Single (19)F probe for simultaneous detection of multiple metal ions using miCEST MRI

Fluorinated 1,7-DO2A-Based Iron(II) Complexes as Sensitive 19F MRI Molecular Probes for Visualizing Renal Dysfunction in Living Mice

Kidney diseases have become an important global health concern due to their high incidence, inefficient diagnosis, and poor prognosis. Devising direct methods, especially imaging means, to assess renal function is the key for better understanding the mechanisms of various kidney diseases and subsequent development of effective treatment. Herein, we developed a fluorinated ferrous chelate-based sensitive probe, 1,7-DO2A-Fe(II)-F18 (Probe 1), for 19F magnetic resonance imaging (MRI). This highly fluorinated probe (containing 18 chemically equivalent 19F atoms with a fluorine content at 35 wt %) achieves a 15-time enhancement in signal intensity compared with the fluorine-containing ligand alone due to the appropriately regulated 19F relaxation times by the ferrous ion, which significantly increases imaging sensitivity and reduces acquisition time. Owing to its high aqueous solubility, biostability, and biocompatibility, this probe could be rapidly cleared by kidneys, which provides a means for monitoring renal dysfunction via 19F MRI. With this probe, we accomplish in vivo imaging of the impaired renal dysfunction caused by various kidney diseases including acute kidney injury, unilateral ureteral obstruction, and renal fibrosis at different stages. Our study illustrates the promising potential of Probe 1 for in vivo real-time visualization of kidney dysfunction, which is beneficial for the study, diagnosis, and even stratification of different kidney diseases. Furthermore, the design strategy of our probe is inspiring for the development of more high-performance 19F MRI probes for monitoring various biological processes.

Radiofrequency labeling strategies in chemical exchange saturation transfer MRI

Chemical exchange saturation transfer (CEST) MRI has generated great interest for molecular imaging applications because it can image low-concentration solute molecules in vivo with enhanced sensitivity. CEST effects are detected indirectly through a reduction in the bulk water signal after repeated perturbation of the solute proton magnetization using one or more radiofrequency (RF) irradiation pulses. The parameters used for these RF pulses-frequency offset, duration, shape, strength, phase, and interpulse spacing-determine molecular specificity and detection sensitivity, thus their judicious selection is critical for successful CEST MRI scans. This review article describes the effects of applying RF pulses on spin systems and compares conventional saturation-based RF labeling with more recent excitation-based approaches that provide spectral editing capabilities for selectively detecting molecules of interest and obtaining maximal contrast.

Water-Soluble Chemically Precise Fluorinated Molecular Clusters for Interference-Free Multiplex 19F MRI in Living Mice

Fluorine-19 magnetic resonance imaging (¹⁹F MRI) is gaining widespread interest from the fields of biomolecule detection, cell tracking, and diagnosis, benefiting from its negligible background, deep tissue penetration, and multispectral capacity. However, a wide range of ¹⁹F MRI probes are in great demand for the development of multispectral ¹⁹F MRI due to the limited number of high-performance ¹⁹F MRI probes. Herein, we report a type of water-soluble molecular ¹⁹F MRI nanoprobe by conjugating fluorine-containing moieties with a polyhedral oligomeric silsesquioxane (POSS) cluster for multispectral color-coded ¹⁹F MRI. These chemically precise fluorinated molecular clusters are of excellent aqueous solubility with relatively high ¹⁹F contents and of single ¹⁹F resonance frequency with suitable longitudinal and transverse relaxation times for high-performance ¹⁹F MRI. We construct three POSS-based molecular nanoprobes with distinct ¹⁹F chemical shifts at -71.91, -123.23, and -60.18 ppm and achieve interference-free multispectral color-coded ¹⁹F MRI of labeled cells in vitro and in vivo. Moreover, in vivo ¹⁹F MRI reveals that these molecular nanoprobes could selectively accumulate in tumors and undergo rapid renal clearance afterward, illustrating their favorable in vivo behavior for biomedical applications. This study provides an efficient strategy to expand the ¹⁹F probe libraries for multispectral ¹⁹F MRI in biomedical research.

In-cell NMR: Why and how?

NMR spectroscopy has been applied to cells and tissues analysis since its beginnings, as early as 1950. We have attempted to gather here in a didactic fashion the broad diversity of data and ideas that emerged from NMR investigations on living cells. Covering a large proportion of the periodic table, NMR spectroscopy permits scrutiny of a great variety of atomic nuclei in all living organisms non-invasively. It has thus provided quantitative information on cellular atoms and their chemical environment, dynamics, or interactions. We will show that NMR studies have generated valuable knowledge on a vast array of cellular molecules and events, from water, salts, metabolites, cell walls, proteins, nucleic acids, drugs and drug targets, to pH, redox equilibria and chemical reactions. The characterization of such a multitude of objects at the atomic scale has thus shaped our mental representation of cellular life at multiple levels, together with major techniques like mass-spectrometry or microscopies. NMR studies on cells has accompanied the developments of MRI and metabolomics, and various subfields have flourished, coined with appealing names: fluxomics, foodomics, MRI and MRS (i.e. imaging and localized spectroscopy of living tissues, respectively), whole-cell NMR, on-cell ligand-based NMR, systems NMR, cellular structural biology, in-cell NMR… All these have not grown separately, but rather by reinforcing each other like a braided trunk. Hence, we try here to provide an analytical account of a large ensemble of intricately linked approaches, whose integration has been and will be key to their success. We present extensive overviews, firstly on the various types of information provided by NMR in a cellular environment (the "why", oriented towards a broad readership), and secondly on the employed NMR techniques and setups (the "how", where we discuss the past, current and future methods). Each subsection is constructed as a historical anthology, showing how the intrinsic properties of NMR spectroscopy and its developments structured the accessible knowledge on cellular phenomena. Using this systematic approach, we sought i) to make this review accessible to the broadest audience and ii) to highlight some early techniques that may find renewed interest. Finally, we present a brief discussion on what may be potential and desirable developments in the context of integrative studies in biology.

