Synthesis of fluorine-18-labelled 5- and 6-fluoro-2-pyridinamine

Article abstract of DOI:10.1002/jlcr.1049

A one-pot radiosynthesis method to prepare the new fluorine-18-labelled fluoropyridine derivatives 5-[¹⁸F]fluoro-2-pyridinamine and 6-[ ¹⁸F]fluoro-2-pyridinamine in two to three reaction steps was developed. The first step consisted of no-carrier-added nucleophilic aromatic substitution of commercially available halogen-substituted 2-pyridinecarboxamide or 2-pyridinecarbonitrile derivatives with K[¹⁸F]F-K₂₂₂ in DMSO at 150-180°C. The [¹⁸F]fluoride incorporation yields ranged from 67 to 98% for all studied precursor molecules. It is remarkable that 5-bromo-2-pyridinecarbonitrile gave almost quantitative [¹⁸F] fluoride incorporation at the meta-position (5-position) of the pyridine ring after only 5 min of heating at 150°C. After base-catalysed hydrolysis of the [¹⁸F]fluorinated pyridinecarbonitriles into their corresponding carboxamides, the latter were transformed in a Hofmann-type rearrangement reaction into the respective amines by treatment of crude reaction mixtures with bromine and aqueous base (20-30% conversion yield). Reaction mixtures were purified by reversed-phase semipreparative HPLC followed by strong cation exchange solid-phase extraction to afford 5-[¹⁸F]fluoro-2- pyridinamine and 6-[¹⁸F]fluoro-2-pyridinamine in non-decay-corrected radiochemical yields of 6-10% in a total synthesis time of 83-112 min. The preparation of 5-[¹⁸F]fluoro-2-pyridinamine is one of very few examples demonstrating the feasibility of nucleophilic meta-[ ¹⁸F]fluorination of a pyridine derivative. Both 5-[ ¹⁸F]fluoro-2-pyridinamine and 6-[¹⁸F]fluoro-2-pyridinamine are new potentially useful radiolabelled synthons for radiopharmaceutical chemistry. Copyright

Full text of DOI:10.1002/jlcr.1049

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2006; 49: 345–356.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.1049
Research Article
Synthesis of fluorine-18-labelled 5- and 6-fluoro-
2-pyridinamine
Aiman Abrahim1, Peter Angelberger2, Kurt Kletter3, Markus Muller1,
¨
Christian Joukhadar1, Thomas Erker4 and Oliver Langer1,2,
*
1 Department of Clinical Pharmacology, Division of Clinical Pharmacokinetics, Medical
University of Vienna, Austria
2 Department of Radiopharmaceuticals, Austrian Research Centers, Seibersdorf, Austria
3 Department of Nuclear Medicine, Medical University of Vienna, Austria
4 Department of Medicinal Chemistry, University of Vienna, Austria
Summary
A one-pot radiosynthesis method to prepare the new fluorine-18-labelled fluoro-
pyridine derivatives 5-[18F]fluoro-2-pyridinamine and 6-[18F]fluoro-2-pyridinamine in
two to three reaction steps was developed. The first step consisted of no-carrier-added
nucleophilic aromatic substitution of commercially available halogen-substituted 2-
pyridinecarboxamide or 2-pyridinecarbonitrile derivatives with K[18F]F-K222 in DMSO
at 150–1808C. The [18F]fluoride incorporation yields ranged from 67 to 98% for all studied
precursor molecules. It is remarkable that 5-bromo-2-pyridinecarbonitrile gave almost
quantitative [18F]fluoride incorporation at the meta-position (5-position) of the pyridine
ring after only 5 min of heating at 1508C. After base-catalysed hydrolysis of the
[18F]fluorinated pyridinecarbonitriles into their corresponding carboxamides, the latter
were transformed in a Hofmann-type rearrangement reaction into the respective amines by
treatment of crude reaction mixtures with bromine and aqueous base (20–30% conversion
yield). Reaction mixtures were purified by reversed-phase semipreparative HPLC followed
by strong cation exchange solid-phase extraction to afford 5-[18F]fluoro-2-pyridinamine
and 6-[18F]fluoro-2-pyridinamine in non-decay-corrected radiochemical yields of 6–10% in
a total synthesis time of 83–112 min. The preparation of 5-[18F]fluoro-2-pyridinamine is
one of very few examples demonstrating the feasibility of nucleophilic meta-[18F]fluorina-
tion of a pyridine derivative. Both 5-[18F]fluoro-2-pyridinamine and 6-[18F]fluoro-2-
pyridinamine are new potentially useful radiolabelled synthons for radiopharmaceutical
chemistry. Copyright # 2006 John Wiley & Sons, Ltd.
