Dissociation of gadolinium chelates in mice: relationship to chemical characteristics

Precipitation of gadolinium from magnetic resonance imaging contrast agents may be the Brass tacks of toxicity

The formation of gadolinium-rich nanoparticles in multiple tissues from intravenous magnetic resonance imaging contrast agents may be the initial step in rare earth metallosis. The mechanism of gadolinium-induced diseases is poorly understood, as is how these characteristic nanoparticles are formed. Gadolinium deposition has been observed with all magnetic resonance imaging contrast agent brands. Aside from endogenous metals and acidic conditions, little attention has been paid to the role of the biological milieu in the degradation of magnetic resonance imaging contrast agents into nanoparticles. Herein, we describe the decomposition of the commercial magnetic resonance imaging contrast agents Omniscan and Dotarem in the presence of oxalic acid, a well-known endogenous compound. Omniscan dechelated rapidly and preluded measurement by the means available, while Dotarem underwent a two-step decomposition process. The decomposition of both magnetic resonance imaging contrast agents by oxalic acid formed gadolinium oxalate (Gd2[C2O4]3, Gd2Ox3). Furthermore, both observed steps of the Dotarem reaction involved the associative addition of oxalic acid. Adding protein (bovine serum albumin) increased the rate of dechelation. Displacement reactions could occur at lysosomal pH. Through these studies, we have demonstrated that magnetic resonance imaging contrast agents can be dissociated by endogenous molecules, thus illustrating a metric by which gadolinium-based contrast agents (GBCAs) might be destabilized in vivo.

Gadopiclenol: A q = 2 Gadolinium-Based MRI Contrast Agent Combining High Stability and Efficacy

OBJECTIVES

Gadopiclenol is a q = 2 pyclen gadolinium-based contrast agent (GBCA) recently approved by the Food and Drug Administration, European Medicines Agency, and other European countries. The aim of this report is to demonstrate its stability in multiple stressed in vitro conditions and in vivo, in rat kidney, while maintaining its higher relaxivity compared with conventional GBCAs on the market.

MATERIALS AND METHODS

Both gadopiclenol and its chemical precursor Pi828-Gd were characterized and compared with q = 1 gadolinium (Gd) complexes. The number of water molecules coordinated to the Gd (the hydration number, q) was determined by luminescence. 17 O NMR (Nuclear Magnetic Resonance) measurements gave access to the water residence time τ M . These parameters were used for the fitting of the nuclear magnetic relaxation dispersion profiles in water. Proton relaxivities of the complexes were determined in different media at 60 MHz (1.4 T), at different pH and temperature. The kinetic inertness was investigated in human serum, acidic media, under zinc competition in the presence of phosphate, and under ligand competition. The in vivo stability was evaluated in rat kidneys 12 months after repeated injections.

RESULTS

The presence of 2 inner-sphere water molecules per Gd complex was confirmed for both pyclen derivatives. The high relaxivity of the complexes in water is maintained under physiological conditions, even under stressed conditions (ionic media, extreme pH, and temperature), which guarantees their efficiency in a large range of in vivo situations. Gd release from the q = 2 complexes was investigated in different potentially destabilizing conditions. Either no Gd release or a slower one than with "q = 1" stable macrocyclic GBCA (acidic conditions) was observed. Their kinetic inertness was demonstrated in physiological conditions, and the Gd release was below the lower limit of quantification of 0.1 μM after 12 days at 37°C in human serum. It was also demonstrated that gadopiclenol is stable in vivo in rat kidney 12 months after repeated injections.

CONCLUSIONS

Thanks to its optimized structural design, gadopiclenol is a highly stable and effective macrocyclic q = 2 GBCA.

Mapping the distribution of electronic states within the 5D4 and 7F6 levels of Tb3+ complexes with optical spectroscopy

The Tb(III) ion has the most intense luminescence of the trivalent lanthanide(III) ions. In contrast to Eu(III), where the two levels only include a single state, the high number of electronic states in the ground (7F6) and emitting (5D4) levels makes detailed interpretations of the electronic structure-the crystal field-difficult. Here, luminescence emission and excitation spectra of Tb(III) complexes with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA, [Tb(DOTA)(H2O)]-), ethylenediaminetetraacetic acid (EDTA, [Tb(EDTA)(H2O)3]-) and diethylenetriaminepentaacetic acid (DTPA, [Tb(DTPA)(H2O)]2-) as well as the Tb(III) aqua ion ([Tb(H2O)9]3+) were recorded at room temperature and in frozen solution. Using these data the electronic structure of the 5D4 multiplets of Tb(III) was mapped by considering the transitions to the singly degenerate 7F0 state. A detailed spectroscopic investigation was performed and it was found that the 5D4 multiplet could accurately be described as a single band for [Tb(H2O)9]3+, [Tb(DOTA)(H2O)]- and [Tb(EDTA)(H2O)3]-. In contrast, for [Tb(DTPA)(H2O)]2- two bands were needed. These results demonstrated the ability of describing the electronic structure of the emitting 5D4 multiplet using emission spectra. This offers an avenue for investigating the relationship between molecular structure and luminescent properties in detailed photophysical studies of Tb(III) ion complexes.

Monolacunary Wells-Dawson Polyoxometalate as a Novel Contrast Agent for Computed Tomography: A Comprehensive Study on In Vivo Toxicity and Biodistribution

Polyoxotungstate nanoclusters have recently emerged as promising contrast agents for computed tomography (CT). In order to evaluate their clinical potential, in this study, we evaluated the in vitro CT imaging properties, potential toxic effects in vivo, and tissue distribution of monolacunary Wells-Dawson polyoxometalate, α2-K10P2W17O61.20H2O (mono-WD POM). Mono-WD POM showed superior X-ray attenuation compared to other tungsten-containing nanoclusters (its parent WD-POM and Keggin POM) and the standard iodine-based contrast agent (iohexol). The calculated X-ray attenuation linear slope for mono-WD POM was significantly higher compared to parent WD-POM, Keggin POM, and iohexol (5.97 ± 0.14 vs. 4.84 ± 0.05, 4.55 ± 0.16, and 4.30 ± 0.09, respectively). Acute oral (maximum-administered dose (MAD) = 960 mg/kg) and intravenous administration (1/10, 1/5, and 1/3 MAD) of mono-WD POM did not induce unexpected changes in rats' general habits or mortality. Results of blood gas analysis, CO-oximetry status, and the levels of electrolytes, glucose, lactate, creatinine, and BUN demonstrated a dose-dependent tendency 14 days after intravenous administration of mono-WD POM. The most significant differences compared to the control were observed for 1/3 MAD, being approximately seventy times higher than the typically used dose (0.015 mmol W/kg) of tungsten-based contrast agents. The highest tungsten deposition was found in the kidney (1/3 MAD-0.67 ± 0.12; 1/5 MAD-0.59 ± 0.07; 1/10 MAD-0.54 ± 0.05), which corresponded to detected morphological irregularities, electrolyte imbalance, and increased BUN levels.

