The Role of Equilibrium and Kinetic Properties in the Dissociation of Gd[DTPA-bis(methylamide)] (Omniscan) at near to Physiological Conditions

Protein MRI Contrast Agents as an Effective Approach for Precision Molecular Imaging

ABSTRACT

Cancer and other acute and chronic diseases are results of perturbations of common molecular determinants in key biological and signaling processes. Imaging is critical for characterizing dynamic changes in tumors and metastases, the tumor microenvironment, tumor-stroma interactions, and drug targets, at multiscale levels. Magnetic resonance imaging (MRI) has emerged to be a primary imaging modality for both clinical and preclinical applications due to its advantages over other modalities, including sensitivity to soft tissues, nondepth limitations, and the use of nonionizing radiation. However, extending the application of MRI to achieve both qualitative and quantitative precise molecular imaging with the capability to quantify molecular biomarkers for early detection, staging, and monitoring therapeutic treatment requires the capacity to overcome several major challenges including the trade-off between metal-binding affinity and relaxivity, which is an issue frequently associated with small chelator contrast agents. In this review, we will introduce the criteria of ideal contrast agents for precision molecular imaging and discuss the relaxivity of current contrast agents with defined first shell coordination water molecules. We will then report our advances in creating a new class of protein-targeted MRI contrast agents (ProCAs) with contributions to relaxivity largely derived from the secondary sphere and correlation time. We will summarize our rationale, design strategy, and approaches to the development and optimization of our pioneering ProCAs with desired high relaxivity, metal stability, and molecular biomarker-targeting capability, for precision MRI. From first generation (ProCA1) to third generation (ProCA32), we have achieved dual high r1 and r2 values that are 6- to 10-fold higher than clinically approved contrast agents at magnetic fields of 1.5 T, and their relaxivity values at high field are also significantly higher, which enables high resolution during small animal imaging. Further engineering of multiple targeting moieties enables ProCA32 agents that have strong biomarker-binding affinity and specificity for an array of key molecular biomarkers associated with various chronic diseases, while maintaining relaxation and exceptional metal-binding and selectivity, serum stability, and resistance to transmetallation, which are critical in mitigating risks associated with metal toxicity. Our leading product ProCA32.collagen has enabled the first early detection of liver metastasis from multiple cancers at early stages by mapping the tumor environment and early stage of fibrosis from liver and lung in vivo, with strong translational potential to extend to precision MRI for preclinical and clinical applications for precision diagnosis and treatment.

Relationship between gadolinium-based MRI contrast agent consumption and anthropogenic gadolinium in the influent of a wastewater treatment plant

Gadolinium-based contrast agents (GBCAs) used in magnetic resonance imaging (MRI) are highly resistant in the environment. They pass through wastewater treatment plants (WWTPs) unhindered escaping degradation. Although GBCAs are subjects of intensive research, we recognized that a quantitative approach to the mass balance of gadolinium, based on known input and output data, is missing. The administered amount of Gd as GBCAs, the number of out- and inpatients and the concentration of rare earth elements (REEs) in wastewater were monitored for 45 days in a medium sized city (ca. 203,000 inhabitants) with two MRI centres. An advection-dispersion type model was established to describe the transport of Gd in the wastewater system. The model calculates with patient locality, excretion kinetics of Gd and the yield of wastewater. The estimated and measured daily amount of anthropogenic gadolinium released to the WWTP were compared. GBCAs (Omniscan and Dotarem) were administered to 1008 patients representing a total of 700 ± 1 g Gd. The amount of total Gd entering the WWTP was 531 ± 2 g, of which the anthropogenic contribution (i.e. GBCAs) was 261 ± 6 g (49 ± 1 % of the total Gd) during the sampling campaign. Local residents and inpatients should fully release Gd in the city, but outpatients only partially. Overall, 37 ± 1 % of the total administered Gd was recovered in the wastewater, so the remaining 63 ± 1 % of administered Gd is expected to be dispensed outside of the sewer system. Our approach enables to better understand the dispersion of GBCAs originated Gd in an urban environment.

