The importance of water exchange rates in the design of responsive agents for MRI

Unraveling the Stability and Magnetic Properties of Bis-Hydrated Mn(II) Complexes via Tailored Ligand Design

Exploring the electronic structure and dynamic behavior of Mn(II) complexes reveals fascinating magnetic properties and prospective biomedical applications. In this study, we investigate the solvent phase dynamics of heptacoordinated Mn(II) complexes through ab initio molecular dynamics simulations and density functional theory (DFT) calculations with effectively varying temperatures. We observed that the complex with high stability ([Mn(pmpa)(H2O)2]) remains relatively rigid as the temperature increases to 90 °C, with only a minor change in its radial distribution functions (RDFs), compared to the RDF peaks at 25 °C. To elucidate the impact of halogens on the magnetic anisotropy of seven-coordinated Mn(II) complexes, we performed both DFT and multireference calculations. This shows that the zero-field splitting (ZFS) parameter D follows the order D(I)> D(Br)> D(Cl). We observed a significant increase in the D-value following the substitution of soft Se-donors in the equatorial position and heavier halogens in the axial position. The D-value of halogen derivatives of Se-analogues varies in the order of D(Cl) < D(I) < D(Br), deviating from the regular spectrochemical series with the discrepancy between the covalency of the Mn(II)-Se bond and the ligand field strength. We anticipate that this study will enhance our understanding of the solvent phase dynamics and structural aspects of ZFS in various Mn(II) complexes with different electronic environments.

TEMPO-conjugated tobacco mosaic virus as a magnetic resonance imaging contrast agent for detection of superoxide production in the inflamed liver

Superoxide, an anionic dioxygen molecule, plays a crucial role in redox regulation within the body but is implicated in various pathological conditions when produced excessively. Efforts to develop superoxide detection strategies have led to the exploration of organic-based contrast agents for magnetic resonance imaging (MRI). This study compares the effectiveness of two such agents, nTMV-TEMPO and kTMV-TEMPO, for detecting superoxide in a mouse liver model with lipopolysaccharide (LPS)-induced inflammation. The study demonstrates that kTMV-TEMPO, with a strategically positioned lysine residue for TEMPO attachment, outperforms nTMV-TEMPO as an MRI contrast agent. The enhanced sensitivity of kTMV-TEMPO is attributed to its more exposed TEMPO attachment site, facilitating stronger interactions with water protons and superoxide radicals. EPR kinetics experiments confirm kTMV-TEMPO's faster oxidation and reduction rates, making it a promising sensor for superoxide in inflamed liver tissue. In vivo experiments using healthy and LPS-induced inflamed mice reveal that reduced kTMV-TEMPO remains MRI-inactive in healthy mice but becomes MRI-active in inflamed livers. The contrast enhancement in inflamed livers is substantial, validating the potential of kTMV-TEMPO for detecting superoxide in vivo. This research underscores the importance of optimizing contrast agents for in vivo imaging applications. The enhanced sensitivity and biocompatibility of kTMV-TEMPO make it a promising candidate for further studies in the realm of medical imaging, particularly in the context of monitoring oxidative stress-related diseases.

α-Aryl substituted GdDOTA derivatives, the perfect contrast agents for MRI?

Enhancing the performance of Gd3+ chelates as relaxation agents for MRI has the potential to lower doses, improving safety and mitigating the environmental impact on our surface waters. More than three decades of research into manipulating the properties of Gd3+ have failed to develop a chelate that simultaneously optimizes all relevant parameters and affords maximal relaxivity. Introducing aryl substituents into the α-position of the pendant arms of a GdDOTA chelate affords chelates that, for the first time, simultaneously optimize all physico-chemical properties. Slowing tumbling by binding to human serum albumin affords a relaxivity of 110 ± 5 mM-1 s-1, close to the maximum possible. As discrete chelates, these α-aryl substituted GdDOTA chelates exhibit relaxivities that are 2-3 times higher than those of currently used agents, even at the higher fields (1.5 & 3.0 T) used in modern clinical MRI.

Gold nanoparticles conjugated with epidermal growth factor and gadolinium for precision delivery of contrast agents in magnetic resonance imaging

The utilization of contrast agents in magnetic resonance imaging (MRI) has become increasingly important in clinical diagnosis. However, the low diagnostic specificity of this technique is a limiting factor for the early detection of tumors. To develop a new contrast agent with a specific target for early stage tumors, we present the synthesis and characterization of a nanocontrast composed of gold nanoparticles (AuNPs), gadopentetic acid (Gd-DTPA), and epidermal growth factor (EGF). Carbodiimide-based chemistry was utilized to modify Gd-DTPA for functionalization with AuNPs. This resulted in the formation of the Au@Gd-EGF nanocontrast. The relaxation rate (1/T1) of the nanocontrast was analyzed using MRI, and cytotoxicity was determined based on cell viability and mitochondrial activity in a human breast adenocarcinoma cell line. Fourier-transform infrared spectroscopy analysis confirmed the effectiveness of carbodiimide in the formation of the Gd-DTPA-cysteamine complex in the presence of bands at 930, 1042, 1232, 1588, and 1716 cm-1. The complexes exhibited good interactions with the AuNPs. However, the signal intensity of the Au@Gd-EGF nanocontrast was lower than that of the commercial contrast agent because the r1/r2 relaxivities of the Gd-DTPA-based contrast agents were lower than those of the gadoversetamide-based molecules. The Au@Gd-EGF nanocontrast agent exhibited good biocompatibility, low cytotoxicity, and high signal intensity in MRI with active targeted delivery, suggesting significant potential for future applications in the early diagnosis of tumors.

