Isolation and characterization of echinocandin resistant Candida albicans mutants

Abstract

LY303366 (LY) is an investigational echinocandin that has demonstrated activity against Candida species. We attempted to isolate an LY resistant mutant by plating the C. albicans clinical strain Y41 at a concentration of 10$\sp7$ CFU/plate on Brain Heart Infusion agar (BHI) plates supplemented with LY at concentrations of 1 to 40 $\mu$g/ml. After 7 days of incubation at 35$\sp\circ$C very small colonies were observed. One colony was selected and repeatedly subcultured on BHI with 1 $\mu$g/ml LY. After 5 passages the MIC was demonstrated to be 1.28 $\mu$g/ml LY, (wild type MIC = 0.04 $\mu$g/ml, an increase of 32$\times$). However, when 6 colonies were individually subcultured to Sabouraud dextrose agar (SDA) plates, only 3 remained resistant to LY. One of the transient mutant strains was evaluated in terms of: antifungal susceptibility (amphotericin B, 5-flucytosine, fluconazole, ketoconazole), growth, stability, and pharmacodynamic time-kill curves. There were no differences observed between parent and transient mutant strains in antifungal susceptibility, or growth patterns. Transient resistant strains reverted back to the parent MIC after 3 passages on SDA. The resistance phenotype was restored when this strain was cultured on BHI with 1 $\mu$g/ml LY. The transient resistant strain exhibit regrowth during pharmacodynamic time-kill curves at 1$\times$ MIC of LY after this strain had reverted to the wild type MIC. One of the stable mutant strains was evaluated in terms of: antifungal susceptibility (amphotericin B, 5-flucytosine, fluconazole, ketoconazole), morphology, growth, stability, pharmacodynamic time kill-curves, and germ tube formation. (Abstract shortened by UMI.)