Quick assembly of 1,4-diphenyltriazoles as probes targeting β-amyloid aggregates in Alzheimer's disease

Article abstract of DOI:10.1021/jm070467l

Accumulation of β-amyloid aggregates (Aβ) in the brain is linked to the pathogenesis of Alzheimer's disease (AD). We report a novel approach for producing 1,4-diphenyltriazoles as probes for targeting Aβ aggregates in the brain. The imaging probes, a seri

Full text of DOI:10.1021/jm070467l

3380
J. Med. Chem. 2007, 50, 3380-3387
Quick Assembly of 1,4-Diphenyltriazoles as Probes Targeting â-Amyloid Aggregates in
Alzheimer’s Disease
Wenchao Qu,† Mei-Ping Kung,† Catherine Hou,† Shunichi Oya,† and Hank F. Kung*,†,‡
Departments of Radiology and Pharmacology, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104
ReceiVed April 19, 2007
Accumulation of â-amyloid aggregates (Aâ) in the brain is linked to the pathogenesis of Alzheimer’s disease
(AD). We report a novel approach for producing 1,4-diphenyltriazoles as probes for targeting Aâ aggregates
in the brain. The imaging probes, a series of substituted tricyclic 1,4-diphenyltriazoles showing excellent
binding affinities to Aâ aggregates (Ki ) 4-30 nM), were conveniently assembled by “click chemistry.”
Two radioiodinated probes, [125I]10a and [125I]10b, and two radiofluorinated probes, [18F]17a and [18F]17b,
exhibited moderate lipophilicities and showed excellent initial brain penetrations and fast washouts from
the normal mouse brain. In vitro autoradiography of postmortem AD brain sections and homogenates showed
that these triazoles were binding to Aâ plaques. Preliminary results strongly suggest that use of click chemistry,
which led to a 1,4-diphenyltriazole-based core, is a highly convenient and flexible approach for assembling
novel imaging agents for targeting Aâ aggregates in senile plaques in the living human brain.
Introduction
or p-N,N-dimethylaminophenyl- group (Figure 2).22 Some of
these diphenylthiophene compounds displayed excellent binding
affinities (ranging from 3.9 to 31 nM, depending on the
substitution patterns on the phenyl ring). The fluoroethyl-
substituted 2,5-diphenylthiophene, 5c, showed excellent binding
affinity. However, due to the high lipophilicity associated with
thiophenes, they were not evaluated further. To reduce the
lipophilicity, we modified the thiophene ring by substituting
the ring with triazoles, which can be quickly assembled by “click
chemistry”. The use of “click chemistry”, modified Huisgen
condition,23 to quickly assemble various interesting molecules
has attracted a lot of interest as an approach for drug
development.24-30 Click chemistry reactions, which commonly
employ a Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC),
have several distinctive advantages, including use of modular
and fragment-based reagents, wide scope, stereospecificity, easy
separation, and high yield.31-33 Resveratrol, 6, is a natural
antioxidant normally present in red wine, which may modify
the metabolism of lipoproteins and may have beneficial effects
of antioxidants.29 A group of compounds, 7, triazole-modified
analogues of 6, were prepared by click chemistry and tested
(Figure 2). This quick assembly method is particularly suited
for generating diversified substitution groups and developing
novel agents for studying biologically important processes and
targeted binding sites. The reaction shows great promise for
discovery applications because it combines the synthesis and
screening of libraries, which can be done by microfluidics.24,34
Alzheimer’s disease (AD)a is a neurodegenerative disorder
of the brain predominantly affecting the older population.
Diagnosis of this disease is based on neurological observations,
which are often difficult and unreliable. There is an urgent unmet
need for specific probes targeting markers in the brain, which
can facilitate the diagnosis of this disease. It is generally believed
that the accumulation of â-amyloid (Aâ) aggregates (major
protein aggregates of senile plaques) in the brain is the hallmark
of AD.1-3 Probes targeting these Aâ aggregates in the brain
may greatly facilitate the diagnosis of AD. Indeed, radionuclide-
labeled agents targeting the Aâ aggregates have already been
developed.4,5 A diverse group of core structures (Figure 1)
have demonstrated high in vivo and in vitro binding to the Aâ
aggregates present in the AD brain, including 4-N-methyl-
amino-4′-hydroxystilbene (1, SB-13),6,7 6-iodo-2-(4′-dimethy-
lamino)phenyl-imidazo[1,2-a]pyridine (2, IMPY),8,9 and
2-(4′-methylaminophenyl)-6-hydroxybenzothiazole (3, PIB).10,11
2-[1-(6-[(2-Fluoroethyl)(methyl)amino]naphthalen-2-yl)ethylidene]-
malononitrile (4, FDDNP)12,13 binds to both plaques and tangles
(τ aggregates). Additional core structures have been
reported,5,10,14-21 suggesting that there are a number of potential
small molecules that can fit into the binding pockets of Aâ
aggregates in the senile plaques of AD patients. It is generally
assumed that the p-N-methyl- or p-N,N-dimethylaminophenyl-
group is critical for successful binding affinity. Modifications
of the phenyl group on the other end of structures 1-3 are
tolerated and may not alter the binding affinity.
Recently, “hot click reactions” have been applied in the
preparation of radiopharmaceuticals in situ.35-37 In these reports,
radiolabeling fragments were “clicked” to form the F-18 labeled
radiopharmaceuticals. Besides, the click reaction have also been
utilized to assemble the chelates, which were further used for
the radiolabeling.38,39 Herein we used “cold” click chemistry
to assemble a novel series of diphenyltriazoles as probes for
the investigation of specific binding site(s) on Aâ protein
aggregates. Following a “cold” click approach, several diphe-
nyltriazole derivatives as analogues of stilbenes (previously
tested Aâ plaque-specific imaging agents) were prepared,
subsequently radiolabeled, and evaluated as potential candidates
for positron emission tomography (PET) or single photon
Recently, we have reported a series of novel tricyclic 2,5-
diphenylthiophene derivatives, 5a-d, that contain a p-N-methyl-
* Corresponding author: Department of Radiology, University of
Pennsylvania, 3700 Market Street, Room 305, Philadelphia, PA 19104; tel
(215) 662-3096; fax (215) 349-5035; e-mail kunghf@sunmac.spect.upenn.edu.
† Department of Radiology.
‡ Department of Pharmacology.
a Abbreviations: SPECT, single photon emission computed tomography;
PET, positron emission tomography; AD, Alzheimer’s disease; Aâ, â-amy-
loid; PIB, 2-(4′-methylaminophenyl)-6-hydroxybenzothiazole; SB-13, 4-N-
methylamino-4′-hydroxystilbene; FDDNP, 2-[1-(6-[(2-fluoroethyl)(methyl)-
amino]naphthalen-2-yl)ethylidene]malononitrile; IMPY, 6-iodo-2-(4′-
dimethylamino-)phenylimidazo[1,2-a]pyridine; CuAAC, Cu(I)-catalyzed
azide-alkyne cycloaddition; PC, partition coefficient.
