Acidic elements in histamine H3 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2010.01.089

Antagonists of the human histamine H₃ receptor (hH₃R) often contain a second basic moiety, which is well known to boost affinity on this histamine receptor subtype. Here, we prepared compounds with acidic moieties of different pK_a values to figure out that the hH₃R tolerates these functionalities when added to a common pharmacophore blueprint. Depending on the acidic, electronic and steric features the designed ligands showed hH₃R affinities in the nanomolar concentration range. Additionally, selected ligands were tested but failed as dual acting hH₃R/hPPAR (human peroxisome proliferator-activated receptor) ligands.

Full text of DOI:10.1016/j.bmcl.2010.01.089

Bioorganic & Medicinal Chemistry Letters 20 (2010) 1581–1584
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Acidic elements in histamine H3 receptor antagonists
Kerstin Sander a, Yvonne von Coburg a, Jean-Claude Camelin b, Xavier Ligneau b, Oliver Rau a,
Manfred Schubert-Zsilavecz a, Jean-Charles Schwartz b, Holger Stark a,
*
a Johann Wolfgang Goethe University, Institute of Pharmaceutical Chemistry, ZAFES/LiFF/CMP, Max-von-Laue-Str. 9, 60438 Frankfurt/Main, Germany
b Bioprojet-Biotech, 4 Rue du Chesnay-Beauregard, 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:
Antagonists of the human histamine H3 receptor (hH3R) often contain a second basic moiety, which is
well known to boost affinity on this histamine receptor subtype. Here, we prepared compounds with
acidic moieties of different pKa values to figure out that the hH3R tolerates these functionalities when
added to a common pharmacophore blueprint. Depending on the acidic, electronic and steric features
the designed ligands showed hH3R affinities in the nanomolar concentration range. Additionally, selected
ligands were tested but failed as dual acting hH3R/hPPAR (human peroxisome proliferator-activated
receptor) ligands.
Received 4 December 2009
Revised 12 January 2010
Accepted 13 January 2010
Available online 21 January 2010
Keywords:
GPCR
Histamine
Antagonists
Pharmacophore
PPAR
Ó 2010 Elsevier Ltd. All rights reserved.
Metabolic diseases
Ligands
Human histamine H3 receptors (hH3R) in the central nervous
system (CNS) regulate the synthesis and liberation of the neuro-
transmitter histamine (autoreceptor function). Additionally, as
heteroreceptors, they control the concentration of various other
biogene amines and neuropeptides. Hence, these G protein-cou-
pled receptors (GPCR) play a major role in keeping a certain neuro-
transmitter balance in regions of co-localized neurons and affect
central functions like vigilance, learning and attention.1 Antago-
nists/inverse agonists of the hH3R have well advanced in their
way from bench to bedside and some have passed clinical phase
II.2 Besides their main potential indications in the broad field of
neurodegenerative diseases some have shown efficacy in models
of metabolic diseases, for example, obesity and diabetes mellitus.
In rodents, caloric intake was reduced by administration of hH3R
antagonists, like A-3314403 or NNC-381202.4 Histaminergic neu-
rons in the CNS interfere with orexinergic pathways. Elevated lev-
els of brain histamine reduce food intake and weight gain in
several species.5 Most probably, this occurs via H1R-mediated
downstream signalling because H1R antagonists or antipsychotics
evoke the contrary effect.6 However, contradictory results have
been reported2 and further studies are needed to clear up this com-
plex mechanism.
onists consist of a basic moiety, which is coupled via an alkyl
linker to an aromatic core. The latter might be substituted with
a variety of basic, lipophilic or polar moieties of different sizes.7
A second basic centre usually boosts hH3R affinity due to addi-
tional beneficial interactions with the receptor’s binding pocket
(Glu206)8 but also covers the risk of central accumulation and
induction of phospholipidosis.9 One of the most prominent
examples of these diamine-based structures is JNJ-5207852 (1,
Fig. 1),10 which has become a parent structure for this class of
dibasic compounds.6
Recently, we reported a series of diamine-containing multiple-
acting hH3R ligands with antipsychotic properties, amongst it the
amitryptiline hybrid 2. The main focus was on the modification
of the in vitro receptor profile of neuroleptic drugs. Additionally
we were able to show the robustness of the H3R binding pocket
with regard to bulky ligands.6,11 To expand this series with another
piperazine-containing derivative we perchance prepared a cipro-
floxacin hybrid (3) in an analogue manner and determined its
hH3R affinity.
