N-8-substituted benztropinamine analogs as selective dopamine transporter ligands

Article abstract of DOI:10.1016/j.bmcl.2005.08.111

A series of N-8-substituted benztropinamines was synthesized and evaluated for binding at the dopamine (DAT), serotonin (SERT), norepinephrine (NET) transporters, and muscarinic M1 receptors. In general, the isosteric replacement of the C-3 benzhydrol eth

Full text of DOI:10.1016/j.bmcl.2005.08.111

Bioorganic & Medicinal Chemistry Letters 15 (2005) 5419–5423
N-8-Substituted benztropinamine analogs as selective
dopamine transporter ligands
Peter Grundt,a Theresa A. Kopajtic,b Jonathan L. Katzb and Amy Hauck Newmana,*
aMedicinal Chemistry Section, National Institute on Drug Abuse—Intramural Research Program,
National Institutes of Health, 5500 Nathan Shock Drive, Baltimore, MD 21224, USA
bPsychobiology Section, National Institute on Drug Abuse—Intramural Research Program, National Institutes of Health,
5500 Nathan Shock Drive, Baltimore, MD 21224, USA
Received 9 August 2005; revised 31 August 2005; accepted 31 August 2005
Available online 6 October 2005
Abstract—A series of N-8-substituted benztropinamines was synthesized and evaluated for binding at the dopamine (DAT), sero-
tonin (SERT), norepinephrine (NET) transporters, and muscarinic M1 receptors. In general, the isosteric replacement of the C-3
benzhydrol ether of benztropine by a benzhydryl amino group was well tolerated at the DAT. However, for certain N-8 substituted
derivatives, selectivity over muscarinic M1 receptor affinity was reduced.
Ó 2005 Elsevier Ltd. All rights reserved.
O
Cocaine (1) is an effective psychomotor stimulant drug
that is subject to widespread abuse. Among the multiple
actions of cocaine, the inhibition of dopamine uptake
via the dopamine transporter (DAT) is thought to be
the primary mechanism underlying its stimulant and
reinforcing effects.1–4
CH3N
N8
CH3
7
OCH3
2
3
O
6
H
H
O
O
The dopamine uptake inhibitor benztropine (2, BZT)
has structural similarities to cocaine (1), however, the
behavioral effects of BZT and many of its analogs differ
from those of cocaine.5 Structure–activity relationships
(SAR) for benztropine-based analogs have been devel-
oped in an attempt to provide insight into dopamine
transporter function and the neurobiological substrates
involved in cocaine abuse6,7 and these efforts may also
provide leads for the discovery of medications for the
treatment of cocaine abuse (see Fig. 1).
1: cocaine
2: BZT (benztropine)
Figure 1. Structures of cocaine and benztropine (BZT).
been introduced into the 2-position,10,11 and the conse-
quence of substituents at the 6,7-bridgehead has been
reported.12–15 Furthermore, the SAR of analogs bearing
different substituents on the N-8 nitrogen have been
explored.16–19
A variety of structural analogs of BZT have been syn-
thesized to improve affinity for the DAT and selectivity
among the sites at which the analogs bind. For example,
the effect of various substituents on the C-3 benzhydrol
ether moiety has been studied,8,9 an ester function has
In continuation of the efforts to explore the SAR of BZT
analogs, this report examines the isosteric replacement
of the benzhydrol ether moiety by a benzhydryl amino
group. If such a modification is tolerated at the DAT,
it is expected that, compared to their corresponding
BZT analogs, the resulting benztropinamine derivatives
4a–m might be more chemically stable and the water sol-
ubility might be improved.
Keywords: Monoamine transporters; Dopamine transporters; Cocaine;
Muscarinic M1 receptor; Benztropine; Benztropinamine; Ligand
binding.