A Manganese Porphyrin Platform for the Design and Synthesis of Molecular and Targeted MRI Contrast Agents

Magnetic resonance imaging (MRI) contrast agents, in contrast to the plethora of fluorescent agents available to target disease biomarkers or exogenous implants, have remained predominantly non-specific. That is, they do not preferentially accumulate in specific locations in vivo because doing so necessitates longer contrast retention, which is contraindicated for current gadolinium (Gd) agents. This double-edge sword implies that Gd agents can offer either rapid elimination (but lack specificity) or targeted accumulation (but with toxicity risks). For this reason, MRI contrast agent innovation has been severely constrained. Gd-free alternatives based on manganese (Mn) chelates have been largely ineffective, as they are inherently unstable. In this study, we present a Mn(III) porphyrin (MnP) platform for bioconjugation, offering the highest stability and chemical versatility compared to any other T₁ contrast agent. We exploit the inherent metal stability conferred by porphyrins and the absence of pendant bases (found in Gd or Mn chelates) that limit versatile functionalization. As proof-of-principle, we demonstrate labeling of human serum albumin, a model protein, and collagen hydrogels for applications in in-vivo targeted imaging and material tracking, respectively. In-vitro and in-vivo results confirm unprecedented metal stability, ease of functionalization, and high T₁ relaxivity. This new platform opens the door to ex-vivo validation by fluorescent imaging and multipurpose molecular imaging in vivo.

Use of Real-Life Safety Data From International Pharmacovigilance Databases to Assess the Importance of Symptoms Associated With Gadolinium Exposure

OBJECTIVE

Recent scientific publications have reported cases of patients who complained from a variety of symptoms after they received a gadolinium-based contrast agent (GBCA). The aim of this study was to appreciate the importance of these clinical manifestations in the overall population by assessing the weight of "symptoms associated with gadolinium exposure" (SAGE) among the bulk of safety experiences reported to major health authorities.

MATERIALS AND METHODS

Symptoms associated with gadolinium exposure were identified from a review of the scientific literature, and the corresponding preferred terms were searched in each system organ class (SOC) category recorded in the European and North American pharmacovigilance databases EudraVigilance (EV) and FDA Adverse Event Reporting System (FAERS), respectively. The numbers of SAGE per preferred term, and cumulatively per SOC, were recorded and their weights in the overall spectrum of adverse events (AEs) were determined for each GBCA.

RESULTS

The analysis of the selected AEs revealed a significantly higher SAGE weight for gadobenate dimeglumine (EV: 25.83%, FAERS: 32.24%) than for gadoteridol (EV: 15.51%; FAERS: 21.13%) and significantly lower SAGE weights for gadobutrol (EV: 7.75%; FAERS: 13.31%) and gadoterate meglumine (EV: 8.66%; FAERS: 12.99%). A similar ranking was found for most of the SOCs except for "nervous system disorders," probably owing to a limitation in the methods of data selection. Furthermore, this analysis showed a greater percentage of reports mentioning a decrease in the quality of life of the patients when they were exposed to gadobenate dimeglumine or gadoteridol than to gadobutrol or gadoterate meglumine.

CONCLUSION

This study showed that SAGE represent a significant percentage of the bulk of AEs reported to the health authorities for each GBCA. It provided real-life arguments suggesting that SAGE may be more prevalent with linear than macrocyclic GBCAs and that gadoteridol may present a higher SAGE risk than the other macrocyclic contrast agents.

Modeling Gd3+ Complexes for Molecular Dynamics Simulations: Toward a Rational Optimization of MRI Contrast Agents

The correct parametrization of lanthanide complexes is of the utmost importance for their characterization using computational tools such as molecular dynamics simulations. This allows the optimization of their properties for a wide range of applications, including medical imaging. Here we present a systematic study to establish the best strategies for the correct parametrization of lanthanide complexes using [Gd(DOTA)]^- as a reference, which is used as a contrast agent in MRI. We chose the bonded model to parametrize the lanthanide complexes, which is especially important when considering the study of the complex as a whole (e.g., for the study of the dynamics of its interaction with proteins or membranes). We followed two strategies: a so-called heuristic approach employing strategies already published by other authors and another based on the more recent MCPB.py tool. Adjustment of the Lennard-Jones parameters of the metal was required. The final topologies obtained with both strategies were able to reproduce the experimental ion to oxygen distance, vibrational frequencies, and other structural properties. We report a new strategy to adjust the Lennard-Jones parameters of the metal ion in order to capture dynamic properties such as the residence time of the capping water (τ_m). For the first time, the correct assessment of the τ_m value for Gd-based complexes was possible by recording the dissociative events over up to 10 μs all-atom simulations. The MCPB.py tool allowed the accurate parametrization of [Gd(DOTA)]^- in a simpler procedure, and in this case, the dynamics of the water molecules in the outer hydration sphere was also characterized. This sphere was divided into the first hydration layer, an intermediate region, and an outer hydration layer, with a residence time of 18, 10 and 19 ps, respectively, independent of the nonbonded parameters chosen for Gd³⁺. The Lennard-Jones parameters of Gd³⁺ obtained here for [Gd(DOTA)]^- may be used with similarly structured gadolinium MRI contrast agents. This allows the use of molecular dynamics simulations to characterize and optimize the contrast agent properties. The characterization of their interaction with membranes and proteins will permit the design of new targeted contrast agents with improved pharmacokinetics.

Hyperaccumulation of Gadolinium by Methylorubrum extorquens AM1 Reveals Impacts of Lanthanides on Cellular Processes Beyond Methylotrophy

Lanthanides (Ln) are a new group of life metals, and many questions remain regarding how they are acquired and used in biology. Methylotrophic bacteria can acquire, transport, biomineralize, and use Ln as part of a cofactor complex with pyrroloquinoline quinone (PQQ) in alcohol dehydrogenases. For most methylotrophic bacteria use is restricted to the light Ln, which range from lanthanum to samarium (atomic numbers 57–62). Understanding how the cell differentiates between light and heavy Ln, and the impacts of these metals on the metabolic network, will advance the field of Ln biochemistry and give insights into enzyme catalysis, stress homeostasis, and metal biomineralization and compartmentalization. We report robust methanol growth with the heavy Ln gadolinium by a genetic variant of the model methylotrophic bacterium Methylorubrum extorquens AM1, named evo-HLn, for “evolved for Heavy Lanthanides.” A non-synonymous single nucleotide polymorphism in a cytosolic hybrid histidine kinase/response regulator allowed for sweeping transcriptional alterations to heavy metal stress response, methanol oxidation, and central metabolism. Increased expression of genes for Ln acquisition and uptake, production of the Ln-chelating lanthanophore, PQQ biosynthesis, and phosphate transport and metabolism resulted in gadolinium hyperaccumulation of 36-fold with a trade-off for light Ln accumulation. Gadolinium was hyperaccumulated in an enlarged acidocalcisome-like compartment. This is the first evidence of a bacterial intracellular Ln-containing compartment that we name the “lanthasome.” Carotenoid and toblerol biosynthesis were also upregulated. Due to its unique capabilities, evo-HLn can be used to further magnetic resonance imaging (MRI) and bioremediation technologies. In this regard, we show that gadolinium hyperaccumulation was sufficient to produce MRI contrast in whole cells, and that evo-HLn was able to readily acquire the metal from the MRI contrast agent gadopentetic acid. Finally, hyperaccumulation of gadolinium, differential uptake of light and heavy Ln, increased PQQ levels, and phosphate transport provide new insights into strategies for Ln recovery.

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.

