# Decoding Immunotherapy Response in Colorectal Cancer: Translational Insights Beyond MSI

## TL;DR

This review explores new biomarkers to improve immunotherapy outcomes in colorectal cancer beyond existing methods.

## Contribution

The paper highlights emerging biomarkers and strategies to refine immunotherapy response prediction in colorectal cancer.

## Key findings

- Deficient mismatch repair and high microsatellite instability remain the best-known biomarkers for immunotherapy response.
- New biomarkers like tumor mutational burden and microbiome connections show promise for predicting treatment outcomes.
- Combining multiple biomarkers may improve patient selection and treatment strategies in both MSI-H and MSS CRC.

## Abstract

Immunotherapy is redefining the clinical management of colorectal cancer. However, its broader application remains limited to a well-defined subgroup of patients. As an emerging therapeutic strategy in this context, further investigation is warranted to fully realize its therapeutic potential. This review aims to provide a comprehensive overview of biomarkers for immune checkpoint inhibitors, evolving beyond the established mismatch repair status to refine patient selection, maximize clinical benefits, minimize toxicities, and circumvent the initiation of suboptimal therapies.

Background/Objectives: Immune checkpoint inhibitors (ICIs) are among the transformative and manageable systemic therapies for several cancer types, including colorectal cancer (CRC). Nevertheless, their clinical benefit is limited to mismatch-deficient or microsatellite instability-high diseases, which represent only a small percentage of cases. Despite this initial major and stringent selection, primary and acquired resistance remain clinically relevant. Therefore, the identification of additional biomarkers is essential to refine patient selection and guide rational combinational strategies. This review aims to summarize the current evidence regarding established and emerging biomarkers of response and resistance to ICIs in CRC. Methods: This narrative review identified and synthesized relevant clinical trials, translational studies, and reviews through a literature search of emerging biomarkers of immunotherapy response in colorectal cancer. Results: Deficient mismatch repair/high microsatellite instability remains the most reliable predictive biomarker of ICI response, emphasized by high tumor mutational burden, POLE/POLD mutations, and specific tumor microenvironment features. Emerging indicators, including molecular alterations, antigen presentation machinery integrity, PD-L1-mediated signaling, microbiome connections, and circulating tumor DNA kinetics, have demonstrated significant potential as sources for therapeutic response prediction and have informed the development of innovative combination strategies in both MSI-H and MSS CRCs. Conclusions: Immunotherapy response in CRC is determined by a complex interplay between tumor-intrinsic, immune, microenvironmental, and systemic factors. Integrating multiple biomarkers may provide superior stratification and guide therapeutic strategies. Prospective validation and standardized biomarker assessment will be imperative to translate these insights into clinical practice.