The synthesis of the N-8 substituted benztropinamine
analogs is depicted in Scheme 1. In short, the a-tropin-
*
Corresponding author: Tel.: +1 410 550 6568; fax: +1 410 550
6855; e-mail: anewman@intra.nida.nih.gov
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.08.111

5420
P. Grundt et al. / Bioorg. Med. Chem. Lett. 15 (2005) 5419–5423
CH
3N
F
CH3N
a
H
H
R4 = CH3
N
NHR4
H3C
3a: R4 = H
3b: R4 = CH3
F
4e
R4 = H
a
R3N
HN
CH
3N
F
F
R2
c or d
R3Br
b
H
H
H
R1 = R2 = 4-F
HN
HN
HN
R1
F
F
4a: R1 = R2 = 4-F
4b: R1 = R2 = 4-Cl
4c: R1 = 4-Cl, R2 = H
4d: R1 = R2= 3,4-Cl
4f
4g: R3 = allyl
4h: R3 = n-butyl
4i: R3 = 4-phenylbutyl
4j: R3 = [3-(N-phenyl)-propionamido]-
4k:R3 = 2-ethylamino
4l: R3 = [2-(1H-indol-3-yl)-ethyl]-
CH3N
CH3N
F
H
a
N
NH2
H
H
F
5
4m
Scheme 1. Synthesis of the benztropinamine analogs. Reagents and conditions: (a) 1—1 eq. benzhydryl chloride analogue, acetonitrile, reflux, 34–
71%. (b) 1—1-Chloroethyl chloroformate, 1,2-dichloroethane, Na2CO3, reflux. 2—Methanol, reflux, 92%. (c) 1.0 eq. alkyl bromide, NaHCO3,
acetonitrile, sealed tube, 120 °C, 58–65%; 4k: 1. 2-(2-bromoethyl)-isoindoline-1,3-dione, 2. hydrazine, 51%. (d) 4l: 1—2-indoleacetic acid, DCC,
HOBt, 2—LiAlH4, 67%.
amines 320 were alkylated with the appropriate benzhy-
dryl chloride to give the benztropinamines 4a–e in low
to moderate yields. The N-nor-derivative 4f was pre-
pared in excellent yield from 4a by N-demethylation
with 1-chloroethyl chloroformate followed by methanol-
ysis. The N-8 substituted derivatives 4g–j were prepared
from 4f by alkylation with the appropriate alkylbro-
mide. Side products resulting from potential alkylation
at the benzhydryl nitrogen were not observed, presum-
ably due to steric hindrance. Compounds 4k and 4l were
prepared using standard procedures. In order to test the
influence of the configuration at C-3 of the tropane moi-
ety on binding at the monoamine transporters and the
M1 receptors, the b analog 4m was synthesized from
the b-tropinamine 5.21 All final compounds were puri-
fied by column chromatography and crystallization of
their oxalate or tartrate salts.22
strates a ꢀ3-fold lower affinity at the DAT than the
ether derivative 6a (Ki = 4.11 nM). Under these assay
conditions 6a exhibits DAT over SERT and DAT over
NET selectivities of 794- and 148-fold, respectively.14
For 4a the binding selectivities of DAT over SERT
and DAT over NET (Table 2) appear to be preserved.
4a (Ki = 7.81 nM) exhibits a very similar binding affinity
at the M1 receptor as 6a (Ki = 11.6 nM).14 Thus, due to
the lower binding affinity of 4a at the DAT, the DAT
over M1 receptor selectivity for this compound is some-
what reduced as compared to 6a.