10.1021/jm070467l CCC: $37.00 © 2007 American Chemical Society
Published on Web 06/15/2007

1,4-Diphenyltriazoles as Probes for Aâ Aggregates
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 14 3381
Figure 1. Structures of four agents (1-4) reported for imaging Aâ aggregates in the brain. Ki values indicate the in vitro binding affinities to AD
brain homogenates.9
Figure 2. Diphenylthiophenes, a tricyclic ring system, also displayed excellent binding affinities to Aâ aggregates (Ki ) 3.9-31 nM). It was
recently reported29 that, by use of click chemistry, 6 was modified. The tricyclic structure diphenyltriazoles, 7, retained many aspects of interesting
antioxidant properties. Using the same concept, the click chemistry derivatives of 1 led to the formation of tricyclic diphenyltriazoles that showed
excellent high binding affinities to Aâ aggregates comparable to those observed for 1 and 5.
Scheme 1a
a Reagents and conditions: (a) CuSO4, sodium ascorbate, t-BuOH/H2O, rt, 24 h; (b) (Bu3Sn)2, Pd(PPh3)4, toluene, 100 °C, 4 h; (c) H2O2, Na125I, HCl,
EtOH.
emission computerized tomography (SPECT) imaging of amy-
loid plaques.
8a,b, and sodium azide in one pot, the desired diphenyltriazoles
13a,b were synthesized with acceptable yields (62% and 65%).
Furthermore, the reaction between 13b and diethylaminosulfur
trifluoride (DAST), which was performed under 0 °C over 0.5
h, provided the fluoropegylated ligand 13c in 20% yield.
The desired diphenyltriazole derivatives 15a,b were prepared
by a modified one-pot two-step approach (Scheme 3), which
was developed by Liang and co-workers.41 In this reaction, a
chelating ligand trans-N,N′-dimethyl-1,2-cyclohexanediamine,
copper(I) iodide, and an equal amount of sodium ascorbate were
used as the catalysts. The desired reactions were accomplished
at room temperature in 3 h, with yields of 99% and 92%,
respectively. The alcohols 15a,b were then converted to the
tosylates 16a,b (96% and 90%), and a microwave-assisted
fluorination reaction afforded final products 17a,b (80% and
99%).
Chemistry. The rationale for using “cold” click chemistry
for developing new radiopharmaceuticals is based on several
unique features: (1) synthesis can be broken down to fragments
and the key assembling step can readily be accomplished by
“click” reaction; (2) the reaction can be adapted to fragments
with various substitution groups and therefore facilitate the
assembly of diversified groups of substituting groups; (3) the
core structure (tricyclic ring system of diphenyltriazole) contains
three nitrogens, which reduce the lipophilicity as compared to
thiophene analogues; (4) additional variations of the triazole
ring, containing combinations of an assortment of nitrogen and
oxygen atoms, may further extend the range of five-membered
rings suitable for providing probes with high binding affinities;
and (5) various substitution groups on the tricyclic ring systems
may modulate the biological kinetics, thus leading to probes
with improved signal-to-noise ratios in PET or SPECT imaging.
The Cu-catalyzed method reported by Sharpless and co-
workers28 was introduced to assemble the iodinated ligands 10a
(70%) and 10b (98%) (Scheme 1). A combination of copper-
(II) sulfate/sodium ascorbate was utilized in situ to prepare the
copper(I) species, and a “click reaction” was achieved in 1 day
at the room temperature. Subsequently, a palladium-catalyzed
trans-stannylation afforded the tributyltin precursors 11a and
11b for radiolabeling with 45% and 66% yields, respectively.
In order to radioiodinate the ligands, the standard iododestan-
nylation reactions, using sodium [125I]iodide, hydrogen peroxide,
and hydrochloric acid, were successfully applied to yield [125I]-
10a and [125I]10b with excellent yields (80-85%) and greater
than 95% radiochemical purities.
To obtain [18F]17a and [18F]17b, the corresponding tosylated
precursors 16a and 16b were reacted with [18F]fluoride in the
presence of Kryptofix 222 and potassium carbonate in DMSO
at 120 °C for 4 min. The resulting F-18 labeled crude products
were purified by HPLC (Figure 3). The desired products were
obtained in 70 min, and the specific activities at the end of
synthesis were 600 and 800 mCi/µmol for [18F]17a and [18F]-
17b, respectively. The radiochemical purities were >99% for
both ligands, and the radiochemical yields were 50% for [18F]-
17a and 30% for [18F]17b (decay-corrected).
Biological Studies. Several of the diphenyltriazoles here
synthesized displayed excellent to good binding affinities (Table
1). Two iodophenyltriazole derivatives, 10a and 10b, effectively
competed with [125I]2 binding, showing comparable and lower
Ki values of 4.0 ( 0.4 and 8.0 ( 1.6 nM, respectively.
In order to simplify the radiolabeling of F-18 agents and
modulate lipophilicity of potential probes targeting Aâ ag-
gregates, we had previously developed a series of fluoropegy-
lated stilbene and styrylpyridine derivatives.18,42,43 The “hot”
A one-pot two-step approach reported by Fokin and co-
workers40 was utilized to synthesize the fluoro- and hydroxyp-
egylated diphenyltriazole derivatives 13a,b (Scheme 2). By
directly reacting alkylated iodobenzene 12a,b, terminal alkyne

3382 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 14
Qu et al.
Scheme 2a
a Reagents and conditions: (a) Na2CO3, sodium ascorbate, CuSO4, L-proline, NaN3, DMSO/H2O, 65 °C, 24 h; (b) DAST, CH2Cl2, 0 °C,
0.5 h.
Scheme 3a
a Reagents and conditions: (a) NaN3, trans-N,N′-dimethyl-1,2-cycloexanediamine, CuI, sodium ascorbate, DMSO/H2O, rt, 3 h; (b) TsCl, Et3N, DMAP,
CH2Cl2, 0 °C to rt, 3 h; (c) TBAF, THF, 110 °C, 0.5 h, microwave heating; (d) [18F]HF, K222, K2CO3, DMSO, 120 °C, 4.0 min.
Table 1. Potencies (Ki) of Compounds Competing with [125I]2 Binding
to Amyloid Plaques in AD Brain Homogenatesa
compound
Ki (nM ( SEM)
10a
10b
13a
13b
13c
15a
15b
17a
17b
4.0 ( 0.4
8.0 ( 1.6
30.0 ( 6.0
16.5 ( 3.3
8.4 ( 1.7
10.0 ( 2.0
75 ( 10
5.0 ( 1.0
6.2 ( 1.2
a Each value was determined three times with duplicates for each
measurement.
( 2.0 nM for 15a) to a significant reduction (Ki ) 75.0 ( 10
nM for 15b).
Figure 3. HPLC profiles of [18F]17a (top) and 17a (bottom). HPLC
conditions: Agilent 1100 series; Gemini C-18 analytical column (4.6
× 250 mm, 5 µm), CH3CN/ammonium formate (10 mM) 8/2, 1 mL/
min, 265 nm, tR ) (UV) 4.5 min, (γ) 4.8 min. The slight difference in
retention time between the radioactive peak and the UV peak is due to
the configuration of the detector system).