Endeavor toward Redox-Responsive Transition Metal Contrast Agents Based on the Cross-Bridge Cyclam Platform

We present the synthesis and characterization of a series of Mn(III), Co(III), and Ni(II) complexes with cross-bridge cyclam derivatives (CB-cyclam = 1,4,8,11-tetraazabicyclo[6.6.2]hexadecane) containing acetamide or acetic acid pendant arms. The X-ray structures of [Ni(CB-TE2AM)]Cl2·2H2O and [Mn(CB-TE1AM)(OH)](PF6)2 evidence the octahedral coordination of the ligands around the Ni(II) and Mn(III) metal ions, with a terminal hydroxide ligand being coordinated to Mn(III). Cyclic voltammetry studies on solutions of the [Mn(CB-TE1AM)(OH)]2+ and [Mn(CB-TE1A)(OH)]+ complexes (0.15 M NaCl) show an intricate redox behavior with waves due to the MnIII/MnIV and MnII/MnIII pairs. The Co(III) and Ni(II) complexes with CB-TE2A and CB-TE2AM show quasi-reversible features due to the CoIII/CoII or NiII/NiIII pairs. The [Co(CB-TE2AM)]3+ complex is readily reduced by dithionite in aqueous solution, as evidenced by 1H NMR studies, but does not react with ascorbate. The [Mn(CB-TE1A)(OH)]+ complex is however reduced very quickly by ascorbate following a simple kinetic scheme (k0 = k1[AH-], where [AH-] is the ascorbate concentration and k1 = 628 ± 7 M-1 s-1). The reduction of the Mn(III) complex to Mn(II) by ascorbate provokes complex dissociation, as demonstrated by 1H nuclear magnetic relaxation dispersion studies. The [Ni(CB-TE2AM)]2+ complex shows significant chemical exchange saturation transfer effects upon saturation of the amide proton signals at 71 and 3 ppm with respect to the bulk water signal.

Coating influence on inner shell water exchange: An underinvestigated major contributor to SPIONs relaxation properties

Superparamagnetic iron oxide nanoparticles (SPIONs) are heavily studied as potential MRI contrast enhancing agents. Every year, novel coatings are reported which yield large increases in relaxivity compared to similar particles. However, the reason for the increased performance is not always well understood mechanistically. In this review, we attempt to relate these advances back to fundamental models of relaxivity, developed for chelated metal ions, primarily gadolinium. We focus most closely on the three-shell model which considers the relaxation of surface-bound, entrained, and bulk water molecules as three distinct contributions to total relaxation. Because SPIONs are larger, more complex, and entrain significantly more water than gadolinium-based contrast agents, we consider how to adapt the application of classical models to SPIONs in a predictive manner. By carefully considering models and previous results, a qualitative model of entrained water interactions emerges, based primarily on the contributions of core size, coating thickness, density, and hydrophilicity.

Di-Pyridine-Containing Macrocyclic Triamide Fe(II) and Ni(II) Complexes as ParaCEST Agents

Fe(II) and Ni(II) paraCEST contrast agents containing the di-pyridine macrocyclic ligand 2,2',2″-(3,7,10-triaza-1,5(2,6)-dipyridinacycloundecaphane-3,7,10-triyl)triacetamide (DETA) are reported here. Both [Fe(DETA)]²⁺ and [Ni(DETA)]²⁺ complexes were structurally characterized. Crystallographic data revealed the seven-coordinated distorted pentagonal bipyramidal geometry of the [Fe(DETA)]·(BF₄)₂·MeCN complex with five coordinated nitrogen atoms from the macrocyclic ring and two coordinated oxygen atoms from two amide pendant arms. The [Ni(DETA)]·Cl₂·2H₂O complex was six-coordinated in nature with a distorted octahedral geometry. Four coordinated nitrogen atoms were from the macrocyclic ring, and two coordinated oxygen atoms were from two amide pendant arms. [Fe(DETA)]²⁺ exhibited well-resolved sharp proton resonances, whereas very broad proton resonances were observed in the case of [Ni(DETA)]²⁺ due to the long electronic relaxation times. The CEST peaks for the [Fe(DETA)]²⁺ complex showed one highly downfield-shifted and intense peak at 84 ppm with another shifted but less intense peak at 28 ppm with good CEST contrast efficiency at body temperature, whereas [Ni(DETA)]²⁺ showed only one highly shifted intense peak at 78 ppm from the bulk water protons. Potentiometric titrations were performed to determine the protonation constants of the ligand and the thermodynamic stability constant of the [M(DETA)]²⁺ (M = Fe, Co, Ni, Cu, Zn) species at 25.0 °C and I = 0.15 mol·L^-1 NaClO₄. Metal exchange studies confirmed the stability of the complexes in acidic medium in the presence of physiologically relevant anions and an equimolar concentration of Zn(II) ions.

Analysis of Stable Chelate-free Gadolinium Loaded Titanium Dioxide Nanoparticles for MRI-Guided Radionuclide Stimulated Cancer Treatment

Background

Recent studies demonstrate that titanium dioxide nanoparticles (TiO₂ NPs) are an effective source of reactive oxygen species (ROS) for photodynamic therapy and radionuclide stimulated dynamic therapy (RaST). Unfortunately tracking the in vivo distribution of TiO₂ NPs noninvasively remains elusive.

Objective

Given the use of gadolinium (Gd) chelates as effective contrast agents for magnetic resonance imaging (MRI), this study aims to (1) develop hybrid TiO₂-Gd NPs that exhibit high relaxivity for tracking the NPs without loss of ROS generating capacity; and (2) establish a simple colorimetric assay for quantifying Gd loading and stability.

Methods

A chelate-free, heat-induced method was used to load Gd onto TiO₂ NPs, which was coated with transferrin (Tf). A sensitive colorimetric assay and inductively coupled plasma mass spectrometry (ICP-MS) were used to determine Gd loading and stability of the TiO₂-Gd-Tf NPs. Measurement of the relaxivity was performed on a 1.4 T relaxometer and a 4.7 T small animal magnetic resonance scanner to estimate the effects of magnetic field strength. ROS was quantified by activated dichlorodihydrofluorescein diacetate fluorescence. Cell uptake of the NPs and RaST were monitored by fluorescence microscopy. Both 3 T and 4.7 T scanners were used to image the in vivo distribution of intravenously injected NPs in tumor-bearing mice.

