Design, synthesis, and biological evalution of bifunctional inhibitors against Hsp90-HDAC6 interplay

HDAC6 and HSP90, existing as a cytosolic complex play an important role in maintaining the protein homeostasis. The interplay of HDAC6 and HSP90 has attracted wide attention due to their important role and promise as therapeutic targets in malignant cancers. Therefore, the discovery of dual inhibitors targeting HDAC6 and HSP90 is of high importance. In the present study, we describe the design, synthesis, and biological evaluation of bifunctional inhibitors against HDAC6 and HSP90 interplay. In particular, compound 6e shows a significant inhibitory activity against both HDAC6 and HSP90 with IC₅₀ values of 106 nM and 61 nM, respectively. Compound 6e promotes the acetylation of HDAC6 substrate proteins such as α-tubulin and HSP90 via HDAC6 inhibition, and also induces the degradation of HSP90 clients such as Her2, EGFR, Met, Akt, and HDAC6 via HSP90 inhibition. Compound 6e consequently furnishes potent antiproliferative effect on gefitinib-resistant H1975 non-small cell lung Cancer (NSCLC) with a GI₅₀ value of 1.7 μM. In addition, compound 6e successfully achieved significant tumor growth inhibition in H1975 NSCLC xenograft model without noticeable abnormal behavior, body weight changes, and apparent ocular toxicity. We conclude that compound 6e constitutes an excellent tool as well as a valuable lead for assessment of HSP90 and HDAC6 dual inhibition with a single molecule.