"Click to chelate": Synthesis and installation of metal chelates into biomolecules in a single step

Article abstract of DOI:10.1021/ja066779f

Click chemistry has been employed for the assembly of novel and efficient triazole-based multidentate chelating systems while simultaneously attaching them to molecules of biological interest. The "click-to-chelate" approach offers a powerful new tool for

Full text of DOI:10.1021/ja066779f

Published on Web 11/03/2006
“Click to Chelate”: Synthesis and Installation of Metal Chelates into
Biomolecules in a Single Step
Thomas L. Mindt,† Harriet Struthers,§ Luc Brans,‡ Todor Anguelov,§ Christian Schweinsberg,§
Veronique Maes,‡ Dirk Tourwe´,‡ and Roger Schibli*,†,§
Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, Switzerland, Center for
Radiopharmaceutical Science, ETH-PSI-USZ, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland, and
Faculty of Sciences, Vrije UniVersiteit Brussels, 1050 Brussels, Belgium
Received September 24, 2006; E-mail: roger.schibli@pharma.ethz.ch
Scheme 1 a
The radiolabeling of biologically active molecules has become
an indispensable tool for the assessment of novel drug candidates.
To keep pace with the growing number of new targets, innovative
and efficient methodologies are needed for the firm attachment of
molecules of biomedical interest to readily available radionuclides
with suitable decay characteristics.
In recent years, new cores of technetium-99m (T1/2 ) 6 h, 140
keV γ-radiation) have been explored for the radiolabeling of
biomolecules.1 The organometallic precursor [Tc(OH2)3(CO)3]+ is
a prominent example.2 A variety of bifunctional tridentate ligand
systems (e.g., based on amino acid scaffolds like cysteine, lysine,
and histidine) have been designed for the tricarbonyl precursor.3-6
However, the preparation of such chelators generally requires
multiple-step syntheses, and their incorporation into biomolecules
often lacks efficiency and is complicated by cross-reactivity with
other functional groups present. To circumvent the problems
a Conditions: (a) Cu(OAc)2, Na(ascorbate), water, 25 °C (15 h) or 100
°C (30 min); (b) M ) 99mTc: [99mTc(OH2)3(CO)3]+, PBS, pH 7.4, 30 min,
100 °C; M ) Re: [ReBr3(CO)3]2-, water or alcohols, 50-65 °C, 1-4 h.
associated with current strategies for functionalizing (bio)molecules
with metal chelators, we set out to investigate the use of Sharpless’
click chemistry in this context.7,8
Click chemistry (the Cu(I)-catalyzed [3 + 2] cycloaddition of
alkynes and azides forming stable 1,2,3-triazole linkages) meets
the requirements of an innovative functionalization strategy for bio-
molecules because it is efficient, selective, and devoid of side reac-
tions. The mild reaction conditions are well suited for the modi-
fication of a wide variety of (bio)molecules, into which the required
azide and alkyne functionalities can be incorporated by standard
synthetic transformations or biochemical methods.9 In addition, we
recognized that 1,4-disubstituted 1,2,3-triazoles share structural and
electronic features with 1,4-disubstituted imidazoles of Nꢀ-deriva-
tized histidines, which have been shown to be extraordinarily good
Figure 1. Derivatives of biomolecules functionalized via click chemistry
with a tridentate 1,2,3-triazole containing metal chelator.
chelators, particularly for organometallic cores of Mo, Tc, and
Re.10-12 Surprisingly few examples of 1,2,3-triazole chelators
obtained via click chemistry are reported,13,14 even though triazoles
Unlike the multistep syntheses required for the preparation of
are known to be potent ligand systems for various transition
Nꢀ-derivatized histidines,18 the click strategy avoids protective
metals.15
groups, which allowed the synthesis of optically pure histidine-
To probe the viability of a “click-to-chelate” approach, various
like, 1,4-functionalized triazoles 5-8 and 10-12 in a single step
commercial and noncommercial azide and alkyne derivatives were
and in quantitative yields.19 These features are particularly appealing
reacted with either L-propargyl glycine (1) or L-azido alanine (2)16
to form 1,2,3-triazole-4-yl alanines 5/6 (referred to as a “regular”
