Reactive species responsible for biological actions of photoexcited fullerenes

Abstract

Fullerene (C60, C70, etc.) is an effective photosensitizer and its utilization as a pharmacophore for photo-chemotherapy of tumors has received considerable attention. We developed a method to solubilize fullerenes into water-with polyvinylpyrrolidone (PVP) as a detergent. By using thus prepared aqueous fullerene solutions, we have clarified a series of biological activities of fullerene under photoirradiation which include DNA-cleavage, hemolysis, mutagenicity, cancer-initiation, and cell-toxicity. A newly synthesized C60 derivative with an acridine moiety as a DNA-chelating function showed much more effective DNA-cleaving activity in the presence of NADH. Visible-light irradiation of PVP-solubilized C60 in water in the presence of NADH as a reductant and molecular oxygen resulted in the formation of O2·-, which was detected by the EPR spin-trapping method. Formation of O2·- was also evidenced by the direct observation of a characteristic signal of O2·- by the use of a low-temperature EPR technique at 77 K. On the other hand, no formation of 1O2 was observed by the use of TEMP as a 1O2 trapping agent. No near-IR luminescence of 1O2 was also observed in the aqueous C60/PVP/O2 system. These results suggest that photoinduced bioactivities of the PVP-solubilized fullerene are caused not by 1O2, but by reduced oxygen species (O2·-, ·OH) which are generated by the electron-transfer reaction of C60·- with molecular oxygen.