Abstract 3758: Discovery of a bridged proteolysis targeting chimera (PROTAC) recruiting the SPOP E3 ubiquitin ligase for targeted protein degradation

Abstract Proteolysis Targeting Chimeras (PROTACs) are promising therapeutic modalities for eliminating disease-causing proteins, particularly in oncology. However, the widespread applicability of PROTAC technology is limited by the paucity of suitable E3 ubiquitin ligases available for targeted protein degradation (TPD) despite the existence of over 600 such ligases in the human genome. Many E3 ligases lack small-molecule ligands, rendering them inaccessible to recruitment by traditional PROTAC design methodology. To overcome this challenge, we instead leverage a high-affinity binder of the histone methyltransferase GLP, an endogenous substrate of the yet unharnessed E3 ligase SPOP, to facilitate PROTAC discovery and targeted degradation of the epigenetic regulator BRD4. Through the application of this novel substrate-bridged PROTAC strategy we discovered the compound MS479, which represents the first molecule of its class capable of recruiting SPOP for TPD and the first to do so by taking advantage of the endogenous interaction between an E3 ligase and one of its protein substrates. This approach successfully facilitates the polyubiquitination of BET-family proteins (i.e., BRD4/3/2) and their subsequent degradation of by the 26S proteasome. In particular, MS479 demonstrates significant efficacy in reducing the post-translational expression of BRD4 short isoform in HT29 cells in a time-, concentration-, GLP-, SPOP-, and ubiquitin-proteasome system- (UPS) dependent manner. Additionally, MS479 exhibits potent inhibition of colorectal cancer (CRC) cell proliferation. Our substrate-bridged PROTAC strategy will ultimately increase the available repertoire of E3 ligases that are tractable for TPD, serving as proof-of-concept for a novel and generalizable platform to target the large suite of E3 ubiquitin ligases that lack pharmacological ligands. Citation Format: Jerrel Lewis Catlett, Zhijie Deng, Youngeun Lee, Yan Xiong, Husnu Ü. Kaniskan, Jian Jin. Discovery of a bridged proteolysis targeting chimera (PROTAC) recruiting the SPOP E3 ubiquitin ligase for targeted protein degradation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 3758.