Synthesis, pharmacological evaluation and molecular modeling studies of triazole containing dopamine D3 receptor ligands

Article abstract of DOI:10.1016/j.bmcl.2014.12.023

A series of 2-methoxyphenyl piperazine analogues containing a triazole ring were synthesized and their in vitro binding affinities at human dopamine D₂ and D₃ receptors were evaluated. Compounds 5b, 5c, 5d, and 4g, demonstrate high a

Full text of DOI:10.1016/j.bmcl.2014.12.023

Bioorganic & Medicinal Chemistry Letters 25 (2015) 519–523
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis, pharmacological evaluation and molecular modeling
studies of triazole containing dopamine D3 receptor ligands
Xin Peng a, Qi Wang a, Yogesh Mishra d, Jinbin Xu a, David E. Reichert a, Maninder Malik d, Michelle Taylor d,
Robert R. Luedtke d, Robert H. Mach a,b,c,
⇑
a Department of Radiology, Washington University School of Medicine, St. Louis, MO 63110, United States
b Department of Cell Biology & Physiology, Washington University School of Medicine, St. Louis, MO 63110, United States
c Department of Biochemistry & Molecular Biophysics, Washington University School of Medicine, St. Louis, MO 63110, United States
d Department of Pharmacology and Neuroscience, University of North Texas Health Science Center, Fort Worth, TX 76107, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 2-methoxyphenyl piperazine analogues containing a triazole ring were synthesized and their
in vitro binding affinities at human dopamine D2 and D3 receptors were evaluated. Compounds 5b, 5c, 5d,
and 4g, demonstrate high affinity for dopamine D3 receptors and moderate selectivity for the dopamine
D3 versus D2 receptor subtypes. To further examine their potential as therapeutic agents, their intrinsic
efficacy at both D2 and D3 receptors was determined using a forskolin-dependent adenylyl cyclase inhi-
bition assay. Affinity at dopamine D4 and serotonin 5-HT1A receptors was also determined. In addition,
information from previous molecular modeling studies of the binding of a panel of 163 structurally-
related benzamide analogues at dopamine D2 and D3 receptors was applied to this series of compounds.
The results of the modeling studies were consistent with our previous experimental data. More impor-
tantly, the modeling study results explained why the replacement of the amide linkage with the het-
ero-aromatic ring leads to a reduction in the affinity of these compounds at D3 receptors.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 7 November 2014
Accepted 9 December 2014
Available online 17 December 2014
Keywords:
Dopamine receptors
Dopamine D3 receptor selective ligands
Triazole
Molecular modeling
Dopamine is a neurotransmitter that plays a number of impor-
tant roles in the central nervous system (CNS) by activating both
presynaptic and postsynaptic dopamine receptors. Dopamine
receptors belong to the family of seven transmembrane domain
G-protein coupled receptors (GPCRs) and are classified into two
types: D1-like and D2-like receptors. D1-like receptors consist of
D1 and D5 receptors, and D2-like receptors consist of D2, D3 and
D4 receptors. Agonist activation of D1-like receptors leads to an
increase of adenylyl cyclase activity, whereas activation of D2-like
receptors leads to adenylyl cyclase inhibition.1
human cocaine overdose victims when compared with age-
matched controls.2 In addition, several reports suggest that D3
receptor antagonists and partial agonists attenuate cocaine’s
rewarding effects.3–5 An up-regulation of D3 receptors has been
found in animal models of dyskinesia6–8 and D3 receptor selective
compounds were found to attenuate
L-DOPA-induced dyskinetic-
like involuntary movements in hemi-parkinsonian rats, suggesting
that D3 receptors could be therapeutic targets for L-DOPA-induced
dyskinesia.9 Therefore, highly selective D3 receptor ligands repre-
sent potential pharmacotherapeutic agents for the treatment of
D3-related CNS disorders.