Key Words: fluorine-18; fluoropyridine; no-carrier-added nucleophilic aromatic
substitution; meta-position; Hofmann rearrangement
*Correspondence to: O. Langer, Department of Clinical Pharmacology, Division of Clinical Pharma-
cokinetics, Medical University of Vienna, Wahringer-Gurtel 18-20, 1090 Vienna, Austria.
E-mail: oliver.langer@meduniwien.ac.at
¨
¨
Copyright # 2006 John Wiley & Sons, Ltd.
Received 6 December 2005
Revised 16 December 2005
Accepted 18 December 2005

346
A. ABRAHIM ET AL.
Introduction
The fluoropyridinyl moiety labelled with fluorine-18 (18F, half-life: 109.8 min)
is a commonly encountered substituent in radiochemistry for positron
emission tomography (PET).1 The [18F]fluorine is in most cases located in
the ortho-position (2-position) of the pyridine nitrogen, since this position is
well activated for no-carrier-added nucleophilic aromatic substitution with the
potassium [18F]fluoride-kryptofix 2.2.2 (K[18F]F-K222) complex.2 In a recent
study, Karramkam et al. have compared the reactivity of ortho-, meta- and
para-nitropyridine (2-, 3- and 4-nitropyridine) to nucleophilic substitution
with [18F]fluoride and obtained good yields for ortho- and para-[18F]fluoro-
pyridine (70–90%), whereas no reaction was observed for substitution at the
meta-position.3 In contrast to ortho- and para-substitution, nucleophilic
[18F]fluorination at the meta-position requires an additional electron-
withdrawing substituent in the pyridine ring. To our knowledge, only one
example of meta-[18F]fluorination of a pyridine derivative has been reported
in the literature. In this example, the potential MAO-B imaging radiotracer
N-(2-aminoethyl)-5-[18F]fluoro-2-pyridinecarboxamide was obtained in 35%
decay-corrected radiochemical yield by nucleophilic aromatic nitro-for-fluoro
substitution.4
In our ongoing research program to synthesize 18F-labelled peptide
deformylase inhibitors, such as LBM-415 (1, Figure 1),5 we needed to prepare
a meta-18F-substituted pyridine derivative, i.e. 5-[18F]fluoro-2-pyridinamine
([18F]-2, Figure 3), as a radiolabelled intermediate. Here, we report a one-pot
method to synthesize [18F]-2 and its regioisomer 6-[18F]fluoro-2-pyridinamine
([18F]-3, Figure 3) in practical radiochemical yields via no-carrier-added
nucleophilic aromatic substitution of commercially available halogen-sub-
stituted 2-pyridinecarboxamide or 2-pyridinecarbonitrile derivatives with
K[18F]F-K222, followed by conversion of the carboxamide into the amine
function by a Hofmann-type rearrangement reaction.
Results and discussion
Our initial strategy to synthesize the target molecule [18F]-2 aimed at reacting
5-bromo-2-nitropyridine (4) with K[18F]F-K222 to obtain 5-[18F]fluoro-2-
Figure 1. Chemical structure of the peptide deformylase inhibitor LBM-415 (1)
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nitropyridine ([18F]-5, Figure 2), which we then planned to reduce to the
desired amine [18F]-2. Unfortunately, the only radiolabelled product that
was formed in the substitution reaction – in excellent radiochemical yield
of about 85% – was 5-bromo-2-[18F]fluoropyridine ([18F]-6, Figure 2) as
identified by HPLC co-elution with the commercially available unlabelled
reference compound. These results indicate that in the presently employed
nucleophilic substitution reaction, the 2-nitro group preferably acted as
a leaving group for 18F-substitution rather than as an activating group
for substitution at the meta-position. We therefore searched for a substrate
that possesses an electron-withdrawing substituent in the 2-position that
would not be capable of acting as a leaving group in the substitution reaction.
