Prospective National Cohort Evaluating Predictive Biomarkers of Resistance to Immunotherapy in Patients With MSI/dMMR Metastatic Colorectal Cancer (CORESIM)

Inclusion Criteria: * Patients over 18 years old * Histologically confirmed colorectal adenocarcinoma with unresectable metastasis(s) receiving immunotherapy as first-line treatment with pembrolizumab * Tumor with microsatellite instability determined by immunohistochemistry (loss of expression of MLH1, MSH2, MSH6 and/or PMS2) and/or by molecular biology (MSI-H on microsatellite analysis from tumor DNA according to practice routine of the center) Exclusion Criteria: * Patients with another concomitant cancer at the time of diagnosis requiring systemic treatment or impacting prognosis according to the medical team. * Previous treatment with anti-PD1 or anti-PDL1. * Previous treatment with chemotherapy +/- targeted therapy for MSI/dMMR metastatic colorectal cancer. * Contraindication due to psychological or social reasons that may hinder follow-up (cognitive deficit, psychological disorders incompatible with obtaining non-opposition or consent; inability to be followed in the same center throughout the follow-up period for geographical reasons). * Pregnant women * persons under court protection or under protective supervision (guardianship or curatorship) * Opposition to participation in the study.