One-pot synthesis of an 125I-labeled trifunctional reagent for multiscale imaging with optical and nuclear techniques

Article abstract of DOI:10.1002/anie.201102072

A highly efficient and rapid Cu^II-mediated three-component "click reaction" allows one-pot assembly of dual optical and nuclear labeling reagents. Proof-of-concept imaging studies demonstrate that the distribution of the dual-labeled antibody A

Full text of DOI:10.1002/anie.201102072

DOI: 10.1002/anie.201102072
Imaging Agents
One-Pot Synthesis of an 125I-Labeled Trifunctional Reagent for
Multiscale Imaging with Optical and Nuclear Techniques**
Ran Yan, Ethaar El-Emir, Vineeth Rajkumar, Mathew Robson, Amit P. Jathoul,
R. Barbara Pedley, and Erik ꢀrstad*
Molecular imaging has transformed biomedical research,
drug development, and clinical practice. Yet, our under-
standing of biological processes in vivo remains limited
because no imaging modality meets the need for high
resolution, high sensitivity, and deep tissue penetration. The
ability to study biological processes across the cellular and
macroscopic scales therefore remains a fundamental goal for
molecular imaging. Tracers equipped with dual reporter
groups can overcome some of the current restraints by
allowing modalities with overlapping strengths to be used in
combination.[1–3] Here, we report a new approach to multi-
scale multimodal imaging based on one-pot synthesis of dual
optical and nuclear labeling reagents.
The ideal labeling method should be technically simple
and versatile, and should preserve the biological properties of
the parent molecule. To date, attempts to prepare dual optical
and nuclear imaging agents have relied on stepwise conjuga-
tion of a fluorescent group and a radionuclide to the
biological “hook”, using nanoparticles or small-molecule
linkers as the central building block.[4,5] This modular
approach is technically challenging, and in the case of
nanoparticle-based probes, often results in unfavorable
pharmacokinetic properties.[6,7] Efficient assembly of multi-
functional biocompatible small-molecule scaffolds is there-
fore key to overcome the experimental hurdles of current
techniques.
step, providing a more suitable platform for tracer develop-
ment (Figure 1). One chemical reaction that can achieve this
is the copper-catalyzed reaction of azides, alkynes, and
Figure 1. Reaction of a fluorescent group (red), a radioactive element
(yellow), and a group for bioconjugation (blue) to yield a dual-labeling
reagent. Subsequent reaction with biomolecules (green) provides dual
optical and nuclear tracers.
electrophilic iodine to give 5-iodo-1,2,3-triazoles.[8,9] Iodine
is attractive for multiscale imaging as its numerous radio-
isotopes enable a wide range of applications: the low-energy
g-emission of 125I (t1/2 = 60 days) for autoradiography, the
more energetic g-rays of 123I (t1/2 = 13.2 h) for single photon
emission computed tomography (SPECT), b particles from
131I (t1/2 = 8.0 days) for radioimmunotherapy (RIT), and b+
from 124I (t1/2 = 4.18 days) for positron emission tomography
(PET).
We demonstrated our new approach to multifunctional
labeling reagents by using the chemical route depicted in
Scheme 1. Rhodamine B was chosen as the fluorescent group
as it has excellent properties for imaging, including broad
fluorescence in the red spectrum of visible light, high
quantum yield, and good photostability.[10] The alkyne-
We envisaged that a three-component reaction combining
a fluorescent group, a radioactive element, and a group for
bioconjugation could yield dual-labeling reagents in a single
[*] Dr. R. Yan, Dr. E. ꢀrstad
Institute of Nuclear Medicine and Department of Chemistry
University College London
235 Euston Road (T-5), NW1 2BU, London (UK)
Fax: (+44)207-679-2344
E-mail: e.arstad@ucl.ac.uk
Dr. E. El-Emir, Dr. V. Rajkumar, M. Robson, Dr. A. P. Jathoul,
Prof. R. B. Pedley
UCL Cancer Institute, University College London
72 Huntley Street, London WC1E 6BT (UK)
[**] We acknowledge support by Cancer Research UK (R.B.P., M.R.),
European Union Seventh Framework Programme Grant 201342,
and ADAMANT (E.El-E.) and by King’s College London, UCL
Comprehensive Cancer Imaging Centre CR-UK, and EPSRC, in
association with the MRC and DoH (England) (V.R.). This work was
undertaken at UCLH/UCL which is funded in part by the Depart-
ment of Health’s NIHR Biomedical Research Centres funding
scheme (E.ꢀ., R.Y.).
