Elsevier

Journal of Hazardous Materials

Volume 365, 5 March 2019, Pages 952-961
Journal of Hazardous Materials

Research Article
Thorium decorporation efficacy of rationally-selected biocompatible compounds with relevance to human application

https://doi.org/10.1016/j.jhazmat.2018.11.038Get rights and content

Highlights

  • Hemolysis assay identified Th decorporation potential of tiron, SLB, PA and L52.

  • ICP-MS validated their decorporation potential in human blood and liver cells.

  • TEM images of liver cells revealed the decorporation effect of these agents.

  • DFT and EXAFS studies provided evidence about their Th chelation properties.

Abstract

During civil, nuclear or defense activities, internal contamination of actinides in humans and mitigation of their toxic impacts are of serious concern. Considering the health hazards of thorium (Th) internalization, an attempt was made to examine the potential of ten rationally-selected compounds/formulations to decorporate Th ions from physiological systems. The Th-induced hemolysis assay with human erythrocytes revealed good potential of tiron, silibin (SLB), phytic acid (PA) and Liv.52® (L52) for Th decorporation, in comparison to diethylenetriaminepentaacetic acid, an FDA-approved decorporation drug. This was further validated by decorporation experiments with relevant human cell models (erythrocytes and liver cells) and biological fluid (blood) under pre-/post-treatment conditions, using inductively coupled plasma mass spectrometry (ICP-MS) and transmission electron microscopy (TEM). Furthermore, density functional theory-based calculations and extended X-ray absorption fine structure (EXAFS) spectroscopy confirmed the formation of Th complex by these agents. Amongst the chosen biocompatible agents, tiron, SLB, PA and L52 hold promise to enhance Th decorporation for human application.

Introduction

Despite good safety standards in nuclear industries, a risk of internalization of actinides in humans exists during handling of radioactive materials or accidents at nuclear facilities like Chernobyl and Fukushima Dai-ichi reactors [[1], [2], [3]]. In addition, nuclear war or terrorism also present serious concerns of internal contamination of actinides to general public. The radioactive actinides enter the body through inhalation, absorption through skin breaks/open wounds or by ingestion of contaminated water/food [4]. Studies on French CEA-AREWA centres (Alternative Energies and Atomic Energy Commission) between 1970–2003 revealed that both inhalation and wound routes are responsible for ∼81% of total internal contamination in humans [4]. Within few hours (1–3 h) of internalization, depending on the size (1–20 μm) and solubility/chemical form (nitrate, chloride, dioxide), actinides are distributed by the blood and deposited in the liver, bone and lungs (in case of inhalation), where they are retained by the tissues for several years (biological half-life ∼5 to 20 years) [5]. They cause both chemical as well as radiological toxicity due to their complexation with biomolecules and emission of high energy alpha-particles (4–5 Mev) [6]. Therefore, there is a need to develop efficient countermeasures for medical management of actinide contamination.

Among the actinides, uranium (U) has been extensively used for nuclear energy generation either due to the natural occurrence of its fissile form (U-235) or its potential to be converted into fissile fuel (U-238 to Pu-239) [3]. Internal contamination with U and plutonium (Pu) in humans have been reported under occupational, accidental or as a result of nuclear weapons test fallout [2,5,7]. In addition, the threat of nuclear terrorism involving Pu poses a significant risk of contamination to the general public [8]. In view of depletion of natural U-235, several countries including India have planned to develop thorium (Th)-based nuclear reactors due to several potential advantages such as i) large natural Th-232 reserves, ii) resistance for weapons use, and iii) significantly less generation of fission products and long-lived alpha-emitters in the nuclear waste [9,10]. The large-scale handling of Th-containing materials such as monazite [ThO2 ∼2.5–10%] may increase the risks of occupational and accidental exposures of Th to nuclear workers and general public in future [[11], [12], [13], [14], [15]]. Notably, the long-term inhalation of Th-containing dust was found to cause pneumoconiosis and lung carcinogenesis in Chinese miners (average Th in lung, 0.4 mg) as compared to unexposed workers [15].

