Synthesis and Physicochemical Characterisation of Gd-DTPA Derivatives as Contrast Agents for MRI

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.

Enhanced relaxivity of GdIII-complexes with HP-DO3A-like ligands upon the activation of the intramolecular catalysis of the prototropic exchange

The simple modification of the hydroxypropyl arm in Gd(HP-DO3A) complex allows to achieve an increased relaxivity by the activation of the intramolecular catalysis of the proton exchange process.

Recent advances in improving tumor-targeted delivery of imaging nanoprobes

Tumor-targeted delivery of imaging nanoprobes provides a promising approach for the precision imaging diagnosis of cancers. Nanoprobes with desired bio-nano interface properties can preferably enter tumor tissues through the vascular endothelium, penetrate into deep tissues, and detect target lesions. Surface engineering of nanoparticles offers a critical strategy to improve tumor-targeting capacities of nanoprobes. Improvements to the efficacy of targeted nanoprobes have been intensively explored and much of this work centers on the selection of suitable targeting ligands. Herein, in this review, various recent strategies based on different targeting ligands to improve tumor-targeting of imaging nanoprobes have been developed, ranging from small molecule ligands to biomimetic coatings, with highlights on emerging coating techniques using cell membranes and dual-targeting ligands. In particular, construction and surface modification methods, targeting capacities, and imaging/theranostic performance with key issues and potential questions have been described and discussed together with considerations for future development and innovations.

New polyaminocarboxylate macrocycles containing phenolate binding units: synthesis, luminescent and relaxometric properties of their lanthanide complexes

The synthesis of two new polyaminocarboxylate macrocycles incorporating one or two intracyclic phenol units (H₄L¹ and H₈L², respectively) is described. The 12-membered H₄L¹ macrocycle leads to soluble and stable mononuclear Ln^III complexes of [(L¹)Ln(H₂O)₂]^- composition (Ln = Eu, Tb and Gd) in aqueous solutions. In Tris buffer (pH 7.4), the [(L¹)Tb(H₂O)₂]^- complex displays a suitable efficiency for sensitized emission (η_sens = 48%) and a high luminescence quantum yield (Φ = 22%), which is worthy of note for a bis-hydrated terbium complex. Besides, luminescence experiments show that bidentate endogenous anions (citrate, carbonate, and phosphate) do not displace the two inner-sphere water molecules of this complex. In contrast, the possible presence of LMCT states causes the europium complex to be weakly luminescent. The [(L¹)Gd(H₂O)₂]^- complex is characterized by high relaxivity (r = 7.2 s^-1 mM^-1 at 20 MHz) and a very short water residence time of the coordinated water molecules (τ = 9 ns), promising values for the realisation of macromolecular systems with high relaxivities. Thus, the Tb and Gd complexes of the H₄L¹ macrocycle exhibit several improvements in terms of luminescent (lower excitation energy, higher brightness) and relaxometric (shorter τ_M) properties compared to the corresponding LnPCTA complexes, where a phenol moiety substitutes a pyridine ring. On the other hand, the 24-membered H₈L² macrocycle including two phenol units in its structure leads to dinuclear complexes of [(L²)Ln₂]^2- composition. Its terbium complex shows a long luminescence lifetime (2 ms) and a high quantum yield (43%) in aqueous solutions, making this compound a new promising candidate for time-resolved applications.

Dual modal imaging agents based on chromophore-bearing DTPA analogues

Two new DTPA analogues, centrally (L1) and terminally (L2) functionalised with a 1,8-naphthalimide chromophore, have been successfully prepared and fully characterized.

(1) H-NMR relaxometric studies of interaction between apoptosis specific MRI paramagnetic contrast agents and micellar models of apoptotic cells

