The 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 at concentrations of 1 mg/ml or more. 5-FC has about 1% of the immunosuppressive activity of 5-fluorouracil in vivo and 0.1% in vitro. Deamination of small amounts of 5-FC to 5-FU or conversion to other toxic metabolities may account for its immunosuppressive effects. The in vitro effects of 5-FC could also be accounted for by the presence of traces (0.1%) of 5-FU as a contaminant.