Objectives: To determine the activity of newer triazoles against strains of Histoplasma capsulatum resistant to fluconazole.
Methods: Susceptibility testing was performed on 17 paired pre- and post-treatment H. capsulatum isolates from patients with AIDS who failed fluconazole.
Results: The median MICs of fluconazole, voriconazole, and posaconazole and ravuconazole for the pre-treatment isolates were 1 mg/L, 0.015 mg/L and <0.007 mg/L, respectively. A 4-fold or greater increase in the MIC of fluconazole and voriconazole was observed in 12 and 7 of the post-treatment isolates, respectively; the median fold increases in MIC were 8 and 2.1, respectively. No MIC increases were observed for posaconazole and ravuconazole. One pair of isolates exhibiting reduced susceptibility was examined in more detail. A single amino acid substitution (at tyrosine 136) was identified in the active site of the CYP51 protein from the post-treatment isolate, which is presumed to be responsible for reduced susceptibility to voriconazole and fluconazole, analogous to recent observations in Candida albicans.
Conclusions: These findings support careful monitoring for relapse in patients receiving voriconazole treatment for histoplasmosis, particularly in those who were previously treated with fluconazole.