Challenges for molecular magnetic resonance imaging

A Responsive Mesoporous Silica Nanoparticle Platform for Magnetic Resonance Imaging-Guided High-Intensity Focused Ultrasound-Stimulated Cargo Delivery with Controllable Location, Time, and Dose

Magnetic resonance imaging (MRI) is an essential modality for clinical diagnosis, and MRI-guided high-intensity focused ultrasound (MRgHIFU) is a powerful technology for targeted therapy. Clinical applications of MRgHIFU primarily utilize hyperthermia and ablation to treat cancerous tissue, but for drug delivery applications thermal damage is undesirable. A biofriendly MRgHIFU-responsive mesoporous silica nanoparticle (MSN) platform that is stimulated within a physiological safe temperature range has been developed, reducing the possibility of thermal damage to the surrounding healthy tissues. Biocompatible polyethylene glycol (PEG) was employed to cap the pores of MSNs, and the release of cargo molecules by HIFU occurs without substantial temperature increase (∼4 °C). To visualize by MRI and measure the stimulated delivery in situ, a U.S. Food and Drug Administration (FDA)-approved gadolinium-based contrast agent, gadopentetate dimeglumine (Gd(DTPA)^2-), was used as the imageable cargo. Taking advantage of the three-dimensional (3-D) imaging and targeting capabilities of MRgHIFU, the release of Gd(DTPA)^2- stimulated by HIFU was pinpointed at the HIFU focal point in 3-D space in a tissue-mimicking gel phantom. The amount of Gd(DTPA)^2- released was controlled by HIFU stimulation times and power levels. A positive correlation between the amount of Gd(DTPA)^2- released and T₁ was found. The MRgHIFU-stimulated cargo release was further imaged in a sample of ex vivo animal tissue. With this technology, the biodistribution of the nanocarriers can be tracked and the MRgHIFU-stimulated cargo release can be pinpointed, opening up an opportunity for future image-guided theranostic applications.

Artificial Engineered Natural Killer Cells Combined with Antiheat Endurance as a Powerful Strategy for Enhancing Photothermal-Immunotherapy Efficiency of Solid Tumors

Although photothermal therapy (PTT) is preclinically applied in solid tumor treatment, incomplete tumor removal of PTT and heat endurance of tumor cells induces significant tumor relapse after treatment, therefore lowering the therapeutic efficiency of PTT. Herein, a programmable therapeutic strategy that integrates photothermal therapeutic agents (PTAs), DNAzymes, and artificial engineered natural killer (A-NK) cells for immunotherapy of hepatocellular carcinoma (HCC) is designed. The novel PTAs, termed as Mn-CONASHs, with 2D structure are synthesized by the coordination of tetrahydroxyanthraquinone and Mn²⁺ ions. By further adsorbing polyetherimide/DNAzymes on the surface, the DNAzymes@Mn-CONASHs exhibit excellent light-to-heat conversion ability, tumor microenvironment enhanced T₁ -MRI guiding ability, and antiheat endurance ability. Furthermore, the artificial engineered NK cells with HCC specific targeting TLS11a-aptamer decoration are constructed for specifically eliminating any possible residual tumor cells after PTT, to systematically enhance the therapeutic efficacy of PTT and avoid tumor relapse. Taken together, the potential of A-NK cells combined with antiheat endurance as a powerful strategy for immuno-enhancing photothermal therapy efficiency of solid tumors is highlighted, and the current strategy might provide promising prospects for cancer therapy.

Magnetic susceptibility and paramagnetism-based NMR

The magnetic interactions between the nuclear magnetic moment and the magnetic moment of unpaired electron(s) depend on the structure and dynamics of the molecules where the paramagnetic center is located and of their partners. The long-range nature of the magnetic interactions is thus a reporter of invaluable information for structural biology studies, when other techniques often do not provide enough data for the atomic-level characterization of the system. This precious information explains the flourishing of paramagnetism-assisted NMR studies in recent years. Many paramagnetic effects are related to the magnetic susceptibility of the paramagnetic metal. Although these effects have been known for more than half a century, different theoretical models and new approaches have been proposed in the last decade. In this review, we have summarized the consequences for NMR spectroscopy of magnetic interactions between nuclear and electron magnetic moments, and thus of the presence of a magnetic susceptibility due to metals, and we do so using a unified notation.

Coordination Behavior of 1,4-Disubstituted Cyclen Endowed with Phosphonate, Phosphonate Monoethylester, and H-Phosphinate Pendant Arms

Three 1,4,7,10-tetraazacyclododecane-based ligands disubstituted in 1,4-positions with phosphonic acid, phosphonate monoethyl-ester, and H-phosphinic acid pendant arms, 1,4-H4do2p, 1,4-H2do2pOEt, and 1,4-H2Bn2do2pH, were synthesized and their coordination to selected metal ions, Mg(II), Ca(II), Mn(II), Zn(II), Cu(II), Eu(III), Gd(III), and Tb(III), was investigated. The solid-state structure of the phosphonate ligand, 1,4-H4do2p, was determined by single-crystal X-ray diffraction. Protonation constants of the ligands and stability constants of their complexes were obtained by potentiometry, and their values are comparable to those of previously studied analogous 1,7-disubstitued cyclen derivatives. The Gd(III) complex of 1,4-H4do2p is ~1 order of magnitude more stable than the Gd(III) complex of the 1,7-analogue, probably due to the disubstituted ethylenediamine-like structural motif in 1,4-H4do2p enabling more efficient wrapping of the metal ion. Stability of Gd(III)-1,4-H2do2pOEt and Gd(III)-H2Bn2do2pH complexes is low and the constants cannot be determined due to precipitation of the metal hydroxide. Protonations of the Cu(II), Zn(II), and Gd(III) complexes probably takes place on the coordinated phosphonate groups. Complexes of Mn(II) and alkali-earth metal ions are significantly less stable and are not formed in acidic solutions. Potential presence of water molecule(s) in the coordination spheres of the Mn(II) and Ln(III) complexes was studied by variable-temperature NMR experiments. The Mn(II) complexes of the ligands are not hydrated. The Gd(III)-1,4-H4do2p complex undergoes hydration equilibrium between mono- and bis-hydrated species. Presence of two-species equilibrium was confirmed by UV-Vis spectroscopy of the Eu(III)-1,4-H4do2p complex and hydration states were also determined by luminescence measurements of the Eu(III)/Tb(III)-1,4-H4do2p complexes.

