5-FLUOROCYTOSINE-INDUCED ERADICATION OF MURINE ADENOCARCINOMAS ENGINEERED TO EXPRESS THE CYTOSINE DEAMINASE SUICIDE GENE REQUIRES HOST IMMUNE COMPETENCE AND LEAVES AN EFFICIENT MEMORY

The nonmammalian cytosine deaminase (CD) enzyme converts the nontoxic prodrug 5-fluorocytosine (5-FC) to the toxic metabolite 5-fluorouracil . Parental cells of a mammary adenocarcinoma (TSA-pc) of BALB/c mice w ere transfected with the CD gene (TSA-CD), and the ability of 5-FC to hamper their growth was evaluated. A quantity amounting to 0.5 mg of 5 -FC/0.3 ml of medium inhibits the proliferation of TSA-CD cells, but n ot that of TSA-pc, nor that of TSA-pc transfected with neomycin-resist ance gene only (TSA-neo). In BALB/c mice, 800 mg 5-FC/kg of body weigh t injected daily i.p. for 30 days causes total regression of incipient (1-day-old), and established (3- and 7-day-old) TSA-CD tumors, and of 3-day-old experimental lung metastases, but does not impair TSA-pc no r TSA-neo cell growth. Because in CD8(+) T lymphocyte- and granulocyte -depleted mice 5-FC no longer impairs TSA-CD growth, immune mechanisms appear to play an important role in this regression. Following regres sion, all mice are resistant to subsequent s.c. or i.v. lethal challen ges with TSA-pc. The induction of this immune memory is dependent on C D4(+) lymphocytes, whereas its effector phase depends on both CD4(+) a nd CD8(+) lymphocytes. The memory elicited in tumor-bearing mice by th e 5-FC-dependent regression of TSA-CD tumors cures a significant numbe r of mice with 4-day-old TSA-pc metastases, but does not impair the gr owth of 4-day-old solid s.c. tumors. The reliability of this regressio n and the subsequent establishment of an efficient immune memory again st poorly immunogenic TSA-pc offer the prospect that CD-transduced tum or cells and 5-FC can be used as components of a live antitumor vaccin e.