Automated radiosynthesis of no-carrier-added 4-[18F] fluoroiodobenzene: A versatile building block in 18F radiochemistry

Article abstract of DOI:10.1002/jlcr.3137

4-[¹⁸F]Fluoroiodobenzene ([¹⁸F]FIB) is a versatile building block in ¹⁸F radiochemistry used in various transition metal-mediated C-C and C-N cross-coupling reactions and [¹⁸F] fluoroarylation reactions. Various

Full text of DOI:10.1002/jlcr.3137

Note
Received 10 September 2013,
Revised 7 October 2013,
Accepted 8 October 2013
Published online in 26 November 2013 Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.3137
Automated radiosynthesis of no-carrier-added
18
4-[ F]fluoroiodobenzene: a versatile building
18
block in F radiochemistry
*
Jenilee Dawn Way and Frank Wuest
4-[18F]Fluoroiodobenzene ([18F]FIB) is a versatile building block in 18F radiochemistry used in various transition metal-
mediated C–C and C–N cross-coupling reactions and [18F]fluoroarylation reactions. Various synthesis routes have been
described for the preparation of [18F]FIB. However, to date, no automated synthesis of [18F]FIB has been reported to allow
access to larger amounts of [18F]FIB in high radiochemical and chemical purity. Herein, we describe an automated synthesis
of no-carrier-added [18F]FIB on a GE TRACERlab™ FX automated synthesis unit starting from commercially available
(4-iodophenyl)diphenylsulfonium triflate as the labelling precursor. [18F]FIB was prepared in high radiochemical yields of
89 10% (decay-corrected, n = 7) within 60 min, including HPLC purification. The radiochemical purity exceeded 95%, and
specific activity was greater than 40 GBq/μmol. Typically, from an experiment, 6.4 GBq of [18F]FIB could be obtained starting
from 10.4 GBq of [18F]fluoride. Copyright © 2013 John Wiley & Sons, Ltd.
Keywords: 4-[18F]fluoroiodobenzene; automation; sulfonium salts; building block
labelling precursors. Moreover, novel technologies like microwave
Introduction
activation and microfluidic devices have also been applied to
prepare 4-[18F]fluorohalobenzenes. A comprehensive summary of
The success of positron emission tomography (PET) for
functional molecular imaging depends largely on the availability
of suitable radiotracers. Recent advancements in radionuclide
methods and technologies for the preparation of 4-[18F]
fluorohalobenzenes has recently been published.20 This review
production,1 automation of radiotracer synthesis,2,3 and
also discusses all the advantages and disadvantages for the
significant improvement of PET scanner instrumentation,4–7
including small animal PET scanners,8 have further stimulated
clinical and preclinical research activities focused on the
visualization and assessment of biochemical processes in living
organisms. However, the design and synthesis of innovative
PET radiotracers remain a special challenge, and as such, PET
chemistry has evolved into a complex chemical science. Special
attention is attributed to radiochemistry with the short-lived
positron emitter fluorine-18 (18F, t1/2 = 109.8min). The relatively
long half-life, the low maximal positron energy (0.635 MeV), and
the ease of large scale cyclotron production make 18F an ideal
radionuclide for the design and synthesis of PET radiotracers.
The PET chemistry of [18F]fluoride9 and the applications of
18F-labelled radiotracers10,11 have been reviewed frequently over
the last decades. Within the plethora of 18F-labelled radiotracers,
only a few have been prepared by the use of transition metal-
mediated cross-coupling reactions. These transition metal-
mediated cross-coupling reactions mainly exploited palladium
complexes and 4-[18F]fluorohalobenzenes as an electrophilic
coupling partners for the preparation of PET radiotracers
containing a 4-[18F]fluorophenyl group.
selections of available precursors for the preparation of 4-[18F]
fluorohalobenzenes. Based on the reported radiochemical yields
and the availability of starting materials, nucleophilic aromatic
radiofluorination reactions using sulfonium salts as labelling
precursors seem to be the most promising synthesis route for
the preparation of 4-[18F]fluorohalobenzenes. Therefore, we
decided to adapt sulfonium salt-based chemistry to an automated
synthesis procedure enabling the preparation of large amounts of
no-carrier-added (n.c.a.) 4-[18F]fluorohalobenzenes with special
focus on 4-[18F]fluoroiodobenzene ([18F]FIB).
