Non-Gadolinium-Based MRI Contrast Agents

In Situ Generated Novel 1H MRI Reporter for β-Galactosidase Activity Detection and Visualization in Living Tumor Cells

For wide applications of the lacZ gene in cellular/molecular biology, small animal investigations, and clinical assessments, the improvement of noninvasive imaging approaches to precisely assay gene expression has garnered much attention. In this study, we investigate a novel molecular platform in which alizarin 2-O-β-d-galactopyranoside AZ-1 acts as a lacZ gene/β-gal responsive 1H-MRI probe to induce significant 1H-MRI contrast changes in relaxation times T 1 and T 2 in situ as a concerted effect for the discovery of β-gal activity with the exposure of Fe3+. We also demonstrate the capability of this strategy for detecting β-gal activity with lacZ-transfected human MCF7 breast and PC3 prostate cancer cells by reaction-enhanced 1H-MRI T 1 and T 2 relaxation mapping.

Fe-HBED Analogs: A Promising Class of Iron-Chelate Contrast Agents for Magnetic Resonance Imaging

Contrast-enhanced magnetic resonance imaging is an essential tool for disease diagnosis and management; all marketed clinical magnetic resonance imaging (MRI) contrast agents (CAs) are gadolinium (Gd) chelates and most are extracellular fluid (ECF) agents. After intravenous injection, these agents rapidly distribute to the extracellular space and are also characterized by low serum protein binding and predominant renal clearance. Gd is an abiotic element with no biological recycling processes; low levels of Gd have been detected in the central nervous system and bone long after administration. These observations have prompted interest in the development of new MRI contrast agents based on biotic elements such as iron (Fe); Fe-HBED (HBED = N,N′-bis(2-hydroxyphenyl)ethylenediamine-N,N′-diacetic acid), a coordinatively saturated iron chelate, is an attractive MRI CA platform suitable for modification to adjust relaxivity and biodistribution. Compared to the parent Fe-HBED, the Fe-HBED analogs reported here have lower serum protein binding and higher relaxivity as well as lower relative liver enhancement in mice, comparable to that of a representative gadolinium-based contrast agent (GBCA). Fe-HBED analogs are therefore a promising class of non-Gd ECF MRI CA.

Design and Control of Nanoconfinement to Achieve Magnetic Resonance Contrast Agents with High Relaxivity

The enhanced relaxation of hydrogen atoms of surrounding water from suitable contrast agent promotes magnetic resonance imaging as one of the most important medical diagnosis technique. The key challenge for the preparation of performant contrast agents for magnetic resonance imaging with high relaxivity is to ensure a high local concentration of contrast agent while allowing a contact between water and the contrast agent. Both requirements are answered by tailoring a semipermeable confinement for a gadolinium complex used as contrast agent. A locally high concentration is achieved by successfully encapsulating the complex in polymer nanocontainers that serves to protect and retain the complex inside a limited space. The access of water to the complex is achieved by carefully controlling the chemistry of the shell and the core of the nanocontainers. The confinement of the nanocontainers enables an increased relaxivity compared to an aqueous solution of the contrast agent. The nanocontainers are successfully applied in vivo to yield enhanced contrast in magnetic resonance imaging.

89 Zr- and Fe-Labeled Polymeric Micelles for Dual Modality PET and T1 -Weighted MR Imaging

In this study, a new ⁸⁹ Zr- and Fe³⁺ -labeled micelle nanoplatform (⁸⁹ Zr/Fe-DFO-micelles) for dual modality position emission tomography/magnetic resonance (PET/MR) imaging is investigated. The nanoplatform consists of self-assembling amphiphilic diblock copolymers that are functionalized with ⁸⁹ Zr-deferoxamine (⁸⁹ Zr-DFO) and Fe³⁺ -deferoxamine (Fe-DFO) for PET and MR purposes, respectively. ⁸⁹ Zr displays favorable PET imaging characteristics with a 3.3 d half-life suitable for imaging long circulating nanoparticles. The nanoparticles are modified with Fe-DFO as MR T₁ -contrast label instead of commonly used Gd³⁺ -based chelates. As these micelles are cleared by liver and spleen, any long term Gd- related toxicity such as nephrogenic systemic fibrosis is avoided. As a proof of concept, an in vivo PET/MR study in mice is presented showing tumor targeting of ⁸⁹ Zr/Fe-DFO-micelles through the enhanced permeability and retention (EPR) effect of tumors, yielding high tumor-to-blood (10.3 ± 3.6) and tumor-to-muscle (15.3 ± 8.1) ratios at 48 h post injection. In vivo PET images clearly delineate the tumor tissue and show good correspondence with ex vivo biodistribution results. In vivo magnetic resonance imaging (MRI) allows visualization of the intratumoral distribution of the ⁸⁹ Zr/Fe-DFO-micelles at high resolution. In summary, the ⁸⁹ Zr/Fe-DFO-micelle nanoparticulate platform allows EPR-based tumor PET/MRI, and, furthermore, holds great potential for PET/MR image guided drug delivery.