Synthesis of Water-Soluble Chiral DOTA Lanthanide Complexes with Predominantly Twisted Square Antiprism Isomers and Circularly Polarized Luminescence

One-step cyclization of a tetraazamacrocycle 5 with 70% yield in a 25-g scale was performed. Its chiral DOTA derivatives, L4, has ∼93% of TSAP coordination isomer in its Eu(III) and Yb(III) complexes in aqueous solution. [GdL4]^5- exhibits a high relaxivity, making it a promising and efficient MRI contrast agent. High luminescence dissymmetry factor (g_lum) values of 0.285 (ΔJ = 1) for [TbL3]^- and 0.241 (ΔJ = 1) for [TbL4]^5- in buffer solutions were recorded.

In Vivo Tumor Visualization through MRI Off-On Switching of NaGdF4 -CaCO3 Nanoconjugates

The development of high-performance contrast agents in magnetic resonance imaging (MRI) has recently received considerable attention, as they hold great promise and potential as a powerful tool for cancer diagnosis. Despite substantial achievements, it remains challenging to develop nanostructure-based biocompatible platforms that can generate on-demand MRI signals with high signal-to-noise ratios and good tumor specificity. Here, the design and synthesis of a new class of nanoparticle-based contrast agents comprising self-assembled NaGdF₄ and CaCO₃ nanoconjugates is reported. In this design, the spatial confinement of the T₁ source (Gd³⁺ ions) leads to an "OFF" MRI signal due to insufficient interaction between the protons and the crystal lattices. However, when immersed in the mildly acidic tumor microenvironment, the embedded CaCO₃ nanoparticles generate CO₂ bubbles and subsequently disconnect the nanoconjugate, thus resulting in an "ON" MRI signal. The in vivo performance of these nanoconjugates shows more than 60-fold contrast enhancement in tumor visualization relative to the commercially used contrast agent Magnevist. This work presents a significant advance in the construction of smart MRI nanoprobes ideally suited for deep-tissue imaging and target-specific cancer diagnosis.

The Transition from Metal-Based to Metal-Free Contrast Agents for T1 Magnetic Resonance Imaging Enhancement

Magnetic resonance imaging (MRI) has received significant attention as the noninvasive diagnostic technique for complex diseases. Image-guided therapeutic strategy for diseases such as cancer has also been at the front line of biomedical research, thanks to the innovative MRI, enhanced by the prior delivery of contrast agents (CAs) into patients' bodies through injection. These CAs have contributed a great deal to the clinical utility of MRI but have been based on metal-containing compounds such as gadolinium, manganese, and iron oxide. Some of these CAs have led to cytotoxicities such as the incurable Nephrogenic Systemic Fibrosis (NSF), resulting in their removal from the market. On the other hand, CAs based on organic nitroxide radicals, by virtue of their structural composition, are metal free and without the aforementioned drawbacks. They also have improved biocompatibility, ease of functionalization, and long blood circulation times, and have been proven to offer tissue contrast enhancement with longitudinal relaxivities comparable with those for the metal-containing CAs. Thus, this Review highlights the recent progress in metal-based CAs and their shortcomings. In addition, the remarkable goals achieved by the organic nitroxide radical CAs in the enhancement of MR images have also been discussed extensively. The focal point of this Review is to emphasize or demonstrate the crucial need for transition into the use of organic nitroxide radicals-metal-free CAs-as against the metal-containing CAs, with the aim of achieving safer application of MRI for early disease diagnosis and image-guided therapy.

Calcium phosphate nanocarriers for drug delivery to tumors: imaging, therapy and theranostics

Calcium phosphate (CaP) was engineered as a drug delivery nanocarrier nearly 50 years ago due to its biocompatibility and biodegradability. In recent years, several approaches have been developed for the preparation of size-controllable, stable and multifunctional CaP nanocarriers, and several targeting moieties have also been decorated on the surface of these nanocarriers for active targeting. The CaP nanocarriers have been utilized for loading probes, nucleic acids, anticancer drugs and photosensitizers for cancer imaging, therapy and theranostics. Herein, we reviewed the recent advances in the preparation strategies of CaP nanocarriers and the applications of these nanocarriers in tumor diagnosis, gene delivery, drug delivery and theranostics and finally provided perspectives.

Recent progress in the imaging detection of enzyme activities in vivo

Enzymatic activities are important for normal physiological processes and are also critical regulatory mechanisms for many pathologies. Identifying the enzyme activities in vivo has considerable importance in disease diagnoses and monitoring of the physiological metabolism. In the past few years, great strides have been made towards the imaging detection of enzyme activity in vivo based on optical modality, MRI modality, nuclear modality, photoacoustic modality and multifunctional modality. This review summarizes the latest advances in the imaging detection of enzyme activities in vivo reported within the past years, mainly concentrating on the probe design, imaging strategies and demonstration of enzyme activities in vivo. This review also highlights the potential challenges and the further directions of this field.

