A simple and rapid automated radiosynthesis of [18F] fluoroacetate

Article abstract of DOI:10.1002/jlcr.1520

Two fully automated synthetic procedures of [¹⁸F]fluoroacetate ([¹⁸F]FAC) have been developed using a modified commercial TRACERlab FX_FN synthesizer. One was a two-step one-pot procedure, consisting of nucleophilic [¹⁸F]fluorination of benzyl-2-bromoacetate as a precursor with no-carrier-added [¹⁸F]fluoride, hydrolysis within the same [¹⁸F]fluorination reaction vessel, and purification with/without high-performance liquid chromatography (HPLC). The second procedure consisted of nucleophilic [¹⁸F]fluorination, hydrolysis on the column, and purification with SEP-PAK cartridges instead of HPLC. The radiochemical purity of [¹⁸F]FAC was >95% by the two procedures. The second procedure was a simple, rapid, and fully automated synthesis of [¹⁸F]FAC with a high and reproducible radiochemical yield exceeding 60% (decay uncorrected) within the total synthesis time less than 20 min. The new, simple, and rapid on-column hydrolysis procedure should be adaptable to the fully automated synthesis of [¹⁸F]FAC at a commercial fluoro-deoxyglucose synthesis module. Copyright

Full text of DOI:10.1002/jlcr.1520

Note
Received 17 December 2007,
Revised 20 February 2008,
Accepted 2 April 2008
Published online 6 June 2008 in Wiley Interscience
(www.interscience.wiley.com) DOI: 10.1002/jlcr.1520
A simple and rapid automated radiosynthesis
of [18F]fluoroacetate
Ã
Ganghua Tang,a,b Xiaolan Tang,c Mingfang Wang,a Baoyuan Li,a
Mingquan Liang,a and Quanshi Wanga
Two fully automated synthetic procedures of [18F]fluoroacetate ([18F]FAC) have been developed using a modified
commercial TRACERlab FXFN synthesizer. One was a two-step one-pot procedure, consisting of nucleophilic [18F]fluorination
of benzyl-2-bromoacetate as a precursor with no-carrier-added [18F]fluoride, hydrolysis within the same [18F]fluorination
reaction vessel, and purification with/without high-performance liquid chromatography (HPLC). The second procedure
consisted of nucleophilic [18F]fluorination, hydrolysis on the column, and purification with SEP-PAK cartridges instead of
HPLC. The radiochemical purity of [18F]FAC was 495% by the two procedures. The second procedure was a simple, rapid,
and fully automated synthesis of [18F]FAC with a high and reproducible radiochemical yield exceeding 60% (decay
uncorrected) within the total synthesis time less than 20 min. The new, simple, and rapid on-column hydrolysis procedure
should be adaptable to the fully automated synthesis of [18F]FAC at a commercial fluoro-deoxyglucose synthesis module.
Keywords: [18F]fluoroacetate; fully automated synthesis; on-column hydrolysis; solid phase purification
using commercial benzyl bromoacetate as a precursor via a two-
step one-pot procedure with a commercial TRACERlab FXFN
Introduction
[11C]Acetate ([11C]AC) has been used as a tracer for positron
emission tomography (PET) to study myocardial metabolism and
cerebral oxidative metabolism. PET with [11C]AC also has a high
sensitivity for detection of prostate cancer and several other
cancers that are poorly detected with [18F]-2-fluoro-deoxyglu-
cose ([18F]FDG).1 However, the potential to widespread use of
[11C]AC is limited by the short radioactive half-life (20.4 min) of
11C, which necessitates production with an in-house cyclotron.
[18F]Fluoroacetate ([18F]FAC) is an analog of [11C]AC with a
longer radioactive half-life (110 min); therefore, it can be a
potential tracer for tumor imaging. Recently, [18F]FAC has been
reported to be a useful PET tracer for prostate cancer imaging.
