Synthesis and biological evaluation of indole-chalcone derivatives as β-amyloid imaging probe

Article abstract of DOI:10.1016/j.bmcl.2010.12.045

A series of chaclone derivatives containing an indole moiety were evaluated in competitive binding assays with Aβ_1-42 aggregates versus [¹²⁵I]IMPY. The affinity of these compounds ranged from 4.46 to >1008 nM, depending on the substi

Full text of DOI:10.1016/j.bmcl.2010.12.045

Bioorganic & Medicinal Chemistry Letters 21 (2011) 980–982
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and biological evaluation of indole-chalcone derivatives
as b-amyloid imaging probe
Mengchao Cui a,b, Masahiro Ono a, , Hiroyuki Kimura a, Bo Li Liu b, Hideo Saji a,
⇑
⇑
a Graduate School of Pharmaceutical Sciences, Kyoto University, 46-29 Yoshida Shimoadachi-cho, Sakyo-ku, Kyoto 606-8501, Japan
b Key Laboratory of Radiopharmaceuticals, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of chaclone derivatives containing an indole moiety were evaluated in competitive binding
assays with Ab1-42 aggregates versus [125I]IMPY. The affinity of these compounds ranged from 4.46 to
>1008 nM, depending on the substitution on the phenyl ring. Fluorescent staining in vitro showed that
one compound with a N,N-dimethylamino group intensely stained Ab plaques within brain sections of
AD transgenic mice. The radioiodinated probe [125I]-(E)-3-(1H-indol-5-yl)-1-(4-iodophenyl)prop-2-en-
1-one, [125I]4, was prepared and autoradiography in sections of brain tissue from an animal model of
AD showed that it labeled Ab plaques specifically. However, experiments with normal mice indicated that
Received 13 October 2010
Accepted 8 December 2010
Available online 13 December 2010
Keywords:
Alzheimer’s disease
b-Amyloid
Imaging agent
Binding assay
Autoradiography
[
125I]4 exhibited a low uptake into the brain in vivo (0.41% ID/g at 2 min). Additional chemical modifica-
tions of this indole-chalcone structure may lead to more useful imaging agents for detecting b-amyloid
plaques in the brains of AD patients.
Ó 2010 Elsevier Ltd. All rights reserved.
Alzheimer’s disease (AD) is a progressive and fatal brain disease
which accounts for the majority of dementia cases. Although the
cause and progression of AD are not well understood, research
indicates that the disease is associated with b-amyloid plaques
(Ab) and neurofibrillary tangles (NFTs). The amyloid cascade
hypothesis indicates that the deposition of amyloid plaques consti-
tutes a central and probably early event in the pathogenesis of
AD.1–3 Therefore, a positron emission tomography (PET) or single
photon emission computed tomography (SPECT) tracer agent spe-
cifically targeting Ab plaques would provide an important tool for
the early and non-invasive diagnosis of this disease.
from Congo Red or thioflavin-T, and few other core structures for
imaging Ab have been found.
In the search for more useful candidates for Ab imaging probes,
we found iodinated and fluorinated chalcones to function as a new
scaffold. The chalcone derivatives showed strong binding to Ab
aggregates and high brain penetration, but also slow clearance
from the brain, resulting in low background-to-noise ratios.13–15
To develop more useful probes based on the chalcone structure,
we decided to further extend our research using the new chaclone
core, which possesses an indole ring (Fig. 1). The indole structure is
often used as a fused ring of N-methyl- or N,N-dimethylamino-
groups, and expected to maintain the affinity for Ab aggregates.16
In the present study, we synthesized 10 new indole-chalcone
derivatives and evaluated their utility as Ab imaging probes. To
our knowledge, this is the first time indole-chalcone derivatives
have been proposed as Ab imaging probes for detecting AD.
The procedures used to produce the indole-chalcone derivatives
are outlined in Scheme 1. The key step in the synthesis was the
base-catalyzed Claisen condensation reaction. Compounds 1–10
were obtained from substituted acetophenones and 1H-indole-5-
carbaldehyde in the presence of a basic catalyst (28% CH3ONa) in
ethanol at room temperature (yield, 5.8–59.8%). The tributyltin
precursor 11 was prepared from the corresponding bromo com-
pound in a bromo to tributyltin exchange reaction catalyzed by
(PPh3)4Pd (yield 18.4%).
During the last decade, a number of Ab plaque-specific imaging
agents for PET or SPECT have been reported. Among them, 2-(40-
[
11C]methylaminophenyl)-6-hydroxybenzothiazole ([11C]PIB),4,5
4-N-[11C]methylamino-40-hydroxystilbene ([11C]SB-13),6,7 18F]-4-
[
(N-methylamino)-40-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)-stilbene
([18F]BAY94-9172)8 and [123I]-6-iodo-2-(40-dimethylamino)-phe-
nyl-imidazo[1,2]pyridine ([123I]IMPY)9,10 have been tested clini-
cally and demonstrated potential for imaging Ab plaques in vivo.
Recently, a styryl pyridine derivative, [18F]-(E)-4-(2-(6-(2-(2-(2-
fluoroethoxy)ethoxy)ethoxy)pyridin-3-yl)vinyl)-N-methylaniline
([18F]AV-45), has shown promise as a probe in living human brain
tissue in phase II and III clinical trials.11,12 These probes are derived
The affinity of these indole-chalcone ligands (1–10) for Ab1-42
aggregates was tested with a competition binding assay using
[
⇑
Corresponding authors. Tel.: +81 75 753 4608; fax: +81 75 753 4568 (M.O.); tel.:
+81 75 753 4556; fax: +81 75 753 4568 (H.S.).
E-mail addresses: ono@pharm.kyoto-u.ac.jp (M. Ono), hsaji@pharm.kyoto-u.ac.jp
(H. Saji).
125I]IMPY as the competing radioligand. IMPY was also screened
using the same system for comparison. As shown in Table 1,
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.12.045

