Reduced susceptibility in laboratory-selected mutants of Aspergillus fumigatus to itraconazole due to decreased intracellular accumulation of the antifungal agent

Abstract

To study the mechanism of resistance of Aspergillus fumigatus to itraconazole, spontaneous mutants with reduced susceptibility were selected by spreading 2×10 8 conidia from a clinical isolate (W73355) susceptible to miconazole (MIC 2 mg/l), itraconazole (MIC 0.25 mg/l) and amphotericin B (MIC 0.5 mg/l) on 40 peptone yeast extract glucose agar plates containing miconazole (32 mg/l). The 19 colonies that grew (frequency 0.95×10 −7) in the presence of miconazole were screened by broth macrodilution technique for their susceptibility to itraconazole. A total of two isolates (frequency 1×10 −8) MCZ14 and MCZ15 had MICs of 16 and 8 mg/l, respectively, for itraconazole. Both MCZ14 and MCZ15 showed concomitant increases in MICs for ketoconazole and miconazole, but not for amphotericin B. Growth inhibition studies as well as kill curve experiments revealed that MCZ14 and MCZ15 were less susceptible to itraconazole compared to the parental strain. The intracellular accumulation of itraconazole in A. fumigatus was time and concentration dependent. Maximum accumulation was obtained within 30 min at 5 μM itraconazole. In MCZ14 and MCZ15 intracellular accumulation of [ 3H]itraconazole was reduced by approximately 80 and 60%, respectively, compared to the susceptible parent. The respiratory inhibitor carbonyl cyanide m-chlorophenyl hydrazone at 200 μM reduced the intracellular accumulation of itraconazole by approximately 36.2% ( P≤0.05) in the parent and in the mutant strains. These results suggest that (i) the reduced accumulation of itraconazole in MCZ14 and MCZ15 is due to diminished permeability of the drug and perhaps not due to efflux, (ii) the uptake of itraconazole in A. fumigatus may be an energy dependent process, and (iii) decreased accumulation of itraconazole is at least in part responsible for the reduced susceptibility of the mutant isolates to itraconazole.