Chelating Polymers for Targeted Decontamination of Actinides: Application of PEI-MP to Hydroxyapatite-Th(IV)

In case of an incident in the nuclear industry or an act of war or terrorism, the dissemination of plutonium could contaminate the environment and, hence, humans. Human contamination mainly occurs via inhalation and/or wounding (and, less likely, ingestion). In such cases, plutonium, if soluble, reaches circulation, whereas the poorly soluble fraction (such as small colloids) is trapped in alveolar macrophages or remains at the site of wounding. Once in the blood, the plutonium is delivered to the liver and/or to the bone, particularly into its mineral part, mostly composed of hydroxyapatite. Countermeasures against plutonium exist and consist of intravenous injections or inhalation of diethylenetetraminepentaacetate salts. Their effectiveness is, however, mainly confined to the circulating soluble forms of plutonium. Furthermore, the short bioavailability of diethylenetetraminepentaacetate results in its rapid elimination. To overcome these limitations and to provide a complementary approach to this common therapy, we developed polymeric analogs to indirectly target the problematic retention sites. We present herein a first study regarding the decontamination abilities of polyethyleneimine methylcarboxylate (structural diethylenetetraminepentaacetate polymer analog) and polyethyleneimine methylphosphonate (phosphonate polymeric analog) directed against Th(IV), used here as a Pu(IV) surrogate, which was incorporated into hydroxyapatite used as a bone model. Our results suggest that polyethylenimine methylphosphonate could be a good candidate for powerful bone decontamination action.

Selective electrochemical capture and release of uranyl from aqueous alkali, lanthanide, and actinide mixtures using redox-switchable carboranes

We report the selective electrochemical biphasic capture of the uranyl cation (UO₂²⁺) from mixed-metal alkali (Cs⁺), lanthanide (Nd³⁺, Sm³⁺), and actinide (Th⁴⁺, UO₂²⁺) aqueous solutions to an organic, 1,2-dichloroethane (DCE), phase using the ortho-substituted nido-carborane anion, [1,2-(Ph₂PO)₂-1,2-C₂B₁₀H₁₀]²⁻ (^POCb²⁻). The reduced ^POCb²⁻ is generated by electrochemical reduction of the closo-carborane, ^POCb, prior to mixing with the aqueous mixed-metal solution. Subsequent UO₂²⁺ release from the captured product, [UO₂(^POCb)₂]²⁻, was performed by galvanostatic bulk electrolysis of the DCE phase and back-extraction of UO₂²⁺ to a fresh aqueous phase. The selective capture and release of UO₂²⁺ was confirmed by combined ICP-OES and NMR spectral analyses of the aqueous and organic phases, respectively, against the newly synthesized nido-carborane complexes, [[CoCp*₂][Cs(^POCb)]]₂, [CoCp*₂]₃[Nd(^POCb)₃], [CoCp*₂]₃[Sm(^POCb)₃], and [CoCp*₂]₂[Th(^POCb)₃].

Redox-switchable carboranes electrochemically capture and release UO₂²⁺ selectively from mixed metal aqueous solutions, mimicking in part spent nuclear fuel.

Structural and Thermodynamics Studies on Polyaminophosphonate Ligands for Uranyl Decorporation

The development of actinide decorporation agents with high complexation affinity, high tissue specificity, and low biological toxicity is of vital importance for the sustained and healthy development of nuclear energy. After accidental actinide intake, sequestration by chelation therapy to reduce acute damage is considered as the most effective method. In this work, a series of bis- and tetra-phosphonated pyridine ligands have been designed, synthesized, and characterized for uranyl (UO₂²⁺) decorporation. Owing to the absorption of the ligand and the luminescence of the uranyl ion, UV-vis spectroscopy and time-resolved laser-induced fluorescence spectroscopy (TRLFS) were used to probe in situ complexation and structure variation of the complexes formed by the ligands with uranyl. Density functional theory (DFT) calculations and X-ray absorption fine structure (XAFS) spectroscopy on uranyl-ligand complexes revealed the coordination geometry around the uranyl center at pH 3 and 7.4. High affinity constants (log K ∼17) toward the uranyl ion were determined by displacement titration. A preliminary in vitro chelation study proves that bis-phosphonated pyridine ligands can remove uranium from calmodulin (CaM) at a low dose and in the short term, which supports further uranyl decorporation applications of these ligands.

Polyethyleneimine methylenecarboxylate: a macromolecular DTPA analogue to chelate plutonium(iv)

Up until now, molecular chelating agents, such as diethylenetriamine pentaacetic acid (DTPA), have been the standard method for actinide human decorporation. Mainly active in blood serum, their distribution within the body is thus limited. To treat a wider range of organs affected by plutonium contamination, a potential new class of macromolecular decorporation agents is being studied. Polyethyleneimine methylenecarboxylate (PEI-MC) is one such example. It is being considered here because of its capacity for targeting the liver and bones.

Towards the development of chitosan nanoparticles for plutonium pulmonary decorporation

Since the 1940s, great amounts of Plutonium (Pu) have been produced for both military and civil purposes. Until now, the standard therapy for decorporation following inhalation has been the intravenous injection of diethylenetriaminepentaacetic acid ligand (Ca-DTPA form). This method offers a strong complexing constant for Pu(iv) but has poor chemical specificity, therefore its efficacy is limited to actinides present in the blood. Consequently, there is no decorporation treatment currently available which efficiently removes the intracellular Pu(iv) trapped in the pulmonary macrophages. Our research shows that a nanoparticle approach could be of particular interest due to large contact area and ability to target the retention compartments of the lungs. In this study, we have focused on the inhalation process involving forms of Pu(iv) with poor solubility. We explored the design of biocompatible nanoparticles able to target the macrophages in the lung alveoli and to chelate the forms of Pu(iv) with poor solubility. Nanoparticle formation was achieved through an ionic cross-linking concept using a polycationic polymer and an anionic chelate linker. We chose N-trimethyl chitosan, for its biocompatibility, as the polycationic polymer base of the nanoparticle and the phosphonic analogue of DTPA, diethylenetriamine-pentamethylenephosphonic acid (DTPMP) as the anionic chelating linker in forming NPs TMC-DTPMP. The synthesis and physico-chemical characterization of these NPs are presented. Secondly, the complexation mechanisms of TMC-DTPMP NPs with Thorium (Th(iv)) are discussed in terms of efficiency and structure. The Extended X-Ray Absorption Fine Structure (EXAFS) of the TMC-DTPMP complex with Th(iv) as well as Pu(iv) are defined and completed with DFT calculations to further delineate the plutonium coordination sphere after complexation. Finally, preliminary cytotoxicity tests onto macrophages were assayed.