Radioiodinated flavones for in vivo imaging of β-amyloid plaques in the brain

Article abstract of DOI:10.1021/jm050635e

In vivo imaging of β-amyloid (Aβ) peptide aggregates in the brain may lead to early detection of Alzheimer's disease (AD) and monitoring of the progression and effectiveness of AD treatment. The purpose of this study was to develop novel amyloid imaging a

Full text of DOI:10.1021/jm050635e

J. Med. Chem. 2005, 48, 7253-7260
7253
Radioiodinated Flavones for in Vivo Imaging of â-Amyloid Plaques in the Brain
Masahiro Ono,*,† Naoko Yoshida,† Kenichi Ishibashi,‡ Mamoru Haratake,† Yasushi Arano,§ Hiroshi Mori,‡ and
Morio Nakayama†
Department of Hygienic Chemistry, Graduate School of Biomedical Sciences, Nagasaki University, 1-14 Bunkyo-machi,
Nagasaki 852-8521, Japan, Department of Neuroscience, Osaka City University Medical School, 1-4-3 Asahi-machi, Abeno-ku,
Osaka 545-8585, Japan, Department of Molecular Imaging and Radiotherapy, Graduate School of Pharmaceutical Sciences,
Chiba University, 1-8-1 Inohana, Chuo-ku, Chiba 260-8675, Japan
Received July 4, 2005
In vivo imaging of â-amyloid (Aâ) peptide aggregates in the brain may lead to early detection
of Alzheimer’s disease (AD) and monitoring of the progression and effectiveness of AD treatment.
The purpose of this study was to develop novel amyloid imaging agents based on flavone as a
core structure. Radioiodinated flavone derivatives were designed and synthesized. The binding
affinities of flavone derivatives for Aâ aggregates varied from 13 to 77 nM. When in vitro plaque
labeling was carried out using post-mortem AD brain sections, all flavones intensely stained
not only amyloid plaques but also cerebrovascular amyloids. In biodistribution studies using
normal mice, they displayed high brain uptakes ranging from 3.2 to 4.1% ID/g at 2 min
postinjection. The radioactivity washed out from the brain rapidly (0.5-1.9% ID/g at 30 min),
which is highly desirable for amyloid imaging agents. The results in the study suggest that
these classes of radioiodinated flavones may be useful candidates as potential imaging agents
for amyloid plaques.
Introduction
for SPECT has been highly anticipated. In an attempt
to develop 123I tracers for SPECT imaging, many radio-
iodinated probes based on various core structures have
been studied. Among the radioiodinated ligands evalu-
ated, 6-iodo-2-(4′-dimethylamino)phenylimidazo[1,2-a]-
pyridine (IMPY) (Figure 1) has been characterized as a
potential SPECT imaging agent for amyloid plaques.11,12
IMPY displayed selective amyloid plaque labeling in ex
vivo autoradiography using double transgenic mice
(PSAPP) as a model of AD.13 In addition, 2-(3′-iodo-4′-
aminophenyl)-6-hydroxybenzothiazole (6-OH-BTA-O-3′-
I) (Figure 1) also showed desirable in vitro and in vivo
properties.14 However, no SPECT imaging agents have
been evaluated in humans until now.
Recently, the effects of polyhydroxyflavones on the
formation, extension, and destabilization of Aâ ag-
gregates were studied in vitro.15 These flavones dose-
dependently inhibited the formation of Aâ aggregates,
as well as destabilizing preformed Aâ aggregates,
indicating that they could directly interact with Aâ
aggregates. The findings in this previous report prompted
us to apply flavones as a novel core structure of amyloid
imaging agents. Furthermore, some recent studies have
shown that electron-donating groups such as methyl-
amino, dimethylamino, methoxy, and hydroxy groups
play a critical role in the binding affinity to Aâ
aggregates.5,6,12,16-19 With these considerations in mind,
we designed four radioiodinated flavones with a radio-
iodine at the 6 position and an electron-donating group
at the 4′ position (Figure 2).
Alzheimer’s disease (AD) is a progressive neurode-
generative disorder characterized by abundant senile
plaques (SPs) composed of â-amyloid (Aâ) peptides and
numerous neurofibrillary tangles (NFTs) formed by
filaments of highly phosphorylated τ proteins in the
brain.1 Currently, the definitive confirmation of AD is
dependent on only post-mortem histopathological ex-
amination of SPs and/or NFTs in the brain. Therefore,
in vivo imaging of amyloid plaques in the living brain
may be useful for early detection of AD or monitoring
of the progression and effectiveness of novel treatments
that are currently being investigated.2-4
Extensive research has been undertaken to develop
radiolabeled amyloid-specific imaging agents by several
research groups (Figure 1). Several agents including
[11C]-4-N-methylamino-4′-hydroxystilbene ([11C]SB-13),5
[11C]-2-(4-(methylamino)phenyl)-6-hydroxybenzothiaz-
ole ([11C]-6-OH-BTA-1),6 and [18F]-2-(1-(2-(N-(2-fluoro-
ethyl)-N-methylamino)naphthalene-6-yl)ethylidene)ma-
lononitrile ([18F]FDDNP)7 have been reported for positron
emission tomography (PET) imaging of amyloid plaques
in AD patients. Encouraging results reported with these
agents in AD patients suggest that the development of
imaging agents for detecting amyloid plaques in the
living human brain is possible.8-10 It is well-known that
PET provides better functional information with higher
resolution and sensitivity than single photon emission
computed tomography (SPECT). However, since SPECT
imaging is more practical as a routine clinical diagnostic
procedure, the development of amyloid imaging agents
In the present study, we synthesized a series of
flavone derivatives and evaluated their usefulness as
in vivo amyloid imaging agents. To our knowledge, this
is the first time flavones have been proposed as amyloid
imaging agents for detecting AD.
* To whom correspondence should be addressed. Phone: 81-95-819-
2443. Fax: 81-95-819-2442. E-mail: mono@net.nagasaki-u.ac.jp.
† Nagasaki University.
‡ Osaka City University Medical School.
§ Chiba University.
10.1021/jm050635e CCC: $30.25 © 2005 American Chemical Society
Published on Web 10/19/2005

7254 Journal of Medicinal Chemistry, 2005, Vol. 48, No. 23
Ono et al.
Figure 1. Chemical structures of amyloid imaging agents previously reported.
