Receptor recognition of transferrin bound to lanthanides and actinides: a discriminating step in cellular acquisition of f-block metals

Cisplatin/Apo-Transferrin Adduct: X-ray Structure and Binding to the Transferrin Receptor 1

Here, we report the X-ray structure of the adduct formed upon reaction of cisplatin, one of the most prescribed anticancer agents for the clinic treatment of solid tumors, with the apo-form of human serum transferrin (hTF). Two Pt binding sites were identified in both molecules of the adduct present in the crystal asymmetric unit: Pt binds close to the side chains of Met256 and Met499 at the N- and C-lobe, respectively. In the crystal structure, the cisplatin moiety bound to Met256 also interacts with Ser616 from a symmetry related molecule. Structural analyses, together with in solution data, demonstrate that the presence of iron does not affect the ability of hTF to bind cisplatin and that the cisplatin binding does not significantly alter the overall conformation of the different forms of the protein that remain able to form a complex with the transferrin receptor 1 (TfR1). These data suggest that the different hTF forms can be used as nanocarriers for targeted (combined) metallodrug delivery.

Biogeochemistry of Actinides: Recent Progress and Perspective

Actinides are elements that are often feared because of their radioactive nature and potentially devastating consequences to humans and the environment if not managed properly. As such, their chemical interactions with the biosphere and geochemical environment, i.e., their "biogeochemistry," must be studied and understood in detail. In this Review, a summary of the past discoveries and recent advances in the field of actinide biogeochemistry is provided with a particular emphasis on actinides other than thorium and uranium (i.e., actinium, neptunium, plutonium, americium, curium, berkelium, and californium) as they originate from anthropogenic activities and can be mobile in the environment. The nuclear properties of actinide isotopes found in the environment and used in research are reviewed with historical context. Then, the coordination chemistry properties of actinide ions are contrasted with those of common metal ions naturally present in the environment. The typical chelators that can impact the biogeochemistry of actinides are then reviewed. Then, the role of metalloproteins in the biogeochemistry of actinides is put into perspective since recent advances in the field may have ramifications in radiochemistry and for the long-term management of nuclear waste. Metalloproteins are ubiquitous ligands in nature but, as discussed in this Review, they have largely been overlooked for actinide chemistry, especially when compared to traditional environmental chelators. Without discounting the importance of abundant and natural actinide ions (i.e., Th4+ and UO2 2+), the main focus of this review is on trivalent actinides because of their prevalence in the fields of nuclear fuel cycles, radioactive waste management, heavy element research, and, more recently, nuclear medicine. Additionally, trivalent actinides share chemical similarities with the rare earth elements, and recent breakthroughs in the field of lanthanide-binding chelators may spill into the field of actinide biogeochemistry, as discussed hereafter.

Thorium Alters Lung Surfactant Protein Expression in Alveolar Epithelial Cells: In Vitro and In Vivo Investigation

Thorium-232 (Th), the most abundant naturally occurring nuclear fuel, has been identified as a sustainable source of energy. In view of its large-scale utilization and human evidence of lung disorders and carcinogenicity, it is imperative to understand the effect of Th exposure on lung cells. The present study investigated the effect of Th-dioxide (1-100 μg/mL, 24-48 h) on expression of surfactant proteins (SPs) (SP-A, SP-B, SP-C, and SP-D, which are essential to maintain lung's surface tension and host-defense) in human lung cells (WI26 and A549), representative of alveolar cell type-I and type-II, respectively. Results demonstrated the inhibitory effect of Th on transcriptional expression of SP-A, SP-B, and SP-C. However, Th promoted the mRNA expression of SP-D in A549 and reduced its expression in WI26. To a significant extent, the effect of Th on SPs was found to be in accordance with their protein levels. Moreover, Th exposure altered the extracellular release of SP-D/A from A549, which remained unaltered in WI26. Our results suggested the differential role of oxidative stress and ATM and HSP90 signaling in Th-induced alterations of SPs. These effects of Th were found to be consistent in lung tissues of mice exposed to Th aerosols, suggesting a potential role of SPs in Th-associated lung disorders.