Ratiometric Strategy Based on Intramolecular Internal Standard for Reproducible and Simultaneous Fingerprint Recognition of Diols via 19F NMR Spectroscopy

¹⁹F NMR spectroscopy has been widely used as a convenient and noninvasive analytical technique for understanding complex natural phenomena at the atomic level. However, current NMR referencing techniques are most optimized for ¹H NMR, which causes some limitations while referencing heteronuclear NMR. Despite its promising advantages, ¹⁹F NMR spectroscopy often exhibits large variations in experimental results and lacks consistency compared with ¹H NMR. Herein, we propose a new strategy to improve the consistency of ¹⁹F NMR referencing using an internal standard method. As a proof-of-concept, BA-Py-TFP was applied as a sensor for diols via ¹⁹F NMR spectroscopy. This strategy proved to be a robust and reproducible referencing method with acceptable deviation (Δδ_F = 43-58 ppb) across diverse NMR spectrometers at different institutions. In particular, this new strategy allows reliable fingerprint recognition for analytes and enables qualitative and quantitative analyses of mixtures of multiple analytes simultaneously. The high recovery rates for d-glucose in the human serum matrix suggest its potential suitability for a diverse range of applications, such as in diabetes-related diagnostics.

Insights into the Responding Modes of Highly Potent Gadolinium-Based Magnetic Resonance Imaging Probes Sensitive to Zinc Ions

Zn ions (Zn²⁺) play an important biological role in many diseases; hence, an imaging method for monitoring the Zn²⁺ distribution in tissues could provide important clinical insights. Recently, we reported a potent Zn-sensitive probe based on the Gd-DO3A (DO3A = 1,4,7,10-tetraazacyclododecane-1,4,7-tricarboxylic acid), modified tyrosine. and di(2-picolyl)amine chelator for this metal cation, which generates an outstanding magnetic resonance imaging (MRI) response. Here we further explored the origin of this unprecedented response and expanded the choice of potential MRI probes by preparing the free acid version of the initial MRI sensor. We report a detailed investigation of the ¹H NMR dispersion, ¹⁷O NMR, and isothermal titration calorimetry properties of these two MRI probes upon interaction with Zn²⁺. The performed experiments confirm selective interaction of the MRI probes and target metal cation, which causes substantial changes in the coordination sphere of the paramagnetic center. It also evidenced some aggregation, which enhances the relaxivity response. Interestingly, conversion of the methyl ester to the free carboxylic acid of the tyrosine moiety changes the nature of the aggregates and leads to a smaller relaxivity response. The probes interact with human serum albumin (HSA) in the absence of Zn²⁺, which leads to a possible modification of the coordination sphere of Gd³⁺ or a substantial change in the exchange rate of second-sphere water molecules. In the presence of Zn²⁺, the interaction with HSA is very weak, demonstrating the importance of the Zn²⁺ coordination sphere in the behavior of these systems.

Fluorinated Ionic Liquid Based Multicolor 19 F MRI Nanoprobes for In Vivo Sensing of Multiple Biological Targets

Multicolor imaging, which maps the distribution of different targets, is important for in vivo molecular imaging and clinical diagnosis. Fluorine 19 magnetic resonance imaging (¹⁹ F MRI) is a promising technique because of unique insights without endogenous background or tissue penetration limit. Thus multicolor ¹⁹ F MRI probes, which can sense a wide variety of molecular species, are expected to help elucidate the biomolecular networks in complex biological systems. Here, a versatile model of activatable probes based on fluorinated ionic liquids (ILs) for multicolor ¹⁹ F MRI is reported. Three types of ILs at different chemical shifts are loaded in nanocarriers and sealed by three stimuli-sensitive copolymers, leading to "off" ¹⁹ F signals. The coating polymers specifically respond to their environmental stimuli, then degrade to release the loaded ILs, causing ¹⁹ F signals recovery. The nanoprobes are utilized for non-invasive detection of tumor hallmarks, which are distinguished by their individual colors in one living mouse, without interference between each other. This multicolor imaging strategy, which adopts modular construction of various ILs and stimuli-responsive polymers, will allow more comprehensive sensing of multiple biological targets, thus, opening a new realm in mechanistic understanding of complex pathophysiologic processes in vivo.

