LAUSR

Abstract Background Molecular mechanisms driving acquired resistance to anti-EGFR therapies in metastatic Colorectal Cancer (mCRC) are complex but generally involve the activation of the downstream RAS-RAF-MEK-MAPK pathway. Methods In order to understand the mechanism underlying MEK inhibitor (MEKi) resistance, we generated three different MEKi resistant models, two all RAS WT (SW48-MR and LIM1215-MR) and one mutated in RAS (HCT116-MR). Results These models showed features related to the gene signature of Colorectal Cancer CMS4 with upregulation of immune pathway as confirmed by Microarray and Western blot analysis. In particular, moving forward the MEKi phenotype we assisted to the loss of epithelial features and acquisition of mesenchymal markers and morphology. Moreover, this change in the morphology is accompanied by up-regulation of PD-L1 expression and activation of EGFR and its downstream pathway, independently of cell line status mutation. To extend these in vitro findings, we performed an in vivo study using MC38 and CT26 MEKi resistant syngeneic models that we have previously generated. Combined treatment of MEKi, EGFR inhibitor (EGFRi) and PD-L1inhibitor (PD-L1i) resulted in a marked inhibition of tumor growth in both MC38–MR and CT26-MR xenograft model. Conclusions These results suggest a strategy to potentially overcome immunotherapy resistance in CMS4-like tumors and to improve the efficacy of MEK inhibition by co-treatment with other agents providing an additional therapeutic strategy via modulation of host immune responses. Legal entity responsible for the study The authors. Funding Has not received any funding. Disclosure All authors have declared no conflicts of interest.