Fluoroalkyl azides for rapid radiolabeling and (Re)investigation of their potential towards: In vivo click chemistry

Article abstract of DOI:10.1039/c7ob00880e

In recent years, radiofluorinated alkyl azides have been reported for click radiolabeling and pretargeted PET imaging, but only little is known about the biodistribution and metabolism of these compounds. In this work, we present a significantly improved procedure for the synthesis of [¹⁸F]fluoroethyl azide and reinvestigated this radiolabeled probe in detail showing poor stability and very restricted suitability for in vivo application. Therefore, modified low-molecular-weight [¹⁸F]fluoroalkyl azides were developed. Propargyl-tagged endomorphin-1 (as model compound) was successfully radiolabeled in high yield and short reaction time making these probes useful and efficient bioorthogonal tools for rapid radiolabeling. Biodistribution, pharmacokinetics and in vivo stability were studied by preclinical PET/MR scanning and metabolite analysis. The results of this study revealed only limited applicability of [¹⁸F]fluoroalkyl azides for in vivo application.

Full text of DOI:10.1039/c7ob00880e

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Org. Biomol. Chem., 2017, DOI: 10.1039/C7OB00880E.
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[18F]Fluoroalkyl azides for Rapid Radiolabeling and (Re)Investigation of
their Potential Towards In Vivo Click Chemistry
Christoph Denka, Martin Wilkovitscha, Philipp Skrinjara, Dennis Svatuneka, Severin Mairingerb, Claudia
Kuntnerb, Thomas Filipb, Johannes Fröhlicha, Thomas Wanekb and Hannes Mikulaa,*
a) Institute of Applied Synthetic Chemistry, Vienna University of Technology (TU Wien), Austria; E-mail: hannes.mikula@tuwien.ac.at
b) Health and Environment Department, Biomedical Systems, Austrian Institute of Technology (AIT), Seibersdorf, Austria
Graphical Abstract
Abstract
In recent years, radiofluorinated alkyl azides have been reported for click radiolabeling and pretargeted PET
imaging, but only little is known about the biodistribution and metabolism of these compounds. In this
work, we present a significantly improved procedure for the synthesis of [18F]fluoroethyl azide and
reinvestigated this radiolabeled probe in detail showing poor stability and very restricted suitability for in
vivo application. Therefore, modified low-molecular-weight [18F]fluoroalkyl azides were developed.
Propargyl-tagged endomorphin-1 (as model compound) was successfully radiolabeled in high yield and
short reaction time making these probes useful and efficient bioorthogonal tools for rapid radiolabeling.
Biodistribution, pharmacokinetics and in vivo stability were studied by preclinical PET/MR scanning and
metabolite analysis. The results of this study revealed only limited applicability of [18F]fluoroalkyl azides for
in vivo application.
Introduction
Positron emission tomography (PET) allows non-invasive molecular imaging and quantification of metabolic
processes. High sensitivity, spatial and temporal resolution made PET an indispensable diagnostic tool in
the clinic and in biomedical research. The versatility of this imaging modality is highly dependent on the
availability of specifically radiolabeled probes. Short-lived nuclides are generally favored due to low patient
radiation doses and high specific activities.1 However, sufficient time is still necessary for production,
quality control, delivery and application of radiolabeled probes. Fluorine-18 (half-life of 109.8 min) is
considered to be the ideal PET radioisotope (for high resolution imaging) enabling the radiochemist to
conduct multi-step synthesis and purifications using chromatographic methods.2-4
[18F]Fluoroethyl azide ([18F]1-azido-2-fluoroethane, [18F]FEA) is reported to be a highly versatile tool for PET
imaging and radiochemistry, and has frequently been used as a prosthetic group for rapid radiolabeling of
precursor compounds modified with terminal alkynes through copper-catalyzed alkyne azide cycloadditions
(CuAAC).5–7 These reactions are fast, highly selective, high yielding and - in contrast to direct 18F-labeling -
can be carried out under mild reaction conditions without the need of protecting groups. Hence, this kind

