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.

Analysis methodology and development of a statistical tool for biodistribution data from internal contamination with actinides.

The aim of this work was to develop a computational tool that integrates several statistical analysis features for biodistribution data from internal contamination experiments. These data represent actinide levels in biological compartments as a function of time and are derived from activity measurements in tissues and excreta. These experiments aim at assessing the influence of different contamination conditions (e.g. intake route or radioelement) on the biological behavior of the contaminant. The ever increasing number of datasets and diversity of experimental conditions make the handling and analysis of biodistribution data difficult. This work sought to facilitate the statistical analysis of a large number of datasets and the comparison of results from diverse experimental conditions. Functional modules were developed using the open-source programming language R to facilitate specific operations: descriptive statistics, visual comparison, curve fitting, and implementation of biokinetic models. In addition, the structure of the datasets was harmonized using the same table format. Analysis outputs can be written in text files and updated data can be written in the consistent table format. Hence, a data repository is built progressively, which is essential for the optimal use of animal data. Graphical representations can be automatically generated and saved as image files. The resulting computational tool was applied using data derived from wound contamination experiments conducted under different conditions. In facilitating biodistribution data handling and statistical analyses, this computational tool ensures faster analyses and a better reproducibility compared with the use of multiple office software applications. Furthermore, re-analysis of archival data and comparison of data from different sources is made much easier. Hence this tool will help to understand better the influence of contamination characteristics on actinide biokinetics. Our approach can aid the optimization of treatment protocols and therefore contribute to the improvement of the medical response after internal contamination with actinides.

Study of the distribution of actinides in human tissues using synchrotron radiation micro X-ray fluorescence spectrometry.

This study aims at evaluating the capabilities of synchrotron radiation micro X-ray fluorescence spectrometry (SR micro-XRF) for qualitative and semi-quantitative elemental mapping of the distribution of actinides in human tissues originating from individuals with documented occupational exposure. The investigated lymph node tissues were provided by the United States Transuranium and Uranium Registries (USTUR) and were analyzed following appropriate sample pre-treatment. Semi-quantitative results were obtained via calibration by external standards and demonstrated that the uranium concentration level in the detected actinide hot spots reaches more than 100 µg/g. For the plutonium hot spots, concentration levels up to 31 µg/g were found. As illustrated by this case study on these unique samples, SR micro-XRF has a high potential for this type of elemental bio-imaging owing to its high sensitivity, high spatial resolution, and non-destructive character.

Actinide chelation: biodistribution and in vivo complex stability of the targeted metal ions.

Because of the continuing use of nuclear fuel sources and heightened threats of nuclear weapon use, the amount of produced and released radionuclides is increasing daily, as is the risk of larger human exposure to fission product actinides. A rodent model was used to follow the in vivo distribution of representative actinides, administered as free metal ions or complexed with chelating agents including diethylenetriamine pentaacetic acid (DTPA) and the hydroxypyridinonate ligands 3,4,3-LI(1,2-HOPO) and 5-LIO(Me-3,2-HOPO). Different metabolic pathways for the different metal ions were evidenced, resulting in intricate ligand- and metal-dependent decorporation mechanisms. While the three studied chelators are known for their unrivaled actinide decorporation efficiency, the corresponding metal complexes may undergo in vivo decomposition and release metal ions in various biological pools. This study sets the basis to further explore the metabolism and in vivo coordination properties of internalized actinides for the future development of viable therapeutic chelating agents.

Bone as target organ for metals: the case of f-elements.

The skeleton is a target organ for most metals. This leads to their bioaccumulation, either as storage of useful oligoelements or as a protection against damage by toxic elements. The different events leading to their accumulation in this organ, under constant remodeling, are not fully understood, nor the full subsequent impact on bone metabolism. This lack of knowledge is particularly true for lanthanides and actinides, whose use has been increasing over recent decades. These metals, known as f-elements, present chemical similarities and differences. After a comparison of the biologically relevant physicochemical properties of lanthanides and actinides, and a brief reminder of the main events of bone metabolism, this review considers the results published over the past decade regarding the interaction between bones and f-elements. Emphasis will be given to the molecular events, which constitute the basis of the most recent toxicological studies in this domain but still need further investigation. Ionic exchanges with the inorganic matrix, interactions with bone proteins, and cellular mechanism disturbances are mainly considered in this review.

