Synthesis and evaluation of amides surrogates of dopamine D3 receptor ligands

Article abstract of DOI:10.1016/j.bmcl.2010.07.096

Isosteric replacement of the amide function and modulation of the arylpiperazine moiety of known dopamine D3 receptor ligands led to potent and selective compounds. Enhanced bioavailability and preferential brain distribution make compound 6c a good candi

Full text of DOI:10.1016/j.bmcl.2010.07.096

Bioorganic & Medicinal Chemistry Letters 20 (2010) 5376–5379
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
BMCL Digest
Synthesis and evaluation of amides surrogates of dopamine D3 receptor ligands
Mickaël Jean a, Jacques Renault a, Nicolas Levoin b, Denis Danvy b, , Thierry Calmels b,
Isabelle Berrebi-Bertrand b, Philippe Robert b, J. C. Schwartz b, J. M. Lecomte b, Philippe Uriac a, Marc Capet b,
*
a UPRES EA 4090, Substances Lichéniques et Photoprotection, Université de Rennes I, Faculté des Sciences Biologiques et Médicales, 2, Av. du Pr. Léon Bernard, 35043 Rennes, France
b Bioprojet-Biotech, 4 rue du Chesnay Beauregard, BP 96205, 35762 Saint-Grégoire, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Isosteric replacement of the amide function and modulation of the arylpiperazine moiety of known dopa-
mine D3 receptor ligands led to potent and selective compounds. Enhanced bioavailability and preferen-
tial brain distribution make compound 6c a good candidate for pharmacological and clinical evaluation.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 24 June 2010
Revised 21 July 2010
Accepted 23 July 2010
Available online 29 July 2010
Keywords:
Dopamine D3 receptor ligand
Amide isoster
Selectivity
Since its discovery in 1990,1 the dopamine D3 receptor (D3R)
has been widely studied. Its localization in the limbic area of the
brain2 as well as the early investigations with agonists3 or antago-
nists4 led to the current view that the therapeutic use of such com-
pounds should be directed toward drug abuse. More recent studies
have shown that therapeutic potential uses could lie in other neu-
rological and neuropsychiatric disorders.5 Hence, there are at least
two new chemical entities currently in phase I development: ABT-
614 and GSK618334, planned to be used for alcoholism, compul-
sive disorder and substance abuse.
Isosteric replacement of the aromatic amide with pyrimidone
(A-690344)12 or incorporation into a benzodiazepinedione (A-
706149)13 or a benzolactam14 has been reported (Fig. 2), but the
effect of these substitutions on
a1R affinity was not reported.
N
O
N
O
N
H
BP 897
BP 8974 (Fig. 1) is the first representative of potent selective
antagonists or partial agonists for this receptor. It has represented
a prototype for further analogs by many groups. The structural
modifications addressed all parts of the molecule: modification of
the naphthyl part, homologation, and rigidification of the linker,
substitution of the arylpiperazine and its replacement with other
heterocycles.
Modification of the aromatic amide has mainly focused toward
its replacement with other polyaromatics such as fluorene in
NGB29046 and biphenyl in GR103691,7 or heteroaromatics like
benzothiophene in FAUC3468 or benzofurane in A9 and, even, large
ferrocenyl derivatives.10 Aromatic amides bearing an ortho-
methoxyphenylpiperazine have been found to be generally partial
agonists.11 Such substitution however is also linked to strong affin-
Figure 1. Structure of BP 897.
N
O
Cl
O
N
O
N
Cl
N
O
N
N
H
H
NGB2904
GR 103691
O
N
Cl
O
O
N
N
O
N
Cl
O
S
N
N
H
H
FAUC349
A
N
N
ity for adrenergic
a1 receptor (a1R), which represents a serious
F
F
N
N
O
N
N
drawback for clinical candidates.
F
N
F
N
O
F
N
F
N
N
O
* Corresponding author. Tel.: +33 299280440; fax: +33 299380444.
E-mail address: m.capet@bioprojet.com (M. Capet).
HO
A-706149
A-690344
Present address: Interor, IRCOF, UMR 6014 CNRS LFAOC, Faculté des Sciences et
Techniques, rue Tesnières, 76821 Mont Saint-Aignan cédex, France.
Figure 2. Structure of D3R ligands derived from BP 897.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.07.096

