New halogenated tris-(phenylalkyl)amines as h5-HT2Breceptor ligands

Article abstract of DOI:10.1016/j.bmcl.2016.05.079

A series of compounds in which various halogen substituents were incorporated into a phenyl ring of a tris-(phenylalkyl)amine scaffold, was synthesized and evaluated for affinity to h5-HT₂receptors. In general, all compounds were found to have

Full text of DOI:10.1016/j.bmcl.2016.05.079

Bioorganic & Medicinal Chemistry Letters 26 (2016) 3216–3219
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
New halogenated tris-(phenylalkyl)amines as h5-HT2B receptor
ligands
Nirav Kapadia a,b, Shahrear Ahmed a, Wayne W. Harding a,b,
⇑
a Chemistry Dept., Hunter College, City University Of New York, 695 Park Ave, New York, NY 10065, USA
b CUNY Graduate Center, 365 5th Avenue, New York, NY 10016, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of compounds in which various halogen substituents were incorporated into a phenyl ring of a
tris-(phenylalkyl)amine scaffold, was synthesized and evaluated for affinity to h5-HT2 receptors. In gen-
eral, all compounds were found to have good affinity for the 5-HT2B receptor and were selective over 5-
HT2A and 5-HT2C receptors. Compound 9i was the most selective compound in this study and is the high-
est affinity 5-HT2B receptor ligand bearing a tris-(phenylalkyl)amine scaffold to date.
Ó 2016 Elsevier Ltd. All rights reserved.
Received 4 April 2016
Revised 25 May 2016
Accepted 27 May 2016
Available online 28 May 2016
Keywords:
5-HT2A
5-HT2B
CNS
Tris-(phenylalkyl)amine
5-HT2B receptors are known to play an important role in cardiac
function,1–4 regulation of gastrointestinal motility,5,6 growth and
differentiation7,8 and in regulation of the CNS.9–11 Selective
5-HT2B receptor antagonists are pursued as therapeutics for the
treatment of migraine,12 irritable bowel syndrome,6,13 pulmonary
hypertension14 and cardiac failure.2,15–17 Antagonism at the
5-HT2B receptor is also suggested as a therapeutic approach for
the treatment of MDMA abuse.10 Although a few selective 5-HT2B
antagonists have been advanced to clinical trials, there are no such
compounds clinically approved up to now. SB-200646 and
RS-127445 (Fig. 1) are perhaps the most well-known selective
5-HT2B receptor antagonists; these compounds have found utility
as biological tools rather than as drugs. Identification of new selec-
tive 5-HT2B antagonists is of current importance because of their
potential clinical applications. However, identifying selective
5-HT2B antagonists is challenging because most 5-HT2B receptor
ligands also have affinity for the closely related 5-HT2A and 5-
HT2C receptors.
template that are of importance to 5-HT2B receptor affinity and
selectivity.
From the previous study, we found that the introduction of
methoxy substituents at the ortho, meta or para positions in ring
C of compound 1 (Fig. 2), resulted in compounds with higher affin-
ity for the 5-HT2B receptor as compared to 1. Indeed, these com-
pounds had very similar 5-HT2B affinities (4.6–6.8 nM). Since
only methoxy substituents were investigated, it remains unclear
how other functionalities with varied electronic and steric proper-
ties in ring C may influence 5-HT2B affinity. Therefore, we decided
to synthesize and evaluate a series of ring C halogenated com-
pounds in order to decipher the tolerance of ring C for such
substituents.
Scheme 1 outlines our synthetic approach to the target com-
pounds. We followed a similar route as described in our previous
report. Here, commercially available amine 3 was coupled with
acid 4 to afford amide 5; compound 5 was subsequently reduced
to amine 6. EDCI coupling of 6 with various halo-acids (7a–7l) fur-
nished amides 8a–8l. Reduction of 8a–8l afforded the target com-
pounds 9a–9l.
Compounds 9a–9l were submitted to the Psychoactive Drug
Screening Program (PDSP), where they were evaluated for affinity
at h5-HT2 receptor sites.19 Details on the experimental procedures
employed by the PDSP may be found in the assay protocolbook:
https://pdspdb.unc.edu/pdspWeb/content/PDSP%20Protocols%20II
%202013-03-28.pdf. The data for affinity (Ki) measurements are
compiled in Table 1. A discussion of the structure-affinity results
follows, with a focus on comparisons to compound 1.
We recently reported the discovery of tris-(phenylalkyl)amines
as a novel 5-HT2B receptor-preferring antagonist scaffold.18 This
fortuitous finding, revealed a number of compounds with high
5-HT2B receptor affinity and good selectivity versus 5-HT2A and
5-HT2C receptors. The present report describes follow-up struc-
ture-affinity relationship (SAR) studies in order to glean further
insights into the structural features of the tris-(phenylalkyl)amine
⇑
Corresponding author.
http://dx.doi.org/10.1016/j.bmcl.2016.05.079
0960-894X/Ó 2016 Elsevier Ltd. All rights reserved.

