F-18 stilbenes as PET imaging agents for detecting β-amyloid plaques in the brain

Article abstract of DOI:10.1021/jm050166g

Imaging agents targeting β-amyloid (Aβ) may be useful for diagnosis and treatment of patients with Alzheimer's disease (AD). Compounds 3e and 4e are fluorinated stilbene derivatives displaying high binding affinities for Aβ plaques in AD brain homogenates (K_i = 15 ± 6 and 5.0 ± 1.2 nM, respectively). In vivo biodistributions of [ ¹⁸F]Se and [¹⁸F]4e in normal mice exhibited excellent brain penetrations (5.55 and 9.75% dose/g at 2 min), and rapid brain washouts were observed, especially for [¹⁸F]4e (0.72% dose/g at 60 min). They also showed in vivo plaque labeling in APP/PS1 or Tg2576 transgenic mice, animal models for AD. Autoradiography of postmortem AD brain sections and AD homogenate binding studies confirmed the selective and specific binding properties to Aβ plaques. In conclusion, the preliminary results strongly suggest that these fluorinated stilbene derivatives, [¹⁸F]3e and [¹⁸F]4e, are suitable candidates as Aβ plaque imaging agents for studying patients with AD.

Full text of DOI:10.1021/jm050166g

5980
J. Med. Chem. 2005, 48, 5980-5988
F-18 Stilbenes as PET Imaging Agents for Detecting â-Amyloid Plaques in the
Brain
Wei Zhang,† Shunichi Oya,† Mei-Ping Kung,† Catherine Hou,† Donna L. Maier,‡ and Hank F. Kung*,†,§
Departments of Radiology and Pharmacology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, and
Department of Neuroscience, AstraZeneca, Wilmington, Delaware 19850
Received February 21, 2005
Imaging agents targeting â-amyloid (Aâ) may be useful for diagnosis and treatment of patients
with Alzheimer’s disease (AD). Compounds 3e and 4e are fluorinated stilbene derivatives
displaying high binding affinities for Aâ plaques in AD brain homogenates (Ki ) 15 ( 6 and
5.0 ( 1.2 nM, respectively). In vivo biodistributions of [18F]3e and [18F]4e in normal mice
exhibited excellent brain penetrations (5.55 and 9.75% dose/g at 2 min), and rapid brain
washouts were observed, especially for [18F]4e (0.72% dose/g at 60 min). They also showed in
vivo plaque labeling in APP/PS1 or Tg2576 transgenic mice, animal models for AD. Autorad-
iography of postmortem AD brain sections and AD homogenate binding studies confirmed the
selective and specific binding properties to Aâ plaques. In conclusion, the preliminary results
strongly suggest that these fluorinated stilbene derivatives, [18F]3e and [18F]4e, are suitable
candidates as Aâ plaque imaging agents for studying patients with AD.
Introduction
Alzheimer’s disease (AD) is a brain disorder associ-
ated with progressive memory loss and decrease of
cognitive function. Currently, a definitive diagnosis of
AD can only be established by demonstrating the
presence of abundant senile plaques and neurofibrillary
tangles in the postmortem brain.1,2 The senile plaques
are extracellular deposits of amyloid fibrils of â-amyloid
(Aâ), and their presence is strongly associated with
Figure 1. Structures of PET imaging agents proven to target
amyloid plaques in AD patients.
pathogenesis of the disease. Thus, specific in vivo
imaging agents targeting Aâ plaques may serve as
suitable markers for monitoring the amyloid burden
[11C]PIB,6,11,12 and a 18F-labeled probe, [18F]FDD-
following the disease progression and further provide
NP,9,13,14 for plaque and tangle visualization in living
supporting evidence for therapeutic intervention.3 Cur-
AD patients have demonstrated the potential utility of
rently, many efforts focusing on development of thera-
in vivo imaging (Figure 1). Similarly, our group has
peutic approach targeting Aâ plaques or reversing the
also prepared a 11C-labeled PET ligand, [11C]SB-13
effects of the plaque-deposition have been reported.4,5
(4-N-methylamino-4′-hydroxystilbene), a stilbene de-
Development of imaging agents for direct mapping of
rivative,7,15 and a 123I-labeled SPECT ligand, IMPY
Aâ aggregates in the living brain is an active research
(6-iodo-2-(4′-N,N-dimethylamino)phenylimidazo[1,2-a]-
area in recent years. Several research groups have
pyridine).16-18 Both tracers showed selective and high
reported biomarkers for imaging Aâ plaques in the
binding affinities to Aâ plaques.19 As expected, [11C]SB-
brain.6-10 The Aâ-plaque-specific imaging agents can be
13 displayed a high accumulation in the frontal cortex
labeled with short-lived isotopes suitable for in vivo
(presumably an area containing a high density of Aâ
imaging studies.10 Two isotopes,99mTc (T1/2, 6 h, 140 keV)
plaques) in mild to moderate AD patients, but not in
and 123I (T1/2, 13 h, 159 keV), are commonly used for
age-matched control subjects;7 while the SPECT ligand,
single photon emission computed tomography (SPECT);
[
123I[IMPY, is currently under active clinical evaluation.
while 11C (T1/2, 20 min, 511 keV) and 18F (T1/2, 110 min,
511 keV) are often used for positron emission tomogra-
phy (PET). It is generally accepted that SPECT is more
convenient and cost-effective, but PET gives a better
imaging resolution. Recently, successful preliminary
reports using a 11C-labeled benzothiazole derivative,
Other structurally similar compounds have been re-
cently reported for targeting Aâ plaques in the brain.20-22
Initially, we reported an iodinated derivative of stil-
bene, 3-iodo-4′-diaminostilbene, targeting amyloid plaques
for SPECT imaging.23 However, the unfavorable in vivo
kinetic properties make it unattractive for further
development as a useful in vivo SPECT tracer for
amyloid plaques in the brain. We have subsequently
identified another stilbene derivative, SB-13, labeled
with C-11 as a potential PET tracer for plaque imag-
ing.15 It was clinically demonstrated that [11C]SB-13 can
detect senile plaques present in AD patients.7 However,
* Hank F. Kung, Ph.D., Department of Radiology, University of
Pennsylvania, 3700 Market St., Rm. 305, Philadelphia, PA 19104.
Tel:
(215)662-3096, Fax:
(215)349-5035; e-mail:
kunghf@
sunmac.spect.upenn.edu.
† Department of Radiology, University of Pennsylvania.
‡ AstraZeneca.
§ Department of Pharmacology, University of Pennsylvania.
10.1021/jm050166g CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/23/2005

F-18 Aâ Imaging Agents
Journal of Medicinal Chemistry, 2005, Vol. 48, No. 19 5981
Scheme 1a
a (a) SnCl2, HCl(c), EtOH; (b) (CHO)n, NaBH3CN, AcOH, rt; (c) (1) NaOMe, MeOH, (CHO)n; (2) NaBH4; (d) 8m, K2CO3, DMF, 100 °C;
(e) HCl, CH3COCH3, rt; (f) TsCl, Py, 0 °C; (g) TBAF, THF, reflux; (h) 8n, K2CO3, DMF, 100 °C; (i) (BOC)2O, THF, reflux; (j) TBAF (1 M),
THF, rt; (k) DHP, PPTS, DCM, rt; (l) MsCl, Et3N, DCM, rt.