Experimental evidence has suggested that changes in the
expression of D3 receptors may be associated with several CNS dis-
eases and behavioral disorders, including psychostimulant abuse.
For instance, increased CNS D3 receptor expression was found in
There is substantial amino acid sequence homology between
the transmembrane helices of the D2 and D3 receptors (78% homol-
ogy),10 which makes it difficult to develop highly selective D3
receptor ligands. Our group has developed several benzamide ana-
logues as D3-selective ligands as candidates including (Fig. 1): WC
10 and WC 44.11 Tritium-labeled WC 10 has been used for
quantitative autoradiography studies.12
In this work, we explored new synthons as replacements for
the amide linkage in benzamide based D3 receptor ligands.
Five-membered heterocycle ring, such as triazole, has been
reported to represent an isosteric substitution of an amide
Abbreviations: 3D-QSAR models, three dimensional quantitative structure–
activity relationship models; CoMSIA1, Comparative Molecular Similarity Index
Analysis; EL1, extracellular loop 1; EL2, extracellular loop 2; GPCR, G-protein
coupled receptor; RMSD, Root Mean-Square Deviation; SAR, structure–activity
relationship; TM, transmembrane.
⇑
Corresponding author at present address: University of Pennsylvania, Chemis-
try Building, Room 283, 231 S. 34th St., Philadelphia, PA 19104, United States.
E-mail address: rmach@mail.med.upenn.edu (R.H. Mach).
http://dx.doi.org/10.1016/j.bmcl.2014.12.023
0960-894X/Ó 2014 Elsevier Ltd. All rights reserved.

520
X. Peng et al. / Bioorg. Med. Chem. Lett. 25 (2015) 519–523
F
N
H
N
H
N
N
N
N
O
O
N
O
O
WC 10
WC 44
D
D
3 = 0.8 0.1 nM
2 = 34.4 3.3 nM
D
D
3 = 2.4 0.4 nM
2 = 54.5 4.4 nM
D2/D3 = 43
D2/D3 = 23
Figure 1. Structure and D2/D3 receptor binding data for compounds WC 10 and WC 44.
linkage.13,14 In addition, the hetero atoms in the triazole ring may
participate in hydrogen bonding and dipole–dipole interactions
with receptors,14,15 which could result in a more favorable receptor
binding affinity. Gmeiner et al. first synthesized a series of
analogues with 1,2,3-triazole linkers as ligands for GPCR recep-
tors.16 Micheli et al. reported 1,2,4-triazol-3-yl-thiopropyl-tetra-
hydrobenzazepines as D3 antagonists.17 Carro et al. has explored
1,4-disubstituted triazole analogues as D3 ligands.18 We have syn-
thesized a series of 1,2,3-triazole based D3 receptor ligands with
reversed substitution pattern from Carro et al.’s Letter. The aryl
substitutions are on N1 position of the triazole in our series of
ligands. Moreover, we explored varied carbon linkages in conjunc-
tion with triazole moiety. This Letter reports the synthesis, evalu-
ation, and SAR studies of 1,2,3-triazole based D3 receptor ligands
and molecular modeling studies with these ligands.
the presence of sodium nitrite. The transformation of diazoniums
to azides was then achieved by treating with sodium azide, using
sodium acetate to neutralize the excess HCl.
The substituted 4-phenypiperazinyl aryl-1-ynes with different
carbon spacers were synthesized by N-alkylation of 1-(2-meth-
oxylphenyl)-piperazine and corresponding tosylates, using aceto-
nitrile as the solvent and sodium carbonate as the base. Tosylates
were converted from corresponding substituted alcohols by
stirring with p-TsCl and triethylamine in dichloromethane.
