Novel styrylpyridines as probes for SPECT imaging of amyloid plaques

Article abstract of DOI:10.1021/jm070025+

We report a series of radioiodinated styrylpyridines as single photon emission computed tomography probes for imaging Aβ plaques in the brain of patients with Alzheimer's disease (AD). In vitro binding showed that all of the styrylpyridines displayed very good binding affinities in postmortem AD brain homogenates (K_i = 3.6 to 15.5 nM). No-carrier-added samples of 13a, 13b, 16a, 16b, and 16e (radioiodinated with ¹²⁵I) were successfully prepared. The in vivo biodistribution in normal mice, at 2 min after injection, showed excellent initial brain penetrations (4.03, 6.22, 5.43, and 8.04% dose/g for [¹²⁵I]13a, 13b, 16a, and 16b, respectively). Furthermore, in vitro autoradiography of AD brain sections showed that the high binding signal was specifically due to the presence of Aβ plaques. Taken together, these results strongly suggest that these styrylpyridines are useful for imaging Aβ plaques in the living human brain.

Full text of DOI:10.1021/jm070025+

J. Med. Chem. 2007, 50, 2157-2165
2157
Novel Styrylpyridines as Probes for SPECT Imaging of Amyloid Plaques
Wenchao Qu,† Mei-Ping Kung,† Catherine Hou,† Tyler E. Benedum,‡ and Hank F. Kung*,†,§
Departments of Radiology and Pharmacology, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104, and AVid Radiopharmaceuticals,
Inc., Philadelphia, PennsylVania 19104
ReceiVed January 9, 2007
We report a series of radioiodinated styrylpyridines as single photon emission computed tomography probes
for imaging Aâ plaques in the brain of patients with Alzheimer's disease (AD). In vitro binding showed that
all of the styrylpyridines displayed very good binding affinities in postmortem AD brain homogenates (Ki
) 3.6 to 15.5 nM). No-carrier-added samples of 13a, 13b, 16a, 16b, and 16e (radioiodinated with 125I) were
successfully prepared. The in vivo biodistribution in normal mice, at 2 min after injection, showed excellent
initial brain penetrations (4.03, 6.22, 5.43, and 8.04% dose/g for [125I]13a, 13b, 16a, and 16b, respectively).
Furthermore, in vitro autoradiography of AD brain sections showed that the high binding signal was
specifically due to the presence of Aâ plaques. Taken together, these results strongly suggest that these
styrylpyridines are useful for imaging Aâ plaques in the living human brain.
Introduction
lower 1 uptake in the cortex appear to be less likely to convert
to AD.19,21,24-26 In addition to the promising data obtained with
[11C]1, in human PET studies using [11C]SB-13,27 2, and
[18F]FDDNP,28-30 3, significant differences in the labeling of
AD and age-matched control patients have been observed.
Alzheimer's disease (ADa) is an insidious neurodegenerative
disease of the brain. It is an increasingly significant medical
problem with a high prevalence in millions of elderly people.
Major neuropathological observations of postmortem AD brains
reveal the presence of senile plaques (containing â-amyloid (Aâ)
aggregates) and neurofibrillary tangles (highly phosphorylated
tau proteins).1-6 An excessive burden of Aâ produced by various
normal or abnormal mechanisms may represent the starting point
of neurodegenerative events. The formation of soluble and
diffusible Aâ7 and Aâ aggregates associated with the neuritic
plaques in the brain produce various toxic effects in neuronal
cells.6,8-11 Currently, it is difficult for clinicians to differentiate
between the cognitive decline associated with normal aging and
the cognitive decline associated with AD. Aside from postmor-
tem pathological staining of the brain tissue, there is no simple
and definitive diagnostic method to detect Aâ plaques in the
brain. Thus, development of positron emission tomography
(PET) or single photon emission computed tomography (SPECT)
imaging agents, which could measure Aâ plaques in the living
brain may assist with early diagnosis of AD.
Currently, there are extensive infrastructures set up for SPECT
imaging studies in major medical centers and community
hospitals. If appropriate, SPECT imaging agents targeting Aâ
plaques could be developed; they would be useful for wide-
spread diagnosis and monitoring of AD patients. The develop-
ment of this series of novel SPECT imaging agents would
therefore benefit a large number of patients. We, and others,
have reported several radioiodinated ligands based on ben-
zothiazole backbone structures.31-33 However, most of these
ligands had slow in vivo washouts resulting in high background-
to-noise ratios, thus preventing their further development as
potential in vivo imaging agents. Subsequently, our search for
potential SPECT imaging agents extended from thioflavins to
the stilbene series34 and to the highly rigid tricyclic fluorene
series,35 with relatively limited success.
The development of [123I]IMPY, 4, a unique thioflavin
derivative with a [1,2,a]imidazopyridine ring, showed the
feasibility of developing SPECT imaging agents for targeting
Aâ plaques.36-38 However, ligand [123I]4 had certain undesirable
characteristics that prompted us to pursue the development of
a second generation of 123I SPECT imaging agents. In prelimi-
nary clinical data, AD patients showed a higher uptake of [123I]-
4 in comparison with control subjects.39 However, the target-
to-background ratio for plaque labeling was not as high as that
of 1. In AD patients, ligand 1 showed a S/N ratio of about
2.5,15,25 while 4 displayed a ratio of 1.4 to 1.6 (30-50 min after
an iv injection).40 This lower specific signal may be due to the
relatively fast brain and plasma clearance observed in AD as
well as in normal subjects. It could also be the result of the in
vivo metabolism or the in vivo instability of [123I]4 (unpublished
data). While the clinical study of [123I]4 in normal and AD
patients is ongoing, we are seeking to improve the signal-to-
noise ratio by searching for a second generation of 123I SPECT
agents with better kinetics and an increased target-to-background
ratio.
In the past few years, successful PET imaging studies with
[11C]PIB, 1, in AD patients have been reported.12-21 Recently,
ligand [11C]1 (Figure 1) has also been used in a limited number
of patients with mild cognitive impairment (MCI).15,22,23 Using
1/PET, it is possible to study the relationship between Aâ plaque
burden and neurological symptoms of AD. The results seem to
suggest that while some MCI cases convert to AD, those with
* To whom correspondence should be addressed. Hank F. Kung, Ph.D.,
Department of Radiology, University of Pennsylvania, 3700 Market Street,
Room 305, Philadelphia, Pennsylvania 19104. Tel.: (215) 662-3096. Fax:
(215) 349-5035. E-mail: kunghf@sunmac.spect.upenn.edu.
† Department of Radiology, University of Pennsylvania.
‡ Avid Radiopharmaceuticals, Inc.
§ Department of Pharmacology, University of Pennsylvania.
a Abbreviations: SPECT, single photon emission computed tomography;
PET, positron emission tomography; AD, Alzheimer's disease; MCI, mild
cognitive impairment; Aâ, â-amyloid; PIB, 2-(4′-(methylaminophenyl)-6-
hydroxybenzothiazole; SB-13, 4-N-methylamino-4′-hydroxystilbene; FDD-
NP,
2-(1-{6-[(2-fluoroethyl)methyl-amino]-2-naphthyl}ethylidend)-
malononitrile; IMPY, 6-iodo-2-(4′-dimethylamino-)phenyl-imidazo[1,2-
a]pyridine; PEGN3-SB, {4-[2-(4-{2-[2-(2-Fluoro-ethoxy)-ethoxy]-ethoxy}-
phenyl)-vinyl]-phenyl}-methylamine; Styrylpyridine, (E)-2-(2-(2-(2-
fluoroethoxy)ethoxy)ethoxy)-5-(4-dimethylaminostyryl)-pyridine; FPEG,
fluoro-polyethyleneglycol.
We have recently developed two new PET tracers based on
stilbene and styrylpyridine core structures containing fluoro-
10.1021/jm070025+ CCC: $37.00 © 2007 American Chemical Society
Published on Web 04/06/2007

2158 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 9
Qu et al.
Figure 1. Chemical structures of various PET and SPECT ligands targeting Aâ plaque. 6-OH-BTA-1 (PIB), 2-(4′-(methylaminophenyl)-6-
hydroxybenzothiazole; SB-13, 4-N-methylamino-4′-hydroxystilbene; FDDNP, 2-(1-[6-[(2-fluoroethyl)methyl-amino]-2-naphthyl]ethylidend)ma-
lononitrile; IMPY, 6-iodo-2-(4′-dimethylamino-)phenyl-imidazo[1,2-a]pyridine; PEGN3-SB, (E)-4-(4-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)styryl)-
N-methylbenzenamine; styrylpyridine, (E)-4-(2-(6-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)pyridin-3-yl)vinyl)-N,N-dimethylbenzenamine.
Table 1. Potencies (Ki) of Compounds on Competition of [125I]4
Scheme 1a
Binding to Amyloid Plaques in AD Brain Homogenatesa
cmpd
Ki (nM ( SEM)
11a
11b
11e
13a
13b
13e
14a
14b
14e
16a
16b
16e
6.8 ( 1.4
4.5 ( 0.9
14.2 ( 3.8
7.5 ( 0.8
9.0 ( 1.0
21 ( 8.0
3.6 ( 0.8
5.0 ( 1.6
6.8 ( 0.8
7.5 ( 1.5
8.5 ( 2.5
15.5 ( 0.5
a Reagentsandconditions: (a)NIS,CH3CN,reflux,1h;(b)F(CH2CH2O)3H,
Ph3P, DIAD, THF, -5 °C to rt, 2 h; (c) (1) HOCH2CH2OTBDMS, Ph3P,
DIAD, THF, -5 °C to rt, 2 h; (2) 1% HCl in 95% EtOH, rt, 1 h.
polyethyleneglycol (FPEG) substituents41-43 (5 and 6) showing
highly desirable Aâ-targeting properties for PET imaging. The
in vivo biodistribution studies showed excellent brain penetration
after iv injection in normal mice. We were able to modulate
the lipophilicity and preserve the ability to penetrate the blood-
brain barrier by using end-capped FPEG groups or simply an
ethylene glycol derivative.41,42 On the basis of these new
findings, we hypothesized that if we created iodinated com-
pounds with a pyridine ring instead of the phenyl ring of the
stilbene, they would have good binding affinities, while the
pyridine ring modulates the lipophilicity or improves brain
penetration.44 Initially, we tested the strategy by comparing
iodine derivatives of stilbene and styrylpyridine derivatives.
