The Zinc-Binding Group Effect: Lessons from Non-Hydroxamic Acid Vorinostat Analogs.

Histone deacetylases (HDACs) are enzymes pursued as drug targets in various cancers and several non-oncological conditions, such as inflammation and neurodegenerative disorders. In the past decade, HDAC inhibitors (HDACi) have emerged as relevant pharmaceuticals, with many efforts devoted to the development of new representatives. However, the growing safety concerns regarding the established hydroxamic acid-based HDAC inhibitors tend to drive current research more toward the design of inhibitors bearing alternative zinc-binding groups (ZBGs). This Perspective presents an overview of all non-hydroxamic acid ZBGs that have been incorporated into the clinically approved prototypical HDACi, suberoylanilide hydroxamic acid (vorinostat). This provides the unique opportunity to compare the inhibition potential and biological effects of different ZBGs in a direct way, as the compounds selected for this Perspective differ only in their ZBG. To that end, different strategies used to select a ZBG, its properties, activity, and liabilities are discussed.

Identification of mercaptoacetamide-based HDAC6 inhibitors via a lean inhibitor strategy: screening, synthesis, and biological evaluation.

Non-selective inhibition of different histone deacetylase enzymes by hydroxamic acid-based drugs causes severe side effects when used as a (long-term) cancer treatment. In this work, we searched for a potent zinc-binding group able to replace the contested hydroxamic acid by employing a lean inhibitor strategy. This instructed the synthesis of a set of HDAC6-selective inhibitors containing the more desirable mercaptoacetamide moiety. Biological evaluation of these new compounds showed an IC50 in the nanomolar range, dose-dependent HDAC6 inhibition in MM1.S cells and improved genotoxicity results, rendering these new inhibitors valuable hits for applications even beyond oncology.

Selective pharmacological inhibitors of HDAC6 reveal biochemical activity but functional tolerance in cancer models.

Our study investigates the biochemical and functional impact of selective histone deacetylase 6 (HDAC6) inhibitors, a promising class of novel therapeutics, in several cancer models. Selective HDAC6 inhibitors (Tubathian A, Tubastatin A, Tubacin and Ricolinostat) and a non-selective HDAC inhibitor (Vorinostat) were evaluated on cancer cell lines derived from multiple tumour types in both an in vitro and in vivo setting as potential cancer therapeutics. Selective HDAC6 inhibitors resulted in α-tubulin acetylation with no impact on histone acetylation but failed to show any anti-cancer properties. Only the use of high concentrations of selective HDAC6 inhibitors resulted in co-inhibition of other HDAC enzymes and consequently in reduced growth, migratory and/or invasive activity of cancer cells in vitro as well as in vivo. The specificity of HDAC6 inhibition was confirmed using a CRISPR/Cas9 knockout cell line. Our results suggest that selective HDAC6 inhibitors may fall short as potential single agent anti-cancer drugs and prove that many previous data regarding this promising class of compounds need to be interpreted with great care due to their use in high concentrations resulting in low selectivity and potential off-target effects.

Assessment of the trifluoromethyl ketone functionality as an alternative zinc-binding group for selective HDAC6 inhibition.

Recent studies point towards the possible disadvantages of using hydroxamic acid-based zinc-binding groups in HDAC inhibitors due to e.g. mutagenicity issues. In this work, we elaborated on our previously developed Tubathian series, a class of highly selective thiaheterocyclic HDAC6 inhibitors, by replacing the benzohydroxamic acid function by an alternative zinc chelator, i.e., an aromatic trifluoromethyl ketone. Unfortunately, these compounds showed a reduced potency to inhibit HDAC6 as compared to their hydroxamic acid counterparts. In agreement, the most active trifluoromethyl ketone was unable to influence the growth of SK-OV-3 ovarian cancer cells nor to alter the acetylation status of tubulin and histone H3. These data suggest that replacement of the zinc-binding hydroxamic acid function with a trifluoromethyl ketone zinc-binding moiety within reported benzohydroxamic HDAC6 inhibitors should not be considered as a standard strategy in HDAC inhibitor development.