Photodynamic Oxidative Stress Targets Cancer as Well as Cancer Stem Cells

Cancer cells contain more reactive oxygen species (ROS) than normal cells; therefore, ROS is closely related to tumorigenesis. While cancer cells control ROS levels through powerful antioxidant defense mechanisms, they are observed to remain higher than in normal cells. Often, cancer cells maintain an effective redox homeostasis by reprogramming metabolic machinery. Increased oxidative stress due to exogenous ROS generation therapy has been proposed to selectively kill cancer cells without affecting normal cells. During cancer treatment, it is well recognized that certain chemotherapy agents and radiation therapy can contribute to an accumulation of ROS. Recent studies have shown that the degree of oxidative damage will assess the mode of death of the cells. Free radicals, especially ROS, have been documented to be common mediators of apoptosis. It is believed that since cancer cell lines contain a subpopulation of cancer stem cells (CSCs), the anti-cancer activity of ROS would be even more pronounced, and thus, oxidative stress can be used as a possible tumor-containing CSC therapy technology. Photodynamic therapy (PDT) is one such technology. PDT is based on a cascade of synergistic effects between light, a photosensitizer (PS), and oxygen, which significantly facilitates regulation of the procedure. Therefore, anticancer drugs that kill malignant cells that induce the development of intracellular ROS, such as PDT, can be developed to treat cancer by targeting CSCs.