5-Fluorocytosine

5-Fluorocytosine (5-FC), a fluorinated pyrimidine analog, is a synthetic antimycotic prodrug that is converted by cytosine deaminase to 5-fluorouracil.¹ 5-Fluorouracil, a widely used cytotoxic drug, is further metabolized to fluorinated ribo- and deoxyribonucleotides, resulting in the inhibition of DNA and protein synthesis, which has multiple effects including inhibition of Candida species and C. neoformans infections and cytotoxicity towards cancer cells.^1,2 In combination with a retroviral replicating vector carrying a cytosine deaminase prodrug-activating gene, 5-FC has been shown to selectively eliminate CT26 and Tu-2449 tumor cells in vitro (IC₅₀s = 4.2 and 1.5 μM, respectively) and to significantly improve survival and reduce tumor size (at a dose of 500 mg/kg) in two different syngeneic mouse glioma models.³

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1. Herbrecht, R., Nivoix, Y., Fohrer, C., et al. Management of systemic fungal infections: Alternatives to itraconazole. J. Antimicrob. Chemother. 56(Suppl 1), i39-i48 (2005).

2. Fuerer, C., and Iggo, R. 5-Fluorocytosine increases the toxicity of Wnt-targeting replicating adenoviruses that express cytosine deaminase as a late gene. Gene Ther. 11(2), 142-151 (2004).

3. Ostertag, D., Amundson, K.K., Lopez Espinoza, F., et al. Brain tumor eradication and prolonged survival from intratumoral conversion of 5-fluorocytosine to 5-fluorouracil using a nonlytic retroviral replicating vector. Neuro Oncol. 14(2), 145-159 (2012).