M. Jean et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5376–5379
5377
These first two compounds showed a modest affinity for D3
receptor16 (1a 19 nM, 1b 52 nM) with a 20-fold selectivity over
D2 receptor. Although, their D3R affinity is clearly not sufficient,
the initial isosteric replacement was shown to be valid. We thus
turned toward other ring size incorporating the amidine ring.
A second set of compounds was synthesized with pyridine as
heteroaromatics. Substituents on the arylpiperazine were ortho-
methoxy (as in BP 897, GR103691, and FAUC349), ortho,meta-di-
chloro (as in NGB2904 and benzofurane A). We also added meta-
trifluoromethyl (strong electrowithdrawing substituent) and
ortho-fluoro (methoxy surrogate) as further variations.
Synthesis was achieved by condensing 2-chloropyridine
derivative onto appropriate 4-(4-arylpiperazinyl)butylamines
(scheme 4).17
Binding experiments (see Table 1) showed that substitutions
with methoxy (2a) or dichloro (2b) lead to potent ligands with
subnanomolar affinity at dopamine D3 receptor. Modulation of
Figure 3. Parmacophore representation of D3 receptor ligands. Grey spheres
represent lipophilic groups, black sphere is for positive ionisable function, and the
vector symbolizes the hydrogen bond acceptor.
The current pharmacophore15 for D3 ligands is depicted in Fig-
ure 3 with BP 897.
D2 receptor recognition was observed, but selectivity for
a1 was
not reached. Replacement of ortho-methoxy by ortho-fluoro (2d)
did not improve this last parameter. Further replacement with
methyl (2e–f) showed that hydrogen or halogen bonding is not
implicated in adrenergic activity as it is for D3R. Methylated com-
pound cannot build hydrogen bond, but showed an improvement
As its congeners, BP 897 has a potent activity toward human
a1R (Ki = 9 nM). Thus we tackled this aspect of D3R ligands, while
maintaining selectivity versus D2 receptor. Rigidification is a well-
known strategy to improve selectivity, but it was shown detrimen-
tal for D3R affinity when applied to the linker.15a We thus turned
toward incorporating the hydrogen bond acceptor and the aro-
matic moiety into an heterocycle. The rotatable bond between
naphthyl and carbonyl thus disappears. Moreover, rigidification
can improve bioavailability and suppression of the hydrophile
amide bond should be beneficial to brain permeation.
in the
a1R binding. Interestingly, meta-trifluoromethylation (2c)
strongly diminished binding to
a1 receptor, though affinity for
D3R was insufficient.
We then turned toward extending the aromatic part of the pyr-
idine. This approach led to quinoleine (3a–c) or isoquinoleine
A first set of compounds was synthesized with a benzo[f]aze-
pine as replacement of the naphthamide part of BP 897. For com-
parison purpose, we prepared compounds bearing the same
chain as BP 897 and a fluoro-substituted analog as fluorine is sup-
posed to be able to get involved in hydrogen bonding.
X
X
ArX
N
N
N
Ar
neat
rfx
N
N
H2N
H
The compounds were prepared by reacting 2-chlorobenzo
[f]azepine with 4-arylpiperazinebutanamines (Scheme 1).
Starting 2-chlorobenzo[f]azepine was prepared in two steps
from tetralone oxime: Beckman rearrangement and chlorination
with phosphoryl chloride (Scheme 2).
The 4-arylpiperazinebutanamines were prepared by reacting
the corresponding substituted arylpiperazine with 4-bromobuty-
ronitrile and reducing the nitrile with Raney nickel (Scheme 3).
Scheme 4. Synthesis of 4-(4-arylpiperazinyl)butylaminoheterocycles.
Table 1
Binding and selectivity ratio of compounds 2–4 a–f at dopamine D2 and D3 receptors
and adrenergic 1 receptor
a
X
N
N
Ar
N
H
Entry
Ar
X
Ki (nM)
Ratio
D2/D3 a1/D3
Cl
N
N
N
N
X
D2
D3
a
1
N
X
N
N
H2N
neat
rfx
H
2a
2b
2c
2d
2e
2f
3a
3b
3c
4a
4b
4c
2-Pyridyl
2-Pyridyl
2-Pyridyl
2-Pyridyl
2-Pyridyl
2-Pyridyl
2-Quinolyl
2-Quinolyl
2-Quinolyl
1-Isoquinolyl
1-Isoquinolyl
1-Isoquinolyl
2-OMe
2,3-Cl2
3-CF3
2-F
2-Me
2,3-Me2
2-OMe
2,3-Cl2
3-CF3
2-OMe
2,3-Cl2
3-CF3
198
25
0.8
0.4
17
11
32
23
33
593
17
14
40
247
62
12
12
12
17
29
84
35
1.5
0.4
3
1a X=OMe
1b X=F
198
133
383
207
15
23
53
70
19
Scheme 1. Synthesis of 4-(4-arylpiperazinyl)butylaminoheterocycles.
12
5.6
0.6
1.6
1.0
3.1
1.0
6.0
2.6
1.1
46
12
39
33
70
77
7.5
6.3
OH
H
Cl
N
O
POCl3
N
N
6.0
60
68
6.2
58
19
68
58
Scheme 2. Synthesis of 2-chlorobenzo[f]azepine.
Br(CH2)3CN
KI cat
H2
K2CO3
3 bar
X
X
X
N
N
N
N
CH3CN
rfx, 1 night
HN
N
N
NH3
EtOH
30ºC, 3 hr
H2N
Scheme 3. Synthesis of 4-(4-arylpiperazinyl)butylamines.

