The combination of an intensified regimen using the triple-drug chemotherapy regimen FOLFOXIRI with either the anti-EGFR cetuximab or panitumumab has been investigated in the MACBETH and VOLFI trials, respectively, showing a higher ORR. as monotherapy has not conferred any major clinical benefit to patients with MMR-proficient (pMMR) or microsatellite stable (MSS) mCRC, reflecting 95% of the CRC population. There thus remains a high unmet medical need for the development of novel immunotherapy approaches for the vast majority of patients with pMMR or MSS/MSI-low (MSI-L) mCRC. Defining the molecular mechanisms for immunogenicity in mCRC and mediating immune resistance in MSS mCRC is needed to develop predictive biomarkers and Afegostat effective therapeutic combination strategies. Here we review available clinical data from combinatorial therapeutic approaches using immunotherapy-based strategies for MSS mCRC. mutations conferring resistance to anti-EGFR agents and amplifications. Microsatellite instability (MSI) status is currently the main biomarker for immunotherapeutic response in mCRC. The advent of immunotherapyincluding programmed cell death 1 (PD-1), programmed cell death-ligand 1 (PD-L1), and cytotoxic T-lymphocyte antigen 4 (CTLA-4) immune checkpoint inhibitorshas revolutionized the therapeutic paradigm across a wide range of cancers. It has increased treatment options, leading to remarkable improvements in terms of response rate, survival, and quality of life in Afegostat many different tumor types, and has been positioned as frontline therapy, second-line therapy, and beyond in many solid tumors. The underlying mechanism of action relies on the upregulation of the immune system to reverse the immunosuppression Afegostat provoked by cancer cells to evade an antitumor T cell response. In CRC, the beneficial effect of immune checkpoint inhibitors observed in patients with mCRC harboring deficient mismatch repair (dMMR) and MSI has been ascribed to the abundance of DNA replication errors due to the loss of function of any of the DNA mismatch machinery proteins, including MSH2, MSH6, MLH1, and PMS2 , that leads to a hypermutated phenotype. These tumors are characterized by high tumor mutational burden (TMB) and by consequence an inflammatory tumor microenvironment (TME), comprising infiltrating lymphocytes (TILs), and notably memory cells and cytotoxic T lymphocytes (CTLs) . Otherwise, CRC is considered to be a cold tumor with a low number of neoantigens and an immune-excluded or immune-desert TME with absent or inactive TILs . Therefore, while immune checkpoint inhibitorssuch as anti-PD-1, anti-PD-L1, and anti-CTLA-4 agentshave demonstrated efficacy as frontline therapy as reported in the KEYNOTE-177 study in patients with previously-treated dMMR CRC who have progressed on fluoropyrimidines, oxaliplatin, and irinotecan , they fail to provide benefit to 95% of patients with mCRC defined as proficient mismatch repair (pMMR) or microsatellite stable (MSS) tumors. Five single-arm trials (phase Ib KEYNOTE 012 and 028, and Rabbit Polyclonal to RGS10 phase Afegostat II KEYNOTE 016, 164, and Afegostat 158) investigated the anti-PD-1 agent pembrolizumab across different tumor types, including 149 patients with previously-treated advanced CRC, 59 of whom presented MSS/pMMR tumors. Forty-one patients with metastatic MSI-H or MSS carcinoma included in the phase II KEYNOTE-016 clinical trial received treatment with pembrolizumab . The overall response rate (ORR) in patients with dMMR was 71% (5 of 7 patients) and 40% (4 of 10 patients) for non-CRC and CRC, respectively, while the ORR in patients with pMMR was 0% (0 of 18 patients). The immune-related progression-free survival (PFS) rate at 20 weeks was 67% (4 of 6 patients), 78% (7 of 9 patients), and 11% (2 of 18 patients), for dMMR non-CRC, dMMR CRC, and pMMR CRC, respectively. These data support the hypothesis that, regardless of the primary tumor origin, the large proportion of mutant neoantigens in dMMR tumors make them sensitive to immune checkpoint inhibitors. . However, it should be noted that, similarly to dMMR/MSI tumors, response to pembrolizumab has been reported in a patient with MSS mCRC harboring mutations (which encodes the DNA polymerase responsible for lead strand DNA replication) . This antitumor activity might be mediated by an increase in the expression of cytotoxic T-cell markers and effector cytokines and a higher intratumoral CD8+ lymphocyte infiltration . Thus, given that most patients with MSS mCRC do not respond to immunotherapy or do not have durable clinical responses, it is necessary to obtain predictors of response to immunotherapy and rational immunotherapy-based combination therapies. The.