Structure-activity relationships of 2,4-diphenyl-1H-imidazole analogs as CB2 receptor agonists for the treatment of chronic pain

Article abstract of DOI:10.1016/j.bmcl.2010.11.044

A series of 2,4-diphenyl-1H-imidazole analogs have been synthesized and displayed potent human CB2 agonist activity. Many of these analogs showed high functional selectivity over human CB1 receptors. The syntheses, structure-activity relationships, and selected pharmacokinetic data of these analogs are described.

Full text of DOI:10.1016/j.bmcl.2010.11.044

Bioorganic & Medicinal Chemistry Letters 21 (2011) 182–185
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Structure–activity relationships of 2,4-diphenyl-1H-imidazole analogs
as CB2 receptor agonists for the treatment of chronic pain
Shu-Wei Yang a, , Jennifer Smotryski a, Julius Matasi a, Ginny Ho a, Deen Tulshian a, William J. Greenlee a,
⇑
Rossella Brusa b, Massimiliano Beltramo b, Kathleen Cox c
a Department of Chemical Research—CV & CNS, Merck Research Laboratory, 2015 Galloping Hill Road, Kenilworth, NJ 07033, USA
b Neurobiology Department of Biological Research, Merck Research Laboratory, 2015 Galloping Hill Road, Kenilworth, NJ 07033, USA
c Department of Drug Metabolism, Merck Research Laboratory, 2015 Galloping Hill Road, Kenilworth, NJ 07033, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 2,4-diphenyl-1H-imidazole analogs have been synthesized and displayed potent human CB2
agonist activity. Many of these analogs showed high functional selectivity over human CB1 receptors.
The syntheses, structure–activity relationships, and selected pharmacokinetic data of these analogs are
described.
Received 19 August 2010
Revised 3 November 2010
Accepted 5 November 2010
Available online 11 November 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
CB2
Agonist
Chronic pain
Cannabinoid
Cannabinoid receptors, members of the superfamily of G pro-
tein-coupled receptors, mediate many of the CNS effects of 9-tet-
rahydrocannabinol ( 9-THC), the active component of Cannabis
CB2 receptors are considered an attractive analgesic target, more
studies are needed to elaborate their clinical potential.
D
D
During high-throughput screening of our chemical library, some
analogs with the 2,4-diphenyl-1H-imidazole skeleton were identi-
fied to display high human CB2 (hCB2) affinity and good selectivity
against human CB1 (hCB1). Among them, compound 1 displayed
high hCB2 affinity (Ki 42 nM) and potent agonist activity (EC50
9 nM, Emax 80%) in the cAMP assay. Compound 1 displayed decent
selectivity over hCB1. In the pharmacokinetic assay, 1 showed rea-
sonable plasma exposure after oral administration in rats (AUC (0–
6 h) 866 nM h, 10 mg/kg oral administration).
sativa (marijuana).1 The medical and psychotropic properties of
marijuana have been known for centuries. Up to now, two cannab-
inoid receptors, CB1 and CB2, have been cloned and characterized.2
Although both receptors and their ligands have been implicated in
pain perception, CB1 is expressed predominantly in the brain and
its activation is linked to the CNS effects of cannabinoids.3 In con-
trast, CB2 is mainly found in peripheral immune cells, sensory
afferents, and the spinal cord. Therefore selective CB2 agonists
have potential to retain analgesic efficacy with fewer adverse ef-
fects associated with CB1 receptors, such as catalepsy, ataxia, seda-
tion, and undesired psychotropic effects.
