Fancy bioisosteres: Metallocene-derived G-protein-coupled receptor ligands with subnanomolar binding affinity and novel selectivity profiles

Article abstract of DOI:10.1021/jm050170s

Metallocene-derived bioisosteres lead to exceptionally strong binding G-protein-coupled receptor ligands, indicating substantial plasticity of the receptor excluded volume. Novel binding profiles of ferrocenylcarboxamides combining subnanomolar K_i

Full text of DOI:10.1021/jm050170s

3696
J. Med. Chem. 2005, 48, 3696-3699
Chart 1
Fancy Bioisosteres: Metallocene-Derived
G-Protein-Coupled Receptor Ligands
with Subnanomolar Binding Affinity and
Novel Selectivity Profiles
Karin Schlotter, Frank Boeckler, Harald Hu¨bner, and
Peter Gmeiner*
Department of Medicinal Chemistry, Emil Fischer Center,
Friedrich Alexander University, Schuhstrasse 19,
D-91052 Erlangen, Germany
Received February 22, 2005
Abstract: Metallocene-derived bioisosteres lead to exception-
ally strong binding G-protein-coupled receptor ligands, indicat-
ing substantial plasticity of the receptor excluded volume.
Novel binding profiles of ferrocenylcarboxamides combining
subnanomolar Ki values for the dopamine D4 receptor (1a,
0.52 nM; 1b, 0.63 nM) with superpotent serotonin 5-hydroxy-
tryptamine1A (1a, 0.50 nM) and dopamine D3 receptor binding
(1b, 0.64 nM) and selective D4 agonist properties of the
ruthenocene 1c may be a starting point for highly beneficial
central nervous system active drugs.
69-95% yield of the ferrocenyl- and ruthenocenylcar-
boxamides 1a,b and 1c,d, respectively (Scheme 1). For
the preparation of the structural hybrid 1e, a practical
synthetic route to the building block 3c had to be
elaborated when we took advantage of a Buchwald-
Hartwig cross-coupling9,10 as a key reaction step. Thus,
palladium promoted amination of 2,6-dichloroanisole (4)
with piperazine gave the intermediate 5. Subsequent
treatment with bromobutyronitrile and reduction with
LiAlH4 resulted in formation of the primary amine 3c.
Coupling with ferrocenylcarboxylic acid was induced by
TBTU to furnish the final product 1e.
To properly adjust the relative disposition of the
pharmacophoric elements, the distance between the
double-layered arene bioisostere and the carboxamide
moiety should be increased by insertion of small spacer
elements. Thus, ferrocenylacetic acid (6) and the double-
layered cinnamic acid analogue 7,11 which we prepared
by treatment of ferrocene with acetoacetic acid ethyl
ester under acidic conditions and subsequent saponifi-
cation, should be used as additional molecular scaffolds.
In fact, activation by TBTU or HATU and subsequent
coupling with the primary amine 3a gave access to the
carboxamides 1f and 1g in 59% and 69% yield, respec-
tively (Scheme 2).
Bioisosteric replacement is commonly accepted as a
valuable strategy in drug discovery, facilitating an
improvement of pharmacodynamic and pharmacokinetic
properties.1 Taking advantage of the structural variety
of heteroarene-derived scaffolds, we developed subtype-
specific dopamine D3 receptor ligands2 and selective D4
modulators.3 An extension of our SAR studies demon-
strated that the utilization of uncommon structural
moieties (fancy bioisosteres) substantially expands the
chemical diversity space of bioactive compounds. Be-
sides the successful introduction of enynes and endiynes
into dopaminergics,4 we found that paracyclophane
derivatives can be used as double-layered aryl bioiso-
steres.5 To learn more about the obvious plasticity of
G-protein-coupled receptors (GPCRs) and to approach
central nervous system (CNS) active drugs with novel
subtype selectivity profiles, these studies were contin-
ued. Employing the D3 partial agonists BP 8976 and
FAUC 3462 as lead compounds, we herein report the
evaluation of metallocenes of type 1 as potential ligands
of GPCRs involving dopamine D1, D2short, D1long, D3,
D4, serotonin 5-HT1A and 5-HT2 subtypes, and adren-
ergic R1-receptors (Chart 1).
