Initiation of apoptosis versus necrosis by photodynamic therapy with chloroaluminum phthalocyanine.

While chloroaluminum phthalocyanine is a highly effective photosensitizer of murine leukemia P388 or L1210 cells, the mode of cell death varies as a function of the PDT dose. When cells were incubated with 0.3 microM of the sensitizer, a light dose of 45 mJ cm-2 (670 +/- 5 nm) yielded a 90% apoptotic cell population within 60 min. The sensitizer localized throughout the cytoplasm and catalyzed both lysosomal and mitochondrial photodamage at this light dose. Higher light doses yielded progressively more membrane photodamage and inhibited the apoptotic response as determined by the examination of Höchst dye HO 33342-labeled nuclei, DNA fragmentation on gels and a poly(adenosylribose) polymerase (PARP)-cleavage assay. Pulse-field gel electrophoresis revealed nonspecific DNA degradation to particles > or = 50 kbp at the higher PDT doses but neither PARP cleavage nor apoptotic nuclei.