Synthesis and in vitro binding studies of piperazine-alkyl-naphthamides: Impact of homology and sulphonamide/carboxamide bioisosteric replacement on the affinity for 5-HT1A, α2A, D4.2, D3 and D2L receptors

Article abstract of DOI:10.1016/j.bmcl.2010.07.002

A series of carboxamide and sulphonamide alkyl(ethyl to hexyl)piperazine analogues were prepared and tested for their affinity to bind to a range of receptors potentially involved in psychiatric disorders. These chemical modifications led us to explore th

Full text of DOI:10.1016/j.bmcl.2010.07.002

Bioorganic & Medicinal Chemistry Letters 20 (2010) 5199–5202
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and in vitro binding studies of piperazine-alkyl-naphthamides:
Impact of homology and sulphonamide/carboxamide bioisosteric
replacement on the affinity for 5-HT1A, a2A, D4.2, D3 and D2L receptors
*
Mélissa Résimont, Jean-François Liégeois
Laboratory of Medicinal Chemistry and C.I.R.M., University of Liège, Avenue de l’Hôpital, 1 (B36), B-4000 Liège 1, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of carboxamide and sulphonamide alkyl(ethyl to hexyl)piperazine analogues were prepared and
tested for their affinity to bind to a range of receptors potentially involved in psychiatric disorders. These
chemical modifications led us to explore the impact of homology and bioisosteric replacement of the
amide group. All of these compounds possessed a high affinity for 5-HT1A receptors, irrespective of the
Received 19 May 2010
Revised 30 June 2010
Accepted 1 July 2010
Available online 6 July 2010
size of the linker, the carboxamide derivative with a pentyl linker had the highest affinity for a2A receptor
sites and also a high affinity for 5-HT1A and D3 receptors. The sulphonamide analogue with a hexyl linker
possessed a high affinity for 5-HT1A, D4.2 and D3 receptors.
Keywords:
Naphthamide
Receptors
Dopamine
Serotonin
Ó 2010 Elsevier Ltd. All rights reserved.
Binding
Schizophrenia
The multiple receptor strategy remains a valuable approach for
developing CNS drugs1,2 as a complex circuitry exists in vivo as dem-
onstrated by dopamine release studies.3,4 Conversely, it is clear that
a pharmacological tool must be selective for a specific target to pre-
vent problems interpreting both pharmacological and psychophar-
macological data. For example, WAY-100,635 is only relatively
selective for 5-HT1A receptors, as it also displays a reasonable affinity
to interact with D4 receptors.5 Recently, we showed that some pip-
erazinyl-ethyl carboxamide derivatives (Fig. 1) possessed a signifi-
cant affinity for both 5-HT1A and D4.2 receptors and found that
some chemical modulations tended to increase selectivity.6
for receptors that are potentially targets for the development of
putative antipsychotic drugs (e.g., 5-HT1A a2A or D4.2 receptors).
,
The target compounds were synthesized by reaction of the
appropriate primary amine with the appropriate acyl chloride
(2-naphthoyl or 2-naphthalenesulfonyl) as mentioned in Scheme
1. The crude amines were obtained following a Gabriel procedure
using the appropriate N-substituted phthalimides which were syn-
thesized by reaction of 1-(2-methoxy-phenyl) piperazine with
N-(
bonate. The N-arylpiperazine derivative was obtained from com-
mercial sources, while N-( -bromoalkyl)phthalimide analogues
x-bromoalkyl)phthalimide in the presence of potassium car-
x
In the present work, we were interested in extending this series
by exploring two concepts of medicinal chemistry namely homol-
ogy and bioisosteric replacement. Homology in this study is repre-
sented by modifying the size of the linker between the amide
moiety and the basic heterocycle from an ethyl up to a hexyl chain
(Fig. 1). The linker size is limited by the expected low solubility re-
lated to an increased lipophilicity. Bioisosterism in the present
work concerns the replacement of a carboxamide group possessing
a sp2 hybridization and thus a trigonal planar configuration by a
sulphonamide moiety with a sp3 hybridization that presents a tet-
rahedral configuration. The preliminary biological evaluation was
done by measuring the in vitro binding affinity of these molecules
were purchased or prepared in-house following classical methods
and further characterised. All intermediates were used after purifi-
cation and crystallization followed by a classical analytical charac-
terisation (e.g., 1H NMR, elemental analysis). Target compounds
were mainly isolated as a base and further characterised.
In vitro binding experiments were conducted on membrane
preparations isolated from cells transfected and expressing cloned
human or rat receptors. The radioligands used were [3H]-8-OH-
DPAT (ꢀ0.25 nM), [3H]-MK912 (ꢀ0.7 nM), [3H]-YM-09151-2
(ꢀ0.2 nM), [3H]-spiperone for 5-HT1A
, a2A, D4.2 and D3 and D2L
receptors, respectively. Experimental procedures for filtration on
GF/C glass microfibre filters and radioactivity counting were as pre-
viously described6 and are summarised in Table 1, the biological re-
sults are reported in Table 2.
The most significant finding is that all compounds interacted
with 5-HT1A receptors irrespective of the size of the linker or the
* Corresponding author. Tel.: +32 43 66 43 77; fax: +32 43 66 43 62.
E-mail address: JF.Liegeois@ulg.ac.be (J.-F. Liégeois).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.07.002

