N-{4-[4-(2,3-dichlorophenyl)piperazin-1-yl]butyl, butenyl and butynyl}arylcarboxamides as novel dopamine D3 receptor antagonists

Article abstract of DOI:10.1016/S0960-894X(03)00389-5

The dopamine D₃ receptor subtype has been targeted as a potential neurochemical modulator of the behavioral actions of psychomotor stimulants, such as cocaine. Previous synthetic studies provided structural requirements for high affinity bindin

Full text of DOI:10.1016/S0960-894X(03)00389-5

Bioorganic & Medicinal Chemistry Letters 13 (2003) 2179–2183
N-{4-[4-(2,3-Dichlorophenyl)piperazin-1-yl]butyl, Butenyl and
Butynyl}arylcarboxamides as Novel Dopamine D3 Receptor
Antagonists
Amy Hauck Newman,a,* Jianjing Cao,a Christina J. Bennett,a Michael J. Robarge,a,y
b
Rebekah A. Freemanb andRobert R. Luedtke
aMedicinal Chemistry Section, National Institute on Drug Abuse-Intramural Research Program, Baltimore, MD 21224, USA
bDepartment of Pharmacology and Neurosciences, University of North Texas Health Science Center, Fort Worth, TX 76107, USA
Received7 February 2003; accepted28 March 2003
Abstract—The dopamine D3 receptor subtype has been targetedas a potential neurochemical modulator of the behavioral actions
of psychomotor stimulants, such as cocaine. Previous synthetic studies provided structural requirements for high affinity binding to
D3 receptors which included a 2,3-dichloro-phenylpiperazine linked to an arylamido function via a butyl chain. To reduce lipo-
philicity of these agents and further investigate optimal conformation, a second series of 15 novel ligands was designed that inclu-
ded heteroaromatic substitution and unsaturated alkyl linkers. These compounds were synthesized and evaluated for binding at rat
D3 andD 2 receptors stably expressedin Sf9 cells. D 3 binding affinities ranged from Ki=0.6–1080 nM, with a broadrange of D 3/D2
selectivities (2–97). The discovery of potent, selective and bioavailable D3 receptor ligands will provide essential molecular probes to
elucidate the role D3 receptors play in the psychomotor stimulant andreinforcing effects of cocaine.
# 2003 Elsevier Science Ltd. All rights reserved.
The dopamine D3 receptor, a member of the D2 recep-
tor family, resides in brain regions associated with
emotional andcognitive function, such as the nucleus
accumbens (for review see refs 1 and2 ). Efforts to fur-
ther elucidate the function and potential therapeutic
advantages of targeting D3 receptors have resultedin
compounds that may be useful in treating Parkinson’s
disease, schizophrenia and drug abuse.2À4 The discovery
of D3 receptor selective antagonists andpartial agonists
has receivedparticular attention for potential treatment
of cocaine abuse since the compoundBP 897 was first
reportedto block cue-controlledcocaine-seeking in
rats.5 Additional studies with BP 8976 andthe potent
to an aryl amide via a butyl chain (Fig. 1). However, the
2- or 4-substitutedfluorenyl analogues, which gave the
highest D3 binding affinities and selectivities rendered
these molecules highly lipophilic andpossibly limited
their bioavailability.9 In vivo investigation of these
ligands will be required to elucidate mechanisms asso-
ciatedwith reduction in cocaine-seeking behavior. Thus,
improving physical properties of the molecules by redu-
cing lipophilicity andfurther iedntifying structural
modifications that would yield highly potent and selec-
tive D3 ligands, was prioritized. In this pursuit, hetero-
aromatic replacement of the fluorenyl ring system and
conformational optimization by adding unsaturation to
the butyl linker was explored. The target molecules are
shown in Figures 2 and3 .
andD selective antagonist SB-2770117,8 further sup-
3
port the development of D3 selective ligands as potential
cocaine abuse medications.
Synthesis of novel amides 21–35 was achievedas depic-
As we have described9 andothers have more recently
confirmed10À12 optimal D3 receptor binding affinities are
obtainedwhen a 2,3-dichlorophenylpiperazine is linked
tedin
linkedto the
Scheme 1. 2,3-Dichlorophenypiperazine was
N-phthalimido-protected butyl (11),
butenyl (cis; 16a or trans; 16b) or butynyl (19) amines
using standard N-alkylation conditions followed by
deprotection with hydrazine.9 Amidation via the acid
chloride using Schotten–Baumann conditions (Method
A) or directly using CDI (Method B) gave the desired
*Corresponding author. Fax: +1-410-550-1621; e-mail: anewman@
intra.nida.nih.gov
yCurrent address: Celgene Corp., Warren, NJ 07059, USA.
0960-894X/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0960-894X(03)00389-5

