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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.

Background Historically, immunotherapies have been tested in syngeneic mouse models and until now only limited CRC syngeneic cells are available. Animal models with functioning human immune systems are critically needed to more accurately evaluate checkpoint blockers delivery, therapeutic response, and to better define biomarker expression in the presence of a competent immune system. Methods To better address the efficacy of immune checkpoint therapy specifically in relation to presence of a BRAFmutation, we developed a novel platform for the generation of somatic mosaic models of CRC enabling (i) high-throughput generation of genetically complex syngeneic models of cancer, (ii) tracing studies through fluorescence reporters. Results Cdx2Cre/+ mice, expressing the Cre recombinaseunder the control of a human Cdx2 promoter/enhancer sequence have been crossed with the R26LSL-Cas9-Gfp strain to generate models allowing for tissue specific activation of Cas9 and Gfpreporter only in CDX2 positive cells. This strain has been crossed with BRAFFSF-V600E mice to generate the final model. 6-8 weeks old mice have then been transduced with AAV constructs expressing the FLPO recombinasethat can be activated by Cre recombinase and sgRNAs targeting APC, TP53, MLH1, MSH2and ARID1Aalone or combined, in order to model MSI CRC (APC-TP53-MLH1and ARID1A-MLH1conditional mosaic knock-outs) and MSS CRC (APC-TP53 conditional mosaic knock-out). Viral particles have been surgically delivered via subserosal cecal injection and mice monitored for tumor formation by IVIS imaging. 35 mice injected with 3 combinations of tumor suppressors provide a diverse immunology repetoire. Cell lines isolated from genetically modified mice will provide a physiologic relevant and feasible means to study the mechanisms of response and resistance to immunotherapies and to understand biological and molecular differences between BRAFmutated MSI and MSS tumors. Conclusions This project focuses on the identification and characterization of the functional drivers of CMS1CRC tumors leveraging in vivomosaic genome editing technologies and functional genetic labeling of malignant sub-populations emerging during disease progression and in response to therapy. Legal entity responsible for the study The authors. Funding MD Anderson Cancer Center. Disclosure All authors have declared no conflicts of interest.

Background Anti-PD-1 antibodies represent nowadays a first-choice therapy for metastatic melanoma patients. Despite impressive results in terms of PFS and OS, a proportion of patients does not respond to anti-PD-1 therapy with an overall poor prognosis. Identification of predictive biomarkers is considered an important unmet clinical need to avoid expensive and potentially harmful drugs in patients who will not respond to them. In the last years, many studies have evaluated the role of cytokines on both blood and tissue samples as predictive biomarkers for immunotherapy, with encouraging results. Methods Blood samples from 18 patients with metastatic melanoma treated with anti-PD-1 antibodies as first line therapy were collected at baseline. 8 patients were classified as non-responders (best response: PD excluding pseudo-progression with median PFS of 2 months) and 10 patients as responders (best response: PR or CR, with median PFS of 17 months). mRNA expression levels of the main pro- and anti-inflammatory cytokines were evaluated by Real time quantitative PCR in PBMCs obtained from baseline blood samples. Unpaired two-tailed t-test was used for statistical analysis. Results IFN-γ mRNA expression levels were higher in responder patients (p 0.05). Combining data for each patient, we noticed a correlation between higher levels of IFN-γ and lower levels of IL-10 for responders and vice versa. Starting from these findings, we observed that the IFN-γ/IL-10 ratio was higher (median: 43,3 vs 5,2) in responders (p Conclusions Our data suggest an interesting correlation between IFN-γ/IL-10 ratio and response to anti-PD-1 therapy in melanoma patients. This correlation seems to be stronger than using IFN-γ expression levels alone probably because of the influence of anti-inflammatory cytokines. Since this is an exploratory and retrospective analysis of 18 patients, a larger population should be tested to validate our results. Legal entity responsible for the study Dipartimento di Medicina di Precisione, Universita degli studi della Campania Luigi Vanvitelli. Funding Has not received any funding. Disclosure F. Ciardiello: Advisory / Consultancy: Roche; Advisory / Consultancy: Amgen; Advisory / Consultancy: Merck; Advisory / Consultancy: Sanofi; Advisory / Consultancy: Bayer; Advisory / Consultancy: Pfizer; Advisory / Consultancy: Servier; Advisory / Consultancy: BMS; Advisory / Consultancy: Cellgene; Advisory / Consultancy: Lilly; Research grant / Funding (institution): Bayer; Research grant / Funding (institution): Roche; Research grant / Funding (institution): Merck; Research grant / Funding (institution): AstraZeneca; Research grant / Funding (institution): Amgen; Research grant / Funding (institution): Takeda. T. Troiani: Research grant / Funding (institution): Roche; Research grant / Funding (institution): Sanofi; Research grant / Funding (institution): Merck; Research grant / Funding (institution): Amgen; Research grant / Funding (institution): Servier; Research grant / Funding (institution): Novartis; Research grant / Funding (institution): Bayer. All other authors have declared no conflicts of interest.