Activatable 19 F MRI Nanoprobes for Visualization of Biological Targets in Living Subjects

Visualization of biological targets such as crucial cells and biomolecules in living subjects is critical for the studies of important biological processes. Though ¹ H magnetic resonance imaging (MRI) has demonstrated its power in offering detailed anatomical and pathological information, its capacity for in vivo tracking of biological targets is limited by the high biological background of ¹ H. ¹⁹ F distinguishes itself from its competitors as an exceptional complement to ¹ H in MRI through its high sensitivity, low biological background, and broad chemical shift range. The specificity and sensitivity of ¹⁹ F MRI can be further boosted with activatable nanoprobes. The advantages of ¹⁹ F MRI with activatable nanoprobes enable in vivo detection and imaging at the cellular or even molecular level in deep tissues, rendering this technique appealing as a potential solution for visualization of biological targets in living subjects. Here, recent progress over the past decades on activatable ¹⁹ F MRI nanoprobes made from three major ¹⁹ F-containing compounds, as well as present challenges and potential opportunities, are summarized to provide a panoramic prospective for the people who are interested in this emerging and exciting field.

Solution NMR of synthetic cavity containing supramolecular systems: what have we learned on and from?

NMR has been instrumental in studies of both the structure and dynamics of molecular systems for decades, so it is not surprising that NMR has played a pivotal role in the study of host-guest complexes and supramolecular systems. In this mini-review, selected examples will be used to demonstrate the added value of using (multiparametric) NMR for studying macrocycle-based host-guest and supramolecular systems. We will restrict the discussion to synthetic host systems having a cavity that can engulf their guests thus restricting them into confined spaces. So discussion of selected examples of cavitands, cages, capsules and their complexes, aggregates and polymers as well as organic cages and porous liquids and other porous materials will be used to demonstrate the insights that have been gathered from the extracted NMR parameters when studying such systems emphasizing the information obtained from somewhat less routine NMR methods such as diffusion NMR, diffusion ordered spectroscopy (DOSY) and chemical exchange saturation transfer (CEST) and their variants. These selected examples demonstrate the impact that the results and findings from these NMR studies have had on our understanding of such systems and on the developments in various research fields.

Fast Ion-Chelate Dissociation Rate for In Vivo MRI of Labile Zinc with Frequency-Specific Encodability

Fast ion-chelate dissociation rates and weak ion-chelate affinities are desired kinetic and thermodynamic features for imaging probes to allow reversible binding and to prevent deviation from basal ionic levels. Nevertheless, such properties often result in poor readouts upon ion binding, frequently result in low ion specificity, and do not allow the detection of a wide range of concentrations. Herein, we show the design, synthesis, characterization, and implementation of a Zn²⁺-probe developed for MRI that possesses reversible Zn²⁺-binding properties with a rapid dissociation rate (k_off = 845 ± 35 s^–1) for the detection of a wide range of biologically relevant concentrations. Benefiting from the implementation of chemical exchange saturation transfer (CEST), which is here applied in the ¹⁹F-MRI framework in an approach termed ion CEST (iCEST), we demonstrate the ability to map labile Zn²⁺ with spectrally resolved specificity and with no interference from competitive cations. Relying on fast k_off rates for enhanced signal amplification, the use of iCEST allowed the designed fluorinated chelate to experience weak Zn²⁺-binding affinity (K_d at the mM range), but without compromising high cationic specificity, which is demonstrated here for mapping the distribution of labile Zn²⁺ in the hippocampal tissue of a live mouse. This strategy for accelerating ion-chelate k_off rates for the enhancement of MRI signal amplifications without affecting ion specificity could open new avenues for the design of additional probes for other metal ions beyond zinc.

Single Fluorinated Agent for Multiplexed 19 F-MRI with Micromolar Detectability Based on Dynamic Exchange

The weak thermal polarization of nuclear spins limits the sensitivity of MRI, even for MR-sensitive nuclei as fluorine-19. Therefore, despite being the source of inspiration for the development of background-free MRI for various applications, including for multiplexed imaging, the inability to map very low concentrations of targets using ¹⁹ F-MRI raises the need to further enhance this platform's capabilities. Here, we employ the principles of CEST-MRI in ¹⁹ F-MRI to obtain a 900-fold signal amplification of a biocompatible fluorinated agent, which can be presented in a "multicolor" fashion. Capitalizing on the dynamic interactions in host-guest supramolecular assemblies in an approach termed GEST, we demonstrate that an inhalable fluorinated anesthetic can be used as a single ¹⁹ F-probe for the concurrent detection of micromolar levels of two targets, with potential in vivo translatability. Further extending GEST with new designs could expand the applicability of ¹⁹ F-MRI to the mapping of targets that have so-far remained non-detectable.

Versatile non-luminescent color palette based on guest exchange dynamics in paramagnetic cavitands

Multicolor luminescent portrayal of complexed arrays is indispensable for many aspects of science and technology. Nevertheless, challenges such as inaccessible readouts from opaque objects, a limited visible-light spectrum and restricted spectral resolution call for alternative approaches for multicolor representation. Here, we present a strategy for spatial COlor Display by Exploiting Host-guest Dynamics (CODE-HD), comprising a paramagnetic cavitand library and various guests. First, a set of lanthanide-cradled α-cyclodextrins (Ln-CDs) is designed to induce pseudo-contact shifts in the ¹⁹F-NMR spectrum of Ln-CD-bound guest. Then, capitalizing on reversible host-guest binding dynamics and using magnetization-transfer ¹⁹F-MRI, pseudo-colored maps of complexed arrays are acquired and applied in molecular-steganography scenarios, showing CODE-HD’s ability to generate versatile outputs for information encoding. By exploiting the widely shifted resonances induced by Ln-CDs, the guest versatility and supramolecular systems' reversibility, CODE-HD provides a switchable, polychromatic palette, as an advanced strategy for light-free, multicolor-mapping.

