Tetrahydrobenzindoles: Selective antagonists of the 5-HT7 receptor [1]

Full text of DOI:10.1021/jm980519u

© Copyright 1999 by the American Chemical Society
Volume 42, Number 4
February 25, 1999
Communications to the Editor
Ta ble 1. 5-HT7 and 5-HT2 Receptor Affinities of Compounds
1-3
Tetr a h yd r oben zin d oles: Selective
An ta gon ists of th e 5-HT7 Recep tor
Chika Kikuchi,* Hiroshi Nagaso,
Toyokazu Hiranuma, and Masao Koyama
Pharmaceutical Research Center, Meiji Seika Kaisha Ltd.,
760 Morooka-cho, Kohoku-ku, Yokohama 222-8567, J apan
pKi ( SEa
b
c
Received September 14, 1998
compd
n
5-HT7
5-HT2
The neurotransmitter serotonin (5-hydroxytryptamine,
5-HT) plays important roles in a variety of physiological
and pathophysiological processes through the activation
of seven types of 5-HT receptors, 5-HT1-5-HT7. The
5-HT7 receptor was found by the application of molec-
ular cloning and has been identified in the rat,1-3
mouse,4 human,5 and guinea pig.6 The deduced amino
acid sequences of 5-HT7 receptors show a high degree
of interspecies homology, but only a limited homology
with those of other types of 5-HT receptors. All four
species homologues of the 5-HT7 receptor have high
affinity for 5-HT, 5-carboxyamidotryptamine (5-CT),
5-methoxytryptamine (5-MeOT), and methiothepin and
moderate affinity for 8-hydroxy-2-dipropylaminotetralin
(8-OH-DPAT), clozapine, and a number of other psy-
choactive drugs.7 Four 5-HT7 splice variants exist in
human and rat. Both the long (5-HT7a) and short (5-
HT7b) forms of the human receptor exhibit similar
distribution patterns and pharmacology.8-10 5-HT7 re-
ceptors are coupled to stimulation of adenylyl cyclase,
and 5-HT, 5-CT, and 5-MeOT display agonist activity.
8-OH-DPAT is a partial agonist, and methiothepin and
clozapine are antagonists. Recently, the first selective
5-HT7 receptor antagonist, SB-258719, was reported,11
but no selective agonist for the 5-HT7 receptor is yet
available.
1
2
3
2
3
4
6.99 ( 0.14
8.28 ( 0.15
8.48 ( 0.02
8.27 ( 0.19
7.79 ( 0.08
7.37 ( 0.05
a
All values represent the mean of at least two determinations,
with each determination lying within 0.2 log unit of the mean.
b
Binding affinity (human recombinant receptors in mammalian
cells; [3H]5-CT). c Binding affinity (rat cerebral cortex membranes;
[3H]ketanserin).
mus, thalamus, brain stem, and hippocampus),1,3,5,16 and
the distribution of 5-HT7 receptor binding sites in rat
and guinea pig brain was essentially the same as the
mRNA distribution.16-18 The 5-HT7 receptor is involved
in the control of circadian rhythms of spontaneous
electrical activity in the suprachiasmatic nucleus (SCN)
of the hypothalamus.3,19-21 The 5-HT7 receptor may be
involved in disturbance of circadian rhythms, such as
jet lag, delayed sleep-phase syndrome (DSPS), and non-
24-h sleep-wake disorder (non-24).22 The affinity of a
number of antipsychotic agents for the 5-HT7 receptor
also suggests that this receptor may mediate the
therapeutic actions of these compounds.7 Clearly, the
5-HT7 receptor may be a valuable novel drug target. To
examine this possibility, the development of potent and
selective ligands for the 5-HT7 receptor is highly desir-
able.
In the present study, we have synthesized highly
potent and selective 5-HT7 receptor ligands. Screening
of our compound library against the cloned human
5-HT7 receptor resulted in the identification of tetrahy-
drobenzindole derivatives as ligands.23 Although these
compounds have high affinity for the 5-HT7 receptor,
they also show high affinity for the 5-HT2 receptor. To
find derivatives with lower affinity for the 5-HT2 recep-
The biological functions of the 5-HT7 receptor have
not been fully clarified. Early pharmacological data
suggested that the 5-HT7 receptor may be involved in
the vasodilation of blood vessels.12-15 Indeed, the highest
levels of 5-HT7 receptor mRNA have been found in
human coronary arteries.5 High levels of 5-HT7 receptor
mRNA have also been observed in the brain (hypothala-
10.1021/jm980519u CCC: $18.00 © 1999 American Chemical Society
Published on Web 01/30/1999