Our metallocene-derived target compounds should
involve ferrocenyl- and ruthenocenylcarboxamides. For
the ferrocenes,7 we additionally planned to modify the
distance between the double-layered system and the
phenylpiperazine moiety by inserting a methylene or a
(methyl)ethenylene group. Finally, different phenyl
substituents were evaluated. In detail, HATU promoted
coupling of ferrocenylcarboxylic acid (2a) and the ru-
thenocene-derived analogue 2b8 with the 2-methoxy-
phenylpiperazinyl-substituted butylamine 3a and its
2,3-dichlorophenyl analogue 3b, being readily prepared
following previously described protocols, which gave a
Radioligand binding assays, mitogenesis, and
[35S]GTPγS binding experiments were employed to
analyze affinity, selectivity profiles, and ligand efficacy
of target compounds of type 1. The Ki and EC50 values
of the test compounds were compared to those of the
D3 partial agonist BP 897, which is known for highly
interesting antidyskinetic properties and for beneficial
effects on cocaine-seeking behavior.6b The binding data
were generated by measuring their ability to compete
with [3H]spiperone for the cloned human dopamine
12
receptor subtypes D2long, D2short
,
D3,13 and D4.414
stably expressed in Chinese hamster ovary cells (CHO).4a
D1 receptor affinities were determined utilizing porcine
striatal membranes and the D1 selective radioligand
[3H]SCH 23390.4a Because of the observation that the
lead compound BP 897 reveals serotoninergic and
adrenergic activity,6a the test compounds were investi-
gated for their potency to displace [3H]-8-OH-DPAT,
[3H]ketanserin, and [3H]prazosin when employing por-
cine 5-HT1A, 5-HT2, and R1-receptors, respectively.
* To whom correspondence should be addressed. Phone:
+49-(0)9131-8529383. Fax: +49-(0)9131-8522585. E-mail: gmeiner@
pharmazie.uni-erlangen.de.
10.1021/jm050170s CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/29/2005

Letters
Journal of Medicinal Chemistry, 2005, Vol. 48, No. 11 3697
Scheme 1a
significantly modified the binding profile of the test
compound 1b when D3 affinity substantially increased
(Ki ) 0.64 nM). On the other hand, reduction of affinity
for 5-HT1A and an unchanged behavior toward D4 were
observed when we determined a subnanomolar Ki value
(0.63 nM). In fact, FAUC 382 (1b) turned out to be a
superpotent D3/D4 modulator with more than 30-fold
selectivity over D1, D2long, D2short, 5-HT1A, 5-HT2, and
R1. The structural similarity of the 3-chloro-2-methoxy-
phenyl derivative 1e displayed a binding profile with
Ki values between those of the 2-methoxyphenylpipera-
zine 1a and the 2,3-dichloro analogue 1b. Formal
exchange of the Fe2+ central ion by Ru2+ led to ad-
ditional highly potent GPCR ligands when the affinity
patterns of the ruthenocenes 1c and 1d were very
similar to those of the ferrocene analogues 1a and 1b,
respectively. However, a highly significant exchange
was noticed for the 5-HT1A binding of the methoxy-
phenyl derivatives. Thus, the lower Ki value (20 nM)
for the ruthenocene 1c yielded an enhanced D4 selectiv-
ity (Ki ) 0.37 nM for D4; selectivity over 5-HT1A is >50).
The binding affinities of the ferrocene 1g as a structur-
ally extended homologue of 1a clearly indicate a moder-
ate ability to compete with the radioligands in the
binding assay. Thus, insertion of a two-carbon spacer
between the double-layered π-system and the carbox-
amide moiety induced a reduced target recognition. On
the other hand, formal insertion of a methylene unit
leading to the ferrocenylacetamide 1f significantly
caused a decrease of binding affinity for only D4 (∼6-
fold) and 5-HT1A (∼25-fold).
a Reagents and conditions: (a) for 1a,b, HATU, DIPEA, DMF,
3a,b, 0 °C to room temp, 1-2 h (69-71%); for 1c,d, HATU, DIPEA,
DMF, CH2Cl2, 3a,b, CH2Cl2, 0 °C to room temp, 1-2h (83-95%);
for 1e, TBTU, DIPEA, DMF, CH2Cl2, 3c, CH2Cl2, 0 °C to room
temp, 30 min (79%); (b) Pd(OAc)2, (o-biphenyl)P(t-Bu)2, NaOt-Bu,
toluene, piperazine, 80 °C, 21 h (37%); (c) (1) Br(CH2)3CN, Na2CO3,
MeCN, reflux, 15 h (77%); (2) LiAlH4, Et2O, 0 °C to room temp, 1
h (95%).
