Preparation of 3′-deoxy-3′-[18F]fluorothymidine ([18F]FLT) in ionic liquid, [bmim][OTf]

Article abstract of DOI:10.1002/jlcr.1046

Although 3′-deoxy-3′-[¹⁸F]fluorothymidine ([ ¹⁸F]FLT) is a prospective radiopharmaceutical for the imaging of proliferating tumor cell, it is difficult to prepare large amount of [ ¹⁸F]FLT, We herein describe the preparati

Full text of DOI:10.1002/jlcr.1046

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2006; 49: 287–293.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.1046
Research Article
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0
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Preparation of 3 -deoxy-3 -[ F]fluorothymidine
1
8
[ F]FLT) in ionic liquid, [bmim][OTf]
(
1
Byung Seok Moon , Kyo Chul Lee , Gwang Il An , Dae Yoon Chi ,
1
1
2
1
3
3
1,
Seung Dae Yang , Chang Woon Choi , Sang Moo Lim and Kwon Soo Chun *
1
Laboratory of Radiopharmaceuticals, Korea Institute of Radiological and
Medical Sciences, Seoul 139-706, Korea
2
Department of Chemistry, Inha University, 253 Yonghyundong Namgu,
Inchon 402-751, Korea
3
Department of Nuclear Medicine, Korea Institute of Radiological and
Medical Sciences, Seoul 139-706, Korea
Summary
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0
18
18
Although 3 -deoxy-3 -[ F]fluorothymidine ([ F]FLT) is a prospective radiopharma-
ceutical for the imaging of proliferating tumor cell, it is difficult to prepare large
1
8
18
amount of [ F]FLT. We herein describe the preparation of [ F]FLT in an ionic
liquid, [bmim][OTf] (1-butyl-3-methyl-imidazolium trifluoromethanesulfonate). At
1
8
optimized condition, [ F]fluorinationin ionic liquid with 5 ml of 1 M KHCO and
3
5
mg of the precursor yielded 61:5 ꢀ 4:3% (n ¼ 10). Total elapsed time was about
0 min including HPLC purification. The rapid synthesis of [ F]FLT can be achieved
1
8
7
by removing all evaporation steps. Overall radiochemical yield and radiochemical
purity were 30 ꢀ 5% and >95%, respectively. This method can use a small amount
of a nitrobenzenesulfonate precursor and can be adapted for automated produc-
tion. Copyright # 2006 John Wiley & Sons, Ltd.
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8
0
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Key Words: [ F]-3 -Deoxy-3 -fluorothymidine ([ F]FLT); ionic liquids; tumor
imaging; PET
Introduction
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18
F]Fluorodeoxyglucose ([ F]FDG) has been the most widely used in
[
positron emission tomography (PET) for tumor imaging. As glucose is
1
8
utilized by benign cells such as activated macrophage, [ F]FDG gives little
11
non-specific results for tumor imaging. [ C]Thymidine has been used for the
*Correspondence to: K. S. Chun, Laboratory of Radiopharmaceuticals, Korea Institute of Radiological and
Medical Sciences, 215-4 Gongneung-Dong, Nowon-ku, Seoul 139-706, Korea. E-mail: kschun@kcch.re.kr
Contract/grant sponsor: Nuclear R&D Program from the Ministry of Science & Technology.
Copyright # 2006 John Wiley & Sons, Ltd.
Received 16 June 2005
Revised 23 November 2005
Accepted 23 November 2005