H
N
N
hCB2 EC50: 9 nM (Emax 80%)
hCB2 Ki: 42 nM
Selective CB2 agonists have been reported to be efficacious in ro-
dent models of neuropathic or inflammatory pain.4 Some CB2 ago-
nists including Hu-308,5 AM1241,6 GW405833,7 GW842166X,8 and
JWH1339 have been investigated in various acute and chronic pain
models. The analgesic effects of these selective CB2 agonists were
blocked by a CB2 antagonist, but not by a CB1 antagonist. The anal-
gesic effects of AM1241 and GW405833 were not observed in CB2
knockout mice.6,7 These evidences demonstrated that the analgesic
effect against inflammatory pain was mediated by CB2. Although
N
O
hCB1 EC50: >1 uM (Emax 67%)
hCB1 Ki: 1 uM
CF3
rat PK: AUC (0-6 h) 866 nM.h
(oral admin., 10 mg/kg, 6 h)
1
In this Letter, we will describe syntheses and structure–activity
relationships of this series of 2,4-diphenyl-1H-imidazole analogs.
The pharmacokinetic data of selected compounds will be
discussed.
The synthetic route to the 2,4-diphenyl-1H-imidazole analogs is
summarized in Scheme 1. The commercially available substituted
acetophenone was brominated with bromine and then cyclized
with formamide to form 4-phenyl-1H-imidazole 2.10 Subsequent
⇑
Corresponding author. Tel.: +1 908 740 7291; fax: +1 908 740 7164.
E-mail address: shu-wei.yang@merck.com (S.-W. Yang).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.11.044

S.-W. Yang et al. / Bioorg. Med. Chem. Lett. 21 (2011) 182–185
183
R
a, b
c, d
NSEM
O
R1
NH
R1
R
N
R1
N
N
N
N
a
I
b
1
3
N
2
N
O
O
N
O
c
d
CF3
R
O S
N
CF3
9
10
SEM
N
H
N
N
R
O
O
R2
e
f, g
N
N
N
N
R1
R3
R1
N
O
5
4
N
O
N
CF3
CF3
Scheme 1. Reagents and conditions: (a) Br2, CHCl3, rt; (b) HCONH2, neat, 185 °C,
2 h; (c) SEM-Cl, NaH, THF, rt; (d) n-BuLi, I2, THF, À78 °C to rt; (e) 3-(CHO)Ph-B(OH)2,
Pd(dppf)2Cl2, dioxane–H2O, 95 °C; (f) amine, NaBH(OAc)3, DCE, rt; (g) 6 N HCl,
MeOH, 80 °C.
12
11
Scheme 3. Reagents and conditions: (a) NaH, X-R, DMF, 100 °C; (b) R-Ph-B(OH)2,
Cu(OAc)2, 4 Å mol. sieve, pyridine, DCM, rt; (c) Ac2O, pyridine, 0 °C; (or) Ph-COCl,
Et3N, DCM, rt; (d) PhSO2Cl, Et3N, DCM, rt.
N-protection with 2-(trimethylsilyl)ethoxymethyl (SEM) chloride
and iodination using iodine were achieved under basic conditions
to afford 3. The iodo displacement with 3-phenyl aldehyde was
conducted under Suzuki coupling condition using Pd(dppf)2Cl2 as
the catalyst to give 4. Reductive amination and final removal of
the SEM yielded the target compounds (5).11
When R1 was bromine, Suzuki coupling (step e, in Scheme 1)
was not successful with 3. Both bromine and iodine atoms could
be displaced with the phenyl group, and this made it difficult to ob-
tain the desired target. Therefore, an alternative route was imple-
mented to prepare target 5 (Scheme 2). Commercially available
methyl 3-carbamimidoylbenzoate hydrochloride (6) was coupled
with 3-bromophenacyl bromide to give the 2,4-diphenyl-1H-imid-
azole moiety in one step. The methyl ester was reduced to the alco-
hol and then oxidized to aldehyde. Reductive amination then
afforded target 8.
The first SAR study focused on N-1 substitution of the middle
imidazole ring to identify whether the N-1 proton is important
for hCB2 agonist activity. Alkyl substitution of N-1 led to signifi-
cantly reduced hCB2 functional activity. Examples including the
i-propyl, c-propylmethyl, benzyl, and phenyl analogs (13–16) are
shown in Table 1. The acylated analogs (17 and 18) were evaluated
for the CB2 agonist activity, and displayed a similar potency to that
of 1. However, both acetyl and benzoyl analogs (17 and 18) were
unstable under aqueous acidic conditions (1 N HCl, 37 °C). The acyl
group was hydrolyzed, and parent 1 was detected. Therefore, these
analogs were not further evaluated because of their instability. The
N-1 substitution with a phenyl-sulphonyl group (19) totally erad-
icated the hCB2 functional activity. From these SAR data, the NH
group was identified to be critical for the hCB2 agonist activity,
and preferred not to be substituted.
