Abstract 1578: Novel Cereblon mediated bifunctional degraders of SOS1 for treatment of pan-KRAS mutant tumors
Authors: Begovich Kyle , Schoolmeesters Angela , Rajapakse Navin , Kumar Maneesh , Shakya Arvind , Whitefield Brandon , Martinez-Terroba Elena , Pagadala Nataraj , Huang Shenlin , Chourasia Aparajita H. , Fung Leah 
Published: April 2023

Abstract
KRAS, a GTPase that regulates cellular growth, is the most frequently mutated oncogene with implications in non-small cell lung, pancreatic and colorectal cancers. Hyperactivation of KRAS in mutant alleles favors KRAS in its active GTP-bound state over its inactive GDP-bound state. 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 dual approach would be to couple KRAS inhibition with a blockade of its guanine exchange factor (GEF), SOS1, to keep KRAS in its inactive state as well as prevent upstream pathway reactivation. 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 Cereblon (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. Additionally, treatment of SOS1 degraders resulted in suppression of downstream signaling markers, pERK and pS6, in both 3D immunoblots and high-throughput assays with pERK IC50s &lt;10nM. Chemical competitor assays also revealed that inhibition of pERK and pS6 was dependent on CRBN-mediated competition. To determine functional activity of our SOS1 degraders, we developed and optimized 3D proliferation assays in multiple KRAS mutant (G12C, G12D, 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;75% degradation of SOS1 in tumors and subsequently led to significant tumor growth inhibition as a single agent. SOS1 degradation also provided synergistic anti-proliferation effects with other RAS/MAPK/AKT inhibition in in vitro studies. For example, combination treatment of AMG510 and SOS1 degraders resulted in strong synergistic effects in KRAS G12C H358 cells. Together, our data SOS1 degradation could be a valuable potential treatment option for a range of KRAS mutant cancers.
Citation Format: Kyle Begovich, Angela Schoolmeesters, Navin Rajapakse, Maneesh Kumar, Arvind Shakya, Brandon Whitefield, Elena Martinez-Terroba, Nataraj Pagadala, Shenlin Huang, Aparajita H. Chourasia, Leah Fung. Novel Cereblon mediated bifunctional degraders of SOS1 for treatment of pan-KRAS mutant tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1578.