Recognition of Actinides by Siderocalin

Plain simulations and enhanced sampling unveil a novel siderocalin (Scn) recognition mode for An-Ent (where An = actinides and Ent = enterobactin) complexes and identify a "seesaw" relationship between actinide affinity to Ent and Scn recognition to an An-Ent complex. Electrostatic interactions predominantly govern competitive binding in both processes. Additionally, hydrolysis-induced negative charge, water expulsion-driven entropy, and Ent's conformational adaptability collectively enhance high-affinity recognition.

Mechanistic insights into Thorium-232 induced liver carcinogenesis: The driving role of Wnt/β-catenin signaling pathway

Thorium-232 (Th-232), a naturally-occurring radioactive element with high potential of nuclear fuel is now being utilized in advanced nuclear reactors for CO2-free energy generation. To achieve all-round capability in Th-fuel cycle for health and environment, understanding the biological effects of Th-232 at cellular and molecular level are extremely important. The present study investigated long-term effects (6 and 12 months) of Th-232 (4, 10 and 20 mg/kg) on gene expression in mice liver (major target organ). Analysis of differentially expressed genes (DEGs, ≥2.0 folds, p < 0.05) showed that with the increase of Th dose (4 to 20 mg/kg), the number of upregulated DEGs increased and the number of downregulated DEGs decreased significantly. A significant number of upregulated DEGs (10 genes in 6 months and 14 genes in 12 months) were found common between 4 and 20 mg/kg. Gene Ontology analysis revealed significant (Padj ~ 10-6-10-28) enrichment of upregulated DEGs for metabolic process, signal transduction, cell death, cell cycle and cell proliferation. KEGG pathway analysis showed DEGs significantly enriched in several cancer-related pathways including hepatocellular carcinoma (HCC). Protein-protein interaction analysis further revealed statistically significant functional interaction (p-value ~10-6-10-10) among the proteins of HCC, which identified β-catenin as one of the most significant signaling nodes in association with myc, an oncogene and p53, a tumor suppressor. Importantly, these results were corroborated by quantitative real time-polymerase chain reaction and western blotting in liver tissues of animals exposed to Th-232. This study insights Wnt/β-catenin signaling network attributable to drive Th-induced liver carcinogenesis, which may have significant implications for management of long-term effects of Th-232.

Macrocyclic 1,2-Hydroxypyridinone-Based Chelators as Potential Ligands for Thorium-227 and Zirconium-89 Radiopharmaceuticals

Thorium-227 (227Th) is an α-emitting radionuclide that has shown preclinical and clinical promise for use in targeted α-therapy (TAT), a type of molecular radiopharmaceutical treatment that harnesses high energy α particles to eradicate cancerous lesions. Despite these initial successes, there still exists a need for bifunctional chelators that can stably bind thorium in vivo. Toward this goal, we have prepared two macrocyclic chelators bearing 1,2-hydroxypyridinone groups. Both chelators can be synthesized in less than six steps from readily available starting materials, which is an advantage over currently available platforms. The complex formation constants (log βmlh) of these ligands with Zr4+ and Th4+, measured by spectrophotometric titrations, are greater than 34 for both chelators, indicating the formation of exceedingly stable complexes. Radiolabeling studies were performed to show that these ligands can bind [227Th]Th4+ at concentrations as low as 10-6 M, and serum stability experiments demonstrate the high kinetic stability of the formed complexes under biological conditions. Identical experiments with zirconium-89 (89Zr), a positron-emitting radioisotope used for positron emission tomography (PET) imaging, demonstrate that these chelators can also effectively bind Zr4+ with high thermodynamic and kinetic stability. Collectively, the data reported herein highlight the suitability of these ligands for use in 89Zr/227Th paired radioimmunotheranostics.

