Decoration of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) with N-oxides increases the T1 relaxivity of Gd-complexes

High complex stability and longitudinal relaxivity of Gd-based contrast agents are important requirements for magnetic resonance imaging (MRI) because they ensure patient safety and contribute to measurement sensitivity. Charged and zwitterionic Gd3+-complexes of the well-known chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) provide an excellent basis for the development of safe and sensitive contrast agents. In this report, we describe the synthesis of DOTA-NOx, a DOTA derivative with four N-oxide functionalities via "click" functionalization of the tetraazide DOTAZA. The resulting complexes Gd-DOTA-NOx and Eu-DOTA-NOx are stable compounds in aqueous solution. NMR-spectroscopic characterization revealed a high excess of the twisted square antiprismatic (TSAP) coordination geometry over square antiprismatic (SAP). The longitudinal relaxivity of Gd-DOTA-NOx was found to be r1=7.7 mm-1 s-1 (1.41 T, 37 °C), an unusually high value for DOTA complexes of comparable weight. We attribute this high relaxivity to the steric influence and an ordering effect on outer sphere water molecules surrounding the complex generated by the strongly hydrated N-oxide groups. Moreover, Gd-DOTA-NOx was found to be stable against transchelation with high excess of EDTA (200 eq) over a period of 36 h, and it has a similar in vitro cell toxicity as clinically used DOTA-based GBCAs.

Rapid In Vitro Quantification of a Sensitized Gadolinium Chelate via Photoinduced Triplet Harvesting

Gadolinium (Gd) based contrast agents (GBCAs) are widely used in magnetic resonance imaging (MRI) and are paramount to cancer diagnostics and tumor pharmacokinetic analysis. Accurate quantification of gadolinium concentration is essential to monitoring the biodistribution, clearance, and pharmacodynamics of GBCAs. However, current methods of quantifying gadolinium in blood or plasma (biological media) are both low throughput and clinically unavailable. Here, we have demonstrated the use of a sensitized gadolinium chelate, Gd[DTPA-cs124], as an MRI contrast agent that can be used to measure the concentration of gadolinium via luminescence quantification in biological media following transmetalation with a terbium salt. Gd[DTPA-cs124] was synthesized by conjugating carbostyril-124 (cs124) to diethylenetriaminepentaacetic acid (DTPA) and chelating to gadolinium. We report increases in both stability and relaxivity compared to the clinically approved analog Gd[DTPA] (gadopentetic acid or Magnevist). In vivo MRI experiments were conducted using C57BL6 mice in order to further illustrate the performance of Gd[DTPA-cs124] as an MRI contrast agent in comparison to Magnevist. Our results indicate that similar chemical modification to existing clinically approved GBCA may likewise provide favorable property changes, with the ability to be used in a gadolinium quantification assay. Furthermore, our assay provides a straightforward and high-throughput method of measuring gadolinium in biological media using a standard laboratory plate reader.

Coordination-Directed Self-Assembly of Functional Polynuclear Lanthanide Supramolecular Architectures

Lanthanide supramolecular chemistry is a fast growing and intriguing research field due to the unique photophysical, magnetic, and coordination properties of lanthanide ions (Ln^III). Compared with the intensively investigated mononuclear Ln-complexes, polymetallic lanthanide supramolecular assemblies offer more structural superiority and functional advantages. In recent decades, significant progress has been made in polynuclear lanthanide supramolecules, varying from structural evolution to luminescent and magnetic functional materials. This review summarizes the design principles in ligand-induced coordination-driven self-assembly of polynuclear Ln-structures and intends to offer guidance for the construction of more elegant Ln-based architectures and optimization of their functional performances. Design principles concerning the water solubility and chirality of the lanthanide-organic assemblies that are vital in extending their applications are emphasized. The strategies for improving the luminescent properties and the applications in up-conversion, host-guest chemistry, luminescent sensing, and catalysis have been summarized. Magnetic materials based on supramolecular assembled lanthanide architectures are given in an individual section and are classified based on their structural features. Challenges remaining and perspective directions in this field are also briefly discussed.

