Photodynamic Efficacy and Spectroscopic Properties of 5-ALA Induced Protoporphyrin IX in Human Glioblastoma Cells

One of the most promising clinical applications of 5-aminolevulinic acid (5-ALA) induced protoporphyrin IX (PP IX) is the fluorescence diagnosis and photodynamic treatment of malignant brain tumours. In order to obtain a deeper understanding of the cellular processes involved, U373-MG human glioblastoma cells were used as model system to study intracellular location, fluorescence properties and light-induced reactions of the photosensitizer. In comparison with ovarian cancer or breast cancer cell lines, glioblastoma cells accumulated PP IX only to a moderate extent, but were most effectively inactivated (highest photodynamic efficacy per photosensitizer molecule). In contrast to breast cancer cells, which showed a rather granular PP IX fluorescence pattern, PP IX fluorescence in U373-MG cells seemed to originate mainly from cellular membranes. In addition, photobleaching and lowering of fluorescence lifetimes during irradiation were comparably small for this cell line. Combining the results of fluorescence lifetime imaging microscopy (FLIM) and photobleaching measurements, we deduced that those cellular sites where PP IX fluorescence was most pronounced, contributed rather little to its photodynamic efficacy.