The following SAR was focused on morpholine side chain
replacement, and the data are summarized in Table 2. Replacement
of the morpholine with thio-morpholine (20) retained the hCB2
agonist activity (EC50 6 nM, Emax 100%). Methyl substitution on
the 2- or 3-position of the morpholine ring led to slightly and sig-
nificantly reduced functional activity (25 and 169 nM for 21 and
22, respectively). The piperidinyl analog (23) displayed potent full
hCB2 agonist activity (EC50 14 nM, Emax 104%), indicating that the
morpholine oxygen atom was not crucial for potent activity. Hence
a series of substituted piperidinyl analogs were prepared for the
SAR study. Compound 24 with the 4-methyl piperidinyl moiety
displayed similar hCB2 agonist activity compared to that of 23. In
contrast, the 3-methyl piperidinyl analog (25) showed a ꢀfivefold
reduction in the hCB2 agonist activity, compared to that of 23. Ana-
logs 26 and 27 with the 4-fluoro or 4,4-di-fluoro piperidinyl moiety
retained potent CB2 functional activity with EC50 values of 9 and
Several N-1 substituted analogs of 1 were synthesized as de-
scribed in Scheme 3. Alkylation of 1 using alkyl halide and sodium
hydride gave 9. Compound 1 was coupled with phenyl boronic acid
using Cu(OAc)2 to generate 10. Acylation or sulfonylation of 1
afforded analogs 11 and 12, respectively.
Target compounds were tested for their affinity at the cloned
hCB1 or hCB2 receptors expressed in CHO cell membranes by mea-
suring their ability to compete with [H3]-CP55,940, a non-selective
CB1 and CB2 agonist. The functional activities of all synthesized
compounds were evaluated by their ability to inhibit forskolin-
mediated cAMP accumulation, using CHO cells expressing recom-
binant hCB1 or hCB2 receptors. Functional efficacy (EC50) and max-
imum percent activation (Emax) of hCB2 were used to evaluate the
compound’s potency and agonist activity. WIN55212-2 was em-
ployed as a reference compound with an EC50 value of 4 nM for
hCB2 agonist activity.
Table 1
HCl
NH
NH2
H
N
SAR of N-1 substituted analogs of 1
R
N
a
N
CO2Me
N
CO2Me
b, c, d
O
N
Br
7
6
CF
3
H
N
R
hCB2, EC50 (nM) (Emax
)
hCB1, EC50 (nM)
N
13
14
15
16
17
18
19
i-Propyl
218
412 (68%)
>1000
>1000
>1000
>1000
ND
>1000
ND
N
O
Cyclopropyl-methyl
Benzyl
Br
8
Phenyl
Ac
Benzoyl
–SO2Ph
>1000
9 (102%)
11 (101%)
>1000
Scheme 2. Reagents and conditions: (a) 3-bromophenacyl bromide, Na2CO3, DMF,
rt; (b) LiAlH4, THF, 0 °C; (c) Dess–Martin, DCM, rt; (d) morpholine, NaBH(OAc)3,
DCE, rt.
ND

184
S.-W. Yang et al. / Bioorg. Med. Chem. Lett. 21 (2011) 182–185
Table 2
Table 3
SAR of analogs with morpholine, piperidine or thio-morpholine side chain
SAR of substituted 4-phenyl piperidinyl analogs
H
N
H
N
N
N
N
R
R
CF3
R
hCB2, EC50 (nM) (Emax
)
hCB1, EC50 (nM)
R
hCB2, EC50 (nM) (Emax
)
hCB1, EC50 (nM)
>10,000
32
33
34
35
36
3-Cl
10 (114%)
8 (115%)
24 (104%)
4 (108%)
87 (102%)
1055
12,550
1800
320
2,3-di-Cl
2,4-di-Cl
2-F, 5-OMe
2-F, 5-OEt
1
8 (80%)
N
O
20
6 (100%)
>1000
N
S
8500
N
O
O
21
22
25 (97%)
>10,000
ND
of the 3-trifluoromethyl group was tolerated and retained potent
CB2 agonist activity.
