Immunotherapy for microsatellite-stable colorectal cancer: overcoming resistance and exploring novel therapeutic strategies

Sun Young Kim

doi:10.3393/ac.2025.01354.0193

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Review Colorectal cancer Immunotherapy for microsatellite-stable colorectal cancer: overcoming resistance and exploring novel therapeutic strategies: Sun Young Kim; Annals of Coloproctology 2026;42(1):47-57.
DOI: https://doi.org/10.3393/ac.2025.01354.0193
Published online: February 19, 2026

Department of Oncology, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea

Correspondence to: Sun Young Kim, MD, PhD Department of Oncology, Asan Medical Center, University of Ulsan College of Medicine, 88 Olympic-ro 43-gil, Songpa-gu, Seoul 05505, Korea Email: sunyoungkim@amc.seoul.kr

• Received: November 9, 2025 • Accepted: December 8, 2025

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Abstract
INTRODUCTION: THE UNMET NEED FOR IMMUNOTHERAPY IN MSS CRC
CURRENT LANDSCAPE OF IMMUNOTHERAPY FOR MSS CRC
PREDICTIVE BIOMARKERS FOR IMMUNOTHERAPY RESPONSE
INNOVATIVE STRATEGIES TO OVERCOME IMMUNE RESISTANCE IN MSS CRC
CONCLUSION
ARTICLE INFORMATION
REFERENCES

Abstract

Microsatellite-stable (MSS) colorectal cancer (CRC), comprising 85% to 95% of all CRC cases, represents a significant therapeutic challenge in the era of cancer immunotherapy. Unlike microsatellite instability-high tumors that demonstrate remarkable responses to immune checkpoint inhibitors, MSS CRC exhibits profound resistance due to low tumor mutational burden, minimal T-cell infiltration, and an immunosuppressive tumor microenvironment. This article reviews the current landscape of immunotherapy trials in MSS CRC, including the recently reported STELLAR-303 study, discusses emerging predictive biomarkers such as tumor mutational burden, Immunoscore Immune Checkpoint (Immunoscore-IC), and artificial intelligence-driven tools like Lunit SCOPE, and explores innovative strategies to overcome immune resistance, including next-generation anti–cytotoxic T-lymphocyte–associated protein 4 (anti–CTLA-4) antibodies, programmed cell death-ligand 1 (PD-L1)/interleukin 2 (IL-2) bispecific antibodies, CD47-targeting strategies, vaccines, and chimeric antigen receptor T (CAR-T) cell therapy. Understanding these evolving strategies is critical for advancing precision immunotherapy in this challenging patient population.
Keywords: Colorectal neoplasms; Immunotherapy; Microsatellite instability

INTRODUCTION: THE UNMET NEED FOR IMMUNOTHERAPY IN MSS CRC

Colorectal cancer (CRC) is the third most common cancer worldwide and a leading cause of cancer-related mortality [1]. The discovery that microsatellite instability-high (MSI-H) or mismatch repair-deficient (dMMR) CRC responds dramatically to immune checkpoint inhibitors (ICIs) has revolutionized treatment for approximately 5% to 15% of CRC patients [2]. Anti–programmed cell death protein 1 (anti–PD-1) antibody, pembrolizumab, achieves objective response rates (ORRs) of 40% to 50% and durable disease control in MSI-H metastatic CRC (mCRC), leading to US Food and Drug Administration (FDA) approval in 2017 [3].

However, the remaining 85% to 95% of CRC cases are microsatellite stable (MSS) or mismatch repair-proficient (pMMR), and these tumors exhibit profound resistance to ICI monotherapy. This dichotomy creates a substantial unmet medical need, as the vast majority of CRC patients cannot benefit from the immunotherapy revolution that has transformed outcomes in MSI-H disease.

The resistance of MSS CRC to checkpoint inhibitor therapy stems from multiple interconnected biological mechanisms, as follows:

(1) Low mutational burden and neoantigen paucity: MSS CRC typically harbors 1 to 35 mutations per megabase compared to ≥50 mutations in MSI-H tumors [4]. This low tumor mutational burden (TMB) results in fewer neoantigens available for immune recognition, reducing the probability of spontaneous T-cell priming and limiting the substrate for checkpoint inhibitor activity [5].

(2) Immunologically “cold” tumor microenvironment: MSS CRC demonstrates minimal baseline T-cell infiltration, creating an immune-excluded or immune-desert phenotype [6]. These tumors lack the preexisting antitumor immune response that checkpoint inhibitors require to unleash therapeutic activity.

(3) Active immunosuppression: The tumor microenvironment in MSS CRC is characterized by multiple immunosuppressive mechanisms: High proportions of regulatory T cells (Tregs), myeloid-derived suppressor cells, and tumor-associated macrophages create barriers to effective cytotoxic T-cell activity [7]; soluble immunosuppressive factors including transforming growth factor β (TGF-β), interleukin 10 (IL-10), and vascular endothelial growth factor (VEGF) family members further dampen antitumor immunity [8]; activation of oncogenic pathways such as WNT/β-catenin signaling prevents CD8+ T-cell infiltration into tumor parenchyma [9].

(4) Anatomic considerations: Colorectal liver metastases (LMs) present particularly challenging biology, with the liver microenvironment providing additional layers of systemic and local immunosuppression that further limit checkpoint inhibitor responses [10].

