Stereoselective Covalent Inhibitor of the Ovarian Cancer-Driving Transcription Factor PAX8

Abstract

Transcription factors remain among the most challenging therapeutic targets in part because they lack well-defined ligandable binding pockets. We recently showed that aberrantly reactive cysteines in transcription factors can be directly targeted with electrophilic small molecules to induce selective transcription factor destabilization and degradation. Here, we extend this strategy to the lineage-defining oncogenic transcription factor PAX8, a critical driver of ovarian cancer. Screening of a chemically diverse library of more than 3,000 cysteine-reactive compounds against an endogenously HiBiT-tagged PAX8 reporter identified a sulfinyl aziridine chemotype that selectively reduced PAX8 abundance. Structure-activity and stereochemical analyses revealed highly enantio- and diastereoselective activity, identifying KL6-159A as the lead compound. Quantitative proteomics demonstrated selective loss of PAX8, while cellular thermal shift analysis and chemoproteomic profiling established direct covalent engagement of PAX8 at cysteine C57. Mutation of C57 completely abolished KL6-159A-induced PAX8 depletion, demonstrating that this residue is essential for compound activity. Transcriptomic profiling revealed broad suppression of the PAX8 transcriptional program, with FOXM1 emerging as the most significantly downregulated regulatory network together with numerous established PAX8 target genes. Collectively, these studies establish direct covalent engagement, transcriptional inhibition, and destabilization of PAX8 and further demonstrate the generality of covalent chemoproteomic approaches for drugging previously intractable transcription factors.