Screening the complex biological behavior of late lanthanides through genome-wide interactions

Despite their similar physicochemical properties, recent studies have demonstrated that lanthanides can display different biological behaviors. Hence, the lanthanide series can be divided into three parts, namely early, mid, and late lanthanides, based on their interactions with biological systems. In particular, the late lanthanides demonstrate distinct, but poorly understood biological activity. In the current study, we employed genome-wide functional screening to help understand biological effects of exposure to Yb(III) and Lu(III), which were selected as representatives of the late lanthanides. As a model organism, we used Saccharomyces cerevisiae, since it shares many biological functions with humans. Analysis of the functional screening results indicated toxicity of late lanthanides is consistent with disruption of vesicle-mediated transport, and further supported a role for calcium transport processes and mitophagy in mitigating toxicity. Unexpectedly, our analysis suggested that late lanthanides target proteins with SH3 domains, which may underlie the observed toxicity. This study provides fundamental insights into the unique biological chemistry of late lanthanides, which may help devise new avenues toward the development of decorporation strategies and bio-inspired separation processes.

Thorium(IV) triggers ferroptosis through disrupting iron homeostasis and heme metabolism in the liver following oral ingestion

Thorium is a byproduct of the rare earth mining industry and can be utilized as fuel for the next-generation nuclear power facilities, which may pose health risks to the population. Although published literature has shown that the toxicity of thorium possibly originates from its interactions with iron/heme-containing proteins, the underlying mechanisms are still largely unclear. Since the liver plays an irreplaceable role in iron and heme metabolism in the body, it is essential to investigate how thorium affects iron and heme homeostasis in hepatocytes. In this study, we first assessed the liver injury in mice exposed to tetravalent thorium (Th(IV)) in the form of thorium nitrite via the oral route. After a two-week oral exposure, thorium accumulation and iron overload were observed in the liver, which are both closely associated with lipid peroxidation and cell death. Transcriptomics analysis revealed that ferroptosis, which has not previously been documented in cells for actinides, is the main mechanism of programmed cell death induced by Th(IV). Further mechanistic studies suggested that Th(IV) could activate the ferroptotic pathway through disrupting iron homeostasis and generating lipid peroxides. More significantly, the disorder of heme metabolism, which is crucial for maintaining intracellular iron and redox homeostasis, was found to contribute to ferroptosis in hepatocytes exposed to Th(IV). Our findings may shed light on a key mechanism of hepatoxicity in response to Th(IV) stress and provide in-depth understanding of the health risk of thorium.

Cisplatin Binding to Human Serum Transferrin: A Crystallographic Study

The molecular mechanism of how human serum transferrin (hTF) recognizes cisplatin at the atomic level is still unclear. Here, we report the molecular structure of the adduct formed upon the reaction of hTF with cisplatin. Pt binds the side chain of Met256 (at the N-lobe), without altering the protein overall conformation.

Development of radiopharmaceuticals for targeted alpha therapy: Where do we stand?

Targeted alpha therapy is an oncological treatment, where cytotoxic doses of alpha radiation are locally delivered to tumor cells, while the surrounding healthy tissue is minimally affected. This therapeutic strategy relies on radiopharmaceuticals made of medically relevant radionuclides chelated by ligands, and conjugated to targeting vectors, which promote the drug accumulation in tumor sites. This review discusses the state-of-the-art in the development of radiopharmaceuticals for targeted alpha therapy, breaking down their key structural components, such as radioisotope, targeting vector, and delivery formulation, and analyzing their pros and cons. Moreover, we discuss current drawbacks that are holding back targeted alpha therapy in the clinic, and identify ongoing strategies in field to overcome those issues, including radioisotope encapsulation in nanoformulations to prevent the release of the daughters. Lastly, we critically discuss potential opportunities the field holds, which may contribute to targeted alpha therapy becoming a gold standard treatment in oncology in the future.

