Singlet oxygen production by amphiphilic C60 derivatives and its correlation to cell cytotoxicity in vitro

Abstract

Fullerene derivatives have appealing properties that can potentially be used in materials science and medical applications. In particular, fullerenes are known to produce reactive oxygen species upon their excitation with light. This makes them particularly attractive as photosensitizers for photodynamic therapy (PDT). Photodynamic therapy is a new modality of treatment of cancer as well as some non-cancerous conditions. It involves the combined actions of a drug (photosensitizer) and light to produce a cytotoxic effect. Water-soluble hexa(sulfo- n -butyl)[60]fullerenes (FC 4 S) was reported recently to generate singlet oxygen ( 1 O 2 ) and superoxide radical (O 2 - ·) upon its excitation with light, making it a promising candidate for PDT treatments. Recently, we synthesized new amphiphilic fullerene derivatives, namely, [60]fullerene-diphenylaminofluorene-oligo(ethylene glycol) conjugates, C 60 (>DPAF-PEG600) and C 60 (>DPAF-PEG2000), as potential photosensitizers. In this paper we compare FC 4 S to PEG-based fullerenes in terms of their singlet oxygen photosensitization ability. We measured time-resolved kinetics of singlet oxygen luminescence photosensitized by excitation of fullerenes via a 10 ns pulsed laser at 523 nm. For FC 4 S we observed normal kinetics with a monoexponential decay profile giving a time constant 3.8 us in water. In contrast, for the case of C 60 (>DPAF-PEG600) and C 60 (>DPAF-PEG2000), a non-monoexponential decay profile with a long tail (~ 10 2 μs) in water was observed. We hypothesize that this is due to formation of vesicles by PEG fullerenes in aqueous solution. To investigate photodynamic activity of these fullerene derivatives in vitro , we used HeLa human adenocarcinoma and B16 mouse melanoma cell lines. FC 4 S showed clear photodynamic effects in both cell lines. The total fluence required to kill 50% of the cells at the drug concentration of 20 μM was 36 Jcm -2 for HeLa cells and 72 Jcm -2 for B16 cells. Neither PEG-based fullerene derivatives showed any appreciable photodynamic activity, possibly, due to low efficiency of singlet oxygen generation.