Fast production of highly reactive no-carrier-added [18F] fluoride for the labeling of radiopharmaceuticals

Article abstract of DOI:10.1002/anie.200906341

(Chemical equation presented) Doing without water: The ¹⁸F labeling of radiopharmaceuticals requires nearly anhydrous solutions of [ ¹⁸F]fluoride. Aqueous K₂CO₃ is generally used to elute [¹⁸F]fluorid

Full text of DOI:10.1002/anie.200906341

Angewandte
Chemie
DOI: 10.1002/anie.200906341
Radiofluorination
Fast Production of Highly Reactive No-Carrier-Added [18F]Fluoride
for the Labeling of Radiopharmaceuticals**
Christian F. Lemaire,* Joꢀl J. Aerts, Samuel Voccia, Lionel C. Libert, Frꢁdꢁric Mercier,
David Goblet, Alain R. Plenevaux, and Andrꢁ J. Luxen
Fluorine-18 has become the most widely used short-lived
radioisotope for the labeling of radiopharmaceuticals for
positron emission tomography (PET; 18F, t1/2 = 109.7 min).[1]
Depending on the mode of production, the no-carrier-added
[18F]fluoride ion can be obtained in aqueous solution. How-
ever, under these conditions it is strongly hydrated and
therefore unreactive for nucleophilic substitution. Several
methods have been developed to increase its reactivity.[2,3]
Currently, the most well-established procedure requires
trapping of the [18F]fluoride ion on an anion-exchange resin
and its subsequent elution with a small volume of an organic–
aqueous solution (CH3CN/H2O, 50:50 v/v) of an inorganic
weak base (potassium carbonate) and a cryptand (kryptofix
K222).[4] After two or three azeotropic evaporation steps, the
cryptand enables solubilization of the [18F]fluoride ion in an
active form in a polar aprotic solvent suitable for the
subsequent labeling reaction (e.g. CH3CN, dimethyl sulfox-
ide). However, this process, which requires several minutes
(5–10 min) and complex automation, consumes radiochem-
ical yield (3.1–6.1%) and is not suited to the miniaturization
of PET equipment.
considered as the main limitation to the size reduction of such
systems.
Herein, we evaluate a new method for elution from the
anion-exchange resin which would avoid the aforementioned
azeotropic evaporation step with acetonitrile. This step is
typically very difficult to implement on a microchip device.
We examined a selection of organic bases as potential
additives to replace the inorganic bases or salts classically
used in the resin eluent.
A large variety of organic bases that differ, for example, in
terms of strength, nucleophilicity, and steric hindrance, are
commercially available (some are listed in Table 1 according
to their pKa value). These organic bases usually contain
nitrogen atoms, the protonation of which can lead to highly
reactive anions.[7]
Table 1: Effect of the nature of the base on the radiochemical yield of 2.[a]
Entry
Base
pKa[b]
RY[c] [%]
Water[d] [ppm]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
none
Et3N
sparteine
TMG
BTMG
DBN
–
15
30
25
27
95
13
47
87
93
22
81
93
92
76
17
14
6
4958
2392
2912
1746
3912
6641
3572
2621
4755
4899
3543
2748
446
18.46
21.66
23.3
23.56
23.89
24.3
25.1
25.44
25.98
26.5
26.9
27.6
28.4
32.9
32.9
41.9
As an example, the synthesis of [18F]fluorodeoxyglucose
(FDG), the most used PET metabolic tracer, with micro-
reactors or microfluidic chips was recently reported.[5] The
extremely high surface-area-to-volume ratio of these micro-
reactors enables experiments to be performed on a far lower
scale than is possible with conventional-scale reactors; lower
starting amounts of very expensive precursors are required,
the products are purer and obtained in higher yield and in
shorter reaction times, and there is less waste. Moreover, hot-
cell shielding is greatly facilitated. However, despite all these
advantages, the [18F]fluoride is always dried by the conven-
