Agenus has reported deep tumour responses and no observed colorectal cancer recurrences in longer follow-up from a Phase 2 study of botensilimab and balstilimab before surgery for resectable colon cancer.

The updated results, published in Clinical Cancer Research, showed a 32% pathologic complete response rate among patients with mismatch repair proficient and microsatellite stable (pMMR/MSS) tumours, a group that has historically responded poorly to immunotherapy.

The NEST study also found that 88% of evaluable patients with detectable circulating tumour DNA (ctDNA) at baseline cleared it before surgery, while all patients proceeded to planned surgical resection without treatment-related delays.

Deep responses reported in pMMR/MSS tumours

NEST evaluated the combination of botensilimab, an anti-CTLA-4 antibody, and balstilimab, an anti-PD-1 antibody, as neoadjuvant treatment before surgery in patients with resectable colorectal cancer.

Among 22 pMMR/MSS tumours, 59% achieved a pathologic response. This included a 41% major pathologic response rate, defined as 10% or less viable tumour remaining, and a 32% pathologic complete response rate, where no viable tumour was detected in the resected tumour or adjacent lymph nodes.

The updated analysis had a data cut-off of 31 March 2026. No colorectal cancer recurrences had been observed at that point, with median follow-up of 32.2 months in NEST-1 and 23.5 months in NEST-2.

The study was small, however, enrolling 24 eligible patients with 26 resectable colorectal tumours. It was also single-centre, open-label and single-arm, meaning the results cannot establish whether the treatment reduces recurrence compared with standard treatment.

ctDNA clearance before surgery

The researchers also assessed circulating tumour DNA, which can provide an indication of residual cancer that may not be detected through conventional imaging or pathology.

Among patients with detectable ctDNA at baseline and an available sample before surgery, 88% cleared the circulating tumour DNA before resection. ctDNA remained undetectable after surgery in all evaluated patients.

Patients received botensilimab and balstilimab for approximately four weeks in NEST-1 or approximately eight weeks in NEST-2 before undergoing planned surgery.

Pashtoon M Kasi, medical director of GI medical oncology at City of Hope Orange County and originator of the NEST study, said: “The NEST results reinforce a major opportunity in colorectal cancer to use immunotherapy earlier, before surgery, when the primary tumor and immune system are still positioned to generate a coordinated anti-tumor response.”

He added that the combination of pathologic responses, ctDNA clearance and immune changes was encouraging in pMMR/MSS disease.

Findings support larger Phase 3 study

Analysis of paired tumour samples found changes in the tumour immune microenvironment in responding tumours, including increased CD8+ T-cell infiltration and reduced regulatory T cells.

The findings are being used to support Agenus’ plans for ROBBIN, a global randomised Phase 3 trial of neoadjuvant botensilimab and balstilimab followed by standard of care compared with standard of care alone in previously untreated patients with high-risk Stage 2 or Stage 3 MSS colon cancer.

The planned Phase 3 study will use event-free survival as its primary endpoint.

Steven O’Day, chief medical officer of Agenus, said: “With longer follow-up now extending beyond two years across both NEST cohorts, the findings show BOT+BAL can generate deep tumor responses and immune activation before surgery, without delaying surgery.”

The NEST findings therefore provide early clinical evidence for investigating the combination before surgery in pMMR/MSS colon cancer, but the small Phase 2 study means the potential effect on long-term recurrence and survival remains to be established in larger controlled trials.