Surface Modification of Gd Nanoparticles with pH-Responsive Block Copolymers for Use As Smart MRI Contrast Agents

Putting Hybrid Nanomaterials to Work for Biomedical Applications

Hybrid nanomaterials have found use in many biomedical applications. This article provides a comprehensive review of the principles, techniques, and recent advancements in the design and fabrication of hybrid nanomaterials for biomedicine. We begin with an introduction to the general concept of material hybridization, followed by a discussion of how this approach leads to materials with additional functionality and enhanced performance. We then highlight hybrid nanomaterials in the forms of nanostructures, nanocomposites, metal-organic frameworks, and biohybrids, including their fabrication methods. We also showcase the use of hybrid nanomaterials to advance biomedical engineering in the context of nanomedicine, regenerative medicine, diagnostics, theranostics, and biomanufacturing. Finally, we offer perspectives on challenges and opportunities.

Recent development of pH-responsive theranostic nanoplatforms for magnetic resonance imaging-guided cancer therapy

The acidic characteristic of the tumor site is one of the most well-known features and provides a series of opportunities for cancer-specific theranostic strategies. In this regard, pH-responsive theranostic nanoplatforms that integrate diagnostic and therapeutic capabilities are highly developed. The fluidity of the tumor microenvironment (TME), with its temporal and spatial heterogeneities, makes noninvasive molecular magnetic resonance imaging (MRI) technology very desirable for imaging TME constituents and developing MRI-guided theranostic nanoplatforms for tumor-specific treatments. Therefore, various MRI-based theranostic strategies which employ assorted therapeutic modes have been drawn up for more efficient cancer therapy through the raised local concentration of therapeutic agents in pathological tissues. In this review, we summarize the pH-responsive mechanisms of organic components (including polymers, biological molecules, and organosilicas) as well as inorganic components (including metal coordination compounds, metal oxides, and metal salts) of theranostic nanoplatforms. Furthermore, we review the designs and applications of pH-responsive theranostic nanoplatforms for the diagnosis and treatment of cancer. In addition, the challenges and prospects in developing theranostic nanoplatforms with pH-responsiveness for cancer diagnosis and therapy are discussed.

Metal-organic frameworks: A promising option for the diagnosis and treatment of Alzheimer's disease

Beta-amyloid (Aβ) peptide is one of the main characteristic biomarkers of Alzheimer's disease (AD). Previous clinical investigations have proposed that unusual concentrations of this biomarker in cerebrospinal fluid, blood, and brain tissue are closely associated with the AD progression. Therefore, the critical point of early diagnosis, prevention, and treatment of AD is to monitor the levels of Aβ. In view of the potential of metal-organic frameworks (MOFs) for diagnosing and treating the AD, much attention has been focused in recent years. This review discusses the latest advances in the applications of MOFs for the early diagnosis of AD via fluorescence and electrochemiluminescence (ECL) detection of AD biomarkers, fluorescence detection of the main metal ions in the brain (Zn²⁺, Cu²⁺, Mn²⁺, Fe³⁺, and Al³⁺) in addition to magnetic resonance imaging (MRI) of the Aβ plaques. The current challenges and future strategies for translating the in vitro applications of MOFs into in vivo diagnosis of the AD are discussed.

Engineering nanosystems to overcome barriers to cancer diagnosis and treatment

Over the past two decades, multidisciplinary investigations into the development of nanoparticles for medical applications have continually increased. However, nanoparticles are still subject to biological barriers and biodistribution challenges, which limit their overall clinical potential. This has motivated the implementation of innovational modifications to a range of nanoparticle formulations designed for cancer imaging and/or cancer treatment to overcome specific barriers and shift the accumulation of payloads toward the diseased tissues. In recent years, novel technological and chemical approaches have been employed to modify or functionalize the surface of nanoparticles or manipulate the characteristics of nanoparticles. Combining these approaches with the identification of critical biomarkers provides new strategies for enhancing nanoparticle specificity for both cancer diagnostic and therapeutic applications. This review discusses the most recent advances in the design and engineering of nanoparticles as well as future directions for developing the next generation of nanomedicines.