Host-guest supramolecular chemistry can be used as a tool to develop multicolor displays. Here, the authors present a system based on lanthanide-cradled cyclodextrins that allows to construct MRI-readable and erasable artificial non-luminescent color palettes.

Enhanced Ca2+ influx in mechanically distorted erythrocytes measured with 19F nuclear magnetic resonance spectroscopy

We present the first direct nuclear magnetic resonance (NMR) evidence of enhanced entry of Ca²⁺ ions into human erythrocytes (red blood cells; RBCs), when these cells are mechanically distorted. For this we loaded the RBCs with the fluorinated Ca²⁺ chelator, 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid (5FBAPTA), and recorded ¹⁹F NMR spectra. The RBCs were suspended in gelatin gel in a special stretching/compression apparatus. The 5FBAPTA was loaded into the cells as the tetraacetoxymethyl ester; and ¹³C NMR spectroscopy with [1,6-¹³C]D-glucose as substrate showed active glycolysis albeit at a reduced rate in cell suspensions and gels. The enhancement of Ca²⁺ influx is concluded to be via the mechanosensitive cation channel Piezo1. The increased rate of influx brought about by the activator of Piezo1, 2-[5-[[(2,6-dichlorophenyl)methyl]thio]-1,3,4-thiadiazol-2-yl]-pyrazine (Yoda1) supported this conclusion; while the specificity of the cation-sensing by 5FBAPTA was confirmed by using the Ca²⁺ ionophore, A23187.

19F NMR as a tool in chemical biology

We previously reviewed the use of 19F NMR in the broad field of chemical biology [Cobb, S. L.; Murphy, C. D. J. Fluorine Chem. 2009, 130, 132-140] and present here a summary of the literature from the last decade that has the technique as the central method of analysis. The topics covered include the synthesis of new fluorinated probes and their incorporation into macromolecules, the application of 19F NMR to monitor protein-protein interactions, protein-ligand interactions, physiologically relevant ions and in the structural analysis of proteins and nucleic acids. The continued relevance of the technique to investigate biosynthesis and biodegradation of fluorinated organic compounds is also described.

Elucidating dissociation activation energies in host-guest assemblies featuring fast exchange dynamics

The ability to mediate the kinetic properties and dissociation activation energies (E _a) of bound guests by controlling the characteristics of "supramolecular lids" in host-guest molecular systems is essential for both their design and performance. While the synthesis of such systems is well advanced, the experimental quantification of their kinetic parameters, particularly in systems experiencing fast association and dissociation dynamics, has been very difficult or impossible with the established methods at hand. Here, we demonstrate the utility of the NMR-based guest exchange saturation transfer (GEST) approach for quantifying the dissociation exchange rates (k _out) and activation energy (E _a,out) in host-guest systems featuring fast dissociation dynamics. Our assessment of the effect of different monovalent cations on the extracted E _a,out in cucurbit[7]uril:guest systems with very fast k _out highlights their role as "supramolecular lids" in mediating a guest's dissociation E _a. We envision that GEST could be further extended to study kinetic parameters in other supramolecular systems characterized by fast kinetic properties and to design novel switchable host-guest assemblies.

From Zn(II) to Cu(II) Detection by MRI Using Metal-Based Probes: Current Progress and Challenges

Zinc and copper are essential cations involved in numerous biological processes, and variations in their concentrations can cause diseases such as neurodegenerative diseases, diabetes and cancers. Hence, detection and quantification of these cations are of utmost importance for the early diagnosis of disease. Magnetic resonance imaging (MRI) responsive contrast agents (mainly Lanthanide(+III) complexes), relying on a change in the state of the MRI active part upon interaction with the cation of interest, e.g., switch ON/OFF or vice versa, have been successfully utilized to detect Zn2+ and are now being developed to detect Cu2+. These paramagnetic probes mainly exploit the relaxation-based properties (T1-based contrast agents), but also the paramagnetic induced hyperfine shift properties (paraCEST and parashift probes) of the contrast agents. The challenges encountered going from Zn2+ to Cu2+ detection will be stressed and discussed herein, mainly involving the selectivity of the probes for the cation to detect and their responsivity at physiologically relevant concentrations. Depending on the response mechanism, the use of fast-field cycling MRI seems promising to increase the detection field while keeping a good response. In vivo applications of cation responsive MRI probes are only in their infancy and the recent developments will be described, along with the associated quantification problems. In the case of relaxation agents, the presence of another method of local quantification, e.g., synchrotron X-Ray fluorescence, single-photon emission computed tomography (SPECT) or positron emission tomography (PET) techniques, or 19F MRI is required, each of which has its own advantages and disadvantages.

Efficient temperature-feedback liposome for 19F MRI signal enhancement

A new non-encapsulated fluorinated liposome (TSL) was developed, which showed instantaneous temperature-induced 19F MR signal enhancement and excellent stability under reversible signal transition at different conditions.