The clinical consequences of this immune resistance are profound. While MSI-H CRC patients can achieve years of disease control with well-tolerated immunotherapy, MSS CRC patients face limited options beyond conventional chemotherapy and targeted therapies. For refractory metastatic MSS CRC, median overall survival (OS) remains under 12 months with current salvage therapies [11]. This disparity underscores the urgent need for innovative therapeutic strategies specifically designed to overcome the inherent immune resistance mechanisms in MSS CRC.

CURRENT LANDSCAPE OF IMMUNOTHERAPY FOR MSS CRC

Early phase 2 studies established that checkpoint inhibitor monotherapy produces minimal benefit in unselected MSS CRC populations. Anti–PD-1 or anti–programmed death-ligand 1 (anti–PD-L1) monotherapy typically achieves ORR of 0% to 3% with median progression-free survival (PFS) of 1.8 to 2.0 months [12]. These disappointing results confirmed the need for rational combination strategies to convert “cold” MSS tumors into immunologically “hot” tumors susceptible to checkpoint blockade. The clinical development of immunotherapy for MSS CRC has been marked by numerous attempts to overcome immune resistance through combination approaches. However, several combination strategies including checkpoint inhibitors and tyrosine kinase inhibitors (TKIs) or dual checkpoint inhibitors have shown modest efficacy signals in randomized trials, as shown in Table 1 [13–18].

The phase 2 CCTG CO.26 trial, which evaluated dual checkpoint inhibition with durvalumab (anti–PD-L1) and tremelimumab (anti–cytotoxic T-lymphocyte–associated protein-4 [anti–CTLA-4]), demonstrated a statistically significant improvement in median OS compared to best supportive care (6.6 months vs. 4.1 months; hazard ratio [HR], 0.72; 95% confidence interval [CI], 0.54–0.97; P=0.03). However, the ORR was extremely low (1% vs. 0%), and the median PFS was similar between arms (1.8 months vs. 1.9 months), leaving the observed OS benefit unexplained and limiting the clinical impact of these findings [14].

The phase 3 LEAP-017 trial compared pembrolizumab (anti–PD-1) plus lenvatinib (a multitargeted TKI) versus standard of care (regorafenib or TAS-102) in previously treated metastatic colorectal cancer. The combination improved the ORR (10.4% vs. 1.7%) and showed a trend toward improved median OS (9.8 months vs. 9.3 months; HR, 0.83; 95% CI, 0.68–1.02; P=0.038). Median PFS was also modestly prolonged (3.8 months vs. 3.3 months). Although results trended favorably, the clinical benefit remained marginal [16]. Subgroup analysis revealed that patients without LMs derived greater benefit from pembrolizumab plus lenvatinib (HR, 0.65; 95% CI, 0.42–0.99) compared to those with liver involvement, and patients from Asia also demonstrated more favorable OS (HR, 0.66; 95% CI, 0.45–0.96) than other geographic regions. However, analysis of PD-L1 expression showed limited utility as a predictive biomarker.

As of October 2025, only one phase 3 trial, STELLAR-303, has met its primary endpoint of OS. The study evaluated zanzalintinib (a multitarget tyrosine kinase inhibitor) in combination with atezolizumab (anti–PD-L1) versus regorafenib in refractory MSS CRC [17]. However, the magnitude of benefit was modest, with a median OS only 1.5 months longer than that achieved with the standard of care, regorafenib, making it unlikely to change clinical practice.

PREDICTIVE BIOMARKERS FOR IMMUNOTHERAPY RESPONSE

The modest efficacy of immunotherapy combinations in unselected MSS CRC populations underscores the critical need for predictive biomarkers to identify patients most likely to benefit. Several biomarker approaches are under active investigation.

Tumor mutational burden

TMB quantifies the total number of somatic mutations per megabase of DNA and serves as a surrogate for neoantigen load. A higher TMB increases the likelihood that tumors will generate immunogenic neoantigens capable of eliciting antitumor T-cell responses [19]. Although high TMB largely explains the prominent benefit of checkpoint inhibitors in MSI-H CRC, it does not appear to predict the benefit within MSS CRC. While pan-cancer analyses have shown that high TMB (≥10 mutations per megabase) predicts response to checkpoint inhibitors across multiple tumor types [20], this threshold has not been a reliable indicator of immunotherapy benefit in CRC when MSI-H tumors are excluded [21, 22]. In the high-TMB cohort of the TAPUR Study, CRC patients achieved only modest outcomes with pembrolizumab (ORR 11% and disease-control rate 31%), which was suboptimal compared to other high-TMB tumor types (ORR 26% and disease-control rate 45%) [23].