Scheme 1. Synthesis of the non-radioactive labeling reagent 3.
Reagents: a) oxalyl chloride, CH2Cl2; b) N-methylpropargylamine,
Na2CO3, CH2Cl2; c) CuI, NIS, TEA, CH3CN.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102072.
Angew. Chem. Int. Ed. 2011, 50, 6793 –6795
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6793

Communications
functionalized rhodamine derivative 1 was obtained in 72%
overall yield by treatment of rhodamine B with oxalyl
chloride, and reaction of the resulting acid chloride with N-
methylpropargylamine. 4-Azidomethyl-N-succinimidyl ben-
zoate (2) was prepared in 65% yield in two steps from 4-
chloromethylbenzoic acid as previously described.[11] Cou-
pling of the two “click partners” 1 and 2 in the presence of
CuI, N-iodosuccinimide (NIS), and triethylamine (TEA)
provided the nonradioactive labeling reagent 3 in 92% yield.
For the radiochemical reaction we carried out a series of
experiments with the aim to identify a new catalytic system
that would allow formation of [125I]-3 directly from aqueous
non-carrier-added (n.c.a.) [125I]NaI. Initially, we used N-
bromosuccinimide (NBS) to oxidize n.c.a. [125I]NaI and
treated the resulting mixture with solutions of CuCl, TEA,
rhodamine 1 and azide 2 in DMF (Table 1, entry 1). Under
Figure 2. HPLC trace of labeling reaction mixture (Table 1, entry 6)
showing radioactivity (positive trace) and UV absorption at 254 nm
(negative trace) Azide 2 at 5.47 min, alkyne 1 at 9.90 min, and [125I]-3
at 13.4 min. Non-radioactive side products at 10.62, 12.24, and
19.59 min.
Table 1: Optimization of the radiochemical reaction conditions.
sharp drop in yields, whereas 1 equivalent gave the desired
product only in trace amounts (Table 1, entries 7 and 8).
For proof-of-concept imaging studies we coupled our
dual-labeling reagent [125I]-3 to the carcinoembryonic antigen
(CEA) specific antibody A5B7 (150 kDa). CEA is expressed
by most gastrointestinal tumors, and A5B7 and its fragments
are in regular preclinical and clinical use.[12–14] Antibody
labeling was achieved by incubating a solution of A5B7 with a
mixture of the non-radioactive labeling reagent 3 (20 equiv)
and [125I]-3 (20 MBq) at room temperature for 1 hour. The
resulting solution was purified with a size-exclusion column to
give the dual-labeled imaging agent [125I]-3/A5B7 in 22%
RCY with an average of six to eight fluorescent groups per
antibody.[125I]-3/A5B7 was subsequently evaluated in mice
bearing human colorectal xenografts.[13] The isotype-matched
dual-labeled antibody [125I]-3/MOPC was included as a
negative control. The overall organ distribution of [125I]-3/
A5B7 (see Figure S7 in the Supporting Information) was
similar to that of the antibody when it was labeled with 125I
alone using the Chloramine-T method.[13,15] The tumor uptake
of [125I]-3/A5B7 24 h and 48 h after injection was (13.9 Æ
4.5)% IDgÀ1 (IDgÀ1: injected dose per gram of tissue) and
(12.3 Æ 1.3)% IDgÀ1, respectively. Good clearance was
observed for all organs apart from the liver, which had an
increased uptake, (6.99 Æ 0.16)% IDgÀ1 at 48 h). Interest-
ingly, the increased uptake by the liver was accompanied by a
more rapid blood clearance, resulting in tumor-to-blood ratios
of 3.5:1 and 4.8:1 at 24 h and 48 h postinjection, respectively.
