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

This study follows patients with a specific type of colorectal cancer — one that has a genetic marker called MSI/dMMR — who are receiving immunotherapy (pembrolizumab) as their first treatment. Researchers want to identify biological markers in the blood and tumor that predict who will or won't respond well to this immunotherapy. **You may be eligible if...** - You are 18 or older with confirmed colorectal cancer that has spread and cannot be surgically removed - Your tumor has been tested and shows the MSI-H (microsatellite instability-high) marker - You are starting pembrolizumab as your first-line treatment **You may NOT be eligible if...** - You have another active cancer requiring treatment at the same time - You have previously received anti-PD-1 or anti-PD-L1 immunotherapy - You have previously received chemotherapy or targeted therapy for your metastatic colorectal cancer - You are pregnant - You have cognitive or psychological conditions that would prevent you from giving consent or following up Talk to your doctor to see if this trial is right for you.