IMMUNOSUPPRESSIVE EFFECTS OF 5-FLUOROCYTOSINE AND 5-FLUOROURACIL

5-Fluorocytosine (5-FC) is weakly immunosuppressive when given in lethal doses to mice, and inhibits the responses of mouse and human lymphoid cells to plant lectins [phytohemagglutinin and concanavalin A] at concentrations of 1 mg/ml or more. 5-FC has about 1% of the immunosuppressive activity of 5-fluorouracil (5-FU) in vivo and 0.1% in vitro. Deamination of small amounts of 5-FC to 5-FU or conversion to other toxic metabolites may account for its immunosuppressive effects. The in vitro effects of 5-FC could be accounted for by the presence of traces (0.1%) of 5-FU as a contaminant. [These results have relevance to the possible immunosuppression which may occur in patients with fungal infections who are treated with 5-FC.].