Bioorganic & Medicinal Chemistry Letters 13 (2003) 513 517
Pharmacophore-Based Discovery of Substituted Pyridines as
Novel Dopamine Transporter Inhibitors
Istvan J. Enyedy,a Sukumar Sakamuri,a Wahiduz A. Zaman,b
Kenneth M. Johnsonb and Shaomeng Wanga,*
a
Departments of Internal Medicine and Medicinal Chemistry, University of Michigan, Ann Arbor, MI 48109-0934, USA
b
Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston, TX 77555-1031, USA
Received 31 July 2002; accepted 6 October 2002
Abstract Abnormal dopamine signaling in brain has been implicated in several conditions such as cocaine abuse, Parkinson s
disease and depression. Potent and selective dopamine transporter inhibitors may be useful as pharmacological tools and ther-
apeutic agents. Simple substituted pyridines were discovered as novel dopamine transporter (DAT) inhibitors through pharmaco-
phore-based 3D-database search. The most potent compound 18 has a Ki value of 79 nM in inhibition of WIN35,248 binding to
dopamine transporter and 255 nM in inhibition of dopamine reuptake, respectively, as potent as cocaine. Preliminary structure
activity relationship studies show that the geometry and the nature of the substituents on the pyridine ring determine the inhibitory
activity and selectivity toward the three monoamine transporters. The substituted pyridines described herein represent a class of
novel DAT inhibitors with simple chemical structures and their discovery provides additional insights into the binding site of DAT.
# 2002 Elsevier Science Ltd. All rights reserved.
Dopamine (DA) is a neurotransmitter crucial for nor-
mal brain function. The dopamine transporter (DAT)
plays a critical role in terminating DA neurotransmis-
sion by taking up DA released into the synapse.1,2
Abnormal DA signaling in brain has been implicated in
many pathological conditions such as cocaine abuse,
Parkinson s disease and depression.1 3 The ability of
cocaine to bind to the DAT and to inhibit the reuptake Our group has recently employed a pharmacophore-
of DA has been strongly implicated in the reinforcing based 3D-database searching approach for the dis-
properties of cocaine.2 As such, considerable emphasis covery of novel DAT inhibitors.4,6 9 Extensive struc-
has been directed toward DAT as a molecular target for ture activity relationship studies on cocaine (1) and
developing a pharmacotherapy for the treatment of other tropane analogues showed that a tertiary amine at
cocaine addiction and abuse.1 4 Novel DAT inhibitors the 8-position, a phenyl group at the 3-position and a
may function as mild and long-lasting stimulants, which carbonyl group at the 2-position may be crucial for their
may be used as replacement therapy for cocaine addic- binding to the DAT.4 Based upon these data we con-
tion.2,5 These compounds can also function as cocaine structed the first pharmacophore model.4 3D-database
antagonists or partial agonists in behavioral models, searching using the first pharmacophore model led to
and may be useful as potential therapeutic agents for the discovery of 4-hydroxy-1-methyl-4-(4-methylphenyl)-
the treatment of certain aspects of cocaine abuse and 3-piperidyl 4-methylphenyl ketone (2)4,9 and 2-alkyl-
addiction.2 DAT inhibitors with truly novel chemical 3aryl quinuclidines10 (3) among others as novel DAT
scaffolds will also provide new insights into the binding inhibitors.
site in DAT.
Although our very first pharmacophore model was suc-
cessful in identification of novel DAT inhibitors, several
known potent DAT inhibitors like 4, 5, 6, and 711,12
*Corresponding author. Tel.: +1-734-615-0362; fax: +1-734-647-
9647; e-mail: shaomeng@umich.edu (Fig. 1) did not have a carbonyl group as specified in the
0960-894X/03/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00943-5
514 I. J. Enyedy et al. / Bioorg. Med. Chem. Lett. 13 (2003) 513 517
Our goal is to identify novel DAT inhibitors with a dif-
ferent binding mode from that of cocaine which would
potentially function as cocaine antagonists. Recent site-
directed mutagenesis experiments showed that mutation
of aspartate 79 (Asp79) to alanine residue strongly
affects the binding affinity of both substrate and cocaine
analogues containing an amine nitrogen.14,15 The site-
directed mutational experiments thus suggest a direct
interaction of Asp79 with the basic amine groups of
cocaine analogues. The basic amine group in cocaine
analogues and DA becomes protonated (positively
charged) under physiological conditions, thus having a
strong interaction with the negatively charged Asp79.
