Macromolecules, dendrimers, and nanomaterials in magnetic resonance imaging: the interplay between size, function, and pharmacokinetics

Optimizing Water Exchange Rates and Rotational Mobility for High-Relaxivity of a Novel Gd-DO3A Derivative Complex Conjugated to Inulin as Macromolecular Contrast Agents for MRI

Thanks to the understanding of the relationships between the residence lifetime τ_M of the coordinated water molecules to macrocyclic Gd-complexes and the rotational mobility τ_R of these structures, and according to the theory for paramagnetic relaxation, it is now possible to design macromolecular contrast agents with enhanced relaxivities by optimizing these two parameters through ligand structural modification. We succeeded in accelerating the water exchange rate by inducing steric compression around the water binding site, and by removing the amide function from the DOTA-AA ligand [1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid mono(p-aminoanilide)] (L) previously designed. This new ligand 10[2(1-oxo-1-p-propylthioureidophenylpropyl]-1,4,7,10-tetraazacyclodecane-1,4,7-tetraacetic acid (L₁ ) was then covalently conjugated to API [O-(aminopropyl)inulin] to get the complex API-(GdL₁ )x with intent to slow down the rotational correlation time (τ_R ) of the macromolecular complex. The evaluation of the longitudinal relaxivity at different magnetic fields and the study of the ¹⁷ O-NMR at variable temperature of the low-molecular-weight compound (GdL₁ ) showed a slight decrease of the τ_M value (τM310 = 331 ns vs. τM310 = 450 ns for the GdL complex). Consequently to the increase of the size of the API-(GdL₁ )x complex, the rotational correlation time becomes about 360 times longer compared to the monomeric GdL₁ complex (τ_R  = 33,700 ps), which results in an enhanced proton relaxivity.

A New Bis(aquated) High Relaxivity Mn(II) Complex as an Alternative to Gd(III)-Based MRI Contrast Agent

Disclosed here are a piperazine, a pyridine, and two carboxylate groups containing pentadentate ligand H₂pmpa and its corresponding water-soluble Mn(II) complex (1). DFT-based structural optimization implied that the complex had pentagonal bipyramidal geometry where the axial positions were occupied by two water molecules, and the equatorial plane was constituted by the ligand ON₃O donor set. Thus, a bis(aquated) disc-like Mn(II) complex has been synthesized. The complex showed higher stability compared with Mn(II)-EDTA complex [log K_MnL = 14.29(3)] and showed a very high r₁ relaxivity value of 5.88 mM^-1 s^-1 at 1.41 T, 25 °C, and pH = 7.4. The relaxivity value remained almost unaffected by the pH of the medium in the range of 6-10. Although the presence of 200 equiv of fluoride and bicarbonate anions did not affect the relaxivity value appreciably, an increase in the value was noticed in the presence of phosphate anion due to slow tumbling of the complex. Cell viability measurements, as well as phantom MR images using clinical MRI imager, consolidated the possible candidature of complex 1 as a positive contrast agent.

Coordination-Triggered Hierarchical Folate/Zinc Supramolecular Hydrogels Leading to Printable Biomaterials

Printable hydrogels desired in bioengineering have extremely high demands on biocompatibility and mechanic strength, which can hardly be achieved in conventional hydrogels made with biopolymers. Here, we show that on employment of the strategy of coordination-triggered hierarchical self-assembly of naturally occurring small-molecule folic acid, supramolecular hydrogels with robust mechanical elastic modulus comparable to synthetic double-network polymer gels can be made at concentrations below 1%. A sequence of hierarchical steps are involved in the formation of this extraordinary hydrogel: petrin rings on folate form tetramers through hydrogen bonding, tetramers stack into nanofibers by π-π stacking, and zinc ions cross-link the nanofibers into larger-scale fibrils and further cross-link the fibril network to gel water. These supramolecular qualities endow the hydrogel with shear-thinning and instant healing ability, which makes the robust gel injectable and printable into various three-dimensional structures. Owing to the excellent biocompatibility, the gel can support cells three-dimensionally and can be used as an ideal carrier for imaging agent (Gd³⁺), as well as chemodrugs. In combination with its easy formation and abundant sources, this newly discovered metallo-folate supramolecular hydrogel is promising in various bioengineering technological applications.