Scheme 2a
As a measure of functional activity, ligand efficacy of
the ferrocenylcarboxamides 1a,b,e and the ruthenocyl
derivatives 1c,d was determined by a mitogenesis assay
measuring the rate of [3H]thymidine incorporation into
growing CHO dhfr- cells and a CHO10001 cell line
stably expressing the human D3 and D4.2 receptor,
respectively.13,15,16 The data listed in Table 2 clearly
show substantial D3 ligand efficacy of the test com-
pounds 1a-e (24-50%, EC50 ) 2.0-9.8 nM) that proved
to be similar to the data we determined for the partial
agonist BP 897 with respect to intrinsic activity and
EC50 values. For the D4 subtype, the intrinsic activity
of the ferrocene-derived 2-methoxyphenyl derivative 1a
(67%, EC50 ) 0.55 nM) was superior to the respective
effects of the 2,3-dichloro analogue 1b and the 3-chloro-
2-methoxy-substituted phenylpiperazine 1e, whereas for
the ruthenocenyl type compounds both derivatives (the
2-methoxyphenyl derivative 1c (60%, EC50 ) 1.2 nM)
and the 2,3-dichloro analogue 1d (60%, EC50 ) 35 nM))
showed an intrinsic activity similar to that of 1a. In
contrast, the reference compound BP 897 displayed only
weak D4 partial agonist properties.
To prove the functional activity of the test compounds
detected in the mitogenesis experiments, a [35S]GTPγS
binding assay was established. When membrane
preparations from human D4.4 receptor expressing
CHO cells were used, the dose-dependent stimulation
of [35S]GTPγS binding was determined to distinguish
between agonist and antagonist properties and to
measure the potency and efficacy of agonists to the D4
receptor.17,18 The intrinsic effects of 1a-e listed in Table
2 indicate that the 2-methoxyphenylpiperazine moiety
is a privileged structural feature to stimulate the D4
a Reagents and conditions: (a) TBTU, DIPEA, DMF, 3a, CH2Cl2,
0 °C to room temp, 1 h (59%); (b) HATU, DIPEA, DMF, 3a, CH2Cl2,
0 °C to room temp, 21 h (69%).
The characterization of the test compounds was
initiated by investigating the ferrocenylcarboxamides
1a,b,e when the 2-methoxyphenylpiperazine 1a showed
the highest degree of similarity with the target com-
pound BP 897. In fact, the binding data depicted in
Table 1 clearly indicate that the double-layered metal-
locene partial structure is tolerated by the binding sites
of the GPCRs investigated. Significant radioligand
displacement was observed, leading to Ki values in the
nanomolar or single-digit micromolar range. Compared
to the D3 selective lead compound BP 897, the ferroce-
nylcarboxamide 1a displayed a similar affinity pattern
for D1, D2long, D2short, 5-HT2, and R1. Competition
experiments at the D3 subtype led to a Ki of 6.5 nM,
indicating only a 5-fold lower target recognition when
compared to BP 897 (Ki ) 1.3 nM). The most exciting
binding data were observed for the D4 and the 5-HT1A
receptors resulting in Ki values of 0.52 and 0.50 nM.