On the basis of the encouraging in vitro binding data obtained
for 10a, 10b, 17a, and 17b, we chose to carry out further
biological evaluations on the corresponding I-125 or F-18
labeled probes. It was anticipated that eventually the I-125 or
F-18 labeling probes would be suitable for imaging with SPECT
or PET. Because the probes need to penetrate the brain easily
after iv injection, the partition coefficient (PC, commonly
measured as log P) is an important factor for consideration. A
moderate lipophilicity (log P ) 1-3.5) is highly desirable. The
radiofluorinated diphenyltriazoles [18F]17a and [18F]17b had
lower PCs (log P ) 2.7 for both) as compared to the two
radioiodinated triazoles, [125I]10a and [125I]10b, which had log
P values of 3.0 and 3.4, respectively. When normal mice were
evaluated for whole animal distribution, these radiolabeled
triazoles displayed excellent initial brain penetrations (6-9.5%
ID/g at 2 min) (Figure 4). The initial high brain uptakes (at 2
min after the injection) observed for these radiolabeled probes,
especially for the radioiodinated probes [125I]10a and [125I]10b,
were subsequently followed by rapid washout with less than
0.5% ID/g remaining in the brain at 2 h after the injection. [18F]-
17a and [18F]17b appeared to show fast washouts initially, but
fluorination reaction can be readily accomplished by a nucleo-
philic substitution of a mesylated or tosylated terminal leaving
group by an activated fluoride ion. When a fluoropegylated chain
was added to one end of the phenyl group, it led to two active
diphenyltriazoles, 13a and 13c. A higher binding affinity was
observed for 13c (Ki ) 8.4 ( 1.7 nM) as compared to 13a (Ki
) 30.0 ( 6.0 nM). Compared to 13c, a slightly lower binding
affinity for the hydroxypegylated derivative 13b was obtained
(Ki ) 16.5 ( 3.3 nM). Similarly, two fluoroalkyl triazoles 17a
and 17b had Ki values of 5.0 ( 1.0 and 6.2 ( 1.2 nM,
respectively, indicating that they could effectively compete with
[
125I]2 binding to amyloid plaques. After replacement of the
fluoro group of 17a or 17b with a hydroxy group, the binding
affinities of the resulting triazoles showed a slight (Ki ) 10.0

1,4-Diphenyltriazoles as Probes for Aâ Aggregates
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 14 3383
Figure 5. In vitro film autoradiography of macroarray brain sections
constructed from six AD and one control (marked by arrowhead) cases.
The Aâ plaques were clearly visualized with low background labeling
with the two radiofluorinated probes [18F]17a and [18F]17b. High white
matter labeling was observed with the iodinated probe [125I]10a in
addition to plaque labeling.
To further characterize the specificity of plaque binding, we
evaluated [125I]10a and [18F]17a with a direct in vitro binding
assay using homogenates (from different brain regions) prepared
from AD and control brain tissues. For both ligands the signal
for specific binding was detected predominantly in the gray
matter homogenates of AD. In contrast, in white matter
homogenates of AD brain, the binding signals for both [125I]-
10a and [18F]17a were very low or nonexistent (Figure 6). In
the homogenates of control brain tissues (both gray and white
matters), the specific binding signal was low, suggesting that
the specific binding was highly selective to the AD brain due
to the presence of Aâ plaques in these brain samples.
In conclusion, by use of convenient click chemistry, a series
of substituted tricyclic diphenyltriazoles were successfully
prepared as potential PET and SPECT imaging agents for
targeting Aâ plaques in the brains of patients with AD. These
diphenyltriazole derivatives showed excellent binding affinities
to Aâ plaques (Ki in the nanomolar range). The radiolabeled
diphenyltriazoles, especially I-125 labeled 10a and 10b, also
had very good in vivo propertiesshigh initial uptake and fast
washout in normal mice. Specific plaque-binding was clearly
delineated by I-125 or F-18 labeled diphenyltriazoles. Taken
together, these diphenyltriazole probes demonstrate promising
in vitro and in vivo characteristics and they may provide a
convenient platform for development of new imaging agents
targeting amyloid plaques in the brain.
Figure 4. Brain uptakes and washouts of radiolabeled triazole probes
in normal mice. Data are presented as % ID/g of three mice ( standard
deviation.
this was followed by gradually slower rates of clearance from
the mouse brain, resulting in higher residual radioactivities (1-
2% ID/g at 2 h after tracer injection). Thus, the two iodinated
probes [125I]10a and [125I]10b had higher and better brain
washout indexes (brain2min/brain30min) of 5.66 and 3.72, respec-
tively. [18F]17a and [18F]17b had lower indexes of 2.34 and
2.31, respectively, which is less favorable for Aâ plaque
detection.5,10
It is worth noting that there was a significant amount of in
vivo defluorination (reflected by bone uptake) observed with
[18F]17a (4.64% ID/g, Supporting Information) and [18F]17b
(18.59% ID/g, Supporting Information). These values are 5-10
times higher as compared to the values reported for previous
PET ligands.18,20,43 The unfavorable in vivo stability of these
two fluoroalkylated triazoles make them less attractive as
potential imaging agents. It has been previously demonstrated
that F-18 labeled stilbene and styrylpyridine derivatives contain-
ing three PEG units are not significantly defluorinated in
vivo.18,20,42,43 We are currently evaluating [18F]13c (which
contains three PEG units) as an improved PET imaging agent.
The preliminary data clearly indicated the more stable nature
of [18F]13c. Detailed evaluations of this ligand as a more suitable
PET ligand will be reported in a separate publication.
To investigate the ability of these probes to bind with Aâ
aggregates in human brain tissues, we have carefully constructed
a macroarray block of postmortem human brain samples
consisting of seven confirmed AD cases. After sectioning of
this macroarray block, adjacent sections, which reflect a
comparable pathophysiology, were used. In vitro film autora-
diography was carried out using these novel I-125 or F-18
labeled diphenyltriazole probes. Among the probes examined,
[18F]17a and [18F]17b exhibited the most distinctive Aâ plaque-
labeling and a minimal level of background in the white matter
areas of AD brain (Figure 5). The labeling pattern was consistent
with that observed by immunohistochemical labeling with an
antibody (4G8) specific for Aâ (data not shown). In addition
to plaque labeling, [125I]10a displayed significant white matter
labeling under a similar incubation condition. A similar pattern
was also observed for [125I]10b (data not shown). The higher
background labeling observed for [125I]10a and [125I]10b may
be related to their higher lipophilicity (log P ) 3.0 and 3.4,
respectively).
Experimental Section
General Information. All reagents used were commercial
products and were used without further purification unless otherwise
indicated. Flash chromatography (FC) was performed on silica gel
60 (230-400 mesh, Sigma-Aldrich). Preparative thin-layer chro-
matography (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 and 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. Microwave-
assisted reactions were performed in a Initiator microwave reactor
1
(Biotage). H NMR spectra were obtained at 200 MHz, and 13C
NMR spectra were recorded at 50 MHz (Bruker DPX spectrometer).
Chemical shifts were reported as δ values (parts per million) relative
to internal tetramethylsilane (TMS). Coupling constants are 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 on a Micromass/Waters GCT instrument (GC-EI/CI

3384 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 14
Qu et al.
Figure 6. Specific binding of (A) [125I]10a and (B) [18F]17a to pooled AD and control brain tissue homogenates. Gray and white matter was
dissected from the cortical regions. High specific binding was detected mainly in the gray matter of AD brain. Relatively low binding was measured
in the white matter homogenates of AD brain as well as in gray and white matter homogenates of the control brain.