Manganese (II) Complex of 1,4,7-Triazacyclononane-1,4,7-Triacetic Acid (NOTA) as a Hepatobiliary MRI Contrast Agent

Magnetic resonance imaging (MRI) is increasingly used to diagnose focal and diffuse liver disorders. Despite their enhanced efficacy, liver-targeted gadolinium-based contrast agents (GBCAs) raise safety concerns owing to the release of toxic Gd³⁺ ions. A π-conjugated macrocyclic chelate, Mn-NOTA-NP, was designed and synthesized as a non-gadolinium alternative for liver-specific MRI. Mn-NOTA-NP exhibits an r₁ relaxivity of 3.57 mM^-1 s^-1 in water and 9.01 mM^-1 s^-1 in saline containing human serum albumin at 3 T, which is significantly greater than the clinically utilized Mn²⁺-based hepatobiliary drug, Mn-DPDP (1.50 mM^-1 s^-1), and comparable with that of GBCAs. Furthermore, the in vivo biodistribution and MRI enhancement patterns of Mn-NOTA-NP were similar to those of the Gd³⁺-based hepatobiliary agent, Gd-DTPA-EOB. Additionally, a 0.05 mmol/kg dose of Mn-NOTA-NP facilitated high-sensitivity tumor detection with tumor signal enhancement in a liver tumor model. Ligand-docking simulations further indicated that Mn-NOTA-NP differed from other hepatobiliary agents in their interactions with several transporter systems. Collectively, we demonstrated that Mn-NOTA-NP could be a new liver-specific MRI contrast agent.

MRI Contrast Agents in Glycobiology

Molecular recognition involving glycoprotein-mediated interactions is ubiquitous in both normal and pathological natural processes. Therefore, visualization of these interactions and the extent of expression of the sugars is a challenge in medical diagnosis, monitoring of therapy, and drug design. Here, we review the literature on the development and validation of probes for magnetic resonance imaging using carbohydrates either as targeting vectors or as a target. Lectins are important targeting vectors for carbohydrate end groups, whereas selectins, the asialoglycoprotein receptor, sialic acid end groups, hyaluronic acid, and glycated serum and hemoglobin are interesting carbohydrate targets.

Surface Accessibility of an Intrinsically Disordered Protein Probed by 2D Time-Resolved Laser-Assisted NMR Spectroscopy

Probing the protein surface accessibility of different residues is a powerful way of characterizing the overall conformation of intrinsically disordered proteins (IDPs). We present a two-dimensional (2D) time-resolved photo-CIDNP (TR-CIDNP) experiment suitable for IDP analysis. Pulse stretching of high-power laser pulses, band-selective decoupling of ¹³C^α, and simultaneous application of radiofrequency and laser pulses were implemented to quantitatively analyze the IDP surface at ultrahigh resolution. Comparative analysis with other methods that measure protein surface accessibility validated the newly developed method and emphasized the importance of dye charge in photo-CIDNP. Using the neutral riboflavin dye, surface accessibilities were measured to be nearly identical for the four Tyr residues of α-synuclein (α-Syn), whose ¹H^α-¹³C^α correlations were well-resolved in the 2D TR-CIDNP spectrum. Having confirmed the similarity between the time-resolved and steady-state photo-CIDNP results for α-Syn, we used the more sensitive latter method to show that divalent cations induce compaction of the C-terminal region and release of the N-terminal region of α-Syn. The photo-CIDNP method presented herein can be used as an orthogonal and independent method for investigating important biological processes associated with changes in the overall IDP conformation.

Rigidified Derivative of the Non-macrocyclic Ligand H4OCTAPA for Stable Lanthanide(III) Complexation

The stability constants of lanthanide complexes with the potentially octadentate ligand CHXOCTAPA^4–, which contains a rigid 1,2-diaminocyclohexane scaffold functionalized with two acetate and two picolinate pendant arms, reveal the formation of stable complexes [log K_LaL = 17.82(1) and log K_YbL = 19.65(1)]. Luminescence studies on the Eu³⁺ and Tb³⁺ analogues evidenced rather high emission quantum yields of 3.4 and 11%, respectively. The emission lifetimes recorded in H₂O and D₂O solutions indicate the presence of a water molecule coordinated to the metal ion. ¹H nuclear magnetic relaxation dispersion profiles and ¹⁷O NMR chemical shift and relaxation measurements point to a rather low water exchange rate of the coordinated water molecule (k_ex²⁹⁸ = 1.58 × 10⁶ s^–1) and relatively high relaxivities of 5.6 and 4.5 mM^–1 s^–1 at 20 MHz and 25 and 37 °C, respectively. Density functional theory calculations and analysis of the paramagnetic shifts induced by Yb³⁺ indicate that the complexes adopt an unprecedented cis geometry with the two picolinate groups situated on the same side of the coordination sphere. Dissociation kinetics experiments were conducted by investigating the exchange reactions of LuL occurring with Cu²⁺. The results confirmed the beneficial effect of the rigid cyclohexyl group on the inertness of the Lu³⁺ complex. Complex dissociation occurs following proton- and metal-assisted pathways. The latter is relatively efficient at neutral pH, thanks to the formation of a heterodinuclear hydroxo complex.