Immunoscore and Immunoscore-IC

The Immunoscore is a validated immune-based scoring system that quantifies the density and location of CD3+ and CD8+ T cells in the tumor center and invasive margin [24]. Originally developed as a prognostic marker, Immunoscore has been extensively validated across multiple cohorts and is now recognized as a powerful predictor of recurrence risk and survival in stage I–III CRC. However, it is only evaluable for surgical specimen, so practical utility for biopsy specimen from metastatic disease has been limited. Furthermore, to predict efficacy of immunotherapy, checkpoint markers such as PD-L1 also need to be evaluated. Immunoscore Immune Checkpoint (Immunoscore-IC) is an evolution of the original Immunoscore specifically designed to predict response to ICIs [25]. It is a synthetic measure of CD8+ T lymphocyte infiltration, PD-L1+ cell abundance, and the proximity between PD-L1+ and CD8+ cells. Immunoscore-IC has undergone rigorous analytical validation in non-small cell lung cancer cohorts demonstrating high reproducibility (94%–100% concordance) and superior predictive performance compared to standard PD-L1 assessment [26]. In the AtezoTRIBE study, a randomized phase 2 trial of bevacizumab+FOLFOXIRI (fluorouracil, leucovorin, oxaliplatin, and irinotecan)±atezolizumab, Immunoscore-IC was predictive of the benefit from atezolizumab; it identified approximately 32% of pMMR/MSS patients as high Immunoscore-IC, who achieve substantial survival benefit from ICI-combined first-line therapy (21.2 months in high Immunoscore-IC- patients vs. 9.1 months in low Immunoscore-IC patients). HR in terms of OS with atezolizumab was 0.41 in high Immunoscore-IC group, while 0.96 in low Immunoscore-IC group (P for interaction=0.089) [27]. A prospective validation of bevacizumab plus FOLFOXIRI±atezolizumab in high Immunoscore-IC MSS CRC population is ongoing through AtezoTRIBE2 trial (ClinicalTrials.gov identifier: NCT06733038).

AI-driven biomarkers

Artificial intelligence (AI) models applying deep learning to histopathology have demonstrated high accuracy in distinguishing MSI-H from MSS CRC by extracting spatial and morphological features, showing promise for clinic-ready MSI status prediction while potentially reducing the need for costly molecular testing [28, 29]. On top of that, AI models are also emerging as promising tools for predicting immunotherapy benefit in MSS CRC. Recent advances leverage deep learning models applied to routine hematoxylin-eosin–stained slides to characterize the tumor microenvironment, immune cell infiltration, and spatial cellular interactions that underpin response to ICIs. Platforms like Lunit SCOPE IO (Lunit Inc) utilize AI to numerically quantify complex immune-tumor features such as density of lymphocytes, fibroblasts, macrophages, tumor, endothelial and mitotic cells in cancer area and stroma and have shown predictive value for immunotherapy efficacy in MSS CRC within large clinical trials such as the AtezoTRIBE and the AVETRIC (FOLFOXIRI+cetuximab+avelumab) [30]. This AI approach complements traditional biomarkers (e.g., PD-L1, TMB, Immunoscore-IC), which often exhibit limited predictive power in MSS CRC, by providing a more holistic, multiparametric, and spatially resolved immune signature. Beyond MSI status prediction, the integration of AI-based spatial analyses enhances patient stratification, revealing subsets of MSS CRC patients likely to benefit from immunotherapy combinations. Moreover, AI facilitates discovery of novel histopathologic biomarkers, such as tumor-adipocyte interactions [31], which may characterize distinct tumor biology associated with immune response and therapy outcomes. While still awaiting wide clinical adoption and regulatory approval, these AI-driven biomarkers represent a transformative step toward precision immuno-oncology in MSS CRC, enabling more personalized and effective treatment selection based on comprehensive tumor-immune profiling.

Absence of LMs

LM is a well-recognized poor prognostic factor in mCRC. Animal models showed that LMs generate CD11b+ suppressive macrophages that induce antigen-specific apoptosis of CD8+ T cells, while also activating Tregs that foster distal immunosuppression. Clinical data have demonstrated that patients with LM across multiple cancers, including melanoma, non-small cell lung cancer, and CRC, have decreased response rates and poorer PFS and OS with ICI treatment compared to patients without LM [32]. Also, most randomized trials in MSS CRC showed no benefit with ICIs in patients with LM, while those without LM more benefitted from ICIs [16, 33, 34]. Translational studies support these clinical findings, revealing that LMs are characterized by impaired infiltration of effector immune cells and increased presence of immunoregulatory cell populations, which dampen the antitumor immune response [35].

Recently, ICI-based combination therapies showed LM-free MSS CRC showed response rate ranged from 10% to 30% [36–38], while STELLAR-303 trial, of which one of co-primary endpoints was the OS in the subset of patients without LM, showed no interaction between treatment arm and the presence of LM [17]. Overall, the current evidence underscores that LM likely acts as a significant predictive biomarker of poor response to immunotherapy in MSS CRC, but tailored approaches targeting the unique immunosuppressive hepatic microenvironment in LM hold promise for expanding therapeutic benefits in this resistant subgroup.

INNOVATIVE STRATEGIES TO OVERCOME IMMUNE RESISTANCE IN MSS CRC

The modest efficacy of ICIs targeting PD-1/PD-L1, CTLA-4, or lymphocyte activation gene-3 (LAG-3) in MSS CRC has driven development of novel therapeutic approaches specifically designed to address the unique resistance mechanisms in MSS CRC. Several noteworthy ongoing clinical trials are listed in Table 2.

Next-generation anti–CTLA-4 antibodies

In MSS CRC, Tregs are abundant and induce apoptosis of CD8+ T cells, resulting in immunosuppressive microenvironment. Enhancing Treg depletion might be one of the ways to overcome intrinsic immune resistance of MSS CRC [39]. Tregs express high levels of CTLA-4, making them targets for Fc-mediated depletion by anti–CTLA-4 antibodies [40]. CTLA-4 functions as a critical IC by competing with CD28 for binding to B7 ligands (CD80/CD86) on antigen-presenting cells, thereby inhibiting T-cell activation and proliferation. This mechanism maintains peripheral tolerance and prevents autoimmunity. Ipilimumab, the first approved anti–CTLA-4 antibody, demonstrated remarkable efficacy in melanoma but showed limited activity in MSS CRC when combined with anti–PD-1/PD-L1 agents. The modest therapeutic benefit was accompanied by significant immune-related adverse events, which limited the treatment window.

