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.

Evidence has been provided that circulating cancer-associated macrophage-like cell (CAM-L) numbers increase in response to chemotherapy, with an inverse trend compared to circulating tumor cells (CTCs). In the era of evolving cancer immunotherapy, whether CAM-Ls might have a potential role as predictive biomarkers of response has been unexplored. We evaluated whether a serial blood evaluation of CTC to CAM-L ratio might predict response to immune checkpoint inhibitors in a cohort of non-small-cell lung cancer patients. At baseline, CTCs, CAM-Ls, and the CTC/CAM-L ratio significantly correlate with both progression-free survival (PFS) and overall survival (OS). The baseline CTC/CAM-L ratio was significantly different in early progressors (4.28 ± 3.21) compared to long responders (0.42 ± 0.47) (p = 0.001). In patients treated with immune checkpoint inhibitors, a CTC/CAM-L ratio ≤ 0.25 at baseline is associated with better PFS and OS. A baseline CTC/CAM-L ratio ≤ 0.25 is statistically significant to discriminate early progressions from durable response. The results of the present pilot study suggest that CAM-Ls together with CTCs could play an important role in evaluating patients treated with cancer immunotherapy.

Oxidative stress is considered one of the cancer hallmarks, influencing tumor initiation, progression, and metastasis. High levels of reactive oxygen species (ROS) impair the effectiveness of the immune response in cancer patients. We examined changes in oxidative stress during immunotherapy, exploring the relationship between the immune system and clinical parameters related to oxidative burden. Several T-cell and myeloid subsets from 79 metastatic non-oncogene-addicted non-small-cell lung cancer (NSCLC) patients were analyzed using flow cytometry. Additionally, 20 cytokines were measured in serum samples, and sNox2-dp levels, an indicator of NOX2 activity, were assessed by ELISA. Seventy-nine healthy donors served as controls. The data showed that cancer patients had higher levels of sNox2-dp compared to healthy donors (p < 0.0001). Elevated sNox2-dp levels were associated with inflammation-related comorbidities (p = 0.008) and platelet counts (p = 0.03) in NSCLC patients. Furthermore, sNox2-dp displayed a negative correlation with immune cells involved in activation, such as proliferating (Ki67⁺) CD8⁺, PD1⁺ and effector lymphocytes, and a positive correlation with immunosuppressive PMN-MDSCs and inflammatory soluble immune factors, including IL1α, IL1β, IL6, IL10, CCL3, and CCL4. Oxidation levels decreased after immunotherapy (p = 0.04) and increased only in non-responder patients (p = 0.02). Oxidative stress may be indirectly affected by immunotherapy and could serve as a novel tool to identify responding patients in the NSCLC setting.