Mechanism of Photofrin-enhanced Ultrasound-induced Human Glioma Cell Death

Background: Low-intensity ultrasound showed tumor cell killing by a non-thermal effect in human leukemia cells. The aim of our study was to investigate the efficacy of low-intensity ultrasound on malignant astrocytic tumor cells with the photosensitizer, Photofrin, which is taken up by the cell surface receptor, low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor (LRP/alpha 2MR). Materials and Methods: Cells were sonicated with continuous wave ultrasound with or without the presence of Photo in (75 mg/ml) at an intensity of 0.3 W/cm(2) for a duration of 5, 15, or 30 s. Results: Ultrasound alone induced instant cell killing immediately after sonication in both U251MG and U105MG malignant gliomas cells. In U251MG cells, which expressed LRP/alpha 2MR, significant enhancement of cell killing was observed following Photofrin pretreatment, 52.7 +/- 17.5%, 13.0 +/- 4.6% and 3.9 +/- 0.9% for 5, 15, and 30 s respectively (p<0.05). This enhancement of cell killing was abolished by preincubarion with receptor-associated protein (RAP) which binds specifically to LRP/alpha 2MR. This enhancement by Photofrin was not achieved in U105MG which did not express LRP/alpha 2MR. U251MG cells accumulated 2.43 +/- 0.25 Photofrin mg/mg protein, which significantly decreased with RAP pretreatment (1.38 +/- 0.22 Photofrin mg/mg protein) (p<0.05). U105MG cells accumulated 1.31 +/- 0.16 Photofrin mg/mg protein, which was significantly less than in U251MG cells. Photofrin uptake was not altered by RAP pretreatment in U105MG cells. U251MG cells exposed to ultrasound in the presence of Photofrin showed multiple sin face pores and dimple-like craters. Conclusion: This is the first report to demonstrate the usefulness of low-intensity ultrasound for the cell killing of malignant glioma cells. Antitumor activity might be enhanced by combination with photosensitizer, which is transported by cell surface LRP/alpha 2-MR to some degree.