Scheme 2a
the short half-life (20 min) of C-11 may limit the
usefulness of [11C]SB-13 or [11C]PIB for a widespread
clinical application. One major focus of our effort is in
development of 18F-labeled plaque-specific imaging
agents, because the longer half-life of F-18 and stability
in solution allow the use of the radioligand over a long
period of time. While this work was in progress a series
of 18F-labeled styrylbenzoxazole compounds, such as
2-(4-methylaminostyryl)-6-(2-[18F]fluoroethoxy) benzox-
azole, were reported to show promise as potential PET
imaging agents targeting amyloid plaques in the brain.8
Reported herein are the synthesis and in vitro and
in vivo evaluations of a novel series of two 18F-labeled
stilbene derivatives as prospective PET radiotracers for
imaging amyloid plaques in the brain.
a (a) 7m: (CH3O)2C(CH3)2, TsOH, reflux; 7n: HOBT, Si(t-
Bu)2Cl2, Et3N, DCM, reflux; (b) CBr4, PPh3, Py, DCM, rt.
compound 1e was similarly synthesized by a coupling
reaction of 1a with 8m24 followed by tosylation and
fluorination. The synthesis of compound 4e was ac-
complished by reduction of the nitro group of 1e with
SnCl2/EtOH followed by a monomethylation of the
amino group with (CHO)n, NaOCH3, and NaBH4. The
nitro group of the acetal-protected intermediate 1b was
reduced to amine 2c and then monomethylated to give
compound 4c. Similarly, 2a was first monomethylated
to give the compound 4a, after which 4a was coupled
with 8n to give 5f. The di-tert-butylsilyl group of 5f was
removed with 1 N TBAF in THF at room temperature
to give diol 5c. The obtained 5c could be either mono-
tosylated to give 5d or monoprotected by THP26 to give
5g. From 5g, the desired mesylated precusor 5h for
radiolabeling can thus be obtained.
A related p-bromobenzyl compound 3j was also syn-
thesized as shown in Scheme 3. The substituted mal-
onate 927 was reduced to diol 10 with DIBALH and then
reacted with 1 equiv of TBSCl to give 11. The unpro-
tected OH was then converted into bromide 12 with
CBr4/PPh3. Compound 12 was reacted with 3a to give
3i which was treated with TBAF to remove TBS group
to yield 3j.
Results and Discussion:
Chemistry. Syntheses of compounds 3e, 4e and
radiolabelings of precursors 3d, 5h to prepare [18F]3e
and [18F]4e are shown in Scheme 1 and Scheme 4. To
prepare compound 3a the nitro group of 4-nitro-4′-
hydroxystilbene, 1a, was reduced with SnCl2/HCl(c) in
ethanol to give the corresponding amine 2a. Following
a treatment with (CHO)n and NaBH3CN, the dimethyl-
amino compound 3a was obtained. Compound 3b was
obtained by reacting the hydroxylstilbene 3a with
bromide 8m,24 which was separately prepared as shown
in Scheme 2, and potassium carbonate in anhydrous
DMF. Compound 3c was obtained by treatment of 3b
with 1 N HCl in acetone to remove the protected group.
Reacting diol 3c with 1.5 equiv of tosyl chloride in
pyridine gave a mixture; however, monotosylate 3d
could be isolated from the mixture by silica gel chro-
matography with 5% methanol in dichloromethane as
the eluent. The tosylate group of 3d was converted to
fluoride 3e by refluxing with anhydrous TBAF in THF.25
The tosyl compound 3d was also used as the starting
material to obtain radiolabeled compound, [18F]3e. Nitro
Several starting materials were evaluated in a ra-
diofluoride displacement reaction. To obtain dimethyl-
amino derivative [18F]3e, the tosylated precursor 3d was

5982 Journal of Medicinal Chemistry, 2005, Vol. 48, No. 19
Zhang et al.
Scheme 3a
a (a) DIBALH, THF, 0 °C; (b) TBSCl, Et3N, DCM, rt; (c) CBr4, PPh3, DCM, rt; (d) K2CO3, 12, DMF, 100 °C; (e) TBAF(1 M), THF, rt.
Scheme 4a
a (a) [18F]HF/K2CO3/K222 DMSO, (b) aq HCl.
Table 1. Inhibition Constants (Ki, nM) of Compounds on
mixed with [18F]fluoride/potassium carbonate and Krypto-
fix 222 in DMSO and heated at 120 °C for 4 min
(Scheme 4). Crude product was purified by HPLC to
I-125-IMPY Binding to Amyloid Plaques in AD Brain
Homogenates
compound
Ki ( SEM (nM)a
compound
Ki ( SEM (nM)a
attain >99% of the radiochemical purity with 10%
radiochemical yield (decay corrected). The procedure
took 90 min, and specific activity was estimated to be
70 Ci/mmol at the end of synthesis. To obtain the
N-monomethyl derivative [18F]4e, tosylate 5d was first
prepared as the precursor for radiolabeling. However,
the purification of [18F]4e was tedious and time-
consuming resulting in a low yield. The situation was
greatly improved when using 5h as the precursor for
labeling, of which mesylate was used instead of tosylate,
and the free OH was protected with THP. An advantage
of using mesylate 5h as the radiolabeling precursor is
that reaction proceeded more cleanly. As a result, the
amount and the number of unknown side-products were
significantly reduced. Specific activity estimated by
comparing UV peak intensity of labeled compound with
reference nonradioactive compound of known concentra-
tion improved at least 10 times at the end of synthesis
comparing with that obtained from tosylated precursor
5d. A similar procedure was carried out to obtain [18F]-
4e from the mesylated precursor 5h. After the initial
reaction in DMSO, the mixture was treated with aque-
ous HCl to remove BOC and THP groups. Radiochemi-
cal purity was >99% after HPLC purification and the
radiochemical yield was 20% (decay corrected). The total
syntheses took 110 min and specific activity was esti-
mated to be 900-1000 Ci/mmol at the end of synthesis.
Biological Studies. Binding affinities of the new
series of stilbene derivatives were examined in a binding
assay using AD brain homogenates and [125I]IMPY as
the radioligand.19 Compound 4a (SB-13), as shown
2a
2e
3a
3b
3c
3d
95 ( 8.0
15 ( 4.0
1.1 ( 0.2
59 ( 10
38 ( 5
3e
15 ( 6
80 ( 20
1.2 ( 0.2*
32.5 ( 5.0
5.0 ( 1.2
2.8 ( 0.5
3j
4a (SB-13)
4c
4e
150 ( 30
PIB
a Each value was determined three times with duplicate for each
measurement. b The value was reported previously.19
previously, displayed a high binding affinity.19 With an
additional methyl group attached to the nitrogen atom
(dimethylamino vs monomethylamino in 3a vs 4a), a
similar high binding affinity was observed (1.1 nM and
1.2 nM for 3a and 4a, respectively) (Table 1). Further
modifications of 3a result in epoxy and diol derivatives
3b and 3c, but they showed lower binding affinities (Ki
) 59 and 38 nM, respectively). A relatively high binding
affinity (Ki ) 15 nM) was obtained with 3e (a fluorinated
derivative).