Long-term safety of Gadofosveset in clinical practice

PURPOSE

The purpose of this study was to systematically search for long-term complications, including Nephrogenic Systemic Fibrosis (NSF), in patients who were previously administered the gadolinium-based contrast agent Gadofosveset at our institute.

MATERIALS AND METHODS

All patients who were administered Gadofosveset at our institute between 2006 and 2009 were identified in our Radiological Information System (RIS). Clinical data such as cause of death during follow-up, and dermatological or nephrological diseases were systematically searched for in electronic patient records (EPR).

RESULTS

During 2006-2009, Gadofosveset was administered a total of 67 times to 62 patients. One patient was unavailable for follow-up. The remaining 61 patients were followed up for up to 14 (median 12) years based on RIS and EPR data. There were 13 deaths among the 61 patients, all assessed as unrelated to Gadofosveset administration. No dermatological or renal disease suggestive of NSF, or potentially related to Gadofosveset administration, was found. At the time of examination, six patients were diagnosed with various stages of renal insufficiency, three of whom were on hemodialysis. Another three patients were diagnosed with renal insufficiency during the follow-up period, but none of these diagnoses were suspected to be related to the administration of Gadofosveset.

CONCLUSIONS

Based on the results of this retrospective safety analysis of up to 14 years following 1-2 exposures, we conclude that Gadofosveset in clinical practice is safe in the long-term.

Effects of Gadolinium Deposition in the Brain on Motor or Behavioral Function: A Mouse Model

Background Recent studies showing gadolinium deposition in multiple organs have raised concerns about the safety of gadolinium-based contrast agents (GBCAs). Purpose To explore whether gadolinium deposition in brain structures will cause any motor or behavioral alterations. Materials and Methods This study was performed from July 2019 to December 2020. Groups of 17 female BALB/c mice were each repeatedly injected with phosphate-buffered saline (control group, group A), a macrocyclic GBCA (group B), or a linear GBCA (group C) for 8 weeks (5 mmol per kilogram of bodyweight per week for GBCAs). Brain MRI studies were performed every other week to observe the signal intensity change caused by the gadolinium deposition. After the injection period, rotarod performance test, open field test, elevated plus-maze test, light-dark anxiety test, locomotor activity assessment test, passive avoidance memory test, Y-maze test, and forced swimming test were performed to assess the locomotor abilities, anxiety level, and memory. Among-group differences were compared by using one-way or two-way factorial analysis of variance with Tukey post hoc testing or Dunnett post hoc testing. Results Gadolinium deposition in the bilateral deep cerebellar nuclei was confirmed with MRI only in mice injected with a linear GBCA. At 8 weeks, contrast ratio of group C (0.11; 95% CI: 0.10, 0.12) was higher than that of group A (-2.1 × 10^-3; 95% CI: -0.011, 7.5 × 10^-3; P < .001) and group B (2.7 × 10^-4; 95% CI: -8.2 × 10^-3, 8.7 × 10^-3; P < .001). Behavioral analyses showed that locomotor abilities, anxiety level, and long-term or short-term memory were not different in mice injected with linear or macrocyclic GBCAs. Conclusion No motor or behavioral alterations were observed in mice with brain gadolinium deposition. Also, the findings support the safety of macrocyclic gadolinium-based contrast agents. © RSNA, 2021 Online supplemental material is available for this article. See also the editorial by Chen in this issue.

MR Imaging Safety Considerations of Gadolinium-Based Contrast Agents: Gadolinium Retention and Nephrogenic Systemic Fibrosis

Gadolinium (Gd)-based contrast agents (GBCAs) have revolutionized of MR imaging, enabling physicians to obtain life-saving medical information that often cannot be obtained with unenhanced MR imaging or other imaging modalities. Since regulatory approval in 1988, more than 450 million intravenous GBCA doses have been administered worldwide, with an extremely favorable pharmacologic safety profile. Recent evidence has demonstrated, however, that a small fraction of Gd is retained in human tissues. No direct correlation between Gd retention and clinical effects has been confirmed; however, a subset of patients have attributed various symptoms to GBCA exposure. This review details current knowledge regarding GBCA safety.

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.

Musculoskeletal magnetic resonance imaging in the DE50-MD dog model of Duchenne muscular dystrophy

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Normalized muscle volumes distinguish between wild type and DE50-MD dogs.

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Global muscle T2 signal intensities discriminate between wild type and DE50-MD dogs.

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Musculoskeletal changes detected by MRI reflect those seen in human DMD patients.

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Musculoskeletal MRI in this model will be useful to assess treatment efficacy.

The DE50-MD canine model of Duchenne muscular dystrophy (DMD) has a dystrophin gene splice site mutation causing deletion of exon 50, an out-of-frame transcript and absence of dystrophin expression in striated muscles. We hypothesized that the musculoskeletal phenotype of DE50-MD dogs could be detected using Magnetic Resonance Imaging (MRI), that it would progress with age and that it would reflect those in other canine models and DMD patients. 15 DE50-MD and 10 age-matched littermate wild type (WT) male dogs underwent MRI every 3 months from 3 to 18 months of age. Normalized muscle volumes, global muscle T2 and ratio of post- to pre-gadolinium T1-weighted SI were evaluated in 7 pelvic limb and 4 lumbar muscles bilaterally. DE50-MD dogs, compared to WT, had smaller volumes in all muscles, except the cranial sartorius; global muscle T2 was significantly higher in DE50-MD dogs compared to WT. Muscle volumes plateaued and global muscle T2 decreased with age. Normalized muscle volumes and global muscle T2 revealed significant differences between groups longitudinally and should be useful to determine efficacy of therapeutics in this model with suitable power and low sample sizes. Musculoskeletal changes reflect those of DMD patients and other dog models.

Pyclen-Based Ligands Bearing Pendant Picolinate Arms for Gadolinium Complexation

We report the synthesis of two pyclen-based regioisomer ligands (pyclen = 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene) functionalized with picolinic acid pendant arms either at positions 3,9-pc2pa (L5) or 3,6-pc2pa (L6) of the macrocyclic fragment. The ligands were prepared by the regiospecific protection of one of the amine nitrogen atoms of the macrocycle using Boc and Alloc protecting groups, respectively. The X-ray structure of the Gd(III) complex of L5 contains trinuclear [(GdL5)₃(H₂O)₃]³⁺ entities in which the monomeric units are joined by μ₂-η¹:η¹-carboxylate groups. However, the ¹H and ⁸⁹Y NMR spectra of its Y(III) analogue support the formation of monomeric complexes in solution. The Tb(III) complexes are highly luminescent, with emission quantum yields of up to 28% for [TbL5]⁺. The luminescence lifetimes recorded in H₂O and D₂O solutions indicate the presence of a water molecule coordinated to the metal ion, as also evidenced by the ¹H relaxivities measured for the Gd(III) analogues. The Gd(III) complexes present very different exchange rates of the coordinated water molecule (k_ex²⁹⁸ = 87.1 × 10⁶ and 1.06 × 10⁶ s^-1 for [GdL5]⁺ and [GdL6]⁺, respectively). The very high water exchange rate of [GdL5]⁺ is associated with the steric hindrance originating from the coordination of the ligand around the water binding site, which favors a dissociatively activated water exchange process. The Gd(III) complexes present rather high thermodynamic stabilities (log K_GdL = 20.47 and 19.77 for [GdL5]⁺ and [GdL6]⁺, respectively). Furthermore, these complexes are remarkably inert with respect to their acid-assisted dissociation, in particular the complex of L5.