(1) H-NMR was previously used to analyze the interaction between peptides (E3 and R826) selected by phage display to target apoptotic cells and phospholipidic models of these cells. In order to avoid the use of apoptotic cells and to obtain a fast evaluation of the efficiency of the potential MRI contrast agents obtained by grafting these peptides and their scramble analogs on a paramagnetic gadolinium complex, their proton relaxometric behavior was investigated in the presence of micelles mimicking healthy and apoptotic cells. Their preferential interaction with 1,2-dipalmitoyl-sn-glycero-3-phospho-l-serine micelles mimicking apoptotic cells as compared with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine micelles modeling healthy cells was shown by nuclear magnetic relaxation dispersion profiles and the enhancement of the transverse proton relaxation rates at 60 MHz. The association constant values confirm the stronger interaction of the selected conjugated peptides (Ka Gd-PMN-E3(gadolinium 2,2',2'',2'''-[((4-carboxy)pyridine-2,6-diyl)bis(methylenenitrilo)]-tetrakis acetate) grafted with E3 peptide): 2.43 10(4)  m(-1) ; Ka Gd-DTPA-R826(gadolinium ((1-p-isothiocyanatobenzyl)-diethylenetriaminepentaacetate) grafted with R826 peptide): 2.91 10(4)  m(-1) ) as compared with their conjugated scrambles (Ka Gd-PMN-E3sc(gadolinium 2,2',2'',2'''-[((4-carboxy)pyridine-2,6-diyl)bis(methylenenitrilo)]-tetrakis acetate) grafted with E3 scramble peptide): 0.18 10(4)  m(-1) ; Ka Gd-DTPA-R826sc(gadolinium ((1-p-isothiocyanatobenzyl)-diethylenetriaminepentaacetate) grafted with R826 scramble peptide): 0.32 10(4)  m(-1) ) even if the conjugation of E3 and R826 seems to decrease their interaction. Copyright © 2015 John Wiley & Sons, Ltd.

Synthesis of an Amphiphilic Bis-Aqua Gd(OBETA) Complex for the Preparation of High-Relaxivity Supramolecular Magnetic Resonance Imaging Probes

Prompted by the favourable relaxometric, thermodynamic and kinetic properties of the bis-hydrated Gd(OBETA) (OBETA=2,2'-oxybis(ethylamine)-N,N,N',N'-tetraacetic acid) complex, a novel derivative tailored with an n-hexadecyl chain was synthesised. The amphiphilic gadolinium complex was designed and prepared with the aim of obtaining high relaxivity supramolecular aggregates by self-assembly in micelles and liposomes. Thus, lipidic nanoparticles were prepared and characterised by dynamic light scattering and ¹ H NMR relaxometry. Relaxivity values of up to 48.3 mm^-1  s^-1 (20 MHz and 298 K) were registered in liposomal aggregates. The binding to human serum albumin (HSA), evaluated both in terms of affinity and relaxometric properties of the supramolecular adduct, yielded exceptionally high relaxivity values (71.4 mm^-1  s^-1 at 30 MHz and 298 K).

Optical and relaxometric properties of monometallic (EuIII, TbIII, GdIII) and heterobimetallic (ReI/GdIII) systems based on a functionalized bipyridine-containing acyclic ligand

The photophysical and relaxometric properties, in aqueous solution, of Ln-BPMNTA complexes and a derived Re^I/Gd^III dinuclear complex are reported in this paper.

r1andr2Relaxivities of Dendrons Based on a OEG-DTPA Architecture: Effect of Gd3+Placement and Dendron Functionalization

In magnetic resonance imaging, contrast agents are employed to enhance the signal intensity. However, current commercial contrast agents are hindered by a low relaxivity constant. Dendrimers can be employed to create higher molecular weight contrast agents which have an increased relaxivity due to a lower molecular rotation. In this study, dendrimers containing DTPA derivatives as cores and/or branching units were used to chelate gadolinium ions. Locating the gadolinium ions inside the dendrimers results in higher relaxivity constants, possibly because the paramagnetic center is closer to the rotational axis of the macromolecule. The highest gain in relaxivity was produced by decorating the dendron surface with peptide sequences, which could be explained by the presence of more second-sphere water molecules attracted by the peptides. These findings could contribute to the development of more effective contrast agents, either by placing the paramagnetic gadolinium ion in a strategic position or through functionalization of the dendron surface.

Silica-based nanoparticles: a versatile tool for the development of efficient imaging agents

In this review a selection of the most recent examples of imaging techniques applied to silica-based NPs for imaging is reported.

Spectrophotometric determination and removal of unchelated europium ions from solutions containing Eu-diethylenetriaminepentaacetic acid chelate-peptide conjugates

Europium chelates conjugated with peptide ligands are routinely used as probes for conducting in vitro binding experiments. The presence of unchelated Eu ions in these formulations gives high background luminescence and can lead to poor results in binding assays. In our experience, the reported methods for purification of these probes do not achieve adequate removal of unchelated metal ions in a reliable manner. In this work, a xylenol orange-based assay for the quantification of unchelated metal ions was streamlined and used to determine levels of metal ion contamination as well as the success of metal ion removal on attempted purification. We compared the use of Empore chelating disks and Chelex 100 resin for the selective removal of unchelated Eu ions from several Eu-diethylenetriaminepentaacetic acid chelate-peptide conjugates. Both purification methods gave complete and selective removal of the contaminant metal ions. However, Empore chelating disks were found to give much higher recoveries of the probes under the conditions used. Related to the issue of probe recovery, we also describe a significantly more efficient method for the synthesis of one such probe using Rink amide AM resin in place of Tentagel S resin.