TPS789 Background: Treatment of metastatic colorectal cancer (mCRC) has improved due to the introduction of more active chemotherapies (CT) and novel targeted agents that have significantly increased response rate (RR), progression free survival (PFS) and overall survival (OS). Recently, CORRECT and CONCUR trials have demonstrated both activity and efficacy of regorafenib, a small multi-kinase inhibitor, as monotherapy in pretreated mCRC. The wide range of action of regorafenib makes it an ideal candidate for monotherapy in earlier disease treatment lines in which different pathways could be involved in the acquisition of resistance. To improve long term efficacy of first line therapy several therapeutic approaches of maintenance treatment have been explored in mCRC. Methods: RAVELLO is an academic randomized, double-blind, placebo-controlled, multi-center, phase III study designed to evaluate efficacy and safety of regorafenib as maintenance treatment after first line therapy. Eligible patients: pathologically confirmed mCRC RAS wild type (KRAS and NRAS genes) treated with a first line fluoropyrimidine-based CT in combination with an anti-EGFR (epidermal growth factor receptor) monoclonal antibody for a minimum of 4 to a maximum of 8 months, with a stratification by response to the first line treatment (complete response/partial response or stable disease). 480 patients will be enrolled and randomly assigned in a 1:1 ratio to receive 160 mg regorafenib or placebo per os, every day for 3 weeks of every 4 weeks cycle, until disease progression or unacceptable toxicity. Primary endpoint is PFS. With a two-tailed alpha error of 0.05, the study will have 90% power to detect a 3-month prolongation of median PFS from randomization (corresponding to a hazard ratio of progression of 0.67 with 6-month median PFS expected in the control arm). Secondary endopoint are OS, safety, and biomarker correlative studies. Currently, one patient has been enrolled and is on treatment. EudraCT number: 2013-005428-41. Clinical trial information: 2013-005428-41.

Background Cetuximab, a monoclonal antibody targeting the epidermal growth factor receptor (EGFR), is active in K-RAS wild type colorectal cancer (CRC). However, initially in responding patients cancer cells would become resistant to EGFR inhibition by the activation of alternative growth controlling pathways including the hepatocyte growth factor (HGF)/MET-dependent signal. Methods mRNA expression profiles of cetuximab-sensitive human GEO colon cancer cells and of their cetuximab-resistant derived GEO-CR cells were analyzed by microarrays. Protein expression levels were evaluated by western blot (WB). Growth factors were measure in the culture medium by Luminex technology. The in vitro antitumor activity of MET inhibitor (METi), was tested in a panel of ten human colon cancer cell lines by MTT assay. Results Evaluation of gene expression profile identified a series of genes that were up-regulated in GEO-CR cells possibly involved in acquired resistance to EGFR inhibition. Among these we found several genes involved in the MET pathway. WB analysis detected activated, phosphorylated MET in GEO-CR but not in GEO and in the other colon cancer cell lines tested. We also found in GEO-CR cells up-regulation of EGFR ligands such as transforming growth factor – α (TGFa) and Heparin Binding- Epidermal Growth Factor (HB-EGF). We further observed expression of HGF in GEO-CR cells, supporting HGF/MET autocrine activation following acquired resistance to cetuximab treatment. In fact, the RAS/RAF/MEK/MAPK pathway was constitutively active despite of EGFR inhibition by cetuximab in GEO-CR cells possibly due to HGF-induced MET activation. Treatment with a potent and selective METi was able to overcome cetuximab resistance in GEO-CR cells and causes cell growth inhibition. Conclusion These results suggest that autocrine activation of HGF/MET could be a relevant therapeutic target in colorectal cancer patients that become resistant to anti-EGFR treatment. Disclosure All authors have declared no conflicts of interest.