N. Kapadia et al. / Bioorg. Med. Chem. Lett. 26 (2016) 3216–3219
3217
O
O
N
NH2
O
O
H
N
H
N
B
B
N
O
N
N
p
X
C
C
O
O
N
O
O
N
m
o
A
A
F
SB-200646
RS-127445
1
2 : X = halogen atom
Figure 1. Structures of the selective 5-HT2B antagonists SB-200646 and RS-127445.
Figure 2. SAR strategy.
Compound 9a which has a fluorine atom in the para-position of
ring C, displayed slightly improved affinity (Ki = 14 nM) at the 5-
HT2B receptor as compared to compound 1 (Ki = 26 nM). This
change also led to an increase in selectivity over the 5-HT2A recep-
tor (13-fold vs 7-fold), but a decrease in selectivity over the 5-HT2C
receptor (9-fold vs 15-fold). Interestingly, the para-chloro (9b,
Ki = 54 nM), para-bromo (9c, Ki = 45 nM) and para-iodo (9d,
Ki = 51 nM) compounds, had a slight reduction in 5-HT2B receptor
affinity. Compounds 9b–9d also showed lower selectivity over 5-
HT2A and 5-HT2C receptors as compared to 1.
The reason for this trend in affinity for the para-substituted halo
derivatives is not clear at the moment (assuming that the com-
pounds all have similar binding orientations in the receptor). On
steric grounds, it is tempting to postulate that the para-fluoro
group is smaller than the other para-halo groups and thus may
be accommodated more readily in the binding pocket into which
the halo group protrudes. However, the corresponding para-meth-
oxy compound (identified previously) has a higher 5-HT2B affinity
than 9a (Ki = 6.8 nM). Thus a steric reasoning of the trend is elusive.
The trend also does not fit any obvious rationale based on the elec-
tronic properties of the halogen groups.
receptors in this series. Thus, 9e–9h are generally less selective
for the 5-HT2B receptor than 1. All the analogues in the 9e–9h ser-
ies had similar affinities, indicative of a good tolerance for halogen
atoms at this position.
In the ortho-substituted series (9i–9l), introduction of a fluorine
atom resulted in a 15-fold increase (9i, Ki = 1.7 nM) in affinity at
the 5-HT2B receptor, when compared with compound 1. In a simi-
lar comparison at 5-HT2A and 5-HT2C receptors, the affinity of 9i
was found to increase by 2.5 and 5-fold respectively. Moreover,
the selectivity of 9i at both these receptors was better as compared
to 1 (38- vs 6-fold over 5-HT2A and 46- vs 15-fold over 5-HT2C).