Four-in-One: Advanced Copper Nanocomposites for Multianalyte Assays and Multicoding Logic Gates

The usage of non-noble-metal nanomaterials for nanoprobes or functional modules is still a big challenge because of their poor stability, functionality, and surface plasmon resonance property. In this work, copper ion, mercaptosuccinic acid, and nanocrystalline cellulose are combined for facile one-step synthesis and self-assembly of ultrasmall copper nanoparticles to produce supercolloidal particles (NCC@MSA-Cu SPs). Cu SPs show advanced multifunctionality for fast point-of-care tests (POCTs) of four metal ions (Hg²⁺, Pb²⁺, Ag⁺, and Zr⁴⁺). These selective recognitions integrate four different chemical reaction mechanisms (ion etching, core-shell deposition, templated synthesis, and precipitation) to produce four distinct readout signals. The multisignal mode-guided multianalyte sensing strategy can effectively avoid interference that affects single signal mode-based sensing. Benefiting from the creative multi-input and multireadout abilities, the visual multicoding logic gates of OR, NOR, AND, and INHIBIT are built based on optical responses of Cu SPs.

Harnessing chemical exchange: 19F magnetic resonance OFF/ON zinc sensing with a Tm(iii) complex

A fluorinated, thulium(iii) complex (Tm-PFZ-1) serves as an off-on ¹⁹F magnetic resonance probe for Zn(ii). Rapid exchange among different conformations combined with paramagnetic relaxation and chemical shift effects of Tm(iii) effectively eliminate the ¹⁹F NMR/MRI signal in Tm-PFZ-1. Chelation of Zn(ii) induces increased structural rigidity and reduces exchange rate, affording a robust ¹⁹F NMR/MRI signal. Tm-PFZ-1 represents a first-in-class paramagnetic ¹⁹F MR agent that exploits a novel sensing mechanism for Zn(ii) and is the first ¹⁹F MR-based scaffold to provide an "off-on" response to Zn(ii) in aqueous solution.

A ratiometric 19F MR-based method for the quantification of Ca2+ using responsive paramagnetic probes

A custom made lanthanide complex with a high number of fluorine atoms was used to develop a quantitative method to assess calcium ion concentration by means of ¹⁹F chemical shift imaging.

Performance of compressed sensing for fluorine-19 magnetic resonance imaging at low signal-to-noise ratio conditions

PURPOSE

To examine the performance of compressed sensing (CS) in reconstructing low signal-to-noise ratio (SNR) ¹⁹ F MR signals that are close to the detection threshold and originate from small signal sources with no a priori known location.

METHODS

Regularization strength was adjusted automatically based on noise level. As performance metrics, root-mean-square deviations, true positive rates (TPRs), and false discovery rates were computed. CS and conventional reconstructions were compared at equal measurement time and evaluated in relation to high-SNR reference data. ¹⁹ F MR data were generated from a purpose-built phantom and benchmarked against simulations, as well as from the experimental autoimmune encephalomyelitis mouse model. We quantified the signal intensity bias and introduced an intensity calibration for in vivo data using high-SNR ex vivo data.

RESULTS

Low-SNR ¹⁹ F MR data could be reliably reconstructed. Detection sensitivity was consistently improved and data fidelity was preserved for undersampling and averaging factors of α = 2 or = 3. Higher α led to signal blurring in the mouse model. The improved TPRs at α = 3 were comparable to a 2.5-fold increase in measurement time. Whereas CS resulted in a downward bias of the ¹⁹ F MR signal, Fourier reconstructions resulted in an unexpected upward bias of similar magnitude. The calibration corrected signal-intensity deviations for all reconstructions.

CONCLUSION

CS is advantageous whenever image features are close to the detection threshold. It is a powerful tool, even for low-SNR data with sparsely distributed ¹⁹ F signals, to improve spatial and temporal resolution in ¹⁹ F MR applications.

Development of Zinc-Specific iCEST MRI as an Imaging Biomarker for Prostate Cancer

The healthy prostate contains the highest concentration of mobile zinc in the body. As this level decreases dramatically during the initial development of prostate cancer, in vivo detection of prostate zinc content may be applied for diagnosis of prostate cancer. Using 19 F ion chemical exchange saturation transfer magnetic resonance imaging (iCEST MRI) and TF-BAPTA as a fluorinated Zn-binding probe with micromolar sensitivity, we show that iCEST MRI is able to differentiate between normal and malignant prostate cells with a 10-fold difference in contrast following glucose-stimulated zinc secretion in vitro. The iCEST signal decreased in normal prostate cells upon downregulation of the ZIP1 zinc transporter. In vivo, using an orthotopic prostate cancer mouse model and a transgenic adenocarcinoma of the mouse prostate (TRAMP) model, a gradual decrease of >300 % in iCEST contrast following the transition of normal prostate epithelial cells to cancer cells was detected.

Wavelength distinguishable signal quenching and enhancing toward photoactive material 3,4,9,10-perylenetetracarboxylic dianhydride for simultaneous assay of dual metal ions

