Zn2+ Responsive Bimodal Magnetic Resonance Imaging and Fluorescent Imaging Probe Based on a Gadolinium(III) Complex

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.

Imaging Tissue Physiology In Vivo by Use of Metal Ion-Responsive MRI Contrast Agents

Paramagnetic metal ion complexes, mostly based on gadolinium (Gd3+), have been used for over 30 years as magnetic resonance imaging (MRI) contrast agents. Gd3+-based contrast agents have a strong influence on T1 relaxation times and are consequently the most commonly used agents in both the clinical and research environments. Zinc is an essential element involved with over 3000 different cellular proteins, and disturbances in tissue levels of zinc have been linked to a wide range of pathologies, including Alzheimer's disease, prostate cancer, and diabetes mellitus. MR contrast agents that respond to the presence of Zn2+ in vivo offer the possibility of imaging changes in Zn2+ levels in real-time with the superior spatial resolution offered by MRI. Such responsive agents, often referred to as smart agents, are typically composed of a paramagnetic metal ion with a ligand encapsulating it and one or more chelating units that selectively bind with the analyte of interest. Translation of these agents into clinical radiology is the next goal. In this review, we discuss Gd3+-based MR contrast agents that respond to a change in local Zn2+ concentration.

Trivalent metal complex geometry of the substrate governs cathepsin B enzymatic cleavage rate

The identity of the trivalent metal ion controls the rate of the enzymatic cleavage of a series of metal-complexed cathepsin B substrates. Increasing the distance between the metal complex and the enzyme cleavage site diminishes this effect.

Molecular Magnetic Resonance Imaging with Gd(III)-Based Contrast Agents: Challenges and Key Advances

In an era of personalized medicine, the clinical community has become increasingly focused on understanding diseases at the cellular and molecular levels. Magnetic resonance imaging (MRI) is a powerful imaging modality for acquiring anatomical and functional information. However, it has limited applications in the field of molecular imaging due to its low sensitivity. To expand the capability of MRI to encompass molecular imaging applications, we introduced bioresponsive Gd(III)-based magnetic resonance contrast agents (GBCAs) in 1997. Since that time, many research groups across the globe have developed new examples of bioresponsive GBCAs. These contrast agents have shown great promise for visualizing several biochemical processes, such as gene expression, neuronal signaling, and hormone secretion. They are designed to be conditionally retained, or activated, in vivo in response to specific biochemical events of interest. As a result, an observed MR signal change can serve as a read-out for molecular events. A significant challenge for these probes is how to utilize them for noninvasive diagnostic and theranostic applications. This Perspective focuses on the design strategies that underlie bioresponsive probes, and describes the key advances made in recent years that are facilitating their application in vivo and ultimately in clinical translation. While the field of bioresponsive agents is embryonic, it is clear that many solutions to the experimental and clinical radiologic problems of today will be overcome by the probes of tomorrow.

Bimodal Fluorescence-Magnetic Resonance Contrast Agent for Apoptosis Imaging

Effective cancer therapy largely depends on inducing apoptosis in cancer cells via chemotherapy and/or radiation. Monitoring apoptosis in real-time provides invaluable information for evaluating cancer therapy response and screening preclinical anticancer drugs. In this work, we describe the design, synthesis, characterization, and in vitro evaluation of caspase probe 1 (CP1), a bimodal fluorescence-magnetic resonance (FL-MR) probe that exhibits simultaneous FL-MR turn-on response to caspase-3/7. Both caspases exist as inactive zymogens in normal cells but are activated during apoptosis and are unique biomarkers for this process. CP1 has three distinct components: a DOTA-Gd(III) chelate that provides the MR signal enhancement, tetraphenylethylene as the aggregation induced emission luminogen (AIEgen), and DEVD peptide which is a substrate for caspase-3/7. In response to caspase-3/7, the water-soluble peptide DEVD is cleaved and the remaining Gd(III)-AIEgen (Gad-AIE) conjugate aggregates leading to increased FL-MR signals. CP1 exhibited sensitive and selective dual FL-MR turn-on response to caspase-3/7 in vitro and was successfully tested by fluorescence imaging of apoptotic cells. Remarkably, we were able to use the FL response of CP1 to quantify the exact concentrations of inactive and active agents and accurately predict the MR signal in vitro. We have demonstrated that the aggregation-driven FL-MR probe design is a unique method for MR signal quantification. This probe design platform can be adapted for a variety of different imaging targets, opening new and exciting avenues for multimodal molecular imaging.

