The use of calix[6]arene molecules for actinides analysis in urine and drinking water: an alternative to current procedures

Compared in vivo efficiency of nanoemulsions unloaded and loaded with calixarene and soapy water in the treatment of superficial wounds contaminated by uranium

No emergency decontamination treatment is currently available in the case of radiological skin contamination by uranium compounds. First responders in the workplace or during an industrial nuclear accident must be able to treat internal contamination through skin. For this purpose, a calixarene nanoemulsion was developed for the treatment of intact skin or superficial wounds contaminated by uranium, and the decontamination efficiency of this nanoemulsion was investigated in vitro and ex vivo. The present work addresses the in vivo decontamination efficiency of this nanoemulsion, using a rat model. This efficiency is compared to the radio-decontaminant soapy water currently used in France (Trait rouge^®) in the workplace. The results showed that both calixarene-loaded nanoemulsion and non-loaded nanoemulsion allowed a significant decontamination efficiency compared to the treatment with soapy water. Early application of the nanoemulsions on contaminated excoriated rat skin allowed decreasing the uranium content by around 85% in femurs, 95% in kidneys and 93% in urines. For skin wounded by microneedles, mimicking wounds by microstings, nanoemulsions allowed approximately a 94% decrease in the uranium retention in kidneys. However, specific chelation of uranium by calixarene molecules within the nanoemulsion was not statistically significant, probably because of the limited calixarene-to-uranium molar ratio in these experiment conditions. Moreover, these studies showed that the soapy water treatment potentiates the transcutaneous passage of uranium, thus making it bioavailable, in particular when the skin is superficially wounded.

Novel drug delivery systems for actinides (uranium and plutonium) decontamination agents

The possibility of accidents in the nuclear industry or of nuclear terrorist attacks makes the development of new decontamination strategies crucial. Among radionuclides, actinides such as uranium and plutonium and their different isotopes are considered as the most dangerous contaminants, plutonium displaying mostly a radiological toxicity whereas uranium exhibits mainly a chemical toxicity. Contamination occurs through ingestion, skin or lung exposure with subsequent absorption and distribution of the radionuclides to different tissues where they induce damaging effects. Different chelating agents have been synthesized but their efficacy is limited by their low tissue specificity and high toxicity. For these reasons, several groups have developed smart delivery systems to increase the local concentration of the chelating agent or to improve its biodistribution. The aim of this review is to highlight these strategies.

Impact of the uranium (VI) speciation in mineralised urines on its extraction by calix[6]arene bearing hydroxamic groups used in chromatography columns

Actinides determination in urine samples is part of the analyses performed to monitor internal contamination in case of an accident or a terrorist attack involving nuclear matter. Mineralisation is the first step of any of these analyses. It aims at reducing the sample volume and at destroying all organic compounds present. The mineralisation protocol is usually based on a wet ashing step, followed by actinides co-precipitation and a furnace ashing step, before redissolution and the quantification of the actinides by the appropriate techniques. Amongst the existing methods to perform the actinides co-precipitation, alkali-earth (typically calcium) precipitation is widely used. In the present work, the extraction of uranium(VI), plutonium(IV) and americium(III) from the redissolution solutions (called "mineralised urines") on calix[6]arene columns bearing hydroxamic groups was investigated as such an extraction is a necessary step before their determination by ICP-MS or alpha spectrometry. Difficulties were encountered in the transfer of uranium(VI) from raw to mineralised urines, with yield of transfer ranging between 0% and 85%, compared to about 90% for Pu and Am, depending on the starting raw urines. To understand the origin of such a difficulty, the speciation of uranium (VI) in mineralised urines was investigated by computer simulation using the MEDUSA software and the associated HYDRA database, compiled with recently published data. These calculations showed that the presence of phosphates in the "mineralised urines" leads to the formation of strong uranyl-phosphate complexes (such as UO2HPO4) which compete with the uranium (VI) extraction by the calix[6]arene bearing hydroxamic groups. The extraction constant of uranium (VI) by calix[6]arene bearing hydroxamic groups was determined in a 0.04 mol L(-1) sodium nitrate solution (logK=4.86±0.03) and implemented in an extraction model taking into account the speciation in the aqueous phase. This model allowed to simulate satisfactorily the experimental uranium extraction data and to support the preliminary conclusions about the role of the phosphates present in mineralised urines. These calculations also showed that the phosphate/calcium ratio is a key parameter as far as the efficiency of the uranium (VI) extraction by the calix[6]arene columns is concerned. It predicted that the addition of CaCl2 in mineralised urines would release uranium (VI) from phosphates by forming calcium (II)-phosphate complexes and thus facilitate the uranium (VI) extraction on calix[6]arene columns. These predictions were confirmed experimentally as the addition of 0.1 mol L(-1) CaCl2 to a mineralised urine containing naturally a high concentration of phosphate (typically 0.04 mol L(-1)) significantly increased the percentage of uranium (VI) extraction on the calix[6]arene columns.