Scheme 1
Results and Discussion
leads to generation of the desired flavone (4, 15). The
free amino derivative 5 was readily prepared from 4 by
reduction with SnCl2 (84% yield). Conversion of 5 to the
monomethylamino derivative 6 was achieved by a
method previously reported21 (39% yield). Compound 5
was also converted to the dimethylamino derivative 7
by an efficient method5,19 with paraformaldehyde, so-
dium cyanoborohydride, and acetic acid (86% yield).
Compound 15 was converted to 16 by demethylation
Chemistry. The most common method of synthesiz-
ing flavones is known as the Baker-Venkataraman
transformation.20 Schemes 1 and 2 illustrate an ap-
plication of the conventional Baker-Venkataraman
synthesis. In this process, a hydroxyacetophenone (1,
12) is first converted into a benzoyl ester (2, 13), and
this species is then treated with a base, forming a 1,3-
diketone (3, 14). Treatment of this diketone with acid

Flavone Derivatives as â-Amyloid Imaging Agents
Journal of Medicinal Chemistry, 2005, Vol. 48, No. 23 7255
125I]10, [125I]11, [125I]19, and [125I]20. Novel radioiodi-
[
nated flavones were achieved by an iododestannylation
reaction using hydrogen peroxide as the oxidant, which
produced the desired radioiodinated ligands (Scheme 3).
It was anticipated that the no-carrier-added preparation
would result in a final product bearing a theoretical
specific activity similar to that of 125I (2200 Ci/mmol).
The radiochemical identities of the radioiodinated ligands
were verified by co-injection with nonradioactive com-
pounds by their HPLC profiles. The final radioiodinated
compounds [125I]10, [125I]11, [125I]19, and [125I]20 showed
a single radioactivity peak at retention times of 9.3,
19.9, 13.7, and 4.2 min, respectively, identical to those
of nonradioactive compounds 10, 11, 19, and 20 (Table
1). Four radioiodinated products were obtained in 55-
70% radiochemical yields with radiochemical purities
of >95% after purification by HPLC.
Figure 2. Chemical structure of radioiodinated flavone
derivatives. Compounds reported here include the following:
R ) NHMe (10), NMe2 (11), OMe (19), OH (20).
with BBr3 in CH2Cl2 (13% yield). The tributyltin deriva-
tives (8, 9, 17, and 18) were prepared from the corre-
sponding bromo compounds (6, 7, 15, and 16) using a
bromo to tributyltin exchange reaction catalyzed by Pd-
(0) for yields of 37%, 17%, 69%, and 43%, respectively.
The tributyltin derivatives (8, 9, and 17) were readily
reacted with iodine in chloroform at room temperature
to give the iodo derivatives, compounds 10, 11, and 19
at yields of 29%, 47%, and 72%, respectively. Compound
20 was obtained in 44% yield from 19 by the same
reaction to prepare 16. Furthermore, these tributyltin
derivatives (8, 9, 17, and 18) can be also used as the
starting materials for radioiodination in preparation of
Binding Studies Using Aâ Aggregates in Solu-
tion. The binding studies of [125I]11 to aggregates of Aâ-
(1-40) and Aâ(1-42) were carried out. Transformation
of the saturation binding of [125I]11 to Scatchard plots
Scheme 2
Scheme 3

7256 Journal of Medicinal Chemistry, 2005, Vol. 48, No. 23
Table 1. HPLC Data and Partition Coefficients for [125I]10,
Ono et al.
Table 2. Inhibition Constants of Compounds on Ligand
[
125I]11, [125I]19, and [125I]20
compd
Binding to Aggregates of Aâ(1-40) and Aâ(1-42)
retention time (min)a
log PCb
Ki (nM)
[
125I]10
125I]11
125I]19
125I]20
9.3
19.9
13.7
4.2
2.15(0.01)
2.69(0.01)
2.41(0.03)
1.92(0.01)
compd
Aâ(1-40)
Aâ(1-42)
[
[
[
10
11
19
20
22.6 ( 3.4
13.2 ( 0.2
29.0 ( 3.2
72.5 ( 8.2
>1000
30.0 ( 3.4
15.6 ( 2.4
38.3 ( 8.1
77.2 ( 9.2
>1000
a Using a mixture of H2O/acetonitrile (2/3) as the mobile phase.
b Octanol/buffer (0.1 M phosphate buffered saline, pH 7.4) partition
coefficients. Each value represents the mean (SD in parentheses)
of three experiments.
thioflavin T
Congo Red
>1000
>1000
> 10 > 19 > 20. No marked difference between Aâ(1-
40) and Aâ(1-42) aggregates was observed in their Ki
values. It is especially noteworthy that compounds 10,
11, 19, and 20 could bind not only Aâ(1-40) aggregates
but also Aâ(1-42) aggregates, as we aim to develop
novel probes that can detect diffuse plaques mainly
composed of Aâ(1-42). More interestingly, when thiofla-
vin T and Congo Red were evaluated for their competi-
tion against [125I]11 binding on Aâ(1-40) and Aâ(1-
42) aggregates, high Ki values (> 1000 nM) were
observed (Table 2), indicating poor binding competition.
This finding suggests that these flavones may have a
binding site on Aâ aggregates different from that of
thioflavin T and Congo Red, although additional studies
regarding the selectivity of binding affinity for Aâ
aggregates are required.
Neuropathological Staining on AD Brain Sec-
tions. Since the in vitro binding assays demonstrated
the high binding affinity of the flavone derivatives for
Aâ(1-40) and Aâ(1-42) aggregates (Table 2), com-
pounds 10, 11, 19, and 20 were investigated for their
neuropathological staining of amyloid plaques and NFTs
in human AD brain sections (Figure 4). Compounds 10,
11, 19, and 20 intensely stained amyloid plaques (Figure
4A,E,I,M), neuritic plaques (Figure 4B,F,J,N), and cere-
brovascular amyloids (Figure 4C,G,K,O) with nearly the
same staining pattern. However, as seen in Figure
4A,E,I,M, these flavone compounds did not intensely
stain the core region in so-called classical amyloid
plaques, unlike the thioflavin T and Congo Red deriva-
tives previously reported as amyloid imaging probes,
indicating that flavone derivatives may have somewhat
distinct binding characteristics for amyloid fibrils. The
binding characteristics of the flavone derivatives for
amyloid plaques in human AD sections may be relevant
to the phenomenon observed in the in vitro binding
assay; they may occupy a binding site on Aâ aggregates
different from that of thioflavin T and Congo Red. Such
a binding characteristic of the present flavone deriva-
tives is thought to be related to the recent finding that
a similar compound with a flavone backbone structure
has a binding affinity with amyloid fibrils and the APP
sequence.22 However, this issue regarding the binding
property of the flavone derivatives remains unresolved.