MEDOR, a didactic tool to support interpretation of bioassay data after internal contamination by actinides.

A didactic software, MEthodes DOsimètriques de REférence (MEDOR), is being developed to provide help in the interpretation of biological data. Its main purpose is to evaluate the pertinence of the application of different models. This paper describes its first version that is focused on inhalation exposure to actinide aerosols. With this tool, sensitivity analysis on different parameters of the ICRP models can be easily done for aerosol deposition, in terms of activity and particle number, actinide biokinetics and doses. The user can analyse different inhalation cases showing either that dose per unit intake cannot be applied if the aerosol contains a low number of particles or that an inhibition of the late pulmonary clearance by particle transport can occur which contributes to a 3-4 fold increase in effective dose as compared with application of default parameters. This underlines the need to estimate systematically the number of deposited particles, as well as to do chest monitoring as long as possible.

Limits of DPUI application associated with the number of particles within actinide aerosols.

Dose per unit intake (DPUI) of radionuclides is obtained using International Commission on Radiological Protection (ICRP) models. After inhalation exposure, the first model calculates the fraction of activity deposited within the different regions of the respiratory tract, assuming that the aerosol contains an infinite number of particles. Using default parameters for workers, an exposure to one annual limit of intake (ALI) corresponds to an aerosol of 239PuO2 containing approximately 1 x 10(6) particles. To reach such an exposure, very low particle number might be involved especially for compounds having a high specific activity. This study provides examples of exposures to actinide aerosols for which the number of particles is too low for a standard application of the ICRP model. These examples, which involve physical studies of aerosols collected at the workplace and interpretation of bioassay data, show that the number of particles of the aerosol can be the main limit for the application of DPUI after inhalation exposure.

Uncertainties in estimation of intakes of actinides for dose reconstruction cases.

Intakes and doses arising from exposure to actinides must be reconstructed from historical bioassay data for the purposes of worker compensation and for epidemiology studies. The usual default assumption is that a series of urine activities is the result of a constant chronic intake. In reality, the urine activities will most likely arise from a random sequence of discrete intakes. In order to investigate the accuracy of the constant chronic assumption, we have created virtual urine datasets using Monte Carlo modelling and these were used as input to the code IMBA(1). Comparisons of estimated intakes with those used as input allow the uncertainties in the procedure to be estimated. The effects of incorrect assumptions about the scattering factors, activity median aerodynamic diameter (AMAD) and solubility can also be examined. The results show that the constant chronic assumptions leads to remarkably reliable estimates of intake, even for datasets generated by just a few intakes per year. The estimate of intake is fairly robust against mis-assignment of scattering factor and AMAD. However, as is well-known, the correct assignment of solubility is crucial in obtaining reliable estimates of intake and dose.

Treatment of actinide exposures: a review of Ca-DTPA injections inside CEA-COGEMA plants.

Calcium diethylenetriamine pentacetate (Ca-DTPA) has been used for medical treatment of plutonium and americium contaminations in the CEA and COGEMA plants from 1970 to 2003. This paper is a survey of the injections Ca-DTPA administered as a chelating molecule and it will be a part of the authorisation process for Ca-DTPA by intravenous administration. Out of 1158 injections administered to 469 persons, 548 events of possible or confirmed contamination were reported. These employees were followed by occupational physicians according to the current French regulations. These incidents took place at work, were most often minor, not requiring follow-up treatment. The authors present (1) a synthesis of the most recent findings. Due to its short biological half-time and its limited action in the blood, Ca-DTPA does not chelate with plutonium and americium as soon as these elements are deposited in the target organs. It justifies an early treatment, even in cases of suspected contamination followed by additional injections if necessary (2) data concerning these 1158 injections (route of contamination, dosage, adverse effects, etc.) The authors also investigated a study on the efficacy of the product on a group of persons having received five or more injections. These results were compared with the efficacy estimated theoretically. Dosages and therapeutic schemes were proposed based on these observations. This synthesis is the result of a collective work having mobilised the occupational medicine departments, the medical laboratories inside a working group CEA-COGEMA-SPRA.