Tuning the ultrasonic and photoacoustic response of polydopamine-stabilized perfluorocarbon contrast agents

Contrast-enhanced ultrasound (CEUS) offers the exciting prospect of retaining the ease of ultrasound imaging while enhancing imaging clarity, diagnostic specificity, and theranostic capability. To advance the capabilities of CEUS, the synthesis and understanding of new ultrasound contrast agents (UCAs) is a necessity. Many UCAs are nano- or micro-scale materials composed of a perfluorocarbon (PFC) and stabilizer that synergistically induce an ultrasound response that is both information-rich and easily differentiated from natural tissue. In this work, we probe the extent to which CEUS is modulated through variation in a PFC stabilized with fluorine-modified polydopamine nanoparticles (PDA NPs). The high level of synthetic tunability in this system allows us to study signal as a function of particle aggregation and PFC volatility in a systematic manner. Separation of aggregated and non-aggregated nanoparticles lead to a fundamentally different signal response, and for this system, PFC volatility has little effect on CEUS intensity despite a range of over 50 °C in boiling point. To further explore the imaging tunability and multimodality, Fe3+-chelation was employed to generate an enhanced photoacoustic (PA) signal in addition to the US signal. In vitro and in vivo results demonstrate that PFC-loaded PDA NPs show stronger PA signal than the non-PFC ones, indicating that the PA signal can be used for in situ differentiation between PFC-loading levels. In sum, these data evince the rich role synthetic chemistry can play in guiding new directions of development for UCAs.

A dual-responsive probe for detecting cellular hypoxia using 19F magnetic resonance and fluorescence

We report the first dual-responsive 19F MRI and fluorescence imaging probe for cellular hypoxia. The Cu2+-based probe exhibits no 19F MR signal and reduced fluorescence signal due to paramagnetic quenching; however, the probe turns-on in both modes following reduction to Cu+. This bimodal agent can differentiate hypoxic and normoxic cells in both modalities.

Relaxivity of Gd-Based MRI Contrast Agents in Crosslinked Hyaluronic Acid as a Model for Tissues

The efficiency of MRI contrast agents depends on the relaxation rate enhancement that they can induce at imaging fields. It is well known that, at these fields, large relaxation rates are obtained by binding of gadolinium(III) ions to large molecules. By the same token, the interaction of the gadolinium(III) complexes with macromolecules that are found in biological tissues can be responsible for an increase of the relaxation rate with respect to the value observed in liquids. We investigate here the relaxation enhancement of gadoteridol (Gd-HP-DO3A) in crosslinked hyaluronic acid, taken as model tissue, using fast field-cycling relaxometry. The analysis of the relaxation profiles as a function of the magnetic fields indicates that a sizable increase in the relaxation rates is due to a modest interaction of the contrast agent with the hydrogel and to the slower mobility of the water molecules outside the first-coordination sphere of the gadolinium(III) ion.

Hydroxyl-PEG-Phosphonic Acid-Stabilized Superparamagnetic Manganese Oxide-Doped Iron Oxide Nanoparticles with Synergistic Effects for Dual-Mode MR Imaging

The T₁-T₂ dual-mode contrast agents for magnetic resonance imaging (MRI) can generate self-complementary confirmed T₂ and T₁ images, hence greatly improving the reliability. Facilely synthesizing nanoparticles with the ultrasensitive contrast property remains extremely challenging in nanoscience. Moreover, uncovering the mechanism correlating the signal enhancements and chemical constituents is vital for designing novel efficient synergistically enhanced T₁-T₂ dual-mode MRI nanoprobes. Herein, we report a one-pot facile method to synthesize the superparamagnetic manganese oxide-doped iron oxide (Fe₃O₄/MnO) nanoparticles for T₁-T₂ dual-mode MR imaging. Under external magnetic field, the local magnetic field intensities of MnO and Fe₃O₄ could be simultaneously enhanced through embedding MnO into Fe₃O₄ nanoparticles and hence can cause synergistic T₁ and T₂ contrast enhancements. Moreover, a novel and facile cost-effective method for large-scale synthesis of hydroxyl-polyethylene glycol-phosphonic acid-stabilizing ligands is designed. The facile synthetic method and surface coating strategy of superparamagnetic Fe₃O₄/MnO nanoparticles offer an idea for the chemical design and preparation of superparamagnetic nanoparticles with ultrasensitive MRI contrast abilities for disease evaluation and treatment.

Manganese dioxide nanosheets: from preparation to biomedical applications

Advancements in nanotechnology and molecular biology have promoted the development of a diverse range of models to intervene in various disorders (from diagnosis to treatment and even theranostics). Manganese dioxide nanosheets (MnO₂ NSs), a typical two-dimensional (2D) transition metal oxide of nanomaterial that possesses unique structure and distinct properties have been employed in multiple disciplines in recent decades, especially in the field of biomedicine, including biocatalysis, fluorescence sensing, magnetic resonance imaging and cargo-loading functionality. A brief overview of the different synthetic methodologies for MnO₂ NSs and their state-of-the-art biomedical applications is presented below, as well as the challenges and future perspectives of MnO₂ NSs.

Magnetic Nanoparticle Relaxation Dynamics-Based Magnetic Particle Spectroscopy for Rapid and Wash-Free Molecular Sensing

Magnetic nanoparticles (MNPs) have been extensively used as contrasts and tracers for bioimaging, heating sources for tumor therapy, carriers for controlled drug delivery, and labels for magnetic immunoassays. Here, we describe a MNP Brownian relaxation dynamics-based magnetic particle spectroscopy (MPS) method for the quantitative detection of molecular biomarkers. In MPS measurements, the harmonics of oscillating MNPs are recorded and used as a metric for the freedom of rotational motion, which indicates the bound states of the MNPs. These harmonics can be collected from microgram quantities of iron oxide nanoparticles within 10 s. As the harmonics are largely dependent on the quantity of the MNPs in the sample, the MPS bioassay results could be biased by the deviations of MNP quantities in each sample, especially for the very low-concentration biomarker detection scenarios. Herein, we report three MNP concentration/quantity-independent metrics for characterizing the bound states of MNPs in MPS. Using a streptavidin-biotin binding system as a model, we demonstrate the feasibility of using MPS and MNP concentration/quantity-independent metrics to sense these molecular interactions, showing that this method can achieve rapid, wash-free bioassays, and is suitable for future point-of-care, sensitive, and versatile diagnosis.

