Novel human histamine H(3) receptor antagonists.

Article abstract of DOI:10.1016/S0960-894X(02)00738-2

High throughput screening, using the recombinant human H(3) receptor, was used to identify novel histamine H(3) receptor antagonists. Evaluation of the lead compounds ultimately afforded potent, selective, orally bioavailable compounds (e.g., 38) with fav

Full text of DOI:10.1016/S0960-894X(02)00738-2

Bioorganic & Medicinal Chemistry Letters 12 (2002) 3309–3312
Novel Human Histamine H3 Receptor Antagonists
Chandra Shah, Laura McAtee, J. Guy Breitenbucher, Dale Rudolph, Xiaobing Li,
Timothy W. Lovenberg, Curt Mazur, Sandy J. Wilson and Nicholas I. Carruthers*
Johnson and Johnson Pharmaceutical Research and Development, L.L.C., 3210 Merryfield Row, San Diego, CA 92121, USA
Received 1 July 2002; accepted 15 August 2002
Abstract—High throughput screening, using the recombinant human H3 receptor, was used to identify novel histamine H3 receptor
antagonists. Evaluation of the lead compounds ultimately afforded potent, selective, orally bioavailable compounds (e.g., 38) with
favorable blood–brain barrier penetration.
# 2002 Elsevier Science Ltd. All rights reserved.
The monoamine histamine (1) exerts a physiological
effect via four distinct G-protein coupled receptors.
Thus histamine plays a role, via the H1 receptor, in
immediate hypersensitivity reactions upon IgE mediated
release from mast cells.1,2 Histamine is also an impor-
tant regulator of gastric acid secretion through its action
upon H2 receptors expressed in parietal cells.3 The third
histamine receptor (H3) identified in 1983, was first
described as a pre- and postsynaptic autoreceptor in the
brain4 and subsequently as a presynaptic heteroreceptor
on non-histamine containing neurons in both the cen-
tral and peripheral nervous system.5 Recently, a fourth
histamine receptor (H4) was described with an expres-
sion profile that suggests a role in immune function.6ꢀ10
Our interests relate to ligands for the histamine H3
receptor, in particular histamine H3 receptor antago-
nists, and follows from the successful cloning of the
human H3 receptor.11 Histamine H3 ligands are postu-
lated to have a range of therapeutic applications and
both H3 agonists (e.g., R-a-methylhistamine 2) and
antagonists (e.g., thioperamide 3) have undergone
pharmacological evaluation.12
natural ligand. This results in compounds with poor
blood–brain barrier (BBB) penetration13 and metabolic
liabilities associated with the imidazole ring.14 Although
compounds such as clozapine15 4 and the marine nat-
ural product aplysamine16 5 were reported to be H3
antagonists, earlier efforts to identify imidazole replace-
ments were disappointing.17
Only very recently have several groups described series
of potent non-imidazole based histamine H3 antago-
nists. Thus these groups have used low affinity H3
ligands sabeluzole18 6, dimaprit19 7 and Na-(4-phenyl-
butyl)histamine20 8 to obtain antagonists 9, 10, and 11,
respectively.
Until recently, a common feature of these molecules is
their retention of the imidazole nucleus present in the
*Corresponding author. Fax: +1-858-450-2049; e-mail: ncarruth@
prdus.jnj.com
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00738-2

