Optimization of an Hsp90β-selective inhibitor via exploration of the Hsp90 N-terminal ATP-binding pocket.

The 90-kDa heat shock protein (Hsp90) promotes the maturation of >400 client protein substrates, many of which are implicated in the development/progression of cancer. Although 22 Hsp90 N-terminal inhibitors have undergone clinical evaluation, the toxicities that arose from pan-inhibition have hindered their development as chemotherapeutic agents. Hence, Hsp90 isoform-selective inhibition represents a promising alternative to overcome these detriments. We recently described the structure-based design of an isoquinolin-1(2H)-one-containing scaffold and produced several highly potent Hsp90β-selective inhibitors, such as KUNB106. But there are limitations to these compounds that require further optimization. Herein, we describe structure-activity relationship (SAR) studies on the KUNB106 indazolone ring system to assess the structural limits of Hsp90β binding. Among the alterations explored in this work, alkyl chain homologation, fluorination, and spirocyclization were most effective at retaining high affinity and selectivity towards Hsp90β. Subsequent biological characterization revealed these derivatives to promote the degradation of Hsp90β-dependent clients while avoiding the induction of Hsp90 levels, which is consistent with prior studies. Altogether, the work presented in this study supports the therapeutic advantages of Hsp90β-selective inhibition over Hsp90 pan-inhibition.