Transduction of cytosine deaminase gene makes rat glioma cells highly sensitive to 5‐fluorocytosine

To investigate the potential use of E. coli cytosine deaminase (CD) gene instead of the commonly used HSV‐TK gene in the gene therapy of brain tumors, we constructed a retrovirus vector carrying the CD gene. We then transduced a rat glioma cell line C6 with CD gene by the retrovirus vector. Transduction of the CD gene made C6 cells become highly sensitive to the anti‐fungi drug 5‐fluorocytosine (5FC). IC50 for 5FC was 6,000 μM in CD‐negative cells, while it was 3 μM in Cd ‐positive cells. Mixed cellular assay showed that CD‐positive cells had a strong “bystander effect” on CD‐negative cells when exposed to 5FC. Significant anti‐tumor effects were observed in nude mice bearing s.c. tumors derived from CD‐positive cells when these animals were given 250 mg/kg 5FC twice a day for 20 consecutive days. A marked decrease in tumor weight occurred when a mixture containing 50% CD‐positive and 50% CD‐negative C6 cells was injected s.c., followed by 5FC treatment, suggesting the bystander effect in vivo. Concerning the pharmacokinetics of 5FC, especially its high oral bio‐availability and good penetration into cerebrospinal fluid, we suppose that the combination of CD‐gene transfer and 5FC oral administration may have potential use in the gene therapy of brain tumors. Int. J. Cancer 71:675‐679, 1997. © 1997 Wiley‐Liss, Inc.

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