Albumin-based nanoparticles as contrast medium for MRI: vascular imaging, tissue and cell interactions, and pharmacokinetics of second-generation nanoparticles

This multidisciplinary study examined the pharmacokinetics of nanoparticles based on albumin-DTPA-gadolinium chelates, testing the hypothesis that these nanoparticles create a stronger vessel signal than conventional gadolinium-based contrast agents and exploring if they are safe for clinical use. Nanoparticles based on human serum albumin, bearing gadolinium and designed for use in magnetic resonance imaging, were used to generate magnet resonance images (MRI) of the vascular system in rats ("blood pool imaging"). At the low nanoparticle doses used for radionuclide imaging, nanoparticle-associated metals were cleared from the blood into the liver during the first 4 h after nanoparticle application. At the higher doses required for MRI, the liver became saturated and kidney and spleen acted as additional sinks for the metals, and accounted for most processing of the nanoparticles. The multiple components of the nanoparticles were cleared independently of one another. Albumin was detected in liver, spleen, and kidneys for up to 2 days after intravenous injection. Gadolinium was retained in the liver, kidneys, and spleen in significant concentrations for much longer. Gadolinium was present as significant fractions of initial dose for longer than 2 weeks after application, and gadolinium clearance was only complete after 6 weeks. Our analysis could not account quantitatively for the full dose of gadolinium that was applied, but numerous organs were found to contain gadolinium in the collagen of their connective tissues. Multiple lines of evidence indicated intracellular processing opening the DTPA chelates and leading to gadolinium long-term storage, in particular inside lysosomes. Turnover of the stored gadolinium was found to occur in soluble form in the kidneys, the liver, and the colon for up to 3 weeks after application. Gadolinium overload poses a significant hazard due to the high toxicity of free gadolinium ions. We discuss the relevance of our findings to gadolinium-deposition diseases.

Manganese Complex of a Rigidified 15-Membered Macrocycle: A Comprehensive Study

Owing to the increasing importance of manganese(II) complexes in the field of magnetic resonance imaging (MRI), large efforts have been devoted to find an appropriate ligand for Mn(II) ion encapsulation by providing balance between the seemingly contradictory requirements (i.e., thermodynamic stability and kinetic inertness vs low ligand denticity enabling water molecule(s) to be coordinated in its metal center). Among these ligands, a large number of pyridine or pyridol based open-chain and macrocyclic chelators have been investigated so far. As a next step in the development of these chelators, 15-pyN₃O₂Ph and its transition metal complexes were synthesized and characterized using established methods. The 15-pyN₃O₂Ph ligand incorporates both pyridine and ortho-phenylene units to decrease ligand flexibility. The thermodynamic properties, protonation and stability constants, were determined using pH-potentiometry; the solid-state structures of two protonation states of the free ligand and its manganese complex were obtained by single crystal X-ray diffractometry. The results show a seven-coordinate metal center with two water molecules in the first coordination sphere. The longitudinal relaxivity of [Mn(15-pyN₃O₂Ph)]²⁺ was found to be 5.16 mM^-1 s^-1 at 0.49 T (298 K). Furthermore, the r_2p value of 11.72 mM^-1 s^-1 (0.49 T), which is doubled at 1.41 T field, suggests that design of this Mn(II) complex does achieve some characteristics required for contrast imaging. In addition, ¹⁷O NMR measurements were performed in order to access the microscopic parameters governing this key feature (e.g., water exchange rate). Finally, manganese complexes of ligands with analogous polyaza macrocyclic scaffold have been investigated as low molecular weight Mn(CAT) mimics. Here, we report the H₂O₂ disproportionation study of [Mn(15-pyN₃O₂Ph)]²⁺ to demonstrate the versatility of this ligand scaffold as well.

Brain tissue gadolinium retention in pediatric patients after contrast-enhanced magnetic resonance exams: pathological confirmation

BACKGROUND

Retained gadolinium from gadolinium-based contrast agents (GBCAs) used in MR exams has been inferred based on signal changes on serial brain MRI and subsequently demonstrated pathologically in adults. Retention has been similarly inferred in children but pathological demonstration in pediatric patients is limited. The long-term effects of retained gadolinium are unknown but are potentially of greater concern in children given their increased vulnerability from continuing development and their expected longer period of exposure. Several factors can influence gadolinium retention. In adults as well as in children, greater accumulation has been demonstrated based on MR signal changes with linear compared with macrocyclic gadolinium chelates, attributed to lower chelate affinity with linear agents. Effects of age at exposure on retention are unknown, while differences in GBCA washout rates are still under investigation and might affect gadolinium retention relative to time of GBCA administration.

OBJECTIVE

The purpose of this study was to confirm whether gadolinium brain deposits are present in pediatric patients who received GBCAs and to quantify the amounts present.

MATERIALS AND METHODS

Brain autopsy specimens from 10 pediatric patients between 1 year and 13 years of age who underwent at least one contrast-enhanced MR exam were analyzed for elemental gadolinium using inductively coupled plasma mass spectrometry. Brain samples included white matter, basal ganglia (putamen, globus pallidus), thalamus, dentate nucleus and tumor tissue as available. Type and dose of contrast agent, number and timing of contrast-enhanced MR exams and renal function (estimated glomerular filtration rate [eGFR]) were documented for each child.

RESULTS

Patient exposures ranged from 1 dose to 20 doses of GBCAs including both macrocyclic and linear ionic agents. Gadolinium was found to be present in brain tissue in all children and was generally highest in the globus pallidus. Those who received only macrocyclic agents showed lower levels of gadolinium retention.

CONCLUSION

This study demonstrates pathological confirmation of gadolinium retention in brain tissue of a series of pediatric patients exposed to GBCAs including not only linear ionic agents but also macrocyclic agents with both nonionic and ionic compounds. The distribution and deposition levels in this small pediatric population are comparable with the findings in adults. While the clinical significance of these deposits remains unknown, at this point it would be prudent to exert caution and avoid unnecessary use of GBCAs in pediatric patients.

Self-Immolative Activation of β-Galactosidase-Responsive Probes for In Vivo MR Imaging in Mouse Models

Our lab has developed a new series of self-immolative MR agents for the rapid detection of enzyme activity in mouse models expressing β-galactosidase (β-gal). We investigated two molecular architectures to create agents that detect β-gal activity by modulating the coordination of water to GdIII . The first is an intermolecular approach, wherein we designed several structural isomers to maximize coordination of endogenous carbonate ions. The second involves an intramolecular mechanism for q modulation. We incorporated a pendant coordinating carboxylate ligand with a 2, 4, 6, or 8 carbon linker to saturate ligand coordination to the GdIII ion. This renders the agent ineffective. We show that one agent in particular (6-C pendant carboxylate) is an extremely effective MR reporter for the detection of enzyme activity in a mouse model expressing β-gal.

Stability evaluation of Gd chelates for macromolecular MRI contrast agents

OBJECTIVE

We try to establish designs for the macromolecular agents possessing high Gd³⁺-chelating stability, because free Gd³⁺ ion released from Gd chelates is known as a risk factor to cause toxic side effects and a safety concern.

