Proteolysis Targeting Chimeras With Reduced Off-targets

Proteolysis Targeting Chimeras (PROTACs), a class of heterobifunctional molecules that recruit target proteins to E3 ligases, have gained traction for targeted protein degradation. However, pomalidomide, a widely used E3 ligase recruiter in PROTACs, can independently degrade other targets, such as zinc-finger (ZF) proteins, that hold key functions in normal development and disease progression. This off-target degradation of pomalidomide-based PROTACs raises concerns about their therapeutic applicability and long-term side effects. Therefore, there is a crucial need to develop rules for PROTAC design that minimize off-target degradation. In this study, we developed a high-throughput platform that interrogates the off-target degradation of ZF domains and discovered, using this platform, that PROTACs with the current design paradigm induce degradation of several ZF proteins. To identify new rules for PROTAC design, we generated a library of pomalidomide analogs that allowed systematic exploration of the impact of positional isomerism (e.g., C4 and C5 positions of the phthalimide ring), hydrogen bonding, steric and hydrophobic effects on propensities for ZF protein degradation. We found that modifications of appropriate size on the C5 position reduced off-target ZF degradation. We validated these results using immunoblotting, target engagement, and global mass spectrometric studies. We applied our newfound design principles on a previously developed ALK oncoprotein-targeting PROTAC and generated PROTACs with enhanced potency and minimal off-target degradation. We envision the reported off-target profiling platform and pomalidomide analogs will find utility in design of specific PROTACs.