5200
M. Résimont, J.-F. Liégeois / Bioorg. Med. Chem. Lett. 20 (2010) 5199–5202
O
N
R'
N
N
H
Y
O
X
R'
N
N
N
H
reported in ref. 6
N
R
N
OMe
N
H
n
(1 - 10)
Figure 1. Chemical modulations in a series of piperazine-alkyl-naphthamide derivatives (R = CO or SO2; R0 = H, 2-Cl, 3-Cl, 4-Cl, 2-F, 3-F, 4-F, 2-Me, 3-Me, 4-Me, 2-OMe,
3-OMe, 4-OMe, 3-CF3; n = 1–5; Y@X: CH2–N or CH@C).
O
O
(a)
N
N
N
N
Br
n
n
O
O
MeO
H2N
(b)
(c)
N
N
n
MeO
N
R
N
OMe
N
H
n
Scheme 1. Reagents and conditions: (a) 1-(2-methoxy-phenyl) piperazine, K2CO3, reflux; (b) NH2NH2, EtOH, reflux; (c) 2-naphthoyl chloride or 2-naphthalenesulfonyl
chloride, Et3N, EtOAc, rt; n = 1–5; R = –CO– or –SO2–.
nature of the amide group. The high 5-HT1A affinity of the sulph-
onamide analogue 4 with a propyl linker was also previously re-
ported in the literature in the development of a series of C3
congeners with serotonergic properties.7 In terms of bioisosterism,
carboxamide derivatives are generally more potent on 5-HT1A
receptors, except the hexyl sulphonamide analogue 10 which ap-
pears to display a higher affinity for 5-HT1A receptors than its car-
boxamide analogue 9. Regarding the impact of the linker on the
affinity for 5-HT1A receptors it is observed an uneven evolution.
Most of the compounds, except the butyl sulphonamide 6, pre-
sented a lower affinity for D2L receptors when compared with
other receptors sites tested. The affinity tended to increase with
an increase in linker size. A small linker in this series led to mole-
cules which did not display significant affinity for the D2L receptor.
This was similar to the affinity previously observed with piperidine
naphthamide derivatives.8,9
Carboxamide and sulphonamide alkyl(ethyl)piperazine ana-
logues displayed a high binding affinity for D4.2 receptors, in par-