A comparison between the a derivative 4a and the b deriv-
ative 4m reveals that configuration at C-3 plays a pivotal
role for the binding affinity at the DAT and the NET, and
to a somewhat lesser extent at the SERT and the M1
receptor. As such, compared to 4a, the binding affinity
of 4m at the DAT is reduced by 58-fold. A similar SAR
at the DAT concerning the configuration at C-3 has been
previously described for the benzhydrol ether analogs.8
Binding affinities of the novel benztropinamines 4 and
selected BZT analogs 6 at the DAT in rat brain mem-
branes are depicted in Table 1. The benztropinamine
derivative 4a is the amine analog of the benzhydrol ether
derivative 6a. Compound 4a (Ki = 11.3 nM) demon-
The introduction of a methyl group at the benzhydryl
nitrogen, resulting in the tertiary amine 4e, reduces the

P. Grundt et al. / Bioorg. Med. Chem. Lett. 15 (2005) 5419–5423
5421
Table 1. Binding data at the DAT for the benztropinamines 4 and selected BZT derivatives 6 in rat brain membranesa
R3N
3N
R
R2
R2
H
H
4N
O
R
R1
R1
4
6
R4
Compound
R1
R2
R3
DAT [3H]WIN 35,428 Ki SEM (nM)
4a
4b
4-F
4-Cl
4-Cl
3,4-Cl
4-F
4-F
4-F
4-F
4-F
4-F
4-F
4-F
4-F
4-F
4-F
4-F
4-F
4-F
4-Cl
H
CH3
CH3
CH3
CH3
CH3
H
H
11.3 1.61
38.1 5.35
36.5 0.66
5.35 0.25
123 15.5
8.45 0.23
26.8 3.43
21.5 2.31
11.7 0.39
4.61 0.54
12.5 1.73
64.5 4.32
661 35
4.11 0.5014
6.22 0.73
5.59 0.62
29.2 3.24
74.3 5.42
H
4c
4d
H
3,4-Cl
4-F
4-F
4-F
4-F
4-F
4-F
4-F
4-F
4-F
4-F
4-F
4-F
4-F
H
4e
CH3
H
4f
4g
Allyl
n-Butyl
4-Phenylbutyl
H
4h
4i
H
H
4j
4k
[3-(N-Phenyl)-propionamido]-
2-Ethylamino
H
H
4l
[2-(1H-Indol-3-yl)-ethyl]-
CH3
CH3
H
4mb
6a
H
n/a
n/a
n/a
n/a
6b
6c
[3-(N-Phenyl)-propionamido]-
2-Ethylamino
[2-(1H-Indol-3-yl)-ethyl]-
6d
Cocaine
a Each Ki value represents data from at least three independent experiments, each performed in triplicate. Ki values were analyzed by GraphPad
Prism. A detailed description of the binding assay methods has been previously published.23
b The configuration at C-3 is beta.
Table 2. Binding data and selectivities for the benztropinamines 4 at the SERT, NET and muscarinic M1 receptors
Compound
SERTa
[3H]Citalopram
Ki SEM (nM)
NETa
[3H]Nisoxetine
Ki SEM (nM)
M1a
[3H]Pirenzepine
Ki SEM (nM)
SERT/DAT
NET/DAT
M1/DAT
4a
4b
4c
4d
4e
4f
8685 436
4180 623
13,600 1040
3010 332
17,200 2460
4150 368
3920 581
2640 27.6
502 68.1
1810 269
7580 325
3170 233
259 33.6
7.81 1.17
50.0 7.10
9.3 0.27
17.8 2.27
18.6 2.48
154 19.6
130 10.5
454 36.7
438 56.2
2540 124
2110 109
413 6.55
59.4 7.3
11.6 0.9314
769
110
373
563
140
491
146
123
43
160
199
87
0.7
1
0.3
3
48
68
8410 724
997 107
0.2
18
5
118
208
136
139
536
284
128
230
205
4g
4h
4i
5580 821
2920 209
1630 115
2470 83.3
3550 222
8250 939
15,2000 12,300
844 56.514
21
37
551
169
6
4j
608 41
10,900 1090
347 41.3
132
872
5
4k
4l
4m
6a
48,500 6370
3260 10814
73
793
0.1
3
a A detailed description of the binding assay conditions have been previously published.18 Binding data were analyzed as described in Table 1.
binding affinity at the DAT by 11-fold compared to the
secondary amine 4a. The binding affinities at the SERT
and the M1 receptor (ꢀ2-fold lower) are far less affected
by this modification than the binding affinity at the NET
(ꢀ5-fold lower).
tion patterns on the benzhydryl moiety was evaluated
with analogs 4b–d. Compared to the 4,40-difluoro ana-
log, 4a, the 4,40-dichloro as well as 4-monochloro substi-
tution, compounds 4b and 4c, reduced binding affinity at
the DAT by approximately 3-fold. As with 4a, the M1
over DAT selectivity was reduced compared to the cor-
responding ether derivatives. Only the derivative 4d,
with a 3,4-dichloro substitution on both phenyl rings,
The impact on the binding affinity at the monoamine
transporters and the M1 receptor of different substitu-

5422
P. Grundt et al. / Bioorg. Med. Chem. Lett. 15 (2005) 5419–5423
has a somewhat higher binding affinity at the DAT (2-
fold) and a higher DAT over M1 selectivity than 4a.