2180
A. H. Newman et al. / Bioorg. Med. Chem. Lett. 13 (2003) 2179–2183
Figure 1. Previously synthesizedD 3 ligands.
D3 andD receptor binding affinities for the 15 novel
2
compounds were determined by competitive radioligand
binding experiments using (a) the D2-like receptor
selective radioligand 125I-IABN13 and(b) either rat D
2
or D3 receptors expressedin Sf9 cells using a recombi-
nant baculovirus. The binding methods used are as pre-
viously reported14 andbriefly describedin the
Table 1
legend. Ten previously reported compounds 1–10 and
NGB 2904, shown in Figure 1, were also evaluatedfor
comparison purposes. cLogD values15 are also shown in
Tables 2 and3 .
The binding data in Table 2 revealedthat when the
aromatic ring system contains heteroatoms, high affinity
binding at D3 is retained, while lipophilicity is
decreased, for example compounds 5 and 21 compared
to 23, 25, and 26. Compound 26 demonstrated the
highest D3 binding affinity (Ki=0.6 nM) in this series
andwas 97-foldselective for D
receptors over D2.
3
Recently, similar findings have been reported with other
heteroaromatic substitutions.10,11 However, smaller aryl
ring systems on the saturatedbutyl linker also demon-
strate moderately high affinity binding at D2 receptors,
limiting their utility as selective D3 compounds (e.g., 9
and 10 as comparedto 5). Substitution of a phenyl ring
with the sterically bulky 40-iodo atom in compound 24,
somewhat improvedD 3-selectivity, nevertheless, this
compoundwas still quite lipophilic (cLogD=6.20).
Alternatively, the investigation into structurally more
rigidanalogues obtainedby unsaturation of the butyl
linker ledto a promising lead.
Figure 2. Novel D3 ligands with saturated butyl chain link.
products (21 and 23–35). For compound 22, the 5-
nitrofluorenyl carboxylic acidwas first reducedby cata-
lytic hydrogenation (10% Pd/C) to the intermediate
aniline that was then coupledto amine 13 using DCC
andHOBt (MethodC). All carboxylic acids were either
commercially available or preparedby literature meth-
ods as indicated in Table 1 where physical properties of
all final products are provided.
Whereas the alkyne linker (33–35) servedto significantly
reduce binding affinities at both D3 andD , introduc-
2
tion of a cis, andparticularly a trans, olefin resultedin
retention of high affinity binding at D3 receptors (Table
3). This was particularly true when the lipophilic fluor-
enyl ring system was replacedwith a simple phenyl ring,
where compound 31 showedhigh affinity ( Ki=1.3 nM),
as well as 39-foldselectivity for D over D2 receptors.
3

A. H. Newman et al. / Bioorg. Med. Chem. Lett. 13 (2003) 2179–2183
2181
Figure 3. Structurally rigidD 3 receptor ligands with unsaturated chain link.
Scheme 1. Synthesis of novel D3 ligands.
This modification reduced lipophilicity in this series of
analogues by 2 orders of magnitude (e.g., compounds
27–30 comparedto 31 and 32). In fact, the smaller ring
substitutions in the trans butenyl series, is currently
underway.
system improvedbinidng affinity at D
significantly.
Evaluation of compounds 23–26, 29, 31, and 32 in a
functional assay using stimulation (agonist) or inhibi-
tion of quinpirole stimulation (antagonist) of mitogen-
esis in human D3 transfectedCHO cells is shown in
Table 4. Data to support an antagonist profile for com-
pounds 23, 25, 31 and 32, which was predicted based on
3
Furthermore, for the phenyl-ring substitutedcom-
pounds, 31 comparedto 32, the trans olefin is the pre-
ferredconformation for optimal D 3 binding affinity and
selectivity over D2. This conformational preference is in
accordwith trans-cyclohexamides previously reported
as high affinity D3 ligands.16,17 Investigation into opti-
mal substitution of the phenyl ring as well as heteroaryl
NGB 290418 andother more recently reportedD
3
antagonists having the 2,3-dichlorophenyl moiety10 is