Design of High-Relaxivity Polyelectrolyte Nanocapsules Based on Citrate Complexes of Gadolinium(III) of Unusual Composition

Through nuclear magnetic relaxation and pH-metry, the details of the complexation of gadolinium(III) ions with citric acid (H₄L) in water and aqueous solutions of cationic polyelectrolytes are established. It is shown that the presence of poly(ethylene imine) (PEI) in solution affects magnetic relaxation behavior of gadolinium(III) complexes with citric acid (Cit) to a greater extent than polydiallyldimethylammonium chloride (PDDC). A large increase in relaxivity (up to 50 mM⁻¹s⁻¹) in the broad pH range (4–8) is revealed for the gadolinium(III)–citric acid–PEI system, which is particularly strong in the case of PEI with the molecular weight of 25 and 60 kDa. In weakly acidic medium (pH 3–7), the presence of PEI results in the formation of two tris-ligand associates [Gd(H₂L)₃]³⁻ and [Gd(H₂L)₂(HL)]⁴⁻, which do not exist in aqueous medium. In weakly alkaline medium (pH 7–10), formation of ternary complexes Gd(III)–Cit–PEI with the Gd(III)–to–Cit ratio of 1:2 is evidenced. Using transmission electron microscopy (TEM) and dynamic light scattering techniques (DLS), the formation of the particles with the size of 50–100 nm possessing narrow molecular-mass distribution (PDI 0.08) is determined in the solution containing associate of PEI with tris-ligand complex [Gd(H₂L)₂(HL)]⁴⁻.

Nanoscale metal-organic frameworks for tumor phototherapy

Metal-Organic Frameworks (MOFs) are constructed from metal ions/cluster nodes and functional organic ligands through coordination bonds. Owing to the advantages of diverse synthetic methods, easy modification after synthesis, large adsorption capacity for heavy metals, and short equilibrium time, considerable attention has recently been paid to MOFs for tumor phototherapy. Through rational tuning of metal ions and ligands, MOFs present abundant properties for various applications. Light-triggered phototherapy, including photodynamic therapy (PDT) and photothermal therapy (PTT), is an emerging cancer treatment approach. Nanosized MOFs can be applied as phototherapeutic agents to accomplish phototherapy with excellent phototherapeutic efficacy. This review outlines the latest advances in the field of phototherapy with various metal ion-based MOFs.

Mono-dispersed nano-hydroxyapatite based MRI probe with tetrahedral DNA nanostructures modification for in vitro tumor cell imaging

Taking advantage of the superior biocompatibility, good stability in a wide pH and temperature range, as well as its strong affinity with DNA of hydroxyapatite (HAp), tetrahedral DNA nanostructures (TDNs) conjugated with AS1411 aptamer (anti-nucleolin overexpressed on tumor cell membranes) were employed as affinity ligands to construct a novel mono-dispersed HAp based probe with Gd³⁺ doping (Apt-TDNs-GdHAp) for MR imaging. The adsorption of TDNs on the nano-HAp surface facilely accomplished the construction of the Apt-TDNs-GdHAp probes. Meanwhile, the use of hydrophilic TDNs not only favored the phase-transfer from the oil phase to the aqueous phase, but also enhanced the mono-dispersion of this probe due to the well-ordered distribution of TDNs on the surface of nano-HAp. Moreover, Apt-TDNs-GdHAp probe with a better mono-dispersion and crystalinity achieved twice higher longitudinal relaxivity (r₁ value) than that of GdHAp synthesized by microwave-assisted method (Microwave-GdHAp), exhibiting much more excellent T₁-weighted imaging performance. With the introduction of TDNs, the stability and the tumor-targeting accessibility were also greatly improved, showing its great potential for further bio-applications.

Gadolinium complexes of macrocyclic diethylenetriamine-N-oxide pentaacetic acid-bisamide as highly stable MRI contrast agents with high relaxivity

Gadolinium(iii) complexes are generally considered efficient magnetic resonance imaging (MRI) contrast agents (CAs) and widely used in clinical applications. High relaxivity and stability are two essential criteria for a Gd(iii)-complex to be used as a MRI-CA. One crucial strategy to achieve high relaxivity for small molecular Gd(iii)-based MRI contrast agents is to increase the hydration number q. Meanwhile, metal complexes with macrocyclic ligands have been proved to inherit high thermodynamic stability and kinetic inertness. Herein, a series of macrocyclic ligands based on diethylenetriamine-N-oxide pentaacetic acid-bisamide were synthesized. Among them, cyclo-DTPA-NO-C6O2 (3d) was the strongest ligand for Gd(iii) as confirmed by experimental results. The hydration number of the Gd-cyclo-DTPA-NO-C6O2 (4d) complex was characterized by luminescence measurements to be 3 and the coordination structure was confirmed with computational simulations. Consequently, the relaxivity of this complex (14.3 mM-1 s-1, 1.5 T, 25 °C) is about triple that of commercial MRI CAs. The conditional stability constant of the Gd(iii) complex, pGd, calculated from spectrophotometric titration studies, was comparable to that of one of the most stable commercial MRI-CAs, Gd-DTPA (Magnevist®). Meanwhile, the kinetic inertness of the complex was even higher than that of Gd-DTPA thanks to its macrocyclic coordination structure.

