BAY-805

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.

Prostate cancer (PCa) is one of the prevalent cancers in men. Although deubiquitinating enzymes are implicated in tumorigenesis and progression, the specific role of ubiquitin specific peptidase 21 (USP21) in PCa remains unclear. This study provides the first comprehensive analysis of USP21 as a key oncogene driving PCa progression.

Prognosis-related deubiquitinating enzymes in PCa were initially identified through PCa cohorts from TCGA and GEO databases, followed by experimental validation. USP21 expression in PCa tissues and cell lines using tissue microarray immunohistochemistry, western blotting, and RT-qPCR. The biological functions of USP21 were evaluated through patient-derived organoids, in vitro and in vivo experiments. USP21 substrate proteins were identified by co-immunoprecipitation (Co-IP) coupled with mass spectrometry, revealing Y-box binding protein 1 (YBX1) as the primary target. YBX1 transcriptional activity was quantified using dual-luciferase reporter assays and chromatin immunoprecipitation (ChIP). The effect of USP21 inhibition was evaluated using the USP21-targeted inhibitor Bay-805 in PCa cells.

USP21 is upregulated in PCa and correlates with poor prognosis. Functional assays demonstrated that USP21 knockdown significantly inhibited PCa cells proliferation, migration, and invasion. RNA-sequencing further revealed that USP21 modulates the HIF1 signaling pathway. Mechanistically, USP21 deubiquitinates and stabilizes YBX1, which in turn enhances HIF1A (encoding HIF1-α) transcription. Notably, pharmacological inhibition of USP21 with Bay-805 suppressed PCa progression, highlighting its therapeutic potential.

USP21 promotes PCa malignancy by interacting with YBX1 to upregulate HIF1-α expression. These findings suggest Bay-805 as a promising therapeutic candidate for PCa.