Results

A simple colorimetric assay accurately determined both the loading and stability of the NPs compared with the expensive and complex ICP-MS method. Coating of the TiO₂-Gd NPs with Tf stabilized the nanoconstruct and minimized aggregation. The TiO₂-Gd-Tf maintained ROS-generating capability without inducing cell death at a wide range of concentrations but induced significant cell death under RaST conditions in the presence of F-18 radiolabeled 2-fluorodeoxyglucose. The longitudinal (r1 = 10.43 mM^-1s^-1) and transverse (r2 = 13.43 mM^-1s^-1) relaxivity of TiO₂-Gd-Tf NPs were about twice and thrice, respectively, those of clinically used Gd contrast agent (Gd-DTPA; r1 = 3.77 mM^-1s^-1 and r2 = 5.51 mM^-1s^-1) at 1.4 T. While the r1 (8.13 mM^-1s^-1) reduced to about twice that of Gd-DTPA (4.89 mM^-1s^-1) at 4.7 T, the corresponding r2 (87.15 mM^-1s^-1) increased by a factor 22.6 compared to Gd-DTPA (r2 = 3.85). MRI of tumor-bearing mice injected with TiO₂-Gd-Tf NPs tracked the NPs distribution and accumulation in tumors.

Conclusion

This work demonstrates that Arsenazo III colorimetric assay can substitute ICP-MS for determining the loading and stability of Gd-doped TiO₂ NPs. The new nanoconstruct enabled RaST effect in cells, exhibited high relaxivity, and enhanced MRI contrast in tumors in vivo, paving the way for in vivo MRI-guided RaST.

Methylated tetra-amide derivatives of paramagnetic complexes for magnetic resonance biosensing with both BIRDS and CEST

Paramagnetic agents that utilize two mechanisms to provide physiological information by magnetic resonance imaging (MRI) and magnetic resonance spectroscopic imaging (MRSI) are described. MRI with chemical exchange saturation transfer (CEST) takes advantage of the agent's exchangeable protons (e.g., -OH or -NH_x , where 2 ≥ x ≥ 1) to create pH contrast. The agent's incorporation of non-exchangeable protons (e.g., -CH_y , where 3 ≥ y ≥ 1) makes it possible to map tissue temperature and/or pH using an MRSI method called biosensor imaging of redundant deviation in shifts (BIRDS). Hybrid probes based upon 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate chelate (DOTA^4- ) and its methylated analog (1,4,7,10-tetraazacyclododecane-α, α', α″, α‴-tetramethyl-1,4,7,10-tetraacetate, DOTMA^4- ) were synthesized, and modified to create new tetra-amide chelates. Addition of several methyl groups per pendent arm of the symmetrical chelates, positioned proximally and distally to thulium ions (Tm³⁺ ), gave rise to favorable BIRDS properties (i.e., high signal-to-noise ratio (SNR) from non-exchangeable methyl proton peaks) and CEST responsiveness (i.e., from amide exchangeable protons). Structures of the Tm³⁺ probes elucidate the influence of methyl group placement on sensor performance. An eight-coordinate geometry with high symmetry was observed for the complexes: Tm-L1 was based on DOTA^4- , whereas Tm-L2 and Tm-L3 were based on DOTMA^4- , where the latter contained an additional carboxylate at the distal end of each arm. The distance of Tm³⁺ from terminal methyl carbons is a key determinant for sustaining BIRDS temperature sensitivity without compromising CEST pH contrast; however, water solubility was influenced by introduction of hydrophobic methyl groups and hydrophilic carboxylate. Combined BIRDS and CEST detection of Tm-L2, which features two high-SNR methyl peaks and a strong amide CEST peak, should enable simultaneous temperature and pH measurements for high-resolution molecular imaging in vivo.

Repurposing ICG enables MR/PA imaging signal amplification and iron depletion for iron-overload disorders

Precise and noninvasive theranostic methods to quantify and deplete focal iron are of crucial importance for iron-overload disorders. Here, we developed an indocyanine green (ICG)–based imaging platform to reveal Fe³⁺ in vitro and in vivo. The high sensitivity and specificity of ICG-Fe interaction facilitated MR images with a marked correlation between T₁ signal intensity ratio (T₁SIR) changes and Fe³⁺ concentration in rodent models and humans. On the basis of these findings, a rational design for coordination-driven self-assembly ICG-Lecithin (ICG/Leci) was proposed to determine Fe³⁺. The enhancement of photoacoustic signal at 890 nm with increasing Fe³⁺ concentration showed an over 600% higher linear slope than that of T₁SIR changes in animal models. ICG/Leci also promoted a 100% increase in iron depletion in the liver compared with deferoxamine. The high MR sensitivity and superior photoacoustic contrast, combined with enhanced iron depletion, demonstrate that ICG/Leci is a promising theranostic agent for simultaneous detection and treatment of iron-overload disorders.

Chemical exchange saturation transfer for detection of antiretroviral drugs in brain tissue

OBJECTIVE

Antiretroviral drug theranostics facilitates the monitoring of biodistribution and efficacy of therapies designed to target HIV type-1 (HIV-1) reservoirs. To this end, we have now deployed intrinsic drug chemical exchange saturation transfer (CEST) contrasts to detect antiretroviral drugs within the central nervous system (CNS).

DESIGN AND METHODS

CEST effects for lamivudine (3TC) and emtricitabine (FTC) were measured by asymmetric magnetization transfer ratio analyses. The biodistribution of 3TC in different brain sub-regions of C57BL/6 mice treated with lipopolysaccharides was determined using MRI. CEST effects of 3TC protons were quantitated by Lorentzian fitting analysis. 3TC levels in plasma and brain regions were measured using ultraperformance liquid chromatography tandem mass spectrometry to affirm the CEST test results.

RESULTS

CEST effects of the hydroxyl and amino protons in 3TC and FTC linearly correlated to drug concentrations. 3TC was successfully detected in vivo in brain sub-regions by MRI. The imaging results were validated by measurements of CNS drug concentrations.