Calixarene cleansing formulation for uranium skin contamination

An oil-in-water cleansing emulsion containing calixarene molecule, an actinide specific chelating agent, was formulated in order to improve the decontamination of uranium from the skin. Commonly commercialized cosmetic ingredients such as surfactants, mineral oil, or viscosifying agents were used in preparing the calixarene emulsion. The formulation was characterized in terms of size and apparent viscosity measurements and then was tested for its ability to limit uranyl ion permeation through excoriated pig-ear skin explants in 24-h penetration studies. Calixarene emulsion effectiveness was compared with two other reference treatments consisting of DTPA and EHBP solutions. Application of calixarene emulsion induced the highest decontamination effect with an 87% decrease in uranium diffusion flux. By contrast, EHBP and DTPA solutions only allowed a 50% and 55% reduction of uranium permeation, respectively, and had the same effect as a simple dilution of the contamination by pure water. Uranium diffusion decrease was attributed to uranyl ion-specific chelation by calixarene within the formulation, since no significant effect was obtained after application of the same emulsion without calixarene. Thus, calixarene cleansing emulsion could be considered as a promising treatment in case of accidental contamination of the skin by highly diffusible uranium compounds.

Texturing formulations for uranium skin decontamination

CONTEXT

Since no specific treatment exists in case of cutaneous contamination by radionuclides such as uranium, a nanoemulsion comprising calixarene molecules, known for their good chelation properties, was previously designed. However, this fluid topical form may be not suitable for optimal application on the skin or wounds.

OBJECTIVE

To develop a texturing pharmaceutical form for the treatment of wounded skins contaminated by uranium.

MATERIALS AND METHODS

The formulations consisted in oil-in-water (O/W) nanoemulsions, loaded with calixarene molecules. The external phase of the initial liquid nanoemulsion was modified with a combination of thermosensitive gelifying polymers: Poloxamer and HydroxyPropylMethylcellulose (HPMC) or methylcellulose (MC). These new formulations were characterized then tested by ex vivo experiments on Franz cells to prevent uranyl ions diffusion through excoriated pig ear skin explants.

RESULTS

Despite strong changes in rheological properties, the physico-chemical characteristics of the new nanoemulsions, such as the size and the zeta potential as well as macroscopic aspect were preserved. In addition, on wounded skin, diffusion of uranyl ions, measured by ICP-MS, was limited to less than 5% for both HPMC and MC nanoemulsions.

CONCLUSIONS

These results demonstrated that a hybrid formulation of nanoemulsion in hydrogel is efficient to treat uranium skin contamination.

The extraction of thorium by calix[6]arene columns for urine analysis

Thorium is a natural alpha-emitting element occurring in various ores and has numerous industrial applications. Routine monitoring of potentially exposed workers is generally achieved through radiobioassay (urine and faeces). The procedures currently used for analysing actinides such as thorium in urine require lengthy chemical separation associated with long counting times by alpha-spectrometry due to low activity levels. Thus, their main drawback is that they are time-consuming, which limits the frequency and flexibility of individual monitoring. In this context, this study developed new radiochemical procedures based on the use of tertbutylcalix[6]arenes bearing three carboxylic acid groups or three hydroxamic acid groups. These previous works demonstrated that these macrocyclic molecules immobilised on an inert solid support are excellent extractants for uranium, plutonium and americium. In this study, the authors investigated the thorium extraction by calix[6]arene columns. Experiments were performed on synthetic solutions and on real urine samples. The influence of various parameters, such as the thorium solution pH and the column flow rate on thorium extraction, was studied. The results showed that both calix[6]arenes are efficient to extract thorium. Thorium extraction is quantitative from pH = 2 for synthetic solution and from pH = 3 for real urine samples. This study has demonstrated that the column flow rate is a crucial parameter since its value must not be too high to achieve the steady-state complexation equilibrium. Finally, these results will be compared with those obtained for other actinides (U, Pu and Am) and the conditions of actinides' separation will be discussed.