A non-macrocyclic ligand containing a rigid cyclohexyl spacer forms thermodynamically stable complexes with the lanthanide(III) ions in aqueous solution. The complexes also show remarkable kinetic inertness, though a structural change facilitates dissociation through the metal-assisted mechanism for the small lanthanides. The Gd(III) complex displays a relatively high relaxivity due to the presence of a water molecule coordinated to the metal ion, while the Eu(III) and Tb(III) analogues display strong metal-centered luminescence.

Stable and inert macrocyclic cobalt(II) and nickel(II) complexes with paraCEST response

We report the synthesis of the macrocyclic ligands 3,9-PC2AMH (2,2'-(3,6,9-triaza-1(2,6)-pyridinacyclodecaphane-3,9-diyl)diacetamide) and 3,9-PC2AMtBu (2,2'-(3,6,9-triaza-1(2,6)-pyridinacyclodecaphane-3,9-diyl)bis(N-tert-butyl)acetamide) which contain a pyclen platform functionalized with acetamide or tert-butylacetamide pendant arms at positions 3 and 9 of the macrocyclic unit. The corresponding Co(II) and Ni(II) complexes were prepared, isolated and characterised as potential paramagnetic chemical exchange saturation transfer (paraCEST) agents. The X-ray structures of the Ni(II) complexes reveal six-coordination of the ligands to the metal ion. The Co(II) complex with 3,9-PC2AMtBu shows a similar six-coordinate structure in the solid state, while the Co(II) complex with 3,9-PC2AMH contains a seven-coordinate metal ion, seventh coordination being completed by the presence of an inner-sphere water molecule. The structure of the Co(II) complexes was investigated using ¹H NMR spectroscopy and computational methods. The complexes present a seven-coordinate structure in solution, as demonstrated by the analysis of the paramagnetic shifts using density functional theory. Ligand protonation constants and stability constants of the complexes with 3,9-PC2AMH were determined using potentiometric titrations (I = 0,15 M NaCl). The Co(II) complex was found to be more stable than the Ni(II) analogue (log K_CoL = 14.46(5) and log K_NiL = 13.15(3)). However, the Ni(II) and Co(II) complexes display similar rate constants characterizing the proton-assisted dissociation mechanism. The presence of highly shifted ¹H NMR signals due to the amide protons in slow exchange with bulk water results in sizeable CEST signals, which are observed at +67 and +15 ppm for the Co(II) complex with 3,9-PC2AMH and +42 and +7 ppm for the Ni(II) analogue at 25 °C.

Synthesis and characterization of a stable and inert MnII-based ZnII responsive MRI probe for molecular imaging of glucose stimulated zinc secretion (GSZS)

A ZnII responsive MnII-based MRI contrast agent, [Mn(PC2A-DPA)], has been synthesized, investigated and applied in imaging studies. It shows high stability and excellent inertness and can be used to visualize glucose triggered ZnII release by MRI.

How the Chemical Properties of GBCAs Influence Their Safety Profiles In Vivo

The extracellular class of gadolinium-based contrast agents (GBCAs) is an essential tool for clinical diagnosis and disease management. In order to better understand the issues associated with GBCA administration and gadolinium retention and deposition in the human brain, the chemical properties of GBCAs such as relative thermodynamic and kinetic stabilities and their likelihood of forming gadolinium deposits in vivo will be reviewed. The chemical form of gadolinium causing the hyperintensity is an open question. On the basis of estimates of total gadolinium concentration present, it is highly unlikely that the intact chelate is causing the T1 hyperintensities observed in the human brain. Although it is possible that there is a water-soluble form of gadolinium that has high relaxitvity present, our experience indicates that the insoluble gadolinium-based agents/salts could have high relaxivities on the surface of the solid due to higher water access. This review assesses the safety of GBCAs from a chemical point of view based on their thermodynamic and kinetic properties, discusses how these properties influence in vivo behavior, and highlights some clinical implications regarding the development of future imaging agents.