Herein, we describe a fully automated synthesis of n.c.a. [18F]
FIB on a GE TRACERlab™ FX (General Electric Company, Fairfield,
CT, US) automated synthesis unit (ASU) starting from
commercially available (4-iodophenyl)diphenylsulfonium triflate
as the labelling precursor.
Over the last decades, numerous methods have been reported Department of Oncology, University of Alberta, 11560 University Ave, Edmonton,
for the preparation of 4-[18F]fluorohalobenzenes. Methods include
AB, T6G 1Z2, Canada
hot atom recoil chemistry and direct electrophilic and nucleophilic
*Correspondence to: Frank Wuest, Department of Oncology, University of
Alberta, 11560 University Avenue, Edmonton, AB T6G 1Z2, Canada.
aromatic radiofluorination chemistry.12–16 Significant improvements
were achieved by using iodonium17,18 and sulfonium19 salts as E-mail: wuest@ualberta.ca
J. Label Compd. Radiopharm 2014, 57 104–109
Copyright © 2013 John Wiley & Sons, Ltd.

J. D. Way and F. Wuest
CH3CN (1 mL), and the reactor was sealed. The reaction proceeded for
15 min at 85 °C. Upon completion, the reaction mixture was diluted with
water (20 mL) and trapped on a Waters Sep-Pak® tC18 plus light cartridge.
The solid phase extraction (SPE) cartridge was then washed with additional
water (10 mL), and the final product of [18F]FIB was eluted off in CH3CN (3 mL).
Experimental
General
All chemicals used were obtained from Sigma-Aldrich® (St. Louis, MO, US)
and used as received without further purification. Water was obtained from
a
Barnstead Nanopure water filtration system (Barnstead Diamond
Fully automated synthesis of 4-[18F]fluoroiodobenzene
Nanopure pack organic free RO/DIS, Thermo Scientific™ ( Waltham, MA, US)).
The HPLC purification and analysis of 18F-radiolabelled products
were performed using a Phenomenex (Torrance, CA, US) Luna® C18(2)
column (100 Å, 250 × 10 mm, 10 μm) using gradient elution specific to the
given compound (Gilson (Middleton, WI, US) 321 pump, 171 diode array
detector, Berthold Technologies Herm LC). Radio-thin layer chromatography
(TLC) were performed using either EMD Merck (Darmstadt, Germany) F254
silica gel 60 aluminum-backed TLC plates or Analtech (Newark, DE, US)
RP18 with UV254 aluminum-backed TLC plates (Bioscan AR-2000
(Washington, DC, US)). Quantification of radioactive samples during chemistry
was achieved using a Biodex Atomlab™ 400 dose calibrator (Shirley, NY, US).
Reaction parameters were screened using an IKAMAG® Ret-G stir plate
(IKA® Works Inc., (Wilmington, NC, US)) with an oil bath.
Radiosynthesis of [18F]FIB was performed on a GE TRACERlab™ FX. This
ASU was modified in terms of program and hardware (Figure 1).
Synthetic procedure started with the elution of resin-bound cyclotron-
produced [18F]fluoride from the Waters Sep-Pak® light QMA anion-exchange
column into reactor 1 (R1) of the GE TRACERlab™ FX using a solution of 86%
K(2.2.2.)/K2CO3 (1.5mL). [18F]Fluoride was dried azeotropically under vacuum
under a steady stream of nitrogen at 50 and 95 °C. To dried [18F]fluoride,
(4-iodophenyl)diphenylsulfonium triflate (V3, 10 mg) in CH3CN (1 mL) was
added and reacted for 15 min at 90 °C. Once the reaction was completed,
the mixture was diluted with water (V5, 12 mL) and passed through a Waters
Sep-Pak® tC18 plus light cartridge (300 mg). The cartridge was washed with
additional water (V6, 10 mL), and [18F]FIB was eluted off in CH3CN (V4,
3.0 mL) into a 20 mL sealed sterile collection vial with a vent needle.
