Novel probes for imaging amyloid-β: F-18 and C-11 labeling of 2-(4-aminostyryl)benzoxazole derivatives

Article abstract of DOI:10.1002/jlcr.811

2-(4-Methylaminostyryl)-6-(2-[¹⁸F]fluoroethoxy)benzoxazole ([¹⁸F]BF-168) was prepared and found to be a potential probe for imaging amyloid-β. The precursor, a 6-(2-tosyloxyethoxy)benzoxazole derivative, was fluorinated with [¹⁸

Full text of DOI:10.1002/jlcr.811

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2004; 47: 181–190.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.811
Research Article
Novel probes for imaging amyloid-b: F-18 and C-11
labeling of 2-(4-aminostyryl)benzoxazole derivatives
Hiroshi Shimadzu*, Takahiro Suemoto, Masako Suzuki, Tsuyoshi Shiomitsu,
Nobuyuki Okamura, Yukitsuka Kudo and Tohru Sawada
BF Research Institute, Inc. c/o National Cardiovascular Center 7-1, 5-Chome,
Fujishirodai, Suita, Osaka 565-0873, Japan
Summary
2-(4-Methylaminostyryl)-6-(2-[18F]fluoroethoxy)benzoxazole ([18F]BF-168) was pre-
pared and found to be a potential probe for imaging amyloid-b. The precursor, a 6-(2-
tosyloxyethoxy)benzoxazole derivative, was fluorinated with [18F]KF and Kryptofix
222 in acetonitrile, and the crude product purified by semi-preparative HPLC to give
[18F]BF-168. The radiochemical purity was >95% and the maximum specific activity
was 106 TBq/mmol at the end of synthesis. The synthesis time was 110 min from the
end of bombardment.
2-(4-[N-methyl-11C]methylaminostyryl)-5-fluorobenzoxazole ([11C]BF-145) was
also prepared from 2-(4-aminostyryl)-5-fluorobenzoxazole, [11C]MeI and 5 N NaOH
in DMSO, and purified by semi-preparative HPLC. The radiochemical purity was
>95% and the specific activity was 40–70 TBq/mmol at the end of synthesis. The
synthesis time was 45 min from the end of bombardment. Copyright # 2004 John
Wiley & Sons, Ltd.
Key Words: fluorine-18; 2-(4-methylaminostyryl)-6-(2-[18F]fluoroethoxy)benzoxazole;
BF-168; 2-(4-[N-methyl-11C]methylaminostyryl)-5-fluorobenzoxazole; BF-145; amy-
loid-b; PET
Introduction
Assessing the deposition of amyloid-b (Ab) in the living brain is important for
the diagnosis of Alzheimer’s disease (AD) at an early stage. Radioactive
probes which have a high binding affinity to Ab are required in non-invasive
imaging techniques, such as positron emission tomography (PET) or single
*Correspondence to: H. Shimadzu, BF Research Institute, Inc. c/o National Cardiovascular Center 7-1,
5-Chome, Fujishirodai, Suita, Osaka 565-0873, Japan. E-mail: shimadzu@po.iijnet.or.jp
Contract/grant sponsor: Program for Promotion of Fundamental Studies in Health Sciences of the
Organization for Pharmaceutical Safety and Research of Japan
Copyright # 2004 John Wiley & Sons, Ltd.
Received 29 September 2003
Revised 4 December 2003
Accepted 6 December 2003

182
H. SHIMADZU ET AL.
photon emission computed tomography (SPECT), which are expected to
facilitate the evaluation of AD. Many Ab imaging probes, such as FDDNP,1
BSB,2 TZDM,3 IMPY,3 6-OH-BTA-1,4 and SB-135 have recently been
reported.
Previously, we also reported the synthesis of (2-[18F]fluoroethyl)ethylamino-
6-diethylaminoacridine ([18F]BF-108).6 But this probe was not removed
rapidly enough from the brain of mice. So, we further searched for compounds
which specifically bind to Ab(l-40) with a b-sheet structure using the
fluorescence method that we adapted from LeVine’s7 and Wood’s8 methods,
and found that some 2-(4-aminostyryl)benzoxazole derivatives bind to Ab(1-
40) stronger than thioflavin T, a positive control. Many of these derivatives
also showed a high brain uptake (2–5% ID/g) at 2 min post injection in mice.