MATERIALS AND METHODS

We prepared three types of Gd-based macromolecular MRI contrast agents from a synthetic polymer (poly(glutamic acid) homopolymer or poly(ethylene glycol)-b-poly(lysine) block copolymer) and a chelating moiety (DO3A or DOTA) having two strategic designs for high chelate stability. Then, we examine the in vitro Gd³⁺-chelate stability of these macromolecular MRI contrast agents.

RESULTS

The prepared macromolecular agents exhibited the same or higher Gd³⁺-chelate stability as/than did Gd-DOTA that possesses the highest Gd³⁺-chelate stability among the approved small-MW Gd-chelate MRI contrast agent.

DISCUSSION

Our macromolecular design was considered to work well for high Gd³⁺-chelate stability.

Gadolinium-based contrast agents - review of recent literature on magnetic resonance imaging signal intensity changes and tissue deposits, with emphasis on pediatric patients

Gadolinium has been used as a base for contrast agents in MRI for the last three decades. Numerous studies over the last 4 years have reported increased signal intensity in deep brain nuclei in non-contrast MRI images following gadolinium-based contrast agent (GBCA) administration. Pathology studies performed on adults and children, and rodent necropsy studies have also shown gadolinium deposition in brain and other tissues after GBCA administration. The purpose of this review was to summarize and discuss the knowledge gained from these reports and the relevance for imaging pediatric patients.

Summary of Imaging in 2020: Visualizing the Future of Healthcare with MR Imaging

The Imaging in 2020 meeting convenes biannually to discuss innovations in medical imaging. The 2018 meeting, titled "Visualizing the Future of Healthcare with MR Imaging," sought to encourage discussions of the future goals of MRI research, feature important discoveries, and foster scientific discourse between scientists from a variety of fields of expertise. Here, we highlight presented research and resulting discussions of the meeting.

Ligand design strategies to increase stability of gadolinium-based magnetic resonance imaging contrast agents

Gadolinium(III) complexes have been widely utilised as magnetic resonance imaging (MRI) contrast agents for decades. In recent years however, concerns have developed about their toxicity, believed to derive from demetallation of the complexes in vivo, and the relatively large quantities of compound required for a successful scan. Recent efforts have sought to enhance the relaxivity of trivalent gadolinium complexes without sacrificing their stability. This review aims to examine the strategic design of ligands synthesised for this purpose, provide an overview of recent successes in gadolinium-based contrast agent development and assess the requirements for clinical translation.

Gadolinium Deposition in Neurology Clinical Practice

Background: Magnetic resonance imaging (MRI) enhanced with gadolinium-based contrast agents (GBCAs) is an essential tool in the diagnosis and management of many neurologic diseases, including multiple sclerosis, brain tumors, and infections. The clinical utility of GBCAs is evidenced by their widespread use. GBCAs are produced in macrocyclic and linear forms. Since 2014, evidence has suggested that repeated administration of GBCAs can lead to gadolinium deposition in the brain. Methods: We review the literature on gadolinium deposition, including both animal and human studies, as well as the literature on GBCA-associated health outcomes. Additionally, we summarize and discuss the updated medical society recommendations and perspectives on GBCA use in clinical practice. Results: The first publication reporting gadolinium deposition in the human brain was published in 2014. Since that seminal report, multiple studies have demonstrated that exposure to linear GBCAs is associated with gadolinium deposition in the dentate nucleus and globus pallidus as seen on brain MRI. Macrocyclic GBCA exposure has not convincingly been associated with gadolinium deposition evident on brain MRI. Conclusion: Clear evidence demonstrates that GBCAs lead to gadolinium deposition in the brain in a dose-dependent manner; however, only linear GBCAs have been associated with gadolinium deposition visualized on MRI. To date, no evidence links gadolinium deposition with any adverse health outcome. Updated medical society guidelines emphasize the importance of an individualized risk-benefit analysis with each administration of GBCAs.

Pediatric Brain: Gadolinium Deposition in Dentate Nucleus and Globus Pallidus on Unenhanced T1-Weighted Images Is Dependent on the Type of Contrast Agent

PURPOSE

The aim of this study was to identify the signal intensity (SI) changes in the dentate nucleus (DN) and the globus pallidus (GP) on unenhanced T1-weighted magnetic resonance (MR) images after multiple administration of gadolinium-based contrast agents (GBCAs) in children and compare those changes between linear and macrocyclic GBCAs.

MATERIALS AND METHODS

This retrospective study was approved by the institutional review board and the requirement for informed consent was waived. We identified 92 children who underwent at least 4 consecutive MR examinations exclusively using either linear GBCA (gadodiamide or gadopentetate dimeglumine, n = 41) or macrocyclic GBCA (gadoterate meglumine, n = 51). Signal intensity ratio changes in the DN to pons and GP to thalamus between the first and last MR examinations were calculated.

RESULTS

The SI ratios in the linear group increased significantly between the first and last MR examinations (mean difference: DN to pons, 0.0461 ± 0.0480, P < 0.001; GP to thalamus, 0.0332 ± 0.0385, P < 0.001), but not in the macrocyclic group (mean difference: DN to pons, -0.0010 ± 0.0371, P = 0.855; GP to thalamus, 0.0007 ± 0.0294, P = 0.867). In the linear regression analysis, the numbers of administrations of gadodiamide and gadopentetate dimeglumine were highly associated with the differences in SI ratios (DN to pons, P < 0.001 and P = 0.003; GP to thalamus, P < 0.001 and P = 0.002, respectively).

CONCLUSION

The SIs of the DN and GP on unenhanced T1-weighted images increased after serial administrations of linear GBCA, but not macrocyclic GBCA, in children. The number of linear GBCA administration had a linear association with the SI changes in the DN and GP.

The biological fate of gadolinium-based MRI contrast agents: a call to action for bioinorganic chemists

Gadolinium-based contrast agents (GBCAs) are widely used with clinical magnetic resonance imaging (MRI), and 10 s of millions of doses of GBCAs are administered annually worldwide. GBCAs are hydrophilic, thermodynamically stable and kinetically inert gadolinium chelates. In clinical MRI, 5-10 millimoles of Gd ion is administered intravenously and the GBCA is rapidly eliminated intact primarily through the kidneys into the urine. It is now well-established that the Gd3+ ion, in some form(s), is partially retained in vivo. In patients with advanced kidney disease, there is an association of Gd retention with nephrogenic systemic fibrosis (NSF) disease. However Gd is also retained in the brain, bone, skin, and other tissues in patients with normal renal function, and the presence of Gd can persist months to years after the last administration of a GBCA. Regulatory agencies are restricting the use of specific GBCAs and inviting health care professionals to evaluate the risk/benefit ratio prior to using GBCAs. Despite the growing number of studies investigating this issue both in animals and humans, the biological distribution and the chemical speciation of the residual gadolinium are not fully understood. Is the GBCA retained in its intact form? Is the Gd3+ ion dissociated from its chelator, and if so, what is its chemical form? Here we discuss the current state of knowledge regarding the issue of Gd retention and describe the analytical and spectroscopic methods that can be used to investigate the Gd speciation. Many of the physical methods that could be brought to bear on this problem are in the domain of bioinorganic chemistry and we hope that this review will serve to inspire this community to take up this important problem.

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.