Imaging Insulin Secretion from Mouse Pancreas by MRI Is Improved by Use of a Zinc-Responsive MRI Sensor with Lower Affinity for Zn2+ Ions

It has been demonstrated that divalent zinc ions packaged with insulin in β-cell granules can be detected by MRI during glucose-stimulated insulin secretion using a gadolinium-based Zn²⁺-sensitive agent. This study was designed to evaluate whether a simpler agent design having single Zn²⁺-sensing moieties but with variable Zn²⁺ binding affinities might also detect insulin secretion from the pancreas. Using an implanted MR-compatible window designed to hold the pancreas in a fixed position for imaging, we now demonstrate that focally intense "hot spots" can be detected in the tail of the pancreas using these agents after administration of glucose to stimulate insulin secretion. Histological staining of the same tissue verified that the hot spots identified by imaging correspond to clusters of islets, perhaps reflecting first-responder islets that are most responsive to a sudden increase in glucose. A comparison of images obtained when using a high-affinity Zn²⁺ sensor versus a lower-affinity sensor showed that the lower-affinity sensors produced the best image contrast. An equilibrium model that considers all possible complexes formed between Zn²⁺, the GdL sensor, and HSA predicts that a GdL sensor with lower affinity for Zn²⁺ generates a lower background signal from endogenous Zn²⁺ prior to glucose-stimulated insulin secretion (GSIS) and that the weaker binding affinity agent is more responsive to a further increase in Zn²⁺ concentration near β-cells after GSIS. These model predictions are consistent with the in vivo imaging observations.

The salen based chemosensors for highly selective recognition of Zn2+ ion

Two novel salen based chemosensors have been successfully synthesized. UV-vis absorption, fluorescence emission spectroscopy and cyclic voltammetry (CV) were exploited to investigate their recognition toward various metal ions, including Na⁺, K⁺, Mg²⁺, Al³⁺, Zn²⁺, Ag⁺, Pb²⁺, Co²⁺, Li⁺, Ba²⁺, Ca²⁺, Cd²⁺, La³⁺, Cu²⁺ and Mn²⁺ ions. The results indicated that the sensor L1 and L2 exhibited highly selective and sensitive recognition for Zn²⁺ ions. The binding stoichiometry ratio of L1-Zn²⁺/L2-Zn²⁺ were recognized as 4:1 by the method of Job's plot. Meanwhile, this investigation is confirmed by ¹H NMR. These results indicated that L1 and L2 can be applied as chemosensor for the detection of Zn²⁺ ion.

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.

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.

Heteronuclear Ir(III)-Ln(III) Luminescent Complexes: Small-Molecule Probes for Dual Modal Imaging and Oxygen Sensing

Luminescent, mixed metal d-f complexes have the potential to be used for dual (magnetic resonance imaging (MRI) and luminescence) in vivo imaging. Here, we present dinuclear and trinuclear d-f complexes, comprising a rigid framework linking a luminescent Ir center to one (Ir·Ln) or two (Ir·Ln2) lanthanide metal centers (where Ln = Eu(III) and Gd(III), respectively). A range of physical, spectroscopic, and imaging-based properties including relaxivity arising from the Gd(III) units and the occurrence of Ir(III) → Eu(III) photoinduced energy-transfer are presented. The rigidity imposed by the ligand facilitates high relaxivities for the Gd(III) complexes, while the luminescence from the Ir(III) and Eu(III) centers provide luminescence imaging capabilities. Dinuclear (Ir·Ln) complexes performed best in cellular studies, exhibiting good solubility in aqueous solutions, low toxicity after 4 and 18 h, respectively, and punctate lysosomal staining. We also demonstrate the first example of oxygen sensing in fixed cells using the dyad Ir·Gd, via two-photon phosphorescence lifetime imaging (PLIM).

Oxaazamacrocycles incorporating the quinoline moiety: synthesis and the study of their binding properties towards metal cations

Nitrogen- and oxygen-containing macrocycles with an endocyclic quinoline moiety synthesized via Pd(0)-catalyzed amination were found to be prospective fluorescent chemosensors for Cu(ii).

Triple-stimuli-responsive nanocontainers assembled by water-soluble pillar[5]arene-based pseudorotaxanes for controlled release

Working mechanism of triple-stimuli-responsive nanocontainers: alkaline, acid and Zn²⁺ stimuli can open the advanced supramolecular nanovalves.

