Abstract B20: Unbiased high-throughput screenings to identify combination therapies targeting RAS-mutated colorectal cancer
Authors: Powell Reid , Fan Fan , Sobieski Mary , Brunell David , Wang Rui , Ye Xiangcang , Stephan Clifford , Ellis Lee M. , Bhattacharya Rajat 
Published: June 2021

Abstract
Introduction: Colorectal cancer (CRC) is the second leading cause of cancer death in the US with ~50,000 deaths estimated this year. Although the median overall survival for patients with unresectable metastatic CRC (mCRC) has modestly improved, the response rate to current combination systemic therapies is ~50% and most patients die of their disease within 2.5 years of the diagnosis of metastasis. Among new therapeutic approaches, immunotherapies have been shown to be effective in only a subset of patients (4-5%) with mCRC with DNA mismatch repair defects. Greater than 50% of patients with mCRC harbor mutations in the oncogenic driver RAS (KRAS, NRAS). As targeting RAS directly is technically challenging, efforts have been concentrated on targeting MEK, a downstream mediator of RAS. However, targeting MEK with single-agent therapy is ineffective in patients with mCRC. We hypothesized that MEK-targeted therapy in patients with mCRC will be most effective when combined with other agents, either chemotherapy or targeted therapy, identified by unbiased high-throughput screening (HTS) approaches.
Methods: We performed HTS with CRC cells grown in 2D and 3D cell culture using a MEK inhibitor (approved by the FDA for melanoma) as a backbone, and two different clinically “ready” compound libraries: 1) the NCI oncology set of approved compounds and 2) a custom clinical compound set consisting of compounds either approved by the FDA or in clinical trials. Instead of predicting effective combinations, these unbiased HTS studies allowed the biology of CRC cells to identify the most effective drug combinations. Using the Bliss model of synergy, multiple compounds were identified to be synergistic with the MEK inhibitor. These combinations were further validated in multiple CRC cell lines by examining the effects of the combinations on CRC cell growth and apoptosis.
Results: Our unbiased 2D and 3D HTS approaches demonstrated that combining SRC inhibitors with MEK inhibitors is synergistic in 2D and 3D CRC cell culture. This combination was further validated by various growth assays, including MTT and colony formation assays. Further analyses of markers for apoptosis by FACS and Western blotting indicated that targeting SRC with MEK leads to increases in cell death in multiple CRC cell lines as compared to single agents.
Conclusions: Based on our unbiased HTS and further validation studies in vitro, we conclude that combining SRC and MEK inhibitors can potentially enhance the efficacy of MEK-targeted therapy in patients with RAS-mutated mCRC.
Citation Format: Reid Powell, Fan Fan, Mary Sobieski, David Brunell, Rui Wang, Xiangcang Ye, Clifford Stephan, Lee M. Ellis, Rajat Bhattacharya. Unbiased high-throughput screenings to identify combination therapies targeting RAS-mutated colorectal cancer [abstract]. In: Proceedings of the AACR Special Conference on Targeting RAS-Driven Cancers; 2018 Dec 9-12; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(5_Suppl):Abstract nr B20.