Photodynamic therapy (PDT) is a clinically approved treatment that can selectively destroy tumor cells. The aim of this work was to study the response of cancer cells to PDT with Pheophorbide a (Pba), focusing our attention on the NF-kB/YY1/RKIP loop, normally dysregulated in cancer. The NO induced after PDT, through the up-regulation of nitric oxide synthases (NOS),can modulate the NF-kB/YY1/RKIP loop in a dose dependent way, influencing the outcome of the therapy. The high NO level generated by a high-dose of Pba/PDT treatment can induce cell death, whereas the low-dose of Pba/ PDT causes stimulation of pro-survival pathways. To better understand the mechanisms involved in tumor recurrence we analyzed what happen after repeated low-dose Pba/PDT treatment. The low levels of NO induced by this kind of treatment lead to an activation of NF-kB/YY1/RKIP loop, an increase of colony formation, epithelial mesenchymal transition and resistance to a new PDT treatment. Moreover after repeated treatment we highlighted the presence of a cell population with stemness features (CD44+/CD24+). The findings demonstrate the cytoprotective role of NO following low dose of Pba/PDT treatment that was corroborated by the use of L-NAME, an inhibitor of NOS.
In parallel, we started to analyze the importance of the structure of the photosensitizer (PS) in the development of both CSC population and resistance, using two different PS (m-THPP and Pba).
Investigation of both the mechanisms involved in tumor recurrence after PDT and the role of PS structure is potentially useful for predicting the efficacy of this therapy and for the development of more effective and specific photosensitizers.
In order to follow this strategy, we proposed a new compound: a chemical conjugate between Pba and an NO donor, named DRPDT2. Preliminary data show that DRPDT2 inhibits EMT and induces cell death more efficiently then Pba alone.