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

Background

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

Objective

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

Methods

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

Results

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

Conclusion

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

Uranium chelating ability of decorporation agents in serum evaluated by X-ray absorption spectroscopy

Internal exposure to actinides such as uranium and plutonium has been reduced using chelating agents for decorporation because of their potential to induce both radiological and chemical toxicities. This study measures uranium chemical forms in serum in the presence and absence of chelating agents based on X-ray absorption spectroscopy (XAS). The chelating agents used were 1-hydroxyethane 1,1-bisphosphonate (EHBP), inositol hexaphosphate (IP6), deferoxamine B (DFO), and diethylenetriaminepentaacetate (DTPA). Percentages of uranium-chelating agents and uranium-bioligands (bioligands: inorganic and organic ligands coordinating with uranium) dissolving in the serum were successfully evaluated based on principal component analysis of XAS spectra. The main ligands forming complexes with uranium in the serum were estimated as follows: IP6 > EHBP > bioligands > DFO ≫ DTPA when the concentration ratio of the chelating agent to uranium was 10. Measurements of uranium chemical forms and their concentrations in the serum would be useful for the appropriate treatment using chelating agents for the decorporation of uranium.

Engineering lanmodulin's selectivity for actinides over lanthanides by controlling solvent coordination and second-sphere interactions

Developing chelators that combine high affinity and selectivity for lanthanides and/or actinides is paramount for numerous industries, including rare earths mining, nuclear waste management, and cancer medicine. In particular, achieving selectivity between actinides and lanthanides is notoriously difficult. The protein lanmodulin (LanM) is one of Nature's most selective chelators for trivalent actinides and lanthanides. However, mechanistic understanding of LanM's affinity and selectivity for f-elements remains limited. In order to decipher, and possibly improve, the features of LanM's metal-binding sites that contribute to this actinide/lanthanide selectivity, we characterized five LanM variants, substituting the aspartate residue at the 9th position of each metal-binding site with asparagine, histidine, alanine, methionine, and selenomethionine. Spectroscopic measurements with lanthanides (Nd3+ and Eu3+) and actinides (243Am3+ and 248Cm3+) reveal that, contrary to the behavior of small chelator complexes, metal-coordinated water molecules enhance LanM's affinity for f-elements and pH-stability of its complexes. Furthermore, the results show that the native aspartate does not coordinate the metal directly but rather hydrogen bonds to coordinated solvent. By tuning this first-sphere/second-sphere interaction, the asparagine variant nearly doubles LanM's selectivity for actinides versus lanthanides. This study not only clarifies the essential role of coordinated solvent for LanM's physiological function and separation applications, but it also demonstrates that LanM's preference for actinides over lanthanides can be further improved. More broadly, it demonstrates how biomolecular scaffolds possess an expanded repertoire of tunable interactions compared to most small-molecule ligands - providing an avenue for high-performance LanM-based actinide/lanthanide separation methods and bio-engineered chelators optimized for specific medical isotopes.

Thorium inhibits human respiratory chain complex IV (cytochrome c oxidase)

Thorium is a radioactive heavy metal and an emerging environmental pollutant. Ecological and human health risks from thorium exposure are growing with the excavation of rare earth metals and implementation of thorium-based nuclear reactors. Thorium poisoning is associated with carcinogenesis, liver impairments, and congenital anomalies. To date, the biomolecular targets that underlie thorium-induced toxicity remain unknown. Here, we used in vitro enzymatic activity assays to comprehensively evaluate the effects of thorium on the mitochondrial respiration process. Thorium was found to inhibit respiratory chain complex IV (cytochrome c oxidase) at sub-micromolar concentrations (IC₅₀ ~ 0.4 μM, 90 μg/L). This is lower than the thorium level limit (246 μg/L) in drinking water specified by the World Health Organization. The inhibitory effects were further verified in mitochondria from human bone and liver cells (thorium mainly deposits in these organs). The inhibition of cytochrome c oxidase can readily rationalize well-documented cellular toxicities of thorium, such as alteration of mitochondrial membrane potential and production of reactive oxygen species. Therefore, cytochrome c oxidase is potentially a key molecular target underlying thorium-induced toxicological effect.

