Benzimidazole-substituted (3-phenoxypropyl)amines as histamine H3 receptor ligands

Article abstract of DOI:10.1016/j.bmcl.2008.08.008

A series of non-imidazole histamine H₃ receptor antagonists based on the (3-phenoxypropyl)amine motif, which is a common pharmacophore for H₃ antagonists, has been identified. A preliminary SAR study around the amine moiety has identified 8a as a potent H₃ antagonist possessing a good pharmacokinetic profile in the rat.

Full text of DOI:10.1016/j.bmcl.2008.08.008

Bioorganic & Medicinal Chemistry Letters 18 (2008) 5032–5036
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Benzimidazole-substituted (3-phenoxypropyl)amines as histamine H3
receptor ligands
*
Robert Aslanian , Xiaohong Zhu, Henry A. Vaccaro, Neng-Yang Shih,
John J. Piwinski, Shirley M. Williams, Robert E. West, Jr.
The Schering Plough Research Institute, 2015 Galloping Hill Road, Kenilworth, NJ 07033-1300, 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 non-imidazole histamine H3 receptor antagonists based on the (3-phenoxypropyl)amine
motif, which is a common pharmacophore for H3 antagonists, has been identified. A preliminary SAR
study around the amine moiety has identified 8a as a potent H3 antagonist possessing a good pharmaco-
kinetic profile in the rat.
Received 5 June 2008
Revised 31 July 2008
Accepted 4 August 2008
Available online 7 August 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Histamine
H3
The endogenous amine, histamine, is a ligand for four distinct
histaminergic receptor subtypes, namely the H1, H2, H3, and H4
receptors.1 H1 receptors are located in the CNS, and on skin and
smooth muscle in the airways; H2 receptors are located in the gas-
trointestinal tract. Antagonists of the H1 and H2 receptors are well-
known therapeutic agents, and have been used clinically for many
years for the treatment of allergic diseases and ulcers, respectively.
H3 receptors are located primarily in the CNS where they act as
both autoreceptors for histamine as well as heteroreceptors for
other neurotransmitters. Evidence from in vivo studies in animals
indicates that H3 receptor ligands may be useful for the treatment
of a variety of CNS-related disorders like ADHD, Alzheimer disease,
sleep disorders, and neuropathic pain.2 Additionally, given the
known role of central histamine in the control of appetite, H3
receptor ligands are also reported to be active in animal models
of obesity.2,3 The H4 receptor is located on various hematopoietic
cells and functions in chemotaxis of these cells.4 There are cur-
rently no marketed drugs that act at either the H3 or H4 receptors,
although several companies have entered clinical trials with H3
antagonists/inverse agonists for the treatment of cognition, pain,
and narcolepsy.5 Recently, positive clinical data in a small clinical
trial for narcolepsy have been reported.6
derivatives make poor drug candidates due to their propensity to
inhibit numerous mammalian CYP450 isozymes,7 and in the case
of a CNS drug, their poor brain penetration. Therefore, a significant
synthetic effort was undertaken in academia and industry to iden-
tify non-imidazole-derived H3 antagonists. Although the natural
product Aplysamine-1, identified by scientists at Harbor Branch
Oceanographic Institute and Schering-Plough, was known as a
weak H3 receptor ligand,8 the first real breakthrough came from
Ganellin and co-workers who identified the phenoxyalkyl amine
motif of Aplysamine-1 as a potent H3 pharmacophore (UCL 1972,
Fig. 2).9 Subsequently, a large number of groups have utilized this
pharmacophore as the basis of their own programs (Fig. 2).2a More
recently, other H3 pharmacophores have been described, but none
have been as thoroughly examined as the phenoxyalkyl amine.10,11
When examining the structures of compounds based on this
pharmacophore described in the primary and patent literature,
we were struck not only by the similarity of many of the structures
but also by the apparent promiscuity of the receptor toward moi-
eties on the phenyl ring. Simple moieties like the nitro group as
well as basic groups, fused rings, and heterocycles are all tolerated.
Based on a series of benzimidazole-substituted analogs previously
prepared in our laboratories, we decided to investigate a series of
compounds in which the phenyl ring was substituted by an N-
linked 5-fluorobenzimidazole moiety (Fig. 3).12 The results are de-
scribed herein.
Structurally, the first generation of H3 ligands were analogs of
histamine in that they were based on a 4-substituted imidazole
motif. Typical examples of agonists and antagonists are given in
Figure 1. These compounds were useful tools in delineating some
of the pharmacology of the H3 receptor, but in general imidazole
The syntheses of the analogs in Tables 1 and 2 are described in
Schemes 1 and 2.13 Initially, we decided to focus on analogs incor-
porating a 2-pyridyl, thioether, or ether moiety at the 2-position of
the benzimidazole ring based on our previous experience.12 Briefly,
the benzimidazole moiety is constructed by reacting amine 1 with
* Corresponding author. Tel.: +1 908 740 3514; fax: +1 908 740 7152.
E-mail address: robert.aslanian@spcorp.com (R. Aslanian).
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.08.008

