Synthesis and monoamine transporter affinity of 2β-carbomethoxy-3β-(2-,3- or 4-substituted) biphenyltropanes

Article abstract of DOI:10.1016/S0960-894X(00)00317-6

A series of 11 novel 3β-substituted biphenyltropanes was synthesized and evaluated by selective radioligand binding assays for affinity to monoamine transporters. Both 5-HTT potency and selectivity for 5-HTT over DAT was greatest with electron withdrawing group at the 3_-position. (C) 2000 Elsevier Science Ltd. All rights reserved.

Full text of DOI:10.1016/S0960-894X(00)00317-6

Bioorganic & Medicinal Chemistry Letters 10 (2000) 1783±1785
Synthesis and Monoamine Transporter Anity of
2ꢀ-Carbomethoxy-3ꢀ-(200-, 300- or 400-substituted) Biphenyltropanes
Gilles Tamagnan,a,* Ronald M. Baldwin,a Nora S. Kula,b Ross J. Baldessarinib
and Robert B. Innisa
aYale University, School of Medicine, VA Connecticut HCS (116A2), 950 Campbell Avenue, West Haven, CT 06516, USA
bDepartment of Psychiatry & Neuroscience Program, Harvard Medical School, Mailman Research Center,
McLean Division of Massachusetts General Hospital, Belmont, MA 02478, USA
Received 4 April 2000; accepted 26 May 2000
AbstractÐA series of 11 novel 3b-substituted biphenyltropanes was synthesized and evaluated by selective radioligand binding
assays for anity to monoamine transporters. Both 5-HTT potency and selectivity for 5-HTT over DAT was greatest with electron
withdrawing group at the 300-position. # 2000 Elsevier Science Ltd. All rights reserved.
Introduction
the phenyl ring and removing the N-Me group enhance
selectivity for 5-HTT over DAT. We therefore hypothe-
sized that adding a second substitued phenyl moiety at
position 40 of the 3b-phenyl ring would increase selectivity
for the 5-HTT over DAT.
Tropane (8-azabicyclo[3.2.1]octane) derivatives have
been studied extensively in recent years, particularly in
relation to the pharmacology of the benzoyloxytropane
stimulant cocaine (1), encouraged by strong interest in
developing treatments for cocaine addiction. Several 3b-
phenyltropanes have been used as radiotracers for quan-
ti®cation of DAT in brain tissue, with clinical applications
in SPECT or PET brain scanning.1 4 In addition, radi-
olabeled derivatives of 2b-carbomethoxy-3-b-(4-iodo-
phenyl) tropane (b-CIT)5 and its precursor norCIT have
been used to quantify the serotonin (5-hydro-
ytryptamine) transporter (5-HTT), with limited success
because of inadequate selectivity for 5-HTT over DAT.
The proposed biphenyltropanes have the e€ect of adding
unsaturation to the 3b-phenyl ring, which is believed to
interact with speci®c amino acid residues in the binding
region of the transporter peptide chains. For example, in
the 5-HTT protein, tyrosine residue Tyr-95 in the putative
®rst hydrophobic transmembrane domain and Tyr-176 in
the third transmembrane domain have been proposed as
critical sites of molecular interaction with 5-HT and aro-
matic tropanes.10,11 For DAT, phenylalanine residues
play the same role.12 Interactions of the aromatic rings
of these amino acid residues with the phenyl ring of the
phenyltropane would explain the absolute requirement for
the 3b-phenyl substitution for DAT and 5-HTT potency.
In addition, the presence of an OH group in tyrosine resi-
dues in 5-HTT and its absence in the phenylalanine resi-
dues of DAT suggests that appropriate substituents on
a 40-phenyl substituted-tropane might yield di€erential
anity for 5-HTT over DAT. These aromatic residues in
transporter polypeptides may interact with 40-substituted
phenyltropanes by both p-stacking and hydrogen bonding
to increase potency and selectivity for 5-HTT. In this
study, we evaluated the e€ect of aromatic substitution on
the 3b-phenyl ring of 2b-carbomethoxy-3b-(40-iodophe-
nyl)tropane (b-CIT; RTI-55) on anity and selectivity
for 5-HTT over DAT.
Information is accumulating on structure±activity rela-
tionships of tropanes that bind at speci®c membrane
transporter proteins for DA, 5-HT and norepinephrine
(NE). Transporters are critical for the physiological inac-
tivation of monoamine neurotransmitters following their
extraneuronal release at synapses, and they are blocked by
cocaine and other stimulants. Assessment of substitution
at the 2b, 3b, and 8 positions of the tropane ring indicates
that high anity for DAT or 5-HTT requires an aromatic
group at the 3b position.6 Observations by Davies7,8 and
Carroll9 suggest that having unsaturated substituents on
*Corresponding author. Tel.: +1-203-932-5711 ext 3109; fax: +1-203-
937-3897; e-mail: gilles.tamagnan@yale.edu
0960-894X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00317-6

