A new ditopic Gd(III) complex functionalized with an adamantyl moiety as a versatile building block for the preparation of supramolecular assemblies

Cyclodextrins: promising scaffolds for MRI contrast agents

Magnetic resonance imaging (MRI) is a powerful tool for non-invasive, high-resolution three-dimensional medical imaging of anatomical structures such as organs and tissues. The use of contrast agents based on gadolinium chelates started in 1988 to improve the quality of the image, since researchers and industry focused their attention on the development of more efficient and stable structures. This review is about the state of the art of MRI contrast agents based on cyclodextrin scaffolds. Chemical engineering strategies are herein reported including host-guest inclusion complexation and covalent linkages. It also offers descriptions of the MRI properties and in vitro and in vivo biomedical applications of these emerging macrostructures. It highlights that these supramolecular associations can improve the image contrast, the sensitivity, and the efficiency of MRI diagnosis by targeting cancer tumors and other diseases with success proving the great potential of this natural macrocycle.

Rigid and Compact Binuclear Bis-hydrated Gd-complexes as High Relaxivity MRI Agents

The first binuclear Gd‐complex of the 12‐membered pyridine‐based polyaminocarboxylate macrocyclic ligand PCTA was synthesized by C−C connection of the pyridine units through two different synthetic procedures. A dimeric AAZTA‐ligand was also synthesized with the aim to compare the relaxometric results or the two ditopic Gd‐complexes. Thus, the ¹H relaxometric study on [Gd₂PCTA₂(H₂O)₄] and on [Gd₂AAZTA₂(H₂O)₄]²⁻ highlighted the remarkable rigidity and compactness of the two binuclear complexes, which results in molar relaxivities (per Gd), at 1.5 T and 298 K of ca. 12–12.6 mM⁻¹ s⁻¹ with an increase of ca. 80 % at 1.5 T and 298 K (+70 % at 310 K) with respect to the corresponding mononuclear complexes.

Optimizing the relaxivity at high fields: systematic variation of the rotational dynamics in polynuclear Gd-complexes based on the AAZTA ligand

A homogeneous series of polynuclear Gd-complexes (n = 1–8) based on a stable and bis-hydrated [Gd(AAZTA)]− chelate shows high relaxivity values at high fields (1.5–7 T), per Gd, particularly pronounced for the more rigid and compact members.

Novel paramagnetic clays obtained through intercalation of Gd3+-complexes

Novel paramagnetic lanthanide-exchanged saponite clays, bearing in the interlamellar region positively charged Gd3+-complexes, were synthesized using a hydrothermal approach followed by a classical ion exchange reaction. A detailed characterization was performed to assess the physico-chemical properties of the samples, which showed a hydrodynamic diameter between 50 and 90 nm and good thermal stability. 1H-NMR relaxometric studies in aqueous solution as a function of magnetic field and temperature were carried out to evaluate the local chemical environment of the intercalated paramagnetic centres and their interaction with water molecules. The data indicate a strong interaction of the confined complexes with the lamellae, resulting in a restriction of the local rotational dynamics and of the exchange process of water molecules between the inner coordination sphere of Gd3+ and the bulk. The stability over time in aqueous solutions of varying complexity was also studied by relaxometric techniques.

Contrast agents for MRI: 30+ years and where are we going?

Thirty years ago, Schering filed the first patent application for a contrast agent for magnetic resonance imaging (MRI) covering the forefather of the gadolinium contrast agents and still the most widely used gadolinium probe: gadolinium(III) diethylenetriaminepentaacetate (Magnevist). To date, 11 contrast agents have been approved by the US Food and Drug Administration for intravenous use. Coordination chemists have done a great deal to move the field forward. Our understanding of lanthanide chemistry now makes possible the design of complexes with long rotational correlation times, fast or slow water-exchange rates, high thermodynamic stabilities, and kinetic inertness, leading to sensitive and nontoxic contrast agents. Chemists did not stop there. The last few decades has seen the development of novel classes of probes that yield contrast through different mechanisms, such as paramagnetic chemical exchange saturation transfer agents. Thirty years since the first patent, chemists are still leading the way. The development of high-sensitivity contrast agents for high magnetic fields, safe probes for patients with kidney disorders, and multimodal, targeted, and responsive agents demonstrates that the field of contrast agents for MRI still has much to offer.