Gadolinium-conjugated star-block copolymer polylysine-modified polyethylenimine as high-performance T 1 MR imaging blood pool contrast agents

Core-shell copolymers have received widespread attention because of their unique properties, such as suitable for surface modification and increasing the functionality. Thus, they have been increasingly used in many fields including biomedical, pharmaceutical, electronics and optics. Here, a new core-shell copolymer system was developed to synthesize potential blood pool contrast agent (CA) for magnetic resonance imaging (MRI). The novel CA with high T ₁ relaxivity was synthesized by conjugating gadolinium (Gd) chelators onto star-block copolymer polyethylenimine-grafted poly(l-lysine) (PEI-PLL) nanoparticles (NPs). The T ₁ relaxivity of PEI-PLL-DTPA-Gd NPs measured on a 7.0 T small animal MRI scanner was 8.289 mM^-1 s^-1, higher than that of T ₁ contrast agents widely used in the clinic, such as Gd-DTPA (also known as Magnevist, r ₁ = 4.273 mM^-1 s^-1). These results show that PEI-PLL-DTPA-Gd exhibits more efficient T ₁ MR contrast enhancement compared to Gd-DTPA. More importantly, the PEI-PLL-DTPA-Gd core-shell NPs exhibited extremely low toxicity when measured against the HepG2 cell line over a similar concentration rang of Magnevist. In in vivo experiments, PEI-PLL-DTPA-Gd not only displayed good T ₁ contrast enhancement for the abdominal aorta, but also showed prolonged blood circulation time compared with Gd-DTPA, which should enable longer acquisition time, for MR and MR angiographic images, with high resolution in clinical practice. PEI-PLL-DTPA-Gd NPs have potential to serve as high T ₁ relaxivity blood pool MRI CA in the clinic.

Gadolinium-Labeled Aminoglycoside and Its Potential Application as a Bacteria-Targeting Magnetic Resonance Imaging Contrast Agent

Magnetic resonance imaging (MRI) is a powerful diagnostic technique that can penetrate deep into tissue providing excellent spatial resolution without the need for ionizing radiation or harmful radionuclides. However, diagnosing bacterial infections in vivo with clinical MRI is severely hampered by the lack of contrast agents with high relaxivity, targeting capabilities, and bacterial penetration and specificity. Here, we report the development of the first gadolinium (Gd)-based bacteria-specific targeting MRI contrast agent, probe 1, by conjugating neomycin, an aminoglycoside antibiotic, with Dotarem (Gd-DOTA, an FDA approved T₁-weighted MRI contrast agent). The T₁ relaxivity of probe 1 was found to be comparable to that of Gd-DOTA; additionally, probe 1-treated bacteria generated a significantly brighter T₁-weighted MR signal than Gd-DOTA-treated bacteria. More importantly, in vitro cellular studies and preliminary in vivo MRI demonstrated probe 1 exhibits the ability to efficiently target bacteria over macrophage-like cells, indicating its great potential for high-resolution imaging of bacterial infections in vivo.

Cross-Linked and Biodegradable Polymeric System as a Safe Magnetic Resonance Imaging Contrast Agent

Owing to the low efficacy of clinically used small-molecule gadolinium (Gd)-based magnetic resonance imaging (MRI) agents, we designed and explored biodegradable macromolecular conjugates as MRI contrast agents. The linear polymeric structure and core-cross-linked formulation possessed different characteristics and features, so we prepared and comparatively studied the two kinds of Gd-based N-(2-hydroxypropyl) methacrylamide (HPMA) polymeric systems (the core-cross-linked pHPMA-DOTA-Gd and the linear one) using the clinical agent diethylene-triamine pentaacetic acid-Gd(III) (DTPA-Gd) as a control. This study was aimed to find the optimal polymeric formulation as a biocompatible and efficient MRI contrast agent. The high molecular weight (MW, 181 kDa) and core-cross-linked copolymer was obtained via the cross-linked block linear copolymer and could be degraded to low-MW segments (29 kDa) in the presence of glutathione (GSH) and cleaned from the body. Both core-cross-linked and linear pHPMA-DOTA-Gd copolymers displayed 2-3-fold increased relaxivity (r₁ value) than that of DTPA-Gd. Animal studies demonstrated that two kinds of macromolecular systems led to much longer blood circulation time, higher tumor accumulation, and much higher signal intensity compared with the linear and clinical ones. Finally, in vivo and in vitro toxicity studies indicated that the two macromolecular agents had great biocompatibility. Therefore, we performed preliminary but important studies on the Gd-based HPMA polymeric systems as biocompatible and efficient MRI contrast agents and found that the biodegradable core-cross-linked pHPMA-DOTA-Gd copolymer might have greater benefits for the foreground.

Ultrasound promoted one-pot synthesis of 3,4-dihydropyrimidin-2(1H)-ones/thiones using dendrimer-attached phosphotungstic acid nanoparticles immobilized on nanosilica

A highly efficient and improved synthetic methodology for the preparation of 3,4-dihydropyrimidin-2(1H)-ones and thiones in the Biginelli reaction by condensation of β-dicarbonyl compounds, urea/thiourea and aromatic aldehydes using Dendrimer-PWAⁿ as a nanocatalyst under ultrasonic irradiation is reported. Compared with classical Biginelli reaction conditions, this new method has the advantage of easy reaction condition, short reaction time, excellent yields, recyclable green catalyst, reduced environmental impacts and absence of any tedious workup or purification. The catalyst can be easily recovered by filtration and reused for eight consecutive reaction cycles without significant loss of activity. Also, SEM and DLS images of the catalyst after reaction cycle were investigated.