Th-232, an alpha particle emitter (radiological half-life 14 × 109 years) may internalize in the form of nitrate or oxide during various steps of fuel cycle (mining, milling, extraction, fabrication and disposal) [5,16]. Under physiological conditions, Th is stable in + IV oxidation state, which forms strong complexes with biological ligands in blood/tissues [16]. Due to the closeness of charge-to-ionic-radii-ratio of Th (0.042) with iron (0.046), Th binds iron(Fe)-binding sites and mimics the mechanism of Fe-transport and Fe-storage/utilization in liver and bone [17]. Our recent study showed that the charge-to-ionic-radii-ratio of actinides plays an important role in determining their interactions with Fe-binding sites in hemoglobin (Hb) [18]. Our previous animal studies and other’s report on human data showed that following injection, Th mainly accumulates in liver, spleen and bone [19]. Studies on patients, receiving Thorotrast (a contrast agent used in 1930–1950) injections were found to have liver and bone cancers after ∼20 years [20]. Our animal studies showed that Th induces oxidative-stress-mediated toxicity in liver [19]. Together, these results suggested a clinical need for efficient decorporation of Th to significantly reduce chemical and radiation toxicities. For internal contamination, the FDA-approved diethylenetriaminepentaacetic acid (DTPA), which forms stable complex with Pu4+, americium (Am3+) and curium (Cm3+), is recommended to enhance the excretion of actinides [3,4]. However, it has several limitations: i) short retention time in blood plasma (T1/2 ∼1.5 h), ii) inability to reach actinide-depository organs and iii) needs intravenous administration. Also, its long-term use leads to calcium and magnesium deficiency, which causes neurotoxicity, nephrotoxicity, hematopoiesis suppression, etc [5]. Although, some national [21] and international guidelines [22] still recommend DTPA to treat contamination with Th, DTPA is reportedly ineffective for Th decorporation [23]. Therefore, there is a need to develop efficient, and less toxic Th-chelators, which can be used alone or in combination with DTPA.

In recent past, attempts have been made to improve decorporation efficacy of DTPA by either chemical or pharmacological modifications [4,24,25]. Alternatively, a new class of chelating agent i.e. siderophore analogue has also been synthesized and tested for actinide (mainly Pu) decorporation [26]. However, no efforts have been made to explore the Th-decorporation potential in rationally-selected natural or synthetic compounds, which could be efficiently evaluated for clinical application. In the present study, in order to explore more efficient and biocompatible (less toxic) agent(s) than DTPA, ten natural/synthetic compounds/formulation (Scheme 1) were selected on the basis of the following rationales: i) compounds with known Fe-binding or other metal-binding property viz. tiron, desferioxamine, d-penicillamine [27], as Th behaves similar to Fe(III) in vivo; or ii) phosphate-containing biomolecule, phytic acid (PA) as Th forms strong complexes with phosphate ligands [28]; or iii) compounds/formulations with known hepatoprotective action for human viz. Liv.52® (L52), silibin (SLB) [29,30] as Th preferentially accumulates in liver; or iv) compounds structurally similar to SLB viz. rutin, quercetin, myricetin and kaempferol [31]. Since, Th induces lysis of erythrocytes (hemolysis) [32], this biological end point was used to screen the relative efficacies of the selected compounds. The effective compounds were further assessed for their ability to decorporate Th from human erythrocytes, blood and liver cells, which were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) and transmission electron microscopy (TEM). Furthermore, density functional theory (DFT)-based calculations and extended X-ray absorption fine structure (EXAFS) spectroscopy were performed to investigate the complexation of Th with the most effective agents.

Section snippets

Materials

232Th-nitrate was obtained from the Radiochemistry Division of our Centre. DTPA (Cat. No. 6518), desferioxamine (D9533), d-penicillamine (P4875), tiron (172553), silibin (S0417), rutin (R5143), kaempferol (60010), quercetin (Q0125), myricetin (M6760), phytic acid (68388) were purchased from Sigma, MO, USA. Polyherbal hepatoprotective formulation, Liv.52® syrup (L52) was purchased from The Himalaya Drug Company, India.