Compared with [11C]AC PET, [18F]FAC offers the possibility of
delayed imaging, with potential to further increase the tumor-
to-background ratios.2
synthesizer.9,10 We obtained an uncorrected radiochemical yield
of 45% in about 50 min with preparative HPLC purification 9 and
got an uncorrected radiochemical yield of 440% within 40 min
with SEP-PAK cartridges instead of preparative HPLC purifica-
tion.10 However, all of the previously reported syntheses still
needed further optimization to achieve radiochemical yield
within a shorter synthesis time.
Here, we report two new automated methods to produce
[18F]FAC using commercial benzyl bromoacetate as a precursor
with TRACERlab FXFN synthesizer. One is a two-step one-pot
procedure with the preparative HPLC or SEP PAK purification,
the other is a two-step on-column hydrolysis procedure with
SEP-PAK purification. The latter procedure can obtain high
radiochemical yield within very short synthesis time.
In the past few years, production of [18F]FAC was reported by
several groups.3–7 However, most of these syntheses needed a
long synthesis time (460 min) and provided a low radio-
Results
Automated synthesis of [18F]FAC was carried out using the two-
step one-pot protocol and the two-step on-column hydrolysis
procedure starting from benzyl 2-bromoacetate as a precursor
with TRACERlab FXFN synthesizer.
7
chemical yield (o34%).3–6 Padgett et al. reported a chemistry
process control unit for the automated production of [18F]FAC
using commercial benzyl bromoacetate as a precursor with a
high corrected radiochemical yield (460%). However, this two-
step two-pot procedure still needed a long synthesis time
(440 min). More recently, a fully automated synthesis of
[18F]FAC via on-column hydrolysis has been reported using
ethyl O-mesyl glycolate as a precursor and the high corrected
radiochemical yield (about 50%) was achieved within a short
synthesis time (32 min) without the need for preparative high-
aNan Fang PET Centre, Department of Nuclear Medicine, Nan Fang Hospital,
Southern Medical University, Guangzhou 510515, China
bDepartment of Nuclear Medicine, The First Affiliated Hospital, Sun Yat-Sen
University, Guangzhou 510080, China
cScience College, South China Agricultural University, Guangzhou 510642, China
performance liquid chromatography (HPLC) using a commercial
[18F]FDG synthesizer, the TRACERlab MXFDG
*Correspondence to: Ganghua Tang, Nanfang PET Centre, Nan Fang Hospital,
Southern Medical University, Guangzhou 510515, China.
E-mail: gtang0224@yahoo.com.cn
8
.
Our group
previously carried out a fully automated synthesis of [18F]FAC
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G. Tang et al.
The two-step one-pot protocol included nucleophilic fluor- thin layer chromatography (TLC) and HPLC. We did not find any
ination, aqueous basic hydrolysis in the same reaction vessel, radiochemical or chemical impurities in the final product by
and purification with the preparative HPLC or SEP-PAK analytical HPLC chromatogram and radio-TLC. The average
cartridges in series. The uncorrected radiochemical yield of specific activity of the final product ranged from 70 to 140 GBq/
[18F]FAC by HPLC purification was 445% (n = 5) after a synthesis mmol. The final product was analyzed with gas chromatography
time of about 50 min, while the uncorrected radiochemical yield to show that it contained less than 10 ppm acetonitrile. Color
of [18F]FAC by SEP-PAK purification was 40% (n = 5) after a spot test for the detection of K222 by TLC also showed no trace
synthesis time o30 min. The HPLC purification gave higher of K222 in the final injectable [18F]FAC solution. With high initial
uncorrected radiochemical yield than the SEP-PAK purification, radioactivity, [18F]FAC showed good stability, of more than 95%
but the SEP-PAK purification could reduce the total synthesis radiochemical purity, at 6 h after synthesis.
time. HPLC purification of crude products showed only two
radioactive peaks as free [18F]FÀ peak and [18F]FAC peak with a
retention time of 8 and 10.2 min, respectively. For HPLC
Discussion
purification, the extent of acetonitrile evaporation also affected To perform automated synthesis of [18F]FAC, the most
the radiochemical yield. Usually, a half of residual acetonitrile important issues, such as simple and rapid [18F]fluorination,
(0.5 mL) left after [18F]fluorination would not significantly affect hydrolysis, and purification, must be solved. A comparative
the radiochemical yield. For SEP-PAK purification, complete study of the two-step one-pot protocol and the on-column
acetonitrile evaporation after [18F]fluorination would reduce hydrolysis protocol was described in this article.