5378
M. Jean et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5376–5379
(4a–c), these bicyclic aromatics being reminiscent of the naphthyl
part of BP 897. Here again, ortho-methoxylated derivatives (3a and
4a) showed adrenergic affinity. Extension of the heteroaromatic
part allowed wider substitution onto the arylpiperazine as prod-
ucts substituted with meta-trifluoromethyl (3c and 4c) displayed
nanomolar affinity. Unfortunately, all these compounds were less
2250
2000
1750
1500
1250
1000
750
Oral route
Intravenous route
selective toward D2R and a1R.
The precise location of the aromatic part is thought to be a key
factor for both D2R and D3R recognition.15a Therefore this param-
eter had to be investigated in order to get a good selectivity for D3R
over D2R. We thus turned toward compounds in which the exten-
sion of the aromatic part is realized through a phenyl substituent
appended onto the pyridine.
500
250
These compounds were synthesized using the same strategy as
for pyridine derivatives. Pyridine bearing a phenyl in position 5
showed subnanomolar affinity for dopamine D3 receptor
0
0
5
10
15
20
25
Time (hr)
(Table 2). Interestingly, selectivity over D2R and
a1R was im-
Figure 4. Absolute bioavailability of 6c in mice plasma administered at 10 mg/kg
dose po and iv.
proved, but only in a sufficient manner for the meta-trifluorome-
thylated arylpiperazine. Further improvement was reached when
phenyl was located on position 4.
Isosteric replacement of the pyridine nucleus with oxazole and
thiazole has also been investigated leading to compounds 7 and 8.
Interestingly, selectivity over dopamine D2 receptor is retained
regardless the observed decrease of affinity for dopamine D3
receptor (Table 3).
Compound 6c was shown to be a potent and selective dopamine
D3R ligand. Furthermore, this compound showed no strong inter-
action with human cytochromes. The bioavailability and distribu-
tion of this compound were further investigated in mice (Fig. 4)
and rat (Fig. 5). Absolute bioavailability was rather satisfactory
(0.40 in mice and 0.65 in rat). Compared to other investigated or-
gans, brain exposure was particularly high (Fig. 6). This is clearly
a major advantage for this CNS-oriented class of molecules.
Rigidification of the amide part of BP 897 by incorporation into
a heterocycle has led to a new scaffold. Further optimization of the
1000
900
800
700
600
500
Oral route
Intravenous route
400
300
200
100
0
0
5
10
15
20
25
Time (hr)
Table 2
Binding and selectivity ratio of compounds 5 and 6 a–c at dopamine D2, dopamine D3
Figure 5. Absolute bioavailability of 6c in rat plasma administered at 10 mg/kg
and adrenergic
a1 receptors
dose po and 3 mg/kg iv.
X
N
N
Ph
N
7000
6000
N
H
Entry
Ar
X
Ki (nM)
Ratio
D2/D3
D2
D3
a
1
a
1/D3
4
5000
Plasma
Brain
Heart
5a
5b
5c
6a
6b
6c
5-Phenyl
5-Phenyl
5-Phenyl
4-Phenyl
4-Phenyl
4-Phenyl
2-F
122
41
143
309
184
194
2.5
0.59
1.1
1.5
0.55
0.76
10
49
69
130
206
334
256
2,3-Cl2
3-CF3
2-F
2,3-Cl2
3-CF3
21
227
9
58
119
35
207
6
4000
3000
2000
1000
0
106
157
Table 3
Binding and selectivity ratio of compounds 7 and 8 at dopamine D2, dopamine D3 and
adrenergic
a
1 receptors
0
5
10
15
20
25
Time (hr)
F
F
N
N
F
N
N
F
N
7
F
Figure 6. Tissue distribution of 6c in mice administered at 10 mg/kg po.
N
8
F
O
N
S
N
H
H
aromatic part together with the modulation of the substitution
onto the arylpiperazine led to potent and selective compounds.
As expected, the overall reduction in aqueous solubility of the mol-
ecules had enhanced both bioavailability and brain permeation.
The optimized compound 6c is a very potent D3 ligand which set-
Entry
Ki (nM)
Ratio
D2
D3
a
1
D2/D3
a
1/D3
28
5.4
7
8
2322
2431
9.6
13
272
72
242
183