tional method with K2CO3.[6] This unavoidable step can be
DBU
TMGN
MTBD
TBD
P1tOct
P1tBu
BEMP
BTPP
P2Et
Verkade iPr
P4tBu
3877
3536
1960
2720
[a] Reaction conditions: 1a (40 mg), CH3CN (1 mL), solution of 18FÀ
(20 mL), base (50 mmol), 1008C, 5 min. [b] pKa value of the conjugate
acid in CH3CN. [c] Radiochemical yield (decay-corrected). [d] Water
content after labeling. BEMP=2-tert-butylimino-2-diethylamino-1,3-
dimethylperhydro-1,3,2-diazaphosphorine, BTMG=2-tert-butyl-1,1,3,3-
tetramethylguanidine, BTPP=tert-butylimino-tri(pyrrolidino)phosphor-
ane, DBN=1,5-diazabicyclo[4.3.0]non-5-ene, DBU=1,8-diazabicyclo-
[5.4.0]undec-7-ene, TMG=1,1,3,3-tetramethylguanidine, MTBD=7-
methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene, P1tBu=tert-butylimino-tris-
[*] Dr. C. F. Lemaire, Dr. J. J. Aerts, L. C. Libert, Dr. F. Mercier, D. Goblet,
Dr. A. R. Plenevaux, Prof. A. J. Luxen
University of Liꢀge, B30—Cyclotron Research Center
Sart Tilman, 4000 Liꢀge (Belgium)
Fax: (+32)4-366-2946
E-mail: christian.lemaire@ulg.ac.be
Dr. S. Voccia
Trasis sa
Voie de Liꢀge, 2, 4053 Embourg (Belgium)
(dimethylamino)phosphorane,
P1tOct=tert-octylimino-tris(dimethyl-
amino)phosphorane, P1tBu=tert-butylimino-tris(dimethylamino)phos-
phorane, P2Et=N’’’-ethyl-N,N,N’,N’-tetramethyl-N’’-[tris(dimethylami-
no)phosphoranylidene]phosphorimidic triamide, P4tBu=N-[[tert-butyl-
imino-bis[tris(dimethylamino)phosphoranylideneamino]phosphoranyl]-
imino-bis(dimethylamino)phosphoranyl]-N-methylmethanamine, TBD=
1,5,7-triazabicyclo[4.4.0]dec-5-ene, TMGN=1,8-bis(tetramethylguani-
dino)naphtalene, Verkade iPr=2,8,9-triisopropyl-2,5,8,9-tetraaza-1-
phosphabicyclo[3.3.3]undecane.
[**] This project was supported by the Fonds de la Recherche
Scientifique, the University of Liꢀge (Fonds spꢁciaux, Crꢁdit
classique, and Fonds Rahier). A.R.P. is a Senior Research Associate
of the Fonds de la Recherche Scientifique (F.R.S.–FNRS, Brussels,
Belgium).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906341.
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Communications
Acetonitrile is the solvent of choice for the
the base increased from the low pKa value of TMG to the high
pKa value of P4tBu (Figure 2). However, pKa values of around
30 were required for quantitative elution of the [18F]fluoride
trapped on the cartridge. Thus, Verkade superbases can also
be used for elution.
Similar effects were observed when water was replaced
with other dry compounds containing an acidic hydrogen
atom, such as alcohols (e.g. methanol, 2-propanol; Figure 3).
In this case, the [18F]fluoride is displaced from
the cartridge by the alkoxide anions generated
by deprotonation of the alcohol, as is well-
illustrated by curve A (Figure 3), which shows
that no elution occurs until the addition of a
small amount of dry methanol to the eluent.
[18F]radiolabeling of aliphatic compounds. Generally, this
polar aprotic solvent contains a small amount of water (0.01–
0.5%). In the presence of a Schwesinger base, such as P4tBu
(pKa = 41.9) or P4tOct (pKa = 42.7), we believed that the
deprotonation of water should occur and that a sufficient
amount of hydroxide anion should be present at equilibrium
(Scheme 1).[8] In this case, the elution of [18F]fluoride trapped
To evaluate the reactivity of the eluted
[18F]fluoride, we selected the labeling of the
mannose triflate precursor to FDG as a model
reaction (Scheme 2, Table 1). Low radiochemi-
cal yields were observed when either the P2Et
organic base was used or when no organic bases
were used (Table 1, entries 1 and 15). However,
Scheme 1. Possible pathway for the generation of hydroxide anions with the strong
organic base P4tBu.
on the Sep-Pak QMA (silica-based hydrophilic strong anion
exchange surface, Waters) support should be induced by this
hydroxide generated in solution.
