A fluorous and click approach for screening potential PET probes: Evaluation of potential hypoxia biomarkers

Article abstract of DOI:10.1016/j.bmc.2011.10.084

Radiopharmaceuticals for nuclear imaging are essentially targeting molecules, labeled with short-lived radionuclides (e.g., F-18 for PET). A significant drawback of radiopharmaceuticals development is the difficulty to access radiolabeled molecule libraries for initial in vitro evaluation, as radiolabeling has to be optimized for each individual molecule. The present paper discloses a method for preparing libraries of ¹⁸F-labeled radiopharmaceuticals using both the fluorous-based ¹⁸F-radiochemistry and the Huisgen 1,3-dipolar (click) conjugation reaction. As a proof of concept, this approach allowed us to obtain a series of readily accessible ¹⁸F-radiolabeled nitroaromatic molecules, for exploring their structure-activity relationship and further in vitro evaluation of their hypoxic selectivity.

Full text of DOI:10.1016/j.bmc.2011.10.084

Bioorganic & Medicinal Chemistry 20 (2012) 324–329
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry
journal homepage: www.elsevier.com/locate/bmc
A fluorous and click approach for screening potential PET probes: Evaluation
of potential hypoxia biomarkers
Romain Bejot a,b, , Laurence Carroll a, Kishore Bhakoo b, Jérôme Declerck c, Veronique Gouverneur a
⇑
a Chemistry Research Laboratory, University of Oxford, Mansfield Road, OX1 3TA Oxford, UK
b Singapore Bioimaging Consortium (A-Star), Helios, 02-02, 11 Biopolis Way, 138667 Singapore, Singapore
c Siemens Molecular Imaging, 23-38 Hythe Bridge Street, OX1 2EP Oxford, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Radiopharmaceuticals for nuclear imaging are essentially targeting molecules, labeled with short-lived
radionuclides (e.g., F-18 for PET). A significant drawback of radiopharmaceuticals development is the dif-
ficulty to access radiolabeled molecule libraries for initial in vitro evaluation, as radiolabeling has to be
optimized for each individual molecule. The present paper discloses a method for preparing libraries
of 18F-labeled radiopharmaceuticals using both the fluorous-based 18F-radiochemistry and the Huisgen
1,3-dipolar (click) conjugation reaction. As a proof of concept, this approach allowed us to obtain a series
of readily accessible 18F-radiolabeled nitroaromatic molecules, for exploring their structure–activity rela-
tionship and further in vitro evaluation of their hypoxic selectivity.
Received 9 September 2011
Revised 26 October 2011
Accepted 29 October 2011
Available online 6 November 2011
Keywords:
Radiochemistry
PET
Hypoxia
Click
Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.
FMISO
1. Introduction
freshly produced libraries of radiolabeled candidates that could be
evaluated in vitro at the trace level.2 For this purpose we propose
Radiopharmaceutical and drug development methodologies are
similar: both relying on the identification of a lead compound and
later optimization via the generation of analog libraries. However
chemical libraries synthesized for drug screening can not be di-
rectly used for identification of targeted nuclear imaging probes,
due to the numerous challenges in radiochemistry: direct
18F-labeling of radiopharmaceuticals is especially tedious and lim-
ited to specific substrates,1 furthermore, the relative short half-
lives of nuclear imaging isotopes (e.g., 109.7 min for F-18) limit
the synthesis and evaluation of the radiopharmaceutical. We have
identified a need for identifying candidates for nuclear imaging
(PET, SPECT) that can be readily radiolabeled: the present article
hence describes the use of the fluorous technology and click chem-
istry for the development of such radiopharmaceuticals.
The ‘click chemistry’ and more specifically the practical and
reliable Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition between
an alkyne and an azide is being used increasingly in drug discov-
ery.2–4 The approach is restricted to easy-to-make molecules but
provides a means for the rapid discovery of lead compounds as
well as the lead optimization (enables rapid structure–activity
relationship profiling through the generation of large screening li-
braries). Similarly radiopharmaceutical development could benefit
from the very efficient ‘click chemistry’ approach, readily providing
using a simple radiolabeled synthon to tag libraries of chemicals.
