Synthesis and monoamine transporter affinity of 3′-analogs of 2-β-carbomethoxy-3-β-(4′-iodophenyl)tropane (β-CIT)

Article abstract of DOI:10.1016/j.bmcl.2004.02.043

The 3′-iodo positional isomer of 2-β-carbomethoxy-3-β- (4′-iodophenyl)tropane (β-CIT) and other 3′-substituted analogs were synthesized and evaluated for binding to monoamine transporters in rat forebrain and membranes of cell lines selectively expressing human transporter genes. All 3′-substituted compounds displayed affinity for both serotonin (SERT) and dopamine (DAT), but much less for norepinephrine transporters (NET), with selectivity for rat (r) or human (h) SERT over NET, but only 3′-iodo-substituted phenyltropanes showed selectivity for SERT versus DAT. The 3′-iodo, N-methyl analog of β-CIT (7) displayed 29-fold selectivity and high affinity for hSERT (K_i=9.6nM) over hDAT (K _i=279nM), and its nor-congener (8) showed even higher hSERT potency (K_i=1.2nM) and selectivity over DAT (415-fold).

Full text of DOI:10.1016/j.bmcl.2004.02.043

Bioorganic & Medicinal Chemistry Letters 14 (2004) 2117–2120
Synthesis and monoamine transporter affinity of 30-analogs of
2-b-carbomethoxy-3-b-(40-iodophenyl)tropane (b-CIT)
Frederic Bois,a Ronald M. Baldwin,a Nora S. Kula,b Ross J. Baldessarini,b
Robert B. Innisa and Gilles Tamagnana,c,
*
aYale School of Medicine, VA Connecticut HCS (116A2), 950 Campbell Avenue, CT 06516, USA
bDepartment of Psychiatry & Neuroscience Program, Harvard Medical School, Mailman Research Center
McLean Division of Massachusetts General Hospital, Belmont, MA 02478, USA
cInstitute for Neurodegenerative Disorders, suite 8A, 60 Temple Street, New Haven, CT 06510, USA
Received 22 January 2004; revised 5 February 2004; accepted 9 February 2004
Abstract—The 30-iodo positional isomer of 2-b-carbomethoxy-3-b-(40-iodophenyl)tropane (b-CIT) and other 30-substituted analogs
were synthesized and evaluated for binding to monoamine transporters in rat forebrain and membranes of cell lines selectively
expressing human transporter genes. All 30-substituted compounds displayed affinity for both serotonin (SERT) and dopamine
(DAT), but much less for norepinephrine transporters (NET), with selectivity for rat (r) or human (h) SERT over NET, but only
30-iodo-substituted phenyltropanes showed selectivity for SERT versus DAT. The 30-iodo, N-methyl analog of b-CIT (7) displayed
29-fold selectivity and high affinity for hSERT (Ki ¼ 9.6 nM) over hDAT (Ki ¼ 279 nM), and its nor-congener (8) showed even
higher hSERT potency (Ki ¼ 1.2nM) and selectivity over DAT (415-fold).
Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction
neuronally released DA, and so potentiates dopami-
nergic neurotransmission to result in the behaviorally
reinforcing, stimulating, and euphoriant effects associ-
ated with abuse of these drugs.5–9 Although effects of
cocaine on the DAT have received a great deal of re-
search attention, cocaine is an even more potent inhib-
itor of the transporters selective for 5-HT (SERT) and
norepinephrine (NET) on central neurons that produce
these monoamine neurotransmitters. Moreover, poten-
tiation of 5-HT probably contributes to the behaviorally
reinforcing effects of cocaine, indicating that effects at
both SERT and DAT may be important for the neu-
ropharmacodynamics and potential treatment of co-
caine addiction.10–12
Cocaine (1) and other psycho-stimulants exert multiple
physiological and behavioral effects by inhibiting the
presynaptic neuronal transport of dopamine (DA),
serotonin (5-HT), and norepinephrine (NE) in mam-
malian central nervous systems. Altered abundance or
functioning of these monoaminergic neurons is impli-
cated in the pathophysiology or treatment of several
neuropsychiatric disorders, including ParkinsonÕs dis-
ease, and schizophrenia, major mood disorders, and
attention deficit-hyperactivity disorder, as well as vari-
ous forms of substance abuse.1–4
Cocaine binds to a stereoselective recognition site of the
membrane DA transporter protein (DAT), a specific
molecular marker of DA neurons in mid- and forebrain
tissues. Neuronal reuptake by the DAT is the principal
physiological means of regulating synaptic availability
of DA. Inhibition of DA transport by stimulants
increases and prolongs extracellular concentrations of
Studies of the design of novel antagonists or high-
affinity partial-agonists for the DAT have focused
mainly on modifications of the nonhydrolyzable phen-
yltropane 2b-carbomethoxy-3b-phenyltropane (WIN
35,065-2; 2, Fig. 1).
This and similar stable analogs replace cocaineÕs meta-
bolically labile benzoate ester with a substituted aro-
matic ring at the 3-position of the tropane moiety.13
Substituents in position-2( 3, 3b, 3c), as well as
replacement of the bridge heteroatom with oxygen or
Keywords: Tropane; Cocaine; DAT; SERT; Monoamine.
* Corresponding author. Tel.: +1-2034014309; fax: +1-2037892119;
e-mail: gtamagnan@indd.org
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.02.043

