Abstract 550: Determining efficacy of drug combinations identified by unbiased high throughput screening for RAS-mutated colorectal cancer

Background: Metastatic colorectal cancer (mCRC) is the second leading cause of cancer death in the US. The response rate to current systemic therapies is ~50% and most patients die within 2.5 years from diagnosis of metastasis. With ~5% of patients with mCRC benefiting from immunotherapies, effective therapies for ~95% of patients with mCRC are urgently required. Greater than 50% of patients with mCRC harbor mutations in the oncogenic driver RAS (KRAS, NRAS). As targeting RAS directly is technically challenging for the majority of RAS mutated sites, 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. Aims: We aimed to perform unbiased high throughput screening (HTS) to identify drugs that, when combined to MEK inhibitors, would enhance their efficacy. Methods: We performed HTS with CRC cells grown in 2D and 3D cell culture using the MEK inhibitor trametinib as a backbone, and two different compound libraries composed of drugs either approved by the FDA or in clinical trials; 1) the NCI oncology set V, and, 2) a custom clinical compound set. Using the Bliss model of synergy, multiple compounds were identified to be synergistic with trametinib. In vitro and in vivo validation of synergy for a specific drug combination, dasatinib with trametinib, were performed. Results: Our unbiased HTS studies demonstrated that combining the SRC inhibitor dasatinib with trametinib was synergistic in both, 2D and 3D, CRC cell culture. This combination was further validated by MTT and colony formation assays. RPPA and western blot analyses of markers for apoptosis and various cell proliferation markers demonstrated that targeting SRC with MEK inhibits cell proliferation and increases cell death in multiple CRC cell lines as compared to single agents. In vivo studies in mice using RAS mutated CRC PDXs and cell lines with these drugs at clinically relevant doses, however, failed to demonstrate significant tumor growth inhibition or tumor regression. Conclusions: Although our unbiased HTS and other in vitro validation studies demonstrated that combining dasatinib and trametinib can enhance the efficacy of MEK targeted therapy in RAS mutated mCRC cells, careful in vivo preclinical studies failed to demonstrate enhanced efficacy of combining these two drugs. These studies highlight the need for performing careful in vivo preclinical studies as a prerequisite to translate in vitro findings into clinical studies. Further, our studies emphasize the importance for performing in vivo studies using drug doses that reflect clinically achievable doses in humans to identify therapies, either single or in combination, that may finally benefit our patients. This study also exemplifies the importance of publishing negative data so that others can pursue alternative strategies. Citation Format: Fan Fan, Reid Powell, Jason Roszik, Rui Wang, Xiangcang Ye, Clifford Stephan, Lee Ellis, Rajat Bhattacharya. Determining efficacy of drug combinations identified by unbiased high throughput screening for RAS-mutated colorectal cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 550.