Defining the Mechanism of Action and Enzymatic Selectivity of Psammaplin A against Its Epigenetic Targets

Psammaplin A (11c) is a marine metabolite
previously
reported to be a potent inhibitor of two classes of epigenetic enzymes:
histone deacetylases and DNA methyltransferases. The design and synthesis
of a focused library based on the psammaplin A core has been carried
out to probe the molecular features of this molecule responsible for
its activity. By direct in vitro assay of the free thiol generated
upon reduction of the dimeric psammaplin scaffold, we have unambiguously
demonstrated that 11c functions as a natural prodrug,
with the reduced form being highly potent against HDAC1 in vitro (IC50 0.9 nM). Furthermore, we have shown it to have high isoform
selectivity, being 360-fold selective for HDAC1 over HDAC6 and more
than 1000-fold less potent against HDAC7 and HDAC8. SAR around our
focused library revealed a number of features, most notably the oxime
functionality to be important to this selectivity. Many of the compounds
show significant cytotoxicity in A549, MCF7, and W138 cells, with
the SAR of cytotoxicity correlating to HDAC inhibition. Furthermore,
compound treatment causes upregulation of histone acetylation but
little effect on tubulin acetylation. Finally, we have found no evidence
for 11c functioning as a DNMT inhibitor.