Novel anti–CTLA-4 antibodies have been engineered with enhanced Fc region functionality to provide additional mechanisms of action beyond simple checkpoint blockade, suggesting potential to overcome intrinsic immune resistance [41]. Selective depletion of intratumoral Tregs, while preserving peripheral Tregs, may improve the therapeutic index. Botensilimab (AGEN1181), a Fc-enhanced anti–CTLA-4 antibody designed for potent antibody-dependent cellular cytotoxicity activity, selectively depletes CTLA-4–high Tregs in the tumor microenvironment [36]. Early-phase clinical data combining botensilimab with balstilimab (anti–PD-1) in heavily pretreated MSS CRC patients without LM suggested promising activity, with ORRs up to 19%, and a phase 3 trial is planned [42]. Muzastotug (ADG126), another next-generation anti–CTLA-4 antibody engineered using SAFEbody technology (AbbVie Inc), features a covalently linked N-terminal masking peptide cleaved preferentially in tumor tissue by tumor-associated proteases [43]. This selective unmasking is expected to improve target specificity and therapeutic index. Dose expansion in the phase 1 trial of muzastotug plus pembrolizumab in MSS CRC patients without LM showed an ORR up to 20% and a favorable safety profile [38].

PD-L1/IL-2 bispecific antibodies

To overcome immunologically “cold” tumor microenvironment in MSS CRC, next-generation immunotherapies aim to deliver pro-inflammatory cytokines directly into the tumor, where they can activate tumor-reactive T cells without provoking systemic toxicity. One of the most promising strategies is PD-L1–targeted delivery of engineered IL-2. One of the strategies is the bispecific fusion protein MB2033. This molecule uses PD-L1 as a tumor anchor to localize IL-2 signaling, while incorporating IL-2 mutations that reduce binding to IL-2 receptor α (IL-2Rα) to limit Treg expansion and systemic adverse effects. In preclinical colorectal cancer models, MB2033 demonstrated superior tumor control compared to anti–PD-L1 or recombinant IL-2 alone, significantly enhancing CD8 T-cell activation within tumors and improving the CD8: Treg ratio, a key metric for productive antitumor immunity. Importantly, PD-L1 targeting concentrated IL-2 activity to the tumor, reducing peripheral cytokine release—addressing one of the chief historical barriers to IL-2 therapeutics [44]. The clinical proof-of-concept for targeted IL-2 delivery is also currently being established by related PD-1–IL-2 fusion programs, such as Innovent’s IBI363, now in phase 1 testing. IBI363 is a novel IL-2Rβγ–attenuated, IL-2Rα–biased agonist combined with anti–PD-1 antibody, which preserves an indispensable role of IL-2α stimulating tumor-specific T cells [45]. Early data from these agents show feasible safety and preliminary antitumor activity in MSS CRC; in IBI363+bevacizumab cohort, objective response was seen in 15% in 73 patients, and 31.3% in 32 patients without LM [46]. Together, these developments suggest that PD-L1 or PD-1/ IL-2 bispecifics may represent a promising next wave of immune conversion strategies for MSS CRC, with substantial potential when combined with agents that remodel the tumor microenvironment to support effector T-cell recruitment and function.

Targeting CD47

CD47 blockade—targeting the “do-not-eat-me” signal on tumor cells that interacts with macrophage signal regulatory protein α (SIRPα)—has emerged as a rational innate-immune strategy to sensitize otherwise immune-cold MSS colorectal cancers to immunotherapy. Preclinical studies show that CD47/SIRPα disruption promotes macrophage-mediated phagocytosis of CRC cells, increases antigen cross-presentation, and can drive downstream adaptive T-cell priming, converting anergic microenvironments into ones permissive for antitumor immunity [47–49]. Studies in colorectal models and other solid tumor systems also demonstrates synergy when CD47 blockade is combined with tumor-opsonizing antibodies (for example anti–epidermal growth factor receptor [anti-EGFR] agents) or with agents that remodel the tumor microenvironment, supporting combination approaches to overcome the low baseline T-cell infiltration characteristic of MSS tumors [48, 50, 51].

These translational data have rapidly moved to the clinic: anti-CD47 antibodies and SIRPα-Fc decoys (for example magrolimab/Hu5F9-G4 and TTI-621/TTI-622) have entered phase 1/2 studies in solid tumors including CRC, frequently in combination with monoclonal antibodies such as cetuximab. Early clinical reports describe pharmacodynamic evidence of target engagement and manageable, on-target hematologic effects (transient anemia), with preliminary antitumor activity (stable disease and occasional partial responses) observed in heavily pretreated CRC cohorts when CD47 blockade is combined with an opsonizing antibody [51, 52]. Ongoing development is focusing on optimizing combination partners, mitigating hematologic toxicity (dose schedules, priming strategies), and identifying biomarkers (tumor CD47 expression, myeloid signatures) that predict which MSS CRC patients are most likely to benefit. CD47-binding 4-1BB T-cell engager, DSP107, also showed favorable toxicity profile and efficacy signal of disease control in 62% of patients including those with LM in the phase 2 study [53].