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of bioorthogonal ligation is a very useful tool for the radiolabeling of structurally complex compounds like
peptides and other biomolecules (Fig. 1a).5,6,8–12
Furthermore, [18F]FEA has been proposed as a promising secondary agent for pretargeted PET imaging
applying in vivo bioorthogonal ligation to cyclooctyne-tagged probes (e.g. antibodies).11 This two-step
strategy is supposed to enable imaging of long circulating nanomedicines with short half-life nuclides such
as fluorine-18. First, the probe modified with a reactive tag (reporter) is administered and allowed to
accumulate in target tissue. After clearance of excess probe a radiolabeled click imaging agent is given that
is supposed to react with the pre-administered reporter in vivo at the target site (Fig. 1b). The strain
promoted alkyne-azide cycloaddition (SPAAC) using organic azides and cyclooctynes as reactants has
frequently been applied in vitro and in vivo.13–17
a) Rapid Radiolabeling for PET
N
N
N
in vitro click
N
N
N
N
N
N
Target
N3
Cu(I)
b) Bioorthogonal PET Imaging
N3
click
in vivo
Target
accumulation
?
+
=
N3
= cyclooctyne
Figure 1. Radiolabeled alkyl azides as (a) prosthetic groups for rapid radiolabeling (in vitro click) and (b) secondary imaging agents
for pretargeted bioorthogonal PET imaging (in vivo click).
Moreover, promising results on pretargeted PET applying pre-administered cyclooctyne-labeled
nanoparticles followed by administration of an azide-functionalized fluorine-18-labeled mini-PEG have
been reported.13 Nevertheless, this highly polar radiolabeled probe is unlikely to cross cell membranes and
thus is not suitable in combination with internalizing chemical reporters. Hence, a metabolically stable
probe with fast pharmacokinetics and medium polarity (cell permeability) is supposed to be of high value
for SPAAC-mediated in vivo applications.
In recent years, the inverse electron demand Diels-Alder (IEDDA) reactions between 1,2,4,5-tetrazines and
trans-cyclooctenes (TCOs) has successfully been applied in rapid radiolabeling and bioorthogonal PET
imaging.7,18–23 Rate constants of this ligation are particularly high, however, in addition to increased steric
demand, tetrazine (radio)chemistry does not meet the robustness of low-molecular-weight [18F]fluoroalkyl
azides.
Despite the applicability of [18F]FEA several problems are associated with this low-molecular weight agent.
[18F]FEA is highly volatile and usually purified from the labeling reaction mixture by co-distillation with
acetonitrile.5 However this purification is time-consuming and care must be taken not to exceed a
distillation temperature of 90 °C. Otherwise, the precursor is carried over causing impurities and
interferences in subsequent cycloaddition reactions.24 Moreover, the tosylated precursor is highly viscous
making it difficult to handle in small quantities. Stable 2-fluoroethyl azide required for synthesis of cold
reference materials is potentially explosive, gaseous and difficult to handle in isolated form.5
In this paper we aim to shed light on the applicability of [18F]FEA and related compounds as click imaging
agents by detailed investigation of biodistribution and metabolism. Furthermore, we present new tools

