Effect of Modification Manner on the Photodynamic Antitumor Activity of C60 Modified with Pullulan

To design a novel cytospecific photosensitizer for photodynamic antitumor therapy, a fullerene (C₆₀) was chemically modified with pullulan, a water-soluble polysaccharide with a high affinity for asialoglycoprotein receptors (ASGPRs). The effect of the molecular weight of pullulan and the modification manner to C₆₀ on the photodynamic antitumor activity of C₆₀ modified with pullulan was evaluated. In this study, two modification manners were selected. First, ethylene diamine was chemically introduced to the hydroxyl groups of pullulan with different molecular weights. Then, C₆₀ was coupled to pullulan through the amino groups introduced (pendant type). Second, ethylene diamine was introduced to the terminal aldehyde groups of pullulan by a reductive amination reaction, and then the pullulan with the terminal amino groups was coupled to C₆₀ (terminal type). Irrespective of the pullulan molecular and the modification manner, the C₆₀–pullulan conjugates exhibited a similar ability to generate superoxide anions upon light irradiation. Comparing the C₆₀–pullulan conjugates of pendant and terminal types, a high lectin affinity was observed for the latter conjugates. The conjugates showed a high affinity for HepG2 cells with ASGPRs and, consequently, a strong in vitro antitumor activity on the cells. It is concluded that the manner of pullulan modification is a key factor contributing to the photodynamic antitumor activity of modified C₆₀.