3310
C. Shah et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3309–3312
Recognizing the deficiencies of the existing imidazole
based ligands and the need to identify additional non-
imidazole small molecule lead compounds, we under-
took a high throughput screening (HTS) approach using
the recombinant human receptor. Thus, HTS was initi-
ated with the objective of finding small molecule leads
that could ultimately afford compounds that were orally
bioavailable and had good BBB penetration. A second-
ary objective was to evaluate a H3 antagonist in a range
of central nervous system (CNS) disorders.
so a limited number of substitutions in the 2-phenyl-
imidazopyridine nucleus were examined whilst main-
taining the piperidinylpropyl side chain. This indicated
that small alkyl groups were tolerated in the imidazo-
pyridine nucleus (Table 3). Simultaneously we observed
that the cyclic amine could be piperidine, pyrrolidine or
cycloheptylamine (Table 4).
At this juncture, we selected the 7-methylimidazo-
pyridine analogue 38 for more detailed evaluation. The
7-methyl substituent was retained since it was known to
reduce both calcium channel affinity and local anes-
thetic activity in the original series of compounds.21
Syntheses of the compounds were accomplished
Results
To our satisfaction, HTS afforded several lead series
with good affinity for the human H3 receptor including
Table 2. Amino group substitutions
a
series of 2-phenyl-imidazo[1,2-a]pyridines. These
compounds were first prepared as calcium channel
blockers and subsequently found to exhibit local anes-
thetic properties independent of their calcium blocking
activity (e.g., RWJ-20085, 12).21,22 Thus, 12 was found
to be a weak histamine H3 receptor ligand (Ki=4 mM).
No.
NR2
Ki (nM)
No.
NR2
Ki (nM)
N-(nC5H11 2
)
34
7900
1000
28
12
30
31
N-(nC4H9)2
N-(nC3H7)2
N-(CH3)2
2700
43
13
35
36
80
3
Screening analogues of 12 retaining the di-n-butylamino
terminus, which had been essential for local anesthetic
activity, indicated that a range of substituents were tol-
erated in the 2-phenyl-imidazo[1,2-a]pyridine fragment
with minimal effect on potency (Table 1).
32
33
N-(CH(CH3)Ph)2
NHCH2Ph
4000
5400
Table 3. Imidazopyridine and aryl ring substituents, piperidino ter-
minus
However, when we turned our attention to analogues in
which the size of the terminal amino group was reduced
a dramatic increase in potency was observed. In parti-
cular, analogues containing cyclic amines provided sig-
nificantly more potent compounds (Table 2).
Thus these early screening results quickly demonstrated
the favorable effect of a piperidinylpropyl side chain and
No.
R
Ki (nM)
No.
R
Ki (nM)
37
36
38
39
H
6
3
2
28
40
41
42
20,7-di-CH3
20F, 7-CH3
30-OCH3, 7-CH3
2
10
69
8-CH3
7-CH3
30-CH3
Table 1. Imidazopyridine and aryl ring substituents, di-n-butylamino
terminus
Table 4. Alkylamine ring size
No.
R
Ki (nM) No.
R
Ki (nM)
12
13
14
15
16
17
18
19
8-CH3
H
7-CH3
6-CH3
5-CH3
3,8-di-CH3
5,7-di-CH3
8-NO2
2700
1300
1100
1900
2100
870
20
21
22
23
24
25
26
27
6,8-di-Br
8-PhCH2O
6-Br
4800
2400
2900
1500
4200
8000
1700
3300
No.
–NR2
Ki (nM)
8-OH
20,8-di-CH3
