aryl or hetero-
aryl showed moderate potency for CB1R binding. So
far, the best results in this series were obtained when
cycloalkyl tetrazoles were attempted. Thus, cyclopen-
tyl-tetrazole 9a exhibited high level activity as well as
good selectivity of CB1R over CB2R. Additional PK
and in vivo efficacy studies in addition to further SAR
studies of the novel tetrazole-based series will be the sub-
ject of future publications.
4.5d
26.5
26.7
11.6
16.5
22.6
14.2
1760d
>10,000
818
Cyclopentyl Me Cl 6s
Cycloheptyl Me Cl 6u
Cyclopentyl Me Br 9a
Cycloheptyl Me Br 9b
4240
>10,000
840
1930
Cyclopentyl Et
Cyclopentyl Et
Cl 9c
Br 9d
a CB1 receptor was collected from brain tissue of SD rat.
b CB2 receptor was recombinant human receptor expressed in CHO
cell.
c These data were obtained by single determinations.
d These data were obtained by in-house assay.
Acknowledgments
Financial support was provided by Green Cross Corpo-
ration (GCC). We thank Drs. Chong-Hwan Chang and
Jae-Wook Huh for their many helpful discussions
throughout small molecule programs at GCC. Also we
are grateful to Dr. Eun Chul Huh, Mr. Jung Ho Kim
and Ms. Jae-Young Jang at GCC Office of R&D plan-
ning and coordination for their supports and services.
suggested that there might be a size requirement for the
tetrazole alkyl region to attain good binding to the CB1
receptor. As shown in the case of cyclopentyl (6s, 7s), 1-
substituted 1H-tetrazoles often turned out to be more
potent than 2-substituted 2H-tetrazoles.
In order to improve inhibition levels to the CB1 recep-
tor, a structural replacement unit A to 4-bromophenyl
was undertaken. In addition, extension of methyl to
ethyl on pyrazole core was also studied. A structurally
related series of tetrazole-biarylpyrazole derivatives
was prepared in an analogous fashion previously de-
scribed in Scheme 3. The inhibitions of binding of these
tetrazole analogues are shown in Table 2.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.bmcl.2008.02.061.
References and notes
With observation of inhibition activity of CB2 receptor,
interesting compounds were further evaluated. The IC50
values were measured for the recombinant human CB2
receptor expressed in CHO cells and employing
[3H]WIN-55,212-2 as a radio-ligand.22 Replacement of
5-(4-chlorophenyl) 6s (IC50 = 26.5 nM) with 5-(4-
bromophenyl) 9a (IC50 = 11.6 nM) improved CB1
receptor binding affinity in more than twofold. This phe-
nomenon is clearly demonstrated by comparing 6u
(IC50 = 26.7 nM) versus 9b (IC50 = 16.5 nM) and 9c
(IC = 22.6 nM) versus 9d (IC50 = 14.2 nM), indicating
the importance of bromine on the phenyl ring for the
improvement of activity of CB1 receptor inhibition.
Regarding CB2 receptor activity, there appeared to be
no improvement as chlorines were switched to the corre-
sponding bromides. On the other hand, extension of
methyl to ethyl on pyrazole ring in this series was not
as sensitive as the halogen substitution of phenyl ring
for the inhibition of CB1 receptor, as exemplified by
two pairs of compounds involving 6s (IC50 = 26.5 nM)
to 9c (IC50 = 22.6 nM) and 9a (IC50 = 11.6 nM) to 9d
(IC50 = 14.2 nM).
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