Responsive crosslinked polymer nanogels for imaging and therapeutics delivery

Nanogels are water-soluble crosslinked polymer networks with tremendous potential in targeted imaging and controlled drug and gene delivery.

Supramolecular assemblies based on amphiphilic Mn2+-complexes as high relaxivity MRI probes

The Mn²⁺ complexes of amphiphilic derivatives of EDTA and 1,4-DO2A ligands show a strong increase in relaxivity upon micellar aggregation and human serum albumin binding.

Recent progress in the electron paramagnetic resonance study of polymers

This review article provides an overview of the contemporary research based on a tailor-made technique to understand the paramagnetic behavior of different polymer classes.

Nitroxide radical polymers – a versatile material class for high-tech applications

A comprehensive summary of synthetic strategies for the preparation of nitroxide radical polymer materials and a state-of-the-art perspective on their latest and most exciting applications.

Metal–organic framework nanoparticles for magnetic resonance imaging

This review aims to integrate the state-of-the-art of MOF nanoparticles and their use in MRI. It gives an overview of the work done so far, focusing especially on the clinical applicability. Furthermore, it summarises the different factors for MR signal formation mechanisms important for the development of MR active nanoparticles and provides suggestions for a better comparison between different studies.

A new heptadentate picolinate-based ligand and its corresponding Gd(iii) complex: the effect of pendant picolinate versus acetate on complex properties

Replacing one picolinate pendant by acetate group in H₄bpeda ligand, the synthesised bis(aquated) Gd(iii) complex of ligand H4peada showed better stability and r₁ relaxivity for its potential use as MRI contrast agent.

Amphiphilic complexes of Ho(iii), Dy(iii), Tb(iii) and Eu(iii) for optical and high field magnetic resonance imaging

Amphiphilic lanthanide(iii) complexes self-assemble into monodisperse micelles with favourable properties for optical and high field magnetic resonance imaging.

Zwitterionic Manganese and Gadolinium Metal-Organic Frameworks as Efficient Contrast Agents for in Vivo Magnetic Resonance Imaging

Two water-stable three-dimensional Mn- and Gd-based metal-organic frameworks (MOFs), {[Mn₂(Cmdcp)₂(H₂O)₂]·H₂O}_n (1) and {[Gd(Cmdcp)(H₂O)₃](NO₃)·3H₂O}_n (2, H₃CmdcpBr = N-(4-carboxy benzyl)-(3,5-dicarboxyl)pyridinium bromide), have been prepared and analyzed. In vitro magnetic resonance imaging indicated that MOFs 1 and 2 possess relaxivity r₁ values of 17.50 and 13.46 mM^-1·S^-1, respectively, which are superior to that of the control Gd-DTPA (r₁ = 4.87 mM^-1·S^-1, DTPA = diethylene triamine pentaacetate). MOFs 1 and 2 also possessed good biocompatibility and low cytotoxicity against a model cell line. In vivo magnetic resonance images of treated Kunming mice indicated that kidneys showed remarkably positive signal enhancement after 15 min with intravenous administration of MOF 1 and the hyperintensity of both kidneys persisted for about 240 min with no obvious tissue damage. MOF 1 is therefore promising in vivo probes for imaging intravascular diseases and renal dysfunction.

High Relaxivity Gadolinium-Polydopamine Nanoparticles

This study reports the preparation of a series of gadolinium-polydopamine nanoparticles (GdPD-NPs) with tunable metal loadings. GdPD-NPs are analyzed by nuclear magnetic relaxation dispersion and with a 7-tesla (T) magnetic resonance imaging (MRI) scanner. A relaxivity of 75 and 10.3 mM^-1 s^-1 at 1.4 and 7 T is observed, respectively. Furthermore, superconducting quantum interference device magnetometry is used to study intraparticle magnetic interactions and determine the GdPD-NPs consist of isolated metal ions even at maximum metal loadings. From these data, it is concluded that the observed high relaxivities arise from a high hydration state of the Gd(III) at the particle surface, fast rate of water exchange, and negligible antiferromagnetic coupling between Gd(III) centers throughout the particles. This study highlights design parameters and a robust synthetic approach that aid in the development of this scaffold for T₁ -weighted, high relaxivity MRI contrast agents.