Impact of chelation timing on gadolinium deposition in rats after contrast administration

OBJECTIVE

To determine if gadolinium (Gd) can be rechelated once released from Gd-based contrast agents (GBCAs) and deposited in vivo. Despite extensive research comparing GBCAs and GBCA formulations as well as the ongoing debate about their risks of deposition and the role of Gd release, it remains unknown if retained Gd can be eliminated by administering chelating agents.

MATERIALS AND METHODS

Rats were injected intravenously with 10 doses of 1 mmol/kg gadodiamide and treated with intravenous Zn-DTPA (30 μmol/kg) concomitantly or 1, 4 or 8 h after GBCA administration (N = 3 rats per group). After euthanization, tissues were harvested three days after the last dose of gadodiamide and tissue Gd concentrations were assessed by ICP-MS. Additionally, a simulation of a single 0.1 mmol/kg gadopentetate dose with 30 μmol/kg DTPA given either concomitantly or within the first 24 h after GBCA was run; simulated tissue Gd concentrations were compared with those observed in rats to determine if simulated trends were accurate.

RESULTS

Concomitant DTPA did not produce a significant reduction in Gd concentration in any organ for rats. There was a time-dependent trend in liver Gd reduction. The 1 h timepoint was associated with a non-significant increase in kidney, brain and femur Gd relative to untreated controls. There were no significant deviations from the model-predicted Gd changes.

DISCUSSION

Both the simulation and rat study did not identify major benefits for chelation at the doses given, despite the simulation assuming all Gd deposited in tissues is unchelated. The potential redistribution in the rat study provide a compelling result that may impact the clinical relevance of further work investigating rechelation of Gd. Future work should further describe the three-dimensional dose-time-response relationship for preventing Gd deposition, and how that relates to long-term Gd toxicities.

Influence of excess ligand on Nephrogenic Systemic Fibrosis associated with nonionic, linear gadolinium-based contrast agents

BACKGROUND

The molecular structure, charge, thermodynamic and kinetic stability are approximately the same for gadodiamide and gadoversetamide, the main substantive difference is that gadodiamide is manufactured with 5% free ligand to form Omniscan® and gadoversetamide with 10% free ligand to form OptiMARK®.

PURPOSE

To determine the relative risk of Nephrogenic Systemic Fibrosis (NSF) between gadodiamide (Omniscan®) and gadoversetamide (OptiMARK®) and to explore the potential contribution of the amount of excess ligand added to their commercial formulations.

MATERIALS AND METHODS

In this retrospective observational study, the number of doses and NSF cases associated with these agents were calculated based on two different approaches: the number of doses was determined based on pharmaceutical companies' information, and the number of unconfounded NSF cases was obtained from the previously published literature based on a legal database. A second analysis estimates the number of doses and NSF cases from the Food and Drug Administration (FDA) Adverse Event Reporting System (FAERS).

RESULTS

Approximately 87 million and 12 million doses of Omniscan® and OptiMARK®, respectively, have been administered worldwide since their original approval for use in the various countries throughout the world. A total of 197 and 8 unconfounded cases of NSF have been reported with Omniscan® and OptiMARK®, rendering an incidence of 2.3/million and 0.7/million for these agents, respectively. The FAERS analysis suggested reported incidences of 13.1/million and 5.0/million.

CONCLUSION

There is an approximately 3-fold greater incidence of NSF from Omniscan® than OptiMARK®. The difference in incidence might reflect the lesser quantity of added free ligand to the formulation of Omniscan®.

Gadolinium Retention: A Research Roadmap from the 2018 NIH/ACR/RSNA Workshop on Gadolinium Chelates

Gadolinium-based contrast agents (GBCAs) have revolutionized MRI, enabling physicians to obtain crucial life-saving medical information that often cannot be obtained with other imaging modalities. Since initial approval in 1988, over 450 million intravenous GBCA doses have been administered worldwide, with an extremely favorable pharmacologic safety profile; however, recent information has raised new concerns over the safety of GBCAs. Mounting evidence has shown there is long-term retention of gadolinium in human tissues. Further, a small subset of patients have attributed a constellation of symptoms to GBCA exposure, although the association of these symptoms with GBCA administration or gadolinium retention has not been proven by scientific investigation. Despite evidence that macrocyclic GBCAs show less gadolinium retention than linear GBCAs, the safety implications of gadolinium retention are unknown. The mechanism and chemical forms of gadolinium retention, as well as the biologic activity and clinical importance of these retained gadolinium species, remain poorly understood and underscore the need for additional research. In February 2018, an international meeting was held in Bethesda, Md, at the National Institutes of Health to discuss the current literature and knowledge gaps about gadolinium retention, to prioritize future research initiatives to better understand this phenomenon, and to foster collaborative standardized studies. The greatest priorities are to determine (a) if gadolinium retention adversely affects the function of human tissues, (b) if retention is causally associated with short- or long-term clinical manifestations of disease, and (c) if vulnerable populations, such as children, are at greater risk for experiencing clinical disease. The purpose of the research roadmap is to highlight important information that is not known and to identify and prioritize needed research. ©RSNA, 2018 Online supplemental material is available for this article .

Radioactive Main Group and Rare Earth Metals for Imaging and Therapy

Radiometals possess an exceptional breadth of decay properties and have been applied to medicine with great success for several decades. The majority of current clinical use involves diagnostic procedures, which use either positron-emission tomography (PET) or single-photon imaging to detect anatomic abnormalities that are difficult to visualize using conventional imaging techniques (e.g., MRI and X-ray). The potential of therapeutic radiometals has more recently been realized and relies on ionizing radiation to induce irreversible DNA damage, resulting in cell death. In both cases, radiopharmaceutical development has been largely geared toward the field of oncology; thus, selective tumor targeting is often essential for efficacious drug use. To this end, the rational design of four-component radiopharmaceuticals has become popularized. This Review introduces fundamental concepts of drug design and applications, with particular emphasis on bifunctional chelators (BFCs), which ensure secure consolidation of the radiometal and targeting vector and are integral for optimal drug performance. Also presented are detailed accounts of production, chelation chemistry, and biological use of selected main group and rare earth radiometals.

Gadolinium deposition and the potential for toxicological sequelae - A literature review of issues surrounding gadolinium-based contrast agents

Every year, approximately 30 million magnetic resonance imaging scans are enhanced with gadolinium-based contrast agents (GBCAs) worldwide. Although the development of nephrogenic systemic fibrosis in patients with renal impairment is well-documented, over recent years it has become apparent that exposure to GBCAs can potentially result in gadolinium deposition within human bone and brain tissue even in the presence of normal renal function. This review will address some of the controversies surrounding the safety of GBCA administration based on evidence from in vivo experiments, animal studies and clinical studies. We additionally evaluate the potential risk of toxicity from exposure to gadolinium in light of new guidance published by the US Food and Drug Administration and the European Medicines Agency, and discuss whether gadolinium deposition disease exists as a new diagnosis.