1
time-of-flight mass spectrometer). Elemental analyses were per-
formed by Atlantic Microlab INC. Analytical HPLC analysis was
carried out on an Agilent 1100 series LC. Two systems were used
to confirm the purity of some compounds listed in this section.
System A conditions: Phenomenex Gemini 5µ C18 110A reverse-
phase analytical column (250 × 4.6 mm, 5 µm), 70/30 CH3CN/1
mM ammonium formate (pH ) 7) water buffer, 1.0 mL/min, UV
292 nm. System B conditions: Phenomenex silica column (4.6 ×
250 mm, 5 µm), EtOAc/hexanes (from 30/70 to 80/20), 1.0 mL/
min, UV 292 nm. All compounds reported in this paper showed
greater than 95% purity in both systems.
4-[1-(4-Iodophenyl)-1H-1,2,3-triazol-4-yl]-N-methylbenze-
namine (10a). Alkyne 8a (0.042 g, 0.32 mmol), azide 9 (0.32 mmol,
0.079 g), and sodium ascorbate (0.16 mL, fresh prepared 1.0 M
solution) were added into t-butanol/H2O (1/1 v/v, 2 mL) and the
whole mixture was degassed with nitrogen for 10 min. Copper(II)
sulfate (CuSO4, 1.0 M aqueous solution, 16 µL) was added and
the reaction mixture was vigorously stirred at room temperature
(rt) for 24 h. After dilution with ice-cold water (10 mL), the mixture
was filtered and washed with cold water and ice-cold Et2O. The
solid was dried under vacuum to afford 10a (0.084 g, 70%) as a
pale green solid. 1H NMR [(CD3)2CO] δ 8.75 (s, 1H), 7.99 (d, 2H,
J ) 8.8 Hz), 7.82-7.72 (m, 4H), 6.69 (d, 2H, J ) 8.7 Hz), 5.19
(br s, 1H), 2.85, 2.83 (s, 3H, -NCH3). 13C NMR (DMSO-d6) δ
150.0, 148.4, 138.5, 136.4, 126.4, 121.6, 117.3, 117.0, 111.7, 93.7,
29.6. HRMS calcd for C15H13IN4 (M+), 376.0185; found, 376.0168.
4-[1-(4-Iodophenyl)-1H-1,2,3-triazol-4-yl]-N,N-dimethylben-
zenamine (10b). Following the procedure in the preparation of 10a,
compound 10b was prepared from alkyne 8b (0.073 g, 0.50 mmol)
and 9 (0.147 g, 0.60 mmol) as a pale yellow solid (0.191 g, 98%
yield). H NMR [(CD3)2CO] δ 8.81 (s, 1H), 7.99 (d, 2H, J ) 8.9
Hz), 7.86-7.73 (m, 4H), 6.84 (d, 2H, J ) 8.9 Hz), 3.00 (s, 6H).
13C NMR (DMSO-d6) δ 150.3, 148.1, 138.5, 136.4, 126.2, 121.6,
121.5, 117.7, 117.3, 112.3, 93.8. HRMS calcd for C16H15IN4 (M+),
390.0341; found, 390.0332. Anal. (C16H15IN4) C, H. N: calcd,
14.36; found, 14.47.
N-Methyl-4-[1-(4-tributylstannylphenyl)-1H-1,2,3-triazol-4-
yl]benzenamine (11a). A mixture of 10a (0.041 g, 0.11 mmol),
bis(tributyltin) [(Bu3Sn)2, 0.319 g, 0.55 mmol], and palladium
tetrakistriphenylphosphine [Pd(PPh3)4, 0.013 g, 10 mol %] in
toluene was heated at 100 °C for 4 h. The reaction solution was
cooled to rt and treated with 5 mL of 10% KF. After vigorous
stirring for an additional 0.5 h, the standard workup with EtOAc
and following PTLC (EtOAc/hexanes, 30/70) afforded 11a as a
pale yellow solid (0.026 g, 45%). 1H NMR (CDCl3) δ 8.05 (s, 1H),
7.77-7.70 (m, 4H), 7.62 (d, 2H, J ) 8.3 Hz), 6.70 (d, 2H, J ) 8.6
Hz), 3.86 (br s, 1H), 2.91, 2.90 (s, 3H, -NCH3), 1.66-1.50 (m,
6H), 1.45-1.27 (m, 6H), 1.15-1.01 (m, 6H), 0.91 (t, 9H, J ) 7.2
Hz). 13C NMR (CDCl3) δ 149.7, 149.1, 143.4, 138.1, 137.8, 137.4,
137.3, 127.2, 120.3, 119.9, 119.6, 119.5, 116.1, 112.7, 30.8, 29.5,
29.3, 29.1, 28.1, 27.5, 27.0, 13.9, 13.4, 13.2, 10.0, 6.7, 6.5. HRMS
calcd for C27H41N4Sn (M+ + H+), 541.2353; found, 541.2366.
N,N-Dimethyl-4-[1-(4-tributylstannylphenyl)-1H-1,2,3-triazol-
4-yl]benzenamine (11b). In accordance with the procedure for the
preparation of 11a, compound 11b was prepared from 10b (0.039
g, 0.1 mmol) as a light yellow solid (0.037 g, 66% yield). 1H NMR
(CDCl3) δ 8.06 (s, 1H), 7.81-7.72 (m, 4H), 7.62 (d, 2H, J ) 8.4
Hz), 6.81 (d, 2H, J ) 8.9 Hz), 3.02 (s, 6H), 1.66-1.50 (m, 6H),
1.45-1.24 (m, 6H), 1.15-1.02 (m, 6H), 0.91 (t, 9H, J ) 7.1 Hz).
13C NMR (CDCl3) δ 150.7, 149.0, 143.4, 138.1, 137.7, 137.4, 137.3,

1,4-Diphenyltriazoles as Probes for Aâ Aggregates
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 14 3385
127.2, 127.0, 120.3, 119.9, 119.5, 118.7, 116.1, 112.7, 40.6, 29.4,
29.2, 29.0, 28.1, 27.5, 26.9, 13.8, 13.3, 13.2, 9.9, 6.7, 6.5. HRMS
calcd for C27H40N4Sn (M+), 554.2431; found, 554.2433.
2H, J ) 6.3 Hz), 3.75 (t, 2H, J ) 6.0 Hz), 2.99 (s, 6H), 2.00 (pentet,
2H, J ) 6.2 Hz). 13C NMR [(CD3)2CO] δ 160.2, 151.6, 131.8,
127.4, 122.6, 120.0, 117.7, 116.3, 113.4, 66.2, 59.1, 58.9, 40.6,
33.4. HRMS calcd for C19H23N4O2 (M + H+), 339.1822; found,
339.1825.