1784
G. Tamagnan et al. / Bioorg. Med. Chem. Lett. 10 (2000) 1783±1785
Synthesis
Binding and Discussion
Cocaine (1) was treated with 6 N HCl at re¯ux to give
ecgonine (2; Scheme 1). Treatment with POCl3 and MeOH
converted 2 to ecgonidine methyl ester (3). Grignard addi-
tion of phenyl-magnesium bromide to 3 led to compound
4. Treatment of 4 with iodine gave b-CIT (5). The tin
derivative 6 was obtained by treatment of b-CIT with
hexamethylditin in the presence of tetrakistriphenylphos-
phine palladium (Scheme 1).
The potency (Ki, nM) of the substituted biphenyltropane
compounds synthesized was evaluated by competition
against radiolabeled ligands selective for 5-HTT, DAT,
and NET in rat forebrain tissue evaluated by competition
against radiolabeled ligands selective for 5-HTT, DAT,
and NET in rat forebrain tissue (Table 1).14 17 In general,
40-substitution on the primary phenyl ring with phenyl
(i.e., p-biphenyl derivatives) led to much lower anity at
NET than at either 5-HTT or DAT. To retain high
potency for 5-HTT, meta (300)-substitution was preferred
over ortho (200) and para (400), particularly with small elec-
tronegative substituents (NO2, OH, and OCH3), which
yielded Ki values <2 nM, (Table 1). Moreover, when
the meta position was substituted with a bulkier electro-
negative group (300-COCH3 compound 7h), 5-HTT
potency fell by more than 22-fold to Ki=42 nM compared
to 7e (Table 1). In contrast, substitution to the para (400)
position on the secondary phenyl ring favored binding
to DAT over 5-HTT.
A series of 40-phenyl-substituted aromatic phenyltropanes
was synthesized by Stille coupling between 6 and a desired
bromo or iodo phenyl derivative (Scheme 2).13
Phenol 7j was obtained as the major product of reaction
of 3-bromophenol trimethylsilyl ether with 6 after work
up and puri®cation on silica gel. Compound 7k was
obtained by catalytic reduction of 7e under hydrogen
with 5% Pd/C.
Scheme 1.
Scheme 2.
Table 1. Anity of biphenyltropane derivatives to transporters in rat forebrain tissue for dopamine, serotonin and norepinephrine transporter
(Ki Æ SE, nM)
Compound
Substitution
5-HTTa
[3H]Paroxetine
DATa
[3H]GBR-12935
NET
[3H]Nisoxetine
Selectivity
5-HTT/DAT
b-CITb
7e
7j
7b
7f
7c
7h
7k
7a
7i
7g
I
0.46 Æ 0.06
0.80 Æ 0.08
1.39 Æ 1.10
1.89 Æ 0.20
29.7 Æ 0.9
32.0 Æ 4.5
42.0 Æ 3.0
67.3 Æ 2.9
79.5 Æ 4.1
179 Æ 20
5000
0.96 Æ 0.15
9.54 Æ 2.32
15.2 Æ 3.2
58.7 Æ 10.0
5.10 Æ 0.17
13.2 Æ 0.9
12.4 Æ 0.8
24.2 Æ 3.9
357 Æ 49
2.80 Æ 0.40
963 Æ 110
Ð
2.08
11.9
11.0
31.1
0.17
0.41
0.30
0.36
4.49
0.11
0.20
1.00
300-NO2
300-OH
300-OCH3
400-NO2
400-OCH3
300-COCH3
300-NH2
200-OCH3
400-COCH3
200-COCH3
200-NO2
>3000
370 Æ 13
352 Æ 35
>50,000
Ð
=10,000
754 Æ 80
>20000
>3000
19.1 Æ 1.8
1016 Æ 431
53000
7d
53000
aSelectivity 5-HTT/DAT: higher numbers are more selective for 5-HTT.
bb-CIT is 4-iodophenyltropane.5