Thus, ferrocene 1a (FAUC 378) proved to be a super-
affinity ligand for 5-HT1A and D4, displaying also
substantial affinity for D3 and R1. Exchange of the
2-methoxyphenyl group to a 2,3-dichlorophenyl moiety

3698 Journal of Medicinal Chemistry, 2005, Vol. 48, No. 11
Letters
Table 1. Receptor Binding Data of 1a-g Compared to the Reference Compound BP 897 Utilizing Human D2long, D2short, D3, and
D4.4 Receptors and Porcine D1, 5-HT1A, 5-HT2, and R1 Receptors
Ki a (nM)
[3H]spiperone
[3H]SCH 23390
D1
[3H]8-OH-DPAT [3H]ketanserin [3H]prazosin
compd Me
A
R
R′
D2long
D2short
D3
D4.4
5-HT1A
5-HT2
R1
1a
1b
1c
1d
1e
1f
Fe CdO
Fe CdO
Ru CdO
Ru CdO
Fe CdO
Fe CH2CdO OMe
Fe OMe
OMe
Cl
OMe
Cl
H
1500 ( 150
630 ( 25
110 ( 10 78 ( 1.5 6.5 ( 0.54 0.52 ( 0.086
31 ( 2.0 19 ( 0.50 0.64 ( 0.11 0.63 ( 0.027
120 ( 24 60 ( 6.7 10 ( 0.93 0.37 ( 0.012
43 ( 12 12 ( 0.58 0.84 ( 0.11 0.60 ( 0.025
99 ( 12 72 ( 9.1 3.5 ( 0.54 0.91 ( 0.19
66 ( 5.0 45 ( 14 2.0 ( 0.17 3.3 ( 0.45
0.50 ( 0.080
27 ( 10
20 ( 1.9
77 ( 2.0
14 ( 3.0
12 ( 3.9
210 ( 0.0
310 ( 10
250 ( 15
370 ( 20
610 ( 30
110 ( 0.0
700 ( 30
2500 ( 300
9.7 ( 0.30
73 ( 5.0
5.5 ( 0.25
26 ( 0.0
35 ( 1.0
5.1 ( 0.80
26 ( 1.0
Cl
H
720 ( 25
Cl
330 ( 25
OMe Cl
2200 ( 300
750 ( 330
1700 ( 500
H
H
1g
170 ( 5.0 120 ( 5.0 20 ( 4.5
21 ( 2.5
BP897
760 ( 60
220 ( 19 200 ( 5.0 1.3 ( 0.096 44 ( 7.6
81 ( 8.0
840 ( 120
5.0 ( 0.40
a Ki ( SEM are based on the mean of two to five experiments each done in triplicate.
Table 2. Intrinsic Activities of 1a-e and of the Reference
Compounds Quinpirole and BP 897 Derived from the
Stimulating Effect on Mitogenesis of D3 and D4 Receptor
Expressing Cells and from the D4 Receptor Mediated Binding
of [35S]GTPγS
[3H]thymidine uptake (mitogenesis)
D3 receptora D4 receptorb
agonist EC50 agonist EC50
[
35S]GTPγS binding
D4 receptorc
agonist
EC50
compd
effectd (%) (nM)e effectd (%) (nM)e effectf (%) (nM)g
1a
38
28
2.0
3.5
3.9
9.1
9.8
2.7
1.4
67
37
0.55
7.6
74
43
2.5
13
1.9
14
23
nd
49
1b
1c
24
60
1.2
94
1d
50
61
35
43
33
nd
100
1e
38
45
22
49
19
BP 897
quinpirole
38
22
100
100
Figure 1. Investigation of the cytotoxic effect of the ferrocenyl
derivative 1a tested with nontransfected CHO K1 cells in the
presence of 100 µM (not filled), 10 µM (light-gray), 1 µM (dark-
gray), and 0.01 µM (black) compared to the number of living
cells without any test compound (odd lines). Significance was
derived from an unpaired t-test: (/) no significant difference;
(//) p < 0.05; (///) p < 0.0001.
a Determined with CHO dhfr- mutant cells stably expressing
the human D3 receptor. b Determined with CHO10001 cells stably
expressing the human D4.2 receptor. c Determined with CHO K1
cells stably expressing the human D4.4 receptor. d Rate of incor-
poration of [3H]thymidine as evidence for mitogenesis activity
relative to the maximal effect of the full agonist quinpirole ()100%)
used as a reference. e EC50 values derived from the mean of three
or four independent experiments. f Maximum binding of [35S]GTPγS
induced by receptor activation at 1 µM relative to the effect of the
reference agonist quinpirole ()100%) (at 10 µM) derived from six
independent experiments. g EC50 values derived from the mean
of two independent experiments.
receptor when the ferrocenylcarboxmide 1a (74%, EC50
) 2.5 nM) and the ruthenocene analogue 1c (94%, EC50
) 1.9 nM) is superior to the 2,3-dichlorophenylpipera-
zine derivatives 1b and 1d (43%, EC50 ) 13 nM; 43%,
EC50 ) 14 nM) and to the 3-chloro-2-methoxypiperazine
1e (33%, EC50 ) 23 nM). Interestingly, the ruthenocene
Figure 2. Electrostatic potentials mapped onto the van der
Waals surfaces of the N-methylcarboxamide fragments A-C
of the lead compound BP 897 and the ferrocene and ru-
thenocene derived bioisosteres of type 1.