The target triazole analogues were synthesized by click
reactions using classic Cu(I)-catalyzed conditions reported by
Fokin et al.21
The triazole analogues were evaluated for affinity at human D2
and D3 dopamine receptors expressed in stably transfected HEK
cells (Table 1) using a competitive inhibition binding assay, which
was previously reported.22 The ligand binding selectivity is
calculated as Ki (D2)/Ki (D3). Analogues that exhibited >10-fold
binding selectivity for dopamine D3 receptors compared to the
D2 receptor subtype were further evaluated for (a) affinity at D4
dopamine receptors and 5-HT1A serotonin receptors; (b) intrinsic
efficacy at D2 and D3 receptors using a forskolin-dependent aden-
ylyl cyclase inhibition assay.
The radioligand binding analysis identified several triazole ana-
logues with P10-fold selectivity for D3 versus D2 receptors includ-
ing compounds 4b, 5b, 3c, 4c, 5c, 4f, 4g, and 5g. Among these
compounds, 4b, 5b, 4g, and 5g have the highest affinity, with Ki
values for the D3 receptor 64 nM.
Compound 4a, which is the reversed substituted analogue of
compound 15 in Carro et al.’s Letter,18 shows a much higher affin-
ity at D3 receptor and higher D2/D3 selectivity compared to 15
(Fig. 2).
These analogues were evaluated using in vitro receptor binding
assays to determine the binding affinity and selectivity at D2 and
D3 receptors. Additional assays, such as dopamine D4, 5-HT1A and
the intrinsic efficacy of selected ligands at D2 and D3 receptors
were also conducted at D3-selective compounds.
Our group had previously generated comprehensive three
dimensional quantitative structure–activity relationship (3D-QSAR)
models for a set of 163 D2/D3 benzamide compounds based on
homology models of the D2 and D3 receptors.19 The homology model
of the D3 receptor is consistent with the D3 X-ray crystal structure20
(PDB code: 3PBL) (RMSD of the model to the X-ray structure
is 2.88 Å, which is lower than the resolution of the
X-ray structure). The 3D-QSAR models for the compound library also
showed excellent correlation and predictive power. Thus, we used
the new series of triazole phenylpiperazine analogues as another
external test set to evaluate the accuracy of our QSAR models. More-
over, QSAR has provided insights on the different binding mode of
this series of ligands versus our previous benzamide ligands.
The target compounds were synthesized using click chemistry
as depicted in Scheme 1. Anilines were first converted to the
corresponding diazoniums using 1:1 HCl/H2O as the solvent, in
Another observation from the experimental data is that varying
the length of the carbon spacer group while maintaining the same
aromatic substituents changes both D2 and D3 receptor affinity. It
was found that triazole analogues with a three-carbon spacer
generally exhibits lower D2 and D3 affinity than their four or
a
N3
H2N
Ar
1a 1h
Ar
-
c
b
N
N [spacer]
HO [spacer]
TsO [spacer]
O
a
2a s
( =3),
2b s
( =4),
2d s
( =5)
2e s 2f
( =3),
s
2g s
spacer(s)=3,4,5
( =4), ( =5)
Ar
d
N
N
N
N
[spacer]
N
O
Triazoles
Scheme 1. Synthesis of triazole analogues. a Synthesis of 2d is different from that of 2a and 2b. The procedure is described in Supplementary information. Reagents and
conditions: (a) 1:1 HCl, NaNO2; CH3COONa, NaN3. (b) p-TsCl, Et3N, CH2Cl2, 0 °C. (c) 1-(2-Methoxylphenyl)piperazine, NaCO3, CH3CN, reflux. (d) Azides (1a–1h), 5 mol % CuSO4,
10 mol % sodium ascorbate, 1:1 tert-butanol/H2O.

