Real-world outcomes of neoadjuvant immunotherapy for mismatch repair-deficient colorectal cancer: A single center experience in Russia.

e15647 Background: Microsatellite instability (MSI) and mismatch repair deficiency (dMMR) are key biomarkers predicting positive responses to immunotherapy in colorectal cancer (CRC). The NICHE-2 study showed promising outcomes with neoadjuvant immunotherapy in MSI/dMMR CRC. However, real world data at this point are limited. This study aims to assess the clinical response and feasibility of neoadjuvant immunotherapy in MSI/dMMR CRC in a single community based comprehensive cancer center in Russia, providing insights into its efficacy and potential broader application in this population. Methods: We identified 37 patients who received immunotherapy for locally advanced colorectal cancer between 2019 and 2023: 32 patients with colon cancer and 5 patients with rectal cancer. Treatment effects, pathological outcomes, disease-free survival, and overall survival were calculated and analyzed. Results: The patient characteristics are listed in the table below. 37 patients received immunotherapy: 25 (67.6%) – Ipi-Nivo and 12 (32.4%) – Mono Pembro. Surgery was performed in 17 patients (53.1%). 15 patients did not receive surgery: 3 patients due to comorbidities, 11 patients refused surgery and 1 patient progressed. pCR was achieved in 82.4% of patients who underwent surgical resection. With a median follow-up of 14 months, the 2 year EFS was 85% and median was not reached, 2 year OS was 84% Conclusions: Neoadjuvant immunotherapy demonstrated a high rate of pathological complete response (pCR) in patients with MSI/dMMR colorectal cancer, with 82.4% of surgical patients achieving pCR. The treatment resulted in a promising 2-year event-free survival (EFS) of 85% and overall survival (OS) of 84%. These results confirm that neoadjuvant immunotherapy is an effective approach for locally advanced MSI/dMMR colorectal cancer in a real-world, community-based setting in Russia. Characteristics Patients Age Median age 70 (34-85) Female 59,4% Male 40.6% Left colon 11 (27%) Right colon 21 (56.5%) Rectum 5 (13,5%) Stage III 86,4% T3 11 (29.7%) T4 22 (59.5%) KRAS 4 (19%) BRAF 47.6%