Synthesis of potent and selective serotonin 5-HT1B receptor ligands

Article abstract of DOI:10.1016/j.bmcl.2005.07.024

A series of serotonin 5-HT_1B ligands were synthesized and evaluated for their potency and selectivity against other 5-HT receptor subtypes. Many of these new compounds displayed high affinity and selectivity for the 5-HT_1B receptor a

Full text of DOI:10.1016/j.bmcl.2005.07.024

Bioorganic & Medicinal Chemistry Letters 15 (2005) 4786–4789
Synthesis of potent and selective serotonin 5-HT1B receptor ligands
a
c
a,b
Yiyun Huang,a,b, Sung-A Bae, Bryan L. Roth and Marc Laruelle
*
aDepartment of Psychiatry, Columbia University College of Physicians and Surgeons, New York, NY 10032, USA
bDepartment of Radiology, Columbia University College of Physicians and Surgeons, New York, NY 10032, USA
cDepartments of Biochemistry, Neurosciences and Psychiatry, Case Western Reserve University Medical School,
Cleveland, OH 44106, USA
Received 3 May 2005; revised 17 July 2005; accepted 21 July 2005
Available online 6 September 2005
Abstract—A series of serotonin 5-HT1B ligands were synthesized and evaluated for their potency and selectivity against other 5-HT
receptor subtypes. Many of these new compounds displayed high affinity and selectivity for the 5-HT1B receptor and compound 6c
was found to have the in vitro binding profile necessary for development as a PET radioligand.
Ó 2005 Elsevier Ltd. All rights reserved.
The serotonin 5-HT1B receptors are autoreceptors that
regulate the release of serotonin (5-HT) from the 5-HT
terminals. Historically, these receptors were included
in the class of 5-HT1D receptors. The human 5-HT1D
fast-acting antidepressants or as an adjunct treatment
of depression with SSRIs.13,14 Along with the report of
enhanced cocaine intake in 5-HT1B knockout mice,
Koob et al. have reported that stimulation of 5-HT1B
receptor potentiated cocaine reinforcement in rats, thus
implicating the involvement of this receptor subtype in
cocaine abuse.15 Furthermore, biological and genetic
studies have implicated the 5-HT1B receptors and the
receptor genes in alcoholism, especially antisocial alco-
holism, further underscoring the importance of the
5-HT1B receptor.11,16
receptors have been divided into 5-HT1Da and
1
5-HT1Db
,
which were then renamed 5-HT1D and 5-
HT1B receptors, respectively.2 The 5-HT1B is the pre-
dominant form of the 5-HT1B/D receptor family in the
human brain, accounting for about 90% of the receptor
3,4
population formerly designated as 5-HT1D
.
Over the
last few years, biological and pharmacological studies
have noted the importance of this receptor subtype
and implicated the involvement of 5-HT1B receptors in
depression, aggressive behavior, suicide, substance
abuse, and alcoholism.5–12 For example, studies in 5-
HT1B knockout animals have shown that transgenic
mice devoid of the 5-HT1B receptors displayed enhanced
aggressiveness, increased cocaine intake, and heightened
susceptibility to alcohol over-consumption.6–8 Since the
5-HT1B receptor is an autoreceptor located on the nerve
terminal and stimulation of this receptor inhibits the re-
lease of serotonin from the neurons, antagonism of the
5-HT1B receptor would, in theory, produce an immedi-
ate increase in the extracellular 5-HT concentrations,
an effect induced by prolonged treatment with the most
widely used antidepressants: the selective serotonin
reuptake inhibitors (SSRIs). Therefore, it was proposed
that 5-HT1B antagonists could probably function as
Despite the recent advance in elucidating the role of the
5-HT1B receptor in the CNS and in psychiatric diseases,
there is still a lack of specific pharmacological tools to
probe this receptorÕs functions in vivo. Selective agonists
and antagonists for the 5-HT1B receptor, among them
GR127935 and SB-224289 (Fig. 1) are only recently
available. GR127935 is a high affinity ligand (Ki
0.14 nM for the cloned human 5-HT1B (h5-HT1B),
0.70 nM for h5-HT1D, and 70 nM for h5-HT1A recep-
tors).17 However, this compound has been shown in
some tests to be a partial agonist. SB-224289 was report-
ed to be a 5-HT1B full antagonist with a 26-fold selectiv-
ity for the h5-HT1B over h5-HT1D receptor.18,19
Nevertheless, its affinity for the h5-HT1B receptor (Ki
10 nM) seems to be relatively low. Yet another com-
pound (1, Fig. 1) was reported to be a full antagonist
with a Ki of 2 nM for the 5-HT1B receptor. However,
in our hands this compound also displayed partial ago-
nist activity. In view of the importance of the 5-HT1B
receptor in psychiatric diseases, we aim to develop a
Keywords: Synthesis; Ligand; Serotonin 5-HT1B receptor.
*
Corresponding author. Tel.: +1 212 543 6629; fax: +1 212 568
6171; e-mail: hh285@columbia.edu
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.07.024