The elution of [18F]fluoride was initially attempted with
acetonitrile containing different amounts of water (ppm) and
the less strong organic base P2Et (pKa = 32.9), which was
chosen over the P4tBu base for ease of handling (Figure 1).
Figure 2. Elution profile for the elution of [18F]fluoride from a QMA
cartridge with CH3CN containing water (10900 ppm) and an organic
base (90 mmol). P5F=tetrakis[tris(dimethylamino)phosphoranyliden-
amino]phosphonium fluoride.
Figure 1. Elution profile for the elution of [18F]fluoride from a QMA
cartridge with CH3CN containing the P2Et base (90 mmol) and variable
amounts of water (20–25000 ppm).
From this data it appears that dry acetonitrile does not enable
elution of the radioactive fluoride. With amounts of water
between 3100 and 25000 ppm, [18F]fluoride was eluted nearly
quantitatively, and the activity on the support was decreased
to less than 3%. However, the elution volume was smaller
with eluents containing greater amounts of water.
We also evaluated the influence of the strength of several
organic bases. Matching amounts of bases with pKa values
between 18 and 42 were used.[9–13] The percentage of elution
with a small volume (0.5–1 mL) increased as the pKa value of
Figure 3. Elution profile for the elution of [18F]fluoride from a QMA
cartridge with dry CH3CN containing a dry alcohol (840 mmol) and
P2Et (90 mmol).
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Scheme 2. Labeling step in the synthesis of [18F]FDG (Tf=trifluorome-
thanesulfonyl).
high (> 85%) and reproducible radiochemical yields were
only observed upon the addition of an organic base (Table 1,
entries 5, 8, 9, 12, and 13). These high yields were obtained
only with bases having either a guanidine or phosphazene
structure, no labile hydrogen atoms, and a pKa value between
23 and 28. Although aliphatic nucleophilic substitutions are
usually performed under basic conditions, the reason for such
prerequisite features of these organic bases remains unclear.
Some applications of this new strategy to the synthesis of
18F-labeled aliphatic and aromatic compounds are summar-
ized in Table 2. All precursors investigated were converted
into the corresponding 18F-fluorinated compounds
(Scheme 3) in the presence of BTMG in high radiochemical
yields. In contrast to the conventional K222/K2CO3 drying
procedure, during which 10–30% of the activity can be lost by
adsorption on the glass reactor surfaces, this method enables
the retention in solution of more than 97% of the activity of
the [18F]fluoride eluted from the cartridge until the end of the
process. In microsystems, the retention of this activity in
solution will positively impact both the radiochemical yield
and the radioprotection problems resulting from the residual
reactor activity.
Scheme 3. Structures of the labeled compounds (Ts=p-toluenesul-
fonyl).
modification could aid the development of new applications
in [18F]fluorine radiochemistry. Radiochemical TLC purities
of the different labeled products (Table 2) were confirmed by
HPLC analysis.
Finally, in another set of experiments, we demonstrated
that this approach can be used for the synthesis of various
PET compounds, 1b–6 (Table 2). The syntheses were carried
out either with 20 mg of the precursor or on a scale more
adapted to the potential use of microreactors (0.5–4 mg).
Even with these low amounts of precursors, good radio-
chemical yields were observed for all the compounds inves-
tigated. Moreover, in these experiments, compounds 2, 3, and
4 were obtained with at least 600 MBq after purification, with
both high chemical and high radiochemical purities.
