Discovery of the first potent, selective 5-hydroxytryptamine1D receptor antagonist

Article abstract of DOI:10.1021/jm049039v

A series of 5-(piperidinylethyloxy)quinoline 5-hydroxytryptamine _1D (5-HT_1D) receptor antagonists have been discovered from elaboration of the series of dual 5-hydroxytryptamine₁-selective serotonin reuptake inhibitors (5HT₁-SSRIs) reported previously. This is the first report of highly potent, selective antagonists for the 5-HT _1D receptor, which represents an extremely useful set of pharmacological tools for further understanding the roles of the 5-HT ₁ receptor subtypes.

Full text of DOI:10.1021/jm049039v

3
478
J. Med. Chem. 2005, 48, 3478-3480
Discovery of the First Potent, Selective
5
-Hydroxytryptamine1D Receptor
Antagonist
Simon E. Ward,* Frank P. Harrington,
Laurie J. Gordon, Susanna C. Hopley,
Claire M. Scott, and Jeannette M. Watson
Psychiatry Centre of Excellence for Drug Discovery,
GlaxoSmithKline, New Frontiers Science Park,
Third Avenue, Harlow, Essex, CM19 5AW, U.K.
11
Figure 1. Structures of GR127935 (mixed 5-HT1B/D ligand)
12
and SB-224289, the first selective 5-HT1B tool compound.
Received November 29, 2004
Abstract: A series of 5-(piperidinylethyloxy)quinoline 5-
hydroxytryptamine1D (5-HT1D) receptor antagonists have been
discovered from elaboration of the series of dual 5-hydroxy-
1 1
tryptamine -selective serotonin reuptake inhibitors (5HT -
SSRIs) reported previously. This is the first report of highly
potent, selective antagonists for the 5-HT1D receptor, which
represents an extremely useful set of pharmacological tools
Figure 2. Structure of SB-649915, a dual 5-HT1A receptor
antagonist and 5-HT reuptake inhibitor.
Scheme 1a
for further understanding the roles of the 5-HT
subtypes.
1
receptor
The 5-hydroxytryptamine (5-HT) family of receptors
comprises 14 distinct subtypes that have been exten-
sively studied and categorized into seven main families
(
5-HT1-5-HT7) on the basis of their operational, struc-
tural, signal transduction pathways and pharmacologi-
cal attributes.1 The largest subtype group, the 5-HT1
receptors, has been further divided into 5-HT1A, 5-HT1B,
,2
5
-HT1D, 5-HT1E, and 5-HT1F receptors, which show
approximately 40-60% homology between members.
Literature concerning the 5-HT1B and 5-HT1D receptors
is further complicated by their nomenclature because
a
Reagents and conditions: (i) BrCH2CH2Br, K2CO3, butan-2-
5
-HT1D receptors were formerly classified as 5-HT1DR
one, 80 °C; (ii) 1-Boc-piperazine, K2CO3, DMF, 70 °C; (iii) 1 M HCl
in Et O, EtOH; (iv) X-PhCHO, NaBH(OAc) , dichloroethane.
3
and 5-HT1B receptors as 5-HT1Dâ. Significantly, the rat
orthologue of the human 5-HT1B receptor has a key
single amino acid modification leading to differential
2
3
because of the lack of suitable, selective molecules,
although recent publications describing combined 5-HT1D
antagonist/5-HT reuptake blockers are providing in-
creased evidence for the role of the 5-HT1D receptor as
a presynaptic autoreceptor.1
Previous publications have described the development
of novel selective serotonin reuptake inhibitors (SSRIs)
with additional 5-HT1A receptor antagonist properties
as potentially therapeutically useful compounds that
should have a shorter latency to onset for antidepres-
sant action.16 In particular, the compounds described
4
pharmacology. The 5-HT1B receptor is located princi-
pally in the striatum, frontal cortex, and basal ganglia,5
whereas the 5-HT1D receptor is found most abundantly
in the hippocampus, cortex, caudate putamen, and
4,15
6
nucleus accumbens.