R. Aslanian et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5032–5036
5033
Imidazole-derived Agonists
NH2
H3C
NH
NH2
CH3
HN
HN
HN
N
N
N
Histamine
Imidazole-derived H3 Antagonists
R-α-methylhistamine
Sch 50971
S
NH
N
H
N
N
H
S
HN
N
HN
N
Thioperamide
Clobenpropit
Cl
HN
H
N
H
N
N
Cl
O
Sch 79687
Cl
Figure 1. Examples of imidazole-derived agonists and antagonists.
CH3
N
Br
O
H3C
CH3
N
Br
Aplysamine-1
CN
CH3
NO2
O
N
N
O
Abbott
N
UCL1972
H3C
CH3
N
H3C
N
N
O
N
O
Johnson & Johnson
N
OH
Schering-Plough
Figure 2. Examples of non-imidazole-derived H3 antagonists.
R1
benzimidazole 6, which was alkylated on the phenol to give key
intermediate 7. Chloride 7 was easily converted to a variety of
amines 8 utilizing parallel synthesis techniques.
O
N
R3
R2
The synthesis of analogs incorporating either an ether or a thi-
oether in the 2-position of the benzimidazole moiety is given in
Scheme 2.
N
N
Treatment of phenol 9 with base and 1-bromo-3-chloropropane
followed by displacement of the chlorine with piperidine gave 11
in excellent yield for the two steps. Reduction of the nitro group
of 11 with Ra-Ni and reaction with 2 gave compound 12. Reduction
of the nitro group, again with Ra-Ni, gave the amine which was
reacted with either thiocarbonyl diimidazole (X@S) or carbonyl
diimidazole (X@O) to give 14a and 14b. Treatment of 14a or 14b
with base and an alkyl halide gave the ethers 15a and 15b. Human
H3 binding data for analogs 8, 14, and 15 are given in
Tables 1 and 2.14
F
Figure 3.
difluoronitrobenzene 2 to yield 3. Reduction of the nitro group of 3
gave compound 4, which was coupled with picolinic acid to give
amide 5. Heating 5 in acetic acid results in ring closure to give

