Photodynamic killing of retinoblastoma cells with hematoporphyrin and light.

Hematoporphyrin and white light exerted a lethal effect on two cell lines of retinoblastoma cells. The lowest values of dye concentration and light exposure capable of killing an entire cell population were, respectively, 2 X 10(-5) M and 6 min exposure to an irradiance of 6.0 microwatts/sq mm. Cells exposed to light in the presence of the dye did not require lengthy incubation periods but were rapidly killed with increasing periods of light exposure. Cells washed free of the dye, however, required a minimum sensitizing period of 3.5 hr to achieve a value close to 100% cell death. An inhibitor of this photodynamic process was demonstrated in normal serum. When the concentration of either fetal calf serum, rabbit serum, or rabbit plasma was increased to 25% from a standard 10%, there was as much as a 100-fold greater requirement of hematoporphyrin concentration to produce the same lethal response. The suggested explanation for this phenomenon is a porphyrin-binding plasma factor, hemopexin, which is a natural beta-glycoprotein believed to be responsible for the transport of circulating porphyrins to the liver for their elimination. Retinoblastoma cells grow in suspension and thereby provide an excellent tool for study of photosensitive dyes, especially in the case of the rapidly growing Y79 cell line.