Relation between intracellular location and photodynamic efficacy of 5-aminolevulinic acid-induced protoporphyrin IX in vitro. Comparison between human glioblastoma cells and other cancer cell lines.

A promising clinical application of 5-aminolevulinic acid (5-ALA)-induced protoporphyrin IX (PP IX) is fluorescence detection and photodynamic treatment of residual tumour tissue during surgical resection of high grade malignant glioma. U373 MG human glioblastoma cells were used as a model system to study the relation between intracellular location and photodynamic efficacy of 5-ALA-induced PP IX in more detail. Therefore, ultra-sensitive fluorescence microscopy, using either optical excitation of whole cells or selective excitation of the plasma membrane by an evanescent electromagnetic field, was combined with quantitative measurements of intracellular porphyrin amount and phototoxicity. Glioblastoma cells accumulated PP IX to a moderate extent as compared to T47D breast cancer cells (high accumulation) or OV2774 ovarian cancer cells (low accumulation). Although photodynamic inactivation of the different cell lines (decreasing in the order T47D > U373 MG > OV2774) seemed to be directly related to PP IX accumulation, examination of the data in more detail revealed that photodynamic efficacy per photosensitizer molecule (PE) was about two times higher in glioblastoma and ovarian cancer cells as compared to breast cancer cells. The different photodynamic efficacy of PP IX was related to the different intracellular location. In contrast to breast cancer cells where PP IX fluorescence was localized in small granules, PP IX fluorescence in glioblastoma cells and ovarian cancer cells originated mainly from cellular membranes. Thus, the intracellular location of PP IX in a predominantly lipophilic environment, characterized by a comparably high photostability (probed by photobleaching and photoproduct formation) and a lower degree of porphyrin aggregation (probed previously by fluorescence decay kinetics), seems to be the key factor for high photodynamic efficacy of 5-ALA-induced PP IX. In the case of OV2774 ovarian cancer cells, however, a low PP IX accumulation limited cell inactivation upon irradiation, whereas the results obtained for glioblastoma cells are encouraging to develop PDT to an additional therapeutic option for the treatment of tumour margins in patients who underwent fluorescence-guided resection of high grade malignant glioma after 5-ALA administration.