LEXINGTON, Mass., July 6, 2026
Agenus announced new three-year follow-up results from the fully enrolled Phase 1b C-800-01 study evaluating botensilimab (BOT), an Fc-enhanced multifunctional anti-CTLA-4 antibody, in combination with balstilimab (BAL), an anti-PD-1 antibody, in patients with refractory microsatellite-stable (MSS) metastatic colorectal cancer (mCRC) without active liver metastases. The findings were presented at the European Society for Medical Oncology Gastrointestinal (ESMO GI) Congress 2026 in Munich, Germany, demonstrating durable long-term survival in a patient population that historically has limited treatment options and poor outcomes after progression on standard therapies.
Phase 1b Study Demonstrated Durable Survival Beyond Historical Expectations
The fully enrolled trial included 123 heavily pretreated patients, with participants receiving a median of three prior lines of therapy. Extended follow-up showed a median overall survival of 21.2 months, substantially exceeding historical survival benchmarks of approximately 10–14 months for comparable patient populations. The study reported a 33% three-year overall survival rate, while the Kaplan-Meier survival curve demonstrated a plateau beyond two years, suggesting sustained long-term benefit for a subset of patients. Investigators also reported a 21% confirmed objective response rate, including three complete responses and 23 partial responses, while the median duration of response had not yet been reached, with responses continuing beyond 37 months in some patients.
Treatment-Free Survival Highlights Lasting Clinical Benefit
One of the study’s most notable findings was the durability of treatment benefit after therapy completion. At the latest follow-up, 17% of enrolled patients remained alive without receiving any systemic anticancer therapy, including 13 patients who had achieved confirmed responses. Tumor regression was observed in more than 40% of participants, while disease control was achieved in 69% of patients at six weeks and the clinical benefit rate reached 28% at 24 weeks. A post hoc analysis of patients previously treated with multiple late-line therapies, including regorafenib, trifluridine/tipiracil, or fruquintinib, also demonstrated encouraging activity, with a 30% three-year overall survival rate and a median overall survival of 16.2 months, supporting the regimen’s activity even in highly refractory disease.
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Extended Safety Data Support Ongoing Phase 3 BATTMAN Trial
The extended safety analysis identified no new safety signals and no treatment-related deaths following longer follow-up. Immune-mediated diarrhea and colitis remained the most common immune-related adverse event but resolved in 98% of affected patients, with a median resolution time of 14 days. The selected Phase 3 dosing regimen of BOT 1 mg/kg plus BAL demonstrated improved tolerability compared with the higher-dose regimen, reducing rates of severe immune-mediated diarrhea and colitis. The encouraging efficacy, durable treatment-free survival, and manageable safety profile provide strong support for the ongoing Phase 3 BATTMAN trial, which is evaluating botensilimab plus balstilimab in patients with refractory MSS/proficient mismatch repair (pMMR) metastatic colorectal cancer. The latest results further reinforce the potential of the BOT+BAL combination to redefine immunotherapy outcomes in one of the most treatment-resistant forms of colorectal cancer.
Source: Agenus press release