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F. Bois et al. / Bioorg. Med. Chem. Lett. 14 (2004) 2117–2120
2. Chemistry
H3C
H3C
8
N
N
CO2Me
CO2Me
O
1
7
6
R
2
As summarized in Scheme 1, cocaine (1) was hydrolyzed
by refluxing in HCl to give ecgonine 4 (98% yield), which
was transformed into the corresponding ecgonidine
methyl ester (5, 54% yield) with POCl3 and MeOH.
Grignard addition to 5 with 3-trimethylsilylphenylmag-
nesium bromide in anhydrous ether at )78 °C gave
silane 6 (43%). Treatment of 6 with I2 and AgBF4 in
MeOH at rt yielded the 30-isomer of b-CIT 7 (60%).
Demethylation of the nitrogen was achieved with
1-chloroethyl chloroformate in 1,2-dichloroethane at
reflux followed by refluxing in MeOH to produce the
nortropane derivative 8 (62%).
5
4
3
O
1
2, R=H
3, R=I
3b R=Phenyl
3c R=3-thiophenyl
Figure 1.
R1
N
CO2Me
3'
R2
Additional five- and six-membered ring substituents
(Scheme 2) were introduced by means of Rieke coupling
between 7 and 3-thiophenylzinc iodide in THF at rt (9a,
97%). Stille coupling of 7 with tributylfuran-2-yl-
stannane in DMF at rt gave 9b (67%) and with tri-
methylphenylstannane in refluxing toluene gave 9c (30%
yield).
7, R1= CH3, R2=I
8, R1= H, R2=I
9a,R1=CH3, R2=3-thiophenyl
9b,R1=CH3, R2=2-furanyl
9c,R1=CH3, R2=phenyl
Figure 2.
carbon, also can be tolerated without significant loss of
binding affinity for the DAT (Fig. 1).14;15
3. Transporter affinities
Radiolabeled phenyltropane derivatives have been used
successfully for SPECT and PET radionuclide imaging
to visualize and quantify DAT in human brain tis-
sue.16–20 Derivatives of b-CIT (RTI-55; 3) and its
N-desmethyl analog nor-b-CIT21 have also been con-
sidered as potential radiotracers to quantify SERT, but
with limited success owing to their poor selectivity for
SERT versus DAT. To seek more SERT-selective
ligands, we built on observations reported by Davies
and colleagues22;23 and Blough et al.24 indicating that
selectivity for SERT over DAT was enhanced by
unsaturated substituents on the phenyl ring and by
N-demethylation of phenyltropanes. Our previous
findings confirmed these trends with a novel series of 200-,
300-, or 400-substituted biphenyl tropanes.25 We now
report on the synthesis of novel 3Õ-substituted analogs
(7–9; Fig. 2) and their binding affinity to the three major
monoamine transporters, SERT, DAT, and NET, in
rat forebrain and in membranes of cells selectively
expressing the human genes for these membrane
proteins.
Transporter potency (Ki, nM) of the novel 30-substituted
phenyltropanes was evaluated by their competition
against radioligands selective for SERT, DAT, and
NET, using rat (r) forebrain tissue and membranes of
cell lines transfected to express human (h) genes for
these transporter proteins selectively (Table 1). Selec-
tivity for SERT versus DAT or NET is expressed as Ki
ratios, with larger values indicating greater selectivity
(Table 2).
The 30-iodo or meta-substituted positional isomer 7 was
similarly less potent than the 40-iodo or para-congener
b-CIT (3) at both rSERT (21-fold) and hSERT (14-fold)
proteins, but was >3571 times less potent at NET, and
104–581 times less potent at rDAT and hDAT sites
(Table 1), with substantial selectivity for SERT over
DAT (by 10- to 29-fold) or NET proteins (108- to
>1018-fold). Similar to our previous findings,25 selec-
tivity for SERT was increased even further by
N-demethylation, such that compound 8 exhibited
H3C
N
H3C
H3C
.HCl
N
N
CO2Me
c
CO2Me
CO2H
a
b
O
O
OH
5
4
1
H3C
H
H3C
N
N
N
CO2Me
CO2Me
CO2Me
d
e
Si
I
I
6
7
8
Scheme 1. Reagents and conditions: (a) HCl aq, D; (b) POCl3, MeOH, D; (c) 3-SiMe3–C6H5MgBr, Et2O; (d) I2/AgBF4, MeOH; (e) (1) ACE-Cl,
C2H4Cl2, D, (2) MeOH, D.