Photoelectrochemical (PEC) assay with low background, simple instrumentation and high sensitivity has deemed as one of the most potential strategies to simultaneous multi-component detection. How to distinguish photocurrent changes caused by various targets on a single sensing platform thus becomes the key issue to be resolved. Herein, we innovatively proposed a multiplex PEC biosensor based on wavelength distinguishable signal quenching and enhancing toward photoactive material 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) for simultaneous assay of dual metal ions. Briefly, S1 and S2 ssDNA containing sensitizer methylene blue and quencher ferrocene (termed as MB-S1 and Fc-S2), respectively, were first generated through target Pb²⁺ and Mg²⁺-induced DNAzyme-assisted target recycling, which thereafter were modified on PTCDA sensing platform specifically via host-guest recognition with β-cyclodextrin (β-CD). Interestingly, the sensitizer MB could enhance photocurrent of PTCDA under the excitation wavelength of 623 nm and 590 nm, respectively, while the quencher Fc just quencher the photocurrent of PTCDA under the excitation wavelength of 590 nm, thereby achieving wavelength distinguishable signal quenching and enhancing toward photoactive material PTCDA for simultaneous assay of dual metal ions. As a result, the conceived biosensor for Mg²⁺ and Pb²⁺ detection realized high sensitivity with detection limit of 0.3 pM and 0.3 nM, respectively. The proposed strategy not only for the first time achieved the discrimination of varied PEC signal caused by two targets with usage of sole photoelectric material, but also realized the simultaneous multiplex assay on a single sensing platform, providing a new way for constructing effective and sensitive PEC biosensor for multi-component detection.

Fluorinated MRI contrast agents and their versatile applications in the biomedical field

MRI has been recognized as one of the most applied medical imaging techniques in clinical practice. However, the presence of background signal coming from water protons in surrounding tissues makes sometimes the visualization of local contrast agents difficult. To remedy this, fluorine has been introduced as a reliable perspective, thanks to its magnetic properties being relatively close to those of protons. In this review, we aim to give an overall description of fluorine incorporation in contrast agents for MRI. The different kinds of fluorinated probes such as perfluorocarbons, fluorinated dendrimers, polymers and paramagnetic probes will be described, as will their imaging applications such as chemical exchange saturation transfer (CEST) imaging, physico-chemical changes detection, drug delivery, cell tracking and inflammation or tumors detection.

Fluorine polymer probes for magnetic resonance imaging: quo vadis?

Over the last few years, the development and relevance of 19F magnetic resonance imaging (MRI) for use in clinical practice has emerged. MRI using fluorinated probes enables the achievement of a specific signal with high contrast in MRI images. However, to ensure sufficient sensitivity of 19F MRI, fluorine probes with a high content of chemically equivalent fluorine atoms are required. The majority of 19F MRI agents are perfluorocarbon emulsions, which have a broad range of applications in molecular imaging, although the content of fluorine atoms in these molecules is limited. In this review, we focus mainly on polymer probes that allow higher fluorine content and represent versatile platforms with properties tailorable to a plethora of biomedical in vivo applications. We discuss the chemical development, up to the first imaging applications, of these promising fluorine probes, including injectable polymers that form depots that are intended for possible use in cancer therapy.

19F-GEST NMR: studying dynamic interactions in host–guest systems

GEST NMR provides dynamic information on host–guest systems. It allows signal amplification of low concentrated complexes, detection of intermolecular interactions and quantification of guest exchange rates.

Perfluorocarbon-Based 19 F MRI Nanoprobes for In Vivo Multicolor Imaging

In vivo multicolor imaging is important for monitoring multiple biomolecular or cellular processes in biology. ¹⁹F magnetic resonance imaging (MRI) is an emerging in vivo imaging technique because it can non‐invasively visualize ¹⁹F nuclei without endogenous background signals. Therefore, ¹⁹F MRI probes capable of multicolor imaging are in high demand. Herein, we report five types of perfluorocarbon‐encapsulated silica nanoparticles that show ¹⁹F NMR peaks with different chemical shifts. Three of the nanoprobes, which show spectrally distinct ¹⁹F NMR peaks with sufficient sensitivity, were selected for in vivo multicolor ¹⁹F MRI. The nanoprobes exhibited ¹⁹F MRI signals with three colors in a living mouse. Our in vivo multicolor system could be utilized for evaluating the effect of surface functional groups on the hepatic uptake in a mouse. This novel multicolor imaging technology will be a practical tool for elucidating in vivo biomolecular networks by ¹⁹F MRI.

Probing the brain with molecular fMRI

One of the greatest challenges of modern neuroscience is to incorporate our growing knowledge of molecular and cellular-scale physiology into integrated, organismic-scale models of brain function in behavior and cognition. Molecular-level functional magnetic resonance imaging (molecular fMRI) is a new technology that can help bridge these scales by mapping defined microscopic phenomena over large, optically inaccessible regions of the living brain. In this review, we explain how MRI-detectable imaging probes can be used to sensitize noninvasive imaging to mechanistically significant components of neural processing. We discuss how a combination of innovative probe design, advanced imaging methods, and strategies for brain delivery can make molecular fMRI an increasingly successful approach for spatiotemporally resolved studies of diverse neural phenomena, perhaps eventually in people.

Comparing Strategies in the Design of Responsive Contrast Agents for Magnetic Resonance Imaging: A Case Study with Copper and Zinc