In vitro evidence of the influence of complexation of Pu and Am on uptake by human lung epithelial cells Calu-3

Understanding the mechanisms involved in retention and clearance of actinides from the lungs after accidental intake is essential for the evaluation of the associated radiological risks. Although the absorption of radioelements has been shown in vivo to depend on their nature and physico-chemical properties, their mechanisms of translocation remain unknown. In this study, we have evaluated in vitro the binding and uptake by bronchial epithelial cells Calu-3 of 2 transuranic actinides, plutonium (Pu) and americium (Am), as the first steps of translocation across the pulmonary barrier. For this purpose, Calu-3 cells grown to confluence in 24-well plates were exposed to the radioelements for 24 h under various culture conditions. Two compartments were identified for the association of actinides to cells, corresponding to the membrane bound and internalized fractions. Binding of Pu was slightly higher than of Am, and depended on its initial chemical form (nitrate, citrate, colloids). Uptake of Pu and Am nitrate was higher in serum-free conditions than in supplemented medium, with an active mechanism involved in Pu internalization. Overall, our results suggest that complexation of actinides to bioligands may have an influence on their uptake by pulmonary epithelial cells, and therefore possibly on their subsequent absorption into blood.

Capturing an elusive but critical element: Natural protein enables actinium chemistry

Actinium-based therapies could revolutionize cancer medicine but remain tantalizing due to the difficulties in studying and limited knowledge of Ac chemistry. Current efforts focus on small synthetic chelators, limiting radioisotope complexation and purification efficiencies. Here, we demonstrate a straightforward strategy to purify medically relevant radiometals, actinium(III) and yttrium(III), and probe their chemistry, using the recently discovered protein, lanmodulin. The stoichiometry, solution behavior, and formation constant of the ²²⁸Ac³⁺-lanmodulin complex and its ⁹⁰Y³⁺/^natY³⁺/^natLa³⁺ analogs were experimentally determined, representing the first actinium-protein and strongest actinide(III)-protein complex (sub-picomolar K_d) to be characterized. Lanmodulin’s unparalleled properties enable the facile purification recovery of radiometals, even in the presence of >10⁺¹⁰ equivalents of competing ions and at ultratrace levels: down to 2 femtograms ⁹⁰Y³⁺ and 40 attograms ²²⁸Ac³⁺. The lanmodulin-based approach charts a new course to study elusive isotopes and develop versatile chelating platforms for medical radiometals, both for high-value separations and potential in vivo applications.

Mechanism of thorium-nitrate and thorium-dioxide induced cytotoxicity in normal human lung epithelial cells (WI26): Role of oxidative stress, HSPs and DNA damage

Inhalation represents the most prevalent route of exposure with Thorium-232 compounds (Th-nitrate/Th-dioxide)/Th-containing dust in real occupational scenario. The present study investigated the mechanism of Th response in normal human alveolar epithelial cells (WI26), exposed to Th-nitrate or colloidal Th-dioxide (1-100 μg/ml, 24-72 h). Assessment in terms of changes in cell morphology, cell proliferation (cell count), plasma membrane integrity (lactate dehydrogenase leakage) and mitochondrial metabolic activity (MTT reduction) showed that Th-dioxide was quantitatively more deleterious than Th-nitrate to WI26 cells. TEM and immunofluorescence analysis suggested that Th-dioxide followed a clathrin/caveolin-mediated endocytosis, however, membrane perforation/non-endocytosis seemed to be the mode of Th internalization in cells exposed to Th-nitrate. Th-estimation by ICP-MS showed significantly higher uptake of Th in cells treated with Th-dioxide than with Th-nitrate at a given concentration. Both Th-dioxide and nitrate were found to increase the level of reactive oxygen species, which seemed to be responsible for lipid peroxidation, alteration in mitochondrial membrane potential and DNA-damage. Amongst HSPs, the protein levels of HSP70 and HSP90 were affected differentially by Th-nitrate/dioxide. Specific inhibitors of ATM (KU55933) or HSP90 (17AAG) were found to increase the Th- cytotoxicity suggesting prosurvival role of these signaling molecules in rescuing the cells from Th-toxicity.

