Hypericin-PDT-induced rapid necrotic death in human squamous cell carcinoma cultures after multiple treatment

PDT (photodynamic therapy) has been used for the treatment of NMCC (non-melanoma cutaneous cancer) particularly, human SCC (squamous cell carcinoma). However, the nature of the photosensitizer, the activation light source and the mode of cell death induced post-PDT remains elusive. We tried to optimize PDT using the light-activated (320-400 nm) St John's Wort-derived compound, Hyp (hypericin). The work highlights the potential mode of cell death and the increased efficacy of the technique associated with multiple Hyp-PDT treatment. SCC cells were exposed to different concentrations of Hyp and activated with light at 1 J/cm2 for 1 or 2 days. Thereafter with the optimum dose of Hyp proliferation, ROS (reactive oxygen species), and apoptosis were analysed by XTT [2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide] assay, FACS analysis and Fluorescent/Phase contrast microscopy was carried out for morphological studies. Hyp-PDT produces more ROS after 1 day compared with 2 days and the mode of cell death is a necrotic caspase-independent mechanism. We propose a novel 'double-hit/2-day' strategy to reduce the viability in SCC using Hyp-based PDT as an adjunctive treatment modality.