Since the binding affinity of 3e appeared promising,
the corresponding tosylated derivative 3d was prepared
as a starting material for radiolabeling with 18F. Under
the chosen HPLC conditions (Hamilton PRP-1 column,
CH3CN/dimethyl glutarate buffer (5 mM, pH 7) ) 9/1),
the tosylated and the diol derivatives can be clearly
separated from the desired 18F-labeled stilbene [18F]3e.
This HPLC system has been successfully applied to
remove the potential pseudo-carrier, the hydroxyl com-
pound 3c. Similarly, the monomethylamino compounds
displayed the same trend with a high affinity for the
fluorinated derivative 4e (Ki ) 5 nM) and a lower

F-18 Aâ Imaging Agents
Journal of Medicinal Chemistry, 2005, Vol. 48, No. 19 5983
Table 2. Biodistribution in ICR Mice after an iv Injection of
[
18F]Tracers in 0.1% BSA (%dose/g, avg of three mice ( SD)
organ
2 min
30 min
1 h
2 h
[
18F]3e (Partition Coefficient ) 1375)
blood
heart
1.79 ( 0.25
10.43 ( 0.96
1.65 ( 0.15 1.51 ( 0.20 1.18 ( 0.08
2.76 ( 0.26 1.45 ( 0.17 0.91 ( 0.04
1.53 ( 0.30 1.22 ( 0.20 0.66 ( 0.13
muscle 0.84 ( 0.30
lung
93.40 ( 16.91 12.93 ( 4.75 5.00 ( 0.84 3.74 ( 0.35
kidney 13.32 ( 1.41
4.64 ( 0.26 2.97 ( 0.29 2.19 ( 0.26
2.45 ( 0.22 1.24 ( 0.11 0.99 ( 0.14
Figure 3. In vivo plaque labeling visualized by ex vivo
autoradiogram: (A) [18F]3e in a APP/PS1 mouse brain; (B) [18F]-
4e in a Tg2576 mouse. The animal was injected via tail vein
with 270 µCi [18F]3e or 260 µCi [18F]4e and sacrificed 2 h (for
[18F]3e) or 1 h (for [18F]4e) after tracer injection.
spleen
liver
skin
brain
bone
4.43 ( 0.26
14.13 ( 0.73 14.65 ( 1.73 13.46 ( 2.62 9.51 ( 1.04
0.67 ( 0.02
5.55 ( 0.64
1.57 ( 0.12
1.69 ( 0.38 1.65 ( 0.15 1.00 ( 0.07
5.21 ( 0.66 2.97 ( 0.43 1.37 ( 0.14
2.06 ( 0.35 6.50 ( 2.92 8.63 ( 1.78
[
18F]4e (Partition Coefficient ) 889)
blood
heart
2.60 ( 0.23
7.49 ( 0.95
2.47 ( 0.25 1.82 ( 0.30 0.78 ( 0.02
2.06 ( 0.24 1.54 ( 0.12 0.65 ( 0.02
1.31 ( 0.45 0.85 ( 0.41 0.41 ( 0.04
8.90 ( 1.39 7.51 ( 1.01 4.15 ( 0.61
4.13 ( 0.35 3.06 ( 0.17 1.50 ( 0.08
2.41 ( 0.57 1.37 ( 1.08 1.22 ( 0.34
muscle 0.74 ( 0.11
lung
26.13 ( 1.91
kidney 11.70 ( 1.49
spleen
liver
skin
brain
bone
5.01 ( 1.64
20.61 ( 2.00 19.60 ( 1.40 14.58 ( 1.35 8.14 ( 2.33
0.92 ( 0.23
9.75 ( 1.38
1.53 ( 0.32
1.49 ( 0.25 1.22 ( 0.21 0.57 ( 0.08
1.70 ( 0.41 0.72 ( 0.13 0.31 ( 0.04
2.13 ( 0.19 5.30 ( 1.94 7.76 ( 0.29
affinity for the diol derivative 4c (Ki ) 32.5 nM). In
addition, other derivatives such as p-bromobenzyl-
substituted 3j showed a lower binding affinity (Ki ) 80
nM). Interestingly, the free amino derivative 2e, under
a similar binding assay condition, showed a Ki value of
15 ( 4.0 nM which was comparable to the dimethyl-
amino (3e) and monomethylamino (4e) derivatives
(competing with [125I]IMPY binding) (Table 1). This
result is different from the original stilbene compounds
showing a much lower binding affinity for the free amino
compound (Ki ) 1.1, 1.2, and 95 nM for 3a, 4a (SB-13),
and 2a, respectively). Surprisingly, despite the fact that
2e showed reasonably good in vitro binding affinity (a
relatively low Ki value), [18F]2e exhibited no specific
binding signal for amyloid plaques (using either AD
brain section labeling or AD homogenate binding) (data
not shown). Therefore, we have abandoned this free
amino compound and focused only on [18F]3e and [18F]-
4e as likely candidates. As expected, both [18F]3e and
[18F]4e clearly label plaques by in vitro autoradiography
using postmortem AD brain sections showing a high
plaque labeling and a low background (see Figure 2).
The same results were obtained using brain sections of
double transgenic (APP/PS1) mice (data not shown).
Previously, using an in vitro binding assay we have
reported saturable and high binding affinities in post-
mortem brain homogenates of AD patients for two
potential tracers, [3H]SB-13 (a N-methylamino deriva-
tive) and [125I]IMPY (a N,N-dimethylamino deriva-
Figure 4. [18F]4e showed binding to AD and control tissue
homogenates prepared from dissected gray and white matter
of cortical regions. High specific binding was detected mainly
in gray matter with relatively low binding in white matter
homogenates. In contrast, homogenates from the control
showed significantly lower specific binding of [18F]4e.
tive).19 Interestingly, [18F]3e contains a N,N-dimethyl-
amino group, but it did not display any specific Aâ
plaque binding signals in the brain homogenates bind-
ing assay (data not shown). However, using a mono-N-
methylated derivative, [18F]4e, in the same binding
assay, a distinct binding signal was observed in the gray
matter homogenates of AD patients. In the white matter
of AD patients, where Aâ plaque deposits were low to
nonexistent, the binding signal was significantly lower
(6-8 times lower) (Figure 4), whereas in assays using
brain homogenates from control subjects, the binding
signals in both gray and white matters were low,
suggesting that the binding was highly selective to the
presence of Aâ plaque deposits. The specific binding
observed for [18F]4e in AD brain homogenates was
saturable, and the binding capacity (Bmax) was in the
range of 10-20 pmol/mg tissue (data not shown).
Detailed in vitro binding characterization of [18F]4e will
be published elsewhere as a separate paper.
After an iv injection, both [18F]3e and [18F]4e dis-
played good initial brain uptakes in normal mice (5.55%
and 9.75% ID/g at 2 min postinjection for [18F]3e and
[18F]4e, respectively). The rate of washout of [18F]3e
from the brain was slower as compared to that of [18F]-
4e (1.37% and 0.31%ID/g at 120 min postinjection for
[18F]3e and [18F]4e, respectively). A relatively higher
lipophilicity was observed for [18F]3e (partition coef-
ficient ) 1375 and 889 for [18F]3e and [18F]4e, respec-
tively). This disparity may account for the longer brain
retention observed for [18F]3e. Initial blood levels were
relatively low for both tracers (1.79% and 2.60% ID/g
for [18F]3e and [18F]4e, respectively), and there was a
Figure 2. In vitro autoradiography of brain sections from AD
patients labeled with [18F]3e (A) and [18F]4e (B). The Aâ
plaques were clearly labeled with both 18F tracers with low
background labeling.

