N-[(3S)-1-Benzylpyrrolidin-3-yl]-(2-thienyl)benzamides: Human dopamine D4 ligands with high affinity for the 5-HT2A receptor

Article abstract of DOI:10.1016/j.bmcl.2005.08.051

A series of N-[(3S)-1-benzylpyrrolidin-3-yl]-(2-thienyl)benzamides 8 has been prepared and found to bind with high affinity to the human D₄ (hD₄) and 5-HT_2A receptors. Several compounds displayed selectivity for these rece

Full text of DOI:10.1016/j.bmcl.2005.08.051

Bioorganic & Medicinal Chemistry Letters 15 (2005) 5253–5256
N-[(3S)-1-Benzylpyrrolidin-3-yl]-(2-thienyl)benzamides:
Human dopamine D4 ligands with high affinity
for the 5-HT2A receptor
Jalaj Arora, Michel Bordeleau, Laurence Dube, Keith Jarvie, Lucy Mazzocco,
Jack Peragine, Ashok Tehim and Ian Egle*
NPS Pharmaceuticals, 6850 Goreway Dr., Mississauga, Ont., Canada L4V 1V7
Received 27 July 2005; revised 15 August 2005; accepted 15 August 2005
Available online 15 September 2005
Abstract—A series of N-[(3S)-1-benzylpyrrolidin-3-yl]-(2-thienyl)benzamides 8 has been prepared and found to bind with high
affinity to the human D4 (hD4) and 5-HT2A receptors. Several compounds displayed selectivity for these receptors versus hD2
and a1 adrenergic receptors of over 500-fold.
Ó 2005 Elsevier Ltd. All rights reserved.
The diverse pharmacology of the atypical antipsychotic
clozapine1 has complicated the search for newer agents
with more tolerable side-effect profiles. The efforts to
dissect the pharmacology required for novel antipsy-
chotics have led to the conclusion that D4 antagonism
alone may not be sufficient for clinical efficacy2 however,
such compounds might yet find use in the treatment of
attention deficit hyperactivity disorder or mood disor-
ders.3,4 There is evidence that a D4 antagonist that also
possesses serotonergic activity (i.e., 5-HT2A antagonism)
might function as a useful antipsychotic.5 There has
been considerable interest in the development of tool
compounds with the appropriate pharmacology to yield
answers to this question.6–9
systems,12,13 suggesting a synergistic relationship be-
tween the dopaminergic and serotonergic systems.
Fananserin 1 was the first selective D4/5-HT2A antago-
nist to undergo clinical trials for schizophrenia. It has
high affinity for D4 (Ki 2.9 nM) and 5-HT2A (Ki
0.37 nM) receptors, and is over 100-fold selective versus
H1, a1 adrenergic, 5-HT1A and D2 dopamine recep-
tors.14 Development of this compound was halted fol-
lowing phase II clinical trials due to lack of efficacy.15
The extent to which this result precludes the use of a
D4/5-HT2A antagonist is difficult to discern because D4
receptor blockade in vivo was not demonstrated at
clinically relevant doses.16
The combination of D4 and 5-HT2A receptor blockade is
attractive for a number of reasons. A favourable 5-HT2/
D2 ratio may limit the propensity of a compound to in-
duce extrapyramidal symptoms (EPS).10 5-HT2A antag-
onists are also known to be efficacious in the treatment
of negative symptoms of schizophrenia.11 In addition,
cortical dopaminergic systems are regulated by
5-HT indirectly via glutamatergic and GABAergic
N
O
S
N
O
N
N
O
N
O
H
F
O
Cl
1
N
2
H
Herein we wish to report the discovery of a novel class
of compounds with D4/5-HT2A activity that is an exten-
sion of previous work.17 The broad-spectrum dopamine
ligand YM-43611 2 from Yamanouchi Pharmaceuti-
cals18 was used as the starting point for the design of
compounds for this study. YM-43611 has affinities for
the D2, D3 and D4 receptors of 220, 21 and 2.1 nM,
Keywords: Dopamine; Serotonin; Schizophrenia; D4; 5-HT2A.
*
Corresponding author. Tel.: +1 90 56 7 70 831; fax: +1 90 56 77 95
95; e-mail: iegle@npsp.com
Present address: Memory Pharmaceuticals Corp., 100 Philips
Parkway, Montvale, NJ 07645, USA.
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.08.051

