Structure-activity relationships of non-imidazole H3 receptor ligands. Part 3: 5-Substituted 3-phenyl-1,2,4-oxadiazoles as potent antagonists

Article abstract of DOI:10.1016/j.bmcl.2003.11.038

Further SAR studies on novel histamine H₃ receptor antagonists are presented. Compound 14bb is a potent antagonist of both the rat cortical and human clone receptors, and is demonstrated to act functionally as an antagonist in an in vivo mouse dipsogenia model.

Full text of DOI:10.1016/j.bmcl.2003.11.038

Bioorganic & Medicinal Chemistry Letters 14 (2004) 673–676
Structure–activity relationships of non-imidazole H3 receptor
ligands. Part 3: 5-Substituted 3-phenyl-1,2,4-oxadiazoles as
potent antagonists
Gregory A. Gfesser,* Henry Zhang, Jurgen Dinges, Gerard B. Fox, Jia Bao Pan,
Timothy A. Esbenshade, Betty B. Yao, David Witte, Thomas R. Miller, Chae-Hee Kang,
Kathy M. Krueger, Youssef L. Bennani,y Arthur A. Hancock and Ramin Faghih
Neuroscience Research, Global Pharmaceutical Research and Development, Abbott Laboratories, 100 Abbott Park Road,
Abbott Park, IL 60064, USA
Received 18 August 2003; accepted 17 November 2003
Abstract—Further SAR studies on novel histamine H3 receptor antagonists are presented. Compound 14bb is a potent antagonist
of both the rat cortical and human clone receptors, and is demonstrated to act functionally as an antagonist in an in vivo mouse
dipsogenia model.
# 2003 Elsevier Ltd. All rights reserved.
The histamine H3 receptor has been described as a pre-
synaptic autoreceptor in the brain, where it regulates
the synthesis and release of histamine, as well as the
presynaptic release of acetylcholine, g-aminobutyric
acid, dopamine, noradrenaline, and serotonin.1 H3
antagonists have been studied in cognitive models and
they may yet prove useful for the treatment of a wide
range of central nervous system disorders, including
attention-deficit disorder, Alzheimer’s disease, and
schizophrenia.2
Oxadiazoles have been shown to serve as bioisosteres of
carbonyls and thioguanidines in imidazole-containing
H3 antagonists.5 We sought to apply this approach to
non-imidazole H3 antagonists. Initial results from bind-
ing assays with 1,2,4-oxadiazole analogues of 2, 4a and
4b (Table 1), revealed that these were as potent as the
parent molecule at rH3 and encouraged development of
the series. Oxadiazole analogues of amide 3, 5a and 5b,
were also comparably potent, though it is worth noting
that the (l)-enantiomer of 5b was considerably less
potent (rH3 Ki=110 nM; hH3 Ki ꢀ1000 nM). Oxadia-
zole 5a was further tested in two in vitro functional
assays in order to determine whether the antagonist
properties of the parent molecules remained intact. It
effectively blocked (R)-a-methylhistamine-induced inhi-
bition of forskolin-stimulated cAMP levels in a dose-
dependent manner, giving a pKb value of 7.22Æ0.17
(n=3) in a clonal C6 cell line expressing the long form
of hH3.4b In addition, 5a effectively blocked (R)-a-
methylhistamine-induced increases in intracellular cal-
cium ([Ca2+]i) in a dose-dependent manner, giving a
pKb value of 6.81Æ0.20 (n=3) using a Fluorescence
Imaging Plate Reader method and a clonal cell line co-
expressing hH3 and the G-protein chimera, Gqi5.6 These
results showed that 5a behaved as a functional antago-
nist at hH3 receptors coupled to Gi-mediated inhibition
of adenylate cyclase and Gqi5-mediated stimulation of
[Ca2+]i.
Our research began with the identification of the high
throughput screening lead 1 (Fig. 1) and soon pro-
gressed to the identification of 2.3 This amino acid deri-
vative bound tightly (Ki=1.0 nM) to the rat cortical
histamine H3 receptor (rH3), but once the human hista-
mine H3 receptor (hH3) was cloned1d our assays for 2
and its analogues demonstrated a binding difference of
over two orders of magnitude between the receptors
(hH3 Ki=760 nM).4 Converting the free amine to the
amide of a furyl or 4-substituted phenyl ring improved
binding to hH3, and addition of a fluorine atom to pro-
vide compounds such as 3 increased binding at the hH3
clone further (rH3 Ki=1.3 nM; hH3 Ki=43 nM).4
* Corresponding author. Fax: +1-847-937-9195; e-mail: greg.gfesser@
abbott.com
y
Present address: Athersys, Inc., Cleveland, OH, USA.
