Novel imaging agents for β-amyloid plaque based on the N-benzoylindole core

Article abstract of DOI:10.1016/j.bmcl.2011.06.077

We report the synthesis and evaluation of a series of N-benzoylindole derivatives as novel potential imaging agents for β-amyloid plaques. In vitro binding studies using Aβ_1-40 aggregates versus [ ¹²⁵I]TZDM showed that all these deri

Full text of DOI:10.1016/j.bmcl.2011.06.077

Bioorganic & Medicinal Chemistry Letters 21 (2011) 5594–5597
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Novel imaging agents for b-amyloid plaque based on the N-benzoylindole core
Yang Yang a, Xin-Hong Duan a,b, Jun-Yuan Deng a, Bing Jin a, Hong-Mei Jia a, Bo-Li Liu a,
⇑
a Key Laboratory of Radiopharmaceuticals, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing, PR China
b College of Science, Beijing Forestry University, Beijing, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the synthesis and evaluation of a series of N-benzoylindole derivatives as novel potential imag-
ing agents for b-amyloid plaques. In vitro binding studies using Ab1–40 aggregates versus [125I]TZDM
showed that all these derivatives demonstrated high binding affinities (Ki values ranged from 8.4 to
121.6 nM). Moreover, two radioiodinated compounds [125I]7 and [125I]14 were prepared. Autoradiogra-
phy for [125I]14 displayed intense and specific labeling of Ab plaques in the brain sections of AD model
mice (C57, APP/PS1) with low background. In vivo biodistribution in normal mice exhibited sufficient ini-
tial brain uptake for imaging (2.19% and 2.00% ID/g at 2 min postinjection for [125I]7 and [125I]14, respec-
tively). Although additional modifications are necessary to improve brain uptake and clearance from the
brain, the N-benzoylindole may be served as a backbone structure to develop novel b-amyloid imaging
probes.
Received 24 February 2011
Revised 13 June 2011
Accepted 17 June 2011
Available online 7 July 2011
Keywords:
Alzheimer’s disease
b-Amyloid
Binding affinity
Imaging agent
Ó 2011 Elsevier Ltd. All rights reserved.
Alzheimer’s disease (AD), the most common form of dementia
in elderly people, is characterized by cognitive impairment and
memory loss. Currently, a definite confirmation of AD is attained
only by post-mortem histopathology of the brain. It is generally ac-
cepted that the accumulation of senile plaques (SPs) and neurofi-
brillary tangles (NFTs) are two pivotal clinical pathological
features of AD, and the formation of amyloid plaques is prior to
the onset of clinical symptoms.1,2 Therefore, noninvasive detection
of Ab plaques in vivo by PET (positron emission tomography) or
SPECT (single photon emission computed tomography) would be
very useful for early diagnosis of AD.3,4
use. Therefore, the development of more useful Ab plaques imag-
ing agents for SPECT has been a critical issue. During the past
