Neoadjuvant botensilimab plus balstilimab in resectable mismatch repair proficient and deficient colorectal cancer: NEST-1 clinical trial.

117 Background: There is an unmet need to have an effective systemic immunotherapeutic option for patients with mismatch repair proficient/microsatellite stable (pMMR/MSS) colorectal cancer. The neoadjuvant platform presents an ideal setting as a window of opportunity to evaluate new drugs. The NEST-1 trial explored the safety and efficacy of neoadjuvant botensilimab (BOT), an Fc-enhanced next-generation anti–CTLA–4 antibody, alongside balstilimab (BAL; an anti-PD-1 antibody) in patients with colon and rectal cancer who were candidates for surgery. Methods: NEST-1, was a single-arm trial testing the feasibility, safety, and efficacy of the BOT/BAL regimen in neoadjuvant settings for patients with colorectal cancer before resection. All patients received 1 fixed dose of 75 mg of BOT and 2 fixed doses of 240 mg of BAL 2 weeks apart. Patients could proceed to surgery 1 week after completion of the 2 nd dose of BAL. Less than 25% of patients were allowed to be mismatch repair deficient/microsatellite instability-high (dMMR/MSI-H). Other inclusion and exclusion criteria were in accordance with ongoing immunotherapy-based clinical trials. Results: The study met its primary endpoints. A total of 12 patients with colon and rectal cancers were safely treated with the BOT/BAL combination without delaying surgery or increasing the risk of any severe adverse events. Significant regression of tumors was noted (Table). Spatial biology analyses on pre-treatment biopsy and post-treatment surgical samples using RareCyte Inc. revealed novel mechanisms of action and patterns of response/regression; most of the viable tumor if present was superficially oriented near the luminal surface. ctDNA reductions correlated with pathological responses. Conclusions: BOT/BAL appears to be a safe and active regimen both in pMMR/MSS and dMMR/MSI-H colon as well as rectal cancers. The downstaging observed may spare surgery and/or adjuvant chemotherapy. Given the findings, the study has now expanded to two additional cohorts that are now enrolling to study further the dosing, and timing and/or need for surgery in these patients. Clinical trial information: NCT05571293 . [Table: see text]