Potential utility of histamine H3 receptor antagonist pharmacophore in antipsychotics

Article abstract of DOI:10.1016/j.bmcl.2008.09.012

Histamine H₃ receptor (H₃R) antagonists have some antipsychotic properties although the clear molecular mechanism is still unknown. As actually the most effective and less side effective antipsychotics are drugs with multiple targets

Full text of DOI:10.1016/j.bmcl.2008.09.012

Bioorganic & Medicinal Chemistry Letters 19 (2009) 538–542
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Potential utility of histamine H3 receptor antagonist pharmacophore
in antipsychotics
Y. von Coburg a, T. Kottke a, L. Weizel a, X. Ligneau b, H. Stark a,
*
a Johann Wolfgang Goethe Universität, Institut für Pharmazeutische Chemie, Biozentrum, ZAFES/CMP, Max-von-Laue-Strasse 9, 60438 Frankfurt, Germany
b Bioprojet-Biotech, 4 rue du Chesnay Beauregard, BP 96205, 35762 Saint Grégoire Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Histamine H3 receptor (H3R) antagonists have some antipsychotic properties although the clear molecu-
lar mechanism is still unknown. As actually the most effective and less side effective antipsychotics are
drugs with multiple targets we have designed typical and atypical neuroleptics with an additional hist-
amine H3 pharmacophore. The 4-(3-piperidinopropoxy)phenyl pharmacophore moiety has been linked to
amitriptyline, maprotiline, chlorpromazine, chlorprothixene, fluphenazine, and clozapine. Amide, amine
and ester elements have been used generally to maintain or slightly shift affinity at dopamine D2-like
receptors (D2 and D3), to decrease affinity at histamine H1 receptors, and to obtain H3R ligands with
low nanomolar or subnanomolar affinity. Change of effects at D1-like receptors (D1 and D5) were heter-
ogeneous. With these newly profiled compounds different antipsychotic properties might be achieved.
Ó 2008 Elsevier Ltd. All rights reserved.
Received 15 August 2008
Revised 1 September 2008
Accepted 3 September 2008
Available online 7 September 2008
Schizophrenia and related mood disorders are diseases of the
central nervous system (CNS), regarded to be neurodevelopmental
with epigenetic and genetic factors.1–3 In general, dysregulations in
different neurotransmitter systems mainly dopamine, serotonin,
GABA, and glutamate are hypothesized.4 Moreover, there is indi-
rect evidence that histaminergic pathways may play a role in
schizophrenia and that histamine H3 receptor (H3R) antagonists
could lead to therapeutic effects, namely on cognitive deficits.5–7
Today, typical and atypical neuroleptics are used for treatment of
schizophrenia. Their effects are mainly mediated by inhibition of
dopamine D2-like receptors (D2, D3 (and may be D4)) and some
other aminergic receptors.
of patients.16,17 Reduction of undesirable side effects connected to
antipsychotic therapy like weight gain, somnolence, and cognitive
impairment have been demonstrated.17
The data mentioned above indicate that a drug combination of
H3R antagonist and neuroleptic properties may be beneficial in
therapy. Instead of a combination of two drugs, we used the multi-
ple target approach designing one new drug by combination of two
related pharmacophore elements in one structure.18,19 This ap-
proach has been realized by linking the known antagonist H3R
pharmacophore 4-(3-piperidinopropoxy)phenyl to known neuro-
leptics like amitriptyline (1), maprotiline (2), chlorpromazine
(3), chlorprothixene (4), clozapine (5), and fluphenazine (6)
(Figs. 1 and 2).
For introduction of H3R affinity all novel antipsychotic deriva-
tives need to follow a broad general construction pattern shown
inFigure 220,21, bearing a relatively flexible part at the ‘eastern’ part
of the molecule. The lead structure of 4-(3-piperidinoprop-
oxy)phenyl was taken for further derivatization by linking the neu-
roleptic compounds 1–6 by amide, amine or ester functionalities.
Related hybrid structure in the class of H3R antagonists have been
described with H1, ACh, BuCh, NO, and SSRI (for reviews see Refs.
12–14).
The objective of our study was the development of novel multi-
acting antipsychotic drugs showing an optimized target profile
with potent action, fast onset and reduced side effects by maintain-
ing D2/D3 affinity, reducing H1 affinity and introducing high H3R
affinity (Fig. 2). The novel compounds were tested in vitro for sev-
eral aminergic G-protein-coupled receptors (GPCRs) to examine
their enlarged pharmacological profiles. One selected compound
was chosen for further H3R in vivo screening.
Extrapyramidal side effects and weight gain problems are some
of the most important side effects which are related to their antag-
onist properties at dopamine D2-like and histamine H1 receptors,
respectively.8–11
The H3R is acting as presynaptic auto- and heteroreceptor mainly
in the CNS12 controlling the synthesis and the release of histamine,
but also modulating several other neurotransmitter systems e.g.,
dopamine, serotonin, GABA, noradrenalin, or ACh. H3R antagonists
have shown distinct pharmacological actions in preclinical and clin-
ical trials revealing the importance for diverse CNS-related thera-
peutic applications like schizophrenia, depression, sleep–wake
disorders, dementia, or epilepsy.13–15 In fact, H3R inverse agonists/
antagonists displayed significant inhibitory activity in several
s
rodent models of schizophrenia, a disease in which N -methylhista-
minelevel is usually significantlyhigh in cerebralcerebrospinalfluid
* Corresponding author. Fax: +49 69 798 29258.
E-mail address: h.stark@pharmchem.unifrankfurt.de (H. Stark).
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.09.012