1c (FAUC 413) shows full agonist effects (94%, EC50
)
1.9 nM) but a 25-fold lower EC50 value when compared
to the reference agent quinpirole.
To investigate the chemical stability of metallocene-
derived receptor ligands, we observed the representative
1a and 1c under conditions of biological testing (37 °C,
incubation up to 20 h) when HPLC-MS analysis did
not give evidence of any degradation. Furthermore, we
were interested in investigating a probable cytotoxic
effect of the test compounds in a cell-based experiment
utilizing nontransfected CHO K1 cells that were incu-
bated with different concentrations of 1a. As presented
in Figure 1, analysis of the number of living cells
indicated a significant cytotoxic effect only at 100 µM
(76% loss of cells) and 10 µM (32% loss of cells), whereas
0.01 and 1 µM did not result in statistically significant
cell toxicity. Nevertheless, since metallocene-derived
bioisosteres have not been investigated clinically yet,
their utility as drugs remains unclear.
To understand the molecular similarity of the naph-
thaline-2-carboxamide moiety of BP 897 (A) and the
respective ferrocene and ruthenocene derived bioiso-
steric elements B and C, we investigated the steric
demand and the charge distribution by mapping the
electrostatic potentials onto van der Waals surfaces.19
In detail, the structures were derived from X-ray data20
of suitable precursors. Electrostatic potential charges
were calculated using the PM3(tm) Hamiltonian in the
program package Spartan.21 The distribution of charge
on the molecular surfaces was visualized with MOLCAD
implemented in SYBYL 6.9.1.22 Figure 2 shows a high
degree of similarity when the molecular electrostatic
properties are compared. This might be the structural
origin for the remarkable high target binding of the

Letters
Journal of Medicinal Chemistry, 2005, Vol. 48, No. 11 3699
(5) (a) Ortner, B.; Waibel, R.; Gmeiner, P. Indoloparacyclophanes:
synthesis and dopamine receptor binding of a novel arylbioisos-
tere. Angew. Chem., Int. Ed. 2001, 40, 1283-1285. (b) Ortner,
B.; Hu¨bner, H.; Gmeiner, P. Planar chiral indoles: synthesis and
biological effects of the enantiomers. Tetrahedron: Asymmetry
2001, 12, 3205-3208.
(6) (a) Pilla, M.; Perachon, S.; Sautel, F.; Garrido, F.; Mann, A.;
Wermuth, C. G.; Schwartz, J.-C.; Everitt, B. J.; Sokoloff, P.
Selective inhibition of cocaine-seeking behaviour by a partial
dopamine D3 receptor agonist. Nature 1999, 400, 371-375. (b)
Bezard, E.; Ferry, S.; Mach, U.; Stark, H.; Leriche, L.; Boraud,
T.; Gross, C.; Sokoloff, P. Attenuation of levodopa-induced
dyskinesia by normalizing dopamine D3 receptor function. Nat.
Med. 2003, 9, 762-767.
(7) (a) van Staveren, D. R.; Metzler-Nolte, N. Bioorganometallic
chemistry of ferrocene. Chem. Rev. 2004, 104, 5931-5985. (b)
Wenzel, M.; Wu, Y. Ferrocene, ruthenocene and rhodocene
analogs in haloperidol synthesis and organ distribution after
labeling with 103Ru and 103mRh. Int. J. Radiat. Appl. Instrum.,
Part A 1988, 39, 1237-1241.
(8) (a) Rausch, M. D.; Fischer, E. O.; Grubert, H. The aromatic
reactivity of ferrocene, ruthenocene, and osmocene. J. Am. Chem.
Soc. 1960, 82, 76-82. (b) Reeves, P. C. Carboxylation of aromatic
compounds: ferrocenecarboxylic acid. Org. Synth. 1977, 56, 28-
31.
double-layered bioisosteres. Nevertheless, the ability of
the double-layered ligands of type 1 to specifically bind
to the GPCRs investigated is surprising because the
height of the metallocene-derived π-systems is more
than doubled (6.2 and 6.7 Å) when compared to the
naphthalene substructure of BP 897 (3.0 Å). Obviously,
the plasticity of the binding region is substantially
higher than expected.