Preparation of kryptofix 2.2.2. solution
Into a 50 mL volumetric flask, K2CO3 (92.3 mg, 668 μmol) in water (7.0 mL) HLPC purification of 4-[18F]fluoroiodobenzene
is added. Next, kryptofix 2.2.2. (TCI America, (Portland, OR, US)) (500 mg,
The HPLC purification of the crude SPE purified [18F]FIB was performed using
1.38 mmol) is added with CH3CN (43.0 mL) to the mark of the flask. The
a gradient elution as follows: (A: water; B: CH3CN; 0min 50% B, 8min 50% B,
solution is then sealed in an amber vial and stored at 4 °C.
21 min 80% B, 33 min 100% B). The flow rate of the system was 3 mL/min,
which gave a retention time of approximately 22min for the [18F]FIB product
as confirmed with the use of a commercially available reference compound.
Manual synthesis of 4-[18F]fluoroiodobenzene
The n.c.a. [18F]fluoride was produced via the 18O(p,n)18F nuclear reaction
The radiochemical purity was determined by the area under the peak of
from [18O]H2O (Rotem Industries Ltd (Mishor, Israel), Hyox oxygen-18
interest compared with the rest of the radiochromatogram; the specific
activity of [18F]FIB was calculated against a standard curve as well.
enriched water, min. 98%) on an ACSI TR19/9 Cyclotron (Advanced
Cyclotron Systems Inc., Richmond, Canada). Cyclotron-produced [18F]
fluoride was then trapped on a Waters (Milford, MA, US) Sep-Pak® light
QMA anion-exchange cartridge (filled with quaternary ammonium
Results and discussion
chloride polymer) and eluted off with 86% K(2.2.2.)/K2CO3 (1.5 mL) into Optimization of reaction parameters for manual synthesis of
4-[18F]fluoroiodobenzene
a long screw top test tube. This solution was then dried azeotropically
with additional CH3CN (6 mL) under nitrogen at 95 °C in an oil bath. Once
fully dried, the reaction vessel was allowed to cool for 5 min, the labelling
Radiosynthesis of [18F]FIB
2
starting from (4-iodophenyl)
precursor (4-iodophenyl)diphenylsulfonium triflate (7 mg) was added in diphenylsulfonium triflate 1 is depicted in Figure 2. Manual
Figure 1. Scheme of the automated synthesis unit for the synthesis of 4-[18F]fluoroiodobenzene.
J. Label Compd. Radiopharm 2014, 57 104–109
Copyright © 2013 John Wiley & Sons, Ltd.
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J. D. Way and F. Wuest
Figure 2. Synthetic procedure for the radiosynthesis of 4-[18F]fluoroiodobenzene 2.
syntheses were performed to optimize reaction conditions by comparable 40% radiochemical yield. However, radiochemical
screening various reaction parameters (solvent, temperature, purity of [18F]FIB in the reaction mixture was higher in CH3CN
synthesis time, and precursor concentration) and SPE methods (91%) compared with that of using DMF as the solvent (70%).
aimed at increasing the radiochemical yield and radiochemical
Also, the UV traces as determined by HPLC (displayed in the
purity of [18F]FIB 2 and decreasing the amount of inevitably right panels) clearly demonstrate that there are more possible
formed by-product [18F]fluorobenzene 3.
contaminants pushed near the retention time of the peak of
interest in DMF as the solvent versus CH3CN. Overall, CH3CN
was chosen to be the solvent of choice, and it was used in all
Influence of solvent on radiosynthesis of 4-[18F]fluoroiodobenzene
Based on the publication of Linjing et al.19 as a general guide, further experiments.
