Synthesis and structure-activity relationships of 2-(1,4′-bipiperidin-1′-yl)thiazolopyridine as H3 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2009.09.006

A series of 2-(1,4′-bipiperidine-1′-yl)thiazolopyridines was synthesized and evaluated as a new lead of non-imidazole histamine H₃ receptor antagonists. Introduction of diversity at the 6-position of the pyridine ring was designed to enhance in vitro potency and decrease hERG activity. The structure-activity relationships for these new thiazolopyridine antagonists are discussed.

Full text of DOI:10.1016/j.bmcl.2009.09.006

Bioorganic & Medicinal Chemistry Letters 19 (2009) 6176–6180
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and structure–activity relationships of 2-(1,40-bipiperidin-10-yl)
thiazolopyridine as H3 receptor antagonists
*
Ashwin U. Rao , Anandan Palani, Xiao Chen, Ying Huang, Robert G. Aslanian, Robert E. West Jr.,
Shirley M. Williams, Ren-Long Wu, Joyce Hwa, Christopher Sondey, Jean Lachowicz
Schering-Plough Research Institute, 2015 Galloping Hill Road, K-15-1-1800, Kenilworth, NJ 07033, USA
a r t i c l e i n f o
a b s t r a c t
A series of 2-(1,40-bipiperidine-10-yl)thiazolopyridines was synthesized and evaluated as a new lead of
non-imidazole histamine H3 receptor antagonists. Introduction of diversity at the 6-position of the pyr-
idine ring was designed to enhance in vitro potency and decrease hERG activity. The structure–activity
relationships for these new thiazolopyridine antagonists are discussed.
Article history:
Received 29 July 2009
Revised 31 August 2009
Accepted 2 September 2009
Available online 6 September 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Histamine
H3
2-(1,40-Bipiperidin-10-yl)thiazolopyridine
Histamine plays a variety of physiological roles in the central
nervous system (CNS) and peripheral tissues through the four
known G protein-coupled receptors, H1, H2, H3, and H4.1 The his-
tamine H3 receptor located on the presynaptic nerve terminals
inhibits release of the neurotransmitters histamine, dopamine,
norepinephrine, acetylcholine, glutamate, and serotonin.2 Hista-
mine H3 receptor antagonist-enhanced neurotransmitter release
offers a promising approach to the treatment of several CNS dis-
orders,3,4 including attention deficit hyperactivity disorder,5 sleep
disorders,6 epilepsy,7 and schizophrenia.8 Additionally, given the
role of histamine in regulating appetite, H3 receptor ligands are
also reported to be active in various preclinical models of
obesity.9
inhibition.15 Most of the reported non-imidazole H3 antagonists
possess an aromatic ring-linker-basic amine motif. Notable exam-
ples include ABT-239,16 GSK-189254,17 UCL-2190,18 A-331440,19
and JNJ-5207852.20
´
Walczynski and co-workers identified a structurally new non-
imidazole histamine H3 receptor antagonist I (Fig. 1). Compound
I showed pA2 = 7.25 0.07 (electric field stimulation assay on
guinea-pig jejunum).21 Relative to compound I, we envisioned
that substitution on the pyridine motif might enhance the H3
activity with a belief that higher lipophilicity would increase
drug distribution to the brain, potentially resulting in improved
access to the H3 receptors in the CNS. In this work, we report
on synthesis and SAR of 2-(1,40-bipiperidine-10-yl)thiazolopyri-
dines II as non-imidazole histamine H3 receptor antagonists hav-
ing substitution (R) at the C-6 position of the pyridine ring
bearing the 1,40-bipiperidine side chain. The results are described
herein.
