Synthesis, biological evaluation and SAR studies of benzimidazole derivatives as H1-antihistamine agents

Article abstract of DOI:10.1016/j.cclet.2012.04.020

A series of benzimidazole derivatives have been designed, synthesized and evaluated for H₁ antihistamine activity. Six compounds have showed potent antihistamine H₁ activity. The primary SAR analysis indicated that benzyl or benzylidinyl substituted on the exo-nitrogen atom and C2 of the benzimidazole were significant. Further experiments indicated that compound 17d displayed excellent activity to reduce mast cell degranulation, moderate anti-PAF activity and decreased potency on hERG compared to astermizole. Hence compound 17d could serve as anti-allergic agent for further development.

Full text of DOI:10.1016/j.cclet.2012.04.020

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 707–710
www.elsevier.com/locate/cclet
Synthesis, biological evaluation and SAR studies of benzimidazole
derivatives as H1-antihistamine agents
Xiao Jian Wang a, Mei Yang Xi b, Ji Hua Fu b, Fu Rong Zhang a, Gui Fang Cheng a,
b,
Da Li Yin a, , Qi Dong You
*
*
a Department of Medicinal Chemistry, Beijing Key Laboratory of Active Substances Discovery and Drugability Evaluation,
State Key Laboratory of Bioactive Substances and Functions of Natural Medicines, Institute of Materia Medica,
Peking Union Medical College and Chinese Academy of Medical Sciences, Beijing 100050, China
b School of Pharmacy, China Pharmaceutical University, Nanjing 210009, China
Received 31 March 2012
Available online 11 May 2012
Abstract
A series of benzimidazole derivatives have been designed, synthesized and evaluated for H1 antihistamine activity. Six
compounds have showed potent antihistamine H1 activity. The primary SAR analysis indicated that benzyl or benzylidinyl
substituted on the exo-nitrogen atom and C2 of the benzimidazole were significant. Further experiments indicated that compound
17d displayed excellent activity to reduce mast cell degranulation, moderate anti-PAF activity and decreased potency on hERG
compared to astermizole. Hence compound 17d could serve as anti-allergic agent for further development.
# 2012 Da Li Yin. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Benzimidazole derivatives; Antihistamine activity; SAR; Anti-PAF activity; hERG
Histamine has been widely recognized as an intercellular messenger, which plays an important role in several
physiological processes [1,2]. Four histamine receptors have been identified (H1–4), characterized as critical members
of the G protein coupled receptors (GPCRs) superfamily [3–5]. Among them, H1 receptor protein that has a molecular
weight of 53–58 kDa in different animal tissues is regarded as responsible to histamine mediated allergy syndrome and
therefore as the target of anti-allergy drugs since it controls the contraction of smooth muscle and is responsible for
allergy and immunity courses [2,6,7].
The first generation H1 antagonists such as mepyramine and diphenhydramine were proven to possess significant
antihistamine effects. However, most of them also displayed the side effect of sedation. The central depressant effects
have been devoid by the second generation H1 antagonists due to the low penetration of the agents into CNS [6,8].
However, the substantial clinical results indicated that some of the second generation agents (astermizole, terfenadine)
that cause potentially fatal arrhythmia as side effect have been withdrawn from the market [9].
Astermizole is a potent histamine H1-receptor antagonist (Ki = 2 nmol/L). However, it causes acquired long QT
syndrome cardiac arrhythmias including torsades de pointes, and sudden death due to blockage of the rapidly
* Corresponding authors.
E-mail addresses: yindali@imm.ac.cn (D.L. Yin), youqidong@gmail.com (Q.D. You).
1001-8417/$ – see front matter # 2012 Da Li Yin. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2012.04.020