5984 Journal of Medicinal Chemistry, 2005, Vol. 48, No. 19
Zhang et al.
Lab) (3.0 g, 0.012 mol) in ethanol (100 mL) followed by the
addition of concentrated hydrochloric acid (5.0 mL). The
solution was brought to reflux for 3 h and cooled to room
temperature stirring overnight. Aqueous sodium hydroxide (1
N) was added to adjust the pH to 8-9. After standard workup
with dichloromethane, crude product 2a was obtained and was
used in the following step without further purifications. 2a
(2.6 g, ∼100%): 1H NMR (DMSO-d6) δ 9.39 (s, 1H), 7.30 (d,
2H, J ) 8.5 Hz), 7.20 (d, 2H, J ) 8.5 Hz), 6.80 (m, 2H), 6.72
(d, 2H, J ) 8.5 Hz), 6.53 (d, 2H, J ) 8.5 Hz), 5.19 (s, 2H).
4-N,N′-Dimethylamino-4′-hydroxystilbene (3a). To a
mixture of 2a (211 mg, 1.0 mmol), paraformaldehyde (300 mg,
10 mmol), and sodium cyanoborohydride (189 mg, 3.0 mmol)
was added acetic acid (10 mL). The whole mixture was stirred
at room-temperature overnight and then poured into 100 mL
of water. Sodium carbonate was added to adjust the pH to 8-9.
After standard workup with 5% methanol in dichloromethane,
the residue was purified by silica gel column chromatography
(2.5% methanol in dichloromethane) to afford 3a as a white
solid (214 mg, 89.5%): 1H NMR δ 7.37 (m, 4H), 6.87 (s, 2H),
6.75 (m, 4H), 4.68 (s, 1H), 2.98 (s, 6H).
(4-{2-[4-(2,2-Dimethyl-[1,3]dioxan-5-ylmethoxy)phenyl]-
vinyl}phenyl)dimethylamine (3b). Under a nitrogen at-
mosphere, 3a (100 mg, 0.38 mmol) was dissolved in anhydrous
DMF (5.0 mL). Potassium carbonate (140 mg, 1.0 mmol) was
added to this solution followed by 5-bromomethyl-2,2-dimethyl-
[1,3]dioxane 8m24 (105 mg, 0.5 mmol). The mixture was heated
to 100 °C and stirred overnight. After cooling to room tem-
perature, standard workup with dichloromethane was applied
and the residue was purified by silica gel preparative TLC (1%
methanol in dichloromethane) to afford compound 3b (100 mg,
72%): 1H NMR δ 7.38 (m, 4H), 6.88 (m, 4H), 6.70 (d, 2H, J )
8.7 Hz), 4.08 (m, 4H), 3.87 (m, 2H), 2.96 (s, 6H), 2.13 (m, 1H),
1.46 (s, 3H), 1.42 (s, 3H). Anal. (C23H29NO3) C, H, N.
2-{4-[2-(4-Dimethylaminophenyl)vinyl]phenoxymethyl}-
propane-1,3-diol (3c). Compound 3b (180 mg, 0.49 mmol)
was suspended in acetone (5.0 mL) and cooled to 0 °C with an
ice bath. 1 N HCl (5.0 mL, 5.0 mmol) was slowly added over
20 min. The suspension turned into a clear solution during
the addition. The solution was stirred at 0 °C for an additional
1.5 h and then warmed to room temperature in 0.5 h.
Saturated sodium bicarbonate was added to adjust pH to 8-9.
After standard workup with dichloromethane, the residue was
purified by silica gel preparative TLC (5% methanol in
dichloromethane) to afford compound 3c as a white solid (140
mg, 87%): 1H NMR δ 7.40 (m, 4H), 6.88 (m, 4H), 6.74 (m, 2H),
4.10 (d, 2H, J ) 5.47 Hz), 3.89 (d, 4H, J ) 5.28 Hz), 2.98 (s,
6H), 2.22 (m, 1H). Anal. (C20H25NO3) C. H. N.
Toluene-4-sulfonicAcid3-{4-[2-(4-(Dimethylamino)phen-
yl)vinyl]phenoxy}-2-hydroxymethylpropyl Ester (3d).
Compound 3c (158 mg, 0.49 mmol) was dissolved in anhydrous
pyridine (15 mL) and cooled to 0 °C with an ice bath. Tosyl
chloride (137 mg, 0.72 mmol) was added, and the solution was
stirred at 0 °C for 2 h. After standard workup with dichlo-
romethane, the residue was purified by silica gel preparative
TLC (5% methanol in dichloromethane) to afford monotosylate
compound 3d as a white solid (95 mg, 41%): 1H NMR δ 7.75
(d, 2H, J ) 8.26 Hz), 7.37 (m, 4H), 7.26 (m, 2H), 6.88 (m, 2H),
6.72 (m, 4H), 4.26 (d, 2H, J ) 5.66 Hz), 3.97 (d, 2H, J ) 5.96
Hz), 3.79 (d, 2H, J ) 5.24 Hz), 2.95 (s, 6H), 2.38 (m, 4H). Anal.
(C27H31NO5S) C, H, N.
3-{4-[2-(4-Dimethylaminophenyl)vinyl]phenoxy}-2-flu-
oromethylpropan-1-ol (3e). Compound 3d (40 mg, 0.083
mmol) was dissolved in anhydrous THF (5.0 mL). Under a
nitrogen atmosphere, anhydrous TBAF 25 (150 mg, 0.5 mmol)
in anhydrous THF (1.0 mL) was slowly added. The solution
was then heated to reflux for 3 h. After cooling to room
temperature, standard workup with dichloromethane was
applied and the residue was applied for silica gel preparative
TLC (5% methanol in dichloromethane) to afford product 3e
(17 mg, 62%): 1H NMR δ 7.40 (m, 4H), 6.89 (m, 4H), 6.70 (d,
2H, J ) 8.82 Hz), 4.67 (d d, 2H, J1 ) 47.1 Hz, J2 ) 5.46 Hz),
4.10 (d, 2H, J ) 5.86 Hz), 3.88 (d, 2H, J ) 5.24 Hz), 2.97 (s,
6H), 2.40 (m, 1H), 1.76 (s, 1H). Anal. (C20H24FNO2) C, H, N.
significant reduction at 2 h postinjection (1.18 and
0.78%ID/g). In vivo defluorination was likely for both
tracers, because the bone uptake showed increasing
uptake reaching 7-8%ID/g at 2 h postinjection.
To examine the in vivo labeling of Aâ plaques in a
living brain, we have tested both candidates using
mouse models, APP/PS18,28 or Tg2576 transgenic mice,29
which are specifically engineered to over produce the
Aâ plaques in the brain. When subjected to in vivo
labeling of Aâ plaques (after an iv injection) in these
models, distinctive Aâ plaque labeling can be observed
for both [18F]3e and [18F]4e (Figure 3A and 3B). The
specific in vivo targeting for Aâ plaques, especially for
[18F]4e, demonstrates the feasibility of using it as an
in vivo PET imaging agent for detecting senile plaques.