Hydration number: crucial role in nuclear magnetic relaxivity of Gd(III) chelate-based nanoparticles

Today, nanostructure-based contrast agents (CA) are emerging in the field of magnetic resonance imaging (MRI). Their sensitivity is reported as greatly improved in comparison to commercially used chelate-based ones. The present work is aimed at revealing the factors governing the efficiency of longitudinal magnetic relaxivity (r₁) in aqueous colloids of core-shell Gd(III)-based nanoparticles. We report for the first time on hydration number (q) of gadolinium(III) as a substantial factor in controlling r₁ values of polyelectrolyte-stabilized nanoparticles built from water insoluble complexes of Gd(III). The use of specific complex structure enables to reveal the impact of the inner-sphere hydration number on both r₁ values for the Gd(III)-based nanoparticles and the photophysical properties of their luminescent Tb(III) and Eu(III) counterparts. The low hydration of TTA-based Gd(III) complexes (q ≈ 1) agrees well with the poor relaxivity values (r₁ = 2.82 mM^-1s^-1 and r₂ = 3.95 mM^-1s^-1), while these values tend to increase substantially (r₁ = 12.41 mM^-1s^-1, r₂ = 14.36 mM^-1s^-1) for aqueous Gd(III)-based colloids, when macrocyclic 1,3-diketonate is applied as the ligand (q ≈ 3). The regularities obtained in this work are fundamental in understanding the efficiency of MRI probes in the fast growing field of nanoparticulate contrast agents.

Metabolomic Analysis of N-acetylcysteine Protection of Injury from Gadolinium-DTPA Contrast Agent in Rats with Chronic Renal Failure

Gadolinium-based contrast agents (GBCAs) are frequently used to enhance the diagnostic efficacy of magnetic resonance imaging. On the other hand, the association between GBCA administration in patients with advanced renal disease and nephrogenic systemic fibrosis (NSF) was also noted. NSF is a systemic disorder characterized by widespread tissue fibrosis that may lead to death. N-acetylcysteine (NAC) protects rats from injury induced by gadolinium-based contrast agents, but the underlying mechanisms remain unclear. In this study, a nuclear magnetic resonance-based metabolomic approach was used to systematically investigate the protective effects of NAC on Gd-DTPA-induced injury. Thirty-two male Sprague-Dawley rats were given adenine (200 mg·kg^-1 body weight) by oral gavage once a day for 3 weeks to induce chronic renal failure (CRF). NAC (600 mg/L in drinking water for 9 days) pretreatment was initiated 2 days before Gd-DTPA injection (a single tail vein injection, 2 mmol/kg body weight). Serum and liver samples were collected on day 7 after Gd-DTPA injection. By study design, the serum and hepatic metabolic changes of rats were measured in four groups of eight each: CRF, CRF-Gd, CRF-Gd-NAC, and CRF-NAC. Gd-DTPA administration to rats with CRF resulted in disturbances of several metabolic pathways, including glucose, lipid, glutamate, choline, gut microbiota, one-carbon, and purine metabolism. NAC pretreatment reversed the abundance changes of high-density lipoprotein, low-density lipoprotein, very low-density lipoprotein, glutamate, glutamine, oxidized glutathione, choline, phosphocholine, glycerophosphocholine, trimethylamine, and trimethylamine-N-oxide induced by Gd-DTPA. It is noteworthy, however, that the ameliorating effects of NAC on the disturbance of glutamate, choline, and gut microbiota metabolism may be specific to Gd-DTPA. In all, these findings could be potentially useful to decipher the underlying mechanisms of NAC protective effects from the injury induced by gadolinium-based contrast agents.

Nitroxide-Based Macromolecular Contrast Agents with Unprecedented Transverse Relaxivity and Stability for Magnetic Resonance Imaging of Tumors

Metal-free magnetic resonance imaging (MRI) agents could overcome the established toxicity associated with metal-based agents in some patient populations and enable new modes of functional MRI in vivo. Herein, we report nitroxide-functionalized brush-arm star polymer organic radical contrast agents (BASP-ORCAs) that overcome the low contrast and poor in vivo stability associated with nitroxide-based MRI contrast agents. As a consequence of their unique nanoarchitectures, BASP-ORCAs possess per-nitroxide transverse relaxivities up to ∼44-fold greater than common nitroxides, exceptional stability in highly reducing environments, and low toxicity. These features combine to provide for accumulation of a sufficient concentration of BASP-ORCA in murine subcutaneous tumors up to 20 h following systemic administration such that MRI contrast on par with metal-based agents is observed. BASP-ORCAs are, to our knowledge, the first nitroxide MRI contrast agents capable of tumor imaging over long time periods using clinical high-field 1H MRI techniques.

Vesicles from Amphiphilic Dumbbells and Janus Dendrimers: Bioinspired Self-Assembled Structures for Biomedical Applications

The current review focuses on vesicles obtained from the self-assembly of two types of dendritic macromolecules, namely amphiphilic Janus dendrimers (forming dendrimersomes) and amphiphilic dumbbells. In the first part, we will present some synthetic strategies and the various building blocks that can be used to obtain dendritic-based macromolecules, thereby showing their structural versatility. We put our focus on amphiphilic Janus dendrimers and amphiphilic dumbbells that form vesicles in water but we also encompass vesicles formed thereof in organic solvents. The second part of this review deals with the production methods of these vesicles at the nanoscale but also at the microscale. Furthermore, the influence of various parameters (intrinsic to the amphiphilic JD and extrinsic-from the environment) on the type of vesicle formed will be discussed. In the third part, we will review the numerous biomedical applications of these vesicles of nano- or micron-size.