Efficient discrimination of transplutonium actinides by in vivo models

Transplutonium actinides are among the heaviest elements whose macroscale chemical properties can be experimentally tested. Being scarce and hazardous, their chemistry is rather unexplored, and they have traditionally been considered a rather homogeneous group, with most of their characteristics extrapolated from lanthanide surrogates. Newly emerged applications for these elements, combined with their persistent presence in nuclear waste, however, call for a better understanding of their behavior in complex living systems. In this work, we explored the biodistribution and excretion profiles of four transplutonium actinides (248Cm, 249Bk, 249Cf and 253Es) in a small animal model, and evaluated their in vivo sequestration and decorporation by two therapeutic chelators, diethylenetriamine pentaacetic acid and 3,4,3-LI(1,2-HOPO). Notably, the organ deposition patterns of those transplutonium actinides were element-dependent, particularly in the liver and skeleton, where lower atomic number radionuclides showed up to 7-fold larger liver/skeleton accumulation ratios. Nevertheless, the metal content in multiple organs was significantly decreased for all tested actinides, particularly in the liver, after administering the therapeutic agent 3,4,3-LI(1,2-HOPO) post-contamination. Lastly, the systematic comparison of the radionuclide biodistributions showed discernibly element-dependent organ depositions, which may provide insights into design rules for new bio-inspired chelating systems with high sequestration and separation performance.

Exploring Serum Transferrin Regulation of Nonferric Metal Therapeutic Function and Toxicity

Serum transferrin (sTf) plays a pivotal role in regulating iron biodistribution and homeostasis within the body. The molecular details of sTf Fe(III) binding blood transport, and cellular delivery through transferrin receptor-mediated endocytosis are generally well-understood. Emerging interest exists in exploring sTf complexation of nonferric metals as it facilitates the therapeutic potential and toxicity of several of them. This review explores recent X-ray structural and physiologically relevant metal speciation studies to understand how sTf partakes in the bioactivity of key non-redox active hard Lewis acidic metals. It challenges preconceived notions of sTf structure function correlations that were based exclusively on the Fe(III) model by revealing distinct coordination modalities that nonferric metal ions can adopt and different modes of binding to metal-free and Fe(III)-bound sTf that can directly influence how they enter into cells and, ultimately, how they may impact human health. This knowledge informs on biomedical strategies to engineer sTf as a delivery vehicle for metal-based diagnostic and therapeutic agents in the cancer field. It is the intention of this work to open new avenues for characterizing the functionality and medical utility of nonferric-bound sTf and to expand the significance of this protein in the context of bioinorganic chemistry.

Determining Metal Ion Complexation Kinetics with Fluorescent Ligands by Using Fluorescence Correlation Spectroscopy

Fluorescence correlation spectroscopy (FCS) has been extensively used to measure equilibrium binding constants (K) or association and dissociation rates in many reversible chemical reactions across chemistry and biology. For the majority of investigated reactions, the binding constant was on the order of ∼100 M^-1 , with dissociation constants faster or equal to 10³  s^-1 , which ensured that enough association/dissociation events occur during the typical diffusion-determined transition time of molecules through the FCS detection volume. However, complexation reactions involving metal ions and chelating ligands exhibit equilibrium constants exceeding 10⁴  M^-1 . In the present paper, we explore the applicability of FCS for measuring reaction rates of such complexation reactions, and apply it to binding of iron, europium and uranyl ions to a fluorescent chelating ligand, calcein. For this purpose, we exploit the fact that the ligand fluorescence becomes strongly quenched after binding a metal ion, which results in strong intensity fluctuations that lead to a partial correlation decay in FCS. We also present measurements for the strongly radioactive ions of ²⁴¹ Am³⁺ , where the extreme sensitivity of FCS allows us to work with sample concentrations and volumes that exhibit close to negligible radioactivity levels. A general discussion of the applicability of FCS to the investigation of metal-ligand binding reactions concludes our paper.