These flavone derivatives appeared to stain not only
neuritic amyloid plaques but also diffuse amyloid plaques,
which are known to be mainly composed of Aâ(1-42)23
and to be the initial pathological change of AD.24 Thus,
flavone derivatives with high binding affinity for Aâ-
(1-42)-positive diffuse plaques may be more useful for
presymptomatic detection of AD pathology. Further-
more, compounds 10, 11, 19, and 20 also showed high
binding affinity for NFTs in the AD sections (Figure
4D,H,L,P). A previous study reported that a remarkable
Figure 3. Scatchard plots of [125I]11 binding to aggregates of
Aâ(1-40) and Aâ(1-42). [125I]11 displayed one-site binding.
High-affinity binding with Kd values in a nanomolar range was
obtained (Kd ) 12.3 and 17.6 nM for Aâ(1-40) and Aâ(1-42)
aggregates, respectively).
gave linear plots, suggesting one binding site. [125I]11
showed excellent binding affinity for both Aâ(1-40) (Kd
) 12.4 ( 2.3 nM) and Aâ(1-42) (Kd ) 17.4 ( 5.7 nM)
aggregates (Figure 3). Binding affinities of nonradioac-
tive flavone derivatives (compound 10, 11, 19, and 20)
were evaluated with inhibition studies against [125I]11
binding on Aâ(1-40) and Aâ(1-42) aggregates. As
shown in Table 2, all flavone derivatives competed well
with [125I]11 binding on Aâ(1-40) and Aâ(1-42) ag-
gregates. The Ki values estimated for 10, 11, 19, and
20 were 23, 13, 29, and 73 nM for Aâ(1-40) aggregates
and 30, 16, 38, and 77 nM for Aâ(1-42) aggregates,
respectively. These Ki values suggested that the new
series of flavones had high binding affinity for Aâ(1-
40) and Aâ(1-42) aggregates in the following order: 11

Flavone Derivatives as â-Amyloid Imaging Agents
Journal of Medicinal Chemistry, 2005, Vol. 48, No. 23 7257
Figure 4. Neuropathological staining of compounds 10 (A-D), 11 (E-H), 19 (I-L), and 20 (M-P) on 5 µm AD brain sections
from the temporal cortex. Amyloid plaques (A, E, I, M) are clearly stained with compounds 10, 11, 19, and 20 (40× magnification).
Clear staining of neuritic plaques (B, F, J, N) and cerebrovascular amyloid (C, G, K, O) was also obtained. Many NFTs (D, H, L,
P) are intensely stained with compounds 10, 11, 19, and 20 (40× magnification).
accumulation of NFTs in the hippocampus and entorhi-
nal cortex has been observed at an early stage of AD.25
Since flavone derivatives can strongly bind NFTs, they
would detect an increase of NFT accumulation in the
hippocampus and entorhinal cortex of the AD brain.
These findings suggested that the flavone derivatives
10, 11, 19, and 20 can bind amyloid fibrils and NFTs
without the backbone structure of thioflavin T or Congo
Red and that quantitative evaluation of their cerebral
localization may provide useful information on Aâ and
τ pathology.
Biodistribution Studies. Four radioiodinated fla-
vone ligands ([125I]10, [125I]11, [125I]19, and [125I]20) were
evaluated for their in vivo biodistribution in normal
mice. A biodistribution study provides critical informa-
tion on brain penetration. Generally, a freely diffusible
compound with an optimal log PC value of 2-3 will have
an initial brain uptake of 2-3% dose/whole brain at 2
min after iv injection.26 All four ligands examined in this
study displayed optimal lipophilicities as reflected by
their log PC values of 1.94, 2.69, 2.41, and 1.92 for [125I]-
10, [125I]11, [125I]19, and [125I]20, respectively (Table 1).
As expected, these ligands displayed high brain uptakes
ranging from 3.2 to 4.1% ID/g brain at 2 min postinjec-
tion, indicating a level sufficient for brain imaging
probes (Table 3). In addition, they displayed good
clearance from the normal brain with 1.2, 1.0, 0.17, and
0.08% ID/g at 60 min postinjection for [125I]10, [125I]11,
[
125I]19, and [125I]20, respectively (Table 3). These values
were equal to 29%, 27%, 4.3%, and 2.4% of initial brain
uptake peak for [125I]10, [125I]11, [125I]19, and [125I]20,
respectively. Since the normal brain has no Aâ plaques
to trap the agent, the washout from the normal brain
should be fast. These desirable pharmacokinetics dem-
onstrated by radioiodinated flanones are critical to
obtain a higher signal-to-noise ratio earlier in the AD
brain. Radioiodinated amyloid imaging agents such as
[ [ [
125I]TZDM,16 125I]IBOX,17 125I]m-I-stilbene,18 and [125I]-
benzofuran19 reported previously showed high brain
uptakes, but the washout rates from the normal brain
were relatively slow. The slow washout rate from the
brain leads to a high background activity and prevents
the visualization of amyloid plaques in the AD brain.
Currently, few reports describing radioiodinated amy-
loid agents with both high initial brain uptake and rapid
washout from the normal brain have been available
except for IMPY11-13 and 6-OH-BTA-O-3′-I.14 The ap-
propriate in vivo properties observed for radioiodinated
flavones in the present study (higher brain uptake and
faster washout from the normal brain) make them
useful candidates as SPECT tracers for amyloid imag-
ing.

7258 Journal of Medicinal Chemistry, 2005, Vol. 48, No. 23
Ono et al.
Table 3. Biodistribution of Radioactivity after Intravenous
Administration of [125I]10, [125I]11, [125I]19, and [125I]20 in Mice
s (singlet), d (doublet), t (triplet), br (broad), and m (multiplet).
Mass spectra were obtained on a JEOL IMS-DX instrument.
Chemistry. 4-Nitrobenzoic Acid 2-Acetyl-4-bromophe-
nyl Ester (2). To a stirring solution of 4-nitrobenzoyl chloride
(1.00 g, 4.65 mmol) in pyridine (20 mL) in an ice bath was
added 5′-bromo-2′-hydroxyacetophenone (863 mg, 4.65 mmol).
