Correlation between metastatic site and immunotherapy efficacy in patients with advanced dMMR/MSI colorectal cancer: Real-World Italian multicentric retrospective study (Colon-MSI).

e15544 Background: Immune checkpoint inhibitors (ICIs) are the gold standard therapy in metastatic colorectal cancer (mCRC) patients (pts) with microsatellite instability (MSI). The Colon-MSI study aimed to describe a cohort of MSI mCRC pts treated with ICIs before Italian pembrolizumab approval and to evaluate the efficacy, safety, and possible predictive factors of response in a real-world setting. Methods: We retrospectively collected data of pts with MSI mCRC treated with ICIs before June 30, 2023, in 16 Italian Cancer Centers. The OS was calculated from the diagnosis of metastatic disease, and the PFS from the beginning of ICIs. We explored the correlation between clinicopathological factors, survival outcomes, and treatment response. Results: Between January 2021 and June 2023, we reported 83 consecutive pts with data available. Among all, the median age was 61 years (23-86), 42 (51%) were female, and 45 (54%) had a ECOG PS 0; 35 (42%) pts were at stage IV at diagnosis, and 61 (74%) had right colon tumor site. The most common metastatic sites were liver (35%), lymph nodes (43%), peritoneum (31%), and lung (15%); 48 (58%) pts had a single metastatic site. 33 (39%) pts had a mucinous adenocarcinoma, 32 (39%) BRAF mutated, 32 (39%) RAS mutated, and 26% presented a Lynch syndrome. The majority of pts (69%) were pre-treated, and 26 pts (31%) received ICIs in first line. Nivolumab alone or plus Ipilimumab, and Pembrolizumab, were administered in 23 (28%), 3 (4%), and 57 (68%) pts, respectively. The median follow-up was 23 (1-65) months (m) and the median duration of treatment was 23 m (0-65) with 27 pts still ongoing. Treatment-related severe toxicities (G3 or G4) were observed only in 5.6% of pts. The median OS and PFS were not reached; the mean OS was 45.6 m (IC 95% 39.6-51.6) and the mean PFS was 38.8 m (IC 95% 32.5-45.1). Complete response (CR) was observed in 20 pts (24%), partial response (PR) in 28 (34%), stable disease (SD) in 19 (23%) and progression disease (PD) in 16 (19%). Overall response (ORR) was observed in 48 pts (57.8%; 95 %IC 0.31- 0.53), and disease control rate in 67 pts (80.7%; 95% IC 0.11– 0.29). Lung metastatic site was associated with poorer outcome in terms of OS (p = 0.001, HR 3.88 IC 95% 1.7-8.9), PFS (p = 0.045 HR = 2.31 IC95% 1-5.2), and ORR (p = 0.015 OR = 6.19 IC95% 1.4-26.9). Significantly worse PFS was also observed in pts with liver metastasis (p = 0.009 HR = 2.5 IC 95% 1.3-4.9). Conclusions: Our data confirm that ICIs are effective and well-tolerated treatment in MSI mCRC pts, with long-term clinical outcomes reported. These results suggest an unfavorable correlation between lung and liver metastasis and the ICI response. Further biomolecular analysis will be provided in the future.