Agenus Inc. announced three-year Phase 1b data from the fully enrolled C-800-01 cohort evaluating botensilimab (BOT) plus balstilimab (BAL) in patients with refractory microsatellite-stable (MSS) metastatic colorectal cancer (mCRC) without active liver metastases. The data were presented at the European Society for Medical Oncology Gastrointestinal Cancers (ESMO GI) Congress 2026 on July 2 in Munich, Germany. The combination demonstrated clinically meaningful long-term survival in a heavily pretreated population where durable long-term survival is rarely reported.
BOT+BAL showed a median overall survival of 21.2 months and a three-year overall survival rate of 33%, with the Kaplan-Meier curve indicating a plateau beyond two years. Available later-line standards in this setting have historically reported median overall survival of approximately 10–14 months. The dataset includes 26 confirmed responses, with a median duration of response not reached. Additionally, 21 patients, or 17%, were alive and off all systemic anticancer therapy at last follow-up, including 13 responders.
Key Efficacy Results
The Phase 1b cohort included 123 patients who had received a median of three prior lines of therapy. Key efficacy outcomes from the study are detailed below:
| Metric |
Result |
| Median overall survival |
21.2 months |
| 24-month overall survival rate |
41% |
| 36-month overall survival rate |
33% |
| Confirmed objective response rate |
21% (3 complete, 23 partial) |
| Disease control rate (6 weeks) |
69% |
| Clinical benefit rate (24 weeks) |
28% |
In a post hoc late-line–exposed subgroup of 37 patients who had received at least one prior regimen of regorafenib, trifluridine/tipiracil with or without bevacizumab, or fruquintinib, BOT+BAL showed a confirmed objective response rate of 22% and a median overall survival of 16.2 months. The three-year overall survival rate in this subgroup was 30%.
Safety Profile
Extended safety follow-up showed no new safety signals and no treatment-related deaths. Immune-mediated diarrhea/colitis, the most common immune-mediated adverse event, resolved in 98% of affected patients with a median time to resolution of 14 days. The selected Phase 3 regimen of BOT 1 mg/kg plus BAL demonstrated improved tolerability, with lower rates of immune-mediated diarrhea/colitis compared to the 2 mg/kg regimen.
"These three-year data are important because they show a pattern of benefit that is not typically expected in refractory MSS colorectal cancer," said Benjamin L. Schlechter, M.D., of Dana-Farber Cancer Institute and presenting author of the study. "Seeing a subset of patients remain alive and off systemic anticancer therapy after treatment speaks to the clinical relevance of these results."
The findings support continued evaluation of BOT+BAL in MSS mCRC and provide rationale for the ongoing randomized Phase 3 BATTMAN trial. The C-800-01 study is a first-in-human Phase 1b clinical trial evaluating botensilimab with or without balstilimab in patients with advanced solid tumors.