COmplexes of LANTHAnide ions for radical redox sensing – Co-lantha

Our approach consists in using lanthanide cations for imaging. These metal ions exhibit remakable properties, widely used for biomedical imaging: The gadolinium(III), with its 7 unpaired electrons is a major contrast agent in MRI. The europium cation is luminescent and found in our screens, while the ytterbium(III) is very promising for NIR fluorescence imaging.
The redox imaging implies that the agents must change of oxidation state. However most of the lanthanides exhbit a single oxidation state (+III) in physiological conditions. In other terms they cannot be used, in principle, for redox imaging. Our approach consists in associating a redox-active ligand with a lanthanide cation. We hypothesize that a change in the redox state of the ligand will influence the coordinating ability, the electronic levels of the ligand, as well as the relaxivity of the coordinated water molecules (is applicable). Hence the complex will be sensitive to the redox status and its response should be detectable by conventional imaging equipments. In this project polydentate pro-phenoxyl, pro-iminosemiquinonate and nitroxide ligands will be prepared and chelated to a series of lanthanide ions: TbIII, EuIII for luminescence detection in the visible region, NdIII and YbIII for a luminescence in the NIR region and GdIII for application as contrast agents.

We focused our efforts on ligands based on the macrocyclic DOTA platforms. It is indeed the closest to Dotarem®, which is a clinically approved contrast agent. We optimized the synthesis of redox-active precursors based on nitroxide or pro-nitroxide units, combined with sensitizing groups and used them as building blocks for preparing DOTA-based ligands. The lanthanide complexes (Ln = Gd, Eu, Yb) were prepared and fully characterized. We established that the sensitizing unit, when present, acts as a antenna, allowing Ln-based luminescence. The electrochemical investigation confirmed the correct range of operating redox potentials. We therefore investigates the response of the probes to a change in oxidation state. Depending on the ligand we observed distinct changes of fluorescence upon formation of the radical. It is in general a quenching and its ranges between 95 % and 15 % for both the visible and NIR emitters. We also confirmed that radical formation has a dramatic impact on the relaxivity, with changes up to 40 %.
Finally, we observed a remarkable reactivity of hydroxylamine/nitrone towards ROS and designed a dual mode (EPR/luminescence) probe for detecting the hydroxyl radical.

We envision investigating in details the physicochemical properties of the compounds under various oxidation states. This fundamental study is indispensable for rationalizing the behaviour and design new generations of probes. We will also test other redox active moieties and functionalize the probes for adressing them towards biological targets (cells, vectorization). The coupling with peptides is the solution that we will use. All these results will be valorized by biological studies and of course imaging.

1. J. K. Molloy, C. Philouze, L. Fedele, D. Imbert, O. Jarjayes and F. Thomas, «Seven-coordinate lanthanide complexes with a tripodal redox active ligand: structural, electrochemical and spectroscopic investigations« Dalton Trans., 2018, 47, 10742-10751 [on invitation, special issue “Europe: New Talents 2018”]
2. J. K. Molloy, L. Fedele, O. Jarjayes, C. Philouze, D. Imbert and F. Thomas, «Structural and spectroscopic investigations of redox active seven coordinate luminescent lanthanide complexes« Inorg. Chim. Acta, 2018, 483, 609-617

This project targets the synthesis of lanthanide(III) complexes with redox-sensitive ligands for monitoring of the redox status. A major downstream application is the non-invasive preclinical and biomedical imaging. We hypothesize that a change in the redox state of the ligand will influence both the lanthanide-based luminescence and the relaxation times (coordinated water molecules) for magnetic resonance imaging (MRI). Thus, the ligand will be the probe of the redox-status and the lanthanide will be the reporter. Polydentate ligands appended by pro-phenoxyl, pro-iminosemiquinonate and nitronyl nitroxide units will be prepared and chelated to a series of lanthanide ions: TbIII, EuIII for luminescence detection in the visible region, NdIII and YbIII for a luminescence in the NIR region and GdIII for application as contrast agents. The stability constants of the complexes will be determined and structural characterizations will be conducted. With the aim of improving the solubility and targeting the ligands will be appended by cell penetrating peptides or integrin recognition peptides. The operating potentials of the probes will be next investigated by electrochemical techniques (cyclic voltammetry, electrolysis). The complexes under their different redox states (radical or not) will be prepared both electrochemically and in vitro with biologically relevant reactants, and characterized by luminescence and relaxometric techniques. Having established that the oxidation state of the complexes could be monitored by luminescence and relaxivity, biological studies will be undertaken. They include MTT assays in order to ensure that the probes are non-toxic. Further in cellulo luminescence measurements (2D/3Dfluorescence microscopy) and relaxivity studies will be conducted on cells under various redox status (oxidative stress or hypoxia) for assessing the sensitivity of the probes. Finally, a thorough investigation of the magnetic coupling between the ligand radical and the lanthanide ions will give us major insight onto the operating mode of the complexes.