radiochemical yield of about 5–20% compared with the case
As the TRACERlab FXFN synthesizer was designed for the
that left 0.5–1.0 mL of residual acetonitrile after [18F]fluorination. automated radiosynthesis of 18F-labeled compounds via a one-
The on-column hydrolysis procedure included nucleophilic pot procedure using HPLC purification, some modifications were
fluorination, basic hydrolysis and purification on four SEP-PAK required for the production of [18F]FAC. For simplification of the
cartridges in series (an integrated short-column system). A high delivery of radioactivity from cyclotron, [18F]fluoride was directly
uncorrected radiochemical yield of 460% (n = 5) was obtained delivered to a SEP-PAK QMA cartridge, where [18F]fluoride was
within the short total synthesis time of o20 min. Two SEP-PAK trapped and 18O-water was collected for recycling (Figures 1 and
C18 cartridges were used to trap and hydrolyze intermediate 2). The fully automated synthesis of [18F]FAC via the two-step
benzyl 2-[18F]FAC and also used to remove the intermediate and one-pot protocol using HPLC purification is depicted in Figure 1.
the unreacted precursor. A TSCX cartridge was used to remove For the on-column hydrolysis protocol, TRACERlab FXFN
K222 and neutralize the basic solution; a SEP-PAK alumina N synthesizer was substantially modified and adapted to the
cartridge was used to remove the unreacted [18F]fluoride.
production of [18F]FAC as shown in Figure 2. For the two-step
Purified [18F]FAC solution produced by the two procedures one-pot protocol using SEP-PAK purification, some necessary
was made to isotonic injection by adding enough saline to vial changes of chemical reagents arrangement and SEP-PAK
14 to achieve a 0.9% concentration. The pH of the final solution cartridges in Figure 2 were required. In addition, a new program
resulting from each of the procedures was 6.0–7.0. The identity file was written to control the automated synthesis of [18F]FAC
of the final product was confirmed by comparison of the via the different procedures.
chromatograms with unlabeled reference materials. The reten-
The automated synthesis of [18F]FAC was modified from the
tion time for [18F]FAC, the intermediate benzyl 2-[18F]FAC, and TRACERlab FXFN synthesizer via a one-pot procedure and an on-
free [18F]fluoride, determined by isocrateic HPLC system with a column hydrolysis procedure that consisted of 18F-fluorination
HypersilsAPS2 column and MeCN/H2O (60/40, v/v) as mobile of benzyl 2-bromoacetate as a precursor and subsequent
phase, was 5.0, 2.3, and 8.0 min, respectively. The radiochemical hydrolysis of the benzyl-protected intermediate as shown in
purity of the final product using both one-pot procedure and Scheme 1. Steps for the preparation of [18F]FAC consisted of
on-column hydrolysis procedure was over 95%, confirmed by three categories: fluorination of the precursor, hydrolysis of the
Figure 1. Schematic diagram of the automated synthesis of [18F]FAC using HPLC purification.
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G. Tang et al.
Figure 2. Schematic diagram of the automated synthesis of [18F]FAC using on-column hydrolysis and SEP-PAK purification.
protective group, and neutralization/HPLC purification or SEP-
Most researchers and our previous work reported that
PAK purification. For the one-pot procedure, hydrolysis reaction hydrolysis reaction usually took place at high temperature.3,9,10
was carried out in the same [18F]fluorination reaction vessel and However, hydrolysis reaction on the column at low temperature
purification was performed with preparative HPLC or SEP-PAK was also reported with ethyl O-mesyl glycolate as the precursor
purification. For the on-column hydrolysis procedure, hydrolysis in the recent published papers.1,8 In the present work, we found
reaction was carried out in SEP-PAK plus C18 cartridges and that the intermediate benzyl 2-[18F]FAC in 2–3 M NaOH solution
purification was performed with SEK-PAK plus C18 cartridges, was also completely hydrolyzed at room temperature within
TSCX cartridge, and SEP-PAK light Alumina cartridge connected 2 min. In addition, hydrolysis on the short column (SEP-PAK C18
in series. After synthesis, cleaning procedures were carried out in cartridge) with 2–3 M NaOH solution proceeded almost quanti-
between the runs.
