Effect of a lower NPe6 concentration together with a higher light dose on the ability of lysosomal photodamage to potentiate the loss of ΔΨm. Cells were labeled with mitotracker orange (MTO) as described in the text. A, control (untreated) cells; B, after a photodynamic therapy (PDT) dose involving benzoporphyrin derivative (BPD) alone (37.5 mJ/cm2); C, NPe6 (20 μM, 180 mJ/cm2); D, sequential PDT: both agents present with 660 nm light preceding 690 nm light. (White bar in panel A = 20 μm).