Magnetic resonance imaging (MRI) has emerged over the years as one of the preferred modalities for medical diagnostic and biomedical research. It has the advantage over other imaging modalities such as positron emission tomography and X-ray of affording high resolution three-dimensional images of the body without using harmful radiation. The use of contrast agents has further expanded this technique by increasing the contrast between regions where they accumulate and background tissues. As MRI most often measures the relaxation rate of water throughout the body, contrast agents function by modulating the intensity of the water signal either via improved relaxation or via saturation transfer to selected exchangeable proton. Among the growing class of MRI contrast agents, a subset of them called "smart" contrast agents function as responsive probes. Their ability to increase or decrease their signal intensity is modulated by the presence of an analyte. These probes offer the unique ability to image the distribution of an analyte in vivo, thereby opening new possibilities for diagnostics and for elucidating the role of specific analytes in various pathologies or biological processes. A number of different strategies can be exploited to design responsive MRI contrast agents. The majority of contrast agents are based on Gd^III complexes. These complexes can be rendered responsive in either of two ways: either by modulating the number of inner-sphere water molecules, q, or via modulating the rotational correlation time, τ_R, of the contrast agent upon substrate binding. The longitudinal relaxivity increases with the number of inner-sphere water molecules. Gd^III complexes can be rendered responsive if they contain a recognition moiety that can bind to both the open coordination site of Gd^III and to the analyte. When the recognition moiety leaves the lanthanide ion to bind to the analyte, q increases and therefore so does the relaxivity. The dependence of relaxivity on rotational correlation time is more complex and more pronounced at lower magnetic fields. In general, slower tumbling macromolecules have longer rotational correlation times and higher relaxivities. Analyte-triggered formation of macromolecules thus also increases relaxivity. Such macromolecules can either be analyte-templated supramolecular assemblies, or analyte-enhanced protein-contrast agent complexes. Chemical Exchange Saturation Transfer (CEST) agents are a newer class of contrast agents that offer the possibility of multifrequency and thus ratiometric imaging, which in turn enables quantitative mapping of the concentration of an analyte in vivo under conditions where the concentration of the contrast agent is not known. Such agents can be rendered responsive if the analyte changes the number of exchangeable proton(s), its exchange rate, or its chemical shift. All of these approaches have been successfully employed for detecting and imaging both copper and zinc, including in vivo. Magnetic Iron Oxide Nanoparticles (MIONs) are powerful MRI transverse relaxation agents. They can also be rendered responsive to an analyte if the latter can control the aggregation of the nanoparticles. For metal ions, this can be achieved via chemical functionalities that only react to form conjugates in the presence of the metal ion analyte.

19F CEST imaging probes for metal ion detection

For detecting metal ions with ¹⁹F chemical exchange saturation transfer magnetic resonance imaging (¹⁹F CEST MRI), a class of novel fluorinated chelators with diverse fluorine contents and chelation properties were conveniently synthesized on gram scales. Among them, a DTPA-derived chelator with high sensitivity and selectivity was identified as a novel ¹⁹F CEST imaging probe for simultaneously detecting multiple metal ions.

Local Intratracheal Delivery of Perfluorocarbon Nanoparticles to Lung Cancer Demonstrated with Magnetic Resonance Multimodal Imaging

Eighty percent of lung cancers originate as subtle premalignant changes in the airway mucosal epithelial layer of bronchi and alveoli, which evolve and penetrate deeper into the parenchyma. Liquid-ventilation, with perfluorocarbons (PFC) was first demonstrated in rodents in 1966 then subsequently applied as lipid-encapsulated PFC emulsions to improve pulmonary function in neonatal infants suffering with respiratory distress syndrome in 1996. Subsequently, PFC nanoparticles (NP) were extensively studied as intravenous (IV) vascular-constrained nanotechnologies for diagnostic imaging and targeted drug delivery applications.

Methods: This proof-of-concept study compared intratumoral localization of fluorescent paramagnetic (M) PFC NP in the Vx2 rabbit model using proton (¹H) and fluorine (¹⁹F) magnetic resonance (MR) imaging (3T) following intratracheal (IT) or IV administration. MRI results were corroborated by fluorescence microscopy.

Results: Dynamic ¹H-MR and ¹⁹F-MR images (3T) obtained over 72 h demonstrated marked and progressive accumulation of M-PFC NP within primary lung Vx2 tumors during the first 12 h post IT administration. Marked ¹H and ¹⁹F MR signal persisted for over 72 h. In contradistinction, IV M-PFC NP produced a modest transient signal during the initial 2 h post-injection that was consistent circumferential blood pool tumor enhancement. Fluorescence microscopy of excised tumors corroborated the MR results and revealed enormous intratumor NP deposition on day 3 after IT but not IV treatment. Rhodamine-phospholipid incorporated into the PFC nanoparticle surfactant was distributed widely within the tumor on day 3, which is consistent with a hemifusion-based contact drug delivery mechanism previously reported. Fluorescence microscopy also revealed similar high concentrations of M-PFC NP given IT for metastatic Vx2 lung tumors. Biodistribution studies in mice revealed that M-PFC NP given IV distributed into the reticuloendothelial organs, whereas, the same dosage given IT was basically not detected beyond the lung itself. PFC NP given IT did not impact rabbit behavior or impair respiratory function. PFC NP effects on cells in culture were negligible and when given IV or IT no changes in rabbit hematology nor serum clinical chemistry parameters were measured.

Conclusion: IT delivery of PFC NP offered unique opportunity to locally deliver PFC NP in high concentrations into lung cancers with minimal extratumor systemic exposure.

Complete on/off responsive ParaCEST MRI contrast agents for copper and zinc

Two thulium-based paraCEST contrast agents enable detection and imaging of copper and zinc by MRI with a complete on/off response.

