Despite the age-old belief that most anti-cancer agents kill tumor cells by necrosis, recent findings have demonstrated that photosensitizers could also kill tumor cells by triggering genetically programmed series of events termed apoptosis. Cell death by apoptosis is a very neat way to eliminate unwanted cells: no traces are left and the cell contents are never released or accessible to the immune system. Hence there is no inflammation. This is in contrast to death by necrosis. Under these conditions, normally the cell swells and then, when membrane integrity comes under attack, the cell collapses like a balloon and the contents spill out into the extracellular milieu. This may result in an inflammatory response. Because of the relatively clean nature of the apoptotic process, it is desirable to identify compounds that effectively activate the apoptotic pathway. Photodynamic therapy (PDT), a new mode of treatment, is based on the combined use of light-absorbing compounds and light irradiation. Recent developments in understanding the mechanisms of the PDT effect of photosensitizers indicate that a critical factor in the success of the agent is the ability to induce apoptosis in the malignant cell population. Hypericin and Hypocrellins are perylquinones, which are novel natural photosensitizers characterized by high absorption around 470 nm and high singlet oxygen yield. To study the signaling mechanism in vitro we have investigated uptake kinetics, intracellular localization, mode of cell death and mechanisms involved in the photodynamic action following PDT in human cell lines of poorly differentiated (CNE2) and moderately differentiated (TW0-1) nasopharyngeal carcinoma (NPC) and also poorly differentiated colon (CCL-220.1) and bladder (SD) cells.

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