Synthesis and monoamine transporter affinity of new 2β-carbomethoxy- 3β-[4-(substituted thiophenyl)]phenyltropanes: Discovery of a selective SERT antagonist with picomolar potency

Article abstract of DOI:10.1016/j.bmcl.2004.12.014

Preparation of cocaine analogues has been aimed largely at development of stable compounds with high affinity and selectivity for the dopamine transporter (DAT). We now report the synthesis and monoamine transporter affinity of 10 new 2β-carbomethoxy-3β-[

Full text of DOI:10.1016/j.bmcl.2004.12.014

Bioorganic & Medicinal Chemistry Letters 15 (2005) 1131–1133
Synthesis and monoamine transporter affinity of new
2b-carbomethoxy-3b-[4-(substituted thiophenyl)]phenyltropanes:
discovery of a selective SERT antagonist with picomolar potency
a
a
b
b
Gilles Tamagnan,a,c, David Alagille, Xing Fu, Nora S. Kula, Ross J. Baldessarini,
*
Robert B. Innisa and Ronald M. Baldwina
aDepartment of Psychiatry, Yale University and VA Connecticut/116A2, 950 Campbell Avenue, West Haven, CT 06516, USA
bDepartment of Psychiatry and Neuroscience Program, Harvard Medical School and Mailman Research Center,
McLean Division of Massachusetts General Hospital, Belmont, MA 02478-9106, USA
cInstitute for Neurodegenerative Disorders, 60 Temple Street, Suite 8A, New Haven, CT 06510, USA
Received 23 September 2004; revised 1 December 2004; accepted 6 December 2004
Available online 29 December 2004
Abstract—Preparation of cocaine analogues has been aimed largely at development of stable compounds with high affinity and
selectivity for the dopamine transporter (DAT). We now report the synthesis and monoamine transporter affinity of 10 new 2b-carbo-
methoxy-3b-[4-(substituted thiophenyl)]phenyltropanes. Among these, compound 4b exhibited very high affinity for the serotonin
transporter (SERT: Ki = 17 pM) and good selectivity over dopamine (DAT: 710-fold) and norepinephrine transporters (NET:
11,100-fold).
Ó 2004 Elsevier Ltd. All rights reserved.
The naturally occurring alkaloid (ꢀ)-cocaine isolated
from Erythroxylom coca remains one of the most abused
drugs worldwide due to its powerful psychostimulant
and self-reinforcing effects. Considerable effort has been
directed toward understanding the neuropharmacologi-
cal mechanisms of action and behavioral effects of
cocaine.
cocaine with DAT, even though cocaine has multiple
other neuropharmacological actions. Notably, cocaine
binds with similar affinity to transporters for norepi-
nephrine (NET) and serotonin (SERT) as well as dop-
amine, to inhibit the neuronal reuptake and
inactivation of all three major monoamine neurotrans-
mitters. Studies employing gene-knockout mice lacking
expression of DAT proteins have found that cocaine
provides its rewarding cues through its effects on others
systems, and particularly through its interactions with
the SERT.6–8 Both DAT and SERT knockout mice have
been generated, as well as combined DAT and SERT
knockout mice. Whereas conditioned place preference
was achieved with cocaine in mice lacking DAT and
SERT,8 no place preference for cocaine was observed
in combined DAT and SERT knockout mice.9 These
findings suggest that, in the absence of DAT, the inter-
action of cocaine at the SERT might be sufficient to
induce cocaine reward.
It is hypothesized that stimulant and behavior-reinforc-
ing effects of cocaine are mediated importantly by the
central dopaminergic system.1–3 Cocaine binds at the
dopamine transporter (DAT) protein to result in inhibi-
tion of dopamine reuptake into presynaptic neurons, in-
creased synaptic dopamine concentration, and enhanced
dopaminergic neurotransmission.1–4 Most efforts to de-
velop a pharmacological treatment for cocaine abuse
and addiction have targeted either dopamine receptors
or DAT without clinical success to date.5 The dopamine
hypothesis of the actions of cocaine is a simplified model
that focuses virtually exclusively on the interaction of
To further study the roles of different transporter
systems in the behavioral effects of cocaine, ligands
exhibiting selectivity for specific monoamine transport-
ers would be valuable. The present study sought to
provide additional tools for use in investigations of the
Keywords: Serotonin transporter; Tropane; Affinity.
*
Corresponding author. Tel.: +1 203 401 4309; fax: +1 203 789
2119; e-mail: gtamagnan@indd.org
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.12.014