Transferrin Cycle and Clinical Roles of Citrate and Ascorbate in Improved Iron Metabolism

Fe(III) delivery from blood plasma to cells via the transferrin (Tf) cycle was studied intensively due to its crucial role in Fe homeostasis. Tf-cycle disruptions are linked to anemia, infections, immunodeficiency, and neurodegeneration. Biolayer interferometry (BLI) enabled direct kinetic and thermodynamic measurements for all Tf-cycle steps in a single in vitro experiment using Tf within blood serum or released into the medium by cultured liver cells. In these media, known Tf cycle features were reproduced, and unprecedented insights were gained into conditions of rapid endosomal (pH 5.6) Fe(III) release from the Tf-Tf receptor 1 (TfR1) adduct. This release occurred via synergistic citrate and ascorbate effects, which pointed to respective roles as the likely elusive Fe chelator and reductant within the Tf cycle. These results explain enhanced cellular Fe uptake by ascorbate, the clinical efficacy of anemia treatment with Fe citrate and ascorbate, and dietary effects associated with loss of Fe homeostasis, including the large health burden of infections and neurodegeneration.

Evaluating the potential of chelation therapy to prevent and treat gadolinium deposition from MRI contrast agents

Several MRI contrast agent clinical formulations are now known to leave deposits of the heavy metal gadolinium in the brain, bones, and other organs of patients. This persistent biological accumulation of gadolinium has been recently recognized as a deleterious outcome in patients administered Gd-based contrast agents (GBCAs) for MRI, prompting the European Medicines Agency to recommend discontinuing the use of over half of the GBCAs currently approved for clinical applications. To address this problem, we find that the orally-available metal decorporation agent 3,4,3-LI(1,2-HOPO) demonstrates superior efficacy at chelating and removing Gd from the body compared to diethylenetriaminepentaacetic acid, a ligand commonly used in the United States in the GBCA Gadopentetate (Magnevist). Using the radiotracer ¹⁵³Gd to obtain precise biodistribution data, the results herein, supported by speciation simulations, suggest that the prophylactic or post-hoc therapeutic use of 3,4,3-LI(1,2-HOPO) may provide a means to mitigate Gd retention in patients requiring contrast-enhanced MRI.

Are clinical findings of systemic titanium dispersion following implantation explained by available in vitro evidence? An evidence-based analysis

Although the presence of titanium wear particles released into tissues is known to induce local inflammation following the therapeutic implantation of titanium devices into humans, the role that titanium ions play in adverse tissue responses has received little attention. Support that ongoing titanium ion release occurs is evidenced by the presence of ionic titanium bound to transferrin in blood, and ongoing excretion in the urine of patients with titanium devices. However, as reports documenting the presence of titanium within tissues do not distinguish between particulate and ionic forms due to technical challenges, the degree to which ionic titanium is released into tissues is unknown. To determine the potential for titanium ion release into tissues, this study evaluates available in vitro evidence relating to the release of ionic titanium under physiological conditions. This is a systematic literature review of studies reporting titanium ion release into solutions from titanium devices under conditions replicating the interstitial pH and constituents. Inclusion and exclusion criteria were defined. Of 452 articles identified, titanium ions were reported in nine media relevant to human biology in seventeen studies. Only one study, using human serum replicated both physiological pH and the concentration of constituents while reporting the presence of titanium ions. While there is insufficient information to explain the factors that contribute to the presence of titanium ions in serum of humans implanted with titanium devices, currently available information suggests that areas of future inquiry include the role of transferrin and organic acids.