The reaction mixture was left in the ice bath for 5 min, stirred
at room temperature for 30 min, and poured into 1 N aqueous
HCl/ice solution with vigorous stirring. The precipitate that
formed was filtered and washed with water to yield acetophe-
biodistribution of radioactivitya
for time after injection
tissue
2 min
10 min
125I]10
1.39(0.10)
30 min
60 min
[
blood
liver
1.89(0.28)
1.34(0.07)
1.50(0.09)
16.28(0.90) 25.28(0.31) 18.61(1.81) 15.14(0.89)
kidney
intestine
spleen
heart
8.13(1.28)
3.10(0.61)
2.57(1.54)
4.87(0.66)
0.78(0.02)
4.12(0.15)
5.21(0.44)
7.91(1.05) 12.84(1.18) 21.48(3.17)
2.31(0.01)
2.66(0.12)
0.87(0.22)
3.68(0.18)
3.85(0.33)
3.05(0.25)
1
none (1.57 g, 92.7% yield). H NMR (300 MHz, CDCl3) δ 2.55
1.76(0.23)
1.67(0.14)
1.44(0.69)
1.84(0.12)
1.52(0.29)
1.28(0.12)
1.80(0.84)
1.19(0.04)
(s, 3H), 7.16 (d, J ) 8.7 Hz, 1H), 7.76-7.72 (m, 1H), 8.00 (s,
1H), 8.37 (s, 4H). MS m/z 365 (MH+).
stomachb
brain
1-(5-Bromo-2-hydroxyphenyl)-3-(4-nitrophenyl)pro-
pane-1,3-dione (3). A solution of acetophenone 2 (2.31 g, 6.34
mmol) and pyridine (40 mL) was heated to 50 °C, and to it
was added pulverized potassium hydroxide (530 mg, 9.45
mmol). The reaction mixture was stirred for 15 min, and when
it was cooled, 20 mL of 10% aqueous acetic acid solution was
added. The pale-yellow precipitate that formed was filtered
[
125I]11
1.07(0.08)
blood
1.87(0.18)
1.20(0.15)
1.15(0.16)
liver
15.41(0.98) 21.85(2.14) 15.71(0.96) 12.40(2.38)
kidney
intestine
spleen
heart
8.33(1.47)
2.24(0.24)
2.72(0.20)
5.63(0.80)
0.73(0.17)
3.22(0.15)
4.31(0.28)
3.40(0.31)
2.32(0.45)
6.56(0.83) 12.97(1.15) 18.64(2.05)
1.92(0.33)
2.47(0.14)
0.63(0.16)
3.61(0.60)
1.58(0.31)
1.69(0.06)
1.17(0.40)
1.89(0.21)
1.18(0.17)
1.07(0.17)
1.06(0.27)
0.99(0.10)
1
to yield 3 (2.01 g, 87.1% yield). H NMR (300 MHz, CDCl3) δ
6.83 (s, 2H), 6.95 (d, J ) 9.0 Hz, 1H), 7.57 (d, J ) 9.0 Hz, 1H),
7.88 (s, 1H), 8.13 (d, J ) 9.0 Hz, 2H), 8.36 (d, J ) 9.0 Hz, 2H),
11.85 (s, 1H). MS m/z 365 (MH+).
stomachb
brain
[
125I]19
6-Bromo-4′-nitroflavone (4). A mixture of the diketone 3
(2.00 g, 5.49 mmol), concentrated sulfuric acid (0.5 mL), and
glacial acetic acid (40 mL) was heated at reflux for 1 h and
cooled to room temperature. The mixture was poured onto
crushed ice, and the resulting precipitate was filtered to yield
4 (1.79 g, 94.2% yield). MS m/z 317 (MH+).
6-Bromo-4′-aminoflavone (5). A mixture of 4 (100 mg,
0.29 mmol), SnCl2 (275 mg, 1.45 mmol), and EtOH (7 mL) was
stirred under reflux for 1 h. After the mixture was cooled to
room temperature, 1 M NaOH (50 mL) was added until the
mixture became alkaline. After extraction with ethyl acetate
(25 mL × 2), the combined organic layers were washed with
brine, dried over Na2SO4, and evaporated to give 77 mg of 5
(84.3%). 1H NMR (300 MHz, CDCl3) δ 4.15 (s, 2H), 6.70 (s,
1H), 6.72 (d, J ) 8.7 Hz, 2H), 7.44 (d, J ) 9.0 Hz, 1H), 7.72-
7.76 (m, 3H). MS m/z 317 (MH+).
blood
1.87(0.21)
8.96(1.48)
7.99(1.08)
1.19(0.17)
9.01(0.97)
6.30(1.02)
0.40(0.01)
3.75(0.47)
4.51(1.59)
0.23(0.09)
1.88(0.61)
1.46(1.12)
liver
kidney
intestine
spleen
heart
3.52(0.29) 14.39(0.80) 22.51(1.11) 30.05(3.61)
2.70(0.08)
4.98(0.41)
0.68(0.06)
4.00(0.18)
1.38(0.37)
2.25(0.40)
0.45(0.18)
2.36(0.33)
0.55(0.30)
0.84(0.14)
0.55(0.33)
0.51(0.07)
3.67(5.89)
0.47(0.22)
0.31(0.07)
0.17(0.05)
stomachb
brain
[
125I]20
blood
2.77(0.43)
9.77(1.89)
1.58(0.18)
8.24(0.50)
0.66(0.03)
6.80(0.86)
6.45(0.84)
0.20(0.02)
4.78(1.09)
1.66(0.62)
liver
kidney
intestine
spleen
heart
14.79(2.59) 15.11(2.00)
3.12(0.37) 11.26(0.63) 22.01(1.34) 27.28(0.48)
3.92(1.18)
5.51(0.71)
0.89(0.09)
3.31(0.32)
1.55(0.15)
1.60(0.18)
0.59(0.16)
1.90(0.07)
0.56(0.13)
0.53(0.04)
1.56(0.50)
0.52(0.03)
0.17(0.06)
0.12(0.02)
0.81(0.36)
0.08(0.02)
stomachb
brain
6-Bromo-4′-methylaminoflavone (6). To a mixture of 5
(300 mg, 0.95 mmol) and paraformaldehyde (154 mg, 5.13
mmol) in MeOH (15 mL) was added a solution of NaOMe (28
wt % in MeOH, 0.27 mL) dropwise at 0 °C. The mixture was
stirred under reflux for 1 h. After addition of NaBH4 (180 mg,
4.75 mmol), the solution was heated under reflux for 2 h. To
the cold mixture 1 M NaOH (30 mL) was added followed by
extraction with CH2Cl2 (30 mL × 2). The organic phase was
dried over Na2SO4 and filtered. The solvent was removed, and
the residue was purified by silica gel chromatography (3:5
hexane/ethyl acetate) to give 121 mg of 6 (38.6%). 1H NMR
(300 MHz, CDCl3) δ 2.91 (s, 3H), 4.18 (s, 1H), 6.62-6.66 (m,
3H), 7.39 (d, J ) 8.7 Hz, 1H), 7.69-7.75 (m, 3H), 8.31 (s, 1H).