4-(1-[4-(2-[2-(2-Fluoroethoxy)ethoxy]ethoxy)phenyl]-1H-1,2,3-
triazol-4-yl)-N-methylbenzenamine (13a). To a 20 mL scintillation
vial were added alkyne 8a (0.040 g, 0.3 mmol), iodobenzene 12b
(0.106 g, 0.3 mmol), Na2CO3 (0.003 g, 0.03 mmol), L-proline
(0.0035 g, 0.003 mmol), NaN3 (0.029 g, 0.45 mmol), sodium
ascorbate (0.006 g, 0.03 mmol), CuSO4 (1.0 M aqueous solution,
0.015 mL), and 1.0 mL of mixed solvent DMSO and H2O (9/1
v/v). After being purged by nitrogen 10 min, the reaction mixture
was heated to 65 °C for 24 h. It was cooled down to rt, poured
into diluted ammonia (20 mL), and extracted with EtOAc (3 × 15
mL). The combined organic phase was washed with brine (2 × 10
mL), dried over Na2SO4, filtered, and concentrated. The residue
was submitted to FC (EtOAc/hexanes, 70/30) to afford a light brown
solid 13a (0.745 g, 62%). 1H NMR (CDCl3) δ 7.97 (s, 1H), 7.76-
7.63 (m, 4H), 7.06 (dt, 2H, J1 ) 9.0 Hz, J2 ) 2.7 Hz), 6.72(d, 2H,
J ) 8.6 Hz), 4.72-4.68 (m, 1H), 4.49-4.44 (m, 1H), 4.20 (t, 2H,
J ) 2.5 Hz), 3.94-3.68 (m, 8H), 2.90 (s, 3H). 13C NMR (CDCl3)
δ 159.0, 149.6, 148.9, 131.0, 127.1, 122.1, 119.5, 116.4, 115.6,
112.6, 85.0, 81.6, 71.04, 71.00, 70.8, 70.4, 69.8, 68.0, 30.7. HRMS
calcd for C21H25FN4O3 (M+), 400.1911; found, 400.1895.
2-(2-[2-(4-[4-(4-Dimethylaminophenyl)-1H-1,2,3-triazol-1-yl]-
phenoxy)ethoxy]ethoxy)ethanol (13b). In accordance with the
procedure for the preparation of 13a, compound 13b was prepared
from 12a (0.176 g, 0.5 mmol) as a light yellow solid (0.135 g,
65% yield). 1H NMR (CDCl3) δ 7.97 (s, 1H), 7.76 (d, 2H, J ) 8.7
Hz), 7.64 (d, 2H, J ) 8.9 Hz), 7.02 (d, 2H, J ) 8.9 Hz), 6.79(d,
2H, J ) 8.6 Hz), 4.17 (t, 2H, J ) 4.6 Hz), 3.85 (t, 2H, J ) 4.6
Hz), 3.72-3.60 (m, 8H), 2.99 (s, 6H), 2.61 (br s, 1H). 13C NMR
(CDCl3) δ 159.0, 150.5, 148.8, 131.0, 129.4, 126.9, 122.1, 116.5,
115.6, 112.8, 72.6, 71.0, 70.5, 69.8, 68.0, 61.9, 40.7. HRMS calcd
for C22H28N4O4 (M+), 412.2111; found, 412.2106.
2-(4-[4-(4-Dimethylaminophenyl)-1H-1,2,3-triazol-1-yl]phe-
noxy)ethyl 4-Methylbenzenesulfonate (16a). To a stirred solution
of 15b (0.162 g, 0.5 mmol) in CH2Cl2 (5 mL) cooled with an ice
bath (0 °C) were added Et3N (0.35 mL, 2.5 mmol), p-toluenesulfo-
nyl chloride (TsCl, 0.143 g, 0.75 mmol) and a catalytic amount of
4-dimethylaminopyridine (DMAP, 0.005 g). After the solution was
maintained at 0 °C for 10 min, the ice bath was removed, and the
reaction was kept at rt for 3 h and then submitted to standard workup
(solvent, CHCl3/MeOH 90/10). The crude product was purified by
FC (CHCl3/MeOH 97/3) to provide a pale white solid (0.228 g,
1
96%). H NMR (CDCl3) δ 8.01 (s, 1H), 7.86-7.79 (m, 4H), 7.67
(d, 2H, J ) 9.0 Hz), 7.37 (d, 2H, J ) 8.5 Hz), 6.96-6.91 (m, 4H),
4.44-4.40 (m, 2H), 4.25-4.21 (m, 2H), 3.05 (s, 6H), 2.47 (s, 3H).
13C NMR (CDCl3) δ 158.3, 148.8, 145,3, 132,8, 131.3, 130.0, 128.1,
126.9, 122.2, 116.8, 115.6, 113.2, 68.1, 66.0, 40.8, 21.7.
3-(4-[4-(4-Dimethylaminophenyl)-1H-1,2,3-triazol-1-yl]phe-
noxy)propyl 4-methylbenzenesulfonate (16b). In accordance with
the procedure for the preparation of 16a, compound 16b was
prepared from alcohol 15c (0.169 g, 0.5 mmol) as a pale white
1
solid (0.221 g, 90% yield). H NMR [(CD3)2CO] δ 8.65 (s, 1H),
7.84-7.77 (m, 6H), 7.40 (d, 2H, J ) 8.0 Hz), 7.03 (dt, 2H, J1 )
9.0 Hz, J2 ) 2.8 Hz), 6.83 (d, 2H, J ) 8.9 Hz), 4.29 (t, 2H, J )
6.0 Hz), 4.08 (t, 2H, J ) 5.9 Hz), 2.99 (s, 6H), 2.38 (s, 3H), 2.16
(pentet, 2H, J ) 6.0 Hz). 13C NMR [(CD3)2CO] δ 159.6, 151.6,
145.9, 131.0, 128.7, 127.4, 126.1, 122.5, 120.0, 117.7, 116.2, 113.4,
68.2, 64.7, 40.6, 21.6.
4-(1-[4-(2-Fluoroethoxy)phenyl]-1H-1,2,3-triazol-4-yl)-N,N-
dimethylbenzenamine (17a). To a solution of tosylate 16a (0.096
g, 0.20 mmol) in THF (1 mL) was added TBAF solution (1.0 M in
THF, 1.0 mL). The reaction solution was heated in the microwave
reactor at 110 °C for 0.5 h. After cooling and standard workup
with EtOAc, the residue was purified by PTLC (EtOAc/hexanes,
50/50) to afford 8c as a light brown solid (0.052 g, 80%). 1H NMR
[(CD3)2CO] δ 8.65 (s, 1H), 7.90-7.78 (m, 4H), 7.20 (dt, 2H, J1 )
9.1 Hz, J2 ) 2.8 Hz), 6.83 (d, 2H, J ) 8.9 Hz), 4.96-4.923 (m,
1H), 4.72-4.68 (m, 1H), 4.48-4.44 (m, 1H), 4.33-4.29 (m, 1H),
3.00 (s, 6H). 13C NMR [(CD3)2CO] δ 159.6, 151.7, 149.4, 132.3,
127.4, 122.6, 120.0, 117.7, 116.4, 113.4, 84.6, 81.3, 69.0, 68.6,
40.6. HRMS calcd for C18H19FN4O (M+), 326.1543; found,
326.1532.