Dual T1/T2 MRI contrast agent based on hybrid SPION@coordination polymer nanoparticles

A novel dual T₁/T₂¹H-MRI contrast agent based on the encapsulation of super-paramagnetic iron oxide nanoparticles (SPIONs) with an iron coordination polymer is proposed.

Paramagnetic relaxation based biosensor for selective dopamine detection

We report a new NMR relaxation time-based method for sensitive and selective dopamine detection using paramagnetic nanoparticles.

Complementary strategies for developing Gd-free high-field T₁ MRI contrast agents based on Mn(III) porphyrins

Mn(III) porphyrin (MnP) holds the promise of addressing the emerging challenges associated with Gd-based clinical MRI contrast agents (CAs), namely, Gd-related adverse effect and decreasing sensitivity at high clinical magnetic fields. Two complementary strategies for developing new MnPs as Gd-free CAs with optimized biocompatibility were established to improve relaxivity or clearance rate. MnPs with distinct and tunable pharmacokinetic properties can consequently be constructed for different in vivo applications at clinical field of 3 T.

Manganese-impregnated mesoporous silica nanoparticles for signal enhancement in MRI cell labelling studies

Mesoporous silica nanoparticles (MSNs) are used in drug delivery and cell tracking applications. As Mn(2+) is already implemented as a "positive" cell contrast agent in preclinical imaging procedures (in the form of MnCl2 for neurological studies), the introduction of Mn in the porous network of MSNs would allow labelling cells and tracking them using MRI. These particles are in general internalized in endosomes, an acidic environment with high saline concentration. In addition, the available MSN porosity could also serve as a carrier to deliver medical/therapeutic substances through the labelled cells. In the present study, manganese oxide was introduced in the porous network of MCM-48 silica nanoparticles (Mn-M48SNs). The particles exhibit a narrow size distribution (~140 nm diam.) and high porosity (~60% vol.), which was validated after insertion of Mn. The resulting Mn-M48SNs were characterized by TEM, N2 physisorption, and XRD. Evidence was found with H2-TPR, and XPS characterization, that Mn(II) is the main oxidation state of the paramagnetic species after suspension in water, most probably in the form of Mn-OOH. The colloidal stability as a function of time was confirmed by DLS in water, acetate buffer and cell culture medium. In NMR data, no significant evidence of Mn(2+) leaching was found in Mn-M48SNs in acidic water (pH 6), up to 96 hours after suspension. High longitudinal relaxivity values of r1 = 8.4 mM(-1) s(-1) were measured at 60 MHz and 37 °C, with the lowest relaxometric ratios (r2/r1 = 2) reported to date for a Mn-MSN system. Leukaemia cells (P388) were labelled with Mn-M48SNs and nanoparticle cell internalization was confirmed by TEM. Finally, MRI contrast enhancement provided by cell labelling with escalated incubation concentrations of Mn-M48SNs was quantified at 1 T. This study confirmed the possibility of efficiently confining Mn into M48SNs using incipient wetness, while maintaining an open porosity and relatively high pore volume. Because these Mn-labelled M48SNs express strong "positive" contrast media properties at low concentrations, they are potentially applicable for cell tracking and drug delivery methodologies.

In vivo bio-safety evaluations and diagnostic/therapeutic applications of chemically designed mesoporous silica nanoparticles