6-Bromo-4′-dimethylaminoflavone (7). To a stirred mix-
ture of 5 (75 mg, 0.24 mmol) and paraformaldehyde (71 mg,
2.37 mmol) in AcOH (10 mL) was added NaCNBH3 (74.5 mg,
1.19 mmol) in one portion at room temperature. The resulting
mixture was stirred at room temperature for 3 h, and 1 M
NaOH (50 mL) was added followed by extraction with CH3Cl
(25 × 2 mL). The organic phase was dried over Na2SO4. The
solvent was removed, and the residue was purified by silica
gel chromatography (4:1 hexane/ethyl acetate) to give 70 mg
of 7 (85.7%). 1H NMR (300 MHz, CDCl3) δ 3.08 (s, 6H), 6.70-
6.77 (m, 3H), 7.43 (d, J ) 9.0 Hz, 1H), 7.72-7.82 (m, 3H), 8.34
(s, 1H). MS m/z 345 (M+).
a Expressed as % injected dose per gram. Each value represents
the mean (SD in parentheses) for three to five animals at each
interval. b Expressed as % injected dose per organ.
Conclusions
In conclusion, we successfully designed and synthe-
sized novel radioiodinated flavones as amyloid imaging
agents with high in vitro binding affinity for Aâ ag-
gregates. These flavones bound not only amyloid plaques
but also NFTs in human AD sections in vitro. In
addition, they displayed excellent brain uptake and
rapid washout from the brain after injection in normal
mice. The combination of high binding affinity for
amyloid plaques, high brain uptake, and good clearance
in mice of flavone derivatives 10, 11, 19, and 20 provides
a series of promising in vivo amyloid imaging agents
for SPECT. A more detailed biodistribution study using
transgenic mice containing an excess amount of Aâ
aggregate deposition in the brain is currently underway.
Moreover, additional structural changes on the flavone
backbone may lead to more useful amyloid imaging
agents for SPECT and PET.
6-(Tributylstannyl)-4′-methylaminoflavone (8). A mix-
ture of 7 (286 mg, 0.87 mmol), bis(tributyltin) (0.55 mL), and
(Ph3P)Pd (41 mg, 0.035 mmol) in a mixed solvent (28 mL, 3:1
dioxane/triethylamine mixture) was stirred at 90 °C for 6 h.
The solvent was removed, and the residue was purified by
silica gel chromatography (4:1 hexane/ethyl acetate) to give
Experimental Sections
All reagents used in syntheses were commercial products
and were used without further purification unless otherwise
indicated. 1H NMR spectra were obtained on a Varian Gemini
300 spectrometer with TMS as an internal standard. Coupling
constants are reported in hertz. The multiplicity is defined by
1
174 mg of 8 (37.2%). H NMR (300 MHz, CDCl3) δ 0.86-1.35
(m, 27H), 2.92 (s, 3H), 4.14 (s, 1H), 7.46 (d, J ) 9.0 Hz, 1H),
7.65-7.78 (m, 3H), 8.30 (s, 1H). MS m/z 541 (MH+).

Flavone Derivatives as â-Amyloid Imaging Agents
Journal of Medicinal Chemistry, 2005, Vol. 48, No. 23 7259
6-(Tributylstannyl)-4′-dimethylaminoflavone (9). The
same reaction as described above to prepare 8 was employed,
and 54 mg of 9 was obtained in a 16.8% yield from 7. 1H NMR
(300 MHz, CDCl3) δ 0.86-1.56 (m, 27H), 6.72-6.77 (m, 3H),
7.49 (d, J ) 9.0 Hz, 1H), 7.71-7.75 (m, 1H), 7.82 (d, J ) 9.0
Hz, 2H), 8.30 (s, 1H). MS m/z 555 (MH+).
6-Iodo-4′-methyaminoflavone (10). To a solution of 8 (100
mg, 0.19 mmol) in CHCl3 (20 mL) was added a solution of
iodine in CHCl3 (1.5 mL, 1 M) at room temperature. The
mixture was stirred at room temperature for 10 min, and
NaHSO3 solution (15 mL) was added. The mixture was stirred
for 5 min, and the organic phase was separated. The aqueous
phase was extracted with CH3Cl, and the combined organic
phase was dried over Na2SO4 and filtered. The solvent was
removed, and the residue was purified by silica gel chroma-
tography (2:1 hexane/ethyl acetate) to give 20 mg of 10 (28.7%).
1H NMR (300 MHz, CDCl3) δ 2.98 (s, 3H), 4.15 (s, 1H), 6.64-
6.69 (m, 3H), 7.28 (d, J ) 10.8 Hz, 1H), 7.75 (d, J ) 8.4 Hz,
2H), 7.90 (d, J ) 9.0 Hz, 1H), 8.53 (s, 1H). MS m/z 377 (MH+).
Anal. (C16H12INO2) C, H, N.
7.32 (d, J ) 8.7 Hz, 1H), 7.86-7.93 (m, 3H), 8.55 (s, 1H). MS
m/z 378 (M+). Anal. (C16H11IO3) C, H.
6-Iodo-4′-hydroxyflavone (20). The same reaction as
described above to prepare 16 was used, and 79 mg of 20 was
obtained in a 44.3% yield from 19. 1H NMR (300 MHz, CDCl3)
δ 6.93-6.95 (m, 3H), 7.61 (d, J ) 8.7 Hz, 1H), 7.98 (d, J ) 8.4
Hz, 2H), 8.28-8.30 (m, 1H), 8.27 (s, 1H), 10.37 (s, 1H). MS
m/z 364 (M+). Anal. (C15H9IO3) C, H.
