Metallofullerenes: a new class of MRI agents and more?

Systematic overview of soft materials as a novel frontier for MRI contrast agents

Magnetic resonance imaging (MRI) is a well-known diagnostic technique used to obtain high quality images in a non-invasive manner. In order to increase the contrast between normal and pathological regions in the human body, positive (T1) or negative (T2) contrast agents (CAs) are commonly intravenously administered. The most efficient class of T1-CAs are based on kinetically stable and thermodynamically inert gadolinium complexes. In the last two decades many novel macro- and supramolecular CAs have been proposed. These approaches have been optimized to increase the performance of the CAs in terms of the relaxivity values and to reduce the administered dose, decreasing the toxicity and giving better safety and pharmacokinetic profiles. The improved performances may also allow further information to be gained on the pathological and physiological state of the human body. The goal of this review is to report a systematic overview of the nanostructurated CAs obtained and developed by manipulating soft materials at the nanometer scale. Specifically, our attention is centered on recent examples of fibers, hydrogels and nanogel formulations, that seem particularly promising for overcoming the problematic issues that have recently pushed the European Medicines Agency (EMA) to withdraw linear CAs from the market.

Gd(iii)-nanostructurated Constrast Agents (CAs) for Magnetic Resonance Imaging (MRI) can be designed and developed by manipulating soft material, including fibers, hydrogels and nanogels, in the nanometer scale.

Positron Emission Tomography/Magnetic Resonance Imaging of Glioblastoma Using a Functionalized Gadofullerene Nanoparticle

Water-soluble gadofullerene nanomaterials have been extensively investigated as magnetic resonance imaging (MRI) contrast agents, radical scavengers, sensitizers for photodynamic therapy, and inherent antineoplastic agents. Most recently, an alanine-modified gadofullerene nanoparticle (Gd@C₈₂-Ala) with excellent anticancer activity has been reported; however, the absolute tumor uptake of Gd@C₈₂-Ala is still far from being satisfactory, and its dynamic pharmacokinetics and long-term metabolic behaviors remain to be elucidated. Herein, Gd@C₈₂-Ala was chemically modified with eight-arm polyethylene glycol amine to improve its biocompatibility and provide the active sites for the attachment of a tumor-homing ligand (cRGD) and positron emission tomography (PET) isotopes (i.e., ⁶⁴Cu or ⁸⁹Zr). The physical and chemical properties (e.g., size, surface functionalization condition, radiochemical stability, etc.) of functionalized Gd@C₈₂-Ala were properly characterized. Also, its glioblastoma cell targeting capacity was evaluated in vitro by flow cytometry, confocal fluorescence microscopy, and dynamic cellular interaction assays. Because of the presence of gadolinium ions, the gadofullerene conjugates can act simultaneously as T₁* MRI contrast agents and PET probes. Thus, the pharmacokinetic behavior of functionalized Gd@C₈₂-Ala was investigated by PET/MRI, which combines the merits of high resolution and excellent sensitivity. The functionalized Gd@C₈₂-Ala-PEG-cRGD-NOTA-⁶⁴Cu (NOTA stands for 1,4,7-triazacyclononane-triacetic acid) demonstrated much higher accumulation in U87-MG tumor than its counterpart without cRGD attachment from in vivo PET observation, consistent with observation at the cellular level. In addition, Gd@C₈₂-Ala-PEG-Df-⁸⁹Zr (Df stands for desferrioxamine) was employed to investigate the metabolic behavior of gadofullerene conjugates in vivo for up to 30 days. It was estimated that nearly 70% of Gd@C₈₂-Ala-PEG-Df-⁸⁹Zr was excreted from the test subjects primarily through renal pathways within 24 h. With proper surface engineering, functionalized Gd@C₈₂-Ala nanoparticles can show an improved accumulation in glioblastoma. Pharmacokinetic studies also confirmed the safety of this nanoplatform, which can be used as an image-guidable therapeutic agent for glioblastoma.

Peptide-based building blocks as structural elements for supramolecular Gd-containing MRI contrast agents

Magnetic resonance imaging (MRI) is one of the most important clinic diagnostic tool used to obtain high-quality body images. The administration of low-molecular-weight Gd complex-based MRI contrast agents (CAs) permits to increase the ¹ H relaxation rate of nearby water molecules, thus modulating signal intensity and contrast enhancement. Even if highly accurate, MRI modality suffers from its low sensitivity. Moreover, low-molecular-weight CAs rapidly equilibrate between the intravascular and extravascular spaces after their administration. In order to improve their sensitivity and limit the extravasation phenomenon, several macromolecular and supramolecular multimeric gadolinium complexes (dendrimers, polymers, carbon nanostructures, micelles, and liposomes) have been designed until now. Because of their biocompatibility, low immunogenicity, low cost, and easy synthetic modification, peptides are attractive building blocks for the fabbrication of novel materials for biomedical applications. We report on the state of the art of supramolecular CAs obtained by self-assembly of three different classes of building blocks containing a peptide sequence, a gadolinium complex, and, if necessary, a third functional portion achieving the organization process.