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2003.11.038

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G. A. Gfesser et al. / Bioorg. Med. Chem. Lett. 14 (2004) 673–676
The aim of these studies was to increase affinity for hH3,
while maintaining potency at rH3, by modifying the
substituents on both the oxadiazole and the terminal
amide. The syntheses were accomplished by a straight
forward route (Scheme 1), and resulted in a variety of
compounds potent at both receptors. Commercially
available 2-fluoro-4-hydroxybenzonitrile reacted with
1-bromo-3-chloropropane under basic conditions to
afford alkyl chloride 6. Displacement of the chloride
with either Boc-protected piperazine or homopiperazine
provided intermediate 7, at which point the nitrile was
converted to an oxadiazole by a three-step procedure.
Treatment of 7 with hydroxylamine formed N-hydroxy-
benzamidine 8, and slow addition of acyl chloride fol-
lowed by heating provided 9. After the protecting group
was removed, the free amine was subjected to peptide
coupling conditions to give 10 and ultimately the target
series 11.
The first set of compounds derived from 5b retained the
furyl amide while the oxadiazole substituent was varied
(Table 2). In vitro data consistently demonstrated that
hydrophobic substituents gave the greatest potency,
especially at the hH3 receptor. Replacing one methylene
unit in 12g (rH3 Ki=0.56 nM; hH3 Ki=8.6 nM) with an
oxygen atom to provide 12h (rH3 Ki=3.7 nM; hH3
Ki=110 nM) significantly dropped potency at both the
rat cortical and human cloned receptors. Similarly,
compounds 12z and 12aa bearing a pyridyl group were
much less potent at hH3 (rH3 Ki=6.9–11 nM; hH3
Ki=190–520 nM) than 12t, which possessed a simple
phenyl moiety (rH3 Ki=4.1 nM; hH3 Ki=8.6 nM).
Carbamates 12o–q (rH3 Ki=8.2–28 nM; hH3 Ki=140–
520 nM) each had poor activity, again particularly at
hH3. The greatest activity was demonstrated by those
compounds bearing moderately large hydrophobic sub-
stituents. In the alkyl series, 12a bearing a small ethyl
group had the lowest activity (rH3 Ki=4.1 nM; hH3
Ki=120 nM), while compounds with larger groups,
such as 12g, 12k, and 12r, possessed the best potencies
(rH3 Ki=0.56–0.94 nM; hH3 Ki=7.5–9.3 nM). How-
ever, a plateau appeared to be reached upon the accu-
mulation of five or six carbons.
Figure 1. Structural leads.
Table 1. Binding affinities (Ki)a of aryloxadiazole leads
R
n
rH3
hH3
We then chose compounds with one of the best aryl
(phenyl) and alkyl (cyclopentylmethyl) substituents in
the above series and used these in our studies of the
terminal amide (Table 3) among both piperazine- and
homopiperazine-containing compounds. Compounds 13
possessing a phenyl group on the oxadiazole showed no
strong trend when the amide was varied. Aromatic
moieties appeared to be somewhat superior to alkyl
groups, but in our hands two were inexplicably poor.
Molecules 14 and 15 possessing a cyclopentylmethyl
4a
4b
5a
5b
H
H
1
2
1
2
3.1
3.0
1.1
1.5
560
240
64
C(O)-(2-furyl)
C(O)-(2-furyl)
42
a Values are reported as nanomolar potencies; nꢀ3 (SEM typically
<0.2).
Scheme 1. Preparation of target molecules.

