In vitro studies with combinations of 5-fluorocytosine and amphotericin B

Synergistic antifungal activity of 5-fluorocytosine (5-FC) and amphotericin B was studied using an abbreviated checkerboard titration scheme. 5-FC was titrated in twofold increments (100 to 0.05 mug/ml) in the absence and presence of graded increments of amphotericin B (1.0. 0.5, 0.1, 0.05, and 0.01 mug/ml) in buffered yeast nitrogen base. A limited number of experiments were performed using expanded dual titration checkerboard schemes and growth curve studies. Forty-eight isolates of yeastlike organisms were tested; two were inhibited by the buffer system. Evidence of synergy, as indicated by a fourfold or greater reduction of the minimal inhibitory concentration of 5-FC in the presence of subinhibitory concentrations of amphotericin B, was seen with 11 of 46 isolates, or 24%, at the fungistatic level and with three isolates, or 7% at the fungicidal level. Indifferent results were obtained for 44 and 74% of the isolates, respectively, at the fungistatic and fungicidal levels. Antagonism was observed with three isolates.