tatively at room temperature within 2 min. For the one-pot
procedure, complete acetonitrile evaporation before hydrolysis
resulted in the loss of much radioactivity and radiochemical
yield. For the on-column hydrolysis, the trapping efficiency was
about 90% by using one SEP-PAK C18 cartridge. While two SEP-
PAK C18 cartridges were used, the trapping efficiency improved
to almost 100%. In the present experiments, 2 M NaOH solution,
a reaction temperature at room temperature, and a reaction
time of 2 min were chosen for the hydrolysis with a yield of
almost 100%.
Purification was a key factor for automated radiosynthesis of
[18F]FAC. For the one-pot procedure, HPLC purification gave a
good radiochemical yield, but there were limitations such as
being time consuming and having difficulty in the automated
synthesis, while SEP-PAK purification was easy for the auto-
mated synthesis of [18F]FAC with a short synthesis time,
but complete acetonitrile evaporation before hydrolysis sig-
nificantly reduced radiochemical yield. For the on-column
hydrolysis, basic hydrolysis and purification on an integrated
short-column system consisting of four SEP-PAK cartridges in
series greatly improved uncorrected radiochemical yield
(460%) and significantly reduced the total synthesis time
(o20 min).
Scheme 1. Synthetic scheme for the automatic synthesis of [18F]FAC with benzyl
2-bromoacetate as a precursor.
Several precursors were reported to the automated production
8
of [18F]FAC.1,3,7,8 Ethyl O-mesyl glycolate (a synthetic precursor)
7
and benzyl bromoacetate
(a commercial precursor) were
reported to prepare [18F]FAC in good radiochemical yields. In
our work, we chose benzyl 2-bromoacetate as the precursor to
produce [18F]FAC using the two protocols described above.
[18F]Fluorination reaction of benzyl 2-bromoacetate with [18F]fluo-
ride formed benzyl 2-[18F]FAC very easily with a good labeling
yield 490%, using more than 3 mg of benzyl bromoacetate
dissolved in 1 mL of anhydrous acetonitrile at 80–901C for more
than 5 min. In our present experiments, 10 mg of benzyl 2-
bromoacetate, a reaction temperature of 851C, and a reaction time
of 5 min were chosen for [18F]fluorination with a labeling yield of
495% measured by analytic HPLC ([18F]fluoride adsorption on
their column was not taken into account).
In our previous work, the fully automated synthesis of
[18F]FDG using the on-column hydrolysis procedure in TRACER-
lab FXFN synthesizer was carried out in a high uncorrected
radiochemical yield (460%) within a short total synthesis time
(o20 min),11 which should be adaptable to the fully automated
synthesis of [18F]FAC at a commercial FDG synthesis module,
such as TRACERlab MXFDG synthesizer, because the synthetic
steps to produce [18F]FAC were similar to those used to prepare
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G. Tang et al.