Non-Chromatographic Purification of Endohedral Metallofullerenes

The purification of endohedral metallofullerenes by high performance liquid chromatography is very time-consuming and expensive. A number of rapid and inexpensive non-chromatographic methods have thus been developed for large-scale purification of metallofullerenes. In this review, we summarize recent advances in non-chromatographic purification methods of metallofullerenes. Lewis acid-based complexation is one of the most efficient and powerful methods for separation of metallofullerenes from empty fullerenes. The first oxidation potential of metallofullerenes is a critical factor that affects the separation efficiency of the Lewis acid-based method. Supramolecular methods are effective for separation of fullerenes and metallofullerenes that are different in size and shape. Chemical/electrochemical reduction and exohedral functionalization are also utilized to separate and purify metallofullerenes on a large scale.

Biomedical Applications of Metal-Encapsulated Fullerene Nanoparticles

The carbonaceous nanomaterials known as metallofullerenes have attracted considerable attention due to their attractive properties. The robust nature of the "Trojan Horse" fullerene cage provides an important structural component, which isolates the metal cluster from the bioenvironment. The large carbon surface area is ideally suited for multiple exo-functionalization approaches to modify the hydrophobic cage for a more hydrophilic bioenvironment. Additionally, peptides and other agents are readily covalently attached to this nanoprobe for targeting applications. The recent progress in developing metallofullerenes for next-generation biomedical applications is described. Of special interest are magnetic resonance imaging (MRI) contrast agents. Several recent studies reported cumulative gadolinium deposition in the brain and bones of individuals using commercial clinical MRI contrast agents. Gadolinium-based metallofullerenes provide 2-3 orders of magnitude improvement in MRI relaxivity and potentially lower clinical levels of toxic Gd³⁺ ions deposited. Other potential biomedical applications are also reviewed herein.

A Versatile and Clearable Nanocarbon Theranostic Based on Carbon Dots and Gadolinium Metallofullerene Nanocrystals

Nanocarbons such as carbon nanotubes, graphene derivatives, and carbon nanohorns have illustrated their potential uses as cancer theranostics owing to their intrinsic fluorescence or NIR absorbance as well as superior cargo loading capacity. However, some problems still need to be addressed, such as the fates and long-term toxicology of different nanocarbons in vivo and the improvement of their performance in various biomedical imaging-guided cancer therapy systems. Herein, a versatile and clearable nanocarbon theranostic based on carbon dots (CDs) and gadolinium metallofullerene nanocrystals (GFNCs) is first developed, in which GFNCs enhance the tumor accumulation of CDs, and CDs enhance the relaxivity of GFNCs, leading to an efficient multimodal imaging-guided photodynamic therapy in vivo without obvious long-term toxicity. Furthermore, biochemical analysis reveals that the novel nanotheranostic can harmlessly eliminate from the body in a reasonable period of time after exerting diagnostic and therapeutic function.

Synthesis of a UCNPs@SiO2@gadofullerene nanocomposite and its application in UCL/MR bimodal imaging

A novel bimodal imaging agent (GdF–UCNPs) has been designed through conjugating the water-soluble polyhydroxy Gd@C₈₂-PCBM with silica coated UCNPs.

Classification of carbon nanostructure families occurring in a chemically activated arc discharge reaction

Controlling the generation of empty cages, endohedral metallofullerenes and carbon nanotubes is an important challenge for the tailored synthesis of functional materials and their scaled up production.

Advances in nanoparticle imaging technology for vascular pathologies

Nanoparticle imaging agents for vascular pathologies are in development, and some agents are already in clinical trials. Untargeted agents, with long circulation, are excellent blood-pool agents, but molecularly targeted agents have significant advantages due to the signal enhancement possible with nanoparticle presentation of the contrast agent molecules. Molecular targets that are accessible directly from the vasculature are optimal for such agents. Targets that are removed from the vasculature, such as those on tumor cell surfaces, have limited accessibility owing to the enhanced permeation and retention effect. Yet, efforts at molecular targeting have tested small molecules, peptides, antibodies, and most recently aptamers as possible targeting ligands. The future is bright for nanoparticle-based imaging of vascular pathologies.

