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

Abstract

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.

Effects of Hsp90 Inhibitor Ganetespib on Inhibition of Azole-Resistant Candida albicans

“…Thus, further structural optimization of ganetespib is required to reduce the cytotoxicity. Recently, fungal-selective HSP90 inhibitors have been reported (Huang et al, 2020 ; Marcyk et al, 2021 ). However, the improvement of selectivity seems to have little influence on reducing cytotoxicity.…”

Treatment of Cryptococcal Meningitis: How Have We Got Here and Where are We Going?

“…Mycograb initially generated considerable interest, and phase II studies were planned [ 92 ], but unfortunately, all Hsp90 inhibitors developed thus far have proved too immunosuppressive to be used as antifungals. However, recent research suggests that intrinsic differences in protein flexibility can confer selective inhibition of fungal versus human Hsp90 isoforms, raising the possibility this drug class may yet have a role [ 93 ].…”

Synthesis, Structural Characterization, and In Vitro and In Silico Antifungal Evaluation of Azo-Azomethine Pyrazoles (PhN2(PhOH)CHN(C3N2(CH3)3)PhR, R = H or NO2)

“…For C. albicans we used the sterol 14-alpha demethylase (CaCYP51) protein with PDB code 5FSA [31], the thymidylate kinase protein (CaTMPK) with PDB code 5UIV [49], and the N-myristoyltransferase protein (CaNmt) with PDB code 1IYL [50]. For C. neoformans we used the farnesyltransferase protein (CnFTase) with PDB code 3SFX [51], the Hsp90 nucleotide binding domain (CnHsp90) with PDB code 7K9S [52], and the adenylosuccinate synthetase protein (CnAdSS) with PDB code 5I34 [53].…”