Prodrugs in genetic chemoradiotherapy

Improvements in the radiotherapeutic management of solid tumors through the concurrent use of gene therapy is a realistic possibility. Of the broad array of candidate genes that have been evaluated, those encoding prodrug-activating enzymes are particularly appealing since they directly complement ongoing clinical chemoradiation regimes. Gene-Directed Enzyme-Prodrug Therapy (GDEPT) only requires a fraction of the target cells to be genetically modified, providing that the resultant cytotoxic prodrug metabolites redistribute efficiently (the bystander effect). This transfer of cytotoxicity to neighboring non-targeted cancer cells is central to the success of any gene therapy strategy, irrespective of the therapeutic gene employed. In the context of genetic chemoradiotherapy, efficient prodrug metabolite diffusion will be a prerequisite for efficient radiosensitization. Some, but not all GDEPT approaches have been analysed in combination with radiotherapy. Examples of prodrugs of clinically established chemotherapeutic agents currently used in conjunction with radiotherapy include: 5-fluorocytosine (5FC), cyclophosphamide (CPA), irinotecan (CPT-11), gemcitabine (dFdC), capecitabine, mitomycin C (MMC) and AQ4N. Other GDEPT paradigms, such as ganciclovir (GCV) and Herpes Simplex thymidine kinase (HSV-tk), dinitrobenzamide (DNB) mustard or aziridinyl analogs and the E. coli nitroreductase (NTR), CMDA or ZP2767P with Pseudomonas aeruginosa carboxypeptidase G2 (CPG2), and indole-3-acetic acid (IAA) activated by horseradish peroxidase (HRP) have no clinically established chemotherapeutic counterpart. Each prodrug is discussed in this review in the context of GDEPT, with a particular attention to translational research and clinical utility in combination with radiotherapy.