Y. Huang et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4786–4789
4787
Me
OMe
N
Me
N
Me
N
Me
H
H
N
N Me
N
N
Me
O
N
N
N
Me
N
O
N
N
O
OMe
OMe
O
O
N
O
GR127935
SB-224289
1
Figure 1. Structural representation of GR127935, SB-224289, and compound 1.
radioligand for the positron emission tomography
(PET) imaging studies of the 5-HT1B receptor in vivo.
To this end, we have synthesized a series of compounds
in search of a high affinity 5-HT1B antagonist for poten-
tial development as PET radiotracer.
were fully characterized by 1H NMR, MS, HRMS,
and/or elemental analysis.
The newly synthesized compounds were tested in recep-
tor binding assays in vitro. Binding to the 5-HT1B and 5-
HT1D receptors was carried out by displacement of the
radioligand [3H]5-carboximidotryptamine (5-CT) at the
cloned human 5-HT1B and 5-HT1D receptors with the
test compounds. In addition, binding affinities of these
compounds at various cloned human 5-HT receptors,
including 5-HT1A, 5-HT2A, 5-HT2C, 5-HT5A, 5-HT6
and 5-HT7, were also determined, as previously de-
scribed.22 Selected binding data are given in Table 1.
The synthetic program was initiated using compound 1
as a lead.20 Based on this lead compound, a number
of derivatives bearing different functional groups were
prepared and screened in comprehensive in vitro binding
assays. The new compounds 6a–g were prepared accord-
ing to the synthetic pathway outlined in Scheme 1. Brief-
ly, nitration of 1-methyl-4-(2-methoxyphenyl)piperazine
(2) gave compound 3, which was then reduced with Ra-
ney nickel and hydrazine to the aniline 4. The reaction
of 4 with triphosgene and coupling of the resulting carb-
amyl chloride with various monosubstituted phenylpip-
erazines (5) provided the final products 6a–g. The
monosubstituted phenylpiperazines, if not commercially
available, were prepared by the palladium-catalyzed
aromatic amination of substituted phenylbromides with
piperazine.21 Monosubstituted phenylpiperazines 5e–g
were obtained in 32–75% yield. The desired compounds
6a–g were prepared in 45–82% yield, following purifica-
tion by column chromatography, salt formation with
fumaric acid, and recrystallization from acetone. In
addition, we have also prepared compound 11 to eluci-
date further the structure–activity relationship of this
class of compounds (Scheme 2). All new compounds
As is evident from Table 1, all the test compounds dem-
onstrated high affinity for the serotonin 5-HT1B recep-
tor. Compounds 6a, 6c, and 11 also displayed high
affinity (Ki < 100 nM) for the 5-HT1D receptor. In addi-
tion, compounds 6a–d, 6f, and 6g had greater than 13-
fold selectivity for 5-HT1B over the other 5-HT receptor
subtypes. All compounds display negligible affinities
(Ki > 10 lM) for the following cloned receptors: dopa-
mine D1, D2, D3, D4 and D5, j, d, and l-opiod, hista-
mine-H1, muscarinic M1, M2, M3, M4 and M5,
nicotinic acetylcholine receptors, and the PCP site.
Judging from the binding data, it is clear that substitu-
ents as diverse as fluorine, methoxy, thiomethyl, and tri-
fluoromethyl, placed at either the ortho or para position
Raney Ni
N
N
Me
O2N
N
N
Me
H2N
N
N
Me
H2SO4, KNO3
OMe
OMe
OMe
H2NNH2, EtOH
2
3
4
R
R
N
5
NH
6a: R = 2-Cl;
6b: R = 2-F;
6c: R = 4-Cl;
6d: R = 4-F;
6e: R = 4-Ph
6f: R = 4-CF3
6g: R = 4-SMe
H
N
N
Me
N
N
N
(CCl3O)2CO, CH2Cl2
O
OMe
Scheme 1. Synthetic route for compounds 6a–g.
OMe
OMe
Pd[(o-tolyl)3P]4
1. aq. NaOH, THF
2. SOCl2, reflux
N
OMe
HN
OMe
+
Br
NaOt-Bu, toluene
O
O
7
8
9
OMe
OMe
N
4, THF, r.t.
H
N
N
OMe
N
Me
N
Cl
O
O
10
11
Scheme 2. Synthetic route for compound 11.

