Colorectal cancer (CRC) is a major cause of cancer-related death, with increasing incidence among younger adults. This thesis focuses on microsatellite stable (MSS) metastatic CRC, a form largely resistant to immunotherapy. Using data from the AVETUXIRI trial (NCT03608046), it explores new therapeutic strategies combining immunotherapy, targeted agents, and chemotherapy. The study highlights the role of immune biomarkers—such as the biopsy-adapted Immunoscore, T cell proximity to tumour cells, and genetic immunoediting—in predicting treatment benefit. While only a subset of patients showed meaningful immune changes during treatment, these findings suggest that immune profiling can help tailor therapies. This work underscores the need for biomarker-driven, personalized approaches to improve outcomes in MSS CRC and lays the groundwork for future clinical trials.