Thus replacing the basic amine group in cocaine ana-
logues and DA with a functional group that cannot
have a strong interaction with Asp79 may lead to DAT
inhibitors with novel binding mode to DAT. A recent
study has showed that 8-oxa-2-carbomethoxynorbenzo-
tropine (9) is a potent DAT inhibitor.16 In compound 9,
an oxygen atom which can function only as a hydrogen
bonding acceptor replaces the basic nitrogen atom in
cocaine analogues, indicating that the presence of a
basic amino group is not an absolute requirement for
high affinity binding to the DAT and a hydrogen bond
acceptor in this position of the basic nitrogen can be
equally effective for binding to the DAT and for inhibi-
tion of DA reuptake. This prompted us to propose a
new pharmacophore model (Fig. 1) in which the tertiary
nitrogen (sp3) atom was replaced with a hydrogen-bond
acceptor N (sp2) atom in a ring system. Distance para-
meters between the nitrogen atom and the center of the
two aromatic rings were established based upon low
energy structures obtained from the conformational
Figure 1. A new pharmacophore model derived from several known
DAT inhibitors. analysis of compounds 2, 4, 5, 6, and 7.
Using this new pharmacophore model we searched the
National Cancer Institute (NCI)17 3D-database that
first pharmacophore model. Furthermore, in our contained 206,876 open compounds accessible by the
design of quinuclidines as a novel class of DAT public. The program Chem-X18 was used for identifying
inhibitors, we also found that the carbonyl group in compounds that fit our pharmacophore query. A total
the original lead compound was not essential.13 This of 1104 (0.5%) compounds ( hits ) met the pharmaco-
carbonyl group could be replaced with either an ali- phore requirements as specified in the Figure 1. These
phatic or an aromatic hydrophobic group. Taken hits were only potential DAT inhibitors and they needed
together, these data suggested that the carbonyl to be confirmed for their activity to inhibit the
group was not absolutely required for binding to the reuptake of DA into striatal nerve endings (synapto-
DAT and inhibition of DA reuptake. Thus, more than somes) and to displace the binding of [3H]WIN35,428
one pharmacophore model can be proposed and used (10) to DAT.4 Since the binding and uptake assays were
for the discovery of novel DAT inhibitors. Based on this quite time-consuming, we used filters for selecting a
idea, we proposed our second pharmacophore model limited number of hits for testing. The molecular
that had the carbonyl group replaced with a phenyl weight of the selected compounds had to be below 500,
group.7 3D-database pharmacophore searching using the number of rotatable bonds had to be below five, the
this second pharmacophore model led to the identifica- chemical matter had to be novel and attractive for chem-
tion of 3,4-disubstituted pyrrolidines (8) as novel DAT ical modifications. We selected 10 structurally diverse
inhibitors.7 compounds for preliminary testing.
I. J. Enyedy et al. / Bioorg. Med. Chem. Lett. 13 (2003) 513 517 515
These 10 compounds were first screened in DA uptake Therefore, we were able to use the Cheng-Prusoff equa-
assay. Since we were only interested in fairly potent tion for classic, competitive inhibition to calculate the Ki
compounds, we have screened them with a concen- values from IC50 values in these experiments. Their IC50
tration of 1 mM for each compound. Two compounds values were determined using the computer program
17 and 18 were found to have significant inhibition of LIGAND. The Km values used were 67 nM for [3H]DA,
DA reuptake. The IC50 value was estimated by adding 53 nM for [3H]5-HT, and 54 nM for [3H]NE.19 While 17
the radiolabeled neurotransmitter following equilibra- has a Ki value of 2.3 mM, 18 has a Ki value of 0.2 mM, as
tion between the test compounds and the transporter. potent as cocaine (Table 1). Interestingly, both of these
compounds belong to substituted pyridines.
Table 1. Preliminary results of selected hits from pharmacophore-
Compound 18 represents a novel class of DAT inhibi-
based virtual screening in dopamine reuptake assay
tors with very simple chemical structure and fairly good
potency. We have tested six additional new analogues to
Compd Structure DA Ki (mM)
gain insights into structure activity relationship for this
class of DAT inhibitors (Table 2). Compounds 18, 21,
22, and 23 show quite potent activities in binding and
11 >1.0
uptake assays, with Ki values 0.079 0.780 mM in bind-
ing, and 0.255 1.067 mM in inhibition of DA reuptake,
respectively. Compound 18 is the most potent among
tested with Ki values of 79 nM in binding and of 255 nM
12 >1.0
in inhibition of DA reuptake. Despite its very simple
chemical structure, 18 is as potent as cocaine in the DA
uptake assay. The activities of compounds 18, 21, and
22 show that the position of the nitrogen on the pyridine
13 >1.0
ring (or the position of the diphenylmethyl substituent
on the pyridine ring) affects both inhibition of DA
uptake and WIN binding (Table 2). Compounds 21 and
22 with the diphenylmethyl substituent at either the
meta- or the ortho-position on the pyridine ring are
14 >1.0
about 10-fold less potent in WIN binding and 4-fold less
potent in inhibition of DA reuptake, respectively, when
compared to 18. Compound 23, which has a benzyl
group in the position of the phenyl group in 18, is only
slightly less potent than 18. This suggests that a larger
15 >1.0
16 >1.0
17 2.3
18 0.2
19 >1.0
20 >1.0
Figure 2. Superposition of the lowest energy conformations: (A) com-
pound 11 (green) and compound 15 (grey). (B) Compound 11 (green)
and compound 17 (grey).
