Photodynamic therapy (PDT) with the phthalocyanine photosensitize PC 4 causes rapid generation of reactive oxygen species (ROS), mitochondrial permeability transition (MPT), depolarization and swelling, release of cytochrome c, and activation of both necrosis and caspase-dependent apoptosis in tumor cells. However, PC 4 and a recently synthesized PC 4 derivative PC 181 also localize to endoplasmic reticulum (ER) and lysosomes. We hypothesize that damage to these organelles by PC 4 and PC 181 leads to additional perturbations, such as Ca2+, iron and protease release that ultimately promote MPT-dependent cell killing after PDT. Our goal is to further characterize the role of the MPT in PDT-induced killing of cancer cells and to determine the interactions of damage to ER and lysosomes in promotion of death pathways. In Aim 1, we will characterize changes of cytosolic, mitochondrial and ER Ca2+ after PDT using compartmentally localized fluorescence indicators and Confocal microscopy. Using specific interventions, we will determine whether depletion of ER Ca2+ stores, inhibition of mitochondrial Ca2+ uptake and intramitochondrial chelation act to suppress MPT after PDT and prevent subsequent cell killing. We also determine whether increased cytosolic Ca2+ may activate Ca2+-dependent calpains, to cause Bid processing and translocation to mitochondria with con- sequent mitochondrial dysfunction, MPT, and cell death. In Aim 2, we will investigate the contribution of lysosomal iron and protease release in relation to the MPT and cell death. We will measure cytosolic and mitochondrial chelatable iron using specific fluorescent indicator and determine the effects of compartmentally loaded iron chelators. Similarly, we will assess protease release into the cytosol and investigate the protection conferred by specific cathepsin indicators and knock-out cells. We expect iron and protease release from lysosomes to synergistically promote mitochondrial permeabilization by activating Bid cleavage and translocation to mitochondria. In Aim 3, we will investigate strategies to enhance PDT toxicity in cultured cells and in isolated mitochondria. We will evaluate the ability of salicylate to decrease the threshold of MPT onset and to enhance tumor killing after PDT. The proposed studies will enhance our understanding how to enhance PDT-induced killing of cancer cells and further increase the efficacy of PDT translation to the benefit for cancer patients.