Commercially available 5-bromo-2-pyridinecarboxamide (7) seemed to meet
these requirements. The use of 7 was further supported by the fact that in
an earlier published study a carboxamide group has been successfully
employed as an activating group for meta-18F-substitution of a pyridine
derivative.4
Derivative 7 was reacted with K[18F]F-K222 in dimethylsulphoxide (DMSO)
for 20 min at 1808C, which afforded the desired product, i.e. 5-[18F]fluoro-
2-pyridinecarboxamide ([18F]-8), in an average radiochemical yield of
67 ꢀ 14% (n ¼ 9) (Figure 3). The reaction kinetics shown in Figure 4(A)
indicate that heating times longer than 20 min – both at 150 and 1808C – lead
to decomposition of the radiolabelled product. Analysis of the crude reaction
mixture by HPLC and TLC revealed the presence of a hydrophilic by-product,
that was not identical with unreacted [18F]fluoride. In our attempts to identify
this hydrophilic product we co-eluted the reaction mixture on HPLC (system
B) with two potential, commercially available by-products, i.e. 5-fluoro-
2-pyridinecarboxylic acid and 3-fluoropyridine. We assumed that the first
compound might be formed by hydrolysis of the carboxamide function as
promoted by traces of water contained in the alkaline reaction mixture,6
whereas the latter compound might be generated by 2-decarboxylation.
Interestingly, even though the chromatographic behaviour of the radiolabelled
Figure 2. Attempt to synthesize 5-[18F]fluoro-2-nitropyridine ([18F]-5) by
reaction of 5-bromo-2-nitropyridine (4) with K[18F]F-K222, which afforded 5-
bromo-2-[18F]fluoropyridine ([18F]-6) as the only radiolabelled product
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(a)
(b)
Figure 4. Reaction kinetics for the formation of the meta-substituted pyridines 5-
[18F]fluoro-2-pyridinecarboxamide ([18F]-8) and 5-[18F]fluoro-2-pyridinecarboni-
trile ([18F]-10) (A) and the ortho-substituted pyridines 6-[18F]fluoro-2-pyridine-
carboxamide ([18F]-12) and 6-[18F]fluoro-2-pyridinecarbonitrile ([18F]-14) (B) by
nucleophilic substitution with K[18F]F-K222 at two different reaction tempera-
tures (150 and 1808C). Radiochemical yields were calculated by TLC analysis
(system B)
by-product pointed to the presence of a carboxylic acid function (prolongation
of HPLC retention time when using dilute phosphoric acid instead of water as
mobile phase in HPLC system B) the observed by-product was not identical
with any of the suspected by-products. We tried to suppress by-product
formation by employing less alkaline 18F-fluorination conditions (i.e. the use
of potassium oxalate instead of potassium carbonate as a base) that have
previously been successfully employed for the 18F-labelling of base-sensitive
substrates.7 In this case, however, we observed that precursor molecule 7 was
considerably less reactive to substitution by [18F]fluoride as compared to the
standard potassium carbonate system (i.e. about 10% incorporation yield
versus 67%).
The reaction solution containing crude [18F]-8 was then directly treated
with an ice-cooled mixture of bromine and aqueous potassium hydroxide to
afford the desired amine [18F]-2 in 20–30% conversion yield based on [18F]-8
(Figure 3). This type of reaction, which involves the conversion of an amide
into an amine containing one fewer carbon upon treatment with bromine in
aqueous base, is generally referred to as the Hofmann rearrangement8 and is
commonly employed in preparative organic synthesis.9,10 Importantly, in the
case of our radiosynthesis, the rearrangement reaction was compatible with
the reagents of the nucleophilic substitution reaction and could therefore
be performed as a one-pot reaction without intermediate purification of the
18F-labelled carboxamide. In the crude reaction mixture, the remainder of
radioactivity was distributed between unreacted [18F]-8 and an unidentified
hydrophilic by-product, that comprised about 50% of total radioactivity and
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A. ABRAHIM ET AL.
that was presumably identical with the by-product that had already been
generated in the nucleophilic substitution reaction. The identity of product
[18F]-2 was confirmed by HPLC co-elution with unlabelled 2. The crude
reaction mixture was then purified by reversed-phase semipreparative HPLC
(system C). Pyridinecarboxamide [18F]-8 and pyridinamine [18F]-2 co-eluted
on the employed HPLC system (Figure 5) and were thus collected in one single
fraction. Quantitative separation of [18F]-8 and [18F]-2 was achieved by a
strong cation exchange (SCX) solid-phase extraction procedure, which
afforded radiochemically and chemically pure, no-carrier-added [18F]-2. It is
important to note that the pseudo-carrier 5-bromo-2-pyridinamine, which
was formed by Hofmann rearrangement of precursor pyridinecarboxamide 7,
was removed during the semipreparative HPLC purification of the crude
reaction mixture (Figure 5) and therefore not present in purified [18F]-2.