In conclusion, we demonstrate that the high steric
demand of ferrocene and ruthenocene derivatives of type
1 is well tolerated by the binding sites of a number of
family 1 GPCRs. The novel binding profiles including
subnanomolar Ki values of D4/5-HT1A (for 1a) and
D3/D4 (for 1b) and the D4 selective agonist properties
of 1c may be a starting point for highly beneficial CNS
active drugs because these subtypes are extraordinarily
relevant for neuropathological diseases such as schizo-
phrenia, L-dopa-induced dyskinesia, attention deficit
hyperactivity disorder, psychostimulant abuse, and
sexual dysfunction. The high binding affinity of fancy
bioisosteres of type 1 indicating substantial plasticity
of the receptor excluded volumes of family 1 GPCRs will
be a valuable tool for the investigation of GPCR-ligand
interactions.
(9) (a) Wolfe, J. P.; Tomori, H.; Sadighi, J. P.; Yin, J.; Buchwald, S.
L. Simple, efficient catalyst system for the palladium-catalyzed
amination of aryl chlorides, bromides, and triflates. J. Org.
Chem. 2000, 65, 1158-1174. (b) Hartwig, J. F. Transition metal
catalyzed synthesis of arylamines and aryl ethers from aryl
halides and triflates: scope and mechanism. Angew. Chem., Int.
Ed. 1998, 37, 2046-2067.
(10) (a) Nishiyama, M.; Yamamoto, T.; Koie, Y. Synthesis of N-
arylpiperazines from aryl halides and piperazine under a pal-
ladium tri-tert-butylphosphine catalyst. Tetrahedron Lett. 1998,
39, 617-620. (b) Torisawa, Y.; Nishi, T.; Minamikawa, J.-i.
Progress in arylpiperazine synthesis by the catalytic amination
reaction. Bioorg. Med. Chem. 2002, 10, 4023-4027.
(11) Plazuk, D.; Zakrzewski, J. Friedel-Crafts Type reaction of
ferrocene with â-ketoesters. J. Org. Chem. 2002, 67, 8672-8674.
(12) Hayes, G.; Biden, T. J.; Selbie, L. A.; Shine, J. Structural
subtypes of the dopamine D2 receptor are functionally distinct:
expression of the cloned D2A and D2B subtypes in a heterologous
cell line. Mol. Endocrinol. 1992, 6, 920-926.
(13) Sokoloff, P.; Andrieux, M.; Besanc¸on, R.; Pilon, C.; Martres, M.-
P.; Giros, B.; Schwartz, J.-C. Pharmacology of human dopamine
D3 receptor expressed in a mammalian cell line: comparison
with D2 receptor. Eur. J. Pharmacol. 1992, 225, 331-337.
(14) Asghari, V.; Sanyal, S.; Buchwaldt, S.; Paterson, A.; Jovanovic,
V.; Van Tol, H. H. M. Modulation of intracellular cyclic AMP
levels by different human dopamine D4 receptor variants. J.
Neurochem. 1995, 65, 1157-1165.
Acknowledgment. The authors thank Dr. H. H. M.
Van Tol (Clarke Institute of Psychiatry, Toronto,
Canada), Dr. J.-C. Schwartz and Dr. P. Sokoloff
(INSERM, Paris), and Dr. J. Shine (The Garvan In-
stitute of Medical Research, Sydney) for providing
dopamine D4, D3, and D2 receptor expressing cell lines,
respectively, and Dr. R. Huff (Pfizer, Kalamazoo, MI)
for providing dopamine D4.2 receptor expressing cells.
This work was supported by the BMBF and Fonds der
Chemischen Industrie.
Supporting Information Available: Experimental Sec-
tion, including details on synthesis, analytical characterization,
biological studies, and literature references. This material is
available free of charge via the Internet at http://pubs.acs.org.
(15) (a) Mierau, J.; Schneider, F. J.; Ensinger, H. A.; Chio, C. L.;
Lajiness, M. E.; Huff, R. M. Pramipexole binding and activa-
tion of cloned and expressed dopamine D2, D3 and D4 receptors.