However, with 4d DAT selectivity over NET was re-
duced by 3-fold.
References and notes
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5. Katz, J. L.; Izenwasser, S.; Kline, R. H.; Allen, A. C.;
Newman, A. H. J. Pharmacol. Exp. Ther. 1999, 288, 302.
6. Katz, J. L.; Kopajtic, T.; Agoston, G. E.; Newman, A. H.
J. Pharmacol. Exp. Ther. 2004, 288, 302.
In the benzhydrol ether (BZT) series, it has been shown
that N-demethylation at the N-8 nitrogen and alkylation
lead to higher DAT over muscarinc M1 selectivities.
Likewise, in compounds 4f–k this modification is gener-
ally well tolerated at the DAT and leads to a higher
DAT over M1 selectivity profile than the parent com-
pound 4a. Furthermore, these N-8 modified compounds
show a retained if not higher DAT over NET selectivity.
However, with the exception of 4k, all of these com-
pounds demonstrate a somewhat lower DAT over
SERT selectivity than 4a.
7. Newman, A. H.; Kulkarni, S. Med. Res. Rev. 2002, 22,
429.
8. Newman, A. H.; Kline, R. H.; Allen, A. C.; Izenwasser, S.;
George, C.; Katz, J. L. J. Med. Chem. 1995, 38, 3933.
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10. Meltzer, P. C.; Liang, A. Y.; Madras, B. K. J. Med. Chem.
1994, 37, 2001.
11. Zou, M. F.; Kopajtic, T.; Katz, J. L.; Newman, A. H.
J. Med. Chem. 2003, 46, 2908.
As such, the anilino amide derivative 4j exhibits the
highest DAT binding affinity (Ki = 4.61 nM) and the
highest DAT over NET and M1 selectivities in this series
(535- and 551-fold, respectively). Incidentally, the corre-
sponding ether derivative 6b (Ki = 4.61 nM) has a simi-
12. Meltzer, P. C.; Wang, B.; Chen, Z. M.; Blundell, P.;
Jayaraman, M.; Gonzalez, M. D.; George, C.; Madras, B.
K. J. Med. Chem. 2001, 44, 2619.
13. Simoni, D.; Roberti, M.; Rondanin, R.; Baruchello, R.;
Rossi, M.; Invidiata, F. P.; Merighi, S.; Varani, K.; Gessi,
S.; Borea, P. A.; Marino, S.; Cavallini, S.; Bianchi, C.;
Siniscalchi, A. Bioorg. Med. Chem. Lett. 2001, 11, 823.
14. Grundt, P.; Kopajtic, T. A.; Katz, J. L.; Newman, A. H.
Bioorg. Med. Chem. Lett. 2004, 14, 3295.
15. Simoni, D.; Rossi, M.; Bertolasi, V.; Roberti, M.;
Pizzirani, D.; Rondanin, R.; Baruchello, R.; Invidiata,
F. P.; Tolomeo, M.; Grimaudo, S.; Merighi, S.; Varani,
K.; Gessi, S.; Borea, P. A.; Marino, S.; Cavallini, S.;
Bianchi, C.; Siniscalchi, A. J. Med. Chem. 2005, 48,
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lar binding affinity at the DAT and
a similar
selectivity profile over the NET and the muscarinic
M1 receptor.17 However, the benzhydrol ether deriva-
tive has a somewhat higher DAT over SERT selectivity
than 4j, (239-fold compared to 132-fold).17
The N-8 ethylamino derivative 4k shows a similar bind-
ing affinity at the DAT as the parent compound 4a. The
DAT over SERT and NET selectivity profile is retained,
if not somewhat improved. Compared to 4a, the binding
affinity at the muscarinic M1 receptor is greatly reduced
(170-fold).
The indole derivative 4l differs from the rest of the N-8
modified derivatives (compounds 4f–k) in that its bind-
ing affinity at the DAT is 6-fold lower than that of the
parent compound 4a. Further, its SERT affinity is the
highest within the series, presumably due to the indole
moiety as a serotonin-like pharmacophore, resulting in
only a 5-fold DAT over SERT selectivity. Indeed, the
DAT/SERT dual activity has been suggested to also
have promise as a cocaine-abuse medication target.24
Interestingly, the corresponding benzhydrol ether deriv-
ative 6d demonstrated a 5-fold lower binding affinity at
the DAT compared to that of 6a, but showed a 160-fold
DAT over SERT selectivity (Ki (SERT) = 4600 nM).