CONCLUSION

CEST contrasts can be used to detect antiretroviral drugs using MRI. Such detection can be used to assess spatial--temporal drug biodistribution. This is most notable within the CNS where drug biodistribution may be more limited with the final goal of better understanding antiretroviral drug-associated efficacy and potential toxicity.

Iron oxide nanoparticles for immune cell labeling and cancer immunotherapy

Cancer immunotherapy is a novel approach to cancer treatment that leverages components of the immune system as opposed to chemotherapeutics or radiation. Cell migration is an integral process in a therapeutic immune response, and the ability to track and image the migration of immune cells in vivo allows for better characterization of the disease and monitoring of the therapeutic outcomes. Iron oxide nanoparticles (IONPs) are promising candidates for use in immunotherapy as they are biocompatible, have flexible surface chemistry, and display magnetic properties that may be used in contrast-enhanced magnetic resonance imaging (MRI). In this review, advances in application of IONPs in cell tracking and cancer immunotherapy are presented. Following a brief overview of the cancer immunity cycle, developments in labeling and tracking various immune cells using IONPs are highlighted. We also discuss factors that influence the effectiveness of IONPs as MRI contrast agents. Finally, we outline different approaches for cancer immunotherapy and highlight current efforts that utilize IONPs to stimulate immune cells to enhance their activity and response to cancer.

A DNA-Based MRI Contrast Agent for Quantitative pH Measurement

Extracellular pH is important in clinical measurements due to its correlation to cell metabolism and disease progression. In MRI, T1/T2 ratiometric analysis and other methods have been previously applied to quantify pH using conventional pulse sequences. However, for nanoparticle-based approaches, heterogeneity in size and surface functionalization tends toward qualitative rather than quantitative results. To address this limitation, we developed a novel DNA-based MRI contrast agent, pH-DMRCA, which utilizes a highly programmable and reproducible nanostructure. The pH-DMRCA is a dendritic DNA scaffold that is functionalized with a pH-responsive MRI-sensitive construct, Gd(NP-DO3A), at the end of each DNA arm. We first evaluated the r1 and r2 response of our pH-DMRCA over a range of pH values (pH = 5-9) to establish a relaxometric model of pH. These MRI-based assessments of pH were validated in a separate set of samples using a pH electrode (n = 18) and resulted in a good linear correlation (R2 = 0.99, slope = 0.98, intercept = 0). A Bland-Altman analysis of the results also showed reasonable agreement between the calculated pH and measured pH. Moreover, these pH comparisons were consistent across three different pH-DMRCA concentrations, demonstrating concentration-independence of the method. This MRI-based pH quantification methodology was further verified in human blood plasma. Given the versatility of the DNA-based nanostructures, the contrast agent has a potential to be applied to a wide variety of imaging applications where extracellular pH is important including cancer, stroke, cardiovascular disease, and other important diseases.

Rational Design of High-Relaxivity EuII -Based Contrast Agents for Magnetic Resonance Imaging of Low-Oxygen Environments

Metal-based contrast agents for magnetic resonance imaging present a promising avenue to image hypoxia. EuII -based contrast agents have a unique biologically relevant redox couple, EuII/III , that distinguishes this metal for use in hypoxia imaging. To that end, we investigated a strategy to enhance the contrast-enhancing capabilities of EuII -based cryptates in magnetic resonance imaging by controlling the rotational dynamics. Two dimetallic, EuII -containing cryptates were synthesized to test the efficacy of rigid versus flexible coupling strategies. A flexible strategy to dimerization led to a modest (114 %) increase in contrast enhancement per Eu ion (60 MHz, 298 K), but a rigid linking strategy led to an excellent (186 %) increase in contrast enhancement despite this compound's having the smaller molecular mass of the two dimetallic complexes. We envision the rigid linking strategy to be useful in the future design of potent EuII -based contrast agents for magnetic resonance imaging.

A T2ex MRI Dy-based contrast agent for direct pH imaging using a ratiometric approach

We developed a series of T_2ex MRI probes which helped achieve concentration-independent and direct pH mapping in physiological pH ranges.

5-HT1A targeting PARCEST agent DO3AM-MPP with potential for receptor imaging: Synthesis, physico-chemical and MR studies

Contrast enhancement in MRI using magnetization or saturation transfer techniques promises better sensitivity, and faster acquisition compared to T₁ or T₂ contrast. This work reports the synthesis and evaluation of 5-HT_1A targeted PARACEST MRI contrast agent using 1,4,7,10-tetraazacycloDOdecane-4,7,10-triacetAMide (DO3AM) as the bifunctional chelator, and 5-HT_1A-antagonist methoxyphenyl piperazine (MPP) as a targeting unit. The multi-step synthesis led to the MPP conjugated DO3AM with 60% yield. CEST-related physicochemical parameters were evaluated after loading DO3AM-MPP with paramagnetic MRI active lanthanides: Gadolinium (Gd-DO3AM-MPP) and Europium (Eu-DO3AM-MPP). Luminescence lifetime measurements with Eu-DO3AM-MPP and computational DFT studies using Gd-DO3AM-MPP revealed the coordination of one water molecule (q = 1.43) with metal-water distance (r_M-H₂O) of 2.7 Å and water residence time (τ_m) of 0.23 ms. The dissociation constant of K_d 62 ± 0.02 pM as evaluated from fluorescence quenching of 5-HT_1A (protein) and docking score of -4.81 in theoretical evaluation reflect the binding potential of the complex Gd-DO3AM-MPP with the receptor 5-HT_1A. Insights of the docked pose reflect the importance of NH₂ (amide) and aromatic ring in Gd-DO3AM-MPP while interacting with Ser 374 and Phe 370 in the antagonist binding pocket of 5-HT_1A. Gd-DO3AM-MPP shows longitudinal relaxivity 5.85 mM^-1s^-1 with a water residence lifetime of 0.93 ms in hippocampal homogenate containing 5-HT_1A. The potentiometric titration of DO3AM-MPP showed strong selectivity for Gd³⁺ over physiological metal ions such as Zn²⁺ and Cu²⁺. The in vitro and in vivo studies confirmed the minimal cytotoxicity and presential binding of Gd-DO3AM-MPP with 5-HT_1A receptor in the hippocampus region of the mice. Summarizing, the complex Gd-DO3AM-MPP can have a potential for CEST imaging of 5-HT_1A receptors.