M. Cui et al. / Bioorg. Med. Chem. Lett. 21 (2011) 980–982
981
O
O
R = NH2
A
R = NHCH3
R = N(CH3)2
R = OH
18F
125I
R
O
N
3
R = OCH3
O
O
B
N
H
R
R
NH2
Figure 1. (A) Radiolabeled chalcone derivatives as b-amyloid imaging probes; (B) The ring-fusing indole-chalcone derivatives designed based on the chalcone structure.
binding was dependent on the substitution patterns of the phenyl
ring. Compared with the fluorinated ligand 1, halogenation at the
para-position (Cl, Br or I) on the phenyl ring enhanced the binding
affinity (2, 3 and 4). Replacing the para-halogen with a hydroxy
group decreased the affinity (5) dramatically, while methylation
of the hydroxy group restored it (6). Binding was totally abolished
when an amino group was placed at the para-position. However, if
gradually increased with the methylation of the amino group, as
reflected by the following order: N,N-dimethylated derivative
(9) > N-monomethylated derivative (8) ꢀ primary amino deriva-
Scheme 1. Reagents and conditions: (a) ethanol, NaOMe (1 M in MeOH), r.t.;
tive (7). Enlargement of the aromatic conjugation system also in-
creased the affinity as shown by 11 with an enlarged
naphthalene ring. Also, compared with IMPY, the iodinated com-
pound 4 had a lower Ki value. Interestingly, all of these indole-chal-
cone ligands showed competition with [125I]IMPY for binding to
Ab1-42 aggregates. This means that the derivatives and IMPY share
the same binding pocket on Ab fibers. Previously, our research indi-
cated chaclones to have a binding site on Ab aggregates different
from that of thioflavin-T and Congo Red, because neither of them
could inhibit the binding of a radioiodinated chaclone to Ab aggre-
gates.13 Initially, we considered that enlargement of the aromatic
conjugation system (phenyl ring changed to a indole ring) may
alter the binding. So we chose a chaclone compound, (E)-3-(4-
(dimethylamino)phenyl)-1-(4-iodophenyl)prop-2-en-1-one (DMIC),
for comparison. To our surprise, DMIC inhibited the binding of
(b) (Bu3Sn)2, (PPh3)4Pd, toluene.
Table 1
Inhibition constants (Ki) for binding to Ab1-42 aggregates versus [125I]IMPY
Compound
R
Ki (nM)
1
2
3
4
5
6
7
8
9
4-Fluorophenyl
4-Chorophenyl
4-Bromophenyl
4-Iodophenyl
4-Hydroxyphenyl
4-Methoxyphenyl
4-Aminophenyl
4-Methylaminophenyl
4-Dimethylaminophenyl
35.06 6.21
8.43 2.13
8.96 0.92
8.22 1.46
>360
8.52 2.15
>1008
51.09 7.71
5.17 0.32
[
125I]IMPY in a dose-dependent manner with high affinity for
Ab1-42 aggregates (Ki = 1.97 nM). These findings suggested
chaclone and IMPY to share the same binding site (thioflavin-T)
on Ab aggregates or at least have overlapping binding sites.
Due to the encouraging results obtained for the iodinated ligand
4, further biological evaluations were carried out with an [125I]-la-
beled probe. [125I]4 was prepared from the corresponding tributyltin
precursor through an iododestannylation reaction in two steps.
HPLC-purified [125I]4 showed greater than 95% purity with 21.1%
yield. The specific activity of the no-carrier-added preparation was
comparable to that of Na125I, 2200 Ci/mmol. Finally, the identity of
10
4.46 0.87
DMIC-14
IMPY
—
—
1.97 0.26
10.5 1.0
[
125I]4 was verified by a comparison of retention time with the
nonradioactive compound 4(Scheme 2) (see Supplementary data).
To confirm the specific binding of these indole-chalcone deriv-
atives to Ab plaques, we performed fluorescent staining and auto-
Scheme 2. Radiolabeling of 4.
Figure 2. Fluorescence staining of 9 (A) in a section of brain tissue from a Tg-C57 (APP/PS1) mouse (6 lm thick) with a filter set for GFP. The labeled plaques were confirmed
by staining of the adjacent section with thioflavin-S (B) using a filter set for DAPI.

