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TPS870 Background: Approximately 96% of CRCs have an MSS phenotype, which results in more immunologically quiescent tumors for which immunotherapies are largely ineffective (Lee et al. 2015; Overman et al. 2016). Pts with MSS CRC and activating RAS mutation (35%–45% of CRCs) have treatment options limited still further because anti-EGFR monoclonal antibodies (eg, cetuximab) are ineffective owing to dominant activation of RAS in the MAPK pathway (Douillard et al. 2014). However, preclinical and preliminary clinical data suggest that MAPK pathway inhibition enhances antigen presentation and T-cell cytotoxicity to positively modulate the efficacy of checkpoint inhibitors (Brea et al. 2016; Bendell et al. 2014). The main objective of this open-label multicenter phase 1b/2 study is to evaluate whether the potential positive modulation of NIVO or NIVO plus IPI, when combined with BINI, translates into clinically meaningful overall response in pts with MSS mCRC and RAS mutation. Methods: The study will enroll ~90 previously treated pts (1 or 2 prior regimens), ~42 in phase 1b and ~48 in phase 2. The primary objective of phase 1b will be to determine the recommended phase 2 dose (RP2D) of BINI in combination with NIVO ± IPI. Dose finding in the doublet arm will begin with BINI 45 mg BID + NIVO 480 mg Q4W; the triplet arm will begin with the BINI RP2D from the doublet arm + NIVO 480 mg Q4W + IPI 1 mg/kg Q8W. In phase 2, pts will be randomized 1:1 to doublet or triplet arms, incorporating the BINI RP2Ds found in phase 1b; treatment will continue in 28-day cycles until disease progression, unacceptable toxicity, withdrawal of consent, initiation of subsequent anticancer therapy, loss to follow-up, or death. The primary objective for phase 2 will be to assess response by RECIST version 1.1. The study will also characterize safety and PK. CT.gov Identifier: Clinical trial information: NCT03271047.