LONDON, Boston & Budapest, July 2, 2026
Treos Bio announced new translational findings from its Phase Ib/II OBERTO-301 clinical study demonstrating that PolyPEPI1018, combined with anti-PD-L1 immunotherapy (atezolizumab), remodeled the tumor immune microenvironment in patients with microsatellite-stable metastatic colorectal cancer (MSS mCRC). The data, presented at the ESMO Gastrointestinal Cancers Congress 2026, were generated through a retrospective translational analysis conducted in collaboration with Mayo Clinic. Researchers evaluated paired tumor biopsies and peripheral blood samples collected before and after treatment and observed significant increases in intratumoral CD8+ T-cell infiltration, PD-L1 expression, and expansion of tumor-reactive T-cell receptor (TCR) clonotypes. The findings provide mechanistic evidence that PolyPEPI1018 may convert immunologically “cold” MSS colorectal tumors into a more inflamed and potentially checkpoint inhibitor-responsive tumor microenvironment, supporting continued clinical development of the combination therapy.
Immune Remodeling Linked to Improved Clinical Outcomes
The translational analysis demonstrated that treatment with PolyPEPI1018 plus atezolizumab increased CD8+ T-cell density and PD-L1 expression within tumors, indicating enhanced immune activation following therapy. T-cell receptor sequencing further revealed selective expansion of tumor-reactive TCR clonotypes, including immune cells targeting PolyPEPI1018 antigens, additional tumor-associated antigens, and newly recognized neoantigens, suggesting evidence of antigen spreading beyond the vaccine’s original targets. Importantly, patients with higher numbers of tumor-reactive TCR clonotypes and greater post-treatment CD8+ tumor-infiltrating lymphocyte (TIL) density experienced longer progression-free survival and overall survival, reinforcing the relationship between immune activation and favorable clinical outcomes. These results strengthen the biological rationale for combining therapeutic cancer vaccines with checkpoint inhibitors in difficult-to-treat solid tumors.
PolyPEPI1018 Addresses a Major Challenge in MSS Colorectal Cancer
Microsatellite-stable metastatic colorectal cancer (MSS mCRC) has historically shown limited response to immune checkpoint inhibitors because these tumors typically lack sufficient immune cell infiltration to generate durable anti-tumor responses. According to Treos Bio, the new findings suggest that PolyPEPI1018 can reshape the tumor immune environment by stimulating broad multi-antigen T-cell responses, increasing tumor inflammation, and potentially improving sensitivity to anti-PD-L1 therapy. The observed immune remodeling supports the concept that therapeutic vaccination may overcome one of the biggest barriers to immunotherapy in MSS colorectal cancer, offering a potential strategy to expand treatment options for patients with this common and challenging disease.
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Treos Bio Advances PEPI Technology Across Solid Tumors
Treos Bio is a clinical-stage biotechnology company developing off-the-shelf and personalized cancer immunotherapies using its proprietary PEPI Technology, which is designed to identify promiscuous epitopes capable of generating broad T-cell responses across diverse patient populations. PolyPEPI1018, the company’s lead immunotherapy candidate, is being developed specifically for MSS colorectal cancer, while additional programs are advancing across multiple solid tumor indications. The latest translational data presented at ESMO GI 2026 provide further validation of the PEPI platform by demonstrating measurable immune remodeling within tumors and its association with improved patient outcomes. These findings support continued clinical evaluation of PolyPEPI1018 in combination with checkpoint inhibitors and reinforce Treos Bio’s strategy of developing next-generation cancer immunotherapies designed to improve treatment responses in immunologically resistant cancers.
Source: Treos Bio press release