Stability of MRI contrast agents in high-energy radiation of a 1.5T MR-Linac

BACKGROUND

Gadolinium-based contrast is often used when acquiring MR images for radiation therapy planning for better target delineation. In some situations, patients may still have residual MRI contrast agents in their tissue while being treated with high-energy radiation. This is especially true when MRI contrast agents are administered during adaptive treatment replanning for patients treated on MR-Linac systems.

PURPOSE

The purpose of this study was to analyze the molecular stability of MRI contrast agents when exposed to high energy photons and the associated secondary electrons in a 1.5T MR-Linac system. This was the first step in assessing the safety of administering MRI contrast agents throughout the course of treatment.

MATERIALS AND METHODS

Two common MRI contrast agents were irradiated with 7 MV photons to clinical dose levels. The irradiated samples were analyzed using liquid chromatography-high resolution mass spectrometry to detect degradation products or conformational alterations created by irradiation with high energy photons and associated secondary electrons.

RESULTS

No significant change in chemical composition or displacement of gadolinium ions from their chelates was discovered in samples irradiated with 7 MV photons at relevant clinical doses in a 1.5T MR-Linac. Additionally, no significant correlation between concentrations of irradiated MRI contrast agents and radiation dose was observed.

CONCLUSION

The chemical composition stability of the irradiated contrast agents is promising for future use throughout the course of patient treatment. However, in vivo studies are needed to confirm that unexpected metabolites are not created in biological milieus.

Interaction of macrocyclic gadolinium-based MR contrast agents with Type I collagen. Equilibrium and kinetic studies

The interactions of gadoterate meglumine, gadobutrol, gadoteridol and Gd(HB-DO3A) with bovine Type I collagen were investigated by ultrafiltration and dialysis. The affinity of the four agents to collagen is similar. However, the maximum adsorbed amount of GdIII-complexes decreases in the following order: gadoterate meglumine > gadobutrol > gadoteridol > Gd(HB-DO3A). Calculations with the open three-compartment model reveal that the structural homologs gadoteridol and Gd(HB-DO3A) have a lower adsorption onto collagen, which may explain the less prolonged in vivo retention of gadoteridol observed in soft tissues of rats.

Gadolinium-Based Contrast Media Nephrotoxicity in Kidney Impairment: The Physio-Pathological Conditions for the Perfect Murder

Gadolinium-based contrast media (GBCM) toxicity in patients with kidney disease is a concern for the possible development of systemic nephrogenic fibrosis and possible renal complications. This review focuses on the pathological mechanisms underlying the potential kidney toxicity of gadolinium. Gadolinium, as a free compound (Gd3+), is highly toxic in humans because it competes with divalent calcium (Ca2+) and magnesium (Mg2+) ions, interfering in some relevant biologic processes. Its toxicity is blunted by the complexing of Gd3+ with a carrier, allowing its use in magnetic resonance imaging. The binding reaction between gadolinium and a carrier is thermodynamically reversible. Consequently, under some conditions, gadolinium can be released in the interstitial space as a free Gd3+ compound with the possibility of toxicity. Other metals such as iron, copper, and calcium can interfere with the binding between gadolinium and its carrier because they compete for the same binding site. This process is known as transmetallation. In patients with kidney impairment, conditions such as low clearance of the Gd-carrier complex, acid-base derangements, and high serum phosphorous can increase the presence of free Gd3+, leading to a higher risk for toxicity.

