PDT with ALA/PPIX enhanced by prolonged light exposure putatively by targeting mitochondria

The synthesis of protoporphyrin IX (PPIX), a photosensitizer, occurs in the mitochondria of cells. Continuous light exposure activates the PPIX while it still resides in the mitochondria. The mitochondria are especially sensitive sites for lethal damage. In tissue culture experiments, cells were treated by PDT in two ways: (1) the cells were preloaded with PPIX by overnight incubation in medium containing ALA then exposed to light for various time periods, and (2) cells were not preloaded but rather were placed in medium containing ALA at the start of light exposure. Light exposure was 100 mW/cm². After exposure the cells were plated and incubated for seven days to watch for colony formation. The results of experiment 1 showed that in preloaded cells, the large amount of PPIX that accumulates was easily photobleached in just 5 minutes which yielded a drop in cell survival to a level of 2 percent. Further light exposure up to an hour had no additional effect on cell survival. But when light exposure was continued past one hour, cell survival began to drop again. The results of experiment 2 showed that light exposure had no effect in the first hour, but thereafter continued exposure caused continued exponential drop in cell survival. In both experiments, cell survival dropped to 1/e its value for every 8 minutes of continued light exposure.