MnII-containing coordination nanoparticles as highly efficient T1 contrast agents for magnetic resonance imaging

Nanoparticles of Mn^II-containing Prussian blue analogues reveal large longitudinal relaxivities exceeding that of clinical contrast agents.

Upconverting and NIR emitting rare earth based nanostructures for NIR-bioimaging

In recent years, significant progress was achieved in the field of nanomedicine and bioimaging, but the development of new biomarkers for reliable detection of diseases at an early stage, molecular imaging, targeting and therapy remains crucial. The disadvantages of commonly used organic dyes include photobleaching, autofluorescence, phototoxicity and scattering when UV (ultraviolet) or visible light is used for excitation. The limited penetration depth of the excitation light and the visible emission into and from the biological tissue is a further drawback with regard to in vivo bioimaging. Lanthanide containing inorganic nanostructures emitting in the near-infrared (NIR) range under NIR excitation may overcome those problems. Due to the outstanding optical and magnetic properties of lanthanide ions (Ln(3+)), nanoscopic host materials doped with Ln(3+), e.g. Y2O3:Er(3+),Yb(3+), are promising candidates for NIR-NIR bioimaging. Ln(3+)-doped gadolinium-based inorganic nanostructures, such as Gd2O3:Er(3+),Yb(3+), have a high potential as opto-magnetic markers allowing the combination of time-resolved optical imaging and magnetic resonance imaging (MRI) of high spatial resolution. Recent progress in our research on over-1000 nm NIR fluorescent nanoprobes for in vivo NIR-NIR bioimaging will be discussed in this review.

Cholesterol-diethylenetriaminepentaacetate complexed with thulium ions integrated into bicelles to increase their magnetic alignability

Lanthanides have been used for several decades to increase the magnetic alignability of bicelles. DMPE-DTPA (1,2-dimyristoyl-sn-glycero-3-phospho-ethanolamine-diethylenetriaminepentaacetate) is commonly applied to anchor the lanthanides into the bicelles. However, because DMPE-DTPA has the tendency to accumulate at the highly curved edge region of the bicelles and if located there does not contribute to the magnetic orientation energy, we have tested cholesterol-DTPA complexed with thulium ions (Tm(3+)) as an alternative chelator to increase the magnetic alignability. Differential scanning calorimetric (DSC) measurements indicate the successful integration of cholesterol-DTPA into a DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) bilayer. Cryo transmission electron microscopy and small-angle neutron scattering (SANS) measurements show that the disklike structure, that is, bicelles, is maintained if cholesterol-DTPA·Tm(3+) is integrated into a mixture of DMPC, cholesterol, and DMPE-DTPA·Tm(3+). The size of the bicelles is increased compared to the size of the bicelles obtained from mixtures without cholesterol-DTPA·Tm(3+). Magnetic-field-induced birefringence and SANS measurements in a magnetic field show that with addition of cholesterol-DTPA·Tm(3+) the magnetic alignability of these bicelles is significantly increased compared to bicelles composed of DMPC, cholesterol, and DMPE-DTPA·Tm(3+) only.

Modulating water-exchange rates of lanthanide(III)-containing polyaminopolycarboxylate-type complexes using polyethylene glycol

We have synthesized a series of Ln(III)-containing polyethylene glycol conjugates and studied the structural and electronic properties of these complexes. These studies demonstrate that polyethylene glycol can be used to fine-tune water-exchange rates of Ln(III)-containing polyaminopolycarboxylate-type complexes; this control is desirable in developing Ln(III)-containing contrast agents for magnetic resonance imaging.