Iododestannylation Reaction. The radioiodinated forms
of compounds 10, 11, 19, and 20 were prepared from the
corresponding tributyltin derivatives by an iododestannylation.
Briefly, to initiate the reaction, 50 µL of H2O2 (3%) was added
to a mixture of a tributyltin derivative (100 µg/50 µL EtOH),
1.5 mCi sodium [125I]iodide (specific activity 2200 Ci/mmol),
and 100 µL of 1 N HCl in a sealed vial. The reaction was
allowed to proceed at room temperature for 10 min and
terminated by addition of NaHSO3. The reaction, after neu-
tralization with sodium bicarbonate, was extracted with ethyl
acetate. The extract was dried by passing through an anhy-
drous Na2SO4 column and was then blown to dryness with a
stream of nitrogen gas. The radioiodinated ligand was purified
by HPLC on a Cosmosil C18 column with an isocratic solvent
of H2O/acetonitrile (2/3) at a flow rate of 1.0 mL/min. The
purified ligand was stored at -20 °C for in vitro binding and
biodistribution studies.
6-Iodo-4′-dimethyaminoflavone (11). The same reaction
as described above to prepare 10 was used, and 10 mg of 11
was obtained in a 47.3% yield from 9. 1H NMR (300 MHz,
CDCl3) δ 3.09 (s, 6H), 6.71 (d, J ) 8.6 Hz, 2H), 6.81 (d, J )
16.3 Hz, 1H), 7.04 (d, J ) 16.3 Hz, 1H), 7.21 (d, J ) 8.2 Hz,
1H), 7.38 (d, J ) 8.2 Hz, 2H), 7.63 (d, J ) 8.2 Hz, 2H). MS m/z
351 (MH+). Anal. (C17H14INO2) C, H, N.
Binding Assays Using the Aggregated Aâ Peptide in
Solution. A solid form of Aâ(1-40) and Aâ(1-42) was
purchased from Peptide Institute (Osaka, Japan). Aggregation
of peptides was carried out by gently dissolving the peptide
(0.5 mg/mL) in a buffer solution (pH 7.4) containing 10 mM
sodium phosphate and 1 mM EDTA. The solutions were
incubated at 37 °C for 36-42 h with gentle and constant
shaking. Binding studies were carried out in 12 mm × 75 mm
borosilicate glass tubes according to the procedure described
before16 with some modification. For saturation studies, a
solution of [125I]11 (final concentration, 0.78-100 nM) was
prepared by mixing nonradioactive 11. Nonspecific binding was
defined in the presence of 100 nM nonradioactive 11. For
inhibition studies, 1 mL of the reaction mixture contained 50
µL of inhibitors (10-5-10-10 M in 10% EtOH) and 0.05 nM of
radiotracer in 10% EtOH. The binding assay was performed
by mixing 50 µL of Aâ(1-40) or Aâ(1-42) aggregates (57 nM
in the final assay mixture), an appropriate concentration of
50 µL of [125I]11, and 900 µL of 10% ethanol. After incubation
for 3 h at room temperature, the binding mixture was filtered
through GF/B filters (Whatman, Kent, U.K.) using an M-24
cell harvester (Brandel, Gaithersburg, MD). Filters containing
the bound 125I ligand were counted in a γ camera counter. The
dissociation constant (Kd) of compound 11 was determined by
Scatchard analysis using GraphPad Prism (GraphPad Soft-
ware, San Diego, CA). For inhibition studies, a mixture
containing 50 µL of test compounds (8 pM to 12.5 µM in 10%
ethanol), 50 µL of 0.02 nM [125I]11, 50 µL of Aâ(1-40) or Aâ-
(1-42) aggregates, and 850 µL of 10% ethanol was incubated
at room temperature for 3 h. The mixture was then filtered
through Whatman GF/B filters using a Brandel M-24 cell
harvester, and the filters containing the bound 125I ligand were
counted in a γ counter. Values for the half-maximal inhibitory
concentration (IC50) were determined from displacement curves
of three independent experiments using GraphPad Prism, and
those for the inhibition constant (Ki) were calculated using the
Cheng-Prusoff equation:27 Ki ) IC50/(1 + [L]/Kd), where [L] is
the concentration of [125I]11 used in the assay, and Kd is the
dissociation constant of compound 11.
4-Methoxybenzoic Acid 2-Acetyl-4-bromophenyl Ester
(13). The same reaction as described above to prepare 2 was
used, and 3.15 g of 13 was obtained in a 96.5% yield from 12.
1H NMR (300 MHz, CDCl3) δ 7.72 (d, J ) 7.5 Hz, 1H), 8.06 (d,
J ) 7.2 Hz, 1H), 8.86 (s, 1H), 10.13 (s, 1H).
1-(5-Bromo-2-hydroxyphenyl)-3-(4-methoxyphenyl)-
propane-1,3-dione (14). The same reaction as described
above to prepare 3 was used, and 1.42 g of 14 was obtained in
an 85.2% yield from 13. 1H NMR (300 MHz, CDCl3) δ 3.90 (s,
3H), 6.68 (s, 2H), 6.90 (d, J ) 8.7 Hz, 1H), 7.00 (d, J ) 9.3 Hz,
2H), 7.53 (d, J ) 9.0 Hz, 1H), 7.83 (s, 1H), 7.94 (d, J ) 9.3 Hz,
2H), 12.01 (s, 1H).
6-Bromo-4′-methoxyflavone (15). The same reaction as
described above to prepare 4 was used, and 2.01 g of 15 was
obtained in a 79.4% yield from 14. 1H NMR (300 MHz, CDCl3)
δ 3.90 (s, 3H), 6.75 (s, 1H), 7.03 (d, J ) 9.0 Hz, 2H), 7.45 (d, J
) 9.3 Hz, 1H), 7.77 (d, J ) 9.0 Hz, 1H), 7.87 (d, J ) 9.0 Hz,
2H), 8.35 (s, 1H).
6-Bromo-4′-hydroxyflavone (16). To a solution of 15 (400
mg, 1.21 mmol) in CH2Cl2 (215 mL) at 0 °C was added BBr3
(12 mL, 1 M solution in CH2Cl2) dropwise in an ice bath. The
mixture was allowed to warm to room temperature and was
stirred for 6 h. Water (100 mL) was added while the reaction
mixture was cooled in an ice bath to keep the reaction
temperature at 0 °C. After extraction with CH2Cl2, the
combined organic phase was dried over Na2SO4. The filtrate
was concentrated and the residue was chromatographed on a
silica gel column (eluted with 2:5 ethyl acetate/ hexane) to give
1
50 mg of 16 (13.1%). H NMR (300 MHz, CDCl3) δ 6.93-6.96
(m, 3H), 7.76 (d, J ) 9.0 Hz, 1H), 7.97-8.00 (m, 3H), 8.09 (s,
1H), 10.38 (s, 1H).
