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BOT+BAL is an investigational combination of botensilimab, an anti-CTLA-4 antibody, and balstilimab, an anti-PD-1 antibody. A small, nonrandomized phase 1b ovarian-cancer study found confirmed tumor responses in some heavily pretreated patients, but it cannot show whether the combination works better than other treatments or establish it as standard care. Agenus reported longer-term follow-up from the same patients in 2026; the combination is not FDA-approved.
What are botensilimab and balstilimab?
Botensilimab (BOT) and balstilimab (BAL) are immune checkpoint inhibitors being studied together. BAL blocks PD-1 from interacting with PD-L1 and PD-L2, signals that can restrain immune activity. BOT blocks CTLA-4 and is engineered with an enhanced Fc region, intended to engage activating Fc gamma receptors and influence T-cell priming, regulatory T cells, and myeloid cells.
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The proposed rationale is that the drugs may affect different parts of the immune response. These mechanisms explain why the combination is being studied; they do not prove that a particular person’s tumor will respond.
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The peer-reviewed report describes C-800-01, an open-label, multicenter phase 1b study of botensilimab with or without balstilimab. The ovarian-cancer cohort included people with confirmed recurrent disease whose standard therapy had failed or was unavailable. Patients enrolled at nine U.S. sites from April 1, 2019, through November 8, 2023; some had previously received checkpoint inhibitors.
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In the combination cohort, the study protocol used intravenous botensilimab at 1 or 2 mg/kg every six weeks and intravenous balstilimab at 3 mg/kg every two weeks, for up to two years. These are trial-protocol details, not approved prescribing instructions or a recommendation for an individual patient.
What results have been reported?
Published phase 1b results
The 2025 peer-reviewed report included 44 patients in its safety analysis and 35 in its efficacy analysis. In the safety group, patients had received a median of three prior lines of therapy; median follow-up for the primary report was 9.6 months.
| Measure | Result in the published report |
|---|---|
| Confirmed objective response | 23% (8 of 35; 95% confidence interval [CI], 10%–40%): one complete response and seven partial responses |
| Clinical benefit | 31% (11 of 35; 95% CI, 17%–49%). Defined as a complete or partial response, or stable disease for at least 24 weeks. |
| Median duration of response | 9.7 months (95% CI, 2.8 months to not reached) |
| Median progression-free survival | 2.8 months (95% CI, 1.4–5.5) |
| Median overall survival | 14.8 months (95% CI, 12.1 months to not reached) |
| Overall survival at 12 months | 75% (95% CI, 55%–86%) |
These results come from a small, single-arm study: there was no comparison group receiving another treatment. The response and survival figures therefore describe outcomes in this cohort, not proof that BOT+BAL caused better outcomes than an alternative would have.
Longer-term update reported in 2026
At the October 3, 2026, International Gynecologic Cancer Society (IGCS) meeting, Agenus reported follow-up from the same cohort, with a data cutoff of December 13, 2025. In the 35-patient efficacy group, estimated overall survival was 48% at both two and three years; median overall survival remained 14.8 months. The company also reported that 11 of the 44 treated patients (25%) were alive and off treatment at last follow-up. This was a company-reported conference update, not an independent replication or a new randomized study.
Agenus also reported estimated three-year overall survival of 47% among 25 patients with platinum-resistant or platinum-refractory disease and 45% among 10 with platinum-sensitive disease. The resistant/refractory subgroup had an objective response rate of 20% (5 of 25); three of the 10 patients in the platinum-sensitive subgroup had partial responses. These small subgroup estimates are exploratory and do not establish equal benefit across platinum-sensitivity groups. In the 2026 company update, the cohort had received a median of four prior lines of therapy; all had received platinum, 77% bevacizumab, and 57% a PARP inhibitor.
What side effects and treatment risks were reported?
All 44 patients in the published safety analysis experienced at least one treatment-emergent adverse event, and 89% had an event considered treatment-related. Diarrhea or colitis was the most common treatment-related event, reported in 43% of patients and at grade 3 in 16%. Fatigue and nausea each occurred in 36%. Grade 3 or higher treatment-related adverse events occurred in 41%.
Nineteen patients (43%) stopped botensilimab and/or balstilimab because of a treatment-related adverse event; immune-mediated enterocolitis and colitis were notable reasons. The report recorded no treatment-related deaths, and Agenus said its 2026 update showed no new safety signals or treatment-related deaths. The frequency and severity of gut-related immune toxicity make it important to discuss warning signs, monitoring, and treatment-discontinuation risks with an oncology team.
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Do biomarkers show who might benefit?
Exploratory analyses associated response with higher levels of FcγRIIIA-positive/CD11c-positive cells and higher PD-L1 expression; tumors with T-cell infiltration were associated with clinical benefit. The study also found differences in immune architecture by histologic subtype. These findings are hypothesis-generating, not a validated test for choosing patients, and should not be used alone to predict an individual’s outcome.
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Is BOT+BAL approved, and how can patients ask about access?
Agenus stated in its October 2026 release that neither botensilimab nor balstilimab is approved by the U.S. Food and Drug Administration. The company describes access as limited to clinical trials and authorized early-access mechanisms where permitted by local rules. It says eligible patients treated in French hospitals who meet predefined criteria may receive reimbursed treatment through the AAC pathway. Availability and costs elsewhere depend on local rules and programs.
People considering the combination can ask their oncologist about standard treatment options, clinical-trial eligibility, and whether an authorized early-access route exists where they live. Trial-protocol dosing and early results are not individualized treatment advice.
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