5034
R. Aslanian et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5032–5036
Table 1
Table 2
Human H3 binding data for benzimidazoles 814
H3 binding data for 14 and 1514
Benzimidazole
Amine
H3 Kia (nM)
Compound
X or RX
H3 Kia (nM)
14a
14b
15a
15b
O
S
8.7
8.0
2.6
15
8a
1.2
N
SCH2CH3
OCH2CH3
H3C
CH3
a
8b
8c
11
H3 binding Ki values are the average of at least two independent
determinations.
N
3.4
O
N
A number of conclusions regarding the SAR of this series of
benzimidazole-derived H3 ligands can be drawn from these
data. First, five-, six-, and seven-membered monocyclic amines
give good to excellent H3 binding activity, and a second het-
eroatom is tolerated within the ring (entries 8a–k). Bulkier,
fused bi- and tricyclic amines, however, are less active (entries
8l and 8m) although some steric bulk is tolerated (8d). It is
interesting to note that a simple methylamine substituent does
not result in good binding affinity (8r). Polar substituents like
amino, hydroxyl, ester, and amide are tolerated on the pyrrol-
idine and piperidine ring (entries 8h and 8n–8q). The position
of the substituent on the ring does not appear to be critical to
the binding affinity in the case that was examined (8p
vs 8q).
8d
8e
2.0
13
N
H3CN
N
8f
1.2
5.1
N
8g
N
The nature of the substituents on the benzimidazole moiety
does not seem to play a major role in the binding profile. Hetero-
atomic, heteroaroyl, and ether substituents are tolerated at the 2-
position of the benzimidazole ring (Table 2). For example, ana-
logs 14a and 14b, featuring either oxygen or sulfur in the 2-posi-
tion, or analogs 15a and 15b are good ligands for the H3 receptor.
Taken in aggregate, these data are consistent with the idea that
the phenoxyalkyl amine H3 pharmacophore used in this study
is, in general, very tolerant of different substituents on the phe-
nyl ring.
H3C
N
CH3
8h
4.8
N
N
S
8i
8j
20
5.3
N
In addition to H3 binding activity, hERG activity was mea-
sured using a high-throughput rubidium efflux assay.15 Repre-
sentative data are given in Table 3. In general, hERG activity
as measured by this assay is quite high regardless of the nat-
ure of the amine present on the terminal position. This obser-
vation is consistent with other similar series of H3 ligands
incorporating this pharmacophore that also displays hERG
activity.2b Interestingly, introduction of polar substituents, an
approach that has frequently demonstrated a positive impact
on hERG activity, could improve the hERG profile of some
analogs in this series as well (Table 3, compounds
8h and 8n).
H3C
N
8k
8l
3.6
N
N
107
8m
713
N
Because of its excellent activity at the human H3 receptor, 8a
was further profiled. Compound 8a had a Ki of 1.8 nM when
tested against the mouse H3 receptor, and its in vitro functional
activity as measured in a human cAMP assay was 0.1 nM.16 It
was a full antagonist. It did not inhibit CYP3A4 or 2D6 when
8n
8o
1.8
16
HO
N
O
O
N
tested at concentrations up to 30
sessed reasonable oral pharmacokinetic profile in
throughput rat pharmacokinetic assay: AUC = 2027 h ng/mL;
max = 393 ng/mL (10 mg/kg in methyl cellulose, n = 2, 0–6 h time
points).18
In conclusion, a new series of H3 ligand based on a well-known
l
M.17 Furthermore, 8a pos-
high-
H3C
a
a
O
8p
8q
20
N
C
H2N
N
H2N
O
11
H3 pharmacophore, the phenoxyalkyl amine, have been identified.
Many of these analogs possess excellent H3 binding affinity, but
like many analogs that incorporate this motif, they also have the
potential to interact with the hERG channel. This series also helps
to demonstrate the promiscuous nature of the receptor toward this
pharmacophore.
H3C
8r
101
NH
a
H3 binding Ki values are the average of at least two independent
determinations.

R. Aslanian et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5032–5036
5035
OH
F
NO2
NH2
H
H
NO2
N
N
a
b
+
F
OH
F
OH
NH2
F
1
2
3
4
N
N
N
d
e
c
N
OH
O
NH
H
N
F
OH
F
5
6
N
N
N
NR2
Cl
f
N
N
O
N
O
F
F
7
8
Scheme 1. Reagents and conditions: (a) Dioxane, reflux, 64%; (b) Ra-Ni, H2, MeOH, 83%; (c) picolinic acid, DCC, HOBT, CH2Cl2, 48%; (d) Acetic acid, 120 °C, 100%; (e) 1-bromo-
3-chloropropane, K2CO3, acetone, reflux, 93%; (f) R2NH, CH3CN, i-Pr2NEt, 80 °C, 12–98%.
N
OH
O
a, b
c, d
Br
Cl
+
O2N
11
9
10
O2N
N
N
O
O
X
HN
e, f
HN
O2N
14a X = O
14b X = S
N
12
F
F
N
O
g
X
R
15a X = SCH2CH3
15b X = OCH2CH3
N
N
F
Scheme 2. Reagents and conditions: (a) Acetone, K2CO3, R.T., 97%; (b) Piperidine, n-butanol, NaI, Na2CO3, 100 °C, 100%; (c) Ra-Ni, H2, MeOH, 85%; (d) 2, dioxane, reflux, 68%;
(e) Ra-Ni, H2, MeOH, 94%; (f) Carbonyl diimidazole or thiocarbonyl diimidazole, 70 °C, 100% for 14a, 98% for 14b; (g) CH3CH2I, DMF, K2CO3, 60% for 15a, 65% for 15b.

