SHIELDING EFFECTS AND HYPOXIA IN PHOTODYNAMIC THERAPY

Photodynamic therapy (PDT) is a new modality to treat cancer. In clini cal use, the procedure is limited by the poor penetration depth of the curative light, which ranges between 3 and 6 mm. The cause of this si tuation is not clear. Two hypotheses are discussed: shielding effect o r hypoxia. To test both hypotheses, we designed an in vitro model. Sus pensions and sediments of OAT-75 cells were incubated at 37-degrees-C with pheophorbide a or 13(2)-hydroxy-bacteriopheophorbide a methyl est er, both second-generation sensitizers. Irradiation was done with lase r light of suitable wavelength. In all experiments, cell death was see n on the surface of the sediment, i.e., near the border of the oxygen- containing atmosphere and opposite the site of the irradiation beam. T herefore, we should accept the hypoxia thesis.