Efficient radiosynthesis of 2-[18F]fluoroadenosine: A new route to 2-[18F]fluoropurine nucleosides

Article abstract of DOI:10.1021/ml100055m

An efficient method to incorporate the fluorine-18 radionuclide in 2-nitropurine-based nucleosides was developed. The nucleophilic radiofluorination of the labeling precursor with [¹⁸F]KF under aminopolyether-mediated conditions (Kryptofix 2.2.2/K₂CO₃) followed by deprotection was straightforward and, after formulation, gave 2-[¹⁸F]fluoroadenosine, ready for injection with a radiochemical yield of 45 ± 5%, a radiochemical purity of >98%, and a specific radioactivity up to 148 GBq/μmol. A micropositron emission tomography imaging and biodistribution study on rodents was reported.

Full text of DOI:10.1021/ml100055m

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Efficient Radiosynthesis of 2-[18F]Fluoroadenosine:
A New Route to 2-[18F]Fluoropurine Nucleosides
ꢀ €
Patrice Marchand, Christophe Lorilleux, Gwenaelle Gilbert, Fabienne Gourand,
ꢀ
Franck Sobrio, Damien Peyronnet, Martine Dhilly, and Louisa Barre*
ꢀ
ꢀ
Laboratoire de Developpements Methodologiques en Tomographie par Emission de Positons, CEA/DSV/I2BM, CI-NAPS UMR
ꢀ
6232, Universite de Caen Basse Normandie, Cyceron, Caen, France
ABSTRACT An efficient method to incorporate the fluorine-18 radionuclide in
2-nitropurine-based nucleosides was developed. The nucleophilic radiofluorina-
tion of the labeling precursor with [18F]KF under aminopolyether-mediated con-
ditions (Kryptofix 2.2.2/K2CO3) followed by deprotection was straightforward and,
after formulation, gave 2-[18F]fluoroadenosine, ready for injection with a radio-
chemical yield of 45 ( 5%, a radiochemical purity of >98%, and a specific
radioactivity up to 148 GBq/μmol. A micropositron emission tomography imaging
and biodistribution study on rodents was reported.
KEYWORDS [18F]Fluorine, nucleoside, purine, 2-[18F]fluoroadenosine, PET
ositron emission tomography (PET) is based on the use
of βþ-emitting radionuclides and is now a widely used
technique for noninvasive medical imaging at clinical
Moreover, when considering the radiochemistry of [18F]-6,
only limited methods have been so far described: The
Schiemann reaction widely used for the classical fluorination
of purines16 was proven to be inapplicable to radiochemistry,
and the conventional methods using substitution of iodine or
fluorine by [18F]KF or [18F]AgF were of low efficiency (low
yields and low specific radioactivities were obtained).10,11
Our strategy to introduce a fluorine-18 atom is illustrated
in Figure 1. This approach is based on the use of a nitro group
at the 2-position of purines to activate the purine ring and to
act as a leaving group for the [18F] nucleophilic aromatic
substitution. The radical nitration step necessitates the full
protection of the starting material as the reaction does not
tolerate any labile protons.
Previously reported methods proved the benzoyl group to
be the most suitable for the full protection of adenosine and
subsequent nitration.17 Radical nitration was carried out
according to the described procedure using TBAN/TFAA to
afford 2-nitro-pentabenzoyl adenosine 3 as the labeling pre-
cursor.17,18 In radiochemistry, for a complete analysis of
radiochemical reactions and identification of radiofluori-
nated products, nonradioactive fluorinated compounds
were needed as standard references. Substitution of the
nitro group of 3 by an excess of Bu4NF in dimethyl forma-
mide was previously reported, but the resulting 2-fluoroade-
nosine derivatives were not isolated.18 Modification of this
protocol, using 3 in acetonitrile with only 1.3 equiv of Bu4NF,
enabled us to isolate the fluorinated derivatives 4 and 5 in 35
and 58%, respectively, after purification (Scheme 1). Under
P
or preclinical stages.1-3 From the chemistry point of view,
the synthesis, purification, and formulation of the radio-
pharmaceutical should be efficient within a short time and
a limited number of steps, high yielding, and reproducible to
ensure the feasibility of PET studies.
Many fluorinated nucleosides have been synthesized and
studied as potential antitumor and antiviral agents.4 The
development of labeling methods for introducing a fluorine-
18 atom provides the possibility of access to novel PET
probes for in vivo imaging.
Most of the strategies used for the [18F] radiolabeling of
nucleosides and purine derivatives are based on a fluorina-
tion of the sugar moiety at the 20-, 30-, or even 50-position or
on a side chain.5,6 In contrast, the direct [18F] labeling of the
purine ring has been scarcely described. The [18F] labeling at
the 8-position was reported using [18F]F2 but in low yield
(less than 2% decay-corrected).7 The 6-position was effi-
ciently labeled with [18F]F- via substitution of the trimethy-
lammonium salt on various substrates,8,9 while for the
2-position radiolabeling was reported only in low yield (5%