In contrast, the tumor uptake of [125I]-3/MOPC was (1.10 Æ
0.02)% IDgÀ1 at 48 h postinjection, with a tumor-to-blood
ratio of 0.28:1. Frozen tumor sections were used to visualize
antibody localization across the whole tumor 6 h after
injection of 50 mg of [125I]-3/A5B7. The distribution of radio-
activity (Figure 3A) was in good agreement with the fluores-
cence signal (Figure 3B), and demonstrated antibody uptake
across the viable areas of the tumor (Figure 3D). High-
magnification fluorescence imaging (Figure 3C) showed that
the antibody was associated mainly with tumor cells around
perfused blood vessels, but also had started to diffuse away
from vessels into the tumor mass at this early time post-
injection.
Entry Solvent/water Catalyst system
10:1
Radiochemical yield[e]
1[a]
2[a]
3[a]
4[a]
5[b]
6[b]
DMF
DMF
DMF
MeCN
MeCN
MeCN
CuCl/NBS/TEA (1:1:1.5) 0%
CuCl/CuCl2/TEA(1:1:2.2) 21%
CuCl2/TEA (1:1.5)
CuCl2/TEA (1:1.5)
CuCl2/TEA (1:1.5)
CuCl2/TEA (1:1.5)
14%
(33Æ15)% (n=3)
(56Æ7)% (n=3)
(80Æ3)% (n=8)[c]
(72Æ4)% (n=5)[c,d]
13%
7[b]
8[b]
MeCN
MeCN
CuCl2/TEA(1:2.5)
CuCl2/TEA(1:1.0)
<5%
[a] A solution of rhodamine 1 (1 mmol) and azide 2 (1 mmol) was added
to a mixture of the copper catalyst, TEA, and [125I]NaI and left to react for
30 min. [b] A solution of rhodamine 1 (1 mmol), CuCl2 (1 mmol), and TEA
was added to a mixture of azide 2 (1 mmol) and [125I]NaI and left to react
for 30 min. [c] Reaction time 90 min. [d] RCY of isolated product. [e] RCY
is a mean value of n experiments Æstandard deviation; when not
specified n=1.
these conditions no radiochemical reaction was observed.
When investigating other oxidizing reagents we discovered
that the use of CuCl2 led to formation of the desired product
[
125I]-3 in 21% analytical radiochemical yield (RCY) within
30 min at room temperature (Table 1, entry 2). Encouraged
by these results we attempted to use CuCl2 as the sole source
of copper. In DMF the reaction proceeded to give [125I]-3 in
14% analytical RCY, whereas moderate to good yields were
obtained in acetonitrile (Table 1, entries 3 and 4). The
addition sequence was found to influence both the efficiency
and reproducibility of the reaction, and when alkyne 1 was
combined with CuCl2 and TEA before addition to [125I]NaI,
[
125I]-3 was consistently obtained in good analytical yield
(Table 1, entry 5). Increasing the reaction time to 90 min
increased the analytical RCY to (80 Æ 3)% (Table 1, entry 6;
Figure 2). Following purification of the reaction mixture with
radio-HPLC,[125I]-3 was isolated in (72 Æ 4)% RCY with
excellent radiochemical purity (> 98%). When we started
with 20–25 MBq of [125I]NaI, the specific activity of [125I]-3
was in the range of 2–3 GBqmmolÀ1. Further investigation of
the reaction revealed that the ratio of TEA to CuCl2 was
critical. The use of more than 2 equivalents of TEA led to a
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Angew. Chem. Int. Ed. 2011, 50, 6793 –6795

tained [125I]-3 was diluted with water (12 mL), the solution was passed
though a Sep-Pak C18 light cartridge (Waters), the cartridge was
washed with water (5 mL), and the radioactive product was eluted
with acetonitrile (0.5 mL). The solvents were removed by a stream of
nitrogen prior to labeling.