Although ciprofloxacin features not only a second basic moiety
but also a weak carboxylic acid it showed a surprising high affinity
(pKi = 8.7). We took this result as a rationale to investigate if the
aminergic hH3R tolerates acidic moieties added to the common
1-(3-phenoxypropyl)piperidine pharmacophore. Starting from the
dibasic 4,5-dihydro-1H-imidazole derivative 7 a series of ligands
with different acidic moieties was prepared by modification of
the heterocycle in the eastern molecule part.
The hH3R targeting domain and the corresponding antagonist
pharmacophore are well investigated. Actually, most hH3R antag-
* Corresponding author. Tel.: +49 69 798 29302; fax: +49 69 798 29258.
E-mail address: h.stark@pharmchem.uni-frankfurt.de (H. Stark).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.01.089

1582
K. Sander et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1581–1584
(i)
JNJ-5207852
N
N
O
O
N
O
N
O
H
1
2
N
N
N
6
7
N
Scheme 1. Synthesis of 4,5-dihydro-1H-imidazole derivative 7. Reagents and
conditions: (i) ethan-1,2-diamine, sulfur, W, 30 min, 110 °C.
Amitryptiline hybrid
CH3
N
l
tives,19,20 whereas thiazolidindiones 10 and 11 are the products of
a Knoevenagel reaction with thiazolidindione21,22 and rhodanine,23
respectively. According to previous publications24 this condensa-
tion exclusively leads to the Z configuration of the exocyclic double
bond, which was confirmed by X-ray investigation showing the ex-
pected configuration and the dimerization of molecules in the crys-
tal structure (CCDC No. 760848, cf. Supplementary data).
Compound 12 was prepared in a catalytic hydrogenation of 10
(Scheme 2).22
With regard to their hH3R affinity compounds 3 and 7–12 were
tested in a [125I]iodoproxyfan displacement assay, which was per-
formed on a membrane preparation of HEK-293 cells stably
expressing the hH3R (Table 1).25–27 Compounds 10 and 12 were
O
N
O
F
OH
Ciprofloxacin hybrid
Rosiglitazone
N
O
O
N
3
4
N
O
N
N
CH3
S
NH
O
Figure 1. Reference structures for the development of acid-containing hH3R
antagonists.
additionally tested in a hPPARa/c reporter gene assay. Receptor
activation was determined by quantification of the luminescence
evoked by the expressed luciferase in Cos-7 cells (Table 2).13,28
The 4,5-dihydro-1H-imidazole-containing compound 7 showed
Drugs that are commonly used in the treatment of metabolic
diseases, that is, fibrates or glitazones (cf. Fig. 1 for rosiglitazone
4),12 usually request an acidic head group to interact with their tar-
gets, that is, human peroxisome proliferator-activated receptors
(hPPARs; cf. Fig. 2).13 A second goal of this study was to explore
if the newly designed compounds with both, basic and acidic moi-
eties, are efficacious as dual acting hH3R/hPPAR ligands. Such com-
pounds could be useful in the treatment of metabolic diseases like
obesity and diabetes mellitus. Additionally, because PPARs induce
neuroprotection14 and reduce neuroinflammation in rodent mod-
els,15 such hybrids might be efficacious in the treatment of neuro-
degenerative diseases.
a
hH3R affinity in the low nanomolar concentration range
(pKi = 8.1) and hence served as a lead structure for the develop-
ment of this series of acidic compounds (Table 1).
