We compare the ability of two structurally different classes of epigenetic modulators, namely, histone deacetylase (HDAC) inhibitors containing either a hydroxamate or a mercaptoacetamide as the zinc binding group, to protect cortical neurons in culture from oxidative stress-induced death. This study reveals that some of the mercaptoacetamide-based HDAC inhibitors are fully protective, whereas the hydroxamates show toxicity at higher concentrations. Our present results appear to be consistent with the possibility that the mercaptoacetamide-based HDAC inhibitors interact with a different subset of the HDAC isozymes [less activity at HDAC1 and 2 correlates with less inhibitor toxicity], or alternatively, are interacting selectively with only the cytoplasmic HDACs that are crucial for protection from oxidative stress.

The histone deacetylases (HDACs) are able to regulate gene expression and inhibitors of the HDACs (HDACIs) hold promise in the treatment of cancer as well as a variety of neurodegenerative diseases. To investigate the possibility to achieve some measure of isoform selectivity in the inhibition of the HDACs, we prepared a small series of 2,4′-diaminobiphenyl ligands functionalized at the para-amino group with an appendage containing either a hydroxamate or a mercaptoacetamide group and coupled to an amino acid residue at the ortho-amino group. A smaller series of substituted phenylthiazoles was also explored. Some of these newly synthesized ligands show low nM potency in the HDAC inhibition assays and display micromolar to low nanomolar IC 50 values when tested against five pancreatic cancer cell lines. The isoform selectivity of these ligands for the Class I HDACs (HDAC1-3 and 8) and Class IIb HDACs (HDAC6 and HDAC10) together with QSAR studies of their correlation with the lipophilicity are presented. Of particular interest is the HDAC6 selectivity of the mercaptoacetamides.