the following solvents were tested for the synthesis of solvents
Influence of temperature on radiosynthesis of 4-[18F]fluoroiodobenzene
of [18F]FIB: acetonitrile (CH3CN), toluene, dioxane, N,N-
dimethylformamide (DMF), and THF. The labelling precursor The reaction between (4-iodophenyl)diphenylsulfonium triflate 1
(4-iodophenyl)diphenylsulfonium triflate 1 was only poorly and n.c.a. [18F]fluoride requires elevated temperatures. However,
soluble in toluene, THF and dioxane. Only CH3CN and DMF proved at an elevated temperature, significant amounts of nonradioactive
to be suitable solvents for (4-iodophenyl)diphenylsulfonium by-products were present in the reaction mixture. Careful testing
triflate 1. Results of using DMF (Figure 3a/3b) and CH3CN of reaction temperature on the radiochemical yield and formation
(Figure 3c/3d) as solvents for the synthesis of [18F]FIB are depicted of nonradioactive by-products is necessary. Various reaction
as respective radio-HPLC and ultraviolet (UV) traces of the reaction temperatures were tested to determine their influence on
mixture (Figure 3). The reaction in both solvents proceeded with a radiochemical yield and the formation of nonradioactive
Figure 3. (a–d) Comparison of N,N-dimethylformamide and CH3CN as solvents for the radiolabelling of (4-iodophenyl)diphenylsulfonium triflate 1 with no-carrier-added
[
18F]fluoride.
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Copyright © 2013 John Wiley & Sons, Ltd.
J. Label Compd. Radiopharm 2014, 57 104–109

J. D. Way and F. Wuest
by-products as represented by their respective HPLC profiles. The Influence of solid phase extraction cartridges on radiosynthesis of
results are summarized in Figure 4a.
4-[18F]fluoroiodobenzene
Reaction temperature of 70 °C resulted in only low
radiochemical yields of 1–2%. Radiochemical yields increased
significantly at elevated temperatures of 85 (61%) to 120 °C
(44%). However, only the reaction temperature of 85 °C gave
sufficient radiochemical yields while showing low amount of
nonradioactive by-products in the UV trace of the HPLC trace.
Therefore, a reaction temperature of 85 °C was used for further
optimization of reaction conditions.
Four different SPE cartridges were tested for their ability to retain
and purify [18F]FIB. SPE cartridges tested included Mackerey-
Nagel (Düren, Germany) Chromafix® HR-P (M), Phenomenex
Strata® C18-U (500 mg), Waters Sep-Pak® tC18 plus light, and
Waters Sep-Pak® C18 plus light.
All cartridges gave comparable results, and we decided to use
Waters Sep-Pak® tC18 plus light cartridges for further experiments.
Summary of optimized reaction parameters for the manual
synthesis of 4-[18F]fluoroiodobenzene
Influence of reaction time on radiosynthesis of
4-[18F]fluoroiodobenzene
Optimal reaction conditions resulted in the following manual
radiosynthesis of [18F]FIB. Radiosynthesis of [18F]FIB was
performed using (4-iodophenyl)diphenylsulfonium triflate as
the labelling precursor (7 mg) in CH3CN (1 mL) at a temperature
of 85 °C for 15 min. Application of these reaction conditions
afforded [18F]FIB in 41 9% decay-corrected radiochemical
yields after SPE and HPLC purification within a total synthesis
time of over a 64 4 min (n = 17). Radiochemical purity was
greater than 99%. In a typical experiment, 235 MBq of [18F]FIB
was prepared starting from 850 MBq of n.c.a. [18F]fluoride. These
optimized reaction parameters are very similar to that of Linjing
et al.16 and provide confirmation that [18F]FIB can be produced
selectively and sufficiently over [18F]fluorobenzene as by-
product. Moreover, application of HPLC purification afforded
[18F]FIB in high radiochemical and chemical purity suitable for
subsequent reactions like transition metal-mediated cross-
coupling reactions.