Then, F-18 labeling was investigated in various ways, and resulted in the
successful preparation of 2-(4-methylaminostyryl)-6-(2-[18F]fluoroethoxy)ben-
zoxazole ([18F]BF-168). It was confirmed that this radioactive probe combines
selectively with the injected Ab peptide in the rat brain. Thus, we have chosen
[18F]BF-168 as a new candidate to image the Ab peptide. C-11 labeling of
2-(4-aminostyryl)benzoxazole derivatives was also performed. 2-(4-[N-
methyl-11C]methylaminostyryl)-5-fluorobenzoxazole ([11C]BF-145) was pre-
pared from 2-(4-aminostyryl-5-fluorobenzoxazole and it was confirmed that
[11C]BF-145 also binds specifically to the injected Ab peptide in the rat brain.
Results and discussion
Several 2-(4-aminostyryl)benzoxazole derivatives have been prepared and
evaluated as precursors for F-18 radiolabeling. Firstly, 5- or 6-nitro-2-(4-
dimethylaminostyryl)benzoxazole was heated with KF and Kryptofix 222
(K.222) in DMSO, without any reaction, only gradual decomposition of the
starting material being noticed at temperatures over 1608C. No 5- or 6-fluoro-
2-(4-dimethylaminostyryl)benzoxazole was formed. Secondly, the 5- or 6-
fluoro-2-(4-dimethylaminostyryl)benzoxazole was heated with [18F]KF (15 mA,
30 min irradiation) and K.222 in DMSO. A 19F-18F exchange reaction
occurred to a small extent in the case of the 6-fluoro derivative, but only gave
3.4 MBq of radiolabeled product with a specific activity of ca. 370 MBq/mmol.
These values showed that this reaction using the fluoro-derivative was not
suitable for practical use. Thirdly, fluorodestannylation of 2-(4-dimethylami-
nostyryl)-5-tributylstannylbenzoxazole was attempted using fluorine- or acetyl
hypofluorite-diluted neon gas in various solvents and at different tempera-
tures. However, the required compound, 5-fluoro-2-(4-dimethylaminostyryl)-
benzoxazole, was not obtained in either cold or hot experiments, while
iododestannylation of the same compound proceeded readily to give 5-iodo-
2-(4-dimethylaminostyryl)benzoxazole by the usual method. So, using the
iodo-derivative obtained, an iodine–fluorine exchange reaction was attemp-
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NOVEL PROBES FOR IMAGINE
183
ted by heating a mixture of the iodo-derivative, KF and K.222 in DMSO,
but none of the required compound, 5-fluoro derivative, was obtained.
Next, the fluorination of 2-(4-dimethylamino)benzoxazole was attem-
pted in both cold and hot conditions using fluorine- or acetyl hypofluo-
rite-diluted neon gas in various solvents and at different temperatures, but
without success. Finally, 2-(4-dimethylaminostyryl)-6-(2-tosyloxyethoxy)
benzoxazole (7) was chosen as a substrate for F-18 labeling of
2-(4-dimethylaminostyryl)-6-(2-fluoroethoxy)benzoxazole (8, BF-168).9 The
synthetic routes to the authentic 8 and the precursor 7 are shown in Figure 1.
The non-radioactive fluorination was successfully carried out by reacting 7
with powdered potassium fluoride in the presence of K.222 in acetonitrile at
808C for 20 min. The reaction mixture was concentrated to dryness and the
residue was purified by column chromatography on silica gel to give 8 in 46%
yield as an E=Z mixture. The authentic 8 (E isomer) also gave an E=Z mixture
on standing at room temperature in solution. The same phenomenon was
observed in the majority of 2-(aminostyryl)benzoxazole derivatives. As it has
been reported that E and Z isomers are almost equal in their binding ability to
Ab,10 the evaluation for Ab recognition using an E=Z mixture is acceptable.
