Abstract 7164: Predictive biomarkers and immune dynamics in MSS metastatic colorectal cancer treated with regorafenib, ipilimumab, and nivolumab

Abstract Background: Microsatellite-stable (MSS) metastatic colorectal cancer (mCRC) is largely resistant to immune checkpoint inhibitors (ICIs), posing a significant therapeutic challenge. Our phase I trial of regorafenib, ipilimumab, and nivolumab (RIN) demonstrated a 27.6% overall response rate (ORR) and a median overall survival (OS) of 20 months, with better outcomes in patients without liver metastases. This study aims to identify predictive biomarkers and elucidate the immunological mechanisms associated with response and resistance to RIN therapy in MSS mCRC patients. Methods: Tumor biopsies from eight patients and peripheral blood samples from 29 MSS mCRC patients undergoing RIN therapy were collected at baseline and during treatment. Comprehensive molecular and immune profiling was performed using RNA sequencing, multiplex imaging (CODEX), flow cytometry, and cytokine assays. Gene expression analyses, immune cell phenotyping, cytokine profiling, and T cell receptor (TCR) repertoire analyses were conducted to characterize the tumor microenvironment (TME) and systemic immune responses. Results: At baseline, tumors from responders exhibited enhanced cellular proliferation, active DNA repair pathways, and increased expression of stimulator of interferon genes (STING) pathway, indicative of a more immunogenic TME. In contrast, non-responders showed aberrant vascular remodeling and a pro-inflammatory TME. In the peripheral blood, responders displayed a Th1-skewed cytokine profile, while non-responders showed markers of DNA damage, elevated immune checkpoint expression, and upregulation of pathogen-response genes, suggesting immune aging and chronic inflammation. Patients with liver metastases displayed significant immune senescence and metabolic hyperactivation, correlating with poor responses to RIN therapy. Following RIN therapy, responders demonstrated increased infiltration of immune effector cells into the TME, associated with a transient reduction of peripheral CD8+ T cells and natural killer (NK) cells, suggesting their migration into the tumor. This was accompanied by increased markers of proliferation, activation, and exhaustion in peripheral immune cells. Furthermore, RIN therapy induced clonal expansion and diversification of CD8+ T cells in responders, along with upregulation of immune activation signatures. Conclusions: The RIN regimen can reshape the immunological landscape of MSS mCRC by modulating the TME and systemic immunity. Distinct immune profiles between responders and non-responders, especially in patients with liver metastases, highlight the importance of predictive biomarkers for personalized therapy. Addressing immune senescence and metabolic dysregulation may be key to overcoming resistance in this challenging patient population. Citation Format: Jian Ye, Chongkai Wang, Colt Egelston, Weihua Guo, Peter Lee, Mawan Fakih. Predictive biomarkers and immune dynamics in MSS metastatic colorectal cancer treated with regorafenib, ipilimumab, and nivolumab [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 7164.