Furthermore, the synthesis of the radiolabeled fragment would
benefit from a simple and reliable synthesis and purification meth-
od, such as the fluorous radiochemistry approach. Fluorous chem-
istry is best considered as solution phase synthesis with the benefit
of separation tagging: rapid purification of crude reaction mixtures
by using fluorous solid-phase extraction (FSPE) is allowed by the
unique separation properties of highly polyfluorinated mole-
cules.5,6 An 18F-labeled synthon can therefore be obtained by
detagging a fluorous-tagged precursor upon nucleophilic 18F-fluo-
rination and subsequent purification by FSPE.7
As a practical demonstration of the approach, we decided to eval-
uate potential PET probes for hypoxia in cancer. Hypoxia is actually
a common attribute of a solid tumor microenvironment and is con-
sidered a significant factor in the hallmarks of cancers.8,9 So far, the
development of hypoxia biomarker has focused on 2-nitroimidazole
derivatives which are reduced and trapped within hypoxic cells.10
Several derivatives, labeled with 18F, have hence been investigated
and reported for molecular imaging of hypoxia using PET: 18FMI-
SO,11 18FAZA,12 18FETNIM,13 18FETA,14 18F]HX4.15 However those
[
compounds exhibit a relatively low uptake in hypoxic tumors (pas-
sive transport across cell membranes), and require long examina-
tion protocols (slow clearance from other tissues), limiting the
clinical potential of these PET biomarkers.
There is considerable evidence that 2-nitroimidazoles have elec-
tron affinity under hypoxic conditions and recently, we reported
that the crucial irreversible binding step occurring after the initial
⇑
Corresponding author. Tel.: +65 64788737; fax: +65 64789957.
E-mail address: romain_bejot@sbic.a-star.edu.sg (R. Bejot).
0968-0896/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2011.10.084

R. Bejot et al. / Bioorg. Med. Chem. 20 (2012) 324–329
325
reduction step may have been overlooked.16 Hence we postulate
that the development of a new hypoxia selective biomarker might
arise from the optimization of the binding rate of the reduced
metabolites within hypoxic cells. We evaluate how a modification
of the structure of the potential biomarkers can improve the binding
of their reduced metabolites within hypoxic cells.
and 50 mM phosphate buffer (0.5–1.0 mL, pH 7.4) for 30 min at
room temperature, after which equal amounts of both phases were
analyzed by HPLC for UV-absorbance quantification (Phenomenex
Gemini NX, C18, 150 ꢀ 4.6 mm, 1 mL/min isocratic ethanol:water
5:95 for 1, 2, 3, 5; 10:90 for 6; 20:80 for 4, 7). The experiment
was carried out in triplicate.
2.3. Electrochemistry
2. Materials and methods
2.1. Chemistry
Voltammetry measurements were recorded in a glass cell located
inside a Faraday cage at room temperature. Cyclic voltammograms
of approximately 5 mM solutions of the nitroaromatic compound
and 0.1 M n-tetrabutyl ammonium tetrafluoroborate in 3.0 mL of
anhydrous and degassed acetonitrile were recorded on an electro-
chemical analyzer (CH Instruments), using a platinum working elec-
trode, a platinum wire counter-electrode/auxiliary electrode, and a
silver/silver ion reference electrode. Ferrocene was used as an inter-
nal reference for which the one electron redox process occurs at
E1/2(Fc/Fc+) = 0.42 versus SCE in anhydrous acetonitrile.
2-Azidoethyl 1H,1H,2H,2H-perfluorodecane-1-sulfonate and
2-fluoroethyl azide were prepared as described previously.7 2-
Nitroimidazole, 2-methyl-4-nitroimidazole, 5-nitrobenzimidazole,
5-nitro-1H-pyrazole, 2-nitropyridine and 2-amino-5-nitrothiazole
were purchased from Sigma–Aldrich, 3-amino-2-nitropyridine
was purchased from Alfa Aeser.
N-Propargyl derivatives of nitroaromatics were prepared by
treating the N-heteroaromatics and aromatic amines with propar-
gyl bromide in the presence of potassium carbonate (Scheme 1).
Primary aromatic amines are protected by N-formylation with ace-
tic formic anhydride generated in situ, prior to the formation of N-
propargyl derivatives. Purifications were performed on silica gel
cartridge using ethyl acetate:hexane as the eluent.
