Benzothieno[3,2-b]indole derivatives as potent selective estrogen receptor modulators

Article abstract of DOI:10.1016/j.bmcl.2005.03.111

A series of estrogen receptor ligands based on benzothieno[3,2-b]indole were synthesized and their binding affinity for estrogen receptor subtypes (ERα and ERβ) and effects on mouse uterus and bone were evaluated. Some of these compounds showed strong bin

Full text of DOI:10.1016/j.bmcl.2005.03.111

Bioorganic & Medicinal Chemistry Letters 15 (2005) 2891–2893
Benzothieno[3,2-b]indole derivatives as potent selective
estrogen receptor modulators
Qinggang Ji,a Jie Gao,b Junbo Wang,a Chunhao Yang,a,* Xin Hui,b Xueming Yan,a
Xihan Wu,a Yuyuan Xiea and Ming-Wei Wangb,*
aState Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Shanghai Institute for Biological Sciences,
Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, 555 Zu Chong Zhi Road, Shanghai 201203, China
bThe National Center for Drug Screening, 189 Guo Shou Jing Road, Shanghai 201203, China
Received 17 December 2004; accepted 22 March 2005
Available online 29 April 2005
Abstract—A series of estrogen receptor ligands based on benzothieno[3,2-b]indole were synthesized and their binding affinity for
estrogen receptor subtypes (ERa and ERb) and effects on mouse uterus and bone were evaluated. Some of these compounds showed
strong binding affinity to ER and significantly increased the bone mineral density of ovariectomized mice.
Ó 2005 Elsevier Ltd. All rights reserved.
Estrogen receptor and endogenous estrogens play
important roles in the development and function of fe-
male reproductive system. At present, estrogen replace-
ment therapy (ERT) is one of the most effective
treatments for menopausal symptoms and postmeno-
pausal osteoporosis arising from lowering levels of cir-
culation estrogen.1 Although ERT is effective, it is also
associated with some serious side-effects. More recently,
selective estrogen receptor modulators (SERMs) which
fully antagonize the effects of estrogen on uterine and
mammary tissues, while mimicking the effects of estro-
gen on the bone and cardiovascular system, have been
investigated as a possible alternative to ERT.2 Synthetic
estrogen receptor ligands, such as tamoxifen and raloxif-
ene, produce biological responses that can be either
estrogenic or anti-estrogenic, depending upon the tissue
in which their action is examined.3 The SERMs pres-
ently used clinically have obvious advantages over the
conventional ERT, but they retain some of the disad-
vantages as well. It is known that in the X-ray crystal
structure of raloxifene bound to ERa, the basic side
chain of the ligand occupied an orthogonal position rel-
ative to the benzothiophene core.4 In an effort to con-
strain the side chain into its active conformation, we
change the benzothiophene core to benzothioph-
ene[3,2-b]indole (Fig. 1), which contains a tetracycle
core-motif that simultaneously restricts rotation of the
two phenolic groups so that the core is more coplanar
and perhaps better mimics the steroidal ligand. In addi-
tion, attachment of the side chain can readily assume the
axial position. This tetracycle core also contains a rigid
stilbene moiety that has pharmaceutical relevance in
many synthetic non-steroidal estrogen antagonists. This
may lead to the discovery of compounds with potential
biological activity in estrogen sensitive tissues.5
The amine-containing side chain of raloxifene is recog-
nized to be critical in binding to the amino acid residue
of the ER, and it also plays a role in determining the
tissue specificity.4 Therefore, it could be valuable to
modify the side chain in order to understand the struc-
ture–activity relationship (SAR) of these compounds.
In this article, we report the synthesis of benzothi-
eno[3,2-b]indole derivatives and their biological
activities.
Scheme 1 depicts the syntheses of the compounds.
Alkylation of thiol 10 gave 11. Compound 11 is
suspended in tetrachloroethane and treated with PCl3
at 90 °C for 3 h. After standing at room temperature
overnight, the reaction mixture was added to a slurry
of aluminum chloride in tetrachloroethane at 60 °C
and kept at that temperature for 30 min. The ketone
12 thus formed was treated with phenylhydrazine or
Keywords: SERM; Benzothieno[3,2-b]indole; Estrogen receptor.
*
Corresponding authors: Tel./fax: +86 21 50806770 (C.Y.); tel./fax:
+86 21 50800721 (M.-W.W.); e-mail addresses: chyang@mail.shcnc.
ac.cn; mwwang@siniwest.com
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.03.111

