New tetrahydrobenzindoles as potent and selective 5-HT7 antagonists with increased In vitro metabolic stability

Article abstract of DOI:10.1016/S0960-894X(02)00842-9

Chemical modifications of compound 1 (DR4004), a potent, selective antagonist of the 5-HT₇ receptor, were conducted with the aim of improving its metabolic stability. Halogenation of putative sites of oxidative metabolism afforded compounds 7-1

Full text of DOI:10.1016/S0960-894X(02)00842-9

Bioorganic & Medicinal Chemistry Letters 13 (2003) 61–64
New Tetrahydrobenzindoles as Potent and Selective 5-HT7
Antagonists with Increased In Vitro Metabolic Stability
Chika Kikuchi,* Hisashi Suzuki, Toyokazu Hiranuma and Masao Koyama
Pharmaceutical Research Center, Meiji Seika Kaisha Ltd., 760 Morooka-cho, Kohoku-ku, Yokohama 222-8567, Japan
Received 21 August 2002;accepted 3 October 2002
Abstract—Chemical modifications of compound 1 (DR4004), a potent, selective antagonist of the 5-HT receptor, were conducted
7
with the aim of improving its metabolic stability. Halogenation of putative sites of oxidative metabolism afforded compounds 7–10,
which retained high affinity and selectivity for the 5-HT
DR4485) showed oral bioavailability, and should be a useful tool for evaluating the therapeutic potential of 5-HT
2002 Elsevier Science Ltd. All rights reserved.
7
receptor, and showed increased in vitro metabolic stability. Compound 10
(
#
7
antagonists.
The neurotransmitter serotonin (5-hydroxytryptamine,
-HT) plays important roles in a variety of physiological
potent antagonist for the 5-HT receptor, with selectiv-
7
5
ity over the 5-HT receptor and other related recep-
2
1
8
and pathophysiological processes through the activation
of seven types of 5-HT receptors, 5-HT –5-HT . The
tors.
1
7
5
of 5-HT receptors.
-HT receptor is the most recent addition to the family
Its biological functions are still
poorly understood. Early pharmacological data sug-
The purpose of this study is to improve the oral bio-
24
7
1À6
availability of compound 1. As shown in Chart 1,
most 5-HT receptor antagonists contain a phenyl ring,
7
gested that the 5-HT receptor may be involved in the
7
and might be expected to be rapidly metabolized in
vivo. Therefore, we modified compound 1 by protection
of putative metabolic sites. On the basis of superior in
vitro metabolic stability and relative affinity for the
5-HT and 5-HT receptors, compound 10 was selected
for further evaluation of binding property for other
related receptors. We also evaluated its in vitro agonist
or antagonist activity.
7À10
vasodilation of blood vessels.
This receptor is
involved in the control of circadian rhythms of sponta-
neous electrical activity in the suprachiasmatic nucleus
,11À13
and may be involved
(
SCN) of the hypothalamus,3
in disturbance of circadian rhythms.
7
2
1
4,15
A recent study
provided preliminary evidence that 5-HT7 receptor
antagonists show an antidepressant-like effect on sleep
parameters in animals and it was suggested that they
might have therapeutic utility in sleep disorders and
1
6
depression. The affinity of a number of antipsychotic
agents for the 5-HT receptor also led to the speculation
7
that this receptor may mediate the therapeutic actions
1
7
of these compounds. Thus, the 5-HT receptor may be
7
of value as a novel therapeutic target.
Only a few selective antagonists for the 5-HT receptor
7
1
8À23
have been reported to date,
is yet available (Chart 1). In previous papers, we repor-
and no selective agonist
ted the synthesis and affinities for the 5-HT receptor
7
and other receptors of
1
a
series of tetrahydro-
Among them, compound 1 is highly
8-20
benzindoles.
*
9
Corresponding author. Tel.: +81-45-545-3133;fax: +81-45-543-
771;e-mail: chika_kikuchi@meiji.co.jp
7
Chart 1. 5-HT antagonists 1–3.
0
960-894X/03/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00842-9

