First-in-Class Potent, Dual HDAC6/Proteasome Inhibitors Lacking a Hydroxamic Acid Motif: Discovery of Novel Anti-Multiple Myeloma Agents

Motivated by promising clinical trial data for the combination of the histone deacetylase 6 (HDAC6) inhibitor ricolinostat with the proteasome inhibitor bortezomib in relapsed/refractory multiple myeloma (MM) patients, we engineered dual HDAC6/proteasome inhibitors. FDA-approved HDAC inhibitors suffer from off-target effects, which have been attributed, in part, to their lack of HDAC isoform selectivity. Furthermore, they are potentially mutagenic, because of their indispensable hydroxamic acid zinc-binding groups (ZBGs). Deploying the HDAC6-selective phenyl-4-hydroxamic acid motif, and <i>O</i>-carbamoylated hydroxamates as hydroxamic acid surrogates, then grafting to the electrophilic boronic acid warhead of bortezomib/ixazomib, we discovered several dual HDAC6/proteasome inhibitors that were potent in cell-free assays, inhibiting the chymotrypsin-like (CL) proteasomal activity on par with that of bortezomib, and many compounds demonstrated selectivity for HDAC6 over HDAC1 as predicted. Moreover, several dual HDAC6/proteasome inhibitors were submicromolar inhibitors of MM cell growth. Of particular interest, <b>AMC-3-030</b> with an <i>O</i>-(<i>N</i>-phenylcarbamoyl)-hydroxamate ZBG emerged as an exciting lead for further studies.