Combining both pegylation and a pyridine ring on stilbenes
resulted in a highly successful method for designing suitable
123I probes to target Aâ plaques in the brain. We report herein
the synthesis and the initial biological characterizations of this
new series of iodinated styrylpyridine derivatives targeting Aâ
plaques in the brain.
a Each value was determined three times with duplicate for each
measurement.
starting material for a radioiodination reaction. The desired
organostannane compounds (12a-c, 12e, 15a,b, and 15e) were
prepared by using Pd(PPh3)4-catalyzed trans-stannylation from
their bromide precursors.50 The subsequent iododestanny-
lation reaction afforded iodinated targets (13a, 13c, 13e, 16a,b,
and 16e). However, we were surprised that we were not
able to make 13b directly by an iododestannylation reaction
from 12b.51 The acidic deprotection of the Boc group on 6c
failed to give the desired product. Alternatively, the same
reaction was carried out in the presence of trimethylsilyl triflate
and 2,6-lutidine,52 which resulted in a clean cleavage of the
N-Boc group and led to the target molecule, 13b, with an
excellent yield (88%).
Radiolabeling. To make the desired 125I-labeled 13a, 13b,
16a, 16b, and 16e, the corresponding tributyltin starting
materials, 12a, 12b, 15a, 15b, and 15e, were prepared as the
precursors for labeling (Scheme 2). Standard iododestannylaltion
reactions, using sodium [125I]iodide, hydrogen peroxide, and
hydrochloric acid, were successfully applied to all five ligands
with 25-50% overall yields (Scheme 3). The chemical identities
of the 125I-labeled ligands were confirmed by using authentic
standards to show identical retention times on HPLC. The
radiochemical purities of all of the purified ligands were greater
than 95% and all had a high specific activity (no carrier added,
approx. 2000 Ci/mmol). Due to the pyridine ring on the
backbone structure for all the ligands, we observed lower
labeling yields (50-70% after the EtOAc extraction) as well
as lower initial radiochemical purities (60-70% before the
HPLC purification). These values were lower as compared to
the preparation of the corresponding stilbene series of 125I-
labeled ligands (unpublished data). Currently, we are trying to
optimize the labeling conditions to increase the overall yield.
It is likely that minor adjustments of the labeling procedure will
result in improved radiochemical yields.
Results and Discussion
Chemistry. The syntheses of styrylpyridine derivatives were
readily accomplished by the reactions shown in Schemes 1 and
2. Iodination of 2-hydroxy-3-bromopyridine 7 with N-iodosuc-
cinimide (NIS) in acetonitrile (MeCN) under a refluxing
condition afforded 8 in 85% yield.45 Mitsunobu reaction of 8
with monofluorated triethylene glycol provided O-alkylated 9
in 75% yield.46 The ethylene glycol tethered intermediate 10
was prepared by a similar Mitsunobu reaction and followed by
an acidic desilylation reaction.47 The combined yield of the two
steps was 68%.
The bromo-substituted styrylpyridine derivatives (11a-d,
14a,b, and 14d) were assembled by a palladium-catalyzed Heck
coupling reaction of iodide 9 and 10 with different 4-substituted
styrenes.48,49 The phenolic intermediates, 11e and 14e,were
obtained by deacetylation of 11d and 14d under a basic
condition. We have also prepared the corresponding tributyltin

Styrylpyridines as Probes of Amyloid Plaques
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 9 2159
Scheme 2a
a Reagents and conditions: (a) 4-substituted styrenes, K2CO3, Bu4NBr, Pd(OAc)2, DMF, 55-65 °C; (b) (Bu3Sn)2, Pd(PPh3)4, toluene, 110 °C; (c) K2CO3,
EtOH/ THF, rt, 2 h; (d) I2, THF, 0 °C to rt; (e) TMSOTf, 2,6-lutidine, DCM, -78 °C to rt.
Scheme 3a
a Reagents and conditions: H2O2, NaI125, HCl, EtOH.
Biological Evaluation. An in vitro binding assay serves as
the first screening step for potential ligands targeting â-amyloid
plaques. Successful assays have been developed with several
PET and SPECT imaging agents with different core structures,
including 2 and 4, as well as several fluoropegylated stilbenes
currently under clinical evaluation. The binding affinities (Ki,
nM) of the iodinated styrylpyridine derivatives were first
evaluated by a competitive binding assay with [125I]4 for Aâ
plaques using homogenates prepared from postmortem AD brain
tissues. All the iodinated styrylpyridine derivatives examined
showed excellent-to-good binding affinities in comparison with
similar binding affinities (Ki values demonstrated in Table 1
between series 11, 13, 14, and 16).
On the basis of the encouraging binding data observed for
13a, 13b, 16a, 16b, and 16e, we chose to carry out further
biological evaluations using the 125I-labeled styrylpyridines. All
of the radioiodinated probes measured under the experimental
conditions showed optimum partition coefficients (log P ) 2.2-
2.6), a desirable property for imaging agents. One exception is
the hydroxy derivative, [125I]16e, which displayed a lower
partition coefficient (Log P ) 1.98), leading to a low brain
uptake (see the results of the animal study shown below). When
evaluated for whole animal biodistribution after an iv injection
in normal mice, [125I]13a, 13b, 16a, and 16b displayed good
penetration of the intact blood-brain barrier with excellent
initial brain uptakes (4.03, 6.22, 5.43, and 8.04% dose/g for
[
125I]4 binding. The addition of a fluoropegylated chain to the
one end of the phenyl group did not affect the binding affinity
to amyloid plaques. Compound 13a showed a Ki value of 7.5
( 0.8 nM (Table 1), which is comparable to a previously
reported pyridine probe (Ki ) 4.8 ( 0.6 nM44) containing a
simple hydroxy substitution instead of an attached fluoropegy-
lated chain. Similarly, the hydroxyl pegylated derivative, that
is, 16a, competed effectively with [125I]4 binding with a Ki value
of 7.5 ( 1.5 nM. There is no significant difference in the binding
affinities between N,N-dimethylamino derivatives, 13a, 16a, and
N-monomethylamino derivatives, 13b, 16b (Ki ) 9.0 ( 1.0 and
8.5 ( 2.5 nM, respectively). After replacing the substituted
amino group with a hydroxy group attached to the 4-position
of one end of the phenyl ring, the binding affinities showed a
slight reduction (Ki ) 21 ( 8.0 and 15.1 ( 0.5 nM for 13e and
16e, respectively). It was also evident that all of the brominated
derivatives and the corresponding iodinated ligands displayed
[
125I]13a, 13b, 16a, 16b, respectively at 2 min after a tracer
injection). Compounds containing the hydroxy- or fluoro-peg
group showed much higher brain penetrations as compared with
the previously reported nonpegylated iodinated ligands.44,53
These series of iodinated stryrylpyridines displayed superior
brain penetration. The high brain uptakes of these iodinated
ligands were subsequently followed by rapid washouts with less
than 0.5% dose/g remaining in the brain at 2 h after the injection
(Table 2). The brain washouts for the N,N-dimethylamino
ligands, that is, [125I]13a and [125I]16a, were slower in com-
parison to the N-methylamino ligands, that is, [125I]13b and [125I]-
16b (Table 2). One way to select a ligand with appropriate in
vivo kinetics is to use brain2min/brain30min as the index to compare

2160 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 9
Qu et al.
Table 2. Biodistribution in Mice after IV Injections of [125I]-Labeled
Tracersa
[
125I]13a (log P ) 2.59)
organ
2 min
30 min
1 h
2 h
blood
heart
muscle
lung
kidney
spleen
liver
skin
brain
thyroid
2.70 ( 0.58
12.76 ( 1.24
0.90 ( 0.20
10.1 ( 2.15
16.6 ( 1.96
4.47 ( 1.28
22.2 ( 4.34
0.54 ( 0.05
4.03 ( 0.43
3.89 ( 0.67
2.05 ( 0.18
1.63 ( 0.03
1.00 ( 0.08
2.50 ( 0.14
3.32 ( 0.11
1.42 ( 0.05
9.54 ( 1.30
1.47 ( 0.26
1.93 ( 0.18
16.2 ( 11.75
1.65 ( 0.45
0.97 ( 0.16
0.59 ( 0.13
1.62 ( 0.46
2.30 ( 0.54
0.99 ( 0.47
5.34 ( 2.22
1.59 ( 0.68
0.68 ( 0.17
24.2 ( 8.26
1.45 ( 0.41
0.73 ( 0.17
0.53 ( 0.08
1.33 ( 0.39
1.71 ( 0.24
0.79 ( 0.27
5.62 ( 1.31
1.23 ( 0.41
0.26 ( 0.04
60.8 ( 6.09
[
125I]13b (log P ) 2.54)
organ
2 min
30 min+
1 h
2 h
blood
heart
muscle
lung
kidney
spleen
liver
skin
Brain
Thyroid
4.37 ( 1.07
9.85 ( 1.78
1.04 ( 0.25
6.85 ( 0.27
9.03 ( 6.81
4.41 ( 1.05
26.2 ( 4.47
1.48 ( 0.07
6.22 ( 1.01
5.74 ( 0.42
3.83 ( 1.11
2.54 ( 0.37
1.11 ( 0.34
3.01 ( 0.96
3.40 ( 0.76
2.49 ( 0.75
11.5 ( 2.10
2.95 ( 0.81
1.23 ( 0.13
24.1 ( 27.4
2.88 ( 0.28
1.75 ( 0.26
0.85 ( 0.06
2.37 ( 0.29
2.81 ( 0.70
1.75 ( 0.33
7.70 ( 1.22
2.46 ( 0.16
0.62 ( 0.17
38.1 ( 6.37
2.21 ( 0.73
1.22 ( 0.28
0.44 ( 0.19
1.85 ( 0.74
1.86 ( 0.36
1.27 ( 0.24
6.25 ( 1.79
1.32 ( 0.41
0.26 ( 0.01
215 ( 74.6
Figure 2. In vitro autoradiography of macroarray brain sections
constructed from six postmortem AD cases plus one control (marked
by an arrowhead). Section labeling was carried out with four 125I-labeled
styrylpyridine ligands. Immunohistochemistry with 4G8 confirmed the
presence and location of plaques in the sections (dots in the tissue
section of the AD brain, but not in the control brain). Ligand [125I]4, a
well characterized SPECT ligand37 targeting Aâ plaques, was included
for comparison. Clearly, the Aâ plaques were successfully targeted by
125I styrylpyridine derivatives 13a, 13b, 16a, and 16b with low
background labeling.
[
125I]16a (log P ) 2.64
organ
2 min
30 min
1 h+
2 h
blood
heart
muscle
lung
kidney
spleen
liver
skin
brain
thyroid
2.71 ( 0.07
10.2 ( 0.45
0.71 ( 0.46
9.41 ( 0.56
14.3 ( 1.98
4.40 ( 1.89
19.1 ( 2.68
0.46 ( 0.13
5.43 ( 0.85
4.15 ( 0.43
2.24 ( 0.38
1.93 ( 0.27
1.05 ( 0.20
3.02 ( 0.38
4.19 ( 0.45
1.94 ( 0.19
12.4 ( 1.29
1.18 ( 0.26
3.56 ( 0.32
11.2 ( 7.88
2.18 ( 0.66
1.12 ( 0.02
0.55 ( 0.03
1.98 ( 0.21
2.49 ( 0.33
1.32 ( 0.10
6.22 ( 0.96
1.16 ( 0.00
1.32 ( 0.00
59.1 ( 6.26
1.01 ( 0.02
0.62 ( 0.12
0.22 ( 0.04
1.00 ( 0.15
1.48 ( 0.20
0.80 ( 0.11
4.87 ( 0.46
0.40 ( 0.05
0.46 ( 0.05
24.8 ( 0.62
have better signal-to-noise ratios and therefore may be better
for Aâ plaque detection. The low brain uptake observed for
[
125I]16e may be due to either the presence of the additional
hydroxy group on the molecule or to a lower partition
coefficient.