The H3 receptor was first pharmacologically identified in 19832a
by Arrang et al. and later cloned in 1999 by Lovenberg et al.10 This
breakthrough sparked a significant synthetic interest in identifying
structurally novel H3 receptor antagonists in academia and indus-
try alike.11,12 Imidazole based H3 antagonists were among the ear-
liest structures investigated.13 Two drawbacks to this class of
compounds are the potential issue for drug–drug interactions
through inhibition of hepatic cytochrome P450 enzymes and also
relatively poor CNS penetration.14
The synthesis of key intermediate 2-(1,40-bipiperidine-10-yl)-
6-bromothiazolo[4,5-b]pyridine 5 began with commercially avail-
able 2-amino-3-chloropyridine 1, which was cyclized to the
thiazolo[4,5-b]pyridine-2(3H)-thione
2 using potassium ethyl
xanthate in refluxing NMP (Scheme 1). Subsequent treatment of 2
with excess sulfuryl chloride provided 2-chlorothiazolo[4,5-b]pyri-
dine 3.22 2-(1,40-Bipiperidine-10-yl)thiazolo[4,5-b]pyridine 4 was
prepared through nucleophilic displacement of the chlorine by
1,40-bipiperidine in DMF in the presence of K2CO3. Bromination of
4 provided 5 in 50% yield.23 The analogous 2-(1,40-bipiperidine-10-
yl)-6-chlorothiazolo[4,5-c]pyridine 7 was similarly obtained from
the readily available 5-amino-2,4-dichloropyridine 6.
More recently, attention in the field has turned to non-imidaz-
ole class of H3 antagonists as these compounds offer improvements
in binding affinity, CNS penetration, and reduced potential for CYP
* Corresponding author. Tel.: +1 908 740 5527; fax: +1 908 740 7664.
E-mail address: ashwin.rao@spcorp.com (A.U. Rao).
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.09.006

A. U. Rao et al. / Bioorg. Med. Chem. Lett. 19 (2009) 6176–6180
6177
R
Z
X
S
N
S
N
N
N
N
N
N
Y
one of X, Y or Z is N and the
other two of X, Y or Z are CH
II
I
Figure 1. Structure of 1-(2-thiazolo[4,5-c]pyridine)-4-n-propylpiperazine and the target molecules of study.
HN
N
Cl
C2H5O(C=S)SK
S
N
S
SO2Cl2
54%
S
N
N
N
S
Cl
N
H
N
NH2
NMP
65%
K2CO3, DMF
45%
N
N
N
1
2
3
4
Br
Cl
Cl
Cl
S
N
Br2, NaOAc
S
N
N
N
N
N
N
N
N
NH2
AcOH, H2O
50%
6
7
5
Scheme 1. Synthesis of 2-(1,40-bipiperidine-10-yl)-6-halothiazolo[4,5-b]pyridine.
Isomeric 2-(1,40-bipiperidine-10-yl)-5-chlorothiazolo[5,4-b]pyr-
idine 11 (Scheme 2) was obtained by treatment of 3-amino-6-
chloropyridine 8 with potassium thiocyanate and bromine to give
as aryl and heteroaryl groups by means of boronic acid or
Buchwald–Hartwig couplings in moderate to excellent yields
(Scheme 3).26,27
5-chlorothiazolo[5,4-b]pyridine-2-amine derivative
9
that was
Table 1 summarizes the human H3 binding SAR of sterically less
demanding substitution at the C-6 position of the 4- and 5-pyridyl
analogs. In general, electron donating substituents (NH2) as in 12b,
and 13b showed less H3 receptor binding affinity when compared
to electron withdrawing substituents (CN, Cl, and NHCOMe) in 12c,
13c, 7, and 12e. The best affinity was shown by 7 (Ki = 10 nM). Ow-
ing to concern over potential reactivity of chloride in 7, it was eval-
uated for glutathione adducts by incubating in human and rat liver
microsomes. However, no glutathione adducts were observed at
the C-6 position.
then converted to the 2-bromo derivative 10 with CuBr2 and
t-butylnitrite.24 Microwave assisted nucleophilic displacement of
the bromine by 1,40-bipiperidine in PhCF3-1,4-dioxane mixture
(1:4, v/v) afforded 2-(1,40-bipiperidine-10-yl)-5-chlorothiazol-
o[5,4-b]pyridine 11.
