Fig. (4).

A) In Gene Directed Enzyme Prodrug Therapy (GDEPT), a viral vector is used to deliver the gene for a prodrug activating enzyme to specific cells, such as cancer cells. Then, a non-bioactive prodrug is administered systemically, but only activated locally in the cancer cells where the gene is expressed [64]. In this example, the enzyme represented is cytosine deaminase which catalyzes the conversion of non-toxic 5-Fluorocytosine (5-FC) to cytotoxic 5-Fluorouracil (5-FU). B) In this application of photodynamic therapy, a protein photosensitizer, KillerRed, is delivered into tumor cells by a viral vector and targeted to the cell membrane. Irradiation of KillerRed leads to production of ROS, which can oxidize the cell membrane and result in cell death. C) A potential use of Chromophore Assisted Light Inactivation (CALI) in which the photosensitizer SuperNova is fused to a protein toxin. Due to the light responsive nature of CALI, the toxin’s activity can be limited to a confined location and duration.