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B. S. MOON ET AL.
diagnosis of tumor or measurement of tumor’s multiplication rate in PET.
1
Because [ C]thymidine has some limitations to track metabolism due to its
1
0
18
rapid in vivo degradation and short half-life of C-11, 3 -[ F]fluoro-3 -
0
1
8
1–4
18
The [ F]FLT
deoxythymidine ([ F]FLT) has recently been developed.
promises to be one of the most promising radiopharmaceuticals due to lack of
in vivo degradation, metabolic trapping in proliferating cells, and a favorable
3
18
half life for PET imaging. Radiochemical yield of [ F]FLT synthesis was less
5
than 10% when anhydrothymidine derivative was used as a precursor. In
1
8
addition, Oh et al. recently reported that [ F]FLT was obtained in 40–60%
radiochemical yield using over 30 mg of p-nitrobenzenesulfonate (nosylate)
6
–7
precursor.
clinical use of [ F]FLT. Recently, ionic liquids have been reported to be
These drawbacks have been the major obstacles for routine
1
8
8
–10
excellent reaction media for many reactions,
and their usefulness for
1
1,12
fluorination has also been published.
nucleophilic fluorination of precursor of [ F]FDG could be carried out with
Interestingly, it was found that a
1
8
13
high efficiency in the presence of water. Recently, we have developed a new
method for fluorine-18 labeling using an ionic liquid as a reaction medium. In
the present study, we eliminated the azeotropic drying procedure, the most
18
time wasting step, of the [ F]FLT synthesis by using a solvent containing an
ionic liquid ([bmim][OTf], 1-butyl-3-methyl-imidazolium trifluoromethanesul-
1
8
fonate) as a reaction medium. For the synthesis of [ F]FLT, N-Boc-protected
analogue was used as a precursor and the amount of precursor, the kind and
equivalent of base, and the amount of ionic liquid ([bmim][OTf]) were
optimized.
Results and discussion
Generally, mesyl-, tosyl-, and triflate (trifluoromethanesulfonyloxy) are typical
18
leaving groups in the nucleophilic [ F]fluorination reactions using fluoride-18
18
18
fluoride obtained from an O(p,n) F reaction, providing the corresponding
1
8
18
F]fluoroalkanes in excellent yields. In [ F]fluorination process using
[
[
1
8
F]fluoride, an anion exchange resin (QMA) is frequently used to recovery
1
8
O-18 water and a small amount of water is also essential to elute [ F]fluoride.
In addition, water has to be removed by azeotropic distillation with
acetonitrile. Although this drying step is necessitated, over-drying or not
enough drying of water makes failure of reaction or decrease of yield
sometimes. It was reported that the labeling process using ionic liquid
1
2
(
[bmim][OTf]) showed tolerance for a small amount of water.
18
In the present study, therefore, the [ F]fluorination was evaluated using a
minimum amount volume of water. Furthermore, the selection of base
appeared to be very important during the reaction. Some metal bases such as
potassium bicarbonate, potassium carbonate, or cesium carbonate have been
shown to give high fluorination yields. However, the metal bases have low
Copyright # 2006 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2006; 49: 287–293

