Targeted degradation approaches such as proteolysis targeting chimeras (PROTACs) offer new ways to address disease through tackling challenging targets and with greater potency, efficacy, and specificity over traditional approaches. However, identification of high-affinity ligands to serve as PROTAC starting points remains challenging. As a complementary approach, we describe a class of molecules termed biological PROTACs (bioPROTACs)—engineered intracellular proteins consisting of a target-binding domain directly fused to an E3 ubiquitin ligase. Using GFP-tagged proteins as model substrates, we show that there is considerable flexibility in both the choice of substrate binders (binding positions, scaffold-class) and the E3 ligases. We then identified a highly effective bioPROTAC against an oncology target, proliferating cell nuclear antigen (PCNA) to elicit rapid and robust PCNA degradation and associated effects on DNA synthesis and cell cycle progression. Overall, bioPROTACs are powerful tools for interrogating degradation approaches, target biology, and potentially for making therapeutic impacts.

Mutations to RAS proteins (H-, N-, and K-RAS) are among the most common oncogenic drivers, and tumors harboring these lesions are some of the most difficult to treat. Although covalent small molecules against KRAS G12C have shown promising efficacy against lung cancers, traditional barriers remain for drugging the more prevalent KRAS G12D and KRAS G12V mutants. Targeted degradation has emerged as an attractive alternative approach, but for KRAS, identification of the required high-affinity ligands continues to be a challenge. Another significant hurdle is the discovery of a hybrid molecule that appends an E3 ligase-recruiting moiety in a manner that satisfies the precise geometries required for productive polyubiquitin transfer while maintaining favorable druglike properties. To gain insights into the advantages and feasibility of KRAS targeted degradation, we applied a protein-based degrader (biodegrader) approach. This workflow centers on the intracellular expression of a chimeric protein consisting of a high-affinity target-binding domain fused to an engineered E3 ligase adapter. A series of anti-RAS biodegraders spanning different RAS isoform/nucleotide-state specificities and leveraging different E3 ligases provided definitive evidence for RAS degradability. Further, these established that the functional consequences of KRAS degradation are context dependent. Of broader significance, using the exquisite degradation specificity that biodegraders can possess, we demonstrated how this technology can be applied to answer questions that other approaches cannot. Specifically, application of the GDP-state specific degrader uncovered the relative prevalence of the “off-state” of WT and various KRAS mutants in the cellular context. Finally, if delivery challenges can be addressed, anti-RAS biodegraders will be exciting candidates for clinical development.

Mutations to RAS proteins (H-, N-, and K-RAS) are amongst the most common oncogenic drivers and tumors harboring these lesions are some of the most difficult to treat. Although the recently discovered covalent small molecules against the KRAS G12C mutant have shown promising efficacy against lung cancers, traditional barriers remain for drugging the more prevalent KRAS G12D and KRAS G12V mutants. Targeted degradation has emerged as an attractive alternative approach but for KRAS, identification of the required high-affinity ligands continues to be a challenge. Another significant hurdle is the discovery of a hybrid molecule that appends an E3 ligase-recruiting moiety in a manner that satisfies the precise geometries required for productive polyubiquitin transfer while maintaining favorable druglike properties. As a tool to gain insights into the advantages and feasibility of KRAS targeted-degradation, we applied the bioPROTAC approach. This workflow centers on the intracellular expression of a chimeric protein consisting of a high-affinity target-binding domain fused to an engineered E3 ligase adapter. We generated a series of anti-RAS bioPROTACs that span different RAS isoform/nucleotide-state specificities and leverage different E3 ligases. Overall, our results provide definitive evidence for the degradability of RAS proteins. We further elucidate the functional consequences of RAS degradation, the susceptibility and degradation kinetics of various mutant KRAS, and the prevalence of different nucleotide-states in WT and mutant KRAS. Finally, if delivery challenges can be addressed, anti-RAS bioPROTACs will be exciting candidates for clinical development.

Targeted degradation approaches have recently generated much excitement as a paradigm shift to address human disease in unprecedented ways. Amongst these, small molecule based approaches such as Proteolysis targeting chimeras (PROTACs) have attracted the lion's share of attention due to their potential to tackle historically intractable targets and achieve greater potency, efficacy, and specificity over traditional small molecule inhibitors. Despite their promise, the identification of high-affinity ligands that can serve as starting points for PROTAC strategies remains challenging. As a complementary approach, we describe herein a class of intracellular biologics termed bioPROTACs. The substrate binding component of these fusion proteins consists of a peptide or an antibody-mimetic which allows for an unprecedented diversity of protein targets that can be addressed. The high-affinity binder is linked directly to an E3 ubiquitin ligase to harness the power of targeted degradation. Using GFP-tagged proteins as model substrates, we show that there is considerable flexibility in both the choice of substrate binders (binding positions, scaffold-class) and the E3 ligases.Indeed, 9 out of 16 binder-E3 combinations tested resulted in greater than 70% target clearance. Through a systematic approach, we then identified a highly effective bioPROTAC against an oncology target, proliferating cell nuclear antigen (PCNA), a sliding DNA clamp with critical roles in DNA replication and repair. The bioPROTAC, termed Con1-SPOP, elicited rapid and robust PCNA degradation and associated effects on DNA synthesis and cell cycle progression. Compared to RNAi-based approaches which typically take days to manifest, PCNA knockdown using Con1-SPOP was evident within 4 h. The advantage of degradation versus stoichiometric inhibition was also clearly demonstrated with bioPROTAC strategies. Combining superior pharmacological inhibition and relative ease of development, bioPROTACs are powerful tools for interrogating the degradability of a substrate, for guiding the identification of the fittest E3 ligase, for studying the functional consequences associated with target protein down-regulation, and potentially for making therapeutic impacts.