9-7

Synthesis of Microcapsules for Medical Applications
-Implantation of RI Ions into Fullerene-



Fig. 9-15 Schematic of 133Xe ion implantation by ISOL

Radioactive 133Xe gas was placed in a Xe gas cylinder along with enriched stable 129Xe gas, which was used as a mass marker in mass separation. Xenon atoms supplied from the cylinder through a stainless steel tube were ionized in an ion source and accelerated to 40 keV. The 133Xe ions were mass-separated and implanted into a fullerene target.



Fig. 9-16 Schematic of the formation of endohedral 133Xe-fullerene by ion implantation

An accelerated 133Xe ion approaches a C60 fullerene molecule, expands a six-membered ring in the molecule, and penetrates into it. The last figure shows the case where the 133Xe is in the center of the molecule.



Fig. 9-17 Elution curves of C60 fullerene and 133Xe

After 133Xe ion implantation, the fullerene portion of the target was dissolved in ortho-dichlorobenzene. The resulting solution was injected into a Cosmosil 5PYE column from Nakalai Tesque. Elution was done with ortho-dichlorobenzene at a flow rate of 1 ml/min. A strong correlation between C60 and 133Xe corroborated the formation of endohedral 133Xe-fullerene.



Regarding fullerene species, one has C60 in the shape of a soccer ball, C70 (a rugby ball), and higher fullerenes such as C82, C84, etc. All of these fullerenes, which are hollow inside, can encapsulate one or more atoms in the interior space. These compounds are called endohedral fullerenes, and in particular the compounds encapsulating radioisotopes are called endohedral radioisotope-fullerenes.
The endohedral radioisotope-fullerenes offer the potential of becoming new radiopharmaceuticals. In order to realize this possibility, hydrophilic fullerene derivatives that are programmed to be transported only to desired tissues such as cancer cells have to be synthesized, in addition to the encapsulation of radioisotopes emitting beta- or gamma-rays suitable for therapy or diagnosis.
By using an Isotope Separator On-Line (ISOL), we have implanted 133Xe ions into fullerene targets made by vacuum evaporation of C60 or C70 onto Ni foils (Figs. 9-15 and 9-16). A high performance liquid chromatography (HPLC) analysis following dissolution of the fullerene targets in ortho-dichlorobenzene has corroborated the formation of endohedral 133Xe-fullerene, showing a strong correlation between C60 determined by ultraviolet spectrophotometry (UV) and 133Xe by gamma-ray spectrometry (Fig. 9-17). To our knowledge, the present work represents the first time that endohedral radioisotope-fullerene has been produced by ion implantation methods.
In order to produce new endohedral radioisotope-fullerenes, not only 133Xe but also other radioisotopes useful for therapy or diagnosis will have to be implanted into a fullerene host. From endohedral radioisotope-fullerenes so created, hydrophilic fullerene derivatives will be synthesized and applied to animal experiments in order to study their behavior in vivo.


Reference
S. Watanabe et al., Production of Endohedral 133Xe-Fullerene by Ion Implantation, J. Radioanal. Nucl. Chem., 255(3), 495, (2003).

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Persistent Quest Research Activities 2002
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