The binding affinities of the isomeric halogenated thiazolopyri-
dines 5, 7, and 11 were determined as Ki values in a human H3
recombinant assay.25 Compound 11 displayed weak H3 receptor
affinity (Ki = 1.7
l
M) when compared to compound
5
(Ki = 141 nM) and compound 7 (Ki = 10 nM) and so was not pur-
sued further. Therefore compounds 5 and 7 provided an excellent
entry into new classes of H3 antagonists with more elaborate
functionalizations of the pyridine core. The halo groups in 5 and
7 were exchanged for sterically less demanding groups as well
Next, we investigated the effect of aryl substitution on the 6-
position to introduce lipophilic groups and the receptor binding
data are summarized in Table 2. The aryl motif showed potency
similar to the smaller substituents (Table 1) with the thiazol-
o[4,5-b]pyridine 12 showing slightly better affinities versus the
HN
N
H2N
Cl
N
CuBr2, t-BuONO Cl
N
Cl
N
S
S
N
S
N
KSCN, Br2
NH2
Br
N
N
N
N
Cl
MeCN
80%
AcOH
48%
DIPEA
70%
8
11
9
10
Scheme 2. Synthesis of 2-(1,40-bipiperidine-10-yl)-5-chlorothiazolo[5,4-b]pyridine.
R
X
Pd catalyzed coupling
S
N
S
N
N
N
N
N
N
N
60 - 90% yields
5
12/13
Scheme 3. Pd catalyzed coupling of 2-(1,40-bipiperidine-10-yl)-6-halothiazolopyridine.

6178
A. U. Rao et al. / Bioorg. Med. Chem. Lett. 19 (2009) 6176–6180
Table 1
Table 3
Binding affinities of 2-(1,40-bipiperidine-10-yl)thiazolopyridines 12 and 13
hERG inhibitory activity and rat exposure data for selected analogs
R
R
S
N
S
N
Compd
hERGa (10
lM) (% inh)
Rat AUC0–6 h (10 mpk, po) (h ng/mL)
N
N
N
N
N
7
12f
12g
59
90
100
56
697
339
N
13
12
a
hERG IonWorks Quattro assay.28
a
a
R
Compd
Human H3 (Ki, nM)
Compd
Human H3 (Ki, nM)
H
4
139
79
36
125
31
13a
13b
13c
7
128
73
35
10
NH2
CN
Cl
12b
12c
12d
12e
NHCOMe
13e
53
exhibited high hERG inhibitory activities. The most potent
derivative 7 displayed moderate hERG inhibitory activity and
poor oral exposure in rat. While aryl substituted analogs 12f
and 12g showed a slightly improved pharmacokinetic (PK) pro-
file, high hERG inhibitory activity precluded further progression
of these compounds.
a
Inhibition of [3H]-N-
a-methylhistamine binding to human brain receptor. H3
binding Ki values are the average of at least two independent determinations. The
assay-to-assay variation was generally 2-fold.25
To develop a successful lead optimization strategy aiming at
overcoming hERG-related safety issues, we focused our modifica-
tion efforts on replacement of the aryl motif with heteroaryl
substituents as summarized in Table 4.29 Both electron with-
drawing (F, CN) and electron donating (OMe) substituents as in
12l, 12n, and 12p, led to a slight increase in H3 potency but also
produced increased hERG inhibitory activities. Polar functional
groups, an approach that has frequently demonstrated a positive
impact on hERG activity, were introduced on the pyridine moi-
ety. To this end compounds that had hydroxy, amino, and car-
bamoyl groups on the pyridine moiety were synthesized.
Compound 13r (Ki = 8 nM), 12s (Ki = 16 nM) and 12t (Ki = 10 nM)
not only showed very good binding affinity but also had the
lowest hERG channel inhibitory activity. In addition to substi-
tuted pyridines, several other heteroaryl motifs were found to
be tolerated albeit with slightly weaker affinity for the histamine
H3 receptor (u, v).
In an effort to further differentiate key compounds with
good binding activity and lower hERG activity, several analogs
were subjected to an ex vivo receptor occupancy study in the
ICR mouse model.30 Four hours following oral administration
of compounds at 10 mg/kg, ex vivo receptor displacement of
control in mouse brain slices was determined. The results of
the correlation between potency (mouse) and H3 receptor occu-
pancy are shown in Table 5. Receptor occupancy was excellent
(>80%) in 7 and 13k and moderate (45%) for 12t. There was al-
most no receptor occupancy for 13r, 12s, and 12u thought to
be a result of poor PK and/or low brain penetration. Although
additional studies are needed, at least a certain level of high
receptor occupancy seems necessary for H3 antagonists to exhi-
bit a significant increase in histamine levels in mouse brain
(Table 5).