2182
A. H. Newman et al. / Bioorg. Med. Chem. Lett. 13 (2003) 2179–2183
Table 1. Synthetic methods and physical properties of novel D3 ligands
e
CompdMethod
YieldSalt/rec solv.
Mp
ꢀC
Formulac
. .
C28H29N3OCl2 C4H4O4 0.5H2O
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
B
C
B
B
B
Aa
B
10b
81
Fumarate/MeOH
Oxalate/2-PrOH
HCl/MeOH
HCl/2-PrOH
HCl/MeOH/ether
diHCl/2-PrOH
187ꢁ190
140ꢁ142
d271 ꢁ274
227ꢁ229
209ꢁ211
234ꢁ237
187ꢁ190
170ꢁ172
174ꢁ176
176ꢁ178
118ꢁ122
100ꢁ102
180ꢁ181
194ꢁ195
126ꢁ130
.
.
C28H28Cl2N4O 2C2H2O4 2H2O
C23H25N4OFCl2 HCl
46b
25b
20b
40b
87
89
99
94
99
.
.
C21H24N3OCl2I HCl
.
C23H25N5OCl2 HCl
C26H28N4OCl2 2HCl
C28H27Cl2N3O C2H2O4 1.5H2O
.
. .
Oxalate/MeOH/ether
Oxalate/MeOH
Oxalate/MeOH/ether
Oxalate/MeOH
Oxalate/Acetone
HCl/2-PrOH
. .
C28H27Cl2N3O C2H2O4 0.5H2O
B
Bd
Bd
A
A
B
C28H27Cl2N3O C2H2O4 0.25H2O
.
.
. .
C28H27Cl2N3O C2H2O4 0.5H2O
.
C21H23Cl2N3O C2H2O4 0.5H2O
.
.
C21H23Cl2N3O HCl
99
69
.
C28H25Cl2N3O C2H2O4 0.25H2O
.
Oxalate/2-PrOH
Oxalate/MeOH
Oxalate/2-PrOH
Bd
A
96
98
C28H25Cl2N3O C2H2O4
.
.
C21H21Cl2N3O C2H2O4
a4-Pyridin-2-yl benzoic acid.21
b%Yieldis basedon the purifiedsalt, all others basedon crude free base.
cAll compounds were purified through their respective salts and the free bases were characterized using 1H and 13C NMR, FT-IR andGC–MS.
Combustion analysis results agreedto ꢂ0.4% of C, H andN with theoretical values.
d2-Fluorenylcarboxylic acid.9
eMethodA 9; B18; C.22
Table 2. D3 andD 2 receptor binding data and clog D values for non-
rigidmolecules
Table 3. D3 andD 2 receptor binding data and ClogD values for rigid
analogues
CompdD
(nM)ꢂSEMa D2 (nM)ꢂSEMa D2/D3 clog Db
CompdD
(nM)ꢂSEMa
D2 (nM)ꢂSEMa
D2/D3
clog D
3
3
1
2
3
4
5
6
7
8
9
10
21
22
23
24
25
26
84.5ꢂ19
34.0ꢂ11
11.1ꢂ3.8
43.0ꢂ23
1.6ꢂ0.9
2.0ꢂ1.1
139ꢂ5.0
199ꢂ52
4.8ꢂ2.2
5.6ꢂ3.9
4.5ꢂ2.2
3.5ꢂ2.7
2.6ꢂ1.4
1.4ꢂ0.5
1.9 ꢂ0.7
0.6ꢂ0.2
1.1ꢂ0.2
1020ꢂ110
250ꢂ82
12
7
5
4.16
6.08
6.36
6.36
6.64
6.85
6.66
6.66
5.70
5.91
6.95
5.66
5.45
6.20
4.92
5.30
6.94
27
28
29
30
31
32
33
34
35
19ꢂ7.8
6.5ꢂ1.8
5.9ꢂ1.8
6.0ꢂ1.6
1.3ꢂ0.4
4.9ꢂ1.9
1080ꢂ380
214ꢂ63
394ꢂ140
106ꢂ22
10.7ꢂ5
6
1.6
34
15
39
4
1
5
4
7.07
7.07
7.07
7.07
5.13
5.13
7.04
7.04
5.10
55.2ꢂ22
153ꢂ57
198ꢂ39
4
87.3ꢂ19
50.1 ꢂ6
20.0ꢂ1.8
891ꢂ250
1000ꢂ180
1700ꢂ450
150ꢂ20
94
18
3
35.3ꢂ9.6
376ꢂ170
396ꢂ170
102ꢂ33
2
21
15
30
23
32
63
58
97
830
86.4ꢂ20
134ꢂ45
aKi values were obtainedas describedin Table 1.
79.3ꢂ18
83.2ꢂ20
87.5 ꢂ33
109.4ꢂ5.3
57.9 ꢂ5.1
911ꢂ190
Table 4. D3 functional assay using stimulation or inhibition of quin-
pirole stimulation of mitogenesis in CHO cells (hD3)
NGB 2904
CompdAgonist EC
%Max
Stim.ꢂSEMa
Antagonist IC50
(nM)ꢂSEMa
50
(nM)ꢂSEMa
aThe methods for the binding assays have been previously described.14
Ki values are the mean of at least three independent determinations.
23
24
25
26
29
31
32
>10,000
31.7ꢂ10
>10,000
6.31ꢂ1.7
173ꢂ9.6
>10,000
>10,000
—
30.0ꢂ0.1
—
29.7ꢂ0.2
44.1ꢂ0.9
—
52.4ꢂ1.2
The IC50 values obtainedfrom competition experiments were con-
23
—
vertedto Ki values using the Cheng andPrusoff correction.
The
26.9ꢂ7.7
—
—
radioligand used for the competitive radioligand binding studies was
125I-IABN. A recombinant baculovirus (Bv) expression system was
usedto express either rat D 2 (BvD2) or rat D3 (BvD3) receptors in Sf9
cells. Competition curves were modeled as a one site fit using the
7.72ꢂ1.6
—
6.00ꢂ0.59
TABLECURVE program. Human D3 andD receptor binding in
2
CHO cells has previously been reportedfor compounds 1–10 and
NGB 2904.9
bSee ref 15.
aThese data were obtained through the service of CTDP, Division of
Treatment Research andDevelopment, NIDA, using a contract
(N01DA-1-8816) protocol.
shown. The trans olefins are particularly potent antago-
nists in this assay with IC50 values in the low nanomolar
range. Nevertheless, compounds 24, 26, and 29 show a
partial agonist profile, which is consistent with the 2-
methoxy phenylpiperazine compoundBP 897 that was
initially testedin a similar in vitro model of D 3 receptor
function.5 However, subsequently BP 897 was shown to
have an antagonist profile in other in vitro models of D3
function.19,20 Although the initial report on BP 897
suggestedthat D 3 partial agonists wouldbe predictedto
block cocaine-seeking, recent reports have shown that
the potent andselective D antagonist SB-277011
showedremarkable inhibition of cocaine seeking and
3
8
cocaine-enhancedbrain rewardin rat.
These studies
point to potential inconsistencies between in vitro and
in vivo models of D3 receptor function andsupport
further investigation of novel D3 compounds in vivo.
Hence, the development of potent and selective D3
receptor ligands, such as compounds 26 and 31, will
provide the pharmacological tools to clarify these

