Development of bifunctional CRBN-SOS1 degraders for treatment of mutant KRAS cancers.
Authors: Begovich Kyle , Schoolmeesters Angela , Rajapakse Navin , Kumar Maneesh , Martinez-Terroba Elena , Shakya Arvind , Whitefield Brandon , Mali Venkat , Pagadala Nataraj , Okano Akinori , Huang Shenlin , Chourasia Aparajita Hoskote , Fung Leah M. 
Published: June 2023

3151 Background: KRAS, a GTPase that regulates cellular growth, is the most frequently mutated oncogene in non-small cell lung, pancreatic and colorectal cancers. Hyperactivation of KRAS in mutant alleles result in upregulated signaling within the MAPK pathway leading to aberrant cell proliferation. In addition to the pan- and mutant-specific KRAS inhibitors, its upstream GEF activator, SOS1, has emerged as an attractive therapeutic target given its ability to regulate the active, GTP-loaded state of KRAS and mitigate MAPK pathway feedback. Methods: Utilizing our PRODEGY platform, we developed bifunctional CRBN-SOS1 degraders for the treatment of patients with KRAS-mutated cancers, independent of mutation type. Results: Our degraders display CRBN- and proteasome-mediated SOS1 degradation with DC50s < 15nM in high-throughput (HiBiT) and traditional Western Blot assays in multiple mutant KRAS cell lines. SOS1 degradation resulted in the reduction of active KRAS, HRAS and MRAS levels when assayed by RAF-RBD pulldowns in MIA PaCa-2 (KRAS G12C) cells. Other downstream signaling markers (pERK and pS6) were also decreased upon SOS1 degradation in KRAS G12C, G12V, G12S and G13D cell lines in 3D Western Blot assays. Notably, the observed pERK and pS6 inhibition was driven by SOS1 degradation and not inhibition as decreases in signaling was no longer observed when SOS1 degraders were outcompeted with CRBN competitors. To assess the antiproliferative effects of our SOS1 degraders, we developed 3D proliferation assays in various KRAS mutant (G12A, G12C, G12D, G12V, G12S, G13C and G13D) as well as KRAS amplified cell lines and observed IC50s ranging from 0.5 - 70nM. Treatment of CRBN KO and SOS1 KO H358 and MIA PaCa-2 cell lines with our SOS1 degraders resulted in 100- to 400-fold IC50 values, validating that the functional activity observed is dependent on its targets. Consistent with our in vitro data, treatment of SOS1 degraders in MIA PaCa-2 xenografts resulted in > 90% degradation and led to significant tumor growth inhibition. Given the diversity of mutations and tumors as well as acquired drug resistance to allele-specific KRAS inhibitors, we sought to determine if coupling SOS1 degradation with other EGFR-RAS-MAPK inhibitors could produce more potent responses to maximize patient benefit. Strong synergistic antiproliferative effects were observed when SOS1 degraders were co-treated with KRAS inhibitors AMG510 (G12C) and MRTX1133 (G12D) in respective KRAS-mutant cell lines as well as with MEK inhibition and EGFR inhibition. Lastly, combination of AMG510 or Trametinib with SOS1 degraders yielded strong synergy and a greater anti-tumor response in MIA PaCa-2 xenografts relative to the individual agents themselves. Conclusions: Together, these results highlight the potential of SOS1 degraders alone and in combination as therapeutic options for a variety of KRAS-driven cancers.