Evaluation of uncertainties in lung measurement of actinides due to non-uniform distribution of activity in lungs.

Various parameters can introduce uncertainties in the lung activity measurements of actinides. In this study, uncertainties due to non-uniform distribution of activity in the lungs are evaluated. To study the effect of non-uniform distribution, lungs of ICRP male thorax voxel and resized phantoms are divided into upper and lower parts of both right and left lungs as well as into anterior and posterior lung regions. Simulation of uniform and non-uniform distribution of activity in lungs is carried out using thorax voxel phantoms in FLUKA for Phoswich and an array of three HPGe detectors for 18-238keV photons. Source sampling for non-uniform distribution of activity is carried out by selecting the source points by varying the weightage to 0.4, 0.5, 0.6 and 1 in different parts of lungs. Uncertainties in lung activity estimation at different energies are quantified in the form of scattering factors (SFs) which are geometric standard deviations. The SFs due to non-uniform distribution of activity of the order of 0.4-0.6 in different parts of the lungs are found to be ~ 1.25 for Phoswich and HPGe array detectors above 18keV.

Skin absorption of actinides: influence of solvents or chelates on skin penetration ex vivo.

PURPOSE: To evaluate skin penetration and retention of americium (Am) and plutonium (Pu), in different chemical forms relevant to the nuclear industry and to treatment by chelation. MATERIALS AND METHODS: Percutaneous penetration of different Am and Pu forms were evaluated using viable pig skin with the Franz cell diffusion system. The behavior of the complex Pu-tributyl phosphate (Pu-TBP), Am or Pu complexed to the chelator Diethylene triamine pentaacetic acid (DTPA) and the effect of dimethyl sulfoxide (DMSO) was assessed. Radioactivity was measured in skin and receiver compartments. Three approaches were used to visualize activity in skin including the recent iQID technique for quantification. RESULTS: Transfer of Am was 24-fold greater than Pu and Pu-TBP complex penetration was enhanced by 500-fold. Actinide-DTPA transfer was greater than the Am or Pu alone (17-fold and 148-fold, respectively). The stratum corneum retained the majority of activity in all cases and both DMSO and TBP enhanced skin retention of Am and Pu, respectively. Histological and bioimaging data confirmed these results and the iQID camera allowed the quantification of skin activity. CONCLUSIONS: Skin penetration and fixation profiles are different depending on the chemical actinide form. Altered behavior of Pu-TBP and actinide-DTPA complexes reinforces the need to address decontamination protocols.

[The parameters of clearance of industrially significant alpha-emitters from the lungs of mammals].

