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Due to their impact on several oncogenic client proteins, the Hsp90 family of chaperones has been widely studied for the development of potential anticancer agents. Although several Hsp90 inhibitors have entered clinical trials, most were unsuccessful because they induced a heat shock response (HSR). This issue can be circumvented by using isoform-selective inhibitors, but the high similarity in the ATP-binding sites between the isoforms presents a challenge. Given that Hsp90 shares a conserved Bergerat fold with bacterial DNA gyrase B and human topoisomerase IIα, we repurposed our ATP-competitive inhibitors of these two proteins for Hsp90 inhibition. We virtually screened a library of in-house inhibitors and identified eleven hits for evaluation of Hsp90 binding. Among these, compound 11 displayed low micromolar affinity for Hsp90 and demonstrated a 12-fold selectivity for Hsp90β over its closest isoform, Hsp90α. Out of 29 prepared analogs, 16 showed a preference for Hsp90β over Hsp90α. Furthermore, eleven of these compounds inhibited the growth of several cancer cell lines in vitro. Notably, compound 24e reduced intracellular levels of Hsp90 client proteins in MCF-7 cells, leading to cell cycle arrest in the G0/G1 phase without inducing HSR. This inhibitor exhibited at least a 27-fold preference for Hsp90β and was selective against topoisomerase IIα, a panel of 22 representative protein kinases, and proved to be non-toxic in a zebrafish larvae toxicology model. Finally, molecular modeling, corroborated by STD NMR studies, and the binding of 24e to the S52A mutant of Hsp90α confirmed that the serine to alanine switch drives the selectivity between the two cytoplasmic isoforms., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Masson SAS.. All rights reserved.)

Heat shock protein 90 (Hsp90) and topoisomerase IIα (TopoIIα) are members of the GHKL protein superfamily, both with clinically validated roles as anticancer drug targets. We report the discovery of the first class of dual inhibitors targeting the ATP-binding site of TopoIIα and the C-terminal domain of Hsp90, displaying potent cancer growth inhibition both in vitro and in vivo. Initially, a known TopoIIα inhibitor, compound 3, was shown to bind to the C-terminal domain of Hsp90, but not to its ATP-binding N-terminal domain. Nineteen analogs were then prepared and evaluated to investigate the structure-activity relationships, several of which inhibited the growth of SK-N-MC Ewing sarcoma cells in vitro. Compound 3 emerged as one of the most potent growth inhibitors (IC 50  = 0.33 ± 0.04 µM), demonstrating the ability to induce apoptosis and cell cycle arrest in SK-N-MC cells in vitro, and to slow the growth of Ewing sarcoma in vivo in a zebrafish model., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)