2.4. Enzymatic reduction
Enzymatic reduction of the nitroaromatic compound by xanthine
oxidase was measured spectrophotometrically using the method
described in the literature.16,17 Specific activity of the enzyme was
assessed by measuring the conversion of xanthine to uric acid in
air. Xanthine oxidase (from bovine milk, grade III, Sigma–Aldrich,
Propargyl-functionalized nitroaromatics were conjugated with
2-fluoroethylazide in the presence of copper sulfate and sodium
ascorbate (Scheme 2). Purifications were performed on silica gel
cartridge using ethyl acetate:hexane as the eluent.
about 0.02 units) in PBS (100
(58 M) in PBS (pH 7.5, 50 mM, 0.6 mL), maintained at 25 °C, and the
formation of uric acid was recorded at 292 nm. One unit converts
1.0 mol/min of xanthine to uric acid at pH 7.5 at 25 °C.
Xanthine and the nitroaromatic compounds were used as the
lL) was added to a solution of xanthine
l
2.2. Partition coefficient
l
The logP values were measured using a standard shake flask
method. Approximately 0.01–0.1 mg/mL of the nitroaromatic
compound was shaken in a mixture of 1-octanol (0.5–1.0 mL)
reducing and oxidizing substrates, respectively. Anoxic conditions
were produced by bubbling humidified argon (BOC Pureshield
NO2
N
NO2
NH
a
N
N
99%
NO2
NH
NO2
N
NO2
N
a
+
89%
N
N
N
95:5
O2N
H
N
N
N
O2N
a
O2N
N
N
+
86%
N
63:37
NO2
NH
NO2
NO2
a
+
N
N
N
N
93%
N
97:3
NO2
NO2
NO2 CHO
N
NO2
H
N
H
N
c
b
d
NH2
H
O
N
N
N
N
93%
98%
92%
NO2
S
NO2
S
H
e
b
N
S
O
O2N
39%
N
67%
N
N
HN
NH2
H
Scheme 1. Propargyl-functionalization of nitroaromatics. Reagents and conditions: (a) Propargyl bromide, K2CO3, acetone, RT, overnight. (b) Acetic formic anhydride, Et2O,
RT, overnight. (c) Propargyl bromide, K2CO3, acetone, RT, overnight. (d) cat. K2CO3, MeOH, RT, 5 min. (e) (1) Propargyl bromide, K2CO3, acetone, RT, overnight. (2) MeOH, RT,
20 min.

326
R. Bejot et al. / Bioorg. Med. Chem. 20 (2012) 324–329
"click"
R
F
N
N
F
R
+
N3
N
1. K18F, K222
1. "click"
R
18F
N3
N
N
N3
18F
Rf SO2
O
2. HPLC purification
N
2. Alumina, Fluorous SPE
18F-, Rf SO2
O
NO2
NO2
O2N
NO2
NO2
NO2
H
N
N
R =
N
N
N
N
N
N
S
N
N
N
NH
C8F17 (CH2)2
Rf =
Scheme 2. Click coupling to 2-fluoroethylazide.
Argon) for 20 min in the substrates and enzyme PBS solutions (pH
7.5, 50 mM). Xanthine oxidase in PBS (0.118 0.002 unit in 100 L)
was cooled down to 50 °C, diluted with 1.5 mL water and eluted
through neutral alumina (Sep-Pak light, Waters) and fluorous silica
l
was added to a solution of xanthine (0.29 mM) and nitroaromatic
substrate (0.12 mM) in PBS (pH 7.5, 50 mM, 0.6 mL), maintained
at 37 °C in a degassed cuvette sealed with a rubber vaccine cap,
and the solution was mixed by inversion. The nitroaromatic sub-
strate reduction was determined from the initial loss of absorption
at the wavelength of maximum absorbance (>325 nm) or at
249 nm, the isosbestic point for the conversion of xanthine to uric
acid in air. The losses of absorbance were converted to nmol/min
using the extinction coefficients.
gel (SPE cartridge, 100 mg, 40 lm, Fluorous Technologies Inc.).