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PREPARATION OF [ F] FLT
289
O
N
O
N
N-Boc
O
N-Boc
O
[18
bmim][OTf]
CH CN, base
F]F-
ONs
DMTrO
DMTrO
O
O
[
18F
3
protected [18F]FLT
1
18
Scheme 1. Synthesis of protected [ F]FLT
Table 1. Labeling yield with various amounts of carbonates
a
Radiolabeling yield (%), detected by TLC (n ¼ 3)
Base (ml)
1 M KHCO
3
1 M Cs
2
CO
3
2 3
1 M K CO
2
5
0.0
5.0
0.0
5.0
.5
.0
20.4 ꢀ 2.7
52.4 ꢀ 3.4
46.6 ꢀ 2.9
34.4 ꢀ 2.0
NA
NA
34.1 ꢀ 4.2
43.0 ꢀ 3.4
18.6 ꢀ 2.8
13.4 ꢀ 2.3
NA
17.7 ꢀ 1.9
33.1 ꢀ 2.7
34.4 ꢀ 3.7
40.1 ꢀ 3.3
51.4 ꢀ 4.2
38.4 ꢀ 3.0
1
1
2
2
b
NA
a
Reaction condition: 10 mg of nosylate precursor 1, 200 ml of acetonitrile, 200 ml of [bmim][OTf], x ml of
1
8
[
F]fluoride in H
2
O+y ml of base ¼ 70 ml for 15 min at 1208C.
b
NA ¼ not attempted.
1
2
solubility in water containing acetonitrile without Kryptofix[2.2.2]. There-
fore, optimization of the metal bases was carried out (Scheme 1).
As shown in Table 1, the amounts of metal bases were optimized with fixed
1
M concentration of metal bases such as potassium bicarbonate, potassium
carbonate, or cesium carbonate and a fixed amount of the precursor (10 mg).
The highest yield was obtained when 5 ml of potassium bicarbonate or 20 ml of
potassium carbonate were used (Table 1).
Under the most optimal condition of potassium bicarbonate (1 M, 5 ml) and
potassium carbonate (1 M, 20 ml), as shown at Table 1, 5 mg of the precursor
and 5 ml of potassium bicarbonate gave the best yield (61:5 ꢀ 4:3%) (Table 2).
Finally, as shown in Table 3, the best yield was obtained when 200 ml of
ionic liquid was used. The optimized labeling conditions were as follows: 5 mg
of nosylate precursor 1, 200 ml of acetonitrile, 200 ml of [bmim][OTf], 65 ml of
1
[
8
F]fluoride in H O and 5 ml of 1 M KHCO for 15 min at 1208C. The reaction
2
3
solvent – ionic liquid – was removed with silica gel cartridge and the fluorine-
1
8 labeled product was deprotected by the treatment with 0.5 N HCl (300 ml)
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for 5 min at 1108C. The synthesis of [ F]FLT was confirmed by radio-TLC
(Figure 1) and separated using HPLC (Figure 2).
Total elapsed time was about 70 min including deprotections and HPLC
purification step. Overall radiochemical yield and purity were 30 ꢀ 5% (decay
Copyright # 2006 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2006; 49: 287–293

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B. S. MOON ET AL.
Table 2. Labeling yield with various amounts of 1
a
Radiolabeling yield (%), detected by TLC (n ¼ 3)
1
(mg)
1 M KHCO (5 ml)
1 M K CO (20 ml)
3
2
3
b
5
61.5 ꢀ 4.3
52.4 ꢀ 3.4
24.1 ꢀ 1.2
20.8 ꢀ 5.1
29.2 ꢀ 3.6
51.4 ꢀ 4.2
53.0 ꢀ 2.8
25.6 ꢀ 6.5
1
1
2
0
5
0
a
18
Reaction condition: In 200 ml of acetonitrile, 200 ml of [bmim][OTf], and 65 or 50 ml of [ F]fluoride in
H2O þ 5 ml of 1 M KHCO
n ¼ 10.
3
or 20 ml of 1 M KHCO
3
, respectively, for 15 min at 1208C.
b
Table 3. Labeling yield with various amounts of [bmim] [OTf]
a
Radiolabeling yield (%), detected by TLC (n ¼ 3)
[
bmim][OTf] (ml)
1 M KHCO (5 ml)
3
100
200
300
400
49.5 ꢀ 5.4b
61.5 ꢀ 4.3
40.3 ꢀ 3.6
37.3 ꢀ 4.5
a
Reaction condition: 5 mg of nosylate precursor 1, 200 ml of acetonitrile, amount of [bmim][OTf], 65 ml of
F]fluoride in H2O þ 5 ml of 1 M KHCO for 15 min at 1208C.
3
n ¼ 10.
1
8
[
b
Figure 1. Monitoring of reaction by radio-TLC [left: fluorination (MeOH/
CHCl /hexane=1:4:5) right. Hydrolysis (5% water: acetonitrile)]
3
1
8
Figure 2. [ F]FLT purification using HPLC (7.8 mm ꢁ 300 mm, lBondapak
RP-18, 10 l, EtOH=H O ¼ 10=90 [v/v], flow rate: 2.5 ml/min)
2
Copyright # 2006 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2006; 49: 287–293