Starting from 2.1–2.7 GBq of K[18F]F-K222, 115–173 MBq of pure [18F]-2
could be obtained in a total synthesis time of about 110 min (see Table 1).
Figure 5. Representative semipreparative HPLC chromatogram for the
purification of 5-[18F]fluoro-2-pyridinamine ([18F]-2). A Waters mBondapak
C18 column (300 mm ꢁ 7.8 mm, 10 mm) was eluted with water/acetonitrile (95/5,
v/v) at a flow rate of 6 ml/min (system C). Under the employed HPLC conditions
precursor 5-bromo-2-pyridinecarboxamide (7) and the corresponding amine
5-bromo-2-pyridinamine did not elute from the HPLC column
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Table 1. Overview of final product amounts (non-decay-corrected) obtained in the
preparative syntheses of 5-[18F]fluoro-2-pyridinamine ([18F]-2) and 6-[18F]fluoro-2-
pyridinamine ([18F]-3)
Start activitya
(MBq)
Precursor
molecule
Product
Amount of
product (MBq)
Synthesis
time (min)
2740
2130
3190
2500
1360
3140
3200
7
7
9
[18F]-2
[18F]-2
[18F]-2
[18F]-2
[18F]-3
[18F]-3
[18F]-3
173b
115b
507b
163b
119c
383c
258c
112
106
83
100
90
9
11
13
13
89
89
aStart activity is based on the dried K[18F]F-K222 complex.
bIsolated product.
cThe amount of [18F]-3 was calculated by HPLC analysis of semipreparative-HPLC-purified mixtures of
[
18F]-3 and [18F]-12.
As a second precursor for the synthesis of [18F]-8 the corresponding
pyridinecarbonitrile 5-bromo-2-pyridinecarbonitrile 9 was employed (Fig-
ure 3). As can be seen in the reaction kinetics depicted in Figure 4(A),
derivative 9 was considerably more reactive to nucleophilic [18F]fluorination
than the corresponding pyridinecarboxamide 7. After only 5 min of heating at
both 150 and 1808C the 18F-incorporation was almost quantitative (Figure 3
and Figure 4(A)). These findings underline the strong electron-withdrawing
properties of the 2-cyano group in derivative 9, which make this substituent an
excellent activating group for 18F-substitution at the meta-position of the
pyridine ring. Reaction times longer than 5 min resulted in decomposition of
the radiolabelled product into an unidentified hydrophilic by-product.
Interestingly, HPLC analysis (system B) revealed that part of the product,
i.e. pyridinecarbonitrile [18F]-10, was converted into the corresponding
pyridinecarboxamide [18F]-8, particularly in those cases when reaction times
longer than 5 min were chosen. This hydrolysis reaction was desired in view of
the subsequent rearrangement reaction (Figure 3) and presumably took place
due to traces of water contained in the reaction solvent. We were able to drive
this reaction to completion by treating the crude reaction mixture with
aqueous potassium hydroxide followed by 5 min of stirring at room
temperature. Crude pyridinecarboxamide [18F]-8 was then transformed, in
an analogous way as described before, into pyridinamine [18F]-2 by reaction
with Br2/aqueous KOH.