534 J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 4
Ta ble 2. 5-HT7 and 5-HT2 Receptor Affinities of Compounds 4-8
Communications to the Editor
pKi ( SEa
b
c
compd
R
5-HT7
5-HT2
4
5
6
7
8
4-(2-methoxphenyl)piperazinyl
4-(2-cyanophenyl)piperazinyl
4-(2-pyridyl)piperazinyl
4-phenyl-1,2,3,6-tetrahydropyridyl
4-cyclohexylpiperazinyl
8.29 ( 0.08
8.42 ( 0.07
8.73 ( 0.09
8.67 ( 0.07
6.95 ( 0.10
6.98 ( 0.07
7.27 ( 0.06
7.01 ( 0.05
<6
<6
a
b
All values represent the mean of at least two determinations, with each determination lying within 0.2 log unit of the mean. Binding
affinity (human recombinant receptors in mammalian cells; [3H]5-CT). c Binding affinity (rat cerebral cortex membranes; [3H]ketanserin).
Sch em e 1
Ta ble 3. Receptor Binding Profile of 7a
receptor
5-HT1A
5-HT2
5-HT4
5-HT6
5-HT7
dopamine D2
affinity (pKi ( SE)b
6.77 ( 0.05
7.01 ( 0.05
<6
6.28 ( 0.07
8.67 ( 0.07
6.98 ( 0.05
F igu r e 1. 5-HT-induced stimulation of cAMP accumulation
in COS-7 cells expressing the 5-HT7 receptor and its inhibition
by compound 7. Data represent the mean ( SE of at least three
determinations.
a
Binding experiments were conducted as follows. Receptors and
radioligands used in the binding assay: 5-HT1A (human recom-
binant (mammalian); [3H]8-OH-DPAT); 5-HT2 (rat cerebral cortex;
[3H]ketanserin); 5-HT4 (guinea-pig striatum; [3H]GR-113808);
5-HT6 (human recombinant (mammalian); [3H]LSD); 5-HT7 (hu-
man recombinant (mammalian); [3H]5-CT); dopamine D2 (rat
and 2. This result suggested that the carbon chain
length of these compounds is very important in deter-
mining the affinity for the 5-HT7 receptor. As the next
step, we fixed the carbon chain length at the optimum
value (n ) 4) and modified the cyclic amine moiety. The
o-substituted phenylpiperazines 4 and 5 showed high
affinity for the 5-HT7 receptor, with about 20-fold
selectivity over the 5-HT2 receptor. The pyridylpipera-
zine 6 also had high affinity for the 5-HT7 receptor, and
the 4-phenyltetrahydropyridine derivative 7 showed
both high affinity and high selectivity. Compound 7 had
a pKi of 8.67 for the 5-HT7 receptor, with 47-fold
selectivity over the 5-HT2 receptor. Thus, high affinity
for the 5-HT7 receptor did not require both nitrogen
atoms. The cyclohexylpiperazine derivative 8 did not
show affinity for either the 5-HT7 or the 5-HT2 receptor.
An aromatic moiety would thus appear to be required
for affinity for the 5-HT7 and 5-HT2 receptors.
striatum; [3H]spiperone). All values represent the mean of at
b
least two determinations, with each determination lying within
0.2 log unit of the mean.
tor, we synthesized and tested a range of modified
compounds, including the phenylpiperazine derivatives
1-3. The substituted phenylpiperazine derivatives 4
and 5 and the pyridylpiperazine derivative 6 were also
investigated. Preparation of the tetrahydropyridine
analogue, corresponding to 3, led to the identification
of 7 (DR4004) as a highly potent and selective 5-HT7
receptor ligand. A cyclohexylpiperazine derivative 8 was
also synthesized.
Compounds 1-8 were evaluated for affinity for the
5-HT7 and 5-HT2 receptors. The affinity for the 5-HT7
receptor was assayed in terms of the ability to displace
the radioligand [3H]5-CT from cloned human 5-HT7
receptor expressed in COS-7 cells. The results, ex-
pressed as pKi, are summarized in Tables 1 and 2.
Compound 3 was a potent ligand for the 5-HT7 receptor,
being at least 4-fold more selective than compounds 1
The method of synthesis of 7 is shown in Scheme 1.
Compound 10 was prepared by reacting tetrahydroben-
zindole 9 with sodium hydride and 1,4-dibromobutane.
Structure of compound 10 was confirmed by 1H and 13C
NMR. Compound 7 was obtained by allowing compound

Communications to the Editor
J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 4 535
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10 to react with the corresponding amine in the pres-
ence of K2CO3. Compounds 1-6 and 8 were synthesized
in a similar manner to that described above. The
synthesis of compound 7 was accomplished in 30%
overall yield by a two-step procedure.
On the basis of its relative affinity for the 5-HT7 and
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from Table 3, compound 7 was found to be highly
selective for the 5-HT7 receptor compared with the
5-HT1A, 5-HT2, 5-HT4, 5-HT6, and dopamine D2 recep-
tors. Thus, compound 7 was confirmed to be a high-
affinity ligand for the 5-HT7 receptor with high selec-
tivity.
Compound 7 was evaluated for influence on 5-HT-
induced stimulation of cAMP accumulation in COS-7
cells transfected with an expression vector containing
human 5-HT7 receptor cDNA. Intracellular cAMP for-
mation was measured by enzyme-immunoassay (Am-
ersham cAMP EIA kit). Compound 7 did not stimulate
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In summary, we have described the synthesis and the
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receptor antagonist. This compound should be a useful
tool for examining the feasibility of targeting the 5-HT7
receptor with new drugs to obtain novel pharmacological
effects.
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