Both of these transgenic mouse models have been
successfully used in studying relationships between
deposit of Aâ plaques and AD.30,31 For the purpose of
this in vivo Aâ plaque labeling study by a 18F tracer in
a transgenic mouse model we did not make a distinction
between using either one of these models. Recently, the
presence of soluble oligomer of amyloid-â peptides, prior
to their aggregation to fribrillary forms, has attracted
attention as the primary underlying feature leading to
neuronal toxicity and symptoms of AD.32-34 The PET
imaging agents reported in this paper will not be able
to detect the soluble form of amyloid-â peptides; how-
ever, the presence of the oligomers is likely a prelude
leading to the formation of Aâ aggregates in the brain.
Therefore, from a diagnostic perspective, imaging agents
targeting Aâ plaques in the brain will still be very useful
for the diagnosis of AD. Efforts in developing novel
ligands for binding the soluble Aâ oligomers will be an
interesting and highly important research topic.
Conclusion
A new series of novel stilbene derivatives displaying
high binding affinities to Aâ plaques was successfully
prepared as potential PET imaging agents for AD. Both
4-dimethylamino and 4-monomethylamino stilbenes
([18F]3e and [18F]4e) entered the brain of normal mice
readily and quickly. [18F]4e showed a fast washout with
a low normal brain retention. Specific plaque labeling
was demonstrated by in vitro and ex vivo autoradiog-
raphy of brain sections. Favorable pharmacokinetic
properties and specific plaque labeling properties of
these 18F-labeled stilbene derivatives, especially [18F]-
4e, warrant further investigation.
Experimental Section
All reagents used in syntheses were commercial products
and were used without further purification unless otherwise
1
indicated. H NMR spectra were obtained on a Bruker DPX
spectrometer (200 MHz) in CDCl3 unless otherwise indicated.
Chemical shifts are reported as δ values (parts per million)
relative to internal TMS. Coupling constants are reported in
Hertz. The multiplicity is defined by s (singlet), d (doublet), t
(triplet), br (broad), m (multiplet). Elemental analyses were
performed by Atlantic Microlab Inc. For each procedure,
“standard workup” refers to the following steps: addition of
indicated organic solvent, washing the organic layer with
water then brine, separation of the organic layer from the
aqueous layer, drying the combined organic layers with
anhydrous sodium sulfate, filtering off the sodium sulfate, and
removing the organic solvent under reduced pressure.
4-Amino-4′-hydroxystilbene (2a). Stannous chloride (11.8
g, 0.062 mol) was added to a solution of compound 1a (Frinton

F-18 Aâ Imaging Agents
Journal of Medicinal Chemistry, 2005, Vol. 48, No. 19 5985
2,2-Dimethyl-5-{4-[2-(4-nitrophenyl)vinyl]phenoxy-
methyl}-[1,3]dioxane (1b). Compound 1b was prepared from
1a (241 mg, 1.0 mmol) with the same procedure described for
compound 3b. 1b (260 mg, 70%): 1H NMR δ 8.19 (d, 2H, J )
8.80 Hz), 7.49 (m, 4H), 7.07 (m, 2H), 6.90 (d, 2H, J ) 8.80
Hz), 4.12 (m, 4H), 3.89 (d, 2H), 2.10 (m, 1H), 1.48 (s, 3H), 1.43
(s, 3H). Anal. Calcd. (C21H23NO5) C, H, N.
2-{4-[2-(4-Nitrophenyl)vinyl]phenoxymethyl}propane-
1,3-diol (1c). Compound 1c was prepared from 1b (260 mg,
0.7 mmol) with the same procedure described for compound
3c. 1c (190 mg, 82%): 1H NMR (CD3OD) δ 8.19 (d, 2H, J )
8.80 Hz), 7.72 (d, 2H, J ) 8.80 Hz), 7.55 (d, 2H, J ) 8.70 Hz),
7.24 (q, 2H), 6.96 (d, 2H, J ) 8.70 Hz), 4.09 (d, 2H, J ) 5.78
Hz), 3.74 (d, 4H, J ) 5.94 Hz), 2.14 (m, 1H). Anal. (C18H19-
NO5) C, H, N.
(d, 2H, J ) 5.78 Hz), 2.30 (m, 1H), 1.39 (s, 1H), 1.04 (s, 9H),
1.02 (s, 9H).
5-Bromomethyl-2,2-di-tert-butyl-[1,3,2]dioxasilinane
(8n). Compound 7n (123 mg, 0.5 mmol) was dissolved in
dichloromethane (10 mL) and cooled to -10 °C with an
ethanol-ice bath. Pyridine (1 mL) was added followed by carbon
tetrabromide (220 mg, 0.66 mmol). Triphenylphosphine (174
mg, 0.66 mmol) was added in portions, and the solution was
stirred at -10 °C for 2 h then raised to room-temperature
overnight. Solvent was removed under reduced pressure, and
residue was applied for silica gel column chromatography (10%
ethyl acetate in hexane) to afford compound 8n, which was
used in the following step without further purification. 8n (130
mg, 84%): 1H NMR δ 4.20 (m, 2H), 3.93 (t, 2H), 3.20 (d, 2H,
J ) 6.19 Hz), 2.39 (m, 1H), 1.04 (s, 9H), 1.01 (s, 9H).
Toluene-4-sulfonic Acid 2-Hydroxymethyl-3-{4-[2-(4-
nitrophenyl)vinyl]phenoxy}propyl Ester (1d). Compound
1d was prepared from 1c (80 mg, 0.24 mmol) with the same
procedure described for compound 3d. 1d (66 mg, 56%): 1H
NMR δ 8.18 (d, 2H, J ) 8.82 Hz), 7.77 (d, 2H, J ) 8.32 Hz),
7.58 (d, 2H, J ) 8.82 Hz), 7.45 (d, 2H, J ) 8.73 Hz), 7.28 (d,
2H, J ) 8.18 Hz), 7.09 (q, 2H), 6.81 (d, 2H, J ) 8.73 Hz), 4.27
(d, 2H, J ) 5.70 Hz), 4.01 (m, 2H), 3.80 (d, 2H, J ) 5.61 Hz),
2.40 (m, 4H), 2.02 (s, 1H). Anal. (C25H25NO7S) C, H, N.
2-Fluoromethyl-3-{4-[2-(4-nitrophenyl)vinyl]phenoxy}-
propan-1-ol (1e). Compound 1e was prepared from 1d (33
mg, 0.069 mmol) with the same procedure described for
compound 3e. 1e (20 mg, 88%): 1H NMR δ 8.19 (d, 2H, J )
8.83 Hz), 7.58 (d, 2H, J ) 8.84 Hz), 7.48 (d, 2H, J ) 8.74 Hz),
7.10 (q, 2H), 6.94 (d, 2H, J ) 8.68 Hz), 4.69 (d d, 2H, J1 ) 47.1
Hz, J2 ) 5.36 Hz), 4.15 (d, 2H, J ) 5.89 Hz), 3.90 (d, 2H, J )
5.43 Hz), 2.43 (m, 1H), 1.74 (s, 1H). Anal. (C18H18FNO4) C, H,
N.