Table 2
Binding affinities of aryl substituted 2-(1,40-bipiperidine-10-yl)thiazolopyridine
R
R
S
N
S
N
N
N
N
N
N
N
13
12
a
a
R
Compd
Human H3
Compd
Human H3
(Ki, nM)
(Ki, nM)
NC
12f
28
23
13f
126
46
CN
O
F
12g
13g
13h
13i
13j
Me
12h
12i
12j
33
140
59
80
230
213
OMe
HO
The 4-fluoropyridyl analog 13k possessed the best overall pro-
file in the thiazolopyridine series and was selected for further eval-
uation. Compound 13k showed potent H3 functional activity with
Kb value of 0.1 nM in the human cAMP assay.31 13k showed no sig-
nificant competitive inhibitory activity against CYP450. Further-
more, pharmacokinetic parameters of 13k were evaluated in rats
and monkeys. The results are summarized in Table 6. 13k dis-
played moderate profiles in both species.
a
Inhibition of [3H]-N-
a-methylhistamine binding to human brain receptor. H3
binding Ki values are the average of at least two independent determinations. The
assay-to-assay variation was generally 2-fold.25
thiazolo[4,5-c]pyridine 13. As seen earlier electron withdrawing
substituents on the phenyl ring (12f, 12g, and 12h) seem to be
critical in improved binding affinities over electron donating sub-
stituents (12i, 13i, and 13j). The 3-cyano-4-fluoro phenyl deriva-
tive 12g was the most potent derivative (Ki = 23 nM) among the
aryl substituted derivatives.
The hERG inhibitory activity and rat AUC data of three po-
tent derivatives were subsequently assessed, the results of
which are summarized in Table 3.28 All of these compounds
In conclusion, a new series of thiazolopyridine derivatives was
evaluated as H3 antagonists. The nature of the substituent on the
pyridine ring does not seem to play a major role in the binding pro-
file. However, the heteroaryl substituent seems critical in improv-
ing PK and hERG. Among this series, 2-fluoro pyridine 13k showed
the best overall profile. To advance our lead optimization strategy
we intend to further evaluate SAR in this series to improve potency
and pharmacokinetic profile. These results will be reported in due
course.

A. U. Rao et al. / Bioorg. Med. Chem. Lett. 19 (2009) 6176–6180
6179
Table 4
SAR of heteroaryl substituted 2-(1,40-bipiperidine-10-yl)thiazolopyridine
R
R
S
N
S
N
N
N
N
N
N
N
12
13
a
a
R
Compd
Human H3 (Ki, nM)
hERGb (10
97
l
M) (% inh)
Compd
Human H3 (Ki, nM)
hERGb (10
51
lM) (% inh)
F
N
12k
63
13k
35
44
F
12l
13
42
16
98
79
89
13l
85
88
60
N
N
F
12m
12n
13m
13n
20
22
CN
N
MeO
N
12o
12p
35
4
99
98
13o
13p
93
64
90
91
OMe
N
Me
N
N
12q
133
57
13q
104
55
HO
12r
12s
42
16
3
13r
13s
8
5
5
H2N
N
34
25
H
N
Me
N
12t
12u
12v
10
25
66
20
30
20
13t
13u
13v
27
25
17
54
61
7
O
N
N
H2N
N
N
a
Inhibition of [3H]-N-
a-methylhistamine binding to human brain receptor. H3 binding Ki values are the average of at least two independent determinations. The assay-to-
assay variation was generally 2-fold.25
hERG IW Quattro assay.28
b

6180
A. U. Rao et al. / Bioorg. Med. Chem. Lett. 19 (2009) 6176–6180
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13. Stark, H.; Kathmann, M.; Schlicker, E.; Schunack, W.; Schlegel, B.; Wolfgang, S.