Consistently, blood levels for all four radioiodinated ligands
dropped gradually with time. A good initial brain uptake
combined with a rapid washout (high brain2min/brain30min ratio)
in a normal mouse brain (presumably, there are no Aâ plaques
in the normal mouse brain for extra binding by these Aâ plaque-
targeting probes) is a highly desirable property. The radioiodi-
nated styrylpyridine probes reported here, especially [125I]13b
and [125I]16b, met this criteria and may be favorably considered
as potential SPECT imaging agents for targeting Aâ plaques in
vivo.
We also carefully constructed a group of human macro-array
sections consisting of six confirmed AD cases and one control
subject. After sectioning this macro-array, adjacent sections
would reflect a comparable pathophysiology. In vitro film
autoradiography was carried out using these 125I-labeled styrylpy-
ridine probes. Among the four styrylpyridine probes, [125I]13a
and 16a exhibited the most distinctive plaque labeling with a
minimal background (Figure 2). The labeling pattern was
consistent with that observed by immunohistochemical labeling
with an antibody for Aâ (4G8, Sigma). In the control (marked
by the arrowhead, Figure 2), there was no notable Aâ labeling.
The results suggest that these novel 125I-labeled styrylpyridine
probes were specifically labeling Aâ plaque sites. Furthermore,
ligand [125I]4, a well characterized SPECT imaging agent for
amyloid plaques,37 displayed a similar pattern of Aâ plaque
labeling using adjacent sections from the macro-array. These
results confirmed that the binding of these radioiodinated
styrylpyridine probes was specific for Aâ plaques. However,
the N-methylamino ligands, that is, [125I]13b and [125I]16b,
displayed a less intense and moderate plaque labeling (see Figure
[
125I]16b (log P ) 2.20)
organ
2 min
30 min
1 h
2 h
blood
heart
muscle
lung
kidney
spleen
liver
skin
brain
thyroid
4.14 ( 0.41
7.16 ( 1.16
1.15 ( 0.38
7.43 ( 1.21
11.5 ( 1.48
4.08 ( 0.68
20.8 ( 2.38
0.95 ( 0.09
8.04 ( 0.82
6.31 ( 1.59
3.08 ( 0.35
1.50 ( 0.18
0.91 ( 0.06
2.67 ( 0.46
3.73 ( 0.75
1.34 ( 0.29
12.6 ( 3.03
1.86 ( 0.50
0.88 ( 0.30
17.2 ( 14.2
1.81 ( 0.56
0.88 ( 0.30
0.42 ( 0.08
1.76 ( 0.32
2.16 ( 0.08
0.87 ( 0.37
5.62 ( 0.68
1.29 ( 0.51
0.26 ( 0.03
36.7 ( 37.2
1.96 ( 0.14
0.76 ( 0.03
0.38 ( 0.02
1.58 ( 0.10
1.53 ( 0.20
1.08 ( 0.15
3.41 ( 0.20
1.43 ( 0.10
0.15 ( 0.02
99.9 ( 69.5
[
125I]16e (log P ) 1.98)
organ
2 min
30 min
1 h
2 h
blood
heart
muscle
lung
kidney
spleen
liver
skin
brain
thyroid
10.1 ( 1.12
6.66 ( 0.31
1.01 ( 0.34
14.2 ( 0.92
20.4 ( 2.20
4.20 ( 0.31
18.3 ( 1.29
0.64 ( 0.20
0.99 ( 0.24
4.38 ( 0.46
3.92 ( 0.07
1.35 ( 0.16
0.59 ( 0.05
3.10 ( 0.05
10.0 ( 2.12
1.28 ( 0.44
5.15 ( 0.61
1.36 ( 0.07
0.26 ( 0.03
3.99 ( 3.56
1.29 ( 0.05
0.65 ( 0.21
0.21 ( 0.02
1.34 ( 0.11
2.94 ( 0.17
0.50 ( 0.03
2.38 ( 0.58
0.62 ( 0.01
0.09 ( 0.01
13.0 ( 8.11
1.56 ( 0.04
0.51 ( 0.09
0.12 ( 0.01
1.02 ( 0.01
2.50 ( 1.32
0.50 ( 0.06
2.63 ( 1.30
0.37 ( 0.08
0.06 ( 0.01
16.0 ( 11.5
a %dose/g, avg of three mice ( SD.
the washout rate.54,55 The four radioiodinated styrylpyridine
probes showed brain2min/brain30min ratios of 2.09, 5.05, 1.52, and
9.13 for [125I]13a, [125I]13b, [125I]16a, and [125I]16b, respec-
tively. According to this, the N-methylamino- derivatives, [125I]-
13b and [125I]16b, which showed a high washout index, may

Styrylpyridines as Probes of Amyloid Plaques
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 9 2161
(2-fluoroethoxy)ethoxy)ethanol (0.200 g, 1.3 mmol), and PPh3
(0.511 g,1.95 mmol) in 10 mL of THF at -5 °C was added
dropwise diisopropyl azodicaboxylate (DIAD, 0.394 g, 1.95 mmol)
in 5 mL of THF. The ice-salt bath was removed, and the reaction
was kept at room temperature (rt) for 2 h. The reaction solution
was concentrated and purified by FC (MeOH/CHCl3, 1/99) to yield
2). This could be due to the labeling conditions, which were
not currently optimized for individual ligand. In addition,
sections fixed in formalin and subsequently treated with xylene
during plaque labeling may lead to some alternation affecting
the ligand recognition site(s).
To further characterize the specific nature of plaque binding,
we chose [125I]16b to carry out a direct in vitro binding assay
using AD brain homogenates. As expected, [125I]16b dis-
played highly specific and saturable binding (data not shown).
However, there was a large amount of nonspecific filter (glass
fiber GF/B) binding in this assay. The nonspecific binding makes
this assay an unfavorable tool for quantitatively testing the
burden of Aâ plaque in the brain tissue. To solve this problem,
we are currently trying different methods to increase the signal
detection.
In conclusion, a new series of novel radioiodinated styrylpy-
ridine derivatives, containing a pegylated chain, were success-
fully prepared as potential SPECT imaging agents for AD. These
novel styrylpyridine derivatives displayed excellent binding
affinities to Aâ plaques (Ki in the nM range). The radioiodinated
probes showed desirable in vivo brain penetration and fast
washout in the mouse brain. A specific plaque-labeling signal
was clearly indicated for these probes in postmortem AD brain
sections as well as in brain tissue homogenates. When labeled
with 123I, this novel series of styrylpyridine ligands could be
potentially useful for in vivo SPECT imaging of Aâ plaques.
1
9, a colorless viscous liquid (0.423 g, 75%). H NMR δ 8.21 (d,
1H, J ) 2.0 Hz), 8.02 (d, 1H, J ) 2.0 Hz), 4.66 (t, 1H, J ) 4.1
Hz), 4.50-4.39 (m, 3H), 3.89-3.64 (m, 8H). 13C NMR δ 159.4,
151.2, 148.5, 108.5, 84.9, 81.6, 81.5, 71.1, 71.0, 70.8, 70.4, 69.3,
66.9. HRMS calcd for C11H14BrFINO3 (M+), 432.9186; found,
432.9173.
2-(3-Bromo-5-iodopyridin-2-yloxy)ethanol (10). To a stirring
suspension of 8 (0.906 g, 3.0 mmol), 2-(tert-butyl-dimethyl-
silanyloxy)ethanol (0.554 g, 3.15 mmol), and PPh3 (0.944 g, 3.6
mmol) in 20 mL of THF at -5 °C was added dropwise diisopro-
pylazodicarboxylate (DIAD, 0.728 g, 3.6 mmol) in 10 mL of THF.
The ice-salt bath was removed, and the reaction was kept at rt for
2 h. The reaction solution was concentrated and purified by FC
(EtOAc/hexanes, 5/95) to afford 3-bromo-2-(2-(tert-butyl-dim-
ethylsilyloxy)ethoxy)-5-iodopyridine, a colorless viscous liquid
(0.995 g, 72%). 1H NMR δ 8.23 (d, 1H, J ) 2.0 Hz), 8.05 (d, 1H,
J ) 2.0 Hz), 4.42 (t, 2H, J ) 4.9 Hz), 3.98 (t, 2H, J ) 4.9 Hz),
0.90 (s, 9H), 0.10 (s, 6H). HRMS calcd for C12H18BrINO2Si (M -
CH3+), 441.9335; found, 441.9312.
3-Bromo-2-(2-(tert-butyl-dimethylsilyloxy)ethoxy)-5-iodopy-
ridine (0.230 g, 0.50 mmol) was added to 5 mL of 95% EtOH
containing 1% concd HCl and stirred at rt for 0.5 h. The reaction
solution was poured into 20 mL of ice-cold 5% Na2CO3, and the
aqueous layer was extracted with CH2Cl2 (15 mL × 3). The organic
layers were collected, dried, filtered, and concentrated. The residue
was purified by FC (EtOAc/hexanes, 30/70) and afforded 10 as a
Experimental Section
General Information. All reagents used were commercial
products and were used without further purification unless otherwise
indicated. All reactions were carried out in flame-dried glassware
under nitrogen atmosphere. Flash chromatography (FC) was
performed using silica gel 60 (230-400 mesh, Sigma-Aldrich).
Preparative thin layer chromatography (PTLC) was performed on
silica gel plates with a fluorescent indicator that was visualized
with light at 254 nm (Analtech). For each procedure, “standard
workup” refers to the following steps: addition of the indicated
organic solvent, washing the organic layer with water then brine,
separation of the organic layer from the aqueous layer, drying off
the combined organic layers with sodium sulfate or magnesium
sulfate, filtering off the solid, and concentrating the filtrate under
reduced pressure. 1H NMR spectra were obtained at 200 MHz and
13C NMR spectra were recorded at 50 MHz in CDCl3 (Bruker DPX
spectrometer). Chemical shifts were reported as δ values (parts per
million) relative to internal TMS. Coupling constants were reported
in hertz. The multiplicity is defined by s (singlet), d (doublet), t
(triplet), br (broad), or m (multiplet). High-resolution MS experi-
ments were performed at the McMaster Regional Centre for Mass
Spectrometry using a Micromass/Waters GCT instrument (GC-EI/
CI time-of-flight mass spectrometer). Elemental analyses were
performed by Atlantic Microlab Inc. Analytical HPLC analysis was
carried out using an Agilent 1100 series LC. Two systems were
used to confirm the purity of some compounds listed in this
section: system A conditions, Hamilton PRP-1 reverse-phase
analytical column (4.1 × 250 mm, 10 µm), 80/20 CH3CN/1 mM
ammonium formate (pH ) 7) water buffer, 1.0 mL/min, UV 350
nm; system B conditions, Phenomenex silica column (4.6 × 250
mm, 5 µm), 40/60 EtOAc/hexanes, 1.0 mL/min, UV 350 nm. All
compounds reported in this paper showed greater than 95% purity
in both systems.