Recent Advances in Nanotheranostic Agents for Tumor Microenvironment–Responsive Magnetic Resonance Imaging

Nanomaterials integrating a variety of excellent properties (such as controllable/suitable size, surface modifier, and multifunctionality) have attracted increasing attention in the biomedical field and have been considered a new generation of magnetic resonance imaging (MRI) contrast agents (CAs). In recent years, stimuli-responsive nanomaterials with specifically responsive ability have been synthesized as MRI CAs, which can significantly improve the diagnostic sensitivity and accuracy depending on their outstanding performance. Furthermore, the inherent tumor microenvironment (TME) of malignant tumor is considered to possess several unique features, such as low extracellular pH, redox condition, hypoxia, and high interstitial pressure, that are significantly different from healthy tissues. Hence, constructing nanomaterials for TME-responsive MRI as an emerging strategy is expected to overcome the current obstacles to precise diagnosis. This review focuses on recent advances of nanomaterials in their application of TME-responsive MRI that trigger the diagnostic function in response to various endogenous stimulations, including pH, redox, enzyme, and hypoxia. Moreover, the future challenges and trends in the development of nanomaterials serving as TME-responsive MRI CAs are discussed.

Stimulus-Responsive Nanoparticle Magnetic Resonance Imaging Contrast Agents: Design Considerations and Applications

Magnetic resonance imaging (MRI) has been widely used for disease diagnosis because it can noninvasively obtain anatomical details of various diseases through accurate contrast between soft tissues. Over one-third of MRI examinations are performed with the assistance of contrast agents. Traditional contrast agents typically display an unchanging signal, thus exhibiting relatively low sensitivity and poor specificity. Currently, advances in stimulus-responsive contrast agents which can alter the relaxation signal in response to a specific change in their surrounding environment provide new opportunities to overcome such limitation. The signal changes based on stimulus also reflects the physiological and pathological conditions of the site of interests. In this review, how to design stimulus-responsive nanoparticle MRI contrast agents from the perspective of theory and surface design is comprehensively discussed. Key structural features including size, clusters, shell features, and surface properties are used for tuning the T₁ and T₂ relaxation properties. The reversible or non-reversible signal changes highlight the contrast agents have undergone structural changes based on certain stimulus, as an indication for disease diagnosis or therapeutic efficacy.

Redox-triggered aggregation of ESIONPs with switchable T1 to T2 contrast effect for T2-weighted magnetic resonance imaging

Magnetic resonance imaging (MRI) contrast agents (CAs) have drawn increasing attention in cancer diagnosis. However, since the signals they generate are always "on" and may bring interfering background signals to the region of interest, their selectivity and sensitivity need further improvement. Herein, extremely small iron oxide nanoparticles (ESIONPs) conjugated through a disulfide bond with polyethylene glycol (PEG) that is terminally modified with folic acid (FA), namely ESIONPs-s-s-PEG-FA, were designed and synthesized to target tumor tissues and selectively activate the T2 MRI contrast effect in the reducing environment of tumor cells. Due to the breakage of disulfide bonds by the high glutathione (GSH) concentration in tumor cells, the hydrophilic PEG chains detached from the surface of ESIONPs, which led to the aggregation of ESIONPs and the activation of the T2 contrast effect. In vitro results showed that ESIONPs-s-s-PEG-FA could effectively target tumors to assemble in the reductive environment and switch from a T1 contrast agent (CA) to a T2 one. Furthermore, MRI in tumor-bearing mice also indicated the obvious targeting capacity and the "turn on" of the T2 contrast effect. In addition, the results of the biosafety assay suggest that the tumor-targeted T1/T2 switchable CA is equipped with favorable biocompatibility for cancer diagnosis.

