.
Angewandte
Communications
DOI: 10.1002/anie.201303108
Peptidomimetics
1,2,3-Triazoles as Amide Bond Mimics: Triazole Scan Yields Protease-
Resistant Peptidomimetics for Tumor Targeting**
Ibai E. Valverde, Andreas Bauman, Christiane A. Kluba, Sandra Vomstein, Martin A. Walter,
and Thomas L. Mindt*
The targeted delivery of diagnostic and therapeutic agents to
tumors and metastases has emerged as a promising strategy
for the management of cancer.[1] The functionalization of
cytotoxic agents and imaging probes with targeting moieties
(vectors) enables their specific delivery to tumors with an
efficiency higher than in nonconjugated form and may thus
reduce side effects and improve imaging, respectively.[2]
Ideally, a vector should be readily synthesized, offer the
possibility for conjugations, exhibit an appropriate stability,
have a well-defined structure, and display nanomolar affinity
towards a cell surface recognition element (e.g., receptors),
which is overexpressed by tumors but not in nontargeted
tissue.[3] Some naturally occurring, synthetically accessible
small molecules fulfill these criteria and have found applica-
tion in the clinic. For example, cyclic peptides (octreotide,[4]
RGD[5]) labeled with diagnostic or therapeutic radionuclides
are currently used in nuclear medicine. On the other hand, the
employment of linear peptides (e.g., bombesin, neurotensin)
in this context has been hampered despite their high potential
in part due to their low stability in vivo.[6] Considerable
research efforts have been made in the past in order to
stabilize such peptides while not impacting their favorable
biological properties, however, with varying degree of suc-
cess.[6a] Therefore, new approaches for the metabolic stabili-
zation of tumor-targeting peptides are needed.
such backbone modifications on the stability of the peptide.
With the goal to develop a tumor-targeting peptidic vector
with retained high receptor affinity but improved stability, we
set out to replace systematically amide bonds of linear
peptides with 1,4-disubstituted 1,2,3-triazoles by the CuI-
catalyzed azide–alkyne cycloaddition (CuAAC)[7a,9] and study
the effect of the structural changes. For proof of concept of
the new methodology (termed a triazole scan), we chose the
minimal binding sequence of the peptide bombesin (BBN(7-
14)). This short octapeptide (H-QWAVGHLM-NH2) is
a high-affinity agonist of the gastrin-releasing peptide recep-
tor (GRPr), which is overexpressed in a variety of clinically
relevant tumors including, prostate and breast cancer.[10]
BBN(7-14) undergoes cell internalization by endocytosis
upon activation of the GRPr and thus is widely studied for
applications in drug delivery and nuclear oncology.[11] BBN(7-
14) represents not only a good model peptide to illustrate the
potential of this novel stabilization technique but also
a challenge as other backbone-modification strategies (e.g.,
cyclization, carbonyl reduction, N-methylation) have had
only moderate success in providing bombesin analogues with
maintained affinity towards GRPr.[12]
We are particularly interested in BBN(7-14) as a tumor-
targeting vector for the development of novel radiopharma-
ceuticals using radioactive metals (radiometals) in a theranos-
tic approach.[13] For this purpose, the peptide can be
functionalized N-terminally through a spacer unit with an
appropriate chelator for complexation of a radiometal.In this
work, we used the universal macrocyclic chelator 1,4,7,10-
tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA),
a short hydrophilic tetraethylene glycol (PEG4) spacer, and
lutetium-177 (177Lu) as a clinically established therapeutic
radionuclide with a concomitant g-emission for imaging.[14] In
addition, the methionine residue in position 14 of the amino
acid sequence was replaced by norleucine to avoid the
formation of oxidation side products during radiolabeling.[15]
These modifications have all been reported to be tolerated by
the peptide.[16]
It has previously been shown that 1,4-disubstituted 1,2,3-
triazoles can effectively mimic trans-amide bonds because of
their similar size, planarity, H-bonding capabilities, and dipole
moment.[7] It has also been suggested that the replacement of
an amide bond by a 1,2,3-triazole isostere could afford
protease-resistant peptidomimetics.[7a,h,8] However, to the best
of our knowledge, no study has yet reported the influence of
[*] Dr. I. E. Valverde, Dr. A. Bauman, C. A. Kluba, S. Vomstein,
Prof. Dr. T. L. Mindt
Department of Radiology and Nuclear Medicine
Division of Radiopharmaceutical Chemistry
University of Basel Hospital
Petersgraben 4, 4031 Basel (Switzerland)
E-mail: thomas.mindt@usb.ch
The peptide analogues described herein were synthesized
by a solid-phase approach, which includes the CuAAC
reaction. Amino alkyne and azido acid building blocks were
prepared following reported procedures (Scheme 1). In brief,
chiral Fmoc-protected amino alkynes were obtained by
reduction of amino acid derived Weinreb amides followed
by a Seyferth–Gilbert homologation of the in situ generated
a-amino aldehydes using the Bestmann–Ohira reagent.[17]
The enantiomeric purity of the alkyne building blocks was
verified in each case.[18] Azido acids were synthesized from
amino acids with retention of chirality by diazo-transfer
Dr. M. A. Walter
Institute of Nuclear Medicine, Bern University Hospital
Freiburgstrasse 4, 3010 Bern (Switzerland)
[**] This work was supported by the Swiss National Science Foundation
(grant no. 132280) and the Nora van Meeuwen-Hꢀfliger Founda-
tion. We thank A. Mascarin and E. Huxol for technical assistance
and Dr. I. Muckenschnabel (Novartis, Basel, Switzerland) for
assistance with MS analysis.
Supporting information for this article is available on the WWW
2
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
These are not the final page numbers!
Valverde, Ibai E.