1
white solid (0.169 g, 94%). H NMR δ 8.24 (d, 1H, J ) 2.0 Hz),
8.09 (d, 1H, J ) 2.0 Hz), 4.50 (t, 2H, J ) 4.5 Hz), 4.02-3.93 (m,
2H), 2.58-2.52 (m, 1H, -OH). 13C NMR δ 159.5, 151.2, 148.9,
108.7, 81.9, 69.6, 61.7. HRMS calcd for C7H7BrINO2 (M+),
342.8705; found, 342.8707.
General Procedure of the Heck Reaction for the Synthesis
of Bromo-Substituted Styryl-Pyridine Derivatives (11a-d,
14a,b, and 14d). (E)-4-(2-(5-Bromo-6-(2-(2-(2-fluoroethoxy)-
ethoxy)ethoxy)pyridin-3-yl)vinyl)-N,N-dimethylbenzenamine (11a).
A mixture of 4-dimethylaminostyrene (0.110 g, 0.75 mmol), 9
(0.217 g, 0.5 mmol), K2CO3 (0.173 g, 1.25 mmol), tetrabutylam-
monium bromide (TBAB, 0.322 g, 1.0 mmol), and palladium acetate
(Pd(OAc)2, 0.006 g, 0.025 mmol) in 2 mL of DMF was deoxy-
genated by purging into nitrogen for 15 min and then heating to
65 °C for 2 h. The reaction mixture was cooled to rt and submitted
to standard workup with ethyl acetate (EtOAc). The crude product
was purified by FC (EtOAc/hexanes, 30/70) and resulted in 11a as
1
a light yellow solid (0.178 g, 79%). H NMR δ 8.08 (d, 1H, J )
2.1 Hz), 8.00 (d, 1H, J ) 2.1 Hz), 7.39 (d, 2H, J ) 8.8 Hz), 6.92
(d, 1H, J ) 16.3 Hz), 6.74 (d, 1H, J ) 16.3 Hz), 6.72 (d, 2H, J )
8.1 Hz), 4.69 (t, 1H, J ) 4.2 Hz), 4.55 (t, 2H, J ) 4.8 Hz), 4.45
(t, 1H, J ) 4.2 Hz), 3.94-3.68 (m, 8H), 3.00 (s, 6H). 13C NMR δ
158.3, 150.4, 143.5, 138.0, 129.6, 129.5, 127.7, 125.2, 118.8, 112.5,
107.5, 85.0, 81.6, 71.2, 71.0, 70.8, 70.4, 69.6, 66.7, 40.5. HRMS
calcd for C21H26BrFN2O3 (M+), 452.1111; found, 452.1099.
(E)-4-(2-(5-Bromo-6-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)py-
ridin-3-yl)vinyl)-N-methylbenzenamine (11b). Compound 11b
was prepared from 4-methylaminostyrene (0.073 g, 0.55 mmol) and
9 (0.217 g, 0.50 mmol) as a light yellow viscous liquid (0.113 g,
52% yield). 1H NMR δ 8.07 (d, 1H, J ) 2.1 Hz), 8.00 (d, 1H, J )
2.1 Hz), 7.35 (d, 2H, J ) 8.6 Hz), 6.91 (d, 1H, J ) 16.3 Hz), 6.74
(d, 1H, J ) 16.3 Hz), 6.60 (d, 2H, J ) 8.6 Hz), 4.69 (t, 1H, J )
4.2 Hz), 4.55 (t, 2H, J ) 4.8 Hz), 4.45 (t, 1H, J ) 4.2 Hz), 3.94-
3.68 (m, 8H), 2.88 (s, 3H). 13C NMR δ 158.4, 149.5, 143.6, 138.0,
129.8, 129.5, 127.9, 126.1, 118.9, 112.6, 107.5, 85.0, 81.7, 71.2,
71.1, 70.8, 70.4, 69.6, 66.8, 30.7. HRMS calcd for C20H24BrFN2O3
(M+), 438.0954; found, 438.0967.
3-Bromo-5-iodopyridin-2-ol (8). Following a previously re-
ported method,45 compound 8 was prepared from N-iodosuccini-
mide (2.48 g, 11.0 mmol) and 3-bromo-2-hydroxypyridine 7 (1.74
g, 10.0 mmol) as a pale brown solid (2.55 g, 85%). 1H NMR
(DMSO-d6) δ 12.27 (br s, 1H), 8.08 (d, 1H, J ) 2.3 Hz), 7.71 (d,
1H, J ) 2.3 Hz).
3-Bromo-(2-(2-(2-Fluoroethoxy)ethoxy)ethoxy)-5-iodopyri-
dine (9). To a stirring suspension of 8 (0.393 g, 1.3 mmol), 2-(2-
(E)-tert-Butyl 4-(2-(5-Bromo-6-(2-(2-(2-fluoroethoxy)ethoxy)-
ethoxy)pyridin-3-yl)vinyl)phenyl(methyl)carbamate (11c). Com-

2162 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 9
Qu et al.
pound 11c was prepared from 4-N-methyl-4-N-(tert-butyloxycar-
bonyl)aminostyrene (0.219 g, 0.94 mmol) and 9 (0.273 g, 0.63
mmol) as a white viscous liquid (0.319 g, 94% yield). 1H NMR δ
8.12 (d, 1H, J ) 2.1 Hz), 8.03 (d, 1H, J ) 2.1 Hz), 7.44 (d, 2H,
J ) 8.6 Hz), 7.25 (d, 2H, J ) 9.0 Hz), 6.94 (d, 2H, J ) 2.1 Hz),
4.69 (t, 1H, J ) 4.2 Hz), 4.56 (t, 2H, J ) 4.9 Hz), 4.45 (t, 1H, J
) 4.2 Hz), 3.94-3.68 (m, 8H), 3.28 (s, 3H), 1.48 (s, 9H). 13C NMR
δ 158.8, 154.5, 144.0, 143.5, 138.2, 133.6, 128.5, 128.4, 126.8,
126.6, 125.4, 122.9, 107.4, 84.8, 81.4, 80.4, 71.0, 70.9, 70.6, 70.2,
69.4, 66.7, 53.5, 37.1, 28.4. HRMS calcd for C25H32BrFN2O5 (M+),
538.1479; found, 538.1476.
(E)-4-(2-(5-Bromo-6-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)py-
ridin-3-yl)vinyl)phenyl Acetate (11d). Compound 11d was pre-
pared from 4-acetoxystyrene (0.122 g, 0.75 mmol) and 9 (0.217 g,
0.5 mmol) as a white viscous liquid (0.181 g, 77% yield). 1H NMR
δ 8.12 (d, 1H, J ) 2.1 Hz), 8.03 (d, 1H, J ) 2.1 Hz), 7.50 (d, 2H,
J ) 8.6 Hz), 7.10 (d, 2H, J ) 8.6 Hz), 6.94 (d, 2H, J ) 3.3 Hz),
4.69 (t, 1H, J ) 4.2 Hz), 4.56 (t, 2H, J ) 4.9 Hz), 4.45 (t, 1H, J
) 4.2 Hz), 3.94-3.68 (m, 8H), 2.32 (s, 3H), 1.48 (s, 9H). 13C NMR
δ 169.3, 158.9, 150.3, 144.1, 138.2, 134.5, 128.24, 128.16, 127.4,
123.4, 121.9, 107.5, 84.8, 81.5, 71.0, 70.9, 70.6, 70.3, 69.4, 66.7,
21.1. HRMS calcd for C21H23BrFNO5 (M+), 467.0744; found,
467.0731.
(E)-2-(3-Bromo-5-(4-(dimethylamino)styryl)pyridin-2-yloxy)-
ethanol (14a). Compound 14a was prepared from 4-dimethylami-
nostyrene (0.031 g, 0.212 mmol) and 10 (0.073 g, 0.212 mmol) as
a light yellow solid (0.022 g, 29% yield). 1H NMR δ 8.07 (d, 1H,
J ) 2.1 Hz), 8.03 (d, 1H, J ) 2.1 Hz), 7.39 (d, 2H, J ) 8.8 Hz),
6.94 (d, 1H, J ) 16.3 Hz), 6.78-6.69 (m, 3H), 4.57-4.52 (m,
2H), 3.99 (t, 2H, J ) 4.3 Hz), 3.21 (br s, 1H), 3.00 (s, 6H). 13C
NMR δ 158.3, 150.4, 143.0, 138.2, 129.9, 129.8, 127.6, 124.9,
118.3, 112.3, 107.5, 69.6, 62.1, 40.3. HRMS calcd for C17H19-
BrN2O2 (M+), 362.063; found, 362.0629.
129.2, 129.0, 127.8, 119.8, 115.6, 106.7, 68.4, 59.2. HRMS calcd
for C15H14BrNO3 (M+), 335.0157; found, 335.0165.
General Procedure of the Stannylation for the Synthesis of
Tributyltin-Substituted Styryl-Pyridine Derivatives (12a-c, 12e,
and 15a,b, 15e). (E)-4-(2-(6-(2-(2-(2-Fluoroethoxy)ethoxy)ethoxy)-
5-(tributylstannyl)pyridin-3-yl)vinyl)-N,N-dimethylbenze-
namine (12a). A mixture of 11a (0.052 g, 0.115 mmol), bis-
(tributyltin) ((Bu3Sn)2, 0.333 g, 0.57 mmol), and palladium
tetrakistriphenylphosphine (Pd(PPh3)4, 0.013 g, 10 mol %) in
toluene was heated at 110 °C for 18 h. The reaction solution was
cooled to rt and treated with 5 mL of 10% KF. After vigorously
stirring for an additional 0.5 h, the standard workup with EtOAc
and the following FC (EtOAc/hexanes, 25/75) afforded 12a as a
1
light yellow oil (0.052 g, 68%). H NMR δ 8.11 (d, 1H, J ) 2.5
Hz), 7.81 (d, 1H, J ) 2.5 Hz), 7.41 (d, 2H, J ) 8.8 Hz), 6.93 (d,
1H, J ) 16.5 Hz), 6.81 (d, 1H, J ) 16.5 Hz), 6.72 (d, 2H, J ) 8.7
Hz), 4.69 (t, 1H, J ) 4.2 Hz), 4.46 (t, 3H, J ) 4.9 Hz), 3.83 (t,
3H, J ) 4.8 Hz), 3.71-3.66 (m, 5H), 3.00 (s, 6H), 1.68-1.48 (m,
6H), 1.43-1.21 (m, 6H), 1.15-1.02 (m, 6H), 0.91 (t, 9H, J ) 7.1
Hz). 13C NMR δ 166.7, 150.2, 145.4, 143.6, 127.8, 127.7, 127.5,
126.0, 123.7, 121.2, 112.6, 85.0, 81.6, 71.0, 70.8, 70.7, 70.4, 70.0,
65.0, 40.6, 29.5, 29.3, 29.1, 28.1, 27.5, 26.9, 13.9, 13.4, 13.3, 9.9,
6.6, 6.4. HRMS calcd for C33H53FN2O3Sn (M+), 664.3062; found,
664.3037.