Highly fluorinated metal complexes as dual 19F and PARACEST imaging agents

We reported a set of water-soluble transition metal complexes that can serve as both 19F and PARACEST magnetic resonance imaging agents. The high number of equivalent fluorine atoms and the paramagnetic effect of metals offer these complexes high 19F sensitivity as demonstrated by in vitro19F MRI experiments. The complexes contain carboxamide groups appended onto a cyclen macrocycle, which provide 1H CEST peaks well differentiated from bulk water. The Co(ii) agent displays two CEST peaks that can be utilized for ratiometric pH determination and the concept of combining 19F MR and PARACEST as complementary imaging techniques was demonstrated with the Fe(ii) complex.

Exploring Inner-Sphere Water Interactions of Fe(II) and Co(II) Complexes of 12-Membered Macrocycles To Develop CEST MRI Probes

Several paramagnetic Co(II) and Fe(II) macrocyclic complexes were prepared with the goal of introducing a bound water ligand to produce paramagnetically shifted water ¹H resonances and for paramagnetic chemical exchange saturation transfer (paraCEST) applications. Three 12-membered macrocycles with amide pendent groups including 1,7-bis(carbamoylmethyl)-1,4,7,10-tetraazacyclodocane (DCMC), 4,7,10-tris(carbamoylmethyl)-,4,7,10-triaza-12-crown-ether (N3OA), and 4,10-bis(carbamoylmethyl)-4,10-diaza-12-crown-ether (NODA) were prepared and their Co(II) complexes were characterized in the solid state and in solution. The crystal structure of [Co(DCMC)]Br₂ featured a six-coordinated Co(II) center with distorted octahedral geometry, while [Co(NODA)(OH₂)]Cl₂ and [Co(N3OA)](NO₃)₂ were seven-coordinated. The analogous Fe(II) complexes of NODA and NO3A were successfully prepared, but the complex of DCMC oxidized rapidly to the Fe(III) form. Similarly, [Fe(NODA)]²⁺ oxidized over several days, forming crystals of the Fe(III) complex isolated as the μ-O bridged dimer. Magnetic susceptibility values and paramagnetic NMR spectra of the Fe(II) complexes of NODA and N3OA, as well as Co(II) complexes of DCMC, NODA, and N3OA, were consistent with high spin complexes. CEST peaks ranging from 60 ppm to 70 ppm, attributed to NH groups of the amide pendents, were identified. Variable-temperature ¹⁷O NMR spectra of Co(II) and Fe(II) NODA complexes were consistent with rapid exchange of the water ligand with bulk water. Notably, the Co(II) and Fe(II) complexes presented here produced substantial paramagnetic shifts of bulk water ¹H resonances, independent of having an inner-sphere water.

Seven coordinate Co(ii) and six coordinate Ni(ii) complexes of an aromatic macrocyclic triamide ligand as paraCEST agents for MRI

We are reporting Co(ii) and Ni(ii) complexes of a pyridine containing aromatic macrocyclic triamide ligand, 3,6,9,15-tetraazabicyclo(9.3.1)pentadeca-1(15),11,13-triene-3,6,9-triacetamide (TPTA), as paramagnetic chemical exchange saturation transfer (paraCEST) MRI contrast agents. The synthesis and characterization of TPTA and its complexes are reported. The solution chemistry and solid-state structure of Co(ii) and Ni(ii) complexes are studied. Crystallographic data show that the [Co(TPTA)]·Cl₂·2H₂O complex (seven-coordinate, all four N atoms of ring and three amide O atoms) has a distorted pentagonal bipyramidal geometry, however the [Ni(TPTA)Cl]·Cl·0.25H₂O complex (six-coordinate, all four N atoms of the ring, one amide O and one chloride ion) adopts a distorted octahedral geometry. Notably the two pendent amide arms are not coordinated in the [Ni(TPTA)Cl]⁺ complex and one chloride ion fulfils its sixth coordination. The CEST effect of [Co(TPTA)]²⁺ and [Ni(TPTA)Cl]⁺ amide protons is observed at 57 ppm and 78 ppm downfield of the bulk water proton respectively in a buffer solution containing 20 mM N-(2-hydroxyethyl)piperazine-N'-ethanesulfonic acid and 100 mM NaCl at pH 7.4 at 37 °C on a 9.4 T NMR spectrometer. The effects of CEST intensity and exchange rate constant with variation of pH of the solution were studied. The CEST effect and exchange rate constant for the amide protons of the [Co(TPTA)]²⁺ complex have been monitored in HEPES buffer, fetal bovine serum (FBS), rabbit serum and 4% agarose gel (w/w). The stability of the [Co(TPTA)]²⁺ complex in aqueous solution towards oxidation was verified by cyclic voltammetry measurement. The stability of [Co(TPTA)]²⁺ has further been monitored in the presence of biologically relevant ions including HPO₄^2-, CO₃^2-, and Zn²⁺ and under acidic conditions.

Targeted Magnetic Resonance Imaging and Modulation of Hypoxia with Multifunctional Hyaluronic Acid-MnO2 Nanoparticles in Glioma

Manganese dioxide (MnO₂ )-based nanoparticles are a promising tumor microenvironment-responsive nanotheranostic carrier for targeted magnetic resonance imaging (MRI) and for alleviating tumor hypoxia. However, the complexity and potential toxicity of the present common synthesis methods limit their clinical application. Herein, multifunctional hyaluronic acid-MnO₂ nanoparticles (HA-MnO₂ NPs) are synthesized in a simple way by directly mixing sodium permanganate with HA aqueous solutions, which serve as both a reducing agent and a surface-coating material. The obtained HA-MnO₂ NPs show an improved water-dispersibility, fine colloidal stability, low toxicity, and responsiveness to the tumor microenvironment (high H₂ O₂ and high glutathione, low pH). After intravenous injection, HA-MnO₂ NPs exhibit a high imaging sensitivity for detecting rat intracranial glioma with MRI for a prolonged period of up to 3 d. These nanoparticles also effectively alleviate the tumor hypoxia in a rat model of intracranial glioma. The downregulation of VEGF and HIF-1α expression in intracranial glioma validates the sustained attenuation effect of HA-MnO₂ NPs on tumor hypoxia. These results show that HA-MnO₂ NPs can be used for sensitive, targeted MRI detection of gliomas and simultaneous attenuation of tumor hypoxia.