Results of assessment of biokinetic parameters of change in the burden of significant alpha-emitters in the lungs of mammals in various times after inhalation intake (Qt(lung)) were generalized. 1740 Wistar rats of both sex with the initial age of 2-2.5 months and 143 mature mongrel dogs used in 23 and 3 animal tests, respectively, were involved in this work. The analysis of experimental data resulted in selection of three groups of chemically soluble compounds of alpha-emitters that differ in the rate of radionuclide clearance from the lung as well as in integral doses. Stable complex compounds of quadrivalent and of hexavalent nuclides and non-complex salts of quinquivalent and of hexavalent 237Np were assigned to the group of soluble compounds of 239Pu and 237Np. A three-component exponential model of change in Qt(lung) with the prevalence of fast and of intermediate phases (55%, T(eff) = 0.41 days and 35%, T(eff) = 18.1 days respectively) and the presence of a slow clearance phase (10%, T(eff) = 206 days) was developed for these compounds. Complex compounds of quadrivalent 239Pu and 237Np unstable in the environment of pH and of body temperature, their non-complex salts of mineral acids in ionic or polymer form, and submicron plutonium dioxide (SMD = 0.07 mkm) were assigned to the group of relatively soluble compounds includes. An exponential model with 2-3 components with the prevalence of intermediate and of slow clearance phases (71%, T(eff) = 19.3 days and 22%, T(eff) = 169 days respectively) was developed for compounds of this group. The third group of the compounds is presented based on the soluble 241Am compounds that could be typical for stable trivalent compounds of rare-earth and transuranium radionuclides. Their biokinetics is described by a 3-4-component exponential model with the fast phase prevailing (96.7%, T(eff) < or = 6.8 days), and with intermediate (2.6%, T(eff) = 69 days) and with slow (0.7%, T(eff) = 1040 days) phases being negligible. Physical chemicas and biological processes determining nuclides biokinetics in lungs are discussed.

Monte Carlo modelling of a voxel head phantom for in vivo measurement of bone-seeker nuclides.

Whole-body counters (WBCs) are used for the assessment of the internal contamination of actinides in the human body. WBCs require adequate calibration procedures that rely on the use of suitable calibration phantoms. A previous study carried out at the ENEA-Radiation Protection Institute was aimed at designing a head calibration phantom in which a heterogeneous distribution of 241Am point sources could satisfactorily approximate an assumed homogeneous contamination throughout the head bones. Suitable correction factors for the WBC detection efficiencies were evaluated with Monte Carlo. The present paper summarises the main aspects and implications of an advanced modelling technique based on a VOXEL approach. The methodology could be extended to other bone-seeker radionuclides.

Assessment of uncertainties in the lung activity measurement of low-energy photon emitters using Monte Carlo simulation of ICRP male thorax voxel phantom.

Assessment of intake due to long-lived actinides by inhalation pathway is carried out by lung monitoring of the radiation workers inside totally shielded steel room using sensitive detection systems such as Phoswich and an array of HPGe detectors. In this paper, uncertainties in the lung activity estimation due to positional errors, chest wall thickness (CWT) and detector background variation are evaluated. First, calibration factors (CFs) of Phoswich and an array of three HPGe detectors are estimated by incorporating ICRP male thorax voxel phantom and detectors in Monte Carlo code 'FLUKA'. CFs are estimated for the uniform source distribution in lungs of the phantom for various photon energies. The variation in the CFs for positional errors of ±0.5, 1 and 1.5 cm in horizontal and vertical direction along the chest are studied. The positional errors are also evaluated by resizing the voxel phantom. Combined uncertainties are estimated at different energies using the uncertainties due to CWT, detector positioning, detector background variation of an uncontaminated adult person and counting statistics in the form of scattering factors (SFs). SFs are found to decrease with increase in energy. With HPGe array, highest SF of 1.84 is found at 18 keV. It reduces to 1.36 at 238 keV.

Distribution of transuranic elements in bone.

The transport, retention, and excretion of transuranic elements from the body have been widely studied for many years. A summary of the results is given with an emphasis on the distribution of these elements in bone. Implications of these studies for understanding the relationships between lead in blood and lead in bone are presented. The expected distribution of lead at various bone sites is also considered.

Cellular aspects of retention and transport of inhaled soluble and insoluble actinide compounds in the rat lung.