Radiosynthesis was performed in a manner similar to that of the cold synthesis
described above, except that [18F]KF prepared in situ from F-18 fluoride and
Figure 1. Synthetic route to BF-168
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H. SHIMADZU ET AL.
potassium carbonate was used instead of potassium fluoride (Figure 2). The
desired product was isolated by semi-preparative reversed-phase HPLC. The
radioactive fraction containing [18F]8 was collected, concentrated under
reduced pressure and transferred to a product vial. The synthesis time was
110 min from the end of bombardment. The analytical HPLC showed two
peaks corresponding to the E and Z isomers and that the radiochemical purity
was >95% (Total % of E and Z isomers). The specific activity was a
maximum of 106 TBq/mmol. Biodistribution of [18F]8 was studied in Slc:ICR
mice (male, 28–32 g, SLC). The mice were decapitated at specific times (2, 10,
30, 60, 120 and 180 min) after intravenous injection of 0.24–0.38 MBq of
[18F]8. The organs of interest were removed and weighed, and the radioactivity
measured with an automatic g-counter (Wizard 1480, Turku, Finland). The
percentage injection dose per gram (% ID/g) was calculated. Each % ID/g
value is an average ꢀ standard deviation of three separate experiments. A
high brain uptake (1.6% ID/g) at 30 min and low uptake (0.76%) at 180 min
post injection were obtained as shown in Table 1, and a hot spot was clearly
observed at 180 min post i.v. administration on the same site of the rat brain
where Ab(1-42) (and a buffer only on the opposite site) had been injected 6
days before11 (Figure 3). These results suggest that [18F]8 has the potential to
be used as a PET radiotracer for diagnosing AD in its early stage.
In addition to F-18 labeling, we also attempted C-11 labeling and found
that 2-(4-[N-methyl-11C]methylaminostyryl)-5-fluorobenzoxazole ([11C]15,
Figure 2. Synthesis of [18F]BF-168
Table 1. Biodistribution of [18F]BF-168 in mice after i.v. injection
Organ
2 min
10 min
30 min
60 min
120 min
180 min
Blood
Brain
Liver
Kidney
Heart
Lung
2.6 ꢀ 0.46
3.9 ꢀ 0.22
8.7 ꢀ 0.14
9.1 ꢀ 0.34
1.6 ꢀ 0.17
3.2 ꢀ 0.35
7.2 ꢀ 0.74
3.8 ꢀ 0.40
1.4 ꢀ 0.066
1.6 ꢀ 0.007
5.7 ꢀ 0.59
2.3 ꢀ 0.051
1.6 ꢀ 0.062
1.7 ꢀ 0.081
4.1 ꢀ 2.0
1.8 ꢀ 0.35
1.3 ꢀ 0.040 0.93 ꢀ 0.12
5.2 ꢀ 0.083
2.0 ꢀ 0.37
1.4 ꢀ 0.033
1.5 ꢀ 0.044
4.0 ꢀ 1.4
1.3 ꢀ 0.091
1.1 ꢀ 0.16
0.76 ꢀ 0.13
3.1 ꢀ 0.39
0.86 ꢀ 0.11
0.94 ꢀ 0.10
1.4 ꢀ 0.71
6.6 ꢀ 2.9
4.4 ꢀ 0.68
1.1 ꢀ 0.15
1.1 ꢀ 0.11
1.7 ꢀ 0.94
5.4 ꢀ 1.2
0.76 ꢀ 0.087 0.66 ꢀ 0.10
1.1 ꢀ 0.18
4.8 ꢀ 0.090 2.2 ꢀ 0.26
6.2 ꢀ 0.51
1.7 ꢀ 0.40
1.9 ꢀ 0.22
1.2 ꢀ 0.24
3.2 ꢀ 0.13
2.9 ꢀ 0.40
2.7 ꢀ 1.3
1.5 ꢀ 0.38
1.8 ꢀ 0.071 1.7 ꢀ 0.41
1.8 ꢀ 0.28
8.0 ꢀ 1.1
Bone
Muscle
Skin
Spleen
1.2 ꢀ 0.035 0.93 ꢀ 0.19
1.4 ꢀ 0.16
1.1 ꢀ 0.047 0.92 ꢀ 0.15
12 ꢀ 2.3 12 ꢀ 7.1
0.82 ꢀ 0.060
0.71 ꢀ 0.14
12 ꢀ 2.3
1.3 ꢀ 0.11
11 ꢀ 0.55
Intestine 3.3 ꢀ 0.13
All values show the %ID/g: mean of three mice ꢀ standard deviation.
