In vivo characterization of horseradish peroxidase with indole-3-acetic acid and 5-bromoindole-3-acetic acid for gene therapy of cancer.

Gene-directed enzyme prodrug therapy is a form of targeted cancer therapy, in which an enzyme is used to convert a non-toxic prodrug to a cytotoxin within the tumor. Horseradish peroxidase (HRP) is able to convert the indole prodrugs indole-3-acetic acid (IAA) and the halogenated derivative 5-bromo-IAA (5Br-IAA) to toxic agents able to induce cell kill in vitro. This study characterized HRP-directed gene therapy in vivo. Human nasopharyngeal squamous cell carcinoma cells, FaDu, stably expressing HRP were grown as xenografts in SCID mice. Pharmacokinetic analysis of IAA and 5Br-IAA showed satisfactory drug profiles, and millimolar concentrations could be achieved in tumor tissue at non-toxic doses. HRP-expressing tumors showed a modest growth delay when treated with IAA compared with drug-vehicle controls. Treatment response could not be improved using different drug scheduling or drug vehicle, nor by combining HRP-directed gene therapy with fractionated radiotherapy.