Gadolinium-Labeled Biodegradable Dendron-Hyaluronic Acid Hybrid and Its Subsequent Application as a Safe and Efficient Magnetic Resonance Imaging Contrast Agent

Novel magnetic resonance imaging (MRI) contrast agents with high sensitivity and good biocompatibility are required for the diagnosis of cancer. Herein, we prepared and characterized the gadolinium [Gd(III)]-labeled peptide dendron-hyaluronic acid (HA) conjugate-based hybrid (dendronized-HA-DOTA-Gd) by combining the advantages of HA and the peptide dendron. The dendronized-HA-DOTA-Gd hybrid with 3.8% Gd(III) as weight percentage showed a negative zeta potential (-35 mV). The in vitro degradation results indicated that the dendronized-HA-DOTA-Gd hybrid degraded into products with low molecular weights in the presence of hyaluronidase. The dendronized-HA-DOTA-Gd hybrid showed a 3-fold increase in longitudinal relaxivity and much higher in vivo signal enhancement in 4T1 breast tumors of mice compared with clinical Magnevist (Gd-DTPA). The dendronized-HA-DOTA-Gd hybrid had a higher accumulation in tumors than Gd-DTPA; it was 2-3-fold after administration. Meanwhile, the polymeric hybrid resulted in low Gd(III) residue in the body compared with that of Gd-DTPA. The systematic biosafety evaluations, including blood compatibility and toxicity assessments, suggested that the dendronized-HA-DOTA-Gd hybrid exhibited good biocompatibility. Thus, the gadolinium-labeled and dendronized HA hybrid shows promise as a safe and efficient macromolecular MRI contrast agent based on high sensitivity, low residue content in the body, and good biosafety.

T1-T2 Dual-Modal Magnetic Resonance Imaging: From Molecular Basis to Contrast Agents

Multimodal imaging strategies integrating manifold images have improved our ability to diagnose, to guide therapy, and to predict outcomes. Magnetic resonance imaging (MRI) is among the most widely used imaging technique in the clinic and can enable multiparameter anatomical demonstration of diagnosis. Due to the inherent black-and-white production of MR images, however, MRI detection is largely hampered by the occurrence of false-positive diagnoses. In this Perspective, we introduce the paradigm of manipulating the multiparameter MRI, T₁-T₂ dual-modal MRI, along with enhancement by specific contrast agents. We hope this discussion will promote emerging research interest in T₁-T₂ dual-modal MRI and provoke the rational design of contrast agents for sophisticated MRI applications.

In situ targeted MRI detection of Helicobacter pylori with stable magnetic graphitic nanocapsules

Helicobacter pylori infection is implicated in the aetiology of many diseases. Despite numerous studies, a painless, fast and direct method for the in situ detection of H. pylori remains a challenge, mainly due to the strong acidic/enzymatic environment of the gastric mucosa. Herein, we report the use of stable magnetic graphitic nanocapsules (MGNs), for in situ targeted magnetic resonance imaging (MRI) detection of H. pylori. Several layers of graphene as the shell effectively protect the magnetic core from corrosion while retaining the superior contrast effect for MRI in the gastric environment. Boronic-polyethylene glycol molecules were synthesized and modified on the MGN surface for targeted MRI detection. In a mouse model of H. pylori-induced infection, H. pylori was specifically detected through both T₂-weighted MR imaging and Raman gastric mucosa imaging using functionalized MGNs. These results indicated that enhancement of MRI using MGNs may be a promising diagnostic and bioimaging platform for very harsh conditions.

Crosslinked shells for nano-assembled capsules: a new encapsulation method for smaller Gd3+-loaded capsules with exceedingly high relaxivities

Nano-assembled capsules can incorporate large payloads of high relaxivity Gd3+, permitting the development of highly detectable molecular imaging agents for MRI. A new encapsulating shell, based upon cross-linked peptides, is found to afford smaller capsules (127 nm average diameter) with exceptionally high per-Gd3+ relaxivities (70.7 s-1 mmolal-1).

Polymeric 1 H MRI Probes for Visualizing Tumor In Vivo

Magnetic resonance imaging (MRI) has become a prominent non- or low-invasive imaging technique, providing high-resolution, three-dimensional images as well as physiological information about tissues. Low-molecular-weight Gd-MRI contrast agents (CAs), such as Gd-DTPA (DTPA: diethylenetriaminepentaacetic acid), are commonly used in the clinical diagnosis, while macromolecular Gd-MRI CAs have several advantages over low-molecular-weight Gd-MRI CAs, which help minimize the dose of CAs and the risk of side effects. Accordingly, we developed chiral dendrimer Gd-MRI CAs, which showed high r₁ values. The association constant values (K_a ) of S-isomeric dendrimer CAs to bovine serum albumin (BSA) were higher than those of R-isomeric dendrimer CAs. Besides, based on a totally new concept, we developed ¹³ C/¹⁵ N-enriched multiple-resonance NMR/MRI probes, which realized highly selective observation of the probes and analysis of metabolic reactions of interest. This account summarizes our recent study on developing both chiral dendrimer Gd-MRI CAs, and self-traceable ¹³ C/¹⁵ N-enriched phosphorylcholine polymer probes for early detection of tumors.