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Figure 3. Autoradiographic image at 180 min post i.v. administration of 70 MBq
of [18F]BF-168 in rat brain slice injected with Ab(1-42) dissolved in phosphate
buffer (right arrow) and buffer only (left arrow)
[11C]BF-145) was easily synthesized. Thus, [11C]15 was prepared from a
precursor, 2-(4-aminostyryl)-5-fluorobenzoxazole (12), with C-11-labeled
methyl iodide (20 mA, 30 min irradiation) and 5 N NaOH in DMSO12 at
room temperature for 5 min, followed by purification of the product by semi-
preparative HPLC. The synthesis time was 45 min from the end of
bombardment. The radiochemical purity was >95% and the specific activity
was 40–70 TBq/mmol at the end of synthesis. The [11C]15 solution was injected
into the same model rat with which [18F]8 was used, and after 70 min, hot spots
were observed on the same sites of the rat brain slice where Ab(1-42) had been
injected by autoradiography and were well correlated with the result of Ab
immunostaining with the 6F/3D monoclonal antibody using the same slice13
(Figure 4). This result suggests that [11C]15 also has the potential to be used as
a probe for imaging Ab.
Experimental
General method
The authentic compounds, 2-(4-methylaminostyryl)-6-(2-tosyloxyethoxy)ben-
zoxazole (7) and 2-(4-methylaminostyryl)-6-(2-fluoroethoxy)benzoxazole (8,
BF-168) were prepared from 2-methyl-6-methoxybenzoxazole (1) and 4-(N-
butoxycarbonyl-N-methyl)aminobenzaldehyde (2) as shown in Figure 1, and
2-(4-aminostyryl)-5-fluorobenzoxazole (12) and 2-(4-methylaminostyryl)-5-
fluorobenzoxazole (15, BF-145) prepared from 4-fluoro-2-aminophenol (9)
as shown in Figure 5, both of which were supplied by Tanabe R&D Service
Co., Ltd., Osaka City. All other chemicals used in this study were purchased
from commercial suppliers. NMR spectra were recorded on a Varian
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H. SHIMADZU ET AL.
Figure 4. Autoradiographic image (left) at 70 min post i.v. administration of
90 MBq of [11C]BF-145 in rat brain slice injected with Ab(1-42) and
immunostaining (right) with the 6F/3D monoclonal antibody using the same
slice (magnified figure)
Figure 5. Synthetic route to BF-145
GEMINI-300 spectrometer. A JASCO PU-987 pump and a JASCO UV-970
UV/Vis detector were used for the semi-preparative HPLC, and a Shimadzu
C-R4AX Chromatopac, a Shimadzu SPD-6A UV detector, a JASCO PU-980
pump and an Aloka RLC-700 radioanalyzer were used for the analytical
HPLC. Autoradiography was performed by using a Fuji Film BAS-III
imaging film and a Fuji Film BAS2000 bioimaging analyzer.
2-(4-Methylaminostyryl)-6-(2-tosyloxyethoxy)benzoxazole (7)
To a solution of 2-[4-(N-methyl-N-trifluoroacetyl)aminostyryl]-6-hydroxy
benzoxazole (6) (400 mg, 1.1 mmol), triphenylphosphine (637 mg, 2.4 mmol)
and 2-hydroxyethyl tosylate (525 mg, 2.4 mmol) in dry THF (10 ml),
diisopropyl azodicarboxylate (DIAD, 0.48 ml, 2.4 mmol) was added. The
reaction mixture was stirred for 20 h at room temperature and evaporated to
dryness. The residue was purified by silica gel column chromatography (eluent:
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hexane/ethyl acetate 2:1) to give 601 mg (97%) of an intermediate, 2-[4-(N-
methyl-N-trifluoroacetyl)aminostyryl]-6-(2-tosyloxyethoxy)benzoxazole, as lem-
on-yellow crystals which melted at 143–1448C. To a solution of the
intermediate (400 mg, 0.71 mmol) in THF (5 ml), 1 N LiOH (1.43 ml) was
added and stirred for 2 h under ice-water cooling. To the reaction solution, a
saturated aqueous NH4Cl solution and AcOEt were added. The organic phase
was washed with saline solution, dried with anhydrous MgSO4 and evaporated
to dryness. The residue was recrystallized from a mixture of AcOEt and
hexane to give 320 mg (96%) of 7 as an orange crystalline powder. m.p.
