35P Enhancing efficacy of the MEK inhibitor trametinib in KRAS-mutated colorectal cancer cells

Metastatic colorectal cancer (mCRC) is the 2nd leading cause of cancer deaths in the US. >50% of patients with mCRC harbor mutations in KRAS or NRAS. Direct RAS targeting has failed to benefit patients with mCRC. Targeting MEK, a downstream mediator of RAS, failed to demonstrate efficacy in patients with mCRC. We hypothesize that identifying combination therapies using unbiased screening has the potential to improve the efficacy of MEK targeting in patients with KRAS mutated mCRC. The aims of this study were to perform unbiased high-throughput screening (HTS) using KRAS mutated CRC spheroids to identify drugs that, when combined with the MEK inhibitor trametinib, would enhance its efficacy. We performed unbiased HTS using KRAS mutated CRC spheroids to investigate the synergistic effect of trametinib with agents from two distinct “clinically ready” compound library sets: 1) the NCI oncology set V, and 2) a custom compound set composed of FDA approved drugs or drugs in clinical trials. Using the Bliss model of synergy, paclitaxel was identified to be synergistic with trametinib. Effects of combining trametinib with paclitaxel were validated in vitro by cell growth assays and in vivo using KRAS mutated patient derived xenografts (PDXs). HTS studies showed that combining trametinib with paclitaxel was synergistic in CRC spheroids. This combination was validated in vitro by MTT and colony formation assays in multiple CRC cell lines. Analyses of Annexin V/PI staining by flow cytometry demonstrated that the drug combination increased cell death in multiple CRC cell lines when compared to single agents. Importantly, when compared to the monotherapies, combining trametinib with paclitaxel led to significant tumor growth inhibition in PDXs with KRAS G12D and G13D mutations, but not in a PDX with KRAS G12C mutation. Our unbiased HTS and in vitro and in vivo validation studies demonstrated that combining trametinib with paclitaxel can enhance the efficacy of MEK inhibitors in KRAS mutated CRC cells and PDXs. Our in vivo studies using clinically achievable doses may serve as the basis for future clinical studies to determine the efficacy of this drug combination in patients with KRAS mutated mCRC.