516 I. J. Enyedy et al. / Bioorg. Med. Chem. Lett. 13 (2003) 513 517
Table 2. Binding affinities of substituted pyridines to DAT and their update activities at the three monoamine transporters
Compd Structure Ki (mM)
[H3]WIN binding [H3]DA uptake [H3]5-HT uptake [H3]NE uptake
R-cocaine (1) 0.270 0.020 0.155 0.001 0.108 0.004
18 0.079 0.004 0.255 0.008 1.160 0.020 3.46 0.10
21 0.780 0.064 0.860 0.032 12.60 2.700 7.32 0.77
22 0.742 0.026 1.067 0.034 35.00 7.000 5.53 0.45
23 0.099 0.017 0.263 0.003 0.910 0.100 0.393 0.008
24 >10.00 >10.00 >10.00 >10.00
25 >10.00 >10.00 >10.00 >10.00
26 >10.00 >10.00 >10.00 >10.00
Standard deviation was obtained with three experiments.
group can be tolerated at this site for binding to the data showed that 22 has the highest selectivity, 35-fold,
DAT. Compound 24, whose structure may be viewed as between inhibiting DA uptake versus 5-HT uptake.
the diphenyl rings in 18 fused into an anthracene ring, Compound 18 has the highest selectivity, about 14-fold,
has a minimal activity at 10 mM. Likewise, compound between inhibiting DA uptake versus NE uptake.
25, which also has an fused anthracene ring, is also Compound 23, whose structure is more flexible than 18,
inactive. Molecular modeling studies showed that the shows no selectivity between DAT and NET, but has
two phenyl rings in 18 have a quite different relative the same selectivity as 18 between DAT and SERT. The
orientation as compared to the corresponding aromatic difference in selectivity for monoamine transporters
rings in 24 (Fig. 2A). While the two aromatic rings in 24 between 23 and 18 may also be attributed to the size dif-
have to be in the same plane, the two phenyl rings in 18 ference between substituents on the pyridine ring. Com-
cannot be in the same plane in low energy conforma- pounds 24 26 show no appreciable activity for up to 10
tions because they are connected to a sp3 carbon. Com- mM concentration. Our data suggests that the position,
pound 26, which may be viewed as one phenyl ring in 18 size and flexibility of the substituents on the pyridine ring
being replaced with H, is inactive. Molecular modeling are important for their selectivity among these three
shows that the pyridine and the phenyl rings in 26 can monoamine transporters for this class of compounds.
be superimposed on the corresponding rings in 18 (Fig.
2B), suggesting that both phenyl rings in 18 are impor- In summary, simple substituted pyridines are discovered
tant for binding to DAT. as a novel class of DAT inhibitors through 3D
database searching using a new pharmacophore
To achieve a further insight into the selectivity of model. The discovery of pyridines as fairly potent
these compounds among the three monoamine trans- DAT inhibitors provides a validation to our pro-
porters (DAT, SERT and NET), we also evaluated posed new pharmacophore model used in our 3D-
the activity of the seven monosubstituted pyridines in database searching and further shows that the proto-
inhibition of 5-HT and NE reuptake (Table 2). Our nated nitrogen and the ester group in cocaine are not
I. J. Enyedy et al. / Bioorg. Med. Chem. Lett. 13 (2003) 513 517 517
absolutely required for binding to DAT and other 8. Enyedy, I. J.; Wang, J.; Zaman, W. A.; Johnson, K. M.;
Wang, S. Bioorg. Med. Chem. Lett. 2002, 12, 1775.
monoamine transporters.
9. Wang, S.; Sakamuri, S.; Enyedy, I. J.; Kozikowski, A. P.;
Zaman, W. A.; Johnson, K. M. Bioorg. Med. Chem. 2001, 9,
1753.
Acknowledgements
10. Sakamuri, S.; Enyedy, I. J.; Kozikowski, A. P.; Wang, S.
Tetrahedron Lett. 2000, 41, 9949.
The financial support (DA R0111545 to S.W.) from the
11. Hoffman, B. T.; Kopajtic, T.; Katz, J. L.; Newman, A. H.
National Institute on Drug Abuse is greatly appreciated.
J. Med. Chem. 2000, 43, 4151.
12. Kozikowski, A. P.; Saiah, M. K. E.; Johnson, K. M.;
Bergmann, J. S. J. Med. Chem. 1995, 38, 3086.
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