Synergistic regulation of longitudinal and transverse relaxivity of extremely small iron oxide nanoparticles (ESIONPs) using pH-responsive nanoassemblies

Extremely small iron oxide nanoparticles (ESIONPs), as a kind of the special T1 magnetic resonance imaging (MRI) contrast agent that can provide T1 contrasting enhancement since their magnetically disordered shells are dominant compared to their magnetic cores and have powerful potential for constructing stimuli-responsive contrast agents (CAs) to realize precise the tumor diagnosis with high specificity and sensitivity. The stimuli-responsive function of ESIONPs-based CAs can be directly endowed through the synergistic regulation of the longitudinal and transverse relaxivity (r1 and r2) of ESIONPs. However, the systematical investigation for the synergistic regulation of r1 and r2 of ESIONPs is quite lacking. Herein, based on the relaxivity theories, three kinds of ESIONPs-based nanoassemblies with pH-responsiveness were designed and constructed to explore the possibility of various synergistic regulations on r1 and r2. When three kinds of ESIONPs-based nanoassemblies were converted to dissociated ones under a weak acid environment, ESIONPs micelle could realize a synergistic regulation of the single r2 decrease along with the stable r1, while gold nanoparticles-ESIONPs (AuNPs-ESIONPs) vesicle could provide a synergistic regulation comprising the single r1 increase along with the stable r2, and ESIONPs vesicle could offer a synergistic regulation involving the r2 decrease together with the r1 increase. Moreover, all the synergistic regulations on r1 and r2 were efficient strategies to fabricate ESIONPs-based CAs with the stimuli-responsive function. These systematic and feasible synergistic regulations of r1 and r2 may guide and promote the development of ESIONPs-based stimuli-responsive CAs for the highly sensitive and specific tumor diagnosis.

Recent Advances of Bioresponsive Nano-Sized Contrast Agents for Ultra-High-Field Magnetic Resonance Imaging

The ultra-high-field magnetic resonance imaging (MRI) nowadays has been receiving enormous attention in both biomaterial research and clinical diagnosis. MRI contrast agents are generally comprising of T1-weighted and T2-weighted contrast agent types, where T1-weighted contrast agents show positive contrast enhancement with brighter images by decreasing the proton's longitudinal relaxation times and T2-weighted contrast agents show negative contrast enhancement with darker images by decreasing the proton's transverse relaxation times. To meet the incredible demand of MRI, ultra-high-field T2 MRI is gradually attracting the attention of research and medical needs owing to its high resolution and high accuracy for detection. It is anticipated that high field MRI contrast agents can achieve high performance in MRI imaging, where parameters of chemical composition, molecular structure and size of varied contrast agents show contrasted influence in each specific diagnostic test. This review firstly presents the recent advances of nanoparticle contrast agents for MRI. Moreover, multimodal molecular imaging with MRI for better monitoring is discussed during biological process. To fasten the process of developing better contrast agents, deep learning of artificial intelligent (AI) can be well-integrated into optimizing the crucial parameters of nanoparticle contrast agents and achieving high resolution MRI prior to the clinical applications. Finally, prospects and challenges are summarized.

To what extent do polymeric stabilizers affect nanoparticles characteristics?

Colloidal synthesis of nanoparticles using polymeric stabilizers as a template of a structure directing agent provided a plethora of opportunities in fabricating nanoparticles (NPs) with controlled size, shape, composition and structural characteristics. To understand the complete potency of polymeric stabilizers during the synthesis of nanoparticles, the relationship between polymer characteristics such as structure, molecular weight and concentration and nanoparticles characteristics is discussed in depth. This review portrays the use of polymers to attain nanostructured materials via covalent and non-covalent approaches. These polymers can also serve as surfaces modifier as well as the growth regulators during the synthesis of nanomaterials. The effect provided by polymers that directs the formation of nanomaterials into desired forms is otherwise hard to achieve. We especially spotlight on the approaches for tuning the characteristic properties of nanoparticles via cautious choice of the polymer system with special focus to stimuli-responsive polymers. This review mainly focusses on answering the main challenging question; what is the ideal polymeric stabilizer system to obtain specific morphology, size and phase structure of nanoparticles? Such vital information will enable rational design of nanoparticles to meet specific needs for different applications.