The binding of 241Am-hydroxide polymers (as models for readily soluble actinide compounds) to the cell components of rat lung was investigated using differential centrifugation, density gradient centrifugation, gel chromatography, carrier-free electrophoresis and electron microscopic autoradiography (with 241Pu). Irregularly shaped and spherical mixed (U, Pu)O2 particles (as models for insoluble actinide compounds) were administered to rats by inhalation and intratracheal installation and the lung and organ retention was determined. Electron microscopic studies were performed with rat lung and with rat and bovine alveolar macrophages exposed to the actinide compounds in vitro. In the case of the mixed (U, Pu)O2 particles the lung retention was independent of the particle shape and route of administration. Whereas 241Am administered as a hydroxide polymer was transferred rapidly from lung to skeleton and liver, only a few percent of the initial alveolar deposit was found in these organs after mixed oxide inhalation. It is concluded that all types of particles are stored primarily within phagolysosomes of alveolar macrophages. In the case of readily soluble compounds, the actinides are solubilized within these lysosomes and become bound to cytosolic ferritin in the alveolar macrophages. They are then released from the macrophages and probably cross the alveolar membranes as transferrin or as low-molecular-weight forms. Insoluble compounds remain within the lysosomes of alveolar macrophages, but there are indications of chemical damage to the lysosomal membranes, causing the particles to lie free in the cytoplasm.

Factors affecting the placental transfer of actinides.

The primary goal of this paper is to consider factors that affect the availability and transport of actinides from maternal blood, through the placenta, to the conceptus. These factors, of particular importance in scaling results from animals to man, include the route and temporal pattern of administration, the mass and physicochemical state of material administered, metabolism of the pregnant animal and fetal organs or tissue, and species-specific changes in placental structure relative to stage of gestation at exposure. Preliminary concepts for descriptive and kinetic models are proposed to integrate these results, to identify additional information required for developing more comprehensive models, and to provide a basis for scaling to human pregnancies for purposes of radiation dosimetry.

The use of animal experiments for assessing annual limits on intake and interpreting chest-monitoring data for workers exposed to industrial actinide-bearing dusts.

The metabolic behavior of 239Pu and 241Am present in three industrial dusts has been studied after their inhalation by the rat. A comparative experiment has also been carried out with a mixture of these actinides, inhaled as their nitrates. The aim of this work was to provide an experimental basis for assessing limits on intake and to establish whether the 239Pu content in the lungs could be interpolated from measurements of 241Am. The results (1) demonstrate the wide differences in the lung retention kinetics of the actinides and in the absolute and relative amounts which translocate to the blood that can occur for industrially produced materials; (2) show that the annual limits on intake (ALI) for the different materials vary between those postulated for class W and Y compounds by the International Commission on Radiological Protection; (3) indicate that, depending on the nature of the dust, acute intakes of 239Pu equivalent to the ALI can be estimated from 241Am chest-monitoring data at times from a few days up to about 3 y after exposure.

Implications of human tissue studies for radiation protection.

Through radiochemical analysis of voluntary tissue donations, the U.S. Transuranium and Uranium Registries (USTR) are gaining improved understanding of the distribution and biokinetics of actinide elements in occupationally exposed persons. Evaluation of the first two whole-body contributions to the USTR revealed an inverse proportionality between actinide concentration and bone ash. The analysis of a whole body with significant 241Am deposition indicated a significantly shorter half-time in liver and a greater fraction resident in the skeleton than predicted by existing models. Other studies with tissues obtained at autopsy suggest that existing biokinetic models for 238Pu and 241Am and the currently accepted models and limits on intake, which use these models as their basis, may be inaccurately implying that revisions of existing safety standards may be necessary. Other studies of the registries are designed to evaluate in-vivo estimates of actinide deposition with those derived from postmortem tissue analysis, to compare results of animal experiments with human data, and to review histopathologic slides for tissue changes that might be attributable to exposure to transuranic elements. The implications of these recent findings and other work of the registries is discussed from the standpoint of this potential effect on biokinetic modeling, internal dose assessment, and safety standards and operational health physics practices.

Biokinetics and absorption of actinides in human volunteers: a review.

Experiments that have been described in the literature are reviewed, with emphasis on the techniques that have been used, rather than a critical appraisal of the results obtained.