A novel polydentate ligand chromophore for simultaneously colorimetric detection of trace Ag+ and Fe3. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2017;186:17-22. [PMID: 28600992 DOI: 10.1016/j.saa.2017.06.007]

A novel polydentate ligand chromophore, 3,6-di-(N-ethyl-N-ethoxyl phenylazo) acridine (EEPA), was identified and synthesized. After its structure was characterized by FTIR, ¹H NMR, mass spectra and element analyses, it was noted to find that there was a simultaneously colorimetric response to Ag⁺ and Fe³⁺ accompanying with different color changes, i.e., from brown to light purple for Ag⁺ and further to purple-red for Fe³⁺, respectively. Their different action mechanisms, a 1:2 complex mode for EEPA-Ag⁺ and 1:1 for EEPA-Fe³⁺, were investigated and confirmed by means of Job's plot and theoretical calculation. EEPA would be a potential colorimetric chemo-dosimeter for simultaneous detection of Ag⁺ and Fe³⁺ with the detection limits of 1.6nmol·L^-1 and 69nmol·L^-1, respectively.

Heading toward Macromolecular and Nanosized Bioresponsive MRI Probes for Successful Functional Imaging

The quest for bioresponsive or smart contrast agents (SCAs) in molecular imaging, in particular magnetic resonance imaging (MRI), is progressively increasing since they allow for the monitoring of essential biological processes on molecular and cellular levels in a dynamic fashion. These are offshoot molecules of common contrast agents that are sensitive to biochemical changes in their environment, capable of reporting on such changes by inducing MRI signal alteration. Various mechanistic approaches and different types of SCAs have been developed in order to visualize desired processes, using diverse imaging protocols and methods. To date, the most frequently exploited probes are paramagnetic molecules that change longitudinal or transverse relaxation at proton frequency, or so-called T₁- and T₂-weighted probes, respectively. Moreover, SCAs operating by the chemical exchange saturation transfer mechanism, suitable for ¹⁹F MRI or possessing hyperpolarized nuclei have also appeared in the past decade, slowly finding their role in functional imaging studies. Following these mechanistic principles, a large number of SCAs suitable for diverse targets have been reported to date. This Account condenses this exciting progress, particularly focusing on probes designed for abundant targets that are suitable for practical, in vivo utilization. To date, the greatest advancements have been certainly made in the preparation of pH sensitive probes, which usually contain protonable groups that interact with paramagnetic centers, or take advantage of supramolecular (dis)assembling to induce the MRI signal change, thereupon enabling pH mapping in vivo. In a complementary approach, a combination of metal chelating ligands for Ca²⁺ or Zn²⁺ with MR reporting units results in a wide variety of SCAs that operate with different contrast mechanisms and can be used for initial functional experiments. Finally, the first examples of molecular sensing by creating host-guest complexes to track neurotransmitter flux have also been recently reported, allowing the study of brain function in an unprecedented manner. Nevertheless, wider SCA utilization in vivo has not yet been achieved. There are a few reasons for this disparity between their nominal potential and practical usage, with one of the major reasons being the low sensitivity of the MRI technique. Subsequently, the production of detectable signal change can be achieved using higher concentrations of the bioresponsive probe; however, the biocompatibility of these probes then starts to play an important role. An elegant solution to these practical challenges has been found with the integration of multiple small-sized SCAs into macromolecular and nanosized probes. In such case, the multivalent SCAs are able to circumvent the sensitivity issue, thus enhancing the MR signal and desired contrast changes. Moreover, they prolong the probe tissue retention time, while often reducing their toxicity. Finally, with altered size and properties, they allow for exploitation of mechanisms that induce the contrast change which is not possible with small-sized SCAs. To this end, this Account also discusses the current approaches that aim to develop macromolecular and nanosized SCAs suitable for practical MRI applications. With these, further progress of this exciting field is affirmed, with remarkable results expected in the near future on both the probe preparation and their utilization in functional molecular imaging.

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.

Cellular and Molecular Imaging Using Chemical Exchange Saturation Transfer

Chemical exchange saturation transfer (CEST) is a powerful new tool well suited for molecular imaging. This technology enables the detection of low concentration probes through selective labeling of rapidly exchanging protons or other spins on the probes. In this review, we will highlight the unique features of CEST imaging technology and describe the different types of CEST agents that are suited for molecular imaging studies, including CEST theranostic agents, CEST reporter genes, and CEST environmental sensors.

Recent progress in the use and tracking of transplanted islets as a personalized treatment for type 1 diabetes

INTRODUCTION

Type 1 diabetes mellitus (T1DM) is an autoimmune disease in which the pancreas produces insufficient amounts of insulin. T1DM patients require exogenous sources of insulin to maintain euglycemia. Transplantation of naked or microencapsulated pancreatic islets represents an alternative paradigm to obtain an autonomous regulation of blood glucose levels in a controlled and personalized fashion. However, once transplanted, the fate of these personalized cellular therapeutics is largely unknown, justifying the development of non-invasive tracking techniques.

AREAS COVERED

In vivo imaging of naked pancreatic islet transplantation, monitoring of microencapsulated islet transplantation, visualizing pancreatic inflammation, imaging of molecular-genetic therapeutics, imaging of beta cell function.

EXPERT COMMENTARY

There are still several hurdles to overcome before (microencapsulated) islet cell transplantation will become a mainstay therapy. Non-invasive imaging methods that can track graft volume, graft rejection, graft function (insulin secretion) microcapsule engraftment, microcapsule rupture, and pancreatic inflammation are currently being developed to design the best experimental transplantation paradigms.

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.