Dual versus single immune checkpoint blockade for patients with metastatic colorectal cancer harboring microsatellite instability: The Saint-Antoine immunoMSI score.

168 Background: Anti-PD1 monotherapy (mono) and anti-PD1 plus anti-CTLA4 therapies (combo) are validated for metastatic colorectal cancer (mCRC) harboring microsatellite instability and/or mismatch repair deficiency (MSI/dMMR). We aimed at assessing the efficacy of combined anti-CTLA4 and anti-PD1 blockade versus anti-PD1 monotherapy and identifying which subgroups benefit more from the combo. Methods: All patients (pts) with MSI/dMMR mCRC treated by anti-PD1 ± anti-CTLA4 included in the prospective monocenter immunoMSI cohort were analyzed. The choice between monotherapy or combination therapy varied according to open studies and regulatory approvals, without taking pt characteristics into account. Datalock date was April 23, 2024. Main endpoints were progression-free survival (PFS) per iRECIST and overall survival (OS) estimated with the Kaplan-Meier method. Univariate Cox proportional hazards models were used to estimate hazard ratios with CI. The interactions between studied subgroups and the treatment group were modelized by adding in a single Cox model the studied subgroups, the treatment group parameters, and their interaction term. Results: Among 210 pts, 97 were treated combo and 113 with mono. Baseline characteristics were similar in both groups. 25% and 17% were in first line, respectively. Median follow-up was 4.4 years: 4.1 years for combo and 5.4 years for mono. Combo was associated with better PFS (HR=0.48; 95%CI 0.31-0.76) and OS (HR=0.49; 0.29-0.84). Best observed response rates were: 78% of objective responses, 17% of stable diseases and 5% disease progressions with combo versus 60%, 25% and 15% with mono. Subgroup analyses showed significant differential effect of the treatment group for gender (PFS HR=0.21 vs 0.87 for females and males, P-interaction: 0.0067), liver metastases (PFS HR=0.30 vs 0.79 for pts without liver mets and those with liver mets, P-interaction: 0.0479) and sidedness (PFS HR=0.27 vs 0.64 for left-sided and right-sided mCRC, P-interaction=0.0922), but not RAS / RAF mutations. We computed a predictive score based on gender (female: 2 points), no liver met (1 point) and left side (1 point). PFS HR was 0.1 (0.03-0.33) for the group ≥3 points (N=67), 0.66 (0.36-1.23) for the group with 1-2 points (N=113) and 1.54 (0.50-4.70) for the group with 0 point (N=30). Conclusions: Combined CTLA4 and PD1 blockade is associated with improved PFS and OS compared to anti-PD1 alone in MSI/dMMR mCRC. The subgroup deriving the highest benefit of the combo seems to be female pts with left sided tumor and no liver met. Data from the randomization between nivolumab versus nivolumab + ipilumumab in the CheckMate-8HW phase III study are expected.