The remarkable progress of nanotechnology and its application in biomedicine have greatly expanded the ranges and types of biomaterials from traditional organic material-based nanoparticles (NPs) to inorganic biomaterials or organic/inorganic hybrid nanocomposites due to the unprecedented advantages of the engineered inorganic material-based NPs. Colloidal mesoporous silica NPs (MSNs), one of the most representative and well-established inorganic materials, have been promoted into biology and medicine, and shifted from extensive in vitro research towards preliminary in vivo assays in small-animal disease models. In this comprehensive review, the recent progresses in chemical design and engineering of MSNs-based biomaterials for in vivo biomedical applications has been detailed and overviewed. Due to the intrinsic structural characteristics of elaborately designed MSNs such as large surface area, high pore volume and easy chemical functionalization, they have been extensively investigated for therapeutic, diagnostic and theranostic (concurrent diagnosis and therapy) purposes, especially in oncology. Systematic in vivo bio-safety evaluations of MSNs have revealed the evidences that the in vivo bio-behaviors of MSNs are strongly related to their preparation prodecures, particle sizes, geometries, surface chemistries, dosing parameters and even administration routes. In vivo pharmacokinetics and pharmacodynamics further demonstrated the effectiveness of MSNs as the passively and/or actively targeted drug delivery systems (DDSs) for cancer chemotherapy. Especially, the advance of nano-synthetic chemistry enables the production of composite MSNs for advanced in vivo therapeutic purposes such as gene delivery, stimuli-responsive drug release, photothermal therapy, photodynamic therapy, ultrasound therapy, or anti-bacteria in tissue engineering, or as the contrast agents for biological and diagnostic imaging. Additionally, the critical issues and potential challenges related to the chemical design/synthesis of MSNs-based "magic bullet" by advanced nano-synthetic chemistry and in vivo evaluation have been discussed to highlight the issues scientists face in promoting the translation of MSNs-based DDSs into clinical trials.

Novel molecular platform integrated iron chelation therapy for 1H-MRI detection of β-galactosidase activity

Targeting the increased Fe(3+) content in tumors, we propose a novel molecular platform integrated cancer iron chelation therapy for (1)H-magnetic resonance imaging (MRI) detection of β-galactosidase (β-gal) activity. Following this idea, we have designed, synthesized, and characterized a series of β-d-galactosides conjugated with various chelators and demonstrated the feasibility of this concept for assessing β-gal activity in solution by (1)H-MRI T1 and T2 relaxation mapping.

Metalloprotein-based MRI probes

Metalloproteins have long been recognized as key determinants of endogenous contrast in magnetic resonance imaging (MRI) of biological subjects. More recently, both natural and engineered metalloproteins have been harnessed as biotechnological tools to probe gene expression, enzyme activity, and analyte concentrations by MRI. Metalloprotein MRI probes are paramagnetic and function by analogous mechanisms to conventional gadolinium or iron oxide-based MRI contrast agents. Compared with synthetic agents, metalloproteins typically offer worse sensitivity, but the possibilities of using protein engineering and targeted gene expression approaches in conjunction with metalloprotein contrast agents are powerful and sometimes definitive strengths. This review summarizes theoretical and practical aspects of metalloprotein-based contrast agents, and discusses progress in the exploitation of these proteins for molecular imaging applications.

Bioengineered probes for molecular magnetic resonance imaging in the nervous system

The development of molecular imaging probes has changed the nature of neurobiological research. Some of the most notable successes have involved the use of biological engineering techniques for the creation of fluorescent protein derivatives for optical imaging, but recent work has also led to a number of bioengineered probes for magnetic resonance imaging (MRI), the preeminent technique for noninvasive investigation of brain structure and function. Molecular MRI agents are beginning to be applied for experiments in the nervous system, where they have the potential to bridge from molecular to systems or organismic levels of analysis. Compared with canonical synthetic small molecule agents, biomolecular or semibiosynthetic MRI contrast agents offer special advantages due to their amenability to molecular engineering approaches, their properties in some cases as catalysts, and their specificity in targeting and ligand binding. Here, we discuss an expanding list of instances where biological engineering techniques have aided in the design of MRI contrast agents and reporter systems, examining both advantages and limitations of these types of probes for studies in the central nervous system.

Infusion-based manganese-enhanced MRI: a new imaging technique to visualize the mouse brain

Manganese-enhanced magnetic resonance imaging is a technique that employs the divalent ion of the paramagnetic metal manganese (Mn(2+)) as an effective contrast agent to visualize, in vivo, the mammalian brain. As total achievable contrast is directly proportional to the net amount of Mn(2+) accumulated in the brain, there is a great interest in optimizing administration protocols to increase the effective delivery of Mn(2+) to the brain while avoiding the toxic effects of Mn(2+) overexposure. In this study, we investigated outcomes following continuous slow systemic infusion of manganese chloride (MnCl(2)) into the mouse via mini-osmotic pump administration. The effects of increasing fractionated rates of Mn(2+) infusion on signal enhancement in regions of the brain were analyzed in a three-treatment study. We acquired whole-brain 3-D T1-weighted images and performed region of interest quantitative analysis to compare mean normalized signal in Mn(2+) treatments spanning 3, 7, or 14 days of infusion (rates of 1, 0.5, and 0.25 μL/h, respectively). Evidence of Mn(2+) transport at the conclusion of each infusion treatment was observed throughout the brains of normally behaving mice. Regions of particular Mn(2+) accumulation include the olfactory bulbs, cortex, infralimbic cortex, habenula, thalamus, hippocampal formation, amygdala, hypothalamus, inferior colliculus, and cerebellum. Signals measured at the completion of each infusion treatment indicate comparable means for all examined fractionated rates of Mn(2+) infusion. In this current study, we achieved a significantly higher dose of Mn(2+) (180 mg/kg) than that employed in previous studies without any observable toxic effects on animal physiology or behavior.