Optimising the relaxivities of Mn2+ complexes by targeting human serum albumin (HSA)

We report two novel macrocyclic ligands based on the 1,4-DO2AM platform (1,4-DO2AM = 2,2'-(1,4,7,10-tetraazacyclododecane-1,4-diyl)diacetamide) and containing two benzyl groups attached either to the nitrogen atoms of the macrocyclic unit (1,4-BzDO2AM) or to the amide pendant arms (1,4-DO2AMBz). The protonation constants of the ligands and the stability constants of their Mn²⁺ complexes were determined using pH potentiometry. The introduction of benzyl groups results in a slight decrease of the stability constants of the Mn²⁺ complexes and a slight increase of their acid-catalysed dissociation reactions. A detailed relaxometric characterisation of the complexes using nuclear magnetic dispersion relaxation (NMRD) and ¹⁷O NMR studies indicated that the increase in molecular weight associated with the presence of benzyl groups results in a remarkable increase of proton relaxivities r_1p, which take values of 3.8, 3.5 and 2.5 mM^-1 s^-1 for [Mn(1,4-BzDO2AM)]²⁺, [Mn(1,4-DO2AMBz)]²⁺ and [Mn(1,4-DO2AM)]²⁺ (at 25 °C and 20 MHz). The [Mn(1,4-BzDO2AM)]²⁺ and [Mn(1,4-DO2AMBz)]²⁺ complexes form relatively strong adducts with Human Serum Albumin (HSA) with association constants of (3.9 ± 0.6) × 10³ and (2.0 ± 0.3) × 10³ M^-1, respectively. The interaction with the protein slows down the rotational tumbling of the complex in solution, which results in adducts endowed with remarkably high proton relaxivities (r_1p^b = 18.5 ± 0.7 and 27.4 ± 1.4 mM^-1 s^-1 for [Mn(1,4-BzDO2AM)]²⁺ and [Mn(1,4-DO2AMBz)]²⁺, respectively).

Signal Enhancement of the Dentate Nucleus at Unenhanced MR Imaging after Very High Cumulative Doses of the Macrocyclic Gadolinium-based Contrast Agent Gadobutrol: An Observational Study

Purpose To test for measurable visual enhancement of the dentate nucleus (DN) on unenhanced T1-weighted magnetic resonance (MR) images in a cohort of patients with a primary brain tumor who had not received linear gadolinium-based contrast agents (GBCAs) but had received many injections of macrocyclic GBCAs. Materials and Methods Seventeen patients with high-grade gliomas who had received 10-44 administrations of the macrocyclic GBCA gadobutrol (0.1 mmol/kg of body weight) were retrospectively included in this regional ethics committee-approved study. Two neuroradiologists inspected T1-weighted MR images with optimized window settings to visualize small differences in contrast at the baseline and at the last examination for the presence of visual DN signal enhancement. Signal intensity (SI) in the DN was normalized to the SI of the pons, and a one-sample t test was used to test for differences between baseline normalized SI (nSI) in the DN (nSI_DN) and the average change in nSI_DN of all postbaseline MR imaging sessions (ΔnSI_DNavg) or the change in nSI_DN from baseline to the last MR imaging session (ΔnSI_DN). Linear and quadratic correlation analyses were used to examine the association between the number of macrocyclic GBCA administrations and ΔnSI_DN or ΔnSI_DNavg. Results The mean ± standard deviation number of macrocyclic GBCA administrations was 22.2 ± 10.6 administered throughout 706 days ± 454. Visually appreciable signal enhancement was observed in two patients who had received 37 and 44 macrocyclic GBCA injections. Mean ΔnSI_DN was greater than zero (0.03 ± 0.05; P = .016), and there was a significant linear association between the number of macrocyclic GBCA injections and ΔnSI_DN (r = 0.69, P = .002) and ΔnSI_DNavg (r = 0.77, P < .001). Conclusion A small but statistically significant dose-dependent T1-weighted signal enhancement was observed in the DN after multiple macrocyclic GBCA injections. Visually appreciable enhancement in the DN was observed on contrast-optimized images in two patients who had received 37 and 44 standard doses of macrocyclic GBCAs. ^© RSNA, 2017 Online supplemental material is available for this article.

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.

The Impact of Excess Ligand on the Retention of Nonionic, Linear Gadolinium-Based Contrast Agents in Patients With Various Levels of Renal Dysfunction: A Review and Simulation Analysis

The role of gadolinium (Gd)-based contrast agents (GBCAs) in the pathophysiology of nephrogenic systemic fibrosis (NSF) is now uncontested. Although the definitive mechanism has not been established, the association with weaker GBCA ligands and with reduced renal clearance supports a hypothesis that Gd release from the GBCAs is a key process in precipitating the disease. Prevention strategies often include the use of more stable GBCA ligands in patients with reduced kidney function, but animal models and some clinical data suggest that better patient outcomes can be achieved when excess ligand is administered with weaker GBCAs; this is particularly significant for OptiMARK, which contains a nonionic, linear ligand similar to gadodiamide, the active ingredient in Omniscan, but contains twice the amount of excess ligand. Here we review evidence regarding the use of OptiMARK over Omniscan for prevention of NSF and perform a pharmacokinetic-based simulation to determine if the presented evidence is consistent with the established kinetics of GBCAs and Gd.

In Vitro Mouse and Human Serum Stability of a Heterobivalent Dual-Target Probe That Has Strong Affinity to Gastrin-Releasing Peptide and Neuropeptide Y1 Receptors on Tumor Cells

Receptor-targeting radiolabeled molecular probes with high affinity and specificity are useful in studying and monitoring biological processes and responses. Dual- or multiple-targeting probes, using radiolabeled metal chelates conjugated to peptides, have potential advantages over single-targeting probes as they can recognize multiple targets leading to better sensitivity for imaging and radiotherapy when target heterogeneity is present. Two natural hormone peptide receptors, gastrin-releasing peptide (GRP) and Y1, are specifically interesting as their expression is upregulated in most breast and prostate cancers. One of our goals has been to develop a dual-target probe that can bind both GRP and Y1 receptors. Consequently, a heterobivalent dual-target probe, t-BBN/BVD15-DO3A (where a GRP targeting ligand J-G-Abz4-QWAVGHLM-NH2 and Y1 targeting ligand INP-K [ɛ-J-(α-DO3A-ɛ-DGa)-K] YRLRY-NH2 were coupled), that recognizes both GRP and Y1 receptors was synthesized, purified, and characterized in the past. Competitive displacement cell binding assay studies with the probe demonstrated strong affinity (IC50 values given in parentheses) for GRP receptors in T-47D cells (18 ± 0.7 nM) and for Y1 receptors in MCF7 cells (80 ± 11 nM). As a further evaluation of the heterobivalent dual-target probe t-BBN/BVD15-DO3A, the objective of this study was to determine its mouse and human serum stability at 37°C. The in vitro metabolic degradation of the dual-target probe in mouse and human serum was studied by using a 153Gd-labeled t-BBN/BVD15-DO3A and a high-performance liquid chromatography/radioisotope detector analytical method. The half-life (t1/2) of degradation of the dual-target probe in mouse serum was calculated as 7 hours and only ∼20% degradation was seen after 6 hours incubation in human serum. The slow in vitro metabolic degradation of the dual-target probe can be compared with the degradation t1/2 of the corresponding monomeric probes, BVD15-DO3A and AMBA: 15, and ∼40 minutes for BVD15-DO3A and 3.1 and 38.8 hours for AMBA in mouse and human serum, respectively. A possible pathway for in vitro metabolic degradation of the t-BBN/BVD15-DO3A in mouse serum is proposed based on the chromatographic retention times of the intact probe and its degradants.

The stability of gadolinium-based contrast agents in human serum: A reanalysis of literature data and association with clinical outcomes

PURPOSE

To reanalyze literature data of gadolinium (Gd)-based contrast agents (GBCAs) in plasma with a kinetic model of dissociation to provide a comprehensive assessment of equilibrium conditions for linear GBCAs.