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that circumvent problems associated with [18F]FEA. A novel crystalline precursor for efficient radiolabeling
is presented enabling the synthesis of [18F]FEA in high yields also avoiding problems related to distillation
carry-over. Moreover, two new low-molecular-weight [18F]fluoroalkyl azides (1-azido-3-[18F]fluoropropane
and 2-azido-1-[18F]fluoropropane) are presented as prosthetic groups with beneficial properties, and
further investigated with emphasis on applicability as click imaging agents for pretargeted PET imaging
showing only very limited applicability.
Results and discussion
Radiosynthesis and rapid radiolabeling
Radiosynthesis of [18F]1 is commonly carried out using K2CO3/Kryptofix-222
(K222)-mediated nucleophilic radiofluorination of the tosylated precursor 2-
azidoethyl 4-methylbenzenesulfonate (2).5,25,26 After short reaction time at
elevated temperature (80-110 °C) the product is isolated by co-distillation with
acetonitrile. However, time-consuming distillation at low temperature, reduced pressure and the use of low
precursor mass are required to minimize carry-over of the volatile tosylate 2. Co-distilled precursor causes
chemical impurities and furthermore competes with [18F]1 within the cycloaddition with alkyne groups
leading to low yields in the ligation reaction.
To circumvent this problem 2-azidoethyl 4-nitrobenzenesulfonate (3) was synthesized as novel precursor
material from 2-azidoethanol and nosyl chloride in 67% yield (Fig. 2). The nosylate was obtained as
crystalline solid, thus making handling of small quantities significantly easier compared to the highly
viscous, commonly used precursor 2-azidoethyl 4-methylbenzenesulfonate (2). [18F]1 was obtained by
reacting 3 with azeotropically dried [18F]fluoride and K2CO3/K222 in acetonitrile for 5 min at 105°C (Scheme
1). Time-efficient distillation at 105-110°C in a stream of argon afforded [18F]1 in 75% decay-corrected yield
and >99% radiochemical purity. Most noteworthy, no precursor was detectable in the distilled product by
HPLC due to reduced volatility of 3.
Improved precursor
Commonly used precursor
Crystalline solid
Non-volatile "distillation-safe"
Viscous Oil, difficult to handle
Volatile, causing distillative carry-over
O
S
O
O
O
O
N3
S
N3
O
O2N
3
2
O
S
O
i
18F
ii
N3
OH
N3
N3
O
67%
up to 75%
18F]1
3
[
O2N
Figure 2. Advantages (in comparison to the commonly used precursor 2) and synthesis of precursor 3, and subsequent radiolabeling
affording [18F]1. [i) NsCl, N(Et)3; ii) 18F-HF, K2CO3, Kryptofix-222]
Due to the toxicity associated with acetonitrile a different method for purification was applied to obtain
[18F]1 for in vivo investigations. Precursor 3 was radiolabeled using an automated GE Tracerlab FX module
with an integrated HPLC purification system. A water/ethanol gradient in combination with a reversed
phase column was used to yield [18F]1 in biocompatible media in 48% decay-corrected radiochemical yield

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(non-optimized) with a radiochemical purity of 99.6%. Identity of [18F]1 was confirmed by HPLC, and
stability of the formulation was assessed after 6 h still revealing > 97% purity.
However, our findings from the murine metabolic study of [18F]1 showed high bone uptake indicating low
metabolic stability and release of free 18F- (for details see in vivo section). In contrast, probes prepared by
rapid radiolabeling using [18F]1 are known to exhibit high metabolic stability in vivo.27 Therefore, we assume
an influence of the azide functionality on defluorination. To minimize in vivo degradation of [18F]fluoroalkyl
azides, we have considered [18F]4, [18F]5 and [18F]6 (Scheme 1) as structurally similar analogs to [18F]1.
We assumed that [18F]1-azido-3-fluoropropane ([18F]4) might show reduced tendency to release fluoride
due to decreased electronic interactions between the azide functionality and 18F. Furthermore, [18F]2-azido-
1-fluoropropane ([18F]5) and [18F]2-azido-2-methyl-1-fluoropropane ([18F]6) were considered to investigate
the influence of increased steric hindrance on in vivo defluorination.
3-Azidopropyl 4-nitrobenzenesulfonate (7) was prepared as precursor compound for [18F]4 in two steps
starting from 3-chloropropan-1-ol. The crude product was treated with p-nitrobenzenesulfonyl chloride and
triethylamine to afford 7 as a crystalline solid in 33% overall yield (Scheme 1). Radiolabeling using
conditions described above for the synthesis of [18F]1 afforded [18F]4 after co-distillation with acetonitrile in
39.3% decay corrected radiochemical yield and with a radiochemical purity of > 98%. Preparative HPLC was
used instead of distillation to obtain pure product (without acetonitrile) for in vivo investigation. Identity of
[18F]4 was confirmed by comparison via HPLC with an authentic standard, and stability of the formulation
was assessed by radio-HPLC three hours post radiosynthesis still revealing > 89% radiochemical purity.
Scheme 1. Synthesis of precursor materials 7, 9, 11 and subsequent radiolabeling. [i) NaN3; ii) NsCl, NEt3; iii) [18F]HF, K2CO3,
Kryptofix-222; iv) 1H-Imidazole-1-sulfonyl azide, K2CO3, CuSO4]
[18F]5 was synthesized from crystalline precursor 9. Commercially available racemic alaninol was subjected
to copper-catalyzed diazotransfer conditions using 1H-imidazole-1-sulfonyl azide to obtain 2-azidopropan-
1-ol (8) in 75% yield.28,29 Nosylation of 8 gave 9 as a racemic mixture in 71% yield (Scheme 1). Noteworthy,
alaninol is commercially available in optically pure form (R or S) making enantiopure [18F]5 accessible
following the presented procedure. However, in this study racemic [18F]5 was further investigated. 9 was
then reacted with azeotropically dried 18F-K222 complex in anhydrous acetonitrile for 7 min at 100 °C
followed by distillation in a stream of argon to obtain [18F]5 as solution in anhydrous acetonitrile with 58%
decay-corrected yield and a radiochemical purity of 99.7%. For in vivo studies acetonitrile-free [18F]5 was
synthesized on an automated GE Tracerlab FX module followed by preparative HPLC purification (27%
decay-corrected radiochemical yield, >96% radiochemical purity).