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X.J. Wang et al. / Chinese Chemical Letters 23 (2012) 707–710
Scheme 1. Reagents and condition: (a) KF, K2CO3, 180 8C, 24 h, 85%; (b) Zn, AcOH, rt, 2 h, 98%; (c) Boc2O, 1 mol/L NaOH, THF, 25 8C, 2 h,
97%; (d) ethyl chloroformate, Et3N, CH2Cl2, 0 8C, 30 min; (e) AcOH, reflux, 2 h, 72%.
activating delayed rectifier K+ current IKr and the human ether-a go-go related gene (hERG) K+ channels that underlie
it (IC50 = 0.9 nmol/L) [10,11].
A systematic structure modification based on astermizole has been made in order to discover the novel H1 receptor
antagonists with reduced hERG blockage activity. We reported herein the synthesis and biological evaluation of a
series of benzimidazole derivatives. The SAR of the in vitro antihistamine effects of these compounds was discussed.
1. Result and discussion
The synthetic route for benzimidazole derivatives 7a, 7b and 17a–17f is summarized in Schemes 1 and 2
respectively. Compounds 3a and 3b were obtained by the amination of 1 with p-fluorine or p-chlorine substituted
aniline (2a and 2b). The further reduction of the nitro with Zn/AcOH in CH2Cl2 afforded 4a and 4b. On the other hand,
Boc-protected 5 gave compound 6 which could acylate 4a and 4b in the presence of ethyl chloroformate. The further
cyclization and deprotection in refluxed acetic acid afforded 7a and 7b (Scheme 1). Compounds 10a and 10b were
obtained by the amination of p-fluorine or p-chlorine substituted benzyl chloride. Additionally, compounds 14 and 16
were synthesized from 11 according to previous literature [12]. 17a–17f were generated by the following acylation,
cyclization and deprotection procedures (Scheme 2).
The antihistamine activity of target compounds 7a, 7b, 17a–17f were assessed by inhibition of histamine-induced
contraction in guinea ileum that was performed according to the method of Magnus [13]. 17c, 17d, 17f showed the
maximum in vitro antihistamine activity which are more potent than astermizole. The introduction of 4-fluoro or
chloro benzyl group onto the exo-nitrogen atom of the benzimidazole system was significantly more active than the 4-
fluoro or chloro phenyl substituted derivatives, which indicated that the flexibility of the group is required for
antihistamine activity. Each compound 17a, 17c, 17e (4-fluoro) was as active as 17b, 17d, 17f (4-chloro), respectively.
H2N HN
NH2
NH2
a
X
+
Cl
9-10a X=F
9-10b X=Cl
8
9a-b
10a-b
X
HN
HCl
O
N
N
HO
b
HO
m
N
Boc
NH
11
12
O
O
c
5
6
NH
h, i
or
Boc
N
Boc
N
d
e
Boc
N
O
O
O
X
17a: X=F, m=0
17b: X=Cl, m=0
HO
EtO
13
14
17c: X=F, m=1, double bond
17d: X=Cl, m=1, double bond
17e: X=F, m=1
Boc
N
g
or
f
Boc
N
O
O
HO
17f: X=Cl, m=1
EtO
15
16
Scheme 2. Reagents and condition: (a) K2CO3, DMF, 50 8C, 6 h, 78%; (b) Boc2O, 1 mol/L NaOH, THF, 25 8C, 2 h, 97%; (c) Boc2O, 2 mol/L
NaHCO3, THF, 25 8C, 6 h, 98%; (d) ethyl diethoxyphosphoryl acetate, K2CO3, DMF, 78 8C, 12 h, 90%; (e) 1 mol/L NaOH, CH3OH, 25 8C, 30 min,
98%; (f) 10% Pd/C, H2, C2H5OH, 25 8C, 7 h, 96%; (g) 1 mol/L NaOH, CH3OH, 25 8C, 30 min, 98%; (h) ethyl chloroformate, Et3N, CH2Cl2, 0 8C,
30 min; (i) AcOH, reflux, 2 h, 74%.

X.J. Wang et al. / Chinese Chemical Letters 23 (2012) 707–710
709
Table 1
Antihistamine, receptor binding and anti-PAF activities of benzimidazole derivatives.
No.
AntiH1 activity ileum
IC50 (mmol/L)a
H1 receptor binding
IC50 (mmol/L)a
PAF-induced platelet
aggregation IC50 (mmol/L)a
7a
7b
4.07
0.337
150
130
200
110
100
78
0.661
0.717
17a
17b
0.0692
0.0457
0.0191
0.00794
0.0174
0.049
0.0213
0.0221
0.00335
0.000881
0.00268
0.00224
0.00588
0.004
17c
17d
17e
17f
100
140
130
ndb
Desloratadine
Astermizole
0.0721
0.0453
a
The IC50 values were determined from six different concentrations of compounds at twofold dilutions, and were the means of two separate
experiments.
b
Not determined.
Compounds 17e and 17f with –CH2– between benzimidazole and piperidine were 2–5-fold more active than 17a and
17b in which benzimidazole directly linked to piperidine, whereas 3–10-fold improvement was observed for 17c and
17d with a carbon–carbon double bond between benzimidazole and piperidine.
The further receptor binding assay [14] indicated that all tested compounds displaced the radioligand (3H-
mepyramine) from H1 receptor with an IC50 ranging from 0.8 to 717 nmol/L (Table 1). In this respect, compounds 17d
are more active than astermizole and desloratadine, while compounds (17c, 17e, 17f) showed the same order of
magnitude of an affinity for the receptor as the controls.
Benzimidazole derivatives were also tested for platelet activating factor receptor (PAF) antagonist activity using in
vitro PAF-induced platelet aggregation assay to fully evaluate their anti-allergy activity [15]. PAF is believed to be a
potent chemotactic stimulus for eosinophils, and it is involved in mediating allergy and inflammatory disease. The
results suggested that only 17d possessed moderate anti-PAF activity.
Compound 17d has been further evaluated for its ability to reduce the number of degranulated mast cells (DMC) of
Wistar rat [16]. 17d displayed excellent inhibitory effect on the numbers of DMC with a concentration-dependent
manner (IC50 = 3.1 nmol/L). The significant protection from mast cell degranulation induced by 17d demonstrated
that it possessed good anti-allergy activity.
The hERG IC50 value for 17d was corroborated by cellular electrophysiology using a standard whole-cell voltage
clamp assay in CHO cells that stably expressed hERG [17]. The results showed that 17d possessed moderate inhibitory
activity on hERG (IC50 = 142.2 nmol/L) (Fig. 1). In comparison, astermizole potently blocked the IKr-like current with
IC50 values of 36 nmol/L.
Fig. 1. The inhibitory activity of 17d on hERG.

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X.J. Wang et al. / Chinese Chemical Letters 23 (2012) 707–710
2. Conclusion
We describe the synthesis and biological activity toward the new benzimidazole derivatives as H1-antihistamine
agents. The SAR study reveals that the flexible substitution of 4-fluoro or chloro benzyl group onto the exo-nitrogen
atom of the benzimidazole, and carbon–carbon double bonds between benzimidazole and piperidine showed increased
antihistamine activity. Compound 17d showed potent histamine H1-receptor antagonistic activity, moderate anti-PAF
activity and displayed significant reduction effect on the numbers of DMC. The hERG affinity of 17d is remarkably
decreased compared with astermizole. It is therefore believed that compound 17d is a more potent, safer and promising
compound for further development.
Acknowledgment
This project was supported by National Major Scientific and Technological Special Project (No. 2009ZX09301-003).
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