30,8-di-CH3
30-OCH3, 8-CH3
30,50-di-OCH3, 8-CH3
2
38
2000
11,000
43
44
5
6
Ki values are the mean of two to six determinations and were deter-
mined in house and calculated according to Cheng and Prusoff 23
where Ki=IC50/(1+[S]/Kd) where [S]=0.8 nM and Kd=0.8 nM for
[3H]-N-methylhistamine.

C. Shah et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3309–3312
3311
Scheme 4. Reagents and conditions: (a) 5% Pd–BaSO4, H2, EtOH, 4.5
h, 100%; (b) MsCl, NEt3, CH2Cl2, 18 h, 100%; (d) piperidine, K2CO3,
CH3CN, reflux, 18 h, 48%.
Scheme 1. Reagents and conditions: (a) K2CO3, 1-bromo-3-chloro-
propane, acetone, reflux, 18 h, 98%; (b) Br2, Et2O, 18 h, 98%; (c) 2-amino-
4-picoline, EtOH, 73 ꢁC, 2 h, 63%; (d) piperidine, 100 ꢁC, 1.5 h, 90%;
(e) MeOH, 2M HCl/Et2O, 100%.
Scheme 5. Reagents and conditions: (a) 1-but-3-enylpiperidine,
Pd(OAc)2, PPh3, NEt3, DMF, 150 ꢁC, 20 h, 14%.
Scheme 2. Reagents and conditions: (a) 3-piperidinyl propan-1-ol,
DEAD, polymer bound PPh3, THF, 20 h, 25%.
Scheme 6. Reagents and conditions: (a) 2-amino-3-picoline, EtOH,
reflux, 1.5 h, 22%; (b) cyclohexadiene, Pd/C, EtOH, reflux, 2 h, 50%;
i
(c) 1-piperidinepropionic acid, EDAC, HOBT, Pr2NEt, DMF, 18 h,
24%; (d) 2 M BH3/DMS, PhCH3, reflux, 18 h, 51%.
Scheme 3. Reagents and conditions: (a) 2-amino-4-picoline, EtOH,
reflux, 18 h, 58%; (b) 3-butyn-1ol, CuI, NEt3, Pd(PPh3)4, CH3CN,
reflux, 18 h, 56%; (c) MsCl, pyridine, CH2Cl2, 80%; (d) piperidine,
K2CO3, CH3CN, reflux, 18 h, 97%.
according to the procedure outlined in Scheme 1 for the
preparation of 38. Thus an appropriately substituted
4-hydroxyacetophenone (45) was alkylated with 1-bromo-
3-chloropropane and brominated to give 46. Conden-
sation of 46 with a 2-aminopicoline gave 47, which was
treated with an amine to afford 38.
Scheme 7. Reagents and conditions: (a) 3-chloropropanesulfonyl
chloride, NEt3, DMF, 1.5 h, 66%; (b) piperidine, 100 ꢁC, 18 h, 88%.
Table 5. Linker variations
The aminoalkyl side chain could also be introduced via
a Mitsunobu reaction as shown for the preparation of
39 (Scheme 2).
No.
R=7-CH3
X–Y
Ki
No.
(nM) R=8-CH3
X–Y
Ki
(nM)
In parallel to the detailed biological evaluation of 38
alternatives to the ether linkage present in 38 were
explored (Table 5). The syntheses of these analogues
were accomplished as shown in Schemes 3–7. Thus the
carbon linked analogues 48–50 were prepared from the
2-(4-bromophenyl)-imidazo[1,2-a]pyridine 51 via either
a Sonogashira coupling (Schemes 3 and 4) or a Heck
coupling reaction (Scheme 5). The nitrogen linked ana-
logues, 52–54 were prepared from the 2-(4-nitrophenyl)
imidazo[1,2-a]pyridine 55 (Scheme 6). The intermediates
51 and 55 were in turn prepared via condensation of an
appropriately substituted acetophenone with a 2-amino-
picoline.
38
48
49
50
OCH2
CꢂC
2
7
17
22
37
52
53
54
OCH2
NH(CO)
NHCH2
6
300
15
CH2CH2
CH¼CH
trans/cis 9:1
NHSO2CH2
500
Although these studies were not exhaustive, they
demonstrated that the ether linkage could be replaced
with a carbon or an amine linkage without significant
loss of affinity. However the amide and sulfonamide
analogues 52 and 54 were considerably less active.