HPLC analysis (Phenomenex Gemini NX, C18, 150 ꢀ 4.6 mm,
water:ethanol gradient 100:0 to 40:60 in 10 min, 1 mL/min) indi-
cated a radiochemical purity of 2-[18F]fluoroethylazide >95%. The
non-decay-corrected radiochemical yield was >50% estimated from
the cyclotron yield and the obtained radioactivity. Noteworthy, the
azide is volatile and cannot be analyzed by radio-TLC.
2.5.3. Conjugation of 2-[18F]fluoroethylazide with nitroaromatics
To propargyl-functionalized nitroaromatics (2.0 0.5 mg) in
2.5. Radiochemistry
acetonitrile (100
per(II) sulfate (20
18F]fluoroethylazide (200
l
for 30 min at room temperature and further purified by HPLC (Phe-
nomenex Gemini NX, C18, 150 ꢀ 4.6 mm, water:ethanol gradient
100:0 to 40:60 in 10 min, 1 mL/min) using an autosampler and a
fraction collector. The collected fraction was directly diluted with
PBS at 0.5–1.0 MBq/mL. Analysis by radio-HPLC indicated >95%
radiochemical purity. Specific activity was determined in the range
l
l
L) was added aqueous solutions of 1 M cop-
L) and 1 M sodium ascorbate (20 L) and 2-
L, 100 MBq). The mixture was stirred
l
HPLC analysis was performed with a Shimadzu Prominence
HPLC system equipped with a NaI/PMT-radiodetector (Gamma-
RAM, IN/US) and a diode array detector (Shimadzu).
[
No-carrier-added [18F] fluoride was produced by the 18O(p,n)18
F
nuclear reaction with a 16 MeV proton beam generated by a GE
PETtrace cyclotron in a silver target using [18O]H2O (Singapore
Radiopharmaceuticals Pte Ltd).
A GE TracerLab FXFN radiochemistry module was used for pro-
cessing [18F]fluoride, nucleophilic fluorination and purification of
2-[18F]fluoroethylazide. Isolation of the [18F]fluoride from [18O]
H2O was achieved by trapping on a QMA ion exchange cartridge
(Sep-Pak light, Waters) and further elution of the column with a
solution containing 1.8 mg K2CO3 and 10 mg of Kryptofix 2.2.2 in
2.0 mL of 22:1 acetonitrile:water into a glassy carbon reaction ves-
sel. Anhydrous [18F]fluoride was obtained after azeotropic evapora-
tion for 7 min at 60 °C under a flow of helium and 7 min at 120 °C
under vacuum.
of 0.1–2 GBq/lmol.
2.6. In vitro assay
MCF-7 cells, a human breast adenocarcinoma cell line, were cul-
tured in RPMI 1640 medium supplemented with 10% fetal bovine
serum (FBS) and 1%penicillin–streptomycin (P/S). Prior to the uptake
experiment, MCF-7 cells (2 ꢀ 105 cells/mL) were seeded in 24-well
plates (0.5 mL, 100,000 cells) and cultured for 24 h in a humidified
atmosphere of 5% CO2 at 37 °C. Cells are cultured at normoxic or
hypoxic conditions for 4 h. Hypoxic conditions were obtained as
described previously19: cells were placed in a humidified air-tight
chambers (Billubs–Rothenberg) and a hypoxic air mixture (5% CO2/
95% N2) was infused into the air-tight chambers at a flow rate of
10 L/min for 15 min. After closing the inflow and outflow valves,
the humidified air-tight chamber was kept at 37 °C for 4 h.
2.5.1. [18F]FMISO
FMISO was prepared as described previously by nucleophilic
fluorination of 1-(20-nitro-10-imidazolyl)-2-O-tetrahydropyranyl-
3-O-toluenesulfonyl-propanediol (5 mg, ABX) at 110 °C for 10 min
in anhydrous acetonitrile (0.8 mL), and further hydrolysis with
HCl 1 N (1 mL) for 5 min at 100 °C, neutralization with 30% sodium
acetate (0.5 mL), dilution with 3 mL water and purification by HPLC
18F-labeled nitroaromatics (0.5–1.0 MBq/mL in PBS; 0.1–2 GBq/
l
mol,20 were added to the normoxic and hypoxic cells (50
lL) and
(Phenomenex Luna 5
l
m C18(2), 100 Å, 150 ꢀ 10 mm, 5 mL/min,
incubated for 1 h at 37 °C: normoxic cells in a humidified atmo-
sphere of 5% CO2 and hypoxic cells in a humidified air-tight cham-
bers after infusion of the hypoxic air mixture at a flow rate of 10 L/
min for 15 min. Subsequently, cells were washed twice with PBS
and lysed in 0.5 mL 1 M sodium hydroxide for 30 min. Cell lysates
were counted with an automatic gamma counter (2470 Wizard2,
Perkin Elmer).