4-(1-[4-(2-[2-(2-Fluoroethoxy)ethoxy]ethoxy)phenyl]-1H-1,2,3-
triazol-4-yl)-N,N-dimethylbenzenamine (13c). To a stirred solu-
tion of 13b (0.062 g, 0.15 mmol) in CH2Cl2 (5 mL) cooled with an
ice bath (0 °C) was added diethylaminosulfur trifluoride (DAST,
0.039 mL, 0.30 mmol) dropwise. After addition, the reaction
mixture was maintained at 0 °C for 0.5 h and was submitted to
PTLC (EtOAc/hexanes, 70/30) to provide 13c as a light brown solid
(0.013 g, 21%). 1H NMR (CDCl3) δ 7.99 (s, 1H), 7.79 (d, 2H, J )
8.7 Hz), 7.68 (d, 2H, J ) 9.0 Hz), 7.06 (d, 2H, J ) 9.0 Hz), 6.86
(d, 2H, J ) 8.1 Hz), 4.70 (t, 1H, J ) 4.2 Hz), 4.46 (t, 1H, J ) 4.2
Hz), 4.21 (t, 2H, J ) 4.7 Hz), 3.93-3.68 (m, 8H), 3.02 (s, 6H).
13C NMR (CDCl3) δ 159.2, 148.8, 131.1, 127.1, 122.2, 116.6, 115.7,
113.4, 85.0, 81.7, 71.2, 71.1, 70.9, 70.5, 70.0, 69.8, 68.1, 41.1.
HRMS calcd for C22H27FN4O3 (M+), 414.2067; found, 414.2067.
2-(4-[4-(4-Dimethylaminophenyl)-1H-1,2,3-triazol-1-yl)phenoxy]-
ethanol (15a). To a 15 mL two-neck flask were added alkyne 8b
(0.145 g, 1.0 mmol), iodobenzene 14a (0.264 g, 1.0 mmol)), trans-
N,N′-dimethyl-1,2-cyclohexanediamine (0.024 mL, 0.15 mmol),
NaN3 (0.072 g, 1.1 mmol), sodium ascorbate (0.02 g, 0.10 mmol),
CuI (0.019 g, 0.10 mmol), and 3 mL of mixed solvent DMSO and
H2O (5/1 v/v). The reaction mixture was purged by nitrogen for
10 min and then was vigorously stirred at rt for 3 h. After dilution
with ice-cold water (15 mL), the solution was filtered and washed
with ice-cold water and ice-cold Et2O. The solid was dried to afford
4-(1-[4-(3-Fluoropropoxy)phenyl]-1H-1,2,3-triazol-4-yl)-N,N-
dimethylbenzenamine (17b). In accordance with the procedure
for the preparation of 17a, compound 17b was prepared from
tosylate 16b (0.099 g, 0.2 mmol) as a white solid (0.068 g, 100%
1
yield). H NMR [(CD3)2CO] δ 8.64 (s, 1H), 7.88-7.78 (m, 6H),
7.18 (dt, 2H, J1 ) 6.8 Hz, J2 ) 2.8 Hz), 6.83 (d, 2H, J ) 8.9 Hz),
4.79 (t, 1H, J ) 5.9 Hz), 4.56 (t, 1H, J ) 5.9 Hz), 4.23 (t, 2H, J
) 6.2 Hz), 2.99 (s, 6H), 2.28 (pentet, 1H, J ) 6.0 Hz), 2.15 (pentet,
1H, J ) 6.0 Hz). 13C NMR [(CD3)2CO] δ 159.8, 151.6, 149.4,
132.1, 127.4, 122.6, 120.0, 117.7, 116.3, 113.4, 83.1, 79.9, 65.0,
40.6, 31.4, 31.0. HRMS calcd for C19H22FN4O (M + H+), 341.1779;
found, 341.1776.
Radiolabeling: (1) Radioiodination. The radioiodinated com-
pounds [125I]10a and [125I]10b were prepared via iododestannylation
reactions from the corresponding tributyltin precursors 11a and 11b
according to the method described previously.44,45 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 10 min at room temperature
and terminated by the addition of 100 µL of saturated NaHSO3
solution. The reaction mixture neutralized with 1.5 mL of saturated
NaHCO3 solution was loaded on a small preconditioned C-4
minicolumn. After sequential rinsing with 10% and 20% EtOH
solutions, the desired products [125I]10a and [125I]10b were obtained.
The radiochemical purity was checked by HPLC on a reversed-
1
a pale yellow solid 15a (0.323 g, 98% yield). H NMR [(CD3)2-
CO] δ 8.64 (s, 1H), 7.87-7.76 (m, 4H), 7.16 (dt, 2H, J1 ) 9.1 Hz,
J2 ) 2.8 Hz), 6.83 (dt, 2H, J1 ) 9.0 Hz, J2 ) 2.1 Hz), 4.17 (t, 2H,
J ) 4.8 Hz), 4.03-3.88 (m, 3H, -OH, -CH2), 3.00 (s, 6H). 13C
NMR [(CD3)2CO] δ 160.1, 151.6, 149.4, 131.9, 127.4, 122.5, 120.0,
117.6, 116.4, 113.4, 71.2, 61.4, 61.3, 40.6. HRMS calcd for
C18H20N4O2 (M+), 324.1586; found, 324.1583.
3-(4-[4-(4-Dimethylaminophenyl)-1H-1,2,3-triazol-1-yl]phe-
noxy)propan-1-ol (15b). In accordance with the procedure for the
preparation of 15a, compound 15b was prepared from iodobenzene
14b (0.278 g, 1.0 mmol) as a pale yellow solid (0.310 g, 92% yield).
1H NMR [(CD3)2CO] δ 8.64 (s, 1H), 7.85-7.78 (m, 4H), 7.15 (dt,
2H, J1 ) 9.0 Hz, J2 ) 2.7 Hz), 6.83 (d, 2H, J ) 8.9 Hz), 4.20 (t,

3386 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 14
Qu et al.
phase column (Phenomenex Gemini C18 analytical column, 4.6 ×
250 mm, 5 µm, CH3CN/ammonium formate buffer (1 mM) 8/2;
flow rate 0.5 mL/min). The no-carrier-added products were stored
at -20 °C up to 6 weeks for animal studies, homogenate binding,
and autoradiography studies.
assembled. The presence and localization of plaques on the sections
was confirmed with immunohistochemical staining with monoclonal
Aâ antibody 4G8 (Sigma). The frozen sections were incubated with
[
125I] and [18F]tracers (200 000-250 000 cpm/200 µL) for 1 h at
room temperature. The sections were then dipped in saturated LiOH
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 I-125 or F-18 labeled sections were exposed to Kodak
Biomax MR film overnight.
(2) Radiofluorination. [18F]Fluoride was produced by the JSW
typeBC3015 cyclotron via O-18(p,n) F-18 reaction and passed
through a Sep-Pak Light QMA cartridge (Waters) as an aqueous
solution in 18O-enriched water. The cartridge was dried by airflow,
and the F-18 activity was eluted with 1.3 mL of Kryptofix 222
(K222)/K2CO3 solution (11 mg of K222 and 2.6 mg of K2CO3 in
CH3CN/H2O 1.12/0.18). The solvent was removed at 120 °C under
an argon stream. The residue was azeotropically dried with 1 mL
of anhydrous CH3CN twice at 120 °C under a nitrogen stream. A
solution of tosylate precursor 16a or 16b (2 mg) in DMSO (0.2
mL) was added to the reaction vessel containing the dried F-18
activities. The mixture was heated at 120 °C for 4 min. Water (5
mL) was added and the mixture was passed through a precondi-
tioned Oasis HLB cartridge (3 cm3) (Waters). The cartridge was
washed with 10 mL of water and the labeled compound was eluted
with 2 mL of CH3CN. The eluted compound was purified by HPLC
[Phenonemex Gemini C18 semipreparative column (10 × 250 mm,
5 µm), CH3CN/water 7/3, flow rate 3 mL/min, tR ) 11 min]. The
radiochemical purity and specific activity were determined by
analytical HPLC (Phenomenex Gemini C18 analytical column (4.6
× 250 mm, 5 µm), CH3CN/ammonium formate buffer (10 mM)
8/2; flow rate 1 mL/min; tR ) 4.8 min for [18F]17a, 5.7 min for
[18F]17b). Specific activity was estimated by comparing the UV
peak intensity of the purified F-18 labeled compound with reference
nonradioactive compound of known concentration.
