AN IODINE LABELED PORPHYRIN AS A NEW RADIATION SENSITIZER IN HUMAN BLADDER CANCER CELLS IN VITRO AND IN VIVO, COMBINING PHOTODYMAMIC THERAPY (PDT) WITH PHOTON ACTIVATION THERAPY (PAT)

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Purpose: ¹²⁴Iodine-isotope labeled porphyrin pyropheophorbide HPPH (¹²⁴I-HPPH) has been developed for photodynamic therapy (PDT) and positron emission tomography (PET) imaging. We hypothesize that HPPH with cold iodine (¹²⁷I-HPPH, 717), which occurs as an intermediate product in ¹²⁴I-HPPH synthesis, can act as a new specific and selective radiosensitizer, because iodine is used as a contrast agent in clinical X-ray imaging. The peak X-ray energy was set at 33keV (the K-edge absorption energy of iodine), with the advantages of both PDT and Photon Activation Therapy (PAT). In this study, we evaluated the radiosensitizing activity of 717 in a human bladder cancer cell line. Methods and Materials: The in vitro radiosensitization activity of 717 was tested in T24 human bladder cancer cell lines using the WST and colony formation assays. Subcellular localization of 717 was investigated using confocal microscopy with organelle probes and High Performance Liquid Chromatography (HPLC). Generation of reactive oxygen species (ROS) after 717 treatment combined with X-ray irradiation was measured by hydroxyl radical (OH.) detection using the fluorescence reagent 2-(6-(4-amino)phenoxy-3H-xanthen-3-on-9-yl) benzoic acid (APF). To evaluate the radiosensitizing effect of 717 in vivo, X-ray irradiation with or without prior injection of 717 was applied to mice treated with subcutaneous injection of T24 cells on their back. Results: T24 cells treated with 717 prior to radiation showed lower cell survival than cells irradiated without 717 treatment with the WST8 assay. In the colony formation assay, only treatment with 717 prior to radiation exhibited any radiosensitization effect. 717 was localized mainly in the Golgi apparatus and mitochondria. Under X-ray irradiation, APF with 717 showed a greater increase of fluorescence compared with APF without 717, indicating that 717 could generate ROS in response to X-ray irradiation. Conclusions: 717 can enhance tumor radiosensitivity based upon the WST and colony formation assays. Iodine may play a crucial role in the radiosensitizing activity of 717. The radiosensitizing effect of 717 may be caused by ROS generation following X-ray irradiation. 717 is oncotropic with minimal toxicity. Thus, utilization of 717 has a great advantage as a radiosensitizer.