Water-Mediated Nanostructures for Enhanced MRI: Impact of Water Dynamics on Relaxometric Properties of Gd-DTPA

Recently, rational design of a new class of contrast agents (CAs), based on biopolymers (hydrogels), have received considerable attention in Magnetic Resonance Imaging (MRI) diagnostic field. Several strategies have been adopted to improve relaxivity without chemical modification of the commercial CAs, however, understanding the MRI enhancement mechanism remains a challenge. Methods: A multidisciplinary approach is used to highlight the basic principles ruling biopolymer-CA interactions in the perspective of their influence on the relaxometric properties of the CA. Changes in polymer conformation and thermodynamic interactions of CAs and polymers in aqueous solutions are detected by isothermal titration calorimetric (ITC) measurements and later, these interactions are investigated at the molecular level using NMR to better understand the involved phenomena. Water molecular dynamics of these systems is also studied using Differential Scanning Calorimetry (DSC). To observe relaxometric properties variations, we have monitored the MRI enhancement of the examined structures over all the experiments. The study of polymer-CA solutions reveals that thermodynamic interactions between biopolymers and CAs could be used to improve MRI Gd-based CA efficiency. High-Pressure Homogenization is used to obtain nanoparticles. Results: The effect of the hydration of the hydrogel structure on the relaxometric properties, called Hydrodenticity and its application to the nanomedicine field, is exploited. The explanation of this concept takes place through several key aspects underlying biopolymer-CA's interactions mediated by the water. In addition, Hydrodenticity is applied to develop Gadolinium-based polymer nanovectors with size around 200 nm with improved MRI relaxation time (10-times). Conclusions: The experimental results indicate that the entrapment of metal chelates in hydrogel nanostructures offers a versatile platform for developing different high performing CAs for disease diagnosis.

Structure-Relaxivity Relationships of Magnetic Nanoparticles for Magnetic Resonance Imaging

Magnetic nanoparticles (MNPs) have been extensively explored as magnetic resonance imaging (MRI) contrast agents. With the increasing complexity in the structure of modern MNPs, the classical Solomon-Bloembergen-Morgan and the outer-sphere quantum mechanical theories established on simplistic models have encountered limitations for defining the emergent phenomena of relaxation enhancement in MRI. Recent progress in probing MRI relaxivity of MNPs based on structural features at the molecular and atomic scales is reviewed, namely, the structure-relaxivity relationships, including size, shape, crystal structure, surface modification, and assembled structure. A special emphasis is placed on bridging the gaps between classical simplistic models and modern MNPs with elegant structural complexity. In the pursuit of novel MRI contrast agents, it is hoped that this review will spur the critical thinking for design and engineering of novel MNPs for MRI applications across a broad spectrum of research fields.

Water gated contrast switching with polymer–silica hybrid nanoparticles

The polymeric capping of a paramagnetically doped MSN enables the generation of high T₁ MRI contrast which is highly pH responsive through a fully reversible change in polymer conformation.

Recent advancements in biocompatible inorganic nanoparticles towards biomedical applications

Due to their intrinsic physical properties potentially useful for imaging and therapy as well as their highly engineerable surface, biocompatible inorganic nanoparticles offer novel platforms to develop advanced diagnostic and therapeutic agents for improved detection and more efficacious treatment of major diseases. The in vivo application of inorganic nanoparticles was demonstrated more than two decades ago, however it turns out to be very complicated as nanomaterials exhibit much more sophisticated pharmacokinetic properties than conventional drugs. In this review, we first discuss the in vivo behavior of inorganic nanoparticles after systematic administration, including the basic requirements for nanoparticles to be used in vivo, the impact of the particles' physicochemical properties on their pharmacokinetics, and the effects of the protein corona formed across the nano-bio interface. Next, we summarize the state-of-the-art of the preparation of biocompatible inorganic nanoparticles and bioconjugation strategies for obtaining target-specific nanoprobes. Then, the advancements in sensitive tumor imaging towards diagnosis and visualization of the abnormal signatures in the tumor microenvironment, together with recent studies on atherosclerosis imaging are highlighted. Finally, the future challenges and the potential for inorganic nanoparticles to be translated into clinical applications are discussed.