Preparation of solutions

232Th-nitrate stock solutions (10–100 mM), prepared in 0.01 N HNO3 were

Screening of agents for Th decorporation using hemolysis assay

The IC50 values (i.e. concentration of test agent required to prevent hemolysis by 50%) of the anti-hemolysis activities of the test agents, under both pre- and post-treatment conditions were determined from the graphs shown in Fig. 1 and listed in Table 1. The results revealed that among the selected agents, tiron, SLB, L52 and PA, but not others (based on their IC50 values) were more effective than the FDA-approved actinide decorporation agent, DTPA, under the pre-treatment conditions (Table 1

Discussion

In the present study, screening of ten rationally-selected compounds/formulation using hemolysis assay has identified tiron, SLB, PA and L52 as potential physiological Th decorporating agents. Our previous work suggested that Th induces hemolysis by acting on surface sialic acid of human erythrocytes [32]. Therefore, this assay was applied to evaluate the relative Th-complexing abilities of test agents vis-à-vis that of DTPA. In the pre-treatment experiments (Fig. 1), the effectiveness of test

Conclusions

The present study screened the ten rationally-selected compounds/formulation, which has identified tiron, PA, SLB and L52, as potential biocompatible agents for Th decorporation. The Th-decorporation ability of these agents was validated using relevant human cell models and blood as compared to DTPA, a FDA-approved actinide decorporation drug. TEM studies provided the evidence about their Th decorporation ability in liver cells. Furthermore, EXAFS and DFT studies confirmed their Th complexation

Conflicts of interest

There are no conflict of interest.

Acknowledgements

BARC, Department of Atomic Energy, Government of India supported the research work. Authors thank Dr. Vinay Kumar, Head, RB&HSD, BARC and Dr. S. Chattopadhyay, Ex-Director, Biosciences Group, BARC for their intellectual support and guidance during the work. We acknowledge Dr. S. N. Jha, AMPD, BARC for his support in EXAFS at RRCAT, Indore and Smt. Sharda Sawant, ACTREC, Navi Mumbai for TEM. BS thanks Dr. R. K. Gopalakrishnan and Dr. K. S. Pradeepkumar, HSEG, BARC for their support.

References (48)

  • S. Schneider et al.

    Plutonium release from Fukushima Daiichi fosters the need for more detailed investigations

    Sci. Rep.

    (2013)
  • D. Williams

    Cancer after nuclear fallout: lessons from the Chernobyl accident

    Nat. Rev. Cancer

    (2002)
  • D. Albright et al.

    Plutonium and Highly Enriched Uranium 1996: World Inventories, Capabilities, and Policies

    (1997)
  • International Commission on Radiological Protection

    Individual Monitoring for Internal Exposure of Workers, ICRP Publication, 78

    (1997)
  • B.N. Pandey et al.

    Radiobiological basis in management of accidental radiation exposure

    Int. J. Radiat. Biol.

    (2010)
  • National Council on Radiation Protection and Measurements (NCRP)

    Population Monitoring and Radionuclide Decorporation Following a Radiological or Nuclear Accident

    (2010)
  • T. Schneider et al.

    Nuclear and radiological preparedness: the achievements of the european research project PREPARE

    Radiat. Prot. Dosimetry

    (2017)
  • P. Bagla

    Thorium seen as nuclear’s new frontier

    Science

    (2015)
  • H.S. Dang et al.

    Studies on intake and body fluid concentration of thorium for subjects working and living in thorium rich environments

    J. Radioanalyt. Nucl. Chem.

    (2000)
  • E.E. Zapadinskaia et al.

    Analysis of health state in individuals exposed to thorium and chemical hazards in occupational environment

    Med. Tr. Prom. Ekol.

    (2005)
  • B.A. Ulsh et al.

    Establishing bounding internal dose estimates for thorium activities at Rocky Flats

    Health Phys.

    (2008)
  • X.A. Chen et al.

    Health effects following long-term exposure to thorium dusts: a twenty-year follow-up study in China

    Radioprotection

    (2004)
  • DACTARI

    A Database for Chemical Toxicity and Radiotoxicity Assesement of Radionuclides

    (2018)
  • A.E.V. Gorden et al.

    Rational design of sequestering agents for plutonium and other actinides

    Chem. Rev.

    (2003)
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