Isolation and in vitro susceptibility to amphotericin B, itraconazole and posaconazole of voriconazole-resistant laboratory isolates of Aspergillus fumigatus

Objectives To select voriconazole-resistant mutants of Aspergillus fumigatus in the laboratory from drug-susceptible clinical isolates and er;amine their in vitro susceptibility to amphotericin B and investigational azoles, and to compare the intramycelial accumulation of voriconazole in the resistant isolates crith that in the susceptible parent. Methods Voriconazole-resistant Aspergillus fumigatus isolates were selected in the laboratory from three highly susceptible (MIC less than or equal to 0.5 mg/L) clinical isolates Ly stepwise selection on peptone yeast extract glucose (PYG) agar containing 0.5 mg and 4 mg voriconazole/L. Twenty-three colonies that grew in the presence of 4 mg voriconazole/L on PYG agar (frequency 1.9 x 10(-8)) were tested for their in vitro susceptibility to amphotericin B, itraconazole, voriconazole and posaconazole by a broth macrodilution technique. The accumulation of voriconazole in the mycelia of two representative resistant isolates (VCZ-W42 and VCZ-W45) was determined by a previously described bioassay. Results The geometric mean MICs (mg/L) of amphotcricin B, itraconazole, voriconazole and posaconazole for these isolates were 0.45 +/- 0.19, 0.69 +/- 0.45, 5.24 +/- 3.74 and 0.27 +/- 0.18, respectively A comparison of the geometric mean MICs of the antifungals obtained for the resistant isolates to those of the susceptible parents showed 1.15-, 2.76-, 16.90- and 1.42-fold increases, respectively, for amphotcricin B, itraconazole, voriconazole and posaconazole, suggesting that low-level cross-resistance exists between the azole antifungals. The susceptible parent and the resistant isolates accumulated similar amounts of voriconazole. Conclusions These results suggest that spontaneous mutants of Aspergillus fumigatus resistant to voriconazole could emerge among clinical isolates under selection pressure and that the observed reduced in vitro susceptibility to voriconazole may not be due to reduced accumulation of the drug in the mycelia.