Rapid aqueous [18F]-labeling of a bodipy dye for positron emission tomography/fluorescence dual modality imaging

Article abstract of DOI:10.1039/c1cc13089g

We report the rapid nucleophilic [¹⁸F]-radiolabeling of a bodipy dye in aqueous solutions. This radiolabeled dye, whose biodistribution and clearance has been studied in mice, is stable in vivo and can be used as a positron emission tomography/

Full text of DOI:10.1039/c1cc13089g

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Rapid aqueous [18F]-labeling of a bodipy dye for positron emission
tomography/fluorescence dual modality imagingw
Zibo Li,z*a Tzu-Pin Lin,yb Shuanglong Liu,ya Chiun-Wei Huang,a Todd W. Hudnall,b
Franc¸
ois P. Gabbaı
¨
z*b and Peter S. Contia
Received 26th May 2011, Accepted 2nd July 2011
DOI: 10.1039/c1cc13089g
We report the rapid nucleophilic [18F]-radiolabeling of a bodipy
dye in aqueous solutions. This radiolabeled dye, whose
biodistribution and clearance has been studied in mice, is stable
in vivo and can be used as a positron emission tomography/
fluorescence dual modality imaging agent.
high quantum yields and their emission can be tuned into the
NIR by simple variation of the molecular structure.7 They also
typically possess a BF2 unit which could in principle provide a
site for [18F]-radiofluorination. To explore this possibility, we
have now embarked on a project aimed at the radiofluorination
of bodipy dyes for biological imaging. In this paper, we
provide an original illustration of this approach.
[18F]-Positron emission tomography (PET) is a powerful
imaging technique1 which provides in vivo information on
the distribution of radiolabeled biomolecules. Despite numerous
advantages, this technique remains affected by two major
limitations. First of all, the short-lived 18F-radionuclide needs
to be incorporated into molecules as expediently as possible.
However, 18F is typically prepared by proton bombardment of
[18O]-water and is thus obtained as the anion in an aqueous/
non-nucleophilic form.2 A second limitation of PET imaging
relates to the relatively low spatial resolution (1–2 mm)
provided by this technique.2 To tackle the first challenge, a
great deal of effort has been devoted to the development of
aqueous fluorination protocols based on fluorophilic elements
such as silicon and aluminum.3 Another elegant approach was
developed by Perrin who showed that electron deficient
arylboronic acid or esters can be radiofluorinated in aqueous
solutions to afford [18F]-labeled aryltrifluoroborates. The second
challenge can be addressed by combining PET imaging with a
second imaging technique such as fluorescence which offers
much higher spatial and temporal resolution.4 This approach
necessitates the synthesis of PET/fluorescence dual modality
agents,5 which is typically achieved by introduction of a
fluorophore and a radiolabeled component as two separate
entities.6
As part of our work8 on the reactivity of the B–F bond of
bodipy dyes,9 we recently observed that the hydroxo derivative
1-OH could be easily converted into the corresponding fluoride
1-F by simple reaction with KHF2 in THF (Fig. 1).10 Encouraged
by this observation, we decided to determine if similar reactions
could also be implemented in aqueous solution. To this end,
we targeted the cationic derivative [2-OH]+ as a triflate salt.
This compound, which has been thoroughly characterized by
NMR spectroscopy and mass spectrometry, was synthesized
by alkylation of the dimethylamino precursor (Scheme 1).11
Owing to its cationic nature,12 this derivative can be dissolved
in water at 10ꢀ3–10ꢀ2 M concentrations. Under these conditions,
however, [2-OH]+ does not react with KHF2, presumably
because of the strength of the B–OH bond. Fortunately, we
found that conversion into [2-F]+ could be achieved by
addition of KHF2 to a deutero hydrochloric acid (DCl)
solution (0.95 M) in MeOD/D2O (1/1 vol.) (Scheme 1).
This reaction, which can be monitored by the appearance a
19F NMR signal at ꢀ173 ppm, is complete in less than 2 min
(see Supporting Informationw). The triflate salt of [2-F]+ has
been fully characterized. The photophysical properties of this
derivative are typical of other bodipy dyes. It features a broad
absorption band at 506 nm and an emission band centered at
528 nm (F = 14.3% in CH2Cl2). Encouraged by these
synthetic and spectroscopic results, we investigated the radio-
fluorination of [2-OH]+ in aqueous solution. The optimized
radiolabeling procedure is described below: 30 mCi of
[18F]-fluoride (100 mL unfixed target water) was directly added
In search of an integrated solution to these two problems,
we were drawn by the appealing properties of bodipy
dyes which have been used for the fluorescent labelling of
biomolecules.7 In addition to being very stable, such dyes have
a Molecular Imaging Center, Department of Radiology,
University of Southern California, Los Angeles 90033, USA.
E-mail: ziboli@usc.edu
b Department of Chemistry, Texas A&M University, College Station,
Texas 77843, USA. E-mail: gabbai@mail.chem.tamu.edu