A. H. Newman et al. / Bioorg. Med. Chem. Lett. 13 (2003) 2179–2183
2183
mechanistic questions andrelate chemical structure, D
receptor function andbehavior in models of cocaine
abuse.
8. Vorel, S. R.; Ashby, C. R.; Paul, M.; Liu, X. H.; Hayes, R.;
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9. Robarge, M. J.; Husbands, S. M.; Kieltyka, A.; Brodbeck,
R.; Thurkauf, A.; Newman, A. H. J. Med. Chem. 2001, 44,
3175.
10. Bettinetti, L.; Schlotter, K.; Hubner, H.; Gmeiner, P. J.
Med. Chem. 2002, 45, 4594.
11. Leopoldo, M.; Berardi, F.; Colabufo, N. A.; De Giorgio,
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3
Acknowledgements
C.J.B. andM.J.R. were supportedby NIH Intramural
Research Program fellowships. The authors acknowl-
edge the support and helpful discussions with Dr.
Jonathan Javitch. We also acknowledge Dr. Peter
Grundt for critically reading an earlier version of this
manuscript. This work was funded by the National
Institute on Drug Abuse-Intramural Research Program.
15. Calculatedpartition coefficient at physiological pH 7.4;
ACD/LogD Suite, Advanced Chemistry Development Inc.:
Toronto, Canada.
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D. J.; Georgic, L. M.; Zoski, K. T.; Akunne, H. C.; Wise, L. D.
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