We have discovered a set of small molecules that are selective
for only two HDAC isoforms belonging to distinct
phylogenetic classes. Our biochemical and computational data
provide evidence that evolutionary relationships between
HDACs cannot always predict molecular recognition or ligand
binding similarities. Potency and selectivity for HDAC6 seems
to be driven by close contacts between the linking phenyl motif
and an optimal hydroxamic acid chelating geometry at the zinc
coordination center. The meta-substituents are primarily
oriented toward solvent and play a marginal role in binding.
However, for HDAC8, potency and selectivity seem to be
strongly dependent on the presence of a hydrophobic metasubstituent
binding in a well-defined secondary pocket. These
dual HDAC6/8-selective inhibitors are active in cells, and the
results reported here will help guide future efforts toward
developing novel HDAC inhibitors with optimized selectivity
profiles.