1
118–1208C. H-NMR (CDCl3) d: 2.44 (3H, s) 2.89 (3H, s) 4.05(1H, br) 4.20
(2H, t, J=4.8 Hz) 4.40 (2H, t, J=4.8 Hz) 6.61 (2H, d, J=8.6 Hz) 6.77
(1H, dd, J=8.7, 2.3 Hz) 6.78(1H, d, J=16.2 Hz) 6.91 (1H, d, J=2.3 Hz) 7.33
(2H, d, J ¼ 8.5 Hz) 7.43 (2H, d, J=8.5 Hz) 7.49 (1H, d, J=8.7 Hz) 7.62 (1H,
d, J=16.2 Hz) 7.82 (2H, d, J=8.6 Hz); Elemental anal. Calculated for
C25H24N2O5S ꢁ 1/10H2O: C, 64.39; H, 5.23; N, 6.01; S, 6.87. Found: C, 64.18;
H, 5.12; N, 6.01; S, 6.86.
2-(4-Methylaminostyryl)-6-(2-fluoroethoxy)benzoxazole (8, BF-168)
Method A. The title compound 8 was prepared similarly to 7 from 6, using
2-fluoroethanol instead of 2-hydroxyethyl tosylate. An intermediate, 2-[4-(N-
methyl-N-trifluoroacetyl)aminostyryl]-6-(2-fluoroethoxy)benzoxazole, was ob-
tained in 90% yield as lemon-yellow crystals which melted at 124–1258C.
Deprotection of the intermediate by LiOH gave 8 in 87% yield as an orange
1
crystalline powder. m.p. 136–1378C. H-NMR (CDCl3) d: 2.89 (3H, s) 4.06
(1H, br) 4.27 (2H, dt, J=27.8, 4.2 Hz) 4.79 (2H, dt, J=47.4, 4.2 Hz) 6.61 (2H,
d, J=8.6 Hz) 6.79 (1H, d, J=16.2 Hz) 6.93 (1H, dd, J=8.8, 2.3 Hz) 7.06 (1H,
d, J=2.3 Hz) 7.43 (2H, d, J=8.6 Hz) 7.54 (1H, d, J=8.8 Hz) 7.62 (1H, d,
J=16.2 Hz); Elemental anal. Calculated for C18H17FN2O2: C, 69.22; H, 5.49;
N, 8.97; F, 6.08. Found: C, 68.95; H, 5.40; N, 8.99; F, 5.80.
Method B14. To a solution of 7 (10 mg) in acetonitrile (2 ml), powdered
potassium fluoride (10 mg) and K.222 (20 mg) were added, and the mixture
stirred at 808C for 20 min. The reaction mixture was evaporated to dryness
under reduced pressure, and the residue purified by silica gel column
chromatography (eluent: chloroform only) to give 3.1 mg (46%) of 8 as a
yellowish orange solid. However, the compound was obtained as an E=Z
mixture and it was then shown that the authentic sample (E-isomer) of 8
prepared by method A was also converted to an identical E=Z mixture after
being allowed to stand in solution, especially in polar solvents. The
compounds were identified by means of NMR, HPLC and TLC. The
1
retention times of the E and Z isomer were 6.1 and 6.6 min, respectively. H-
NMR (CDCl3, E=Z=7/3 mixture) d: 2.89 (2.1H, s) 2.90 (0.9H, s) 4.04 (1H, br)
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4.20–4.24 (1H, m) 4.30–4.33 (1H, m) 4.69–4.73 (1H, m) 4.85–4.89 (1H, m) 6.25
(0.3H, d, J=13.2 Hz) 6.61 (2H, d, J=8.7 Hz) 6.80 (0.7H, d, J=16.5 Hz) 6.83
(0.3H, d, J=12.9 Hz) 6.92–6.98 (1H, m) 7.02 (0.7H, d, J=2.4 Hz) 7.07 (0.3H,
d, J=2.4 Hz) 7.44 (1.4H, d, J=8.4 Hz) 7.55 (1H, d, J=9.0 Hz) 7.60 (0.3H, d,
J=8.7 Hz) 7.63 (0.7H, d, J=16.2 Hz) 7.82 (0.6H, d, J=8.4 Hz).
2-(4-Methylaminostyryl)-6-(2-[18F]fluoroethoxy)benzoxazole ([18F]8)
[18F]Fluoride was prepared by the 18O(p,n)18F nuclear reaction (15 mA, 30 min
irradiation) in a CYPRIS HM-18 cyclotron (Sumitomo Heavy Industries Ltd.,
Tokyo) on an enriched water (ca. 95% 18O, ISOTEC) target which was
adsorbed onto an anion exchange resin (AG1-X8, BIO-RAD) and was then
eluted with 7.5 mM potassium carbonate (0.3 ml ꢂ 2) and a solution of K.222
(10 mg) in acetonitrile (l.5 ml) was added. The resulting solution was
transferred to a glassy carbon vessel. Then, water and acetonitrile were
removed in a stream of helium at 1008C under reduced pressure for 10 min. To
the residue, acetonitrile (1.5 ml) was added and the resultant mixture was dried
again in a stream of helium at 100–1158C under reduced pressure for 20 min.