(E)-4-(2-(6-(2-(2-(2-Fluoroethoxy)ethoxy)ethoxy)-5-(tributyl-
stannyl)pyridin-3-yl)vinyl)-N-methylbenzenamine (12b). Com-
pound 12b was prepared from 11b (0.069 g, 0.156 mmol) as a
light yellow oil (0.068 g, 68% yield). 1H NMR δ 8.10 (d, 1H, J )
2.5 Hz), 7.80 (d, 1H, J ) 2.5 Hz), 7.36 (d, 2H, J ) 8.6 Hz), 6.92
(d, 1H, J ) 16.3 Hz), 6.80 (d, 1H, J ) 16.3 Hz), 6.61 (d, 2H, J )
8.6 Hz), 4.69 (t, 1H, J ) 4.2 Hz), 4.45 (t, 3H, J ) 5.1 Hz), 3.83
(t, 3H, J ) 4.4 Hz), 3.71-3.66 (m, 5H), 2.88 (s, 3H), 1.68-1.48
(m, 6H), 1.43-1.25 (m, 6H), 1.15-1.02 (m, 6H), 0.91 (t, 9H, J )
7.1 Hz). 13C NMR δ 166.8, 149.1, 145.4, 143.6, 127.8, 127.7, 127.0,
123.8, 121.2, 112.6, 85.0, 81.6, 71.1, 70.9, 70.8, 70.5, 70.1, 65.0,
30.8, 29.5, 29.3, 29.1, 28.1, 27.5, 26.9, 13.9, 13.4, 13.3, 9.9, 6.6,
6.4. HRMS calcd for C32H51FN2O3Sn (M+), 650.2906; found,
650.2894.
(E)-tert-Butyl 4-(2-(6-(2-(2-(2-Fluoroethoxy)ethoxy)ethoxy)-
5-(tributylstannyl)pyridin-3-yl)vinyl)phenyl(methyl)carbam-
ate (12c). Compound 12c was prepared from 11c (0.072 g, 0.133
mmol) as a white viscous liquid (0.077 g, 77% yield). 1H NMR δ
8.14 (d, 1H, J ) 2.5 Hz), 7.83 (d, 1H, J ) 2.5 Hz), 7.46 (d, 2H,
J ) 8.6 Hz), 7.23 (d, 2H, J ) 8.5 Hz), 6.96 (s, 2H), 4.70-4.66 (m,
1H), 4.49-4.42 (m, 3H), 3.86-3.66 (m, 8H), 3.28 (s, 3H), 1.80-
1.02 (m, 27H), 0.90 (t, 9H, J ) 7.1 Hz). 13C NMR δ 167.3, 146.1,
143.8, 143.2, 134.6, 127.0, 126.8, 126.6, 125.7, 125.4, 124.1, 85.0,
81.6, 80.6, 71.1, 70.9, 70.8, 70.5, 70.0, 65.1, 37.4, 29.5, 29.3, 29.1,
28.1, 27.5, 26.9, 13.9, 13.4, 9.9, 6.4. HRMS calcd for C37H59FN2O5-
Sn (M+), 750.343; found, 750.3425.
(E)-2-(3-Bromo-5-(4-(methylamino)styryl)pyridin-2-yloxy)-
ethanol (14b). Compound 14b was prepared from 4-methylami-
nostyrene (0.140 g, 1.05 mmol) and 10 (0.241 g, 0.7 mmol) as a
1
light yellow viscous liquid (0.149 g, 61% yield). H NMR δ 8.07
(d, 1H, J ) 2.1 Hz), 8.03 (d, 1H, J ) 2.1 Hz), 7.35 (d, 2H, J ) 8.6
Hz), 6.93 (d, 1H, J ) 16.3 Hz), 6.74 (d, 1H, J ) 16.3 Hz), 6.61 (d,
2H, J ) 8.6 Hz), 4.57-4.52 (m, 2H), 3.99 (br s, 2H), 3.18 (br s,
1H), 2.88 (s, 3H). 13C NMR δ 149.6, 143.3, 138.5, 130.1, 130.0,
128.0, 126.0, 118.6, 112.6, 107.7, 69.8, 62.2, 30.7. HRMS calcd
for C17H19BrN2O2 (M+), 348.0473; found, 348.0468.
(E)-4-(2-(5-Bromo-6-(2-hydroxyethoxy)pyridin-3-yl)vinyl)-
phenyl Acetate (14d). Compound 14d was prepared from 4-ac-
etoxystyrene (0.130 g, 0.80 mmol) and 10 (0.244 g, 0.7 mmol) as
1
a white viscous liquid (0.031 g, 12% yield). H NMR δ 8.12 (d,
1H, J ) 2.1 Hz), 8.08 (d, 1H, J ) 2.1 Hz), 7.50 (d, 2H, J ) 6.8
Hz), 7.11 (d, 2H, J ) 6.8 Hz), 6.95 (d, 2H, J ) 5.2 Hz), 4.58-
4.54 (m, 2H), 4.01 (br s, 2H), 3.08 (br s, 1H), 2.32 (s, 3H).
(E)-4-(2-(5-Bromo-6-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)py-
ridin-3-yl)vinyl)phenol (11e). Acetate 11d (0.145 g, 0.31 mmol)
and K2CO3 (0.064 g, 0.465 mmol) were placed in EtOH/THF (5
mL/5 mL), and the reaction mixture was stirred at rt for 2 h. After
standard workup with EtOAc, the crude product was purified by
(E)-4-(2-(6-(2-(2-(2-Fluoroethoxy)ethoxy)ethoxy)-5-(tributyl-
stannyl)pyridin-3-yl)vinyl)phenol (12e). Compound 12e was
prepared from 11e (0.032 g, 0.075 mmol) as a white viscous liquid
1
(0.040 g, 84% yield). H NMR δ 8.11 (d, 1H, J ) 2.5 Hz), 7.82
(d, 1H, J ) 2.5 Hz), 7.39 (d, 2H, J ) 8.6 Hz), 6.98-6.74 (m, 4H),
5.19 (br s, 1H), 4.71-4.66 (m, 1H), 4.48-4.43 (m, 3H), 3.90-
3.62 (m, 8H), 1.70-1.02 (m, 18H), 0.91 (t, 9H, J ) 7.1 Hz). 13C
NMR δ 166.9, 156.0, 145.4, 144.0, 130.1, 127.9, 127.6, 127.4,
124.3, 123.0, 115.9, 85.0, 81.6, 71.0, 70.9, 70.7, 70.5, 70.0, 65.2,
29.5, 29.3, 29.1, 28.0, 27.5, 26.9, 13.9, 13.4, 13.3, 9.9, 6.6, 6.4.
HRMS calcd for C31H48FNO4Sn (M+), 637.2589; found, 637.2573.
(E)-2-(5-(4-(Dimethylamino)styryl)-3-(tributylstannyl)-pyri-
din-2-yloxy)ethanol (15a). Compound 15a was prepared from 14a
(0.100 g, 0.275 mmol) as a light yellow oil (0.105 g, 66% yield).
1H NMR δ 8.10 (d, 1H, J ) 2.5 Hz), 7.85 (d, 1H, J ) 2.4 Hz),
7.41 (d, 2H, J ) 8.7 Hz), 6.95 (d, 1H, J ) 16.3 Hz), 6.81 (d, 1H,
J ) 16.6 Hz), 6.73 (d, 2H, J ) 8.8 Hz), 4.48-4.44 (m, 2H), 3.96-
3.92 (m, 2H), 2.99 (s, 6H), 1.68-1.01 (m, 18H), 0.92 (t, 9H, J )
7.2 Hz). 13C NMR δ 166.6, 150.1, 144.5, 144.1, 128.2, 128.1, 127.4,
125.6, 124.0, 120.5, 112.4, 69.4, 63.0, 40.4, 29.0, 27.2, 13.6, 9.8.
HRMS calcd for C29H46N2O2Sn (M+), 574.2581; found, 574.2584.
1
PTLC to give 11e as a white solid (0.128 g, 97%). H NMR δ
8.07 (d, 1H, J ) 2.1 Hz), 7.99 (d, 1H, J ) 2.1 Hz), 7.35 (d, 2H,
J ) 8.6 Hz), 6.96-6.74 (m, 4H), 5.22 (br s, 1H), 4.69 (t, 1H, J )
4.2 Hz), 4.54 (t, 2H, J ) 4.8 Hz), 4.45 (t, 1H, J ) 4.2 Hz), 3.94-
3.68 (m, 8H). 13C NMR δ 158.5, 156.4, 143.6, 138.2, 129.2, 129.0,
127.9, 120.7, 116.0, 107.6, 84.9, 81.6, 71.1, 71.0, 70.8, 70.4, 69.6,
66.8. HRMS calcd for C19H21BrFNO4 (M+), 425.0638; found,
425.0651.
(E)-4-(2-(5-Bromo-6-(2-hydroxyethoxy)pyridin-3-yl)vinyl)-
phenol (14e). In a similar procedure as described in the preparation
of 11e, compound 14e was prepared from acetate 14d (0.031 g,
1
0.082 mmol) as a white solid (0.020 g, 73%). H NMR (DMSO-
d6) δ 9.60 (br s, 1H), 8.31 (s, 1H), 8.23 (s, 1H), 7.39 (d, 2H, J )
8.3 Hz), 7.19 (d, 1H, J ) 16.8 Hz), 6.94 (d, 1H, J ) 16.6 Hz),
6.77 (d, 2H, J ) 8.3 Hz), 4.35 (t, 2H, J ) 5.1 Hz), 3.73 (t, 2H, J
) 5.1 Hz). 13C NMR (DMSO-d6) δ 157.9, 157.4, 143.7, 138.1,

Styrylpyridines as Probes of Amyloid Plaques
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 9 2163
(E)-2-(5-(4-(Methylamino)styryl)-3-(tributylstannyl)-pyridin-
2-yloxy)ethanol (15b). Compound 15b was prepared from 14b
(0.052 g, 0.15 mmol) as a light yellow oil (0.059 g, 64% yield).
