Shares of Agenus Inc. (AGEN) traded at $7.67, down 1.79%, as newly published colorectal cancer data for the company's BOT+BAL combination drew attention. Agenus reported mature Phase 1b results from 123 heavily pretreated patients with microsatellite-stable (MSS) metastatic colorectal cancer, with the cohort achieving a median overall survival of 21.2 months and a three-year overall survival rate of 33%. The confirmed response rate reached 21%, comprising three complete and 23 partial responses, and responses were observed even in tumors with low tumor mutational burden and no detectable PD-L1. Extended follow-up turned up no new safety signals and no treatment-related deaths.

In the fully enrolled C-800-01 study, patients received botensilimab (BOT) at 1 mg/kg or 2 mg/kg every six weeks in combination with balstilimab (BAL) at 3 mg/kg every two weeks. Safety and tolerability served as the trial's primary endpoint, while secondary endpoints covered response, response duration, disease control, and progression-free survival. Overall survival remained an exploratory measure.

Durable Survival in Refractory MSS Colorectal Cancer

Agenus evaluated botensilimab plus balstilimab in patients without active liver metastases who had limited remaining treatment options. Before entering the fully enrolled C-800-01 cohort, participants had undergone a median of three prior lines of therapy. The findings appear in Clinical Cancer Research, following an earlier presentation of the three-year efficacy data at the ESMO GI Congress 2026.

MSS tumors account for most cases of metastatic colorectal cancer — one of the most commonly diagnosed cancers worldwide — and generally respond poorly to conventional checkpoint immunotherapy. Among comparable refractory patients, available later-line treatments have reported median overall survival in the range of roughly 10 to 14 months. Measured against that historical backdrop, BOT+BAL produced a median overall survival of 21.2 months across the Phase 1b cohort. Because C-800-01 is a single-arm study without a randomized comparator, those benchmark figures provide context rather than a direct head-to-head comparison.

The confirmed objective response rate reached 21%, including three complete responses and 23 partial responses. Median response duration was not reached, and reported responses continued for at least 37.4 months during follow-up. Disease control stood at 69% at six weeks, with a 24-week clinical benefit rate of 28%.

Heavily Pretreated Patients Retain Clinical Benefit

A separate analysis covered 37 patients who had already exhausted available later-line therapies before starting BOT+BAL. Having received a median of five earlier treatment lines, this subgroup represented a particularly refractory disease setting. Even in these patients, the confirmed objective response rate reached 22% and median overall survival came to 16.2 months.

The subgroup recorded a three-year overall survival rate of 30%, indicating continued activity after several earlier treatments. Median response duration reached 16.6 months, and disease control reached 70% in the exploratory subgroup analysis. The 24-week clinical benefit rate of 27% further pointed to preserved activity despite extensive prior therapy.

Across the broader cohort, 17% of patients remained alive without any systemic anticancer therapy at the last follow-up. That treatment-free group included 13 responders, showing that a number of patients sustained clinical benefit after study treatment ended. Notably, patients received a median of just two BOT doses and six BAL doses, despite the durable activity reported.

Biomarker and Safety Findings Support Further Development

Exploratory biomarker analyses detected responses in tumors with low tumor mutational burden and no detectable PD-L1 expression. Across the evaluable population included in the publication, neither biomarker was associated with response, meaning BOT+BAL activity was not confined to tumors carrying the conventional biological markers linked with checkpoint sensitivity.

Botensilimab is designed by Agenus as an Fc-enhanced anti-CTLA-4 antibody intended to support both innate and adaptive immune responses. Its mechanism aims to activate Fc-gamma receptors, promote T-cell priming, reduce regulatory T cells, and reshape the tumor environment. Balstilimab blocks PD-1 interactions. Botensilimab has also received U.S. Food and Drug Administration Fast Track designation in relapsed or refractory MSS metastatic colorectal cancer, a status intended to speed the development and review of therapies for serious conditions with unmet medical need. The combined evidence supports planned Phase 3 ROBBIN testing alongside neoadjuvant NEST data, the next steps toward confirming these findings in larger patient populations.

Extended follow-up identified no new safety signals, and researchers reported no treatment-related deaths across the cohort. Immune-mediated diarrhea or colitis resolved in 98% of affected patients, with median resolution taking 14 days. Both BOT dose levels produced 21% response rates, while the 1 mg/kg regimen showed fewer immune-mediated adverse events.