Albumin-based nanoparticles loaded with hydrophobic gadolinium chelates as T1-T2 dual-mode contrast agents for accurate liver tumor imaging

Magnetic resonance contrast agents with T₁-T₂ dual mode contrast capability have attracted considerable interest because they offer complementary and synergistic diagnostic information, leading to high imaging sensitivity and accurate diagnosis. Here, we reported a facile strategy to construct albumin based nanoparticles loaded with hydrophobic gadolinium chelates by hydrophobic interaction for magnetic resonance imaging (MRI). We synthesized a glycyrrhetinic acid-containing Gd-DOTA derivative (GGD) and loaded GGD molecules into BSA nanoparticles to form GGD-BSA nanoparticles (GGD-BSA NPs). The large size and porous structure endow GGD-BSA NPs with geometrical confinement, which restricts the tumbling of GGD and the diffusion of surrounding water molecules. As a result, GGD-BSA NPs exhibit ultrahigh T₁ and T₂ relaxivities, which are approximately 8-fold higher than those of gadolinium-based clinical contrast agents at 0.5 T. Besides, due to the intrinsic properties of their components, GGD-BSA NPs show good biocompatibility in vitro and in vivo, which warrants their great potential in clinical translation. Furthermore, GGD-BSA NPs show remarkable sensitivity in noninvasive detection of liver tumors by self-confirmed T₁-T₂ dual-mode contrast-enhanced MRI. All of these merits make GGD-BSA NPs a potential candidate for fruitful biomedical and preclinical applications.

Expanding the potential of MRI contrast agents through multifunctional polymeric nanocarriers

MRI is a sought-after, noninvasive tool in medical diagnostics, yet the direct application of contrast agents to tissue suffers from several drawbacks. Hosting the contrast agents in polymeric nanocarriers can solve many of these issues while creating additional benefit through exploitation of the intrinsic characteristics of the polymeric carriers. In this report, the versatility is highlighted with recent examples of dendritic and hyperbranched polymers, polymer nanoparticles and micelles, and polymersomes as multifunctional bioresponsive nanocarriers for MRI contrast agents.

Gadolinium-Loaded Poly(N-vinylcaprolactam) Nanogels: Synthesis, Characterization, and Application for Enhanced Tumor MR Imaging

We report the synthesis of poly(N-vinylcaprolactam) nanogels (PVCL NGs) loaded with gadolinium (Gd) for tumor MR imaging applications. The PVCL NGs were synthesized via precipitation polymerization using the monomer N-vinylcaprolactam (VCL), the comonomer acrylic acid (AAc), and the degradable cross-linker 3,9-divinyl-2,4,8,10-tetraoxaspiro-[5,5]-undecane (VOU) in aqueous solution, followed by covalently binding with 2,2',2″-(10-(4-((2-aminoethyl)amino)-1-carboxy-4-oxobutyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl) triacetic acid (NH₂-DOTA-GA)/Gd complexes. We show that the formed Gd-loaded PVCL NGs (PVCL-Gd NGs) having a size of 180.67 ± 11.04 nm are water dispersible, colloidally stable, uniform in size distribution, and noncytotoxic in a range of the studied concentrations. The PVCL-Gd NGs also display a r₁ relaxivity (6.38-7.10 mM^-1 s^-1), which is much higher than the clinically used Gd chelates. These properties afforded the use of the PVCL-Gd NGs as an effective positive contrast agent for enhanced MR imaging of cancer cells in vitro as well as a subcutaneous tumor model in vivo. Our study suggests that the developed PVCL-Gd NGs could be applied as a promising contrast agent for T₁-weighted MR imaging of diverse biosystems.

Gd-DTPA-induced dynamic metabonomic changes in rat biofluids

OBJECTIVES

The purposes of this study were (1) to detect the dynamic metabonomic changes induced by gadopentetate dimeglumine (Gd-DTPA) and (2) to investigate the potential metabolic disturbances associated with the pathogenesis of nephrogenic systemic fibrosis (NSF) at the early stage.

METHODS

A nuclear magnetic resonance (NMR)-based metabolomics approach was used to investigate the urinary and serum metabolic changes induced by a single tail vein injection of Gd-DTPA (dosed at 2 and 5mmol/kg body weight) in rats. Urine and serum samples were collected on days 1, 2 and 7 after dosing.