Manganese(II)-containing MRI contrast agent employing a neutral and non-macrocyclic ligand

The ligand N,N'-bis(2-pyridylmethyl)-bis(ethylacetate)-1,2-ethanediamine (debpn) coordinates divalent transition metal ions in either a pentadentate or hexadentate fashion. The coordination number correlates with the ionic radius of the metal ion, with larger cations being heptacoordinate as assessed by solid-state analysis. With Mn(II), the debpn ligand is hexadentate and remains bound to the oxophilic metal ion, even when dissolved in water. The ligand's incomplete coordination of the manganous ion allows water molecules to coordinate to the metal center. These two properties, coupled with the high paramagnetism associated with the S = 5/2 metal center, enable [Mn(debpn)(H(2)O)](ClO(4))(2) to serve as a stable and effective magnetic resonance imaging contrast agent despite the ligand's lack of both a macrocyclic component and an anionic charge.

Photodynamic therapy and the development of metal-based photosensitisers

Photodynamic therapy (PDT) is a treatment modality that has been used in the successful treatment of a number of diseases and disorders, including age-related macular degeneration (AMD), psoriasis, and certain cancers. PDT uses a combination of a selectively localised light-sensitive drug (known as a photosensitiser) and light of an appropriate wavelength. The light-activated form of the drug reacts with molecular oxygen to produce reactive oxygen species (ROS) and radicals; in a biological environment these toxic species can interact with cellular constituents causing biochemical disruption to the cell. If the homeostasis of the cell is altered significantly then the cell enters the process of cell death. The first photosensitiser to gain regulatory approval for clinical PDT was Photofrin. Unfortunately, Photofrin has a number of associated disadvantages, particularly pro-longed patient photosensitivity. To try and overcome these disadvantages second and third generation photosensitisers have been developed and investigated. This Review highlights the key photosensitisers investigated, with particular attention paid to the metallated and non-metallated cyclic tetrapyrrolic derivatives that have been studied in vitro and in vivo; those which have entered clinical trials; and those that are currently in use in the clinic for PDT.

Nucleic acid delivery using magnetic nanoparticles: the Magnetofection™ technology

In recent years, gene therapy has received considerable interest as a potential method for the treatment of numerous inherited and acquired diseases. However, successes have so far been hampered by several limitations, including safety issues of viral-based nucleic acid vectors and poor in vivo efficiency of nonviral vectors. Magnetofection™ has been introduced as a novel and powerful tool to deliver genetic material into cells. This technology is defined as the delivery of nucleic acids, either ‘naked’ or packaged (as complexes with lipids or polymers, and viruses) using magnetic nanoparticles under the guidance of an external magnetic field. This article first discusses the principles of the Magnetofection technology and its benefits as compared with standard transfection methods. A number of relevant examples of its use, both in vitro and in vivo, will then be highlighted. Future trends in the development of new magnetic nanoparticle formulations will also be outlined.

Correlation of relaxivity with coordination number in six-, seven-, and eight-coordinate Mn(II) complexes of pendant-arm cyclen derivatives

The syntheses and characterization of several complexes of Mn(II) with cyclen derivatives having variable numbers of pendant N-acetic acid or N-acetamide arms are reported. X-ray crystallographic results are presented for Mn(DOTAM)Cl(2) x 2 H(2)O (monoclinic C2/c, a = 18.5798(15), b = 13.6006(11), c = 10.5800(8) A, beta = 110.490(1) degrees, Z = 4), [Mn(DO3AM)][MnCl(4)] x EtOH (monoclinic P2(1)/n, a = 8.366(8), b = 19.483(2), c = 16.3627(16) A, beta = 99.254(2) degrees, Z = 4), and Mn(H(2)DOTA) (monoclinic C2/c, a = 16.374(3), b = 6.6559(13), c = 16.750(3) A, beta = 98.381(3) degrees, Z = 4), which exhibit 8-, 7-, and 6-coordinate Mn(II), respectively. (1)H relaxivity data in water at 20 MHz and 37 degrees C is presented and interpreted in terms of a mechanism involving transient binding of water in an associative intermediate. Relaxivity studies in mixed water/methanol solvents are consistent with this interpretation.