1H NMR δ 8.08 (d, 1H, J ) 2.5 Hz), 7.84 (d, 1H, J ) 2.4 Hz),
7.37 (d, 2H, J ) 8.6 Hz), 6.93 (d, 1H, J ) 16.3 Hz), 6.80 (d, 1H,
J ) 16.4 Hz), 6.61 (d, 2H, J ) 8.6 Hz), 4.48-4.43 (m, 2H), 3.95-
3.91 (m, 2H), 2.88 (s, 3H), 1.69-1.01 (m, 18H), 0.91 (t, 9H, J )
7.1 Hz). 13C NMR δ 166.9, 149.2, 144.7, 144.3, 128.4, 128.3, 127.8,
126.7, 124.2, 120.7, 112.6, 69.6, 63.2, 30.8, 29.5, 29.3, 29.1, 28.0,
27.5, 26.9, 13.9, 13.5, 13.4, 10.0, 6.6, 6.5. HRMS calcd for
C28H44N2O2Sn (M+), 560.2425; found, 560.2419.
(m, 2H), 4.01-3.96 (m, 2H), 2.96 (s, 1H), 2.89 (s, 3H). HRMS
calcd for C16H17IN2O2 (M+), 396.0335; found, 396.0335.
(E)-4-(2-(6-(2-Hydroxyethoxy)-5-iodopyridin-3-yl)vinyl)phe-
nol (16e). Compound 16e was prepared from 15e (0.009 g, 0.017
mmol) as a white solid (0.003 g, 50%). 1H NMR δ 8.26 (d, 1H, J
) 2.1 Hz), 8.10 (d, 1H, J ) 2.1 Hz), 7.38 (d, 2H, J ) 8.6 Hz),
6.98-6.73 (m, 4H), 4.55-4.47 (m, 2H), 4.02-3.98 (m, 2H), 3.25
(br s, 1H). HRMS calcd for C16H17IN2O2 (M+), 383.0018; found,
383.0022.
(E)-4-(2-(6-(2-(2-(2-Fluoroethoxy)ethoxy)ethoxy)-5-iodopyri-
din-3-yl)vinyl)-N-methylbenzenamine (13b). To a stirred solution
of 13c (0.014 g, 0.024 mmol) and 2,6-lutidine (28 µL, 0.24 mmol)
in 2 mL of CH2Cl2 at 0 °C was added trimethylsilyl triflate (34
µL, 0.19 mmol). After 15 min, the reaction solution was submitted
to the standard workup with CH2Cl2. The crude product was purified
by PTLC to give a light yellow viscous liquid 13b (0.010 g, 88%).
1H NMR δ 8.22 (d, 1H, J ) 2.1 Hz), 8.10 (d, 1H, J ) 2.1 Hz),
7.34 (d, 2H, J ) 8.6 Hz), 6.91 (d, 1H, J ) 16.3 Hz), 6.70 (d, 1H,
J ) 16.3 Hz), 6.60 (d, 2H, J ) 8.6 Hz), 4.71-4.67 (m, 1H), 4.54-
4.43 (m, 3H), 3.94-3.69 (m, 9H), 2.88 (s, 3H). 13C NMR δ 160.5,
149.5, 144.6, 129.8, 129.7, 128.0, 126.3, 118.9, 112.6, 85.1, 81.7,
80.6, 77.4, 71.3, 71.2, 70.9, 70.5, 69.7, 67.2, 30.8. HRMS calcd
for C20H24FIN2O3 (M+), 486.0816; found, 486.0818.
Radioiodination. Radioiodinated compounds, [125I]13a, 13b,
16a, 16b, and 16e, were prepared via iododestannylation reactions
from the corresponding tributyltin precursors according to the
method described previously.38,56 Hydrogen peroxide (50 µL, 3%
w/v) was added to a mixture of 50 µL of the tributyltin precursor
(4 µg/µL EtOH), 50 µL of 1 N HCl, and [125I]NaI (1-5 mCi
purchased from Perkin-Elmer) in a sealed vial. The reaction was
allowed to proceed for 5-10 min at rt and terminated by addition
of 100 µL of satd NaHSO3. The reaction mixture was extracted
with ethyl acetate (3 × 1 mL) after neutralization with 1.5 mL of
satd sodium bicarbonate solution. The combined extracts were
evaporated to dryness. The residues were dissolved in 100 µL of
EtOH and purified by HPLC using a reversed-phase column
(Phenomenex Gemini C18 analytical column , 4.6 × 250 mm, 5
mm, CH3CN/ammonium formate buffer (1 mM) 8/2 or 7/3; flow
rate 0.5-1.0 mL/min). The no-carrier-added products were evapo-
rated to dryness and redissolved in 100% EtOH (1 µCi/µL) to be
stored at -20 °C up to 6 weeks for animal studies and autorad-
iography studies.
Preparation of Brain Tissue Homogenates. AD postmortem
brain tissues were obtained from University of Washington Alzhe-
imer’s Disease Research Center. The neuropathological diagnosis
was confirmed by current criteria (NIA-Reagan Institute Consensus
Group, 1997). Homogenates were then prepared from dissected gray
matters from four pooled AD patients in phosphate buffered saline
(PBS, pH 7.4) at the concentration of approximately 100 mg wet
tissue/mL (motor-driven glass homogenizer with setting of 6 for
30 s). The homogenates were aliquoted into 1 mL portions and
stored at -70 °C for up to 2 years without loss of binding signal.
(E)-4-(2-(6-(2-Hydroxyethoxy)-5-(tributylstannyl)-pyridin-3-
yl)vinyl)phenol (15e). Compound 15e was prepared from 14e
(0.031 g, 0.092 mmol) as a white viscous liquid (0.012 g, 24%
1
yield). H NMR δ 8.07 (d, 1H, J ) 2.5 Hz), 7.85 (d, 1H, J ) 2.5
Hz), 7.39 (d, 2H, J ) 8.6 Hz), 6.99-6.80 (m, 4H), 5.97 (br s, 1H),
5.01 (br s, 1H), 4.50-4.46 (m, 2H), 3.98-3.94 (m, 2H), 1.69-
1.01 (m, 18H), 0.91 (t, 9H, J ) 7.1 Hz). 13C NMR δ 167.2, 156.0,
144.9, 144.7, 144.5, 130.1, 128.0, 127.96, 124.7, 122.8, 116.0, 69.9,
63.4, 29.9, 29.5, 29.3, 29.1, 28.1, 27.5, 26.9, 13.9, 13.6, 13.5, 10.1,
6.7, 6.6. HRMS calcd for C27H41NO3Sn (M+), 547.2108; found,
547.2112.
General Procedure of the Iodo-Tributyltin Exchange Reac-
tion for the Synthesis of Iodo-Substituted Styryl-Pyridine
Derivatives (13a, 13c, 13e and 16a,b, 16e). (E)-4-(2-(6-(2-(2-(2-
Fluoroethoxy)ethoxy)ethoxy)-5-iodopyridin-3-yl)vinyl)-N,N-dim-
ethylbenzenamine (13a). A solution of iodine (I2, 0.063 g, 0.24
mmol) in THF (2 mL) was added dropwise to an ice bath cooled
solution of 12a (0.114 g, 0.172 mmol) in THF (3 mL). After the
addition, the reaction was stirred at 0 °C for 1 h. Following standard
workup with CH2Cl2, the crude product was purified by FC (EtOAc/
hexanes, 25/75) to give a light yellow solid 13a (0.037 g, 48%).
1H NMR δ 8.22 (d, 1H, J ) 2.1 Hz), 8.10 (d, 1H, J ) 2.1 Hz),
7.38 (d, 2H, J ) 8.8 Hz), 6.92 (d, 1H, J ) 16.3 Hz), 6.72 (d, 1H,
J ) 16.3 Hz), 6.71 (d, 2H, J ) 8.8 Hz), 4.72-4.67 (m, 1H), 4.54-
4.44 (m, 3H), 3.93-3.69 (m, 8H), 3.00 (s, 6H). 13C NMR δ 160.4,
150.5, 144.6, 144.55, 129.8, 129.5, 127.8, 125.3, 118.8, 112.6, 85.1,
81.7, 80.6, 71.3, 71.1, 70.8, 70.5, 69.6, 67.1, 40.6. HRMS calcd
for C21H26FIN2O3 (M+), 500.0972; found, 500.0959.
(E)-tert-Butyl 4-(2-(6-(2-(2-(2-Fluoroethoxy)ethoxy)ethoxy)-
5-iodopyridin-3-yl)vinyl)phenyl(methyl)carbamate (13c). Com-
pound 13c was prepared from 12c (0.024 g, 0.032 mmol) as a white
1
viscous liquid (0.018 g, 98%). H NMR δ 8.25 (d, 1H, J ) 1.6
Hz), 8.13 (d, 1H, J ) 1.6 Hz), 7.44 (d, 2H, J ) 8.4 Hz), 7.24 (d,
2H, J ) 8.4 Hz), 6.97 (d, 1H, J ) 16.4 Hz), 6.86 (d, 1H, J ) 16.4
Hz), 4.69 (t, 1H, J ) 4.1 Hz), 4.53 (t, 2H, J ) 4.8 Hz), 4.45 (t,
1H, J ) 4.1 Hz), 3.94-3.69 (m, 8H), 3.28 (s, 3H), 1.47 (s, 9H).
13C NMR δ 161.0, 154.8, 145.3, 144.9, 143.7, 133.9, 128.9, 128.6,
126.8, 125.7, 123.1, 85.1, 81.7, 80.7, 77.4, 71.3, 71.1, 70.9, 70.5,
69.6, 67.2, 37.4, 28.6. HRMS calcd for C21H26FIN2O3 (M+),
500.0972; found, 500.0959.
(E)-4-(2-(6-(2-(2-(2-Fluoroethoxy)ethoxy)ethoxy)-5-iodopyri-
din-3-yl)vinyl)phenol (13e). Compound 13e was prepared from
Binding Studies. Ligand [125I]4 with 2200 Ci/mmol specific
activity and greater than 95% radiochemical purity was prepared
using the standard iododestannylation reaction and purified by a
simplified C-4 mini-column, as described previously.38 Competition
binding assays were carried out in 12 × 75 mm borosilicate glass
tubes. The reaction mixture contained 50 mL of pooled AD brain
homogenates (20-50 mg), 50 mL of [125I]4 (0.04-0.06 nM diluted
in PBS), and 50 mL of inhibitors (10-5-10-10 M diluted serially
in PBS containing 0.1% bovine serum albumin) in a final volume
of 1 mL. Nonspecific binding was defined in the presence of 600
nM 4 in the same assay tubes. The mixture was incubated at 37 °C
for 2 h and the bound and the free radioactivity were separated by
vacuum filtration through Whatman GF/B filters using a Brandel
M-24R cell harvester followed by 2 × 3 mL washes of PBS at rt.
