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 (C(60)) 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 C(60) on the photodynamic antitumor activity of C(60) 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, C(60) 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 C(60) (terminal type). Irrespective of the pullulan molecular and the modification manner, the C(60)-pullulan conjugates exhibited a similar ability to generate superoxide anions upon light irradiation. Comparing the C(60)-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 C(60).