M. Jean et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5376–5379
16. Determination of dopamine receptor subtype and adrenergic
5379
a
1 receptor binding:
tled D2R and a1R selectivity (Ki ratio of 250 and 150, respectively).
Biochemical assays for the human D3 and human D2S dopamine receptors
It represents thus a good candidate for further pharmacological
were performed as described previously with slight modifications.15a Briefly,
for the D3 receptor, binding was performed with [3H] spiperone using 5
lg of
and clinical investigations.
membrane suspension (prepared from stable human D3 transfected CHO cell
line). Binding of the D2S receptor was performed with [3H] spiperone using
Acknowledgments
10
HEK293 cell line). For the human adrenergic
with [3H] prazosin using 5
g of membrane suspension (prepared from stable
l
g of membrane suspension (prepared from stable human D2S transfected
a1 receptor, binding was achieved
l
The authors gratefully acknowledge Jean-Claude Camelin,
Myriam Le Roch, Marcel Morvan, and Benoît Messager for technical
assistance.
human hADRA1A transfected cell line). All assays were performed at least in
duplicate. Mean values are given.
17. Step A: preparation of 4-[4-(3-trifluoromethylphenyl)piperazin-1-yl]butyro-
nitrile.
A mixture of 12.02 g (52 mmol) of 1-(3-trifluoromethylphenyl)
piperazine, 7.9 g (57 mmol) of potassium carbonate, 8.5 g (57 mmol) of 4-
bromobutyronitrile and 120 mL of acetonitrile was refluxed overnight. The
reaction medium was concentrated, taken up with ethyl acetate and washed
with water. After separation of the aqueous phase, the organic phase was dried
over magnesium sulfate, filtered and concentrated under reduced pressure. In
this manner, 15 g (97%) of 4-[4-(3-trifluoromethylphenyl)piperazin-1-
yl]butyronitrile were obtained as a viscous oil which was used without any
further purification.
References and notes
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Schwartz, J. C.; Everitt, B. J.; Sokoloff, P. Nature (London, U.K.) 1999, 400, 371.
5. Luedtke, R. R.; Mach, R. H. Curr. Pharm. Des. 2003, 9, 643.
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12. (a) Geneste, H.; Backfisch, G.; Braje, W.; Delzer, J.; Haupt, A.; Hutchins, C. W.;
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L.; Wernet, W. Bioorg. Med. Chem. Lett. 2006, 16, 1934.
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Step B: preparation of 4-[4-(3-trifluoromethylphenyl)piperazin-1-yl]butyl-
amine. Approximately 1 g of Raney nickel washed beforehand with ethanol
was added to
a solution of 15 g (37.4 mmol) of 4-[4-(3-trifluorophenyl)
piperazin-1-yl]butyronitrile obtained previously, in a mixture of 100 mL of an
aqueous solution of concentrated ammonia and 100 mL of an approx. 8 N
solution of ammoniacal ethanol. The suspension was hydrogenated overnight
under 3 bar of hydrogen at 30 °C. The mixture was filtered on Celite, rinsed with
ethanol and concentrated under a vacuum. The oily residue was taken up with
50 mL of ethanol and concentrated. This operation was repeated once to afford
14 g (92%) de 4-[4-(3-trifluorophenyl)piperazin-1-yl]butylamine as a viscous oil
which was used without any further purification.
Step C: preparation of 2-{4-[4-(3-trifluoromethylphenyl)piperazin-1-yl]butyl}
amino-4-phenylpyridine. In
a test tube, 0.38 g (2.0 mmol) of 2-chloro-4-
phenylpyridine, 0.6 g (2.0 mmol) of 4-[4-(3-trifluoromethylphenyl)piperazin-
1-yl]butylamine and a spatula tipful of 4-dimethylaminopyridine were heated
to approximately 300–350 °C for 3 min. The mixture was diluted with ethyl
acetate and chromatographed on silica gel (eluent: dichloromethane/ethanol
90:10). The product crystallized upon concentration of collected fractions.
Trituration in diethyl oxide, filtration and drying gave 80 mg of 2-{4-[4-(3-
trifluoromethylphenyl)piperazin-1-yl]butyl}amino-4-phenylpyridine as
a
white solid melting at 120 °C (tube).
1H NMR (CDCl3): 8.1 (doublet, 1H); 7.65–7.55 (unresolved peaks, 2H); 7.5–7.25
(unresolved peaks, 4H); 7.2–7.0 (unresolved peaks, 3H); 6.8 (doublet, 1H); 6.55
(singlet, 1H); 4.8 (wide triplet, 1H); 3.35 (multiplet, 2H); 3.35–3.15 (unresolved
peaks, 4H); 2.7–2.55 (unresolved peaks, 4H); 2.45 (triplet, 2H); 1.85–1.65
(unresolved peaks, 4H).