Complexation of Mn(II) by Rigid Pyclen Diacetates: Equilibrium, Kinetic, Relaxometric, Density Functional Theory, and Superoxide Dismutase Activity Studies

We report the Mn(II) complexes with two pyclen-based ligands (pyclen = 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene) functionalized with acetate pendant arms at either positions 3,6 (3,6-PC2A) or 3,9 (3,9-PC2A) of the macrocyclic fragment. The 3,6-PC2A ligand was synthesized in five steps from pyclen oxalate by protecting one of the secondary amine groups of pyclen using Alloc protecting chemistry. The complex with 3,9-PC2A is characterized by a higher thermodynamic stability [log K_MnL = 17.09(2)] than the 3,6-PC2A analogue [log K_MnL = 15.53(1); 0.15 M NaCl]. Both complexes contain a water molecule coordinated to the metal ion, which results in relatively high ¹H relaxivities (r_1p = 2.72 and 2.91 mM^-1 s^-1 for the complexes with 3,6-PC2A and 3,9-PC2A, respectively, at 25 °C and 0.49 T). The coordinated water molecule displays fast exchange kinetics with the bulk in both cases; the rates (k_ex²⁹⁸) are 140 × 10⁶ and 126 × 10⁶ s^-1 for [Mn(3,6-PC2A)(H₂O)] and [Mn(3,9-PC2A)(H₂O)], respectively. The two complexes were found to be remarkably inert with respect to their dissociation, with half-lives of 63 and 21 h, respectively, at pH = 7.4 in the presence of excess Cu(II). The r_1p values recorded in blood serum remain constant at least over a period of 120 h. Cyclic voltammetry experiments show irreversible oxidation features shifted to higher potentials with respect to [Mn(EDTA)(H₂O)]^2- (H₄EDTA = ethylenediaminetetraacetic acid) and [Mn(PhDTA)(H₂O)]^2- (H₄PhDTA = phenylenediamine-N,N,N',N'-tetraacetic acid), indicating that the PC2A complexes reported here have a lower tendency to stabilize Mn(III). The superoxide dismutase activity of the Mn(II) complexes was tested using the xanthine/xanthine oxidase/p-nitro blue tetrazolium chloride assay at pH = 7.8. The Mn(II) complexes of 3,6-PC2A and 3,9-PC2A are capable of assisting decomposition of the superoxide anion radical. The kinetic rate constant of the complex of 3,9-PC2A is smaller by 1 order of magnitude than that of 3,6-PC2A.

[Gd(AAZTA)]- Derivatives with n-Alkyl Acid Side Chains Show Improved Properties for Their Application as MRI Contrast Agents*

Herein, the synthesis and an extensive characterization of two novel Gd(AAZTA) (AAZTA=6-amino-6-methylperhydro-1,4-diazepine tetra acetic acid) derivatives functionalized with short (C₂ and C₄ ) n-alkyl acid functions are reported. The carboxylate functionality is the site for further conjugations for the design of more specific contrast agents (CAs). Interestingly, it has been found that the synthesized complexes display enhanced properties for use as MRI contrast agents on their own. The stability constants determined by using potentiometric titration and UV/Vis spectrophotometry were slightly higher than the one reported for the parent Gd(AAZTA) complex. This observation might be accounted for by the larger sigma-electron donation of the acyl substituents with respect to the one provided by the methyl group in the parent complex. As far as concerns the kinetic stability, transmetallation experiments with endogenous ions (e.g. Cu²⁺ ) implied that the Gd³⁺ ions present in these Gd(AAZTA) derivatives show somewhat smaller susceptibility to chemical exchange towards these ions at 25 °C, close to the physiological condition. The ¹ H NMR spectra of the complexes with Eu^III and Yb^III displayed a set of signals consistent with half the number of methylene protons present on each ligand. The number of resonances was invariant over a large range of temperatures, suggesting the occurrence of a fast interconversion between structural isomers. The relaxivity values (298 K, 20 MHz) were consistent with q=2 being equal to 8.8 mm^-1  s^-1 for the C₂ derivative and 9.4 mm^-1  s^-1 for the C₄ one, that is, sensibly larger than the one reported for Gd(AAZTA) (7.1 mm^-1  s^-1 ). Variable-temperature (VT)-T₂ ¹⁷ O NMR measurements showed, for both complexes, the presence of two populations of coordinated water molecules, one in fast and one in slow exchange with the bulk water. As the high-resolution ¹ H NMR spectra of the analogs with Eu^III and Yb^III did not show the occurrence of distinct isomers (as frequently observed in other macrocyclic lanthanide(III)-containing complexes), we surmised the presence of two fast-interconverting isomers in solution. The analysis of the ¹⁷ O NMR VT-T₂ profiles versus temperature allowed their relative molar fraction to be established as 35 % for the isomer with the fast exchanging water and 65 % for the isomer with the water molecules in slower exchange. Finally, ¹ H NMRD profiles over an extended range of applied magnetic field strengths have been satisfactory fitted on the basis of the occurrence of the two interconverting species.