6-(Tributylstannyl)-4′-methoxyflavone (17). The same
reaction as described above to prepare 8 was used, and 562
mg of 9 was obtained in a 68.8% yield from 15. 1H NMR (300
MHz, CDCl3) δ 0.86-1.57 (m, 27H), 3.90 (s, 3H), 6.77 (s, 1H),
7.02-7.05 (m, 2H), 7.56 (m, 1H), 7.68-7.75 (m, 1H), 7.88-
7.92 (m, 2H), 8.31 (s, 1H). MS m/z 542 (MH+).
6-(Tributylstannyl)-4′-hydroxyflavone (18). The same
reaction as described above to prepare 8 was used, and 36 mg
of 18 was obtained in a 43.3% yield from 16. 1H NMR (300
MHz, CDCl3) δ 0.86-1.59 (m, 27H), 6.31 (s, 1H), 6.77 (s, 1H),
7.00 (d, J ) 8.4 Hz, 2H), 7.50-7.55 (m, 1H), 7.85 (d, J ) 8.4
Hz, 2H), 8.31 (s, 1H). MS m/z 527 (M+).
Partition Coefficient Determination. Partition coef-
ficients were measured by mixing the radioiodinated tracers
with 3 mL each of 1-octanol and buffer (0.1 M phosphate, pH
7.4) in a test tube. The test tube was vortexed for 3 min at
room temperature, followed by centrifugation for 5 min. Two
weighed samples (0.5 mL each) from the 1-octanol and buffer
layers were counted in a well counter. The partition coefficient
was determined by calculating the ratio of cpm/0.5 mL of the
1-octanol to that of buffer. A sample from the octanol layer
was repartitioned with the same volume of buffer until
6-Iodo-4′-methoxyflavone (19). The same reaction as
described above to prepare 10 was used, and 227 mg of 19
1
was obtained in a 72.2% yield from 17. H NMR (300 MHz,
CDCl3) δ 3.90 (s, 3H), 6.75 (s, 1H), 7.03 (d, J ) 8.7 Hz, 2H),

7260 Journal of Medicinal Chemistry, 2005, Vol. 48, No. 23
Ono et al.
(9) Klunk, W. E.; Engler, H.; Nordberg, A.; Wang, Y.; Blomqvist,
G.; et al. Imaging brain amyloid in Alzheimer’s disease with
Pittsburgh compound-B. Ann. Neurol. 2004, 55, 306-319.
(10) Shoghi-Jadid, K.; Small, G. W.; Agdeppa, E. D.; Kepe, V.; Ercoli,
L. M.; et al. Localization of neurofibrillary tangles and beta-
amyloid plaques in the brains of living patients with Alzheimer
disease. Am. J. Geriatr. Psychiatry 2002, 10, 24-35.
(11) Kung, M. P.; Hou, C.; Zhuang, Z. P.; Zhang, B.; Skovronsky, D.;
et al. IMPY: an improved thioflavin-T derivative for in vivo
labeling of beta-amyloid plaques. Brain Res. 2002, 956, 202-
210.
(12) Zhuang, Z. P.; Kung, M. P.; Wilson, A.; Lee, C. W.; Plossl, K.; et
al. Structure-activity relationship of imidazo[1,2-a]pyridines as
ligands for detecting â-amyloid plaques in the brain. J. Med.
Chem. 2003, 46, 237-243.
(13) Kung, M. P.; Hou, C.; Zhuang, Z. P.; Cross, A. J.; Maier, D. L.;
et al. Characterization of IMPY as a potential imaging agent
for beta-amyloid plaques in double transgenic PSAPP mice. Eur.
J. Nucl. Med. Mol. Imaging 2004, 31, 1136-1145.
(14) Wang, Y.; Klunk, W. E.; Huang, G. F.; Debnath, M. L.; Holt, D.
P.; et al. Synthesis and evaluation of 2-(3′-iodo-4′-aminophenyl)-
6-hydroxybenzothiazole for in vivo quantitation of amyloid
deposits in Alzheimer’s disease. J. Mol. Neurosci. 2002, 19, 11-
16.
(15) Ono, K.; Yoshiike, Y.; Takashima, A.; Hasegawa, K.; Naiki, H.;
et al. Potent anti-amyloidogenic and fibril-destabilizing effects
of polyphenols in vitro: implications for the prevention and
therapeutics of Alzheimer’s disease. J. Neurochem. 2003, 87,
172-181.
consistent partitions coefficient values were obtained. The
measurement was done in triplicate and repeated three times.
Staining of Senile Plaques in Human AD Brain Sec-
tions. Post-mortem brain tissues from AD cases (74-79 years
old) were confirmed by conventional neuropathology, including
amyloid and τ staining in addition to silver staining. Experi-
ments were performed under the regulations of the ethics
committee of Osaka City University Medical School. Five-
micrometer-thick serial sections of paraffin-embedded blocks
from the temporal cortex were used for staining. Paraffin
sections were first taken through five 10-min incubations in
xylene and five 10-min incubations in 100% EtOH to com-
pletely deparaffinize them, followed by two 5-min washes in
water and then PBS. Tissue sections were immersed in the
compound solution at various concentrations. At greater than
0.5 mM concentration, we used 50% EtOH in PBS to ensure
the full solubilization of the compound. Finally, the sections
were washed in PBS for 15 min. Thereafter, the sections were
incubated in EtOH and xylene and embedded in Entellan Neu
(Merck, Darmstadt, Germany). Fluorescent sections were
viewed using a BX50 fluoromicroscope with a M-3204C CCD
camera (Olympus, Tokyo) equipped with a G-filter/rhodamine.
The sections were also immunostained with DAB as a chro-
mogen using monoclonal antibodies against Aâ28 and tau29 as
previously described.
In Vivo Biodistribution in Normal Mice. Animal studies
were conducted in accordance with our institutional guidelines
and were approved by Nagasaki University Animal Care
Committee. A saline solution (100 µL) containing radiolabeled
agents (2 µCi) was injected directly into the tail vein of ddY
mice (5 weeks old, average weight of 25-30 g). The mice were
sacrificed at various time points postinjection. The organs of
interest were removed and weighed, and the radioactivity was
counted with an automatic γ counter (Aloka 3000).
