[Biological activity of photoexcited fullerene]

Abstract

Fullerene (C60, C70, etc.) is a third carbon allotrope discovered in 1985, and a great deal of attention has been focused on its physical and chemical properties in recent years. We are very interested in its biological properties for use fullerene as a pharmacophore. We first developed a method of solubilizing fullerene itself in water to perform in vitro biological screening. The concentrations of aqueous C60 and C70 solution with 5% poly(vinylpyrorridone) (PVP) are 400 and 200 micrograms/mL, respectively. By using aqueous fullerene solutions prepared in this manner, we have clarified a series of biological activities of fullerene, consisting of DNA-cleavage, hemolysis, cancer-initiation, and cell-toxicity under photoirradiation, and chondrogenesis and inhibition of glutathione S-transferase activity without photoirradiation. The biological activity of photo-excited fullerene was found to be promising, because fullerene is a highly efficient photo-sensitizer. We synthesized a C60 derivative with an acridine moiety as a DNA-chelating function and assessed its effective DNA-cleaving activity. What kind of active species is involved in the biological action of photo-excited fullerene is our next concerns. Two pathways have been reported for the photo-excitation of fullerene. The so-called Type II energy transfer pathway generates singlet oxygen (1O2), while the Type I electron transfer pathway gives a fullerene radical anion (C60.-, C70.-). In order to clarify the effective oxygen species actually responsible for the biological action of photo-excited fullerene, we performed DNA-cleaving tests and EPR spectroscopic analyses under several conditions. The results showed that the photo-induced biological activity of fullerene is not caused by 1O2, but by reduced oxygen species (O2.-, .OH) generated by the electron transfer reaction of C60.-, with molecular oxygen. Its specificity is thought to be mainly attributed to the high-reducible property of fullerene. Since the reductive activation of molecular oxygen by photo-excited fullerene was observed at physiological concentrations of NADH as the reductant, fullerene can be classified as an oxyl-radical-generating photosensitizer. Pharmaceutical application of fullerene to cancer photo-dynamic therapy appears promising.