Synthesis and biological evaluation of <i>ortho</i>-phenyl phenylhydroxamic acids containing phenothiazine with improved selectivity for class IIa histone deacetylases

Class IIa histone deacetylases (HDACs) have been linked to tumorigenesis in various cancers. Previously, we designed phenylhydroxamic acid <b>LH4f</b> as a potent class IIa HDAC inhibitor. However, it also unselectively inhibited class I and class IIb HDACs. To enhance the compound’s selectivity towards class IIa HDACs, the <i>ortho</i>-phenyl group from the selective HDAC7 inhibitor <b>1</b> is incorporated into <i>ortho</i> position of the phenylhydroxamic acid in <b>LH4f</b>. Compared to <b>LH4f</b>, most resulting compounds displayed substantially improved selectivity towards the class IIa HDACs. Notably, compound <b>7 g</b> exhibited the strongest HDAC9 inhibition with an IC₅₀ value of 40 nM. Molecular modelling further identified the key interactions of compound <b>7 g</b> bound to HDAC9. Compound <b>7 g</b> significantly inhibited several human cancer cells, induced apoptosis, modulated caspase-related proteins as well as p38, and caused DNA damage. These findings suggest the potential of class IIa HDAC inhibitors as lead compounds for the development of cancer therapeutics.