Considerable research effort has been expended on
the 5-HT1 receptor family, and a number of reviews have
covered the potential therapeutic applications of ago-
7
,8
nists and antagonists of these receptors. In brief,
central 5-HT release is regulated by inhibitory 5-HT
autoreceptors, including 5-HT1A, 5-HT1B, and 5-HT1D
(
Figure 2) show potent activity against 5-HT1 receptors
and inhibition of the serotonin transporter. Continued
SAR studies in this area have led to the identification
of a range of compounds with specific activities against
the 5-HT1 receptor subtypes and the 5-HT transporter.
Herein, we describe a series of piperazines that consti-
tute the first report of highly selective 5-HT1D receptor
antagonists as valuable pharmacological tools.
The general synthetic approach employed is outlined
in Scheme 1. 1,2-Dibromoethane was reacted sequen-
tially with 5-hydroxy-2-methylquinoline17 and then
receptors, and potentially, antagonism of these receptors
could result in acute increases in extracellular 5-HT.9,10
Consequently antagonists of the 5-HT1B receptor, or
mixed 5-HT1B/D ligands (Figure 1), have been studied
in detail alone and in combination with other seroton-
ergic agents for their potential utility in depression and
anxiety.
Despite the high level of interest in the members of
the 5-HT receptor family, including many known 5-HT1D
1
3
agonists, there is little known directly about the
potential utility of a 5-HT1D receptor antagonist in part
1
-(tert-butoxycarbonyl)piperazine to afford intermediate
A. Deprotection of this product afforded key intermedi-
ate B, which was further elaborated by reductive
alkylation to afford a set of substituted benzylamines
*
To whom correspondence should be addressed. Phone: 44 (0)1279
6
22894. Fax: 44 (0)1279 622790. E-mail: Simon.E.Ward@gsk.com.
1
0.1021/jm049039v CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/23/2005

Letters
Journal of Medicinal Chemistry, 2005, Vol. 48, No. 10 3479
Table 1. Receptor Binding Affinity (pKi a) for 5-HT1A, 5-HT1B, 5-HT1D, and SerT for Novel Compoundsb
pKi
selectivity
5-HT1D/5-HT1B
compd
R
5-HT1A
5-HT1B
5-HT1D
SerT
SB-649915
8.6
7.2
8.0
7.0
6.6
6.4
6.5
6.6
7.1
6.7
5.7
<5.0
6.2
6.8
6.6
6.5
6.7
6.4
7.2
8.8
8.0
8.8
8.0
7.7
8.1
8.7
8.3
5.6
<5.0
8.4
8.5
8.6
8.6
9.1
8.5
9.6
8.4
7.6
6
10
158
40
16
32
40
40
1
1
2
3
4
5
6
7
8
9
6-benzoxazinonyl
3-NH2, 4-OH-phenyl
3-NO2, 4-OH-phenyl
3-NO2, 4-OMe-phenyl
3-NO2, 4-F-phenyl
3-NO2, 4-Me-phenyl
3-NO2, 4-Cl-phenyl
3,4-dichlorophenyl
2,6-dichlorophenyl
2,5-dichlorophenyl
5-indolyl
<5.9
6.1
6.0
6.4
6.3
6.5
7.3
6.7
6.3
5.9
5.9
7.4
7.2
<5.0
<5.0
7.1
1
1
1
1
1
1
1
0
158
50
100
126
251
126
251
1
6.0
2
5-quinolyl
6.4
3
5-quinazolyl
6.4
4 (SB714786)
8-quinolyl
6.5
6.5
6.4
6.1
5
6
7-indolyl
6.4
4-benzimidazolyl
6.8
a
Radioligand binding assay from cloned human receptors for 5-HT receptors and rat cortical synaptosomes for SerT. Each determination
lies within 0.3 log units of the mean. b All compounds were characterized, and purity was assessed using H NMR and LC-MS.
1
C, affording easy access to explore the chemical space16
previously thought to be important for selectivity)
occupied by the benzoxazinone moiety in Figure 2.
All compounds prepared were screened against h5-
knowledge that the nitro group was not essential for
activity, we prepared the 3,4-dichloro derivative 8.
Comparison of this compound with the 3-chloro, 4-nitro
derivative 7 described above showed a significant loss
in affinity for all 5-HT1 receptors. Further dichloro
derivatives were prepared in this exploration, and
suprisingly, although the 2,6-dichlorophenyl 9 was also
inactive, the 2,5-dichlorophenyl 10 provided a marked
restoration of affinity for the 5-HT1A and 5-HT1D recep-
tors.