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M. Cui et al. / Bioorg. Med. Chem. Lett. 21 (2011) 980–982
[
125I]4 displayed a lower partition coefficient (log D = 2.03 0.10)
which may explain the low brain uptake.
In conclusion, we have successfully designed and synthesized a
series of indole-chalcone derivatives, whose affinity for Ab aggre-
gates depends on the substitution at the phenyl ring. We also
found that the derivatives likely bind to Ab aggregates at the thio-
flavin-T site. A radioiodinated ligand, [125I]4, showed specific label-
ing of Ab plaques in sections of brain tissue from an animal model
of AD. However, [125I]4 displayed a relatively low initial uptake
into the brain. Further chemical modifications of the indole-chal-
cone structure may lead to a useful b-amyloid imaging agent.
Acknowledgments
This study was supported by a Grant-in-aid for Young Scientists
(A) and Exploratory Research from the Ministry of Education, Cul-
ture, Sports, Science and Technology, Japan, as well as by the China
Scholarship Council (CSC).
Figure 3. In vitro labeling of brain sections from Tg-C57(APP/PS1) mice (6 lm
thick) and wild-type controls by autoradiography. [125I]4 labeled the Ab plaques in
the cortex of the brain (A), while the control case was clearly void of any notable Ab
labeling (B). The same sections were also stained with thioflavin-S (C, D) and the
distribution of Ab plaques was consistent with the results of autoradiography (red
arrows).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2010.12.045.
Table 2
Biodistribution in normal ddY mice after iv injection of [125I]4a
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a
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% injected dose per gram. Average for five mice standard
b
Expressed as % injected dose per organ.
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[
125I]4 initially accumulated in the blood, lungs and kidneys. The
proportion taken up into the brain was 0.41% ID/g at 2 min postin-
jection, and radioactivity was slowly washed out from the brain at
30 min (0.20% ID/g) (Table 2). Under the experimental conditions,