A New Interleukin-13 Amino-Coated Gadolinium Metallofullerene Nanoparticle for Targeted MRI Detection of Glioblastoma Tumor Cells

The development of new nanoparticles as next-generation diagnostic and therapeutic ("theranostic") drug platforms is an active area of both chemistry and cancer research. Although numerous gadolinium (Gd) containing metallofullerenes as diagnostic magnetic resonance imaging (MRI) contrast agents have been reported, the metallofullerene cage surface, in most cases, consists of negatively charged carboxyl or hydroxyl groups that limit attractive forces with the cellular surface. It has been reported that nanoparticles with a positive charge will bind more efficiently to negatively charged phospholipid bilayer cellular surfaces, and will more readily undergo endocytosis. In this paper, we report the preparation of a new functionalized trimetallic nitride template endohedral metallofullerene (TNT EMF), Gd3N@C80(OH)x(NH2)y, with a cage surface bearing positively charged amino groups (-NH3(+)) and directly compare it with a similar carboxyl and hydroxyl functionalized derivative. This new nanoparticle was characterized by X-ray photoelectron spectroscopy (XPS), dynamic light scattering (DLS), and infrared spectroscopy. It exhibits excellent (1)H MR relaxivity. Previous studies have clearly demonstrated that the cytokine interleukin-13 (IL-13) effectively targets glioblastoma multiforme (GBM) cells, which are known to overexpress IL-13Rα2. We also report that this amino-coated Gd-nanoplatform, when subsequently conjugated with interleukin-13 peptide IL-13-Gd3N@C80(OH)x(NH2)y, exhibits enhanced targeting of U-251 GBM cell lines and can be effectively delivered intravenously in an orthotopic GBM mouse model.

Theranostic applications of carbon nanomaterials in cancer: Focus on imaging and cargo delivery

Carbon based nanomaterials have attracted significant attention over the past decades due to their unique physical properties, versatile functionalization chemistry, and biological compatibility. In this review, we will summarize the current state-of-the-art applications of carbon nanomaterials in cancer imaging and drug delivery/therapy. The carbon nanomaterials will be categorized into fullerenes, nanotubes, nanohorns, nanodiamonds, nanodots and graphene derivatives based on their morphologies. The chemical conjugation/functionalization strategies of each category will be introduced before focusing on their applications in cancer imaging (fluorescence/bioluminescence, magnetic resonance (MR), positron emission tomography (PET), single-photon emission computed tomography (SPECT), photoacoustic, Raman imaging, etc.) and cargo (chemo/gene/therapy) delivery. The advantages and limitations of each category and the potential clinical utilization of these carbon nanomaterials will be discussed. Multifunctional carbon nanoplatforms have the potential to serve as optimal candidates for image-guided delivery vectors for cancer.

Recent advances in biological applications of cage metal complexes

This review highlights advances in biochemical and medical applications of cage metal complexes (clathrochelates) and related polyhedral compounds.

Gd3N@C84(OH)x: a new egg-shaped metallofullerene magnetic resonance imaging contrast agent

Water-soluble derivatives of gadolinium-containing metallofullerenes have been considered to be excellent candidates for new magnetic resonance imaging (MRI) contrast agents because of their high relaxivity and characteristic encapsulation of the lanthanide ions (Gd(3+)), preventing their release into the bioenvironment. The trimetallic nitride template endohedral metallofullerenes (TNT EMFs) have further advantages of high stability, high relative yield, and encapsulation of three Gd(3+) ions per molecule as illustrated by the previously reported nearly spherical, Gd3N@I(h)-C80. In this study, we report the preparation and functionalization of a lower-symmetry EMF, Gd3N@C(s)-C84, with a pentalene (fused pentagons) motif and an egg-shaped structure. The Gd3N@C84 derivative exhibits a higher (1)H MR relaxivity compared to that of the Gd3N@C80 derivative synthesized the same way, at low (0.47 T), medium (1.4 T), and high (9.4 T) magnetic fields. The Gd3N@C(s)-C84 derivative exhibits a higher hydroxyl content and aggregate size, as confirmed by X-ray photoelectron spectroscopy (XPS) and dynamic light scattering (DLS) experiments, which could be the main reasons for the higher relaxivity.

Modified Seed Growth of Iron Oxide Nanoparticles in Benzyl Alcohol - Optimization for Heating and Broad Stability in Biomedical Applications

Iron oxide nanoparticles have received sustained interest for biomedical applications as synthetic approaches are continually developed for control of nanoparticle properties. However, many approaches focus solely on the material, rather than the complete optimization of synthesis and functionalization together to enhance translation into biological systems. Presented herein is a modified seed growth method designed for obtaining optimal nanoparticle properties and ease of surface functionalization for long term stability. With a one or two addition process, iron oxide nanoparticles were produced in crystallite sizes ranging from 5–15 nm using only benzyl alcohol and an iron precursor. In the functionalization process, concentration variations were required for stabilizing different nanoparticle sizes. Radio frequency induction heating experiments of various crystallite and hydrodynamic sizes verified that the heating efficiency greatly increased while approaching the 15 nm crystallite, and suggested an important role of the overall particle size on heating efficiency. Initial in vitro experiments with the functionalized nanoparticles showed success in providing hyperthermia-induced tumour cell killing without an increase in the temperature of the cell suspension medium. This demonstrates the potential for nanoparticle-based hyperthermia to provide a therapeutic effect while limiting normal tissue damage.

A theoretical study on ionic liquid endohedral C540 fullerene

The effect of the confinement of ionic liquid (choline benzoate) cluster inside C540 fullerene has been studied through both molecular dynamic and density functional theory simulations.