F. Bois et al. / Bioorg. Med. Chem. Lett. 14 (2004) 2117–2120
2119
H3C
N
CO2Me
I
7
c
a
b
H3C
H3C
H3C
N
N
N
CO2Me
CO2Me
CO2Me
O
9c
9b
9a
S
Scheme 2. Reagents and conditions: (a) 3-thiophenylzinc iodide, PdCl2(PPh3)2, THF, rt; (b) tributylfuran-2-stannane, PdCl2(PPh3)2, DMF, rt;
(c) trimethylphenylstannane, Pd(PPh3)4, toluene, D.
Table 1. Binding affinity to monoamine transporters [Ki SEM (nM)]
Compound
rDAT
hDAT
rSERT
hSERT
rNET
hNET
7* (YP 241)
8 (YP 256)
9a* (YP-242)
9b* (YP 243)
9c* (YP 244)
b-CIT26
100 11
182
8
9.82 0.80
1.35 0.13
82.5 4.8
9.63 0.4
1.23 0.04
74.1 5.1
55.1 2.2
38.6 2.0
N/D
>10,000
282 12
>10,000
>10,000
>10,000
2.80 0.40
1.85 0.21
1039 53
118 3
>20,000
21.2 4.6
23.7 1.3
30.8 4.9
9.59 0.34
1.33 0.15
0.64 0.097
438 52
29.8 2.6
54.0 1.3
8.83 0.98
0.48 (2)
41.7 2.2
35.2 1.1
3024 438
1360 183
11.2 0.7
0.41 0.03
0.062 0.001
Nor-b-CIT26
*Tested as HCl salt;26 (2) Kd value from Scatchard plot.
Table 2. Binding selectivity ratios
ylated compound 8 (nor-7) as a potentially useful SERT
radioligand are now required.
Compound rSERT/
rDAT
hSERT/
hDAT
rSERT/
rNET
hSERT/
hNET
7
10.218.9
15.7
0
>102108
356
0.402
8
241
>121
>240
>283
95.9
>270
54.9
Acknowledgements
9a
9b
9c
0.280
0.740
0.270
0.980
0.229
Supported in part by NARSAD (Young Investigator
Award), USDVA Schizophrenia Research Center, an
award from the Bruce J. Anderson Foundation, and by
the McLean Private Donors Neuropharmacology
Research Fund.
35.2
16-fold (r) or 415-fold (h) selectivity toward SERT over
DAT, and 96 (h) to 241-fold (r) selectivity toward SERT
over NET (Table 2).
Substitution at position 30 with larger, aromatic groups,
including the 30-phenyl, thiophenyl, and furanyl deriv-
atives (compounds 9a–9c) resulted in 4- to 8-fold lower
SERT potency and 3- to 10-fold higher DAT affinity,
with loss of selectivity for SERT over DAT, all com-
pared to the corresponding N-methyl, 30-iodo congener
7, and in the descending rank order of rSERT potency:
9c (30-furanyl) P 9b (30-thiophenyl) > 9a (30-phenyl;
Tables 1 and 2).
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