1132
G. Tamagnan et al. / Bioorg. Med. Chem. Lett. 15(2005) 1131–1133
H C
3
H C
N
H C
3
O
3
N
O
O
4 steps
N
O
a
O
O
I
SnMe
3
O
O
1 (-)-Cocaine
2 β -CIT
4a-h
3
H C
3
O
N
R
b
O
S
4a-h
Scheme 1. Reagents and conditions: (a) (SnMe3)2, Pd(PPh3)4, DME, reflux; (b) halogenothiophene, Pd(PPh3)4, toluene, reflux.
role of selective SERT inhibitors in the pharmacology of
cocaine and potential agents for use in the treatment of
stimulant addiction. We report the synthesis and in vitro
evaluation of 2b-carbomethoxy-3b-[4-(substituted-thio-
phenyl)]phenyltropanes as potent and selective SERT
inhibitors.
b-CIT was selectively demethylated by treatment with 1-
chloroethyl chloroformate (ACE-Cl) and methanol in
75% yield. Treatment of nor-b-CIT with trichloroethyl
chloroformate (Troc-Cl) led to the corresponding N-
protected carbamate 7 (Scheme 2). The same sequence
of stannylation (compound 8) and Stille cross-coupling
led to 9a–b, and subsequent treatment with diluted
NaOH in methanol led to the N-desmethyl analogues
of 4a–b (Scheme 2, Table 1).
b-CIT (2) was synthesized in four steps starting from co-
caine (1) in good yield.10 The key intermediary 3 was ob-
tained by treatment of 2 with hexamethyl-ditin in
presence of a catalytic amount of tetrakis(triphenyl-
phosphine)palladium (Scheme 1). The desired 40-thio-
phenylphenyltropane 4 was obtained by Stille cross-cou-
pling between 3 and the desired halogenothiophene
(Scheme 1, Table 1).
All final compounds were tested for their ability to inhi-
bit high-affinity binding of selective index radioligands
to SERT, DAT, or NET using cell membrane-contain-
ing homogenates of rat caudate (DA) and rat cere-
bral cortex (SERT, NET) as transporters sources.
Table 1. Neuropharmacological characterization of novel and comparison compoundsa
R
N
4
CO2Me
X
3
5
S
2
Compound
Substitution
R
SERT
DAT
NET
Selectivity SERT
versus DAT
Selectivity SERT
versus NET
4b
5a
4a
5b
4c
2
3-Thiophene
2-Thiophene
CH3
H
0.017 0.004
0.11 0.006
0.15 0.015
0.23 0.02
0.38 0.008
0.46 0.06
0.64 0.04
1.60
12.1
3
189 82
75.3 9.6
158 12
39 0.8
324 19
2.80 0.40
311 25
6190
710
111
346
28
11,118
685
12.2 0.9
52 12.8
6.4 0.27
6.43 0.9
0.96 0.15
4.42 1.64
16,540
2-Thiophene
3-Thiophene
CH3
H
1053
170
2-(5-Br)-Thiophene
b-CIT
CH3
17
2.1
853
6.1
4d
2-(5-Cl)-Thiophene
R,S-Citalopram
3-(4-Br)-Thiophene
2-(5-I)-Thiophene
2-(5-NH2)-Thiophene
2-(4,5-NO2)-Thiophene
(ꢀ)-Cocaine
CH3
6.9
486
10,338
46
4.9
3869
>2488
249
4h
4e
4f
4g
1
CH3
CH3
CH3
CH3
4.02 0.34
4.56 0.84
64.7 3.7
5,000
183 69
22.1 3.2
>10,000
>30,000
89
>10,000
1137 123
>30,000
>10,000
3320
>155
>6.0
0.08
>464
>2.0
3.2
1050
a Assays of novel compounds employed cell membrane-containing homogenates of rat forebrain (frontoparietal cerebral cortex for SERT and NET,
caudate-putamen for DAT), with [3H]paroxetine (0.17 nM) for SERT (blank = 10 lM R,S-fluoxetine, Sigma-RBI), [3H]GBR-12935 (0.20 nM) for
DAT (blank = 10 lM GBR-12909, Sigma-RBI), and [3H]nisoxetine (0.40 nM) for NET (blank = 10 lM desipramine, Sigma-RBI); potencies are as
inhibitory constant (Ki) SE, in nanomolar (nM), based on at least three separate determinations. Test agents are ranked by SERT-potency. Note
that for comparison, (ꢀ)-cocaine was evaluated in very similar assays with rat forebrain tissue, using [3H]citalopram for SERT, the phenyltropane
[3H]WIN-35,428 for DAT, and [3H]nisoxetine for NET, as reported elsewhere.11 R,S-Citalopram was evaluated with cell membranes from cell lines
genetically transfected with human genes for each transporter protein, assaying with [3H]citalopram for SERT, the phenyltropane [3H]RTI-55 for
DAT, and again [3H]nisoxetine for NET.12