M. Résimont, J.-F. Liégeois / Bioorg. Med. Chem. Lett. 20 (2010) 5199–5202
5201
Table 1
Experimental conditions of in vitro binding procedures
Cloned
receptors
Ligand
Non-specific
Incubation
buffer
Incubation time Filtration
and temperature and counting
h-5-HT1A (Perkin Elmer 6110501) [3H]-8-OH-DPAT (0.25 nM) WB4101 (10
lM)
50 mM Tris–HCl buffer
at pH 7.4 + 10 mM
60 min at 27 °C
GF/C 0.3% PEI; Ecoscint A;
TRI-CARB 1600TR
MgSO4, 0.5 mM EDTA and
0.1% ascorbic acid
M) 50 mM Tris–HCl buffer at
h-a2A (Perkin Elmer 6110113)
[3H]-MK912 (0.7 nM)
Metergoline (10
l
60 min at 27 °C
60 min at 27 °C
GF/C 0.3% PEI; Ecoscint A;
TRI-CARB 1600TR
pH 7.4 + 12.5 mM
MgCl2, and 2 mM EDTA
50 mM Tris–HCl buffer at
pH 7.4 + 5 mM MgCl2,
h-D4.2 (Sigma D2439)
[3H]-YM-09151-2 (0.2 nM) Clozapine (10
l
M)
GF/C 0.3% PEI; Ecoscint A;
TRI-CARB 1600TR
5 mM KCl, 1.5 mM CaCl2,
and 5 mM EDTA
r-D3 (Perkin Elmer 6110139)
h-D2L (RBI D-180)
[3H]-Spiperone (0.5 nM)
[3H]-Spiperone (0.2 nM)
Haloperidol (10
l
M) 50 mM Tris–HCl buffer at
pH 7.4 + 10 mM MgCl2,
1 mM EDTA and 120 mM NaCl
M) 50 mM Tris–HCl buffer at pH
7.4 + 10 mM MgCl2
60 min at 27 °C
60 min at 27 °C
GF/C 0.3% PEI; Ecoscint A;
TRI-CARB 1600TR
Haloperidol (10
l
GF/C 0.3% PEI; Ecoscint A;
TRI-CARB 1600TR
and 1 mM EDTA
Table 2
Affinity of piperazine-alkyl-naphthamide derivatives for human cloned D4.2, 5-HT1A, D3,
a
2A, and D2L receptors
N
R
N
N
OMe
n
H
Compound
R
n
D4.2
5-HT1A
D3
66
56 9.9
175 109
58.5 5.0
0.77 0.11
27.5 2.1
13 5.7
a2A
D2L
1
2
3
4
5
6
7
8
9
CO
SO2
CO
SO2
CO
SO2
CO
SO2
CO
SO2
1
1
2
2
3
3
4
4
5
5
2.32 0.66
24.4 1.2
17.8 0.1
112 25
174 25
62 14
1.69 0.18
5.19 0.49
4.59 1.07
4.64 1.49
2.75 0.25
4.1 0.73
4
113 20
348
71 7.4
101
20.2 11.6
79 0.3
9.36 4.73
84 12
96 7.4
890 122
44*
5
46*
1
132 11
30*
46.5 6.4
254 35
438 79
121 6.4
99 17
54 10
11.1 2.5
27.4 2.7
102 29
9.62 2.63
8.53 2.28
6.15 1.92
2.87 0.52
12.5 6.4
0.56 0.19
1.97 0.15
10
45 15.5
*
Ki (in nM, mean SD; n P 2 if unspecified) or percentage of inhibition at 1 lM.
allel to the differing affinity for 5-HT1A receptors. This is most nota-
ble for the ethyl carboxamide 1 and hexyl sulphonamide 10 ana-
logues. Carboxamide analogues displayed higher affinity for D4.2
receptors than sulphonamide analogues, when the linker was
small (ethyl and propyl). However, the opposite was true for longer
chains (butyl to hexyl).
In contrast, the affinity for a2A receptors was enhanced when
the size of the linker increased. In a series of molecules with an
ethyl linker that we recently reported in the literature, the affinity
for a2A receptors was effectively weak.6 The present compounds
D3 receptors. We observed a high affinity for compound 5 at D3
receptors as previously reported, but unlike previous reports13
compound 5 also presented a similar binding affinity to 5-HT1A
receptors. Studies on molecules related to BP897 (compound 5)
have indicated that compounds with an extended and more linear
conformation in the aliphatic or aryl spacer are more selective for
D3 over D2 receptors.14 Complete classification of the affinities of
these compounds at other receptors has yet to be determined,
and the length of the spacer in these studies has been limited to
propyl or butyl.13,14 Similarly, we observed that compounds with
a small linker (ethyl and propyl) displayed reduced interaction
with D3 receptors, when compared with compounds possessing a
longer linker (butyl to hexyl). Moreover, the hexyl analogues 9
and 10 also presented a high affinity for this receptor. Although
the difference is low the carboxamide 9 has more affinity than
the carboxamide 5.
are unlike the
a2A-adrenoceptor ligands recently reported in the
literature, because they lack a guanidine or 2-aminoimidazoline
moiety.10 Nevertheless, a number of piperazine-alkyl-naphtha-
mides are shown to possess a significant affinity for the a2A recep-
tor. Compound 7, with a pentyl moiety, had the highest affinity for
this receptor site. Further increase of the size of the linker is
unfavourable.
In this study we observe that the impact of homology within the
linker is different for the examined receptors. A parabolic curve is
not observed for chain length against affinity for these receptors
Compound 5 is the D3 receptor partial agonist BP897.11,12
Therefore, we examined the interaction of compounds 1–10 with