G. A. Gfesser et al. / Bioorg. Med. Chem. Lett. 14 (2004) 673–676
675
Table 2. Binding affinitiesa (Ki) of substituted oxadiazoles at rat cortical H3 receptors and cloned human H3 receptors
R
rH3
hH3
R
rH3
hH3
R
rH3
hH3
12a
12b
12c
12d
12e
12f
12g
12h
12i
Et
CHMe2
CH2CHMe2
CMe3
CH2CMe3
4.1
2.1
1.7
4.1
2.4
2.3
0.56
3.7
7.7
120
46
33
85
40
34
8.6
110
210
12j
12k
12l
12m
12n
12o
12p
12q
12r
Cyclopentyl
Cyclopentylmethyl
Cyclohexyl
Cyclohexylmethyl
Cycloheptyl
(S)-CH(Me)(NHBoc)
(R)-CH(Me)(NHBoc)
rac-CH(iPr)(NHBoc)
(CH2)2Ph
0.86
0.83
2.4
2.6
3.3
8.2
28
9.8
0.94
15
9.3
19
21
15
140
520
170
7.5
12s
12t
CH2Ph
Ph
3-Thienyl
2-Thienyl
3-Furyl
1.6
4.1
2.6
6.2
1.7
5.0
6.2
6.9
11
40
8.6
49
160
85
170
240
190
520
12u
12v
12w
12x
12y
12z
12aa
CHEt2
2-Furyl
(CH2)2CHMe2
CH2OCHMe2
rac-2-Tetrahydrofuryl
2-Thiazolyl
3-Pyridinyl
4-Pyridinyl
a Values (nM) were estimated from at least three separate competition experiments (SEM<0.3).
Table 3. Binding affinitiesa (Ki) of substituted amides at rat cortical H3 receptors and cloned human H3 receptors
R
rH3
HH3
R
rH3
hH3
R
rH3
hH3
R
rH3
HH3
13a–t: n=1; R0=Ph
CHMe2
14
11
14
7.0
6.9
110
56
68
70
Cyclopentylmethyl
CH2OEt
(S)-Tetrahydrofuranyl
(R)-Tetrahydrofuranyl
Morpholinomethyl
21
10
8.4
4.2
8.2
130
100
80
92
92
Morpholinoethyl
Ph
4-NC–C6H4
2-Thienyl
3-Thienyl
6.4
4.6
2.6
5.0
4.2
88
39
20
40
28
2-Furyl
2-Furyl
5-Isooxazolyl
3-Pyridyl
2-Pyrazinyl
2.4
13
13
2.3
1.7
43
83
86
27
29
CH2CHMe2
(CH2)2CHMe2
Cyclopentyl
Cyclohexyl
49
14a–ff: n=1; R0=Cyclopentylmethyl
Me
Et
CHMe2
CH2CHMe2
Bu
CMe3
CH2CMe3
(CH2)2CHMe2
8.9
7.2
8.0
5.0
2.6
7.0
6.4
6.1
200
170
230
110
79
130
100
98
Morpholinoethyl
2-Me–C6H4
3-Me–C6H4
2,5-diMe–C6H3
2-MeO–C6H4
3-MeO–C6H4
4-MeO–C6H4
3,4-diMeO–C6H3
1.1
2.4
4.7
6.6
7.0
3.6
1.9
2.5
37
13
31
75
54
34
11
24
3-F3C–C6H4
3-F3CO–C6H4
2-F–C6H4
3-F–C6H4
4-F–C6H4
10
24
2.9
3.8
1.5
2.8
6.7
2.8
87
65
16
18
17
8.6
36
24
3,4-diCl–C6H3
4-Br–C6H4
1.8
6.2
2.6
2.4
1.5
1.2
1.7
1.3
15
18
47
15
35
9.5
20
16
3-Me2N–C6H4
4-Me2N–C6H4
3-NC–C6H4
4-NC–C6H4
3-Pyridyl
2-Cl–C6H4
3-Cl–C6H4
4-Cl–C6H4
4-HO–C6H4
15a–o: n=2; R0=Cyclopentylmethyl
Me
Et
CHMe2
CH2CHMe2
2.0
1.1
1.3
1.1
40
26
14
13
Bu
CMe3
CH2CMe3
(CH2)2CHMe2
1.4
2.0
4.9
2.8
18
36
74
23
Morpholinoethyl
4-Me–C6H4
4-MeO–C6H4
4-NC–C6H4
0.58
1.4
1.1
1.5
13
17
9.8
12
4-F–C6H4
4-Cl–C6H4
3,4-diCl–C6H3
1.2
5.2
31
14
31
140
a Values (nM) were estimated from at least three separate competition experiments (SEM<0.3).
group on the oxadiazole showed a greater difference
between alkyl and aryl substituents, though only in the
piperazine series; in the homopiperazine series the dif-
ference was negligible. A variety of aryl groups with
small substituents conferred slightly better activity than
other aromatic groups, and the morpholinoethyl group
was the most potent of non-aromatic substituents.
substituents (R=alkyl), compounds 15 bound with the
greatest affinity for the receptors.