RESULTS

Metabolic responses of rats to Gd-DTPA administration were systematic involving changes in lipid metabolism, glucose metabolism, TCA cycle, amino acid metabolism and gut microbiota functions. Urinary and serum metabonomic recovery could be observed in both the 2 and 5mmol/kg body weight group, but the metabolic effects of high-dosed (5mmol/kg body weight) Gd-DTPA lasted longer. It is worth noting that hyperlipidemia was observed after Gd-DTPA injection, and nicotinate might play a role in the subsequent self-recovery of lipid metabolism. The disturbance of tyrosine, glutamate and gut microbiota metabolism might associate with the progression of NSF.

CONCLUSION

These findings offered essential information about the metabolic changes induced by Gd-DTPA, and could be potentially important for investigating the pathogenesis of NSF at the early stage. Moreover, the recovery of rats administrated with Gd-DTPA may have implications in the treatment of early stage NSF.

Geometrical confinement directed albumin-based nanoprobes as enhanced T1 contrast agents for tumor imaging

We report a facile strategy to assemble geometrically confined albumin-based nanoparticles as T₁ contrast agents for sensitive tumor imaging.

A colloidally stable water dispersion of Ni nanowires as an efficient T2-MRI contrast agent

A colloidally stable dispersion of anisotropic Ni nanowires in water has been achieved showing good performance as a T₂-contrast agent in MRI.

Synthetic methodologies and spatial organization of metal chelate dendrimers and star and hyperbranched polymers

The synthetic methodologies, physico-chemical peculiarities, properties, and structure of metal chelate dendrimers and star and hyperbranched polymers are considered.

Synthesis and Evaluation of a Biocompatible Macromolecular Gadolinium Compound as a Liver-Specific Contrast Agent for MRI

A new macromolecular biocompatible gadolinium chelate complex (PAI-N2-DOTA-Gd) as a liver-specific magnetic resonance imaging (MRI) contrast agent was synthesised and evaluated. An aspartic acid–isoleucine copolymer was chemically linked with Gd-DOTA via ethylenediamine to give PAI-N2-DOTA-Gd. In vitro, the T1-relaxivity of PAI-N2-DOTA-Gd (14.38 mmol–1⋅L⋅s–1, 0.5 T) was much higher than that of the clinically used Gd-DOTA (4.96 mmol–1⋅L⋅s–1, 0.5 T), with obvious imaging signal enhancement. In the imaging experiments in vivo, PAI-N2-DOTA-Gd exhibited good liver selectivity, and had a greater intensity enhancement (68.8 ± 5.6 %) and a longer imaging window time (30–70 min), compared to Gd-DOTA (21.1 ± 5.3 %, 10–30 min). Furthermore, the in vivo histological studies of PAI-N2-DOTA-Gd showed a low acute toxicity and desirable biocompatibility. The results of this study indicate that PAI-N2-DOTA-Gd is a feasible liver-specific contrast agent for MRI.

Stimuli-responsive biodegradable and gadolinium-based poly[N-(2-hydroxypropyl) methacrylamide] copolymers: their potential as targeting and safe magnetic resonance imaging probes

Functionalized and biodegradable block pHPMA copolymer–gadolinium conjugates demonstrated good biocompatibility, high T₁ relaxivity, and enhanced tumor signal intensity for MRI.

Multi-arm star-branched polymer as an efficient contrast agent for tumor-targeted magnetic resonance imaging

Contrast agents with high efficiency and safety are excellent candidates as magnetic resonance imaging probes.

Optical and Magnetic Resonance Imaging Using Fluorous Colloidal Nanoparticles

Improved imaging of cancerous tissue has the potential to aid prognosis and improve patient outcome through longitudinal imaging of treatment response and disease progression. While nuclear imaging has made headway in cancer imaging, fluorinated tracers that enable magnetic resonance imaging (¹⁹F MRI) hold promise, particularly for repeated imaging sessions because nonionizing radiation is used. Fluorine MRI detects molecular signatures by imaging a fluorinated tracer and takes advantage of the spatial and anatomical resolution afforded by MRI. This manuscript describes a fluorous polymeric nanoparticle that is capable of ¹⁹F MR imaging and fluorescent tracking for in vitro and in vivo monitoring of immune cells and cancerous tissue. The fluorous particle is derived from low-molecular-weight amphiphilic copolymers that self-assemble into micelles with a hydrodynamic diameter of 260 nm. The polymer is MR-active at concentrations as low as 2.1 mM in phantom imaging studies. The fluorinated particle demonstrated rapid uptake into immune cells for potential cell-tracking or delineation of the tumor microenvironment and showed negligible toxicity. Systemic administration indicates significant uptake into two tumor types, triple-negative breast cancer and ovarian cancer, with little accumulation in off-target tissue. These results indicate a robust platform imaging agent capable of immune cell tracking and systemic disease monitoring with exceptional uptake of the nanoparticle in multiple cancer models.