5254
J. Arora et al. / Bioorg. Med. Chem. Lett. 15 (2005) 5253–5256
respectively. The compounds for this study were pre-
pared as illustrated in Scheme 1, starting from commer-
cially available (3S)-3-amino-1-benzylpyrrolidine 3.
Compound 3 was acylated with either 3- or 4-iodo
benzoyl chloride, followed by a Suzuki coupling with
thiophene-2-boronic acid under typical conditions to
provide 5a (meta isomer) or 5b (para isomer). N-deben-
zylation was best accomplished in a two-step procedure
using 2,2,2-trichloroethyl chloroformate, followed by
zinc reduction to provide the key intermediates 7a, b.19
Hydrogenolysis of the N-benzyl protecting group failed,
presumably due to the thiophene moiety present in the
compounds, and other chloroformate deprotections
provided less satisfactory chemical yields. Finally, 7a,
b were alkylated with the various benzylic halides under
typical basic conditions in parallel to provide the com-
pounds for this study, 8a–o (see Table 1). Following col-
umn chromatography, they were determined to be of
sufficient purity (by 1H NMR) for pharmacological
assessment. For the purposes of this study enantiomeric
purity was not assessed, as retention of chirality was
expected under the reaction conditions employed.
8b, 8c and 8d, and 8e and 8f). There is a strong prefer-
ence for the para isomer of these pairs over the meta
isomer. In the case of the positional isomers 8a and
8b, the ratio of affinities for the 5-HT2A receptor is
256-fold in favour of the para isomer. The para isomer
also exhibits lower affinity for the dopamine D2 and a1
adrenergic receptors, while dopamine D4 receptor affin-
ity remains unaffected. This results in ligands with high
affinity for D4/5-HT2A receptors with good selectivity
over dopamine D2 and a1 adrenergic receptors (e.g.,
8a, 8k). The nature and position of the substituent
on the benzyl group had a less dramatic effect on the
affinity. There was a slight preference for para substitu-
tion over meta or ortho (compare 8c to 8g, and 8j to
8k), but in all cases the unsubstituted derivative 8a
had the highest affinity. In our previous paper in this
area, we have shown that there is a preference for
the S enantiomer over the R, and that methylation of
the amide nitrogen or replacement of the amide with
a sulfonamide was not tolerated.17
Previous work has demonstrated that a similar com-
pound was a dopamine D4 antagonist;17,21 however,
we do not have knowledge of the functional role of
the compounds disclosed herein on either the D4 or
5-HT2A receptors.
The hD2, hD4 and h5HT2A receptor binding profiles of
compounds 8a to 8o were evaluated by their ability to
displace [3H]spiperone (dopamine receptors) or [3H]ke-
tanserin (h5HT2A receptor) binding to heterologous cells
expressing the cloned receptors. Non-specific binding
was determined using 30 lM methysergide. For the pur-
poses of this assay, human embryonic kidney 298 cells
were stably transfected with hD4 (D4.2 subtype) and
h5HT2A receptor, and GH4C1 (rat pituitary) cells were
stably transfected with hD2 (short isoform) receptor.
Rat frontal cortex tissue was used for the a1 adrenergic
receptor assay, using 7-methoxy-[3H]prazosin as the
radioligand. Clozapine was used as a reference in all of
the assays. Non-specific binding was determined using
30 lM methysergide. Ki values for each compound were
calculated by the Cheng and Prusoff transformation.20
These compounds display a favourable selectivity profile
for further development as antipsychotics. Their lack of
affinity for the a1 adrenergic receptor suggests that they
may be free of undesirable cardiovascular effects such as
orthostatic hypotension; however, it is possible that
there are beneficial effects of a1 antagonism in an anti-
psychotic medication.16 The large D4/D2 and 5-HT2A
/
D2 ratios indicate that these compounds are likely to
be free of EPS.10
This study has successfully identified a novel class of
highly potent dopamine D4 ligands that also display
high affinity for the serotonin 5-HT2A receptor. They
are selective over the dopamine D2 and a1 adrenergic
receptors. The utility of such compounds for the treat-
ment of schizophrenia remains to be determined.
The affinities of these compounds for the 5-HT2A
receptor have a clear dependence on the point of
attachment of the 2-thiophene ring (see pairs 8a and
N
N
O
O
N
(a)
(c)
(b)
N
N
H
H
H2N
S
I
3
4a meta
4b para
5a meta
5b para
O
N
O
CCl3
NH
O
N
Ar
O
O
(d)
(e)
N
H
N
N
H
H
S
S
S
6a meta
6b para
7a meta
7b para
8a-o
Scheme 1. Reagents and conditions: (a) 3- or 4-iodobenzoyl chloride, Et3N, CH2Cl2, 0 °C; (b) thiophene-2-boronic acid, Pd(PPh3)4, DME, 2 M
Na2CO3; (c) 2,2,2-trichloroethyl chloroformate, MeCN; (d) Zn, AcOH; (e) ArCH2Cl, K2CO3, KI, MeCN, 90 °C.

J. Arora et al. / Bioorg. Med. Chem. Lett. 15 (2005) 5253–5256
5255
Table 1. Binding profile of series 8 at the serotonin 5-HT2A, dopamine hD4 and hD2, and a1 adrenergic receptors
Compound
Biaryl isomer
Ar
D4 Ki (nM)a
5-HT2A Ki (nM)a
D2 Ki (nM)
a1 Ki (nM)a
1
2.9
3.0
0.37
1.8
8a
8b
para
meta
Ph
Ph
980
60 20
1200
160 50
1.5 0.3
460
8c
8d
8e
8f
para
meta
para
meta
14
130
740
140
5600
7% at 100 nM
1300 100
2% at 1 lM
CF3
21
78
52
3
6
360
6
CF3
7% at 100 nM
500 100
16,000
OCF3
1200 300
OCF3
CF3
8g
8h
para
para
22
15
76% at 100 nM
22
0% at 100 nM
29,000
0% at 100 nM
5000 2000
Cl
6
4
Cl
F
8i
8j
8k
8l
para
para
para
para
14
33
5
16
18,000
10,000 2000
6900
F
89% at 100 nM
14
38,000
F
3
0
1500 400
1800 400
420
F
O
7
89% at 100 nM
400 30
O
O
8m
para
14
2
2
22
4460 30
230 70
920 70
8n
8o
meta
meta
16
4
1100
610
220 60
80 10
Cl
2
60
6
a Ki values are reported as means of at least two independent determinations SEM. Where no SEM is reported, only a single determination was
made.
mann, B.; Teschendorf, H.-J.; Traut, M.; Unger, L.;
Gross, G. Drugs Future 1998, 23, 191.
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(1.97 g, 5.43 mmol) in EtOAc (40 mL) was added 2,2,2-
trichloroethyl chloroformate (11.5 g, 54.3 mmol). After
1 h, the mixture was poured into water and extracted
three times with EtOAc, dried (MgSO4), filtered and
concentrated. Column chromatography (50% EtOAc/
hexanes) provided carbamate 6 as a colourless solid. To
a solution of 6 (100 mg, 0.223 mmol) in MeOH (3 mL)
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