Abstract 1056: Determining efficacy of combining the MEK inhibitor trametinib with vincristine identified by unbiased high throughput screening in RAS-mutated colorectal cancer cells
Authors: Ghosh Susmita , Fan Fan , Roszik Jason , Powell Reid , Park Yong , Stephan Clifford , Ellis Lee M. , Bhattacharya Rajat 
Published: June 2022

Abstract
Background: Colorectal cancer (CRC) is a heterogeneous disease with various driver genetic mutations. Metastatic colorectal cancer (mCRC) is the second leading cause of cancer related deaths in the US. Greater than 50% of patients with mCRC harbor mutations in the oncogenic drivers KRAS or NRAS. While direct targeting of specific mutations in RAS is in the early stages of development in patients with mCRC, targeting most mutations in RAS is technically challenging. Hence, prior efforts have been focused on targeting MEK, a downstream mediator of RAS. Unfortunately, several MEK inhibitors that have entered clinical trials have failed to exhibit clinical efficacy as a monotherapy or in combination with select agents in patients with mCRC. Thus, identifying combination therapies with unbiased screening methods has the potential to improve the efficacy of MEK targeting in patients with mCRC.
Aims: We aimed to perform unbiased high throughput screening (HTS) to identify drugs that, when combined with the MEK inhibitor trametinib, would enhance its efficacy.
Methods: We performed unbiased HTS with KRAS mutated CRC cells grown in 3D using the MEK inhibitor trametinib as the base drug, and two different “clinically ready” compound libraries; 1) the NCI oncology set V, and, 2) a custom clinical compound set composed of drugs either approved by the FDA or in clinical trials. Using the Bliss model of synergy, vincristine was identified to be synergistic with trametinib. Effects of combining trametinib with vincristine were analyzed in vitro by 1) cell growth assays, and 2) measuring changes in cell signaling and apoptosis mediators by RPPA and western blotting. The efficacy of this combination was also examined in vivo using KRAS mutated patient derived xenografts (PDXs).
Results: HTS studies demonstrated that combining trametinib with vincristine was synergistic in 3D CRC cell culture. This combination was further validated by MTT and colony formation assays. Analyses of markers for apoptosis by RPPA and western blotting demonstrated that the combination increases cell death in multiple CRC cell lines when compared to single agents. Furthermore, combining trametinib with vincristine led to significant inhibition in tumor growth when compared to the monotherapies using RAS-mutated PDXs in vivo. The combination was well tolerated by mice in vivo with minor decreases in body weight.
Conclusions: Our unbiased HTS along with in vitro and in vivo validation studies demonstrated that combining trametinib with vincristine can enhance the efficacy of MEK targeted therapy in RAS mutated CRC cells. Furthermore, our in vivo studies were performed using drug doses that reflect clinically achievable doses in humans and thus serve as basis for future clinical studies to determine the efficacy of this drug combination in patients with RAS mutated mCRC.
Citation Format: Susmita Ghosh, Fan Fan, Jason Roszik, Reid Powell, Yong Park, Clifford Stephan, Lee M. Ellis, Rajat Bhattacharya. Determining efficacy of combining the MEK inhibitor trametinib with vincristine identified by unbiased high throughput screening in RAS-mutated colorectal cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1056.