yield with low specific radioactivity).10,11
Introduction of a fluorine atom at the 2-position on the
purine ring often results in beneficial effects from metabolic
stability due to enzymatic resistance, especially toward
adenosine deaminase, and therefore has a longer in vivo
lifetime.12,13 Thus, an efficient and direct [18F] labeling of
purine at the 2-position might provide access to highly
valuable tools for PET imaging.
Among the potent targets, 2-[18F]fluoroadenosine ([18F]-6)
is an attractive candidate as 2-fluoroadenine-based nucleo-
sides are potential tracers in cardiology and oncology.11,14,15
Received Date: March 15, 2010
Accepted Date: May 10, 2010
Published on Web Date: May 28, 2010
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Scheme 2. Radiosynthesis of [18F]-6a
Figure 1. Retrosynthesis of [18F]-6.
a Reagents and conditions: (a) Compound 3, [18F]KF, K2CO3, K222
CH3CN, 55-60 °C, 8 min. (b) MeOH/NH3 H2O (28%), 70 °C, 20 min.
,
3
The deprotection of the intermediate [18F]-5 was found to
be a critical step. Removal of K2CO3/K222 with a Sep-Pak
silica gel cartridge was found to be mandatory for subse-
quent deprotection, as the use of a crude solution led mainly
to defluorination. The use of the NH3/MeOH system was
found to be the most efficient method20 for the deprotection
reaction, and [18F]-5 was converted into the desired product
Figure 2. Influence of potassium salts on the fluorination reac-
tion.
[
18F]-6 in 70-80% after heating the solution at 70 °C for 20
min. A higher temperature was found to be unfavorable as
defluorination increased rapidly.
Scheme 1. Compounds 4 and 5a
After neutralization, the crude solution was purified by
semipreparative high-performance liquid chromatography,
and the collected fraction [18F]-6 was adsorbed on a C18
Sep-Pak. After the cartridge was dried, [18F]-6 was eluted
with CH3CN, evaporated, and formulated in a sterile saline
solution.
From this route, [18F]-6 was obtained in 45 ( 5% overall
radiochemical yield, with a radiochemical purity of >98%
and a specific radioactivity up to 148 GBq/μmol.21 This high-
yielding radiosynthesis enabled us to evaluate in vivo [18F]-6
under baseline conditions.
a Reagents and conditions: (a) Bu4NF, CH3CN, 0 °C.
the reaction conditions, 4 was formed by a selective mono-
deprotection at N-6 of the purine ring.
First attempts to synthesize [18F]-5 using [18F]KF under
19
classical conditions with K2CO3 and 3 in dimethyl sulf-
2-Fluoroadenosine is known to be a ligand of adenosine
receptors (A1AR, A2AAR, A2BAR, and A3AR), a subclass of the
super family of G-protein-coupled receptors involved in
various pathologies.22 A recent review highlighted the inter-
est of PET imaging agents for mapping of adenosine recep-
tors in CNS.23 The adenosine receptors also mediate a wide
range of responses in the heart,24 and the use of labeled
adenosine with βþ-emitting atoms for PET studies was
envisaged as an attractive candidate for local blood flow
measurement.15
oxide (DMSO) at 140 °C proved to be inefficient, and no
labeled compound was observed. Further attempts and
modifications of the reaction conditions demonstrated that
the amount and the nature of the potassium salt used to
generate [18F]KF were crucial for the success of the fluorina-
tion step. When using a nonbasic potassium salt such as
K2SO4 (even in excess), the reaction led to the formation of
the fluorinated products in high yield (up to 80%). On the
other hand, the use of K2CO3 was found to be efficient only
when used in low quantities (excess of precursor 3 as
compared to K2CO3). Surprisingly, the composition of the
products was found to be dependent on the nature of the
potassium salt, and the selectivity of the reaction toward
A preliminary study was to evaluate [18F]-6 in male
Sprague-Dawley rats. After iv injection, the in vivo [18F]-6
biodistribution was visualized by microPET scans over a 60
min period. A high uptake in lung, heart, spleen and kidneys
was observed (Figure 3). These images were in agreement
with the post-mortem biodistribution study performed at 60
min after microPET imaging. The measurement of the radio-
activity in selected organs showed the highest values in the
lung . kidney > heart > spleen along with an important
urinary elimination (Table 1). The high level of radioactivity
[
18F]-4 or [18F]-5 could be shifted when using K2SO4 instead
of K2CO3 (Figure 2). Optimization of the reaction condi-
tions (molar ratio 3/K2CO3 = 1.6) and the use of CH3CN
instead of DMSO enabled a very efficient radiolabeling
of [18F]-5 in 92-98% in 8 min with less than 5% of [18F]-4
(Scheme 2).
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ꢁ
Funding Sources: We thank the Commissariat a l'Energie
Atomique for financial support (grant to P.M.).
ꢀ ^
ACKNOWLEDGMENT We thank Ahmed Abbas, Jerome Dela-
ꢀ
mare, Meziane Ibazizene, and Olivier Tirel for their technical
assistance.
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