Y. von Coburg et al. / Bioorg. Med. Chem. Lett. 19 (2009) 538–542
539
Figure 1. Structures of antipsychotics used in this study.
nation of 9 with nortryptiline or 2 receiving the corresponding
amitriptyline or maprotiline derivatives, respectively (Fig. 4).23
The tertiary amine functionality of chlorpromazine, chlorpro-
thixene and clozapine was conveniently demethylated by reaction
of a-chloroethyl chloroformate (ACE-Cl) and following stirring and
heating in methanol. The intermediate carbamates are cleaved
leaving the secondary amines as well as gaseous by-products,
which can easily be removed under reduced pressure.24,25 Then
the resulting secondary amines could be used for reductive amina-
tion with 9 as described before (Fig. 5).
In addition to amines and amides as nitrogen connection, we
extended these linking elements to oxygen ports. Therefore, we
decided that an ester group may be a suitable functionality. Flu-
phenazine hybrid 17 was synthesized by simple esterification of
activated 8 (Fig. 6).
Figure 2. General construction pattern of histamine H3 receptor antagonists with
neuroleptics and H3 pharmacophore starting structures.
All of the drugs 1–6 and new multiple target compounds 10–17
were first tested for their histamine H3 receptor activity obtained
by [125I]iodoproxyfan binding assay on CHO/HEK293 cells stably
expressing the hH3 receptor (Table 1).26 To enlarge their pharma-
cological profile, we also investigated their binding affinities in
competition binding experiments with seven-point measurements
in at least duplicates (n P 2). Displacement assay were carried out
using membrane suspension of cell lines stably expressing the hu-
man histamine hH1 receptor (CHO) with [3H]pyrilamine, dopamine
hD1 and hD5 receptors (HEK) against [3H]SCH23390 and hD2S, hD3
receptors (CHO) using [3H]spiperone, respectively (Table 1).27–29
All new compounds revealed high H3R binding affinities in low
nanomolar to even picomolar concentration range (17) whereas
For the modular approach with the final combination of differ-
ent antipsychotic structures to H3R pharmacophores functional-
ized benzoic acid, phenyl acetic acid and benzaldehyde
derivatives 7–9 were prepared according to literature methods
(Fig. 2).22
The acid derivates 7ÂHCl and 8ÂHCl were transformed into
reactive carboxylic chlorides for convenient synthesis using the
commercially available desmethylated amitriptyline derivative,
nortriptyline (Fig. 1), resulting in amide derivatives 10 and 11,
respectively (Fig. 3).
For comparison of amide and amine linker H3R hybrids with a
second basic amine functionality were prepared by reductive ami-
Figure 3. Synthesis of amide derivatives 10 and 11. Reagents and conditions: (a) i—SOCl2, toluene, 70 °C, 3 h; ii—1, CH2Cl2, N(Et)3, rt, 2 h.

