Angewandte
Communications
Chemie
Antibiotic Conjugates
Hot Paper
Multivalent Siderophore–DOTAM Conjugates as Theranostics for
Imaging and Treatment of Bacterial Infections
Kevin Ferreira+, Hai-Yu Hu+, Verena Fetz+, Hans Prochnow, Bushra Rais, Peter P. Mꢀller, and
Abstract: There is a strong need to better diagnose infections at
deep body sites through noninvasive molecular imaging
methods. Herein, we describe the synthesis and character-
ization of probes based on siderophore conjugates with
catechol moieties and a central DOTAM scaffold. The
probes can accommodate a metal ion as well as an antibiotic
moiety and are therefore suited for theranostic purposes. The
translocation of the conjugates across the outer and inner cell
membranes of E. coli was confirmed by growth recovery
experiments with enterobactin-deficient strains, by the anti-
bacterial activity of ampicillin conjugates, and by confocal
imaging using a fluorogen-activating protein–malachite green
system adapted to E. coli. The suitability of the probes for
in vivo imaging was demonstrated with a Cy5.5 conjugate in
mice infected with P. aeruginosa.
(e.g., through biofilm formation) or a systemic spread of the
infection.[2]
The positron emission tomography (PET) agent
2-[18F]fluorodeoxyglucose (18F-FDG) is in clinical use, but
cannot differentiate between sterile inflammation and bacte-
rial infections. To address this issue, several probes for
bacterial imaging have recently been published that are
based on prothrombin,[3] maltodextrin,[4] sorbitol,[5] vancomy-
cin, or substrates for the micrococcal nuclease (MN) and for
the b-lactamases of M. tuberculosis.
Herein, we targeted the bacterial iron transport machi-
nery to monitor and treat infections. To satisfy their iron
demand, bacteria biosynthesize and secrete siderophores,
small molecules with a high affinity for iron that capture the
metal from the environment, followed by an active uptake of
the iron–siderophore complex.[6] Multiple transporters enable
bacteria to import not only endogenous siderophores, but also
so-called xenosiderophores, thereby guaranteeing iron supply
even in the absence of the endogenous molecules.[6b] Various
groups have exploited this property for the design of actively
transported antibacterial conjugates of an antibiotic effector
coupled to an iron-binding motif. Achieving reliable efficacy
towards adapting bacterial populations is seen as a major
challenge of this approach for treatment[7] and imaging[8]
applications.
We searched for a small-molecule scaffold that enables
bacterial imaging and treatment (i.e., theranostic)[9] applica-
tions at the same time by accommodating 1) an iron-binding
motif, 2) variable imaging modalities, and 3) an optional
effector antibiotic. The tetrapodal 1,4,7,10-tetraazacyclodo-
decane-1,4,7,10-tetraacetic amide (DOTAM) moiety was
selected[10] because it fulfills all functional requirements and
has various advantages such as straightforward synthetic
access, high solubility, low toxicity, and proven biocompati-
bility.[11] This scaffold has also been widely applied for the
chelation of metals, a property that is utilized in Ln-based
magnetic resonance imaging, 68Ga- or 64Cu-based nuclear
imaging, and 90Y-based radiotherapy.[12]
For siderophore-based bacterial imaging, three of the four
arms branching from the DOTAM core were functionalized
with catechol moieties for iron binding while the fourth arm
was coupled to a fluorophore through various linkers (Fig-
ure 1a). The imaging probes, antibiotics, and control com-
pounds 1–13 were prepared in three to eight steps from cyclen
(Scheme 1; see also the Supporting Information, Schemes S1–
S7). The iron-binding properties of the compounds were
determined by a colorimetric titration assay based on chrome
azurol S (CAS).[13] Whereas 1 bound one equivalent of ferric
I
nfections caused by pathogenic bacteria represent a major
health threat that is expected to aggravate further in the
future,[1] fueled by the facilitated spread of pathogens through
international mobility and an increase in modern medical
procedures that disrupt physical biological barriers and the
immune system. The fact that a growing proportion of
multidrug-resistant pathogens meet a declining pipeline of
novel antibiotic drugs raises serious concerns. There is also
a lack of methods to detect infections at sites that are
unknown or inaccessible for sampling; this concerns in
particular biomaterial-associated infections or endocarditis.
For this purpose, noninvasive imaging methods that can
detect early-stage infections are highly desirable as they
would enable timely interventions before local manifestations
[*] M. Sc. K. Ferreira,[+] Dr. H.-Y. Hu,[+] Dr. V. Fetz,[+] Dr. H. Prochnow,
Dr. B. Rais, Prof. Dr. P. P. Mꢀller, Prof. Dr. M. Brçnstrup
Department of Chemical Biology
Helmholtz Centre for Infection Research and German Centre for
Infection Research (DZIF)
Inhoffenstrasse 7, 38124 Braunschweig (Germany)
E-mail: mark.broenstrup@helmholtz-hzi.de
Dr. H.-Y. Hu[+]
State Key Laboratory of Bioactive Substances and Function of Natural
Medicine, Institute of Materia Medica, Chinese Academy of Medical
Sciences and Peking Union Medical College
1 Xiannongtan Street, 100050 (P.R. China)
Dr. V. Fetz[+]
School of Engineering and Science
Jacobs University Bremen (Germany)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
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