Zn2+-Responsive Bimodal Magnetic Resonance Imaging and Fluorescence Imaging Agents and Their Interaction with Human Serum Albumin

Gd3+ Complexes for MRI Detection of Zn2+ in the Presence of Human Serum Albumin: Structure-Activity Relationships

Zn2+-responsive magnetic resonance imaging (MRI) contrast agents are typically composed of a Gd chelate conjugated to a Zn2+-binding moiety via a linker. They allow for Zn2+ detection in the presence of human serum albumin (HSA). In order to decipher the key parameters that drive their Zn2+-dependent MRI response, we designed a pyridine-based ligand, PyAmC2mDPA, and compared the properties of GdPyAmC2mDPA to those of analogue complexes with varying Gd core, Zn-binding moiety, or linker sizes. The stability constants determined by pH potentiometry showed the good selectivity of PyAmC2mDPA for Gd3+ (log KGd = 16.27) versus Zn2+ (log KZn = 13.58), proving that our modified Zn2+-binding DPA moiety prevents the formation of previously observed dimeric species. Paramagnetic relaxation enhancement measurements indicated at least three sites that are available for GdPyAmC2mDPA binding on HSA, as well as a 2-fold affinity increase when Zn2+ is present (KD = 170 μM versus KDZn = 60 μM). Fluorescence competition experiments provided evidence of the higher affinity for site II vs site I, as well as the importance of both the Zn-binding part and the Gd core in generating enhanced HSA affinity in the presence of Zn2+. Finally, an analysis of nuclear magnetic relaxation dispersion (NMRD) data suggested a significantly increased rigidity for the Zn2+-bound system, which is responsible for the Zn2+-dependent relaxivity response.

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.

Accelerated 19F·MRI Detection of Matrix Metalloproteinase-2/-9 through Responsive Deactivation of Paramagnetic Relaxation Enhancement

Paramagnetic gadolinium ions (Gd^III), complexed within DOTA-based chelates, have become useful tools to increase the magnetic resonance imaging (MRI) contrast in tissues of interest. Recently, "on/off" probes serving as ¹⁹F·MRI biosensors for target enzymes have emerged that utilize the increase in transverse (T ₂ ^∗ or T ₂) relaxation times upon cleavage of the paramagnetic Gd^III centre. Molecular ¹⁹F·MRI has the advantage of high specificity due to the lack of background signal but suffers from low signal intensity that leads to low spatial resolution and long recording times. In this work, an "on/off" probe concept is introduced that utilizes responsive deactivation of paramagnetic relaxation enhancement (PRE) to generate ¹⁹F longitudinal (T ₁) relaxation contrast for accelerated molecular MRI. The probe concept is applied to matrix metalloproteinases (MMPs), a class of enzymes linked with many inflammatory diseases and cancer that modify bioactive extracellular substrates. The presence of these biomarkers in extracellular space makes MMPs an accessible target for responsive PRE deactivation probes. Responsive PRE deactivation in a ¹⁹F biosensor probe, selective for MMP-2 and MMP-9, is shown to enable molecular MRI contrast at significantly reduced experimental times compared to previous methods. PRE deactivation was caused by MMP through cleavage of a protease substrate that served as a linker between the fluorine-containing moiety and a paramagnetic Gd^III-bound DOTA complex. Ultrashort echo time (UTE) MRI and, alternatively, short echo times in standard gradient echo (GE) MRI were employed to cope with the fast ¹⁹F transverse relaxation of the PRE active probe in its "on-state." Upon responsive PRE deactivation, the ¹⁹F·MRI signal from the "off-state" probe diminished, thereby indicating the presence of the target enzyme through the associated negative MRI contrast. Null point ¹H·MRI, obtainable within a short time course, was employed to identify false-positive ¹⁹F·MRI responses caused by dilution of the contrast agent.

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.

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.