decade, many SPECT tracers with new backbone structures have
been reported as Ab probes, such as radio-iodinated aza-diphe-
nylacetylenes16, styrylpyridines17, flavone derivatives18,19, aurone
derivatives20,21
,
chalcones derivatives22,23
,
(E)-5-styryl-2,20-
bithiophen derivatives24, as well as 99mTc labeled chalcone25, fla-
vone and aurone.26
In the search for novel Ab imaging agents with improved proper-
ties, many small-molecule Ab inhibitors, including nonsteroidal
anti-inflammatory drugs (NSAIDs), have been developed with nec-
essary modifications in order to improve the affinities to Ab plaques
and enhance permeability through the BBB.27 Agdeppa et al. have
reported that naproxen and ibuprofen shared the same binding sites
of [18F]FDDNP on Ab fibrils. The Ki values of (S)-naproxen, (R)-ibu-
Currently, a lot of radiolabeled ligands derived from Congo
Red (CR) or Thioflavin T (ThT) have been developed as Ab pla-
ques imaging probes. Several PET tracers, such as 2-(40-
[
11C]methylaminophenyl)-6-hydroxybenzothiazole ([11C]PIB)5,6
,
4-N-[11C]methylamino-40-hydroxystilbene ([11C]SB-13),7,8
4-(N-methylamino)-40-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)-stil-
bene ([18F]BAY94-9172)9, 18F]-2-(3-fluoro-4-(methylamino)
phenyl)benzo[d]thiazol-6-ol ([18F]GE-067)10 18F]-(E)-4-(2-(6-
[
18F]-
profen, and (S)-ibuprofen were 5.70 1.31 nM, 44.4 17.4
lM and
11.3 5.20
l
M, respectively.28 Indomethacin has been known to in-
[
hibit Ab fibril formation from Ab at required concentrations, and the
anti-aggregation effect of it may due to its interaction with Ab.29 In
our present study, we selected indomethacin as the lead compound
aiming to develop radiotracers with a new core structure to image
Ab plaques. In order to simplify the synthesis process and enhance
permeability through the BBB, methyl and CH2COOH groups were
removed from the chemical structure. Furthermore, ‘C@C’ was
introduced to improve the affinities to Ab plaques (Scheme 1). A ser-
ies of the resulted N-benzoylindole-based compounds were synthe-
sized and their binding affinities for Ab1–40 aggregates were
measured. Moreover, two radio-iodinated derivatives were pre-
pared and evaluated as potential SPECT tracers for imaging amyloid
plaques in the brain.
,
[
(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)pyridin-3-yl)vinyl)-N-
methylaniline ([18F]AV-45)11,12 have been evaluated in human
studies. However, [123I]-6-iodo-2-(40-dimethylamino-)phenyl-imi-
dazo[1,2]pyridine ([123I]IMPY)13–15, the only SPECT tracer tested
in human studies, has failed because of its high lipophilicity,
in vivo instability and insufficient target-to-background ratio
(Fig. 1). In comparison with PET, SPECT is a more widely acces-
sible and cost-effective technique in terms of routine diagnostic
⇑
Corresponding author.
E-mail address: liuboli@bnu.edu.cn (B.-L. Liu).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.06.077