In Vivo Uranium Sequestration using a Nanoscale Metal-Organic Framework

An agent for actinide sequestration with fast uranium uptake kinetics and efficient in vivo uranium removal using a nanoscale metal-organic framework (nano-MOF) is proposed. UiO-66 nanoparticles post-synthetically functionalized with carboxyl groups, UiO-66-(COOH)₄ -180, exhibit the fastest uranium uptake kinetics reported with more than 65 % of uranyl in fetal bovine serum (FBS) removed within 5 min. Moreover, the in vivo bio-distribution studies show that the material partially accumulates in kidneys and femurs where uranium mainly deposits facilitating the in vivo sequestration of uranium. The results of the in vivo uranium decorporation assays with mice show that UiO-66-(COOH)₄ -180 could successfully reduce the amounts of uranyl deposited in kidneys and femurs by up to 55.4 % and 36.5 %, respectively, and is significantly more efficient than the commercial actinide decorporation agent, ZnNa₃ -DTPA.

Relaxivity of manganese ferrite nanoparticles

Manganese ferrite nanoparticles are superparamagnetic and have very high saturation magnetization, which makes them candidates for application as MRI contrast agents. Because these nanoparticles are very effective enhancers of transverse relaxation, they are particularly suitable as negative (T₂-weighted) contrast agents. The magnitude of the relaxivity of nanoparticulate Mn ferrites seems to be determined mainly by the method of preparation, their dimensions, and their saturation magnetization.

Gadolinium Complexes of Highly Rigid, Open-Chain Ligands Containing a Cyclobutane Ring in the Backbone: Decreasing Ligand Denticity Might Enhance Kinetic Inertness

In an effort to explore novel ligand scaffolds for stable and inert lanthanide complexation in magnetic resonance imaging contrast agent research, three chiral ligands containing a highly rigid (1S,2S)-1,2-cyclobutanediamine spacer and different number of acetate and picolinate groups were efficiently synthesized. Potentiometric studies show comparable thermodynamic stability for the Gd³⁺ complexes formed with either the octadentate (L3)^4- bearing two acetate or two picolinate groups or the heptadentate (L2)^4- analogue bearing one picolinate and three acetate groups (log K_GdL = 17.41 and 18.00 for [Gd(L2)]^- and [Gd(L3)]^-, respectively). In contrast, their dissociation kinetics is revealed to be very different: the monohydrated [Gd(L3)]^- is considerably more labile, as a result of the significant kinetic activity of the protonated picolinate function, as compared to the bishydrated [Gd(L2)]^-. This constitutes an uncommon example in which lowering ligand denticity results in a remarkable increase in kinetic inertness. Another interesting observation is that the rigid ligand backbone induces an unusually strong contribution of the spontaneous dissociation to the overall decomplexation process. Thanks to the presence of two inner-sphere water molecules, [Gd(L2)]^- is endowed with high relaxivity (r₁ = 7.9 mM^-1 s^-1 at 20 MHz, 25 °C), which is retained in the presence of large excess of endogenous anions, excluding ternary complex formation. The water exchange rate is similar for [Gd(L3)]^- and [Gd(L2)]^-, while it is 1 order of magnitude higher for the trishydrated tetraacetate analogue [Gd(L1)]^- (k_ex²⁹⁸ = 8.1, 10, and 127 × 10⁶ s^-1, respectively). A structural analysis via density functional theory calculations suggests that the large bite angle imposed by the rigid (1S,2S)-1,2-cyclobutanediamine spacer could allow the design of ligands based on this scaffold with suitable properties for the coordination of larger metal ions with biomedical applications.