(16) Zhuang, Z. P.; Kung, M. P.; Hou, C.; Skovronsky, D. M.; Gur, T.
L.; et al. Radioiodinated styrylbenzenes and thioflavins as probes
for amyloid aggregates. J. Med. Chem. 2001, 44, 1905-1914.
(17) Zhuang, Z. P.; Kung, M. P.; Hou, C.; Plossl, K.; Skovronsky, D.;
et al. IBOX(2-(4′-dimethylaminophenyl)-6-iodobenzoxazole):
a
ligand for imaging amyloid plaques in the brain. Nucl. Med. Biol.
2001, 28, 887-894.
(18) Kung, H. F.; Lee, C. W.; Zhuang, Z. P.; Kung, M. P.; Hou, C.; et
al. Novel stilbenes as probes for amyloid plaques. J Am Chem
Soc 2001, 123, 12740-12741.
(19) Ono, M.; Kung, M. P.; Hou, C.; Kung, H. F. Benzofuran
derivatives as Abeta-aggregate-specific imaging agents for Alzhe-
imer’s disease. Nucl. Med. Biol. 2002, 29, 633-642.
(20) Ares, J. J.; Outt, P. E.; Randall, J. L.; Murray, P. D.; Weisshaar,
P. S.; et al. Synthesis and biological evaluation of substituted
flavones as gastroprotective agents. J. Med. Chem. 1995, 38,
4937-4943.
(21) Barluenga, A. M.; Bayron, G.; Asensio, A. A new and specific
method for the monomethylation of primary amine. J. Chem.
Soc., Chem. Commun. 1984, 1334-1335.
(22) Espeseth, A. S.; Xu, M.; Huang, Q.; Coburn, C. A.; Jones, K. L.;
et al. Compounds that bind APP and inhibit Abeta processing
in vitro suggest a novel approach to Alzheimer disease thera-
peutics. J. Biol. Chem. 2005, 280, 17792-17797.
(23) Iwatsubo, T.; Odaka, A.; Suzuki, N.; Mizusawa, H.; Nukina, N.;
et al. Visualization of A beta 42(43) and A beta 40 in senile
plaques with end-specific A beta monoclonals: evidence that an
initially deposited species is A beta 42(43). Neuron 1994, 13, 45-
53.
(24) Morris, J. C.; Storandt, M.; McKeel, D. W., Jr.; Rubin, E. H.;
Price, J. L.; et al. Cerebral amyloid deposition and diffuse
plaques in “normal” aging: Evidence for presymptomatic and
very mild Alzheimer’s disease. Neurology 1996, 46, 707-719.
(25) Price, J. L.; Morris, J. C. Tangles and plaques in nondemented
aging and “preclinical” Alzheimer’s disease. Ann. Neurol. 1999,
45, 358-368.
(26) Dishino, D. D.; Welch, M. J.; Kilbourn, M. R.; Raichle, M. E.
Relationship between lipophilicity and brain extraction of C-11-
labeled radiopharmaceuticals. J. Nucl. Med. 1983, 24, 1030-
1038.
Acknowledgment. The authors thank Drs. Mei-
Ping Kung and Hank F. Kung (University of Pennsyl-
vania) for helpful discussions. This work was supported
in part by a Grant-in-Aid for Young Scientists (B) (Grant
No. 16790734) and Scientific Research (B) (Grant No.
15390014) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
Note Added after ASAP Publication. This manu-
script was released ASAP on October 19, 2005, with
errors in the caption for Figure 4. The correct version
was posted on October 27, 2005.
References
(1) Selkoe, D. J. Alzheimer’s disease: genes, proteins, and therapy.
Physiol. Rev. 2001, 81, 741-766.
(2) Selkoe, D. J. Imaging Alzheimer’s amyloid. Nat. Biotechnol.
2000, 18, 823-824.
(3) Mathis, C. A.; Wang, Y.; Klunk, W. E. Imaging beta-amyloid
plaques and neurofibrillary tangles in the aging human brain.
Curr. Pharm. Des. 2004, 10, 1469-1492.
(4) Nordberg, A. PET imaging of amyloid in Alzheimer’s disease.
Lancet Neurol. 2004, 3, 519-527.
(5) Ono, M.; Wilson, A.; Nobrega, J.; Westaway, D.; Verhoeff, P.; et
al. 11C-labeled stilbene derivatives as Abeta-aggregate-specific
PET imaging agents for Alzheimer’s disease. Nucl. Med. Biol.
2003, 30, 565-571.
(27) Cheng, Y.; Prusoff, W. H. Relationship between the inhibition
constant (K1) and the concentration of inhibitor which causes
50% inhibition (I50) of an enzymatic reaction. Biochem. Phar-
macol. 1973, 22, 3099-3108.
(28) Mori, H.; Takio, K.; Ogawara, M.; Selkoe, D. J. Mass spectrom-
etry of purified amyloid beta protein in Alzheimer’s disease. J.
Biol. Chem. 1992, 267, 17082-17086.
(29) Endoh, R.; Ogawara, M.; Iwatsubo, T.; Nakano, I.; Mori, H. Lack
of the carboxyl terminal sequence of tau in ghost tangles of
Alzheimer’s disease. Brain Res. 1993, 601, 164-172.
(6) Mathis, C. A.; Wang, Y.; Holt, D. P.; Huang, G. F.; Debnath, M.
L.; et al. Synthesis and evaluation of 11C-labeled 6-substituted
2-arylbenzothiazoles as amyloid imaging agents. J. Med. Chem.
2003, 46, 2740-2754.
(7) Agdeppa, E. D.; Kepe, V.; Liu, J.; Flores-Torres, S.; Satyamurthy,
N.; et al. Binding characteristics of radiofluorinated 6-dialkyl-
amino-2-naphthylethylidene derivatives as positron emission
tomography imaging probes for beta-amyloid plaques in Alzhe-
imer’s disease. J. Neurosci. 2001, 21, RC189.
(8) Verhoeff, N. P.; Wilson, A. A.; Takeshita, S.; Trop, L.; Hussey,
D.; et al. In-vivo imaging of Alzheimer disease beta-amyloid with
[
11C]SB-13 PET. Am. J. Geriatr. Psychiatry 2004, 12, 584-595.
JM050635E