Selective radiosensitization of 9L glioma in the brain transduced with double suicide fusion gene

PURPOSE Suicide gene therapy has proved to be successful in enhancing the therapeut ic index by sensitizing genetically modified tumor cells to prodrugs. Two o f the most widely studied suicide genes, herpes simplex virus type I thymid ine kinase and Escherichia coli cytosine deaminase, have proved effective a t selectively eliminating malignant tumor cells. We previously demonstrated that transduced 9L glioma cells expressing E. coli cytosine deaminase and herpes simplex virus type I thymidine kinase concomitantly as a fusion prot ein exhibited greater levels of targeted cytotoxicity and radiosensitizatio n than could be achieved by single suicide gene therapy. The present in viv o studies were carried out to determine whether double suicide gene therapy would enhance the tumor control rate of orthotopically implanted malignant glioma growing in the brain when coupled with. radiotherapy. MATERIALS AND METHODS Rat 9L gliosarcoma cells were transfected with retroviral vectors containin g an E. coli cytosine deaminase and herpes simplex virus type 1 thymidine k inase fusion gene and maintained in Dulbecco's modified Eagle's medium. The antitumor response of 9L, E. coli cytosine deaminase and herpes simplex vi rus type I thymidine kinase tumors growing in the brain of Fischer rats was evaluated with small tumors (6-day-old tumors) versus large tumors (14-day -old tumors) against single versus double prodrug treatments. In the large brain tumors, the therapeutic efficacy of the combined single and double pr odrugs coupled with radiotherapy was evaluated. RESULTS Double suicide gene therapy using two prodrugs, 5-fluorocyosine (500 mg/kg) and ganciclovir (30 mg/kg), was effective in achieving long-term tumor con trol (50% survival) against early-stage brain tumors (6 days after implanta tion) but was only marginally effective against advanced stage tumors (14 d ays old). However, when these prodrugs were combined with radiotherapy and double suicide gene therapy against advanced-stage tumors, more than 70% of the animals were cured, whereas radiotherapy alone (20 Gy) failed to achie ve any cue at all. Combined radiotherapy and single prodrug therapy showed a moderate increase in the animal survival rate (17% and 40% for 5-fluorocy tosine and ganciclovir, respectively) but was inferior to the combination t herapy of radiation and double prodrugs. CONCLUSION The present in vivo results indicate that double suicide gene therapy combi ned with radiotherapy may represent a new, effective approach to achieve a high tumor cure rate without producing any excessive normal tissue damage.