Preparation of Brain Tissue Homogenates. AD postmortem
brain tissues were obtained from the University of Washington
Alzheimer’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 of 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.
Organ Distribution in Normal Mice. While the mice were
under isoflurane anesthesia, 0.15 mL of a 0.1% bovine serum
albumin solution containing [125I] or [18F]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 γ-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 % ID/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] or [18F] 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 room temperature, 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.
Acknowledgment. This work was supported by grants from
the National Institutes of Health (AG022559 to H.F.K and AG-
021868 to M.P.K). We thank Dr. Carita Huang for her editorial
assistance.
Supporting Information Available: Procedures for synthesiz-
ing some intermediates; elemental analysis results and HPLC purity
analysis data for all bioassay-involved compounds in two different
HPLC systems; and full sets of biodistribution data in normal mice
for [125I]10a, [125I]10b, [18F]17a, and [18F]17b. This material is
available free of charge via the Internet at http://pubs.acs.org.
Binding Studies. The ligand [125I]2, with 2200 Ci/mmol specific
activity and greater than 95% radiochemical purity, was prepared
by the standard iododestannylation reaction and purified on a
simplified C-4 minicolumn as described previously. Binding assays
were carried out in 12 × 75 mm borosilicate glass tubes. For
competition studies, the reaction mixture contained 50 µL of pooled
AD brain homogenates (20-50 µg), 50 µL of [125I]2 (0.04-0.06
nM diluted in PBS) and 50 µL 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 2 in the same assay tubes. The mixture was incubated
at 37 °C for 2 h, and the bound and free radioactivity were separated
by vacuum filtration through Whatman GF/B filters by use of a
Brandel M-24R cell harvester followed by 2 × 3 mL washes of
PBS at room temperature. Filters containing the bound 125I ligand
were counted in a γ-counter (Packard 5000) with 70% counting
efficiency. Under the assay conditions, the fraction that was
nonspecifically bound was less than 20% of the total radioactivity.
The results of inhibition experiments were subjected to nonlinear
regression analysis by use of equilibrium binding data analysis,
with which Ki values were calculated.
Similarly, the specific binding of radioiodinated and radioflu-
orinated ligands (0.06 nM for [125I]probe and 0.5 nM for [18F]probe)
to homogenates, prepared from the gray and white matters of AD
and control brain tissues, were carried out as described above.
Nonspecific binding was determined in the presence of 2 µM of
the corresponding nonradioactive probes.
In Vitro autoradiography. To compare different probes using
similar sections of human brain tissue, human macroarray brain
sections from six confirmed AD cases and one control subject were
References
(1) Selkoe, D. J. Imaging Alzheimer’s amyloid. Nat. Biotechnol. 2000,
18, 823-824.
(2) Hardy, J. Alzheimer’s disease: The amyloid cascade hypothesis: An
update and reappraisal. J. Alzheimers Dis. 2006, 9, 151-3.
(3) Goedert, M.; Spillantini, M. G. A century of Alzheimer’s disease.
Science 2006, 314, 777-81.
(4) 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.
(5) 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.
(6) 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.
(7) Ono, M.; Wilson, A.; Nobrega, J.; Westaway, D.; Verhoeff, P.;
Zhuang, Z. P.; Kung, M. P.; Kung, H. F. 11C-labeled stilbene
derivatives as Abeta-aggregate-specific PET imaging agents for
Alzheimer’s disease. Nucl. Med. Biol. 2003, 30, 565-571.
(8) 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 â-amyloid
plaques in the brain. J. Med. Chem. 2003, 46, 237-243.
(9) 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.

1,4-Diphenyltriazoles as Probes for Aâ Aggregates
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 14 3387
(10) 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.
(28) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. A
stepwise huisgen cycloaddition process: copper(I)-catalyzed regi-
oselective “ligation” of azides and terminal alkynes. Angew. Chem.,
Int. Ed. 2002, 41, 2596-9.
(11) Klunk, W. E.; Engler, H.; Nordberg, A.; Wang, Y.; Blomqvist, G.;
Holt, D. P.; Bergstrom, M.; Savitcheva, I.; Huang, G.-f.; Estrada,
S.; Ausen, B.; Debnath, M. L.; Barletta, J.; Price, J. C.; Sandell, J.;
Lopresti, B. J.; Wall, A.; Koivisto, P.; Antoni, G.; Mathis, C. A.;
Langstrom, B. Imaging Brain Amyloid in Alzheimer’s Disease with
Pittsburgh Compound-B. Ann. Neurol. 2004, 55, 306-319.
(12) Small, G. W.; Kepe, V.; Ercoli, L. M.; Siddarth, P.; Bookheimer, S.
Y.; Miller, K. J.; Lavretsky, H.; Burggren, A. C.; Cole, G. M.; Vinters,
H. V.; Thompson, P. M.; Huang, S. C.; Satyamurthy, N.; Phelps, M.
E.; Barrio, J. R. PET of brain amyloid and tau in mild cognitive
impairment. N. Engl. J. Med. 2006, 355, 2652-63.
(13) Agdeppa, E. D.; Kepe, V.; Liu, J.; Flores-Torres, S.; Satyamurthy,
N.; Petric, A.; Cole, G. M.; Small, G. W.; Huang, S. C.; Barrio, J.
R. Binding characteristics of radiofluorinated 6-dialkylamino-2-
naphthylethylidene derivatives as positron emission tomography
imaging probes for â-amyloid plaques in Alzheimer’s disease. J.
Neurosci. 2001, 21, RC189.
(14) Lockhart, A.; Imaging Alzheimer’s disease pathology: one target,
many ligands. Drug DiscoVery Today 2006, 11, 1093-9.
(15) Okamura, N.; Suemoto, T.; Furumoto, S.; Suzuki, M.; Shimadzu,
H.; Akatsu, H.; Yamamoto, T.; Fujiwara, H.; Nemoto, M.; Maruyama,
M.; Arai, H.; Yanai, K.; Sawada, T.; Kudo, Y. Quinoline and
benzimidazole derivatives: candidate probes for in vivo imaging of
tau pathology in Alzheimer’s disease. J. Neurosci. 2005, 25, 10857-
62.
(16) Shimadzu, H.; Suemoto, T.; Suzuki, M.; Shiomitsu, T.; Okamura,
N.; Kudo, Y.; Sawada, T. Novel probes for imaging amyloid-â: F-18
and C-11 labeling of 2-(4-aminostyryl)benzoxazole derivatives. J.