169 (78%)
N
N
The biological data of some selected potent hCB2 agonists from
hybrid molecules are outlined in Table 4. The bromo analog (8) of 1
displayed similar potent CB2 agonist activity. Two 3-chloro analogs
(37 and 38) retained potent hCB2 agonist activity. Conversely, the
selectivity ratio of hCB1/hCB2 was significantly reduced to only
ꢀ16-fold for 38. All three 2,3-dichloro-substituted piperidinyl ana-
logs (39–41) exhibited potent and selective hCB2 agonist activity.
In the 2-fluoro-(4 or 5)-methoxy series (42–45), potent hCB2 ago-
nist activity was retained, but selectivity over hCB1 was dimin-
ished (ꢀ5–35-fold).
23
24
25
26
14 (104%)
21 (97%)
70 (87%)
9 (107%)
>10,000
>10,000
ND
N
N
N
N
2290
F
F
Several ortho-substituted 2-phenyl analogs were synthesized
using the same methods described in Scheme 1 to compare with
the meta-substituted analogs. In general, hCB2 functional activity
was significantly reduced for ortho-substituted analogs. Two
examples (46 and 47) are shown in Table 5.
Because compound 1 did not possess high plasma exposure
(AUC 866 nM h) in rats, selected potent analogs were investigated
for their pharmacokinetic properties, mainly AUC (0–6 h) in the rat
27
28
5 (100%)
16 (98%)
2550
ND
F
N
N
F
29
30
31
25 (98%)
113 (92%)
61 (91%)
ND
F
F
OH
>10,000
>1000
N
N
Table 4
SAR of compounds with hCB2 agonist activities <25 nM
H
N
OH
N
R2
1
R
R1
R2
hCB2, EC50 (nM) (Emax
)
hCB1, EC50 (nM)
665
5 nM, respectively. Fluoro substitution at the 3-position of the
piperidine led to less potent analogs (28 and 29). Hydroxyl or
hydroxylmethyl substitution at the 3-position of piperidine (30
and 31) resulted in an ꢀeightfold or fourfold decrease in potency,
respectively, compared to that of 23. Overall, the methyl (24) or
fluoro substitution (26–29) at the 4-position of piperidine was al-
lowed and retained potent hCB2 agonist activity and good selectiv-
ity over hCB1.
The SAR of substitution on the 4-phenyl ring was further inves-
tigated. The hCB2 functional activity of these piperidinyl analogs
(32–36) were compared to that of 23 (Table 3). Replacement of
the 3-trifluoromethyl group of 23 with a chlorine atom retained
potent hCB2 agonist activity (32, EC50 10 nM). Additional chlorine
substitution on 2-position of the phenyl did not affect the potency
(33, EC50 8 nM). However the 2,4-dichloro analog (34) showed a
threefold reduction in the hCB2 agonist activity, compared to that
of 33. When the substitution was changed to 2-fluoro and 5-meth-
oxy groups on the 4-phenyl ring, functional activity was improved
to EC50 4 nM (35, Emax 108%). However hCB2/hCB1 selectivity of 35
declined to ꢀ80-fold. Increasing the size of the alkoxyl group from
methoxy to ethoxy group resulted in a significant reduction of the
hCB2 activity (36, EC50 87 nM). In this preliminary study, variation
8
3-Br
5 (112%)
N
N
O
O
37 3-Cl
14 (102%)
3250
38 3-Cl
8 (107%)
130
N
F
39 2,3-di-Cl
25 (107%)
>10,000
N
N
N
N
40 2,3-di-Cl
41 2,4-di-Cl
5 (119%)
1294
2450
F
F
10 (107%)
F
42 2-F, 4-OMe
43 2-F, 5-OMe
44 2-F, 5-OMe
45 2-F, 5-OMe
3 (107%)
4 (109%)
5 (107%)
5 (106%)
39
140
25
F
N
N
O
F
F
87
N
F

S.-W. Yang et al. / Bioorg. Med. Chem. Lett. 21 (2011) 182–185
185
Table 5
could be substituted with a methyl group or replaced with a
(un)substituted piperidine ring. Changing the substitution group
or pattern of 4-phenyl group of 1 was tolerated. However, some
disubstituted 4-phenyl analogs (42–45) displayed diminished
selectivity over hCB1. Moving the saturated-heterocyclic methyl
moiety from the meta to the ortho position led to reduced hCB2
agonist activity, and was not desired. During efforts to improve
the pharmacokinetics of 1, our screening results indicated that
the methyl substitution at the 2-position of the morpholine ring
could enhance compound exposure to twofold in rats, compared
to that of 1. Several analogs from this 2,4-diphenyl-1H-imidazole
series exhibited efficacy in neuropathic pain models in rodents.
