Synthesis and structure-activity relationship (SAR) study of 4-azabenzoxazole analogues as H3 antagonists

Article abstract of DOI:10.1016/j.bmcl.2012.01.020

The synthesis and SAR of a novel series of 4-azabenzoxazole histamine H₃ antagonists is described. Introduction of substituted phenyl, pyridyl and fused heterocyclic groups to the 6-position of the 4-azabenzoxazole core gave a series of compounds with good H₃ antagonist activity in both ex vivo and in vivo assays.

Full text of DOI:10.1016/j.bmcl.2012.01.020

Bioorganic & Medicinal Chemistry Letters 22 (2012) 2075–2078
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and structure–activity relationship (SAR) study of 4-azabenzoxazole
analogues as H3 antagonists
Ning Shao a, , Robert Aslanian a, Robert E. West Jr. b, Shirley M. Williams b, Ren-Long Wu b, Joyce Hwa b,
⇑
Christopher Sondey b, Jean Lachowicz b, Anandan Palani a
a Chemical Research Department, Department of Chemical Research, Merck Research Laboratories, 2015 Galloping Hill Road, Kenilworth, NJ 07033, USA
b Cardiovascular/Metabolic Disease, Department of Chemical Research, Merck Research Laboratories, 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:
The synthesis and SAR of a novel series of 4-azabenzoxazole histamine H3 antagonists is described. Intro-
duction of substituted phenyl, pyridyl and fused heterocyclic groups to the 6-position of the 4-azabenz-
oxazole core gave a series of compounds with good H3 antagonist activity in both ex vivo and in vivo
assays.
Received 19 October 2011
Revised 3 January 2012
Accepted 9 January 2012
Available online 24 January 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Histamine
H3 antagonists
SAR
4-Azabenzoxazole
Histamine plays very important roles in physiological pro-
cesses. H1 and H2 receptors have been successfully targeted, and
therapeutic agents have been developed to treat conditions such
as allergies (H1 receptor antagonist) and gastric ulcers (H2 receptor
antagonist). The newly discovered H4 receptor is found in immune
cells, suggesting its role in regulating inflammatory responses.1
The H3 receptor, widely distributed in the central nervous system,
modulates the release of histamine and other neurotransmitters
such as acetylcholine, dopamine, norepinephrine and serotonin.2
Central administration of a histamine H3 antagonist leads to
increased central histamine levels and hence may be useful for
the treatment of a variety of CNS disorders,3 such as attention def-
icit and hyperactivity disorder (ADHD),4 sleep disorders,5 schizo-
phrenia6 or obesity.7 Early generations of H3 antagonists, based
on structures containing the imidazole moiety found in histamine,8
suffered from CYP450 inhibition and drug–drug interactions.9 For
this reason, recent efforts have been directed to the non-imidazole
H3 antagonists, such as ABT-239,10 GSK-189254,11 UCL-2190,12
A-331440 and JNJ-5207852.13,14
series of derivatives of II as potent H3 antagonists has been
published recently.15 Herein we describe our effort in developing
4-azabenzoxazole derivatives III as non-imidazole antagonists.
Synthesis of the key intermediate began with the commercially
available 2-aminopyridin-3-ol 1 (Scheme 1). Condensation of 1
with carbon disulfide gave compound 2. Methylation and substitu-
tion with piperidinopiperidine, which was identified in the
previous study as a privileged scaffold, afforded compound 4. Reg-
ioselective bromination at the 6-position gave compound 5, which
served as the key intermediate for the subsequent analogue synthe-
sis. Suzuki coupling or Buchwald–Hartwig couplings gave a series
of derivatives, and their activity was evaluated.16
The binding affinities of the analogues were determined as Ki
values in the human H3 recombinant assay.17 Compound 4 (human
Ki = 1200 nM) and 4a (human Ki = 4000 nM) showed weak binding
affinity, but putting substitutions groups onto the 6-position of
benzoxazole core increased the binding activity (Table 1). We first
added smaller groups such as vinyl (compound 5), acetamido (com-
pound 6), and small cyclic amino (compound 7) groups, and the
binding affinities improved to as potent as 150 nM (compound 7).
Further extension of the substituent either by fusing a phenyl ring
(compound 8) or attaching a heterocycle (compound 9) reduced
binding affinities. Aromatic rings with different substitution groups
were then directly attached to the core. Polar substituents (com-
pounds 10, 11, 12, 14) resulted in compounds which have binding
affinities at around 300 nM, with an exception (compound 13) that
the ortho acetamido substitution has a fivefold drop in binding
affinity. Presumably the appended phenyl group is twisted out of
´
Walczynski et al. identified a structurally novel non-imidazole
histamine H3 receptor antagonist I (Fig. 1), which has 5-azabenzo-
thiazole moiety as the core structure.14 Compound I showed
pA2 = 7.25 0.07 (electric field stimulation assay on guinea-pig
jejunum). In our own medicinal chemistry effort, we became inter-
ested in structurally similar cores II and III. The development of a
⇑
Corresponding author.
E-mail address: Ning.shao@merck.com (N. Shao).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2012.01.020

