First-line (1L) nivolumab (NIVO) plus ipilimumab (IPI) in patients (pts) with microsatellite instability-high/mismatch repair deficient (MSI-H/dMMR) metastatic colorectal cancer (mCRC): 64-month (mo) follow-up from CheckMate 142

3550 Background: NIVO + IPI demonstrated robust, durable clinical benefit, and was well tolerated as a 1L therapy in pts with MSI-H/dMMR mCRC in the phase 2 CheckMate 142 study (NCT02060188), leading to the inclusion of NIVO + IPI in the NCCN guidelines as an initial therapy option for these pts. At 52-mo median follow-up, 1L NIVO + IPI continued to demonstrate durable clinical benefit, and no new safety signals were identified. Here we report longer follow-up results. Methods: Pts with MSI-H/dMMR mCRC and no prior treatment for metastatic disease received NIVO 3 mg/kg Q2W + IPI 1 mg/kg Q6W until disease progression or unacceptable toxicity. The primary endpoint was objective response rate (ORR) by investigator assessment (INV) per RECIST v1.1. Other key endpoints were disease control rate (DCR), duration of response (DOR), progression-free survival (PFS), all by INV; overall survival (OS); and safety. Results: In total, 45 pts received 1L NIVO + IPI. With median follow-up of 64.2 mo (range, 59.4–68.9 mo), ORR by INV was 71% (95% CI, 56–84%). The proportion of pts with a best overall response of complete response (CR) was 20%, partial response (PR) was 51%, stable disease (SD) was 13%, and progressive disease was 16%. Median DOR (mDOR) was not reached, and the 60-mo DOR rate was 72%. Median PFS (mPFS) by INV and median OS (mOS) were not reached, with 60-mo PFS and OS rates of 55% and 67%, respectively (Table). Among pts alive at the data cutoff (n = 31), 30 remained treatment-free after initial study treatment without receiving any subsequent systemic therapy, with a median treatment-free interval of 34.7 mo (range, 1.6–61.4 mo). Exploratory analysis by tumor mutational burden status will be presented. Safety data are shown in the Table. Conclusions: At 64-mo follow-up, NIVO + IPI continued to demonstrate clinically meaningful survival and durable responses, with mPFS, mOS, and mDOR still not reached, suggesting the potential for long-term clinical benefit. Safety remained consistent with previous data. These findings further support current recommendations for NIVO + IPI as a 1L treatment for pts with MSI-H/dMMR mCRC. Clinical trial information: NCT02060188 . [Table: see text]