Antibody labeling:
A solution of non-radioactive labeling
reagent 3 (62 mg, 20 equiv) in DMF (13 mL) was added to [125I]-3
(20 MBq) followed by a solution of A5B7 (507 mg) in phosphate
buffer (130 mL, pH 7) and sodium carbonate buffer (130 mL, 1.0m,
pH 9.0). The reaction mixture was incubated at room temperature for
one hour and purified using a PD MiniTrap G-25 column (GE
Healthcare) following the manufacturerꢁs instructions. The dual-
labeled [125I]-3/A5B7 (4.74 MBq, six to eight fluorescent groups per
antibody) was collected in a total volume of 0.8 mL. The recovery
efficiency of the purification step was determined to be (80 Æ 7)%
(n = 3) in separate experiments. Under the conditions used the dual-
labeling reagent [125I]-3 was completely retained on the size-exclusion
column. The number of fluorescent groups incorporated was calcu-
lated by taking into account the specific activity of the dual-labeling
reagent [125I]-3, and correcting the RCY for residual activity in the
reaction vial and loss of the antibody on the size-exclusion column.
Figure 3. A) Radioluminograph (color bar shows counts per pixel from
0 (blue) to 6000 (white)) and B) fluorescence image of adjacent tumor
sections demonstrating uptake of [125I]-3/A5B7 6 h postinjection of
50 mg antibody; C) high-power image; red: distribution of [125I]-3/A5B7
antibody; blue: Hoechst staining showing perfused blood vessels;
green: CD31 staining showing blood vessel distribution. D) H&E
(haematoxylin and eosin) staining showing tumor histology: viable
tumor (V); necrosis (N). Colored arrows (color definitions in (A))
indicate that the distribution of radioactivity colocalizes with the
fluorescence signal (B) and demonstrates antibody uptake in viable
tumor (D).
Received: March 23, 2011
Published online: June 14, 2011
Keywords: click chemistry · fluorescence · imaging agents ·
.
iodotriazoles · radiochemistry
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multiscale imaging based on the one-pot formation of dual
optical and nuclear labeling reagents. The concept has been
demonstrated using a highly efficient and rapid CuII-mediated
three-component radiochemical reaction that yields 5-
[
125I]iodo-1,4,5-trisubstituted-1,2,3-triazoles directly from
aqueous
[
125I]NaI. The method reported is technically
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Experimental Section
Synthesis of [125I]-3: A solution of copper(II) chloride (134 mg,
1.0 mmol) and triethylamine (151 mg, 1.5 mmol) in acetonitrile (40 mL)
was added to rhodamine 1 (530 mg, 1.0 mmol). After 5 min the
resulting solution was added to a mixture of the azide 2 (275 mg,
1.0 mmol) in acetonitrile (20 mL) and [125I]NaI (20–40 MBq) in water
(6.0 mL). After 90 min the reaction mixture was diluted with water
and acetonitrile (10:1, 1.0 mL) and the resulting solution was purified
by HPLC using a ZORBAX column (300SB-C18, 9.4 ꢀ 250 mm,
5 mm) with the following eluent: water (0.1% formic acid) as
solvent A and methanol (0.1% formic acid) as solvent B, going
from 60% of B to 70% of B in 30 min and going back to 60% of B in
2 min and remaining at 60% of B for an additional 3 min with a flow
rate of 3 mLminÀ1. The retention time of the compound [125I]-3 was
13.40 min. The labeled compound co-eluted with the non-radioactive
reference compound. For antibody labeling the fraction that con-
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A. Green, R. H. Begent, R. B. Pedley, Nucl. Med. Biol. 2009, 36,
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Angew. Chem. Int. Ed. 2011, 50, 6793 –6795
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6795