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N. Shao et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2075–2078
6
1
3
R3
1
3
6
O
N
1
R3
S
S
N
N
N
N
N
N
N
N
N
N
I
5
3
II
N
III
Figure 1. Structures of target molecules of study.
CS2, KOH
EtOH, reflux
MeI, K2CO3
HN
N
OH
O
N
O
N
DMF
SMe
SH
90%
85%
DMF 100 °C
90%
N
1
NH2
N
N
3
2
Br
O
N
O
Br2, NaOAc/HOAc
82%
N
N
N
N
N
N
N
4
4a
Suzuki coupling or
R
O
N
N
Buchwald-Hartwig couplings
N
40-92%
N
Scheme 1. Synthesis of the 4-azabenzoxazole derivatives.
Table 1
R
O
N
N
N
N
Compound
R
Ki (H3, nM)
Compound
R
Ki (H3, nM)
5
6
370
12
13
AcHN
233
O
NHAc
657
150
1526
288
N
H
O
N
7
14
H2N
F3C
8
260
357
249
379
15
16
17
18
238
58
N
N
N
N
O
9
H
F
HO
HO
10
11
36
NC
NC
37
plane with the azabenzoxazole ring due to the presence of the ortho
acetamido group, thus reducing binding affinity for the H3 receptor.
Smaller substituents on the phenyl ring resulted in compounds
(16–18) with 10-fold increased binding activity.
The active compounds were further tested in the ex vivo occu-
pancy assay in the ICR mouse model.18 Four hours following the
oral administration of compounds at 10 mg/kg, ex vivo receptor
displacement of control in mouse brain slices was determined.
Compound 18 exhibited good receptor occupancy (59%) in mouse
at the 4 h time point. It also has favourable rat pK, with an AUC0–
In order to retain activity and reduce the hERG inhibition (Table
2) we next evaluated replacing the phenyl group with heteroclyes
at the 6-position. This may also be beneficial to the binding activity
since it is well documented that introducing a second amine into
the molecule normally improves the binding activity as potent
H3 antagonist.20 However, one drawback of those dibasic amine
compounds is that it may result in phospholipidolysis, a type of
toxicity that occurs when molecules partition into lipid bilayer
and impair normal phospholipid turn over.21 We envisioned that
heterocycles containing a less basic nitrogen would retain the
similar activity-boosting effect, and in addition, should also have
a better hERG profile due to the its reduced lipophilicity.
of 706 ngÁh/mL (10 mg/kg) and a brain/plasma ratio of 11.3 at
6h
the 6 h time point. This is very important since H3 activity is
centrally mediated. However, compounds 17 and 18 showed sig-
Pyridine was first evaluated as the phenyl replacement with
2-, 3- and 4-pyridyl substituents attached to the 6 position of the
core structure (19, 20, 21). All three compounds retained good
binding affinity (95, 37, 47 nM), with the 3-pyridyl slightly less
active than the 2- and 4-pyridyl substituents. Then we appended
nificant inhibition of the hERG channel (96% and 92% at 10
lM),
a classic problem associated with historic H3 antagonists.19a The
less active compound 13 displayed no hERG inhibition at 10
(À2.0%), probably due to its polar substituent group.19b
lM