Vaccines and adaptive cell therapies

Several cancer vaccine programs have shed light on treatment of MSS CRC. Personalized neoantigen vaccines administered as maintenance or in minimal residual disease (MRD) settings (the GRANITE program) have induced durable antigen-specific T-cell responses in several types of tumors including MSS CRC and reported durable PFS and circulating tumor DNA (ctDNA)—clearance signals when combined with standard maintenance and checkpoint blockade [54]. Also, lymph node–targeted amphiphile KRAS vaccines (ELI-002) have produced high rates of mutant KRAS-specific CD4 and CD8 responses and declines in ctDNA and tumor markers in adjuvant setting [55]. These results support both individualized and shared-antigen vaccine approaches for MSS CRC and highlighting MRD/low-burden settings and combination with PD-1 axis blockade as the most promising contexts.

Adoptive cellular therapies have seen parallel advances: chimeric antigen receptor T (CAR-T) cell therapy, engineered T-cell receptor T-cell therapies, or tumor-infiltrating lymphocytes programs in CRC are moving beyond proof-of-concept to show objective responses and disease control in selected early-phase cohorts, driven by improved target selection (e.g., guanyl cyclase 2C [GUCY2C], carcinoembryonic antigen [CEA], claudin 18.2), next-generation CAR designs (bispecific/logic-gated or “armored” constructs), and locoregional delivery strategies to increase intratumoral exposure and reduce systemic toxicity [56]. Early phase IM96 (GUCY2C) and CEA CAR-T programs report encouraging disease-control rates in heavily pretreated MSS CRC patients, and multiple reviews and trial reports emphasize that durable benefit will likely require antigen selection, combinational approaches to reverse myeloid/Treg-mediated suppression, and biomarker-guided patient selection (including ctDNA for MRD) [57, 58]. These convergent vaccine and adoptive cell strategies suggest a pragmatic path forward: generate high-quality tumor-specific T cells (vaccines or engineered cells), protect and expand them in the hostile tumor microenvironment with targeted combination therapy, and deploy them where tumor burden and antigen expression maximize the chance of clinical benefit.

Neoadjuvant approach to nonmetastatic MSS CRC

Despite modest efficacy in advanced disease, immunotherapy has greater translational potential in nonmetastatic MSS colorectal cancer, where the biological and clinical context favors immune modulation. Early-stage tumors present with lower antigenic heterogeneity and smaller tumor burden, reducing the immunosuppressive cytokine milieu and physical barriers that characterize metastatic lesions. Patients with localized disease also retain a more intact and functional immune system, enabling durable priming of de novo tumor-specific T-cell responses once effective antigens are exposed. In neoadjuvant setting, cytotoxic chemotherapy and radiotherapy synergize with immune checkpoint blockade by inducing immunogenic cell death, enhancing major histocompatibility complex expression, and promoting dendritic-cell activation, thereby transforming “cold” MSS tumors into transiently inflamed, immune-accessible lesions. These mechanistic advantages might translate clinically into higher rates of tumor regression when PD-1 inhibitors are integrated with chemoradiation or neoadjuvant chemotherapy regimens, as illustrated by recent TORCH and NICHE trials [59, 60]. Furthermore, postoperative MRD monitoring via ctDNA provides a unique opportunity to enable biomarker-guided immunotherapy escalation or de-escalation. Collectively, these features make nonmetastatic MSS CRC the optimal setting for immunotherapy innovation—where combination strategies can feasibly induce immune memory, achieve curative outcomes, and prevent metastatic dissemination that remains largely intractable once established.

CONCLUSION

MSS CRC represents one of the most formidable challenges in cancer immunotherapy. The clinical trial landscape over the past decades has underscored the importance of rational combination approaches and clinicopathologic biomarkers in MSS CRC. The LEAP-017 trial demonstrated that combining pembrolizumab with lenvatinib can marginally improve response rates and OS compared to standard care. Most recently, the STELLAR-303 trial met its primary endpoint, showing that zanzalintinib plus atezolizumab significantly improves OS compared to regorafenib in previously treated non-MSI-H mCRC. While these advances represent important steps forward, the modest absolute benefit highlights the continued need for more effective strategies.

The evolution of predictive biomarkers represents a critical parallel development. Traditional approaches including TMB and PD-L1 expression have limited utility in MSS CRC given the intrinsically low levels of both markers. The Immunoscore and its evolution into Immunoscore-IC offer validated tools to quantify immune infiltration and potentially identify the subset of MSS tumors with sufficient baseline immunity to respond to ICIs. The emergence of AI-driven biomarkers such as Lunit SCOPE, which leverage deep learning to extract complex, multidimensional information from routine histopathology images, is gathering interest in MSS CRC. These tools promise to democratize access to sophisticated biomarker assessment while potentially capturing patterns invisible to human observers.

The next generation of immunotherapeutic agents specifically addresses the unique resistance mechanisms in MSS CRC. Fc-enhanced or masking peptide-linked anti–CTLA-4 antibodies combine checkpoint blockades with selective depletion of intratumoral regulatory T cells, potentially overcoming a key immunosuppressive mechanism while maintaining an acceptable safety profile. PD-L1 or PD-1/IL-2 bispecific antibodies aim to convert “cold” tumors into “hot” tumors by providing localized T-cell activation signals while simultaneously blocking inhibitory checkpoints. CD47-targeting T-cell engagers engage the innate immune system by blocking the “don’t eat me” signal that allows tumor cells to evade macrophage-mediated destruction, with particular promise in colorectal LM where CD47 expression is elevated.