All reactions were performed at 85 °C using different reaction
times varying from 5 to 30 min. At each time point, aliquots of
the reaction mixture were taken and analyzed via radio-TLC with
50% EtOAc/hexane as the solvent to develop TLC plates. As shown
in Figure 4b, no significant improvement of radiochemical yield
was observed at reaction times longer than 15 min.
Influence of labelling precursor amount on radiosynthesis of 4-[18F]
fluoroiodobenzene
After the optimization of reaction temperature, solvent, and
reaction time, the needed amount of the labelling precursor 1
was tested using the reaction conditions of 85 °C in CH3CN for
15 min. Progress of the reaction was monitored using radio-
TLC. Results are summarized in Figure 4c.
Results in Figure 4c indicate that 5 to 7 mg of the labelling
precursor 1 provided sufficient radiochemical yields of [18F]FIB
(50–59%). Further increases in the mass of sulfonium salt 1
did not result in significantly higher radiochemical yields. To
reduce the amount of potential cold contaminations in the
Automated synthesis of 4-[18F]fluoroiodobenzene
reaction mixture, a labelling precursor amount of 5–7 mg Reaction conditions from optimized manual synthesis were directly
seems to be optimal.
transferred to the GE TRACERlab FX fully ASU. Radiosynthesis of
a
b
100
100
80
60
40
20
0
80
60
40
20
0
60
80
100
120
140
0
10
20
30
40
Temperature (celsius)
Time (minutes)
c
aRadiochemical yield determined by radio-TLC
representing the percentage of 4-
100
80
60
40
20
0
[
18F]fluoroiodobenzene present in the reaction
mixture.
0
5
10
15
Mass of precurssor (mg)
Figure 4. (a–c) Influence of reaction temperature (n = 3), reaction time (n = 3), and concentration of labelling precursor 1 (n = 1) on radiochemical yielda of 4-[18F]
fluoroiodobenzene.
J. Label Compd. Radiopharm 2014, 57 104–109
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J. D. Way and F. Wuest
Figure 5. HPLC trace of reaction mixture of 4-[18F]fluoroiodobenzene synthesis in an automated synthesis unit. This figure is available in colour online at
wileyonlinelibrary.com/journal/jlcr
Figure 6. HPLC purified 4-[18F]fluoroiodobenzene with co-injection of [19F]fluoroiodobenzene. This figure is available in colour online at wileyonlinelibrarycom/journal/jlcr
[18F]FIB in the ASU gave comparable results as found for the suitable for a broad variety of different buildup syntheses,
manual synthesis. The reaction temperature was slightly increased including transition metal-mediated cross-coupling reactions.
to 90 °C because larger amounts of nonreacted [18F]fluoride were
found in the reaction mixture, when a reaction temperature of
85 °C was applied. In summary, the ASU synthesis provided [18F] Acknowledgements
FIB in decay-corrected radiochemical yields of 89 10% (n= 7)
within a reaction time of 59 2 min including HPLC purification. The authors would like to thank John Wilson, David Clendening,
Radiochemical purity was 97 3%. Specific activity of [18F]FIB was
and Blake Lazurko from the Edmonton PET Center for
radionuclide production and excellent technical support. We also
gratefully acknowledge the Dianne and Irving Kipnes
Foundation and the National Science and Engineering Research
Council of Canada (NSERC) for supporting this work.
greater than 40 GBq/μmol. In a typical ASU synthesis, 6.4GBq of
[18F]FIB could be prepared from 10.4 GBq of n.c.a. [18F]fluoride.
HPLC purification of 4-[18F]fluoroiodobenzene
Conflict of Interest
Purification was completed as described in the experimental
section, eluting [18F]FIB at 22 min as shown in Figure 5. Co-injection
with cold reference compound as displayed in Figure 6 confirmed
the identity of [18F]FIB.
The authors did not report any conflict of interest.
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