γ-Glutamyltranspeptidase-Triggered Intracellular Gadolinium Nanoparticle Formation Enhances the T2-Weighted MR Contrast of Tumor

Magnetic resonance imaging (MRI) is advantageous in the diagnosis of deep internal cancers, but contrast agents (CAs) are always needed to improve MRI sensitivity. Gadolinium (Gd)-based agents are routinely used as T₁-dominated CAs in clinic but using intracellularly formed Gd nanoparticles to enhance the T₂-weighted MRI of tumor in vivo at high magnetic field has not been reported. Herein, we rationally designed a "smart" Gd-based probe Glu-Cys(StBu)-Lys(DOTA-Gd)-CBT (1), which was subjected to γ-glutamyltranspeptidase (GGT) cleavage and an intracellular CBT-Cys condensation reaction to form Gd nanoparticles (i.e., 1-NPs) to enhance the T₂-weighted MR contrast of tumor in vivo at 9.4 T. Living cell experiments indicated that the 1-treated HeLa cells had an r₂ value of 27.8 mM^-1 s^-1 and an r₂/r₁ ratio of 10.6. MR imaging of HeLa tumor-bearing mice indicated that the T₂ MR contrast of the tumor enhanced 28.6% at 2.5 h post intravenous injection of 1. We anticipate that our probe 1 could be employed for T₂-weighted MRI diagnosis of GGT-related cancers in the future when high magnetic field is available in clinic.

A dual-modal nanoprobe based on Eu(iii) complex-MnO2 nanosheet nanocomposites for time-gated luminescence-magnetic resonance imaging of glutathione in vitro and in vivo

Dual-modal fluorescence-magnetic resonance imaging (MRI) techniques have gained great interest in biomedical research and clinical practice, since they integrate the advantages of both imaging techniques and provide a useful approach to simultaneously investigate both molecular and anatomical information at the same biological structures. Herein, we report the construction of a dual-modal time-gated luminescence (TGL)/MRI nanoprobe, BHHBB-Eu3+@MnO2, for glutathione (GSH) by anchoring luminescent β-diketone-Eu3+ complexes on layered MnO2 nanosheets. The fabricated nanoprobe exhibited very week luminescence and MR signals since the luminescence of the Eu3+ complex was quenched by MnO2 nanosheets and Mn atoms were isolated from water. Upon exposure to GSH, the MnO2 nanosheets were rapidly and selectively reduced to Mn2+ ions, resulting in remarkable enhancements of TGL and MR signals simultaneously. The combination of TGL and MR detection modes enables the nanoprobe to be used for detecting GSH in a wide concentration range (1-1000 μM) and imaging GSH at different resolutions and depths ranging from the subcellular level to the whole body. Furthermore, the as-prepared nanoprobe exhibited a low cytotoxicity and good biocompatibility, rapid response rate, long-lived luminescence, and high sensitivity and selectivity for responding to GSH. These features allowed it to be successfully used for the TGL detection of GSH in human sera, TGL imaging of GSH in living cells and zebrafish, as well as dual-modal TGL/MR imaging of GSH in tumor-bearing mice. All of these results highlighted the applicability and advantages of the nanoprobe for detecting GSH in vitro and in vivo.

Self-Assembling Endogenous Biliverdin as a Versatile Near-Infrared Photothermal Nanoagent for Cancer Theranostics

Photothermal nanomaterials that integrate multimodal imaging and therapeutic functions provide promising opportunities for noninvasive and targeted diagnosis and treatment in precision medicine. However, the clinical translation of existing photothermal nanoagents is severely hindered by their unclear physiological metabolism, which makes them a strong concern for biosafety. Here, the utilization of biliverdin (BV), an endogenic near-infrared (NIR)-absorbing pigment with well-studied metabolic pathways, to develop photothermal nanoagents with the aim of providing efficient and metabolizable candidates for tumor diagnosis and therapy, is demonstrated. It is shown that BV nanoagents with intense NIR absorption, long-term photostability and colloidal stability, and high photothermal conversion efficiency can be readily constructed by the supramolecular multicomponent self-assembly of BV, metal-binding short peptides, and metal ions through the reciprocity and synergy of coordination and multiple noncovalent interactions. In vivo data reveal that the BV nanoagents selectively accumulate in tumors, locally elevate tumor temperature under mild NIR irradiation, and consequently induce efficient photothermal tumor ablation with promising biocompatibility. Furthermore, the BV nanoagents can serve as a multimodal contrast for tumor visualization through both photoacoustic and magnetic resonance imaging. BV has no biosafety concerns, and thereby offers a great potential in precision medicine by integrating multiple theranostic functions.

Chemical exchange saturation transfer (CEST) as a new method of signal obtainment in magnetic resonance molecular imaging in clinical and research practice

The work describes the physical basis of the chemical exchange saturation transfer (CEST) technique; it presents the beginnings of the implementation of the method and its possible applications. The principles of correct data acquisition and possible solutions used during the design of the CEST sequence are shown. The main problems related to data analysis are indicated, and an example Z-spectrum from in vivo study of the rat brain is introduced. Furthermore, the parameters related to spectrum analyses such as magnetisation transfer asymmetry (MTRasym) and amide proton transfer asymmetry (APTasym) are presented. In the following part, different types of the CEST method often mentioned in the literature are discussed. Subsequently, the possible applications of the CEST method in both clinical and experimental practice are described.

Linear PVA–DTPA–Gd conjugate for magnetic resonance imaging

In this study, we report the preparation and characterization of the PVA–DTPA–Gd conjugate as a potential MRI contrast agent (CA). The r₁ value and the r₂/r₁ ratio were about 5.6 mM⁻¹ s⁻¹ and 1.31, respectively. In vitro toxicity studies not only demonstrated that the polymeric system possessed good biocompatibility, but also proved that the conjugate could be an attractive candidate for CA.