Beyond these specific agents, a diverse portfolio of innovative approaches is under clinical investigation, including oncolytic viruses, therapeutic cancer vaccines, adoptive cell therapies, innate immune agonists, metabolic modulators, and microbiome interventions. The optimal strategy likely involves rational combinations that address multiple resistance mechanisms simultaneously while maintaining acceptable toxicity profiles.

The exploration of neoadjuvant checkpoint inhibitor approaches in nonmetastatic MSS CRC offers additional promise. By treating patients when their immune systems are intact and tumor burden is lower, neoadjuvant strategies may achieve higher response rates than in the heavily pretreated metastatic setting. The ability to assess pathological response provides an early readout of treatment efficacy and enables biomarker discovery. However, substantial work remains to identify which MSS CRC patients will benefit from neoadjuvant immunotherapy and which combination approaches are most effective.

Several key principles have emerged from the clinical experience to date:

(1) Monotherapy is insufficient: Single-agent checkpoint inhibitors produce minimal activity in unselected MSS CRC populations.

(2) Mechanism-based combinations are essential: Success requires partner agents that address specific resistance mechanisms rather than empiric combinations.

(3) Biomarkers are critical: The modest ORRs demand predictive biomarkers to identify patients most likely to benefit.

(4) LM matter: The immunosuppressive liver microenvironment requires specific consideration in trial design and patient selection.

(5) Earlier intervention may be better: The neoadjuvant setting offers biological advantages that may translate to improved efficacy.

Looking forward, the path to effective immunotherapy for MSS CRC requires continued innovation across multiple fronts. Clinically, we need larger randomized trials of the most promising combination approaches with appropriate biomarker stratification. Scientifically, we need deeper understanding of resistance mechanisms and how they vary across patient populations and anatomic sites. Technologically, we need validation and implementation of next-generation biomarkers including AI-driven tools. Therapeutically, we need continued development of novel agents with distinct mechanisms of action.

The ultimate goal is to achieve for MSS CRC what has been accomplished in MSI-H disease: durable responses and prolonged survival with manageable toxicity. While we have not yet reached that goal, the trajectory of progress over the past 5 years provides genuine optimism. The combination of mechanistically distinct therapeutic agents, refined biomarker strategies, and earlier intervention in the disease course holds significant promise for transforming outcomes in this challenging patient population.

The journey toward effective immunotherapy for MSS CRC exemplifies the evolution of precision oncology. Rather than applying a one-size-fits-all approach, success requires understanding the specific biological barriers in each tumor type and developing rational strategies to overcome them. For MSS CRC, this means converting immunologically “cold” tumors into “hot” tumors, depleting immunosuppressive cells while activating effector populations, engaging both innate and adaptive immunity, and selecting patients most likely to benefit based on sophisticated biomarker assessment.

As we move forward, the integration of novel therapeutics, validated biomarkers, and innovative trial designs will be essential to realizing the promise of immunotherapy for patients with MSS CRC. While challenges remain, the current momentum in both clinical and translational research provides hope that MSS CRC will transition from an immunotherapy-resistant disease to one where meaningful and durable responses can be achieved through precision, biology-driven approaches.

ARTICLE INFORMATION

Conflict of interest

Sun Young Kim participates in advisory boards for Roche Korea, AstraZeneca, Natera, Guardant Health, and Merck Sharp & Dohme Korea. No other potential conflict of interest relevant to this article was reported.

Funding

This study was supported by a research funding from Roche Korea.

Table 1.

Phase 2/3 randomized trials of immunotherapy in microsatellite-stable metastatic colorectal cancer

Trial	Phase	Treatment	No. of patients		ORR (%)		Median PFS (mo)		Median OS (mo)		HR for OS (95% CI)	P-value	Status/outcome
Trial	Phase	Treatment	Immunotherapy	Control	Immunotherapy	Control	Immunotherapy	Control	Immunotherapy	Control	HR for OS (95% CI)	P-value	Status/outcome
IMblaze370 [13]	3	Atezolizumab+cobimetinib vs. Regorafenib	183	90	3	2	1.9	2.0	8.9	8.5	1.00 (0.73–1.38)	NS	Negative
CCTG CO.26 [14]	2	Durvalumab+tremelimumab vs. BSC	90	90	1	0	1.8	1.9	6.6	4.1	0.72 (0.54–0.97)	0.03	Modest OS benefit, minimal response
AtezoTRIBE [15]	2	FOLFOXIRI+bevacizumab±atezolizumab	73	145	59	64	13.1	11.5	33.0	27.2	0.78 (0.61–0.98^a)	0.084	Primary end point (PFS) met in the first-line setting
LEAP-017 [16]	3	Pembrolizumab+lenvatinib vs. Regorafenib or TAS-102	350	350	10.4	1.7	3.8	3.3	9.8	9.3	0.83 (0.68–1.02)	0.038^b	Positive trend; improved ORR
RELATIVITY-123 [18]	3	Nivolumab+relatlimab vs. SOC	NR	NR	NR	NR	NR	NR	NR	NR	NR	NR	Discontinued for futility (December 2023)
STELLAR-303 [17]	3	Zanzalintinib+atezolizumab vs. Regorafenib	451	450	4	1	3.7	2.0	10.9	9.4	0.80	0.0045	Positive (met primary OS endpoint)

ORR, objective response rate; PFS, progression-free survival; OS, overall survival; HR, hazard ratio; CI, confidence interval; NS, not significant; BSC, best supportive care; FOLFOXIRI, fluorouracil, leucovorin, oxaliplatin, and irinotecan; SOC, standard of care; NR, not reported.