(
HT1A receptors expressed in Chinese hamster ovary
3
(
CHO) cells using displacement of [ H]WAY100635 and
against h5-HT1B and h5-HT1D receptors expressed in
3
CHO cells using displacement of [ H]-5-HT. Affinity for
the 5-HT reuptake site was assessed by measurement
3
of the inhibition of [ H]-5-HT into rat cortical synapto-
somes, with data expressed as pKi (Table 1). The
Although the substituted phenyl derivatives had led
to an improvement in selectivity for the 5-HT1D receptor,
the majority also led to a reduction in affinity for the
5-HT1D receptor from parent benzoxazinone derivative
1. Consequently we resumed exploration of this tem-
plate by investigation of other bicyclic systems. Encour-
agingly, the 5-indolyl derivative 11 showed good 5-HT1D
receptor affinity and furthermore exhibited significantly
improved selectivity over 5-HT1A and 5-HT1B receptors.
From this lead, we widened our exploration of the
aromatic ring systems to encompass derivatives 12-
16 and saw a consistent pattern of high 5-HT1D affinity
with excellent selectivity over the other receptors
screened. In particular, 4-benzimidazolyl derivative 16
has outstanding affinity for the 5-HT1D receptor and
selectivity against the 5-HT1B receptor in this assay.
To assess the functional activity of these compounds,
intrinsic activity measurements were determined as
intrinsic activity of the compounds was determined
3
5
using a [ S]GTPγS binding assay in cells expressing
the h5-HT1A, h5-HT1B, or h5-HT1D receptors, with data
reported relative to the maximum response elicited by
the endogenous agonist 5-HT. A 10-point half-log serial
dilution was used to generate a concentration response
for each compound.
Historically, no selective antagonists of the 5-HT1D
receptor have been reported, although mixed 5-HT1D
antagonist/5-HT reuptake blockers and selective ago-
nists are well documented. Most commonly, compounds
with low or no intrinsic activity for the 5-HT1D receptor
are also high-affinity ligands for the closely related
5
-HT1B receptor as previously described.
The piperazine analogue 1 of SB-649915 showed
1
6
reduced 5-HT1A, 5-HT1B, and 5-HT1D affinities relative
to the parent piperidine but allowed rapid exploration
of the SAR, which led to the identification of selective
35
described. Because this [ S]GTPγS binding assay was
run in a recombinant cell line with high receptor
density, it is anticipated that weak partial agonists
5
-HT1D receptor antagonists.
Interestingly, molecules 2-4, which contained some
features of the benzoxazinone but lacked the carbonyl
group, maintained or increased affinity for the 5-HT1D
receptor but lost affinity for 5-HT1A and 5-HT1B recep-
tors and the serotonin transporter. Further investiga-
tion of the nature of the group at the 4-position revealed
that fluorine 5, chlorine 7, and methyl 6 were all
tolerated for 5-HT1D affinity, with the 4-methyl affording
the highest 5-HT1A and 5-HT1D affinity. With the
identified from this screen would behave as antagonists
in the native system.1
8,19
Gratifyingly, all compounds
that were assessed for their intrinsic activity had IA <
0.55, implying that all were antagonists or partial
agonists.
From those compounds assayed, a proportion had no
intrinsic activity. These are the first examples of potent,
selective, full antagonists at the 5-HT1D receptor to be

3
480 Journal of Medicinal Chemistry, 2005, Vol. 48, No. 10
Letters
Table 2. In Vitro Cross-Screening Characterization (pKi) of
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S.; Gozlan, H.; Hamon, M. Quantitative autoradiography of
multiple 5-HT1 receptor subtypes in the brain of control or 5,7-
dihydroxytryptamine-treated rats. J. Neurosci. 1986, 6, 3474-
SB-714786a
5-HT
adrenergic dopamine
3
482.
1
E
1F 2A 2B 2C
4
5A
6
7
R1B â2 D2 D3 D4
(
6) Bruinvels, A. T.; Palacios, J. M.; Hoyer, D. Autoradiographic
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macol. 1993, 347, 569-582.
<5.0 5.7 <5.6 5.9 5.5 <5.3 5.5 <5.0 6.3 6.0 5.7 5.5 6.0 <5.0
a
Radioligand binding assays from cloned human receptors.
