Discovery and Development of a Selective Inhibitor of the ER Resident Chaperone Grp78

A recent study illustrated that a fluorescence polarization
assay
can be used to identify substrate-competitive Hsp70 inhibitors that
can be isoform-selective. Herein, we use that assay in a moderate-throughput
screen and report the discovery of a druglike amino-acid-based inhibitor
with reasonable specificity for the endoplasmic reticular Hsp70, Grp78.
Using traditional medicinal chemistry approaches, the potency and
selectivity were further optimized through structure–activity
relationship (SAR) studies in parallel assays for six of the human
Hsp70 isoforms. The top compounds were all tested against a panel
of cancer cell lines and disappointingly showed little effect. The
top-performing compound, 8, was retested using a series
of endoplasmic reticulum (ER) stress-inducing agents and found to
synergize with these agents. Finally, 8 was tested in
a spheroid tumor model and found to be more potent than in two-dimensional
models. The optimized Grp78 inhibitors are the first reported isoform-selective
small-molecule-competitive inhibitors of an Hsp70–substrate
interaction.