4788
Y. Huang et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4786–4789
Table 1. In vitro binding affinities of compounds 6a–g and 11 at selected 5-HT receptors
Compound
Ki (nM)a
5-HT2A
5-HT1B
5-HT1D
5-HT1A
5-HT2C
1937 794
5-HT6
5-HT7
1
6a
12.4 1.1
11.3 0.9
18.1 2.7
1.82 0.35
38.3 9.0
13.7 1.4
28.4 2.2
16.7 4.1
9.68 0.74
4.31 0.50
105.9 20.1
34.7 12.1
103.1 39.2
6.51 1.69
168.2 40.4
209.2 14.6
345.1 65.6
328.7 62.5
15.4 2.8
641 147
335 74
315 85
6739 1146
5988 958
>10,000
2410 409
469 80
841 404
5089 1272
>10,000
1412 762
342 195
>10,000
>10,000
>10,000
>10,000
>10,000
>10,000
nd
1241 521
1077 409
267.0 98.8
>10,000
6b
6c
52.5 14.2
1079 291
405 182
382 187
882 432
15.4 2.8
80.0 42.4
1008 60
4451 490
171 19
6d
6e
6.50 2.47
2833 3395
1145 1054
9954 2688
105 70
1429 286
2863 830
>10,000
6f
6g
614 117
1335 280
>10,000
11
GR127935
nd
>10,000
12.3 1.85
42.7 9.0
>10,000
nd, not determined.
a All assays were conducted in duplicate. Data are means SD of computer-derived estimates for n = 4 separate determinations.
of the phenylpiperazine moiety in the lead compound,
are well-tolerated by the 5-HT1B binding site and the
resulting compounds all retain their high binding affinity
for the 5-HT1B receptor. Replacement of the methoxy
group in the lead compound with a bulky substituent,
such as the phenyl group, also results in a compound
with high affinity and selectivity for the 5-HT1B receptor
(6e). However, strongly electron-withdrawing substitu-
ents, such as fluorine or trifluoromethyl group, placed
at the 4-position of the phenyl ring (6d and 6f) appear
to diminish the compoundsÕ affinities for 5-HT1B and
5-HT1D receptors.
receptor in vivo. Another compound, 11, was found
to be a 5-HT1B full antagonist devoid of any agonist
activity.
Acknowledgments
This study was supported by the National Alliance
for Research in Schizophrenia and Depression
(NARSAD), the Lieber Center for Schizophrenia
Research at Columbia University and the NIMH Psy-
choactive Drug Screening Program.
Among the compounds synthesized, it is notable that
the 4-chlorophenyl derivative (6c) displays a higher
affinity for the 5-HT1B receptor than both the lead
and GR127935, a compound that has been shown in
the literature to be the ligand with the highest affinity
for the 5-HT1B receptor. In our binding assays, com-
pound 6c is twice as potent as GR127935, and thus
appears to be the most potent 5-HT1B ligand reported
to date.
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