Resistance of lung cancer cells grown as multicellular tumour spheroids to zinc sulfophthalocyanine photosensitization

Photodynamic therapy (PDT) is phototherapeutic modality used in the treatment of neoplastic and non-neoplastic diseases. The photochemical interaction of light, photosensitizer (PS) and molecular oxygen produces singlet oxygen which induces cell death. Zinc sulfophthalocyanine (ZnPcS_mix) has been shown to be effective in A549 monolayers, multicellular tumor spheroids (MCTSs) (250 μm) and not on MCTSs with a size of 500 μm. A549 cells used in this study were grown as MCTSs to a size of 500 μm in order to determine their susceptibility to PDT. ZnPcS_mix distribution in MCTSs and nuclear morphology was determined using a fluorescent microscope. Changes in cellular responses were evaluated using cell morphology, viability, proliferation, cytotoxicity, cell death analysis and mitochondrial membrane potential. Untreated MCTSs, showed no changes in cellular morphology, proliferation, cytotoxicity and nuclear morphology. Photoactivated ZnPcS_mix also showed no changes in cellular morphology and nuclear morphology. However, photoactivated ZnPcS_mix resulted in a significant dose dependant decrease in viability and proliferation as well as an increase in cell membrane damage in MCTSs over time. ZnPcS_mix photosensitization induces apoptotic cell death in MCTSs with a size of 500 μm and more resistantance when compared to monolayer cells and MCTSs with a size of 250 μm.