Nano-assemblies of cationic mPEG brush block copolymers with gadolinium polyoxotungstate [Gd(W5O18)2]9- form stable, high relaxivity MRI contrast agents

Polyoxometalates (POMs) incorporating paramagnetic ions, such as gadolinium, show promise as contrast agents for application in magnetic resonance imaging (MRI). Specifically, [Gd(W5O18)2]9- (denoted as GdWO) has been reported to have a higher relaxivity than commercially available contrast agents, but it's clinical utility has been limited by the intrinsic instability of POMs at physiological pH (7.4). In the current report we present a stability study on neat GdWO and nano-assemblies of block copolymers with GdWO in the pH range 5.0-7.4 to assess their suitability as MRI contrast agents. Neat GdWO only maintained structural stability between pH 5.4 and 6.4, and demonstrated poor MRI contrast at pH 7.4. To address this pH instability, GdWO was self-assembled with cationic mPEG brush block copolymers containing 20 or 40 units derived from the cationic monomer, 2-dimethylaminoethyl methacrylate (DMAEMA). Nano-assemblies with different charge ratios were synthesised and characterised according to their size, stability, contrasting properties and toxicity. The longitudinal relaxivity (r1) of the nano-assemblies was found to be dependent on the charge ratio, but not on the length of the cationic polymer block. Further investigation of PDMAEMA20 nano-assemblies demonstrated that they were stable over the pH range 5.0-7.4, exhibiting a higher r1 than either neat GdWO (2.77 s-1 mM-1) or clinical MRI contrast agent Gd-DTPA (4.1 s-1 mM-1) at pH 7.4. Importantly, the nano-assembly with the lowest charge ratio (0.2), showed the highest r1 (12.1 s-1 mM-1) whilst, stabilising GdWO over the pH range studied, eliciting low toxicity with MDA-MB231 cells.

Big Potential from Small Agents: Nanoparticles for Imaging-Based Companion Diagnostics

The importance of medical imaging in the diagnosis and monitoring of cancer cannot be overstated. As personalized cancer treatments are gaining popularity, a need for more advanced imaging techniques has grown significantly. Nanoparticles are uniquely suited to fill this void, not only as imaging contrast agents but also as companion diagnostics. This review provides an overview of many ways nanoparticle imaging agents have contributed to cancer imaging, both preclinically and in the clinic, as well as charting future directions in companion diagnostics. We conclude that, while nanoparticle-based imaging agents are not without considerable scientific and developmental challenges, they enable enhanced imaging in nearly every modality, hold potential as in vivo companion diagnostics, and offer precise cancer treatment and maximize intervention efficacy.

Polymer Assembly Encapsulation of Lanthanide Nanoparticles as Contrast Agents for In Vivo Micro-CT

Despite recent technological advancements in microcomputed tomography (micro-CT) and contrast agent development, preclinical contrast agents are still predominantly iodine-based. Higher contrast can be achieved when using elements with higher atomic numbers, such as lanthanides; lanthanides also have X-ray attenuation properties that are ideal for spectral CT. However, the formulation of lanthanide-based contrast agents at the high concentrations required for vascular imaging presents a significant challenge. In this work, we developed an erbium-based contrast agent that meets micro-CT imaging requirements, which include colloidal stability upon redispersion at high concentrations, evasion of rapid renal clearance, and circulation times of tens of minutes in small animals. Through systematic studies with poly(ethylene glycol) (PEG)-poly(propylene glycol), PEG-polycaprolactone, and PEG-poly(l-lactide) (PLA) block copolymers, the amphiphilic block copolymer PEG₁₁₄-PLA₅₃ was identified to be ideal for encapsulating oleate-coated lanthanide-based nanoparticles for in vivo intravenous administration. We were able to synthesize a contrast agent containing 100 mg/mL of erbium that could be redispersed into colloidally stable particles in saline after lyophilization. Contrast enhancement of over 250 HU was achieved in the blood pool for up to an hour, thereby meeting the requirements of live animal micro-CT.