To the residue, a solution of precursor 7 (ca. 2 mg) in acetonitrile (1.5 ml) was
added, and the mixture stirred at 808C for 20 min. After cooling, CH3CN/H2O
60:40 (1.0 ml) was added and the mixture injected onto a semi-preparative
HPLC column (YMC, ODS-AQ-323, 10 ꢂ 250 mm + SHISEIDO CAPCEL
PAK C18 10 ꢂ 150 mm; eluent: CH3CN/H2O 60:40; flow rate: 4.0 ml/min).
The eluate was monitored by both UV (254 nm) and radioactivity detectors,
and the fraction containing the desired product eluted at 18–23 min was
collected in a flask which contained 0.1 N HCl (0.5 ml), and concentrated
under reduced pressure to a volume of ca. 5 ml. The resultant solution was
transferred to a vial. The radioactivity of the product obtained was 120–
950 MBq at the end of synthesis. The synthesis time was 110 min from the end
of bombardment. HPLC analysis (Wakosil-II5C18 HG 4.6 ꢂ 150 mm;
CH3CN/0.01 M sodium phosphate buffer (pH6.5) 60:40; 2.0 ml/min: UV
detector at 254 nm; radioactivity detector) showed that the radiochemical
purity was >95% (E=Z total) and the maximum specific activity was
106 TBq/mmol at the end of synthesis.
2-(4-Methylaminostyryl)-5-fluorobenzoxazole (15, BF-145)
To a solution of 12 (2.7 mg) in DMSO (1.0 ml), 5 N NaOH (27 ml) and methyl
iodide (2 ml) were added and the mixture stirred at room temperature for
5 min. HPLC analysis (conditions are as above) of the reaction mixture
showed three peaks corresponding to the starting material (12, Rt: 4.8 min,
52%), required compound (15, Rt: 8.2 min, 15%) and N,N-dimethylated
compound (Rt: 14.4 min, 19%) respectively. In this case, the Z isomers were
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Figure 6. Synthesis of [11C]BF-145
not detectable or had the same retention time as E isomers. The isolation of 15
was not attempted.
2-(4-[N-Methyl-11C]methylaminostyryl)-5-fluorobenzoxazole([11C]15,
[11C]BF-145)
In a reaction vessel were placed a solution of the precursor (12, ca. 1 mg) in
DMSO (0.5 ml) and 5 N NaOH (10 ml). [11C]Methyl iodide (prepared from
[11C]CO2 (20 mA, 30 min irradiation), 0.1 M LAH/THF and 57%HI) were
transferred into the solution via a stream of nitrogen (Figure 6). The
radioactivity of the vessel was monitored, and when the numeric value of
radioactivity was maximized, the flow rate of nitrogen was lowered and then
kept at a low value (ca. 30 ml/min). After 5 min at room temperature, the
reaction mixture was injected onto a semi-preparative HPLC column (YMC,
ODS-AQ-323. 10 ꢂ 250 mm; eluent: CH3CN/0.01 M sodium phosphate buffer
(pH6.5) 60:40; flow rate: 6.0 ml/min). The eluate was monitored with both UV
(254 nm) and radioactivity detectors, and the fraction containing the desired
product, which eluted at ca. 15–16 min, was collected in a flask which
contained EtOH (0.2 ml), and concentrated under reduced pressure. The
resulting solution was transferred to a vial. The radioactivity of the product
obtained was 198–362 MBq at the end of synthesis. HPLC analysis (the same
conditions) showed that the radiochemical purity was >95% and the specific
activity was 40–70 TBq/mmol at the end of synthesis.
Acknowledgements
This study was supported by the Program for Promotion of Fundamental
Studies in Health Sciences of the Organization for Pharmaceutical Safety and
Research of Japan. The authors thank Mr. Norimasa Ejima of the Institute for
Biofunctional Research Ltd. for operating the cyclotron and for his assistance
in dealing with the automated synthesis apparatus.
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