62
C. Kikuchi et al. / Bioorg. Med. Chem. Lett. 13 (2003) 61–64
ꢀ
ꢀ
2 2 2 2 2 3
Cl , CH Cl , 0 C (87%);(c) corresponding amine, K CO , DMF,
Scheme 1. (a) 1,4-Dibromobutane, 55% NaH, DMF, À40 to 0 C (54%);(b) SO
ꢀ
6
0 C (7=32%, 8=64%, 9=60%, 10=64%).
The synthetic procedures are shown in Scheme 1. Com-
pound 5 was prepared by treating tetrahydrobenzindole
4
(M+16), and one of the major metabolites was identi-
2
7
fied as the 6-hydroxy-tetrahydrobenzindole derivative.
with sodium hydride and 1,4-dibromobutane.19 Com-
pound 6 was obtained from compound 5 by reaction
These results suggested the need of blocking the metabolic
oxidation, and we modified compound 1 by hologenation
of putative metabolic sites. The monohalogenated com-
pounds 7 and 8 were slightly more stable than com-
pound 1, and the dihalogenated compounds 9 and 10
were more stable than compounds 7 and 8. Compounds
1 and 7–10 were further evaluated for metabolic stabi-
ꢀ
-chlorobenzindole 6 in a high yield, with no detectable
with SO Cl at 0 C. The reaction furnished the
2
2
6
formation of the 8-chlorobenzindole. Chlorination of
compound 5 at room temperature mainly provided the
2
5
6
,8-dichlorinated compound. Compounds 1 and 7–10
2
6
were obtained from compounds 5 and 6 by the reaction
with the corresponding amines in the presence of
K CO .
lity in the presence of human liver microsomes. As can
be seen Table 1, compounds 9 and 10 were more stable
than compound 1 for both rat and human liver micro-
somes. Increased stability of compounds 9 and 10 could
be explained by the protection of metabolic hydroxyl-
ation. As we had expected, the in vitro metabolic stabi-
lity was improved by the protection of putative sites of
oxidative metabolism.
2
3
Compounds 1 and 7–10 were evaluated for metabolic
2
6
stability in the presence of rat liver microsomes. These
results are summarized in Table 1. Compound 1 was
almost completely lost after 30-min incubation with rat
microsomes, and its several metabolites were detected.
Some metabolites were hydroxylated derivatives
Next, compounds 7–10 were evaluated for affinity for
the 5-HT and 5-HT receptors. The affinity for the
7
2
5-HT receptor was assayed in terms of the ability to
displace the radioligand [ H]5-carboxyamidotryptamine
7
7 2
Table 1. Metabolic stability and 5-HT and 5-HT receptor affinities
of compounds 1, 7–10
3
3
([ H]5-CT) from cloned human 5-HT receptor expres-
sed in COS-7 cells. The results, expressed as pK , are
7
i
also summarized in Table 1.
As we have reported in the previous paper,19 the 4-fluoro-
phenyl derivative 7 is as potent as compound 1. The
6-chloro-tetrahydrobenzindole 8 was as potent and
Compd R1 R2
d
selective as compound 1, and had slightly higher affinity
for the 5-HT7 receptor than the 6-bromo-tetra-
hydrobenzindole analogue19 (pK =7.91 for the 5-HT
receptor). This result is consistent with our previous
speculation that a large substituent at the 6-position of
tetrahydrobenzindole reduces the affinity for the 5-HT7
receptor. The dihalogenated compounds 9 and 10 also
Metabolic rate
pmol/min/mg protein)
pK
i
(
i
7
5HT e
5HT
2
f
Rat
Human
7
635–885a
141–213a
1
7
8
9
1
H
F
H
F
H
H
Cl
Cl
8.67Æ0.07 7.01Æ0.05
8.45Æ0.05 7.18Æ0.04
8.30Æ0.05 6.50Æ0.02
8.69Æ0.06 6.64Æ0.13
b
b
493
418, 423
323
228
131
103, 150
93
51
c
c
b
b
showed high affinity for the 5-HT receptor. Moreover,
7
b
b
0
Cl Cl
8.14Æ0.06
<6
compounds 9 and 10 were at least 2-fold more selective
for the 5-HT receptor than compound 1.
a
n=4.
n=1.
n=2.
7
b
c
On the basis of its metabolic stability and relative affi-
nity for the 5-HT and 5-HT receptors, the dihalogen-
ated compound 10 was selected for evaluation of
d
e
The pK
i
values are meansÆSE of 8–12 values.
7
2
Binding affinity (human recombinant receptors in mammalian cells;
3
[
f
H]5-CT).
Binding affinity (rat cerebral cortex membranes;[ H]ketanserin).
3
28
binding ability to other 5-HT receptor subtypes. As can

C. Kikuchi et al. / Bioorg. Med. Chem. Lett. 13 (2003) 61–64
63
Table 2. Receptor binding profile of 10a
(bioavailability=18% in rats), with high selectivity over
the 5-HT receptor and other receptors, should be a
2
Receptor
Affinity (pK
i
)b
useful tool for evaluating the therapeutic potential of
5-HT antagonists.
5
5
5
5
5
5
5
5
-HT1A
-HT1B
-HT2C
6.50Æ0.14
<6
<6
7
-HT
-HT
-HT
-HT
-HT
2
3
4
6
7
<6
<6
<6
<6
References and Notes
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a
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3
29
5
-HT1A [human recombinant (mammalian);[ H]8-OH-DPAT];
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5
[
-HT1B [human recombinant (mammalian);[ H]5-CT]; 5-HT2C
[rat
[human recombinant (mam-
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28
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64
C. Kikuchi et al. / Bioorg. Med. Chem. Lett. 13 (2003) 61–64
2
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for the 5-HT receptor, but this derivative had lower selectivity
1
2
5. New compounds were characterized by H NMR and MS.
7
2
chloro-2a,3,4,5-tetrahydrobenzo[cd]indol-2(1H)-one (10):
a-[4-[4-(4-Chlorophenyl)-1,2,3,6-tetrahydropyridyl]butyl]-6-
1
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NMR (CDCl ) d 1.06–1.18 (1H, m), 1.30–1.53 (4H, m), 1.71–
3
2
(
.00 (3H, m), 2.08–2.20 (2H, m), 2.30–2.42 (2H, m), 2.47–2.53
2H, m), 2.59–2.66 (2H, m), 2.73–2.85 (2H, m), 3.04–3.11 (2H,
m), 6.02 (1H, br s), 6.62 (1H, d, J=8.0 Hz), 7.15 (1H, d), 7.26
+
(
2
1H, br s), 7.26–7.28 (4H, m);EIMS m/z 454, 456, 458 (M ).
6. All incubations [rat liver or human liver microsomes,
+
substrate, glucose-6-phosphate, b-NADP , G-6-P dehy-
drogenase, MgCl2 6H O, phosphate buffer (pH 7.4) and
] were performed on a gently shaking platform
2
EDTA Na
2
ꢀ
maintained at 37 C. Incubations were started by the addition
of substrate and were stopped after 0, 10, 30 and 60 min by
addition of DMF. Precipitated proteins were removed by
centrifugation and supernatants were injected into the HPLC
system to determine the remaining amount of each compound.
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