click ligand) or the isomeric products 7/8 (“inverse” click ligand,
for the functionalization of biomolecules such as carbohydrates with
polydentate metal chelates, which is notoriously inefficient using
common synthetic strategies as we and others have experienced.20,21
Scheme 1). In addition, representatives of four different classes of
The azido bombesin derivative was prepared and clicked on solid
biomolecules (tumor affine peptide bombesin 9, thymidine 10,
support22 to afford product 9.
carbohydrate 11,17 and phospholipid 12) were selected to demon-
Reaction of ligands 5-12 with [ReBr3(CO)3]2- in aqueous or
strate the versatility of the new approach (Figure 1).
alcoholic media yielded the well-defined neutral complexes
1
[Re(CO)3(5-12)] (Scheme 1). H and 13C NMR analyses indi-
† ETH Zurich.
cate that metal chelation occurs via N(3) (regular click) or N(2)
(inverse click) of the 1,2,3-triazole heterocycle, the NR-amine, and
§ Paul Scherrer Institute.
‡ Vrije Universiteit Brussels.
9
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J. AM. CHEM. SOC. 2006, 128, 15096-15097
10.1021/ja066779f CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
We have shown that click chemistry both simplifies the synthesis
of efficient bifunctional ligands in which 1,4-disubstituted triazoles
form an integral part of the metal chelating system and facilitates
their incorporation into (bio)molecules. Labeling of the chelators
and bioconjugates with fac-“Tc/Re(CO)3” is efficient and results
in complexes that are stable in vitro and in vivo. The one-pot
procedure represents a remarkable improvement for the synthesis
of metal-labeled conjugates, for example, for diagnostic purposes.
Extension of the “click-to-chelate” approach to the preparation of
structurally diverse ligand systems suitable for complexation of
other metals is currently being investigated.
Figure 2. Labeling capacities of ligands 5-8 and Nꢀ-methyl histidine (Me-
His) with [99mTc(OH2)3(CO)3]+.
Acknowledgment. We thank Judith Stahel and Elisa Garcia-
Garayoa for technical assistance, and Jason Holland for DFT.
Scheme 2. One-Pot Procedure to Yield Radiolabeled Conjugates
Directly
Supporting Information Available: Experimental procedures and
analytical data for all compounds, DFT calculations, and in vitro and
in vivo data for 99mTc-labeled bombesin derivative 9. This material is
available free of charge via the Internet at http://pubs.acs.org.
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isolation of the functionalized (bio)molecule prior to labeling with
[M(OH2)3(CO)3]+. Thus, aqueous solutions of 3′-azido thymidine
or 1-azido-1-deoxy-â-D-galactopyranose and alkyne 1, Cu(OAc)2,
and Na(ascorbate) were heated to 100 °C for 30 min. [99mTc(OH2)3-
(CO)3]+ was added, and the mixtures were heated for further 30
min (Scheme 2). HPLC analysis confirmed the clean formation of
complexes [99mTc(CO)3(10)] and [99mTc(CO)3(11)], identical to the
products obtained with presynthesized and purified ligands 10 and
11.19
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The radiolabeled bombesin derivative [99mTc(CO)3(9)] was
assessed in vitro and in vivo for its stability and receptor affinity.19
These preliminary experiments revealed a high in vivo stability of
the conjugate and an almost identical pharmacological profile to
3064.
(23) The identity of the technetium species was verified by comparison of the
γ-HPLC trace (99mTc) with the UV-HPLC trace (254 nm) of the
corresponding Re complexes. See Supporting Information.
(24) We were able to show that [99mTc(CO)3(5-12)] can be quantitatively
synthesized directly, by reacting 5-12 in presence of [99mTcO4]- in a
vial containing the IsoLink components necessary for formation of the
[
99mTc(CO)3NRAcHis-bombesin],25 a previously tested stable bomb-
[
99mTc(OH2)3(CO)3]+. See Supporting Information for details.
esin analogue with sequence homology but a histidine chelate
(detailed results will be published elsewhere). These results suggest
that the new triazole ligands represent a valuable alternative to
histidine-derived chelators.
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