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PREPARATION OF [ F] FLT
291
corrected) and >95%, respectively. Furthermore, we effectively decreased the
amount of nosylated precursor and the method can be adapted to routine
1
production of [ F]FLT.
8
The above optimized condition was satisfactory when a small amount of
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8
water – 65 ml of [ F]fluoride in O-18 water and 5 ml of 1 M KHCO was used.
3
However, when this condition was applied to a large amount of water such as
over 100 ml, the labeling yield was dramatically dropped. Therefore, to reduce
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total volume of [ F]fluoride in O-18 water, QMA (SPE cartridge Chromafix
0-PS-HCO ) cartridge was used. When we extracted the activity from QMA
3
3
cartridge using the best conditions described in Tables 1 and 2, about 15% of
the total activity loaded was eluted with 80 ml of KHCO (5 ml of 1 M KHCO3
3
þ75 ml of H O) and 250 ml of acetonitrile. However, when 80 ml of K CO
2
2
3
(
20 ml of 1 M K CO þ 60 ml of H O) were used, over 90% of the activity was
2
3
2
extracted. Therefore, we adopted 1 M K CO as the eluting base for a large
2
3
scale of activity (over 37.0 GBq); the optimized labeling conditions would be
1
2
4
0 mg of the precursor, 80 ml of K CO (20 ml of 1 M K CO þ 60 ml of H O),
2
3
2
3
2
1
8
50 ml of CH CN and 200 ml of ionic liquid. [ F]Fluorination yield was
3
8 ꢀ 6:0% (n ¼ 3) and overall radiochemical yield and purity were 25 ꢀ 5%
and >95%, respectively.
Conclusion
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Based on a recent report of nucleophilic [ F]fluorination of some halo- and
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18
mesyloxyalkanes to corresponding [ F]fluoroalkanes using [ F]fluoride ion
and metal carbonate in ionic liquid [bmim][OTf] in the presence of some water,
1
we prepared [ F]FLT and optimized synthetic condition in ionic liquid. This
8
1
8
method not only improved radiochemical yield of [ F]FLT, but also reduced
the amount of precursor. Besides this method using ionic liquid is easy to
synthesis, it has shorter synthetic time and less eliminated by-product
formation relative to the previous literature. But, this method is slightly
decreased radiochemical yield in scale-up condition as well as difficult in
separation of polar compounds. The optimized labeling conditions were 5 mg
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8
of precursor, 5 ml of 1 M KHCO (for a small amount [ F]fluoride in O-18
3
1
8
water) or 20 ml of 1 M K CO (for a large amount [ F]fluoride in O-18 water),
2
3
2
50 ml of CH CN, and 200 ml of ionic liquid. When used 5 ml of 1 M KHCO ,
3
3
the labeling yield was 61:5 ꢀ 4:3% (n ¼ 10) and overall radiochemical yield
and radiochemical purity were 30 ꢀ 5% and >95%, respectively. In case of
1
8
using 20 ml of 1 M K CO , [ F]fluorination yield was 48 ꢀ 6:0% (n ¼ 3) and
2
3
overall radiochemical yield and purity were 25 ꢀ 5% and >95%, respectively.
The specific activity after HPLC purification was >37.0 GBq/mmol in a large
scale activity condition. We effectively reduced the amount of nosylated
1
8
precursor and this method could be used to routine production of [ F]FLT.
Copyright # 2006 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2006; 49: 287–293