X. Peng et al. / Bioorg. Med. Chem. Lett. 25 (2015) 519–523
521
Table 1
D2, D3, D3, 5-HT1A affinities and LogD values of triazole analogues
Ar
Carbon spacer
Compound number
D2 (Ki nM)
D3 (Ki nM)
D2/D3 ratio
D4 (Ki nM)
D4/D3 ratio
5-HT1A (Ki nM)
LogDb
WC 10
WC 44
34.4 3.3
54.5 4.4
0.8 0.1
2.4 0.4
43
23
896 193
804 33
1120
335
7.5
2.4
3.1
2.9
3
4
3a
4a
64.0 11.4
30.9 3.9
22.2 3.8
4.7 0.4
2.9
6.6
3.8
3.9
5
5a
19.4 2.5
3.1 0.2
6.3
4.7
3
4
5
3b
4b
5b
79.3 5.3
42.3 5.2
17.7 0.7
36.3 1.3
3.7 0.5
1.8 0.3
2.2
11.5
9.8
3.5
3.7
4.4
875 117
186 23
237
103
6.0 1.3
5.8 1.5
N
3
4
3c
4c
158 15
94.3 12.7
8.9 2.1
8.2 1.8
17.7
11.6
90 19
395 13
10
48
7.4 0.5
1.3 0.2
5.2
5.4
S
5
5c
109 19
6.3 1.2
17.3
163 10
26
153 12
6.1
3
4
3d
4d
73.5
46.7 1.2
9
26.5 1.1
6.5 0.7
2.8
7.2
3.8
4.0
F
5
5d
23.7 4.2
3.2 0.5
7.4
4.8
3
4
3e
4e
272 42
93.1 8.8
25.4 2.3
2.9
5.5
3.7
3.8
MeO
139
3
5
5e
62.7 7.6
21.1 1.9
3.0
4.6
3
4
3f
4f
162 13
171 13
29.6 5.2
13.9 1.9
5.5
12.4
3.9
4.0
N
758 23
55
9.4 1.1
5
5f
62.6 6.1
6.7 0.6
9.3
4.8
3
4
5
3g
4g
5g
44.1 2.0
37.7 4.2
26.6 3.6
9.6 2.3
2.1 0.3
2.3 0.3
4.6
18.1
11.6
4.2
4.4
5.2
SMe
200 26
227 14
95
99
5.3 0.6
3.9 0.9
3
4
3h
4h
12.8 1.7
9.4 1.1
6.8 0.84
2.9 0.6
1.9
3.2
3.0
3.1
HO
b
Calculated using ACD logD software, Advanced Chemistry Development, Toronto, Canada. All Ki values for the affinity at D2 and D3 receptors are reported as nanomolar
values and are the mean value SEM for n > 3.
Ar
N
N
N
N [spacer]
N
O
N
N
N
4a
WC 10 (Ki D3 = 0.8 nM, D2/D3 = 43) versus the triazole analogue
4f (Ki D3 = 13.9 nM, D2/D3 = 12).
N
D3 = 4.7 nM
2 = 30.9 nM
D2/D3 = 6.9
N
N
D
N
O
Compounds 4b, 5b, 3c, 4c, 5c, 4f, 4g, and 5g were selected for
further binding assays because their selectivity for the D3 versus
the D2 receptor P10-fold. The affinity of these compounds was
found to be low at the D4 dopamine receptor subtype. Additionally,
as we previously reported, D3-selective benzamide analogues can
exhibit high binding affinity at 5-HT1A receptors.11 Unfortunately,
the triazole analogues did not improve this property. All of the
eight compounds we tested except 3c bind to 5-HT1A receptors
with high affinity. It is necessary to point out that the replacement
of amide with triazoles increases the ligands’ logD value.
4a
15
N
N
15
D3 = 81.2 nM
2 = 228.3 nM
D2/D3 = 2.8
D
N
O
Figure 2. Comparison between compounds 4a and 15.