In situ crystal growth of gold nanocrystals on upconversion nanoparticles for synergistic chemo-photothermal therapy

A multifunctional cancer therapy nanocomposite was proposed and synthesized by linking the pH-responsive SH-PEG-DOX prodrug onto gold nanocrystals that were grown in situ on the surface of upconversion nanoparticles (UCNPs). In the structure of the SH-PEG-DOX prodrug, a hydrazone bond was utilized for subsequent pH-responsive drug release in the intracellular acidic microenvironment of cancer cells. This innovative assembly method is facile and mild, and can be used to obtain nanocomposites of UCNPs and gold, which show excellent photostability and biocompatibility. The final UCNPs@Au-DOX nanocomposites offer efficient treatment effects in vitro under irradiation with an 808 nm laser due to the synergistic effect of chemotherapy and photothermal therapy. In addition, the UCNPs@Au-DOX nanocomposites show excellent intracellular locating ability via upconversion luminescence (UCL) imaging with Er³⁺ ions and magnetic resonance imaging (MRI) with Gd³⁺ ions, indicating that they have potential as a visual tracking agent in cancer treatment. Therefore, the presented bioimaging-guided multifunctional synergistic therapy nanocomposites are promising tools for imaging-guided cancer therapy.

Renal Clearable Peptide Functionalized NaGdF4 Nanodots for High-Efficiency Tracking Orthotopic Colorectal Tumor in Mouse

The effective delivery of bioimaging probes to a selected cancerous tissue has extensive significance for biological studies and clinical investigations. Herein, the peptide functionalized NaGdF₄ nanodots (termed as, pPeptide-NaGdF₄ nanodots) have been prepared for highly efficient magnetic resonance imaging (MRI) of tumor by formation of Gd-phosphonate coordinate bonds among hydrophobic NaGdF₄ nanodots (4.2 nm in diameter) with mixed phosphorylated peptide ligands including a tumor targeting phosphopeptide and a cell penetrating phosphopeptide. The tumor targeting pPeptide-NaGdF₄ nanodots have paramagnetic property with ultrasmall hydrodynamic diameter (HD, c.a., 7.3 nm) which greatly improves their MRI contrast ability of tumor and facilitates renal clearance. In detail, the capability of the pPeptide-NaGdF₄ nanodots as high efficient contrast agent for in vivo MRI is evaluated successfully through tracking small drug induced orthotopic colorectal tumor (c.a., 195 mm³ in volume) in mouse.

A pH-Responsive Yolk-Like Nanoplatform for Tumor Targeted Dual-Mode Magnetic Resonance Imaging and Chemotherapy