The present synthetic pathway was then extended to the preparation of the
ortho-[18F]fluorinated regioisomer of [18F]-2, i.e. 6-[18F]fluoro-2-pyridinamine
([18F]-3) (Figure 3). As expected, the employed precursor molecules, that were
bearing a halogen leaving group in the ortho-position of the pyridine ring,
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i.e. 6-chloro-2-pyridinecarboxamide (11) and 6-bromo-2-pyridinecarbonitrile
(13), were better activated for 18F-fluorination than the corresponding
meta-substituted series (Figure 4(B)). In contrast to the meta-substituted
series, pyridinecarboxamide 11 appeared to be equally well activated for
18F-substitution as pyridinecarbonitrile 13, which resulted in almost quanti-
tative incorporation yields after 5 min of heating for both precursor molecules
(Figure 3 and Figure 4(B)). In addition to that, we observed less hydrophilic
by-product (i.e. 510% of total radioactivity) on HPLC – even at longer
heating times – as compared to the meta-[18F]fluorinated compounds, which
pointed to a higher chemical stability of the ortho-[18F]fluorinated molecules
under the employed reaction conditions. The [18F]fluorinated pyridinecarbox-
amide [18F]-12, which was obtained after reaction of 11 with K[18F]F-K222
,
was converted into target molecule [18F]-3 by employing the same conditions
(i.e. Br2/aqueous KOH) as for the synthesis of [18F]-2 (Figure 3). Pyridinamine
[18F]-3, which co-eluted on HPLC (system B) with unlabelled 3, was formed in
a similar conversion yield (20–30%) based on [18F]-12 as derivative [18F]-2. As
an alternative precursor to 11, pyridinecarbonitrile 13 was reacted with
K[18F]F-K222 to afford the [18F]fluorinated pyridinecarbonitrile [18F]-14 in a
yield of 98 ꢀ 1% (n ¼ 6) (Figure 3). [18F]-14 was then hydrolysed with
aqueous KOH into the corresponding pyridinecarboxamide [18F]-12 and
subsequently subjected to Hofmann rearrangement conditions to give [18F]-3
as a product (Figure 3).
As for [18F]-2, crude [18F]-3 was purified by semipreparative HPLC (sys-
tem C). However, in contrast to the purification of [18F]-2, the mixture of
pyridinecarboxamide [18F]-12 and pyridinamine [18F]-3, which was collected in
one HPLC fraction, could not be resolved by the SCX procedure, which was
most likely related to the fact that pyridinamine [18F]-3 was significantly
less basic than the corresponding meta-[18F]fluorinated pyridinamine [18F]-2
(pKa [18F]-3: 4:63 ꢀ 0:13; pKa [18F]-2: 2:44 ꢀ 0:24).11 Despite this separation
problem, the overall radiochemical yield of [18F]-3, as calculated by HPLC
analysis of unresolved [18F]-12 and [18F]-3, was higher as compared to [18F]-2
(6 ꢀ 1% (n ¼ 3) for [18F]-2 versus 10 ꢀ 2% (n ¼ 3) for [18F]-3, Table 1). These
higher total radiochemical yields were most likely related to the higher yields
obtained in the ortho-18F-substitution and/or the better stability of the ortho-
[18F]fluorinated pyridine derivatives (less hydrophilic by-product formation).
Experimental
General
Chemicals. 5-Bromo-2-pyridinecarboxamide (7) was purchased from Aurora
Fine Chemicals Ltd. (Graz, Austria), 6-chloro-2-pyridinecarboxamide (11)
from Chembridge Corporation (San Diego, CA, USA), 6-bromo-2-pyridine-
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carbonitrile (13) from Chempacific (Baltimore, MD, USA), and 6-fluoro-
2-pyridinamine (3) and 5-fluoro-2-pyridinecarboxylic acid from Synchem
Laborgemeinschaft (Kassel, Germany). All other chemicals were obtained
from Sigma-Aldrich Chemie GmbH (Schnelldorf, Germany) or Merck KGaA
(Darmstadt, Germany) and used without further purification.
Aqueous [18F]fluoride was produced in a General Electrics PETtrace
cyclotron (General Electrics, USA) via the 18O(p,n)18F nuclear reaction by
irradiation of a 1.5 ml water target containing 95.9% enriched [18O]water
(Hyox18, Rotem Industries, Beer Sheva, Israel) with a 16.5 MeV proton beam.
Analytical procedures. For analytical high performance liquid chromatogra-
phy (HPLC) a Luna 5m Phenyl-Hexyl column (150 mm ꢁ 4:6 mm) (Phenom-
enex Ltd., USA) was eluted with a mixture of H2O (A) and acetonitrile (B) at a
flow rate of 1.0 ml/min. The following binary gradient time programs were
used:
HPLC system A: 0–3 min, (A:B, v:v) 80:20 isocratic; 3–12 min, (A:B) 80:
20–50:50; 12–15 min, (A:B) 50:50 isocratic; 15–16 min, (A:B) 50:50–80:20;
16–17 min, (A:B) 80:20 isocratic.
