Bone marrow toxicity associated with 5-fluorocytosine therapy.

Bone marrow toxicity occurred in 4 of 15 patients treated with 5-fluorocytosine (5-FC) for serious fungal infections. The development of marrow toxicity appeared to be related to serum 5-FC levels of 125 mug/ml or greater. In three patients, accumulation of toxic levels of 5-FC was related to diminished renal function. One patient with acute renal failure and prolonged high levels of 5-FC developed marrow aplasia and died of bacterial sepsis. Three patients experienced leukopenia, which was readily reversed when the dosage of 5-FC was decreased and the serum concentration was lowered. With careful monitoring of serum 5-FC concentration and renal function, the dose-related toxic effects of 5-FC on the marrow can be avoided.