Shielding effects and hypoxia in photodynamic therapy.

Photodynamic therapy (PDT) is a new modality to treat cancer. In clinical 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 situation is not clear. Two hypotheses are discussed: shielding effect or hypoxia. To test both hypotheses, we designed an in vitro model. Suspensions and sediments of OAT-75 cells were incubated at 37 degrees C with pheophorbide a or 13(2)-hydroxy-bacteriopheophorbide a methyl ester, both second-generation sensitizers. Irradiation was done with laser light of suitable wavelength. In all experiments, cell death was seen on the surface of the sediment, i.e., near the border of the oxygen-containing atmosphere and opposite the site of the irradiation beam. Therefore, we should accept the hypoxia thesis.