Preparation and In Vitro Characterization of Dendrimer-based Contrast Agents for Magnetic Resonance Imaging

Paramagnetic complexes of gadolinium(III) with acyclic or macrocyclic chelates are the most commonly used contrast agents (CAs) for magnetic resonance imaging (MRI). Their purpose is to enhance the relaxation rate of water protons in tissue, thus increasing the MR image contrast and the specificity of the MRI measurements. Current clinically approved contrast agents are low molecular weight molecules that are rapidly cleared from the body. The use of dendrimers as carriers of paramagnetic chelators can play an important role in the future development of more efficient MRI contrast agents. Specifically, the increase in local concentration of the paramagnetic species results in a higher signal contrast. Furthermore, this CA provides a longer tissue retention time due to its high molecular weight and size. Here, we demonstrate a convenient procedure for the preparation of macromolecular MRI contrast agents based on poly(amidoamine) (PAMAM) dendrimers with monomacrocyclic DOTA-type chelators (DOTA - 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate). The chelating unit was appended by exploiting the reactivity of the isothiocyanate (NCS) group towards the amine surface groups of the PAMAM dendrimer to form thiourea bridges. Dendrimeric products were purified and analyzed by means of nuclear magnetic resonance spectroscopy, mass spectrometry, and elemental analysis. Finally, high resolution MR images were recorded and the signal contrasts obtained from the prepared dendrimeric and the commercially available monomeric agents were compared.

Octreotide Functionalized Nano-Contrast Agent for Targeted Magnetic Resonance Imaging

Reversible addition-fragmentation chain transfer (RAFT) polymerization has been employed to synthesize branched block copolymer nanoparticles possessing 1,4,7,10-tetraazacyclododecane-N,N,'N,″N,‴-tetraacetic acid (DO3A) macrocycles within their cores and octreotide (somatostatin mimic) cyclic peptides at their periphery. These polymeric nanoparticles have been chelated with Gd³⁺ and applied as magnetic resonance imaging (MRI) nanocontrast agents. This nanoparticle system has an r₁ relaxivity of 8.3 mM^-1 s^-1, which is 3 times the r₁ of commercial gadolinium-based contrast agents (GBCAs). The in vitro targeted binding efficiency of these nanoparticles shows 5 times greater affinity to somatostatin receptor type 2 (SSTR2) with K_i = 77 pM (compared to somatostatin with K_i = 0.385 nM). We have also evaluated the tumor targeting molecular imaging ability of these branched copolymer nanoparticle in vivo using nude/NCr mice bearing AR42J rat pancreatic tumor (SSTR2 positive) and A549 human lung carcinoma tumor (SSTR2 negative) xenografts.

Superparamagnetic Fe3O4-PEG2K-FA@Ce6 Nanoprobes for in Vivo Dual-mode Imaging and Targeted Photodynamic Therapy

The development of targeted nanoprobes is a promising approach to cancer diagnostics and therapy. In the present work, a novel multifunctional photo/magnet-diagnostic nanoprobe (MNPs-PEG_2K-FA@Ce6) has been developed. This nanoprobe is built using folic acid (FA), bifunctional polyethylene glycol (PEG_2K) and photosensitizer chlorin e6 (Ce6). The MNPs-PEG_2K-FA@Ce6 nanoprobes are superparamagnetic, can be synthesized on a large scale by a one-pot hydrothermal process without further surface modification and are stable in an aqueous environment for eight months. Compared with free Ce6 nanoprobes in vitro studies, the MNPs-PEG_2K-FA@Ce6 nanoprobes significantly enhance cellular uptake efficiency and promote the effectiveness of photodynamic therapy (PDT) with the assistance of 633 nm laser irradiation. The unique nanoprobes show superior penetration and a retention time of more than six days with less accumulation in the liver allowing highly effective tumor recognition and monitoring. Additionally, there was little damage to healthy organs or tissues. These exciting new nanoprobes could be potential building blocks to develop new clinical therapies and translational medicine.

A superparamagnetic polymersome with extremely high T2 relaxivity for MRI and cancer-targeted drug delivery

Improving the relaxivity of magnetic resonance imaging (MRI) contrast agents is an important challenge for cancer theranostics. Herein we report the design, synthesis, characterization, theoretical analysis and in vivo tests of a superparamagnetic polymersome as a new MRI contrast agent with extremely high T₂ relaxivity (611.6 mM^-1s^-1). First, a noncytotoxic cancer-targeting polymersome is synthesized based on a biodegradable diblock copolymer, folic acid-poly(l-glutamic acid)-block-poly(ε-caprolactone) [FA-PGA-b-PCL]. Then, ultra-small superparamagnetic iron oxide nanoparticles (SPIONs) are in situ generated in the hydrophilic PGA coronas of polymersomes to afford magnetic polymersomes. The in vivo MRI assay revealed prominent negative contrast enhancement of magnetic polymersomes at a very low Fe dose of 0.011 mmol/kg. Moreover, this cancer-targeting magnetic polymersome can effectively encapsulate and deliver anticancer drug to inhibit the tumor growth, demonstrating promising theranostic applications in biomedicine.