Interaction of europium and curium with alpha-amylase

Batch sorption experiments, potentiometric and spectroscopic titration investigations revealed a fast and strong interaction of Eu(iii) and Cm(iii) with the digestive enzyme α-amylase.

Siderocalin-mediated recognition, sensitization, and cellular uptake of actinides

Synthetic radionuclides, such as the transuranic actinides plutonium, americium, and curium, present severe health threats as contaminants, and understanding the scope of the biochemical interactions involved in actinide transport is instrumental in managing human contamination. Here we show that siderocalin, a mammalian siderophore-binding protein from the lipocalin family, specifically binds lanthanide and actinide complexes through molecular recognition of the ligands chelating the metal ions. Using crystallography, we structurally characterized the resulting siderocalin-transuranic actinide complexes, providing unprecedented insights into the biological coordination of heavy radioelements. In controlled in vitro assays, we found that intracellular plutonium uptake can occur through siderocalin-mediated endocytosis. We also demonstrated that siderocalin can act as a synergistic antenna to sensitize the luminescence of trivalent lanthanide and actinide ions in ternary protein-ligand complexes, dramatically increasing the brightness and efficiency of intramolecular energy transfer processes that give rise to metal luminescence. Our results identify siderocalin as a potential player in the biological trafficking of f elements, but through a secondary ligand-based metal sequestration mechanism. Beyond elucidating contamination pathways, this work is a starting point for the design of two-stage biomimetic platforms for photoluminescence, separation, and transport applications.

The amount of injected 177Lu-octreotate strongly influences biodistribution and dosimetry in C57BL/6N mice

BACKGROUND

(177)Lu-octreotate therapy has proven to give favorable results after treatment of patients with neuroendocrine tumors. Much focus has been on the binding and uptake of (177)Lu-octreotate in tumor tissue, but biodistribution properties in normal tissues is still not fully understood, and the effect of receptor saturation may be important. The aim of this study was to investigate the influence of the amount of (177)Lu-octreotate on the biodistribution of (177)Lu-octreotate in normal tissues in mice.

MATERIAL AND METHODS

C57BL/6N female mice were intravenously injected with 0.1-150 MBq (177)Lu-octreotate (0.039 μg peptide/MBq). The mice were killed 0.25 h to 14 days after injection by cardiac puncture under anesthesia. Activity concentration was determined in blood, bone marrow, kidneys, liver, lungs, pancreas, and spleen, and mean absorbed doses were calculated.

RESULTS

The activity concentration varied with time and amount of injected activity. At 4-8 h after injection, a local maximum in activity concentration was found for liver, lungs, pancreas, and spleen. With the exception for the lower injected activities (0.1-1 MBq), the overall highest uptake was found in the kidneys (%IA/g). Large variations were found and the activity concentration in kidneys was 11-23%IA/g at 4 h, and 0.22-1.9%IA/g at 7 days after injection. Furthermore, a clear reduction in activity concentration with increased injected activity was observed for lungs, pancreas and spleen.

CONCLUSION

The activity concentration in all tissues investigated was strongly influenced by the amount of (177)Lu-octreotate injected. Large differences in mean absorbed dose per unit injected activity were found between low (0.1-1 MBq, 0.0039-0.039 μg) and moderate amounts (5-45 MBq, 0.2-1.8 μg). Furthermore, the results clearly showed the need for better ways to estimate absorbed dose to bone marrow other than methods based on a single blood sample analysis. Since the absorbed dose to critical organs will limit the amount of (177)Lu-octreotate administered, these findings must be taken into consideration when optimizing this type of therapy.

“Anion clamp” allows flexible protein to impose coordination geometry on metal ions

X-ray crystal structures of human serum transferrin (77 kDa) with Yb^III or Fe^III bound to the C-lobe and malonate as the synergistic anion show that the large Yb^III ion causes the expansion of the metal binding pocket while octahedral metal coordination geometry is preserved, an unusual geometry for a lanthanide ion.