5% ethanol).18 The collected peak (tR: 7 min, 3.5–4.0 mL) was diluted
with 0.1 mL 35% NaCl solution and 5 mL PBS, pH 7. Analysis by
radio-HPLC indicated >99% radiochemical purity and specific activ-
ity was 100–200 GBq/lmol.
2.5.2. 2-[18F]Fluoroethylazide
In the sealed reaction vessel, 2-azidoethyl 1H,1H,2H,2H-perfluo-
rodecane-1-sulfonate (10 mg) in 0.6 mL anhydrous acetonitrile was
added to and heated for 10 minutes at 110 °C. The crude solution
The radioactivity associated with the cell was calculated as the
percentage of added radiopharmaceuticals. Each compound was
tested using both normoxic and anoxic conditions in 6ꢀ replicates.

R. Bejot et al. / Bioorg. Med. Chem. 20 (2012) 324–329
327
The washout assay was performed by replacing the radioactive
media after 1 h incubation with fresh RPMI 1640 (10% FBS, 1% P/S)
and incubation for 1 h under normoxic or hypoxic conditions. Sub-
sequently, cells were washed with PBS and cell lysate counted with
an automatic gamma counter as described previously.
screening for their ability to bind hypoxic cells. In detail, the radi-
olabeled synthon 2-[18F]fluoroethylazide was synthesized by
nucleophilic 18F-fluorination of the polyfluorinated sulfonate pre-
cursor and purified rapidly by elution through alumina and fluor-
ous silica gel.7 The synthon was subsequently conjugated to
propargyl-functionalized molecules in the copper(I) catalyzed
Huisgen [3+2] cycloaddition (‘click reaction’). Various substrates
were hence successfully 18F-radiolabeled without the need for pro-
tective groups nor intensive radiochemical optimization, and could
be further evaluated in vitro. Six nitroaromatic substrates have
3. Results and discussion
The fluorous-click approach is exemplified in this paper by the
readily preparation of a library of 18F-radiolabeled chemicals and
Table 1
Physico-chemical properties of various fluorinated nitroaromatics
Entry Structure
Lipophilicity Redox potential E1/2
XOD reduction rate (nmol/min/unit enzyme)a
[k/extinction coefficient]
logPa
(vs SCE) (V)a
1
ꢁ0.33 0.02 ꢁ0.98 0.02
37 7 [325 nm/7.8 mMꢁ1 cmꢁ1
]
2
3
ꢁ0.41 0.01 ꢁ1.05 0.01
36 2 [325 nm/7.8 mMꢁ1 cmꢁ1
]
ꢁ0.38 0.02 ꢁ1.37 0.01
5
1 [325 nm/6.2 mMꢁ1 cmꢁ1
]
4
0.94 0.03 ꢁ1.21 0.01
11 2 [325 nm/6.6 mMꢁ1 cmꢁ1
]
5
6
0.03 0.05 ꢁ1.21 0.01
20 10 [278 nmb/6.4 mMꢁ1 cmꢁ1
]
0.04 0.01 ꢁ1.12 0.01
58 11 [415 nm/7.0 mMꢁ1. cmꢁ1
]
7
0.79 0.06 ꢁ1.07 0.02
277 38 [395 nm/9.2 mMꢁ1 cmꢁ1
]
a
Carried out in triplicates (mean value SD).