Table 1. Automated synthesis of [18F]FAC via different approaches
Synthesis
time (min)
Radiochemical yield
(%)
Radiochemi-
cal purity (%)
Method
Reference
One-pot—HPLC purification
One-pot procedure—
SEP-PAK purification
Two-pot procedure
On-column hydrolysis
On-column hydrolysis
50
o30
445 (Uncorrected)
40 (Uncorrected)
499
495
This work and Tang et al.9
This work and Tang et al.10
42
o20
32
66 (Corrected)
460 (Uncorrected)
50 (Corrected)
499
495
499
Padgett et al.7
This work
Sun et al.8
[18F]FDG. For automated synthesis of [18F]FAC, our on-column dissolved in 1 mL of acetonitrile, vial 4 was filled with 0.8 mL of
hydrolysis procedure had some advantages over the hydrolysis 3 M HCl solution or 410 mL of water, vial 5 was filled with 1 mL
method on four Oasis HLB plus cartridges reported by Sun of 3 M NaOH solution, and vial 6 was filled with 1 mL of HPLC
et al.8 For example, we could obtain a higher radiochemical eluent (5% EtOH) or 10 mL of water. For isotonic adjustment, a
yield within a shorter total synthesis time; Toxic matter calculated amount of NaCl was added to vial 14. The radio-
K222 could be completely removed by using TSCX cartridge. activity was collected on a SEP-PAK QMA cartridge, where
In addition, hydrolysis and neutralization were carried out [18F]fluoride was trapped and 18O-water was collected for
on an integrated short-column system, which could recycling. A volume of 1.5 mL of a solution of K2CO3 and K222
further simplify the automated procedure of [18F]FAC from vial 1 was eluted through the SEP-PAK QMA cartridge, in
using TRACERlab MXFDG synthesizer. A comparison of auto- which the trapped 18FÀ was eluted into the reaction vessel. The
mated synthesis of [18F]FAC using several methods is shown in solvent was evaporated under a stream of helium at 851C. After
Table 1.
complete removal of the solvent, the precursor in vial 3 was
added to the reaction vessel containing the dried [K/K222] 118FÀ
complex and the vessel was heated for 5 min at 851C. Then, the
reaction mixture was cooled, concentrated, and the resulting
reaction mixture was added with 3 M NaOH from vial 5. The
mixture was hydrolyzed to remove the benzyl-protected group
at room temperature for 2 min. After cooling the mixture was
neutralized with 3 M HCl from vial 4 and passed through a SEP-
PAK alumina cartridge. The eluate was collected in a glass vial.
Before HPLC purification, 1 mL of HPLC eluent from vial 6 was
added to the reaction vessel and the solution was then passed
through the same SEP-PAK alumina cartridge. The eluate was
collected in the same glass vial. Finally, [18F]FAC was purified by
HPLC system consisting of a pump, an automatic sample
injector, a reverse-phase C18 column (250 Â 10 mm), a UV
absorption detector (220 nm) and radiodetector. The mobile
phase used was H2O/C2H5OH (95/5, v/v) at a flow rate of 8 mL/
min. The peak corresponding to [18F]FAC was collected and
passed through 0.22 mm sterile filter into a sterile vial to obtain
the final formulation. The radiochemical yield was expressed as
the amount of radioactivity in the [18F]FAC fraction divided by
the total 18F-radioactivity.
For SEP-PAK purification, the fluorination reaction mixture
was evaporated to dryness and cooled. Then removal of the
protective group was achieved by hydrolysis with 1 mL of 3 M
NaOH solution (vial 5) and kept at room temperature for 2 min.
The mixture was neutralized with 0.8 mL of 1 M HCl (vial 4) and
passed through a SEP-PAK plus C18 cartridge, a TSCX cartridge,
and a SEP-PAK light alumina N cartridge in series. The SEP-PAK
cartridges were washed with 10 mL of sterile water (vial 6). The
eluates were combined and further passed through 0.22 mm
sterile filter into a sterile vial to obtain the final [18F]FAC.
Experimental
Reagents and apparatus
Benzyl 2-bromoacetate and 4,7,13,16,21,24-hexaoxa-1,10-diazabi-
cyclo[8.8.8]hexacosane (Kryptofix 222, K222) were obtained from
Sigma-Aldrich. SEP-PAK light QMA cartridge, SEP-PAK plus C18
cartridge, and SEP-PAK light Alumina cartridge were obtained from
Waters (Milford, MA). TSCX cartridge was prepared by opening SEP-
PAK C18 cartridge (Waters) and filling them with AG 50W-X8 (H1
form, Bio-Rad). The cartridges were washed with 10 mL of sterile
water before use. All reagents were used without further
purification. Radioactivity was determined using a calibrated ion
chamber (Capintec CRC-15R). TRACERlab FXFN synthesizer was
purchased from GE Medical System (GEMS). HPLC for [18F]FAC
purification was carried out in the TRACERlab FXFN synthesizer
built-in HPLC system with a semi-preparative reverse-phase C18
column (10 mm  250 mm, Waters) and C18 precolumn equipped
with a UV detector and a radioactivity detector. For the quality
control, HPLC analysis was carried out on a modular HPLC system
with a HypersilsAPS2 column (4.6 mm  250 mm, ThermoQuest
Company, UK), consisting of two LC-10ATvp pumps (Shimadzu
Corporation of Japan) and a variable wavelength SPD-10ATvp UV
detector (Shimadzu Corporation of Japan), a LB 508 Radioflow
detector with a two-channel analyzer (EG &G, Germany) and a
computer (Japan). The UV signal was monitored by a UV Lambda
Max detector at 220 nm.
