Metal complexes and MRI

This thematic team integrates expertise in probe chemistry and small animal MR imaging.

A. Probe chemistry

Our chemistry is devoted to the design, synthesis and characterization of innovative probes based on metal complexes for applications in magnetic resonance imaging. We apply coordination chemistry concepts to create molecular MR imaging probes. We are also interested in fundamental aspects, such as optimizing complex stability, kinetic inertness and understanding MRI efficiency of paramagnetic systems. We cover various steps of probe development: (i) design and synthesis of novel ligands, (ii) full physico-chemical characterization of the complexes, involving the determination of individual microscopic parameters that govern relaxivity or chemical exchange saturation transfer (CEST), (iii) assessment of stability and kinetic inertness related to in vivo toxicity, (iv) in vitro/in vivo evaluation of the agents.

Major chemistry projects involve:

Mn-based MRI probes. Manganese is an essential element and Mn²⁺ has high electron spin (S = 5/2) and slow electronic relaxation, thus Mn²⁺ complexes represent good alternatives to replace Gd³⁺-based MRI contrast agents currently used in the clinics. In this context, we have explored open-chain and macrocyclic ligands for Mn²⁺ complexation (Inorg. Chem. 2017, 56, 7746-7760, New J. Chem. 2018, 42, 8012 – 8020, J. Med. Chem. 2020, 63, 6057-6065; collab. G. Tircso, Univ. Debrecen, Hungary). Recently, we have shown that highly rigid and preorganized bispidine ligands are well adapted to chelate Mn²⁺. We have identified Mn²⁺ bispidine complexes with excellent MRI properties and unprecedented kinetic inertness (Angew. Chem. Int. Ed. 2020, 59, 11958-11963, collab. L. Charbonnière, A. Nonat, IPHC Strasbourg) or exceptional thermodynamic stability and Mn²⁺/Zn²⁺ selectivity (Angew. Chem. Int. Ed. 2022, coll. P. Comba, Univ. Heidelberg).

Smart MRI contrast agents for Zn²⁺ detection. Gd³⁺ complexes are designed via a versatile approach, including Gd³⁺ chelates linked to DPA derivatives, which possess an appropriate affinity for Zn²⁺, through spacers of different sizes. Our aim is to decipher the microscopic parameters governing the Zn²⁺-dependent MRI response of these systems allowing their further optimization. These complexes have an optimal response at intermediate magnetic fields but thanks to the FFC-MRI technique (Fast-field-cycling MRI, collab. H. Scharfetter, Gräz Universität, Austria), we could detect Zn²⁺ at higher fields, where the resolution is better (Chem. Eur. J. 2014, 20, 10959 – 10969 ; Coord. Chem. Rev. 2018, 369, 91 ; Pharmaceuticals 2020,13, 436 ; Chem. Eur. J. 2019, 25, 8236-8239).

Peptidic systems for Zn²⁺ detection. Zinc fingers are small protein domains which complex Zn²⁺ with high affinity and good selectivity vs other physiological cations. The peptide folds upon Zn²⁺ and becomes more rigid. By cleverly linking a Gd³⁺ chelate to this peptide, we obtained the first peptidic MRI contrast agent sensitive to Zn²⁺, which was successfully used for MRI detection of Zn²⁺. Thanks to the presence of a Tryptophan, which acts as an antenna to sensitize the luminescence of Tb³⁺, a bimodal contrast agent is obtained (Chem. Commun. 2018, 54, 7350-7353, collaboration O. Sénèque, LCBM Grenoble).

Quantitative detection of Zn²⁺. The quantification of any physiological parameter by MRI is complicated as it is necessary to know the local concentration of the contrast agent, which is not possible in vivo. We have decided to combine MRI (responsive technique) with SPECT (Single-Photon Emission Tomography ; quantitative technique) by using a radioactive Ln³⁺, which enables a similar biodistribution. In collaboration with I. Da Silva (CEMHTI, Orléans), ¹⁶⁵Er was produced in a cyclotron and separated from its father element (Ho), which is of paramount importance for a reliable quantification. It was the first time that ¹⁶⁵Er was produced in France. By using a cocktail of Gd³⁺ and ¹⁶⁵Er³⁺ adapted to each technique, a contrast agent sensitive to Zn²⁺ and a calibration curve, the in vitro proof-of-concept of Zn²⁺ quantification was performed. It was the first quantification method for physiological cations and this work was highlighted by Chem. Comm (Chem. Commun. 2018, 54, 7597-7600 ; collaboration I. Da Silva, CEMHTI Orléans and J. Sobilo et S. Lerondel, CIPA, Orléans).