w Electronic supplementary information (ESI) available: Experimental
and characterization data. See DOI: 10.1039/c1cc13089g
z Jointly conceived the study.
y Contributed equally to the work.
Fig. 1 The conversion of 1-OH to 1-F by simple reaction with KHF2.
c
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To circumvent this difficulty, we decided to repeat the reaction
in the presence of an activating/water scrubbing agent such as
TMSOTf. With this in mind, [2-OH]+ (500 mg, 0.85 mmol in
100 mL MeCN) was pretreated with TMSOTf (20 eq.) and then
subsequently mixed with a MeCN solution (100 mL) of
azeotropically dried [18F]-TBAF (10 mCi). Gratifyingly, this
reaction, which was allowed to proceed for 5 min at 60 1C,
afforded [18F]–[2-F]+ (specific activity Z 1.4 Ci/mmol) in 61%
yield as indicated by HPLC.
Although B–F bonds are among the strongest bonds
known,13 the stability of 18F–B compound is very critical as
free [18F]-fluoride ions could bind to the bones giving rise to
unwanted and interfering background signals.14 We first
studied the stability of [18F]–[2-F]+ in PBS buffer at pH 7.5
over a period of several hours. HPLC analysis carried out at
different time intervals indicated that the concentration of
intact [18F]–[2-F]+ decreased from about 99% after 1 h, to
97% after 3 h and 95% after 6 h thus pointing to the
remarkable resistance of this derivative to hydrolysis at
physiological pH (see Supporting Informationw). In order to
validate our approach and demonstrate its potential for in vivo
PET imaging, [18F]–[2-F]+ (prepared via the aqueous route)
was injected into normal nude mice that were imaged using a
microPET scanner 1 h, 2 h, and 4 h post injection. We did not
observe bone uptake even at 4 h post injection, which indicates
that the hydrolytic release of free [18F]-fluoride from [18F]–[2-F]+
is essentially negligible on the time scale of the 18F-nuclear
decay (Fig. 3). More interestingly, we observed a clear
accumulation of the radiolabeled probe primarily in the liver
and kidneys but also in the gall bladder 2 h post injection.
Bearing in mind that the probe [18F]–[2-F]+ lacks any specific
targeting functionalities, its accumulation in these organs
constitutes a normal phenomenon, in line with its hydrophobic
and cationic nature.
Scheme 1 Synthesis of bodipy [2-OH]+. ArN = [4-(Me2N)-C6H4];
ArN+ = [4-(Me3N)-C6H4]+. F* = 18F or 19F. Reagents and reaction
conditions: (i) p-chloranil, Et3N, and PhBCl2 in CH2Cl2 followed by
aqueous workup; (ii) MeOTf in CH2Cl2; (iii) For [19F]–[2-F]+: KHF2,
0.95 M DCl in MeOD/D2O (1/1 vol.); For [18F]–[2-F]+ 18Fꢀ/KHF2,
:
H2O/MeOH, pH = 2–3.
to 5 mL of KHF2 (0.1 mol Lꢀ1). The mixture was heated at
70 1C for 10 min to ensure a complete homogenization. After
cooling down to room temperature, compound [2-OH]+
(500 mg, 0.85 mmol in 100 mL MeOH) was added and the
labeling was performed at room temperature for 15 min in the
2–3 pH range (Scheme 1). After dilution with 800 mL of water,
the crude mixture was loaded onto a reverse phase HPLC and
[
18/19F]–[2-F]+ was obtained in 22 ꢁ 3% yield (decay corrected,
based on separation, n = 4). The specific activity of the final
product was calculated to be 25 ꢁ 4 mCi/mmol by comparing
its UV absorption with the standard titration curve. The
identity of [18F]–[2-F]+ was confirmed by the co-injection
with the non-radiolabeled standard (Fig. 2). To broaden the
scope of our approach, we have also investigated the formation
of [18F]–[2-F]+ under no carrier added conditions. We first
tested the reaction of [2-OH]+ with azeotropically dried
[18F]-TBAF in acetonitrile, which, however, did not afford
any detectable yield of the target radiolabeled compound. The
failure of this reaction to proceed can be assigned to the
stability of the B–OH bond in [2-OH]+ as well as the presence
of residual water in the [18F]-TBAF and/or the acetonitrile.
Next, we endeavoured to confirm the dual modality poten-
tial of the probe. To this end, the animal was euthanized and
selected organs were harvested for ex vivo fluorescence and
microPET imaging (Fig. 4). The fluorescence images were
obtained by irradiation of the organs at l = 500 nm. This
wavelength was chosen because it falls within the absorption
band of [2-F]+ (see Supporting Informationw). The fluorescence
image was reconstructed based on the emission intensity
measured at l = 580 ꢁ 20 nm, a wavelength which is within
Fig. 2 (A) HPLC traces showing the formation of [18F]–[2-F]+
;
Fig. 3 PET images of a mouse injected with purified [18F]–[2-F]+. No
detectable bone uptake is observed up to 4 h post-injection. All images
shown are 2D projection instead of a single slice of the scan.
(B) HPLC trace obtained for the non-radiolabeled standard [2-F]+
and the radio trace obtained for purified [18F]–[2-F]+
.
c
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This work was supported by the USC Department of
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National Cancer Institute (P30CA014089), the National
Science Foundation (CHE-0952912) and the Welch Foundation
(A-1423).
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