These results will be published in due course.
SAR of ortho-substituted phenyl analogs
H
N
N
R2
CF3
R2
hCB2, EC50 (nM) (Emax
)
hCB1, EC50 (nM)
>10,000
46
47
130 (108%)
N
N
F
O
510 (102%)
>10,000
Acknowledgments
The authors wish to acknowledge Drs. Ying Huang and Andrew
Stamford for providing information of hits from the high-through-
put screening, Dr. Jesse Wong and his group for preparation of key
intermediates, Dr. Tze-Ming Chan and his group for structure con-
firmation of some analogs, and the DMPK group for acquiring phar-
macokinetic data.
Table 6
Rat pharmacokinetic data (AUC) of selected compounds
AUC, (nM h), rat 0–6 h
1
8
866
158
42
20
21
23
24
25
26
27
28
29
1989
537
881
1411
524
1111
682
851
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model with 10 mg/kg oral administration. Their AUC data are listed
in Table 6. Replacing the 3-CF3 group with a bromine atom (8) or
replacing the oxygen atom with a sulfur atom (20) led to reduced
plasma exposure (AUC 158 and 42 nM h, respectively). Implement-
ing a methyl group next to the morpholine oxygen atom (21) re-
sulted in a twofold enhancement of the plasma exposure (AUC:
1989 nM h), compared to that of 1. This may be due to the blockage
of the metabolic potential site of the morpholine ring. The piperid-
inyl analog (23) displayed slightly reduced AUC (537 nM h) in com-
parison to that of 1. However, compounds (24 and 25) with methyl
substitution at the 4- or 3-position of the piperidine ring improved
the exposure in rats (AUC: 881 and 1411 nM h, respectively), rela-
tive to that of 23. Four fluoro-substituted piperidinyl analogs (26–
29) were evaluated for their pharmacokinetic properties. Their
AUC values were comparable or slightly better than that of 1. In
general, methyl substitution at the 2-position of the morpholine
ring or 3- or 4-position of the piperidine ring could increase the
compound exposure in rats, compared to those of the unsubstitut-
ed analogs (1 and 23).
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In conclusion, a novel series of potent and selective CB2 recep-
tor agonists based on the 2,4-diphenyl-1H-imidazole scaffold were
discovered. The preliminary SAR studies are summarized as fol-
lows. The N-1 substitution was not desirable. The morpholine ring
11. Analytical data for compound 1: 1H NMR (CDCl3): d 2.50 (m, 4H), 3.58 (s, 2H),
3.74 (t, J = 4.6 Hz, 4H), 7.36 (d, J = 7.7 Hz, 1H), 7.42 (t, J = 7.7 Hz, 1H), 7.46 (br s,
1H), 7.51 (m, 2H), 7.82 (d, J = 7.7 Hz, 1H), 7.92 (s, 1H), 8.03 (m, 1H), 8.11 (m,
1H), 9.70 (br s, NH). ESI-MS: m/z 388 [M+H]+, C21H20F3N3O.