An adenylyl cyclase inhibition assay was used to determine the
intrinsic activity of compounds 4b, 5b, 3c, 4c, 5c, 4f, 4g, and 5g. All
test compounds were used at a concentration equal to 10Â the cal-
culated Ki value; therefore 90% of the D2 or D3 binding sites were
occupied (Table 2). For each compound, the inhibition was com-
pared to the intrinsic activity of quinpirole (full agonist) and halo-
peridol (antagonist). All of the compounds were found to be partial
agonists at D2 dopamine receptors, with the intrinsic activity rang-
ing from approximately 31–75% of the full agonist. However, for D3
receptor a broader range of intrinsic activity was observed (from
13% efficacy to a 107% full agonist). Interestingly, compound 4f is
a full agonist at D3 receptors, while its benzamide analogue, WC
10 is a D3 receptor antagonist/weak partial agonist (18.7%).11
five-carbon-spacer analogues. When the carbon spacer group
increases from four to five, D2 receptor affinities generally increase
to greater extent than D3 receptor affinities. It leads to the fact that
although most of the five-carbon spacer compounds have the high-
est D3 receptor affinity, they exhibit lower D2 versus D3 selectivity.
The structure–activity relationship was further explored by
changing the substituents on the aromatic ring. It was found that
compounds with highest affinity and/or selectivity are para-substi-
tuted. For example: 3c, 4c, 5c, 5d, 5f, 4g, and 5g.
Unfortunately, when amide bonds were replaced with triazoles,
both D3 receptor affinity and selectivity decreased. For example,

522
X. Peng et al. / Bioorg. Med. Chem. Lett. 25 (2015) 519–523
Computational molecular modeling studies were initiated to
begin to define at a molecular level how triazole phenylpiperazines
bind to the D2 or D3 receptors. The Comparative Molecular Similar-
ity Index Analysis (CoMSIA1) for both D2 and D3 provided results in
good agreement with the experimental values for ligand–receptor
affinity (Supplementary Table 2). Because their values only span
one to two orders of magnitude, no statistical analysis was per-
formed. However, (a) the residuals are all reasonably small and
(b) overall trends for the compounds’ predicted affinities agree
with the experimental binding data on both D2 and D3. These mod-
eling studies indicate that the compounds have similar binding
modes at the D2 receptor site as their benzamide counterparts
(WC series), while losing about 10-fold affinity at D3 site.
In the molecular modeling study, both CoMSIA1 models utilized
a combination of steric, electrostatic, hydrophobic and hydrogen
bond donor and acceptor fields. An analysis of the contribution of
each of these components suggests that hydrophobic interactions
and hydrogen binding are the major determinant factors for the
compounds’ binding affinities.
Figure 3. Binding of WC 10 to the human D3 dopamine receptor. The binding of WC
10 and the human D3 receptor model is shown. A stick model is shown for the
backbone atoms of the D3 receptor. Oxygen is depicted in red and the nitrogen
atoms are depicted in blue. The hydrogen bond between the C181 carbonyl and the
amide nitrogen of WC 10 is shown as a dashed green line. This hydrogen bond
appears to be a pivotal interaction for the binding of arylamide phenylpiperazines.
The triazole and isoxazole analogues are unable to make this pivotal hydrogen
bond. Therefore an interaction that is essential for guiding the aryl moiety of the
benzamide towards a second binding pocket has been eliminated.
The replacement of the amide bond in WC series with triazole
was found to reduce both the affinity and selectivity to D3 recep-
tors compared to the WC series (WC 10 vs 4f). A closer examina-
tion of our D3 receptor–ligand binding model reveals that,
besides the salt bridge between the highly conserved residue
Asp3.32 in the transmembrane (TM) 3 helix of the receptor and
the protonated nitrogen on the piperidine scaffold, the benzamide
group on WC 10 potentially forms a critical hydrogen bond with
the D3 receptor (Fig. 3). This hydrogen bond is between the amine
on the benzamide group and the carbonyl of the backbone from
C181 on extracellular loop (EL) 2 of D3. C181 is a highly conserved
cysteine residue, which is involved in the formation of a disulfide
bond between EL2 and the third transmembrane (TM 3) spanning
helix. This disulfide bond plays an important role in the stabiliza-
tion of the GPCR structure. Consequently, the ligand is securely
anchored at the binding site, with its benzamide group extending
toward the extracellular opening of the binding pocket. The exten-
sion of this amine-containing scaffold into a second binding pocket
is crucial for high D3 receptor binding affinity.20
pocket,20 and recently, as a allosteric site for dopamine D3 recep-
tors.23 The four-carbon linker on the ligand scaffold is conforma-
tionally flexible. The benzamide hydrogen bond anchors the
ligand and helps to orient it toward the secondary binding pocket.