Gd(3+)-Based Magnetic Resonance Imaging Contrast Agent Responsive to Zn(2+)

We report the heteroditopic ligand H5L, which contains a DO3A unit for Gd(3+) complexation connected to an NO2A moiety through a N-propylacetamide linker. The synthesis of the ligand followed a convergent route that involved the preparation of 1,4-bis(tert-butoxycarbonylmethyl)-1,4,7-triazacyclononane following the orthoamide strategy. The luminescence lifetimes of the Tb((5)D4) excited state measured for the TbL complex point to the absence of coordinated water molecules. Density functional theory calculations and (1)H NMR studies indicate that the EuL complex presents a square antiprismatic coordination in aqueous solution, where eight coordination is provided by the seven donor atoms of the DO3A unit and the amide oxygen atom of the N-propylacetamide linker. Addition of Zn(2+) to aqueous solutions of the TbL complex provokes a decrease of the emission intensity as the emission lifetime becomes shorter, which is a consequence of the coordination of a water molecule to the Tb(3+) ion upon Zn(2+) binding to the NO2A moiety. The relaxivity of the GdL complex recorded at 7 T (25 °C) increases by almost 150% in the presence of 1 equiv of Zn(2+), while Ca(2+) and Mg(2+) induced very small relaxivity changes. In vitro magnetic resonance imaging experiments confirmed the ability of GdL to provide response to the presence of Zn(2+).

Dual-modal magnetic resonance/fluorescent zinc probes for pancreatic β-cell mass imaging

Despite the contribution of changes in pancreatic β-cell mass to the development of all forms of diabetes mellitus, few robust approaches currently exist to monitor these changes prospectively in vivo. Although magnetic-resonance imaging (MRI) provides a potentially useful technique, targeting MRI-active probes to the β cell has proved challenging. Zinc ions are highly concentrated in the secretory granule, but they are relatively less abundant in the exocrine pancreas and in other tissues. We have therefore developed functional dual-modal probes based on transition-metal chelates capable of binding zinc. The first of these, Gd⋅1, binds Zn(II) directly by means of an amidoquinoline moiety (AQA), thus causing a large ratiometric Stokes shift in the fluorescence from λem =410 to 500 nm with an increase in relaxivity from r1 =4.2 up to 4.9 mM(-1)  s(-1) . The probe is efficiently accumulated into secretory granules in β-cell-derived lines and isolated islets, but more poorly by non-endocrine cells, and leads to a reduction in T1 in human islets. In vivo murine studies of Gd⋅1 have shown accumulation of the probe in the pancreas with increased signal intensity over 140 minutes.

Long N. Towards understanding the design of dual-modal MR/fluorescent probes to sense zinc ions

A series of new gadolinium complexes have been synthesised to test the design of dual-modal probes that can display a change in fluorescence or relaxivity response upon zinc binding. By an iterative change in parameters of the probes, the compounds give insight into the design protocols required for successful imaging of zinc ions.

Water soluble, cyclometalated Pt(ii)–Ln(iii) conjugates towards novel bimodal imaging agents

A heterometallic, water soluble Pt^II–Gd^III complex possesses visible luminescence and enhanced water relaxivity whilst showing promise and application in cell imaging studies.

A new ligand skeleton for imaging applications with d–f complexes: combined lifetime imaging and high relaxivity in an Ir/Gd dyad

The rigid dinuclear complexes Ir·Ln (Ln = Eu, Gd) show potential for use in dual magnetic resonance + time-resolved luminescence imaging (Ir·Gd) and d → f energy-transfer (Ir·Eu).

Construction of helical coordination polymers via flexible conformers of bis(3-pyridyl)cyclotetramethylenesilane: metal(ii) and halogen effects on luminescence, thermolysis and catalysis

Construction and physicochemical properties of rectangular-tubular-helical coordination polymers of [M(ii)X₂L] (M = Zn(ii), Hg(ii); X⁻ = Cl⁻, Br⁻) were investigated.

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

The local presence and concentration of metal ions in biological systems has been extensively studied ex vivo using fluorescent dyes. However, the detection of multiple metal ions in vivo remains a major challenge. We present a magnetic resonance imaging (MRI)-based method for noninvasive detection of specific ions that may be coexisting, using the tetrafluorinated derivative of the BAPTA (TF-BAPTA) chelate as a ¹⁹F chelate analogue of existing optical dyes. Taking advantage of the difference in the ion-specific ¹⁹F nuclear magnetic resonance (NMR) chemical shift offset (Δω) values between the ion-bound and free TF-BAPTA, we exploited the dynamic exchange between ion-bound and free TF-BAPTA to obtain MRI contrast with multi-ion chemical exchange saturation transfer (miCEST). We demonstrate that TF-BAPTA as a prototype single ¹⁹F probe can be used to separately visualize mixed Zn²⁺ and Fe²⁺ ions in a specific and simultaneous fashion, without interference from potential competitive ions.

