Porfimer-sodium (Photofrin-II) in combination with ionizing radiation inhibits tumor-initiating cell proliferation and improves glioblastoma treatment efficacy

Tumor relapse and tumor cell repopulation has been explained partially by the drug-free break period between successive conventional treatments. Strategies to overcome tumor relapse have been proposed, such as the use of chemotherapeutic drugs or radiation in small, frequent fractionated doses without an extended break period between treatment intervals. Yet tumors usually acquire resistance and eventually escape the therapy. Several mechanisms have been proposed to explain the resistance of tumors to therapy, one of which involves the cancer stem cell or tumor-initiating cell (TIC) concept. TICs are believed to resist many conventional therapies, in part due to their slow proliferation and self-renewal capacities. Therefore, emerging efforts to eradicate TICs are being undertaken. Here we show that treatment with Photofrin II®, among the most frequently used photosensitizers, sensitized a TIC-enriched U-87MG human glioblastoma cell to radiation, and improve treatment outcome when used in combination with radiotherapy. A U-87MG tumor cell population enriched with radiation-resistant TICs becomes radio-sensitive, and an inhibition of cell proliferation and an increase in apoptosis are found in the presence of Photofrin II. Furthermore, U-87MG tumors implanted in mice treated with Photofrin II and radiation exhibit a significant reduction in angiogenesis and vasculogenesis, and an increased percentage of apoptotic TICs when compared with tumors grown in mice treated with radiation alone. Collectively, our results offer a new possible explanation for the therapeutic effects of radiosensitizing agents, and suggest that combinatorial treatment modalities can effectively prolong treatment outcome of glioblastoma tumors by inhibiting tumor growth mediated by TICs.