Enzyme-responsive MRI agents. We work on MRI probes containing a self-immolative linker between the Ln³⁺ complex and the enzyme-specific substrate. Enzymatic cleavage initiates an electronic cascade that leads to structural changes in the complex, with concomitant relaxivity or PARACEST response. This platform has been extended to optical detection by incorporating a pyridine chromophore in the ligand and constitutes the first example where a single molecular system can be used as a responsive imaging agent in three different independent modalities (T₁-weighted and Chemical Exchange Saturation Transfer MRI, optical imaging). These results have been recently published (J. Am. Chem. Soc. 2016, 138, 2913−2916) and highlighted on the cover of JACS (collaboration with P. Durand, ICSN, and S. Petoud).

Detection of amyloid peptides. We have been long interested in benzothiazole-derivative macrocyclic complexes for multimodal imaging of amyloid peptides. Recently we evidenced that nanomolar binding affinities, unprecedented for a metal chelate, are possible to attain with both Ab_1-40 and amylin when the amyloid recognition unit is separated from the chelate by a long, hydrophobic C10 spacer. We also evidenced that different binding affinity regimes exist in solution as a function of GdL concentration, related to the presence of different micellar aggregates, and micellar aggregation of metal chelates should always be kept in mind when interactions with amyloid peptides are considered. (coll. C. Geraldes, Coimbra; C. Hureau, Toulouse, Chem. Eur. J. 2021, 27, 2009 – 2020, Nanotheranostics, 2018, 2, 168-183).

Theranostic probes. We are combining MRI and photodynamic therapy via molecular or nanoparticle approaches. Gd³⁺ chelates were conjugated to porphyrin-photosenzitizers, some for two-photon excitation. We showed that their chemical association can be beneficial for both the relaxation properties and the photosensitizer (collab. V. Heitz, Strasbourg; Chem. Eur. J. 2016, 22, 2775–2786, Inorg. Chem. 2016, 55, 4545–4554; Bioconj. Chem. 2018, 29, 3726−3738, Inorg. Chem. 2020, 59, 14389−14398). In a proof of concept X-ray induced PDT study, we took advantage of the X-ray excited luminescence of lanthanides to locally generate light in a micellar system comprising amphiphilic Ln-chelates and integrating hypericin as photosensitizer in its hydrophobic core. This circumvents intrinsic depth limitations of PDT and could be synergistically combined with classical radiotherapy and MRI tracking of the probe (collab. M. Réfrégiers, Nano Research, 2015, 8, 2373-2379).

Drug-loaded liposomes are typical examples of nanomedicine. We have shown that doxorubicin, the anti-cancer agent in the liposomal drug Doxil®, can sensitize Ytterbium (Yb³⁺) and generate its near infrared (NIR) emission. When doxorubicin and amphiphilic Yb³⁺ chelates are incorporated into liposomes, the sensitized emission of Yb³⁺ is dependent on the integrity of the particles which can be used to monitor drug release. We also demonstrated for the first time that the NIR Yb³⁺ emission signal is observable in living mice following intratumoral injection of the Yb³⁺-doxorubicin-liposomes, using a commercial macroscopic setup equipped with a near-infrared camera (coll. S. Petoud, C. Pichon, CBM, Angew. Chem. Int. Ed. 2021, 60, 23574-23577).

Targeted probes for multimodal Imaging. We are also interested in the development of peptide-based imaging probes that specifically target proteins which are biomarkers of early stages of disease. These proteins are often overexpressed, and thus ideal targets for imaging. In a nanoparticle approach we showed that gold nanoparticles simultaneously decorated with Gd³⁺ and the radioisotope ⁶⁷Ga, and carrying a bombesin, can be internalised by PC3 prostate tumors in vivo. Their Gd³⁺ content enables their detection by MRI, but also promotes therapeutic features together with gold via radiosensitisation (coll. MP. Campello, Lisbon, Materials 2020, 13, 513).

In a molecular approach, we have explored probes specific for tropoelastin, implicated in cardiovascular complications (coll. A. Phinikaridou & R. Botnar, London, Cardiovasc. Res. 2020, 116, 995–1005). An improved tetrameric version has recently been validated in an atherosclerotic murine model (coll. A. Phinikaridou & R. Botnar, London; G. Digilio & S. Aime, Turin, J. Med. Chem. 2021, 64, 15250-15261).