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Ex vivo displacement by H3 antagonists (10 mpk) and brain/plasma ratio in ICR
mouse
Compd
R
Mouse
H3
(Ki, nM)
Ex vivo
displacement
of control
Brain/
plasma
ratio @
a
@10 mpk (%)
10 mpk; 4 h
F
N
14. (a) Lin, J.; Lu, A. Y. H. Clin. Pharmacokinet. 1998, 35, 361–390; (b) Zhang, M.;
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13k
7
89
75
95
23.6
Cl
80b
4.52c
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Rev. Med. Chem. 2004, 979–992.
H
N
Me
N
12t
20
45
3.0
16. (a) Cowart, M. D.; Faghih, R.; Curtis, M. P.; Gfesser, G. A.; Bennani, Y. L.; Black, L.
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M.; Miller, T. R.; Kang, C. H.; Denny, L. I.; Witte, D. G.; Yao, B. B.; Pan, L.; Wetter,
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Pharmacol. Exp. Ther. 2005, 313, 165–175; (c) Fox, G. B.; Esbenshade, T. A.; Pan,
J. B.; Radek, R. J.; Krueger, K. M.; Yao, B. B.; Browman, K. E.; Buckley, M. J.;
Ballard, M. E.; Komater, V. A.; Miner, H.; Zhang, M.; Faghih, R.; Rueter, L. E.;
Bitner, R. S.; Drescher, K. U.; Wetter, J.; Marsh, K.; Lemaire, M.; Porsolt, R. D.;
Bennani, Y.; Sullivan, J. P.; Cowart, M. D.; Decker, M. W.; Hancock, A. A. J.
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O
H2N
HO
N
12s
32
3
0.92
N
17. 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.
P.; Dawson, L. A.; Foley, A. G.; Gartlon, J.; Gonzalez, M. I.; Heslop, T.; Hirst, W.
D.; Jennings, C.; Jones, D. N. C.; Lacroix, L. P.; Martyn, A.; Ociepka, S.; Ray, A.;
Regan, C. M.; Roberts, J. C.; Schogger, J.; Southam, E.; Stean, T. O.; Trail, B. K.;
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13r
20
66
1
0
0.14
1.04
N
N
12u
Inhibition of [3H]-N-
a-methylhistamine binding to mouse brain receptor. H3
binding Ki values are the average of at least two independent determinations. The
assay-to-assay variation was generally 2-fold.
a
b
% Inhibition at 30 mpk.
Dosed at 30 mpk.
c
´
15–23; (b) Walczynski, K.; Guryn, R.; Zuiderveld, O. P.; Timmerman, H. Farmaco
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Pharmacokinetic profile of compound 13k
Species
AUCa (h ng/mL)
Cmax (ng/mL)
Rat
Monkey
1069
624
234
33
24. Okafor, C. O. Tetrahedron 1988, 44, 1187–1194.
a
25. For binding assays, membranes (P2 pellet) from rHu H3-HEK cells (3
lg
AUC0–6 h, po, 10 mpk (rat) and AUC0–24 h, po, 3 mpk (monkey).
protein) were incubated in 200 a-
l
l 50 mM Tris–HCl, pH 7.4, with 1 nM [3H]N-
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 10ꢀ5 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.
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, Steve Sorota for providing the
hERG data, and the reviewers of this letter, Jared Cumming and
Sunil Paliwal for their valuable comments and suggestions. Thanks
are also due to Dr. Malcolm MacCoss for his support and
encouragement.
26. (a) Wagaw, S.; Buchwald, S. L. J. Org. Chem. 1996, 61, 7240–7241;(b) Torraca, K. E.;
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M.; Tucker, K.; Priestley, T. Assay Drug Dev. Technol. 2005, 3, 47–57.
29. Black, L. A.; Liu, H.; Diaz, G. J.; Fox, G. B.; Browman, K. E.; Wetter, J.; Marsh, K.
C.; Miller, T. R.; Esbenshade, T. A.; Brioni, J.; Cowart, M. D. Inflamm. Res. 2008,
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References and notes
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ꢀ80 °C for at least 12 h. Protein concentration was determined by BCA Protein
Assay. A 30 min room temperature incubation was performed, containing
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