Isosbestic point.
b

328
R. Bejot et al. / Bioorg. Med. Chem. 20 (2012) 324–329
been selected according to their reported properties and their com-
mercial availability. Noteworthy, an 18F-labeled derivative of
2-methyl-4-nitroimidazole had been used for biodistribution stud-
ies21; a 99mTc complex of 6-nitrobenzoimidazole had been evalu-
ated in vitro as a hypoxia marker,22 and a derivative had been
evaluated as a radiosensitizing agent in vitro23; finally 2-amino-
5-nitrothiazole and derivatives of 3-nitropyrazole had been evalu-
ated as radiosensitizing agents in vitro.24–26
lipophilicity of such compounds was actually proven to be affect-
ing the pharmakinetics (e.g., tumor penetration) of radiosensitizers
and radiopharmaceuticals.27,28 The enzymatic anaerobic reduction
is also correlated to the reduction potential of the one-electron
reduction to the nitro-radical anion.29 With redox potential E1/2
(vs SCE) comprised between ꢁ1.37 and ꢁ0.98 V, it is likely that
those compounds can accept electrons in anaerobic conditions,
depending on the enzyme-substrate affinity (Km not measured).
Hence the compound with highest electron affinity amongst the
evaluated ones was determined to be the 2-amino-5-nitrothiazole
(7) with a nitroreduction rate of 277 38 nmol minꢁ1 Uꢁ1 xanthine
Non-radioactive reference compounds were synthesized and
their physico-chemical properties determined: lipophilicity, redox
potential and xanthine oxidase (XOD) reduction rate (Table 1). The
Figure 1. Cellular uptake of 18F-labeled nitroaromatics in normoxic and hypoxic conditions (⁄⁄⁄P <0.0001; ⁄⁄P <0.001).
Figure 2. Time course uptake of 2 and 7 (left). Wash-out of 2 and 7 after 60 min incubation (right). (⁄⁄⁄P <0.0001; ⁄P <0.05).

R. Bejot et al. / Bioorg. Med. Chem. 20 (2012) 324–329
329
oxidase. Compounds 1, 2 and 6 proved to have a medium electron
affinity, and compounds 3, 4 and 5 to have the lowest.
of concept hypoxic-selectivity assay, the fluorous-click method
could eventually allow for identification of membrane permeable
and oxygene-sensitive biomarker candidates for 18F-PET.
2-[18F]Fluoroethylazide, whose synthesis and purification were
recently reported using the fluorous technology,7 was used to ac-
cess 18F-labeled compounds for screening. A single batch of 2-
Acknowledgment
[
18F]fluoroethylazide could be readily synthesized and purified by
FSPE within a shielded hot cell, using a remotely controlled module
(GE Tracerlab FXFN). FSPE-purified aliquots of 2-[18F]fluoroethylaz-
ide could then be used for parallel conjugation with various prop-
argyl-functionalized nitroaromatics, within a shielded fume hood.
Further purification with an automated HPLC afforded the various
18F-labeled nitroaromatic-based molecules for evaluation of their
uptake within both hypoxic and normoxic cells (Fig. 1). 18FMISO
was also prepared and used to validate the cell uptake assay.
From the in vitro cell assays, it appears that three compounds are
worth mentioning. The substrates with a medium-to-high electron
affinity (2, 6 and 7) display a significant uptake difference between
hypoxic and normoxic cells, whereas those with a lower electron
affinity (3, 4 and 5) don’t display any hypoxic selectivity (Fig. 1). As
expected from the various well-validated 2-nitroimidazolyl hypoxia
biomarkers (e.g., 18FMISO, [18F]HX4), the 2-nitroimidazole deriva-
tive (2) has significant hypoxia selectivity. The new substrate 3-ami-
no-2-nitropyridine (6) exhibits hypoxia selectivity as well but the
uptake is much lower than the reference compound 18FMISO. This
lead compound could eventually be a candidate for optimization,
especially improving the transport through the cell membrane. 2-
Amino-5-nitrothiazole (7) had been shown to have radiosensitizing
properties due to its electron affinity.26 In this study, the 2-amino-5-
nitrothiazole derivative displays a high uptake within both hypoxic
and normoxic cells. But it turns out that most of the 5-nitrothiazole-
related radioactivitywithin cells isn’t irreversibly bound as exempli-
fied by the washout experiment (Fig. 2). Finally, the high electron
affinity of 7 may explain the low selectivity between hypoxic and
normoxic cells
This work was supported by the DIUS and Siemens Molecular
Imaging (Project No. TP/16636).
Supplementary data
Supplementary data (details of synthesis and characterization)
associated with this article can be found, in the online version, at
doi:10.1016/j.bmc.2011.10.084.
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18F]fluoroethylazide synthon has actually proven valuable in the
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l
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