Self-organized nanogel from pullulan/pheophorbide-A conjugate as a macromolecular photodynamic agent

Abstract

The potential of pullulan(PL)/pheophorbide-A(phA) nanogel (PL/phA) was evaluated as a macromolecular photosensitizer (PS) for photodynamic therapy (PDT). Two samples with different degrees of substitution of phA (PL/phA1 and 2) were synthesized and analyzed by 1H NMR. The mean diameters of self-organized nanogels from PL/phA1 and 2 in aqueous solution were approximately 100 and 160 nm with mono-size distribution, respectively. The changes in nanogel photoactivity such as fluorescence intensity and singlet oxygen (1O2) production were observed in the solvents, dimethyl sulfoxide (DMSO) and phosphate buffered saline (PBS). Although, their photoactivity in DMSO, which cannot form self-organized nanogels, was similar to that of free phA, activities were suppressed in the aqueous solution due to self-quenching between phAs similar to a fluorescence resonance energy transfer (FRET) effect. Moreover, as the nanogels were co-incubated with HeLa cells, the self-quenching effect gradually disappeared as a function of time. Based upon confocal microscopy and cytotoxicity results, we infer that the nanogels were internalized in cancer cells by endocytosis and then degraded by various enzymes in the endosome and lysosome, leading to the restoration of photoactivity. Therefore, the self-organized PL/phA nanogels may be recommended for development of new PDT with minimal unfavorable phototoxic affects.