N. Shao et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2075–2078
2077
Table 2
R
O
N
N
N
N
Compound
R
Ki (H3, nM)
Compound
R
Ki (H3, nM)
N
N
N
19
20
21
95
26
27
28
43
23
37
O
N
37
47
O
N
Cl
N
N
22
253
29
15
N
NC
H2N
N
23
24
25
278
70
30
31
32
15
N
F
143
79
N
N
F3C
N
196
N
Table 3
R
O
N
N
N
N
R
Ki (H3, nM)
hERG (10
95
lM) (%)
R
Ki (H3, nM)
hERG (10 lM) (%)
N
N
N
N
33
34
117
38
33
5
O
O
H
N
O
N
109
153
N.D.
15
39
40
119
25
23
O
N
O
O
N
35
20
N
N
H
O
36
37
79
35
96
92
41
42
4
62
17
N
N
N
N
18
N
N
N
different substituents onto the pyridines. Substitution of 3-pyri-
dine with cyano and amino groups (22, 23) were detrimental to
activity while the F/H replacement (24) did not offer much benefit
either. For the 4-substituted pyridine, adding methyl or methoxyl
group (compounds 25, 26) gave compounds with inferior/compa-
rable binding activity. Gratifyingly, substitution of 2-pyridyl with
methyl, chloro, or methoxyl groups gave compounds with im-
proved potency (27–30, compared to 21). Changing methyl to ethyl
or a trifluoromethyl group reduced the potency (31, 32 compared
to 29), suggesting there may be a size limit at the ortho position.
Two of the most active compounds 29 and 30 were evaluated in
the hERG binding assay and they demonstrated a better profile
To improve ex vivo activity and further improve hERG profile of
these H3 antagonists, we then turned to fused heterocycles to com-
bine the beneficial effect of pyridine basic nitrogen and the small
non-polar substitution groups (Table 3). Phenyl fused heterocycles
such as compounds 33–35 showed no dramatic improvement in
binding activity, but compounds 36 and 37 showed improved
binding activity probably due to the basic nitrogen at the ortho
position. Pyridine-fused heterocycles (38–41) generally exhibited
better binding activity, such as compound 41 with binding affinity
of 4 nM and compound 42 with binding of 18 nM. The hERG activ-
ity of these compounds was also tested, and they showed better
hERG profile compared to the previous compounds (Table 3). Com-
pound 41 had a moderate hERG inhibition of 62% while 42 showed
only 17% inhibition.
(49% and 32% inhibition at 10 lM). Unfortunately, these com-
pounds showed very low receptor occupancy in the ex vivo mouse
study, with compound 29 showing 17% occupancy and compound
30 18% at 4 h time point, probably due to poor pK and/or low brain
penetration.
Compound 41 was further evaluated in the ex vivo study. Fortu-
nately, it exhibited very good receptor occupancy (65%) at 4 h time
point with a dose of 10 mg/kg. We then studied it in the in vivo

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N. Shao et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2075–2078
STZ-DIO mouse assay.22 After a two-day feed with 30 mg/kg dose,
41 demonstrated both antidiabetic (63 mg/L, glucose level reduc-
tion) and anti-obesity effect (3.85% food inhibition, 0.64% body
weight loss).
In summary, a new series of 4-azabenzoxazole derivatives was
evaluated as H3 antagonists. Appending the phenyl, pyridyl or
fused heterocyclic moieties to the 6-position of the core gave a ser-
ies of H3 antagonists with good in vitro binding activity. The hERG
issues within this series were also addressed by incorporation of
fused pyridine derived heterocycles. Compound 41 was identified
as an active H3 antagonist, with single digit in vitro binding activity
and good ex vivo binding activity. It also demonstrated good
in vivo activity in the rodent biological assay for obesity.
11. Medhurst, A. D.; Atkins, A. R.; Beresford, I. J.; Brackenborough, K.; Briggs, M. A.;
Calver, A. R.; Cilia, J.; Cluderay, J. E.; Crook, B.; Davis, J. B.; Davis, R. K.; Davis, R.
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Acknowledgments
15. Rao, A. U.; Palani, A.; Chen, X.; Huang, Y.; Aslanian, R. G.; West, R. E.; Williams,
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The authors thank Drug Metabolism and Pharmacokinetics
groups for providing the pharmacokinetic data, Steve Sorota for
providing the hERG data, and Dr. Jared Cumming, Dr. Pauline Ting,
and Dr. Xianhai Huang for their comments and suggestions on this
manuscript.
17. For binding assays, membranes (P2 pellet) from rHu H3-HEK cells (3
lg
protein) were incubated in 200 l 50 mM Tris–HCl, pH 7.4, with 1 nM [3H]N-a-
l
methylhistamine (82 Ci/mmol) and compounds at concentrations equivalent
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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140
l
g/assay homogenized cortex sample, 0.1% BSA, 1 nM 3H-N-
a-
methylhistamine and 10
l
M thioperamide for non-specific binding or assay
buffer for total binding. Incubations were performed in quadruplet and
stopped by rapid filtration on a Brandel Harvester using Unifilter-96, GF/B
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D04012801) for 3 weeks, and were given streptozotocin (STZ)
intraperitoneally at 80 mg/kg to induce type
2 diabetes. T2D mice were
chosen for the study two weeks after STZ injection (n = 12 per group, with non-
fasting glucose between 250 and 500 mg/dl) and were balance into groups.
Compounds were administered by PO dosing with 0.4% MC daily before the
dark onset. Non-fasting glucose was monitored daily at 4 h after the lights on.
Individual body weight and food intake were monitored daily.