^a80% CI. ^bThe P-value did not meet prespecified threshold for statistical significance but showed favorable trend.

Table 2.

Ongoing clinical trials of immunotherapy for MSS CRC as of October 2025

ClinicalTrials.gov identifier	Country	Investigational drug	Phase	Intervention strategy	Indication/eligibility
NCT06829355	China	Thymalfasin+regorafenib+tislelizumab	2	Triple immunotherapy (cytokine+ICI+TKI)	Advanced/MSS mCRC; ≥2 prior lines
NCT06336902	USA	Botensilimab+balstilimab+fasting-mimicking diet+vitamin C	1/2	Dual ICI+metabolic+antioxidant	KRAS-mutant MSS mCRC; prior chemotherapy
NCT06530303	China	TIL therapy+pembrolizumab+IL-2	1/2	TIL therapy+ICI+cytokine	Advanced/metastatic refractory CRC
NCT06300463	USA	Randomized trial of botensilimab+balstilimab vs. botensilimab+balstilimab+AGEN1423 vs. botensilimab+balstilimab+radiation	2	IO combinations (PD-1, CTLA-4, CD73, and TGF-β) and radiotherapy	Liver-limited/predominant mCRC; non-MSI-H
NCT06846268	Singapore	ADG126+pembrolizumab	2	IO combination	Stage II–III CRC
NCT06792695	International, multicenter (phase 2 open-label)	Volrustomig (PD-1/VEGF)+FOLFIRI+bevacizumab	2	PD-1/VEGF bispecific+chemotherapy	MSS mCRC, front-line
NCT05319314	USA	GCC19CART	1	CAR-T cell targeting GUCY2C	Refractory mCRC; GUCY2C+ by IHC; progressed after 3+ lines
NCT06718738	China	IM96: CAR-T targeting GUCY2C	1	CAR-T cell targeting GUCY2C	Advanced CRC with ≥2 prior lines; GUCY2C+ by IHC
NCT05240950	China	Anti-CEA CAR-T cells	1	CAR-T targeting CEA	CRC with LMs; postoperative MRD/ctDNA+
NCT05089266	USA	CAR-T cells	1	PD-1 expressing mesothelin CAR-T	Advanced solid tumors including CRC; MSLN+; phase 1 dose-escalation; not yet recruiting
NCT04503980	USA	αPD1-MSLN-CAR-T cells	1 (Early)	PD-1 expressing mesothelin CAR-T	MSLN+ advanced CRC or ovarian cancer; early phase 1; recruiting
NCT05028933	China	EpCAM CAR-T	1	EpCAM-targeted CAR-T	EpCAM+advanced malignancy; phase 1 dose-escalation; recruiting
NCT05415475	China	CEA CAR-T cells	1	CEA-targeted CAR-T	CEA+advanced CRC
NCT06653010	China	Universal GCC CAR-T (REVO-UWD-01)	1 (Early)	Universal CAR-T targeting GCC	GCC+mCRC; LMs; progressed after 3+ lines
NCT05759728	Australia	LGR5 CAR-T (CNA3103)	1/2	LGR5-targeted CAR-T	LGR5+ mCRC; phase 1/2; recruiting
NCT06821048	China	CEA CAR-T targeting multiple GI tumors	1	CEA-targeted CAR-T	CEA+advanced GI tumors; ≥2–3 prior lines
NCT07152210	China	CDH17/GUCY2C CAR-T dual-target	1	Dual-target CAR-T (CDH17/GUCY2C)	CDH17/GUCY2C+ advanced CRC; failed standard therapy
NCT06946745	China	IBI363+bevacizumab+chemotherapy	Observational	PD-1/IL-2 bispecific	MSS CRC

MSS, microsatellite stable; CRC, colorectal cancer; ICI, immune checkpoint inhibitor; TKI, tyrosine kinase inhibitor; mCRC, metastatic colorectal cancer; TIL, tumor-infiltrating lymphocytes; IL-2, interleukin 2; IO, immuno-oncology; PD-1, programmed cell death 1; CTLA-4, cytotoxic T-lymphocyte–associated protein 4; TGF-β, transforming growth factor β; MSI-H, microsatellite instability-high; VEGF, vascular endothelial growth factor; FOLFIRI, irinotecan, 5-FU, and leucovorin; CAR-T, chimeric antigen receptor T; GUCY2C, guanyl cyclase 2C; IHC, immunohistochemical staining; CEA, carcinoembryonic antigen; LM, liver metastasis; MRD, minimal residual disease; ctDNA, circulating tumor DNA; MSLN, mesothelin; EpCAM, epithelial cell adhesion molecule; GCC, guanyl cyclase; LGR5, leucine-rich repeat–containing G protein-coupled receptor 5; GI, gastrointestinal.