METHODS

Data for the release of Gd from GBCAs in human serum was extracted from a previous report in the literature and fit to a kinetic dissociation/association model. The conditional stabilities (logK_cond) and percent intact over time were calculated using the model rate constants. The correlations between clinical outcomes and logK_cond or other stability indices were determined.

RESULTS

The release curves for Omniscan®, gadodiamide, OptiMARK®, gadoversetamide Magnevist® and Multihance® were extracted and all fit well to the kinetic model. The logK_conds calculated from the rate constants were on the order of ~4-6, and were not significantly altered by excess ligand or phosphate. The stability constant based on the amount intact by the initial elimination half-life of GBCAs in plasma provided good correlation with outcomes observed in patients.

CONCLUSIONS

Estimation of the kinetic constants for GBCA dissociation/association revealed that their stability in physiological fluid is much lower than previous approaches would suggest, which correlates well with deposition and pharmacokinetic observations of GBCAs in human patients.

Potential Safety Issues Related to the Use of Gadolinium-based Contrast Agents

PURPOSE OF REVIEW

This article reviews recent research on gadolinium deposit formation in the brain linked to contrast-enhanced MRI studies.

RECENT FINDINGS

Human and animal studies have confirmed the presence of gadolinium in the brain following the serial administration of gadolinium-based contrast agents. This is a relatively new and growing field of research primarily driven by concerns regarding unknown and potentially harmful side effects of gadolinium-based contrast agents. Retrospective observational in vivo studies in humans demonstrated T1 shortening effects in the brain parenchyma resulting from gadolinium exposure. These studies were followed by postmortem human and animal studies. Evidence exists that gadolinium may cause deposits in the brain and that this may occur independently of impaired renal function and in the presence of an intact blood-brain barrier. Gadolinium deposition has been linked primarily with the use of linear, rather than macrocyclic, gadolinium-based contrast agents.

SUMMARY

The formation of gadolinium deposits and its implications have been the focus of only a small number of research groups. The currently available data must be verified, and the potential factors that may be linked to this phenomenon and the clinical significance must be explored. Depending on future findings, changes in the clinical application of gadolinium-based contrast agents may be expected.

Brain Metastases — Comparison of Gadodiamide Injection-Enhanced MR Imaging at Standard and High Dose, Contrast-Enhanced CT and Non-Contrast-Enhanced MR Imaging

The aim was to compare the abilities of contrast-enhanced CT, non-contrast-enhanced MR imaging and contrast-enhanced MR imaging using standard (0.1 mmol/kg b.w.) and high (0.3 mmol/kg b.w.) doses of Gadodiamide injection to detect brain metastases (i.e. blood-brain barrier damage). Sixteen patients with at least 2 metastases found by CT were evaluated by MR imaging using non-contrast-enhanced spin-echo, T1-weighted, T2-weighted sequences, and contrast-enhanced spin-echo T1-weighted sequences at 2 dose levels. Gadodiamide injection was first given at the dose of 0.1 mmol/kg b.w. After imaging, another 0.2 mmol/kg b.w. was given, yielding a cumulative dose of 0.3 mmol/kg b.w. No contrast media-related adverse events were recorded. The images were evaluated openly by one and blindly by 2 investigators and the number of metastases, size, delineation (open study) and diagnostic certainty (blind study) of each individual metastasis noted. High-dose MR imaging showed significantly more and smaller metastases than any other examination, and gave a higher diagnostic certainty. All high-dose images were superior to those with the standard dose MR imaging when compared blindly in pairs. We conclude that spin-echo MR imaging with a high dose of Gadodiamide injection is an efficient way to improve the detection of brain metastases, in particular of small ones.

Immediate Adverse Reactions to Gadolinium-Based MR Contrast Media: A Retrospective Analysis on 10,608 Examinations

Background and Purpose. Contrast media (CM) for magnetic resonance imaging (MRI) may determine the development of acute adverse reactions. Objective was to retrospectively assess the frequency and severity of adverse reactions associated with gadolinium-based contrast agents (GBCAs) injection in patients who underwent MRI. Material and Methods. At our center 10608 MRI examinations with CM were performed using five different GBCAs: Gd-BOPTA (MultiHance), Gd-DTPA (Magnevist), Gd-EOBDTPA (Primovist), Gd-DOTA (Dotarem), and Gd-BTDO3A (Gadovist). Results. 32 acute adverse reactions occurred, accounting for 0.3% of all administration. Twelve reactions were associated with Gd-DOTA injection (0.11%), 9 with Gd-BOPTA injection (0.08%), 6 with Gd-BTDO3A (0.056%), 3 with Gd-EOB-DTPA (0.028%), and 2 with Gd-DTPA (0.018%). Twenty-four reactions (75.0%) were mild, four (12.5%) moderate, and four (12.5%) severe. The most severe reactions were seen associated with use of Gd-BOPTA, with 3 severe reactions in 32 total reactions. Conclusion. Acute adverse reactions are generally rare with the overall adverse reaction rate of 0.3%. The most common adverse reactions were not severe, consisting in skin rash and hives.

Can gadolinium be re-chelated in vivo? Considerations from decorporation therapy

Gadolinium (Gd) and Gd-based contrast agents (GBCAs) have been observed to deposit in tissues of patients following contrast enhanced MR imaging procedures. A conservative approach for chelation therapy of this toxic metal dictates the assumption that minimal intact GBCAs are present. Currently the extent to which these deposits are primarily de-chelated Gd remains uncertain, prevailing knowledge suggests that for linear agents much of the Gd is de-chelated, while for the macrocyclic agents, the Gd may be still largely chelated. To extract Gd from tissues and facilitate its release, chelation therapy should be both safe and effective. Here we discuss chelation therapy as it relates to Gd deposition. The principles of chelation are reviewed, initially with reference to ligand stability in complex biological fluids. A model of decorporation and how it relates to elimination of Gd deposits is also reviewed. When more is learned about Gd deposition, optimal removal strategies must be developed using basic thermodynamic and kinetic principles.

Cellular internalization and morphological analysis after intravenous injection of a highly hydrophilic octahedral rhenium cluster complex - a new promising X-ray contrast agent

The octahedral cluster compound Na₂ H₈ [{Re₆ Se₈ }(P(C₂ H₄ CONH₂ )(C₂ H₄ COO)₂ )₆ ] has been shown to be highly radio dense, thus becoming a promising X-ray contrast agent. It was also shown that this compound had low cytotoxic effect in vitro, low acute toxicity in vivo and was eliminated rapidly from the body through the urinary tract. The present contribution describes a more detailed cellular internalization assay and morphological analysis after intravenous injection of this hexarhenium cluster compound at different doses. The median lethal dose (LD₅₀ ) of intravenously administrated compound was calculated (4.67 ± 0.69 g/kg). Results of the study clearly indicated that the cluster complex H_n [{Re₆ Se₈ }(P(C₂ H₄ CONH₂ )(C₂ H₄ COO)₂ )₆ ]^n-10 was not internalized into cells in vitro and induced only moderate morphological alterations of kidneys at high doses without any changes in morphology of liver, spleen, duodenum, or heart of mice. Copyright © 2016 John Wiley & Sons, Ltd.