Lab. Compds. Radiopharm. 2004, 47, 181-190.
(29) Pagliai, F.; Pirali, T.; Del Grosso, E.; Di Brisco, R.; Tron, G. C.;
Sorba, G.; Genazzani, A. A. Rapid Synthesis of Triazole-Modified
Resveratrol Analogues via Click Chemistry. J. Med. Chem. 2006,
49, 467-470.
(30) Mocharla, V. P.; Colasson, B.; Lee, L. V.; Roper, S.; Sharpless, K.
B.; Wong, C. H.; Kolb, H. C. In situ click chemistry: enzyme-
generated inhibitors of carbonic anhydrase II. Angew. Chem., Int.
Ed. 2004, 44, 116-20.
(31) Yan, Z.-Y.; Niu, Y.-N.; Wei, H.-L.; Wu, L.-Y.; Zhao, Y.-B.; Liang,
Y.-M. Combining proline and ‘click chemistry’: a class of versatile
organocatalysts for the highly diastereo- and enantioselective Michael
addition in water. Tetrahedron: Asymmetry 2007, 17, 3288-3293.
(32) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Click Chemistry: Diverse
Chemical Function from a Few Good Reactions. Angew. Chem., Int.
Ed. 2001, 40, 2004-2021.
(33) Kolb, H. C.; Sharpless, K. B. The growing impact of click chemistry
on drug discovery. Drug DiscoVery Today 2003, 8, 1128-37.
(34) Wang, J.; Sui, G.; Mocharla, V. P.; Lin, R. J.; Phelps, M. E.; Kolb,
H. C.; Tseng, H. R. Integrated microfluidics for parallel screening
of an in situ click chemistry library. Angew. Chem., Int. Ed. 2006,
45, 5276-81.
(35) Kolb, H. C.; Mocharla, V. P.; Walsh, J. C. In situ click chemistry
method for screening high affinity molecular imaging probes. U.S.
Patent 2006-US16428 (2006116736), 20060427, 2006.
(36) Kolb, H. C.; Walsh, J. C.; Chen, K. Click chemistry method for
synthesizing 18F-labeled AZT as molecular imaging probes. U.S.
Patent 2006-US16088 (2006116629), 20060427, 2006.
(17) Okamura, N.; Suemoto, T.; Shimadzu, H.; Suzuki, M.; Shiomitsu,
T.; Akatsu, H.; Yamamoto, T.; Staufenbiel, M.; Yanai, K.; Arai, H.;
Sasaki, H.; Kudo, Y.; Sawada, T. Styrylbenzoxazole derivatives for
in vivo imaging of amyloid plaques in the brain. J. Neurosci. 2004,
24, 2535-2541.
(37) Marik, J.; Sutcliffe, J. L. Click for PET: rapid preparation of [18F]-
fluoropeptides using CuI catalyzed 1,3-dipolar cycloaddition. Tet-
rahedron Lett. 2006, 47, 6681-6684.
(38) Mindt, T. L.; Struthers, H.; Brans, L.; Anguelov, T.; Schweinsberg,
C.; Maes, V.; Tourwe, D.; Schibli, R., \“Click to Chelate\”: Synthesis
and Installation of Metal Chelates into Biomolecules in a Single Step.
J. Am. Chem. Soc. 2006, 128, 15096-15097.
(39) Dijkgraaf, I.; Rijnders, A. Y.; Soede, A.; Dechesne, A. C.; van Esse,
G. W.; Brouwer, A. J.; Corstens, F. H.; Boerman, O. C.; Rijkers, D.
T.; Liskamp, R. M. Synthesis of DOTA-conjugated multivalent
cyclic-RGD peptide dendrimers via 1,3-dipolar cycloaddition and
their biological evaluation: implications for tumor targeting and
tumor imaging purposes. Org. Biomol. Chem. 2007, 5, 935-44.
(40) Feldman, A. K.; Colasson, B.; Fokin, V. V. One-Pot Synthesis of
1,4-disubstituted 1,2,3-triazoles from in situ generated azides. Org.
Lett. 2004, 6, 3897-3899.
(18) Zhang, W.; Kung, M. P.; Oya, S.; Hou, C.; Kung, H. F., (18)F-
labeled styrylpyridines as PET agents for amyloid plaque imaging.
Nucl. Med. Biol. 2007, 34, 89-97.
(19) Cai, L.; Innis, R. B.; Pike, V. W. Radioligand Development for PET
Imaging of beta-Amyloid (Abeta)-Current Status. Curr. Med. Chem.
2007, 14, 19-52.
(20) Chandra, R.; Oya, S.; Kung, M. P.; Hou, C.; Jin, L. W.; Kung, H. F.
New diphenylacetylenes as probes for PET imaging of amyloid
plaques. J. Med. Chem. 2007, In Press.
(21) Zhuang, Z. P.; Kung, M. P.; Kung, H. F. Synthesis of Biphenyltrienes
as Probes for â-Amyloid Plaques. J. Med. Chem. 2006, 49, 2841-
2844.
(22) Chandra, R.; Kung, M. P.; Kung, H. F. Design, synthesis, and
structure-activity relationship of novel thiophene derivatives for beta-
amyloid plaque imaging. Bioorg. Med. Chem. Lett. 2006, 16, 1350-
2.
(41) Andersen, J.; Bolvig, S.; Liang, X. Efficient one-pot synthesis of
1-aryl 1,2,3-triazoles from aryl halides and terminal alkynes in the
presence of sodium azide. Synlett 2005, 2941-2947.
(23) Huisgen, R. Introduction, survey, mechanism. In 1,3-Dipolar cy-
cloaddition; Padwa, A., Ed.; John Wiley & Sons: New York, 1984;
Vol. 1, pp 76-147.
(42) Stephenson, K. A.; Chandra, R.; Zhuang, Z. P.; Hou, C.; Oya, S.;
Kung, M. P.; Kung, H. F. Fluoro-pegylated (FPEG) Imaging Agents
Targeting Abeta Aggregates. Bioconjugate Chem. 2007, 18, 238-
46.
(43) 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.
(44) 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.
(24) Sharpless, K. B.; Manetsch, R. In situ click chemistry: a powerful
means for lead discovery. Expert Opin. Drug DiscoVery 2006, 1,
525-538.
(25) Roper, S.; Kolb, H. C. Click chemistry for drug discovery. In
Fragment-Based Approaches in Drug DiscoVery; Jahnke, W., Er-
lanson, D. A., Eds.; Wiley-VCH: Weinheim, Germany, 2006; Vol.
34, pp 313-339.
(26) Fokin, V. V.; Sharpless, K. B. A practical and highly efficient
aminohydroxylation of unsaturated carboxylic acids. Angew. Chem.,
Int. Ed. 2001, 40, 3455-3457.
(27) Liang, B.; Huang, M.; You, Z.; Xiong, Z.; Lu, K.; Fathi, R.; Chen,
J.; Yang, Z. Pd-catalyzed copper-free carbonylative Sonogashira
reaction of aryl iodides with alkynes for the synthesis of alkynyl
ketones and flavones by using water as a solvent. J. Org. Chem.
2005, 70, 6097-100.
(45) 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, 844-53.
JM070467L