Automated synthesis of [18F]FAC via a one-pot procedure
[18F]fluoride was obtained through the nuclear reaction 18O(p,
n)18F by irradiation of a 95% 18O-enriched water target with a
16.5 MeV proton beam at the PETtrace cyclotron (GEMS). Before
delivery of [18F]fluoride to the synthesizer, vial 1 was filled with a
mixture of 15 mg of K222, 3 mg of K2CO3, 1 mL of acetonitrile,
and 0.5 mL of water, vial 3 was filled with benzyl 2-bromoacetate
Automated synthesis of [18F]FAC via on-column hydrolysis
procedure
The [18F]fluorination reaction steps were carried out according
to the one-pot procedure described above. After fluorination,
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J. Label Compd. Radiopharm 2008, 51 297–301

G. Tang et al.
the reaction mixture was concentrated, cooled, diluted with CERlab FXFN synthesizer, the final product can be obtained in
410 mL of water (vial 4), and passed through two SEP-PAK plus o20 min after the end of bombardment with a radiochemical
C18 cartridges, a TSCX cartridge, and a SEP-PAK light alumina N yield of 460% (decay uncorrected). Radiochemical purity of
cartridge in series. The SEP-PAK cartridges were dried under a [18F]FAC is 495%. The on-column hydrolysis procedure can be
flow of helium and the eluate was collected in a waste vial. adaptable to the fully automated synthesis of [18F]FAC in a
Removal of the protective group trapped in the first two Sep- commercial FDG synthesis module, such as TRACERlab MXFDG
Pak plus C18 cartridges was achieved by hydrolysis on the synthesizer.
cartridges with 0.9 mL 2 M NaOH (vial 5) at room temperature for
2 min. The SEP-PAK cartridges were washed with 10 mL of sterile Acknowledgement
water (vial 6) and the eluate was collected in sterile vessel 14.
Finally, the solution in vessel 14 containing solid NaCl was
further passed through 0.22 mm sterile filter into a sterile vial 18
to obtain the final [18F]FAC (pHꢀ7.0) under a flow of helium.
This work was supported by a grant of the Office of Science and
Technology in Guangdong Province, China (Grant No.
2003C34304).
Analysis of [18F]FAC
The aforementioned analytical HPLC system with a Hypersil-
sAPS2 column (4.6 mm  250 mm, ThermoQuest Company, UK)
was used for the checking of radiochemical purity and specific
activity eluted with MeCN/H2O (60/40, v/v) at a flow rate of
1 mL/min, and a radio-TLC developed with 90% MeCN as the
solvent system was also used for the checking of radiochemical
purity. K222 detection test was performed on the silica gel 60
coated plate developed with methanol/ammonium hydroxide
(90/10, v/v) as the solvent system, and iodine vapor was used for
staining the spots to render them visible.12
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The automated synthesis of [18F]FAC has been successfully
accomplished via the two-step one-pot procedure and the on-
column hydrolysis procedure. The on-column hydrolysis proce-
dure is a very simple and rapid automated radiosynthesis of
[18F]FAC. Using the on-column hydrolysis procedure in TRA- [12] Chaly T, Dahl JR. Nucl Med Biol 1989, 16, 385–387.
J. Label Compd. Radiopharm 2008, 51 297–301
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