Clinical Trials With GDEPT: Cytosine Deaminase and 5-Fluorocytosine

One of the major goals for cancer therapies is to target toxic agents to tumor cells in a selective and specific manner, avoiding damage to normal tissues. One approach is to use “suicide” gene therapy or GDEPT (gene-directed enzyme-prodrug therapy) where the gene delivered encodes an enzyme that can activate a nontoxic prodrug into a cytotoxin. Expression of the “suicide” gene can be made selective by taking advantage of transcriptional differences between normal and neoplastic cells. Several combinations have been described for GDEPT, one of these being the cytosine deaminase/5-fluorocytosine system. Cytosine deaminase (CD) is a nonmammalian enzyme that catalyses the hydrolytic deamination of cytosine to uracil. Therefore, it is capable of converting the nontoxic prodrug 5-fluorocytosine (5-FC) to 5-fluorouracil (5-FU), which inhibits RNA and DNA synthesis during the S-phase of the cell cycle (1 ). This system has been used both in vitro and in preclinical studies showing antitumor activity against many tumor types (2 –4 ). However, its use in clinical studies to date is rather limited. A recent phase I clinical trial has tested the safety and efficacy of the CD/5-FC system under the transcriptional control of the erbB-2 promoter to treat breast cancer (5 ). This chapter describes the use of immunocytochemistry to determine the erbB-2 status of each patient and to determine CD protein expression following intratumoural gene delivery. In situ hybridization with a ³⁵ S-labeled riboprobe was used to assess CD RNA expression and, finally, thin-layer chromatography was utilized to determine CD enzyme activity.