Photodynamic therapy: apoptosis, paraptosis and beyond.

Photodynamic therapy (PDT) is a light-catalyzed process that can initiate cellular death pathways from the formation of cytotoxic reactive oxygen species at sub-cellular sites. Apoptosis was the first such pathway to be identified. Autophagy can also occur and is often found to be cytoprotective. Another process termed paraptosis can also have lethal consequences, even in cells with an impaired apoptotic pathway. PDT in vivo can evoke other potentially cytotoxic processes including vascular shutdown and enhanced immunologic recognition of neoplastic cells. Using appropriate photosensitizing agents, sub-cellular PDT targeting can be directed so that the resulting interplay among assorted death and survival pathways will result in an enhanced level of photokilling.