Therefore, interactions between the substituents on the benzene
ring with amino acid residues in the secondary binding site
become possible.
The loss of this hydrogen bonding in the triazole analogues
eliminates an important interaction that is essential for guiding
the aryl moiety (of the arylamide) towards a second binding pocket
(Fig. 4). The importance of the carboxamide linker for achieving D3
versus D2 receptor binding affinity and selectivity has been previ-
ously reported.24 The current work further defines the role that this
pivotal pharmacophore plays in the development of dopamine D3
receptor selective ligands.
This second binding pocket is delineated by the residues at the
junction of EL1, EL2 and the interfaces of TM helices 1, 2 and 7 and
opens to the extracellular regions. This region was described in the
dopamine D3 crystal structure paper as a secondary binding
Alignments for the analogues WC 10, 3f, 4f and 5f on the molec-
ular field maps of CoMSIA1 derived for D3 shows that longer car-
bon spacer compounds (4f and 5f) have the aromatic ring
sticking into the hydrophobic favorable regions within the second
Table 2
Intrinsic activity of selected analogues at dopamine D2 and D3 receptorsc
Compound
hD2 HEK
hD3 HEK
Haloperidol
WC 10
WC 44
3c
4b
4c
4f
4g
À0.6 1.6
33.5 3.1
35.3 1.0
53.8 6.1
49.0 3.2
67.8 5.3
38.8 7.6
41.1 8.8
43.1 5.4
31.4 5.0
75.4 5.5
100
4.0 5.5
18.7 2.2
96.2 4.2
50.5 9.9
48.2 5.4
72.4 4.4
107.0 19.3
39.4 5.0
25.9 6.3
67.8 15.6
82.2 10.3
100
5b
5c
5g
Quinpirole
c
The intrinsic activity of the test compounds was evaluated by determining the
percent inhibition of a forskolin-dependent whole cell adenylyl cyclase assay. The
results were normalized to the percent inhibition obtained using the full agonist
quinpirole at human D2 (1 lM) and D3 (100 nM) receptors expressed in stably
transfected HEK 293 cells. For D2 receptors the maximum inhibition was >90% and
for D3 receptors the maximum inhibition ranged from 38% to 53%. The test drug was
used at a concentration equal to approximately 10Â the Ki value that was deter-
mined from the radioligand binding analysis. The mean the SEM values are
reported for n P 3.
Figure 4. Binding modes for triazole analogues of WC 10 with varying carbon
spacer lengths in the human dopamine D3 receptor binding site. A comparison of
the alignments of the triazole analogues (3f, 4f, 5f), with WC 10 in the D3 dopamine
receptor binding site is shown. Oxygen is depicted in red and the nitrogen atoms are
depicted in blue. The hydrogen bond between the carbonyl group of C181 and the
amide nitrogen of WC 10 is shown as a dashed green line. A composite comparison
of the binding 3f, 4f and 5f in the D3 receptor binding site is shown.

X. Peng et al. / Bioorg. Med. Chem. Lett. 25 (2015) 519–523
523
D3-selective ligands and our knowledge on the binding modes of
dopamine receptors. The development of new class of compounds
for dopamine D3 receptors is ongoing.
Acknowledgments
This work was supported by NIH grants DA29840 (R.H.M.),
DA23957 and DA13584 (R.R.L.) and Integrated Research Training
Program of Excellence in Radiochemistry (60096D).