G. Tamagnan et al. / Bioorg. Med. Chem. Lett. 10 (2000) 1783±1785
1785
Scheme 3.
The most 5-HTT-selective compounds were also those
with the highest 5-HTT potency (Table 1). They ranked:
300-OCH3 > 300-NO2 > 300-OH, with lesser selectivity
found with substitution at the 200 or 400 positions (Table 1).
The most 5-HTT-over-DAT selective compound, the 30-
OCH3 derivative (7b), had a Ki ratio >30, representing
the greatest selectivity reported to date for an N-methyl
tropane derivative.
Research Center) and the US Public Health service
(NIMH Grants MH-34006, MH-47370), an award from
the Bruce J. Anderson Foundation, and by the McLean
Private Donors' Neuropharmacology Research Fund.
References
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B.; Innis, R. B. J. Nucl. Med. 1997, 38, 1453.
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The present results, combined with previous ®ndings,7,10
suggest a possible packing e€ect between a transporter
amino acid such as Tyr-95 or Tyr-176 and the aromatic
ring of the phenyltropane. The high anity and selectiv-
ity of 300-electronegative-substituted (NO2, OH, OCH3)
compounds (7e, 7j, and 7b) may also suggest hydrogen
bonding of this compound may also occur at the trans-
porter, again perhaps Tyr-95 or Tyr. The possibility of
hydrogen bonding between a 300-methoxy substituent on
the secondary phenyl ring of phenyltropanes and Tyr-
176 or 95 of the transporter protein also seems consistent
with the rank order of selectivity of the phenyltropanes
tested: 300-OCH3 >300-NO2=300-OH > 200-OCH3; other
compounds were not 5-HTT-selective (Table 1). A 300-
methoxy group yielded higher 5-HTT-over-DAT selec-
tivity than a 300-nitro or acyl substitution. To further test
the hydrogen bonding hypothesis, we evaluated the 300-
hydroxy and 300-amino derivatives 7j and 7k (Scheme 3).
High 5-HTT anity and selectivity were still observed
with the 300-hydroxy compound. While hydrogen bonding
may exist other factors also must play a role in binding.
In general, the present results indicate that secondary 40-
phenyl substitution was tolerated on the phenyl ring
attached at position 3b of the tropane ring with reten-
tion of high 5-HTT anity. Moreover, meta- (300)-sub-
stitution on the secondary phenyl ring appears favored
over ortho-(20) and para-(40) to retain 5-HTT anity.
13. Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508.
14. Tejani-Butt, S. M. J. Pharmacol. Exp. Ther. 1992, 260, 427.
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30, 89.
16. Andersen, P. H. J. Neurochem. 1987, 1887.
17. Habert, E.; Graham, D.; Tahraoui, L.; Claustre, Y.; Lan-
ger, S. Z. Eur. J. Pharmacol. 1985, 118, 107.
Acknowledgements
The authors thank Dr Gary Rudnick for helpful discus-
sions. This work was supported in part by funds from the
US Department of Veterans A€airs (Schizophrenia