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M. Résimont, J.-F. Liégeois / Bioorg. Med. Chem. Lett. 20 (2010) 5199–5202
but this relationship is not always found among homologous
series.15
research into more selective ligands is required to explore and clar-
ify the physiology of different neuronal systems.
Although bioisosteric replacement of the carboxamide by a sul-
phonamide strongly modified the spatial orientation of the side
chain as we previously reported,16 the impact of such modification
is differentially observed. In the present work, the presence of this
sulphonamide moiety was favourable for some interactions. The
success of this chemical modification was varied in the literature.
The hypoglycaemic sulfonyl isostere of glybenclamide was found
to be more potent,17 however sulfonyl isosteres of orthopramides
did not interact with D2 and 5-HT2 receptors and lost the prokinet-
ic activity of the reference compounds.16 The sulphonamide moiety
is also frequently found in molecules interacting with a high affin-
ity for 5-HT7 receptors but usually these molecules also possess a
higher 5-HT1A receptor affinity.7,18–20
In conclusion, the systematic nature of the present study gives
an interesting overview of the impact of these two chemical mod-
ifications of piperazine-alkyl-naphthamides on the interactions
with a number of receptors implicated in psychiatric disorders. It
is clear that further investigation of the quantitative structure–
activity relationship (QSAR) of these compounds is required, as
all biological data has been reported by one group6,8,9 this would
provide an improved binding profile of such simple molecules in
the context of developing pharmacological tools. The biological
evaluation will be extended in two directions. Firstly, the determi-
nation of their binding affinity for receptors such as 5-HT2A and
5-HT7 that have been frequently implicated in CNS disorders
should be done. Secondly, since bioisosterism or homologation
might have important effects on functional behaviours, testing of
these compounds in functional assays either in vitro or in electro-
physiological bioassays should be informative. In addition, further
study of different molecules like pentyl or hexyl analogues with a
multi-receptor binding profile should be carried out using behav-
ioural models to detect potential antipsychotic effects. The major
drawback detected during in vitro binding investigations is the im-
pact of lipophilicity in terms of solubility for long linker com-
pounds. The preparation of compounds will need an appropriate
vehicle, so further chemical developments should be taken into ac-
count to limit problems associated with excessive lipophilicity, and
reduce possible excessive fat storage when chronically adminis-
tered. Finally, such data confirmed that several current pharmaco-
logical tools are not as selective as previously believed and further
Acknowledgements
The technical assistance of Y. Abrassart and S. Counerotte for IR
measurements and elemental analyses respectively is gratefully
acknowledged. Supported in part by grants of the Fonds de la
Recherche Scientifique-FNRS (F.R.S.-FNRS) and the ‘Fonds Spéciaux
pour la Recherche’ of the University of Liège (Belgium). J.-F.L is a
Research Director of the F.R.S.-FNRS.
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