Eur. J. Pharmacol. 1995, 290, 29-36. (b) Hu¨bner, H.; Kraxner,
J.; Gmeiner, P. Cyanoindole derivatives as highly selective
dopamine D4 receptor partial agonists: solid-phase synthesis,
binding assays, and functional experiments. J. Med. Chem. 2000,
43, 4563-4569.
References
(1) Chen, X.; Wang, W. The use of bioisosteric groups in lead
optimization. Annu. Rep. Med. Chem. 2003, 38, 333-346.
(2) (a) Bettinetti, L.; Schlotter, K.; Hu¨bner, H.; Gmeiner, P. Interac-
tive SAR studies: rational discovery of super-potent and highly
selective dopamine D3 receptor antagonists and partial agonists.
J. Med. Chem. 2002, 45, 4594-4597. (b) Boeckler, F.; Leng, A.;
Mura, A. Bettinetti, L.; Feldon, J.; Gmeiner, P.; Ferger, B.
Attenuation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine
(MPTP) neurotoxicity by the novel selective dopamine D3-
receptor partial agonist FAUC 329 predominantly in the nucleus
accumbens of mice. Biochem. Pharmacol. 2003, 66, 1025-1032.
(3) (a) Lo¨ber, S.; Hu¨bner, H.; Utz, W.; Gmeiner, P. Rationally based
efficacy tuning of selective dopamine D4 receptor ligands leading
to the complete antagonist 2-[4-(4-chlorophenyl)piperazin-1-
ylmethyl]pyrazolo[1,5-a]pyridine (FAUC 213). J. Med. Chem.
2001, 44, 2691-2694. (b) Boeckler, F.; Russig, H.; Zhang, W.;
Lo¨ber, S.; Schetz, J.; Hu¨bner, H.; Ferger, B.; Gmeiner, P.; Feldon,
J. FAUC 213, a highly selective dopamine D4 receptor full
antagonist, exhibits atypical antipsychotic properties in behav-
ioural and neurochemical models of schizophrenia. Psychophar-
macology (Berlin) 2004, 175, 7-17.
(4) (a) Hu¨bner, H.; Haubmann, C.; Utz, W.; Gmeiner, P. Conjugated
enynes as nonaromatic catechol bioisosteres: synthesis, binding
experiments, and computational studies of novel dopamine
receptor agonists recognizing preferentially the D3 subtype. J.
Med. Chem. 2000, 43, 756-762. (b) Lenz, C.; Haubmann, C.;
Huebner, H.; Boeckler, F.; Gmeiner, P. Fancy bioisosteres:
synthesis and dopaminergic properties of the endiyne FAUC 88
as a novel non-aromatic D3 agonist. Bioorg. Med. Chem. 2004,
13, 185-191.
(16) Chio, C.; Lajiness, M. E.; Huff, R. M. Activation of heterologously
expressed D3 dopamine receptors: comparison with D2 dopam-
ine receptors. Mol. Pharmacol. 1994, 45, 51-60.
(17) Gardner, B. R.; Hall, D. A.; Strange, P. G. Pharmacological
analysis of the dopamine stimulation of [35S]GTPγS binding via
human D2short and D2long dopamine receptors expressed in
recombinant cells. Br. J. Pharmacol. 1996, 118, 1544-1550.
(18) Newman-Tancredi, A.; Audinot, V.; Chaput, C.; Verriele, L.;
Millan, M. J. [35S]Guanosine-5′-O-(3-thio)triphosphate binding
as a measure of efficacy at human recombinant dopamine D4.4
receptors: actions of antiparkinsonian and antipsychotic agents.
J. Pharmacol. Exp. Ther. 1997, 282, 181-191.
(19) Meyer, E. A.; Castellano, R. K.; Diederich, F. Interactions with
aromatic rings in chemical and biological recognition. Angew.
Chem., Int. Ed. 2003, 42, 1210-1250.
(20) Allen, F. H. The Cambridge Structural Database: a quarter of
a million crystal structures and rising. Acta Crystallogr. 2002,
B58, 380-388.
(21) PC Spartan Plus; Wavefunction Inc.: Irvine, CA, 1998.
(22) SYBYL 6.9.1, Tripos Inc. (1699 South Hanley Rd, St. Louis, MO,
63144).
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