Natural photosensitizers in photodynamic therapy: In vitro activity against monolayers and spheroids of human colorectal adenocarcinoma SW480 cells

Photodynamic Therapy (PDT), is a treatment option for cancer.It involves the photochemical interaction of light, photosensitizer (PS) and molecular oxygen to produce radical species as well as singlet oxygen which induce cell death. Anthraquinones (AQs) have been extensively studied with respect to their UV/Vis absorption characteristics and their photosensitizing properties in photodynamic reactions. We study the photoactivity of different natural AQs (Parietin, Soranjidiol and Rubiadin) in treating monolayers and multicellular tumor spheroids (MCTSs). Rubiadin and soranjidiol were isolated and purified from the stem and leaves of Heterophyllae pustulata, and PTN was from the liquen Teloschistes flavicans by using repeated combination of several chromatographic techniques. Monolayer and spheroids of human colorectal adenocarcinoma SW480 cells were incubated with different concentrations of the AQs and then irradiated at room temperature. 24 h post-PDT cell viability, nuclear morphology and type of cell death were analyzed. We observed that Soranjidiol and Rubiadin showed no significant difference in the photosensitizing ability on monoculture of colon cancer cells (LD80 at 50 mu M and 10 J/cm(2), for both AQs). Nevertheless, for Parietin (PTN) LD80 was achieved at (20 mu M using the same light dose (10 J/cm(2)). The death mechanism induced post-PDT was necrosis by use of Soranjidol and Rubiadin and apoptosis by use of PTN. Furthermore, in MCTSs of 300 and 900 mu m, the treatment PTN- PDT produces the greatest cytotoxic effect. The three AQs analyzed could be promising chemotherapeutic candidates as anticancer PDT agents.