In this study, we report the preparation and characterization of the PVA–DTPA–Gd conjugate as a potential MRI contrast agent (CA).

A Hyperfluorinated Hydrophilic Molecule for Aqueous 19F MRI Contrast Media

Fluorine-19 (¹⁹F) magnetic resonance imaging (MRI) has the potential for a wide range of in vivo applications but is limited by lack of flexibility in exogenous probe formulation. Most ¹⁹F MRI probes are composed of perfluorocarbons (PFCs) or perfluoropolyethers (PFPEs) with intrinsic properties which limit formulation options. Hydrophilic organofluorine molecules can provide more flexibility in formulation options. We report herein a hyperfluorinated hydrophilic organoflourine, ET1084, with ∼24 wt. % ¹⁹F content. It dissolves in water and aqueous buffers to give solutions with ≥8 M ¹⁹F. ¹⁹F MRI phantom studies at 9.4T employing a 10-minute multislice multiecho (MSME) scan sequence show a linear increase in signal-to-noise ratio (SNR) with increasing concentrations of the molecule and a detection limit of 5 mM. Preliminary cytotoxicity and genotoxicity assessments suggest it is safe at concentrations of up to 20 mM.

SPIO-loaded nanostructured lipid carriers as liver-targeted molecular T2-weighted MRI contrast agent

Background

Superparamagnetic iron oxide (SPIO) acts as a negative contrast agent in magnetic resonance imaging (MRI), and is widely used in clinical applications, including the diagnosis of hepatic diseases. Hepatocyte-targeted magnetic resonance contrast agents (MRCAs) can provide useful information for evaluating hepatic diseases. We prepared targeted magnetic nanostructured lipid carriers (MNLCs) to enhance the hepatocytes targeting efficiency.

Methods

In vitro characterizations of MNLCs were determined by transmission electron microscopy (TEM). The cytotoxicity assay of the MNLCs was measured by methyl tetrazolium (MTT) method. The uptaken study was measured by confocal microscopy, flow cytometry and MRI in vitro. The enhanced liver-targeting efficiency of MNLCs was measured by fluorescence imaging and MRI in vivo.

Results

Gal-NLC-SPIO was prepared successfully. The cytotoxicity assay of the MNLCs demonstrated that the MNLC had relatively low cytotoxicity and high biocompatibility for LO2 cells. More importantly, we confirmed that Gal-NLC-SPIO had greater uptake by LO2 cells than Gal-NLC-SPIO/PEG and free Gal in vitro. A liver distribution study of MNLCs in normal mice demonstrated that the fluorescent signal values to livers of the Gal-NLC-SPIO were significantly stronger than those of NLC-SPIO and Gal-NLC-SPIO/PEG. The liver targeting efficiency of Gal-NLC-SPIO was confirmed both in vitro and in vivo.

Conclusions

We successfully developed liver-targeting MNLCs, which showed accurate hepatocytes targeting, and thus have the potential to be a new MRI contrast agent to help the diagnosis of liver diseases.

A Markedly Improved Synthetic Approach for the Preparation of Multifunctional Au-DNA Nanoparticle Conjugates Modified with Optical and MR Imaging Probes

We describe a new, and vastly superior approach for labeling spherical nucleic acid conjugates (SNAs) with diagnostic probes. SNAs have been shown to provide the unique ability to traverse the cell membrane and deliver surface conjugated DNA into cells while preserving the DNA from nuclease degradation. Our previous work on preparing diagnostically labeled SNAs was labor intensive, relatively low yielding, and costly. Here, we describe a straightforward and facile preparation for labeling SNAs with optical and MR imaging probes with significantly improved physical properties. The synthesis of Gd(III) labeled DNA Au nanoparticle conjugates is achieved by sequential conjugation of 3'-thiol-modified oligonucleotides and cofunctionalization of the particle surface with the subsequent addition of 1,2 diothiolate modified chelates of Gd(III) (abbreviated: DNA-Gd^III@AuNP). This new generation of SNA conjugates has a 2-fold increase of DNA labeling and a 1.4-fold increase in Gd(III) loading compared to published constructs. Furthermore, the relaxivity ( r₁) is observed to increase 4.5-fold compared to the molecular dithiolane-Gd(III) complex, and 1.4-fold increase relative to previous particle constructs where the Gd(III) complexes were conjugated to the oligonucleotides rather than directly to the Au particle. Importantly, this simplified approach (2 steps) exploits the advantages of previous Gd(III) labeled SNA platforms; however, this new approach is scalable and eliminates modification of DNA for attaching the contrast agent, and the particles exhibit improved cell labeling.

A non-synthetic approach to extending the lifetime of hyperpolarized molecules using D2O solvation

Although dissolution dynamic nuclear polarization is a robust technique to significantly increase magnetic resonance signal, the short T₁ relaxation time of most ¹³C-nuclei limits the timescale of hyperpolarized experiments. To address this issue, we have characterized a non-synthetic approach to extend the hyperpolarized lifetime of ¹³C-nuclei in close proximity to solvent-exchangeable protons. Protons exhibit stronger dipolar relaxation than deuterium, so dissolving these compounds in D₂O to exchange labile protons with solvating deuterons results in longer-lived hyperpolarization of the ¹³C-nucleus 2-bonds away. ¹³C T₁ and T₂ times were longer in D₂O versus H₂O for all molecules in this study. This phenomenon can be utilized to improve hyperpolarized signal-to-noise ratio as a function of longer T₁, and enhanced spectral and imaging resolution via longer T₂.