Charge-conversional polyethylenimine-entrapped gold nanoparticles with 131I-labeling for enhanced dual mode SPECT/CT imaging and radiotherapy of tumors

Novel theranostic nanosystems demonstrate great potential to achieve timely diagnosis and effective therapy at the same time. However, due to the relatively low accumulation of theranostic nanosystems at the tumor site, the theranostic efficiency is limited. In this study, a novel theranostic nanosystem with a pH-responsive charge conversion property was constructed to improve the cellular uptake towards cancer cells for enhanced single photon emission computed tomography (SPECT)/computed tomography (CT) dual mode imaging and radiotherapy of tumors. In detail, polyethylenimine (PEI) was utilized as a nanoplatform to link with polyethylene glycol (PEG) monomethyl ether with one end of N-hydroxylsuccinimide (mPEG-NHS), PEG with ends of maleimide and succinimidyl valerate (MAL-PEG-SVA), alkoxyphenyl acylsulfonamide (APAS), 3-(4'-hydroxyphenyl)propionic acid-OSu (HPAO), and fluorescein isothiocyanate (FI), successively. The formed functionalized PEI was then utilized to entrap gold nanoparticles, acetylate the remaining amines of PEI and label with radioactive iodine-131 (¹³¹I) to build theranostic nanosystems. The result demonstrated that the theranostic nanosystem has a 3.8 nm Au core and showed excellent colloidal stability. On account of the charge conversion property of APAS, the APAS linked PEI entrapped gold nanoparticles could switch from neutral to positive in a slightly acidic microenvironment, which induced improved cellular uptake. Above all, after ¹³¹I labeling, the generated theranostic nanosystem could achieve enhanced SPECT/CT dual mode imaging and radiotherapy of cancer cells in vitro and a xenograft tumor model in vivo. The constructed APAS-linked PEI nanosystem has great potential to be used as a model for SPECT/CT imaging and radiotherapy of various types of cancer.

Gadolinium complexes of diethylenetriamine-N-oxide pentaacetic acid-bisamide: a new class of highly stable MRI contrast agents with a hydration number of 3

Diethylenetriamine-N-oxide pentaacetic acid-bisamide-based Gd(iii) complexes with 3 coordinated water molecules have been synthesized to achieve high stability and over three times of the relaxivities of commercial MRI contrast agents.

Gadolinium-containing polymer microspheres: a dual-functional theranostic agent for magnetic resonance imaging and cancer therapy

Preparation of poly(gadolinium methacrylate-co-methacrylic acid) copolymer microspheres with high MRI contrast efficiency and controlled anti-cancer drug loading and release capability.

MRI Contrast Agents Based on Conjugated Polyelectrolytes and Dendritic Polymers

Three complexes of gadolinium-based on dentritic molecules are reported as magnetic resonance imaging (MRI) contrast agents. Their ligands feature four carboxylate groups, which contribute to good water solubility and a strong combination with metal ions. As a new attempt, coupling polymerization is carried out to make a combination of conjugated polyelectrolytes and dendrimers for MRI contrast agents. For comparison, mononuclear and binuclear complexes are also reported. The investigation suggests that the contrast agent with the newly designed macromolecular skeleton provides higher longitudinal relaxivity value (36.2 mm -1 s-1 ) and more visible enhancement in in vivo and in vitro MR images than the small molecular ones. In addition, extremely low cytotoxicity and main clearance via hepatobiliary are confirmed, which reduces the deterioration of chronic kidney disease. All the results indicate that these three complexes are generally applicable as promising clinical contrast agents.

Electrostatic self-assembled nanoparticles based on spherical polyelectrolyte brushes for magnetic resonance imaging

Electrostatic self-assemblies based on SPBs and Gd-DTPA-NO-C₄ exhibit perfect relaxometric performance.

A poly(ε-caprolactone)–poly(glycerol)–poly(ε-caprolactone) triblock copolymer for designing a polymeric micelle as a tumor targeted magnetic resonance imaging contrast agent

Gadolinium-based macromolecular contrast agents (CAs) with favorable biocompatibility, targeting specificity, and high relaxivity properties are desired for magnetic resonance imaging (MRI) of tumors.