5036
R. Aslanian et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5032–5036
13. The syntheses of amines 8 are given as an example: to a 1,4-dioxane (30 mL)
Table 3
Rubidium efflux data for selected analogsa
solution of 1 (3.0 g, 27.50 mmol) at 25 °C was added 2 (4.4 g, 27.50 mmol). The
mixture was refluxed under N2 for 48 h. After cooling to room temperature, the
reaction was concentrated in vacuo and purified by 40 M Biotage cartridge to
give 3 (64%). To a MeOH (50 mL) solution of 3 (4.4 g, 17.73 mmol) was added
Ra-Ni (2.0 g) and the mixture was hydrogenated at 50 psi H2 for 20 h. The
reaction mixture was filtered through Celite, concentrated in vacuo, and
purified by 40 M Biotage cartridge to give 4 (83%, MH+ = 219). To a CH2Cl2
(50 mL) solution of 4 (3.2 g, 14.66 mmol) and picolinic acid (1.7 g, 14.66 mmol)
were added DCC (3.9 g, 20.34 mmol) and HOBT (2.7 g, 20.34 mmol) at 25 °C.
After stirring under N2 for 20 h, water was added, the reaction was extracted
with CH2Cl2 (2Â), combined, washed with brine, dried over Na2SO4, filtered,
concentrated in vacuo, and purified by 40 M Biotage cartridge to give 5 (48%
MH+ = 324). Compound 5 (2.1 g, 6.50 mmol) in 15 mL of acetic acid was heated
at 120 °C under N2 for 20 h. After cooling to room temperature, the reaction
was concentrated in vacuo to give 6 (100%, MH+ = 306). To an acetone (20 mL)
solution of 6 (1.9 g, 6.22 mmol) was added K2CO3 (4.5 g, 32.48 mmol) at 25 °C.
After stirring under N2 for 40 min, 1-bromo-3-chloropropane (1.3 mL,
12.99 mmol) was added, and the mixture was refluxed for 20 h. After cooling
to room temperature, the reaction mixture was filtered, concentrated in vacuo,
and purified by 40 M Biotage cartridge to give 7 (93%, MH+ = 382). To 1-mL
glass tubes were added compound 7 (0.01 g, 0.026 mmol), MeCN (0.5 mL), and
diisopropylethylamine (0.104 mmol). One molar stock solutions of each of the
individual amines (0.053 mL, 0.053 mmol) were added to the tubes, which
were then sealed and heated at 80 °C for 3 days. After cooling to room
temperature, the solutions were transferred into the wells of a deep well
polypropylene microtiter plate containing polystyrene isocyanate resin
(2.5 equiv, 0.066 mmol) and MP-carbonate resin (4 equiv, 0.106 mmol). The
microtiter plate was then sealed and shaken at 25 °C for 16 h. The solutions
were filtered through a polypropylene frit into a collection plate. The wells of
the top plate were then washed with MeCN (0.5 mL), and the plate was
removed. After an aliquot of each solution was removed for LC/MS analysis, the
remaining solutions in the collection plate were transferred into vials and the
solvents removed in vacuo via a SpeedVac to provide amines 8. Yield: 12–98%;
purity >80% by LC/MS. Compounds 8b and 8n–q were obtained as mixtures of
isomers.
Compound
Rb efflux (%)
8a
8c
8h
8i
8l
8n
8o
70
62
26
42
94
16
49
a
Compounds were tested at a concentration of 5 lg/mL.
Acknowledgments
We thank the Drug Metabolism and Pharmacokinetics group of
the Schering-Plough Research Institute, for providing the in vitro
and in vivo pharmacokinetic data, and Steve Sorota for providing
the hERG data.
References and notes
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14. For binding assays, membranes (P2 pellet) from rHu H3–HEK cells (3
protein) were incubated in 200
l of 50 mM Tris–HCl, pH 7.4, with 1 nM [3H]N-
-methylhistamine (82 Ci/mmol) and compounds at concentrations equivalent
lg
l
a
to half orders of magnitude over a five-order of magnitude range. Nonspecific
binding was determined in the presence of 10À5 M thioperamide. After 30 min
at 30 °C, assay mixtures were filtered through 0.3% polyethylenimine-soaked
GF/B glass fiber filters, which were rinsed thrice with buffer, dried,
impregnated with Meltilex wax scintillant, and counted. Ki values were
determined from curves fit to the data using GraphPad Prism nonlinear, least-
squares, curve-fitting program.
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16. HEK293 cells expressing recombinant human histamine H3-receptor were
stimulated 30 min with 3
lM forskolin and compounds were characterized for
their ability to reverse the inhibition of cAMP formation caused by 10À5 M N-
a-
methylhistamine over this time. cAMP assays were performed with an
AlphaScreen cAMP assay kit.
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