Filters containing the bound 125I ligand were counted in a gamma
counter (Packard 5000) with 70% counting efficiency. Under the
assay conditions, the non-specifically bound fraction was less than
15% of the total radioactivity. The results of inhibition experiments
1
12e (0.012 g, 0.019 mmol) as a white solid (0.008 g, 90%). H
NMR δ 8.21 (d, 1H, J ) 2.1 Hz), 8.08 (d, 1H, J ) 2.1 Hz), 7.33
(d, 2H, J ) 8.6 Hz), 6.94-6.69 (m, 4H), 4.71-4.67 (m, 1H), 4.53-
4.43 (m, 3H), 3.94-3.69 (m, 8H). HRMS calcd for C19H21FINO4
(M+), 473.0499; found, 473.0498.
(E)-2-(5-(4-(Dimethylamino)styryl)-3-iodopyridin-2-yloxy)-
ethanol (16a). Compound 16a was prepared from 15a (0.011 g,
1
0.019 mmol) as a light yellow solid (0.004 g, 50%). H NMR δ
8.25 (s, 1H), 8.10 (s, 1H), 7.39 (d, 2H, J ) 8.6 Hz), 6.94 (d, 1H,
J ) 16.3 Hz), 6.76-6.70 (m, 3H), 4.51 (t, 2H, J ) 4.2 Hz), 4.02-
3.95 (m, 2H), 3.19 (s, 1H), 3.00 (s, 6H). HRMS calcd for C17H19-
IN2O2 (M+), 410.0491; found, 410.0489.
(E)-2-(3-Iodo-5-(4-(methylamino)styryl)pyridin-2-yloxy)etha-
nol (16b). Compound 16b was prepared from 15b (0.032 g, 0.057
mmol) as a light yellow solid (0.005 g, 21%). 1H NMR δ 8.24 (d,
1H, J ) 2.1 Hz), 8.09 (d, 1H, J ) 2.0 Hz), 7.36 (d, 2H, J ) 8.5
Hz), 6.92 (d, 1H, J ) 16.3 Hz), 6.76-6.64 (m, 3H), 4.53-4.49

2164 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 9
Qu et al.
(8) Hardy, J.; Selkoe, D. J. The amyloid hypothesis of Alzheimer’s
disease: Progress and problems on the road to therapeutics. Science
2002, 297, 353-356.
(9) Rosenberg, R. N. Explaining the cause of the amyloid burden in
Alzheimer disease. Arch. Neurol. 2002, 59, 1367-1368.
(10) Thal, D. R.; Capetillo-Zarate, E.; Del, Tredici, K.; Braak, H. The
development of amyloid beta protein deposits in the aged brain. Sci.
Aging Knowledge EnViron. 2006, 2006, re1.
(11) Blennow, K.; de Leon, M. J.; Zetterberg, H. Alzheimer’s disease.
Lancet 2006, 368, 387-403.
(12) Klunk, W. E.; Lopresti, B. J.; Ikonomovic, M. D.; Lefterov, I. M.;
Koldamova, R. P.; Abrahamson, E. E.; Debnath, M. L.; Holt, D. P.;
Huang, G. F.; Shao, L.; DeKosky, S. T.; Price, J. C.; Mathis, C. A.
Binding of the positron emission tomography tracer Pittsburgh
compound-B reflects the amount of amyloid-beta in Alzheimer’s
disease brain but not in transgenic mouse brain. J. Neurosci. 2005,
25, 10598-10606.
(13) Lopresti, B. J.; Klunk, W. E.; Mathis, C. A.; Hoge, J. A.; Ziolko, S.
K.; Lu, X.; Meltzer, C. C.; Schimmel, K.; Tsopelas, N. D.; Dekosky,
S. T.; Price, J. C. Simplified quantification of Pittsburgh compound
B amyloid imaging PET studies: A comparative analysis. J. Nucl.
Med. 2005, 46, 1959-1972.
(14) Mathis, C. A.; Klunk, W. E.; Price, J. C.; DeKosky, S. T. Imaging
technology for neurodegenerative diseases: Progress toward detection
of specific pathologies. Arch. Neurol. 2005, 62, 196-200.
(15) Price, J. C.; Klunk, W. E.; Lopresti, B. J.; Lu, X.; Hoge, J. A.; Ziolko,
S. K.; Holt, D. P.; Meltzer, C. C.; DeKosky, S. T.; Mathis, C. A.
Kinetic modeling of amyloid binding in humans using PET imaging
and Pittsburgh compound B. J. Cereb. Blood Flow Metab. 2005, 25,
1528-1547.
(16) Ziolko, S. K.; Weissfeld, L. A.; Klunk, W. E.; Mathis, C. A.; Hoge,
J. A.; Lopresti, B. J.; DeKosky, S. T.; Price, J. C. Evaluation of
voxel-based methods for the statistical analysis of PIB PET amyloid
imaging studies in Alzheimer’s disease. Neuroimage 2006, 33, 94-
102.
were subjected to nonlinear regression analysis using equilibrium
binding data analysis which Ki values were calculated.
In Vitro Autoradiography. To compare different probes using
similar sections of human brain tissue, human macro-array brain
sections from six confirmed AD cases and one control subject were
assembled. The presence and localization of plaques on the sections
was confirmed with immunohistochemical staining with monoclonal
Aâ antibody 4G8 (Sigma). The sections were incubated with [125I]-
tracers (200 000-250 000 cpm/200 mL) for 1 h at rt. The sections
were then dipped in satd lithium carbonate in 40% EtOH (two 2
min washes) and washed with 40% EtOH (one 2 min wash),
followed by rinsing with water for 30 s. After drying, the 125I-
labeled sections were exposed to Kodak Biomax MR film
overnight.
Organ Distribution in Normal Mice. While under isoflurane
anesthesia, 0.15 mL of a 0.1% bovine serum albumin solution
containing [125I]tracers (5-10 µCi) was injected directly into the
tail vein of ICR mice (22-25 g, male). The mice (n ) 3 for each
time point) were sacrificed by cervical dislocation at designated
time points postinjection. The organs of interest were removed and
weighed, and the radioactivity was counted with an automatic
gamma counter. The percentage dose per organ was calculated by
a comparison of the tissue counts to suitably diluted aliquots of
the injected material. The total activity of the blood was calculated
under the assumption that it is 7% of the total body weight. The %
dose/g of samples was calculated by comparing the sample counts
with the count of the diluted initial dose.
Partition Coefficient. Partition coefficients were measured by
mixing the [125I]tracer with 3 g 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 rt, followed by centrifugation for 5 min. Two weighed
samples (0.5 g 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/g of 1-octanol to that of buffer.
Samples from the 1-octanol layer were repartitioned until consistent
partitions of coefficient values were obtained. The measurement
was done in triplicate and repeated three times.
(17) Edison, P.; Archer, H. A.; Hinz, R.; Hammers, A.; Pavese, N.; Tai,
Y. F.; Hotton, G.; Cutler, D.; Fox, N.; Kennedy, A.; Rossor, M.;
Brooks, D. J. Amyloid, hypometabolism, and cognition in Alzheimer
disease. An [11C]PIB and [18F]FDG PET study. Neurology 2007,
68, 501-508.
(18) Johnson, K. A. Amyloid imaging of Alzheimer’s disease using
Pittsburgh compound B. Curr. Neurol. Neurosci. Rep. 2006, 6, 496-
503.
(19) Engler, H.; Forsberg, A.; Almkvist, O.; Blomquist, G.; Larsson, E.;
Savitcheva, I.; Wall, A.; Ringheim, A.; Langstrom, B.; Nordberg,
A. Two-year follow-up of amyloid deposition in patients with
Alzheimer’s disease. Brain 2006, 129, 2856-2866.
(20) Kemppainen, N. M.; Aalto, S.; Wilson, I. A.; Nagren, K.; Helin, S.;
Bruck, A.; Oikonen, V.; Kailajarvi, M.; Scheinin, M.; Viitanen, M.;
Parkkola, R.; Rinne, J. O. Voxel-based analysis of PET amyloid
ligand [11C]PIB uptake in Alzheimer disease. Neurology 2006, 67,
1575-1580.
Acknowledgment. This work was supported by grants from
the National Institutes of Health (AG-021868 to M.P.K). The
authors thank the Pathology Core laboratories at Childrens
Hospital of Philadelphia for assembling the human macro-array
brain sections. The authors also thank Drs. Daniel Skovronsky
and Rajesh Manchanda for their helpful discussion.
(21) Mintun, M. A.; Larossa, G. N.; Sheline, Y. I.; Dence, C. S.; Lee, S.
Y.; Mach, R. H.; Klunk, W. E.; Mathis, C. A.; DeKosky, S. T.;
Morris, J. C. [11C]PIB in a nondemented population: Potential
antecedent marker of Alzheimer disease. Neurology 2006, 67, 446-
452.
(22) Nordberg, A. PET imaging of amyloid in Alzheimer’s disease. Lancet
Neurol. 2004, 3, 519-527.
Supporting Information Available: The procedures for syn-
thesizing some intermediates, elemental analysis results for some
bromo-, tributyltin-, and iodo-substituted styryl-pyridine derivatives,
and HPLC purity analysis data for all bio-assay involved compounds
in two different HPLC systems. This material is available free of
charge via the Internet at http://pubs.acs.org.
(23) Buckner, R. L.; Snyder, A. Z.; Shannon, B. J.; LaRossa, G.; Sachs,
R.; Fotenos, A. F.; Sheline, Y. I.; Klunk, W. E.; Mathis, C. A.; Morris,
J. C.; Mintun, M. A. Molecular, structural, and functional charac-
terization of Alzheimer’s disease: evidence for a relationship between
default activity, amyloid, and memory. J. Neurosci. 2005, 25, 7709-
7717.
(24) Rentz, D. M.; Becker, J. A.; Moran, E.; Sperling, R. A.; Manning,
L.; Bonab, A.; Mathis, C. A.; Klunk, W.; Fischman, A. J.; Johnson,
K. A. Amyloid imaging in AD, MCI, and highly intelligent older
adults with Pittsburgh compound B (PIB). J. Nucl. Med. 2006, 289p
(abstract).
(25) Price, J. C.; Ziolko, S. K.; Weissfeld, L. A.; William, K. E.; Bi, W.;
Hoge, J. A.; Lopresti, B. J.; DeKosky, S. T.; Mathis, C. A. [O-15]
Water and PIB PET imaging in Alzheimer’s disease and mild
cognitive impairment. J. Nucl. Med. 2006, 75p (abstract).
(26) Villemagne, V. L.; Ng, S.; Gong, S. J.; Ackermann, U.; Pike, K.;
Savage, G.; Klunk, W.; Mathis, C.; Masters, C. L.; Rowe, C. C. 11C-
PIB PET imaging in the differential diagnosis of dementia. J. Nucl.