3312
C. Shah et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3309–3312
Biological Results and Discussion
References and Notes
Following the successful identification of small molecule
histamine H3 ligands a more detailed biological evalua-
tion was undertaken. Thus in vitro 38 demonstrated
high affinity for the human H3 receptor (Ki=2 nM) with
slightly reduced affinity for the recombinant rat H3
receptor (Ki=10–20 nM). In addition 38 exhibited
approximately 1000-fold selectivity over the H1, H2, and
H4 receptors. Broad screening against a panel of over 50
receptor targets representing the major classes of bio-
genic amine and neuropeptide receptors, ion channels
and neurotransmitter transporters indicated affinities
greater than Ki=1 mM. Functional activity versus the
human H3 receptor was determined using SKNMC cells
stably transfected with the human H3 receptor. These
compounds, exemplified by 38, produced a rightward
shift in the histamine dose–response curve yielding a
pA2=8.69. The corresponding rat pA2 was weaker at
8.33. In vivo blood–brain barrier penetration (BBB) was
measured following peripheral administration. Thus, 38
was administered (10 mg/kg ip) to rats which afforded a
brain Cmax=10.8ꢃ0.8 mM on a weight/volume basis.
Brain receptor occupancy was determined via ex-vivo
autoradiography and 38 exhibited an ED50=0.2 mg/kg
sc using [3H]-R-a-methylhistamine to define sites not
blocked by the test compound. In the same protocol
thioperamide (3) had an ED50=2 mg/kg. Single dose rat
pharmacokinetics confirmed that 38 had good oral
bioavailability (57%) with a moderate half life (t1/2
=5.2ꢃ1.2 h). Thus, with the objective of identifying
small molecule H3 antagonists having good oral bio-
availability and favorable BBB penetration achieved,
pharmacological evaluation was initiated in a range of
behavioral and cognitive models, details of which will
be reported elsewhere.
1. Dale, H. H.; Laidlaw, P. P. J. Physiol. 1910, 41, 318.
2. Dale, H. H.; Laidlaw, P. P. J. Physiol. 1911, 43, 182.
3. Black, J. W.; Duncan, W. A. M.; Durant, G. J.; Ganellin,
C. R.; Parsons, M. E. Nature 1972, 236, 385.
4. Arrang, J.-M.; Garbarg, M.; Schwartz, J.-C. Nature 1983,
302, 832.
5. Hill, S. J.; Ganellin, C. R.; Timmerman, H.; Schwartz,
J.-C.; Shankley, N. P.; Young, J. M.; Schunack, W.; Levi, R.;
Haas, H. L. Pharmacol. Rev. 1997, 49, 253.
6. Oda, T.; Morikawa, N.; Saito, Y.; Masuho, Y.; Matsu-
moto, S.-I. J. Biol. Chem. 2000, 275, 36781.
7. Liu, C.; Ma, X.-J.; Jiang, X.; Wilson, S. J.; Hofstra, C. L.;
Blevitt, J.; Pyati, J.; Li, X.; Chai, W.; Carruthers, N.; Loven-
berg, T. W. Mol. Pharmacol. 2001, 59, 420.
8. Nguyen, T.; Shapiro, D. A.; George, S. R.; Setola, V.; Lee,
D. K.; Cheng, R.; Rauser, L.; Lee, S. P.; Lynch, K. R.; Roth,
B. L.; O’Dowd, B. F. Mol. Pharmacol. 2001, 59, 427.
9. Zhu, Y.; Michalovich, D.; Wu, H.-L.; Tan, K. B.; Dytko,
G. M.; Mannan, I. J.; Boyce, R.; Alston, J.; Tierney, L. A.; Li,
X.; Herrity, N. C.; Vawter, L.; Sarau, H. M.; Ames, R. S.;
Davenport, C. M.; Hieble, J. P.; Wilson, S.; Bergsma, D. J.;
Fitzgerald, L. R. Mol. Pharmacol. 2001, 59, 434.
10. Morse, K. L.; Behan, J.; Laz, T. M.; West, R. E., Jr.;
Greenfeder, S. A.; Anthes, J. C.; Umland, S.; Wan, Y.; Hip-
kin, R. W.; Gonsiorek, W.; Shin, N.; Gustafson, E. L.; Qiao,
X.; Wang, S.; Hedrick, J. A.; Greene, J.; Bayne, M.; Monsma,
F. J., Jr. J. Pharmacol. Exp. Ther. 2001, 296, 1058.
11. Lovenberg, T. W.; Roland, B. L.; Wilson, S. J.; Jiang, X.;
Pyati, J.; Huvar, A.; Jackson, M. R.; Erlander, M. G. Mol.
Pharmacol. 1999, 55, 1101.
12. The Histamine H3 Receptor; Leurs, R., Timmerman, H.,
Eds. Elsevier: Amsterdam, 1998.
13. Garbarg, M.; Tuong, M. D. T.; Gros, C.; Schwartz, J.-C.
Eur. J. Pharmacol. 1989, 154, 1.
14. Alves-Rodrigues, A.; Leurs, R.; Wu, T.-S.; Prell, G. D.;
Foged, C.; Timmerman, H. Br. J. Pharmacol. 1996, 118, 2045.
15. Kathman, M.; Schlicker, E.; Gothert, M. Psychopharma-
cology 1994, 116, 464.
16. Pompni, S. A., Gullo, V. P., Horan, A. C., Patel, M. G.,
Coval, S. US Patent 5,352,707, 1994, Chem. Abstr. 1994, 121,
272182.
17. Leurs, R.; Vollinga, R. C.; Timmerman, H. Prog. Drug
Res. 1995, 45, 107.
18. Walczynski, K.; Guryn, R.; Zuiderweld, O. P.; Timmer-
man, H. Il Farmaco 1999, 54, 684.
19. Linney, I. D.; Buck, I. M.; Harper, E. A.; Kalindjian,
B. S.; Pether, M. J.; Shankley, N. P.; Watt, G. F.; Wright, P. T.
J. Med. Chem. 2000, 43, 2362.
In summary, proceeding from the cloning of the human
H3 receptor and initiation of a HTS screen a series of
novel histamine H3 ligands were identified. The SARs
for the leads were promptly delineated to obtain potent,
orally bioavailable, brain penetrating compounds that
will aid in the elucidation of the role of central hista-
mine H3 receptors and determine the therapeutic
potential of H3 receptor antagonists.
20. Ganellin, C. R.; Leurquin, F.; Piripitsi, A.; Arrang, J.-M.;
Garbarg, M.; Ligneau, X.; Schunack, W.; Schwartz, J.-C.
Arch. Pharm. Med. Chem. 1998, 331, 395.
Acknowledgements
21. Sanfilippo, P. J.; Urbanski, M.; Press, J. B.; Dubinsky, B.;
Moore, J. B., Jr. J. Med. Chem. 1988, 31, 2221.
22. Dubinsky, B.; Shriver, D. A.; Sanfilippo, P. J.; Press, J. B.;
Tobia, A. J.; Rosanthale, M. E. Drug Dev. Res. 1990, 21, 277.
23. Cheng, Y.-C.; Prusoff, W. H. Biochem. Pharmacol. 1973,
22, 3099.
The authors are grateful to Ms. Paku Desai for the
determination of H4 receptor affinity and to Dr. Xavier
Langlois, Johnson and Johnson Pharmaceutical
Research and Development, Beerse, Belgium, for per-
forming the brain receptor occupancy studies described
in this manuscript.