3-{4-[2-(4-Aminophenyl)vinyl]phenoxy}-2-fluorometh-
ylpropan-1-ol (2e). Compound 2e was prepared from 1e (37
mg, 0.11 mmol) with the same procedure described for
compound 2a. 2e (24 mg, 71%): 1H NMR δ 7.35 (m, 4H), 6.90
(m, 4H), 6.66 (d, 2H, J ) 8.54 Hz), 4.69 (d d, 2H, J1 ) 47.1 Hz,
J2 ) 5.46 Hz), 4.12 (d, 2H, J ) 5.84 Hz), 3.90 (d, 2H, J ) 5.56
Hz), 3.70 (s, 2H), 2.39 (m, 1H), 1.71 (s, 1H). Anal. (C18H20FNO2)
C, H, N.
2-Fluoromethyl-3-{4-[2-(4-methylaminophenyl)vinyl]-
phenoxy}propan-1-ol (4e). Under a nitrogen atmosphere,
sodium methoxide (22 mg, 0.4 mmol) was added to a suspen-
sion of compound 2e (24 mg, 0.08 mmol) in methanol (6 mL)
followed by paraformaldehyde (12 mg, 0.4 mmol). The solution
was heated to reflux for 2 h and cooled to 0 °C with an ice
bath. Sodium borohydride (15 mg, 0.4 mmol) was added in
portions. Reaction mixture was brought to reflux again for 1
h and poured onto crushed ice. After standard workup with
dichloromethane, the residue was applied for silica gel pre-
parative TLC (4.5% methanol in dichloromethane) to afford
product 4e (23 mg, 92%): 1H NMR δ 7.37 (m, 4H), 6.87 (m,
4H), 6.59 (d, 2H, J ) 8.56 Hz), 4.69 (d, d, 2H, J1 ) 47.1 Hz, J2
) 5.44 Hz), 4.12 (d, 2H, J ) 5.86 Hz), 4.00 (s, 1H), 3.89. (d,
2H, J ) 5.52 Hz), 2.86 (s, 3H), 2.41 (m, 1H), 1.75 (s, 1H). Anal.
(C19H22FNO2) C, H, N.
4-N-Methylamino-4′-hydroxystilbene (4a). Compound
4a was prepared from 2a (105 mg, 0.5 mmol) with the same
procedure as described for compound 4e. 4a (100 mg, 89%):
1H NMR δ 7.34 (m, 4H), 6.86 (s, 2H), 6.79 (d, 2H, J ) 8.58
Hz), 6.60 (d, 2H, J ) 8.58 Hz), 2.85 (s, 3H).
(2,2-Di-tert-butyl-[1,3,2]dioxasilinan-5-yl)-methanol (7n).
To the solution of 2-hydroxypropyl-1,3-diol 6 (500 mg, 4.7
mmol) in anhydrous dichloromethane (15 mL) was added
HOBT(135 mg, 1.0 mmol). Under a nitrogen atmosphere,
triethylamine (6.5 mL, 4.9 g, 48 mmol) was added via a syringe
followed by di-tert-butyldichlorosilane (1.05 g, 5.0 mmol). The
solution was gently refluxed for 1 h and cooled to room
temperature. After standard workup with dichloromethane,
the residue was applied for silica gel column chromatography
(1% methanol in dichloromethane) to afford product 7n, which
was used for the following step without further purification.
7n (1.03 g, 89%): 1H NMR δ 4.17 (m, 2H), 3.92 (t, 2H), 3.50
(4-{2-[4-(2,2-Di-tert-butyl-[1,3,2]dioxasilinan-5-ylmeth-
oxy)phenyl]vinyl}phenyl)methylamine (4f). Under a ni-
trogen atmosphere, compound 4a (90 mg, 0.4 mmol) was
dissolved in anhydrous DMF (15.0 mL). Potassium carbonate
(560 mg, 4.0 mmol) was added followed by 5-bromomethyl-
2,2-di-tert-butyl-[1,3,2]dioxasilinane, 8n (127 mg, 0.4 mmol).
The suspension was heated to 100 °C and stirred overnight.
After cooling to room temperature, standard workup with
dichloromethane was applied and the residue was purified by
silica gel preparative TLC (dichloromethane) to afford com-
pound 4f (115 mg, 63%): 1H NMR δ 7.38 (m, 4H), 6.88 (s, 2H),
6.82 (d, 2H, J ) 8.64 Hz), 6.73 (d, 2H, J ) 8.42), 5.80 (s, 1H),
4.26 (m, 2H), 4.04 (t, 2H), 3.81 (d, 2H, J ) 5.82 Hz), 2.89 (s,
3H), 2.58 (m, 1H), 1.06 (s, 9H), 1.04 (s, 9H). Anal. (C27H39NO3-
Si) C, H, N.
(4-{2-[4-(2,2-Di-tert-butyl-[1,3,2]dioxasilinan-5-ylmeth-
oxy)phenyl]vinyl}phenyl)methylcarbamic Acid tert-Bu-
tyl Ester (5f). BOC anhydride (320 mg, 1.46 mmol) was added
to a solution of compound 4f (110 mg, 0.24 mmol) in anhydrous
THF (10 mL). Under the protection of nitrogen, triethylamine
(1.0 mL) was added via a syringe. The solution was then
refluxed for 34 h. After cooling to room temperature, standard
workup with dichloromethane was applied. Organic solvent
was removed under reduced pressure, and the residue was
purified through silica gel column chromatography to afford
compound 5f, which was used in the following step without
further purification. 5f (122 mg, 91%): 1H NMR δ 7.42 (d, 4H,
J ) 7.52 Hz), 7.20 (d, 2H, J ) 8.52 Hz), 6.98 (m, 2H), 6.84 (d,
2H, J ) 8.72 Hz), 4.26 (m, 2H), 4.05 (t, 2H), 3.82 (d, 2H, J )
5.84 Hz), 3.27 (s, 3H), 2.58 (m, 1H), 1.46 (s, 9H), 1.06 (s, 9H),
1.04 (s, 9H).
(4-{2-[4-(3-Hydroxy-2-hydroxymethylpropoxy)phenyl]-
vinyl}phenyl)methylcarbamic Acid tert-Butyl Ester (5c).
Compound 5f (120 mg, 0.22 mmol) was dissolved in anhydrous
THF (10 mL), and the solution was cooled to 0 °C with an ice
bath. Under a nitrogen atmosphere, TBAF (0.44 mL, 1 M in
THF, 0.44 mmol) was added via a syringe. The solution was
stirred at 0 °C for 0.5 h and then brought to room temperature
for another 2 h. After standard workup with dichloromethane,
the residue was applied for silica gel preparative TLC (5%
methanol in dichloromethane) to afford compound 5c (89 mg,
99%): 1H NMR δ 7.43 (d, 4H, J ) 8.68 Hz), 7.20 (d, 2H, J )
8.56 Hz), 6.98 (m, 2H), 6.90 (d, 2H, J ) 8.74 Hz), 4.14 (d, 2H,
J ) 5.96 Hz), 3.95 (d, 4H, J ) 5.24 Hz), 3.27 (s, 3H), 2.27 (m,
1H), 1.70 (s, 2H), 1.46 (s, 9H). Anal. (C24H31NO5) C, H, N.