Insights into 2D MXenes for Versatile Biomedical Applications: Current Advances and Challenges Ahead

Great and interdisciplinary research efforts have been devoted to the biomedical applications of 2D materials because of their unique planar structure and prominent physiochemical properties. Generally, ceramic-based biomaterials, fabricated by high-temperature solid-phase reactions, are preferred as bone scaffolds in hard tissue engineering because of their controllable biocompatibility and satisfactory mechanical property, but their potential biomedical applications in disease theranostics are paid much less attention, mainly due to their lack of related material functionalities for possibly entering and circulating within the vascular system. The emerging 2D MXenes, a family of ultrathin atomic nanosheet materials derived from MAX phase ceramics, are currently booming as novel inorganic nanosystems for biologic and biomedical applications. The metallic conductivity, hydrophilic nature, and other unique physiochemical performances make it possible for the 2D MXenes to meet the strict requirements of biomedicine. This work introduces the very recent progress and novel paradigms of 2D MXenes for state-of-the-art biomedical applications, focusing on the design/synthesis strategies, therapeutic modalities, diagnostic imaging, biosensing, antimicrobial, and biosafety issues. It is highly expected that the elaborately engineered ultrathin MXenes nanosheets will become one of the most attractive biocompatible inorganic nanoplatforms for multiple and extensive biomedical applications to profit the clinical translation of nanomedicine.

Exogenous/Endogenous-Triggered Mesoporous Silica Cancer Nanomedicine

Recent advances in nanomedicine-based theranostic platforms have catalyzed the generation of new theranostic modalities for pathological abnormalities, such as cancer. Mesoporous silica-based nanomedicines, which feature unique physicochemical properties and specific applicability, are extensively explored for numerous oncological applications. Due to the well-defined morphology, specific surface area, and pore volume, mesoporous silica nanoparticle (MSN)-based theranostic platforms have provided unprecedented opportunities for the development of next-generation cancer nanomedicine. However, current understanding on the underlying mechanisms of how these feasible theranostic platforms interact with exogenous/endogenous triggers and how this unique responsiveness for optimized cancer therapy can be taken advantage of is still preliminary. In this progress report, efforts are made to give a comprehensive overview of the development of MSN-based "smart" theranostic platforms, from exogenous physical irradiation-triggered platforms for localized therapy to endogenous biological stimulus-triggered platforms for tumor microenvironment responsiveness. It is highly expected that these elaborately fabricated MSN-based nanoformulations will play an indispensable role in the efficient cancer therapy based on their high therapeutic outcome and reduced side effects.

Magnetic resonance imaging contrast enhancement in vitro and in vivo by octanuclear iron-oxo cluster-based agents

A water-soluble octanuclear cluster, [Fe₈], was studied with regard to its properties as a potential contrast enhancing agent in magnetic resonance imaging (MRI) in magnetic fields of 1.3, 7.2 and 11.9 T and was shown to have transverse relaxivities r₂ = 4.01, 10.09 and 15.83 mM s^-1, respectively. A related hydrophobic [Fe₈] cluster conjugated with 5 kDa hyaluronic acid (HA) was characterized by ⁵⁷Fe-Mössbauer and MALDI-TOF mass spectroscopy, and was evaluated in aqueous solutions in vitro with regard to its contrast enhancing properties [r₂ = 3.65 mM s^-1 (1.3 T), 26.20 mM s^-1 (7.2 T) and 52.18 mM s^-1 (11.9 T)], its in vitro cellular cytotoxicity towards A-549 cells and COS-7 cells and its in vivo enhancement of T₂-weighted images (4.7 T) of a human breast cancer xenografted on a nude mouse. The physiologically compatible [Fe₈]-HA conjugate was i.v. injected to the tumor-bearing mouse, resulting in observable, heterogeneous signal change within the tumor, evident 15 min after injection and persisting for approximately 30 min. Both molecular [Fe₈] and its HA-conjugate show a strong magnetic field dependence on r₂, rendering them promising platforms for the further development of T₂ MRI contrast agents in high and ultrahigh magnetic fields.

Bimodal Phosphorescence-Magnetic Resonance Imaging Nanoprobes for Glutathione Based on MnO2 Nanosheet-Ru(II) Complex Nanoarchitecture

Bimodal fluorescence-magnetic resonance imaging (MRI) technique has shown great utilities in bioassays because it combines the advantages of both optical imaging and MRI to provide more sufficient information over any modality alone. In this work, on the basis of a MnO₂ nanosheet-Ru(II) complex nanoarchitecture, a bimodal phosphorescence-MRI nanoprobe for glutathione (GSH) has been constructed. The nanoprobe, Ru(BPY)₃@MnO₂, was constructed by integrating MnO₂ nanosheets with a phosphorescent Ru(II) complex [Ru(BPY)₃](PF₆)₂ (BPY = 2,2'-bipyridine), which resulted in complete phosphorescence quenching of the Ru(II) complex, accompanied by very low longitudinal and transverse relaxivity. Upon exposure to GSH, the reduction of MnO₂ nanosheets by GSH triggers a recovery of phosphorescence and simultaneously produces a number of Mn²⁺ ions, a perfect MRI contrast agent. The as-prepared nanoprobe showed good water dispersion and biocompatibility and a rapid, selective, and sensitive response toward GSH in the phosphorescence and MR detection modes. The practicability of the nanoprobe was proved by time-gated luminescence assay of GSH in human serum, phosphorescent imaging of endogenous GSH in living cells, zebrafish, and tumor-bearing mice, as well as the MRI of GSH in tumor-bearing mice. The research outcomes suggested the potential of Ru(BPY)₃@MnO₂ for the bimodal phosphorescence-MRI sensing of GSH in vitro and in vivo.

T1-T2 molecular magnetic resonance imaging of renal carcinoma cells based on nano-contrast agents

BACKGROUND

The development of T₁-T₂ dual contrast agent (CA) favors the visualization of the lesion in a more accurate and reliable manner by magnetic resonance imaging (MRI). The relaxivity and the interference between T₁ and T₂ CA are the main concerns for their design.