Supramolecular adducts between macrocyclic Gd(iii) complexes and polyaromatic systems: a route to enhance the relaxivity through the formation of hydrophobic interactions

The set-up of reversible binding interactions between the hydrophobic region of macrocyclic GBCAs (Gadolinium Based Contrast Agents) and SO₃ ^-/OH containing pyrene derivatives provides new insights for pursuing relaxivity enhancements of this class of MRI contrast agents. The strong binding affinity allows attaining relaxation enhancements up to 50% at pyrene/GBCA ratios of 3 : 1. High resolution NMR spectra of the Yb-HPDO3A/pyrene system fully support the formation of a supramolecular adduct based on the set-up of hydrophobic interactions. The relaxation enhancement may be accounted for in terms of the increase of the molecular reorientation time (τ _R) and the number of second sphere water molecules. This effect is maintained in blood serum and in vivo, as shown by the enhancement of contrast in T _1w-MR images obtained by simultaneous injection of GBCA and pyrene derivatives in mice.

Mn2+ Complexes Containing Sulfonamide Groups with pH-Responsive Relaxivity

We present two ligands containing a N-ethyl-4-(trifluoromethyl)benzenesulfonamide group attached to either a 6,6'-(azanediylbis(methylene))dipicolinic acid unit (H₃DPASAm) or a 2,2'-(1,4,7-triazonane-1,4-diyl)diacetic acid macrocyclic platform (H₃NO2ASAm). These ligands were designed to provide a pH-dependent relaxivity response upon complexation with Mn²⁺ in aqueous solution. The protonation constants of the ligands and the stability constants of the Mn²⁺ complexes were determined using potentiometric titrations complemented by spectrophotometric experiments. The deprotonations of the sulfonamide groups of the ligands are characterized by protonation constants of log K_i^H = 10.36 and 10.59 for DPASAm^3- and HNO2ASAm^2-, respectively. These values decrease dramatically to log K_i^H = 6.43 and 5.42 in the presence of Mn²⁺, because of the coordination of the negatively charged sulfonamide groups to the metal ion. The higher log K_i^H value in [Mn(DPASAm)]^- is related to the formation of a seven-coordinate complex, while the metal ion in [Mn(NO2ASAm)]^- is six-coordinated. The X-ray crystal structure of Na[Mn(DPASAm)(H₂O)]·2H₂O confirms the formation of a seven-coordinate complex, where the coordination environment is fulfilled by the donor atoms of the two picolinate groups, the amine N atom, the N atom of the sulfonamide group, and a coordinated water molecule. The lower conditional stability of the [Mn(NO2ASAm)]^- complex and the lower protonation constant of the sulfonamide group results in complex dissociation at relatively high pH (<7.0). However, protonation of the sulfonamide group in [Mn(DPASAm)]^- falls into the physiologically relevant pH window and causes a significant increase in relaxivity from r_1p = 3.8 mM^-1 s^-1 at pH 9.0 to r_1p = 8.9 mM^-1 s^-1 at pH 4.0 (10 MHz, 25 °C).

Isomeric Co(ii) paraCEST agents as pH responsive MRI probes

A newly discovered isomer of Co(ii) (1,4,8,11-tetrakis(carbamoylmethyl)-1,4,8,11-tetraazacyclotetradecane = CCRM) produces four highly paramagnetically shifted chemical exchange saturation transfer (CEST) peaks. The 1,8-pendants of the complex are bound in a trans-arrangement to produce a Co(ii) complex of increased kinetic inertness. The isomers have a stabilized Co(ii) center (E1/2 of 540 to 550 mV versus SHE). Both the 1,8 and the 1,4-isomer are excellent pH probes in solution and in tissue homogenate by virtue of their highly paramagnetically shifted amide protons. These isomers produce both a ratiometric pH readout as well as amide proton exchange rate constants that correlate to pH.

Macrocyclic Pyclen-Based Gd3+ Complex with High Relaxivity and pH Response

We report the synthesis and characterization of the macrocyclic ligand 2,2'-((2-(3,9-bis(carboxymethyl)-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane-6-yl)ethyl)azanediyl)diacetic acid (H₄L) and several of its complexes with lanthanide ions. The structure of the free ligand was determined using X-ray diffraction measurements. Two N atoms of the pyclen moiety in the trans position are protonated in the solid state, together with the exocyclic N atom and one of the carboxylate groups of the ligand. The relaxivity of the Gd³⁺ complex was found to increase from 6.7 mM^-1 s^-1 at pH 8.6 to 8.5 mM^-1 s^-1 below pH ≈ 6.0. Luminescence lifetime measurements recorded from H₂O and D₂O solutions of the Eu³⁺ complex evidence the presence of a single complex species in solution at low pH (∼5.0) that contains two inner-sphere water molecules. DFT calculations suggest that the coordination environment of the Ln³⁺ ion is fulfilled by the four N atoms of the pyclen unit, two oxygen atoms of the macrocyclic acetate groups, and an oxygen atom of an exocyclic carboxylate group. The two inner-sphere water molecules complete coordination number nine around the metal ion. At high pH (∼9.3), the lifetime of the excited ⁵D₀ level of Eu³⁺ displays a biexponential behavior that can be attributed to the presence of two species in solution with hydration numbers of q = 0 and q = 1. The ¹H NMR and DOSY spectra recorded from solutions of the Eu³⁺ and Y³⁺ complexes reveal a structural change triggered by pH and the formation of small aggregates at high pH values.

Single- and Multi-Arm Gadolinium MRI Contrast Agents for Targeted Imaging of Glioblastoma

Background

Position of gadolinium atom(s) plays a key role in contrast enhancement of gadolinium-based contrast agents. To gain a better understanding of effects of distance of gadolinium in relation to the nanoconjugate platform, we designed and synthesized single- and multi-arm (“star”) gadolinium conjugates equipped with antibody and peptides for targeting. The contrast agents were studied for their tumor imaging performance in a glioma mouse model.

