Nanocrystalline zirconia: A novel sorbent for the preparation of 188W/188Re generator

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Abstract

Nanocrystalline zirconia, a novel high capacity sorbent material was synthesized and tested for its utility in the preparation of 188W/188Re generators. The structural investigation of the material was carried out using X-ray diffraction, surface area determination, FTIR and TEM micrograph analysis. Various experimental parameters were optimized to separate 188Re from 188W. The capacity of the material was found to be ∼325 mg W/g at the optimum pH. A chromatographic 188W/188Re generator was developed using this material from which >80% of 188Re generated could be eluted with 0.9% saline solution, with high radionuclidic, radiochemical and chemical purity and appreciably high radioactive concentration suitable for radiopharmaceutical applications.

Introduction

Rhenium-188 (188Re) is an attractive therapeutic radionuclide due to its reasonable half life (16.9 h), high-energy beta radiation (Eβ max=2.118 MeV), low abundance (15.8%) of 155 keV photons and convenient availability in the form of 188W/188Re generator in high specific activity. Being a congener of 99mTc, the chemistry of Re is similar to Tc, which is an additional advantage for working with molecules that have shown promising results as 99mTc-radiopharmaceuticals. The low energy (155 keV) gamma emission enables imaging for monitoring in-vivo localization and dosimetric calculations (Jeong and Knapp, 2008; Knapp, 1998). The attractive physical properties of 188Re and its production from a long-lived parent Tungsten-188 (188W, T1/2=69.2 d), from a generator with an adequate shelf-life, makes it an interesting option for clinical use. Unfortunately, 188W can be produced only by double neutron capture with low absorption cross-sections [186W(n,γ)187W (σ=37.9±0.6 b); 187W(n,γ)188W (σ=64±10 b)]. Further, owing to rather long half-life of 188W, relatively long irradiation periods are required even for the production of 188W of modest specific activity (Knapp et al., 1994a, Knapp et al., 1994b). However, 188W, from the high flux reactors (φ∼1015 n cm−2 s−1) can be used to make 188W/188Re generators to obtain ‘no carrier added’ 188Re. Most of the commercially available 188W/188Re generators are akin to the 99Mo/99mTc generators using alumina columns, where tungsten is retained on the alumina column and 188Re is eluted with 0.9% NaCl solution (Perego et al., 2007; Dadachova et al., 1995; Kamioki et al., 1994; Knapp et al., 1994; Knapp and Mirzadeh, 1994; Coursey et al., 1990; Callahan et al., 1989). However due to the low specific activity of 188W, the 188Re obtained from 188W/188Re generators is dilute with low radioactivity concentration. Consequently, it often requires an additional concentration step (Sarkar et al., 2009; Mansur et al., 2006; Jackel et al., 2005; Mushtaq, 2004; Guhlke et al., 2000; Tanase et al., 1997) for clinical applications.

Although preparation of 188W/188Re generators using alumina sorbent remains the main commercial procedure, alternative pathways such as 188W/188Re gel generators based on matrices such as zirconium or titanium tungstate (Dadachov et al., 1994, Dadachov et al., 2002; Dadachov and Lambrecht, 1995) have also been exploited. In the last few years, a number of alternative sorbents with higher capacity for W such as gel metal oxide composite, hydroxyapatite and polymeric zirconium compound (PZC), have been developed and exploited for the preparation of 188W/188Re generators (Monroy-Guzman et al., 2009; Iller et al., 2007, Iller et al., 2007; Matsuoka et al., 2005). Recently, our group (Chakravarty et al., 2009, Chakravarty et al., 2009) has demonstrated the feasibility of developing an electrochemical 188W/188Re generator. However, these recent techniques have yet to reach the commercial stage for clinical applications. Development of alternative sorbents for 188W/188Re generators still remains an interesting challenge. Thus, in the search for a novel high capacity sorbent for use in 188W/188Re generators, we carried out the work reported here.

Nanoparticles are expected to provide unprecedented opportunities to develop and hold the promise of creating new materials for chromatographic applications. One of the specific properties of nanomaterials is that a high percent of the atoms reside on the surface. These surface atoms are unsaturated, exhibit intrinsic surface reactivity and have a tendency to chemisorb charged species in aqueous solution in order to achieve surface stabilization. The potential of such nanomaterials as new generation of sorbents in the chromatographic separation of metal ions (Cumbal et al., 2003; Okuyama and Lenggoro, 2003; Sarkar et al., 2003; Vassileva and Furuta, 2001; Manning et al., 1998; Hiraide et al., 1997; Ragai and Selim, 1987) have been exploited. However, researches about the preparation of radionuclidic generators using nanoparticles are seldom reported. Keeping parity with the modern trends and to tap the potential of nanomaterials as a sorbent in the relatively unexplored field of radionuclide generators, we earlier synthesized and characterized a noble sorbent, polymer embedded nanocrystalline titania (TiP) for its possible application in the preparation of 99Mo/99mTc (Chakravarty et al., 2008) and 188W/188Re (Chakravarty et al., 2009, Chakravarty et al., 2009) generators. In this communication, we report the synthesis and characterization of nanocrystalline zirconia and its utilization as a column material in the preparation of 188W/188Re generator.

Section snippets

Chemicals

Reagents such as hydrochloric acid, ammonium hydroxide, etc. were of analytical grade and were procured from SD Fine Chemicals, Mumbai or BDH (India). Zirconyl chloride (ZrOCl2·8H2O) and isopropyl alcohol (A.R grade) were obtained from E. Merck, Mumbai, India. Stannous chloride and DMSA were obtained from Sigma Chemical Company, USA. Na2H2HEDP was obtained from Sigma (St. Louis, MO). Paper chromatography strips were purchased from M/s. Whatman, UK. Flexible silica plates (coating thickness 0.25 

Results

The goal of the present study was to evaluate the merits of nanocrystalline zirconia sorbent in the preparation of 188W/188Re generator system. In order to demonstrate the reliability of this new sorbent and to validate the parameters for sorption of 188W and separation of 188Re, we have optimized various process parameters.

Discussion

The overall objective of the present work is to demonstrate the potential utility of nanocrystalline zirconia as an alternative sorbent, in the preparation of 188W/188Re generator to obtain clinical grade 188Re. The synthesis process reported here is inexpensive, simple, carried out at ambient conditions and amenable for large-scale production. All precursors of the synthesis process are commercially available. The material obtained is suitable for chromatographic applications. Initially, most

Conclusions

This investigation demonstrates that nanocrystalline zirconia is a promising sorbent with significant ion-exchange capacity for 188W and could be an attractive option for the preparation of 188W/188Re generators. The 188W/188Re generator prepared using this nanocrytalline zirconia sorbent provides carrier-free 188Re (as 188ReO4-) on elution with 0.9% NaCl solution, which is physiologically compatible. The sorbent was stable and the performance of the generator remained consistent over a period

Acknowledgements

The authors are grateful to Dr. V. Venugopal, Director, Radiochemistry and Isotope Group, Bhabha Atomic Research Centre for his support to this program. The authors also acknowledge Dr. V.K. Manchanda, Head, Radiochemistry Division and Dr. (Ms.) S. Ray, Head, Uranium Extraction Division of this centre for providing their facilities for the determination of zeta potential and ICP analyses respectively.

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