16. Agoston, G. E.; Wu, J. H.; Izenwasser, S.; George, C.;
Katz, J.; Kline, R. H.; Newman, A. H. J. Med. Chem.
1997, 40, 4329.
17. Robarge, M. J.; Agoston, G. E.; Izenwasser, S.; Kopajtic,
T.; George, C.; Katz, J. L.; Newman, A. H. J. Med. Chem.
2000, 43, 1085.
18. Newman, A. H.; Robarge, M. J.; Howard, I. M.;
Wittkopp, S. L.; George, C.; Kopajtic, T.; Izenwasser,
S.; Katz, J. L. J. Med. Chem. 2001, 44, 633.
19. Kulkarni, S. S.; Grundt, P.; Kopajtic, T.; Katz, J. L.;
Newman, A. H. J. Med. Chem. 2004, 47, 3388.
20. Berdini, V.; Cesta, M. C.; Curti, R.; DÕAnniballe, G.; Di
Bello, N.; Nano, G.; Nicolini, L.; Topai, A.; Allegretti, M.
Tetrahedron 2002, 58, 5669.
21. Lewin, A. H.; Sun, G. B.; Fudala, L.; Navarro, H.; Zhou,
L. M.; Popik, P.; Faynsteyn, A.; Skolnick, P. J. Med.
Chem. 1998, 41, 988.
In summary, these 3-amino derivatives of BZT show a
novel structural approach to the development of DAT
selective compounds. In general, the isosteric replace-
ment of the benzhydrol ether moiety by an amino benz-
hydryl group is well tolerated at the DAT and compared
to their ether counterparts, these derivatives show a sim-
ilar SAR. In vivo studies are currently underway.
22. Note: All new compounds have been fully characterized
by spectroscopic means. Representative data: 4a—IR: m
1
3303. H NMR (CDCl3): d 1.57 (d, J 13.2, 2H), 1.96–2.05
(m, 6H), 2.25 (s, 3H), 2.74 (t, J 6.6, 1H), 3.09 (s, 2H), 4.85
(s, 1H), 6.93–6.97 (m, 4H), 7.18–7.22 (m, 4H). 13C NMR
(CDCl3): d 26.97, 37.05, 40.92, 46.92, 60.84, 63.29, 115.31
(JCF 21), 128.91 (JCF 8), 139.67 (JCF 3), 161.44 (JCF 243).
4j—IR: m 3313, 1674. 1H NMR (CDCl3): 1.44 (s, 1H), 1.72
(d, J 14.0, 2H), 2.00–2.08 (m, 4H), 2.18 (m, 2H), 2.45 (t, J
5.6, 2H), 2.69 (t, J 5.6, 2H), 2.90 (t, J 6.0, 1H), 3.33 (s, 2H),
4.89 (s, 1H), 7.19–7.25 (m, 6H), 7.47 (d, J 7.6, 2H), 7.98–
7.25 (m, 5H), 11.53 (s, 1H). 13C NMR (CDCl3): d 26.99,
34.52, 37.27, 47.22, 48.78, 58.55, 63.27, 115.74 (JCF 21),
Acknowledgments
This work was funded by the National Institute on Drug
Abuse-Intramural Research Program. P.G. was sup-
ported by a NIH Visiting Fellowship.

P. Grundt et al. / Bioorg. Med. Chem. Lett. 15 (2005) 5419–5423
5423
119.78, 123.82, 129.30, 129.31 (JCF 8), 139.14, 139.85 (JCF
4), 162.1 (JCF 245), 171.43.
23. Houlihan, W. J.; Boja, J. W.; Kopajtic, T. A.; Kuhar, M.
J.; Degrado, S. J.; Toledo, L. Med. Chem. Res. 1998, 8, 77.
24. Sora, I.; Hall, F. S.; Andrews, A. M.; Itokawa, M.; Li,
X. F.; Wei, H. B.; Wichems, C.; Lesch, K. P.; Murphy,
D. L.; Uhl, G. R. Proc. Natl. Acad. Sci. U.S.A. 2001,
98, 5300.