Activation of caspases 3/7 as a function of the oxygenation level during irradiation indicating partial reversal of effects of hypoxia by prior lysosomal photodamage. Cells were loaded with NPe6 (20 μM, lanes 5–8) and benzoporphyrin derivative (BPD) (0.5 μM, lanes 3, 4, 7, 8) and irradiated as specified in the legend to Fig. 1. Lanes 1 and 2 were untreated controls. Irradiation then occurred in an atmosphere of 20% (lanes 1, 3, 5, 7) or 1% (lanes 2, 4, 6, 8) oxygen. DEVDase activity was assessed by a fluorescence assay 2 h later. Data represent average ± SD for 3 determinations. *Statistically different from BPD PDT in 20% oxygen (p < 0.001); † statistically different from BPD PDT in 1% oxygen (p < 0.01) by unpaired Student’s t-test.