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Over and above what is visible and conventional: development of new territories in colorectal cancer management
In Ja Park
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Immunotherapy for microsatellite-stable colorectal cancer: overcoming resistance and exploring novel therapeutic strategies

Ann Coloproctol. 2026;42(1):47-57. Published online February 19, 2026

DOI: https://doi.org/10.3393/ac.2025.01354.0193

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Immunotherapy for microsatellite-stable colorectal cancer: overcoming resistance and exploring novel therapeutic strategies

Trial	Phase	Treatment	No. of patients		ORR (%)		Median PFS (mo)		Median OS (mo)		HR for OS (95% CI)	P-value	Status/outcome
Trial	Phase	Treatment	Immunotherapy	Control	Immunotherapy	Control	Immunotherapy	Control	Immunotherapy	Control	HR for OS (95% CI)	P-value	Status/outcome
IMblaze370 [13]	3	Atezolizumab+cobimetinib vs. Regorafenib	183	90	3	2	1.9	2.0	8.9	8.5	1.00 (0.73–1.38)	NS	Negative
CCTG CO.26 [14]	2	Durvalumab+tremelimumab vs. BSC	90	90	1	0	1.8	1.9	6.6	4.1	0.72 (0.54–0.97)	0.03	Modest OS benefit, minimal response
AtezoTRIBE [15]	2	FOLFOXIRI+bevacizumab±atezolizumab	73	145	59	64	13.1	11.5	33.0	27.2	0.78 (0.61–0.98^a)	0.084	Primary end point (PFS) met in the first-line setting
LEAP-017 [16]	3	Pembrolizumab+lenvatinib vs. Regorafenib or TAS-102	350	350	10.4	1.7	3.8	3.3	9.8	9.3	0.83 (0.68–1.02)	0.038^b	Positive trend; improved ORR
RELATIVITY-123 [18]	3	Nivolumab+relatlimab vs. SOC	NR	NR	NR	NR	NR	NR	NR	NR	NR	NR	Discontinued for futility (December 2023)
STELLAR-303 [17]	3	Zanzalintinib+atezolizumab vs. Regorafenib	451	450	4	1	3.7	2.0	10.9	9.4	0.80	0.0045	Positive (met primary OS endpoint)

ClinicalTrials.gov identifier	Country	Investigational drug	Phase	Intervention strategy	Indication/eligibility
NCT06829355	China	Thymalfasin+regorafenib+tislelizumab	2	Triple immunotherapy (cytokine+ICI+TKI)	Advanced/MSS mCRC; ≥2 prior lines
NCT06336902	USA	Botensilimab+balstilimab+fasting-mimicking diet+vitamin C	1/2	Dual ICI+metabolic+antioxidant	KRAS-mutant MSS mCRC; prior chemotherapy
NCT06530303	China	TIL therapy+pembrolizumab+IL-2	1/2	TIL therapy+ICI+cytokine	Advanced/metastatic refractory CRC
NCT06300463	USA	Randomized trial of botensilimab+balstilimab vs. botensilimab+balstilimab+AGEN1423 vs. botensilimab+balstilimab+radiation	2	IO combinations (PD-1, CTLA-4, CD73, and TGF-β) and radiotherapy	Liver-limited/predominant mCRC; non-MSI-H
NCT06846268	Singapore	ADG126+pembrolizumab	2	IO combination	Stage II–III CRC
NCT06792695	International, multicenter (phase 2 open-label)	Volrustomig (PD-1/VEGF)+FOLFIRI+bevacizumab	2	PD-1/VEGF bispecific+chemotherapy	MSS mCRC, front-line
NCT05319314	USA	GCC19CART	1	CAR-T cell targeting GUCY2C	Refractory mCRC; GUCY2C+ by IHC; progressed after 3+ lines
NCT06718738	China	IM96: CAR-T targeting GUCY2C	1	CAR-T cell targeting GUCY2C	Advanced CRC with ≥2 prior lines; GUCY2C+ by IHC
NCT05240950	China	Anti-CEA CAR-T cells	1	CAR-T targeting CEA	CRC with LMs; postoperative MRD/ctDNA+
NCT05089266	USA	CAR-T cells	1	PD-1 expressing mesothelin CAR-T	Advanced solid tumors including CRC; MSLN+; phase 1 dose-escalation; not yet recruiting
NCT04503980	USA	αPD1-MSLN-CAR-T cells	1 (Early)	PD-1 expressing mesothelin CAR-T	MSLN+ advanced CRC or ovarian cancer; early phase 1; recruiting
NCT05028933	China	EpCAM CAR-T	1	EpCAM-targeted CAR-T	EpCAM+advanced malignancy; phase 1 dose-escalation; recruiting
NCT05415475	China	CEA CAR-T cells	1	CEA-targeted CAR-T	CEA+advanced CRC
NCT06653010	China	Universal GCC CAR-T (REVO-UWD-01)	1 (Early)	Universal CAR-T targeting GCC	GCC+mCRC; LMs; progressed after 3+ lines
NCT05759728	Australia	LGR5 CAR-T (CNA3103)	1/2	LGR5-targeted CAR-T	LGR5+ mCRC; phase 1/2; recruiting
NCT06821048	China	CEA CAR-T targeting multiple GI tumors	1	CEA-targeted CAR-T	CEA+advanced GI tumors; ≥2–3 prior lines
NCT07152210	China	CDH17/GUCY2C CAR-T dual-target	1	Dual-target CAR-T (CDH17/GUCY2C)	CDH17/GUCY2C+ advanced CRC; failed standard therapy
NCT06946745	China	IBI363+bevacizumab+chemotherapy	Observational	PD-1/IL-2 bispecific	MSS CRC

Table 1. Phase 2/3 randomized trials of immunotherapy in microsatellite-stable metastatic colorectal cancer

^a80% CI. ^bThe P-value did not meet prespecified threshold for statistical significance but showed favorable trend.

Table 2. Ongoing clinical trials of immunotherapy for MSS CRC as of October 2025