Materials and Methods

Antibody- and peptide-targeted nano contrast agents (NCAs) were synthesized using polymalic acid platforms of different sizes. Gadolinium-DOTA and intermediates were attached as amides and targeting agents such as antibodies and peptides as thioethers. For in vivo experiments, we used human U87MG xenografts as glioma models. Magnetic resonance imaging (MRI) was performed on a Bruker BioSpec 94/20USR 9.4 T small-animal scanner. Delivery of contrast agents across the blood–brain barrier was studied by fluorescent microscopy.

Results

All contrast agents accumulated into tumor and showed composition-dependent imaging performance. Peptide-targeted mini-NCAs had hydrodynamic diameters in the range 5.2–9.4 nm and antibody-targeted NCAs had diameters in the range 15.8–20.5 nm. Zeta potentials were in the range of –5.4–−8.2 mV and −4.6–−8.8 mV, respectively. NCAs showed superior relaxivities compared to MultiHance at 9.4 T. The signal enhancement indicated maximum accumulation in tumor 30–60 minutes after intravenous injection of the mouse tail vein. Only targeted NCAs were retained in tumor for up to 3 hours and displayed contrast enhancement.

Conclusion

The novel targeted NCAs with star-PEG features displayed improved relaxivity and greater contrast compared with commercial MultiHance contrast agent. The enhancement by mini-NCAs showed clearance of tumor contrast after 3 hours providing a suitable time window for tumor diagnosis in clinics. The technology provides a great tool with the promise of differential MRI diagnosis of brain tumors.

Towards an Improved Design of MRI Contrast Agents: Synthesis and Relaxometric Characterisation of Gd-HPDO3A Analogues

The properties of Ln^III -HPDO3A complexes as relaxation enhancers and paraCEST agents are essentially related to the hydroxylpropyl moiety. A series of three HPDO3A derivatives, with small modifications to the hydroxyl arm, were herein investigated to understand how heightened control can be gained over the parameters involved in the design of these agents. A full ¹ H and ¹⁷ O-NMR relaxometric analysis was conducted and demonstrated that increasing the length of the OH group from the lanthanide centre significantly enhanced the water exchange rate of the gadolinium complex, but with a subsequent reduction in kinetic stability. Alternatively, the introduction of an additional methyl group, which increased the steric bulk around the OH moiety, resulted in the formation of almost exclusively the TSAP isomer (95 %) as identified by ¹ H-NMR of the europium complex. The gadolinium analogue of this complex also exhibited a very fast water exchange rate, but with no detectable loss of kinetic stability. This complex therefore demonstrates a notable improvement over Gd-HPDO3A.

Dynamic Quantum Sensing of Paramagnetic Species Using Nitrogen-Vacancy Centers in Diamond

Naturally occurring paramagnetic species (PS), such as free radicals and paramagnetic metalloproteins, play an essential role in a multitude of critical physiological processes including metabolism, cell signaling, and immune response. These highly dynamic species can also act as intrinsic biomarkers for a variety of disease states, while synthetic paramagnetic probes targeted to specific sites on biomolecules enable the study of functional information such as tissue oxygenation and redox status in living systems. The work presented herein describes a new sensing method that exploits the spin-dependent emission of photoluminescence (PL) from an ensemble of nitrogen-vacancy centers in diamond for rapid, nondestructive detection of PS in living systems. Uniquely this approach involves simple measurement protocols that assess PL contrast with and without the application of microwaves. The method is demonstrated to detect concentrations of paramagnetic salts in solution and the widely used magnetic resonance imaging contrast agent gadobutrol with a limit of detection of less than 10 attomol over a 100 μm × 100 μm field of view. Real-time monitoring of changes in the concentration of paramagnetic salts is demonstrated with image exposure times of 20 ms. Further, dynamic tracking of chemical reactions is demonstrated via the conversion of low-spin cyanide-coordinated Fe3+ to hexaaqua Fe3+ under acidic conditions. Finally, the capability to map paramagnetic species in model cells with subcellular resolution is demonstrated using lipid membranes containing gadolinium-labeled phospholipids under ambient conditions in the order of minutes. Overall, this work introduces a new sensing approach for the realization of fast, sensitive imaging of PS in a widefield format that is readily deployable in biomedical settings. Ultimately, this new approach to nitrogen vacancy-based quantum sensing paves the way toward minimally invasive real-time mapping and observation of free radicals in in vitro cellular environments.

A Ln(III)-3-hydroxypyridine pH responsive probe optimized by DFT

Differences in tissue pH can be diagnostic of cancer and other conditions that shift cell metabolism. Paramagnetic probes are promising tools for pH mapping in vivo using magnetic resonance spectroscopy (MRS) as they provide uniquely shifted MR signals that change with pH. Here, we demonstrate a 3-hydroxy-6-methylpyridyl coordinating group as a new pH-responsive reporter group for Ln(III) MRS probes. The pH response of the complex was observed by UV-Vis, fluorescence, and NMR spectroscopies, and modeled using DFT. These results provide insight into the observed pH-dependent NMR spectrum of the complex. The protonation state of the hydroxypyridine changes the coordinating ability of the ligand, affecting the dipolar field of the lanthanide and the chemical shift of nearby reporter nuclei. The favorable pH response and coordination properties of the 3-hydroxypyridyl group indicates its potential for further development as a dual responsive-reporter group. Incorporation into optimized scaffolds for MRS detection may enable sensitive pH-mapping in vivo.