Metal-carbonyl units for vibrational and luminescence imaging: towards multimodality

Metal-carbonyl complexes are attractive structures for bio-imaging. In addition to unique vibrational properties due to the CO moieties enabling IR and Raman cell imaging, the appropriate choice of ancillary ligands opens up the opportunity for luminescence detection. Through a classification by techniques, past and recent developments in the application of metal-carbonyl complexes for vibrational and luminescence bio-imaging are reviewed. Finally, their potential as bimodal IR and luminescent probes is addressed.

Combined two-photon excitation and d→f energy transfer in a water-soluble Ir(III)/Eu(III) dyad: two luminescence components from one molecule for cellular imaging

The first example of cell imaging using two independent emission components from a dinuclear d/f complex is reported. A water-stable, cell-permeable Ir(III) /Eu(III) dyad undergoes partial Ir→Eu energy transfer following two-photon excitation of the Ir unit at 780 nm. Excitation in the near-IR region generated simultaneously green Ir-based emission and red Eu-based emission from the same probe. The orders-of-magnitude difference in their timescales (Ir ca. μs; Eu ca. 0.5 ms) allowed them to be identified by time-gated detection. Phosphorescence lifetime imaging microscopy (PLIM) allowed the lifetime of the Ir-based emission to be measured in different parts of the cell. At the same time, the cells are simultaneously imaged by using the Eu-based emission component at longer timescales. This new approach to cellular imaging by using dual d/f emitters should therefore enable autofluorescence-free sensing of two different analytes, independently, simultaneously and in the same regions of a cell.

Non-covalent self assembly controls the relaxivity of magnetically active guests

The relaxivity of a magnetically responsive Gd complex can be controlled by non-covalent molecular recognition with a water-soluble deep cavitand. Lowered relaxivity is conferred by a self-assembled micellar "off state", and the contrast can be regenerated by addition of a superior guest.

Halogen effects on photoluminescence and catalytic properties: a series of spatially arranged trimetallic zinc(ii) complexes

Halogen effects of a unique columnar ensemble of discrete trimetallic zinc(ii) complexes formed via π⋯π interactions and hydrogen-bonds were investigated.

Naphthyridine-based lanthanide complexes worked as magnetic resonance imaging contrast for guanosine 5'-monophosphate in vivo

New lanthanide complex Gd-ANAMD containing 2-amino-7-methyl-1,8-naphthyridine was achieved for selective magnetic resonance imaging towards guanosine 5'-monophosphate over other ribonucleotide polyphosphates in aqueous media and in vivo. The formation of strong multi-hydrogen bonds between naphthyridine and guanosine made the phosphate in guanosine 5'-monophosphate positioned on a suitable site to coordinate with the lanthanide ion. The substitution of the coordination naphthyridine by the phosphate oxygen atoms caused obvious relaxivity decrease. The negligible cytotoxicity and appropriate blood circulation time of Gd-ANAMD allow potential application of Magnetic Resonance Imaging in vivo. (1)H NMR confirmed that the selectivity of these lanthanide complexes towards guanosine was attributed to the formation of hydrogen bonds between the guanine moeity and the naphthyridine. The fluorescence detection and lifetime measurement of Tb-ANAMD and Eu-ANAMD suggested that the decrease of the relaxivity is not attributed to the change of the q value, but caused by the prolonging of the residence lifetime of inner-sphere water.

Promising strategies for Gd-based responsive magnetic resonance imaging contrast agents

Magnetic resonance imaging is a powerful imaging modality that is often coupled with paramagnetic contrast agents based on gadolinium to enhance sensitivity and image quality. Responsive contrast agents are key to furthering the diagnostic potential of MRI, both to provide anatomical information and to discern biochemical activity. Recent design of responsive gadolinium-based T₁ agents has made interesting progress, with the development of novel complexes which sense their chemical environment through changes in the coordination of water molecules, the molecular tumbling time or the number of metal centres. Particular promising design strategies include the use of multimeric systems, and the development of dual imaging probes.