Controlling water exchange rates in potential Mn2+-based MRI agents derived from NO2A2

We report a series of pentadentate ligands based on a 1,4,7-triazacyclononane-1,4-diacetic acid (H2NO2A) containing different substituents attached to the third nitrogen atom of the macrocyclic unit. Detailed 1H Nuclear Magnetic Relaxation Dispersion (NMRD) characterisation of the corresponding Mn2+ complexes suggests the formation of six-coordinate species in solution containing an inner-sphere water molecule. This was confirmed by recording the transverse 17O relaxation time and chemical shift measurements. The water exchange rate of the coordinated water molecule was found to be strongly influenced by the nature of the substituent R at position 7 of the triazacyclononane unit (R = Me, k298ex = 62.6 × 107 s-1; R = Bz, k298ex = 4.4 × 107 s-1; R = 1-phenylethyl, k298ex = 2.6 × 107 s-1). The decreasing exchange rates are explained by the increasing bulkiness of the substituent, which hinders the approach of the entering water molecule in an associatively activated water exchange mechanism. This is supported by DFT calculations (M062X/TZVP), which confirm the associative nature of the water exchange reaction. A potentially decadentate ligand containing two NO2A units linked by a xylenyl spacer in the meta position was also synthesised. The corresponding binuclear Mn2+ complex contains two metal ions with different hydration numbers, as evidenced by 1H NMRD and 17O NMR measurements. DFT calculations show that this is related to the presence of a bridging bidentate μ-η1-carboxylate group connecting the two metal centers. The results reported in this work provide a straightforward strategy to control the exchange rate of the coordinated water molecule in this family of MRI contrast agent candidates.

Gadolinium complexes of diethylenetriamine-N-oxide pentaacetic acid-bisamide: a new class of highly stable MRI contrast agents with a hydration number of 3

Diethylenetriamine-N-oxide pentaacetic acid-bisamide-based Gd(iii) complexes with 3 coordinated water molecules have been synthesized to achieve high stability and over three times of the relaxivities of commercial MRI contrast agents.

Gadolinium Retention in the Rat Brain: Assessment of the Amounts of Insoluble Gadolinium-containing Species and Intact Gadolinium Complexes after Repeated Administration of Gadolinium-based Contrast Agents

Purpose To evaluate the speciation of gadolinium-containing species after multiple administrations of the gadolinium-based contrast agents (GBCAs) gadodiamide and gadoteridol and to quantify the amount of intact gadolinium complexes and insoluble gadolinium-containing species. Materials and Methods A total dose of 13.2 mmol per kilogram of body weight of each GBCA was administered in healthy Wistar rats over a period of 8 weeks. Three days after the final administration, rats were sacrificed, and the brains were excised and divided into three portions. Each portion of brain homogenate was divided into two parts, one for determination of the total gadolinium concentration with inductively coupled plasma mass spectrometry and one for determination of the amount of intact GBCA and gadolinium-containing insoluble species. Relaxometric measurements of gadodiamide and gadolinium trichloride in the presence of polysialic acid were also performed. Results The mean total gadolinium concentrations for gadodiamide and gadoteridol, respectively, were 0.317 μg/g ± 0.060 (standard deviation) and 0.048 μg/g ± 0.004 in the cortex, 0.418 μg/g ± 0.078 and 0.051 μg/g ± 0.009 in the subcortical brain, and 0.781 μg/g ± 0.079 and 0.061 μg/g ± 0.012 in the cerebellum. Gadoteridol comprised 100% of the gadolinium species found in rats treated with gadoteridol. In rats treated with gadodiamide, the largest part of gadolinium retained in brain tissue was insoluble species. In the cerebellum, the amount of intact gadodiamide accounts for 18.2% ± 10.6 of the total gadolinium found therein. The mass balance found for gadolinium implies the occurrence of other soluble gadolinium-containing species (approximately 30%). The relaxivity of the gadolinium polysialic acid species formed in vitro was 97.8 mM/sec at 1.5 T and 298 K. Conclusion Gadoteridol was far less retained, and the entire detected gadolinium was intact soluble GBCA, while gadodiamide yielded both soluble and insoluble gadolinium-containing species, with insoluble species dominating. ^© RSNA, 2017 Online supplemental material is available for this article.

