Proteolysis targeting chimeras (PROTACs) represent a new class of promising therapeutic modalities. PROTACs hijack E3 ligases and the ubiquitin-proteasome system (UPS), leading to selective degradation of the target proteins. However, only a very limited number of E3 ligases have been leveraged to generate effective PROTACs. Herein, we report that the KEAP1 E3 ligase can be harnessed for targeted protein degradation utilizing a highly selective, noncovalent small-molecule KEAP1 binder. We generated a proof-of-concept PROTAC, MS83, by linking the KEAP1 ligand to a BRD4/3/2 binder. MS83 effectively reduces protein levels of BRD4 and BRD3, but not BRD2, in cells in a concentration-, time-, KEAP1-and UPS-dependent manner. Interestingly, MS83 degrades BRD4/3 more durably than the CRBN-recruiting PROTAC dBET1 in MDA-MB-468 cells and selectively degrades BRD4 short isoform over long isoform in MDA-MB-231 cells. It also displays improved antiproliferative activity than dBET1. Overall, our study expands the limited toolbox for targeted protein degradation.

Previously, we reported a first-in-class von Hippel–Lindau (VHL)-recruiting mitogen-activated protein kinase kinases 1 and 2 (MEK1/2) degrader, MS432. To date, only two MEK1/2 degrader papers have been published and very limited structure–activity relationships (SAR) have been reported. Here, we describe our extensive SAR studies exploring both von Hippel–Lindau (VHL) and cereblon (CRBN) E3 ligase ligands and a variety of linkers, which resulted in two novel, improved VHL-recruiting MEK1/2 degraders, 24 (MS928) and 27 (MS934), and the first CRBN-recruiting MEK1/2 degrader 50 (MS910). These compounds potently and selectively degraded MEK1/2 by hijacking the ubiquitin-proteasome system, inhibited downstream signaling, and suppressed cancer cell proliferation. Furthermore, concurrent inhibition of BRAF or PI3K significantly potentiated the antitumor activity of degrader 27, suggesting that the combination of MEK1/2 degradation with BRAF or PI3K inhibition may provide potential therapeutic benefits. Finally, besides being more potent, degrader 27 displayed improved plasma exposure levels in mice, representing the best MEK1/2 degrader to date for in vivo studies.

Proteolysis Targeting Chimeras (PROTACs) are attractive therapeutic modalities for degrading disease-causing proteins. While many PROTACs have been developed for numerous protein targets, current small-molecule PROTAC approaches cannot target undruggable proteins that do not have small-molecule binders. Here, we present a novel PROTAC approach, termed bridged PROTAC, which utilizes a small-molecule binder of the target protein’s binding partner to recruit the protein complex into close proximity with an E3 ubiquitin ligase to target undruggable proteins. Applying this bridged PROTAC strategy, we discovered MS28, the first-in-class degrader of cyclin D1, which lacks a small-molecule binder. MS28 effectively degrades cyclin D1, with faster degradation kinetics and superior degradation efficiency than CDK4/6, through recruiting the CDK4/6-cyclin D1 complex to the von Hippel–Lindau E3 ligase. MS28 also suppressed the proliferation of cancer cells more effectively than CDK4/6 inhibitors and degraders. Altogether, the bridged PROTAC strategy could provide a generalizable platform for targeting undruggable proteins.

The highly homologous protein lysine methyltransferases G9a and GLP, which catalyze mono- and dimethylation of histone H3 lysine 9 (H3K9), have been implicated in various human diseases. To investigate functions of G9a and GLP in human diseases, we and others reported several noncovalent reversible small-molecule inhibitors of G9a and GLP. Here, we report the discovery of the first-in-class G9a/GLP covalent irreversible inhibitors, 1 and 8 (MS8511), by targeting a cysteine residue at the substrate binding site. We characterized these covalent inhibitors in enzymatic, mass spectrometry based and cellular assays and using X-ray crystallography. Compared to the noncovalent G9a/GLP inhibitor UNC0642, covalent inhibitor 8 displayed improved potency in enzymatic and cellular assays. Interestingly, compound 8 also displayed potential kinetic preference for covalently modifying G9a over GLP. Collectively, compound 8 could be a useful chemical tool for studying the functional roles of G9a and GLP by covalently modifying and inhibiting these methyltransferases.