Each determination lies within 0.3 log units of the mean.
(
(
(
7) Slassi, A. Recent advances in 5-HT1B/1D receptor antagonists
and agonists and their potential therapeutic applications. Curr.
Top. Med. Chem. 2002, 2, 559-574.
8) Clitherow, J. W.; King, F. D.; Middlemiss, D. N.; Wyman, P. A.
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antagonists in the rat dorsal raphe nucleus: an in vitro fast
cyclic voltammetry study. Neurosci. Lett. 2001, 300, 45-48.
Table 3. Intrinsic Activity of Piperazinesa
IA
5-HT1A
5-HT1B
5-HT1D
5
1
1
1
-HT
1.0
0
1.0
0.3
0.1
0.3
1.0
0
0.5
0.5
4, SB-714786
5
6
0
(
10) Roberts, C.; Price, G. W.; Middlemiss, D. N. Ligands for the
0
0.44
investigation of 5-HT autoreceptor function. Brain Res. Bull.
SB-272183
GR127935
2001, 56, 463-469.
0.47
0.86
(
11) Watson, J. M.; Burton, M. J.; Price, G. W.; Jones, B. J.;
Middlemiss, D. N. GR127935 acts as a partial agonist at
recombinant human 5-HT1D alpha and 5-HT1D beta receptors.
Eur. J. Pharmacol. 1996, 314, 365-372.
a
[35
S]GTPγS binding assay in the human receptor cell lines.
Intrinsic activity of 0: full antagonist. Intrinsic activity of 1: full
agonist. 5-HT, SB-27213, and GR127935 are standards.
(
12) Gaster, L. M.; Ham, P.; Joiner, G. F.; King, F. D.; Mulholland,
K. R.; Wyman, P. A.; Hagan, J. J.; Price, G. W.; Roberts, C.;
Routledge, C.; Selkirk, J.; Slade, P. D.; Middlemiss, D. N. The
selective 5-HT1B receptor inverse agonist SB-224289, potently
blocks terminal 5-HT autoreceptor function both in vitro and in
vivo. Ann. N.Y. Acad. Sci. 1998, 861, 270-271.
reported. As can be seen (Table 3), compounds typically
exhibited low or no intrinsic activity against the 5-HT1A
and 5-HT1B receptors, with intrinsic activity varying
from antagonist to partial agonist against the 5-HT1D
receptor. In particular, 14 (SB-714786) was found to
have no or low intrinsic activity against all three
receptors (Table 3). Further in vitro characterization of
this compound across a range of other monoamine
receptors has confirmed its exquisite selectivity for the
(13) Tepper, S. J.; Rapoport, A. M.; Sheftell, F. D. Mechanisms of
action of the 5-HT1B/1D receptor agonists. Arch. Neurol. 2002,
5
9, 1084-1088.
(
14) Timms, G. H.; Boot, J. R.; Broadmore, R. J.; Carney, S. L.;
Cooper, J.; Findlay, J. D.; Gilmore, J.; Mitchell, S.; Moore, N.
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5
-HT1D receptor, with at least 250-fold separation over
(15) Pullar, I. A.; Boot, J. R.; Broadmore, R. J.; Eyre, T. A.; Cooper,
J.; Sanger, G. J.; Wedley, S.; Mitchell, S. N. The role of the
all of the 16 other receptors studied (Table 2).
In conclusion, we have identified a novel series of
potent, selective 5-HT1D receptor antagonists and partial
agonists that are currently being used as tools to
increase our understanding of the 5-HT1B and 5-HT1D
receptor systems. Further chemical exploration around
this template is also in progress and will be disclosed
at a later date.
5
-HT1D receptor as a presynaptic autoreceptor in the guinea
pig. Eur. J. Pharmacol. 2004, 493, 85-93.
(16) Atkinson, P. J.; Bromidge, S. M.; Coleman, T.; Duxon, M. S.;
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G. J.; Scott, C. M.; Shaw, T. E.; Starr, K. R.; Stemp, G.; Thewlis,
K. M.; Thomas, D. R.; Thompson, M.; Vong, A. K. K.; Watson, J.
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(
(
(
17) Choi, H. Y.; Chi, D. Y. A facile debromination reaction: can
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JM049039V