Physico-chemical properties of MnII complexes formed with cis- and trans-DO2A: thermodynamic, electrochemical and kinetic studies

SYNOPSIS

Mn^II complexes formed with cis- and trans-DO2A (DO2A=1,4,7,10-tetraazacyclododecane-1,4 (or 1,7) -diacetic acid) chelators were investigated by pH-potentiometry, ¹H relaxometry, UV-vis spectrophotometry and cyclic voltammetry. The physico-chemical characteristics of Mn^II complexes of these structure isomers do not differ dramatically, however the cis-DO2A platform has better potential for further development. Manganese (Mn^II) is a promising alternative to gadolinium (Gd^III) as a magnetic resonance imaging (MRI) agent. Unlike gadolinium, this biogenic metal might be better tolerated by the body, reducing the risk of toxicity associated with dissociation of the complex. Herein we report detailed equilibrium and kinetic studies performed with Mn^II complexes of 1,4,7,10-tetraazacyclododecane-1,4-diacetic acid (1,4-DO2A or cis-DO2A) and 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (1,7-DO2A or trans-DO2A). The protonation constants of the ligands as well as stability constants of their Mn^II complexes have been determined by pH-potentiometry. The stability constants of [Mn(cis-DO2A)] are slightly higher than those of [Mn(trans-DO2A)] (log K_MnL=15.68 and 15.22, respectively). Cyclic voltammetric (CV) experiments performed on [Mn(cis-DO2A)] and [Mn(trans-DO2A)] revealed quasireversible systems with a half-wave potential of +636 and +705mV versus Ag/AgCl, respectively. These values indicate that the Mn^II ion in these complexes is more stabilized against the oxidation than in [Mn(EDTA)]^2-. The kinetic inertness of the complexes has been studied in transmetallation reactions with Cu^II or Zn^II ions. Kinetic measurements indicate that both Mn^II complexes primarily undergo acid catalyzed dissociation and positions of the acetate pendant arms do not influence kinetic inertness. The inertness of these complexes is comparable to that of [Mn(NOTA)]^- (NOTA=1,4,7-triazacyclononane-1,4,7-triacetic acid) and about twenty times lower than that of [Mn(DOTA)]^2- (DOTA=1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). In conclusion, [Mn(cis-DO2A)] displays some very interesting features (thermodynamic and redox stability as well as kinetic inertness) which makes this complex a promising platform for the development of more efficient Mn^II complexes as alternatives to Gd-based MRI agents.

The gadolinium-based contrast agent Omniscan® promotes in vitro fibroblast survival through in situ precipitation

The current study aims to explore how the gadolinium (Gd)-based contrast agent (GBCA) Omniscan® enhanced cell viability of murine fibroblasts. The results of scanning electron microscopy showed that Omniscan® can precipitate in cell culture media and deposit on cell membranes. Energy-dispersive X-ray analysis and Fourier-transform infrared spectroscopy demonstrated the presence of Gd and phosphates in the agglomerated particles. By filtering the Omniscan®-containing medium through a 220 nm filter, it can be clearly found that the increased cell viability should be mainly attributed to the insoluble species of gadolinium rather than to chelated gadolinium. Moreover, the effects of other gadolinium-based contrast agents, Magnevist® and Dotarem®, were compared with that of Omniscan®. It is noted that the three contrast agents differed in their ability to induce cell viability, which is possibly ascribed to the different chemical stabilities of gadolinium chelates as demonstrated by the attenuation in cell growth upon the addition of excess ligands to the compounds. The results of flow cytometry analysis also showed that Omniscan® can promote cell growth via an increase in the S-phase cell population as evidenced by the elevated levels of cell cycle associated proteins cyclin D, cyclin A and the phosphorylated Rb protein. Furthermore, our results revealed that integrin-mediated signaling may play an important role in both Omniscan® and Magnevist®-enhanced focal adhesion formation since the blockade of integrins decreased the level of ERK phosphorylation induced by the two GBCAs. Taken together, these data suggested that in situ gadolinium phosphate precipitation formation mediated Omniscan®-promoted fibroblast survival, which is similar to that of gadolinium chloride. It was demonstrated that the application of GBCAs with more stable thermodynamic stability may cause less dissociation of the gadolinium ion and thus resulted in less precipitation, finally leading to lower occurrence of nephrogenic systemic fibrosis. The obtained results would also be helpful for the development of safe gadolinium-based contrast agents.

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+).