Y. Yang et al. / Bioorg. Med. Chem. Lett. 21 (2011) 5594–5597
5595
Figure 1. Chemical structure of Ab imaging probes for clinical study.
Scheme 1. Design considerations of N-benzoylindole-based derivatives.
The synthetic route of N-benzoylindole derivatives (3–17) is
outlined in Scheme 2. The key step was the Schotten–Baumann
reaction between the suitable 5-substituted indoles and 2. The tri-
butyltin derivatives (18, 19) were prepared from the bromo-pre-
cursors (6, 13) using an exchange reaction catalyzed by Pd(0)
with the yields of 15.8% and 22.6%, respectively. [125I]7 and
[
125I]14 were prepared via the iododestannylation reaction using
hydrogen peroxide as the oxidant (Scheme 3). The reaction was
quenched with saturated NaHSO3 solution. The resulting mixture
was purified by HPLC using a reverse-phase column and mobile
phase consisting of acetonitrile with a flow rate of 1 mL/min. The
radio-ligands were co-injected and co-eluted with the correspond-
ing nonradioactive 7 and 14. The radiochemical yields of [125I]7
and [125I]14 were 57–72% and 50–71%, respectively. The radio-
chemical purities of [125I]7 and [125I]14 were both greater than
98%. The specific activity of the no-carrier-added [125I]NaI was
>2200 Ci/mmol at time of delivery. The specific activities of the
products were not determined. The log D values of [125I]7 and
Scheme 3. Reagents and conditions: (c) [125I]NaI, H2O2, HCl, rt.
1, the binding affinities of these N-benzoylindole derivatives for
Ab1–40 aggregates varied from 8.4 to 121.6 nM, suggesting that
all these compounds share the same binding site with ThT. (E)-1-
(1H-Indol-1-yl)-3-phenylprop-2-en-1-one (10) without any sub-
stituents showed good affinity (Ki = 15.9 nM). The introduction of
methoxy group into the R2 group in the phenyl-ring effected the
affinities slightly (3, Ki = 27.3 nM). The introduction of halogen,
such as F, Cl, Br, into the R1 group in the indole-ring resulted in
slight difference in affinities (Ki = 32.4, 18.1, 25.4, 32.7, 36.4 and
18.9 nM for compounds 4, 5, 6, 11, 12 and 13, respectively), while
modification with methoxy or methyl group at the above position
resulted to a significant reduction in binding affinities (Ki = 103.3,
60.8, 127.7 and 68.2 nM for compounds 8, 9, 15 and 16, respec-
tively). Since derivatives 7 and 14 with iodine-substituent on the
indole-ring displayed the highest binding affinities (Ki = 10.2 and
8.4 nM, respectively), we prepared [125I]7 and [125I]14 for further
biological studies.
[
125I]14 were 2.32 0.02 and 2.53 0.04 (measured by a partition
between n-octanol and pH 7.4 PBS buffer), respectively, which
indicate a potential BBB permeability.
The affinities of these N-benzoylindole derivatives (3–17) for
Ab1–40 aggregates were determined by competition binding assay
using [125I]TZDM as the radiolabeled standard.30 As shown in Table
The radioiodinated probes [125I]14 was investigated by in vitro
autoradiography in the brain sections of a transgenic model mouse
(C57, APP/PS1, 12 months). As shown in Figure 2, autoradiographic
Table 1
Ki values of N-benzoylindole derivatives on [125I]TZDM binding to the aggregated
Ab1–40 peptide in solution
Compound
Kia (nM)
Compound
Kia (nM)
3
4
5
6
7
8
9
10
27.3 1.3
32.4 1.8
18.1 2.0
25.4 1.6
10.2 1.7
103.3 1.7
60.8 1.2
15.9 1.6
11
12
13
14
15
16
17
32.7 1.2
36.4 1.6
18.9 1.2
8.4 1.4
127.7 1.5
68.2 1.3
21.6 4.6
Scheme 2. Reagents and conditions: (a) Et3N, CH2Cl2, reflux; (b) (Bu3Sn)2,
(Ph3P)4Pd, toluene, reflux.
a
Measured in triplicate with results given as the mean SD

