Abstract B002: 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 , Pagadala Nataraj , Chourasia Aparajita , Fung Leah 
Published: May 2023

Abstract
Although oncogenic Kirsten rat sarcoma virus (KRAS) was once deemed undruggable, recent advances have revealed promising anti-tumor effects with pan-KRAS and KRAS-mutant specific inhibitors in both preclinical and clinical settings. Despite the success of these inhibitors, the diversity of tumors as well as tumor sensitivity results in acquired drug resistance suggesting that combinational therapeutic approaches would be necessary to avoid this problem. An attractive combination strategy would be to couple KRAS inhibition with a blockade of its guanine exchange factor, Son of Sevenless (SOS1). This would keep KRAS in its inactive state as well as mitigate upstream MAPK pathway reactivation notoriously associated with inhibition in this pathway. Utilizing our PRODEGY platform, we developed bifunctional SOS1 degraders for the treatment of patients with KRAS mutant cancers. These degraders have shown rapid and potent CRBN-mediated degradation which exhibit DC50s &lt;10nM in a H358 SOS1-HiBiT cell line. Our high-throughput HiBiT data translated into effective SOS1 degradation in KRAS mutant cells harboring G12C, G12D, G12V, G12S, or G13D mutations. SOS1 degradation was validated to be dependent on the proteasome and CRBN as cotreatment with proteasomal inhibitors (MG132 or MLN4924) or treatment in a CRBN KO cells abolished degradation. Treatment of our SOS1 degraders resulted in the suppression of multiple signaling markers downstream of SOS1. Given its role in RAS activation, we observed a reduction in active KRAS, HRAS and MRAS protein upon treatment of SOS1 degraders in KRAS G12C MIA PaCa-2 cells. Furthermore, pERK and pS6 levels were decreased in a more potent or comparable manner compared to the SOS1 inhibitor, BI3406, in multiple KRAS mutant cell lines. Out competing SOS1 degraders with other CRBN binders highlighted that inhibition of pERK and pS6 was due to CRBN-mediated degradation. To determine functional activity of our SOS1 degraders, we developed and optimized 3D proliferation assays in multiple KRAS mutant (G12C, G12V, G12S, G13D) cell lines which yielded IC50s ranging from 0.5 – 70nM. Similarly, 100- to 300-fold IC50 value shifts were observed in SOS1 KO and CRBN KO H358 cell lines validating that the anti-proliferative activity of SOS1 degraders is specific to its targets. Treatment of SOS1 degraders in KRAS-mutant xenograft models resulted in &gt;90% degradation of SOS1 in tumors and subsequently led to significant tumor growth inhibition as a single agent. Combination treatment of the KRAS G12C inhibitor AMG510 and SOS1 degraders produced strong synergistic antiproliferative effects in H358 and MIA PaCa-2 cell lines. Additionally, synergy was observed with SOS1 degraders and the EGFR inhibitor, Erlotinib, as well as the MEK inhibitor, Trametinib, in KRAS G13D mutant LoVo cells. The ability of our SOS1 degraders to have potency alone as well as in combination with multiple types of different RTK/RAS/MAPK pathway inhibitor highlights the value of SOS1 degraders as a potential therapeutic option for a range of KRAS mutant cancers.
Citation Format: Kyle Begovich, Angela Schoolmeesters, Navin Rajapakse, Maneesh Kumar, Elena Martinez-Terroba, Arvind Shakya, Brandon Whitefield, Nataraj Pagadala, Aparajita Chourasia, Leah Fung. Development of bifunctional CRBN-SOS1 degraders for treatment of mutant KRAS cancers [abstract]. In: Proceedings of the AACR Special Conference: Targeting RAS; 2023 Mar 5-8; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Res 2023;21(5_Suppl):Abstract nr B002.