2892
Q. Ji et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2891–2893
Figure 1. Structure of raloxifene and target compounds.
Scheme 1. Synthesis of target compounds. Reagents and conditions: (a) ClCH2COOH, 10% NaOH (aq), reflux, 2 h; (b) PCl3, CHCl2CHCl2, AlCl3,
in two steps; (c) phenylhydrazine or p-methoxyphenylhydrazine, acetic acid, 80 °C; (d) aminoethoxybenzyl chloride, NaH, DMF, rt; (e) NaSEt,
DMF, reflux.
p-methoxyphenylhydrazine in acetic acid to afford
7-methoxy-benzothiophene[3,2-b]indole
dimethoxy-benzothiophene[3,2-b]indole,
through the Fischer indole synthesis.6 The indole nu-
cleus and relevant aminoethoxybenzyl chloride were
coupled in DMF in the presence of NaH, which after
deprotection with NaSEt in dry DMF gave the target
compounds.
NaBH4 in methanol at room temperature gave alcohol
9, which was subsequently reacted with amines to give
15–20. A solution of one compound of 15–20 in THF
was cooled to 0 °C and gaseous HCl bubbled through
until no more thickening was observed, then SOCl2
was added and the mixture was heated at 50 °C until
the mixture became clear. The reaction mixture was
cooled to give the target compounds 21–26, respectively.
and
respectively,
3,7-
The synthesis of side chains is shown in Scheme 2. The
coupling reaction between 4-hydroxybenzaldehyde and
1,2-dibromoethane afforded aldehyde 8. Reduction with
The compounds were primarily tested for intrinsic activ-
ity in an ER binding assay with [3H]17b-estradiol and
full-length recombinant human ERa and ERb proteins.7
Scheme 2. Synthesis of side chain. Reagents and conditions: (a) K2CO3, DMF, 120 °C; (b) NaBH4, methanol; (c) amine, THF, reflux; (d) HCl,
SOCl2, THF in two steps.

Q. Ji et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2891–2893
Table 1. Binding affinitiesa and in vivo examination datab
2893
Compounds
R
Z
Binding affinity (IC50 (nmol))
BMD (mg/cm3)
Uterine weight assay
Wet weight (mg) Agonist (%)
53.8 7.3***
ERa
ERb
Raloxifene
2a
0.89 0.02
39.0 2.2
2.84 0.1
27.2 1.5
6.0 0.7
11.6 2.2
35.0 3.0
11.0 0.2
53.9 8.0
20.2 6.5
16.6 2.8
7.26 0.4
45.6 2.6
31.0 3.7
25.0 7.0
16.4 7.1
215.0 11.9
21.4 0.3
565 21***
118.0
37.7
79.0
11.7
24.6
15.6
79.3
14.1
6.1
###
H
Piperidine
Piperidine
436 41*###
34.0 4.3**
44.2 4.9***
#
#
2b
3a
OH
H
548 47***
Pyrrolidine
Pyrrolidine
505 74*#
469 36*###
444 15*#
27.6 3.0*###
30.7 5.8*###
28.5 4.8*###
3b
4a
OH
H
Hexamethyleneimine
Hexamethyleneimine
Dimethylamine
Dimethylamine
Diethylamine
Diethylamine
Morpholine
18.0 7.76
3.34 1.79
51.4 3.62
18.0 3.37
57.4 10.6
31.2 8.62
126.0 60.5
10.0 0.7
#
#
4b
5a
OH
H
534 25***
44.2 4.8***
474 73*##
480 33*###
448 31*###
479 47*##
455 56*###
470 28*###
434 38
28.2 2.4*###
26.2 5.3*###
5b
6a
OH
H
###
34.9 3.7***
41.4
52.9
9.4
###
6b
7a
OH
H
37.7 8.1**
27.0 3.5*###
26.3 1.2*###
27.2 4.7
7b
OVX + DW
Sham + DW
OH
Morpholine
6.5
###
152 51.8***
569 63***
a The compounds were evaluated for their binding ability to ERa and ERb in competition with H-estradiol. Data were analyzed using GraphPad
Prism software. Each value represents the median observed in three independent experiments ( SD).
3
b Two months old ovariectomized Kun-Ming mice were treated (s.c.) with the test compounds at a dose of 4 lmol/kg q.d. for 4 weeks. The bone
mineral density was determined by peripheral quantitative computed tomography (pQCT, Stratec, XCT Research SA, FRG). The uterine wet
weights were measured on the day when mice were sacrificed [n = 5 per group; mean SD, *p >0.05, **p <0.05, ***p <0.001 vs distilled water (DW);
#p >0.05, ##p <0.05, ###p <0.01 vs raloxifene].
They were then examined for effects on stimulation of
the bone density and uterine weight.8 It is clear from
the data shown in Table 1 that the 3-hydroxyl group is
essential to the pharmacological properties of these
compounds. Comparison of the binding activities of
compounds a and b reveal 2- to 13-fold decrease in rel-
ative binding affinity when the 3-hydroxy group is re-
placed with a proton. Although the same phenomenon
was not observed in the bone mineral density (BMD)
measurement, the effects of compounds 2b and 4b on
bone density are comparable with raloxifene (p >0.05),
while the uterus effects are lower (p <0.05).
of Science and Technology of China (2002AA2Z343A to
MWW), and the Chinese Academy of Sciences (KSCX1-
SW-11-2 to MWW).
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This work was financially supported in part by National
Natural Science Foundation of China (No. 20302009 to
XHW and No. 30371653 to MWW), Shanghai Rising-
Star Program (No. 03QD14064 to XHW), the Ministry