Incorporation of T₁ and T₂ contrast material in one nanosystem performing their respective MR contrast role and simultaneously serving as an efficient drug delivery system (DDS) has a significant potential application for clinical diagnosis and chemotherapy of cancer. However, inappropriate incorporation always encountered many issues, such as low contact area of T₁ contrast material with water-proton, inappropriate distance between T₂ contrast material and water molecule, and undesirable disturbance of T₂ contrast material for T₁ imaging. Those issues seriously limited the T₁ or T₂ contrast effect. In this work, we developed a yolk-like Fe₃O₄@Gd₂O₃ nanoplatform functionalized by polyethylene glycol and folic acid (FA), which could efficiently exert their tumor targeted T₁-T₂ dual-mode MR imaging and drug delivery role. First, this nanoplatform possessed a high longitudinal relaxation rate (r₁) (7.91 mM^-1 s^-1) and a stronger transverse relaxation rate (r₂) (386.5 mM^-1 s^-1) than that of original Fe₃O₄ (268.1 mM^-1 s^-1). Second, cisplatin could be efficiently loaded into this nanoplatform (112 mg/g) and showed pH-responsive release behavior. Third, this nanoplatform could be effectively internalized by HeLa cells with time and dosage dependence. Fourth, the FA receptor-mediated nanoplatform displayed excellent T₁-T₂ dual mode MR contrast enhancement and anticancer activity both in vitro and in vivo. Fifth, no apparent toxicity for vital organs was observed with systemic delivery of the nanoplatform in vivo. Thus, this nanoplatform could be a potential nanotheranostic for tumor targeted T₁-T₂ dual-mode MR imaging and chemotherapy.

Activatable interpolymer complex-superparamagnetic iron oxide nanoparticles as magnetic resonance contrast agents sensitive to oxidative stress

Magnetic resonance contrast agents that can be activated in response to specific triggers hold potential as molecular biosensors that may be of great utility in non-invasive disease diagnosis. We developed an activatable agent based on superparamagnetic iron oxide nanoparticles (SPIOs) that is sensitive to oxidative stress, a factor in the pathophysiology of numerous diseases. SPIOs were coated with poly(ethylene glycol) (PEG) and complexed with poly(gallol), a synthetic tannin. Hydrogen bonding between PEG and poly(gallol) creates a complexed layer around the SPIO that decreases the interaction of solute water with the SPIO, attenuating its magnetic resonance relaxivity. The complexed interpolymer nanoparticle is in an OFF state (decreased T₂ contrast), where the contrast agent has a low T₂ relaxivity of 7±2mM^-1s^-1. In the presence of superoxides, the poly(gallol) is oxidized and the polymers decomplex, allowing solute water to again interact with the SPIO, representing an ON state (increased T₂ contrast) with a T₂ relaxivity of 70±10mM^-1s^-1. These contrast agents show promise as effective sensors for diseases characterized in part by oxidative stress such as atherosclerosis, diabetes, and cancer.

Activatable T1 Relaxivity Recovery Nanoconjugates for Kinetic and Sensitive Analysis of Matrix Metalloprotease 2

Sensitive detection of matrix metalloproteinase 2 (MMP-2, an important cancer marker associated with tumor invasion and metastasis) activity in vitro and at cellular level is of great significance to clinical diagnosis and medical treatment. With unique physical properties, nanoparticles are emerging as a platform for the construction of conjugates of various biological molecules, which can be expected to generate new types of biosensors. In this work, Fe₃O₄ NPs were modified with Gd chelates via linking peptides to construct NP-substrate (Fe₃O₄-pep-Gd) conjugates for kinetic MMP-2 activity assessment in vitro at the cellular level and in vivo. Superparamagnetic Fe₃O₄ quenched the longitudinal relaxation effect (T₁ relaxivity) of the attached Gd chelates by perturbing proton relaxation process under an external magnetic field. MMP-2 cleaved the peptide substrates and released Gd chelates from the local magnetic fields accompanied by T₁ relaxivity recovery and T₁ contrast enhancement. Benefiting from signal amplification through binding multiple Gd chelates to one linking peptide, Fe₃O₄-pep-Gd conjugates exhibited high sensitivity for the detection of MMP-2 (as low as 0.5 nM). Enzymatic processes were in good agreement with the integrated Michaelis-Menten model, revealing an unexpected activity enhancement in the initial stage. Fe₃O₄-pep-Gd conjugates could also probe MMP-2 at cellular level and in vivo that indicates a great promise in in vitro diagnosis (IVD) and disease monitoring.

