Activity of newer triazoles against Histoplasma capsulatum from patients with AIDS who failed fluconazole

To determine the activity of newer triazoles against strains of Histoplasma capsulatum resistant to fluconazole.

Susceptibility testing was performed on 17 paired pre- and post-treatment H. capsulatum isolates from patients with AIDS who failed fluconazole.

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.

These findings support careful monitoring for relapse in patients receiving voriconazole treatment for histoplasmosis, particularly in those who were previously treated with fluconazole.