Med. 2006, 74p (abstract).
(27) Verhoeff, N. P.; Wilson, A. A.; Takeshita, S.; Trop, L.; Hussey, D.;
Singh, K.; Kung, H. F.; Kung, M.-P.; Houle, S. In vivo imaging of
Alzheimer disease beta-amyloid with [11C]SB-13 PET. Am. J.
Geriatr. Psychiatry 2004, 12, 584-595.
References
(1) Tsuboi, Y. Neuropathology of familial tauopathy. Neuropathology
2006, 26, 471-474.
(2) Roberson, E. D.; Mucke, L. 100 years and counting: Prospects for
defeating Alzheimer’s disease. Science 2006, 314, 781-784.
(3) Goedert, M.; Spillantini, M. G. A century of Alzheimer’s disease.
Science 2006, 314, 777-781.
(4) Hasegawa, M. Biochemistry and molecular biology of tauopathies.
Neuropathology 2006, 26, 484-490.
(5) Braak, H.; Alafuzoff, I.; Arzberger, T.; Kretzschmar, H.; Del Tredici,
K.; Staging of Alzheimer disease-associated neurofibrillary pathology
using paraffin sections and immunocytochemistry. Acta Neuropathol.
2006, 112, 389-404.
(6) Jicha, G. A.; Parisi, J. E.; Dickson, D. W.; Johnson, K.; Cha, R.;
Ivnik, R. J.; Tangalos, E. G.; Boeve, B. F.; Knopman, D. S.; Braak,
H.; Petersen, R. C. Neuropathologic outcome of mild cognitive
impairment following progression to clinical dementia. Arch. Neurol.
2006, 63, 674-81.
(7) Catalano, S. M.; Dodson, E. C.; Henze, D. A.; Joyce, J. G.; Krafft,
G. A.; Kinney, G. G. The role of amyloid-beta derived diffusible
ligands (ADDLs) in Alzheimer’s disease. Curr. Top. Med. Chem.
2006, 6, 597-608.

Styrylpyridines as Probes of Amyloid Plaques
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 9 2165
(28) Small, G. W. Diagnostic issues in dementia: neuroimaging as a
surrogate marker of disease. J. Geriatr. Psychiatry Neurol. 2006,
19, 180-185.
(43) Zhang, W.; Kung, M. P.; Oya, S.; Hou, C.; Kung, H. F. F-18 labeled
styrylpyridines as PET agents for amyloid plaque imaging. Nucl. Med.
Biol. 2007, in press.
(29) Kepe, V.; Huang, S. C.; Small, G. W.; Satyamurthy, N.; Barrio, J.
R. Visualizing pathology deposits in the living brain of patients with
Alzheimer’s disease. Methods Enzymol. 2006, 412, 144-160.
(30) Shoghi-Jadid, K.; Barrio, J. R.; Kepe, V.; Wu, H. M.; Small, G. W.;
Phelps, M. E.; Huang, S. C. Imaging beta-amyloid fibrils in
Alzheimer’s disease: A critical analysis through simulation of
amyloid fibril polymerization. Nucl. Med. Biol. 2005, 32, 337-351.
(31) Zhuang, Z.-P.; Kung, M.-P.; Hou, C.; Skovronsky, D.; Gur, T. L.;
Trojanowski, J. Q.; Lee, V. M.-Y.; Kung, H. F. Radioiodinated
styrylbenzenes and thioflavins as probes for amyloid aggregates. J.
Med. Chem. 2001, 44, 1905-1914.
(32) Zhuang, Z.-P.; Kung, M.-P.; Hou, C.; Plossel, K.; Skovronsky, D.;
Gur, T. L.; Trojanowski, J. Q.; Lee, V. M.-Y.; Kung, H. F. IBOX-
(2-(4′-dimethylaminophenyl)-6-iodobenzoxazole): A ligand for imag-
ing amyloid plaques in the brain. Nucl. Med. Biol. 2001, 28, 887-
894.
(33) Wang, Y.; Klunk, W. E.; Debnath, M. L.; Huang, G. F.; Holt, D. P.;
Shao, L.; Mathis, C. A. Development of a PET/SPECT agent for
amyloid imaging in Alzheimer’s disease. J. Mol. Neurosci. 2004,
24, 55-62.
(34) Kung, H. F.; Lee, C.-W.; Zhuang, Z. P.; Kung, M. P.; Hou, C.; Plossl,
K. Novel stilbenes as probes for amyloid plaques. J. Am. Chem. Soc.
2001, 123, 12740-12741.
(35) Lee, C.-W.; Kung, M.-P.; Hou, C.; Kung, H. F. Dimethylamino-
fluorenes: Ligands for detecting â-amyloid plaques in the brain. Nucl.
Med. Biol. 2003, 30, 573-580.
(36) Zhuang, Z. P.; Kung, M. P.; Wilson, A.; Lee, C. W.; Plossl, K.;
Hou, C.; Holtzman, D. M.; Kung, H. F. Structure-activity relation-
ship of imidazo[1,2-a]pyridines as ligands for detecting beta-amyloid
plaques in the brain. J. Med. Chem. 2003, 46, 237-243.
(37) Kung, M.-P.; Hou, C.; Zhuang, Z.-P.; Skovronsky, D.; Kung, H. F.
Binding of two potential imaging agents targeting amyloid plaques
in postmortem brain tissues of patients with Alzheimer’s disease.
Brain Res. 2004, 1025, 89-105.
(38) Kung, M.-P.; Hou, C.; Zhuang, Z.-P.; Cross, A. J.; Maier, D. L.;
Kung, H. F. Characterization of IMPY as a potential imaging agent
for â-amyloid plaques in double transgenic PSAPP mice. Eur. J. Nucl.
Med. Mol. Imaging 2004, 31, 1136-1145.
(39) Newberg, A. B.; Wintering, N. A.; Plossl, K.; Hochold, J.; Stabin,
M. G.; Watson, M.; Skovronsky, D.; Clark, C. M.; Kung, M. P.;
Kung, H. F. Safety, biodistribution, and dosimetry of 123I-IMPY:
A novel amyloid plaque-imaging agent for the diagnosis of Alzhe-
imer’s disease. J. Nucl. Med. 2006, 47, 748-754.
(40) Newberg, A. B.; Wintering, N. A.; Clark, C. M.; Plossl, K.;
Skovronsky, D.; Seibyl, J. P.; Kung, M. P.; Kung, H. F. Use of 123I
IMPY SPECT to differentiate Alzheimer’s disease from controls. J.
Nucl. Med. 2006, 47, p. 78 (Abstract # 220).
(44) Ono, M.; Haratake, M.; Nakayama, M.; Kaneko, Y.; Kawabata, K.;
Mori, H.; Kung, M. P.; Kung, H. F. Synthesis and biological
evaluation of (E)-3-styrylpyridine derivatives as amyloid imaging
agents for Alzheimer’s disease. Nucl. Med. Biol. 2005, 32, 329-
35.
(45) Meana, A.; Rodriguez, J. F.; Sanz-Tejedor, M. A.; Garcia-Ruano, J.
L. Efficient regioselective preparation of monobromo- and bro-
moiodo-hydroxypyridines from dibromo derivatives via bromine-
lithium exchange. Synlett 2003, 1678-1682.
(46) Comins, D. L.; Jianhua, G. N-vs. O-alkylation in the Mitsunobu
reaction of 2-pyridone. Tetrahedron Lett. 1994, 35, 2819-2822.
(47) Cunico, R. F.; Bedell, L. The triisopropylsilyl group as a hydroxyl-
protecting function. J. Org. Chem. 1980, 45, 4797-4798.
(48) Jeffery, T.; Ferber, B. One-pot palladium-catalyzed highly chemo-,
regio-, and stereoselective synthesis of trans-stilbene derivatives. A
concise and convenient synthesis of resveratrol. Tetrahedron Lett.
2002, 44, 193-197.
(49) Jeffery, T. On the efficiency of tetraalkylammonium salts in Heck
type reactions. Tetrahedron 1996, 52, 10113-10130.
(50) Hervet, M.; Thery, I.; Gueiffier, A.; Enguehard-gueiffier, C. Com-
parative study on the reactivity of 6-haloimidazo[1,2-a]pyridine
derivatives towards Negishi- and Stille-coupling reactions. HelV.
Chim. Acta 2003, 86, 3461-3469.
(51) Wnuk, S. F.; Yuan, C.-S.; Borchardt, R. T.; Balzarini, J.; De Clercq,
E.; Robins, M. J. Nucleic acid-related compounds. 84. Synthesis of
6′-(E and Z)-halohomovinyl derivatives of adenosine, inactivation
of S-adenosyl-L-homocysteine hydrolase, and correlation of anticancer
and antiviral potencies with enzyme inhibition. J. Med. Chem. 1994,
37, 3579-3587.
(52) Bastiaans, H. M. M.; van der Baan, J. L.; Ottenheijm, H. C. J. Flexible
and convergent total synthesis of cyclotheonamide B. J. Org. Chem.
1997, 62, 3880-3889.
(53) Ono, M.; Wilson, A.; Nobrega, J.; Westaway, D.; Verhoeff, P.;
Zhuang, Z. P.; Kung, M. P.; Kung, H. F. 11C-Labeled stilbene
derivatives as Aâ-aggregate-specific PET imaging agents for Alzhe-
imer’s disease. Nucl. Med. Biol. 2003, 30, 565-571.
(54) Mathis, C. A.; Wang, Y.; Holt, D. P.; Huang, G.-f.; Debnath, M. L.;
Klunk, W. E. Synthesis and evaluation of 11C-labeled 6-substituted
2-arylbenzothiazoles as amyloid imaging agents. J. Med. Chem. 2003,
46, 2740-2754.
(55) Mathis, C. A.; Wang, Y.; Klunk, W. E. Imaging â-amyloid plaques
and neurofibrillary tangles in the aging human brain. Curr. Pharm.
Des. 2004, 10, 1469-1492.
(56) Kung, M. P.; Hou, C.; Oya, S.; Mu, M.; Acton, P. D.; Kung, H. F.
Characterization of [123I]IDAM as a novel single-photon emission
tomography tracer for serotonin transporters. Eur. J. Nucl. Med. 1999,
26, 844-853.
(41) Zhang, W.; Oya, S.; Kung, M. P.; Hou, C.; Maier, D. L.; Kung, H.
F. F-18 Stilbenes as PET imaging agents for detecting beta-amyloid
plaques in the brain. J. Med. Chem. 2005, 48, 5980-5988.
(42) Zhang, W.; Oya, S.; Kung, M. P.; Hou, C.; Maier, D. L.; Kung, H.
F. F-18 PEG stilbenes as PET imaging agents targeting Aâ aggregates
in the brain. Nucl. Med. Biol. 2005, 32, 799-809.
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