Photodynamic Therapy Overcoming the Hypoxia Microenvironment in Tumor Tissues

Photodynamic therapy (PDT) has been broadly exploited as an substitute therapeutic for cancer treatment since it was first allowed for the treatment of bladder cancer in 1993 [1]. It depends on the ability of photo sensitizers (PS) to transfer energy from light irradiation to tumor-dissolved oxygen (O2) to produce cytotoxic reactive oxygen species (ROS) for killing cells [2,3]. In the presence of molecular oxygen (O2), PS photo activation results in the creation of reactive oxygen species (ROS) like singlet oxygen (O2) and damage to tumor tissues. Compared with other traditional cancer therapies such as chemotherapy, PDT is invasive and negligibly toxic [4]. Since treatment occurs only where light is delivered, it avoids systematic treatment. Moreover, PDT can cause an inflammation immune response and enlargement of anti-tumor immune surveillance. These primary and secondary reaction mechanisms provide great inspiration for developing PDT for cancer treatment [5].