To insert acidic elements cyclic cysteine derivatives 8 and 9
were prepared implying a heteroatom exchange and the complete
O
N
O
F
OH
N
O
N
Starting from piperidine the preparation of the precursors, alde-
hyde 5 and nitrile 6, was performed as described before.11,16 Cycli-
zation of nitrile 6 by refluxing it in ethane-1,2-diamine under
sulfur catalysis in the microwave oven resulted in 4,5-dihydro-
1H-imidazole derivative 7 (Scheme 1).17 All other compounds were
prepared using aldehyde 5 as reactant. Ciprofloxacin hybrid 3 was
synthesized in a reductive amination with the piperazine moiety of
the fluoroquinolone.18 Thiazolidines 8 and 9 were obtained in
reductive nucleophilic substitution reactions with cysteine deriva-
N
3
(i)
(ii)
N
O
N
O
H
N
O
O
5
8: R = CH3
: R = H
S
OR
9
(iii)
hH3R pharmacophore
PPARα/γ pharmacophore
O
S
Basic
head
group
Linker
Aro-
matic
core
Lipophilic
or basic
moiety
Hydro-
phobic
tail
Linker
Aro-
matic
core
Acidic
head
group
N
O
N
O
S
S
NH
NH
O
O
10
11
O
O
N
O
(iv)
N
N
CH3
S
NH
N
O
6
4
O
N
O
S
NH
O
12
O
NH
O
N
O
S
Scheme 2. Synthesis of ciprofloxacin hybrid 3, thiazolidines 8 and 9 as well as
thiazolidinediones 10–12. Reagents and conditions: (i) ciprofloxacin, NaBH(OAc)3,
1,2-dichloro ethane, 16 h, rt; (ii) cysteine methylester (8)/cysteine (9), ethanol/H2O,
3 h, rt; (iii) thiazolidindione (10)/rhodanine (11), piperidine, glacial acetic acid,
toluene, 6 h, reflux. (iv) Pd(OH)2/C, H2, 5 bar, 48 h, 60 °C.
12
Figure 2. Development of the potential merged hH3R/hPPAR hybrid 12.

K. Sander et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1581–1584
1583
Table 1
(pKi values of 8.4 and 8.0, respectively). Nevertheless, the amino
acid moiety with estimated pKa values of 2.2 and 8.7 (acid and
base, respectively) was well tolerated. The thiazolidine-containing
compounds 8 and 9 represent new lead structures for the develop-
ment of neuroprotective agents. Thiazolidines coupled to aromatic
(hetero)cycles have shown some potential to modify the conforma-
tional properties of amyloid beta peptides and to prevent their
aggregation.30 A combination with the hH3R pharmacophore might
be a new approach to design ligands for the treatment of Alzhei-
mer’s disease.31
Acidity and hH3R affinities of references 1,2 and compounds 3, 7–12
N
O
pKa1 = 9.1
R
Compds
R
Acidity of Ra
hH3R affinityb
pKic
pKa2
Base
8.6
Acid
(nM)
1d
9.2
The attenuation of acidity by implementation of NH-acidic
groups slightly improved receptor binding as could be seen in
methylene-thioxo/azolidinone structures 10 and 11. Comparing
the characteristics of these two compounds the less electronega-
tive and therefore less acidic and more lipophilic thioxothiazolidi-
none 11 evoked a stronger affinity. This small series of compounds
shows a tendency regarding the binding potency of acid-contain-
ing ligands: the less acidic the more affine. However, differences
in affinity values of all compounds 9–11 range within half an order
of magnitude (pKi values between 8.0 and 8.3) indicating that the
hH3R tolerates acidic moieties very well. Regarding its affinity but
not its acidity compound 12 constitutes an exception within this
series. The pKa value resembled that of 10 due to similar structural
features of the thiazolidindione moiety. Compared to the receptor
binding of compounds 10 and 11 the hydrogenated derivative 12
showed a decrease of hH3R affinity in the range of almost one order
of magnitude (pKi = 7.3). This might be caused by electronic and
steric effects. Because of the saturation of the methylene linker
the high electronegativity of the thiazolidindione moiety cannot
be attenuated by the phenyl ring, which creates a molecule part
of high electron density. In contrast, the double bond in com-
pounds 10 and 11 generates a conjugated planar system with im-
proved electron distribution.