Analogously, 9j (5-HT2B Ki = 3.1 nM) and 9k (5-HT2B Ki = 3.4 nM)
were found to be 8 times more potent than the parent compound
1, whereas 9l (5-HT2B Ki = 13 nM) was found to be 2 times potent
than 1 at the 5-HT2B receptor. Notably, 9i is 12 times more potent
than the standard 5-HT2B ligand SB-206553 and was the strongest
binder identified in this series.
The affinity of compound 9i at the 5-HT2B receptor was found to
be comparable to methoxyphenyl-containing analogues from our
previous study (e.g. the 3,4,5-trimethoxyphenyl analogue, Ki = 4.1
nM and the 2-methoxyphenyl analogue, Ki = 5.8 nM). Similar
bioisosteric effects between C–F and C–OMe groups have been
reported before; this biological resemblance may have its origins
in the similarity in polarity between fluorine and oxygen.20
Although no clear trend was seen in the para and meta series of
compounds, a trend was observed in the ortho series (9i–9l). Here,
Introduction of a halogen atom at the meta position in ring C
(9e–9h) resulted in a moderate increase in affinity at the 5-HT2B
receptor relative to compound 1. The meta-fluoro (9e, Ki = 13 nM)
and meta-iodo (9h, Ki = 14 nM) derivatives exhibited a 2-fold
increase in affinity at the 5-HT2B receptor. However, in most cases,
an increased affinity was also observed for 5-HT2A and 5-HT2C
COOH
O
O
O
O
HN
HN
4
O
b
O
O
a
NH2
O
O
O
O
O
O
3
5
6
O
O
O
O
COOH
R3
R1
R3
R2
R3
R2
R2
7a-7l
d
O
O
N
O
O
N
c
O
R1
R1
8
9
8a: R1 = F, R2 = R3 =H
8b: R1 = Cl, R2 = R3 =H
8c: R1 = Br, R2 = R3 =H
8d: R1 = I, R2 = R3 =H
8e: R2 = F, R1 = R3 =H
8f: R2 = Cl, R1 = R3 =H
8g: R2 = Br, R1 = R3 =H
8h: R2 = I, R1 = R3 =H
8i: R3 = F, R2 = R3 =H
9a: R1 = F, R2 = R3 =H
9b: R1 = Cl, R2 = R3 =H
9c: R1 = Br, R2 = R3 =H
9d: R1 = I, R2 = R3 =H
9e: R2 = F, R1 = R3 =H
9f: R2 = Cl, R1 = R3 =H
9g: R2 = Br, R1 = R3 =H
9h: R2 = I, R1 = R3 =H
9i: R3 = F, R2 = R3 =H
8j:
R
3 = Cl, R2 = R3 =H
9j: R3 = Cl, R2 = R3 =H
8k: R3 = Br, R2 = R3 =H
8l: R3 = I, R2 = R3 =H
9k: R3 = Br, R2 = R3 =H
9l: R3 = I, R2 = R3 =H
Scheme 1. Reagents & conditions: (a) 3,4-methylenedioxyphenylacetic acid, CDI, THF, 0 °C-rt, 16 h; (b) BH3–THF, BF3ÁEt2O, THF, rt-reflux, 4 h; (c) appropriate acid, EDCI,
HOBt, DMF, 0 °C-rt, 6 h; (d) BH3-THF, BH3ÁEt2O, THF, rt-reflux, 4 h.

