Chelating decorporation agents for internal contamination by actinides: Designs, mechanisms, and advances

During the wide utilization of the actinides in medicine, energy, military, and other fields, internal contaminations can profoundly endanger human health and public security. Chelating decorporation agents are the most effective therapies to reduce internal contamination that includes radiological and chemical toxicities. This review introduces the structures of chelating decorporation agents including inorganic salts, polyaminocarboxylic acids, peptides, polyphosphonates, siderophores, calixarenes, polyethylenimines, and fullerenes, and highlights ongoing advances in their designs and mechanisms. However, there are still numerous challenges that block their applications including coordination properties, pharmacokinetic properties, oral bioavailability, limited timing of administration, and toxicity. Therefore, additional efforts are needed to push novel decorporation agents with high efficiency and low toxicity for the treatment of internal contamination by actinides.

Structural and thermodynamic stability of uranyl-deferiprone complexes and the removal efficacy of U(vi) at the cellular level

Deferiprone (3-hydroxy-1,2-dimethyl-4(1H)-pyridone, DFP), which is a drug clinically used for removing heavy metals in vivo, was explored for its removal efficiency towards uranium. The reaction of uranyl nitrate hexahydrate with DFP at room temperature yielded the compound [(UO2)(H2O)(C7NO2H8)2]·4H2O (1), which crystallizes from a mixed solution of methanol and water (pH = 7.0). X-ray diffraction shows that the stable complexation of uranyl occurs from the coordination of two bidentate DFP ligands perpendicular to the O[double bond, length as m-dash]U[double bond, length as m-dash]O unit with a fifth coordinating oxygen atom coming from one water molecule, resulting in a pentagonal bipyramidal geometry. The formation constants of uranyl and DFP complexes were measured and the species distribution diagram illustrates that UO2L2 (94.6%) is the dominant uranyl-DFP complex in 0.1 M KCl solution at physiological pH = 7.4. The results from both crystallographic and potentiometric studies imply that the metal : ligand ratio is 1 : 2. The effectiveness of using DFP to remove uranium was examined at the cellular level, and the results suggest that it can significantly reduce the cellular uptake and increase the cellular release of U(vi) in renal proximal tubular epithelial cells (NRK-52E).

Chelation therapy for treatment of systemic intoxication with uranium: A review

Elevated levels of naturally occurring uranium have been found in small geographic areas throughout the world. Exposure of the general public to uranium is most often by the ingestion of food and water containing natural uranium from the hydrogeological environment, but this likelihood is remote. However, the risk is increased in regions where uranium is mined, milled, processed and/or fabricated as well as in the vicinity of former battlefields where depleted uranium munitions were deployed. Exposure in such cases is by the inhalation route. Internalized uranium is a long-term hazard the toxicity of which depends upon the dose and the dose rate as well as other parameters such as the chemical form and site of deposition of the uranium and the physiology of the host. The radiological toxicity and the chemical toxicity of uranium and its compounds are responsible for kidney damage and lung cancer. The vulnerable groups are the very young and the very old, individuals predisposed to hypertension or osteoporosis and individuals with chronic kidney disease. Those subject to long-term exposure from internalized uranium are a greater risk for the long-term implications. The accumulation of uranium may be mitigated by decreasing its absorption, distribution and deposition and increasing its elimination with chelating agents. The formation of soluble chelates may enhance the mobilization of uranium deposited in tissue and expedite its transport to and elimination from the renal system. The focus of this review is on the use of chelating agents to enhance decorporation of uranium thereby reducing the risk of intoxication.

Extraction of lanthanides using 1-hydroxy-6-N-octylcarboxamido-2(1H)-pyridinone as an extractant via competitive ligand complexations between aqueous and organic phases

Increased lanthanide extraction by octyl-HOPO improves the TALSPEAK process and allowed for a greater separation of lanthanides from actinides.

Solution thermodynamic evaluation of hydroxypyridinonate chelators 3,4,3-LI(1,2-HOPO) and 5-LIO(Me-3,2-HOPO) for UO2(VI) and Th(IV) decorporation

Solution thermodynamic studies were performed to characterize the coordination of U(VI) and Th(IV) by the multidentate hydroxypyridinonate chelating agents 3,4,3-LI(1,2-HOPO) and 5-LIO(Me-3,2-HOPO). Species distribution and conditional stability pM profiles were calculated based on the determined formation constants, showing that both ligands form extremely stable uranyl and thorium complexes in solution. At physiological pH, the formation of 1:1 ligand:uranyl complexes is favored for both ligands, while a 2:1 ligand:thorium complex is predominant with the tetradentate 5-LIO(Me-3,2-HOPO). Comparisons with functionally similar bidentate ligands as well as with the common actinide chelator diethylenetriamine pentaacetic acid emphasized the superior affinity for U(VI) and Th(IV) of both experimental compounds over a wide pH range. These analytical results corroborate the in vivo chelation efficacy of 3,4,3-LI(1,2-HOPO) and 5-LIO(Me-3,2-HOPO) and validate their selection for further development as therapeutic actinide decorporation agents.

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.

Steric effects on uranyl complexation: synthetic, structural, and theoretical studies of carbamoyl pyrazole compounds of the uranyl(VI) ion

New bifunctional pyrazole based ligands of the type [C(3)HR(2)N(2)CONR'] (where R = H or CH(3); R' = CH(3), C(2)H(5), or (i)C(3)H(7)) were prepared and characterized. The coordination chemistry of these ligands with uranyl nitrate and uranyl bis(dibenzoyl methanate) was studied with infrared (IR), (1)H NMR, electrospray-mass spectrometry (ES-MS), elemental analysis, and single crystal X-ray diffraction methods. The structure of compound [UO(2)(NO(3))(2)(C(3)H(3)N(2)CON{C(2)H(5)}(2))] (2) shows that the uranium(VI) ion is surrounded by one nitrogen atom and seven oxygen atoms in a hexagonal bipyramidal geometry with the ligand acting as a bidentate chelating ligand and bonds through both the carbamoyl oxygen and pyrazolyl nitrogen atoms. In the structure of [UO(2)(NO(3))(2)(H(2)O)(2)(C(5)H(7)N(2)CON {C(2)H(5)}(2))(2)], (5) the pyrazole ligand acts as a second sphere ligand and hydrogen bonds to the water molecules through carbamoyl oxygen and pyrazolyl nitrogen atoms. The structure of [UO(2)(DBM)(2)C(3)H(3)N(2)CON{C(2)H(5)}(2)] (8) (where DBM = C(6)H(5)COCHCOC(6)H(5)) shows that the pyrazole ligand acts as a monodentate ligand and bonds through the carbamoyl oxygen to the uranyl group. The ES-MS spectra of 2 and 8 show that the ligand is similarly bonded to the metal ion in solution. Ab initio quantum chemical studies show that the steric effect plays the key role in complexation behavior.