N
CH3
N
2e
8.6
8.6
9.5
8.7
O
N
O
F
OH
3
6.1
N
N
H
N
7
8
11.2
6.6
8.1
8.4
N
O
H
N
CH3
O
S
O
H
N
OH
9
8.7
2.2
6.2
8.0
8.1
S
O
NH
O
S
10
Compound 12 incorporates parts of both pharmacophores, the
hH3R phenoxypropylpiperidine and the (alkoxybenzyl)thiazolidin-
dione moiety of glitazones. Hence, it exhibits the structural fea-
tures of a merged hybrid ligand (Fig. 2).32 To investigate if the
moderate affine hH3R ligand 12 and its precursor 10 interact with
S
NH
O
S
S
11
12
7.8
6.5
8.3
7.3
O
hPPAR subtypes, especially hPPARc, the main target of the glitaz-
NH
O
one class, these two compounds were tested in a transactivation
assay but none of them was able to activate hPPARs (Table 2).
The main reason for this might be the aliphatic piperidino moiety,
which exhibits a considerably higher pKa value than the aromatic
pyridine structures of the glitazones (cf. Supplementary data).
The presented results confirm that hPPARs do not interact with
compounds that contain strong basic moieties.34 Because the clo-
sely related compounds did not show PPAR activity testing of the
other compounds was closed. Hence, it has not been possible to de-
sign dual acting hH3R/hPPAR ligands with the actual approach be-
cause the exchange of tertiary amines in the basic head of the hH3R
pharmacophore into weaker basic moieties, that is, aromatic nitro-
gen-containing heterocycles found in the glitazone class, leads to a
loss of hH3R affinity.35 Nevertheless, the dual approach may be
useful in the therapy of metabolic diseases. Considering the other
side of the coin we successfully implemented acidic moieties into
the hH3R antagonist pharmacophore, which until now was only
known to tolerate basic, polar or lipophilic moieties of different
sizes. This small series of compounds clearly shows that the
hH3R is able to bind acid-containing ligands in the low nanomolar
concentration range as long as the ionic bond between the basic
pKs Values were assessed with ALOGPS 2.1 program.29
a
b
c
[
125I]Iodoproxyfan binding assay (HEK-293 cells stably expressing hH3R).25–27
Values were determined in one experiment, at least in triplicates for each
concentration tested.
d
Ref. 10.
Ref. 6.
e
saturation of the heterocyclic system of 7. Although both deriva-
tives showed good hH3R affinities, the more lipophilic ester 8
evoked slightly stronger receptor interactions than acid 9 did
Table 2
hPPAR
a
/c
transactivation activities of compounds 10 and 12
activationa EC50
(
lM)
hPPAR
c
activationa EC50
(lM)
b
b
Compds
hPPAR
a
4c
> 10
c
c
1: 0.03
2: 0.1
11
13
> 10
> 10
> 100
> 100
head and the receptor binding pocket (Asp114) as well as the
p
a
Luciferase reporter gene assay (Cos-7 cells transfected with Lipofectamine™
and edge-to-face interactions of the aromatic core (Tyr115 and
Trp371) are kept intact.8,36 With these results the so far broadly ac-
cepted doctrine among hH3R researchers is broadened offering
new possibilities for the development of hH3R antagonists espe-
cially targeted on the treatment of metabolic diseases.
reagent (Invitrogen) according to the manufacturer’s protocol with pFR-Luc
(Stratagene), pRL-SV40 (Promega) and the Gal4-fusion receptor plasmids (pFA-
CMV-hPPAR-LBD) of the respective subtype).13,28
b
Values are means of three independent experiments, each in triplicates.
c
Ref. 33.

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K. Sander et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1581–1584
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