Supplementary data
Supplementary data (experimental procedures, characteriza-
tion of all compounds and biological assays protocols) associated
with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.bmcl.2014.12.023. These data include MOL
files and InChiKeys of the most important compounds described
in this article.
References and notes
Figure 5. Structural alignment of WC and the triazole analogues as they occupy the
D2 and D3 dopamine receptor binding sites. A structural alignment of WC 10 with
triazole compounds 3f, 4f and 5f are shown as they occupy the D2 and D3 dopamine
receptor binding sites. Oxygen is depicted in red and nitrogen atoms are depicted in
blue. Carbon atoms are colored in white for WC 10, in cyan for 3f, in yellow for 4f,
and in green for 5f. Hydrogen atoms are undisplayed for clarity. The contours are
displayed at the 80% favored and 20% disfavored level. (A) The triazole analogues 3f,
4f, and 5f aligned together with WC 10 at the D2 receptor binding pocket. Each of
the ligands assumes a bent configuration. (B) The triazole analogues (3f, 4f, and 5f)
together with WC 10 are illustrated by a stick model in the hydrophobic molecular
field map based on our previously generated CoMSIA1 model for D3 receptor. When
bound to the D3 receptor, each of the ligands assume a more linear configuration
than that observed for D2 receptor binding. Increased binding affinity is correlated
with more hydrophobic near orange, less hydrophobic near violet.
1. Joyce, J. N.; Millan, M. J. Drug Discovery Today 2005, 10, 917.
2. Staley, J. K.; Mash, D. C. J. Neurosci. 1996, 16, 6100.
3. Dalley, J. W.; Fryer, T. D.; Brichard, L.; Robinson, E. S.; Theobald, D. E.; Laane, K.;
Pena, Y.; Murphy, E. R.; Shah, Y.; Probst, K.; Abakumova, I.; Aigbirhio, F. I.;
Richards, H. K.; Hong, Y.; Baron, J. C.; Everitt, B. J.; Robbins, T. W. Science 2007,
315, 1267.
4. Di Ciano, P. Behav. Neurosci. 2008, 122, 129.
5. Chen, L.; Xu, M. J. Neurochem. 2010, 114, 530.
6. Bordet, R.; Ridray, S.; Carboni, S.; Diaz, J.; Sokoloff, P.; Schwartz, J. C. Proc. Natl.
Acad. Sci. U.S.A. 1997, 94, 3363.
7. Bordet, R.; Ridray, S.; Schwartz, J. C.; Sokoloff, P. Eur. J. Neurosci. 2000, 12, 2117.
8. Bordet, R. Rev. Neurol. (Paris) 2004, 160, 862.
9. Kumar, R.; Riddle, L. R.; Griffin, S. A.; Chu, W.; Vangveravong, S.; Neisewander,
J.; Mach, R. H.; Luedtke, R. R. Neuropharmacology 2009, 56, 956.
10. Shi, L.; Javitch, J. A. Annu. Rev. Pharmacol. Toxicol. 2002, 42, 437.
11. Chu, W.; Tu, Z.; McElveen, E.; Xu, J.; Taylor, M.; Luedtke, R. R.; Mach, R. H.
Bioorg. Med. Chem. 2005, 13, 77.
binding pocket (Fig. 5B). Since hydrophobic interactions are a
major contributor to the ligand affinity, we would expect the com-
pounds with the longer spacer chain to bind with higher affinity.
This also partially explains why a substituent in the para position
of the aromatic ring enhances activity. However, in our D2 receptor
model, this class of compounds binds to the D2 receptor with an L-
shaped carbon spacer chain. As a result, the alignment of the tria-
zole analogues overlaps quite well with WC 10 (Fig. 5A). Therefore,
these analogues have comparable binding affinities with the WC
series at the D2 receptor. These modeling studies suggest that the
origins of D2 versus D3 receptor subtype selectivity for the ligands
arise primarily from contour differences of the two binding sites.
Therefore ligands must exploit subtle topographical differences
within the two ligand binding sites to achieve high selectivity
between the D2 and D3 dopamine receptor subtypes.