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Applying a similar synthetic strategy towards [18F]6, 2-amino-2-
methylpropan-1-ol was subjected to copper-catalyzed diazotransfer
conditions to obtain 2-azido-2-methylpropanol (10). The nosylated
compound 11 was obtained in 70% yield by reaction of alcohol 10 with
nosyl chloride and triethylamine (Scheme 1). Unfortunately,
radiolabeling attempts using the K2CO3/K222 method resulted in
formation of an unknown radiolabeled species of unexpected
lipophilicity. Its HPLC retention time did not match the retention time of
an authentic standard. Although the identity of the formed compound
could not be clarified, modeling the labeling reaction in deuterated
acetonitrile using (“cold”) potassium fluoride followed by 19F-NMR
spectroscopy indicated the absence of a primary alkyl fluoride (chemical
shift in the range of -220 ppm). Instead signals at -105 ppm indicated the
formation of undesired products due to elimination and/or
rearrangement.
Scheme 3. SPAAC reaction of 1, 5
and 6 with the cyclooctyne BCN
yielding the ligation products 16, 17
and 18
Access to non-radioactive (“cold”) reference compounds is essential for
identification of the radiolabeled agent by HPLC. 1-Azido-2-fluoroethane
(1) is highly volatile and potentially explosive and should thus not be isolated and handled in pure form.5
The previously reported30 tosylation of 2-fluoroethan-1-ol followed by reaction with sodium azide and
subsequent filtration yielded a 1.36 M solution of 1 in DMF that was used successfully for cycloaddition to
alkynes. Furthermore, DMF-free 1 was obtained by reacting 3 with anhydrous potassium fluoride and
Kryptofix-222 in acetonitrile followed by evaporation at low pressure and trapping of volatiles in a liquid
nitrogen trap (Scheme 2).
1-Azido-3-fluoropropane (4) was synthesized starting from 1,3-dibromopropane that was converted to 1-
bromo-3-fluoropropane (13). Although 13 was isolated in only 14% yield its synthesis from cheap starting
materials is still economically favorable.31 Nucleophilic substitution with sodium azide afforded 3 in 36%
yield. Due to a higher boiling point compared to 1 we were able to isolate neat 3 as colorless liquid by
careful distillation; a clear benefit in case subsequent chemical modification is intended. Although we did
not encounter any problems, safety precautions such as blast shields and performing the reaction in small
scale are highly recommended. Maximum scale in this study was less than 1 g. Moreover, all reactions
involving azide as a nucleophile must be free of alkyl halides including dichloromethane. We observed the
formation of potentially dangerous (explosive) diazidomethane when traces of dichloromethane were
present in the reaction mixture. 2-Azido-1-fluoropropane (5) was also obtained as a solution in DMF (1.05
M) by mesylation of commercially available 1-fluoropropan-2-ol followed by reaction with sodium azide in
anhydrous DMF.32 No attempts were made to isolate neat 5. The tertiary azide 6 was obtained as a solution
in ethyl acetate after applying diazotransfer conditions to 2-amino-2-methyl-1-fluoropropane hydrochloride
(15),29 which was prepared following a known procedure.33