G. Tamagnan et al. / Bioorg. Med. Chem. Lett. 15(2005) 1131–1133
1133
H3C
Troc
H
N
O
O
O
N
N
a
b
O
O
O
I
I
I
2 β-CIT
6
7
Troc
N
Troc
H
O
O
O
N
N
c
d
e
O
O
O
SnMe3
S
S
8
9a-b
5a-b
Scheme 2. Reagents and conditions: (a) (i) ACE-Cl, CH2Cl2, reflux; (ii) methanol, reflux; (b) Troc-Cl, Et3N, THF; (c) (SnMe3)2, Pd(PPh3)4, DME,
reflux; (d) bromothiophen, Pd(PPh3)4, toluene, reflux; (e) MeOH, NaOH 0.01 M.
Transporter potencies and SERT-selectivities of com-
pounds tested are provided in Table 1, along with data
for (ꢀ)-cocaine 1, b-CIT 2, and R,S-citalopram for com-
parison. All compounds exhibited greater affinity for
SERT than DAT and NET. The most potent com-
pounds at SERT were non-substituted thiophenes (com-
pounds 4a–b and 5a–b). The most potent compound, the
N-methyltropane 4b (Ki = 17 pM), was 710-times more
potent at SERT than at DAT, and over 11,000-times
more selective for SERT than NET. Its nor-analogue
5b showed somewhat less potency at SERT
(Ki = 230 pM) and much less selectivity (only 28-fold
vs DAT). This contrast appears to contradict the general
observation of higher SERT affinity and selectivity over
DAT for nortropanes than their corresponding tro-
panes.11 In the 2-thiophene series the 2-thiophene-tro-
pane 4a was slightly less potent for SERT than the
nortropane 5a (110 vs 150 pM) and 4a had somewhat
greater selectivity for SERT over DAT (111- vs 346-
times) and NET (685- vs 1050-times). Introduction of
4- or 5-substituents yielded compounds with relatively
lower potency and selectivity for SERT than the corre-
sponding non-substituted thiophenes.
Acknowledgements
This work was supported by the National Institutes of
Health (MH67066-01) and from Department of Veter-
ans Affairs, National Center for PTSD Alcohol Re-
search Center and NARSAD to G.D.T. and by an
award from the Bruce J. Anderson Foundation and
the McLean Private Donors Neuropharmacology Re-
search Fund to R.J.B.
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In conclusion, we report the synthesis of 10 new 40-thio-
phenephenyltropanes and their evaluation for the mono-
amine transporter potencies and selectivity. The 3-
thiophenphenyltropane 4b exhibited high potency
(Ki = 17 pM) and selectivity for SERT over DAT (710-
fold) and NET (over 11,000-fold). These new ligands
provide additional pharmacological tools of potential
value in attempting to correlate structure and trans-
porter selectivity with in vivo studies of behavioral
outcomes.
12. Dutta, A. K.; Fei, X.-S.; Beardsley, P. M.; Newman, J. L.;
Reith, M. E. A. J. Med. Chem. 2001, 44, 937.