HPLC system B: 0–7 min, (A:B, v:v) 95:5 isocratic; 7–9 min, (A:B) 95:
5–60:40; 9–16 min, (A:B) 60:40 isocratic; 16–17 min, (A:B) 60:40–95:5;
17–19 min, (A:B) 95:5 isocratic.
For detection a Merck Hitachi L-4000 UV detector (wavelength: 254 nm) in
series with a Packard Radiomatic Flo-one Beta Flow scintillation analyser
(PerkinElmer Life Sciences Inc., Boston, USA) were employed.
For thin layer chromatography (TLC) analysis Merck silica gel 60 F254 TLC
aluminium sheets (layer thickness: 0.2 mm) were used with one of the following
two mobile phases:
TLC system A: ethyl acetate/hexane 50/50 (v/v).
TLC system B: dichloromethane/methanol/triethylamine 90/10/1 (v/v/v).
UV detection was performed using a standard UV lamp at a wavelength of
350 nm. For analysis of radioactive spots a Berthold digital autoradiograph
LB 286-20 (Berthold Australia Pty Ltd., Bundoora, Australia) was used.
Radiochemistry
General procedure for the nucleophilic exchange reaction on pyridine
derivatives. Aqueous [18F]fluoride ion from the cyclotron target was collected
into a 3 ml Wheaton V-vial (Wheaton Science Products, Millville, USA)
containing a solution of kryptofix 2.2.2 in acetonitrile (120 mg/ml, 100 ml,
12.0 mg, 32.7 mmol) and a solution of potassium carbonate in water (200 mg/
ml, 20 ml, 4.0 mg, 28.9 mmol). The mixture was exposed to a stream of nitrogen
and concentrated to dryness at 1508C or 1808C (see below) under repeated
addition of acetonitrile (3 ꢁ 0:5 ml). To the dried K[18F]F-K222 complex, the
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respective precursor pyridine derivative (15–20 mmol) dissolved in dimethyl-
sulphoxide (0.5 ml) was added and the resulting solution stirred for 20 min at
1508C (precursor 4) or for 5 min at 1508C (precursors 9 and 13) or for 20 min
at 1808C (precursors 7 and 11). An aliquot of the reaction mixture was
analysed by TLC and HPLC, whereby TLC analysis was used to estimate the
amount of unreacted [18F]fluoride present in the reaction mixture.
Retention times: HPLC system A: precursor 4: 11–12 min and product
[18F]-6 : 12–13 min; HPLC system B: precursor 7: 14–15 min and pro-
duct [18F]-8: 12–13 min; precursor 9: 17–18 min and product [18F]-10:
13.5–14.5 min; precursor 11: 13.5–14.5 min and product [18F]-12: 11.5–
12.5 min; precursor 13: 18–19 min and product [18F]-14: 14.5–15.5 min.
Retention factors (Rf): TLC system A: [18F]fluoride: 0.0; [18F]-6: 0.5-0.6.
TLC system B: [18F]fluoride: 0.0; [18F]-8, [18F]-10, [18F]-12 and [18F]-14:
0.5–0.8.
General procedure for the hydrolysis of 18F-labelled pyridinecarbonitriles. The
reaction mixture containing crude [18F]-10 or [18F]-14, which was obtained
after nucleophilic substitution, was cooled in an ice/water bath. Then
potassium hydroxide (5 mg, 89 mmol) in water (200 ml) was added and the
solution stirred for 5 min at room temperature. An aliquot of the reaction
mixture was analysed by HPLC.
Retention times: HPLC system B: [18F]-10 (pyridinecarbonitrile): 13.5–14.5
and [18F]-8 (pyridinecarboxamide): 12–13 min; [18F]-14 (pyridinecarbonitrile):
14.5–15.5 min and [18F]-12 (pyridinecarboxamide): 11.5–12.5 min.
General procedure for the Hofmann rearrangement of 18F-labelled pyridinecar-
boxamides. Bromine (2.0 ml, 39 mmol) was dissolved in a solution of potassium
hydroxide (12 mg, 214 mmol) in water (1 ml) and stirred for 15 min in an ice/
water bath. This solution was then added to the ice-cooled reaction mixture
obtained after nucleophilic substitution or hydrolysis, which resulted in slight
but clearly visible gas formation. The reaction mixture was first stirred for
15 min in the ice/water bath and then heated for 20 min at 758C. An aliquot
of the reaction mixture was analysed by HPLC.
