Studies on the mode of action of 5-fluorocytosine in Aspergillus species.

The mode of action of 5-fluorocytosine (5-FC) was studied in three isolates of pathogenic Aspergillus with varying degrees of susceptibility to the drug. Distribution studies showed that susceptibility or resistance to 5-FC was not dependent on uptake of the drug. While only a small percentage of the total 5-FC taken up was found in the RNA fraction of the cells, most remained in the acid-soluble intracellular pool. 5-FC, 5-fluorouracil (5-FU) and 5-fluorodeoxyuridine monophosphate (5-Fd-UMP) were among metabolites identified in the pool. In addition, fluoroorotic acid appeared to be a major constituent of the metabolites derived from 5-FC. The aspergilli also were capable of utilizing cytosine as a nitrogen source and this is suggested as a possible mechanism of resistance. A dual mode of action for 5-FC in the aspergilli is proposed. This consists of, first, incorporation of 5-FU into RNA and, second, inhibition of DNA synthesis by production of 5-FdUMP.