Discovery and Characterization of BAY-805, a Potent and Selective Inhibitor of Ubiquitin-Specific Protease USP21

USP21 belongs to the ubiquitin-specific protease (USP)
subfamily
of deubiquitinating enzymes (DUBs). Due to its relevance in tumor
development and growth, USP21 has been reported as a promising novel
therapeutic target for cancer treatment. Herein, we present the discovery
of the first highly potent and selective USP21 inhibitor. Following
high-throughput screening and subsequent structure-based optimization,
we identified BAY-805 to be a non-covalent inhibitor with low nanomolar
affinity for USP21 and high selectivity over other DUB targets as
well as kinases, proteases, and other common off-targets. Furthermore,
surface plasmon resonance (SPR) and cellular thermal shift assays
(CETSA) demonstrated high-affinity target engagement of BAY-805, resulting
in strong NF-κB activation in a cell-based reporter assay. To
the best of our knowledge, BAY-805 is the first potent and selective
USP21 inhibitor and represents a valuable high-quality in vitro chemical
probe to further explore the complex biology of USP21.