Fungal-Selective Resorcylate Aminopyrazole Hsp90 Inhibitors: Optimization of Whole-Cell Anticryptococcal Activity and Insights into the Structural Origins of Cryptococcal Selectivity

The
essential eukaryotic chaperone Hsp90 regulates the form and
function of diverse client proteins, many of which govern thermotolerance,
virulence, and drug resistance in fungal species. However, use of
Hsp90 inhibitors as antifungal therapeutics has been precluded by
human host toxicities and suppression of immune responses. We recently
described resorcylate aminopyrazoles (RAPs) as the first class of
Hsp90 inhibitors capable of discriminating between fungal (Cryptococcus neoformans, Candida albicans) and human isoforms of Hsp90 in biochemical assays. Here, we report
an iterative structure–property optimization toward RAPs capable
of inhibiting C. neoformans growth
in culture. In addition, we report the first X-ray crystal structures
of C. neoformans Hsp90 nucleotide binding
domain (NBD), as the apoprotein and in complexes with the non-species-selective
Hsp90 inhibitor NVP-AUY922 and three RAPs revealing unique ligand-induced
conformational rearrangements, which reaffirm the hypothesis that
intrinsic differences in protein flexibility can confer selective
inhibition of fungal versus human Hsp90 isoforms.