[4-(2-{4-[2-Hydroxymethyl-3-(tetrahydropyran-2-yloxy)-
propoxy]phenyl}vinyl)phenyl]methylcarbamic Acid tert-
butyl Ester (5g). A solution of 5c (53 mg, 0.13 mmol) and 3,
4-dihydropyran (12.9 mg, 0.15 mmol) in dry dichloromethane
(12 mL) containing pyridinium p-toluenesulfonate26 (3.3 mg,
0.013 mol) was stirred at room temperature for 4 h. After
standard workup with dichloromethane, the residue was
applied for silica gel preparative TLC (5% methanol in dichlo-
romethane) to afford compound 5g, which was used in the
following step without further purifications. 5g (43 mg, 67%):
1H NMR δ 7.43 (d, 4H, J ) 8.46 Hz), 7.20 (d, 2H, J ) 8.46
Hz), 6.97 (m, 2H), 6.90 (d, 2H, J ) 8.62 Hz), 4.60(b, 1H), 3.95
(m, 6H), 3.70 (m, 1H), 3.54 (m, 1H), 3.26 (s, 3H), 2.34 (m, 1H),
1.70 (m, 6H), 1.46 (s, 9h).

5986 Journal of Medicinal Chemistry, 2005, Vol. 48, No. 19
Zhang et al.
Methanesulfonic Acid 3-(4-{2-[4-(tert-Butoxycarbonyl-
methylamino)phenyl]vinyl}phenoxy)-2-(tetrahydropyran-
2-yloxymethyl)propyl Ester (5h). Triethylamine (0.2 mL)
was added to a solution of compound 5g (43 mg, 0.087 mmol)
and methanesulfonyl chloride (29.7 mg, 0.26 mmol) in dry
dichloromethane (10 mL). The solution was stirred at room
temperature for 3.5 h. After standard workup with dichlo-
romethane, the residue was applied for silica gel preparative
TLC (2% methanol in dichloromethane) to afford compound
5h (43 mg, 86%): 1H NMR δ 7.43 (d, 4H, J ) 8.58 Hz), 7.20
(d, 2H, J ) 8.46 Hz), 6.97 (m, 2H), 6.90 (d, 2H, J ) 8.62 Hz),
4.59(b, 1H), 4.46 (d, 2H, J ) 5.66 Hz), 4.11 (m, 2H), 3.85 (m,
2H), 3.55 (m, 2H), 3.27 (s, 3H), 3.00 (s, 3H), 2.59 (m, 1H), 1.70
(m, 6H), 1.46 (s, 9h). HRMS m/Z calcd. For C30H41NO8S (M+-
Na+): 598.2451. Found: 598.2444.
2-{4-[2-(4-Aminophenyl)vinyl]phenoxymethyl}propane-
1,3-diol (2c). Compound 2c was prepared from 1b (200 mg,
0.54 mmol) with the same procedure described for 2a and was
used in the following step without further purifications. 2c (144
mg, 89%): 1H NMR (DMSO-d6) δ 7.40 (d, 2H, J ) 8.58 Hz),
7.22 (d, 2H, J ) 8.30 Hz), 6.91 (m, 4H), 6.54 (d, 2H, J ) 8.30
Hz), 5.22 (s, 2H), 4.51 (t, 2H, J ) 5.11 Hz), 3.97 (d, 2H, J )
5.85 Hz), 3.51 (t, 4H), 1.96 (m, 1H).
7.07 (d, 2H, J ) 8.2 Hz), 6.87 (m, 4H), 6.70 (d, 2H, J ) 8.2
Hz), 3.94(m, 2H), 3.75 (b, 2H), 2.97 (s, 6H), 2.76 (d, 2H, J )
7.4 Hz), 2.23 (m, 1H), 1.78 (s, 1H). Anal. (C26H28BrNO2) C, H,
N.
[18F]3-{4-[2-(4-Dimethylaminophenyl)vinyl]phenoxy}-
2-fluoromethylpropan-1-ol ([18F]3e). [18F]Fluoride, pro-
duced by a cyclotron using 18O(p,n)18F reaction, was passed
through a Sep-Pak Light QMA cartridge as an aqueous
solution in [18O]-enriched water. The cartridge was dried by
airflow, and the 18F activity was eluted with 2 mL of Kryptofix
222 (K222)/K2CO3 solution (22 mg of K222 and 4.6 mg of K2-
CO3 in CH3CN/H2O 1.77/0.23). 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
an argon stream. A solution of tosylate precursor 3d (4 mg) in
DMSO (0.2 mL) was added to the reaction vessel containing
the dried 18F activities. The solution was heated at 120 °C for
4 min. Water (2 mL) was added, and the mixture was extracted
with ethyl acetate (1 mL × 2). The combined organic layer
was dried (Na2SO4), and the solvent was removed using an
argon stream with gentle heating (55-60 °C). The residue was
dissolved in CH3CN and injected to HPLC for purification
[Hamilton PRP-1 column (7.0 × 305 mm, 10 µm); CH3CN/
dimethyl glutarate buffer (5 mM, pH 7) ) 9/1; flow rate ) 2
mL/min). Retention time of 3e was 11 min and well separated
from precursor 3d (tR ) 13 min). The same HPLC condition
was used for quality control (RCP > 99%). Specific activity
was estimated by comparing UV peak intensity of purified
2-{4-[2-(4-Methylaminophenyl)vinyl]phenoxymethyl}-
propane-1,3-diol (4c). Compound 4c was prepared from 2c
(100 mg, 0.33 mmol) with the same procedure described for
4a. 4c (104 mg, 99%): 1H NMR (DMSO-d6) δ 7.42 (d, 2H, J )
8.58 Hz), 7.30 (d, 2H, J ) 8.46 Hz), 6.88 (m, 4H), 6.52 (d, 2H,
J ) 8.42 Hz), 5.80 (m, 1H), 4.51 (t, 2H), 3.97 (d, 2H, J ) 5.85
Hz), 3.51 (t, 4H, J ) 5.95 Hz), 2.68 (d, 3H, J ) 4.7 Hz), 1.95
(m, 1H). Anal. (C19H23NO3) C, H, N.
[
18F]3e with a reference nonradioactive compound of known
concentration. Specific activity was estimated to be 70 Ci/mmol
after the preparation.
[
18F]2-Fluoromethyl-3-{4-[2-(4-methylaminophenyl)-
2-(4-Bromobenzyl)propane-1,3-diol (10). 2-(4-Bromoben-
27
vinyl]phenoxy}propan-1-ol ([18F]4e). The labeling reaction
was carried out as described above for dimethylamino com-
pound. The mesylate 5h (4 mg) was used as the precursor for
the labeling. After the initial reaction at 120°C in DMSO, 1
mL of H2O was added and the solution was cooled for 1 min.