5596
Y. Yang et al. / Bioorg. Med. Chem. Lett. 21 (2011) 5594–5597
Figure 2. Autoradiography of [125I]14 in vitro in Tg model mouse (C57-APP/PS1, 12 months old, male) brain sections (A). The presence and distribution of plaques in the
sections were confirmed with thioflavin-S (B) (red arrows).
Table 2
Biodistribution in normal mice after iv injection of [125I]7 and [125I]14 (% ID/g, avg of 5 mice SD) and its partition coefficient (D)
Organ
2 min
15 min
30 min
60 min
120 min
240 min
[
125I]7(log D = 2.32 0.02)
Blood
Brain
Heart
Liver
Spleen
Lung
Kidney
Stomacha
Muscle
13.91 1.86
2.19 0.16
12.64 0.78
24.76 3.16
6.00 1.74
21.76 4.31
12.01 0.28
0.71 0.18
2.73 0.37
5.82 0.31
2.29 0.13
3.52 0.26
18.05 1.12
8.04 0.45
11.13 0.60
9.31 0.38
4.06 0.09
2.62 0.38
4.90 0.44
1.60 0.14
2.45 0.21
14.05 1.09
7.12 2.39
9.05 1.44
8.21 0.93
4.92 1.06
1.63 0.32
3.79 0.71
1.04 0.17
1.53 0.19
12.30 1.32
7.16 3.55
6.39 1.19
5.71 0.36
5.00 1.27
1.30 0.20
3.12 0.48
0.42 0.08
1.21 0.45
9.30 1.12
6.18 2.39
4.31 0.71
4.21 0.66
5.31 1.12
0.65 0.15
2.78 0.35
0.21 0.04
1.17 0.07
7.65 1.12
4.45 1.22
2.95 0.57
4.05 0.72
4.14 1.23
0.92 0.18
[
125I]14(log D = 2.53 0.04)
Blood
Brain
Heart
Liver
Spleen
Lung
Kidney
Stomacha
Muscle
14.13 0.44
2.00 0.17
10.72 0.48
15.89 1.38
4.08 0.60
14.50 3.50
11.22 1.08
1.09 0.53
3.97 0.32
3.91 0.48
1.98 0.13
2.90 0.42
7.73 0.83
2.44 0.62
6.73 0.89
9.68 1.03
1.52 0.21
2.47 0.55
3.75 0.29
1.57 0.22
2.07 0.11
6.48 0.52
1.71 0.23
4.88 0.78
10.16 1.12
1.72 0.42
2.06 0.53
2.88 0.92
0.77 0.12
1.53 0.23
4.09 0.85
1.85 1.31
2.66 1.00
6.14 0.30
2.08 0.68
1.06 0.16
1.61 0.24
0.33 0.04
0.99 0.12
4.86 1.84
0.63 0.54
2.10 0.22
3.54 0.33
2.29 0.67
0.84 0.53
1.07 0.14
0.17 0.03
0.61 0.16
2.23 0.48
0.59 0.12
1.27 0.48
2.22 0.72
1.30 0.38
0.19 0.16
a
Expressed as % ID/organ.
images of [125I]14 showed excellent labeling of Ab plaques in the
cortex region of the brain sections, and no remarkable accumula-
tion of radioactivity were observed in white matter. The same sec-
tions were also stained with thioflavin-S and the localizations of Ab
plaques were in accord with the results of autoradiography. These
results demonstrated that [125I]14 was specific for Ab plaques,
which was consistent with high bind affinity of compound 14 to
Ab1–40 aggregates.
In summary, N-benzoylindole-based compounds have been
synthesized and evaluated as novel imaging probes for Ab plaques.
They showed high binding affinities with Ki values in the nM range
in vitro. Autoradiography for [125I]14 indicated that it stained Ab
plaques in Tg2576 mouse brain clearly. N-benzoylindole-based
derivatives penetrated the brain was encouraging. Although addi-
tional modifications are necessary to improve the in vivo proper-
ties of these derivatives, N-benzoylindole may be served as a
new backbone structure to develop b-amyloid imaging probes.
Biodistribution experiments were performed in normal mice in
order to evaluate the pharmacokinetic properties of these deriva-
tives. [125I]7 and [125I]14 were injected intravenously into normal
mice for biodistribution studies. As shown in Table 2, the initial
brain uptake for [125I]7 and [125I]14 were 2.19 and 2.00% ID/g at
2 min postinjection, respectively, indicating a level sufficient for
brain imaging probe. But the washout of these probes from the
brain in normal mice appears to be relatively slow (1.60% and
1.57% ID/g at 30 min postinjection, respectively), suggesting the
high nonspecific binding of these probes. In addition, the initial
blood uptake of the two probes was higher (13.91% ID/g for
Acknowledgments
This work was funded by NSFC (20871021) and the Fundamen-
tal Research Funds for the Central Universities. The authors thank
Dr. Mengchao Cui (College of Chemistry, Beijing Normal Univer-
sity) for his kindness to provide the Paraffin-embedded brain sec-
tions of AD model mice. The authors also thank Mr. Jin Liu (College
of Life Science, Beijing Normal University) for providing the equip-
ment of the Zeiss Oberver Z1.
[
125I]7 and 14.13% ID/g for [125I]14), which may be due to the rel-
atively high lipophilicity. It has been suggested that the lipophilic-
ity of Ab imaging agents may play an important role in uptake and
washout. Additional structural modifications, such as introducing a
hydrophilic group to decrease the lipophilicity, are needed to
achieve the fast washout rate from brain and blood for the N-ben-
zoylindole-based compounds.
Supplementary data
Supplementary data (procedure for the preparation of new N-
benzoylindole-based derivatives, in vitro binding studies, in vitro
autoradiography using Tg 2576 mouse brain sections and biodis-
tribution studies in normal mice) associated with this article can

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5597
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