Compound 14bb was selective against hH1 (Ki=890
nM), hH2 (Ki>10 mM), and hH4 (Ki>10 mM), and
FLIPR and cAMP assays showed it bound to hH3 as a
competitive antagonist (data not shown). Recent clon-
ing and pharmacological analysis of the mouse hista-
mine H3 receptor show a high degree of similarity to
rH3,7 and previous studies of neurotransmitter release
have demonstrated analogous functional roles across
species.8 With this idea in mind, 14bb was then utilized
in vivo in a previously characterized mouse dipsogenia
A possible trend was also seen between the three series.
Just as the phenyl derivative 12k was more potent at
rH3 than cyclopentylmethyl 12t, compounds 14 and 15
seemed to show more potency than their analogues 13.
Furthermore, among those compounds bearing alkyl

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G. A. Gfesser et al. / Bioorg. Med. Chem. Lett. 14 (2004) 673–676
merman, H. Trends Pharmacol. Sci. 1998, 19, 177. (d)
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.
2. (a) Leurs, R.; Blandina, P.; Tedford, C.; Timmerman, H.
Trends Pharmacol. Sci. 1998, 19, 177. (b) Stark, H. Exp.
Opin. Ther. Pat. 2003, 13, 851. (c) Halpern, M. T. Curr.
Opin. Cent. Peripher. Nerv. Syst. Invest. Drugs 1999, 1,
524. (d) Ali, S. M.; Tedford, C. E.; Gregory, R.; Handley,
M. K.; Yates, S. L.; Hirth, W. W.; Phillips, J. G. J. Med.
Chem. 1999, 42, 903. (e) Fox, G. B.; Pan, J. B.; Esben-
shade, T. A.; Bennani, Y. L.; Black, L. A.; Faghih, R.;
Hancock, A. A.; Decker, M. W. Behav. Brain Res. 2002,
131, 151.
3. Part 2: Faghih, R.; Dwight, W.; Black, L.; Liu, H.; Gen-
tles, R.; Phelan, K.; Esbenshade, T. A.; Ireland, L.;
Miller, T. R.; Kang, C. H.; Krueger, K. M.; Fox, G. B.;
Hancock, A. A.; Bennani, Y. L. Bioorg. Med. Chem. Lett.
2002, 12, 2035.
4. (a) Faghih, R.; Phelan, K.; Esbenshade, T. A.; Miller,
T. R.; Kang, C. H.; Krueger, K. M.; Yao, B. B.; Fox,
G. B.; Bennani, Y. L.; Hancock, A. A. Inflam. Res. 2003,
52, S47. (b) Esbenshade, T. A.; Krueger, K. M.; Miller,
T. R.; Kang, C. H.; Denny, L. I.; Witte, D. G.; Yao, B. B.;
Fox, G. B.; Faghih, R.; Bennani, Y. L.; Williams, M.;
Hancock, A. A. J. Pharm. Exp. Ther. 2003, 305, 887.
5. (a) Clitherow, J. W.; Beswick, P.; Irving, W. J.; Scopes,
D. I. C.; Barnes, J. C.; Clapham, J.; Brown, J. D.; Evans,
D. J.; Hayes, A. G. Bioorg. Med. Chem. Lett. 1996, 6, 833.
(b) Graßmann, S.; Sadek, B.; Ligneau, X; Elz, S.; Ganel-
lin, C. R.; Arrang, J.-M.; Schwartz, J.-C.; Stark, H.;
Schunack, W. Eur. J. Pharm. Sci. 2002, 15, 367.
6. Fox, G. B.; Pan, J. B.; Esbenshade, T. A.; Bitner, R. S.;
Nikkel, A. L.; Miller, T.; Kang, C. H.; Bennani, Y. L.;
Black, L. A.; Faghih, R.; Hancock, A. A.; Decker, M. W.