Gadolinium-based nanoscale MRI contrast agents for tumor imaging

Gadolinium-based nanoscale magnetic resonance imaging (MRI) contrast agents (CAs) have gained significant momentum as a promising nanoplatform for detecting tumor tissue in medical diagnosis, due to their favorable capability of enhancing the longitudinal relaxivity (r₁) of individual gadolinium ions, delivering to the region of interest a large number of gadolinium ions, and incorporating different functionalities. This mini-review highlights the latest developments and applications, and simultaneously gives some perspectives for their future development.

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.

Activatable probes for diagnosis and biomarker detection by MRI

Magnetic resonance imaging (MRI) is a non-invasive imaging technique with widespread use in diagnosis. Frequently, contrast in MRI is enhanced with the aid of a contrast agent, among which smart, responsive, OFF/ON or activatable probes are of particular interest. These kinds of probes elicit a response to selective stimuli, evidencing the presence of enzymes or acidic pH, for instance. In this review, we will focus on smart probes that are detectable by both ¹H and ¹⁹F MRI, frequently based on nanomaterials. We will discuss the triggering factors and the strategies employed thus far to activate each probe.

Eu(III) Complex with DO3A-amino-phosphonate Ligand as a Concentration-Independent pH-Responsive Contrast Agent for Magnetic Resonance Spectroscopy (MRS)

A new DOTA-like ligand H₅do3aNP with a 2-[amino(methylphosphonic acid)]ethyl-coordinating pendant arm was prepared, and its coordinating properties were studied by NMR spectroscopy and potentiometry. The study revealed a rare slow exchange (on the ¹H and ³¹P NMR time scale) between protonated and unprotonated complex species with a corresponding acidity constant pK_A ∼ 8.0. This unusually slow time scale associated with protonation is caused by a significant geometric change from square-antiprismatic (SA) arrangement observed for protonated complex SA-[Eu(Hdo3aNP)]^- to twisted-square-antiprismatic (TSA) arrangement found for deprotonated complex TSA-[Eu(do3aNP)]^2-. This behavior results in simultaneous occurrence of the signals of both species in the ³¹P NMR spectra at approximately -118 and +70 ppm, respectively. Such an unprecedented difference in the chemical shifts between species differing by a proton is caused by a significant movement of the principal magnetic axis and by a change of phosphorus atom position in the coordination sphere of the central Eu(III) ion (i.e., by relative movement of the phosphorus atom with respect to the principal magnetic axis). It changes the sign of the paramagnetic contribution to the ³¹P NMR chemical shift. The properties discovered can be employed in the measurement of pH by MRS techniques as presented by proof-of-principle experiments on phantoms.

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.

Luminescence Enhanced Eu(3+)/Gd(3+) Co-Doped Hydroxyapatite Nanocrystals as Imaging Agents In Vitro and In Vivo

Biocompatible, biodegradable, and luminescent nano material can be used as an alternative bioimaging agent for early cancer diagnosis, which is crucial to achieve successful treatment. Hydroxyapatite (HAP) nanocyrstals have good biocompatibility and biodegradability, and can be used as an excellent host for luminescent rare earth elements. In this study, based on the energy transfer from Gd(3+) to Eu(3+), the luminescence enhanced imaging agent of Eu/Gd codoping HAP (HAP:Eu/Gd) nanocrystals are obtained via coprecipitation with plate-like shape and no change in crystal phase composition. The luminescence can be much elevated (up to about 120%) with a nonlinear increase versus Gd doping content, which is due to the energy transfer ((6)PJ of Gd(3+) → (5)HJ of Eu(3+)) under 273 nm and the possible combination effect of the cooperative upconversion and the successive energy transfer under 394 nm, respectively. Results demonstrate that the biocompatible HAP:Eu/Gd nanocrystals can successfully perform cell labeling and in vivo imaging. The intracellular HAP:Eu/Gd nanocrystals display good biodegradability with a cumulative degradation of about 65% after 72 h. This biocompatible, biodegradable, and luminescence enhanced HAP:Eu/Gd nanocrystal has the potential to act as a fluorescent imaging agent in vitro and in vivo.