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Scheme 2. Synthesis of reference compounds 1, 4, 5 and 6. [i) KF, Kryptofix-222; ii) TsCl, pyridine; iii) NaN3; iv) KF; v) DMAP, MsCl,
NEt3; vi) 1H-imidazole-1-sulfonyl azide, K2CO3, CuSO4; n.d.= not determined]
HPLC retention times of radiolabeled species (in acetonitrile for rapid radiolabeling or in aqueous medium
for in vivo application) were compared with retention times of cold reference compounds to confirm the
identity of radiolabeled agents (see ESI for chromatographic data). Compounds that were not isolated (1, 5
and 6) were ligated with (1R,8S,9s)-bicyclo[6.1.0]non-4-yn-9-ylmethanol (BCN)34 as cyclooctyne in a strain-
promoted alkyne-azide cycloaddition. The resulting triazoles 16, 17 and 18 were isolated and fully
characterized by NMR and MS (Scheme 3). [18F]1 and [18F]5 were reacted with BCN and formation of the
ligation products [18F]16 and [18F]17 was confirmed by comparison with their non-radioactive counterparts.
Reacting the product afforded after radiolabeling of precursor 11 with BCN led to formation of an unknown
compound not matching 18 and thus confirming the formation of an undesired re-arrangement or
elimination product during the radiolabeling reaction of 11 (as mentioned above).
Application of [18F]1 as prosthetic group for rapid “click” radiolabeling is well documented.5,6,8–10,12,26,35
Several modifications of the original procedure from Arstad and Glaser were reported to shorten reaction
times and eliminate the need for heating. Noteworthy, addition of the water-soluble copper-ligand
bathophenanthroline sodium sulfonate reduces the required amount of copper and sodium ascorbate
while maintaining or even enhancing ligation rates at room temperature.6
To test and compare [18F]4 and [18F]5 as prosthetic groups against already established [18F]1 we have
selected the tetrapeptide endomorphin-1 (EM-1) as substrate. EM-1 is an endogenous substance and
powerful µ-opioid receptor agonist (Ki = 360 pM) that is of potential interest for imaging purposes.36 EM-1
is commercially available and possesses only one nucleophilic amine to allow for site specific modification.
EM-1 was treated with commercially available 2,5-dioxopyrrolidin-1-yl-3-(prop-2-yn-1-yloxy)propanoate
and triethylamine to introduce an terminal alkyne (Scheme 6). Propargyl endomorphin-1 19 was obtained
in 42% yield after purification by HPLC. Ligation products 20, 21 and 22 were obtained by reaction of 19
with 1, 4 and 5 using the CuSO4/sodium ascorbate/bathophenantroline procedure, and isolated by RP-
HPLC. [18F]20, [18F]21 were obtained in good radiochemical yields within 10 minutes by reacting 1 mg of 19
with [18F]1 or [18F]4 using the same procedure (Scheme 4).
As [18F]5 was prepared as a racemic mixture, it needs to be mentioned that radiolabeling of chiral
substances produces a diastereomeric mixture potentially complicating analysis. In case of required
optically pure material enantiopure (2S)-1-azido-2-propanol37 can be used for the synthesis of precursor 9.
Subjecting [18F]5 to similar experimental conditions yielded [18F]22 as an inseparable mixture of
diastereomers in excellent radiochemical yield of 98%. Retention times of the ligation products were
compared by radio-HPLC with authentic reference compounds (20-22). In addition, the plasma stability of

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all ligation products was determined by incubation in human blood plasma for 2 hours followed by radio-
TLC analysis. Similar stabilities (≥ 80%) were observed for all ligation products further indicating the
applicability of [18F]4 and [18F]5 as prosthetic groups for rapid radiolabeling.
Scheme 4. Synthesis of propargyl-modified endomorphin-1 (19), and ligation with [18F]1, [18F]4 and [18F]5 yielding [18F]20, [18F]21
and [18F]22 (as an inseparable mixture of diastereomers), respectively.
In vivo investigations
[18F]1, [18F]4, and [18F]5 were injected into female BALB/c mice and dynamic PET (n=4 for [18F]1, n = 2 for
[18F]4 and [18F]5) or PET/MR (n=1 for all test substances) scanning was performed for 2 hours. Coronal
PET/MR images are shown in Fig. 3a. Analysis of organ time activity curves (Fig. 3b) reveals a uniform
uptake of all agents in most analyzed organs that slowly decreases from ~1.5 to ~1 SUV (standardized
uptake value) during the course of the experiment. High uptake in the urinary bladder suggests renal
elimination as main excretion route for all investigated substances. The secondary azide [18F]5 exhibits the
fastest clearance, a clear benefit when application as click imaging agent in bioorthogonal PET imaging is
intended. Interestingly, brain uptake shows a slight but steady increase for all analyzed tracers.
[18F]4 exhibits a higher muscular uptake compared to [18F]1 and [18F]5 that increased throughout the
experiment (Fig. 3b). Most noteworthy, an increase of radioactivity concentration in bone was observed – a
good indicator for metabolic instability and in vivo defluorination of the tracer molecules. Activity
concentration in bone peaked at 3.6 SUV for [18F]1 and 6.5 SUV for [18F]4, thus showing that in vivo stability
could not be improved by insertion of one extra methylene group. [18F]5 showed lowest bone uptake (peak
SUV = 2.7) compared to [18F]1 and [18F]4 indicating improved in vivo stability over [18F]1 and [18F]4 (Fig. 3b).
In addition, coronal PET/MR images (Fig. 3a) show skull uptake in the 5-120 min summation images, which
is least pronounced for [18F]5 while [18F]4 exhibits the highest uptake. Placing a “total box” ROI over the
entire animal revealed constant activity over the entire imaging period showing that [18F]1, [18F]4 and [18F]5
are not excreted by respiration.