1 mL of 10% HCl was then added, and the mixture was heated
at 120°C again for 10 min. Aqueous solution of NaOH was
added to adjust the pH to basic. The mixture was extracted
with ethyl acetate (1 mL × 2), the combined organic layer was
dried (Na2SO4), and the solvent was removed under argon
stream with gentle heating (55-60 °C). The residue was
dissolved in CH3CN and injected to HPLC for purification
[Hamilton PRP-1 column (7.0 × 305 mm, 10 µm); CH3CN/
dimethyl glutarate buffer (5 mM, pH 7) ) 9/1; flow rate ) 2
mL/min). Retention time of 4e was 10 min and well separated
from precursor 5h (tR ) 13 min) as well as the hydrolysis
byproduct of precursor (tR ) 8 min). The same HPLC condition
was used for quality control (RCP > 99%). Specific activity
was estimated by comparing UV peak intensity of purified
zyl)malonic acid diethyl ester 9
(1.5 g, 3.8 mmol) was
dissolved in 5 mL of THF, and the solution was added slowly
to DIBALH (1 M in toluene, 25 mL) via a syringe at 0 °C and
stirred at the same temperature for 3 h. HCl (2 N, 50 mL)
was then added to break the complex. After standard workup
with ethyl acetate, crude product 10 (0.9 g, 99%) was obtained,
which was used directly for the next step without further
purification: 1H NMR δ 7.39 (d, 2H, J ) 8.2 Hz), 7.04 (d, 2H,
J ) 8.2 Hz), 3.60 (m, 4H), 2.55 (d, 2H, J ) 7.4 Hz), 1.96 (m,
1H).
3-(4-Bromophenyl)-2-(tert-butyldimethylsilanyloxy-
methyl)propan-1-ol (11). Under a nitrogen atmosphere, tert-
butyl-chloro-dimethylsilane (246 mg, 1.63 mmol) was added
to a solution of compound 10 (400 mg, 1.63 mmol) in dichlo-
romethane (10 mL) at 0 °C, followed by triethylamine (412
mg, 4.07 mmol). The solution was gradually warmed to room
temperature and stirred overnight. After standard work up
with dichloromethane, the residue was purified by silica gel
preparative TLC (40% ethyl acetate in hexane) to afford
compound 11 (370 mg, 63.2%): 1H NMR δ 7.41 (d, 2H, J )
6.6 Hz), 7.08 (d, 2H, J ) 6.6 Hz), 3.66 (m, 4 H), 2.60 (m, 2H),
1.94 (m, 1H), 0.91 (s, 9H), 0.06 (s, 6H).
[
18F]4e with reference nonradioactive compound of known
concentration. Specific activity was estimated to be 900-1000
Ci/mmol after the preparation.
[2-Bromomethyl-3-(4-bromophenyl)propoxy]-tert-bu-
tyldimethylsilane (12). Compound 12 was prepared from 11
(80 mg, 0.22 mmol) with the same procedure described for 8m
24 and was used in the following step without further purifica-
tions. 12 (70 mg, 75%): 1H NMR δ 7.40 (d, 2H, J ) 6.6 Hz),
7.07 (d, 2H, J ) 6.6 Hz), 3.46 (m, 4H), 2.73 (d, 2H, J ) 7.2
Hz), 2.03 (m, 1H), 0.94 (s, 9H), 0.06 (s, 6H).
Preparation of Brain Tissue Homogenates. Postmortem
brain tissues were obtained from control and AD patients at
autopsy, and neuropathological diagnosis was confirmed by
current criteria (NIA-Reagan Institute Consensus Group,
1997). Homogenates were then prepared from dissected gray
and white matters from pooled control and 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 3-6
months without loss of binding signal.
Binding Studies. [125I]IMPY 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 minicolumn as described previ-
ously.18 Binding assays were carried out in 12 × 75 mm
borosilicate glass tubes. The reaction mixture contained 50 µL
of brain homogenates (20-50 µg), 50 µL of [125I]IMPY (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
[4-(2-{4-[2-(4-Bromobenzyl)-3-(tert-butyldimethylsi-
lanyloxy)propoxy]phenyl}vinyl)phenyl]dimethylamine
(3i). Compound 3i was prepared from 12 (50 mg, 0.12 mmol)
and 3a (28 mg, 0.12 mmol) with the same procedure for 3b.
This product was used in the following step without further
1
purifications. 3i (40 mg, 59%). H NMR δ 7.40 (m, 6H), 7.09
(d, 2H, J ) 8.2 Hz), 6.84 (m, 4H), 6.72 (d, 2H, J ) 8.8 Hz),
3.91 (d, 2H, J ) 5.4 Hz), 3.67 (m, 2H), 2.99 (s, 6H), 2.75 (d,
2H, J ) 7.4 Hz), 2.20 (m, 1H), 0.91 (s, 9H), 0.04 (s, 6H).
2-(4-Bromobenzyl)-3-{4-[2-(4-(dimethylamino)phenyl)-
vinyl]phenoxy}propan-1-ol (3j). Compound 3j was prepared
from 3i (40 mg, 0.069 mmol) with the same procedure
described for 5c. 3j (19 mg, 59%): 1H NMR δ: 7.40 (m, 4H),

F-18 Aâ Imaging Agents
Journal of Medicinal Chemistry, 2005, Vol. 48, No. 19 5987
albumin, BSA) in a final volume of 1 mL. Nonspecific binding
was defined in the presence of IMPY (600 nM) in the same
assay tubes. A similar assay condition for [18F]4e was used
for binding in a range of concentration between 0.3 and 0.5
nM. The nonspecific binding was defined in the presence of
nonradioactive 4e (1000 nM). 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 room temperature. Filters containing the
bound 125I or 18F ligand were assayed for radioactivity content
in a gamma counter (Packard 5000) with 70% counting
efficiency. Under the assay conditions, the specifically bound
fraction was less than 15% of the total radioactivity. The
results of inhibition experiments were subjected to nonlinear
regression analysis using EBDA by which Ki values were
calculated.
Film Autoradiography. Brain sections from AD subjects
were obtained by freezing the brain in powdered dry ice and
cut into 20-µm thick sections. The sections were incubated with
[18F]tracers (200 000-250 000 cpm/200 µL) for 1 h at room
temperature. The sections were then dipped in saturated Li2-
CO3 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 18F-labeled sections were exposed to Kodak
MR film overnight.
In Vivo Plaque Labeling with [18F]3e and [18F]4e. The
in vivo evaluation was performed using either double trans-
genic APP/PS1 or single transgenic APP2576 mice which were
kindly provided by AstraZeneca. After anesthetizing with 1%
isoflurane, 250-300 µCi of [18F]3e or [18F]4e in 200 µL of 0.1%
BSA solution was injected through the tail vein. The animals
were allowed to recover for 60-120 min and then killed by
decapitation. The brains were immediately removed and frozen
in powdered dry ice. Sections of 20 µm were cut and exposed
to Kodak MR film for overnight. Ex vivo film autoradiograms
were thus obtained.
Organ Distribution in Normal Mice. While under iso-
flurane anesthesia, 0.15 mL of a 0.1% bovine serum albumin
solution containing [18F]tracers (5-10 µCi) were 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 120 min postinjection. The organs of interest
were removed and weighed, and the radioactivity was assayed
for radioactivity content 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. Total activities of blood were calculated under the
assumption that they were 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.
is available free of charge via the Internet at http://
pubs.acs.org.
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Acknowledgment. This work was supported by the
grant from the National Institute of Health (AG022559
to H.F.K). APP/PS1 and Tg2576 transgenic mice were
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Supporting Information Available: Elemental analysis
data are available for all prepared compounds. This material

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