Retention times: HPLC system B: [18F]-8 (pyridinecarboxamide): 12–13 min
and [18F]-2 (pyridinamine): 10–11 min; 5-bromo-2-pyridinecarboxamide (7):
14–15 min; 5-bromo-2-pyridinamine: 13.5–14.5 min; [18F]-12 (pyridinecarbox-
amide): 11.5–12.5 min and [18F]-3 (pyridinamine): 12–13 min.
General procedure for the purification of 5-[18F]fluoro-2-pyridinamine ([18F]-2)
and 6-[18F]fluoro-2-pyridinamine ([18F]-3). The purification was performed
on a semipreparative HPLC system consisting of a Rheodyne 7010 titanium
injector equipped with a Rheodyne 2 ml sample loop mounted on a Besta
motor valve (Besta-Technik GmbH, Wilhelmsfeld, Germany), a Jasco 880-PU
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HPLC pump (Jasco Corporation, Tokyo, Japan) and a Jasco 875-UV detector
(wavelength: 280 nm) in series with a Berthold LB508 C-1 radioactivity
detector. A Waters mBondapak C18 column (300 mm ꢁ 7:8 mm, 10 mm)
(Waters Corporation, Milford, USA) equipped with a Phenomenex Secur-
ityGuard C18 (10 ꢁ 10 mm) pre-column was isocratically eluted with a
mixture of water and acetonitrile (95/5, v/v) at a flow rate of 6 ml/min (HPLC
system C). Data were collected on an Axxiom Chromatography 747
chromatography data system (Axxiom Chromatography Inc., Moorpark,
USA).
The crude product was siphoned from the reaction vial into the sample loop
via standard tubing and a disposable 5 ml syringe and injected on to the HPLC
system. Since the 18F-labelled pyridinecarboxamides and corresponding
pyridinamines co-eluted on this HPLC system (retention time: 6–9 min,
Figure 5), both compounds were collected in one fraction. The mixture of
pyridinamine [18F]-2 and pyridinecarboxamide [18F]-8 was resolved by the
following solid-phase extraction procedure: the HPLC fraction was acidified
by addition of concentrated phosphoric acid (50 ml), homogenized and then
passed over a strong cation exchange cartridge (Isolute 100 mg SCX,
International Sorbent Technology Ltd., Hengoed, UK), that had been pre-
conditioned with absolute ethanol (5 ml) and 10 mM aqueous phosphoric
acid (10 ml). The pyridinamine [18F]-2 was retained on the cartridge. The
cartridge was washed with water (3 ml) and [18F]-2 was then eluted with
aqueous potassium hydroxide (10 mg/ml, 3 ml). An aliquot of the eluate was
analysed by HPLC.
Retention times: HPLC system B: [18F]-2: 10–11 min; [18F]-3: 12–13 min.
Retention factors (Rf): TLC system B: [18F]-2: 0.35; [18F]-3: 0.5.
Conclusion
We have developed a one-pot synthesis method to prepare no-carrier-added
5-[18F]fluoro-2-pyridinamine ([18F]-2) and 6-[18F]fluoro-2-pyridinamine
([18F]-3) in practical radiochemical yields of 6–10% (non-decay-corrected).
The preparation of [18F]-2 is one of very few examples demonstrating
the feasibility of nucleophilic meta-[18F]fluorination of a pyridine derivative.
It is remarkable that the use of a 2-cyano-substituted precursor molecule
(i.e. 5-bromo-2-pyridinecarbonitrile, 9) gave almost quantitative meta-
18F-incorporation after only 5 min of heating at 1508C. Both [18F]-2 and
[18F]-3 are new potentially useful radiolabelled synthons for radiopharmaceu-
tical chemistry.
Acknowledgements
The authors wish to thank Dr Frederic Dolle from the SHFJ (Orsay, France)
´ ´ ´
for sharing his experience with [18F]fluoropyridines with them. The PET group
Copyright # 2006 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2006; 49: 345–356

356
A. ABRAHIM ET AL.
from the Department of Nuclear Medicine (Medical University of Vienna) is
gratefully acknowledged for continuous support and Novartis Institutes for
Biomedical Research, Inc. (USA) for a generous gift of unlabelled LBM-415.
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