In summary, we have synthesized and evaluated a series of tri-
azole analogues which showed moderate to high affinity for D3
receptors and variable selectivity for D3 versus D2 receptors. For
this series of compounds we identified several compounds that
exhibited high D3 binding affinity (<4.0 nM) with moderate D3 ver-
sus D2 receptor binding selectivity. The SAR could be partially
explained by our previously generated 3D-QSAR models for selec-
tive dopamine receptor ligands. This study expands the SAR of the
12. Xu, J.; Chu, W.; Tu, Z.; Jones, L. A.; Luedtke, R. R.; Perlmutter, J. S.; Mintun, M. A.;
Mach, R. H. Synapse 2009, 63, 717.
13. Tron, G. C.; Pirali, T.; Billington, R. A.; Canonico, P. L.; Sorba, G.; Genazzani, A. A.
Med. Res. Rev. 2008, 28, 278.
14. Bock, V. D.; Hiemstra, H.; van Maarseveen, J. H. Eur. J. Org. Chem. 2006, 51.
15. Kolb, H. C.; Sharpless, K. B. Drug Discovery Today 2003, 8, 1128.
16. Loaiza, P. R.; Lober, S.; Hubner, H.; Gmeiner, P. J. Comb. Chem. 2006, 8, 252.
17. Micheli, F.; Bonanomi, G.; Blaney, F. E.; Braggio, S.; Capelli, A. M.; Checchia, A.;
Curcuruto, O.; Damiani, F.; Fabio, R. D.; Donati, D.; Gentile, G.; Gribble, A.;
Hamprecht, D.; Tedesco, G.; Terreni, S.; Tarsi, L.; Lightfoot, A.; Stemp, G.;
Macdonald, G.; Smith, A.; Pecoraro, M.; Petrone, M.; Perini, O.; Piner, J.; Rossi,
T.; Worby, A.; Pilla, M.; Valerio, E.; Griffante, C.; Mugnaini, M.; Wood, M.; Scott,
C.; Andreoli, M.; Lacroix, L.; Schwarz, A.; Gozzi, A.; Bifone, A.; Ashby, C. R., Jr.;
Hagan, J. J.; Heidbreder, C. J. Med. Chem. 2007, 50, 5076.
18. Insua, I.; Alvarado, M.; Masaguer, C. F.; Iglesias, A.; Brea, J.; Loza, M. I.; Carro, L.
Bioorg. Med. Chem. Lett. 2013, 23, 5586.
19. Wang, Q.; Mach, R. H.; Luedtke, R. R.; Reichert, D. E. J. Chem. Inf. Model. 2010, 50,
1970.
20. Chien, E. Y.; Liu, W.; Zhao, Q.; Katritch, V.; Han, G. W.; Hanson, M. A.; Shi, L.;
Newman, A. H.; Javitch, J. A.; Cherezov, V.; Stevens, R. C. Science 2010, 330,
1091.
21. Himo, F.; Lovell, T.; Hilgraf, R.; Rostovtsev, V. V.; Noodleman, L.; Sharpless, K.
B.; Fokin, V. V. J. Am. Chem. Soc. 2005, 127, 210.
22. Luedtke, R. R.; Freeman, R. A.; Boundy, V. A.; Martin, M. W.; Huang, Y.; Mach, R.
H. Synapse 2000, 38, 438.
23. Lane, J. R.; Chubukov, P.; Liu, W.; Canals, M.; Cherezov, V.; Abagyan, R.; Stevens,
R. C.; Katritch, V. Mol. Pharmacol. 2013, 84, 794.
24. Banala, A. K.; Levy, B. A.; Khatri, S. S.; Furman, C. A.; Roof, R. A.; Mishra, Y.;
Griffin, S. A.; Sibley, D. R.; Luedtke, R. R.; Newman, A. H. J. Med. Chem. 2011.