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B. S. MOON ET AL.
Experimental
0
0
0
0
3
-N-tert-Butoxycarbonyl-(5 -O-(4,4 -dimethoxytriphenylmethyl)-2 -deoxy-3 -
O-(4-nitrobenzenesulfonyl)-b-D-threopentofuranosyl)thymine
6
This was synthesized by the modified method of known procedures. Sep-Pak
Silica light cartridge was purchased from Waters (USA), QMA cartridge (SPE
cartridge Chromafix 30-PS-HCO ) was from Macherey-Nagel Ins. (USA) and
3
[
bmim][OTf] was from Chem Tech Research Incorporation (C-TRI Co.
Korea). Thin layer chromatography (TLC) was performed on Merck 60 F254
silica plates and radio-TLC was monitored on a Bioscan AC-3000 scanner
(Washington DC, USA). High Performance Liquid Chromatography (HPLC)
was carried out on a Young-Lin System (Young-Lin Instrument, Korea) with
a semi preparative column (mBondapak RP-18, 10 m, 7.8 ꢁ 300 mm) and
simultaneously monitored by a Young-Lin UV instrument (267 nm) and
Raytest GABI g-detector. F-18 was produced with MC-50 cyclotron by
1
8
irradiation of H O at Korea Institute of Radiological and Medical Sciences
2
(KIRAMS).
Typical procedure
F-18 containing water (about 55.0 GBq, 1.0–1.5 ml) was extracted from QMA
cartridge using 80 ml of K CO (20 ml of 1 M K CO þ 60 ml of H O and 250 ml
2
3
2
3
2
of acetonitrile. This solution was added to the mixture of nosylate precursor 1
(10 mg) in [bmim][OTf] (200 ml). The mixture was stirred at 1208C for 15 min
under open system. After 2 ml of dichloromethane was added, the reaction
mixture was loaded on two silica Sep-Pak, and then eluted with
MeOH:CH Cl (1:9). Collected eluate was evaporated with N and diluted
2
2
2
with 1 ml of acetonitrille and 0.5 N HCl (300 ml). The mixture was stirred at
108C for 5 min. After cooling to room temperature, the mixture was purified
by HPLC using a semi-preparative column (mBondapak RP-18, 5 m, EtOH=
1
1
8
H O ¼ 10=90 [v/v], flow rate: 2.5 ml/min). Collected activity of pure [ F]FLT
2
was 8–10 GBq. Total elapsed time was about 70 min including HPLC
purification. Overall radiochemical yield and purity were 25 ꢀ 5% (decay
corrected) and >95%, respectively. The specific activity after HPLC
purification was >37.0 GBq/mmol.
References
1
. Goethals P, Lameire N, van Eijkeren M, Kesteloot D, Thierens H, Dams R.
J Nucl Med 1996; 37: 1048–1052.
2
. Grierson JR, Shields AF, Zheng M, Kozawa SM, Courter JH. Nucl Med Biol
1
995; 22: 671–678.
Copyright # 2006 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2006; 49: 287–293

1
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PREPARATION OF [ F] FLT
293
3
4
. Mankoff DA, Shields AF, Link JM, Graham MM, Muzi M, Peterson LM,
Eary JF, Krohn KA. J Nucl Med 1999; 40: 614–624.
. Shields AF, Grierson JR, Dohmen BM, Machulla HJ, Stayanoff JC,
Lawhorn-Crews JM, Obradovich JE, Muzik O, Mangner TJ. Nat Med 1998; 4:
1334–1336.
5
. Machulla H-J, Blocher A, Kuntzsch M, Piert M, Wei R, Grierson JR. J Radioanal
Nucl Chem 2000; 243: 843–846.
6
. Yun MY, Oh SJ, Ha H-J, Ryu JS, Moon DH. Nucl Med Biol 2003; 30: 151.
. Oh SJ, Mosdzianowski C, Chi DY, Kim JY, Kang SH, Ryu JS, Yeo JS. Nucl Med
Biol 2004; 31: 803–809.
7
8
. Welton T. Chem Res 1999; 99: 2071–2083.
9
. Holbrey JD, Seddon KR. J Chem Soc Dalton Trans 1999; 2133–2139.
1
1
1
1
0. Zhao H, Malhotra SV. Aldrichim Acta 2002; 35: 75–83.
1. Kim DW, Song CE, Chi DY. J Am Chem Soc 2002; 124: 10278–10279.
2. Kim DW, Choe YS, Chi DY. Nucl Med Biol 2003; 30: 345–350.
3. Kim HW, Jeong JM, Lee Y-S, Chi DY, Chung K-H, Lee DS, Chung J-K,
Lee MC. Appl Radiat Isot 2004; 61: 1241–1246.
Copyright # 2006 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2006; 49: 287–293