Gd-labeled glycol chitosan as a pH-responsive magnetic resonance imaging agent for detecting acidic tumor microenvironments

Neoplastic lesions can create a hostile tumor microenvironment with low extracellular pH. It is commonly believed that these conditions can contribute to tumor progression as well as resistance to therapy. We report the development and characterization of a pH-responsive magnetic resonance imaging contrast agent for imaging the acidic tumor microenvironment. The preparation included the conjugation of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid 1-(2,5-dioxo-1-pyrrolidinyl) ester (DOTA-NHS) to the surface of a water-soluble glycol chitosan (GC) polymer, which contains pH-titrable primary amines, followed by gadolinium complexation (GC-NH2-GdDOTA). GC-NH2-GdDOTA had a chelate-to-polymer ratio of approximately1:24 and a molar relaxivity of 9.1 mM(-1) s(-1). GC-NH2-GdDOTA demonstrated pH-dependent cellular association in vitro compared to the control. It also generated a 2.4-fold enhancement in signal in tumor-bearing mice 2 h postinjection. These findings suggest that glycol chitosan coupled with contrast agents can provide important diagnostic information about the tumor microenvironment.

Strategies for optimizing water-exchange rates of lanthanide-based contrast agents for magnetic resonance imaging

This review describes recent advances in strategies for tuning the water-exchange rates of contrast agents for magnetic resonance imaging (MRI). Water-exchange rates play a critical role in determining the efficiency of contrast agents; consequently, optimization of water-exchange rates, among other parameters, is necessary to achieve high efficiencies. This need has resulted in extensive research efforts to modulate water-exchange rates by chemically altering the coordination environments of the metal complexes that function as contrast agents. The focus of this review is coordination-chemistry-based strategies used to tune the water-exchange rates of lanthanide(III)-based contrast agents for MRI. Emphasis will be given to results published in the 21st century, as well as implications of these strategies on the design of contrast agents.

Introductory Chemistry: A Molar Relaxivity Experiment in the High School Classroom

Dotarem and Magnevist, two clinically available magnetic resonance imaging (MRI) contrast agents, were assessed in a high school science classroom with respect to which is the better contrast agent. Magnevist, the more efficacious contrast agent, has negative side effects because its gadolinium center can escape from its ligand. However, Dotarem, though a less efficacious contrast agent, is a safer drug choice. After the experiment, students are confronted with the FDA warning on Magnevist, which enabled a discussion of drug efficacy versus safety. We describe a laboratory experiment in which NMR spin lattice relaxation rate measurements are used to quantify the relaxivities of the active ingredients of Dotarem and Magnevist. The spin lattice relaxation rate gives the average amount of time it takes the excited nucleus to relax back to the original state. Students learn by constructing molar relaxivity curves based on inversion recovery data sets that Magnevist is more relaxive than Dotarem. This experiment is suitable for any analytical chemistry laboratory with access to NMR.

High-relaxivity and luminescent silica nanoparticles as multimodal agents for molecular imaging

The design and synthesis of a new bimodal contrast agent for magnetic resonance imaging and optical imaging is reported. Tunable-sized silica nanoparticles were synthesized by a microemulsion-mediated pathway and used as carriers for paramagnetic and luminescent probes. The near-infrared luminescent agent was a ruthenium complex that was directly entrapped in the silica shell to provide photoluminescence enhancement and to make it highly photostable as it was protected from the surrounding environment. The paramagnetic activity came from a Gd-DTPA derivative that was grafted on the silica surface. NMRD profiles showed a strong relaxivity enhancement (increase of 432% in the r1 value at 20 MHz) when the paramagnetic complex was grafted at the nanoparticle surface, because of a reduction of its mobility. Polyethylene glycol was also grafted at the nanoparticle surface to enhance the nanoparticle residence time in the bloodstream. A thorough characterization of the material confirmed its potential as a very effective bimodal contrast agent.

Biodegradable polydisulfide dendrimer nanoclusters as MRI contrast agents

Gadolinium-conjugated dendrimer nanoclusters (DNCs) are a promising platform for the early detection of disease; however, their clinical utility is potentially limited due to safety concerns related to nephrogenic systemic fibrosis (NSF). In this paper, biodegradable DNCs were prepared with polydisulfide linkages between the individual dendrimers to facilitate excretion. Further, DNCs were labeled with premetalated Gd chelates to eliminate the risk of free Gd becoming entrapped in dendrimer cavities. The biodegradable polydisulfide DNCs possessed a circulation half-life of >1.6 h in mice and produced significant contrast enhancement in the abdominal aorta and kidneys for as long as 4 h. The DNCs were reduced in circulation as a result of thiol-disulfide exchange, and the degradation products were rapidly excreted via renal filtration. These agents demonstrated effective and prolonged in vivo contrast enhancement and yet minimized Gd tissue retention. Biodegradable polydisulfide DNCs represent a promising biodegradable macromolecular MRI contrast agent for magnetic resonance angiography and can potentially be further developed into target-specific MRI contrast agents.