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Figure 3. (a) Coronal PET/MR slices of test substances [18F]1, [18F]4 and [18F]5; (b) time activity curves of analyzed tracers in brain,
liver, muscle, urinary bladder, bone in female BALB/c mice; (c) biodistribution of [18F]1, [18F]4 and [18F]5 obtained by γ-counting of
organ samples 120 min post tracer administration.
Following imaging, blood and urine samples were collected and the mice were sacrificed. Organ samples
were collected and measured for radioactivity in a gamma counter. Data obtained at 120 min post injection
of the tracers match the results of the PET experiment (Fig. 3c). Murine plasma was obtained by
centrifugation of whole blood in heparin tubes, and plasma proteins were precipitated by addition of an
equal volume of acetonitrile. Following centrifugation, the supernatant was injected into the HPLC system
for analysis of plasma metabolites. Due to the volatile nature of the test substances radio-HPLC was carried
out instead of radio-TLC. Urine samples were injected after dilution with water. HPLC data shows
diminutive (<4% of total radioactivity) amounts of intact compounds while the main fraction of radioactivity
(> 90%) was detected as highly polar compound, most likely free [18F]fluoride ions confirming the metabolic
instability of [18F]1, [18F]4 and [18F]5. Considering the rate of bone uptake (Figure 2b) [18F]5 shows increased
stability compared to [18F]1 and [18F]4. This data is also reflected by plasma metabolite HPLC analysis,
wherein [18F]4 displayed the highest percentage of intact tracer 120 min post administration. Despite
nuances in metabolic stability, degradation seems to happen on a time scale that is not compatible with
reaction kinetics of the SPAAC ligation. In addition, high bone uptake will contribute to background signal
hindering image interpretation. Hence, [18F]fluoroalkyl azides do not represent suitable click imaging agents
for in vivo application.

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Although our findings and conclusion regarding the in vivo applicability of [18F]1 deviate from previously
published results,11 it needs to be mentioned that time activity curves, biodistribution data, and assessment
of in vivo stability of [18F]1 have not been shown so far. In general, click imaging agents for pretargeted PET
imaging need to show good metabolic stability to allow for sufficient reaction and diffusion time in target
tissue. This is of particular importance for chemistries with only slow to intermediate reaction kinetics such
as SPAAC. Furthermore, metabolic degradation products contribute to background signal, and might show
specific uptake (e.g. [18F]fluoride in bone), thus complicating image interpretation. In this regard, tetrazine
ligations are preferred and advantageous over SPAAC.38
Conclusion
In summary, we present an improved synthesis of [18F]fluoroethyl azide based on a novel precursor
substance that is crystalline, easier to handle, and less volatile thus preventing the problem of distillative
carry-over. In addition, two novel [18F]fluoroalkyl azide agents ([18F]4, [18F]5) were developed expanding the
library of prosthetic groups for 18F-labeling. Rapid radiolabeling was demonstrated on endogenous µ-opioid
agonistic peptide endomorphin-1. The labeling was successfully carried out in short time (5-10 min)
achieving high radiochemical yields. Despite very limited applicability for in vivo reaction our [18F]fluoroalkyl
azides were shown to be useful and efficient alternatives for rapid radiolabeling compared to tetrazine-
based agents (Fig. 4).
Figure 4. CuAAC and IEDDA as complementary tools for rapid radiolabeling.

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