5-fluorocytosine: inhibition of hematopoiesis in vitro and reversal of inhibition by uracil.

The in vitro effect of 5-fluorocytosine on human and murine hematopoiesis was studied by means of soft-gel assays for erythroid and myeloid colony-forming cells. The drug consistently inhibited colony formation by granulocyte-monocyte and erythroid precursor cells. Clear effects were observable at concentrations of 5-fluorocytosine of 5 x 10(-5) M (6.5 micrograms/ml), and concentrations of 5 x 10(-4) M (65 micrograms/ml) caused approximately 90% inhibition of cloning of bone marrow myeloid colony-forming cells from mice and normal humans. Greater concentrations of 5-fluorocytosine were required to inhibit erythroid progenitors than to inhibit myeloid precursors. Uracil competitively reversed the toxicity of 5-fluorocytosine. Our data strongly suggest that the hematopoietic toxicity of 5-5-fluorocytosine is mediated through cellular metabolism of the drug. The reversal of mammalian, but not of fungal, cytotoxic effects of 5-fluorocytosine by uracil may have important clinical applications.