Gadolinium(III)-based blood-pool contrast agents for magnetic resonance imaging: status and clinical potential

Blood-pool MRI contrast agents have enormous potential to aid sensitive magnetic resonance detection and yield definitive diagnostic data of cancer and diseases of the cardiovascular system. Many attempts have been initiated to design macromolecular gadolinium (Gd[III]) complexes as magnetic resonance imaging blood-pool contrast agents, as macromolecules do not readily diffuse across healthy vascular endothelium, and remain intravascular. Although extremely efficacious in detecting and characterizing pathologic tissue, clinical development of these agents has been limited by potential toxicity concerns from incomplete Gd(III) clearance. Recent innovative technologies, such as reversible protein-binding contrast agents and biodegradable macromolecular contrast agents, may be valuable alternatives that combine the effective imaging characteristics of an intravascular contrast agent and the safety of clinically approved low-molecular-weight Gd(III) chelates.

MAGfect: a novel liposome formulation for MRI labelling and visualization of cells

Cellular entry of imaging probes, such as contrast agents for magnetic resonance imaging (MRI), is a key requirement for many molecular imaging studies, particularly imaging intracellular events and cell tracking. Here, we describe the successful development and in vitro analysis of MAGfect, a novel liposome formulation containing a lipidic gadolinium contrast agent for MRI, Gd-DOTA-Chol , designed to enter and label cells. Liposome formulation and cell incubation time were optimised for maximum cellular uptake of the imaging probe in a variety of cell lines. MRI analysis of cells incubated with MAGfect showed them to be highly MRI active. This formulation was examined further for cytotoxicity, cell viability and mechanism of cell labelling. One of the key advantages of using MAGfect as a labelling vehicle arises from its potential for additional functions, such as concomitant drug or gene delivery and fluorescent labelling. The gadolinium liposome was found to be an effective vehicle for transport of plasmid DNA (pDNA) into cells and expression levels were comparable to the commercial transfection agent Trojene.

Euterpe Olerácea (Açaí) as an alternative oral contrast agent in MRI of the gastrointestinal system: preliminary results

Using contrast agents is a common practice in medical imaging protocols. Paramagnetic properties of certain compounds present in contrast agents can affect magnetic resonance imaging (MRI) signals. For abdominal applications, they are usually injected, but may also be administered orally. However, their use as a routine technique is limited, mainly due to the lack of appropriate oral contrast agents. We herein present the preliminary characterization and results for implementation of Euterpe Olerácea (popularly named Açaí) as a possible clinical oral contrast agent for MRI of the gastrointestinal tract. The pulp of Açaí, a fruit from the Amazon area, presented an increase in T(1)-weighted MRI signal, equivalent to that of gadolinium-diethyltriamine pentaacetic acid, and a decrease in T(2)-weighted images. We looked for intrinsic properties that could be responsible for the T(1) signal enhancement and T(2) opacification. Atomic absorption spectra revealed the presence of Fe, Mn and Cu ions in Açai. The presence of such ions contribute to the susceptometric value found of chi = -4.83 x 10(-6). This finding assents with the hypothesis that image contrast changes were due to the presence of paramagnetic material. The first measurements in vivo demonstrate a clear increase of contrast, in T(1)-weighted images, due to the presence of Açaí. Consistently, the opacification in a T(2)-weighted acquisition was evident, revealing a good contrast on bowel walls of gastric tissues.

A novel series of complexones with bis- or biazole structure as mixed ligands of paramagnetic contrast agents for MRI

We describe the syntheses, physicochemical properties and biological evaluation of a novel series of complexones containing bis- or biazoles moieties and two iminodiacetic acid units as novel ligands for paramagnetic lanthanides. The complexones were prepared by reaction of the corresponding 1,1'-bishaloethylbi- or bispyrazoles with methyl iminodiacetate and subsequent NaOH hydrolysis. 1,1'-Bisbromoethyl precursors were obtained by direct alkylation with an excess of 1,2-dibromoethane, or by heating the corresponding alcohol in HCl. Sigmoidal binding isotherms and MO calculations supported as most stable structures in solution, those containing two Gd(III) atoms bound per molecule of complexone with half saturation values S(0.5) (M(-1), 22 degrees C, pH 7.2) in the range 6.5 10(-6)<S(0.5)<36.1 10(-6). Relaxivity properties [r(1), r(2), s(-1) mM(-1) Gd(III)] determined at 1.5 Tesla gave values (12.0<r(1)<17.7, 12.2<r(2)<20), improving significantly the relaxivities of reference compounds such as Gd(III)EDTA (5.2, 5.6) or Gd(III)DTPA (4.30, 4.30). These improvements involve mainly increased hydration and slower rotational motions. In vitro toxicity experiments are reported.