Imaging tumor acidosis: a survey of the available techniques for mapping in vivo tumor pH

Cancer cells are characterized by a metabolic shift in cellular energy production, orchestrated by the transcription factor HIF-1α, from mitochondrial oxidative phosphorylation to increased glycolysis, regardless of oxygen availability (Warburg effect). The constitutive upregulation of glycolysis leads to an overproduction of acidic metabolic products, resulting in enhanced acidification of the extracellular pH (pHe ~ 6.5), which is a salient feature of the tumor microenvironment. Despite the importance of pH and tumor acidosis, there is currently no established clinical tool available to image the spatial distribution of tumor pHe. The purpose of this review is to describe various imaging modalities for measuring intracellular and extracellular tumor pH. For each technique, we will discuss main advantages and limitations, pH accuracy and sensitivity of the applied pH-responsive probes and potential translatability to the clinic. Particular attention is devoted to methods that can provide pH measurements at high spatial resolution useful to address the task of tumor heterogeneity and to studies that explored tumor pH imaging for assessing treatment response to anticancer therapies.

Magnetic Nanoparticles Supporting Bio-responsive T1/T2 Magnetic Resonance Imaging

: The use of nanoparticulate systems as contrast agents for magnetic resonance imaging (MRI) is well-established and known to facilitate an enhanced image sensitivity within scans of a particular pathological region of interest. Such a capability can enable both a non-invasive diagnosis and the monitoring of disease progression/response to treatment. In this review, magnetic nanoparticles that exhibit a bio-responsive MR relaxivity are discussed, with pH-, enzyme-, biomolecular-, and protein-responsive systems considered. The ability of a contrast agent to respond to a biological stimulus provides not only enriched diagnostic capabilities over corresponding non-responsive analogues, but also an improved longitudinal monitoring of specific physiological conditions.

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.

Smart MRI Agents for Detecting Extracellular Events In Vivo: Progress and Challenges

Many elegant inorganic designs have been developed to aid medical imaging. We know better now how to improve imaging due to the enormous efforts made by scientists in probe design and other fundamental sciences, including inorganic chemistry, physiochemistry, analytical chemistry, and biomedical engineering. However, despite several years being invested in the development of diagnostic probes, only a few examples have shown applicability in MRI in vivo. In this short review, we aim to show the reader the latest advances in the application of inorganic agents in preclinical MRI.

Chemistry of MRI Contrast Agents: Current Challenges and New Frontiers

Tens of millions of contrast-enhanced magnetic resonance imaging (MRI) exams are performed annually around the world. The contrast agents, which improve diagnostic accuracy, are almost exclusively small, hydrophilic gadolinium(III) based chelates. In recent years concerns have arisen surrounding the long-term safety of these compounds, and this has spurred research into alternatives. There has also been a push to develop new molecularly targeted contrast agents or agents that can sense pathological changes in the local environment. This comprehensive review describes the state of the art of clinically approved contrast agents, their mechanism of action, and factors influencing their safety. From there we describe different mechanisms of generating MR image contrast such as relaxation, chemical exchange saturation transfer, and direct detection and the types of molecules that are effective for these purposes. Next we describe efforts to make safer contrast agents either by increasing relaxivity, increasing resistance to metal ion release, or by moving to gadolinium(III)-free alternatives. Finally we survey approaches to make contrast agents more specific for pathology either by direct biochemical targeting or by the design of responsive or activatable contrast agents.

Development and characterization of lanthanide-HPDO3A-C16-based micelles as CEST-MRI contrast agents

The synthesis and characterization of a novel HPDO3A-based ligand having a C16 alkyl chain and its Eu³⁺, Gd³⁺and Yb³⁺complexes are reported.

Water exchange in lanthanide complexes for MRI applications. Lessons learned over the last 25 years

Coordination chemistry offers convenient strategies to modulate the exchange of coordinated water molecules in lanthanide-based contrast agents.

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.

Characterization of D-maltose as a T2 -exchange contrast agent for dynamic contrast-enhanced MRI

Purpose

We sought to investigate the potential of D-maltose, D-sorbitol, and D-mannitol as T2 exchange magnetic resonance imaging (MRI) contrast agents. We also sought to compare the in vivo pharmacokinetics of D-maltose with D-glucose with dynamic contrast enhancement (DCE) MRI.

Methods

T1 and T2 relaxation time constants of the saccharides were measured using eight pH values and nine concentrations. The effect of echo spacing in a multiecho acquisition sequence used for the T2 measurement was evaluated for all samples. Finally, performances of D-maltose and D-glucose during T2-weighted DCE-MRI were compared in vivo.

Results

Estimated T2 relaxivities (r2) of D-glucose and D-maltose were highly and nonlinearly dependent on pH and echo spacing, reaching their maximum at pH=7.0 (~0.08mM−1 s−1). The r2 values of D-sorbitol and D-mannitol were estimated to be ~0.02mM−1 s−1 and were invariant to pH and echo spacing for pH ≤7.0. The change in T2 in tumor and muscle tissues remained constant after administration of D-maltose, whereas the change in T2 decreased in tumor and muscle after administration of D-glucose. Therefore, D-maltose has a longer time window for T2-weighted DCE-MRI in tumors.

Conclusion

We have demonstrated that D-maltose can be used as a T2 exchange MRI contrast agent. The larger, sustained T2-weighted contrast from D-maltose relative to D-glucose has practical advantages for tumor diagnoses during T2-weighted DCE-MRI.

Toward inert paramagnetic Ni(ii)-based chemical exchange saturation transfer MRI agents

The Ni²⁺ complexes with hexadentate ligands containing two 6-methylpicolinamide groups linked by ethane-1,2-diamine (dedpam) or cyclohexane-1,2-diamine (chxdedpam) spacers were investigated as potential contrast agents in magnetic resonance imaging (MRI). The properties of the complexes were compared to that of the analogues containing 6-methylpicolinate units (dedpa^2- and chxdedpa^2-). The X-ray structure of the [Ni(dedpam)]²⁺ complex reveals a six-coordinated metal ion with a distorted octahedral environment. The protonation constants of the dedpa^2- and dedpam ligands and the stability constants of their Ni²⁺ complexes were determined using pH-potentiometry and spectrophotometric titrations (25 °C, 0.15 M NaCl). The [Ni(dedpa)] complex (log K_NiL = 20.88(1)) was found to be considerably more stable than the corresponding amide derivative [Ni(dedpam)]²⁺ (log K_NiL = 14.29(2)). However, the amide derivative [Ni(chxdedpam)]²⁺ was found to be considerably more inert with respect to proton-assisted dissociation than the carboxylate derivative [Ni(chxdedpa)]. A detailed ¹H NMR and DFT study was conducted to assign the ¹H NMR spectra of the [Ni(chxdedpa)] and [Ni(chxdedpam)]²⁺ complexes. The observed ¹H NMR paramagnetic shifts were found to be dominated by the Fermi contact contribution. The amide resonances of [Ni(chxdedpam)]²⁺ at 91.5 and 22.2 ppm were found to provide a sizeable chemical exchange saturation transfer effect, paving the way for the development of NiCEST agents based on these rigid non-macrocyclic platforms.

