5-Fluorocytosine Resistance in Cryptococcus neoformans

Isolates ofCryptococcus neoformans from six patients were obtained before and after unsuccessful therapy with 5-fluorocytosine (5-FC). Post-therapy isolates exhibited massive and stable 5-FC resistance. The frequency of drug-resistant mutants in susceptible isolates ofC. neoformans was <0.001% (70.4 ± 17.9 per 107 cryptococci), whereas mutant frequencies in resistant isolates approached 100%. Non-drug-induced, spontaneously appearing 5-FC resistant mutants were documented in four susceptible isolates ofC. neoformans by use of the statistical method of fluctuation analysis. Mutation rates on these same four isolates ranged from 1.2 × 10−7 to 4.8 × 10−7 . Total intracellular uptake and incorporation of cytosine-5 -3 H (CyH3 ) and 5-fluorocytosine-2 -14 C (5-FC14 ) into a trichloroacetic acid-insoluble fraction were markedly reduced in six isolates with in vivo-acquired resistance when compared with susceptible pretreatment strains from the same patients. Five of these six isolates also had acquired massive resistance to 5-fluorouracil (5-FU), suggesting that a mutation in the uridine-5′-monophosphate pyrophosphorylase was responsible for drug resistance. The sixth isolate, which remained susceptible to 5-FU, appeared to have a defect in a cytosine-specific permease accounting for 5-FC resistance. A single isolate with in vitro-acquired 5-FC and 5-FU resistance had no reduction in uptake or incorporation of CyH3 or 5-FC14 . The mechanism of resistance in this isolate is discussed.