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Y. von Coburg et al. / Bioorg. Med. Chem. Lett. 19 (2009) 538–542
expected we were pleasantly surprised by the relative high affinity
of these amides for D3R.
Comparing the results of the amides 10 and 11 with those of the
related amine hybrid 12, which can be seen as amitriptyline hybrid
as well as nortriptyline hybrid, a tenfold increase to subnanomolar
H3R affinity could be observed simultaneously with increased
affinities at D2, D3, and H1 receptors.
Encouraged by these outcomes, further amine derivatives 13–
16 have been synthesized and tested, resulting in comparable
binding affinities at H3R. The ester hybrid of fluphenazine, 17, even
showed another 100-fold increase compared to that of the amides,
to picomolar affinity (Ki = 42 pM). An additional amine group can
increase binding affinity.30 In contrast to the other amine hybrids
12–16, the distances on all of the basic moieties of compound 17
from the phenyl group as central core are roughly comparable to
each other suggesting another pharmacophore imitation with
additional lipophilic binding areas. This may lead to the assump-
tion that compound 17 could have more interaction possibilities
with additional H3R binding pockets than the other hybrids.
In comparison to the antipsychotic binding patterns, the amine
compounds (12–17) in general show different but still moderate
binding profile for the dopamine D2-like receptor subtypes (Ki val-
ues (D2): 285 to 41 nM; Ki values (D3): 223 to 15 nM). The amine
hybrids of amitriptyline (or nortriptyline), 12, and of maprotiline,
13, even showed slight enhancements at D2-like receptor affinities
(factors of about 1.5–4.5).
Figure 4. Synthesis of tertiary amine amitriptyline hybrid 12 and maprotiline
hybrid 13. Reagents and conditions: (a) 9, NaBH(OAc)3, 1,2-dichloroethane, rt, 18 h.
the marketed antipsychotics do not have any H3R affinity (Table 1).
For clozapine some moderate affinity for rodent H3R has been
reported whereas for hH3R affinity is in the micromolar concentra-
tion range.
The easily prepared amide hybrids of nortriptyline, 10 and 11,
were first screened as an early proof-of-concept. Although
decrease in D2 receptor affinities was observed, D3 receptor affini-
ties were maintained, H1 receptor affinities decreased and H3R
affinities introduced. Whereas the data for D2 were more or less
Figure 5. Synthesis of tertiary amine derivatives after previous demethylation (14–16). Reagents and conditions: (a) i—3, 4, or 5, ACE-Cl, 1,2-dichloroethane, reflux, 24 h;
ii—MeOH, 50 °C, 2 h; (b) 9, NaBH(OAc)3, 1,2-dichloro ethane, rt, 18 h.
Figure 6. Synthesis of ester fluphenazine hybrid (17). Reagents and conditions: (a) i—(8ÂHCl), SOCl2, toluene, 70 °C, 3 h; ii—6, CH2Cl2, N(Et)3, rt, 2 h.

Y. von Coburg et al. / Bioorg. Med. Chem. Lett. 19 (2009) 538–542
541
Table 1
Pharmacological binding profile of compounds 1–17 at selected human dopamine and histamine receptor subtypes
Compound
Histamine/dopamine receptor subtypes binding affinities, Ki (nM)
a
b
c
c
d
d
hH3
hH1
hD2
hD3
hD1
hD5
Amitriptyline (1)
10
11
12
Maprotiline (2)
13
Chlorpromazine (3)
14
Chlorprothixene (4)
15
Clozapine (5)
16
Fluphenazine (6)
17
>1000
4.90 2.6
3.50 1.9
0.250
>1000
0.358
>1000
1.21 0.04
>1000
1.54
>1000
3.27
1.12 0.21
559 40
686 144
196 40
904 185
>1000
101 49
665 197
146 33
4.06 0.16
41 17
206
326 84
243 70
89 31
879 69
>1000
305 49
402 62
203 68
170 49
>1000
>1000
79
9
67
9
273
1.67
40 16
4.25 0.23
504 282
149 27
6.90 1.18
50 19
4.56 0.77
50 13
295 159
223
3.21 1.81
15
429 199
265 28
172 43
153 36
96
7
205
4
232 74
18 12
248 101
89 67
866 470
179
3.75 0.16
295
2.38 0.40
190 17
40
2.96 1.73
9
2.68
52
83
5
6
297
6
198 41
921 171
285
1.44 0.52
47
>1000
0.042
21
6
390 57
203 68
265 28
a
Ref. 26.
Ref. 27.
Ref. 28.
Ref. 29.
b
c
d
Concerning H1 receptor affinity which may be responsible for
weight gain and sleep-inducing effects a reduction in affinity was
observed for the hybrid structures by a factor of 10–600.
The effects at D1-like receptor were heterogeneous, with the
trend that a decrease in affinity was observed with the hybrids
(with the exception of 13 (D1 and D5) and 14 (D5)). The potential
effects in therapy of these changes are unclear.
Acknowledgments
We thank Prof. R. Leurs (Amsterdam), Prof. J. Shine (Sydney), Dr.
P. Sokoloff (Paris), and Prof. J. Lehmann (Jena) for providing cell
lines expressing the histamine H1 receptor and dopamine D2S, D3
and D1, D5 receptors, respectively. We thank C. Hertzsch for the
preparation of 10 and 11.
One amine derivative, hybrid 12, with high H3R receptor affin-
ity and a good profile was selected for early in vivo screening for
central H3R antagonist potency. Potency was determined 90 min
after oral application of the compound to male swiss mice by
Supplementary data
Supplemental material with analytical data of target com-
pounds will be freely available, in the online version, at
doi:10.1016/j.bmcl.2008.09.012.
s
measurement of the increase in N -methylhistamine level in
brain.31 Unfortunately, this compound seems to be inactive
(ED50 > 10 mg/kg po). It is unclear if pharmacokinetic reasons like
absorption, distribution or metabolism or pharmacodynamic rea-
sons like cross reactivity are responsible for this lack of in vivo
potency.
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