Nature Communications | Phase I Trial of mKRAS Peptide Vaccine Combined with Dual Immune Checkpoint Blockade in Metastatic Colorectal Cancer

This study provides a novel combination strategy for immunotherapy in MMRp/MSS colorectal cancer, suggesting that KRAS mutant vaccines can enhance T-cell responses and promote tumor infiltration, offering direct reference value for vaccine design and clinical intervention strategies in the field of tumor immunology.

 

Literature Overview

The article titled "Mutant KRAS peptide vaccine with dual checkpoint blockade in metastatic colorectal cancer: a phase I trial," published in Nature Communications, systematically investigates the safety, immunogenicity, and preliminary efficacy of mKRAS-VAX combined with nivolumab and ipilimumab in patients with metastatic MMRp/MSS colorectal cancer who have undergone multiple lines of prior therapy. Using a single-arm phase I design and enrolling 13 patients, the study demonstrates that the combination regimen is well tolerated and successfully induces broad KRAS-specific T-cell responses. Notably, tumor regression and disease control were observed even in refractory patients with liver metastases, offering new hope for individuals traditionally unresponsive to immunotherapy.

Background Knowledge

Colorectal cancer (CRC) is the second leading cause of cancer-related deaths worldwide, with approximately 95% of metastatic cases exhibiting the MMRp/MSS phenotype. These tumors are typically unresponsive to immune checkpoint inhibitors (ICIs), primarily due to low tumor mutational burden (TMB) and a highly immunosuppressive tumor microenvironment (TME), which hinder the activation and infiltration of tumor-specific T cells. Although dual immune checkpoint blockade targeting PD-1 and CTLA-4 has shown limited efficacy in some high-TMB patients, it is nearly ineffective in those with liver metastases, highlighting the urgent need for new strategies to enhance anti-tumor immunity.
KRAS is the most common driver gene in CRC, with approximately 40% of cases harboring KRAS mutations, mostly clonal events, making it an ideal therapeutic target. However, KRAS has long been considered 'undruggable,' with small-molecule inhibitors only effective against the KRAS G12C variant—a mutation rare in MMRp/MSS CRC. Thus, therapeutic vaccines targeting KRAS mutations represent a potential pathway to overcome ICI resistance. Previous vaccine studies have often been limited by insufficient immunogenicity or by masking KRAS-specific responses due to inclusion of multiple neoantigens. This study addresses these challenges by designing an 'off-the-shelf' peptide vaccine, mKRAS-VAX, covering six common KRAS mutations, aiming to efficiently activate mutant KRAS-specific T cells without requiring personalized manufacturing and to synergize with ICIs to break immune tolerance.

 

 

Research Methods and Experiments

The study employed a single-arm phase I clinical trial design (NCT04117087), enrolling 13 patients with previously treated MMRp/MSS KRAS-mutant mCRC. The intervention consisted of mKRAS-VAX (comprising six 21-mer peptides covering G12V, G12A, G12R, G12C, G12D, and G13D mutations) in combination with nivolumab and ipilimumab. Primary endpoints were safety and immunogenicity within 17 weeks. KRAS-specific T-cell responses induced by the vaccine were assessed using IFNγ ELISpot assays on peripheral blood mononuclear cells (PBMCs), with in vitro stimulation (IVS) applied to enhance detection sensitivity. Paired tumor biopsies from 5 patients were analyzed using TCRβ sequencing to track the dynamics of vaccine-induced T-cell clones within tumor tissue.

Key Conclusions and Perspectives

• All adverse events related to mKRAS-VAX were grade 1–2, with no increase in severe immune-related adverse events (irAEs) associated with dual ICI therapy, indicating acceptable safety of the combination regimen and supporting further exploration in advanced patients.
• Among 12 evaluable patients, 75% (8/12) showed KRAS-specific T-cell responses via direct peripheral blood ELISpot within 17 weeks post-vaccination, and 100% tested positive after in vitro expansion, demonstrating that mKRAS-VAX effectively activates mutant KRAS-specific T cells and provides a reliable biomarker for subsequent immune monitoring.
• TCR sequencing revealed that vaccine-induced KRAS-specific T-cell clones could migrate into tumor tissue, with tumor regression primarily driven by de novo clonal expansion, directly linking tumor shrinkage to vaccine-induced T-cell clonal expansion and providing strong mechanistic evidence for cancer immunotherapy.
• Among 13 patients, the objective response rate (ORR) was 15% and disease control rate (DCR) was 38%, including one patient with liver metastasis achieving partial response—challenging the traditional notion of 'absolute resistance to ICIs in liver metastases'—suggesting that KRAS vaccination may overcome specific immunosuppressive microenvironments.
• The vaccine primarily induced CD4+ T-cell responses, many of which were multifunctional. Despite MHC-II not being expressed on tumor cells, these CD4+ T cells may indirectly exert anti-tumor effects by assisting CD8+ T cells or modulating myeloid cells, offering insights for optimizing future vaccine designs to enhance CD8+ responses.

Research Significance and Prospects

This study opens a new avenue for treating MMRp/MSS colorectal cancer, demonstrating that off-the-shelf vaccines targeting KRAS can synergize effectively with ICIs to activate systemic and tumor-infiltrating specific T cells, thereby creating a 'hot' microenvironment within traditionally 'cold' tumors. This strategy avoids the high cost and long development timelines associated with personalized vaccines, enhancing clinical accessibility. Future studies should validate these findings in larger cohorts and explore its potential as a first-line maintenance therapy to maximize immune activation when tumor burden is low.

From a translational medicine perspective, the study underscores the importance of analyzing intratumoral T-cell clonal dynamics, suggesting that peripheral blood monitoring alone may be insufficient to reflect true anti-tumor immune status. Therefore, multimodal monitoring strategies combining liquid biopsy with tissue-based TCR sequencing may more accurately predict treatment response. Additionally, the observed pseudoprogression indicates that more flexible efficacy criteria may be needed when evaluating mKRAS vaccine combined with ICIs.

 

 

Conclusion

This study establishes the safety and immunogenicity of mKRAS-VAX combined with dual immune checkpoint blockade in patients with MMRp/MSS metastatic colorectal cancer, and for the first time demonstrates that this combination can induce KRAS-specific T cells to infiltrate tumors and mediate tumor regression—even showing efficacy in patients with liver metastases. These findings provide a crucial clinical translation pathway for MMRp/MSS CRC patients who have long lacked effective immunotherapies. The study not only validates the feasibility of KRAS as an immunotherapeutic target but also reveals the association between de novo T-cell clonal expansion and tumor regression, offering key insights for future mechanistic studies and biomarker development. Moving forward, this strategy could be combined with chemotherapy, targeted therapy, or novel immune modulators to further improve outcomes. From bench to bedside, this work provides a blueprint for developing immune interventions against 'undruggable' targets, advancing the field of precision cancer immunotherapy and potentially reshaping the standard of care for colorectal cancer.

 

Hejia Henry Wang, Amanda L Huff, S Daniel Haldar, Nilofer S Azad, and Neeha Zaidi. Mutant KRAS peptide vaccine with dual checkpoint blockade in metastatic colorectal cancer: a phase I trial. Nature Communications.