The high reactivity of [18F]fluoride is well-demonstrated in
the case of 2, for which a radiochemical yield of 60% was
observed at room temperature (Table 2). Besides acetonitrile/
water, classical solvents, such as toluene, with small amounts
of an alcohol can be used to elute the fluoride (e.g. toluene/
CH3OH). The use of nonpolar and aprotic solvents, such as
toluene, in this context has seldom been described. This
In conclusion, this new elution strategy appears promising
when compared to the classical K222/K2CO3 method. When
one of a variety of organic bases is used, the azeotropic
evaporation step is no longer required, and the automation
Table 2: Radiochemical applications.[a]
Labeled
compound
Precursor[b]
(mg)
Water in
the eluent
[ppm][c]
QMA elution
yield [mL][d]
18FÀ solution
used for
labeling [mL]
BTMG
added
[mmol]
t [min], T [8]
TLC purity
[%]
RLY [%][e]
Water
content
[ppm][f]
1b
3
OTs (20)
OTs (4)
OTs (20)
OTs (0.5)
OTs (20)
5200
6300
6300
6300
10900
6300
10900
6300[i]
5275
0.8 (97.2)
0.9 (97)
0.7 (97.9)
0.9 (98.3)
0.7 (99)
0.9 (98.3)
1 (99)
0.9 (91)
1 (98.7)
0.9 (98.3)
1 (98.2)
1 (93)
500
200
700
50
700
50
500
500
1000
50
100
300
800
100
73
25
83
15
98
15
73
25
49
15
98
98
98
15
5, 100
5, 120
5, 110
5, 120
5, 110
4,120
5, 100
4, 130
5, 100
3, 120
5, RT
65
85
90
88
89
86
75
78
91
74
62
93
65
71
63.4[g]
83.3[g]
86.8[h]
86.4[g]
86.1[h]
83.5[g]
73.4[g]
75.7[g]
88.9[h]
71.2[g]
60.8[g]
90.4[g]
61.9[h]
68.9[g]
3700
3200
3333
3150
4002
2750
4800
–
2442
–
2100
–
4
OTs (1)
5
OMs (20)
OMs (3.5)
OTf (20)
OTf (1.8)
OTf (20)
2
6300
2
2
6
6300
[j]
OTf (20)
10, 100
5, 120
3, 140
Me3N+ TfOÀ (20)
Me3N+ TfOÀ (2.5)
6300
6300
0.8 (98.2)
0.9(94)
2641
3235
[a] Reaction conditions: precursor (0.5–20 mg), CH3CN. [b] Leaving group in the precursor to the labeled compound. [c] Eluent: CH3CN/P2Et
(45 mmol)/water. [d] The yield is given as a percentage in parentheses. [e] Radiolabeling yield (decay-corrected). [f] Amount of water at the end of the
labeling process. [g] RLY=activity (%) in solution after labelingꢂTLC purity. [h] RLY=QMA elution yield (%)ꢂactivity (%) in solution after labelingꢂ
TLC purity). [i] P2Et: 30 mmol. [j] Toluene/CH3OH (6000 ppm). Ms=methanesulfonyl.
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3163

Communications
steps are greatly facilitated. An appropriate selection of
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solvent, protic additive, organic base, and temperature should
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volumes and is well-suited to the labeling of various starting
substrates (aliphatic and aromatic) without the need for the
classical water-evaporation step. This approach should be
useful for both the direct labeling of sensitive compounds,
such as protein and peptides, and the miniaturization of PET
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Experimental Section
Typical procedure: [18F]fluoride was trapped on a QMA cartridge
(CO32À, Waters). The cartridge was then washed with dry CH3CN (2–
5 mL), and the [18F]fluoride was eluted with a freshly prepared
CH3CN solution (1 mL) containing water (20–25000 ppm) and a
strong organic base (45 mmol). 18F fluorination was performed directly
with a precursor (0.5–40 mg) previously solubilized in dry CH3CN
(0.5–1 mL), BTMG base (15–98 mmol), and the solution of
[18F]fluoride (10–1000 mL). The mixture was heated at 1008C for
5 min and then cooled to room temperature. The radiochemical yield
was determined by TLC and HPLC.
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Received: November 10, 2009
Revised: February 5, 2010
Published online: March 25, 2010
Keywords: [18F]fluoride · phosphazenes · radiochemistry ·
.
radiofluorination · solid-phase extraction
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