METHODS

In this work, we constructed an Fe₃O₄@mSiO₂/PDDA/BSA-Gd₂O₃ nanocomplex where BSA-Gd₂O₃ NPs and Fe₃O₄ NPs were chosen as T₁ and T₂ MRI CAs and a 20 nm mesoporous silica (mSiO₂) nanoshell was introduced to reduce the interference between them. We performed transmis sion electron microscopy, X-ray powder diffraction, UV-vis absorption spectra, and Fourier transform infrared absorption (FTIR) spectra to characterize the prepared nanocom-plex and MRI scanning to evaluate their MRI behaviors. Furthermore, 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and hematologic and biochemical analyses were introduced to evaluate their in vitro and in vivo toxicity. Finally, the specific MRI of 786-0 cells with Fe₃O₄@mSiO₂/PDDA/BSA-Gd₂O₃-AS1411 nanoprobe in vitro was realized. In vivo biodistribution of Fe₃O₄@mSiO₂/PDDA/BSA-Gd₂O₃ nanocomplex in the mouse was determined by the quantification of the Gd element by inductively coupled plasma-mass spectrometry.

RESULTS

The prepared Fe₃O₄@mSiO₂/PDDA/BSA-Gd₂O₃ nanocomplex possessed high longitudinal (r₁=11.47 mM s^-1 Gd) and transverse (r₂=195.1 mM s^-1 Fe) relaxivities, enabling its use as a T₁-T₂ dual contrast agent for MRI. MTT testing and hematologic and biochemical analysis indicated the good biocompatibility of Fe₃O₄@mSiO₂/PDDA/BSA-Gd₂O₃ nanocomplex in vitro and in vivo. After further conjugation with AS1411 aptamer, they could target tumor cells successfully by T₁ and T₂ MRI in vitro. The possible metabolic pathway of the tail vein-injected Fe₃O₄@mSiO₂/PDDA/BSA-Gd₂O₃ nanocomplex in mouse was mainly via kidney.

CONCLUSION

A T₁-T₂ dual-mode contrast agent, Fe₃O₄@mSiO₂/PDDA/BSA-Gd₂O₃ nano-complex, was developed and its good performance for tumor cell targeting in vitro and kidney contrast-enhanced MRI in mice indicated its promising potential as an effective T₁-T₂ dual-mode contrast agent for in vivo MRI with self-confirmation.

Peripherally Metalated Porphyrins with Applications in Catalysis, Molecular Electronics and Biomedicine

Porphyrins are conjugated, stable chromophores with a central core that binds a variety of metal ions and an easily functionalized peripheral framework. By combining the catalytic, electronic or cytotoxic properties of selected transition metal complexes with the binding and electronic properties of porphyrins, enhanced characteristics of the ensemble are generated. This review article focuses on porphyrins bearing one or more peripheral transition metal complexes and discusses their potential applications in catalysis or biomedicine. Modulation of the electronic properties and intramolecular communication through coordination bond linkages in bis-porphyrin scaffolds is also presented.

Ultrathin Cu-TCPP MOF nanosheets: a new theragnostic nanoplatform with magnetic resonance/near-infrared thermal imaging for synergistic phototherapy of cancers

Nanostructures based on metal-organic frameworks (MOFs) have promising potential as theragnostic nanoplatforms for phototherapy of cancer cells. However, the MOFs alone are seldom reported to be used as photothermal agents mainly due to their poor near-infrared (NIR) light absorption.

Methods: Ultrathin copper-tetrakis (4-carboxyphenyl) porphyrin (Cu-TCPP) MOF nanosheets were prepared by a facile solvothermal route. The photothermal therapy (PTT), photodynamic therapy (PDT), and T₁-weighted magnetic resonance (MR) imaging capabilities and the high biocompatibility of these composite nanosheets were evaluated in vitro as well as in vivo in a mouse tumor model.

Results: The ultrathin Cu-TCPP MOF nanosheets exhibited 1) strong NIR absorption because of the d-d energy band transition of Cu²⁺ and the ultrathin characteristic translating into excellent photothermal performance, 2) ability to produce singlet oxygen because of the inherent characteristic of TCPP, and 3) capability for MR imaging because of the unpaired 3d electrons of copper.

Conclusion: Our study demonstrated that the Cu-TCPP MOF nanosheets are a promising phototherapy nanoplatform with the synergistic ability for PTT and PDT of cancer, guided by MR and infrared thermal imaging.

Interplay of Through-Bond Hyperfine and Substituent Effects on the NMR Chemical Shifts in Ru(III) Complexes

The links between the molecular structure and nuclear magnetic resonance (NMR) parameters of paramagnetic transition-metal complexes are still relatively unexplored. This applies particularly to the contact term of the hyperfine contribution to the NMR chemical shift. We report combining experimental NMR with relativistic density functional theory (DFT) to study a series of Ru(III) complexes with 2-substituted β-diketones. A series of complexes with systematically varied substituents was synthesized and analyzed using ¹H and ¹³C NMR spectroscopy. The NMR spectra recorded at several temperatures were used to construct Curie plots and estimate the temperature-independent (orbital) and temperature-dependent (hyperfine) contributions to the NMR shift. Relativistic DFT calculations of electron paramagnetic resonance and NMR parameters were performed to interpret the experimental observations. The effects of individual factors such as basis set, density functional, exact-exchange admixture, and relativity are analyzed and discussed. Based on the calibration study in this work, the fully relativistic Dirac-Kohn-Sham (DKS) method, the GIAO approach (orbital shift), the PBE0 functional with the triple-ζ valence basis sets, and the polarizable continuum model for describing solvent effects were selected to calculate the NMR parameters. The hyperfine contribution to the total paramagnetic NMR (pNMR) chemical shift is shown to be governed by the Fermi-contact (FC) term, and the substituent effect (H vs Br) on the through-bond FC shifts is analyzed, interpreted, and discussed in terms of spin-density distribution, atomic spin populations, and molecular-orbital theory. In contrast to the closed-shell systems of Rh(III), the presence of a single unpaired electron in the open-shell Ru(III) analogs significantly alters the NMR resonances of the ligand atoms distant from the metal center in synergy with the substituent effect.