Advances in gadolinium-based MRI contrast agent designs for monitoring biological processes in vivo

The gadolinium-based contrast agents widely used in diagnostic MRI exams for 30 years are all small molecule agents that distribute into all extracellular spaces in tissues without providing any specific biological information. Although many 'responsive agent' designs have been presented over the past 20 years or so, none have found use in clinical diagnostic medicine at this point. This review summarizes some recent approaches taken to enhance the sensitivity of such gadolinium-based agents, to target them to specific tissue components, and to create new systems for monitoring specific biological processes.

Chiral DOTA chelators as an improved platform for biomedical imaging and therapy applications

Despite established clinical utilisation, there is an increasing need for safer, more inert gadolinium-based contrast agents, and for chelators that react rapidly with radiometals. Here we report the syntheses of a series of chiral DOTA chelators and their corresponding metal complexes and reveal properties that transcend the parent DOTA compound. We incorporated symmetrical chiral substituents around the tetraaza ring, imparting enhanced rigidity to the DOTA cavity, enabling control over the range of stereoisomers of the lanthanide complexes. The Gd chiral DOTA complexes are shown to be orders of magnitude more inert to Gd release than [GdDOTA]⁻. These compounds also exhibit very-fast water exchange rates in an optimal range for high field imaging. Radiolabeling studies with (Cu-64/Lu-177) also demonstrate faster labelling properties. These chiral DOTA chelators are alternative general platforms for the development of stable, high relaxivity contrast agents, and for radiometal complexes used for imaging and/or therapy.

MRI contrast agents containing the rare earth metal gadolinium are very effective, yet unstable and thus potentially hazardous. Here, the authors developed complexes between gadolinium and the scaffolding compound DOTA with increased stability, which also lend themselves to radiometal labelling.

Optimizing Water Exchange Rates and Rotational Mobility for High-Relaxivity of a Novel Gd-DO3A Derivative Complex Conjugated to Inulin as Macromolecular Contrast Agents for MRI

Thanks to the understanding of the relationships between the residence lifetime τ_M of the coordinated water molecules to macrocyclic Gd-complexes and the rotational mobility τ_R of these structures, and according to the theory for paramagnetic relaxation, it is now possible to design macromolecular contrast agents with enhanced relaxivities by optimizing these two parameters through ligand structural modification. We succeeded in accelerating the water exchange rate by inducing steric compression around the water binding site, and by removing the amide function from the DOTA-AA ligand [1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid mono(p-aminoanilide)] (L) previously designed. This new ligand 10[2(1-oxo-1-p-propylthioureidophenylpropyl]-1,4,7,10-tetraazacyclodecane-1,4,7-tetraacetic acid (L₁ ) was then covalently conjugated to API [O-(aminopropyl)inulin] to get the complex API-(GdL₁ )x with intent to slow down the rotational correlation time (τ_R ) of the macromolecular complex. The evaluation of the longitudinal relaxivity at different magnetic fields and the study of the ¹⁷ O-NMR at variable temperature of the low-molecular-weight compound (GdL₁ ) showed a slight decrease of the τ_M value (τM310 = 331 ns vs. τM310 = 450 ns for the GdL complex). Consequently to the increase of the size of the API-(GdL₁ )x complex, the rotational correlation time becomes about 360 times longer compared to the monomeric GdL₁ complex (τ_R  = 33,700 ps), which results in an enhanced proton relaxivity.

Flip-angle based ratiometric approach for pulsed CEST-MRI pH imaging

Several molecules have been exploited for developing MRI pH sensors based on the chemical exchange saturation transfer (CEST) technique. A ratiometric approach, based on the saturation of two exchanging pools at the same saturation power, or by varying the saturation power levels on the same pool, is usually needed to rule out the concentration term from the pH measurement. However, all these methods have been demonstrated by using a continuous wave saturation scheme that limits its translation to clinical scanners. This study shows a new ratiometric CEST-MRI pH-mapping approach based on a pulsed CEST saturation scheme for a radiographic contrast agent (iodixanol) possessing a single chemical exchange site. This approach is based on the ratio of the CEST contrast effects at two different flip angles combinations (180°/360° and 180°/720°), keeping constant the mean irradiation RF power (B_{avg power}). The proposed ratiometric approach index is concentration independent and it showed good pH sensitivity and accuracy in the physiological range between 6.0 and 7.4.

Lanthanide-Based T2ex and CEST Complexes Provide Insights into the Design of pH Sensitive MRI Agents

The CEST and T1 /T2 relaxation properties of a series of Eu3+ and Dy3+ DOTA-tetraamide complexes with four appended primary amine groups are measured as a function of pH. The CEST signals in the Eu3+ complexes show a strong CEST signal after the pH was reduced from 8 to 5. The opposite trend was observed for the Dy3+ complexes where the r2ex of bulk water protons increased dramatically from ca. 1.5 mm-1  s-1 to 13 mm-1  s-1 between pH 5 and 9 while r1 remained unchanged. A fit of the CEST data (Eu3+ complexes) to Bloch theory and the T2ex data (Dy3+ complexes) to Swift-Connick theory provided the proton-exchange rates as a function of pH. These data showed that the four amine groups contribute significantly to proton-catalyzed exchange of the Ln3+ -bound water protons even though their pKa 's are much higher than the observed CEST or T2ex effects. This demonstrated the utility of using appended acidic/basic groups to catalyze prototropic exchange for imaging tissue pH by MRI.