3218
N. Kapadia et al. / Bioorg. Med. Chem. Lett. 26 (2016) 3216–3219
Table 1
Binding affinities of 9a–9l at the 5-HT2A, 5-HT2B and 5-HT2C receptors
O
O
R3
R2
O
O
N
R1
9a-9l
Compounds
Kia,b (nM)
Selectivity
R1
R2
R3
5-HT2A
5-HT2B
5-HT2C
5-HT2A/5-HT2B
5-HT2C/5-HT2B
1
9a
9b
9c
9d
9e
9f
9g
9h
9i
9j
9k
H
H
H
H
H
H
H
H
H
F
H
H
H
H
H
F
Cl
Br
I
H
H
H
H
H
F
Cl
Br
I
H
H
H
H
H
H
H
H
165 25
185 24
365 47
221 29
288 37
185 24
125 16
103 13
169 22
65 8.4
43 5.5
54 7.0
75 9.7
15 1.9
ndc
26 2.3
14 1.8
54 7.0
45 5.8
51 6.6
18 2.3
17 2.2
14 1.8
18 2.3
1.7 0.2
3.1 0.4
3.4 0.4
13 1.7
ndc
399 75
126 1.6
235 3.0
219 2.8
224 2.9
167 2.2
206 27
143 18
154 20
79 10
92 12
124 16
68 8.8
ndc
6
13
7
5
6
10
7
7
15
9
4
5
4
9
12
10
9
46
30
36
5
9
38
14
16
6
Cl
Br
I
9l
Clozapine
SB-206553
Ritanserin
21 2.7
ndc
ndc
ndc
1.8 0.2
a
b
c
Radioligands are [3H]ketanserin, [3H]LSD and [3H]mesulergine for 5-HT2A, 5-HT2B and 5-HT2C respectively.
Values represent mean of three experiments.
nd = not determined.
5-HT2B receptor affinity was found to increase with decreasing
size/increasing electronegativity of the halogen atom. With regards
to the position of substitution in ring C, with the exception of the
fluoro substituted compounds (9a, 9e and 9i), affinity at the 5-
HT2B receptor was in the following order: ortho-substituted com-
pounds > meta-substituted compounds > para-substituted com-
pounds. For example, 9j (Ki = 3.1 nM) was 5 times potent than 9f
(Ki = 17 nM), which in turn was 3 times more potent than 9b
(Ki = 54 nM). This trend was also observed for the affinities of the
same compounds at the 5-HT2A and 5-HT2C receptors.
To characterize the 5-HT2B activity of the analogues, the most
potent compounds, 9i–9k were tested in 5-HT2B functional assays
by the PDSP. These assays revealed that all three compounds were
5-HT2B antagonists with the following IC50 values: 9i (135 nM); 9j
(99 nM); 9k (326 nM).
in future. In that regard, deciphering the bioactive conformation of
these highly flexible ligands is a goal.
In conclusion, we report the synthesis and evaluation of a series
of halo-substituted analogues of the tris-(phenylalkyl)amine 1 at
human 5-HT2 receptors. All the analogues displayed high affinities
for the 5-HT2B receptor and had good selectivity over 5-HT2A and 5-
HT2C receptors. In general, compounds with ortho–halo substitu-
tions in ring C displayed higher 5-HT2B affinities than the corre-
sponding meta or para analogues. Compounds 9i–9k were found
to be 5-HT2B receptor antagonists. Compound 9i is the most selec-
tive 5-HT2B ligand identified in this study and has the highest 5-
HT2B receptor affinity of the tris-(phenylalkyl)amines evaluated
up to now.
Acknowledgements
Introduction of a halogen atom can have beneficial effects on
the affinities of a ligand. There are several plausible ways in which
halogen atoms can interact with a protein or other relevant target.
Classically halogens are considered to be hydrophobic moieties
and Lewis bases in accordance with their electronegativities.21
This publication was made possible by Grant Numbers
GM092282 and MD007599 from the National Institutes of Health.
Its contents are solely the responsibility of the authors and do not
necessarily represent the official views of the NIH or its divisions.
Ki determinations, and receptor binding profiles were generously
provided by the National Institute of Mental Health’s Psychoactive
Drug Screening Program, Contract # HHSN-271-2008-00025-C
(NIMH PDSP). The NIMH PDSP is directed by Bryan L. Roth MD,
PhD at the University of North Carolina at Chapel Hill and Project
Officer Jamie Driscol at NIMH, Bethesda MD, USA. For experimental
details please refer to the PDSP website http://pdsp.med.unc.edu/
and click on ‘Binding Assay’ or ‘Functional Assay’ on the menu bar.
However, the halogen atom can also act as a Lewis acid (via
r-
hole) and interact with an electron donating residue of a protein.
Bromine and chlorine atoms may be involved in bonding interac-
tions with backbone amino acids that contain carbonyl oxygen
atoms.4 Additionally, halogen atoms are also involved in bond for-
mation with side chain groups such as hydroxyls in serine, thre-
onine and tyrosine, carboxylate groups in aspartate and
glutamate, sulfur atoms in cysteine and methionine, nitrogen
atoms in histidine and the
p surfaces of phenylalanine, tyrosine,
histidine and tryptophan.4 In the case of tris-(phenylalkyl)amines
in this study, it is not clear what receptor ligand interactions are
responsible for the enhanced affinity of the ortho halo-substituted
ring C derivatives. Molecular docking studies may shed light on
their possible modes of interaction and aid in ligand design efforts
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2016.05.
079.

N. Kapadia et al. / Bioorg. Med. Chem. Lett. 26 (2016) 3216–3219
3219
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