Pharmacol. Biochem. Behav. 2002, 72, 741.
7. Chen, J.; Liu, C.; Lovenberg, T. W. Eur. J. Pharmacol.
2003, 467, 57.
Figure 2. Dipsogenia results for 14bb.
model6 (Fig. 2). Peripheral or central administration of
the H3 receptor agonist (R)-a-methylhistamine induced
a rapid and pronounced increase (up to 10 times basal
levels) in water consumption that could be assessed over
discrete time periods in mice. Prior peripheral or central
administration of H3 antagonists blocked this response,
which is believed to be mediated by H3 receptors in the
brain. Compound 14bb potently blocked agonist-
induced dipsogenia at 0.01 mmol/kg, ip without affecting
basal water intake when given alone. These results sug-
gest that 14bb acts functionally as an antagonist at H3
receptors in the CNS.
8. Schlicker, E.; Kathman, M. In The Histamine H3 Recep-
tor: A Target for New Drugs; Leurs, R., Timmerman, H.,
Eds.; Elsevier Science: Amsterdam, 1998; p 15.
In summary, we have developed a new series of non-
imidazole H3 ligands, which expands on previous
examples.9 Compounds previously reported from our
laboratories3 had first been tested only against the rat
cortical receptor and later proved substantially less
potent against the human cortical receptor, and subse-
quently at the cloned human receptor. Our new com-
pounds have proven to be potent antagonists at both
the rat cortical and cloned human receptors, and pos-
sess the balance between the receptors which we had
sought. Furthermore, we have demonstrated that 14bb
acts effectively at low dose in an in vivo dipsogenia
model. Additional in vivo data on these or related series
will be reported elsewhere.
9. (a) Aslanian, R.; wa Mutahi, M.; Shih, N.-Y.; Piwinski,
J. J.; West, R.; Williams, S. M.; She, S.; Wu, R.-L.; Hey,
J. A. Bioorg. Med. Chem. Lett. 2003, 13, 1959. (b) Chai,
W.; Breitenbucher, J. G.; Kwok, A.; Li, X.; Wong, V.;
Carruthers, N. I.; Lovenberg, T. W.; Mazur, C.; Wilson,
S. J.; Axe, F. U.; Jones, T. K. Bioorg. Med. Chem. Lett.
2003, 13, 1767. (c) Apodaca, R.; Dvorak, C. A.; Xiao, W.;
Barbier, A. J.; Boggs, J. D.; Wilson, S. J.; Lovenberg,
T. W.; Carruthers, N. I. J. Med. Chem. 2003, 46, 3938. (d)
Turner, S. C.; Esbenshade, T. A.; Bennani, Y. L.; Han-
cock, A. A. Bioorg. Med. Chem. Lett. 2003, 13, 2131. (e)
Faghih, R.; Dwight, W.; Vasudevan, A.; Dinges, J.; Con-
ner, S. E.; Esbenshade, T.; Bennani, Y. L.; Hancock, A. A.
Bioorg. Med. Chem. Lett. 2002, 12, 3077. (f) Ting, P. C.;
Lee, J. F.; Albanese, M. M.; Tom, W. C.; Solomon,
D. M.; Aslanian, R.; Shih, N.-Y.; West, R. Bioorg. Med.
Chem. Lett. 2002, 12, 2643. (g) Apelt, J.; Ligneau, X.;
Pertz, H. H.; Arrang, J.-M.; Gannelin, R. C.; Schwartz,
J.-C.; Schunack, W.; Stark, H. J. Med. Chem. 2002, 45,
1128. (h) Meier, G.; Ligneau, X.; Pertz, H. H.; Ganellin,
C. R.; Schwartz, J.-C.; Schunack, W.; Stark, H. Bioorg.
Med. Chem. 2002, 10, 2535.
References and notes
1. (a) Hill, S. J.; Gannelin, C. R.; Timmerman, H.; Schwartz,
J.-C.; Shankley, N. P.; Young, J. M.; Shunack, W.; Levi,
R.; Haas, H. L. Pharmacol. Rev. 1997, 49, 253. (b)
Arrang, J.-M.; Garbarg, M.; Schwartz, J.-C. Nature 1983,
302, 832. (c) Leurs, R.; Blandina, P.; Tedford, C.; Tim-